Electrospun Electrodes Research Articles

Electrochemical and Rheological Investigations of Catalyst-Solvent-Polymer Ink Formulations for Electrospun Fuel Cell Electrode Structures

The rational design of electrode structures and interfaces has emerged as a potential route for increasing the efficiency of electrochemical devices. As a fabrication technique, electrospinning allows for the creation of electrode structures possessing purported improvements in proton conductivity, gas transport and durability[1], while economic assessments [2] have shown electrospinning to be cost competitive with other Roll-to-Roll manufacturing techniques.[3] However, at present, very little is known about the particle-solvent-polymer interactions which occur at the ink level. Understanding how the electrospun electrode morphology relies not only on precursor material properties, but also on the interactions between the catalyst, ionomer and solvents being utilized can help improve the ability to dictate and tune relevant interactions within the electrode structure. Unfavorable interactions between ionomer, carrier polymer and Pt active sites can lead to reductions in activity and fuel cell performance. Hence, its imperative to understand the properties that dictate the fundamental chemistry and materials properties of various electrospun ink formulations in order to rationally design electrode structures. In support of this process-focused study, detailed exploration of particle-polymer-solvent interactions in the subject inks and slurries will be undertaken utilizing both electrochemical in-situ diagnostics as well as ex-situ advanced characterization techniques to achieve a fundamental understanding of various ink fabrication processes and colloidal structure of inks and dispersions as a function of solids content, solvent type, and particle size. In particular, we will investigate different electrospun ink formulations for fuel cell electrodes including small-scale studies of various material sets, carrier polymers, slurry compositions and excitation conditions. The resulting ink level interactions will be discussed in conjunction with varying synthetic and fabrication parameters. The resulting electrode structure, ionomer-Pt interactions, and coverage of ionomer on both Pt and carbon, will be related to performance related phenomena at high current density (i.e. oxygen transport resistance and proton conductivity). [4] The electrospinning inks will be characterized via ink/dispersion surface charge stabilization (zeta potential measurements) and rheological studies as function of ink composition and processing variables.[5] Morphological properties of electrospun nanofiber electrodes will also be characterized using various spectroscopic and microscopic techniques (USAXS, TEM, SEM, EDS). Following ink/fiber optimization, electrodes will be conditioned [6] and examined with in-situ electrochemical diagnostics such as electrochemical surface area (ECA), CO-displacement measurements, oxygen transport resistances (Rnf) and electrochemical impedance spectroscopy (EIS) in the catalyst layers will be measured to elucidate the effects of different ink compositions, excitation parameters and electrode structures on PEMFC performance. Preliminary results have demonstrated significant improvements in mass activity, up to a factor of 2, for electrospun electrodes compared to traditional ultrasonically sprayed catalyst coated membrane (CCM) electrodes. Moreover, rheological measurements of Pt/Vu-ionomer (Nafion), Pt/Vu-polymer(PAA) and ionomer-polymer ink slurries have shown that the concentration of carrier polymer (PAA) in the ink slurries plays a significant role in determining rheological properties, which may ultimately play a role in the final electrode structure, ionomer coverage and interaction with the Pt-surface.

Electrospinning and MEA Conditioning As a Tool for Designing Optimized Electrode Structures for Fuel Cell Applications

