Through-plane Electrical Conductivity Research Articles

Investigation of electrical-mechanical performance of epoxy-based nanocomposites filled with hybrid electrically conductive fillers

This study aimed at investigating the influence of utilizing electrically conductive nanofillers such as graphene nanoplatelets (GNP) and carbon nanotubes (CNT) as secondary fillers on the electrical-mechanical behavior of epoxy/synthetic graphite (epoxy/SG) composites where SG was a microfiller. The composites were prepared by an internal mixer and compression molding and the electrical-mechanical behavior of the composites was characterized by the measurements of the in-plane and through-plane electrical conductivity as well as the flexural strength and modulus. The microstructure of the composites was examined by scanning electron microscopy (SEM). The results showed that the addition of GNP to epoxy/SG composites in the expense of SG had a negative impact on the electrical conductivity as the GNP content increased up to 10 wt%. Composites containing M-grade GNP with a particle size of 15 μm showed higher in-plane and through-plane conductivity values compared to that having the C-grade GNP with particle size less than 2 μm. The incorporation of CNT to the epoxy/SG/GNP composites had a pronounced improvement to the electrical and flexural properties. The maximum values of in-plane and through-plane conductivity reported were almost equal of about 41 S cm−1 for composites containing a GNP:CNT weight ratio of 4:6. The effect of fillers morphologies and distribution on the behavior of the composites was also investigated.

Injection-molded lightweight and high electrical conductivity composites with microcellular structure and hybrid fillers

Polypropylene/carbon black (PP/CB) and PP/CB/multiwalled carbon nanotube (PP/CB/MWCNT) composites were fabricated by solid and foam injection molding, with the goal of enhancing the electrical conductivity of the composites while decreasing the cost of the final product. The foaming behavior and through-plane (T-P) electrical conductivity of the composites were characterized and analyzed. Cell growth increased the interconnection of the conductive fillers, changed the filler orientation, and enhanced the T-P electrical conductivity of the composites. Under appropriate processing conditions (200°C melt temperature, 70 cm3/s injection flow rate, and 5% void fraction), the T-P electrical conductivity of the foam PP/CB composites was 5 orders of magnitude higher than that of the solid composites (from 5.877 × 10−12 S/m to 1.010 × 10−7 S/m). Moreover, the T-P electrical conductivity values of the PP/CB and PP/CB/MWCNT were compared at the same conductive fillers content (15 wt%). The results showed that the T-P electrical conductivity of the PP/CB/MWCNT composites was far higher than that of the PP/CB composites by almost five orders of magnitude because the MWCNT acted as a bridge between CB particles, and a unique geometric shape was formed in the system. The T-P electrical conductivity of the foam PP/CB/MWCNT composites with 15 wt% carbon fillers was higher than that of the solid PP/CB composites with 20 wt% carbon fillers. This study reveals that the effect of foaming and the addition of hybrid fillers can improve the T-P electrical conductivity of plastic products, which is very important for the development of lightweight conductive plastics.

Comparative Investigation of Polyphenylene Sulfide Polymer-Graphite Bipolar Plates for Fuel Cell Application

