Integrated platinum carbon nanotube based electrocatalyst for high efficiency proton exchange membrane fuel cells 5

Chapter Electrochemical Stability of Integrated Pt/CNT-based Electrocatalyst 5.1 Introduction Long-term stability of PEMFC electrocatalysts has been recently recognized as one of the major barriers to the commercialization of PEMFCs [1]. The commercial viability of PEMFCs requires a long operation life over 40,000 hours for PEMFC stacks in the stationary cogeneration system, and at least 5,000 hours for automotive applications [2]. Therefore, performance degradation of PEMFCs and degradation mechanisms of their component materials have received extensive attention recently in PEMFC research [1]. Many PEMFC researchers have reported their studies in the failure modes of PEMFCs and the degradation causes and mechanisms of PEMFC components, such as membrane degradation, Pt dissolution and precipitation, carbon support corrosion and so forth [3-7]. Recently Shao et al. [8] reviewed the current understanding of the durability issues of Pt-based electrocatalysts under real or simulated PEMFC conditions. In addition, the approaches to improve the long-term stability of PEMFC electrocatalysts and the experimental methods to scrutinize this durability issue are also discussed in their review. According to previous durability studies on PEMFCs, it is believed that the corrosion of carbon black support is one of the major degradation mechanisms for the long-term stability of PEMFC electrocatalysts [9]. Given the relatively high temperature, high potential, oxygen abundant, and aqueous acidic environment at PEMFC cathode, electrochemical oxidation of carbon black support into CO2 can take place in the cathode catalyst layer, causing electrical isolation and dissolution of the 134    Fig. 5.1 Illustration of carbon support corrosion in PEMFCs. C + 2H2O CO2 + 4H+ + 4e- E0 = 0.207 V (5-1) Pt catalyst particles as illustrated in Fig. 5.1. The corrosion reaction of carbon materials in aqueous acid electrolytes is generalized as Eq. 5-1 [10]. Generally, this reaction is thermodynamically possible at the PEMFC operating potentials whereas it is almost negligibly slow within that potential range to cause severe performance degradation [1]. However, this reaction can be greatly expedited by the supported Pt catalysts and it has been reported to be a prominent degradation mode during the startup and shut-down processes of PEMFCs [11]. Due to the presence of Pt catalysts, oxidation peaks of CO2 evolution were observed not only at potentials above 0.9 V but also at 0.6 V during potential cycling oxidation [12]. Another observation by Kangasniemi et al. [11] indicated that progressive oxidations of carbon black were detected in H2SO4, suggesting inside oxidations of the carbon black support under acidic environment. Therefore, CNT support with higher graphitic content has been developed in recent years for PEMFC electrocatalysts based on the speculation that CNTs are more electrochemically stable than carbon black [13]. Compared with carbon black, several research groups have reported that CNTs are more resistant to electrochemical oxidation than carbon black with or without Pt on them [11-14]. Thus CNTs have been proposed as a promising alternative support material for resolving the carbon corrosion problem in PEMFCs. 135    To evaluate long-term durability of PEMFCs under real operating conditions, in industry the main methodologies usually involve a long-term real-time operation and post-mortem examination of individual component after operation. However, these durability studies and testings are very time-consuming and also costly to be commonly employed [15]. Currently, several types of so called accelerated degradation tests (ADT) have been developed to expedite the life testing of PEMFCs, including (i) thermal degradation under hot air conditions, (ii) reduced humidity, (iii) open circuit cell operation and (iv) electrochemically forced aging under simulated cell conditions [16, 17]. Although these ADT tests have been extensively used to perform durability studies on conventional carbon black support, only a few publications treated the durability issues of CNTs as catalyst support for PEMFCs [11-14]. In 2006, Shao et al. [13] investigated the durability of CNT support