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Bridging polymer design with catalyst surface science is a promising direction for tuning and optimizing electrochemical reactors that could impact long-term goals in energy and sustainability. Particularly, the interaction between inorganic catalyst surfaces and organic-based ionomers provides an avenue to both steer reaction selectivity and promote activity. Here, we studied the role of imidazolium-based ionomers for electrocatalytic CO2 reduction to CO (CO2R) on Ag surfaces and found that they produce no effect on CO2R activity yet strongly promote the competing hydrogen evolution reaction (HER). By examining the dependence of HER and CO2R rates on concentrations of CO2 and HCO3-, we developed a kinetic model that attributes HER promotion to intrinsic promotion of HCO3- reduction by imidazolium ionomers. We also show that varying the ionomer structure by changing substituents on the imidazolium ring modulates the HER promotion. This ionomer-structure dependence was analyzed via Taft steric parameters and density functional theory calculations, which suggest that steric bulk from functionalities on the imidazolium ring reduces access of the ionomer to both HCO3- and the Ag surface, thus limiting the promotional effect. Our results help develop design rules for ionomer-catalyst interactions in CO2R and motivate further work into precisely uncovering the interplay between primary and secondary coordination in determining electrocatalytic behavior.

Tuning bimetallic effects is a promising strategy to guide catalytic properties. However, the nature of these effects can be difficult to assess and compare due to the convolution with other factors such as the catalyst surface structure and morphology and differences in testing environments. Here, we investigate the impact of atomic-scale bimetallic effects on the electrochemical CO2 reduction performance of Cu-based catalysts by leveraging a systematic approach that unifies protocols for materials synthesis and testing and enables accurate comparisons of intrinsic catalytic activity and selectivity. We used the same physical vapor deposition method to epitaxially grow Cu(100) films decorated with a small amount of noble or base metal atoms and a combination of experimental characterization and first-principles calculations to evaluate their physicochemical and catalytic properties. The results indicate that the metal atoms segregate to under-coordinated Cu sites during physical vapor deposition, suppressing CO reduction to oxygenates and hydrocarbons and promoting competing pathways to CO, formate, and hydrogen. Leveraging these insights, we rationalize bimetallic design principles to improve catalytic selectivity for CO2 reduction to CO, formate, oxygenates, or hydrocarbons. Our study provides one of the most extensive studies on Cu bimetallics for CO2 reduction, establishing a systematic approach that is broadly applicable to research in catalyst discovery.

Summary We report an active zinc-copper (Zn-Cu) bimetallic electrocatalyst for CO2 reduction to CO, prepared by a facile galvanic procedure. Under moderate overpotentials, Zn-Cu catalysts that are Zn rich exhibit intrinsic activity for CO formation superior to that of pure Zn, Cu, and Ag, the last of which is the state-of-the-art catalyst in CO2 electrolyzers. Combinatorial experiments involving catalysts prepared by physical vapor deposition reveal trends across the Zn-Cu system, corroborating the high CO selectivity unrivaled by other alloys and intermetallics. Physical and electrochemical characterization and first principles theory reveal that the origin of this synergy in intrinsic activity is an electronic effect from bimetallic Zn-Cu sites that stabilizes the carboxyl intermediate during CO2 reduction to CO. Furthermore, by integrating Zn-Cu into gas-diffusion electrodes, we demonstrate that bimetallic effects lead to improved electrocatalytic performance at industrially relevant currents. These insights provide catalyst design principles that can guide future development of efficient and earth-abundant CO-producing electrocatalysts.

Understanding the differences between reactions driven by elevated temperature or electric potential remains challenging, largely due to materials incompatibilities between thermal catalytic and electrocatalytic environments. We show that Ni, N-doped carbon (NiPACN), an electrocatalyst for the reduction of CO2 to CO (CO2 R), can also selectively catalyze thermal CO2 to CO via the reverse water gas shift (RWGS) representing a direct analogy between catalytic phenomena across the two reaction environments. Advanced characterization techniques reveal that NiPACN likely facilitates RWGS on dispersed Ni sites in agreement with CO2 R active site studies. Finally, we construct a generalized reaction driving-force that includes temperature and potential and suggest that NiPACN could facilitate faster kinetics in CO2 R relative to RWGS due to lower intrinsic barriers. This report motivates further studies that quantitatively link catalytic phenomena across disparate reaction environments.

Advanced manufacturing (AM) represents an appealing approach for creating novel electrochemical systems for chemicals synthesis. In this work, we demonstrate AM for rapid development and testing for improved performance for the carbon dioxide reduction reaction across an evolution of vapor-fed reactor designs. In our final design, we observe activation- and mixed-control regimes for a variety of operating conditions via inlet CO2 flow rate and electrochemical potential. Furthermore, we define a dimensionless number (Da) to identify mass transport regimes by exploring the impact of hypothesized underlying mass transport mechanisms, including consumption of CO2via OH−, increased local temperatures, and partial penetration of electrolyte into gas diffusion layer. The accelerated pace of reactor design and development led to high geometric current densities (500 mA cm−2), heightened selectivity (85.5% FE C2+ products), and increased carbon dioxide conversion (16.6%) and cathodic energy efficiency (49.6% CO2R). Using AM vapor-fed reactors, we attain high ethylene (3.67%) and record ethanol (3.66%) yields compared to the literature. This work underscores the promise of AM for accelerating reactor design, understanding of governing phenomena, and improving the performance of catalytic systems.

