Thermodynamics of Counterion Release Is Critical for Anion Exchange Membrane Conductivity

Alkaline Membranes toward Electrochemical Energy Devices: Recent Development and Future Perspectives

Anion-exchange membranes (AEMs) that can selectively transport OH-, namely, alkaline membranes, are becoming increasingly crucial in a variety of electrochemical energy devices. Understanding the membrane design approaches can help to break through the constraints of undesired performance and lab-scale production. In this Outlook, the research progress of alkaline membranes in terms of backbone structures, synthesis methods, and related applications is organized and discussed. The evaluation of synthesis methods and description of membrane stability enhancement strategies provide valuable insights for structural design. Finally, to accelerate the deployment of relevant technologies in alkaline media, the future priority of alkaline membranes that needs to be addressed is presented from the perspective of science and engineering.

Tuning Alkaline Anion Exchange Membranes through Crosslinking: A Review of Synthetic Strategies and Property Relationships

Alkaline anion exchange membranes (AAEMs) are an enabling component for next-generation electrochemical devices, including alkaline fuel cells, water and CO2 electrolyzers, and flow batteries. While commercial systems, notably fuel cells, have traditionally relied on proton-exchange membranes, hydroxide-ion conducting AAEMs hold promise as a method to reduce cost-per-device by enabling the use of non-platinum group electrodes and cell components. AAEMs have undergone significant material development over the past two decades; however, challenges remain in the areas of durability, water management, high temperature performance, and selectivity. In this review, we survey crosslinking as a tool capable of tuning AAEM properties. While crosslinking implementations vary, they generally result in reduced water uptake and increased transport selectivity and alkaline stability. We survey synthetic methodologies for incorporating crosslinks during AAEM fabrication and highlight necessary precautions for each approach.

Influence of Headgroups in Ethylene-Tetrafluoroethylene-Based Radiation-Grafted Anion Exchange Membranes for CO2 Electrolysis

The performance of zero-gap CO₂ electrolysis (CO₂E) is significantly influenced by the membrane's chemical structure and physical properties due to its effects on the local reaction environment and water/ion transport. Radiation-grafted anion-exchange membranes (RG-AEM) have demonstrated high ionic conductivity and durability, making them a promising alternative for CO₂E. These membranes were fabricated using two different thicknesses of ethylene-tetrafluoroethylene polymer substrates (25 and 50 μm) and three different headgroup chemistries: benzyl-trimethylammonium, benzyl-N-methylpyrrolidinium, and benzyl-N-methylpiperidinium (MPIP). Our membrane characterization and testing in zero-gap cells over Ag electrocatalysts under commercially relevant conditions showed correlations between the water uptake, ionic conductivity, hydration, and cationic-head groups with the CO₂E efficiency. The thinner 25 μm-based AEM with the MPIP-headgroup (ion-exchange capacities of 2.1 ± 0.1 mmol g^-1) provided balanced in situ test characteristics with lower cell potentials, high CO selectivity, reduced liquid product crossover, and enhanced water management while maintaining stable operation compared to the commercial AEMs. The CO₂ electrolyzer with an MPIP-AEM operated for over 200 h at 150 mA cm^-2 with CO selectivities up to 80% and low cell potentials (around 3.1 V) while also demonstrating high conductivities and chemical stability during performance at elevated temperatures (above 60 °C).

