Solar-driven (photo)electrochemical devices for green hydrogen production and storage: Working principles and design

The large-scale deployment of technologies that enable energy from renewables is essential for a successful transition to a carbon-neutral future. While photovoltaic panels are one of the main technologies commonly used for harvesting energy from the Sun, storage of renewable solar energy still presents some challenges and often requires integration with additional devices. It is believed that hydrogen - being a perfect energy carrier - can become one of the broadly utilised storage alternatives that would effectively mitigate the energy supply and demand issues associated with the intermittent nature of renewable energy sources. Current pathways in the development of green technologies indicate the need for more sustainable material utilisation and more efficient device operation. To address this requirement, integration of various technologies for renewable energy harvesting, conversion, and storage in a single device appears as an advantageous option. From the hydrogen economy perspective, systems driven by green solar electricity that allow for (photo)electrochemical water splitting would generate hydrogen with the minimal CO2 footprint. If, at the same time, one of the device electrodes could store the generated gas and release it on demand, the utilisation of critical and often costly elements would be reduced, with possible gain in more effective device operation. Although conceptually attractive, this cross-disciplinary concept has not gained yet enough attention and only limited number of experimental setups have been designed, tested, and reported. This review presents the first exhaustive overview and critical examination of various laboratory-scale prototype setups that attempt to combine both the hydrogen production and storage processes in a single unit, via integration of a metal hydride-based electrode into a photoelectrochemical cell. The architectures of presented configurations enables direct solar energy to hydrogen conversion and its subsequent storage in a single device, which - in some cases - can also release the stored (hydrogen) energy on demand. In addition, this work explores perspectives and challenges related with the potential upscaling of reviewed solar-to-hydrogen storage systems, trying to map and indicate the main future directions of their technological development and optimization. Finally, the review also combines information and expertise scattered among various research fields with the aim of stimulating much-needed exchange of knowledge to accelerate the progress in the development and deployment of optimum green hydrogen-based solutions.