supercapacitors ( strictly , 
 electric double - layer capacitors ) 
 store charge at the interface between porous carbon electrodes and 
 an electrolyte solution ( figure 1 ) .
in contrast to batteries , charge storage in supercapacitors 
 is non - faradaic and occurs by the physical adsorption and desorption 
 of ions inside the pores of the carbon electrodes when an external 
 voltage is applied .
as electronic charge accumulates in an electrode , 
 it is balanced at the interface by an equal and opposite ionic charge 
 in the electrolyte .
this physical mechanism of charge storage gives 
 rise to fast charge and discharge times and long cycle lives , characteristic 
 properties that make supercapacitors attractive devices to complement 
 batteries ( which can store and deliver more energy but with slower 
 charge and discharge times ) .
today , supercapacitors are used in a 
 range of industrial , automotive , and electric utility applications 
 including electric buses , trains , uninterruptible power supply systems , 
 elevators , camera flashes , cranes , and engine starters
. their more widespread use could be facilitated by the development 
 of new devices with improved energy densities , which retain the high 
 power densities and long cycle lives that are characteristic of supercapacitors .
porous carbon materials with 
 disordered structures are used as the electrodes , and the cell is 
 soaked with an electrolyte that may be organic , aqueous or ionic liquid - based , 
 with some typical electrolytes shown .
note , for simplicity the separator 
 ( which prevents short circuit ) , the binder that holds the electrode 
 materials together and the current collectors are not shown .
schematic 
 porous carbon structure adapted from ref ( 17 ) with permission from springer .
porous 
 carbon electrode 
 materials are generally prepared by the heat treatment and subsequent 
 chemical activation of organic materials , such as coconut shells and 
 wood , while a related class of materials , 
  carbide - derived carbons  ( cdcs ) , are obtained from 
 metal carbides by extracting the metal atoms .
more exotic materials such as carbon nanotubes ( cnts ) and graphenes 
 are also being developed for supercapacitor application , but here 
 we will focus our attention on disordered porous carbons ( activated 
 carbons ) as they are well - studied and widely used in commercial devices 
 due to their cheap price , facile synthesis , and sustainability . for 
 the electrolyte ,
the most widely used systems are comprised of salts 
 dissolved in organic solvents ( e.g. , tetraethylammonium tetrafluoroborate 
 in acetonitrile solvent , net4bf4/acn ) . 

such organic electrolytes offer a good balance of relatively large 
 maximum operating voltages ( 2.5 v ) and high ionic conductivities 
 ( 2060 mscm ) .
the stored 
 energy , e , is given by1where c is the cell capacitance , 
 and v is the operating voltage .
thus , organic electrolytes 
 are typically preferred to aqueous electrolytes that are limited to 
 1 v before water decomposes .
aqueous - based 
 systems are , however , being studied for applications where cost is 
 a critical parameter ( e.g. , on the electricity grid ) .
room - temperature ionic liquids are also emerging as alternative 
 electrolytes for supercapacitors , with operating voltages as high 
 as 4 v achievable . however , the 
 increase in energy by eq 1 comes at a cost , as slower ionic transport ( ionic conductivities 
 are typically below 20 mscm ) results in 
 poorer device power performances .
most efforts to increase the 
 energy density of supercapacitors 
 have focused on the development of new carbon materials with increased 
 capacitances , so that more energy can be stored , as shown by eq 1 .
however , the success 
 of such an approach requires an understanding of the carbon structure , 
 and how this in turn affects the charge storage mechanism and the 
 capacitance .
this is a complex problem , as porous carbon materials 
 lack long - range order , making their characterization 
 challenging .
these materials do , however , exhibit order on a local scale ( at length scales up to 10 or 20  ) . while 
 analysis of the broad bragg peaks in diffraction experiments offers 
 little information , pair distribution function ( pdf )
analysis and 
 nuclear magnetic resonance ( nmr ) experiments show that typical activated 
 carbons and cdcs consist of predominantly sp - hybridized 
 carbon atoms organized in a hexagonal arrangement .
recent studies have suggested 
 that non - hexagonal rings , such as 5-membered and 7-membered rings , are also present , 
 giving rise to curvature in the carbon sheets ( see schematic structure 
 in figure 1 ) .
the curved and defective carbon sheets and fragments do not pack 
 together well ( structures are typically non - graphitizing , with graphite 
 not formed even on heating to temperatures as high as 3000 c ) , and nanometer - sized pores exist between the various carbon surfaces 
 ( see figure 1 ) .
this 
 porosity is typically characterized by pore size distributions obtained 
 from the analysis of gas sorption isotherms .
state of the art methods 
 use classical density functional theory ( dft ) to obtain the pore size 
 distribution .
the recently developed nldft ( non - local dft ) and 
 qsdft ( quenched solid dft ) methods both rely on the assumption that 
 the porous material is a collection of pores with identical shapes 
 ( typically slit - shaped pores ) , and with different pore widths .
classical 
 dft is used to determine gas adsorption profiles inside the pores , 
 and a set of partial isotherms corresponding to different pore sizes 
 are generated .
the pore size distribution is then obtained by fitting 
 the experimental isotherm to a sum of partial isotherms .
owing to 
 the relative ease of this technique , pore size distributions are the 
 most frequently used metric in the characterization of carbon structure , 
 and have been used as the primary tool in the search for relationships 
 between carbon structure and capacitance .
seminal studies in 
 2006 showed that the carbon capacitance could 
 be increased by optimizing the pore size of the carbon electrodes .
an  anomalous  increase in capacitance was observed 
 as the carbon pore size was decreased below 1 nm .
these results showed 
 the importance of pore size , and challenged the previous view that 
 pores smaller than the solvated electrolyte ions do not contribute 
 significantly to the capacitance .
it was hypothesized that in small 
 pores ion desolvation allows a closer approach of charge centers at 
 the electrode  electrolyte interface , which increases the capacitance .
however , the capacitance increase was also observed in experimental , and theoretical , studies of ionic liquids , with the capacitance maximized 
 when the pore size matched the ion size .
it was even shown that the 
 capacitance varies in an oscillatory fashion as the ( slit-)pore width 
 is varied .
these findings indicate that factors beyond simple ion desolvation 
 arguments are responsible for the anomalous capacitance increase . 

interestingly , some experimental studies have reported that there 
 is no correlation between pore size and capacitance , with the origin of these differences currently unclear
. needless 
 to say , the observation of pore size effects led to a wave of interest 
 in the mechanisms of charge storage in porous carbon electrodes . in the past 10 years , pioneering experimental and theoretical studies 
 have led to an improved understanding of the molecular mechanisms 
 of charge storage in supercapacitors .
this perspective seeks to bring 
 together recent findings from a range of studies to provide a coherent 
 and detailed view of modern theories of supercapacitor charging mechanisms . 

