Owing to these attractive mechanical and electrical properties of FSG material, it played vital role in flexible device technologies based on electrochemical energy storage and generation, actuators, sensors, and catalysts. Based on the attractive graphene properties and its nomenclature, the graphene oxide has fascinating properties which has layered structure similar to graphene that containing oxygen functional groups such as carboxyl, hydroxyl and epoxy. These functional groups were highly dispersed in DI water; hence, it is well aligned over vacuum filtration process. The GO paper is peeled off after vacuum drying and subjected to reducing treatment, as synthesized FSG material is directly utilized as current collector in

Battery is an electrochemical energy storage device that is cost-effective and ecofriendly and with cyclic durability, excellent overall performance, and long-term stability. In this decade, lithium ion battery (LIB) is successfully commercialized worldwide for portable electronic devices, and it has approximately 200 kWh scale for transportation and stationary storage [43]. On comparison with other secondary-based batteries such as sodium sulfur, redox flow, Ni-Cd, etc., Li ion cells have gathered the most commercial interest because they provide high energy and power densities, respectively. In contrast, other secondary batteries are under development stage for consideration in commercial package over LIB due to its major drawback as follows: large scale storage, cost of materials, toxicity, cyclic performance, or stability issues. However, the better system in secondary batteries credited for LIB because the redox potential of −3.04 V vs. SHE (standard hydrogen electrode) for Li/Li+ which has high electropositive in periodic table and light weight material with small ionic radius. Henceforth, the charge-discharge rates enhance and power densities vary in the ranges of 500–2000 W/kg [44]. In commercialized LIBs, the existing negative electrode is a graphite-layered structure material coupled with the host material and LiCoO2 has positive electrodes. Similar to LIBs, the other systems were also focused since it lags behind to reach the theoretical specific capacity (400 Wh/kg) that requires for electric vehicles for long term usage. Hence, other kinds of secondary batteries have been discovered such as Li-sulfur, sodium-ion

*DOI: http://dx.doi.org/10.5772/intechopen.84275*

#### *Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario DOI: http://dx.doi.org/10.5772/intechopen.84275*

Recently, a rapid reduction treatment was proposed by immersing the GO papers in hydrohalic acids, viz., HI and HBr, which shows a remarkable electrical conductivity around 298 and 3220 S/cm, respectively [32, 42]. Based on the facile chemical treatment, the electrical conductivity of FSG improvement was shown by treating the GO papers in metal halides like MgI2, AlI3, ZnI2, and FeI2 that exhibit 550 S/cm [33].

Owing to these attractive mechanical and electrical properties of FSG material, it played vital role in flexible device technologies based on electrochemical energy storage and generation, actuators, sensors, and catalysts. Based on the attractive graphene properties and its nomenclature, the graphene oxide has fascinating properties which has layered structure similar to graphene that containing oxygen functional groups such as carboxyl, hydroxyl and epoxy. These functional groups were highly dispersed in DI water; hence, it is well aligned over vacuum filtration process. The GO paper is peeled off after vacuum drying and subjected to reducing treatment, as synthesized FSG material is directly utilized as current collector in place of Al, Cu, Ni foam, etc., for energy storage applications.

#### **4. Free-standing graphene electrodes for batteries**

Battery is an electrochemical energy storage device that is cost-effective and ecofriendly and with cyclic durability, excellent overall performance, and long-term stability. In this decade, lithium ion battery (LIB) is successfully commercialized worldwide for portable electronic devices, and it has approximately 200 kWh scale for transportation and stationary storage [43]. On comparison with other secondary-based batteries such as sodium sulfur, redox flow, Ni-Cd, etc., Li ion cells have gathered the most commercial interest because they provide high energy and power densities, respectively. In contrast, other secondary batteries are under development stage for consideration in commercial package over LIB due to its major drawback as follows: large scale storage, cost of materials, toxicity, cyclic performance, or stability issues. However, the better system in secondary batteries credited for LIB because the redox potential of −3.04 V vs. SHE (standard hydrogen electrode) for Li/Li+ which has high electropositive in periodic table and light weight material with small ionic radius. Henceforth, the charge-discharge rates enhance and power densities vary in the ranges of 500–2000 W/kg [44]. In commercialized LIBs, the existing negative electrode is a graphite-layered structure material coupled with the host material and LiCoO2 has positive electrodes. Similar to LIBs, the other systems were also focused since it lags behind to reach the theoretical specific capacity (400 Wh/kg) that requires for electric vehicles for long term usage. Hence, other kinds of secondary batteries have been discovered such as Li-sulfur, sodium-ion battery (SIB), sodium-sulfur, Li-air, Zn-air, and flow batteries.

#### **4.1 Li-ion battery**

Conventionally, LIBs are made up of graphite anode and LiCoO2 layered material as cathode sandwiched between LiPF6 (1.0 mol/L) as an organic electrolyte dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) in 1:1 volume ratio [45]. While LIB is charging, deintercalation happens at cathode, where the Li ions are removed from the layered LiCoO2 by releasing electrons to cathode. The released Li ions are transported to anode with the help of the electrolyte system and finally intercalated into graphite by gaining electrons. The same process is reversed during the discharging process.

Designing of anode materials for LIBs has focused much attention on retaining large reversible specific capacity. Beyond the graphite anode, few metal oxides and

*Energy Storage Devices*

**98**

**Figure 3.**

restoration of sp2

the two sheets as GO has several functional groups that covalently attached to other molecules [36, 37]. The intercalation, functionalization, and interaction between the GO sheets provide high mechanical stiffness for paper-like material. Moreover, the atmospheric humidity affects the mechanical property of the GO paper, increase in the relative humidity to 100%, the GO colloidal solution absorbs water from moisture and it bulges to 70% which decreases the tensile strength [34]. The functionalization on graphene surface also affects the mechanical properties depending on the functional moieties as well as the bonding nature [38–40]. The electrical properties of GO papers depend on the synthetic methods as several changes were observed in structures and reduction ratios of C/O. Upon exposing to the hydrazine vapor, the conductivity of GO papers increased by four order of magnitude from 8.5 × 10−4 to 170 S/cm. Further enhancement in conductivities of GO paper was developed by treating the paper with mixture of argon/hydrogen/hydrazine vapors [41]. The

*(a) Photograph of flexible graphene oxide paper, (b) comparison chart of mechanical properties of GO paper with other flexible paper materials, (c) effect of FSG electrical conductivity changes w.r.t its properties upon HI treatment in different scale of time, and (d) electrical conductivity versus the Raman and XPS data of GO*

chemical or thermal treatment. The chemical reductive treatment efficiently removes the oxygen moieties from the GO paper, whereas the thermal treatment shows high

carbon network but less removal of oxygen functional groups.

carbon network by

removal of the oxygen group is the main factor to restore the sp2

*paper reduced by different metal halides (without permission from Refs. [31–33]).*

metal alloys were developed as anode material, and the lithiation and delithiation processes were investigated. Specifically, FSG paper outpaces the other candidates such as carbon nanotube (CNT) paper or graphite foil due to their tremendous properties as discussed earlier. Importantly, the electrical and mechanical properties of FSG are potentially applied for flexible device application. However, the FSG electrode itself does not provide higher capacity (approximately 100 mAh/g), which is not applicable as anode in LIB; instead, it has good cycling stability. Therefore, the host material that has high electrochemical active sites is incorporated into FSG for improvement of capacity in the device. This extends the large volume expansion in FSG electrodes for an efficient Li ions intercalation. One of the advantages of this FSG hybrid electrode is that it excludes the nonconducting polymer binders as additives. Conventional electrode-based materials were obtained as powders and coated on the metal foils in the form of ink using additives like polymer binders and conducting additive, whereas the FSG hybrid electrode plays dual role as a current collector and conductive additive.

In 2005, LIBs were fabricated with free-standing electrode based on CNTs prepared by vacuum filtration method [46]. Significantly, the free-standing electrode fabrication is a facile route in comparison with the conventional electrode since the mixture of active material, polymer binder, and conductive additive in solvent coated on metal foils. The CNT free-standing electrode provides reversible discharge capacity of 200 mAh/g at 0.08 mA/cm2 . Further, the specific capacity was enhanced by the CVD grown free-standing CNT that delivers 572 mAh/g at 0.2 mA/cm2 [47]. This is a quite interesting result obtained for free-standing electrodes rather than the conventional electrodes. Meanwhile, the usage of high-cost material CNTs as free-standing electrodes lags behind manufacturing process. From this point of view, inexpensive material graphene prepared by chemical methods provides large scale production as dispersion in many solvents. This dispersion is readily subjected to vacuum filtration to prepare FSG paper with desired thickness. Usually, the discharge capacity of 298 mAh/g decreased to 240 mAh/g after 50 cycles for graphite electrodes with 81% retention capacity. But the FSG paper itself as anode provides huge irreversible discharge capacity, i.e., 680 mAh/g at initial cycle dropped to 84 mAh/g second cycle. The retention capacity is very poor compared to graphite electrode and therefore it is concluded to be not a suitable candidate for anode material [48]. This helps infer that solid electrolyte interface (SEI) formation is a significant parameter to reduce the storage capacity in FSG electrodes.

To potentially apply FSG as anode material in LIBs, the second phase material with highly electrochemical active sites should be composited to enhance the capacity. In this regard, Lee et al. composited Si NPs on GO sheets, vacuum filtered, and followed by thermal treatment to produce FSG/Si nanoparticle (NP) paper. This work delivers high Li ion storage when compared to pristine FSG electrodes. Si NPs intercalated between the graphene sheets of FSG paper that facilitates good 3D graphite-like framework and provides high Li ion storage even at high current density [49]. Another work has been reported with similar hybrid FSG/Si NPs, whereas a facile route has been introduced to fabricate. The specific capacity of 708 mAh/g was observed without any loss even after 100 cycles and this is mainly due to the larger volume change in graphene-Si composite. It also denotes the performance of device with an efficient electron and charge transfer contributed by graphene sheets that minimize the internal resistance of the electrodes [50]. Zhang et al. prepared Si hollow nanosheets using Mg as template and connected with graphene sheets to obtain free-standing electrodes by layer-by-layer method followed by HI reduction treatment. The specific capacity was examined during flat and bent state, which delivers similar results without any loss. Remarkably, Si/FSG paper anodes

**101**

efficient Li ion storage.

lower capacity of 43 mAh/cm3

shows higher volumetric capacity of 260 mAh/cm3

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario*

retain high reversible capacities even at long cycles, which reveals their retention capacity. They exhibit specific capacity of 660 mAh/g at 0.2 A/g current density after 150 cycles with 99% coulombic efficiency [51]. As mentioned earlier, all the Si NPs are highly expensive in terms of manufacturing process and hence a low cost method plays a significant factor. To tackle this issue, Cai et al. prepared Si NPs on CNT surface using low-cost Al-Si alloy as starting material and further inserted with graphene sheets to form a self-standing hybrid anodes for LIBs. Comparing with bare Si/CNT or Si/Graphene anodes, Si-CNT/FSG hybrid electrode, it delivers 1100 mAh/g at 0.2 A/g current density after 100 cycles. Addition of CNT was involved to disperse the Si NPs on the surface and provide network between the graphene sheets for conductivity enhancement as well as improved Li ion intercala-

Metal oxides (MOs) play an important role in LIBs as anode material and their poor conductivity restricts their application. Hence, introducing the conductive phase into MOs provides high retention capacity with long-life cycling stability. The theoretical reversible capacity of SnO2 is 782 mAh/g and its poor performance is due to low cycling with serious volume expansion. With this regard, SnO2 NPs dispersed on GO surface, followed by vacuum filtration to obtain free-standing electrodes and used as two different LIB anodes by thermally reduced and chemically reduced respectively [53, 54]. The specific capacity of 438.5 mAh/g at 0.1 A/g and 700 mAh/g at 0.2 A/g has been delivered for the two different reduction methods for SnO2 NPs/FSG electrodes. In both the cases, capacity fading is not observed even after the 50 cycles owing to the good anchoring of SnO2 and graphene sheets. Further, other metal oxides TiO2, Mn3O4, Fe3O4, and CuO nanostructured materials are incorporated into the FSG and are investigated for their performance in anode application for LIBs that delivers 269 mAh/g at 0.2 A/g, 692 mAh/g at 0.05 A/g, 544 mAh/g at 10 A/g, and 698.7 mAh/g at 0.67 A/g capacities, respectively [55–59]. Commonly, all these metal oxides' specific capacity shows a reasonable capacity with the long-life cycling after incorporating the MOs into FSG electrodes due to the following aspects: (1) Interaction of GO and MO precursors increases, which enhances the well dispersive growth of MO NPs on graphene sheets. (2) Anchoring of MOs and graphene enhances the volume expansion/contraction for lithiation/delithiation process. (3) The cycling stability increases compared to pristine MO anodes even after several cycles owing to its structural phase remain stable after alloying/de-alloying process of lithium ions. (4) MOs avoid the aggregation of graphene stacking that leads to larger void space to penetrate the electrolyte and make a strong interface with the electrochemical active MOs for an

Further, with the controlled synthesis of oxygen, functionalized CNT/FSG electrodes were fabricated for anode application in LIBs. The battery performance is based on the oxygen functional groups in the electrodes that have been investigated. An optimization in weight ratios of CNT/FSG and heat treatment improves the volumetric and gravimetric capacitances. The CNT/GO hybrid at a ratio of 1:1

current densities, the role of oxygen in capacity role suppress for 200°C larger than the 900°C [60]. This implies the importance of CNT intercalation between the graphene sheets of FSG electrodes. Zhang et al. demonstrated the defect-rich MoS2 NSs/graphene/CNT hybrid paper as anode material for LIBs. In this design, MoS2 facilitates the lithium ion storage due to the high active sites at the edges and the electrical conductivity improved by the network of CNTs attached to the graphene sheets. In addition to the conductivity enhancement, the porosity of the FSG electrodes increased by the network of CNT sandwiched graphene sheets. On the

that reduced at 200°C, while

for 900°C treated CNT/GO. Whereas, at high

*DOI: http://dx.doi.org/10.5772/intechopen.84275*

tion for efficient charge transfer [52].

#### *Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario DOI: http://dx.doi.org/10.5772/intechopen.84275*

retain high reversible capacities even at long cycles, which reveals their retention capacity. They exhibit specific capacity of 660 mAh/g at 0.2 A/g current density after 150 cycles with 99% coulombic efficiency [51]. As mentioned earlier, all the Si NPs are highly expensive in terms of manufacturing process and hence a low cost method plays a significant factor. To tackle this issue, Cai et al. prepared Si NPs on CNT surface using low-cost Al-Si alloy as starting material and further inserted with graphene sheets to form a self-standing hybrid anodes for LIBs. Comparing with bare Si/CNT or Si/Graphene anodes, Si-CNT/FSG hybrid electrode, it delivers 1100 mAh/g at 0.2 A/g current density after 100 cycles. Addition of CNT was involved to disperse the Si NPs on the surface and provide network between the graphene sheets for conductivity enhancement as well as improved Li ion intercalation for efficient charge transfer [52].

