Improvement of Current Efficiency for a Hydrogenation Electrolyzer with Low Concentration of Toluene
Authors: Sawaguchi Yuki , Morita Naoto , Nagasawa Kensaku , Matsuzawa Koichi , Kato Akihiro , Nishiki Yoshinori , Mitsushima Shigenori 
Published: February 2020

Introduction
Usage of renewable energy has been got a lot of attention to find a way to sustainable society against environment and energy issues. To supply of renewable energy on demand, large-scale of energy storage and transportation is needed. Hydrogen is expected to apply for the energy storage and transportation. We have been paid attention the organic hydride, especially toluene (TL)-methylcyclohexane (MCH) system as a carrier for energy storage and transportation, because it is higher energy density than hydrogen with easy to handle, and can use the petroleum infrastructure in existence.
Direct electrohydrogenation of TL applied polymer electrolyte fuel cell and industrial electrolysis technologies is a simple process with high energy conversion efficiency [1-3]. In this process, hydrogen is a byproduct to decrease current efficiency of chemical hydrogen. Therefore, the suppression of hydrogen evolution is needed.
In this study, we have investigated the effect of catalyst-loaded backing of electrocatalyst layer on the polarization and the current efficiency with low concentration TL feed to improve cell performance.

Experimental
Mixture of ethanol, H2PtCl6・6H2O and H2O ( the ratio is 3.0 : 0.3 : 1.0 ) were used as a precursor of Pt catalyst on a carbon paper (35BC, SGL Carbon). The paper was dipped in the mixture, and dried at 60oC for 10 min. Then it was heated at 240oC for 6 h in nitrogen atmosphere to prepare Pt loaded cathode backing (called CPt). The carbon paper backing with no Pt is called Cnon-Pt.
An anode was the DSE® (De Nora Permelec Ltd.) electrode for oxygen evolution. A cathode was 0.5 mg cm-2 of PtRu/C (TKK) coated on a CPt or Cnon-Pt, and it was hot-pressed(120oC, 0.27 MPa) on a Nafion® 117(DuPont). The anode side of the membrane was made hydrophilic before the hot-pressing. Geometrical electrode area was 11.3 cm-2. 10 ml min-1 of 1 M (=mol dm-3) sulfuric acid, and 5 ml min-1 of 10 mol% TL / MCH were circulated to the anode and the cathode compartments, respectively. 10 mol% concentration of TL feed simulates at the vicinity of outlet in a practical electrolyzer. A Luggin capillary for a RHE was placed near the anode. Polarization was evaluated with constant cell voltage measurement for 5 min at 1.3-5.0 V and 60oC. During the constant cell voltage measurement, the volume of generated hydrogen was determined to evaluate the current efficiency. Resistance of electrolyte evaluated from high frequency intercept in AC impedance at 10-1~105Hz.
Results and discussion
Figure 1 shows the IR corrected polarization curves with 10 mol% TL feed with CPt (circular) that was loaded about 0.8 mg cm-2 of Pt and Cnon-Pt (triangle, diamond). Upper and lower vertical axes are anode and cathode potentials, respectively. Dashed dotted line shows the theoretical potential of 1.23 V and 0.15 V vs. RHE for anode and cathode reaction, respectively. The polarization curves of the anode with the CPt and Cnon-Pt were almost the same, while the potential of Cnon-Pt cathodes were lower than that of CPt cathode above 300 mA cm-2of current density region. The former showed diffusion limitation, while the latter did not show.
Figure 2 shows the current efficiency (WTL
→MCH) with 10 mol% concentration of TL feed at 60OC. Side bar shows the fluctuation of current density during constant cell voltage operation. At the low current density below 300 mA cm-2, the WTL
→MCH of the CPt was significantly higher than that of the Cnon-Pt. The WTL
→MCH of Cnon-Pt rapidly decreased when the current reached to 400 mA cm-2, which related to the large change of the cathode potential as shown in Figure 1. In all the current regions, the fluctuation of the CPt was significantly smaller than that of the Cnon-Pt. Therefore, the CPtwould promote the catalytic hydrogenation of TL with hydrogen bubble generated as side reaction in cathode. The decrease of hydrogen gas in the backing would lead to improve the TL mass transfer.

Acknowledgments

This work was supported by Cross-ministerial Strategic Innovation Promotion Program (SIP), “energy carrier” (Funding agency: JST). The Institute of Advanced Sciences (IAS) in Yokohama National University was supported by the MEXT Program for Promoting Reform of National Universities. We appreciate the person concerned them.
References

K. Ota, A. Ishihara, K. Matsuzawa, and S. Mitsushima, Electrochemistry, 78, 970 (2010).

N. Itou, Hydrogen Energy Systems Society, 33, 9 (2008).

S. Mitsushima, Y. Takakuwa, K. Nagasawa, K. Matsuzawa, Z. Awaludin, A. Kato, Y. Nishiki, Electrocatalysis, 7, 127 (2016).

Figure 1