The potentiostatic deposition mode was used to prepare a catalyst layer of Pd on a gas diffusion substrate (carbon paper) for a polymer electrolyte membrane fuel cell (PEMFC). FE-SEM and TEM analysis revealed that innovative structures are formed on shifting the overpotential deposition from 250 mV to 500 mV: a flower like microstructure with a cubic core evolves to a cubic agglomeration of tiny spherical particles of Pd. At an intermediate overpotential, 400 mV, the deposit exhibits a cubic agglomeration with triangles coming out from their edges. The catalytic activity of these different nanostructures was evaluated towards the oxygen reduction reaction in 0.1 M H2SO4 and 0.1 M HClO4 solutions. The most active Pd nanostructure, the one obtained at 400 mV overpotential, was tested as a cathode on a H-2 vertical bar O-2 feed PEMFC to demonstrate its feasibility in a real fuel cell environment. (C) 2013 Elsevier Ltd. All rights reserved.