2D non-carbide MXenes: an emerging material class for energy storage and conversion

Since the first discovery of MXenes in 2011, various compositions of MXenes have been developed based on the combinations of different transition metals. Their exceptional and intriguing properties, such as excellent electronic conductivity, hydrophilic nature, and ease of large-scale synthesis, enabled them to be candidates for diverse applications, including energy storage, EMI shielding, biomedical, transparent conductive films, electrocatalysis, etc. However, most efforts have been focused on carbide MXenes and not much on non-carbide (carbonitride and nitride) MXenes. With electronegative nitrogen in non-carbide MXenes, an additional negative charge is induced in the conductive network, enhancing their properties such as wettability, conductivity, and pseudocapacitance. The difficulty in their synthesis has been a significant stopgap in the research progress of non-carbide MXenes. In this review, we have provided a comprehensive overview of the synthesis, properties, and applications of non-carbide MXenes, emphasizing the difference between carbide and non-carbide MXenes for better understanding. The challenges and limitations of non-carbide MXenes and the ways to circumvent them are briefly discussed. The future research directions and the opportunities are also discussed to encourage researchers to investigate non-carbide MXenes further.