Alkali Metal Cations Research Articles

Synthesis and Structural Studies of Nucleobase Functionalized Hydrogels for Controlled Release of Vitamins.

The development of biomolecule-derived biocompatible scaffolds for drug delivery applications is an emerging research area. Herein, we have synthesized a series of nucleobase guanine (G) functionalized amino acid conjugates having different chain lengths to study their molecular self-assembly in the hydrogel state. The gelation properties have been induced by the correct choice of chain lengths of fatty acids present in nucleobase functionalized molecules. The effect of alkali metal cations, pH, and the concentration of nucleobase functionalized amino acid conjugates in the molecular self-assembly process has been explored. The presence of Hoogsteen hydrogen bonding interaction drives the formation of a G-quadruplex functionalized hydrogel. The DOSY nuclear magnetic resonance is also performed to evaluate the self-assembling behavior of the newly formed nucleobase functionalized hydrogel. The nanofibrillar morphology is responsible for the formation of a hydrogel, which has been confirmed by various microscopic experiments. The mechanical behaviors of the hydrogel were evaluated by rheological experiments. The in vitro biostability of the synthesized nucleobase amino acid conjugate is also investigated in the presence of hydrolytic enzymes proteinase K and chymotrypsin. Finally, the nucleobase functionalized hydrogel has been used as a drug delivery platform for the control and sustained pH-responsive release of vitamins B2 and B12. This synthesized nucleobase functionalized hydrogel also exhibits noncytotoxic behavior, which has been evaluated by their in vitro cell viability experiment using HEK 293 and MCF-7 cell lines.

Enhancing the Cation-Binding Ability of Fluorescent Calixarene Derivatives: Structural, Thermodynamic, and Computational Studies.

Novel fluorescent calix[4]arene derivatives L1 and L2 were synthesized by introducing phenanthridine moieties at the lower calixarene rim, whereby phenanthridine groups served as fluorescent probes and for cation coordination. To enhance the cation-binding ability of the ligands, besides phenanthridines, tertiary-amide or ester functionalities were also introduced in the cation-binding site. Complexation of the prepared compounds with alkali metal cations in acetonitrile (MeCN), methanol (MeOH), ethanol (EtOH), N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) was investigated at 25 °C experimentally (UV spectrophotometry, fluorimetry, microcalorimetry, and in the solid state by X-ray crystallography) and by means of computational techniques (classical molecular dynamics and DFT calculations). The thermodynamic parameters (equilibrium constants and derived standard reaction Gibbs energies, reaction enthalpies, and entropies) of the corresponding reactions were determined. The tertiary-amide-based compound L1 was found to have a much higher affinity toward cations compared to ester derivative L2, whereby the stabilities of the ML1+ and ML2+ complexes were quite solvent-dependent. The stability decreased in the solvent order: MeCN ≫ EtOH > MeOH > DMF > DMSO, which could be explained by taking into account the differences in the solvation of the ligands as well as free and complexed alkali metal cations in the solvents used. The obtained thermodynamic quantities were thoroughly discussed regarding the structural characteristics of the studied compounds, as well as the solvation abilities of the solvents examined. Molecular and crystal structures of acetonitrile and water solvates of L1 and its sodium complex were determined by single-crystal X-ray diffraction. The results of computational studies provided additional insight into the L1 and L2 complexation properties and structures of the ligands and their cation complexes.

Gas-Phase Alkali-Metal Cation Affinities of Stabilized Enolates.

The chemistry of alkali-metal enolates is dominated by ion pairing. To improve our understanding of the intrinsic interactions between the alkali-metal cations and the enolate anions, we have applied Cooks' kinetic method to determine relative M+ (M=Li, Na, K) affinities of the stabilized enolates derived from acetylacetone, ethyl acetoacetate, diethyl malonate, ethyl cyanoacetate, 2-cyanoacetamide, and methyl malonate monoamide in the gas phase. Quantum chemical calculations support the experimental results and moreover afford insight into the structures of the alkali-metal enolate complexes. The affinities decrease with increasing size of the alkali-metal cations, reflecting weaker electrostatic interactions and lower charge densities of the free M+ ions. For the different enolates, a comparison of their coordinating abilities is complicated by the fact that some of the free anions undergo conformational changes resulting in stabilizing intramolecular interactions. If these complicating effects are disregarded, the M+ affinities correlate with the electron density of the chelating functionalities, that is, the carbonyl and/or the nitrile groups of the enolates. A comparison with the known association constants of the corresponding alkali-metal enolates in solution points to the importance of solvation effects for these systems.

New Amino Acid-Based Thiosemicarbazones and Hydrazones: Synthesis and Evaluation as Fluorimetric Chemosensors in Aqueous Mixtures.

