Dendritic Silver/Palladium Alloy Arrays for Electrochemical CO2 Reduction
Authors: Geng Dongsheng , Han Jie 
Published: May 2020

Increased fossil fuel consumption has sparked concerns about developing new energy sources and reducing carbon dioxide emissions. The electrochemical reduction of CO2 using renewable energy sources, e.g. wind and solar is an appealing method for making a range of carbon-based products such as CO even more valuable hydrocarbons.1 There have been several significant advances in CO2 electrochemical reduction in recent years.2 However, currently there are no efficient catalysts for CO2 reduction as activation of CO2 requires a large input of energy; and the reaction usually results in the broad product distribution.3 Thus, the search for new catalysts, which can promote this reaction along a specific reaction pathway, is one of the most important tasks to make electrochemical CO2 reduction successful.
Herein, we developed dendritic silver/palladium (Ag/Pd) alloys nanoarrays by a simple galvanic replacement reaction. Figure 1a demonstrates a typical SEM image of Ag/Pd dendrites with feather-like branches. The earlier study proposed the vacancies generated by the displacement reaction attract silver diffusing to the surface and form alloys with Pd.4 Compared with the alloys of Ag80Pd20 or Ag60Pd40, Ag70Pd30 shows the better performance that can selectively convert CO2 to CO with a maximum faradaic efficiency (FE) of 98.6% and a current density of ~ 9 mA/cm2 at the potential of −0.8 V vs. RHE (Figure 1b, c).
The high performance of Ag/Pd alloy can be ascribed to the synergistic effects between Pd and Ag. Alloying can adjust electronic state to optimize the adsorption and coupling of intermediate. The calculated Gibbs free energy (ΔG) diagram is depicted in Figure 1d, representing CO2 reduction to CO through elementary reaction steps involving the intermediates of *COOH and *CO according to first principles calculations. The formation of *COOH with an energy barrier of 0.81 eV on Ag70Pd30 alloy surface, which is ~ 0.1 eV lower than on the pure Ag(111) surface. In addition, dendrite with multiple active sites and hydrophobic array might play an indispensable role in improving the catalytic performance. The dendritic vertical arrays structure has an increased active surface area, which will expose more low-coordinated surface sites. Contact angle measurements have confirmed the hydrophobic surface feature of this alloy (Figure 1e), which can enhance the capture of gas, in turn increase the concentration of CO2 on the electrode surface, therefore improve CO2 reduction selectivity.5

Figure 1. (a) SEM image of Ag70Pd30 sample. (b) Potential-dependent faradaic efficiency of different samples on electrocatalytic reduction of CO2 to CO. (c) Stability performance of Ag70Pd30 for CO2 reduction operated at the potential of −0.8 V vs. RHE for 8 h. (d) Free energy diagram for electrochemical CO2 reduction to CO. (e) Contact angel of silver piece and silver/palladium dendrites.

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Figure 1