recent studies have shown that charging is not simply driven by adsorption 
 of counter - ions into the electrode pores , as was previously believed 
 ( note , counter - ions are defined as having opposite charge to the electrode ) . 

nmr experiments and molecular dynamics ( md ) simulations show that 
 porous carbon electrodes typically contain a large number of ions 
 in the absence of an applied potential , such that a range of different 
 charging mechanisms , based on different amounts of ion adsorption 
 and desorption , are then possible when a voltage is applied .
new in situ experimental techniques ( nmr spectroscopy , electrochemical 
 quartz crystal microbalance ( eqcm ) , infrared ( ir ) spectroscopy , and 
 scattering approaches ) have shown that the charging mechanism is sensitive 
 to the electrode and electrolyte materials used , as well as the polarization 
 of the electrode . understanding and
controlling the charge storage 
 mechanism of supercapacitors may hold the key to developing next generation 
 devices with enhanced properties , and we identify new areas of research 
 that can facilitate this process .
before embarking on the study of supercapacitor 
 charging mechanisms , 
 it is crucial to have a detailed understanding of the structure of 
 the electrode  electrolyte interface in the absence of an applied 
 potential ( i.e. , at 0 v ) . as illustrated by figure 2a
, the carbon nanopores may be either filled 
 with electrolyte ions , or they may contain no ions .
these two possibilities 
 have been described as  ionophilic  and  ionophobic  

together , md 
 simulations and nmr spectroscopy experiments have shown that the pores 
 of the carbon electrodes are generally filled at 0 v. coarse - grained 
 md simulations of the ionic liquid butylmethylimidazolium hexafluorophosphate 
 ( bmi  pf6 ) showed a large number of ions inside the 
 carbon nanopores of realistic carbon ( cdc ) 
 structures at zero applied potential ( table 1 ) . with the 
 md approach
, subtly different local adsorption environments could 
 be identified for confined ions , sitting at sheet edges , on top of 
 sheet planes , inside curved hollows , and inside small pockets .
pf6 dissolved in acn to 
 1.5 m ( i.e. , an organic electrolyte ) has also been studied under nanoconfinement , 
 with a considerable number of in - pore ions again observed ( figure 2b ) . in this case 

the solvent molecules replace some of the in - pore ions , leading to 
 a lower in - pore population relative to the neat ionic liquid study , 
 with the small solvent molecules tending to occupy the most confined 
 pore sites .
other theoretical studies based on idealized electrode 
 geometries , such as cnts and slit pores , have also shown spontaneous pore - filling behavior .
( a ) schematic illustrating 
 the concept of filled and empty carbon 
 pores at 0 v. the carbon ( slit-)pore walls are represented by black 
 rectangles .
( b ) snapshot of an md simulation showing the presence 
 of in - pore ions and solvent molecules at 0 v. red , blue , and green 
 correspond to cations , anions , and solvent molecules , respectively , 
 while the carbon surfaces are gray .
( c ) nmr ( 9.4 t ) measurements of yp50f activated carbon soaked 
 with a typical supercapacitor electrolyte , net4bf4/dacn ( 1.5 m ) , recorded with magic angle spinning at a frequency 
 of 5 khz .
note , deuterated acetonitrile ( dacn ) was used here to allow a convenient 
 separation of the signals from the cations and the solvent molecules . 

md simulations and nmr experiments both reveal a significant number 
 of in - pore ions in the absence of an applied potential . in the nmr studies the commercially 
 activated carbon yp50f
was studied in all cases , except for in ref ( 55 ) , where activated carbons 
 derived from poly(ether - ether - ketone ) ( peek ac ) were studied . in
most 
 of the md simulations listed , idealized carbon geometries such as 
 slit pores and multi - walled carbon nanotubes ( mwcnts ) were studied , 
 with the exception of the studies of model cdc electrodes in refs 
 ( 39 ) and ( 40 ) . for the calculation of the gravimetric 
 in - pore population from ref ( 41 ) , only the inner wall of the mwcnt
note : pyr13tfsi is 1-methyl-1-propylpyrrolidinium 
 bis(trifluoromethanesulfonyl)imide , and emi  tfsi is 1-ethyl-3-methylimidazolium 
 bis(trifluoromethanesulfonyl)imide .
experimental results from nmr spectroscopy are in 
 excellent accord 
 with the findings from md simulations , also revealing the predominance 
 of filled pores in the absence of an applied potential ( see table 1 ) . in nmr experiments , 
 ions and solvent molecules inside carbon pores (
give rise to peaks which are distinct to those from species in bulk 
 electrolyte between the carbon particles (  ex - pore  ) . by studying different nmr nuclei ,
the anions , cations , and solvent 
 molecules can be studied separately . for example , for yp50f ( a commercial 
 activated carbon ) soaked with the electrolyte net4bf4 in acn ( 1.5 m )
, clear resonances can be distinguished for 
 in - pore anions ( f nmr ) , cations ( h nmr ) , 
 and solvent ( h nmr ) ( figure 2c ) . in each case
the in - pore peak appears 
 at a negative chemical shift ( relative to the ex - pore ) due to the 
 local magnetic field originating from the circulation of the carbon s 
 delocalized  electrons in the applied magnetic field .
these effects are largely independent 
 of the choice of nmr nucleus , such that anions , cations , and solvent 
 molecules all experience a similar change of chemical shift upon adsorption 
 in a given carbon . since nmr is quantitative , the number of ions inside 
 the carbon pores can be readily determined by fitting and integration 
 of the spectra .
nmr studies have been carried out on carbons 
 soaked with a range 
 of organic electrolytes .
for pet4bf4 in acn ( 1.5 m ) and the 
 carbon yp50f , there was 0.86 mmol of in - pore anions and cations per 
 gram of carbon , with this number decreasing at lower electrolyte concentrations 
 ( table 1 ) .
nmr studies of ionic liquids and yp50f carbon have revealed larger numbers of in - pore ions at 
 0 v ( table 1 ) .
when the ionic liquid samples were diluted with 
 acn ( to 1.8 m ) , the in - pore ion populations dropped as solvent 
 molecules displaced some of the ions ( table 1 ) .
this is consistent 
 with md simulations on model cdc electrodes with and without a solvent 
 present ( table 1 ) . 

the nmr approach has also recently been applied to study the confinement 
 of aqueous electrolytes , highlighting the versatility 
 of the method .
it was shown that for a carbon 
 with a relatively small average pore width ( 0.58 nm ) soaked with naf 
 ( 0.8 mol kg aq ) , the ions were unable to enter 
 the carbon pores in the absence of an applied potential , despite the 
 presence of in - pore water .
measurements 
 on a carbon with larger pores ( average width 1.55 nm ) showed there 
 were a considerable number of in - pore ions , confirming that the absence 
 of in - pore ions in the former carbon arose from steric factors . in 
 the absence of any steric effects , it is currently unknown if it is 
 possible to synthesize carbons with ionophobic pores ( see later ) . 

