Layer‐By‐Layer Self‐Assembled Nickel Hexacyanoferrate Electrodes Coupled With Polypyrrole for Battery‐Type Sodium Ion Exchange Systems

The growing demand for sustainable energy storage and efficient desalination technologies has driven the development of electrochemical systems capable of addressing both challenges. In this work, nickel hexacyanoferrate (NiHCF) electrodes were supported on graphite felt using the layer‐by‐layer (LbL) electrostatic self‐assembly technique and compared with electrodes obtained by ink impregnation. The LbL electrodes show a significant improvement in electrochemical performance, reaching up to 83% of the theoretical capacity, compared to 53% in electrodes prepared by ink impregnation. This improvement is attributed to a more homogeneous and conformal distribution of the active material, which favors accessibility to the active sites and reduces polarization, consistent with electron microscopy analyses and charge–discharge profiles. The influence of the counter electrode and the number of deposited bilayers was also evaluated. The use of polypyrrole (PPy) as a chloride reversible counter electrode increases the capacity of the system, while symmetrical NiHCF/NiHCF “rocking‐chair” configurations exhibit greater cyclic stability. Specific capacity scales with the number of layers without significant diffusion limitations. LbL electrodes show good capacity at various currents and stability over 200 cycles and perform ion removal in desalination tests in undivided, membrane‐less hybrid LbL NiHCF‐PPy flow system, highlighting their potential for sodium storage and electrochemical desalination.