Enhanced performance for early transition metal nitrides via pseudocapacitance in protic ionic liquid electrolytes

Accepted Manuscript Enhanced performance for early transition metal nitrides via pseudocapacitance in protic ionic liquid electrolytes

Abdoulaye Djire, Jean Yves Ishimwe, Saemin Choi, Levi T. Thompson PII: DOI: Reference:

S1388-2481(17)30033-4 doi: 10.1016/j.elecom.2017.02.001 ELECOM 5873

To appear in:

Electrochemistry Communications

Received date: Revised date: Accepted date:

31 January 2017 3 February 2017 3 February 2017

Please cite this article as: Abdoulaye Djire, Jean Yves Ishimwe, Saemin Choi, Levi T. Thompson , Enhanced performance for early transition metal nitrides via pseudocapacitance in protic ionic liquid electrolytes. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Elecom(2016), doi: 10.1016/j.elecom.2017.02.001

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ACCEPTED MANUSCRIPT Enhanced Performance for Early Transition Metal Nitrides via Pseudocapacitance in Protic Ionic Liquid Electrolytes

Department of Chemical Engineering, University of Michigan, Ann Arbor 48109-2136, USA

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Department of Electrical Engineering, University of Michigan, Ann Arbor 48109-2316, USA

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Department of Mechanical Engineering, University of Michigan, Ann Arbor 48109-2316, USA d

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Abdoulaye Djirea,d, Jean Yves Ishimweb, Saemin Choia,d, Levi T. Thompsona,c,d*

Hydrogen Energy Technology Laboratory, University of Michigan, Ann Arbor 48109-2136,

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USA

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Abstract

Early transition metal nitrides achieve high capacitances via a pseudocapacitive mechanism that

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involves redox reactions with protons at their surfaces. Typically aqueous electrolytes are the source of protons, therefore the operating voltages are limited to ~1.2 V. Protic ionic liquid (PIL)

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based electrolytes offer the possibility of significantly higher operating voltages and energy

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densities. This paper describes the behavior of VN and TiN in a PIL consisting of 2methylpyridine and trifluoroacetic acid. These nitrides can be cycled up to 2.0 V in this electrolyte. Voltammograms for VN and TiN in the PIL and aqueous electrolytes were similar suggesting similar pseudocapacitive mechanisms. The use of PIL electrolytes instead of aqueous electrolytes could significantly increase the energies of nitride-based supercapacitors without significant losses in power. Keywords: Nitride electrode materials • Non-aqueous pseudocapacitance • Protic ionic liquid

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ACCEPTED MANUSCRIPT electrolytes *Corresponding author. Tel: + 1 734 936 2015; Email address: [email protected] (L.T. Thompson).

1. Introduction

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Electrochemical capacitors (ECs) or supercapacitors are being developed for a variety of

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applications including use in electric vehicles, portable electronic devices, and uninterruptible

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power supplies due to their high power densities and long cycle lifes [1,2]. In terms of their specific energy and power densities, they fill the gap between conventional capacitors and

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batteries, and can be used with batteries in hybrid configurations to manage short, high power

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pulses, thereby minimizing stress on the primary energy-storage device. Markets for supercapacitors, however, remain small due to their modest energy densities.

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For commercially available, carbon-based supercapacitors, charge is stored in the

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electrochemical double layer that forms at the electrode/electrolyte interface [1,2]. These electrochemical double-layer capacitors (EDLCs) typically incorporate organic solvents and

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exhibit capacitances of ~100 F g-1 with cell voltages of ~2.7 V and energy densities up to ~5 Wh

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kg-1 [2]. Pseudocapacitors store charge through fast surface redox reactions and can possess capacitances that are significantly higher than those for EDLCs [1,2]. Materials that exhibit

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pseudocapacitive behavior include metal oxides such as RuO2 [3,4] and MnO2 [5,6], and conducting polymers such as polyaniline and polypyrrole [7,8]. Hydrous RuO2 has been reported to yield specific capacitances of 1300 F g-1 [4] over voltage windows up to 1.1 V, but the high cost of Ru limits its large-scale use. Less expensive oxides of first-row metals including Fe, V, Ni, Co and Mn have been reported to exhibit pseudocapacitance but have low electronic conductivities limiting their power densities [9]. Early-transition metal nitrides have metal-like

