Accelerated discovery of CO2 electrocatalysts using active machine learning.

The rapid increase in global energy demand and the need to replace carbon dioxide (CO2)-emitting fossil fuels with renewable sources have driven interest in chemical storage of intermittent solar and wind energy1,2. Particularly attractive is the electrochemical reduction of CO2 to chemical feedstocks, which uses both CO2 and renewable energy3-8. Copper has been the predominant electrocatalyst for this reaction when aiming for more valuable multi-carbon products9-16, and process improvements have been particularly notable when targeting ethylene. However, the energy efficiency and productivity (current density) achieved so far still fall below the values required to produce ethylene at cost-competitive prices. Here we describe Cu-Al electrocatalysts, identified using density functional theory calculations in combination with active machine learning, that efficiently reduce CO2 to ethylene with the highest Faradaic efficiency reported so far. This Faradaic efficiency of over 80 per cent (compared to about 66 per cent for pure Cu) is achieved at a current density of 400 milliamperes per square centimetre (at 1.5 volts versus a reversible hydrogen electrode) and a cathodic-side (half-cell) ethylene power conversion efficiency of 55 ± 2 per cent at 150 milliamperes per square centimetre. We perform computational studies that suggest that the Cu-Al alloys provide multiple sites and surface orientations with near-optimal CO binding for both efficient and selective CO2 reduction17. Furthermore, in situ X-ray absorption measurements reveal that Cu and Al enable a favourable Cu coordination environment that enhances C-C dimerization. These findings illustrate the value of computation and machine learning in guiding the experimental exploration of multi-metallic systems that go beyond the limitations of conventional single-metal electrocatalysts.

Molecular tuning of CO2-to-ethylene conversion.

The electrocatalytic reduction of carbon dioxide, powered by renewable electricity, to produce valuable fuels and feedstocks provides a sustainable and carbon-neutral approach to the storage of energy produced by intermittent renewable sources1. However, the highly selective generation of economically desirable products such as ethylene from the carbon dioxide reduction reaction (CO2RR) remains a challenge2. Tuning the stabilities of intermediates to favour a desired reaction pathway can improve selectivity3-5, and this has recently been explored for the reaction on copper by controlling morphology6, grain boundaries7, facets8, oxidation state9 and dopants10. Unfortunately, the Faradaic efficiency for ethylene is still low in neutral media (60 per cent at a partial current density of 7 milliamperes per square centimetre in the best catalyst reported so far9), resulting in a low energy efficiency. Here we present a molecular tuning strategy-the functionalization of the surface of electrocatalysts with organic molecules-that stabilizes intermediates for more selective CO2RR to ethylene. Using electrochemical, operando/in situ spectroscopic and computational studies, we investigate the influence of a library of molecules, derived by electro-dimerization of arylpyridiniums11, adsorbed on copper. We find that the adhered molecules improve the stabilization of an 'atop-bound' CO intermediate (that is, an intermediate bound to a single copper atom), thereby favouring further reduction to ethylene. As a result of this strategy, we report the CO2RR to ethylene with a Faradaic efficiency of 72 per cent at a partial current density of 230 milliamperes per square centimetre in a liquid-electrolyte flow cell in a neutral medium. We report stable ethylene electrosynthesis for 190 hours in a system based on a membrane-electrode assembly that provides a full-cell energy efficiency of 20 per cent. We anticipate that this may be generalized to enable molecular strategies to complement heterogeneous catalysts by stabilizing intermediates through local molecular tuning.

Catalyst Regeneration via Chemical Oxidation Enables Long-Term Electrochemical Carbon Dioxide Reduction.

