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Electrochemical conversion of CO(2) into hydrocarbons and oxygenates is envisioned as a promising path toward closing the carbon cycle in modern technology. To date, however, the reaction mechanisms toward the plethora of products are disputed, complicating the search for alternative catalyst materials. To conclusively identify the rate-limiting steps in CO reduction on Cu, we analyzed the mechanisms on the basis of constant-potential density functional theory (DFT) kinetics and experiments at a wide range of pH values (3-13). We find that *CO dimerization is energetically favored as the rate-limiting step toward multicarbon products. This finding is consistent with our experiments, where the reaction rate is nearly unchanged on a standard hydrogen electrode (SHE) potential scale, even under acidic conditions. For methane, both theory and experiments indicate a change in the rate-limiting step with electrolyte pH from the first protonation step under acidic/neutral conditions to a later one under alkaline conditions. We also show, through a detailed analysis of the microkinetics, that a surface combination of *CO and *H is inconsistent with the measured current densities and Tafel slopes. Finally, we discuss the implications of our understanding for future mechanistic studies and catalyst design.

Nanostructured Cu catalysts have increased the selectivities and geometric activities for high value C-C coupled (C2) products (ethylene, ethanol, and acetate) in the electrochemical CO(2) reduction reaction (CO(2)RR). The selectivity among the high-value C2 products is also altered, where for instance the yield of acetate increases with alkalinity and is dependent on the catalyst morphology. The reaction mechanisms behind the selectivity towards acetate vs. other C2 products remain controversial. In this work, we elucidate the reaction mechanism for acetate formation by using ab initio simulations, a coupled kinetic-transport model, and loading dependent experiments. We find that trends in acetate selectivity can be rationalized from variations in electrolyte pH and the local mass transport properties of the catalyst and not from changes in Cu's intrinsic activity. The selectivity mechanism originates from the transport of ketene, a stable (closed shell) intermediate, away from the catalyst surface into solution where it reacts to form acetate. While this type of mechanism has not yet been discussed in the CO(2)RR, variants of it may explain similar selectivity fluctuations observed for other stable intermediates like CO and acetaldehyde. Our proposed mechanism suggests that acetate selectivity increases with increasing pH, decreasing catalyst roughness and significantly varies with the applied potential.

As we are in the midst of a climate crisis, there is an urgent need to transition to the sustainable production of fuels and chemicals. A promising strategy towards this transition is to use renewable energy for the electrochemical conversion of abundant molecules present in the earth’s atmosphere such as H2O, O2, N2 and CO2, to synthetic fuels and chemicals. A cornerstone to this strategy is the development of earth abundant electrocatalysts with high intrinsic activity towards the desired products. In this perspective, we discuss the importance and challenges involved in the estimation of intrinsic activity both from the experimental and theoretical front. Through a thorough analysis of published data, we find that only modest improvements in intrinsic activity of electrocatalysts have been achieved in the past two decades which necessitates the need for a paradigm shift in electrocatalyst design. To this end, we highlight opportunities offered by tuning three components of the electrochemical environment: cations, buffering anions and the electrolyte pH. These components can significantly alter catalytic activity as demonstrated using several examples, and bring us a step closer towards complete system level optimization of electrochemical routes to sustainable energy conversion.

Iron−nitrogen-doped graphene (FeNC) has emerged as an exciting earth-abundant catalyst for electrochemical CO2 reduction (CO2R). However, standard theoretical approaches based on density functional theory (DFT) suggest complete poisoning of the active sites and are unable to rationalize the experimentally observed dramatic pH dependence and Tafel slopes, which have a critical impact on the electrocatalytic activity. In this work, we overcome these challenges through a rigorous theoretical investigation of FeNC single-atom catalysts using a combination of several state-of-the-art methods: hybrid functionals, continuum solvation, and potential-dependent electrochemical reaction energetics. Our model shows dipole-field interactions in CO2 adsorption to determine the overall activity, which resolves the contentious origin of experimentally observed pH dependence and rationalizes differences in activity and Tafel slopes among different samples in experimental work. A critical conclusion of our study is that single-atom catalysts can be tuned for electrocatalytic activity not only through the traditionally considered binding energies but also through the corresponding surface dipole moment of rate-determining surface intermediates. Our presented methodology paves the way for accurate mechanistic studies as well as the computational catalyst design of general single-atom catalysts.

A critical step toward the systematic development of electrocatalysts is the determination of the microscopic structure and processes at the electrified solid/electrolyte interface. The major challenges toward this end for experiment and computations are achieving sufficient cleanliness and modeling the complexity of electrochemical systems, respectively. In this sense, benchmarks of well-defined model systems are sparse. This work presents a rigorous joint experimental-theoretical study on the single-crystal (SC) Cu/aqueous interface. Within typical computational uncertainties, we find quantitative agreement between simulated and experimentally measured voltammograms, which allows us to unequivocally identify the *OH adsorption feature in the fingerprint region of Cu(110), Cu(100), and Cu(111) SCs under alkaline conditions. We find the inclusion of hydrogen evolution reaction kinetics in the theoretical model to be crucial for an accurate steady-state description that gives rise to a negligible H* coverage. A purely thermodynamic description of the H* coverage through a Pourbaix analysis would incorrectly lead to a H* adsorption peak. The presented results establish a fundamental benchmark for all electrochemical applications of Cu.

One of the major open challenges in ab initio simulations of the electrochemical interface is the determination of electrochemical barriers under a constant driving force. Existing methods to do so include extrapolation techniques based on fully explicit treatments of the electrolyte, as well as implicit solvent models which allow for a continuous variation in electrolyte charge. Emerging hybrid continuum models have the potential to revolutionize the field, since they account for the electrolyte with little computational cost while retaining some explicit electrolyte, representing a "best of both worlds" method. In this work, we present a unified approach to determine reaction energetics from both fully explicit, implicit, and hybrid treatments of the electrolyte based on a new multi-capacitor model of the electrochemical interface. A given electrode potential can be achieved by a variety of interfacial structures; a crucial insight from this work is that the effective surface charge gives a good proxy of the local potential, the true driving force of electrochemical processes. In contrast, we show that the traditionally considered work function gives rise to multi-valued functions depending on the simulation cell size. Furthermore, we show that the reaction energetics are largely insensitive to the countercharge distribution chosen in hybrid implicit/explicit models, which means that any of the myriad implicit electrolyte models can be equivalently applied. This work thus paves the way for the accurate treatment of ab initio reaction energetics of general surface electrochemical processes using both implicit and explicit electrolyte.