2.6 V Aqueous Battery with a Freely Diffusing Electron Acceptor - The

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A 2.6 V Aqueous Battery with a Freely Diffusing Electron Acceptor Ravikumar Thimmappa, Mruthyunjayachari Chattanahalli Devendrachari, Alagar Raja Kottaichamy, Shambhulinga Aralekallu, Manu Gautam, Shahid Pottachola Shafi, Zahid Manzoor Bhat, and Musthafa Ottakam Thotiyl J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11180 • Publication Date (Web): 02 Feb 2017 Downloaded from http://pubs.acs.org on February 3, 2017

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The Journal of Physical Chemistry

A 2.6 V Aqueous Battery with a Freely Diffusing Electron Acceptor Ravikumar Thimmappa, Mruthyunjayachari Chattanahalli Devendrachari, Alagar Raja Kottaichamy, Shambhulinga Aralekallu, Manu Gautam, Shahid Pottachola Shafi, Zahid Manzoor Bhat, Musthafa Ottakam Thotiyl* Department of Chemistry and Centre for Energy Science, Indian Institute of Science Education Research Pune, Dr. Homibaba Road, Pune, 411008 India.

Abstract: Here we show a strategy to expand the working voltage of aqueous metal batteries beyond the thermodynamic limit of 1.23 V by modifying the interfacial chemistry existing at the cathode/electrolyte interface. Highly non-wettable carbon nanoparticle cathode/electrolyte interface with a freely diffusing electron acceptor kinetically muted water decomposition due to reduced contact between water and the electrode, expanding the working voltage far beyond 1.23 V. Zn battery equipped with hydrophobic carbon nanoparticle cathode delivered an open circuit voltage (OCV) of 2.6 V with capacity (930 mAh/g), energy (2420 Wh/kg @ 50mA/cm2) and power densities (83 W/kg) remarkably higher than conventional Pt-based aqueous Zn-air batteries (OCV=1.5 V, ~650 mAh/g, 1161 Wh/kg and 43W/kg). When probed by in-situ and ex-situ FTIR spectroelectrochemistry and galvanostatic intermittent titration technique wettable carbon particles (contact angle = 20) are found to catalyze parasitic oxygen evolution reactions (OER), while their non-wettable counterpart (contact angle = 117) dominantly catalyzed electron acceptor’s redox reaction by inhibiting any such parasitic chemistry. Zn batteries equipped with carbon cathode contribute to a Pt-free battery having a closed cathode addressing the

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complexity of carbonate clogging and electrolyte evaporation often encountered in open air batteries and could be used to power electrical appliances. 1. Introduction In the context of global warming and its associated impacts on the flora and the fauna thriving on this planet, metal-air and metal ion batteries are considered as suitable power sources for developing zero emission electric vehicles.1-5 Metal air batteries especially Zn-air batteries possess longer driving range per charge because of their projected energy density.1-4 The architectural components of a zinc-air battery generally consist of a Zn metal (or powder) as the anode, concentrated alkali (6 M KOH) as the electrolyte and oxygen as the oxidant on a suitable precious metal electrocatalyst such as Pt.6,7 Oxygen in principle does not have to be carried on board the cell as it can be accessed directly from the air, however an open cathode structure is inevitable to negate fuel starvation in such air batteries.8,9 From engineering perspective the porous composite cathode architecture of air batteries has several bottlenecks. Firstly an open cathode architecture poses the alkaline electrolyte with serious issues of CO2 contamination, and the generated carbonate is known to clog the pores of the separator and the gas diffusion layer, significantly decreasing the ionic conductivity and oxygen accessibility in the long run.10,11 The electrolyte evaporation in such open cathode architectures may demand frequent refilling of battery tanks. There are possibilities of using gas selective membranes; however similarity in molecular sizes of CO2 and O2 is a real challenge to develop size selective membranes with sufficient selectivity and permeability.12,13 The air electrode in zinc-air battery require an expensive Pt electrocatalyst at a significantly higher loading (1 mg/cm2) to catalyze the oxygen reduction reaction (ORR) at a current density required for practical applications and there appeared a concourses of articles contending with various catalysts and their combinations to reduce catalyst 2


