Vacuum-Assisted Layer-by-Layer Nanocomposites for Self-Standing

Aug 29, 2014 - George W. Woodruff School of Mechanical Engineering, Georgia Institute of ... The electronic conductivity and mechanical stability are ...
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Vacuum-Assisted Layer-by-Layer Nanocomposites for Self-Standing 3D Mesoporous Electrodes Md Nasim Hyder,†,‡,∥ Reza Kavian,§,∥ Zakia Sultana,‡ Kittipong Saetia,† Po-Yen Chen,† Seung Woo Lee,§ Yang Shao-Horn,*,‡ and Paula T. Hammond*,† †

Department of Chemical Engineering and ‡Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States § George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States S Supporting Information *

ABSTRACT: Electrochemical energy storage devices will play a critical role for efficient storage and reliable on-demand supply to portable electronics, electric and/or plug-in-electric vehicles that entail rapid charging/discharging with long cycle life. The design and assembly of nanoscale materials is critical for developing high performance mesoporous electrodes for energy storage devices that can be scaled-up for manufacturing. To address the challenge of nanostructured electrode development, this work reports a layer-by-layer (LbL) fabrication technique based on electrostatic self-assembly coupled with vacuum assisted filtration. By combining electrostatic interactions with vacuum force, thick electrodes (4−50 μm) of electroactive polyaniline (PANi) nanofibers and oxygen functionalized multiwalled carbon nanotubes (MWNT) are assembled in tens of minutes. The electronic conductivity and mechanical stability are further improved through controlled heat treatment of these electrodes that shows high surface area with interpenetrating networks of nanofibers and nanotubes. Electrochemical measurements reveal high specific capacity of 147 mAh/g originating from the MWNTs and redox active PANi nanofibers that store charges through both electrical double layer and faradaic mechanism with excellent charge/discharge stability over 10,000 cycles. The precise control over the electrode thickness and rapid assembly from this VA-LbL technique show promise for the development of binder-free mesoporous electrodes for next generation electrochemical energy storage devices.

1. INTRODUCTION

des for efficient electron and ion transport to deliver high performance with a long cycle life. In the present study, we report the design and assembly of binder-free mesoporous thick electrodes using vacuum-assisted layer-by-layer assembly (VA-LbL) of CNTs with conjugated polymer nanofibers. Layer-by-layer (LbL) assembly is a bottom-up nanoassembly which consists of sequential immersion of a charged substrate into aqueous solutions of complementarily charged materials in which synergies between different nanomaterials can be achieved due to the intimate contact of individual components.4,14,15 The ability to control a diverse range of composition and transport properties within LbL films has led to investigations into their utility for a number of electrochemical devices.16,17 One unique feature of LbL assembly is the ability to directly incorporate nanomaterials with a variety of geometries into cohesive thin films without the use of a polymeric binder, thus permitting the generation of high porosity, high surface area thin films without blocking the

Nanoscale materials have opened up new opportunities for the development of electrochemical capacitors (EC) with high energy and power densities due to their unique physicochemical and surface properties.1 Among various nanomaterials, carbon nanotubes (CNTs) have been utilized extensively due to their high aspect ratio, electrical conductivity, and exceptional stability.2−4 For example, functionalized multiwalled carbon nanotubes (MWNT) have attracted a new surge of research interest by mixing with other exotic metal oxides or conjugated polymers as nanostructured electrode materials for high-power ECs.5−8 Incorporation of CNTs into an electroactive polymer matrix improves the electrical conductivity as well as the mechanical properties of the original polymer matrix.9 CNT based nanostructured electrodes assembled from chemical vapor deposition (CVD),10 Meyer rod,11 plasmaspray,12 and vacuum-assisted filtration (VA)13 have demonstrated excellent electronic and ionic transport behavior for energy storage devices. However, some of the above-mentioned techniques are expensive and not suitable to assemble nanoscale materials for developing conducting porous electro© 2014 American Chemical Society

Received: June 27, 2014 Revised: August 26, 2014 Published: August 29, 2014 5310

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with ammonium peroxydisulfate (99% purity).30 Briefly, aniline hydrochloride (purum; 2.59 g, 20 mmol) was dissolved in distilled 1 M HCl in a volumetric flask to 50 mL of solution. Ammonium peroxydisulfate (purum; 5.71 g, 25 mmol) was dissolved in 1 M HCl also to 50 mL of solution. Both solutions were kept for 1 h at room temperature (∼18−24 °C), then mixed in a beaker, briefly stirred for several minutes and left at rest to polymerize for 24 h at 4 °C. After that the PANi was collected as a fiber suspension using a centrifuge (at 10,000 rpm for 15 min) by washing the supernatant a couple of times with 100 mL of deionized Milli-Q water (pH ∼ 2.5). Scanning electron microscopy in Figure 1a shows the microstructure of the