The electrochemical CO2 reduction reaction (CO2RR) using Cu-based catalysts holds great potential for producing valuable multi-carbon products from renewable energy. However, the chemical and structural state of Cu catalyst surfaces during the CO2RR remains a matter of debate. Here, we show the structural evolution of the near-surface region of polycrystalline Cu electrodes under in situ conditions through a combination of grazing incidence X-ray absorption spectroscopy (GIXAS) and X-ray diffraction (GIXRD). The in situ GIXAS reveals that the surface oxide layer is fully reduced to metallic Cu before the onset potential for CO2RR, and the catalyst maintains the metallic state across the potentials relevant to the CO2RR. We also find a preferential surface reconstruction of the polycrystalline Cu surface toward (100) facets in the presence of CO2. Quantitative analysis of the reconstruction profiles reveals that the degree of reconstruction increases with increasingly negative applied potentials, and it persists when the applied potential returns to more positive values. These findings show that the surface of Cu electrocatalysts is dynamic during the CO2RR, and emphasize the importance of in situ characterization to understand the surface structure and its role in electrocatalysis.

Rigorous in situ studies of electrocatalysts are required to enable the design of higher performing materials. Nonplatinum group metals for oxygen reduction reaction (ORR) catalysis containing ligh...

Ni,N-doped carbon catalysts have shown promising catalytic performance for CO2 electroreduction (CO2 R) to CO; this activity has often been attributed to the presence of nitrogen-coordinated, single Ni atom active sites. However, experimentally confirming Ni-N bonding and correlating CO2 reduction (CO2 R) activity to these species has remained a fundamental challenge. We synthesized polyacrylonitrile-derived Ni,N-doped carbon electrocatalysts (Ni-PACN) with a range of pyrolysis temperatures and Ni loadings and correlated their electrochemical activity with extensive physiochemical characterization to rigorously address the origin of activity in these materials. We found that the CO2 R to CO partial current density increased with increased Ni content before plateauing at 2 wt % which suggests a dispersed Ni active site. These dispersed active sites were investigated by hard and soft X-ray spectroscopy, which revealed that pyrrolic nitrogen ligands selectively bind Ni atoms in a distorted square-planar geometry that strongly resembles the active sites of molecular metal-porphyrin catalysts.

The electrochemical oxygen evolution reaction (OER) is the half-cell reaction for many clean-energy production technologies, including water electrolyzers and metal-air batteries. However, its sluggish kinetics hinders the performance of those technologies, impeding them from broader implementation. Recently, we reported the use of zirconium phosphate (ZrP) as a support for transition metal catalysts for the oxygen evolution reaction (OER). These catalysts achieve promising overpotentials with high mass activities. Herein, we synthesize ZrP structures with controlled morphology: hexagonal platelets, rods, cubes, and spheres, and subsequently modify them with Co(ii) and Ni(ii) cations to assess their electrochemcial OER behavior. Through inductively coupled plasma mass-spectrometry measurements, the maximum ion exchange capacity is found to vary based on the morphology of the ZrP structure and cation selection. Trends in geometric current density and mass activity as a function of cation selection are discussed. We find that the loading and coverage of cobalt and nickel species on the ZrP supports are key factors that control OER performance.

Utilizing electrochemical conversion of CO(2) into hydrocarbons and oxygenates is envisioned as a promising path towards closing the carbon cycle in modern technology. To this day, however, the exact reaction mechanisms towards the plethora of single and multi-carbon products on Cu electrodes are still disputed. This uncertainty even extends to the rate-limiting step of the respective reactions. Since multi-carbon products do not show a dependence on the electrolyte pH in neutral and alkaline media, CO dimerization on the Cu surface has been proposed as the rate-limiting step. However, other elementary steps would lead to the same pH dependence, namely the proton-electron transfer to *CO followed by subsequent coupling or the protonation of the *OCCO dimer. The pH dependence of methane production on the other hand suggests that the rate limiting step is located beyond the first proton-electron transfer to *CO. In order to conclusively identify the rate limiting steps in CO reduction, we analyzed the mechanisms on the basis of constant potential DFT calculations, CO reduction experiments on Cu at varying pH values (3 - 13) and fundamental rate theory. We find that, even in acidic media, the reaction rate towards multi-carbon products is nearly unchanged on an SHE potential scale, which indicates that its rate limiting step does not involve a proton donor. Hence, we deduce that the rate limiting step can indeed only consist of the coupling of two CO molecules on the surface, both in acidic and alkaline conditions. For methane, on the other hand, the rate-limiting step changes with the electrolyte pH from the first protonation step in acidic/neutral conditions to a later step in alkaline conditions. Finally, based on an in-depth kinetic analysis, we conclude that the pathway towards CH4 involving a surface combination of *CO and *H is unlikely, since it is unable to reproduce the measured current densities and Tafel slopes.

Iridium-based mixed metal oxide phases have been shown as promising electrocatalysts for the oxygen evolution reaction (OER) because of their ability to stabilize unique Ir-based surface sites with...

