Interaction Regulation Between Ionomer Binder and Catalyst: Active Triple-Phase Boundary and High Performance Catalyst Layer for Anion Exchange Membrane Fuel Cells

Polymerized and Colloidal Ionic Liquids─Syntheses and Applications

The breadth and importance of polymerized ionic liquids (PILs) are steadily expanding, and this review updates advances and trends in syntheses, properties, and applications over the past five to six years. We begin with an historical overview of the genesis and growth of the PIL field as a subset of materials science. The genesis of ionic liquids (ILs) over nano to meso length-scales exhibiting 0D, 1D, 2D, and 3D topologies defines colloidal ionic liquids, CILs, which compose a subclass of PILs and provide a synthetic bridge between IL monomers (ILMs) and micro to macro-scale PIL materials. The second focus of this review addresses design and syntheses of ILMs and their polymerization reactions to yield PILs and PIL-based materials. A burgeoning diversity of ILMs reflects increasing use of nonimidazolium nuclei and an expanding use of step-growth chemistries in synthesizing PIL materials. Radical chain polymerization remains a primary method of making PILs and reflects an increasing use of controlled polymerization methods. Step-growth chemistries used in creating some CILs utilize extensive cross-linking. This cross-linking is enabled by incorporating reactive functionalities in CILs and PILs, and some of these CILs and PILs may be viewed as exotic cross-linking agents. The third part of this update focuses upon some advances in key properties, including molecular weight, thermal properties, rheology, ion transport, self-healing, and stimuli-responsiveness. Glass transitions, critical solution temperatures, and liquidity are key thermal properties that tie to PIL rheology and viscoelasticity. These properties in turn modulate mechanical properties and ion transport, which are foundational in increasing applications of PILs. Cross-linking in gelation and ionogels and reversible step-growth chemistries are essential for self-healing PILs. Stimuli-responsiveness distinguishes PILs from many other classes of polymers, and it emphasizes the importance of segmentally controlling and tuning solvation in CILs and PILs. The fourth part of this review addresses development of applications, and the diverse scope of such applications supports the increasing importance of PILs in materials science. Adhesion applications are supported by ionogel properties, especially cross-linking and solvation tunable interactions with adjacent phases. Antimicrobial and antifouling applications are consequences of the cationic nature of PILs. Similarly, emulsion and dispersion applications rely on tunable solvation of functional groups and on how such groups interact with continuous phases and substrates. Catalysis is another significant application, and this is an historical tie between ILs and PILs. This component also provides a connection to diverse and porous carbon phases templated by PILs that are catalysts or serve as supports for catalysts. Devices, including sensors and actuators, also rely on solvation tuning and stimuli-responsiveness that include photo and electrochemical stimuli. We conclude our view of applications with 3D printing. The largest components of these applications are energy related and include developments for supercapacitors, batteries, fuel cells, and solar cells. We conclude with our vision of how PIL development will evolve over the next decade.

Investigating Electrode-Ionomer Interface Phenomena for Electrochemical Energy Applications

The endeavor to develop high-performance electrochemical energy applications has underscored the growing importance of comprehending the intricate dynamics within an electrode's structure and their influence on overall performance. This review investigates the complexities of electrode-ionomer interactions, which play a critical role in optimizing electrochemical reactions. Our examination encompasses both microscopic and meso/macro scale functions of ionomers at the electrode-ionomer interface, providing a thorough analysis of how these interactions can either enhance or impede surface reactions. Furthermore, this review explores the broader-scale implications of ionomer distribution within porous electrodes, taking into account factors like ionomer types, electrode ink formulation, and carbon support interactions. We also present and evaluate state-of-the-art techniques for investigating ionomer distribution, including electrochemical methods, imaging, modeling, and analytical techniques. Finally, the performance implications of these phenomena are discussed in the context of energy conversion devices. Through this comprehensive exploration of intricate interactions, this review contributes to the ongoing advancements in the field of energy research, ultimately facilitating the design and development of more efficient and sustainable energy devices.

