Green energy harvesting from CO2 and NOx 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 NOx 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 NOxRR 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. [Display omitted] •MXenes are emerging as a promising class of photocatalysts, co-catalysts, and electrocatalysts.•MXene exhibit viable and adjustable band gaps, and their surface can be modified.•A critical review was conducted on MXenes as catalysts for NOxRR and CO2RR.•Surface modified MXenes showed efficient electrochemical reduction into clean energy.•The potential and challenges of MXene-based materials were explored.