Through-plane Direction Research Articles

Towards uniform distributions of reactants via the aligned electrode design for vanadium redox flow batteries

Enhancing the hydraulic permeability of electrodes along both the through-plane and in-plane directions is essential in flow-field structured vanadium redox flow batteries, as it can promote uniform distributions of reactants, lower the concentration overpotential, and therefore improve battery performances. In this work, uniaxially-aligned carbon fiber electrodes with the fiber diameter ranging from 7 to 12 µm (average ~10 µm) are fabricated by electrospinning method. Attributed to the enhanced permeability of the aligned structure, the battery assembled with the prepared electrodes exhibits an energy efficiency of 84.4% at a current density of 100 mA cm−2, which is 13.2% higher than that with conventional electrospun fiber electrodes. The permeability in the in-plane direction is further tailored by adjusting the orientation of aligned fibers against the flow channels. Results show that when the orientation of aligned fibers is perpendicular to the direction of flow channels, the battery delivers the largest discharge capacity and the highest limiting current density (~900 mA cm−2). Such an enhancement in the battery performance can be ascribed to the more uniform in-plane distribution of reactants and current by maximizing the permeability along the direction vertical to the flow channels, as evidenced by a three-dimensional model.

The effect of different GNPs addition on the electrical conductivities and percolation thresholds of the SiAlON matrix composites

Insulating SiAlON ceramics may become electrically conductive with the addition of a conductive phase such as GNPs and can be used more widely. However, the differences in the properties of the used GNPs significantly affect the amount of electrical conductivity that they provide to the matrix. In this study, four different GNPs with different properties such as lateral dimension, thickness and aspect ratio were added to SiAlON in the amount of 1.5, 2, 3 and 4 wt. % and the effects of different properties on the conductivity of composites were investigated. The thinnest GNPs with largest dimension and aspect ratio among the used GNPs provided the highest electrical conductivity and lowest percolation thresholds to SiAlON. The decrease in dimension, aspect ratios and the increase in thickness decreased the electrical conductivity of GNPs. Composites exhibited anisotropic behavior with better conductivity and percolation threshold values in the in-plane direction than through-plane direction.

Effect of clamping pressure on liquid-cooled PEMFC stack performance considering inhomogeneous gas diffusion layer compression

Gas diffusion layer (GDL) is a critical component of fuel cells, since it directly determines the transfer rate of mass, charge and heat that in turn affects the cell performance and stability. Within a fuel cell stack, assembled by over hundreds of single cells, GDL plays a more dominant role since the GDL deformation caused by assembly pressure would result in significant changes in its thermal, electrical and transport properties, thus impacting the uniformity, consistency and overall performance of the stack. Here, a numerical study is performed to investigate the effect of clamping pressure on the performance of liquid-cooled PEMFC stacks, which can be utilized as a guidance for stack assembly process in practical application. In particular, inhomogeneous compression as well as nonlinear strain-stress behavior of GDL are taken into consideration in the fuel-cell stack model, thus the GDL at under-rib and under-channel regions owns different physical properties, i.e., thickness, porosity, effective diffusivity, permeability, thermal conductivity, electrical conductivity, and contact resistance. It is found that the stack power output can be maximized when clamping pressure ranging from 1.5 MPa and 3.5 MPa owing to the balanced mass transport and contact resistance, meanwhile, 1.5 MPa enables the lowest voltage difference among different single cells that is beneficial in cell-to-cell consistency. Moreover, the inhomogeneous compression of GDL is found to be capable of facilitating water removal via enhanced convection at both in-plane and through-plane directions, and avoiding local hotspots with the increased in-plane thermal conductivity.

Facile Liquid-Exfoliation Process of Boron Nitride Nanosheets for Thermal Conductive Polyphthalamide Composite.

In this study, we describe the fabrication of thermally conductive composites based on a polyphthalamide (PPA) matrix by the exfoliation of hexagonal BN nanosheets (BNNs) via the melt-mixing method. Boron nitride (BN) particles were hydroxyl groups surface-treated with sodium hydroxide (NaOH). Compared with existing BN peeling experiments, we successfully produced BNNs that are simpler, more economical, and have an excellent aspect ratio. For the same weight content of BN and BNNs, PPA/BN composites surface-treated with high aspect ratio BNNs have a high in-plane and through-plane thermal conductivity because of the intercalation of the hydroxyl group surface treatments between BN and PPA, which not only increases the wettability but also provides a good heat transfer path. Moreover, wide and thin BNNs are evenly dispersed inside the PPA/BN composite to provide excellent heat transfer paths in both in-plane and through-plane directions.

