Layers of 150 nm wide and 0.5-1.5 mu m long carbon nanorods were grown by glancing angle deposition on Si substrates, sputter-coated with 0.10 mg/cm(2) Pt, and transferred to polymer electrolyte membranes for testing as cathode electrodes in fuel cells. The rods were etched within fully assembled cells by applying a potential above the reversible H-2/O-2 voltage, which leads to polarization curves that show a 4-7 times higher current at 0.40 V. The current increase is attributed to the opening of pores within the electrode, which facilitates easy oxygen transport and leads to a reduction in mass transport resistance by a factor of 360, as determined by electrochemical impedance spectroscopy. Etching sequences with increasing voltage V-E indicate that V-E <= 1.6 V yields water electrolysis and Pt oxidation that facilitates Pt agglomeration and migration of Pt ions into the electrolyte, while V-E = 1.7 V results in removal of C and the formation of pores within rods that facilitate oxygen transport to reaction sites, yielding a 400-700% increase in fuel cell output current at low potential. These results suggest that the controlled etching of temporary scaffolds to create pores in an operating fuel cell may be an effective approach to reduce mass transport limitations. (C) 2009 The Electrochemical Society. [DOI:10.1149/1.3244589] All rights reserved.