RESEARCH ARTICLE

Ti4O7 supported IrOx for anode reversal tolerance in proton exchange membrane fuel cell

  • Yonghuan LI 1,2 ,
  • Wei SONG , 3 ,
  • Guang JIANG 1,2 ,
  • Yue YANG 1,2 ,
  • Hongmei YU 3 ,
  • Zhigang SHAO , 3 ,
  • Fangwei DUAN 4 ,
  • Yingxuan YANG 4
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  • 1. Fuel Cell System and Engineering Laboratory, Key Laboratory of Fuel Cells and Hybrid Power Sources, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
  • 2. University of the Chinese Academy of Sciences, Beijing 100039, China
  • 3. Fuel Cell System and Engineering Laboratory, Key Laboratory of Fuel Cells and Hybrid Power Sources, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
  • 4. State Grid Liaoning Electric Power Research Institute, Shenyang 110055, China

Received date: 16 Jul 2021

Accepted date: 27 Sep 2021

Published date: 15 Oct 2022

Copyright

2021 Higher Education Press

Abstract

Fuel starvation can occur and cause damage to the cell when proton exchange membrane fuel cells operate under complex working conditions. In this case, carbon corrosion occurs. Oxygen evolution reaction (OER) catalysts can alleviate carbon corrosion by introducing water electrolysis at a lower potential at the anode in fuel shortage. The mixture of hydrogen oxidation reaction (HOR) and unsupported OER catalyst not only reduces the electrolysis efficiency, but also influences the initial performance of the fuel cell. Herein, Ti4O7 supported IrOx is synthesized by utilizing the surfactant-assistant method and serves as reversal tolerant components in the anode. When the cell reverse time is less than 100 min, the cell voltage of the MEA added with IrOx/Ti4O7 has almost no attenuation. Besides, the MEA has a longer reversal time (530 min) than IrOx (75 min), showing an excellent reversal tolerance. The results of electron microscopy spectroscopy show that IrOx particles have a good dispersity on the surface of Ti4O7 and IrOx/Ti4O7 particles are uniformly dispersed on the anode catalytic layer. After the stability test, the Ti4O7 support has little decay, demonstrating a high electrochemical stability. IrOx/Ti4O7 with a high dispersity has a great potential to the application on the reversal tolerance anode of the fuel cell.

Cite this article

Yonghuan LI , Wei SONG , Guang JIANG , Yue YANG , Hongmei YU , Zhigang SHAO , Fangwei DUAN , Yingxuan YANG . Ti4O7 supported IrOx for anode reversal tolerance in proton exchange membrane fuel cell[J]. Frontiers in Energy, 2022 , 16(5) : 852 -861 . DOI: 10.1007/s11708-021-0811-7

