Electrolysis offers an attractive route to upgrade greenhouse gases such as carbon dioxide (CO2) to valuable fuels and feedstocks; however, productivity is often limited by gas diffusion through a liquid electrolyte to the surface of the catalyst. Here, we present a catalyst:ionomer bulk heterojunction (CIBH) architecture that decouples gas, ion, and electron transport. The CIBH comprises a metal and a superfine ionomer layer with hydrophobic and hydrophilic functionalities that extend gas and ion transport from tens of nanometers to the micrometer scale. By applying this design strategy, we achieved CO2 electroreduction on copper in 7 M potassium hydroxide electrolyte (pH ≈ 15) with an ethylene partial current density of 1.3 amperes per square centimeter at 45% cathodic energy efficiency.

CO2 electrolysis to multicarbon products at activities greater than 1 A cm−2

BACKGROUND Catalysis has had a transformative impact on society, playing a crucial role in the production of modern materials, medicines, fuels, and chemicals. Precious metals have been the cornerstone of many industrial catalytic processes for decades, providing high activity, stability, and tolerance to poisons. In stark contrast, redox catalysis essential to life is carried out by metalloenzymes that feature exclusively Earth-abundant metals (EAMs). The terrestrial abundance of some EAMs is 104 times that of precious metals, and thus their increased use would lead to reduced cost and environmental footprint. In addition to these practical considerations, EAMs display distinct reactivity profiles that originate from their characteristic electronic structure, thermochemistry, and kinetics. The behavior of EAMs provides compelling scientific opportunities for catalyst design. We assert that nature’s blueprint provides essential principles for vastly expanding the use of EAMs in sustainable catalysis. ADVANCES Exquisite tuning of the local environment around EAM active sites is key to enabling their use in catalysis. Such control is achieved in enzymatic catalysis by directed evolution of the amino acid environment, resulting in engineered enzymes with extraordinary catalytic performance. Similarly in molecular catalysis, modifying the steric and electronic properties of ligands can lead to some EAM catalysts with performance superior to that obtained from precious metal catalysts. In addition, for heterogeneous catalysts, the local environment and electronic structure of active sites can be modified by bonding to other metals or main-group elements, facilitating reaction pathways distinct from those involving precious metals. Innovations in the design of EAM catalysts demonstrate their potential to catalyze many of the reactions that traditionally relied on precious metals, although further improvements are needed in activity, selectivity, lifetime, or energy efficiency. The characteristics of EAMs point to an overarching need for improved theories and computational methods that accurately treat their multiconfigurational electronic structure. OUTLOOK The remarkable ability of enzymes to catalyze a variety of reactions under mild conditions, using only EAMs, highlights compelling opportunities for the discovery of new catalysis. Although enzymes are versatile platforms for harnessing the properties of EAMs, they are insufficiently robust under the harsh pH, temperature, pressure, and solvent conditions required for some industrial catalytic processes. Thus, systematic strategies are needed for directed evolution to extend the reactivity and persistence of engineered enzymes. For molecular catalysts, the tunability of the ligands provides opportunities for systematically varying the activities of EAMs. Key challenges include enhancing metal-ligand cooperativity, controlling transport to EAM active sites, and mastering the interactions of EAM centers with both metal-based and organic-based redox-active ligands. In heterogeneous catalysis, tuning the lattice environment of EAMs offers new opportunities for catalyst discovery, but for practical applications EAM catalysts should exhibit long-term stability and high active-site density. Thus, advances are needed in the synthesis of materials with tunable phase and nanostructure, as well as insights into how EAM catalysts undergo electronic and structural changes under sustained catalytic turnover. Strategies for controlling EAM reactivity patterns, coupled with advances in synthetic methods and spectroscopic and computational techniques, are critical for the systematic use of EAMs in sustainable catalysis.

/pdf/using-nature-s-blueprint-to-expand-catalysis-with-earth-453io2dbiw.pdf

Using nature's blueprint to expand catalysis with Earth-abundant metals

Electroreduction of carbon dioxide (CO2) over copper-based catalysts provides an attractive approach for sustainable fuel production. While efforts are focused on developing catalytic materials, it is also critical to understand and control the microenvironment around catalytic sites, which can mediate the transport of reaction species and influence reaction pathways. Here, we show that a hydrophobic microenvironment can significantly enhance CO2 gas-diffusion electrolysis. For proof-of-concept, we use commercial copper nanoparticles and disperse hydrophobic polytetrafluoroethylene (PTFE) nanoparticles inside the catalyst layer. Consequently, the PTFE-added electrode achieves a greatly improved activity and Faradaic efficiency for CO2 reduction, with a partial current density >250 mA cm-2 and a single-pass conversion of 14% at moderate potentials, which are around twice that of a regular electrode without added PTFE. The improvement is attributed to a balanced gas/liquid microenvironment that reduces the diffusion layer thickness, accelerates CO2 mass transport, and increases CO2 local concentration for the electrolysis.

