Recently, with the rapid development of consumer electronics, electric vehicles, and new energy systems, the requirement for high-property energy storage equipment has shown explosive growth [
1,
2]. Among them, Li-ion batteries (LIBs) have become the most extensively utilized commercial energy storage systems on account of their high energy density and high energy efficiency. However, the scarcity of lithium resources and safety problems have severely limited their further exploitation and application [
3,
4]. Among the new energy storage systems, aqueous Zn-ion hybrid capacitors (ZHCs) have drawn extensive concern from researchers owing to their large theoretical capacity (820 mAh/g), splendid stability, and low redox potential (−0.76 V
vs. SHE) [
5,
6]. Unfortunately, the relatively small energy density of ZHCs impedes their practical applications. Generally, the zinc anodes have high theoretical specific capacitance, while the capacitive cathode materials have lower specific capacitance. According to the short-board effect, the design and development of high-capacitance cathode materials are crucial for boosting the energy density of ZHCs.
Carbon-based capacitive cathode materials, including porous carbon [
7,
8], activated carbon [
9], carbon nanotubes [
10], metal organic framework (MOF) derived carbon [
11-
13], and graphene [
14-
16], have been widely applied in ZHCs. Among them, researchers have modified carbon-based materials through strategies such as structural design and surface modification, which significantly improve their specific surface area, electronic conductivity and electrochemical reactivity, shorten their ion transport paths, and effectively improve their electrochemical performance [
17,
18]. However, the ZHCs with carbon-based cathode [
19] still exhibit limited energy density arising from the energy storage process dominated by the electric double-layer (EDL) capacitance. In view of this, the exploitation of pseudocapacitive cathode materials with large theoretical specific capacitance is a good strategy to further boost the energy density of ZHCs.
Various typical pseudocapacitive cathode materials, including MXenes [
20,
21], conductive polymers [
22], and transition metal oxides [
23], exhibit higher theoretical specific capacitance due to their unique Faradaic reaction mechanisms. These materials are able to undergo reversible redox reactions with zinc ions, thereby storing more charge and enhancing the energy density of ZHCs. Among various pseudocapacitive cathode materials, RuO
2, especially hydrated amorphous RuO
2, is regarded as an ideal energy storage material due to its large theoretical specific capacitance, splendid stability, and highly reversible electrochemical oxidation/reduction characteristics compared with other cathode materials [
23]. Unfortunately, the insufficient electrochemical reaction between RuO
2 and the electrolyte results in limited specific capacitance, as well as poor rate performance and cyclic lifespan. The RuO
2 nanomaterials have been proved to have the characteristics of small particle size and high surface/volume ratio, which can sufficiently improve the utilization of electrode materials and enhance their electrochemical redox behavior [
24]. However, the high surface energy of nanomaterials can cause severe agglomeration, resulting in the inhomogeneity and densification of morphology and structure, thereby seriously affecting their electrochemical performance. The combination of RuO
2 nanomaterials and reduced graphene oxide (rGO) is a feasible method to prevent their agglomeration [
25]. The introduction of rGO can reduce the proportion of RuO
2 in the composite, consequently decreasing its cost. Moreover, rGO has excellent electronic conductivity and can be used as a conductive matrix to boost the conductivity and electrochemical reactivity of RuO
2, thus boosting the overall electrochemical performance of the composite. Therefore, the construction of RuO
2 nanodots (NDs)/rGO composite is a feasible strategy to achieve efficient energy storage of ZHCs. As we know, there is no study on the application of RuO
2 NDs/rGO in ZHCs, and its energy storage mechanism of RuO
2-based pseudocapacitive cathode materials in ZHCs has not been reported yet. Therefore, it is highly significant to study the electrochemical behavior and energy storage mechanism of the RuO
