(<i>Invited</i>) Design Tradeoffs and Model-Based Optimization for Photoelectrochemical Water Splitting and CO₂ Reduction

In the design of cells for photelectrochemical water reduction (i.e., hydrogen generation) and CO 2 reduction, there are various tradeoffs that must be understood. These stem from kinetic, thermodynamic, and especially transport ohenomena that control the local environment and hence overall overpotentials. Mathematical modeling is ideally suited to explore these tradeoffs endemic to photoelectrochemical systems. In this talk, we will detail our recent modeling work that explores different archietctures for hydrogen generation such as the metal-insulator-semiconductor (MIS) structure. In this architecture, the semiconductor absorbs photons, creating electron-hole pairs; the insulator facilitates the separation of these excited electron-hole pairs; and the metal collects the desired charge carrier and facilitates its use in the fuel-forming reaction. The ultrathin insulator also stabilizes the semiconductor by serving as a physical barrier that mitigates corrosion by the electrolyte. Despite these attractive features, the MIS fuel-formation rates are significantly limited by the reaction driving force, i.e., the MIS photovoltage. Previous work has shown that the MIS photovoltage can be improved substantially by tuning the insulator properties ( e.g. , thickness and band structure) and by using metal nanoparticles instead of a planar metal film. These studies explain that tuning the interfacial properties affect the selectivity of carrier transmission into the metal, but there lacks a clear understanding, which will be discussed including how interfacial properties affect the MIS photovoltage. In addition, we will also detail simulations focused on optimizing photovoltaic-powered membrane-electrode-assemblies (PV-MEA) for CO 2 reduction to highly valuable multicarbon (C 2+ ) chemicals. In particular, we will analyze how the configuration of the photovoltaic (PV) element and the design of the catalyst layer in the MEA influence the STC 2+ efficiency and overall selectivity. We will describe how four silicon solar cells connected in series delivers sufficient photovoltage to drive the conversion of CO 2 to C 2+ products with ~ 5% STC 2+ efficiency. By delineating and optimizing the design and operating conditions, we will show how one should be able to have a PV-MEA devices that camn achieve STC 2+ efficiencies as high as ~ 7%. Finally, we will also examine possible combinations of tandem photoelectrochemical reactors with photochemical ones to obtain C4+ products. Overall, the understanding gained from the mathematical modeling is vital for the development of next-generation photoelectrodes and cells that can enable attainment of high rates of solar-to-chemical energy conversion. This work is partially funded by the Liquid Sunlight Alliance, which is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Fuels from Sunlight Hub under Award Number DE-SC0021266.