Anion Exchange Ionomers: Design Considerations and Recent Advances - An Electrochemical Perspective

Closed-Loop Recyclable Lithium and Sodium Conducting Covalent Adaptable Networks

Within the past two decades, covalent adaptable networks (CANs) have emerged as a novel class of dynamically crosslinked polymers, combining the benefits of thermosets and thermoplastics. Although some CANs with charged side chains have been reported, CANs with negatively charged backbones remain very limited. The integration of permanent charge into the backbones upon their formation could open up important new applications. Here, we introduce a series of aliphatic spiroborate-linked ionic covalent adaptable networks (ICANs), representing a new category of dynamic ionomer thermosets. These ICANs were synthesized using a catalyst-free, scalable, and environment-friendly method. Incorporating lithium or sodium as counter cations in these networks yielded promising ion conductivity without the need of plasticizers. The dynamic nature of the spiroborate linkages in these materials allows for rapid reprocessing and recycling under moderate conditions. Furthermore, their potential as flexible solid-state electrolytes is demonstrated in a device that maintained robust conducting performance under extreme physical deformation, coupled with effective self-healing properties. This research opens new possibilities for future development of dynamic ionomer thermosets and their potential applications in flexible electronic devices.

Influence of Commercial Ionomers and Membranes on a PGM-Free Catalyst in the Alkaline Oxygen Reduction

Hitherto, research into alkaline exchange membrane fuel cells lacked a commercial benchmark anionomer and membrane, analogous to Nafion in proton-exchange membrane fuel cells. Three commercial alkaline exchange ionomers (AEIs) have been scrutinized for that role in combination with a commercial platinum-group-metal-free Fe-N-C (Pajarito Powder) catalyst for the cathode. The initial rotating disc electrode benchmarking of the Fe-N-C catalyst's oxygen reduction reaction activity using Nafion in an alkaline electrolyte seems to neglect the restricted oxygen diffusion in the AEIs and is recommended to be complemented by measurements with the same AEI as used in the alkaline exchange membrane fuel cell (AEMFC) testing. Evaluation of the catalyst layer in a gas-diffusion electrode setup offers a way to assess the performance in realistic operating conditions, without the additional complications of device-level water management. Blending of a porous Fe-N-C catalyst with different types of AEI yields catalyst layers with different pore size distributions. The catalyst layer with Piperion retains the highest proportion of the original BET surface area of the Fe-N-C catalyst. The water adsorption capacity is also influenced by the AEI, with Fumion FAA-3 and Piperion having equally high capabilities surpassing Sustainion. Finally, the choice of the membrane influences the ORR performance as well; particularly, the low hydroxide conductivity of Fumion FAA-3 in the room temperature experiments mitigates the ORR performance irrespective of the AEI in the catalyst layer. The best overall performance at high current densities is shown by the Piperion anion exchange ionomer matched with Sustainion X37-50 membrane.

NiFe on CeO2, TiO2, and ZrO2 Supports as Efficient Oxygen Evolution Reaction Catalysts in Alkaline Media

The high cost and low energy efficiency of conventional water electrolysis methods continue to restrict the widespread adoption of green hydrogen. Anion exchange membrane (AEM) water electrolysis is a promising technology that can produce hydrogen using cost-effective transition-metal catalysts at high energy efficiency. Herein, we investigate the catalytic activity of nickel and iron nanoparticles dispersed on metal-oxide supports for the oxygen evolution reaction (OER), employing electrochemical testing with an anion exchange ionomer to evaluate their potential for application in AEM electrolyzers. We report the electrochemical performance of NiFe nanoparticles of varying Ni:Fe ratios on CeO2 for OER reaction, assessing the overpotential, Tafel slope, and electrochemical stability of the catalysts. Our findings indicate that Ni90Fe10 has the highest catalytic activity as well as stability. To further understand the role of different supports, we assess the electrocatalytic performance of Ni90Fe10 nanoparticles on two more supports - TiO2 and ZrO2. While CeO2 has the lowest overpotential, the other supports also show high activity and good performance at high current densities. TiO2 exhibits superior stability and its overpotential after chronopotentiometry measurements approaches that of CeO2 at high current densities. These results underscore the critical role of iron addition in enhancing nickel nanoparticles' catalytic activity and further emphasize the importance of metal oxide supports in improving catalyst stability and performance.

