Mesoporous Iron Doped Carbons Based on Protic Ionic Salt with Tunable Oxygen Reduction Activity
Authors: Hoque Mahfuzul , Zhang Shiguo , Thomas Morgan L. , Dokko Kaoru , Watanabe Masayoshi 
Published: February 2020

Electrochemical energy systems such as fuel cells and metal–air batteries [1, 4] are based on oxygen reduction reaction (ORR) which is intrinsically sluggish in kinetics. So, the importance of efficient, low-cost and stable electrocatalysts for such systems is paramount. In this study, we prepared metal (Fe) and nitrogen (N) doped mesoporous carbons (Fe/N/C) as an ORR electrocatalyst by using low cost precursors, which are protic ionic salt, 1,10-phenanthrolinium dibisulfate [Phen][2HSO4], and iron (III) chloride (FeCl3) via facile and scalable strategy termed as nanocasting. Colloidal silica was used as a hard template for the introduction of mesopores [2] in the catalyst structure. Structural characterizations and electrochemical features i.e., performance and stability were investigated and compared with the state of the art Pt/C.

Mesoporous Fe/N/C catalysts were synthesized by direct carbonization (Scheme 1: PR-I & PR-II) of the mixture of [Phen][2HSO4], FeCl3 and silica for 2h under an Ar flow of 100 mL/min from 600 oC to1000 oC. Composition of iron precursor was also varied. Also, after the removal of template by HF the second carbonization (selected samples) was done for 1h. To test the catalyst in acidic media, it was washed with 6M HCl; dried and carbonized. Electrochemical measurements were carried out in 0.1 M KOH and 0.1M HClO4(in progress) using a temperature controlled water-jacketed three-electrode cell. Ag/AgCl and Pt coil were used as the reference and counter electrodes, respectively. The mesoporous catalyst was dispersed in a mixture of deionized water, ethanol, and Nafion (5 wt%) and sonicated for 1h. Definite amount of the ink was then dropped for the desired loading onto the surface of a glassy carbon electrode (4 mm) and dried naturally.

Successful fabrication of mesopores was observed by HR-TEM and BET. XRD pattern of Fe/N/C showed (002) and (100) diffraction peaks at 2θ = 24.7o and 43.7o, respectively for the graphitic structure. In addition, broad Raman bands were observed at ν =1589 and 1349 cm-1 which could be attributed to the G mode and D mode peak, respectively, and the low value of I
G/I
D ratio indicates the presence of defects and disorders in the carbons. EDX elemental analysis revealed the presence of C, N, O, S, and Fe in the prepared mesoporous carbon also supported by XPS survey scan. HR-XPS exhibited that the N1s spectrum of Fe/N/C is comprised of pyridinic N, pyrrolic/pyriodonic N and graphitic-type quaternary N structures. The slight impurity that appeared at a higher binding energy was assigned to oxidized N species [2]. The S2p spectrum was also resolved into well-defined peaks related to S2p3/2 and S2p1/2 in the lower binding energy and those at the higher binding energies were ascribed to oxidized species stemming from the anion. But, it was not possible to obtain well-resolved spectra for iron owing to the low amount of iron confirmed from ICP-AES measurements. Electrochemical measurements revealed that the Fe/N/C has an electrocatalytic activity for oxygen reduction reaction (ORR) in alkaline solution. Figure 1 shows rotating disk electrode (RDE) voltammograms of Fe/N/C prepared under carbonization profile, i.e., PR-I and PR-II by keeping other parameters constant in 0.1M KOH solution under O2 atmosphere at 30 oC. In comparison with Pt/C (Sigma-Aldrich) catalyst, mesoporous Fe/N/C synthesized by following PR-II showed improved activity in terms of onset potential and half-wave potential under the same catalyst loading. Chronoamperometry measurements (20000s) and cyclic performance (1500 cycles) study vindicated that the Fe/N/C mesoporous carbon was stable in alkaline media. Electrochemical measurement in acidic media is currently under study. We found that in alkaline electrolyte the feed composition of iron, carbonization procedure (temperature, profile, and second carbonization) and acid treatment played pivotal role to fine-tunethe ORR activity of the metal doped carbons. The performance of Fe/N/C electrocatalyst was affected by the balance between surface area, graphitic degree, and Fe/N doping [1-3] which could be controlled by the synthetic parameters. Moreover, protic ionic salt platform could be considered as a viable candidate for utilization of non-precious metal based carbon materials for fuel cells.

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Figure 1