Bipolar plates (BPPs) are one of the key components of the polymer electrolyte membrane fuel cell (PEM FC) which play an important role in the development of fuel cell application. They should fulfil many functions in a fuel cell stack such as feeding H2 and O2 gases through the flow field upon operation, heat transfer, physical separation of individual fuel cells in series, collection and transport of electrons from anode to cathode of adjacent cells [1]. Among the BPPs used in PEM FC the polyphenylene sulfide (PPS) polymer-graphite BPP has a good electrical conductivity, lower cost, higher flexibility and chemical stability, in comparison to metallic and graphite plates. However, some challenges such as higher thickness and weight in comparison to metallic bipolar plates are limiting their use in automotive application [1]. In addition, it was reported that corrosion of carbon is taking place under operation conditions of fuel cells [2]. Therefore, in this work two compression molded commercial polyphenylene sulfide (PPS) polymer-graphite composite bipolar plates BPP-20 and BPP-15 from two different manufacturers with amount of PPS binder equal to 20 and 15 %, respectively, were employed for an analytical comparative investigation using physical and electrochemical analysis. Chemical stability of BPPs was investigated after storage in phosphoric acid. Corrosion resistance and electrochemical aging of the BPPs were studied by using linear sweep voltammetry, cyclic voltammetry and electrochemical impedance spectroscopy in phosphoric acid. In order to characterize the difference in physical properties water contact angle measurement and through-plane electrical conductivity experiments were performed. The morphology of the BPPs surfaces was determined by scanning electron and confocal microscopy. The chemical composition was investigated by X-ray diffraction and energy dispersive X-ray spectroscopy, respectively. Comparison of BPPs revealed that the surface roughness and electrical conductivity is lower while acid uptake and hydrophilicity is higher for BPP with higher amount of PPS. Moreover, there was a trend of reduced corrosion rate for BPP with increased PPS content. These properties could be explained based on structure and electrical characteristics of the materials. [1] S. S. Araya, F. Zhou, V. Liso, S. L. Sahlin, J. R. Vang, S. Thomas, X. Gao, C. Jeppesen, S. K. Kær, International Journal of Hydrogen Energy 2016, 41, 21310-21344. [2] T. Engl, L. Gubler, T. J. Schmidt, Journal of the Electrochemical Society 2015, 162, F291-F297. Figure 1

Comparative Investigation of Polyphenylene Sulfide Polymer-Graphite Bipolar Plates for Fuel Cell Application

In this work two compression molded commercial polyphenylene sulfide (PPS) polymer-graphite composite bipolar plates (BPPs) with different amount of polymer binder material from two different manufacturers were employed for an analytical comparative investigation using physical and electrochemical analysis. Chemical stability of BPPs was investigated after storage in phosphoric acid. Corrosion resistance and electrochemical aging of the BPPs were studied by using linear sweep voltammetry, cyclic voltammetry and electrochemical impedance spectroscopy in phosphoric acid. In order to characterize the difference in physical properties water contact angle measurement and through-plane electrical conductivity experiments were performed. The morphology of the BPPs surface was determined by scanning electron and confocal microscopy. The chemical composition was investigated by X-ray diffraction and energy dispersive X-ray spectroscopy, respectively. Comparison of BPPs revealed that the surface roughness and electrical conductivity is lower while acid uptake and hydrophilicity is higher for BPP with higher amount of PPS. Moreover, there was a trend of corrosion rate reducing for BPP with increased PPS content. This behaviour could be explained based on structure and electrical properties of the materials.

Extruded polycarbonate/Di-Allyl phthalate composites with ternary conductive filler system for bipolar plates of polymer electrolyte membrane fuel cells

Here, we report multifunctional polycarbonate (PC)-based conductive polymer composites (CPCs) with outstanding performance manufactured by a simple extrusion process and intended for use in bipolar plate (BPP) applications in polymer electrolyte membrane (PEM) fuel cells. CPCs were developed using a ternary conductive filler system containing carbon nanotube (CNT), carbon fiber (CF), and graphite (G) and by introducing di-allyl phthalate (DAP) as a plasticizer to PC matrix. The samples were fabricated using twin-screw extrusion followed by compression molding and the microstructure, electrical conductivity, thermal conductivity, and mechanical properties were investigated. The results showed a good dispersion of the fillers with some degree of interconnection between dissimilar fillers. The addition of DAP enhanced the electrical conductivity and tensile strength of the CPCs. Due to its plasticizing effect, DAP reduced the processing temperature by 75 °C and facilitated the extrusion of CPCs with filler loads as high as 63 wt% (3 wt% CNT, 30 wt% CF, 30 wt% G). Consequently, CPCs with the through-plane electrical, in-plane electrical and thermal conductivities and tensile strength of 4.2 S cm−1, 34.3 S cm−1, 2.9 W m−1 K−1, and 75.4 MPa, respectively, were achieved. This combination of properties indicates the potential of PC-based composites enriched with hybrid fillers and plasticizers as an alternative material for BPP application.