Improved electrochemical oxygen evolution catalysis is crucial for many clean-energy production technologies. Recently, transition-metal-modified zirconium phosphate (ZrP) catalysts were studied for the oxygen evolution reaction (OER) in alkaline media. These studies suggest that the OER occurs preferentially on the surface of the layered ZrP nanoparticles rather than the interlayer gallery. Herein, ZrP nanoparticles are exfoliated with tetrabutylammonium hydroxide (TBA+OH–) to further expose surface sites which are subsequently modified with Co and Ni cations by an ion-exchange reaction. Because of the greater surface accessibility of the exfoliated ZrP support, higher loadings of catalyst material were achieved along with improved site access for catalysis. These new composite materials have improved geometric area normalized overpotentials than metal-adsorbed ZrP nanoparticles without exfoliation. Specifically, Co-modified and Ni-modified exfoliated ZrP show a reduction in overpotential at a current de...

The present work was undertaken to elucidate the facet-dependent activity of Ag for the electrochemical reduction of CO2 to CO. To this end, CO2 reduction was investigated over Ag thin films with (111), (100), and (110) orientations prepared via epitaxial growth on single-crystal Si wafers with the same crystallographic orientations. This preparation technique yielded larger area electrodes than can be achieved using single-crystals, which enabled the electrocatalytic activity of the corresponding Ag surfaces to be quantified in the Tafel regime. The Ag(110) thin films exhibited higher CO evolution activity compared to the Ag(111) and Ag(100) thin films, consistent with previous single-crystal studies. Density functional theory calculations suggest that CO2 reduction to CO is strongly facet-dependent, and that steps are more active than highly coordinated terraces. This is the result of both a higher binding energy of the key intermediate COOH as well as an enhanced double-layer electric field stabilizati...

We demonstrate that applying atomic layer deposition-derived molybdenum disulfide (MoS2) catalytic coatings on copper gallium diselenide (CGSe) thin film absorbers can lead to efficient wide band gap photocathodes for photoelectrochemical hydrogen production. We have prepared a device that is free of precious metals, employing a CGSe absorber and a cadmium sulfide (CdS) buffer layer, a titanium dioxide (TiO2) interfacial layer, and a MoS2 catalytic layer. The resulting MoS2/TiO2/CdS/CGSe photocathode exhibits a photocurrent onset of +0.53 V vs RHE and a saturation photocurrent density of −10 mA cm–2, with stable operation for >5 h in acidic electrolyte. Spectroscopic investigations of this device architecture indicate that overlayer degradation occurs inhomogeneously, ultimately exposing the underlying CGSe absorber.

Accurately quantifying applied potential is important to ensuring the comparability, accuracy, and precision of electrochemical studies. Reference electrodes (REs) enable knowledge/determination of the applied potential at electrodes in electrochemical systems. Ultimately, the choice of RE will depend on the particular requirements of a given electrochemical system, however, we note it is imperative to ensure the accuracy of the RE potential and its proper translation to a standardized scale. In this work, we highlight that while there are many commercially available REs, these must be experimentally calibrated to a reliable and practical standard potential scale, for instance the reversible hydrogen electrode (RHE) scale for aqueous systems. With representative data, we provide streamlined instructions on how to calibrate any RE to the RHE scale. We also provide guidance to mitigate and/or avoid possible electrolyte contamination issues arising from REs. Moreover, we offer a step-by-step guide on how to build a practical RHE RE, which may be a suitable and desirable option in certain applications. Our work emphasizes the need for the continuous adoption of standardized reference potential scales and demonstrates the versatility of the RHE scale, particularly in aqueous electrochemistry.

Alloying is a powerful tool that can improve the electrocatalytic performance and viability of diverse electrochemical renewable energy technologies. Herein, we enhance the activity of Pd-based electrocatalysts via Ag-Pd alloying while simultaneously lowering precious metal content in a broad-range compositional study focusing on highly comparable Ag-Pd thin films synthesized systematically via electron-beam physical vapor co-deposition. Cyclic voltammetry in 0.1 M KOH shows enhancements across a wide range of alloys; even slight alloying with Ag (e.g. Ag0.1Pd0.9) leads to intrinsic activity enhancements up to 5-fold at 0.9 V vs. RHE compared to pure Pd. Based on density functional theory and x-ray absorption, we hypothesize that these enhancements arise mainly from ligand effects that optimize adsorbate–metal binding energies with enhanced Ag-Pd hybridization. This work shows the versatility of coupled experimental-theoretical methods in designing materials with specific and tunable properties and aids the development of highly active electrocatalysts with decreased precious-metal content., Electrocatalyst development is key to improving the performance and viability of many electrochemical energy technologies. Here, the authors design Ag-Pd alloys with specifically tuned electronic structures to have enhanced oxygen reduction electrocatalysis and decreased precious metal content.

Electrochemical CO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{2}$$\end{document}2 reduction is a potential route to the sustainable production of valuable fuels and chemicals. Here, we perform CO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{2}$$\end{document}2 reduction experiments on Gold at neutral to acidic pH values to elucidate the long-standing controversy surrounding the rate-limiting step. We find the CO production rate to be invariant with pH on a Standard Hydrogen Electrode scale and conclude that it is limited by the CO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{2}$$\end{document}2 adsorption step. We present a new multi-scale modeling scheme that integrates ab initio reaction kinetics with mass transport simulations, explicitly considering the charged electric double layer. The model reproduces the experimental CO polarization curve and reveals the rate-limiting step to be *COOH to *CO at low overpotentials, CO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{2}$$\end{document}2 adsorption at intermediate ones, and CO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{2}$$\end{document}2 mass transport at high overpotentials. Finally, we show the Tafel slope to arise from the electrostatic interaction between the dipole of *CO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{2}$$\end{document}2 and the interfacial field. This work highlights the importance of surface charging for electrochemical kinetics and mass transport., Electrochemical CO2 reduction is a potential route to the sustainable production of valuable fuels and chemicals. In this joint experimental-theoretical work, the authors address the issue of the rate-limiting step on Gold and present insights from multi-scale simulations into the importance of the electric double layer on reaction kinetics and mass transport.

