Polymer electrolyte fuel cells (PEFCs) have attracted attention as an energy system with low environmental pollution because they emit only water. However, due to its high overpotential of the oxygen reduction reaction (ORR), a large amount of Pt is used as an electrocatalyst of PEFCs. We must reduce the amount of Pt loading to promote the use of PEFCs. Therefore, it is necessary to develop electrocatalysts with higher activity for the ORR and durability.
One of the methods of developing electrocatalysts with higher ORR activity is the control of the surface structures of the electrodes [1–8]. Marković et al. found that the ORR activity of the low index planes of Pt increases in the order Pt(100) < Pt(111) < Pt(110) in 0.1 M HClO4 [1]. Systematic study using the high–index planes can determine the structure of the active site of the ORR at atomic level. Feliu et al. investigated the ORR on the high index planes of Pt in acidic solutions and found that the ORR activity increases with increasing step atomic density [2, 3]. Hoshi et al. found that the (111) terrace edge enhances the ORR activity of the Pt electrode [5]. Theoretical calculations predicted that the enhancement of the ORR is attributed to the change in the structure of adsorbed water at the (111) terrace edge, resulting in the suppression of Pt oxide formation [9].
Pt alloy catalysts have higher ORR activity than pure Pt catalysts [6–8, 10–21]. Due to the difference in atomic size between the Pt skin layer formed on the topmost surface and the second layer containing a transition metal, the interatomic distance becomes shorter in the Pt skin and the d–band center is downshifted in the Pt alloys. The change of the electronic state weakens the adsorption strength of the blocking species and improves the ORR activity [10–13].
Stamenkovic et al. found that the ORR activity of Pt3Ni(111) is 10 times higher than that of Pt(111) [8]. Wakisaka et al. investigated the effect of Co ratio on the ORR activity of Pt100–xCox and found that Pt100–xCox(111) with x = 27 has 27 times higher ORR activity than pure Pt(111) [15, 16]. Surface X–ray scattering (SXS) found that Co was enriched up to 98% in the second layer of Pt75Co25(111). The positively charged Co in the second layer promotes strong electron transfer to the Pt skin, resulting in the high ORR activity [17].
The ORR of PtFe alloys was studied by several groups expecting higher activity. Wang et al. found that multi–metallic Au/FePt3 nanoparticles, in which FePt thin film was deposited on Au(111) surface, had three times higher ORR activity than Pt/C. After the 6000 potential cycles, Au/FePt3 became 7 times more active than Pt/C, and gave higher durability [18]. It is also reported that the ORR activity of interconnected surface–vacancy–rich PtFe nanowires is 10 times higher than that of commercial Pt/C. The activity decreased only 8.1% after 10000 potential cycles, indicating extremely higher durability compared to Pt/C, of which activity was halved after 5000 cycles [19].
The study on Pt alloy catalysts was extended to the high–index planes with (111) terraces, and the structural effects differ from those of the high–index planes of Pt [6, 7]. Hereafter the high index planes are notated as n(hkl)–(mno), where n, (hkl) and (mno) show the number of terrace atomic rows, structures of terrace and step, respectively. Takesue et al. reported that Pt3Co(331) = 3(111)–(111) and Pt3Co(544) = 9(111)–(100) are 1.3 times more active than Pt(331) that has the highest activity in the single crystal electrodes of Pt [6]. The ORR activity of Pt3Ni(211) = 3(111)–(100) and Pt3Ni(322) = 5(111)–(100) are 1.4 times as high as that of Pt(331) [7].
We studied the ORR activity on single crystal electrodes of Pt3Fe. The ORR activity increases as Pt3Fe(100) < Pt3Fe(110) < Pt3Fe(111). Pt3Fe(111) has 20 times higher activity than Pt(111) [20]. In the n(111)–(111) series of Pt3Fe, the ORR activity reaches the upper limit at 4 ≤ n. In the n(111)–(100) series, the ORR activity plotted against step atom density dS gives volcano–shape with the highest activity at Pt3Fe(544) = 9(111)–(100). The ORR activities of the surfaces with (100) terrace are much lower than those with (111) terrace, giving no structural effects [21], as are the cases of the high index planes of Pt3Co and Pt3Ni [6, 7].
Pt–OH is a blocking species of the ORR on Pt electrodes [22–24]. Pt–OH is stabilized by hydrogen bonding with water molecules. Density functional theory (DFT) calculation predicted the adsorption forms of Pt–OH and Pt–O on Pt(111) and Pt(332) surfaces [9]. These Pt oxides were destabilized due to the change of water structure around the step. This prediction is consistent with the experimental ORR activity order: Pt(111) < Pt(332).
Hydrophobic species can also change water structure around electrode surfaces; the ORR activity can be improved by modifying the electrode surface with hydrophobic species [25–31]. Miyabayashi et al. found that the ORR activity and durability of Pt nanoparticles were enhanced by the modification with octylamine (OA) and alkyl amine containing pyrene ring (PA) [25]. Saikawa et al. studied the ORR activity on n(111)–(111) series of Pt modified with OA/PA = 9/1 and found that the activity increases on the surfaces with 7 ≤ n [26]. Infrared reflection absorption spectroscopy (IRAS) indicates that ice–like water with smaller cluster size increases the ORR activity on Pt(111) modified with OA/PA [27]. Modification of Pt and PtPdCo nanoparticles with melamine enhances the ORR activity [28]. Melamine also increases the ORR activity of n(111)–(111) series of Pt where Pt(111) gives the highest increase ratio [29]. Tetra‒n‒hexylammonium cation (THA+) enhances the ORR activity of Pt(111) by 8 times [30]. Modification with ionic liquids (ILs) such as [MTBD][beti] improves the ORR activity of Pt nanoparticles [31]. These results suggest that further improvement of ORR activity of Pt alloys is expected by the modification with these hydrophobic species.
In this study, we aim to improve the ORR activity and durability of Pt3Fe single crystal electrodes by the modification with hydrophobic species such as THA+, melamine and ionic liquid ([MTBD][beti]) (Fig. 1).
We examined Pt3Fe(111), Pt3Fe(775) = 7(111)–(111) and Pt3Fe(544) = 9(111)–(100) single crystal electrodes, which gives the highest activity in the low index planes, n(111)–(111) and n(111)–(100) series of Pt3Fe, respectively [20, 21]. Hard sphere models for the surfaces are shown in Fig. 2.