Complimentary to materials improvements, the rational design of electrode structures and interfaces have emerged as potential routes for increasing fuel cell efficiency. Traditional MEA fabrication methods are dependent upon the deposition of uniform ink compositions that couple the integration of ionomer and catalyst. Subsequent electrode morphology then depends solely on the material properties and interactions of the catalyst, ionomer and solvents being utilized. In Pt based systems, unfavorable interactions between ionomer and Pt active sites can lead to reductions in activity and additional oxygen transport resistance. [1,2] Additionally, in Platinum Group Metal (PGM)-free electrodes, inhomogeneous coverage of larger particles results in poor catalyst utilization and reduced proton conductivity throughout the layer. Electrospinning can be used to combat such limitations. Electrospinning can produce nanofiber threads (typically ~500 nm in diameter) formed by applying an electrostatic voltage from the deposition needle to the grounded substrate.[3] Electrospun electrode development gives an opportunity for us to dictate electrode properties and interactions that are more defined and desirable, by allowing us to decouple electrode structural aspects from solution or ink-based behavior. Such advantages include: (i) the ability to produce homogeneous ionomer distributions across both electrocatalyst agglomerates and the electrode thickness, (ii) greatly improve the ionomer distribution for larger catalysts structures/agglomerates, such as MOF based PGM-free electrocatalysts and (iii) utilize multiple ionomers, each of which may be specifically tailored based on local or bulk electrode requirements. Here, we will discuss the incorporation of two classes of electrocatalysts; (i) PGM based (e.g. Pt/C and Pt alloy/C)s and (ii) and PGM-free (e.g. FeNx/C based) . The electrospun electrodes were fabricated with different ionomer (Nafion), carrier polymer, (PVDF, PAA) and solvent blends (IPA, DMF, Acetone). The effect of varying synthetic and fabrication parameters (flow rates, humidity, and operating potential) on electrode structure were investigated in order to improve high current density performance. The electrodes were characterized using various microscopic techniques (TEM, SEM, EDS), and in-situ diagnostics such as oxygen reduction reaction (ORR) mass activity, electrochemical surface area (ECA) and H2-Air/O2 performance, oxygen transport resistances (Rnf) and Electrochemical Impedance Spectroscopy (EIS) in the catalyst layers were measured to elucidate the effects of different electrode structures on PEMFC performance. Preliminary results show improvement in mass activity (determined at 0.9V, 150 kPaabs, 80oC and 100% RH) when compared to traditional ultrasonically sprayed catalyst coated membrane (CCM) electrodes. Reductions in non-Fickian transport resistances (RnF) were also observed on nanofiber electrodes, suggesting a re-arrangement in ionomer coverage and distribution on Pt-surface. The results from this study were subsequently applied to thicker (~100 μm) platinum group metal free (PGM-Free) electrodes where optimization of gas and proton transport are more impactful.

New Si Nanofiber Mat Designs for Li-Ion Battery Anodes

Electrospinning is a convenient technique for the fabrication of non-woven mats of micron/sub-micron diameter fibers. Electrospun mats of nanoparticle/polymer fibers have a broad range of applications, including their use as fuel cell and battery electrodes.1,2 Electrospun fiber electrodes for Li-ion batteries exhibit characteristics superior to those of conventional slurry cast electrodes, including: (i) a high interfacial electrode/electrolyte area with short Li+ ion diffusion paths in the radial-fiber direction and (ii) an adjustable inter-fiber void space (porosity) which enhances electrolyte infiltration and allows for the fabrication and use of thick electrodes. Pintauro and co-workers reported on several types of electrospun electrodes for Li-ion batteries: anodes3-5 with TiO2/C, Si/C or carbon powder and a cathode with LiCoO2/C particles1, where the polymer binder was either poly(acrylic acid) (PAA) or poly(vinylidene fluoride). The present work extends the above studies by focusing on dual-fiber systems where separate Si/PAA and C/polyacrylonitrile (PAN) fibers are co-electrospun to form a porous mixed fiber electrode. The dual interpenetrating fiber network morphology is expected to have increased robustness due to interlocking of the dimensionally variable Si/PAA fibers within an electrically conductive cage of C/PAN fibers. Preliminary results are very encouraging. For example, an electrode with 57 wt.% Si-PAA fibers (40 wt.% PAA) and 43 wt.% C-PAN fibers (39 wt.% PAN) cycled in a half cell at 0.1C showed a terminal discharge capacity of 1000 mAh/g (gravimetric) at 2.4 mAh/cm2. The terminal discharge capacity at 1C was 611 mAh/g (gravimetric) and 1.5 mAh/cm2 (areal). During this talk, details will be given on the fabrication methods of these new composite anodes, along with their electrochemical performance and capacity during charge/discharge cycling at various C-rates. The effects of fiber composition (particle/binder weight ratio and the relative weights of Si and C fibers in a dual fiber anode) on gravimetric, volumetric, and areal capacities will be discussed. Acknowledgements: This work was funded by the United States Department of Energy, Vehicle Technologies Office, grant No. DE-EE0007215.

