Windowless thin layer electrochemical Raman spectroscopy of Ni Fe oxide electrocatalysts during oxygen evolution reaction

Abstract In-situ Raman spectroelectrochemistry is a powerful tool to understand the structures and reaction mechanisms of electrochemical interfaces yet usually suffers from a low sensitivity and detection efficiency. Herein, we develop a windowless thin layer electrochemical cell design to improve the detection efficiency in-situ Raman spectroscopy for practical nanoscale electrocatalysts. This was achieved by perforating a hole in the optical window of the cell, which not only eliminates the refraction of light by the window but also induces a meniscus thin electrolyte layer on the working electrode, thus significantly improving the detection efficiency of Raman spectra. We successfully applied this method to conduct in-situ Raman spectroscopy of structural evolutions of two representative Ni Fe oxide catalysts (layered double hydroxides and spinel oxides) during oxygen evolution reaction (OER) electrocatalysis. It was shown that both catalysts underwent surface transformation to oxyhydroxides and formed active oxygen species at OER relevant potentials. However, the extent of the surface transformation was much lower on the Ni Fe spinel catalyst, accounting for its lower OER activity. We further show that adding a carbon support can promote the surface transformation of Ni Fe spinel to oxyhydroxide phase, thus significantly increasing the electrocatalytic activities. The established windowless thin layer method provides a new way to conduct in-situ electrochemical Raman spectroscopy of a wide range of materials and may be also combined with surface-enhanced Raman spectroscopic methods.