Importance of Hydroxide Ion Conductivity Measurement for Alkaline Water Electrolysis Membranes

Alkaline water electrolysis (AWE) refers to a representative water electrolysis technology that applies electricity to synthesize hydrogen gas without the production of carbon dioxide. The ideal polymer electrolyte membranes for AWE should be capable of transporting hydroxide ions (OH−) quickly in harsh alkaline environments at increased temperatures. However, there has not yet been any desirable impedance measurement method for estimating hydroxide ions’ conduction behavior across the membranes, since their impedance spectra are significantly affected by connection modes between electrodes and membranes in the test cells and the impedance evaluation environments. Accordingly, the measurement method suitable for obtaining precise hydroxide ion conductivity values through the membranes should be determined. For this purpose, Zirfon®, a state-of-the-art AWE membrane, was adopted as the standard membrane sample to perform the impedance measurement. The impedance spectra were acquired using homemade test cells with different electrode configurations in alkaline environments, and the corresponding hydroxide ion conductivity values were determined based on the electrochemical spectra. Furthermore, a modified four-probe method was found as an optimal measurement method by comparing the conductivity obtained under alkaline conditions.

Stability Analysis of Substituted Cobaltocenium [Bis(cyclopentadienyl)cobalt(III)] Employing Chemistry-Informed Neural Networks

Cationic cobaltocenium derivatives are promising components of the anion exchange membranes because of their excellent thermal and alkaline stability under the operating conditions of a fuel cell. Here, we present an efficient modeling approach to assessing the chemical stability of substituted cobaltocenium CoCp₂⁺, based on the computed electronic structure enhanced by machine learning techniques. Within the aqueous environment, the positive charge of the metal cation is balanced by the hydroxide anion through formation of the CoCp₂⁺OH^- complexes, whose dissociation is studied within the implicit solvent employing the density functional theory. The data set of about 118 the CoCp₂⁺OH^- complexes based on 42 substituent groups characterized by a range of electron-donating (ED) and electron-withdrawing (EW) properties is constructed and analyzed. Given 12 carefully chosen chemistry-informed descriptors of the complexes and relevant fragments, the stability of the complexes is found to strongly correlate with the energies of the highest occupied and lowest unoccupied molecular orbitals, modulated by a switching function of the Hirshfeld charge. The latter is used as a measure of the electron-withdrawing-donating character of the substituents. On the basis of this observation from the conventional regression analysis, two fully connected, feed-forward neural network (FNN) models with different unit structures, called the chemistry-informed (CINN) and the quadratic (QNN) neural networks, together with a support vector regression (SVR) model are developed. Both deep neural network models predict the bond dissociation energies of the cobaltocenium complexes with mean relative errors less than 10.56% and average absolute errors less than 1.63 kcal/mol, superior to the conventional regression and the SVR model prediction. The results show the potential of QNN to efficiently capture more complex relationships. The concept of incorporating the domain (chemical) knowledge/insight into the neural network structure paves the way to applications of machine learning techniques with small data sets, ultimately leading to better predictive models compared to the classical machine learning method SVR and conventional regression analysis.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

Electrophilic and nucleophilic displacement reactions at the bridgehead borons of tris(pyridyl)borate scorpionate complexes

Although a wide variety of boron-based "scorpionate" ligands have been implemented, a modular route that offers facile access to different substitution patterns at boron has yet to be developed. Here, we demonstrate new reactivity patterns at the bridgehead positions of a ruthenium tris(pyrid-2-yl)borate complex that allow for facile tuning of steric and electronic properties.

Tunable Anion Exchange Membrane Conductivity and Permselectivity via Non-Covalent, Hydrogen Bond Cross-Linking

Ion exchange membranes (IEMs) are a key component of electrochemical processes that purify water, generate clean energy, and treat waste. Most conventional polymer IEMs are covalently cross-linked, which results in a challenging tradeoff relationship between two desirable properties─high permselectivity and high conductivity─in which one property cannot be changed without negatively affecting the other. In an attempt to overcome this limitation, in this work we synthesized a series of anion exchange membranes containing non-covalent cross-links formed by a hydrogen bond donor (methacrylic acid) and a hydrogen bond acceptor (dimethylacrylamide). We show that these monomers act synergistically to improve both membrane permselectivity and conductivity relative to a control membrane without non-covalent cross-links. Furthermore, we show that the hydrogen bond donor and acceptor loading can be used to tune permselectivity and conductivity relatively independently of one another, escaping the tradeoff observed in conventional membranes.

