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Oxygen reduction reaction on smooth single crystal electrodes

Electrocatalysis

The oxygen reduction/evolution reaction

  1. P. N. Ross Jr

Published Online: 15 DEC 2010

DOI: 10.1002/9780470974001.f205035

Handbook of Fuel Cells

Handbook of Fuel Cells

How to Cite

Ross, P. N. 2010. Oxygen reduction reaction on smooth single crystal electrodes. Handbook of Fuel Cells. .

Author Information

  1. University of California, Berkeley, CA, USA

Publication History

  1. Published Online: 15 DEC 2010

Abstract

New in-situ techniques for structure determination at the electrochemical interface have made it possible to use single crystal electrodes in kinetic studies in a definitive way. Use of techniques like surface X-ray scattering (SXS) and/or scanning tunneling microscopy (STM) under reaction conditions assures that the single crystal surface being studied has a stable and known atomic arrangement. Specifically, this capability has made it possible to determine definitively the structure sensitivity of the oxygen reduction reaction on Pt and Au surfaces. For Pt in sulfuric and phosphoric acid electrolytes, the structure sensitivity is dominated by the inhibiting effects of anion adsorption, which is strongly structure sensitive. In nonadsorbing acids like perchlorate or triflate, the structure sensitivity is weak, probably due to the single-site nature of the rate-determining step, i.e., electron transfer to form adsorbed superoxide radical anion. Thus, a particle size effect is not expected for supported Pt catalyst in a PEM fuel cell cathode if the geometric change in particle shape with size is the only factor contributing, i.e., absent electronic effects like quantum confinement. For Au in alkaline electrolyte, oxygen reduction is strongly structure sensitive, with the (100) surface the only Au surface with direct 4e oxidation at potentials near 0.9 V. There is no explanation available for the unique catalytic activity of Au(100) for the ORR in alkaline electrolyte. In acid electrolyte, the ORR polarization curves on Au surfaces are shifted negatively (on a RHE scale) by hundreds of mV, and the reaction becomes structure insensitive. The large effect of pH on the overpotential for ORR on Au is due to the absence of chemisorption of either O2 or O2. The rate-determining step on Au is inner sphere electron transfer to O2 from O2, and this reaction step is independent of pH, i.e., the reaction occurs at a fixed potential on a SCE scale, so the overpotential (on an RHE scale) changes with pH.

Keywords:

  • oxygen reduction;
  • platinum;
  • gold;
  • acid electrolyte;
  • alkaline electrolyte;
  • single crystals