Particle-Based Photoelectrodes for PEC Water Splitting: Concepts and Perspectives

Engineering perovskite solar cells for photovoltaic and photoelectrochemical systems: strategies for enhancing efficiency and stability

Solar-driven fuel production, including photovoltaic-electrochemical (PV-EC) and photoelectrochemical (PEC) water splitting as well as CO2 reduction reaction (CO2RR), presents a viable approach to mitigating carbon emissions. One of the major obstacles in developing efficient PV-EC and PEC systems lies in identifying suitable photoabsorbers that can effectively harness solar energy while maintaining stability under operating conditions. Despite their intrinsic instability in such environments, halide perovskites have garnered significant attention as promising photoabsorbers due to their exceptional optoelectronic properties, which are essential for facilitating efficient electrochemical reactions. This review first provides a concise overview of the mechanisms underlying water splitting and the CO2RR, followed by an examination of the structural configurations and performance evaluation metrics of PV-EC and PEC systems. Next, the design and engineering of perovskite solar cells (PSCs) are explored, with an emphasis on optimizing light absorption, charge transport layer engineering, and addressing stability issues. Recent advancements in enhancing the efficiency and operational stability of PV-EC and PEC systems incorporating PSCs are then summarized. Finally, key challenges currently being addressed in the field are discussed, along with perspectives on future research directions. This review aims to support researchers in further advancing this technology toward the commercial production of green hydrogen.

Paired Chemical Upgrading in Photoelectrochemical Cells

Photoelectrochemical (PEC) technology has emerged as a promising platform for sustainable energy conversion and chemical synthesis, utilizing solar energy to facilitate redox reactions. While PEC systems have been extensively studied for water splitting, CO2 reduction, nitrogen reduction for value-added compounds synthesis, the sluggish oxygen evolution reaction (OER) on the anode side and the less economic value of O2 limit system efficiency. To address this, researchers have explored paired chemical upgrading strategies, coupling selective anodic organic oxidation reactions (OORs) with cathodic reduction reactions. This approach enabled the simultaneous production of high-value chemicals and fuels, enhancing the PEC system efficiency and economic viability. In this Perspective, we highlight the latest advancements and milestones in coupling anode OORs and cathode reduction reactions within PEC cells. Particular emphasis is placed on the key design principles, catalyst development, reaction mechanisms, and the performance of paired PEC cells. In addition, challenges and perspectives are provided for the future development of this emerging and sustainable technology.

Substitutional Mo doping in a Ta3N5 photoanode: mitigating native defects through engineering and enhancing water-splitting performance

Ta3N5, with its 2.1 eV bandgap and favorable band edge positions, is a promising compound for solar water splitting. However, its performance is limited by defective states introduced during high-temperature nitridation, particularly those based on reduced Ta species that act as electron recombination centers and can pin the Fermi level. Increasing electron density to extend the conduction band may suppress the formation of these states. Here, we introduce a combined theoretical and experimental study on Mo doping in Ta3N5, aiming to inhibit structural defects and enhance photoelectrochemical activity. Theoretical calculations reveal that Mo doping in Ta3N5 not only decreases the bandgap but also transforms the material from an indirect to a direct bandgap semiconductor. This transformation is attributed to the ability of Mo4+ ions, with comparable ionic radii and oxidation states for substitutional doping. This substitution introduces neutralizing acceptor states, effectively mitigating the formation of reduced Ta3+/Ta4+ states and nitrogen vacancies. As a result, charge carrier transport is enhanced, and recombination is suppressed. Additionally, the refractive index increases from 2.65 to 2.89 upon Mo doping, demonstrating improved optical performance for photoelectrochemical applications. Experimental results demonstrate a 4.3-fold enhancement in photoelectrochemical activity, alongside a 150 mV cathodic shift in the onset potential with substitutional Mo doping in Ta3N5. Moreover, the substitutional Mo doping does not induce lattice strain. These findings suggest that precise Mo doping in Ta3N5 has the potential to drive the development of innovative photoelectrochemical systems for practical solar fuel applications.

