Understanding the Impact of Porous Transport Layers on Anion Exchange Membrane Electrolyzer Performance
Authors: Kreider Melissa E , Clauser Arielle L , Hu Leiming , Volk Emily K. , Chan Ai-Lin , Sugar Josh D , Alia Shaun M 
Published: December 2024

Anion exchange membrane water electrolysis (AEMWE) is a promising technology for the low-cost production of green hydrogen. However, significant improvements in components and component integration are needed to improve the efficiency and lifetime of AEMWE devices. [1] The anode catalyst layer is critical to activity due to the large overpotentials associated with the sluggish kinetics of the oxygen evolution reaction (OER) and to durability due to the highly oxidizing environment that can lead to catalyst and polymer degradation. The anodic overpotential can be improved by increasing the number of catalyst sites that are utilized for the reaction, which requires the formation of uniform catalyst layers with good interfaces with both the membrane and the porous transport layer (PTL). [2] The PTL serves several important roles in the electrolyzer, including the transport of electrons between catalyst sites and the current collector and the transport of gaseous products out of the catalyst layer. The PTLs used for AEMWE are typically Ni-based, with no precious metal coating or microporous layer. The morphology of the PTL impacts its interface with the catalyst layer, which affects in-plane electronic resistance and the ability to utilize catalyst sites effectively. [3] The porosity and pore size of the PTL also affect bubble removal, which can contribute significantly to mass transport overpotential, particularly at high current densities. Commercially available PTLs are often not designed for the AEMWE application, meaning that their properties are not optimized for these two critical functions. In addition, in a supporting electrolyte like potassium hydroxide, hydroxide ions are not restricted to the catalyst layer-membrane interface or to the ionomer network such that the PTL also contributes to catalytic activity. Ni alloy PTLs can provide an advantage of higher initial catalytic activity compared to standard Ni, but many dopant metals (i.e., Fe, Cr, etc.) are prone to dissolution under AEMWE operating conditions, which can damage the membrane and lead to higher rates of degradation. Improved PTL designs are needed to improve electrolyzer performance and durability.

In this study, we investigate the impact of PTLs with varying morphology and composition on kinetics, catalyst layer resistance, and durability. The properties of the PTLs, including composition, crystal structure, thickness, porosity, and fiber dimensions are determined using spectroscopy, diffraction, electron microscopy, and micro X-ray tomography. These material properties are then correlated with the performance of the PTL, with and without an additional catalyst layer. As shown in Figure 1, the PTLs have a significant effect on device performance and often have significant electrochemical activity in the absence of a catalyst layer. The impact of the PTL on performance trends with catalyst conductivity and loading is also explored. Electrochemical impedance spectroscopy (EIS) is further used to understand the effect of the PTL on interfacial charge resistance and resistances within the catalyst layer. Finally, ex situ characterization of the catalyst layer and PTL with scanning electron microscopy and X-ray spectroscopy, as well as analysis of dissolved species in the electrolyte using an inductively coupled-mass spectrometer (ICP-MS), are used to understand the material stability of the PTL and impacts of PTL degradation on overall durability. This work provides insight into PTL design principles, which can be used to improve the performance and lifetime of AEM electrolyzers.

References

[1] Santoro, C.; Lavacchi, A.; Mustarelli, P.; Di Noto, V.; Elbaz, L.; Dekel, D. R.; Jaouen, F. ChemSusChem
2022, 15 (8), e202200027.

[2] E.K. Volk, M.E. Kreider, S. Kwon, S.M. Alia. EES Catal.
2024, 2 (1), 109-137.

[3] Mo, J.; Kang, Z.; Retterer, S. T.; Cullen, D. A.; Toops, T. J.; Green, J. B.; Mench, M. M.; Zhang, F.-Y. Sci. Adv.
2016, 2 (11), e1600690.

Figure 1