Integrated Electrode‐Electrolyte Optimization to Manufacture a Real‐Life Applicable Aqueous Supercapacitor with Record‐Breaking Lifespan

Aqueous supercapacitors (SCs) have been regarded as a promising candidate for commercial energy storage device due to their superior safety, low cost, and environmental benignity. Unfortunately, an age‐old challenge of achieving both long electrode lifespan and qualified energy‐storage property blocks their practical application. Herein, we develop an electrode‐electrolyte integrated optimization strategy to fulfill the real‐life device requirements. Electrode optimization simultaneously regulates the nanomorphology and surface chemistry of the tungsten oxide anode, resulting in superior electrochemical performance given by an ideal “bird‐nest” structure with optimal oxygen vacancy status; the anodes interact with and are protected from dissolution and structural collapse by the rationally designed hybrid electrolyte with optimized pH and facilitated cation desorption behavior. Collaboratively, a record‐breaking durability of no capacitive decay after 250 000 cycles is achieved. On the basis of this integrated optimization, the first aqueous pouch SCs with real‐life practicability were manufactured by a soft‐package encapsulation technique, which can steadily power commercial 3 C products such as tablets and smartphones and maintain safely working against extreme conditions. This work demonstrates the possibility of using aqueous energy storage devices with enhanced safety and lower cost to replace the commercial organic counterparts for wide range of daily applications.


Introduction
Aqueous SCs, compared with commercial organic supercapacitors (SCs), present distinguished merits of non-flammability, non-toxicity, and low material and manufacturing cost. [1][2][3][4] It is worth noting that their superior safety attracts more and more attention due to the frequent accidents with organic electrochemical energy storage (EES) devices in recent years. Nonetheless, their energy density has not been comparable with practical organic SCs due to the confined voltage window caused by water decomposition. [5][6][7][8][9] It is difficult to improve the narrow voltage of aqueous SCs with low-cost electrolytes or simple techniques. [10] Based on E ¼ 1=2CV 2 , the energy density (E) can be improved by either increasing the voltage window (V) or by enlarging the specific capacitance (C). Accordingly, pseudocapacitive materials with higher capacitance, which combine electrical double-layer and Faradic reactions, were widely employed in aqueous SCs to replace the pure carbon materials adopted in conventional commercial SCs. [11,12] However, typical pseudocapacitive electrodes cannot simultaneously achieve long-term stability as well as high performance. [13] That is, lowcrystalline or amorphous pseudocapacitive materials usually achieve better cycling stability whereas highly crystalline counterparts have better charge storage performance. [14,15] Consequently, the balance of electrode service life and superior electrochemical properties has become the core bottleneck that hinders the practical application of aqueous SCs.
Tungsten oxide (WO 3-x ), as a series of complex materials with polymorphism and defect chemistry nature (0 ≤ x < 3), used to be extensively studied in optoelectronic regions. [16] Furthermore, tungsten oxide has been more adopted to EES devices owing to the high theoretical specific capacitance and multiple oxidation states. [17][18][19] The aqueous tungsten oxide-based electrodes typically suffered from the narrow potential window, and/or unsatisfied electrochemical performance with high mass loading, which restricted their applications for commercial SCs. [20,21] On the other hand, electrolyte plays a vital role in electrode stability in aqueous EES devices. Severe chemical dissolution and structural pulverization of electrode materials happen in the unfit electrolyte during charging-discharging cycles, bringing obvious capacity loss and electrochemical instability. [22,23] Thus, the well-matching of electrode and electrolyte is also a non-ignorable factor for aqueous SCs.
To acquire a more flexible size and lighter weight, pouch-type devices have been applied to the EES field by the soft-package encapsulating technique. [24] The energy density of pouch-type battery can be increased up to 50% compared to commercial shell-type battery. To date, some corporations, e.g., GM and Nissan, have even introduced pouch cell technics into the battery development of the electric vehicle. However, the quantitative study of pouch-type SC is very limited. The reported pouch SCs lack mature overall optimization for key components, which cannot operate real-life devices. [25,26] Consequently, it is very urgent to design a practical SC by rational integrated optimization.
