Long-term continuous ammonia electrosynthesis.

Ammonia is crucial as a fertilizer and in the chemical industry and is considered to be a carbon-free fuel1. Ammonia electrosynthesis from nitrogen under ambient conditions offers an attractive alternative to the Haber-Bosch process2,3, and lithium-mediated nitrogen reduction represents a promising approach to continuous-flow ammonia electrosynthesis, coupling nitrogen reduction with hydrogen oxidation4. However, tetrahydrofuran, which is commonly used as a solvent, impedes long-term ammonia production owing to polymerization and volatility problems. Here we show that a chain-ether-based electrolyte enables long-term continuous ammonia synthesis. We find that a chain-ether-based solvent exhibits non-polymerization properties and a high boiling point (162 °C) and forms a compact solid-electrolyte interphase layer on the gas diffusion electrode, facilitating ammonia release in the gas phase and ensuring electrolyte stability. We demonstrate 300 h of continuous operation in a flow electrolyser with a 25 cm2 electrode at 1 bar pressure and room temperature, and achieve a current-to-ammonia efficiency of 64 ± 1% with a gas-phase ammonia content of approximately 98%. Our results highlight the crucial role of the solvent in long-term continuous ammonia synthesis.

Calcium-mediated nitrogen reduction for electrochemical ammonia synthesis.

Ammonia (NH3) is a key commodity chemical for the agricultural, textile and pharmaceutical industries, but its production via the Haber-Bosch process is carbon-intensive and centralized. Alternatively, an electrochemical method could enable decentralized, ambient NH3 production that can be paired with renewable energy. The first verified electrochemical method for NH3 synthesis was a process mediated by lithium (Li) in organic electrolytes. So far, however, elements other than Li remain unexplored in this process for potential benefits in efficiency, reaction rates, device design, abundance and stability. In our demonstration of a Li-free system, we found that calcium can mediate the reduction of nitrogen for NH3 synthesis. We verified the calcium-mediated process using a rigorous protocol and achieved an NH3 Faradaic efficiency of 40 ± 2% using calcium tetrakis(hexafluoroisopropyloxy)borate (Ca[B(hfip)4]2) as the electrolyte. Our results offer the possibility of using abundant materials for the electrochemical production of NH3, a critical chemical precursor and promising energy vector.

Phase Engineering of Intermetallic PtBi2 Nanoplates for Formic Acid Electrochemical Oxidation.

Phase engineering of Pt-based intermetallic catalysts has been demonstrated as a promising strategy to optimize catalytic properties for a direct formic acid fuel cell. Pt-Bi intermetallic catalysts are attracting increasing interest due to their high catalytic activity, especially for inhibiting CO poisoning. However, the phase transformation and synthesis of intermetallic compounds usually occurring at high temperatures leads to a lack of control of the size and composition. Here, we report the synthesis of intermetallic ß-PtBi2 and γ-PtBi2 two-dimensional nanoplates with controlled sizes and compositions under mild conditions. The different phases of intermetallic PtBi2 can significantly affect the catalytic performance of the formic acid oxidation reaction (FAOR). The obtained ß-PtBi2 nanoplates exhibit an excellent mass activity of 1.1 ± 0.01 A mgPt-1 for the FAOR, which is 30-fold higher than that of commercial Pt/C catalysts. Moreover, intermetallic PtBi2 demonstrates high tolerance to CO poisoning, as confirmed by in situ infrared absorption spectroscopy.

Theories for Electrolyte Effects in CO2 Electroreduction.