Electrical Properties and Electromagnetic Interference Shielding Effectiveness of Interlayered Systems Composed by Carbon Nanotube Filled Carbon Nanofiber Mats and Polymer Composites.

The demand for multifunctional requirements in aerospace, military, automobile, sports, and energy applications has encouraged the investigation of new composite materials. This study focuses on the development of multiwall carbon nanotube (MWCNT) filled polypropylene composites and carbon nanofiber composite mats. The developed systems were then used to prepare interlayered composites that exhibited improved electrical conductivity and electromagnetic interference (EMI) shielding efficiency. MWCNT-carbon nanofiber composite mats were developed by centrifugally spinning mixtures of MWCNT suspended in aqueous poly(vinyl alcohol) solutions. The developed nanofibers were then dehydrated under sulfuric acid vapors and then heat treated. Interlayered samples were fabricated using a nanoreinforced polypropylene composite as a matrix and then filled with carbon fiber composite mats. The in-plane and through-plane electrical conductivity of an eight-layered flexible carbon composite (0.65 mm thick) were shown to be 6.1 and 3.0 × 10−2 S·cm−1, respectively. The EMI shielding effectiveness at 900 MHz increased from 17 dB for the one-layered composite to 52 dB for the eight-layered composite. It was found that the reflection of the electromagnetic waves was the dominating mechanism for EMI shielding in the developed materials. This study opens up new opportunities for the fabrication of novel lightweight materials that are to be used in communication systems.

Comparison of Carbon Based Bipolar Plate Materials for Polymer Electrolyte Membrane Fuel Cells

Two types of graphite composite based materials, BPP-01 with higher amount of polymer binder in comparison to BPP-02, are investigated and reported for their suitability as bipolar plates (BPPs) in polymer electrolyte membrane fuel cells (PEMFC). Through-plane electrical conductivity of pristine BPP-01 and BPP-02 are 10.31 and 11.38 S cm-1, respectively. The electrochemical behaviour of aged BPPs is examined by cyclic voltammetry (CV) in sulfuric acid. Chemically aged BPP-01 exhibits lower double layer capacitance than BPP-02. In order to observe cathodic and anodic corrosion behaviour of BPPs, potentiostatic polarisation experiments are also performed. BPP-02 undergoes higher corrosion rate than BPP-01 during cathodic polarisation. BPP-02 surface change can be observed even with bare eyes. Conversely, physical changes do not occur after anodic polarisation for both BPPs which were thoroughly characterised by profilometer, optical microscope and micro-computed tomography (μ-CT). The higher polymer content based graphite composite bipolar plate (BPP-01) endures the potential applied in PEMFC applications.

Multi-component MWCNT/NG/EP-based bipolar plates with enhanced mechanical and electrical characteristics fabricated by compression moulding

In this work, multi-walled carbon nanotubes (MWCNTs), natural graphite (NG) and epoxy (EP) were used to fabricate bipolar plates by the compression moulding technique. The mechanical and thermal behaviours of the as-developed material were investigated in terms of the effects of temperature on the morphological, mechanical and electrical properties. It was observed that the flexural strength of the MWCNT/NG/EP composite decreased with increasing temperature. The highest flexural strength (27.05 MPa) was recorded at 25 °C. When heated to 200 °C, the flexural strength of the material decreased by up to 2.40 MPa. The shore hardness of the MWCNT/NG/EP composite decreased with increasing temperature. The highest shore hardness (69.7 HD) was recorded at a temperature of 25 °C, while the lowest shore hardness (31.3 HD) was recorded at 200 °C. The temperature-dependent through-plane and in-plane electrical conductivities of the MWCNT/NG/EP composite were also investigated, although its contact, bulk and forward resistivity values were found to decrease. The experimental results revealed that the effect of temperature on the thermal and mechanical properties of the bipolar plate material was different at different temperatures. The synergistic electrical nature of the as-designed material may be ascribed to the phenomenon of electron transfer among the channels of natural graphite and MWCNTs. The enhanced mechanical behaviour was attributed to the presence of EP in the composite material.

