Iron catalysts prepared by high-temperature pyrolysis of tetraphenylporphyrins adsorbed on carbon black for oxygen reduction in polymer electrolyte fuel cells

Different quantities of iron tetraphenylporphyrin (FeTPP) have been adsorbed onto carbon black (XC) and pyrolyzed at 1000 degrees C to produce catalysts containing iron loadings of 2, 4 and 6 wt%. The relative catalytic activities for oxygen reduction in polymer electrolyte Fuel cells and in rotating disk electrode cells was: 4 > 2 > 6 wt% Fe. All these catalysts demonstrated stable behavior in a fuel cell element at 0.5 V vs RHE and at 50 degrees C until the 10th h of operation when slow decaying of the catalytic activity began. Reloading of the catalysts containing 2 and 4 wt% Fe with further quantities of FeTPP followed by pyrolysis at 1000 degrees C to increase the iron content to 4 and 8 wt% Fe, respectively, produced catalysts having smaller catalytic activities than the starting products. Efforts to remove iron-based material [alpha-Fe, Fe(C) and various carbides] from the catalysts by acid digestion (HCl, pH = 0.5, T = 20 degrees C, t = 6 weeks) only succeeded in removing a small part (< 25%) of the bulk iron content from the catalyst. Most of the iron remained encapsulated in an acid resistant graphite-like protective coating. The catalytic activities of the acid washed catalysts are superior to those of the starting products, but showed the same decaying catalytic activity after spending 10 h in a fuel cell environment as did the nonacid washed catalysts. The role of the acid digestion as an important step in removing material which would otherwise block the access of the oxygen molecules to the active site of the FeTPP/XC materials is hypothesized. The material obtained following adsorption of hydrogen tetraphenylporphyrin onto the carbon black support and pyrolysis at 1000 degrees C showed negligible catalytic activity. Exposure to an aqueous solution of FeSO4 caused the adsorption of iron ions onto the nitrogen containing surface of the product, but did not improve the catalytic activity towards oxygen reduction. Improved catalytic activity is only observed after pyrolizing the C-N-X-Fe material at 1000 degrees C. The occurrence of catalytic activity requires, therefore, a carbon black support, a source of iron and nitrogen as well as thermal treatment at elevated temperature. (C) 1997 Elsevier Science Ltd