Electrochemical CO2 reduction (eCO2R) enables the conversion of waste CO2 to high-value fuels and commodity chemicals powered by renewable electricity, thereby offering a viable strategy for reaching the goal of net-zero carbon emissions. Research in the past few decades has focused both on the optimization of the catalyst (electrode) and the electrolyte environment. Surface-area normalized current densities show that the latter can affect the CO2 reduction activity by up to a few orders of magnitude.In this Account, we review theories of the mechanisms behind the effects of the electrolyte (cations, anions, and the electrolyte pH) on eCO2R. As summarized in the conspectus graphic, the electrolyte influences eCO2R activity via a field (ε) effect on dipolar (µ) reaction intermediates, changing the proton donor for the multi-step proton-electron transfer reaction, specifically adsorbed anions on the catalyst surface to block active sites, and tuning the local environment by homogeneous reactions. To be specific, alkali metal cations (M+) can stabilize reaction intermediates via electrostatic interactions with dipolar intermediates or buffer the interfacial pH via hydrolysis reactions, thereby promoting the eCO2R activity with the following trend in hydrated size (corresponding to the local field strength ε)/hydrolysis ability: Cs+ > K+ > Na+ > Li+. The effect of the electrolyte pH can give a change in eCO2R activity of up to several orders of magnitude, arising from linearly shifting the absolute interfacial field via the relationship USHE = URHE - (2.3kBT)pH, homogeneous reactions between OH- and desorbed intermediates, or changing the proton donor from hydronium to water along with increasing pH. Anions have been suggested to affect the eCO2R reaction process by solution-phase reactions (e.g., buffer reactions to tune local pH), acting as proton donors or as a surface poison.So far, the existing models of electrolyte effects have been used to rationalize various experimentally observed trends, having yet to demonstrate general predictive capabilities. The major challenges in our understanding of the electrolyte effect in eCO2R are (i) the long time scale associated with a dynamic ab initio picture of the catalyst|electrolyte interface and (ii) the overall activity determined by the length-scale interplay of intrinsic microkinetics, homogeneous reactions, and mass transport limitations. New developments in ab initio dynamic models and coupling the effects of mass transport can provide a more accurate view of the structure and intrinsic functions of the electrode-electrolyte interface and the corresponding reaction energetics toward comprehensive and predictive models for electrolyte design.

Enriching Surface-Accessible CO2 in the Zero-Gap Anion-Exchange-Membrane-Based CO2 Electrolyzer.

Zero-gap anion exchange membrane (AEM)-based CO2 electrolysis is a promising technology for CO production, however, their performance at elevated current densities still suffers from the low local CO2 concentration due to heavy CO2 neutralization. Herein, via modulating the CO2 feed mode and quantitative analyzing CO2 utilization with the aid of mass transport modeling, we develop a descriptor denoted as the surface-accessible CO2 concentration ([CO2 ]SA ), which enables us to indicate the transient state of the local [CO2 ]/[OH- ] ratio and helps define the limits of CO2 -to-CO conversion. To enrich the [CO2 ]SA , we developed three general strategies: (1) increasing catalyst layer thickness, (2) elevating CO2 pressure, and (3) applying a pulsed electrochemical (PE) method. Notably, an optimized PE method allows to keep the [CO2 ]SA at a high level by utilizing the dynamic balance period of CO2 neutralization. A maximum jCO of 368±28 mA cmgeo -2 was achieved using a commercial silver catalyst.

A Cation Concentration Gradient Approach to Tune the Selectivity and Activity of CO2 Electroreduction.

The linear scaling relationship of the binding energies of different intermediates limits the catalyst performance in CO2 electroreduction. Here we demonstrate a cation concentration gradient strategy to promote the activity and tune the selectivity of CO2 electroreduction, thereby breaking the scaling relationship. In optimal concentrations of the potassium acetate (KAc) electrolyte, Cu, Ag and In catalysts deliver current densities that are 7.1, 3.2, 2.7 times higher than those obtained in 0.5 M KAc for C2 H4 , CO, and formate production, respectively. Increasing the concentration of KAc also changes the selectivity from CO to formate on Ag, and from CO to C2 products on Cu. In situ surface-enhanced Raman spectroscopy and computational simulations reveal that the binding energies of intermediates are changed at different electrolyte concentrations, which is due to a local electrostatic interaction modulated by potassium cations at the electrode surface.

Efficient Methane Electrosynthesis Enabled by Tuning Local CO2 Availability.

The electroreduction of carbon dioxide (CO2RR) to valuable chemicals is a promising avenue for the storage of intermittent renewable electricity. Renewable methane, obtained via CO2RR using renewable electricity as energy input, has the potential to serve as a carbon-neutral fuel or chemical feedstock, and it is of particular interest in view of the well-established infrastructure for its storage, distribution, and utilization. However, CO2RR to methane still suffers from low selectivity at commercially relevant current densities (>100 mA cm-2). Density functional theory calculations herein reveal that lowering *CO2 coverage on the Cu surface decreases the coverage of the *CO intermediate, and then this favors the protonation of *CO to *CHO, a key intermediate for methane generation, compared to the competing step, C-C coupling. We therefore pursue an experimental strategy wherein we control local CO2 availability on a Cu catalyst by tuning the concentration of CO2 in the gas stream and regulate the reaction rate through the current density. We achieve as a result a methane Faradaic efficiency (FE) of (48 ± 2)% with a partial current density of (108 ± 5) mA cm-2 and a methane cathodic energy efficiency of 20% using a dilute CO2 gas stream. We report stable methane electrosynthesis for 22 h. These findings offer routes to produce methane with high FE and high conversion rate in CO2RR and also make direct use of dilute CO2 feedstocks.

