Space Resolved, in Operando X-ray Absorption Spectroscopy: Investigations on Both the Anode and Cathode in a Direct Methanol Fuel Cell

In operando space resolved XAS measurements on a catalysts coated membrane anode and cathode during methanol oxidation were performed through three different XAS windows at each electrode. Preheated methanol was pumped to the anode (PtRu/VC), and two different levels of nonhumidified O2 (providing O2 excess or O2 starvation) were pumped to the cathode (Pt/VC). At the anode, the methanol flowed upward; i.e., it entered the cell from near the bottom window and CO2 and other products exited near the top. At the cathode, oxygen was in counter flow, i.e., downward with inlet near the top. The relative OH and CO coverages on both the anode and cathode were followed using the Δμ-XANES technique at each window during operation at several current levels. A strong spatial correlation between the OH adsorbate coverage on the cathode and that on the anode located just across the proton exchange membrane (PEM) is found, this arising from the H+ “cross-talk” between the anode and cathode through the PEM. To our knowledge, these are the first results to show such direct correlation in space and potential between OH adsorbates on the cathode with H+ production at the anode, and it points directly to the importance of the H+ concentration in the kinetics of the oxygen reduction reaction. Finally, the electrode with the slowest reaction rate (methanol oxidation at the anode at high O2 flow or oxygen reduction at the cathode at low O2 flow) dictates the local potential and therefore in part the local adsorbate coverage on the opposite electrode.