High-Power and Ultralong-Life Aqueous Zinc-Ion Hybrid Capacitors Based on Pseudocapacitive Charge Storage
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.
KeywordsZinc-ion hybrid capacitor Hydrous ruthenium oxide Ultralong life Redox pseudocapacitance High power
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, multivalent-ion (e.g., Zn2+, Ca2+, Mg2+, and Al3+) 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 ZnSO4 solutions in ZIBs and ZICs is beneficial for achieving high power output. The electrochemical properties of ZIBs and ZICs are strongly dependent on the Zn2+-storage behaviors in cathode materials.
Several cathode materials have been developed for ZIBs and ZICs, including manganese oxides, vanadium oxides, Prussian blue analogs, conductive polymers, and carbon materials. Zn2+ insertion/extraction in manganese oxides, especially tunnel-structured MnO2, creates high specific capacities. However, the poor electrical conductivity of manganese oxides and manganese dissolution issues cause unsatisfactory rate performance and poor cycling stability [12, 13, 14, 15]. Vanadium oxides possess high capacities and fast kinetics for Zn2+ storage [16, 17, 18, 19, 20, 21, 22], whereas their high toxicity impedes their practical applications. Besides, most of the Prussian blue analogs show low capacities of about 50 mAh g−1 when used as cathode materials for ZIBs [23, 24, 25, 26]. Although conductive polymers (e.g., polyaniline and polypyrrole) and carbon materials (e.g., activated carbon, denoted as “AC”) generally have a Zn2+-storage capacity of 100–150 mAh g−1 and better rate performance compared to manganese oxides [11, 27, 28, 29, 30], their low density of about 0.3–1 mg cm−2 is unfavorable for the volumetric energy density of corresponding batteries. Therefore, seeking high-performance Zn2+-storage materials is still a big challenge.
Herein, for the first time, we demonstrate that fast, ultralong-life, and safe Zn2+ storage can be realized in amorphous RuO2·H2O cathode materials based on a pseudocapacitive storage mechanism. The constructed RuO2·H2O||Zn ZICs can reversibly store Zn2+ in a voltage window of 0.4–1.6 V (vs. Zn/Zn2+), delivering a capacity of about 122 mAh g−1, an excellent rate capability and an ultralong cycle life exceeding 10,000 cycles.
2.1 Electrochemical Measurements
Amorphous ruthenium oxide hydrate (RuO2·xH2O) powder was obtained from Sigma-Aldrich Corporation. To synthesize anhydrous RuO2, the RuO2·xH2O powder was heat-treated in air at 300 °C for 1 h with a heating rate of 5 °C min−1. The amorphous RuO2·xH2O power (or anhydrous RuO2 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 RuO2·xH2O (or RuO2) 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 Zn2+ storage was evaluated by assembling CR2032 coin cells, in which RuO2·xH2O (or RuO2) 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(CF3SO3)2 or 2 M ZnSO4 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.)
2.2 Material and Electrode Characterizations
We used scanning electron microscopy (SEM; model: Zeiss Supra 55VP) and transmission electron microscopy (TEM; 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 RuO2·xH2O 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 surface-adsorbed electrolyte.
3 Results and Discussion
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 . As a comparison, ZIB cathode materials such as MnO2 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 RuO2·H2O for Zn2+ storage. According to Dunn’s method (Fig. 4c, d) , 79.0–96.4% capacitance originates from the surface-controlled capacitive process, i.e., redox pseudocapacitance and electric double-layer capacitance. Considering that the specific surface area of the RuO2·H2O is only 57 m2 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 RuO2·H2O and the charge stored at the so-called outer surface (easily accessible to electrolyte ions) of the RuO2·H2O are 502.5 and 428.4 C g−1, respectively . 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) , benefits for the ultrafast charging/discharging of the RuO2·H2O cathode .
In summary, amorphous RuO2·H2O was employed to achieve fast, ultralong-life, and safe Zn2+ storage. In the RuO2·H2O||Zn zinc-ion hybrid capacitors with aqueous Zn(CF3SO3)2 electrolyte, the RuO2·H2O cathode reversibly stores Zn2+ in a voltage window of 0.4–1.6 V (vs. Zn/Zn2+), displaying a discharge capacity of 122 mAh g−1 and an outstanding high rate performance. The zinc-ion hybrid capacitors can be rapidly charged/discharged within 36 s, in which case a very high power density of 16.74 kW kg−1 and a high energy density of 82 Wh kg−1 are delivered. Such an excellent high rate performance originates from redox pseudocapacitive reactions of the RuO2·H2O by storing Zn2+. Besides, the zinc-ion hybrid capacitors exhibit superior cycling stability with 87.5% capacity retention over 10,000 charge/discharge cycles. This work could greatly facilitate the development of ultrafast and safe aqueous electrolyte-based electrochemical energy storage.
The authors acknowledge the financial support by the Australian Research Council through the ARC Discovery projects (DP160104340 and DP170100436) and Rail Manufacturing Cooperative Research Centre (RMCRC 1.1.1 and RMCRC 1.1.2 projects). This work was also financially supported by the International Science & Technology Cooperation Program of China (No. 2016YFE0102200) and Shenzhen Technical Plan Project (No. JCYJ20160301154114273).
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