Design Strategies for Carbon-based Electrocatalysts and Application to Oxygen Reduction in Fuel Cells

Fuel cells have attracted much attention because of their high specific energy and low environmental load, but their commercial application is limited by the poor performance and high cost of the relevant electrode catalysts.The oxygen reduction reaction (ORR) is the key cathodic reaction in a fuel cell, and it plays an important role in the chemical energy conversion.However, the slow reaction kinetics, large reaction energy barrier, and low selectivity deteriorate the energy efficiency of fuel cells.Thus, rational design of low-cost electrocatalysts that show good activity is highly desirable for improving the performance of fuel cells.Although noble-metal-based electrocatalysts (e.g., Pt/C) show excellent catalytic activity for the ORR, their limited resources, high price, and low stability caused by the migration and agglomeration of nanoparticles on the surface of carbon supports have hindered their extensive application.Because of their excellent electrical conductivity and stability, carbon-based materials are widely used as substrates for electrode materials in the ORR.Heteroatom (e.g., nitrogen, phosphorus, sulfur)-doped carbon materials can influence the adsorption state of oxygen molecules and intermediates by changing the charge distribution of adjacent carbon atoms because of the difference in electronegativity and atomic radius between the heteroatoms and carbon atoms, thus promoting the ORR activity.Optimization of the structure and surface properties of carbon-based electrocatalysts has helped accelerate the four-electron reaction and reduce the overpotential in the ORR.Therefore, non-noble metal and heteroatom-doped carbon-based catalysts exhibit improved ORR activity.The dispersion of non-noble metals on carbon materials via the interaction of metal atoms with the neighboring nitrogen atoms or other heteroatoms produces high-density active sites in the carbon support, thus leading to high atomic utilization and significantly improving the electrocatalytic activity owing to the synergistic effect.This review focuses on the applications of carbon-based electrocatalysts in fuel cells, summarizing the design strategies and electrocatalytic activities of heteroatom-doped carbon-based catalysts with non-noble metals toward improving their ORR activity.Furthermore, the latest research progress in the field of carbon-based catalysts used as cathode catalysts in proton exchange membrane fuel cells (PEMFCs) and anion-exchange membrane fuel cells (AEMFCs) is integrated, and the direction of future development is addressed.