(Invited) Modelling and Experimental Results for the Hydrogen Oxidation/Hydrogen Evolution Reactions: New Insights into the Performance of Polymer Electrolyte Fuel Cells and Electrolyzers

Abstract

An alternative approach to the rotating disk electrode (RDE) for characterising fuel cell electrocatalysts has been recently demonstrated1. The approach combines high mass transport with a flat, uniform, and homogeneous catalyst deposition process, well suited for studying intrinsic catalyst properties at realistic operating conditions of a polymer electrolyte fuel cell (PEFC). Uniform catalyst layers were produced with loadings as low as 0.16 μgPt cm-2 and thicknesses as low as 200 nm. Such ultra thin catalyst layers are considered advantageous to minimize internal resistances and mass transport limitations leading to very high performance at low platinum loadings. Modelling of the associated diffusion field suggests that such high performance is enabled by fast lateral diffusion within the electrode. The electrodes operate over a wide potential range with insignificant mass transport losses, allowing the study of the hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) at high overpotentials. For the HOR, geometric current densities as high as 5.7 A cm-2 Geo were experimentally achieved at a loading of 10.15 μgPt cm-2 at room temperature (561 A mgPt -1), which is three orders of magnitude higher than current densities achievable with the RDE. For the HER specific current densities greater than 5 A cm-2 Specific have been achieved. The latter corresponds to a turnover frequency of almost 20,000 hydrogen molecules per surface platinum site per second.We have taken these results and applied a new microkinetic model for hydrogen evolution/oxidation based around the classical Heyrovsky-Tafel-Volmer mechanistic steps. This new mechanistic formalism is easy to implement and provides insights into the HOR/HER. For each of the different mechanisms, an “atlas” of Hads coverage with overpotential and corresponding current density is provided, allowing an understanding of all possible responses depending on the dimensionless parameters. Analysis of these mechanisms provides the limiting reaction orders of the exchange current density for protons and bimolecular hydrogen for each of the different mechanisms, as well as the possible Tafel slopes as a function of the molecular symmetry factor, b. Only the HV mechanism is influenced by pH whereas the TV,HT, and HTV mechanisms are not. The cases where the equations simplify to limiting forms are discussed. Analysis of the exchange current density from experimental data is discussed, and it is shown that fitting the linear region around the equilibrium potential underestimates the true exchange current density for all of the mechanisms studied. Furthermore, estimates of exchange current density via back-extrapolation from large overpotentials is also shown to be highly inaccurate. Analysis of Tafel slopes is discussed along with the mechanistic information which can and cannot be determined. Finally, some interesting effects are discussed which show that the HER can be accelerated under certain conditions.