Electrochemical CO2 reduction (ECR) with industrially relevant current densities, high product selectivity, and long-term stability has been a long-sought goal. Unfortunately, copper (Cu) catalysts for producing valuable multicarbon (C2+) products undergo structural and morphological changes under ECR conditions, especially at high current densities, resulting in a rapid decrease in product selectivity. Herein, we report a catalyst regeneration strategy, one that employs an electrolysis method comprising alternating "on" and "off" operating regimes, to increase the operating stability of a Cu catalyst. We find that it increases operating lifetime many times, maintaining ethylene selectivity ≥40% for at least 200 h of electrolysis in neutral pH media at a current density of 150 mA cm-2 using a flow cell. We also demonstrate ECR to ethylene at a current density of 1 A cm-2 with ethylene selectivity ≥40% using a three-dimensional Cu gas diffusion electrode, finding that this system under these conditions is rendered stable for greater than 36 h. This work illustrates that Cu-based catalysts, once they have entered into the state conventionally considered to possess degraded catalytic activity, may be recovered to deliver high C2+ selectivity. We present evidence that the combination of short periods of electrolysis, which minimizes the morphological changes during "on" segments, with the progressive chemical oxidation of Cu atoms on the catalyst surface during "off" segments, united with the added effects of washing the accumulated salt and decreasing the catholyte temperature prolong together the catalyst's operating lifetime.

N-Heterocyclic Carbene-Stabilized Hydrido Au24 Nanoclusters: Synthesis, Structure, and Electrocatalytic Reduction of CO2.

Atomically precise hydrido gold nanoclusters are extremely rare but interesting due to their potential applications in catalysis. By optimization of molecular precursors, we have prepared an unprecedented N-heterocyclic carbene-stabilized hydrido gold nanocluster, [Au24(NHC)14Cl2H3]3+. This cluster comprises a dimer of two Au12 kernels, each adopting an icosahedral shape with one missing vertex. The two kernels are joined through triangular faces, which are capped with a total of three hydrides. The hydrides are detected by electrospray ionization mass spectrometry and nuclear magnetic resonance spectroscopy, with density functional theory calculations supporting their position bridging the six uncoordinated gold sites. The reactivity of this Au24H3 cluster in the electrocatalytic reduction of CO2 is demonstrated and benchmarked against related catalysts.

Enhanced electrocatalytic CO2 reduction via field-induced reagent concentration.

Electrochemical reduction of carbon dioxide (CO2) to carbon monoxide (CO) is the first step in the synthesis of more complex carbon-based fuels and feedstocks using renewable electricity. Unfortunately, the reaction suffers from slow kinetics owing to the low local concentration of CO2 surrounding typical CO2 reduction reaction catalysts. Alkali metal cations are known to overcome this limitation through non-covalent interactions with adsorbed reagent species, but the effect is restricted by the solubility of relevant salts. Large applied electrode potentials can also enhance CO2 adsorption, but this comes at the cost of increased hydrogen (H2) evolution. Here we report that nanostructured electrodes produce, at low applied overpotentials, local high electric fields that concentrate electrolyte cations, which in turn leads to a high local concentration of CO2 close to the active CO2 reduction reaction surface. Simulations reveal tenfold higher electric fields associated with metallic nanometre-sized tips compared to quasi-planar electrode regions, and measurements using gold nanoneedles confirm a field-induced reagent concentration that enables the CO2 reduction reaction to proceed with a geometric current density for CO of 22 milliamperes per square centimetre at -0.35 volts (overpotential of 0.24 volts). This performance surpasses by an order of magnitude the performance of the best gold nanorods, nanoparticles and oxide-derived noble metal catalysts. Similarly designed palladium nanoneedle electrocatalysts produce formate with a Faradaic efficiency of more than 90 per cent and an unprecedented geometric current density for formate of 10 milliamperes per square centimetre at -0.2 volts, demonstrating the wider applicability of the field-induced reagent concentration concept.

Stabilizing Highly Active Ru Sites by Suppressing Lattice Oxygen Participation in Acidic Water Oxidation.

