Electrocatalysis of Ketjenblack-Supported Ru-Ir Cathode for the Electrohydrogenation of Toluene to Methylcyclohexane in the Organic Hydride System

Hydrogenation of toluene (TL) and dehydrogenation of methylcyclohexane (MCH) has been paid great attention for a highly efficient H2 storage technology [1,2]. At energy supply area, TL is hydrogenated to MCH by the catalytic hydrogenation using H2 produced by the water-electrolysis. We can easily transport and store MCH using the current infrastructures, because MCH is a component of petroleum and liquid state at ambient conditions. At energy demanding area, the catalytic dehydrogenation of MCH to TL and H2 (47.4 kg-H2/m3-MCH) is conducted. H2 can be used as an energy source, and TL is reused as the H2 carrier, repeatedly. To improve the H2 storage system to more efficient one, a direct electrohydrogenation of TL with water using a solid polymer electrolyte (SPE) electrolysis cell and 50 wt% Pt/Ketjenblack (KB) cathode has been proposed (Fig. 1) [3]. In those system, water is oxidized at the anode, and TL is electrochemically reduced to MCH at the cathode; however, the high loading of Pt is disadvantage from the viewpoints of system cost. For the efficient hydrogenation of TL, it is essential to reduce Pt or other precious metal loadings with keeping suppression of the competitive H2 evolution. In this study, we found that Ru-Ir alloy supported on KB (Ru-Ir/KB) electrocatalyst showed superior electrocatalytic activity for the electrohydrogenation of TL to MCH under galvanostatic electrolysis conditions at 0.20 A cm-2. 5 wt% Ru-5 wt% Ir/KB loaded on a gas-diffusion layer (GDL) cathode (0.2 mgRu-Ir cm-2) showed a higher faradic efficiency of MCH formation than the 50 wt% Pt/KB loaded on the GDL cathode (1.0 mgPt cm-2), though the working potential of -0.183 V (SHE) at the former was slightly negative than that of -0.071 V (SHE) at the later. This indicated the significant reduction of amount of precious metals used for the electrohydrogenation of TL. Characterization studies for the Ru-Ir/KB electrocatalyst were conducted by using X-ray diffraction (XRD), transmission electron microscopy with energy dispersive spectroscopy (TEM-EDS) and X-ray absorption fine structure spectroscopy (XAFS), and a formation of hcp-structured Ru-Ir alloy was revealed. Based on the electrochemical and kinetic studies, synergy mechanisms of Ru and Ir for the electrohydrogenation of TL were proposed. To be concrete, a fast hydrogenation of TL to MCH on the Ru site, a fast electrocatalytic reduction of H+ to H-species on the Ir site, and a fast spillover of H-species from Ir to Ru, these three steps cooperate. References [1] J. Gretz, J. P. Baselt, O. Ullmann, H. Wendt, Int. J. Hydrogen Energy, 15, 419 (1990) [2] Y. Okada, E. Sasaki, E. Watanabe, S. Hyodo, H. Nishijima, Int. J. Hydrogen Energy, 31, 1347 (2006) [3] S. Mitsushima, Y. Takakuwa, K. Nagasawa, Y. Sawaguchi, Y. Kohno, K. Matsuzawa, Z. Awaludin, A. Kato, Y. Nishiki, Electrocatalysis, 7, 127 (2016) Figure 1