The stability upon cycling of Fe₂WO₆ used as a negative electrode material for electrochemical capacitors was investigated. The material was synthesized using low temperature conditions for the first time (220 °C). The electrochemical study of Fe₂WO₆ in a 5 M LiNO₃ aqueous electrolyte led to a specific and volumetric capacitance of 38 F g⁻¹ and 240 F cm⁻³ when cycled at 2 mV·s⁻¹, respectively, associated with a minor capacitance loss after 10,000 cycles. In order to investigate this very good cycling stability, both surface and bulk characterization techniques (such as Transmission Electron Microscopy, Mössbauer spectroscopy, and magnetization measurements) were used. Only a slight disordering of the Fe³⁺ cations was observed in the structure, explaining the good stability of the Fe₂WO₆ upon cycling. This study adds another pseudocapacitive material to the short list of compounds that exhibit such a behavior up to now.

Metal organic frameworks (MOFs) built up from metal–sulfur (M–S) bonds have shown great promise in the last decade thanks to their impressive electronic properties arising from the more covalent nature of the M–S bond when compared to M–O. In this study, we aim to expand the scope of available S-based MOFs to high oxidation state cations Mⁿ⁺ (n ≥ 3). Through the systematic exploration of the reactivity of the mixed thiolate-carboxylate ligand 2,5-disulfhydrylbenzene-1,4-dicarboxylic acid (H₄DSBDC) with Fe(III) salts, three new compounds were isolated. The structure of these materials consists of the same hybrid layers made of chains of edge-sharing FeS₄O₂ octahedra connected by fully deprotonated DSBDC⁴⁻, alternating with either organic (dimethyammonium, H₂N(CH₃)₂ ⁺) or inorganic (Na⁺ or K⁺) cations. The Fe–S connection leads to a strong absorption in the visible range, while the +III oxidation state of Fe, evidenced by Mössbauer spectroscopy, ensures a fairly good stability towards oxygen and water, higher than that typically observed with +II cations. Furthermore, Na{Fe(DSBDC)} exhibits an interesting reversible electrochemical activity when cycled both versus Li, Na and K in half-cells with good capacity retention, confirming the potential of this family of materials for applications involving electron transfer.

The common approach to modify the thermoelectric activity of oxides is based on the concept of selective metal substitution. Herein, we demonstrate an alternative approach based on the formation of multiphase composites, at which the individual components have distinctions in the electric and thermal conductivities. The proof-of-concept includes the formation of multiphase composites between well-defined thermoelectric Co-based oxides: Ni, Fe co-substituted perovskite, LaCo_0.8Ni_0.1Fe_0.1O₃ (LCO), and misfit layered Ca₃Co₄O₉. The interfacial chemical and electrical properties of composites are probed with the means of SEM, PEEM/XAS, and XPS tools, as well as the magnetic susceptibility measurements. The thermoelectric power of the multiphase composites is evaluated by the dimensionless figure of merit, ZT, calculated from the independently measured electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (λ). It has been demonstrated that the magnitude’s electric and thermal conductivities depend more significantly on the composite interfaces than the Seebeck coefficient values. As a result, the highest thermoelectric activity is observed at the composite richer on the perovskite (i.e., ZT = 0.34 at 298 K).