Ion-conducting polymer thin films via chemical vapor deposition polymerization

Ion-conducting polymers (ICPs), benefiting from the movement of ions instead of electrons, have attracted significant interest in various scientific and technological fields, including drug delivery, water purification, and electrochemical devices. This review aims to highlight recent advances in the synthesis of ICP thin films, with a particular focus on chemical vapor deposition (CVD) technologies. Traditional solution-based methods for ICP thin film deposition face challenges, including non-uniformity, low-throughput manufacturing, and the generation of hazardous wastes. In comparison, CVD eliminates the drawbacks associated with solution-based processes. They offer precise control film properties, including high purity, conformal coating, delicate control over thickness, etc. This review organizes the latest developments in CVD-based ICP synthesis, based on material properties and the synthesis strategy, into direct deposition and post-polymerization modification, ionogels, hydrogels, and ultrathin siloxane or silazane-based polymer films. By providing an up-to-date review of the materials and synthesis, we aim to position CVD polymerization as an effective strategy for future materials development/production and device fabrication in energy, sustainability, and healthcare where ion conductivity is desired.

Recent Progress and Perspectives on Functional Materials and Technologies for Renewable Hydrogen Production

Scientists worldwide have been inspecting hydrogen production routes and showing the importance of developing new functional materials in this domain. Numerous research articles have been published in the past few years, which require records and analysis for a comprehensive bibliometric and bibliographic review of low-carbon hydrogen production. Hence, a data set of 297 publications was selected after filtering journal papers published since 2010. The search keywords in the Scopus Database were "green hydrogen" and "low carbon hydrogen production and materials". The data were analyzed using the R Bibliometrix package. This analysis made it possible to determine the total annual publication rate and to segregate it by country, author, journal, and research institution. With a general upward trend in the total number of publications, China was identified as the leading country in research on the subject, followed by Germany and Korea. Keyword analysis and the chronological evolution of several important publications related to the topic showed the focus was on water splitting for low-carbon H2 production. Finally, this review provides future directions for technologies and functional materials for low-carbon hydrogen production.

Multiscale Understanding of Anion Exchange Membrane Fuel Cells: Mechanisms, Electrocatalysts, Polymers, and Cell Management

Anion exchange membrane fuel cells (AEMFCs) are among the most promising sustainable electrochemical technologies to help solve energy challenges. Compared to proton exchange membrane fuel cells (PEMFCs), AEMFCs offer a broader choice of catalyst materials and a less corrosive operating environment for the bipolar plates and the membrane. This can lead to potentially lower costs and longer operational life than PEMFCs. These significant advantages have made AEMFCs highly competitive in the future fuel cell market, particularly after advancements in developing non-platinum-group-metal anode electrocatalysts, anion exchange membranes and ionomers, and in understanding the relationships between cell operating conditions and mass transport in AEMFCs. This review aims to compile recent literature to provide a comprehensive understanding of AEMFCs in three key areas: i) the mechanisms of the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) in alkaline media; ii) recent advancements in the synthesis routes and structure-property relationships of cutting-edge HOR and ORR electrocatalysts, as well as anion exchange membranes and ionomers; and iii) fuel cell operating conditions, including water management and impact of CO2. Finally, based on these aspects, the future development and perspectives of AEMFCs are proposed.

Understanding the Roles and Regulation Methods of Key Adsorption Species on Ni-Based Catalysts for Efficient Hydrogen Oxidation Reactions in Alkaline Media

This review focuses on recent advancements in the development and understanding of nickel-based catalysts for the hydrogen oxidation reaction in alkaline media. Given the economic and environmental limitations associated with platinum group metals, nickel-based catalysts have emerged as promising alternatives due to their abundance, lower cost, and comparable catalytic properties. The review begins with an exploration of the fundamental HOR mechanisms, emphasizing the key roles of the reactive species in optimizing the catalytic activity of Ni-based catalysts. Thermodynamic and stability optimizations of nickel-based catalysts are thoroughly examined, focusing on alloying strategies, heteroatom incorporation, and the use of various support materials to enhance their catalytic performance and durability. The review also addresses the challenge of catalyst poisoning, particularly by carbon monoxide, and evaluates the effectiveness of different approaches to improve poison resistance. Finally, the review concludes by summarizing the key findings and proposing future research directions to further enhance the efficiency and stability of nickel-based catalysts for practical applications in anion exchange membrane fuel cells. The insights gained from this comprehensive analysis aim to contribute to the development of cost-effective and sustainable catalysts and facilitate the broader adoption of AEMFCs in the quest for clean energy solutions.

Impact of ionomers on porous Fe-N-C catalysts for alkaline oxygen reduction in gas diffusion electrodes

Alkaline exchange membrane fuel cells (AEMFCs) offer a promising alternative to the traditional fossil fuel due to their ability to use inexpensive platinum group metal (PGM)-free catalysts, which could potentially replace Platinum-based catalysts. Iron coordinated in nitrogen-doped carbon (Fe-N-C) single atom electrocatalysts offer the best Pt-free ORR activities. However, most research focuses on material development in alkaline conditions, with limited attention on catalyst layer fabrication. Here, we demonstrate how the oxygen reduction reaction (ORR) performance of a porous Fe-N-C catalyst is affected by the choice of three different commercial ionomers and the ionomer-to-catalyst ratio (I/C). A Mg-templated Fe-N-C is employed as a catalyst owing to the electrochemical accessibility of the Fe sites, and the impact of ionomer properties and coverage were studied and correlated with the electrochemical performance in a gas-diffusion electrode (GDE). The catalyst layer with Nafion at I/C = 2.8 displayed the best activity at high current densities (0.737 ± 0.01 VRHE iR-free at 1 A cm⁻²) owing to a more homogeneous catalyst layer, while Sustainion displayed a higher performance in the kinetic region at the same I/C. These findings provide insights into the impact of catalyst layer optimization to achieve optimal performance in Fe-N-C based AEMFCs.