In hydrogen production, the anodic oxygen evolution reaction (OER) limits the energy conversion efficiency and also impacts stability in proton-exchange membrane water electrolyzers. Widely used Ir-based catalysts suffer from insufficient activity, while more active Ru-based catalysts tend to dissolve under OER conditions. This has been associated with the participation of lattice oxygen (lattice oxygen oxidation mechanism (LOM)), which may lead to the collapse of the crystal structure and accelerate the leaching of active Ru species, leading to low operating stability. Here we develop Sr-Ru-Ir ternary oxide electrocatalysts that achieve high OER activity and stability in acidic electrolyte. The catalysts achieve an overpotential of 190 mV at 10 mA cm-2 and the overpotential remains below 225 mV following 1,500 h of operation. X-ray absorption spectroscopy and 18O isotope-labeled online mass spectroscopy studies reveal that the participation of lattice oxygen during OER was suppressed by interactions in the Ru-O-Ir local structure, offering a picture of how stability was improved. The electronic structure of active Ru sites was modulated by Sr and Ir, optimizing the binding energetics of OER oxo-intermediates.

Gas diffusion electrode design for electrochemical carbon dioxide reduction.

Anthropogenic carbon dioxide (CO2) emissions contribute to the greenhouse effect and global warming, which can lead to undesirable climate change and extinction of species. Besides the ongoing efforts to develop environmentally benign sources of energy and to advance technologies for the capture and sequestration of CO2, the transformation of emitted CO2 into valuable products is a pragmatic solution to curb its accumulation in the atmosphere. In this regard, electrochemical CO2 reduction (ECR) powered by renewable electricity provides an attractive approach because it not only converts CO2 to valuable fuels and chemicals but also offers a solution for the long-term storage of intermittent renewable energies. In ECR, the gas diffusion electrode (GDE) is the most critical component and has been the subject of intensive research in the last few years. This tutorial review provides an insightful guide to developing GDEs with high activity, selectivity, and stability, the three important performance metrics in ECR. First, we introduce critical fundamentals of ECR, including the chemical and physical phenomena at the electrodes as well as the electrochemical cell configurations. Next, we discuss recent advances in GDE design, focusing on their structure-performance correlation and fabrication techniques for each component of GDEs. Finally, we discuss the remaining challenges and propose promising research directions for the design of efficient GDEs. This review aims at promoting the development of industrially relevant ECR systems to bring this technology to practical applications.

Enhanced Nitrate-to-Ammonia Activity on Copper-Nickel Alloys via Tuning of Intermediate Adsorption.

Electrochemical conversion of nitrate (NO3-) into ammonia (NH3) recycles nitrogen and offers a route to the production of NH3, which is more valuable than dinitrogen gas. However, today's development of NO3- electroreduction remains hindered by the lack of a mechanistic picture of how catalyst structure may be tuned to enhance catalytic activity. Here we demonstrate enhanced NO3- reduction reaction (NO3-RR) performance on Cu50Ni50 alloy catalysts, including a 0.12 V upshift in the half-wave potential and a 6-fold increase in activity compared to those obtained with pure Cu at 0 V vs reversible hydrogen electrode (RHE). Ni alloying enables tuning of the Cu d-band center and modulates the adsorption energies of intermediates such as *NO3-, *NO2, and *NH2. Using density functional theory calculations, we identify a NO3-RR-to-NH3 pathway and offer an adsorption energy-activity relationship for the CuNi alloy system. This correlation between catalyst electronic structure and NO3-RR activity offers a design platform for further development of NO3-RR catalysts.

Binding Site Diversity Promotes CO2 Electroreduction to Ethanol.

