Hydrogen bonding steers the product selectivity of electrocatalytic CO reduction

Photocatalytic CO2 reduction on Cu single atoms incorporated in ordered macroporous TiO2 toward tunable products

The Cu/3DOM-TiO2 photocatalyst exhibits high performance toward CO2 to CH4 conversion in a gas–solid system while producing C2H4 in a liquid–solid system.

Influence of solvent structure and hydrogen bonding on catalysis at solid-liquid interfaces

Solvent molecules interact with reactive species and alter the rates and selectivities of catalytic reactions by orders of magnitude. Specifically, solvent molecules can modify the free energies of liquid phase and surface species via solvation, participating directly as a reactant or co-catalyst, or competitively binding to active sites. These effects carry consequences for reactions relevant for the conversion of renewable or recyclable feedstocks, the development of distributed chemical manufacturing, and the utilization of renewable energy to drive chemical reactions. First, we describe the quantitative impact of these effects on steady-state catalytic turnover rates through a rate expression derived for a generic catalytic reaction (A → B), which illustrates the functional dependence of rates on each category of solvent interaction. Second, we connect these concepts to recent investigations of the effects of solvents on catalysis to show how interactions between solvent and reactant molecules at solid-liquid interfaces influence catalytic reactions. This discussion demonstrates that the design of effective liquid phase catalytic processes benefits from a clear understanding of these intermolecular interactions and their implications for rates and selectivities.

Designing Copper-Based Catalysts for Efficient Carbon Dioxide Electroreduction

The electroreduction of carbon dioxide (CO₂ ) has been emerging as a high- potential approach for CO₂ utilization using renewables. When copper (Cu) based catalysts are used, this platform can produce multi-carbon (C₂₊ ) fuels and chemicals with almost net-zero emission, contributing to the closure of the anthropogenic carbon cycle. Nonetheless, the rational design and development of Cu-based catalysts are critical toward the realization of highly selective and efficient CO₂ electroreduction. In this review, first the latest advances in Cu-catalyzed CO₂ electroreduction in the product selectivity and electrocatalytic activity are briefly summarized. Then, recent theoretical and mechanistic studies of CO₂ electroreduction on Cu-based catalysts are investigated, which serve as programs to design catalysts. Strategies for devising Cu catalysts that aim at promoting different key elementary steps for hydrocarbon and C₂₊ oxygenates production are further summarized. Moreover, challenges in understanding the mechanism, operando investigation of Cu catalysts and reactions, and systems' influences are also presented. Finally, the future prospects of CO₂ electroreduction are discussed.

Recent Progress toward Ab Initio Modeling of Electrocatalysis

Electrode potential is the key factor for controlling electrocatalytic reactions at electrochemical interfaces, and moreover, it is also known that the pH and solutes (e.g., cations) of the solution have prominent effects on electrocatalysis. Understanding these effects requires microscopic information on the electrochemical interfaces, in which theoretical simulations can play an important role. This Perspective summarizes the recent progress in method development for modeling electrochemical interfaces, including different methods for describing the electrolytes at the interfaces and different schemes for charging up the electrode surfaces. In the final section, we provide an outlook for future development in modeling methods and their applications to electrocatalysis.

Modeling Electrified Pt(111)-Had/Water Interfaces from Ab Initio Molecular Dynamics

Unraveling the atomistic structures of electric double layers (EDL) at electrified interfaces is of paramount importance for understanding the mechanisms of electrocatalytic reactions and rationally designing electrode materials with better performance. Despite numerous efforts dedicated in the past, a molecular level understanding of the EDL is still lacking. Combining the state-of-the-art ab initio molecular dynamics (AIMD) and recently developed computational standard hydrogen electrode (cSHE) method, it is possible to realistically simulate the EDL under well-defined electrochemical conditions. In this work, we report extensive AIMD calculation of the electrified Pt(111)-Had/water interfaces at the saturation coverage of adsorbed hydrogen (Had) corresponding to the typical hydrogen evolution reaction conditions. We calculate the electrode potentials of a series of EDL models with various surface charge densities using the cSHE method and further obtain the Helmholtz capacitance that agrees with experiment. Furthermore, the AIMD simulations allow for detailed structural analyses of the electrified interfaces, such as the distribution of adsorbate Had and the structures of interface water and counterions, which can in turn explain the computed dielectric property of interface water. Our calculation provides valuable molecular insight into the electrified interfaces and a solid basis for understanding a variety of electrochemical processes occurring inside the EDL.

