A Catalyst Based on Silver-Zinc Bimetal for Electrochemical Reduction of CO2 to CO
Authors: Shim Kyubin , Kim Yong-Tae 
Published: December 2020

Energy and environmental issues have become the most significant challenges for human beings in the 21st century [1]. Among these issues, the continuing use of fossil fuels has led to increase the concentration of carbon dioxide (CO2) in the atmosphere, which has caused global climate change [2]. However, the CO2 reduction reaction have challenges due to the high overpotential and kinetically sluggish multi-electron transfer process. To solve these disadvantages, various bimetallic-based catalytic materials have reported. Because it provides a great potential for catalytic activity owing to their synergetic effects [3]. In the present study, we prepared an AgZn bimetallic catalysts by physical vapor deposition (PVD), which provides an ideal model for the electrochemical reduction of CO2 to CO. Three samples with different thicknesses of 2 nm, 5 nm, and 10 nm Ag layers, deposited on polycrystalline Zn pellets (poly Zn), were used for the electroreduction of CO2.
All electrochemical experiments were conducted in a three-electrode system employing a potentiostat. An H-type electrochemical cell separated by a slice of Nafion 211 membrane was used for the CO2 electrochemical reduction. The electrolyte (0.1 M KHCO3) was saturated with CO2 (pH 6.8). The produced gas was directly introduced into the gas sampling loop of a gas chromatograph, which was equipped with a flame ionization detector (FID) for CO analysis and a thermal conductivity detector (TCD) for H2 analysis.
The morphologies of Ag sputtered on the poly Zn were determined by TEM. It is observed that a 2 nm thickness Ag film forms Ag clusters of nearly circular shapes, which is likely an isolated island. As the thickness of Ag increases, the surface area covered by Ag also increases. The 10 nm sample shows the surface almost fully covered by Ag. All the XRD peaks of the Zn-based catalysts are indexed to the hexagonal wurtzite structure of Zn. However, the diffraction pattern of the 2AZ catalyst matches well with that of the poly Zn pellets and does not match with the diffraction pattern of Ag. It is because the amount of silver is too small to be detected. However, for the 5AZ and 10AZ catalysts, the diffraction peaks of Ag and Zn are observed. XPS was performed to demostrate the synergetic effect. Interestingly, the binding energy of Ag 3d of the AgZn compared to that for poly Ag is shifted towards a lower binding energy. Synergetic effect of AgZn catalyst could be attributed to the electron transfer from Ag to Zn, because Ag has a lower work function than Zn.
To confirm the performance of CO2 reduction, the electrochemical experiments were conducted in CO2 saturated 0.1 M KHCO3 (pH =6.8) solution using a homemade H-type cell with a three-electrode system. The total current density of the 2AZ catalyst (3.53 mA cm−2) at −1.0 V is 2.05, 1.17, 1.38, and 1.28-fold higher than the total current densities of the poly Zn (1.72 mA cm−2), poly Ag (3.02 mA cm−2), 5AZ (2.56 mA cm−2), and 10AZ (2.76 mA cm−2), respectively. The 2AZ catalyst has the maximum CO conversion FE of 84.2% at a potential of −1.0 V vs. RHE, which is approximately 2.3-fold higher than that of poly Zn (35.9%), 1.5-fold higher than that of poly Ag (55.3%). the 2AZ catalyst exhibited excellent activity and selectivity for CO2 reduction to CO, even better than poly Au.
Conclusively, we prepared AgZn bimetallic catalysts using a physical vapor deposition sputtering system. The 2 nm Ag deposited Zn catalyst selectively catalyzed the electrochemical CO2 reduction to CO with the FE of 84.2% at −1.0 V. The XPS analysis demonstrates a red-shift of the binding energy in the Ag 3d spectra, which results from the charge transfer from Ag to Zn. This indicates that the AgZn bimetallic structure can improve the surface charge transfer for CO2 reduction. This study can provide a new direction for further study on the electrochemical reduction of CO2 to CO.
[1] H.-T. Pao, C.-C. Chen, J. Clean. Prod. 206 (2019) 907–919.
[2] D. D. Zhu, J. L. Liu, S. Z. Qiao, Adv. Mater. 28 (2016) 3423-3452.
[3] J.-H. Kim, S.-W. Yun, K. Shim, S.-H. You, S.-M. Jung, H. Kweon, S. H. Joo, Y. H. Moon, Y.-T. Kim, ACS Appl. Energy Mater. 3 (2020) 1423–1428.