by means of an ADT test in a three-electrode half-cell setup. In the ADT test, they first prepared a Pt/CNT-based electrode via the conventional ink-spread process and then placed it vertically in a chamber filled with 0.5 M H2SO4 solution. The Pt/CNT-based electrode was used as the working electrode and the H2SO4 solution was working as the electrolyte that mimics the PEMFC environment. To carry out the ADT test, a fixed potential of 1.2 V vs. RHE was applied onto the working electrode for 192 h and the electrochemical surface area loss of the electrode was characterized by cyclic voltammetry along the ADT test. It was found that after the 192 h ADT test, a total loss of 26.1% in electrochemical surface area was observed for the Pt/CNT catalyst, compared to 49.8% of the Pt/VXC72R reference catalyst. This dramatic difference was mainly due to the more prominent Pt particle growth and Pt dissolution from the oxidized VXC72R support. In another ADT test for CNT support by Wang et al. [14], the Pt/CNT catalysts were immersed in a N2 purged H2SO4 solution and applied a 136    potential of 0.9 V for different periods at 60 °C. The electrochemical stability of the Pt/CNT catalyst was also evaluated by CV characterization, which revealed only 37% electrochemical surface area loss for the Pt/CNT catalyst after 168 h oxidation while almost 80% of Pt surface area was lost for the Pt/VXC72R catalyst. Despite the higher electrochemical stability of CNT support claimed in previous durability studies, it should be noted that these studies were always performed under a simulated PEMFC environment while little evidence is shown in literature that CNT support is more oxidation resistant than carbon black under real PEMFC conditions. The longterm stability information of Pt/CNT-based electrocatalysts from a fuel cell under real operating conditions would be very useful for the full evaluation of these electrocatalysts in terms of their electrochemical performance. In this study, a series of in situ accelerated degradation tests (ADT) were performed on the integrated Pt/CNT-based electrode to examine the electrochemical stability of the Pt/CNT-based electrocatalyst. The ADT tests were conducted in the real fuel cell test system by using an asymmetric MEA with a Pt/VXC72R-based anode and a Pt/CNT-based cathode, operating at the conditions described in Section 2.3.6. During the ADT test, the cathode was purged with 300 sccm N2 flow to serve as the working electrode whereas the anode was fed with 50 sccm H2 to work as both the reference and the counter electrodes simultaneously. Different ADT tests, including both static and dynamic potential oxidation processes, were carried out in this study to investigate their effectiveness for evaluating the electrochemical stability of the in situ grown CNTs. In situ CV characterization was used to determine the electrochemical surface area loss after the ADT tests. In order for comparison, 20 wt% Pt/VXC72R 137    (E-TEK) and 40 wt% Pt/VXC72R (Johnson Matthey) commercial catalysts were also investigated for their electrochemical stability via the ADT tests. 5.2 Comparison of Electrochemical Stability of Pt/CNT and Pt/VXC72R-based Electrocatalysts This section mainly concentrates on the electrochemical stability evaluation of the Pt/CNT-based electrocatalyst by means of a series of ADT tests under real fuel cell conditions. The ADT tests were carried out in three different modes: CV cycling between 0.1−1.2 V vs. DHE, potentiostatic oxidation at 1.5 V vs. DHE, and potential cycling between 0.6 and 1.8 V vs. DHE. The electrochemical stability evaluation was performed based on the loss of electrochemical surface area revealed by in situ CV characterization. The stability data of the Pt/CNT-based electrocatalyst are shown in the following subsections in terms of the different ADT methods. 