The CuO@CuFe₂O₄ core–shell structure represents a new family of photocatalysts that can be used as photoelectrodes that are able to produce hydrogen under a broad spectrum of visible light. Herein, we report a novel approach for the production of this active film by the thermal conversion of CuFe Prussian Blue Analogues. The outstanding photoelectrochemical properties of the photocathodes of CuO@CuFe₂O₄ were studied with the use of combinatory photo-electrochemical instrumental techniques which proved that the electrodes were stable over the whole water photolysis run under relatively positive potentials. Their outstanding performance was explained by the coupling of two charge transfer mechanisms occurring in core–shell architectures.

The relentless consumption of fossil fuels and soaring CO₂ emissions have plunged the world into an energy and environmental crisis. As society grapples with these challenges, the demand for clean, renewable, and sustainable energy solutions has never been more urgent. However, even though many efforts have been made in this field, there is still room for improvement concerning efficiency, material stability, and catalytic enhancement regarding kinetics and selectivity of photoelectrochemical (PEC) processes. Herein, we provide the experimental proof for the enhancement of the photocurrent efficiency by the critical focus on semiconductor–electrolyte interface (SEI) properties. By tailoring electrolyte composition, researchers can unlock significant improvements in catalytic efficiency and stability, paving the way for advanced PEC technologies. In this study, we investigate the influence of electrolyte composition on SEI properties and its impact on PEC performance. By employing electrolytes enriched with carbonates, borates, sulphates, and alkali cations, we demonstrate their profound role in optimising photoelectrochemical CO₂ reduction reaction (CO₂RR) efficiency. Central to this work is Cu₂O—an affordable, highly promising photocatalyst. While its potential is undeniable, Cu₂O’s inherent instability and diverse reduction products, ranging from CH₃OH to CO, HCOOH, CH₃COOH, and CH₃CH₂OH, have hindered its widespread adoption in PEC CO₂ reduction (CO₂RR). Our approach leverages a straightforward yet powerful electrodeposition method, enabling a deeper exploration of SEI dynamics during photocatalysis. Key parameters, such as carbonate concentration, local pH, alkali cation presence, anionic geometry, CO₂ solubility, and electrolyte conductivity, are systematically investigated. The findings reveal the formation of a unique “rigid layer” at the photocatalyst surface, driven by specific cation–anion interactions. This rigid layer plays a pivotal role in boosting PEC performance, offering a new perspective on optimising, among other PEC processes, CO₂RR catalytic efficiency. This profound study bridges a critical knowledge gap, shedding light on the dual influence of cations and anions on SEI properties and PEC CO₂RR. By unravelling these intricate interactions, we provide a roadmap for designing next-generation PEC systems. These insights pave the way for sustainable energy advancements, inspiring innovative strategies to tackle one of the most pressing challenges of our time.

The modular photoelectrochemical (PEC) reactor accommodating eight photoelectrodes with a total active area of up to 46 cm² has been designed and manufactured using the fused deposition modeling method. The device was equipped with an electrolyte flow system, a relay module for the photoelectrode connection, and a feedback-loop module for switching between counter electrodes. The performance and durability of the system were tested within three case study experiments. The water splitting process was successfully combined with an in situ hydrogen storage in the form of metal hydride phases (confirmed by powder X-ray diffraction) using Fe₂O₃- or WO₃-based photoanodes and LaNi₅-based cathodes. The PEC water oxidation at the anodes was realized either in a strongly alkaline electrolyte (pH > 13.5) or in acidified synthetic seawater (pH < 2) for Fe₂O₃ and WO₃ electrodes, respectively. In the latter case, the photoresponse of the anodes decreased the cell charging voltage by 1.7 V at the current density of 60 mA∙g⁻¹. When the seawater was used as an anolyte, the oxygen evolution reaction was accompanied by the chlorine evolution reaction. The manufactured PEC-metal hydride reactor revealed mechanical and chemical stability during a prolonged operation over 300 h and in the broad range of pH values.