The Effect of Flow Field Property in Toluene Direct Electro-Hydrogenation Electrolyzer

Abstract

Introduction Recently, the utilization of renewable energy has been expected to decrease the carbon dioxide emission. For effective utilization of the renewable energy that fluctuates and uneven distributes, the large-scale storage and transportation of hydrogen as secondary energy is important technology. Toluene (TL)/ methylcyclohexane (MCH) organic hydride system is the candidates of energy carrier, and have the advantages of efficiency, safety and handling. We have studied the high-efficient electrolytic direct-hydrogenation of toluene with water decomposition for the organic hydride system as an efficient energy carrier process.1 In these studies, the importance of mass transfer in porous carbon flow field for TL/MCH has been clarified. In this study, we have investigated the effect for electrochemical property on the thickness of the porous carbon flow field to improve mass transfer for the cathode reaction in the toluene direct electro-hydrogenation electrolyzer. Experimental Figure 1 shows the schematic drawing of toluene direct electro hydrogenation electrolyzer. A DSE® electrode (De Nora Permelec ltd) for the oxygen evolution reaction, a carbon paper (29BC, thickness=235 μm or 10BC, thickness=385 μm, SGL carbon ltd.) applied 0.5 mg-PtRu cm-2 of PtRu/C (TEC61E54, TKK) catalyst with ionomer, and Nafion 117® (Du Pont) were used for the anode, the cathode, and the proton exchange membrane (PEM), respectively. The carbon paper as the flow field was loaded 0.02 mg-Pt cm-2 of Pt particles by the impregnation method for the chemical-hydrogenation catalyst.2 The cathode was hot-pressed on the PEM at 120oC and 0.1 MPa for 3 min to fabricate a membrane cathode assembly. Geometrical electrode area was 11.6 cm-2. Operation temperature of the electrolyzer conducted at 60oC. The anode and cathode compartments were circulated 1 mol dm-3 H2SO4 and 1, 5 and 10% toluene diluted by methylcyclohexane, respectively. The tip of the Luggin capillary of a reversible hydrogen electrode (RHE) was placed near the anode. The electrochemical measurements were conducted for the linear sweep voltammetry (LSV), the electrochemical impedance spectroscopy (EIS), the chronoamperometry (CA) and the current efficiency measurement. Results and discussion Figure 2 shows the cell voltage and the internal resistance of R­1 (see inset of Fig. 2) as a function of the current density curves for two types thickness of flow field with 1, 5 and 10% TL feed at 60oC. Internal resistance of R­1 was obtained from the high frequency intercept of the Nyquist plot of the EIS mesurement. The cell voltage and R­1 of 10BC flow field was lower than that of 29BC at all TL concentration. Especially, the cell voltage of 10BC exhibited high performance about 1.83 V at 400 mA cm-2. Figure 3 shows the iR corrected anode and cathode polarization curves of with 1, 5 and 10% TL feed at 60 oC. The anode polarization curves were not affected for the change of the TL concentration and the type of flow field. On the other hand, the cathode overpotential of 10BC was lower than that of 29BC at all TL concentration. This difference was remarkable at high current density region, and it would be affected by the improvement of TL mass transfer in the flow field. Figure 4 shows the current efficiency with 1, 5 and 10% TL feed at 60oC. At 1% TL feed, obvious difference for current efficiency between 10BC and 29BC were not observed. In the initial stage of current efficiency drop at 5 and 10 % TL feed, the current efficiency of 29BC was higher than that of 10BC. However, in higher current region, 10BC showed higher current efficiency than 29BC. In this experiment, the flow rate for two types of flow field was same. At this condition, the thinner 29BC has higher linear velocity than 10BC, and is easy to supply with toluene to the catalyst layer. Hence, 29BC might exhibit the high current efficiency in lower current density region. Under 90-95% current efficiency, which the hydrogen generation increased, the thicker 10BC exhibited the higher current efficiency than the 29BC. It would be considered that the thinner 29BC is easy to occupy the hydrogen gas in the flow field, and the toluene supply to the catalyst layer was insufficient. Acknowledgements This work was partially supported by the Toyota Mobility Foundation and Core Research for Evolutional Science and Technology (CREST) “innovative reaction” (Funding agency: JST). Anode was supplied by De Nora Permelec Ltd. Reference Mitsushima, Y. Takakuwa, K. Nagasawa, Y. Sawaguchi, Y. Kohno, K. Matsuzawa, Z. Awaludin, A. Kato, and Y. Nishiki, Electrocatalysis, 7, 127 (2016). Nagasawa, Y. Sawaguchi, A. Kato, Y. Nishiki, and S. Mitsushima, Electrochemistry 86, 339 (2018). Figure 1