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cost to remarkably low levels.14,15 It should be noted that, despite some success in the construction of electrically rechargeable Zn-air batteries by integrating oxygen evolution catalysts 16-20, they are primarily mechanically rechargeable, due to the complexity of 4 electron oxygen evolution reaction (OER). First rechargeable zinc-air battery was constructed by Slovenian innovator Miro Zoric and seminal works of Li. et al and Zhang et al are noteworthy in this direction.21-25 Here we demonstrate a Pt-free aqueous Zn battery (Scheme 1) with a closed cathode architecture containing only carbon nanoparticles as the cathode electrocatalyst, nonetheless with capacity, energy and power density remarkably higher than conventional Pt based aqueous Zn-air batteries. The air oxidant in conventional zinc-air batteries are replaced by inexpensive and environmentally benign electron acceptor Na2S2O8 with highly positive standard reduction potential (2.08 V vs. SHE), negating the requirement of an expensive metal based electrocatalyst to catalyse the cathode half-cell reaction. Since the standard reduction potential of persulphate redox reaction is highly positive, in order to circumvent catalytic water oxidation we have altered the surface chemistry of carbon nanoparticles to near super hydrophobicity so as to reduce water contact area on the electrode. We will further show that remarkable performance of proposed Zn battery is possible only with a hydrophobic carbon cathode/electrolyte interface and not with the corresponding hydrophilic carbon/electrolyte interface revealing the interplay between battery relevant characteristics and interfacial chemistry. 2. Experimental section 2.1 Chemicals and reagents: Chemicals such as sodium sulfate, sodium persulfate, sodium hydroxide, nitric acid, sulfuric acid, Nafion solution (5 %), zinc metal foil and Ketjen black were procured from Alfa Aesar India. Pt@C (20 wt %) catalyst was procured from Jhonson-Mathey and Nafion 117 membrane was procured from Sigma Aldrich India. 3



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2.2 Modification of carbon nanoparticles: Ketjen black carbon was modified to hydrophobic and hydrophilic as follows.26-28 About 1.0 g of Ketjen black was taken in a quartz boat and heat treated in a furnace at 900 ºC under Ar: H2 (95:5 v/v) for about 5 h in order to remove hydrophilic functionalities. After completion of heat treatment, obtained Ketjen black was stored in a desicator without exposing to the atmosphere. Hydrophilic Ketjen black was prepared by stirring a solution of 100 ml of 6 M HNO3 containing 1.0 g of Ketjen black in a round bottom flask for an hour. The mixture was then sonicated for an hour followed by refluxing for about 24 h. Obtained mass was collected by filtration and washed thoroughly with water followed by ethanol and dried in an oven. 2.3 Physicochemical characterization. FTIR was collected to characterize the carbon nanoparticles using ATR-FTIR (Bruker Alpha). Gas chromatography was carried out using Thermoscientific TRACE 1110 gas chromatograph. Water contact angles were measured by contact angle analyzer (HOLMARC opto-mechanics). BET surface area measurements were carried out using BelSorpmax (Bel Japan). Electrochemical measurements were conducted using VMP 300 Biologic electrochemical work station. In-situ FTIR spectra were acquired in thin layer configuration in respective half cells using ATR-FTIR spectrometer (Bruker Alpha), ATR accessory (with a diamond Crystal) and an electrochemical cell. In situ FTIR spectra under applied bias was acquired using a three electrode cell consisting of glassy carbon disk (coated with Zn or carbon nanoparticles) as the working electrode, Pt wire as the counter electrode and Hg/HgO (1M NaOH) as the reference electrode. The working electrode was contacted to the ATR crystal with the help of leveling screws to achieve contact with the crystal. The spectra at desired potentials were recorded by holding at that potential. Each spectrum was the average of 32 scans. The spectral resolution was 4 cm−1 and a potential sweep rate of 1 mV/s was used for all the studies. The spectra at the open circuit voltage was taken as the background in all the electrolytes and with all the

