Through-plane Conductivity Research Articles

Charge transfer mechanisms in 40SiO2-40P2O5-20ZrO2 /sulfonated styrene-ethylene-butylene-styrene hybrid membranes for low temperature fuel cells

A series of hybrid membranes synthetized via sol-gel chemistry and direct infiltration method have been prepared, consisting of sulfonated styrene-ethylene-butylene-styrene block copolymer (sSEBS), as polymeric matrix, and a zirconia modified phosphosilicate (40SiO2–40P2O5–20ZrO2) as inorganic component. The infiltration procedure has been carried out by immersion of sSEBS membranes in a 40SiO2–40P2O5–20ZrO2 sol solution for 5, 10, 20, and 40 min. The hybrid infiltrated membranes (sSEBS-Zr) have been thermally characterized to further investigate their suitability as electrolytes for low temperature fuel cells. TGA thermograms showed that sSEBS-Zr were more thermally stable than sSEBS. DSC thermograms showed that the addition of inorganic component decreases the Tg of the polystyrene block in hybrid membranes sSEBS-Zr. DETA showed significant differences in the charge transfer mechanisms between low and high temperature regions. The through-plane proton conductivity analysis showed that the sSEBS-Zr infiltrated 10 min had a better proton conductive capacity at 333K, thus showing that longer infiltration times might induce excessive M-O-M′ bonds, causing competition for the available proton sites. These results indicated that the proposed methodology shows good agreement with experimental performance data in hydrogen PEMFCs. Nonetheless, when DMFCs are considered, minimizing the permeability of methanol enhances more the performance than increasing the proton conductivity.

Development of Corrosion Resistant and Electrically Conductive Coatings on Metallic Bipolar Plates for Applications in PEMFC

Proton exchange membrane fuel cell (PEMFC) is considered as one of the most promising alternative energy devices due to its high efficiency, low pollutants emission and possible application in automobiles. However, the application of PEMFC is still hindered by the high cost of some of its components such as bipolar plates (BPP) which are a key component in PEMFC to facilitate electron transfer, gas flow, heat and water removal. To reduce the cost, increase conductivity and durability, metallic bipolar plates, typically made of stainless steel, are normally used but they could suffer from severe corrosion in the acidic environment of PEMFC; the formation of corrosion products on the metal surface could reduce the through-plane electrical conductivity and increase the interfacial contact resistance (ICR) between the bipolar plates and gas diffusion layer, eventually causing high power loss in the fuel cell stack. Therefore, developing corrosion resistant and electrically conductive bipolar plates is of vital importance for the practical applications of PEMFC.Transition nitride coated stainless steel, such as titanium nitride (TiN) coated 316L SS, is considered as a possible solution to improve the performance metallic bipolar plates. In this work, TiN coated 316L SSs with heat treatment at different temperatures were prepared. The corrosion behaviours of the prepared samples were investigated in the simulated working environments of fuel cell. The heat treated samples exhibit the improved corrosion resistance compared with the pristine TiN coated samples and with an increase of the heat treatment temperature, the corrosion resistance tends to increase due to the formation of oxide and oxynitride on the sample surface. The ICR test results indicate that high temperature (450 ℃) heat treatment has detrimental effect on the electrical conductivity of samples due to the formation of a thick oxide dominated layer, while the samples heat treated at 300 ℃ only form the graded layers with suitable oxide amount which endows the coated specimens with a very low ICR value both before and after the corrosion tests.Moreover, TiN coatings with different amounts of Ta addition were also prepared. After the corrosion tests in the H2SO4 solution with pH=3 at 70 ℃, the results reveal that Ti150Ta30N samples exhibit the highest corrosion resistance compared with the pristine TiN coated samples and the samples with different amount of Ta addition. The steady state current density of Ti150Ta30N samples after long term potentiostatic polarization at 0.6 V (vs Ag/AgCl) is 0.02 μA/cm2 which is much lower than that of TiN coated samples. The Ti150Ta30N samples also presents a good electrical conductivity with low ICR values before and after the corrosion tests. These studies suggest that a suitable amount of oxygen or Ta incorporation into TiN is a feasible strategy to improve the corrosion resistance while maintaining considerable electrical conductivity of TiN samples.

