Investigation of sustainable ionic liquids for the electrochemical CO2 conversion to syngas.

Nowadays, the main reason for the increment of the CO2 presence in the atmosphere, which favors the consistency of the layer of gases responsible for the Greenhouse Effect, is the production of energy from fossil fuels. In addition, there are not enough industrial CO2 conversion processes that can overweight the increasingly amount of the gas generated as waste from many industries like thermal power plants or oil/gas refinery. In response to such problematic, the SunCoChem European Project in which this thesis work is involved, proposes an alternative for a better utilization of carbon dioxide, that includes its photo-electrochemical reduction into CO as an intermediate for the generation of oxo-products for the chemical industry. It has been reported that the first step in the electrochemical reduction (ECR) of CO2 to CO is the formation of the CO2- anion radical that occurs at quite negative potentials. The main objective of this thesis work is to explore the performance of five different ionic liquids made of imidazolium salts as helper catalysts in lowering the potential required to overcome the first barrier in the previous cited transformation, using a silver cathode that is well known to be selective towards the CO2 to CO conversion. In order to accomplish the purpose of the work, firstly the miscibility of the imidazolium salts with three different solvents as water, an aqueous solution of potassium hydroxide and pure acetonitrile (MeCN), has been tested for two concentrations. Showing that because of its organic nature, these salts are more miscible in acetonitrile than in the other solvents. Then, the electrochemical stability window of such organic solutions of MeCN was examined in a single electrochemical cell using a working and counter electrode of platinum. Once the potential range in which these five solutions of imidazolium salts do not react has been defined, the ECR of CO2 was performed in a two-compartment cell separated by a membrane with a silver electrode as cathode and a potassium hydroxide solution as anolyte. The onset potential of each reduction has been registered through linear sweep voltammetry (LSV) and the concentration of the gases produced in the reaction has been examined with a µ-GC. Particular attention is paid to the electrochemical setup designed for the ECR of CO2 while using imidazolium salts solutions since the formation of a precipitate was evidenced in each one of the experiments. Ion chromatography and 1HNMR confirmed that it was formed by the cation of the anolyte (K+) and the anion of the imidazolium salt. Many possibilities have been tried in order to reduce its production, related to the change of the anodic solution from a basic to an acid one, substituting the Nafion® membrane for a bipolar membrane, and finally alternating the concentration of the basic anolyte while using the second type of membrane. Further calculations were made in order to obtain the faradaic efficiency and the H2/CO ratio in each test. Based on these results, the imidazolium salt solution with the highest production ratio and lowest onset potential has been chosen as the most suitable one in the transformation of CO2 to CO, as well as the conditions at which the electrochemical cell is optimized.