Surface oxidation/spin state determines oxygen evolution reaction activity of cobalt-based catalysts in acidic environment.

Co-based catalysts are promising candidates to replace Ir/Ru-based oxides for oxygen evolution reaction (OER) catalysis in an acidic environment. However, both the reaction mechanism and the active species under acidic conditions remain unclear. In this study, by combining surface-sensitive soft X-ray absorption spectroscopy characterization with electrochemical analysis, we discover that the acidic OER activity of Co-based catalysts are determined by their surface oxidation/spin state. Surfaces composed of only high-spin CoII are found to be not active due to their unfavorable water dissociation to form CoIII-OH species. By contrast, the presence of low-spin CoIII is essential, as it promotes surface reconstruction of Co oxides and, hence, OER catalysis. The correlation between OER activity and Co oxidation/spin state signifies a breakthrough in defining the structure-activity relationship of Co-based catalysts for acidic OER, though, interestingly, such a relationship does not hold in alkaline and neutral environments. These findings not only help to design efficient acidic OER catalysts, but also deepen the understanding of the reaction mechanism.

Molecular regulation of electrolytes for enhancing anode interfacial stability in lithium-sulfur batteries.

We addressed the poor interfacial stability of the Li metal anode in Li-S batteries through molecular regulation of electrolytes using arylthiol additives with various numbers of anchoring sites. The dual functional tetrathiol additive markedly enhanced the Li anode interfacial stability, controlled the sulfur redox kinetics and suppressed side reactions towards polysulfides, thus leading to an improved capacity retention of 70% after 500 cycles at 1 C.

Visible-Light-Driven Water Oxidation on Self-Assembled Metal-Free Organic@Carbon Junctions at Neutral pH.

Sustainable water oxidation requires low-cost, stable, and efficient redox couples, photosensitizers, and catalysts. Here, we introduce the in situ self-assembly of metal-atom-free organic-based semiconductive structures on the surface of carbon supports. The resulting TTF/TTF•+@carbon junction (TTF = tetrathiafulvalene) acts as an all-in-one highly stable redox-shuttle/photosensitizer/molecular-catalyst triad for the visible-light-driven water oxidation reaction (WOR) at neutral pH, eliminating the need for metallic or organometallic catalysts and sacrificial electron acceptors. A water/butyronitrile emulsion was used to physically separate the photoproducts of the WOR, H+ and TTF, allowing the extraction and subsequent reduction of protons in water, and the in situ electrochemical oxidation of TTF to TTF•+ on carbon in butyronitrile by constant anode potential electrolysis. During 100 h, no decomposition of TTF was observed and O2 was generated from the emulsion while H2 was constantly produced in the aqueous phase. This work opens new perspectives for a new generation of metal-atom-free, low-cost, redox-driven water-splitting strategies.

Sodium chromium hexacyanoferrate as a potential cathode material for aqueous sodium-ion batteries.

Sodium chromium hexacyanoferrate (NaCrHCF) is obtained here using a facile co-precipitation method at room temperature. The powder was investigated in terms of potential use as a cathode material for aqueous sodium-ion batteries under neutral conditions. The highest achieved discharge capacity of NaCrHCF was around ∼64 mA h g-1 at C/3 current rate.

Anomaly in the electronic structure of the Na(x)CoO(2-y) cathode as a source of its step-like discharge curve.

In this paper we would like to show a new approach to an explanation of the nature of the discharge-charge curve of Na/Na(+)/NaxCoO2-y batteries, which can justify the existence of the step-like characteristics. This is still an open problem, which until now had no proper description in the literature. On the basis of comprehensive experimental studies of physicochemical properties of NaxCoO2-y cathode material (XRD, electrical conductivity, thermoelectric power, electronic specific heat) supported by calculations performed using the DFT method with accounting for chemical disorder, it has been shown that the observed step-like character of the discharge curve reflects the variation of the chemical potential of electrons (Fermi level) in the density of states of NaxCoO2-y, which is anomalously perturbed by the presence of the oxygen vacancy defects and sodium ordering. Our studies of structural, electronic and thermal properties of NaxCoO2-y cathode material as a function of concentration of electrochemically intercalated sodium document strong and step-like shift of the position of the Fermi level during introduction of electrons in this process. This effect is coherently supported by the shape of calculated density of states (DOS) of NaxCoO2-y having included oxygen defects and sodium ordering.