electrolyte interface 
 in aqueous systems is highly complex , as the chemical nature of the 
 carbon surface will depend on the ( de)protonation of the various functional 
 groups , and oh and h3o ions 
 are present alongside the main electrolyte ions . beyond nmr 
 spectroscopy and md simulations , small - angle x - ray scattering 
 ( saxs ) and small - angle neutron scattering ( sans ) have also emerged 
 as experimental methods to probe the wetting of carbon pores in the 
 absence of an applied potential .
changes of the scattered neutron intensity after 
 the addition of acn to activated carbon fabric suggested that the 
 solvent had wetted the carbon nanopores .
interestingly , this approach allows wetting to be studied as a function 
 of pore size , and it was shown that the pore wetting of activated 
 carbon fabrics was incomplete for the smallest nanopores .
in contrast to nmr and md simulations , however , 
 these methods have not yet allowed an absolute quantification of the 
 numbers of in - pore anions , cations , and solvent molecules .
given the large 
 number of ions inside the carbon pores at 0 v , a 
 range of different charging mechanisms are possible ( figure 3 ) .
a second possibility is 
 that counter - ion adsorption is accompanied by simultaneous co - ion desorption 
 from the pores , which we refer to as ion exchange ( where co - ions are 
 defined as having charge with the same sign as the electrode ) .
a third 
 possibility is that charging is driven purely by the desorption of 
 co - ions ( figure 3 ) . 
 in each case
the excess ionic charge inside the carbon 
 pores is equal and opposite to the electronic charge 
 stored in the carbon . in practice , charging may involve a combination 
 of the different mechanisms shown in figure 3 .
for example , ion - exchange and counter - ion 
 adsorption could occur simultaneously .
different possible charging mechanisms for carbon 
 pores that are 
 initially filled with electrolyte : counter - ion adsorption , ion exchange , 
 and co - ion desorption .
the different charging mechanisms may be described 
 by the charging mechanism parameter , x , as defined 
 in eq 2 .
example calculations 
 of x ( using eq 2 ) are shown for the three depicted charging mechanisms , with v0 taken as 0 v. a value of x = + 1 is obtained for charging solely by counter - ion adsorption , while x = 0 is obtained for ion exchange , and x = 1 for co - ion desorption . as indicated by the scale on 
 the right , x is a continuous variable ( e.g. , an x value intermediate between 1 and 0 would indicate that 
 both io - exchange and counter - ion adsorption processes occur during 
 charging ) .
part of this figure is adapted from ref ( 49 ) with permission from the 
 royal society of chemistry .
here we 
 introduce the charging mechanism parameter , x(v , v0 ) , defined 
 as2where n(v ) is the total number of in - pore ions at a charging voltage v , n(v0 ) is 
 the total number of in - pore ions at some initial voltage v0 ( typically 0 v ) , and ncounter(v ) and nco(v ) are the number of in - pore counter - ions and co - ions , respectively , 
 at a voltage v. qionic(v ) and qionic(v0 ) are then the net in - pore ionic charges at 
 the two voltages , and e is the elementary charge . 

put simply , x gives a measure of the roles of counter - ion 
 adsorption , ion exchange , and co - ion desorption in the charging mechanism . 

for the classical charging mechanism of counter - ion adsorption , x = + 1 . for ion exchange ,
x = 0 , while 
 for co - ion desorption , x = 1 ( see example 
 calculations in figure 3 ) .
intermediate values of x are also possible ; for 
 example , x = 0.3 would indicate that both ion exchange 
 and counter - ion adsorption occur during charging , with ion exchange 
 dominating ( as 0.3 is closer to 0 than it is to 1 ) .
we stress that 
 this parameter refers to charging ; i.e. , |v| must 
 be greater than |v0| .
it is also important 
 to realize that x can take different values in the 
 positive and negative electrodes , such that separate calculations 
 must be performed for each , and that x may also depend 
 on the studied voltage range .
we note that for pores that are initially empty , charging 
 must 
 at least initially proceed by counter - ion adsorption ( x = 1 ) , as there are no in - pore co - ions available for desorption . 
 for such ionophobic pores , charging may also involve the net movement 
 of pairs of ions into the carbon pores . in this unusual case ,
as we will see in the 
 next section , the different charging mechanisms can be probed with 
 computational and experimental techniques , and the mechanism depends 
 on the electrode and electrolyte materials studied .
a schematic 
 illustrating some of the main approaches that have been developed 
 to study the charging mechanisms of supercapacitors is shown in figure 4 .
computational methods 
 have been extremely successful in this area as they allow one to probe 
 local length scales .
the simulation of supercapacitors and their charging 
 mechanisms presents a number of challenges which have been tackled 
 through different approaches ranging from mean - field theories to md 
 simulations .
one of the main difficulties is related to the complex 
 structure of the carbon electrodes . in mean - field theories ,
electrodes 
 are usually represented as single slit or cylindrical pores . in molecular 
 simulations , it is also difficult to account for the complexity of 
 the carbon structure as ( i ) an accurate description of the structure 
 at the atomistic scale is not readily available and ( ii ) a realistic 
 representation of the carbon structure requires the use of a large 
 number of atoms in the simulations , which is time consuming
. nevertheless , 
 md simulations are valuable as they provide a precise description 
 of the ion  ion correlations and packing effects that are important 
 in the context of the concentrated electrolytes used in supercapacitors .
schematic 
 showing the different approaches for studying the charging 
 mechanisms of supercapacitors , and their advantages and challenges .
experiments and simulations have shown that the 
 materials and conditions used in the carbon synthesis have a large 
 effect on the ordering of the carbon sheets .
this ordering , as well as the 
 presence of functional groups , impacts the electronic conductivity 
 of the carbons and their performances as electrode materials . in molecular simulations , there 
 are two main approaches to deal with the electrode charge : ( i ) constant 
 charge and ( ii ) constant potential . in the constant 
 charge approach ,
charges are assigned to the carbon atoms at the beginning 
 of the simulations and kept constant throughout . as such
the converse approach is to consider the electrode as metallic and 
 include the fluctuation of carbon atom charges in the simulations .
while this approach is likely to be more realistic , 
 especially for carbons with high degrees of order , it is much more computationally expensive and has been 
 used in only a limited number of studies .
further work is needed to 
 assess the validity of these different approaches and also to develop 
 quantum mechanical methods to treat the carbon charge .
one way 
 of circumventing the problem of computational cost is to 
 explore the effects of confinement and polarization through analytical 
 expressions .
indeed , while early theories considered ions as point 
 charges and electrolytes as dilute , new advances in this field have 
 allowed the inclusion of steric interactions and charge screening 
 in the representation of the systems .
superionic effect  which states 
 that the packing of ions of the same charge is easier in confined 
 spaces because of an exponential screening of the electrostatic interactions 
 by the charged pore wall .
this charge screening has recently been 
 explored in more detail by inserting atoms in gold nanotubes and cnts using 
 dft calculations .
the authors showed that the atoms were fully ionized 
 upon insertion , turning the initially semi - conducting nanotubes into 
 conductors .
both analytical theories and quantum chemistry simulations 
 have provided valuable insights into the understanding of charging 
 mechanisms by investigating the impact of charge screening .
nevertheless , 
 the specific performances of different electrolytes , with or without 
 solvent , can be understood only via techniques that explicitly account 
 for ionic correlations , and the size and shape of the electrolyte 
 molecules .
classical md simulations are currently the method of choice 
 here , although md - dft simulations will become increasingly important 
 as computational power increases .
ideally , md simulations of 
 supercapacitors would include a realistic 
 representation of the carbon electrode structure , as well as a description 
 of its electronic conductivity . as mentioned above , this is very challenging , 
 as it involves using a large number of atoms for the carbon porous 
 structure and a constant potential approach , which are both computationally 
 expensive . to date , only a few studies have included both of these 
 characteristics .
effects in nanoporous electrodes contributes to the anomalous capacitance 
 increase observed in nanometer - sized pores . in nanoporous electrodes , only a single layer ( or a few layers ) 
 of adsorbed ions
this is in stark contrast to planar electrode surfaces , 
 such as the graphite surface , where several layers of ions of alternating 
 charge extend from the charged surface into the bulk . at planar surfaces , the first layer of adsorbed counter - ions carries 
 a greater charge than the electrode surface (  overscreening  
 the surface charge ) , and subsequent layers of co -
ions and counter - ions 
 then balance the excess charge in the first layer .
the lack of overscreening 
 in nanoporous electrodes does not , however , account for all of the 
 anomalous capacitance , and hence a more atomistic / molecular understanding 
 of charge storage is required . to this end ,
md simulations of a pure 
 ionic liquid ( bmi  pf6 ) and cdc electrodes have shown 
 that charge storage does not occur by counter - ion adsorption alone 
 ( table 2 ) . in the positive electrode , charging occurred 
 by a combination of ion exchange and counter - ion adsorption ( x = 0.34 ) , while in the negative electrode charging was 
 mainly by ion exchange , with a small amount of co - ion desorption ( x = 0.11 ) .
thus , it appears that the pf6 anions are the more  active  species here , playing 
 the dominant role in the charge storage process in both electrodes . 

