Correlating Effects of Catalyst Loading and Porous Transport Layer Morphologies on Operation of Polymer Electrolyte Water Electrolyzers

Abstract

Polymer electrolyte water electrolyzers (PEWEs) are a promising technology to produce green hydrogen with high efficiencies at low temperature [1]. The widespread deployment of this technology is hindered by the catalyst cost and durability issues [2]. Higher anode catalyst loadings are currently used to enable longer operating time of the PEWEs. Interface between catalyst layer and porous transport layer (PTL) is not well understood, especially for novel PTLs with varied morphological properties, such as sintered vs. fiber. [3] The PTLs’ bulk properties, interface, catalyst loading, and catalyst distribution all impact cell performance and possible correlation between these properties needs further investigation.This study focuses on catalyst layers with different loadings, and the resulting catalyst layer-PTL interfaces with two different types of PTLs (sintered Ti and fiber Ti). For a given loading, two different electrode configurations have been compared namely the catalyst coated membrane (CCM) and gas diffusion electrode (GDE). The current density was applied from 1 to 5 A/cm2 with a constant liquid water flow rate and temperature of 60oC. We categorized the PEWEs into three categories according to catalyst loading, (1) high loadings: 1.75– 2.20 mg/cm2, (2) medium loadings: 1.00– 1.26 mg/cm2, and (3) low loadings: 0.50– 0.65 mg/cm2. Operando x-ray computed tomography (CT) and radiography were used to observe the catalyst distribution and oxygen transport in all the samples. The interfacial analysis using the tomography data and electrochemical testing gives insight into the percentage triple phase contact area (%TPCA) and the cell performance, respectively. Herein, we present a comprehensive comparative analysis of the catalyst loadings as well as the correlate cell performance with the %TPCA and the double layer capacitance.We observed a Tafel slope difference of over 10 mVdec-1 between sintered CCMs and sintered GDEs at low loadings suggesting proton or electron transport limitations at the interface. We then tested the cells at steady state conditions for 90 hours and compared the initial and final performances. Over 600 mV increase in the overpotential for low loaded fiber CCMs was observed, compared to < 200 mV increase for its high loaded counterpart at 1.5 A/cm2 after the steady state holds (Figure 1c-d). Computational fluid dynamics (CFD) with the Lattice Boltzmann Method (LBM) was used to simulate the oxygen transport in the PTLs with varying catalyst loadings, which showed underutilization of transport pathways for lower loaded fiber PTLs and showing that sintered PTLs have more uniform oxygen distribution due to more uniform through-plane morphological properties. Additionally, with the x-ray radiography study, we showed that having large oxygen slugs in the channel is preferred, as long as oxygen content in the channels is below 80 %.