Enhanced through-plane thermal conductivity and high electrical insulation of flexible composite films with aligned boron nitride for thermal interface material

Thermal interface materials (TIMs) with high thermal conduction and electrical insulation properties in through-plane direction are highly desirable in high-power and dense electronic devices. Thus a generalizable step-by-step orientation method without chemical modification or special treatment is adopted to prepare the composite films (boron nitride/poly (vinylidene fluoride) (i.e. x-BN/PVDF)). Scanning electron microscopy (SEM) and X-ray diffraction (XRD) results indicate that quite part of the BN platelets (BNs) are aligned well along the through-plane direction. A thermal conductivity of 3.50 W m−1 K−1 is obtained at a BN loading of 30 wt%, which has been improved by 1448% compared with the pure PVDF (0.226 W m−1 K−1). The volume resistivity and alternating current (AC) resistivity of 30-BN/PVDF sample are 6.51 × 1013 Ω·cm and 4.56 × 107 Ω·m (1 kHz), respectively, indicating much potential as high-frequency insulation material. In addition, the strong thermal conduction capability of the composite film is demonstrated by comparing temperature changes, which shows the importance of higher through-plane thermal conductivity in TIM applications.

High-Quality Boron Nitride Nanosheets and Their Bioinspired Thermally Conductive Papers

Hexagonal boron nitride has displayed increased potential in heat dissipation applications due to its desirable high thermal conductivity and remarkable thermal stability. However, the large-yield and high-quality preparation of boron nitride nanosheets (BNNSs) has been still an enormous challenge. In present work, we developed a universal exfoliation strategy to synthesize few-layer and defect-free BNNSs, which involved the intercalation of hexafluorosilicates/sodium hydroxide into BN crystals followed by exfoliation through a mild stirring process. The yield and concentration of as-obtained BNNS reached up to 78.5% and 12.78 mg/mL, respectively. More importantly, this method has been proven to exfoliate other layered materials like graphene (G), MoS2, and WS2. These as-obtained BNNSs can be directly used for constructing freestanding papers with high thermal conductivities. Typically, the thermal conductivities of the BNNS-G hybrid paper were up to 63.5 W/mK along the in-plane direction and 7.4 W/mK along the through-plane direction. According to the thermal interface materials performance measures, BNNS-G hybrid paper shows great promising applications for heat transfer in integrated circuit packaging.

Enhancement in thermal conductivity of polymer composites through vertically parallel multilayered distribution of microdiamonds

We propose a strategy to realize a vertically parallel multilayered distribution of microdiamonds in a polymeric matrix by pre-constructing layers with abundantly stacked microdiamonds. Silicone-based composites with this microdiamond arrangement exhibit interesting material properties in the through-plane direction: significant increments in thermal conductivity (335% that of composites with a random structure at a similar low loading of 24.7 vol%), and excellent dielectric properties as thermal interface materials for applications such as radars. Novel numerical models focused on this design of microdiamond arrangement are proposed to further understand mechanism of thermal conductivity increment and are found to match thermal conductivity and tendency of composites with this type of microdiamond distribution. It is revealed that adjusting microstructures in pre-constructed microdiamond layers is regarded as an effective strategy to improve further thermal conductivity of composites.

Effect of cell compression on the water dynamics of a polymer electrolyte fuel cell using in-plane and through-plane in-operando neutron radiography

Water dynamics in the membrane electrode assembly (MEA) and flow channels of polymer electrolyte fuel cells (PEFCs) is governed by the complex interplay of many physical and operational factors. The chemical nature and structure of the gas diffusion layer (GDL) plays a large part in this and is affected by the extent to which is mechanically compressed. Here, X-ray computed tomography shows the effect of cell compression on the MEA, and how it differs under the land and channel regions. Multi-orientation neutron radiography reveals the effect of compression on the way in which water accumulates and is transported between land and channel and between cathode and anode. By performing neutron imaging in both the in-plane and through-plane directions, it is possible to determine what constitutes a given ‘thickness’ of water mapped across the extent of an MEA. Changing MEA compression from 25% to 35% has a significant effect on water distribution and dynamics in operational cells. The effect of compression on performance is most marked in the mass transport region, and there are consequences for liquid accumulation in channels and back-diffusion of water from the cathode to the anode.

