On the Operando Structure of Ruthenium Oxides during the Oxygen Evolution Reaction in Acidic Media.

In the search for rational design strategies for oxygen evolution reaction (OER) catalysts, linking the catalyst structure to activity and stability is key. However, highly active catalysts such as IrOx and RuOx undergo structural changes under OER conditions, and hence, structure-activity-stability relationships need to take into account the operando structure of the catalyst. Under the highly anodic conditions of the oxygen evolution reaction (OER), electrocatalysts are often converted into an active form. Here, we studied this activation for amorphous and crystalline ruthenium oxide using X-ray absorption spectroscopy (XAS) and electrochemical scanning electron microscopy (EC-SEM). We tracked the evolution of surface oxygen species in ruthenium oxides while in parallel mapping the oxidation state of the Ru atoms to draw a complete picture of the oxidation events that lead to the OER active structure. Our data show that a large fraction of the OH groups in the oxide are deprotonated under OER conditions, leading to a highly oxidized active material. The oxidation is centered not only on the Ru atoms but also on the oxygen lattice. This oxygen lattice activation is particularly strong for amorphous RuOx. We propose that this property is key for the high activity and low stability observed for amorphous ruthenium oxide.

Assessment of the Degradation Mechanisms of Cu Electrodes during the CO2 Reduction Reaction.

Catalyst degradation and product selectivity changes are two of the key challenges in the electrochemical reduction of CO2 on copper electrodes. Yet, these aspects are often overlooked. Here, we combine in situ X-ray spectroscopy, in situ electron microscopy, and ex situ characterization techniques to follow the long-term evolution of the catalyst morphology, electronic structure, surface composition, activity, and product selectivity of Cu nanosized crystals during the CO2 reduction reaction. We found no changes in the electronic structure of the electrode under cathodic potentiostatic control over time, nor was there any build-up of contaminants. In contrast, the electrode morphology is modified by prolonged CO2 electroreduction, which transforms the initially faceted Cu particles into a rough/rounded structure. In conjunction with these morphological changes, the current increases and the selectivity changes from value-added hydrocarbons to less valuable side reaction products, i.e., hydrogen and CO. Hence, our results suggest that the stabilization of a faceted Cu morphology is pivotal for ensuring optimal long-term performance in the selective reduction of CO2 into hydrocarbons and oxygenated products.

Revealing the Active Phase of Copper during the Electroreduction of CO2 in Aqueous Electrolyte by Correlating In Situ X-ray Spectroscopy and In Situ Electron Microscopy.

The variation in the morphology and electronic structure of copper during the electroreduction of CO2 into valuable hydrocarbons and alcohols was revealed by combining in situ surface- and bulk-sensitive X-ray spectroscopies with electrochemical scanning electron microscopy. These experiments proved that the electrified interface surface and near-surface are dominated by reduced copper. The selectivity to the formation of the key C-C bond is enhanced at higher cathodic potentials as a consequence of increased copper metallicity. In addition, the reduction of the copper oxide electrode and oxygen loss in the lattice reconstructs the electrode to yield a rougher surface with more uncoordinated sites, which controls the dissociation barrier of water and CO2. Thus, according to these results, copper oxide species can only be stabilized kinetically under CO2 reduction reaction conditions.