/pdf/enhancing-carbon-dioxide-gas-diffusion-electrolysis-by-4xnwe4qe0c.pdf

Enhancing carbon dioxide gas-diffusion electrolysis by creating a hydrophobic catalyst microenvironment.

Ammonia, as a significant chemical for fertilizer production and also a promising energy carrier, is mainly produced through the traditional energy-intensive Haber–Bosch process. Recently, the electrocatalytic N2 reduction reaction (NRR) for ammonia synthesis has received tremendous attention with the merits of energy saving and environmental friendliness. To date, the development of the NRR process is primarily hindered by the competing hydrogen evolution reaction (HER), whereas the corresponding strategies for inhibiting this undesired side reaction to achieve high NRR selectivity are still quite limited. Furthermore, for such a complex reaction involving three gas–liquid–solid phases and proton/electron transfer, it is also rather meaningful to decouple and summarize the current strategies for suppressing H2 evolution in terms of NRR mechanisms, kinetics, thermodynamics, and electrocatalyst design in detail. Herein, on the basis of the NRR mechanisms, we systematically summarize the recent strategies to inhibit the HER for a highly selective electrocatalytic NRR, focusing on limiting the proton- and electron-transfer kinetics, shifting the chemical equilibrium, and designing the electrocatalysts. Additionally, insights into boosting the NRR selectivity and efficiency for practical applications are also presented in detail with regard to the determination of ammonia, the activation of the N2 molecule, the regulation of the gas–liquid–solid three-phase interface, the coupled NRR with value-added oxidation reactions, and the development of flow cell reactors.

Strategies to suppress hydrogen evolution for highly selective electrocatalytic nitrogen reduction: challenges and perspectives

The electrochemical reduction of CO2 stores intermittent renewable energy in valuable raw materials, such as chemicals and transportation fuels, while minimizing carbon emissions and promoting carbon-neutral cycles. Recent technoeconomic reports suggested economically feasible target products of CO2 electroreduction and the relative influence of key performance parameters such as faradaic efficiency (FE), current density, and overpotential in the practical industrial-scale applications. Furthermore, fundamental factors, such as available reaction pathways, shared intermediates, competing hydrogen evolution reaction, scaling relations of the intermediate binding energies, and CO2 mass transport limitations, should be considered in relation to the electrochemical CO2 reduction performance. Intensive research efforts have been devoted to designing and developing advanced electrocatalysts and improving mechanistic understanding. More recently, the research focus was extended to the catalyst environment, because the interfacial region can delicately modulate the catalytic activity and provide effective solutions to challenges that were not fully addressed in the material development studies. Herein, we discuss the importance of catalyst-electrolyte interfaces in improving key operational parameters based on kinetic equations. Furthermore, we extensively review previous studies on controlling organic modulators, electrolyte ions, electrode structures, as well as the three-phase boundary at the catalyst-electrolyte interface. The interfacial region modulates the electrocatalytic properties via electronic modification, intermediate stabilization, proton delivery regulation, catalyst structure modification, reactant concentration control, and mass transport regulation. We discuss the current understanding of the catalyst-electrolyte interface and its effect on the CO2 electroreduction activity.

Catalyst–electrolyte interface chemistry for electrochemical CO2 reduction

Over the past decade, electrochemical carbon dioxide reduction has become a thriving area of research with the aim of converting electricity to renewable chemicals and fuels. Recent advances through catalyst development have significantly improved selectivity and activity. However, drawing potential dependent structure-activity relationships has been complicated, not only due to the ill-defined and intricate morphological and mesoscopic structure of electrocatalysts, but also by immense concentration gradients existing between the electrode surface and bulk solution. In this work, by using in situ surface enhanced infrared absorption spectroscopy (SEIRAS) and computational modeling, we explicitly show that commonly used strong phosphate buffers cannot sustain the interfacial pH during CO2 electroreduction on copper electrodes at relatively low current densities, <10 mA/cm2. The pH near the electrode surface was observed to be as much as 5 pH units higher compared to bulk solution in 0.2 M phosphate buffer at potentials relevant to the formation of hydrocarbons (-1 V vs RHE), even on smooth polycrystalline copper electrodes. Drastically increasing the buffer capacity did not stand out as a viable solution for the problem as the concurrent production of hydrogen increased dramatically, which resulted in a breakdown of the buffer in a narrow potential range. These unforeseen results imply that most of the studies, if not all, on electrochemical CO2 reduction to hydrocarbons in CO2 saturated aqueous solutions were evaluated under mass transport limitations on copper electrodes. We underscore that the large concentration gradients on electrodes with high local current density (e.g., nanostructured) have important implications on the selectivity, activity, and kinetic analysis, and any attempt to draw structure-activity relationships must rule out mass transport effects.

