Fractal flow field architectures for enhanced transport and performance in electrochemical CO₂ reduction flow cells

Electrochemical CO₂ reduction offers a promising route for converting CO₂ into value-added fuels and chemicals using renewable electricity, but its practical implementation remains limited by CO₂ mass transport and non-uniform reactant distribution at high current densities. Recent reactor-scale analysis has shown that gas-fed CO₂ electrolysers behave as reacting-flow electrochemical reactors, in which electrochemical conversion reshapes the cathode gas phase through coupled changes in composition, density and velocity [Alyahya et al., Advanced Science, 2026, https://doi.org/10.1002/advs.78274]. This study investigates transport enhancement through flow-field engineering by evaluating lung-inspired fractal gas flow-field architectures in a gas-fed CO₂ reduction flow cell. Using this previously validated cathode-side, flow-direction-resolved modelling framework, different fractal flow fields were assessed under identical operating and material conditions to isolate the effect of gas-distributor geometry on reactant transport, electrochemical performance and hydrodynamic resistance; the study is therefore framed as a design-oriented modelling analysis of local terminal-region transport, rather than as a full-device optimisation or experimental demonstration of a fractal CO₂ electrolyser. The model describes CO₂ transport from the gas phase to the electrode-electrolyte interface, where CO₂ is reduced to CO, which then acts as an intermediate for downstream C₂₊ product formation in competition with the hydrogen evolution reaction. The results show that higher-generation fractal designs improve local CO₂ availability, produce a more uniform reactant distribution and increase attainable CO₂-reduction current densities under transport-limited conditions. These architectures also reduce the local pressure drop in the terminal gas-delivery region, although the dependence on generation number is non-monotonic because of the trade-off between outlet density and per-outlet flow strength. Overall, this work identifies fractal gas flow fields as a promising strategy for mitigating near-electrode mass-transport limitations and controlling the local reacting gas environment in gas-fed CO₂ electrolysers, providing design guidance for future optimisation and experimental implementation.