Amorphous Transition Metal Dichalcogenides for Electrochemical CO2 Reduction

Electrochemical CO2 reduction (ECR) is a promising solution for utilizing carbon dioxide, converting it into value-added chemicals, while exploiting excess energy from intermittent renewable sources. The state-of-art electrocatalysts for ECR are transition metals (TMs), such as Ag for CO production, but the existence of linear energy scaling relations among the different reaction intermediates limits the maximum catalytic efficiency achievable with these materials. Instead, transition metal dichalcogenides (TMDs) are a promising alternative to TMs as they overcome these linear energy scaling relations and can, in principle, achieve higher CO2 reduction activity to complex products. Crystalline TMDs (c-TMDs) exhibit high catalytic activity in the edge sites, where CO is produced with high selectivity, when the undesired side reaction - Hydrogen Evolution Reaction (HER) - is suppressed. Additionally, defective-basal sites reduce CO2 to complex products via different reaction mechanisms from the edge sites, where reduction beyond CO is hindered by kinetic limits. Amorphous TMDs (a-TMDs) lack a long-range ordered structure and have many defective sites, thus formation of complex products is more plausible. However, their utilization for ECR is still not thoroughly investigated. This thesis work focuses on the study of group VI a-TMDs catalysts for ECR, starting from the work carried out with these materials at the Center for Nano Science and Technology (CNST-IIT@PoliMi). We tested four different nanostructured TMDs films, i.e. molybdenum selenide, molybdenum sulfide, tungsten selenide and tungsten sulfide, grown by Pulser Laser Deposition (PLD), a technique that allows to control the nanoscale features of the films, creating short-range ordered structures. The morphology of the films is chosen to investigate the intrinsic activity of the amorphous catalysts. The pristine films, irrespective of the material, share a nanocrystalline structure embedded in an amorphous matrix, which is a peculiarity of this deposition method. The actual catalytic phase of all four TMDs is formed through an electrochemical “activation” process, consisting in a reduction of the chalcogen ligands and in an oxidation of the metal centers, modifying the structure of the films. The activated sulfur-based films are unstable, while the selenium-based films resist in the operating condition of ECR. The activity of selenium-based films is investigated by time-dependent measurements in an ionic liquid-based (IL) and an aqueous electrolyte - more promising for large scale application - and by analyzing the reaction products. The films are stable for more than one hour, but the undesired HER prevails in IL electrolyte. Differently, in the aqueous electrolyte, despite the competitive HER, methane is also produced, which has never been observed on a-TMDs in protic solvents. A methane Faraday efficiency up to around 30% is obtained in both cases at low overpotential and a yield up to 0.9 umol/h/cm2 at -0.8 VRHE. Methane production is interesting since it suggests that a-TMDs have many defective active sites produced by PLD deposition and through the activation process, where ECR takes place with different mechanism than c-TMDs. These results are promising in the framework of employing defective a-TMDs structure for ECR to complex value-added products, which are unfeasible on c-TMDs.