Oxide-Metal Transition Processes of CuxO Foams during CO2RR Probed by Operando Quick-XAS
Authors: Blaseio Sonja , Dutta Abhijit , Dosche Carsten , Rahaman Motiar , Kiran Kiran , Dworzak Alexandra , Mahrt Björn , Broekmann Peter , Oezaslan Mehtap 
Published: December 2024

Electrochemical CO2 reduction reaction (CO2RR) is a promising alternative for large-scale production of hydrocarbons. However, there are still some challenges including poor product selectivity and highly complex multiple-step reaction mechanisms.[1,2] To enhance the electrochemically active surface area and create more active surface sites, one of the promising approaches is the use of partial or complete (surface) oxidation of nanostructured copper materials. These Cu oxide precursor catalyst materials are not stable during the cathodic potentials applied for CO2RR, but still show an enhanced selectivity for the formation of C2+ products.[3,4] Interestingly, few studies proposed that oxygen can be present under CO2RR conditions in a few nm thick amorphous copper layer, while DFT calculations indicate that no subsurface oxygen remains thermodynamically stable at these highly cathodic potentials.[5-7] So far, only Valesco-Vélez et al. studied the Cu oxidation state changes (Cu2+ and Cu+ to metallic Cu) during CO2RR in CO2-saturated 0.1 M KHCO3 probed by operando X-ray absorption spectroscopy (XAS).[8] However, for pure CuO no reduction to metallic copper or Cu2O even at highly cathodic conditions is found, due to the formation of a copper carbonate passivation layer.[8] Thus, it is still poorly understood how the structure of the Cu/CuxO precursor materials and their simultaneous reduction processes to metallic copper influence the CO2RR product distribution and the overpotential required for hydrocarbon formation. Recently, we have shown the critical potential of oxide-metal transition processes for a Cu oxide foam annealed at 300 °C in air probed by operando XAS, X-ray diffraction (XRD), and Raman spectroscopy techniques.[9,10]
Using Cu K-edge Quick-XAS technique, we aim to understand the potential-dependent reduction of different Cu2+:Cu+ ratios to Cu0 for nanoporous CuxO foam precursor catalysts under CO2RR conditions. The CuxO foams were prepared by electrodeposition and subsequent thermal annealing between 100 and 450 °C in air to generate the different Cu2+:Cu+:Cu0 ratios. Initially, from ex-situ XANES, XRD, and XPS analyses, we found that a rise in annealing temperature leads to an increase in the proportion of Cu2+ species and their degree of crystallinity within the CuxO foams. After annealing at 100 °C and 200 °C, the CuO phase/species are amorphous and mainly found at the surface of the foam. Once the annealing temperature reaches 300 °C or higher, the CuO phase is entirely crystalline.
With these different chemical states of the precursor catalyst, the Cu oxide-metal transition kinetics during cathodic potential increment (ΔE = 100 mV), step (ΔE > 100 mV), and jump (ΔE > 500 mV) experiments in CO2-saturated 0.5 M KHCO3 were comprehensively investigated using Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) and Linear Combination Fit (LCF) analyses of the operando Cu K-edge Quick-XANES data. Our results demonstrate that different oxide-metal transition kinetics were found strongly dependent on the initial abundance of Cu2+ species and precursor structure (ordered vs. amorphous) as well as on the applied potential protocol to the CuxO foam precursor catalysts. More precisely, a high initial population of crystalline CuO, which is found for the CuxO foams annealed at 300 °C and 450 °C, leads to a significant shift of the oxide-metal transition potential towards lower cathodic overpotentials. For example, for the CuxO foam annealed at 450 °C (rich in crystalline CuO phases) the oxide-metal transition potential is shifted to 0 VRHE, while for the foam annealed at 100 °C this transition occurs at – 0.6 VRHE. Furthermore, smaller potential increments increase the formation and accumulation of Cu+ species, thereby slowing down the reduction kinetics. For the CuxO foam annealed at 200 °C, we could observe a full reduction to metallic Cu if jumping from OCP directly to – 0.5 VRHE, whereas no full reduction took place if using incremental potentials before applying – 0.5 VRHE.
Overall, we show the substantial influence of crystalline CuOand the applied potential protocol on the reduction of CuxO foam precursor materials to metallic Cu during CO2RR.
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