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.

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.

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...

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