Probing the Dynamics of Low-Overpotential CO₂-to-CO Activation on Copper Electrodes with Time-Resolved Raman Spectroscopy

Abstract

Oxide-derived copper electrodes have displayed a boost in activity and selectivity toward valuable base chemicals in the electrochemical carbon dioxide reduction reaction (CO2RR), but the exact interplay between the dynamic restructuring of copper oxide electrodes and their activity and selectivity is not fully understood. In this work, we have utilized time-resolved surface-enhanced Raman spectroscopy (TR-SERS) to study the dynamic restructuring of the copper (oxide) electrode surface and the adsorption of reaction intermediates during cyclic voltammetry (CV) and pulsed electrolysis (PE). By coupling the electrochemical data to the spectral features in TR-SERS, we study the dynamic activation of and reactions on the electrode surface and find that CO2 is already activated to carbon monoxide (CO) during PE (10% Faradaic efficiency, 1% under static applied potential) at low overpotentials (−0.35 VRHE). PE at varying cathodic bias on different timescales revealed that stochastic CO is dominant directly after the cathodic bias onset, whereas no CO intermediates were observed after prolonged application of low overpotentials. An increase in cathodic bias (−0.55 VRHE) resulted in the formation of static adsorbed CO intermediates, while the overall contribution of stochastic CO decreased. We attribute the low-overpotential CO2-to-CO activation to a combination of selective Cu(111) facet exposure, partially oxidized surfaces during PE, and the formation of copper-carbonate-hydroxide complex intermediates during the anodic pulses. This work sheds light on the restructuring of oxide-derived copper electrodes and low-overpotential CO formation and highlights the power of the combination of electrochemistry and time-resolved vibrational spectroscopy to elucidate CO2RR mechanisms.

Near-Unity Electrochemical CO2 to CO Conversion over Sn-Doped Copper Oxide Nanoparticles

“…Scans were acquired from 1.0 V versus RHE to −0.95 V versus RHE under in situ conditions. As shown in Figure c, a similar change in the electrocatalyst structure is found for the pristine CuO nanoparticles during the cathodic scan from 1.0 to −0.95 V versus RHE: Initially, CuO is reduced to Cu 2 O, followed by Cu 2 O reduction to metallic Cu . Reduction to metallic Cu is directly followed by the appearance of the band associated with Cu–C at 360 cm –1 , evidencing the presence of *CO as confirmed by the spectra in the CO region (Figure d, around 2000 cm –1 ).…”

Correlating CO Coverage and CO Electroreduction on Cu via High-Pressure in Situ Spectroscopic and Reactivity Investigations

“…Meanwhile, positions of the CO L band under 20 and 40 barg of CO almost overlap at all potentials, suggesting similar absolute CO coverages. We note that the CO L band can consist of the low frequency band (LFB) (∼2073 cm –1 ) and high frequency band (HFB) (∼2093 cm –1 ) attributed to the CO adsorbed on terrace and undercoordinated sites, respectively. − Our spectra show that the CO L band mainly consists of LFB, indicating a major CO L population on terrace sites.…”

The Necessity for Multiscale In Situ Characterization of Tailored Electrocatalyst Nanoparticle Stability

“…Although special cell designs are necessary for the proper use of infrared spectroscopy, which relies on ATR crystals, Raman spectroscopy and X-ray spectroscopy and diffraction can be readily applied. Raman spectroscopy is of particular interest in our research endeavors, because it allows to probe the catalyst structure (<700 cm –1 Raman shift) and electrolyte species (1000–1500 cm –1 ) as well as key reaction intermediates (>2000 cm –1 ) simultaneously . However, due to the low Raman scattering probability, the signal intensity is relatively low, which hampers the time and space resolution.…”

“…Every technique has its pros and cons in terms of time and space resolution but also sensitivity and reaction conditions. For example, Raman spectroscopy is frequently applied to study the adsorbed intermediates at the electrocatalyst surface but relies heavily on signal enhancement through the surface enhanced Raman spectroscopy (SERS) effect. , Therefore, special care needs to be taken to ensure that the SERS effect is homogeneous and that the probed surface is representative for the catalytic activity. This can only be achieved through the use of complementary techniques or through an internal standard in the Raman spectra (e.g., a vibration of an electrolyte species that does not change over time).…”

“…For example, Raman spectroscopy is often performed in a microscope, which poses limitations for the characterization of GDEs and MEAs. In our current setup, we utilize a water immersion objective to boost the signal-to-noise ratio and measure selectively at the electrode/electrolyte interface. , In a GDE configuration, it should be possible to apply Raman spectroscopy and measure the electrode/electrolyte interface in a similar fashion with a water immersion objective, but the in situ Raman cell needs to be carefully designed to ensure optimal electrocatalytic performance (high current densities). Potentially, Raman spectroscopy should be performed in transmission mode for in situ GDE characterization, for which high intensity laser setups should be developed without the need for objectives.…”