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electrodes. All the potentials are converted to reversible hydrogen electrode (RHE) scale by calibrating the reference electrode with respect to RHE. 2.4 Fabrication of zinc battery. A two compartment cell was fabricated with 3 mm thick polyacrylic sheet. The anode and cathode compartments were separated by Na ion conducting Nafion 117 polymer electrolyte. The carbon ink was prepared by mixing the required amount with 5% PTFE solution as binder and 2-propanol as solvent and the resulting mixture was ultrasonicated for 30 min and brush coated on the Toray carbon paper with a loading of 3 mg/cm2. Anodic and cathodic compartments were filled with 6 M NaOH and freely diffusing sodium persulfate (saturated solution) solution respectively. The battery was assembled with zinc foil as anode and hydrophobic (or hydrophilic) carbon nanoparticles (3 mg/cm2) coated Toray carbon paper as cathode. 2.5 Electrochemical characterization. The sodium persulfate (SPS) reduction reaction on modified carbon nanoparticles was performed using linear sweep voltammetry (LSV) in a three electrode configuration. The carbon ink was prepared by mixing the required amount with 5% PTFE solution as binder and 2-propanol as solvent and the resulting mixture was ultrasonicated for 30 min. The ink was drop casted on to the glassy carbon electrode at a loading of 25 μg/cm2. The electrochemical cell consisted of glassy carbon as the working electrode, Pt foil as the counter electrode and Ag/AgCl (3 M KCl) as the reference electrode in N2 saturated 0.5 M Na2SO4. The zinc battery was tested with VMP 300 Biologic electrochemical workstation. Polarization data were collected using linear sweep voltammetry at a scan rate of 5 mV/s. The battery was discharged with different loads galvanostatically to measure the capacity values. Galvanostatic intermittent titration technique (GITT) was carried out at a discharge current density of 25 mA/cm2 for 1 hour. The cell was allowed to relax to its open circuit at 0 mA/cm2 (relaxation time) for 1 5



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hour and the process was repeated for 60 hours. Zn-air (oxygen was fed to the cathode continuously) battery was assembled using 20% Pt@C (0.5mg/cm2) as air cathode and zinc foil as anode in 6 M NaOH for comparative purpose. Electrochemically active area of carbon nanoparticles were estimated from the peak currents (Figure S1, Supporting Information) of a fast moving redox couple using Randles-Sevcik equation. 3. Results and Discussion

Scheme 1. Schematic representation of Zn-HPho-C battery The battery consists of Zn metal as the anode immersed in 6 M NaOH electrolyte and hydrophobic carbon nanoparticles (HPho-C) as the cathode electrocatalyst immersed in aqueous 3 M Na2S2O8 (SPS) oxidant. The anolyte and catholyte were separated by a sodium ion conducting Nafion 117 membrane, scheme 1. Since the oxygen half-cell reaction in conventional Zn-air battery is replaced by persulphate reduction in proposed Zn battery, it is imperative to investigate its chemistry on a carbon nanoparticle electrode. Since the standard reduction potential of 6


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persulphate is very high (E0 = 2.08 V)