similar results were found for the same disordered carbon with the 
 organic electrolyte ( bmi  pf6/acn ) , showing that 
 the solvent does not significantly affect the capacitance or charging 
 mechanism ( table 2 ) . for a model supercapacitor system with slit - pore electrodes and the ionic liquid 1-ethyl-3-methylimidazolium 
 bis(trifluoromethanesulfonyl)imide ( emi  tfsi ) , charging in 
 the positive electrode involved both ion exchange and co - ion desorption 
 ( x = 0.36 ) , while charging in the negative 
 electrode involved both ion exchange and counter - ion adsorption ( x = 0.45 ) ; i.e. , the emi cations are more  active  
 in this system .
these observations for 
 the different md studies indicate that ion packing effects and ion  ion 
 interaction energies influence the charging mechanism that operates . 

we note that , for an identical potential difference and a similar 
 pore size , the difference between the number of anions and cations 
 in the charged electrodes ( which is proportional to the capacitance ) 
 is 4 times larger in the disordered cdc electrodes compared to the 
 single slit pores ( table 2 ) .
this is partly because both sides of the carbon sheets 
 are accessible to ions in the disordered electrodes while only a single 
 surface is accessible in the slit - pore system .
accounting for this , 
 the capacitance is still twice as large for the disordered electrodes , 
 indicating that the curved and defective nature of the carbon sheets 
 facilitates charge separation .
it has been shown that more confined 
  pocket - like  ion adsorption sites allow the more effective 
 storage of charge , than  plane  sites , presumably because the curved surfaces maximize favorable 
 coulombic interactions between the carbon surfaces and the counter - ions 
 ( while also screening repulsive interactions between different counter - ions ) . 

these effects result in a much larger capacitance for disordered porous 
 electrodes and highlight the importance of using complex porous carbon 
 structures in simulations of supercapacitors .
ncounter - ion  nco - ion is proportional to 
 the excess ionic charge ( which is in turn directly proportional to 
 gravimetric capacitance ) .
besides 
 providing a microscopic picture and fundamental understanding 
 of the local processes occurring during supercapacitor charging , both 
 mean - field theories and molecular simulations can be used to explore 
 new ideas to improve energy storage efficiency in supercapacitors . 

