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

Diverse Ag–MnOx surface sites/structures in Ag–Mn electrocatalysts afford robust local electronic structures tuned for efficient oxygen reduction.