1. Introduction

Improvement in ORR activity of Pt-based catalysts for use in PEFCs of FCVs is crucial for cost reduction and widespread use [1]. We reported that surface modification of Pt and Pt-based catalysts with melamine molecules enhances ORR activity [2, 3]. However, it is predicted that the melamine is easily washed off by water generated at the cathode in PEFCs because melamine is soluble in water. In this study, organic salts composed of protonated melamine derivatives and bis(nonafluorobutanesulfonyl)imide were newly synthesized to suppress the washing-off effects. Pt and Pd@Pt core-shell catalysts were modified with the organic salts, and the ORR activity and the I-V performance were investigated.

2. Experimental

Ketjen Black EC-600JD (KB, S_BET: 1,345 m²/g, LION) and mesoporous carbon (MPC, CNovel MH-18^®, S_BET 1,334 m²/g, TOYO TANSO) were used as carbon supports for Pd@Pt core-shell and Pt catalysts, respectively. The Pt/Pd/KB core-shell catalyst was synthesized by a direct displacement reaction method [4] and the Pt/MPC catalyst was synthesized by an impregnation/thermal reduction method. The organic salts were synthesized by mixing equimolar melamine derivatives and Li bis(nonafluorobutanesulfonyl)imide in 0.1 M HNO₃ aqueous solution at 80^oC. Pt/Pd/KB catalyst was loaded onto a glassy carbon electrode and immersed in acetone containing 0.1 mM organic salts for 10 min. to modify the catalyst surface with the organic salts. CV of Pt/Pd/KB catalyst was recorded at 25^oC in Ar-saturated 0.1 M HClO₄ with a potential scan rate of 50 mV/s and LSV was measured by RDE method at 25^oC in O₂-saturated 0.1 M HClO₄ with a potential scan rate of 10 mV/s at a rotation speed of 1,600 rpm.

Pt/MPC catalyst was dispersed in acetone containing 20 wt.% organic salt and acetone was slowly evaporated by using an evaporator to modify the catalyst with organic salt, and used for 1 × 1 cm² MEA tests. The MEA was fabricated by a decal method using Nafion^® membrane (25 µm in thickness) and a GDL (SGL). Nafion^® DE2020 was used as an ionomer for catalyst ink preparation (I/C: 0.83). The Pt loading of the cathode catalyst layer in the MEA was set to 0.1 mg-Pt/cm². I-V performance of the MEA was evaluated at 80^oC, 75% RH. H₂ gas (418 NmL) and air (998 NmL) were supplied to the anode and the cathode, respectively, and the H₂ gas was pressurized to 150 kPa (ambient + 50 kPa) at the gas-outlet (H₂ utilization: 5% at a current density of 3.0 A/cm²).

3. Results and Discussion

Figure 1 shows molecular structure of an organic salt of [diethylmelamine+H]⁺[bis(nonafluorobutanesulfonyl)imide]^- ([DEM]⁺[NFSI]^-). The salt was synthesized in 0.1 M HNO₃ (pH 1) at 80^oC and was obtained in a solid state in the solution with a yield > 90%. CVs of Pt/Pd/KB catalyst and after modification with [DEM]⁺[NFSI]^- are depicted in Figure 2. Hydrogen adsorption/ desorption waves observed in the potential range of 0.05-0.4 V were slightly suppressed by the modification, indicating that surface Pt active sites were partially occupied by the salt. Simultaneously, the onset potential for Pt-OH formation was positively shifted and the formation of Pt-oxygenated species was suppressed in the high potential range of 0.7-1.2 V, showing that the modified Pt surface retained more metallic state at high potentials. Figure 3 demonstrates LSVs of the Pt/Pd/KB catalyst and after modification with [DEM]⁺[NFSI]^-. The LSV of the catalyst positively shifted by the modification and the ORR mass activity was enhanced from 1,223 to 2,234 A/g-Pt at 0.9 V. These results indicated that the modification of the Pt/Pd/KB catalyst with [DEM]⁺[NFSI]^- was effective in enhancing ORR activity and the results are almost the same as the activity enhancement observed by melamine modification [3].

Figure 4 shows I-V performance of the MEA using Pt/MPC as the cathode catalyst. The cell voltage of the MEA using the modified Pt/MPC exceeded that using the unmodified catalyst in the whole current density region, indicating that ORR activity enhancement by the organic salt modification was also demonstrated in the MEA tests. Furthermore, the resistance and the diffusion overpotentials as well as the activation overpotential were all reduced by the modification with [DEM]⁺[NFSI]^-. Thus, the organic salt could interact with ionomers or membrane and decrease the resistance and the diffusion overpotentials.

This study was partly supported by NEDO, Japan.

References

[1] M. Suzuki, FCCJ, https://www.nedo.go.jp/content/100895111.pdf, June 2019, in Japanese.

[2] M. Asahi et al., J. Electrochem. Soc., 166, F498 (2019).

[3] S. Yamazaki et al., ACS Catal., 10, 14567 (2020).

[4] N. Aoki et al., J. Electrochem. Soc., 167, 044513 (2020).