Publication date: 7th June 2020
Investigating synergetic effects in Cu-Sn mixed metal oxide CO2 reduction electrocatalysts by hard and soft X-ray spectroscopy
Alexander Arndt, Laura Pardo-Perez, Sasho Stojkovikj, Götz Schuck, Lifei Xi, Matthew T. Mayer
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Young Investigator Group: Electrochemical Conversion of CO2
Electrochemical reduction of carbon dioxide can produce a wide variety of products with poor control over selectivity and poor efficiency. The nature of the catalyst surface plays a central role in dictating the reaction activity, and researchers are working to develop new catalysts and uncover design principles enabling selective and efficient production of a desired product. A key challenge is that electrocatalyst structure is highly dependent on the applied electrochemical potential and the chemical environment, especially under the particularly reducing conditions required for CO2 reduction. Methods for in situ study of electrodes under operating conditions are therefore crucial.
In seeking to modify catalyst activity by surface functionalization, we discovered that sub-nm ALD coatings of SnO2 films onto CuO nanostructure electrodes led to huge changes in catalyst selectivity, converting CO2 to CO (carbon monoxide) with selectivity exceeding 90%. Since this behavior is atypical for either CuO or SnO2 electrodes themselves, a synergetic effect between the two components is likely. We employed X-ray spectroscopy to investigate the electronic states and chemical environments of Cu and Sn using soft and hard X-ray techniques at BESSY II. We discovered that, during electrochemical operation, the Sn migrates away from the surface and into the CuO bulk. The presence of Sn influences the observed oxidation state of Cu near the surface, which remains partially oxidized (in comparison to fully reduced Cu which is observed in the absence of Sn). This result shows that electrocatalytic surfaces can be significantly affected by the presence of trace sub-surface dopants, causing changes in electronic structure which affect catalytic mechanisms and the resulting product selectivity.
The research group EE-NECC is supported by the Helmholtz Association Initiative and Networking Fund and by Helmholtz-Zentrum Berlin für Materialien und Energie.
Hard X-ray data was collected on beamline KMC-2, and soft X-ray measurements were done on beamline UE56-2 PGM-2 with the LiXEdrom experimental station at BESSY II.
We are thankful for the support of Dr. Götz Schuck, Dr. Ronny Golnak and Dr. Jie Xiao for BESSY II support, as well as Eike Köhnen and Philipp Tockhorn for ALD support at the HySPRINT laboratories.
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