The inhibition of electrochemical carbon corrosion in polymer electrolyte membrane fuel cells using iridium nanodendrites
Highlights
► Carbon corrosion is inhibited in the presence of water electrolysis catalysts. ► 2 wt% of Ir nanoparticles is required to inhibit carbon corrosion. ► Ir nanodendrites showed higher activity of oxygen evolution reaction. ► 1 wt% of Ir nanodendrites showed equivalent effect with 2 wt% of Ir nanoparticles.
Introduction
In recent years, electrochemical corrosion of carbon supports has been considered to be one of the critical factors limiting the durability of polymer electrolyte membrane fuel cells (PEMFCs) [1], [2], [3], [4]. The mechanism of electrochemical carbon corrosion suggests that carbon can be oxidized to CO2 in the presence of water, as shown in the following reaction [5]:
Even though the relatively low standard potential favors thermodynamic electrochemical carbon oxidation, carbon corrosion is not observed under normal PEMFC operational conditions due to slow electrochemical kinetics. However, it has been reported that severe carbon corrosion could occur, especially when the potential of electrodes rises higher than 1.4 VNHE, during abnormal operational conditions, such as fuel starvation and repetitive start-up/shut-down processes [6], [7], [8]. Such a high potential quickly oxidizes carbon supports and results in a substantial decrease in the performance of the PEMFC. To solve this problem, several recent studies have focused on developing corrosion-resistant materials, such as carbon nanofibers [9], [10], carbon nanotubes [11], [12] and carbon nanocages [13]. It has been reported that rolled graphene sheets with less dangling bonds or defects would significantly improve the resistance to carbon corrosion because it is difficult for oxidative atoms/groups to attack the closed structure [14]. However, there are still no carbon materials providing sufficient corrosion resistance at a highly corrosive potential.
Our previous study demonstrated that the existence of water is required for carbon corrosion, and no electrochemical carbon corrosion occurred under dry conditions [15]. In this regard, we have recently reported the effect of water electrolysis catalysts on electrochemical carbon corrosion [16]. The iridium oxide (IrO2) in the catalyst layer decomposes water molecules around the carbon supports when the potential is increased under carbon corrosion conditions. As a result, the addition of 2 wt% IrO2 (0.016 mg cm−2) to the catalyst layer of the cathode reduced the electrochemical corrosion of carbon by 76% at 1.6 VNHE compared with a conventional membrane electrode assembly (MEA). In this system, iridium oxides were adopted for oxygen evolution reaction (OER), which is the anodic reaction during water electrolysis, due to their high activity and stability in acidic electrolyte. For use of Ir, it is critical to decrease the cost because Ir is a noble metal. One of the approaches is to develop cheaper OER electrocatalyst materials. The other one is to reduce Ir usage through increasing OER catalytic activity.
In this study, we attempted to decrease the amount of Ir used while preserving the resistance to electrochemical carbon corrosion. The effect of IrO2 content on the electrochemical carbon corrosion was investigated and optimized. The Ir content was decreased further by improving OER catalytic activity, which is achieved by shape control of Ir nanoparticles. The electrochemical carbon corrosion was systematically evaluated by measuring CO2, which is direct evidence of carbon corrosion, using a mass spectrometry.
Section snippets
Synthesis of water electrolysis catalysts
IrO2 was prepared by the Adams-type fusion of iridium salt in nitrate flux [17]. The metal precursors (H2IrCl6·xH2O) were dissolved in 10 mL of ethanol/isopropanol (volume ratio 1:1) and vigorously stirred for 1 h. Then, 10 g of finely ground NaNO3 (Duksan pure chemical Co.) was added into solution. This slurry was heated at 60 °C for 1 h and further dried in a convection oven at 80 °C for 30 min. The dry salt mixture was finely ground and heat-treated at 500 °C for 30 min in a continuous flow of air.
Results and discussion
Fig. 1 shows a mass spectrogram indicating CO2 production during the corrosion test. It was conducted using the MEAs prepared with different loadings of IrO2 as a water electrolysis catalyst in the cathode catalyst layer. Because the generation of CO2 is direct evidence of carbon corrosion, it can be used to evaluate the effect of a water electrolysis catalyst on electrochemical carbon corrosion effectively. As shown in Fig. 1, the generation of CO2 depends strongly on the amount of IrO2 in the
Conclusion
The effect of OER electrocatalysts on carbon corrosion was investigated. It was found that the addition of IrO2 to the catalyst layer of PEMFCs prevents electrochemical carbon corrosion under severely corrosive conditions by eliminating water, which was essential for electrochemical carbon corrosion. It was found that the optimum amount of IrO2 was 2 wt% (0.016 mg cm−2). However, when the more active Ir dendrite was used, 1 wt% of Ir dendrite (0.008 mg cm−2) showed similar corrosion resistance of 2
Acknowledgments
This work was supported by the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Knowledge Economy (No. 20104010100500) and the Priority Research Centers Program through the National Research Foundation of Korea (2009-0093823) and the National Research Foundation of Korea (NRF-2009-C1AAA001-0092926) funded by the Ministry of Education, Science and Technology.
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