an interesting example of these hypothetical explorations is the concept 
 of ionophobicity and ionophilicity .
simulations can be used to investigate 
 the effects of having initially empty ( ionophobic ) pores , on both the energy stored 
 and the charging rate .
both mean - field theories and md simulations for single 
 slit pores have shown that ionophobic pores perform better in terms 
 of charging rate , and suggest that , in 
 some cases , the quantity of energy stored is higher for ionophobic 
 pores .
one interesting aspect is that 
 the charging of initially filled pores usually leads to an overfilling 
 of the porosity , corresponding to a temporary state where the density 
 of ions is higher than the final density at the end of charging .
this 
 type of phenomena raises the issues of kinetic barriers and the difference 
 between static and dynamic charging of supercapacitors .
while the 
 effects of ionophobicity and ionophilicity are now well studied in 
 idealized slit - pore geometries , it would be interesting to study similar 
 ideas in realistic three - dimensional porous geometries where it was 
 shown that charging mechanisms are different from slit pores due to 
 local heterogeneities in the carbon structure .
experiments using an eqcm were some of the first to 
 investigate the molecular mechanisms of supercapacitor charging .
levi 
 et al . demonstrated that by depositing porous carbon electrodes on 
 a quartz crystal , it was possible to measure changes of electrode 
 mass during the operation of a supercapacitor cell .
in practice , variations of the resonance frequency of the 
 quartz crystal are measured , and these are converted into mass changes 
 using the sauerbrey equation .
these experiments 
 have mostly been performed with dynamic charging conditions , with 
 the voltage continuously swept between the voltage limits at a constant 
 rate ( i.e. , cyclic voltammetry ) .
the extracted mass changes are then 
 typically compared to the predicted mass changes assuming the adsorption 
 of a single counter - ion to balance each electronic charge in the electrode 
 ( i.e. , assuming x = 1 ) .
while this assumption generally 
 does not hold , deviations from the predicted mass changes can be used 
 to infer the presence of ion - exchange , and qualitative studies of 
 the supercapacitor charging mechanism are possible .
initial 
 studies of activated carbon electrodes with an aqueous cscl ( 0.5 m ) 
 electrolyte suggested that counter - ion adsorption processes dominated 
 the charging mechanism .
in contrast , further experiments on tetrabutylammonium 
 chloride ( nbu4cl , 0.5 m aq ) , with the bulky nbu4 cation , suggested that charging was brought about predominantly 
 by the migration of chloride anions ( i.e. , counter - ion adsorption in 
 the positive electrode , and co - ion desorption in the negative electrode ) , 
 with these experiments providing the first evidence that the charging 
 mechanism depends on the electrolyte . in follow - on studies on organic electrolytes , different charging 
 mechanisms
could be discerned depending on the magnitude of the charge 
 stored in the electrode ( i.e. , depending on the cell voltage ) , see figure 5a . for low charge densities ( region i ) , the measured mass 
 change was smaller than that expected for pure counter - ion adsorption 
 processes , indicating the presence of ion exchange . for larger ( intermediate ) 
 charge densities ( region ii ) , the measured mass changes were consistent 
 with a counter - ion adsorption mechanism . finally , for high charge densities 
 ( region iii ) , the measured mass changes were larger than those predicted 
 for pure counter - ion adsorption , and it was suggested the source of 
 the additional mass was extra solvent molecules that entered the electrode 
 pores . this is surprising given that the pores are already densely 
 packed with ions and solvent molecules , and we note that md simulations 
 have suggested that the in - pore solvent population does not significantly 
 change during charging .
further studies are 
 thus needed to investigate how the in - pore solvent population varies 
 during charging .
interestingly , when electrolytes with different cations 
 were studied with eqcm ( figure 5b ) , it was shown that the ion - exchange processes in the negative 
 electrode became more significant for electrolytes with bulkier cations 
 ( e.g. , nbu4bf4 ) , with the role of desorption 
 of the smaller anions becoming more important .
 gives the moles of ions per surface area of quartz crystal , 
 obtained from the measured electrode mass changes with the assumption 
 of charge storage solely by counter - ion adsorption processes .
dashed 
 lines showing theoretical  values for charging by pure counter - ion 
 adsorption .
( a ) measurements for yp-17 activated carbon and net4bf4 in propylene carbonate ( pc ) solvent 
 at two different concentrations .
( b ) further measurements for yp-17 
 carbon supercapacitors with different electrolyte salts ( 0.1 m in 
 pc ) : tea , tba , and toa are tetraethylammonium , 
 tetrabutylammonium , and tetraoctylammonium ( noc4 ) , respectively . 

again , 
 the charging mechanism was found to depend on the magnitude of the 
 charge stored on the electrodes . in the positive electrode ion exchange 

was observed at low charge densities , with counter - ion adsorption processes 
 then dominating at higher charge densities . in the negative electrode
, 
 on the other hand , counter - ion adsorption appeared to dominate over 
 the full range of studied charge densities .
interestingly , the charging mechanism 
 was largely unaffected by the presence of solvent , though additional 
 mass changes were detected in the negative electrode , assigned to 
 the solvent molecules that the emi cations carried into the pores . 

