This invention relates generally to a fuel storage device and electrochemical fuel cell system employing the same. Fuel cells are efficient energy conversion devices established on electrochemistry principles. Fuel cells directly convert chemical energy stored in fuels and oxidants to electrical energy. The general oxidation and reduction can be expressed as:[O]+[R]→P
[O] represents oxidant or oxidants; [R] represents reductant or reductants; P represents product or products.
Reductant or reductants act as fuel or fuels during fuel cell reaction processes. Fuel cells are well-known electrochemical devices and avoid Carnot cycle energy losses compared with combustion engines. Generally, fuel cells can reach 45-65% electrical efficiency in practice.
In the past, several types of fuel cells have been developed and progress has been made. Fuel cells can be generally categorized by the electrolytes that the reaction uses. Table 1 lists the most commonly seen types of fuel cells developed by industry and the academic world.
TABLE 1Different Type of Fuel CellsWorkingConductiveTemperatureDevelopmentTypeElectrolyteIon(° C.)StageSystem SizeAlkaline FuelKOH/NaOHOH−Room temp.-200Well developed1-100 kWCell (AFC)and appliedMoltenMoltenCO32−500-700Developing andCarbonate Fuelcarbonate saltdemonstratingCell (MCFC)PhosphoricH3PO4H+100-200Well developed1 kW-2 MWAcid Fuel Celland applied(PAFC)ProtonPerfluorinatedH+Room temp.-125Developing and1-300 kWExchangepolymer (e.g.demonstratingMembrane FuelNafion)Cell (PEMFC)Solid OxideCeramic (e.g.O2−600-1000Developing and1-200 kWFuel CellY2O3/ZrO2)demonstrating
Alkaline fuel cells (“AFC”) and Proton Exchange fuel cells (“PEMFC”) are two kinds of fuel cells that can be operated in a relatively low temperature range. PEMFCs use solid electrolytes such as E. I. DuPont Nafion® membrane, a fully fluorinated Teflon-based polymeric material that is able to conduct proton ions. Due to their moderate working temperature range and performance characteristics, PEMFCs have received much attention as a potential alternative power source for automotive applications in the last decades. Hydrogen gas is the most popular fuel associated with PEMFC applications. Due to the absence of H2 infrastructure worldwide and catalysts associated with noble metals such as Pt, Rh, and more, the PEMFC adoption is very slow and much promise has disappeared in the past few years.
Traditional AFCs suffer low tolerance of CO2 because they react with the electrolytes to form carbonate salt. This has limited AFCs to use pure H2 as a fuel and pure O2 as an oxidant. AFCs exhibit excellent reaction kinetics in the oxidant side (cathode) and can use nickel, cobalt, and other low-cost metals as catalysts. If a non-CO2 fuel and a suitable oxidant can be identified, one can take full advantage of the AFC-specific characteristics.
US 2008/0145733 A1 and CN 101138112 A patent applications report the use of hydrazine (N2H4) or diamine as the fuel and oxygen (O2) as the oxidant. Co, Co/C, and Pt/C catalysts were used at fuel side (anode) as catalyst, respectively. The over-potential phenomenon (voltage loss) is reduced when Co and Co/C catalysts were used. This result confirms that a non-noble catalyst is a good option to catalyze hydrazine oxidation at the basic condition.
Hydrazine, or diamine, in the form of propellant for thrusters, is by far the most common means of spacecraft propulsion and altitude control. Based on the prior art, hydrazine-oxygen fuel cells are considered superior to ammonia (NH3) and methanol (CH3OH) fuel cells, and next to hydrogen units in specific power (see S. S. Tomter and A. P. Anthony, The Hydrazine Fuel Cell System in Fuel Cells, American Institute of Chemical engineers, New York (1963), pp. 22-31). Hydrazine is reactive and highly soluble in the electrolyte, yielding high current densities. As of the late 1960s, hydrazine fuel cells awaited a significant cost reduction to see their widespread application. Hydrazine monopropellant systems have also been used as auxiliary power units on aircraft.
Conventional fuel cells that use liquid fuels such as methanol and ethanol have poor reactivity and thus cannot produce the power output necessary and sufficient for automotive applications. Using hydrazine hydrate, which possesses excellent reactivity with an oxygen oxidant, the new fuel cell can produce a high output of 0.5 W/cm2 (as reported by Daihatsu in “Daihatsu Develops Platinum-Free, Direct Hydrazine Fuel Cell Technology”, 14 Sep. 2007), which is comparable to the output obtained from a hydrogen/oxygen fuel cell using a platinum catalyst in a PEMFC based system.
Fuel Handling and Safety Issues
Concentrated hydrazine hydrate (N2H4.H2O) is designated as a poisonous oxidant, and it must be handled under the same safety standards applicable to gasoline and most toxic industrial chemicals.
Hydrazine is also listed among shock-sensitive chemicals, as a chemical prone to rapidly decompose or explode when struck, vibrated, or otherwise agitated. It is flammable in mixtures with air from 4.7% to 100% hydrazine. Without addressing the safety concerns of fuel storage and delivery, fuel cells using hydrazine or its similar derivatives or other chemical compounds as fuels will be impractical, especially in automotive applications.
With the objective of ensuring safe fuel use, storage, and delivery, US 2009/0318662 A1 patent application discloses a hydrazine storage resin that is able to fix the hydrazine hydrate into the fuel tank through the use of the resin. Its chemistry principle is believed to be based on the following chemical reaction:CH3—CO—CH3+N2H4→CH3—CN2H2—CH3+H2O
The resin or polymer in the fuel tank acts as a medium allowing hydrazine to be properly and safely stored, and in the meantime to be able to convert or supply hydrazine in a timely manner for fuel cell system. Although this disclosure addresses on-board fuel tank safety in the case of tank damage during automobile collisions or other incidents, it suffers a few drawbacks such as the kinetics of fuel conversion and system complexity. Particularly, the power density will be decreased due to the slow or sluggish fuel/chemicals conversion step.