Nano-scale Electrode Engineering for Alkaline Membrane Water Electrolyzers: From Design, Optimization to Scalable Implementation

Electrochemical water splitting is regarded as a promising technology for producing large-scale green hydrogen. Among the many electrolysis technologies, Anion Exchange Membrane Water Electrolysis (AEMWE) has garnered special attention due to its advantages such as use of low-cost materials and higher efficiencies over conventional approaches such as Alkaline Water Electrolysis (AWE) or Proton Exchange Membrane Water Electrolysis (PEMWE). Most improvements are mainly on electrocatalyst level, wherein low-cost nickel-based materials are typically utilized in AEMWE for the rate-limiting anodic oxygen evolution reaction (OER). The conventional method of catalyst preparation involves the use of powder catalysts that are coated on porous transport layers (PTLs) using organic ionomers/binders. However, during operation, this design suffers from catalyst detachment, delamination, and ionomer oxidation. The PTLs used in AEMWE are composed of elements recognized for their high anodic activity. This inherent activity was the driving force to explore the potential of engineering PTL properties, opening new avenues to enhance performance and efficiency. This thesis investigates the effect of surface modification on the electrochemical properties of nickel-based materials. Helium plasma exposure, which results in surface nanostructuring is showcased as a novel method. This technique has been extensively investigated in the frame of fusion research to determine the surface erosion and embrittlement of fusion reactors. While considered as key challenge in the fusion field, it offers a great opportunity for catalysis where surface modifications are critical for performance optimization. Chapter 1 outlines the scientific motivation and key research questions of this thesis, while introducing helium plasma treatment for AEMWE. Chapter 2 outlines the applicability of helium plasma irradiation on planar nickel-iron (NiFe) surfaces. Helium plasma treatment resulted in significant increase in the electrochemical surface area (ECSA), with a corresponding increase in activity. Additionally, the physicochemical stability of the nanostructured electrodes during long-term operation also demonstrated the advantages of plasma irradiation over conventional electrode design techniques. In Chapter 3, the operational regime for fabricating self-supported nanostructures was identified. While surface temperature was an important lever to induce morphological changes, the interplay between two counteracting processes: sputtering and annealing determined the growth towards a specific morphology. Additionally, the electrochemical performance was found to be dependent not only on the ECSA, but also on the size, shape and thickness of the nano/micro features. Following the design and optimization of helium plasma nanostructuring for electrocatalysis, the research reported in Chapter 4 focuses on the applicability of helium plasma treatment on commercial nickel-based PTLs, namely Stainless Steel, Inconel, and Hastelloy. At the device level, the modified PTLs decreased the interfacial contact resistances, significantly improved mass transfer and provided additional active sites due to nanostructuring. The most promising performance in terms of activity and long-term stability was realized with an AEM electrolyzer employing nanostructured Hastelloy PTL as the anode. Furthermore, the performance at elevated temperatures surpassed the technical targets for AWE and AEMWE, and aligned closely with the PEMWE targets.Chapter 5 focuses on scaling an AEM electrolyzer into a stack configuration, offering initial insights into the challenges and factors to consider when scaling up AEMWE systems. Furthermore, Chapter 6 explores the impact of operational parameters on the performance of AEMWE. In particular, the effect of electrolyte concentration, catalyst conductivity is investigated. In conclusion, this thesis provides valuable insights into the role of PTL surface modification which considerably impacts the electrochemical performance. It demonstrates the applicability of helium plasma treatment as a sustainable and scalable alternative for fabricating next-generation electrocatalyst architectures. Additionally, it also highlights the role of operational parameters that impact the electrolyzer performance. Combining the versatile nature of plasma technology and the desirable characteristics of mixed-metal oxides can be utilized in next-generation energy systems.