Metal oxides (MOs) play an important role in LIBs as anode material and their poor conductivity restricts their application. Hence, introducing the conductive phase into MOs provides high retention capacity with long-life cycling stability. The theoretical reversible capacity of SnO2 is 782 mAh/g and its poor performance is due to low cycling with serious volume expansion. With this regard, SnO2 NPs dispersed on GO surface, followed by vacuum filtration to obtain free-standing electrodes and used as two different LIB anodes by thermally reduced and chemically reduced respectively [53, 54]. The specific capacity of 438.5 mAh/g at 0.1 A/g and 700 mAh/g at 0.2 A/g has been delivered for the two different reduction methods for SnO2 NPs/FSG electrodes. In both the cases, capacity fading is not observed even after the 50 cycles owing to the good anchoring of SnO2 and graphene sheets. Further, other metal oxides TiO2, Mn3O4, Fe3O4, and CuO nanostructured materials are incorporated into the FSG and are investigated for their performance in anode application for LIBs that delivers 269 mAh/g at 0.2 A/g, 692 mAh/g at 0.05 A/g, 544 mAh/g at 10 A/g, and 698.7 mAh/g at 0.67 A/g capacities, respectively [55–59]. Commonly, all these metal oxides' specific capacity shows a reasonable capacity with the long-life cycling after incorporating the MOs into FSG electrodes due to the following aspects: (1) Interaction of GO and MO precursors increases, which enhances the well dispersive growth of MO NPs on graphene sheets. (2) Anchoring of MOs and graphene enhances the volume expansion/contraction for lithiation/delithiation process. (3) The cycling stability increases compared to pristine MO anodes even after several cycles owing to its structural phase remain stable after alloying/de-alloying process of lithium ions. (4) MOs avoid the aggregation of graphene stacking that leads to larger void space to penetrate the electrolyte and make a strong interface with the electrochemical active MOs for an efficient Li ion storage.

Further, with the controlled synthesis of oxygen, functionalized CNT/FSG electrodes were fabricated for anode application in LIBs. The battery performance is based on the oxygen functional groups in the electrodes that have been investigated. An optimization in weight ratios of CNT/FSG and heat treatment improves the volumetric and gravimetric capacitances. The CNT/GO hybrid at a ratio of 1:1 shows higher volumetric capacity of 260 mAh/cm3 that reduced at 200°C, while lower capacity of 43 mAh/cm3 for 900°C treated CNT/GO. Whereas, at high current densities, the role of oxygen in capacity role suppress for 200°C larger than the 900°C [60]. This implies the importance of CNT intercalation between the graphene sheets of FSG electrodes. Zhang et al. demonstrated the defect-rich MoS2 NSs/graphene/CNT hybrid paper as anode material for LIBs. In this design, MoS2 facilitates the lithium ion storage due to the high active sites at the edges and the electrical conductivity improved by the network of CNTs attached to the graphene sheets. In addition to the conductivity enhancement, the porosity of the FSG electrodes increased by the network of CNT sandwiched graphene sheets. On the

*Energy Storage Devices*

collector and conductive additive.

at 0.2 mA/cm2

FSG electrodes.

discharge capacity of 200 mAh/g at 0.08 mA/cm2

metal alloys were developed as anode material, and the lithiation and delithiation processes were investigated. Specifically, FSG paper outpaces the other candidates such as carbon nanotube (CNT) paper or graphite foil due to their tremendous properties as discussed earlier. Importantly, the electrical and mechanical properties of FSG are potentially applied for flexible device application. However, the FSG electrode itself does not provide higher capacity (approximately 100 mAh/g), which is not applicable as anode in LIB; instead, it has good cycling stability. Therefore, the host material that has high electrochemical active sites is incorporated into FSG for improvement of capacity in the device. This extends the large volume expansion in FSG electrodes for an efficient Li ions intercalation. One of the advantages of this FSG hybrid electrode is that it excludes the nonconducting polymer binders as additives. Conventional electrode-based materials were obtained as powders and coated on the metal foils in the form of ink using additives like polymer binders and conducting additive, whereas the FSG hybrid electrode plays dual role as a current

In 2005, LIBs were fabricated with free-standing electrode based on CNTs prepared by vacuum filtration method [46]. Significantly, the free-standing electrode fabrication is a facile route in comparison with the conventional electrode since the mixture of active material, polymer binder, and conductive additive in solvent coated on metal foils. The CNT free-standing electrode provides reversible

was enhanced by the CVD grown free-standing CNT that delivers 572 mAh/g

electrodes rather than the conventional electrodes. Meanwhile, the usage of high-cost material CNTs as free-standing electrodes lags behind manufacturing process. From this point of view, inexpensive material graphene prepared by chemical methods provides large scale production as dispersion in many solvents. This dispersion is readily subjected to vacuum filtration to prepare FSG paper with desired thickness. Usually, the discharge capacity of 298 mAh/g decreased to 240 mAh/g after 50 cycles for graphite electrodes with 81% retention capacity. But the FSG paper itself as anode provides huge irreversible discharge capacity, i.e., 680 mAh/g at initial cycle dropped to 84 mAh/g second cycle. The retention capacity is very poor compared to graphite electrode and therefore it is concluded to be not a suitable candidate for anode material [48]. This helps infer that solid electrolyte interface (SEI) formation is a significant parameter to reduce the storage capacity in

To potentially apply FSG as anode material in LIBs, the second phase material with highly electrochemical active sites should be composited to enhance the capacity. In this regard, Lee et al. composited Si NPs on GO sheets, vacuum filtered, and followed by thermal treatment to produce FSG/Si nanoparticle (NP) paper. This work delivers high Li ion storage when compared to pristine FSG electrodes. Si NPs intercalated between the graphene sheets of FSG paper that facilitates good 3D graphite-like framework and provides high Li ion storage even at high current density [49]. Another work has been reported with similar hybrid FSG/Si NPs, whereas a facile route has been introduced to fabricate. The specific capacity of 708 mAh/g was observed without any loss even after 100 cycles and this is mainly due to the larger volume change in graphene-Si composite. It also denotes the performance of device with an efficient electron and charge transfer contributed by graphene sheets that minimize the internal resistance of the electrodes [50]. Zhang et al. prepared Si hollow nanosheets using Mg as template and connected with graphene sheets to obtain free-standing electrodes by layer-by-layer method followed by HI reduction treatment. The specific capacity was examined during flat and bent state, which delivers similar results without any loss. Remarkably, Si/FSG paper anodes

[47]. This is a quite interesting result obtained for free-standing

. Further, the specific capacity

**100**

whole, the binder-free and substrate-free hybrid anode papers deliver high reversible capacity of 1137.2 mAh/g at 0.1 A/g current density with good cycling stability [61]. This framework induces a novel pathway to incorporate other host materials to understand the CNT/FSG electrodes. Recently, several transition metal oxides provide high reversible theoretical capacities compared with the commercialized graphite anode. To the CNT/FSG electrode network, transition metal oxides such as Fe2O3 [62], CuO [63], MnO [64], and CoSnO3 [65] were incorporated as electrochemical active phase into the framework and investigated as anode material performance for LIBs. All these hybrid papers exhibit high reversible capacity of 716 and 600 mAh/g at 0.5 A/g current density more than 50 cycles for Fe2O3 and CuO nanobox, respectively. Apart from this, an enhanced capacity was observed for CoSnO3 and MnO NPs at high current density of 2 A/g, which delivers 676 and 530 mAh/g, respectively. Individually, the CNT/FSG and transition metal oxide anodes were found to have a drastic decrease of specific capacity upon increasing the current density, whereas a slight decrease of specific capacity was observed after hosting the metal oxides into CNT/FSG framework. Reasons for high reversible capacity and good cyclic stability of metal oxide-CNT/FSG electrodes are very similar due to the following merits: (1) incorporation of metal oxides improves the Li ion kinetics and enhances the charge transfer due to highly conductive CNT network between the graphene sheets; (2) 3D framework of CNT/FSG has highly porous nature, large specific surface area, and large volume change, which has well dispersion of metal oxide NPs onto the carbon surfaces; and (3) long cycling due to good attachment of metal oxide with CNT/FSG, whereas greater the volume expansion, higher the Li ion intercalation.

Interestingly, Cao et al. designed a unique layered nanostructure of porous ternary ZnCo2O4 on graphene sheets and fabricated as flexible anode and investigated its electrochemical performance. And also they constructed full cell with LiFePO4 as cathode material that deposited on FSG paper as slurry by homogenous mixing of conductive additive and polymer binder [66]. **Figure 4a** shows the photograph of flexible Li-ion battery fabricated by FSG hybrid electrodes. The half-cell of ZnCo2O4/FSG anode delivers higher specific capacity of 791 mAh/g at 1 A/g after 1000 cycles with 97.3% of capacity retention and concludes that it has an excellent cycling stability. **Figure 4b** shows the rate capability of the flexible battery with different current densities ranging from 0.5 to 10 C. This full cell delivers 40 mAh/g even at 10 C rate and the specific capacitance remains the same after the current density decreased to 2 C, which shows a good reversibility. The full cell has FSG paper as current collector for both the anode and cathode that are composited with ZnCo2O4 and LiFePO4 as host materials, respectively. It operates at 2 V with initial charge of 143 mAh/g and coulombic efficiency of 97.2%, which is comparable to existing LIB. The specific capacity is maintained at 90 mAh/g with high capacity retention under flat and bent states over 100 cycling process, which implies the flexibility of the device as shown in **Figure 4d**. It represents that graphene conductivity is unchanged while bending the device.

#### **4.2 Sodium-ion battery**

Ahead of LIBs, SIBs have attracted the research community as the resources of Na are inexhaustible across the globe. In comparison with LIBs, the redox potential is −2.71 V vs. SHE and only the radius is 55% larger than the Li ions. Larger radius influences to focus on suitable material for insertion/extraction of Na ions effectively. The researchers focused on developing an efficient anode material for SIBs that involves carbon-based families and Na intermetallic compounds. The first cycle-specific capacity of sodium-antimony and sodium-phosphorous shows 600 and 2596 mAh/g,

**103**

**Figure 4.**

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario*

respectively [67–69]. Specific capacities drop after first cycles due to the internal cracking in the electrodes upon Na ion insertion. It leads to hinder the electrical properties and dissolution of electrode materials to electrolyte. The hard carbon with large interlayer distance that functions as anode material for SIBs and delivers more

*(a) Photograph of flexible full cell Li-ion battery with FSG/ZnCo2O4 as anode and FSG/LiFePO4 as cathode, (b) charge-discharge curve of full cell at 0.5 C rate, (c) charge-discharge rate capability at different rates, and (d) capacity variation on flat and bent state during cycling at 2 C rate (without permission from Ref. [66]).*

The porous nature and structure of the FSG could facilitate the accommodation of host materials such as transition metal chalcogenides (TMCs), which are electrochemically active for the Na ions for alloying process. David et al. reported that the MoS2/FSG composite papers exhibit an excellent cyclic stability with high reversible capacity of 338 mAh/g at 0.025 A/g. It is the first report and opens the pathway to apply free-standing electrodes for SIB anode [70]. The cyclic stability was enhanced in flower-like MoS2 incorporated on graphene foam prepared by onestep microwave-assisted synthesis. It offers stable capacity of 290 mAh/g at 0.1 A/g after 50 cycles compared to previous MoS2/FSG electrode. The cycling performance is enhanced due to highly conductive 3D graphene foam and well-dispersed MoS2, which shields as well as avoids the strain during the sodiation/desodiation process at anode [71]. With the significance of MoS2 TMC for SIB anodes, further investigation was followed by incorporating other TMCs such as WS2 and Co0.85Se into FSG [72, 73]. As mentioned in LIBs, the electrochemical behavior can be increased by introducing the heteroatoms into the graphene sheets. Heteroatom-doped FSG electrode performance was investigated for SIB anode, where the nitrogen improves the electronic conductivity and fluorine expands the interlayer for an efficient accommodation of Na ions. This delivers a reversible capacity of 56.3 mAh/g at 1 A/g for 5000 cycles. It indicates that the doping of heteroatoms enhances the cycling stability of SIB anodes. **Figure 5a** shows the discharge/charge profile before and after the bent state, which remains with the same capacity at current density

than 200 mAh/g of capacity even after 100 cycles was reported elsewhere.

*DOI: http://dx.doi.org/10.5772/intechopen.84275*

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario DOI: http://dx.doi.org/10.5772/intechopen.84275*

**Figure 4.**

*Energy Storage Devices*

ion intercalation.

is unchanged while bending the device.

**4.2 Sodium-ion battery**

whole, the binder-free and substrate-free hybrid anode papers deliver high reversible capacity of 1137.2 mAh/g at 0.1 A/g current density with good cycling stability [61]. This framework induces a novel pathway to incorporate other host materials to understand the CNT/FSG electrodes. Recently, several transition metal oxides provide high reversible theoretical capacities compared with the commercialized graphite anode. To the CNT/FSG electrode network, transition metal oxides such as Fe2O3 [62], CuO [63], MnO [64], and CoSnO3 [65] were incorporated as electrochemical active phase into the framework and investigated as anode material performance for LIBs. All these hybrid papers exhibit high reversible capacity of 716 and 600 mAh/g at 0.5 A/g current density more than 50 cycles for Fe2O3 and CuO nanobox, respectively. Apart from this, an enhanced capacity was observed for CoSnO3 and MnO NPs at high current density of 2 A/g, which delivers 676 and 530 mAh/g, respectively. Individually, the CNT/FSG and transition metal oxide anodes were found to have a drastic decrease of specific capacity upon increasing the current density, whereas a slight decrease of specific capacity was observed after hosting the metal oxides into CNT/FSG framework. Reasons for high reversible capacity and good cyclic stability of metal oxide-CNT/FSG electrodes are very similar due to the following merits: (1) incorporation of metal oxides improves the Li ion kinetics and enhances the charge transfer due to highly conductive CNT network between the graphene sheets; (2) 3D framework of CNT/FSG has highly porous nature, large specific surface area, and large volume change, which has well dispersion of metal oxide NPs onto the carbon surfaces; and (3) long cycling due to good attachment of metal oxide with CNT/FSG, whereas greater the volume expansion, higher the Li

Interestingly, Cao et al. designed a unique layered nanostructure of porous ternary ZnCo2O4 on graphene sheets and fabricated as flexible anode and investigated its electrochemical performance. And also they constructed full cell with LiFePO4 as cathode material that deposited on FSG paper as slurry by homogenous mixing of conductive additive and polymer binder [66]. **Figure 4a** shows the photograph of flexible Li-ion battery fabricated by FSG hybrid electrodes. The half-cell of ZnCo2O4/FSG anode delivers higher specific capacity of 791 mAh/g at 1 A/g after 1000 cycles with 97.3% of capacity retention and concludes that it has an excellent cycling stability. **Figure 4b** shows the rate capability of the flexible battery with different current densities ranging from 0.5 to 10 C. This full cell delivers 40 mAh/g even at 10 C rate and the specific capacitance remains the same after the current density decreased to 2 C, which shows a good reversibility. The full cell has FSG paper as current collector for both the anode and cathode that are composited with ZnCo2O4 and LiFePO4 as host materials, respectively. It operates at 2 V with initial charge of 143 mAh/g and coulombic efficiency of 97.2%, which is comparable to existing LIB. The specific capacity is maintained at 90 mAh/g with high capacity retention under flat and bent states over 100 cycling process, which implies the flexibility of the device as shown in **Figure 4d**. It represents that graphene conductivity

Ahead of LIBs, SIBs have attracted the research community as the resources of Na are inexhaustible across the globe. In comparison with LIBs, the redox potential is −2.71 V vs. SHE and only the radius is 55% larger than the Li ions. Larger radius influences to focus on suitable material for insertion/extraction of Na ions effectively. The researchers focused on developing an efficient anode material for SIBs that involves carbon-based families and Na intermetallic compounds. The first cycle-specific capacity of sodium-antimony and sodium-phosphorous shows 600 and 2596 mAh/g,

**102**

*(a) Photograph of flexible full cell Li-ion battery with FSG/ZnCo2O4 as anode and FSG/LiFePO4 as cathode, (b) charge-discharge curve of full cell at 0.5 C rate, (c) charge-discharge rate capability at different rates, and (d) capacity variation on flat and bent state during cycling at 2 C rate (without permission from Ref. [66]).*

respectively [67–69]. Specific capacities drop after first cycles due to the internal cracking in the electrodes upon Na ion insertion. It leads to hinder the electrical properties and dissolution of electrode materials to electrolyte. The hard carbon with large interlayer distance that functions as anode material for SIBs and delivers more than 200 mAh/g of capacity even after 100 cycles was reported elsewhere.

