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Insight into Nature of Iron Sulfide Surfaces during the Electrochemical Hydrogen Evolution and CO2 Reduction Reactions

Research output: Contribution to journalArticle

Siti N. A. Zakaria, Nathan Hollingsworth, Husn Islam, Anna Roffey, David Santos-Carballal, Alberto Roldan, Wim Bras, Gopinathan Sankar, Graeme Hogarth, Katherine B. Holt, Nora H. de Leeuw

Original languageEnglish
JournalACS Applied Materials and Interfaces
Early online date20 Jul 2018
DOIs
Publication statusE-pub ahead of print - 20 Jul 2018

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Abstract

Greigite and other iron sulfides are potential cheap, earth-abundant electrocatalysts for the hydrogen evolution reaction (HER), yet little is known about the underlying surface chemistry. Structural and chemical changes to a greigite (Fe3S4) modified electrode were determined at −0.6 V vs. SHE at pH 7, under conditions of the HER. In situ X-ray Absorption Spectroscopy (XAS) was employed at the Fe K-edge to show that iron-sulfur linkages were replaced by iron-oxygen units under these conditions. The resulting material was determined as 60% greigite and 40% iron hydroxide (goethite) with a proposed core-shell structure. A large increase in pH at the electrode surface (to pH 12) is caused by the generation of OH− as a product of the HER. Under these conditions iron sulfide materials are thermodynamically unstable with respect to the hydroxide. In situ IR spectroscopy of the solution near the electrode interface confirmed changes in the phosphate ion speciation consistent with a change in pH from 7 to 12 when −0.6 V vs. SHE is applied. Saturation of the solution with CO2 resulted in inhibition of the hydroxide formation, potentially due to surface adsorption of HCO3−. This study shows that the true nature of the greigite electrode under conditions of the HER is a core-shell greigite-hydroxide material and emphasises the importance of in situ investigation of the catalyst under operation in order to develop true and accurate mechanistic models.

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