Short communication
Dependence of current distribution on water management in PEFC of technical size

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Abstract

Measuring local currents in PE-fuel cells is an important tool for diagnostics and development. A semi-segmented cell has been developed, which can serve as a key instrument to investigate different phenomena in cells and stacks of technical relevance. Data with respect to water management is presented. These results show, that the local current distribution is strongly influenced by the dew point of the process air, the stoichiometry of the process air and the mode of operation.

Introduction

The performance of a PEM-fuel cell depends on many interdependent parameters. These are on the one hand the properties of the electrochemical components such as activity of the electro-catalyst, transport properties of the electrodes (transport of gases and liquid water through the porous structures) and conductivity of the membrane. On the other hand the properties of the bipolar plate with respect to distribution of the gases over the active area, heat dissipation properties and electric conductivity in the bipolar arrangement influence the performance. Finally also the operation parameters such as gas pressures, gas stoichiometries, gas dew points and cell temperatures are important for the overall performance. A number of the parameters are interlinked, and especially the water management depends on several parameters such as water sorption and transport properties of the membrane and the electrodes, transport of water in the flow field, the cell temperature, gas dew points, gas stoichiometries and global as well as local current density.

In a standard fuel cell experiment changing voltage or changing average current density of the cell is the only measurable response on variation of any of the parameters influencing the water management. The interpretation of the measurement with regard to optimization of the cell performance remains difficult. This is because the reasons for the change in cell performance are not easily deductible in a straight forward manner.

There are several options to gain more information from such experiments. One of these is measuring the current density locally resolved at different locations of the active area of the cell. Knowledge of the current distribution over the active area of the cell can provide useful information on the underlying mechanisms and can therefore be used for optimizing components or as validating input to support modeling efforts.

For local current density measurements, in principle, two different approaches are feasible: (i) construction of a model-cell, optimized for local current density measurements with respect to accuracy and resolution [1], [2]; (ii) adopting a real cell with the necessary instrumentation [3]. In the first case accurate measurements are possible, but these results would be difficult to be transposed to cells of technical importance, mainly due to scaling factors and thermal management differences. In the second case the challenge is, to include the instrumentation for locally resolved current measurement in the cell without making changes to the gas flow field to conserve fluid dynamic properties, and to the thermal properties of the cell (cooling, temperature profiles). In this work, we pursued the approach to use a technical cell (developed for an automotive power train [4], [5]) in order to gain information on the “real” system. The construction described in detail below would be suitable to be included in a stack. The results presented however, are measured in a single cell.

Section snippets

Semi-segmented plate principle

The principle of adopting a real cell for the local current density measurements was chosen, because inclusion of the measurement principle into a big stack should be possible. For this purpose a “semi-segmented” cell, including an unchanged air flow field plate, as used in the stacks [4], [5], was developed which allows for local current density measurements but conserves the fluid dynamic properties, as well as the electrical, thermal, and mechanical properties exactly equivalent to those of

Experimental

Nafion 112® (Dupont) and standard Elat (E-Tek Inc.) electrodes (1 mg Pt cm−2 MEA) were used as electrochemical components. The single cell was assembled with a standard anode flow field plate and the semi-segmented cathode flow field plate.

The cell measurements were made in a microprocessor controlled test station, which accurately controls gas flows, gas pressures, cell current or cell voltage. The cell is water cooled, with an integrated cooling flow field on anode and cathode side. An enhanced

Results

Single cells can be operated in different regimes with respect to electrical and gas flow conditions. Constant current (CC) or constant voltage (CV) modes are applicable as the electrical regimes and constant flow (CF) or stoichiometric flow (SF) as the mass flow regimes. Three, out of the four possible combinations, have been investigated. The CC/SF regime is the one seen by a cell in a stack, and therefore of practical interest, the modes CV/SF and CC/CF are of academic interest in order to

Conclusion

The current density distribution has been investigated in a PE-fuel cell of technical interest. For this purpose a semi-segmented plate with exactly the same properties and cell design as the one employed to build stacks of up to 125 cells, was used. Therefore, information was obtained from a cell set-up completely identical to the one having been used for construction of a fuel cell system for an automotive power train [6]. Results with respect to humidification show, that in operation modes

Acknowledgment

Development of multi-channel current measurement instrumentation by R. Eckl, technical support from C. Marmy and financial support from the Swiss Federal Office of Energy (BFE) for ABG are gratefully acknowledged. All work was carried out at PSI during a leave of RPN from IST, which was financially supported by the Portuguese Foundation for Science and Technology grant no. 15805/98.

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1

Present address: Soluções Racionais de Energia, 2565-641 Ramalhal Torres Vedras, Portugal.

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