Surfactants as Performance-Enhancing Additives in Supercapacitor Electrolyte Solutions: History
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Xuecheng Chen
,
Rudolf Holze

Wetting the surface area of an electrode material as completely as possible is desirable to achieve optimum specific capacity of an electrode material. Keeping this surface area utilized even at high current densities and even when inside small pores is required for high capacitance retention. The addition of surfactants at small concentrations to aqueous supercapacitor electrolyte solutions has been suggested as a way to improve performance in terms of capacitance, capacitance retention at increased current density and stability. Effects are pronounced with carbon materials used in electrochemical double-layer capacitors; they are also observed with redox materials. 

  • supercapacitor
  • ultracapacitor
  • electrochemical capacitor
  • surfactant

1. Introduction

Supercapacitors (for the sometimes confusing usage of their names, trademarks, etc., see [1][2]) are devices in electrical engineering and electronics that accept and deliver extremely high currents, i.e., high power, like a large conventional capacitor. They store electric energy by separating electric charges across electrochemical interfaces at the boundary between an ionically conducting phase, the electrolyte or electrolyte solution; and an electronically conducting phase, the electrode. Two such interfaces or electrodes joined together by the common ionically conducting phase form the complete device. These operating principles characteristic of a supercapacitor of the electrochemical double-layer capacitor type (EDLC-type) and of a redox-type supercapacitor are sketched schematically in Figure 1. Different from the conventional dielectric capacitor, the extremely large surface areas of the participating electrodes, the amount of stored charge and, thus, the amount of stored energy is much larger for a supercapacitor. Further increase of the amount of stored charge is possible when redox reactions at the materials surface or near to it are used. The associated electrochemical behavior is frequently called pseudocapacitive because it appears capacitor-like but is caused actually by a Faradaic redox reaction (see also below). The respective device may thus be called redox-capacitor.
Figure 1. Schematic principles of operation of supercaps.
The cell voltage of a supercapacitor is limited by the stability of the ionically conducting phase; too-high voltages will result in electrolytic decomposition of the solvent or the electrolyte. A water-based electrolyte solution limits the cell voltage to values in the range of 1.2 V; sometimes, higher values are claimed because said decomposition may be extremely slow at one or both electrodes because of, e.g., kinetic hindrances. With organic-based solvents, larger values are possible. Relationships between the properties of the employed materials and the considered range of possible electrode potentials and cell voltages have been critically examined [3]. Too-large values may result in overoxidation of the electrode material at the positive electrode (sometimes slightly confusingly called the cathode) [4].
Because the amount of stored energy depends on the amount of stored charge and the electrode potentials/cell voltage associated with this charge separation, it is possible to store larger amounts of energy, preferably by storing larger amounts of charge; the accessible potentials and voltages are limited as described. The charge per unit area of a typical electrochemical interface (the double layer) and voltage difference (i.e., electrode potential) can be expressed in terms of an interfacial capacitance. Taking the Helmholtz model [5] and the common equation for a parallel-plate condenser
 
with permittivity ε = 6 F·m−1 and distance d = 300 pm, a value C = 18 μF·cm−2 is calculated as the capacitance. The capacitance of the electrochemical double layer CDL can subsequently be considered either in differential form [6][7]:
 
or in integral form:
 
with the electrode potential E, the electrode potential of zero charge Epzc and q = ΔQ. They are related to each other according to:
 
