Zakaria, Siti Nurul Azian binti; (2017) Insight into the surface structure of iron (-nickel) sulfides in aqueous environments for CO2 reduction applications using in-situ spectroelectrochemistry. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Iron sulfides and iron nickel sulfides, specifically greigite, (Fe3S4), and violarite, (FeNi2S4), have been shown to reduce CO2 on application of low electrode potentials (up to −1.3 V vs AgCl|Ag). However, at such low potentials, metal sulfides are predicted to undergo reductive decomposition and competing reactions such as water reduction may also take place. The aim of the work in this thesis was to determine the stability of the iron (-nickel) sulfides under conditions where CO2 reduction has been demonstrated to take place. In-situ Attenuated Total Reflectance Fourier Transformed Infrared Spectroscopy (ATRFTIR) and X-ray Absorption Spectroscopy (XAS) techniques were used to monitor and characterise the structural transformations of iron and iron-nickel sulfides in aqueous solutions on application of an external potential. Additional iron (- nickel) sulfides investigated in this work include pyrrhotite (Fe7S8) and pentlandite ((Fe,Ni)9S8). Pourbaix diagrams were used to preliminarily assign the redox features in the cyclic voltammograms of these iron and iron-nickel sulfides. These assignments were then correlated with the spectroscopic data to verify the assignments. Iron hydroxide / oxyhydroxide was detected on applying negative potentials on both the iron and iron-nickel sulfides, which was not predicted by the Pourbaix diagrams at constant solution pH. The formation of the iron hydroxide / oxyhydroxide was proposed to be instigated by water reduction occurring at low electrode potentials, causing an increase in the local pH. The XAS spectra showed the transformation to iron hydroxide / oxyhydroxide was surface confined as a high percentage of sulfide character remained within the nanoparticles. The significant shift in the local pH near the electrode, due to water reduction, contributed to the stability of the iron hydroxide / oxyhydroxide formed at pH 4 and 7. On introduction of CO2, the formation of iron hydroxide / oxyhydroxide was halted. This was attributed to the adsorption of CO2 and its dissolved species HCO3 −, based on the spectroscopic evidence collected. Current suppression was observed in the voltammograms of the iron sulfides, while current enhancement was observed in the voltammograms of the iron-nickel sulfides. These differences were linked to the different nature and transformations of the electrode surface of the iron and iron-nickel sulfides on application of potential with electrolyte media. On variation of the Fe:S and Fe:Ni stoichiometries of the iron and iron-nickel sulfides, the voltammetric responses remained similar with small differences in the anodic currents attributed to different amounts of sulfates and elemental sulfur produced. The work described in this thesis provides a new insight into the stability and potential thermodynamic transformations of iron (-nickel) sulfides, on application of an applied electrode potential, in aqueous environments essential for future CO2 reduction applications.

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