The direct and partial oxidation of methane to value-added chemical fuels, such as methanol, remains a major yet lucrative challenge in catalytic chemistry. Herein, we introduce engineered plasma-catalyst-liquid interfaces (PCLIs) that enable a one-step, ambient-pressure, electrified pathway for methane oxidation to methanol and higher-order hydrocarbons. By integrating a CuO-infused porous glass frit coupled with a nonthermal methane plasma at an aqueous interface, we demonstrate the importance of mass transfer of plasma-activated species to the catalyst surface in controlling oxidative selectivity. Following systematic experiments of reaction conditions, we report an optimized liquid-phase methanol selectivity of 96.8 ± 0.6% (highest total selectivity = 57.9 ± 5.5%) with a simultaneous production rate of 51.8 ± 1.5 mmolMeOH gCuO-1 hr-1. Additional gas-phase production of H2 and C2+ hydrocarbons (e.g., ethane, ethylene, propane, propylene) was measured with a notable absence of overoxidized products (i.e., CO2) under optimized reaction conditions. A specific electricity consumption of 46.7 kWh/kgMeOH indicates competitive efficiency for electrified methane upgrading. Plasma diagnostics, including charge-voltage Lissajous analysis, optical emission spectroscopy, and plasma modeling, reveal a complex mechanistic picture where CuO-stabilized biradical coupling, gas-phase radical recombination, and vibrationally "hot" methane compete for overall reaction selectivity as a function of the pulsed plasma discharge. This study demonstrates the importance of modulating plasma chemistry and transport between plasma, catalyst, and liquid to improve reaction outcomes under complex multiphase environments.

Electrochemical CO2 reduction reactions face activity, selectivity, and stability challenges as this technology moves towards commercialization. Pulsed electrolysis (PE) has been shown to improve selectivity and stability at the cost of a negligible energy increase. The effectiveness of PE was assessed in a zero-gap membrane electrode assembly with a Cu catalyst between 50 and 500 mA cm-2 with various, widely adopted binders including Nafion, Sustainion, and fluorinated ethylene propylene. PE suppresses H2 production at 50-300 mA cm-2 and nearly doubles the faradaic efficiency of multi-carbon products when this approach is combined with the use of Sustainion-incorporating electrodes. We find that the notable improvement can be accounted for via the increased local CO2 concentration observed using in situ surface enhanced Raman spectroscopy.

The formation and study of oxygen vacancies are critical for the development of enhanced functional oxides; in the oxygen evolution reaction (OER), oxygen vacancies are proposed to influence the activity and degradation of electrocatalysts. We use thin films of state-of-the-art OER catalyst SrIrO 3 deposited on crystal substrates with varied lattice parameters to demonstrate the effect of epitaxial strain on oxygen vacancy formation. Through in situ x-ray diffraction under elevated temperatures and reducing conditions of 3% H 2 / balance N 2 , we show that tensile epitaxial strain makes oxygen vacancy formation more favorable, whereas compressive epitaxial strain has no significant effect compared with an unstrained film. We further use in situ and ex situ x-ray absorption spectroscopy to reveal the effect of strain on the favorability of full reduction of Ir species within SrIrO 3 films from Ir 4 + to Ir 0 and on the energy levels of unoccupied electronic states in the out -of -plane direction, respectively. This study adds experimental evidence for the link between strain and oxygen vacancy formation in 5 d thin film perovskites, for which the discussion has been dominated by theory -based approaches.

Perovskite oxynitrides, in addition to being promising electrocatalysts and photoabsorbers, present an interesting case study in crystal symmetry. Full or partial ordering of the O and N anions affects global symmetry and influences material performance and functionality; however, anion ordering is challenging to detect experimentally. In this work, we synthesize a novel perovskite oxynitride CaW(O,N)(3) and characterize its crystal structure using both X-ray and neutron diffraction. Through co-refinement of the diffraction patterns with a range of literature and theory-derived model structures, we demonstrate that CaW(O,N)(3) adopts an orthorhombic Pnma average structure and exhibits octahedral distortion with evidence for preferred anion site occupancy. However, through comparison with a large, low-symmetry unit cell, we identify the presence of disorder that is not fully accounted for by the high-symmetry model. We compare CaW(O,N)(3) with SrW(O,N)(3) to demonstrate the broader presence of such disorder and identify contrasting features in the electronic structures. This work signifies an updated perspective on the inherent crystal symmetry present in perovskite oxynitrides.

Linking fundamental insights with high performance for electrochemical hydrogen peroxide production using boron/nitrogen co-doped carbon catalysts in neutral pH.

