Quantitative Mapping of Local Mass Transfer Based on Thin‐Film Interference in Electrodeposited Copper Oxide Films

Mass transfer at liquid–solid interfaces and its spatial distribution at electrodes and membranes strongly affects the performance of electrochemical reactors, electrodialysis cells and membrane modules. However, local mass‐transfer measurements are not readily accessible and are limited in spatial resolution. This work introduces a colour‐based method to measure local mass transfer by analysing thin‐film interference in electrodeposited copper oxide films. A quantitative link between mass transfer and colour is created in a three‐step approach: Copper is electrochemically deposited under strict mass‐transfer limitation to ensure that the copper loading reflects the local transport rate. Subsequent thermal oxidation converts the copper layer into a transparent thin film. Thin‐film interference creates a thickness‐dependent colour pattern on the electrode as an optical readout. A calibrated colour scale is obtained by registering a photograph of the electrode with an electron probe microanalysis (EPMA) reference, linking red, green and blue intensities to copper mass per area. Combined with an image‐analysis algorithm, this calibration enables pixel‐wise reconstruction of copper loading from photographs and, in turn, reconstruction of local mass transfer coefficients without further EPMA measurements. We demonstrate the proposed method by analysing and comparing an open‐channel configuration with a mesh‐spacer configuration in an electrochemical flow cell. The resulting mass‐transfer maps reveal pronounced local non‐uniformity for both configurations, showing inlet effects and spacer‐induced local enhancement with high spatial resolution. The proposed method provides an accessible and scalable tool for characterizing and optimizing local mass transfer in electrochemical reactors and related systems.