Measurement of ion transport properties in ion exchange membranes for photoelectrochemical water splitting

Photoelectrochemical (PEC) water-splitting systems have the unique ability to produce renewable hydrogen directly from sunlight, independent of the electrical grid. These systems are therefore appealing technological options for resilient long-term energy storage. Ion selective membranes, such as monopolar and bipolar membranes, are a vital component of PEC water-splitting systems. These membranes allow for ionic conduction between the cathode and anode chambers, separation of products, and improved catalyst environments for reactions. In order to measure key properties and to study the performance of these ion exchange membranes, it is imperative to develop a robust testing protocol that can be used across the field. This paper introduces two standard electrochemical cells designed to directly measure ion transport properties in monopolar and bipolar membranes. The first electrochemical cell uses commercially available Pt disk electrodes to preform electrochemical impedance spectroscopy (EIS) and reliably measure through-plane conductivity of monopolar membranes. The second electrochemical cell uses four-point measurements with Luggin capillaries and a series of membrane configurations to perform current density-voltage and Faradaic efficiency (FE) measurements for water dissociation (WD) reactions on bipolar membranes. The cell designs and techniques laid out below allow for accurate measurement of ion transport parameters in ion exchange membranes, direct comparison of membranes being developed across the field, and in turn, greater advancements in ion exchange membranes and PEC water-splitting systems.

The effects of GDL anisotropic transport properties on the PEFC performance

PurposeThis paper aims to numerically investigate the impact of gas diffusion layer (GDL) anisotropic transport properties on the overall and local performance of polymer electrolyte fuel cells (PEFCs).Design/methodology/approachA three-dimensional numerical model of a polymer electrolyte fuel cell with a single straight channel has been developed to investigate the sensitivity of the fuel cell performance to the GDL anisotropic transport properties – gas permeability, diffusivity, thermal conductivity and electrical conductivity. Realistic experimentally estimated GDL transport properties were incorporated into the developed PEFC model, and a parametric study was performed to show the effect of these properties on fuel cell performance and the distribution of the key variables of current density and oxygen concentration within the cathode GDL.FindingsThe results showed that the anisotropy of the GDL must be captured to avoid overestimation/underestimation of the performance of the modelled fuel cell. The results also showed that the fuel cell performance and the distributions of current density and oxygen mass fraction within the cathode GDL are highly sensitive to the through-plane electrical conductivity of the GDL and, to a lesser extent, the through-plane diffusivity, and the thermal conductivity of the GDL. The fuel cell performance is almost insensitive to the gas permeability of the GDL.Practical implicationsThis study improves the understanding of the importance of the GDL anisotropy in the modelling of fuel cells and provides useful insights on improving the efficiency of the fuel cells.Originality/valueRealistic experimentally estimated GDL transport properties have been incorporated into the PEFC model for the first time, allowing for more accurate prediction of the PEFC performance.

Processing Effects on the Through-Plane Electrical Conductivities and Tensile Strengths of Microcellular-Injection-Molded Polypropylene Composites with Carbon Fibers.

Polymers reinforced with conducting fibers to achieve electrical conductivity have attracted remarkable attention in several engineering applications, and injection molding provides a cost-effective way for mass production. However, the electrical performance usually varies with the molding conditions. Moreover, high added content of conducting fibers usually results in molding difficulties. In this study, we propose using microcellular (MuCell) injection molding for polypropylene (PP)/carbon fiber (CF, 20, and 30 wt%) composites and hope that the MuCell injection molding process can improve both electrical and mechanical performance as compared with conventional injection molded (CIM) parts under the same CF content. Both molding techniques were also employed with and without gas counter pressure (GCP), and the overall fiber orientation, through-plane electrical conductivity (TPEC), and tensile strength (TS) of the composites were characterized. Based on the various processing technologies, the results can be described in four aspects: (1) Compared with CIM, microcellular foaming significantly influenced the fiber orientation, and the TPECs of the samples with 20 and 30 wt% CF were 18–78 and 5–8 times higher than those of the corresponding samples molded by CIM, respectively; (2) when GCP was employed in the CIM process, the TPEC of the samples with 20 and 30 wt% CF increased by 3 and 2 times, respectively. Similar results were obtained in the case of microcellular injection molding—the TPEC of the 20 and 30 wt% composites increased by 7–74 and 18–32 times, respectively; (3) although microcellular injection molding alone (i.e., without GCP) showed the greatest influence on the randomness of the fiber orientation and the TPEC, the TS of the samples was the lowest due to the uncontrollable foaming cell size and cell size uniformity; (4) in contrast, when GCP was employed in the microcellular foaming process, high TS was obtained, and the TPEC was significantly enhanced. The high foaming quality owing to the GCP implementation improved the randomness of fiber orientation, as well as the electrical and mechanical properties of the composites. Generally speaking, microcellular injection combined with gas counter pressure does provide a promising way to achieve high electrical and mechanical performance for carbon-fiber-added polypropylene composites.