Fast Bulky Anion Conduction Enabled by Free Shuttling Phosphonium Cations

Highly conductive anion-exchange membranes (AEMs) are desirable for applications in various energy storage and conversion technologies. However, conventional AEMs with bulky HCO₃ ^- or Br^- as counterion generally exhibit low conductivity because the covalent bonding restrains the tethered cationic group's mobility and rotation. Here, we report an alternative polyrotaxane AEM with nontethered and free-shuttling phosphonium cation. As proved by temperature-dependent NMR, solid-state NMR, and molecular dynamics simulation, the phosphonium cation possesses a thermally trigged shuttling behavior, broader extension range, and greater mobility, thus accelerating the diffusion conduction of bulky anions. Owing to this striking feature, high HCO₃ ^- conductivity of 105 mS cm^-1 at 90°C was obtained at a relatively lower ion-exchange capacity of 1.17 mmol g^-1. This study provides a new concept for developing highly conductive anion-exchange membranes and will catalyze the exploration of new applications for polyrotaxanes in ion conduction processes.

Phenolphthalein Anilide Based Poly(Ether Sulfone) Block Copolymers Containing Quaternary Ammonium and Imidazolium Cations: Anion Exchange Membrane Materials for Microbial Fuel Cell

A class of phenolphthalein anilide (PA)-based poly(ether sulfone) multiblock copolymers containing pendant quaternary ammonium (QA) and imidazolium (IM) groups were synthesized and evaluated as anion exchange membrane (AEM) materials. The AEMs were flexible and mechanically strong with good thermal stability. The ionomeric multiblock copolymer AEMs exhibited well-defined hydrophobic/hydrophilic phase-separated morphology in small-angle X-ray scattering and atomic force microscopy. The distinct nanophase separated membrane morphology in the AEMs resulted in higher conductivity (IECw = 1.3-1.5 mequiv./g, σ(OH^-) = 30-38 mS/cm at 20 °C), lower water uptake and swelling. Finally, the membranes were compared in terms of microbial fuel cell performances with the commercial cation and anion exchange membranes. The membranes showed a maximum power density of ~310 mW/m² (at 0.82 A/m²); 1.7 and 2.8 times higher than the Nafion 117 and FAB-PK-130 membranes, respectively. These results demonstrated that the synthesized AEMs were superior to Nafion 117 and FAB-PK-130 membranes.

Study of Anion Exchange Membrane Properties Incorporating N-spirocyclic Quaternary Ammonium Cations and Aqueous Organic Redox Flow Battery Performance

Flexible cross-linked anion exchange membranes (AEMs) based on poly (p-phenylene oxide) grafted with N-spirocyclic quaternary ammonium cations were synthesized via UV-induced free-radical polymerization by using diallylpiperidinium chloride as an ionic monomer. Five membranes with ion exchange capacity (IEC) varying between 1.5 to 2.8 mmol Cl-·g-1 polymer were obtained and the correlation between IEC, water uptake, state of water in the membrane and ionic conductivity was studied. In the second part of this study, the influence of properties of four of these membranes on cell cycling stability and performance was investigated in an aqueous organic redox flow battery (AORFB) employing dimethyl viologen (MV) and N,N,N-2,2,6,6-heptamethylpiperidinyl oxy-4-ammonium chloride (TMA-TEMPO). The influence of membrane properties on cell cycling stability and performance was studied. At low-current density (20 mA·cm-2), the best capacity retention was obtained with lower IEC membranes for which the water uptake, freezable water and TMA-TEMPO and MV crossover are low. However, at a high current density (80 mA·cm-2), membrane resistance plays an important role and a membrane with moderate IEC, more precisely, moderate ion conductivity and water uptake was found to maintain the best overall cell performance. The results in this work contribute to the basic understanding of the relationship between membrane properties and cell performance, providing insights guiding the development of advanced membranes to improve the efficiency and power capability for AORFB systems.

