In-situ methods for the determination of current distributions in PEM fuel cells
Introduction
PEM fuel cells show great promise in portable[1], automotive[2]and stationary[3]applications because of their high power density and adaptability for different system requirements[4]. Further advantages compared to for example phosphoric acid or alkaline fuel cells include significantly reduced corrosion problems due to the limited operating temperature and the use of a solid electrolyte. In order to reach optimum performance in commercial scale fuel cells, the optimisation of electrochemical activity over the whole electrode area is of prime importance.
This also requires efficient water management in particular on the oxidant side (cathode) which is a function of cell design as well as the reactant flow distribution and the temperature distribution. Water management in solid polymer fuel cells has a major impact on cell performance and lifetime5, 6. Water accumulates on the cathode side mainly through two mechanisms: (i) reaction water and (ii) water transport from the anode to the cathode by the electro-osmotic drag. These effects can lead to significant loss in performance through the filling of pores in the gas diffusion layer with liquid water inhibiting access of the oxidant gas to the catalyst. On the other hand, insufficient water content of the polymer electrolyte membrane will increase the membrane resistance leading to higher ohmic losses. Some advanced strategies for efficient water management in fuel cells have recently been discussed and involve anode- and cathode gas humidification, temperature profile optimisation and cell design[7]. Therefore, there is a need for in-situ techniques which can provide information on the current distribution in full scale operational fuel cells and fuel cell stacks. Most of the work in the past has relied on mathematical modeling8, 9, 10, 11with only limited activity in the development of experimental techniques such as multiple reference electrodes[12]and multiple reference electrodes in combination with subdivided electrodes13, 14.
In this paper we will compare three novel in-situ methods which give information on the current density distribution: the partial membrane electrode assembly (MEA) method, the subcell method and the current mapping technique. Typical experimental results are shown and the benefits and limitations of each approach are discussed; however, for all three approaches different MEA's were used, therefore, despite similar operating conditions the observed performances may be different.
Section snippets
Experimental
In the partial MEA approach portions or segments of the MEA are tested independently either by masking areas or my making several partially catalysed MEAs. The specific performance of regions of the MEA can be determined by difference. Masking with thin PTFE sheet has been shown to give similar results to partial catalysation for operation on air. In the examples presented here, three cathode electrodes were made by spreading Pt black and PTFE suspension as a paste onto hydrophobic Toray TGP090
Subcells
In this section we will describe another method useful for the optimisation of fuel cell performance. This method allows the determination of localised currents and localised electrochemical activity in a full scale operational cell.
Current distribution mapping
The third and most advanced method which provides spatially resolved performance data on a commercial fuel cell is the current mapping technique. It involves the use of a passive resistor network distributed over the whole MEA area in order to map the current density and combines the advantages of both approaches previously discussed, namely good spatial resolution and coverage of the whole electrode area. Further advantages lie in the fact that time dependent phenomena can be monitored in real
Acknowledgements
The authors acknowledge the assistance of James Dudley and Olen Vanderleeden in preparing the current mapping plots.
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