transport of ions/electrons or electrochemical activity at material interfaces.17 In previous work, we demonstrated the power of this technique by generating MWNT thin-film electrodes with unusually high power and energy density.18,19 Recently, we have designed highly porous multilayer electrodes of polyaniline nanofibers (PANi) and MWNTs that allow rapid electronic and ionic transport.16,17,20 These electrodes deliver high-power and high-energy density (∼220 Wh/Lelectrode at ∼100 kW/Lelectrode) and are stable over 2000 cycles. While these thin-film nanostructured electrodes are suitable for microscale devices, challenges still remain to design a fabrication technique that can generate thick mesoporous electrodes on a larger scale and at a pace suitable for commercial translation.1,21−25 To address the speed of assembly and increase the thickness of electrodes, we recently reported a spray-LbL technique based on the use of a porous substrate to develop all-MWNT electrodes that are tens of microns thick.3 Another method that can address the speed and scale-up concerns, the vacuumassisted (VA) filtration technique, has shown promise for assembling nanomaterials for energy and environmental applications.13,26−29 However, simple VA deposition involves premixing of the nanomaterials, which can result in aggregation, and leads to electrodes with lower surface area.20 While sprayLbL assembly is faster than the VA-LbL technique, the latter allows developing binder-less free-standing all-active nanostructured electrodes including efficient use of nanomaterials with minimum waste. Here, we report an electrode nanofabrication technique, vacuum assisted layer-by-layer (VA-LbL) assembly for the development of binder-free thick electrodes in short time scales. Using the VA-LbL nanoassembly, we have designed tens of microns thick mesoporous electrodes of PANi/MWNT that deliver high-energy, high-power density for thousands of cycles with a little drop in specific capacitance. The vacuum-assisted LbL (VA-LbL) technique adapts the concept from electrostatic LbL assembly of depositing nanoscale materials in a multilayer architecture by adding shorter processing times through the physical trapping of materials by removal of solvents using vacuum force, thus presenting a means for scaled-up electrode manufacturing. In traditional layer-by-layer self-assembly (Dip-LbL), film builds up on the substrate based on diffusion and subsequent selflimited electrostatic adsorption of nanometer scale polyelectrolyte layers. This mechanism usually requires tens of minutes per adsorption cycle to achieve films with equilibrium amounts adsorbed and uniform morphologies using traditional dip-LbL methods. For example, fabricating a 1.5 μm thick CNT electrode by Dip-LbL takes 4 days of processing time, which is not viable from a manufacturing perspective despite the excellent electrochemical performance of the electrode.20 On the other hand, using the VA-LbL technique a 1.5 μm electrode of PANi/MWNT has been assembled in a few hours that shows excellent porosity and mechanical integrity suitable for highperformance energy storage devices. To be competitive for manufacturing for next generation energy storage devices, it is critical to design high performance electrodes with tens of microns (thickness) in hours rather than days, and the VA-LbL assembled PANi/MWNT electrodes described below fulfill the need for rapidly assembled high-power ECs.

Figure 1. Schematics of the vacuum-assisted layer-by-layer (VA-LbL) assembly. The multilayer films of PANi nanofibers (cationic) and MWNT (anionic) are assembled based on the electrostatic LbL technique on a porous filter substrate for the deposition of complementary charges nanomaterials and removal of the solvent. PANi nanofibers; the diameters were found to be 25−70 nm and the length 0.3−4 μm which is supported by the transmission electron microscopy (TEM) (Figure S1). MWNT Functionalization. MWNTs prepared by a conventional chemical vapor deposition (CVD) method were purchased from NANOLAB (95% purity, length 1−5 μm, outer diameter 15 ± 5 nm). MWNTs were refluxed in concentrated H2SO4/HNO3 (3/1 v/v, 96% and 70%, respectively) at 70 °C to prepare oxygen functionalized MWNTs (MWNT) described elsewhere and then washed with deionized Milli-Q water (18.2 MΩ·cm) water several times using a nylon membrane filter (0.2 μm). As can be seen from the SEM microstructure in Figure 1b, the MWNTs have diameters around 15 nm (as supported by the TEM morphology in Figure S2) with length ∼1−5 μm. Vacuum-Assisted Filtration Setup for the LbL Assembly. The schematic of the vacuum-filtration setup shown in Figure 1 consists of a substrate holder, vacuum filtering flask, rubber gasket connector, and house vacuum connector. For the present study, we used a substrate holder that can produce free-standing electrodes of 2.5 cm in diameter (effective electrode area of 4.91 cm2). The polyelectrolytes were deposited manually to repeat the desired number of bilayers to fabricate the electrodes. The substrate was allowed to dry at least 6 h before peeling off the free-standing electrode from the substrates. Depending on the vacuum applied and concentration of the nanomaterials, the typical processing time for a bilayer of PANi and MWNTs can take approximately 20−30 s which is significantly shorter compared to dipping LbL assembly of 900−1200 min. In addition, the nanomaterial suspensions can be spread uniformly producing a homogeneous layer of deposited nanoscale materials. Assembly of VA-LbL-PANi/MWNT Electrodes. The dialyzed PANi nanofiber (pH 2.5; 0.02 mg/mL) and MWNT (pH 3.5; 0.025 mg/mL) suspensions were sonicated using a Branson 3510 ultrasonic cleaner (40 kHz) in Milli-Q water (18.2 MΩ·cm) for 30 min at room temperature to form stable dispersions. The nanosuspensions were then vacuum-filtrated alternately on porous substrates (Carbon paper, Electrospun mat, Whatman PC track-etch polycarbonate filtration membrane with pore diameter 200 nm) (Figure S3). The concentrations and the volume of the cationic and anionic suspensions

2. EXPERIMENTAL SECTION Polyaniline Synthesis. Polyaniline (PANi) nanofibers were obtained from the rapid polymerization of aniline (99.5% purity) 5311