Liquid metal incorporated graphene oxide films with enhanced through-plane thermal conductivity and flame resistance

With the recent upsurge in electronic and telecommunications industries, there is an extensive demand for thermal interface materials (TIMs) with integrated high thermal conductivity and flame resistance to ensure the performance, lifetime, and safety of electronic devices. Traditional polymer-based TIMs have failed to meet this demand owing to their inherent weaknesses like the utilization of rigid fillers, low through-plane conductivity, and poor thermal stability. Recently, graphene-based films have attracted considerable attention to use as TIMs due to pristine graphene's excellent in-plane thermal conductivity (2000–5000 Wm−1K−1). However, unsatisfactory through-plane thermal conductivity (<0.1 Wm−1K−1), resulting from the horizontally stacked graphene sheets, remains a major challenge in designing graphene-based TIMs for practical applications. Herein, we design a liquid metal (Galinstan) incorporated graphene oxide (GO) films with improved through-plane thermal conductivity and flame resistance via a combined solution mixing and vacuum filtration method. Gallium from the liquid metal Galinstan reacts with GO and reduces it while constructing heat transfer pathways along the through-plane direction. The resultant composite film demonstrates a through-plane thermal conductivity of 0.42 Wm−1K−1, which is much better than graphene oxide films (0.01 Wm−1K−1) and previous similar studies. Moreover, it has excellent thermal stability and can withstand an alcohol flame for 30 s, proving its potential to meet the ever-increasing demand for TIMs.

Incoherent phonon transport dominates heat conduction across van der Waals superlattices

Heat conduction mechanisms in superlattices could be different across different types of interfaces. Van der Waals superlattices are structures physically assembled through weak van der Waals interactions by design and may host properties beyond the traditional superlattices limited by lattice matching and processing compatibility, offering a different type of interface. In this work, natural van der Waals (SnS)1.17(NbS2)n superlattices are synthesized, and their thermal conductivities are measured by time-domain thermoreflectance as a function of interface density. Our results show that heat conduction of (SnS)1.17(NbS2)n superlattices is dominated by interface scattering when the coherent length of phonons is larger than the superlattice period, indicating that incoherent phonon transport dominates through-plane heat conduction in van der Waals superlattices even when the period is atomically thin and abrupt, in contrast to conventional superlattices. Our findings provide valuable insights into the understanding of the thermal behavior of van der Waals superlattices and devise approaches for effective thermal management of superlattices depending on the distinct types of interfaces.

How Electrochemical Impedance Spectroscopy Helps Drive Innovation in Fully Hydrocarbon, Reinforced Polymer Electrolyte Membranes

The realm of hydrocarbon-based polymer electrolyte membranes (PEMs) has undergone substantial changes over the past decade. A new era has emerged, based upon strong, fundamental R&D emphasizing both improved performance across a wider range of operating conditions, and greater combined chemical-mechanical stability, when integrated into a state-of-the-art fuel cell. The groundwork has been laid for commercialization of highly promising, fluorine-free technologies which simultaneously outperform previous generation materials, and address weaknesses historically attributed to all hydrocarbon-based membranes. Hydrocarbons address critical issues with perfluorinated sulfonic acid (PFSA) materials. Reduced gas crossover allows longer component lifetimes, operational window for system pressures, and reduced hydrogen venting. Higher temperature tolerances allow system operation or excursions to ≥ 100 °C, which affords a myriad of potential benefits such as reduction in heat exchanger volume, improved system water management, and increased catalyst activity due to mitigated activation losses. Furthermore, higher operating temperatures go hand-in-hand with greater pollution tolerance, and hence reduced hydrogen purity requirements.The award-winning hydrocarbon-based PEM, Pemion®, produced by Ionomr Innovations, is nested at the center of these exciting developments. Mechanically reinforced composite membranes are produced at scale utilizing advanced roll-to-roll manufacturing techniques, affording a hydrocarbon-based PEM that, for the first time, exhibits performance parallel to or greater than incumbent perfluorinated sulfonic acid (PFSA)-based PEMs. These developments, however, would not be possible without powerful characterization methods such as electrochemical impedance spectroscopy (EIS). Assessments of a membrane’s in-plane and through-plane proton conductivity are used to derive valuable insight to its constituents, the ion-conducting ionomer and chemically-inert mechanical reinforcement, as well as their interrelation. The anisotropic nature of the composite membranes is probed via comparison of in-plane and through-plane conductivity. The sensitivity of through-plane conductivity measurements to experimental parameters such as cell pressure are probed, and a new cell design is realized to reduce measurement error. These data help identify favorable and unfavorable material processing and manufacturing techniques, and their implications to in-situ fuel cell performance. The net consequence of these findings is establishing EIS as a core component of an iterative material design and production process for output of consistent, high-performance PEMs.