Fully stretchable triboelectric nanogenerator for energy harvesting and self-powered sensing

In this work, a fully stretchable triboelectric nanogenerator (FSTENG) composed of an electrospun electrode and a porous PDMS friction layer with anti-nickel foam structure was fabricated. This FSTENG can deliver an optimal output voltage of 92 V, which is ten times greater than that of the TENG based on flat PDMS film. The superior performance is attributed to the use of anti-nickel foam structure-based porous PDMS film in FSTENG, which enhances the surface-to-volume ratio and surface roughness. Importantly, the FSTENG could maintain stable output performance even under a stretching ratio of 50%. With these excellent features, the developed FSTENG was designed as a self-driven sensor for finger bending and walking status monitoring. Furthermore, to expand the driving mode of FSTENG, FSTENG was successfully attached to the leaf for monitoring the wind speed in the natural environment without external power supply. The excellent flexibility, stretchability, and versatility of FSTENG make it as a promising self-driven sensor for human behavior and environmental monitoring.

A Theoretical Model for the Triple Phase Boundary of Solid Oxide Fuel Cell Electrospun Electrodes

Electrospinning is a new state-of-the-art technology for the preparation of electrodes for solid oxide fuel cells (SOFC). Electrodes fabricated by this method have been proven to have an experimentally superior performance compared with traditional electrodes. However, the lack of a theoretic model for electrospun electrodes limits the understanding of their benefits and the optimization of their design. Based on the microstructure of electrospun electrodes and the percolation threshold, a theoretical model of electrospun electrodes is proposed in this study. Electrospun electrodes are compared to fibers with surfaces that were coated with impregnated particles. This model captures the key geometric parameters and their interrelationship, which are required to derive explicit expressions of the key electrode parameters. Furthermore, the length of the triple phase boundary (TPB) of the electrospun electrode is calculated based on this model. Finally, the effects of particle radius, fiber radius, and impregnation loading are studied. The theory model of the electrospun electrode TPB proposed in this study contributes to the optimization design of SOFC electrospun electrode.

Formation of electrodes by self-assembling porous carbon fibers into bundles for vanadium redox flow batteries

Electrospinning has been employed to fabricate carbonaceous materials with larger surface areas for vanadium redox flow batteries. However, the woven carbon nanofibers prepared with conventional electrospinning methods are plagued by the low porosity and poor permeability, thereby causing a significant mass-transport resistance during the operation of batteries. To tackle this problem, we report a novel method by self-assembling porous carbon fibers into large bundles to form electrodes. This electrode is fabricated by electrospinning polyacrylonitrile and polystyrene binary solutions. Instead of forming single fibers, the individual fibers are self-assembled into fiber bundles by properly managing the viscosity of the precursor solution. The formation of large fiber bundles significantly enlarges the pore size while retaining large specific surface areas. The single cell with the as-prepared electrodes achieves an energy efficiency of 87.7% at a current density of 100 mA cm−2, which is 15.2% higher than that of the single cell with conventional electrospinning electrodes. The energy efficiency still maintains over 80% at 200 mA cm−2. More importantly, the discharge capacity and electrolyte utilization are nearly doubled. All these results demonstrate that this electrode preparation method is effective to improve the mass transport properties of traditional electrospun electrodes in vanadium redox flow batteries.

