Short CommunicationNitrogen-doped carbon supported platinum catalyst via direct soft nitriding for high-performance polymer electrolyte membrane fuel cell
Introduction
Polymer electrolyte membrane fuel cells (PEMFCs) are highly efficient and green energy-conversion device, regarded as one of the most promising next-generation energy technologies [1]. However, PEMFCs still face several technological challenges, despite enduring periods of research. In particular, the sluggish oxygen reduction reaction (ORR) of the carbon supported Pt (Pt/C) catalyst in the cathode plays an important role in impeding the fuel cell performance [2]. Hence, the development of electrocatalyst with high catalytic activity towards ORR has been one of the major issues that limit the commercialization.
Among many, the nitrogen-doped Pt/C catalyst has been widely accepted as one of the highly efficient electrocatalysts towards ORR [3], [4], [5], [6]. Although the active sites of nitrogen-doped carbon materials are unclear, it is suggested that the carbon atoms adjacent to pyridinic nitrogen may play a key part in promoting ORR under acidic conditions, which may be synergistic to the Pt catalysts [7], [8]. In this regard, several efforts have been made to control the doped nitrogen content and nitrogen atom configuration. Li et al. first reported that the nitrogen doping starts at 300 °C and reaches the highest doping level of 5 atom% at 500 °C when ammonia (NH3) is used [9]. Luo et al. suggested that the surface area of the nitrogen-doped carbons increases with higher temperature and longer time while the nitrogen content decreases with increasing temperature [10]. Similarly, Zhang et al. found that the nitrogen content in the nitrogen-doped graphene varied at different temperatures, with the highest nitrogen content obtained at 500 °C and the lowest at 800 °C, and concluded that the temperatures ranging from 500 to 600 °C may be acceptable for stabilizing all the nitrogen-containing species (pyrrolic, pyridinic and graphitic nitrogen) [11]. Later, Zhao et al. reported N-doped carbon nanotubes and nanofibers interacting with various metal catalysts for ORR [12]. Furthermore, the impact of different nitrogen-containing precursors on the nitrogen contents and configurations has been studied. For instance, Lai et al. revealed that the annealing of graphene oxide with NH3 preferentially leads to the formation of pyridinic and graphitic nitrogens [13]. Despite these efforts, the current synthetic process of the nitrogen-doped carbons, without exception, has suffered from at least partially using toxic nitrogen sources, e.g. ammonia. In addition, mass-production of such materials is still critical challenge for the practical use.
Recently, Liu et al. have synthesized the nitrogen-doped carbon supports via “soft nitriding” technique, which introduces nitrogen onto the carbon surfaces by employing NH3 and isocyanic acid (HCNO) obtained from thermal decomposition of urea [14]. After the heat treatment, the precious metal can be loaded onto the nitrogen-doped carbon support. Their results set a useful pathway to manufacture the nitrogen-doped carbon supports for noble metal catalysts, yet have been employed in the further applications with Pt/C.
In this work, we, for the first time, fabricated the nitrogen-doped Pt/C catalyst via direct soft nitriding of the commercial Pt/C catalyst for promoting the ORR. Unlike the previous work [14], the commercial 40 wt% Pt/C is first physically mixed with urea by vigorous grinding and then mildly heat-treated up to 300 °C to facilitate the fabrication process, thus enabling efficient nitrogen doping in a large quantity. Neither high annealing temperatures above 500 °C [9], [10] nor toxic gas such as ammonia [11] is required for nitrogen doping. First, the surface state of the nitrogen-doped carbon is investigated. Subsequently, the particle diameter and crystallinity of Pt catalyst are explored to examine the chemical stability during the soft nitriding. Finally, the electrochemical performance of the nitrogen-doped Pt/C catalyst and fuel cell performance are evaluated in comparison to those of the as-received Pt/C.
Section snippets
Soft nitriding by urea
Soft nitriding of the commercial Pt/C catalyst (HiSPEC 4000, Johnson Matthey) was conducted through a mild reaction between the carbon supports and ammonia/isocyanic acid generated by thermal decomposition of urea (Fig. 1). The direct soft nitriding may facilitate the catalyst preparation with reduced fabrication steps: 3 g of the Pt/C catalyst and 4.5 g of urea were physically grinded. Subsequently, the mixture was heat-treated at 300 °C for 1 h under nitrogen atmosphere, with the temperature
Results and discussion
Fig. 2 shows that the Pt/C catalyst maintains the average particle size and its distribution of Pt after the thermal treatment with urea. The TEM images are carefully examined as the thermal annealing may result in the formation of Pt aggregates to a certain extent [17]. The average diameter of the Pt catalyst was initially 3.84 nm (Fig. 2A) and only slightly increased to 3.98 nm after urea treatment (Fig. 2B). It is thus suggested that there may be only negligible thermal and chemical
Conclusions
We synthesized the nitrogen-doped Pt/C catalysts via soft nitriding technique. The nitrogen species were directly introduced onto the carbon surfaces of the Pt/C catalyst by thermal decomposition of urea at a relatively lower temperature of 300 °C as compared to the conventional method proceeded with ammonia gas at 900 °C. As a result, the electro-catalytic performance has been notably improved with the half-wave potential increase in the RDE test reaching approximately 39 mV in the acidic
Acknowledgements
This work was conducted under the framework of the research and development program of the Korea Institute of Energy Research (B8-2423).
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D. Seo and M. Kim contributed equally to this work.