Self-Assembled NbOPO₄ Nanosheet/Reduced Graphene Oxide Heterostructure for Capacitive Desalination

Capacitive deionization (CDI) is a promising and energy-efficient technology for desalination. The development of high-performance capacitive electrodes is essential for enhancing the CDI properties for practical applications. Here, a 2D heterostructure was rationally designed and synthesized by face-to-face restacking of NbOPO₄ nanosheets and reduced graphene oxide (rGO) via an electrostatic self-assembly process. The as-prepared 2D NbOPO₄/rGO heterostructure achieved an excellent ion storage capacity, electronic conductivity, and unimpeded ion kinetics. When applied as electrodes for CDI, the 2D NbOPO₄/rGO heterostructure delivered a high specific capacitance of 258.3 F g^–1 and an electrosorption capacity of 73 mg g^–1 for NaCl solution of 10 000 mg L^–1 at an applied voltage of 1.2 V, which is more than five times larger than that of activated carbon. The heterostructure electrode also showed high desalination stability for up to 50 adsorption/desorption cycles. The high CDI performance is attributed to the strong 2D/2D coupling between NbOPO₄ nanosheets and rGO. The strong 2D/2D coupling reduced the charge transfer resistance, affirmed via the electrochemical impedance spectroscopy technique, attesting to the enhanced charge transportation across the heterointerface. The robust 2D/2D coupling was affirmed via the uniform and identical Raman shifts at various random regions, and larger XPS binding energy shifts for the self-assembled NbOPO₄/rGO heterostructure. This work demonstrated the potential of self-assembled nanoheterostructures for water desalination via capacitive deionization.