Tuning the Ligament Size and the Content of the Foreign Atom of the Nanoporous Copper for the Electrochemical Carbon Dioxide Reduction
Authors: Hecker Burkhard , Oezaslan Mehtap 
Published: February 2020

During the electrochemical reduction reaction of carbon dioxide (CO2RR) on metallic copper surfaces several C1 and C2 products can be formed. The reaction mechanisms and kinetics, yet, are unclear to date.  Different reaction mechanisms for CO2RR are postulated.1-3 For instance, the adsorption of negatively charged intermediates at the catalytically active surface might play a very important role in the formation of C2 products.3 Further critical issues for the CO2RR are addressed: (i) high overpotentials of up to 1 V, (ii) broad product distribution, (iii) fast degradation by catalyst poisoning and (iv) competition reaction at high cathodic potentials, referred to as hydrogen evolution reaction. Thus, tailoring the structure, morphology and chemical composition of Cu-based materials by adding other metals is one of the promising strategies to control the catalytic activity and product distribution during the CO2RR.4-5

In our study, we investigated the catalytic properties of a novel nanoporous copper film (np-Cu) for the CO2RR. The np-Cu films were prepared by de-alloying process of the less noble metal (Zn) from a Zn-Cu alloy. This technique allows us to tailor the ligament and pore size as well as the content of the Zn in the np-Cu. For the characterization of the np-Cu we used SEM, TEM, EDX, XRD and XPS. The catalytic performance of different np-Cu materials for the CO2RR was evaluated by linear sweep voltammetry and chronoamperometry. To correlate the measured current during the CO2RR polarization curves, the formed products were quantified by using gas chromatography and ion chromatography.

Our results show how the ligament structure and the Zn content of the np-Cu have an effect on the activity and selectivity during the CO2RR. Depending on the experimental conditions, the dealloying in alkaline media largely forms copper oxide with very low content of Zn. In contrast, after dealloying in acidic media the ligament size and pore distribution of np-Cu are more uniform and contains a higher amount of Zn. The polarization curves for both np-Cu materials showed higher activity for the CO2RR compared to that for a flat Cu surface. We showed that a high content of Zn at the surface promotes the formation of hydrogen, while the copper oxide leads to higher selectivity to C2 products.

Based on our results, we provide a deeper insight into the mechanism and kinetics for the CO2RR on np-Cu materials.

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