Suppressing Self-discharge of Vanadium Diboride by Zwitterionicity of

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Suppressing Self-discharge of Vanadium Diboride by Zwitterionicity of Polydopamine Coating Layer Fanqi Wang, Meifen Wu, Tao Zhang, and Zhaoyin Wen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20112 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

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ACS Applied Materials & Interfaces

Suppressing Self-discharge of Vanadium Diboride by Zwitterionicity of Polydopamine Coating Layer Fanqi Wanga,c, Meifen Wua,c, Tao Zhangb,c, Zhaoyin Wen*a,c a

CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China. b

State Key Lab of High Performance Ceramics and Superfine Microstructure,

Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai, 200050, P.R. China. c

University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

*Email address: [email protected] Keywords: Vanadium diboride air battery; VB2 anode; Self-discharge; The polydopamine (PDA) membrane; pH-switching characteristics.

Abstract The vanadium boride (VB2) air battery is currently known as a primary battery with the highest theoretical specific capacity, 4060 mAh g-1, which originates from extraordinary 11 electron per VB2 molecule oxidation process. However, the parasitical reaction between VB2 and hydroxide ions in the alkaline electrolyte leads to obvious self-discharge, which is presented in severe capacity loss during discharge. In this work, we applied the polydopamine (PDA) membrane to modify the surface of VB2 particles, which contains amine groups and phenolic hydroxyl groups exhibiting fully reversible, pH-switchable

permselectivity. The

‘‘smart’’ membrane

with

pH-switching

characteristics successfully coordinated the conflict between the electrolyte and VB2 in

ACS Paragon Plus Environment

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the open circuit to avoid corrosion, but also ensured that the hydroxide ions can enter the VB2 particle surface to participate in the reaction during discharge. According to the corrosion suppression test, the remaining amount of VB2@PDA is 90 wt.% stored at 65 ℃ for two weeks, which is 10 wt.% more than the uncoated VB2. The assembled pouch cell with the VB2@PDA anode can deliver a high capacity of 325 mAh at 250 mA g-1, retaining an improved Coulombic efficiency of 86.3%, which is 18.7% higher than that of cell with the raw VB2 anode. Moreover, the 0.05 V higher discharge voltage of the VB2@PDA based cell further shows that the PDA membrane can effectively conduct hydroxide ions during discharge. 1. Introduction In order to build a powerful energy system, energy storage materials realizing multielectron reactions have drawn the attention for researchers. And now multi-electron reactions are involved in various battery systems (e.g. lithium storage alloys1-3, oxides4, sulfides5-6 and metal-air7-9, transition metal boride–air battery10-11 etc.). The above systems all exhibit several times more specific capacity than conventional lithium-ion batteries with carbon-based anode, 372 mAh g-1. In a MO/Li battery made with Co, Ni or Fe oxide, about 2 Li per M could be removed, leading to reversible capacities ranging from 600 to 800 mAh g-1.4 In a metal-air battery such as the zinc-air battery, it can release two electrons per zinc and exhibit an intrinsic capacity of 820 mAh g-1.9 While in all the studies, the transition metal boride-air batteries, especially the vanadium diboride air battery, shows the highest intrinsic specific capacity of 4060 mAh g-1. It stems from that the vanadium diboride (VB2) undergoes an extraordinary 11 electron