Charge Carrier Separation Enhancement Mechanism in Eco-Friendly CZTSSe/(Zn, Sn)O Thin-Film Photocathodes for Highly Efficient Solar-To-Hydrogen Evolution

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe)-based photocathodes present promising solutions for solar hydrogen evolution, owing to their non-toxic, cost-effective nature and exceptional photoelectrochemical (PEC) properties. Traditionally, the development of CZTSSe-based photocathodes for PEC water splitting have utilized CdS as the electron transport layer (ETL) owing to its favorable band alignment with the CZTSSe light-absorbing thin film. However, its environmental concerns pose a significant challenge. Therefore, it is crucial to identifying an eco-friendly ETL that ensures effective band alignment with kesterite materials. In this study, the Zn/Sn ratio and the thickness of the (Zn,Sn)O buffer layer is optimized to fabricate a Cd-free CZTSSe/(Zn,Sn)O/TiO₂/Pt photocathode. This design not only promotes environmental safety but also establishes optimal spike-like band alignments with the CZTSSe thin film. The optimized photocathode demonstrates excellent charge carrier separation and transfer, resulting in a photocurrent density of 29.80 mA cm- 2 at 0 VRHE and a half-cell solar-to-hydrogen (HC-STH) conversion efficiency of 4.0% in a 0.5 M H₂SO₄ electrolyte. As research continues to optimize the alternative materials, Cd-free CZTSSe/(Zn, Sn)O-based photocathodes along with their eco-friendly nature hold great promise for achieving competitive efficiencies in sustainable solar-to-hydrogen energy applications.

Partial Ni-Embedded NiO/g-C3N4 Nanocomposite Utilizing g-C3N4 as a Sacrificial Support and Intrinsic Reducing Agent for Sustainable Photoelectrochemical Hydrogen Production

Layered g-C3N4 holds great potential for photoelectrochemical (PEC) water splitting, but its performance is restricted by significant charge recombination and slow reaction kinetics. To address this, we demonstrate a novel approach to enhance PEC efficiency using a Ni/NiO/g-C3N4 nanocomposite, synthesized by leveraging g-C3N4 as both a sacrificial support and an intrinsic reducing agent. Pyrolysis of g-C3N4 with NiO in air, without external reducing agents, enabled the formation of metallic Ni and NiO within the composite. Comprehensive analyses, including XRD, FTIR, TEM, XPS, PL, and TRPL, confirmed the structural and electronic properties, highlighting improved charge separation and transport. Optimization of the NiO to g-C3N4 ratio revealed a synergistic effect of Ni and NiO, significantly enhancing PEC water splitting performance. This sustainable and efficient synthesis strategy underscores the dual functionality of g-C3N4 and advances the development of eco-friendly catalytic materials.

Semiconductor-Based Photoelectrocatalysts in Water Splitting: From the Basics to Mechanistic Insights-A Brief Review

Hydrogen and oxygen serve as energy carriers that can ease the transition of energy due to their high energy densities. Nonetheless, their production processes entail the development of efficient and low-cost storage and conversion technologies. In this regard, photoelectrocatalysts are materials based on the photoelectronic effect where electrons and holes interact with H2O, producing H2 and O2, and in some cases, this is achieved with acceptable efficiency. Although there are several reviews on this topic, most of them focus on traditional semiconductors, such as TiO2 and ZnO, neglecting others, such as those based on non-noble metals and organic ones. Herein, semiconductors like CdSe, NiWO4, Fe2O3, and others have been investigated and compared in terms of photocurrent density, band gap, and charge transfer resistance. In addition, this brief review aims to discuss the mechanisms of overall water-splitting reactions from a photonic point of view and subsequently discusses the engineering of material synthesis. Advanced composites are also addressed, such as WO3/BiVO4/Cu2O and CN-FeNiOOH-CoOOH, which demonstrate high efficiency by delivering photocurrent densities of 5 mAcm-2 and 3.5 mA cm-2 at 1.23 vs. RHE, respectively. Finally, the authors offer their perspectives and list the main challenges based on their experience in developing semiconductor-based materials applied in several fields. In this manner, this brief review provides the main advances in these topics, used as references for new directions in designing active materials for photoelectrocatalytic water splitting.

A nearly transparent Ni-based oxygen-evolving catalyst for photoelectrocatalysis

An adaptive Ni-based catalyst derived from soluble nickel-bipyridine [Ni(bpy)3]2+ exhibits increased active sites, and yet is nearly transparent to solar light; hence, this catalyst on nickel-sputtered Si exhibits a 250 mV negative shift of overpotential at 20 mA cm-2 relative to the pristine Ni/Si photoanode.