Matti Noponen

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.

Analysis of a Two-Phase Non-Isothermal Model for a PEFC

A non-isothermal, two-phase model for a polymer electrolyte fuel cell (PEFC) is presented, analyzed, and solved numerically under three different thermal, and two hydrodynamic, modeling assumptions ...

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.

Feasibility of Autothermally Reformed Natural Gas on Anode Supported Solid Oxide Fuel Cells

Three different planar anode supported solid oxide fuel cells (SOFC) were tested with hydrogen, with autothermally prereformed natural gas from which sulfur was removed, and with autothermally prereformed natural gas that contained sulfur. The cells were obtained from Forschungszentrum Julich (FZJ), Energy research Centre of the Netherlands (ECN), and HTceramix SA (HTc). All cells were so called Real-SOFC first generation cells. Cell polarizations were first measured with hydrogen, followed by a 200 h test (25 A, 800°C) with a selected fuel, and finally cell polarizations were measured with hydrogen. When hydrogen was used as the fuel in the 200 h test, the performance for all cells was comparable and no degradation was observed. All cells underwent an initial deactivation process when reformate fuels were used but their cell voltage stabilized during the first 50 h. All cells also showed deactivation after the reformate tests when the area specific resistance values were compared to the values obtained from the hydrogen tests. The deactivation was comparable between the sulfur-free and sulfur-rich reformate tests. Sulfur-rich reformate, however, caused oscillation in cell voltages as the sulfur level in natural gas was not constant.

Dynamic Model of 5kW SOFC CHP Test Station

A dynamical model for a 5 kW class solid oxide fuel cell (SOFC) combined heat and power (CHP) test station has been composed using the APROS® environment. The model is based on a real test station being constructed and operated at VTT Technical Research Centre of Finland and it comprises the following main components of the real test system: the autothermal reforming unit, SOFC stack situated inside a furnace, catalytic afterburner, and three heat exchangers. The constructed model has been verified against experimental results obtained from the autothermal reforming unit, catalytic afterburner, and the two cathode side heat exchangers. The model has been used for the phenomenological studies of the system during current transient simulations using a simplified and fast zero-dimensional model including internal reforming reactions for the SOFC stack. The test station model was capable of operating at a speed of 18 times the real time using a standard personal computer.

Solid Oxide Fuel Cell System Development in VTT

The Finnish solid oxide fuel cell (SOFC) project (FINSOFC) was initiated in 2002 as a five-year project. It forms the core of the publicly funded SOFC research in Finland. The purpose of the project is to support the industry in its development of SOFC systems and components and other possible SOFC-based business to be created in the future. The project is coordinated by the VTT Technical Research Centre of Finland in cooperation with universities and industrial enterprises. The project is executed in close cooperation with several European partners both bilaterally and within Real-SOFC. The focus is to construct and run a natural gas-fueled 5 kW e SOFC power plant demonstration connected to heat and electricity grids. The power plant demonstration contains a stack and all BOP components from fuel processing to power conditioning and grid connections. The aim is also to thoroughly understand the behavior of the system. The subprojects needed to do this are (i) fuel processing, (ii) testing of fuel cells and stacks, (iii) construction of a 5 kW e power station demonstration, and (iv) system modeling.

Effect of Ethene Impurity on Performance of Solid Oxide Fuel Cell

A pre-reforming unit is needed if liquid hydrocarbon fuels, such as gasoline or diesel, are used in solid oxide fuel cell (SOFC) systems. Although pre-reforming removes most of the higher hydrocarbons, there may still be some fractions left in the reformate flow. Ethene was chosen to be the subject of this work because it has been analyzed to be the most significant impurity in diesel reformate. In this work, an anode supported unit cell was used to examine the behavior and performance of a SOFC when ethene was added to hydrogen feed. The ethene concentration in the feed was increased stepwise from 0 ppm to 1000 ppm and the current was kept constant for 100 hours at each concentration level. Changes in the cell performance were monitored by impedance spectroscopy and polarization measurements. It was detected that ethene does not accelerate cell degradation.