The Bartlett group focuses on inorganic synthesis to prepare compositionally complex materials for applications in renewable energy. The two main thrusts of our program are electrical energy storage in ion insertion materials and solar energy conversion in light-harvesting metal oxides. A central theme of our program is the interplay between structure, composition, and physical properties such as electrochemistry and absorption. Our current efforts are aimed at controlling morphology and composition in nanoscale materials.
In the 21st century, one of the most important problems facing humanity is the development of an enduring, sustainable energy economy. Although fossil fuels are in sufficient supply to meet the estimated global energy demand at least until the year 2050 (~30 TW annually where 1 TWh = 61 Mtons H2/year), this strategy has catastrophic societal implications due to carbon dioxide (CO2) emissions, a leading contributor to the greenhouse gas effect. Sunlight is the only known sustainable and practical source of energy slated to replace our current carbon-based fuel sources, delivering more energy to earth in one hour than consumed globally per year. Solar energy conversion to chemical energy in the form of hydrogen fuel eliminates the consumption of carbon-based precursors as well as minimizes CO2 production. Furthermore, production of hydrogen from the direct splitting of water using sunlight is completely renewable: combustion of hydrogen with oxygen in the atmosphere simply regenerates the water from which we started. Our research program addresses three chemical challenges in sustainable energy. First, we are preparing semiconducting oxides to harvest solar energy to convert it to hydrogen fuel efficiently. Second, we are synthesizing Li-ion battery nanostructures to store chemical energy, and afford its rapid conversion to electrical energy for high power applications such as automobiles. Third, we are creating small molecule catalysts that store and transfer charge rapidly for making and breaking chemical bonds in overall water splitting. Our progress and current efforts in each of these challenges are described in the following sections.
Solar Energy Conversion
The growing demand for energy worldwide has brought increasing attention to alternative fuels that are both clean and efficient. One highly sought avenue of renewable energy exploration is hydrogen production via the photocatalytic splitting of water. Traditionally, closed-shell metal oxide semiconductors such as TiO2 have been the focus of such research. However, binary oxides have O(2p) valence band-edge energies of roughly 3 eV, poorly matched to the chemical potential of the water oxidation half-reaction (1.23 V). An important problem in science is identifying and preparing compositions with lower band gaps and high conversion efficiency. Our group is synthesizing thin films composed of a mixture of CuWO4 by electrodeposition starting from Cu2+ and W2O112– under acidic conditions, followed by annealing at 500 ºC. This is the first example of using electrodeposition to prepare metal tungstate thin films.
In addition to metal tungstates, we have lowered the band gap of anatase TiO2 by co-alloying the material with Nb and N. Although we do not observe water oxidation with this material, it effectively degrades methylene blue dye, a common test-reagent toward promising photocatalysts for multi-electron transformations.
Complex Spinels for Electrical Energy Storage
Li-ion batteries with cathodes composed of manganese oxides having the spinel structure (LiMn2O4) are attractive for hybrid electric vehicles due to their high energy storage capacity and rapid Li-ion transport. The Mn4+/3+ redox couple is the most studied system in the cubic crystal structure, and the theoretical gravimetric energy storage capacity of this material is 248 mAh/g. However, phase transitions to a double hexagonal cell at high voltage or to a tetragonal cell at low voltage limit the useable energy density of the material. Our group has developed a low-temperature hydrothermal synthesis starting from potassium permanganate and an organic reductant (acetone) that affords 30-50 nm particles, necessary for rapid Li-ion diffusion. An important chemistry problem in these materials is that traditional high temperature synthesis routes give materials that are replete with oxide anion vacancies. An anion vacancy is positively charged. Therefore, to obey the principle of electroneutrality, these materials are rich in Jahn-Teller active Mn3+, which leads to surface fracture. Most important, we have extended this beyond LiMn2O4 to include routes toward the higher-voltage LiNi0.5Mn1.5O4 composition.