The electrochemical reduction of CO2 has seen many record-setting advances in C2 productivity in recent years. However, the selectivity for ethanol, a globally significant commodity chemical, is still low compared to the selectivity for products such as ethylene. Here we introduce diverse binding sites to a Cu catalyst, an approach that destabilizes the ethylene reaction intermediates and thereby promotes ethanol production. We develop a bimetallic Ag/Cu catalyst that implements the proposed design toward an improved ethanol catalyst. It achieves a record Faradaic efficiency of 41% toward ethanol at 250 mA/cm2 and -0.67 V vs RHE, leading to a cathodic-side (half-cell) energy efficiency of 24.7%. The new catalysts exhibit an in situ Raman spectrum, in the region associated with CO stretching, that is much broader than that of pure Cu controls, a finding we account for via the diversity of binding configurations. This physical picture, involving multisite binding, accounts for the enhanced ethanol production for bimetallic catalysts, and presents a framework to design multimetallic catalysts to control reaction paths in CO2 reductions toward desired products.

Hydronium-Induced Switching between CO2 Electroreduction Pathways.

Over a broad range of operating conditions, many CO2 electroreduction catalysts can maintain selectivity toward certain reduction products, leading to materials and surfaces being categorized according to their products; here we ask, is product selectivity truly a property of the catalyst? Silver is among the best electrocatalysts for CO in aqueous electrolytes, where it reaches near-unity selectivity. We consider the hydrogenations of the oxygen and carbon atoms via the two proton-coupled-electron-transfer processes as chief determinants of product selectivity; and find using density functional theory (DFT) that the hydronium (H3O+) intermediate plays a key role in the first oxygen hydrogenation step and lowers the activation energy barrier for CO formation. When this hydronium influence is removed, the activation energy barrier for oxygen hydrogenation increases significantly, and the barrier for carbon hydrogenation is reduced. These effects make the formate reaction pathway more favorable than CO. Experimentally, we then carry out CO2 reduction in highly concentrated potassium hydroxide (KOH), limiting the hydronium concentration in the aqueous electrolyte. The product selectivity of a silver catalyst switches from entirely CO under neutral conditions to over 50% formate in the alkaline environment. The simulated and experimentally observed selectivity shift provides new insights into the role of hydronium on CO2 electroreduction processes and the ability for electrolyte manipulation to directly influence transition state (TS) kinetics, altering favored CO2 reaction pathways. We argue that selectivity should be considered less of an intrinsic catalyst property, and rather a combined product of the catalyst and reaction environment.

Metal-Organic Frameworks Mediate Cu Coordination for Selective CO2 Electroreduction.

The electrochemical carbon dioxide reduction reaction (CO2RR) produces diverse chemical species. Cu clusters with a judiciously controlled surface coordination number (CN) provide active sites that simultaneously optimize selectivity, activity, and efficiency for CO2RR. Here we report a strategy involving metal-organic framework (MOF)-regulated Cu cluster formation that shifts CO2 electroreduction toward multiple-carbon product generation. Specifically, we promoted undercoordinated sites during the formation of Cu clusters by controlling the structure of the Cu dimer, the precursor for Cu clusters. We distorted the symmetric paddle-wheel Cu dimer secondary building block of HKUST-1 to an asymmetric motif by separating adjacent benzene tricarboxylate moieties using thermal treatment. By varying materials processing conditions, we modulated the asymmetric local atomic structure, oxidation state and bonding strain of Cu dimers. Using electron paramagnetic resonance (EPR) and in situ X-ray absorption spectroscopy (XAS) experiments, we observed the formation of Cu clusters with low CN from distorted Cu dimers in HKUST-1 during CO2 electroreduction. These exhibited 45% C2H4 faradaic efficiency (FE), a record for MOF-derived Cu cluster catalysts. A structure-activity relationship was established wherein the tuning of the Cu-Cu CN in Cu clusters determines the CO2RR selectivity.

Tunable Cu Enrichment Enables Designer Syngas Electrosynthesis from CO2.

