Enhanced activity and durability of Pt nanoparticles supported on reduced graphene oxide for oxygen reduction catalysts of proton exchange membrane fuel cells
Graphical abstract
Introduction
Recently, air pollution caused by automobile exhaust fumes has been a critical social issue, so that eco-friendly vehicles such as hydrogen fuel cell vehicles are attracting enormous attention in the automotive industry. As one of the promising candidates, proton exchange membrane fuel cells (PEMFCs) are mainly used as a power source of hydrogen fuel cell vehicles, because PEMFCs have lots of advantages including compactness, light weight, cost, rapid start/stop, low operation temperature, and high power density [[1], [2], [3]]. However, there are two intrinsic drawbacks which restrict maximizing the performance of the PEMFCs; (1) The kinetics of the oxygen reduction reaction (ORR) is relatively slower than that of the hydrogen oxidation reaction (HOR). The kinetics of the HOR on the Pt supported catalyst is very fast and the voltage losses are negligible, while the lethargic kinetics of ORR results in large voltage loss for the PEMFCs system. (2) The performance of catalysts deteriorates during the long operation time [4,5]. Since the general automotive operates for 5,000–20,000 h, the long-term durability of PEMFCs is an essential issue for the commercial applications of PEMFCs [6,7].
As PEMFCs operate at low temperatures (below 100 ℃), highly active catalysts such as Pt and its compounds have been used for the ORR electrodes. Also, the carbon black (CB) with benefits of the low cost and high surface area has been generally used as a support material of the Pt supported catalyst (Pt/CB) to reduce the Pt consumption. However, when the Pt/CB catalyst is exposed to oxygen atmosphere for a long operation time, the Pt/CB catalyst could be suffered from various degrading durability issues such as carbon corrosion, sintering and agglomeration of the Pt nanoparticles (Pt NPs), which might lead to the irreversible deterioration of the PEMFCs performance [1,[8], [9], [10], [11]].
The requirements for a suitable catalyst support are high specific surface, electrical conductivity and electrochemical stability under the operating condition [12,13]. Graphene, as one of the carbon allotropes, is a two-dimensional and one-atomic-thick material with hexagonal lattice pattern, which has many advantages such as superior strength, physicochemical stability, high specific surface area, electrical, and thermal conductivity [14,15]. From the catalytic advantage point of view, the high specific surface area of graphene can provide uniform dispersion of the Pt NPs with reduced amount of Pt usage, and also the high physicochemical stability of graphene could improve the electrochemical stability under the carbon corrosion condition of PEMFCs. Recently, many researches have been conducted to apply graphene as a durable support of ORR catalyst. B.Y. Kaplan et al. examined performance of the Pt NPs supported on a hybrid carbon support comprising the CB and graphene [16]. In addition, Y. Shao et al. reported the graphene nanoplatelets (GNPs) as the support of durable electrocatalyst [17] and A. Ghosh et al. investigated differences in characteristics of the graphene and functionalized graphene as supports of the Pt supported catalyst [18].
In this paper, we report the enhanced performance of oxygen reduction catalysts composed of the Pt NPs supported on the reduced graphene oxide. For more comprehensive understanding of carbon surface effect, the Pt NPs were decorated on the various carbonaceous supports. First of all, the Pt NPs were directly synthesized on the graphene oxide (GO) and the reduced graphene oxide (rGO). And then, the Pt NPs on the partially reduced graphene oxide (Pt/GO-r) catalyst, which might show an intermediate degree of oxygen functional groups between the Pt/GO and Pt/rGO catalysts was prepared by heat treatment of the Pt/GO catalyst. The characteristics of the synthesized catalysts were analyzed with transmission electron microscopy, X-ray diffraction, thermogravimetric analysis, X-ray photoelectron spectroscopy, linear sweep voltammetry, cyclic voltammetry, electrochemical impedance spectroscopy, and Raman spectroscopy. The results confirmed that the rGO was a superior support comparing to the CB in terms of the oxygen reduction activity and the long-term durability. Here, the reduced concentration of oxygen related functional groups on the carbon support was the key factor to enhance the oxygen reduction properties and the long-term durability of the Pt/rGO catalyst.
Section snippets
Catalysts preparation
The GO and rGO were obtained from Angstron Materials Inc. The Pt/GO and Pt/rGO catalysts were synthesized by the polyol method using a microwave heating apparatus and the amounts of Pt among the catalysts was fixed as 20 wt%. 55 mg of the GO and 55 mg of the rGO were respectively mixed with 50 ml of diethylene glycol (DEG, 99%, Junsei Co) with ultrasonication for 2 h. The Pt precursor of H2PtCl6∙6H2O (Sigma-Aldrich Co.) and NaOH (Junsei Chemical Co.) solution were added to the mixed solution,
Morphologies and crystal structures of the catalysts
The morphologies of the Pt/CB, Pt/GO, Pt/GO-r, and Pt/rGO catalysts were observed from transmission electron microscopy (TEM) (Fig. 1 a-d). The TEM images demonstrate that the Pt NPs are uniformly distributed on the surface of all of the support materials, and the sizes of the Pt NPs are approximately 1.5–3 nm for all catalysts. However, for the Pt/GO-r catalyst, it was obtained from the heat treatment process of the Pt/GO catalyst for partial chemical reduction of the GO surface, whereby the
Conclusions
We demonstrated the Pt NPs supported on the conventional CB support can be easily removed by carbon oxidation during the ORR, and that the rGO can be used as a good support for Pt NPs in fabricating a highly efficient and durable ORR catalyst for PEMFCs. The Pt/CB catalyst exhibited the lowest ORR activity and poor long-term durability. The Pt/GO and Pt/GO-r catalysts showed favorable ORR activity compared to Pt/CB catalyst, however, they showed a large loss of the Pt NPs in the long-term
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We gratefully acknowledge financial supports from National Research Foundation of Korea (NRF-2017M3A7B4049466, NRF-2018R1C1B5045721), Ministry of Trade, Industry and Energy of Knowledge Economy (MOTIE, 10067386), and Ministry of Economy and Finance (MOEF, EO190011). We also appreciate experimental supports from Busan Center of Korea Basic Science Institute.
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