In Situ Infrared Spectroscopy Reveals Persistent Alkalinity near Electrode Surfaces during CO2 Electroreduction.

The deployment of gas diffusion electrodes (GDEs) for the electrochemical CO2 reduction reaction (CO2RR) has enabled current densities an order of magnitude greater than those of aqueous H cells. The gains in production, however, have come with stability challenges due to rapid flooding of GDEs, which frustrate both laboratory experiments and scale-up prospects. Here, we investigate the role of carbon gas diffusion layers (GDLs) in the advent of flooding during CO2RR, finding that applied potential plays a central role in the observed instabilities. Electrochemical characterization of carbon GDLs with and without catalysts suggests that the high overpotential required during electrochemical CO2RR initiates hydrogen evolution on the carbon GDL support. These potentials impact the wetting characteristics of the hydrophobic GDL, resulting in flooding that is independent of CO2RR. Findings from this work can be extended to any electrochemical reduction reaction using carbon-based GDEs (CORR or N2RR) with cathodic overpotentials of less than -0.65 V versus a reversible hydrogen electrode.

/pdf/role-of-the-carbon-based-gas-diffusion-layer-on-flooding-in-2s3cr5b9lv.pdf

Role of the Carbon-Based Gas Diffusion Layer on Flooding in a Gas Diffusion Electrode Cell for Electrochemical CO 2 Reduction

Electrochemical CO2 reduction has received an increased amount of interest in the last decade as a promising avenue for storing renewable electricity in chemical bonds. Despite considerable progress on catalyst performance using nanostructured electrodes, the sensitivity of the reaction to process conditions has led to debate on the origin of the activity and high selectivity. Additionally, this raises questions on the transferability of the performance and knowledge to other electrochemical systems. At its core, the discrepancy is primarily a result of the highly porous nature of nanostructured electrodes, which are vulnerable to both mass transport effects and structural changes during the electrolysis. Both effects are not straightforward to identify and difficult to decouple. Despite the susceptibility of nanostructured electrodes to mass transfer limitations, we highlight that nanostructured silver electrodes exhibit considerably higher activity when normalized to the electrochemically active surface in contrast to gold and copper electrodes. Alongside, we provide a discussion on how active surface area and thickness of the catalytic layer itself can influence the onset potential, selectivity, stability, activity and mass transfer inside and outside of the three dimensional catalyst layer. Key parameters and potential solutions are highlighted to decouple mass transfer effects from the measured activity in electrochemical cells utilizing CO2 saturated aqueous solutions.

/pdf/electrochemical-co2-reduction-on-nanostructured-metal-jw2z21zaaz.pdf

Electrochemical CO2 reduction on nanostructured metal electrodes: fact or defect?

Cation-Driven Increases of CO2 Utilization in a Bipolar Membrane Electrode Assembly for CO2 Electrolysis

Results of a 2-D transport model for a gas diffusion electrode performing CO₂ reduction to CO with a flowing catholyte are presented, including the concentration gradients along the flow cell, spatial distribution of the current density and local pH in the catalyst layer. The model predicts that both the concentration of CO₂ and the buffer electrolyte gradually diminish along the channels for a parallel flow of gas and electrolyte as a result of electrochemical conversion and nonelectrochemical consumption. At high single-pass conversions, significant concentration gradients exist along the flow channels leading to large local variations in the current density (>150 mA/cm²), which becomes prominent when compared to ohmic losses. In addition, concentration overpotentials change dramatically with CO₂ flow rate, which results in significant differences in outlet concentrations at high conversions. The outlet concentration of CO attains a maximum of 80% along with 5% CO₂ and 15% H₂, although the maximum single-pass conversion is limited to below 60% due to homogeneous consumption by the electrolyte. Fundamental and practical implications of our findings on electrochemical CO₂ reduction are discussed with a focus on the trade-off between high current density operation and high single-pass conversion efficiency.

Kailun Yang

Papers

In Situ Infrared Spectroscopy Reveals Persistent Alkalinity near Electrode Surfaces during CO2 Electroreduction.

Role of the Carbon-Based Gas Diffusion Layer on Flooding in a Gas Diffusion Electrode Cell for Electrochemical CO 2 Reduction

Electrochemical CO2 reduction on nanostructured metal electrodes: fact or defect?

Cation-Driven Increases of CO2 Utilization in a Bipolar Membrane Electrode Assembly for CO2 Electrolysis

Along the Channel Gradients Impact on the Spatioactivity of Gas Diffusion Electrodes at High Conversions during CO2 Electroreduction