Using renewable energy to recycle CO2 provides an opportunity to both reduce net CO2 emissions and synthesize fuels and chemical feedstocks. It is of central importance to design electrocatalysts that both are efficient and can access a tunable spectrum of products. Syngas, a mixture of carbon monoxide (CO) and hydrogen (H2), is an important chemical precursor that can be converted downstream into small molecules or larger hydrocarbons by fermentation or thermochemistry. Many processes that utilize syngas require different syngas compositions: we therefore pursued the rational design of a family of electrocatalysts that can be programmed to synthesize different designer syngas ratios. We utilize in situ surface-enhanced Raman spectroscopy and first-principles density functional theory calculations to develop a systematic picture of CO* binding on Cu-enriched Au surface model systems. Insights from these model systems are then translated to nanostructured electrocatalysts, whereby controlled Cu enrichment enables tunable syngas production while maintaining current densities greater than 20 mA/cm2.

High-Density Nanosharp Microstructures Enable Efficient CO2 Electroreduction.

Conversion of CO2 to CO powered by renewable electricity not only reduces CO2 pollution but also is a means to store renewable energy via chemical production of fuels from CO. However, the kinetics of this reaction are slow due its large energetic barrier. We have recently reported CO2 reduction that is considerably enhanced via local electric field concentration at the tips of sharp gold nanostructures. The high local electric field enhances CO2 concentration at the catalytic active sites, lowering the activation barrier. Here we engineer the nucleation and growth of next-generation Au nanostructures. The electroplating overpotential was manipulated to generate an appreciably increased density of honed nanoneedles. Using this approach, we report the first application of sequential electrodeposition to increase the density of sharp tips in CO2 electroreduction. Selective regions of the primary nanoneedles are passivated using a thiol SAM (self-assembled monolayer), and then growth is concentrated atop the uncovered high-energy planes, providing new nucleation sites that ultimately lead to an increase in the density of the nanosharp structures. The two-step process leads to a new record in CO2 to CO reduction, with a geometric current density of 38 mA/cm2 at -0.4 V (vs reversible hydrogen electrode), and a 15-fold improvement over the best prior reports of electrochemical surface area (ECSA) normalized current density.

ZnFe2 O4 Leaves Grown on TiO2 Trees Enhance Photoelectrochemical Water Splitting.

TiO2 has excellent electrochemical properties but limited solar photocatalytic performance in light of its large bandgap. One important class of visible-wavelength sensitizers of TiO2 is based on ZnFe2 O4 , which has shown fully a doubling in performance relative to pure TiO2 . Prior efforts on this important front have relied on presynthesized nanoparticles of ZnFe2 O4 adsorbed on a TiO2 support; however, these have not yet achieved the full potential of this system since they do not provide a consistently maximized area of the charge-separating heterointerface per volume of sensitizing absorber. A novel atomic layer deposition (ALD)-enhanced synthesis of sensitizing ZnFe2 O4 leaves grown on the trunks of TiO2 trees is reported. These new materials exhibit fully a threefold enhancement in photoelectrochemical performance in water splitting compared to pristine TiO2 under visible illumination. The new materials synthesis strategy relies first on the selective growth of FeOOH nanosheets, 2D structures that shoot off from the sides of the TiO2 trees; these templates are then converted to ZnFe2 O4 with the aid of a novel ALD step, a strategy that preserves morphology while adding the Zn cation to achieve enhanced optical absorption and optimize the heterointerface band alignment.

Visible light induced hydrogen generation using a hollow photocatalyst with two cocatalysts separated on two surface sides.

A hollow Fe2O3-TiO2-PtOx photocatalyst for visible light H2 generation was prepared from nanosized MIL-88B consisting of coordinatively unsaturated metal centers as a hard template. This photocatalyst is composed of hybrid metal oxide-TiO2 with controllable wall thickness and two different cocatalysts that are separately located on two surface sides.

Three-dimensional ordered assembly of thin-shell Au/TiO2 hollow nanospheres for enhanced visible-light-driven photocatalysis.

