Importance of Substrate Metals in Cu-based Catalysts for Electroreduction of CO2
Alexandros N. Karaiskakis, Sujan Shrestha and Elizabeth J. Biddinger*
Department of Chemical Engineering, City College of New York, New York 10031(USA)
The utilization of carbon dioxide to valuable carbon based fuels and chemicals through electrochemical reduction combined with renewable resources could contribute to the achievement of a carbon-neutral cycle. The biggest hindrances of CO2 electrochemical reduction are related with the characteristics of the electrocatalysts due to their low performance1 (catalyst activity, the product selectivity, the catalyst stability and faradaic efficiency). Copper (Cu) is the only metal, until now, that was proven to effectively yield hydrocarbons and alcohols2. Surface morphology3 attracts the efforts of recent research activity as a solution to the aforementioned obstacles and in particular the switch from Cu foil to the construction of Cu particle-based catalysts with higher surface area.
One way to change the morphology of Cu particles is by changing the substrate they grow on. The electrochemical techniques used for the growth of Cu particles use Cu as substrate. In this report, the Cu-based catalysts were constructed with electrodeposition on different substrates metals (Cu, Ni, Ti), and their reduction efficiency of CO2 and products selectivity were investigated. Two main deposition parameters were evaluated: the charge (Q) of the Cu deposited on the substrate metals and the potential during the deposition. A range of charge (1.5C to 15C) was used representing the film thickness of Cu deposited and a potential range (-0.6V to -1V), which influences the size of the Cu particles4.
The evaluation of each catalyst involved the examination of surface morphology, the efficiency during the CO2 reduction, and the products selectivity evaluation. For that purpose, scanning electron microscopy (SEM), X-ray diffraction (XRD) and capacitance measurements with cyclic voltammetry were used. The products analysis was conducted with micro gas spectrometry (microGS) for gas products and with high pressure liquid chromatography (HPLC) for liquid products.
The results of the optimal experimental conditions regarding the performance of the catalysts are presented under the use of cyclic voltammograms in CO2-saturated electrolyte. The following order of overpotential reduction occurred: Cu/Ni> Cu/Cu> Cu/Ti > Cu foil based on the observed potentials for specific current density. Surface evaluation experiments gave the following roughness factor order: Cu/Ni> Cu/Cu > Cu/Ti> Cu foil. From XRD experiments performed for copper characterization under same deposition conditions (-1V and Q= 15C), the results illustrate that Cu crystal orientation is dependent upon the electrodeposition substrate; Cu/Cu gave Cu (111) and Cu (200), whereas Cu/Ti and Cu/Ni gave Cu (111) and Cu (220). The impact of the changes in substrate material on CO2 electroreduction performance and suggestions for further modifications to the Cu electrocatalyst will be presented.