Bearing in mind the interest in the development and application of amino acids/peptides as bioinspired systems for sensing, a series of new phenylalanine derivatives bearing thiosemicarbazone and hydrazone units at the side chain were synthesised and evaluated as fluorimetric chemosensors for ions. Thiosemicarbazone and hydrazone moieties were chosen because they are considered both proton-donor and proton-acceptor, which is an interesting feature in the design of chemosensors. The obtained compounds were tested for the recognition of organic and inorganic anions (such as AcO-, F-, Cl-, Br-, I-, ClO4-, CN-, NO3-, BzO-, OH-, H2PO4- and HSO4-) and of alkaline, alkaline-earth, and transition metal cations, (such as Na+, K+, Cs+, Ag+, Cu+, Cu2+, Ca2+, Cd2+, Co2+, Pb2+, Pd2+, Ni2+, Hg2+, Zn2+, Fe2+, Fe3+ and Cr3+) in acetonitrile and its aqueous mixtures in varying ratios via spectrofluorimetric titrations. The results indicate that there is a strong interaction via the donor N, O and S atoms at the side chain of the various phenylalanines, with higher sensitivity for Cu2+, Fe3+ and F- in a 1:2 ligand-ion stoichiometry. The photophysical and metal ion-sensing properties of these phenylalanines suggest that they might be suitable for incorporation into peptide chemosensory frameworks.

Modulating electron distributions by integrating ligands with metal molecules in THF

While the complex structures and dynamic behavior of alkali metal cations in solvents have been fully understood, insights into the structures, properties and dynamic behaviors of metal dimers in aprotic solvents (e.g. tetrahydrofuran/THF) are quite scarce, and especially the bonding characteristics, valence electron distribution and effect of co-solvents (e.g. crown ether/18-crown-6 (18C6)) are poorly understood. We here perform ab initio molecular dynamics simulation characterization on K2 molecular solute in THF with and without co-solvent molecule (18C6). Our findings indicate that K2 can be integrated by different numbers of THF molecules and 18C6 (if available), forming various chemical identities with unique structures and electron distributions in THF solvent ((THF)xK-K(THF)y, x + y ≤ 9; (THF)x(18C6)K-K(THF)y, x + y ≤ 4). These identities have considerable lifetimes in time evolution and the structure fluctuation dynamics of solution can realize interconversion among these identities through removing and/or adding THFs from/to two K subcluster moieties. In general, compared with the gaseous K2, integration of THFs considerably expands the K2 electron cloud and exhibit much more active states featuring distorted diffuse distributions which can be viewed as electron-expanded chemical entities. Further 18C6 coordination can tightly bind an end K and keep one THF coordinating a K at the axis direction and another K keeps a low coordination status. Such a coordination mode largely forces valence electrons to THF bulk acting as the electron-expanded complexes. In particular, the coordination of 18C6 greatly polarizes the bonding electron, changing the K-K covalent bond into highly ionic one with a Kδ+-Kδ− core structure. All these identities present lower ionization potentials than that of the gaseous K2, and thus have highly reductive reactivity, in agreement with the experiments. This work provides a promising strategy for how to modulate the bonding electrons around the electron-flexible metal–metal bonds.

Comparison of Density-Functional Theory Dispersion Corrections for the DES15K Database.

While density-functional theory (DFT) remains one of the most widely used tools in computational chemistry, most functionals fail to properly account for the effects of London dispersion. Hence, there are many popular post-self-consistent methods to add a dispersion correction to the DFT energy. Until now, the most popular methods have never been compared on equal footing due to not being implemented in the same electronic structure packages. In this work, we performed a large-scale benchmarking study, directly comparing the accuracy of the exchange-hole dipole moment (XDM), D3BJ, D4, TS, MBD, and MBD-NL dispersion models when applied to the recent DES15K database of nearly 15,000 molecular complexes at both expanded and compressed geometries. Our study showed similarly good performance for all dispersion methods (except TS) when applied to neutral complexes. However, they all performed worse for ionic complexes, particularly those involving dications of alkaline earth metals, due to systematic overbinding by the base PBE0 density functional. Investigation of the largest outliers also revealed that only the MBD and MBD-NL methods demonstrate surprising errors for complexes involving alkali metal cations at compressed geometries where they tended to significantly overbind. As we would expect minimal dispersion binding for such complexes, we further investigated the origins of these errors for the potential energy curve of a model cation-π complex. Overall, there is little choice between the XDM, D3BJ, D4, MBD, and MBD-NL dispersion methods for most systems. However, the MBD-based methods are not recommended for complexes involving organic species and alkali or alkaline earth metal cations, for example when modeling Li+ intercalation into graphite.

Electrocatalytic Reduction of Dinitrogen to Ammonia with Water as Proton and Electron Donor Catalyzed by a Combination of a Tri-ironoxotungstate and an Alkali Metal Cation.