1 Introduction

Proton exchange membrane fuel cell (PEMFC) has attracted wide attention in recent years due to its advantage of high conversion efficiency, high power density, and environmentally friendliness [1]. However, its cost, performance, and durability are the three major challenges in the process of large-scale commercialization [2,3].
The supply of hydrogen may be delayed or insufficient when the fuel cell is operating under complicated conditions, such as start-up [4], rapid load change [5], and when flooded [6,7]. In particular, the anode potential increases rapidly to about 1.5 V or higher when fuel starvation happens in a cell of the stack. In this case, other reactions like water electrolysis and carbon corrosion will occur to provide protons and electrons for the current output [8,9]. The reaction formulas can be expressed as [6,10,11]
2H 2O O2+ 4H ++ 4e,
E0 = 1. 229 V ( vs. SHE) ;
C + 2H 2O CO2+ 4H ++ 4e,
E0 = 0.207 V (vs. SHE);
C + H2O CO + 2H++ 2e ,
E0= 0. 518 V ( vs. SHE) .
It is well known that the reaction rate of carbon corrosion is much slower than that of water electrolysis [1,12,13]. Therefore, water electrolysis usually takes place first during cell reversal process. Unfortunately, the hydrogen oxidation reaction (HOR) catalyst like Pt/C will lose its function after a period of time due to the poisoning caused by carbon oxidation products [1416]. Subsequently, an anode potential will rise rapidly attributing to the dominance of carbon corrosion [17]. The decay of carbon support would cause the aggregation or shedding of Pt particles, thus seriously damaging the anode catalytic layer (CL) [18]. The heat generated by carbon corrosion will lead to the formation of pinholes in proton exchange membrane [1]. In addition, the fuel and oxidant will mix in the reaction chamber, resulting in a catastrophic failure [19]. System control strategies are proposed in order to monitor the health of the stack system and find abnormalities in time, such as detecting the emitted CO2 [8,9] and using it as an indicator to diagnose the abnormality of the system, detecting the localized anode potential [20,21], and monitoring the change in the stoichiometric ratio of the reactant gas [22], etc. However, these strategies require the support of additional systems so that the cost and complexity of the stack system will be increased. Reversal tolerance anode (RTA) is a reliable method based on materials to replace system strategies. The anode catalyst layer is usually doped with OER catalysts to extending the reaction time of water electrolysis in cell reverse, so as to alleviate the rate of carbon corrosion [23,24].
In recent years, a variety of OER catalysts have been used in the RTA, such as IrO2 [1], RuO2 [6], IrRu alloy [25,26], etc. Of these OER catalysts, IrO2 has the best stability, and its activity is second only to RuO2 [2729]. Therefore, IrO2 can decompose water for a long time at a relatively low potential. However, it is usually added directly into the anode in the form of particulate matter in most literature. In this case, IrO2 may agglomerate in the catalytic layer so that it cannot protect all the carbon support [30]. Based on this situation, Roh et al. [30] deposited monodisperse IrO2 on commercial Pt/C, and effectively improved the dispersion of IrO2 in the catalytic layer. However, the preparation steps are cumbersome. Jang et al. [31] added IrO2/C to the anode by improving the dispersion of IrO2 on carbon, which showed less cell performance degradation than that of adding IrO2 after frequent cell reverse. However, the additional carbon introduction would increase the possibility of corrosion. Consequently, it is necessary to improve the dispersion of IrO2 in the catalytic layer and develop antioxidant catalyst supports at the same time. Ti4O7 has been widely regarded as a corrosion-resistant and electrically conductive material [32]. There are some reports utilizing Ti4O7 as a support material. For example, Ioroi and Yasuda [33] used Ti4O7 as the support of Pt, which greatly improved the reversal tolerance of the cell under the condition of fuel starvation. Won et al. [34] used Ti4O7 supported PtIr alloy as a bifunctional catalyst, showing excellent oxygen reduction reaction (ORR) and OER performance compared with single metal. However, there is no report on using Ti4O7-supported iridium oxide as the reversal tolerant components in the presence of Pt/C.
In this study, IrOx/Ti4O7 was synthesized by utilizing the surfactant-assistant method. For comparison, IrOx without support was prepared in the same way. The electrochemical performance of OER catalysts was characterized by half-cell and full cell tests. The dispersity of the catalysts which contribute to the electrolysis performance was analyzed by transmission electron microscopy (TEM), scanning electron microscope (SEM), and energy dispersive spectroscopy (EDS). In addition, the electrochemical stability of Ti4O7 in the three electrode system was studied. The results show that IrOx particles have a good dispersity on the surface of Ti4O7 and IrOx/Ti4O7 particles are uniformly dispersed on the anode catalytic layer, which make the reverse tolerance of MEA with IrOx/Ti4O7 much better than MEA with IrOx.

2 Experiment

2.1 Preparation of IrOx and IrOx/Ti4O7

The IrOx/Ti4O7 was synthesized by the surfactant-assistant method [35]. Typically, 328 mg of Ti4O7 (Changsha Purong Chemical Engineering Inc.) was ultrasonically dispersed in 60.8 mL of distilled water (18.2 mΩ). Then, 153 mg of Pluronic® F127 (Sigma-Aldrich) was dissolved in the mixture and stirred for 30 min. After adding 626.3 mg of H2IrCl6·xH2O (Tianjin Jinbolan Fine Chemical Co., Ltd.), the mixture was vigorously stirring at 60°C for 5 h. After that, the sample was added with 12.2 mL of NaBH4 solution (1 mol/L) at the same concentration as that of F127. Ultimately, the mixture was stirring overnight at 60°C for 12 h. After the reaction, the product was washed several times with anhydrous ethanol and dried at 60°C. The mass content of IrOx in IrOx/Ti4O7 is 32% (mass fraction). For comparison, the IrOx without support was prepared in the same way.

