The improvement of catalysts for the four-electron oxygen-reduction reaction (ORR; O(2) + 4H(+) + 4e- -> 2H(2)O) remains a critical challenge for fuel cells and other electrochemical-energy technologies. Recent attention in this area has centred on the development of metal alloys with nanostructured compositional gradients (for example, core-shell structure) that exhibit higher activity than supported Pt nanoparticles (Pt-C; refs 1-7). For instance, with a Pt outer surface and Ni-rich second atomic layer, Pt(3)Ni(111) is one of the most active surfaces for the ORR (ref. 8), owing to a shift in the d-band centre of the surface Pt atoms that results in a weakened interaction between Pt and intermediate oxide species, freeing more active sites for O(2) adsorption(2,9). However, enhancements due solely to alloy structure and composition may not be sufficient to reduce the mass activity enough to satisfy the requirements for fuel-cell commercialization(10), especially as the high activity of particular crystal surface facets may not easily translate to polyfaceted particles. Here we show that a tailored geometric and chemical materials architecture can further improve ORR catalysis by demonstrating that a composite nanoporous Ni-Pt alloy impregnated with a hydrophobic, high-oxygen-solubility and protic ionic liquid has extremely high mass activity. The results are consistent with an engineered chemical bias within a catalytically active nanoporous framework that pushes the ORR towards completion.