Electrochemical CO₂ conversion is emerging as a promising route for the electrification of carbon-based chemical production. While high current densities and Faradaic efficiencies are often demonstrated in laboratory-scale cells with active areas of only a few square centimetres, the transition toward larger electrolyzer systems introduces additional challenges. In extended flow cells, reactant depletion, heat management, pressure effects, and mass transport limitations can generate spatial inhomogeneities in current density and product selectivity. As a result, scaled electrolyzers cannot always be treated as spatially uniform systems, and outlet-averaged measurements provide only a black-box description of the underlying local processes.

Previously, we reported spatial gradients in current density and selectivity along the flow direction in a linear CO₂ electrolyzer, showing how CO₂ depletion and changes in the local reaction environment drive a shift from CO formation toward hydrogen evolution. In this work, we extend this concept from one-dimensional flow-direction analysis to two-dimensional X–Y spatial mapping, providing a more complete picture of the reaction environment across the electrode surface.

Using a diagnostic zero-gap CO₂ electrolyzer, we correlate local current density with local product distribution and reveal position-dependent performance under industrially relevant operating conditions. This approach enables the mapping of partial current densities toward different products across the electrode, identifying where parasitic hydrogen evolution emerges and how it relates to local transport limitations. The platform is also used to investigate how different flow-field architectures influence the current density distribution, linking cell design directly to spatially resolved electrochemical performance. Importantly, the system enables long-term operation, allowing the evolution of spatial gradients to be followed over several days.

In this talk, I will discuss how two-dimensional spatial diagnostics can reveal hidden heterogeneities in CO₂-to-CO electrolyzers and how flow-field design affects the distribution of reaction rates across the electrode. These results emphasize that efficient operation must be achieved across the entire electrode area, not only at the outlet, and highlight the importance of local chemical analysis for guiding the design of efficient, stable, and scalable CO₂ electrolysis systems.