287039 Network Topology and Kinetics of Hydrogen Oxidation and Oxygen Reduction Reactions
Although, due to their central importance, the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) are by far the most extensively investigated of all electrocatalytic reactions, their mechanistic and kinetic understanding is still far from complete. Even in the case of HOR, even though only three mechanistic steps (Tafel, Volmer and Heyrovsky steps) are involved, how these steps affect the HOR kinetics under different conditions is not adequately appreciated. The ORR constitutes the single biggest overpotential and efficiency loss in fuel cells. An improved understanding these reactions is clearly essential for a better understanding of fuel cell performance and how progress might be made on developing better and cheaper catalysts.
We utilize our graph-theoretical reaction network approach, namely, the Reaction Route (RR) Graph framework for a topological and kinetic analysis of the HOR and ORR networks. In the RR Graph approach, reaction steps are depicted as directed edges, connected at nodes that conform to the steady-state condition (Kirchhoff flux law, KFL). All possible reaction pathways may be traced as walks on the RR Graph. Thermodynamic functions such as Gibbs free energy change of reaction steps in closed walk, or a cycle, add up to zero, in conformation to the Kirchhoff’s potential law, or KPL. In addition to consistence with KFL and KPL much like electrical circuits, the RR Graph approach visualizes the steps as resistances, so that flux analysis and comparison of resistances allows a transparent analysis of the reaction network in analogy with electrical circuits.
For the case of HOR on Pt in acidic systems of relevance to fuel cells, we consider the usual three-step mechanism ,and find that the Volmer-Heyrovsky mechanism adequately describes the experimental findings in the potential region of interest to PEM fuel cells. Moreover, the exchange current density for HOR in the acidic system is two orders of magnitude higher than what was assumed many decades. A 4-step mechanism for ORR is also analyzed using the to deduce an explicit rate expression which is in good agreement with the experimental data from the literature. Reduced rate expression based on the dominant reaction pathway is next obtained in a logical manner by comparing the resistance across different possible pathways.