High-Power and Ultralong-Life Aqueous Zinc-Ion Hybrid Capacitors Based on Pseudocapacitive Charge Storage

Highlights This work starts the research of pseudocapacitive oxide materials for multivalent Zn2+ storage. The constructed RuO2·H2O||Zn systems exhibit outstanding electrochemical performance, including a high discharge capacity, ultrafast charge/discharge capability, and excellent cycling stability. The redox pseudocapacitive behavior of RuO2·H2O for Zn2+ storage is revealed. Electronic supplementary material The online version of this article (10.1007/s40820-019-0328-3) contains supplementary material, which is available to authorized users.


Introduction
Novel energy storage systems with the merits of high safety, fast charge-discharge capability, and high energy density are highly demanded with the rapid development of electric vehicles and customer electronics. Recently, multivalention (e.g., Zn 2+ , Ca 2+ , Mg 2+ , and Al 3+ ) storage systems have emerged and exhibited unique electrochemical behaviors [1][2][3][4][5][6]. During various multivalent-ion storage systems, zinc metal anode-based aqueous rechargeable zinc-ion batteries (ZIBs) and zinc-ion hybrid capacitors (ZICs) are particularly attractive [1,[7][8][9][10][11], due to their high safety, low cost, abundant natural resource of zinc, and unique electrochemical features of zinc metal anodes such as low redox potential of − 0.76 V (vs. standard hydrogen electrode) and ultrahigh volumetric capacity of 5845 Ah L −1 . Furthermore, the high ionic conductivity of aqueous electrolytes such as ZnSO 4 solutions in ZIBs and ZICs is beneficial for achieving high power output. The electrochemical properties of ZIBs and ZICs are strongly dependent on the Zn 2+ -storage behaviors in cathode materials.
Herein, for the first time, we demonstrate that fast, ultralong-life, and safe Zn 2+ storage can be realized in amorphous RuO 2 ·H 2 O cathode materials based on a pseudocapacitive storage mechanism. The constructed RuO 2 ·H 2 O||Zn ZICs can reversibly store Zn 2+ in a voltage window of 0.4-1.6 V (vs. Zn/Zn 2+ ), delivering a capacity of about 122 mAh g −1 , an excellent rate capability and an ultralong cycle life exceeding 10,000 cycles.

Electrochemical Measurements
Amorphous ruthenium oxide hydrate (RuO 2 ·xH 2 O) powder was obtained from Sigma-Aldrich Corporation. To synthesize anhydrous RuO 2 , the RuO 2 ·xH 2 O powder was heat-treated in air at 300 °C for 1 h with a heating rate of 5 °C min −1 . The amorphous RuO 2 ·xH 2 O power (or anhydrous RuO 2 powder) was mixed with conductive black and polyvinylidene fluoride binder in N-methyl-pyrrolidone solutions, then coated on a stainless steel foil, and finally dried at 100 °C in vacuum to obtain RuO 2 ·xH 2 O (or RuO 2 ) electrodes. Mass loading of active materials in the prepared cathodes was 2.5-3.0 mg cm −2 . Electrochemical performance of these ruthenium oxides for Zn 2+ storage was evaluated by assembling CR2032 coin cells, in which RuO 2 ·xH 2 O (or RuO 2 ) electrode was used as the cathode, commercial Zn foil was used as the anode, air-laid paper was used as separator, and 2 M Zn(CF 3 SO 3 ) 2 or 2 M ZnSO 4 aqueous solution served as the electrolyte. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests were performed on a Bio-Logic VMP3 electrochemical station. An AC amplitude of 5 mV and a frequency range of 0.1-100 kHz were applied for the EIS test at open-circuit voltage (OCV). For galvanostatic charge-discharge (GCD) measurements, when the applied current was 0.1-3 A g −1 , they were performed on a LAND battery testing instrument, and when the current was 5-20 A g −1 , the GCD measurements were completed on the Bio-Logic VMP3 electrochemical station. (This is because for fast charge/discharge tests, Bio-Logic VMP3 electrochemical station is more sensitive and accurate.)

