Electrochemical Energy Storage with an Aqueous QuinoneâAir

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Electrochemical Energy Storage with an Aqueous Quinone-Air Chemistry Xingwen Yu, and Arumugam Manthiram ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00536 • Publication Date (Web): 18 May 2018 Downloaded from http://pubs.acs.org on May 19, 2018

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ACS Applied Energy Materials

Electrochemical Energy Storage with an Aqueous Quinone-Air Chemistry Xingwen Yu and Arumugam Manthiram* Materials Science and Engineering Program and Texas Materials Institute, The University of Texas at Austin, Austin, TX 78712, United States

ABSTRACT: Organic electrode materials such as quinones are drawing rising attention as promising redox-active materials for the development of rechargeable batteries. In aqueous solutions, the redox potential of quinones is dependent on the alkalinity and acidity of the medium. Under an alkaline condition, the oxidation potential of hydroquinone (existing as diphenolate) is ca. 0.8 V lower than that under an acidic condition. On the other hand, under an acidic condition, the reduction potential of oxygen is ca. 0.8 V higher than that under an alkaline condition. By taking these advantages, a Quinone-air cell with a rational voltage is strategically demonstrated with an alkaline anode electrolyte and an acidic cathode electrolyte, which are physically separated by a Na+ion conductive solid-state electrolyte membrane. The Na+-ions shuttling through the solid-state membrane act as ionic mediators/messengers to sustain and link the redox reactions at the two electrodes.

trode, it would be an attractive option to develop an environmentally benign and low-cost secondary battery system with a hydroquinone-air chemistry. Both the gaseous air and the liquid quinone are interesting electrode materials for the development of redox flow batteries.14-16 However, in addition to the electrochemical capacity of the electrode, a practical battery requires a reasonable electromotive force (an acceptable cell voltage) between the positive and negative electrodes.

KEYWORDS: energy storage, quinone-air battery, organic electrode material, solid electrolyte, mediator-ion electrolyte

Rechargeable battery technologies play an important role toward the efficient utilization of renewable energy resources.1,2 Among the various types of rechargeable batteries, air cathode batteries offer a series of technical merits, such as the reduced cost, volume, and weight. These advantages can be easily achieved through the use of free oxygen in the atmosphere as the cathode reactant.3,4 The air cathode batteries have mostly been developed and investigated with metal anodes resulting in a class of primary and secondary metal-air batteries.5,6 However, the use of metal anodes is sometimes constrained by environmental concerns and material abundance. Recently, redox-active organics have attracted much attention as environmentally benign electrode materials for the development of sustainable and safe energy storage systems.7-9 Quinones, a class of electrochemically active moieties, have recently been explored as liquid electrode materials for the development of secondary batteries.10,11 Hydroquinone, a member of quinoneseries compounds, has previously been used as a safe organic for biological applications in controlling electron-transport processes.12,13 From an electrochemical energy storage point of view, hydroquinone can deliver a specific capacity of 487 mA h g-1 based on a two-electron charge transfer process through the transformation of an aromatic ring into a cyclic diketone core (oxidation process) or the transformation of a cyclic diketone core into an aromatic ring (reduction process) in accordance with the redox reaction shown in Figure 1a. Based on the electrochemicalcapacity features of the air electrode and the hydroquinone elec-

Figure 1. (a) Redox reaction and electrochemical transition of hydroquinone (or diphenolate) and benzoquinone. (b) Cyclic voltammograms (CVs) of a carbon paper electrode in 0.1 M hydroquinone solution with different pH values. The pH of the solution was adjusted by H2SO4 or NaOH. O2 was removed from the solution by a continuous bubbling of N2 into the solution for 20 min before each experiment. (c) Redox potential of hydroquinone (or diphenolate) as a function of the pH value of the solution. (d) The pH-dependent potential of oxygen reduction reaction (ORR) calculated with Nernst equation. It has been known that in aqueous solutions, the redox potential of quinones is dependent sensitively on the alkalinity and acidity of the solution.17,18 In an alkaline solution, hydroquinone generally exists as diphenolate. Figure 1b shows the cyclic voltammograms (CVs) of hydroquinone in aqueous solutions with different pH values. To avoid any parasitic reactions between hydroquinone (or diphenolate) and oxygen, all solutions for the CV experiments were saturated with nitrogen. As seen in Figure 1b, the redox potential of hydroquinone (or diphenolate) shifts to negative values as the solution pH increases. The correlation be-

