Electrocatalysis in Alkaline Membrane Fuel Cells

In a way of introduction relevant to this special symposium,  I like to mention first the review on Electrocatalysis in Alkaline Media by Spendelow and Wieckowski ,  published right at the onset of enhanced R&D activity in the new area of alkaline membrane fuel cells ( AMFCs). This coincidence indicated that from Andrzey’s had projected several years earlier, that  alkaline fuel cells may stage a “comeback”. This projection was, in fact,  correct , with a new form of cells based on an  alkaline membrane electrolytes ( AMFCs)  entering the stage  in the mid 2000’s as alternatives for liquid alkaline electrolyte based  fuel cells (AFCs). 

This coincidence between the onset of enhanced AMFC activity and the publication of the review [1] , made it an important source of knowledge and insight for the parties involved in AMFC R&D, certainly so for the AMFC technology team at Cellera Technologies that started it’s activity early in 2008.  One interesting use we made early on of some information discussed in [1] , was the strong dependence of the ORR rate at silver catalysts on pH. This d¹⁰metal has low affinity to oxygen and indeed exhibits low ORR activity at non-alkaline pH values. However, the ORR activity increases substantially, so as  to match that of Pt at pH 13 and above.  This rate dependence on the electrolyte pH was used by us as a highly localized , in situ pH probe,  for probing the value of the pH at  the catalyst surface in contact with a hydrated membrane carrying (TAA+)(OH-) functional groups (no aqueous base added ).  The value of the pH in an ionomer phase of given population of such functional groups is  determined by the dissociation constant ( base strength ) of the  (TAA+)(OH-) base which is not documented for polymer bound base groups.  Rather than getting involved with direct sensing of the pH within the polymer phase by means of a pH sensor, we opted to probe it by measuring the ORR activity at a silver catalyst in contact with the recast ionomer.  The results , to be presented, suggested an effective pH at the catalyst ionomer interface near 13.     

The case of the silver ORR catalyst is quite  interesting well beyond the probing of interfacial pH(…) and this  ties with an attempt I made over the last few years  to come up with  a general expression for the ORR process , that  recognizes the generality of active surface redox mediation in this electrochemical surface process. This insight was offered before in a Foreword for Andrzej’s  2010 book on Fuel Cell Science [2] , that  he invited me to write.  

This  general rule for active fuel cell electrocatalysts was proposed in the hope that it can provide  common ground for the evaluation  and development of new electrocatalysts.  The  rule is based on the recognition that a wide variety of electrocatalytic processes  ,taking place at either  redox-functionalized or metal  surfaces,  are “surface redox mediated”, leading, in turn, to the pursuit of an  optimum value for the potential gap:    E⁰ cell  process - E⁰  surface redox,

as  a  guideline for maximizing  electrocatalytic activity.

  An optimized gap between these  two standard potentials   will best address  the conflicting demands  of  a minimum overpotential  for surface activation  by population of  the active form of the surface redox mediator ( e.g., generating more metal sites from the Pt/PtOH surface redox system )  and  a high rate of the reaction between the reactant molecule and the active surface site ( i.e., high rate of the reaction between molecular oxygen and a Pt metal site formed at cathode potential E ).  The  plot of the rate of the  electrocatalytic  process  vs E⁰ cell  process - E⁰ surface redox, will take the famous form of a  “volcano” , however with this typical  shape now explained in terms of a redox mediation mechanism and the  need to optimize the  difference between E⁰ cell  process  and  E⁰ surface redoxto achieve the highest  rate at low overpotentials .

Putting it in the form of an equation , the general expression proposed for the ORR process, as example, has the form:

J (E) = k⁰ A^*_cat f(E - E⁰ surface redox  ) Creact.^g  x

x  exp{-DH*_act/RT}  exp{-(E- E⁰cell  process) /b}

 

where , in the simplest case ,

f(E - E⁰ surface redox  ) =  1/(Z+1)  , where ,

 

 Z= exp{ ( F/RT ) (E - E⁰ surface redox  )}

 

The main unique element in this proposed general expression for the rate of an electrocatalytic process, is recognition of the high significance of  the pre-exponential factor in determining the variation of the current density with electrode potential and the quantification of such dependence based on  the mechanism involving surface redox mediation.

The above treatment will be used in this talk for clarification of  electrocatalysis challenges in AMFCs.

 References:    

[1].Jacob S. Specdelow and Andrzej Wieckowski, PCCP, May 2007 [2]. Fuel Cell Science: Theory, Fundamentals, and Biocatalysis, A. Wieckowski and J. Norskov , Eds., John Wiley, 2010