Objective evaluation of the performance of electrocatalysts for CO 2 reduction has been complicated by a lack of standardized methods for measuring and reporting activity data. In this perspective, we advocate that standardizing these practices can aid in advancing research efforts toward the development of efficient and selective CO 2 reduction electrocatalysts. Using information taken from experimental studies, we identify variables that influence the measured performance of CO 2 reduction electrocatalysts and propose procedures to improve the accuracy and reproducibility of reported data. We recommend that catalysts be measured under conditions which do not introduce artifacts from impurities, either from the electrolyte or counter electrode, and advocate the acquisition of data measured in the absence of mass transport effects.Furthermore, measured rates of electrochemical reactions should be normalized to both the geometric electrode area as well as the electrochemically active surface area to facilitate the comparison of reported catalysts with those previously known. We demonstrate that when these factors are accounted for, the CO 2 reduction activity of Ag and Cu measured in different laboratories exhibit little difference. Adoption of the recommendations presented in this perspective would greatly facilitate the identification of superior catalysts for CO 2 reduction arising solely from changes in their composition and pretreatment.

The photoelectrochemical splitting of water into hydrogen and oxygen requires a semiconductor to absorb light and generate electron-hole pairs, and a catalyst to enhance the kinetics of electron transfer between the semiconductor and solution. A crucial question is how this catalyst affects the band bending in the semiconductor, and, therefore, the photovoltage of the cell. We introduce a simple and inexpensive electrodeposition method to produce an efficient n-Si/SiOx/Co/CoOOH photoanode for the photoelectrochemical oxidation of water to oxygen. The photoanode functions as a solid-state, metal-insulator-semiconductor photovoltaic cell with spatially non-uniform barrier heights in series with a low overpotential water-splitting electrochemical cell. The barrier height is a function of the Co coverage; it increases from 0.74 eV for a thick, continuous film to 0.91 eV for a thin, inhomogeneous film that has not reached coalescence. The larger barrier height leads to a 360 mV photovoltage enhancement relative to a solid-state Schottky barrier.

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