Oxidation State and Surface Reconstruction of Cu under CO₂ Reduction Conditions from In Situ X-ray Characterization

Abstract

The electrochemical CO 2 reduction reaction (CO 2 RR) using Cu-based catalysts holds great potential for producing valuable multi-carbon products from renewable energy. However, the chemical and structural state of Cu catalyst surfaces during the CO 2 RR remains a matter of debate. Here, we show the structural evolution of the near-surface region of polycrystalline Cu electrodes under in situ conditions through a combination of grazing incidence X-ray absorption spectroscopy (GIXAS) and X-ray diffraction (GIXRD). The in situ GIXAS reveals that the surface oxide layer is fully reduced to metallic Cu before the onset potential for CO 2 RR, and the catalyst maintains the metallic state across the potentials relevant to the CO 2 RR. We also find a preferential surface reconstruction of the polycrystalline Cu surface toward (100) facets in the presence of CO 2 . Quantitative analysis of the reconstruction profiles reveals that the degree of reconstruction increases with increasingly negative applied potentials, and it persists when the applied potential returns to more positive values. These findings show that the surface of Cu electrocatalysts is dynamic during the CO 2 RR, and emphasize the importance of in situ characterization to understand the surface structure and its role in electrocatalysis. 47 migrate. CO, which is a key intermediate in the CO 2 RR, has 48 been shown to exacerbate this reconstruction in near-ambient 49 pressure conditions. 15 Surface reconstructions can affect 50 product selectivity because the Cu(111) surface preferentially 51 yields CH 4 , whereas the Cu(100) surface produces C 2 H 4 with 52 a lower onset potential. 16 To probe the surface structure under 53 CO 2 RR conditions, electrochemical scanning tunneling mi-54 croscopy (ECSTM) has been utilized to image Cu surfaces 55 with atomic resolution and has successfully demonstrated that 56 polycrystalline Cu (hereafter referred to as Cu(pc)) 57 reconstructs into Cu(100) surfaces in N 2 -purged electrolytes. 17 58 However, one of the limitations of ECSTM is its limited field 59 of view, and it is unclear whether these changes occur globally. 60 Therefore, to understand the structural dynamics of Cu 61 surfaces more fully, it is imperative to elucidate both the local 62 atomic structure and long-range order under realistic CO 2 RR 63 conditions. Here, we characterize the near-surface structure of 64 a Cu(pc) thin film (50 nm thickness) under CO 2 RR 65 conditions by utilizing in situ grazing incidence X-ray 66 absorption spectroscopy (GIXAS) and X-ray diffraction 67 (GIXRD). The Cu(pc) thin film is utilized as an electrocatalyst 68 because it has been demonstrated that the roughness of the Cu 69 thin film is low enough to allow sensitivity to a few nm of the

Tracking Structure and Composition Changes in Oxide Derived Cu-Sn Catalysts During CO2 Electroreduction Using X-Ray Spectroscopy

“…Spectra are presented with a vertical offset to facilitate comparison. observation of full reduction to Cu° under CO2ER relevant potentials is in agreement with recent investigations performed by in situ XAS 35,51,52 and quasi in situ XPS 53 for pure Cu catalysts. In the context of a bimetallic Cu-Sn structure, it is interesting to note that the functionalization of the Cu(OH)2 nanowires with SnO2 does not affect their full reduction to Cu°.…”

“…In the EXAFS analysis of as-synthesized samples (Figure 3d) the peak 1.6 Å corresponds to the Cu-O distance in both CuO and Cu(OH)2, while the peak at 2.3 Å corresponds to the Cu-Cu distance in the underlying sputtered metallic Cu layer, in agreement with previous reports. 35,36 With the aim of investigating possible effects of Sn content on the progressive reduction of Cu, several Cu K-edge XANES spectra (each acquired for ~20 min) were collected during chronopotentiometric (CP) pre-reduction of bare CuNW as well as samples CuNW-SnLOW and CuNW-SnHIGH. Although under standard conditions this pre-reduction step was typically carried out at -2 mA cm -2 , taking on average 8-10 min (see ESI Figure S2), here the activation was carried out at lower current density of -0.2 mA cm -2 in order to allow sufficient time to collect a series of XANES spectra at different stages during the activation step.…”

Electrochemical C–N coupling with perovskite hybrids toward efficient urea synthesis

“… 18–22 In comparison, energy-saving and environmentally benign electrocatalysis technologies such as the nitrogen reduction reaction (NRR) and the carbon dioxide reduction reaction (CO 2 RR) are drawing growing attention. 23–30 Besides, the emerged electrochemical C–N coupling reaction may provide a new possibility of enhancing the spectrum of products of CO 2 by using CO 2 and amine derivatives/nitrogen sources (nitrate, nitrite, NO, and even N 2 ) as feedstock. 9,31,32 …”

Linking the Dynamic Chemical State of Catalysts with the Product Profile of Electrocatalytic CO2 Reduction

“…[12,66,67] Fore xample, Jaramillo and co-workers reported that the surface oxide layer was fully reduced to metallic Cu 0 before the CO 2 RR/ CORR through acombination of grazing incidence XAS and XRD. [68,69] TheC O 2 RR selectivity was found result from the surface reconstruction of the polycrystalline Cu surface toward (100) facets during the CO 2 electroreduction. Similarly,H an and co-workers have also observed the complete reduction of oxides/hydroxides to Cu 0 during CO 2 RR.…”

“…On the other hand, in some studies high selectivity toward C 2 or CO products during CO 2 RR was observed on the Cu x O catalyst that was reduced to the metallic Cu 0 state at cathodic potentials (hollow squares in Figure 4). [12, 66, 67] For example, Jaramillo and co‐workers reported that the surface oxide layer was fully reduced to metallic Cu 0 before the CO 2 RR/CORR through a combination of grazing incidence XAS and XRD [68, 69] . The CO 2 RR selectivity was found result from the surface reconstruction of the polycrystalline Cu surface toward (100) facets during the CO 2 electroreduction.…”