Relationship between Hydrophilicity of Cathode Porous Flow Field and Current Efficiency for Toluene Direct Electro-Hydrogenation Cells
Authors: Shinohara Rio , Ikegami Kaoru , Kuroda Yoshiyuki , Mori Hirotaka , Kanada Yuka , Yamashita Junya , Mitsushima Shigenori 
Published: December 2024

Introduction

Organic chemical hydrides have been attracted as an energy carrier for mass storage and transportation of renewable energy. In particular, a toluene (TOL) / methylcyclohexane (MCH) system has advantages in terms of using conventional petroleum infrastructure and relatively low toxic properties. A TOL direct electro-hydrogenation cell has a cathode catalyst layer and electrolyte which are similar to proton exchange membrane fuel cells. Anode reaction is oxygen evolution. An issue of the cell is the electro-osmosis of water through the electrolyte membrane to the cathode catalyst layer. It inhibits the diffusion of TOL into the catalyst layer and promotes H2 evolution as a side reaction.1 Therefore, it is important for a cathode porous flow field that not only the flow of TOL and MCH but also the enhancement of water and H2 discharge. In this study, we focused on the hydrophilicity of the cathode porous flow field and investigated the relationship between current efficiency and hydrophilicity of the porous flow field.

Experiments

Nafion115® (DuPont Inc.) was used as the electrolyte membrane. A DSE® for oxygen evolution (De Nora Permelec Ltd.) was used as an anode and 1 mol dm-3 H2SO4 was circulated as anolyte at 10 ml min-1. As a cathode, 1 mg-PtRu cm-2 of PtRu/C (TEC61E54, TKK) with 0.8 of ionomer/carbon weight ratio, was coated on the membrane. 10 vol. %TOL - MCH was circulated at 10 ml min-1. As the cathode porous flow field, carbon papers with various contact angles and thicknesses were used as shown in Table 1. The substrates were named "c"-contact angle-(thickness). Here, the C0(660), C0(320), and C145(280) had the same pore structure with various wettability and thickness. The C138MPL(280) is a commercially available carbon diffusion layer (39BB, SGL carbon Ltd.), which had a contact angle of 138° on both with and without a microporous layer (MPL). The internal resistance was measured by the high-frequency intercept of the Nyquist plot of impedance measurements after 3 minutes of constant current electrolysis. IR-free cell voltage was determined by subtraction of multiplied by the resistance and the current value from the measured cell voltage. Current efficiency was calculated with Faraday's law from the volume ratio of TOL/MCH solution to H2 gas after 3 minutes of constant current electrolysis.

Results and discussion

Figure 1 shows the dependence of cell voltages (open plots) and current efficiencies (close plots) on current density for TOL direct electro-hydrogenation cells with various cathode porous carbon flow fields. There were no significant differences in cell voltages for the cells. On the other hand, current efficiencies were significantly affected by the property of the porous carbon flow field. The C0(320), which was hydrophilic, had the highest current efficiency. Following the C0(320), the C69(220) and the C145(280) were high in this order. So, the more hydrophilic carbon papers showed the higher current efficiency. This implies that the use of a hydrophilic porous flow field promotes water removal in the cathode catalyst layer. As the effect of thickness in the same contact angle, the thinner cathode porous flow field of the C0(320) was higher than thicker that of the C0(660). This implies that a thin flow field would enhance TOL supply, and this means the flow velocity in line also affects current efficiency. C138MPL(280) was the lowest current efficiency. This may be due to the pore size of the MPL on the porous flow field in contact with the catalyst layer. The pores in the porous carbon flow field between the fibers and the MPL were fine nanocarbon particles with Teflon bonding; therefore, the pore size and the porosity of MPL are much smaller than those of the flow field. Then The lowest current efficiency of the C138MPL(280) would be affected by slow transport of TOL supply with slow discharge of MCH and water.
In conclusion, it was suggested that the use of hydrophilic, thin, and large pore size cathode porous flow field facilitated the mass transportation of TOL into the catalyst layer and the emission of water and MCH from the catalyst layer and suppressed side reactions.

References

(1) Kensaku Nagasawa, Yuki Sawaguchi, Akihiro Kato, Yoshinori Nishiki and Shigenori Mitsushima, Electrocatalysis, 8, 164-169 (2017).

Figure 1