In this work, a real-life applicable aqueous SC was fabricated, for the first time, by using the integrated electrode-electrolyte optimization ( Figure 1). First, the structural engineering of morphology, and stoichiometry by oxygen-vacancy regulation for tungsten oxide anode were synchronously optimized via a series of facile one-step hydrothermal reactions. Subsequently, constructing a hybrid electrolyte attenuates the activity of hydrogen evolution reaction (HER), resulting in a lower cutoff window. More importantly, a record-high anode operation life of 250 000 cycles was achieved by virtue of the unique pH effect and ionic adsorption mechanism in the optimized electrolyte. The fabricated pouch SCs can effectively charge smartphones or tablets, and also exhibit excellent safety and reliability when suffering from physical destruction.

Electrode Structure Optimization
In this work, two series of WO 3-x were prepared on a carbon cloth (CC) substrate via a facile hydrothermal synthesis method (see Experimental Section, Supporting Information). The samples are denoted as WO 3-x -Ny-Hz, according to the reactant content of NH 4 Ac and H 2 O 2 , i.e., Ny and Hz refer to y mg NH 4 Ac and z mL H 2 O 2 , respectively. To explore the function of various reagents on morphology, a multi-series of scanning electron microscopy (SEM) characterizations were conducted. Wherein, a set of successive-magnification images of WO 3-x -Ny-H8 samples reveals the morphology evolution from 2D nanosheets to 1D nanowires when adding the NH 4 Ac reagent. On the basis of the theory von Weimarn, their morphological formation can be assumed as a competition of nucleation reaction (i.e., horizontal growth) and crystal growth (i.e., vertical growth). [27] For WO 3-x -H8 (Figure 2a), the speed of nucleation reaction precedes than that of crystal growth, resulting in the horizontal-orientation growing of WO 3-x into 2D nanosheets. Accompanying with the increase of NH 4 Ac content, 1D vertical-orientation growing of WO 3-x took the lead gradually, in which WO 3-x -N7-H8 (Figure 2b) shows transitional morphology from nanosheets to nanowires. Notably, WO 3-x -N14-H8 (Figure 2c) presents a porous and robust 3D "bird-nest" structure entirely built with 1D nanowires. However, this "bird-nest" structure collapses with higher content NH 4 Ac reactant since the 1D nanowires are too slim and loose to establish an effective structural framework as exhibited in the WO 3-x -N56-H8 sample displayed in Figure 2d. From these cases, it is indicated that NH 4 Ac can effectively manipulate the morphology and structure of WO 3-x by attenuating the speed of the nucleation reaction. On the other hand, H 2 O 2 shows a negligible function on the structural engineering of WO 3-x -N14-Hz samples, which all represented a similar "bird-nest" structure ( Figure S1, Supporting Information).
The detailed growth process of the morphology of WO 3-x was examined in three time-dependent experiments to determine the Notably, the growth speed of these materials in the vertical 1D direction kept synchronously, which can be identified from the overall contrast of SEM images versus time. After 30 min, the WO 3-x particles began to appear on the CC surface, in which WO 3-x -N56-H8 showed a much smaller size than those of the others. After 45 min, nanosheets (WO 3-x -H8) and nanorods (WO 3-x -N14-H8) have totally covered the surface of the CC substrate. Thereafter, the evolution of the growing structures concentrated on the length dimension. For example, WO 3-x -N14-H8 developed from a "coral reef" formed with dense nanorods to the extremely porous "bird-nest" structure comprising nanowires, which is extremely beneficial for electrolyte penetration. In the case of WO 3-x -N56-H8, the ultimate structure is limited by the smaller diameter and density of nanowires; a longer reaction time (over 60 min) was required for its nanowires to cover the substrate surface completely. Accordingly, its architectural foundation was formed with longer and looser nanowires, which resulted in eventual structural collapse at the end of the hydrothermal reaction, leading to a lower porosity "compressed mat" structure than that of WO 3-x -N14-H8. The above phenomena further demonstrate that NH 4 Ac indeed restrains the nucleation rate of WO 3-x on the surface. In addition, this capability to inhibit nucleation is stronger with a higher concentration of NH 4 Ac reactant. Beyond a hydrothermal reaction time of 180 min, the morphology, architecture, and mass loading of WO 3-x change little. Thus, given the observations from SEM experiments, 3 h is considered to be an appropriate hydrothermal duration.