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Figure 2. Morphologies of the polyaniline nanofibers, multiwalled carbon nanotubes, and their layer-by-layer electrodes: (a) scanning electron microscopy (SEM) of the PANi nanofibers; the inset shows stable suspensions of PANi at pH ∼ 2.5 (0.01 mg/mL), (b) SEM morphology of the oxygen functionalized MWNTs; the inset shows stable suspension of the MWNTs at pH ∼ 3.5 (0.025 mg/mL), (c) multilayer electrode thickness as a function of the number of bilayers deposited on porous substrates using the VA-LbL technique with PANi nanofibers (pH ∼ 2.5) and MWNTs (pH ∼ 3.5), and (d) comparison of multilayer electrode buildup between VA-LbL and Dip-LbL on the PC membrane substrate. were adjusted with specific deposition time and repeated sequentially for multiple deposition cycles (number of bilayers, n). Therefore, for a fixed concentration and volume, the electrode thickness can be controlled easily just by varying the number of bilayers deposited. There is no rinsing step necessary for the VA-LbL technique since the process relies on electrostatic interaction coupled with convective force for electrode buildup. To assemble VA-LbL electrodes with a thickness of 10−30 μm (with a filter holder of 25 mm diameter), the processing time could be somewhere around 1−3 h depending on the vacuum applied, filter substrate porosity, the concentration of the nanomaterials, and the number of bilayers deposited (longer processing time with the growth of layer numbers). Following filtration, the air-dried LbL-PANi/MWNT film was lifted off the filtration PC membrane for further heat treatment at 180 °C for 12 h under vacuum. Physicochemical Characterization. Zeta potentials of PANi and MWNT suspensions were measured as a function of pH using a Nano ZS-90 (Malvern Instruments Inc.). Thicknesses of the free-standing PANi/MWNT electrodes were measured using a Mituyoto micrometer as well as a scanning electron microscopy (JEOL 6700). Mechanical testing on the VA-LbL electrode samples were performed using a Hysitron Triboindenter with a blunt tip (diameter ∼10 μm) that was progressively pressed onto the sample surface with predetermined depths. For each sample, 25 load-penetration curves were performed with maximum loads of 200 μN. Before each experiment, a fused quartz standard sample was used to calibrate the instrument and calculate the tip area function. The electrode microstructures were investigated using a high resolution scanning electron microscope (JEOL 6700 Field-Emission HR-SEM) with operating voltage at 2.5 kV and a transmission electron microscope (JEOL 2010 Advanced High Performance TEM) for medium and high resolution imaging. Thermal behavior of the LbL electrodes was analyzed with a thermogravimetric analyzer (TA Instruments Q50) in air/N2 at a heating rate of 10 °C/min. Brunauer−Emmett−Teller (BET) surface area of the VA-LbL electrode was measured by ASAP 2020 accelerated surface area and porosimetry instrument (Micro-

metitics, Norcross, GA) with N2 physisorption. The samples were degassed overnight at 60 °C, and the adsorption−desorption isotherms were obtained at −196 °C (77 K). Electrode surface chemistry was probed with an X-ray Photoelectron Spectrometer (PHI Versa Probe II, Physical Electronics) equipped with a monochromatic Al Kα X-ray source with a highly focused beam size that can be set from 10 to 300 μm. Sheet-resistance was measured by a 4-point probe (Signatone S-302−4) with a device analyzer (Keithley 4200). A series of 3−4 measurements were taken on each film, and the measurements then were averaged to give the final reported value with the standard deviation as an error range. Electrochemical Measurements. The lithium-cell test was conducted using a two-electrode electrochemical cell (Tomcell Co. Ltd.) consisting of multilayer films as the positive electrodes and lithium foils as the negative electrodes. The electrolyte solutions were 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) 3:7 vol % (Novolyte) contained within two Celgard 2500 microporous separators. The electrode area of LbL-PANi/MWNT electrodes was 1.05 cm2, and the loading density was 0.15−0.23 mg/ cm2. Li-cell testing was performed using a Solartron 1470 in the voltage range 1.5−4.5 V vs Li at room temperature. For galvanostatic rate capability tests, the gravimetric current ranged from 0.05 to 250 A/g, and the voltage was held constant at either 1.5 or 4.5 V vs Li for 30 min prior to charge or discharge, respectively. Cycling tests consisted of accelerated galvanostatic cycling at 10 A/g for 99 cycles, followed by a slower charge and discharge (0.1 A/g) every 100 cycles up to 1000 cycles, with a 30 min voltage hold at 1.5 or 4.5 V vs Li prior to low-rate charge or discharge, respectively. For cycle numbers between 1000 and 11,000, 499 accelerated cycles were performed for every slower charge and discharge cycle. Only low-rate (0.1 A/g) discharge capacity is reported. In all cases, assembled cells were allowed to rest for at least 8 h prior to testing in order to allow full wetting of porous films by the electrolyte. 5312

