Hybrid electrochemical capacitors in aqueous electrolytes: Challenges and prospects

In an internal hybrid capacitor, at least one electrode displays battery-like charge/discharge and the other electrode stores charge reversibly at the electric double layer (EDL). Recently, a plethora of hybrid cells in aqueous electrolytes have been proposed by coupling an EDL electrode with a battery electrode, the latter made from a variety of redox-active/redox-mediator species either dissolved in the electrolyte or adsor-bed/immobilized in nanoporous electrodes. This review presents current opinions, discusses challenges, and supplies recommendation about the hybrid cells with aqueous electro-lytes and carbon electrodes.


Introduction
Electrical energy storage systems are essential support in modern devices to ensure reliability and uninterrupted energy supply [1e3].Electrochemical capacitors are adapted for systems where bursts of energy are stored and delivered in short periods of time, and the process is repeated for thousands of cycles [4e6].The capacitance of an electrical double-layer capacitors (EDLCs) is dependent on the electrode/electrolyte interface represented by surface area 'S' in equation C = 3S/d.Therefore, carbon with high surface area (up to w3500 m 2 g À1 ) is the preferred electrode material to achieve high values.Obviously, the cell capacitance of an EDLC represents a low value which is half of the individual electrode capacitance (1 To compensate for low capacitance, commercial capacitors implement organic electrolytes which enable reaching high voltage and improve the energy performance.Nevertheless, use of organic electrolytes raises environmental concerns regarding toxicity of solvents and cost issues due to the extensive drying of carbon electrodes. In this regard, aqueous electrolytesebased capacitors offer great opportunity owing to low assembling cost, eco-friendliness and reasonably large potential window [7e11].Thanks to the universal property of water for dissolving salts, redox-active species can be easily mixed to prepare electrolytes with high ionic conductivity which favors fast redox reactions.Figure 1a shows the difference between the charge/discharge curves of an EDLC and a hybrid capacitor.In accordance with E = 1 / 2 CU 2 , energy density of electrochemical capacitors in aqueous electrolytes can be enhanced by increasing capacitance or/and voltage, and an effective way to enhance capacitance is hybridization of battery-like electrodes with an EDL one [12,13].Owing to almost constant potential of battery-like electrodes, the potential window of the EDL electrode during charge/ discharge of the hybrid capacitor is approximately twice longer than in a symmetric EDLC, leading to approximately twice higher discharge capacity (Q hybrid ): Historically, hybrid capacitors are comprised of metal oxideebased redox electrodes coupled with nanoporous carbonebased EDL electrodes [14e17].Razumov et al. [16] and Pell and Conway et al. [14] proposed hybrid capacitors using activated carbon as an EDL electrode with NiOOH/Ni(OH) 2 (alkaline media) or PbO 2 / PbSO 4 (acidic media) as the battery-like electrode, and the key aspect in these devices has been the linear charge/discharge curves on a voltageetime scale.In the last Q2 decade, a number of hybrid capacitors have been proposed in acidic or basic media by dissolving redox species in the electrolyte or covalently bonding them at the carbon material to transform charging to a battery-