5.2.1 ADT – CV Cycling Oxidation In this ADT test, the Pt/CNT-based electrode was subjected to CV cycling oxidation between 0.1 and 1.2 V vs. DHE at a scan rate of 100 mV s-1. A total number of 500 cycles were performed continuously, corresponding to a continuous oxidation process of about h. The in situ cyclic voltammograms of the Pt/CNT-based electrode were recorded before and after every 100 CV cycles. In this test, a 40 wt% Pt/VXC72R-based electrode (Johnson Matthey) was used as a reference undergoing the same oxidation process. Figure 5.2 shows the CVs of the Pt/CNT and Pt/VXC72R-based electrodes during the CV cycling oxidation. It was observed that the electrochemical surface area 138    of the Pt/CNT-based electrode showed only slight shrinking after the h oxidation process. The calculated ECSA of the Pt/CNT-based electrode was 17.5 m2 gPt-1 before oxidation and it changed to 16.2 m2 gPt-1 after 500 CV cycles, giving a 7.4% loss of Pt surface area. By contrast, it is noticeable that the ECSA of the Pt/VXC72R- 80 (a) Before CV cycling After 100 CV cycles After 200 CV cycles After 300 CV cycles After 400 CV cycles After 500 CV cycles Current density / mA mgPt -1 60 40 20 -20 -40 -60 -80 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Potential / V   (b) 80 Before CV cycling After 100 CV cycles After 200 CV cycles After 300 CV cycles After 400 CV cycles After 500 CV cycles Current density / mA mgPt -1 60 40 20 -20 -40 -60 -80 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Potential / V Fig. 5.2 500-cycle CV cycling oxidation of (a) Pt/CNT-based catalyst and (b) 40 wt% Pt/VXC72R-based catalyst (Johnson Matthey).   139    based electrode exhibited a visible drop after the same oxidation process, which was calculated as 39.2%. It suggests that the in situ grown CNTs with sputter-deposited Pt catalysts are more oxidation resistant under real fuel cell operating conditions, comparing the commercial Pt/VXC72R catalyst. It also suggests that the corrosion reaction of the VXC72R support was notably accelerated by the dynamic CV cycling within 0.1−1.2 V vs. DHE. However, the low oxidation rate of the CNT support revealed that the oxidation potential should be elevated to further accelerate the CNT oxidation thus reducing the ADT test duration. 5.2.2 ADT – Potentiostatic Oxidation The second ADT test was basically a potentiostatic oxidation process in which the Pt/CNT-based electrode experienced a constant potential of 1.5 V vs. DHE for h, instead of a dynamic potential cycling as in the CV cycling oxidation process. In this ADT test, the CV characterization for electrode electrochemical surface area was performed before and after oxidation in a 30 interval. To compare with its electrochemical stability, a 20 wt% Pt/VXC72R-based electrode (E-TEK) was used as a reference in this test. Figure 5.3 shows the oxidation current of the two electrodes during the oxidation process. It can be clearly seen that the Pt/VXC72R-based electrode exhibited a notably higher oxidation current than that of the Pt/CNT-based electrode. It is known that this oxidation current consists of four main components: oxidation of permeated hydrogen from anode, oxide formation on Pt surface, double-layer charging and carbon corrosion [18]. The oxidation of permeated hydrogen and oxide formation on Pt surface usually take pace instantly at potentials higher than 1.0 V, revealing as 140    sharp current pulses at the beginning of oxidation after each CV characterization. The double-layer charging also occurs rapidly and remains unvaried under potential oxidation. Therefore, the oxidation current change during the oxidation process can be mainly attributed to the variation of the carbon corrosion current [19]. As can be seen in Fig. 5.3, at a high oxidation potential of 1.5 V, the carbon corrosion current for the commercial Pt/VXC72R catalyst gradually increased in the first 30 oxidation, and then it slowly decreased in the subsequent 30 oxidation while a small oxidation current was maintained in the following oxidation process. This current behavior implies that the corrosion of the VXC72R support mostly occurred in the first hour of the oxidation process. By contrast, the oxidation current of the CNT support remained very small while little variation was observed during the whole oxidation process. It suggests that the in situ grown CNT support is more oxidation resistant than the VXC72R support under the potentiostatic oxidation at 1.5 V vs. DHE. Pt/CNT catalyst 0.10 Current density / A cm -2 20 wt% Pt/VXC72R catalyst (E-TEK) 0.08 0.06 0.04 0.02 0.00 1800 3600 5400 7200 Time / s Fig. 5.3 Potentiostatic oxidation of Pt/CNT and 20 wt% Pt/VXC72Rbased electrodes at 1.5 V vs. DHE.   141    120 (a) Before potentiostatic oxidation After 30min oxidation at 1.5V After 60min oxidation at 1.5V After 90min oxidation at 1.5V After 120min oxidation at 1.5V Current density / mA mgPt -1 100 80 60 40 20 -20 -40 -60 -80 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Potential / V   800 (b) Current density / mA mgPt -1 600 400 Before potentiostatic oxidation After 30min oxidation at 1.5V After 60min oxidation at 1.5V After 90min oxidation at 1.5V After 120min oxidation at 1.5V 200 -200 -400 -600 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Potential / V Fig. 5.4 CVs of (a) Pt/CNT and (b) 20 wt% Pt/VXC72R-based electrodes before and after potentiostatic oxidation process.   