Applications of isothermal titration calorimetry in pure and applied research from 2016 to 2020

The last 5 years have seen a series of advances in the application of isothermal titration microcalorimetry (ITC) and interpretation of ITC data. ITC has played an invaluable role in understanding multiprotein complex formation including proteolysis-targeting chimeras (PROTACS), and mitochondrial autophagy receptor Nix interaction with LC3 and GABARAP. It has also helped elucidate complex allosteric communication in protein complexes like trp RNA-binding attenuation protein (TRAP) complex. Advances in kinetics analysis have enabled the calculation of kinetic rate constants from pre-existing ITC data sets. Diverse strategies have also been developed to study enzyme kinetics and enzyme-inhibitor interactions. ITC has also been applied to study small molecule solvent and solute interactions involved in extraction, separation, and purification applications including liquid-liquid separation and extractive distillation. Diverse applications of ITC have been developed from the analysis of protein instability at different temperatures, determination of enzyme kinetics in suspensions of living cells to the adsorption of uremic toxins from aqueous streams.

Expeditious synthesis of aromatic-free piperidinium-functionalized polyethylene as alkaline anion exchange membranes

Alkaline anion exchange membranes (AAEMs) with high hydroxide conductivity and good alkaline stability are essential for the development of anion exchange membrane fuel cells to generate clean energy by converting renewable fuels to electricity. Polyethylene-based AAEMs with excellent properties can be prepared via sequential ring-opening metathesis polymerization (ROMP) and hydrogenation of cyclooctene derivatives. However, one of the major limitations of this approach is the complicated multi-step synthesis of functionalized cyclooctene monomers. Herein, we report that piperidinium-functionalized cyclooctene monomers can be easily prepared via the photocatalytic hydroamination of cyclooctadiene with piperidine in a one-pot, two-step process to produce high-performance AAEMs. Possible alkaline-degradation pathways of the resultant polymers were analyzed using spectroscopic analysis and dispersion-inclusive hybrid density functional theory (DFT) calculations. Quite interestingly, our theoretical calculations indicate that local backbone morphology-which can potentially change the Hofmann elimination reaction rate constant by more than four orders of magnitude-is another important consideration in the rational design of stable high-performance AAEMs.

Metallo-Polyelectrolytes: Correlating Macromolecular Architectures with Properties and Applications

Since the middle of the 20th century, metallopolymers have represented a standalone subfield with a beneficial combination of functionality from inorganic metal centers and processability from the organic polymeric frameworks. Metallo-polyelectrolytes are a new class of soft materials that showcase fundamentally different properties from neutral polymers due to their intrinsically ionic behaviors. This review describes recent trends in metallo-polyelectrolytes and discusses emerging properties and challenges, as well as future directions from a perspective of macromolecular architectures. The correlations between macromolecular architectures and properties are discussed from copolymer self-assembly, metallo-enzymes for biomedical applications, metallo-peptides for catalysis, crosslinked networks, and metallogels.

Introducing Seven Transition Metal Ions into Terpyridine-Based Supramolecules: Self-Assembly and Dynamic Ligand Exchange Study

In coordination-driven self-assembly, 2,2':6',2″-terpyridine (tpy) has gained extensive attention in constructing supramolecular architectures on the basis of ⟨tpy-M-tpy⟩ connectivity. In direct self-assembly of large discrete structures, however, the metal ions were mainly limited to Cd(II), Zn(II), and Fe(II) ions. Herein, we significantly broaden the spectrum of metal ions with seven divalent transition metal ions M(II) (M = Mn, Fe, Co, Ni, Cu, Zn, Cd) to assemble a series of supramolecular fractals. In particular, Mn(II), Co(II), Ni(II), and Cu(II) were reported for the first time to form such large and discrete structures with ⟨tpy-M-tpy⟩ connectivity. In addition, the structural stabilities of those supramolecules in the gas phase and the kinetics of the ligand exchange process in solution were investigated using mass spectrometry. Such a fundamental study gave the relative order of structural stability in the gas phase and revealed the inertness of coordination in solution depending on the metal ions. Those results would guide the future study in tpy-based supramolecular chemistry in terms of self-assembly, characterization, property, and application.

