Platinum-based catalysts have been frequently used for proton exchange membrane fuel cells (PEMFCs) over the decades [1]. However, its high cost and scarcity led to attention to non-precious metal catalysts (NPMCs) with anion exchange membrane [2]. One of the most promising NPMCs is iron and nitrogen co-doped carbon (Fe-N-C). Since nitrogen (N), iron (Fe) and carbon (C) are plentiful on the Earth, the cost of Fe-N-C is much lower than that of Pt-based catalysts. In order to improve the activity of Fe-N-C, various transition metals have been introduced. Among them, the introduction of copper (Cu) is expected to be able to achieve higher performance by imitating the cytochrome-c-oxidase structure [3]. In this study, the effect of different types of Cu precursors on the physical and electrochemical properties of Fe-N-C catalysts was investigated. Copper chloride, nitrate, sulfate, and acetate with different contents were applied to the preparation of Fe-N-C catalysts.
Cu introduced Fe-N-C catalysts were named as FexCuy-N-C (x and y are a molar ratio of Fe and Cu, respectively.). FeCu-N-C was synthesized by nano-replication of SBA-15 using iron chloride (FeCl3), 1,10-phenanthroline, and Cu precursors, phosphoric acid and citric acid. For P doping, phosphoric acid was impregnated into silica pores. Citric acid was exploited as a carbon precursor and acid catalyst for polymerization simultaneously. After pyrolysis at 900 ˚C under Ar atmosphere for 3 h, silica was etched by 20 wt.% HF, followed by acid leaching with 0.5 M sulfuric acid at 80 ˚C for 8 h. After dried at 80 ˚C overnight, it was heat treated again under Ar+NH3 atmosphere at 900 ˚C for 1 h.
From X-ray photoelectron spectroscopy, N 1s spectra of all catalyst showed that the contents of graphitic N were about 50% due to two heat treatment steps. And amount of pyridinic N was about 30% of entire surface nitrogen. From elemental analysis technique, N content was about to be 3 at.%. During acid leaching and heat treatment most of unwanted iron clusters such as iron oxides and iron carbides with low number of carbon shells were eliminated from the surface. Therefore, less than 0.1 at.% of Fe and Cu were confirmed from Fe 2p and Cu 2p spectra. Hence, there were no obvious peaks from Fe or Cu crystal while only carbon peaks existed in X-ray diffraction pattern. Surface area and porosity of catalysts were investigated by N2 adsorption and desorption technique. Surface areas were in the range of 1000-1050 m2/g, and pore volumes were about to be 1.1 cm3/g. Interestingly, with increasing content of citric acid, enlargement of mesopores from 4 to 5 nm was shown and transition of pore shape from slit to cylinder as confirmed by hysteresis of N2 ads/des isotherm curve. It infers that citric acid can effectively help the nano-replication process by its bifunctional role.
With the different molar ratios of Fe and Cu, oxygen reduction activity was measured by a rotating disk electrode technique. Catalyst using CuSO4 was used since its catalytic activity was superior to those of catalyst which used copper chloride or acetate. The half-wave potential of catalyst with Cu1.0 was higher than other catalysts (918 mV), which is 16 mV higher than that (902 mV) of Fe-N-C. Cu-N-C had the lowest oxygen reduction activity (824 mV). Therefore, with proper content of Cu and type of precursor, optimized performance could be achieved.
References
[1] D.R. Dekel, J. Power Sources, 375, 158 (2018).
[2] X. Zhang, Y.B. Mollamahale, D. Lyu, L. Liang, F. Yu, M. Qing, Y. Du, X. Zhang, Z.Q. Tian, P.K. Shen, J. Catal.
372, 245 (2019).
[3] J. Li, J. Chen, H. Wan, J. Xiao, Y. Tang, M. Liu, H. Wang, Appl. Catal. B, 242, 209 (2019).
