The electrochemical reduction of carbon dioxide (CO₂RR) is crucial for storing renewable energy as valuable chemicals and fuels. Copper and its alloys are particularly attractive catalysts due to their ability to produce multi-carbon products. However, their efficiency and selectivity are insufficient for industrial applications, requiring optimization of catalysts and reaction conditions. Understanding the nanoscale morphology of copper catalysts under reaction conditions is essential for knowledge-based catalyst design and understanding reaction pathways. Recent research focuses on controlling CO₂RR catalyst reactivity and selectivity using potentiodynamic operation, where pulsed operation between CO₂RR and Cu oxidation regimes creates active states steering the reaction towards desired products. This typically involves periodic reversal of electrode polarity, which might be challenging for large-scale applications. The potential for similar selectivity gains using potential pulses restricted to the cathodic regime remains unclear. Copper's high surface mobility at room temperature and its tendency for surface restructuring under various conditions are central to the success of these potentiodynamic modes. Previous studies have shown hydrogen-induced surface reconstruction and CO-induced nanocluster formation on Cu surfaces, but under conditions differing from those found during CO₂ electroreduction. Existing studies on Cu electrodes under CO₂RR conditions have shown larger-scale morphological changes, often starting with an initially oxidized sample. This research aims to directly investigate atomic-scale dynamic restructuring during CO₂RR in neutral carbonate electrolyte, utilizing high-resolution in situ STM, SXRD, and SERS to provide direct evidence of CO-promoted surface restructuring and its implications for CO₂RR.

Several studies have investigated the surface restructuring of copper electrodes under various conditions. In situ STM studies have shown hydrogen-induced surface reconstruction of Cu(100) and Cu(111) in acidic media at pH < 3. Research in alkaline media demonstrated that Cu(111) reconstructs at the potential of zero free charge, undergoing full reconstruction at higher potentials. Restructuring of polycrystalline Cu, resulting in preferential exposure of (100) facets, has also been observed. Studies using STM and X-ray photoelectron spectroscopy in the presence of CO gas have revealed the formation of triangular and hexagonal nanoclusters on Cu(111) and square-shaped nanoclusters on Cu(100). More recently, reversible cluster formation on Cu(111) in CO-saturated alkaline electrolyte during CO electro-oxidation was reported, highlighting the role of low-coordinated Cu surface species. Studies of Cu electrodes under CO₂RR conditions, mostly in CO₂-saturated bicarbonate solutions at pH = 7, have mainly revealed larger-scale morphological changes, such as reconstruction to Cu(100) and the formation of Cu nanocuboids. In situ AFM studies demonstrated Cu(100) surface roughening through oxidation and subsequent formation of Cu islands. Indirect evidence for surface restructuring was also found in in situ vibrational spectroscopy studies. Previous research proposed reversible Cu cluster formation at high CO coverage during CO₂RR in KHCO₃, suggesting a link between surface restructuring and catalytic activity. However, these studies often involved pre-oxidation of the Cu sample or did not examine atomic-scale dynamic restructuring directly under CO₂RR conditions.

The experiments employed three primary techniques: in situ STM, in situ SXRD, and in situ SERS. STM studies were performed using an electrochemical cell with a Cu(100) single crystal as the working electrode. To avoid surface oxidation, a careful procedure involving immersion in 0.1 M H₂SO₄ and potential control was implemented before introducing the KHCO₃ electrolyte. STM images were recorded at various potentials, allowing the observation of nanoscale surface changes. SXRD measurements were conducted at the P23 beamline of the PETRA III synchrotron at DESY, utilizing a home-built electrochemical flow cell to maintain constant CO₂ saturation and minimize bubble formation effects. Full sets of crystal truncation rods (CTRs) were recorded at different potentials, providing quantitative data on surface structural changes. A model incorporating lattice defects (adatom-vacancy pairs) and vertical expansion of the top Cu layers was used to fit the CTR data. In situ SERS experiments were performed using a Raman spectrometer coupled to an optical microscope. Roughened Cu(100) electrodes were used to enhance Raman signals. The potential was varied stepwise during the measurements, and the resulting SERS spectra provided information on the potential-dependent composition of the adlayer. Sample preparation for each technique involved specific electropolishing cycles to achieve the desired surface conditions. Data analysis included quantitative analysis of cluster size and density from STM images, fitting of CTR data using the SuPerRod program, and analysis of Raman peaks to identify adsorbed species.

