A Rechargeable Hydrogen Battery - The Journal of Physical Chemistry

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Energy Conversion and Storage; Plasmonics and Optoelectronics

A Rechargeable Hydrogen Battery Neethu Christudas Dargily, Ravikumar Thimmappa, Zahid Manzoor Bhat, Mruthyunjayachari Chattanahalli Devendrachari, Alagar Raja Kottaichamy, Manu Gautam, Shahid Pottachola Shafi, and Musthafa Ottakam Thotiyl J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00858 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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The Journal of Physical Chemistry Letters

A Rechargeable Hydrogen Battery Neethu Christudas Dargily†, Ravikumar Thimmappa†, Zahid Manzoor Bhat, Mruthunjayachari Chattanahalli Devendrachari, Alagar Raja Kottaichamy, Manu Gautam, Shahid Pottachola Shafi and Musthafa Ottakam Thotiyl* Department of Chemistry and Centre for Energy Science Indian Institute of Science Education and Research (IISER)-Pune, Dr. Homi Bhabha Road, Pashan, Pune 411008, India

Corresponding Author E-mail: [email protected]

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ABSTRACT: We utilize the proton coupled electron transfer in hydrogen storage molecules to unlock a rechargeable battery chemistry based on the cleanest chemical energy carrier molecule hydrogen. Electrochemical, spectroscopic and spectroelectrochemical analysis evidence the participation of protons during the charge-discharge chemistry and extended cycling. In an era of anthropogenic global climate change and paramount pollution, a battery concept based on virtually non-polluting energy carrier molecule demonstrates a distinct progress in the sustainable energy landscape.

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Even though the electricity production cost from renewable energy resources have significantly come down, its large scale storage is limited by efficient energy storage modules.1-17 Energy storage is largely dominated by metal ion batteries and is well known that the state of the art energy storage technologies have issues related to safety, cost and environmental compatibility.1,9,11,14-21 New inventions in flow batteries, organic electrodes, supercapacitors, and air batteries are right steps in making the energy output much cleaner.2-5,7-10,14,15,21-25 Here we show an electrically rechargeable battery based on hydrogen, one of the most abundant element in the universe and cleanest chemical energy carrier molecule26-28 and electrochemical hydrogenation/dehydrogenation of hydrogen storage molecules (Scheme 1). Various physico chemical techniques reveal the involvement of protons at both interfaces during the charge-discharge chemistry and extended cyclability.

Scheme 1. Schematic representation of rechargeable hydrogen battery based on hydrogen storing quinone/hydroquinone redox system. Proposed battery consists of Pt supported on carbon (Pt@C) at a loading of 0.5 mg/cm2 as the anodic electrocatalyst for the ionization of H2 molecules and bond formation between H+ ions during 3 ACS Paragon Plus Environment

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discharge and charge chemistry respectively. Positive electrode is based on the well-known hydrogen storage molecule benzoquinone (BQ), and the two electrodes are separated by an H+-ion conducting Nafion 212 membrane (Scheme 1 and Scheme S1, Supporting Information). We chose Pt electrode as negative electrode due to its well-known electrocatalytic ability in hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR)29,30, (Figure S1a, Supporting Information), and BQ as positive electrode because of its well-known proton coupled electron transfer with electrochemically assisted formation and decomposition of hydroquinone (QH2), cyclic voltammogram, investigation