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.

Oxygen-induced degradation of the electronic properties of thin-layer InSe.

Thin-layer indium selenide (InSe) compounds, as two-dimensional (2D) semiconductors, have been widely and intensively studied due to their high electron mobility and environmental stability. Here, we report a study demonstrating the oxygen-induced degradation of monolayer and bilayer InSe nanosheets using first-principles calculations and deformation potential theory. It is evident that O atoms prefer to substitute Se atoms instead of undergoing adsorption onto surfaces, while interstitial sites are the most stable adsorption sites of O atoms in the interior for both monolayer and bilayer InSe. Using the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional, we calculated band structures and carrier mobility. The band gaps of a monolayer or bilayer InSe nanosheet with O atoms remained unchanged and corresponded to the pristine structure except for a slight decrease in the substituted cases. Additionally, no impurity levels are observed, indicating that the addition of O atoms has little effect on carrier concentrations. With the calculated mobility of monolayer InSe with and without O atoms, we show that the degradation is governed by the interstitial impurity of O atoms, whose electron mobility can decrease by 3-4 orders of magnitude. As for bilayer InSe, there is a one order of magnitude decrease at most, which indicates a stronger resistance to oxidation than that of the monolayer structure. Our calculations provide a detailed understanding of the degradation induced by O atoms from the aspects of structures and electronic properties, which is a foundation for the application and modification of thin-layer InSe.

Unraveling the rate-determining step of C2+ products during electrochemical CO reduction.

The electrochemical reduction of CO has drawn a large amount of attention due to its potential to produce sustainable fuels and chemicals by using renewable energy. However, the reaction's mechanism is not yet well understood. A major debate is whether the rate-determining step for the generation of multi-carbon products is C-C coupling or CO hydrogenation. This paper conducts an experimental analysis of the rate-determining step, exploring pH dependency, kinetic isotope effects, and the impact of CO partial pressure on multi-carbon product activity. Results reveal constant multi-carbon product activity with pH or electrolyte deuteration changes, and CO partial pressure data aligns with the theoretical formula derived from *CO-*CO coupling as the rate-determining step. These findings establish the dimerization of two *CO as the rate-determining step for multi-carbon product formation. Extending the study to commercial copper nanoparticles and oxide-derived copper catalysts shows their rate-determining step also involves *CO-*CO coupling. This investigation provides vital kinetic data and a theoretical foundation for enhancing multi-carbon product production.

Phenol as proton shuttle and buffer for lithium-mediated ammonia electrosynthesis.

Ammonia is a crucial component in the production of fertilizers and various nitrogen-based compounds. Now, the lithium-mediated nitrogen reduction reaction (Li-NRR) has emerged as a promising approach for ammonia synthesis at ambient conditions. The proton shuttle plays a critical role in the proton transfer process during Li-NRR. However, the structure-activity relationship and design principles for effective proton shuttles have not yet been established in practical Li-NRR systems. Here, we propose a general procedure for verifying a true proton shuttle and established design principles for effective proton shuttles. We systematically evaluate several classes of proton shuttles in a continuous-flow reactor with hydrogen oxidation at the anode. Among the tested proton shuttles, phenol exhibits the highest Faradaic efficiency of 72 ± 3% towards ammonia, surpassing that of ethanol, which has been commonly used so far. Experimental investigations including operando isotope-labelled mass spectrometry proved the proton-shuttling capability of phenol. Further mass transport modeling sheds light on the mechanism.

Continuous-flow electrosynthesis of ammonia by nitrogen reduction and hydrogen oxidation.

Ammonia is a critical component in fertilizers, pharmaceuticals, and fine chemicals and is an ideal, carbon-free fuel. Recently, lithium-mediated nitrogen reduction has proven to be a promising route for electrochemical ammonia synthesis at ambient conditions. In this work, we report a continuous-flow electrolyzer equipped with 25-square centimeter-effective area gas diffusion electrodes wherein nitrogen reduction is coupled with hydrogen oxidation. We show that the classical catalyst platinum is not stable for hydrogen oxidation in the organic electrolyte, but a platinum-gold alloy lowers the anode potential and avoids the decremental decomposition of the organic electrolyte. At optimal operating conditions, we achieve, at 1 bar, a faradaic efficiency for ammonia production of up to 61 ± 1% and an energy efficiency of 13 ± 1% at a current density of -6 milliamperes per square centimeter.

How pH affects electrochemical processes.

[Figure: see text].

A metal-supported single-atom catalytic site enables carbon dioxide hydrogenation.

