On the Origin of Enhanced ORR Activity of RuO2 Nanosheet Modified Pt Catalysts: An Impedance Spectroscopy Approach

Numerous approaches have been proposed to increase the ORR activity of Pt, including the use of alloys or core-shell structures, oxide additives, graphitized carbon, and others. Although these catalysts have improved activity and/or durability, the preparation scheme for such state-of-the-art catalysts is often multi-process and difficult to produce in large scale. In terms of simplicity of synthesis, the ideal method may be to have a catalyst with high initial activity and durability by making a composite with a commercial Pt/C by a simple one-step process. We have recently reported such a catalyst by mixing ruthenium oxide nanosheets with practical Pt/C [1-7]. Catalysts composed of RuO₂ nanosheet and Pt/C (RuO₂ns-Pt/C) can be prepared by simply mixing and drying of the two colloids and exhibits higher initial ORR activity and at the same time shows improved performance after accelerated durability cycling tests. However, the reason(s) for the initial activity improvement is poorly understood. For example, the increase (30%) in electrochemically active surface area can only partially explain the initial activity increase.

In this study, we focus on the clarifications of the increase in the initial activity of the RuO₂ns-Pt/C catalysts with composites prepared with nanosheets having different lateral size by electrochemical impedance spectroscopy and transmission line model simulations.

Nanosheets with smaller size resulted in higher activity, which was correlated to a decrease in pore resistance. The initial mass activity of Pt/C is j_k=285 A (g-Pt)^-1 at 0.85 V vs RHE in 0.5M H₂SO₄ at 60°C. Addition of 100 nm RuO₂ nanosheet improved the initial mass activity to j_k=412 A (g-Pt)^-1. The impedance spectra were conducted at open charge potential in N₂ saturated 0.5M H₂SO₄ electrolyte at 60°C and simulated with the transmission line model. From the simulation, resistivity in the porous Pt/C was 625 Ω cm^-1 whereas with the addition of 100 nm RuO₂ nanosheet, resistivity was only 125 Ω cm^-1. Addition of nanosheet improved the conductivity in the catalyst layer. The impedance spectra conducted at 0.85V vs RHE in O₂ saturated 0.5M H₂SO₄ electrolyte at 60°C showed that addition of smaller size nanosheet improved the charge transfer reaction of the ORR.

This work was supported in part by the “Polymer Electrolyte Fuel Cell Program” from the New Energy and Industrial Technology Development Organization (NEDO), Japan.

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