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3. RESULTS AND DISCUSSION A unique electrode fabrication technique based on the principle of layer-by-layer (LbL) assembly has been developed for electrode fabrication on porous substrates using vacuumassisted (VA) filtration, as illustrated in Figure 1. The anionic (MWNTs) and cationic (PANi) nanomaterials were deposited alternately on a porous substrate, and the solvent was removed continuously using a vacuum trap. Therefore, by simply controlling the number of deposition cycles (i.e., number of bilayers, n) on a filter membrane (porous), multilayer electrodes can be assembled with varying thicknesses from 4−50 μm in hours. The strength of the VA-LbL assembly technique lies in its simplicity of bringing nanomaterials through a convective force that also induces electrostatic interaction due to the functional moieties or charges in the molecules. Previously, we have successfully assembled traditional thin-film LbL-PANi/MWNT directly onto ITO-coated glass substrates with PANi nanofiber (at pH of 2.5, cationic nature) and multiwalled carbon nanotube (at pH of 3.5 as anionic counterpart) suspensions, respectively.20 On the other hand, to use vacuum filtration assembly, it is necessary to have a porous support that can facilitate removal of the aqueous phase quickly. Figure 1 shows schematics of the VA-LbL electrode architecture of PANi nanofibers and MWNTs that does not require any binder materials for the film buildup. In traditional electrostatic LbL self-assembly, it is essential to maintain optimum surface charge of the nanomaterials in the suspensions to build conformal electrodes based on electrostatic interaction. For VA-LbL, the deposition of nanomaterials on the porous membrane is assisted by convection in addition to the electrostatic interactions. In addition, the charge reversal and overcompensation processes are not necessarily achieved due to the lack of rinsing steps, in contrast to traditional LbL assembly. As a result, ionically charged nanomaterials deposited at different pH’s can be assembled based on the VA-LbL technique for multilayer electrode buildup. In this study, an HCl doped PANi suspension was kept at pH ∼ 2.5 (zeta potential: +44 mV) with the acid functionalized MWNTs at pH ∼ 3.5 (zeta potential: −48 mV). As shown in Figure 2a-b, the lengths of the PANi nanofibers and MWNTs are comparable (∼1−4 μm), and the diameters of the PANi nanofibers (∼40 nm) are slightly larger than the MWNTs (∼15 nm); the multiscale dimensions of the nanofibers and nanotubes allow the design of mesoporous electrodes with high surface area, and the surface charge allows for electrostatic assembly of the multilayer electrode. Stable suspensions of PANi nanofibers and MWNTs are shown in the inset of Figures 2a and 2b; the suspensions were sonicated prior to use for the electrode buildup, for which the concentrations were 0.02 and 0.025 mg/ mL, respectively. A fine control over the composition and thickness can be achieved simply by controlling the desired number of bilayers (n) deposited. Here we assembled PANi nanofibers and MWNTs using the vacuum assisted layer-bylayer (VA-LbL) technique for rapid assembly of thick multilayer electrodes. The dried VA-LbL samples were further heattreated at 180 °C for 12 h under vacuum to improve the mechanical stability and electrical conductivity as discussed later. Multilayer VA-LbL electrodes of PANi nanofibers and MWNTs were assembled on various porous substrates, from highly porous carbon paper (CP) to electrospun mats (ES) to track-etch polycarbonate (PC) membranes (Figure S3). As can

be observed from the digital image in Figure S4, multilayered electrode buildup on the porous substrate creates a visibly dark film due to the presence of MWNTs. In addition, with the ES mat and PC membrane, it is possible to peel off the film to obtain a free-standing electrode (Figure S4), since their surfaces are relatively smooth compared to the porous carbon paper (Figure S5). In Figure 2c, the electrode growth curve as a function of the bilayers deposited on various porous substrates is shown. Interestingly, for the first 100 bilayers, the thickness growth behavior shows an exponential trend after which it becomes linear with the number of bilayers deposited; while for polyelectrolytes this behavior is often interpreted as a sign of interdiffusion,31,32 this might not be the case here. Initially, the nanotubes and nanofibers could be randomly distributed on the substrates that later become confined and pressed (flexible PANi nanofibers) due to the increase loading of materials and applied vacuum (convective force). As for the thickness growth behavior, it is slightly higher for the ES mat and PC membrane compared to the carbon paper. Since the carbon paper has higher porosity (and uneven big pores, Figure S5), the thickness growth is lower than other substrates due to the loss of some PANi and MWNTs and difficulty in measuring the average film thickness (high standard deviation). A relevant comparison of the multilayer electrode buildup using VA-LbL and traditional Dip-LbL (as shown in Figure 2d) by keeping the concentration of the charged nanomaterials similar and only varying the number of bilayers. The VA-LbL technique always builds up thicker films compared to the Dip-LbL sample, e.g., for 60 deposition cycles of PANi/MWNT, a VA-LbL technique generates a film of 2.2 μm in about 20 min compared to a 0.95 μm Dip-LbL film at a much longer period of time of 30 h (∼1800 min). Compared to the Dip-LbL assembly where the charged nanomaterials build the film based on electrostatic interaction, in VA-LbL, the addition of a convective force (vacuum) along with electrostatic interaction allows rapid deposition of materials and thereby higher film growth rate (i.e., thickness). It should be noted that the processing time increases with the growth of bilayers deposited which can arise from the increase in solvent flow resistance due to the presence of entangled networks of nanomaterials as well as deposition time. Depending on the filter/substrate size (pore diameter) the amount of waste generated is significantly low, about 4−11% when compared to Dip-LbL (>70%) and SprayLbL (>80%); therefore, the VA-LbL technique shows great promise for excellent yield for nanomaterials loading in electrode assembly. The relative contribution of convective force and electrostatic interaction for film growth is beyond the scope of this paper, and a fundamental study is necessary. A free-standing PANi/MWNT electrode in Figure 3a shows the flexibility of the VA-LbL sample even without the presence of any inactive binder materials. The presence of PANi nanofibers in the electrode allows simple handling and processing for device buildup thereby showing promise for manufacturing. The VA-LbL electrodes were further heattreated to improve the mechanical integrity and electrochemical stability. Mechanical properties such as elastic modulus and hardness of the VA-LbL electrode samples were tested by nanoindentation measurements for the as-prepared and heattreated samples. As shown in Figure 3b, the heat-treated sample requires higher force (to indent the same depth) and slope (solid line, the unloading portion of the force-depth data) compared to the as-prepared sample; the elastic modulus and hardness of the heat-treated sample was found to be 4.01 5313