Structure-property-processing investigation of electrically conductive polypropylene nanocomposites

Abstract This paper investigates the structure-property-processing correlations of electrically conductive polypropylene (PP) nanocomposites. The process parameters and fabrication techniques of PP-based composite materials were studied. Various structures of carbon allotrope-based materials, including synthetic graphite (SG), exfoliated graphene nanoplatelets (xGnP), multi-walled carbon nanotubes (MWCNTs) and carbon black (CB), were used to fabricate the PP-based nanocomposites. The nanocomposites were prepared by either direct melt mixing using an internal mixer or by ball milling of components before the melt mixing process. The electrical and flexural properties were measured. In order to understand the conductivity behavior, both in-plane and through-plane electrical conductivities were measured. The results showed that the incorporation of the xGnP into PP/60 wt.% SG composites resulted in a slight increase of the in-plane conductivities and had a minimal effect on the through-plane conductivities. The addition of MWCNTs and CB to the PP/SG/xGnP composites had a significant effect on the electrical properties and was more pronounced in the case of MWCNTs. The flexural properties of all samples were much lower than those of pure PP. The interface between the filler and the PP matrix and the morphology of the composite materials were observed from the fracture surfaces of the composites using scanning electron microscopy (SEM). In addition, SEM was employed to observe adhesion, microstructural homogeneity, orientation of the xGnP platelets and agglomeration in the composites.

Highly Conductive and Strong Graphite-Phenolic Resin Composite for Bipolar Plate Applications

Composite materials for bipolar plate applications in proton exchange membrane fuel cells were fabricated from synthetic graphite (SG), natural graphite (NG), or expanded graphite (EG) and novolac phenolic resin using compression molding. In comparing the resultant samples, the EG composite exhibited the best properties with a density of ∼1.55 g/cm3, flexural strength of 109 MPa, and modulus of 24 GPa even with a high graphite loading of 80 wt % and a low plate thickness of ∼0.9 mm, as well as in-plane conductivity of 182 S/cm. The EG content was further varied to find the optimal composition. The effects of using carbon nanotube sheets (buckypapers) or multiwalled carbon nanotubes as reinforcements to the EG-resin composite were investigated. A modified approach of measuring through-plane electrical conductivity capable of separated analysis of bulk resistance and interfacial contact resistance was attempted. The corrosion resistance, thermal conductivity, and gas permeability of EG-based composites were...

In-plane and through-plane electrical conductivities and contact resistances of a Mercedes-Benz catalyst-coated membrane, gas diffusion and micro-porous layers and a Ballard graphite bipolar plate: Impact of humidity, compressive load and polytetrafluoroethylene

By precisely deconvoluting the bulk from the contact and residual resistances, the in-plane electrical conductivities of virgin (dry or as-is), wet and dried catalyst-coated membranes (CCMs), a micro-porous layer (MPL) and several dry and wet gas diffusion layers (GDLs) with different PTFE loading are accurately determined. The through-plane conductivities of the GDLs, MPL and a bipolar plate (BPP) are rigorously measured using the two-thickness method. The contact resistances of the BPP with the GDLs (3–25 mΩ cm2) and MPL (14–57 mΩ cm2) are precisely deconvoluted as well. Moreover, the impact of pressure, PTFE and humidity on the resistances are thoroughly investigated.This study reveals that the in-plane electrical conductivity of a wet CCM (70 S m−1) is three times lower than its dry version (210 S m−1). It is found that the in-plane conductivity of GDLs (∼6000 S m−1) does not change with PTFE, MPL and humidity and is significantly higher than those of wet and dry CCMs. The through-plane conductivities of BPP (1935 S m−1), untreated and PTFE-treated GDLs (300–1500 S m−1), and MPL (50–350 S m−1) are in descending order. The through-plane GDL conductivity increases with pressure and decreases with PTFE, MPL and cyclic loading but it is not affected by humidity. It is also found that MPL and high PTFE loading increase, but pressure decreases, the GDL-BPP contact resistance dramatically.

