We present a novel apparatus for gathering in situ measurements of through-plane, ionic potential distributions in the porous electrodes of a polymer electrolyte membrane (PEM) fuel cell. Our diagnostic method uses a micro-structured electrode scaffold (MES) that is comprised of alternating layers of insulating and sensing materials into which a 100 μm diameter hole is micro-milled and then filled with catalyst ink. Using the MES, we performed a polarization curve experiment where the ionic potential was measured within a 50 μm thick catalyst layer at 8 and 24 μm from membrane. Our results show that there are significant ionic potential variations within the electrode. Such data is valuable in the electrochemical characterization of electrodes and catalysts. The MES potential measurements also provide insight into reaction distributions across the thickness of the electrode, which is valuable in the validation of porous electrode models.

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