Mikko Mikkola

Flooding of Gas Diffusion Backing in PEFCs Physical and Electrochemical Characterization

In polymer electrolyte fuel cells (PEFCs) gas diffusion backings (GDBs) have a significant effect on water management and cell performance. In this study, methods for characterizing GDB performance by fuel cell testing and ex situ measurements are presented. The performance of four different commercial GDB materials was tested and significant differences were found between the materials. While the performance and behavior are almost similar in the single-phase region, the flooding behavior of different GDBs in the two-phase region varies widely. The results show that using high clamping pressures increases cell flooding, but the increase varies from material to material. Increased flooding is caused by the combination of decreased porosity and a temperature difference between GDB and current collector. Furthermore, it was observed that the decrease in porosity due to cell compression and corresponding increase in mass-transfer resistance should be studied in the single-phase region, because flooding of the GDB easily becomes the dominating source of mass-transfer resistance. In addition, a literature review on GDB studies and characterization methods was carried out. The review revealed a lack of an established GDB testing regime and the absence of a relation between physical properties of the GDB and fuel cell performance.

Measurement of current distribution in a free-breathing PEMFC

A measurement system for the mapping of current distribution in a free-breathing polymer electrolyte membrane fuel cell (PEMFC) is introduced. In the measurement system, the ridges of the flow-field are made of gold-plated stainless steel and the rest of the measurement plate is made of a non-conducting material. The gas diffusion layer is not segmented and the error resulting from this is analyzed computationally. The effect of the cell temperature on the current distribution is studied with the measurement system. It appears that the measurement system is useful for PEMFC characterization and even large spatial variations of current density can be measured with it. According to the results, the optimum operating temperature for the studied cell is around 60 °C without external humidification. In addition, it is concluded that the molecular diffusion is dominating mass transport mechanism at low temperatures but the current density profile is more homogeneous at elevated temperatures.

Measurement of ohmic voltage losses in individual cells of a PEMFC stack

The ohmic voltage loss in a fuel cell can be determined with the current interruption method. The method was utilized to measure the ohmic voltage loss in an individual cell of a fuel cell stack. This was achieved by producing voltage transients and monitoring them with a digital oscilloscope connected in parallel with the individual cell. In this study, the method was applied to a small polymer electrolyte membrane fuel cell (PEMFC) stack in which different air supply levels were employed on the cathode side. In the case of higher air-feed rate, the results revealed an increase of ohmic losses in the middle of the stack by up to 21% at 400 mA cm � 2 , compared to the unit cell with the lowest ohmic loss. This probably resulted from the decrease of membrane conductivity because of drying. Comparison to individual cell voltages showed that the decrease of conductivity would not be observed if only the individual cell voltages alone were measured. The total ohmic loss in the stack was measured using the same method to verify the reliability of the measurement system. The results indicate a good agreement between the total ohmic loss and the combined ohmic losses in the individual cells. # 2002 Elsevier Science B.V. All rights reserved.

Effect of ambient conditions on performance and current distribution of a polymer electrolyte membrane fuel cell

The performance and current distribution of a free-breathing polymer electrolyte membrane fuel cell (PEMFC) was studied experimentally in a climate chamber, in which temperature and relative humidity were controlled. The performance was studied by simulating ambient conditions in the temperature range 10 to 40 °C. The current distribution was measured with a segmented current collector. The results indicated that the operating conditions have a significant effect on the performance of the fuel cell. It was observed that a temperature gradient between the fuel cell and air is needed to achieve efficient oxygen transport to the electrode. Furthermore, varying the air humidity resulted in major changes in the mass diffusion overpotential at higher temperatures.

Determination of mass diffusion overpotential distribution with flow pulse method from current distribution measurements in a PEMFC

The mass diffusion overpotential distribution in a free-breathing proton exchange membrane fuel cell (PEMFC) was determined from current distribution measurements using a flow pulse approach. The current distribution measurements were conducted with a segmented flow-field plate. Flow pulses were fed to the cathode channels to form a uniform oxygen concentration distribution along the channels. Simultaneously, the cell resistance was monitored using the current interruption method. From the experimental data, the mass diffusion overpotential distribution was calculated using the Tafel equation. The results show that the mass diffusion overpotential in different parts of the cell may vary considerably, for example, at 180 mA cm−2 the mass diffusion overpotential difference between the bottom and top part of the cell was 0.1 V.

Mass transport in the cathode of a free-breathing polymer electrolyte membrane fuel cell

In small fuel cell applications, it is desirable to take care of the management of reactants, water and heat by passive means in order to minimize parasitic losses. A polymer electrolyte membrane fuel cell, in which air flow on the cathode was driven by free convection, was studied by experimental and modelling methods. The cathode side of the cell had straight vertical channels with their ends open to the ambient air. A two-dimensional, isothermal and steady state model was developed for the cathode side to identify the limiting processes of mass transport. The modelled domain consists of the cathode gas channel and the gas diffusion layer. Experimental data from current distribution measurements were used to provide boundary conditions for oxygen consumption and water production. The model results indicate that at the cell temperature of 40 °C the performance of the cell was limited by water removal. At the cell temperature of 60 °C, the current distribution was determined by the partial pressure of oxygen.

Titanium sinter as gas diffusion backing in PEMFC

The use of a titanium sinter material as gas diffusion backing (GDB) in a polymer electrolyte membrane fuel cell (PEMFC) has been evaluated. The study focused on reducing the high contact resistance between the sinter and MEA using platinum and carbon coatings evaporated on the surface of the sinter. The results show that the titanium sinters are a promising solution for gas diffusion backing in a fuel cell and that the initially high contact resistance observed can be effectively reduced with platinum coating. The coatings also reduce the mass diffusion overpotential observed with the sinter. # 2003 Elsevier Science B.V. All rights reserved.