The interaction between catalyst surfaces and adsorbed oxygen intermediates is critical to catalytic performance for electrochemical water oxidation to oxygen. However, the relationship between adsorption energetics and electrocatalytic activity is primarily assessed for pristine catalyst materials, which leaves much unknown about the dynamics of these properties in relationship to catalyst performance during long-term operation. In this work, we experimentally assess OH and O adsorption on Ca2IrO4 nanoparticles and monitor their evolution during extensive chronoamperometry tests at highly oxidizing potentials in a range of low pH electrolytes. In situ x-ray absorption spectroscopy reveals changes for surface adsorbate energetics and local iridium structures with applied potentials. Increasingly unfavorable adsorption of OH and formation of O intermediates after long-term operation is correlated with severe metal dissolution, distorted [IrO6] octahedral linkages, and a decreased average Ir valence. This work establishes connections between surface adsorption energetics, Ir structure, OER kinetics, and material stability outcomes.

Water electrolysis using proton exchange membrane technology offers an ideal process for green hydrogen production, but widespread deployment is inhibited by insufficient catalyst activity, stability and affordability. Iridium-based oxides provide the best overall performance for acidic water oxidation, the limiting reaction for this process, but further improvements are impeded by poor understanding of the restructured active catalyst surface that forms under reaction conditions. Here we present a combination of X-ray and electron scattering data that reveals direct evidence for three paracrystalline structural motifs at the restructured surfaces of highly active catalysts (including rutile IrO2 and perovskite SrIrO3) that have previously been described as amorphous. These insights enable the design of a paracrystalline IrOx catalyst that is independent of the bulk crystalline support and maintains higher activity, longer stability and more effective use of iridium to promote the production of green hydrogen. Iridium-based oxides are the most active catalysts for acidic water oxidation, but a complete understanding of their surface reconstruction under operation remains elusive. Now three key paracrystalline structural motifs are identified on the seemingly amorphous reconstructed IrOx surface.

An improved understanding of catalyst dynamics for the oxygen evolution reaction (OER) in acid is critical for informing the development of highly efficient, stable, and cost-effective OER catalysts for proton exchange membrane water electrolysis applications. Herein highly tunable, active, and dynamic Ir 5+ materials are studied, Ln(3)IrO(7) (Ln = Pr, Nd, Sm, and Eu). Leveraging a combination of in situ and ex situ characterization, as well as an advanced mercury underpotential deposition technique for Ir surface site quantification, the dynamic nature of Ln(3)IrO(7) materials throughout electrochemical activation under OER conditions is characterized. The trends are elucidated between intrinsic OER activity, surface Ir site quantity, and metal site dissolution behavior as tuned by the Ln site's atomic number. A critical relationship is uncovered to show that maintenance of excellent OER activity throughout performance testing is correlated with a catalysts' ability to preserve a high degree of Ir enrichment, where heightened stability of Ir sites interestingly parallels reduced stability of Ln sites throughout testing. It is found that as the Ln site's atomic number is decreased, the materials' intrinsic OER performance improves, due to an increased thermodynamic driving force for Ln dissolution, which is hypothesized to enable the maintenance of highly active Ir-based surface motifs.

Earth-abundant manganese-based oxides have emerged as promising alternatives to noble-metal-based catalysts for the oxygen evolution reaction (OER) in acidic conditions; however, their inferior activity and stability present critical challenges for the sustainable production of hydrogen via water electrolysis. Moving beyond oxides, heteroanionic materials, which incorporate anions with lower electronegativity than oxygen, have shown potential for improving the OER performance, but a detailed understanding of the underlying mechanisms is lacking. Here, we investigate manganese-based oxychlorides (Mn8O10Cl3 and FeMn7O10Cl3) that exhibit excellent activity and stability for acidic OER to elucidate material property dynamics and correlate them with OER behaviors. Our rigorous electrochemical stability testing reveals that the high operating potential mitigates Mn dissolution over prolonged exposure to the OER conditions. Through a combination of ex situ and in situ surface and bulk-sensitive X-ray spectroscopy analyses, we observe a trade-off between increasing Mn valence and maintaining structural integrity, which results in dynamic bond length changes within the [MnCl6] octahedra during the activation and degradation processes of these oxychloride catalysts. This study provides insights into the fundamental relationships between the chemical, electronic, and geometric properties of the catalysts and their electrocatalytic outcomes.

