Host/Guest Nanostructured Photoanodes Integrated with Targeted Enhancement Strategies for Photoelectrochemical Water Splitting

Metal-insulator-semiconductor photoelectrodes for enhanced photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting provides a scalable and integrated platform to harness renewable solar energy for green hydrogen production. The practical implementation of PEC systems hinges on addressing three critical challenges: enhancing energy conversion efficiency, ensuring long-term stability, and achieving economic viability. Metal-insulator-semiconductor (MIS) heterojunction photoelectrodes have gained significant attention over the last decade for their ability to efficiently segregate photogenerated carriers and mitigate corrosion-induced semiconductor degradation. This review discusses the structural composition and interfacial intricacies of MIS photoelectrodes tailored for PEC water splitting. The application of MIS heterostructures across various semiconductor light-absorbing layers, including traditional photovoltaic-grade semiconductors, metal oxides, and emerging materials, is presented first. Subsequently, this review elucidates the reaction mechanisms and respective merits of vacuum and non-vacuum deposition techniques in the fabrication of the insulator layers. In the context of the metal layers, this review extends beyond the conventional scope, not only by introducing metal-based cocatalysts, but also by exploring the latest advancements in molecular and single-atom catalysts integrated within MIS photoelectrodes. Furthermore, a systematic summary of carrier transfer mechanisms and interface design principles of MIS photoelectrodes is presented, which are pivotal for optimizing energy band alignment and enhancing solar-to-chemical conversion efficiency within the PEC system. Finally, this review explores innovative derivative configurations of MIS photoelectrodes, including back-illuminated MIS photoelectrodes, inverted MIS photoelectrodes, tandem MIS photoelectrodes, and monolithically integrated wireless MIS photoelectrodes. These novel architectures address the limitations of traditional MIS structures by effectively coupling different functional modules, minimizing optical and ohmic losses, and mitigating recombination losses.

Improved Conductivity and in Situ Formed Heterojunction via Zinc Doping in CuBi2O4 for Photoelectrochemical Water Splitting

As a photocathode with a band gap of about 1.8 eV, copper bismuthate (CuBi2O4) is a promising material for photoelectrochemical (PEC) water splitting. However, weak charge transfer capability and severe carrier recombination suppress the PEC performance of CuBi2O4. In this paper, the conductivity and carriers transport of CuBi2O4 are improved via introducing Zn2+ into the synthesis precursor of CuBi2O4, driving a beneficial 110 mV positive shift of onset potential in photocurrent. Detailed investigations demonstrate that the introduction of an appropriate amount of zinc leads to in situ segregation of ZnO which serves as an electron transport channel on the surface of CuBi2O4, forming heterojunctions. The synergistic effect of heterojunctions and doping simultaneously promotes the charge transfer and the carrier concentration. OCP experiment proves that ZnO/Zn-CuBi2O4 possesses better charge separation; the Mott-Schottky curve shows that the doping of Zn significantly enhances the carrier concentration; carrier lifetime calculated from time-resolved photoluminescence confirms faster extraction of carriers.

Enhanced Charge Separation in Nanoporous BiVO4 by External Electron Transport Layer Boosts Solar Water Splitting

The optimization of charge transport with electron-hole separation directed toward specific redox reactions is a crucial mission for artificial photosynthesis. Bismuth vanadate (BiVO4 , BVO) is a popular photoanode material for solar water splitting, but it faces tricky challenges in poor charge separation due to its modest charge transport properties. Here, a concept of the external electron transport layer (ETL) is first proposed and demonstrated its effectiveness in suppressing the charge recombination both in bulk and at surface. Specifically, a conformal carbon capsulation applied on BVO enables a remarkable increase in the charge separation efficiency, thanks to its critical roles in passivating surface charge-trapping sites and building external conductance channels. Through decorated with an oxygen evolution catalyst to accelerate surface charge transfer, the carbon-encased BVO (BVO@C) photoanode manifests durable water splitting over 120 h with a high current density of 5.9 mA cm-2 at 1.23 V versus the reversible hydrogen electrode (RHE) under 1 sun irradiation (100 mW cm-2 , AM 1.5 G), which is an activity-stability trade-off record for single BVO light absorber. This work opens up a new avenue to steer charge separation via external ETL for solar fuel conversion.