Promoting electrocatalytic carbon monoxide reduction to ethylene on copper-polypyrrole interface

The renewable energy-powered electroreduction of carbon dioxide or monoxide (CO) has been emerging as an attractive means to decarbonize the emission-intensive chemical manufacturing, which heavily relies on fossil fuels nowadays. One potential approach to promote the activity of electrocatalysts is to construct hybrid interface that can increase the stability of intermediates on electrode surfaces. Herein we developed a copper nanoparticle/polypyrrole (Cu-Ppy) nanowire composite as an efficient electrocatalyst for electrochemical CO reduction reaction. Compared to pure Cu nanoparticles, the Cu-Ppy composite exhibited a dramatically enhanced Faradaic efficiency of converting CO to ethylene (C₂H₄) from 34% to 69% at -0.78 V vs. reversible hydrogen electrode (RHE) in KOH electrolyte, and an excellent C₂H₄ partial current density of 276 mA·cm^-2 at -1.18 V vs. RHE. Density functional theory calculations showed that the Cu-Ppy composite could bind CO more strongly as compared to pure Cu. As the Ppy coating allowed to stabilize OCCO*, a key intermediate in the C₂H₄ formation, both the activity and selectivity of Cu-Ppy for CO-to-C₂H₄ were increased. Our work suggests that constructing rationally designed hybrid interface can tune the local environment of catalyst surface toward enhanced activity and product selectivity.

Hidden Mechanism Behind the Roughness-Enhanced Selectivity of Carbon Monoxide Electrocatalytic Reduction

High roughness has been proved to be an effective design strategy for electrocatalyst in many systems. Especially, high selectivity of carbon monoxide reduction (CORR) in competition with the hydrogen evolution reaction has been observed on high roughness electrocatalysts. However, the two well-known mechanisms, i.e., decreasing the energy barrier of CORR and increasing local pH, failed to understand the roughness-enhanced selectivity in a recent experiment. Herein we unravel the hidden mechanism by establishing a comprehensive kinetic model for CORR on catalysts with different roughness factors. We conclude that the roughness-enhanced CORR selectivity is actually kinetic controlled by local-electric-field-directed mass transfer of adsorbed species on the electrode surface. Several ways to optimize CORR selectivity are predicted. Our work highlights the kinetics in electrocatalysis on nanocatalysts, and provides a conceptually new principle for future catalyst design.

Determining intrinsic stark tuning rates of adsorbed CO on copper surfaces

This work reports a general and effective strategy of determining the intrinsic Stark tuning rate by removing the impact of the dynamical coupling of adsorbed CO on the Cu surface with surface enhanced infrared absorption spectroscopy (SEIRAS).

Bridging the Gap in the Mechanistic Understanding of Electrocatalysis via In Situ Characterizations

Electrocatalysis offers a promising strategy to take advantage of the increasingly available and affordable renewable energy for the sustainable production of fuels and chemicals. Attaining this promise requires a molecular level insight of the electrical interface that can be used to tailor the selectivity of electrocatalysts. Addressing this selectivity challenge remains one of the most important areas in modern electrocatalytic research. In this Perspective, we focus on the use of in situ techniques to bridge the gap in the fundamental understanding of electrocatalytic processes. We begin with a brief discussion of traditional electrochemical techniques, ex situ measurements and in silico analysis. Subsequently, we discuss the utility and limitations of in situ methodologies, with a focus on vibrational spectroscopies. We then end by looking ahead toward promising new areas for the application of in situ techniques and improvements to current methods.