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per molecule oxidation, involving oxidation of the tetravalent transition metal ion, V (+4 → +5), and each of the two boron’s 2 ×B (−2 → +3)12. Meanwhile, as calculated, VB2-Air battery can reach a theoretical discharge potential 1.55 V, which is promising as an air battery with high practical potential13. Similar to metal air batteries, VB2-Air battery also adopts oxygen from ambient atmosphere as the cathode reactant material. The electrochemical reactions of the cell are displayed as follows14: Anode: 𝑉𝐵2 + 11𝑂𝐻 − → 1⁄2 𝑉2 𝑂5 + 𝐵2 𝑂3 + 1⁄2 𝐻2 𝑂 + 11𝑒 − (1) Cathode: 𝑂2 + 2𝐻2 𝑂 + 4𝑒 − → 4𝑂𝐻 − (2) Cell: 2𝑉𝐵2 + 11⁄2 𝑂2 → 2𝐵2 𝑂3 + 𝑉2 𝑂5 (3) As shown in Equation (1), VB2-Air battery adopts alkali media as electrolyte, such as KOH, NaOH or a mixture of NaOH and KOH. Just like metal anodes, the VB2 anode can chemically react with hydroxide ions in the alkaline electrolyte, which involves corrosion reaction and hydrogen evolution reaction (HER).15 This self-discharge phenomenon of VB2 anode causes a direct loss of battery capacity, and predominantly impedes the practical application of VB2-Air battery. The current effort on stabilizing VB2 against corrosion was conducted by Licht et al16. A thin zirconia coating on VB2 not only guarantees charge transfer during the anodic discharge process, but can effectively prevent corrosion at the surface of the boride17-18. After storage in the alkaline electrolyte at 45 ℃ or 70 ℃, obvious side reactions of VB2 lead to sever capacity loss of 10 wt.% and 35 wt.%, respectively. In comparison, after modified by 1 wt.% ZrO2, the capacity retention of VB2 anode has been greatly improved to 100 wt.%



and 85 wt.%, respectively16. Therefore, the zirconia coating has been treated as an effective method to suppress the anode corrosion. However, the principle of corrosion inhibition is by reducing the direct contact area between the electrolyte and the active material. This will inevitably affect the activity of the VB2 particles and obstruct the anode electrochemical reaction to some extent. In this work, we applied the polydopamine (PDA) membrane to modify the surface of the VB2 particles. The PDA membrane contains abundant amine groups and phenolic hydroxyl groups exhibiting fully reversible, pH-switchable permselectivity for both cationic and anionic molecules in specific pH range. At high pH, the membrane has a net negative charge that excludes anions but passes cations, while at low pH it is positively charged and excludes cations but passes anions19. The powder corrosion tests on VB2 and VB2@PDA reveal that the PDA membrane effectively restrains the corrosion against the electrolyte. The result is consistent with the membrane character at high pH, (Figure 1a). During the discharge, the anode protected by the PDA membrane delivers most of the intrinsic specific capacity, which is higher than the raw VB2. This can be attributed to that the rapid consumption of hydroxide ions during discharge decreases the electrolyte pH around the anode. The PDA membrane tends to be positively charged and the hydroxide ions can enter the VB2 particle surface to participate in the discharge reaction, (Figure 1b). This work provides a fresh insight into the anodic protection for transition metal boride–air battery, even for metal air batteries.


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Figure 1. Schematic illustration of pH-switchable function and structure of the PDA membrane on VB2 in solutions (a) the PDA membrane exhibits negative charge during the open circuit; (b) the PDA membrane exhibits positive charge during discharge.

2. Experimental 2.1 Synthesis of VB2@PDA Vanadium diboride powder (VB2, 99.97 wt.%, 5~10 μm particles) was dispersed in the deionized water by ultrasonication. Then dopamine hydrochloride was added to the resulting dispersion. After stirring for 12 h, the VB2@PDA products were washed by centrifugation with the deionized water three times. Figure 2 shows the synthesis process of [email protected],21



Figure 2. Schematic illustration of the synthesis route for VB2@PDA.

2.2 Structural characterization of materials The synthesis of the PDA membrane on the surface of VB2 particles was characterized by Fourier Transform Infrared Spectroscopy (FTIR, Tensor 27) and Xray photoelectron spectroscopy (XPS) using an ESCAlab250 system with a monochromatic Al Kα X-ray source. And the FTIR was further conducted on the stabilization study of the PDA membrane during discharge. Thickness and morphology of the PDA membrane were measured by a high resolution transmission electron microscope (HRTEM). The VB2@PDA particles were calcined at 700 °C for 2 hours in argon atmosphere to obtain the VB2@C particles. Then the carbon content in VB2@C was detected using thermogravimetric analyzer (Netzsch STA 409PC) in air atmosphere from room temperature to 800 ℃ at a heating rate of 5 ℃ min-1. Based on


