The transition to green hydrogen production is a cornerstone for reducing fossil fuel dependence and enabling sustainable energy systems. Photoelectrochemical (PEC) water splitting relies on the complementary operation of photoanodes and photocathodes integrated within a complete PEC cell [1]. In this context, this work focuses on the development and characterization of two representative photocathodic materials, CuBi₂O₄ and CuFeO₂ (belonging to the delafossite class), together with the archetypal photoanode material BiVO₄ [2]. All materials are composed of earth-abundant elements and are investigated with the aim of enabling scalable, efficient, and durable PEC devices [3].

CuBi₂O₄ and CuFeO₂ were selected as candidate photocathodes due to their visible-light absorption, chemical stability in aqueous environments, and tunable electronic structure. Among the two materials, CuBi₂O₄ emerges as the most promising photocathode, which should exhibit superior photoresponse and optimal compatibility with BiVO₄ for complementary solar spectrum utilization in a tandem PEC configuration [3,4]. CuFeO₂ also demonstrates potential, although lower photocurrent values are usually obtained under comparable conditions [3], [5]. A comparative screening was performed to evaluate phase purity, morphology, porosity, optical properties, and preliminary PEC performance.

BiVO₄ photoanodes were synthesized via electrodeposition, followed by vanadium precursor drop-casting and thermal annealing [6]. This strategy yielded transparent and uniform films with controlled thickness and fine nanogranular microstructure. Scanning electron microscopy confirmed the formation of continuous coatings, while X-ray diffraction verified the formation of the monoclinic scheelite phase. Optical absorption measurements revealed a clear correlation between film thickness and visible-light absorption, with excessive thickness leading to increased light scattering and reduced transparency, thus highlighting the importance of deposition time optimization.

Photoelectrochemical measurements under AM 1.5 G illumination showed that pristine BiVO₄ films achieved photocurrents of approximately 0.4 mA cm⁻² at 1.23 V_RHE, maintained for 2 h of continuous operation. Molybdenum doping significantly enhanced the photoanodic performance, increasing the photocurrent up to 1.2 mA cm⁻² at 1.23 V_RHE. This improvement is attributed to increased charge carrier density, reduced film resistance, and enhanced separation of photogenerated electron–hole pairs. Optimal deposition times in the range of 0.75–1.5 h were identified, providing an effective balance between optical transparency and PEC activity and offering practical guidelines for scalable fabrication.

All possible combinations between the investigated photocathodes and BiVO₄ as the photoanodic counterpart will be evaluated with the aim of identifying the most promising electrode pairing for tandem PEC devices. The results highlight the effectiveness of electrodeposition for BiVO₄ synthesis, the beneficial role of Mo doping, and the critical importance of photocathode selection in determining overall device performance. This study establishes a solid experimental groundwork for the assembly of fully functional PEC cells and guides future improvements through heterojunction engineering, multilayer architectures, and co-catalyst integration.

In conclusion, this work advances the understanding of material–structure–function relationships in photoelectrodes and provides valuable insights for the rational design of efficient, durable, and scalable PEC systems for solar-driven hydrogen production. Future efforts will focus on integrating the optimized BiVO₄ photoanode with the selected photocathode into a complete tandem cell, performing long-term stability tests, and exploring further material enhancements to maximize solar-to-hydrogen conversion efficiency.