In situ STM revealed CO-promoted nanoscale Cu cluster formation at potentials as positive as -0.22 V versus RHE. These clusters, predominantly ≤1.5 nm in diameter, were reversible at potentials above the CO₂RR onset. Below -0.2 V, cluster formation was irreversible, with the clusters persisting even after increasing the potential. SXRD measurements showed that surface defects, likely arising from the formation of adatom-vacancy pairs, increased at potentials ≤-0.2 V. The increase in surface defects was consistent with the STM observations of cluster formation. Vertical expansion of the top Cu layers was observed at more negative potentials, attributed to changes in the adsorbate layer. STM and SXRD data showed that cluster formation induced irreversible changes in the molecular adlayer structure. The initial ordered carbonate adlayer transitioned to a disordered phase containing various molecular species, including Cu adatoms and vacancies. SERS data showed a potential-dependent change in the adlayer composition. At potentials above -0.05 V, bidentate carbonates and (bi)carbonate anions were dominant. Below -0.05 V, these bands decreased in intensity, while new bands corresponding to adsorbed carboxylate/carboxy groups emerged, which eventually are further reduced to CO. The formation of CO intermediates coincided with the appearance of Cu nanoclusters, indicating a direct link between CO formation and surface restructuring. The irreversible changes in the molecular adlayer structure were attributed to the presence of Cu adatoms and surface vacancies which persisted after cluster dissolution, preventing the reordering of the carbonate adlayer. Analysis of CO bands in the high and low Raman shift region showed a transition from low coverage bridge-bound CO to a higher coverage atop configuration. The broad band at 2900 cm⁻¹ indicated the irreversible formation of various species containing C-H bonds. A second potential cycle showed a significantly higher cluster density, suggesting a process of self-activation.

The findings demonstrate that the onset of CO formation triggers a dynamic restructuring of the Cu(100) surface in bicarbonate solution, leading to the spontaneous and irreversible formation of low-coordinated Cu species. The limited Cu cluster density compared to CO-induced cluster formation in gas-phase studies is likely due to kinetic limitations imposed by the presence of co-adsorbates such as (bi)carbonate and carboxylates, which may interact with the undercoordinated Cu atoms and influence the CO-Cu interactions. The irreversible nature of the adlayer changes arises from the mutual stabilization of the Cu adatoms and CO₂RR reaction intermediates, preventing adatom recombination with vacancies. The presence of these low-coordinated Cu sites may significantly enhance CO₂RR activity and selectivity. This self-activation mechanism may play a role in the formation of multi-carbon products. The kinetic limitations indicate that potential pulses into the cluster formation range could further enhance or regenerate active sites, potentially leading to improved CO₂RR performance without requiring anodic polarization.

This study shows that CO formation induces Cu nanocluster formation on Cu(100) electrodes in bicarbonate solution. This leads to the irreversible formation of low-coordinated Cu species that are stable over the entire CO₂RR potential range. This self-induced surface restructuring is a new mechanism complementing previously reported larger-scale morphological changes. The resulting undercoordinated Cu sites are likely crucial for specific CO₂RR reaction pathways. Kinetic limitations suggest that potentiodynamic protocols targeting the cluster formation potential range might offer strategies for catalyst activation and enhanced performance.

The STM studies were limited by the instability of the STM tip at highly negative potentials during CO₂RR, restricting the direct observation of surface changes at the most negative potentials. The SXRD measurements, while providing quantitative data on surface defects, were less sensitive to other morphological changes. The SERS experiments provided valuable insights into the adlayer composition, but the assignment of Raman bands can be ambiguous, potentially affecting interpretation. Finally, the study focused on a specific crystal orientation (Cu(100)) and electrolyte concentration, limiting the generality of the findings to other systems.