(Digital Presentation) Oxygen Evolution Activity of RuO2/MO2/TiO2(110) (M = Ir, Sn) Surfaces in Acidic Electrolyte
Authors: Todoroki Naoto , Kudo Ryutaro , Hayashi Kenta , Yokoi Mizuho , Wadayama Toshimasa 
Published: November 2022

Introduction

Developing highly active and durable electrocatalysts for oxygen evolution reaction (OER) are required for efficient hydrogen production by polymer electrolyte membrane water electrolysis (PEMWE). While RuO2 shows the highest OER activity among pure transition metal oxides catalysts, the low durability hinders its practical application as OER catalyst for PEMWE. Recent studies demonstrated that RuO2 mixed with other metal oxides, such as IrO2 and SnO2 etc. showed higher activity and durability than pure RuO2 (1-3). When the RuO2 deposited on the other metal oxides, interfacial resistance and lattice strain which derives from the lattice mismatch should influence on the OER properties. In this study, to clarify the influence of interface between different rutile-type oxides, we fabricated RuO2/MO2/TiO2(110) (M = Ir or Sn) single crystal model catalyst and evaluated the oxygen evolution activity and durability.

Experimental

Nb-doped TiO2(110) (Nb: 1 wt%) single crystal substrate was etched in 20 % HF solution for 10 min and annealed at 1000 ℃ in air. After the sample transfer to the APD chamber, 2 nm-thick Ir or Sn was deposited onto the substrate at room temperature under 0.5 Pa-O2 partial pressure and annealed at 350 ℃ for 30 min under the same O2 pressure. After that, 4 nm-thick Ru was deposited on the MO2/TiO2(110) and was annealed under the same condition of the MO2 layer. Hereafter, the sample was denoted as RuO2/MO2/TiO2.

Oxygen evolution activity was measured by linear sweep voltammetry (LSV) at a scan rate of 1 mV/s in N2-purged 0.1 M HClO4 at room temperature. The electrochemical impedance spectroscopy was conducted at 1.5 V vs. reversible hydrogen electrode (RHE) to measure the resistance of solution (Rs), electrodes (Re), and charge transfer for OER (Rct). The LSV curves were corrected using the measured Rs. Chronopotentiometry was conducted at 0.5 mA/cm2 for 120 min to evaluate the durability. In-plane XRD analysis was performed for estimating lattice strain in the surface RuO2 layer.

Results and Discussions

XRD analysis showed that both of the MO2 interlayer and RuO2 catalyst layer grow with (110) orientation on the TiO2(110) substrate. Figure 1(a) shows initial LSV curves for OER. The onset potential of RuO2/SnO2/TiO2 samples clearly shifted to lower potential side relative to the RuO2/IrO2/TiO2 and RuO2/TiO2 samples. The overpotential at 0.1 mA cm-2 of the RuO2/SnO2/TiO2 (b) was ca. 25 mV lower than the other two samples. The chronopotentiometric curves (c) showed the overpotential of RuO2/TiO2 increased by 250 mV after 120 min. On the other hand, change in the overpotential of the RuO2/SnO2/TiO2 was only 25 mV, indicating the high durability.

Cole-cole plots at 1.5 V (d) revealed that both of the Re and Rct were significantly reduced owing to SnO2 interlayer, while the IrO2 layer slightly increased the Re and decreased the Rct. In-plane XRD results showed the compressive lattice strain was induced in [001] direction in RuO2 layer on TiO2 substrate and decreased by insertion of IrO2 and SnO2 layer. However, the lattice spacing in [-110] direction was almost the same for all the samples. Figure 1 (e) shows the relation between Rct and the lattice strain in [001] direction. The Rct tended to decrease with decreasing the compressive strains, suggesting the OER activity of RuO2(110) surface is related to the anisotropic strains in [001] direction. The results demonstrated that the activity and durability of RuO2/TiO2 catalysts can be improved by insertion of SnO2 layer at the catalyst/substrate interface.

Acknowledgments

This study was partly supported by JSPS KAKENHI Grant Number 21H01661 and Toyota Mobility Foundation Hydrogen Initiative.

References

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Figure 1