Maximizing the transport properties of two-dimensional fibrous materials

Fibrous materials show inferior anisotropic properties and inadequate performance due to their non-optimized microstructures despite the tremendous advances evolved in the manufacturing of customized geometries. The present work provides the optimized microstructures of fibrous materials to maximize their transport properties. By presenting easy-to-use closed-form formulae, the study further reveals the realistic functionality of the in-plane and through-plane transport properties of two-dimensional fibrous media with the key parameters namely fiber diameter, fiber spacing and fiber angle at a non-varying porosity. The results show that equal fiber spacings in different layers enhance all the through-plane and in-plane bulk properties. However, the optimal fiber spacings that maximize or minimize a bulk property in the in-plane direction of interest can vary depending on the other parameters including porosity. It is also found that for high heat transfer and low pressure drop applications, the minimum and maximum fiber diameters are respectively required. The orthogonal arrangement of fibers with equal spacings results in the maximum through-plane conductivity and heat transfer coefficient. The provided Excel spreadsheet calculators can be readily used for predicting and optimizing the conductivity, permeability, pressure drop and Nusselt number of two-dimensional fibrous materials in the in-plane and through-plane directions.

Towards n-type conductivity in hexagonal boron nitride

Asymmetric transport characteristic in n- and p-type conductivity has long been a fundamental difficulty in wide bandgap semiconductors. Hexagonal boron nitride (h-BN) can achieve p-type conduction, however, the n-type conductivity still remains unavailable. Here, we demonstrate a concept of orbital split induced level engineering through sacrificial impurity coupling and the realization of efficient n-type transport in 2D h-BN monolayer. We find that the O 2pz orbital has both symmetry and energy matching to the Ge 4pz orbital, which promises a strong coupling. The introduction of side-by-side O to Ge donor can effectively push up the donor level by the formation of another sacrificial deep level. We discover that a Ge-O2 trimer brings the extremely shallow donor level and very low ionization energy. By low-pressure chemical vapor deposition method, we obtain the in-situ Ge-O doping in h-BN monolayer and successfully achieve both through-plane (~100 nA) and in-plane (~20 nA) n-type conduction. We fabricate a vertically-stacked n-hBN/p-GaN heterojunction and show distinct rectification characteristics. The sacrificial impurity coupling method provides a highly viable route to overcome the n-type limitation of h-BN and paves the way for the future 2D optoelectronic devices.

High-density and low-density gas diffusion layers for proton exchange membrane fuel cells: Comparison of mechanical and transport properties

Gas diffusion layers (GDL) play multi-roles in proton exchange membrane fuel cells, including gas-water transport, thermal-electron conduction and mechanical support. Mechanical strength and transport properties are essential for GDLs. In this work, high-density (paper-type) and low density (felt-type) GDLs are scanned and reconstructed using X-ray computed tomography. Porosities under different compression ratios are compared and discussed. Effective diffusivity and liquid water permeability are calculated using pore-scale modeling and lattice Boltzmann method. Mechanical strength, anisotropic thermal-electrical resistivity for two types of GDLs are obtained using compression tests and thermal-electrical conductivity measurements. Results show that the porosity, diffusivity, permeability, and through-plane thermal-electrical conductivity of felt-type GDL are significantly higher than that of paper-type GDL owing to the higher porosity and fiber-clusters oriented along the through-plane direction. The in-plane electrical resistivity of paper-type GDL is lower than that of felt-type GDL. The mechanical strength of felt-type GDL is much lower, but the fibers of paper-type GDL are more easily to be broken because of its lower elasticity. The results obtained may guide microstructure optimization and performance improvement of GDLs.

