Selective Electrified Propylene-to-Propylene Glycol Oxidation on Activated Rh-Doped Pd.

Renewable-energy-powered electrosynthesis has the potential to contribute to decarbonizing the production of propylene glycol, a chemical that is used currently in the manufacture of polyesters and antifreeze and has a high carbon intensity. Unfortunately, to date, the electrooxidation of propylene under ambient conditions has suffered from a wide product distribution, leading to a low faradic efficiency toward the desired propylene glycol. We undertook mechanistic investigations and found that the reconstruction of Pd to PdO occurs, followed by hydroxide formation under anodic bias. The formation of this metastable hydroxide layer arrests the progressive dissolution of Pd in a locally acidic environment, increases the activity, and steers the reaction pathway toward propylene glycol. Rh-doped Pd further improves propylene glycol selectivity. Density functional theory (DFT) suggests that the Rh dopant lowers the energy associated with the production of the final intermediate in propylene glycol formation and renders the desorption step spontaneous, a concept consistent with experimental studies. We report a 75% faradic efficiency toward propylene glycol maintained over 100 h of operation.

Acidic media enables oxygen-tolerant electrosynthesis of multicarbon products from simulated flue gas.

Renewable electricity powered electrochemical CO2 reduction (CO2R) offers a valuable method to close the carbon cycle and reduce our overreliance on fossil fuels. However, high purity CO2 is usually required as feedstock, which potentially decreases the feasibility and economic viability of the process. Direct conversion of flue gas is an attractive option but is challenging due to the low CO2 concentration and the presence of O2 impurities. As a result, up to 99% of the applied current can be lost towards the undesired oxygen reduction reaction (ORR). Here, we show that acidic electrolyte can significantly suppress ORR on Cu, enabling generation of multicarbon products from simulated flue gas. Using a composite Cu and carbon supported single-atom Ni tandem electrocatalyst, we achieved a multicarbon Faradaic efficiency of 46.5% at 200 mA cm-2, which is ~20 times higher than bare Cu under alkaline conditions. We also demonstrate stable performance for 24 h with a multicarbon product full-cell energy efficiency of 14.6%. Strikingly, this result is comparable to previously reported acidic CO2R systems using pure CO2. Our findings demonstrate a potential pathway towards designing efficient electrolyzers for direct conversion of flue gas to value-added chemicals and fuels.

Nanocurvature-induced field effects enable control over the activity of single-atom electrocatalysts.

Tuning interfacial electric fields provides a powerful means to control electrocatalyst activity. Importantly, electric fields can modify adsorbate binding energies based on their polarizability and dipole moment, and hence operate independently of scaling relations that fundamentally limit performance. However, implementation of such a strategy remains challenging because typical methods modify the electric field non-uniformly and affects only a minority of active sites. Here we discover that uniformly tunable electric field modulation can be achieved using a model system of single-atom catalysts (SACs). These consist of M-N4 active sites hosted on a series of spherical carbon supports with varying degrees of nanocurvature. Using in-situ Raman spectroscopy with a Stark shift reporter, we demonstrate that a larger nanocurvature induces a stronger electric field. We show that this strategy is effective over a broad range of SAC systems and electrocatalytic reactions. For instance, Ni SACs with optimized nanocurvature achieved a high CO partial current density of ~400 mA cm-2 at >99% Faradaic efficiency for CO2 reduction in acidic media.

Electrophotochemical Synthesis Facilitated Trifluoromethylation of Arenes Using Trifluoroacetic Acid.

The trifluoromethyl (CF3) group is an essential moiety in medicinal chemistry due to its unique physicochemical properties. While trifluoroacetic acid (TFA) is an inexpensive and easily accessible reagent, its use as a source of CF3 is highly challenging due to its high oxidation potential. In this study, we present a novel electrophotochemical approach that enables the use of TFA as the CF3 source for the selective, catalyst- and oxidant-free trifluoromethylation of (hetero)arenes. Key to our approach is the selective oxidation of TFA over arenes, generating CF3 radicals through oxidative decarboxylation. This strategy enables the sustainable and environmentally-friendly synthesis of CF3-, CF2H- and perfluoroalkyl-containing (hetero)arenes with a broad range of substrates. Importantly, our results demonstrate significantly improved chemoselectivity by light irradiation, opening up new possibilities for the synthetic and medicinal applications of TFA as an ideal yet underutilized CF3 source.

