In English

Production of green fuels from CO2

Pascal Gebauer
Göteborg : Chalmers tekniska högskola, 2015. 60 s.
[Examensarbete på avancerad nivå]

Abstract Electrochemical reduction of CO2 is a research direction within electrochemistry that adresses environmental probems related to CO2emissions.CO2 is reduced in the presence of a catalyst material and an electrolyte to hydrocarbons by providing energy to the process. The technique promises a pathway towards a carbon neutral energy cycle by using renewable as an energy source. In this Master’s thesis a set-up for electrochemical reduction of CO2 has been assembled, calibrated and its successful operation verified by repetition of literature results on a Cu electrode. Additionally, Au electrodes with different roughness factors have been produced in order to analyse the influence of the surface structure on the selectivity of the reduction of CO2 to CO and the decrease of overpotential for this reaction as reported in literature before. The reduction products of CO2 electrolysis are dependent on the electrode material, topography and other electrochemical parameters like pH, mass transport, pressure and temperature. The gas products are measured in a closed system with a gas chromatograph. In this work copper and gold electrodes were used with a limitation of gaseous products that are mainly hydrogen, carbon monoxide, methane, ethane, ethylene and propylene for copper and only carbon monoxide for gold. Gold was chosen as electrode due to its low overpotential to form CO that is used as a bulk chemical for methanol production and the Fischer–Tropsch process. Verification of the set-up has been done by comparing results from literature on a plane copper electrode in aqueous 0,1 M KHCO3 solution with the performance in this set-up. The results differ from Hori’s results what was traced back to contaminations on the copper electrode and changed conditions at the electrode surface. A carbon coverage of the copper surface was observed after electrolysis. Au electrodes were produced by beam evaporation of a 100 nm Au film on a silicon substrate. Nanoporous gold (NPG) electrodes have been used to estimate the dependence of the surface roughness and structure on the selectivity of gold towards CO synthesis. The NPG films were produced with an electroplating technique based on the reduction of gold oxide in the anodic potential region and deposition of dissolved Au in the cathodic region. The method to control the roughness was based on slow cathodic sweeps to different low potentials and the number of sweeps. 5 different gold electrodes (RF2, RF6, RF10, RF15, RF17) with different roughness factors (RF) have been produced and the evaporated gold film was measured to have a roughness factor of 1.05. The RF was measured with underpotential deposition of copper ions and with analysis of the gold-oxide reduction peak in acidic solution. The results for these electrodes were contradictory. Whereas it was possible to measure a reduction series for RF1 and RF10 without obvious problems between 0.25 V and 0.6 V vs. RHE, other electrodes showed signs of contaminations. The selectivity towards CO for RF6 dropped after 40 min electrolysis and remained on a low level. Anodic polarization was used to remove contaminants on the surface after each potential step and electropolishing was conducted prior to the experiments. Whereas this method seemed to be successful for RF10, RF15 showed a very low selectivity for CO. In order to analyse the type of contamination XPS measurements of the electrodes were conducted. The results showed a contamination with copper and carbon on the gold electrodes. Copper ions could have remained in the cell after electrolysis with Cu electrodes or they could come from a copper foil which is used to mount the Au samples on a metal conductor. Carbon contamination was already observed on the copper electrode but the Au sample were also exposed to air before XPS measurements. The topography of the electrodes was analysed with a scanning electron microscope (SEM). The images showed an increasing film thickness for rougher electrodes from several 100 nm to about 2μm for RF17. Whereas the evaporated samples are very flat with a packed and even structure, the rougher electrodes show distinctive clusters and particles. The particle size does not alter with RF and the oval shape is about 200 nm in length and 150 nm in width whereas the cluster size increases with roughness. This suggests that the dimension of the nano particles might be the dominant factor rather than the roughness factor.

Publikationen registrerades 2015-06-15. Den ändrades senast 2015-06-15

CPL ID: 218346

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