To highlight the best-performance sample, the following study of material characterization focused on WO 3-x -N14-H8 with a 3D porous "bird-nest" structure. Energy dispersive X-ray spectroscopy elemental mapping images indicates the uniform distribution of W and O elements in the WO 3-x -N14-H8 material. In addition, the scattered C elements that come from the CC substrate can be clearly observed after WO 3-x -N14-H8 fully covers the surface, verifying its structural uniformity and porosity (Figure 2e). A transmission electron microscopy (TEM) image shows the diameter of the nanowires is equal to about 9 nm (Figure 2f). The lattice fringe spacings of 0.37 and 0.39 nm can be indexed to (110) and (001) planes of the hexagonal-WO 3-x crystal (JCPDS No. . Furthermore, the sharp and orderly diffraction spots in the selected area electron diffraction (SAED) pattern reveal the single crystalline nature of the WO 3-x -N14-H8 material and reconfirm the existence of (110) and (001) planes ( Figure 2g). The TEM and SAED results of the WO 3-x -N14-H8 sample are consistent with that of its X-ray diffraction (XRD) pattern ( Figure 2h).

Oxygen Vacancy Regulation
The stoichiometric WO 3 crystal consists of corner-edge sharing WO 6 octahedra unit representing a cubic framework ( Figure 3a). However, this ideal structure has been proved to be extremely unstable due to the unavoidable tilting and rotation of WO 6 octahedra in practical experiments. [16] Furthermore, nonstoichiometric WO 3-x would manifest stronger metallicity and enhanced conductivity with the raising of oxygen vacancy concentration. [21] Accordingly, our work took advantage of the reducibility of H 2 O 2 to regulate the oxygen vacancy of the WO 3-x samples.
The XRD patterns of WO 3-x -Ny-H8 and WO 3-x -N14-Hz series samples all match well with the standard XRD card of JCPDS No. 33-3187 of h-WO 3-x (Figure 2h). [28] It is worth noting that the crystal structure of WO 3-x -H8 (without NH 4 Ac additive) is identical to those of other samples, indicating that NH 4 Ac had not directly participated in the redox reaction of WO 3-x formation as a reactant. Consequently, the speculated hydrothermal reaction for generating WO 3-x is as follows: wherein NH 4 A C plays the role of a template reagent (TMPL*) affecting morphology development and H 2 O 2 acts as a reducing agent. To validate the modification of H 2 O 2 for the oxidation state of WO 3-x , a set of X-ray photoelectron spectroscope (XPS) tests for WO 3-x -N14-Hz samples was conducted (Figure 3d-k). The XPS survey spectrum of WO 3-x -N14-H8 reveals that the sample is comprised of three element components (W, O, and C) without other unmistakable impurities ( Figure S3, Supporting Information). The core level W 4f spectra of all samples were well fitted with two W 6+ and two W 5+ subpeaks. The peak centers of W 6+ locate at 35.75 and 37.85 eV, respectively, corresponding to 4f 7/2 and 4f 5/2 . [29,30] The fitted peaks centered at~34.85 and 36.9 eV are attributed to 4f 7/2 and 4f 5/2 of W 5+ , respectively. [31] The calculated contents of W 6+ and W 5+ , along with their compositions, are both based on the corresponding peak area sum ( Figure 3b and  structural water. [32][33][34] As expected, the WO 3-x -N14-H8 sample shows the maximal O 2 percentage (Figure 3c and Table S2, Supporting Information), i.e., defect oxygen, in agreement with the suggestions of the W 4f spectra. The electron paramagnetic resonance (EPR) spectrum of WO 3-x -N14-H8 further confirms the existence of oxygen vacancy, which exhibits a representative EPR signal of oxygen vacancy at a g value of 2.003 ( Figure S4, Supporting Information). [35] Importantly, it is generally accepted that oxygen defects with higher concentration, which serve as shallow donors, can theoretically promote the charge storage capability of metal oxides by improving the carrier density. [36,37]

Electrolyte Optimization
Based on electrode optimization, the electrolyte collaborative optimization can further promote the electrochemical performance of EES device, e.g., Wang and co-workers reported that a tailored electrolyte reduced the solubility and mobility of LiPSs in Li-S battery, leading to an improved cathode cycling stability. [38] Furthermore, constructing a hybrid electrolyte with a rational configuration has been proved to effectively enlarge the electrode potential window in aqueous SCs. [39] Therefore, it is necessary to design an optimized electrolyte suitable for tungsten oxide electrodes.
