Highly Stable Hybrid Capacitive Deionization with a MnO₂ Anode and a Positively Charged Cathode

Abstract

Performance degradation caused by the oxidation of carbon anodes during capacitive deionization (CDI) remains a major problem that may greatly restrict the practical application of CDI. To improve the cyclic stability of CDI, carbon-based anode materials were replaced by pseudocapacitive MnO 2 in this work. The cation-selective MnO 2 anode was assembled with an anion-selective quaternized poly(4-vinylpyridine)coated activated carbon cathode into a hybrid CDI cell. The cell exhibited inverted CDI performance with a wide operating voltage window of 1.4 V and a salt adsorption capacity (SAC) of 14.9 mg/g in 500 mg/L NaCl. The SAC retention ratio of the cell can be as high as 95.4% after 350 adsorption−desorption cycles at 1.0/0 V, while that of the CDI cell consisting of activated carbon electrodes was only 15.7% after 285 cycles. The enhanced cyclic stability of the hybrid CDI cell is attributed to the employment of the MnO 2 anode, which avoided the use and oxidation of carbon anodes.

Recent advances in ion selectivity with capacitive deionization

“…Interestingly, this mechanism results in the absence of co-ion repulsion, 89 enhancing the charge efficiency of electrosorption of intercalation materials without the use of membranes, as reported in literature. 78,80,90 Research into intercalation materials has also been catalyzed by their inherent selectivity towards ions, usually by size. 98 This is especially true for PB and PBAs, that have a cubic lattice structure and store cations in their interstitial sites.…”

Faradaic Electrodes Open a New Era for Capacitive Deionization

“…Some HCDI cells more recently reported still use a Na + insertion Faradaic electrode (such as MnO 2 and NaTi 2 (PO 4 ) 3 ) but adopt positively charged carbon electrodes instead of carbon-AEM suit for selective Cl − adsorption, which also generated good desalination results. [76,77] In contrast to the aforementioned familiar configuration for HCDI, a less common configuration inverts configuration by employing a Faradaic electrode for Cl − capture and a capacitive carbon electrode for Na + capture, such as Ag paired with carbon-cation exchange membrane (CEM), [78] or calcined layered double hydroxide matched with carbon electrode (see Section 3.2.3). [57,79,80] However, this configuration remains poorly studied as identifying acceptable Cl − storage electrode materials that can meet key criteria (low cost, insolubility, stability, and reversibility within a limited voltage range) remains a research challenge.…”

“…Apart from manganese oxides with unique crystalline structures, various amorphous manganese oxides have been applied as Faradaic electrodes for CDI as well. [76,108,[159][160][161][162] For instance, Wu et al [76] employed an amorphous MnO 2 as a Faradaic electrode for a HCDI cell, which exhibited an ion removal capacity of 14.9 mg g −1 in 500 mg L −1 NaCl solution and maintained 95.4% of the initial capacity after 350 cycles. Moreover, considering that the electronic conductivity of carbon materials (about 50 S cm −1 ) [163] is much higher than that of manganese oxides (10 −7 -10 −3 S cm −1 ), [164,152] carbon materials have been incorporated into manganese oxides to further improve their Faradaic charge-transfer.…”

“…To date, 1D tunnel structured MnO 2 , 2D δ ‐MnO 2 , 3D δ ‐MnO 2 , and amorphous MnO 2 have been explored as Faradaic electrodes in CDI, and their crystallographic microstructure heterogeneity coupled with the various valence states of Mn in MnO 2 significantly affect the ion insertion/extraction behavior and corresponding desalination performance. [ 76,103,109,128 ] 1)1D tunnel structured MnO 2 (TuMOs) …”

Electrochemical supercapacitors: Operation, components and materials

“…Манган-оксид се често користи уместо рутенијум-оксида, због релативно ниске цене, мале токсичности, прихватљивости за становишта животне средине, уз одлична капацитивна својства -теоријска капацитивност је око 1200 F g -1 [161][162][163][164][165][166][167][168][169][170][171]. Као и код свих металних оксида, механизам складиштења енергије код оксида мангана је сачињен од низа реверзибилних редокс прелаза који укључују размену протона и/или катјона са електролитом.…”