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the formula of PDA22 (Figure S1), the mass fraction of PDA in VB2@PDA can be calculated. Field emission scanning electron microscopy (FESEM, SU8200 and Magellan 400) was performed to characterize the morphology of the anodic active material VB2 before and during discharge. 2.3 Batteries assembly The electrochemical evaluation of VB2-Air batteries was conducted in coin-type cells (CR2032) and pouch cells. And the electrodes were cut into a diameter of 9 mm disc for coin-type cells and a 3.3×3.3 cm2 square for pouch cells. Batteries were fabricated in the ambient atmosphere. Both of the battery’s cathode and anode adopted the modus of thin film electrode manufactured by rolling depression. The anode was prepared firstly by mixing 85 wt.% powders (VB2 and VB2@PDA), 7 wt.% polytetrafluoroethylene (in emulsion, 2 wt.%) and 8 wt.% KB (Ketjen black carbon, ECP-600JD) into paste, then roll-pressing the paste into thick film (optimized surface density, VB2 8.0~10.0 mg cm-2 and VB2@PDA 10.0~12.5 mg cm-2). For the cathode, there are two main components, the catalyst layer and the gas diffusion layer. The catalyst layer was composed of the MnO2 catalyst (20 wt.%), KB (65 wt.%) and polytetrafluoroethylene (PTFE) binder (15 wt.%). The gas diffusion layer was formed with acetylene black (60 wt.%) and PTFE (40 wt.%). After mixing the composing materials, the two layers’ pastes were rolled into thick film, respectively (optimized surface density, the catalyst layer 9.5~10.5 mg cm-2 and the gas diffusion layer 4.5~5.5 mg cm-2). Finally, the anode film, the catalyst layer and the gas diffusion layer were placed in a vacuum oven (P2F-6050) for 48 h at 65 ℃ to remove organic residues and



water. With the assembly of the battery, the anode was made by pressing the VB2 film onto a stainless steel mesh (SUS304) at a pressure of 8 MPa and the cathode used the same pressure to press the parts together (The structure of cathode is shown in Figure S2). Moreover, we used the 8 M KOH solution as electrolyte, which was added to the separator (BSA-PST-100A). 2.4 Electrochemical test Batteries were tested on a LAND CT2001A battery test system (Wuhan, China). And these batteries were placed in a Temperature and Humidity Camber (LHU-114) set at a temperature of 25 ℃ and a humidity of 60%. The current density was at 500 mA g-1 to evaluate the coin-type cells and at 250 mA g-1 for pouch cells evaluation, which were based on the load of VB2, with the cut-off voltage of 0.1 V. 3. Results and discussion 3.1 Characterization on VB2@PDA anode


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Figure 3. (a) FTIR spectra of VB2@PDA; (b) Thermogravimetric analysis curves of VB2@C; (c) full XPS spectra of PDA coating and corresponding high resolution element spectra of (d) O1s, (e) C1s, (f) N1s.

In order to ensure the coating of the PDA membrane on the surface of VB2 particles, the FTIR evaluation was conducted. The Fourier transform infrared (FTIR) spectra of VB2@PDA are shown in Figure 3a. The broad strong peak at 3300~3500 cm-1 is attributed to the stretching vibration of O-H and N-H bonds. The absorption peak at 1620 cm-1~1650 cm-1 is consistent with the stretching vibration of C=N bond22. The other bonds can be assigned to CH2 bending vibration (1345 cm-1) and C-O shear



vibration (1120 cm-1), respectively, which are also correlated with characteristic groups of the PDA. These results preliminarily indicate the successful polymerization of the dopamine to the PDA on the surface of VB2. XPS analysis was conducted to further verify the PDA coating layer on the VB2 surface, and the results are exhibited in Figure 3c-3f. The cleavage peaks of O1s, (Figure 3d), at the binding energy of 531.2 eV and 533 eV demonstrate that the oxygen bond in the PDA structure may be quinone and C=O bond, which determines the existence of catechol or hydroxyl groups. Besides, C1s spectra are divided into four peaks in Figure 3c, including sp2C at 284.2 eV, sp3C at 284.8 eV, C-N bond at 285.3 eV and C-O bond at 286.6 eV. The most crucial N1s spectra, as shown in Figure 3f, indicate that there are two nitrogen-containing species in the PDA membrane, i.e., the pyridine N at 398.2 eV and the N-H bond at 399.5 eV on the heterocycle. They correspond well to the molecular structure of the PDA. The XPS analysis suggests that the PDA membrane certainly covers VB2 particles as FTIR analysis demonstrated. The mass ratio of the PDA membrane in VB2@PDA can be calculated from the carbon content which can be detected through thermogravimetric analysis (TGA) (Figure 3b). The carbon content was calculated to be 12.2 wt.%, reflecting 18.6 wt.% content of the PDA in VB2@PDA. This result is the basis for setting the discharge current density and calculating the actual specific capacity of the VB2-Air battery.


