Enhanced electrocatalysis of the oxygen reduction reaction in phosphoric acid fuel cells

Research group led by Nenad M. Markovic of Argonne National Laboratory (Illinois, USA) has succeeded in developing new concepts for designing oxygen reduction reaction (ORR) catalysts with potential use in phosphoric acid fuel cells (PAFCs) or any environments containing strongly adsorbing tetrahedral anions. The findings have been published online on 15 August 2010 in Nature Chemistry.

Reduction of the considerably high ORR overpotential on cathode side is still remaining as one of the major hindrances for quick penetration of hydrogen-based fuel cells into the market. Development of suitable catalysts for ORR reactions is challenging as the catalysts need to be both noble (to minimize adsorption of poisonous “spectator” species) and catalytically active (to break oxygen molecule at potentials close to the ORR reversible potential, i.e. 1.23 V). The approaches of i) fine-tuning of the electronic properties of metal surface atoms and ii) systematic alteration of the components and structure of the electrochemical double layer have recently demonstrated some success in polymer electrolyte fuel cells (PEFCs) and alkaline fuel cells (AFCs), respectively. However, these approaches cannot be effective for designing catalysts for PAFCs due to the fact that ORR in PAFCs is governed by the surface coverage of spectator phosphoric acid anions.

Therefore, this research has come up with an approach based on molecular patterning of platinum surfaces with cyanide adsorbates forming Pt(111)-CNad. Modification of platinum by adsorbed cyanide adlayer helps in blocking the sites for adsorption of covalently bonded spectator anions while providing a sufficient number of free metal sites to chemisorb the oxygen molecule and break the O-O bond.

The research shows the enhancement of ORR activity of Pt(111)-CNad by 25 times in the presence of sulfuric acid while by 10 times in the presence of phosphoric acid.