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ORGANIC ELECTRODES FOR STRUCTURAL ENERGY AND POWER Jodie L. Lutkenhaus, Assistant Professor

Artie McFerrin Department of Chemical Engineering, Texas A&M University Reporting period 3/15/2013 – 3/14/2016

FA9550-13-1-0147 This research project aims to design a simple yet novel approach to fabricate

structural power systems, specifically “structural electrodes,” capable of simultaneously providing power and protection to the Air Force warfighter. As components in structural power systems, these electrodes must address both energy and power needs as well as mechanical stress in one unit. We are investigating a novel collection of structural electrodes based upon graphene, polyaniline:polyacid colloid, and aramid (Kevlar®) nanofibers. We hypothesize that achievements in energy storage and mechanical properties can be simultaneously realized through control of composition, secondary interactions, and structure through processing.

The objectives of this work are listed below: o Objective 1: Explore Moldable Structural Electrodes of Functionalized

Graphene Sheets and Polyaniline:Polyacid Colloids on Aramid Fabric o Objective 2: Investigate Paintable Structural Electrodes of Functionalized

Graphene Sheets, Polyaniline:Polyacid Colloids, and Aramid Nanofibers o Objective 3: Examine Structural Paper Electrodes of Functionalized

Graphene Sheets, Polyaniline:Polyacid Colloids, and Aramid Nanofibers Progress To-Date

In our initial investigation, we explored the electrochemistry of PANI:PAAMPSA within layer-by-layer (LbL) assemblies. We previously reported that PANI:PAAMPSA alone was electrochemically stable up to 4.5 V vs. Li/Li+ because of stabilizing electrostatic interactions between PANI and PAAMPSA.1 From this study, we concluded that PANI:PAAMPSA does indeed retain its stability in albeit with reduced conductivity in the overall electrode. Figure 1 shows cycling behavior for a layer-by-layer electrode consisting of PANI:PAAMPSA and poly(ethyleneimine). This study resulted in one publication in ACS Applied Materials & Interfaces.2

Because the PANI:PAAMPSA system showed only moderate electrochemical properties, we changed our focus to polyaniline nanofibers. We specifically investigated LbL assembly of polyaniline nanofibers and functionalized graphene sheets, Figure 2.

Figure 1. Electrochemical testing of PANI:PAAMPSA/poly(ethyleneimine) layer-by-layer electrodes. (a) Capacity vs. cycle number and (b) cyclic voltammetry. From these results, it can be concluded that PANI:PAAMPSA retains its electrochemical stability within the layer-by-layer electrode, even up to 4.5 V vs. Li/Li+.

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Polyaniline nanofibers were synthesized and characterized, in which it was determined that they bore a positive charge, a diameter of about 50 nm, and a length of about 500 nm, Figure 2c. We found that assembly proceeded most robustly with graphene oxide (GO) sheets rather than reduced graphene oxide sheets, because GO sheets have a strong net negative charge, Figure 2b. Top-view and cross-sectional views of the dip-assisted assembly show evidence of both materials, assembled into a very porous electrode (62.5 % void). After assembly, the GO sheets within the electrode were reduced electrochemically to functionalized graphene sheets by application of 1.5 V vs. Li/Li+ for ten hours. Raman and X-ray photoelectron spectroscopy confirmed the successful reduction of the GO sheets. The morphology of the reduced electrode, Figure 2f, appears unchanged.

Figure 2. (a) Schematic of layer-by-layer assembly of polyaniline nanofibers (PANI NF) and GO sheets. (b) PANI NF/GO thickness vs. number of layer pairs or cycles measured using profilometry for varying GO pH values. The pH of PANI NFs was fixed at 2.5. (c) Top-view of PANI NFs, (d) top-view and (e) cross-sectional SEM images of (PANI NF/GO) electrodes. (f) Top-view of (PANI NF/GO) electrodes after electrochemical reduction.