EDL electrode charging mechanisms and enhanced cell voltage
Because the capacitance of a hybrid capacitor is controlled by the EDL electrode, understanding its charging mechanisms is important.Nanoporous carbons adsorb the partially desolvated ions electrostatically [32,33].In situ Q3 electrochemical quartz crystal microbalance and in situ nuclear magnetic resonance applied for neat EMI-TFSI and EMI-TFSI in acetonitrile-and carbide-derived carbons with precise pore sizes [34e37] have shown linear relationship between charge and weight difference at negative polarization of carbon electrode, indicating a permselective (only cations) adsorption/desorption mechanism (Figure 1c).In addition, a larger slope than the theoretical one proves adsorption of additional molecules at negative polarity, proportional to the charge of the electrode (3.7 solvent molecules per cation).Electrochemical dilatometry has also confirmed that cations carry additional solvent molecules into the carbon pores during negative charging with the relative strain increase from 3.4% to 4.2% after adding acetonitrile to the ionic liquid [38].Charging mechanism at the positive polarization is mainly ion exchange, confirmed by in situ nuclear magnetic resonance with PEt 4 eBF 4 ionic liquid [37].Under positive polarization, a loss of mass due to the exchange of the heavier cations (147.2 g/mol) with the lighter anions (86.8 g/mol) confirms that the weight change depends on the molecular weight difference per charge of the cation and the anion.Ion exchange at positive polarity goes on until the depletion of cations, which is then dominated by adsorption of anions (Figure 1d).Prehal et al. have demonstrated with small-angle X-ray scattering the change of ion concentration inside carbon pores due to the local ion rearrangements in aqueous media [39,40], and the charging mechanisms are found to be dependent on the experimental condition, e.g., electrolyte concentration, charging rate, and cell design [41].Besides EDL charging, additional redox reactions at the negative electrode in aqueous electrolyte take place, e.g., reversible hydrogen adsorption/desorption.The simplified Nernst equation E H = À0.059pH suggests that reduction potential of aqueous electrolyte is determined by the pH of the electrolyte [9], and water reduction causes nascent hydrogen generation which chemisorbs within carbon pores [42].In addition, the OH À anions accumulate within carbon pores resulting in local pH shift [43].The pH shift Q4 is significant for neutral electrolytes, while for acidic solutions, pH does not change during charging, as confirmed by comparing 1 mol L À1 Li 2 SO 4 (pH = 6.5) and 1 mol L À1 BeSO 4 (pH = 2.1), where the reduction potential (E H ) differs significantly (À0.383V vs SHE for Li 2 SO 4 and -0.124 V vs SHE for BeSO 4 ) [9].The dihydrogen evolution in Li 2 SO 4 starts at ca. À0.8 V vs SHE, meaning an overpotential of ca.À0.42 V vs SHE is achievable, while for BeSO 4 , it starts at ca. À0.3 V vs SHE very close to the E H , with less overpotential.The advantageous effects of aqueous electrolyte pH have been harnessed in bifunctional electrolytes either to enhance capacitance or to access large voltage for hybrid capacitors.Clearly Q5 , hydroquinone-based, methylene blueebased, and iodide-based hybrid cells have exhibited enhanced capacitance when assembled with acidic media H 2 SO 4 [20,21,44], iodides with MnSO 4 (pH = 3.0) [45], or pphenylenediamine (PPD) in aqueous KOH [26].On the other hand, hybrid capacitors using bifunctional electrolytes with neutral solution acting as supporting electrolyte (pH = 6.5e7.0)exhibit enhanced voltage up to 1.5e1.6V [29,46e48], and with concentrated 20 mol kg À1 NaTFSI þ 0.8 mol kg À1 KI up to 1.8 V [49].

Equilibrium potential and hybrid cell performance
The battery-like electrode in a hybrid cell works in narrow potential range, e.g., in bromine system, around the redox potential of Br À /Br 3 -, which is E 0 = 0.89 V vs. Ag/AgCl [50].However, the large potential window of positive electrodes to reach bromide redox potential implies the equilibrium potential or potential of zero charge must be lifted such that the positive electrode works in a small potential range and the negative electrode can go through a large potential window.Yamazaki et al. used 1% bromine water to preimpregnate the positive carbon electrode which worked in narrow potential window and the hybrid capacitor was discharged down to 0.5 V with improved capacitance and coulomb efficiency.Indeed, the adsorption of species on carbon decreases the redox potential by w0.1 V [24].Yoo et al. and Evanko et al. used phase change inside the pores to keep the redox species at the electrodes by implementing tertiary aminesecomplexing agents such as 1ethyl-1-methylpyrrolidiniumbromide (MEPBr) producing second liquid phase [51,52].TBABr (n-Bu 4 NBr) [51] and HVBr 2 (1,1 0 -diheptyl-[4,4 0 -bipyridine]-dibromide) were also used [52] to produce solid-phase TBABr 3 which is retained in the pores of carbon upon charging because of strong interaction of Br 3 -, improving cycling stability and slowing self-discharge.
In contrast to bromides, equilibrium potential of hybrid cell in aqueous iodides is close to the redox-active potential region (Figure 2a,d), which makes the cell an almost-perfect hybrid capacitor with linear charge/ discharge curves [18,29].Although hybrid capacitor with 0.08 M KI þ H 2 SO 4 exhibited twice higher capacitance than a symmetric one with H 2 SO 4 [21], the issue of low voltage in these systems was only resolved by using bifunctional aqueous KI þ Li 2 SO 4 which worked at 1.6 V and exhibited capacitance comparable with the organic electrolyteebased EDLC charged up to 2.5 V [29].Carbon electrodes in hybrid capacitors with asymmetric configuration improved long-term performance owing to better iodide immobilization in mesoporous positive electrodes and enhanced EDL capacitance of microporous negative electrodes [47].Lately, a hybrid capacitor showing high capacitance of 50 F g À1 at 1.5 V and À40 C was realized with choline cationebased electrolyte (choline nitrate þ choline iodide) [53].Similar to the Q6 strategy of Yamazaki et al. with bromide system, shuttling of iodides was prevented by electrochemically immobilizing them in the nanopores of carbon which was then used as a positive battery-like electrode in neat NaNO 3 aqueous electrolyte [54].Such approach is well justified as iodides adsorb deep in the nanopores [55], which is confirmed also by the gas adsorption analysis on the floated/cycled carbon electrode [45,56].Redox potential of iodides located close to equilibrium potential also requires careful selection of operation parameters (Figure 2b,e &  2c,f) where a slight capacity imbalance of electrodes may drive negative electrode to work in the iodide-redox region, which could hamper the hybrid cell performance [57]; this can be avoided with a voltage cut at appropriate values [58].
Hydroquinone dissolved in H 2 SO 4 makes good redoxactive electrolyte for hybrid capacitors to achieve high capacitance [20]; however, the self-discharge and the loss of capacitance during cycling is quite high [19].To improve cycling characteristics, ion-exchange membranes were used [22] or the carbon electrode grafted with quinones was implemented [23]; the latter strategy results in strong covalent bonding between quinones and carbon which prevents shuttling.Composite of anthraquinone with reduced graphene oxide and carbons with quinone functionalities have demonstrated enhanced capacitance as electrode materials [59e61].Hybrid device based on LiMn 2 O 4 as a positive electrode and anthraquinone-modified carbon as a negative electrode in 1 mol L À1 Li 2 SO 4 is a good strategy to enhance energy and power parameters [62], and determining correct amount of anthraquinone loading on carbon appears to be key for achieving optimized performance [63].Redox activity of PPD in aqueous KOH greatly enhances the delivered capacitance of the hybrid cell [26], and high self-discharge in PPD-based cells could be solved by its covalent bonding to carbon surface [64].Polyoxomettalate-based Q7 redox species as part of electrode or electrolyte gave high capacitance owing to redox reactions which can be useful for improving hybrid capacitance [65e68].Ferricyanide/ferrocyanide redox couple showed promising results in neutral aqueous electrolyteebased hybrid capacitors up to 1.8 V, and the diffusion of redox species was prevented with ionexchange membranes [28].