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ACCEPTED MANUSCRIPT electronic conductivities [10,11], and exhibit pseudocapacitive behavior in aqueous electrolytes [12-25]. These materials can also be produced inexpensively with high surface areas [10-16]. For nitrides in aqueous electrolytes, the pseudocapacitance has been attributed to redox reactions between the surface and protons [15-18]. The theoretical operating voltage window of cells

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incorporating aqueous electrolytes is limited by the hydrogen and oxygen evolution reactions

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resulting in modest energy densities (up to ~10 Wh/kg) [1-4].

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which occur at ~0 and ~1.23 V vs. NHE, respectively, at ambient temperature and pressure,

Like other non-aqueous electrolytes, ionic liquids offer the possibility of significantly higher

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operating voltages than those for aqueous electrolytes [27,28]. Protic ionic liquids (PILs) should

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support pseudocapacitive charge storage for early transition metal nitrides. In this paper we report the performance of high surface area VN and TiN in PILs that can be cycled up to 2.0 V.

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Comparative results from cyclic voltammetry (CV) and electrochemical impedance spectroscopy

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(EIS) were used to assess the potential benefits of using the PIL electrolyte over aqueous

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electrolytes.

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2. Experimental

2.1 Electrolyte and Electrode Preparation

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The PIL system consisted of a mixture of 2-picoline (base) and trifluoroacetic acid (99.99% Sigma Aldrich). The base:acid molar ratio of 1:2 was chosen because it yields good ionic conductivity (9.07 mS.cm-1 at 27 °C). To prepare the PIL electrolytes, trifluoroacetic acid was slowly added to 2-picoline with constant stirring until the desired molar composition is achieved. In order to eliminate traces of water coming from either the starting materials or the atmosphere, the resulting mixture was heated at 85 °C for 48 h under vacuum [27,28]. The ionic liquid

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ACCEPTED MANUSCRIPT samples were stored in sealed vials and transferred to the glove box for the electrochemical measurements. The aqueous electrolyte contained 0.1 mol dm-3 H2SO4; the solutions were prepared using ultrapure water (18 MΩ cm, Millipore Milli-Q Advantage A10). The high surface area VN and TiN materials were synthesized via the temperature-

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programmed reaction of V2O5 (99.9%, Sigma Aldrich) or TiO2 (99.9%, Sigma Aldrich) with

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anhydrous NH3 (99.99%, Cryogenic Gases). Details regarding the synthesis are reported

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elsewhere [10-18]. Disc electrodes were prepared by coating titanium foils (99.7%, Aldrich) with slurries containing 92% of the active material, 5% carbon black (Super P Li) and 3%

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polyvinylidene fluoride (Kynar, Arkema) in N-methyl-2-pyrrolidone (99.95%, Alfa Aesar)

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solvent. The target loading was 1.5 mg cm-2 (~ 3 mg) of active material. The electrodes were dried under vacuum at 80 °C for 8 hr. The mass of the active material was determined by

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subtracting the mass of the Ti substrate from the mass of the coated electrode.

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2.2 Physical and Electrochemical Characterization Physical surface areas were measured by N2 physisorption using the Brunauer-Emmett-Teller

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method. Pore size distributions were determined using the Barrett-Joyner-Helenda (BJH) and

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Horvath-Kawazoe (HK) methods. All measurements were performed using a Micromeritics ASAP 2020 analyzer. Prior to analysis, the materials were degassed under vacuum at 350 °C for

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5 hr. X-ray diffraction (XRD) patterns were collected using a Rigaku Miniflex diffractometer with a Cu Kα (λ = 0.15404 nm) source. Crystalline phases were identified using JADE 10.0 software. The surface morphologies of coated electrodes were examined using scanning electron microscopy (SEM). The microscopy was carried out using a FEI Nova 200 Nanolab field emission electron microscope operating at an accelerating voltage of 10.0 kV. Electrochemical characterization was carried out in a three-electrode electrochemical cell

