Porous Electrode Theory Historical Perspective

The importance of structural effects in gas diffusion electrodes was realized long before the development of the current generation of CLs for PEFCs. The basic theory of gas diffusion electrodes, including the interplay of reactant transport through porous networks and electrochemical processes at highly dispersed electrode I electrolyte interfaces, dates back to the 1940s and 50s [13, 14]. Later work realized the importance of surface area and utilization of electrocatalysts in porous electrodes [15]. A series of seminal contributions by R. De Levie opened 

Major contributions to the development of the macrokinetic or macrohomogeneous theory of porous electrodes were made by Yu, A, Chizmadzhev and Yu, G, Chirkov [25-27], In these works the importance of the interplay of oxygen diffusion and interfacial kinetics had already been realized. For oxygen reduction electrodes, a large electrocatalytically active surface area per unit volume,, has to compensate for the smallness of the intrinsic activity per unit 

Similar agglomerate approaches were adopted by Iczkowski and Cutlip [30] and by Bjbmbom [31], Those works already identified the doubling of the apparent Tafel slope as a universal signature of the interplay of mass transport limitations and interfacial electrochemical kinetics. Flooded agglomerate models have been employed since then to analyze sources of irreversible voltage losses, optimum electrode thickness, and effectiveness of catalyst utilization. Moreover, it was 

Since then, other groups have adopted similar models. Assets of this approach are versatile (i) it relates global performance of CCLs to immeasurable local distributions of reactants, electrode potential and reaction rates (ii) it defines a penetration depth of the active zone and (iii) it suggests an optimum range of current density and catalyst layer thickness with minimal performance losses and highest utilization of the catalyst. 

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