2.2 Characterizations

The morphology of the synthesized catalysts was characterized by transmission electron microscopy (TEM, JEM 2100) at 120 kV. The cross-section morphology was observed by field emission scanning electron microscopy (SEM, JSM-7800F). Energy dispersive X-ray spectroscopy (EDS) mapping was conducted by the detectors on SEM at an accelerated voltage of 20 kV to observe the distributions of Pt, Ir, and Ti on the electrode surface. The crystal structure of the catalysts was characterized by X-ray diffraction (XRD, PANalytical Empyrean) with a Cu-Kα tube. X-ray photoelectron spectroscopy (XPS, ESCALABXi) with an Mg anode was utilized to reveal the chemical state of the prepared IrOx. The catalyst loading in terms of Ir content was measured via an inductively coupled plasma optical emission spectrometer (ICP-OES, 7300DV).

2.3 Electrochemical measurements

Electrochemical measurements were performed in the N2-saturated 0.5 mol/L H2SO4 solution at 30°C with a rotating disk electrode (RDE) system in the traditional three electrode system, which consisted of a glassy carbon electrode (GCE, 0.1256 cm2), a saturated calomel electrode (SCE), and a Pt tablet as the working, counter, and reference electrode, respectively. All potentials used in this study were calibrated by reversible hydrogen electrode (RHE). The catalyst ink was prepared by mixing 5 mg of powder, 1 mL of isopropyl alcohol (Aladdin), and 50 μL of Nafion solution (5%, Dupont). Then, 10 μL of the ink was deposited onto the GCE, drying naturally in air.
The stability of Ti4O7 was tested by potentiostatic method at 1.5 V for 5 h in a CHI 760 E electrochemical system. The linear sweep voltammetry (LSV) was carried out from 0 V to 1.4 V at 2 mV/s with a rotation rate of 1600 r/min, using the Gamry Interface 1000 E. Before the test, the catalyst was activated by cyclic voltammetry (CV) from 0 V to 1.2 V at 100 mV/s for 30 cycles.

2.4 MEA fabrication and single cell test

First, catalyst coated membrane (CCM) was prepared [36], and then MEA was prepared by hot pressing. Typically, the catalyst ink of cathode was prepared by mixing Pt/C (70% (mass fraction), Johnson Matthey) with isopropyl alcohol, deionized water, and Nafion solution. The specific composition of the anode catalyst is listed in Table 1. The Pt loadings of the cathode and anode were 0.4 and 0.2 mg/cm2, respectively. The mass ratio of the ionomer/carbon (I/C ratio) was 0.7. The catalyst inks were directly sprayed on both sides of the Nafion 212 membrane. Finally, the MEA was fabricated by hot pressing the CCM with two gas diffusion layers (GDL) at 140°C, 0.1 MPa for 2 min. The effective area was 5 cm2.
Tab.1 Composition of catalysts in the anode
Catalyst/(mg·cm–2) MEA-1 MEA-2 MEA-3
Pt/C 0.2 0.2 0.2
IrOx 0 0.1 0
IrOx/Ti4O7 0 0 0.3
The single cell test was conducted on a computer-controlled home-made test bench. The polarization curve (I-V) and the electrochemical impedance spectroscopy (EIS) were measured at 75°C, with a back pressure of 0.05 MPa (gauge pressure). The flow rates of fully humidified H2 (anode) and air (cathode) were 120 and 800 mL/min, respectively. Before testing, all the single cells were activated for 4 h. The I-V curve was measured by KFM2030 (Kikusui, Japan). The EIS was measured by Gamry Interface 5000 E under the same condition of the I-V curve with the frequency ranging from 0.1 Hz to 10 kHz. The current density was kept at 0.2 A/cm2.
The cell reversal test was conducted without back pressure at 75°C and the relative humidity (RH) of anode and cathode was both 100%. First, H2 and air were supplied to the anode and cathode for about 30 min, respectively. Then, the H2 was suddenly replaced by N2. After that, a constant current of 0.2 A/cm2 was supplied to the cell until the cell voltage reaches –2 V [6,11,27]. The schematic diagram of the voltage reversal test station is shown in Fig. 1. The I-V curve and EIS after the cell reversal test were performed. The reversal time is defined as the time that the voltage drops from 0 to –2 V.
Fig.1 Schematic diagram of voltage reversal test station.