Activation and degradation of electrospun LiFePO4 battery cathodes

Electrodes prepared by electrospinning often exhibit an activation behavior, i. e. they are reaching their full capacity only after numerous charge and discharge cycles. The activation mechanism can be explained by the improvement of the accessibility of Li+ ions to the active particles of the cathode, which increases with the number of cycles. It is assumed that, as an effect of cycling, the dense, impermeable carbon layer which covers the active material due to the carbonization step during processing cracks and delaminates, allowing this way the Li+ ions to access the active material and to intercalate into it. This has been confirmed by scanning and transmission electron microscopy performed in correlation with the electrochemical performance of electrospun electrodes. However, with even further cycling a decrease in capacity is observed. The microscopic results suggest that this is partly caused by cracks at the carbon-LiFePO4 interfaces. Thus, the cracking responsible for the activation of the electrospun electrodes at the beginning of cycling seems also to cause a part of their degradation at the end of their life. Another slow degradation mechanism confirmed by scanning electron microscopy and by X-ray photoelectron spectroscopy is the ongoing formation of a cathode electrolyte interphase.

Unveiling Polyindole: Freestanding As-electrospun Polyindole Nanofibers and Polyindole/Carbon Nanotubes Composites as Enhanced Electrodes for Flexible All-solid-state Supercapacitors

Polyindole(Pind) is one of the conducting polymers (CPs) which previously was less studied but of recent is gaining attention for energy storage applications. In all the few previous reports, when Pind was employed as electrode active material in supercapacitors, the capacitance was reported low with reasonable values only being obtained as a composite with other materials. The reasons underlying the poor performance of Pind and Pind nanocomposites are thought to be: 1) inactive morphology and limited surface area, 2) poor conductivity, and 3) poor electrode fabrication techniques. To address the trio, we employed the traditional, easy and scalable electrospinning technique to fabricate high surface area electroactive Pind nanofibers. Further, a little percentage (10wt.%) of carbon nanotubes (CNTs) were added to enhance the conductivity of Pind and to study the effect of our fabrication route on the nanocomposites. Significant capacitance improvements of up to 238Fg−1 and 476Fg−1 at 1.0Ag−1 for Pind and Pind/CNT freestanding electrospun electrodes, respectively were achieved. Moreover, we report the significant performance of the all-solid-state symmetric, flexible and binder-free supercapacitor fabricated by a one-step and scalable method of as-electrospun Pind/CNT nanofibers on the stainless steel fabric current collector. The supercapacitor showed a high energy density of 17.14W h kg−1 at a power density of 426Wkg−1 and capacitance retention of 95% after 2000 cycles. We strongly believe that we have set a stage for Pind to compete in a healthy race with other CPs as a next generation electrode material for supercapacitors.

Fluid Transport Properties from 3D Tomographic Images of Electrospun Carbon Electrodes for Flow Batteries

Three-dimensional x-ray computer tomography images were obtained of electrospun poly(acrylonitrile) electrodes for a flow battery. The materials were imaged before and after carbonization. Information about the internal morphology; local fiber size and porosity, was analyzed and provided key insights into both the electrospinning and carbonizing processes. It was found that traditional imaging techniques may not be suitable for materials generated through electrospinning as it is a highly dynamic process. The fiber size tended to vary throughout the process while the porosity was relatively constant. Viscous flow was modelled through the material using the Lattice Boltzmann Method and the 3D flow fields that resulted provided further information about the role of heterogenous features on the performance of an electrospun electrode in a flow battery. The local porosity of the material had the largest effect on the material’s flow dynamics.

Fluid Transport Properties from 3D Tomographic Images of Electrospun Carbon Electrodes for Flow Batteries

