Complex oxide electrocatalyst surfaces and interfaces – how physical and magnetic properties dictate electrochemical performance

The ‘power-to-hydrogen’ strategy aims at splitting water into O₂ and H₂ via the oxygen and hydrogen evolution reactions. The four-step oxygen evolution reaction (OER) limits the overall efficiency of hydrogen production. Complex oxides are attractive electrocatalysts to catalyze the OER because they promise optimized performance by tuning electronic and atomic structure through compositional variations. In this talk, I will highlight (1) developments in the exploration of the vast compositional space for earth-abundant electrocatalysts, (2) the importance of surface transformations that occur under reaction conditions, and (3) optimization of the activity via fine-tuning of physical properties such as magnetic order. In all three cases, interface effects in epitaxial model systems and advanced in situ or operando characterization are key.

First, I will discuss how the multi-cation composition in so-called high entropy perovskite oxides (HEO) can maximize the catalytic activity. The HEO LaCr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2O_3-δ outperforms all of its parent compounds.¹ X-ray photoemission studies reveal a synergistic effect of simultaneous oxidation and reduction of different transition metal cations during adsorption of reaction intermediates. HEOs are thus found to be a highly attractive new material class for high-activity OER electrocatalysts.

Secondly, I will describe the role of structural and chemical transformations of the outermost surface layer.² Using the example of LaNiO₃, we demonstrated that Ni-(001)-facets are approximately twice as active as the La-(001)-facets. Based on ex situ, in situ and operando spectroscopy tools, we found that the reason for the activity enhancement lies in a surface transformation of the Ni-rich perovskite surface towards a catalytically active Ni hydroxide-type surface, revealing the importance of phase transformations down to a single atomic layer.

Lastly, I will demonstrate the effect of intrinsic magnetic order on the OER performance. Using the ferromagnetic to paramagnetic transition at the Curie temperature in La_0.67Sr_0.33MnO₃, the magnetic order of the catalysts was switched in situ during the OER by changing the temperature. Our observations indicate that the presence of ferromagnetic ordering below Curie temperature enhances OER activity.³

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