Enhanced thermomechanical and electrical properties of multiwalled carbon nanotube paper reinforced epoxy laminar composites

Thermomechanical and electrical properties of multiwalled carbon nanotubes (MWCNTs) bucky paper reinforced epoxy laminar composites have been studied. Incorporation of bucky paper in epoxy matrix led to improvement in thermomechanical properties but it reduced through-plane electrical conductivity. Therefore, 0.05 wt.% of MWCNTs is incorporated as secondary network in epoxy matrix to improve the electrical conductivity. The storage modulus for a 0.05 wt.% dispersed MWCNTs in epoxy resin impregnated 20 plies of bucky paper based composites (20Ply.05) was 4.84 GPa as compared to 2.24 GPa of pure epoxy. The glass transition temperature of 20Ply.05 laminar composite reaches to 191.6 °C as compared to pure epoxy (168.5 °C). The increment in electrical conductivity (792%) is reflected in improved electromagnetic shielding effectiveness (SE) over a wide frequency range of X & Ku band. The SE of more than 50 dB for 20Ply.05 laminar composite in both the bands was obtained.

Ultra-large sized graphene nano-platelets (GnPs) incorporated polypropylene (PP)/GnPs composites engineered by melt compounding and its thermal, mechanical, and electrical properties

Abstract In this study, polypropylene (PP)/graphene nanoplatelet (GnPs) nanocomposites with very large sized GnPs (∼150 μm) are prepared by melt extrusion followed by injection molding. A number of characteristics including thermal, mechanical, and electrical properties are analyzed. DSC shows that the introduction of GnPs facilitates the crystallization of polymer matrix due to a role of GnPs that serves as seeds for heterogeneous nucleation, and XRD reveals that GnPs have a minor induction effect of β crystals. Taking advantage of the large size and high aspect ratio of GnPs, a relatively low percolation threshold of ∼2.99 vol% is obtained with highly increased in-plane and through-plane electrical conductivity. The fitting of experimental data to the percolation theory indicates that GnPs are three dimensionally dispersed within the polymer matrix. The composites exhibit relatively limited mechanical enhancement due to compromising of GnPs by the shear force introduced during the compounding process. Overall, the usage of large sized GnPs is clearly beneficial for obtaining high electrical conductivity with a less amount of filler, but an enhanced dispersion of fillers with controlled morphology is required to achieve great physical and mechanical properties of the PP/GnPs composites.

Surface coating and texturing on stainless-steel plates to decrease the contact resistance by using screen printing

Stainless steel is an attractive material for use in bipolar plates of polymer-electrolyte-membrane fuel cells, except for its high interfacial contact resistance (ICR). Inexpensive surface treatment is required to decrease the ICR. A carbonaceous conductive composite was coated on stainless-steel plate surfaces by using a screen-printing technique. A grid-like texture of the same material as the coating was also printed on the coated plate surface. The cross section showed that conductive carbon particles were well dispersed in the coating layer, which favors through-plane electrical conductivity. The coated and textured plates exhibited a much lower ICR than that of bare stainless steel. The ICR of textured plates was lower than that of coated plates under lower compaction pressures. A single cell with coated and textured bipolar plates exhibited higher power densities than that of bare stainless-steel bipolar plates.

Impact of the spatial distribution of high content of carbon nanotubes on the electrical conductivity of glass fiber fabrics/epoxy composites fabricated by RTM technique

To incorporate high content of carbon nanotubes (CNTs) into a polymer matrix is greatly desired to obtain highly conductive structural composites, but it is difficult in practice to be realized due to increased resin viscosity along with dispersion difficulty. In this study, we demonstrate the production of carbon nanotubes (CNTs)-integrated glass fiber fabrics (GFF)/epoxy composites by resin transfer molding (RTM), in which the mass fraction of CNTs on GFF varies from 0 to as high as 8 wt%. Three different methods were compared to load CNTs on GFFs, which include one-step in-situ CVD growth, catalyst pre-deposited two-step CVD growth and CNT suspension casting. For each type of CNTs-GFFs, a series of RTM composites with varied CNT contents were prepared. The impact of the CNT spatial distribution on the composite in-plane and through-plane electrical conductivities was compared at different CNT loading levels by subtracting contact resistance between the sample and the electrodes. It was found that the composites reinforced with one-step CVD-grown CNTs/GFFs show the best electrical conductivity among the three types of the composites studied, due to good CNT structure and their alignment on GF surface. A detailed discussion was made on the composite conductive behavior with the different CNTs-GFFs. This study helps to provide insight to the production of high-performance structural composites functionalized with high contents of CNTs at low cost.