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electrochemical reduction of persulphate can be

achieved even without a precious metal electrocatalyst. Further, the very positive standard reduction potential of SPS may pose the issue of direct water oxidation catalyzed by the carbon electrode. To circumvent the catalytic water oxidation by SPS on a carbon electrode we have engineered the carbon surface to make it hydrophobic (see experimental section for more details). The non-catalytic nature of HPho-C as opposed to catalytic nature of hydrophilic carbon (HPhiC) towards water oxidation in the presence of SPS is revealed by a range of electrochemical and spectroscopic techniques as discussed below. The cyclic voltammogram (Figure 1a) of persulphate reduction in neutral media demonstrate no redox wave in the negative going scan on a bare glassy carbon electrode (GCE). With hydrophilic carbon particles on GCE (HPhi-C@GCE) a clear irreversible wave was present, Figure 1a, which was negatively shifted by 200 mV compared to its hydrophobic (HPho-C@GCE) counterpart. Further, the peak currents at 0.6 V vs RHE was amplified on HPho-C@GCE compared to HPhi-C@GCE. All these demonstrate electrocatalytic effect on HPho-C/electrolyte interface compared to its hydrophilic counterpart. The reduction wave on HPho-C@GCE demonstrated a linear increase with respect to the concentration of persulphate (Figure 1b), indicating the reduction wave stemming from electrochemical reduction of persulphate. Exchange current densities normalized to true surface area (Sac, see experimental section for more details) demonstrated 53 times enhancement on HPho-C (2.05x10-5 A cm-2) compared to HPhi-C (3.86x10-7 A cm-2), Figure 1c, indicating intrinsic electrocatalytic activity associated with HPho-C particles. Further, the Tafel slope (Figure 1c) observed on HPho-C was 172 mV/dec as opposed to 181 mV/dec on HPhi-C. The significant improvement of parameters displaying the true rate of the reaction on HPho-C clearly demonstrates a beneficial interfacial effect prevailing when the carbon electrode is made hydrophobic. Stability and longtime activity

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were probed by chronoamperometric studies and it demonstrated after the double layer decay higher activity for longer duration on HPho-C compared to HPhi-C, Figure 1d. This is further supported by the electrochemical impedance spectroscopy data, Figure 1e and Table S1 (Supporting Information), wherein a significant decrease in charge transfer resistance is observed on HPho-C compared to corresponding hydrophilic counterpart. The plot of concentration vs. peak current (Figure S2a, Supporting Information) and the plot of peak current vs. square root of scan rate (Figure S2b, Supporting Information) on HPho-C followed a linear relationship suggesting a diffusion controlled electrochemical process which is further clear from a near 0.5 slope of log (i) vs. log () plot (Figure S2c, Supporting Information). This is further supported by the gas chromatography (GC) data on HPhi-C electrode, Figure S3 (Supporting Information) demonstrating the clear presence of oxygen, suggesting that O2 evolution (OER) in the presence of SPS is catalyzed by HPhi-C electrodes. The inertness of HPho-C electrode towards oxygen evolution could be due to the interfacial chemistry of respective electrodes wherein the water contact angle on HPho-C is found to be 117 compared to 20 on HPhi-C, Figure 2a and 2b. The higher contact angles on HPho-C could be due to the reduction of hydrophilic functionalities compared to HPhi-C, FTIR spectra, Figure S4 (Supporting Information). The lower contact angles on HPhi-C electrode make the water to wet the carbon electrodes and this may induce a catalytic effect towards OER (Figure S3, Supporting Information). The higher contact angles on HPho-C restrict the accessibility of water on the electrodes, minimizing parasitic chemistry. Taken together, HPho-C is found to be a better electrode for SPS reduction without the complexity of parasitic OER chemistry based on CV, impedance spectroscopy, gas chromatography and contact angle measurements, and therefore this system is investigated further for spectroelectrochemistry and battery studies. Ex-situ FTIR spectroelectrochemistry on HPho-C, Figure 1f, reveals the formation

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of sulphate species during electrochemical reduction of persulphate. Based on these studies, the following chemistry (equation 1) is proposed for persulphate reduction on HPho-C electrode. In essence, wetting of the hydrophilic carbon cathode surface by electrolytes (Figure 2) makes the competing parasitic reaction of O2 evolution more dominant compared to its hydrophobic counterpart making the latter more active towards the redox chemistry of peroxydisulphate.

