Abstract Commercial‐scale generation of carbon‐containing chemicals and fuels by means of electrochemical CO 2 reduction (CO 2 R) requires electrolyzers operating at high current densities and product selectivities. Membrane‐electrode assemblies (MEAs) have been shown to be suitable for this purpose. In such devices, the cathode catalyst layer controls both the rate of CO 2 R and the distribution of products. In this study, we investigate how the ionomer‐to‐catalyst ratio (I:Cat), catalyst loading, and catalyst‐layer thickness influence the performance of a cathode catalyst layer containing Ag nanoparticles supported on carbon. In this paper, we explore how these parameters affect the cell performance and establish the role of the exchange solution (water vs. CsHCO 3 ) behind the anode catalyst layer in cell performance. We show that a high total current density is best achieved using an I:Cat ratio of 3 at a Ag loading of 0.01–0.1 mg Ag /cm 2 and with a 1.0 M solution of CsHCO 3 circulated behind the anode catalyst layer. For these conditions, the optimal CO partial current density depends on the voltage applied to the MEA. The work also reveals that the performance of the cathode catalyst layer is limited by a combination of the electrochemically active surface area and the degree to which mass transfer of CO 2 to the surface of the Ag nanoparticles and the transport of OH − anions away from it limit the overall catalyst activity. Hydration of the ionomer in the cathode catalyst layer is found not to be an issue when using an exchange solution. The insights gained allowed for a Ag CO 2 R MEA that operates between 200 mA/cm 2 and 1 A/cm 2 with CO faradaic efficiencies of 78–91%, and the findings and understanding gained herein should be applicable to a broad range of CO 2 R MEA‐based devices.
Isolated hafnium (Hf) sites were prepared on Silicalite-1 and SiO<sub>2</sub> and investigated for acetone conversion to isobutene. Characterization by IR, <sup>1</sup>H MAS NMR, and UV-vis spectroscopy suggests that Hf atoms are bonded to the support via three O atoms and have one hydroxyl group, i.e, (≡SiO)<sub>3</sub>Hf-OH. In the case of Hf/Silicalite-1, Hf-OH groups hydrogen bond with adjacent Si-OH to form (≡SiO)<sub>3</sub>Hf-OH···HO-Si≡ complexes. The turnover frequency for isobutene formation from acetone is 4.5 times faster over Hf/Silicalite-1 than Hf/SiO<sub>2</sub>. Lewis acidic Hf sites promote the aldol condensation of acetone to produce mesityl oxide (MO), which is the precursor to isobutene. For Hf/SiO<sub>2</sub>, both Hf sites and Si-OH groups are responsible for the decomposition of MO to isobutene and acetic acid, whereas for Hf/Silicalite-1, the (≡SiO)<sub>3</sub>Hf-OH···HO-Si≡ complex is the active site. Measured reaction kinetics show that the rate of isobutene formation over Hf/SiO<sub>2</sub> and Hf/Silicalite-1 is nearly second order in acetone partial pressure, suggesting that the rate-limiting step involves formation of the C-C bond between two acetone molecules. The rate expression for isobutene formation predicts a second order dependence in acetone partial pressure at low partial pressures and a decrease in order with increasing acetone partial pressure, in good agreement with experimental observation. The apparent activation energy for isobutene formation from acetone over Hf/SiO<sub>2</sub> is 116.3 kJ/mol, while that for Hf/Silicalite-1 is 79.5 kJ/mol. The lower activation energy for Hf/Silicalite-1 is attributed to enhanced adsorption of acetone and formation of a C-C bond favored by the H-bonding interaction between Hf-OH and an adjacent Si-OH group.