Understanding the electric and nonelectric field components of the cation effect on the electrochemical CO reduction reaction

Electrolyte cations affect the activity of surface-mediated electrocatalytic reactions; however, understanding the modes of interaction between cations and reaction intermediates remains lacking. We show that larger alkali metal cations (excluding the thickness of the hydration shell) promote the electrochemical CO reduction reaction on polycrystalline Cu surfaces in alkaline electrolytes. Combined reactivity and in situ surface-enhanced spectroscopic investigations show that changes to the interfacial electric field strength cannot solely explain the reactivity trend with cation size, suggesting the presence of a nonelectric field strength component in the cation effect. Spectroscopic investigations with cation chelating agents and organic molecules show that the electric and nonelectric field components of the cation effect could be affected by both cation identity and composition of the electrochemical interface. The interdependent nature of interfacial species indicates that the cation effect should be considered an integral part of the broader effect of composition and structure of the electrochemical interface on electrode-mediated reactions.

From Molecules to Porous Materials: Integrating Discrete Electrocatalytic Active Sites into Extended Frameworks

Metal-organic and covalent-organic frameworks can serve as a bridge between the realms of homo- and heterogeneous catalytic systems. While there are numerous molecular complexes developed for electrocatalysis, homogeneous catalysts are hindered by slow catalyst diffusion, catalyst deactivation, and poor product yield. Heterogeneous catalysts can compensate for these shortcomings, yet they lack the synthetic and chemical tunability to promote rational design. To narrow this knowledge gap, there is a burgeoning field of framework-related research that incorporates molecular catalysts within porous architectures, resulting in an exceptional catalytic performance as compared to their molecular analogues. Framework materials provide structural stability to these catalysts, alter their electronic environments, and are easily tunable for increased catalytic activity. This Outlook compares molecular catalysts and corresponding framework materials to evaluate the effects of such integration on electrocatalytic performance. We describe several different classes of molecular motifs that have been included in framework materials and explore how framework design strategies improve on the catalytic behavior of their homogeneous counterparts. Finally, we will provide an outlook on new directions to drive fundamental research at the intersection of reticular-and electrochemistry.

Direct Observation of Carbon Dioxide Electroreduction on Gold: Site Blocking by the Stern Layer Controls CO2 Adsorption Kinetics

Directly observing active surface intermediates represents a major challenge in electrocatalysis, especially for CO₂ electroreduction on Au. We use in-situ, plasmon-enhanced vibrational sum frequency generation spectroscopy, which has detection limits of <1% of a monolayer and can access the Au/electrolyte interface during active electrocatalysis in the absence of mass transport limitations. Measuring the potential-dependent surface coverage of atop CO confirms that the rate-determining step for this reaction is CO₂ adsorption. An analysis of the interfacial electric field reveals the formation of a dense cation layer at the electrode surface, which is correlated to the onset of CO production. The Tafel slope increases in conjunction with the field saturation due to active site blocking by adsorbed cations. These findings show that CO₂ reduction is extremely sensitive to the potential-dependent structure of the electrochemical double layer and provides direct observation of the interfacial processes that govern these kinetics.

Strong Propensity of Ionic Liquids in Their Aqueous Solutions for an Organic-Modified Metal Surface

Understanding ionic structure and electrostatic environments near a surface has both fundamental and practical value. In electrochemistry, especially when room temperature ionic liquids (ILs) are involved, the complex ionic structure near the interface is expected to crucially influence reactions. Here we report evidence that even in dilute aqueous solutions of several ILs, the ions aggregate near the surface in ways that are qualitatively different from simple electrolytes. We have used a vibrational probe molecule, 4-mercaptobenzonitrile (MBN), tethered to a metal surface to monitor the behavior of the ionic layers. The characteristic nitrile vibrational frequency of this molecule has distinct values in the presence of pure water (∼2232 cm^-1) and pure IL (for example, ∼2226 cm^-1 for ethylmethylimidazolium tetrafluoroborate, [EMIM][BF₄]). This difference reflects the local electrostatic field and the hydrogen-bonding variations between these two limiting cases. We tracked this frequency shift as a function of IL concentration in water all the way from pure water to pure IL. We report two important findings. First, only one nitrile peak is observed for the entire concentration range, indicating that at least on the length scale of the probe molecule water and ILs do not phase separate within the interface, and no heterogeneously distinct electrostatic environments are formed. Second, and more importantly, we find that even up to a significant mole fraction of bulk water (x ∼ 0.95), the nitrile frequency does not change from that indicative of a pure IL for [EMIM][BF₄], indicating preferential aggregation of the ions near the surface. Because this behavior is very similar to surfactants, we chose an imidazolium cation with a longer side chain which resulted in behavior expected from a surfactant, with a preferential layer of the ions on the surface even in dilute water solutions (x ∼ 0.995). This observation indicates that even those ILs that are not nominally categorized as surfactants have a strong tendency to aggregate at the surface. Because ILs serve as electrolytes in a range of electrochemical reactions, including those requiring water, our results are likely useful for mechanistic understanding and tuning of such reactions.

