Permethyl Cobaltocenium (Cp*2Co+) as an Ultra-Stable Cation for Polymer Hydroxide-Exchange Membranes

Tetraarylphosphonium Cations with Excellent Alkaline-Resistant Performance for Anion-Exchange Membranes

To realize the robust anion exchange membrane (AEM)-based water splitting modules and fuel cells, the design and synthesis of tetraarylphosphonium (TAP) cations are described as a new class of cationic building blocks that exhibit remarkable alkaline stability under harsh conditions. TAP cations with highly sterically demanding aromatic substituents were efficiently synthesized from triarylphosphine derivatives and highly reactive arynes, whose alkaline degradation proved to be suppressed dramatically by the sterically demanding substituents. In the case of bis(2,5-dimethylphenyl)bis(2,4,6-trimethylphenyl)phosphonium, for example, approximately 60% of the cation survived for 27 d under the forced conditions (i.e., in 4 M KOH/CD3OH at 80 °C), while tetraphenylphosphonium degraded completely within 10 min in 1 M KOH/CD3OH at that temperature. Through the decomposition of the alkaline-stable TAP cations, not only triarylphosphine oxides, which are often reported to form via the nucleophilic attack toward the cationic phosphorus center, but also triarylphosphines were detected, which suggested the presence of other degradation mechanisms due to the sterically demanding aromatic substituents. In kinetic analyses, bis(2,5-dimethylphenyl)bis(2,4,6-trimethylphenyl)phosphonium was found to exhibit 52 times higher stability compared to benzyltrimethylammonium, which is often employed as the cationic building block for AEMs.

Aryl ether-free polymer electrolytes for electrochemical and energy devices

Anion exchange polymers (AEPs) play a crucial role in green hydrogen production through anion exchange membrane water electrolysis. The chemical stability of AEPs is paramount for stable system operation in electrolysers and other electrochemical devices. Given the instability of aryl ether-containing AEPs under high pH conditions, recent research has focused on quaternized aryl ether-free variants. The primary goal of this review is to provide a greater depth of knowledge on the synthesis of aryl ether-free AEPs targeted for electrochemical devices. Synthetic pathways that yield polyaromatic AEPs include acid-catalysed polyhydroxyalkylation, metal-promoted coupling reactions, ionene synthesis via nucleophilic substitution, alkylation of polybenzimidazole, and Diels-Alder polymerization. Polyolefinic AEPs are prepared through addition polymerization, ring-opening metathesis, radiation grafting reactions, and anionic polymerization. Discussions cover structure-property-performance relationships of AEPs in fuel cells, redox flow batteries, and water and CO2 electrolysers, along with the current status of scale-up synthesis and commercialization.

A New 2-Aminospiropyrazolylammonium Cation with Possible Uses in the Topical Areas of Ionic Liquids

Based on the fact that 2-aminospiropyrazolinium compounds and structurally related azoniaspiro compounds belong, in a broad sense, to the class of ionic liquids, we have reviewed them and studied their practical applications. To search for possible uses of a new 2-aminospiropyrazolinium compounds, it is necessary to undertake a comparison with the related class of azoniaspiro compounds based on available information. The structures of the well-studied class of azoniaspiro compounds and the related but little-studied class of 2-aminospiropyrazolinium have rigid frameworks, limited conformational freedom, and a salt nature. These properties give them the ability to organize the nearby molecular space and enable the structure-forming ability of azoniaspiro compounds in the synthesis of zeolites, as well as the ability to act as phase-transfer catalysts and have selective biological effects. Additionally, these characteristics enable their ability to act as electrolytes and serve as materials for anion exchange membranes in fuel cells and water electrolyzers. Thus, the well-studied properties of azoniaspiro compounds as phase-transfer catalysts, structure-directing agents, electrolytes, and materials for membranes in power sources would encourage the study of the similar properties of 2-aminospiropyrazolinium compounds, which we have studied in relation to in vitro antitubercular, antidiabetic, and antimicrobial activities.

Examining the Alkaline Stability of Tris(dialkylamino)sulfoniums and Sulfoxoniums

Herein, a synthetic method was developed to prepare a series of tris(dialkylamino)sulfonium and sulfoxonium cations from sulfur monochloride. Alkaline stability studies of these two cation families in 2 M KOH/CD₃OH solution at 80 °C revealed how degradation pathways change as a function of the oxidation state of the S center, as determined by ¹H NMR spectroscopy. The sulfonium cations (⁺S(NR₂)₃) typically degrade by nucleophilic attack at the sulfur atom with loss of an amino group and a proton transfer reaction to produce sulfoxides, while the sulfoxoniums (⁺O═S(NR₂)₃) tend to degrade by loss of an R group to form sulfoximines. From the group of sulfoniums and sulfoxoniums explored in this work, the tris(piperidino)sulfoxonium cation was noted to have excellent alkaline stability. This sulfoxonium should be suitable for future examination as a tethered cation in anion-exchange membranes (AEMs), or as a phase-transfer catalyst in biphasic reactions.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

