Translating laboratory success into the large-scale implementation of photocatalytic overall water splitting

As global energy consumption continues to rise, mitigating anthropogenic climate change demands the search for carbon-neutral energy sources to cut down on our reliance on fossil fuels. Sunlight-driven water splitting using particulate photocatalysts represents a promising and sustainable approach for the large-scale production of green H2. In their Frontiers in Science Lead Article, Hisatomi and colleagues [1] provide comprehensive yet succinct insights that lay a crucial foundation for the development of future H2 generation systems through particulate photocatalysis. They focus on key aspects such as photocatalyst stability, efficient H2 recovery, the safe design of reactor systems, and advancement in technologies for the separate production of H2 and O2 from H2O. The authors have developed heterogeneous photocatalysts capable of efficiently splitting H2O into H2 and O2 in the near-UV region, highlighting the ideal structure and properties of cocatalyst/photocatalyst hybrid systems for overall water splitting (OWS) [2]. Photocatalyst materials have also been identified to split H2O with longer wavelengths of visible light, but their apparent quantum yield (AQY) values remain low, mainly due to difficulties in fine-tuning their material properties [3]. New techniques have been introduced to harness visible light for Z-scheme OWS by combining distinct photocatalysts. In addition, efforts are underway to develop reactors for large-scale applications and routes to extract pure H2 from oxyhydrogen mixture saturated with H2O vapor [4]. The authors finally emphasized the importance of creating standardized methods for measuring solar-to-hydrogen (STH) efficiency, stressing that leading researchers in photocatalysis should drive this initiative, given its current low technology readiness level. The notable progress highlighted largely stems from over four decades of research in the field of OWS, with much of this work spearheaded by the authors themselves. At this pivotal juncture, where there is an unprecedented demand for scalable solar fuel production technologies, the influence of their research is anticipated to extend far beyond the scope of solar-driven catalysis and engineering, impacting a broad spectrum of industries. This viewpoint endeavors to spark a critical discussion on the challenges associated with translating laboratory success into largescale implementation of photocatalytic OWS.Current maximum STH efficiencies for photocatalytic OWS systems are close to 1%, attained under laboratory conditions with constrained testing durations and remain considerably distant from real-world settings. In 2021, Domen et al. conducted a large-scale experiment (on a 100 m 2 scale) by immobilizing Al doped SrTiO3 photocatalysts on panels and generating 600 L of H2 on a sunny day using a commercial separation membrane, reaching an optimal STH efficiency of 0.76%. This work highlights both the potential for safely generating H2 on a large scale and the significant challenges facing its actual application [4]. While the reported system can continuously produce H2, the current efficiency levels are still insufficient to make this technology viable for large-scale H2 production at this stage. Thus, realizing higher solar-toenergy conversion efficiencies remain the primary focus to make practical implementation of photocatalytic OWS feasible. In the search for suitable photocatalysts, an absorption limit over 600 nm coupled with an AQY value exceeding 60% are the necessary benchmarks for reaching the goal of 10% STH efficiency [5]. Conversely, band gap (Eg) reduction diminishes the ability of photocatalysts to facilitate redox reactions. Hence, it is crucial to strike a balance between lowering the Eg to extend light absorption and increasing it to generate stronger driving potential to promote H2O splitting. Metal (oxy)nitrides and oxysulfides have been shown to be effective for photocatalytic OWS under light with wavelengths up to 600 nm. However, these photocatalysts generally display low efficiencies and limited chemical stability. Recent progress on metal-free carbon nitride photocatalysts for H2O splitting presents a promising alternative to these inorganic semiconductors [6,7].Enhancing solar energy conversion efficiency typically involves hybridizing photocatalysts with cocatalysts. These cocatalyst materials assist in carrier separation and transport while providing active sites and speeding up water-splitting reactions [8]. However, most photocatalysts for OWS rely on rare or expensive materials, such as platinum-group metals and other toxic inorganic components, as cocatalysts. This dependence on scarce resources limits scalability and increases costs, raising sustainability issues. For instance, Rh@CrOx (or Rh-Cr mixed oxide) and Ru/Ir-based oxides serve as cocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in these systems. Another concern is that the CrOx shell could potentially leach Cr 6+ ions during prolonged photo-irradiation, leading to not only catalyst deactivation but also environmental harm and health risks [9]. Although these rare and hazardous materials are used in minimal amounts, their inclusion still presents economic and environmental challenges. Given the limited production capacities of noble metals, widespread adoption of photocatalytic OWS technology could cause substantial market price increases due to rising demand and expectations. As highlighted in the lead article, precious cocatalyst materials can be recycled using standard metal recovery processes once the photocatalyst's lifecycle is complete. Nonetheless, there is a pressing need to develop cost-effective, earthabundant cocatalysts that offer performance similar to noble metals to eliminate the need for additional recycling steps. If using noble metals is necessary, one possible solution is to alloy at least one platinum-group metal with another or with a more widely available metal.Bimetallic systems can alter their electronic structure, resulting in a catalytic performance that surpasses that of their individual parent metals [10].Author 1 et al. Short title/running header 6 Similar to the previous concerns with cocatalysts, although immobilizing photocatalysts on substrates or sheets has demonstrated scalability and effectiveness in advancing OWS, attention must also be directed toward the disposal problems related to future water-splitting systems. Although the scalability and efficiency of photocatalysts are well established, it is of equal importance to probe how to address their environmental impact. Again, the key question is that can photocatalysts and other essential components in these systems be restored or regenerated? More specifically, the photocatalysts should endure multiple operational cycles with only minor losses in efficiency. Developing recycling protocols that do not involve dismantling the entire system can be an excellent solution to minimize operating and maintenance costs. Materials approaching the end of their service life must be disposed of correctly to mitigate environmental and health risks.Lastly, one of the biggest challenges in the practical implementation of photocatalytic OWS systems is integrating them with existing energy infrastructures, especially when it comes to safely storing and transporting generated H2 to avoid contamination and operational risks.Hisatomi et al. emphasized that after H2 is separated and recovered, it can be used in various catalytic reactions to create other value-added chemicals and fuels. This approach is more practical for safe handling and transportation compared to managing H2 as the final product [1]. In this context, governments will play a crucial role in setting safety and environmental standards for the secure production, storage, and potential transport of surplus H2. Investments in key infrastructures by governments will further accelerate the adoption of photocatalytic OWS. Strong political commitment is crucial for implementing regulations and policies, such as setting H2 production standards and providing tax incentives, to make green H2 more competitive-similar to the approach taken with other emerging technologies. For example, Germany's National Hydrogen Strategy envisions a comprehensive H2 infrastructure by 2030. This involves plans to build over 1,800 kilometers of new and repurposed pipelines dedicated In summary, although significant progress has been made in photocatalytic OWS technology, challenges remain, particularly in achieving higher STH efficiencies and addressing both economic and environmental sustainability concerns. The development of cost-effective, earthabundant photocatalysts and cocatalysts, alongside standardized methods for efficiency measurement, will be crucial for the future scalability of these systems. Furthermore, the environmental impact of the future water-splitting systems must be thoroughly investigated in order to ensure their sustainability, minimize potential ecological risks, and create efficient recycling and disposal protocols that comply with environmental safety regulations. With continued research, political support, and partnerships between industries, the shift from laboratory breakthroughs to scalable photocatalytic H2 production systems holds promise for contributing to global decarbonization efforts.