MXenes represent an emerging family of conductive two-dimensional materials. Their representative, TiCT, has been recognized as an outstanding member in the field of electrochemical energy storage. However, an in-depth understanding of fundamental processes responsible for the superior capacitance of TiCT MXene in acidic electrolytes is lacking. Here, to understand the mechanism of capacitance in TiCT MXene, we studied electrochemically the charge/discharge processes of TiCT electrodes in sulfate ion-containing aqueous electrolytes with three different cations, coupled with in situ Raman spectroscopy. It is demonstrated that hydronium in the HSO electrolyte bonds with the terminal O in the negative electrode upon discharging while debonding occurs upon charging. Correspondingly, the reversible bonding/debonding changes the valence state of Ti element in the MXene, giving rise to the pseudocapacitance in the acidic electrolyte. In stark contrast, only electric double layer capacitance is recognized in the other electrolytes of (NH)SO or MgSO. The charge storage ways also differ: ion exchange dominates in HSO, while counterion adsorption in the rest. Hydronium that is characterized by smaller hydration radius and less charge is the most mobile among the three cations, facilitating it more kinetically accommodated on the deep adsorption sites between the MXene layers. The two key factors, i.e., surface functional group-involved bonding/debonding-induced pseudocapacitance, and ion exchange-featured charge storage, simultaneously contribute to the superior capacitance of TiCT MXene in acidic electrolytes.

Graphene oxide (GO) sheets, considered as “soft” two-dimensional macromolecules containing multiple aromatic regions and hydrophilic oxygen groups, can adsorb the pristine multiwalled carbon nanotubes (MWNTs) through the π-stacking interaction, thus causing pristine MWNTs to stably disperse and fractionate in aqueous media. The water-soluble and insoluble complexes can be obtained by changing the initial proportion of MWNTs to GO sheets, which are important for noncovalent approaches toward solubilizing CNTs and open a new way for GO applications in colloidal chemistry. Solubility results indicate that the GO sheets are prone to stabilize MWNTs with larger diameters. When the diameters of CNTs decrease to a critical value, the π-stacking interactions are weakened by large surface energy among CNTs themselves; the CNTs are inclined to form bundles or twisted structures instead of CNT−GO complexes. This can be probably developed into a practical method to fractionate MWNTs with different outer diameters in water.

Polyaniline hollow spheres (PANI-HS)@electrochemical reduced graphene oxide (ERGO) hybrids with core-shell structures have been fabricated via a solution-based coassembly process. The hollow nanostructured designing for the PANI-HS greatly enlarges the specific surface area, providing high electroactive regions and short diffusion lengths for both charge and ion transport. The wrapping of ERGO sheets on the PANI-HS can offer highly conductive pathways by bridging individual PANI-HS together, thus facilitating the rate and cycling performance of supercapacitors. The specific capacitance of PANI-HS36@ERGO hybrids can reach 614 F g(-1) at a current density of 1 A g(-1). Furthermore, the capacitance of the PANI-HS36@ERGO hybrids maintains 90% after 500 charging/discharging cycles at a current density of 1 A g(-1), indicating a good cycling stability. The greatly enhanced electrochemical performance can be ascribed to the synergic effects of the two components of PANI-HS and ERGO, suggesting that the PANI-HS@ERGO hybrids as novel electrode materials may have potential applications in high-performance energy storage devices.

Layered double hydroxides (LDHs) are generally expressed as [M(2+)(1-x)M(3+)(x) (OH)(2)] [A(n-)(x/n)·mH(2)O], where M(2+) and M(3+) are divalent and trivalent metal cations respectively, and A is n-valent interlayer guest anion. Co-Al layered double hydroxides (LDHs) with different sizes have been grown on graphene oxide (GO) via in situ hydrothermal crystallization. In the synthesis procedure, the GO is partially reduced in company with the formation of Co-Al LDHs. The morphology and structure of LDHs/GO hybrids are characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The growth mechanism of LDHs on GO nanosheets is discussed. Moreover, both LDHs and LDHs/graphene nanosheets (GNS) hybrids are further used as electrochemical supercapacitor materials and their performance is evaluated by cyclic voltammetry (CV) and galvanostatic charge/discharge measurements. It is shown that the specific capacitances of LDHs are significantly enhanced by the hybridization with GNS.

A Co@N-CNTF electrocatalyst derived from a metal–organic framework exhibits high catalytic activity in the ORR, OER and HER.