Scalable hybrid electrode-based capacitive deionization modules for practical water softening

Conventional water softening technologies suffer from high energy consumption and reliance on chemicals, while existing capacitive deionization (CDI) approaches still struggle to sufficiently remove hardness ions from domestic tap water. Herein, we demonstrate roll-to-roll fabricated scalable vanadium-oxide-based hybrid (VH) electrodes for the construction of bipolar multistack CDI modules toward practical water softening technology. The VH electrode couples faradaic V⁵⁺/V⁴⁺ ion storage with electrical double layer adsorption, delivering high divalent capacity and rate capability in the hard-water feed together with 300-cycle stability (93.2% retention). Based on the optimized module design via COMSOL simulations, the multistack CDI modules were assembled with 105 stacks of bipolar VH electrodes, corresponding to a total active area of 42,168 cm². These modules achieved high ion removal efficiencies of 89.9 and 83.7% at flow rates of 0.5 and 2.5 L min⁻¹, respectively, validating both scalability and real-world applicability. This work confirms the strong potential of redox-active hybrid materials for practical CDI module applications and establishes a viable strategy for sustainable hardness removal.