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Abstract (Mg,Fe)O ferropericlase‐magnesiowüstite has been proposed to host the majority of Earth's sodium, but the mechanism and capacity for incorporating the alkali cation remain unclear. In this work, experiments in the laser‐heated diamond anvil cell and first‐principles calculations determine the solubility of sodium and favorability of sodium incorporation in iron‐rich magnesiowüstite relative to (Mg,Fe)SiO3 bridgmanite. Reaction of Mg/(Mg + Fe) (Mg#) 55 and 28 olivine with NaCl at 33–128 GPa and 1600–3000 K produces iron‐rich magnesiowüstite containing several percent sodium, while iron‐rich bridgmanite contains little to no detectable sodium. In sodium‐saturated magnesiowüstite, sodium number [Na/(Na + Mg + Fe)] is 2–5 atomic percent at pressures below 60 GPa and drastically increases to 10–20 atomic percent at deep lower mantle pressures. For these two compositions, there is no significant dependence of the results on Mg#. Our calculations not only show consistent results with experiments but further indicate that such an increase in solubility and partitioning of Na into magnesiowüstite is driven by the spin transition in iron. These results provide fundamental constraints on the crystal chemistry of sodium at lower‐mantle conditions. If the sodium capacity of (Mg,Fe)O is not strongly dependent on Mg#, (Mg,Fe)O in the lower mantle may have the capacity to store the entire sodium budget of the Earth.

Iron spin transition occurs at ultrahigh pressure. The total electron spin increases from 0 to 1 as the structural transition of (Mg,Fe)O occurs (~0.6 TPa) and drops back to 0 at higher pressure. Its effects on exoplanet interiors are anticipated.

The molecular single-source precursor (SSP) route has emerged as a promising avenue for synthesizing highly efficient electro(pre)catalysts tailored for both oxygen evolution (OER) and organic electrooxidation reactions. This study introduces a novel [CoII(PyHS)4(OTf)2] molecular complex, offering a facile route to access the nanocrystalline Co9S8 phase. Upon application in alkaline OER, Co9S8 displayed remarkably high electrocatalytic activity across various metrics, including overpotentials, Tafel slopes, faradaic efficiency, charge transfer resistance, turnover frequency, electrochemical surface area, Co-redox active sites, and long-term stability at industrially relevant current densities (for 80 h at 100 mA cm−2) outperforming its Co-based counterparts under identical conditions. In-depth analysis employing several ex situ techniques revealed the complete leaching of sulfur and irreversible reconstruction of Co9S8 into a cobalt oxyhydroxide active phase during the OER. Moreover, quasi in situ Raman spectroscopy provided insights into the presence of CoIVO2 active species under operational OER conditions, along with the formation of the Co superoxide intermediate. Beyond OER applications, the Co9S8-derived active phase demonstrated notable efficiency in catalyzing the selective oxidation of biomass-derived glycerol and furan-2-carboxaldehyde to formate and furan-2-carboxylic acid, respectively, achieving yields exceeding 80%, with excellent reusability. A full hybrid-water electrolysis cell has been developed, wherein biomass valorization with Co9S8, coupled with H2 production, resulted in a significant improvement in energy efficiency compared to conventional water-splitting. [ABSTRACT FROM AUTHOR]