Arrays of 1 m long C nanorods were grown by glancing angle deposition on flat and patterned Si wafers, coated with 0.1 mg/cm 2 Pt catalyst by magnetron sputtering, removed from the substrates, and tested as cathode electrodes in proton exchange membrane ͑PEM͒ fuel cells. Deposition on flat substrates yields a nearly fully dense nucleation layer with Ͻ5 nm wide pores, followed by the formation of separated rods with an average width that strongly increases with rod height, from Ͻ30 to 190 nm. In

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... contrast, deposition on a patterned surface results in regularly spaced 50 nm wide pores and a rod width that only moderately increases with height, from 95 to 155 nm. Polarization curves on pure H 2 and O 2 for the two sample types are identical at high potential E Ͼ 0.55 V. However, the cathodes deposited on the patterned substrates yield considerably higher currents at low potential, with a 2 times higher limiting current density i L = 0.73 A/cm 2 than those grown on flat substrates. The higher current in the mass-transport-limited regime is attributed to the 10 times wider engineered pores that facilitate O 2 transport to the active catalyst sites, resulting in a 5 times lower mass transport resistance R MT = 1.5 ⍀ cm 2 at E = 0.50 V, as quantified by electrochemical impedance spectroscopy.