Development of ion conductive nanofibers for polymer electrolyte fuel cells

Surface proton hopping conduction mechanism dominant polymer electrolytes created by self-assembly of bicontinuous cubic liquid crystals

For the development of the next generation of fuel cells, it is essential to create an innovative design principle of polymer electrolytes that is beyond extension of the existing strategy. In the present study, we focused on the surface hopping proton conduction mechanism where an activation energy for proton conduction is greatly reduced by decreasing the distance between SO3- groups. Our gyroid nanostructured polymer film (Film-G), with a hydrophilic surface where the SO3- groups are aligned densely and precisely, shows high proton conductivity of the order of 10-2 S cm-1 when the water content is about 15 wt%. We reveal that the high proton conductivity of Film-G is attributed to the exhibition of an extremely-fast surface hopping conduction mechanism due to the reduced activation energy barrier along the gyroid minimal surface. This finding should introduce a game-changing novel opportunity in polymer electrolyte design.

De Novo Ion-Exchange Membranes Based on Nanofibers

The unique functions of nanofibers (NFs) are based on their nanoscale cross-section, high specific surface area, and high molecular orientation, and/or their confined polymer chains inside the fibers. The introduction of ion-exchange (IEX) groups on the surface and/or inside the NFs provides de novo ion-exchangers. In particular, the combination of large surface areas and ionizable groups in the IEX-NFs improves their performance through indices such as extremely rapid ion-exchange kinetics and high ion-exchange capacities. In reality, the membranes based on ion-exchange NFs exhibit superior properties such as high catalytic efficiency, high ion-exchange and adsorption capacities, and high ionic conductivities. The present review highlights the fundamental aspects of IEX-NFs (i.e., their unique size-dependent properties), scalable production methods, and the recent advancements in their applications in catalysis, separation/adsorption processes, and fuel cells, as well as the future perspectives and endeavors of NF-based IEMs.

Fabrication and Electrolyte Characterizations of Nanofiber Framework-Based Polymer Composite Membranes with Continuous Proton Conductive Pathways

For future fuel cell operations under high temperature and low- or non-humidified conditions, high-performance polymer electrolyte membranes possessing high proton conductivity at low relative humidity as well as suitable gas barrier property and sufficient membrane stability are strongly desired. In this study, novel nanofiber framework (NfF)-based composite membranes composed of phytic acid (Phy)-doped polybenzimidazole nanofibers (PBINf) and Nafion matrix electrolyte were fabricated through the compression process of the nanofibers. The NfF composite membrane prepared from the pressed Phy-PBINf showed higher proton conductivity and lower activation energy than the conventional NfF composite and recast-Nafion membranes, especially at low relative humidity. It is considered that the compression process increased the nanofiber contents in the composite membrane, resulting in the construction of the continuously formed effective proton conductive pathway consisting of the densely accumulated phosphoric acid and sulfonic acid groups at the interface of the nanofibers and the Nafion matrix. Since the NfF also improved the mechanical strength and gas barrier property through the compression process, the NfF composite polymer electrolyte membranes have the potential to be applied to future fuel cells operated under low- or non-humidified conditions.

Crosslinked Sulfonated Poly(vinyl alcohol)/Graphene Oxide Electrospun Nanofibers as Polyelectrolytes

Taking advantage of the high functionalization capacity of poly(vinyl alcohol) (PVA), bead-free homogeneous nanofibrous mats were produced. The addition of functional groups by means of grafting strategies such as the sulfonation and the addition of nanoparticles such as graphene oxide (GO) were considered to bring new features to PVA. Two series of sulfonated and nonsulfonated composite nanofibers, with different compositions of GO, were prepared by electrospinning. The use of sulfosuccinic acid (SSA) allowed crosslinked and functionalized mats with controlled size and morphology to be obtained. The functionalization of the main chain of the PVA and the determination of the optimum composition of GO were analyzed in terms of the nanofibrous morphology, the chemical structure, the thermal properties, and conductivity. The crosslinking and the sulfonation treatment decreased the average fiber diameter of the nanofibers, which were electrical insulators regardless of the composition. The addition of small amounts of GO contributed to the retention of humidity, which significantly increased the proton conductivity. Although the single sulfonation of the polymer matrix produced a decrease in the proton conductivity, the combination of the sulfonation, the crosslinking, and the addition of GO enhanced the proton conductivity. The proposed nanofibers can be considered as good candidates for being exploited as valuable components for ionic polyelectrolyte membranes.

