Step-by-step synthesis of non-noble metal electrocatalysts for O-2 reduction under proton exchange m
Fe-based catalysts for O2 reduction under proton exchange membrane fuel cell conditions were prepared on a commercial N234 carbon black support using both a “classical” and a step-by-step procedure to determine if parameters other than microporosity and nitrogen loading of the carbon support are important in the synthesis of Fe/N/C electrocatalysts. The “classical” procedure for obtaining Fe/N/C electrocatalysts is to use a single-step synthesis, in which a carbon support loaded with a metal precursor is heat treated at high temperatures (900−950 °C) in pure NH3. In the step-by-step procedure, microporosity is first etched into the carbon support followed, if necessary, by the addition of N-bearing functionalities and, last, the loading of the metal precursor. Similar maximum microporous contents can be etched into N234, using either NH3 or O2 (air). However, unlike O2 (air), etching with NH3 has the added benefit of creating N-bearing functionalities on the carbon surface. For carbon supports etched in O2 (air), it is possible to add N-bearing functionalities either by N2 plasma treatment or by a subsequent, short pyrolysis in NH3. In the case of the multistep procedure, a second heat treatment is essential for activating the catalytic sites. This demonstrates the importance of a third factor controlling the activity of the catalysts. The duration and temperature of the activation step depend on the ambient gas used. This activation step, during which a C−Nx−Fe complex is transformed into a catalytic site, is unapparent in the “classical” procedure. Both “classical” and step-by-step syntheses yield the same maximum catalytic activity when measured using either the rotating disk electrode method or by fuel cell testing. The similarity in microporous specific area of catalysts made using these two synthesis methods explains this finding.