Continuum Modeling of Porous Electrodes for Electrochemical Synthesis

Utilizing an Integrated Flow Cathode-Membrane Filtration System for Effective and Continuous Electrochemical Hydrodechlorination

Pd-based electrodes are recognized to facilitate effective electrochemical hydrodechlorination (EHDC) as a result of their superior capacity for atomic hydrogen (H*) generation. However, challenges such as electrode stability, feasibility of treating complex matrices, and high cost associated with electrode synthesis hinder the application of Pd-based electrodes for EHDC. In this work, we investigated the feasibility of degrading 2,4-dichlorophenol (2,4-DCP) by EHDC employing Pd-loaded activated carbon particles, prepared via a simple wet-impregnation method, as a flow cathode (FC) suspension. Compared to other Pd-based EHDC studies, a much lower Pd loading (0.02-0.08 mg cm-2) was used. Because of the excellent mass transfer in the FC system, almost 100% 2,4-DCP was hydrodechlorinated to phenol within 1 h. The FC system also showed excellent performance in treating complex water matrices (including hardness ion-containing wastewater and various other chlorinated organics such as 2,4-dichlorobenzoic acid and trichloroacetic acid) with a relatively low energy consumption (0.26-1.56 kW h m-3 mg-1 of 2,4-DCP compared to 0.32-7.61 kW h m-3 mg-1 of 2,4-DCP reported by other studies). The FC synthesized here was stable over 36 h of continuous operation, indicating its potential suitability for real-world applications. Employing experimental investigations and mathematical modeling, we further show that hydrodechlorination of 2,4-DCP occurs via interaction with H*, with no role of direct electron transfer and/or HO•-mediated processes in the removal of 2,4-DCP.

Advancements in Asymmetric Supercapacitors: From Historical Milestones to Challenges and Future Directions

Numerous challenges, like the uninterrupted supply of electricity, stable and reliable power, and energy storage during non-operational hours, arise across various industries due to the absence of advanced energy storage technologies. With the continual technological advancements in portable electronics, green energy, and transportation, there are inherent limitations in their innovative production. Thus, ongoing research is focused on pursuing sustainable energy storage technologies. An emerging solution lies in the development of asymmetric supercapacitors (ASCs), which offer the potential to extend their operational voltage limit beyond the thermodynamic breakdown voltage range of electrolytes. This is achieved by employing two distinct electrode materials, presenting an effective solution to the energy storage limitations faced by ASCs. The current review concentrates on the progression of working materials to develop authentic pseudocapacitive energy storage systems (ESS). Also, evaluates their ability to exceed energy storage constraints. It provides insights into fundamental energy storage mechanisms, performance evaluation methodologies, and recent advancements in electrode material strategies. The review approaches developing high-performance electrode materials and achieving efficient ASC types. It delves into critical aspects for enhancing the energy density of ASCs, presenting debates and prospects, thereby offering a comprehensive understanding and design principles for next-generation ASCs in diverse applications.

Understanding Photovoltage Enhancement in Metal-Insulator Semiconductor Photoelectrodes with Metal Nanoparticles

A metal-insulator-semiconductor (MIS) structure holds great potential to promote photoelectrochemical (PEC) reactions, such as water splitting and CO2 reduction, for the storage of solar energy in chemical bonds. The semiconductor absorbs photons, creating electron-hole pairs; the insulator facilitates charge separation; and the metal collects the desired charge and facilitates its use in the electrochemical reaction. Despite these attractive features, MIS photoelectrodes are significantly limited by their photovoltage, a combination of the voltage generated from photon absorption minus the potential drop across the insulator. Herein, we use multiscale continuum modeling of the carrier, electrolyte, and interfacial transport to identify strategies for mitigating the deleterious potential drop across the insulator and enabling high MIS photovoltages. To this end, we model Ni/SiO2/n-Si photoanodes that employ a planar Ni film or Ni nanoparticles (np-MIS) and validate both models using experimental polarization curves and photovoltage measurements from the literature. The simulations reveal that the insulator potential drop is lower and hence achieves higher photovoltages for np-MIS structures than MIS structures because the electrolyte screens charge trapped at defect states between the semiconductor and the insulator. This electrolyte charge screening phenomenon can be further leveraged by using low loadings or small nanoparticles, which not only minimize the interfacial potential drop but also improve the photocurrent by enabling more light absorption. These insights contribute to the optimization of the np-MIS structures for sustainable energy conversion.

