Key considerations and strategies for safe and sustainable scale-up of MOF-based photocatalytic hydrogen production : a practical implementation perspective

The hydrogen (H2) economy is a growing industry with the potential to decarbonize various sectors. While numerous production pathways exist for H2 production, technologies enabling on-site production are increasingly favored for their ability to mitigate environmental bottlenecks (including risks due to leakage, fire, and explosion) and high insurance costs (fixed upfront premiums) associated with liquid H2 transportation and distribution. Although on-site H2 production technologies, such as electric grid-powered and off-grid photovoltaic (PV) and other renewables-assisted electrolyzers, along with EAT’s Si+ pathway, are demonstrating market viability through commercial and pilot-scale projects, photocatalytic overall water splitting (PC-OWS) faces challenges due to its low solar-to-H2 conversion efficiencies, limiting its potential for widespread scale-up even with improved arrayed panel reactor designs. This performance gap necessitates the exploration of advanced photocatalytic materials that enhance efficiency and strategies for effective practical deployment. In this context, we investigate metal-organic frameworks (MOFs) as promising materials for enhancing on-site H2 production via PC-OWS pathway, introducing MOF-based PC-OWS where MOFs act as efficient catalysts. The unique properties of MOFs, including high surface area, tunable pore sizes, and diverse metal centers, facilitate improved light absorption, charge separation, and catalytic activity (both as main and co-catalysts, particularly with design innovations involving metal nanoparticles (MNPs) doping). This offers a potential pathway to overcome the efficiency limitations of PC-OWS systems, especially in enhancing the H2 evolution reaction (HER) activity. Despite these advantages, concerns regarding the safety and sustainability of MOF-based PC-OWS system design and operation remain and warrant careful consideration.

Addressing these concerns is crucial for realizing the full potential of MOF-based PC-OWS innovation for on-site H2 production. Therefore, we aim to identify and analyze the key considerations for the safe and sustainable scale-up. To achieve this, we applied the European Commission’s Safe and Sustainable by Design
(SSbD) framework’s (re)-design phase 8 principles, recognizing that MOF-based PC-OWS is still in its early stages of development. This approach allows us to explore potential risks and guide the design for safety and sustainability across the technology readiness levels. Drawing upon our experiences in the MOF2H2 European Union Project, insights from our diverse consortia partners, and a comprehensive review of the literature spanning scientific, engineering, and practical deployment perspectives, we have developed a scale-up design
framework accounting key considerations tailored for real-world operational setting.

Our analysis has identified several key considerations, for instance, from MOF side, material selection, synthesis procedures, degradation and toxicity related considerations, and circularization options especially for the reuse and recycling of spent MOFs (for synthesis of MNPs and nanocomposites). From toxicity point
of view, the crystal size effects on MOFs toxicity, and toxicity of metal ions, organic linkers, solvents used for synthesis, and toxic effects due to solvents used in the recycling/reuse of spent MOFs. From a PC-OWS side, several critical factors emerge: PC sheet design and material selection, PC sheet-based array reactor structural design and installation, and operational complexities. These complexities span the entire process, from water injection to HER and oxygen (O2) evolution reaction (OER), and then to gas separation and distribution. To optimize performance, and reduce the risk of explosion timely venting, separation, and collection of H2 and O2 are paramount. Furthermore, accounting for the influence of weather parameters on PC-OWS performance is essential for reliable operation. Given the nature of PC-OWS systems as practical installations akin to solar PV power plants, factors such as deployment of both logistics and reverse logistics, installation variants and locations, land-use conflicts (as they need large areas for installation), and other potential risks of decommissioning were also found to be crucial and points to be considered especially when it is scaled. Based on the identified considerations, we also explored strategies to enhance material efficiency, minimize hazardous chemical usage (particularly in MOFs and arrayed panel reactor design), optimize energy and water consumptions and prioritize localized renewable resources and operating site-specific integration approaches to mitigate intermittent issues in H2 production at scale serving the real use case. These considerations may pave the way for the safe, sustainable, and systematic adoption of MOF-based PC-OWS in a decarbonized energy landscape, fostering further innovation.