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Fuel Cells with no Precious Metals and no Liquid Electrolyte . Shimshon Gottesfeld Cellera Technologies, Ltd Caesaria Industrial Park, Israel TAU, Feb 5, 2010. Outline. Brief report on AMFC development at Cellera
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Fuel Cells with no Precious Metals and no Liquid Electrolyte Shimshon Gottesfeld Cellera Technologies, Ltd Caesaria Industrial Park, Israel TAU, Feb 5, 2010
Outline • Brief report on AMFC development at Cellera • Is there a common “activity yardstick” which applies to all fuel cell elecrtocatalysts ?
Outline • Brief report on AMFC development at Cellera • Is there a common “activity yardstick” which applies to all fuel cell elecrtocatalysts ?
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
Alkaline Membrane FC Materials and Componenets Simplified thermal management Non-acidic membrane Non-platinum catalysts Light metal hardware 5
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 ?
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 )
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
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)}
log JORR 1.23V Ecath E0H2O/ O2 E0Red/Ox
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 ?
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” )
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
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
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
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 ”
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
Co3O4/Co(OH)2 ; E0 = 1.0V vs. RHE ; stable in air pH>8 Example of catalyst of choice : “CoNx/C”
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
Acknowledgements of Support*Israel Cleantech Ventures*Office of the Chief Scientist Ministry of Commerce & Industry