Our electrochemical tests were conducted in a three-electrode cell, which included WO 3-x -Ny-Hz as the working electrode, graphite foil as a counter electrode, saturated calomel electrode (SCE) as a reference electrode, and an optimized hybrid electrolyte. In typical acidic electrolyte, the lower cutoff potentials of tungsten oxide-based electrodes were limited to~−0.6 V (vs SCE), since HER starts to occur under lower potential. [40] On the other hand, HER can be pushed to lower potential in a neutral environment. However, no satisfactory capacitive and cycling performance was achieved. To address the limitation of lower cutoff potential for tungsten oxide anodes in SCs without sacrificing the electrochemical performance, a novel mixed electrolyte, which combines acidic H 2 SO 4 and neutral Na 2 SO 4 , was employed. To determine the optimized composition of the mixed electrolyte, our experiments first estimated the HER activity of the WO 3-x -N14-H8 electrode in H 2 SO 4 electrolytes with different concentrations, which were tested by cyclic voltammetry (CV) between 0 and −0.8 V (Figure 4a).
The clear HER polarization appears below −0.6 and −0.7 V in 1 M and 0.5 M H 2 SO 4 electrolytes, respectively. In contrast, the CV profile in 0.1 M H 2 SO 4 electrolyte shows quasi-rectangle without obvious polarized deformation over the whole voltage range. For the sake of stabilizing this extensive potential window, the mixed electrolyte of 0.1 M H 2 SO 4 and 1 M Na 2 SO 4 was applied to obtain a more neutral pH circumstance, which is beneficial to maintain a lower HER potential (i.e., lower cutoff potential). Figure 4b compares the CV performances of the WO 3-x -N14-H8 electrode in 0.1 M H 2 SO 4 , 1 M Na 2 SO 4 , and 0.1 M H 2 SO 4 and 1 M Na 2 SO 4 hybrid electrolyte. The CV curve in 1 M Na 2 SO 4 indicates a steady potential window, however, much smaller capacity than that in 0.1 M H 2 SO 4 , corresponding to previous reports. [20] Remarkably, the CV curve in the hybrid electrolyte displays a stronger redox couple and larger capacitance than that in the 0.1 M H 2 SO 4 case due to increased cation concentration from Na 2 SO 4 doping suggesting better pseudocapacitive behavior. Meanwhile, a more stable potential window is observed thanks to no polarization phenomenon emerging. Eventually, a wide voltage range (−0.8 to 0 V) with excellent electrode capacitive performance is realized by means of the electrolyte optimization, that is, in the 0.1 M H 2 SO 4 and 1 M Na 2 SO 4 hybrid electrolyte case (Figure 4c).