Reorganization of Interfacial Water by an Amphiphilic Cationic Surfactant Promotes CO2 Reduction

The presence of cetyltrimethylammonium bromide (CTAB) near the surface of a Cu electrode promotes the electrochemical reduction of CO₂ to fuels. CTAB increases the CO₂ reduction rate by as much as 10× and decreased the HER rate by 4×, leading to ∼75% selectivity toward the reduction of CO₂. Surface enhanced infrared absorption spectroscopy (SEIRAS) was used to probe the effects of CTAB adsorption on the structure of interfacial water and CO₂ reduction intermediates. HER suppression was found to arise from the displacement of interfacial water molecules from CTAB adsorption within the double layer. The enhanced CO₂ reduction rate can be correlated to an increased population of atop-bound CO and the emergence of a low frequency atop-CO band. These results unravel the role of additives in improving CO₂-to-fuels electrocatalysis and establishing this as a powerful methodology for directing product selectivity.

Hydroxide Is Not a Promoter of C2+ Product Formation in the Electrochemical Reduction of CO on Copper

Highly alkaline electrolytes have been shown to improve the formation rate of C₂₊ products in the electrochemical reduction of carbon dioxide (CO₂ ) and carbon monoxide (CO) on copper surfaces, with the assumption that higher OH^- concentrations promote the C-C coupling chemistry. Herein, by systematically varying the concentration of Na⁺ and OH^- at the same absolute electrode potential, we demonstrate that higher concentrations of cations (Na⁺ ), rather than OH^- , exert the main promotional effect on the production of C₂₊ products. The impact of the nature and the concentration of cations on the electrochemical reduction of CO is supported by experiments in which a fraction or all of Na⁺ is chelated by a crown ether. Chelation of Na⁺ leads to drastic decrease in the formation rate of C₂₊ products. The promotional effect of OH^- determined at the same potential on the reversible hydrogen electrode scale is likely caused by larger overpotentials at higher electrolyte pH.

Heterogeneous electrocatalytic reduction of CO2 promoted by secondary coordination sphere effects

The incorporation of secondary coordination sphere effects to rationally modulate electrochemical CO₂ reduction on heterogeneous catalysts is reviewed.

Role of H2O in CO2 Electrochemical Reduction As Studied in a Water-in-Salt System

CO₂ electrochemical reduction is of great interest not only for its technological implications but also for the scientific challenges it represents. How to suppress the kinetically favored hydrogen evolution in the presence of H₂O, for instance, has attracted significant attention. Here we report a new way of achieving such a goal. Our strategy involves a unique water-in-salt electrolyte system, where the H₂O concentration can be greatly suppressed due to the strong solvation of the high-concentration salt. More importantly, the water-in-salt electrolyte offers an opportunity to tune the H₂O concentration for electrokinetic studies of CO₂ reduction, a parameter of critical importance to the understanding of the detailed mechanisms but difficult to vary previously. Using Au as a model catalyst platform, we observed a zeroth-order dependence of the reaction rate on the H₂O concentration, strongly suggesting that electron transfer, rather than concerted proton electron transfer, from the electrode to the adsorbed CO₂ is the rate-determining step. The results shed new light on the mechanistic understanding of CO₂ electrochemical reduction. Our approach is expected to be applicable to other catalyst systems, as well, which will offer a new dimension to mechanistic studies by tuning H₂O concentrations.