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

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

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) O 2 + X- Fe(II) = X- Fe(III)-O-O(+e) (b2) X- Fe(III)-O-O(+e) +3e +4H + = X- Fe(II) + 2H 2 O Dual function of the cathode potential: * Surface Activation (step a) : Generate minimal steady state population of Fe(II) to trigger process (b) * Lowering of  H act (step(s) b2) at the activated surface

9. Rate Expression for ORR Assisted by a Surface Redox Function General: J (E) = Fk 0 A* cat f(E - E 0 surface redox ) C r  exp{-  H*act/RT} exp{-(E- E 0 cell process ) /b} Surface populations of the Red and Ox obey a simple Nernst relationship: J (E) = Fk 0 A* cat (1/Z+1)C r  exp{-  H*act/RT} exp{-(E- E 0 cell process ) /b} where, Z= exp{ ( F/RT) (E - E o surface redox )}

10. log J ORR E 0 Red/ Ox E 0 H 2 O / O 2 E cath 1.23V

11. The Effect of Cathode Potential in ORR at Metal Electrocatalysts A typical rate expression used for metal electrocatalysts: J (E) = Fk 0 A* cat C r  exp{-  H* act /RT} exp{- (E-E 0 cell 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 – A cat is not a function of E : Is this assumption defensible ?

12. there is high chemisorbed O/OH coverage derived from water on Pt in the operating cathode : formed by: Pt ss + H 2 0 = OH- Pt ss + (H + +e) (“water discharge” ) Answer: The accepted expression for J ORR assumes a metal surface fully available, however:

13. Consideration of Metal Site Availability in the expression for ORR: The pre-exponential factor (1) J 0RR dependence on N ss, total and  OX : J ORR (E) = k N total (1-  OX ) P O2 exp{-  H* act /RT}exp{-(E-E 0 O2/H2O ) /b} (2)  OX dependence on E cath :  OH /(1-  OH )= exp{ ( RT/F) (E - E o Pt(H2O)/Pt-OHads )} (1) +(2) :J orr (E)= k N total (1/Z+1) P O2 exp{-  H*act/RT}exp{-(E-E 0 O2/H2O ) /b} where Z= exp{ ( RT/F) (E - E o Pt(H2O)/Pt-OHads )} E-E 0 O2/H2O =1.23V vs. hydrogen ; E o Pt(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 = 2Pt surface + 2H 2 O (b) faradaic reaction of O 2 at/with the reduced site O 2 +2Pt surface + 2e + 2H + = 2Pt-OH surface

15. log J ORR E 0 Pt / PtOx E 0 H 2 O /O 2 E cath 1.23V 0.80V Uribe et al, LANL,1992 (ECS Proc Vol) J ORR = Jo(1-  ox ) exp [(E o -E) /b int ], d(log J ORR )/d( Eo-E) = 1/ b int + [1/(1-  ox )] (d  ox /dE ) Surface Redox Mediation (a)active site generation 2Pt-OH surface + 2H + + 2e = 2Pt surface + 2H 2 O (b) faradaic reaction of O 2 at/with the reduced site O 2 +2Pt surface + 2e + 2H + = 2Pt-OH surface

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 O 2 +M ss 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-OH ads standard 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-OH ads,1/3 ML ; E 0 = 0. 8V, ( stable in air pH 0-14 )

19. Co 3 O 4 /Co(OH) 2 ; E 0 = 1.0V vs. RHE ; stable in air pH>8 Example of catalyst of choice : “CoN x /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 E 0 cell process - E 0 surface redox (  E 0 ) is a guideline for maximizing the electrocatalytic activity, because An optimized  E 0 addresses 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  E 0 of 0.3V-0.4V

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