Electrolytes For Supercapacitors Research Articles

Fabrication of high performance supercapacitors based on ethyl methyl imidazolium bis(trifluoromethylsulfonyl) imide (EMIMTFSI)-decorated reduced graphene oxide (rGO)

In this paper, we outline the construction of symmetric supercapacitors comprised of reduced graphene oxide (rGO) and hydrophobic ionic liquid (IL) (1-ethyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide (EMIMTFSI) acting as physical spacer and electrolyte between rGO plates. To increase the porosity and interlayer spacing between the graphene plates, different solvents are analyzed by Hansen solubility parameters’ approach (HSP). Accordingly, the solubility performances of two solvents, namely Tween 80 and N, N-dimethylformamide (DMF), coupled with rGO and IL, are deemed to offer the most beneficial results, and they are the solvents used in this study to develop rGO/Tween 80/IL and rGO/DMF/IL electrodes. The comparison of the electrodes’ performances shows that the consumption of 80 wt percent IL would deliver the optimum electrochemical performance and capacitance results. At room temperature, the prepared rGO/Tween 80/IL80% and rGO/DMF/IL80% deliver capacitances of 375 and 213 F/g, respectively. The maximum energy density of rGO/Tween 80/IL electrode is 187.5 Wh/kg at a power density of 1125 W/kg, while for the rGO/DMF/IL electrode the maximum energy density reaches 106.5 Wh/kg at a power density as high as 639 W/Kg. The supercapacitors prepared based on rGO/Tween 80/IL80% and rGO/DMF/IL80% electrodes show capacitance retention of 90% and 84% after 10,000 charge-discharge cycles, respectively. This design approach eliminates the need for the prevalant stand-alone type of electrolyte in supercapacitors, and instead, the electrolyte is incorporated as a component of electrode’s structure.

Fiber supercapacitor using epoxy-based gel polymer electrolyte with high ionic conductivity and mechanical flexibility

Recently, flexible energy storage devices have become an important issue for the smart textile market and flexible electric devices. Among them, fiber supercapacitors are one of the most promising candidates due to their light weight and mechanical flexibility. Traditional liquid electrolytes for fiber supercapacitors exhibit critical problems such as leakage when the device is deformed. On the other hand, solid electrolytes have a trade-off relationship between mechanical property for flexibility and ionic conductivity. Therefore, gel polymer electrolytes (GPEs) have attracted attention because of their sufficient ionic conductivity and high flexibility for maintaining their structure under bending, stretching, or twisting. However, liquid electrolytes in GPEs have a safety issue caused by narrow window voltage and flammability. Electrolytes with narrow window voltages could not provide the appropriate capacitance. To overcome this problem, ionic liquids that featured high electrochemical and thermal stability are used for GPEs. However, GPEs with ionic liquid have limited flexibility and ionic conductivity. In this work, we report a GPE comprising an ionic liquid and polyethylene oxide-abundant epoxy. The epoxy is synthesized from the crosslinking reaction between the amine with three-block copolymer and bisphenol A diglycidyl ether. The ionic conductivity of the fabricated GPE increased 2.0 × 10−3 S cm−1 at room temperature with increasing contents of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI). Further, EMImTFSI gives an improvement of flexibility up to 107%. A fiber supercapacitor using developed GPE shows excellent electrochemical performance approaching using liquid electrolyte. In short, our result demonstrates that the GPE is a promising candidate for various applications involving multidirectional deformation such as fiber-type electric and flexible devices.

Tough and Antifreezing Organohydrogel Electrolyte for Flexible Supercapacitors with Wide Temperature Stability

Hydrogels bridged by some alcohol compounds are reported to be useful for high tough and antifreezing properties of electronics and energy storage devices. However, the relationship between the str...

