Role of Nafion Ionomer in Electrochemical Reduction of CO2 on Au Clusters
Authors: Sharma Shailendra Kumar , Golovko Vladimir , Marshall Aaron Timothy 
Published: July 2022

More than 80% of the global energy demand is produced by burning fossil fuels, which leads to the emission of environmentally unfriendly greenhouse gases that adversely affect climate and human health. Electrochemical reduction of CO2 into high-value chemicals or fuels can be a promising way to store renewable energy sources such as solar and wind energies into chemical form, along with alleviating the environmental concerns.1 Recently, atomically precise metal clusters have attracted increasing interest in the research field of catalysis due to their unique size-dependent electronic and chemical properties.2 For the robust application and reusability of catalysts, the metal clusters are generally deposited onto a support, and the electrode is prepared by mixing a binder with the active catalyst. The binder remains in the secondary/outer coordination sphere of the supported catalyst; thus, it can interact with the nanoparticles and modulate their chemical and electronic properties resulting in a change in the catalytic activity.3

In this work, we have studied the role of Nafion binder in the electrochemical reduction of CO2 on [Au9(PPh3)8](NO3)3 clusters supported onto a carbon black support. The atomically precise [Au9(PPh3)8](NO3)3 clusters were chemically synthesised, characterised, and deposited on a carbon black support via wet impregnation. The cathode material for the electrochemical CO2 reduction was prepared by spraying Au9/C catalyst along with Nafion ionomer onto a carbon felt. A potentiostatic electrochemical reduction of CO2 at -1.3 V vs. Ag|AgCl was studied in a three-electrode based electrochemical flow cell in 0.2 M KHCO3 electrolyte. The Au9/C electrocatalyst exhibited a gradual increase in the current density and CO Faradaic efficiency over time when studied for potentiostatic (-1.3 V vs. Ag|AgCl) electrochemical CO2RR suggesting activation of the catalyst. Besides, the process of activation was found to be accelerated when the electrode was treated at a higher potential (-1.7 V vs. Ag|AgCl).

The physical mixing of Au9 clusters with the Nafion ionomer results in dilution of the solution colours, and a precipitate was obtained within seconds. A concentration-dependent investigation of the physical mixture of Au9 clusters and the Nafion ionomer using UV-Vis absorption and X-ray absorption spectroscopy indicated the formation of chemical bonds between the cluster metallic core and the Nafion ionomer. The X-ray absorption studies on the as-made Au9/C electrode, the electrode after 2 h of electrolysis and the electrode after electrochemical activation at higher potential (-1.7 V vs. Ag|AgCl) suggested that the interaction between the Nafion ionomer and Au9 clusters results in shielding of active sites in the as-made electrode (Figure). The EXAFS fitting of the Au9/C electrode after electrochemical activation show a lower metal ligand coordination number. The results demonstrate that the Nafion-cluster interface restructures itself upon applying an electrochemical potential for a long time or applying a higher potential. This restructuring of the interface allows the reactants to access the active sites resulting in an improvement in the catalytic activity during the electrolysis.

An alternative approach for the preparation of cluster-based electrodes was developed to minimise the interactions between the Au9 clusters and the Nafion ionomer by first depositing a layer of carbon black then dropcasting Au9 clusters (Au9 DC electrodes). In addition to that, the effect of the amount of the Nafion ionomer in the catalytic layer and the role of solvent used in the fabrication of the catalytic layer on the electrocatalytic performance were studied.

The work shows that the Nafion ionomer, one of the most commonly used binders, has a profound influence on the catalytic behaviour of ultra-small Au clusters. A detailed discussion on the experimental design, and the analysis of electrochemical and X-ray absorption studies will be given in the conference contribution.

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Figure 1