The reversible Alkaline Fuel Cell (rAFC)

Low temperature fuel cells will become essential for the utilisation of hydrogen as clean fuel in a few hundred W to a few kW devices.

Low temperature fuel cells will become essential for the utilisation of hydrogen as clean fuel in a few hundred W to a few kW devices. Furthermore the production of hydrogen from water in Electrolysers with a renewable electricity source (e.g. solar energy) has to be permitted when peak energy is provided. The stored chemical potential (hydrogen) will be available for the consumer when required. Such a reversible hydrolyser-fuel cell system can be operated as rechargeable battery as well.

At present, neither of the two low temperature fuel cell types - the polymer electrolyte membrane fuel cell (PEFC) and the alkaline fuel cell (AFC) - allows commercialisation while alkaline electrolysers are a well-established technology. In comparison both fuel cells have specific advantages and disadvantages.

The advantages of the AFC are in summary a higher cell voltage, lower costs, near-atmospheric operation of the system and less sensitivity to impurities. Electrochemical reaction kinetics of several conversion processes are generally faster and more efficient in alkaline electrolytes compared to acidic systems. Alkaline Fuel Cells (AFC) can reach extraordinarily high system efficiency (60-65%) for the reversible chemical conversion processes of water to hydrogen and oxygen and vice versa (hydrogen + oxygen <-> water).

Challenges in materials research are to develop an electrode material as an alternative to noble metals that is low in cost and highly efficient, which is bi-functional for the water generating process (during fuel cell operation) and the water splitting process (during electrolyser operation).

Today’s electrode materials in alkaline fuel cells are made of expensive, precious-metal electro catalysts (e.g. Raney-Ag) and large-surface carbon as a conductive support. This composite material is used in a highly alkaline solution which causes corrosion problems concerning mainly the carbon support. In order to optimise the interface between the carbon and the electro catalyst, as well as the stability of the electrode, a new composite material based on multiwalled carbon nanotubes (MWCNT) as replacement for carbon black will be developed. The tubular structure of these graphitic materials offers interesting electronic properties combined with low weight, enhanced chemical and thermal stability, good electrical conductance and a large surface area.

Conductive and alkaline stable perovskite-type oxides (ABO3) with rare earth and/or alkaline earth ion in A and transition metals in B position are promising candidates to be used as catalysts in AFC to catalyse the oxygen evolution and the oxygen reduction processes on the air electrode.

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