The porous nature and structure of the FSG could facilitate the accommodation of host materials such as transition metal chalcogenides (TMCs), which are electrochemically active for the Na ions for alloying process. David et al. reported that the MoS2/FSG composite papers exhibit an excellent cyclic stability with high reversible capacity of 338 mAh/g at 0.025 A/g. It is the first report and opens the pathway to apply free-standing electrodes for SIB anode [70]. The cyclic stability was enhanced in flower-like MoS2 incorporated on graphene foam prepared by onestep microwave-assisted synthesis. It offers stable capacity of 290 mAh/g at 0.1 A/g after 50 cycles compared to previous MoS2/FSG electrode. The cycling performance is enhanced due to highly conductive 3D graphene foam and well-dispersed MoS2, which shields as well as avoids the strain during the sodiation/desodiation process at anode [71]. With the significance of MoS2 TMC for SIB anodes, further investigation was followed by incorporating other TMCs such as WS2 and Co0.85Se into FSG [72, 73]. As mentioned in LIBs, the electrochemical behavior can be increased by introducing the heteroatoms into the graphene sheets. Heteroatom-doped FSG electrode performance was investigated for SIB anode, where the nitrogen improves the electronic conductivity and fluorine expands the interlayer for an efficient accommodation of Na ions. This delivers a reversible capacity of 56.3 mAh/g at 1 A/g for 5000 cycles. It indicates that the doping of heteroatoms enhances the cycling stability of SIB anodes. **Figure 5a** shows the discharge/charge profile before and after the bent state, which remains with the same capacity at current density

#### **Figure 5.**

*(a) Discharge/charge profile of heteroatoms (N and F)-doped FSG electrode at bent and normal state for SIBs. (inset: The photograph of FSG pouch cell illuminated with LED), (b) comparison of specific capacity and coulombic efficiency of bare FSG and N-doped FSG for Li-S battery. Cross-sectional SEM images of (c) discharged and (d) re-charged macroporous FSG electrodes (without permission from Refs. [74–76]).*

of 0.05 A/g. It reveals the mechanical strength of the FSG electrodes that is suitable to fabricate flexible pouch cell [74]. Even though the above said materials show an excellent cyclic stability, still it is necessary to improve the specific capacity of SIBs. It is well known that Na3P has theoretical capacity of 2600 mAh/g, where its demerits are very similar to those of Si electrode in LIBs. Because of high pulverization, fast capacity fading and also it hinders the electrical contact which lags behind in the electrochemical stability. Lots of effort have been made by assembling red P into carbon matrix to overcome these problems. Red P was composited on carbon nanofibers (CNFs) and dipped in GO solution followed by HI treatment providing P-CNF/FSG electrodes. In this architecture, CNF network enhances the pathway of electron transport rapidly and the role of graphene sheets to improve the conductivity as well as to avoid the breakup of bonds P–P from electrodes. This work demonstrates a significant capacity of 406.6 mAh/g at 1 A/g after 180 cycles [77]. Moreover, the graphene sheets have been utilized as a multifunctional conductive binder, and hard carbon/FSG as anodes for SIBs was constructed. It delivers high reversible capacity of 372.4 mAh/g and shows capacity retention of 90% over 200 cycling. A superior performance is observed in the absence of PVDF binder with higher rate capabilities and converting the rigid nature of hard carbon into flexible graphene sheets [78].

#### **4.3 Li-S battery**

Akin to SIBs, FSG electrodes play a major role in other rechargeable secondary batteries such as Li-S, Li-air, and Zn-air. The higher specific energy is a significant parameter for transportation and stationary applications, and in that case, Li-S batteries offer advantages but it is limited with few challenges discussed later. The

**105**

**4.4 Metal-air battery**

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario*

change and thus improves the cycling stability of Li-S battery [82].

Recently, metal-air batteries have inspired much attention apart from the above said battery systems due to their high theoretical capacity than the metal-ion and Li-S batteries. The metal-air batteries can be operated in aqueous or nonaqueous medium based on the selection of metals. The nonaqueous medium is well suited for the Li-air batteries that deliver high capacity than in aqueous medium but still there are some issues when it comes to the practical application. The development of cathode in Li-air is significant as it is the main compartment to breathe oxygen

highest theoretical capacity of Li-S system is 2600 Wh/kg, which is highest than the LIB due to highest capacity of Li-S cathode sulfur has 1675 mAh/g. The most challenging part is to improve the electronic conductivity of cathodes of Li-S as the sulfur exhibits poor conductivity of 10–17 S/cm as well as the formation of polysulfides at cathodes. These polysulfides oxidize the Li anode and get back to cathodes and re-oxidize, thus lowering the performance of Li-S system. An extensive effort has been made to improve the cathodes by incorporating the carbon additives to sulfur to minimize the unnecessary reactions. Initially, mesoporous FSG was prepared and the sulfur was deposited by vapor treatment and was utilized as cathodes for Li-S system. It delivers charging capacity of 1288 mAh/g with high coulombic efficiency that reveals the restriction of sulfur to dissolute polysulfides in mesoporous FSG framework [79]. Similar to LIB and SIBs, the electrochemical behavior of cathode in Li-S system enhanced for heteroatom-doped FSG electrodes. **Figure 5b** shows the comparison of FSG and N-doped FSG capacity and coulombic efficiency with different cycle number. The heteroatom-doped FSG shows superior performance than the bare FSG due to the high interaction of polysulfides with heteroatoms that increase specific capacity. The nitrogen doping effect in FSG minimizes the concentration of polysulfides and forms a uniform layer of Li2S at cathode. This system delivers 1000 mAh/g at 0.335 A/g after 100 cycles [75]. In another work, Zhu et al. developed free-standing cathodes by CNTs that were interconnected with the sulfur-graphene walls and investigated the electrochemical behavior that delivers 1346 mAh/g at 0.17 A/g current density. It is due to sulfur at graphene walls that deals to provide dual response as follows: (i) hinder the dissolution of polysulfides minimizing the shuttle phenomenon and (ii) offer volume expansion even at high quantity of sulfur. Moreover, its capacity retention shows 40% when current density is increased to 16.7 A/g owing to the good electron pathway by CNTs connected with graphene nanosheets [80]. Further, nanosized Li2S (25–50 nm) particles incorporated into FSG papers by vacuum filtration process demonstrated an excellent cycling and rate capability with reversible capacity of 816.1 mAh/g at 0.1675 A/g (150 cycles) and 597 mAh/g at 11.7 A/g (200 cycles). This shows excellent performance in electrochemical behavior due to the uniform distribution of Li2S particles on graphene sheets that minimize the barrier for Li ion transport and particularly it has superior wetting nature to interconnect the polysulfides with graphene network into the paper electrodes [81]. Similarly, Chen et al. designed an efficient hierarchical nanostructure like nanobundled forest with Li2S/few-walled CNTs at FSG obtained solution processing followed by self-assembly method as cathodes. In this design, CNTs assembled in shaft-like structure and Li2S as active material, whereas the graphene sheets act as barrier for Li2S. It achieves high capacity of 868 and 433 mAh/g at current density of 335 and 16.7 A/g, respectively. This originates from the good framework between CNTs and graphene sheets as well as the uniform distribution of Li2S, and moreover, the barrier of graphene sheets for Li2S reduces the dissolution of polysulfides. Overall, the influence of void space enhances the volume

*DOI: http://dx.doi.org/10.5772/intechopen.84275*

#### *Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario DOI: http://dx.doi.org/10.5772/intechopen.84275*

*Energy Storage Devices*

of 0.05 A/g. It reveals the mechanical strength of the FSG electrodes that is suitable to fabricate flexible pouch cell [74]. Even though the above said materials show an excellent cyclic stability, still it is necessary to improve the specific capacity of SIBs. It is well known that Na3P has theoretical capacity of 2600 mAh/g, where its demerits are very similar to those of Si electrode in LIBs. Because of high pulverization, fast capacity fading and also it hinders the electrical contact which lags behind in the electrochemical stability. Lots of effort have been made by assembling red P into carbon matrix to overcome these problems. Red P was composited on carbon nanofibers (CNFs) and dipped in GO solution followed by HI treatment providing P-CNF/FSG electrodes. In this architecture, CNF network enhances the pathway of electron transport rapidly and the role of graphene sheets to improve the conductivity as well as to avoid the breakup of bonds P–P from electrodes. This work demonstrates a significant capacity of 406.6 mAh/g at 1 A/g after 180 cycles [77]. Moreover, the graphene sheets have been utilized as a multifunctional conductive binder, and hard carbon/FSG as anodes for SIBs was constructed. It delivers high reversible capacity of 372.4 mAh/g and shows capacity retention of 90% over 200 cycling. A superior performance is observed in the absence of PVDF binder with higher rate capabilities and converting the rigid nature of hard carbon into flexible

*(a) Discharge/charge profile of heteroatoms (N and F)-doped FSG electrode at bent and normal state for SIBs. (inset: The photograph of FSG pouch cell illuminated with LED), (b) comparison of specific capacity and coulombic efficiency of bare FSG and N-doped FSG for Li-S battery. Cross-sectional SEM images of (c) discharged and (d) re-charged macroporous FSG electrodes (without permission from Refs. [74–76]).*

Akin to SIBs, FSG electrodes play a major role in other rechargeable secondary batteries such as Li-S, Li-air, and Zn-air. The higher specific energy is a significant parameter for transportation and stationary applications, and in that case, Li-S batteries offer advantages but it is limited with few challenges discussed later. The

**104**

graphene sheets [78].

**4.3 Li-S battery**

**Figure 5.**

highest theoretical capacity of Li-S system is 2600 Wh/kg, which is highest than the LIB due to highest capacity of Li-S cathode sulfur has 1675 mAh/g. The most challenging part is to improve the electronic conductivity of cathodes of Li-S as the sulfur exhibits poor conductivity of 10–17 S/cm as well as the formation of polysulfides at cathodes. These polysulfides oxidize the Li anode and get back to cathodes and re-oxidize, thus lowering the performance of Li-S system. An extensive effort has been made to improve the cathodes by incorporating the carbon additives to sulfur to minimize the unnecessary reactions. Initially, mesoporous FSG was prepared and the sulfur was deposited by vapor treatment and was utilized as cathodes for Li-S system. It delivers charging capacity of 1288 mAh/g with high coulombic efficiency that reveals the restriction of sulfur to dissolute polysulfides in mesoporous FSG framework [79]. Similar to LIB and SIBs, the electrochemical behavior of cathode in Li-S system enhanced for heteroatom-doped FSG electrodes. **Figure 5b** shows the comparison of FSG and N-doped FSG capacity and coulombic efficiency with different cycle number. The heteroatom-doped FSG shows superior performance than the bare FSG due to the high interaction of polysulfides with heteroatoms that increase specific capacity. The nitrogen doping effect in FSG minimizes the concentration of polysulfides and forms a uniform layer of Li2S at cathode. This system delivers 1000 mAh/g at 0.335 A/g after 100 cycles [75]. In another work, Zhu et al. developed free-standing cathodes by CNTs that were interconnected with the sulfur-graphene walls and investigated the electrochemical behavior that delivers 1346 mAh/g at 0.17 A/g current density. It is due to sulfur at graphene walls that deals to provide dual response as follows: (i) hinder the dissolution of polysulfides minimizing the shuttle phenomenon and (ii) offer volume expansion even at high quantity of sulfur. Moreover, its capacity retention shows 40% when current density is increased to 16.7 A/g owing to the good electron pathway by CNTs connected with graphene nanosheets [80]. Further, nanosized Li2S (25–50 nm) particles incorporated into FSG papers by vacuum filtration process demonstrated an excellent cycling and rate capability with reversible capacity of 816.1 mAh/g at 0.1675 A/g (150 cycles) and 597 mAh/g at 11.7 A/g (200 cycles). This shows excellent performance in electrochemical behavior due to the uniform distribution of Li2S particles on graphene sheets that minimize the barrier for Li ion transport and particularly it has superior wetting nature to interconnect the polysulfides with graphene network into the paper electrodes [81]. Similarly, Chen et al. designed an efficient hierarchical nanostructure like nanobundled forest with Li2S/few-walled CNTs at FSG obtained solution processing followed by self-assembly method as cathodes. In this design, CNTs assembled in shaft-like structure and Li2S as active material, whereas the graphene sheets act as barrier for Li2S. It achieves high capacity of 868 and 433 mAh/g at current density of 335 and 16.7 A/g, respectively. This originates from the good framework between CNTs and graphene sheets as well as the uniform distribution of Li2S, and moreover, the barrier of graphene sheets for Li2S reduces the dissolution of polysulfides. Overall, the influence of void space enhances the volume change and thus improves the cycling stability of Li-S battery [82].

#### **4.4 Metal-air battery**

Recently, metal-air batteries have inspired much attention apart from the above said battery systems due to their high theoretical capacity than the metal-ion and Li-S batteries. The metal-air batteries can be operated in aqueous or nonaqueous medium based on the selection of metals. The nonaqueous medium is well suited for the Li-air batteries that deliver high capacity than in aqueous medium but still there are some issues when it comes to the practical application. The development of cathode in Li-air is significant as it is the main compartment to breathe oxygen

for delivering high capacity of the system. There are a lot of reports for cathode development based on metal oxides grown on Ni foam as binder-free electrodes. The role of FSG electrodes was also investigated as cathodes for Li-air batteries. First, Kim et al. developed graphene nanoplates (GNP)/GO composite paper-like electrodes as cathodes for Li-air battery system. The wrinkled nature of the paper electrodes induces the high surface area and also delivers higher discharge capacity of 9760 mAh/g at 0.1 A/g current density. This superior performance is due to the reduced overpotential, and the difference in consumption/evolution of O2 is minimized. On the whole, the system exhibits higher efficiency in OER (oxygen evolution reaction)/ORR (oxygen reduction reaction) of 87% [83]. The same group developed macroporous FSG paper with surface area of 373 m2 /g and pore volume of 10.9 cm3 /g with 91.6% of porosity that exhibits a high specific capacity of 12,200 mAh/g at 0.2 A/g. The rate capability is enhanced where it shows high cycling performance even at higher current density of 0.5 and 2 A/g that delivers approximately 1000 mAh/g. This is attributed to the minimized volume expansion that limits the decomposition and formation of Li2O2 at the macroporous nature of FSG. While discharging/charging the macroporous FSG, the nature of FSG electrode decomposes the discharge products completely that reveals its highly porous structure as shown in the **Figure 5c** and **d** [76]. Researchers investigated the effect of FSG cathodes in Li-air upon introduction of metal oxides, namely, α-MnO2 and NiCo2O4. Upon insertion of α-MnO2 into FSG electrodes, the overpotential decrease was caused during charge/discharge process. It delivers 2900 mAh/g for the higher content of α-MnO2 that was reported and shows the catalytic improvement in this study [84]. And Jiang et al. reported an excellent reversible capacity of 5000 mAh/g at 0.4 A/g by incorporating mesoporous NiCo2O4 into macropores of FSG. It also lowers about 0.18 and 0.54 V of overpotential for discharge and charge, respectively [85].

### **5. Conclusions**

In this chapter, FSG electrodes in battery applications signify their potential advantages to the fabrication technology. The fabrication of FSG electrode is facile as well as it excludes some additives applied in conventional electrodes. At present, the electrode of spent batteries contains active materials, binder, and metal foil, which set hurdles for recycling process. Herein, the FSG hybrid electrodes provide good capacity and cycling for battery application without binder and metal current collector. This exclusion provides light weight and flexible batteries and also there is a pathway to discover a facile route to recover the materials from FSG hybrid–based spent batteries in future.

