Simultaneous Measurements of Water Distribution and Electrochemical Characteristics in Polymer Electrolyte Fuel Cell

In this study, neutron radiography and electrochemical impedance spectroscopy (EIS) were simultaneously used to evaluate the relation between the water amount and the electro chemical characteristics in a polymer electrolyte fuel cell (PEFC). Two-dimensional water distributions in the through-plane direction of the proton exchange membrane (PEM) were measured every 60 s during the PEFC operation. The results were compared with ionic and the polarization resistances obtained from EIS. The ionic conductivity through the PEM increased with an increase in the liquid-water content in the membrane. The effects of water content on the ionic conductivity were much smaller in comparison to the Springer’s model at a water content was less than 1. The polarization resistance increased with an increasing in liquid-water accumulation in the gas diffusion layer. Introduction Water management is a key topic of a polymer electrolyte fuel cell (PEFC). If condensed water exists in the gas diffusion layer (GDL) and the gas channel, it may depress the gas diffusion as flooding. However, the generated water must be appropriately supplied to the proton exchange membrane (PEM) for proton conduction. Hence, water management is significantly important for PEFC performance, and clarification of the water-transport mechanisms between the PEM, GDL, and gas channels is of great concern. Several investigations on water management have been carried out concerning water movement inside the PEM, water flooding in the GDL, and water plugging in the channel [1, 2]. Loss in electric power generation in the PEFC is due to resistances such as ionic, activation, concentration, etc. Many of these resistances are related to the mass transport in the PEFC. To evaluate the resistances in a PEFC, electrochemical impedance spectroscopy (EIS) has been widely used [3]. However, the effect of water in the PEFC on the resistances has not been fully understood. Aim of this study is to clarify the effect of water in the PEM and the GDL on the resistances. Simultaneous measurements of the water distribution and the electrical impedance were carried out by using neutron radiography and EIS. Changes in water accumulation in the PEM and the GDL were compared with the resistances. Experimental setup and method A schematic diagram of the PEFC is shown in Fig. 1. A proton exchange membrane was sandwiched between the gas diffusion layers and the gas channels. The separators were made of gold-plated aluminum having nine-parallel gas-channels with a cross-sectional area of 1 × 1 Neutron Radiography WCNR-11 Materials Research Forum LLC Materials Research Proceedings 15 (2020) 268-273 https://doi.org/10.21741/9781644900574-42 269 mm. The length of the channel was 10 mm. Nafion NR-212 was used as the PEM with a thickness of approximately 90 μm, having catalyst layers on both the anode and the cathode sides. The electrode area was 10 × 19 mm. The GDL was carbon paper (Toray Ind.) with thickness of 190 μm at the cathode side and 280 μm at the anode side. The porosity of the GDL was approximately 78%. Two-dimensional water distributions were measured using neutron radiography during PEFC operation. The neutron radiography facility at B-4 port in the Kyoto University Reactor (KUR) was used in this study. The total neutron flux at the beam exit was approximately 5 × 10 /cm s with a nominal thermal output of 5 MW [4]. Neutrons entered from the side-view direction and were attenuated by the PEFC including the accumulated water. The transmitted neutrons were converted to visible rays using a scintillator screen, and the 16-bit gray-scale radiographs were taken using a cooled CCD camera (PIXIS 1024, Princeton Instruments) with an array of 1024 × 1024 pixels. Exposure time for obtaining an image was set at 1 min. Obtained water-distribution represents the average value over 1 min exposure time. The details of the measurement methods are described in reference [2]. The equivalent electric circuit for the EIS analysis used is shown in Fig. 2. As the polarization resistance of the cathode is much higher than that of the anode, associated elements of the anode can be neglected [5]. Cdl is the double-layer capacitance, RPEM is the ionic resistance through the PEM, and Rpol is the polarization resistance consisting activation and concentration polarization resistances at the cathode. Impedance analysis took approximately 40 s for one measurement with a frequency range of 0.5 to 30 kHz. Therefore, an impedance measurement was complete within the time taken for a neutron radiograph measurement. Comparisons of the water distribution and the electro chemical characteristics were carried out over 1 min. Impedance analysis can be applied for steady state conditions, however, if the water distribution changes rapidly, results might be affected. Therefore, to change the water distribution gradually, the experiments were carried out at a relatively lower current density, i = 158 mA/cm and 316 mA/cm. The air and hydrogen flow rates were 66 Ncc/min and 28 Ncc/min, respectively. The oxygen utilization was 7.5% at i =158 mA/cm and 15% at i =316 mA/cm and a hydrogen utilization was 7.5% at i =158 mA/cm and 15% at i =316 mA/cm. The experiments were carried out at room temperature, while the temperature of the PEFC varied between 25 – 30 °C. Fig. 1. Schematic diagram of the polymer electrolyte fuel cell and geometry of the gas channel. Neutron Radiography WCNR-11 Materials Research Forum LLC Materials Research Proceedings 15 (2020) 268-273 https://doi.org/10.21741/9781644900574-42 270 Evaluation of the ionic conductivity Springer et al. [6] proposed a correlation between ionic conductivity of the PEM, σ, and the water content in the PEM as ( ) ( )            