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3 Results and discussion

3.1 Physical characterizations of the catalysts

Physical characterizations of the catalysts were conducted by XRD, XPS, and TEM. The crystal structure of the catalysts was performed by XRD. Figure 2 depicts the XRD pattern of the synthesized IrOx and IrOx/Ti4O7. The two peaks in the spectrum of IrOx are attributable to the most intense peak of the hollandite or rutile phases of IrO2 [37]. It can be found that there is no obvious peak of Ir and IrO2 in the spectrum of IrOx/Ti4O7 compared to the standard JCPDS cards (JCPDS No. 15-0780; JCPDS No. 87-0715). These results indicate that the IrOx is amorphous [38,39].
Fig.2 XRD patterns of synthesized IrOx, IrOx/Ti4O7 and initial Ti4O7 with the standard card for IrO2 and Ti4O7.

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Fig.3 XPS spectrum of IrOx/Ti4O7.

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Fig.4 TEM images and particle sizes.

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XPS was performed to investigate the electronic state of the Ir element. The XPS spectrum of Ir 4f for IrOx and IrOx/Ti4O7, as well as the full XPS spectrum of IrOx/Ti4O7 is presented in Fig. S1 in Electronic Supplementary Material (ESM). Figures 3(a) and 3(b) manifest the fitted Ir 4f and O 1s spectra, respectively. Figure 3(a) shows the high resolution Ir 4f spectrum of IrOx/Ti4O7. The two peaks correspond to the Ir 4f5/2 and Ir 4f7/2, respectively. Then, each peak was deconvoluted into two peaks for the analysis of Ir element on the IrOx/Ti4O7 in detail. The most intense doublet at 62.38 and 65.28 eV can be attributed to Ir 4f7/2 and Ir 4f5/2, corresponding to the Ir3+. The weaker doublet at 63.38 and 66.48 eV can be assigned to the Ir4+ [4042]. According to the area integral results of each fitting curve, the content of Ir in different valence states is estimated. Ir3+ and Ir4+ account for 47% and 53% (atomic percentage), respectively. Therefore, the synthesized IrOx/Ti4O7 is likely to provide a sufficient OER activity with a perfect durability due to the increased Ir4+ states [43]. Figure 3(b) exhibits the peaks of different kinds of oxygen in the XPS spectrum of O 1s. In the catalyst, about 91% (atomic percentage) of the oxygen exists in the form of metal oxide while the rest is surface species.
Figure 4 shows the TEM images and particle sizes of Ti4O7 and IrOx/Ti4O7. Figure 4(a) is the image of Ti4O7 which has irregular shapes, such as particles and blocks. The particle size of Ti4O7 ranges from 80 nm to 150 nm. As shown in Fig. 4(c), IrOx nanoparticles are well dispersed on the Ti4O7 (the red mark in the picture). The perfect dispersion is beneficial to improve the mass specific activity of the catalyst and the electrolysis efficiency compared to the form of aggregation. The particle size and distribution of IrOx on the Ti4O7 are displayed in Fig. 4(d). The particle size of IrOx ranges from 1 to 2 nm and the average particle size is 1.53 nm. Such a small particle size is due to the effect of Pluronic®F127, which plays a huge role as a template and stabilizer. Specifically, it can trigger seeds formation and control the size, shape and dispersion of IrOx amorphous nanoparticles [35].

