Controllable Product Selectivity on Polycrystalline Cu through Tunable Surface Morphology
Authors: Karaiskakis Alexandros N , Biddinger Elizabeth J 
Published: February 2020

Carbon dioxide (CO2) electroreduction (CO2ELR) has the advantage of using a gaseous waste stream accompanied with water, a catalyst and electricity from renewables and turn it to carbon neutral chemicals and fuels. The most valuable products of CO2ELR are formed predominantly on Cu-based catalysts, such as methane (CH4), ethylene (C2H4), ethanol, and formic acid. However, there are still hindrances that prevent the broad commercial use of the process and are associated mainly with catalyst performance, such as current efficiency, selectivity, and stability1, 2.

Recent studies highlighted the importance of surface morphology which can improve faradaic efficiency (FE) and activity towards more desirable products (hydrocarbons) 3-6. Our recent study showed that Cu undergoes reconstruction under CO2 reduction conditions that influences the surface morphology and the crystal orientation7 which added more complexity to understanding morphology-driven performance. Understanding and identifying the factors behind the activity and selectivity improvement is of great importance since it will allow us to control and direct the reactions towards the targeted products though the optimal design of catalysts.

This work examines the morphology-related factors (particle size, roughness, and crystal orientation) and copper oxidation state that drive the selectivity on rough Cu-based catalysts surfaces. Rough Cu-based catalysts were synthesized by electrodeposition and their morphological aspects were tuned through control of the parameters of potential applied and charge passed. A surface current distribution due to morphology was performed for each catalyst. The evaluation of each synthesized catalyst involved the examination of their surface morphology, reaction selectivity, and current efficiency. The surface morphology of each Cu-based catalyst was examined with capacitance measurements using cyclic voltammetry techniques, scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM). Micro gas chromatography (microGC) was used for the gaseous product analysis. The results present the factors that influence the selectivity and illustrate a direct relation between specific morphological aspects, current distribution, and product selectivity. In particular, lower roughness catalysts (1.6 roughness factor) covered with larger particles (3μm size particles) produced methane as the main hydrocarbon product; whereas, higher surface roughness catalysts (7.8 roughness factor) covered with smaller particles (300nm size) were associated with the higher formation of ethylene. Current distribution analysis illustrated lower current intensity on low roughness surfaces in comparison with higher roughness surfaces where the current intensity was higher. We will report that the crystal orientation of polycrystalline Cu is not the main factor that drives the selectivity and the oxidized Cu doesn’t seem to enhance the selectivity to C2H4 in comparison with metallic Cu.

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