Controlling the Surface Active Site Geometry for Electrochemical Catalytic Reactions

Proton exchange membrane fuel cells (PEMFCs) are considered as one of the most promising alternative clean and sustainable energy sources to fossil fuels. In general, PEMFC is consisted of anodic and cathodic electrode assembly, electrolyte, and proton-exchange membrane. While renewable fuels, such as hydrogen gas and formic acid, get oxidized at the anode to produce protons, oxygen molecules are reduced to form water at the cathode. Platinum has been widely used for both anodic and cathodic reactions due to its excellent catalytic reactivity. Significant effort has been devoted to improving the reactivity and selectivity of Pt-based catalysts by alloying with a second metal. AuPt alloy nanoparticles have been studied extensively for electrochemical formic acid oxidation reaction, and isolated Pt species are recognized as the most active sites. While the majority body of literature focuses on structure-reactivity relationships based on as-synthesized materials, less attention is paid to the structural evolution during electrochemical catalysis. In this work, we develop a colloidal synthetic method to deposit Pt shell onto Au nanoparticles with variable thickness to study the microstructural evolution under electrocatalytic formic oxidation. We find that Pt atoms are submerged from the surface to form isolated Pt species in the first 100 cycles, which show enhanced FAO activity by shifting the reaction pathway. Additional CV scanning causes further depletion of Pt from the surface, resulting in the deactivation of the catalyst. Despite the high activity of Pt-based catalysts, the use of these materials is limited by its high cost. Recently, transition metal sulfides such as cobalt sulfides have been found to show comparable activity to Pt-based catalysts in pH 7 ORR. However, it is challenging to isolate the role of coordination environment amidst multiple geometries and oxidation states that exist within any given phase. In this effort, we synthesize isolated Co atoms supported on colloidal WS2 nanosheets. By doing post synthetic treatment on these nanosheets, we are able to achieve a range of Co-S coordination number. Correlating Co-S CN to their ORR activities, we find the optimal active sites for ORR in neutral media possess a Co-S coordination number of 3-4