In this paper, a method for estimating the radial profile of electron density n e using a single line-of-sight signal by the He I line intensity ratio method is proposed. By applying this method to cylindrical helium plasma, in which electron temperature was almost uniform and density was uniform in the center, we tried to estimate the parameters representing spatial distribution. It was confirmed that a good distribution estimation result could be obtained by considering the sensitivity factor, the rate at which the line intensity ratio changes as the parameters change, during optimization. Two methods of considering a sensitivity factor are proposed: using the best combination of intensity ratios for analysis in terms of the sensitivity factor, and weighting the objective function using the sensitivity factor. The former method can be analyzed in short computational time, although its applicability is limited. The latter method can be used when it is not obvious which set of intensity ratios is best to use, although it takes more computational time compared with the former method. Both methods reproduce the parameter of a radial density profile.
In converging field configuration of cylindrical plasma following electron cyclotron resonance plasma source, intermittent variation of floating potential, which is a negative spike with about ten microsecond duration, is observed. The event is ubiquitous; independent of measurement position and magnetic field. Cross correlation analysis for two floating potentials at separate positions reveals that strong correlation exists within the event timescale. The correlation along the magnetic field line suggests axially elongated structure. Radial extent of the structure is clarified by the radial correlation profile. Different FWHM of the radial profile at the plasma production region and the downstream converging field region is essentially explained by field line tracing.
1. Introduction It is of great importance to decrease Pt usage in PEFCs for cost reduction. Carbon supported Pt shell-Pd core structured catalyst (Pt/Pd/C) is a promising candidate for the decrease owing to high Pt utilization and ORR activity [1]. We found that ORR specific activity of the catalyst drastically enhanced with an accelerated durability test (ADT), while electrochemical surface area (ECSA) largely decreased due to agglomeration of the catalyst nanoparticles (NPs), resulting in a moderate increase of ORR mass activity [2]. Thus, it is essential to mitigate the ECSA decay for further enhancement of the ORR mass activity. In the present study, poly-dopamine coating was investigated to suppress the catalyst NPs’ agglomeration with the ADT and further enhance the ORR mass activity of the catalyst. 2. Experimental Pt/Pd/C catalyst was synthesized by a modified Cu-UPD/Pt displacement method [2]. 200 mg of Pt/Pd/C catalyst was added in 200 mL of 10 mM tris-HCl buffer solution (pH8.5) containing 2 mg/mL of dopamine hydrochloride and stirred at 30°C for 6 h under O2 gas bubbling (500 mL/min.), followed by filtering, washing and drying at 60°C in air [3]. ADT was performed at 80°C for 10,000 cycles in Ar saturated 0.1 M HClO4 by using a rectangular potential cycling of 0.6 V (3 s)-1.0 V (3 s) vs. RHE. Characterization of the catalyst was carried out by using TG, XRD, XRF, TEM and CV techniques. ORR activity of the catalyst was measured by RDE technique performed at 1,600 rpm in O2 saturated 0.1 M HClO4with positive scan rate of 10 mV/s at 25°C. 3. Results and Discussion It has been reported that polymerization of the dopamine was accelerated by O2and that poly-dopamine (PD) can be coated on various materials such as metals, metal oxides, organic films and plastics at low temperature (30°C) [3, 4]. For carbon coating, carbonization of the PD is carried out by heat treatment in non-oxidative atmosphere [5] and it has been demonstrated that size of the PD coated PtFe NPs (6.5 nm) did not change before and after the heat treatment at 700°C, indicating that the carbon coating using the PD precursor well suppressed coalescence and sintering of the NPs against the heat treatment [6]. In this study, we investigated effect of the PD coating on durability of the Pt/Pd/C catalyst without the heat treatment. Figure 1 depicts TEM images of the Pt/Pd/C catalysts before and after the PD coating and after the ADT performed at 80°C for 10,000 cycles. Little morphological changes were observed before and after the PD coating and agglomeration of the catalyst NPs was well suppressed after the ADT, which mitigated the ECSA decay of the catalyst with the ADT (Figure 2). Figure 3 summarizes change in ORR mass activity of the catalyst with the ADT. The ORR mass activity of the PD coated Pt/Pd/C catalyst was enhanced by 3.8-fold of a reference Pt/C catalyst (TEC10E50E, TKK), implying that the PD coating without the heat treatment, i.e., not a carbon coating, effectively mitigated the catalyst NPs’ agglomeration and improved the durability of the Pt/Pd/C core-shell structured catalyst. In the meeting, the durability of the heat treated catalyst will be also presented. Acknowledgement This study was supported by NEDO, Japan. References [1] J. Zhang et al., J. Phys. Chem. B, 108, 10955 (2004). [2] M. Inaba and H. Daimon., J. Jpn. Petrol. Inst., 58, 55 (2015). [3] H. Lee et al., Science, 318, 426 (2007). [4] H. W. Kim et al., ACS Appl. Mater. Interfaces, 5, 233 (2013). [5] R. Liu et al., Angew. Chem. Int. Ed., 50, 6799 (2011). [6] D. Y. Chung et al., J. Am. Chem. Soc., 137, 15478 (2015). Figure 1
