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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 2017.121:3707-3713. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 11/02/18. For personal use only.

Department of Chemistry and Centre for Energy Science, Indian Institute of Science Education Research, Pune, Dr. Homibaba Road, Pune 411008, India W Web-Enhanced Feature * S Supporting Information *

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 nonwettable 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 @ 50 mA/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 ∼43 W/kg). When probed with 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 reaction, while their nonwettable 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 complexity of carbonate clogging and electrolyte evaporation often encountered in open-air batteries, and could be used to power electrical appliances. and permeability.12,13 The air electrode in zinc-air battery requires 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 a concourse of articles appeared contending with various catalysts and their combinations to reduce catalyst 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 four-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 is replaced by inexpensive and environmentally benign electron acceptor Na2S2O8 with highly positive standard reduction potential (2.08 V vs SHE), negating

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 because it can be accessed directly from the air; however, an opencathode structure is inevitable to negate fuel starvation in such air batteries.8,9 From an engineering perspective the porous composite cathode architecture of air batteries has several bottlenecks. First, 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 © 2017 American Chemical Society

Received: November 7, 2016 Revised: February 2, 2017 Published: February 2, 2017 3707

DOI: 10.1021/acs.jpcc.6b11180 J. Phys. Chem. C 2017, 121, 3707−3713

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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 (1 M 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 of the studies. The spectra at the open-circuit voltage were taken as the background in all of the electrolytes and with all of the electrodes. All of 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-ionconducting 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 SPS (saturated solution) solution, respectively. The battery was assembled with zinc foil as anode and hydrophobic (or hydrophilic) carbon-nanoparticle-coated (3 mg/cm2) Toray carbon paper as cathode. 2.5. Electrochemical Characterization. The 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 2propanol as solvent, and the resulting mixture was ultrasonicated for 30 min. The ink was drop-casted onto the glassy carbon electrode (GCE) 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 LSV 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 h. The cell was allowed to relax to its open circuit at 0 mA/ cm2 (relaxation time) for 1 h, and the process was repeated for ∼60 h. Zn-air (oxygen was fed to the cathode continuously) battery was assembled using 20% Pt@C (0.5 mg/cm2) as air cathode and zinc foil as anode in 6 M NaOH for comparative purpose. Electrochemically active areas of carbon nanoparticles were estimated from the peak currents (Figure S1, Supporting Information) of a fast moving redox couple using the Randles− Sevcik equation.

Scheme 1. Schematic Representation of Zn-HPho-C Battery

the requirement of an expensive metal-based electrocatalyst to catalyze the cathode half-cell reaction. Because the standard reduction potential of persulfate redox reaction is highly positive, 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 (SPS), 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 Johnson-Mathey, and Nafion 117 membrane was procured from Sigma-Aldrich India. 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 °C under Ar/H2 (95:5 v/v) for ∼5 h to remove hydrophilic functionalities. After completion of heat treatment, as-obtained Ketjen black was stored in a desiccator without exposure 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-bottomed flask for 1 h. The mixture was then sonicated for 1 h, followed by refluxing for ∼24 h. As-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 ATRFTIR (Bruker Alpha). Gas chromatography (GC) was carried out using a 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 workstation. In situ FTIR spectra were acquired in thin-layer configuration in respective half cells using an ATR-FTIR spectrometer (Bruker Alpha), ATR accessory (with a diamond crystal), and an electrochemical cell. In situ FTIR spectra under applied bias were acquired

3. RESULTS AND DISCUSSION 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). Because the oxygen half-cell reaction in conventional Zn-air battery is replaced by persulfate reduction in proposed Zn battery, it is imperative to investigate its chemistry 3708

DOI: 10.1021/acs.jpcc.6b11180 J. Phys. Chem. C 2017, 121, 3707−3713

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

Figure 2. Water contact angle images of (a) HPho-C and (b) HPhi-C electrodes.