Acidic Media Impedes Tandem Catalysis Reaction Pathways in Electrochemical CO2 Reduction.

Electrochemical CO2 reduction (CO2 R) in acidic media with Cu-based catalysts tends to suffer from lowered selectivity towards multicarbon products. This could in principle be mitigated using tandem catalysis, whereby the *CO coverage on Cu is increased by introducing a CO generating catalyst (e.g. Ag) in close proximity. Although this has seen significant success in neutral/alkaline media, here we report that such a strategy becomes impeded in acidic electrolyte. This was investigated through the co-reduction of 13 CO2 /12 CO mixtures using a series of Cu and CuAg catalysts. These experiments provide strong evidence for the occurrence of tandem catalysis in neutral media and its curtailment under acidic conditions. Density functional theory simulations suggest that the presence of H3 O+ weakens the *CO binding energy of Cu, preventing effective utilization of tandem-supplied CO. Our findings also provide other unanticipated insights into the tandem catalysis reaction pathway and important design considerations for effective CO2 R in acidic media.

Stepwise on-demand functionalization of multihydrosilanes enabled by a hydrogen-atom-transfer photocatalyst based on eosin Y.

Organosilanes are of vital importance for modern human society, having found widespread applications in functional materials, organic synthesis, drug discovery and life sciences. However, their preparation remains far from trivial, and on-demand synthesis of heteroleptic substituted silicon reagents is a formidable challenge. The generation of silyl radicals from hydrosilanes via direct hydrogen-atom-transfer (HAT) photocatalysis represents the most atom-, step-, redox- and catalyst-economic pathway for the activation of hydrosilanes. Here, in view of the green characteristics of neutral eosin Y (such as its abundance, low cost, metal-free nature, absorption of visible light and excellent selectivity), we show that using it as a direct HAT photocatalyst enables the stepwise custom functionalization of multihydrosilanes, giving access to fully substituted silicon compounds. By exploiting this strategy, we realize preferable hydrogen abstraction of Si-H bonds in the presence of active C-H bonds, diverse functionalization of hydrosilanes (for example, alkylation, vinylation, allylation, arylation, deuteration, oxidation and halogenation), and remarkably selective monofunctionalization of di- and trihydrosilanes.

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 .

Machine learning for a sustainable energy future.

Transitioning from fossil fuels to renewable energy sources is a critical global challenge; it demands advances - at the materials, devices and systems levels - for the efficient harvesting, storage, conversion and management of renewable energy. Energy researchers have begun to incorporate machine learning (ML) techniques to accelerate these advances. In this Perspective, we highlight recent advances in ML-driven energy research, outline current and future challenges, and describe what is required to make the best use of ML techniques. We introduce a set of key performance indicators with which to compare the benefits of different ML-accelerated workflows for energy research. We discuss and evaluate the latest advances in applying ML to the development of energy harvesting (photovoltaics), storage (batteries), conversion (electrocatalysis) and management (smart grids). Finally, we offer an overview of potential research areas in the energy field that stand to benefit further from the application of ML.

High-Rate and Selective CO2 Electrolysis to Ethylene via Metal-Organic-Framework-Augmented CO2 Availability.

