A Fuel Exhaling Fuel Cell - The Journal of Physical Chemistry Letters

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A Fuel Exhaling Fuel Cell Zahid Manzoor Bhat, Ravikumar Thimmappa, Mruthyunjayachari Chattanahalli Devendrachari, Alagar Raja Kottaichamy, Shahid Pottachola Shafi, Swapnil Varhade, Manu Gautam, and Musthafa Ottakam Thotiyl J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b03100 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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The Journal of Physical Chemistry Letters

A Fuel Exhaling Fuel Cell Zahid Manzoor Bhat[a], Ravikumar Thimmappa[a], Mruthyunjayachari Chattanahalli Devendrachari, Alagar Raja Kottaichamy, Shahid Pottachola Shafi, Swapnil Varhade, Manu Gautam, 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: State-of the art proton exchange membrane fuel cells (PEMFC) anodically inhale H2 fuel and cathodically expel water molecules. We show an unprecedented fuel cell concept exhibiting cathodic fuel exhalation capability of anodically inhaled fuel, driven by the neutralization energy on decoupling the direct acid-base chemistry. The fuel exhaling fuel cell delivered a peak power density of 70 mW/cm2 at a peak current density of 160 mA/cm2 with a cathodic H2 output of 80 mL in 1 hour. We illustrate that the energy benefits from the same fuel stream can at least be doubled by directing it through proposed neutralization electrochemical cell prior to PEMFC in a tandem configuration.

TOC GRAPHICS

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KEYWORDS: Energy Storage and Conversion• Proton Exchange Membrane Fuel Cell• Sustainable Energy• Acid-Base Neutralization • Tandem Fuel Cells


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21st century is witnessing serious energy crisis and electrochemical energy storage and conversion devices assumed a larger space due to their near zero emission energy output.1-10 Proton exchange membrane fuel cells (PEMFC) have been a forerunner in the domain of sustainable energy chain as it can convert the chemical energy stored in H2 and O2 into electricity at higher efficiency.11-13 In the state of the art of PEMFC, O2 is the electron acceptor and the reaction generates water and heat as the byproducts.14-16 We show that by designing a system with solvent as the electron acceptor it is possible to design a H2 producing H2/hydronium ion fuel cell with concomitant energy and fuel outputs. To do so, differing from state of the art of PEMFC, the redox power of the electron donor should be negatively enhanced which is quite possible given the pH dependent redox energy of H2/H+ redox couple.17-20 By coupling a half cell with H2 as the electron donor in an alkaline pH and hydronium ion as the electron acceptor in an acidic pH, we demonstrate a H2 producing H2/hydronium ion fuel cell, scheme 1 with the net cell reaction being the acid base neutralization reaction. We further demonstrate that by connecting a PEMFC in series with the H2 producing H2/hydronium ion fuel cell, the voltage generation, power and energy output can be at least doubled using the same fuel streams.

Scheme 1. Scheme of fuel exhaling fuel cell by harnessing the energy of acid-base neutralization.


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The proposed fuel cell consists of H2 in an alkaline environment (pH=14) with a Pt/C anode and a Pt/C cathode immersed in an acidic pH (pH=0) separated by a Nafion 117 membrane. Use of an alkaline pH for housing the H2 fuel is justified by the H2 evolution curves as a function of pH and the corresponding Pourbaix diagram, Figure 1a and 1b. The shift of the hydrogen oxidation reaction (HOR) voltage as a function of pH with a near Nernstian voltage shift of 60 mV/pH is indicative of a pH dependent redox reaction and an alkaline pH render H2/H+ with a negative redox energy. It is observed that HOR peak intensity decreases noticeably in alkaline pH compared to acidic pH which is attributed to sluggish HOR kinetics in alkaline medium due to the higher metalhydrogen binding energy in the rate limiting Volmer step.21-26 This explains the decrease in the HOR peak intensity in alkaline pH compared to acidic pH. The choice of an acidic pH for hydrogen evolution reaction (HER) is justified in similar lines, Figure 1c and 1d with its pH dependent behaviour with a voltage shift of 60 mV/pH, suggesting the HER in acidic pH, relatively has a positive redox energy. This is further clear from the cyclic voltammograms of Pt/C electrode as a function of pH, (Figure S1, Supporting Information) wherein Hads and Hdes regions as well oxide formation and reduction regions shift as a function of pH. These demonstrate that when H2 is housed in an alkaline pH (pH=14) it can act as an electron donor and hydronium ion at acidic pH (pH=0) can act as an electron acceptor, and the resulting cell should demonstrate an electromotive force of 826 mV which is close to the voltage in the state of the art PEMFC. Since hydronium ion is the electron acceptor, the product of discharge reaction from such a cell will be H2 fuel. The insitu cyclic voltammogram of Pt/C anode and Pt/C cathode in the proposed H2 producing H2 fuel cell, (Figure S2, Supporting Information), demonstrate H2 oxidation near 0 V vs. open circuit voltage (OCV) in alkaline pH and H2 evolution near 840 mV vs. OCV in acidic pH. A lower area under the Hupd regions in alkaline media compared to acidic media is attributed to the proximity of H adsorption and oxygen adsorption in alkaline medium.27,28 ACS Paragon Plus Environment