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tween the redox potential of hydroquinone (or diphenolate) and the pH value of the solution is summarized in Figure 1c. The potential values in Figure 1c were determined by averaging the onset reduction potential of the cathodic scan and the onset oxidation potential of the anodic scan in each voltammogram profile. The linear regression slope of the data in Figure 1c is ca. -0.06 V pH-1, which is consistent with the Nernst equation.19 The electrochemistry of oxygen is well understood in the literature. Equations 1 and 2 present the redox reactions as well as the corresponding electrochemical potentials of oxygen under, respectively, alkaline (1.0 M OH-) and acidic (1.0 M H+) conditions. O2 + 2H2O  4OH- + 4e-

E0 = 0.40 V vs. SHE

(1)

O2 + 4H+ + 4e- 2H2O

E0 = 1.23 V vs. SHE

(2)

The pH-dependent potential of oxygen redox reaction is calculated with Nernst equation as summarized in Figure 1d. According to the data presented in Figure 1c and d, if a quinone-air cell is operated with the traditional cell development strategy by employing either an acidic electrolyte or an alkaline electrolyte, the theoretical voltage of the resulting quinone-air cell would be ca. 0.53 V (This number is obtained by subtracting the potential values in Figure 1c from those potential values in Figure 1d). Under the practical working conditions, the cell voltage will be even lower. Such a low cell voltage makes the quinone-air chemistry unattractive for the practical development of electrochemical energy storage systems. Therefore, although hydroquinone delivers a reasonably high electrochemical capacity, it is not practically feasible to develop a quinone-air cell due to the low electromotive force of the quinone-air chemistry. By taking the advantages of the data presented in Figure 1c and d, however, the pH-dependent potentials of hydroquinone (or diphenolate) and oxygen provide a possibility to develop a quinone-air cell with a reasonable voltage. If a cell is operated with a high-pH (alkaline) diphenolate negative electrode and a low-pH (acidic) oxygen positive electrode, the theoretical voltage of the resulting hybrid-electrolyte quinone-air cell can be tuned up to a reasonably high value (ca. 1.3 V). However, the traditional cell development strategy based on a liquid-electrolyte-integrated porous polymer membrane would not allow to develop such a hybrid-electrolyte cell since the alkaline anode electrolyte and the acidic cathode electrolyte would mix with each other during cell operation. Herein we present a hybrid-electrolyte quinone-air cell with a mediator-ion strategy as schematized in Figure 2a. The quinone-air cell is demonstrated with an alkaline anode electrolyte (anolyte) and an acidic cathode electrolyte (catholyte), which are separated by a sodium-ion conductive solidstate membrane (Na-SSM). Separation of a liquid-phase catholyte and a liquid-phase anolyte has previously been investigated with either a lithium-ion (Li+-ion) or a sodiumion (Na+-ion) solid electrolyte.20-24 The Na-SSE usually offers a higher ionic conductivity than Li-SSE.22 In this study, A NASICON-type Na3Zr2Si2PO12 solid electrolyte was employed for the demonstration of the quinone-air cell. The NaSSM electrically and physically separates the alkaline anolyte and the acidic catholyte. Na +-ions shuttling through the Na-SSM act as an ionic mediator (or messenger) to sustain the redox reactions of the alkaline diphenolate negative electrode and the acidic air positive electrode. In order to minimize the overpotential of the air positive electrode, the quinone-air cell was operated with a decoupled positive electrode configuration by employing a carbon supported platinum (Pt/C) catalyst for the oxygen reduction reaction (ORR) and a titanium mesh supported iridium oxide (IrO 2/Ti) catalyst for the oxygen evolution reaction (OER). Such an as-

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assembled cell is accordingly annotated as quinone (alkaline) ǁ Na-SSM ǁ air (acid). Figure S1provides a picture of the quinone (alkaline) ǁ Na-SSM ǁ air (acid) cell. The assembly and fabrication of the cell are described in the experimental section in the Supporting Information.