De Novo Design of Aminopropyl Quaternary Ammonium-Functionalized Covalent Organic Frameworks for Enhanced Polybenzimidazole Anion Exchange Membranes

Quaternary ammonium functionalized covalent organic frameworks (COFs) have great potential to enhance hydroxide transport owing to crystalline ordered 1D nanochannels, however, suffer from limited quaternary ammonium functional monomers and poor membrane-forming ability. In this work, a novel aminopropyl quaternary ammonium-functionalized COF (DCOF) is designed and synthesized via a bottom-up strategy. The self-supporting DCOF membrane exhibits high crystallinity with a dense and orderly arrangement of quaternary ammonium groups (IEC, 2.07 mmol g-1), achieving a high hydroxide conductivity of 172.5 mS cm-1 and an extremely low water swelling of 5.3% at 80 °C. The exfoliated DCOF colloidal suspension is further incorporated into quaternary ammonium di-cation grafted polybenzimidazoles (DPBI) matrix. Molecular simulations reveal strong electrostatic and van der Waals interfacial interactions between DCOF and DPBI, which enable a high doping content of 20 wt.% and interconnected ionic channels through the surface and nanochannels of the DCOF. The DCOF/DPBI-20% membrane exhibits a tensile strength of 29.7 MPa, a hydroxide conductivity of 135.3 mS cm-1, and a low swelling ratio of 37.2% at 80 °C. A H2/O2 single cell assembled with the membrane reaches a peak power density of 323 mW cm- 2, surpassing most recently reported COF-based membranes.

Rechargeable Hydrogen Gas Batteries: Fundamentals, Principles, Materials, and Applications

The growing demand for renewable energy sources has accelerated a boom in research on new battery chemistries. Despite decades of development for various battery types, including lithium-ion batteries, their suitability for grid-scale energy storage applications remains imperfect. In recent years, rechargeable hydrogen gas batteries (HGBs), utilizing hydrogen catalytic electrode as anode, have attracted extensive academic and industrial attention. HGBs, facilitated by appropriate catalysts, demonstrate notable attributes such as high power density, high capacity, excellent low-temperature performance, and ultralong cycle life. This review presents a comprehensive overview of four key aspects pertaining to HGBs: fundamentals, principles, materials, and applications. First, detailed insights are provided into hydrogen electrodes, encompassing electrochemical principles, hydrogen catalytic mechanisms, advancements in hydrogen catalytic materials, and structural considerations in hydrogen electrode design. Second, an examination and future prospects of cathode material compatibility, encompassing both current and potential materials, are summarized. Third, other components and engineering considerations of HGBs are elaborated, including cell stack design and pressure vessel design. Finally, a techno-economic analysis and outlook offers an overview of the current status and future prospects of HGBs, indicating their orientation for further research and application advancements.

Ionomer and Membrane Designs for Low-temperature CO2 and CO Electrolysis

Low-temperature electroreduction of CO2 and CO (CO(2)RR) into valuable chemicals and fuels offers a promising pathway to reduce greenhouse gas emissions and achieve carbon neutrality. Today's low-temperature CO(2)RR technology relies on the use of ionomers, polymers with ionized groups, primarily as catalyst layer (CL) additives. In the meantime, ionomers can assemble into ion-exchange membranes (IEMs), serving as important components of electrolyzers. According to the ion-exchange functions, ionomer additives are classified as cation-exchange ionomers (CEIs) and anion-exchange ionomers (AEIs); similarly, IEMs are divided into cation-exchange membranes (CEMs) and anion-exchange membranes (AEMs), as well as the multilayer polymer electrolytes (MPEs). Recent studies show that ionomer additives can regulate the catalytic microenvironment and thereby enhance performance towards desired products. This Review discusses the roles of ionomer additives and IEMs in CO2 and CO reduction reactions, highlighting the latest mechanistic insights and performance advances. It outlines challenges in designing ionomer additives and IEMs to improve product selectivity, energy efficiency (EE), and operational lifetime of CO(2)RR electrolyzers, while also providing perspectives on future research directions. The aim is to connect the current status of ionomer and membrane development with performance metrics analysis, offering insights for the advancement of commercially relevant low-temperature CO(2)RR electrolyzers.