Electrochemical transformation of potent greenhouse gases such as CO2 and CH4 to produce useful carbon-based products is a highly desirable sustainability goal. However, selectivity challenges remain in aqueous electrochemical processes as selective CO2 reduction to desired products is difficult and electrochemical CH4 oxidation often proceeds at very low rates. The formation of C-C coupled products in these fields is particularly desirable as this provides a path for the production of high-value fuels and chemicals. We have developed a cyclic electrochemical strategy which can produce acetylene, a C-C coupled product, from such carbon sources and water, with favorable current density and selectivity. This strategy is exemplified with a lithium-mediated cycle: an active Li0 surface is electrochemically generated from LiOH, the newly formed Li0 reacts with a carbon source to form Li2C2, and Li2C2 is hydrolyzed to form acetylene and regenerate LiOH. We demonstrate this process primarily using CO2 gas, achieving a current efficiency of 15% to acetylene (which represents 82% of the maximum based on stoichiometric production of oxygenated byproducts, e.g.LiCO3 and/or Li2O), as verified by gas chromatography and Fourier transform infrared radiation studies. We also explore CH4, CO, and C as alternative precursors in the acetylene synthesis. Notably, the use of graphitic carbon at higher temperatures resulted in over 55% current efficiency to acetylene, with opportunity for further optimization. Importantly, this cycling method avoids the formation of common side products observed during aqueous electrochemical CO2 and CH4 redox reactions, such as H2, CO, HCO2−, or CO2. Theoretical considerations elucidate the feasibility and general applicability of this cycle and the process steps have been characterized with specific electrochemical and materials chemistry techniques. The continued development of this strategy may lead to a viable route for the sustainable production of C-C coupled carbon fuels and chemicals.

Catalytic systems whose properties can be systematically tuned via changes in synthesis conditions are highly desirable for the next-generation catalyst design and optimization. Herein, we present a two-dimensional (2D) conductive metal-organic framework consisting of M-N4 units (M = Ni, Cu) and a hexaaminobenzene (HAB) linker as a catalyst for the oxygen reduction reaction. By varying synthetic conditions, we prepared two Ni-HAB catalysts with different crystallinities, resulting in catalytic systems with different electric conductivities, electrochemical activity, and stability. We show that crystallinity has a positive impact on conductivity and demonstrate that this improved crystallinity/conductivity improves the catalytic performance of our model system. Additionally, density functional theory simulations were performed to probe the origin of M-HAB's catalytic activity, and they suggest that M-HAB's organic linker acts as the active site with the role of the metal being to modulate the linker sites' binding strength.

Electrochemical processes for ammonia synthesis could potentially replace the high temperature and pressure conditions of the Haber‐Bosch process, with voltage offering a pathway to distributed fertilizer production that leverages the rapidly decreasing cost of renewable electricity. However, nitrogen is an unreactive molecule and the hydrogen evolution reaction presents a major selectivity challenge. An electrode of electrodeposited lithium in tetrahydrofuran solvent overcomes both problems by providing a surface that easily reacts with nitrogen and by limiting the access of protons with a nonaqueous electrolyte. Under these conditions, we measure relatively high faradaic efficiencies (ca. 10 %) and rates (0.1 mA cm−2) toward NH3. We observe the development of a solid electrolyte interface layer as well as the accumulation of lithium and lithium‐containing species. Detailed DFT studies suggest lithium nitride and hydride to be catalytically active phases given their thermodynamic and kinetic stability relative to metallic lithium under reaction conditions and the fast diffusion of nitrogen in lithium.

Improved activity and stability Pt-based catalysts for the oxygen reduction reaction (ORR) are needed to perpetuate the deployment of polymer electrolyte fuel cells (PEFCs) in the transportation sector. Here, we use atomic layer deposition of TiO2 and Pt coupled with thermal reductive annealing to prepare Pt3Ti electrocatalysts. The atomic level synthetic control resulted in Pt3Ti nanoparticles with high ORR performance, including a mass activity of 1.84 A/mgPt and excellent electrochemical stability. The Pt3Ti nanoparticles show excellent specific activity — 5.3-fold higher than commercial Pt/C and 3-fold higher than polycrystalline Pt, exceeding the performance of any PtTi catalysts reported to date. Combined experimental and computational efforts indicate that Pt enrichment on the Pt3Ti enhances the activity, and the intrinsic stability of the Pt3Ti phase provides durability. This knowledge, along with the facile fabrication of alloys by atomic layer deposition, can be leveraged to designed improved performance catalysts.

