Water Electrolysis Using Water Absorbing Porous Electrolyte Cell

Abstract

We have reported so far water electrolysis by use of a water absorbing porous electrolyte that works under similar conditions to a polymer electrolyte electrolysis cell (PEEC) [1]. This cell consist of a surface-proton conducting metal oxide nanoparticles as proton-conducting electrolyte, a controlled-hydrophobic electrocatalytic layer, and fully hydrophobic gas diffusion layer (GDL) to generate pressurized hydrogen and oxygen gases. In this presentation, we will discuss the optimized preparation condition for membrane electrode assembly (MEA), which consist of electrolyte, electrode, and GDL layers. We also show that the hydrogen gas generation rate depend on the supplied water pressure (ΔP).MEA of the water electrolysis cell is prepared as follows: The GDL films were formed by compressing of acetylene black/poly(tetrafluoroethylene) (PTFE) (=1/6, weight ratio) composite powder using Newton Press under 6 kN cm-2 for 1 min and successive heat treatment at 360 °C for 15 min. Electrode part with Pt loading of 2.0 mg cm-2 on top of the GDL films was formed using hot-press at 300 °C for 3 min under 300 kgf cm-2, which is consisted of consisted of platinum carbon (Pt/C) / PTFE / titanium phosphate ionomer composite. Finally, electrolyte layer on top of electrode layer was prepared by coating of titanium sulfonate/Nafion (=9/1, weight ratio) slurry and hot-pressing at 140 °C under 100 kgf cm-2 for 1 min. The fabricated MEAs were different sizes with diameters of 30 mm and 12 mm for anode and cathode sides, respectively. The current-voltage characteristic and generation hydrogen gas is measured by putting the fabricated MEA into the cell as shown Figure 1. The distilled water is supplied from cathode side and is pressurized using nitrogen gas in the ΔP range from 0 MPa to 0.25 MPa. The gas evolution rates of hydrogen during water electrolysis were evaluated by gas chromatography to be determined by 1% hydrogen/argon calibration curve. The gas evolution rate of hydrogen increased with increasing ΔP from 0 MPa to 0.10 MPa, and reaches a maximum value at 0.15 MPa. At ΔP higher than 0.15 MPa, the rates decreased to almost the same value at 0 MPa. We supposed that the generated gas was leaked to outside of the cell in lower pressure region than 0.15 MPa. On the other hand, in higher region than 0.15 MPa, the generated gas in the water phase by water electrolysis was not completely separated to gas phase. The obtained evolution rate was compatible with the theoretical value calculated by Faraday’s law. However, electrolysis voltages were high compared with conventional PEEC. Possible reason is inhomogeneous state of electrode layer and electrolyte layer with lower electrical conductivity. Acknowledgements This work was supported by CREST in JST. References (1) S.-J. Kim, T. Sakai, H. Oda, J.-I Hamagami, Y. Okuyama, M. Matsuka, S. Ohta, Y. Shimizu, T. Ishihara, H. Matsumoto Electrochem. 2012 80(4), 246. Figure 1