The X-ray diffraction (XRD) spectrum of as synthesized Cu/CeO2-δ ("pre-cycling") illustrates several short, broad peaks corresponding to the cubic fluorite phase of CeO2-δ (Figure Indeed, comparisons between pre-and post-cycling XRD (Figure ) indicate that the intensities of CeO2-δ and Cu2O peaks are comparable, suggesting little to no Cu or CeO2-δ dissolution or other forms of degradation, consistent with stable electrochemical cycling (Figure ).