Confinement of ionomer for electrocatalytic CO2 reduction reaction via efficient mass transfer pathways

Gas diffusion electrodes (GDEs) mediate the transport of reactants, products and electrons for the electrocatalytic CO2 reduction reaction (CO2RR) in membrane electrode assemblies. The random distribution of ionomer, added by the traditional physical mixing method, in the catalyst layer of GDEs affects the transport of ions and CO2. Such a phenomenon results in elevated cell voltage and decaying selectivity at high current densities. This paper describes a pre-confinement method to construct GDEs with homogeneously distributed ionomer, which enhances mass transfer locally at the active centers. The optimized GDE exhibited comparatively low cell voltages and high CO Faradaic efficiencies (FE > 90%) at a wide range of current densities. It can also operate stably for over 220 h with the cell voltage staying almost unchanged. This good performance can be preserved even with diluted CO2 feeds, which is essential for pursuing a high single-pass conversion rate. This study provides a new approach to building efficient mass transfer pathways for ions and reactants in GDEs to promote the electrocatalytic CO2RR for practical applications.

Crosslinked Polynorbornene-Based Anion Exchange Membranes with Perfluorinated Branch Chains

To investigate the effect of perfluorinated substituent on the properties of anion exchange membranes (AEMs), cross-linked polynorbornene-based AEMs with perfluorinated branch chains were prepared via ring opening metathesis polymerization, subsequent crosslinking reaction, and quaternization. The crosslinking structure enables the resultant AEMs (CFnB) to exhibit a low swelling ratio, high toughness, and high water uptake, simultaneously. In addition, benefiting from the ion gathering and side chain microphase separation caused by their flexible backbone and perfluorinated branch chain, these AEMs had high hydroxide conductivity up to 106.9 mS cm^-1 at 80 °C even at low ion content (IEC < 1.6 meq g^-1). This work provides a new approach to achieve improved ion conductivity at low ion content by introducing the perfluorinated branch chains and puts forward a referable way to prepare AEMs with high performance.

Impact of Anion Exchange Ionomers on the Electrocatalytic Performance for the Oxygen Reduction Reaction of B-N Co-doped Carbon Quantum Dots on Activated Carbon

Composite electrocatalytic electrodes made from B-N co-doped carbon quantum dots (CQD) and various anion exchange ionomers (AEI) are studied for the oxygen reduction reaction (ORR) in alkaline solutions. The quantity and positions of dopants in CQD, prepared by hydrothermal synthesis, are analyzed by various spectroscopies, including ¹¹B NMR spectroscopy that evidenced boronic acid at edge sites. The AEI are synthesized with various backbones, including more hydrophilic polysulfone, hydrophobic poly(alkylene biphenyl), and poly(2,6-dimethyl-1,4-phenylene oxide) with intermediate hydrophilicity; the functional groups are trimethylammonium moieties grafted on long (LC) or short (SC) side chains. The CQD/AEI ink is drop-casted on activated carbon paper, and the samples are fixed on a rotating disk electrode and studied in three-electrode configuration in oxygen-saturated 0.1 M KOH. The onset potentials are among the best in the literature (E_onset ≈ 0.94 V vs RHE). The highest electrocatalytic activity is observed for electrodes containing AEI with long side chains; the sample containing PPO LC attains excellent ORR currents approaching that of benchmark Pt/C cloth. The electrocatalytic performances are discussed in view of the many relevant AEI parameters, including hydrophilicity, oxygen permeability, catalyst dispersivity, and ionic conductivity.

Fuel cell performance improvement via the steric effect of a hydrocarbon-based binder for cathode in proton exchange membrane fuel cells

In this study, a sulfonated poly(ether sulfone) having cardo-type fluorenyl groups (FL-SPES) was investigated as a cathodic binder to improve fuel cell performance via increased the oxygen diffusion in the cathode. The maximum power density achieved by using the membrane electrode assembly (MEA) prepared with FL-SPES with a low ion exchange capacity (IEC) of 1.31 meq g^-1 was 520 mW cm^-2, which is more than twice as high as that of BP-SPES (210 mW cm^-2) having typical biphenyl groups with a similar IEC. At high IEC of 1.55 meq g^-1, the power density obtained by using BP-SPES was improved to 454 mW cm^-2 but remained lower than that of FL-SPES. In addition, although the IEC, swelling degree, and specific resistance were similar to each other, the gas permeability of FL-SPES was improved by approximately three times compared to that of BP-SPES. The steric structure of cardo-type FL-SPES increased the free volume between the polymer backbones, leading to an increase in gas transfer. Consequently, oxygen diffusion was promoted at the cathode, resulting in improved fuel cell performance.

Anion-exchange membrane water electrolyzers and fuel cells

The key components, working management, and operating techniques of anion-exchange membrane water electrolyzers and fuel cells are reviewed for the first time.