The flow battery remains a cost challenged technology, significant effort has been made into improving performance by way of advanced redox couples, different chemistries, and different electrode materials.[1] One aspect of flow cell performance that has remained relatively overlooked is the effect of different transport properties in the electrode itself. The structure and morphology of the porous media plays an essential role in the electrolyte dispersion and the access the fibers have to the active species. Custom made porous media was electrospun in house and carbonized for use in as a flow battery electrode. 3D tomography images were taken of both the carbonized and ‘as-spun’ polyacrylonitrile (PAN) mats at a variety of different spinning conditions. Lattice-Boltzmann simulations were performed on the tomographic images to determine the flow distribution through the material as well as the permeability of the electrospun mats. The findings were consistent with previous experimentally measured permeabilities of electrospun PAN materials.[2] While the results indicated permeabilities that would be acceptable or even beneficial for a flow battery, the modelling and analysis showed that material inconsistencies in the fibrous mat led to a large variability in local material and transport properties that could have a very negative effect on flow battery performance. There was a small variation in local fiber sizes and a much more significant variation in material porosity. Because these variations only existed in one axis, specifically through the thickness of the material, we were able to determine that the cause must be changing electrospinning conditions as the material was deposited. The findings of this study are being used to develop more optimized flow battery electrodes from electrospun materials. While there is tradeoff in terms of ease of production when utilizing electrospun electrodes versus traditional graphite fibrous electrodes, electrospun materials allow for a greater degree of control of fiber size and general morphology allowing the optimization of permeability, surface area as well as mass transfer properties. The attached figure demonstrates the detail and the accuracy generated through the Lattice-Boltzmann simulations. The fibers and streamlines shown are actual electrospun carbonized PAN with fibers that have a diameter of approximately 750 nm. [1]A. Z. Weber, M. M. Mench, J. P. Meyers, P. N. Ross, J. T. Gostick, and Q. Liu, “Redox flow batteries: a review,” J. Appl. Electrochem., vol. 41, no. 10, pp. 1137–1164, Sep. 2011. [2]M. D. R. Kok and J. T. Gostick, “Transport properties of electrospun fibrous membranes with controlled anisotropy,” J. Membr. Sci., vol. 473, pp. 237–244, Jan. 2015. Figure 1

Performance of electro-spun carbon nanofiber electrodes with conductive poly(3,4-ethylenedioxythiophene) coatings in bioelectrochemical systems

Abstract Bioelectrochemical systems (BESs) employ extracellular electron transfer from bacteria that grow at electrodes. Due to biofilm and electrode limitations, industrial-scale applications require large electrode areas, and thus inexpensive electrode materials. Here, electro-spun polyacrylonitrile (PAN) and carbon nanofiber (CNF) were manufactured. In addition, the conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) was applied as a coating to these materials and to carbon cloth (CC). We tested these materials as electrodes by using physicochemical measurements, cyclic voltammetry, and bioelectrochemical growth-studies with Geobacter sulfurreducens. PAN is a nonconductive material without capacitance, but with PEDOT coating the conductivity and capacitance became sufficient to support electric current production in our BES. CNF outperformed CC in capacitance, but behaved similarly in our BES when normalized to projected surface area. With the PEDOT coating, CNF increased electric current production by 38% in our BES, while this was 64% for CC. When applied to a gold microfluidic electrode, electric current with G. sulfurreducens increased almost three-fold. PEDOT added considerable specific surface area to electrodes possessing a low surface area, but not with a high surface area such as CNF. This work demonstrates that electro-spun electrodes and PEDOT coating are a promising electrode alternative that can be readily implemented into existing BESs.

Investigation of a High‐Performance Nanofiber Cathode with Ultralow Platinum Loading for Proton Exchange Membrane Fuel Cells

AbstractNanofiber electrodes fabricated by using the electrospinning technique in the cathode of proton exchange membrane fuel cells gave a peak power density of 0.692 W cm−2 with an ultralow Pt loading of 0.087 mg cm−2. As the cathode, the electrospun (E_spun) electrodes exposed a high Pt surface to oxygen, with a highly uniform distribution of Pt catalyst and Nafion ionomer, which improved the utilization of Pt. The ionic resistance of the E_spun electrode was decreased owing to the nanofiber structure. The high porosity of the E_spun electrode enhanced the Knudsen diffusion in small pores, which mitigated the oxygen‐transfer resistance. The addition of 5 wt % polytetrafluoroethylene (PTFE) to the E_spun electrode optimized water management, especially at high current densities. The accelerated stability test showed that the long‐term durability of the E_spun electrode is much better than the conventional decal electrode due to the interaction of the carbon support and hydrophilic poly(acrylic acid).

