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Multi-Walled Carbon Nanotubes (MWCNTs) modified with mixtures of polyoxometalate (POM) ions were investigated as pseudocapacitive electrodes for electrochemical capacitors. POMs undergo fast reversible multi-electron transfer reactions, and different POM molecules such as PMo12O40 3- (PMo12) and PW12O40 3- (PW12) demonstrate characteristic electrochemical activity related to their chemical composition. However, we have discovered that when these molecules are combined in aqueous solutions, the resulting mixture shows unique electrochemical properties leading to cyclic voltammograms (CVs) very different from those of the constituent molecules. Further analyses revealed that these POM ions do not just mix physically in aqueous solutions, but instead react spontaneously to form PMo12-xWx mixed addenda chemistries. By simply mixing PMo12 and PW12 in different ratios, a variety of mixed addenda ions, each with unique electrochemical properties, can be easily synthesized. This control over POM redox behavior afforded by the novel mixed addenda synthesis is used along with layer-by-layer (LbL) deposition to design molecular coatings that demonstrate desired pseudocapacitive characteristics. The best performance was achieved with a coating that superimposed a 3:1 PMo12O40 3−-PW12O40 3− mixed layer on a 1:1 GeMo12O40 4−-SiMo12O40 4− mixed layer, which resulted in an 11X capacitance enhancement over unmodified CNT. This dual layer electrode also demonstrated a close to rectangular CV profile due to the overlapping redox features of the POM combination (Figure 1). In this talk we will discuss the unique electrochemistry of various POM mixtures, the mechanisms of the novel mixed addenda synthesis, and our approach for using these molecules to design high performance composite EC electrodes. Figure 1

Printed wiring board (PWB)-based MEMS RF switches were developed using modified processes and similar materials as used for high density interconnect/embedded passives (HDI/EP) components. By constructing cantilever-beam electrostatic mechanisms, single-pole single throw through single-pole four-throw RF switches were developed with suitable performance in the microwave band. Combining these switches with embedded lumped element filter components using the same substrate materials, a switched filter bank was simulated, fabricated, and measured results shown.

Mechanical models and analytical solutions have been developed for the RF-MEMS switches under electrical actuation, forced vibration, and shock/impact loading conditions. The basic structure based on cantilever beam switches has been used as model system to illustrate the static and dynamic responses. Analytical solutions for folded beam structures have been furthermore derived and combined with the model system as an effective beam. These characterizations are used to define reliability and performance criteria for the robust design, performance evaluation, and rapid reliability assessment of RF-MEMS switch devices. It was found that the actuation voltage depends on several key design parameters and material properties. Actual acceleration to tiny switch structure is found only one portion of applied acceleration under shock/impact conditions, depending on system stiffness.Copyright © 2004 by ASME