Figure 5.4 presents the cyclic voltammograms of the two electrodes before and after the potentiostatic oxidation. Unlike by the CV cycling oxidation ADT test, a more severe oxidation was observed for the Pt/CNT-based electrode by the 2-h potentiostatic oxidation as shown in Fig. 5.4 (a), probably due to the higher oxidation potential of 1.5 V. According the CVs in Fig. 5.4 (a), the Pt/CNT-based electrode lost 142    approximately 50−60% of its original electrochemical surface area after the first 30 oxidation process. Nevertheless, further ECSA loss was not quite noticeable in the following oxidation process. It suggests that the corrosion of the CNT support was rather mild after the first 30 oxidation given that most of its vulnerable content was oxidized within the first 30 oxidation. For the Pt/VXC72R-based electrode, it can be clearly seen in Fig. 5.4 (b) that its electrochemical surface area was almost completely lost after the first 30 oxidation at 1.5 V, indicating a high oxidation rate as demonstrated in Fig. 5.3. The post-oxidation CVs of the Pt/VXC72R-based electrode revealed a notably enlarged double-layer charging region, suggesting a severely damaged electrode structure where electron transfer was greatly impaired [11]. Moreover, a pair of redox peaks were found in the potential range of 0.5−0.7 V in the post-oxidation CVs of the Pt/VXC72R-based electrode, corresponding to the oxidation and reduction of the quinone (Q) and hydroquinone (HQ) species formed on the oxidized carbon material surface [11, 20, 21]. The oxygen content on a carbon material surface derived from Q and HQ species can be used to evaluate the oxidation resistivity of carbon materials based on their redox peak areas [11]. The larger the redox peak areas of the Q and HQ species, the more easily the carbon material can be oxidized. It can be clearly seen that the Pt/VXC72R-based electrode exhibited rather pronounced Q-HQ redox peak areas by the potentiostatic oxidation whereas those of the Pt/CNT-based electrode were not visible during the oxidation process. It further confirmed that the Pt/CNT-based catalyst is more oxidation resistant than the Pt/VXC72R-based catalyst under a high oxidation potential of 1.5 V in the real fuel cell environment. The results also suggest that the potentiostatic oxidation test at 1.5 V is more efficient than the CV cycling oxidation test that carbon corrosion was visibly accelerated by the high oxidation potential as shown for both two electrodes. 143    5.2.3 ADT – Potential Cycling Oxidation It has been reported that, when compared with oxidation under constant potentials, the dynamic oxidation would accelerate the corrosion process and more closely resemble the working conditions of PEMFCs [22, 23]. Therefore, based on the results of the potentiostatic oxidation at 1.5 V, further electrochemical stability investigation on the Pt/CNT-based catalyst was carried out by means of a potential cycling oxidation test using oxidation potentials of 0.6 V and 1.8 V vs. DHE. During this ADT test, the electrode was dynamically cycled between 0.6 and 1.8 V for a total of 100 oxidation cycles. For each cycle, the electrode was held at 0.6 V for 40 s and at 1.8 V for 20 s, corresponding to a total oxidation time of 100 for the ADT test. In situ CV curves were measured before oxidation and then again after every 10 oxidation cycles to examine the variation of electrode electrochemical surface area. In this study, both the 20 wt% and 40 wt% Pt/VXC72R commercial catalysts were used as reference catalyst. Figure 5.5 (a) illustrates the first 10 oxidation cycles of the three electrodes. It is noticeable that the oxidation current for the Pt/CNT catalyst showed