Electrochemical Assessment
To gain insight into the charge storage behavior of WO 3-x electrodes in this optimized electrolyte, CV, and galvanostatic charge-discharge (GCD) tests were systematically conducted on the entire WO 3-x -Ny-H8 and WO 3-x -N14-Hz series electrodes. It was found that all the WO 3-x -Ny-Hz electrodes exhibit well pseudocapacitive behavior identified from the symmetrical redox peak in CV profiles, illustrated in Figure  5a,d. Among these electrodes, WO 3-x -N14-H8 performs at maximal capacity in both series electrodes, while the capacitance contribution of the CC substrate can be negligible ( Figure S5, Supporting Information). Likewise, the GCD profiles collected at different current densities also confirm this result ( Figure S6 and Figure S7, Supporting Information). The calculated areal capacitances via discharge time of GCD experiments are shown in Figure 5b,e. The results reveal that the WO 3-x -N14-H8 sample outperforms the others under each current density and delivers a superior areal capacitance of 4.28 F cm −2 (equivalent to the gravimetric capacitance of 522.74 F g −1 standardized to 8.19 mg cm −2 ) at 5 mA cm −2 . Furthermore, the WO 3-x -N14-H8 electrode maintains an outstanding rate capability of 60.18% from 5 to 30 mA cm −2 even at such a high mass loading, as well as excellent coulombic efficiencies ( Figure S8, Supporting Information), suggesting highly effective ion diffusion and charge transfer. Undoubtedly, the interconnected internal voids in the highly porous "bird-nest" structure of WO 3-x -N14-H8, as shown above, provide an effective pathway for sufficient immersion of electrolyte into the mass active material, which is the primary factor of rapid ionic transformation. Figure 5c,f compare the effect of NH 4 AC and H 2 O 2 on mass loading, areal capacitance, and gravimetric capacitance at the same current density of 5 mA cm −2 , respectively. The mass loading of WO 3-x -Ny-H8 electrodes decreases with NH 4 AC concentration rising due to the restriction of NH 4 AC on WO 3-x nucleation as discussed above. When the additive content of NH 4 AC reaches 14 mg (WO 3-x -N14-H8), both areal and gravimetric capacitances achieve peak values, while still holding a rather high mass loading of 8.19 mg cm −2 (Figure 5c). To the best of our knowledge, no tungsten oxide-based electrode with higher mass loading has been reported before. Therefore, the N14 case (i.e., 14 mg NH 4 AC) is deemed to be the desirable NH 4 AC concentration for structural engineering. As for the stoichiometric modification by oxygen vacancy regulation, it is interestingly observed that H8 (i.e., 8 mL H 2 O 2 ) not only provides the supreme reducibility to obtain more oxygen defects but also the most active growth of WO 3-x on the CC substrate, namely, the highest mass loading among WO 3-x -N14-Hz electrodes (Figure 5f). Furthermore, the WO 3-x -N14-H8 electrode also exhibits much better capacitive performance than those of WO 3 without oxygen vacancy ( Figures S5 and S9, Supporting Information), in which the WO 3 sample was effectively obtained by the oxidation treatment of WO 3-x -N14-H8 ( Figure S10, Supporting Information). To sum up, the best capacitive performance with the H8 condition verifies our prediction that more oxygen defects cause improved electrical conductivity and charge storage behavior.
To further study the charge storage mechanism and electrode kinetics, the capacitance contribution of the WO 3-x -N14-H8 electrode was quantitatively separated into capacitive-controlled and diffusioncontrolled sections by Dunn's method, [41,42] in which the former comprises electrical double-layer and fast faradaic capacity from electrode surface or subsurface whereas the latter comes from ion slow de/intercalation of the bulk phase structure. [43] As a result, the capacitivecontrolled processes with faster kinetics dominate the charge storage processes (59.4%, Figure S11, Supporting Information). This result indicates our electrode is a suitable material for SCs with rapid chargedischarge capability. In addition, electrochemical impedance spectroscopy was further utilized to explore the electrochemical performance of the WO 3-x -N14-H8 electrode. The Nyquist plots demonstrate the approximate equivalent series resistance (R s ) and charge transfer resistance (R ct ) of the WO 3-x -N14-H8 electrode and CC substrate (Figure S12, Supporting Information), suggesting the excellent electrical conductivity of the WO 3-x -N14-H8 electrode.
