Exploring the stabilization effect and mechanism of Nafion coating on Prussian blue analogue electrodes

Prussian blue analogues (PBAs), particularly nickel hexacyanoferrate (NiHCF), are promising electrode materials for aqueous sodium-ion batteries but suffer from limited calendar life due to inherent framework instability upon direct contact with aqueous electrolytes. To mitigate this degradation, we investigate the stabilization effects of a multifunctional Nafion coating—exhibiting Na⁺-selective permeability, interfacial bonding enhancement, and electrolyte isolation—applied via drop-casting to NiHCF electrodes derived from powders processed under distinct drying protocols (vacuum vs. air drying). These drying protocols induce divergent physicochemical and electrochemical properties in NiHCF electrodes, critically influencing Nafion-electrode interactions: a discrete Nafion film forms on vacuum-dried NiHCF due to reduced wettability, whereas Nafion infiltrates the porous architecture of air-dried counterparts. Experimentally, Nafion-coated electrodes demonstrate superior cycle stability (83.7 % vs. 68.0 % capacity retention after 300 cycles for coated vs. uncoated vacuum-dried samples), enhanced specific capacity, and improved rate performance. Intriguingly, while numerical simulations confirm that the Nafion coating reduces concentration polarization via selective Na⁺ transport, they fail to predict the observed gains in specific capacity or rate capability. These findings challenge the conventional wisdom that improved ionic transference numbers inherently boost electrode performance by alleviating concentration polarization, revealing instead that such benefits only emerge when concentration polarization dominates over combined ohmic and activation polarization effects.