A Bias-Free Tandem Photoelectrochemical Cell of BiVO4 - CuO for Solar-Driven Water Splitting

Photoelectrochemical (PEC) water splitting has been considered as a promising technique for converting solar energy into clean and renewable hydrogen (H₂) fuel. Recently, Tandem PEC cells based on stable and low-cost metal oxides have attracted attention for generating green H₂ solely through water and solar energy. In this study, we synthesized BiVO₄/FeNiO_x photoanodes and CuO photocathodes using magnetron sputtering deposition. These materials were integrated into a straightforward and cost-efficient Tandem PEC cell to facilitate oxygen and hydrogen evolution reactions (OER and HER), respectively. Notably, the BiVO₄/FeNiO_x photoanode displayed remarkable PEC performance and demonstrated robust chemical stability in OER, achieving a high photocurrent density of +1.22 mA cm^-2 and an impressive charge transfer efficiency of 96% at the water oxidation potential. The CuO photocathode displayed remarkable performance in the PEC process for the hydrogen evolution reaction. It initiated at a starting potential of 1.03 V vs. RHE and achieved a photocurrent density of approximately 0.4 mA cm^-2 at +0.40 V vs. RHE. This outcome underscores the viability of employing these materials in a bias-free PEC cell. Additionally, we introduced a practical model based on classical band theory to assess the interfacial band alignment of photoelectrodes during PEC water splitting under operational conditions. Our energy band diagrams, simulated under illumination, illustrated that the photogenerated (electrons ad holes) in the BiVO₄/FeNiO_x - CuO Tandem PEC film possessed sufficient energy to drive OER and HER without requiring external bias. Encouragingly, our novel BiVO₄/FeNiO_x - CuO Tandem PEC device produced a stable operating photocurrent density of ~50 μA cm^-2 under zero-bias illumination (AM 1.5G) for at least 1000 seconds, evidencing the occurrence of bias-free solar water splitting reactions.