Concentration Bias in Reference Potentials and the pH-Effect on the Electrocatalysis of Hydrogen and Oxygen Evolution for Water Electrolysis

Abstract

The electrolysis of water at low temperatures involves the transition from the liquid to the gas phase. The reactant, liquid water, has a concentration of around 55 mol/L, whereas gases such as hydrogen and oxygen with a nearly ideal behavior correspond to a concentration of only 0.04 mol/L at standard conditions (room temperature, pressure equal to 1 bar). Moreover, the oxygen and hydrogen molecules produced during electrolysis are first formed in a dissolved state in the electrolyte before escaping to the gas phase. The saturation concentrations of these gases in liquid water are around 0.001 mol/L at standard conditions, even lower than the concentrations in the gas phase. Accordingly the water electrolysis reaction involves a concentration change from approx. 55 mol/L for the reactant down to approx. 0.001 mol/L for the products. This dramatic concentration drop strongly contributes to the entropic part of the Gibbs free energy of the water splitting reaction, and it therefore implicitly biases the equilibrium voltage.We discuss why this concentration bias can be obstructive for a clear perspective on the reaction kinetics when using the equilibrium voltage, or potentials, as a reference [1]. To provide an unbiased reference voltage, we propose a definition of reference conditions with balanced concentrations on the reactant and product side. The theoretical equilibrium voltage corresponding to such balanced reactive conditions is an unbiased intrinsic reference for kinetic analysis, for which reason we denote it the kinetic reference voltage Ukin. For the electrochemical water splitting reaction, it has a value of Ukin = 1.441 V, which is in remarkable agreement with commonly observed onset voltages for macroscopic electrolysis rates.Applying the same concept of balanced reactive conditions to half-cell reactions, we define the kinetic reference potential Ekin. For a simple electrochemical redox couple Red ↔ Ox + e− with a 1:1 stoichiometry, the kinetic reference potential Ekin is simply equal to the standard equilibrium potential, because the standard reference concentrations are already balanced (1 mol/L for dissolved species). In contrast, for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), the standard reference concentrations are very different for reactants and products, and the corresponding standard equilibrium potentials suffer from concentration bias. Accordingly, the unbiased kinetic reference potentials of HER and OER are shifted with respect to the standard equilibrium potentials. These shifts are different in acidic and alkaline conditions and reveal an intriguing correlation with the experimentally observed effect of the pH value on the kinetics of HER and OER.The concept of balanced reactive reference conditions thus sheds a new light on the activation overvoltage in water electrolysis and the pH-effect in the electrocatalysis of HER and OER.[1] T. Binninger, A. Heinritz, R. Mohamed, Kinetic Reference Potential, pH-Effect, and Energy Recovery in Electrolysis of Water, ChemRxiv (2020), preprint, DOI: 10.26434/chemrxiv.12739091.v1