Monitoring the Activation of a AuCu Aerogel CO₂-Reduction Electrocatalyst via <i>Operando</i> XAS

The electrochemical reduction of CO₂ is a promising approach to mitigate global warming by converting CO₂ into valuable industrial chemicals such as CO. Among the various CO₂-electroreduction catalysts investigated, AuCu alloys have proven to be particularly promising as they exhibit even higher activity and selectivity toward CO production compared to pure Au, which can be considered as one of the state-of-the-art catalysts for this reaction. In a recent study, we showed that unsupported AuCu aerogels feature an appealing CO₂-to-CO activity and selectivity, even if in their as-synthesized form they were not phase-pure but instead contained Cu oxide. Thus, in this work, we aim at understanding how the transformation of this bimetallic and compositionally heterogeneous aerogel induced by a cyclic voltammetry (CV) treatment leads to this enhanced CO₂-electroreduction performance. This was done by applying three different experimental protocols, implying (i) the absence of this CV treatment, (ii) the completion of the CV treatment without exchanging the electrolyte prior to the CO₂-reduction test, or (iii) the CV treatment and exchanging the electrolyte before performing the CO₂-reduction potential hold. These three protocols were complemented with <i>operando</i> grazing incidence X-ray absorption spectroscopy (GIXAS) measurements that revealed the structural and compositional changes undergone by the AuCu aerogel during CV treatment. The latter is then shown to lead to the removal of Cu oxide side phases and the enrichment of the aerogel's surface with Au atoms and a AuCu alloy phase, which in turn results in a significant increase in the faradaic efficiency toward CO, from 23 to 81% when this CV treatment is overlooked vs performed, respectively.