(Invited) Fuel Cell Electrocatalysis in Acid and Alkaline Media: The Role of the Electrode Potential in Active Site Generation

This talk will open with presentation of a general aspect of electrocatalysis which has not been fully recognized, particularly so when it comes to theoretical derivations of the dependence of the rate of electrocatalystic processes on the electrode potential. Description of this prevalent and important pattern of potential-generated active sites in electrocatalysis,  will be followed by specific examples of ORR and HOR electrocatalysts in acid and in alkaline fuel cells , including the case of bifunctional HOR catalysts in alkaline membrane fuel cells.

 A wide variety of electrocatalytic processes taking place at either functionalized surfaces or “metal”  surfaces,  are all , in fact,  surface redox mediated . This means  that the electrode potential defines the steady state population of active sites and suggests that the role of the potential  in active site generation must  be incorporated in the pre-exponential factor of the overall rate expression.  In fact, the   potential driven active site generation serves to “ignite” the faradaic process.   This explains why the E⁰  _{redoc  system}  of M/Mox , or  MOX⁺ⁿ/MOX^+n+m  surface couples , must be as close as possible to  E⁰ _{cell process} , to enable high conversion efficiency as well as higher rates at low overpotentials.  

In acid systems, strong proximity of E⁰  _{redoc  system}  to E⁰ _{cell process} , is difficult to implement for the ORR , because the reduced form of the relevant redox mediator must also serve as stabilizer of the intermediate, OOH_ads, requiring significant oxygen affinity of the reduced form of the redox system ( e.g., Pt metal)    The coinage metals are active ORR catalysts at high pH apparently because a different early step in the faradaic process results in full highlighting of the benefit of active  site generation at the lowest possible  overpotential.

Bifunctional metal catalysts have been recognized as key to high HOR activity in alkaline membrane fuel cells. The last part of this talk will analyze this observation in terms of the wide principle of potential generated active sites.