Construction of Photothermo-Electro Coupling Field Based on Surface Modification of Hydrogenated TiO2 Nanotube Array Photoanode and Its Improved Photoelectrochemical Water Splitting

Solar water splitting has gained increasing attention in converting solar energy into green hydrogen energy. However, the construction of a photothermo-electro coupling field by harnessing light-induced heat and its enhancement on solar water splitting were seldom studied. Herein, we developed a full-spectrum responsive photoanode by depositing CdxZn1-xS onto the surface of hydrogenated TiO2 nanotube array (H-TNA), followed by modification with Ni2P. The resulting ternary photoanode exhibits a photocurrent density of 4.99 mA·cm-2 at 1.23 V vs. RHE with photoinduced heating, which is 11.9-fold higher than that of pristine TNA, with an optimal ABPE of 2.47%. The characterization results demonstrate that the ternary photoanode possesses superior full-spectrum absorption and efficient photogenerated carrier separation driven by the interface electric fields. Additionally, Ni2P reduces the hole injection barrier and increases surface active sites, accelerating the consumption of holes accumulating on the relatively unstable CdxZn1-xS to simultaneously improve the activity and stability of water splitting. Moreover, temperature-dependent measurements reveal that H-TNA and Ni2P significantly motivate the photothermal conversion to construct a photothermo-electro coupling field, optimizing photoelectric conversion and charge carrier-induced surface reactions. This work contributes to understanding the synergistic effect of the photothermo-electro coupling field on the photoelectrochemical water splitting.

Imperfect makes perfect: defect engineering of photoelectrodes towards efficient photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting for hydrogen evolution has been considered as a promising technology to solve the energy and environmental issues. However, the solar-to-hydrogen (STH) conversion efficiencies of current PEC systems are far from meeting the commercial demand (10%) due to the lack of efficient photoelectrode materials. The recent rapid development of defect engineering of photoelectrodes has significantly improved the PEC performance, which is expected to break through the bottleneck of low STH efficiency. In this review, the category and the construction methods of different defects in photoelectrode materials are summarized. Based on the in-depth summary and analysis of existing reports, the PEC performance enhancement mechanism of defect engineering is critically discussed in terms of light absorption, carrier separation and transport, and surface redox reactions. Finally, the application prospects and challenges of defect engineering for PEC water splitting are presented, and the future research directions in this field are also proposed.

Hierarchical Co-Pi Clusters/Fe2O3 Nanorods/FTO Micropillars 3D Branched Photoanode for High-Performance Photoelectrochemical Water Splitting

In this study, an efficient hierarchical Co-Pi cluster/Fe₂O₃ nanorod/fluorine-doped tin oxide (FTO) micropillar three-dimensional (3D) branched photoanode was designed for enhanced photoelectrochemical performance. A periodic array of FTO micropillars, which acts as a highly conductive "host" framework for uniform light scattering and provides an extremely enlarged active area, was fabricated by direct printing and mist-chemical vapor deposition (CVD). Fe₂O₃ nanorods that act as light absorber "guest" materials and Co-Pi clusters that give rise to random light scattering were synthesized via a hydrothermal reaction and photoassisted electrodeposition, respectively. The hierarchical 3D branched photoanode exhibited enhanced light absorption efficiency because of multiple light scattering, which was a combination of uniform light scattering from the periodic FTO micropillars and random light scattering from the Fe₂O₃ nanorods. Additionally, the large surface area of the 3D FTO micropillar, together with the surface area provided by the one-dimensional Fe₂O₃ nanorods, contributed to a remarkable increase in the specific area of the photoanode. Because of these enhancements and further improvements facilitated by decoration with a Co-Pi catalyst that enhanced water oxidation, the 3D branched Fe₂O₃ photoanode achieved a photocurrent density of 1.51 mA cm^-2 at 1.23 V_RHE, which was 5.2 times higher than that generated by the non-decorated flat Fe₂O₃ photoanode.

Optoelectrical Regulation of CuBi2O4 Photocathode via Photonic Crystal Structure for Solar-Fuel Conversion

Metal oxide semiconductors have been regarded as ideal candidates for photoelectrochemical (PEC) CO₂ reduction if the contradiction between photon harvesting and photocarrier collection can be resolved. The novel three-dimensional structure provides an available approach to balancing the above-mentioned contradiction. In this work, CuBi₂O₄ photonic crystal photocathodes with different feature sizes were developed to realize the regulation of optoelectrical properties. The resulted photocathode displays promoted PEC activity as the enhanced photocurrent and CO₂ reduction activity. Such an excellent performance was attributed to the improved efficiency of charge carrier generation and collection through extending the optical path and shortening the carrier transport distance inside films. COMSOL simulations and PEC spectroscopy analysis confirmed the promoted photon harvesting capacity and carrier dynamics. This work demonstrates a feasible strategy for developing novel photocathodes with modulated microstructures in solar-fuel conversion.