Honeysuckle flowers-derived hierarchical porous carbon matching with ionic liquid electrolyte for high-energy supercapacitors

As an electrode material, biomass-derived porous carbon has been extensively investigated for supercapacitors (SCs) due to its abundant resources, low cost, and sustainability. It is an important strategy to boost the energy capacity of SCs by using high-capacitance electrodes and widening electrochemical stability potential window (ESPW). In this work, a carbon material with hierarchical porous structure has been prepared by using honeysuckle flowers as the raw material and KOH as the chemical activator. The porous size distribution can be easily controlled by regulating the dosage of KOH to match the size of ionic liquid (IL) electrolyte. The honeysuckle-derived carbon (HC) electrode material exhibits a high specific capacitance of 186 F g−1 at 1 A g−1 in 1-ethyl-3-methylimidazolium tetrafluoroborate (EmimBF4) IL electrolyte. Moreover, the assembled symmetric SCs can provide a high specific energy of 93 Wh kg−1 at a specific power of 954 W kg−1, and its specific energy still remains 64 Wh kg−1 at 23 kW kg−1. Benefiting from the persevering architecture, the fabricated SCs also show a superior cyclability (83% of its initial value after 7500 cycles). Consequently, the investigation proves honeysuckle flowers are a promising sustainable carbon source for producing efficient electrode materials of high-performance supercapacitors.

Nonhalogenated Surface-Active Ionic Liquid as an Electrolyte for Supercapacitors

We report a nonhalogenated surface-active ionic liquid (SAIL) that consists of the surface-active anion 2-ethylhexyl sulfate and the tetraoctylammonium cation ([N8,8,8,8][EHS]). We explored the thermal and electrochemical properties, i.e., degradation, melting and crystallization temperatures, ionic conductivity, and electrochemical potential window of neat SAIL and its binary mixture with acetonitrile. This SAIL was tested as an electrolyte in a multiwalled carbon nanotube (MWCNT)-based supercapacitor at various temperatures from 298 to 373 K. In addition, we also tested the binary mixture of SAIL with acetonitrile as an electrolyte at lower temperatures (253–298 K). The electrochemical performance of SAIL and the SAIL/acetonitrile binary mixture as a function of temperature was compared with that of a standard electrolyte, an aqueous solution of 6 M KOH, in the same MWCNT-based supercapacitor. The solution resistance (Rs), charge transfer resistance (Rct), and equivalent series resistance (ESR) decreased with an increase in temperature for all SAIL-based electrolytes. We found that the supercapacitor cell with SAIL as an electrolyte has a high specific capacitance (Celec in F g–1), a high energy density (E in Wh kg–1), and a high power density (in W kg–1) compared to those for the binary mixture of SAIL with acetonitrile and for the 6 M KOH aqueous electrolytes, particularly at elevated temperatures. For the SAIL/MWCNT-based supercapacitor, Celec increased from 75 F g–1 at 298 K to 169 F g–1 at 373 K, whereas the energy density increased from 42 Wh kg–1 (at 298 K) to 94 Wh kg–1 (at 373 K) and the power density increased from 75 kW kg–1 (at 298 K) to 169 kW kg–1 (at 373 K) at a scan rate of 2 mV s–1 (potential window = 4 V). This study reveals that SAIL can potentially be used as an electrolyte for high-temperature electrochemical applications for energy storage devices.

MODERN RESEARCH METHODS OF PHYSICOCHEMICAL AND ELECTROCHEMICAL PROPERTIES OF ELECTROLYTES IN FOR Li-ION BATTERIES AND HYBRID SUPERCAPACITIES

In review examineі base properties of modern non-aqueous electrolytes for li-ion batteries and hybrid supercapacities taking part in the formation of power density, electrochemical and thermal stability. Discussed such aspects as the electrolytes functions in electrochemical power sources, physicochemical and electrochemical properties of electrolytes for supercapacitors, the physicochemical and electrochemical properties of electrolytes for primary and secondary batteries, and methods of electrolytes research. As the base methodі for electrolytes studies considered: electrochemical impedance spectroscopy, voltammetry, amperometry, viscosimetry, and combined Ramman spectroscopy.

Preparation of porous polyacrylonitrile membrane-based gel polymer electrolyte for solid-state supercapacitor

Recently, solid polymer electrolytes (SPEs) are of great current interest as substitutes to conventional liquid type electrolytes. Although the SPEs significantly alleviate the safety issues of the lithium-ion batteries based on conventional liquid electrolytes, they intrinsically exhibit low ion conductivity. In this study, a bicontinuous solid polymer electrolyte containing polyacrylonitrile (PAN) membrane and a mixture of succinonitrile (SN) and lithium bis(trifluoromethanesulfonyl)imide was prepared with different molarities of the lithium salt. Porous PAN membrane was fabricated via a thermally induced phase separation method (TIPS). The PAN-based SPE exhibited the maximum ionic conductivity of ∼5.0 10−3 S/cm at the salt molarity of 1.0 M.