2 NDs/rGO cathode in ZHCs.
In this work, the new RuO2 NDs/rGO composite was prepared through sol-gel method combined with low-temperature anneal treatment, and utilized as a cathode for ZHC for the first time. Thanks to the synergism of nanoscale design and composite engineering, the assembled RuO2 NDs/rGO//Zn ZHCs based on the RuO2 NDs/rGO cathode exhibits large specific capacitance (169.5 mAh/g at 0.1 A/g), superior rate property (74.4 mAh/g at 20 A/g), remarkable cyclic performance (90.4% capacity retention after 10,000 cycles at 5 A/g), large energy and power densities (101.7 Wh/kg at 60 W/kg and 41.7 Wh/kg at 12 kW/kg). More impressively, the energy storage mechanism of the EDL and pseudocapacitive reactions of electrolyte ions in the RuO2 NDs/rGO cathode was elucidated for the first time through electrochemical kinetic analyses based on kinetic analyses (cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS)) and systematic ex-situ tests (X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and corresponding energy-dispersive X-ray spectroscopy (EDS), EIS, and X-ray photoelectron spectroscopy (XPS)). This research not only affords a good strategy for the design and development of ZHCs pseudocapacitive cathode, but also elucidates the energy storage mechanism of the RuO2 NDs/rGO cathode for the first time.
The synthesis process of the RuO
2 NDs/rGO composite is presented in
Fig. 1a. The RuO
2 NDs/rGO composite was prepared by sol-gel method combined with low-temperature calcination treatment using RuCl
3·nH
2O and GO as precursors. The morphology, microstructure and elemental composition and distribution of the prepared RuO
2 NDs/rGO composite were evaluated by FESEM, (high-resolution) transmission electron microscopy (TEM/HRTEM) and elemental mapping tests. As presented in the FESEM images, compared with the agglomerated pure RuO
2 (
Fig. 1b and Fig. S1 in Supporting information), the RuO
2 NDs/rGO composite presents a rGO sheet structure with some folds and the presence of uniformly distributed RuO
2 nanodots on the rGO sheet structure (
Fig. 1c and Fig. S2 in Supporting information). By the further TEM test (
Fig. 1d), it is observable that the RuO
2 nanodots are even distributed on the rGO sheets with a size of 5–8 nm. As presented in the HRTEM image (
Fig. 1e), a lattice spacing of 0.26 nm can be observed, belonging to the (101) crystal plane of RuO
2, which is consistent with XRD analysis results [
26]. Moreover, the co-presence and uniform distribution of Ru, O, and C elements in the RuO
2 NDs/rGO composite are clearly observed in the elemental mapping test, as demonstrated in Fig. S3 (Supporting information). Besides, N
2 adsorption-desorption test (Fig. S4 in Supporting information) reveals that the RuO
2 NDs/rGO composite possesses a high BET surface area of 54.9 m
2/g and abundant mesopores in the range of 2.7–4.2 nm, which is conducive to zinc storage and fast ion transport.
In addition, the structure of the pure RuO
2 and RuO
2 NDs/rGO composite was examined by XRD test. As shown in
Fig. 1f, it is clear that no strong characteristic peaks of RuO
2 crystal planes are observed, and two weak and broad diffraction peaks emerge at 2
θ = 32.9° and 56.2°, corresponding to the (101) and (220) planes of RuO
2, respectively, which indicates that the RuO
2 formed during the low-temperature annealing at 150 ℃ is typically amorphous [
27,
28]. The TG curves of the pure RuO
2 and RuO
2 NDs/rGO composite are shown in
Fig. 1g, from which the mass percentage of rGO in the RuO
2 NDs/rGO composite is calculated to be approximately 10 wt%. In order to further investigate the elemental composition and chemical valence of the RuO
2 NDs/rGO composite, XPS test was executed. As manifested in
Fig. 1h, the survey spectrum of the RuO
2 NDs/rGO composite confirms the existence of Ru, O, and C elements. In the high-resolution C 1s/Ru 3d spectrum (
Fig. 1i), the peaks situated at 287.2, 284.8, 282.2, and 281.1 eV are indexed to the C—O-H, C—C, Ru 3d
3/2, and Ru 3d
5/2 [
29,
30], respectively, and the peak position of Ru 3d
5/2 indicates the existence of Ru
4+, further verifying the successful preparation of RuO
2 [
31]. Additionally, in the high-resolution Ru 3p spectrum (
Fig. 1j), the main peaks situated at 484.8 and 462.6 eV match with Ru 3p
1/2 and Ru 3p
3/2, respectively, which belong to ruthenium (Ⅳ) oxide [
26]. The high-resolution O 1s spectrum (
Fig. 1k) shows three characteristic peaks at 532.1, 530.0, and 528.8 eV, which are relevant to bound water, Ru-O-H, and Ru-O-Ru, respectively [
32,
33].
