Investigations of interlayer chemistry in layered metal oxides for energy conversion and storage

Abstract

The overall goal of this dissertation research was to design, tailor and understand layered metal oxides in the context of electrocatalytic energy conversion and storage processes. To accomplish this goal the thesis research combined electrochemistry, state-of-the-art structural characterization and theoretical calculations. The hypothesis examined in this dissertation is that incorporation of metal atoms or metal ions into the sheets and/or interlayer region of the layered materials will enhance the properties of selected 2D materials for chemistry relevant to electrochemical energy conversion (i.e. electrochemical water splitting catalysis; H2O ® H2 + 1/2O2) and energy storage (i.e., as pseudocapacitors). The primary 2D layered materials investigated in this thesis research were birnessite (nominally MnO2) and Fe:Ni double hydroxide materials. Metals (cations) used to modify the geometric and electronic structure of the layered materials include Cu, Ni, and Co. Perhaps the result with broadest impact to result from the integration of experimental and theoretical studies in the thesis research was that the confinement of solvated redox active metals within the interlayer region of 2D layered materials can be used to facilitate their electron transfer reaction rates (relative to the respective unconfined metal) and energy related electrochemistry. This new paradigm for electron transfer has implications for the development of novel electrocatalytic materials for energy conversion. Research showed that the electrocatalytic activity of birnessite toward water oxidation (2H2O® 4H+ + 4e- + O2) was increased by intercalating zero valent copper into the interlayer region of the layered manganese oxide. Electrocatalytic studies showed that the Cu-modified birnessite exhibited an overpotential for water oxidation of ∼490 mV (at a current density of 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. It was experimentally shown that the oxygen evolution reaction (OER; water oxidation) catalysis of redox active transition metal ions (Ni2+ and Co2+) can be enhanced by individually confining them in the interlayer region of birnessite. It was demonstrated that the metal confined electrocatalyst reached a current density of 10 mA cm−2 at much lower overpotentials than pure Ni and Co oxides, and pristine birnessite. For example, with interlayer nickel and cobalt, overpotentials of 400 and 360 mV, respectively, were achieved for the OER. Molecular dynamics (MD) simulations suggested that electron transfer reaction rates relevant to OER and involving Ni or Co were enhanced when the metal cations were confined in the interlayer of birnessite. The strategy of metal confinement, which was successfully applied to layered manganese oxide to improve OER activity was extended to Ni-Fe based layered double hydroxide. It was demonstrated that the electrocatalytic activity of NiFe layered double hydroxides (NiFe LDHs) for the OER could be significantly enhanced by systematic cobalt incorporation using coprecipitation and/or intercalation. Electrochemical measurements showed that cobalt modified NiFe LDH possessed an enhanced activity for the OER relative to pristine NiFe LDH. The cobalt doped NiFe LDH exhibited overpotentials in the range of 290−322 mV (at 10 mA cm−2), depending on the degree of cobalt content. The cobalt intercalated NiFe LDH achieved a current density of 10 mA cm−2 at a much lower overpotential of ∼265 mV (compared to 310 mV for NiFe LDH). With regard to energy storage, it was shown that the pseudocapacitive charge storage in layered manganese oxide was a sensitive function of interlayer composition and distance. Even though pristine layered manganese oxide shows a 7 Å interlayer spacing, the interlayer engineering via metal (Mg2+) intercalation and thermal annealing led to layered manganese oxide materials with variable interlayer spacings of 10 and 5.6 Å respectively. The interlayer expanded layered manganese oxide (10 Å interlayer spacing) exhibited an improved specific capacitance of 380 Fg-1, in comparison to synthetic Na-birnessite (specific capacitance of 200 F g-1). Dehydrated Na-birnessite (~5.6 Å spacing) produced by annealing to expel interlayer water, showed the lowest specific capacitance of 50 Fg-1. Experimental results showed that interlayer expanded manganese oxide (with intercalated Mg2+) was unstable if exposed to a solution containing only Na+ cation electrolyte. In this circumstance, the interlayer distance decreased from the expanded 10 Å value back to an interlayer distance of 7 Å and a specific capacitance of ~200 F g-1; values associated with synthetic Na-birnessite. Finally, a highly active alkaline medium hydrogen evolving electrocatalyst based on earth abundant materials (Co, Mo and P) was developed and the catalyst exhibited a ~0 V onset for the hydrogen evolution reaction (HER; 2H+ + 2e- ® H2). This value was comparable to that of the precious metal platinum. The Co-Mo-P catalyst was prepared by room temperature electrodeposition and it exhibited an overpotential of ~ 25-30 mV for HER at a geometrical current density of 10 mA cm-2 in an alkaline medium. A DFT theoretical investigation revealed that a Co-Mo center acts as the water-dissociation site enhancing the alkaline medium HER.