Photovoltaics (PV) are a major and rapidly expanding source of renewable electricity, yet their output fluctuates over daily and seasonal cycles. As PV penetration grows, the resulting mismatch between generation and demand leads to curtailment and grid instability. Addressing these fluctuations requires storage solutions capable of capturing excess PV power across timescales from seconds to seasons. Batteries provide effective short-term storage on the order of hours, while long-duration storage is more efficiently achieved by converting surplus PV electricity into solar fuels via electrochemical cells. In particular, CO₂ reduction to carbon-based products offers a promising pathway for seasonal energy storage and industrial decarbonization.

We previously demonstrated that directly coupled PV-EC systems targeting CO₂ to CO can maintain high solar-to-chemical (STC) efficiency under realistic fluctuating irradiance and temperature¹. However, the PV-driven operating profile inherently forces the electrolyzer to cycle between low and high voltages and currents, leading to corresponding variations in electrochemical voltage efficiency. As a result, catalyst selection becomes restricted to materials capable of maintaining robust selectivity over a wide dynamic range.

Herein, we introduce a hybrid PV-battery-EC (PV-B-EC) architecture in which a compact Li-ion battery is connected in parallel with both the PV module and a CO₂ electrolyzer equipped with a CO-selective Ag catalyst. This simple configuration stabilizes the operating voltage and current, enabling uninterrupted, self-sustained CO₂ reduction without external control electronics or DC–DC conversion. Under realistic dynamic PV profiles, the battery buffers PV power fluctuations, stores excess energy during peak irradiance, and releases it to the electrolyzer during low-light and nighttime periods. This extends the operating window of the electrolyzer while maintaining lower and more consistent operating power (voltage and current) compared with a system without a battery. As a result, adding the battery to the PV-EC system increases the electrochemical voltage efficiency from 47% to 55% and delivers a 2.3% absolute improvement in solar-to-chemical (STC) efficiency, rising from 10.2% to 12.5%. This gain directly translates either to higher product yield or to reduced electrolyzer size and cost at equal overall efficiency.

Notably, the measured STC efficiency in PV-B-EC exceeds the theoretical limit of the optimally coupled PV-EC system, validating a previously predicted synergy in battery-mediated PV-driven electrocatalysis process such as water splitting and CO₂ reduction under ideal On/Off operating cycle^2-5. This synergy arises from lower overpotential losses and the stabilization of operating voltage and current provided by the battery.

Overall, this work demonstrates that the simple integration of a battery into directly coupled PV-EC systems offers a powerful and scalable approach to stabilize and further enhance electrocatalytic CO₂ reduction under realistic renewable-energy conditions. Hybrid PV-battery-electrochemical architectures can accelerate the deployment of electrocatalytic technologies essential for decarbonizing fuels and chemical production.