The ZHC (denoted as rGO//Zn, RuO
2//Zn and RuO
2 NDs/rGO//Zn ZHC) was constructed utilizing Zn foil as the anode, rGO, RuO
2 and RuO
2 NDs/rGO as the cathode, and 2 mol/L ZnSO
4 solution as the electrolyte (
Fig. 2a). Fig. S5 (Supporting information) displays the CV profiles of the RuO
2 NDs/rGO//Zn ZHC at different voltages, indicating that they can operate stably in the voltage range of 0.4–1.6 V.
Figs. 2b and
c compare the CV profiles (5 mV/s) and GCD profiles (0.3 A/g) of the rGO//Zn, RuO
2//Zn and RuO
2 NDs/rGO//Zn ZHCs. Compared with rGO//Zn and RuO
2//Zn ZHC, the RuO
2 NDs/rGO ZHC presents larger CV curve area and higher reversible specific capacity. As presented in
Figs. 2d and
e and Fig. S6 (Supporting information), all GCD curves are approximately symmetric at current densities of 0.1–20 A/g, indicating that the ZHCs have good capacitive behavior. The specific capacities of the RuO
2 NDs/rGO//Zn ZHC at current densities of 0.1, 0.2, 0.3, 0.5, 1, 2, 3, 5, 10, and 20 A/g are 169.5, 158.5, 153.2, 147.9, 141.4, 133.0, 126.1, 114.7, 95.0, and 74.4 mAh/g, respectively, exhibiting larger specific capacity and better rate performance compared to rGO//Zn and RuO
2//Zn ZHC (
Fig. 2f), which is ascribed to the synergism of nanoscale design and composite engineering on the RuO
2 cathode [
34]. It is worth noting that the RuO
2 NDs/rGO//Zn ZHC exhibits an energy density of 101.7 Wh/kg @60 W/kg based on the RuO
2 NDs/rGO cathode, and still retains an energy density of 44.7 Wh/kg even at 12 kW/kg, outperforming most of previously reported ZHCs such as GH//Zn (76.2 Wh/kg @147.6 W/kg, 39 Wh/kg @6.54 kW/kg) [
35], AC//Zn (84 Wh/kg @66.6 W/kg, 30 Wh/kg @14.9 kW/kg) [
9], ACC//Zn (100 Wh/kg @43.9 W/kg, 20.4 Wh/kg @1.69 kW/kg) [
36], EG/PANI//POP-TAPP-NTCA (48 Wh/kg @85 W/kg, 12 Wh/kg @2.01 kW/kg) [
37], NPC//Zn (82 Wh/kg @167.7 W/kg, 35 Wh/kg @12.5 kW/kg) [
38], N, P-MXene//Zn (43.9 Wh/kg @117 W/kg, 19 Wh/kg @29 kW/kg) [
39], and H-MXene//Zn (53.6 Wh/kg @110 W/kg, 28 Wh/kg @2.3 kW/kg) [
40], as presented in
Fig. 2g.
Fig. 2h exhibits a small fan powered by two RuO
2 NDs/rGO//Zn ZHCs in series, confirming their potential practical application, which further indicates their superior energy storage property. In addition, the cycling performance of the RuO
2 NDs/rGO//Zn ZHC was tested at 5 A/g (
Fig. 2i). The capacity retention of the RuO
2 NDs/rGO//Zn ZHC reaches up to 90.4% after 10, 000 charge-discharge cycles, and its coulombic efficiency approaches 100% during the cycling process, demonstrating its remarkable cycling stability and reversibility.