Increased demand for green hydrogen as part of a global effort to decarbonize will place a significant strain on global precious metal resources and drive up the price of electrolyzers. This study explores a new way of producing oxide electrode materials for proton exchange membrane electrolyzer cells (PEMECs) that may help decrease material and processing costs while potentially improving catalyst lifetime. The method is the so-called “exsolution” technique, previously used in high temperature catalysts and fuel cell electrodes (1). In exsolution, a high-temperature reduction process causes one or more cations in an oxide to be reduced to form surface metal nanoparticles. For example, Sr0.95Ti0.3Fe0.63Ru0.07O3-δ (STFR) has been shown to be an effective solid oxide cell fuel electrode where exsolution forms Ru-Fe nanoparticles that substantially enhance electrode performance (2). Here, we compare the ambient temperature catalytic activity for the acidic hydrogen evolution reaction (HER) of pristine STFR and STFR exsolved for varying durations at temperatures ranging from 500 to 800℃ in a gas mixture of 97% H2/3% H2O. The exsolved materials demonstrate superior HER activity and stability compared to pristine STFR. As shown in Figure 1, the initial overpotential required to achieve a current density of -10mA/cm2 geo was decreased from 388 mV without exsolution to 141 mV after 4h exsolution at 700 ℃. Overpotentials generally decrease further after 1000 LSV cycles. This paper will also show the effect of varying reduction times and gas compositions on the resulting electrocatalytic activity. Results for a different electrode composition, Sr0.95Ti0.3Fe0.63Ni0.07O3-δ (STFN), will also be presented. The electrochemical results will be related to the size and density of exsolved nanoparticles. Figure 1: Linear Sweep Voltammetry polarization curves measured at 5 mV/s in 0.5M H2SO4 at room temperature for as-prepared STFR electrodes compared with STFR electrodes that had been reduced at various temperatures for 4 hours in 97% H2, 3% H2O prior to testing. Solid lines show the first sweep and dashed lines show the polarization after 300 voltammetry cycles for STFR without exsolution, and 1000 CVs for the exsolved STFR electrodes. D. Neagu, J. T. S. Irvine, J. Wang, B. Yildiz, A. K. Opitz, J. Fleig, Y. Wang, J. Liu, L. Shen, F. Ciucci, B. A. Rosen, Y. Xiao, K. Xie, G. Yang, Z. Shao, Y. Zhang, J. M. Reinke, T. A. Schmauss, S. Barnett, R. Maring, V. Kyriakou, U. Mushtaq, M. N. Tsampas, Y. Kim, R. O'Hayre, A. J. Carrillo, T. Ruh, L. Lindenthal, F. Schrenk, C. Rameshan, E. I. Papaioannou, K. Kousi, I. Metcalfe, X. Xu and G. Liu, Journal of Physics: Energy (2023). R. Glaser, T. Zhu, H. Troiani, A. Caneiro, L. Mogni and S. Barnett, Journal of Materials Chemistry A, 6, 5193 (2018). Figure 1

Hydrogen peroxide (H2O2) can be produced electrochemically as a replacement for conventional anthraquinone oxidation routes. From a sustainability perspective, it would be best to use it in chemical applications without further purification or added cosolvents. Here, cyclohexene oxidation is carried out with dilute (0.08-0.25 M) aqueous H2O2 containing sulfate salts as a probe of the direct use of electrochemically produced oxidants. As catalysts, tungsten and molybdenum salts were combined with ammonium and imidazolium phase transfer agents. A mixture of [CH3(n-C8H17)(3)N][H2PO4] and [Et3NH][H2PO4] with Na2WO4 resulted in 95% overall yield of cyclohexene oxidation products-cyclohexene oxide (80%) and cyclohexene-1,2-diol (15%)-using a solution of 0.25 M H2O2 and 0.5 M K2SO4 in pH 5 water, with no added cosolvent. Results were validated with authentic electrochemically produced H2O2, demonstrating the practical applicability of this approach.

Using electron- and X-ray-based characterization techniques, three paracrystalline structural motifs are shown to form at the surface of amorphized iridium oxide catalysts upon use for water electrolysis in acidic conditions. An iridium oxide catalyst containing only these paracrystalline structural motifs achieves enhanced performance, making more efficient use of its limited iridium content.

