Full Length ArticleTunable oxidation state of Co in CoOx@N-doped graphene derived from PANI/Co3O4 and the enhanced oxygen reduction catalysis
Introduction
The consumption of fossil fuel plays a vital part in the development of modern civilization [1], [2]. Fuel cells are one of the most promising alternatives to conventional energy conversion devices due to its high energy density and environmental friendliness [3], [4], [5].
Most commonly used catalysts for electro reduction of oxygen in the cathode is Pt. However, its high cost and poor stability seriously limits the commercialization of fuel cells [6], [7], [8], [9]. Heteroatom doped-carbon [10], [11], [12], [13] can be utilized as metal-free ORR catalysts, but their performance are not so good, because the relatively weak adsorption of reactant [14], [15]. A good idea to manipulate the electron structure of carbon is through electronic interaction with metal species. Metal-doped carbon materials are developed, and some present similar or even better performance than Pt/C [16], [17], [18], [19], [20].
Thanks to the easy fabrication and its stability under harsh elecreochemical environment, transition metal oxides is regarded as a promising metal species used for electro catalysis [21], [22], [23], [24], [25]. Co3O4 has the potential to catalyze ORR by its surface Co3+ cations, which act as donor-acceptor reduction sites [26]. However, Co3O4 nanoparticles alone, present negligible ORR activity, because of there are not enough pathway for electron to transfer [27]. Carbon is usually chosen to guarantee electron transfer within the catalyst and between the catalyst and the electrode substance, and much enhanced activity can be observed [27], [28], [29].
However, during the operation conditions of fuel cells, metal elements in the cathode leach out inevitably due to harsh conditions (high potential, high humidity and O2 concentration). When using the most studied Fe-based non-precious metal catalysts, Fe2+ can dissolve into the electrolyte, and when meeting H2O2, an intermediate product of 2 electron pathway of ORR, H2O2 undergoes Fenton’s reaction under catalysis of Fe2+, producing highly oxidative and chemically active hydroxyl radical and hydroperoxyl radical useful in decompose organic pollutions. But because the Nafion membrane can also be destroyed by radicals, such reaction is not desired and should be avoided. By wrapping metal or metal oxide nanoparticles within graphene shell, not only can the dissolve of metal be avoided, manipulation of carbon electron structure can be achieved through strong interactions between inorganic nanoparticles and carbon layers. Similar structure are reported in oxygen reduction [28], [30], [31], [32], [33], [34], hydrogen evolution [35], [36], [37], [38], [39] and oxygen evolution reaction [38], [40], [41], [42], [43], [44]. However, in the traditional fabrication of metal/nitrogen-doped carbon (MNC), metal ions are reduced into metallic particles by organic carbon/nitrogen precursors or their derived carbons at high temperature, and it’s impossible to stop the reduction procedure to some extent, giving metals tunable oxidation state, which is crucial to high ORR performance.
Herein, we report CoOx@NG with controllable the oxidation state of Co simply by changing the pyrolysis temperature of Co3O4/PANI. Under high temperature, PANI decompose and release various organic gases, which can partially reduce Co3O4 and in the meantime in-situ grow into graphene. The balance of these two reactions are crucial to control the oxidation number of Co. Thus, ORR activity can be easily tuned by inner CoOx core with different electron structure. The catalyst achieve remarkable stability and anti-poisoning ability comparing with commercial Pt/C.
Section snippets
Chemicals
Aniline (C6H5NH2, AR), ammonium peroxydisulfate ((NH4)2S2O8, AR), cobaltous acetate tetrahydrate (Co(CH3COO)2·4H2O, AR), ammonium hydroxide(NH3·H2O, AR), sulfuric acid (H2SO4, AR), ethanol (C2H5OH, AR), potassium hydroxide (KOH, GR), Nafion (5 wt% in ethanol), isopropyl alcohol ((CH3)2CHOH, AR). All the chemicals were used as purchased without further treatment. Deionized water (18.2 MΩ cm, Milli-Q Corp.) was used in the whole procedure.
Preparation of polyaniline (PANI)
2 mL aniline and 3 g ammonium peroxydisulfate (APS) were
Physical characterization
The successful polymerization of aniline is confirmed by Fourier transform infrared spectroscopy (FTIR). From the FTIR spectrum shown in Fig. 1, the as-prepared PANI has characteristic peak at 1135.8 cm−1, related to Q = NH+-B (where Q and B represent quinoid ring and benzene ring, respectively), and the peak at 1490.7 cm−1 represent C C stretching deformation of quinoid ring [45]. The peaks at 1300 cm−1 and 1537 cm−1 represent C-N stretching of secondary aromatic amine, and CC stretching
Conclusions
In summary, we synthesized PANI/Co3O4-derived CoOx@NG as efficient ORR catalysts using a simple pyrolysis procedure. Under high temperature, PANI decompose and release various organic gases, which can partially reduce Co3O4 and in the meantime in-situ grow into graphene. Balancing these two reactions are crucial to control the oxidation state of Co, which can then tune the electron structure of outer carbon shell, facilitating ORR. The unique core-shell structure also endows CoOx@NG remarkable
Acknowledgement
This work is supported by talent introduction program of Yangtze Normal University.
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