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Figure 1. (a) HOR on platinum electrode in different pH solutions, (b) Pourbaix diagram for HOR, (c) HER on platinum electrode in different pH solutions and (d) Pourbaix diagram for HER. The proposed H2 producing H2 fuel cell yielded an open circuit voltage of 900 mV (Figure S3, Supporting Information), which is close to the value expected based on Figure 1. Slightly higher OCV than that expected based on Nernstian equilibrium of H2/H+ could be due to the slight contamination of oxygen in the acidic compartment. The polarization curve demonstrates a peak power density of 70 mW/cm2 at 160 mA/cm2 peak current, Figure 2a. The galvanostatic discharge curve at the peak rate (160 mA/cm2) demonstrate a steady state voltage of 375 mV resulting in the production of 80 mL of H2 gas from the cathodic compartment in 1 h time, Figure 2b. The experimental and theoretical values matched quite closely indicating the near absence of any parasitic chemistry. ACS Paragon Plus Environment

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Figure 2. (a) polarization curve for H2 producing H2 fuel cell, with a H2 flow rate of 100 mL/minute, (b) galvanostatic polarization at the peak current density (160 mA/cm2) with H2 quantification at the cathode, (c) galvanostatic polarization of fuel producing fuel cell at 160 mA/cm2 when it is connected to air breathing PEMFC (discharged at 30 mA/cm2) and (d) pH measurements at anodic and cathodic compartments of fuel exhaling fuel cell during galvanostatic polarization. When the resulting H2 produced at the cathode of H2 producing fuel cell is pumped to a PEMFC in an air breathing configuration, it delivered an OCV of 900 mV, and a galvanostatic discharge at 30 mA /cm2 demonstrated a steady state voltage close to 750 mV, Figure 2c. It should be noted that the PEMFC discharge lasted for nearly 18 hours by which the cell voltage of H2 producing fuel cell fell to zero due to neutralization reactions, Figure 2d and equation 3, confirming


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that H2 produced at the cathode of H2 producing fuel cell is the fuel for air breathing PEMFC. This further suggest that energy benefits from same fuel stream can at least be doubled by directing it through the neutralization cell in a tandem configuration with a PEMFC. The accompanying video demonstrate that the same fuel stream can be used to drive multiple devices simultaneously (Videos 1 and 2, Supporting Information). The pH of the anolyte decreased and the pH of the catholyte increased during the discharge chemistry, Figure 2d, confirming that hydronium ion is the electron acceptor and the reaction is driven by the free energy of neutralization reaction. Based on these the following reaction are suggested as anodic chemistry, cathodic chemistry and complete cell chemistry (equation 1-3) with salt formation (Na2SO4) at the cathode. For acid-base neutralization reaction, the entropy change is 80.60 J/K. mole and enthalpy change is -57.62 kJ/mole at 298 K. Using these thermodynamic parameters, the free energy change is calculated to be -81.47 kJ/mole which corresponds to an electromotive force of ~ 0.84 V closely matching with that in eq.3. Anodic half-cell reaction: H2O + 2e− ↔ H2 + 2OH− ………. ESHE = -0.826 V (pH=14)

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Cathodic half-cell reaction: 2H+ +2e- ↔ H2

…………………… ESHE = 0 V (pH=0)

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Net cell reaction: 2H+ + 2OH− ↔ H2O………………….Ecell = 0.826V

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The pH decrease observed in Figure 2d is not due to diffusion driven chemical reactions of H+ and OH- when they are separated by an ion conductor. This is confirmed by monitoring the pH


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of anodic and cathodic compartments at the open circuit as a function of time (Figure S4, Supporting Information). No obvious decrease in pH and voltage was observed even after 35 hours suggesting the pH change observed in Figure 2d is the result of electrochemical reactions as shown in equations 1-3. Further individual half-cell polarization studies demonstrate that anodic part limit the overall performance, Figure 3a, which is due to sluggish electrode kinetics of HOR in alkaline medium as explained earlier.21-26

