In recent years, with the increasing demand for green energy and clean environment, development of new nanomaterials towards highly efficient energy storage and selective radioactive removal have received much attention, and various nanomaterials have been reported previously to solve the necessity. However, most of the existing materials have problems in synthetic strategy, high prices, and mass production. To solve these problems, it is imperative to develop cost-effective two-dimensional (2D) composite materials in large scale towards highly efficient energy storage and selective radioactive ions removal. Also, it is of prime importance to study the energy storage mechanisms of energy storage materials thorough charge/discharge and electrochemical analyses. Pertaining to the radionuclide adsorption studies, the experimental data obtained through batch adsorption experiments should be compared with that of the theoretical values in determining the adsorption mechanisms.
First. Two-dimensional (2D) titanium carbide (Ti3C2Tx), MXene, is an attractive pseudocapacitive electrode material that exhibits the highest capacitance at a negative potential in aqueous protic (H₂SO₄) electrolyte. The best way to realize a material with higher energy density in aqueous electrolytes is to develop hybrid supercapacitor (HSC) using diverse cathode materials. However, it is difficult to find redox-active positive cathode materials that are suitable for use with MXene and that can be work well in H₂SO₄ electrolyte. 2D nanocomposite of reduced graphene oxide (rGO) decorated phosphomolybdic acid (PMo₁₂), POMs are reported here as promising cathode materials suitable for use against MXene anode. The complementary potential window of MXene and rGO-POMs along with the redox activity and 2D nanostructured features of these materials can significantly enhance the electrochemical properties of a HSC cell. All-redox active HSC cell with an rGO-POM cathode and an MXene anode can deliver a maximum specific energy of 50.46 Wh/kg with 87.12% capacitance retention over 10000 cycles and with superb energy and coulombic efficiency over all applied current densities. This result demonstrates that 2D redox-active electrode materials are superior to many reported carbon-based asymmetric and hybrid capacitors.
Second. Although exfoliated MoS₂ sheets have recently demonstrated their great potential for use as electrodes for supercapacitors, they suffer from poor intrinsic electrical conductivity and strong tendency for inter-sheet aggregation. Here, we develop a synthetic method for the preparation of N-doped carbon-coated MoS₂ (NC-MoS₂) nanosheets composed of coating of polydopamine using post carbonization process. Construction of carbon layers on the MoS₂ surface enables effective prevention of sheet aggregation and provide a rapid pathway for electron transfer. As-prepared NC-MoS₂ materials are then characterized by electrochemical measurements to demonstrate the superior performance of the supercapacitor electrodes. The NC-MoS₂ electrode exhibits improved electrochemical performance relative to the exfoliated MoS₂ sheets, with a high specific capacitance (158 F g-1), high rate capability (83% retention), and good cycling life (89% retention) during 1000 cycles.
Third. We prepared two-dimensional (2D) stack-structured aminopropyl Isobutyl polyhedral oligomeric silsesquioxane (POSS-NH₂) intercalated titanium carbide (Ti3C2Tx) MXene material (Ti3C2Tx/POSS-NH₂) using a post-intercalation strategy as a potential adsorbent for the removal of cesium (Cs+) and strontium (Sr2+) ions from aqueous solutions. Ti3C2Tx/POSS-NH₂ exhibited unprecedented adsorption capacities of 148 and 172 mg g-1 for Cs+ and Sr2+ ions, respectively. Batch adsorption experimental data well fitted the Freundlich isotherm model, which revealed multilayer adsorption of Cs+ and Sr2+ ions onto heterogeneous -OH, -F, -O, and -NH₂ adsorption sites of Ti3C2Tx/POSS-NH₂ with different energies. Ti3C2Tx/POSS-NH₂ exhibited rapid Cs+/Sr2+ ions adsorption kinetics and attained equilibrium within 30 min. Also, Ti3C2Tx/POSS-NH₂ exhibited recyclable capability over three cycles and remarkable selectivities of 89% and 93% for Cs+ and Sr2+ ions, respectively, in the presence of co-existing mono- and divalent cations. We suggest the high adsorption capacity of Ti3C2Tx/POSS-NH₂ might be due to the synergistic effects of (i) increased inter-lamellar distance between Ti3C2Tx galleries due to POSS-NH₂ intercalation, enable diffusion and encapsulation of large numbers of Cs+/Sr2+ ions, (ii) strong complexation of amine (-NH₂) groups of POSS-NH₂ with Cs+/Sr2+ ions, and (iii) the presence of large numbers of heterogeneous surface functional groups (e.g., -OH, -F, and -O), which resulted in the adsorptions of Cs+/Sr2+ ions through electrostatic, ion exchange, and surface complexation mechanisms. Given the extraordinary adsorption capacities observed, intercalation appears to be a promising strategy for the effective removal of radioactive Cs+ and Sr2+ ions from aqueous media.