Development and performance evaluation of zinc oxide/graphene/nickel foam photoanode for photoelectrochemical hydrogen production

Abstract

Efficient photoelectrochemical (PEC) water splitting lies in the photoelectrode, a crucial component for directly converting solar energy into chemical energy in the form of hydrogen (H2). The application of semiconductor materials as photoelectrodes is often suffer by rapid charge recombination and poor stability in electrolytes. Meanwhile, zinc oxide (ZnO) offers high electron mobility and environmental friendliness, its wide band gap (3.37 eV) limits ultraviolet light absorption and causes rapid electron-hole recombination. Integrating graphene can significantly enhance the performance, but its tendency toward aggregation due to its intrinsic two-dimensional nature remains a critical challenge in the fabrication process. Although nickel foam (Ni-foam) can support graphene, the charge transfer mechanism among ZnO, graphene, and Ni-foam remains poorly understood. This work aimed to systematically investigate the potential of ZnO/graphene/Ni-foam composite photoelectrode for enhanced PEC performance, as well to elucidate the underlying charge transfer mechanism using experimental and density functional theory (DFT) approaches. Graphene was first synthesized on Ni-foam via chemical vapor deposition (CVD), tailoring annealing time (0 – 60 minutes), methane flow rate (0 – 30 sccm), and growth time (0 – 30 minutes) for high quality graphene. The best graphene/Ni-foam were then used to hydrothermally grow ZnO on graphene/Ni-foam substrate, varying reaction temperatures (150 – 220 °C) for 8 hours. The morphological structure and PEC performance of ZnO/graphene/Ni-foam was characterized, and DFT calculations provided insights into charge transfer. The best CVD parameters produced high-quality graphene on Ni-foam at annealing time: 50 minutes, methane flow rate: 10 sccm, and growth time: 20 minutes. Hydrothermal synthesis at 200 °C formed highly crystalline ZnO nanorods with an optimized length-to-diameter ratio and high surface coverage across the graphene/Ni-foam. The best ZnO/graphene/Ni-foam composite achieved an outstanding photocurrent density of 29.04 mA/cm2 at 1.23 V vs. RHE with a H2 production rate of 1.72 mL/hour. This superior performance is attributed to the synergistic effects of each component within the composite system. The porous structure of the Ni-foam improves electrolyte accessibility and provides higher surface area for photocatalytic reactions, while the integration of graphene successfully lowers the photocurrent onset potential from 0.40 V to 0.20 V and facilitates more efficient charge separation and transfer. Furthermore, the presence of Na2SO3 as a hole scavenger suppresses electron-hole recombination by rapidly consuming photogenerated holes, thereby further enhancing the PEC performance. DFT studies revealed that the Zn-terminated ZnO/Top-fcc graphene/Ni configuration is the most stable regarding interfacial binding energy, and facilitating efficient electron transfer pathways, align with experimental result. These findings clearly provide the fundamental understanding of charge transfer mechanisms that highlight the high potential of ZnO/graphene/Ni-foam composites for efficient and sustainable solar H2 production.