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

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

doi:10.1021/acsami.8b08612

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 versus standard hydrogen electrode (SHE) at pH 7, under conditions of the HER. In situ X-ray absorption spectroscopy 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 infrared 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 versus SHE is applied. Saturation of the solution with CO2 resulted in the 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 emphasizes the importance of in situ investigation of the catalyst under operation to develop true and accurate mechanistic models.

Highlights

linear combination fitting (LCF) analysis confirms that the only contribution to the X-ray absorption near-edge structure (XANES) spectrum for both dry and immersed samples is from greigite
In situ XANES spectroscopy has revealed that at −0.6 V versus standard hydrogen electrode (SHE) in a pH 7 electrolyte, a greigite electrode loses sulfur, which is replaced by oxygen, resulting in a 60% greigite 40% Fe hydroxide product
high-resolution transmission electron microscopy (HRTEM) along with computational calculations suggest a greigitehydroxide core−shell structure for the product. This transformation is proposed to take place due to the large increase in local pH at the electrode caused by the hydrogen evolution reaction (HER) taking place at this potential

Summary

Introduction

Metal sulfides find extensive use as electrode materials in Liion batteries and as electrocatalysts.[1−3] In aqueous media, sulfides of molybdenum,[4,5] nickel,[6] and cobalt[7] among others have been shown to catalyze the hydrogen evolution reaction (HER) and are considered as nonprecious metal alternatives to platinum for electrocatalytic hydrogen generation from water.[2,3] The cheap and earth-abundant iron sulfide materials such as pyrite[8−11] (FeS2), pyrrhotite[12] (Fe1−xS), and greigite (Fe3S4)[13] have been demonstrated as stable electrodes for the HER. For iron sulfides, depending on factors such as sulfide phase, morphology, solution composition, and pH, water reduction is reported to take place at potentials negative of −0.1 V versus standard hydrogen electrode (SHE). X-ray diffraction (XRD) and Raman and X-ray photoelectron spectroscopy, in situ studies to determine the nature of the catalyst surface under operating conditions are still lacking. The iron sulfide material has been re-examined after use with scanning electron microscopy or XRD to determine if structural or chemical changes have taken place under catalytic conditions. These techniques would not necessarily reveal the presence of amorphous materials or changes confined to the surface of the catalyst

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Conclusion