Monitoring the Activation of a AuCu Aerogel CO2-Reduction Electrocatalyst via Operando 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 operando 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.

Operando Fe dissolution in Fe-N-C electrocatalysts during acidic oxygen reduction: impact of local pH change

Atomic Fe in N-doped C (Fe-N-C) catalysts provide the most promising non-precious metal O2 reduction activity at the cathodes of proton exchange membrane fuel cells. However, one of the biggest remaining challenges to address towards their implementation in fuel cells is their limited durability. Fe demetallation has been suggested as the primary initial degradation mechanism. However, the fate of Fe under different operating conditions varies. Here, we monitor operando Fe dissolution of a highly porous and >50% FeN x electrochemical utilization Fe-N-C catalyst in 0.1 M HClO4, under O2 and Ar at different temperatures, in both flow cell and gas diffusion electrode (GDE) half-cell coupled to inductively coupled plasma mass spectrometry (ICP-MS). By combining these results with pre- and post-mortem analyses, we demonstrate that in the absence of oxygen, Fe cations diffuse away within the liquid phase. Conversely, at -15 mA cm-2 geo and more negative O2 reduction currents, the Fe cations reprecipitate as Fe-oxides. We support our conclusions with a microkinetic model, revealing that the local pH in the catalyst layer predominantly accounts for the observed trend. Even at a moderate O2 reduction current density of -15 mA cm-2 geo at 25 °C, a significant H+ consumption and therefore pH increase (pH = 8-9) within the bulk Fe-N-C layer facilitate precipitation of Fe cations. This work provides a unified view on the Fe dissolution degradation mechanism for a model Fe-N-C in both high-throughput flow cell and practical operating GDE conditions, underscoring the crucial role of local pH in regulating the stability of the active sites.

Effect of Cobalt Speciation and the Graphitization of the Carbon Matrix on the CO2 Electroreduction Activity of Co/N-Doped Carbon Materials

The electroreduction of carbon dioxide is considered a key reaction for the valorization of CO₂ emitted in industrial processes or even present in the environment. Cobalt-nitrogen co-doped carbon materials featuring atomically dispersed Co-N sites have been shown to display superior activities and selectivities for the reduction of carbon dioxide to CO, which, in combination with H₂ (i.e., as syngas), is regarded as an added-value CO₂-reduction product. Such catalysts can be synthesized using heat treatment steps that imply the carbonization of Co-N-containing precursors, but the detailed effects of the synthesis conditions and corresponding materials' composition on their catalytic activities have not been rigorously studied. To this end, in the present work, we synthesized cobalt-nitrogen co-doped carbon materials with different heat treatment temperatures and studied the relation among their surface- and Co-speciation and their CO₂-to-CO electroreduction activity. Our results reveal that atomically dispersed cobalt-nitrogen sites are responsible for CO generation while suggesting that this CO-selectivity improves when these atomic Co-N centers are hosted in the carbon layers that cover the Co nanoparticles featured in the catalysts synthesized at higher heat treatment temperatures.