A high-performance hydroxide exchange membrane enabled by Cu2+-crosslinked chitosan

Ion exchange membranes are widely used to selectively transport ions in various electrochemical devices. Hydroxide exchange membranes (HEMs) are promising to couple with lower cost platinum-free electrocatalysts used in alkaline conditions, but are not stable enough in strong alkaline solutions. Herein, we present a Cu²⁺-crosslinked chitosan (chitosan-Cu) material as a stable and high-performance HEM. The Cu²⁺ ions are coordinated with the amino and hydroxyl groups of chitosan to crosslink the chitosan chains, forming hexagonal nanochannels (~1 nm in diameter) that can accommodate water diffusion and facilitate fast ion transport, with a high hydroxide conductivity of 67 mS cm^-1 at room temperature. The Cu²⁺ coordination also enhances the mechanical strength of the membrane, reduces its permeability and, most importantly, improves its stability in alkaline solution (only 5% conductivity loss at 80 °C after 1,000 h). These advantages make chitosan-Cu an outstanding HEM, which we demonstrate in a direct methanol fuel cell that exhibits a high power density of 305 mW cm^-2. The design principle of the chitosan-Cu HEM, in which ion transport channels are generated in the polymer through metal-crosslinking of polar functional groups, could inspire the synthesis of many ion exchange membranes for ion transport, ion sieving, ion filtration and more.

Correlation between the Stability of Substituted Cobaltocenium and Molecular Descriptors

Metallocenium cations, used as a component in an anion exchange membrane of a fuel cell, demonstrate excellent thermal and alkaline stability, which can be improved by the chemical modification of the cyclopentadienyl rings with substituent groups. In this work, the relation between the bond dissociation energy (BDE) of the cobaltocenium (CoCp₂⁺) derivatives, used as a measure of the cation stability, and chemistry-informed descriptors obtained from the electronic structural calculations is established. The analysis of 12 molecular descriptors for 118 derivatives reveals a nonlinear dependence of the BDE on the electron donating-withdrawing character of the substituent groups coupled to the energy of the frontier molecular orbitals. A chemistry-informed feed-forward neural network trained using k-fold cross-validation over the modest data set is able to predict the BDE from the molecular descriptors with the mean absolute error of about 1 kcal/mol. The theoretical analysis suggests some promising modifications of cobaltocenium for experimental research. The results demonstrate that even for modest data sets the incorporation of the chemistry knowledge into the neural network architecture, e.g., through mindful selection and screening of the descriptors and their interactions, paves the way to gain new insight into molecular properties.

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.

Are Radicals Formed During Anion-Exchange Membrane Fuel Cell Operation?

In this paper we present a study on stable radicals and short-lived species generated in anion-exchange membrane (AEM) fuel cells (AEMFCs) during operation. The in situ measurements are performed with a micro-AEMFC inserted into a resonator of an electron paramagnetic resonance (EPR) spectrometer, which enables separate monitoring of radicals formed on the anode and cathode sides. The creation of radicals is monitored by the EPR spin trapping technique. For the first time, we clearly show the formation and presence of stable radicals in AEMs during and after long-term AEMFC operation. The main detected adducts during the operation of the micro-AEMFC are DMPO-OOH and DMPO-OH on the cathode side, and DMPO-H on the anode side. These results indicate that oxidative degradation involving radical reactions has to be taken into account when stability of AEMFCs is investigated.

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 Anion-Exchange Membrane Fuel Cells: Challenges in Electrocatalysis and Interfacial Charge Transfer

Alkaline anion-exchange membrane (AAEM) fuel cells have attracted significant interest in the past decade, thanks to the recent developments in hydroxide-anion conductive membranes. In this article, we compare the performance of current state of the art AAEM fuel cells to proton-exchange membrane (PEM) fuel cells and elucidate the sources of various overpotentials. While the continued development of highly conductive and thermally stable anion-exchange membranes is unambiguously a principal requirement, we attempt to put the focus on the challenges in electrocatalysis and interfacial charge transfer at an alkaline electrode/electrolyte interface. Specifically, a critical analysis presented here details the (i) fundamental causes for higher overpotential in hydrogen oxidation reaction, (ii) mechanistic aspects of oxygen reduction reaction, (iii) carbonate anion poisoning, (iv) unique challenges arising from the specific adsorption of alkaline ionomer cation-exchange head groups on electrocatalysts surfaces, and (v) the potential of alternative small molecule fuel oxidation. This review and analysis encompasses both the precious and nonprecious group metal based electrocatalysts from the perspective of various interfacial charge-transfer phenomena and reaction mechanisms. Finally, a research roadmap for further improvement in AAEM fuel cell performance is delineated here within the purview of electrocatalysis and interfacial charge transfer.

