Co2 valorization by electrochemical reduction using catalysts supported on nonastructured carbon materials

Resumen

During the last years, CO2 concentration in the atmosphere has been increasing, being the major responsible of global warming. Therefore, several mitigation technologies have been proposed, such as the chemical use of CO2, the carbon capture and storage or its conversion. Among them, the CO2 reutilization by electrochemical route seems to be of particular interest since it could both mitigate the greenhouse gas emission and use CO2 as a carbon source for producing a variety of useful products. The electrochemical reduction of CO2 has been widely studied on several bulk metals suspended on different electrolytes. However, the development of new catalysts with high activity and selectivity continues being a key challenge. This process could be improved using gas diffusion electrodes (GDEs) based on carbon-supported catalysts, which allow a better distribution of CO2, enhancing the rate of CO2 conversion. This PhD Thesis focuses on the CO2 valorization by electrochemical route using catalysts supported on nanostructured carbon materials (NCMs). First, different NCMs were synthesized to evaluate the influence of the carbon nature on its activity toward the CO2 electrochemical reduction. Concretely, ordered mesoporous carbons due to their high surface area, as well as carbon nanofibers and carbon nanocoils because of their graphitic character were prepared. In addition, the material Vulcan XC-72R (supplied by Cabot) was also used for comparison with the commercial carbon conventionally employed for electrochemical applications. A similar behavior for CO2 reduction was obtained for the NCMs. In general, a lower cathodic current and an inhibition of the hydrogen evolution reaction (HER, which takes places by the reduction of water) was obtained in presence of CO2, which may be explained by the adsorption of species derived from CO2 reduction at the carbon surface. This adsorption took place at more negative potentials in the case of Vulcan. In addition, a study of the impact of oxygen functionalization of Vulcan on the electrochemical activity was also performed. It was found that the presence of oxygenated species at the surface carbon and their distribution may change the activity toward the HER, modifying the performance for CO2 conversion. After that, different metals (Pt, Pd, Ni, Cu, Co and Fe) were deposited on the NCMs with the aim of obtaining catalysts for CO2 reduction. Electrochemical studies showed that CO2 was effectively reduced to other species at the surface of carbon-supported electrocatalysts. Additionally, it was probed that these species were adsorbed on the electrodes, which could be promoted by the carbon support. GDEs were also prepared from the carbon-supported electrocatalysts and their activity and selectivity for CO2 conversion was analysed by differential electrochemical mass spectrometry (DEMS), i.e. a mass spectrometer in situ coupled to an electrochemical system. This technique allows following in situ the electrochemical properties of the electrodes and, simultaneously detecting the products/intermediates from CO2 reduction. However, DEMS is not commercial and an adaptation of the experimental setup was necessary in order to characterize GDEs and elucidating the reaction mechanism. Formic acid/formaldehyde was obtained as the main product on electrodes based on Fe oxides supported on Vulcan at room conditions. This result is really promising since formic acid can be used as fuel or as raw material in many applications for agriculture, chemical or textile industries. On the other hand, it was found that the activity of the CO2 reduction and the product distribution were strongly influenced by the surface chemistry of the support. Finally, a novel electrochemical reactor was designed to perform the electrochemical reduction of CO2 in gas phase. Operating in gas phase presents many advantages in comparison to the conventional aqueous devices since the problem of the low solubility of CO2 is suppressed and the HER prevented due to the lack of electrolyte. The reactor, which presents a technology similar to a polymer electrolyte membrane fuel cell, consists of two GDEs, anode and cathode, separated by a polymer membrane. Electrodes based on the carbon-supported electrocatalysts previously synthesized were used as cathode. The reactor did not present a strong polarization, and the results were in correlation with the measurements in liquid phase, showing its potential for characterizing GDEs.