Trends for Proton Transport Activity and Stability in Turnbull's Blue Analogues: Theory and Experiments

Prussian Blue analogues (PBAs) are promising materials for energy storage and conversion because their robust open-framework structures enable efficient ion intercalation. Among them, Turnbull's Blue analogues (TBAs), a subclass featuring ferrocyanide vacancies, exhibit exceptional rate capability in acidic electrolytes by facilitating rapid proton transport. To optimize TBA performance, understanding how the metal identity influences both proton mobility and structural stability is essential. Here, we combined density functional theory (DFT) calculations with electrochemical characterization experiments to investigate the proton transport activity and stability of TBAs incorporating six metals (Cu, Zn, Mn, Fe, Co, and Ni) under acidic conditions. Our theoretical analysis revealed that the inclusion of Mn, Fe, Co, and Ni enhances proton transport via electron spin-flipping at low-spin Fe sites. Furthermore, the presence of ferromagnetic metals promotes the reduction of Fe ions located farther from dense ferrocyanide vacancy regions during proton intercalation, destabilizing the initial protonated configuration and lowering the reaction energy. Stability trends, assessed through Gibbs free energy changes for dissolution in acidic electrolytes, aligned with experimental capacity retention. Through this integrated study, Ni-TBA emerged as a promising candidate, combining high proton transport activity with superior stability, offering valuable guidance for the rational design of high-performance proton-ion battery materials for energy storage.