Chinese Chemical Letters  2019, Vol. 30 Issue (6): 1186-1189   PDF    
New strategy for reversal tolerant anode for automotive polymer electrolyte fuel cell
Chanho Paka,*, Seung Woo Leea, Chaekyung Baika, Bong Ho Leea, Dae Jong Youb, Eunyoung Youb,*     
a Graduate Program of Energy Technology, School of Integrated Technology, Institute of Integrated Technology, Gwangju Institute of Science and Technology, Gwangju 61005, Republic of Korea;
b Fuel Cell Engineering Core Technology Team, Division of Electric Powertrain, Hyundai Mobis, Co., Ltd., Yongin, Gyeonggi-do 16891, Republic of Korea
Abstract: Since the commercialization of the fuel cell electric vehicles (FCEVs), the polymer electrolyte membrane fuel cell system has been actively improved as a powertrain for ultimate environment-friendly vehicle. During the FCEV operation, various transient conditions such as start-up/shut-down and fuel starvation occur irregularly, which deteriorates the durability of the membrane electrode assembly. In particular, when fuel starvation occurs, the carbon support in the anode is oxidized within few minutes, thus the mitigation of this phenomenon is essential for securing the durability. This short review introduces the concept of reversal tolerant anode (RTA), which is a mitigation method using an oxygen evolution catalyst and reviews some previous reports. In addition, new approach for RTA suggested by authors recently, which is the replacement of Pt anode catalyst by multifunctional IrRu alloy catalyst that simultaneously exhibits the activities for the hydrogen oxidation reaction and the oxygen evolution reaction is introduced.
Keywords: Multifunctional IrRu alloy catalyst     Reversal tolerant anode     Hydrogen oxidation reaction catalyst     Oxygen evolution reaction catalyst     Cell reversal durability     Polymer electrolyte membrane fuel cell    
1. Introduction

Inrecentdecades, to decrease the carbon dioxide emission, which contributes greatly to global warming, there is a growing interest in environmentally friendly vehicles(greencar)such as hybrid vehicles (HVs), batteryelectric vehicles (BEVs) and hydrogen fuel cell electric vehicles (FCEVs). Among them, FCEV is considered as an ultimate green car because it can be operated in similar manner to the present internal combustion engine cars [1-3]. Since Hyundai Motor Company (HMC) started selling the world's first mass-produced Tucson ix35 FCEV in Feb. 2013, Toyota Motor Corporation and Honda Motor Company has launched the Mirai in Dec. 2014 and the Clarity FCEV in Mar. 2016, respectively. In 2018, HMC introduced the second generation FCEV, Nexo in March and Mercedes Benz started to join the competition for commercial FCEV market by launching the GLCF cell in the late 2018 [4, 5]. Despite the substantial improvement of polymer electrolyte membrane fuel cell (PEMFC) systems as a powertrain for the FCEV, there are still three major challenges such as cost, performance and durability [4, 6]. Inparticular, the degradation and durability of membrane electrode assemblies (MEAs) in the stack under the transient conditions such as start-up/shut-down (SU/SD) [7, 8], potential cycling [9, 10], high potential loading [11, 12], fuel starvation [13-15], etc., during the FCEV operation are the main subjects of many researches to secure the life time of fuel cell system.

Of these conditions, hydrogen fuel starvation caused by hasty load change and uncommon fuel supply malfunction [16, 17] resulted in cell reversal (CR) phenomena and the detrimental degradation of anode of the MEA. When the hydrogen feed is ceased to the anode, the cell terminal voltage is steeply decreased in few seconds due to the increase the anode voltage over than cathode voltage as shown in Fig. 1. CR occurs when the anode voltage become more positive than the cathode voltage.

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Fig. 1. The change of the anode and cathode voltage during the fuel starvation condition. Reproduced with permission [16]. Copyright 2004, Elsevier.

