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Insights on the Proton Insertion Mechanism in the Electrode of Hexagonal Tungsten Oxide Hydrate Heng Jiang, Jessica J Hong, Xianyong Wu, T. Wesley Surta, Yitong Qi, Shengyang Dong, Zhifei Li, Daniel P. Leonard, John J Holoubek, Jane C Wong, Joshua James Razink, Xiaogang Zhang, and Xiulei Ji J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03959 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 5, 2018

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Journal of the American Chemical Society

Insights on the Proton Insertion Mechanism in the Electrode of Hexagonal Tungsten Oxide Hydrate Heng Jiang,#,a Jessica J. Hong,#,a Xianyong Wu,a T. Wesley Surta,a Yitong Qi,a Shengyang Dong,a,b Zhifei Li,a Daniel P. Leonard,a John J. Holoubek,a Jane C. Wong,a Joshua James Razink,c Xiaogang Zhang,b and Xiulei Jia,* a. Department of Chemistry, Oregon State University, Corvallis, 97331-4003, United States. b. Jiangsu Key Laboratory of Materials and Technology for Energy Conversion, College of Material Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, P. R. China c. CAMCOR, University of Oregon, Eugene, OR 97403, United States Supporting Information Placeholder ABSTRACT: This study reveals the transport behavior of lattice water during proton (de)insertion in the structure of the hexagonal WO3·0.6H2O electrode. By monitoring the mass evolution of this electrode material via electrochemical quartz crystal microbalance, we discovered (1) WO3·0.6H2O incorporates additional lattice water during initial cycling; (2) The reductive proton insertion in WO3 hydrate is a three-tier process, where in the first stage 0.25 H+ is inserted per formula while simultaneously 0.25 lattice water is expelled; then in the second stage 0.30 naked proton is inserted, followed by the third stage with 0.17 H3O+ inserted. Ex situ XRD reveals that protonation of the WO3 hydrate causes consecutive anisotropic structural changes—it first contracts along the c-axis but later expands along the ab planes. Furthermore, WO3·0.6H2O exhibits impressive cycle life over 20,000 cycles, together with appreciable capacity and promising rate performance.

There exists tremendous demand for novel energy storage systems, especially sustainable rechargeable batteries with low cost and long cycle life.1 Recently, much attention has been paid to earth-abundant metal ions as batteries’ charge carriers, such as Na+,2 K+,3 Zn2+,4 Mg2+,5 and Al3+.6 However, limited attention has been given to non-metal charge carriers, particularly the attractive proton and its simplest hydrate — hydronium.7 It is generally accepted that proton as the charge carrier facilitates (pseudo)capacitive behavior of metal oxides in the near-surface regions of electrode phases.8,9 Nevertheless, the nature of topotactic reactions for (de)protonation in metal oxides has yet to be well understood. Herein, we select a crystalline hydrous WO3 as the model electrode material to study the topochemistry of proton/hydronium in particular about the transport behavior of lattice water. WO3 is a well-known electrode material for batteries or supercapacitors.10 Recently, the role of lattice water in WO3’s structure on the electrochemical performance of proton storage has attracted attention. Li et al. reveals that the high rate capability and long cycle life can be promoted by the water chains inside the structural channels of hydrous hexagonal WO3.11 Augustyn et al. reported that the lattice water in layered WO3·2H2O facilitates the significantly

faster proton storage than the anhydrous WO3.12 Kaner et al. systematically studied the water tunnel in the lattice, which facilitates the insertion of proton.13 Nevertheless, to date the understanding on the transport behavior of lattice water during the proton (de)insertion in WO3 has remained limited, although such behavior is, however, central to the electrochemical performance of proton topochemistry. In this study, we uncover the mechanism of proton storage in WO3·nH2O by studying the mass evolution of the electrode with electrochemical quartz crystal microbalance (EQCM) as well as its structural evolution by ex situ X-ray diffraction.

