Proton migration through hydrated Nation membranes in polymer electrolyte membrane (PEM) fuel cells occurs both in the aqueous phase of the membrane and on the sulfonate groups on the surface of the membrane pores. Here we show using D-2 and H-2 fuel and basic quantum mechanical equations that this surface proton migration is largely due to proton tunneling between adjacent sulfonate groups, leading to an exponential variation of Nation conductivity with cell potential. This amphibious mode of proton migration, particle-like in the aqueous phase and wave-like in the narrow pores, is shown to be the major cause of cell overpotential, bistability and oscillations of state-of-the-art PEM fuel cells operating on H-2, reformate or methanol fuel. We also show that this phenomenon can be exploited via introduction of a third auxiliary electrode to independently control the anode-cathode potential difference and dramatically enhance fuel cell power output even in absence of noble metals at the anode. (C) 2004 Elsevier B.V. All rights reserved.