High Performance Capacitive Deionization Cathode of Nickel Hexacyanoferrate Doped with Trace Molybdenum: Breaking the Capacity‐Stability Trade‐Off

Nickel hexacyanoferrate (NiHCF) is considered one of the most promising electrode materials for capacitive deionization (CDI) because of its excellent cycling stability and other advantages. However, the poor desalination ability and low adsorption capacity caused by a single reaction site seriously restrict its practical application. Here, a high-valence transition metal element Mo-doped strategy is proposed to utilize traces of Mo⁶⁺ to activate the inert-sites within NiHCF. The Mo-doped NiHCF electrodes (NiMo_x-HCF) with multiple pairs of redox active sites and long cycling capability were designed, breaking the trade-off between high adsorption capacity and cycling stability. The results show that the preferred NiMo_0.3-HCF not only retains the excellent cycling stability (91% after 200 cycles), but also possesses a high adsorption capacity of 90.92 mg g^-1 at 1.2 V, which is much better than those of NiHCF under the same conditions. In addition, it has good practical application in simulated brackish water and circulating cooling water. Structural characterization and theoretical calculations indicate that the Mo doping strategy enhanced electronic conductivity while maintaining structural stability. This research proposes a new method to activate inert sites using high-valence elements, thus effectively balancing the adsorption capacity and cycling stability of the electrode.