Abstract Cation exchanged-zeolites are functional materials with a wide range of applications from catalysis to sorbents. They present a challenge for computational studies using density functional theory due to the numerous possible active sites. From Al configuration, to placement of extra framework cation(s), to potentially different oxidation states of the cation, accounting for all these possibilities is not trivial. To make the number of calculations more tractable, most studies focus on a few active sites. We attempt to go beyond these limitations by implementing a workflow for a high throughput screening, designed to systematize the problem and exhaustively search for feasible active sites. We use Pd-exchanged CHA and BEA to illustrate the approach. After conducting thousands of individual calculations, we identify the sites most favorable for the Pd cation and discuss the results in detail. The high throughput screening identifies many energetically favorable sites that are non-trivial. Lastly, we employ these results to examine NO adsorption in Pd-exchanged CHA, which is a promising passive NOx adsorbent (PNA) during the cold start of automobiles. The results shed light on critical active sites for NOx capture that were not previously studied.
Propene and 1,3-butadiene are important building-block chemicals that can be produced by dehydrogenation of propane and butane on Pt catalysts. A challenge is to develop highly active and selective catalysts that are resistant to deactivation by Pt sintering and coke formation. We have recently shown (Qi , J. Am. Chem. Soc. 2021, 143, 21364−21378) that these objectives can be met for propane dehydrogenation (PDH) using atomically dispersed Pt anchored to neighboring ≡SiOZn-OH groups bonded to the framework of dealuminated zeolite BEA. In the present study, we demonstrate that significantly superior performance can be achieved using self-pillared pentasil (SPP) zeolite nanosheets as supports. Following catalyst reduction in H2, atomic resolution, scanning transmission electron microscopy (STEM), and X-ray absorption spectroscopy (XAS) indicate that Pt is stabilized in structures well approximated as (≡Si-O-Zn)4-5Pt. These species are highly active, selective, and stable for PDH to give propene and for n-butane dehydrogenation (BDH) to give 1,3-butadiene. No catalyst deactivation was observed after 12 days of time on stream, and the selectivity remained at nearly 100% for PDH conducted at 823 K and a weight hourly space velocity (WHSV) of 1350 h–1. The apparent rate coefficient for PDH on this catalyst is significantly higher than that reported previously for Pt-containing catalysts. For BDH at 823 K and a WHSV of 3560 h–1, the selectivity to butene isomers and 1,3-butadiene is 98.9%, and the selectivity to 1,3-butadiene is 45%. We propose that the high catalyst stability observed during PDH and BDH is a consequence of a large fraction of the Pt-containing centers being located on the external surface of the zeolite nanosheets, where nascent coke precursors can desorb before condensing to form coke.
Zeolites are widely used as catalysts for the processing of petroleum to produce transportation fuels, the synthesis of a wide variety of chemicals, and for the abatement of automotive emissions. These applications have stimulated an interest in describing the mechanism and kinetics for zeolite-catalyzed reactions using theoretical methods. This Mini-review summarizes the author's efforts towards this goal. It is shown that accurate predictions of adsorption and activation enthalpies and entropies requires that several criteria be met. The first is a correct description of the structure of the catalytically active center, as well as the portion of the zeolite framework immediately surrounding the active center and that located far from the active center. Second, the level of density functional theory (DFT) must be sufficiently high to account for the effects of dispersive interactions between the adsorbate, the active center, and the immediately surrounding zeolite atoms. Third, dispersive and coulombic interactions between the atoms in the vicinity of the active center and the balance of the zeolite framework must also be accounted for. It is shown that these conditions can be met using hybrid quantum mechanics/molecular mechanics (QM/MM) together with a high-level exchange-correlation functional and a large basis set. The success of our QM/MM approach is illustrated for reactions of light alkanes in H-MFI, as well as other protonated zeolites, and in Ga/H-MFI. We show that for low temperatures (<400 K), the QM/MM approach gives good predictions of molecular adsorption enthalpies and activation enthalpies for elementary reactions. This is also true for higher temperatures (>400 K) if the effects of configuration are considered using a correction obtained from configurationally biased Monte Carlo (CBMC) calculations. Calculations of the molecular adsorption entropy and the activation entropy for elementary reactions are more difficult to predict accurately. Application of the quasi-rigid rotor harmonic approximation overpredicts the loss of entropy of adsorption from the gas phase, particularly for zeolites containing large cavities and channels. CBMC corrections capture this deviation well for molecular adsorption and for early transition states resembling the adsorbed state but are inadequate for late transition states involving two loosely associated fragments.
