Application of synthetic tricopper complexes and NOx in energy conversion and storage

Ever-increasing energy consumption and rising public awareness for environmental protection require the development of new technologies for renewable energy conversion and storage. The renewable energy, abundant but intermittent, is usually stored in stable chemicals that can be easily converted back to electrical energy. This process is similar to how Nature harvests and utilizes solar energy through photosynthesis and respiration.For energy conversion, one of the major technologies is fuel cells. A Fuel cell is an electrochemical device that converts chemical energy in fuels directly into electrical energy. A fuel cell separates a combustion reaction into two half-reactions (fuel oxidation and oxygen reduction), and these two half-reactions occur in two physically separated chambers. In commercialized fuel cells, both reactions require platinum catalysts. Oxygen reduction reaction requires large amounts of platinum catalysts to achieve high current density. Therefore, developing alternative, cheap catalysts for ORR is important for lowering the price of fuel cells. In nature, life-sustaining ORR is catalyzed by multicopper oxidases (MCOs), where O2 is captured and reduced at a tricopper cluster. MCOs are a group of enzymes which can selectively catalyze the reduction of oxygen to H2O with a high turnover frequency (560 s-1). In Chapter 1 and Chapter 2, a biomimetic tricopper complex, resembling the active site in multicopper oxidases (MCOs), was developed to understand how nature employs tricopper clusters to catalyze ORR at near-zero overpotential. In order to mimic MCO’s ability to reversibly store and deliver multiple electrons, a fully encapsulated tricopper complex was synthesized. Site isolation and intramolecular proton transfer site are further shown to be the two critical factors in lowering the overpotential of oxygen reduction reaction. Our work represents a significant step toward inexpensive and efficient copper catalysts for ORR.Another major technology that stores energy on a large scale is redox flow batteries (RFBs).3 In RFBs, at least one electrode contains redox-active liquid electrolyte that can be reduced or oxidized. After electrochemical reaction, the reduced or oxidized materials are pumped out of the electrochemical cell and stored in a large tank. The energy density of RFBs, one of the most important factors in evaluating the performance, is usually limited by the solubility of the energy storage material and the volume of the electrolyte. In Chapter 3, a new type of catholyte utilizing low-cost gaseous nitrogen oxides (NOx) was developed and investigated. With this material, the energy can be stored in a redox-active gas, which allows scale-up of storage capacity without increasing the concentration of the redox-active solute or the amount of electrolyte employed, which is one of the main cost drivers in non-aqueous RFBs.