Material and Electrode Characterizations
We used scanning electron microscopy (SEM; model: Zeiss Supra 55VP) and transmission electron microscopy (TEM; 1 3 model: Tecnai G2 F30) to observe the micromorphologies of samples and used a Brunauer-Emmett-Teller (BET) analyzer to characterize the specific surface area. X-ray diffraction (XRD; model: Bruker D8 Discover Diffractometer) and X-ray photoelectron spectroscopy (XPS; model: MDTC-EQ-M20-01) were applied to analyze the phase and compositions. Thermogravimetric (TG)-differential scanning calorimeter was utilized to determine the water content in amorphous RuO 2 ·xH 2 O powder. Note that to characterize the electrodes at various charge/discharge states, corresponding cells were charged/discharged, then disassembled, and washed using deionized water five times to remove surfaceadsorbed electrolyte. Figure 1 shows the physicochemical characteristics of the RuO 2 ·xH 2 O sample. The RuO 2 ·xH 2 O is irregular-shaped particles with a size of about 100-500 nm (Fig. 1a). Its selected-area electron diffraction (SAED) image (inset in Fig. 1b) exhibits a characteristic halo ring pattern, revealing the amorphous feature of the RuO 2 ·xH 2 O. Correspondingly, the high-resolution TEM image does not show clear lattice fringes (Fig. S1). The amorphous feature of the RuO 2 ·xH 2 O is also confirmed by the XRD result (Fig. 1c), from which only several broad diffraction peaks are observed. To determine the structural water content in the RuO 2 ·xH 2 O, TG analysis was performed, as shown in Fig. 1d. Mass loss in the temperature range of 100-300 °C originates from the structural water of the RuO 2 ·xH 2 O [31], which is ~ 12.4 wt%. This means that x in the RuO 2 ·xH 2 O is 1.0. Therefore, the RuO 2 ·xH 2 O is denoted as RuO 2 ·H 2 O. In the XPS spectrum of Ru 3d (Fig. 1e), the Ru 3d 3/2 peak at 285.3 eV and Ru 3d 5/2 peak at 281.1 eV are observed, corresponding well to the previously reported hydrous RuO 2 [32,33]. Ru 3p XPS spectrum shown in Fig. S2 (Fig. S3). In the CV curves, there is one pair of broad redox peaks. Even at high scan rates such as 100 mV s −1 , the redox peaks remain, suggesting good rate performance of the RuO 2 ·H 2 O||Zn system. As shown in the GCD profiles (Fig. 2b), the charge curves and discharge curves deviate from linear shapes without flat voltage plateaus. This is consistent with the broad redox peaks observed in the CV curves. At a charge/discharge current of 0.1 A g 1 , the RuO 2 ·H 2 O cathode shows a discharge capacity of 122 mAh g −1 with a coulombic efficiency of 86%. When the current increases for 200 times (to 20 A g −1 ), in which the RuO 2 ·H 2 O||Zn system is charged/discharged within 36 s, the discharge capacity still reaches 98 mAh g −1 . In fact, considering that the coulombic efficiency of the RuO 2 ·H 2 O||Zn system at low current densities of 0.1-1 A g −1 and high current densities of 3-20 A g −1 is 86-98% and 99-100%, respectively, the RuO 2 ·H 2 O||Zn system is more suitable for fast charging/ discharging. For comparison, rate performance of some typical cathode materials for Zn 2+ storage is summarized in Fig. 2c [34], polyaniline [28], and AC [11]. Figure 2c intuitively shows the excellent rate capability of the RuO 2 ·H 2 O cathode, compared with the other cathode materials. It should be noted that the RuO 2 ·H 2 O exhibits similar superior performance in 2 M ZnSO 4 aqueous electrolyte (Fig.  S4). Furthermore, according to the Ragone plot shown in Fig. 2d, the RuO 2 ·H 2 O cathode can provide a maximum energy density of 119 Wh kg −1 . More importantly, it keeps a high energy density of 82 Wh kg −1 under the condition of delivering an ultrahigh power output of 16.74 kW kg −1 . Such a high power output with considerable energy density is almost impossible for most of the current electrochemical energy storage systems [9,16,35]. For instance, the maximum power density of currently reported lithiumion batteries and aqueous ZIBs is generally smaller than 1-10 kW kg −1 .