Figure 2. (a) Schematic of a quinone-air cell with an alkaline anolyte, an acidic catholyte, and a Na+-ion conductive solidstate membrane (Na-SSM). The cell is operated with a decoupled configuration for the air electrode. A carbon supported platinum (Pt/C) catalyst is used for the oxygen reduction reaction (ORR) and a titanium mesh supported iridium oxide (IrO2/Ti) catalyst is used for the oxygen evolution reaction (OER). (b) Scanning electron microscopy (SEM) image of the carbon cloth matrix for the redox reaction of diphenolate/benzoquinone. (c) SEM image of the IrO 2/Ti OER catalyst. The inset shows the high-magnification image of the IrO2/Ti catalyst with a small patch of the IrO2 layer being intentionally scratched off. During discharge of a quinone (alkaline) ǁ Na-SSM ǁ air (acid) cell, diphenolate is oxidized to benzoquinone and releases two electrons at the negative electrode. At the positive electrode, oxygen is reduced to water on the Pt/C ORR catalyst by combining with four protons and four electrons received via the external circuit from the negative electrode. Meanwhile, Na+-ions migrate from the anolyte to the catholyte through the Na-SSM toward balancing the ionic charge between the two electrodes. During the charge, the processes are reversed. At the positive electrode, the water is oxidized to oxygen on the IrO2/Ti OER catalyst and releases four electrons and four protons. At the negative electrode, benzoquinone is reduced to diphenolate by receiving the electrons through the external circuit. To maintain the ionic charge balance between the two electrodes, Na +-ions migrate back to the anolyte from the catholyte through the Na-SSM. A NASICON-type Na3Zr2Si2PO12 pellet provided by 421 Energy Corporation, South Korea was used as a sodium mediator-ion solid-state membrane. Na3Zr2Si2PO12 exhibits an ionic conductivity of ca. 1.0 x 10 -3 S. cm-1 at ambient temperature. A piece of carbon cloth (supplied by Fuel Cell Store) was used as the negative electrode matrix of the liquid diphenolate/benzoquinone active material. Figure 2b shows a scanning electron microscope (SEM) image of the carbon


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Figure 3. (a, b) A discharge-charge profile of the quinone (alkaline) ǁ Na-SSM ǁ air (acid) cell. (c) Ultraviolet-visible (UV-Vis) spectra of the fresh anolyte and the anolyte after discharging or charging the cell to different states indicated with the violet open circles in the discharge-charge curves in Figure 3a and b.

cloth. This high surface fabric carbon material can provide a facile environment for the electrochemical reaction of diphenolate/benzoquinone. Preparation of the titanium mesh supported iridium oxide OER catalyst is described in the experimental section of Supplementary Information. The surface morphology of IrO2/Ti is shown in Figure 2c. The wire diameter of the Ti mesh is ca. 100 µm. On the Ti wire surface, a thin layer of IrO 2 was deposited as shown in Figure 2c. To show the contrast between the Ti substrate and the IrO2 coating, a small patch of the IrO2 layer was intentionally scratched off (Figure 2c inset). The obtained IrO 2 film is amorphous as characterized with X-ray diffraction (XRD, Figure S2), which is consistent with the reported information in the literature.25,26 This IrO2/Ti material has previously been proved as a stable and active OER catalyst in acidic solutions.27 Figure 3a and b show the first charge-discharge curve of the quinone (alkaline) ǁ Na-SSM ǁ air (acid) cell. Upon deep discharge, the negative electrode material could deliver a specific capacity of ca. 410 mA h g-1 (based on the mass of hydroquinone). According to the 2-electron redox reaction illustrated in Figure 1a and based on the molecular weight of hydroquinone (110 g mol-1), the theoretical capacity of the active hydroquinone is 487 mA h g-1. Comparison of the practical discharge capacity and the theoretical capacity indicates that the utilization of hydroquinone active material is ca. 87%. The utilization of hydroquinone was also correlated with the depth of discharge of the quinone (alkaline) ǁ NaSSM ǁ air (acid) cell with ultraviolet-visible spectroscopy (UV-Vis) analyses. Figure S3 shows the UV-Vis spectra of the hydroquinone in a weakly acidic solution and that in an alkaline solution (exists as diphenolate). The absorption peak shifts from 295 nm to 287 nm when hydroquinone in acidic solution is converted to diphenolate in alkaline solution. The UV-Vis spectra of the diphenolate anolyte at various chargedischarge states are presented in Figure 3c. The anolyte samples were collected after the quinone (alkaline) ǁ Na-SSM ǁ air (acid) cell was discharged-charged to a half-discharged state, a full-discharged state, a half-charged state, and a full charged state, as indicated in Figure 3a and b with the violet circles. As expected, the fresh anolyte shows an absorption peak of diphenolate at a wavelength of 287 nm. When the quinone (alkaline) ǁ Na-SSM ǁ air (acid) cell was discharged to a specific capacity of 200 mA h g-1 (half discharged state), mixed absorption characteristics of the diphenolate and benzoquinone are observed in the UV-Vis spectrum.28 The characteristic absorption peak of benzoquinone is located at 244

