Internal Resistance Reduction of a Membrane Electrolyzer for Electrohydrogenation of Toluene As Hydrogen Carrier Synthesis
Potential of renewable energy is uneven distribution with fluctuation; therefore, to increase renewable energies, energy carrier technology is needed for storage and transportation. Hydrogen can be produced with various energies, but its volumetric energy density is too low to storage and transport. Toluene-methylcyclohexane organic chemical hydride system is one of promising technologies as hydrogen storage and transportation by invention of the dehydrogenation catalyst (1). Electrohydrogenation of toluene with water splitting has higher theoretical energy conversion efficiency compare to a series process of water electrolysis and hydrogenation. Direct and indirect electrohydrogenation processes had been proposed (2, 3). However, further challenge is needed to improve efficiency, durability, and increasing of the scale.
We focused on a membrane electrolysis as a direct process with water splitting. Cathode side is a cathode membrane assembly, which is applied PEFC technology with precious metal loaded carbon electrocatalyst. Anode is a dimensionally stable electrode for oxygen evolution reaction in acidic electrolyte using industrial electrolysis technology. In our previous study, cell voltage of electrohydrogenation of toluene was almost as same as water electrolysis, but internal resistance of the electrolyzer was not small enough and conversion was very low (4).
In this study, the membrane of anode side was hydrophilized to release oxygen from anode to reduce internal resistance, and characteristics of low concentration of toluene has been evaluated to reach high conversion to methylcyclohexane.
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® 212CS, DuPont) for a cathode membrane assembly. The membrane of the cathode side was hydrophilized by ZrO2 power coating or mechanically roughing. A non-treated membrane was also used for comparison. A DSE® anode with IrO2 based electrocatalyst is used for oxygen evolution.
Toluene/methylcyclohexane mixture and 1M (=moldm-3) of H2SO4 were supplied to the cathode and anode for hydrogenation of toluene, respectively. The toluene concentration was in the range from 1 to 100 %.
Cell voltage was determined with 4 mVs-1 of voltage sweep from 0.6 for toluene hydrogenation. 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 in the cell voltage range from 1.0 to 2.5 V.
Results and discussion
Figure 1 shows the internal resistance and cell voltage as a function of current density for 100 % TL fed with various membrane treatments of anode side. The output of H2SO4 pulsed especially in high current density region, and current fluctuated above 2.2 V of the cell voltage with hydrogen evolution from cathode. This behavior would be affected by oxygen release between the membrane and the anode. The cell voltage and resistance of the hydrophilized membranes was smaller than those of non-treated membrane. At this moment, the output of H2SO4 and oxygen mixture was smooth, and there was no gas evolution reaction from cathode.
Figure 2 shows the cell voltage (a) and current efficiency of hydrogenation as a function of current density for 1 to 100 % of TL-MCH fed with the rough surface membrane cell. The side reaction is only hydrogen evolution for this system. Error bars were current variation during constant cell voltage electrolysis for current and current efficiency measurements. The lines show the polarization curves with 4 mVs-1 of cell voltage sweep. For 100 and 50 % of TL fed, the current efficiency of electrohydrogenation was above 97 % up to the limit of current source of 0.44 Acm-2. At this moment, electrolysis was stable, and the cell voltage was around 2.0 V at 0.4 Acm-2. Below 10 % of TL fed, simultaneously hydrogen evolution proceeded at higher current density region with current variation. Therefore, this process should be feasible, and improvement of mass transfer in the cathode catalyst layer is important issue for real application.
This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “energy carrier” (Funding agency: JST). We applicate the person concerned them.
1) Y. Okada, E. Sasaki, E. Watanabe, S. Hyodo, H. Nishijima, J. Hydrogen Energy, 2006, 31, 1348.
2) T. Hirashige, T. Ishikawa, M. Sugimasa, WO 2011/122155 Al (PCT/JP2011/053478) 2011.
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4) S. Mitsushima, Y. Takakuwa, Y. Kohno, K. Matsuzawa, A. Kato, Y. Nishiki, Abstract of 226th meeting of the ECS, #619 (2014).