The B-site cation ordering of Ba(Mg 1/3 Ta 2/3 )O 3 microwave dielectrics with the complex perovskite structure has been studied using a combination of first-principles calculations, a cluster expansion technique, and Monte Carlo simulations. Our calculations confirm the experimentally observed hexagonal superstructure with space group P3m1 (D 3 3d ) as the ground state. The order-disorder transition between the low-temperature 1:2 ordered hexagonal phase (P3m1) and high-temperature simple perovskite phase (Pm3m) is predicted to occur at ∼3770 K. This indicates that Ba(Mg 1/3 Ta 2/3 )O 3 in equilibrium should be fully ordered at all practical temperatures. Sintering at high temperature for a long time or prolonging the anneal should therefore be effective in enhancing the degree of cation order in Ba(Mg 1/3 Ta 2/3 )O 3 . The charge density distribution and one electron density of states (DOS) for the 1:2 ordered structure indicate that Ta and O atoms possess some degree of covalency with some overlap between the O-2 p orbitals and the Ta-5 d orbitals.
The adsorption of <em>n</em>-alkanes onto Brønsted-acid sites is a key step in the catalytic cracking of alkanes. Employing configurational-bias Monte Carlo simulations, we have investigated how the ratio of equilibrium adsorption constants for central C-C bonds relative to terminal bonds of <em>n</em>-alkanes (i.e., the adsorption selectivity ratio) in Brønsted-acid zeolites is influenced by the Si/Al ratio and the Al distribution. A new computational approach was implemented, and the developed force field was validated by a comprehensive comparison between simulation results and experimental data for a number of Brønsted-acid zeolites. While the adsorption selectivity seems to be relatively insensitive to the Si/Al ratio, our results reveal that the Al distribution plays a crucial role in determining the adsorption selectivity. Changes in the Al distribution result in a change of as much as 2-fold in the adsorption selectivity ratio for <em>n</em>-hexane. The selectivity generally shows larger variations with respect to Al distribution in zeolites with a larger Si/Al ratio. The two factors identified by this work that substantially influence the selectivity ratio are the siting of Al atoms among T-sites and their spatial proximity, and an atomic-level understanding of each of these effects was achieved. The siting of Al atoms at more or less selective T-sites significantly influences the overall selectivity ratio, and Al atoms in close proximity can synergistically enhance the adsorption of central C-C bonds, leading to a higher selectivity ratio relative to isolated Al atoms. Finally, we anticipate that these results will have important implications for future large-scale computational screenings and the development of advanced synthesis approaches to target certain Al distributions in zeolites.
This paper focuses on the migration patterns of residents of the community of San Mateo, San Pedro, Belize.Recent research conducted in the community of San Mateo by Florida State University faculty and students revealed numerous vacant lots along the coast and in areas where water has risen and not been displaced.In total, the vacant lots accounted for nearly a quarter of the community.Drawing from these findings, this paper argues that these vacant lots are due to environmental changes in the community, most likely attributed to climate change.This paper will examine environmental migration in San Mateo as well as availability of resources such as electricity and water.The ultimate goal of this research is to assess how the resettlement of environmental migrants is impacted by the availability of resources.The paper will subsequently illustrate that Central America is especially prone to the effects of climate change, and that this directly impacts the most vulnerable, coastal communities.
