(Invited) Structural Transformations of OER Catalysts during Activation and Operation

Abstract

Electrocatalysis of the oxygen evolution reaction (OER) plays a pivotal role in the hydrogen economy. Optimizing materials for this reaction presents a significant challenge, as many highly active materials exhibit low stability during activation and/or operation. Therefore, operando methods capable of probing catalyst structure and morphology become key tools for monitoring transformations during operation. These transformations not only determine activity but also influence long-term stability. In this context, high-energy X-rays provide a valuable probe, easily penetrating electrolytes and cell walls, offering unprecedented insight into material behavior. This contribution presents several classes of OER catalysts operating in alkaline and acidic environments, with their structural transformations studied using high-energy synchrotron scattering techniques.Layered double hydroxides (LDH) represent an important class of catalysts functioning in alkaline environments. Notably, NiFe and CoFe serve as exemplary materials due to their high activity and stability. However, in many cases, the structure during the OER reaction differs from the as-prepared materials, undergoing an α-phase to γ-phase transition, where the latter is the active phase. This transition, at times reversible with potential, is challenging to observe without high-energy wide-angle X-ray scattering (WAXS) techniques [1,2]. Another interesting example involves the origin of high activity in spinel-type Co3O4 thin film catalysts, elucidated by high-energy surface X-ray diffraction (HE-SXRD). Here, the superior performance is attributed to the formation of a 3D CoOx(OH)y active phase skin layer during activation, and its thickness defines the final activity [3].In the case of catalysts operating in an acidic environment, RuOx and IrOx are acknowledged as promising materials. While RuOx exhibits high activity, it is prone to instability, whereas IrOx, although stable, demonstrates lower activity and have significantly higher price. A prevailing trend is to create solid solution alloys of Ir and Ru to capitalize on the strengths of both materials [4]. This contribution details structural investigations of a series of IrRu alloys with varying compositions prepared through DC magnetron sputtering. The study investigates the unusual stability of these materials at high current densities using operando WAXS and a custom-built electrolyzer cell [5]. The findings reveal that these alloys undergo Ru dissolution upon activation, forming a thin amorphous IrOx layer that protects the alloy underneath. The stability is defined by this layer, while the activity is mainly determined by the electronic structure of the IrRu alloy core.The examples presented are a reminder that without operando characterization of the materials it is very difficult to ascribe origins of activity and stability, as the ex-situ characterization might be misleading regarding the structure and electronic structure of the active phase.[1] Dionigi, F. et al. In-situ structure and catalytic mechanism of NiFe and CoFe layered double hydroxides during oxygen evolution. Nature Communications, 2020, 11. https://doi.org/10.1038/s41467-020-16237-1[2] Dresp, S. et al., Molecular Understanding of the Impact of Saline Contaminants and Alkaline pH on NiFe Layered Double Hydroxide Oxygen Evolution Catalysts. ACS Catalysis, 2021, 11, 6800–6809. https://doi.org/10.1021/acscatal.1c00773.[3] Wiegmann, T. et al., OperandoIdentification of the Reversible Skin Layer on Co3O4 as a Three-Dimensional Reaction Zone for Oxygen Evolution. ACS Catalysis, 2022, 12, 3256–3268. https://doi.org/10.1021/acscatal.1c05169.[4] Galyamin, D. et al, Unraveling the Most Relevant Features for the Design of Iridium Mixed Oxides with High Activity and Durability for the Oxygen Evolution Reaction in Acidic Media. JACS Au, 2023, 3, 2336–2355. https://doi.org/10.1021/jacsau.3c00247.[5] Moss, A. B. et al., Versatile High Energy X-Ray Transparent Electrolysis Cell for Operando Measurements. Journal of Power Sources, 2023, 562, 232754. https://doi.org/10.1016/j.jpowsour.2023.232754.