Figure 1. a) Linear sweep voltammetry (LSV) of Na2S2O8 reaction on GCE, HPhi-C@GCE and HPho-C@GCE electrodes at 10 mV/s, b) LSV curves of Na2S2O8 reduction on HPho-C@GCE at different concentrations, c) Tafel plots for Na2S2O8 reaction on HPhi-KB@GCE and HPhoC@GCE electrodes, d) i-t curves for Na2S2O8 reaction on HPhi-C@GCE and HPho-C@GCE electrodes, e) Nyquist plots for Na2S2O8 reaction on HPhi-C@GCE and HPho-C@GCE electrodes. Inset shows the equivalent circuit. f) FTIR spectra of Na2S2O8 electrolyte after electrochemical reduction of Na2S2O8 on HPho-C electrode.

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Figure 2. Water contact angle images of a) HPho-C and b) HPhi-C electrodes. 3.1 Battery Chemistry The battery was assembled as explained in experimental section with Zn metal as the anode immersed in 6 M NaOH and 3 M SPS as the catholyte in contact with an HPho-C (or HPhi-C) electrode. The cathode and anode compartments were separated by a Nafion 117 membrane to avoid direct reduction of persulfate on Zn metal (Scheme 1). It should be noted that persulfate was directly dissolved in distilled water without using any additional electrolytes. The single electrode potentials measured with respect to reversible hydrogen electrode were found to be 1.34 V for HPho-C (S2O82-/SO42-) electrode and -1.26 V for zinc electrode, suggesting major voltage loss occurred on the cathode electrode was possibly due to sluggish electrode kinetics of persulphate reduction on carbon electrode. This suggests that even though HPho-C electrode demonstrates electrocatalytic effect towards persulphate reduction chemistry, plenty of rooms are available for improving its activity. The zinc carbon battery polarisation curve (Figure 3a) with HPho-C cathode demonstrated a peak power density of 420 mW/cm2 at a peak current density of 340 mA/cm2. The performance metrics with HPhi-C was inferior to the performance on HPho-C (Figure 3a and Table S2, Supporting Information) in line with Figure 1a. The lower performance on HPhi-C is due to the direct oxidation of water by SPS catalyzed by HPhi-C, as explained earlier (Figure S3, Supporting Information). This is further investigated by galvanostatic intermittent titration 10


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technique (GITT), Figure 3b, as it is a well-known electrochemical technique to decipher thermodynamic and kinetic parameters. The cell was discharged at a current density of 25 mA/cm2 for 1 hour followed by a relaxation time (at 0 mA/cm2) of 1 hour to relax the system to the open circuit potential and the process was repeated

Figure 3. a) Polarization and power density curves of Zn battery in comparison to Pt based Zn-air battery, b) galvanostatic intermittent titration technique (GITT) plots for Zn-HPho-C (black) and Zn-HPhi-C (red) at 25 mA/cm2, c) galvanostatic discharge curves at different current densities for Zn-HPho-C and d) capacity plots of Zn-HPho-C and Pt based Zn-Air batteries, e) Digital photographs of Zn-HPho-C battery powering commercial LED lamp and f) commercial electric fan with LED lamp. for 60 hours. The quasi OCV (average OCV) during relaxation was found to be higher on hydrophobic carbon electrode compared to its hydrophilic counterpart throughout the cycling process. This clearly demonstrates the dominance of electron acceptors reduction on hydrophobic carbon electrode. Further during the discharge process immediately after the iR drop, the voltage 11