1-Ethyl-3-methyl imidazolium hydrogensulfate (EMIHSO4), a non-fluorinated IL, was studied both as liquid and polymer electrolytes for electrochemical capacitors [1]. While EMIHSO4exhibited a reasonable ionic conductivity, improvement in proton conductivity is possible by altering the structure of the cation, which warrants further investigation. The objectives of this study are twofold: to investigate the effect of cationic functional groups on the properties of the IL; and to select and characterize the IL system with optimum proton conductivity. The electrolytes were prepared using ILs with different cations, namely 1-ethyl-3-methyl imidazolium hydrogensulfate (EMIHSO4), 1-methylimidazolium hydrogensulfate (MIHSO4), and imidazolium hydrogensulfate (ImHSO4) in methanol solutions. For comparison, a solution of EMIHSO4 with propylene carbonate (PC), an aprotic solvent, was prepared with the same concentration as those in methanol-based electrolytes. To demonstrate the proton conductivity of the electrolytes, standard two-electrode cells were assembled utilizing pseudocapacitive electrodes such as RuO2and nanocarbon/polyoxometalate (POMs) composites [2].The cell performance were characterized using cyclic voltammtery (CV) and ac impedance spectroscopy. RuO2 exhibits pseudocapacitance via a coupled proton–electron transfer in a protic electrolyte. Figure 1 shows the CV profiles of the RuO2-based cells in all electrolytes. The ImHSO4 and MIHSO4-based cells demonstrated similar CVs and the highest capacitance performance. Their higher capacitance compared to EMIHSO4-based cell indicates the higher proton conductivity of ImHSO4 and MIHSO4, which can contribute to the electrochemical reactions of RuO2 electrodes. Due to the role of solvent on proton dissociation, the cell performance was superior using EMIHSO4/MeOH electrolytes than that for EMIHSO4/PC. The proton conductivity of PILs was further examined on a carbon/POM electrode and compared to the bare carbon electrodes. POMs involve fast and reversible multielectron transfer reactions in proton containing electrolytes. For a better comparison, the respective carbon/POM cells were also tested in 0.5M H2SO4. The electrochemical performance of the carbon/POM-based cells in the PIL electrolytes followed a similar trend as those for RuO2-based electrolytes. Figure 2 (a) shows the CV profiles of the carbon/POM-based cells and the bare carbon in ImHSO4/MeOH electrolyte. The increased capacitance of the carbon/POM cell over that for the bare carbon implies the proton conducting characteristic of ImHSO4 electrolyte which participates in the redox reaction of the POM. Figure 2 (b) presents the capacitance ratio of the carbon/POM cells in the respective PILs to H2SO4 electrolyte. ImHSO4-based cells showed the highest capacitance ratio followed by MIHSO4 and EMIHSO4in both MeOH and PC solutions. Imidazoles consist of two nitrogen sites, and their protonated and unprotonated nitrogen functions may act as proton donor and acceptor in proton transfer reactions. As the nitrogen sites decrease in MI and EMI cations, the contribution from proton conduction becomes smaller. More details on optimizing the proton conductivity of these PILs for room temperature applications, especially polymer electrolytes will be presented. References: [1] S. Ketabi, Z. Le, and K. Lian, Electrochemical and Solid-State Letters, 15 (2), A19, 2012. [2] G. Bajwa, M. Genovese, and K. Lian, ECS Journal of Solid State Science and Technology, 2 (10), M3046, 2013. Figure 1. CVs of RuO2 cells in ImHSO4, MIHSO4, EMIHSO4 methanol-based, and EMIHSO4/PC electrolytes at a scan rate of 100 mV/s Figure 2. (a) CVs of carbon/POM and bare carbon cells in ImHSO4/MeOH; (b) capacitance ratio of carbon/POM cells in respective PILs to H2SO4 electrolyte (all tests at a scan rate of 100 mV/s)

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The field of energy textiles is growing but continues to face two main challenges: (1) flexible energy storage does not yet exist in a form that is directly comparable with everyday fabrics including their feel, drape and thickness, and (2) in order to produce an “energy textile” as part of a garment, it must be fabricated in a systematic manner allowing for multiple components of e-textiles to be integrated simultaneously. To help address these issues, we have developed textile supercapacitors based on knitted carbon fibers and activated carbon ink. We show capacitances as high as 0.51 F cm−2 per device at 10 mV s−1, which is directly comparable with those of standard activated carbon film electrodes tested under the same conditions. We also demonstrate the performance of the device when bent at 90°, 135°, 180° and when stretched. This is the first report on knitting as a fabrication technique for integrated energy storage devices.

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Polymer electrolytes comprised of silicotungstic acid (SiWA) in cross-linked polyvinyl alcohol have been developed and characterized for ultra-high rate electrochemical capacitors (ECs). The solid polymer electrolytes outperformed liquid H2SO4 electrolytes in a similar cell setup. The polymer electrolyte enabled thin flexible solid double layer ECs demonstrated an exceptionally high rate capability of 50 V s−1 in cyclic voltammograms and a 10 ms time constant in impedance analyses. Enhanced stability and water uptake were observed for the cross-linked polymer electrolytes compared to their non-cross-linked counterparts. Thermal analysis revealed an increase of water content with the percentage of cross-linking. Structural analyses suggested that SiWA in the cross-linked polymer electrolyte was more hydrated than SiWA in the non-cross-linked polymer electrolyte. In addition, a stronger hydrogen bonding interaction was observed due to an increase in the crystallized water content in hydrated SiWA suggesting a more stable proton-hopping network that can be leveraged for high conductivity.

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Surface modifications by pseudocapacitive polyoxometalates (POMs) on multi-walled carbon nanotubes (MWCNTs) have been investigated for electrochemical capacitors (ECs). The electrochemical behavior of two POM molecules, phospho-molybdate (PMo12) and silicomolybdate (SiMo12), have been studied. A Layer-by-Layer (LbL) deposition technique was utilized to coat thin layer PMo12 and SiMo12 on MWCNTs. This method was shown to be very effective and versatile in changing the surface chemistry of the MWCNT substrates. Several deposition sequences for POM layers have been demonstrated by combining the different POM chemistries to greatly enhance the pseudocapacitance on MWCNTs. This approach also enables to tailor design surface chemistries for many other applications.

