Core–Shell MOF@COF Motif Hybridization: Selectively Functionalized Precursors for Titanium Dioxide Nanoparticle-Embedded Nitrogen-Rich Carbon Architectures with Superior Capacitive Deionization Performance

Metal–organic frameworks (MOFs) and covalent–organic frameworks (COFs) are promising precursors for preparing high-performance carbonaceous materials for capacitive deionization (CDI). However, the simple pyrolysis of single MOFs or COFs usually leads to carbonaceous materials with disadvantages in salt adsorption capacity (SAC) and cycling stability, which are unfavorable to the further development of CDI. To address this issue, herein, we report the directed core–shell motif hybridization of COFs on MOFs to obtain selectively functionalized carbonaceous precursors, NH2-MIL-125(Ti)@TP-DQ COF, which then produce titanium dioxide nanoparticle-embedded nitrogen-rich carbon architectures, called TiO2@COF, via pyrolysis. It is evidently expected that the resulting TiO2@COF possesses several advantageous features: (1) the inner core, which contains titanium dioxide nanoparticles, provides abundant faradic active sites for ion accommodation contributing additionally to the high SAC; (2) the outer capacitive shell, which is fibrous nitrogen-rich carbon, not only protects the inner core from the harsh environment of the solution and stabilizes the cycling performance but also affords plentiful nitrogen dopants for enhanced pseudocapacitive capacity and abundant pores for ion adsorption and electrolyte permeation; and (3) the outer COF-derived fibers interconnect with each other, giving rise to increasing electrical conductivity. As a result, TiO2@COF delivers a high SAC of 33.66 mg g–1 and favorable cycling stability over 40 cycles, significantly exceeding those of CDI electrodes derived from single MOFs or COFs. This work is expected to enrich the construction of selectively functionalized carbonaceous particles from MOFs and COFs and may also endow multiple promising applications of core–shell motif hybrids.