Structural effects on the enhancement of ORR activity on Pt single-crystal electrodes modified with alkylamines
Graphical abstract
Introduction
Pt is widely used as an electrocatalyst in polymer electrolyte fuel cells (PEFCs). However, the high overpotential of the oxygen reduction reaction (ORR) and the large amount of Pt loading required prevent their general use. It is therefore necessary to develop electrocatalysts with higher ORR activity that will lead to a reduction in the Pt loading.
One strategy for enhancing ORR activity is to identify the active sites using well-defined single crystal electrodes. Incorporation of nanoparticles in the active structure can also enhance the ORR activity of practical electrocatalysts. Marković et al. reported that ORR activity on the low index planes of Pt in 0.1 M HClO4 increases in the order Pt(100) < Pt(111) < Pt(110) [1]. The study has been extended to the high index planes of Pt: n(hkl)–(mno), where n, (hkl) and (mno) represent the number of terrace atomic rows, and terrace and step structures, respectively. Feliu et al. estimated the ORR activity on n(111)–(100) and n(100)–(111) Pt surfaces using the exchange current density [2]. Later the same group evaluated the activity on n(111)–(111) and n(110)–(111) surfaces using the half wave potential [3]. They found that the introduction of step structures enhances ORR activity. However, the best step structure for ORR cannot be determined because different measures of ORR activity are used in the two papers [2], [3]. We estimated the ORR activity using the cathodic current density at 0.90 V(RHE) on n(111)–(111), n(111)–(100), n(100)–(111) and n(100)–(110) surfaces, and found that the (111) terrace edge enhances ORR activity on Pt electrodes [4], [5].
DFT calculations predict that the (111) terrace edge changes the adsorbed water structure on n(111)–(111) Pt surfaces. The structural change of the adsorbed water hinders the formation of Pt oxides that are known to deactivate the ORR, resulting in a higher ORR activity than Pt(111) electrodes without a terrace edge [6]. Adsorbed organic molecules can also change the adsorbed water structure on Pt electrodes, enhancing their activity for the ORR. Some papers reported that catalytic activity is improved by modification with organic molecules and inorganic species [7], [8], [9], [10], [11], [12]. For example, Pt nanoparticles modified with octylamine (OA) and an alkylamine with a pyrene ring (PA) show higher ORR activity and durability than bare Pt nanoparticles [8], [10]. If we find a surface structure on which ORR activity is greatly enhanced by OA/PA, building up such a structure on Pt nanoparticles would dramatically improve the ORR activity of practical catalysts. However, there has been no report on the effects of structure on the ORR activity of OA/PA modified Pt single-crystal electrodes.
In this paper, we have studied the ORR on OA/PA modified n(111)–(111) surfaces of Pt, which have the highest ORR activity when bare. For comparison, the effects of OA/PA on the ORR have also been studied on Pt(100), which has the lowest ORR activity in the bare state. Fig. 1 shows the structural formulae of the amines used and hard sphere models of the single crystal electrodes examined.
Section snippets
Chemicals
Toluene (99.5%), acetone (99.5%) and OA (98.0%) were purchased from Wako Pure Chemicals Co., Ltd. PA (8–(pyrene-1-ylmetoxy)octane-1-amine) was obtained from the National Institute of Advanced Industrial Science and Technology. Perchloric acid (60%, ultrapure) was purchased from Kanto Chemical Co., Inc. The electrolytic solution was prepared with ultrapure water treated with Milli–Q Advantage A10 (Millipore).
Preparation of the single-crystal electrodes
A single-crystal bead of Pt was prepared with reference to the report of Clavilier et
Results and discussion
Fig. 2(a) shows representative voltammograms for the n(111)–(111) = (n − 1)(111)–(110) surfaces of Pt before and after modification with OA/PA = 9/1 in 0.1 M HClO4 saturated with Ar. The charge densities of adsorbed hydrogen QH and oxide formation regions QOX decrease after the modification. Sharp redox peaks at 0.12 V(RHE) are due to hydrogen adsorption/desorption at the (110) step on the bare Pt electrodes. These redox peaks disappear after the modification, showing that OA/PA is preferentially
Conclusion
Adsorbed OA/PA = 9/1 (molar ratio) enhances the activity for ORR on n(111)–(111) = (n − 1)(111)–(110) surfaces of Pt in 0.1 M HClO4 when the number of terrace atomic rows n is greater than 7. The ratio of the enhancement of flat Pt(111) is the highest: the ORR activity on (OA/PA)/Pt(111) is 2.5 times higher than that on bare Pt(111). However, the ORR on flat Pt(100) is largely deactivated after OA/PA adsorption. It appears that a wide (111) structure is necessary for enhancement of the ORR by adsorbed
Acknowledgements
This work was supported by the New Energy Development Organization (NEDO) (15100789-0). We appreciate Dr. Tsutomu Ioroi and Dr. Shin–ichi Yamazaki at Research Institute of Electrochemical Energy, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology for the supply of PA.
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