Hybrid bio-electrochemical system for CO2 reduction to valuable products

Global warming is an important worldwide problem and the CO2 emissions are one of the main concerns. The use of CO2 as raw material for value added products has not already an important relevance; the small quantity used so far is mainly converted in carbonates and urea. It can be understood since carbon dioxide is a very stable molecule. Catalysts are being investigated to reduce the energy required to transform it to more complex molecules. Electrochemical reactions let to achieve transportation fuels, also have good controllability, efficiency and scalability, but in general have a low selectivity. In contrast, the most remarkable benefits of the use of biocatalysts, in addition to be environmentally friendly, are their high selectivity and specificity, but are more difficult to control, less flexible with the change of conditions and very instable. An example of interest is the production of methanol with three-enzymes system. First the CO2 transforms into formate, then in formaldehyde and finally in methanol, respectively with the aid of Formate Dehydrogenase (FDH), Formaldehyde Dehydrogenase (FaldDH) and Alcohol Dehydrogenase (ADH). However, this technique still is inefficient because needs long times of reactions and has very low productivity, the major problem is the second step, as the FaldDH is highly affected by the substrate – product ratio. About electrochemical methods, the formic acid or formate production with metal surfaces as Sn is remarkable thanks to its high selectivity. Instead, the cupper catalysts are interesting since a lot of different compounds can be obtained, as aldehydes; these reaction pathways are being studying to improve the selectivity and to control better the reaction mechanism. At least, examples of hybrid approaches include the use of microorganisms to take the CO2 and use it in their electron transfer processes and the enzymatic cascade mentioned first, coupled with electrical or photoelectrical systems to regenerate the NADH cofactor, molecule needed to carry out the reactions. This work focusses on the transformation of carbon dioxide in value added chemicals, such as fuels, with an electrocatalytic-enzyme hybrid system. Two main arrangements were tested. First tin oxide was coupled with the FDH, to evaluate the desirability of combining both types of catalyst to increase the production of formate. Then a sequential reaction was searched: the CO2 reduction with cupper to obtain aldehydes and a second step with the ADH to arrive to methanol. The experimental development began with a characterization of the enzymes, measuring the enzyme activity and choosing the more adequate immobilization technique, in part, because of its compatibility with the electrocatalytic spray coated deposition. The material used in all the research was the carbon felt. Then, electrochemical activity tests were done in an electrochemical cell by varying the cathode electrode. The different systems compared were: the bare electrocatalyst electrode, the immobilized enzyme, the deposited electrocatalyst with the enzyme in solution and, finally, both electrocatalyst and enzyme attached to the electrode. Between others, as parameter of valuation, the faradic efficiency was used.