Selective ammonium separation by capacitive deionization using binder-free MXene/PBA-coated activated carbon electrodes

Efficient ammonium (NH₄⁺) recovery from wastewater is essential for a circular water economy. This study presents a high-performance rocking-chair capacitive deionization (RCDI) system featuring a novel binder-free MXene/Prussian Blue Analogue (PBA) composite membrane on activated carbon (AC) electrode. The assembled, 3D interlocking “nano-pier” structure prevents MXene stacking, maintaining open pathways for rapid ion transport. The optimized MXene/PBA@AC-HNO₃ electrode achieves a maximum SAC of 118 mg/g at 1.6 V, while the untreated MXene/PBA@AC reaches 78 mg/g, with an NH₄⁺/Na⁺ selectivity coefficient of 3.3—triple of conventional AC electrodes. Furthermore, the electrode demonstrates outstanding stability, retaining over 90% capacity after 100 cycles. Enhanced selectivity is driven by synergistic Donnan repulsion, size sieving, and specific PBA lattice affinity for partially dehydrated NH₄⁺ ions. This work provides a robust and mechanistic framework for selective nutrient recovery from complex synthetic wastewater matrices.