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of carboxylic acids on the functionalized MWNTs and due to the loss of dopants and oligomers of PANi.35 The subsequent weight loss between 250−550 °C is due to the degradation of the PANi backbone.20 At relatively higher temperatures, between 550−800 °C, oxidation of the MWNTs occurs and complete breakdown of PANi will lead to gaseous byproducts, catalytic impurities of MWNTs. The relative compositions of PANi and MWNTs in the VA-LbL sample were determined from the derivative of weight loss/temperature with pure PANi and MWNT samples as control. For example, a 12 μm thick VA-LbL-PANi/MWNT electrode contains 54 wt % PANi and 46 wt % of MWNTs. The average density of the free-standing VA-LbL electrode was found to be ∼0.48 g/cm3. The incorporation of PANi nanofibers and MWNTs were also confirmed by X-ray photoelectron spectroscopy (XPS) measurements (Figure S7). Compared to the PANi nanofibers sample (Figure S7a), the VA-LbL PANi/MWNT sample shows considerable amounts of oxygen (9.2%) and nitrogen (3.6%) after heat treatment (Figure S7b). Figure 4 shows the

Figure 3. Physicochemical characterizations of VA-LbL-PANi/ MWNT electrodes: (a) free-standing multilayer electrodes of PANi/ MWNT (thickness ∼30 μm) removed from the PC membrane, (b) force-depth plot from the nanoindentation mechanical testing of the VA-LbL samples before and after heat treatment (the solid line shows the unloading part of the test), (c) N2 adsorption−desorption isotherms from BET measurements on VA-LbL samples, and (d) thermogravimetric scans of the VA-LbL electrode along with PANi and MWNT samples.

(±0.44) GPa and 131 (±20) MPa as compared to 2.11 (±0.69) GPa and 59 (±9) MPa for the as-prepared sample. Heat treatment of the VA-LbL sample allows better physical contact of the PANi nanofibers with the MWNTs, thereby creating a robust morphology with nanoporous network and improved mechanical properties. Total surface area was determined using the Brunauer−Emmett−Teller (BET) equation, and pore-size distribution curves were obtained with the Barrett-JyonerHalenda (BJH) method. As can be seen in Figure 3c, the adsorption−desorption isotherm of the VA-LbL sample follows type I/II isotherms that are typical of porous nanomaterials with diameters exceeding micropores (>2 nm) according to IUPAC classification.33 In addition, the hysteresis loop belongs to the type-H3 which is associated with capillary condensation in mesopores structure and does not exhibit any limiting adsorption at high relative pressure (p/pO).33 The BET surface area of the LbL sample was measured to be 103 m2/g which is lower than MWNTs (218 m2/g) but higher than PANi nanofibers 79 m2/g. From the BJH adsorption and desorption measurements, the average pore diameters of the VA-LbL sample were found to be 27.2 and 24.7 nm, respectively. This finding was supported by image analysis (using “ImageJ” software)34 of an SEM surface micrograph of the electrode sample with average pore size to be 25 nm (±8 nm) in diameter (Figure S6). The high surface area and mesoporous electrode architecture arises from the unique VA-LbL electrode assembly technique with PANi nanofibers and MWNTs. In addition, the relative loading of PANi nanofibers and MWNTs in the electrodes was determined from thermogravimetric measurements of the multilayered samples. Figure 3d shows the weight loss of the VA-LbL sample as a function of temperature between 50−850 °C along with PANi and MWNTs as controls. In the multilayered sample, a gradual weight loss can be seen from 50−110 °C from the loss of sorbed water molecules in PANi, between 125−250 °C due to the organic decomposition

Figure 4. Microstructures of the free-standing VA-LbL-PANi/MWNT electrodes: (a) cross-section of a free-standing VA-LbL-PANi/MWNT electrode of selected area (thickness ∼12 μm) with selected magnified areas and (b) surface morphology of the VA-LbL electrode sample showing the distribution of the PANi and MWNTs.

