▎ 摘　　要

Ultrafine catalysts are desirable for the reduction of fuel cell costs but are intrinsically unstable. Here the authors report graphene-nanopocket-encaged PtCo catalysts with exceptional durability under the demanding ultralow-Pt-loading condition while delivering a satisfactory fuel cell performance. The proton exchange membrane fuel cell (PEMFC) as an attractive clean power source can promise a carbon-neutral future, but the widespread adoption of PEMFCs requires a substantial reduction in the usage of the costly platinum group metal (PGM) catalysts. Ultrafine nanocatalysts are essential to provide sufficient catalytic sites at a reduced PGM loading, but are fundamentally less stable and prone to substantial size growth in long-term operations. Here we report the design of a graphene-nanopocket-encaged platinum cobalt (PtCo@Gnp) nanocatalyst with good electrochemical accessibility and exceptional durability under a demanding ultralow PGM loading (0.070 mg(PGM) cm(-2)) due to the non-contacting enclosure of graphene nanopockets. The PtCo@Gnp delivers a state-of-the-art mass activity of 1.21 A mg(PGM)(-1), a rated power of 13.2 W mg(PGM)(-1) and a mass activity retention of 73% after an accelerated durability test. With the greatly improved rated power and durability, we project a 6.8 g(PGM) loading for a 90 kW PEMFC vehicle, which approaches that used in a typical catalytic converter.