The Direct Ammonia Fuel Cell and a Common Pattern of Electrocatalytic Processes

A record power density of 450 mW/cm2 has been demonstrated for a direct ammonia fuel cell using an alkaline membrane electrolyte. The relative activities of Pt-based and Ir-based AOR catalysts are determined by a lower onset potential at Ir, explained by the higher bond strength Ir-N vs. Pt-N. This beneficial effect of the higher bond strength overrides the opposite effect on the rate of surface dimerization. A mechanistic pattern common to the AOR, ORR, and HOR processes is shown to involve a pre-step preceding the RDS, where the former step is the determinant of the onset potential. This pattern and the role of the pre-step in determining both the onset potential and the form of polarization curves observed for electro-catalytic processes are revealed from rate expressions based on absolute rate theory, with a pre-factor reflecting the potential dependence of the surface populations of active sites and/or reactants for the RDS. A brief discussion of "single descriptors" which have been proposed for such complex electrocatalytic processes suggests that an effective description of these processes would require, as a rule, more than a single descriptor.