Electrochemical CO2 reduction
Aachen (2015, 2016) [Dissertation / PhD Thesis]
Page(s): 1 Online-Ressource (xii, 189 Seiten) : Illustrationen, Diagramme
The atmospheric concentration of CO2 has increased significantly during the last two centuries. Since CO2 is considered to be one of the largest contributors to the greenhouse effect and is postulated to cause global warming, it is important to stabilize and/or reduce its concentration. Apart from regulations for the amount of CO2 that may be emmitted, carbon dioxide capture and storage (CCS), biological and chemical conversions are potential ways to stabilize and/or reduce the atmospheric concentration of CO2.In the present thesis, the electrochemical CO2 reduction in an electrochemical membrane reactor (ecMR) was investigated. An ecMR consists of two half-cells separated by a polymer electrolyte membrane. An electrical potential is applied as driving force for the reaction. The production of protons by the electrolysis of water takes place at the anode catalyst. The produced protons are transported through a cation exchange membrane (CEM) and electrons are transported via the external circuit to the cathode to reduce CO2(g) to hydrocarbons as desired products. The main undesired side reaction is the H2 evolution. According to literature, Cu is the most promising catalyst material for the electrochemical CO2 reduction if the focus is on the production of hydrocarbons.The Cu cathode catalyst was prepared by using electroplating on 3D-structured woven stainless steel meshes. In order to receive a thin and uniform Cu layer on the 3D-structured woven stainless steel mesh, the electroplating has to be performed at limiting current density. Higher temperatures and/or rotation speeds of the plating target caused an increase in the limiting current density.The electrochemical experiments for the reduction of CO2 were performed in a G-G and a G-L type ecMR. Membrane electrode assemblies (MEA) were applied when gaseous reactants were involved. A MEA can be used to create a so-called three-phase boundary, whereby (i) the electrocatalyst, (ii) the ionomer/electrolyte and (iii) the reactant are in contact. In the G-G type ecMR a double-sided MEA was used comprising of a copper felt cathode and a Ti felt anode coated with iridium mixed metal oxide (IrMMO) catalysts separated by a proton conductive membrane. Here, IrMMO is used as anode catalyst, since it showed a high activity for the oxygen evolution reaction (OER). In the G-L type ecMR a one-sided MEA was applied that only consists of the copper cathode and the CEM. The production of hydrocarbons with low current efficiency was achieved in both reactor types. The experiments performed with a G-L type ecMR did not show a limiting current density, which was the case when G-G type ecMR was used. A possible reason for the absence of the limiting current density is the conductivity of the membrane, which is increasing at higher relative humidity. Next to the experimental work, the electrochemical CO2 reduction in a G-L type ecMR was modelled in Aspen+.Further, the activity and stability testing of a new Pt/Ir/V catalyst was carried out for the OER in acidic environment. The overpotential of Pt/Ir catalysts for the OER at 2 mA/cm2 decreased with increasing Ir content for all studied temperatures. The V-containing catalysts deactivated during the stability measurements. However, the Pt/Ir catalysts showed only a slight deactivation, whereas pure Pt catalysts were stable after a whole period of stability tests. Besides, the activity for the oxygen reduction reaction (ORR) of the new Pt/Ir/V catalyst was determined. The Pt65Ir22V13 catalyst showed the highest activity for the ORR.
Kriescher, Stefanie M. A.