Control of the Anisotropic Conductivity of Carbon Nanotube Sheet and Their Thermoelectric Properties

Large anisotropy of thermal and electric conductions of the carbon nanotubes (CNTs) paper between in-plane and through-plane direction due to the one-dimensional fibrous structure of CNTs is problematic when the temperature gradient are required for applications such as thermoelectric conversion using Seebeck effect. To control the orientation of the CNT direction, polymer particles are introduced upon paper formation via vacuum filtration method. Thermal and electric conductivity in in-plane direction are decreased as increase the amount of the polymer particles used for the paper formation, while those in through-plane direction are increased. Heat diffusion in the paper are monitored by IR camera and it is found that the larger temperature gradient are observed for the CNT paper with anisotropy control compared to the original paper without anisotropy control.

High-performance polyketone nanocomposites achieved via plasma-assisted mechanochemistry

Polyketone (PK) is an engineering plastic with excellent impact strength, chemical resistance, barrier properties, and flame retardancy. However, the development of PK nanocomposites with further improved properties obtained via compounding is challenging due to the poor compatibility between PK and nanofillers. Also, since PK is not soluble in common organic solvents, it is difficult to enhance the affinity between components by applying wet chemistry processes widely used to make other composites. Herein, we report an effective solution to improve the compatibility of PK and nanofillers via a completely dry process. The plasma-assisted mechanochemistry (PMC) process can form chemical bonds between polymers and nanofillers, thereby promoting the dispersion of the nanofillers in polymer composites. PK was compounded with graphite nanoplatelets (GNPs) using the PMC process, and the structure and properties of the composites were investigated. The composites displayed greatly improved mechanical and gas barrier properties, and thermal conductivity; compared with conventionally prepared composites having the same GNP content (10 wt%), the composites prepared via the PMC process had 9.7 times higher elongation at break (112.1%), 2.2 times higher impact strength (89.2 J/m), 2.2 times better barrier performance (0.9 g/m2·day), and 2.5 times higher thermal conductivity (1.6 and 13.9 W/mK in the through-plane and in-plane directions). This approach is an innovative route to high-performance polymer nanocomposites, even those constructed from insoluble and incompatible polymers such as PK.

The Effect of Fiber Orientation on Stochastic Reconstruction and Permeability of a Carbon Paper Gas Diffusion Layer

By analyzing the three-dimensional digital model of a real carbon paper gas diffusion layer (GDL) reconstructed by X-ray computed tomography (CT), it was found that fibers are not distributed at any angle but within a certain range. The fiber orientation can be represented by fiber pitch (i.e., the angle between a single fiber and the in-plane direction). The effect of fiber orientation on stochastic reconstruction and transport properties (permeability) was investigated in this paper to find which fiber pitch range can achieve a better GDL on fluid flow. First, the actual fiber pitch was measured by analyzing SGL-24BA images obtained by X-ray CT. Also, seven different ranges of fiber pitch were randomly chosen to reconstruct GDL. Then, the permeability of these digital models was calculated using the Lattice Bolzmann Method (LBM) and discussed to obtain the fiber pitch range of the optimal permeability. The results show that the mean fiber pitch of SGL-24BA is 2.40° and the individual values are all less than 6°, also, the permeability of the through-plane direction increases gradually as the range of fiber pitch increases, which can be used for the structural design of carbon paper GDL.

Balance thermal conductivity and thermal expansion in RGO/Cu composites through alignment engineering

Electronic components often exhibit a low service lifetime due to low thermal conductivity (TC) of heat sink plates and large coefficient of thermal expansion (CTE) mismatch between Si heat source and the plate. Herein, high TC and low CTE are achieved simultaneously in Cu composites with aligned reduced graphene oxide (RGO) inside prepared by molecular-level mixing (MLM) combined with spark plasma sintering (SPS). The RGO/Cu pellets exhibit obvious anisotropic alignment morphology between in-plane direction (perpendicular to sintering pressure) and through-plane direction (parallel to sintering pressure). The in-plane TC of RGO/Cu pellets reached up to 332 W m−1K−1, which was close to that of pure Cu. In addition, the through-plane CTE (9.8 ppm K−1) is remarkably smaller than the CTE of pure Cu. On the premise of balancing TC and CTE, this work shows high potential in scalable industrial fabrication for its simple processing, short fabrication duration and low cost.