Rate-Determining Step for Electrochemical Reduction of Carbon Dioxide into Carbon Monoxide at Silver Electrodes

Silver is one of the most studied electrode materials for the electrochemical reduction of carbon dioxide into carbon monoxide, a product with many industrial applications. There is a growing number of reports in which silver is implemented in gas diffusion electrodes as part of a large-scale device to develop commercially relevant technology. Electrochemical models are expected to guide the design and operation toward cost-efficient devices. Despite decades of investigations, there are still uncertainties in the way this reaction should be modeled due to the absence of scientific consensus regarding the reaction mechanism and the nature of the rate-determining step. We review previously reported studies to draw converging conclusions on the value of the Tafel slope and existing species at the electrode surface. We also list conflicting experimental observations and provide leads to tackling these remaining questions.

CO2 Electrolyzers

CO2 electrolyzers have progressed rapidly in energy efficiency and catalyst selectivity toward valuable chemical feedstocks and fuels, such as syngas, ethylene, ethanol, and methane. However, each component within these complex systems influences the overall performance, and the further advances needed to realize commercialization will require an approach that considers the whole process, with the electrochemical cell at the center. Beyond the cell boundaries, the electrolyzer must integrate with upstream CO2 feeds and downstream separation processes in a way that minimizes overall product energy intensity and presents viable use cases. Here we begin by describing upstream CO2 sources, their energy intensities, and impurities. We then focus on the cell, the most common CO2 electrolyzer system architectures, and each component within these systems. We evaluate the energy savings and the feasibility of alternative approaches including integration with CO2 capture, direct conversion of flue gas and two-step conversion via carbon monoxide. We evaluate pathways that minimize downstream separations and produce concentrated streams compatible with existing sectors. Applying this comprehensive upstream-to-downstream approach, we highlight the most promising routes, and outlook, for electrochemical CO2 reduction.

Quantifying mass transport limitations in a microfluidic CO2 electrolyzer with a gas diffusion cathode

A gas diffusion electrode (GDE) based CO2 electrolyzer shows enhanced CO2 transport to the catalyst surface, significantly increasing current density compared to traditional planar immersed electrodes. A two-dimensional model for the cathode side of a microfluidic CO2 to CO electrolysis device with a GDE is developed. The model, validated against experimental data, examines key operational parameters and electrode materials. It predicts an initial rise in CO partial current density (PCD), peaking at 75 mA cm-2 at -1.3 V vs RHE for a fully flooded catalyst layer, then declining due to continuous decrease in CO2 availability near the catalyst surface. Factors like electrolyte flow rate and CO2 gas mass flow rate influence PCD, with a trade-off between high CO PCD and CO2 conversion efficiency observed with increased CO2 gas flow. We observe that a significant portion of the catalyst layer remains underutilized, and suggest improvements like varying electrode porosity and anisotropic layers to enhance mass transport and CO PCD. This research offers insights into optimizing CO2 electrolysis device performance.

Multiscale effects in tandem CO2 electrolysis to C2+ products

CO2 electrolysis is a sustainable technology capable of accelerating global decarbonisation through the production of high-value alternatives to fossil-derived products. CO2 conversion can generate critical multicarbon (C2+) products such as drop-in chemicals ethylene and ethanol, however achieving high selectivity from single-component catalysts is often limited by the competitive formation of C1 products. Tandem catalysis can overcome C2+ selectivity limitations through the incorporation of a component that generates a high concentration of CO, the primary reactant involved in the C-C coupling step to form C2+ products. A wide range of approaches to promote tandem CO2 electrolysis have been presented in recent literature that span atomic-scale manipulation to device-scale engineering. Therefore, an understanding of multiscale effects that contribute to selectivity alterations are required to develop effective tandem systems. In this review, we use relevant examples to highlight the complex and interlinked contributions to selectivity and provide an outlook for future development of tandem CO2 electrolysis systems.

