Effect of temperature and presence of minor amount of metal on porous carbon materials derived from ZIF8 pyrolysis for electrocatalysis

ZIF8 without and with adsorption of minor amounts of Co precursor have been pyrolized at different temperatures studying systematically the effect of carbonization temperature on the texture, composition and ORR performance both in acidic and basic media. The increase of pyrolysis temperature diminished the N content but increased the graphitic character. The trade-off between these two parameters in metal-free electrocatalysts resulted in an improvement of ORR performance, which is met for the highest pyrolysis temperature of 1000 oC. Moreover, the adsorption of minor amount of Co precursor enhanced further the ORR performance, increased the graphitic character and the porosity of the resulting carbon material. Morphologically, these carbon materials kept the polyhedral shape of ZIF8 and were covered by graphitic nanostructures devoid of metal nanoparticles. Co metal catalysed the growth of these graphitic nanostructures and concomitantly is stabilized as monoatomic Co-Nx-C species preventing the agglomeration as metal nanoparticles. Therefore, the need of an acid washing step is avoided, which is usually necessary in the carbonization of other metal containing MOFs.