Development of alkaline fuel cell gas diffusion cathodes using new substrate materials

Abstract

Hydrogen, as a clean and renewable fuel, may play a key role in the near future because of the increasing cost of fossil fuels and the impact of CO2 on the climate. Fuel cells are electrochemical devices which directly convert chemical energy stored in hydrogen into electrical energy at high efficiency with only water and heat as byproducts. A leading candidate fuel cell technology for operation on hydrogen fuels is the proton exchange membrane fuel cell (PEMFC). But today its commercialization remains limited, mainly because of the price of the materials used for electrode manufacture. Catalysts based on precious metals such as platinum, which are currently inherent to PEMFCs, preclude inexpensive mass production. In contrast an alternate fuel cell technology well suited to hydrogen fuels, the alkaline fuel cell (AFC), offers the potential for low cost, mass producible fuel cells, without the need for platinum based catalysts, but has received less attention in recent years. The aim of this work is to develop AFC gas diffusion cathodes using new substrate materials (nickel foam and porous silver membranes) which ally mechanical support, current collection and catalyst support so as to reduce the cost of the electrode. Silver, which is one of the most active materials for the oxygen reduction reaction (ORR) and which is 100 times cheaper than platinum, has been used as the catalyst in this work. The effect of optimising the cathode performance has been monitored using DC polarization curves and electrochemical impedance spectroscopy. Both the nickel foam and porous silver membrane substrates have been successfully developed as the gas diffusion medium in aqueous alkaline media. Silver plated nickel foam showed a decrease in both the Ohmic and charge transfer resistance compared to uncoated nickel foam, leading to improved performance. The porous silver membrane showed good performance in a passive air-breathing cell (50 mW cm-2 at 25 oC) due to its high surface area and optimised hydrophobic properties.