Fabrication and Characterization of Electrospun Electrodes for Flow Battery Electrodes

Flow battery is a promising large scale energy storage technology. Improving electrode material, and therefore transport properties is a possible method to improve performance. Customizing material to target optimal properties via electrospinning was investigated in this study. The effect of carbonization was also studied. It was demonstrated that it was possible to produce thick enough electrode for flow battery electrode with fiber diameter close to 1 µm, and porosity up to 90%. In plane permeability was also measured for electrospun and carbonized materials. The fabrication and characterization methods are described in more detail.

Electrospun Nanomaterials for Supercapacitor Electrodes: Designed Architectures and Electrochemical Performance

Electrospinning is the most facile and highly versatile approach to produce 1D polymeric, inorganic, and hybrid nanomaterials with a small diameter, controllable dimensions, and designed architectures. In particular, with large surface area, high porosity, low density, good directionality, and tunable composition, electrospun nanofibers and mats are regarded as ideal candidates for various kinds of electrochemical energy storage devices such as supercapacitors (SCs). In this review, the recent progress in electrospun electrode materials for SCs is presented, covering the architecture design and their electrochemical performance. After a brief introduction about SCs, the basic principles of the electrospinning technique are discussed. Following, attention is paid to the discussion of various electrospun nanofibers and mats including 1D carbons, metal oxides, metal sulfides, metal nitrides, conducting polymers and composite nanomaterials with various types of architectures as electrodes for SCs. The relationship between the composition, architecture, and the electrochemical performance is discussed in detail. Finally, some challenges and perspectives of future research of the electrospun nanofibers and mats for high performance SCs are highlighted. It is anticipated that this review would provide the researchers some inspiration for constructing new types of energy storage devices.

Fabrication and Characterization of Electrospun Electrodes for Flow Battery Applications

Flow batteries are a promising candidate for grid-level energy storage, due to their decoupled power generation and energy storage [1]. Since they were first proposed by NASA in the 1970’s [2], numerous prototypes have been shown, but commercialization has been hindered by their suboptimal design and high cost. Most research in this field has been focused on adjusting redox chemistry or improving catalytic properties of the electrode. Less effort has been made in addressing the mass transport within the electrochemical cell. The electrode is typically a porous carbon material which supports redox reactions on its surface [1]. Optimizing the hydrodynamic conditions and transport phenomena via structural modifications of the electrode is a promising option to improve cell performance. Fibrous materials are of special interest due to their high surface area, which leads to a higher reaction rate, while at the same time being highly porous to provide good permeability and diffusivity. In this work, a range of fibrous electrodes were produced by electrospinning of polyacrylicnitrile (PAN) followed by carbonization. Electrospinning is a convenient method for making prototype materials since it allows tremendous flexibility in the final product by simply varying processing parameters [3]. The use of novel electrodes to improve mass transport and reactive surface area in flow batteries has started to be the focus of investigations [4], but recent modeling work [5] has shown that significant improvements can be made. Materials were produced with a range of fiber morphology, porosity, pore sizes, and thickness. SEM images of 1 specific electrospun material before and after carbonization are shown in figure 1. In addition to these structural measurements, key transport properties such as diffusivity and permeability were also measured and found to vary widely between materials. It is expected that these properties will correlate closely with cell performance, hence proper characterization of transport parameters will be essential to the further optimization of high performance materials. [1] A. Z. Weber, M. M. Mench, J. P. Meyers, P. N. Ross, J. T. Gostick, and Q. Liu, “Redox flow batteries: a review,” J. Appl. Electrochem., vol. 41, no. 10, pp. 1137–1164, Sep. 2011. [2] L. H. Thaller, “Electrically rechargeable REDOX flow cell,” US3996064 A, 07-Dec-1976. [3] B. Zhang, F. Kang, J.-M. Tarascon, and J.-K. Kim, “Recent advances in electrospun carbon nanofibers and their application in electrochemical energy storage,” Prog. Mater. Sci., vol. 76, pp. 319–380, Mar. 2016. [4] G. Lin, P. Y. Chong, V. Yarlagadda, T. V. Nguyen, R. J. Wycisk, P. N. Pintauro, M. Bates, S. Mukerjee, M. C. Tucker, and A. Z. Weber, “Advanced Hydrogen-Bromine Flow Batteries with Improved Efficiency, Durability and Cost,” J. Electrochem. Soc., vol. 163, no. 1, pp. A5049–A5056, Jan. 2016. [5] M. D. R. Kok and J. T. Gostick, “Multiphysics Simulation of the Bromine Cathode: Cell Architecture and Electrode Optimization,” ECS Trans., vol. 69, no. 1, pp. 21–35, Sep. 2015. Figure 1