Figure 1. (a) The X-ray diffraction (XRD) pattern and the Rietveld refinement of WO3·0.6H2O; (b) A refined crystal structure unit of WO3·0.6H2O, where the red sectors in water “dots” represent the probability of water’s presence at the corresponding sites (Other viewports are shown in Figure S4); (c, d) HRTEM images for WO3·0.6H2O nanorods. The as-synthesized tungsten oxide is grayish yellow, slightly different from the yellow WO3·H2O and WO3·2H2O (Figure S1). The as-prepared tungsten oxide comprises nanofibers, ~ 500 nm long and ~ 40 nm wide, as shown by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images (Figure S2). X-ray photoelectron spectroscopy (XPS) reveals the

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oxidation state of tungsten being W(VI) with the 4f5/2 and 4f7/2 binding energies at 35.7 and 36.8 eV, respectively (Figure S3a). The absence of W(V) indicates a lack of intercalated cationic species within the WO3 phase. Thermogravimetric analyses (TGA) reveal that there exists 0.6H2O per WO3 formula unit based on the mass loss above 100 °C (Figure S3b). Note that we assume the water mass removed below 100 °C as surfaceadsorbed water, and the structural studies of the WO3 hydrate by XRD were carried out after the sample was heated at 100 °C. As shown in Figure 1a, the XRD pattern presents a hexagonal structure (space group P6/mmm) with lattice parameters of a=7.35780 Å, b=7.35780 Å, and c=3.81746 Å. The refined crystallographic structure is depicted in Figure 1b, and the Rietveld refinement results are summarized in Table S1, which are corroborated by the HRTEM images (Figure 1c, d and Figure S2b). In WO3·0.6H2O, the WO6 octahedra form layers along the ab planes via edge-sharing oxygen atoms; WO3·0.6H2O is, thus, a tunnel structure, and the dodecagon tunnels host lattice water molecules along the c axis. Note that the lattice water molecules reside between W-O layers along the c-axis (see Figure S4 and Figure 3c). Rietveld refinement analysis reveals two possible locations of zeolitic water molecules with respect to ab planes within the dodecagon tunnels, centered and off-centered (Figure 1b). The atomic displacement parameters summarized in Table S1 for the lattice water (O3 position in Figure S3) are considerably high, which implies that these lattice water molecules are relatively mobile.

Figure 2. (a) GCD potential profiles of the tungsten oxide hydrate electrode at 200 mA/g; (b) CV curves of the tungsten oxide hydrate electrode at a scan rate of 3 mV/s (top) and EQCM results of the tungsten oxide hydrate (bottom). Blue: cathodic scan; red: anodic scan. We tested the electrochemical properties of WO3·0.6H2O using three-electrode cells (see Supporting Information). Figure 2a reveals the initial discharge (protonation) capacity of 104 mAh/g with the first-cycle Coulombic efficiency (CE) of 77%, where the

galvanostatic charge/discharge (GCD) profiles are generally sloping. The theoretical capacity is 110 mAh/g if one proton/hydronium gets inserted per WO3 formula unit, where W(VI) is reduced to W(V). Such an obtained capacity is higher than WO3 hydrates that have a big number of lattice water, but is on par with the anhydrous WO3. The first cathodic scan in cyclic voltammetry (CV) shows two reduction peaks at -0.1 and -0.45 V and two corresponding oxidation peaks at 0.08 and -0.23 V, respectively (Figure 2b). All potentials are referred to the Ag/AgCl reference electrode, unless noted otherwise. We have measured the Brunauer-Emmett-Teller (BET) specified surface area of WO3·0.6H2O, which is merely 32.74 m2/g, where the N2 isotherm and pore size distribution (PSD) are shown in Figure S5. The pores of WO3·0.6H2O are primarily macropores, which are the voids between the nanofibers. Since the electrical double layer capacitance (EDLC) is proportional to the specific surface area, and the EDLC mechanism of charge storage must play a minor role here. To identify the active charge carriers and the topochemistry mechanism, we conducted operando EQCM measurements during CV tests (See the bottom panel in Figure 2b). Notably, over the initial cycling, the electrode of WO3·0.6H2O absorbs additional lattice water into the structure. EQCM monitored the electrode mass evolution during the initial 5 CV cycles (Figure S6a). We also observed the gradual increase of the electrode mass when it rests at open circuit voltage (OCV) in the electrolyte (Figure S6b). In CV measurements, the initial mass gain corresponds to 0.29 H2O incorporated per formula of WO3, close to a gain of 0.26 H2O caused by immersing at OCV. If we assume the formula of the tungsten oxide hydrate electrode as WO3·0.6H2O when it was just placed in the electrolyte, the mass-stabilized formula of the operating electrode should be WO3·0.89H2O. According to the EQCM results during protonation, the electrode experiences a mass loss at a rate of 18/e-, followed by a stage with nearly constant mass, and later a mass gain at the rate of 18/e-; during the extraction of charge carriers, the direction of electrode’s mass evolution is exactly reversed: mass loss first followed by mass gain. By integrating the area under the CV curves to calculate the capacity and by considering the corresponding EQCM results, during protonation, ~0.25 lattice H2O was expelled first from WO3·0.89H2O along with ~0.25 proton inserted simultaneously (0 to -0.25 V), then ~0.30 naked proton was incorporated (-0.25 to -0.40 V), followed by ~ 0.17H3O+ inserted afterwards (-0.40 to -0.55 V). The (de)insertion of water and protons are highly reversible because the mass of the electrodes recovers to the original value after a full cycle. Intriguingly, the initially opposite transport directions of protons and lattice water in the structure casts a question on whether protons’ migration is mediated via lattice water molecules inside the WO3 structures by the Grotthuss mechanism during the first stage, where in Grotthuss conduction, protons migrate by structural diffusion of water instead of ‘vehicle-like’ transfer. The first-stage behavior of the WO3 electrode seems more aligned with the computational studies by Ozolinš et al. who suggested the proton conduction inside WO3 via bridge oxygens, instead of the Grotthuss mechanism.14 The release of lattice water upon protonation is intriguing but may relate to the structural evolving of the WO3 hydrate electrode. Thus, we analyzed the structural evolution by ex situ XRD studies. Schematics of different crystallographic planes are shown in Figure 3a, b and c. During proton/hydronium insertion, the (001) peak shifts to the right, suggestive of contraction of crystals along the c-axis, which may be due to the strong interaction between the inserted protons and the bridge oxygen atoms (Figure 3f). Also, interestingly, both (100) and (110) peaks shift leftward upon protonation, indicative of the structural elongation