3.2 Electrochemical characterizations of the catalysts

The electrochemical stability of Ti4O7 was tested by utilizing the potentiostatic method at 1.5 V for 5 h. Figure S2 in ESM shows the CV curves of Ti4O7 before and after the stability test. As seen in Fig. S2 in ESM, there is no obvious redox peak in the initial CV curve, nor in that after the test. This shows that Ti4O7 has not been oxidized into TiO2 after the potentiostatic test. Ti4O7 still maintains its original structure and conductivity. This means that the Ti4O7 has an excellent electrochemical stability at 1.5 V, which is expected to be seen in the reverse of fuel cell.
The electrochemical activity of synthesized catalysts was measured by LSV. The curves mainly reveal the performance of IrOx and IrOx/Ti4O7 between 1.4 V and 1.65 V (versus RHE). Figure 5(a) shows the OER activity of the catalysts. The overpotential of IrOx is 303 mV (@ 10 mA/cm2) while that of IrOx/Ti4O7 is only 293 mV. This means that IrOx/Ti4O7 can perform oxygen evolution reaction at a relatively low potential. As shown in Fig. 5(b), the mass specific activity (MA) of IrOx at 1.6 V (versus RHE) is 90 mA/mgIr. The MA of IrOx/Ti4O7 is 273 mA/mgIr, which is three times that of IrOx. This can be attributed to the dispersion effect of IrOx on Ti4O7, which leads to the exposure of more activity sites. The existence of Ti4O7 suppresses the aggregation of IrOx to a certain extent. Besides, Ti4O7 reduces the amount of precious metal iridium under the condition of the same performance.
Fig.5 OER activity of the catalysts.

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3.3 Performance of single cell

The result of the half-cell test indicates that the IrOx/Ti4O7 has an excellent oxygen evolution activity and the Ti4O7 has a good chemical stability under an acidic condition. After this, the initial electrochemical performances of the two MEAs were studied, in which 0.1 mg/cm2 IrOx (MEA-2) and IrOx/Ti4O7 (MEA-3) were added respectively. For comparison, the cell performance of the traditional Pt/C anode (MEA-1) is also given in Fig. 6, from which it can be observed that the initial performances of the anodes with the two self-made OER catalysts and that of the traditional Pt/C without OER catalyst are different from each other. The I-V curves are shown in Fig. 6(a) and the performance parameters are presented in Fig. 6(b). At a current density of 1000 mA/cm2, MEA-1 obtains the highest cell voltage (0.627 V) of the three MEAs, followed by MEA-2 (0.625 V). The cell voltage of MEA-3 is 0.618 V. Besides, the maximum power density of MEA-1, MEA-2, and MEA-3 are 743.4, 725.6, and 747.5 mW/cm2, respectively. There is no obvious difference between the three MEAs at a current density range of less than 1200 mA/cm2. However, the difference in cell voltage is obviously above 1200 mA/cm2. The mass transfer problem of MEA-2 is most serious. This can be attributed to the addition of IrOx into the anode catalyst layer of MEA-2. The resistance of electron transmission in the catalytic layer will be increased because of the poor conductivity of oxides. However, the performance of MEA-3 in the high current density region is normally worse than that of MEA-2 because it has a thicker catalytic layer. As shown in Figs. S3(a) and S3(b) in ESM, the anode catalytic layer of MEA-3 is 2.3 μm, which is 1.6 times that of MEA-2 (1.4 μm). On the contrary, MEA-3 has a better performance than MEA-2. This can be attributed to the excellent electrical conductivity of Ti4O7 [44] so that the adverse effect of thicker catalytic layer will be reduced. To sum up, there is no doubt that the introduction of OER catalysts into the anode catalyst layer will cause the initial performance of the cell to slightly decrease in the high current density region whether the OER catalyst is IrOx or IrOx/Ti4O7. The decrease of IrOx/Ti4O7 is smaller than that of IrOx because of the better electrical conductivity of Ti4O7 and the excellent dispersion of IrOx.
Fig.6 Performance of the three MEAs.

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3.4 Cell reversal tolerance of MEA

