The oxygen reduction reaction (ORR) is a critical but sluggish reaction used for various applications, including fuel cells and metal-oxygen batteries. Currently, the benchmark catalysts for ORR are Pt-based catalysts. However, due to these catalysts being scarce and very expensive, alternative catalysts that are more abundant and less expensive are urgently in demand. MXenes, a new two-dimensional (2D) material based on transition metal carbides and nitrides, have advanced the performance of various existing applications and technologies, including batteries, supercapacitors, sensors, etc. Owing to their unique electronic structure, physical and chemical properties, and tunable morphology, MXenes are expected to thrive as catalysts in ORR electro- and photo-catalytic systems. Additionally, they are promising catalytic supports for ORR, thus significantly reducing the necessary Pt loading. In this work, we carefully review the most pertinent works on ORR using MXenes. We discuss the gaps and challenges in the field, identify key areas that need further attention, and provide directions for future research.

MXenes, a family of two-dimensional (2D) materials based on transition metal carbides and nitrides, show great promise as electrocatalysts for electrochemical reactions but suffer from a lack of fundamental understanding of their electrocatalytic mechanisms. Decoupling the surface and bulk reactivities are key to understanding the electrocatalytic mechanisms of these emerging nanomaterials. Herein, we use Raman spectroscopy to decouple the surface and bulk reactivities of the Ti 2 NT x MXene. By attenuating the Raman intensity, we were able to control the penetration depth of the incident light. We show that with 5−10% laser power, we can restrict the access to the surface of the Ti 2 NT x MXene, which shows predominantly the oxide phase from the passivation layer. With 50−100% laser power, we can penetrate the bulk of the MXene material, as evidenced by the Ti−N bond network characteristics. By using this approach, we can study and understand the surface reactivity of MXenes, which has not been understood. To expand these findings to electrochemical reactions, we have analyzed the surface reactivities of the pristine and NaOH treated Ti 2 NT x MXenes using Raman spectroscopy and hydrogen evolution reaction (HER). The surface chemistry of the NaOH treated Ti 2 NT x MXene was different from that of the pristine ones and showed predominantly −OH termination groups and no −F. This resulted in an excellent HER activity of ∼100 mV overpotential at 10 mA cm −2 and an impressive Tafel slope of 98 mV dec −1 . These results are comparable to those for the benchmark Pt/C in the alkaline electrolyte. To summarize, we propose an approach for decoupling the surface and bulk reactivities of MXenes and a simple method to tune their HER activity to match those for noble metals. This approach can be applied to other materials and systems to accelerate the discovery of electrocatalysts and electrode materials.

Current climate issues can be partially remedied through the inclusion of renewable energy sources. However, these energy sources suffer from the need for highly efficient energy storage systems. To this end, studies have been conducted on developing energy storage materials that can provide high energy and power densities. Two-dimensional (2D) carbide and nitride MXenes have the potential to provide both if their mechanism of charge storage is understood. Here, we use in situ/operando Raman spectroelectrochemistry to investigate in real time the charge storage mechanism of the benchmark Ti 3 C 2 MXene in acidic and neutral media. We found that during charge/discharge cycling in acidic media, protons are pulled towards the surface, leading to a reversible reaction with the surface termination groups leading to a switch from À OÀ to À O(OH) to À OÀ chemistry, indicative of a pseudocapacitive mechanism. In neutral media, no pseudocapacitive behavior is observed and it is found that the material exhibits an electrostatic double layer charge storage mechanism through attracting sodium ions towards the surface for transient adsorption processes. Taking into consideration the mechanism we propose, the Ti 3 C 2 MXene can exhibit a pseudocapacitance of 358 F/g within a voltage window of 1.35 V. Ultimately, these fundamental insights can be used to design electrode materials with both high energy and power densities.

Decarbonizing the chemical industry and achieving carbon-neutral energy is paramount to the sustainability of the human species on earth. Electrocatalysis using transition metalbased catalysts plays a major role in achieving this task. In this work, we present an overview on the application of transition metal-based catalysts in electrocatalytic reactions. We particularly focus on the advancement of the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), nitrogen reduction reaction (NRR), and carbon dioxide reduction reaction (CO 2 RR) using transition metal electrocatalysts. We also aim to highlight the major achievements in these fields and the limitations in their large-scale industrial applications. Different transition metal catalysts are discussed, from 3-dimensional to 1-to 0-dimensional structures. We place special attention to the emerging transition metal catalysts such as 2dimensional carbide and nitride MXenes and single atom catalysts. Lastly, we offer recommendation for future research directions for the aforementioned electrocatalysis fields using transition metal electrocatalysts. This work will ultimately help both existing and incoming researchers in these fields in providing the state-of-the-art research findings and identifying the major challenges that must be addressed in order to attain carbon-neutral energy.

Pt-based catalysts are generally used for the oxygen reduction reaction (ORR) in batteries and fuel cells but due to their high price, low abundance, and poor stability, alternative materials are...