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Journal of the American Chemical Society along the ab planes, where this trend is also observed by ex situ HRTEM studies (Figure S7). This implies that the inserted protons may form strong H-bonding with the bridge oxygens in the structure, which weakens the W-O bonds inside the ab planes and relaxes the structure in these directions. After charging to 0.3 V, all the peaks shift back as the pristine sample, showing structural reversibility of this tungsten oxide hydrate (Figure 3e and Figure 8). The structural contraction along c-axis during initial protonation may cause water expelling; however, the expansion along ab planes may promote hydronium instead of proton insertion. The overall topotactic behavior of the WO3 structure should be a tradeoff of these two battling effects. Notably, the directions of this tradeoff may be switched along the different state of charge (SOC). By inspecting the ex situ XRD patterns closely (Figure 3f), one can see that the (100) peak moves to a low angle slowly from the pristine state to -0.4 V but shifts faster from -0.4 to -0.55 V. Inversely, the (001) peak shifts to a higher angle fast from the pristine to -0.2 V; it shifts slowly from -0.2 to -0.4 V; there is barely any shift from -0.4 to -0.55 V. Therefore, during the first stage, the contraction along the c direction dominates, where the structure squeezes and expels water; from -0.2 to -0.4 V, both contraction and expansion are quite mild, and neither dominates, thus resulting in no net water migration; after discharge below 0.4 V, the rapid expansion along ab planes plays a dominant role, and the structure accommodates hydroniums accordingly.

the capacitive contribution by utilizing the equation: i(V)= k1v + k2v1/2, where the current response, i(V), is a combination of the capacitive (k1v) and diffusion-controlled (k2v1/2) current contributions. The high extent of capacitive contribution of H+ storage corroborates the pseudocapacitive properties (Figure 4c and Figure S10). Furthermore, the small charge-transfer resistance (RCT) of less than 10 Ω and a low activation energy of 8.2 KJ/mol derived from electrochemical impedance spectroscopy (EIS) measurements indicate fast proton (de)insertion reactions (Figure S12).16 Interestingly, proton storage in WO3·0.6H2O delivers a much higher capacity than hosting Na+, K+, and NH4+ albeit within a narrower potential range (Figure S11). This may have to do with the strong binding between proton and the host of the WO3 hydrate.