The electrification of ammonia synthesis is a key target for its decentralization and lowering impact on atmospheric CO2 concentrations. The lithium metal electrochemical reduction of nitrogen to ammonia using alcohols as proton/electron donors is an important advance, but requires rather negative potentials, and anhydrous conditions. Organometallic electrocatalysts using redox mediators have also been reported. Water as a proton and electron donor has not been demonstrated in these reactions. Here a N2 to NH3 electrocatalytic reduction using an inorganic molecular catalyst, a tri-iron substituted polyoxotungstate, {SiFe3W9}, is presented. The catalyst requires the presence of Li+ or Na+ cations as promoters through their binding to {SiFe3W9}. Experimental NMR, CV and UV-vis measurements, and MD simulations and DFT calculations show that the alkali metal cation enables the decrease of the redox potential of {SiFe3W9} allowing the activation of N2. Controlled potential electrolysis with highly purified 14N2 and 15N2 ruled out formation of NH3 from contaminants. Importantly, using Na+ cations and polyethylene glycol as solvent, the anodic oxidation of water can be used as a proton and electron donor for the formation of NH3. In an undivided cell electrolyzer under 1 bar N2, rates of NH3 formation of 1.15 nmol sec-1 cm-2, faradaic efficiencies of ∼25%, 5.1 equiv of NH3 per equivalent of {SiFe3W9} in 10 h, and a TOF of 64 s-1 were obtained. The future development of suitable high surface area cathodes and well solubilized N2 and the use of H2O as the reducing agent are important keys to the future deployment of an electrocatalytic ammonia synthesis.

(Invited) Probing the Specific Adsorption of Alkali Metal Cations on Au Electrodes Under CO2 Reduction

Electrolyte engineering has recently been recognized as an integral component in the design of electrode/electrolyte interfaces for electrocatalysis. In particular, alkali metal cations of the supporting electrolyte are known to influence electrocatalytic processes. A better understanding of these cation effects requires methods capable of probing the distribution of alkali metal cations in the electrochemical double layer under reaction conditions. However, the scarcity of experimental technique capable of probing alkali metal cations greatly hinders further advances in leveraging cation effects in electrolyte engineering. In this talk, we will discuss our novel spectroscopic approach for quantifying the surface coverage of specifically adsorbed alkali metal cations, that is, the cations that are in direct contact with the electrode and therefore most likely to influence electrocatalysis. The technique is based on the use of an organic cation, tetramethylammonium (methyl4N+), as a vibrational probe of the electrode/electrolyte interface in the presence of alkali metal cations. We will discuss how we use this approach to characterize the adsorption of alkali metal cation during CO2-to-CO conversion.

Re-Structuring of Interfacial Water into Strongly Hydrogen Bonded “Ice-like” Structures as the Unifying Descriptor for Improving Sluggish HOR/HER in Alkaline Electrolyte

