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

The surface structure and oxide content near the surface of copper electrodes under CO and CO 2 reduction conditions are debated. By live-monitoring Cu and Cu 2 O Bragg peaks from the surface of a polycrystalline Cu electrode while scanning from open-circuit potential to CO reduction potentials, we show that the near-surface region is fully converted to the metallic phase at approximately +0.3 V vs RHE.

Alloying is a powerful tool that can improve the electrocatalytic performance and viability of diverse electrochemical renewable energy technologies. Herein, we enhance the activity of Pd-based electrocatalysts via Ag-Pd alloying while simultaneously lowering precious metal content in a broad-range compositional study focusing on highly comparable Ag-Pd thin films synthesized systematically via electron-beam physical vapor co-deposition. Cyclic voltammetry in 0.1 M KOH shows enhancements across a wide range of alloys; even slight alloying with Ag (e.g. Ag0.1Pd0.9) leads to intrinsic activity enhancements up to 5-fold at 0.9 V vs. RHE compared to pure Pd. Based on density functional theory and x-ray absorption, we hypothesize that these enhancements arise mainly from ligand effects that optimize adsorbate–metal binding energies with enhanced Ag-Pd hybridization. This work shows the versatility of coupled experimental-theoretical methods in designing materials with specific and tunable properties and aids the development of highly active electrocatalysts with decreased precious-metal content.

Steering the selectivity of Cu-based electrochemical CO 2 reduction (CO 2 R) catalysts toward targeted products will serve to improve the technoeconomic outlook of technologies based on this process. Using physical vapor deposition as a tool to overcome thermodynamic miscibility limitations, CuAg thin films with nonequilibrium Cu/Ag alloying were prepared for CO 2 R performance evaluation. In comparison to pure Cu, the CuAg thin films showed significantly higher activity and selectivity toward liquid carbonyl products, including acetaldehyde and acetate. Suppressed activity and selectivity toward hydrocarbons and the competing hydrogen evolution were also demonstrated on CuAg thin films, with a greater degree of suppression observed at increasing nominal Ag compositions. Compositional-dependent CO 2 R trends coupled with physical characterization and density functional theory suggest that significant miscibility of Ag into the Cu-rich phase of the catalyst underpinned the observed CO 2 R trends through tuning of adsorbate and reaction intermediate binding energies on the surface.

Metal, nitrogen‐doped carbon materials have attracted interest as heterogenous catalysts that contain MNx active sites that are analogous to molecular catalysts. Of particular interest is Ni,N‐doped carbon, a catalyst that is active for the electrochemical reduction of CO2 to CO. Critical to the understanding of these materials is proof of single atomic sites and characterization of the environment surrounding the metal atom; however, directly probing this coordination remains challenging. This challenge is addressed by combining scanning transmission electron microscopy (STEM), single atom electron energy loss spectroscopy (EELS), and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS). Through STEM imaging, atomic dispersion of Ni in the carbon framework is confirmed and image analyses are utilized to give semiquantitative estimates of neighbor distance distributions and site densities of Ni atoms. Atomic resolution EELS demonstrates that N and Ni are colocated at the single Ni atom sites suggesting Ni–N coordination. ToF‐SIMS reveals a distribution of NiNxCy− fragments that reflect the Ni–N bonding environments within Ni,N‐doped carbon. The fragmentation from Ni,N‐doped carbon is similar to Ni phthalocyanine, suggesting the existence of heterogenized, molecular‐like NiN4 active sites which motivates future studies that leverage insight from molecular catalysis design to develop next‐generation heterogeneous catalysts.

A combination of experiment and theory has been used to understand the relationship between the hydrogen evolution reaction (HER) and CO 2 reduction (CO 2 R) on transition metal phosphide and transition metal sulfide catalysts. Although multifunctional active sites in these materials could potentially improve their CO 2 R activity relative to pure transition metal electrocatalysts, under aqueous testing conditions, these materials showed a high selectivity for the HER relative to CO 2 R. Computational results supported these findings, indicating that a limitation of the metal phosphide catalysts is that the HER is favored thermodynamically over CO 2 R. On Ni-MoS 2 , a limitation is the kinetic barrier for the proton−electron transfer to *CO. These theoretical and experimental results demonstrate that selective CO 2 R requires electrocatalysts that possess both favorable thermodynamic pathways and surmountable kinetic barriers.