microstructure of the as-prepared multilayered VA-PANi/ MWNT electrodes with highly mesoporous morphology. Figure 4a shows the cross-section of a free-standing electrode sample, 12 μm thick, where the PANi nanofibers are intricately networked with the MWNTs (creating mesoporous morphologies free of any visible aggregation of the nanomaterials). The high-resolution images of the magnified area confirm the presence of mesoporosity in the multilayered electrode. The surface microstructure in Figure 4b shows similar mesoporous morphology as the cross-section (Figure 4b), confirming a conformal and uniform electrode buildup from the vacuum filtration LbL assembly. In contrast to the VA-LbL electrodes, samples (Figure S8) prepared from the premixed colloidal mixture of PANi nanofibers and MWNTs (using vacuumassisted filtration to remove solvent, hence VA electrode) show aggregation with lower porosity and random morphology. The aggregation can be seen on the surface of the VA-colloidal sample as well as in the cross-section showing globules of 5314

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which increases over two times after the heat treatment at 180 °C under vacuum. The binder-free highly interconnected architecture of nanofibers (PANi) and nanotubes (MWNTs) in the multilayered electrode imparts good electronic conductivity and high surface area. To investigate the Li storage capability, the VA-LbL samples of PANi/MWNT were tested as cathodes in two electrode Li cells with nonaqueous electrolyte using potential dependent cyclic voltammetry (CV). As can be seen in Figure 6a, with the

randomly distributed PANi and MWNTs. Electrochemical CVs clearly show higher peak currents (as well as total area) from the VA-LbL sample compared to the VA-colloidal sample which could arise due to less active electrochemical interface accessible by the electrolyte for efficient electrochemical reaction to store and deliver charges. The advantages of vacuum filtration include the ability to precisely control the thickness through suspension concentration and volume of nanomaterials filtered to create uniform, dense mesoporous electrodes that can offer high surface area for improved electrochemical performance. To investigate the distribution and assembly of nanomaterials during the VA-LbL process, a sample was prepared on a holey carbon TEM grid from 3 bilayers of PANi nanofibers and MWNTs are deposited. Figure 5 shows the detailed

Figure 6. Electrochemical performance of the thick VA-LbL-PANi/ MWNT electrodes: (a) potential-dependent cyclic voltammograms of the electrode from 1.5 to 4.5 V vs Li at a scan rate of 1 mV/s, the potential windows were 1.5−3 V vs Li(black), 1.5−3.2 V vs Li (red), 1.5−3.5 V vs Li(blue), 1.5−4 V vs Li(green), 1.5−4.5 V vs Li(pink), (b) the scan rate dependent CV of electrodes in the range of 1.5−4.5 V vs Li, (c) comparison of cyclic voltammograms of thick spary-LbLMWNT electrodes with thick VA-LbL-PANi/MWNT electrodes at 1 mV/s, and (d) galvanostatic charge−discharge profiles of VA-PANI/ MWNT electrodes in the range of 1.5−4.5 V vs Li and the gravimetric currents ranged from 0.05 A/g to 50 A/g. The electrode shows high capacity of 123 mAh/g at a low discharge rate (0.1 A/g) with a sample thickness of 30 μm (400 bilayers of PANi and MWNTs).

Figure 5. Transmission electron microscopy (TEM) of the VA-LbLPANi/MWNT electrode. The inset shows the high resolution TEM showing the distinct architecture maintained by the MWNTs and PANi nanofibers. Sample was prepared on a holey carbon TEM grid with 3 bilayers of PANi and MWNT using vacuum-assisted filtration.

nanostructure of the sample, where nanoscale networks and pores are present in the multilayered electrodes of PANi nanofibers and MWNTs. In addition, the inset in Figure 5 shows that the MWNTs are not coated with PANi, and the multiwall architecture of the carbon nanotube remains intact. The VA-LbL technique creates nanostructures in which different nanomaterials can be assembled where a synergistic effect can be achieved between the functional nanomaterials for designing hybrid electrodes with well-developed mesopores and pathways for fast ion/electron transport. The interpenetrated PANi nanofibers and carbon nanotube networks help to achieve high porosity and surface area in the LbL electrode. This nanostructured architecture provide fast electronic and ionic conducting channels. The conductivity of the PANi/MWNT thick electrode was obtained by measuring the sheet resistance of the free-standing samples with a 4-point probe. The electronic conductivity of only MWNT electrode ∼ 6.5 S/cm; however, PANi has a lower conductivity of ∼0.01− 0.8 S/cm. For example, for the heat-treated VA-LbL electrode sample (thickness ∼30 μm), we obtained a conductivity of ∼5.7 S/cm. This high conductivity of VA-LbL electrodes arises from the high loading of the MWNTs (46 wt %) in the thicker films with a continuous pathway for fast electron mobility. For the as-prepared PANi/MWNT film, the conductivity is 2.3 S/cm,

increase in the potential window (1.5−4.5 V vs Li) at a scan rate of 1 mV/s, there is a gradual increase in the gravimetric currents due to the redox reaction of PANi in the multilayer electrode. The redox peaks around 3.5 V vs Li originate from the oxidation states of PANi through the doping/undoping by PF6¯ anions. Figure 6b shows the rate dependent CVs of the VA-LbL electrode sample maintaining characteristic redox peaks of PANi up to 25 mV/s. It is interesting to note that relatively small currents were observed below 3 V vs Li, indicating that the major charge storage mechanism in this voltage regime is associated with redox reactions of Li with functional groups of MWNTs. Figure 6c shows an interesting comparison of CVs at a scan rate of 1 mV/s, between two thick LbL electrodes; one with the LbL-MWNT sample (18 μm) only, which was created using the Spray-LbL method,3 and the other with LbL-PANi/MWNT (30 μm). Though the thicknesses are different, on a mass basis both of these electrode samples have an equal amount of MWNTs which is why the CV between 1.5−2.5 V has nearly the same area. On the other hand, the VA-LbL contains additional PANi nanofibers; faradaic contribution from the oxidation and 5315