#### **Acknowledgements**

This work was supported by South China Normal University. F.C. thanks the support from Outstanding Young Scholar Project (8S0256), the Project of Blue Fire Plan (CXZJHZ201709), and the Scientific and Technological Plan of Guangdong Province (2018A050506078).

**107**

**Author details**

P.R. China

Karthick Ramalingam and Fuming Chen\*

provided the original work is properly cited.

\*Address all correspondence to<EMAIL_ADDRESS>
Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, Guangdong Engineering Technology Research Center of Efficient Green Energy and Environment Protection Materials, School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou,

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario*

*DOI: http://dx.doi.org/10.5772/intechopen.84275*

#### **Conflict of interest**

The authors declare that there is no conflict of interest.

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario DOI: http://dx.doi.org/10.5772/intechopen.84275*

### **Author details**

*Energy Storage Devices*

volume of 10.9 cm3

respectively [85].

**5. Conclusions**

spent batteries in future.

**Acknowledgements**

**Conflict of interest**

Province (2018A050506078).

for delivering high capacity of the system. There are a lot of reports for cathode development based on metal oxides grown on Ni foam as binder-free electrodes. The role of FSG electrodes was also investigated as cathodes for Li-air batteries. First, Kim et al. developed graphene nanoplates (GNP)/GO composite paper-like electrodes as cathodes for Li-air battery system. The wrinkled nature of the paper electrodes induces the high surface area and also delivers higher discharge capacity of 9760 mAh/g at 0.1 A/g current density. This superior performance is due to the reduced overpotential, and the difference in consumption/evolution of O2 is minimized. On the whole, the system exhibits higher efficiency in OER (oxygen evolution reaction)/ORR (oxygen reduction reaction) of 87% [83]. The same

group developed macroporous FSG paper with surface area of 373 m2

of 12,200 mAh/g at 0.2 A/g. The rate capability is enhanced where it shows high cycling performance even at higher current density of 0.5 and 2 A/g that delivers approximately 1000 mAh/g. This is attributed to the minimized volume expansion that limits the decomposition and formation of Li2O2 at the macroporous nature of FSG. While discharging/charging the macroporous FSG, the nature of FSG electrode decomposes the discharge products completely that reveals its highly porous structure as shown in the **Figure 5c** and **d** [76]. Researchers investigated the effect of FSG cathodes in Li-air upon introduction of metal oxides, namely, α-MnO2 and NiCo2O4. Upon insertion of α-MnO2 into FSG electrodes, the overpotential decrease was caused during charge/discharge process. It delivers 2900 mAh/g for the higher content of α-MnO2 that was reported and shows the catalytic improvement in this study [84]. And Jiang et al. reported an excellent reversible capacity of 5000 mAh/g at 0.4 A/g by incorporating mesoporous NiCo2O4 into macropores of FSG. It also lowers about 0.18 and 0.54 V of overpotential for discharge and charge,

In this chapter, FSG electrodes in battery applications signify their potential advantages to the fabrication technology. The fabrication of FSG electrode is facile as well as it excludes some additives applied in conventional electrodes. At present, the electrode of spent batteries contains active materials, binder, and metal foil, which set hurdles for recycling process. Herein, the FSG hybrid electrodes provide good capacity and cycling for battery application without binder and metal current collector. This exclusion provides light weight and flexible batteries and also there is a pathway to discover a facile route to recover the materials from FSG hybrid–based

This work was supported by South China Normal University. F.C. thanks the support from Outstanding Young Scholar Project (8S0256), the Project of Blue Fire Plan (CXZJHZ201709), and the Scientific and Technological Plan of Guangdong

The authors declare that there is no conflict of interest.

/g with 91.6% of porosity that exhibits a high specific capacity

/g and pore

**106**

Karthick Ramalingam and Fuming Chen\* Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, Guangdong Engineering Technology Research Center of Efficient Green Energy and Environment Protection Materials, School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou, P.R. China

\*Address all correspondence to<EMAIL_ADDRESS>
© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### **References**

[1] Neelu C, Ru Shi L. Electrochemical technologies for energy storage and conversion. In: Ru Shi L, Lei Z, Xueliang S, Hansan L, Jiujun Z, editors. Electrochemical Technologies for Energy Storage and Conversion. Wiley; 2012. pp. 1-43. DOI: 10.1002/9783527639496

[2] Dai L, Chang DW, Jong-Beom B, Lu W. Carbon nanomaterials for advanced energy conversion and storage. Small. 2012;**8**:1130-1166. DOI: 10.1002/ smll.201101594

[3] Hou J, Shao Y, Ellis MW, Moore RB, Yi B. Graphene-based electrochemical energy conversion and storage: Fuel cells, supercapacitors and lithium ion batteries. Physical Chemistry Chemical Physics. 2011;**13**:15384-15402. DOI: 10.1039/C1CP21915D

[4] Liu J, Xue Y, Zhang M, Dai L. Graphene-based materials for energy applications. MRS Bulletin. 2012;**37**:1265-1272. DOI: 10.1557/ mrs.2012.179

[5] Chen T, Dai L. Carbon nanomaterials for high-performance supercapacitors. Materials Today. 2013;**16**:272-280. DOI: 10.1016/j.mattod.2013.07.002

[6] Choi HJ, Jung SM, Seo JM, Chang DW, Dai L, Baek JB. Graphene for energy conversion and storage in fuel cells and supercapacitors. Nano Energy. 2012;**1**:534-551. DOI: 10.1016/j. nanoen.2012.05.001

[7] Feng L, Yang L, Huang Z, Luo J, Li M, Wang D, et al. Enhancing electrocatalytic oxygen reduction on nitrogen-doped graphene by active sites implantation. Scientific Reports. 2013;**3**:3306(1-8). DOI: 10.1038/ srep03306

[8] Zhai P, Lee CC, Chang YH, Liu C, Wei TC, Feng SP. A significant improvement in the electrocatalytic stability of N-doped graphene nanosheets used as a counter electrode for [Co(bpy)3 ] 3+/2+ based porphyrinsensitized solar cells. ACS Applied Materials and Interfaces. 2015;**7**:2116- 2123. DOI: 10.1021/am5083705

[9] Cai X, Lai L, Shen Z, Lin J. Graphene and graphene-based composites as Li-ion battery electrode materials and their application in full cells. Journal of Materials Chemistry A. 2017;**5**:15423- 15446. DOI: 10.1039/C7TA04354F

[10] Yuan H, Kong L, Li T, Zhang Q. A review of transition metal chalcogenide/ graphene nanocomposites for energy storage and conversion. Chinese Chemical Letters. 2017;**28**:2180-2194

[11] Srivastava M, Singh J, Kuila T, Layek RK, Kim NH, Lee JH. Recent advances in graphene and its metaloxide hybrid nanostructures for lithium-ion batteries. Nanoscale. 2015;**7**:4820-4868. DOI: 10.1039/ C4NR07068B

[12] Han S, Wu D, Li S, Zhang F, Feng X. Porous graphene materials for advanced electrochemical energy storage and conversion devices. Advanced Materials. 2014;**26**:849-864. DOI: 10.1002/adma.201303115

[13] Geim AK, Novoselov KS. The rise of graphene. Nature Materials. 2007;**6**:183- 191. DOI: 10.1038/nmat1849

[14] Wang X, Shi Y. Fabrication techniques of graphene nanostructures. In: Gang Z, Navin M, editors. Nanofabrication and its Application in Renewable Energy. RSC Publishing: London; 2014. pp. 1-30. DOI: 10.1039/9781782623380-00001

[15] Cao MS, Wang XX, Cao WQ, Yuan J. Ultrathin graphene: Electrical properties and highly efficient

**109**

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario*

large area and uniform graphene on tilted copper foil for high performance flexible transparent conductive film. Journal of Materials Chemistry. 2012;**22**:18283-18290. DOI: 10.1039/

[23] Liu Z, Xie Y, Zhao J, Wu S, Zhou Z, Wen M, et al. Rapid preparation of conductive transparent films via

solution printing of graphene precursor. Thin Solid Films. 2018;**657**:24-31. DOI:

[24] Karthick R, Brindha M, Selvaraj M, Ramu S. Stable colloidal dispersion of functionalized reduced graphene oxide in aqueous medium for transparent conductive film. Journal of Colloid and Interface Science. 2013;**406**:69-74. DOI:

[25] Ambrosi A, Chua CK, Latiff NM, Loo AH, Wong CHA, Eng AYS, et al. Graphene and its electrochemistry—An update. Chemical Society Reviews. 2016;**45**:2458-2493. DOI: 10.1039/

[26] Li D, Muller MB, Gilje S, Kaner RB, Wallace GG. Processable aqueous dispersions of graphene nanosheets. Nature Nanotechnology. 2008;**3**:101-105.

[28] Konios D, Stylianakis M, Stratakis E, Kymakis E. Dispersion behaviour of graphene oxide and reduced graphene oxide. Journal of Colloid and Interface Science. 2014;**430**:108-112. DOI:

[29] Rinzler AG, Liu J, Dai H, Nikolaev P, Huffman CB, Rodríguez-Macías FJ, et al. Large-scale purification of single-wall carbon nanotubes: Process, product, and characterization. Applied

DOI: 10.1038/nnano.2007.451

[27] Sun L, Fungetsu B. Mass production of graphene oxide from expanded graphite. Materials Letters. 2013;**109**:207-210. DOI: 10.1016/j.

matlet.2013.07.072

10.1016/j.jcis.2014.05.033

10.1016/j.tsf.2018.05.005

10.1016/j.jcis.2013.06.006

C6CS00136J

C2JM33881E

*DOI: http://dx.doi.org/10.5772/intechopen.84275*

electromagnetic interference shielding. Journal of Materials Chemistry C. 2015;**3**:6589-6599. DOI:

[16] Allen MJ, Tung VC, Kaner RB. Honeycomb carbon: A review of graphene. Chemical Reviews. 2010;**110**:132-145. DOI: 10.1021/

Mohammadhosseini A, Boulet P,

high-surface-area carbons with

and Nanometer Structures— Processing, Measurement, and Phenomena. 2007;**25**:2558-2561. DOI:

[19] Bizao RA, Botari T, Perim E, Nicola MP, Galvao DS. Mechanical properties and fracture patterns of graphene (graphitic) nanowiggles. Carbon. 2017;**119**:431-437. DOI: 10.1016/j.carbon.2017.04.018

[20] Brownson DAC, Banks CE. The Handbook of Graphene Electrochemistry. 1st ed. London: Springer-Verlag; 2014 . 174 p. DOI: 10.1007/978-1-4471-6428-9

[21] Bianco A, Cheng HM, Enoki T, Gogotsi Y, Hurt RH, Koratkar N, et al. All in the graphene family—A recommended nomenclature for two-dimensional carbon materials. Carbon. 2013;**65**:1-6. DOI: 10.1016/j.carbon.2013.08.038

[22] Zhang J, Hu P, Wang X, Wang Z, Liu D, Yang B, et al. CVD growth of

10.1116/1.2789446

Beckner M, Romanos J, et al. Hypothetical

exceptional hydrogen storage capacities: Open carbon frameworks. Journal of the American Chemical Society. 2012;**134**:15130-15137. DOI: 10.1021/

[18] Frank IW, Tanenbaum DM, Van der Zande AM, McEuen PL. Mechanical properties of suspended graphene sheets. Journal of Vacuum Science and Technology, B: Microelectronics

10.1039/C5TC01354B

[17] Kuchta B,Firlej L,

cr900070d

ja306726u

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario DOI: http://dx.doi.org/10.5772/intechopen.84275*

electromagnetic interference shielding. Journal of Materials Chemistry C. 2015;**3**:6589-6599. DOI: 10.1039/C5TC01354B

[16] Allen MJ, Tung VC, Kaner RB. Honeycomb carbon: A review of graphene. Chemical Reviews. 2010;**110**:132-145. DOI: 10.1021/ cr900070d

[17] Kuchta B,Firlej L, Mohammadhosseini A, Boulet P, Beckner M, Romanos J, et al. Hypothetical high-surface-area carbons with exceptional hydrogen storage capacities: Open carbon frameworks. Journal of the American Chemical Society. 2012;**134**:15130-15137. DOI: 10.1021/ ja306726u

[18] Frank IW, Tanenbaum DM, Van der Zande AM, McEuen PL. Mechanical properties of suspended graphene sheets. Journal of Vacuum Science and Technology, B: Microelectronics and Nanometer Structures— Processing, Measurement, and Phenomena. 2007;**25**:2558-2561. DOI: 10.1116/1.2789446

[19] Bizao RA, Botari T, Perim E, Nicola MP, Galvao DS. Mechanical properties and fracture patterns of graphene (graphitic) nanowiggles. Carbon. 2017;**119**:431-437. DOI: 10.1016/j.carbon.2017.04.018

[20] Brownson DAC, Banks CE. The Handbook of Graphene Electrochemistry. 1st ed. London: Springer-Verlag; 2014 . 174 p. DOI: 10.1007/978-1-4471-6428-9

[21] Bianco A, Cheng HM, Enoki T, Gogotsi Y, Hurt RH, Koratkar N, et al. All in the graphene family—A recommended nomenclature for two-dimensional carbon materials. Carbon. 2013;**65**:1-6. DOI: 10.1016/j.carbon.2013.08.038

[22] Zhang J, Hu P, Wang X, Wang Z, Liu D, Yang B, et al. CVD growth of

large area and uniform graphene on tilted copper foil for high performance flexible transparent conductive film. Journal of Materials Chemistry. 2012;**22**:18283-18290. DOI: 10.1039/ C2JM33881E

[23] Liu Z, Xie Y, Zhao J, Wu S, Zhou Z, Wen M, et al. Rapid preparation of conductive transparent films via solution printing of graphene precursor. Thin Solid Films. 2018;**657**:24-31. DOI: 10.1016/j.tsf.2018.05.005

[24] Karthick R, Brindha M, Selvaraj M, Ramu S. Stable colloidal dispersion of functionalized reduced graphene oxide in aqueous medium for transparent conductive film. Journal of Colloid and Interface Science. 2013;**406**:69-74. DOI: 10.1016/j.jcis.2013.06.006

[25] Ambrosi A, Chua CK, Latiff NM, Loo AH, Wong CHA, Eng AYS, et al. Graphene and its electrochemistry—An update. Chemical Society Reviews. 2016;**45**:2458-2493. DOI: 10.1039/ C6CS00136J

[26] Li D, Muller MB, Gilje S, Kaner RB, Wallace GG. Processable aqueous dispersions of graphene nanosheets. Nature Nanotechnology. 2008;**3**:101-105. DOI: 10.1038/nnano.2007.451

[27] Sun L, Fungetsu B. Mass production of graphene oxide from expanded graphite. Materials Letters. 2013;**109**:207-210. DOI: 10.1016/j. matlet.2013.07.072

[28] Konios D, Stylianakis M, Stratakis E, Kymakis E. Dispersion behaviour of graphene oxide and reduced graphene oxide. Journal of Colloid and Interface Science. 2014;**430**:108-112. DOI: 10.1016/j.jcis.2014.05.033

[29] Rinzler AG, Liu J, Dai H, Nikolaev P, Huffman CB, Rodríguez-Macías FJ, et al. Large-scale purification of single-wall carbon nanotubes: Process, product, and characterization. Applied

**108**

srep03306

*Energy Storage Devices*

**References**

smll.201101594

10.1039/C1CP21915D

mrs.2012.179

[4] Liu J, Xue Y, Zhang M,

10.1016/j.mattod.2013.07.002

[6] Choi HJ, Jung SM, Seo JM,

[7] Feng L, Yang L, Huang Z,

[8] Zhai P, Lee CC, Chang YH, Liu C, Wei TC, Feng SP. A significant

nanoen.2012.05.001

Chang DW, Dai L, Baek JB. Graphene for energy conversion and storage in fuel cells and supercapacitors. Nano Energy. 2012;**1**:534-551. DOI: 10.1016/j.

