Reversibility of Ferri-Ferrocyanide Redox during in-Situ Soft X-Ray Spectroscopy

Ferri-ferrocyanide is a common facile redox couple in electrochemistry and related fields; select applications include charge storage in flow batteries, prevention of electrolyte decomposition in batteries, redox shuttling in dye-sensitized solar cells and photoanodes, as well as measuring charge transfer on catalytic surfaces. Ferri-ferrocyanide is also frequently employed as a standard to establish new steady-state and time-resolved in-situ spectroscopy methods. Despite the ubiquity of the ferri-ferrocyanide redox couple in diverse fields, discussion exists as to whether its kinetics may depend on the composition of both the electrode and electrolyte. Furthermore, ferri-ferrocyanide decomposes when exposed to ionizing radiation such as soft X-rays at a synchrotron, which may lead to electrodeposition on the electrode.

We studied the photochemical response of ferri-ferrocyanide to intense synchrotron radiation by in-situ X-ray absorption spectroscopy at the iron L edge. For photon flux densities above a defined threshold, precipitation of ferric (hydr)oxide from both ferricyanide and ferrocyanide solutions was clearly detectable. The formation of the precipitate was accelerated during cyclic voltammetry, which we probed by time-resolved in-operando X-ray absorption spectroscopy. The iron redox was completely inhibited in the probed volume after only two electrochemical cycles (70 min). Our study highlights the importance of considering both electrochemical and spectroscopic conditions when designing in-situ experiments that can shed light on charge transfer and chemistry of active sites in energy and health research.