Steering the selectivity of Cu-based electrochemical CO2 reduction (CO2R) catalysts toward targeted products will serve to improve the technoeconomic outlook of technologies based on this process. Using physical vapor deposition as a tool to overcome thermodynamic miscibility limitations, CuAg thin films with nonequilibrium Cu/Ag alloying were prepared for CO2R performance evaluation. In comparison to pure Cu, the CuAg thin films showed significantly higher activity and selectivity toward liquid carbonyl products, including acetaldehyde and acetate. Suppressed activity and selectivity toward hydrocarbons and the competing hydrogen evolution were also demonstrated on CuAg thin films, with a greater degree of suppression observed at increasing nominal Ag compositions. Compositional-dependent CO2R trends coupled with physical characterization and density functional theory suggest that significant miscibility of Ag into the Cu-rich phase of the catalyst underpinned the observed CO2R trends through tuning of a...

The discovery of materials for the electrochemical transformation of carbon dioxide into liquid fuels has the potential to impact large-scale storage of renewable energies and reduce carbon emissions. Here, we report the discovery of an electrocatalyst composed of gold nanoparticles on a polycrystalline copper foil (Au/Cu) that is highly active for CO2 reduction to alcohols. At low overpotentials, the Au/Cu electrocatalyst is over 100 times more selective for the formation of products containing C–C bonds versus methane or methanol, largely favouring the generation of alcohols over hydrocarbons. A combination of electrochemical testing and transport modelling supports the hypothesis that CO2 reduction on gold generates a high CO concentration on nearby copper, where CO is further reduced to alcohols such as ethanol and n-propanol under locally alkaline conditions. The bimetallic Au/Cu electrocatalyst exhibits synergistic activity and selectivity superior to gold, copper or AuCu alloys, and opens new possibilities for the development of CO2 reduction electrodes exploiting tandem catalysis mechanisms. The electrochemical transformation of CO2 into liquid fuels is a major challenge. Now, Jaramillo, Hahn and co-workers present a Au/Cu catalyst highly active to C2+ alcohols at low overpotentials as a result of a tandem mechanism where CO2 is reduced to CO on Au and further reduced to C2+ alcohols on nearby Cu.

Understanding the surface reactivity of CO, which is a key intermediate during electrochemical CO2 reduction, is crucial for the development of catalysts that selectively target desired products for the conversion of CO2 to fuels and chemicals. In this study, a custom-designed electrochemical cell is utilized to investigate planar polycrystalline copper as an electrocatalyst for CO reduction under alkaline conditions. Seven major CO reduction products have been observed including various hydrocarbons and oxygenates which are also common CO2 reduction products, strongly indicating that CO is a key reaction intermediate for these further-reduced products. A comparison of CO and CO2 reduction demonstrates that there is a large decrease in the overpotential for C–C coupled products under CO reduction conditions. The effects of CO partial pressure and electrolyte pH are investigated; we conclude that the aforementioned large potential shift is primarily a pH effect. Thus, alkaline conditions can be used to inc...

Increasing the activity of Ag-based catalysts for the oxygen reduction reaction (ORR) is important for improving the performance and economic outlook of alkaline-based fuel cell and metal–air battery technologies. In this work, we prepare CuAg thin films with controllable compositions using electron beam physical vapor deposition. X-ray diffraction analysis indicates that this fabrication route yields metastable miscibility between these two thermodynamically immiscible metals, with the thin films consisting of a Ag-rich and a Cu-rich phase. Electrochemical testing in 0.1 M potassium hydroxide showed significant ORR activity improvements for the CuAg films. On a geometric basis, the most active thin film (Cu70Ag30) demonstrated a 4-fold activity improvement vs pure Ag at 0.8 V vs the reversible hydrogen electrode. Furthermore, enhanced ORR kinetics for Cu-rich (>50 at. % Cu) thin films was demonstrated by a decrease in Tafel slope from 90 mV/dec, a commonly observed value for Ag catalysts, to 45 mV/dec. S...

Hydrogen peroxide (H2O2) is a valuable chemical with a wide range of applications, but the current industrial synthesis of H2O2 involves an energy-intensive anthraquinone process. The electrochemical synthesis of H2O2 from oxygen reduction offers an alternative route for on-site applications; the efficiency of this process depends greatly on identifying cost-effective catalysts with high activity and selectivity. Here, we demonstrate a facile and general approach to catalyst development via the surface oxidation of abundant carbon materials to significantly enhance both the activity and selectivity (~90%) for H2O2 production by electrochemical oxygen reduction. We find that both the activity and selectivity are positively correlated with the oxygen content of the catalysts. The density functional theory calculations demonstrate that the carbon atoms adjacent to several oxygen functional groups (–COOH and C–O–C) are the active sites for oxygen reduction reaction via the two-electron pathway, which are further supported by a series of control experiments. The direct synthesis of hydrogen peroxide via oxygen reduction is an attractive alternative to the anthraquinone process. Here, a general trend linking oxygenation of carbon surfaces with electrocatalytic performance in peroxide synthesis is demonstrated, and computational studies provide further insight into the nature of the active sites.

