(554f) Investigation of the Morphological Factors That Drive Selectivity on Polycrystalline Cu during CO2 Electroreduction

Alexandros N. Karaiskakis, Elizabeth J. Biddinger^*

Department of Chemical Engineering, City College of New York, CUNY, New York, NY 10031 (USA)

^*ebiddinger@ccny.cuny.edu

Intensive human activity through industrial processes, combustion and agriculture produce more carbon dioxide (CO₂) than the ability of nature to absorb, leading to disruption of the natural carbon cycle and accumulation in the atmosphere. Carbon dioxide is a known greenhouse gas responsible for the contribution to climate change due to global warming. Electroreduction of CO₂ (CO2ELR) is a promising process that combines the utilization of CO₂ residual gas with renewable electricity and water to form a variety of useful carbon neutral chemicals and fuels such as methane (CH₄)_, ethylene (C₂H₄), ethanol, and formic acid. A key component of the CO2ELR process is the performance of the catalyst in terms of current efficiency, selectivity, and stability^{1, 2} which continue to limit the broader use of the process. Cu was found to be the only metal that catalyzes the reduction towards valuable products (hydrocarbons) in comparison with other metals³.

Recent studies have presented the importance of Cu surface morphology on product selectivity and activity^4-6. The oxidation state of Cu along with several morphological parameters, such as roughness factor, particle sizeand crystal orientation, have been evaluated and related as dominant factors that influence the selectivity and the activity of CO2ELR on Cu-based catalysts. Oxidized Cu (CuO and Cu₂O) and higher roughness factor have been related with C₂H₄ enhancement in comparison with smooth Cu, which produces mainly methane^{5, 7} Particle size has been related with product selectivity, and in particular 2nm diameter nanoparticles were observed to enhance H₂ formation, rather than CO2ELR seen on smooth Cu under similar CO2ELR conditions⁸. The crystal orientation of Cu, on single crystal work, has been related with the selectivity between CH₄ and C₂H₄³. We recently reported that polycrystalline Cu under CO₂ reduction conditions undergoes reconstruction that influences the morphology and the crystal orientation⁹. This work examines the oxidation state and morphological aspects on rough polycrystalline Cu (roughness factor, particle size, and crystal orientation) provides insights regarding which factors seem to control CO2ELR selectivity. A current distribution analysis on the surface due to the morphology of each catalyst was also performed. Electrodeposition of copper was used as the synthesis technique for the catalysts studied for its ability to control catalyst morphology. The evaluation of the synthesized catalysts involved the examination of their morphology, current efficiency, and product selectivity. The morphological aspects on the surface of each catalyst were evaluated with scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and capacitance measurements with cyclic voltammetry. Micro GC was used for gaseous product analysis. The results obtained present a direct relation between roughness, particle size and current distrubution on the surface with the product selectivity. In particular, lower roughness catalysts, with a 1.6 roughness factor and 3μm particle size form methane as the main hydrocarbon product, whereas higher surface roughness catalysts, with a 7.8 roughness factor and 300nm particle size were associated with the higher formation of ethylene. The current distribution analysis illustrates lower current intensity on lower roughness surfaces in comparison with higher roughness surfaces under the same potential.

References

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