Anion Coordination Improves High-Temperature Performance and Stability of NaPF6-Based Electrolytes for Supercapacitors

Electrolyte stability can be improved by incorporating complexing agents that bind key decomposition intermediates and slow down decomposition. We show that hexamethyl-phosphoramide (HMPA) extends both the thermal stability threshold of sodium hexafluorophosphate (NaPF6) in dimethoxyethane (DME) electrolyte and the cycle life of double-layer capacitors. HMPA forms a stable complex with PF5, an intermediate in PF6 anion thermal degradation. Unbound, this intermediate leads to autocatalytic degradation of the electrolyte solution. The results of electrochemical impedance spectroscopy (EIS) and galvanostatic cycling measurements show large changes in the cell without the presence of HMPA at higher temperatures (≥60 °C). Fourier transform infrared spectroscopy (FTIR) on the liquid and gas phase of the electrolyte shows without HMPA the formation of measurable amounts of PF5 and HF. The complimentary results of these measurements proved the usefulness of using Lewis bases such as HMPA to inhibit the degradation of the electrolyte solution at elevated temperatures and potentially lead to improve cycle life of a nonaqueous capacitor. The results showed a large increase in capacitance retention during cycling (72% retention after 750,000 cycles). The results also provide evidence of major decomposition processes (0% capacitance retention after 100,000 cycles) that take place at higher temperatures without the additive of a thermal stability additive such as HMPA.

Solid-State, Single-Anion-Conducting Networks for Flexible and Stable Supercapacitor Electrolytes

Supercapacitors are a complementary energy storage technology to batteries and will be important for next-generation applications such as wearable sensors, soft robotics, roll-up displays, and electric vehicles. Here, we report a flexible supercapacitor with an all solid-state, single-ion-conducting polymer network electrolyte. The electrodes are comprised of reduced graphene oxide held together with a neutral polymer network to stabilize the dispersion and lend flexibility to the overall supercapacitor. Due to the high thermal and electrochemical stability of the network polymerized ionic liquid electrolyte, the resulting device can be charged to high voltages (3 V) and operated at high temperatures (120 °C) with excellent cycling stability. A 300 F/g specific capacitance is achieved by charging the device at 0.57 A/g and 90 °C or at 1.8 A/g and 120 °C. Due to the flexible network, the device can bend up to 180° without a substantial change in capacitance. The electrode/electrolyte interface, network architecture, and single-ion-conducting nature of the electrolyte are shown to be critical for high capacitance through a series of control experiments with ionic liquid supercapacitors.

Study on the themodynamic properties and electrochemical performance of mixture electrolyte for supercapacitor composed of ionic liquid [BMIM][BF4] and SBPBF4/PC

Study on the themodynamic properties and electrochemical performance of mixture electrolyte for supercapacitor composed of ionic liquid [BMIM][BF4] and SBPBF4/PC

Application of aprotic ionic liquids based on bis(trifluoromethylsulfonyl)imide anion as polymer gel electrolytes for cobalt oxide symmetric supercapacitors

Mesoporous Co3O4 nanowires were uniformly deposited onto Ni foam by using a facile hydrothermal route. The as-prepared Co3O4@Ni materials were applied as electrodes for symmetric supercapacitors. The electrochemical performances of Co3O4@Ni were investigated in the presence of aprotic ionic liquids (ILs) and compared to KOH electrolyte. The selected ILs are composed by the same bis(trifluoromethylsulfonyl)imide (TFSI−) anion, paired with 1-Allyl-3-methylimidazolium (AMIM+) or 1-Ethyl-3-methylimidazolium (EMIM+) cations. The electrolytes composed by KOH and ILs were prepared in a polymer gel form. Independently on the electrolyte nature, the profiles of cyclic voltammograms and galvanostatic charge/discharge curves are indicative of an electrochemical double-layer charge storage mechanism. Combining our Co3O4@Ni electrodes with selected ILs polymer gel electrolytes showed good electrochemical stabilities with 85% and 97% retentions of the initial capacitances after 10,000 cycles in [AMIM-TFSI] and [EMIM-TFSI], respectively. At a current rate of 0.2 A g−1, the lowest values of specific energy (2.8 Wh kg−1) and specific power (201.9 W kg−1) were exhibited in the presence of KOH based gel electrolyte. This poor electrochemical performance was considerably improved by using aprotic ILs. For [AMIM-TFSI] and [EMIM-TFSI] based polymer gel electrolytes, high specific powers up to 811 and 680 W kg−1 were delivered at 0.2 A g−1 with enhanced specific energies of 29.4 and 23.3 Wh kg−1, respectively. Hence, the specific energies of the devices assembled by PVDF-HFP/AMIM-TFSI or PVDF-HFP/EMIM-TFSI polymer gel electrolytes were 8 to 10 times higher than that of the device assembled by PVA/KOH gel electrolyte.