Graphene-doped electrospun nanofiber membrane electrodes and proton exchange membrane fuel cell performance

Abstract A rational electrode structure can allow proton exchange membrane (PEM) fuel cells own high performance with a low noble metal loading and an optimal transport pathway for reaction species. In this study, we develop a graphene doped polyacrylonitile (PAN)/polyvinylident fluoride (PVDF) (GPP) electrospun nanofiber electrode with improved electrical conductivity and high porosity, which could enhance the triple reaction boundary and promote gas and water transport throughout the porous electrode. Thus the increased electrochemical active surface area (ECSA) of Pt catalysts and fuel cell performance can be expected. As results, the ECSA of hot-pressed electrospun electrodes with 2 wt% graphene oxide (GO) is up to 84.3 m 2 /g, which is greatly larger than that of the conventional electrode (59.5 m 2 /g). Significantly, the GPP nanofiber electrospun electrode with Pt loading of 0.2 mg/cm 2 exhibits higher fuel cell voltage output and stability than the conventional electrode.

High Performance Nanofiber Electrodes for Li-Ion Batteries Using Particle/Polymer Electrospinning

The demand for rechargeable batteries with high energy and power densities has never been greater. The functionality of many portable electronic devices is limited by battery size and lifetime. Similarly, electric vehicle propulsion systems require significant improvements to satisfy consumer expectations for long drive distances and short recharge times. To meet the demands of energy-hungry consumers, new battery electrodes with high capacities and long cycle life at fast charging rates must be developed.[1-2] These requirements have motivated intense research efforts to develop next-generation Li-ion battery materials, in particular replacement designs for conventional slurry cast electrodes. One approach is the use electrospun electrodes containing engineered void spaces to enhance Li+ transport rates for high capacities at fast charge/discharge rates. Battery researchers largely focus on preparing new electrodes with high gravimetric capacity, but this metric alone is insufficient to assess a material’s applicability for consumer devices. An electrode’s volumetric capacity is another important measure for applications, such as electric vehicle propulsion, where battery space is limited. Likewise, electrodes must have high areal capacities to limit the amount of inactive components (e.g., current collectors and separators) that contribute to the overall battery size and weight. Because gravimetric, areal, and volumetric capacities are important and complementary measures of battery performance, all three should be examined to fully assess the strengths and weaknesses of new electrode materials and designs. The present work is the latest installment by Pintauro and co-workers on the use of polymer/particle electrospun nanofiber mats for fuel cell and Li-ion battery electrodes.[1-5] While previously reported electrospun battery electrodes have shown impressive performance, these materials were prepared by carbonizing polymer fiber precursors at high temperature. In particle/polymer electrospinning, there is no fiber pyrolysis; the electrospinning and post-processing are performed at room temperature to preserve the beneficial characteristics of the polymer binder that holds electrode particles together. Due to the these characteristics, high gravimetric, areal, and volumetric storage capacities can be achieved at fast charge/discharge rates through the use of thick, densely packed particle/polymer nanofiber mats. Scanning electron microscopy images of as-spun and highly compacted carbon/poly(vinylidene fluoride) (C/PVDF) nanofiber anodes are shown in Figures 1a and 1b. For comparison, a slurry cast anode of the same composition has a very dense structure with essentially no visible voids (see Figure 1c). The presence of intra- and interfiber voids in the electrospun mat is important for battery operation; electrolyte intrusion throughout these voids ensures rapid Li+ transport between the electrolyte and the active material nanoparticles in the radial fiber direction. In addition to C/PVDF anodes, particle/polymer electrospinning can be used to prepare nanofiber electrodes from new active materials and binders as they are developed. In this presentation, recent results will be presented on the fabrication and use of several electrospun particle/polymer nanofiber anodes (with titania, carbon, and silicon powders) and cathodes (with LiCoO2 powder). Charge/discharge properties of half and full cells will be shown as a function of C-rate. Electrode capacities for different fiber mat thicknesses and mat porosities will also be shown and discussed. Overall, the results will demonstrate that proper micron-scale design of electrodes can enhance Li+ transport rates and thus improve Li-ion battery performance. Acknowledgements This work was funded in part by the National Science Foundation (NSF EPS-1004083) through the TN-SCORE program under Thrust 2.

