Impact of Cu+ and Cu2+ species on the oxide-metal transition processes of CuxO foams during the CO2RR probed by operando Quick-XAS

Abstract

As a promising electrocatalyst for the CO₂ reduction reaction (CO₂RR), Cu/Cu oxide (Cu_xO) derived materials have been intensively studied in the last few decades. However, it is still poorly understood how the structure of Cu/Cu_xO precursors and their simultaneous reduction process influence CO₂RR product distribution. Using Quick X-ray absorption near edge structure spectroscopy (Quick-XANES), we aim to understand the potential-dependent reduction processes of Cu_xO foam precursors with different Cu⁰ : Cu⁺ : Cu²⁺ ratios to pure metallic Cu during the CO₂RR. Initially, the Cu_xO foams were prepared by thermal annealing of electrodeposited Cu foams at 100, 200, 300, and 450 °C in air to vary the Cu⁰ : Cu⁺ : Cu²⁺ ratio and especially the crystallinity of CuO. With these different chemical states and structures, the oxide-metal transition kinetics during the cathodic potential increment (ΔE = 100 mV), step (ΔE >100 mV), and jump (ΔE >500 mV) experiments were comprehensively investigated using multivariate curve resolution-alternating least squares (MCR-ALS) analysis of the Quick-XANES data. This allows in operando monitoring of the changes in the chemical state of Cu species particularly in relation to the effect of the previously applied potential. In principle, two rate determining steps can be involved in the CuO reduction to Cu⁰via intermediate Cu⁺ formation. First, our results demonstrate that the oxide-metal transition kinetics strongly depend on the initial abundance of Cu²⁺ species and precursor structure (ordered vs. amorphous) as well as on the type of chronoamperometric experiment. More precisely, compared to amorphous CuO, a high initial population of crystalline CuO species leads to a significant shift of the oxide-metal transition potential towards lower cathodic values, signifying a lower energy barrier to reduction. In addition, our work reveals that the different chronoamperometric experiments strongly influence the electrochemical stability of Cu⁺ species within the Cu_xO foams during CO₂ electrolysis. Smaller potential steps increase the formation of Cu⁺ species and lead to a slowdown in the reduction kinetics.