Cycling stability is another critical figure-of-merit for electrode materials in SCs, which is a decisive factor for further practical application. Figure 5g compares the cycle stability of the WO 3-x -N14-H8 electrode in a different electrolyte (evaluated by CV at 100 mV s −1 ). Strikingly, unexpected ultra-long durability was observed in 0.1 M H 2 SO 4 and 1 M Na 2 SO 4 circumstance, which maintained 106.37% capacitance retention after 250 000 cycles. In contrast, only 51.54% capacitance retention was gained in 1 M Na 2 SO 4 circumstance after 10 000 cycles. This result shows the electrolyte component plays a more important role for this system durability while the morphology of the WO 3-x electrode provides a foundation for its stability. To our best knowledge, the lifespan of WO 3-x -N14-H8 electrode over 250 000 cycles is a recordbreaking result, which not only substantially exceeds those of tungsten oxide-based electrodes but also those of carbon-based electrodes with ultra-long cycling stability in the previous reports (Figure 5h and Table  S3, Supporting Information). Overall, the optimizations of electrode and electrolyte dominate in capacitive performance and stability of WO 3-x electrode, respectively.

Electrode-Electrolyte Integrated Optimization Mechanism
The achievement of this ultra-long electrode lifespan can be ascribed to two main reasons: (1) the special high porosity "bird-nest" structure possesses slight mechanical flexibility, which can release the strain caused by constant redox reaction, preserving overall structural stability ( Figure S13, Supporting Information) and (2) the ideal electrolyte characteristics protect the WO 3-x from dissolution or structural collapse. According to previous investigations, tungsten oxide transforms to various W-containing ions under different pH values in redox reactions. Below pH 1, the H + -assisted dissolution of WO 3-x occurs by forming soluble WO 2þ 2Àx . Moreover, more stable WO 2À 4Àx appears when pH > 1 and the H + -assisted dissolution diminishes with pH value increasing, which reduces to the minimum at around 2.6 and then manifests H 2 Oassisted dissolution without diffusion effect (i.e., WO 3- . When the pH further enhances to the range of 4.5-6.5, OH-assisted dissolution with WO 2À 4Àx diffusion effect can be observed (i.e., WO 3-x + OH − = WO 2À 4Àx + H + ). As a result, the most passive dissolution of WO 3-x takes place at pH between 1 and 2.6. [23,44] The measured pH value of different electrolytes in our experiments are summarized in Table S4, Supporting Information. Benefited from the suitable pH value (1.52) of 0.1 M H 2 SO 4 and 1 M Na 2 SO 4 hybrid electrolyte, WO 3-x -N14-H8 demonstrates negligible dissolution after undergoing 250 000 cycles charge-discharge processes. Conversely, the obvious dissolution and peeling of WO 3-x -N14-H8 appear on the CC substrate in 1 M Na 2 SO 4 electrolyte (pH = 5.3) after 10 000 cycles test ( Figure S14, Supporting Information).
