Reversibility of Ferri-/Ferrocyanide Redox during Operando Soft X-ray Spectroscopy

The ferri-/ferrocyanide redox couple is ubiquitous in many fields of physical chemistry. We studied its photochemical response to intense synchrotron radiation by in situ X-ray absorption spectroscopy (XAS). For photon flux densities equal to and above 2 x 10(11) s(-1) mm(-2), precipitation of ferric (hydr)oxide from both ferricyanide and ferro cyanide solutions was clearly detectable, despite flowing fast enough to replace the solution in the flow cell every 0.4 s (flow rate 1.5 mL/min). During cyclic voltammetry, precipitation of ferric (hydr)oxide was promoted at reducing voltages and observed below 10(11) s(-1) mm(-2). This was accompanied by inhibition of the ferri-/ferrocyanide redox, which we probed by time-resolved operando XAS. Our study highlights the importance of considering both electrochemical and spectroscopic conditions when designing in situ experiments.