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profiles dropped and then relaxed to higher values. This behavior was absent in their hydrophobic counterpart with nearly steady voltage profiles indicating the occurrence of some sort of parasitic chemistry at the hydrophilic electrode/electrolyte interface. For this reason in order to compare the performance of proposed zinc battery with conventional Pt-based zinc-air batteries we have chosen the hydrophobic (HPho-C) carbon cathode/electrolyte interface. This comparison is particularly important given the stature of mechanically and electrically rechargeable zinc-air batteries as suitable power sources for zero emission electric vehicles. The performance on all studied carbons are much higher than a conventional Pt-based aqueous zinc-air battery, Figure 3a, even though the direct water oxidation is possible on HPhi-C electrodes. The zinc-HPho-C cell delivered an OCV of 2.6 V and a peak power density of 420 mW/cm2 at a peak current density of 340 mA/cm2 which were higher than those of Pt-based zinc-air batteries (1.6 V, 150 mW/cm2 at 200 mA/cm2), Figure 3a and Table S2 (Supporting Information). The open circuit voltage and battery characteristics were abysmal when SPS was absent (Figure S5, Supporting Information) indicating its role as a powerful oxidant. The galvanostatic polarization (Figure 3c and Figure 3d) at different current densities with SPS on an HPho-C cathode demonstrate that the battery possesses decent long term polarization which is considerably higher than that of Pt-based Zn-air batteries. At discharge current densities of 50 and 100 mA/cm2, the capacities normalized to the weight of consumed Zn for the Zn-HPho-C battery is 930 mAh/g (Figure 3d) which is 1.5 times higher than that of Pt-based zinc-air battery (~600 mAh/g, Figure 3d). Additionally, the capacity values were almost identical at different discharge current densities for Zn-HPho-C battery as opposed to a significant decrease observed in Pt-based Zn-air batteries, Figure 3d. This indicates that even with expensive Pt-based electrocatalyst, higher rate capability is greatly limited in Zn-air batteries by the complex kinetics of 4 electron oxygen reduction reaction. On the other hand the higher

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standard reduction potential of SPS provides extra handles to raise the power capability with a concomitant amplification of its energy (energy per weight of consumed Zn) in Zn-HPho-C batteries even with inexpensive carbon particles. This is further clear from the Ragone plot (Figure S6, Supporting Information) of the two systems wherein the power (per weight of consumed Zn) and the energy in Zn-HPho-C batteries are more than doubled compared to those in Pt-based Znair batteries. The higher energy and power density albeit with a Pt-free cathode with a closed cathode architecture can be desirable for practical applications. We have deployed this high capacity (10.4 V, 6.5 Ah) battery for an LED based table lamp (Figure 3e) and demonstrate that 4 serially connected cells can power the lamp for several hours. Further, this battery can be used for running a commercial fan and agricultural pesticide sprayer for several hours (Figure 3f and Video 1) demonstrating its application as mechanically rechargeable Zn battery for commercial electric appliances. We have calculated the amount of time this primary battery can be operated continuously when Zn strip having a weight of 25 g was used as anode with 3 M of SPS in the cathode compartment and is found to be 234 h at 100 mA/cm2. When electrolytes are mixed without a separator, such batteries furnished lower OCV (2.0 V), current density (88 mA cm-2) and power density (86 mW cm-2), (Figure S7a, Supporting Information) possibly due to direct oxidation of Zn in the presence of persulfate. This aspect is confirmed by measuring the weight loss of Zn in the configurations with and without separator under open circuit conditions over time, Figure S7b (Supporting Information). That points to loss of electrons and zinc in the mixed electrolytes (~120 mg/hr of Zn loss occurred without separator) is severe as opposed to an almost unchanged Zn weight (0.7 mg/hr loss) in separated configurations. Taken together, the presented Pt-free Zn-HPho-C battery with a separator possesses better energy and power density compared with a Pt-free cathode electrocatalyst. Since

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the anolyte is alkaline and catholyte is neutral separated by a Nafion 117 membrane, it is inevitable to investigate the cross over or mixing of electrolytes over time. As shown, the self-discharge rate was in the order 0.01 mV/hr (Figure 4a) and time dependent FTIR spectra of SPS solution (Figure 4b) remained unaffected without noticeable change in relative intensities indicating such issues are not encountered in the present architecture over the time tested. Using the same HPho-C electrode, by replacing the electrolytes, the Zn-HPho-C battery could be cycled almost 500 times with nearly identical discharge capacities, (Figure 4c), and the charge transfer resistances extracted from the Nyquist plots (Figure 4d) of the battery on each mechanical recharging were almost comparable, demonstrating the mechanical rechargeability of the battery architecture and extended cyclability of HPho-C electrode.

