A new era in solar fuels: a battery pushes PV-driven CO2 reduction beyond its limits

Abstract

Photovoltaic (PV) technologies are central to renewable energy generation, yet their intermittent output limits efficient utilization and grid integration. Coupling PV with short- and long-term energy storage at the earliest stage of electricity generation is an attractive but challenging strategy for stabilizing PV output. Using batteries is a feasible way to cover short-term PV output variations. For longer, seasonal variations, an electrochemical (EC) pathway to produce fuels and chemicals is a promising solution. Here, we demonstrate a self-sufficient directly coupled hybrid PV-battery-electrochemical CO₂ reduction (PV-B-EC) system that autonomously and continuously converts PV energy to fuels under realistic daily irradiance and temperature variations. In comparison with a directly coupled PV-EC reference, the PV-B-EC storage combination is capable of covering timescales ranging from seconds to years. This is achieved while operating efficiently without the need for control electronics. Furthermore, the PV-B-EC combination stabilises the CO₂ reduction process by minimising power peaks. In our experiment, the system with a battery achieved near-ideal energy coupling efficiency (0.99 vs. 0.96), higher electrochemical voltage efficiency (57.8% vs. 47%), and a 2.3%_abs. increase in solar-to-chemical efficiency, exceeding even the theoretical limit of the equivalent reference PV-EC system. These results validate a previously predicted synergistic efficiency enhancement arising from the redistribution of photovoltaic energy through the battery. Implications of the power input stabilization provided by the battery on the Ag catalyst structure are studied in a dedicated experiment involving scanning transmission electron microscopy (STEM) in combination with energy-dispersive X-ray spectroscopy (EDX) and 4D-STEM analyses. For the tested voltage profiles, no discernible differences in microstructural features were observed between the PV-B-EC and PV-EC reference systems. The findings demonstrate that the simple addition of a Li-ion battery substantially enhances the PV-to-fuel efficiency under realistic field conditions, offering a simple and scalable route towards stable and efficient solar-driven CO₂ conversion.