(420g) Understanding and Eliminating Interaction of Cells in an Electrochemical Stack

Typically, both the fuel cells and electrolysis cells are stacked in series by means of impermeable graphite or metallic bipolar plates. The open-circuit voltage of most types of fuel cells is approximately 1 V. To support sufficient voltage and power required for commercial applications, cells are assembled in a stack. By connecting multiple cells in series, the voltage of a stack is multiplied, generating the required high power output. Similarly, electrolysis cells are stacked to uptake the high voltages and power, and reach a demanding hydrogen production; the hydrogen generation-rate of a single cell is limited to electrodesâ?? active area.

In an ideal stack, the design and operating conditions for all cells should be identical throughout the stack. Overall stack performance would then be the summation of the equal stand-alone performances of cells. However, in a real stack, the cells are not perfectly identical and they might differ either in design or operating conditions or both for a variety of reasons: imperfections during manufacturing, a non-uniform inlet flow distribution, degradation of one or more cells and external boundary conditions, such as heat transfer with the surroundings. Though such non-uniformities are encountered at a single cell level, the performance differences are translated, very often not to a single cell only, but to a group of cells located in neighborhood. In other words, one poorly operating or perturbed cell could affect the performance of other cells in the stack. Such a coupling of the cells is the focus of investigation here as it could be detrimental for the performance and lifetime of the stack.

As described above, the individual cells in a stack are separated by impermeable bipolar plates. Thus, we can classify the transport phenomena in their stacks according to the region of influence. Mass, momentum and species transport are confined to individual cells, whereas charge and heat transfer take place globally across the cells such that the cells become coupled and influence each other. Since the cells are coupled via heat and charge transport, abnormal behavior of one cell can propagate to the neighboring cells. Two adjacent cells can be said to be (1) electrically decoupled when their local current density distributions do not influence and perturb each other, and (2) thermally decoupled if there is negligible heat transfer between the two cells. We analyze charge and heat transport across the cells in a stack for the above two decoupling conditions. In short, electrical decoupling of the cells in a stack is found to depend on the geometrical and material properties of the bipolar plate and the operating parameters. Similarly, the thermal decoupling is governed by the design and operating parameters of coolant flow fields. We finally identify the design guidelines that ensure decoupling of the cells via both the transport modes. The guidelines could be of use for stack designers when selecting the bipolar plate material and dimensions and coolant flow field conditions, so as to minimize the spread of disturbances in a cell to neighboring cells.

 

Acknowledgements: The research is supported by the National Research Foundation, Singapore under its Competitive Research Program (Award No. NRF-CRP8-2011-04).