Introduction Carbon supported Pd core-Pt shell catalyst (Pt/Pd/C) is a promising alternative to the conventional Pt/C catalyst because of high Pt utilization and enhancement of ORR activity [1]. Recently, we found that ORR specific activity of the Pt/Pd/C catalyst was drastically enhanced with an accelerated durability test (ADT) conducted at 80°C [2]. During the ADT, the Pt shell rearranged associated with the Pd core dissolution and a compressive strain was induced in the Pt shell, which is considered to enhance the ORR specific activity [2]. However, ECSA of the catalyst decreased after the ADT. Thus, we developed a high activation protocol (HAP) using GC electrode to mitigate the ECSA decay and enhance ORR mass activity [3]. In this study, we explored influence of potential range in the HAP on ECSA decay and ORR activity enhancement of the Pt/Pd/C catalyst. Furthermore, we developed a Cu-O2 treatment to scale-up the HAP on the GC electrode for mass-production of highly activated Pt/Pd/C catalyst. Experimental Pt/Pd/C catalyst was synthesized with a modified Cu-UPD/Pt replacement process [2]. A carbon supported Pd core (Pd/C, Pd size: 4.6 nm, Pd loading: 30 wt.%, Ishifuku Metal Industry) was stirred in 50 mM H2SO4 containing 10 mM CuSO4 with co-existence of a metallic Cu sheet at 5°C under Ar atmosphere. After stirring for 5 h, the Cu sheet was removed and K2PtCl4 was added to replace under potentially deposited Cu shell on the Pd core surface with the Pt shell. ADT was carried out using rectangular wave potential cycling of 0.6 (3 s)-1.0 V (3 s) vs. RHE in Ar saturated 0.1 M HClO4 at 80℃ for 10,000 cycles. The Pt/Pd/C catalyst was characterized by TG, XRF, XRD, TEM and CV. ORR activity of the catalyst was evaluated with RDE technique in O2 saturated 0.1 M HClO4 at 25°C. Results and Discussion We explored influence of potential range in the HAP on electrochemical properties of the Pt/Pd/C catalyst using rectangular potential cycling of 0.05~0.8 V (300 s for low potential) to 1.0 V (300 s for high potential) vs. RHE performed in Ar saturated 0.1 M HClO4 at 80℃ for 30 cycles. Changes in ECSA and ORR activity of the catalyst are demonstrated in Fig. 1. In comparison to ADT, ECSA decay of the Pt/Pd/C catalyst was mitigated with the HAP. Interestingly, ORR specific activity of the catalyst was largely enhanced when the low potential range was 0.2~0.6 V, which largely enhanced ORR mass activity of the catalyst. At the low potential range (0.2~0.6 V), it is considered that Pt shell rearrangement associated with Pd core dissolution was advanced due to sufficient oxidation/reduction of Pt and Pd. At low potential of 0.05 V, since hydrogen adsorbs on the Pt, it is considered that hydrogen adsorption hindered rearrangement of the Pt shell [4] and the ORR activity was not largely enhanced. We further developed a Cu-O2 treatment to scale-up the HAP performed on GC electrode. In the Cu-O2 treatment, the Pt/Pd/C catalyst powder (200 mg) is stirred at 80℃ for 300 s in 2 M H2SO4 containing 0.1 M CuSO4 with co-existence of a metallic Cu sheet under N2 atmosphere, where equilibrium potential of Cu2+/Cu (ca. 0.3 V) is applied to the catalyst powder when it contacts with the Cu sheet. Next, the Cu sheet is removed and O2 gas is introduced for 300 s, where equilibrium potential of ORR (ca. 1.0 V) is applied to the catalyst (Fig. 2). Figure 3 summarizes changes in ECSA and ORR mass activity of the Pt/Pd/C catalyst with the HAP and the Cu-O2 treatment (30 cycles). Compared with the HAP, the Cu-O2 treatment equivalently mitigated ECSA decay and enhanced ORR mass activity of the catalyst by ca. 3 times of reference carbon supported Pt catalyst (Pt/C, Pt size: 2.8 nm, Pt loading: 46 wt.%, TEC10E50E, TKK), indicating that the Cu-O2 treatment mimics the HAP on the GC electrode and is suitable for mass-production of highly activated Pt/Pd/C catalyst. Acknowledgement This work was supported by NEDO, Japan. References [1] A. U. Nilekar et al., Top Catal., 46, 276 (2007). [2] M. Inaba and H. Daimon, J. Jpn. Petrol. Inst., 58(2), 55 (2015). [3] H. Daimon et al., The 228th Electrochemical Society Meeting, #1375, Phoenix, USA, October 2015. [4] Q. Xu et al., J. Electrochem. Soc., 155(3), B228 (2008). Figure 1
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