Nitrogen-doped graphene-supported single atoms convert CO2 to CO, but fail to provide further hydrogenation to methane - a finding attributable to the weak adsorption of CO intermediates. To regulate the adsorption energy, here we investigate the metal-supported single atoms to enable CO2 hydrogenation. We find a copper-supported iron-single-atom catalyst producing a high-rate methane. Density functional theory calculations and in-situ Raman spectroscopy show that the iron atoms attract surrounding intermediates and carry out hydrogenation to generate methane. The catalyst is realized by assembling iron phthalocyanine on the copper surface, followed by in-situ formation of single iron atoms during electrocatalysis, identified using operando X-ray absorption spectroscopy. The copper-supported iron-single-atom catalyst exhibits a CO2-to-methane Faradaic efficiency of 64% and a partial current density of 128 mA cm-2, while the nitrogen-doped graphene-supported one produces only CO. The activity is 32 times higher than a pristine copper under the same conditions of electrolyte and bias.

Plasma-Engraved Co2N Nanostructures toward High-Performance Alkaline Hydrogen Evolution.

Hydrogen generated by electrochemical water splitting is an attractive alternative to fossil fuels. Herein, we developed hollow-like Co2N nanoarrays that serve as electrocatalysts for the hydrogen evolution reaction (HER) with surface engineering by argon plasma. The argon plasma-engraved Co2N nanoarrays (Ar-Co2N/CC) represent a dramatic catalytic performance for the HER with an overpotential of 34 mV at a current density of 10 mA cm-2 in an alkaline electrolyte, as well as outstanding durability of 240 h. Characterization experiments and density functional theory (DFT) calculations suggest that the enhanced HER activity is due to the rational coordination environment of Co, which can be tuned by Ar plasma engraving. Based on our research, one new view for conducting exceptional catalyst surface modification engineering via plasma engraving might be established.

Boride-derived oxygen-evolution catalysts.

Metal borides/borates have been considered promising as oxygen evolution reaction catalysts; however, to date, there is a dearth of evidence of long-term stability at practical current densities. Here we report a phase composition modulation approach to fabricate effective borides/borates-based catalysts. We find that metal borides in-situ formed metal borates are responsible for their high activity. This knowledge prompts us to synthesize NiFe-Boride, and to use it as a templating precursor to form an active NiFe-Borate catalyst. This boride-derived oxide catalyzes oxygen evolution with an overpotential of 167 mV at 10 mA/cm2 in 1 M KOH electrolyte and requires a record-low overpotential of 460 mV to maintain water splitting performance for over 400 h at current density of 1 A/cm2. We couple the catalyst with CO reduction in an alkaline membrane electrode assembly electrolyser, reporting stable C2H4 electrosynthesis at current density 200 mA/cm2 for over 80 h.

Low coordination number copper catalysts for electrochemical CO2 methanation in a membrane electrode assembly.

The electrochemical conversion of CO2 to methane provides a means to store intermittent renewable electricity in the form of a carbon-neutral hydrocarbon fuel that benefits from an established global distribution network. The stability and selectivity of reported approaches reside below technoeconomic-related requirements. Membrane electrode assembly-based reactors offer a known path to stability; however, highly alkaline conditions on the cathode favour C-C coupling and multi-carbon products. In computational studies herein, we find that copper in a low coordination number favours methane even under highly alkaline conditions. Experimentally, we develop a carbon nanoparticle moderator strategy that confines a copper-complex catalyst when employed in a membrane electrode assembly. In-situ XAS measurements confirm that increased carbon nanoparticle loadings can reduce the metallic copper coordination number. At a copper coordination number of 4.2 we demonstrate a CO2-to-methane selectivity of 62%, a methane partial current density of 136 mA cm-2, and > 110 hours of stable operation.

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.

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%.

The aggression behavior study of Cl- on the defect structure of passive films on copper.

The destructive role of chloride ions on the defect structure of barrier layers (bl) is vitally important for understanding the initial breakdown of passive films on metals. Here photo-electrochemical and density functional theory (DFT) were applied to investigate the influence of chloride on the defect structure of the bl in passive films. The results show a bl with a narrow band gap, in which the valence band maximum (VBM) increased upon introducing chloride into the electrolyte. DFT calculations indicate that an increase in the copper vacancy concentration, due to cation extraction at the bl/solution interface could increase the VBM while oxygen vacancy generation results in a decrease in the conduction band minimum (CBM). The combination of these results verifies the aggressive role of chloride as proposed by the Point Defect Model (PDM) where an enhancement of the cation vacancy concentration across the bl occurs in response to the absorption of Cl- into oxygen vacancies on the bl.