Abstract
Water phenomena play a crucial role for the operation of polymer electrolyte fuel cells (PEFCs). It is therefore important to develop modeling and diagnostic tools that connect the distribution of water in fuel cell media with the overall water balance and performance. The distribution and state of water not only impact the normal performance of a PEFC cell or stack, but also their degradation rates and freeze-start capability. Unfortunately, transport coefficients reported in the literature vary by several orders of magnitude [Weber2017], thwarting predictions of water distributions and fluxes and their impact on the lifecycle-performance. Accordingly, measurement methods and tools as well as physical models are needed to precisely predict humidity and liquid water profiles in membrane electrode assemblies (MEAs) and stacks.
In the presented work, we have devised a comprehensive test procedure for water transfer measurements in MEAs. In parallel, we develop a mathematical model that is parametrized with these data. The model is able to predict MEA water management in a wide range of operation conditions. To achieve high accuracy, dew point sensors (chilled mirrors) with an uncertainty of 0.15°C are used. The test procedure employs a water flux analysis in a dedicated single cell test station that allows to correlate effective transport coefficients of water to controlled driving forces. The driving forces considered are gradients in concentration, pressure, temperature, and electric potential. Combinations of these driving forces are applied to determine the individual transport coefficients (e.g., Fig. 1a). A key result, illustrating the capabilities of the presented approach, is the prediction of the effective water flow rate out of the MEA with an accuracy of 2σ ≈7µg/(s cm²), Fig. 1b.
[Weber2017] A. Kusoglu, A. Weber, Chem. Rev. 2017, 117, 987−1104: DOI:10.1021/acs.chemrev.6b00159]
Figure 1: a) simulated water flows for various combinations of transport gradients b) comparison of the experimental and the calculated water flow for various operation conditions.
Figure 1