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reduction of PANi nanofibers clearly contributes to the additional area and thereby specific capacity of the electrode. A redox peak with a corresponding high current can be seen from the PANi in the PANi/MWNT sample which is absent in the MWNT electrode, which contributes high charge storage from the double layer and faradaic (from the surface functional group of MWNTs) mechanism.19 It is worthwhile to mention that both of these multilayered electrodes in Figure 6c were assembled by electrostatic interaction based on layer-by-layer assembly although the fabrication processes are different; e.g., in spray-LbL electrodes, materials were directed toward the substrate by air-blasting the MWNTs,3 whereas for the VA-LbL electrode, the nanomaterials are deposited from individual suspensions which allows better loading and less wastage of active materials. From the rate dependent galvanostatic tests in Figure 6d, the gravimetric capacity of the VA-LbL electrode was found to be ∼147 mAh/g at a rate of 0.05 A/g discharge rate in the voltage range of 4.5−1.5 V vs Li for an electrode with a thickness of 30 μm. At a higher current rate of 1 A/g, the VALbL electrode of PANi/MWNT maintained 84% of the capacity (∼123 mAh/g) which is higher compared to a vacuum filtered few walled carbon nanotube (VA-FWNT) electrode (assembled by directly depositing the suspension of few walled carbon nanotubes on porous substrate using vacuum filtration assembly although not in layer-by-layer assembly) showing the capacity of ∼76 mAh/g at a rate of 1 A/g.13 This finding demonstrates the promise of VA-LbL assembly to incorporate various nanomaterials to improve the energy storage capacity for next generation electrodes. As can be seen in Figure 7a, after 100 cycles, the discharge capacity drops to 10% of the original capacity of 128 mAh/g (At 0.1 A/g), while after 11,000 cycles the capacity drops to 21%; while the energy efficiency improves slightly from 77% to 79% after 11,000 cycles. The VA-LbL electrode shows excellent cycle stability up to 10,000 cycles (Figure 7b) after which there

is a 7% drop in capacity. In addition, Coulombic efficiency improved from 88% to 95% after the first 100 cycles and remained stable indicating the VA-LbL electrode based energy storage device can attain the original state of charge (SOC) reversibly. Here, the efficiency for the charging−discharging process is defined as the fraction of the electrical charge stored during charging that is recoverable during discharge. In Figure 7c, the Ragone plot compares energy and power densities between the VA-LbL and spray-LbL electrodes. The VA-LbL sample can deliver higher energy compared to spray-LbL electrodes below 10 KW/kg. This finding clearly shows the benefit of adding PANi in the MWNT electrode architecture to obtain high-power, high-energy density simultaneously. Compared to the performances of other free-standing LbL electrodes presented in the Ragone plot in Figure 7d, at power-density below 10 KW/kg, the thick VA-LbL electrode outperforms the thick MWNT, FWNT electrodes due to the presence of redox active PANi nanofibers.13,20 Interestingly, the thick PANi/MWNT electrode suffers in performance at a high power region due to its poor conductivity compared to the thick MWNT or FWNT electrode. In addition, the thin electrodes of MWNT or the PANi/MWNT electrode always outperforms the thick VA-LbL electrode; the origin of this behavior may result from loss of accessible surface area of the electrode or transport limitations with increasing thicknesses. For the present study, we have used relatively low concentrations of the PANi nanofibers and MWNTs suspension for cationic and anionic counterparts for the VALbL assembly. At this concentration (0.01−0.08 mg/mL), the relative distribution of PANi nanofibers and MWNTs are very uniform and evenly distributed throughout the architecture. However, the microstructure changes drastically at a higher concentration of the suspensions to yield poor device performance. For example, we prepared VA-LbL electrodes assembled at a higher (10 times that of the sample shown in Figure 2) concentration of PANi nanofibers (0.2 mg/mL) and MWNTs (0.25 mg/mL). As can be seen in Figure S9, the SEM cross-section morphology of a 21 μm thick VA-LbL sample shows successful electrode buildup, but the HR-SEM microstructure (Figure S9b) reveals segregated layers of PANi nanofibers and MWNTs that can affect the electrochemical performance. A cyclic voltammetry test of the sample is shown in Figure S9c, where the CV appears to be significantly different than the one obtained in Figure 5a, behaving more like a battery electrode where the Faradaic peaks can be observed at fixed potentials with PANi being the dominant component. The galvanostatic charge−discharge behavior (Figure S9d) also supports the findings, where the discharge curves show lower slopes at various currents, a feature of battery-like charge− discharge. The capacity obtained is ∼98 mAh/g (Figure S9d), much lower compared to ∼123 mAh/g (Figure 5d) at a rate of 0.1 A/g. At higher current density, there is also an imbalance between the charge and discharge capacities. This unusual behavior confirms that the distribution of nanomaterials in the electrode is critical and segregated layers of nanomaterials are undesirable. Finally VA-LbL electrode samples with higher materials loading of 0.87 mg/cm2 were tested at harsh conditions (high rate loadings) from 50 to 250 A/g, and the results are shown in Figure S10. At current loadings from 0.1 to 10 A/g, the VA-LbL sample shows very low capacity ∼95 to 50 mAh/g much lower than the results shown in Figure 6d with capacity of ∼144 mAh/g at 0.1 A/g. When comparing the microstructure as

