Effect of phosphoric acid concentration on electrocatalysis
Phosphoric acid (H3PO4) is the electrolyte used in the most advanced H2/air fuel cell developed for power generation and for on-site integrated energy systems. Oxygen reduction kinetics in this electrolyte are relatively poor and thus limit the efficiency of such fuel cells. Therefore, in order to better understand oxygen interactions with platinum in these cells, oxygen adsorption and the kinetics of oxygen reduction reaction (ORR) were studied utilizing a Pt rotating disk electrode in several concentrations of highly purified H3PO4. Although the kinetics of the ORR remained first order in all concentrations, it was found that its Tafel slope increased from approximately 10 mv/dec in low H3PO4 concentrations to approximately 134 mv/dec in 85 w/o H3PO4. Anodic adsorption isotherms indicated that the higher H3PO4 concentrations also hindered adsorption of oxygen from solution. These effects were attributed to the blockage of electroactive sites by the adsorption of H3PO4 molecules.