Luo J, Li M, Wang D, et al. Enhancing electrocatalytic oxygen reduction on nitrogen-doped graphene by active sites implantation. Scientific Reports. 2013;**3**:3306(1-8). DOI: 10.1038/

Dai L. Graphene-based materials for energy applications. MRS Bulletin. 2012;**37**:1265-1272. DOI: 10.1557/

[5] Chen T, Dai L. Carbon nanomaterials for high-performance supercapacitors. Materials Today. 2013;**16**:272-280. DOI:

[1] Neelu C, Ru Shi L. Electrochemical technologies for energy storage and conversion. In: Ru Shi L, Lei Z, Xueliang S, Hansan L, Jiujun Z, editors. Electrochemical Technologies for Energy Storage and Conversion. Wiley; 2012. pp. 1-43. DOI: 10.1002/9783527639496

improvement in the electrocatalytic stability of N-doped graphene

sensitized solar cells. ACS Applied Materials and Interfaces. 2015;**7**:2116- 2123. DOI: 10.1021/am5083705

]

for [Co(bpy)3

C4NR07068B

nanosheets used as a counter electrode

[9] Cai X, Lai L, Shen Z, Lin J. Graphene and graphene-based composites as Li-ion battery electrode materials and their application in full cells. Journal of Materials Chemistry A. 2017;**5**:15423- 15446. DOI: 10.1039/C7TA04354F

[10] Yuan H, Kong L, Li T, Zhang Q. A review of transition metal chalcogenide/ graphene nanocomposites for energy storage and conversion. Chinese Chemical Letters. 2017;**28**:2180-2194

[11] Srivastava M, Singh J, Kuila T, Layek RK, Kim NH, Lee JH. Recent advances in graphene and its metaloxide hybrid nanostructures for lithium-ion batteries. Nanoscale. 2015;**7**:4820-4868. DOI: 10.1039/

[12] Han S, Wu D, Li S, Zhang F, Feng X. Porous graphene materials for advanced electrochemical energy storage and conversion devices. Advanced Materials. 2014;**26**:849-864.

DOI: 10.1002/adma.201303115

191. DOI: 10.1038/nmat1849

[14] Wang X, Shi Y. Fabrication

In: Gang Z, Navin M, editors. Nanofabrication and its Application in Renewable Energy. RSC Publishing:

London; 2014. pp. 1-30. DOI: 10.1039/9781782623380-00001

[15] Cao MS, Wang XX, Cao WQ, Yuan J. Ultrathin graphene: Electrical properties and highly efficient

[13] Geim AK, Novoselov KS. The rise of graphene. Nature Materials. 2007;**6**:183-

techniques of graphene nanostructures.

3+/2+ based porphyrin-

[2] Dai L, Chang DW, Jong-Beom B, Lu W. Carbon nanomaterials for advanced energy conversion and storage. Small. 2012;**8**:1130-1166. DOI: 10.1002/

[3] Hou J, Shao Y, Ellis MW, Moore RB, Yi B. Graphene-based electrochemical energy conversion and storage: Fuel cells, supercapacitors and lithium ion batteries. Physical Chemistry Chemical Physics. 2011;**13**:15384-15402. DOI:

Physics A. 1998;**67**:29-37. DOI: 10.1007/ s0033900507

[30] Ding W, Pengcheng S, Changhong L, Wei W, Shoushan F. Highly oriented carbon nanotube papers made of aligned carbon nanotubes. Nanotechnology. 2008;**19**:075609. DOI: 10.1088/0957-4484/19/7/075609

[31] Dikin DA, Stankovich S, Zimney EJ, Piner RD, Dommett GHB, Evmenenko G, et al. Preparation and characterization of graphene oxide paper. Nature. 2007;**448**:457-460. DOI: 10.1038/nature06016

[32] Pei S, Zhao J, Du J, Ren W, Cheng HM. Direct reduction of graphene oxide films into highly conductive and flexible graphene films by hydrohalic acids. Carbon. 2010;**48**:4466-4474. DOI: 10.1016/j.carbon.2010.08.006

[33] Liu C, Hao F, Zhao X, Zhao Q, Luo S, Lin H. Low temperature reduction of free-standing graphene oxide papers with metal iodides for ultrahigh bulk conductivity. Scientific Reports. 2014;**4**:3965. DOI: 10.1038/ srep03965

[34] Chen C, Yang QH, Yang Y, Lu W, Wen Y, Hou PX, et al. Self-assembled free-standing graphite oxide membrane. Advanced Materials. 2009;**21**:3007- 3011. DOI: 10.1002/adma.200803726

[35] Huang ZD, Zhang B, Liang R, Zheng QB, Oh SW, Lin XY, et al. Effects of reduction process and carbon nanotube content on supercapacitive performance of flexible graphene oxide papers. Carbon. 2012;**50**:4239-4251. DOI: 10.1016/j.carbon.2012.05.006

[36] Park S, Lee KS, Bozoku G, Cai W, Nguyen SBT, Ruoff RS. Graphene oxide papers modified by divalent ions—Enhancing mechanical properties via chemical cross-linking. ACS Nano. 2008;**2**:572-578. DOI: 10.1021/ nn700349a

[37] Tan Y, Song Y, Zheng Q. Facile regulation of glutaraldehyde-modified graphene oxide for preparing freestanding papers and nanocomposite films. Chinese Journal of Polymer Science. 2013;**31**:399-406. DOI: 10.1007/ s10118-013-1234-7

[38] Marquez-Lamas U, Martinez-Guerra E, Toxqui-Teran A, Aguirre-Tostado FS, Lara-Ceniceros TE, Bonilla-Cruz J. Tuning mechanical, electrical and optical properties of flexible and free-standing functionalized graphene oxide papers having different interlayer d-spacing. The Journal of Physical Chemistry C. 2017;**121**:852-862. DOI: 10.1021/acs. jpcc.6b09961

#### [39] Medhekar NV,

Ramasubramaniam A, Ruoff RS, Shenoy VB. Hydrogen bond networks in graphene oxide composite paper: Structure and mechanical properties. ACS Nano. 2010;**4**:2300-2306. DOI: 10.1021/nn901934u

[40] Alzate-Carvajal N, Acevedo-Guzman DA, Meza-Laguna V, Farıas MH, Perez-Rey LA, Abarca-Morales E, et al. One-step nondestructive functionalization of graphene oxide paper with amines. RSC Advances. 2018;**8**:15253-15265. DOI: 10.1039/c8ra00986d

[41] Vallés C, David NJ, Benito AM, Maser WK. Flexible conductive graphene paper obtained by direct and gentle annealing of graphene oxide paper. Carbon. 2012;**50**:835-844. DOI: 10.1016/j.carbon.2011.09.042

[42] Gao J, Liu C, Miao L, Wang X, Chen Y. Free-standing reduced graphene oxide paper with high electrical conductivity. Journal of Electronic Materials. 2016;**45**:1290-1295. DOI: 10.1007/s11664-015-4000-5

[43] Tarascon JM, Armand M. Issues and challenges facing rechargeable lithium

**111**

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario*

hybrid film anode for stable Li-ion batteries. Journal of Power Sources. 2016;**307**:214-219. DOI: 10.1016/j.

DOI: 10.1016/j.jpcs.2017.05.009

free-standing graphene/SnO2 nanocomposites paper for Li-ion battery. ACS Applied Materials and Interfaces. 2012;**4**:5742-5748. DOI:

[54] Gao T, Huang K, Qi X, Li H, Yang L, Zhong J. Free-standing SnO2 nanoparticles@graphene hybrid paper for advanced lithium-ion batteries. Ceramics International. 2014;**40**:6891-6897. DOI: 10.1016/j.

[55] Hu T, Sun X, Sun H, Yu M, Lu F, Liu C, et al. Flexible free-standing graphene–TiO2 hybrid paper for use as lithium ion battery anode materials. Carbon. 2013;**51**:322-326. DOI: 10.1016/j.

[56] Ren HM, Ding YH, Chang FH, He X, Feng JQ, Wang CF, et al. Flexible free-standing TiO2/graphene/PVdF films as anode materials for lithium-ion batteries. Applied Surface Science. 2012;**263**:54-57. DOI: 10.1016/j.

[57] Park SK, Seong CY, Yoo S,

for lithium ion battery. Energy. 2016;**99**:266-273. DOI: 10.1016/j.

[58] Guo J, Zhu H, Sun Y, Tang L,

Zhang X. Pie-like free-standing paper of

Piao Y. Porous Mn3O4 nanorod/reduced graphene oxide hybrid paper as a flexible and binder-free anode material

10.1021/am301962d

ceramint.2013.12.009

carbon.2012.08.059

apsusc.2012.08.107

energy.2016.01.061

[52] Cai H, Han K, Jiang H, Wang J, Liu H. Self-standing silicon-carbon nanotube/ graphene by a scalable in situ approach from low-cost Al-Si alloy powder for lithium ion batteries. Journal of Physics and Chemistry of Solids. 2017;**109**:9-17.

[53] Liang J, Zhao Y, Guo L, Li L. Flexible

jpowsour.2015.12.107

*DOI: http://dx.doi.org/10.5772/intechopen.84275*

batteries. Nature. 2001;**414**:359-367.

[44] Hadjipaschalis I, Poullikkas A, Efthimiou V. Overview of current and future energy storage technologies for electric power applications. Renewable and Sustainable Energy Reviews. 2009;**13**:1513-1522. DOI: 10.1016/j.

[45] Younesi R, Veith GM, Johansson P, Edström K, Vegge T. Lithium salts for advanced lithium batteries: Li–metal, Li– O2, and Li–S. Energy and Environmental Science. 2015;**8**:1905-1922. DOI: 10.1039/

[46] Ng SH, Wang J, Guo ZP, Chen J, Wang GX, Liu HK. Single wall carbon nanotube paper as anode for lithiumion battery. Electrochimica Acta. 2005;**51**:23-28. DOI: 10.1016/j.

[47] Chen J, Minett AI, Liu Y, Lynam C, Sherrell P, Wang C, et al. Direct growth of flexible carbon nanotube electrodes. Advanced Materials. 2008;**20**:566-570.

[48] Wang C, Li D, Too CO, Wallace GG. Electrochemical properties of graphene paper electrodes used in lithium batteries.

DOI: 10.1002/adma.200701146

Chemistry of Materials. 2009;**21**: 2604-2606. DOI: 10.1021/cm900764n

DOI: 10.1039/b919738a

[49] Lee JK, Smith KB, Hayner CM, Kung HH. Silicon nanoparticles– graphene paper composites for Li ion battery anodes. Chemical Communications. 2010;**46**:2025-2027.

[50] Wang JZ, Zhong C, Chou SL, Liu HK. Flexible free-standing graphene-silicon composite film for lithium-ion batteries. Electrochemistry Communications. 2010;**12**:1467-1470. DOI: 10.1016/j.elecom.2010.08.008

[51] Zhang H, Jing S, Hu Y, Jiang H, Li C. A flexible freestanding Si/rGO

DOI: 10.1038/35104644

rser.2008.09.028

C5EE01215E

electacta.2005.04.045

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario DOI: http://dx.doi.org/10.5772/intechopen.84275*

batteries. Nature. 2001;**414**:359-367. DOI: 10.1038/35104644

*Energy Storage Devices*

[30] Ding W, Pengcheng S,

[31] Dikin DA, Stankovich S,

[32] Pei S, Zhao J, Du J, Ren W,

10.1016/j.carbon.2010.08.006

[33] Liu C, Hao F, Zhao X, Zhao Q, Luo S, Lin H. Low temperature reduction of free-standing graphene oxide papers with metal iodides for ultrahigh bulk conductivity. Scientific Reports. 2014;**4**:3965. DOI: 10.1038/

[34] Chen C, Yang QH, Yang Y, Lu W, Wen Y, Hou PX, et al. Self-assembled free-standing graphite oxide membrane. Advanced Materials. 2009;**21**:3007- 3011. DOI: 10.1002/adma.200803726

[35] Huang ZD, Zhang B, Liang R, Zheng QB, Oh SW, Lin XY, et al. Effects

of reduction process and carbon nanotube content on supercapacitive performance of flexible graphene oxide papers. Carbon. 2012;**50**:4239-4251. DOI: 10.1016/j.carbon.2012.05.006

[36] Park S, Lee KS, Bozoku G, Cai W, Nguyen SBT, Ruoff RS. Graphene oxide papers modified by divalent ions—Enhancing mechanical properties

via chemical cross-linking. ACS Nano. 2008;**2**:572-578. DOI: 10.1021/

10.1038/nature06016

srep03965

Zimney EJ, Piner RD, Dommett GHB, Evmenenko G, et al. Preparation and characterization of graphene oxide paper. Nature. 2007;**448**:457-460. DOI:

Cheng HM. Direct reduction of graphene oxide films into highly conductive and flexible graphene films by hydrohalic acids. Carbon. 2010;**48**:4466-4474. DOI:

s0033900507

Physics A. 1998;**67**:29-37. DOI: 10.1007/

[37] Tan Y, Song Y, Zheng Q. Facile regulation of glutaraldehyde-modified graphene oxide for preparing freestanding papers and nanocomposite films. Chinese Journal of Polymer Science. 2013;**31**:399-406. DOI: 10.1007/

[38] Marquez-Lamas U, Martinez-Guerra E, Toxqui-Teran A,

Aguirre-Tostado FS, Lara-Ceniceros TE, Bonilla-Cruz J. Tuning mechanical, electrical and optical properties of flexible and free-standing

functionalized graphene oxide papers having different interlayer d-spacing. The Journal of Physical Chemistry C. 2017;**121**:852-862. DOI: 10.1021/acs.

