This review presents insights into the fundamental challenges of wet adhesion, and the applications of catechol-functionalized hydrogels in diverse areas.

The MAX phases are a group of layered ternary compounds with the general formula M nz1 AX n (M: early transition metal; A: group A element; X: C and/or N; n51-3), which combine some properties of metals, such as good electrical and thermal conductivity, machinability, low hardness, thermal shock resistance and damage tolerance, with those of ceramics, such as high elastic moduli, high temperature strength, and oxidation and corrosion resistance. The publication of papers on the MAX phases has shown an almost exponential increase in the past decade. The existence of further MAX phases has been reported or proposed. In addition to surveying this activity, the synthesis of MAX phases in the forms of bulk, films and powders is reviewed, together with their physical, mechanical and corrosion/oxidation properties. Recent research and development has revealed potential for the practical application of the MAX phases (particularly using the pressureless sintering and physical vapour deposition coating routes) as well as of MAX based composites. The challenges for the immediate future are to explore further and characterise the MAX phases reported to date and to make further progress in facilitating their industrial application.

Electrically conductive hydrogels (ECHs), combining electrical properties of metals or semiconductors with the unique features of hydrogels, are ideal frameworks to design and construct flexible supercapacitors and batteries. This review summarized the material design and synthetic approach of ECHs, demonstrating the advances of percolation theory in ECH materials, followed by presenting their effective application in flexible energy storage systems, and discussed the challenges and opportunities in this field.

Nitrogen doping has been proven to be a facile modification strategy to improve the electrochemical performance of 2D MXenes, a group of promising candidates for energy storage applications. However, the underlying mechanisms, especially the positions of nitrogen dopants, and its effect on the electrical properties of MXenes, are still largely unexplored. Herein, a comprehensive study is carried out to disclose the nitrogen doping mechanism in Ti 3 C 2 MXene, by employing theoretical simulation and experimental characterization. Three possible sites are found in Ti 3 C 2 T x (T = F, OH, and O) to accommodate the nitrogen dopants: lattice substitution (for carbon), function substitution (for-OH), and surface absorption (on-O). Moreover, electrochemical test results confirm that all the three kinds of nitrogen dopants are favorable for improving the specific capacitance of the Ti 3 C 2 electrode, and the underlying factors are successfully distinguished. By revealing the nitrogen doping mechanisms in Ti 3 C 2 MXene, this work provides theoretical guidelines for modulating the electrochemical properties of MXene materials for energy storage applications.

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