Pseudocapacitance Metal oxide

Pseudocapacitance is used to describe electrical storage devices that have capacitor-like characteristics but that are based on redox (reduction and oxidation) reactions. Examples of pseudocapacitance are the overlapping redox reactions observed with metal oxides (e.g., RuO,) and the p- and n-dopings of polymer electrodes that occur at different voltages (e.g. polythiophene). Devices based on these charge storage mechanisms are included in electrochemical capacitors because of their energy and power profiles. 

In supercapacitors, apart from the electrostatic attraction of ions in the electrode/electrolyte interface, which is strongly affected by the electrochemically available surface area, pseudocapacitance effects connected with faradaic reactions take place. Pseudocapacitance may be realized through carbon modification by conducting polymers [4-7], transition metal oxides [8-10] and special doping via the presence of heteroatoms, e.g. oxygen and/or nitrogen [11, 12]. 

CNTs were also demonstrated to be a perfect support for cheap transition metal oxides of poor electrical conductivity, such as amorphous manganese oxide (a-Mn02 H20) [5,96], The pseudocapacitance properties of hydrous oxides are attributed to the redox exchange of protons and/or cations with the electrolyte as in Equation 8.13 for a-Mn02 H20 [97] ... 

Among the metal oxide pseudocapacitive materials the most representative are the crystalline ruthenium oxide Ru02 [24] and the amorphous hydrous ruthenium oxide Ru02 XH2O [27, 28], although other materials are under study, for example cobalt oxide [29] and vanadiun oxide [30] xerogels, molybdenum-based materials [31, 32], and Ti-V-W-O oxides [33]. 

The two large families of pseudocapacitive materials under study are conducting polymers [14-17] and metal oxides [8,18-20]. In the case of carbon materials, the surface functionality can also participate to pseudocapacitive redox reactions, as it will be shown later. 

Pseudocapacitors store charge based on reversible (faradaic) charge transfer reactions with ions in the electrolyte. For example, in a metal oxide (such as RUO2 or I1O2) electrode, charge storage results from a sequence of redox reactions. Electrochemical capacitors (ECs) based on such pseudocapacitive materials will have both faradaic and nonfaradaic contributions. The optimization of both EDLCs and pseudocapacitors depends on understanding how features at the nanoscale (e.g. pore size distribution, crystaUite or particle size) affect ion and electron transport and the fundamental properties of electrochemical interfaces. 

Yes, through a pseudocapacitive process such as is the case for V2O5, or through conversion reactions where the metal oxide is converted into nanometer-sized metallic particles dispersed in a Li20 matrix. 

Thin films of oxides and sulfides of transition metals, HS2, LiMn204, LiNi02, LiCo02 with the thickness of 5-15 pm deposited onto supports of A1 or stainless steel were studied as electrodes for PsCs. The electrolyte was the solution of LiAsF in PC. The faradaic process determining pseudocapacitance was the reaction of Li intercalation into the matrix of a transition metal oxide. The capacitance of the studied PsCs was 400 pF/cm for FiNi02 and LiCo02 and 1500 pF/cm for 1182. 

The nature of the electrolyte and its interaction with the electrode materials has a strong influence on the pseudocapacitive behavior and thus the pseudocapacitance. Therefore, the pseudocapacitive mechanism is briefly reviewed. As shown in Figure 1.7, there are several types of electrochemical processes that can contribute to the pseudocapacitance, including reversible adsorption of ions from the electrolyte, redox reactions involving ions from the electrolyte for some transition metal oxides, reversible electrochemical doping/dedoping in conductive polymer-based electrodes, and lattice intercalation [11,41,69,71]. 

This type of pseudocapacitance originates from the redox reactions on some electroactive materials such as metal oxides and hydroxides. RUO2 is the first and one of the most extensively studied redox-type pseudocapacitive material for ESs because of its high specific capacitance from 200 to 1000 F g depending on its structure. The redox reactions of RUO2 in the acid solution involve fast reversible electron transfer accompanied by electro-adsorption of protons on the surface or insertion of protons into the bulk RUO2 [68,72,73] ... 

Brezesinski, K., J. Wang, J. Haetge et al. 2010. Pseudocapacitive contributions to charge storage in highly ordered mesoporous group V transition metal oxides with iso-oriented layered nanocrystalhne domains. Journal of the American Chemical Society 132 6982-6990. 

The pseudocapacitance can also be provided by other pseudocapacitive materials such as some metal oxides and electrically conductive polymers (ECPs) that have much higher theoretical capacitance than carbon-based materials. These materials have been reviewed in detail elsewhere [89,90]. Although many materials have been reported to exhibit pseudocapacitive behavior, they are very sensitive to the type and pH of the electrolytes and few of them are suitable for application in strong acid electrolytes. As previously mentioned in Section 1.3.2, RUO2 is one of the most extensively studied pseudocapacitive materials in H2SO4 electrolytes. 

Similar mechanisms have been proposed for other metal oxide-based pseudocapacitive materials, such as molybdenum oxide [202,203]. Since ions of electrolytes... 

FIGURE 2,25 Linear regression fit of (a) capacitances against the pH of H2SO4 electrolyte and (b) capacitances against the pK of KCl electrolyte solutions for RuOj electrode. (Reprinted from Electrochimica Acta, 50, Wen, S. et al.. The role of cations of the electrolyte for the pseudocapacitive behavior of metal oxide electrodes, MnOj and RuOj, 849-855,... 

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