A New Method for Producing Hydrogen Carrier Using Renewable Energy -Direct MCH–

Abstract

New energy sources with lower environmental impact are the focus of increasing attention to enable the sustainable growth of human societies. There is an urgent need for technologies that enable conversion of renewable energy (RE) to a transportable energy carrier and recovery of that energy for later use. We have focused on organic hydrides, specifically the toluene (TOL) / methylcyclohexane (MCH) system, and have been working on a one-step process for producing MCH from TOL via electrochemical hydrogenation. We named this new method “Direct MCHTM”. The equipment needed for Direct MCH is basically the same in structure as that already used for water electrolysis. An electrolyzer with a membrane-electrode-assembly (MEA), similar to those used for polymer electrolyte water electrolysis (PEWE), can be used. The cathode consists of a carbon-supported Pt-Ru alloy catalyst with a high specific surface area and a proton-conductive ionomer. The cathode shows good electron conductivity and high diffusivity of protons and TOL, and provides a three-dimensional network of active sites. On the cathode catalyst, TOL from the diffusion layer is converted directly to MCH via electrochemical hydrogenation by protons coming from the anode through the electrolyte membrane. (Fig. 1) For process commercialization, it will be necessary to get the electrolysis voltage closer to the theoretical voltage and achieve current densities as high as those seen in alkaline water electrolysis. We also looked at how the toluene concentration affects Direct MCHTM. It has been shown that as electrolysis proceeds and the TOL concentration drops, the current density falls because it is a diffusion-limited reaction. But from the standpoint of transport efficiency, the MCH conversion rate needs to be 95% or higher, and the average current density during electrolysis to reach this rate should be comparable to that of water electrolysis, which is 0.4 A/cm2. By studying the catalyst compositions, we have made it so the percolating water through with the protons from the anode drains more easily from the cathode catalyst layer. Moreover, we obtained a cathode catalyst with high TOL diffusivity. (Fig. 2) Finally, we conducted a demonstration of the Direct MCHTM process in Queensland, Australia that was powered only by renewable energy, using electricity generated by photovoltaic cells. Over four liters of MCH obtained via Direct MCHTM was brought to Japan, showing that it is possible to transport CO2-free energy from Australia to Japan. Then, so-called "green hydrogen" was generated via dehydrogenation of the MCH and used to run a miniature car equipped with a fuel cell. Figure 1