Objective evaluation of the performance of electrocatalysts for CO<sub>2</sub> reduction has been complicated by a lack of standardized methods for measuring and reporting activity data. In this perspective, we advocate that standardizing these practices can aid in advancing research efforts toward the development of efficient and selective CO<sub>2</sub> reduction electrocatalysts. Using information taken from experimental studies, we identify variables that influence the measured activity of CO<sub>2</sub> reduction electrocatalysts and propose procedures to account for these variables in order to improve the accuracy and reproducibility of reported data. We recommend that catalysts be measured under conditions which do not introduce artifacts from impurities, from either the electrolyte or counter electrode, and advocate the acquisition of data measured in the absence of mass transport effects. Furthermore, measured rates of electrochemical reactions should be normalized to both the geometric electrode area as well as the electrochemically active surface area to facilitate the comparison of reported catalysts with those previously known. We demonstrate that, when these factors are accounted for, the CO<sub>2</sub> reduction activities of Ag and Cu measured in different laboratories exhibit little difference. Furthermore, adoption of the recommendations presented in this perspective would greatly facilitate the identification of superior catalysts for CO<sub>2</sub> reduction arising solely from changes in their composition and pretreatment.
Scaling up CO 2 electrolysis is a vital aspect in the transition to manufacturing sustainable fuels and chemical compounds, satisfying the demand for chemicals and demand for storing renewable electricity. Depending on the employed catalyst, different products can be produced, such as carbon monoxide and ethylene, by applying a voltage on a CO 2 and H 2 O fed electrolyzer. CO is a desirable product according to techno-economic analysis, because it can be produced selectively using a silver catalyst and is a precursor for hydrocarbons in the Fischer–Tropsch process. In a state-of-the-art CO 2 electrolyzer, two electrodes are directly pressed against an ion-exchange membrane – this is called a zero-gap configuration. Therefore, the membrane is a crucial component for the system, since it has the role of providing a conductive medium between the electrodes. One of the challenges in CO 2 electrolysis is that water is consumed in the reaction. At high current density, this may cause the membrane's surface near the cathode to dry out, lowering efficiency or perhaps stopping the process entirely, since there is no longer a conductive medium. As a result, water management is critical for this process. In this work, we approach the drying-out challenge by studying the novel concept of a membrane with internal microchannels. These channels allow the circulation of water or an electrolyte inside the membrane, which reduces the water diffusion path and affects the membrane’s conductivity. The effects of channel geometry, location, and concentrations of electrolyte inside on water content, conductivity and overall performance are studied in a 2D COMSOL model. In addition, the effect of internal concentration of electrolyte on the membrane’s resistance, on the process performance and the K + cross-over to cathode side were investigated experimentally. Our modeling results prove that the presence of the channels can keep the membrane hydrated. The highest current densities are observed when the channel is closest to the cathode, and with smaller pores. Smaller pores are advantageous due to the trade-off between enhanced membrane conductivity and the lower conductivity of the liquid itself. If water is circulated in a large channel, it increases the membrane’s ionomer conductivity due to hydration but the overall conductivity is decreased since water is not highly conductive. Nonetheless, the results also show that a higher concentration of electrolyte inside the microchannels can significantly increase the total conductivity of the membrane, and therefore the energy efficiency of the process. These effects are most significant at higher current densities. In the experimental results, we’ve observed similar effects in terms of membrane conductivity and current density of the process – the higher the electrolyte concentration the higher the current density. Furthermore, a low concentration decreases the amount of potassium which crosses over to the cathode side, inhibiting salt deposition. We’ve concluded that a small channel, up to 90 µm wide, close to the pore with an electrolyte with a concentration of up to 10 mM could be very beneficial for the water management and energy efficiency of the process. This helps to keep the membrane hydrated at higher current densities, improves the conductivity of the membrane, and it doesn’t have significant impacts on the salt deposition. Figure 1
Recent events highlight the importance of energy generation and distribution of power in today's society. It is clear that while additional large-scale power generation will have to address long-term societal issues such as safety and environmental impact, enhancing the efficiency of our day-to-day devices (transport, heating/refrigeration, home electronics, etc.) requires mainly the development of new materials with improved properties. This MRS volume, first published in 2002, focuses on materials issues in the low-scale generation, storage and transport of energy-photovoltaics, batteries, fuel cells and other small-scale devices. Materials limitations of these technologies are also addressed. Topics include: lithium batteries; hydrogen fuel cells and hydrogen storage; materials for solar energy; solid oxide fuel cells; materials for power in space; disordered and nanoscale materials for energy applications; and thermoelectrics.
