Résumé
Producing renewable energy is one of the most important electrochemical challenges in the 21
st
century. Currently the focus is mainly on enhancing the oxygen reduction reaction (ORR) on the cathode of the fuel cell and to utilize non-noble metal catalysts instead of the expensive and scarce Pt-based catalysts that are currently prevalent. Iron and nitrogen doped nanocarbons have emerged as a possible replacement. Here, we study a catalyst material synthesized from carbide-derived carbon (CDC, a carbon material made by removing the metal atoms from a carbide) and composites of CDC and carbon nanotubes (CNT). To dope this material, 1,10-phenanthroline and iron(II)acetate were used in a ball-milling and pyrolysis procedure.
1,2
The composites were created by adding CNTs, dicyandiamide and additional iron(II)acetate before repeating the ball-milling and pyrolysis process. The surface morphology was examined by scanning electron microscopy (SEM), and elemental composition by X-ray photoelectron spectroscopy (XPS), porosity analysis was done by N
2
adsorption/desorption and the bulk elemental composition was studied by inductively coupled plasma mass spectroscopy (ICP-MS). The speciation of iron was studied using
57
Fe Mössbauer spectroscopy.
These CDC-based Fe-N-C catalysts show promising half-wave potential for ORR of 0.81 V vs RHE in 0.5 M H
2
SO
4
. For a same Fe content of 0.5 wt% in the catalyst precursor, the Fe-N-C catalyst derived from CDC-2 (derived from TiC) showed ca. 5 times higher activity at 0.8 V vs. RHE than that derived from CDC-1 (derived from B
4
C). The residual presence of boron in CDC-1 seems to be the main reason for the lower activity of CDC-1 derived catalysts, leading to the preferential formation of iron boride instead of ORR-active FeN
x
C
y
moieties. Higher Fe contents were investigated for CDC-2, but lead to unmodified activity, which is explained from Mössbauer spectroscopy measurements by the increasing formation of ORR-inactive Fe species at high Fe content. The addition of CNTs to create a core-shell composite with a CNT shell surrounding the CDC cores increased the stability of the catalyst in acidic media.
Figure 1