(Figure S1b, Supporting Information). For fundamental electrochemistry

of quinone redox

reaction

by cyclic voltammetry,

Pourbaix

diagram

and

chronoamperometry, a solution of BQ (1 - 10 mM) in 0.5 M H2SO4 of pH 0.2 is employed. The plot of equilibrium potential vs. pH (Pourbaix diagram) for the redox transition of BQ (Figure S1c, Supporting Information) demonstrates a slope of 55 mV/pH indicating the electron transfer is accompanied by proton transfer.31-33 The square root dependence of the peak current on the scan rate (Figure S1d, Supporting Information) further indicate a diffusion controlled process which is further clear from the near 1/2 slope of log (i) vs. log (scan rate) plot, (Figure S1e, Supporting Information). The diffusion coefficients are estimated from chronoamperometry and the corresponding Cottrell plots (Figures S2a and S2b, Supporting Information). For BQ and QH2 molecules the diffusion coefficients are 5.5810-6 cm2/s and 6.8910-6 cm2/s respectively, in line with the values reported in the literature.34-36 To understand the electron transfer kinetics of quinone redox reaction, rotating disk electrode (RDE) and rotating ring disk electrode studies (RRDE) are carried out with 1 mM of QH2 (or 1 mM BQ) in 0.5 M H2SO4 of pH0.2, Figure 1. The evident presence of ring currents when the Pt ring is biased at the BQ reduction potential and the Pt disk is swept towards QH2 oxidation potential, Figure 1a, suggest that its hydrogenation/dehydrogenation can be electrochemically reversed within the 4 ACS Paragon Plus Environment

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available potential window. The Koutecky-Levich plots (K-L plots) extracted from corresponding linear sweep voltammograms (LSVs), Figures 1b and 1c, on a Pt disk electrode, indicate the number of electrons transferred are close to 2 and the electron transfer rate constant is in the range of 10-3 cm/s, demonstrating a reasonably fast redox reaction, Figure 1d, (Figure S3 and Table S1, supporting information). In essence the redox reactions of quinone are proton coupled and they undergo reasonably fast electron transfer and either redox states exhibit decent stability under chosen experimental conditions.33,37,38

Figure 1. (a) Rotating ring disk electrode (RRDE) studies for quinone redox reaction (1 mM of QH2 dissolved in 0.5 M H2SO4 of pH 0.2) on a Pt disk and a Pt ring electrode at a scan rate of 10 mV/s at different rotations per minute. Oxidation is carried out potentiodynamically on the disk and 5 ACS Paragon Plus Environment

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potentiostatically on the ring by applying a reduction potential of 200 mV. Rotating disk electrode (RDE) studies for (b) QH2 oxidation (1 mM of QH2 in 0.5 M H2SO4 of pH 0.2), (c) BQ reduction (1 mM of BQ in 0.5 M H2SO4 of pH 0.2) and (d) Koutecky-Levich plots extracted from Figure 1b for QH2 oxidation. To further understand the stability and the nature of the species generated at the electrode/electrolyte interface during the redox reactions of quinone, we have carried out in-situ UV-Vis spectroelectrochemical studies (Figure S4a-c, Supporting Information) with a solution of 1 mM QH2 in 0.5 M H2SO4 of pH 0.2. During the oxidation scan, the differential spectra suggests that features corresponding to QH2 disappeared (negative going bands) with concomitant appearance of BQ features (positive bands). On reversing the scan, QH2 bands are restored with gradual disappearance of BQ features. These indicate reversibility of the reaction and the stability of corresponding species at the electrode/electrolyte interfaces.39,40 Proposed hydrogen battery is constructed as shown in (Scheme 1 and scheme S1, Supporting Information) with a Nafion 212 separator between anodic and cathodic compartments. When H2 is filled into negative electrode compartment constituting a Pt catalyst, the open circuit voltage with respect to quinone composite electrode is found to be 0.73 V which correspond to the difference between single electrode potentials measured with a common a reference electrode, (Figure S5, Supporting Information). In this configuration since Pt is well known electro-catalyst for H2 oxidation29,30 (Figure S1a, Supporting Information), generated electrons will move through the external circuit to the cathode, and the H+ ions will migrate through the proton exchange membrane to the hydrogen storage material, prompting its conversion to hydroquinone (QH2), Scheme 1 and Scheme S1, Supporting Information. H2-quinone battery can be made electrically rechargeable as QH2 can be electrochemically oxidized, Figure 1, (Figure S1 and Figure S4, Supporting Information). Further, H2 evolution can be catalyzed by Pt electrode on the anode side during the charge reaction (Figure S1a, Supporting Information). 6 ACS Paragon Plus Environment

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Supporting this, video 1 (Supporting Information) shows that during charge chemistry H2 is exhaled at the anodic compartment of the hydrogen battery.

