Electrolyzers for CO2 reduction can be used to synthesize renewable fuels, as a route to replace fossil fuels in our everyday economy with the purpose of minimizing the effects of climate change. For a high-scale implementation of electrolyzers, high operation current densities a
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Electrolyzers for CO2 reduction can be used to synthesize renewable fuels, as a route to replace fossil fuels in our everyday economy with the purpose of minimizing the effects of climate change. For a high-scale implementation of electrolyzers, high operation current densities and low overpotentials are essential. Currently available options do not offer this and are limited by the low mass transfer of CO2 in the system. We envision using catalyst-coated capacitive particles (slurry electrodes) to be a solution: a system where the reaction site can be brought to the reagent, instead of the other way around.
This thesis is a first step in the direction of a slurry electrode flow cell and aims to produce silver nanoparticles deposited on activated carbon particles to function as catalysts in such a system. The goal of the synthesis procedures was to produce a capacitive powder (activated carbon) with efficient deposition of silver nanoparticles spheres) ranging 5-20 nm in diameter. Based on literature, the most interesting methods for synthesis were selected to be: solution-based methods with only activated carbon, with added polyvinylpyrrolidone, with added sucrose, electrodeposition and impregnation. The resulting powders were analyzed using ICP-OES and TEM, showing that only impregnation and electrodeposition can yield the desired powders with a near-100% silver deposition efficiency and particles < 20 nm in size.
Next, the impregnation and electrodepostion samples were prepared into a 15 wt% slurry, and a third slurry was made with bare AC as a blank. The catalysis experiments were executed in a flow cell (electrode area = 4 cm2), with currents ranging from -5 to -15 mA/cm2. Within this range of currents, extreme potentials up to -10 V (measured over working electrode versus counter electrode) were measured. The potentials on the anode and cathode side of the cell were also measured individually using reference electrodes, showing that the anodic side of the cell reached +5 V, whereas the cathodic side of this cell reached only -1 V. Additionally, CV scans before and after the experiments confirm that the cell’s conductivity decreased within an experiment, however, this degradation is reversed for a new experiment. These observations combined lead us to believe that the cause of the cell problems is located on the anodic side of the system, and is most likely resistance from oxygen bubbles, which needs to be solved to allow cell operation at higher current densities. Since the aim of the project was CO2 reduction on the slurry electrodes, the outgoing gases from the cathode were measured in a GC. The blank was included to demonstrate the catalytic effect of the silver nanoparticles, yet the highest CO flow (around 0.05 ml/min) was measured at the lowest current density during this blank experiment. Calculating the Faradaic efficiency (FE) suggests that 33% of the electrons in that experiment was used for this conversion. The other measurements, at higher currents or with silver present, did not reach an FE of 10%. This combination leads to the conclusion that the observed CO is most likely not produced by CO2 reduction. A hypothesis was made that this CO was already present at the powder’s surface and released during the experiment, however XPS data showed that there were no noticeable differences in the content of oxygen-bound carbon between the coated samples and the bare sample. Thus, the source of the CO is at this stage unknown.
Future work towards proving CO2 taking place on the slurry electrode should first investigate the source of the observed CO, which requires testing the influence of the different carbon sources present in the system. Additionally, the blank experiment should be improved: using a bare carbon slurry that underwent the same procedure as the coated slurry (without the precursor present) eliminates any differences in the powder.