The anode voltage rapidly is increased to around 1.5 V, which implies that the anode potential is raised until the water electrolysis reaction, i.e., oxygen evolution reaction (OER) is initiated to provide the protons and electrons instead of hydrogen oxidation reaction (HOR) [16-18]. The high voltage in the anode by CR could accelerate kinetically the carbon oxidation reaction (COR) in parallel with OER to provide the protons and electrons and deteriorate the anode structure in a short time, by which results in the irreversible fatal degradation of the MEA [16-18].

To mitigate this damage, there are two main strategies such as system control and material-based approaches [13, 17-30]. System control can alleviate the degradation of MEA by monitoring the operation parameters and adjusting the gas flow, load change and gas humidity and so on [19, 20]. However, most of control strategies for avoidance of fuel starvation are focused on the air (oxygen) starvation [19]. In addition, continuous monitoring of various parameters by sensors is cost-ineffective for the FCEVs because cell reversal in the anode by fuel starvation is transient condition.

On the other hand, materials-based approaches are the use of graphitized and modified carbon supports in the anode [21-23], metal oxide and metal carbide supports [24, 25], and OER catalysts [17, 18, 26-28]. While the durability of modified carbon supports is enhanced, carbon oxidation under CR is inevitable by simple modification of carbon surface [21]. In contrast, metal oxide and metal carbide supports are known for their good corrosion resistance. However, these supports have low electric conductivity and low specific surface areas, which are not enough to apply in the practical MEA. In the end, the incorporation of OER catalyst in the Pt/C anode has been developing as a simple method to obtain a reversal tolerant anode (RTA).

In this short review, the recent studies on RTA using OER catalyst are discussed in the next section. Then, the new method for the RTA using multifunctional catalyst having concomitant activities for HOR and OER is introduced.

2. RTA with OER catalyst

Initial studies on the introduction of OER catalyst to the anode to elongate the lifetime of MEA under the CR condition was initiated jointly by the researchers of Johnson Matthey and Ballard Power Systems [17, 31-34]. These earlier works were well presented and reviewed by researchers who conducted the investigation in the literature [28, 34]. They compared the difference of OER catalysts on the CR tolerance using RuO2, RuO2-TiO2 and RuO2-IrO2 supported on Shawinigan carbon black, which displayed a poorly crystalline RuO2 (rutile) phase [28]. The time to reach -2 V under CR is elongated with the order of tolerance of the catalyst, RuO2-IrO2> RuO2-TiO2> RuO2. Combining with new mixed oxide OER catalyst such as RuIrO2, the anode using high loading PtRu supported on Shawinigan under CR condition with recovery periods showed almost 170 times longer durability compared to that of the plain low-loading PtRu catalyst [34].

After the commercialization of FCEVs in 2013, the durability of MEA under the transient condition, in particular the fuel starvation became critical to ensure the lifetime of FCEVs [18, 26, 27]. The vital role of the OER catalyst in the anode for the RTA is demonstrated systematically by monitoring the amount of evolved CO2 and O2 under the CR condition using on-line mass spectroscopy [27]. Prof. Kim in the Yonsei Univ. of Korea reported that the total volume of CO2 and O2 under 0.2 A/cm2 and no hydrogen condition by concurrent COR and OER is maintained in the range of 21-22 cm3, which is close to the theoretical value of 23 cm3 [27]. However, the composition of CO2 and O2 of evolved gas is changed significantly in the presence of IrO2 catalyst in the anode. Even adding only 1 wt% IrO2 in the anode, the amount of evolved O2 is increased to 19 cm3 from 1.8 cm3 in the Pt/C only anode, which corresponds to 91.7% of the total exhaust gas. The ratio of O2 is increased to 98.2% and 98.6% with increasing the IrO2 amount to 5 wt% and 10 wt%, respectively, which provides that the amount of carbon loss due to COR is estimated as less than 10%. Thus, it is concluded that between two possible reactions under CR, the more fatal COR can be suppressed by using the OER catalyst [27].