smaller amplitudes and slighter changes than those of the Pt/VXC72R catalysts under the high step potential of 1.8 V. It suggests that the CNT support may provide higher corrosion resistance than the VXC72R supports of the two commercial catalysts. As the corrosion current decreases with the amount of carbon support oxidized, the relative electrochemical activities of the Pt/CNT and Pt/VXC72R-based catalysts can be visualized by normalizing the last current value at the end of each cycle [19]. As shown in Fig. 5.5 (b), the activity loss of the three electrodes mainly occurred in the first 10 oxidation cycles. After 100 oxidation cycles, the commercial Pt/VXC72R 144    catalysts lost more than 70−80% of its original activity, whereas the Pt/CNT catalyst remained at 50% activity, in excellent agreement with the potentiostatic oxidation results. The normalized activity diagram explicitly demonstrates that the Pt/CNTbased catalyst consisting of in situ grown CNTs and sputter-deposited Pt particles has higher electrochemical stability compared with the two commercial Pt/VXC72Rbased catalysts. 0.8 (a) Pt/CNT catalyst 20 wt% Pt/VXC72R catalyst (E-TEK) 40 wt% Pt/VXC72R catalyst (Johnson Matthey) Current density / A cm -2 0.6 0.4 i0 0.2 i1 0.0 120 240 360 480 600 Time / s 1.2 (b) Pt/CNT catalyst 1.0 20 wt% Pt/VXC72R catalyst (E-TEK) Normailzed activity 40 wt% Pt/VXC72R catalyst (Johnson Matthey) 0.8 0.6 0.4 0.2 0.0 20 40 60 80 100 Number of oxidation cycles Fig. 5.5 (a) The first 10 oxidation cycles for Pt/CNT and Pt/VXC72R catalysts between oxidation potentials of 0.6 and 1.8 V. (b) Normalized activity based on the 100 oxidation cycles. 145    Figure 5.6 shows the in situ CVs measured before and after 10, and 100 oxidation cycles for the Pt/CNT and the two Pt/VXC72R-based catalysts, respectively. It was observed that the ECSAs of the two Pt/VXC72R catalysts were both considerably reduced after the first 10 cycles and were completely lost when a total of 100 oxidation cycles were reached (see Fig. 5.6 (a) & (b)). It can be also observed that the Q-HQ redox peaks of the two Pt/VXC72R catalysts were quite prominent after the first 10 cycles while disappeared after 100 oxidation cycles, indicating a complete oxidation of the VXC72R support after the ADT test. By contrast, the Pt/CNT catalyst exhibited a visibly lower loss of active surface area as shown in Fig. 5.6 (c). The carbon corrosion was observed as an increased peak current and capacitive current similar to the results by Kangasniemi et al. [11], which can be attributed to the oxidation of amorphous carbon formed in the CNT layer. Furthermore, the cyclic voltammogram of the Pt/CNT catalyst only showed slight changes from the first 10 oxidation cycles onwards, which agrees well with its normalized activity changes as illustrated in Fig. 5.5 (b). Particularly, it is worth mentioning that this potential cycling test can accelerate carbon corrosion more effectively than the previous two methods, in that it allows Pt catalysts being reduced at 0.6 V to expedite the carbon oxidation reaction. Moreover, the high potential step at 1.8 V can actually represent an extreme condition for PEMFC operation, which was reported very likely to take place as a transient state during PEMFC start-ups [1]. Therefore, this ADT result substantially verified that the CNTs grown on carbon paper can provide higher electrochemical stability for the integrated Pt/CNT electrocatalyst under real PEMFC conditions, in addition to previous studies where the stability investigations were usually conducted in a simulated PEMFC environment. 146    -1 Current density / mA mgPt Before oxidation (a) 800 After 10 oxidation cycles After 100 oxidation cycles 600 400 200 -200 -400 -600 -800 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Potential / V -1 Current density / mA mgPt Before oxidation (b) 600 After 10 oxidation cycles After 100 oxidation cycles 400 200 -200 -400 -600 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Potential / V 147    (c) 150 Before oxidation After 10 oxidation cycles After 100 oxidation cycles Current density / mA mgPt -1 100 50 -50 -100 -150 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Potential / V Fig. 5.6 Cyclic voltammograms of (a) 20 wt% Pt/VXC72R catalyst, (b) 40 wt% Pt/VXC72R catalyst, and (c) Pt/CNT catalyst before and after 10, and 100 oxidation cycles. 