Copper-Intercalated Birnessite as a Water Oxidation Catalyst

We report a synthetic method to increase the catalytic activity of bimessite 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 Cut-bearing precursor molecule that underwent disproportionation in solution to yield Cu-0 and Cu2+. Electrocatalytic studies showed that the Cu-modified birnessite exhibited an overpotential for water oxidation of similar to 490 mV (at 10 mA/cm(2)) and a Tafel slope of 126 mV/decade compared to similar 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 bimessite 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 Cu2+ in the prepared Cu-modified birnessite. Control experiments using birnessite that was decorated with only Cu2+ 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 similar to 3 times longer for the overpotential for water oxdiation to increase by 100 mV compared to when Cu was not present in the birnessite.