The 2 orders of magnitude loss of Platinum Group Metal (PGM) activity toward Hydrogen Oxidation and Reduction reactions (HOR/HER) has hindered implementation of alkaline based electrolyzers and fuel cells and demonstrated a significant knowledge gap in our fundamental understanding of electrochemical interfaces.1 Beneath the seemingly simple reaction lies a potentially convoluted mechanism, with various researchers assigning the activity loss to shifts in electrode potential of zero free charge,2 changed binding energies of adsorbates and reactive intermediates,3,4 and the re-arrangement of interfacial water.5 Recently, Monteiro et al. showed an activity dependence on interfacial cation concentration and identity, suggesting that charge dense cations are able to better stabilize the OH- produced from the unfavorable but necessary water splitting in the HER direction near the negatively charged surface.6,7 In this work, we challenge this notion by demonstrating that competition for water molecules between Outer Helmholtz Plane (OHP) cations and the interfacial water structure is responsible for the activity loss. Through the use of traditional Rotating Disk Electrode, in situ Attenuated Total Reflection-Surface Enhanced Infrared Reflection Absorption Spectroscopy (ATR-SEIRAS) and judicious choice of crown ether additives to hinder the water-interfacial cation interaction, we show a positive correlation between the strength and number of interfacial water-water hydrogen bonds and platinum’s HOR/HER activity.The typical double layer structure in an aqueous electrochemical system consists of specifically adsorbed species in the Inner Helmholtz Plane (IHP), the first layer of non-adsorbates at the OHP, and water hydrating the charged species. ATR-SEIRAS has previously been used to analyze the structure of these hydration shells on a CO covered Pt surface, demonstrating hydrating waters interacting mainly with alkali metal cations through the increase of a 3600 cm-1 ν(O-H) stretch and weak interactions with surrounding interfacial water.8 Through chelation with crown ether, these hydration shells convert from strongly to weakly cation interacting, favoring instead hydrogen bonding with other water molecules, shown with increasing absorption bands at lower wavenumbers (3400 cm-1). This transition is accompanied by an increase in 3000 cm-1 on the CO-free Pt surface, previously assigned to strongly hydrogen bonded “ice-like” interfacial water.9 We further show a similar increase in the 3000 cm-1 band for other kinetic enhancing additives recently reported in the literature, supporting the notion that increasing water-water hydrogen bonding is the unifying descriptor for promoting a rigid interfacial “ice-like” structure and improving PGM HOR/HER activity. Durst, J. et al. New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism. Energy Environ. Sci. 7, 2255–2260 (2014).Sarabia, F. J., Sebastián-Pascual, P., Koper, M. T. M., Climent, V. & Feliu, J. M. Effect of the Interfacial Water Structure on the Hydrogen Evolution Reaction on Pt(111) Modified with Different Nickel Hydroxide Coverages in Alkaline Media. ACS Appl. Mater. Interfaces 11, 613–623 (2019).McCrum, I. T. & Koper, M. T. M. The role of adsorbed hydroxide in hydrogen evolution reaction kinetics on modified platinum. Nat. Energy 5, 891–899 (2020).Yang, X., Nash, J., Oliveira, N. J., Yan, Y. & Xu, B. Understanding the pH Dependence of Underpotential Deposited Hydrogen on Platinum. Angew. Chemie - Int. Ed. 58, 17718–17723 (2019).Zhao, K. et al. Enhancing Hydrogen Oxidation and Evolution Kinetics by Tuning the Interfacial Hydrogen-Bonding Environment on Functionalized Platinum Surfaces. Angew. Chemie - Int. Ed. 61, (2022).Monteiro, M. C. O., Goyal, A., Moerland, P. & Koper, M. T. M. Understanding Cation Trends for Hydrogen Evolution on Platinum and Gold Electrodes in Alkaline Media. ACS Catal. 11, 14328–14335 (2021).Goyal, A. & Koper, M. T. M. The Interrelated Effect of Cations and Electrolyte pH on the Hydrogen Evolution Reaction on Gold Electrodes in Alkaline Media. Angew. Chemie - Int. Ed. 60, 13452–13462 (2021).Yamakata, A. & Osawa, M. Cation-dependent restructure of the electric double layer on CO-covered Pt electrodes: Difference between hydrophilic and hydrophobic cations. J. Electroanal. Chem. 800, 19–24 (2017).Osawa, M., Tsushima, M., Mogami, H., Samjeské, G. & Yamakata, A. Structure of water at the electrified platinum-water interface: A study by surface-enhanced infrared absorption spectroscopy. J. Phys. Chem. C 112, 4248–4256 (2008). Figure 1

Role of Electrolyte-Adsorbates Interaction in Determining CO2 Reduction Reaction Pathways

Electrochemical CO2 reduction reaction (CO2RR) is a promising strategy to convert atmospheric CO2 into valuable fuels and chemicals. Although the optimization of the catalyst-adsorbates interaction via tuning the binding energies of electrocatalysts has been extensively studied, activity and product selectivity remain a challenge. Recently, there has been a growing recognition that cations in electrolytes influence the activity of surface electrocatalytic reactions.1 A recent density functional theory study suggested that partially desolvated metal cations stabilize CO2RR intermediates via electrostatic interactions.2 Therefore, an experimental understanding of how the cations (and surrounding water molecules) control the intermediates present on the surface and govern the reaction pathway is critical to establish further insights into the design of electrolytes for CO2RR.In this talk, the role of alkali metal cations and water molecules on the CO2RR is revealed by tracking the surface-bound intermediates using operando attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). We will also integrate the operando surface analysis with final product selectivity, further strengthening the understanding of the reaction mechanism. It will also be demonstrated that the holistic information about the electrolyte-adsorbates interaction can provide additional knobs to tune the energetics of key reaction intermediates, leading to further improvements in electrocatalytic activity for the CO2RR.[Reference]1 Ringe, S. et al., Energy Environ. Sci. 2019, 12, 3001–3014.2 Monteiro, MCO. et al., Nat. Catal. 2021, 4, 654–662.