The electrochemical kinetic performance of the RuO
2//Zn and RuO
2 NDs/rGO//Zn ZHCs was compared by EIS and CV tests. As exhibited in the EIS spectra (
Fig. 3a), the RuO
2 NDs/rGO//Zn ZHC presents a smaller internal resistance (
Rs, 0.84 Ω) and charge transfer resistance (
Rct, 63.9 Ω) than the RuO
2 electrode by fitting the equivalent circuit diagram, confirming its faster charge transfer kinetics. Meanwhile,
Fig. 3b presents the linear fitting diagram of Z'
vs. ω−1/2 in the low-frequency region, and the fitted Warburg coefficients (
σ) are 11.1 (the RuO
2 NDs/rGO//Zn ZHC) and 17.9 (the RuO
2//Zn ZHC), respectively. Based on this, the calculated diffusion coefficient of the RuO
2 NDs/rGO//Zn ZHC (4.36 × 10
–12 cm
2/s) is higher than that of the RuO
2//Zn ZHC (1.68 × 10
–12 cm
2/s), which exhibits the same level of ion diffusion capability as supercapacitors, confirming its excellent ion diffusion kinetics [
41].
Figs. 3c and
d show the CV profiles of the RuO
2//Zn and RuO
2 NDs/rGO//Zn ZHCs in the voltage range of 0.4–1.6 V at sweep rates varying from 2 mV/s to 100 mV/s, and a pair of wider redox peaks in the CV profiles are clearly observed. It is noteworthy that the redox peaks still exist at a high sweep rate of 100 mV/s, implying that the RuO
2 NDs/rGO//Zn ZHC has a good rate property. Furthermore, the electrolyte ion diffusion coefficient can be obtained by fitting the linear relationship between the peak current (
Ip) and the sweep rate (
v1/2) [
42-
44], as manifested in
Fig. 3e. The calculated ion diffusion coefficient of the oxidation and reduction peaks is 7.58 × 10
–12 and 4.51 × 10
–12 cm
2/s, respectively, which is consistent with the EIS kinetic analysis, further confirming the excellent ion diffusion kinetics of the RuO
2 NDs/rGO//Zn ZHC. Furthermore, the excellent electrochemical reaction kinetics of the RuO
2 NDs/rGO//Zn ZHC was further elucidated by a semi-qualitative analysis of the linear relationship between the peak current (
Ip) and the sweep rate (
v) [
42-
44]. As demonstrated in
Fig. 3f, the fitted
b-value of the oxidation and reduction peaks of the RuO
2 NDs/rGO//Zn ZHC is 0.86 and 0.83, respectively, which is higher than that of the RuO
2 electrode (0.80 and 0.79), indicating the more surface capacitive behavior and better electrochemical kinetics. Moreover, as shown in
Figs. 3g-i, the RuO
2 NDs/rGO//Zn ZHC exhibits higher capacitive contributions at various scan rates compared with the RuO
2 electrode, indicating the faster electrochemical kinetics and enhanced rate capability of the RuO
2 NDs/rGO electrode.
To explore the charge storage mechanism of the RuO
2 NDs/rGO//Zn ZHC, various
ex-situ tests (XRD, FESEM, EIS, and XPS) were executed on the RuO
2 NDs/rGO cathode and Zn anode at diverse states during the charge-discharge process. As manifested in
Fig. 4a, the RuO
2 NDs/rGO//Zn ZHC was first charged to 1.6 V (State A), and subsequently discharged to 1.0 V (State B) and 0.4 V (State C), followed by recharging to 1.0 V (State D) and 1.6 V (State E). During the charging and discharging process, the
ex-situ XRD patterns of the Zn anode show no additional diffraction peaks of other substances, except for variations in the intensity of a few diffraction peaks, indicating that only the electrochemical reaction of Zn
2+ deposition/stripping occurs on the anode. For the
ex-situ XRD patterns of the cathode (
Fig. 4b), it is observable that the main peak position of the RuO
2 NDs/rGO cathode does not change significantly during the electrochemical reaction, which indicates that the energy storage of the cathode is attributed to the insertion/extraction (or adsorption/desorption) of electrolyte ions. Meanwhile, it is observable that Zn
4SO
4(OH)
6·4H
2O (PDF #44–0673) is formed during the charge/discharge process [
45]. The intensity of its characteristic peaks reaches its maximum at fully discharged state (State C), and subsequently disappears after recharging back to State E (1.6 V), suggesting a reversible precipitation/dissolution process. Furthermore, the formation and dissolution of sheet-like substances during the charging/discharging process are also directly observed by
ex-situ FESEM images (
Figs. 4c-e), and the corresponding EDS (
Figs. 4f and
g) test reveals that the ratio of Zn and S elements in this product is about 4:1, which further confirms the generation of Zn
4SO
4(OH)
6·4H
2O. It is worth noting that the generation of Zn
4SO
4(OH)
6·4H
2O contributes less to the energy storage, and its formation is mainly related to the change of H
+/OH
- on the surface of the RuO
2 NDs/rGO cathode [
46], implying that H
+ or OH
- is also involved in the reaction process besides the adsorption/desorption of SO
42- on the cathode contributing to the EDL capacitance. In addition, as shown in
Fig. 4h, the
ex-situ EIS test indicates that
Rct gradually increases during the discharge process and decreases during the subsequent charge process, further confirming the reversibility of the precipitation/dissolution process of Zn
4SO
4(OH)
6·4H
2O.
