Pergamon zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Electrochimica Acta. Vol. 42. No. 9, pp. 1379-1388. 1997 Copyright sc~ 1997 Published by Elsevier Science Ltd. Printed in Great Britain. All rights reserved PII: zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG S0013-4686(96)00361-1 0013486197 $17.00 + 0.00 Is nitrogen important in the formulation of Fe-based catalysts for oxygen reduction in solid G. Lalande,’ R. CM,” D. Guay,” J. P. Dodelet,*” L. T. Wengb and P. Bertrandb a INRS-l?nergie et MatCriaux, C.P. 1020, Varennes, Qdbec J3X lS2, Canada, _- -polymer fuel cells? bPCPM, UniversitC Catholique de Louvain, 1, Place Croix du Sud. B-1348 Louvain-la-Neuve, Belgium (Received 11 March 1996; in revised form 27 August 1996) Abstract-The role of nitrogen and iron in the generation of catalysts for oxygen reduction in acidic media has been investigated by using two independent organic precursors. The Fe and N precursors were polyvinylferrocene adsorbed on carbon black and acetonitrile vapor, respectively. These precursors were pyrolyzed at 1000 C. A catalyst is obtained only if Fe and N are present together in the reactor during pyrolysis. Inactive Fe clusters surrounded by a protective graphitic envelope are produced when adsorbed polyvinylferrocene is pyrolyzed alone at 1OOO’C. The latter material may, however, be activated by a second pyrolysis in acetonitrile vapor. The characterization of the catalyst indicates that the iron is oxidized (Fe” and Fe”‘), but no strong Fe-N, bonds were unequivocally detected by ToF SIMS. Lifetime testing of the catalyst in a polymer electrolyte fuel cell demonstrated stable currents for at least 300 h. The current density measured with a catalyst containing 1 wt% Fe was about l/3 of that measured with a commercial Pt-based catalyst containing 2 wt% metal loading. #G 1997 Published by Elsevier Science Ltd. All rights reserved. Kr), lords: OZ. electroreduction, ferrocene, polymer electrolyte fuel cell INTRODUCTION Pyrolyzed NJ--M macrocycles supported on carbon are known to be catalysts for the reduction of oxygen in low temperature fuel cells [l-4]. For low and medium pyrolysis temperatures (up to about 8OOC). there is general agreement on the nature of the catalytic site. Van Veen et al. have proposed that the NJ-M moiety of porphyrins and phthalocyanines is the structural feature associated with the activity of these heat-treated materials [S-7]. This model was adopted by other workers in the field [8-181. Ions containing the metal bound to nitrogen are indeed observed by Time of Flight secondary ion mass spectrometry (ToF SIMS) for Co or Fe phthalocya- nineorporphyrinderivativesadsorbedoncarbonblack and heat-treated up to 700 or 8OO’C, respectively, [ 14. 16. 17, 181. The origin of these ions involves either the Nd-M moiety of the precursor or fragments of the NJ-M chelate containing the metal bound to N. The latter may also display catalytic activity. *Author to whom correspondence should be addressed. The same ToF SIMS studies indicate that no ions containing Co or Fe bound to N persist at temperatures above 900°C. The nature of the catalytic site of these high temperature pyrolysis products is still controversial. Metal particles have been detected after high temperature pyrolysis of adsorbed organometallic precursors [ 1, 2, 6, 13-2 11. It has been reported that part of the metallic clusters are surrounded by a protective graphitic layer that would prevent their corrosion in the acidic medium of polymer electrolyte fuel cells. Catalytic activity for the reduction of oxygen has been attributed to these protected metallic clusters [13-18, 20, 211. It was also proposed that the formation of finely divided metal on the carbon could modify the electrocatalytic properties of the support [2]. A third possibility is that the thermal treatment of the high area carbon supported macrocycles could generate nitrogen containing surface groups that might serve as coordination centers for metal ions. M-N, bonds could then be formed after the dissolution in the electrolyte of the metallic particles or of their oxides [22-251. 1379