Both Cint and Cdiff can be established experimentally [8][9][10][11][12][13].
The numerical value calculated with Equation (1) is very close to a repeatedly mentioned value of 20 μF·cm−2 for a very flat metal surface in contact with an electrolyte solution of moderate concentration. This number may not be applicable for carbon materials. Randin and Yeager studied several high-pressure stress-annealed pyrolytic graphite SAPGs. For the basal plane, they reported Cdiff = 3 μF·cm−2 [14][15]. This low number was associated with the space charge region inside the SAPG but not with the electrochemical double layer (see also [16]). With boronated SAPG, numbers varied with the identity of the electrolyte solution composition and were slightly higher [17]. Edge-oriented SAPG yielded Cdiff = 50–70 μF·cm−2 [18]. Corresponding numbers for highly oriented pyrolytic graphite HOPG and natural graphite crystals were much lower [19]. Ordinary pyrolytic graphite with a lower degree of orientation yielded for the polished basal plane Cdiff = 16 μF·cm−2 [14]. Numbers stated elsewhere for ordinary pyrolytic graphite OPG showed significant variations [20]. With respect to the major structural differences in carbon [21], a wide variation in the highest values of Cdiff per m2 of surface area and correspondingly per gram of material may be anticipated [22]. An approach to bridge the mismatch of surface areas determined with the Brunauer–Emmett–Teller (BET) method without/with further backing by density functional theory (DFT) as well as additional treatments considering further possibilities like structural changes during treatment of carbon materials into a concrete electrode and Cdiff has been reported [23]. A specific capacitance of 0.1 F per m2 effective surface area (which may differ considerably from the BET surface area) was tentatively concluded. Exactly the same value was reported earlier by some of these authors [24]. Gagnon stated exactly this value earlier based on a study of carbon blacks with various BET-surface areas in contact with concentrated electrolyte solutions of KOH [25]. A study by Wen et al. added further complications when establishing a correlation [26]. Values of CDL per BET surface area increasing with growing pore size were ascribed to higher utilization of that surface area. Further details are discussed elsewhere [27].

2. Approaches to Increased Material Utilization

A larger storage capability, i.e., a larger capacitance, can accordingly be achieved with a larger interfacial area. The use of highly porous materials as electrodes is thus the central idea in the technical development of the supercapacitor [1]. Larger surface areas can be achieved mainly by utilizing ever-smaller pores, meaning more porous materials must be used. During this step, further aspects of interfacial science come into play: such surfaces inside ever-smaller pores must be accessible for the ionically conducting phase. Surface tension, capillary forces, viscosity and wetting are of growing importance. Initial fears that pores with too-small openings would be inaccessible for electrolyte solutions, in particular for solvated ions needed for charge storage, turned out to be partially incorrect. Starting with early observations [28] in subsequent studies, evidence of at least partial desolvation of ions was reported [29][30][31][32][33]; this extended the range of useful pores to smaller values than initially expected. However, accessibility of the internal pore surface is not sufficient for materials surface utilization. An electrochemical double layer must be established; in addition, for the surface to become an electrochemically active surface area [5], it must come into contact with the electrode (solution): it must be wetted. Moreover—last but not least—ionic movement into and out of even tiny pores must proceed at a rate compatible with large electric currents typical of a supercapacitor. With ever-smaller pore openings, conceivable interactions between moving ions and the electrochemical double layer on the walls of the pore mouth may become influential.
It is possible that not all surfaces of a material in a supercapacitor electrode, whether it is an activated carbon (AC), a chalcogenide or any other redox-active material showing pseudocapacitive behavior, will be immediately wetted and sufficiently hydrophilic. This depends very much on the material properties of the solid and the liquid phases. Wetting of carbon materials has been discussed before (in the context of fuel cell technology and, later, of supercapacitors) [22]. Some carbons (e.g., carbon black and acetylene black) are highly hydrophobic, and some (many activated carbons and some graphitic materials) are more or less hydrophilic. The hydrophilicity of carbon surfaces can be increased by the creation of surface functional groups containing oxygen. In a typical study, carbon fibers created from polyacrylonitrile were exposed to oxygen at elevated temperatures [34]. An increase in Cdiff of only a few percent was noticed, but the Faradaic current, presumably due to surface redox reactions resulting in a pseudocapacitive behavior, increased by several orders of magnitude. Thus, the overall increase in capacitance (about 25%) of a complete device was attributed mostly to redox processes, but only a very small fraction was attributed to increased utilization because of wetted surface area. Unfortunately, such oxidative treatment may contribute to faster self-discharge of devices [35] and faster aging [36].
Nevertheless, wetting behavior beyond surface properties and pore-size considerations was never ignored. For improved wetting of a material’s surface, surfactants may be helpful; their use has been established in various fields and industries for a long time [37][38]. Surfactants of interest in the present context can be classified into non-ionic, cationic and anionic ones. Typical examples and some frequently encountered compounds are depicted in Figure 2. Generally, they contain a hydrophilic section/end containing heteroatoms like oxygen, and a hydrophobic section with aliphatic or aromatic CH-building blocks. The molecular sizes range from about 0.5 to 2 nm.
Figure 2. Molecular formulas, systematical names, tradenames and acronyms of representative surfactants.
The same considerations regarding porosity and wetting apply to redox-active materials showing pseudocapacitive behavior, as first proposed by Conway et al. [39][40][41][42][43], as well as plain battery electrode materials. The particular behavior of some of these materials, showing in CVs a response to a changing electrode potential closely resembling that of a capacitor, has resulted in the designation by Gileadi and Conway [41] of pseudocapacitive behavior; for an update, see [44]. An earlier use of the term “pseudo-capacity” by Grahame [45] refers to redox processes at the mercury electrode/aqueous electrolyte solution interface with reduction and subsequent oxidation of cadmium/cadmium ions, i.e., involving solutions species different from the suggestion by Gileadi and Conway involving redox reactions of surface-attached species.
Complete wetting of the surface area of carbon or any other supercapacitor electrode material may be impeded by insufficient hydrophilicity or pores with openings too small for the electrolyte solution to get inside because of surface tension [46]. To improve this situation, surfactants [38][39] improving wetting by reducing the surface tension of the electrolyte solution and/or reducing surface hydrophobicity [47] have been added to aqueous electrolyte solutions. The first study of surfactant effects has been reported with surfactant-treated carbon materials (no surfactant addition to the electrolyte solution) [48]. The studied materials were soaked in solutions of unspecified surfactants and dried. Enhanced wetting was easily verified visibly. Apparently, and not very surprisingly, some surfactant was trapped in the porous electrode material and was transferred into the device. No experimental results suggesting specific effects of surfactant molecules adsorbed on the electrode material surface were reported. Considering the already mentioned size of surfactant molecules, they may be too large to enter into very small pores, and they may actually block openings of very small pores, resulting in a negative effect on capacitance [49][50][51][52][53].
Most reports on the addition of surfactants into electrolyte solutions (including those inspected within this entry) deal with aqueous electrolyte solutions, they will be the focus of interest in the following sections. Nonaqueous solutions have been studied frequently because they permit a wider cell voltage window of operation. Basically, the same considerations apply, but with an inverted perspective: instead of wetting with an aqueous phase, wetting with a nonaqueous phase is required. Instead of hydrophilic surfaces, hydrophobic surfaces are of interest. Accordingly, hydrophobization (instead of hydrophilization) of surfaces is an option to increase better “wetting” of a surface. Surfactants other than those shown in a representative selection in Figure 1 may be effective. Better hydrophobization by vinyltrimethoxysilane than with sodium oleate has been reported with a carbon aerogel (both as-prepared and activated) as electrode material [54][55]. The same benefits (higher specific capacitance and better capacitance retention with increased current density and lower electric series resistance ESR of a complete cell) achieved with surfactant treatment as observed before with aqueous solutions were found. Different from surfactant addition to the electrolyte solution in this example, modification of the carbon surface via grafting (i.e., chemical attachment via covalent bonding) of the surfactant was achieved. Less pronounced effects were achieved by adsorptive treatment of a carbon aerogel with sodium oleate [56]. Another option of surfactant use has been examined in studies of ACs vacuum-impregnated with fluorinated surfactants [57]. Moderate performance improvements (higher capacitance and better stability) were best with a cationic surfactant.