The researchers of the HMC and Carnegie Mellon Univ. investigated the IrO2 as an OER catalyst for RTA under the various conditions using by the electrochemical diagnostics and nano Xray computed tomography (CT) [18, 26]. They showed that sustainable reversal time of the RTA is improved by increasing the amount of IrO2 in the anode up to 50 wt% as shown in Fig. 2. The first plateau from RTA with 5 wt% and 50 wt% is observed in the ranges of -0.8 V to -1.1 V. The length of the plateau is significantly elongated by addition of IrO2 up to 50 wt%. However, the voltage of the anode with 50 wt% IrO2 went down eventually to the -2 V after the certain time period. The MEA performance after the first cell reversal test is down to 29% of original performance [26]. In addition, the distribution of OER catalysts before and after CR experiment is observed similarly in the case of 5 wt% and 50 wt% added RTA [18, 26].

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Fig. 2. Voltage reversal behavior at 0.2 A/cm2 with 0, 5 wt% and 50 wt% RTA as a function of reversal time during the first reversal time. Reproduced with permission [26]. Copyright 2018, Elsevier.

Based on the findings from the electrochemical impedance spectroscopy (EIS), cyclic voltammogram and Nano-CT, the authors proposed the degradation mechanism of MEA under CR condition for automotive application, which follows the similar process regardless of the presence of OER catalyst in the anode. The COR occurs nearby the polymer membrane due to the Ohmic limited reaction. The disappearance of carbon supports between the membrane and catalyst layer results in the isolation of Pt electronically and the formation of highly resistant residual ionomer binders, causing structural collapse of the anode at last [26], which implies that the combination of non-corrosive support for the anode catalyst with OER catalyst for RTA is essential as suggested in the literature [17, 22-28].

3. New strategy for advanced RTA

In 2017, Zhu et al. [29] suggested alternative additive to the Pt/C anode to improve the CR durability as a new RTA approach, which is the protic ionic liquid (PIL) such as N, N-diethylmethyl ammonium trifluoromethanesulfonate ([dema][TfO]) and N, N-diethylmethy-lammonium nonafluorobutanesulfonate ([dema][NfO]) [35]. Even though the fuel starvation condition adapted in this study is slightly a mild condition because they disconnected thee lectricload for 30s when the cell potential reached -1.4V under CR condition, the decrease of the electrochemical surface area (ECSA) of Pt is mitigated by the addition of PILs in the anode after 50 CR cycles. To elucidate the improvement of durability under CR condition, they investigated the catalytic activity for OER of PIL-Pt/C catalyst by half-cell configuration.Thevoltage at 10 mA/cm2, which is figureof merit for OER activity, is estimated as 1.87V for Pt/C, 1.70V for [dema][TfO]-Pt/Cand 1.81 V for[dema][NfO]-Pt/C.Itwassuggested that the enhancement of OER activity by the introduction of PIL in the Pt/C catalyst is attributed to the high solubility and high diffusivity of oxygen in PILs, which induced the improved OER kinetics as reported in the literature [36-38].

In 2018, a new approach for the RTA using IrRu alloy catalyst is suggested by authors, which is the total replacement of Pt anode catalyst by multifunctional catalyst having activities for HOR and OER [30, 39]. IrRu alloy composition has been previously investigated as OER catalyst for extending the stability of Pt-coated nanostructured thin film catalyst from 3 M company under CR condition [40, 41]. However, HOR activity of IrRu alloy was not taken into account. In addition, Gasteiger group tried to replace the Pt anode catalyst using selective hydrogen oxidation Ir/C catalyst for reducing the SU/SD degradation of the cathode. They proved the improvement of durability under SU/SD condition using Ir/C anode, while the MEA performance is not the same to the Pt anode due to the lower HOR activity of the Ir/C than Pt/C [42].

