Gas Crossover Suppression by Controlling Wettability of Cathode Current Collector

Abstract

INTRODUCTION PEMWE can produce hydrogen gas at high pressussure, directly. The PEMWE at high pressure operation is expected to be a hydrogen gas charger to FCV at car dealer and/or isolated district in prevailing period of hydrogen station. It is expected to be a hydrogen production device to highly utilize sustainable energy. However, hydrogen gas crossover is a problem to address. High pressure difference, such as several tens MPa, imposed through PEM causes a significant amount of crossover, in which hydrogen gas produced at cathode permeates to anode, resulting in decrease of current efficiency. Also, some amount of the hydrogen gas crossed accumulates in anode and anode cylinders, and the accumulation will become safety issue in case of utilizing oxygen gas, such for regenerative fuel cell. In this study, we try to reduce the crossover by controlling the wettability of cathode current collector. The current collector controlled in wettability is expected to enhance the hydrogen bubble separation from there, leading to a less crossover and higher current efficiency. Controlling the wettability changes the contact angle between water, hydrogen bubble and the matrix of current collector, and also changes bubble dynamics, pressure and occupation in cathode current collector, leading to decreasing the crossover and increasing current efficiency. Three current correctors with different wettability are fabricated, and embedded into a PEMWE. Operating the cell under 2 MPa condition and visualizing hydrogen bubble in cathode examines the wettability effect. EXPERIMENTAL APPATUS Table 1 shows the cathode current collectors prepared. Every collector is fabricated based on carbon paper. Among of them, two indicate hydrophobicity, and one does hydrophilicity. They are named in hydrophobic, high-hydrophobic and hydrophilic current collector. Because the wettability changes in elapsed time, contact angle before and after the operation is shown in the table. The PEM used is Nafion324, on which IrO2 and Pt/C layer is formed as for anode and cathode electrode. The electrode area is 15.4 cm2. Titanium sintered compact with coating platinum is used for anode current collector. PEMWE comprising the above components is mounted on a cell evaluation system. Water at 80 and 150 cc/min is supplied into anode and cathode channel, respectively. Produced gas drains through gas-liquid separator. Hydrogen gas produced boosts up to 2 MPaG with back-pressure valve. Applied current is 1 A/cm2, corresponding to 0.065 A/cm2. This density, which is lower than that in practical cases, is expected to highlight the wettability effect. The current efficiency is obtained through the hydrogen flow rate measured with MFM located at lower reach of the back pressure valve and comparison with theoretical flow rate. Hydrogen bubble in cathode channel is visualized by a high speed camera through windows mounted on cathode separator. RESULTS AND DISCUSSION Figure 1 shows the current efficiency measured. Higher operation pressure indicates higher pressure difference of hydrogen gas between cathode and anode, resulting in increasing the crossover and decreasing current efficiency. This figure also indicates the wettability effect, in which current efficiency in the hydrophilic case is highest. Comparing the hydrophobic and high hydrophobic case suggests that later case is higher in current efficiency. The wettability effect confirmed is discussed with visualization simultaneously done. Fig. 2 is frequency and diameter when hydrogen bubble separated from the surface of cathode current collector. Highest current efficiency appeared in the hydrophilic case can be explained by the highest frequency in this case. For example, under 0.3 MPaG condition, the frequency in the hydrophilic and hydrophobic case is 30 and 17 Hz, respectively. High frequency appeared in the hydrophilic case implies that hydrogen bubble formed in catalyst layer immediately moves to channel through the collector and that accumulation of hydrogen bubble at the interface between catalyst layer and current collector is suppressed, leading to a less chance of hydrogen crossover. This mechanism explains that hydrophilic current collector functions to increase current efficiency. The reason why the high hydrophobic case indicates higher current efficiency comparing with the hydrophobic case is possibly explained by the diameter, shown in Fig. 2 (b). The large diameter measured in the high hydrophobic case suggests that hydrogen gas occupies collector pores over a wide region, and that many gas paths form in through-plane direction. The paths result in a smaller pressure difference of hydrogen gas through the collector and smaller local pressure at catalyst layer, where hydrogen gas crossover starts. Thus, high-hydrophobicity contributes to high current efficiency. Figure 1