Effect of ZrO2 Additive for IrO2-Ta2O5/Ti on Activity of Oer in Sulfuric Acid with Toluene Contamination

Introduction

In order to promote the introduction of renewable energy, large-scale energy storage and transportation systems are required. Hydrogen is expected to apply for the energy storage and transportation. Organic chemical hydride materials are prospective energy carriers to solve the issue of hydrogen’s low volumetric energy density. Among them, toluene (TL)/methylcyclohexane (MCH) system has been focused due to the easy handling and the low toxicity. Recently, researches of one-step electrolytic hydrogenation of TL have just been started^[1].

DSA^® type anodes composed of IrO₂-based electrocatalyst coated on Ti which have excellent activity and extremely high durability for oxygen evolution reaction (OER)^[2] is required as a counter electrode of electrolytic process. It is well known that some organic additive lead serious degradation of the anodes^[3]. IrO₂-Ta₂O₅/Ti would be a good candidate, because the overpotential of IrO₂-Ta₂O₅/Ti in sulfuric acid with organic compound contamination is lower than that of IrO₂/Ti ^[3]. However, the OER activity of IrO₂-Ta₂O₅/Ti would not still be enough and the effect of TL and MCH on the activity and durability have not been clarified.

In this study, activity of IrO₂-Ta₂O₅ based Ti electrodes whose Ta is replaced by Zr was investigated in sulfuric acid with TL contamination.

Experimental

The precursor of the electrocatalyst was n-butanol solution of H₂IrCl₆・6H₂O, Ta(C₄H₉O)₅ and Zr(C₄H₉O)₄ with nominal composition of Ir: Ta: Zr= 50: (50-X): X (0 ≦X < 50) in mole fraction. A pretreated Ti plate was dipped in the precursor, and dried at 100 ^oC for 10 min. Then it was thermally decomposed at 500 ^oC for 10 min. The process of the dipping, drying, and decomposition were repeated several times to get around 10 g m^-2 of IrO₂ loading. Finally, it was baked at 500 ^oC for 1h. The loading was evaluated by a X-ray fluorescent analysis.

   Polarization of the OER was evaluated with 5 mV s^-1 of potential sweep between 1.2 and 2.0 V vs. RHE in 1 M H₂SO₄ with and without saturated TL at 60^oC. A counter electrode and a reference electrode were platinum wire and reversible hydrogen electrode, respectively. Resistance of the electrolyte and double layer capacitance was evaluated with AC impedance method.

Results and discussion

   Double layer capacitance (C_dl) of IrO₂/Ti, Ir₅₀Ta₅₀-Oxide/Ti and Ir₅₀Ta₂₀Zr₃₀-Oxide/Ti anodes in 1 M H₂SO₄ at 60 ^oC were 2.1, 25, and 75 mF cm^-2, respectively. The C_dl of the Ir₅₀Ta₂₀Zr₃₀-Oxide/Ti was three times larger than that of Ir₅₀Ta₅₀-Oxide/Ti which was ten times larger than the IrO₂/Ti. The C_dl corresponds to the real surface area for precious metal oxide coated electrode ^[4]. Therefore, Ta and Zr additives increase the real surface area of the IrO₂ based coating, and the ternary system would increase real surface area compared to the binary system.

The polarization curves for OER current density on IrO₂/Ti, Ir₅₀Ta₅₀-Oxide/Ti and Ir₅₀Ta₂₀Zr₃₀-Oxide/Ti in 1 M H₂SO₄ with and without saturated TL were shown in Fig. 1. The current densities in Tafel region were 0.9, 13, and 115 mA cm^-2 at 1.55 V vs. RHE in 1 M H₂SO₄ at 60 ^oC for IrO₂/Ti, Ir₅₀Ta₅₀-Oxide/Ti and Ir₅₀Ta₂₀Zr₃₀-Oxide/Ti, respectively. The large potential reduction more than 100mV at 100 mA cm^-2 was obtained with Ir₅₀Ta₂₀Zr₃₀-Oxide/Ti compared to the others, reflecting their C_dl values. While the current density of IrO₂/Ti decreased by 43% with TL contamination, the others decreased by 10% or less with TL contamination.

The current densities based on C_dl on Ir₅₀Ta_(50-X)Zr_X-Oxide/Ti at 1.55 V vs. RHE in 1 M H₂SO₄ with and without saturated TL were shown in Fig. 2. They increased with Zr content, and the decrement of current density by TL contamination was kept constant in spite of change in Zr content.

TL would decrease the current density of OER with the adsorption onto IrO₂. However, the addition of Ta might prevent the adsorption of TL onto IrO₂ due to the catalytic contribution on Ir and the addition of Zr might mainly increase the real surface area without changing the Ta effect.

Acknowledgments

This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “energy carrier” (Funding agency: JST). We applicate the person concerned them.

References

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