Study of Water Distribution and Transport in a Polymer Electrolyte Fuel Cell Using Neutron Imaging

Participants: M. Mench, Professor of Mechanical and Nuclear Engineering
  Jack S. Brenizer, Professor of Mechanical and Nuclear Engineering
  Kenan Ünlü, Professor of Mechanical and Nuclear Engineering
  P.A. Chuang, Ph.D
   
  N. Pekula, M.S.
  A. Turhan, Graduate Student
  K. Heller, Graduate Student
   
Services Provided: Neutron Beam Laboratory
   
Sponsor: General Motors Corporation
  RSEC

 

Introduction

Due to its high efficiency, low operating temperature (~30-80 oC), and rapid evolution since over the past decade, the polymer electrolyte fuel cell (PEFC) is currently under intense research and development. Compared to present power systems, such as the internal-combustion engine, fuel cells are advantageous for several reasons and present a promising future. The operating efficiencies can reach as high as 50-90% for units and also pollutants such as nitrous oxides and particulate matter are eliminated, while carbon dioxide and carbon monoxide are reduced to near zero.

Figure 1 shows the basic operation of a hydrogen PEFC. Hydrogen is supplied to the anode of the fuel cell while oxygen, usually taken from the air, is supplied to the cathode. The electrochemical oxidation reaction at the anode produces hydrogen ions and electrons.

 

Figure 1: Simplified Schematic of PEFC

The flow of the electrons through an external circuit powers a load. The ions produced at the anode are transported through an ionically conductive polymer electrolyte to the cathode, where water is produced. A ~10-20 m m platinum catalyst layer is typically employed at both electrodes to reduce the activation energy for the electrochemical reactions. Covering each electrode is a 200-400 m m porous carbon fiber gas diffusion layer (GDL). The GDL functions to enable reactant transport to, and product from the catalyst layer, while providing conductivity for electron transport.

In a PEFC, the level of water must be precisely balanced. Adequate water vapor must be available maintain high electrolyte ionic conductivity and ensure suitable performance. However, if excessive water is present in the liquid phase, it can block pores in the catalyst and GDLs, hindering the transport of reactants to the catalyst. This phenomenon is known as “flooding”, and greatly diminishes cell performance. Due to the delicate balance between the benefit of saturated flow and the deleterious effects of flooding concomitant with liquid water accumulation, there is extensive ongoing research to more fundamentally understand two-phase water transport in PEFCs, to enable performance and design optimization.

Although there have been numerous models presented in literature that predict the water production and transport phenomenon in fuel cells, there has been little research in experimental visualization and quantification of the liquid water distribution and transport. Neutron radiography and radioscopy are excellent non-intrusive techniques for visualization and quantification of the two-phase flow within the fuel cell in real time or steady-state.

Experimental Setup

The neutron radioscopy system and thermal neutron beam from the Breazeale Nuclear Reactor at the Penn State Radiation Science and Engineering Center was utilized in this study. Specialized image processing hardware was developed for the analysis, storage and presentation of the collected images.

An integrated test station at the Neutron Beam Lab (NBL) was built to control and monitor the fuel cell operating parameters. The NBL Test Station (NBLTS) is isolated from the neutron beam source, as illustrated in Figure 2. The station can accommodate various sized fuel cells (up to 22.8 cm diameter in a single frame) for neutron imaging processes while the following conditions are controlled by the operator on the station’s control panel:

The visualization and quantification of the water distribution in the fuel cell is performed by neutron radioscopy and radiography techniques. Data acquisition and data presentation details for these techniques are described in the next section of this report.

Figure 2: Test station for fuel cell imaging at the NBL

 

Neutron Radiography Results[1]

A series of neutron radiographs were collected using the fuel cell and imaging setup . The cell temperature (80°C) and gas flow back pressure (0.274 MPa) were maintained constant throughout the experiments whereas the relative humidity of the anode and cathode were maintained at 100% at 80 oC for all t ests . Figure 3 shows substantial water accumulation near the fuel cell exit (lower left corner) for radiographs taken at the low current density condition, i.e. 0.05 A/cm 2. The images show liquid water occupying a large portion of the gas flow channels causing channel flooding inside the fuel cell. From the flow channel geometry (2-channel pass design on anode and 3-channel design on the cathode) it can be concluded that the liquid water is on the anode side of the cell.

Figure 4 shows three separate radiographs taken at the high current density condition, i.e. 1.0 A/cm 2.The images show that highly dispersed liquid droplets are present in the lower half of the fuel cell, and that the flow channels near the cell inlet (upper third) contain almost no liquid water at all. This is most likely due to the water vapor being continually added to the gas flow stream along the flow channel path via generation until saturation. Through the cell exit, water accumulation is observed due to liquid saturation of the gas flow.

 

Real-Time Neutron Radioscopy Results

Real-time (30 fps) neutron radioscopy video of the operating fuel cell was recorded for a wide variety of test conditions . Video was recorded for approximately 20 minutes for each experiment. The high temporal resolution of the radioscopy procedure allowed for the liquid water accumulation and transport in the cell to be directly observed. Further image analysis gave insight into the characteristics of the liquid water droplet behavior and flow velocities.

The results of velocity measurements indicate that the droplet velocity cannot be assumed to be on the same order of magnitude as the gas flow velocity for the fuel cell configuration and conditions tested, and a homogeneous, no-slip model of the two-phase channel flow is inappropriate for channel level two-phase modeling.

 

Conclusions

Neutron radiography and radioscopy yield excellent spatial and temporal resolution for the investigation of water transport phenomenon and the measurement of liquid water inside an operating polymer electrolyte fuel cell. The results showed that the amount of water accumulation in the flow channels highly depends on current density. It was also shown that channel-level liquid droplet velocity is not constant, and changes substantially due to interactions with the flow channel walls and other droplets. The maximum velocity of the droplets are an order of magnitude less than the reactant gas flow.

 

References

[1] N. Pekula , K. Heller, P. A. Chuang, A. Turhan, M.M. Mench, J. S. Brenizer, K. Ünlü, “ Study of Water Distribution and Transport in a Polymer Electrolyte Fuel Cell Using Neutron Imaging,” Nuclear Instrumentation Methods, Section A. (Accepted for publication)