Ionic liquids electrochemical behavior

Rochefort D, Pont AL (2006) Pseudocapacitive behavior of Ru02 in a proton exchange ionic liquid. Electrochem Commun 8 1539-1543... 

Recently, a eutectic mixture of choline chloride and urea (commercially known as Reline) was used as a medium from which CdS, as well as CdSe and ZnS, thin films were electrodeposited for the first time [53]. Reline is a conductive room-temperature ionic liquid (RTIL) with a wide electrochemical window. The voltammetric behavior of the Reline-Cd(II)-sulfur system was investigated, while CdS thin films were deposited at constant potential and characterized by photocurrent and electrolyte electroabsorbance spectroscopies. 

The ability to vary the composition of Lewis or Bronsted acid adds an additional dimension to the tuneability of the eutectic-based ionic liquids. It has been shown that the Lewis acidity of the liquid affects not only the physical properties of the liquids but also the electrochemical behavior. Type I ionic liquids are also clearly... 

Endres et al. [82] have demonstrated the suitability of an air- and water-stable ionic liquid for the electropolymerization of benzene. This synthesis is normally restricted to media such as concentrated sulfuric acid, liquid SO2 or liquid HF as the solution must be completely anhydrous. The ionic liquid used, l-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, can be dried to below 3 ppm water, and this ionic liquid is also exceptionally stable, particularly in the anodic regime. Using this ionic liquid, poly(para-phenylene) was successfully deposited onto platinum as a coherent, electroactive film. Electrochemical quartz crystal microbalance techniques were also used to study the deposition and redox behavior of the polymer from this ionic liquid (Section 7.4.1) [83]. 

Ionic liquids can be compared to any other liquid in that the reactivity of a species will depend upon its relative activity in solution. To this end it is important to consider the relative Lewis and Bronsted acids that can interact with the solutes to affect their activity. It is also important to remember that ionic liquids with discrete anions have wider potential windows and what we therefore hope to achieve with them is more susceptible to the presence of reactive species. The influence of impurities on the electrochemical behavior of an ionic liquid will depend upon the relative Lewis acidity/basicity of the liquid and of the redox process in question. Eutectic-based ionic liquids behave very differently from ionic liquids with discrete... 

The main contaminants in an ionic liquid will be introduced from the synthesis, absorbed from the atmosphere or produced as breakdown products through electrolysis (see above). The main contaminants for eutectic-based ionic liquids will be from the components. These will be simple amines (often trimethylamine is present which gives the liquid a fishy smell) or alkyl halides. These do not interfere significantly with the electrochemical response of the liquids due to the buffer behavior of the liquids. The contaminants can be effectively removed by recrystallization of the components used to make the ionic liquids. For ionic liquids with discrete anions the major contaminants tend to be simple anions, such as Li+, K+ and Cl-, present from the metathesis technique used. These can give significant difficulties for the deposition of reactive metals such as Al, W and Ti as is demonstrated below with the in situ scanning tunnelling microscope. 

The electrochemical behavior of titanium species has been investigated in an acidic EMICI-AICI3 ionic liquid [32]. Titanium dichloride, TiCl2, dissolves up to 60 and 170 mmol dm at 80°C in the acidic ionic liquids of 60.0 and 66.7 mol% AICI3, respectively. The divalent titanium species, Ti(ll), is less stable in the weakly acidic ionic liquid, leading to the disproportionation into insoluble titanium trichloride and metallic Ti. Metallic Ti is impossible to obtain probably because of the large overpotential of the electrodeposition of metallic Ti whose reduction potential is... 

There are some reports on the electrodeposition of cobalt [105], nickel, zinc, and magnesium [106] in N(CF3S02)2 ionic liquids. The electrochemical behavior of lanthanide elements has been reported in EMIN(CF3S02)2 and BMPN(CF3S02)2 ionic liquids, though their metals were not obtained within the electrochemical potential windows of these ionic liquids [107]. 

Trombetta E, de Souza MO, de Souza RE et al (2009) Electrochemical behavior of aluminum in l-n-butyl-3-methylimidazolium tetrafluoroborate ionic liquid electrolytes for capacitor applications. J Appl Electrochem 39 2315-2321... 

The electrochemistry of titanium (fV) has been exanuned in acidic l-ethyl-3-methyUmidazolium chloride/AlClj ([EmimJCl/AlClj) in 1990 by Carlin et al. [180]. It was shown that titaifium is reduced to Ti(lll) and Ti(n) in two one-electron steps, both of which exhibit slow electron-transfer kinetics. Ten years later, Mukhopadhyay et al. smdied the deposition of Ti nanowires at room temperature from 0.24 M TiCl in the ionic liquid l-butyl-3-methylimidazohumbis((trifluoromethyl)sulfonyl)amide [181]. They found that up to six wires grow at constant potential over a period of about 20 min wires exhibit a narrow width distribution of 10 2 nm and have a length of more than 100 nm. The chemical and electrochemical behavior of titanium was examined in the Lewis acidic [EmimlCl/AlClj molten salt at 353.2 K. The electrodeposition of Al-Ti alloys at Cu rotating disk and wire electrodes was investigated by Tsuda et al. [55]. It was found that Al-Ti alloys which contain up to 19% (atomic fraction) titanium, could be electrodeposited from saturated solutions of Ti(II) in the... 

Suarez PAZ, Consort CS, De Souza RF, Dupont J, Goncalves RS (2002) Electrochemical behavior of vitreous glass carbon and platinum electrodes in the ionic liquid l-n-butyl-3-methyUmidazolium trifluoroacetate. J Brazil Chem Soc 13 106-109... 

The term ionic liquid is commonly used for the molten salts whose melting point is below 100°C (48). In particular, the salts that are liquid at room temperature are called room temperature ionic liquids. The earliest known ionic liquid (published in 1914 49, 50) was ethyl ammonium nitrate EtNHfNO, which has a melting point of 12°C. It was these initially developed ionic liquids (molten salts) that were used as electrolytes to study the electrochemical behavior of other compounds. Recently, ionic liquids with interesting properties have been synthesized and used as solvents, and studied in different areas of chemistry (26-44). 

The dynamic behavior of ionic liquids is important for both practical and theoretical reasons. From a practical standpoint, bulk transport properties such as the viscosity, self-diffUsivity, thermal conductivity and electrical conductivity govern the effectiveness of these liquids in any application. For example, mass transfer of reactants and products is critical to the performance of ionic liquid solvents, and is highly correlated with the self-difiiisivity and viscosity. Viscosity also plays a role in the cost of pumping the liquid and its performance as a lubricant. Thermal conductivity is a key parameter for thermal fluid applications, and electrical conductivity is obviously important in electrochemical applications. 

Liu, W. W., X. B. Yan, J. W. Lang, and Q. J. Xue. 2011. Electrochemical behavior of graphene nanosheets in alkylimidazolium tetrafluoroborate ionic liquid electrolytes Influences of organic solvents and the alkyl chains. Journal of Materials Chemistry 21 13205-13212. 

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