Influence of Anode Flow-Field Channel Distance on the Performance of Polymer Electrolyte Membrane Water Electrolyzers

Abstract

Polymer electrolyte membrane water electrolysis (PEMWE) is well suited for flexible and efficient hydrogen production based on electricity from renewable energy sources [1]. Of all PEMWE stack components, flow-fields (FF) or bipolar plates (BP) are currently among the most expensive parts: In an analysis from 2020, FF/BP are reported to account for more than half of current PEMWE stack costs with medium potential for cost reduction towards the next-generation [2]. Another recent study attributes only 28% of current PEMWE stack costs to FF/BP, but claims that next-generation bipolar plates will be 3 times less expensive [3]. In any case, the optimization of flow-fields represents an important lever to improve the economic competitiveness of PEMWE. Although sophisticated designs are reported in the literature, commercial flow-fields tend to follow simpler approaches [4]. For classic design patterns like serpentine or parallel channels, studies compared different channel patterns or assessed the influence of the channel width [5]. Nevertheless, systematic investigations on the influence of the channel distance (FF land width) on the PEMWE performance have not been reported so far. Recently, an optimal FF land width of 4 times the thickness of the porous transport layer (PTL) was postulated [6]. However, it remains unclear if this ratio is universally valid, because only one specific FF configuration and only one PTL type (with variable PTL thickness) was investigated in the study.In this work, we analyze the influence of the anode side flow-field channel distance (land width) on the cell performance of 5 cm2 PEMWE single-cells. Different single-channel serpentine flow fields with land widths ranging from 0.7 mm to 6.2 mm and a constant channel width of 1 mm are characterized as a function of PTL type (powder-sintered- as well as fiber-based titanium PTL). For all FF/PTL combinations, polarization curves up to 6 A/cm2 are performed with electrochemical impedance spectroscopy (EIS) measurements at each current step. The anode proton sheet resistance is evaluated by EIS fitting during blocking conditions. On one side, a voltage loss analysis reveals that mass transfer losses in PEMWE cells are very small for the best suited FF/PTL combinations (<15 mV at 6 A/cm²). On the other side, for large land widths in combination with certain PTL types, severe mass transport losses are observed at high current densities (increased cell as well as iR-free cell voltage, cf. figure 1a), accompanied by a partial dry-out of the membrane that is reflected by an increased high-frequency resistance (HFR, cf. figure 1b). For this case, the remaining water content in the membrane will be estimated. Porosity and pore structure of the PTL used in this study are analyzed ex-situ through mercury intrusion porosimetry (MIP). The PTL samples cover a broad range of pore sizes (≈15-60 µm) and porosities (≈30-70 %). Further analysis will be conducted to elucidate the contribution of PTL bulk properties such as their in-plane permeability to the observed mass transport losses for large flow-field land widths.