following the approach developed by levi for aqueous electrolytes , a solvation number of 3.7 could be estimated 
 for the emi cations by assuming a purely ion - adsorption - driven charging 
 mechanism , and assuming that the unaccounted mass was due to acn solvent 
 molecules .
given the assumptions here , it would be beneficial if solvation 
 numbers could be verified by a second method such as nmr spectroscopy 
 ( see later ) .
, a significant limitation of these studies 
 is that a single parameter is measured ( the electrode mass ) , which 
 depends on the number of cations , the number of anions and the number 
 of solvent molecules .
thus , the populations of the various in - pore 
 species can not be determined .
this makes it difficult to fully quantify 
 the charging mechanism and obtain x values , such 
 that information from other experimental techniques is crucial . as discussed in section 2
, nmr experiments allow the absolute quantification of ions 
 inside the carbon nanopores , with the separate study of anions , cations , 
 and solvent possible . along with co - workers , we have developed the in situ nmr approach for studying supercapacitors , such 
 that in - pore ion populations can be tracked at different charging 
 potentials in working supercapacitor cells .
these measurements 
 are typically performed in constant voltage mode ; i.e. , a fixed voltage 
 is applied to the cell , and nmr spectra are acquired after equilibration 
 of the system .
figure 6a shows in situ nmr spectra for a supercapacitor cell with yp50f 
 activated carbon electrodes and a pet4bf4/acn ( 1.5 m electrolyte ) , with p and f 
 nmr allowing the study of the pet4 cations and bf4 anions , respectively .
changes of in - pore 
 chemical shift during charging arise from the changes of the carbon 
 electronic structure as electrons are added or removed from the electrodes .
crucially , changes of in - pore peak intensities 
 relate directly to changes of the in - pore ion populations . by fitting 
 the spectra the number of in - pore cations and anions
could be determined , 
 revealing that the charge storage mechanism is inherently different 
 depending on the polarization of the electrode ( see figure 6b ) . in the positive electrode , charging in this system occurs mainly by ion 
 exchange ( x = 0 ) , with simultaneous counter - ion adsorption 
 and co - ion desorption .
in contrast , charging in the positive electrode 
 occurs purely by counter - ion adsorption ( x = 1 ) . despite 
 the different charging mechanisms , the excess ionic charge in the 
 carbon pores balances the electronic charge in both electrodes ( figure 6c ) .
interestingly , 
 the charging mechanism was also found to be invariant to the concentration 
 of the electrolyte , with the same mechanisms observed for concentrations 
 of 1.5 , 0.75 , and 0.5 m. this mirrors observations on neat and diluted 
 ionic liquids studied by eqcm and md 
 simulations above , and further confirms the 
 idea that the solvent concentration does not necessarily dictate which 
 charging mechanism operates , but simply modulates the absolute in - pore 
 populations . in our study , the findings from the nmr measurements 
 were corroborated by eqcm measurements , where a small negative mass 
 change was observed in the positive electrode , as the slightly heavier 
 cations were desorbed from the pores while the lighter anions were 
 adsorbed .
meanwhile , a positive mass change was observed in the negative 
 electrode , in agreement with the counter - ion adsorption mechanism revealed 
 by nmr .
the mass increase in this electrode was greater than that 
 expected due to cation adsorption , and the additional mass was ascribed 
 to solvent molecules , with an estimated cation solvation number of 
 5.4 .
h nmr experiments were also performed to study the 
 deuterated acetonitrile solvent , though the large peak line widths 
 precluded the quantification of the in - pore solvent .
future in situ nmr studies may allow quantitative measurement of 
 in - pore ion solvation numbers , allowing comparison with the results 
 from eqcm analysis .
( a ) in situ nmr measurements of a supercapacitor 
 with yp50f electrodes and pet4bf4/acn 
 ( 1.5 m ) electrolyte .
( b ) in - pore ion populations at different charging 
 voltages , obtained by fitting of the spectra in ( a ) .
( c ) ionic charge 
 stored by in - pore ions , obtained from the in - pore ion populations 
 in ( b ) , as well as electronic charge , obtained from electrochemistry 
 data . reprinted with permission from ref ( 52 ) .
further nmr measurements on the 
 same activated carbon ( yp50f ) and 
 a range of different electrolytes have revealed that the charging 
 mechanism can vary significantly when different electrolyte ions are 
 studied .
ex situ nmr studies of supercapacitors with 
 the ionic liquid pyr13tfsi showed 
 that in the positive electrode charging took place by ion exchange 
 and counter - ion adsorption ( x = 0.3 ) , while in the 
 negative electrode ion exchange and co - ion desorption were observed 
 ( x = 0.4 ) . in 
 this system
the tfsi anions are more active in the charge storage 
 than the pyr13 cations . this charging mechanism is markedly 
 different to that observed for the pet4bf4/acn system above , where the bf4 anions played no significant 
 role in charging in the negative electrode ( see schematic in figure 7 ) . similarly , in situf nmr measurements on yp50f with li 
tfsi / acn 
 ( 1.5 m ) and na  tfsi / acn ( 1.5 m ) electrolytes revealed significant 
 co - ion desorption processes in the negative electrode , though the 
 cations were not studied .
the exact origin 
 of these different charging mechanisms is currently unclear , but ion 
 packing and ion  carbon interaction energies are expected to 
 be important .
co - ion ( anion ) desorption was also observed in the positive 
 electrode of a supercapacitor with the electrolyte nbu4bf4 , suggesting that the large size of the nbu4 cations impeded their adsorption into the carbon pores , necessitating 
 desorption of anions to store charge .
this finding is consistent with eqcm studies on activated carbons , 
 where the role of the anions ( co - ions ) in the negative electrode became 
 more significant for electrolytes with larger cations .
it should , however , be kept in mind that the 
 eqcm measurements were performed in a dynamic mode , with the frequency 
 response of the quartz crystal measured during cyclic voltammetry 
 measurements .
such dynamic measurements with eqcm are more likely 
 to reveal kinetic effects ( e.g. , effects due to the different diffusion 
 rates of the anions and cations ) than the in situ nmr measurements that were performed at equilibrium after charging 
 to a specific voltage .
schematics showing charge storage mechanisms determined 
 by nmr 
 experiments . for the same activated carbon ( yp50f ) electrodes ,
the 
 charging mechanism differs depending on the electrolyte : ( a ) pet4bf4/acn ( 1.5 m ) and ( b ) pyr13tfsi ionic liquid .
note in part ( b ) the x value given for the negative electrode is the experimentally 
 measured one , whereas based on the number of ions in the schematic 
 one obtains an x value of 0.3 .
other research groups have also 
 applied nmr methods to the study 
 of supercapacitor charging mechanisms .
ex situ nmr 
 measurements by deschamps et al . on net4bf4/acn showed that ion - exchange processes operated in both the 
 positive and negative electrodes of two different activated carbons .
more recently , luo et al . have applied the in situ nmr methodology to supercapacitors with aqueous 
 electrolytes .
they studied an activated carbon 
 material with small pores that were inaccessible to the ions ( na and f ) in the absence of an applied potential .
however , when a potential above 0.4 v was applied , 
 the f anions were able to enter the pores of the 
 positive electrode , with charging proceeding via counter - ion adsorption . 
 following discharge to 0 v , some ions remained in the carbon pores , 
 suggesting the presence of a hysteresis in the charging mechanism . 

interestingly , changes of in - pore chemical shift at 0.8 v 
 indicated that the solvation number of the ions decreased above this 
 voltage .
the strength of the nmr approach arises from the ability 
 to separately 
 observe and quantify in - pore ions , as well as to separately detect 
 anions and cations .
this allows the absolute quantification of supercapacitor 
 charging mechanisms ( and x values can be determined ) . 