Biomimetic Polyelectrolytes Based on Polymer Nanosheet Films and Their Proton Conduction Mechanism

We report a biomimetic polyelectrolyte based on amphiphilic polymer nanosheet multilayer films. Copolymers of poly( N-dodecylacrylamide- co-vinylphosphonic acid) [p(DDA/VPA)] form a uniform monolayer at the air-water interface. By depositing such monolayers onto solid substrates using the Langmuir-Blodgett (LB) method, multilayer lamellae films with a structure similar to a bilayer membrane were fabricated. The proton conductivity at the hydrophilic interlayer of the lamellar multilayer films was studied by impedance spectroscopy under temperature- and humidity-controlled conditions. At 60 °C and 98% relative humidity (RH), the conductivity increased with increasing mole fraction of VPA ( n) up to 3.2 × 10^-2 S cm^-1 for n = 0.41. For a film with n = 0.45, the conductivity decreased to 2.2 × 10^-2 S cm^-1 despite the increase of proton sources. The reason for this decrease was evaluated by studying the effect of the distance between the VPAs ( l_VPA) on the proton conductivity as well as their activation energy. We propose that for n = 0.41, l_VPA is the optimal distance not only to form an efficient two-dimensional (2D) hydrogen bonding network but also to reorient water and VPA. For n = 0.45, on the other hand, the l_VPA was too close for a reorientation. Therefore, we concluded that there should be an optimal distance to obtain high proton conductivity at the hydrophilic interlayer of such multilayer films.

Impact of the Solid Interface on Proton Conductivity in Nafion Thin Films

Proton conductivity of polyelectrolytes in the interfacial region with a solid is key to the performance of polyelectrolyte-based fuel cells. The proton conductivity of Nafion thin films was examined as a function of the thickness along both directions, normal and parallel to the interface. Neutron reflectivity measurements revealed that a water-containing multilamellar structure was formed at the substrate interface. The presence of the interfacial layer, or the two-dimensional proton-conductive pathway, suppressed and enhanced the out-of-plane and in-plane proton conductivities, respectively. The method of proton conductivity in the interfacial region differed from that in the bulk, namely, the Grotthuss mechanism. Using laminated films, we conclude by showing that the proton conductivity in the Nafion thin film changes on the basis of the interface-to-volume ratio. This knowledge will be helpful for the design of devices containing polyelectrolytes with solid materials.

Polyelectrolyte Composite Membranes Containing Electrospun Ion-Exchange Nanofibers: Effect of Nanofiber Surface Charges on Ionic Transport

Poly(vinyl alcohol) (PVA)-based ion-exchange nanofibers (IEX-NFs) and their composite polyelectrolyte membranes were prepared and characterized. The PVA-based NFs are well dispersed and form a three-dimensional network structure in the polymer matrix, Nafion. All of the prepared membranes show a similar ion-exchange capacity of ∼1.0 mmol g^-1. The ionic conductivities through the PVA- b-PSS-NF/Nafion composite membranes are superior to that of the Nafion membranes, but the conductivity through the PVA-NF/Nafion composite membrane is half that of the Nafion membrane. Our electrokinetic measurements clearly indicate that a high density of ion-exchange groups on the NF surface results in a continuous ionic transport path in the polymer matrix. In addition, the mechanical strength of all of the NF-composite membranes is improved compared with that of the membranes without NF.

