Catalysts for electrochemical conversion of CO2 in aqueous solutions
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An artificial photosynthesis process can 1) utilise renewable energy from the sun, 2) facilitate convenient energy storage in chemical bonds, 3) supply feed-stock to the chemical industry, and 4) recycle CO2 in a CO2-neutral process. This means that the world could be supplied with clean and renewable energy and chemicals for any relevant timescale. With the growth in renewable electricity production, electrolysis of water and CO2 to synthesis gas (H2+CO) or hydrocarbons is a promising strategy for artificial photosynthesis with the benefit of a direct utilization of surplus electricity from the grid. A major obstacle for such technology is the low reaction rate and low efficiency. Thus, the development of a good catalyst is a fundamental task. Copper has been engaging many researchers due to its unique ability to electrochemically produce hydrocarbons from CO2. Its periodic table neighbour zinc, has shown an ability to produce CO, and this makes zinc an interesting low-cost substitute for gold and silver for this reaction. Furthermore, the synergy of copper and zinc causes methanol to be formed at a high rate from gas phase CO, H2 and CO2. Therefore, a copper-zinc material with intimate contact between the two elements should be investigated for aqueous electrochemical CO2 reduction. Another means of increasing the reaction rate of a catalyst is having a high surface area, i.e. a porous structure. Carbon nanomaterials provide an electrically conductive, high surface area support - with a mesoporous structure that can enhance mass transfer. ZnO can easily be deposited on such fibres. The use of a copper-zinc material supported on carbon nanotubes (CNT) is therefore a promising strategy for catalyst design. The aim of this project has been to study CuxZn1-xOCNT catalysts for electrochemical CO2 reduction in aqueous solutions. When the copper content and the temperature for catalyst fabrication were varied, the catalyst morphology changed from crystalline nanoparticles decorating the nanotubes, to a film-like structure. H2 evolution was enhanced on smaller ZnO crystallites. Conversely, a smaller particle size did not lead to an improvement of CO evolution, which indicates that H2 and CO evolution have different structure sensitivity. Copper was incorporated in the ZnO structure such that copper was well dispersed and in intimate contact with zinc. A separate crystalline metallic copper phase emerged when the copper amount was 14 at% (x=0.14) and higher. The addition of a small amounts of copper stabilized the intermediates of CO2 reduction such that formation of CO was enhanced, and the optimal amount around 20 at%. At higher copper contents the activity for CO declined, methane formation was observed, and in particular CuCNT was much more active for H2 evolution. Also, the rate of catalyst deactivation was faster on catalysts with a segregated crystalline copper phase. For the CuxZn1-xOCNT material class, a high faradaic efficiency and moderate energy efficiency for synthesis gas formation was obtained, with compositions suitable for the Fischer-Tropsch process. The activity for synthesis gas production was better than on polycrystalline silver, and at a lower active material cost. The selectivity for CO was potential-dependent and declined at high current densities. Mechanistic investigations were done to elucidate the cause of this decline and to gain insight into the rate-limiting steps of the reaction. This work shows that ZnO doped with copper and supported on carbon is a good candidate for a low cost electrocatalyst for the formation of synthesis gas from CO2 and water.