Local Two-Phase Flow and Performance in Polymer Electrolyte Water Electrolysis Cells

Abstract

Over the past decade, investigations[1-3] have been carried out to better understand the two-phase flow characteristics within a polymer electrolyte water electrolyzer (PEWE) and optimize the porous transport layer (PTL) morphology to improve the system performance and efficiency. Zhang, et al.[3] used a high-speed camera and transparent cell to study the strong influence of temperature and current density on bubble growth rate. Ito, et al. [4] showed that PTL porosity has a minor effect on cell performance while pore diameter greatly influences performance. They categorized the type of two-phase flow experienced by an electrolyzer by using liquid/gas superficial velocity plots. Furthermore, they showed that bubble detachment diameter strongly influenced the two-phase flow regime at any operating point. In a similar direction, Majasan, et al.[5] showed that longer flow path length promotes gas accumulation and results in annular flow at high current density, negatively impacting cell performance. Immerz et al.[6] also suggested that increased local ionic resistance down-the-channel may be attributed to large bubble accumulation. Therefore, the impacts of two-phase flow characteristics on mass transport as well as ohmic losses need to be better understood.Although the serpentine flow-field is very useful to study transport phenomena within a PEWE, it is rarely used as in commercial and large-scale PEWE system; the parallel channel-land configuration is a more conventional flow-field architecture. Through this work we hope to understand how architectural features in the flow field impact local performance to inform next-generation design. Furthermore, this work focuses on characterizing the two-phase flow and bubble accumulation occurring within the PEWE and understanding its relationship with performance. Initial experimental results revealed that the parallel flow-field outperforms the triple serpentine flow-field at high (50 ml/min) flowrates as seen in Figure 1a. This may be caused by the shorter channel lengths that do not exacerbate bubble accumulation near the outlet. Preliminary calculations of the superficial gas/liquid velocities have supported this conclusion (Figure 1c). However, at lower flowrates the parallel flow-field was observed to underperform and exhibit severe mass transport limitations. We believe that the fluid velocity in the channels at these low flowrates (6 ml/min) was insufficient to facilitate proper bubble detachment at the channel/PTL interface. This led to the formation of a large and spread out region of mass transport limited current density as seen in figure 1b. In conclusion, insights into local two-phase flow characteristics and their impact on performance in a PEWE cell will be described in this presentation[1] Lopata, J., et al., Effects of the Transport/Catalyst Layer Interface and Catalyst Loading on Mass and Charge Transport Phenomena in Polymer Electrolyte Membrane Water Electrolysis Devices. Journal of The Electrochemical Society, 2020. 167(6): p. 064507 DOI: 10.1149/1945-7111/ab7f87.[2] Lopata, J.S., et al., Considering Two-Phase Flow in Three-Dimensional Computational Fluid Dynamics Simulations of Proton Exchange Membrane Water Electrolysis Devices. ECS Transactions, 2020. 98(9): p. 653-662 DOI: 10.1149/09809.0653ecst.[3] Li, Y., et al., In-situ investigation of bubble dynamics and two-phase flow in proton exchange membrane electrolyzer cells. International Journal of Hydrogen Energy, 2018. 43(24): p. 11223-11233 DOI: https://doi.org/10.1016/j.ijhydene.2018.05.006.[4] Ito, H., et al., Experimental study on porous current collectors of PEM electrolyzers. International Journal of Hydrogen Energy, 2012. 37(9): p. 7418-7428 DOI: https://doi.org/10.1016/j.ijhydene.2012.01.095.[5] Majasan, J.O., et al., Two-phase flow behaviour and performance of polymer electrolyte membrane electrolysers: Electrochemical and optical characterisation. International Journal of Hydrogen Energy, 2018. 43(33): p. 15659-15672 DOI: https://doi.org/10.1016/j.ijhydene.2018.07.003.[6] Immerz, C., et al., Local Current Density and Electrochemical Impedance Measurements within 50 cm Single-Channel PEM Electrolysis Cell. Journal of The Electrochemical Society, 2018. 165(16): p. F1292-F1299 DOI: 10.1149/2.0411816jes. Figure 1