Acid-Group-Content-Dependent Proton Conductivity Mechanisms at the Interlayer of Poly(N-dodecylacrylamide-co-acrylic acid) Copolymer Multilayer Nanosheet Films

The effect of the content of acid groups on the proton conductivity at the interlayer of polymer-nanosheet assemblies was investigated. For that purpose, amphiphilic poly(N-dodecylacrylamide-co-acrylic acid) copolymers [p(DDA/AA)] with varying contents of AA were synthesized by free radical polymerization. Surface pressure (π)-area (A) isotherms of these copolymers indicated that stable polymer monolayers are formed at the air/water interface for AA mole fraction (n) ≤ 0.49. In all cases, a uniform dispersion of the AA groups in the polymer monolayer was observed. Subsequently, polymer monolayers were transferred onto solid substrates using the Langmuir-Blodgett (LB) technique. X-ray diffraction (XRD) analyses of the multilayer films showed strong Bragg diffraction peaks, suggesting a highly uniform lamellar structure for the multilayer films. The proton conductivity of the multilayer films parallel to the direction of the layer planes were measured by impedance spectroscopy, which revealed that the conductivity increased with increasing values of n. Activation energies for proton conduction of ∼0.3 and 0.42 eV were observed for n ≥ 0.32 and n = 0.07, respectively. Interestingly, the proton conductivity of a multilayer film with n = 0.19 did not follow the Arrhenius equation. These results were interpreted in terms of the average distance between the AA groups (l_AA), and it was concluded that, for n ≥ 0.32, an advanced 2D hydrogen bonding network was formed, while for n = 0.07, l_AA is too long to form such hydrogen bonding networks. The l_AA for n = 0.19 is intermediate to these extremes, resulting in the formation of hydrogen bonding networks at low temperatures, and disruption of these networks at high temperatures due to thermally induced motion. These results indicate that a high proton conductivity with low activation energy can be achieved, even under weakly acidic conditions, by arranging the acid groups at an optimal distance.

Nanotechnology for environmentally sustainable electromobility

Electric vehicles (EVs) powered by lithium-ion batteries (LIBs) or proton exchange membrane hydrogen fuel cells (PEMFCs) offer important potential climate change mitigation effects when combined with clean energy sources. The development of novel nanomaterials may bring about the next wave of technical improvements for LIBs and PEMFCs. If the next generation of EVs is to lead to not only reduced emissions during use but also environmentally sustainable production chains, the research on nanomaterials for LIBs and PEMFCs should be guided by a life-cycle perspective. In this Analysis, we describe an environmental life-cycle screening framework tailored to assess nanomaterials for electromobility. By applying this framework, we offer an early evaluation of the most promising nanomaterials for LIBs and PEMFCs and their potential contributions to the environmental sustainability of EV life cycles. Potential environmental trade-offs and gaps in nanomaterials research are identified to provide guidance for future nanomaterial developments for electromobility.

Fabrication and electrolyte characterization of uniaxially-aligned anion conductive polymer nanofibers

We report the anion transport properties of anion conductive polymer nanofibers fabricated using an electrospinning method. The aligned nanofibers were prepared to evaluate the anion conductivity of the nanofibers. The aligned nanofibers had 10-15 times higher conductivity (up to 160 mS cm^-1 at 90 °C and 95%RH) and lower activation energy (23-25 kJ mol^-1) than the corresponding membranes, even though the nanofibers showed lower water uptake than the corresponding membranes. The anion conductivity measurement of nanofibers with different IEC values and anion species revealed that the dependency of anion conductivity on these factors was smaller in the nanofibers than in the corresponding membranes. These results indicate that effective ion transport pathways were formed in the nanofibers due to the phase separation and the polymer chain orientation along the nanofiber axis. These nanofibers are expected to be useful for future applications in alkaline fuel cells, air batteries, and other energy- and environment-related devices regardless of the ion species.