Shui 1..7

D ow nladed ro The availability of low-cost, efficient, and durable catalysts for oxygen reduction reaction (ORR) is a prerequisite for commercialization of the fuel cell technology. Along with intensive research efforts of more than half a century in developing nonprecious metal catalysts (NPMCs) to replace the expensive and scarce platinum-based catalysts, a new class of carbon-based, low-cost, metal-free ORR catalysts was demonstrated to show superior ORR performance to commercial platinum catalysts, particularly in alkaline electrolytes. However, their large-scale practical application inmore popular acidic polymer electrolytemembrane (PEM) fuel cells remained elusive because they are often found to be less effective in acidic electrolytes, and no attempt has been made for a single PEM cell test. We demonstrated that rationally designed, metal-free, nitrogen-doped carbon nanotubes and their graphene composites exhibited significantly better long-termoperational stabilities and comparable gravimetric power densitieswith respect to thebest NPMC in acidic PEM cells. This work represents a major breakthrough in removing the bottlenecks to translate lowcost, metal-free, carbon-based ORR catalysts to commercial reality, and opens avenues for clean energy generation from affordable and durable fuel cells.