Figure 7. Comparison of the VA-LbL-PANi/MWNT electrode performance; (a) galvanostatic charge−discharge profiles of the VALbL electrode in the 100th and 11,000th cycles in the voltage range of 1.5−4.5 V vs Li as a rate of 0.1 A/g, (b) cycle stability of gravimetric capacities of the thick electrodes up to 11,000 cycles, (c) Ragone plot comparison of the electrodes assembled from spray-LbL-MWNT and VA-LbL-PANi/MWNT, and (d) Ragone plot comparison of the VALbL-PANI/MWNT electrode with VA-FWNT, LbL-MWNT, and LbL-PANi/MWNT electrodes. 5316

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oxygen functionalized multiwalled carbon nanotubes; porous substrates for the VA-LbL filtration assembly to fabricate PANi/MWNT electrodes; vacuum-filtration layer-by-layer assembly setup; VA-LbL electrodes buildup on the porous carbon paper; image analysis for pore size distribution; XPS spectra of the VA-LbL electrode samples; SEM microstructure of a vacuum assisted PANi/MWNT electrode sample prepared from a colloidal mixture; VA-LbL electrodes assembled at higher (10 times) concentration of PANi nanofibers (0.2 mg/ mL) and MWNTs (0.25 mg/mL). This material is available free of charge via the Internet at http://pubs.acs.org.

shown in Figure S10b, it can be clearly seen that electrodes with higher materials loading show very low porosity and aggregation of the nanomaterials. However, it is interesting to note that at harsh conditions i.e., a very high current rate of 50 to 250 A/g, the electrode sample with high loading still shows some capacity of 30 to 6 mAh/g. Rapid fabrication of thick binder-free electrodes with high surface area and conductivity is essential for the development of next generation electrochemical devices.36 Vacuum filtration layer-by-layer allows excellent control over the composition and thickness to design the architecture of multilayered electrodes. By combining organic/inorganic nanomaterials with the VALbL technique, we have assembled binder free electrodes of PANi nanofibers and MWNTs that are tens of microns thick. The physicochemical properties of the multilayered electrodes show well developed mesopores with high surface area where the active materials (PANi and MWNTs) are easily accessible for efficient electrochemical reaction. The self-assembled electrodes create robust architectures that are free-standing, flexible, and easy to handle for device fabrication. The retention of high capacitance even beyond 10,000 cycles for the thick VALbL electrode shows promise for electrodes with high-energy, high-power density. Similar to traditional layer-by-layer (LbL) assembly, vacuum filtration based LbL can be extended to incorporate more than two polyelectrolytes to design hybrid electrodes from a bilayer to a tetralayer architecture. The ease and control of electrode development using the VA-LbL nanofabrication provide promising alternatives for the assembly of nanoscale materials for next generation electrochemical energy storage devices.

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Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ∥

Equally contributing authors.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Contour Energy for funding support of this project. M.N.H. and P.T.H. also gratefully acknowledge the funding from National Science Foundation (NSF) under the NSF Center CHE-1305124. This work made use of the Shared Experimental Facilities supported by the MRSEC Program of the National Science Foundation under award number DMR-0819762. M.N.H. is thankful for the support of a postdoctoral fellowship from the Natural Sciences and Engineering Research Council (NSERC) of Canada. We acknowledge use of facilities supported by the Institute for Soldier Nanotechnologies (ISN) at MIT.

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CONCLUSIONS A nanoassembly technique based on a modified layer-by-layer assembly has been developed for 3D self-standing, multilayered mesoporous electrodes. By combining electrostatic interaction along with vacuum force, thick electrodes (4−50 μm) of electroactive polyaniline (PANi) nanofibers and acid functionalized multiwalled carbon nanotubes (MWNT) were assembled in hours. The electronic conductivity and mechanical stability of these films were further improved through controlled heat treatment under vacuum. These binder-free VA-LbL electrodes show highly mesoporous architectures with interpenetrating networks of nanofibers and nanotubes resulting in high surface area of 103 m2/g. Electrochemical measurements performed on the multilayer electrodes reveal high specific capacity originating from redox active PANi and MWNTs that stores charge through electrostatic and faradaic mechanism. The precise control of the assembly conditions and rapid processing time allows fabricating highly porous thick electrodes that shows excellent discharge stability over 10,000 cycles during the galvanostatic tests. The high specific capacitance of the LbL electrode indicates the potential to gain precise control of charge and energy storage in PANi/MWNT thin films by controlling the number of bilayers and film thickness in the LBL assembly. Our findings show that VA-LbL nanofabrication could be a promising technique for the assembly of nanomaterials for next generation electrochemical energy storage devices.

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AUTHOR INFORMATION

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ABBREVIATIONS VA vacuum assisted LbL layer by layer PANi polyaniline MWNT multiwalled carbon nanotube CNT carbon nanotube Dip dipping EC ethylene carbonate DMC dimethyl carbonate

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Morphology of the polyaniline nanofibers from the transmission electron microscopy; HR-TEM nanostructures of the 5317

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