Separation of Polarization for Proton Exchange Membrane Water Electrolyzers

Abstract

Introduction Proton exchange membrane water electrolysis (PEMWE) has high energy conversion efficiency with very high current density because of the PEM, however reduction of material cost is needed. Therefore, accurate determination of the electrolyzer with separation of polarization is essential to R&D. Electrochemical impedance spectroscopy is widely used to separate resistance and reaction polarization using relaxation behavior, but it’s difficult to separate anode and cathode reaction polarization, ionic resistance and contact resistance of the interface of mass transfer layer and electrocatalyst layer, and the separation of the polarization for PEMWEs is more important for polymer electrolyte fuel cells (PEFCs) because of importance of contact resistance. In this study, we have developed a small size electrolyzer with double reference electrodes and edge arrangement – controlled membrane – electrode assembly to separate polarizations of electrolyte resistance, anode and cathode polarization, and contact resistance. Experimental Figure 1 shows the schematic drawing of 1 cm2 electrolyzer with double references and independent effective area contact control system. It makes easy setup to optimize current supplier and gasket contact pressure independently. Ag/AgCl references placed on the membrane of seal area contacted with electrolyte solution. As inset drawing in Fig. 1, straight and shift electrode edge arrangements were compared to correct ohmic drop between electrodes and references. Hardware of anode side was made of Ti with Pt plate to reduce contact resistance, and that of cathode side was PEFCs grade glassy carbon. Anode and cathode electrocatalyst were fine IrOx (TKK) and PtRu/C (TKK), respectively. The 1.0 mgcm-2-metal of anode catalyst was transcript on Nafion 117 membrane with Nafion ionomer, and the 0.5 mgcm-2-metal of cathode catalyst was assembled on the membrane as a catalyst coated electrode supported a carbon paper (35BC, SGL). When the electrodes were assembled electrodes was fixed straight or 0.5 mm of shift stacking as the inset of Fig. 1. The performance was evaluated linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). Results and discussion Figure 2 shows cell voltage as a function of current density for straight and shift arrangement membrane – electrodes assemblies (MEAs). Measured cell voltage for the shift arrangement MEA was slightly larger than that for straight arrangement MEA, and the AC resistances from the EIS measurements for shift and straight arrangements were 0.27 and 0.20 Wcm-2, respectively. The iR free cell voltages were almost the same, so this difference would be affected with contact resistance of reproductivity level. Figures 3 and 4 shows measured anode and cathode potentials as functions of current density. The iR corrected polarization curves from the shift arrangement are also in Figs. 3 and 4. Here, solid lines are for measured potentials, and thick and thin lines are for shift and straight arrangement, and these values are affected by iR polarization of the electrolyte membrane. Because of theoretical current distribution around electrodes edge, position of the reference electrode of straight arrangement corresponds to the center of the electrolyte membrane, and that of shift arrangement for the Rc and the Ra corresponds to near the cathode and the anode, respectively. Here, the measured potential with straight with Rc was lower than others for both anode and cathode even in low current density, so this would not affected by IR. Therefore, there is no significant measured potential difference between vs. Rc and Ra of the straight arrangement. In the shift arrangement, the measured anode potential vs. Ra was lower than that of Rc, and the measured cathode potential vs. Rc was lower than that of Ra. Apparent DC resistance, which was determined by the measured potential difference between vs. Ra and Rc divided by current for both anode and cathode, was 81% of AC resistance from the EIS measurement. So, the effective distance from anode or cathode to Ra or Rc was assumed 9.5% of the membrane thickness, respectively. Dashed lines in Figs. 3 and 4 were iR corrected anode and cathode potentials and they were almost straight, which follows Tafel equation. In addition, the anode and cathode Tafel slopes were 43 and 62 mV dec-1 of reasonable values, respectively. In conclusion, double reference potential measurement with shift arrangement gives information of electrolyte membrane resistance directly. So, this information is useful to determine anode and cathode overpotential, contact resistance, and electrolyte resistance with the EIS measurements. Acknowledgment This study was based on results obtained from the Development of Fundamental Technology for Advancement of Water Electrolysis Hydrogen Production in Advancement of Hydrogen Technologies and Utilization Project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Figure 1