Effect of Cations on Electrocatalytic CO2-to-Methanol Conversion by Heterogenized Molecular Catalyst

Catalytic benefit and role of electrolyte cations for electrochemical CO2 reduction reaction have recently received significant research attention. It has been experimentally and theoretically demonstrated that cations can stabilize the key intermediate species, for example, *CO2 –, through mid- or short-range electrostatic interactions, facilitating the CO2 activation step.1-2 However, to date, these discussions are mostly limited to CO2-to-CO conversion (a two electron/proton transfer reaction) and electrostatic stabilization of reaction intermediate species.In present work, we show that cations play a crucial role in further reducing CO2 to methanol. We found that cations engage as a promoter in the rate determining step of the CO2-to-methanol conversion, which involves a proton-coupled electron transfer, beyond the first CO2 activation step. For this study, heterogenized molecular catalyst with a cobalt single site (Co-N4) was employed, achieving ~25% methanol selectivity at –0.78 V vs. RHE. Through an isotope labeling experiment (13CO2 reduction reaction), we validated that methanol is produced by the CO2 reduction reaction. Furthermore, electrolytes with different cation species and concentrations were tested in a systematic manner to investigate the promoting effect of cations on methanol production. In addition, using deuterated electrolytes, we studied kinetic isotope effect, proposing a reaction pathway for CO2-to-methanol conversion on the cobalt single site. Our findings offer new insights into the beneficial effect of cations on electrochemical CO2 reduction reaction kinetics besides their widely acknowledged stabilizing effect. Our discovery also opens up new research directions where non-covalent interactions between electrolyte species and reaction intermediates are utilized to enhance electrocatalytic production of a desired product in CO2 reduction reaction beyond two-electron transfer products.In this talk, I will discuss the promoting effect of cations on the electrochemical reduction of CO2 to methanol and conclude by proposing potential strategies to further improve the electrocatalytic CO2-to-methanol conversion.References Monteiro, M. C. O.; Dattila, F.; Hagedoorn, B.; García-Muelas, R.; López, N.; Koper, M. T. , Absence of CO2 electroreduction on copper, gold and silver electrodes without metal cations in solution. Nature Catalysis 2021, 4 (8), 654-662.Resasco, J.; Chen, L. D.; Clark, E.; Tsai, C.; Hahn, C.; Jaramillo, T. F.; Chan, K.; Bell, A. T., Promoter Effects of Alkali Metal Cations on the Electrochemical Reduction of Carbon Dioxide. Journal of the American Chemical Society 2017, 139 (32), 11277-11287.

(Invited) Structure and Dynamics of Interfacial Species in Electrical Double Layer

Understanding electrochemical processes is of importance for many industrial and scientific fields. Especially, interfacial species, such as electrolyte ions and solvent significantly affect electrochemical reaction activity. Surface oxide species also promote or inhibit various electrode reactions; therefore, the oxidation processes of metal electrode have been investigated using in situ vibrational spectroscopy. Infrared (IR) spectroscopy is an important tool because it can detect the complementary vibrational modes of surface oxide species. However, observation of the low-frequency mode is restricted by infusible IR windows such as CaF2, that are generally used in the IR reflection absorption spectroscopy (IRAS). We developed a complementary IR method that can be used to avoid contamination by dissolved ions from window material. The fusible low-pass IR window can be applied to in-situ IRAS measurements [1,2]. The adsorbed hydroxide was observed on the single crystal Pt electrodes depending on the surface orientation, its coverage was correlated with the decrease in oxygen reduction reaction activity [3]. Recent time resolved X-ray diffraction measurements revealed that there is a delay in the approach of alkali metal cations to surface because the interfacial water causes a blocking effect [4]. Therefore, the kinetics of approaching step in the EDL is significantly different from that of migration in the bulk phase, and the adsorption rate depends on the hydration structure, ionic valence, and ionic size. Although X-ray diffraction provides a distribution of interfacial species, it does not provide sufficient information about species identification and molecular orientation. We investigated the coadsorption dynamics of metal cation and (bi)sulfate anion using time resolved IR measurement [5]. Boron doped diamond (BDD) is a promising material of which properties are significantly different from those of metals and other carbon materials. However, the complex atomic arrangement and oxidation state of the polycrystalline BDD surface inhibit an atomic level study on the relationship between the electrochemical response and surface structure. More detailed surface analyses are necessary for the investigation of the electrode surface of BDD. We applied in situ ATR−IR spectroscopy to reveal the oxidative species and dopamine oxidation on the BDD electrode [6,7].[1] M. Nakamura, Y. Nakajima, N. Hoshi, H. Tajiri, O. Sakata, ChemPhysChem, 14, 2426 (2013).[2] M. Nakamura, Y. Nakajima, K. Kato, O. Sakata, N. Hoshi, J. Phys. Chem. C, 119, 23586 (2015).[3] T. Kumeda, H. Tajiri, O. Sakata, N. Hoshi, M. Nakamura, Nat. Commun., 9, 4378 (2018).[4] M. Nakamura, H. Kaminaga, O. Endo, H. Tajiri, O. Sakata, N. Hoshi, J. Phys. Chem. C, 118, 22136 (2014).[5] M. Nakamura, T. Banzai, Y. Maehata, O. Endo, H. Tajiri. O. Sakata, N. Hoshi, Sci. Rep., 7, 914 (2017).[6] T. Ogose, S. Kasahara, N. Ikemiya, N. Hoshi, Y. Einaga, M. Nakamura, J. Phys. Chem. C, 122, 27456 (2018).[7] R. Hosoda, N. Kamoshida, N. Hoshi, Y. Einaga, M. Nakamura, Carbon, 171, 814 (2021).