nm.29 After the cell was deeply discharged to a cutoff voltage of 0.4 V, the benzoquinone absorption peak dominates and the diphenolate absorption peak becomes tiny. When the cell was charged to a half-way state, the mixed absorption characteristics of the diphenolate and benzoquinone are observed again. After the cell was charged to a cutoff voltage of 2.2 V, the diphenolate absorption peak dominates. To quantify the concentrations of the diphenolate species in the anolyte, a series of standard diphenolate solutions were prepared with specific concentrations and characterized with UV-Vis spectroscopy. A calibration curve is provided in Figure S4. The concentrations of diphenolate species at different chargedischarge states are then determined by plugging the obtained absorption values into the calibration curve, as summarized in Table S1. The results obtained from the UV-Vis analyses (Table S1) are basically consistent with the electrochemical charge-discharge results (Figure 3a and b). Electrochemical performances of the quinone (alkaline) ǁ Na-SSM ǁ air (acid) cell is presented in Figure 4. Since the cell was operated with a separate OER electrode and ORR electrode, the discharge and charge curves were recorded individually by employing two channels on a cell testing instrument. A 5-minute resting time was preset between each discharge and charge period. Under such cycling conditions, the cells are tended to be overcharged or over discharged if they were cycled by controlling the discharge and/or charge cutoff voltages. To avoid this problem, we strategically cycled the cells by controlling the discharge and charge capacities to 300 mA h g-1 (based on the mass of hydroquinone) of the negative electrode material. Figure 4a, b, and c show the consecutive chargedischarge curves of the quinone (alkaline) ǁ Na-SSM ǁ air (acid) cell at operating current densities of, respectively, 0.5, 1.0, and 2.0 mA cm-2. Throughout the 50 cycles at each current density, the charge-discharge profiles show consistent shape. There are no significant increase in the voltage polarization with continuous cycling, implying a good compatibility of the Na-SSM with the anolyte and the catholyte. However, the stability of the Na-SSE under different pH values has not yet been established. The detailed pH-dependent stability of the Na3.4Sc2(PO4)2.6(SiO4)0.4 material will be systematically investigated in the future. To avoid the risk of any incompatible issues, we prepared the acidic catholyte electrolyte with a low-concentration H2SO4. By zooming in the charge-discharge profiles of Figure 4, the polarization behavior of the quinone (alkaline) ǁ Na-SSM ǁ air (acid) cell is summarized in Figure S5. After assembling


