Chapter 7 - Oxygen reduction reaction by metal-free catalysts

Abstract

The use of platinum (Pt) catalysts on fuel cells has caused a significant increase in terms of price. So, carbon-based metal-free electrocatalysts are a potential platinum (Pt) substitute for highly efficient oxygen reduction reaction (ORR) activity and heteroatom doping on carbon materials is more efficient than nondoped carbon materials for ORR. The metal-free electrocatalysts have advantages of the price and environmental aspects. Furthermore, it has high selectivity and excellent electrochemical stability. Nitrogen (N) as the most common dopant element, as well as other dopants such as boron (B), phosphorus (P), silicon (Si), sulfur (S), halogens, and their mixtures, plays an important role in ORR performance. In detail, heteroatom doping on carbon materials, both single-atom doping and codoping with different heteroatoms induces synergistic effects, which facilitates oxygen (O2) adsorption and has high methanol and CO tolerance. Moreover, it improves the electron-donation capability and weakens the OO bond. Also, it increases the electron delocalization and induces defects in the carbon framework, resulting in active sites toward ORR. In addition, it follows a favorable four-electron reduction. The development of carbon-based metal-free electrocatalysts is a major challenge for the enhancement of ORR activity.