Insight into the adsorption of Imidazolium-based ionic liquids on graphene by first principles simulation

The ionic liquid (ILs)-graphene interface is important in understanding the behaviors of ILs electrolyte in carbon-based supercapacitors and Li-ion batteries. Molecular simulation is one of the excellent ways to investigate the mechanism. However, due to the multiple interacting sites between ILs and graphene, it demands a lot of computational time to scan. In this work, an easy method was obtained, and it is for predict the adsorption energy and screen the proper ILs on graphene. The behavior of five kinds of ILs, composed by 1-ethyl-3-methylimidazolium cation ([Emim]+) and anions ([BF4]-, [PF6]-, [Cl]-, [Br]-, and [OAc]-) on the graphene surface were studied by using density functional theory calculations. The stable adsorbed structures were optimized and the influence of the anions was further evaluated. Moreover, charge density difference and noncovalent interaction analysis were performed to figure out the adsorption energies difference on ILs. The results illustrated the correlations among cation–anion interaction, charge distribution, charge variation and adsorption energies. The weaker cation–anion interaction, the bigger charge variation of the ILs and the stronger adsorption energy for [Emim][PF6] and [Emim][BF4], while other three ILs are in the opposite law. Based on this, we can quickly screen suitable ionic liquids as electrolytes and apply them to graphene-based supercapacitors. Apart from this, in order to investigate the distribution of the ions, two pairs of ILs on the graphene were studied and different arrangement configurations were compared, respectively. The crossover-like structures are more stable when compared with the line-like structures. The above results could provide a novel sight on the view of the Graphene-ILs (G-ILs) microstructure, which could be helpful to design new electrolyte systems in electrochemistry applications.

Optimization and application of a gel polymer electrolyte in solid state super capacitors with graphite electrodes

The prime objective of the present study is to exploit a user friendly, inexpensive all solid state super capacitor using a suitable electrolyte and electrodes. A gel polymer electrolyte (GPE) consisted with polyvinylidene fluoride-co-hexafluopropylene (PVdF), ethylene carbonate (EC), propylene carbonate (PC) and zinc trifluoromethane sulfonate (Zn(CF3SO3)2-ZnTF) was prepared using solvent casting method and its properties were optimized by varying the composition. The composition of 16 PVdF : 22 ZnTF : 31 EC : 31 PC (weight %) exhibits the optimum room temperature conductivity of 3.09 × 10-3 Scm-1. It is purely an ionic conductor having a negligible electronic conductivity. In addition, it was possible to obtain a thin, mechanically stable film. The electrodes were prepared using Sri Lankan natural graphite (NG) with a polymer binder. Eventhough there are reports about natural graphite based super capacitors, Sri Lankan natural graphite has not been considered for super capacitors at a large scale. The main reason for turning towards natural graphite is to reap the benefits of low cost as well as the safety. In order to optimize the electrode properties, the ratio between graphite and polymer binder (in weight basis) was changed. The polymer binder used was polytetrafluoroethylene (PTFE). Super capacitor fabricated with the electrode of the composition, NG : PTFE = 90:10 shows the maximum single electrode specific capacitance (2.58 Fg-1). The fabricated device retains for continuous charge discharge operation with a minimum performance reduction.

Investigation of H2SO4 and KOH aqueous electrolytes on the electrochemical performance of activated carbon derived from areca catechu husk

Aqueous electrolytes have shown great interest in developing high performance and environmentally friendly supercapacitors. This work focuses on systematic investigations of the symmetrical properties of activated carbon electrode for supercapacitors in different aqueous electrolytes. Here, we compare the performance of supercapacitors using 1M KOH and 1M H2SO4 different electrolytes based on carbon monolith electrodes derived from areca catechu husk waste. Carbon electrodes were synthesized using single-step pyrolysis both carbonization and physical activation without the addition of synthetic adhesive materials. The electrochemical properties were evaluated based on the cyclic voltammetry method at voltage windows of 0-0.5V and 0-1.0V with a scanning rate of 1 mV s−1, 2 mV s−1, 5 mV s−1, and 10 mV s−1. The electrode supercapacitor showed the highest specific capacitance of 150 F g−1 and 112 F g−1 at a scanning rate of 1 mV s−1 for H2SO4 and KOH electrolytes respectively. The maximum specific energy was found as high as 20.97 Wh kg−1 and the specific power of 75.57 kW kg−1 at the voltage window of 0-1.0V were achieved for H2SO4 electrolytes. As addition, the physical properties of carbon electrode also characterized such as mass, volume, and density. This work provides consideration the selected of different aqueous electrolytes for high-performance supercapacitors.

