Ionic liquid carbon anodes

Table 5 shows cathodic and anodic limits of electrochemical stability windows of a number of ionic liquids. The cathodic limit of the stability window of the ILs based on the EMIm+ and BMIm+ cations, investigated at the glassy carbon electrode, is -2.1 V against the Ag/Ag+ (0.01M in DMSO) reference. The BMPy+ cation is reduced at the glassy carbon at considerably more positive potential, at ca. -1.0 V. 

Anodic limit, potential referred to Li+/Li, cutoff current density in parentheses. Scan rate 5 mV s. Activated carbon as working surface. Scan rate 10 mV s. Supporting electrolyte 0.1 M BU4NBF4. Scan rate 100 mV s . The solvent-free condition was realized by using an ionic liquid based on Imldazolium cation, at 80 °C. Scan rate 20 mV s . ... 

Further examples of recent attempts to reduce the consumption of electrical energy are the electrolysis of aqueous solutions of methanol (but CO2 is still produced at the anode) [78, 79] and water electrolysis using ionic liquids as electrolytes [80]. In the latter case, the authors claimed the possibility of obtaining high hydrogen production efficiencies using an inexpensive material such as low-carbon steel. 

Most work to date has either used soluble anodes or has not considered the anodic reaction. A limited amount of information has been collated on the electrochemical windows of ionic liquids but this tends to be on either platinum or glassy carbon, which is not necessarily suitable for practical plating systems [1], The anodic limits of most liquids are governed by the stability of the anion, although pyridinium and EMIM salts are sometimes limited by the stability of the cation. The widest electrochemical windows are obtained with aliphatic quaternary ammonium salts with fluorous anions. A selection of potential windows is given in Chapter 3. 

The limited reversibility of some electrode reactions might require consideration of consumable (cheap) ionic liquids in the anode compartment for technical applications and commercial electroplating. For example, the electrochemical oxidation of oxalate delivers carbon dioxide, hydride could be oxidized to hydrogen, halides to the halogen or trihalide salt in the case of iodide ionic liquids and so on. Since ionic liquids can readily form biphasic systems an alternative may be to have the anodic reaction in an immiscible solvent. In that case a common ion would be needed that can be transferred from one phase to the other. 

Deng and his coworkers found that CO2 was reduced at a cupper cathode at —2.4 V vs. Ag/AgCl [42]. They successfully prepared cyclic carbonates by the reduction of CO2 in the presence of epoxides in various ionic liquids like [EMIM][BF4], [BMIMJiPFg], and A-butylpyridinium tetrafluoroborate [BPy][BF4] using a Cu cathode and Mg or A1 anode as shown in Scheme 8.16. This electrolysis is nonfaradaic reaction in which a small amount of electricity engaged in the electroreduction of CO2 generates catalytic species responsible for the addition of CO2 to epoxides to form cyclic carbonates. 

Lu and his coworkers reported the electroreduction of benzoylformic acid at a glassy carbon cathode and Pt anode in ionic liquid [EMlM][Br] at 80°C giving mandelic acid in high yield and with moderate current efficiency as illustrated in Scheme 8.19 [46]. 

Zheng H, Jiang K, Abe T, Ogumi Z (2006) Electrochemical intercalation of lithium into a natural graphite anode in quaternary ammonium-based ionic liquid electrolyte. Carbon 44 203-208... 

The synthesis of cyclic carbonates in RTILs, via cycloaddition of cathodically activated carbon dioxide to epoxide, has been reported by Deng et al. [139]. Ionic liquids, saturated with CO by bubbling at normal pressure and containing the epoxidic substrate, were electrolyzed in an undivided cell (Cu as cathode. Mg or Al as sacrificial anode). The electrolyses were carried out under potentiostatic conditions at a potential negative enough to the selective reduction of CO to CO (E=-2.4 V vs. 

Zheng H., Liu G., Battaglia V. Film-Forming Properties of Propylene Carbonate in the Presence of a Quaternary Ammonium Ionic Liquid on Natural Graphite Anode, J. Phys. Chem. C 2010, 114,6182-6189. 

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