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Solid-polymer electrolytes comprised of silicotungstic acid (SiWA) in a polyvinyl alcohol (PVA) matrix were developed for electrochemical capacitors (ECs). Addition of phosphoric acid has further enhanced the ionic conductivity of a SiWA-PVA electrolyte. The proton conductivity of the phosphoric acid modified electrolyte reached 0.01 Scm−1 with good stability. Both SiWA-PVA and H3PO4-modified SiWA-PVA electrolytes conducted protons via a hopping mechanism. All-solid ECs relying on the modified polymer electrolyte using graphite electrodes demonstrated high rate performance. The devices were able to charge and discharge up to 100 V/s in cyclic voltammetry, exceeding their liquid-based counterpart. Thermal and structural characterizations revealed that the addition of phosphoric acid enhanced the thermal stability of the polymer electrolyte and promoted the retention of crystallized water without altering the bonding structure of the original SiWA-PVA network. © 2011 The Electrochemical Society. [DOI: 10.1149/2.064112jes] All rights reserved.

The individual potential changes in positive and negative electrodes in symmetric and asymmetric electrochemical capacitors (ECs) were monitored simultaneously with a reference electrode during cyclic voltammetry scans. The tracked potential showed equal changes in both electrodes for symmetric EC and uneven changes for asymmetric EC. An asymmetric EC with graphite-RuO 2 electrodes and H 4 SiW 12 O 40 (SiWA) electrolyte was investigated for its higher voltage window and excess capacitance. The results suggested that excess capacitance could be attributed to a pseudocapacitance from the reduction/oxidation of SiWA. This potential tracking method can also be utilized to exploit a charge storage mechanism of other asymmetric ECs.

The interface between ultraviolet (UV) ozone treated poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and N,N′-diphenyl-N,N′-bis-(1-naphthyl)-1-1′-biphenyl-4,4′-diamine (α-NPD) was investigated using single carrier hole-only devices and in situ ultraviolet and x-ray photoelectron spectroscopy to elucidate the implications for device applications. It is found that although the work function of PEDOT:PSS is increased by UV ozone treatment, the injection barrier to α-NPD is in fact increased, resulting in lower current density in devices. The apparent increase in work function is attributed to a metastable surface dipole as a result of UV ozone treatment, which does not significantly influence the energy-level alignment.

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Asymmetrical electrochemical capacitors using heteropoly acid (HPA) electrolytes and RuO 2 -graphite electrodes were investigated. The fabricated devices have a working voltage window of 0-1.4 V and can be discharged more efficiently than conventional symmetrical capacitors. Pseudocapacitance from HPA has enhanced the total capacitance of these devices. HPAs have been shown to be able to both function as an electrolyte and to contribute to pseudocapacitance.

Chemically modified carbon nanomaterials, such as nanodiamond (ND) soot, onionlike carbon (OLC), and carbon nanotubes (CNTs), were investigated as electrodes for electrochemical capacitors in high rate applications, that is, applications that drain power very fast. A layer of Keggin-type polyphosphomolybdate (PMo 12 ) was coated on carbons through "layer-by-layer" deposition by alternating electrostatic adsorption of cationic [i.e., poly(diallyldimethylammonium)chloride] and anionic (i.e., PMo 12 ) species to add pseudocapacitance to improve current response and increase capacitance. This modification was successful on OLCs and CNTs but not on ND soot. This difference is due to variations in surface chemistry, structure, and sp 2 /sp 3 content of those materials. In particular, the modified OLC exhibited a 20% increase in capacitance with up to 600 mF/cm 2 capacitance at 5 V/s, illustrating its high rate capability.

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Two heteropoly acids, phosphotungstic acid (PWA) and silicotungstic acid (SiWA), were studied as aqueous electrolytes for applications on electrochemical storage devices. Their performance was compared to the H 2 SO 4 benchmark with respect to both double-layer and pseudocapacitance using cyclic voltammetry, electrochemical impedance spectroscopy, and self-discharge measurements. Both heteropoly acids showed a very good proton conductivity in aqueous solutions, with overall performance comparable to H 2 SO 4 . Therefore, in spite of their electrochemical active nature, PWA and SiWA are feasible as electrolytes provided the operation window is chosen appropriately.