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. 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/cm. 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.

We report a synthetic method to enhance the electrocatalytic activity of birnessite for the oxygen evolution reaction (OER) by intercalating Ni(2+) ions into the interlayer region. Electrocatalytic studies showed that nickel (7.7 atomic %)-intercalated birnessite exhibits an overpotential (η) of 400 mV for OER at an anodic current of 10 mA cm(-2) . This η is significantly lower than the η values for birnessite (η≈700 mV) and the active OER catalyst β-Ni(OH)2 (η≈550 mV). Molecular dynamics simulations suggest that a competition among the interactions between the nickel cation, water, and birnessite promote redox chemistry in the spatially confined interlayer region.

We report the polarized absorption spectra of high-quality, thin crystals of a perylene diimide (PDI) species with branched side chains (B2). The absorption spectrum shows exemplary polarization-dependent H-like and J-like aggregate behavior upon orthogonal excitation, with a sizable Davydov splitting (DS) of 1230 cm and peak to peak splitting of 3040 cm. The experimental results are compared to theoretical calculations with remarkable agreement. The theoretical analysis of the polarized absorption spectra shows evidence of a high degree of intermolecular charge transfer, which, along with Coulombic coupling, conspires to create the unprecedented DS for this family of dye molecules. The large polarization dependence of the electronic spectra is attributed to the unique twisted crystal structure, in which a substantial rotational displacement exists between neighboring chromophores within a π-stack. These results highlight the strong sensitivity of the Davydov splitting to intermolecular geometry in PDI systems.

We report a synthetic method to increase the catalytic activity of birnessite toward water oxidation by intercalating copper in the interlayer region of the layered manganese oxide. Intercalation of copper, verified by XRD, XPS, ICP, and Raman spectroscopy, was accomplished by exposing a suspension of birnessite to a Cu(+)-bearing precursor molecule that underwent disproportionation in solution to yield Cu(0) and Cu(2+). Electrocatalytic studies showed that the Cu-modified birnessite exhibited an overpotential for water oxidation of ∼490 mV (at 10 mA/cm(2)) and a Tafel slope of 126 mV/decade compared to ∼700 mV (at 10 mA/cm(2)) and 240 mV/decade, respectively, for birnessite without copper. Impedance spectroscopy results suggested that the charge transfer resistivity of the Cu-modified sample was significantly lower than Cu-free birnessite, suggesting that Cu in the interlayer increased the conductivity of birnessite leading to an enhancement of water oxidation kinetics. Density functional theory calculations show that the intercalation of Cu(0) into a layered MnO2 model structure led to a change of the electronic properties of the material from a semiconductor to a metallic-like structure. This conclusion from computation is in general agreement with the aforementioned impedance spectroscopy results. X-ray photoelectron spectroscopy (XPS) showed that Cu(0) coexisted with Cu(2+) in the prepared Cu-modified birnessite. Control experiments using birnessite that was decorated with only Cu(2+) showed a reduction in water oxidation kinetics, further emphasizing the importance of Cu(0) for the increased activity of birnessite. The introduction of Cu(0) into the birnessite structure also increased the stability of the electrocatalyst. At a working current of 2 mA, the Cu-modified birnessite took ∼3 times longer for the overpotential for water oxdiation to increase by 100 mV compared to when Cu was not present in the birnessite.

We show that the activity of cobalt for the oxygen evolution reaction (OER) can be enhanced by confining it in the interlayer region of birnessite (layered manganese oxide). The cobalt intercalation was verified by employing state-of-the-art characterization techniques such as XRD, Raman and electron microscopy. It is demonstrated that the Co 2+ /birnessite electrocatalyst can reach 10 mA cm-2 at an 10