Figure 4. (a) Rate performance of WO3·0.89H2O from 1 to 100C (1C = 100 mA/g); (b) CV curves of WO3·0.89H2O under different scan rates with b-values. Sweep rates are varied from 1 to 10 mV/s; (c) Contribution of the capacitive charge storage at 3 mV/s; (d) Long cycling performance of WO3·0.89H2O at 2 A for 20,000 cycles.

Figure 3. Schematics of the WO3·0.6H2O structure to display crystal planes (a) (100), (b) (110), and (c) (001); (d) Typical GCD potential profiles of the WO3·0.89H2O anode; (e and f) Corresponding ex situ XRD patterns at different SOC, as marked in d. WO3·0.6H2O shows a promising rate performance of proton storage, where Figure 4a demonstrates deprotonation capacity values of ~90, ~75, ~70, ~60, ~50, and ~30 mAh/g at 1, 5, 10, 20, 50, and 100C, respectively. CV studies can provide information on the charge storage kinetics, which can be derived by analyzing the redox currents at various scan rates according to the equation: i=avb.8c,15 Current i obeys a power-law relationship with scan rate v, a is an adjustment coefficient, and b-value is determined from the slope of log(i) as a function of log(v). A b-value of 0.5 suggests the diffusion-controlled kinetics, whereas b=1.0 indicates the redox behavior being non-diffusion controlled, or capacitive. The b-values of redox peaks demonstrate prominent pseudocapacitive behavior of H+ storage: 0.92, 0.90, 0.87, and 0.94 for R1, R2, O2, and O1 (Figure 4b), which is more ‘capacitive’ than Na+ and K+ (Figure S9). Such non-diffusion-controlled kinetics explain the relatively high rate performance. Furthermore, we calculated

In addition, WO3·0.6H2O shows excellent cycling stability. At the current rate of 2 A/g, the charge capacity increases to above 80 mAh/g in the first 50 cycles, which is stable during the following 20,000 cycles (Figure 4d). The CE is 87% for the first cycle followed by a rapid increase within the first 5 cycles, and stays at an average of 99%. The cycling performance at a lower current density of 200 mA/g is shown in Figure S13, where the CE is less than that observed at 2 A/g. Thus, hydrogen evolution reaction (HER) on this tungsten oxide hydrate is not negligible. In this study, typically we conducted GCD cycling on the electrodes after they rest in coin cells for 0.5 hour. For such electrodes, the capacity increase was typically observed during initial cycling. However, we found that if the electrodes were kept immersed in the electrolyte for 12 hours before cycling, the initial capacity is much higher, which does not increase much during cycling, as shown in Figure S14. The results suggest that the initial capacity increase (Figure 4d) may correlate to the absorption of lattice water into the structure of WO3. In summary, we characterized the structure of a hexagonal tungsten oxide hydrate, WO3·0.6H2O, where Rietveld refinement of the XRD results shows a tunnel structure, and the zeolitic water molecules exist along the dodecagon tunnels. By operando EQCM, we discovered that the proton topochemistry in WO3 hydrate operates with a three-tier process: proton insertion accompanied by lattice water expelling, naked proton insertion, and a subsequent insertion of hydroniums. The opposite transport



directions of water and proton during initial proton insertion may somewhat provide supports that protons migrate inside WO3 via bridge oxygen ions at least for the first stage. Ex situ XRD studies at different SOC reveal unique anisotropic behavior of structural change with some nuance, where the contraction along c-axis dominates at early SOC of protonation corresponding to the water egression, agreeing with the EQCM results, and the structural expansion along ab planes horizontally is dominant during the late SOC, which is associated with water ingression. This suggests the strong interaction, i.e., chemical bonding, between protons and bridge oxygens. WO3·0.6H2O exhibits an appreciable reversible capacity of ~ 90 mAh/g and a long cycle life over 20,000 cycles at 2 A/g, together with promising rate performance.

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ASSOCIATED CONTENT Supporting Information. Supplementary information is available in the online version of the paper. Reprints and permissions information is available online. This material is available free of charge via the Internet at http://pubs.acs.org.

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More details on experimental methods; material characterization, structure information, comparation with other cations and electrochemical performance.

AUTHOR INFORMATION Corresponding Author Correspondence should be addressed to X.J. [email protected]

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Author Contributions Heng Jiang and Jessica J. Hong contributed to the work equally. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS X.J. thanks the United State National Science Foundation, Award Number 1551693 for the financial supports. The authors thank Professor Douglas A. Keszler for XRD measurements.

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