Ramasubramaniam A, Ruoff RS, Shenoy VB. Hydrogen bond networks in graphene oxide composite paper: Structure and mechanical properties. ACS Nano. 2010;**4**:2300-2306. DOI:

[40] Alzate-Carvajal N, Acevedo-Guzman DA, Meza-Laguna V, Farıas MH, Perez-Rey LA,

Abarca-Morales E, et al. One-step nondestructive functionalization of graphene oxide paper with amines. RSC Advances. 2018;**8**:15253-15265. DOI:

[41] Vallés C, David NJ, Benito AM, Maser WK. Flexible conductive graphene paper obtained by direct and gentle annealing of graphene oxide paper. Carbon. 2012;**50**:835-844. DOI: 10.1016/j.carbon.2011.09.042

[42] Gao J, Liu C, Miao L, Wang X, Chen Y. Free-standing reduced graphene

[43] Tarascon JM, Armand M. Issues and challenges facing rechargeable lithium

oxide paper with high electrical conductivity. Journal of Electronic Materials. 2016;**45**:1290-1295. DOI:

10.1007/s11664-015-4000-5

s10118-013-1234-7

jpcc.6b09961

[39] Medhekar NV,

10.1021/nn901934u

10.1039/c8ra00986d

Changhong L, Wei W, Shoushan F. Highly oriented carbon nanotube papers made of aligned carbon nanotubes. Nanotechnology. 2008;**19**:075609. DOI: 10.1088/0957-4484/19/7/075609

**110**

nn700349a

[44] Hadjipaschalis I, Poullikkas A, Efthimiou V. Overview of current and future energy storage technologies for electric power applications. Renewable and Sustainable Energy Reviews. 2009;**13**:1513-1522. DOI: 10.1016/j. rser.2008.09.028

[45] Younesi R, Veith GM, Johansson P, Edström K, Vegge T. Lithium salts for advanced lithium batteries: Li–metal, Li– O2, and Li–S. Energy and Environmental Science. 2015;**8**:1905-1922. DOI: 10.1039/ C5EE01215E

[46] Ng SH, Wang J, Guo ZP, Chen J, Wang GX, Liu HK. Single wall carbon nanotube paper as anode for lithiumion battery. Electrochimica Acta. 2005;**51**:23-28. DOI: 10.1016/j. electacta.2005.04.045

[47] Chen J, Minett AI, Liu Y, Lynam C, Sherrell P, Wang C, et al. Direct growth of flexible carbon nanotube electrodes. Advanced Materials. 2008;**20**:566-570. DOI: 10.1002/adma.200701146

[48] Wang C, Li D, Too CO, Wallace GG. Electrochemical properties of graphene paper electrodes used in lithium batteries. Chemistry of Materials. 2009;**21**: 2604-2606. DOI: 10.1021/cm900764n

[49] Lee JK, Smith KB, Hayner CM, Kung HH. Silicon nanoparticles– graphene paper composites for Li ion battery anodes. Chemical Communications. 2010;**46**:2025-2027. DOI: 10.1039/b919738a

[50] Wang JZ, Zhong C, Chou SL, Liu HK. Flexible free-standing graphene-silicon composite film for lithium-ion batteries. Electrochemistry Communications. 2010;**12**:1467-1470. DOI: 10.1016/j.elecom.2010.08.008

[51] Zhang H, Jing S, Hu Y, Jiang H, Li C. A flexible freestanding Si/rGO hybrid film anode for stable Li-ion batteries. Journal of Power Sources. 2016;**307**:214-219. DOI: 10.1016/j. jpowsour.2015.12.107

[52] Cai H, Han K, Jiang H, Wang J, Liu H. Self-standing silicon-carbon nanotube/ graphene by a scalable in situ approach from low-cost Al-Si alloy powder for lithium ion batteries. Journal of Physics and Chemistry of Solids. 2017;**109**:9-17. DOI: 10.1016/j.jpcs.2017.05.009

[53] Liang J, Zhao Y, Guo L, Li L. Flexible free-standing graphene/SnO2 nanocomposites paper for Li-ion battery. ACS Applied Materials and Interfaces. 2012;**4**:5742-5748. DOI: 10.1021/am301962d

[54] Gao T, Huang K, Qi X, Li H, Yang L, Zhong J. Free-standing SnO2 nanoparticles@graphene hybrid paper for advanced lithium-ion batteries. Ceramics International. 2014;**40**:6891-6897. DOI: 10.1016/j. ceramint.2013.12.009

[55] Hu T, Sun X, Sun H, Yu M, Lu F, Liu C, et al. Flexible free-standing graphene–TiO2 hybrid paper for use as lithium ion battery anode materials. Carbon. 2013;**51**:322-326. DOI: 10.1016/j. carbon.2012.08.059

[56] Ren HM, Ding YH, Chang FH, He X, Feng JQ, Wang CF, et al. Flexible free-standing TiO2/graphene/PVdF films as anode materials for lithium-ion batteries. Applied Surface Science. 2012;**263**:54-57. DOI: 10.1016/j. apsusc.2012.08.107

[57] Park SK, Seong CY, Yoo S, Piao Y. Porous Mn3O4 nanorod/reduced graphene oxide hybrid paper as a flexible and binder-free anode material for lithium ion battery. Energy. 2016;**99**:266-273. DOI: 10.1016/j. energy.2016.01.061

[58] Guo J, Zhu H, Sun Y, Tang L, Zhang X. Pie-like free-standing paper of graphene paper@Fe3O4 nanorod array@ carbon as integrated anode for robust lithium storage. Chemical Engineering Journal. 2017;**309**:272-277. DOI: 10.1016/j.cej.2016.10.041

[59] Liu Y, Wang W, Gu L, Wang Y, Ying Y, Mao Y, et al. Flexible CuO nanosheets/reduced-graphene oxide composite paper: Binder-free anode for high performance lithium-ion batteries. ACS Applied Materials and Interfaces. 2013;**5**:9850-9855. DOI: 10.1021/ am403136e

[60] Byon HR, Gallant BM, Lee SW, Yang SH. Role of oxygen functional groups in carbon nanotube/graphene freestanding electrodes for high performance lithium batteries. Advanced Functional Materials. 2013;**23**:1037-1045. DOI: 10.1002/ adfm.201200697

[61] Zhang L, Fan W, Liu T. A flexible free-standing defect-rich MoS2/ graphene/carbon nanotube hybrid paper as a binder-free anode for highperformance lithium ion batteries. RSC Advances. 2015;**5**:43130-43140. DOI: 10.1039/c5ra05038c

[62] Wang J, Wang G, Wang H. Flexible free-standing Fe2O3/graphene/ carbon nanotubes hybrid films as anode materials for high performance lithium-ion batteries. Electrochimica Acta. 2015;**182**:192-201. DOI: 10.1016/j. electacta.2015.09.080

[63] Liu Y, Cai X, Shi W. Free-standing graphene/carbon nanotubes/CuO aerogel paper anode for lithium ion batteries. Materials Letters. 2016;**172**: 72-75. DOI: 10.1016/j.matlet.2016.02.068

[64] Li Y, Wang P, Bao Y, Huang K. A flexible nanostructured paper of MnO NPs@MWCNTs/r-GO multilayer sandwich composite for high-performance lithium-ion batteries. Ceramics International.

2017;**43**:7588-7593. DOI: 10.1016/j. ceramint.2017.03.051

[65] Zhao X, Wang G, Zhou Y, Wang H. Flexible free-standing ternary CoSnO3/graphene/carbon nanotubes composite papers as anodes for enhanced performance of lithiumion batteries. Energy. 2017;**118**: 172-180. DOI: 10.1016/j.energy. 2016.12.018

[66] Cao H, Zhou X, Deng W, Ma Z, Liu Y, Liu Z. Layer structured graphene/ porous ZnCo2O4 composite film for high performance flexible lithiumion batteries. Chemical Engineering Journal. 2018;**343**:654-661. DOI: 10.1016/j.cej.2018.03.001

[67] Qian J, Chen Y, Wu L, Cao Y, Ai X, Yang H. High capacity Na-storage and superior cyclability of nanocomposite Sb/C anode for Na-ion batteries. Chemical Communications. 2012;**48**: 7070-7072

[68] Darwiche A,Marino C, Sougrati MT, Fraisse B, Stievano L, Monconduit L. Better cycling performances of bulk Sb in Na-ion batteries compared to Li-ion systems: An unexpected electrochemical mechanism. Journal of the American Chemical Society. 2012;**134**:20805- 20811. DOI: 10.1021/ja310347x

[69] Qian J, Wu X, Cao Y, Ai X, Yang H. High capacity and rate capability of amorphous phosphorus for sodium ion batteries. Angewandte Chemie, International Edition. 2013;**52**:4633- 4636. DOI: 10.1002/anie.201209689

[70] David L, Bhandavat R, Singh G. MoS2/Graphene composite paper for sodium-ion battery electrodes. ACS Nano. 2014;**8**:1759-1770. DOI: 10.1021/nn406156b

[71] Xiang J, Dong D, Wen F, Zhao J, Zhong X, Wang L, et al. Microwave

**113**

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario*

ion batteries. Journal of Materials Chemistry A. 2018;**6**:1574-1581. DOI:

[78] Sun N, Guan Y, Liu YT, Zhu Q, Shen J, Liu S, et al. Facile synthesis of free-standing, flexible hard carbon anode for high performance sodium ion batteries using graphene as a multifunctional binder. Carbon. 2018;**137**:475-483. DOI: 10.1016/j.

[79] Huang X, Sun B, Li K, Chen S, Wang G. Mesoporous graphene paper immobilised sulfur as a flexible electrode for lithium-sulfur batteries. Journal of Materials Chemistry A. 2013;**1**:13484-13489. DOI: 10.1039/

[80] Zhu L, Peng HJ, Liang J, Huang JQ, Chen CM, Guo X, et al. Interconnected carbon nanotube/graphene nanosphere

scaffolds as free-standing paper electrode for high-rate and ultra-stable lithium-sulfur batteries. Nano Energy. 2015;**11**:746-755. DOI: 10.1016/j.

[81] Wang C, Wang X, Yang Y, Kushima A, Chen J, Huang Y, et al. Slurryless Li2S/reduced graphene oxide cathode paper for high-performance lithium sulfur battery. Nano Letters. 2015;**15**:1796-1802. DOI: 10.1021/acs.

[82] Chen Y, Lu S, Zhou J, Qin W, Wu X. Synergistically assembled Li2S/FWNTs@ reduced graphene oxide nanobundle

performance Li2S cathodes. Advanced Functional Materials. 2017;**27**:1700987.

[83] Kim DY, Kim M, Kim DW, Suk J, Park OO, Kang Y. Flexible binder-free graphene paper cathodes for highperformance Li-O2 batteries. Carbon. 2015;**93**:625-635. DOI: 10.1016/j.

forest for free-standing high-

DOI: 10.1002/adfm.201700987

carbon.2015.05.097

nanoen.2014.11.062

nanolett.5b00112

10.1039/C7TA07762A

carbon.2018.05.056

C3TA12826A

*DOI: http://dx.doi.org/10.5772/intechopen.84275*

synthesized self-standing electrode of MoS2 nanosheets assembled on graphene foam for high-performance Li-ion and Na-ion batteries. Journal of Alloys and Compounds. 2016;**660**:11-16. DOI: 10.1016/j.jallcom.2015.11.040

[72] Wang Y, Kong D, Shi W, Liu B, Sim G, Ge Q, et al. Ice Templated freestanding hierarchically WS2/CNTrGO aerogel for high-performance rechargeable lithium and sodium ion batteries. Advanced Energy Materials.

2016;**6**:1601057. DOI: 10.1002/

10.1016/j.jallcom.2017.10.094

carbon.2017.01.101

10.1002/cssc.201402329

[77] Ma X, Chen L, Ren X,

[74] An H, Li Y, Gao Y, Cao C, Han J, Feng Y, et al. Free-standing fluorine and nitrogen co-doped graphene paper as a high-performance electrode for flexible sodium-ion batteries. Carbon. 2017;**116**:338-346. DOI: 10.1016/j.

[75] Han K, Shen J, Hao S, Hao S, Ye H, Wolverton C, et al. Free-standing nitrogen-doped graphene paper as electrodes for high-performance lithium/dissolved polysulfide batteries. ChemSusChem. 2014;**7**:2545-2553. DOI:

[76] Kim DY, Kim M, Kim DW, Suk J, Park JJ, Park OO, et al. Graphene paper with controlled pore structure for high-performance cathodes in Li2O batteries. Carbon. 2016;**100**:265-272. DOI: 10.1016/j.carbon.2016.01.013

Hou G, Chen L, Zhang L, et al. Highperformance red phosphorus/ carbon nanofibers/graphene freestanding paper anode for sodium

[73] Zhang G, Liu K, Liu S, Song H, Zhou J. Flexible Co0.85Se nanosheets/ graphene composite film as binder-free anode with high Li- and Na-ion storage performance. Journal of Alloys and Compounds. 2018;**731**:714-722. DOI:

aenm.201601057

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario DOI: http://dx.doi.org/10.5772/intechopen.84275*

synthesized self-standing electrode of MoS2 nanosheets assembled on graphene foam for high-performance Li-ion and Na-ion batteries. Journal of Alloys and Compounds. 2016;**660**:11-16. DOI: 10.1016/j.jallcom.2015.11.040

*Energy Storage Devices*

10.1016/j.cej.2016.10.041

am403136e

adfm.201200697

10.1039/c5ra05038c

electacta.2015.09.080

graphene paper@Fe3O4 nanorod array@ carbon as integrated anode for robust lithium storage. Chemical Engineering Journal. 2017;**309**:272-277. DOI:

2017;**43**:7588-7593. DOI: 10.1016/j.

[66] Cao H, Zhou X, Deng W, Ma Z, Liu Y, Liu Z. Layer structured graphene/ porous ZnCo2O4 composite film for high performance flexible lithiumion batteries. Chemical Engineering Journal. 2018;**343**:654-661. DOI:

[67] Qian J, Chen Y, Wu L, Cao Y, Ai X, Yang H. High capacity Na-storage and superior cyclability of nanocomposite Sb/C anode for Na-ion batteries. Chemical Communications. 2012;**48**:

10.1016/j.cej.2018.03.001

[68] Darwiche A,Marino C,

[70] David L, Bhandavat R,

10.1021/nn406156b

Singh G. MoS2/Graphene composite paper for sodium-ion battery electrodes. ACS Nano. 2014;**8**:1759-1770. DOI:

[71] Xiang J, Dong D, Wen F, Zhao J, Zhong X, Wang L, et al. Microwave

Sougrati MT, Fraisse B, Stievano L, Monconduit L. Better cycling performances of bulk Sb in Na-ion batteries compared to Li-ion systems: An unexpected electrochemical mechanism. Journal of the American Chemical Society. 2012;**134**:20805- 20811. DOI: 10.1021/ja310347x

[69] Qian J, Wu X, Cao Y, Ai X, Yang H. High capacity and rate capability of amorphous phosphorus for sodium ion batteries. Angewandte Chemie, International Edition. 2013;**52**:4633- 4636. DOI: 10.1002/anie.201209689

[65] Zhao X, Wang G, Zhou Y, Wang H. Flexible free-standing ternary CoSnO3/graphene/carbon nanotubes composite papers as anodes for enhanced performance of lithiumion batteries. Energy. 2017;**118**: 172-180. DOI: 10.1016/j.energy.

ceramint.2017.03.051

2016.12.018

7070-7072

[59] Liu Y, Wang W, Gu L, Wang Y, Ying Y, Mao Y, et al. Flexible CuO nanosheets/reduced-graphene oxide composite paper: Binder-free anode for high performance lithium-ion batteries. ACS Applied Materials and Interfaces. 2013;**5**:9850-9855. DOI: 10.1021/

[60] Byon HR, Gallant BM, Lee SW, Yang SH. Role of oxygen functional groups in carbon nanotube/graphene freestanding electrodes for high performance lithium batteries. Advanced Functional Materials. 2013;**23**:1037-1045. DOI: 10.1002/

[61] Zhang L, Fan W, Liu T. A flexible free-standing defect-rich MoS2/ graphene/carbon nanotube hybrid paper as a binder-free anode for highperformance lithium ion batteries. RSC Advances. 2015;**5**:43130-43140. DOI:

[62] Wang J, Wang G, Wang H. Flexible

[63] Liu Y, Cai X, Shi W. Free-standing graphene/carbon nanotubes/CuO aerogel paper anode for lithium ion batteries. Materials Letters. 2016;**172**: 72-75. DOI: 10.1016/j.matlet.2016.02.068

[64] Li Y, Wang P, Bao Y, Huang K. A flexible nanostructured paper of MnO NPs@MWCNTs/r-GO multilayer sandwich composite for high-performance lithium-ion batteries. Ceramics International.

free-standing Fe2O3/graphene/ carbon nanotubes hybrid films as anode materials for high performance lithium-ion batteries. Electrochimica Acta. 2015;**182**:192-201. DOI: 10.1016/j.

