Beyond surface: Correlating CuI /CuII and O-vacancy surface sites in CuxO nanocubes with their activities as noble metal-free anodes for fuel cells

Nanocrystals' surface area and shape significantly impact their activities in fuel cells. On the other hand, additional parameters that may affect their activities cannot be neglected. Here, we demonstrate that the presence of crystalline defects (such as oxygen vacancies) and the relative concentration of CuI/CuII may also vary with the decrease of CuxO nanocubes sizes, contributing to the observed activities as noble metal-free anodes for methanol fuel cells. However, such differences did not follow the size as expected, showing that this parameter does not play a critical role in determining materials' properties. To this end, CuxO nanocubes having controllable sizes (50, 65, and 85 nm) were synthesized by a simple and similar protocol, leading to nanocrystals enclosed by {100} surfaces. When the catalytic activity of the different-sized CuxO nanocubes was performed toward the electrooxidation of methanol in alkaline media, the observed performances decreased as follows: 50 nm > 85 nm > 65 nm-CuxO nanocubes, in which 50 nm-CuxO nanocubes led to the best electrocatalytic results. Interestingly, our results showed that the differences in the catalytic activities of CuxO nanocubes displaying different sizes could not only be assigned to a gain of surface area with a decrease in particle size. More specifically, XPS results indicated that the reduction of particle size led to an increase in both Os/OL and Cu(I)/Cu(II) ratios, demonstrating the enrichment of oxygen vacancies at the surface of CuxO nanocubes, which also contributed for the observed catalytic activities.