Impact of Anodic Oxidation Reactions in the Performance Evaluation of High-Rate CO2 /CO Electrolysis

The membrane-electrode assembly (MEA) approach appears to be the most promising technique to realize the high-rate CO2 /CO electrolysis, however there are major challenges related to the crossover of ions and liquid products from cathode to anode via the membrane and the concomitant anodic oxidation reactions (AORs). In this perspective, by combining experimental and theoretical analyses, several impacts of anodic oxidation of liquid products in terms of performance evaluation are investigated. First, the crossover behavior of several typical liquid products through an anion-exchange membrane is analyzed. Subsequently, two instructive examples (introducing formate or ethanol oxidation during electrolysis) reveals that the dynamic change of the anolyte (i.e., pH and composition) not only brings a slight shift of anodic potentials (i.e., change of competing reactions), but also affects the chemical stability of the anode catalyst. Anodic oxidation of liquid products can also cause either over- or under-estimation of the Faradaic efficiency, leading to an inaccurate assessment of overall performance. To comprehensively understand fundamentals of AORs, a theoretical guideline with hierarchical indicators is further developed to predict and regulate the possible AORs in an electrolyzer. The perspective concludes by giving some suggestions on rigorous performance evaluations for high-rate CO2 /CO electrolysis in an MEA-based setup.

Unveiling the spatially confined oxidation processes in reactive electrochemical membranes

Electrocatalytic oxidation offers opportunities for sustainable environmental remediation, but it is often hampered by the slow mass transfer and short lives of electro-generated radicals. Here, we achieve a four times higher kinetic constant (18.9 min-1) for the oxidation of 4-chlorophenol on the reactive electrochemical membrane by reducing the pore size from 105 to 7 μm, with the predominate mechanism shifting from hydroxyl radical oxidation to direct electron transfer. More interestingly, such an enhancement effect is largely dependent on the molecular structure and its sensitivity to the direct electron transfer process. The spatial distributions of reactant and hydroxyl radicals are visualized via multiphysics simulation, revealing the compressed diffusion layer and restricted hydroxyl radical generation in the microchannels. This study demonstrates that both the reaction kinetics and the electron transfer pathway can be effectively regulated by the spatial confinement effect, which sheds light on the design of cost-effective electrochemical platforms for water purification and chemical synthesis.

Pathways to enhance electrochemical CO2 reduction identified through direct pore-level modeling

Electrochemical conversion of CO2 to fuels and valuable products is one pathway to reduce CO2 emissions. Electrolyzers using gas diffusion electrodes (GDEs) show much higher current densities than aqueous phase electrolyzers, yet models for multi-physical transport remain relatively undeveloped, often relying on volume-averaged approximations. Many physical phenomena interact inside the GDE, which is a multiphase environment (gaseous reactants and products, liquid electrolyte, and solid catalyst), and a multiscale problem, where "pore-scale" phenomena affect observations at the "macro-scale". We present a direct (not volume-averaged) pore-level transport model featuring a liquid electrolyte domain and a gaseous domain coupled at the liquid-gas interface. Transport is resolved, in 2D, around individual nanoparticles comprising the catalyst layer, including the electric double layer and steric effects. The GDE behavior at the pore-level is studied in detail under various idealized catalyst geometries configurations, showing how the catalyst layer thickness, roughness, and liquid wetting behavior all contribute to (or restrict) the transport necessary for CO2 reduction. The analysis identifies several pathways to enhance GDE performance, opening the possibility for increasing the current density by an order of magnitude or more. The results also suggest that the typical liquid-gas interface in the GDE of experimental demonstrations form a filled front rather than a wetting film, the electrochemical reaction is not taking place at a triple-phase boundary but rather a thicker zone around the triple-phase boundary, the solubility reduction at high electrolyte concentrations is an important contributor to transport limitations, and there is considerable heterogeneity in the use of the catalyst. The model allows unprecedented visualization of the transport dynamics inside the GDE across multiple length scales, making it a key step forward on the path to understanding and enhancing GDEs for electrochemical CO2 reduction.