3. Results and their current understanding

Fic et al. [58][59][60] reported on the effect of vari­ous sur­factants added at 5 mM concentration into the 6 M KOH electro­lyte solution of a symmetric supercapacitor of the EDLC-type with activated carbon elec­trodes. Higher surfactant concentrations were inefficient because of mi­celle for­mation. Best improvements including enhanced capacitance at higher current density, i.e., major im­provement of capacitance retention with growing current density, slower self-discharge (of a complete device in terms of e.g. cell voltage or a single electrode in terms of electrode potential) and in­creased sta­bility were found with non-ionic Triton X-100. The claimed main effect was lower surfface tension of the aque­ous phase. Unfortunately, these statements appear to be slightly inconclusive. Upon closer inspection at lowest current density, addition of any surfactant did not show an increase of specific capacitance. Accordingly a simple wetting enhancement is not the full explana­tion – if any explanation at all because it should have resulted in higher values of spe­cific capacitance even at lowest scan rates. The significantly improved capacitance reten­tion with the studied AC electrode material suggests instead better utilization of in­ner pore surface area than without added surfactant. How this can be cause by addition of a surfactant remains to be studied. This conclusion also applies to the noted improved stability.

With redox-active electrode materials, e.g. metal chalcogenides, reported effects are so far small, observations are rare. 

This entry is adapted from the peer-reviewed paper 10.3390/batteries10010004

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