An Au/TiO(2) nanostructure was constructed to obtain a highly efficient visible-light-driven photocatalyst. The design was based on a three-dimensional ordered assembly of thin-shell Au/TiO(2) hollow nanospheres (Au/TiO(2)-3 DHNSs). The designed photocatalysts exhibit not only a very high surface area but also photonic behavior and multiple light scattering, which significantly enhances visible-light absorption. Thus Au/TiO(2)-3 DHNSs exhibit a visible-light-driven photocatalytic activity that is several times higher than conventional Au/TiO(2) nanopowders.

Multi-metallic Layered Catalysts for Stable Electrochemical CO2 Reduction to Formate and Formic Acid.

Electrochemical CO2 reduction (ECR) to value-added products such as formate/formic acid is a promising approach for CO2 mitigation. Practical ECR requires long-term stability at industrially relevant reduction rates, which is challenging due to the rapid degradation of most catalysts at high current densities. Herein, we report the development of a bismuth (Bi) gas diffusion electrode on a polytetrafluoroethylene-based electrically conductive silver (Ag) substrate (Ag@Bi), which exhibits high Faradaic efficiency (FE) for formate of over 90 % in 1 M KOH and 1 M KHCO3 electrolytes. The catalyst also shows high selectivity of formic acid above 85 % in 1 M NaCl catholyte, which has a bulk pH of 2-3 during ECR, at current densities up to 300 mA cm-2. In 1 M KHCO3 condition, Ag@Bi maintains formate FE above 90 % for at least 500 hours at the current density of 100 mA cm-2. We found that the Ag@Bi catalyst degrades over time due to the leaching of Bi in the NaCl catholyte. To overcome this challenge, we deposited a layer of Ag nanoparticles on the surface of Ag@Bi to form a multi-layer Ag@Bi/Ag catalyst. This designed catalyst exhibits 300 hours of stability with FE for formic acid ≥70 % at 100 mA cm-2. Our work establishes a new strategy for achieving the operational longevity of ECR under wide pH conditions, which is critical for practical applications.

High-rate and selective conversion of CO2 from aqueous solutions to hydrocarbons.

Electrochemical carbon dioxide (CO2) conversion to hydrocarbon fuels, such as methane (CH4), offers a promising solution for the long-term and large-scale storage of renewable electricity. To enable this technology, CO2-to-CH4 conversion must achieve high selectivity and energy efficiency at high currents. Here, we report an electrochemical conversion system that features proton-bicarbonate-CO2 mass transport management coupled with an in-situ copper (Cu) activation strategy to achieve high CH4 selectivity at high currents. We find that open matrix Cu electrodes sustain sufficient local CO2 concentration by combining both dissolved CO2 and in-situ generated CO2 from the bicarbonate. In-situ Cu activation through alternating current operation renders and maintains the catalyst highly selective towards CH4. The combination of these strategies leads to CH4 Faradaic efficiencies of over 70% in a wide current density range (100 - 750 mA cm-2) that is stable for at least 12 h at a current density of 500 mA cm-2. The system also delivers a CH4 concentration of 23.5% in the gas product stream.

Coordination Polymer Electrocatalysts Enable Efficient CO-to-Acetate Conversion.

Upgrading carbon dioxide/monoxide to multi-carbon C2+ products using renewable electricity offers one route to more sustainable fuel and chemical production. One of the most appealing products is acetate, the profitable electrosynthesis of which demands a catalyst with higher efficiency. Here, a coordination polymer (CP) catalyst is reported that consists of Cu(I) and benzimidazole units linked via Cu(I)-imidazole coordination bonds, which enables selective reduction of CO to acetate with a 61% Faradaic efficiency at -0.59 volts versus the reversible hydrogen electrode at a current density of 400 mA cm-2 in flow cells. The catalyst is integrated in a cation exchange membrane-based membrane electrode assembly that enables stable acetate electrosynthesis for 190 h, while achieving direct collection of concentrated acetate (3.3 molar) from the cathodic liquid stream, an average single-pass utilization of 50% toward CO-to-acetate conversion, and an average acetate full-cell energy efficiency of 15% at a current density of 250 mA cm-2 .