Electrospun fibrous electrodes with tunable microstructure made of polyaniline/multi-walled carbon nanotube suspension for all-solid-state supercapacitors

Electrospinning technique was used to prepare high performance fibrous electrodes with tunable microstructure for all-solid-state electrochemical supercapacitor. Symmetrically sandwiched supercapacitors consisting of flexible electrospun polyaniline (PANI)/multi-walled carbon nanotube (MWCNT) electrodes and polyvinyl alcohol (PVA)/sulfuric acid (H2SO4) gel electrolyte were assembled. Tunable microstructure of the fibrous electrode was obtained by changing the electrospinning parameters including the collector–needle distance (CND) and the suspension flow rate (SFR). Results show that, higher CND combining with lower SFR can result in a smaller average diameter of the electrospun fibers and hence improve the electrode performance. When the CND changes from 80 to 140mm, the average fiber diameter will decrease from 2.89 to 1.21μm, and the specific surface area of the electrode can increase from 57 to 83m2·g−1. The corresponding specific capacitance of the electrospun electrode will therefore increase from 129.5 to 180F·g−1, leading to a synchronous improvement of the energy density of the supercapacitor from 18 to 25Wh·kg−1. On the other hand, the supercapacitors using fibrous electrodes in this work also show good rate capability and cycling stability. Using the electrode with an average fiber diameter of 1.21μm, the specific capacitances can maintain 131F·g−1 at a current density of 4A·g−1, which is 73% of the specific capacitance of the same sample at a current density of 0.5A·g−1. And the specific capacitance of the electrode can retain 89% after 1500 charge/discharge cycles.

Nanostructured electrode materials for lithium-ion and sodium-ion batteries via electrospinning

Electrospinning has attracted tremendous attention in the design and preparation of 1D nanostructured electrode materials for lithium-ion batteries (LIBs) and sodium-ion batteries (NIBs), due to the versatility and facility. In this review, we present a comprehensive summary of the development of electrospun electrode nanomaterials for LIBs and NIBs, and a brief introduction about electrode materials beyond LIBs and NIBs. By summarizing various electrochemical active materials, this review focuses on the evolution in structures and the constitution of electrospun electrode materials. In detail, a variety of electrospun anode and cathode materials of LIBs and NIBs have been properly discussed, respectively. Finally, the current progress in the electrospun electrode materials is well reviewed and the development direction is also pointed out. We believe that in the nearly future, electrospun electrode materials would be applied in commercial LIBs and promote the advance in NIBs. And we hope that this review could be helpful in the design and fabrication of electrospun hierarchical materials for other advanced energy-storage devices.

Electrospun materials for lithium and sodium rechargeable batteries: from structure evolution to electrochemical performance

This review summarizes the recent progress in electrospun electrode materials for lithium- and sodium-ion batteries.