Study on fiber-reinforced proton exchange membrane using high-surface-energy substrate

A fiber-reinforced composite membrane has been fabricated using a high-surface-energy substrate via a facile one-step doctor blading technique, which would be a perfect match for roll-to-roll process. The nanofiber mat based on sulfonated polyether-ether-ketone and polyvinylidene fluoride acting as reinforcement frame shows ultra-high surface energy, and enables efficient impregnation of perfluorosulfonated ionomer into porous substrate to form a uniform-layer structure with nanofibers interspersed throughout the composite membrane on the thickness direction. As a result, the fiber-reinforced composite membranes display enhanced mechanical stability and reduced swelling rate. With high packing density and continuous PFSI distribution, the as-prepared composite membranes perform better both on through-plane conductivity and cell performance under a low humidification than those of commercial membranes based on porous poly (tetra fluoroethylene). • Membranes reinforced with a high-surface-energy substrate (sulfonated polyether-ether-ketone and polyvinylidene fluoride) were prepared as PRS by one-step doctor blading technique. • PRS demonstrates a uniform-layer structure unlike commercially used PTFE-reinforced membranes. • PRS performs better than commercially used PTFE-reinforced membranes in fuel cell under all humidity conditions and is significantly higher without humidification.

High-performance fuel cells using Nafion composite membranes with alignment of sulfonated graphene oxides induced by a strong magnetic field

Proton exchange membrane (PEM) is a key component of proton exchange membrane fuel cells (PEMFCs) and provides proton conduction between fuel and oxidant, and it is vitally important and practical to develop PEMs with high proton conductivity through membranes. However, it is still a major challenge to develop such membranes with high through-plane proton conductivity. In this study, Nafion composite membranes with sulfonated graphene oxides (SGOs) were prepared from self-assembly sol-gel either in or not in a strong magnetic field. Microstructures, water uptake, and proton conductivity of the SGO/Nafion composite membranes have been investigated extensively. For the SGO/Nafion composite membranes recast without a magnetic field, it is found that the water uptake and in-plane proton conductivity are greatly increased with the increasing SGO content. Interestingly, SGO/Nafion composite membranes with a low SGO content of ∼ 0.5 wt% exhibit lower crystallinity and higher in-plane proton conductivity. For the SGO/Nafion composite membranes recast in a strong through-plane magnetic field, it is very appealing to find their through-plane proton conductivity is significantly higher than that of the composite membranes recast without a magnetic field. This is attributed to that the protons transport in the membranes preferentially along the water channels primarily interconnected by the aligned SGOs through-plane induced by the strong magnetic field, as confirmed by SEM. Applied in a single PEMFC, the resultant SGO/Nafion composite membrane with 1 wt% SGOs, recast in a strong magnetic field of 1.5 T, demonstrates a high power-density of 1.056 W cm-2, which is far superior to those of pristine Nafion and SGO/Nafion composite membranes recast without a magnetic field. The study provides a novel strategy for preparing proton exchanging membranes with high performances for new-generation fuel cells.

Thermally conductive silicone rubber composites with vertically oriented carbon fibers: A new perspective on the heat conduction mechanism

The continual increase in power density and consumption of modern electronic devices calls for high-performance thermal interface materials (TIMs). In this work, silicone rubber composites with vertically oriented magnetic carbon fibers (o-MCF/SR) were successfully prepared via the cast molding and filler orientation upon uniform magnetic field in a vacuum environment. The through-plane thermal conductivity of o-MCF/SR composites showed an unusual power-law growth with increasing MCF loading. At a 9 vol% MCF loading, the o-MCF/SR composite exhibited a maximum through-plane thermal conductivity of 4.72 W/(m·K) with 2045.5 % TCE, about 3.55 W/(m·K) higher than that of SR composites with randomly dispersed MCF (λ = 1.17 W/(m·K), TCE = 431.8 %). To reveal the underlying through-plane heat conduction mechanism of o-MCF/SR composites, finite element simulation combined with classical effective medium theory model and machine learning method was used. A homogeneous unit fiber-filled composite (HUFC) was modelled, and a new parameter influencing the through-plane thermal conductivity, i.e., shape factor (b/a), was introduced here for the first time. Theoretically, this shape factor was dependent on the filler content and indirectly reflected the assembly of matrix and 1D filler in HUFC fit to a parallel-series hybrid model. Furthermore, a decrease of b/a was practically found with increasing MCF loading, and the dominant series-type assembly gradually transformed to parallel-type assembly of the o-MCF/SR composites. This work not only developed a method to fabricate desirable TIMs that hold great promise for advanced thermal management, but also introduced a new parameter to reveal the underlying mechanism for heat conduction capacity of polymer composites.

