Improvement of Current Efficiency of a Membrane Electrolyzer for Electrohydrogenation of Toluene As Hydrogen Carrier Synthesis

Abstract

Introduction In order to reduce carbon dioxide emissions, a significant number of renewable energies that are uneven distribution with fluctuation must be introduced. Therefore, to increase renewable energies, energy carrier technology is needed for storage and transportation. Toluene-methylcyclohexane organic chemical hydride system is one of promising technologies as hydrogen storage and transportation. Electrohydrogenation of toluene with water splitting has higher theoretical energy conversion efficiency compare to a series process of water electrolysis and hydrogenation. Cathode side is a cathode membrane assembly with PtRu/C, which is applied PEFC technology. Anode is a dimensionally stable electrode for oxygen evolution reaction in acidic electrolyte using industrial electrolysis technology. In our previous study, we demonstrated good performance of the electrolyzer with hydrophilized membrane; however, hydrogen generated with low concentration of toluene feed, which should be improved (1). In this study, the effect of the design of toluene feed flow field on the cell voltage and the current efficiency has been investigated to increase conversion ratio from toluene to methylcyclohexane without hydrogen generation. Experimental A single cell electrolyzer made of titanium with 100 cm2of projected electrode area was used to determine the performance. Figure 1 shows the schematic drawing of flow field for the cathode. A parallel flow, which is usual for industrial electrolysis and liquid electrolyte fuel cells, a serpentine flow, which is conventional for polymer electrolyte fuel cells, and an interdigitated flow have been investigated to improve the performance of the electrolyzer. The anode flow field was parallel. A cathode was a carbon paper (35BC, SGL) coated 0.5 mgcm-2 of PtRu (TEC61E54, TKK) with Nafion dispersion. The cathode was pressed on a perfluoroethylene sulfuric acid (PFSA) membrane (Nafion® 117, DuPont) for a cathode membrane assembly. The membrane of the cathode side was mechanically hydrophilized. A DSE® anode with IrO2 based electrocatalyst is used for oxygen evolution. Backing of the anode was titanium felt. The anode was uniformlypressed on the membrane by elastic force of the titanium felt. 10 cm3 min-1 of toluene or 50% toluene-methylcyclohexane mixture and 1M (=moldm-3) of H2SO4were supplied to the cathode and anode for hydrogenation of toluene, respectively. Cell voltage was determined with 4 mVs-1 of voltage sweep from 1 V for toluene hydrogenation up to 0.5 A cm-2of the current density. Current efficiency was determined with constant cell voltage electrolysis with the volume measurement of gas evolution from cathode during the electrolysis.Internal resistance (iR) was determined with higher frequency intercept of AC impedance method. Results and discussion Figure 1 shows the cell voltage and the current efficiency as a function of the current density for electrohydrogenation of toluene at 60oC with various flow fields for the cathode. 100% of toluene or 50% toluene – methylcyclohexane mixture was fed to the cathode chamber. The internal resistances by AC impedance method were 0.25 Ω cm2 for all electrolyzers, which is almost same as the membrane area resistivity (2). The cell voltages for 50 and 100% toluene feed as a function of current density were almost the same for each flow design of the cathode. The cell voltage of parallel and interdigitated flow was 2.0 V at 0.4 Acm-2, and the cell voltage of the serpentine was a little larger than the other flow patterns. The current efficiency of hydrogenation decreased with the increase of current density, and the order from high current efficiency was the serpentine, interdigitated, and parallel flows. The difference among flow patterns was significantly for 100 % toluene feed, but it was very small for 50 % toluene feed. Turbulence flow would be better for mixing than laminar flow to feed toluene to the catalyst layer, and the order seems to be same to the flow velocity. Flow field design is important to increase toluene conversion. In addition, improvement of catalyst layer should also be important, because current efficiency of 50 % toluene feed seems to be controlled by mass transfer in the catalyst layer. Acknowledgment This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “energy carrier” (Funding agency: JST). The Institute of Advanced Sciences (IAS) in YNU is supported by the MEXT Program for Promoting Reform of National Universities. We appreciate the person concerned them. References 1) S. Mitsushima, Y. Takakuwa, K. Nagasawa, Y. Sawaguchi, Y. Kohno, K. Matsuzawa, Z. Awaludin, A. Kato, Y. Nishiki, Electrocatalysis, 2016, 7, 238.2) S. Slade, S. Campbell, T. Ralph, F. Walsh, J. Electrochem. Soc., 2002, 149, A1556. Figure 1