The cell reversal tests of the MEAs with OER catalysts were conducted. The voltage-time (V-T) curves during the cell reverse, the I-V curves, and EIS are presented in Fig. 7. Figure 7(a) is the V-T curve of MEA-2 and MEA-3 during the cell reverse. After H2 was switched to N2, the residual H2 in the pipeline and inside the cell was quickly exhausted and the cell voltage jumped below 0 V at the same time. As observed in Fig. 7(a), MEA-2 and MEA-3 have the same time of cell reverse (about 75 min). The voltage of MEA-2 during water electrolysis drops obviously while that of MEA-3 is relatively stable. This means that the stability of the water electrolysis reaction for MEA-3 is better than that of MEA-2. The I-V curves of MEA-2 and MEA-3 are tested after cell reverse, and the results are shown in Fig. 7(b). The voltage of MEA-3 has almost no attenuation after cell reverse. However, the voltage of MEA-2 at 1000 mA/cm2 is attenuated by 62.5%. Because the stability of the OER for the IrOx is worse than that of the IrOx/Ti4O7, the efficiency of OER is gradually reduced, resulting in a more serious carbon corrosion. This can also be seen from EIS (Fig. 7(c)), the initial ohmic resistances (RΩ, which is determined by the intercept of the real axis) at a high frequency of MEA-2 and MEA-3 are 5.8 and 3.5 mΩ·cm2, respectively. After undergoing the cell reverse, the RΩ of MEA-2 increases to 15.5 mΩ·cm2, while that of MEA-3 is 4.4 mΩ·cm2. Besides, the growth rate of the charge transfer resistances (Rct) of MEA-2 and MEA-3 are 56.0% and 1.4%, respectively. Obviously, the anode catalytic layer of MEA-2 has suffered from a severe carbon corrosion, which leads to the destruction of catalytic layer structure and affects the charge transfer. To further prove the excellent reverse tolerance of MEA-3, the reverse tolerance performance of MEA-1 was tested, as shown in Figs. 7(d) and 7(e). Since the anode catalyst of MEA-1 is only Pt/C and its OER activity is very low, the reverse time is only 22 s. The I-V curves of the MEA before and after the cell reverse test are given in Fig. 7(e). The voltage attenuation at 1000 mA/cm2 is 5.86%. Its reverse time is two orders of magnitude shorter than that of MEA-3, but the voltage attenuation is more serious. Therefore, compared with MEA-1, MEA-3 has an excellent reverse tolerance.
Fig.7 Single cell performance of MEA-1, MEA-2 and MEA-3.

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In addition, the total reverse time of MEA-3 is 530 min (Fig. 7(f)), which is about 7 times that of MEA-2 (75 min, Fig. 7(a)). The longer time is attributed to the excellent dispersion of IrOx on Ti4O7. Based on this, the OER catalyst can expose more active sites, prolonging the reaction time of water electrolysis. SEM and EDS mapping images on the surface of the catalytic layer were obtained for the distribution of OER catalysts. Figure 8(a) is the image of MEA-2, in which the Ir element shows an obvious aggregation on the surface of the catalytic layer from the element mapping. Therefore, only a small area of carbon support can be protected. In contrast, Fig. 8(b) shows that the iridium is uniformly distributed on the surface of the catalytic layer of MEA-3, which can protect most of the carbon support. Figure 8(c) illustrates the distribution of the two catalysts in the CL. The result of EDS intuitively reveals the uniform distribution of IrOx supported by Ti4O7 in the catalytic layer, which provides an evidence for the reason why MEA-3 has a longer time of water electrolysis. Ti4O7 supported IrOx could provide a longer time of water electrolysis at a relatively stable voltage, which is beneficial to the reverse tolerance of the MEA.
Fig.8 Distribution of IrOx and IrOx/Ti4O7 in the catalyst layer.

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4 Conclusions

In this study, Ti4O7 supported IrOx with a small particle size of 1.53 nm was prepared by the surfactant-assistant method. As a catalyst support, Ti4O7 has shown an excellent electrochemical stability at 1.5 V. IrOx supported on Ti4O7 can effectively reduce the amount of precious metal iridium and improve the catalytic efficiency for the RTA compared to the unsupported IrOx. The reverse time of the prepared MEA with IrOx/Ti4O7 in the anode has reached 530 min, which is 7 folds to that with IrOx, indicating a better cell reverse tolerance under actual working conditions. This can be explained by the excellent dispersion of IrOx on Ti4O7 and IrOx/Ti4O7 on the anode catalytic layer. The initial cell performance only decreases by 1.4% at 1000 mA/cm2 with the addition of IrOx/Ti4O7 to the anode of the MEA for the superior conductivity of Ti4O7. Compared to the recently developed RTA based on carbon-free support, it is a desirable choice to add supported OER catalysts to the anode without sacrificing too much initial cell performance. In particular, support materials with a high conductivity and good corrosion resistance should be considered.

Acknowledgments

This work was supported by the National Key Research and Development Program of China (No. 2019YFB1504502), and the Science and Technology Project of State Grid Corporation of China (SGLNDK00KJJS1900037).

Electronic Supplementary Material

ƒSupplementary material is available in the online version of this article at https://doi.org/10.1007/s11708-021-0811-7 and is accessible for authorized users.
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