A 3-D Multiphase Simulation Study Of Proton Exchange Membrane Electrolyzer Based On Open-source CFD

In the background of carbon neutrality,renewable energy sources such as tidal,wind and solar are becoming increasingly attractive and widely used,but their applications are affected by intermittency.Proton exchange membrane electrolytic cells（PEMEC）are becoming a hot topic of research because they can be coupled with hydrogen energy which is an efficient energy carrier and therefore they have high efficiency in coupling intermittent energy sources.Improving the severe two-phase flow in the flow channel at high current densities is currently a challenge for PEMEC performance.In this paper,a 3D full electrolyzer model is developed based on the open source CFD platform Open FOAM that takes into account the complex gas/liquid two-phase flow,electrochemical reactions,heat transfer,electro/ion transfer,phase change and other physicochemical processes in a PEM electrolyzer,and a pseudo-coupled method is proposed that integrates the effects of the detailed two-phase flow in the anode flow channel into the 3-D multi-phase model.The model can be used to propose corresponding optimization strategies for PEM electrolyzer flow field and liquid/gas diffusion layers（L/GDL）design and treatment,which is a guide for the future commercialization of high-performance PEM electrolyzers.Firstly,this paper investigates the mechanism of the effect of different L/GDL with different wettability,porosity and thickness treatments on the performance of the electrolytic cell,considering the influence of two-phase flow within the flow channel as well as the L/GDL.It is found that L/GDL with strong hydrophilicity and less porosity has better performance.The gradient treatment of L/GDL contact angle and porosity with increasing gradient along the flow direction shows better performance.The L/GDL treated with graded increasing contact angle and decreasing porosity in the though-plane direction has the best performance respectively.In different L/GDL thickness designs,the effect of ohmic loss dominates,so that thinner L/GDLs have better performance.Secondly,the performance of traditional flow fields is analyzed in this paper by combining experimental high-speed photographic images of two-phase flow.The internal field distributions of parallel and serpentine small flow field plates are compared and the characteristics and mechanisms of the internal oxygen volume fraction,temperature field and current density field distributions are investigated,and an internal field distribution non-uniformity index is proposed to quantify the non-uniformity of the internal field distribution.Finally,a new flow field design with a double-layer structure is proposed in combination with the VOF method.It is found that this new flow field design helps to draw oxygen from the lower flow channel into the upper flow channel,thus alleviating the oxygen blockage in the flow channel at high current density.In addition,it is found that the new flow field has a more uniform current density and temperature distribution than the conventional parallel flow field and that the performance can be improved by0.171 V at 3 A cm^-2.Next,the proposed flow field design was optimized in terms of structure and surface wettability by setting different structural parameters for the upper and lower flow field connection tubes and by applying different wettability treatments to the connection tubes.It is found that when the lower end of the connecting tube is inserted into the lower channel at a depth of 1/2 the height of the lower channel（in the study of this paper the insertion depth is 1 mm）and the lower end diameter of the connecting tube is 1.2 mm（corresponding to the upper diameter of 1.6 mm）has a better two-phase performance.In addition,the connection tube with a bottom-up hydrophobic to hydrophilic inside surface and hydrophobic outside surface treatment has better oxygen bubble trapping behavior and the ability to drive the drawn-in oxygen bubbles out to the upper flow channel.