Some material on alkaline fuel cell technology from pragma-industries.com
3. The Alkaline Fuel Cell
a. How does an AFC work?
Modern Alkaline Fuel Cells (AFCs) are classified as low-temperature fuel cells. They operate between 60 and 90°C and use an alkaline electrolyte such as potassium hydroxide, usually in a solution of water. In an alkaline fuel cell, hydroxyl ions are produced at the cathode (2) and migrate to the anode side, where they react with hydrogen (1). Part of the water formed at the anode diffuses to the cathode, where it reacts with oxygen to form hydroxyl ions in a continuous process. The overall reaction produces water and heat as by-products and generates four electrons per mole of oxygen (3), which travel via an external circuit producing the electrical current.
Anode 2H2 + 4OH- ? 4H2O + 4e- (1)
Cathode O2 + 2H2O + 4e- ? 4OH- (2)
Overall O2 + 2H2 ? 2H2O (3)
The Oxygen Reduction Reaction (ORR) is a complex process involving several coupled proton and electron transfer steps. In acid solutions, the ORR reaction is electrocatalytic, and as pH becomes alkaline, redox processes involving superoxide and peroxide ions start to play a role and dominate in strongly alkali media. The reaction in alkaline electrolytes may stop with the formation of the relatively stable HO2- solvated ion (4), which instead of being further reduced into hydroxyl ions (5), can also be decomposed to dioxygen and hydroxyl ions (6). Although there is no consensus on the actual reaction sequence, two different pathways take place at the cathode in alkaline media:
Direct 4-electron pathwayO2 + 2H2O + 4e- ? 4OH- (2)
Peroxide or “2+2-electron” pathwayO2 + H2O + 2e- ? HO2- + OH- (4)
with HO2- + H2O + 2e- ? 3OH- (5)
or 2HO2- ? 2OH- + O2 (6)
The kinetics of the ORR reaction is more facile in alkaline medium than in acid media such as H2SO4. Consequently, the use of non-precious metals like nickel is possible. Thanks to this comparatively high electrochemical rate at the cathode, AFCs units can attain overall electrical efficiencies greater than most other fuel cell types (50-70%).
Due to the faster kinetics for the ORR in alkaline media, a wide range of catalysts have been studied including noble metals, non-noble metals, perovskites, spinels, etc. The catalytic activity of the system also depends on the physico-chemical characteristics of the carbon support and the deposition method. High catalytic activity relies on a very fine and well dispersed catalyst particle. In the case of platinum, the particle size is generally in the nanometer range.
Potassium hydroxide solution (KOH) is almost exclusively used as the electrolyte because it has a higher ionic conductivity than sodium hydroxide solution, and potassium carbonate is less likely to precipitate than sodium carbonate. Concentrations typically range between 6 and 8 mol L-1 i.e., 30-40 wt% KOH in water. Concentrated aqueous potassium hydroxide is non-freezing, which allows quick start and operation at sub-zero conditions. However, it is highly corrosive and can cause sealing issues of gas compartments.
The main drawback of AFCs is their sensitivity to carbon dioxide from air. Carbon dioxide is absorbed by the electrolyte to form carbonate ions CO32-, which are less conductive than hydroxyl ions. This leads to an overall degradation of electrolyte properties. The formation of precipitated potassium carbonate K2CO3 can also lead to the blockage of the electrolyte pathways and electrode pores, and decrease cell lifetime. For effective operation it is therefore necessary to purify the gases fed to the cell. Since most methods for generating hydrogen from other fuels produce some carbon dioxide, this need for pure hydrogen has slowed down work on AFCs in recent years.
In most terrestrial applications, the KOH electrolyte is circulated through the stack, an option which has some advantages over the alternative immobilized systems chosen for space applications such as Apollo and the Space shuttle. The use of a circulating electrolyte allows thermal and water management to be easily controlled. It provides a very simple and effective way of cooling the stack via a heat exchanger. Moreover, impurities (e.g. carbon from electrodes or carbonates) can be easily removed, making the circulating electrolyte systems less sensitive to CO2 poisoning than the immobilized electrolyte systems.