Figure 2. (a) Cyclic voltammogram of hydrogen battery at 10 mV/s scan rate with the cathode as the working electrode when the anode is continuously fed with H2 (black trace) and N2 (red trace) at 100 mL/minute. b) Polarization curves, c) galvanostatic charge discharge curves at different rates and d) extended cyclability at 10 mA/cm2 for the hydrogen battery. For the experiments b)-d), a stagnant H2 anode with hydrogen amount 2 times in excess of the cathode is employed. 0.1 M BQ solution in 0.5 M H2SO4 of pH 0.2 is supplied to the cathode in continuous flow mode (at 100 mL/minute flow rate) by recycling the same electrolyte. Pt@C electrode (0.5 mg/cm2) and carbon coated Toray carbon paper (3 mg/cm2) served as anodic electrocatalyst and cathodic current collector respectively. 7 ACS Paragon Plus Environment

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The cyclic voltammogram of the H2-quinone battery with the cathode (0.1 M BQ in 0.5 M H2SO4 of pH 0.2 at 100 mL/minute) as the working electrode demonstrates very clearly electrochemical hydrogenation/dehydrogenation of the quinone electrode when anodic compartment is continuously supplied with hydrogen (at a flow rate of 100 mL/minute), Figure 2a, black trace. When anodic compartment is supplied with N2, the corresponding cyclic voltammogram did not furnish any faradaic behavior, Figure 2a red trace, signaling a counter reaction which is inevitably proton assisted. The polarization curve for rechargeable hydrogen battery with a stagnant H2 anode where the amount of H2 is 2 times in excess of the cathode and a flow through BQ cathode (0.1 M BQ solution in 0.5 M H2SO4 of pH 0.2 at a flow rate of 100 mL per minute) is shown in Figure 2b, demonstrating a power density of 40 mW/cm2 at a peak current of 125 mA/cm2. At a rate of 5 mA/cm2, the battery delivered a discharge capacity of 350 mAh/g with respect to the weight of BQ, Figure 2c, which is 70% of its theoretical discharge capacity (495.8 mAh/g with respect to BQ). This corresponds to an energy density of ~234 Wh/kg which is 67 % of its theoretical specific energy (347.06 Wh/kg). The lower experimental capacity and specific energy compared to theoretical values could be due to higher electronic resistance of BQ/QH2 molecules.4,41 Galvanostatic cycling at 10 mA/cm2 demonstrated extended cyclability and at the end of 200 cycles it retained 92 % of its discharge capacity with respect to the first discharge, Figure 2d. Further, the battery possessed decent coulombic and energy efficiency for over 200 cycles, (Figure S6, Supporting Information). The discharge and charge chemistries are probed by analyzing the cathodes after charge discharge chemistry by various physicochemical techniques. Galvanostatic intermittent titration technique (GITT) is collected for H2-quinone battery to decipher thermodynamic and kinetic parameters. GITT is obtained by applying a discharge current of 250 A to a stagnant BQ cathode (7 mg/cm2) for 5 minutes and then relaxing the system at zero current to the open circuit voltage (OCV) for an hour. A stagnant H2 electrode with hydrogen content 2 times in excess of the cathode is employed as the anode. The processes of discharging and relaxing 8 ACS Paragon Plus Environment

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are repeated for several hours to understand the phase change and chemical diffusion coefficients. As shown by the blue line in (Figure S7, Supporting Information) the quasi OCV demonstrated a decline from 600 mV to 480 mV at 25 hours suggesting a clear phase change at the BQ electrode. Chemical diffusion coefficient (see experimental section for more details) of H +-ion in the BQ composite electrode is found to be 0.2510-7 cm2/s. UV-Vis spectroscopy demonstrates the attenuation between BQ and QH2 during discharge chemistry and its reversal during the charge chemistry, Figure 3a and 3b. The sustainability of the process can be seen over 200 cycles during discharge and charge chemistry. The appearance of a broad

