As the level of deployment and utilization of renewable energy sources continues to rise, large-scale, long-term energy storage technologies that can time-shift the usage of energy daily or seasonally start to play a significant role in the overall development and deployment of renewable energies. Solar energy is considered one of the most abundant and environmentally-friendly energy sources. However, the relatively low power density, typically 200 W m⁻² to 250 W m⁻², as well as the intermittent nature of the source, present significant challenges for producing cost-effective electricity, heat, or fuels from sunlight.¹  As discussed throughout this book, one promising route is to store the solar energy in the form of chemical bonds, i.e., artificial photosynthesis.^2–4  In this scheme, the incident photons are converted into energetic electrons and holes, which transport to the catalytic sites to perform the desired fuel-forming reactions efficiently and selectively. Two key components: a power-generating component and a fuel forming component are usually required and need to be optimized for operation. The sunlight-driven power-generating component is often a semiconductor material, in which the photo-generated carriers are separated and transported selectively into two terminals. Various strategies to create asymmetry in semiconductor materials systems have been used, including semiconductor liquid junctions as well as solid-state heterojunctions and homojunctions. The basic principle of the power-generating component is similar to the one in the photovoltaic devices. For that reason, there are a variety of well established photovoltaic materials, e.g., Si, CdTe, CIGS, III–V compound semiconductors, etc., that are technologically relevant for solar-driven water-splitting applications. The fuel-forming component often consists of two electrochemical reactions and a transport medium/mechanism between the two reactions. For a solar-driven water-splitting device, the anode and cathode reactions are the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. A robust mechanism for separating the product gasses and for providing necessary ionic transport between the cathode and anode chambers is also required. The power-generating component and the fuel-forming component can be, in principle, connected electrically in series in a modular approach, where a PV panel is wired directly to an electrolyzer unit.^5–12  However, the low capacity factor of PV due to the intermittent nature of the solar resource, and high capability factor and high cost of current electrolyzers, present a significant challenge to produce cost-competitive hydrogen.¹³  In contrast, producing fuels directly from sunlight using cost-effective and earth-abundant materials, with scalable processes, offers a unique opportunity and design space for long-term, grid-scale energy storage relative to series connected PV+electrolyzer configurations.^3,14–16