Capture/Release of Metals of Energy Importance
A significant portion of our research program involves investigating the selective, electrochemical capture and release of metals of energy importance from seawater, such as Li+ or uranyl (UO2)2+ for energy storage or low-carbon fuel applications, respectively, using carborane (Cb) chemistry (see figure). Li and U are the only trace metals dissolved in seawater that are proposed to be economical to extract and both are at very low concentrations (Li (0.17 ppm), U (3.3 ppb)); however, their total content are ~ 10,000 and 1,000 times higher than in land-based reserves, respectively, representing huge untapped resources which could be collected in an environmentally friendly manner. With surging demand for Li-ion batteries, it is estimated that current production methods will soon not meet market demands. Low-carbon nuclear energy production is also expected to increase dramatically in major countries, such as India and China, thus increasing the demand for U as well. While the capture of Li+ or (UO2)2+ from biphasic mixtures (UO2)2+ or seawater (Li+, (UO2)2+) have been studied, their controlled release remains challenging and often material and/or energy intensive.
Reduction of closo-Cb to the nido-Cb results in rupture of the C–C bond, cage opening, and an increased bite angle (see figure). Harnessing these redox-switchable chelating properties, we are currently working on several strategies to selectively capture and release these metals from seawater.
Energy Storage: Redox-Flow Batteries
Redox flow batteries (RFBs) are widely applicable energy storage devices that operate using soluble redox-active molecules, known as charge carriers, and which have modular design, fast response times, and are easily scalable to meet the demands of grid-scale energy storage. RFBs contain two electrolyte tanks (catholyte and anolyte) containing these charge carrier molecules capable of accepting or delivering multiple electron equivalents. We are investigating new, symmetric charge carriers using metal-based redox events with tunable redox innocent or non-innocent ligand frameworks. These molecules are being designed to increase solubility and electrochemical stability, while using cheap and readily available starting materials. Along with standard RFB designs, we are also investigating mixed capacitor RFB systems that use conductive carbon in conjunction with charge carriers to increase the performance of the system.
Energy Storage: Probing Ammonia Oxidation
Our group is interested in exploring the electrocatalytic oxidation of ammonia (NH3) in order to use this potentially clean fuel as an effective renewable energy (RE) vector for fuel cells. Central to using NH3 as a RE vector is understanding and developing effective electrocatalysts for this 6 electron oxidation chemistry. We are currently investigating first-row transition metal complexes as potential electrocatalysts for NH3 oxidation. Metal complexes based on ligand platforms such as salen, pthalocyanines, and novel pacman motifs are used to study the mechanism of NH3 oxidation, either through stepwise H+/e- elimination, or through H-atom abstraction routes to generate N2. Compounds currently being investigated include reactive Fe phthalocyanines, high-valent nickel centers, and manganese-salen nitride complexes. Our strategy is to use this mechanistic understanding to guide our electrocatalyst design.
Metal/Main Group Cooperative Chemistry
Part of our research program involves studying metal/main group cooperative reactivity for new bond activation chemistry. This chemistry explores typically redox-innocent main group centers which are rendered "non-innocent" through electronic delocalization to a metal. This reactivity was initially proposed by Goddard who performed DFT studies on the V/P Oxide (VPO) commercial catalyst for the partial oxidation of butane to maleic anhydride. For the first time, it was proposed that initial C–H activation was initiated by a terminal P(+5)=O bond, rather than the terminal vanadyl (V(+5)=O) bonds. For the first time, we provided experimental support for this so-called ROA mechanism using molecular model compounds, H-atom donors, and H-atom surrogates. Since then, we continue to investigate this cooperative reactivity with the goal of accessing multi-electron transformations.
