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Few-layered potassium nickel and cobalt oxides show drastic differences in catalytic activity based on metal ion preorganization. Uniform compositions [(CoO₂/K)₆ or (NiO₂/K)₆] show limited activity, while homogeneously mixed-metal cobalt/nickel oxides [(Co_nNi_(1–n)O₂/K)₆] display moderate improvement. However, a layer-by-layer arrangement of alternating cobalt and nickel oxide sheets [e.g., (CoO₂/K/NiO₂/K)] provides superior catalytic performance, reducing the oxygen evolution overpotential by ∼200–400 mV. Density functional theory simulations provide an illustration of the electronic properties (density of states and localization of orbitals) that promote catalysis in the layer-segregated materials over those of homogeneous composition. This study reveals that atomic preorganization of metal ions within layeredmore » catalysts plays a more crucial role than the overall metal composition in enhancing catalytic efficiency for oxygen evolution.« less

We report that the discovery of solid-phase, inexpensive transition-metal-based water oxidation catalysts is a central goal for renewable energy, and has led to a general consensus that a partially populated metal e_g d-electronic state is desirable, leading to favorable catalysis for certain elements in specific oxidation states. In manganese systems, the key species is manganese(III), whose high-spin d₄ electronic configuration places an unpaired electron in the e_g orbital, which is postulated to contribute to electronic and structural features that support catalysis. Based on density functional theory calculations, it is predicted that electron transfer would be facilitated by a catalyst withmore » alternating low- and high-Mn^III-content sheets, which positions neighboring band edges in closer energetic proximity. The preparation of such catalysts is demonstrated for the first time and it is shown that the catalytic activity is maximized in these systems over more uniform, but more Mn^III-rich systems. The best catalyst possesses alternating high-and low-average oxidation state sheets with interlayer Cs⁺ ions, and has an overpotential of 450 mV at 10 mA, which represents an improvement of 250 mV over the best unmodified synthetic potassium birnessites. Using scanning tunneling spectroscopy, bandgap modulations consistent with the theoretically predicted band edge shifts are detected.« less

Stimuli-responsive “solvate-sponge”-(DMF) ₃ NaClO ₄ exhibits linear chains of DMF–Na ⁺ ions with ClO ₄ ⁻ anions in the interstitial space. At increased pressure or temperature, DMF is expelled (reversibly), resulting in a new stoichiometry-(DMF) ₂ NaClO ₄ .

Sodium perchlorate (NaClO4) crystallizes with adiponitrile (ADN) as a 1:3 solvate to produce (ADN)₃NaClO₄, a solid electrolyte for sodium ion conduction. The solid possesses high thermal stability (up to 150 °C) and the ability to be melt-cast (T_m = 81 °C). The pressed solid has a high ionic conductivity of 2.2 × 10^–4 S cm^–1 at room temperature with a low activation barrier for ion conduction of 22 kJ mol^–1. The high conductivity is the result of low-affinity ion-conduction channels in the bulk based on the X-ray crystal structure, and by low grain-boundary resistance and possibly a grain-boundary percolating networkmore » due to a fluidlike nanoliquid layer between the grains, observable by scanning electron microscopy and differential scanning calorimetry. When the liquid nanolayer is rinsed away or removed by excessive drying, the bulk room temperature ionic conductivity is 4 × 10^–5 S cm^–1, activation energy for ionic conduction for an organic solid is 37 kJ mol^–1, and the sodium ion transference number is 0.71. Scanning electron microscopy and classical molecular dynamics simulations suggest that these cocrystals form a fluid layer of ADN at the surface, which facilitates the Na+ ion migration between the grains. Finally, density functional theory calculations are consistent with the possibility of ion conduction via a solvent–anion coordinated transition state through vacancy defects in the three symmetry-equivalent ion channels along separate directions, suggesting the possibility of ionic conductivity in three dimensions.« less

Significance We propose, and preliminarily confirm with experiments, a theoretical model to understand various structure–performance dependences of layered-structure birnessite as an oxygen evolution reaction (OER) catalyst. Besides the well-accepted importance of Mn(III), we emphasize the critical importance of a nonuniform distribution of Mn(III) to OER catalytic activity. Such a distribution contributes to the reduction of the overpotential by building an internal potential step. We further propose the small polaron as a common concept to link the fields of oxygen evolution catalysis and Li-ion batteries, suggesting a promising candidate space for oxygen evolution reaction catalysts.

The effect on the electrocatalytic oxygen evolution reaction (OER) of cobalt incorporation into the metal oxide sheets of the layered manganese oxide birnessite was investigated. Birnessite and cobalt-doped birnessite were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and conductivity measurements. A cobalt:manganese ratio of 1:2 ended in the most active catalyst for the OER. Specifically, the overpotential (η) for the OER was 420 mV, significantly lower than the η = 780 mV associated with birnessite in the absence of Co. Moreover, the Tafel slope for Co/birnessite was 81 mV/dec, in comparison to a Tafel slope ofmore » greater than 200 mV/dec for birnessite. For chemical water oxidation catalysis, an 8-fold turnover number (TON) was achieved (h = 70 mmol of O₂/mol of metal). Density functional theory (DFT) calculations predict that cobalt modification of birnessite resulted in a raising of the valence band edge and occupation of that edge by holes with enhanced mobility during catalysis. Inclusion of extra cobalt beyond the ideal 1:2 ratio was detrimental to catalysis due to disruption of the layered structure of the birnessite phase.« less

We investigated the dependence of the electrocatalytic activity for the oxygen evolution reaction (OER) on the interlayer distance of five compositionally distinct layered manganese oxide nanostructures. Each individual electrocatalyst was assembled with a different alkali metal intercalated between two nanosheets (NS) of manganese oxide to form a bilayer structure. Manganese oxide NS were synthesized via the exfoliation of a layered material, birnessite. Atomic force microscopy was used to determine the heights of the bilayer catalysts. The interlayer spacing of the supported bilayers positively correlates with the size of the alkali cation: NS/Cs+/NS > NS/Rb+/NS > NS/K+/NS > NS/Na+/NS > NS/Li+/NS.more » The thermodynamic origins of these bilayer heights were investigated using molecular dynamics simulations. The overpotential (η) for the OER correlates with the interlayer spacing; NS/Cs+/NS has the lowest η (0.45 V), while NS/Li+/NS exhibits the highest η (0.68 V) for OER at a current density of 1 mA/cm2. Kinetic parameters (η and Tafel slope) associated with NS/Cs+/NS for the OER were superior to that of the bulk birnessite phase, highlighting the structural uniqueness of these nanoscale assemblies.« less