(308d) Voltage-Breakdown Analyses in Anion Exchange Membrane Electrolyzers –the Contributions of Catalyst Layer Resistance on Overall Overpotentials

Recently, anion exchange membrane (AEM) electrolyzers have emerged as a promising technology to combine the benefits of existing commercial technologies: alkaline and proton exchange membrane (PEM) electrolyzers. AEM electrolyzers utilize a hydroxide-conducting polymer as the electrolyte material, enabling a zero-gap configuration and high operating current densities like PEM systems. However, unlike PEM systems, a near-surface alkaline environment enables the uses of non-platinum group metal (PGM) materials for catalysts and hardware, providing opportunities for large-scale H₂ production at lower costs. Component-level advances have enabled AEM electrolysis runs of up to 10,000 hrs³ and at current densities exceeding 3 A/cm² (2.2 V),⁴ approaching the performance of their PEM counterparts. Yet, observed overpotentials in AEM electrolyzers remain high relative to PEM electrolyzers at the same current densities, limiting their commercial viability.

Voltage breakdown analyses can be utilized to identify the causes of high overpotentials in electrolyzers, including losses from Ohmic (purely resistive), kinetic, mass transport, and catalytic layer resistance contributions. However, voltage breakdown analyses beyond Ohmic and kinetic contributions are rarely performed, even though other losses can often account for 100s of mVs of losses (20-30 % of total losses). Furthermore, while catalyst layer resistance contributions have been calculated for PEM fuel cells,⁵ this contribution has rarely been investigated for electrolyzers and, to the best of our knowledge, never for AEM systems. To bridge this gap, this work utilized electrochemical impedance spectroscopy and Tafel analyses of collected polarization curves to perform voltage breakdown analyses and identify Ohmic, kinetic, catalyst layer resistance, and residual contributions to the overpotential. Specifically, a method for calculating catalyst layer resistance via transmission line impedance modeling⁶ is discussed and methods for elucidating the contributions of this catalyst layer resistance to overpotential in AEM electrolyzers are introduced.

Four anode catalysts were compared for their performance in single-cell membrane electrode assemblies and the contributions of Ohmic, kinetic, catalyst layer resistance, and residual contributions were calculated. The catalyst layer resistance contributions were found to exceed 100 mV, accounting for up to 20 % of the total voltage loss and suggesting that minimizing catalyst layer resistance is a viable avenue to reduce overpotentials and bridge performance gaps between AEM and PEM electrolyzers Assessing different catalyst layer thicknesses and different ionomer contents revealed information regarding the relative importance of ionic versus electronic resistances in the catalyst layer. Investigations of the anode transport layer revealed that this component contributes its own transmission line impedance of nearly 1 Ω-cm², suggesting that optimization of the PTL materials and especially of the PTL-catalyst interface is paramount to achieving high AEM performance. These results will ultimately allow for the design of more efficient catalyst layers, reducing overpotentials and facilitating AEM deployment commercially.

References

(1) Alia, S.; Ding, D.; McDaniel, A.; Toma, F. M.; Dinh, H. N. Chalkboard 2 - How to Make Clean Hydrogen. Electrochem. Soc. Interface 2021, 30 (4), 49. https://doi.org/10.1149/2.F13214IF.

(2) IRENA. Green Hydrogen Cost Reduction - Scaling up Electrolyzers to Meet the 1.5C Climate Goal; ISBN: 978-92-9260-295-6; International Renewable Energy Agency, 2020. https://www.irena.org/publications/2020/Dec/Green-hydrogen-cost-reduction (accessed 2022-08-18).

(3) Motealleh, B.; Liu, Z.; Masel, R. I.; Sculley, J. P.; Richard Ni, Z.; Meroueh, L. Next-Generation Anion Exchange Membrane Water Electrolyzers Operating for Commercially Relevant Lifetimes. Int. J. Hydrog. Energy 2021, 46 (5), 3379–3386. https://doi.org/10.1016/j.ijhydene.2020.10.244.

(4) Caprì, A.; Gatto, I.; Lo Vecchio, C.; Trocino, S.; Carbone, A.; Baglio, V. Anion Exchange Membrane Water Electrolysis Based on Nickel Ferrite Catalysts. ChemElectroChem 2023, 10 (1), e202201056. https://doi.org/10.1002/celc.202201056.

(5) Huang, J.; Gao, Y.; Luo, J.; Wang, S.; Li, C.; Chen, S.; Zhang, J. Editors’ Choice—Review—Impedance Response of Porous Electrodes: Theoretical Framework, Physical Models and Applications. J. Electrochem. Soc. 2020, 167 (16), 166503. https://doi.org/10.1149/1945-7111/abc655.

(6) de Levie, R. On Porous Electrodes in Electrolyte Solutions: I. Capacitance Effects. Electrochimica Acta 1963, 8 (10), 751–780. https://doi.org/10.1016/0013-4686(63)80042-0.