Electrocatalyst Layer Design of Polymer Electrolyte Membrane Electrolyzer for Simultaneous Hydrogenation of Organic Chemical Hydride Energy Carrier and Water Decomposition

Abstract

Introduction To utilize renewable electricity with fluctuation and localization, large-scale energy storage and transportation technologies using hydrogen energy carrier have been expected. In order to improve hydrogen energy carrier synthesis, we have studied one-step electrolysis for toluene electrohydrogenation and water decomposition, which has cathode side of PEFC’s MEA like structure using PtRu/C and anode side of industrial electrolyzer like structure using mesh shape of DSE® electrode for oxygen evolution reaction. This process has 1.09 V of theoretical decomposition voltage for simultaneous water electrolysis and toluene hydrogenation. We had optimized the electrolyzer with stacked anode of fine and conventional mesh to get high faraday efficiency and low cell voltage [1]. However, ultra fine anode, which decreased cell voltage a lot, based on sintered titanium fiber porous plate significantly decreased faraday efficiency.In this study, layered catalyst layer of dot shape and whole surface coating had been investigated to enhance mass transportation of toluene into the catalyst layer to improve faraday efficiency using ultra fine mesh anode, in order to develop efficient toluene direct hydrogenation electrolyzer with low cell voltage and high faraday efficiency. Experimental Figure 1 shows the schematic drawing dot shape cathode catalyst layer of square and diamond type that was consisted of 1.2 x 1.2 mm2 square dots with different direction. The dot shape was formed on a proton exchange membrane (Nafion®117, DuPont) and whole surface catalyst layer was formed on a carbon paper (35BC, SGL carbon ltd.) dispersed 0.5 mg cm-2 of Pt/C for chemical hydrogenation using by-product of hydrogen. The Pt-Ru/C (TEC61E54, TKK) loading was 0.75 mgcm-2 of metal for the layered potion of dot and whole surface coating. The type is indicated with the ratio of metal loading of dot and uniform layers on a dot as x:(10-x).The anode was ultra fine mesh and stacked anode of fine and conventional DSE® for oxygen evolution (De Nora Permelec, Ltd.), which was iridium oxide based electrocatalyst coated sintered titanium fiber plate.During electrolysis, cathode reactant of 10% toluene – methylcyclohexane or 100% toluene, and anolyte of 1 M (=moldm-3) H2SO4 was circulated at a 10 mL min-1 of flow rate, respectively at 60oC. Current efficiency was determined with the volume ratio of generated hydrogen gas and toluene-methylcyclohexane solution. Results and discussion Figure 2 shows the iR-free cell voltage, current efficiency and internal resistance as a function of current density for various electrolyzers. Internal resistances determined by EIS measurement were in the range from 0.4 to 0.5 Wcm2 in variation. The cell voltage of the conventional type with the double mesh anode was significantly higher than others, although current efficiency was higher than others. The uniform electrocatalyst layer electrolyzer with ultra fine mesh anode showed the lowest current efficiency in these. Here, only difference between these two cells were only anode structure, therefore anode structure would affect to mass transfer of toluene into cathode catalyst layer through current distribution corresponded to the anode shape. Patterned catalyst layers make current distribution by themselves, and their current efficiency were higher than that with uniform catalyst layer, so the effectiveness of the dot shape cathode catalyst layer was confirmed. The current efficiency of the diamond shape was higher than that of square shape, especially the current efficiency of 3:7 of the diamond shape was close to the fine mesh electrolyzer. From the comparison among square, diamond, and double layer catalyst, the current efficiency was affected not only current distribution by a shape of dot but also reactant flow distribution in cathode side flow field.Figure 3 shows current density and cathode potential as a function of the metal loading ratio of dot and uniform layers on a dot: r dot at a current hydrogenation current density of 90, 95, and 100%. The current density was the highest at 30% of the ratio with the lowest cathode potential. Therefor, the selectivity of hydrogenation should not be controlled by kinetics of charge transfer of the electrocatalyst, and strongly affected by mass transfer process of toluene in the catalyst layer. The optimized shape catalyst layer of this study was the stack of the dot layer and thinner whole surface layer of 3:7. Acknowledgement DSE® anodes for oxygen evolution were supplied from De Nora Permelec (DNJ). Authors appreciate to Dr. A. Kato and Dr. Y. Nishiki, DNJ for the distribution of the anodes.