The performance of oxygen reduction reaction (ORR) catalysts has been substantially improved over the past several decades. These catalysts are evaluated for electrochemical activity in a rotating disk electrode (RDE) assembly using an oxygen saturated liquid electrolyte. Koutecky-Levich (K-L) analysis provides a simple and effective method to extract electrokinetic information by correcting for mass transport effects. We propose extensions to the K-L analysis to address some of the simplifying assumptions made during its derivation. In particular, we demonstrate that decreased concentrations of surface reactants contribute to measured overpotentials in a Nernst fashion, and can lead to an underestimation of catalytic activity. By applying a Nernst overpotential correction in conjunction with K-L analysis, more accurate measurements of the intrinsic reaction kinetics under mass transport limited conditions are possible. As the K-L method assumes a reaction order of unity, we also consider kinetic reaction order deviation from unity due to measurement conditions. We show that examining the dependence of the reaction order on overpotential can provide a straightforward technique to probe blocking effects of surface absorbents. We propose that these extensions to the K-L method can allow for increased versatility of the RDE technique for extracting electrokinetic parameters for ORR catalysts.

The electrochemical reduction of carbon dioxide using renewably generated electricity offers a potential means for producing fuels and chemicals in a sustainable manner. To date, copper has been found to be the most effective catalyst for electrochemically reducing carbon dioxide to products such as methane, ethene, and ethanol. Unfortunately, the current efficiency of the process is limited by competition with the relatively facile hydrogen evolution reaction. Since multi-carbon products are more valuable precursors to chemicals and fuels than methane, there is considerable interest in modifying copper to enhance the multi-carbon product selectivity. Here, we report our investigations of electrochemical carbon dioxide reduction over CuAg bimetallic electrodes and surface alloys, which we find to be more selective for the formation of multi-carbon products than pure copper. This selectivity enhancement is a result of the selective suppression of hydrogen evolution, which occurs due to compressive strain i...

© 2017 American Chemical Society. Molybdenum sulfides have been identified as promising materials for catalyzing the hydrogen evolution reaction (HER) in acid, with active edge sites that exhibit some of the highest turnover frequencies among nonpreciousmetal catalysts. The thiomolybdate [Mo 3 S 13 ] 2- nanocluster catalyst contains a structural motif that resembles the active site of MoS2 and has been reported to be among the most active forms of molybdenum sulfide. Herein, we improve the activity of the [Mo 3 S 13 ] 2- catalysts through catalyst-support interactions. We synthesize [Mo 3 S 13 ] 2- on gold, silver, glassy carbon, and copper supports to demonstrate the ability to tune the hydrogen binding energy of [Mo 3 S 13 ] 2- using catalyst-support electronic interactions and optimize HER activity.

We report the first synthesis of a fully contiguous large area supported thin film of highly ordered mesoporous Ru and RuO2 and investigate the electrocatalytic properties towards the oxygen evolution reaction (OER). We find that the nanoscale porous network of these catalysts provides significant enhancements in geometric OER activity without any loss in specific activity. This work demonstrates a strategy for engineering materials at the nanoscale that can simultaneously decrease precious metal loading and increase catalytic activity.

We have studied the influence of low concentrations (0.1 M) of the ionic liquid 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl) on suppressing the hydrogen evolution reaction (HER) using polycrystalline Ag, Cu, and Fe electrodes in aqueous acidic and basic media. HER suppression is generally desired when aiming to catalyze other reactions of interests, e.g., CO2 electro-reduction. Cyclic voltammetry and chronoamperometry measurements were performed at potentials between −0.2 and −0.8 V versus the reversible hydrogen electrode (RHE) to investigate HER activity in a simulated CO2 electrolysis environment without the CO2. In an acidic electrolyte, a decrease in HER activity was observed for all three electrodes with the largest effect being that of Fe, where the HER activity was suppressed by 75% at −0.5 V versus RHE. In contrast to the effect of [EMIM]Cl in an acidic electrolyte, no HER suppression was observed in basic media. Using 1H nuclear magnetic resonance spectroscopy on the electrolyte before and af...

Increases in energy demand and in chemical production, together with the rise in CO2 levels in the atmosphere, motivate the development of renewable energy sources. Electrochemical CO2 reduction to fuels and chemicals is an appealing alternative to traditional pathways to fuels and chemicals due to its intrinsic ability to couple to solar and wind energy sources. Formate (HCOO–) is a key chemical for many industries; however, greater understanding is needed regarding the mechanism and key intermediates for HCOO– production. This work reports a joint experimental and theoretical investigation of the electrochemical reduction of CO2 to HCOO– on polycrystalline Sn surfaces, which have been identified as promising catalysts for selectively producing HCOO–. Our results show that Sn electrodes produce HCOO–, carbon monoxide (CO), and hydrogen (H2) across a range of potentials and that HCOO– production becomes favored at potentials more negative than −0.8 V vs RHE, reaching a maximum Faradaic efficiency of 70% a...

We report on CO2 electroreduction activity and selectivity of a polycrystalline AgZn foil in aqueous bicarbonate electrolyte. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) measurements show that the alloy foil was slightly enriched in zinc both at the surface and in the bulk, with a surface alloy composition of 61.3±5.4 at % zinc and with Ag5Zn8 as the most prominent bulk phase. AgZn is active for CO2 reduction; CO is the main product, likely due to the weak CO binding energy of the surface, with methane and methanol emerging as minor products. Compared to pure silver and pure zinc foils, enhancements in activity and selectivity for methane and methanol are observed. A five-fold increase is observed in the combined partial current densities for methane and methanol at −1.43 V vs. the reversible hydrogen electrode (RHE), representing a four- to six-fold increase in faradaic efficiency. Such enhancements indicate the existence of a synergistic effect between silver and zinc at the surface of the alloy that contributes to the enhanced formation of further reduced products.

