Operando surface-sensitive analysis of electrodes is a great challenge, as most surface-sensitive techniques are not compatible with in situ measurements (e.g. high vacuum requirement for XPS). Here, we present an all-optical approach that allows for in situ probing of transformations that occur within top 10 nm of magnetically active electrodes (archetypically: Co, Ni or Fe). The spin carriers information on the electronic structure [1]. Although the interpretation is not straightforward, the advantage of the method lies in the compatibility of optics with an electrochemical setup. Typical electrode materials used in alkaline electrolysis are Ni and Fe, and their combination. A fundamental understanding, why iron impurities improve the electrocatalytic performance of Ni electrodes [2] is lacking, partially due to the fact that the surface oxide structure under operation conditions is unknown.

The measurement principle is based on non-linear electro- and magneto-optical Kerr effect. The polarisation plane of linearly polarised light rotates after it interacts with the birefringent matter (e.g. electrode) exposed to a magnetic field. The degree of rotation depends on intrinsic properties of the matter (e.g. electronic structure) and is directly related to the intensity difference between the incident and reflected beams. The polypropylene electrochemical cell is designed for optical probing with minimal interference from non-sample components (e.g. 0.5 mm optical window and 2 mm probing depth of electrolyte). The integrated CCD-camera allows for spatially resolved imaging over the whole sample surface.

The electrodes are prepared by sputtering on a gold-covered sapphire supports. Gradient-coated samples are used to differentiate between the bulk-influenced spectral response and the chemical changes of the catalytically active surface. Here, we demonstrate the applicability of the technique for determining the thickness oxide layers as a function of applied overpotential using the electrochemical MOKE setup.

[1] M. Weisheit et al., Science, 2007, 315, 349-351.
[2] D. A. Corrigan, J. Electrochem. Soc., 1987, 134, 377-384.