Heterogeneous electrochemical phenomena, such as (photo)electrochemical water splitting to generate hydrogen using semiconductors and/or electrocatalysts, are driven by the accumulated charge carriers and thus the interfacial electrochemical potential gradients that promote charge transfer. However, measurements of the “surface” electrochemical potential during operation are not generally possible using conventional electrochemical techniques, which measure/control the potential of a conducting electrode substrate. Here we show that the nanoscale conducting tip of an atomic force microscope cantilever can sense the surface electrochemical potential of electrocatalysts in operando. To demonstrate utility, we measure the potential-dependent and thickness-dependent electronic properties of cobalt (oxy)hydroxide phosphate (CoPi). We then show that CoPi, when deposited on illuminated haematite (α-Fe2O3) photoelectrodes, acts as both a hole collector and an oxygen evolution catalyst. We demonstrate the versatility of the technique by comparing surface potentials of CoPi-decorated planar and mesoporous haematite and discuss viability for broader application in the study of electrochemical phenomena.
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This work was supported by the Department of Energy, Basic Energy Sciences, Award DE-SC0014279. S.W.B also thanks the Sloan and Dreyfus Foundations for additional support. The atomic force microscope was purchased using funds provided by the NSF Major Research Instrumentation Program, Grant DMR-1532225. The growth of the planar haematite electrodes was supported by NSF Award CHE-1664823. F.A.L.L. acknowledges funding from the NSF GRFP, Grant 1309047. We thank Dr Fuding Lin, Dr Michaela B. Stevens, Dr Matthew G. Kast, Dr Sebastian Oener and Lisa J. Enman for helpful conversations, Dr Christian Dette for assistance in preparing figures, Dr Zhuangqun Huang for technical assistance with the AFM and John Boosinger for help designing the electrochemistry cell.
The authors declare no competing financial interests.
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