Highly Conductive, Strong, Thin, and Lightweight Graphite-Phenolic Resin Composite for Bipolar Plates in Proton Exchange Membrane Fuel Cells

Featuring high efficiency and power density, relatively low operating temperature (60-80°C), quiet operation, and environmental friendliness, proton exchange membrane fuel cells (PEMFCs) have become promising power sources for transportation, stationary, and portable applications (1, 2). Bipolar plate is a vital and multifunctional component in PEMFCs, accounting for 60-80% of fuel cell stack weight and 30-45% of stack cost (3). While conventional bipolar plate materials are graphite or metal, graphite-polymer composites have recently been investigated as bipolar plate materials to maximally utilize the electrical conductivity of graphite and the mechanical strength of polymers and to exploit the benefits of smaller size, lighter weight, easier fabrication, and reduced cost. In this study, different polymer composites for bipolar plates based on synthetic graphite (SG), natural graphite (NG), and expanded graphite (EG) at a fixed graphite to polymer weight ratio were prepared by compression molding and compared in terms of three main properties: flexural strength, flexural modulus, and in-plane electrical conductivity. Novolac phenolic resin was chosen to form the composite because of its low cost and easy processing. EG turned out to be the best candidate, exhibiting flexural strength of 109 MPa and flexural modulus of 24 GPa even with a high graphite loading of 80 wt% and a low plate thickness of 0.9 mm, as well as in-plane conductivity of 182 S/cm. The EG content was then varied in a range of 60-85 wt% to find out the optimal composite composition. After this, a sandwich structure of buckypaper-composite-buckypaper was created to probe the effects of adding buckypaper on the mechanical and electrical properties. In addition, carbon nanotubes (CNTs) were incorporated into the EG-novolac resin composite for the first time, to the best of our knowledge, at low concentration of 0.5-2 wt% in the hope of potential benefits through nanostructuring. Through-plane electrical conductivity and corrosion resistance were also assessed for the composites with MWNTs 0-2 wt%. Previous research efforts on through-plane conductivity measurement either involved use of a reference material (4, 5), or calculated the sample bulk resistance roughly by assuming that all the contact resistances were the same (6), or did not provide specific description capable of separated analysis of interfacial contact resistance and bulk resistance (7). Herein a modified approach is suggested for detailed and reliable evaluation of through-plane resistance and contact resistance. As to corrosion behavior studies, a majority of them concentrated on metallic plates, some emerged in recent years to delve into the corrosion stability of graphite composites (8, 9), while the field of corrosion characteristics of EG-based bipolar plates and the influence that CNTs would have on their corrosion properties remains unexplored. The present work thus aims to shed some light on this aspect as well.

Design of Electrically Conductive Structural Composites by Modulating Aligned CVD-Grown Carbon Nanotube Length on Glass Fibers.

Function-integration in glass fiber (GF) reinforced polymer composites is highly desired for developing lightweight structures and devices with improved performance and structural health monitoring. In this study, homogeneously aligned carbon nanotube (CNT) shell was in situ grafted on GF by chemical vapor deposition (CVD). It was demonstrated that the CNT shell thickness and weight fraction can be modulated by controlling the CVD conditions. The obtained hierarchical CNTs-GF/epoxy composites show highly improved electrical conductivity and thermo-mechanical and flexural properties. The composite through-plane and in-plane electrical conductivities increase from a quasi-isolator value to ∼3.5 and 100 S/m, respectively, when the weight fraction of CNTs grafted on GF fabric varies from 0% to 7%, respectively. Meanwhile, the composite storage modulus and flexural modulus and strength improve as high as 12%, 21%, and 26%, respectively, with 100% retention of the glass transition temperature. The reinforcing mechanisms are investigated by analyzing the composite microstructure and the interfacial adhesion and wetting properties of CNTs-GF hybrids. Moreover, the specific damage-related resistance variation characteristics could be employed to in situ monitor the structural health state of the composites. The outstanding electrical and structural properties of the CNTs-GF composites were due to the specific interfacial and interphase structures created by homogeneously grafting aligned CNTs on each GF of the fabric.