The electrolyte circulation loop consists of a KOH tank, a pump and a heat exchanger. During start-up the KOH is heated to the desired operating temperature, typically 70°C. An air blower forces air into a CO2 scrubber (usually containing soda lime), from where the air is directed to the air inlet. The outlet air is directly exhausted to the atmosphere whereas the hydrogen is recirculated or dead-ended.
Historically, four different types of AFC cells have been developed:
- Cells with free liquid electrolyte;
- Cells with liquid electrolyte in the pore-system;
- Matrix cells where the electrolyte is fixed in the electrode matrix;
- Falling film cells.
All these cells rely on porous electrode architectures like those used in metal-air batteries. More recently, the alkaline anion-exchange membrane (AAEM) has attracted increasing attention. However, while the AAEM fuel cell holds great promise, developments still need to be made to achieve suitably conducting and stable membranes.
The anode electrode for AFCs has been less studied than the cathode, where catalyst containing platinum-group metals such as Pt/Pd has shown good performance and stability. Nickel, and in particular high surface-area Raney nickel, has been shown to be one of the most active catalysts for the hydrogen oxidation reaction (HOR) in alkaline media, despite reported deactivation effects.
b. A short history of AFC
Alkaline fuel cells (AFCs) were the first practical modern fuel cell following the pioneering work of Francis Thomas Bacon in the 1930s to 1950s. After his invention of the double-layer porous gas diffusion electrode made from activated nickel powder and his choice of an alkaline electrolyte(potassium or lithium hydroxide) instead of the sulfuric acid electrolyte employed by Grove, the english scientist developed and presented a series of alkaline fuel cell stacks running at 200°C under pressurized conditions (50 bar!) with increasing output powers up to 6 kW. The use of porous electrodes allowed increasing the surface area in which the reaction between the electrode, the electrolyte and the fuel occurs. Pressure was applied to hinder flooding phenomena in the tiny electrode pores. Electrodes were developed with increasingly stable interfaces and less corrosion problems. In 1961 Bacon founded his own company, Energy Conversion Ltd., and started commercialize AFC products. Meanwhile, the patents for the fuel cell were licensed by Pratt & Whitney, the aircarft engine manufacturer and industrial gas turbine division of United Technologies Corporation (UTC), which then successfully developed an AFC power plant for the NASA manned Apollo space program in the 1960s.
Due to their high CO2 sensitivity, AFCs were demonstrated in special applications where pure hydrogen and pure oxygen are easily supplied, such as space. They were originally used to provide electric power and drinking water to the astronauts in the Apollo and Skylab spacecrafts, and also by the Russian space program. NASA developed an asbestos-based porous matrix in which the KOH electrolyte was immobilized. Later in the 1970s, the 12 kW Orbiter fuel cell system was supplied by UTC for the Space Shuttle, giving impressive performance at 4 bar and a lower operating temperature of 90°C, but at the expense of high noble metal loading of 10 mg cm-2 at the anode (80% Pt–20% Pd), and 20 mg cm-2 at the cathode (90% Au-10% Pt)!
Alkaline fuel cells were also the first fuel cell technology to be put into mobile applications: demonstration of the first fuel cell powered vehicle was made with a farm truck in 1959 by Allis-Chalmers Manufacturing Company. The tractor with power of 5 kW was followed by an AFC-powered golf cart in 1962 where the AFC unit was fuelled by hydrazine and provided 4 kW of continuous power and 10 kW of peak power. Allis Chalmers Manufacturing Company also produced the world’s first fuel cell powered submersible. In the 1960s Dr Karl Kordesch from the Union Carbide Corporation developed the first circulating electrolyte systems leading to the manufacturing of an AFC motorbike running on hydrazine and of an Austin A40-based fuel cell car. Both vehicles were driven for several years on U.S. public roads in the early 1970s. The fuel cell car has been used by Dr Kordesch for his own personal transportation for three years; it had a driving range of 180 miles (300 km).
