Evaluation of Anode Porous Transport Layer Using Polarization Separation Method on PEM Water Electrolysis

Abstract

Introduction Recently, proton exchange membrane water electrolysis (PEMWE) has been expected as the hydrogen production technologies to convert the renewable energy with regional and temporal unevenness into hydrogen, which can be stored and transported. PEMWE has a high energy conversion efficiency at the high current density, however it is required to reduce the high material cost due to the utilization of noble metals. Currently, the development of new components and the reduction the amount of noble metals have been conducted. The porous transport layer (PTL) is one of the most important elements to improve the efficiency of PEMWE. PTLs have the roles such as the transporting electrons between the catalyst layer (CL) and the end plate, the discharging the generated gas, and supplying water to CL. Therefore, the accurate separation and determination of the polarization in the electrolyzer are essential technologies. In that case, the potential measurement using reference electrodes can be useful. In this study, we measured the anode and cathode polarizations by the elimination of membrane resistance, and evaluated the polarization of anode PTLs with a small size electrolyzer using double references. Experimental Figure 1 shows the schematic drawing of 1 cm2 electrolyzer with double references. Hydrogen storage Pd reference (PdH) were placed on the membrane by the contact with 0.5 M H2SO4. Anode and cathode electrocatalysts were IrOx (TKK) and Pt/C (TKK), respectively. The 1.0 mgIr/cm2 of anode catalyst was decaled on PEM (Nafion®115, DuPont) with Nafion ionomer, and the 1.0 mgPt/cm2 of cathode catalyst was assembled on the membrane as a catalyst coated electrode supported a carbon paper (39BC, SIGRACET®). Pt plated Ti (MMC) and the carbon paper were used as the anode and cathode PTL, respectively. These electrodes were fixed straight (x = 0 mm) or shift (x = 0.3, 0.5, 0.7, 1.0 mm) stacking as shown in Fig. 1. Operation temperature of the electrolyzer conducted at 80℃. DI water was pumped and induced at flow rates of 10 ml/min and 2 ml/min to the anode and cathode, respectively. The performance was evaluated by the chronoamperometry (CA) and the electrochemical impedance spectroscopy (EIS). Results and discussion Figure 2 shows the relationship between the inner resistance R ac obtained from high frequency resistance of EIS and the direct current resistance R dc between references. The slopes of line, which passed through the origin, became larger as the increase of shift amount. As the slope represented θ, they can be written by equation (1). Next, using the α, the relationship between R dc and R membrane is defined by equation (2). Here, assuming that the contact resistance between PTL and CL is sufficiently small, R ac is equal to membrane resistance R membrane. Hence, R membrane can be calculated as α is equal to θ.Figure 3 shows the Tafel plots of anode and cathode polarization curves using reference electrodes of anode or cathode sides, and the membrane iR removed polarization curves E corrected , assuming the reference potential was equivalent to 8 % (= (100-84)/2) from the electrode/electrolyte interface (x = 1.0 mm). E corrected on cathode side was approximately identical to the extrapolated Tafel slope, and there was no mass transfer effect, which is the difference between the two curves. The reason is that the cathode side was only hydrogen generation by supplying electrons. On the other hand, E corrected on anode side showed the overpotential for the extrapolation of Tafel slope above anode potential of 1.4 V vs. PdH, and especially in the high current density area, it was affected by larger mass transfer resistance than cathode side. This overpotential was considered the water/oxygen transfer resistance or/and the transfer resistance related to the electrons in CL around PTL.Figure 4 shows the comparison of overpotential for anode PTLs. In this current density range, these plots showed the linear relationship between the overpotential and the current density. To compare the anode PTLs, PTL A indicated smaller overpotential than PTL B with smaller pore size and bigger porosity.In conclusion, the double references potential measurement with shift arrangement of electrodes enabled to separate the polarization into anode and cathode, and remove the membrane iR. Based on this method, this measurement method using the developed electrolyzer indicated to be able to evaluate the difference of PTL overpotential for the several type PTLs. Acknowledgment This study was supported by the development of fundamental technology for advancement of water electrolysis hydrogen production in advancement of hydrogen technologies and utilization project (JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO) in Japan. Figure 1