Photocatalytic CO2 Reduction with Dissolved Carbonates and Near-Zero CO2(aq) by Employing Long-Range Proton Transport

Photocatalytic CO₂ reduction (CO₂R) in ∼0 mM CO₂(aq) concentration is challenging but is relevant for capturing CO₂ and achieving a circular carbon economy. Despite recent advances, the interplay between the CO₂ catalytic reduction and the oxidative redox processes that are arranged on photocatalyst surfaces with nanometer-scale distances is less studied. Specifically, mechanistic investigation on interdependent processes, including CO₂ adsorption, charge separation, long-range chemical transport (∼100 nm distance), and bicarbonate buffer speciation, involved in photocatalysis is urgently needed. Photocatalytic CO₂R in ∼0 mM CO₂(aq), which has important applications in integrated carbon capture and utilization (CCU), has rarely been studied. Using 0.1 M KHCO₃ (aq) of pH 7 but without continuously bubbling CO₂, we achieved ∼0.1% solar-to-fuel conversion efficiency for CO production using Ag@CrO_x nanoparticles that are supported on a coating-protected GaInP₂ photocatalytic panel. CO is produced at ∼100% selectivity with no detectable H₂, even with copious protons co-generated nearby. CO₂ flux to the Ag@CrO_x CO₂R sites enhances CO₂ adsorption, probed by in situ Raman spectroscopy. CO is produced with local protonation of dissolved inorganic carbon species in a pH as high as 11.5 when using fast electron donors such as ethanol. Isotopic labeling using KH¹³CO₃ was used to confirm the origin of CO from the bicarbonate solution. We then employed COMSOL Multiphysics modeling to simulate the spatial and temporal pH variation and the local concentrations of bicarbonates and CO₂(aq). We found that light-driven CO₂R and CO₂ reactive transport are mutually dependent, which is important for further understanding and manipulating CO₂R activity and selectivity. This study enables direct bicarbonate utilization as the source of CO₂, thereby achieving CO₂ capture and conversion without purifying and feeding gaseous CO₂.

Theory and Simulation of Metal-Insulator-Semiconductor (MIS) Photoelectrodes

A metal-insulator-semiconductor (MIS) structure is an attractive photoelectrode-catalyst architecture for promoting photoelectrochemical reactions, such as the formation of H₂ by proton reduction. The metal catalyzes the generation of H₂ using electrons generated by photon absorption and charge separation in the semiconductor. The insulator layer between the metal and the semiconductor protects the latter element from photo-corrosion and, also, significantly impacts the photovoltage at the metal surface. Understanding how the insulator layer determines the photovoltage and what properties lead to high photovoltages is critical to the development of MIS structures for solar-to-chemical energy conversion. Herein, we present a continuum model for charge-carrier transport from the semiconductor to the metal with an emphasis on mechanisms of charge transport across the insulator. The polarization curves and photovoltages predicted by this model for a Pt/HfO₂/p-Si MIS structure at different HfO₂ thicknesses agree well with experimentally measured data. The simulations reveal how insulator properties (i.e., thickness and band structure) affect band bending near the semiconductor/insulator interface and how tuning them can lead to operation closer to the maximally attainable photovoltage, the flat-band potential. This phenomenon is understood by considering the change in tunneling resistance with insulator properties. The model shows that the best MIS performance is attained with highly symmetric semiconductor/insulator band offsets (e.g., BeO, MgO, SiO₂, HfO₂, or ZrO₂ deposited on Si) and a low to moderate insulator thickness (e.g., between 0.8 and 1.5 nm). Beyond 1.5 nm, the density of filled interfacial trap sites is high and significantly limits the photovoltage and the solar-to-chemical conversion rate. These conclusions are true for photocathodes and photoanodes. This understanding provides critical insight into the phenomena enhancing and limiting photoelectrode performance and how this phenomenon is influenced by insulator properties. The study gives guidance toward the development of next-generation insulators for MIS structures that achieve high performance.