More importantly, the ionic adsorption mechanism based on density functional theory (DFT) also confirms the unique stable circumstance provided by the hybrid electrolyte for the Faradaic chargedischarge process of WO 3-x . [45] According to the above-mentioned experimental results, hexagonal (P6/mmm) WO 3 cleaved along the (100) surface was utilized as the model for WO 3 . The optimized structures of oxygen vacancy WO 3-x are shown in Figure 6a. Under high temperature, the vertical W-O bond is broken, and an oxygen vacancy site is created on the surface of WO 3-x . With regards to the Na 2 SO 4 electrolyte, Na + was firmly adsorbed on the surface of WO 3-x to form four bonds with the adjacent oxygen atoms (Figure 6b). Based on the charge density difference analysis in Figure 6c, Na + loses electrons whereas O obtains electrons when Na + is absorbed. Moreover, apparent hybridization peaks exist around the Fermi level in the projected density of states (PDOS) analysis (Figure 6d). The analysis results of charge density difference and PDOS further verify the formation of the Na-O bond. The strong interactions between Na and O make it extremely difficult to desorb Na + . Therefore, the structure of WO 3-x is inevitably changed during long-term Faradaic reactions in 1 M Na 2 SO 4 , leading to a short lifespan caused by the structural collapse and correspondingly altered properties. When H 2 SO 4 was added simultaneously, the addition of one hydrogen formed one H-O bond with the oxygen atom. However, it does not break any Na-O bond (Figure 6e). It is noteworthy that the adsorption of two hydrogens could break the Na-O bond, making it relatively easy to desorb Na + . In addition, the formation of structural H 2 O could break the W-O bond and create more Energy Environ. Mater. 2023, 6, e12520 7 of 11 oxygen vacancy sites on the surface of WO 3-x while the major crystal framework has little change ( Figure 6f). Overall, the synergistic function of H 2 SO 4 and Na 2 SO 4 increases the lifespan of WO 3-x by promoting the desorption of Na + /H + and inducing other vacancy sites. Furthermore, the DFT calculation also verifies that H + is difficult to desorb in a single H 2 SO 4 electrolyte ( Figure S15, Supporting Information). The cycling stability of WO 3-x -N14-H8 in 0.1 M H 2 SO 4 electrolyte has also proved this DFT calculation result ( Figure S16, Supporting Information). Hence, WO 3-x obtains an ultra-long lifespan in 0.1 M H 2 SO 4 and 1 M Na 2 SO 4 hybrid electrolyte benefiting from the pH effect and ionic adsorption mechanism.
On the other hand, the capacitance gradually increases up to 119.38% over the first 140 000 cycles under the hybrid electrolyte suggesting an activation process happens during the slow diffusion of electrolyte into the whole complex structure of electrode materials. The XPS spectrum of core level O 1s implies the stable oxygen vacancy concentration and a certain increase of structural water content in the WO 3-x -N14-H8 sample after 250 000 cycles ( Figure S17 and Table S2, Supporting Information), consistent with the DFT analysis result. The XRD pattern of the identical sample shows that the diffraction peaks shift to a lower 2θ angle, e.g., diffraction peak at~36.9°( Figure S18, Supporting Information), which indicates the expansion of interplanar distance based on Bragg's law. The larger interplanar distance originating from more structural water decreases the activation energy for interfacial charge transfer and ionic diffusion, resulting in an enhanced capacitive behavior. [46] 2.6. Real-Life Applicable Supercapacitor To explore the practical application of the WO 3-x electrode, an experimental asymmetric supercapacitor (ASC), i.e., WO 3-x -N14-H8//functional exfoliated graphite (FEG), was initially fabricated and electrochemically evaluated in the aqueous two-electrode system (see details in Supporting Information and Figures S19-S23). The exceptional performance of ASC inspired us to scale up the experimental result and develop the practical aqueous SC device.
Accordingly, a pouch SC with Ni-Cu tab-lead was designed, and manufactured with the soft-package encapsulation machines ( Figure  7a). After electrode charge-balance optimization, a stable 1.6 V voltage was obtained for one piece pouch SC, consisting of −0.8 to 0 V for WO 3-x -N14-H8 anode and 0-0.8 V for polyaniline (PANI)/FEG cathode ( Figures S24 and S25, Supporting Information). The CV and GCD test curves of the pouch-type SC are illustrated in Figure 7b,c, respectively. Almost no distortion or deformation can be observed for the CV profiles with increasing scan rate, suggesting a good rate capability. Furthermore, symmetric and linear GCD profiles at relatively high current density demonstrate an ideal capacitive behavior and excellent coulombic efficiencies ( Figure S27, Supporting Information). The pouch SC simultaneously delivers superior gravimetric and volumetric capacitance of 141.61 F g −1 at 0.22 A g −1 and 12.91 F cm −3 at 3 mA cm −2 , which preserves 95.84 F g −1 at 1 A g −1 and 8.74 F cm −3 at 13.75 mA cm −2 ( Figure S28, Supporting Information). Consequently, the peak energy density is calculated to be 50.35 Wh kg −1 at a power density of 175.5 W kg −1 or 4.59 mWh cm −3 at 0.016 W cm −3 , which maintains 34.07 Wh kg −1 or 3.11 mWh cm −3 at the peak power density of 804.4 W kg −1 or 0.073 W cm −3 (Figure 7d). This impressive gravimetric energy density is much higher than that of the reported pouch-type SC. [26] Meanwhile, the excellent volumetric energy density enables practical metrics for constructing ultra-thin SCs. In addition, the satisfied self-discharging performance of pouch SC also proves its good potential for actual application (Figure S29, Supporting Information).