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ACCEPTED MANUSCRIPT (ECC-Aqu, EL-Cell FmbH, Germany) using an Autolab PGSTAT302N potentiostat. The cells were assembled with 18 mm diameter glass fiber separators of thickness 1.55 mm. An 18 mm diameter counter electrode (Kynol activated carbon fabric ACC-507-15, 1500 m2g-1, thickness 0.54 mm) was used. The working electrode diameter was restricted to 16 mm to ensure good

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current flow between the counter and working electrodes. A Pt wire (1 mm diameter) was used

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as a quasi-reference electrode. To ensure good wetting, the counter electrode and separator were

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soaked in the electrolyte overnight. All experiments involving the PILs were carried out in a argon-filled glove box. Aqueous electrolytes were deaerated with nitrogen for at least 30 min

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before assembling the cell.

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Cyclic voltammetry was used to determine the voltage window, capacitance, and dependence of the capacitance on scan rate. EIS analysis was used to determine the equivalent series

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resistance (ESR). These measurements were performed at the open circuit voltage (OCV) using a

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3. Results and Discussion

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low amplitude modulation of 10 mV and a frequency range from 10 mHz to 100 kHz.

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The XRD patterns confirmed that the materials were phase-pure VN and TiN (Figure 1a). The surface areas for the VN and TiN materials were 35.0 ± 0.1 and 18.0 ± 0.2 m 2 g-1,

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respectively. Pore size distributions indicated a significant density of micropores (< 2 nm) and some mesoporosity (2-50 nm) (see Figure 1b). The average pore sizes determined using the BJH method were 19 ± 2 nm for VN, and 31 ± 5 nm for TiN. Micrographs for the VN and TiN materials (Figures 1 c and d, respectively) indicated that the materials are highly porous, consistent with the surface area and pore size distribution measurements.

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Figure 1: (a) X-ray diffraction patterns and (b) pore size distributions for the VN (red) and TiN (blue) materials. Micrographs of the VN (c) and TiN (d) electrodes.

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The specific capacitances, C, were estimated by integrating the area under the voltammograms and dividing by the nitride mass loading. The voltage window is taken as the

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width of the voltammogram. Figures 2 a and b illustrate voltammograms collected using a scan rate of 2 mVs-1. The presence of redox peaks is consistent with pseudocapacitance [1-6]. In general, the voltammograms of VN and TiN in the aqueous and PIL-based electrolytes exhibited similar features. For aqueous electrolytes, charge storage for these nitrides, particularly VN, has been attributed to redox reactions involving protons and space charge accumulation within subsurface layer [12-20,26]. We believe that similar processes are responsible for the pseudocapacitance demonstrated in the PIL electrolyte. 6

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2-Picoline:TFA (1:2) 0.1M_H2SO4_H2O

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Figure 2: Cyclic voltammograms for (a) VN and (b) TiN; 2nd cycle at 2 mVs-1. Areal specific capacitances as function of scan rate for (c) VN and (d) TiN. Nyquist plots for (e) VN and (f) TiN; the electrochemical circuit fit for each system is shown in the inset. All data are collected in protic ionic liquid electrolyte (red) or aqueous 0.1 mol dm-3 H2SO4 electrolyte (blue). The Pt wire reference electrode potential was ~ 0.6 V vs. NHE.

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ACCEPTED MANUSCRIPT Operating voltage windows for VN and TiN were wider in the PIL electrolyte (1.7 and 2.0 V for VN and TiN, respectively) than in the aqueous electrolyte (1.1 V for both VN and TiN). Much of the expanded window is a consequence of the anodic limit for the PIL being higher than the limit for water oxidation [27-30]. We believe the cathodic limit reflects the onset of the

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hydrogen evolution reaction (HER). For VN, the cathodic limits were similar in both electrolytes

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suggesting similar kinetics and transport; this is consistent with the capacitance values (see

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Figure 2c). For TiN, cathodic limits in the two electrolytes were significantly different. Hydrogen evolution in the aqueous electrolyte occurred at a much lower potential than in the PIL

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electrolyte. Note that we did not observe any H2 evolution in the region taken as the voltage

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window, however, we did observe evidence of H2 evolution when we tried to go beyond the voltage window as indicated by the formation of bubbles on the electrode surface.