Energy/capacitance metrics for hybrid capacitors
Capacitance in hybrid electrochemical capacitors must be calculated from total stored energy (area under discharge curve) [69e71].Energy efficiency can be calculated from the ratio of charge and discharge surface.Moreover, the mass of electrolyte should be taken into account while performing calculations [31,72], and for highly concentrated electrolytes (e.g., 20 mol kg À1 LiTFSI), the mass contribution of electrolyte could be even higher.Figure 3 shows the percentage contribution of each component to the total mass of an electrochemical capacitor.Energy values obtained by constant power test in Figure 3 are given for hybrid capacitor in 1 mol L À1 sodium iodide, normalized with mass of various cell components.High mass contribution from a current collector could be reduced by finding alternatives with lower density than stainless steel, e.g., surface-treated aluminum or aluminum alloys [73].

Temperature window for hybrid capacitors with aqueous electrolytes
For neutral aqueous electrolytesebased capacitors, freezing at ca. À10 C and enhanced electrochemical reactions due to water oxidation and reduction at high temperature restrict their application spectrum.Additives such as methanol [74] and ethylene glycol [75] improve the capacitor performance down to low temperatures.However, the preferential adsorption of alcohols in carbon porosity [74] modifies the local electrolyte composition in carbon nanopores resulting in performance fade down to low temperature (Figure 4a  and b).Recently, capacitors using aqueous choline chloride at 5 mol kg À1 demonstrated excellent performance down to À40 C [76] because of eutectic-like character of the electrolyte (choline chloride     The hybrid capacitor containing 5 mol kg À1 choline nitrate þ0.5 mol kg À1 choline iodide in water was able to operate down to À40 C with excellent performance of w50 F g À1 at À40 C and reach the power performance of organic electrolyteebased EDLC at low temperatures [53].For aqueous capacitors working at high temperature, concentrated electrolytes could be useful owing to low amount of free water (Figure 4c).In concentrated electrolytes, most of the water is used in solvation of ionic species; less free water molecules mean less electrochemical reduction of water and related hydrogen gas production.In addition, increased local pH inside the nanopores of carbon electrode favors high overpotential [79].

Conclusion and recommendations
1) Nanoporous carbonebased electrodes accommodate well the solid phase that prevents diffusion of redoxactive species.As solid phase tends to dissolve in acidic/alkaline media, neutral aqueous solutions are recommended in the form of bifunctional electrolytes for hybrid capacitors.2) Equilibrium potential can be adjusted close to the redox potential by appropriate balancing of electrodes capacities, e.g., mass balancing; hence, better cell engineering is key to design hybrid capacitors.3) For better power handling, carbon materials with high surface area and developed microstructure should be used for efficient EDL electrodes.4) True performance metrics can be presented by taking into account the mass/volume of each cell component and particularly the electrolyte.5) Choline cation and nitrates/chloride anions greatly differ in hydration strength; these electrolytes with eutectic-like properties could be tuned further to achieve wide temperature window for hybrid capacitors.

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March 2020 ■ 1/8 like electrode [18e26].Hybrid capacitors have also been proposed in aqueous solutions of neutral pH as supporting electrolytes for redox-active halides to achieve extended cell voltage [27e31], among which iodide-based hybrid capacitors are of particular interest owing to redox potential being close to the cell equilibrium potential (potential at discharged state) and the charge transfer between iodides and nanoporous carbon electrode (Figure 1b).