We next investigated the energy storage capabilities of the polyaniline nanofiber / electrochemically reduced GO electrodes made by dip-assisted layer-by-layer assembly. Cyclic voltammetry (Figure 3a) and galvanostatic cycling were performed over various rates in a half-cell configuration with a lithium metal anode. It was found that the maximum capacity was 184 mA h cm-3 (461 mA h g-1) for a 460 nm thick electrode, Figure 3b. The electrode maintained its capacity over 1000 cycles, Figure 3c. Analysis of cyclic voltammograms showed that the majority of charge stored is pseudocapacitive in nature (having a b-value closer to 1), Figure 3d. While these numbers seem impressive, it should be noted that electrodes are exceptionally thin. Unfortunately, it was found that thicker electrodes (> 1 um) suffered from severe diffusion limitations, despite the electrode’s high porosity. Additionally, mechanical properties could not be measured because the electrode was too thin. However, to demonstrate the coatings versatility, we were successful in coating cotton fabric, where the assembly appeared to coat individual cotton fibers, Figure 3e. This work is published in Journal of Materials Chemistry A.,3 where it was on the “most read” list for January 2015. We have also demonstrated a spray-on version of these electrodes, published in RSC Advances.4

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We have initiated an investigation of paper-like electrodes made from aramid (Kevlar)nanofibers and functionalized graphene sheets. These electrodes are made by the flocculation and vacuum-filtration of the materials of interest. The flow induced by filtration orients graphene sheets parallel to the supporting filter paper. The aramid nanofibers were then mixed with graphene oxide sheets in a water/DMSO mixture, followed by vacuum filtration. The graphene oxide/aramid nanofiber mixture was collected on the filter paper, and was then isolated for further analysis. As controls, GO sheets and aramid nanofibers were also filtered separately. In all cases, paper-like materials were obtained, Figure 4. Under SEM magnification, one can clearly see the nanofibrous texture of the aramid nanofiber papers (Figure 4e). The nanofibrous texture is less obvious for the composite paper (Figure 4f), but an increased interlayer spacing is noticeable. The mechanical properties were measured using a dynamic mechanical analyzer in tensile testing mode, Figure 5. (We are currently confirming these results with digital image correlation, to eliminate the effects of grip-slip.) It was generally observed that the Young’s modulus increased, and other mechanical properties increased with slight variation. We believe this positive result arises from secondary interactions between the aramid nanofibers and GO sheets. Our next studies are focused on paper electrodes containing functionalized graphene sheets (reduced GO), and examining the nature of the hypothesized secondary interactions.

Figure 3. (a) Cyclic voltammetry, (b) rate capability, and (c) capacity retention for PANI NF/electrochemically reduced graphene oxide electrodes. The assembly is the working electrode, and lithium metal foils are the reference and counter electrode. The electrolyte is 0.5 M LiClO4 in propylene carbonate. (d) Voltage-dependence of b-value, which describes whether charge storage is capacitive (b=1) or diffusion limited (b=0.5). (e) The assembly coated onto cotton; the insets show SEM images of the bare and coated fibers. The numbers denote the number of layer-by-layer cycles.

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References (1) Jeon, J.-W.; Ma, Y.; Mike, J. F.; Shao, L.; Balbuena, P. B.; Lutkenhaus, J. L. Physical Chemistry Chemical Physics 2013, 15, 9654. (2) Jeon, J.-W.; O’Neal, J.; Shao, L.; Lutkenhaus, J. L. ACS Applied Materials & Interfaces 2013, 5, 10127. (3) Jeon, J.-W.; Kwon, S. R.; Lutkenhaus, J. L. Journal of Materials Chemistry A 2015, 3, 3757. (4) Kwon, S. R.; Jeon, J.-W.; Lutkenhaus, J. L. RSC Advances 2015, 5, 14994.

Figure 4. Graphene oxide (a, d), aramid nanofiber (b, e), and composite (c, f) papers made from vacuum filtration. The composite consists of 25 wt% aramid nanofibers and 75 wt% graphene oxide sheets. (a-c) Digital camera images and (d-f) SEM images at various magnifications.

Figure 5. Mechanical properties of GO/aramid nanofiber paper as a function of aramid nanofiber content. (a) Young’s modulus, (b) Ultimate strength, (c) Ultimate strain, (d) Work of extension measured using tensile testing.