Highly Active, High Specific Surface Area Fe/C/N ORR Electrocatalyst from Liquid Precursors by Combination of CO2 Laser Pyrolysis and Single NH3 Thermal Post-Treatment

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.

Supercritical Deposition Coupled with Ammonia Treatment: A New Route to Co‐Promoted N‐Doped Carbon Aerogels with High Oxygen Reduction Reaction Activity

“…Indeed, the conductivity of the CAs increases with increasing pyrolysis temperature in the range of 700–1000 °C when the pyrolysis medium is an inert gas . On the other hand, although we do not know whether NH 3 is completely dissociated or not, it is well known that ammonia corrodes the carbon at high temperatures, producing gaseous species, such as HCN and H 2 , which would result in lower conductivity in the material. However, NH 3 activation is a complicated process whose mechanism requires a variety of characterization techniques to elucidate .…”

“…The C1s XP spectra of the samples are also consistent with the Raman spectroscopy in terms of the degree of graphitization by showing a decrease in the ratio of sp 2 bonded carbon with increasing temperature. This is because NH 3 corrodes the carbon producing gaseous species such as HCN and H 2 at high temperatures, which results in a more disordered carbon structure . Thus, the Raman and C1s XP spectra imply that the NH 3 corrodes the carbon structure more at higher temperatures.…”

Improving the Activity of Fe/C/N ORR Electrocatalyst Using Double Ammonia Promoted CO2 Laser Pyrolysis

“…This method gave as-formed catalysts with significant ORR performances in acidic media, interestingly much higher than those recently reported elsewhere for Fe/C/N catalysts prepared by the aerosol-based flame spray pyrolysis method [12]. Another work by us [11] mentioned the formation of highly active Fe/C/N ORR electrocatalyst prepared by a two steps synthesis involving CO 2 laser pyrolysis. It consisted of the formation of a catalyst by ammonia-induced laser pyrolysis of an aerosolized solution of iron acetylacetonate solution in toluene, followed by a thermal treatment under ammonia in a tubular oven during 20 min at 1100 • C. The strong improvement of the ORR performances recorded by this two-step synthesis led us to consider an attempt to put the same laser pyrolysis reactor into the two-step synthesis, by involving a two-stage ammonia-induced CO 2 laser pyrolysis reactor.…”

“…For single CO 2 laser pyrolysis, the aerosol was driven using an argon flow mixed with an ammonia flow just before crossing the CO 2 laser beam. As in previous work [10,11], the flows of argon and ammonia define the so-called R parameter as the flow volume fraction of ammonia in argon. For the experiments carried out in this study, the R parameter was set at different values: 0.22, 0.40, and 0.48.…”

“…Besides, chemical approaches involving the synthesis of tailored precursors such as zeolitic imidazolate framework provide highly performing ORR catalysts with an appreciable reduced number of synthesis steps [6,7]. Recently, we reported, for the first time, an original one-step synthesis approach involving ammonia-induced CO 2 laser pyrolysis of aerosolized liquid precursor media, such as pyridine or toluene, in which an iron precursor (iron acetylacetonate) was dissolved [10,11]. This method gave as-formed catalysts with significant ORR performances in acidic media, interestingly much higher than those recently reported elsewhere for Fe/C/N catalysts prepared by the aerosol-based flame spray pyrolysis method [12].…”