Plasma Oxidation of Cu for Electrochemical CO2 Reduction
Authors: Gustavsen Kim Robert , Johannessen Erik Andrew , Wang Kaiying 
Published: May 2020

Tuning the selectivity of copper-based catalysts towards the formation of C2 hydrocarbon products has become the main driving force in the development of CO2 reduction electrocatalysis. In particular, Oxide derived Cu (OD-Cu) has been proven to significantly increase the formation of C2 hydrocarbons, in which ethylene is produced at the expense of methane [1]. Further, the selectivity is tunable by controlling the oxide thickness and the ratio of Cu+ to Cu2+ [2]. Oxygen plasma have been especially successful in developing Cu-based catalysts since it gives a good control of the oxidation conditions. The large increase in surface roughness is also believed to stabilize Cu+ during the CO2 reduction reaction (CO2RR) [3]. The latter has shown to be a prerequisite for CO dimerization, due to the attractive forces resulting from the oppositely charged *CO on the Cu0 and Cu+ sites [4]. In this paper, we present a method for tuning the selectivity of Cu catalysts through a controlled oxidation process using oxygen plasma. The effect of plasma exposure time on the catalytic performance will be investigated.
A plasma cleaner (Plasma Parylene Systems, Alpha Plasma AL18, Germany) was used to oxidize high purity copper foils (Jiarun Metal Material Corporation, China, 99.9999%) to prepare the catalysts. The plasma power was set to a constant 400 W in an O2 atmosphere at a pressure of 60 Pa, while varying the exposure time from 15 to 60 minutes. Prior to plasma oxidation, the Cu foils were cleaned in an ultrasonic bath of acetone for 25 minutes. Then, 10 minutes of Ar plasma (300 W at 38 Pa) were used prior to oxidation in order to remove any surface contaminants.
Visually, the samples appear red, purple, and blue-grey for exposure times of 15, 30, and 60 minutes, respectively (figure 1). The sample exposed to oxygen plasma for 15 minutes has a granular morphology with clearly defined grains (~ 50 nm), which gradually grows together when doubling the exposure time to 30 minutes, and eventually leads to a continuous film when the exposure time reaches 60 minutes (figure 2). Electron Dispersion Spectroscopy (EDS) analysis shows a steady increase in oxygen concentration as a function of plasma time (figure 2). The visually observed color difference can be attributed to the difference in oxide concentration as well as the surface morphology of the samples, which can explain the untraditional colors of our copper oxides.
The electrochemical measurements will be performed using an H-cell equipped with a Nafion 117 proton exchange membrane, where the cathodic and anodic compartments are each filled with 20 mL of 0.1M KHCO3, while CO2 is bubbled through the solution at a constant flow rate of 20 sccm. Catalytic performance is to be determined using an electrochemical workstation (CHI23213). Furthermore, quantification of the reaction products will be accomplished using a gas chromatograph (Shimadzu GC 2010 PLUS) equipped with a gas sampling valve (GSV) and a barrier discharge ionization (BID) detector. The experiments will be performed in an online configuration with a constant flow of CO2. In summary, this work will provide new insight into plasma-oxidized copper catalysts for electrochemical CO2 reduction.

References

1. Kim, D., et al., Insights into an autonomously formed oxygen-evacuated Cu2O electrode for the selective production of C2H4 from CO2. Physical Chemistry Chemical Physics, 2015. 17(2): p. 824-830.
2. Ren, D., et al., Selective Electrochemical Reduction of Carbon Dioxide to Ethylene and Ethanol on Copper(I) Oxide Catalysts. ACS Catalysis, 2015. 5(5): p. 2814-2821.
3. Mistry, H., et al., Highly selective plasma-activated copper catalysts for carbon dioxide reduction to ethylene. Nature Communications, 2016. 7(1): p. 12123.
4. Xiao, H., et al., Cu metal embedded in oxidized matrix catalyst to promote CO2 activation and CO dimerization for electrochemical reduction of CO2. Proceedings of the National Academy of Sciences, 2017. 114(26): p. 6685.

Figure 1