Improvement of Reversal-Tolerance By Ti4O7-C Supported Pt-Ir Composite Catalysts
Authors: Wakita Hidenobu , Ishigaku Yoko , Nagano Waka , Chisaka Mitsuharu , Takeguchi Tatsuya 
Published: December 2024

ABSTRACT
When hydrogen is depleted in PEFCs, the anode potential increases (cell reversal), which involves the oxidation of carbon (COR), resulting in the deactivation of the anode. The addition of the oxygen evolution (OER) catalysts has been reported to suppress the degradation1. Ioroi et al. have reported that the combination of Ti4O7 with Ir catalysts improved the reversal-tolerance2. Li et al. have recently reported that Pt-IrO2 catalyst supported on Ti4O7 exhibited excellent reversal-tolerance3. We have also independently found a similar combination to be effective using Ti4O7-C synthesized by the carbothermal process4 and reported that the composite catalysts (Pt and OER catalysts supported on Ti4O7-C) showed better reversal-tolerance than the physical mixture of Pt/Ti4O7-C and Ti4O7-supported OER catalysts5. In this study, the composite effect of Pt-Ir was studied in terms of the chemical state of iridium catalysts and the effect of Ti4O7-supported catalysts. The reversal-tolerance was evaluated on the hydrogen pump.
Experimental

Preparation of catalysts: 20 wt% Pt/Ti4O7-C (11 wt% C) was prepared as previously reported4. IrO2 catalysts Ti4O7-C or Pt/Ti4O7-C was impregnated in IrCl3 solution, and hydrothermally treated at 180°C. Afterwards, the sediment was washed and dried at 80°C. 3.8 - 5.7 wt% IrO2/Ti4O7-C and 18 wt% Pt-3.7 wt% IrO2/Ti4O7-C were used for the physical mixture catalysts (hereafter denoted as “catalyst name-M”) and the composite catalyst, respectively. Ir0 catalyst Pt/Ti4O7-C was dipped into IrCl3 aqueous solution, and NaBH4 solution was added. It was washed by ethanol and water and dried at 80°C. Electrochemical measurement: The MEA was composed of Nafion NRE211 as the electrolyte membrane, a commercial heat-treated c-Pt/CB as the cathode (0.5 mg-Pt/cm2), and the above-mentioned catalyst or the physical mixture catalyst as the anode (0.2 mg-Pt/cm2). A commercial c-Pt/CB was also used for a physical mixture catalyst. The Ir loadings were 0.036 - 0.047 mg/cm2 and are indicated in parentheses. The performance was firstly evaluated with IV curves, and then the hydrogen pump, CV and LSV were conducted by flowing H2 gas to the cathode under 80°C fully humidified condition. The cell reversal test was performed by switching the anode supply gas to N2 (RH 78%) while the hydrogen pump was carried out at 0.2 A/cm2, and it was kept it was kept for 10 minutes at a cutoff voltage of 2.7 V. After the test, the same check of the characteristics was conducted. The second test was repeated in the same manner, and the third test was performed until it reached the cutoff voltage.
Results and Discussion

Difference in Ir chemical states of composite catalysts: The difference between IrO2 and Ir0 catalysts was studied (Cat 1 and Cat2). Both catalysts exhibited similar behavior. It was inferred that the oxidized Ir on the surface functioned as the active species for both catalysts3. Since 41% of Ir was leached after the tests for Pt-IrO2/Ti4O7-C catalyst (Cat1), the catalyst calcined at 300°C was also tested. Though the amount of leached Ir by the tests was suppressed to 10%, the degradation behavior hardly changed. Effects of Ir catalysts for the physical mixture catalyst: To confirm the effect of Ti4O7-C-supported IrO2 catalyst for the physical mixture catalyst, the same loading of Ir black was added to Pt/Ti4O7-C (Cat4). It showed better reversal-tolerance than the catalyst with IrO2/Ti4O7-C (Cat3), which means that the reversal-tolerance was improved in the case that Ir was supported on Pt/Ti4O7-C. Differences between Pt/Ti
4
O
7
-C and c-Pt/CB for the physical mixture catalysts: For the physical mixture catalyst with IrO2/Ti4O7-C, c-Pt/CB was used (Cat5). c-Pt/CB showed better reversal-tolerance than the catalyst with Pt/Ti4O7-C. Even though the catalyst with c-Pt/CB held below 1.7 V during the second test, the performance on the hydrogen pump deteriorated, because the COR slowly occurred below 1.7 V2. On the other hand, Ti4O7-C supported Pt-based catalysts did not exhibit significant deterioration on the hydrogen pump while they were kept below 1.7 V, and the HOR activity decreased after the anode voltage increased over 1.7 V. We have confirmed that the Ti4O7-C was slightly oxidized above 1.7 V, however the increase in the ohmic resistance was not the main factor for the increase in the voltage.
This work was supported by NEDO.
(1) T. R. Ralph et al., Platinum Metals Rev.
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46, 117 (2002). (2) T. Ioroi et al., J. Power Sources,
450, 227656 (2020). (3) Z. Li et al., Energy Environ
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., 17, 1580 (2024). (4) M. Chisaka et al., Chem. Comm.
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57, 12772 (2021). (5) H. Wakita et al., In Abstracts of Papers, The 91st ECSJ Annual Meeting, S9-1_3_06, Electrochemical Society of Japan, Nagoya (2024).

Figure 1