Pseudocapacitive Deionization of Saltwater by Mn₃O₄@C/Activated Carbon

Capacitive deionization (CDI), a m ethod with notable advantages of relatively low energy consumption and environmental friendliness, has been widely used in desalination of saltwater. However, due to the weak electrical double-layer electrosorption of ions from water, CDI has suffered from low throughput capacity that may limit its commercial applications. Thus, it is of importance to develop a high-efficiency and engineering-feasible CDI process. Manganese and cobalt and their oxides, being faradic materials, have a relatively high pseudocapacitance, which can cause an increase of positive and negative charges on opposing electrodes. However, their low conductivity properties limit their electrochemical applications. Pseudocapacitive Mn₃O₄ nanoparticles encapsulated within a conducting carbon shell (Mn₃O₄@C) were prepared to enhance charge transfer and capacitance for CDI. Desalination performances of the Mn₃O₄@C (5-15 wt %) core-shell nanoparticles on activated carbon (AC) (Mn₃O₄@C/AC) serving as CDI electrodes have thus been investigated. The pseudocapacitive Mn₃O₄@C/AC electrodes with relatively low diffusion resistances have much greater capacitance (240-1300 F/g) than the pristine AC electrode (120 F/g). In situ synchrotron X-ray absorption near-edge structure spectra of the Mn₃O₄@C/AC electrodes during CDI (under 1.2 and -1.2 V for electrosorption and regeneration, respectively) demonstrate that reversible faradic redox reactions cause more negative charges on the negative electrode and more positive charges on the positive electrode. Consequently, the pseudocapacitive electrodes for CDI of saltwater ([NaCl] = 1000 ppm) show much better desalination performances with a high optimized salt removal (600 mg/g·day), electrosorption efficiency (48%), and electrosorption capacity (EC) (25 mg/g) than the AC electrodes (288 mg/g·day, 23%, and 12 mg/g, respectively). The Mn₃O₄@C/AC electrode has a maximum EC of 29 mg/g for CDI under +1.2 V. Also, the Mn₃O₄@C/AC electrodes have much higher pseudocapacitive electrosorption rate constants (0.0049-0.0087 h^-1) than the AC electrode (0.0016 h^-1). This work demonstrates the feasibility of high-efficiency CDI of saltwater for water recycling and reuse using the low-cost and easily fabricated pseudocapacitive Mn₃O₄@C/AC electrodes.