Fuel Cells with no Precious Metals and no Liquid Electrolyte

1. Fuel Cells with no Precious Metals and no Liquid Electrolyte Shimshon Gottesfeld Cellera Technologies, Ltd Caesaria Industrial Park, Israel TAU, Feb 5, 2010

2. Outline • Brief report on AMFC development at Cellera • Is there a common “activity yardstick” which applies to all fuel cell elecrtocatalysts ?

3. Outline • Brief report on AMFC development at Cellera • Is there a common “activity yardstick” which applies to all fuel cell elecrtocatalysts ?

4. PEM FC Cost Barriers 2009 PEM Power System list price - $2,000 / kW Perfluorinated acidic membrane Platinum based electrodes Graphite or high grade stainless steel hardware materials -based barriers – 90% of stack cost Cost volatility - Platinum $500/Oz - $2,500/Oz 4

5. Alkaline Membrane FC Materials and Componenets Simplified thermal management Non-acidic membrane Non-platinum catalysts Light metal hardware 5

6. Outline • Brief report on AMFC development at Cellera • Is there a common “activity yardstick” which applies to all fuel cell elecrtocatalysts ? metal catalysts & non-metal catalysts , in acid media & alkaline media , is there an activity-determining factor common to all ?

7. Turnover Frequencies: Pt nanoparticle 25, PtM nanoparticle 60, structured nano-film & dealloyed PtM nano-particle 160, bulk Pt 250 , larger PtM nano-particles - 2500 ( H. Gasteiger and N. Markovic ,Science,2009 )

8. Effect of Cathode Potential in the case of ORR assisted by Surface Redox Centers (a) X- Fe(III) +e = X-Fe(II) (b1) O2 + X- Fe(II) = X- Fe(III)-O-O(+e) (b2) X- Fe(III)-O-O(+e) +3e +4H+ = X- Fe(II) + 2H2O Dual function of the cathode potential: * Surface Activation (step a) : Generate minimal steady state population of Fe(II) to trigger process (b) *Lowering of DHact(step(s) b2)at the activated surface

9. Rate Expression for ORR Assisted by a Surface Redox Function General: J(E) = Fk0A*cat f(E -E0 surface redox) Crgexp{-DH*act/RT} exp{-(E- E0 cell process)/b} Surface populations of the Red and Ox obey a simple Nernst relationship: J(E) = Fk0A*cat (1/Z+1)Crgexp{-DH*act/RT} exp{-(E- E0 cell process)/b} where, Z= exp{ ( F/RT) (E - Eosurfaceredox)}

10. log JORR 1.23V Ecath E0H2O/ O2 E0Red/Ox

11. The Effect of Cathode Potential in ORR at Metal Electrocatalysts A typical rate expression used for metal electrocatalysts: • J(E) = Fk0A*catCrgexp{-DH*act/RT} exp{- (E-E0cell process)/b} • Two assumptions are involved: • The effect of a change in E is fully accounted for by the exponential term , i.e., by the effect it has on the activation energy of the process • Acat is not a function of E : Is this assumption defensible ?

12. Answer: The accepted expression for JORR assumes a metal surface fully available, however: there is high chemisorbed O/OH coverage derived from water on Pt in the operating cathode : formed by: Ptss + H20 = OH- Ptss + (H+ +e) (“water discharge” )

13. Consideration of Metal Site Availability in the expression for ORR: The pre-exponential factor (1) J0RR dependence on Nss, total and qOX : • JORR(E) = k Ntotal (1- qOX ) PO2exp{-DH*act/RT}exp{-(E-E0O2/H2O)/b} (2) qOX dependence on Ecath : • qOH /(1-qOH)= exp{ ( RT/F) (E - EoPt(H2O)/Pt-OHads)} (1) +(2) :Jorr(E)= kNtotal (1/Z+1) PO2exp{-DH*act/RT}exp{-(E-E0O2/H2O) /b} where Z= exp{ ( RT/F) (E - EoPt(H2O)/Pt-OHads)} E-E0O2/H2O =1.23V vs. hydrogen ; EoPt(H2O)/Pt-OHads = 0.80V vs. hydrogen “Fuel Cell Catalysis: a Surface Science Approach” Ed. M.T.M Koper ( Leiden University) , Chapter 1 , John Wiley ,2009

14. oxygen reduction “redox mediated” by the Pt/Pt-OH redox system (a) active site generation • 2Pt-OH surface + 2H+ + 2e = 2Ptsurface+ 2H2O (b) faradaic reaction of O2 at/with the reduced site • O2+2Pt surface+ 2e + 2H+ = 2Pt-OHsurface

15. Uribe et al, LANL,1992 (ECS Proc Vol) JORR = Jo(1-qox) exp [(Eo-E) /bint], d(log JORR)/d( Eo-E) = 1/bint + [1/(1- qox)] (dqox /dE ) log JORR • Surface Redox • Mediation • active site generation • 2Pt-OH surface + 2H+ + 2e • = 2Ptsurface+ 2H2O (b) faradaic reaction of O2 at/with the reduced site • O2+2Pt surface+ 2e + 2H+ • = 2Pt-OHsurface 0.80V 1.23V Ecath E0H2O/O2 E0Pt/ PtOx

16. ORR “activity” vs. formation energy of M-O from water Reinterpretation Ascending branch -- Pre-exponential Factor Effect : enhancing 1/Z+1 Descending branch-- Exponential Factor Effect : falling rate of faradaic Process O2 +Mss Volcano Peak is where 1/Z+1 has “maxed out ”

17. Good general principle for searching an active ORR catalyst “A Pourbaix guide for electrocatalysis galaxy travel”:match the target electrode potential in the operating fuel cell, with : the M/M-OHadsstandard potential of the metal , or metal alloy catalyst, or with: the M+n/+(n+1)standard potential of the active surface redox couple

18. Pt/Pt-OHads,1/3 ML ; E0 = 0. 8V , ( stable in air pH 0-14 )

19. Co3O4/Co(OH)2 ; E0 = 1.0V vs. RHE ; stable in air pH>8 Example of catalyst of choice : “CoNx/C”

20. Ni – an anode catalyst of choice in AFCs

21. A Yardstick for Electrocatalytic Activity • a wide variety of electrocatalytic processes ,taking place at either redox-functionalized or metal surfaces, are “surface redox mediated” • optimum value for E0cell process - E0surface redox (DE0 ) is a guideline for maximizing the electrocatalytic activity , because • An optimized DE0addresses conflicting demands of (1) minimum overpotential for surface activation and (2) high rate of the faradaic process at the activated surface • Active ORR electrocatalysts are all associated with DE0 of 0.3V-0.4V

22. Acknowledgements of Support*Israel Cleantech Ventures*Office of the Chief Scientist Ministry of Commerce & Industry