Screen-printed nanostructured composites as thermal interface materials for insulated gate bipolar transistors heat dissipation applications

Thermal interface materials (TIMs) are of crucial importance in enhancing heat transfer and minimizing exceedingly high temperatures in high-density electronics. TIMs functionally aim to reduce the microscale crevices by penetrating the gap between the contacting rigid surfaces. We prepared silver nanoparticles (SNPs) and single-wall carbon nanotubes (SWCNTs)-based nanocomposites with graphite nanoplatelets (GNPs) by using a screen printing technique for conformal spreading of SNPs and SWCNTs with various weight-loading ratios on top of a layer containing the GNPs and measured its thermal conductivity and electrical conductivities in both through-plane and in-plane directions. In particular, the 10% SNPs enhanced TIMs showed highly anisotropic behavior in both electrical and thermal conductivities, viz., in-plane electrical conductivity exceeds its through-plane counterpart by three orders of magnitude, the highest in-plane electrical conductivity was 7.85 S/cm, and through-plane electrical conductivity was 0.00287 S/cm. Similarly, anisotropic behavior was found for the in-plane thermal conductivity ∼8.4 W/mK and through-plane thermal conductivity ∼0.35943 W/mK. In addition, scanning electron microscopy (SEM) was performed to reveal the typical morphology and elements’ existence of screen-printed TIMs. The proposed TIMs were put into the actual 15-kW converter to test the thermal management performance.

An accurate method to determine the through-plane electrical conductivity and to study transport properties in film samples

The through-plane conductivity of a film sample is critically important because it largely affects the performance of batteries, capacitors, and thermoelectric devices. In this study, we developed a modified four-probe through-plane electrical conductivity measurement method using a coaxial structure. This method is general and works for free-standing film samples. We studied different samples including a steel sheet, highly oriented pyrolytic graphite, and conducting polymers. We confirmed metallic transportation in the steel sheet and hopping transportation in graphite in the through-plane direction by conducting low temperature measurements at 100 K. In the case of a conducting polymer poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate, the conductivity anisotropic ratio decreases with increasing in-plane conductivity. Temperature dependent measurements show two distinct activation energy regimes in the through-plane direction in PEDOT/PSS but almost no change in the in-plane electrical conductivity activation energy. This could be due to additional carrier paths that occur through the more disordered region (the PSS-rich region) in the through-plane direction. We also examined the Meyer–Neldel rule in PEDOT/PSS and concluded that PEDOT/PSS follows the anti-Meyer–Neldel rule, likely due to the high carrier density in the film.

Development of ultralight and thin bipolar plates using epoxy-carbon fiber prepregs and graphite composites

Using the excellent mechanical strength and in-plane electrical conductivity of epoxy-carbon fiber prepregs, we design and fabricate ultrathin composite bipolar plates (BPs) for fuel cells. For the successful fabrication of prepreg-based BPs, it is essential to increase the through-plane electrical conductivity of pristine prepregs and reduce the electrical contact resistance due to excessive surface resin extruded during compression molding. In addition, the moldability of prepreg layers should be greatly improved for the proper construction of a serpentine flow field on the prepreg surface. To resolve all these technical issues, we suggest a multilayer BP structure in which prepreg layers, pure graphite layers, and graphite-resin composite layers are combined and compression-molded. The multilayered prepreg BP is approximately 0.6 mm thick and exhibits good electrical behavior (in-plane conductivity of 172 S cm−1 and through-plane conductivity of 38 S cm−1), with a high-quality serpentine flow channel configuration. The test results clearly demonstrate that the low through-plane electrical conductivity and poor moldability of pristine prepregs are greatly improved by our proposed BP design and fabrication.