**112**

[72] Wang Y, Kong D, Shi W, Liu B, Sim G, Ge Q, et al. Ice Templated freestanding hierarchically WS2/CNTrGO aerogel for high-performance rechargeable lithium and sodium ion batteries. Advanced Energy Materials. 2016;**6**:1601057. DOI: 10.1002/ aenm.201601057

[73] Zhang G, Liu K, Liu S, Song H, Zhou J. Flexible Co0.85Se nanosheets/ graphene composite film as binder-free anode with high Li- and Na-ion storage performance. Journal of Alloys and Compounds. 2018;**731**:714-722. DOI: 10.1016/j.jallcom.2017.10.094

[74] An H, Li Y, Gao Y, Cao C, Han J, Feng Y, et al. Free-standing fluorine and nitrogen co-doped graphene paper as a high-performance electrode for flexible sodium-ion batteries. Carbon. 2017;**116**:338-346. DOI: 10.1016/j. carbon.2017.01.101

[75] Han K, Shen J, Hao S, Hao S, Ye H, Wolverton C, et al. Free-standing nitrogen-doped graphene paper as electrodes for high-performance lithium/dissolved polysulfide batteries. ChemSusChem. 2014;**7**:2545-2553. DOI: 10.1002/cssc.201402329

[76] Kim DY, Kim M, Kim DW, Suk J, Park JJ, Park OO, et al. Graphene paper with controlled pore structure for high-performance cathodes in Li2O batteries. Carbon. 2016;**100**:265-272. DOI: 10.1016/j.carbon.2016.01.013

[77] Ma X, Chen L, Ren X, Hou G, Chen L, Zhang L, et al. Highperformance red phosphorus/ carbon nanofibers/graphene freestanding paper anode for sodium

ion batteries. Journal of Materials Chemistry A. 2018;**6**:1574-1581. DOI: 10.1039/C7TA07762A

[78] Sun N, Guan Y, Liu YT, Zhu Q, Shen J, Liu S, et al. Facile synthesis of free-standing, flexible hard carbon anode for high performance sodium ion batteries using graphene as a multifunctional binder. Carbon. 2018;**137**:475-483. DOI: 10.1016/j. carbon.2018.05.056

[79] Huang X, Sun B, Li K, Chen S, Wang G. Mesoporous graphene paper immobilised sulfur as a flexible electrode for lithium-sulfur batteries. Journal of Materials Chemistry A. 2013;**1**:13484-13489. DOI: 10.1039/ C3TA12826A

[80] Zhu L, Peng HJ, Liang J, Huang JQ, Chen CM, Guo X, et al. Interconnected carbon nanotube/graphene nanosphere scaffolds as free-standing paper electrode for high-rate and ultra-stable lithium-sulfur batteries. Nano Energy. 2015;**11**:746-755. DOI: 10.1016/j. nanoen.2014.11.062

[81] Wang C, Wang X, Yang Y, Kushima A, Chen J, Huang Y, et al. Slurryless Li2S/reduced graphene oxide cathode paper for high-performance lithium sulfur battery. Nano Letters. 2015;**15**:1796-1802. DOI: 10.1021/acs. nanolett.5b00112

[82] Chen Y, Lu S, Zhou J, Qin W, Wu X. Synergistically assembled Li2S/FWNTs@ reduced graphene oxide nanobundle forest for free-standing highperformance Li2S cathodes. Advanced Functional Materials. 2017;**27**:1700987. DOI: 10.1002/adfm.201700987

[83] Kim DY, Kim M, Kim DW, Suk J, Park OO, Kang Y. Flexible binder-free graphene paper cathodes for highperformance Li-O2 batteries. Carbon. 2015;**93**:625-635. DOI: 10.1016/j. carbon.2015.05.097

[84] Ozcan S, Tokur M, Cetinkaya T, Guler A, Uysal M, Guler MO, et al. Free standing flexible graphene oxide +α-MnO2 composite cathodes for Li–air batteries. Solid State Ionics. 2016;**286**:34-39. DOI: 10.1016/j. ssi.2015.12.016

Chapter 7

Sangwon Kim

peak power density

1. Introduction

115

energy, a shift in the energy mix.

technology should be secured.

Abstract

Vanadium Redox Flow Batteries:

The importance of reliable energy storage system in large scale is increasing to replace fossil fuel power and nuclear power with renewable energy completely because of the fluctuation nature of renewable energy generation. The vanadium redox flow battery (VRFB) is one promising candidate in large-scale stationary energy storage system, which stores electric energy by changing the oxidation numbers of anolyte and catholyte through redox reaction. This chapter covers the basic principles of vanadium redox flow batteries, component technologies, flow configurations, operation strategies, and cost analysis. The thermodynamic analysis of the electrochemical reactions and the electrode reaction mechanisms in VRFB systems have been explained, and the analysis of VRFB performance according to the flow field and flow rate has been described. It is shown that the limiting current density of "flow-by" design is more than two times greater than that of "flowthrough" design. In the cost analysis of 10 kW/120 kWh VRFB system, stack and

Electrochemical Engineering

electrolyte account for 40 and 32% of total cost, respectively.

Keywords: vanadium electrolyte, carbon electrode, overpotential, polarization, state of charge, flow-through, flow-by, flow rate, limiting current density,

The global environmental is changing rapidly. The established world's first energy demand and biggest carbon emitter countries are being replaced by emerging countries. The use of renewable energy is expanding due to technological development and environmental problems. The global energy market is moving toward the reduction of fossil fuels and the expansion of environment friendly

For stable supply of renewable energy with high volatility such as sunlight or wind power, securing stability of power system is the most important. To do this, an intelligent power network should be built up, and grid-based energy storage

The vanadium redox flow battery is one of the most promising secondary batteries as a large-capacity energy storage device for storing renewable energy [1, 2, 4]. Recently, a safety issue has been arisen by frequent fire accident of a largecapacity energy storage system (ESS) using a lithium ion battery. The vanadium electrolyte is a nonflammable aqueous solution and has a high heat capacity to limit the temperature rise. Therefore, VRFB has no risk of ignition and explosion.

[85] Jiang Y, Zou L, Cheng J, Huang Y, Jia L, Chi B, et al. Needle-like NiCo2O4 coated on graphene foam as a flexible cathode for lithium-oxygen batteries. ChemElectroChem. 2017;**4**:3140-3147. DOI: 10.1002/celc.201700864

#### Chapter 7

*Energy Storage Devices*

ssi.2015.12.016

[84] Ozcan S, Tokur M, Cetinkaya T, Guler A, Uysal M, Guler MO, et al. Free standing flexible graphene oxide +α-MnO2 composite cathodes for Li–air batteries. Solid State Ionics. 2016;**286**:34-39. DOI: 10.1016/j.

[85] Jiang Y, Zou L, Cheng J, Huang Y, Jia L, Chi B, et al. Needle-like NiCo2O4 coated on graphene foam as a flexible cathode for lithium-oxygen batteries. ChemElectroChem. 2017;**4**:3140-3147.

DOI: 10.1002/celc.201700864

**114**

## Vanadium Redox Flow Batteries: Electrochemical Engineering

Sangwon Kim

#### Abstract

The importance of reliable energy storage system in large scale is increasing to replace fossil fuel power and nuclear power with renewable energy completely because of the fluctuation nature of renewable energy generation. The vanadium redox flow battery (VRFB) is one promising candidate in large-scale stationary energy storage system, which stores electric energy by changing the oxidation numbers of anolyte and catholyte through redox reaction. This chapter covers the basic principles of vanadium redox flow batteries, component technologies, flow configurations, operation strategies, and cost analysis. The thermodynamic analysis of the electrochemical reactions and the electrode reaction mechanisms in VRFB systems have been explained, and the analysis of VRFB performance according to the flow field and flow rate has been described. It is shown that the limiting current density of "flow-by" design is more than two times greater than that of "flowthrough" design. In the cost analysis of 10 kW/120 kWh VRFB system, stack and electrolyte account for 40 and 32% of total cost, respectively.

Keywords: vanadium electrolyte, carbon electrode, overpotential, polarization, state of charge, flow-through, flow-by, flow rate, limiting current density, peak power density

#### 1. Introduction

The global environmental is changing rapidly. The established world's first energy demand and biggest carbon emitter countries are being replaced by emerging countries. The use of renewable energy is expanding due to technological development and environmental problems. The global energy market is moving toward the reduction of fossil fuels and the expansion of environment friendly energy, a shift in the energy mix.

For stable supply of renewable energy with high volatility such as sunlight or wind power, securing stability of power system is the most important. To do this, an intelligent power network should be built up, and grid-based energy storage technology should be secured.

The vanadium redox flow battery is one of the most promising secondary batteries as a large-capacity energy storage device for storing renewable energy [1, 2, 4]. Recently, a safety issue has been arisen by frequent fire accident of a largecapacity energy storage system (ESS) using a lithium ion battery. The vanadium electrolyte is a nonflammable aqueous solution and has a high heat capacity to limit the temperature rise. Therefore, VRFB has no risk of ignition and explosion.

The power of VRFB depends on the performance of the stack, and the energy storage capacity depends on the electrolyte concentration and the electrolyte reservoir size, which greatly increases the degree of freedom in system design [7, 24]. A schematic diagram of the vanadium redox flow battery is shown in Figure 1.

Flow batteries suffer from the capacity imbalance due to the mixing of the both

side active materials caused by the electrolyte diffusion across the membrane, resulting in an irreversible loss of capacity as well as an efficiency loss [10–14]. Since the vanadium redox flow battery uses vanadium as the active material of both electrolytes, the use of appropriate rebalancing techniques can mitigate capacity

The vanadium ion may have various oxidation numbers from bivalent to pentavalent. Using this property, vanadium is used as the electrolyte redox

+

Negative electrode: V2þ \$ V3þ þ e� E0 ¼ �0:255 V (1)

The permeation of the vanadium ions through the membrane occurs since any membrane cannot block the crossover of the redox species completely. The vanadium ions diffused to the counter electrolyte cause a cross-contamination reaction as below:

The self-discharging reactions caused by the vanadium ions permeated into the

2 þ e� þ 2Hþ \$ VO2þ þ H2O E0 ¼ þ1:004 V (2)

2 þ V2þ þ 2Hþ \$ VO2þ þ V3þ þ H2O E0 ¼ þ1:259 V (3)

VO2þ þ 2Hþ þ e� \$ V3þ þ H2O E0 ¼ þ0:34 V (4)

VO2þ þ V2þ þ 2Hþ ! 2V3þ þ H2O (5)

V2þ þ VO2þ þ 2Hþ ! 2 V3þ þ H2O (10)

2 þ 2V2þ þ 4Hþ ! 3V2þ þ 2H2O (6)

2 þ V3þ ! 2 VO2þ (7)

2 þ 2Hþ ! 3VO2þ þ H2O (8)

2 ! 2 VO2þ (9)

by V(V), V(IV), V(III), and V(II) for explanation. Solution of V(III) is added to the negative electrolyte tank, and solution of V(IV) is added to the positive electrolyte tank as shown in Figure 1. When the electricity is applied to the electrodes, the V(III) ion of the negative electrolyte is reduced to V(II), and the V(IV) ion of the positive electrolyte is oxidized to V(V). This means that when the VRFB is charged, the difference in the oxidation number between the positive electrolyte and negative electrolyte increases from +1 to +3, and it can be understood conceptually that the electric energy is stored in the increased bivalent oxidation number. When the VRFB is discharged, V(II) in negative electrolyte is oxidized to V(III), and V(V) in positive electrolyte is reduced to V(IV). The chemical

, VO2 +, V3 +, and V2 + are represented

loss though vanadium crossovers can lead to loss of efficiency.

2. Electrochemical reactions and kinetics

Vanadium Redox Flow Batteries: Electrochemical Engineering

DOI: http://dx.doi.org/10.5772/intechopen.85166

reactions for charge-discharge are expressed as follows:

counter electrolytes can be described as below:

VOþ

VOþ

V3þ þ VOþ

V2þ þ 2VOþ

couple material of the flow battery. VO2

Positive electrode: VOþ

Overall reaction: VOþ

Negative electrode:

Positive electrode:

117

#### Figure 1.

Schematic of vanadium redox flow batteries: (a) charging and (b) discharging. Reproduced with permission from [3]. Copyright 2017 by Elsevier.

The power of VRFB depends on the performance of the stack, and the energy

reservoir size, which greatly increases the degree of freedom in system design [7, 24]. A schematic diagram of the vanadium redox flow battery is shown in Figure 1.

Schematic of vanadium redox flow batteries: (a) charging and (b) discharging. Reproduced with permission

storage capacity depends on the electrolyte concentration and the electrolyte

Energy Storage Devices

Figure 1.

116

from [3]. Copyright 2017 by Elsevier.

Flow batteries suffer from the capacity imbalance due to the mixing of the both side active materials caused by the electrolyte diffusion across the membrane, resulting in an irreversible loss of capacity as well as an efficiency loss [10–14]. Since the vanadium redox flow battery uses vanadium as the active material of both electrolytes, the use of appropriate rebalancing techniques can mitigate capacity loss though vanadium crossovers can lead to loss of efficiency.

#### 2. Electrochemical reactions and kinetics

The vanadium ion may have various oxidation numbers from bivalent to pentavalent. Using this property, vanadium is used as the electrolyte redox couple material of the flow battery. VO2 + , VO2 +, V3 +, and V2 + are represented by V(V), V(IV), V(III), and V(II) for explanation. Solution of V(III) is added to the negative electrolyte tank, and solution of V(IV) is added to the positive electrolyte tank as shown in Figure 1. When the electricity is applied to the electrodes, the V(III) ion of the negative electrolyte is reduced to V(II), and the V(IV) ion of the positive electrolyte is oxidized to V(V). This means that when the VRFB is charged, the difference in the oxidation number between the positive electrolyte and negative electrolyte increases from +1 to +3, and it can be understood conceptually that the electric energy is stored in the increased bivalent oxidation number. When the VRFB is discharged, V(II) in negative electrolyte is oxidized to V(III), and V(V) in positive electrolyte is reduced to V(IV). The chemical reactions for charge-discharge are expressed as follows:

$$\text{Negative electrode: } \mathbf{V}^{2+} \leftrightarrow \mathbf{V}^{3+} + \mathbf{e}^- \ E^0 = -0.255 \text{ V} \tag{1}$$

$$\text{Positive electrode: } \text{VO}\_2^+ + \text{e}^- + 2\text{H}^+ \leftrightarrow \text{VO}^{2+} + \text{H}\_2\text{O} \, E^0 = +1.004 \text{ V} \tag{2}$$

$$\text{Overall reaction:}\ \mathrm{VO}\_{2}^{+} + \mathrm{V}^{2+} + 2\mathrm{H}^{+} \leftrightarrow \mathrm{VO}^{2+} + \mathrm{V}^{3+} + \mathrm{H}\_{2}\mathrm{O}\ E^{0} = +1.259\,\mathrm{V}\tag{3}$$

The permeation of the vanadium ions through the membrane occurs since any membrane cannot block the crossover of the redox species completely. The vanadium ions diffused to the counter electrolyte cause a cross-contamination reaction as below:

$$\text{VO}^{2+} + 2\text{H}^+ + \text{e}^- \leftrightarrow \text{V}^{3+} + \text{H}\_2\text{O} \, E^0 = +0.34 \text{ V} \tag{4}$$

The self-discharging reactions caused by the vanadium ions permeated into the counter electrolytes can be described as below:

Negative electrode:

$$\text{V}\text{O}^{2+} + \text{V}^{2+} + 2\text{H}^{+} \rightarrow 2\text{V}^{3+} + \text{H}\_{2}\text{O} \tag{5}$$

$$\text{VO}\_2^+ + 2\text{V}^{2+} + 4\text{H}^+ \rightarrow 3\text{V}^{2+} + 2\text{H}\_2\text{O} \tag{6}$$

$$\text{VO}\_2^+ + \text{V}^{3+} \rightarrow 2\text{VO}^{2+} \tag{7}$$

Positive electrode:

$$\text{V}^{2+} + 2\text{VO}\_2^{+} + 2\text{H}^{+} \rightarrow 3\text{VO}^{2+} + \text{H}\_2\text{O} \tag{8}$$

$$\text{V}^{\text{3}^+} + \text{VO}\_2^+ \rightarrow 2\text{VO}^{\text{2}+} \tag{9}$$

$$\text{V}^{2+} + \text{VO}^{2+} + 2\text{H}^{+} \rightarrow 2\text{ V}^{3+} + \text{H}\_{2}\text{O} \tag{10}$$

When the VRFB is overcharged, hydrogen and oxygen gas can be generated at the negative and positive electrodes, respectively. Additionally, the carbon dioxide gas can be generated by corrosion of graphite plate with the produced oxygen gas.