High-rate conversion of carbon dioxide (CO2 ) to ethylene (C2 H4 ) in the CO2 reduction reaction (CO2 RR) requires fine control over the phase boundary of the gas diffusion electrode (GDE) to overcome the limit of CO2 solubility in aqueous electrolytes. Here, a metal-organic framework (MOF)-functionalized GDE design is presented, based on a catalysts:MOFs:hydrophobic substrate materials layered architecture, that leads to high-rate and selective C2 H4 production in flow cells and membrane electrode assembly (MEA) electrolyzers. It is found that using electroanalysis and operando X-ray absorption spectroscopy (XAS), MOF-induced organic layers in GDEs augment the local CO2 concentration near the active sites of the Cu catalysts. MOFs with different CO2 adsorption abilities are used, and the stacking ordering of MOFs in the GDE is varied. While sputtering Cu on poly(tetrafluoroethylene) (PTFE) (Cu/PTFE) exhibits 43% C2 H4 Faradaic efficiency (FE) at a current density of 200 mA cm- 2 in a flow cell, 49% C2 H4 FE at 1 A cm- 2 is achieved on MOF-augmented GDEs in CO2 RR. MOF-augmented GDEs are further evaluated in an MEA electrolyzer, achieving a C2 H4 partial current density of 220 mA cm-2 for CO2 RR and 121 mA cm-2 for the carbon monoxide reduction reaction (CORR), representing 2.7-fold and 15-fold improvement in C2 H4 production rate, compared to those obtained on bare Cu/PTFE.

Theory-guided experimental design in battery materials research.

A reliable energy storage ecosystem is imperative for a renewable energy future, and continued research is needed to develop promising rechargeable battery chemistries. To this end, better theoretical and experimental understanding of electrochemical mechanisms and structure-property relationships will allow us to accelerate the development of safer batteries with higher energy densities and longer lifetimes. This Review discusses the interplay between theory and experiment in battery materials research, enabling us to not only uncover hitherto unknown mechanisms but also rationally design more promising electrode and electrolyte materials. We examine specific case studies of theory-guided experimental design in lithium-ion, lithium-metal, sodium-metal, and all-solid-state batteries. We also offer insights into how this framework can be extended to multivalent batteries. To close the loop, we outline recent efforts in coupling machine learning with high-throughput computations and experiments. Last, recommendations for effective collaboration between theorists and experimentalists are provided.

Effects of surface diffusion in electrocatalytic CO2 reduction on Cu revealed by kinetic Monte Carlo simulations.

Kinetic Monte Carlo (KMC) methods are frequently used for mechanistic studies of thermally driven heterogeneous catalysis systems but are underused for electrocatalysis. Here, we develop a lattice KMC approach for electrocatalytic CO2 reduction. The work is motivated by a prior experimental report that performed electroreduction of a mixed feed of 12CO2 and 13CO on Cu; differences in the 13C content of C2 products ethylene and ethanol (Δ13C) were interpreted as evidence of site selectivity. The lattice KMC model considers the effect of surface diffusion on this system. In the limit of infinitely fast diffusion (mean-field approximation), the key intermediates 12CO* and 13CO* would be well mixed on the surface and no evidence of site selectivity could have been observed. Using a simple two-site model and adapting a previously reported microkinetic model, we assess the effects of diffusion on the relative isotope fractions in the products using the estimated surface diffusion rate of CO* from literature reports. We find that the size of the active sites and the total surface adsorbate coverage can have a large influence on the values of Δ13C that can be observed. Δ13C is less sensitive to the CO* diffusion rate as long as it is within the estimated range. We further offer possible methods to estimate surface distribution of intermediates and to predict intrinsic selectivity of active sites based on experimental observations. This work illustrates the importance of considering surface diffusion in the study of electrochemical CO2 reduction to multi-carbon products. Our approach is entirely based on a freely available open-source code, so will be readily adaptable to other electrocatalytic systems.

Ternary Alloys Enable Efficient Production of Methoxylated Chemicals via Selective Electrocatalytic Hydrogenation of Lignin Monomers.