Results and Discussion
The kinetic analysis was performed to reveal the mechanisms for the superior electrochemical performance of the RuO 2 ·H 2 O cathode. Zn 2+ storage in the RuO 2 ·H 2 O cathode was firstly confirmed by the high-resolution Zn 2p XPS spectra shown in Fig. 3. Zn 2+ ions were stored in the RuO 2 ·H 2 O cathode when the cathode was discharged from pristine state to 0.4 V, and almost all Zn 2+ ions were extracted from the RuO 2 ·H 2 O cathode when the cathode was further charged to 1.6 V, implying highly reversible Zn 2+ storage in the RuO 2 ·H 2 O cathode. Besides, H + from the slightly acid Zn(CF 3 SO 3 ) 2 aqueous electrolyte is also proved to participate in the electrochemical reactions in the RuO 2 ·H 2 O//Zn system and contributes to a small capacity to the RuO 2 ·H 2 O cathode (Figs. S5-S7). For the CV curves at various scan rates of the RuO 2 ·H 2 O||Zn system (Fig. 2a), the relationship between their peak current (i) and scan rate (v) can be depicted through Eq. 1 [36]: where a and b are variable parameters. Particularly, b values of 0.5 and 1.0 represent a diffusion-controlled process and a complete capacitive process, respectively [33]. As shown in Fig. 4a, the b values for the anodic peaks and cathodic peaks are close to 1.0, suggesting that Zn 2+ storage in the RuO 2 ·H 2 O cathode is dominated by a capacitive process.
We further tested CV curves at low scan rates (Fig. 4b). At 0.2-1 mV s −1 , the voltage separation between anodic peaks and cathodic peaks is very small (< 0.08 V), which is a typical feature of pseudocapacitive behavior [37]. As a comparison, ZIB cathode materials such as MnO 2 generally possess a large voltage separation (> 0.3 V; Fig. S8). Furthermore, two capacitance differentiation methods were applied to analyze the pseudocapacitive reaction of the RuO 2 ·H 2 O for Zn 2+ storage. According to Dunn's method (Fig. 4c, d) [37], 79.0-96.4% capacitance originates from the surfacecontrolled capacitive process, i.e., redox pseudocapacitance and electric double-layer capacitance. Considering that the specific surface area of the RuO 2 ·H 2 O is only 57 m 2 g −1 (Fig.  S9), the majority of the capacitance is redox pseudocapacitance, while the electric double-layer capacitance accounts a small fraction. Trasatti's method analysis in Fig. 4e, f points out that the maximum charge that can be stored in the RuO 2 ·H 2 O and the charge stored at the so-called outer surface (easily accessible to electrolyte ions) of the RuO 2 ·H 2 O are 502.5 and 428.4 C g −1 , respectively [38]. This means that 85.3% capacity is from the outer surface, which is consistent with the Dunn's method analysis. Such an energy storage mechanism of redox pseudocapacitive behavior, as well as high conductivity of hydrous ruthenium oxides (higher than 100 S cm −1 ) [31], benefits for the ultrafast charging/ discharging of the RuO 2 ·H 2 O cathode [37]. It should be emphasized that the structural water in the RuO 2 ·H 2 O plays a vital role in Zn 2+ storage. As a comparison, anhydrous RuO 2 sample was synthesized by heat-treating the RuO 2 ·H 2 O in air (Figs. 5a, b and S10). The TG curve confirms that after heat treatment, structural water content of the sample is negligible. As for the anhydrous RuO 2 , its redox pseudocapacitive reactions are notably suppressed, corresponding to very low Zn 2+ -storage capacities of 38-15 mAh g −1 at 0.1-20 A g −1 (Figs. 5c, d and S11, S12), even though anhydrous RuO 2 generally possesses a higher electrical conductivity than hydrous RuO 2 ·xH 2 O [31]. This is because the structural water can facilitate rapid ion transport in the RuO 2 ·H 2 O [34]. Similarly, hydrous ruthenium oxides perform much better than anhydrous RuO 2 in supercapacitors with H 2 SO 4 aqueous electrolytes [31].
Besides the excellent high rate performance, the amorphous RuO 2 ·H 2 O cathode also exhibits superior long-term cyclic stability, with an 87.5% capacity retention over 10,000 charge/discharge cycles (Fig. 6a). Meanwhile, the coulombic efficiency always maintains ~ 100% during the cycling test (except for the initial tens of cycles). Nyquist plots in Fig. 6b of the RuO 2 ·H 2 O||Zn hybrid capacitor reveal a small chargetransfer resistance even after the 10,000 charge/discharge cycles. In addition, the long-term cycling test does not cause an obvious change in the phase composition and micromorphology of the amorphous RuO 2 ·H 2 O cathode ( Fig. 6c and S13). These imply the high electrochemical and structural stability of the RuO 2 ·H 2 O cathode during repeated Zn 2+ storage processes.    Fig. 6 a Cycling stability test at 20 A g −1 of the RuO 2 ·H 2 O||Zn system. b Nyquist plots and c XRD patterns of the RuO 2 ·H 2 O cathode before and after the cycling test