High-Resolution Ion-Flux Imaging of Proton Transport through Graphene|Nafion Membranes.

In 2014, it was reported that protons can traverse between aqueous phases separated by nominally pristine monolayer graphene and hexagonal boron nitride (h-BN) films (membranes) under ambient conditions. This intrinsic proton conductivity of the one-atom-thick crystals, with proposed through-plane conduction, challenged the notion that graphene is impermeable to atoms, ions, and molecules. More recent evidence points to a defect-facilitated transport mechanism, analogous to transport through conventional ion-selective membranes based on graphene and h-BN. Herein, local ion-flux imaging is performed on chemical vapor deposition (CVD) graphene|Nafion membranes using an “electrochemical ion (proton) pump cell” mode of scanning electrochemical cell microscopy (SECCM). Targeting regions that are free from visible macroscopic defects (e.g., cracks, holes, etc.) and assessing hundreds to thousands of different sites across the graphene surfaces in a typical experiment, we find that most of the CVD graphene|Nafion membrane is impermeable to proton transport, with transmission typically occurring at ≈20–60 localized sites across a ≈0.003 mm2 area of the membrane (>5000 measurements total). When localized proton transport occurs, it can be a highly dynamic process, with additional transmission sites “opening” and a small number of sites “closing” under an applied electric field on the seconds time scale. Applying a simple equivalent circuit model of ion transport through a cylindrical nanopore, the local transmission sites are estimated to possess dimensions (radii) on the (sub)nanometer scale, implying that rare atomic defects are responsible for proton conductance. Overall, this work reinforces SECCM as a premier tool for the structure–property mapping of microscopically complex (electro)materials, with the local ion-flux mapping configuration introduced herein being widely applicable for functional membrane characterization and beyond, for example in diagnosing the failure mechanisms of protective surface coatings.

Tungsten nanoparticle anchoring on boron nitride nanosheet-based polymer nanocomposites for complex radiation shielding

The synergistic effect cooperation of hybrids composed of 2D materials and metal nanoparticles have been studied extensively due to their exotic properties leading to various technological applications. We prepared tungsten nanoparticles (W NPs) decorated on boron nitride nanosheets (BNNS) hybrids and demonstrated the enhanced properties of their polyethylene (PE) nanocomposites. The uniformly decorated W NPs on the BNNS surface improved the performance by maximizing the specific surface area of the hybrids by suppressing the restacking of BNNS and aggregation of NPs. The W-BNNS/PE nanocomposites showed enhanced mechanical properties (16.4% higher yield strength and 58.1% higher elastic modulus than pure PE) and conductivity (35% higher through-plane conductivity than pure PE) due to the synergistic effects of nanoparticle anchoring and interconnection. The nanocomposites exhibited outstanding radiation shielding ability for neutron ray (4.80 cm2/g) and gamma (γ) ray (0.093 cm2/g) radiation. The results suggest that nanocomposites containing W-BNNS hybrids have good potential for application as complex radiation shielding materials.

Preparation and characterization of carbon fiber reinforced plastics (CFRPs) incorporating through-plane-stitched carbon fibers

In this study, novel methods are proposed for manufacturing carbon fiber-reinforced plastics (CFRPs) to enhance their thermal and electrical conductivities and mechanical properties. Polyacrylonitrile (PAN)- and pitch-based carbon fibers were used as the stitching materials, to provide conductive paths through the CFRP body in the through-plane direction. The through-plane thermal and electrical conductivities of the stitched CFRPs were drastically enhanced by a maximum of 2.23 times, and 54.7 times compared to non-stitched CFRP, respectively. The mechanical properties of the stitched CFRPs were also investigated using dynamic mechanical analysis. The results indicate that the pitch-based carbon fibers enhanced the storage modulus of CFRPs, while CFRPs stitched using PAN-based carbon fibers showed decreased value. A thin metal coating was applied to both sides of the CFRPs, and the through-plane thermal conductivity of the silver-coated, stitched CFRPs was slightly enhanced (maximum of 9%), while the through-plane electrical conductivity of the CFRPs was dramatically enhanced (maximum of 498 times). The results indicate that a synergistic effect between the stitched carbon fibers and metal coating can improve the through-plane thermal and electrical conductivities of the CFRPs. This implies that CFRPs manufactured using these simple and effective strategies can be used in the space industry.