normalized to true surface area (Sac, see the Experimental Section for more details) demonstrated a ∼53 times enhancement on HPho-C (2.05 × 10−5 A cm−2) compared with HPhiC (3.86 × 10−7 A cm−2) (Figure 1c), indicating intrinsic electrocatalytic activity associated with HPho-C particles. Furthermore, the Tafel slope (Figure 1c) observed on HPhoC 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 were probed by chronoamperometric studies, and it demonstrated after the double-layer decay higher activity for longer duration on HPho-C compared with 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 with corresponding hydrophilic counterpart. The plot of concentration versus peak current (Figure S2a, Supporting Information) and the plot of peak current versus square root of scan rate (Figure S2b, Supporting Information) on HPho-C followed a linear relationship, suggesting a diffusion-controlled electrochemical process that is further clear from a near 0.5 slope of log(i) versus log(υ) plot (Figure S2c, Supporting Information). This is further supported by the GC data on HPhi-C electrode (Figure S3, Supporting Information), demonstrating the clear presence of oxygen,

on a carbon nanoparticle electrode. Because the standard reduction potential of persulfate is very high (E0 = 2.08 V)29,30 electrochemical reduction of persulfate can be achieved even without a precious metal electrocatalyst. Furthermore, 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 the Experimental Section for more details). The noncatalytic nature of HPho-C as opposed to the catalytic nature of hydrophilic carbon (HPhi-C) toward 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 persulfate reduction in neutral media demonstrates no redox wave in the negative going scan on a bare 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 with its hydrophobic (HPho-C@GCE) counterpart. Furthermore, the peak currents at 0.6 V versus RHE were amplified on HPho-C@GCE compared with HPhi-C@GCE. All of these demonstrate an electrocatalytic effect on the HPho-C/electrolyte interface compared with its hydrophilic counterpart. The reduction wave on HPho-C@GCE demonstrated a linear increase with respect to the concentration of persulfate (Figure 1b), indicating the reduction wave is stemming from electrochemical reduction of persulfate. Exchange current densities 3709

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Figure 3. (a) Polarization and power density curves of Zn battery in comparison with 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, (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.

major voltage loss occurred on the cathode electrode was possibly due to sluggish electrode kinetics of persulfate reduction on carbon electrode. This suggests that even though the HPho-C electrode demonstrates electrocatalytic effect toward persulfate reduction chemistry, plenty of room is available for improving its activity. The zinc carbon battery polarization 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 HPhoC (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 previously explained (Figure S3, Supporting Information). This is further investigated by GITT (Figure 3b), as it is a wellknown electrochemical technique to decipher thermodynamic and kinetic parameters. The cell was discharged at a current density of 25 mA/cm2 for 1 h, followed by a relaxation time (at 0 mA/cm2) of 1 h to relax the system to the open-circuit potential, and the process was repeated for ∼60 h. The quasi OCV (average OCV) during relaxation was found to be higher on hydrophobic carbon electrode compared with its hydrophilic counterpart throughout the cycling process. This clearly demonstrates the dominance of electron acceptor reduction on hydrophobic carbon electrode. Furthermore, during the discharge process immediately after the iR drop the voltage 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, to compare the performance of proposed zinc battery with conventional Pt-based zincair 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 performances 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

suggesting that O2 evolution (OER) in the presence of SPS is catalyzed by HPhi-C electrodes. The inertness of HPho-C electrode toward 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 with ∼20° on HPhi-C (Figure 2a,b). The higher contact angles on HPho-C could be due to the reduction of hydrophilic functionalities compared with HPhi-C (FTIR spectra, Figure S4, Supporting Information). The lower contact angles on HPhi-C electrode make the water wet the carbon electrodes, and this may induce a catalytic effect toward 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, GC, and contact-angle measurements, and therefore this system is further investigated for spectroelectrochemistry and battery studies. Ex situ FTIR spectroelectrochemistry on HPho-C (Figure 1f) reveals the formation of sulfate species during electrochemical reduction of persulfate. On the basis of these studies, the following chemistry (eq 1) is proposed for persulfate 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 with its hydrophobic counterpart, making the latter more active toward the redox chemistry of peroxydisulfate. S2 O82 − + 2e− ⇄ 2SO4 2 − (E 0 = 2.08V)

(1)

