Green energy harvesting from CO₂ and NO_x by MXene materials: Detailed historical and future prospective

Improving the carbon dioxide and nitrogen oxide reduction reactions (CO₂RR and NORR) can reduce anthropogenic greenhouse gas emissions while selectively producing chemicals needed for the fuel, plastic, and chemical industries. Efficient CO₂RR can be used to replace fossil fuels as well as repurpose captured CO₂, while new NORR pathways can be used to supplement or replace the energy-intensive Haber–Bosch process for NH₃ generation with no CO₂ emissions. Incorporating cocatalysts is an effective approach to enhance the performance of semiconductors in photocatalysis and electrocatalysis. Noble metals have traditionally been used as cocatalysts in photocatalysis; however, their high cost restricts their widespread application on a large scale. Therefore, this review article focuses on integrated, i.e., (photo)electrocatalytic and pure photocatalytic conversion of CO₂ and NO_x molecules into useful products, such as CO, methanol, formic acid (HCOOH), and ammonia (NH₃), using metal carbides/nitrides (MXenes) due to their advantageous features such as high surface area, tunable chemistry, and easily adjustable elemental compositions. Numerous novel materials, their preparations and performances have been discussed. Depending upon the nature of MXene-based materials, the synthesis techniques and photocatalytic mechanism of MXenes as co-catalyst are also summarized, including the strategies to enhance the catalytic activity of MXenes for target products. Notably, surface modification has emerged as a practical approach to improve the performance of MXenes further. Despite the remarkable developments, several challenges remain in using MXenes as catalysts for NO_xRR and CO₂RR. Addressing these challenges will pave the way for future advancements in this field. This review will shed light on these lingering challenges and provide insights into the future direction of MXenes development as catalysts for sustainable energy and environmental applications.