Water electrolysis is a promising approach of electrochemical energy storage.^[1] The generation of molecular oxygen at the anode limits the efficiency of the water electrolysis by creating a large anodic overpotential during the overall water splitting process. In particular, the bottleneck is the high dissolution rate and instability of the metal/metal oxides under conditions of the oxygen reduction reaction (OER).^[2] In this context, manganese oxides are candidates as earth abundant and low cost water oxidation electrocatalysts.^[3]

This study compares the state of oxides during electrode polarisation up to potentials with OER and transpassive dissolution on two metals, manganese and copper.^[4,5] For this purpose, metallic manganese and copper electrodes were prepared by physical vapor deposition. In situ spectroscopic ellipsometry was used to investigate the oxide growth and stability. From an analysis of ellipsometric spectra, based on a previous analysis procedure,^[4,5] the oxide thickness was determined as a function of electrode potential, using chronoamperometry. The nature of oxides was studied through in situ Raman spectroscopy. Information on electronic structure was derived from analysis of spectroscopic data from ellipsometry, and by UV/VIS reflection measurements. In addition, the photoluminescence background in the Raman spectra contains valuable information. The spectroscopic data was complemented with ex situ analyses, including X-ray photoelectron spectroscopy (XPS). The current results suggest that the defect formation is critical for the oxide stability and that the oxides are highly dynamic under OER condition.

[1] M. G.Walter, E. L.Warren, J. R. McKone, S. W. Boettcher, Q. Mi, E. A. Santori and N. S. Lewis, Chem. Rev., 2010, 110, 6446-6473.

[2] I. Katsounaros, S. Cherevko, A. R. Zeradjanin, and K. J. J. Mayrhofer, Angew. Chem., Int. Ed., 2014, 53, 102-121.

[3] A. Ramírez, P. Hillebrand, D. Stellmach, M. M. May, P. Bogdanoff, and Sebastian Fiechter, J. Phys. Chem. C, 2014, 118 (26), 14073–14081.

[4] C. Toparli, A. Sarfraz, A. Erbe, Phys. Chem. Chem. Phys. 2015, 17, 31670-31679.

[5] C. Toparli, A. Sarfraz, A. D. Wieck, M. Rohwerder and A. Erbe, Electrochim. Acta, 2017, 236, 104–115.