Photoelectrochemical (PEC) water splitting is considered a promising technology to produce renewable hydrogen, a clean fuel or energy carrier to replace conventional carbon‐based fossil‐fuel sources. Nevertheless, the overall solar‐to‐hydrogen efficiency and the cost‐effectiveness of this technology are still unsatisfactory for practical implementation. This can be primarily attributed to the sluggish kinetics of the anodic oxygen evolution reaction (OER) and the relatively low economic value of cogenerated O2 production. Over the past decades, there are extensive efforts to explore more kinetically favorable photooxidation reactions, which coupled with hydrogen evolution reaction (HER) can simultaneously improve H2 production yield and produce higher valuable alternatives to conventional O2. This review aims to present recent progress on the alternative anodic choices to OER. Here, the fundamental of PEC water splitting and the critical components required for this system are first shortly summarized. Then the benefits and issues of alternative photooxidation reactions including photooxidation of water to hydrogen peroxide, chlorine, alcohol, 5‐hydroxymethylfurfural, or urea oxidation when combined with the concurrent HER, are reviewed and analyzed. This review is concluded by presenting a critical evaluation of the challenges and opportunities of these alternative HER‐coupled photooxidation reactions for solar energy production and environmental treatment.

Environmentally friendly routes from "Power-to-X" (P2X) technologies to sustainably harvest and store renewable energy with net-zero CO 2 emission are imperative. The concept of P2X relies on (photo)electrolysis of earthabundant molecules into value-added products. For practical utilization, engineering robust, active, albeit inexpensive (photo)electrocatalysts via industrially compatible technologies is indeed crucial. In this context, flame spray pyrolysis (FSP) stands as an emerging approach for one-step synthesis of ready-to-use (photo)electrocatalysts with production rates of Kg h -1 in lab-scales. While features of FSP to engineer nanomaterials have been summarised, there is a need for more critical discussions on key factors, modulating properties of flame-made catalysts. Therefore, this review article will first provide an overview about the concept of the P2X and catalyst development strategies. Unique characteristic of flame-synthesized nano-catalysts including compositions, fractal morphologies, defects, and active sites will be then critically discussed. Furthermore, a potential of FSP as an electrodeassembly technique for one-step preparation of catalysts on gas diffusion layers for industry-relevant electrolyser testing will be presented. Finally, perspectives on challenges and opportunities of FSP for renewable energies will be raised. This will provide insights into the versatility and commercial viability of the FSP route for engineering novel nanostructured catalysts for renewable energy applications.

Photoelectrochemical water splitting is a promising approach to produce green hydrogen using solar energy. A primary bottleneck remains the lack of efficient photoanodes to catalyze the sluggish water photooxidation reaction. Engineering photoabsorbers with a narrow bandgap and suitable band edge can boost the photoelectrochemical performance. Herein, nanostructured iron tungstate (FeWO4) photoanodes are engineered directly on a fluorine doped tin oxide glass substrate via a scalable and ultra‐fast flame synthesis route in 13 seconds. Physiochemical, optoelectronic, and electrochemical properties of these photoanodes are systematically investigated. The key roles of charge transport, transfer, and dissolution of W and Fe ions from the FeWO4 matrix within long‐term performance are revealed. Optimal FeWO4 photoanode with a bandgap of 1.82 eV and a FeOOH/NiOOH co‐catalyst coating shows an improved water photooxidation performance, reaching a photocurrent density of 0.23 mA cm−2 at 1.4 V versus reversible hydrogen electrode in 1 m potassium hydroxide. It further demonstrates relatively good photostability, maintaining ≈96% of photocurrent density after 1‐hour continuous photooxidation, albeit some trace of Fe, W and Ni elements dissolution. Insights on the photooxidation performance of nanostructured FeWO4 provide promising directions for the engineering of small band‐gap catalysts for a variety of photoelectrochemical applications.