the technique , however , is limited to ions with nmr active nuclei , 
 and if many experiments are to be performed at different voltages , 
 ions containing sensitive nuclei such as h , f , p , li , b , and na must generally be studied .
another limitation is that in some 
 cases the in - pore resonances can not be clearly resolved ( e.g. , due 
 to broad peaks , or small ring current shifts ) , and methods should 
 thus be developed to improve spectral resolution for challenging carbon 
 and electrolyte systems .
for example , we have shown how cross - polarization 
 experiments can be used to edit the nmr spectrum to reveal peaks solely 
 due to ions at the carbon surface ( i.e. , in - pore ions ) . beyond 
 eqcm and nmr ,
ir spectroscopy has also been demonstrated as a useful 
 probe of supercapacitor charging mechanisms . here ,
changes in intensity of the absorbances from bond vibrations 
 in the electrolyte anions and cations are monitored during charging , 
 allowing the behavior of the two ions to be tracked separately .
the 
 ir radiation is directed on the working electrode of a supercapacitor 
 cell ( which is clamped onto the surface of a diamond attenuated total 
 reflectance ( atr ) crystal ) , and the reflected signal intensity is 
 recorded to obtain an absorbance spectrum . in these measurements the 
 ir radiation penetrates to 1 m depth in the working 
 electrode . in experiments on titanium
carbide - derived carbon ( tic - cdc ) supercapacitors with 
 ionic liquid ( emi  tfsi ) electrolyte , approximately equal losses 
 of intensity were observed for both the anion and cation absorbances 
 during charging , suggesting that both ions penetrated deeper into 
 the carbon particles , beyond the depth to which the ir photons were 
 able to penetrate .
this was explained 
 by postulating that both anions and cations had entered the carbon 
 nanopores during charging , with experiments on nonporous onion - like 
 carbons revealing no significant changes of absorbance during charging 
 and supporting the hypothesis .
these experiments demonstrated that 
 ions entered the carbon nanopores during charging , though the charging 
 mechanism was not fully quantified .
tfsi 
 and nanoporous carbon nanofiber ( cnf ) electrodes revealed similar 
 losses of intensity for anions and cations in the positive electrode , though the intensity loss was more significant 
 for the anions than the cations , explaining how an excess of ionic 
 charge could develop inside the carbon pores .
experiments were also 
 performed on modified cnfs , which had been activated with koh at 800 
 c and contained more oxygen - containing functional groups . in 
 contrast to the measurements on the untreated cnfs , an increase in 
 intensity
was observed for the cation absorbances , indicating that 
 cations were desorbed from the nanopores of the cnfs in the positive 
 electrode .
together with the observation of the loss of intensity 
 of the anion absorbance ( as anions entered the pores of the cnfs ) , 
 it was shown that an ion - exchange mechanism was operating .
the measurements 
 intriguingly showed how the charging mechanism of supercapacitors 
 is dependent on the surface chemistry of the carbon materials .
ir spectroscopy experiments have revealed new 
 insights into the charging mechanisms of supercapacitors . however , 
 a key limitation of the ir approach is that the in - pore ions can not 
 be directly detected , and instead one must rely on measurements of 
 the bulk electrolyte surrounding the carbon particles .
it is therefore 
 challenging to make fully quantitative studies of the supercapacitor 
 charging mechanism , and x values can not be readily 
 determined .
scattering - based 
 methods have also been applied to the study of supercapacitor charging 
 mechanisms . in situ sans experiments
utilize the 
 different scattering properties of the various atoms in the electrolyte 
 and electrode . by measuring the scattered neutron intensity from a 
 single ( working ) electrode at different charging potentials ,
for example , 
 in a study of aqueous h2so4 electrolytes in 
 activated carbon fabric electrodes , changes of scattered intensity 
 were dominated by the migration of the hydrogen - containing ions and 
 solvent molecules . on this basis
it was 
 inferred that ion - exchange processes bring about charging in activated 
 carbon fabric electrodes , for example , with h3o replacing hso4 and so4 in the negative electrode .
similar findings were 
 obtained for devices with the organic electrolyte net4bf4 in dacn ( 1 m ) . here , the increase 
 in scattered intensity in the negative electrode indicated that the 
 strong h - containing scatterers net4 were adsorbed 
 into the carbon pores , while neutron - absorbing bf4 were desorbed from the pores , i.e. , an 
 ion - exchange mechanism .
converse effects were observed in the positive 
 electrode , again rationalized by an ion - exchange mechanism .
an interesting 
 feature of small - angle scattering measurements is that adsorption / desorption 
 processes can be monitored as a function of the carbon pore size . 

boukhalfa et al . found that the largest changes of scattered intensity 
 were observed for the smallest pores ( i.e. , at large scattering wavevectors ) . 

these measurements have the potential to complement md simulations , 
 where the relative ion populations in pores of different geometries 
 and sizes can be tracked .
however , in 
 the sans measurements full quantification of the in - pore ionic species 
 at different potentials was not possible as the scattered neutron 
 intensity ( a single measured quantity ) depends on the population of 
 anions , cations , and solvent molecules . a recent study by prehal 
 et al . developed in situ x - ray transmission ( xrt ) 
 and saxs methods to provide deeper insights into the charging mechanisms 
 of supercapacitors with simple aqueous electrolytes ( cscl , kcl , and 
 nacl , all 1 m concentration ) and the activated carbon yp-80 .
saxs measurements on evacuated and electrolyte - soaked 
 carbon electrodes confirmed that the pores were filled with electrolyte 
 at 0 v. variations of the xrt intensity during electrochemical cycling , 
 taken together with the electrochemical data , then allowed semi - quantitative 
 measurements of the changes of ion concentrations at different charging 
 potentials .
the analysis relies on the different x - ray attenuation 
 coefficients of the cations and anions , and it is assumed that changes 
 of the amount of solvent ( water ) have no significant effects on the 
 xrt signal .
this analysis showed that an ion - exchange mechanism ( x = 0 ) occurs for all three electrolytes studied , with the 
 charging mechanism independent of the choice of cation . in 
 situ saxs measurements
were consistent with these findings 
 and also indicated that the counter - ions became more closely associated 
 with the pore surfaces as the potential was increased .
further work with scattering - based approaches 
 stands to further our understanding of supercapacitor charging mechanisms .
in summary , nmr spectroscopy 
 measurements and md simulations have 
 shown that the pores of the carbon electrodes contain a considerable 
 number of electrolyte ions in the absence of an applied potential . 

larger in - pore populations are observed for more concentrated electrolytes , 
 with the largest populations observed for ionic liquids .
given the 
 large in - pore ion population in the absence of an applied potential , 
 different charging mechanisms can operate when a potential is applied : 
 counter - ion adsorption , co - ion desorption , and ion exchange ( and combinations 
 of these ) .
we have introduced the charging mechanism parameter , x , to allow a convenient comparison of different charging 
 mechanisms . in situ characterization experiments 
 ( nmr , eqcm , ir , and scattering methods ) and simulations
have shown 
 that supercapacitor charging does not generally take place by counter - ion 
 adsorption alone , as is the traditional view , and ion exchange plays 
 an important role in the charge storage process . as shown by table 3 , in a wide range 
 of experimental and computational studies ,
charging almost always 
 involved some degree of ion exchange ( x  
 1 ) , and only rarely was charging driven purely by counter - ion adsorption 
 ( x = 1 ) .
experiments have shown that the exact charging 
 mechanism depends on the polarization of the electrode , the choice 
 of electrolyte ions , and the choice of electrode material ( table 3 ) .
surprisingly , the 
 solvent concentration does not appear to significantly influence the 
 charging mechanism that operates ( table 3 ) , and it appears the role of the solvent 
 is less significant than previously thought .
this suggests that ion 
 confinement effects , screening of ions 
 by the carbon surfaces , as well as the 
 lack of overscreening dominate the capacitance 
 increase reported for nanoporous carbons , rather than ion desolvation , while different in - pore ion populations 
 at 0 v and fundamentally different charging mechanisms may also play 
 a role .
the wide range of charging mechanisms ( x values ) 
 presented in table 3 indicate that factors such as ion - packing energies and ion 
carbon 
 interaction energies dictate which charging mechanism is the thermodynamic 
 one . under dynamic ( fast ) charging conditions the charging mechanism 
 may differ , though studies contrasting thermodynamic and kinetic charging 
 mechanisms have not yet been carried out . when studying table 3 it should be kept in mind that 
 only nmr and md simulations allow the absolute quantification of the 
 different electrolyte species at different potentials , and that various 
 assumptions must be made when inferring charging mechanisms from other in situ methods ( e.g. , with eqcm changes of in - pore anion , 
 cation , and solvent populations have to be determined from a single 
 measured parameter , the electrode mass ) .
further in situ characterization of supercapacitors 
 with a wide range of different electrolytes and carbon materials will 
 help to fully elucidate the factors which control supercapacitor charging 
 mechanisms . as discussed above
, each in situ characterization 
 method comes with its limitations , and these methods must be further 
 developed such that they can be applied to a wide range of systems 
 in a quantitative manner , and on a range of charging time scales . 
 beyond experiments ,
advanced theoretical methods that can properly 
 take into account the electronic structure of the ions and carbon 
 electrodes could provide new insights .
if one can properly understand 
 the factors that influence the charging mechanism , it should be possible 
 to tailor the mechanism by choice of the correct electrolyte - electrode 
 combination . in particular , it would be interesting to study the relative 
 magnitudes of ion  ion and ion  carbon interaction energies 
 within the carbon pores .
additionally , one could investigate packing 
 effects by studying ions with different shapes and sizes within carbon 
 pores of different geometries .
it is clear that systematic studies 
 in which the electrolyte and carbon structure are varied independently 
 will allow the understanding to progress here .
this leads to 
 an important question : how does the charging 
 mechanism affect the performance of a supercapacitor ?
the 
 mechanism should have a significant effect on the power that supercapacitors can offer , and tailoring the mechanism may 
 allow us to improve the power performance of devices .
indeed , theoretical 
 work has indicated that initially empty pores ( ionophobic pores ) should 
 charge more quickly than initially filled pores ( ionophilic pores ) , though this effect is yet to be realized experimentally . 