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the cell, the quinone (alkaline) ǁ Na-SSM ǁ air (acid) cell exhibits an open-circuit voltage (OCV) of ca. 1.20 V. After applying a charge-discharge current, the cell voltage responds. With the increase in the cycling current, the charge voltage of the cell increases and the discharge voltage decreases. Figure S6 plots the discharge voltages at different operating current densities and the power densities delivered by the quinone (alkaline) ǁ Na-SSM ǁ air (acid) cell. At ambient temperature, The quinone (alkaline) ǁ Na-SSM ǁ air (acid) cell delivers reasonable cell voltages. However, due to the relatively low Na-ion conductivity of the Na-SSM and the relatively high thickness (ca. 0.7 mm) of the Na-SSM membrane employed, the cells demonstrated here can only be cycled at relatively low current densities. Our ongoing work includes the development of a facile methodology to reduce the thickness of the Na-SSM pellet. With the development of thin Na-SSM membranes and Na-SSM materials with an enhanced Na+-ion conductivity (by the supplier), the quinone (alkaline) ǁ Na-SSM ǁ air (acid) cell is expected to be operated at high current densities and deliver higher power densities. However, toward practical application, many factors need to be optimized. In addition to the thickness of the NaSSM membrane, the solubility of quinone species in aqueous solution is another concern. For instance, it has previously been reported that if the concentration of hydroquinone is over 0.5 M, the reversibility of the quinone electrode becomes poor.11 This issue is expected to be mitigated through the addition of hydrophilic groups to quinone in the future. Operation of a rechargeable air cathode battery also requires highly active ORR and OER catalysts. The relevant progress in the water splitting technology can be adopted to the quinone (alkaline) ǁ Na-SSM ǁ air (acid) cell. With the progress in the solid-state batteries, both the ionic conductivity and the thickness of the solid electrolyte are expected to be improved. Therefore this proof-of-concept battery is promising to meet the demands of practical applications in the future. Furthermore, the “mediator-ion” battery development strategy provides a versatile approach for the development of a broad range of new battery systems with different redox chemistries. In summary, we have demonstrated a novel electrochemical energy storage system with a quinone-air (O2) chemistry. A reasonable cell voltage of the quinone-air cell is enabled by employing an alkaline anode electrolyte (anolyte) and an acidic cathode electrolyte (catholyte). The alkaline anolyte and the acidic catholyte are physically and electrically separated with a sodium-ion conductive solid-state-electrolyte membrane (Na-SSM). The redox reactions of the alkaline diphenolate negative electrode and the acidic oxygen positive electrode are ionically linked by the shuttling of the mediator Na+-ions through the Na-SSM. In order to enhance the redox reactions of oxygen, the cell is operated with decoupled air cathodes, which comprise a carbon supported platinum (Pt/C) catalyst for the oxygen reduction reaction (ORR) and a titanium mesh supported iridium oxide (IrO2/Ti) catalyst for the oxygen evolution reaction (OER). The mediator-ion approach presented here provides a versatile strategy for the development of rechargeable batteries with different electrolytes in the negative and the positive electrodes.

ASSOCIATED CONTENT

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terns of the titanium mesh substrate and the titanium mesh supported iridium oxide (IrO2/Ti) catalyst (Figure S2); UV-Vis spectra of the hydroquinone in a weakly acid solution and that in an alkaline solution (Figure S3); UV-Vis calibration curve of the diphenolate solutions (Figure S4); polarization behavior of the Quinone(alkaline) ǁ Na-SSM ǁ air(acid) cell at different operating current densities (Figure S5); discharge voltages and power densities of the Quinone(alkaline) ǁ Na-SSM ǁ air(acid) cell at different operating current densities (Figure S6)).

Figure 4. Consecutive charge-discharge curves of the quinone (alkaline) ǁ Na-SSM ǁ air (acid) cell at (a) 0.5 mA cm -2, (b) 1.0 mA cm-2, and (c) 2.0 mA cm-2.

AUTHOR INFORMATION Corresponding Author * Email: [email protected].

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT

Supporting Information Available: (Experimental Section; concentrations of diphenolate species in the anolyte of the Quinone(alkaline) ǁ Na-SSM ǁ air(acid) cell at different chargedischarge states (Table S1); picures of a quinone (alkaline) ǁ NaSSM ǁ air (acid) cell (Figure S1); X-ray diffraction (XRD) pat-

This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering under award number DE-SC0005397.


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Electrochemical Energy Storage with an Aqueous QuinoneâAir

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