Multiple Potentials of Maximum Entropy for a Na2Co[Fe(CN)6] Battery Electrode Material: Does the Electrolyte Composition Control the Interface?

Development of efficient schemes of energy storage is crucial for finding a solution for the "generation versus consumption" problem. Aqueous Na-ion batteries have been already recognized as one of the promising candidates for large-scale energy-storage systems. Despite noticeable progress in this field, the actual intercalation mechanisms governing these battery cells are yet to be fully comprehended. In this manuscript, we examine the electrode/electrolyte interface formed between electrodeposited Na2Co[Fe(CN)6] films and aqueous solutions. The investigated systems exhibit up to three potentials of maximum entropy (PMEs). To the best of our knowledge, the existence of multiple PMEs in electrochemical systems has never been reported in the literature. These unexpected results are, however, in line with the theory explaining the correlation between the water structure at the interface and the ease of the interfacial mass and charge transfer. Additionally, the obtained PMEs appear to largely depend on the anions' properties, most probably on the hydration energy of these species. This reveals the impact of the electrolyte composition on the interfacial processes in Na-ion batteries.