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Figure 4. VB2@PDA (a) TEM; (b) HRTEM images; (c) the fast-Fourier transformed (FFT) patterns of area α in (b).

To deeply analyze the PDA coating on VB2 anode, HRTEM was conducted to convey a more intuitive coverage of the coating layer (Figure 4a, b). As displayed in Figure 4b, an amorphous PDA layer of 4-8 nm has been confirmed on the surface of VB2 particles. Meanwhile, in the fast-Fourier transformed (FFT) patterns of area α in (b) (Figure 3c), the (011), (110) and (101) plane of VB2 can be clearly observed. 3.2 Corrosion inhibition assessment for VB2@PDA

Figure 5. (a) the VB2 and VB2@PDA powder remaining mass fraction; (b) FTIR spectra of VB2@PDA before and after electrolyte soaking.



The coating approach aims at protecting VB2 from the electrolyte corrosion. The protection can help to achieve two targets including reducing the loss of active anodic materials and maintaining its structural and chemical integrity under the strong basic conditions. The latter will be discussed in the next section. Herein, we carried out the powder corrosion test to compare the stability of the VB2 with and without the PDA membrane in the alkali electrolyte. Two powders, VB2 and VB2@PDA, with the same amount of VB2 were added into plastic bottles containing the same volume of electrolyte, and kept in the oven at 65 °C for two weeks. Finally, the treated powders were collected and weighed, the results are shown in Figure 5a. The remaining rate of VB2 with the PDA coating is up to 90 wt.% but only 80 wt.% for the uncoated one. As indicated by FTIR spectra in Figure 5b, the PDA membrane on the surface of VB2 particles keeps its chemical integrity. Meanwhile, the XRD patterns of the collected powders in Figure S3 fully matches the VB2. Based on the nature of the PDA19, the membrane has a net negative charge (Figure 1a) and excludes anions at high pH, which may point to the reason of anti-corrosion effect. The following electrochemical experiment will prove this mechanism and exclude the possibility of anti-corrosion by preventing the electrolyte from direct contact with VB2. 3.3 Electrochemical performance


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Figure 6. Galvanostatic discharge curves for different anode, VB2 and VB2@PDA. (a) the discharge performance comparison of different anode and capacity, 10 mAh and 30 mAh, at a current density of 500 mA g−1 by coin cells; (b) the discharge performance comparison of different anode at a current density of 250 mA g−1 by pouch cells; (c) schematic of the pouch cell.

The galvanostatic discharge was conducted to demonstrate the performance of the cells with the VB2@PDA anode. The specific capacities of the cells were calculated based on the weight of VB2. And the discharge efficiencies of the cells were determined as normalized by the intrinsic specific capacity of 4060 mAh g-1. To emphasize the improvement derived from the PDA coating on VB2, 10 mAh and 30 mAh coin cells with the VB2@PDA anode and the raw VB2 anode were assembled for verification, respectively (Figure 6a). The assembled cells were measured at the current density of 500 mA g−1. The VB2@PDA based cell acquires a Columbic efficiency of 81% at the 10 mAh capacity limit, which is 16% higher than the cells without anodic protection.