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To further investigate the nature of charge storage for the VN and TiN electrodes,

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voltammograms were collected at varying scan rates. Typically at low scan rates, rates for pseudocapacitive reactions are not limited by the diffusion of ions [10]. The areal specific

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capacitances for the VN and TiN materials at different scan rates in aqueous and PIL-based

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electrolytes are illustrated in Figures 2 c and d. The capacitances at low scan rates are significantly higher than those expected for double layer capacitance (~40 µFcm -2) [1,2,9,12],

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and decreased logarithmically with increasing scan rate, consistent with a pseudocapacitive charge storage mechanism [17]. At high scan rates, diffusion limits the access of protons to the interior surface of the material thereby reducing the capacitance [30-33]. Slightly higher capacitances were observed for the materials in aqueous electrolytes as compared to the PIL electrolytes, most likely due to the higher ionic conductivity (46 mS cm -1 at room temperature) and lower viscosity (0.83 cP) of the aqueous electrolyte as compared to the PIL-based electrolyte

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ACCEPTED MANUSCRIPT (9.1 mS cm-1 and 13 cP) [27,28]. The higher ionic conductivity and lower viscosity facilitates the mobility of protons into and out of the pores. Impedance spectra for the VN and TiN are presented in Figures 2 e and f in the form of Nyquist plots. The high frequency, semi-circular feature was fit to an equivalent electrochemical

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circuit to determine the ESR [34,35]. Lower ESR values were obtained in aqueous electrolytes

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suggesting faster mobility of protons as expected.

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The electrochemical stability of the materials was further verified by subjecting the electrode to extended cycling. The materials were cycled at 50 mVs-1; the voltage windows were 1.7 and

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2.0 V for VN and TiN, respectively, in PIL electrolytes and 1.1 V for both VN and TiN in the

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aqueous electrolytes. Figures 3 a and b show the capacitances as a function of cycle number for VN and TiN in the aqueous and PIL electrolytes. High stability was observed in both

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electrolytes. The TiN experiences slight fade in the aqueous electrolyte; nevertheless, the

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material retains more than 80% of its initial capacitance after 100 cycles. b

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Table 1 lists the measured specific capacitances, voltage windows and ESRs for the VN and TiN materials in each of the electrolytes. The corresponding energy (E) and power (P), key

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figures of merit for supercapacitors, can be estimated using the following equations [1,2]:

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and

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where C and V are the capacitance and voltage window, respectively. Given the results, a

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significant increase in energy could be achieved by using the PIL electrolyte instead of the aqueous electrolyte with a moderate decrease in power. We note that while relatively low active

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material loadings were used, we expect similar performance at higher loadings due to the high

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porosity of the materials. Also measurements on electrodes do not necessarily translate to device level performance, especially when all the components are included [36]. Consequently,

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charge/discharge measurements under commercially relevant conditions are being performed for

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cells containing the VN and TiN materials.

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Table 1: Capacitances, stable voltage windows, and equivalent series resistances (ESR) for TiN and VN in aqueous and protic ionic liquid electrolytes. Material

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ACCEPTED MANUSCRIPT 4. Conclusions In summary, high surface area vanadium and titanium nitrides were prepared and electrochemically characterized in PIL and aqueous electrolytes. Cyclic voltammograms in the PIL and aqueous electrolytes were similar suggesting similar charge storage mechanisms. For

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both materials, the areal specific capacitance was significantly higher than that expected for

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double layer capacitance suggesting a pseudocapacitive charge storage mechanism. The

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capacitances ranged from 119 to 153 F g-1 for VN, and 76 to 81 F g-1 for TiN and the voltage windows were ~1.7 V and 2.0 V for VN and TiN, respectively, in the PIL electrolyte (1.1 V for

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both materials in aqueous electrolytes). Overall, a significant increase in energy could be

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achieved by using a PIL electrolyte instead of an aqueous electrolyte. Acknowledgements

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The authors acknowledge financial support from the Army Research Office (W911NF-11-1-

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0465), Automotive Research Center (W56HZV-14-2-0001), and the Hydrogen Energy

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Technology Laboratory.

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ACCEPTED MANUSCRIPT Highlights

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Significantly expanded operating voltage windows for VN and TiN Pseudocapacitive charge storage with protic ionic liquid electrolytes Significant increase in energy without sacrificing power

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Graphical abstract

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