Modeling The Oxygen Evolution Reaction For Efficient Photoelectrochemical Hydrogen Production

Sustainable hydrogen is expected to play a key role in tackling climate change. It can be used, among others applications, for long-term energy storage, as a transportation fuel, and in chemical industries such as steel production. Sustainable hydrogen can be produced by splitting water into hydrogen and oxygen in an electrolyzer. In such a device, typically two metal or semiconductor electrodes are placed in a liquid solution. At each electrode, a reaction occurs: the hydrogen evolution reaction (HER), which generates hydrogen, or the oxygen evolution reaction (OER), which generates oxygen. In his research, Bart van den Boorn focuses on improving the understanding of the OER mechanism by developing a modeling framework.

The water splitting process in electrolyzers requires energy. This can be supplied using (renewable) electricity, but it can also be partially provided by shining sunlight directly onto one of the electrodes. Such a system is known as a photoelectrochemical (PEC) cell. Currently, the efficiency of PEC cells is insufficient for commercialization. This limitation is largely related to materials challenges: an ideal semiconductor must efficiently absorb sunlight, be affordable and readily available, and have suitable properties for the OER that occurs at its surface. These challenges are further exacerbated by a limited understanding of the OER mechanism itself.

The CIF-OER model
In addition to experimental measurements, computational models of both the OER and the physics of the semiconductor material can help improve understanding of the OER reaction mechanism and material behavior. However, these models are often treated separately. In his work, Bart van den Boorn couples a model of the OER, consisting of four intermediate reaction steps, with a model describing the behavior of charge carriers in the semiconductor, namely electrons (negatively charged) and holes (positively charged). The resulting model, referred to as the CIF-OER model, captures the interaction between charge carriers and reaction kinetics and shows how light and recombination conditions affect the current density, a key measure for OER efficiency.

Estimating parameters with experimental data
Van den Boorn then examines the parameters of the part of the model that describes the OER reaction. These parameters often cannot be measured directly in experiments and therefore they contain a degree of uncertainty. This research demonstrates that instead it is possible to estimate certain parameters, such as reaction rate constants, with experimental data using the OER model, provided that the measurements are not significantly affected by additional processes such as transport.

Investigating influential parameters
Finally, the uncertainty in the model parameters is analyzed using a global sensitivity analysis. This analysis reveals which parameters most strongly influences the current density. In particular, two energy parameters, the valence band energy level and the solvent reorganization energy, have the greatest impact. These sensitivity analysis tools can therefore be used to efficiently direct experimental resources to further investigate the most influential parameters.

Overall, this research presents a continuum modeling framework for the OER that covers model development, parameter estimation and sensitivity analysis. The framework contributes to an improved understanding of the OER mechanism at the interface of the semiconductor and electrolyte and supports the development of efficient photoelectrode materials for sustainable hydrogen production.