Using a mixture of vinasse-contaminant and H3PO4 as an efficient electrolyte for high performance flexible carbon nanotube-based supercapacitors

The elimination of vinasse contaminant generated by the tequila industry and other alcoholic beverages is of current interest because millions of liters are produced every year and it cannot be released in the environment due to its low pH and high content of organic material. As an effort to reuse/recycle such a contaminant, we report the use of a mixture of vinasse-contaminant + H3PO4 as the electrolyte for flexible supercapacitors (SCs), which are made with carbon nanotube (CNT) electrodes. We obtained a high capacitance value of 679.7 F g−1 in the devices made with both electrodes of CNTs. When the Ag/TiO2 nanoparticles are added in the SC anode, the capacitance is enhanced to 1852.8 F g−1, which is among the highest values reported so far for flexible CNT based SCs. We also compared the performance of the SC made with vinasse to another made with a conventional electrolyte of PVA/H3PO4 and found that the capacitance and energy density values of this last device were 1.2 times lower. All the devices made with CNTs, Ag/TiO2 and vinasse + H3PO4 stored charge by redox reactions and the redox centers were: oxygen vacancies, carboxylic groups, Ag0/Ag2+ and Ti3+/Ti4+ ions. Thus, the vinasse contaminant is a feasible electrolyte for supercapacitors.

EMIMBF4 in ternary liquid mixtures of water, dimethyl sulfoxide and acetonitrile as “tri-solvent-in-salt” electrolytes for high-performance supercapacitors operating at -70 °C

For many years, the performance of Li-ion batteries (LIBs) and supercapacitors (SCs) has relied mainly on two factors, (1) the optimization of electrodes composition and/or structure and (2) the selection of salts or ionic liquids (ILs) matching well with the compositional and/or structural features of electrodes. Solvents included in electrolyte composition have been typically seen as a mere medium where the electrochemically active salts or ILs are dissolved or mixed. More recently, attention has also been paid to specific issues, such as flammability, toxicity, electrical conductivity and/or electrochemical stability window (ESW). Recent reports describing water-in-salt (WIS), solvent-in-salt (SIS) and bi-solvent-in-salt (BSIS) electrolytes demonstrated that solvent molecules may indeed play a more active role in the achievement of high-performance LIBs and SCs. This work accomplished the design of a tri-solvent-in-salt (TSIS) electrolyte where every solvent contributed (with an IL such as EMIMBF4) to the formation of an electrochemically active hydrogen bond (HB) complex structure. Raman and NMR spectroscopies, as well as molecular dynamic (MD) simulations helped elucidate the ratio among all compounds (e.g., solvents and IL) in the HB complex structure that best works as an electrolyte. For instance, one could start from the eutectic mixture of H2O and dimethylsulfoxide (DMSO) in a 2 to 1 molar ratio and then add acetonitrile (CH3CN) in different molar ratios. Thus, the 2H2O:DMSO mixture offers low melting point and low flammability, and CH3CN provides an improvement of the rate capability to the resulting electrolyte. As compared to other electrolytes, the TSIS electrolyte composed of 1.5EMIMBF4:2H2O:DMSO:3.5CH3CN (5.8 m, TSIS-5.8) was cost efficient and exhibited self-extinction rates as low as 40 s g-1. Moreover, SCs operating with TSIS-5.8, at -70 °C and up to 2.7 V provided energy densities of ca. 49 and 18 Wh kg-1 at, respectively, power densities of 10,000 and 17,000 W kg-1, a capacitance retention of ca. 82% after 15,000 cycles at 4 A g-1 and a self-discharge as low as 22%.

"Water-in-Salt" Electrolytes for Supercapacitors: A Review.

