Some Observations on the Oxygen Reduction Reaction (ORR) At Platinum Catalysts based on Post Year 2000 Reports

Abstract

This paper examines theoretical and experimental ORR work performed post year 2000, with the targets of reviewing new insights and new advances and addressing remaining apparent disagreements on proper reading and interpretation of new theoretical and experimental results. It is argued that the description of the ORR rate at a Pt , or Pt alloy catalyst in a fuel cell cathode, has to be relevant to the actual nature of the catalyst surface in the fuel cell-relevant potential range, i.e., a Pt surface with potential dependent coverage by chemisorbed oxygen which blocks metal sites required for the ORR. A way to achieve this goal, is to express the pre-exponential factor in the rate expression in terms of the cathode potential vs. E0Pt-OH2/ Pt-OH , while the exponential factor remains a function of over-potential vs. E0H2O/O2 . It is further suggested, that the surface couple Pt-OH2 / Pt-OH, can be described as redox mediator for the ORR process at a Pt catalyst with a partly oxidized surface and, consequently, ignition of the ORR process at Pt may critically depend on the potential difference E- E0Pt-OH2/Pt-OH .