Effect of Tridimensional Macroporous Copper Electrodes on the Electrochemical Reduction of Carbon Dioxide
Authors: Sanabria-Chinchilla Jean 
Published: February 2020

It is well known now that several experimental parameters can affect both product distribution and Faradaic efficiency over the CO2 reduction reaction (CO2RR). Nature of the material electrode was found to have a profound effect on this reaction; copper stands alone as the only metal in which CO2 can be reduced to hydrocarbons and oxygenated species. CO2RR on other metals yields mostly formate ion and/or CO, and hydrogen gas as a side product. Regarding the electrode material, polycrystalline and monocrystalline electrodes have been tested for this reaction and the majority of the work has been done over flat surfaces or metal particles [1,2].
Tridimensional (3D) macroporous materials have been evaluated as an alternative to flat electrodes due to their extremely large specific surface area for charge and mass transport in electrochemistry. The material consists in an open structure containing a sponge-like network of channels that facilitate the transport of gas and ionic species from the bulk to the internal pores in the structure (funnel effect) [3–5]. The unique characteristics of the 3D macroporous structure in the context of the CO2RR on copper electrodes seem a very interesting aspect to investigate.
Characterization of the 3D macroporous structures is done by SEM images from which pore size and density of pores are obtained. These two parameters are controlled by carefully choosing the experimental conditions during the structure fabrication.
The contact of copper electrodes with aqueous solutions will form a layer of surface oxides and hydroxides; therefore it is necessary to establish under which experimental conditions surface oxides are eliminated leaving a metallic surface available for the reduction of the CO2. Figure 1 shows a cyclic voltammogram of a copper electrode clearly stating the formation of surface oxides at potentials higher than -0.25 V vs. Ag/AgCl. Reduction of surface oxide species can take potentials as far negative as -0,65 V. Maintaining the potential more negative than -0.3 V (see Figure 2) allows keeping the Cu surface in its reduced metallic form which will be used for the CO2 electroreduction process.
Product distribution and Faradaic efficiency from CO2 electrochemical reduction over 2D and 3D electrodes are compared by means of Differential Electrochemical Mass Spectrometry (DEMS). Species being electrogenerated on the electrode surface are detected at the mass spectrometer under operando conditions. Pore size and pore density of the 3D macroporous structures will be correlated with identity and quantity of hydrocarbons and oxygenated species being formed.
References
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Figure 1