Solar-driven CO₂ reduction is a key route toward sustainable chemical and fuel production, yet persistent challenges in catalyst durability, interfacial stability, and reactor engineering continue to hinder practical deployment. Within the EU H2020 SunCOChem project, we pursue an integrated materials-to-device-to-system approach, targeting CO-rich synthesis gas streams via (photo)electrochemical CO₂ reduction, coupled to a downstream hydroformylation unit for value-added chemical synthesis at scale.

Novel core-shell Cu₂O/SnO₂-based gas diffusion electrodes (GDEs) were designed, fabricated, and validated in a continuous-flow photo-anode/dark-cathode photoelectrochemical (PEC) cell operated under simulated photovoltaic-coupled conditions. Spray-coated catalytic layers were optimized for spatial uniformity, triple-phase boundary density, and mechanical robustness across electrode areas from 10 to 120 cm², with a total validated area of 0.24 m². The optimized electrodes achieved 84–90% Faradaic efficiency for CO production at current densities of −20 to −100 mA cm⁻².¹ The 120 cm² PEC cell was operated for over 200 hours under intermittent current profiles mimicking renewable energy supply, delivering CO-rich streams (CO:H₂ = 9:1) and syngas at total currents up to 1.5 A — directly relevant to industrial hydroformylation and carbonylation feedstock specifications.

An operando diagnostic framework was exploited by combining electrochemical impedance spectroscopy (EIS) with accelerated stress testing protocols. Moving beyond conventional equivalent-circuit EIS modeling, we employ distribution of relaxation times (DRT) analysis, giving insights into overlapping physicochemical processes such as charge transfer, ionic transport, flooding, and interfacial passivation, without imposing a priori circuit assumptions. This enables early, quantitative identification of degradation pathways and their kinetic evolution, directly linking impedance signatures to productivity-relevant parameters such as CO partial current density and selectivity retention in the 120 cm² prototype cell, thereby demonstrating a novel approach for real-time, in situ monitoring of failure modes.

Post-operation and operando physicochemical characterization (SEM, TEM, XRD, ICP-MS)² systematically informed degradation mechanisms identified under operating conditions, revealing catalyst phase restructuring and photocatalyst delamination/metal leaching as dominant failure modes, and guiding rational regeneration strategies for extended device lifetime.

Complementing the experimental framework, multiphysics modeling was deployed as a digital twin of the reactor system. Model-guided flow-field optimization addressed CO₂ starvation, inhomogeneous current-density distributions, and bubble-induced mass-transfer limitations at the cathode interface.

This work demonstrates that operando DRT-EIS diagnostics, post-test materials analysis, and multiphysics digital twins can be tightly integrated at the device level, yielding actionable design principles for next-generation PEC and hybrid solar fuels reactors. The results have direct implications for understanding interface stability, guiding CO-productivity recovery strategies, benchmarking solar fuels devices, and advancing the long-term techno-economic viability of solar-to-chemical conversion systems.