To reduce carbon dioxide emissions, much renewable energies that are uneven distribution with fluctuation must be introduced. Toluene-methylcyclohexane organic chemical hydride system is a promising technology of hydrogen storage and transportation. We have focused on membrane electrolysis for hydrogenation of toluene with water oxidation as an efficient energy carrier synthesis process. In this system, we are using PtRu/C, perfluorocarbon sulfuric acid membrane, and DSE^®for oxygen evolution reaction as cathode electrocatalyst, electrolyte membrane, and anode, respectively. For the practical application, higher conversion without side reaction of hydrogen evolution reaction and lower cell voltage was needed (1-3).

In this study, porous flow filed with dispersed hydrogenation catalyst on the porous medium and low overpotential anode have been developed to improve conversion and cell voltage.

Experimental

A single cell electrolyzer made of titanium with 100 cm² of projected electrode area was used. A cathode that was a carbon paper (10BC, SGL) of 0.02 mg cm^-2 of Pt loading which was coated 0.5 mg cm^-2 of PtRu/C (TEC61E54, TKK) with Nafion dispersion was pressed on a perfluoroethylene sulfuric acid (PFSA) membrane (Nafion^® 117, DuPont). PtRu/C on the carbon paper and Pt in the carbon paper were electrocatalyst and non-electrochemical catalyst, respectively. A fine mesh DSE^® anode with IrO₂based electrocatalyst is used for oxygen evolution. The cathode flow field was the carbon paper of the cathode itself. Backing of the anode to push it to the membrane was titanium web.

10 cm³ min^-1 of 50 % toluene – methylcyclohexane or 0.53 cm³ min^-1 of 100 % toluene and 10 cm³ min^-1 of 1 M (=mol dm^-3) of H₂SO₄ were supplied to the cathode and anode for hydrogenation of toluene, respectively. 0.53 cm³ min^-1 of toluene correspond to 100 % of hydrogenation of toluene at 0.4 A cm^-1. Cell voltage was determined with 4 mV s^-1of voltage sweep and steady state measurements. Toluene current efficiency and conversion were determined with the steady state measurements of constant voltage electrolysis with hydrogen generation rate measurement and constant current electrolysis with gas chromatography of the product analysis, respectively.

Results and discussion

Figure 1 shows the cell voltage (a) and the current efficiency (b) as a function of the current density for 10 cm³ min^-1 of 50 % toluene – methylcyclohexane and 0.53 cm³ min^-1 of toluene feed at 60^oC. The open symbols are for the present cell design of this study, and the close symbols are for the previous cell design with the conventional mesh DSE^® and serpentine flow filed without the Pt loading in the cathode carbon paper. The circle and triangle of the cell voltage (a) are the averages of 1 – 10 s and 120 – 180 s of the measurements, respectively. The present design was much lower cell voltage of 1.83 V at 0.4 A cm^-2 and higher current efficiency of 100 % than previous design for those of 2.13 V and 90 % at 0.4 A cm^-2, respectively. Furthermore, the difference between the scan voltammogram and the steady state voltammogram and, also 1 – 10 s and 120 – 180 s of the steady state electrolysis for the present design were smaller than that for the previous design.

The porous flow field would enhance mass transfer of toluene to the electrocatalyst layer, and Pt loading in the flow field would work as hydrogenation catalyst of by-products hydrogen and toluene to increase current efficiency. The fine mesh anode would decrease the overpotential of the anode itself and that of the cathode with uniform current distribution because of the uniform contact with the membrane.

The toluene conversion to methylcyclohexane was more than 90 % at 0.4 A cm^-2 of the constant current electrolysis with 0.53 cm³ min^-1of 100 % toluene feed without significant cell voltage increase as shown in Fig. 1. Therefore, we successfully confirmed that this process will realize one-step continuous electrochemical energy carrier synthesis process with water oxidation.

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.

1) S. Mitsushima, Y. Takakuwa, K. Nagasawa, Y. Sawaguchi, Y. Kohno, K. Matsuzawa, Z. Awaludin, A. Kato, Y. Nishiki, Electrocatalysis, 2016, 7, 238.

2) K. Nagasawa, Y. Sawaguchi, A. Kato, Y. Nishiki, S. Mitsushima, Electrocatalysis, 2017, 8, 164.

3) S. Mitsushima, K. Nagasawa, Y. Sawaguchi, K. Matsuzawa, A. Kato, Y. Nishiki, PRiME2016 abstract, 2016, #2511.