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Figure 4. a) Self discharge study of Zn-HPho-C battery, b) time dependant FTIR spectra of sodium persulfate at open circuit voltage, c) galvanostatic discharge curves (at 50 mA/cm2) during repeated mechanical recharging of Zn-HPho-C battery with the same HPho-C cathode for 500 cycles, and d) impedance spectra of corresponding cycles. To understand the reaction mechanism we further carried out in-situ FTIR spectroelectrochemistry at individual half-cells and the results point the accumulation of zincate (Figure 5b and 5c) and sulphate (Figure 5d) respectively in the anolyte and the catholyte during the discharge chemistry (Figure 5a). This is clear from the increase in intensity of zincate stretching and decrease in intensity of SPS vibration (with a concomitant increase in intensity of sulphate stretching vibrations) during the polarization. Therefore the anodic reaction involves dissolution of zinc as sodium zincate and the cathodic reaction involves the reduction of persulphate to sulphate. The half-cell and complete cell reactions are given in the eq 2 and 3 and the overall cell reaction is presented in eq 4.

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Figure 5. a) Galvanostatic discharge curve of Zn-HPho-C battery at 50 mA/cm2. b) and c) represent in-situ FTIR spectra of the anode/electrolyte interface at the points indicated in Fig. 5a in the range b) 850 to 4000 cm-1 and c) 480 to 600 cm-1. d) Represent in-situ FTIR spectra of the cathode/electrolyte interface at the points indicated in Figure 5a in the range b) 850 to 1850 cm-1. 4. Conclusions In conclusion, we have successfully demonstrated a strategy to expand the working voltage window in aqueous batteries far beyond the thermodynamic limit of 1.23 V by altering the interfacial chemistry at the cathode/electrode interface to highly hydrophobic. The results point to a high performance Zn HPho-C battery with a Pt-free cathode having a closed cathode

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architecture, thereby overcoming several difficulties associated with state of the art Pt-based aqueous zinc-air batteries. In-situ FTIR spectroelectrochemistry, impedance spectroscopy and galvanostatic intermittent titration technique suggest the interfacial chemistry of the cathode/electrolyte interface profoundly influencing the discharge chemistry with non-wettable carbon nanoparticles predominantly catalyzing the reduction of electron acceptor while the corresponding hydrophilic counterpart catalyzing parasitic oxygen evolution reactions. Zinc batteries equipped with only non-wettable carbon nanoparticles as cathode electrocatalysts delivered an OCV of 2.6 V and a peak power density of 420 mW/cm2 at a peak current density of 340 mA/cm2 which are remarkably higher than those of Pt-based Zn-air batteries (1.6 V, 150 mW/cm2 at 200 mA/cm2). The energy density for Zn hydrophobic carbon battery with a freely diffusing electron acceptor is found to 2 times higher than that of Pt-based Zn-air batteries and could be used to power commercial electric appliances. Author Information *Corresponding Author Email: [email protected], Telephone: +91 (20) 2590 8261 Acknowledgements MOT is indebted to DST-SERB, MHRD fast track and DST-Nano mission for financial assistance. Supporting Information Supporting information contains, electrochemical active surface area calculation, chromatogram, FTIR spectra and Ragone plot. The Supporting Information is available free of charge on the ACS Publications website 17



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2.6 V Aqueous Battery with a Freely Diffusing Electron Acceptor - The

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