OH vibration at 3160 cm-1 and the disappearance of CO at 1630 cm-1 in the Fourier transform infrared (FTIR) spectra of the BQ cathode (Figure 3c), signal the formation of QH2 during the discharge chemistry. 42-44 In line with this, Raman spectra (Figure S8a, Supporting Information) demonstrate the appearance of QH2 bands43-44 which further reinforce the hydrogenation of BQ during the discharge chemistry of H2-quinone battery. However during the charge chemistry, the presence of sharp OHO vibration at 3200 cm-1 together with CO at 1630 cm-1 in the FTIR spectra (Figure 3d) evidence for the formation of quinhydrone, the charge transfer complex between BQ and QH2. Raman spectra of charged cathode (Figure S8b, Supporting Information) further indicate the formation of quinhydrone. The formation of charge transfer complexes will be a matter of future investigations and we believe it could be due to exposure of the cathode to air during characterizations since it was not detected during in-situ spectroelectrochemistry studies conducted under a N2 blanket (Figure S4, Supporting Information). Nevertheless, these demonstrate a rechargeable H2-quinone battery where cathodic reaction is electrochemical hydrogenation/dehydrogenation with corresponding hydrogen oxidation/ hydrogen evolution at the anode.

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Figure 3. UV-Vis spectra of H2-quinone battery cathode a) during the discharge and b) during the charge cycles. FTIR spectra of H2-quinone battery cathode c) during the discharge and d) during the charge cycles. Based on these, equations (1) and (2) are proposed for discharge and charge chemistries and it involves the oxidation of H2 at the anode and the migration of H+ through the proton exchange membrane to hydrogen storage material (BQ), prompting its hydrogenation to hydroquinone (QH2). On electrochemical charging, the processes are reversed, equation (2) with dehydrogenation of QH2 at the cathode with simultaneous H2 evolution on the anode. Negative electrode

H2



2H+ + 2e- ………. (1) 10

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Positive electrode Net cell reaction

BQ + 2H+ + 2eH2 + BQ



QH2 ……………. (2)



QH2 ……………. (3)

To understand quinone cross-over from cathode to anode and associated poisoning of anodic Pt electrocatalyst, we have carried out time dependent cyclic voltammetry in a diffusion cell. Stagnant solutions of 0.5 M H2SO4 and 0.1 M BQ in 0.5 M H2SO4 are taken in anodic and cathodic compartments respectively and the cross-over of BQ from cathode to anode is investigated by time dependent cyclic voltammogram with a Pt electrode as the working electrode, Pt foil as the counter electrode and Ag/AgCl (3 M KCl) as the reference electrode. The voltammograms recorded at the Pt working electrode, Figure S9 (Supporting Information), indicate growing signals due to quinone redox behavior with increase in time (inset of Figure S9, Supporting Information). This suggests the cross-over of BQ through the Nafion membrane. The poisoning of Pt electrode by quinone molecules is investigated by monitoring the intensity of hydrogen evolution (HER) and hydrogen oxidation (HOR) currents on the same Pt electrode as a function of time during the quinone cross-over. HER/HOR currents on Pt (at potentials close to 0 V vs. RHE) remained unaffected over a period of 7 hours even though the quinone redox behavior increased with respect to time, Figure S9, Supporting Information. This suggests that the poisoning of Pt electrode is negligible within the time limit of the experiment. We have demonstrated that the proton coupled electron transfer in BQ/QH2 redox couple and the catalytic nature of Pt electrode for H2 redox reaction can be coupled to construct an electrically rechargeable hydrogen battery. UV-Vis, RRDE, GITT, FTIR and Raman spectroscopy techniques evidence the involvement of protons during the charge discharge chemistry and extended cycling. In an era of paramount pollution and global climate change, a battery concept based on virtually nonpolluting chemical energy carrier molecule is a distinct step towards building green energy storage modules. ASSOCIATED CONTENT 11 ACS Paragon Plus Environment

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Supporting Information: Experimental section, Scheme S1, Figures S1-S8, Table S1, References AUTHOR INFORMATION [†] Contributed equally to this work. Corresponding Author E-mail: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT MOT acknowledges DST-SERB, DST-Nanomission and MHRD for financial assistance. NCD acknowldeges Inspire-DST for PhD fellowship. REFERENCES (1)

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