To acquire a deeper comprehension of the ion storage mechanism on the RuO
2 NDs/rGO//Zn ZHC,
ex-situ XPS characterization was further employed to investigate the changes in surface chemical composition of the electrode at various states. As manifested in
Fig. 5a, the high-resolution Zn 2p spectra show that the electrode has a strong Zn characteristic peak at State C, which arises from the generation of Zn
4SO
4(OH)
6·4H
2O and the insertion of Zn
2+ during the discharge process [
47]. After charging to State E, the intensity of characteristic peak of Zn
4SO
4(OH)
6·4H
2O is significantly weakened arising from the dissolution of Zn
4SO
4(OH)
6·4H
2O and the extraction of most Zn
2+. The insertion of Zn
2+ is further confirmed in the high-resolution Ru 3p spectra (
Fig. 5b), and a new Zn characteristic peak appears upon discharging to State C, which is significantly weakened upon recharging to State E, again confirming the possible pseudocapacitive reaction due to the insertion/extraction of Zn
2+ on RuO
2. As manifested in the high-resolution C 1s spectra (
Fig. 5c), the presence of the characteristic peak of C—O-Zn is detected during the charging and discharging process, confirming the presence of chemical adsorption and desorption reactions between Zn
2+ and the oxygen-containing functional groups of rGO, which contributes to a certain amount of pseudocapacitance [
48]. As presented in the high-resolution O 1s spectra (
Fig. 5d), compared with the initial State A, the characteristic peak of Ru-O-H is significantly enhanced at fully discharged state, and returns to its initial level at fully charged state, which can be attributed to the pseudocapacitance generated by the H
+ insertion/extraction during the charging/discharging process, resulting in the improved specific capacitance of the electrode. In summary, the energy storage mechanism of the RuO
2 NDs/rGO//Zn ZHC is shown in
Fig. 5e.
In summary, the RuO2 NDs/rGO composite is prepared by sol-gel method combined with low-temperature calcination treatment, and utilized as a cathode for ZHC. Thanks to the synergistic effect of nanoscale design and composite engineering, the RuO2 NDs/rGO//Zn ZHC achieves large specific capacitance (169.5 mAh/g at 0.1 A/g), excellent rate property (74.4 mAh/g at 20 A/g), long cyclic property (90.4% capacity retention after 10,000 cycles at 5 A/g), and maximum energy and power densities (101.7 Wh/kg at 60 W/kg and 41.7 Wh/kg at 12 kW/kg). Moreover, systematic kinetic analyses based on CV and EIS tests elucidate the boosted ion transport kinetics of the RuO2 NDs/rGO//Zn ZHC compared with the RuO2//Zn ZHC. More encouragingly, systematic ex-situ XRD, FESEM, EDS, EIS, and XPS tests are utilized to reveal the energy storage mechanism of the EDL and pseudocapacitive reactions of electrolyte ions in the RuO2 NDs/rGO cathode. This study provides a good strategy for the design and exploitation of high-performance ZHCs pseudocapacitive composite cathode and reveals the energy storage mechanism of RuO2-based pseudocapacitive cathode for the first time.
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