A parametric study on the performance requirements of key fuel cell components for the realization of high-power automotive fuel cells

This study examined the performance limitations of polymer electrolyte membrane fuel cells (PEMFC) for high power operations. The effects of improving electron, heat, and oxygen transport within a PEMFC on the cell performance were investigated using a three-dimensional, multiscale, two-phase PEMFC model. For more realistic PEMFC simulations at high current densities, the model newly accounts for anisotropic electron and heat transport in porous components of the membrane electrode assembly (MEA) and the non-uniform MEA compression/deformation occurring during PEMFC stack assembly. The simulations revealed improved transport properties that can be realized by optimizing the component material and design, and the degree of performance improvement was examined for a wide range of operating current densities up to 3.0 A/cm2. The simulations showed that improving the through-plane electronic conductivity and thermal conductivity of MEA components is critical for achieving high-power PEMFC performance. By contrast, the properties of the cathode catalyst layer, such as the Pt particle size and oxygen permeation rate through the ionomer film, are relatively less important under high current density PEMFC operations.

Permselectivity of ionene-based, Aemion® anion exchange membranes

This work reports the permselectivity of an ionene-based anion exchange membrane – ionenes are a class of polymer in which cationic charged groups are located directly on the polymer backbone, as opposed to being pendant. The ionone studied, in the form of commercially available Aemion® anion exchange membranes, is based on hexamethyl-p-terphenyl poly (bibenzimidazolium) (HMT-PMBI), which has been explored in several electrochemical devices, yet their transport numbers, permselectivity, and through-plane conductivity, have not been reported. Measurement of transport properties was carried out on membranes (AF1-HNN8-25, AF1-HNN8-50, AF1-HNN5-25, AF1-HNN5-50) of varying ion exchange capacity and thickness, using the static method in which inherent experimental errors due to liquid junction potentials were elucidated and circumvented. Permselectivity, area specific conductance, and conductivity of Aemion® AEMs are correlated using their fixed charge group concentration. Due to its low water content and high fixed group concentration, the permselectivity of AF1-HNN5-X is higher than AF1-HNN8-X AEMs, despite the latter's higher IEC, but its ionic resistance is lower. Thicker AEMs were confirmed to be more permselective, which can be explained by their relatively lower water content and consequently higher fixed charge group concentration. The permselectivity, area specific conductance, and permselectivity to resistance ratio (α/R) of Aemion® AF1-HNN5-50 were found to be comparable to Selemion AMV, a common anion exchange membrane. The permselectivity/resistance (α/R) value of AF1-HNN8-25 was three times higher than Selemion AMV by virtue of its high ionic conductance.

Electrospun Ion-Conducting Composite Membrane with Buckling-Induced Anisotropic Through-Plane Conductivity.

Fuel cell (FC) is an attractive green alternative for today's fuel combustion systems. In common FCs, a polymer electrolyte membrane selectively conducts protons but blocks the passage of electrons and fuel. Nafion, the current benchmark membrane material, has a superior conductivity owing to unique morphology comprising randomly oriented elongated ionic nanochannels within its Teflon-like matrix. Channel orientation enhances Nafion conductivity, yet there has been no facile method to induce a stable alignment in the desired through-plane (TP) direction. Here, we report an approach based on dual electrospun Nafion-PVDF nanofiber composites that yields a stable TP alignment. It utilizes extreme thinness and strong inherent orientation within electrospun nanofibers, which is readily converted to TP alignment by plunging an electrospun nanofiber mat into a thin slit, resulting in nanofiber buckling and subsequent consolidation. Using TEM and SAXS, we demonstrate a pronounced and sustained TP ion channel orientation in prepared membranes, yielding a highly anisotropic swelling and conductivity exceeding that of bulk Nafion when normalized to Nafion content. The analysis also highlights the importance of PVDF as a stabilizing component, preserving orientation upon annealing, while a similarly prepared pure Nafion membrane loses anisotropy. The approach holds potential to advance the FC technology by overcoming current limitations of ionomeric membranes.