In-plane Mass Transport in Polymer Electrolyte Membrane Electrolyzer Porous Transport Layers with Through Pores

The mass transport in porous transport layers (PTLs) with pores exclusively in the through-plane direction was investigated using in operando X-ray radiography and electrochemical performance analysis. We observed via synchrotron X-ray imaging that through pores that are under the lands are inaccessible by liquid water. We thereby observed the limited in-plane mass transport that takes place in PTLs with pores exclusively in the through-plane direction. Additionally, a thicker layer of gas was observed with the use of the PTL with through pores under both the channels and the lands (PTLCh,L) when compared to the PTL with through pores only under the channels (PTLCh). This corresponded to the PTLCh,L exhibiting a higher mass transport overpotential than the PTLCh. Finally, the limited in-plane mass transport in the PTL with through pores led to a relatively dehydrated catalyst layer and higher ionic resistance with increasing current density.

Improving water management in fuel cells through microporous layer modifications: Fast operando tomographic imaging of liquid water

Polymer electrolyte fuel cells (PEFCs) have been actively developed for a wide range of power generation applications. At the high power densities required for automotive applications, sophisticated water management is vital to further improve cell performance. In particular, the gas diffusion layer (GDL), which includes a microporous layer (MPL), plays a crucial role in optimizing both water drainage and gas transport between the catalyst layer and gas channels. The present work studied the effect of cathode MPL porosity on the water distribution in the GDL. The results show that cells in which MPL materials have larger, micron-sized pores exhibit better performance. Advanced 4D operando X-ray imaging (3D structure plus time) was employed to analyse the water content in GDLs, and demonstrated that the superior performance of cells with large MPL pores is due to the efficient formation of water pathways. These pathways are based on water clusters allowing percolation in the through-plane direction from the bottom to the top of the GDL. Such pathways decrease the liquid water level in the entire cathode GDL. Especially, large MPL pores merge numerous small liquid water pathways in the catalyst layer and stabilize them morphologically, thus creating primary pathways for effective water drainage.

Three-dimensional detached serpentine flow field design for redox flow batteries

In the present study, we propose a novel three-dimensional detached serpentine flow field. The flow field applies two staggered serpentine channels to the bipolar plate and electrode (near the membrane side), respectively. The detached inflow and outflow channels aim to enhance convective mass transfer of active species in both in-plane and through-plane directions of the electrode, thereby, leading to enhanced mass transfer, complete electrolyte penetration into the electrode, and reduced pressure loss. The combined effects via the detached flow field lead to the increases of approximately 4.2% and 3.2% in the voltage and energy (pump consumption included) efficiencies of a vanadium redox flow battery cell, respectively, when compared with those of the interdigitated flow field (under the current density corresponding to 100 mA cm−2 and flow rate corresponding to 20 mL min−1). The experimental results also reveal that the performance of the proposed design is more sensitive to the felt compression and electrolyte flow rate compared to the conventional interdigitated and serpentine flow fields. This is potentially due to the presence of the significant pressure gradient in the through-plane direction of the electrode by using the proposed detached serpentine flow field.

Largely enhanced thermal conductivity of ethylene-propylene-diene monomer composites by addition of graphene ball

Abstract Along with the need of the fast development of electric and electronic industries, polymer composites with high thermal conductivity are much more demanded in the field of energy storage systems and electrical devices. In this paper, graphene ball (GB) composed of stacked graphene sheets was selected as filler and introduced into ethylene-propylene-diene monomer (EPDM) for improving its thermal conductivity. Here, a facile method was proposed to make GB uniformly dispersed into EPDM by solution blending and then obtain high thermal conductive EPDM/GB composites through vulcanization. The thermal conductivity of neat EPDM is 0.29 W/(m·K). The addition of the GB leads to the increase of the thermal conductivity of the composites. Thermal conductivity of the EPDM/GB composite with 60 phr GB loading along the in- and through-plane directions are 18.43 and 1.48 W/(m·K), which are approximately to 62 and 4 times higher than that of neat EPDM, respectively. The EPDM/GB composites are expected to play a key role in new-era advanced electronic devices and related applications due to its excellent thermal transport property.

Effects of shape and alignment of reinforcing graphite phases on the thermal conductivity and the coefficient of thermal expansion of graphite/copper composites

To investigate the effects of the shape and alignment of reinforcing graphite phases on the thermal conductivity (TC) and coefficient of thermal expansion (CTE) of graphite/copper (Gr/Cu) composites, test composites were fabricated using the mechanical mixing and electroless plating methods. Graphite flakes with high anisotropy and graphite granules with low anisotropy were used as the reinforcing phases, and the TC and CTE of the manufactured composites were measured in the in-plane and through-plane directions, and were compared with theoretical estimates. The present study revealed that the TC and CTE values of the Gr/Cu composites were significantly affected by the shape and alignments of the reinforcing graphite phases, and by the sizes of the graphite phases. The effects of the size of the reinforcing graphite on the TC value of the composites were more extensive than its alignment, but its alignment was a more dominant factor than its size in the determination of the CTE. Correspondingly, control of the shape, size, and alignment of the reinforcing graphite can potentially reduce the anisotropy of TC and CTE in the Gr/Cu composites.

