Ferromagnetic Catalysts for Spin Dependent OER

A so-called hydrogen economy, where green hydrogen is used as an energy vector for renewable energy, can help provide a sustainable energy infrastructure. To produce green hydrogen, electrocatalytic water splitting can be used, in which H₂ and O₂ are produced from H₂O. However, the dynamics of one of its two half-reactions, the oxygen evolution reaction (OER), still pose challenges regarding the practical application of this technology. The development of an efficient catalyst for the OER can thus be a large step forward towards sustainable energy.

An important hurdle in the OER originates from the fact that the production of triplet O₂ is a spin-forbidden reaction as the reactants, OH^- or H₂O, are diamagnetic, but the final product, O₂ is a paramagnetic molecule with parallel spin alignment in its ground state. Recently, this was well-recognized theoretically and the use of spin selective catalysts was described as a possible way to promote the OER. [1] Accordingly, multiple experimentalists reported a positive effect of external magnetic fields on OER activity of ferromagnetic catalysts. [2] However, it remains a challenge to investigate the influence of the intrinsic magnetic order on catalytic activity.
We use Ferromagnetic La_0.67Sr_0.33MnO₃ epitaxial thin films as model systems for the OER to investigate the intrinsic magnetic order. We tune the Curie temperature to be close to room temperature. This allows us to change the magnetic order of the catalyst from ferromagnetic to paramagnetic during water electrolysis by changing the temperature. Here, we observe an increment in OER activity upon crossing the Curie temperature. Linking the observed OER activity to the intrinsic magnetic order enabled us to identify that intrinsic ferromagnetic ordering enhances OER activity. Moreover, we show a slight enhancement in catalytic activity upon application of an external magnetic field and find that the dependence of magnetic field direction correlates with the magnetic anisotropy in the catalyst film. Our work thus suggests that both the intrinsic magnetic order in La_0.67Sr_0.33MnO₃ films and magnetic domain alignment in external magnetic fields increase their catalytic activity. [3]

However, during electrocatalysis, the catalyst is continuously evolving, leading to a different structure and composition under OER conditions compared to the pristine thin film. Hence, it is of the highest essence to study the composition and structure ‘operando’ under OER conditions as these influence the magnetic properties. [4,5] To be able to fully understand the relationship between changes at the Curie temperature in the long-range magnetic order and the OER activity, we thus investigated the magnetic properties of the catalyst while it interacts with the reactants through a combined in situ electron paramagnetic resonance spectroscopy and ambient pressure X-ray magnetic circular dichroism study. Here, we find a clear correlation between loss of long range magnetic order and the change in activity while observing limited changes in the electronic state of the catalyst. This further confirms that the OER enhancement is induced by magnetic order and that magnetic order is key for the ideal OER activity.

[1] C. Biz et al., ACS Catal, 2021, 11(22),14249–14261
[2] X. Ren et al., Nat Commun, 2021, 12(1), 2608
[3] E. van der Minne et al., Appl. Phys. Rev., 2024, 11, 011420
[4] J. Ge et al., Adv. Mater., 2021, 33(42), 2101091
[5] A. Füngerlings et al., Nat Commun, 2023, 14, 8284