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A laminated Pd-Cu alloy/C/Nafion multilayer was prepared to sense O₂ atmosphere in a metal-air structure. As a matrix, palladium was doped with various amounts of copper to conduct a preliminary test with optimum response, and four compositions, Pd, Pd8Cu2, Pd6Cu4, and Pd5Cu5, were selected as the candidate electrodes. It was found that the Pd6Cu4/C electrode showed higher sensitivity for all the electrodes. According to the phase identification of X-ray diffraction and X-ray photoelectron spectroscopy tests, the high sensitivity was attributed to the doped Cu, which was merged into the Pd matrix to repel the Pd out of the matrix as a Pd-skin layer on the surface. In the Pd-Cu alloy, the Cu site served as a template reaction site to break the O-O bond and reduce the interaction force of adsorbated oxygen on the Pd site. During the oxygen reduction reaction, not only did the decomposition of O₂ molecules occur on the electrode, but the electrode itself proceeded with a phase transformation to high valance of oxide, PdO₃. The sensing potential for O₂ sensing was determined by polarization curves in which the flat region resulting from a diffusion-control was adopted. Chronoamperometric measurements were employed to construct calibration curves for the selected electrodes. A successive response was measured to test the endurance, which showed appreciable sensitivity decay. We also tested the selectivity by introducing a series of disturbance gases, CO, SO₂, and NO₂, in which the Pd6Cu4 electrode prevailed over the other electrodes. Full article