Activation energy PEMFC

The activation polarization region is observed at low current densities. In this region, the voltage losses are associated with the activation energy barrier that must be overcome in the cathode and anode electrodes. The current density dependency on parameters related to overcoming the activation energy barrier is expressed in the Butler-Volmer equation [31]. Thus, activation losses are function of electrode properties, temperature, and pressure conditions. The performance of an HT-PEMFC is represented by (17.1). 

By alloying Pt with transition metals M (M = V, Cr, Co, Ni, Fe, Ti, etc.), the ORR activity can be enhanced remarkably in both phosphoric acid fuel cell (PAFC) and PEMFC [20-22]. The activity enhancement mechanisms have been an open question for more than three decades and ascribed to decreased Pt-ft bond distance [23], enhanced surface roughness [24], increased Pt d-band vacancy [25-27], weakened OH adsorption [28, 29], and downshifted d-band center [30-35]. Nprskov and Mavrikakis et al. combined the stmctural and electronic effects by introducing a d-band model that correlates changes in the energy center of the valence d-band density of states at the surface sites with their ability to form chemisorption bonds. 

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