1D One-Phase Modeling of the Anode Catalyst Layer/Porous Transport Layer Interface Affecting Proton Exchange Membrane Water Electrolysis

Abstract

Optimizing the structure of the porous transport layer (PTL) is crucial for improving the performance of proton exchange membrane water electrolysis (PEMWE), particularly for cells with low anode catalyst loadings. A growing number of studies reveal that catalysts are normally not entirely utilized in PEMWE, whereas the degree of catalyst utilization strongly depends on the structure of the PTL. As the oxygen evolution reaction (OER) at the anode is the limiting reaction step in PEMWE, inefficient use of the anode catalyst layer (aCL) would result in higher activation losses and, thus, cause a waste of unused catalysts [1–3]. This phenomenon becomes more serious when catalyst loading is reduced to meet the eco-friendly requirement of $2 per kgH2 while conserving noble material resources [4].Schuler et al. [1, 5] combined commercial CCMs with different PTL configurations, and found that with different aCL/PTL interface properties, the activation overpotential could have a maximum deviation of 20 mA at 0.1 A/cm2. Peng et al. [2] manufactured an ultra-low Ir loading CCM with 0.03 mgIr/cm2. Optimizing the aCL/PTL interface showed that the activation overpotential can have a maximum deviation of 40 mV at 0.75 A/cm2. Previous studies concluded that the aCL/PTL interface is crucial for improving the catalyst utilization. Consequently, the triple-phase boundary (TPB) site concept at the aCL/PTL interface is gaining more and more attention: the catalysts can only be entirely activated at reaction sites with sufficient water supply, ionic and electronic conductivities, and oxygen removal pathways [3]. According to TPB, not all reaction sites on the aCL meet these requirements. Figure 1 shows the corresponding physical picture. At the contact point of aCL and PTL (S) offers much greater electronic conductivity compared to the pore area (P), where electron removal is limited by the aCL in-plane conductivity (i.e., 1.85×106 S/m vs. 65.8 S/m) [3]. However, beneath the contact area (S), mass transport of oxygen is limited, indicating the necessity of optimizing the aCL/PTL interface.Figure 1: Illustration of physical picture at aCL/PTL interface.In our contribution, we implement a one-dimensional, one-phase model to investigate the interplay between PTL geometry at the aCL/PTL interface and catalyst utilization. The PTL transports water toward the aCL and electrons and oxygen toward the cathode and anode bipolar plates, respectively [6]. To simplify the physical conditions, we assume sufficient water supply and ion removal for the reaction sites on the aCL. Therefore, mass transport and electron transport limitations are only influenced by the aCL/PTL interface properties. Oxygen is assumed to dissolve in water only so that bubble formation does not disturb the reaction sites. Additionally, an ideal heat management condition is assumed in the model to achieve an isothermal condition. Our simulation results are in good agreement with literature, enabling the prediction of activation overpotentials corresponding to the PTL structure at aCL/PTL interface. Moreover, our results illustrate the local aCL behavior, which can be used to optimize the porous structure at the aCL/PTL interface, maximizing catalyst utilization. With our model, we aim to screen for an optimum PTL structure for a given aCL, allowing us to improve PEMWE cell performance by minimization of activation losses.