Synthesis and Characterization of Carbon/Nitrogen/Iron Based Nanoparticles by Laser Pyrolysis as Non-Noble Metal Electrocatalysts for Oxygen Reduction

Abstract

This paper reports original results on the synthesis of Carbon/Nitrogen/Iron-based Oxygen Reduction Reaction (ORR) electrocatalysts by CO 2 laser pyrolysis. Precursors consisted of two different liquid mixtures containing FeOOH nanoparticles or iron III acetylacetonate as iron precursors, being fed to the reactor as an aerosol of liquid droplets. Carbon and nitrogen were brought by pyridine or a mixture of pyridine and ethanol depending on the iron precursor involved. The use of ammonia as laser energy transfer agent also provided a potential nitrogen source. For each liquid precursor mixture, several syntheses were conducted through the step-by-step modification of NH 3 flow volume fraction, so-called R parameter. We found that various feature such as the synthesis production yield or the nanomaterial iron and carbon content, showed identical trends as a function of R for each liquid precursor mixture. The obtained nanomaterials consisted in composite nanostructures in which iron based nanoparticles are, to varying degrees, encapsulated by a presumably nitrogen doped carbon shell. Combining X-ray diffraction and Mossbauer spectroscopy with acid leaching treatment and extensive XPS surface analysis allowed the difficult question of the nature of the formed iron phases to be addressed. Besides metal and carbide iron phases, data suggest the formation of iron nitride phase at high R values. Interestingly, electrochemical measurements reveal that the higher R the higher the onset potential for the ORR, what suggests the need of iron-nitride phase existence for the formation of active sites towards the ORR.

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

“…It is seen that the chemical yields drop when the R parameter increases. This trend is similar to those obtained when pyridine was employed as liquid precursor in previous studies [10]. For a given R, a comparison of the data related to SSCat on one hand and DSCat on another hand (Table 1) shows that the chemical yields are higher for SSCat than for DSCat, although the difference is lower after chemical washing.…”

“…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.…”

“…Cyclic Voltammetry (CV) was recorded using a Bio-Logic VMP3 potentiostat (BioLogic, Seyssinet-Pariset, France). As described in detail in previous papers [10,11,[14][15][16][17], CVs were recorded on porous electrodes prepared by filtration on a carbon felt H2415 I2C3, from Freudenberg (Weinheim, Germany), of liquid dispersions obtained by sonication of catalysts powders in acetone or isopropanol. The amount of catalyst on each electrode was obtained by weighting the carbon felt before and after catalyst deposition.…”

“…The electrodes were then submitted to an extended scan (200 to 250 scans) at 100 mV•s −1 from 1 V vs. SHE to 40 mV vs. SHE. Also described in previous works [10,11,14], since the catalysts were not acid leached before electrode preparation, this conditioning cycling step gives rise to more or less iron removal into the electrolyte. The complete iron removal is shown by the vanishing of iron related redox peaks, providing a stable voltammogram [14].…”

“…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].…”