We explore the selective electrocatalytic hydrogenation of lignin monomers to methoxylated chemicals, of particular interest, when powered by renewable electricity. Prior studies, while advancing the field rapidly, have so far lacked the needed selectivity: when hydrogenating lignin-derived methoxylated monomers to methoxylated cyclohexanes, the desired methoxy group (-OCH3) has also been reduced. The ternary PtRhAu electrocatalysts developed herein selectively hydrogenate lignin monomers to methoxylated cyclohexanes-molecules with uses in pharmaceutics. Using X-ray absorption spectroscopy and in situ Raman spectroscopy, we find that Rh and Au modulate the electronic structure of Pt and that this modulating steers intermediate energetics on the electrocatalyst surface to facilitate the hydrogenation of lignin monomers and suppress C-OCH3 bond cleavage. As a result, PtRhAu electrocatalysts achieve a record 58% faradaic efficiency (FE) toward 2-methoxycyclohexanol from the lignin monomer guaiacol at 200 mA cm-2, representing a 1.9× advance in FE and a 4× increase in partial current density compared to the highest productivity prior reports. We demonstrate an integrated lignin biorefinery where wood-derived lignin monomers are selectively hydrogenated and funneled to methoxylated 2-methoxy-4-propylcyclohexanol using PtRhAu electrocatalysts. This work offers an opportunity for the sustainable electrocatalytic synthesis of methoxylated pharmaceuticals from renewable biomass.

CO2 electrolysis to multicarbon products in strong acid.

Carbon dioxide electroreduction (CO2R) is being actively studied as a promising route to convert carbon emissions to valuable chemicals and fuels. However, the fraction of input CO2 that is productively reduced has typically been very low, <2% for multicarbon products; the balance reacts with hydroxide to form carbonate in both alkaline and neutral reactors. Acidic electrolytes would overcome this limitation, but hydrogen evolution has hitherto dominated under those conditions. We report that concentrating potassium cations in the vicinity of electrochemically active sites accelerates CO2 activation to enable efficient CO2R in acid. We achieve CO2R on copper at pH <1 with a single-pass CO2 utilization of 77%, including a conversion efficiency of 50% toward multicarbon products (ethylene, ethanol, and 1-propanol) at a current density of 1.2 amperes per square centimeter and a full-cell voltage of 4.2 volts.

Promoting CO2 methanation via ligand-stabilized metal oxide clusters as hydrogen-donating motifs.

Electroreduction uses renewable energy to upgrade carbon dioxide to value-added chemicals and fuels. Renewable methane synthesized using such a route stands to be readily deployed using existing infrastructure for the distribution and utilization of natural gas. Here we design a suite of ligand-stabilized metal oxide clusters and find that these modulate carbon dioxide reduction pathways on a copper catalyst, enabling thereby a record activity for methane electroproduction. Density functional theory calculations show adsorbed hydrogen donation from clusters to copper active sites for the *CO hydrogenation pathway towards *CHO. We promote this effect via control over cluster size and composition and demonstrate the effect on metal oxides including cobalt(II), molybdenum(VI), tungsten(VI), nickel(II) and palladium(II) oxides. We report a carbon dioxide-to-methane faradaic efficiency of 60% at a partial current density to methane of 135 milliampere per square centimetre. We showcase operation over 18 h that retains a faradaic efficiency exceeding 55%.

Enhanced multi-carbon alcohol electroproduction from CO via modulated hydrogen adsorption.

Multi-carbon alcohols such as ethanol are valued as fuels in view of their high energy density and ready transport. Unfortunately, the selectivity toward alcohols in CO2/CO electroreduction is diminished by ethylene production, especially when operating at high current densities (>100 mA cm-2). Here we report a metal doping approach to tune the adsorption of hydrogen at the copper surface and thereby promote alcohol production. Using density functional theory calculations, we screen a suite of transition metal dopants and find that incorporating Pd in Cu moderates hydrogen adsorption and assists the hydrogenation of C2 intermediates, providing a means to favour alcohol production and suppress ethylene. We synthesize a Pd-doped Cu catalyst that achieves a Faradaic efficiency of 40% toward alcohols and a partial current density of 277 mA cm-2 from CO electroreduction. The activity exceeds that of prior reports by a factor of 2.

