From concepts to devices: the odyssey of 3d transition metal catalysts in photocatalytic CO2 conversion.

The rapid increase in atmospheric CO2 concentrations driven by human activity necessitates the development of sustainable mitigation strategies. Among the various approaches, photocatalytic CO2 reduction (CO2RR) offers a green, solar-driven route to fuels and value-added chemicals under mild conditions, closely mimicking natural photosynthesis. Unlike thermochemical and electrochemical CO2 reduction, which typically rely on centralized energy inputs, photocatalysis provides a decentralized and energy-efficient alternative. Molecular photocatalysts based on earth-abundant 3d transition metals are emerging as promising candidates due to their tuneable reactivity and reduced reliance on noble metals. However, their practical deployment is constrained by challenges, including inefficient charge separation, limited stability, and poor recyclability. To address these issues, hybrid systems that couple molecular catalysts with semiconductors such as carbon nitride, quantum dots, metal oxides, chalcogenides, and perovskites have been developed, offering improved light harvesting, charge transfer, and robustness. Further incorporation into porous crystalline frameworks, including metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs), enables enhanced reusability, tailored microenvironments, and straightforward heterogenization. Recent advances in integrating these hybrid catalysts into photoelectrochemical cells and engineered photoreactors highlight their potential for scalable, continuous operation, thus advancing carbon capture, utilization, and storage (CCUS) technologies. This review critically examines the progress in 3d metal-based molecular and hybrid photocatalysts for CO2RR, with emphasis on structural design, mechanistic insights, interfacial engineering, and photoreactor development, aiming to provide a roadmap for future innovation in solar-driven carbon utilization.