Toward accelerated stress testing and operando distribution of relaxation times diagnostics for durability assessment of a 120 cm² CO₂ (photo)electrolyzer

Solar-driven CO2 reduction technologies face unique challenges when transitioning from laboratory to prototype scale. In the European Union-funded SunCoChem project, efforts focused on designing and validating a reactor for CO2 conversion to syngas with a suitable ratio, with an area up to 2,400 cm2, for subsequent upgrading into a hydroformylation unit. In addition to selectivity, ensuring process durability remains critical for economic viability. Yet practical implementation at the lab scale remains difficult. To address this, we combined operando electrochemical impedance spectroscopy (EIS) with stress testing, enabling early identification of degradation pathways based on productivity-related parameters. Importantly, we overcame the intrinsic limitations of conventional EIS analysis based on equivalent-circuit modelling by using relaxation-time distribution analysis. We present an integrated approach that successfully couples in-operando EIS with a 120 cm2 prototype, providing a diagnostic framework for real-time monitoring and quantification of failure modes. We discuss practical approaches to addressing current technological bottlenecks and give a perspective on future optimization strategies.