For evaluating the safety of as-fabricated pouch SCs, a series of successional destruction tests were conducted to simulate real-life extreme conditions. The electric fan driven by charged pouch SC worked well Figure 6. a) Optimized structure of WO 3-x . b) Optimized structure of Na + adsorption on WO 3-x . c) Charge density differences of Na + adsorption on WO 3-x with the isovalue of AE0.002 a.u., cyan and yellow indicate the depletion and accumulation of electrons, respectively. d) PDOS analysis of Na + adsorption on WO 3-x . e) Optimized structures of Na + and one H + adsorption on WO 3-x . f) Optimized structure of Na + and two H + adsorption on WO 3-x .
Energy Environ. Mater. 2023, 6, e12520 8 of 11 in the whole destructive process. Moreover, the pouch SC resisted against burning, explosion, or smoking when suffering from weight pressure, flame-treating, striking, and impaling, and maintained good capacitance retention (Figure 7e and Figure S30, Videos S1 and S2, Supporting Information). The result confirms the extraordinary safety and reliability of our pouch-type SC during the actual application, even under a potentially dangerous situation.
To further expand the application, the SC device with a larger voltage was fabricated via a facile series connection of single pouch-type SCs. With increasing SC pouches and corresponding voltage (i.e., two SC pouches: 3.2 V and three SC pouches: 4.8 V), the CV curves are evenly stretched out while keeping a similar symmetric shape and approximate integrated area (Figure 7f). Moreover, the discharge time of GCD tests almost keeps consistent from one to three cells in series (Figure 7g). These test results demonstrate the high consistency of every single pouch-type SC in electrochemical properties, resulting in a stable operating voltage of up to 4.8 V. More importantly, our homemade power bank comprised of a voltage stabilizer and three series pouch-type SC (Figure 7h and Figure S32, Supporting Information) can steadily power practical 3 C products, e.g., a smartphone (Video S3, Supporting Information) and a tablet (Video S4, Supporting Information). This power bank effectively charged the smartphone and tablet over 60 s and 40 s, respectively, with a short self-charging time of about 240 s (Figure 7g, Videos S3 and S4, Supporting Information), exhibiting superior time-saving features than battery-type devices.

Conclusion
A practical aqueous SC, for the first time, was fulfilled by using the rational collaborative optimization of the electrode and electrolyte. The strategy of electrode optimization ensures the superior electrochemical performance of the WO 3-x anode with high mass loading. Benefiting from the unique ionic adsorption mechanism and pH effect of those generated from overall optimization, the WO 3-x anode in the optimized electrolyte, delivers improved potential window and unexpected long lifespan at the same time (maintained 106.37% capacitance retention after 250 000 cycles). Furthermore, the technique of soft-package encapsulation is utilized to effectively scale up the experimental-level achievement to the real-life application by means of fabricating pouchtype aqueous SCs. The general optimization methodology used here opens new approaches for the practical design of aqueous SCs.

Experimental Section
Detailed information related to the synthesis of active electrodes, physicochemical characterization, and electrochemical evaluation of bifunctional electrodes towards UOR and supercapacitor application is provided in Supporting Information.