Effect of Organic Cation on the Surface Oxidation of Pt(111) Electrode in Alkaline Media

Abstract

Platinum (Pt) is used as electrocatalysts for fuel cells and water electrolysis. However, the performance of catalytic activity and corrosion-resistant of Pt is lowered by oxygen species formed during the surface oxidation. The adsorption region of hydroxide (OHad) species overlaps with the kinetically limited region of the oxygen reduction reaction (ORR), then OHad-H2O cluster blocks the Pt active sites for the ORR [1]. Adsorbed oxygen atoms (Oad) exchange with surface Pt atoms, then irreversible surface roughening and dissolution of Pt are induced [2]. The formation of the oxygen species on Pt depends on nonspecifically adsorbed cations in the electrical double layer. In alkaline media, Li+ stabilizes OHad species and preserves an atomically flat surface of Pt(111) electrode. However, Cs+ promotes the oxide formation, resulting in the surface roughening [3]. Organic cations, such as tetraalkylammonium cations, suppress the formation of OHad species and activate the ORR on Pt(111) electrode markedly [4]. In this study, we performed in situ X-ray crystal truncation rod (CTR) and nanoparticle-enhanced Raman spectroscopy measurement to elucidate the surface oxidation/reduction process of Pt(111) in alkaline solutions containing alkali metal and tetraalkylammonium cations. In situ X-ray CTR measurement was performed in 0.1 M KOH and tetramethylammonium hydroxide (TMAOH) at 1.2 V versus reversible hydrogen electrode (RHE). In KOH solution, the CTR intensities between Bragg peaks decrease compared with those for the initial Pt model. For the optimized model, the occupancy factor of the first Pt layer decreases to 0.4, which indicates that surface roughening is occurred by the oxide formation and the place exchange. On the other hand, the surface roughening was not observed in TMAOH solution. In situ nanoparticle-enhanced Raman spectroscopy was performed to detect the Pt oxide species. The potential-dependent band at 580 cm-1 appears above 1.2 V, which is assigned to Pt oxide species. During the potential steps in the negative direction, the Pt oxide band remains down to ≈ 0.8 V in LiOH, NaOH, and KOH solutions. This finding indicates that the surface oxidation/reduction proceeds irreversibly. On the other hand, the Pt oxide band disappears around 1.2 V in TMAOH. This result shows that the surface oxidation/reduction in TMAOH proceeds reversibly, which is consistent to the results of X-ray CTR measurement. These findings show that the selection of appropriate cationic species increases corrosion-resistant as well as catalytic activity in alkaline electrochemical devices. This work was supported by New Energy and Industrial Technology Development Organization (NEDO). [1] D. Strmcnik, K. Kodama, D. van der Vliet, J. Greeley, V. R. Stamenkovic, and N. M. Markovic, Nat. Chem. 1, 466 (2009). [2] M. Ruge, J. Drnec, B. Rahn, F. Reikowski, D. A. Harrington, F. Carla, R. Felici, J. Stettner, and O. M. Magnussen, J. Am. Chem. Soc. 139, 4532 (2017). [3] M. Nakamura, Y. Nakajima, N. Hoshi, H. Tajiri, and O. Sakata, ChemPhysChem 14, 2426 (2013). [4] T. Kumeda, H. Tajiri, O. Sakata, N. Hoshi, and M. Nakamura, Nat. Commun. 9, 4378 (2018).