Negative electrode:

$$\text{2H}^+ + \text{2e}^- \rightarrow \text{H}\_{2(gas)}\tag{11}$$

where φs is the electric potential of the solid electrode and φl is the electrolyte

The standard open-circuit voltage of VRFB, E0 = 1.26 V, can be derived from

However, the actual operating voltage of VRFB differs from this thermodynamic

At discharge, the operating voltage becomes smaller than theoretical value. As the current density increases, the overpotential and iR drop increase, so the charging voltage increases and the discharging voltage decreases as shown in Figure 3c. Energy density and power density can be calculated in Eqs. (25) and (26), respectively.

L � 26:8 Ah

where n is the number of electrons transferred during reactions, C is a vanadium

electrolyte concentration, 1.6 mol/L, Vdischarge is averaged discharge voltage, and

Charge-discharge voltage of vanadium redox flow battery: Current vs. voltage and overpotential and open-

mol � 1:3 V

Power denisty ¼ current density � Vdischarge (26)

overpotential is required in addition to the thermodynamic voltage. Figure 2 shows the relationship of the voltage and current during charging and discharging at the two electrodes of VRFB, assuming that the overall kinetics are determined by the

value. Charging voltage should be larger than 1.26 V since the amount of

Echarge ¼ E0

Edischarge ¼ E0

where ηa is anodic overpotential and ηc is cathodic overpotential.

¼ 1 � 1:6mol

ΔG0 ¼ ΔH0 � TΔS0 ¼ �nFE0 ¼ �119:3 kJ=mol (22)

cell þ ηa þ ηc þ iRtotal (23)

cell � ηa � ηc � iRtotal (24)

2 ¼ 27:872 Wh=L (25)

potential.

Gibbs free energy relation as below:

DOI: http://dx.doi.org/10.5772/intechopen.85166

Energy density ¼ nCFVdis

Figure 2.

119

charge transfer in the electrochemical reaction.

Vanadium Redox Flow Batteries: Electrochemical Engineering

Ntank

circuit voltage at positive electrode and negative electrode.

Positive electrode:

$$\text{C} + 2\text{H}\_2\text{O} \rightarrow \text{CO}\_{2(gas)} + 4\text{H}^+ + 4\text{e}^- \tag{12}$$

$$\text{2H}\_2\text{O} \rightarrow \text{O}\_{2(gas)} + 4\text{H}^+ + 4\text{e}^- \tag{13}$$

$$\text{C} + \text{O}\_{2(gas)} \rightarrow \text{CO}\_{2(gas)}\tag{14}$$

The equilibrium cell potentials, Eeq for each reaction, are calculated using Nernst equation according to

$$E\_{eq, \text{neg}} = E\_{\text{neg}}^0 + \frac{RT}{F} \ln\left(\frac{C\_{V^{\flat+}}}{C\_{V^{2+}}}\right) \tag{15}$$

$$E\_{eq,pos} = E\_{pos}^0 + \frac{RT}{F} \ln\left(\frac{C\_{VO\_2^+} \left(c\_{H^+}\right)^2}{C\_{VO^{2+}}}\right) \tag{16}$$

$$E\_{eq,overall} = E\_{overall}^0 + \frac{RT}{F} \ln\left(\frac{C\_{VO\_2^+} \left(C\_{H^+}\right)^2}{C\_{VO^{2+}}} \frac{C\_{V^{2+}}}{C\_{V^{3+}}}\right) \tag{17}$$

where Ci\* is the concentration of the i species; E0 is the standard cell potential for the electrode reaction; R is the ideal gas constant, 8.314 J/mol K; T is the cell temperature; and F is Faraday's constant, 96,485 As/mol.

The exchange current density is the magnitude of the current when the electrode reactions reach the equilibrium and can be described as

$$i\_{0, \text{neg}} = Fk\_{\text{neg}}^0 \mathbf{C}\_{V^{\beta+}}^{\*\left(1 - a\_{\text{neg}}\right)} \mathbf{C}\_{V^{2+}}^{\*a\_{\text{neg}}} \tag{18}$$

$$\dot{a}\_{0,pos} = Fk\_{pos}^0 \mathcal{C}\_{VO\_2^+}^{\*\left(1-a\_{pu}\right)} \mathcal{C}\_{VO^{2+}}^{\*a\_{pu}} \tag{19}$$

where k0 is the standard rate constant.

Following the Butler-Volmer equation [5, 24], the currents at negative electrode and positive electrode are described as

$$\dot{\mathbf{u}}\_{\text{neg}} = \dot{\mathbf{u}}\_{0,\text{neg}} \left[ \left( \frac{\mathbf{C}\_{V^{\beta+}}(\mathbf{0}, t)}{\mathbf{C}\_{V^{\beta+}}^{\*}} \right) \exp\left( -\frac{a\_{\text{neg}} F}{RT} \eta\_{\text{neg}} \right) - \left( \frac{\mathbf{C}\_{V^{2+}}(\mathbf{0}, t)}{\mathbf{C}\_{V^{2+}}^{\*}} \right) \exp\left( \frac{(\mathbf{1} - a\_{\text{neg}})F}{RT} \eta\_{\text{neg}} \right) \right] \tag{20}$$

$$i\_{pas} = i\_{0, pas} \left[ \left( \frac{\mathbf{C}\_{VO\_2^+} (0, t)}{\mathbf{C}\_{VO\_2^+}^\*} \right) \exp\left( -\frac{a\_{pas} F}{RT} \eta\_{pas} \right) - \left( \frac{\mathbf{C}\_{VO^{2+}} (0, t)}{\mathbf{C}\_{VO^{2+}}^\*} \right) \exp\left( \frac{(1 - a\_{pas}) F}{RT} \eta\_{pas} \right) \right] \tag{21}$$

where α is the transfer coefficient or symmetry factor and η is the overpotential, defined as η ¼ ϕs � ϕl � Eeq:

Vanadium Redox Flow Batteries: Electrochemical Engineering DOI: http://dx.doi.org/10.5772/intechopen.85166

When the VRFB is overcharged, hydrogen and oxygen gas can be generated at the negative and positive electrodes, respectively. Additionally, the carbon dioxide gas can be generated by corrosion of graphite plate with the produced

The equilibrium cell potentials, Eeq for each reaction, are calculated using Nernst

RT

F ln CV3þ CV2þ � �

CVOþ

CVOþ

2 cH ð Þ þ 2 CVO2þ !

> 2 CH ð Þ þ 2 CVO2þ

!

CV2þ CV3þ

V2þ (18)

VO2þ (19)

1 � αneg � �F

1 � αpos � �F RT ηpos

RT ηneg

(20)

(21)

exp

exp

neg þ

RT F ln

RT F ln

where Ci\* is the concentration of the i species; E0 is the standard cell potential for the electrode reaction; R is the ideal gas constant, 8.314 J/mol K; T is the cell

The exchange current density is the magnitude of the current when the electrode

negC∗ ð Þ 1�αneg

posC∗ ð Þ 1�αpos VOþ 2

Following the Butler-Volmer equation [5, 24], the currents at negative electrode

V3þ C∗ αneg

C∗ αpos

� CV2þ ð Þ 0; t C∗ V2þ � �

� CVO2þ ð Þ 0; t C∗ VO2þ

!

� � � �

" # � �

where α is the transfer coefficient or symmetry factor and η is the overpotential,

pos þ

overall þ

i0,neg ¼ Fk0

i0,pos ¼ Fk0

exp � αnegF

exp � αposF

RT ηneg � �

RT ηpos � �

Eeq,neg ¼ E0

Eeq,pos ¼ E0

Eeq, overall ¼ E0

temperature; and F is Faraday's constant, 96,485 As/mol.

reactions reach the equilibrium and can be described as

where k0 is the standard rate constant.

and positive electrode are described as

CV3þ ð Þ 0; t C∗ V3þ � �

!

CVOþ 2 ð Þ 0; t C∗ VOþ 2

defined as η ¼ ϕs � ϕl � Eeq:

ineg ¼ i0,neg

ipos ¼ i0,pos

118

2Hþ þ 2e� ! H2 gas ð Þ (11)

(15)

(16)

(17)

C þ 2H2O ! CO2 gas ð Þ þ 4Hþ þ 4e� (12) 2H2O ! O2 gas ð Þ þ 4Hþ þ 4e� (13) C þ O2 gas ð Þ ! CO2 gas ð Þ (14)

oxygen gas.

Negative electrode:

Energy Storage Devices

Positive electrode:

equation according to

where φs is the electric potential of the solid electrode and φl is the electrolyte potential.

The standard open-circuit voltage of VRFB, E0 = 1.26 V, can be derived from Gibbs free energy relation as below:

$$
\Delta G^{0} = \Delta H^{0} - T\Delta S^{0} = -nFE^{0} = -\mathbf{1} \mathbf{1} \mathbf{9}.\mathbf{3}\text{ kJ/mol} \tag{22}
$$

However, the actual operating voltage of VRFB differs from this thermodynamic value. Charging voltage should be larger than 1.26 V since the amount of overpotential is required in addition to the thermodynamic voltage. Figure 2 shows the relationship of the voltage and current during charging and discharging at the two electrodes of VRFB, assuming that the overall kinetics are determined by the charge transfer in the electrochemical reaction.

$$E\_{charge} = E\_{cell}^{0} + \eta\_a + \eta\_c + iR\_{total} \tag{23}$$

$$E\_{discharge} = E\_{cell}^0 - \eta\_a - \eta\_c - iR\_{total} \tag{24}$$

where ηa is anodic overpotential and ηc is cathodic overpotential.

At discharge, the operating voltage becomes smaller than theoretical value. As the current density increases, the overpotential and iR drop increase, so the charging voltage increases and the discharging voltage decreases as shown in Figure 3c. Energy density and power density can be calculated in Eqs. (25) and (26), respectively.

$$\text{Energy density} = \frac{nCFV\_{di}}{N\_{tank}} = \frac{1 \times \frac{1.6 \text{mol}}{L} \times 26.8 \frac{Ah}{mol} \times 1.3 \text{ V}}{2} = 27.872 \text{ Wh/L} \tag{25}$$

$$\text{Power density} = \text{current density} \times \overline{V}\_{dickary} \tag{26}$$

where n is the number of electrons transferred during reactions, C is a vanadium electrolyte concentration, 1.6 mol/L, Vdischarge is averaged discharge voltage, and

Figure 2.

Charge-discharge voltage of vanadium redox flow battery: Current vs. voltage and overpotential and opencircuit voltage at positive electrode and negative electrode.

EE ¼ discharge energy Wh ð Þ

DOI: http://dx.doi.org/10.5772/intechopen.85166

Ð

disV tð Þdt=tdis Ð chV tð Þdt=tch

Vanadium Redox Flow Batteries: Electrochemical Engineering

¼ Idis ∙tdis Ich ∙tch

vanadium ions [8, 9]:

3. Electrode

charge energy Wh ð Þ � 100% ¼

SOC ¼ CV2þ

stack and overall battery system efficiency [15, 16].

3.1 Reaction mechanism at carbon felt electrode

[17, 18]. Oxidation and reduction mechanisms of the VO2

operating potential window.

121

CV2þ þ CV3þ

in the reaction. The electrode material influences the performance of VRFB diversely. The electrode should be electrochemically stable in the operating potential window of VRFB. The electrochemical activity of electrode affects the chargedischarge voltages and consequently the voltage efficiency during battery cycle operation. The electrode must have high electrical conductivity to increase the charge transfer speed. The charge transfer speed is related the ohmic losses, cell voltage, and energy efficiency. The vanadium can be dissolved in strong acidic aqueous solution; therefore the electrode should be chemically stable in strong acidic condition. The chemical stability of the electrode in acid electrolyte is related to the corrosion resistance when oxygen is generated at the positive electrode during overcharged and determines the lifetime of VRFB. The porosity of the electrode affects the pumping energy loss, which affects pressure drop across the

The electrode provides the active sites for the redox reaction of redox couples dissolved in the electrolyte notwithstanding the electrode itself does not participate

Various carbon materials including carbon felt, graphite felt, and carbon paper have been extensively studied as electrodes for VRFB. Especially, carbon felts are considered to be suitable for use as electrodes of VRFB because of their wide specific surface area, high electrical conductivity, high chemical stability, and wide

Sun and Skyllas-Kazacos reported that the C-OH functional group acts as an active site for oxidation of VO2+ and reduction of V3+ on the surface of the electrode

+

/VO2+ and V2+/V3+

Ð

Ð

¼ CVOþ

CVOþ

2

2 þ CVO2þ

� 100% ¼ CE � VE

Voltage efficiency (VE) is the average discharge voltage to the average charge voltage. Figure 3a shows the charging and discharging curves of VRFB in constant current mode, in which the current is maintained as constant value during chargedischarge cycle. While the current is constant during charge-discharge, the voltage is not constant but gradually changing in the whole cycle. Voltage efficiency represents a measure of electrical resistance loss and the polarization properties of battery. The polarization plot in Figure 3c coincides with the voltage efficiency trend in Figure 3b. Energy efficiency is the ratio of available energy to stored energy, which can be calculated as the product of voltage efficiency and current efficiency. It is important to monitor the charging status of VRFB since especially overcharging the battery results in gas evolution side reactions, cell resistance increase, and capacity loss. Normally, VRFB is operated in charge range of 20–80%. The status of charge (SOC) is defined as the following using the concentrations of

disI tð ÞV tð Þdt

chI tð ÞV tð Þdt � 100%

(29)

(30)

Figure 3.

Vanadium redox flow battery performance: (a) cell voltage and open-circuit voltage profiles at current density of 60 mA/cm2 , (b) efficiencies depending on current densities, (c) polarization plot of the unit cell, and (d) energy density and power density.

Ntank is a number of tank. There are only three variables that contribute to increasing energy density and power density: the vanadium ion concentration, discharging voltage, and current density. However, the concentration of the vanadium ions is limited by low solubilities of vanadium ions in aqueous solution. The discharging voltage and current density are restricted by the electrochemical activities of vanadium electrolyte. Figure 3d shows that as a current density increases, energy density decreases, and power density increases. Normal operating current density range is 50–80 mA/cm2 , and stored energy density is in the range of 25–35 Wh/L or 20–32 Wh/kg. The corresponding power density is less than 0.1 W/cm2 .

The performance of VRFB can be measured with three efficiencies: current efficiency, voltage efficiency, and energy efficiency, which are defined in Eqs. (27), (28), and (29), respectively. The current efficiency (CE, Coulombic efficiency) is defined as the ratio of the amount of usable charge to the stored charge amount, that is, the discharge capacity divided by the charge capacity. CE is a measure of the storage capacity loss during charge-discharge process. The capacity loss is mainly caused by the crossover of the electrolyte ions through the membrane. The mixed active materials result in a capacity imbalance between the anode and cathode electrolytes and an irreversible capacity loss.

$$\begin{split} \text{CE} &= \frac{\text{discharge capacity}}{\text{charge capacity}} \times 100\text{\%} = \frac{\int\_{di} I(t)dt}{\int\_{ch} I(t)dt} \times 100\text{\%} = \frac{I\_{dis} \bullet t\_{dis}}{I\_{ch} \bullet t\_{ch}} \times 100\text{\%} \\ &= \frac{t\_{dis}}{t\_{ch}} \times 100\text{\% (If } I\_{dis} = I\_{ch} \text{ )} \end{split} \tag{27}$$

$$\text{VE} = \frac{\text{average discharge voltage}}{\text{average charge voltage}} \times 100\text{\%} = \frac{\int\_{di} V(t)dt \Big/\_{t\_{dis}}}{\int\_{di} V(t)dt \Big/\_{t\_{dis}}} \times 100\text{\%} \tag{28}$$