Metal Chalcogenide Nanofilms: Platforms for Mechanistic Studies of Electrocatalysis

The systematic development of improved electrocatalysts requires strategies for preparing candidate materials as well-defined thin-film electrodes that are amenable to straightforward characterization of reaction mechanism and catalyst specific activity. While numerous thin film preparation methods are established for transition metals and metal alloys, few strategies exist for transition metal chalcogenides, despite growing recognition of their role as potent electrocatalysts. Herein we show that electrochemical atomic layer deposition (E-ALD) is a powerful tool for accessing well-defined metal chalcogenide electrocatalysts, by synthesizing, for the first time, crystalline conformal films of Co 9 S 8, a promising earth-abundant oxygen reduction catalyst, with tunable nanoscale thickness. The as-prepared nanofilms display relatively high activity for the oxygen reduction reaction and provide a robust platform for detailed mechanistic investigations. Initial mechanistic studies reveal that oxygen reduction on Co 9 S 8 nanofilms proceeds via rate-limiting one-electron transfer to O 2 with a specific activity of 20.6 μA cm –2 at 600 mV vs RHE. This study opens the door to the systematic application of E-ALD to investigate chalcogenide electrocatalysts across the transition series.