Bipolar membranes (BPMs) possess the potential to optimize pH environments for electrochemical synthesis applications when employed in reverse bias. Unfortunately, the performance of BPMs in reverse bias has long been limited by the rate of water dissociation (WD) occurring at the interface of the BPM. Herein, we develop a continuum model of the BPM that agrees with experiment to understand and enhance WD catalyst performance by considering multiple kinetic pathways for WD in the BPM junction catalyst layer. Here, the model reveals that WD catalysts with a more highly alkaline or acidic pH at the point of zero charge (pH<sub>PZC</sub>) exhibit accelerated WD kinetics because the more acidic or alkaline pH<sub>PZC</sub> catalysts possess greater surface charge, enhancing the local electric field and rate of WD. The model is then employed to explore the sensitivity of the BPM performance to various BPM physical parameters. Finally, the model is used to simulate the operation of bimetallic WD catalysts, demonstrating that an optimal bimetallic catalyst has an acidic pH<sub>PZC</sub> catalyst matched with the cation-exchange layer and an alkaline pH<sub>PZC</sub> catalyst matched with the anion-exchange layer. The study provides insight into the operation of BPM WD catalysts and gives direction toward the development of next-generation WD catalysts for optimal BPM performance under water-splitting and related conditions.
Devising a computational tool that assesses the thermodynamic stability of materials is among the most important steps required to build a ``virtual laboratory'', where materials could be designed from first-principles without relying on experimental input. Although the formalism that allows the calculation of solid state phase diagrams from first principles is well established, its practical implementation remains a tedious process. The development of a fully automated algorithm to perform such calculations serves two purposes. First, it will make this powerful tool available to large number of researchers. Second, it frees the calculation process from arbitrary parameters, guaranteeing that the results obtained are truly derived from the underlying first-principles calculations. The proposed algorithm formalizes the most difficult step of phase diagram calculations, namely the determination of the ``cluster expansion'', which is a compact representation of the configurational dependence of the alloy's energy. This is traditionally achieved by a fit of the unknown interaction parameters of the cluster expansion to a set of structural energies calculated from first-principles. We present a formal statistical basis for the selection of both the interaction parameters to include in the cluster expansion and of the structures to use in order to determine them. The proposed method relies on the concepts of cross-validation and variance minimization. An application to the calculation of the phase diagram of the Si-Ge, CaO-MgO, Ti-Al, and Cu-Au systems is presented.
Read moreElectrochemical CO 2 reduction (CO 2 R) is a promising technology that could enable electricity generated from intermittent renewable sources to be stored in the form of carbon-neutral fuels and chemical precursors. Currently, metallic copper (Cu) is the only known electrocatalyst capable of reducing CO 2 to hydrocarbons and alcohols. However, polycrystalline Cu produces up to 16 different reaction products at an applied potential of -1 V vs RHE, with hydrogen, methane, and ethene accounting for the majority of the charge passed. This lack of selectivity has motivated considerable interest in discovering ways to modify the Cu surface that enhance the reaction selectivity to a single desired product. Of particular interest are C 2 products, such as ethanol and ethene, due to their high market value and fuel potential. Prior research efforts have identified several under coordinated Cu single crystal electrodes that exhibit an enhanced selectivity for C 2+ products compared to polycrystalline Cu; however, such electrocatalysts are not scalable and are likely unstable. The selectivity of ethene relative to methane has also been enhanced by nanostructuring the Cu surface by electrochemical cycling and by the electrodeposition of Cu (I) oxide thin-films. However, these electrocatalysts are generally more selective for the formation of hydrogen than hydrocarbons. We note that all reports in the current literature observe a higher selectivity for hydrocarbons than oxygenates over metallic Cu and that there are currently no known methods of enhancing the oxygenate selectivity. Carbon monoxide (CO) reduction has been identified both experimentally and theoretically as the overpotential-determining step in the reduction of CO 2 to hydrocarbons and alcohols over polycrystalline Cu. CO is known to cover a substantial portion of the Cu surface at steady state, resulting in the suppression of the hydrogen evolution reaction (HER) by competitive adsorption for active surface sites. Interestingly, the reaction selectivity observed during CO 2 R over polycrystalline Cu has been reported to change dramatically at potentials negative of -1 V vs RHE, with hydrogen and methane rapidly increasing at the expense of C 2+ products. The onset potential of this selectivity shift agrees well with the calculated