High-Efficiency Solar-to-CO Conversion via Molecular Mn(I) Complex Catalysts in a CO₂ Flow Electrolyzer Coupled with Silicon Photovoltaics

Solar-driven CO₂ conversion combining photovoltaics (PV) and electrochemical systems (EC) aims for negative CO₂ emissions by directly storing solar energy as fuel and chemicals. Present-day PV+EC systems face challenges due to high overvoltages in the EC, even when using precious metal catalysts, necessitating expensive high-voltage PV. Molecular metal complex catalysts can permit lower overvoltages and the use of Earth-abundant metals. In this work, we report the use of non-planar molecular Mn(I) complex catalysts¹⁾ with electron-withdrawing groups in a zero-gap MEA reactor, enabling CO₂ electrolysis at much lower potentials than conventional metallic silver or planar molecular catalysts. The Mn complex with pyrrole groups in the ligand showed excellent activity when incorporated into polypyrrole chains. Adding phenol further enhanced the catalytic activity. The MEA reactor with this Mn complex polymer and an Fe-based anode catalyst in an alkaline anolyte promoted the CO₂ reduction reaction at an extremely low potential of 1.35 V with 94% CO selectivity. A PV+EC system with this configuration, directly coupled to an inexpensive three-series Si PV unit, showed stable operation for 50 hours with a solar-to-CO conversion efficiency of over 20%.²⁾ This advancement in solar CO₂ electrolysis technology based on molecular catalysts represents a promising step toward practical and sustainable carbon-neutral energy solutions, leveraging abundant materials and efficient solar energy utilization.<br/>References:<br/>[1] S. Sato et al., ACS Catal. 8, 4452 (2018).<br/>[2] K. Sekizawa et al , ChemRxiv 10.26434/chemrxiv-2024-brj46.<br/><br/>Acknowledgements<br/>This work was supported by the Ministry of the Environment of the Government of Japan and the Uncharted Territory Challenge 2050 of NEDO, Japan.