Abstract:
Alkaline anion membrane exchange (AAEM) fuel cell technology is a low-cost
alternative to the depleting sources of fossil fuels. AAEM fuel cells operating at
elevated pH values show improved electrode kinetics on non-precious metal
catalysts, better water management and improved fuel crossover effects. However,
the state of art AAEM fuel cells show low performance which is attributed to
carbonate ion poisoning. In this study we have analyzed the effect of carbonate ions
of the membrane, on the anode- membrane interfacial kinetics and on the transport
of electroactive species through the membrane. A solid-state electrochemical cell
employing a 100μm Pt microelectrode, Pt counter electrode and a dynamic hydrogen
reference electrode was constructed in a controlled atmosphere to mimic the fuel cell
anode-membrane interface. Two types of hydroxide-conducting membranes which
mainly differed by the thickness, Tokuyama A201 and A901 were analyzed. The
hydroxide ions in the membrane were exchanged with carbonate ions, and hydrogen
oxidation reaction (HOR) and the methanol oxidation reaction (MOR) were carried
out. Carbonate ions have a negative effect on the electrode reactions, lowering the
pH of the anode reactions as well as by imposing blocking effect on molecule
diffusion through the membrane. Both HOR and MOR reactions showed slow
kinetics in the presence of carbonate ions. Carbonate ions strongly adsorb on to the
electrode surface decreasing the hydroxide ions required for the reaction. Hydrogen
diffusion through the membrane is not significantly affected by the presence of
carbonate ions due the smaller size of the hydrogen molecule. However, hydrogen
concentration in the membrane was drastically decreased. While methanol solubility
in the membrane is minimally affected, the diffusion of bulky methanol molecules
is greatly hindered by the carbonate ions.