pH Effects in a Model Electrocatalytic Reaction Disentangled

Varying the solution pH not only changes the reactant concentrations in bulk solution but also the local reaction environment (LRE) that is shaped furthermore by macroscopic mass transport and microscopic electric double layer (EDL) effects. Understanding ubiquitous pH effects in electrocatalysis requires disentangling these interwoven factors, which is a difficult, if not impossible, task without physical modeling. Herein, we demonstrate how a hierarchical model that integrates microkinetics, double-layer charging, and macroscopic mass transport can help understand pH effects of the formic acid oxidation reaction (FAOR). In terms of the relation between the peak activity and the solution pH, intrinsic pH effects without consideration of changes in the LRE would lead to a bell-shaped curve with a peak at pH = 6. Adding only macroscopic mass transport, we can already reproduce qualitatively the experimentally observed trapezoidal shape with a plateau between pH 5 and 10 in perchlorate and sulfate solutions. A quantitative agreement with experimental data requires consideration of EDL effects beyond Frumkin correlations. Specifically, the peculiar nonmonotonic surface charging relation affects the free energies of adsorbed intermediates. We further discuss pH effects of FAOR in phosphate and chloride-containing solutions, for which anion adsorption becomes important. This study underpins the importance of a full consideration of multiple interrelated factors for the interpretation of pH effects in electrocatalysis.

Disordered Au Nanoclusters for Efficient Ammonia Electrosynthesis

The electrochemical nitrogen (N₂ ) reduction reaction (N₂ RR) under mild conditions is a promising and environmentally friendly alternative to the traditional Haber-Bosch process with high energy consumption and greenhouse emission for the synthesis of ammonia (NH₃ ), but high-yielding production is rendered challenging by the strong nonpolar N≡N bond in N₂ molecules, which hinders their dissociation or activation. In this study, disordered Au nanoclusters anchored on two-dimensional ultrathin Ti₃ C₂ T_x MXene nanosheets are explored as highly active and selective electrocatalysts for efficient N₂ -to-NH₃ conversion, exhibiting exceptional activity with an NH₃ yield rate of 88.3±1.7 μg h^-1  mg_cat. ^-1 and a faradaic efficiency of 9.3±0.4 %. A combination of in situ near-ambient pressure X-ray photoelectron spectroscopy and operando X-ray absorption fine structure spectroscopy is employed to unveil the uniqueness of this catalyst for N₂ RR. The disordered structure is found to serve as the active site for N₂ chemisorption and activation during the N₂ RR process.

Anion-exchange membranes with internal microchannels for water control in CO2 electrolysis

Electrochemical reduction of carbon dioxide (CO2R) poses substantial promise to convert abundant feedstocks (water and CO2) to value-added chemicals and fuels using solely renewable energy. However, recent membrane-electrode assembly (MEA) devices that have been demonstrated to achieve high rates of CO2R are limited by water management within the cell, due to both consumption of water by the CO2R reaction and electro-osmotic fluxes that transport water from the cathode to the anode. Additionally, crossover of potassium (K+) ions poses concern at high current densities where saturation and precipitation of the salt ions can degrade cell performance. Herein, a device architecture incorporating an anion-exchange membrane (AEM) with internal water channels to mitigate MEA dehydration is proposed and demonstrated. A macroscale, two-dimensional continuum model is used to assess water fluxes and local water content within the modified MEA, as well as to determine the optimal channel geometry and composition. The modified AEMs are then fabricated and tested experimentally, demonstrating that the internal channels can both reduce K+ cation crossover as well as improve AEM conductivity and therefore overall cell performance. This work demonstrates the promise of these materials, and operando water-management strategies in general, in handling some of the major hurdles in the development of MEA devices for CO2R.

Electrochemical Reduction of CO2 in Tubular Flow Cells under Gas-Liquid Taylor Flow

Electrochemical reduction of CO₂ using renewable energy is a promising avenue for sustainable production of bulk chemicals. However, CO₂ electrolysis in aqueous systems is severely limited by mass transfer, leading to low reactor performance insufficient for industrial application. This paper shows that structured reactors operated under gas-liquid Taylor flow can overcome these limitations and significantly improve the reactor performance. This is achieved by reducing the boundary layer for mass transfer to the thin liquid film between the CO₂ bubbles and the electrode. This work aims to understand the relationship between process conditions, mass transfer, and reactor performance by developing an easy-to-use analytical model. We find that the film thickness and the volume ratio of CO₂/electrolyte fed to the reactor significantly affect the current density and the faradaic efficiency. Additionally, we find industrially relevant performance when operating the reactor at an elevated pressure beyond 5 bar. We compare our predictions with numerical simulations based on the unit cell approach, showing good agreement for a large window of operating parameters, illustrating when the easy-to-use predictive expressions for the current density and faradaic efficiency can be applied.