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.

Thermodynamics of Counterion Release Is Critical for Anion Exchange Membrane Conductivity

As the field of anion exchange membranes (AEMs) employs an increasing variety of cations, a critical understanding of cation properties must be obtained, especially as they relate to membrane ion conductivity. Here, to elucidate such properties, metal cation-based AEMs, featuring bis(norbornene) nickel, ruthenium, or cobalt complexes, were synthesized and characterized. In addition, isothermal titration calorimetry (ITC) was used to probe counterion exchange thermodynamics in order to understand previously reported differences in conductivity. The ion conductivity data reported here further demonstrated that nickel-complex cations had higher conductivity as compared to their ruthenium and cobalt counterparts. Surprisingly, bulk hydration number, ion concentration, ion exchange capacity, and activation energy were not sufficient to explain differences in conductivity, so the thermodynamics of metal cation-counterion association were explored using ITC. Specifically, for the nickel cation as compared to the other two metal-based cations, a larger thermodynamic driving force for chloride counterion release was observed, shown through a smaller Δ H_tot for counterion exchange, which indicated weaker cation-counterion association. The use of ITC to study cation-counterion association was further exemplified by characterizing more traditional AEM cations, such as quaternary ammoniums and an imidazolium cation, which demonstrated small variances in their enthalpic response, but an overall Δ H_tot similar to that of the nickel-based cation. The cation hydration, rather than its hydration shell or the bulk hydration of the membrane, likely played the key role in determining the strength of the initial cation-counterion pair. This report identifies for the first time how ITC can be used to experimentally determine thermodynamic quantities that are key parameters for understanding and predicting conductivity in AEMs.

Three-Decker Strategy Based on Multifunctional Layered Double Hydroxide to Realize High-Performance Hydroxide Exchange Membranes for Fuel Cell Applications

Herein, we present a three-decker layered double hydroxide (LDH)/poly(phenylene oxide) (PPO) for hydroxide exchange membrane (HEM) applications. Hexagonal LDH is functionalized with highly stable 3-hydroxy-6-azaspiro [5.5] undecane (OH-ASU) cations to promote it's ion-exchange capacity. The ASU-LDH is combined with triple-cations functionalized PPO (TC-PPO) to fabricate a three-decker ASU-LDH/TC-PPO hybrid membrane by an electrostatic-spraying method. Notably, the ASU-LDH layer with a porous structure shows many valuable properties for the ASU-LDH/TC-PPO hybrid membranes, such as improving hydroxide conductivity, dimensional stability, and alkaline stability. The maximum OH^- conductivity of ASU-LDH/TC-PPO hybrid membranes achieves 0.113 S/cm at 80 °C. Only 11.5% drops in OH^- conductivity was detected after an alkaline stability test in 1 M NaOH at 80 °C for 588 h, prolonging the lifetime of the TC-PPO membrane. Furthermore, the ASU-LDH/TC-PPO hybrid membrane realizes a maximum power density of 0.209 W/cm² under a current density of 0.391 A/cm². In summary, the ASU-LDH/TC-PPO hybrid membranes provide a reliable method for preparing high-performance HEMs.

Ultrastable and High Ion-Conducting Polyelectrolyte Based on Six-Membered N-Spirocyclic Ammonium for Hydroxide Exchange Membrane Fuel Cell Applications

In response to prepare high-stable and ion-conducting polyelectrolyte for hydroxide exchange membrane (HEM) applications, we present an ultrastable polyelectrolyte based on six-membered heterocyclic 6-azonia-spiro[5.5]undecane (ASU) and polyphenyl ether (PPO). A series of ASU-functionalized PPO polyelectrolytes (ASU-PPO), which can be easily dissolved in low-boiling pointing solvent, have been successfully synthesized by a remote-grafting method. The ASU precursor is stable in 1 M NaOH/D₂O at 80 °C for 2500 h as well as in 5 M NaOH/D₂O at 80 °C for 2000 h, and the predicted half-life of the ASU precursor would exceed 10 000 h, even higher in the future. Besides, these remote-grafting ASU-PPO polyelectrolytes are stable in 1 M NaOH_(aq) at 80 °C for 1500 h. Robust and pellucid segmented ASU and triple-ammonium-functionalized PPO-based HEMs attach OH^- conductivity of 96 mS/cm at 80 °C and realize maximal power density of 178 mW/cm² under current density of 401 mA/cm².