Efficient Ion-Selective Transport in Sulfonated Diels-Alder Poly(phenylene) Membrane Enables Aqueous Organic Flow Batteries with Undetectable Crossover

In most aqueous redox flow batteries (RFBs), ion exchange membranes are used to suppress crossover of redox active species while enabling conductivity of charge carrying ions. Many aqueous RFB chemistries, such as the all-vanadium RFB, involve metal cations dissolved in acid. In such systems, high proton conductivity affords low ohmic resistance from the membrane, but crossover rates are also high. Organic and metalorganic reactants with extreme chemical stability are an emerging direction for the development of RFB electrolytes, and may enable batteries with long calendar life composed of earth-abundant elements. Moreover, the size and charge numbers of these reactants enable much lower crossover than observed for systems like vanadium RFBs. However, the most stable chemistries use neutral or alkaline electrolytes where protons are not abundant enough to carry the ionic current. Currently, this presents a drawback: the incumbent cation exchange membrane, Nafion, exhibits a factor-of-ten lower conductivity of potassium compared to protons. The present work introduces a new membrane cast from sulfonated Diels-Alder poly(phenylene) to enable improved area-specific resistance while suppressing crossover.Nafion contains a perflourinated hydrophobic backbone with intervals of a flexible side chain that terminates in hydrophilic sulfonic acid moieties. This structure of a hydrophobic backbone with hydrophilic side chains is common to most ion exchange membranes. When a membrane with this polymer structure is hydrated and ion exchanged to an alkali metal cation form, the cations promote aggregation of the ionic groups via rearrangement of the flexible side chains, giving a configuration that is not optimal for ion conductivity. In contrast, the new membrane has a sulfonated hydrophilic backbone and external hydrophobic moieties, which we hypothesize prevents the aggregation effects seen in Nafion to enable efficient ion-selective transport. In the present work, we show that the new membrane affords negligible ferricyanide permeability (< 2 × 10−13 cm2/s) and promising area-specific resistances (< 1 Ω cm2) for a variety of cations. The conductivity of the new membrane in 1 M KCl electrolyte is 3.6 mS cm-1, more than double that of Nafion NR212 measured in the same flow battery cell. We demonstrate the use of this membrane in an alkaline organic RFB, with a ferri-/ferrocyanide posolyte and a 4,4’-((9,10-anthraquinone-2,6-diyl)dioxy)dibutyrate (2,6-DBEAQ) negolyte. Three-electrode voltametric methods for post-cycling detection of crossed-over species will be discussed and their sensitivities compared, in an effort to quantify crossover fluxes during cycling and their possible contributions to capacity fade. We conclude that in this flow battery, the new membrane suppresses crossover of both iron hexacyanide and 2,6-DBEAQ to the extent that any crossed over compounds cannot be detected by our methods, and we assign an upper limit of 2 × 10−7 μmol cm−2 s−1 on the possible crossover flux, amounting to a projected < 1 %/year capacity fade from crossover in the lab-scale cell.