5.3 Summary In this chapter, a comprehensive study on the electrochemical stability of the Pt/CNT-based catalyst was conducted based on three different ADT tests. These ADT tests were performed using a Pt/CNT-based electrode under real PEMFC working conditions. A summary of these ADT tests is given in Table 5.1. When compared with the 20 wt% and 40 wt% Pt/VXC72R commercial catalysts, the integrated Pt/CNT catalyst demonstrated notably higher electrochemical stability in all of the ADT tests. It was found that the Pt/CNT catalyst revealed approximately 50% loss of its electrochemical surface area after oxidation cycling at an extremely high step potential of 1.8 V, while those of the commercial Pt/VXC72R catalysts were completely lost. This study has successfully validated the high electrochemical stability of the integrated Pt/CNT electrocatalyst via a series of in situ ADT tests in 148    real PEMFC environment. It has also provided a substantial support to previous durability studies on CNT catalyst support that were based on ADT tests performed in simulated PEMFC environment. 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Soc., 151 , A48 (2004) [8]Y Shao, G Yin, and Y Gao, J Power Sources, 171, 55 8 (2007) [9] Y Shao, G Yin, J Zhang, and Y Gao, Electrochim Acta, 51 , 58 53 (2006) [10] K.  Kinoshita, Carbon: Electrochemical and Physicochemical Properties,  Wiley,  New York (1988) [11] K H Kangasniemi, D A Condit, and T D Jarvi, J Electrochem Soc., 151 , E1 25 (2004) [12] L M Roen, C H Paik, and  T D Jarvi, Electrochem Solid-State... density / mA mgPt -1 100 50 0 -50 -100 - 150 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Potential / V Fig 5. 6 Cyclic voltammograms of (a) 20 wt% Pt/VXC72R catalyst, (b) 40 wt% Pt/VXC72R catalyst, and (c) Pt/CNT catalyst before and after 10, and 100 oxidation cycles 5. 3 Summary In this chapter, a comprehensive study on the electrochemical stability of the Pt/CNT -based catalyst was conducted based on three different... based on ADT tests performed in simulated PEMFC environment ADT Test ADT 1 ADT 2 CV cycling between 0.1 V and 1.2 V at 100 mV s- Potentiostatic step scan at 1 .5 V Gas Feed Anode: 50 sccm H2 Cathode: 300 sccm N2 Anode: 50 sccm H2 Cathode: 300 sccm N2 Total Time 50 0 cycles ~ 3 hours 2 hours 100 cycles − 100 min Post-ADT CV − Every 30 min Every 10 cycles * Relatively high oxidation rate * High oxidation rate;... 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ADT tests were performed using a Pt/CNT -based electrode under real PEMFC working conditions A summary of these ADT tests is given in Table 5. 1 When compared with the 20 wt% and 40 wt% Pt/VXC72R commercial catalysts, the integrated Pt/CNT catalyst demonstrated notably higher electrochemical stability in all of the ADT tests It was found that the Pt/CNT catalyst revealed approximately 50 % loss of its electrochemical... 1.8 V vs DHE During this ADT test, the electrode was dynamically cycled between 0.6 and 1.8 V for a total of 100 oxidation cycles For each cycle, the electrode was held at 0.6 V for 40 s and at 1.8 V for 20 s, corresponding to a total oxidation time of 100 min for the ADT test In situ CV curves were measured before oxidation and then again after every 10 oxidation cycles to examine the variation of electrode... cycling between 0.6 V (40s) and 1.8 V (20s) Anode: 50 sccm H2 Cathode: 300 sccm N2 Table 5. 1 Summary of ADT tests for electrochemical stability evaluation of the Pt/CNT and Pt/VXC72R -based catalysts 149    References [1] R Borup, J Meyers, B Pivovar, et al., Chemical Reviews, 107 (10), 3904 (2007) [2] D P Wilkinson, and J St-Pierre, Handbook of Fuel Cells – Fundamentals, Technology and Applications, . validated the high electrochemical stability of the integrated Pt/CNT electrocatalyst via a series of in situ ADT tests in 0.0 0.2 0.4 0.6 0.8 1.0 1.2 - 150 -100 -50 0 50 100 150 Current. long- term stability information of Pt/CNT -based electrocatalysts from a fuel cell under real operating conditions would be very useful for the full evaluation of these electrocatalysts in terms. mgPt -1 Potential / V Before potentiostatic oxidation After 30min oxidation at 1.5V After 60min oxidation at 1.5V After 90min oxidation at 1.5V After 120min oxidation at 1.5V  Fig. 5. 4 CVs of (a)