Mode of electroactive species evolution from Fe3X4 (X = Se, S, O): guidelines from the redox chemistry of chalcogen anions

Abstract

Mixed-valence iron chalcogenides with the common formula Fe₃X₄ (X = Se, S, O) exhibit unique electronic properties due to the presence of Fe^II and Fe^III in their sub-lattices, making them an excellent electrode material. Despite Fe₃O₄ and Fe₃S₄ possessing an inverse spinel-type structure with a cubic lattice, Fe₃Se₄ adopts a layered-type structure with a monoclinic crystal system. The bulk morphology and exposed facets of the Fe₃X₄ nanomaterials are however different. Lattice correlation with the exposed facets confirmed the presence of only Fe^III centers in the case of the (002) surface of Fe₃Se₄, while Fe^II and Fe^III centers are present as exposed in the case of the (400) and (311) surfaces of Fe₃S₄ and Fe₃O₄, respectively. The electrochemical study with Fe₃S₄ and Fe₃Se₄ reveals that the Fe^II and chalcogenide anions (S²⁻/Se²⁻) show irreversible oxidation within −0.2 to 1.0 V (vs. RHE) in an alkaline medium which resulted in electrochemical instability of Fe₃Se₄ to form a trigonal-Se (t-Se)/α-FeO(OH) heterostructure on the electrode surface as the electro-derived species, identified through in situ Raman spectra and subsequent post-chronoamperometric characterization. Under similar electrochemical conditions, Fe₃S₄ undergoes surface modification to form only α-FeO(OH) while S²⁻ leaches out as [SO₄]²⁻ to the electrolyte. Nevertheless, the Fe₃O₄ remains stable under anodic conditions despite showing the Fe^II/III redox feature. During the oxygen evolution reaction (OER) with these three iso-stoichiometric pristine Fe₃X₄ materials or their electro-derived reactive species, the α-FeO(OH)/t-Se species remains the most reactive with a 219 mV overpotential at 10 mA cm⁻². The formation of conductive t-Se⁰ nanoparticles and a heterojunction with α-FeO(OH) facilitates electronic conduction for the reasonable OER activity evident from the intrinsic electrochemical parameters. Although Fe₃Se₄ and Fe₃S₄ are iso-stoichiometric and mixed-valence iron-chalcogenides, the difference in their lattice structure, the electrochemical potential of S²⁻ and Se²⁻ and the stability of the elemental state of the chalcogenide counterpart present in the Fe₃X₄ lattice play an intriguing role in the formation of electroactive species and a distinct reconstruction pathway is followed for the Fe₃X₄. In this study, the role of redox-active chalcogenides in the electrochemical activity of Fe₃X₄ has been established.