Cation-Coordinated Inner-Sphere CO2 Electroreduction at Au-Water Interfaces

The electrocatalytic CO2 reduction reaction (CO2RR) is a promising technology to store renewable energy and mitigate the increase of CO2 concentration.1-2 Understanding the electrode-electrolyte interfacial reaction mechanism from the atomic level is instrumental for both catalyst design and catalytic system’s optimization. Electrolyte ions are critical for CO2RR and the competitive hydrogen evolution reaction (HER), however, the role of alkali metal cations is highly controversial. For example, a recent study by Koper and co-workers highlighted the role of alkali metal ion coordination,3 whereas another work by Hu et al. ascribed the cation effect to the modulation of local electric field.4 In order to explore the crucial cation role in electrocatalytic reactions, electrode-electrolyte interfacial models should be properly constructed by including explicit water solvents and alkali metal cations.In our previous work, CO2 activation step at Au-water-2K interfaces has been studied by ab initio molecular dynamics (AIMD) simulations with the kinetic barrier of 0.61 eV.5 To determine the rate-determining step (RDS) during CO2RR on Au surfaces, the full reaction pathway involving the subsequent electron and proton transfer steps is further investigated, and the reaction kinetics is evaluated by the slow-growth sampling approach integrated with AIMD simulations (SG-AIMD). As shown in Figure 1, the complete free energy diagram of CO2RR under electrochemical conditions is constructed for the first time, where the transition state (TS) structures are shown on top. It is illustrated that such a cation-coordinated inner-sphere CO2RR at Au-water interfaces is facile, and the first electron transfer with the concomitant adsorption during CO2 activation is the RDS. Furthermore, HER pathway is also explored, which is highly suppressed by local alkali cations showing much higher kinetic barrier in the rate-limiting Volmer step.Our systematic atomic-scale study via ab initio molecular dynamics simulations demonstrates that CO2RR shows superior catalytic activity under the cation promotion effect which is originated from the short-range coordination interaction between reaction intermediates and cations. This study motivates the development of periodic models mimicking the electrode-electrolyte interface under electrochemical conditions, which can be extended into studying various electrocatalytic reactions via AIMD simulations. References (1) Nitopi, S.; Bertheussen, E.; Scott, S. B.; Liu, X.; Engstfeld, A. K.; Horch, S.; Seger, B.; Stephens, I. E. L.; Chan, K.; Hahn, C.; Nørskov, J. K.; Jaramillo, T. F.; Chorkendorff, I., Chem. Rev. 2019, 119, 7610-7672.(2) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F., Science 2017, 355, eaad4998.(3) Monteiro, M. C., Dattila, F., Hagedoorn, B., García-Muelas, R., López, N. and Koper, M., Nat. Catal., 2021, 4, 654-662.(4) Gu, J., Liu, S., Ni, W., Ren, W., Haussener, S. and Hu, X., Nat. Catal., 2022, 5, 268-276.(5) Qin, X.; Vegge, T.; Hansen, H. A., J. Chem. Phys. 2021, 155, 134703. Figure 1. Complete free energy landscape of CO2RR at Au-water interfaces with two K cations. The structures of transition state (TS) are shown on top. The charges transferred to key intermediates (*CO2, *COOH, *CO) are shown in the insert figure, where the charge transfer in CHE model is indicated by dashed lines. Color code: Au, golden; K, purple; C, blue; O, red; H, white. Figure 1

Effect of alkali-metal cations on synthesis of stoichiometric layered perovskite oxynitrides

Layered perovskite oxynitrides, which contain a stoichiometric lattice N content, are promising visible-light-driven photocatalysts. However, their synthesis remains a challenge for inorganic chemists. Herein, we report the successful synthesis of layered perovskite oxynitrides A2A'2Ta3O9N with different cations at A/A′-sites (A = Na, K, Rb, Cs; A' = Ca, Sr, Ba) via thermal ammonolysis from layered Dion–Jacobson phase oxide precursors (AA'2Ta3O10). We present the first experimental study of layered oxynitride formation across the A and A′ cation size series, revealing that the A-site alkali-metal cation rather than the A′-site alkaline-earth metal cation is critical to the successful synthesis of the stoichiometric layered oxynitrides. Nitrided products with A = Na, K, or Rb exhibited structural transitions, while no structural transition was observed for Cs. Interestingly, nitrided products with A = Rb and K exhibited the same structural transition behavior, i.e., alkali-metal cations entered the interlayer to form an oxynitride with a composition A2A'2Ta3O9N·nH2O. By contrast, with A = Na, it formed an oxynitride that differs from the Ruddlesden–Popper structure but has the same composition of A2A'2Ta3O9N·nH2O. Our finding suggests that, as the cation size decreases from Cs to Na, stoichiometric A2A'2Ta3O9N compositions become more favorable than compounds with A-sites. This work provides a new perspective for the future design and synthesis of novel layered oxynitrides.

Direct microwave energy input on a single cation for outstanding selective catalysis.

Microwave (MW)-driven catalytic systems are attracting attention not only as an aggressive electrification strategy of the chemical industry but also as creating a unique catalytic reaction field that conventional equilibrium heating cannot achieve. This study unlocked direct and selective heating of single alkali metal cations in the pores of aluminosilicate zeolites under MW. Selectively heated Cs+ cations in FAU zeolite exhibited selective CH4 combustion performance, that is, COx generation at the heated Cs+ cations selectively occurred while side reactions in the low-temperature gas phase were suppressed. The Cs-O pair distribution function revealed by synchrotron-based in situ x-ray total scattering gave us direct evidence of peculiar displacement induced by MW, which was consistent with the results of molecular dynamics simulation mimicking MW heating. The concept of selective monoatomic heating by MW is expected to open a next stage in "microwave catalysis" science by providing physicochemical insights into "microwave effects."

An Interfacial View of Cation Effects on Electrocatalysis Systems.