Hydrogen peroxide (H2O2) synthesis via the electrocatalytic reduction of oxygen is a sustainable alternative to the energy-intensive anthraquinone oxidation process. The use of gas diffusion electrodes in dual membrane electrode assembly (MEA) solid electrolyte (SE) electrolyzers has substantially improved H2O2 production, but the influence of mass transport and local reaction environment on H2O2 performance in these cell architectures is still unclear and unoptimized. Herein, we investigate the impacts of electrode components and reactor operating conditions on the H2O2 performance and cell potential required to reach current densities up to 400 mA cm(-2). Results show an intermediate catalyst loading of 2 mg cm(-2) improves H2O2 production through balancing O-2 diffusion and active site exposure. Hydrophobic treatment via fluoropolymer improves electrode stability, but addition of >15 wt% fluoropolymer worsens performance, likely by limiting active site accessibility at the catalyst-membrane interface. Moreover, decreasing O-2 concentration from typical pure streams to match the composition of air has a negligible effect at moderate current densities (similar to 50 mA cm(-2)), but significantly impacts overall performance at higher current densities (similar to 200 mA cm(-2)). This work also highlights benefits of operating the reactor with a recycled product stream rather than tuning the flow rate of water over the SE to extremely low values to obtain high H2O2 concentrations, as the latter likely contributes to exacerbated H2O2 degradation in the electrolyzer. This work provides insights into how macroscale system properties in flow cell electrolyzers impact the local reaction environment and mass transport, which in turn dictate overall catalytic performance.

Research drives development of sustainable electrocatalytic technologies, but efforts are hindered by inconsistent reporting of advances in catalytic performance. Iridium-based oxide catalysts are widely studied for electrocatalytic technologies, particularly for the oxygen evolution reaction (OER) for proton exchange membrane water electrolysis, but insufficient techniques for quantifying electrochemically accessible iridium active sites impede accurate assessment of intrinsic activity improvements. We develop mercury underpotential deposition and stripping as a reversible electrochemical adsorption process to robustly quantify iridium sites and consistently normalize OER performance of benchmark IrO x electrodes to a single intrinsic activity curve, where other commonly used normalization methods cannot. Through rigorous deconvolution of mercury redox and reproportionation reactions, we extract net monolayer deposition and stripping of mercury on iridium sites throughout testing using a rotating ring disk electrode. This technique is a transformative method to standardize OER performance across a wide range of iridium-based materials and quantify electrochemical iridium active sites.

The production of Portland cement, the industry-standard cement, contributes similar to 8% of global CO2 emissions through fossil-fuel heating and decomposition of limestone (the primary cement raw material). Decarbonization, e.g., via direct electrification, of this 200-year-old liming routine is extremely challenging at the industry scale. We propose a scalable electrochemical decarbonization approach to circumvent the limestone use by switching to carbon-free calcium silicates from abundant minerals and recycled concrete. Water electrolysis produces protons and hydroxides to drive a pH gradient that accelerates Ca2+ ion leaching from calcium silicates and captures atmospheric CO2 to form carbon-negative CaCO3, which serves as the feedstock for cement manufacturing or as the carbon-mineralized product for cement substitution with permanent carbon storage. Value-added co-products amorphous silica and green H2 further enhance cement performance and supplant fossil fuels for net-zero transition, respectively. The products readily meet present-day regulatory standards and demands, and the approach readily synergizes with business-as-usual cement manufacturing and concrete construction, which are important for upscaling and structural safety, promising ready reception by the public and industries. Blended Portland cement produced through our approach with carbon-negative CaCO3 and silica demonstrates enhanced resilience and achieves carbon neutrality or negativity when incorporating storage or circulation of CO2 from cement plant flue gas, respectively. This low-cost, electrochemical cement production approach using abundant ubiquitous raw materials enables electrification, transition to clean fuel, and decarbonization at a gigaton scale.

Electrocatalysis is a promising alternative to many essential but unsustainable chemical production processes. One category of such processes is olefin epoxidations, which are necessary to produce important chemicals, including ethylene oxide, propylene oxide, and cyclohexene oxide. Herein, we demonstrate the use of a liquid diffusion electrode (LDE) reactor for the chlorine-mediated epoxidation of cyclohexene at high current densities. The LDE reactor geometry minimizes mass transport limitations that are often present in organic phase electrocatalysis, which must facilitate reactions of immiscible phases. Oxidation via a halide intermediate allows for >90% selectivity toward chlorocyclohexanol with subsequent conversion to cyclohexene oxide through contact with strong base. We report the performance of this reactor configuration as a function of pH, with low pH significantly enhancing Faradaic efficiency toward chlorocyclohexanol, and provide evidence via dynamic electrochemical mass spectroscopy to suggest that Cl2 is the active chlorine species responsible for oxidation. We then explore the effect of chloride concentration on the product profile as well as the effect of current density on product distribution and Faradaic efficiency. At 50 mA/cm(2), the LDE reactor reaches a peak of similar to 80% Faradaic efficiency and similar to 93% selectivity toward chlorocyclohexanol. At the highest operating current tested, 100 mA/cm2, the reactor maintains a Faradaic efficiency of similar to 65% and selectivity of similar to 90%.

