PhD Defence Emma van der Minne | A new spin on electrochemical water splitting | Unraveling Magnetically Enhanced OER using Thin-Film Model Catalysts

A new spin on electrochemical water splitting | Unraveling Magnetically Enhanced OER using Thin-Film Model Catalysts

Emma van der Minne is a PhD student in the department Inorganic Materials Science. (Co)Promotors are prof.dr. C. Baeumer and prof.dr.ir. G. Koster from the faculty Science & Technology (TNW), University of Twente.

A central challenge in water electrolysis is the oxygen evolution reaction (OER), whose efficiency is constrained by the need to conserve angular momentum during O₂ formation. Previous studies have shown that spin order within OER electrocatalysts influences their observed activity, with the hypothesis that favorable spin alignment can reduce the energetic cost of individual reaction steps and facilitate the formation of triplet O₂ by ensuring proper spin orientation of intermediates across different OER mechanisms. Based on this idea, numerous experimental and computational investigations have explored how magnetic properties, spin polarization, and spin–orbit coupling affect catalytic performance and reaction energetics, opening new pathways for designing catalysts that exploit spin-dependent effects to overcome intrinsic kinetic limitations. In parallel, the influence of the application of external magnetic fields has been investigated and shown to increase measured OER currents. However, despite extensive research, these studies have revealed multiple possible mechanistic pathways without conclusively identifying which mechanisms dominate under specific conditions. This highlights the need for a clear and unified understanding. Identifying the relevant mechanisms, determining the key governing factors, and developing robust methods to probe their relative contributions are therefore central goals of this work.

Chapter 2 presents a detailed overview of the field of spin-dependent OER, with particular emphasis on the relationship between magnetic order and catalytic activity. Theoretical insights and experimental observations are summarized, from which key principles underlying spin-induced effects on the OER are derived. The chapter also highlights major open questions and challenges in disentangling the contributions of these effects and outlines potential future directions that provides the framework for the methodology employed in the following chapters.

Chapter 3 evaluates the properties of PLD-grown La_0.67Sr_0.33MnO₃ thin films, focusing on surface features that largely govern catalytic activity. X-ray–based techniques and scanning probe microscopy reveal clear differences between the bulk and surface in chemical composition and electronic disorder. Surface-sensitive measurements indicate Sr/La non-stoichiometry, deviations in Mn oxidation state, and potential oxygen-vacancy accumulation. The disordered surface, composed of stacked nanoscale islands, resulted from a two-step growth mechanism and correlated with electronic phase separation and a mixed ferromagnetic–paramagnetic state near room temperature, as evidenced by temperature-dependent resistivity. These findings highlight a complex magnetic landscape and emphasize that understanding local surface and magnetic properties is crucial for mechanistic insights into spin-dependent OER effects.

Chapter 4 experimentally validates spin-induced enhancements of OER activity in ferromagnetic La_0.67Sr_0.33MnO₃ epitaxial thin films. Utilizing the para- to ferromagnetic transition at T_C in situ during OER it demonstrates that the onset of ferromagnetic order below the Curie temperature increased OER activity. Moreover, a modest rise in current density was observed upon applying an external magnetic field, with the largest increase along the easy axis, consistent with the film’s magnetic anisotropy. Together, these results show that both intrinsic changes in long-range magnetic order and externally applied magnetic fields could improve catalytic behavior at fixed potentials. Supported by detailed ex situ magnetic characterization, this chapter provides an initial picture of how the magnetic structure of La_0.67Sr_0.33MnO₃ films influence their OER performance.

Chapter 5 further investigates the influence of applied magnetic fields on OER. Current pathways were carefully designed in epitaxial La_0.67Sr_0.33Mn_yO_3-δ films utilizing substrates of varying conductivity, and magnetization and electrical resistance were systematically tuned via composition and thickness. These experiments show that field-induced OER modifications occur only under resistively limited currents, with magnitudes correlating to the film’s magnetoresistance.  Operando Kerr measurements demonstrate that removing magnetic domains did not directly affect OER activity, and that field-induced enhancements persist in a monodomain state. Chronoamperometry and chronopotentiometry further confirm that the magnitude of enhancement followed changes in the catalyst’s electrical resistance under the applied field. Overall, this analysis establishes magnetoresistance as the primary driver of magnetic-field-induced OER current enhancement and provided a practical framework to predict such effects.

Chapter 6 explored the influence of the catalysts’ magnetic properties on OER in greater depth using operando techniques. Temperature-dependent operando ferromagnetic resonance, ambient-pressure X-ray magnetic circular dichroism, and operando X-ray absorption spectroscopy reveal a direct correlation between magnetic behavior and electrochemical activity. OER activity is enhanced by ferromagnetic spin-exchange interactions between adsorbates and nearby ferromagnetic regions, with the loss of these regions reducing activity. Long-range magnetic order is hypothesized to further enhance activity via spin-polarized electron transport, providing spin-polarized conduction channels. This chapter demonstrates that long-range magnetic order could directly alter the intrinsic catalytic activity of La_0.67Sr_0.33Mn_yO_3-δ and highlights the value of combining advanced magnetic characterization with operando studies to establish a comprehensive understanding of how magnetic order influences OER activity.

Chapter 7 explores strategies to harness the spin-dependence of the OER established in previous chapters. By employing heterostructures and leveraging the magnetic proximity effect, the goal was to induce long-range magnetic order in active paramagnetic NdNiO₃ OER catalysts to enhance activity. Although coupling to an underlying La_0.67Sr_0.33Mn_yO_3-δ layer occurred at low temperatures, no measurable OER enhancement was observed under operating conditions, likely primarily due to weak or absent induced ferromagnetism at room temperature. Moreover, the complete lack of enhancement, even via ferromagnetic spin-exchange interactions across the paramagnetic layer, highlights the nuanced nature of spin-enhanced OER and suggests that such interactions are spatially limited, material-specific, or relevant only for less active catalysts.

Overall, this thesis demonstrates that observed magnetic field-induced OER activity enhancements in transition-metal oxides arise primarily from magnetoresistive current-transport effects rather than from changes in intrinsic catalytic activity. In contrast, ferromagnetic order within a catalyst is shown to enhance intrinsic OER activity. Moreover, the work highlights the value of integrating multiple operando techniques. By probing electronic, magnetic, and structural properties alongside electrochemical behavior, these approaches provide fundamental mechanistic insights into spin-dependent OER.

By identifying the origins of spin-induced activity enhancements, this thesis provides new perspectives on how such effects can be leveraged in practical catalyst design. This lays the foundation for establishing magnetic property–structure–activity relationships for OER catalysts, ultimately guiding the development of more efficient electrolyzers.