Evaluation of Morphology-Related Factors That Influence the Product Selectivity on Polycrystalline Cu
Authors: Karaiskakis Alexandros N , Biddinger Elizabeth J 
Published: February 2020

Carbon dioxide (CO­2) is a known greenhouse gas and as such, it is considered responsible for climate change. It is believed that human activities, like farming, combustion, and industrial processes contribute an excess CO2 flow to the atmosphere that disturbs the natural carbon cycle. Carbon dioxide electroreduction (CO2ELR) can be seen as a sustainable process which utilizes this waste and harmful gas and at the same time stores the excess intermittent electricity from renewables. A key parameter of CO2ELR is the catalyst that influences the activity, product selectivity and stability of the system1. At this point, the obstacles towards the broader use of CO2ELR are related to the performance of the catalyst.

Copper (Cu) was found to be the only metal that reduces CO2 to hydrocarbons among a variety of metals1. Recent studies showed that the morphology of the Cu surface influences the selectivity and activity of the products2-4. In particular, morphological parameters like roughness factor, crystal orientation, and particle size have been reported to influence the activity and selectivity of the reduction5-7. We recently reported that Cu undergoes reconstruction under CO2ELR conditions that influence the crystal orientation and the morphology something that added more complexity to understanding the morphology-related parameters that drive the performance8. It is critical to understand and identify the factors behind the product selectivity and catalyst activity improvement since it will allow the optimization of the catalyst design towards the desired products.

This work investigates the morphological aspects on rough polycrystalline Cu - roughness factor, particle size, and crystal orientation. An analysis of current distribution on the catalyst surface was also performed. Electrodeposition was selected as the synthesis technique due to its ability to control surface morphology of the catalysts through the manipulation of the potential applied and charge passed. Synthesized catalysts were evaluated in terms of their morphology, faradaic efficiency, and activity. Morphology-related aspects were examined with the use of X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM) and capacitance measurements with cyclic voltammetry. Gaseous product analysis was performed with the use of microGC. The results illustrate a direct relation between particle size, current distribution, and roughness with the product selectivity. In particular, high roughness (7.8 roughness factor) and 300nm particle size were related with ethylene (C2H4) formation, whereas low roughness (1.6 roughness factor) and 3μm size particles were associated with methane (CH4).The current distribution analysis showed higher current intensity on high roughness surfaces in comparison with low roughness surfaces under the same potential.

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