Control of Transport Water in Direct Toluene Electro-Hydrogenation Electrolyzer

Abstract

Introduction Recently, to decrease carbon dioxide emissions, utilization of renewable energy has been expected. Therefore, the large-scale storage and transportation of hydrogen as secondary energy is needed for effective utilization of fluctuated and unevenly distributed renewable energy. Toluene (TL) / methylcyclohexane (MCH) organic hydride system is one of the best candidates of hydrogen energy carrier because TL and MCH are liquid at ambient temperature and pressure, and it would be able to use oil infrastructure. To improve the energy conversion efficiency for TL hydrogenation using renewable electricity, we have studied direct electro-hydrogenation of TL with water decomposition using proton exchange membrane (PEM). The rate-determining step at the cathode for the hydrogenation of TL is the mass transfer of TL to the cathode catalyst layer [1]. One of the major factors that inhibit the supply of TL to the reaction field is transport water from the anode through the membrane. The accumulation of the water in the catalyst layer inhibits the supply of TL and reduces the current efficiency, but its behavior has not been clarified. In this study, the influence of current density, sulfuric acid concentration supplied to the anode and cell temperature on the amount of the transport water were evaluated in order to control the water in direct TL electro-hydrogenation electrolyzer. Experimental A DSE® (De Nora Permelec, Ltd.) for the oxygen evolution and Nafion 117® (Du Pont) were used as the anode and PEM, respectively. Carbon paper (10BC, SGL carbon ltd.) loaded with 0.5 mg cm-2 of PtRu/C (TEC61E54, Tanaka Kikinzoku Kogyo) was also used as the cathode. The cathode was hot-pressed on the PEM at 120 oC and 15 MPa for 3 min to fabricate a membrane cathode assembly. The anode and cathode compartments were circulated 0-1.5 M (= mol dm-3) H2SO4 and 10% TL/MCH, respectively. Operation temperature of the electrolyzer conducted at 50-80 oC. As the electrochemical measurement, chronopotentiometry at 0.1-0.4 A cm-2 was conducted for 20 min, and the amount of water was evaluated by measuring the weight of it in the reservoir after electrolysis. The concentration of sulfuric acid of the water was determined by measuring the pH. Results and discussion The water permeates the PEM by electro-osmosis and diffusion, and the water flux is given by eq. 1 in the images [2]. The linear region of Fig. 1 would mean water flux had linear relation to current density with constant apparent electro-osmotic coefficient with constant back diffusion flux from eq. 1. The constant back diffusion would mean that the aqueous phase is formed in the cathode catalyst layer, and its activity would be constant.Figure 1 shows dependence of cell voltage and water flux at 60 oC with various sulfuric acid concentrations on current density. There was no significant difference in cell voltages except for 0 M. Relationships between the water flux and the current density were linear except 0.1 A cm-2 at 1.5 M. The water flux decreased as the sulfuric acid concentration increases except 0 M. The absolute value of the intercept by extrapolating the linear region increased with sulfuric acid concentration. The slope of the straight line also decreased as the sulfuric acid concentration increased. In addition, the water detected on the cathode side was sulfuric acid whose concentration was about one-tenth that of the anode. The sulfuric acid concentration in the membrane would also change with the anodic sulfuric acid concentration. The electro-osmotic coefficient obtained from the slope of Fig.1 would decrease with the increase of the sulfuric acid concentration in the membrane.Figure 2 shows the diffusion flux and the concentration difference between the anolyte and cathode side sulfuric acid, which transported from anode side, as functions of the anolyte concentration. They increased linearly with anolyte concentration. Correspondingly, the back diffusion flux, which is affected by the sulfuric acid concentration difference, would have increased linearly with increasing anolyte concentration. Acknowledgements This work was supported by the Toyota Mobility Foundation. Anode was supplied by De Nora Permelec Ltd. We appreciate the person concerned them. Reference [1] K. Nagasawa, Y. Sawaguchi, A. Kato, Y. Nishiki, S. Mitsushima, Electrocatalysis, 8, 164, (2017).[2] T. E. Springer, T. A. Zawodzinski, S. Gottesfeld, J. Electrochemical Society, 138, 2334, (1991). Figure 1