Visualized cell characteristics by a two-dimensional model of vanadium redox flow battery with interdigitated channel and thin active electrode

A two-dimensional and steady-state model of the vanadium redox flow battery with an interdigitated channel and a thin active electrode was developed to visualize the flow velocity, concentrations of the reactants, and local current distribution in the through-plane direction of the electrode sheets. The concentration overpotential was increased in the low flow speed region. The low mass transport coefficient by a slow flow speed resulted in a higher concentration overpotential. The mass flux distributions at the state of charge of 20% were strongly dependent on the flow speeds due to low bulk concentration of the reactants. Based on the visualized data, not only the reactivity improvement of the electrode material but also the advanced design of the porous electrode structure for a high flow permeability of the electrolyte solution are essential to improve the cell voltage during discharge.

Enhancement of thermal conductivity of carbon fiber-reinforced polymer composite with copper and boron nitride particles

A hybrid filler system, which combines hexagonal boron nitride (hBN) and copper particles, was applied on the surface of continuous carbon fiber tapes by utilizing electrophoretic deposition to maximize the thermal conductivity of carbon fiber-reinforced polymer (CFRP) composite in this study. The results show that as the deposition time increased, the thermal conductivity increased. The annealing treatment made the copper particles connect with adjacent particles, which highly enhanced the thermal conductivity. The CFRP made of annealed carbon fiber tapes with hBN/Cu coating performed the best thermal conductivity and largest enhancement efficiency in this work for as high as 2.16 W/m·K and 217% in through-plane direction, and 6.14 W/m·K, and 47% in in-plane direction.

Magnetic resonance elastography of the brain: A study of feasibility and reproducibility using an ergonomic pillow-like passive driver

Magnetic resonance elastography (MRE) can be used to noninvasively resolve the displacement pattern of induced mechanical waves propagating in tissue. The goal of this study is to establish an ergonomically flexible passive-driver design for brain MRE, to evaluate the reproducibility of MRE tissue-stiffness measurements, and to investigate the relationship between tissue-stiffness measurements and driver frequencies.An ergonomically flexible passive pillow-like driver was designed to induce mechanical waves in the brain. Two-dimensional finite-element simulation was used to evaluate mechanical wave propagation patterns in brain tissues. MRE scans were performed on 10 healthy volunteers at mechanical frequencies of 60, 50, and 40 Hz. An axial mid-brain slice was acquired using an echo-planar imaging sequence to map the displacement pattern with the motion-encoding gradient along the through-plane (z) direction. All subjects were scanned and rescanned within 1 h. The Wilcoxon signed-rank test was used to test for differences between white matter and gray matter shear-stiffness values. One-way analysis of variance (ANOVA) was used to test for differences between shear-stiffness measurements made at different frequencies. Scan-rescan reproducibility was evaluated by calculating the within-subject coefficient of variation (CV) for each subject.The finite-element simulation showed that a pillow-like passive driver is capable of efficient shear-wave propagation through brain tissue. No subjects complained about discomfort during MRE acquisitions using the ergonomically designed driver. The white-matter elastic modulus (mean ± standard deviation) across all subjects was 3.85 ± 0.12 kPa, 3.78 ± 0.15 kPa, and 3.36 ± 0.11 kPa at frequencies of 60, 50, and 40 Hz, respectively. The gray-matter elastic modulus across all subjects was 3.33 ± 0.14 kPa, 2.82 ± 0.16 kPa, and 2.24 ± 0.14 kPa at frequencies of 60, 50, and 40 Hz, respectively. The Wilcoxon signed-rank test confirmed that the shear stiffness was significantly higher in white matter than gray matter at all three frequencies. The ranges of within-subject coefficients of variation for white matter, gray matter, and whole-brain shear-stiffness measurements for the three frequencies were 1.8–3.5% (60 Hz), 4.7–6.0% (50 Hz), and 3.7–4.1% (40 Hz).An ergonomic pneumatic pillow-like driver is feasible for highly reproducible in vivo evaluation of brain-tissue shear stiffness. Brain-tissue shear-stiffness values were frequency-dependent, thus emphasizing the importance of standardizing MRE acquisition protocols in multi-center studies.