In the future, industrial CO2 electroreduction using renewable energy sources could be a sustainable means to convert CO2 and water into commodity chemicals at room temperature and atmospheric pressure. This study focuses on the electrocatalytic reduction of CO2 on polycrystalline Au surfaces, which have high activity and selectivity for CO evolution. We explore the catalytic behavior of polycrystalline Au surfaces by coupling potentiostatic CO2 electrolysis experiments in an aqueous bicarbonate solution with high sensitivity product detection methods. We observed the production of methanol, in addition to detecting the known products of CO2 electroreduction on Au: CO, H2 and formate. We suggest a mechanism that explains Au's evolution of methanol. Specifically, the Au surface does not favor C–O scission, and thus is more selective towards methanol than methane. These insights could aid in the design of electrocatalysts that are selective for CO2 electroreduction to oxygenates over hydrocarbons.

The development of small-scale, decentralized reactors for H2O2 production that can couple to renewable energy sources would be of great benefit, particularly for water purification in the developing world. Herein, we describe our efforts to develop electrochemical reactors for H2O2 generation with high Faradaic efficiencies of >90%, requiring cell voltages of only ∼1.6 V. The reactor employs a carbon-based catalyst that demonstrates excellent performance for H2O2 production under alkaline conditions, as demonstrated by fundamental studies involving rotating-ring disk electrode methods. The low-cost, membrane-free reactor design represents a step towards a continuous, modular-scale, de-centralized production of H2O2.

The rational improvement of current and developing electrochemical technologies requires atomistic understanding of electrode-electrolyte interfaces. However, examining these interfaces under operando conditions, where performance is typically evaluated and benchmarked, remains challenging, as it necessitates incorporating an operando probe during full electrochemical operation. In this study, we describe a custom electrochemical flow cell that enables near-surface-sensitive operando investigation of planar thin-film catalysts at significant hydrogen evolution reaction (HER) rates (in excess of -100 mA cm-2) using grazing incidence X-ray methods. Grazing-incidence X-ray spectroscopy and diffraction were implemented on the same sample under identical HER conditions, demonstrating how the combined measurements track changing redox chemistry and structure of Cu thin-film catalyst surfaces as a function of electrochemical conditions. The coupling of these methods with improved mass transport and hydrodynamic control establishes a new paradigm for operando measurement design, enabling unique insights into the key fundamental processes occurring at the catalyst-electrolyte interface.

We present a microkinetic model for CO(2) reduction (CO(2)R) on Cu(211) towards C2 products, based on energetics estimated from an explicit solvent model. We show that the differences in both Tafel slopes and pH dependence for C1 vs C2 activity arise from differences in their multi-step mechanisms. We find the depletion in C2 products observed at high overpotential and high pH to arise from the 2nd order dependence of C-C coupling on CO coverage, which decreases due to competition from the C1 pathway. We further demonstrate that CO(2) reduction at a fixed pH yield similar activities, due to the facile kinetics for CO2 reduction to CO on Cu, which suggests C2 products to be favored for CO2R under alkaline conditions. The mechanistic insights of this work elucidate how reaction conditions can lead to significant enhancements in selectivity and activity towards higher value C2 products., CO2 conversion to reduced products provides a use for greenhouse gases, but reaction complexity stymies mechanistic studies. Here, authors present a microkinetic model for CO2 and CO reduction on copper, based on ab initio simulations, to elucidate pH’s impact on competitive reaction pathways.

To date, copper is the only heterogeneous catalyst that has shown a propensity to produce valuable hydrocarbons and alcohols, such as ethylene and ethanol, from electrochemical CO2 reduction (CO2R). There are variety of factors that impact CO2R activity and selectivity, including the catalyst surface structure, morphology, composition, the choice of electrolyte ions and pH, and the electrochemical cell design. Many of these factors are often intertwined, which can complicate catalyst discovery and design efforts. Here we take a broad and historical view of these different aspects and their complex interplay in CO2R catalysis on Cu, with the purpose of providing new insights, critical evaluations, and guidance to the field with regard to research directions and best practices. First, we describe the various experimental probes and complementary theoretical methods that have been used to discern the mechanisms by which products are formed, and next we present our current understanding of the complex reaction networks for CO2R on Cu. We then analyze two key methods that have been used in attempts to alter the activity and selectivity of Cu: nanostructuring and the formation of bimetallic electrodes. Finally, we offer some perspectives on the future outlook for electrochemical CO2R.

Electrochemical oxygen reduction is an important reaction for many sustainable energy technologies, such as fuel cells and metal–air batteries. Kinetic limitations of this reaction, expensive electrocatalysts, and catalyst instability, however, limit the commercial viability of such devices. Herein, we report an active Ir@Pt core–shell catalyst that combines platinum overlayers with nanostructure effects to tune the oxygen binding to the Pt surface, thereby achieving enhanced activity and stability for the oxygen reduction reaction. Ir@Pt nanoparticles with several shell thicknesses were synthesized in a scalable, inexpensive, one-pot polyol method. Electrochemical analysis demonstrates the activity and stability of the Ir@Pt catalyst, with specific and mass activities increasing to 2.6 and 1.8 times that of commercial Pt/C (TKK), respectively, after 10 000 stability cycles. Activity enhancement of the Ir@Pt catalyst is attributed to weakening of the oxygen binding to the Pt surface induced by the Ir core.