Chloride-mediated selective electrosynthesis of ethylene and propylene oxides at high current density.

Chemicals manufacturing consumes large amounts of energy and is responsible for a substantial portion of global carbon emissions. Electrochemical systems that produce the desired compounds by using renewable electricity offer a route to lower carbon emissions in the chemicals sector. Ethylene oxide is among the world's most abundantly produced commodity chemicals because of its importance in the plastics industry, notably for manufacturing polyesters and polyethylene terephthalates. We applied an extended heterogeneous:homogeneous interface, using chloride as a redox mediator at the anode, to facilitate the selective partial oxidation of ethylene to ethylene oxide. We achieved current densities of 1 ampere per square centimeter, Faradaic efficiencies of ~70%, and product specificities of ~97%. When run at 300 milliamperes per square centimeter for 100 hours, the system maintained a 71(±1)% Faradaic efficiency throughout.

Two-Dimensional Titanium and Molybdenum Carbide MXenes as Electrocatalysts for CO2 Reduction.

Electrocatalytic CO2 reduction reaction (CO2RR) is an attractive way to produce renewable fuel and chemical feedstock, especially when coupled with efficient CO2 capture and clean energy sources. On the fundamental side, research on improving CO2RR activity still revolves around late transition metal-based catalysts, which are limited by unfavorable scaling relations despite intense investigation. Here, we report a combined experimental and theoretical investigation into electrocatalytic CO2RR on Ti- and Mo-based MXene catalysts. Formic acid is found as the main product on Ti2CTx and Mo2CTx MXenes, with peak Faradaic efficiency of over 56% on Ti2CTx and partial current density of up to -2.5 mA cm-2 on Mo2CTx. Furthermore, simulations reveal the critical role of the Tx group: a smaller overpotential is found to occur at lower amounts of -F termination. This work represents an important step toward experimental demonstration of MXenes for more complex electrocatalytic reactions in the future.

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.

Hydroxide promotes carbon dioxide electroreduction to ethanol on copper via tuning of adsorbed hydrogen.

Producing liquid fuels such as ethanol from CO2, H2O, and renewable electricity offers a route to store sustainable energy. The search for efficient electrocatalysts for the CO2 reduction reaction relies on tuning the adsorption strength of carbonaceous intermediates. Here, we report a complementary approach in which we utilize hydroxide and oxide doping of a catalyst surface to tune the adsorbed hydrogen on Cu. Density functional theory studies indicate that this doping accelerates water dissociation and changes the hydrogen adsorption energy on Cu. We synthesize and investigate a suite of metal-hydroxide-interface-doped-Cu catalysts, and find that the most efficient, Ce(OH)x-doped-Cu, exhibits an ethanol Faradaic efficiency of 43% and a partial current density of 128 mA cm-2. Mechanistic studies, wherein we combine investigation of hydrogen evolution performance with the results of operando Raman spectroscopy, show that adsorbed hydrogen hydrogenates surface *HCCOH, a key intermediate whose fate determines branching to ethanol versus ethylene.

Dopant-tuned stabilization of intermediates promotes electrosynthesis of valuable C3 products.

The upgrading of CO2/CO feedstocks to higher-value chemicals via energy-efficient electrochemical processes enables carbon utilization and renewable energy storage. Substantial progress has been made to improve performance at the cathodic side; whereas less progress has been made on improving anodic electro-oxidation reactions to generate value. Here we report the efficient electroproduction of value-added multi-carbon dimethyl carbonate (DMC) from CO and methanol via oxidative carbonylation. We find that, compared to pure palladium controls, boron-doped palladium (Pd-B) tunes the binding strength of intermediates along this reaction pathway and favors DMC formation. We implement this doping strategy and report the selective electrosynthesis of DMC experimentally. We achieve a DMC Faradaic efficiency of 83 ± 5%, fully a 3x increase in performance compared to the corresponding pure Pd electrocatalyst.