Mechanisms of Degradation of Na₂Ni[Fe(CN)₆] Functional Electrodes in Aqueous Media: A Combined Theoretical and Experimental Study

Prussian blue analogues (PBAs) are versatile functional materials with numerous applications ranging from electrocatalysis and batteries to sensors and electrochromic devices. Their electrochemical performance involving long-term cycling stability strongly depends on the electrolyte composition. In this work, we use density functional theory calculations and experiments to elucidate the mechanisms of degradation of model Na₂Ni[Fe(CN)₆] functional electrodes in aqueous electrolytes. Next to the solution pH and cation concentration, we identify anion adsorption as a major driving force for electrode dissolution. Notably, the nature of adsorbed anions can control the mass and charge transfer mechanisms during metal cation intercalation as well as the electrode degradation rate. We find that weakly adsorbing anions, such as NO₃^-, impede the degradation, while strongly adsorbing anions, such as SO₄^2-, accelerate it. The results of this study provide practical guidelines for electrolyte optimization and can likely be extrapolated to the whole family of PBAs operating in aqueous media.