Longo Filippo, Lloreda-Jurado Pedro Javier, Gil-Rostra Jorge, Gonzalez-Elipe Agustin R, Yubero Francisco, Thomä Sabrina L J, Neels Antonia, Borgschulte Andreas
Chemical Energy Carriers and Vehicle Systems Laboratory, Empa - Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland.
Department of Chemistry, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland.
ACS Appl Mater Interfaces. 2024 Oct 30;16(43):59516-59527. doi: 10.1021/acsami.4c11902. Epub 2024 Oct 17.
NiFe electrocatalysts are among the most active phases for water splitting with regard to the alkaline oxygen evolution reaction (OER). The interplay between Ni and Fe, both at the surface and in the subsurface of the catalyst, is crucial to understanding such outstanding properties and remains a subject of debate. Various phenomena, ranging from the formation of oxides/(oxy)hydroxides to the associated segregation of certain species, occur during the electrochemical reactions and add another dimension of complexity that hinders the rational design of electrodes for water splitting. In this work, we have developed the procedure for the quantification of chemical depth profiling by XPS/HAXPES measurements and applied it to two NiFe electrodes with different porosities. The main outcome of this study is related to the surface reconstruction of the electrodes during the OER, followed at two different depths by means of X-ray photoelectron spectroscopy. We find that Fe initially segregates at the surface when exposed to ambient conditions, resulting in the formation of an inactive FeO phase. In addition, the porosity of the catalyst plays a significant role in the segregation process and thus in the performance of the electrode. In particular, the higher porosity of the nanostructured sample is responsible for a more pronounced diffusion of Fe from the subsurface to the surface with a more effective suppression of the activity of the NiFeOOH phase. These results highlight the importance of the fact that the chemical state of the surface of a multielement system is a snapshot in time, dependent on both external parameters, such as the applied potential and the adjacent electrolyte, and the underlying bulk properties accessible with HAXPES.
就碱性析氧反应（OER）而言，镍铁电催化剂是水分解最活跃的相之一。催化剂表面和次表面的镍与铁之间的相互作用对于理解这种优异性能至关重要，并且仍然是一个有争议的话题。在电化学反应过程中会出现各种现象，从氧化物/（羟基）氧化物的形成到某些物种的相关偏析，这增加了另一个复杂层面，阻碍了用于水分解的电极的合理设计。在这项工作中，我们开发了通过XPS/HAXPES测量进行化学深度剖析定量的程序，并将其应用于两种不同孔隙率的镍铁电极。这项研究的主要成果与OER过程中电极的表面重构有关，借助X射线光电子能谱在两个不同深度进行了跟踪。我们发现，当暴露于环境条件时，铁最初在表面偏析，导致形成无活性的FeO相。此外，催化剂的孔隙率在偏析过程中以及因此在电极性能中起着重要作用。特别是，纳米结构样品较高的孔隙率导致铁从次表面到表面的扩散更明显，更有效地抑制了NiFeOOH相的活性。这些结果突出了这样一个事实的重要性，即多元素系统表面的化学状态是一个时间快照，既取决于外部参数，如施加的电势和相邻的电解质，也取决于HAXPES可获取的潜在体相性质。
