A binder free hierarchical mixed capacitive deionization electrode based on a polyoxometalate and polypyrrole for brackish water desalination

Abstract

Capacitive deionization technology is an efficient method for brackish water desalination, in which the pseudocapacitive material plays a vital role in determining the desalination performance of the electrode directly. Compared with a traditional double-layer capacitance deionization electrode, a mixed capacitive deionization electrode possesses obvious advantages, because it integrates pseudocapacitance and double-layer capacitance together. A brand-new mixed capacitive deionization electrode is fabricated by co-deposition of P₂Mo₁₈O₆₂⁶⁻ and polypyrrole on a 3D exfoliated graphite matrix using an electrochemical technique. In this electrode, composite particles composed of P₂Mo₁₈O₆₂⁶⁻ and polypyrrole distribute evenly on the 3D exfoliated graphite matrix. At 1 A g⁻¹ current, the specific capacitance of this electrode is 156.2 mA h g⁻¹. Its rate capability is also promising with more than 76.5% of the capacitance being retained when the current increases to 20 A g⁻¹. At 1.2 V voltage, its desalination capacity and rate reach 17.8 mg g⁻¹ and 1.12 mg g⁻¹ min⁻¹ in 600 mg L⁻¹ NaCl. This satisfactory desalination performance is attributed to the unique electrochemical properties of P₂Mo₁₈O₆₂³⁻ and polypyrrole and the binder free character of this electrode. Even after 100 cycles, its desalination ability does not decay, which confirms its excellent stability. This work confirms the prospects for polyoxometalate based electrodes in brackish water desalination.