Figure 3. (a) Individual electrode polarization and (b) H2 quantification from the cathode when anode is supplied with fixed amounts of hydrogen (40 ml) and operated at 160 mA/cm2. Discharge profiles (c) at different rates with a fixed volume of H2 fuel (40 mL) and (d) at a constant rate (160 mA/cm2) with varying volumes of H2 fuel. When a fixed amount of H2 is supplied to the anode (40 mL), almost 20 mL was collected at the cathode by the time the cell voltage fell to zero during galvanostatic discharge at 160 mA/cm2


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(Figure 3b). This indicates that the stoichiometry of H2 gas supplied and reacted is close to 0.5 which is typical of any PEMFC,29,30 and the conversion efficiency is close to 50%. The lower discharge time in Figure 3b compared to Figure 2c could be due to limited supply of H2 and associated concentration polarization at a current of 160 mA/cm2. This could be due to lower solubility of H2 in aquatic environment, trapping of H2 in the head space, supplier tube space etc. In order to prove this concentration polarization effect, discharge experiments were carried out with respect to varying discharge rates and H2 fuel volumes independently (Figures 3c and 3d). The discharge at different rates with the same volume of hydrogen (40 mL) demonstrates an inverse relationship between the rate and the discharge time, Figure 3c. The discharge profiles at the same rate with different volumes of hydrogen demonstrate a monotonic relation between discharge time and the volume of hydrogen, Figure 3d. These two scenarios confirm that the lower discharge time in Figure 3b is due to concentration polarization effect. The interfacial stability of H2 producing battery is further investigated by galvanostatic pulse discharge, (Figure S5, Supporting Information). On increasing the rate, the cell voltage dropped, however on decreasing the rate the cell voltage regained to approximately the same level for each rate suggesting decent interfacial stability. In summary, we have demonstrated a H2 exhaling fuel cell by harnessing the free energy of acid base neutralization reaction as electromotive force. Proposed architecture constitutes H2 as an electron donor in an alkaline pH and hydronium ion playing the crucial role of cathodic electron acceptor so as to exhale the fuel inhaled at the anode. For the first time, we illustrate that the energy benefits from the same fuel stream can at least be doubled by linking the neutralization electrochemical cell in a tandem configuration with a PEMFC. Experimental section


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Sodium hydroxide (97%), sulphuric acid (98%), potassium hydrogen phosphate (98%), potassium phosphate (99%), acetic acid (99.7%), sodium acetate (99%), potassium sulphate (99%) were bought from Sigma Aldrich India and were used as such. Pt@C was procured from Johnson Matthey India. All the electrochemical measurements were done with the VMP-300 Electrochemical Work Station (Biologic, France). Electrochemical experiments were carried in standard three electrode set up with platinum (2 mm diameter) as working electrode, Ag/AgCl (3.5 M KCl) as reference and platinum mesh as the counter electrode. Hydrogen evolution and oxidation on platinum electrode was carried out in different pH solutions. pH solutions were made by using H2SO4 (pH 0), HSO4-/SO42- (pH 1–2), CH3COOH/ACH3COONa ( pH 3–5), H2PO4-/HPO42-(pH 5– 8), HPO42- /PO43- (pH 9–12), and NaOH (1 M, pH 14). The pH of each solution was adjusted with 1 M H2SO4 or 1 M NaOH solutions. Prior to the measurement the electrode was polished with 0.05 mm alumina powder and cycled in the respective solution. The solutions were purged with nitrogen followed by hydrogen for 15 minutes before carrying out the hydrogen evolution or oxidation reactions. For the fuel cell measurements, a closed two compartment cell was made with each compartment having an opening for the gas passage. The two compartments were separated by a pretreated Nafion@ 117 membrane. The alkaline compartment was fitted with the Pt@C electrode as the anodic electrocatalyst. The electrode was fabricated by coating Pt@C (60 wt% Pt) on Toray carbon paper with a loading of 0.2 mg/cm2. The Cathodic electrode was made by electrochemical deposition of platinum on Pt mesh by chronoamperometry (4mA/cm2 for 2 minutes). Anodic compartment was filled with NaOH (pH=14) and cathodic with H2SO4 (pH=0). Each measurement was carried out by passing the hydrogen gas into anodic compartment at 100 ml/min rate and


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collecting the gas from the cathodic compartment. For quantification purpose the gas was collected in a calibrated cylinder by water displacement. During the long-time polarisation gas evolved from the cathodic compartment was passed directly to the commercial air breathing H2/air fuel cell. ASSOCIATED CONTENT Supporting Information. Figures S1-S5, and videos 1 and 2. AUTHOR INFORMATION [a] Contributed equally to the work. Corresponding Author *Email: [email protected]. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT MOT acknowledges DST-SERB, MHRD fast track, and DST Nanomission for financial assistance. ZMB acknowledges CSIR-JRF fellowship from MHRD.

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A Fuel Exhaling Fuel Cell - The Journal of Physical Chemistry Letters

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