The electrochemical CO 2 reduction reaction (CO 2 RR) can couple carbon-capture storage with renewable energy to convert CO 2 into chemical feedstocks. For this process, copper is the only metal known to catalyze the CO 2 RR to hydrocarbons with adequate efficiency, but it suffers from poor selectivity. Copper bimetallic materials have recently shown an improvement in CO 2 RR selectivity compared with that of copper, such that the secondary metal is likely to play an important role in altering inherent adsorption energetics. This review explores the fundamental role of the secondary metal with a focus on how oxygen (O) and hydrogen (H) affinity affect selectivity in bimetallic electrocatalysts. Here, we identify four metal groups categorized by O and H affinities to determine their CO 2 RR selectivity trends. By considering experimental and computational studies, we link the effects of extrinsic chemical composition and physical structure to intrinsic intermediate adsorption and reaction pathway selection. After this, we summarize some general trends and propose design strategies for future electrocatalysts.

Most fundamental studies of electrocatalysis are based on the experimental and simulation results obtained for bulk model materials. Some of these mechanistic understandings are inapplicable for more active nanostructured electrocatalysts. Herein, considering the simplest and most typical electrocatalytic process, the hydrogen evolution reaction, an alternative reaction mechanism is proposed for nanomaterials based on the identification of a new intermediate, which differs from those commonly known for the bulk counterparts. In-situ Raman spectroscopy and electrochemical thermal/kinetic measurements were conducted on a series of nanomaterials under different conditions. In high-pH electrolytes with negligible hydronium (H3O+) concentration in bulk phase, massive H3O+ intermediates are found generating on the catalytic surface during water dissociation and hydrogen adsorption processes. These H3O+ intermediates create a unique acid-like local reaction environment on nanostructured catalytic surfaces and cut the energy barrier of the overall reaction. Such phenomena on nanostructured electrocatalysts explain their widely observed anomalously high activity under high-pH conditions.

Copper is a significant platform for CO2 electroreduction catalysts because it is the only known metal to produce multi-carbon products but suffers from poor selectivity. In the early stages of the reaction pathway, a selectivity-determining step dictates if the pathway leads to formate (a dead-end) or to CO (and on to multi-carbon products). Therefore, controlling the adsorption of key intermediates, in order to steer the reaction pathway as desired, is critical for selective CO2 electroreduction. Alloying copper is a strategy in which the composition and electronic properties of the alloy surface can be finely tuned to alter the reaction intermediate adsorption behavior. Herein, through in situ Raman spectroscopy and density functional theory (DFT) calculations, we investigate a composition-dependent selectivity toward CO and formate during CO2 electroreduction on a range of Cu–Sn alloy catalysts. We find that the selectivity shifts from CO to formate generation as the Sn content in the alloy catalyst increases because of a shift in adsorption preference from the C-bound *COOH intermediate to the O-bound *OCHO intermediate. Theoretical DFT calculation results indicate that this selectivity shift is due to a gradual weakening of *COOH adsorption and strengthening of *OCHO that occurs with increasing Sn content. A combination of theoretical Bader charge analysis and experimental X-ray photoelectron spectroscopy revealed the origin of such transformation: upon alloying, charge is redistributed from Sn to Cu, which creates regions of localized positive charge on the Sn sites. Therefore, with increasing tin content, these localized positive sites hinder the nucleophilic attack of the CO2 carbon, making *COOH adsorption (and the CO pathway) less favorable.

Cu‐based electrocatalysts with high catalytic selectivity for the CO2 reduction reaction present a significant technological challenge. Herein, a catalyst comprised of Cu single atoms in a nitrogen‐doped graphene matrix (Cu–N4–NG) is developed for highly selective electrocatalytic reduction of CO2 to CO. The single‐atom structure and coordination environment of Cu–N4–NG are identified by synchrotron‐based characterization. Compared to a conventional bulk Cu catalyst, Cu–N4–NG achieves a Faradaic efficiency of 80.6% toward CO under a moderate applied potential of −1.0 V versus reversible hydrogen electrode (RHE). Kinetic experiments show that 1) the Cu–N4 moiety favors the CO2 activation step and 2) the moiety‐anchoring graphene facilitates water dissociation, which supplies protons for CO2 reduction. Moreover, density functional theory (DFT) calculations reveal that CO2 reduction is less hindered thermodynamically on Cu–N4–NG compared to the competing hydrogen evolution reaction (HER) due to their limiting potential differences. Therefore, the highest CO selectivity is observed on Cu–N4–NG over the bulk Cu catalyst due to more favorable kinetics and thermodynamics.

An alkaline-stable anionic Cd(ii) boron imidazolate framework (BIF-89) not only exhibits unique mechanochromic behavior, but also shows an enhanced oxygen evolution reaction (OER) catalytic activity due to the presence of uncoordinated −COO groups able to capture Fe at the atomic level.