Full paperFe@C2N: A highly-efficient indirect-contact oxygen reduction catalyst
Graphical abstract
Oxygen reduction catalyst from iron encapsulated in C2N framework (Fe@C2N) exhibits outstanding catalytic activities in both alkaline and acidic media. The Fe@C2N catalyst holds great potential for commercialization.
Introduction
While platinum (Pt) is known as the most efficient catalyst [1], [2] for clean and sustainable energy technologies such as fuel cells [3], [4] and metal-air cells, [5], [6] the annual production of platinum is only a couple of hundred tons. In addition, Pt has an extremely high-cost, is sensitive to impurities, and almost half of the annual production is used for vehicle emission control devices. These factors severely limit its applications in the commercial scale production of energy conversion devices.
To date, there have been two major research thrusts in the search for alternative materials to replace expensive Pt-based catalysts. The first focused on metal-free heteroatom-doped carbon-based materials, which display appreciable catalytic activity and good stability [5], [7], [8]. These materials, however, only work in alkaline media, and their onset potential is not as good compared to a Pt catalyst. The other comprehensive line of research has targeted non-precious metal-based catalysts [9], [10], [11]. Although their onset potential is close to that of Pt, these catalysts have long been associated with unstable performance.
As a solution, researchers have been actively searching for an efficient method of encapsulating non-precious group VIIIB transition metal (e.g., Fe, Co, Ni) nanoparticles, using carbon-based materials, to achieve both catalytic performance and stability [11], [12], [13], [14], [15], [16], [17]. Unfortunately, these catalysts frequently suffer from poor stability in acids due to leaching [9], [14], [18], [19]. This is because pure carbon-based materials such as carbon nanotubes and graphene are not polar or reactive enough to produce a strong (binding) interaction with metal nanoparticles, which are needed to create robust void-free encapsulation.
As an alternative approach, particularly with regard to air-breathing catalysts, hybrid systems consisting of non-precious metal nanoparticles and polar heteroatom-doped carbon-based materials (e.g., Fe-N-C) have attracted recent attention [20], [21], [22]. Nevertheless, due to defective encapsulation, these materials have been too electrochemically unstable to satisfy commercial demands, and their catalytically active sites are unclear.
In addition, one of the major pitfalls associated with all of the metal containing electrocatalyst systems (including Pt) is metal leaching in corrosive electrolytes (which is even more severe in acidic media) during the electrochemical redox process. Such leaching degrades overall catalytic performance by promoting loss of the electrode and contamination of the membrane [23].
As a result, developing methods to achieve void-free encapsulation of unstable group VIIIB transition metal nanoparticles having electrochemically transparent and stable layers has become critically important in energy technologies [11], [22].
Considering the abundance and catalytic properties of Fe, Fe nanoparticles are of general interest for use in electrocatalytic systems [22], [24]. However, Fe is unstable against oxidation (rusting) and poisoning [25]. To be effective, the Fe nanoparticles need protective encapsulation, driven by strong interactions between the catalytically active but unstable Fe cores and electronically conductive but well-defined polar graphitic shells. This configuration would permit indirect-contact electrocatalysis for ORR, and is necessary not only for overall dynamic catalytic activity, but also for durability.
Herein, we report a new structure for an indirect-contact electrocatalyst, based on Fe nanoparticle cores stably encased in well-defined nitrogenated graphitic shells. The core-shell nanoparticles are uniformly distributed in a nitrogenated, holey two-dimensional network (hereafter denoted C2N) [26]. The catalyst was prepared by the in-situ sandwiching of Fe3+ precusors in C2N layers, followed by the subsequent reduction of the Fe3+ precusors into FexOy nanoparticles. The FexOy nanoparticles were converted into Fe0 nanoparticle cores, which catalysed the C2N layers into well-defined nitrogenated graphitic shells during subsequent annealing. The resulting Fe@C2N catalyst displayed superior electrocatalytic activity and with stability as compared to commercial Pt/C in both acidic and alkaline media.
Section snippets
Materials
All the solvents, chemicals and reagents were purchased from Aldrich Chemical Inc., unless otherwise stated. Solvents were degassed with nitrogen purging before use. All reactions were performed under nitrogen atmosphere using oven dried glassware. 1,2,3,4,5,6-hexaaminobenzene (HAB) was synthesized according to a procedure described in the literature [27].
Synthesis of the Fe@C2N catalyst
FeCl3 (1.168 g) was first dissolved in NMP (35 mL) and placed on an ice bath in a 3-necked round bottom flask. Hexaketocyclohexane octahydrate
Results and discussion
As schematically presented in Scheme 1, the nitrogenated holey two-dimensional network (designated C2N for the empirical formula of the repeating unit in its basal area) consists of evenly distributed holes with six nitrogen atoms facing the center of each hole [26]. The polar C2N serves important roles: (i) to provide rich coordination sites for the stable coordination of the Fe precursor (Fe3+), and thus to prevent its leaching out during purification before annealing; (ii) to support the
Conclusions
In summary, we synthesized a new Fe@C2N catalyst, which exhibited stable ORR in both alkaline and acidic media. The structure of the Fe@C2N catalyst was determined using TEM and XANES studies. The results indicated that zero-valent iron (Fe0) nanoparticle cores were stably cocooned in nitrogenated graphitic shells (Fe@C2N nanoparticles). As a result, the Fe@C2N catalyst, comprised of Fe@C2N nanoparticles uniformly dispersed in C2N matrices, displayed unusual ORR performance with outstanding
Acknowledgements
The authors are grateful for financial support from the Creative Research Initiative (CRI, 2014R1A3A2069102), BK21 Plus, (10Z20130011057), Science Research Center (SRC, 2016R1A5A1009405), Mid-Career Researcher Program (NRF-2015R1A2A1A10055886) and Climate Change (2016M1A2940910) programs through the National Research Foundation (NRF) of Korea and by UNIST
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These authors contributed equally to this work.
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Current address: Department of Chemical Engineering, Wonkwang University, Chonbuk 54538, Republic of Korea.