The development of high-performance and low-cost oxygen reduction and evolution catalysts that can be easily integrated into existing devices is crucial for the wide deployment of energy storage systems that utilize O2-H2O chemistries, such as regenerative fuel cells and metal-air batteries. Herein, we report an NH3-activated N-doped hierarchical carbon (NHC) catalyst synthesized via a scalable route, and demonstrate its device integration. The NHC catalyst exhibited good performance for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), as demonstrated by means of electrochemical studies and evaluation when integrated into the oxygen electrode of a regenerative fuel cell. The activities observed for both the ORR and the OER were comparable to those achieved by state-of-the-art Pt and Ir catalysts in alkaline environments. We have further identified the critical role of carbon defects as active sites for electrochemical activity through density functional theory calculations and high-resolution TEM visualization. This work highlights the potential of NHC to replace commercial precious metals in regenerative fuel cells and possibly metal-air batteries for cost-effective storage of intermittent renewable energy.

Oxygen electrochemistry plays a key role in renewable energy technologies such as fuel cells and electrolyzers, but the slow kinetics of the oxygen evolution reaction (OER) limit the performance and commercialization of such devices. Here we report an iridium oxide/strontium iridium oxide (IrO x /SrIrO 3 ) catalyst formed during electrochemical testing by strontium leaching from surface layers of thin films of SrIrO 3 . This catalyst has demonstrated specific activity at 10 milliamps per square centimeter of oxide catalyst (OER current normalized to catalyst surface area), with only 270 to 290 millivolts of overpotential for 30 hours of continuous testing in acidic electrolyte. Density functional theory calculations suggest the formation of highly active surface layers during strontium leaching with IrO 3 or anatase IrO 2 motifs. The IrO x /SrIrO 3 catalyst outperforms known IrO x and ruthenium oxide (RuO x ) systems, the only other OER catalysts that have reasonable activity in acidic electrolyte.

Understanding catalyst function to improve activity for the oxygen evolution reaction (OER) is key to increasing the overall efficiency of electrochemical water splitting, a promising method for sustainable and clean production of hydrogen. Using a straightforward and scalable sol⿿gel synthesis, we explore the effects of metal composition in CoxTi1⿿xOy on electrochemical activity, atomic structure, and electronic state. Physical and electronic characterization reveal that increased amounts of Ti stabilize the 2+ oxidation state of the Co precursor and lead to formation of less active CoO-like catalysts. Conversely, films with Co:Ti ratios of 1:1 or greater result in catalysts with high activity, correlating with greater Co 3+ character, as measured by ex situ XAS for samples as-prepared and after exposure to OER conditions. Additionally, decreasing the Ti content systematically shifts the Co redox potential from approximately 1.5 V vs. RHE with a 1:3 Co:Ti ratio to 1.0 V vs. RHE with no Ti, further evidence that Ti stabilizes Co in a lower oxidation state. Controlling the oxidation state of metals in metal-oxide OER catalysts can have a profound effect on catalytic activity.

Heteroatom-doped carbons have drawn increasing research interest as catalysts for various electrochemical reactions due to their unique electronic and surface structures. In particular, co-doping of carbon with boron and nitrogen has been shown to provide significant catalytic activity for oxygen reduction reaction (ORR). However, limited experimental work has been done to systematically study these materials, and much remains to be understood about the nature of the active site(s), particularly with regards to the factors underlying the activity enhancements of these boron–carbon–nitrogen (BCN) materials. Herein, we prepare several BCN materials experimentally with a facile and controlled synthesis method, and systematically study their electrochemical performance. We demonstrate the existence of h-BN domains embedded in the graphitic structures of these materials using X-ray spectroscopy. These synthesized structures yield higher activity and selectivity toward the 2e– ORR to H2O2 than structures with i...

Improving the performance of oxygen reduction reaction (ORR) electrocatalysts is essential for the commercial efficacy of many renewable energy technologies, including low temperature polymer electrolyte fuel cells (PEFCs). Herein, we report highly active and stable carbon-supported Ru@Pt core-shell nanoparticles (Ru@Pt/C) prepared by a wet chemical synthesis technique. Through rotating disc electrode testing, the Ru@Pt/C achieves an ORR Pt mass-based activity of 0.50 A mgPt−1 at 0.9 V versus the reversible hydrogen electrode (RHE), which exceeds the activity of the state-of-the-art commercial Pt/C catalyst as well as the Department of Energy 2020 PEFC electrocatalyst activity targets for transportation applications. The impact of various synthetic parameters, including Pt to Ru ratios and catalyst pretreatments (i.e., annealing) are thoroughly explored. Pt-based mass activity of all prepared Ru@Pt/C catalysts was found to exceed 0.4 mgPt−1 across the range of compositions investigated, with the maximum activity catalyst having a Ru:Pt ratio of 1:1. This optimized composition of Ru@Pt/C catalyst demonstrated remarkable stability after 30,000 accelerated durability cycles (0.6 to 1.0 V vs. RHE at 125 mV s−1), maintaining 85% of its initial mass activity. Scanning transmission electron microscopy energy dispersive spectroscopy (STEM-EDS) analysis at various stages of electrochemical testing demonstrated that the Pt shell can provide sufficient protection against the dissolution of the otherwise unstable Ru core.