The AFC was developed and studied extensively throughout the 1960s until the 1980s, prior to the emergence of the proton exchange membrane fuel cell (PEMFC), which has subsequently attracted most of the attention from developers. However, primarily because the forecast cost reduction in PEMFCs has not become a reality yet, a renewed interest in AFCs has occurred during the last decade or so. The trend among academic research groups and several fuel cell companies is to check up the existing AFC technology and find potential ways to reduce costs while further improving its versatility.
For example, fuel cell companies such as UK-based Eneco (formerly Zetek Power) have recently focused on the design of circulating electrolyte low temperature unpressurised systems for backup power, stationary and mobile applications. The aim was to achieve a low cost mass production fuel cell. Injection-molded plastic frames were used to house the electrodes and build the stack. Low cost electrode production was ensured by the use of standard industrial processes such as rolling (calendaring) and pressing. A process was developed to eliminate CO2 from the fuel by selective absorption onto a carbon-based composite fiber. Eneco has applied this design to the production of a light AFC-powered bus and a hybrid 70 kW battery-AFC taxi.
c. AFC applications and perspectives
Today, due to the cell’s sensitivity to CO2 and the need to purify the hydrogen fuel, the AFC has only conquered predominantly niche transportation markets, powering forklift trucks, boats and submarines. It is still used in space applications and other controlled aerospace and underwater applications, when price is not an issue and high electrical efficiencies are requested. NASA continues to operate several 12 kW units in the Space Shuttle fleet. AFC systems now need to meet the challenging requirement of low cost, high performance and durability to become competitive in mainstream terrestrial applications.
The AFC technology suffers from the predominance of PEMFC, DMFC and SOFC technologies, which have left only small market opportunities due to their generally better performances. Yet, AFCs still have the potential of major improvements with modest investment: the utilization of non-noble metal catalysts and liquid electrolyte makes the AFC a potentially low-cost technology compared to PEMFCs, which employ platinum catalysts and specifically engineered membrane electrolytes. AFCs can produceup to 20 kW of electrical power and new designs have been reported to operate at temperatures close to ambient 23-70°C. Cost analyses have shown that ambient-air AFC systems for mobile and low power applications are less expensive than their PEMFC equivalents. Moreover, AFC fabrication methods (rolling, pressing, spraying, and screen-printing) among which rolling is the most commonly applied, are easily scaleable for mass production.
The future of the AFC technology will highly depend on the improvement of electrodes, especially thecathode, which causes the most part of cell losses. The development of new catalyst systems is more likely in alkaline media because of the wide range of options for the materials support and catalyst, as compared to acidic media which offer limited materials choice. For example, researchers in Wuhan University (China) have recently developed a cheap AFC prototype that uses a new membrane material, a silver cathode and an anode coated with nickel nanoparticles decorated with chromium ions that is more tolerant to corrosion than previous nickel cathodes. The power output is relatively low (0.05 W/cm²), but it provides a first proof of principle of a potentially much less expensive fuel cell.
The development of circulating electrolyte systems has shown to have advantages compared to the immobilized electrolyte systems for terrestrial applications that could create further commercial applications. Attempts are being made to push the already well developed AFC technology forward on this route. But electrolyte leakage and parasitic power losses are still challenges with the circulating electrolyte system and require the development of improved stack designs.
A 20,000 h lifetime has been achieved by Siemens and 15,000 h by UTC Fuel Cells with AFC stacks running on pure hydrogen and oxygen; but when air is supplied to the system, lifetime is significantly less (8,000 h) due to carbon dioxide poisoning. Durability is therefore a main issue for AFCs, especially when using non-noble metal catalysts and air. The 40,000 h target for stationary applications has not been reached yet, due to material issues.
In summary, the major issue for in the AFC technology today is a lack of R&D: alternative catalysts have been identified to replace platinum, but substantial efforts are now needed to meet the durability targets required for commercial applications. |
