Xiaojun Sun,Mengting Di,Li Gao,Lei Hu,Wenji Zheng,Xuehua Ruan,Xiaoming Yan,Gaohong He
Reverse electrodialysis (RED) is a promising membrane-based technology for salinity gradient power while it faces the challenge of the energy loss caused by the insufficient monovalent/multivalent ion selectivity of the membranes (such as Na + /Mg 2+ ). Here, covalent organic frameworks (COFs) with nanometre-scale pores (pore size of ~8 Å) are designed as composite membranes to selectively conduct monovalent ion (Na + ~7.1 Å vs Mg 2+ ~8.6 Å) by size sieving. Compared with the commercial Neosepta CMX membrane, the COF composite membrane effectively inhibits the penetration of Mg 2+ without sacrificing Na + penetration flux in RED, which to a large extent reduces the energy loss in RED caused by the penetration of Mg 2+ . Even for Mg 2+ solution with high molar fraction of 30 %, the maximum power density (P d, max ) of CCM only shows a slight decrease of 1.7 %. In contrast, the P d, max of the commercial Neosepta CMX membrane severely decreases by 35.8 %. This study highlights the prospects of COFs as the significant material for designing monovalent ion selective membranes which can improve energy efficiency in RED.
Monovalent cation exchange membranes (M-CEMs) have been extensively applied in environmental remediation and energy harvesting such as the extraction of Na + or Li + from brine and seawater. However, owing to the limitations of membrane structures and materials, M-CEMs have a low perm-selectivity issue. Herein, we proposed a facile approach to construct a novel M-CEM with a Janus-charged structure, consisting of a positively-charged trimesic acid/polyethylenimine surface thin layer and a negatively charged commercial cation exchange membrane (CEM). Selectrodialysis results indicated that the Janus-charged M-CEMs could effectively suppress the migration of anions, which often occurred in porous CEMs, thereby enabling the novel Janus-charged M-CEMs to possess high perm-selectivity and high total cation fluxes. Compared with state-of-the-art M-CEMs, the Janus-charged M-CEM exhibited the highest perm-selectivity of 145.77 for Na + /Mg 2+ beyond the contemporary “Upper Bound” plot as well as the excellent perm-selectivity of 14.11 for Li + /Mg 2+ , indicating its great potentials in ion separation. This study can provide novel insights into the design of Janus-charged M-CEMs for ion separation in diverse environmental and energy applications.
The practical uses of magnesium-ion batteries are hindered by their poor rate capability and fast capacity decay. Moreover, traditional sodium ion batteries suffer from serious safety problems resulting from the sodium dendrites formed on the anode. In order to circumvent these problems, we designed a highly reversible Na+/Mg2+ hybrid-ion battery composed of a metallic Mg anode, a TiS2 derived titanium sulfide cathode and a 1.0 M NaBH4 + 0.1 M Mg(BH4)2/diglyme hybrid electrolyte. The battery showed remarkable electrochemical performances with a large discharge capacity (200 mA h g−1 at the 1C rate), high rate capability (75 mA h g−1 at the 20C rate) and long cycle life (90% capacity retention after 3000 cycles). Moreover, it exhibited excellent safety properties due to dendrite-free Mg deposition of the anode and the high thermal stability of the cathode. These merits demonstrate the great potential of the reported Na+/Mg2+ hybrid-ion battery for large-scale energy storage.
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