Pseudocapacitive Deionization of Saltwater by Mn(3)O(4)@C/Activated Carbon

[Image: see text] 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 Mn(3)O(4) nanoparticles encapsulated within a conducting carbon shell (Mn(3)O(4)@C) were prepared to enhance charge transfer and capacitance for CDI. Desalination performances of the Mn(3)O(4)@C (5–15 wt %) core–shell nanoparticles on activated carbon (AC) (Mn(3)O(4)@C/AC) serving as CDI electrodes have thus been investigated. The pseudocapacitive Mn(3)O(4)@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 Mn(3)O(4)@C/AC electrodes during CDI (under 1.2 and −1.2 V for electrosorption and regeneration, respectively) demonstrate that reversible faradic redox reactions [Image: see text] 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 Mn(3)O(4)@C/AC electrode has a maximum EC of 29 mg/g for CDI under +1.2 V. Also, the Mn(3)O(4)@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 Mn(3)O(4)@C/AC electrodes.