The conversion of solar energy into renewable H₂ fuel via photoelectrochemical and photocatalytic water splitting approaches has attracted considerable attention due to its potential to solve significant energy and environmental issues. To achieve reasonable energy conversion efficiency of 10%, which is amenable to the economic feasibility of this technology, semiconductor materials, which are the main carrier for solar H₂ production, must fulfill several important criteria. One of the most important criteria is the band gap of the semiconductor material, which should be low enough to allow the efficient absorption of incident light. This is because the band gap of semiconductor material fundamentally determines the upper limit of the theoretical energy conversion efficiency of the solar conversion system. In this work, recent advances in utilizing semiconductor material with a band gap smaller than 2.1 eV instead of the simply-defined visible-light-responsive semiconductor materials toward solar H₂ production from water splitting was reviewed.

Developing earth-abundant electrocatalysts useful for hydrogen evolution reactions (HER) is critical for electrocatalytic water splitting driven by renewable energy. Molybdenum carbide (Mo₂C) with the crystal structure of hexagonal symmetry has been identified to be an excellent HER catalyst due to its platinum-like electronic structure while the synthesis of Mo₂C is generally time consuming and energy intensive. Herein, we demonstrated the ultrafast synthesis of a Mo₂C-based electrocatalyst with Joule heating at 1473 K for only 6 s. Benefitting from several advantages including efficient catalytic kinetics, enhanced charge transport kinetics and high intrinsic activity, the as-prepared catalyst exhibited drastically enhanced HER performance compared with commercial Mo₂C. It showed an overpotential of 288 mV for achieving a current density of −50 mA cm⁻² and good stability, which highlighted the feasibility of the Joule heating method towards preparing efficient electrocatalysts.

L-cysteine, a component of the symmetric L-cystine, is essential in numerous biological activities. Thus, detecting cysteine rapidly, selectively, and sensitively is of tremendous interest. Herein, g-C₃N₄@CdS composites were employed as sensing elements in a photoelectrochemical platform for L-cysteine sensing. In this system, g-C₃N₄@CdS composites provided much better optoelectronic function than bare CdS materials owing to their high photon-to-current conversion efficiency and excellent anti-photocorrosion properties. The innovative photoelectrochemical sensor has a wide determination range of 5 to 190 µM, a very low detection limit of 1.56 µM, a fast response time, and good long-term stability (ca. 1 month). Without applying any separation procedures, a low concentration of CySH was successfully detected in human urine samples, which is compatible with the results of chemiluminescence.

Ammonia, as an essential chemical, plays an indispensable role in both industry and agriculture. However, the traditional Haber–Bosch technique for ammonia synthesis suffers from high energy consumption and significant CO₂ emissions. Therefore, developing an energy-efficient and eco-friendly method for ammonia production is imperative. Photoelectrochemical (PEC) nitrate reduction to ammonia has emerged as a promising green alternative, which utilizes renewable solar energy to convert nitrate into valuable ammonia, thereby contributing to nitrogen recycling and wastewater remediation. This review systematically summarizes recent advances in PEC nitrate reduction to ammonia, focusing on the rational design of efficient photocathodes with the development of semiconductor materials, cocatalysts, p–n junction and heterostructure strategies. Furthermore, the integration of photocathodes with photoanodes enables the assembly of bias-free PEC systems capable of simultaneously producing ammonia and value-added chemicals, demonstrating the potential for scalable solar-driven ammonia synthesis. The mechanistic studies and future research directions are also discussed. The review aims to offer valuable insights and promote the further development of PEC nitrate reduction to ammonia.

The development of highly efficient and durable electrocatalysts for water electrolysis is fundamentally constrained by sluggish reaction kinetics, limited active-site regulation, and structural degradation under harsh operating conditions. High-entropy materials (HEMs), featuring maximized compositional diversity and configurational entropy, have emerged as versatile platforms for catalytic optimization. However, the rational incorporation of rare-earth (RE) elements into high-entropy systems and their intrinsic roles in regulating catalytic behavior remain largely unexplored. Owing to their unique electronic configurations, large atomic size mismatch, variable oxidation states, and strong affinity toward oxygen species, RE elements provides additional opportunities to manipulate electronic structures, lattice distortion, defect chemistry, and surface reconstruction behaviors within high-entropy frameworks. Rather than cataloguing individual catalyst systems, this review critically organizes RE-regulated HEMs along a structure-activity chain linking the high-entropy host and RE incorporation to local coordination/electronic effects, catalytic pathways, operando reconstruction, and long-term stability. The fundamental design principles and various modulation strategies, along with their catalytic applications toward the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and overall/seawater electrolysis are discussed, along with emphasis on the structure-activity relationships revealed by advanced characterization techniques and theoretical calculations. Finally, the remaining challenges and perspectives associated with RE incorporation regarding the activity-stability trade-off, long-term structural evolution, resource efficiency, and practical scalability are analyzed, aiming to offer guidelines for the rational design of RE-engineered high-entropy electrocatalysts with optimized activity, stability, and practical applicability for sustainable energy conversion technologies.