Strain-Stabilized 1T Phase Ruthenium Sulfoselenide Monolayer Nanotubes for Oxygen Reduction Reaction in Alkaline Medium

Abstract

Backgrounds: In pursuit of sustainable development, a fuel cell become a key device that can convert chemical energy in hydrogen and oxygen into electrical energy. In this field, the sluggish reaction kinetics of the oxygen reduction reaction (ORR) on cathode side of fuel cell has been a key issue hindering large–scale applications. Although many efforts have been devoted to the development of efficient electrocatalysts for ORR, platinum (Pt)–based catalysts remain the most efficient electrocatalysts owing to their high intrinsic activity. However, their reserve–scarcity and low stability limit their widespread use. Strategy: Therefore, it is highly desirable to develop electrocatalysts with high catalytic activities and stability as well. Among various strategies, metal chalcogenides have led to remarkable electrochemical performances beyond those of pure metals. Also, they have unique characteristics such as multiple crystal phases (e.g., 1H, 2H, 1T, and 1T’); the phase of 1H and 2H correspond to semiconductor, whereas the 1T and distorted 1T’ are metallic phases. Generally, the metallic phase has more active sites and higher electrical conductivity, making it favorable for electrochemical reactions compared to other phases. However, the metastability of 1T phase makes it challenging to directly prepare 1T metal chalcogenides; even, the prepared 1T material easily convert back to stable 2H (or cubic) phase under ambient conditions. Results: Herein, we prepared 1T phase Ru(SxSe1-x)2 (x = 0, 0.2, 0.5, 0.8, and 1) with a shape of monolayer nanotubes (NTs), and they showed remarkable ORR activity and stability as well. The Ru(SxSe1-x)2 nanotube containing abundant anion vacancies was generated from Ru seed particle, which is confirmed by ex-situ TEM, EDS, and EPR. Typically, most nanotubes (e.g., TiO2, and MoS2) are generated via a roll-up mechanism, where the large size of nanosheet is necessary to roll up a sheet into a nanotube; however, the process is not suitable for ultra-small (ø <3 nm) nanotubes. The Ru(SxSe1-x)2 nanotube with a diameter of ø = 1.5 nm takes advantages of larger surface area and more catalytic active sites as well. Moreover, a strain induced by curvature of the nanotube and/or anion vacancies can lead to new bond identities (e.g., bond length and bond angle), resulting in phase stabilization of 1T. In effect, the 1T Ru(S0.8Se0.2)2 nanotubes not only exhibited remarkable ORR activities (E1/2 = 0.864) comparable to commercial Pt/C, but also showed superior durability under alkaline ORR conditions. The electrochemical property was maintained after an accelerated durability test (ADT) for 10K cycles or a long-term air exposure for 6 months. Our results pave the way for the synthesis of stable 1T phase metal chalcogenides, particularly for efficient alkaline oxygen reduction reaction. Figure 1