"Water-in-salt" (WIS) electrolytes, which have more salt than the solvent in both mass and volume, show promising prospects for application in supercapacitors due to their wide electrochemical stability window (about 3 V), considerable ion transport, high safety, low cost, and environmental friendliness. This Review summarizes the advances, progress, and challenges of WIS electrolytes in supercapacitors. The working mechanisms, reason for the wide electrochemical stability window, typical systems, challenges, and modification strategies of the WIS electrolytes in supercapacitors are discussed. Moreover, the application of WIS electrolytes in symmetric and asymmetric supercapacitors are presented. Finally, perspectives and the future development direction of WIS electrolytes are given. This Review is expected to provide inspiration and guidance for designing WIS electrolytes with advanced performance and push forward the development of high-performance aqueous supercapacitors with high cell voltage, good rate performance, and thus high energy density and power density.

Culinary inspired electrolytes for textile supercapacitors

The choice of electrolyte for supercapacitors is challenging, with voltage stability, physical properties, environmental concerns and production processes all having to be taken into account. This challenge is greater still when thinking about e-textile applications, as these devices will be in close proximity to the end user. Historically, this challenge has been overcome with sophisticated encapsulation or the dilution of traditional supercapacitor electrolytes. This work looks to challenge these norms by investigating the performance of everyday, cupboard-items as electrolytes. The performance of four electrolytes (Grenade Energy® drink, an isotonic sports drink and LoSalt® solutions) were evaluated in a single layer, activated-carbon textile supercapacitor, against a traditional electrolyte. With areal capacitances of 25.7 mF.cm−2 these electrolytes have been shown to be competitive with more traditional systems and with further refinement, could provide safe and abundant electrolytes for wearable applications.

Nano Carbon Doped Polyacrylamide Gel Electrolytes for High Performance Supercapacitors.

Novel polyacrylamide gel electrolytes (PGEs) doped with nano carbons with enhanced electrochemical, thermal, and mechanical properties are presented. Carboxylated carbon nanotubes (fCNTs), graphene oxide sheets (GO), and the hybrid of fCNT/GO were embedded in the PGEs to serve as supercapacitor (SC) electrolytes. Thermal stability of the unmodified PGE increased with the addition of the nano carbons which led to lower capacitance degradation and longer cycling life of the SCs. The fCNT/GO-PGE showed the best thermal stability, which was 50% higher than original PGE. Viscoelastic properties of PGEs were also improved with the incorporation of GO and fCNT/GO. Oxygen-containing functional groups in GO and fCNT/GO hydrogen bonded with the polymer chains and improved the elasticity of PGEs. The fCNT-PGE demonstrated a slightly lower viscous strain uninform distribution of CNTs in the polymer matrix and the defects formed within. Furthermore, ion diffusion between GO layers was enhanced in fCNT/GO-PGE because fCNT decreased the aggregation of GO sheets and improved the ion channels, increasing the gel ionic conductivity from 41 to 132 mS cm−1. Finally, MnO2-based supercapacitors using PGE, fCNT-PGE, GO-PGE, and fCNT/GO-PGE electrolytes were fabricated with the electrode-specific capacitance measured to be 39.5, 65.5, 77.6, and 83.3 F·g−1, respectively. This research demonstrates the effectiveness of nano carbons as dopants in polymer gel electrolytes for property enhancements.

Humidity-modulated properties of hydrogel polymer electrolytes for flexible supercapacitors

Hydrogel polymer electrolytes (HPEs) are essential components in building flexible solid-state supercapacitors, which are promising power sources for future flexible and wearable electronics. HPEs reported so far have shown a wide range of properties with various compositions. In this paper, we demonstrate that relative humidity (RH) significantly influences the electrochemical and mechanical properties of HPEs, which in turn, determines the power output of supercapacitors assembled with such HPEs. Using polyvinyl alcohol and phosphoric acid hydrogel (PVA-H3PO4) as model HPE, we showed that water content in PVA-H3PO4 increases with RH and can reach equilibrium at each RH level. By using atomic force microscope, electrochemical impedance spectroscopy, and cyclic voltammetry, we found that the mechanical stiffness, ionic conductivity, and interfacial capacitance of PVA-H3PO4 prepared at precisely controlled RH levels can vary orders of magnitude. As a result, the flexibility and the energy and power performance of PVA-H3PO4 based supercapacitors can all be modulated by RH. We believe that the result provides a new approach to optimizing HPEs for flexible supercapacitor applications.