With the capacity limit increasing to 30 mAh, the Columbic efficiency of the VB2@PDA based cell is 15% more than the VB2 anode cell. Moreover, the 30 mAh capacity limit cell with VB2@PDA anode even exhibits 5% higher Columbic efficiency than the 10 mAh limited VB2 anode cell, which relieves the effect of capacity fade with increased anodic load. The above results show that the PDA membrane does not impede VB2 to participate in the reaction discharge but greatly increase the discharging depth. A mechanism based on the pH-switchable permselectivity of the PDA membrane was proposed as follows. During discharge, the hydroxyl ions near the anode would be rapidly consumed by the electrode reaction, thereby reducing the pH of the electrolyte near the membrane. Although hydroxide ions can continuously diffuse toward the anode, the continuous anode discharge reaction causes continuous consumption of the hydroxide ions, leading to a dynamic equilibrium. As a result, the PDA membrane exhibits positive charge and attracts hydroxide ions into the surface of VB2 during discharge19. After 3h of discharge, the remaining anodic components are still VB2, as shown in Figure 7e. Through the SEM analysis for the anodes terminated at discharging 3h, we found that the VB2@PDA particles maintained the initial morphology (as shown in Figure S3a-d), whereas the significant orientation oxidation occurred in the raw VB2 (Figure 7a, b). These two diametrically opposed morphologies are a good illustration of the positive effect of the PDA membrane on the structural and chemical integrity of VB2 during discharge. Therefore, with the PDA membrane the VB2 anode released


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more capacity as illustrated in Figure 6a, b. The stability of the PDA membrane during the discharge was also confined by the FTIR analysis (Figure 7f).

Figure 7. (a-d) SEM images of anode film discharged three hours, VB2 and VB2@PDA;(e) XRD patterns; (f) FTIR spectra of VB2@PDA before and after discharging.

To verify the performance of the PDA membrane under high-load and long-time discharge conditions, further electrochemical experiments were carried on the pouch cells with 325 mAh capacity at a current density of 250 mA g−1. As demonstrated in Figure 6b, the VB2@PDA based cell attains a Columbic efficiency of 87%, 19% higher



than that of the VB2 based cell. Improved discharge depth and smooth discharge curve further prove the active role of the PDA protective membrane in maintaining VB2 structural and chemical integrity. In addition, the discharge voltage platform of VB2@PDA based cell displays an upgrade about 0.05 V. It may be derived from the attraction of the positively charged PDA membrane to the anion, which accelerates the arrival of hydroxide ions at the surface of VB2. While at a current density of 500mA g1

in Figure 6a, the diffusion of hydroxyl ions in aqueous solution determines the anode

reaction rate, which leads to the inconspicuous conductance of the PDA membrane.

4. Conclusions In summary, the PDA membrane with pH-switching characteristics successfully suppresses the self-discharge of the VB2 anode. Comparing with the unprotected VB2 anode at a current density of 500 mA g-1, the Coulomb efficiencies of the 10 mAh and 30 mAh VB2@PDA based cells increase by 16% and 15%, respectively. Moreover, the protected anode exhibits excellent practical potential. In pouch cells of 325mAh, the VB2@PDA based cell delivers a high Coulombic efficiency of 86.3%, which is 18.7% higher than that of cell with the raw VB2 anode. The galvanostatic discharge test verifies that the PDA membrane does not impede the discharge reaction, and maintains the structural and chemical integrity of VB2 during the continuous discharge process. Furthermore the protective effect of the PDA membrane on VB2 in the powder corrosion test is significant for the long-term storage of the battery. The successful application of the ‘‘smart’’ membranes with pH switch function on VB2-Air batteries leads us to believe that it will also perform better in other metal air batteries, such as


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zinc-air batteries (as shown in Figure S6). Therefore, it is of great practical significance to further study the dynamics of smart modules and design low-cost, high-efficiency ‘‘smart’’ membranes.

Acknowledgment This work was financially supported by JCKY2016130B010, National Natural Science Foundation of China (No.51872316) and Shanghai Engineering Research Center of Inorganic Energy Materials and Electric Power Sources (18D22280800).

Supporting Information This includes the formula of polydopamine, the cathode structure, the XRD patterns of the VB2@PDA powder after corrosion inhibition assessment, and SEM images of the VB2 and the VB2@PDA anode films before discharge,

the experiment results

proving that the PDA membrane has no contribution to the discharge capacity, and galvanostatic discharge curves for different anodes, Zn and Zn@PDA, at a current density of 500 mA g−1 by coin cells.

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Suppressing Self-discharge of Vanadium Diboride by Zwitterionicity of

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