Electrochemical reactions involving protons and hydroxide ions are significantly impacted by changes in the local pH near the catalyst surface. Therefore, it is useful to quantify the catalyst local pH to better understand the impact on overall reaction efficiency and selectivity. While it is difficult to experimentally probe the catalyst/electrolyte interface, this regime can be monitored indirectly using pH-sensitive materials. In this work, we investigate the use of a rotating ring-disk electrode coupled with a pH-sensing probe to track changes in proton concentration near the catalyst surface for the oxygen reduction reaction under well-defined mass transport conditions. We further examine the limitations and describe methods for improving the robustness of this experimental platform. Out of the electrode support and probe materials examined, we find that iridium oxide electrodeposited using cyclic voltammetry onto gold substrates exhibiting high surface area and moderate porosity demonstrates the highest, fastest, and most stable pH-potential response, enabling reliable measurements in under 10 s. Using an analytical convective-diffusion equation, we also estimate the disk local pH under varied operating conditions (e.g., current density and rotation rate) and reaction environments (e.g., bulk pH). This work outlines best practices for applying this technique and provides insights into the impact of relevant reaction environment conditions on the catalytic performance.

The development of active and acid-stable iridium-basedcatalystsis crucial to meet the requirements of proton exchange membrane technologiesfor the sustainable production of hydrogen via water electrolysis.However, long-term stability remains a critical challenge. In thiswork, we focus on a Ca2IrO4 catalyst to developa holistic picture of catalyst electronic and geometric structureevolution under various applied potentials by probing electrochemicallyactive surface area, metal dissolution, Ir valence, and surface morphology.We observe an initial activity increase in parallel with increasingcapacitance and minor iridium dissolution. Extensive chronoamperometrytests at oxidizing potentials lead to significant activity loss thatoccurs simultaneously with a dramatic drop in capacitance and a changein impedance. Using a combination of electrochemical and spectroscopictools, we provide fundamental insights to these material degradationprocesses to enable future catalyst design with balanced activityand long-term stability.

As renewably produced electricity becomes increasingly economical and abundant, electrocatalysis is expected to play a key role in the production of societally important fuels and chemicals to enable a sustainable future. In this work, we demonstrate the feasibility of a novel liquid diffusion electrode reactor design that enables the electrooxidation of neat cyclohexene. This design allows for the reaction of two immiscible liquids at an engineered electrode interface, forgoing the need for cosolvents that are necessary in single phase electroorganic reactions as well as facilitating opportunities for enhanced product separation. Cell performance is assessed as a function of operating potential and in the presence of a variety of catalysts shown to be active toward organic substrate oxidation in traditional heterogeneous catalysis. We find that carbon paper alone can oxidize cyclohexene into a variety of products and displays a Faradaic efficiency greater than 50% toward these products when operated at 1.8 V vs the reversible hydrogen electrode. Addition of metal oxide catalysts increases overall cell activity by up to three times without sacrificing Faradaic efficiency. Overall, this work demonstrates and characterizes the capabilities of a liquid diffusion electrode cell for the oxidation of pure organic compounds that are immiscible with water, enabling a new paradigm of organic electrocatalysis.

The stability of electrocatalysts is a concern for nearly all materials; degradation can occur via dissolution, leaching, sintering, amorphization, or reduction/oxidation processes. Extreme pH or large applied potentials often exacerbate these effects, but scant fundamental understanding of these processes exists due to complex structural and nanoscale effects in electrocatalysis. Instead, "catalyst stability" is often reported using broad electrode performance metrics, such as measured activity over time. To advance the fundamental understanding and comparison of catalyst materials, we propose that it is necessary to establish improved benchmarking metrics that reflect intrinsic material dynamics and stabilities of catalysts, supports, and substrates as a function of testing parameters, to complement existing metrics that primarily capture the performance of the complete electrode. We consider many degradation processes of lab-scale aqueous media systems, as well as membrane electrode assemblies and proton exchange membrane water electrolyzers, and consider the relatability between the two systems. Herein, we summarize various approaches to standardizing or benchmarking electrocatalyst performance, consider their strengths and weaknesses, and provide an outlook for advancing the rigor, specificity, and reproducibility of these techniques.