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2020 (1)

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Dissertation
Synthesis and benchmarking of CO2 reduction catalysts for formic acid production

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Abstract

Valorisation of carbon dioxide by electrochemical reduction can play a role in the mitigation of climate change and simultaneously provide useful chemicals. Throughout the years formic acid has emerged as one of the most promising products, since it can be applied as a hydrogen carrier and thereby serve as a storage medium for energy produced by intermittent renewable energy sources. Besides, it can directly produce energy in a direct formic acid fuel cell (DFAFC) and has several other applications in the (bio)chemical industry. To develop towards large-scale CO2 reduction systems, a catalyst displaying a combination of high selectivity, high current density and long stability is crucial. With these requirements in mind, a benchmarking of catalysts was performed. Different support materials, metallic catalysts and synthesis methods were searched. A copper foam support was found beneficial for achieving high current densities (j). Through combining the support with several catalytic metals, such as palladium, bismuth and tin, well-performing electrodes were created. Especially the combination of the copper foam support and tin showed a high selectivity towards formic acid. The straightforward synthesis methods used allow for easy implementation and upscaling. Electrodeposition of tin on copper foam gave a faradaic efficiency (FE) of 66.2 % with a current density of 106.6 mA/cm2 at -2 V vs Ag/AgCl (-0.95 V vs RHE). Tin nanoparticles deposited on copper foam attained 71.2 % FE at a current density of 35.1 mA/cm2 at -1.6 V vs Ag/AgCl (-0.79 V vs RHE). High production rates of respectively 1.31 mmol/cm2h and 0.465 mmol/cm2h were achieved. Although the selectivities are not exceptional compared to the state-of-the-art, current densities and production rates are among the highest values reported in the liquid phase. Attention should now be pointed towards creating reaction conditions that overcome mass transfer limitations by CO2 diffusion, so that the catalysts that were created can live up to their potential. This could be done by, for example, changing the liquid electrolyte conditions or making the step to a zero-gap electrochemical reactor cell.

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