Crosslinked metallo-polyelectrolytes with enhanced flexibility and dimensional stability for anion-exchange membranes

We report a class of crosslinked metallo-polyelectrolytes as anion exchange membranes with exceptional mechanical flexibility, dimensional stability and ionic conductivity.

Alkaline all iron redox flow battery with a polyethylene/poly(styrene-co-divinylbenzene) interpolymer cation-exchange membrane

This work describes the suitability of a polyethylene styrene–DVB based interpolymer cation exchange membrane for use in a highly alkaline redox flow battery (RFB) with a [Fe(TEA)OH]²⁻/[Fe(TEA)OH]⁻ and Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ redox couple. The alkaline stability of the membrane for 1440 h was evaluated in 5 N NaOH containing a 200 mM Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ redox couple. It was assessed according to the changes in the electrochemical and physicochemical properties. The performance of the membrane was evaluated over 40 charge–discharge cycles at a current density of 5 mA cm⁻² current in a designed RFB cell. The obtained average coulombic efficiency (CE) was 92%, energy efficiency (EE) was 75%, voltage efficiency (VE) was 82% and volumetric efficiency was 34%. Under identical experimental conditions, the values of CE, EE, and VE for Nafion®-112 were 99%, 75%, and 76%, respectively. These results indicate the suitability of the polyethylene styrene–DVB based interpolymer cation exchange membrane for use in an alkaline RFB.

A polyethylene styrene–DVB interpolymer cation exchange membrane is reported for use in a highly alkaline all iron redox flow battery.

Highly conductive and chemically stable alkaline anion exchange membranes via ROMP of trans-cyclooctene derivatives

Alkaline anion exchange membranes (AAEMs) are an important component of alkaline exchange membrane fuel cells (AEMFCs), which facilitate the efficient conversion of fuels to electricity using nonplatinum electrode catalysts. However, low hydroxide conductivity and poor long-term alkaline stability of AAEMs are the major limitations for the widespread application of AEMFCs. In this paper, we report the synthesis of highly conductive and chemically stable AAEMs from the living polymerization of trans-cyclooctenes. A trans-cyclooctene-fused imidazolium monomer was designed and synthesized on gram scale. Using these highly ring-strained monomers, we produced a range of block and random copolymers. Surprisingly, AAEMs made from the random copolymer exhibited much higher conductivities than their block copolymer analogs. Investigation by transmission electron microscopy showed that the block copolymers had a disordered microphase segregation which likely impeded ion conduction. A cross-linked random copolymer demonstrated a high level of hydroxide conductivity (134 mS/cm at 80 °C). More importantly, the membranes exhibited excellent chemical stability due to the incorporation of highly alkaline-stable multisubstituted imidazolium cations. No chemical degradation was detected by 1H NMR spectroscopy when the polymers were treated with 2 M KOH in CD3OH at 80 °C for 30 d.

Metallo-polyelectrolytes as a class of ionic macromolecules for functional materials

The fields of soft polymers and macromolecular sciences have enjoyed a unique combination of metals and organic frameworks in the name of metallopolymers or organometallic polymers. When metallopolymers carry charged groups, they form a class of metal-containing polyelectrolytes or metallo-polyelectrolytes. This review identifies the unique properties and functions of metallo-polyelectrolytes compared with conventional organo-polyelectrolytes, in the hope of shedding light on the formation of functional materials with intriguing applications and potential benefits. It concludes with a critical perspective on the challenges and hurdles for metallo-polyelectrolytes, especially experimental quantitative analysis and theoretical modeling of ionic binding.