similarly , we expect that ion adsorption , ion exchange , and ion desorption 
 mechanisms from initially filled pores should each result in different 
 device power performances , and work must be done to establish which 
 mechanism is optimal for fast charging .
purely counter - ion adsorption 
 processes might be expected to allow fast charging in a front - like 
 manner , with net migration of ions into the interior of the carbon 
 nanoporosity , while ion exchange requires ionic migration in opposite 
 directions . at the same time
, these different mechanisms will bring 
 about changes of in - pore ionic density , and therefore packing during 
 charging , which will also affect the charging rate .
for example , counter - ion 
 adsorption mechanisms will increase the number of ions inside the 
 carbon pores , and recent theoretical work has suggested that more 
 densely packed pores result in slower ionic diffusion . as well as ion packing effects , interactions 
 of the different ions with charged carbon surfaces will also affect 
 in - pore transport processes . clearly , 
 experimental measurements and simulations of diffusion and migration 
 processes in charged carbon nanopores represents another exciting 
 area of research . in principle
, the charging mechanism will 
 affect the capacitance , and therefore the energy density , that can be 
 achieved in supercapacitors . under thermodynamic conditions ,
the charge 
 storage mechanism which operates is the one that minimizes the increase 
 in free energy associated with charging , thus minimizing the voltage 
 increase per unit charge ( i.e. , maximizing the capacitance as c = q / v ) .
kondrat and 
 kornyshev point out that counter - ion adsorption is disfavored here , 
 as there is an entropic penalty for an ion entering a pore , and there 
 are also unfavorable electrostatic ( enthalpic ) terms associated with 
 the packing of ions of the same charge inside the carbon pores ( though 
 this is alleviated to some extent by charge screening from the pore 
 walls ) . in principle
, the ion - exchange 
 mechanism reduces the enthalpic penalty associated with denser ion 
 packing because the total in - pore density remains essentially constant 
 during charging , while the entropic penalty associated with charging 
 will also be reduced .
this may help to explain the prevalence of ion 
 exchange mechanisms revealed by in situ characterization 
 methods detailed in this article ( table 3 ) .
charging by co - ion desorption should minimize 
 the enthalpic penalty due to interactions between like charges , while 
 also increasing entropy , and should thus maximize the capacitance .
that said , charging by purely co - ion desorption 
 ( x = 1 ) has not yet been observed , indicating 
 that other factors beyond these simple arguments are important .
it 
 is clear that additional work must be done to understand the interplay 
 between supercapacitor charging mechanisms and capacitance . under 
 kinetic control ,
the charging mechanism will depend on the relative 
 rates of in - pore motion of the anions and cations , and we again stress 
 the need for further experimental and theoretical studies of these 
 effects .
if the diffusion rates of the different in - pore ions can 
 be controlled , then it should be possible to control the kinetic charging 
 mechanism and thus improve the capacitance .
finally , we stress 
 that the structural complexity of porous carbon electrodes 
 poses a significant challenge as we 
 aim to design enhanced supercapacitors .
ideally the electrode structure 
 would be modified in a controlled way to study its performance , though 
 for this to happen new tools must be developed to characterize and 
 model amorphous carbon structures . beyond 
 activated carbons , more ordered carbon materials based on nanotubes , 
 graphenes , and templated materials may serve as model systems to probe 
 structure  property relationships .
 supercapacitors 
 ( or electric double - layer capacitors ) are high - power 
 energy storage devices that store charge at the interface between 
 porous carbon electrodes and an electrolyte solution . these devices 
 are already employed in heavy electric vehicles and electronic devices , 
 and can complement batteries in a more sustainable future . 
 their widespread 
 application could be facilitated by the development of devices that 
 can store more energy , without compromising their fast charging and 
 discharging times . in situ characterization methods 
 and computational modeling techniques 
 have recently been developed 
 to study the molecular mechanisms of charge storage , with the hope 
 that better devices can be rationally designed . in this perspective 
 , 
 we bring together recent findings from a range of experimental and 
 computational studies to give a detailed picture of the charging mechanisms 
 of supercapacitors . 
 nuclear magnetic resonance experiments and molecular 
 dynamics simulations have revealed that the electrode pores contain 
 a considerable number of ions in the absence of an applied charging 
 potential . 
 experiments and computer simulations have shown that different 
 charging mechanisms can then operate when a potential is applied , 
 going beyond the traditional view of charging by counter - ion adsorption . 
 
 it is shown that charging almost always involves ion exchange ( swapping 
 of co - ions for counter - ions ) , and rarely occurs by counter - ion adsorption 
 alone . 
 we introduce a charging mechanism parameter that quantifies 
 the mechanism and allows comparisons between different systems . 
 the 
 mechanism is found to depend strongly on the polarization of the electrode , 
 and the choice of the electrolyte and electrode materials . in light 
 of these advances 
 we identify new directions for supercapacitor research . 
 
 further experimental and computational work is needed to explain the 
 factors that control supercapacitor charging mechanisms , and to establish 
 the links between mechanisms and performance . increased understanding 
 and control of charging mechanisms should lead to new strategies for 
 developing next - generation supercapacitors with improved performances . 
Introduction
Characterizing
the ElectrodeElectrolyte
Interface at 0 V
Studies of Supercapacitor Charging Mechanisms
Summary and Outlook