Abstract
This paper reports original results on the synthesis and characterization of Fe/C/N ORR electrocatalysts obtained by a combination of CO2 laser pyrolysis and thermal post-treatment. The precursor liquid media, consisting in a 14 g·L−1 iron III acetylacetonate solution in toluene, was aerosolized and then exposed to a CO2 laser beam for pyrolysis in continuous flow. Ammonia was used in the pyrolysis process, both as the laser wavelength absorbing gas (i.e., energy transfer agent) and as the sole source of nitrogen. After the laser pyrolysis step, the material was submitted to thermal post-treatment under argon on the one hand, and ammonia on another hand. The three materials—one as-prepared, one thermally treated under argon, and one thermally treated under ammonia—were characterized, in particular, through specific surface area determination, XPS analysis, and ORR measurement. It was found that both kinds of thermal treatment significantly improved the ORR performances, which were evaluated on porous electrodes. Indeed, while the as-prepared material showed an ORR onset potential at ≈790 mV vs. the standard hydrogen electrode (SHE) in HClO4 1M, the argon treatment increased the latter to ≈820 mV, and the ammonia treatment led to a very high value of ≈910 mV. Selectivities of 3.65 and 3.93 were measured for the argon and ammonia treated materials, respectively. The outstanding ORR performance resulting from the ammonia treatment is probably related to the very high BET specific surface area measured at 1130 m2·g−1, which was notably obtained without using any templating or sacrificial component in the precursor media.
Highlights
One of the key points to the widespread development of fuel cell technology is related to the replacement of the platinum-based electrocatalysts widely employed at the cathode of these devices to reduce oxygen
Most of the performing materials recently developed exhibit specific surface areas in the range of 500 to 1100 m2 ·g−1. They are synthesized from precursors containing carbon, nitrogen, and iron, the latter being known for promoting graphitization and the formation of actives sites towards the Oxygen Reduction Reaction (ORR)
The synthesis of the material was conducted in the CO2 laser pyrolysis reactor, schematized in
Summary
One of the key points to the widespread development of fuel cell technology is related to the replacement of the platinum-based electrocatalysts widely employed at the cathode of these devices to reduce oxygen. Most of the performing materials recently developed exhibit specific surface areas in the range of 500 to 1100 m2 ·g−1 They are synthesized from precursors containing carbon, nitrogen, and iron, the latter being known for promoting graphitization and the formation of actives sites towards the Oxygen Reduction Reaction (ORR). A sacrificial metal organic framework with a high specific surface area has been used to obtain highly active materials, involving a multistep procedure, namely: by mixing with an organic nitrogen containing molecule and an iron salt, drying, ball-milling, and two thermal treatments under argon and NH3 . As for the early work mentioned above, it is worth noting that the transition by which the precursors are transformed into a carbon-based material with active sites towards the ORR is still not clearly understood This holds for argon or NH3 treatments, both gases induce some different effects, which will be pointed along the paper. The paper considers, successively, the features of the thermally treated materials, the ORR performances in acidic media evaluated on porous electrodes by cyclic voltammetry in HClO4 1M, and the evaluation of the selectivity of the ORR
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