After initial finding [43] that IrRu alloy has a HOR activity, the HOR activity is investigated according to the IrRu alloy composition as shown in Fig. 3. Relatively, IrRu4 composition displayed the best HOR activity among the alloy catalysts, by which implies that the synergistic enhancement byalloyingof Ir and Rufor HOR activity is observed. The HOR activity of IrRu4/C is improved by 1.2 and ca. 2 times compared to the Ir/C and Ru/C catalysts, respectively. Thus, it was concluded this IrRu4/C catalyst is very suitable for RTA replacing the Pt anode of the automotive PEMFC because it has activities towards HOR and OER simultaneously [30, 39].

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Fig. 3. Relative HOR activity of Ir/22KB, Ru/22KB and IrRu alloy supported on 22KB catalysts [39].

Carbon supported IrRu4 alloy catalyst is prepared by simple impregnation using ethanol solution of metal precursors, which displayed the hexagonal close-packed Ru structure with 4.7 nm average crystalline size [30]. Comparing the ik values (5.1 and 3.9 mA/cm2, respectively) of IrRu4/C and Pt/C for HOR estimated using the rotating disk electrode method [30], the former catalyst showed 1.3 times higher activity than the latter one as previously reported [43]. However, the long-term durability of the two catalysts under HOR condition has not been compared upto date, which is the topic to address for further practical application of IrRu catalyst. The onset potential of OER of IrRu4/C and Pt/C is 1.3–1.4 V and 1.6 V vs. normal hydrogen electrode, respectively, which implies that IrRu catalyst is much superior towardsOER than the Pt catalyst.

As shown in Fig. 4, single cell performance using two catalysts as the anode is almost identical at 90 ℃, which provides that the possibility of total replacement of Pt anode catalyst is very feasible by using the multifunctional IrRu4/C catalyst for HOR and OER. As listed in Table 1 of the reference [30], the RTA durability using the IrRu4/C as the anode is significantly extended by 120 times longer from minute-scale (~1.5 min) of Pt/C catalyst to hour-scale (~3h), which implies that the IrRu4/C has a promising potential to replace the Pt anode catalyst.

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Fig. 4. Polarization curve of commercial Pt/C and IrRu4/C anode MEAs at 90 ℃, 50% relative humidity and 150kPa [30].

In order to apply the developed catalyst to FCEV, it is necessary to compare the prices of Pt/C and IrRu4/C catalysts. Because Ir and Ru are precious metals like Pt, the price hike may appear if mass application is determined as mentioned earlier [44]. Actually, the price of Ir is very rely on the market situation as shown in Fig. S1 (Supporting information). Based on the data provided by Johnson Matthey [45], the price of Ir has been cheaper than Pt on average forlastdecade (Fig. S1b. The steady average price of the last30 days is 26.1 $/g for Pt and 47.6 $/g for Ir. Basedon these value, theprice of 1g Pt and IrRu4, is 26.1 $/g and 21.2 $/g, respectively, due to the Ru (8.7 $/g) occupy 80% in the IrRu4, which implies that developed catalyst has slight price competitiveness.

4. Summary and outlook

Through this brief review, the strategies to overcome the consequences of CR to the anode that occurs occasionally during the operation of an automotive fuel cell are summarized based on recent studies. Introducing OER catalysts rather than the system control such as IrO2, RuO2, RuO2-IrO2, etc., or PIL in the anode are mainly investigated to supplyelectrons and protons through water electrolysis rather than carbon oxidation under CR caused by fuel starvation.

Recent researches on IrRu alloy catalysts that have concurrent HORand OER activities are also presented to show the possibility of replacement of Pt anode catalyst. Combining non-corrosive supports which are non-corrosive under high potential conditions such as the CR with multifunctional catalytic compositions having both HOR and OER activity should be considered in order to develop the advanced RTA meeting the stringent durability targets of FCEVs.

Acknowledgments

This work was supported by Hyundai Mobis (No. GI06280) and the GIST Research Institute (GRI) grant funded by the Gwangju Institute of Science and Technology in 2019.

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2019.02.020.

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