onset potential of CO 2 depletion due to concentration polarization within the hydrodynamic boundary layer at the Cu surface. The local depletion of CO 2 results in a lower steady state coverage of adsorbed CO on the Cu surface, reducing the rate of C-C coupling and enhancing the methanation of CO. This analysis suggests that higher CO coverages on Cu would result in an increase in the rate of C-C coupling between CO-derived intermediates, such as formyl, at the expense of C-H bond formation. Based on the aforementioned trends in the current literature, we hypothesized that a phase segregated bimetallic alloy of Cu with a CO-generating metal would enhance the selectivity to oxygenated multi-carbon products at the expense of hydrogen and hydrocarbons. We expect this selectivity shift to occur because the supply of additional CO by spillover will enhance the steady state coverage of CO adsorbed to the Cu surface. While several different metals have been identified as being CO selective, such as Au, Ag, and Zn, only Ag is completely immiscible with Cu and exclusively produces CO at applied potentials where Cu is capable of reducing >95% of the CO that it produces. The reaction selectivity observed during CO 2 R over the phase segregated CuAg bimetallic electrodes has led to the conclusion that they possess a reaction selectivity unlike either metallic constituent. The individual proficiencies of each metal were utilized synergistically to sequentially reduce CO 2 , first to CO over Ag and then to hydrocarbons and alcohols over Cu. The supply of additional CO to Cu by spillover from Ag increases the surface coverage of this key reaction intermediate, resulting in a selectivity shift that favors the formation of multi-carbon oxygenates at the expense of hydrogen and hydrocarbons. This effect is so pronounced that these CuAg bimetallic electrodes are currently the only electrocatalyst yet discovered that is more selective for the formation of multi-carbon oxygenates than hydrocarbons. The selectivity of oxygenates relative to hydrocarbons was found to scale with the relative distribution of Cu and Ag facet terminations present at the electrode surface, in agreement with the CO spillover hypothesis. By tuning the surface composition of the CuAg bimetallic electrode, the selectivity to multi-carbon oxygenates relative to hydrocarbons was increased by a factor of ~6 as compared to polycrystalline Cu.
Read moreThe next generation of alternative fuels is being investigated through advanced chemical and biological production techniques for the purpose of finding suitable replacements to diesel and gasoline while lowering production costs and increasing process yields. Chemical conversion of biomass to fuels provides a plethora of pathways with a variety of fuel molecules, both novel and traditional, which may be targeted. In the search for new fuels, an initial, intuition-driven evaluation of fuel compounds with desired properties is required. Due to the high cost and significant production time needed to synthesize these materials at a scale sufficient for exhaustive testing, a predictive model would allow chemists to preemptively screen fuel properties of potentially desirable fuel candidates. Recent work has shown that predictive models, in this case artificial neural networks (ANN’s) analyzing quantitative structure property relationships (QSPR’s), can predict the cetane number (CN) of a proposed fuel molecule with relatively small error. A fuel’s CN is a measure of its ignition quality, typically defined using prescribed ASTM standards and a cetane testing engine. Alternatively, the analogous derived cetane number (DCN), obtained using an Ignition Quality Tester (IQT), is a direct measurement alternative to the CN that uses an empirical inverse relationship to the ignition delay found in the constant volume combustion chamber apparatus. DCN data points acquired using an IQT were utilized for model validation and expansion of the experimental database used in this study. The present work improves on an existing model by optimizing the model architecture along with the key learning variables of the ANN and by making the model more generalizable to a wider variety of fuel candidate types, specifically the class of furans and furan derivatives, by including specific molecules for the model to incorporate. The new molecules considered include tetrahydrofuran, 2-methylfuran, 2-methyltetrahydrofuran, 5,5′-(furan-2-ylmethylene)bis(2-methylfuran), 5,5′-((tetrahydrofuran-2-yl)methylene)bis(2-methyltetrahydrofuran), tris(5-methylfuran-2-yl)methane, and tris(5-methyltetrahydrofuran-2-yl)methane. Model architecture adjustments improved the overall root-mean-squared error (RMSE) of the base database predictions by 5.54%. Additionally, through the targeted database expansion, it is shown that the predicted cetane number of the furan-based molecules improves on average by 49.21% (3.74 CN units) and significantly for a few of the individual molecules. This indicates that a selected subset of representative molecules can be used to extend the model’s predictive accuracy to new molecular classes. The approach, bolstered by the improvements presented in this paper, enables chemists to focus on promising molecules by eliminating less favorable candidates in relation to their ignition quality.
