The Gibbs thermodynamics of hydrogen adsorption for C-supported Pt and Pt-Co shell-core nanoparticles was calculated for the first time on practical electrodes in proton exchange membrane fuel cells (PEMFCs). The variations of the cyclic voltammetry with temperature were used to,determine the Gibbs free energy Delta G(ads) (H-UPD), enthalpy Delta H-ads (H-UPD), entropy Delta S-ads (H-UPD) and lateral interaction parameter to of hydrogen underpotential deposition (H-UPD). The bond energy Dpt-H was further calculated, neglecting the slight change of a double layer over the potential range of H adsorption. The results were compared to those obtained for single crystal Pt surfaces. It is established that the hydrogen adsorption and the catalytic activity are driven by entropic effects. The bond enthalpies of Pt-H are generally found to be unaffected by the structure of the Pt catalysts while entropic effects (order-disorder) are important in determining the catalytic activity. It is shown that the exchange current densities on various single crystal surfaces increase with order. For C-supported Pt-Co shell-core nano-catalyst, the presence of Co core beneath the Pt shell increases the order and hence exhibits better catalytic activity than Pt alone. (C) 2014 The Electrochemical Society. All rights reserved.