Sulfonated Microporous Polyxanthene Binder for High-Temperature Hydrogen Fuel Cells

High-temperature proton exchange membrane fuel cells based on phosphoric acid-doped polybenzimidazole (PBI) materials face challenges of low power output and low Pt utilization due to the lack of suitable electrode binders. We have developed a sulfonated microporous polymer material (namely, SPX, i.e., sulfonated polyxanthene) with excellent chemical stability, to be used as the electrode binder. The rigid and contorted polymer structure of SPX reduces the adsorption of the ionomer on the Pt catalyst surface, prevents phosphoric acid loss, and promotes the rapid transport of reactant gases and water molecules within the catalyst layer. The cell performance is thereby significantly improved, with a Pt utilization reaching 42.51%, and a peak power density approaching 805 mW cm-2 at 180 °C, surpassing the performance of cells using PBI as a binder.

Bioadhesive-Inspired Ionomer for Membrane Electrode Assembly Interface Reinforcement in Fuel Cells

Anion exchange membrane fuel cells promise a sustainable and ecofriendly energy conversion pathway yet suffer from insufficient performance and durability. Drawing inspiration from mussel foot adhesion proteins for the first time, we herein demonstrate catechol-modified ionomers that synergistically reinforce the membrane electrode assembly interface and triple-phase boundary inside catalyst layers. The resulting ionomers present exceptional alkaline stability with only slight ionic conductivity declines after treatment in 2 M NaOH aqueous solution at 80 °C for 2500 h. Adopting catechol-modified ionomer as both anion exchange membrane and binder achieves a single-cell performance increase of 34%, and more importantly, endows fuel cell operation at a current density of 0.4 A cm-2 for over 300 h with negligible performance degradation (with a cell voltage decay rate of 0.03 mV h-1). Combining theoretical and experimental investigations, we reveal the molecular adhesion mechanism between the catechol-modified ionomer and Pt catalyst and illuminate the effect on the catalyst layer microstructure. Of fundamental interest, this bioadhesive-inspired strategy is critical to enabling knowledge-driven ionomer design and is promising for diverse membrane electrode assembly configurational applications.

High-Performance Anion Exchange Membrane Water Electrolyzers Enabled by Highly Gas Permeable and Dimensionally Stable Anion Exchange Ionomers

Designing suitable anion exchange ionomers is critical to improving the performance and in situ durability of anion exchange membrane water electrolyzers (AEMWEs) as one of the promising devices for producing green hydrogen. Herein, highly gas-permeable and dimensionally stable anion exchange ionomers (QC6xBA and QC6xPA) are developed, in which bulky cyclohexyl (C6) groups are introduced into the polymer backbones. QC650BA-2.1 containing 50 mol% C6 composition shows 16.6 times higher H2 permeability and 22.3 times higher O2 permeability than that of QC60BA-2.1 without C6 groups. Through-plane swelling of QC650BA-2.1 decreases to 12.5% from 31.1% (QC60BA-2.1) while OH- conductivity slightly decreases (64.9 and 56.2 mS cm-1 for QC60BA-2.1 and QC650BA-2.1, respectively, at 30 °C). The water electrolysis cell using the highly gas permeable QC650BA-2.1 ionomer and Ni0.8Co0.2O in the anode catalyst layer achieves two times higher performance (2.0 A cm-2 at 1.69 V, IR-included) than those of the previous cell using in-house ionomer (QPAF-4-2.0) (1.0 A cm-2 at 1.69 V, IR-included). During 1000 h operation at 1.0 A cm-2, the QC650BA-2.1 cell exhibits nearly constant cell voltage with a decay rate of 1.1 µV h-1 after the initial increase of the cell voltage, proving the effectiveness of the highly gas permeable and dimensionally stable ionomer in AEMWEs.

From ionic clusters dynamics to network constraints in ionic polymer solutions

Physical networks formed by ionizable polymers with ionic clusters as crosslinks are controlled by coupled dynamics that transcend from ionic clusters through chain motion to macroscopic response. Here, the coupled dynamics, across length scales, from the ionic clusters to the networks in toluene swollen polystyrene sulfonate networks, were directly correlated, as the electrostatic environment of the physical crosslinks was altered. The multiscale insight is attained by coupling neutron spin echo measurements with molecular dynamics simulations, carried out to times typical of relaxation of polymers in solutions. The experimental dynamic structure factor is in outstanding agreement with the one calculated from computer simulations, as the networks are perturbed by elevating the temperature and changing the electrostatic environment. In toluene, the long-lived clusters remain stable over hundreds of ns across a broad temperature range, while the polymer network remains dynamic. Though the size of the clusters changes as the dielectric constant of the solvent is modified through the addition of ethanol, they remain stable but morph, enhancing the polymer chain dynamics.