Nanocarbon-Based Nonprecious-Metal Electrocatalysts for Oxygen Reduction in Various Electrolytes

Oxygen reduction reaction (ORR) is one of the most important electrochemical reactions associated with energy conversion and storage technologies, such as fuel cells, metal-air batteries, and electrolyzers. However, the high ORR overpotentials resulting from very slow kinetics require catalysts containing large amount of precious metals, such as Pt, to enhance the reaction activity and durability. Unfortunately, the high cost and limited supply of Pt have become a grand challenge for widespread application of these clean energy technologies. The development of nonprecious-metal catalysts (NPMCs) for ORRs has become a hot topic for electrochemical energy technologies. In the meantime, exploring the advanced catalyst design and synthesis from Earth-abundant elements will be scientifically important areas and able to provide fundamental knowledge and understanding to materials chemistry. Among the catalyst formulations studied so far, carbon-based catalysts have received considerable attention owing to the balance between cost, activity, and durability. However, the mechanisms of ORR in various media (e.g., acidic, alkaline, and nonaqueous) are different and not fully understood, making it challenging to develop universal ORR electrocatalysts. In this chapter, we will account the recent progress of nanocarbon-based NPMCs for the ORR in three different electrolytes, potentially for Nafion®-based polymer electrolyte fuel cells, alkaline fuel cells and metal-air batteries, and nonaqueous Li-air batteries, respectively. Various carbon materials, including metal-free nitrogen-doped nanocarbons, transition metal-derived M-N-C catalysts, and nanocarbon/transition metal compound hybrids, are discussed in terms of their synthesis, structure/morphology, and catalytic activity and durability. The synthesis-structure-activity correlations of these NPMCs are established, aiming to provide guidance for rational design of advanced catalysts. In addition, perspectives of nanocarbon-based catalysts for energy storage and conversion are also outlined, and the possible pathways to address current challenges of these systems are discussed.