3.1. Battery Chemistry. The battery was assembled as explained in the Experimental Section with Zn metal as the anode immersed in 6 M NaOH and 3 M SPS as the catholyte in contact with a 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 RHE were found to be 1.34 V for HPho-C (S2O82−/ SO42−) electrode and −1.26 V for zinc electrode, suggesting 3710

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The Journal of Physical Chemistry C ∼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,d) at different current densities with SPS on a HPho-C cathode demonstrates 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 capacity 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 four-electron oxygen reduction reaction. On the contrary, the higher 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 with those in Pt-based Zn-air 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 four serially connected cells can power the lamp for several hours. Furthermore, 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 a 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 is used as anode with 3 M of SPS in the cathode compartment, and it 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 the fact that loss of electrons and zinc in the mixed electrolytes (∼120 mg/h of Zn loss occurred without separator) is severe, as opposed to an almost unchanged Zn weight (0.7 mg/h loss) in separated configurations. Taken together, the presented Pt-free ZnHPho-C battery with a separator possesses better energy and power density compared with a Pt-based Zn air battery. Because the anolyte is alkaline and catholyte is neutral, separated by a Nafion 117 membrane, it is inevitable to investigate the crossover or mixing of electrolytes over time. As shown, the self-discharge rate was on the order of 0.01 mV/h (Figure 4a), and time dependent FTIR spectra of SPS solution

Figure 4. (a) Self-discharge study of Zn-HPho-C battery, (b) timedependent 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.

(Figure 4b) remained unaffected without noticeable change in relative intensities, indicating that 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. To understand the reaction mechanism, we further carried out in situ FTIR spectroelectrochemistry at individual half-cells, and the results point to the accumulation of zincate (Figure 5a−c) and sulfate (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 sulfate stretching vibrations) during the polarization. Therefore, the anodic reaction involves the dissolution of zinc as sodium zincate and the cathodic reaction involves the reduction of persulfate to sulfate. The half-cell and complete cell reactions are given in eqs 2 and 3, and the overall cell reaction is presented in eq 4. Anode: Zn + 4NaOH → Na 2Zn(OH)4 + 2Na + + 2e− (E = −1.26 V)

(2)

Cathode: Na 2S2 O8 + 2Na + + 2e− → 2Na 2SO4 (E = 2.08 V)

(3)

Overall: Zn + 4NaOH + Na 2S2O8 → Na 2Zn(OH)4 + 2Na 2SO4 (E = 3.34 V)

(4) 3711

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a mechanically rechargeable Zn battery for commercial electric appliances.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91 (20) 2590 8261. ORCID

