Modeling of the electrolyte distribution in representative volumes

在第一个项目阶段，我们可以在孔尺度上模拟电解液对气体扩散电极的渗透。一个特别的困难是混合润湿行为的描述。当银相被电解液润湿时，PTFE部分具有很强的疏水性，并阻止电解液的渗透。相界面可能同时与电解液和聚四氟乙烯接触。数值计算的工作量非常大，因此只能考虑实际结构的小体积。模拟结果表明，电解液呈现指进行为，并形成强烈的分形结构。第二个资助期的目标是模拟电解液在可被认为具有代表性的体积中的分布。为此，提出了一种混合计算方法。自由相界附近将被严格计算，而电解液通过填充的孔的流动将被简化并建模为流动阻力。通过对SPH模拟的全隐式时间积分，以及对移动相界的并行计算，可以加快计算速度。从初步测量可知，电势能更好地使银相被电解液润湿。这种被称为“电润湿”的行为将在第二个资金期被考虑，用于模拟电解液的分布。为了验证该模型，将在GDE上进行毛细管压力饱和测量，从而改变电位。这使得可以将整体电解液持水率与模拟的持液率进行比较。AK Manke对电解液分布的空间分辨测量允许在孔隙尺度上进行详细的验证。

In the first project phase we could show that it is possible to simulate the penetration of an electrolyte into a gas diffusion electrode on the pore scale. A particular difficulty is the description of the mixed wetting behavior. While the silver phase is wetted by the electrolyte, the PTFE part is strongly hydrophobic and prevents the penetration of the electrolyte. The phase boundary may get in contact with both, electrolyte and PTFE. The numerical effort is very high, therefore only small volumens of the real structure could be considered. Simulation results suggest that the electrolyte exhibits fingering behaviour and forms strongly fractal structures.The goal for the second funding period is to simulate the electrolyte distribution in volumes that can be considered representative. For this purpose, a hybrid calculation method is proposed. The vicinity of the free phase boundaries will be rigorously calculated, while the flow of the electrolyte through filled pores is simplified and modelled as flow resistances. A speed up of the calculation shall be achieved by a fully implicit time integration for the SPH simulation on the one hand, and by parallel calculations of the moving phase boundaries on the other hand.From preliminary measurements it is known that an electric potential leads to a better wetting of the silver phase by the electrolyte. This behaviour, known as "electrowetting", will be considered in the second funding period for the simulation of the electrolyte distribution.In order to validate the model, capillary pressure saturation measurements on GDE’s will be carried out, whereby the electric potential is varied. This allows the comparison of the integral electrolyte holdup with the simulated holdup. Spatially resolved measurements of the electrolyte distribution by the AK Manke allows a detailed validation on the pore scale.