Detection of Spontaneous FeOOH Formation at the Hematite/Ni(Fe)OOH Interface During Photoelectrochemical Water Splitting by Operando X‑ray Absorption Spectroscopy

• Published in: ACS catalysis Vol. 11; no. 19; pp. 12324 - 12335
• Main Authors: Ismail, Ahmed S. M, Garcia-Torregrosa, Ivan, Vollenbroek, Jeroen C, Folkertsma, Laura, Bomer, Johan G, Haarman, Ties, Ghiasi, Mahnaz, Schellhorn, Meike, Nachtegaal, Maarten, Odijk, Mathieu, van den Berg, Albert, Weckhuysen, Bert M, de Groot, Frank M. F
• Format: Journal Article
• Language: English
• Published: American Chemical Society 01.10.2021
• Subjects: iron oxyhydroxide; water splitting; catalysts; hematite; interface; operando X-ray spectroscopy; nickel iron oxyhydroxide
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/acscatal.1c02566

The role that the α-Fe2O3/NiFeOOH interface plays in dictating the oxygen evolution reaction (OER) mechanism on hematite has been a source of intense debate for decades, but the chemical characteristics of this interface and its function are still ambiguous and subject to speculation. In this study, we employed operando X-ray absorption spectroscopy to investigate the interfacial dynamics at the α-Fe2O3/NiFeOOH interface. We uncovered the spontaneous formation of a FeOOH interfacial layer under (photo)­electrochemical conditions. This FeOOH interfacial layer plays a role in the surface passivation of hematite and in accumulating the (photo)­generated holes upon external potential application. This hole-accumulation process leads to the extraction of more (photo)­generated holes from hematite before releasing them to NiFeOOH to carry out the water-splitting reaction, and it also explains the reason for the delay in the nickel oxidation process. Based on these observations, we propose a model where NiFeOOH acts mainly as an OER catalyst and a facilitator of holes extraction from hematite, while the interfacial FeOOH layer acts as a surface passivation and hole-accumulation overlayer.