Fig. 1. Characterization of the gas diffusion electrodes. (a) SEM micrographs taken in the low, medium and high loading regions of a Au/C catalyst gradient sprayed on the gas diffusion layer. The zoomed-in image (red box) shows the shape and distribution of the 20 nm particles on the GDE. (b) Gold weight percentage measured with EDX along the loading gradient showing a shallow (GDE-A) and steep (GDE-B) gradient. Each data point is an average of three measurements taken along the Y axis at a given X-position, and error bars are the respective standard deviation. (c) SEM micrograph of the cross-section of GDE-B exposed after milling with a focused ion beam, together with the EDX elemental maps recorded in the same area.