Musthafa Ottakam Thotiyl: 0000-0002-2439-4708 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.O.T. acknowledges DST-SERB, MHRD fast track, and DSTNano mission for financial assistance. (1) Freunberger, S. A. Batteries: Charging Ahead Rationally. Nature Energy 2016, 1, 16074. (2) Dunn, B.; Kamath, H.; Tarascon, J.-M. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928−935. (3) Aricò, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J.-M.; van Schalkwijk, W. Nanostructured Materials for Advanced Energy Conversion and Storage Devices. Nat. Mater. 2005, 4, 366−377. (4) Linden, D.; Reddy, T. B. Handbooks of Batteries; McGraw-Hill, 2001. (5) Reddy, M. V.; Subba Rao, G. V.; Chowdari, B.V. R. Metal Oxides and Oxysalts as Anode Materials for Li Ion Batteries. Chem. Rev. 2013, 113, 5364−5457. (6) Liu, Y.; Chen, S.; Quan, X.; Yu, H.; Zhao, H.; Zhang, Y.; Chen, G. Boron and Nitrogen Codoped Nanodiamond as an Efficient MetalFree Catalyst for Oxygen Reduction Reaction. J. Phys. Chem. C 2013, 117, 14992−14998. (7) Subhramannia, M.; Pillai, V. K. Shape-Dependent Electrocatalytic Activity of Platinum Nanostructures. J. Mater. Chem. 2008, 18, 5858− 5870. (8) Yousfi-Steiner, N.; Mocotéguy, Ph.; Candusso, D.; Hissel, D. A Review on Polymer Electrolyte Membrane Fuel Cell Catalyst Degradation and Starvation Issues: Causes, Consequences and Diagnostic for Mitigation. J. Power Sources 2009, 194, 130−145. (9) Li, Y.; Dai, H. Recent Advances in Zinc−Air Batteries. Chem. Soc. Rev. 2014, 43, 5257−5275. (10) Freunberger, S. A.; Chen, Y.; Peng, Z.; Griffin, J. M.; Hardwick, L. J.; Bardé, F.; Novák, P.; Bruce, P. G. Reactions in the Rechargeable Lithium-O2 Battery with Alkyl Carbonate Electrolytes. J. Am. Chem. Soc. 2011, 133, 8040−8047. (11) Lee, J.-S.; Tai Kim, S.; Cao, R.; Choi, N.-S.; Liu, M.; Lee, K. T.; Cho, J. Metal−Air Batteries with High Energy Density: Li−Air versus Zn−Air. Adv. Energy Mater. 2011, 1, 34−50. (12) Baker, R. W.; Lokhandwala, K. Natural Gas Processing with Membranes: An Overview. Ind. Eng. Chem. Res. 2008, 47, 2109−2121. (13) Jeong, H. K.; Krych, W.; Ramanan, H.; Nair, S.; Marand, E.; Tsapatsis, M. Fabrication of Polymer/Selective-Flake Nanocomposite Membranes and Their Use in Gas Separation. Chem. Mater. 2004, 16, 3838−3845. (14) Morales, L.; Fernandez, A. M. Unsupported PtxRuyIrz and PtxIry as Bi-Functional Catalyst for Oxygen Reduction and Oxygen Evolution Reactions in Acid Media, for Unitized Regenerative Fuel Cell. Int. J. Electrochem. Sci. 2013, 8, 12692−12706. (15) Kraytsberg, A.; Ein-Eli, Y. The Impact of Nano-Scaled Materials on Advanced Metal−air Battery Systems. Nano Energy 2013, 2, 468− 480. (16) Chen, Z.; Yu, A.; Higgins, D.; Li, H.; Wang, H.; Chen, Z. Highly Active and Durable Core−Corona Structured Bifunctional Catalyst for Rechargeable Metal−Air Battery Application. Nano Lett. 2012, 12, 1946−1952. (17) Du, G.; Liu, X.; Zong, Y.; Hor, T. S. A.; Yu, A.; Liu, Z. Co3O4 Nanoparticle-Modified MnO2 Nanotube Bifunctional Oxygen Cath-

Figure 5. (a) Galvanostatic discharge curve of Zn-HPho-C battery at 50 mA/cm2. (b,c) In situ FTIR spectra of the anode/electrolyte interface at time intervals indicated in panel a in the range (b) 850 to 4000 cm−1 and (c) 480 to 600 cm−1. (d) In situ FTIR spectra of the cathode/electrolyte interface at the points indicated in panel a in the range (b) 850 to 1850 cm−1.

4. CONCLUSIONS 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 architecture, thereby overcoming several difficulties associated with state of the art Pt-based aqueous zinc-air batteries. In situ FTIR spectroelectrochemistry, impedance spectroscopy, and GITT suggest the interfacial chemistry of the cathode/ electrolyte interface profoundly influences the discharge chemistry with nonwettable carbon nanoparticles predominantly catalyzing the reduction of electron acceptor while the corresponding hydrophilic counterpart catalyzes parasitic oxygen evolution reactions. Zinc batteries equipped with only nonwettable 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 be two times higher than that of Pt-based Zn-air batteries and could be used to power commercial electric appliances.

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REFERENCES

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b11180. Electrochemical active surface area calculation, chromatogram, FTIR spectra, and Ragone plot. (PDF) W Web-Enhanced Feature *

A movie of a high capacity (10.4 V, 6.5 Ah) battery (Figure 3e) powering an agricultural pesticide sprayer, a commercial fan, and an LED-based table lamp, demonstrating its application as 3712

DOI: 10.1021/acs.jpcc.6b11180 J. Phys. Chem. C 2017, 121, 3707−3713

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DOI: 10.1021/acs.jpcc.6b11180 J. Phys. Chem. C 2017, 121, 3707−3713