The identity of alkali metal cations in the electrolyte of electrocatalysis systems has been recently introduced as a crucial factor to tailor the kinetics and Faradaic efficiency of many electrocatalytic reactions. In this Minireview, we have summarized the recent advances in the molecular-level understanding of cation effects on relevant electrocatalytic processes such as hydrogen evolution (HER), oxygen evolution (OER), and CO2 electroreduction (CO2 RR) reactions. The discussion covers the effects of electrolyte cations on interfacial electric fields, structural organization of interfacial water molecules, blocking the catalytic active sites, stabilization or destabilization of intermediates, and interfacial pHs. These cation-induced interfacial phenomena have been reported to impact the performance (activity, selectivity, and stability) of electrochemical reactions collaboratively or independently. We describe that although there is almost a general agreement on the relationship between the size of alkali cations and the activities of HER, OER, and CO2 RR, however, the mechanism by which the performance of these electrocatalytic reactions is influenced by alkali metal cations is still in debate.

An Interfacial View of Cation Effects on Electrocatalysis Systems

AbstractThe identity of alkali metal cations in the electrolyte of electrocatalysis systems has been recently introduced as a crucial factor to tailor the kinetics and Faradaic efficiency of many electrocatalytic reactions. In this Minireview, we have summarized the recent advances in the molecular‐level understanding of cation effects on relevant electrocatalytic processes such as hydrogen evolution (HER), oxygen evolution (OER), and CO2 electroreduction (CO2RR) reactions. The discussion covers the effects of electrolyte cations on interfacial electric fields, structural organization of interfacial water molecules, blocking the catalytic active sites, stabilization or destabilization of intermediates, and interfacial pHs. These cation‐induced interfacial phenomena have been reported to impact the performance (activity, selectivity, and stability) of electrochemical reactions collaboratively or independently. We describe that although there is almost a general agreement on the relationship between the size of alkali cations and the activities of HER, OER, and CO2RR, however, the mechanism by which the performance of these electrocatalytic reactions is influenced by alkali metal cations is still in debate.

Alkali metal cations modulate the geometry of different binding sites in HCN4 selectivity filter for permeation or block.

Hyperpolarization-activated cyclic-nucleotide gated (HCN) channels are important for timing biological processes like heartbeat and neuronal firing. Their weak cation selectivity is determined by a filter domain with only two binding sites for K+ and one for Na+. The latter acts as a weak blocker, which is released in combination with a dynamic widening of the filter by K+ ions, giving rise to a mixed K+/Na+ current. Here, we apply molecular dynamics simulations to systematically investigate the interactions of five alkali metal cations with the filter of the open HCN4 pore. Simulations recapitulate experimental data like a low Li+ permeability, considerable Rb+ conductance, a block by Cs+ as well as a punch through of Cs+ ions at high negative voltages. Differential binding of the cation species in specific filter sites is associated with structural adaptations of filter residues. This gives rise to ion coordination by a cation-characteristic number of oxygen atoms from the filter backbone and solvent. This ion/protein interplay prevents Li+, but not Na+, from entry into and further passage through the filter. The site equivalent to S3 in K+ channels emerges as a preferential binding and presumably blocking site for Cs+. Collectively, the data suggest that the weak cation selectivity of HCN channels and their block by Cs+ are determined by restrained cation-generated rearrangements of flexible filter residues.

Electrochemical molecular intercalation and exfoliation of solution-processable two-dimensional crystals.

Electrochemical molecular intercalation of layered semiconducting crystals with organic cations followed by ultrasonic exfoliation has proven to be an effective approach to producing a rich family of organic/inorganic hybrid superlattices and high-quality, solution-processable 2D semiconductors. A traditional method for exfoliating 2D crystals relies on the intercalation of inorganic alkali metal cations. The organic cations (e.g., alkyl chain-substituted quaternary ammonium cations) are much larger than their inorganic counterparts, and the bulky molecular structure endows distinct intercalation and exfoliation chemistry, as well as molecular tunability. By using this protocol, many layered 2D crystals (including graphene, black phosphorus and versatile metal chalcogenides) can be electrochemically intercalated with organic quaternary alkylammonium cations. Subsequent solution-phase exfoliation of the intercalated compounds is realized by regular bath sonication for a short period (5-30 min) to produce free-standing, thin 2D nanosheets. It is also possible to graft additional ligands on the nanosheet surface. The thickness of the exfoliated nanosheets can be measured by using atomic force microscopy and Raman spectroscopy. Modifying the chemical structure and geometrical configuration of alkylammonium cations results in different exfoliation behavior and a family of versatile organic/inorganic hybrid superlattices with tunable physical/chemical properties. The whole protocol takes ~6 h for the successful production of stable, ultrathin 2D nanosheet dispersion in solution and another 11 h for depositing thin films and transferring them onto an arbitrary surface. This protocol does not require expertise beyond basic electrochemistry knowledge and conventional colloidal nanocrystal synthesis and processing.