Read moreThe electrochemical reduction of CO<sub>2</sub> is known to be influenced by the identity of the alkali metal cation in the electrolyte; however, a satisfactory explanation for this phenomenon has not been developed. Here we present the results of experimental and theoretical studies aimed at elucidating the effects of electrolyte cation size on the intrinsic activity and selectivity of metal catalysts for the reduction of CO<sub>2</sub>. Experiments were conducted under conditions where the influence of electrolyte polarization is minimal in order to show that cation size affects the intrinsic rates of formation of certain reaction products, most notably for HCOO<sup>-</sup>, C<sub>2</sub>H<sub>4</sub>, and C<sub>2</sub>H<sub>5</sub>OH over Cu(100)- and Cu(111)-oriented thin films, and for CO and HCOO<sup>-</sup> over polycrystalline Ag and Sn. Interpretation of the findings for CO<sub>2</sub> reduction was informed by studies of the reduction of glyoxal and CO, key intermediates along the reaction pathway to final products. Density functional theory calculations show that the alkali metal cations influence the distribution of products formed as a consequence of electrostatic interactions between solvated cations present at the outer Helmholtz plane and adsorbed species having large dipole moments. The observed trends in activity with cation size are attributed to an increase in the concentration of cations at the outer Helmholtz plane with increasing cation size.
Read moreBipolar membranes (BPMs), which have long seen usage in electrodialysis reactors for the generation of acid and base, have recently demonstrated potential to become critical components in electrochemical synthesis devices. Because they can operate under large pH gradients, BPMs enable favorable environments for electrocatalysis at the individual electrodes. Critical to the implementation of BPMs in these devices is understanding the kinetics of water dissociation that occurs within the BPM junction as well as the co- and counter-ion crossover through the BPM, which both present significant obstacles to developing efficient and stable BPM-devices for electrosynthesis applications. Prior work has modeled ion transport in bipolar membranes in neutral salt solutions for electrodialysis. However, no model exists for the BPM under the harsh applied pH gradients that would be present in electrosynthesis, and there is significant need to explore the effects of the internal hydration on the lifetime and performance of BPMs in such environments. Additionally, a mechanistic understanding of water dissociation catalysis will be required to develop interfacial catalysts that enable the high current density operation required for scalable electrosynthesis of fuels. In this talk, we discuss modeling methodologies and physics inherent in BPMs and present our recent model of ion transport and water dissociation catalysis in BPMs across the pH scale. Specifically, we simulate multi-ion transport for a BPM with various electrolyte combinations on each side of the membrane, demonstrating the significance of co- and counter-ion crossover in BPMs operating under harsh pH gradients. We then investigate effects caused by hydration gradients that occur due to internal ion-exchange and examine potential methods for improving performance and mitigating crossover. Finally, we examine the impact of the interfacial water dissociation catalyst and perform sensitivity analysis on the key properties (catalyst point of zero charge and pK a ) that dictate catalyst performance. These results provide information that is critical to developing a comprehensive understanding of multi-component phenomena in BPMs and to informing the design and implementation of BPMs in next-generation devices for the numerous electrosynthesis chemistries that benefit from operation under an applied pH gradient.
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