Synthesis and design of nanostructured materials for electrochemical energy storage and conversion
Tour, James M.
Doctor of Philosophy
The finite nature of fossil fuels and environmental problems caused by traditional energy sources call for renewable and sustainable energy strategies, including energy storage and conversion. The synthesis and design of nanostructured materials play an important role in the advances of alternative energy systems and devices. Lithium-ion batteries (LIBs) are one of the most important energy storage devices that have been commercially used in daily life. LIBs consist of one positive electrode and one negative electrode, which are separated by a lithium-ion conducting electrolyte. The development of LIBs with high energy density and power density mainly relies on the use of advanced nanomaterials in the two electrodes. For energy conversion, water splitting and the fuel cell are two important clean and renewable techniques to interconvert electrical energy and chemical energy. Water splitting, by applying external electrical energy, generates hydrogen gas on one electrode (hydrogen evolution reaction, HER) and oxygen gas on the other electrode (oxygen evolution reaction, OER). By this process, electrical energy is converted to chemical energy stored in hydrogen fuels. Fuel cells, on the other hand, convert chemical energy into electrical energy by combining hydrogen and oxygen gas into water. The two half-reactions involved are the oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR). Due to the presence of kinetic barriers, all of the above four reactions need electrocatalysts to improve their efficiency. This thesis begins with an introduction of energy storage system of LIBs and energy conversion systems of fuel cells and water splitting in Chapter 1. Chapter 2 discusses three different nanomateirals with core-shelled structures and their applications in LIBs and HER. The graphitic carbon shell is demonstrated to improve the cycling stability and rate capability of Fe2O3 as an anode and LiFePO4 as a cathode. In addition, the graphitic carbon shell with a nitrogen dopant can interact with cobalt nanoparticles at the core to give high HER catalytic activity. Chapter 3 describes two different heteroatom-doped nanocarbons for ORR application. One is B, N-doped graphene nanoribbon and the other is B, N-doped graphene quantum dots/graphene hybrid. The edge abundance in the nanoribbon and quantum dots is demonstrated to have a critical role in enhancing the catalytic activity. In Chapter 4, various porous films, including MoS2, WS2, WC, NiCoOx and CoP/CoPO4, are used as binder-free electrodes for water splitting applications. The porous structure is created by the use of anodization technique. Benefited from the high porosity and high surface area, these films show excellent catalytic activity for HER and/or OER. Chapter 5 describes a new type of electrocatalyst for hydrogen generation based on very small amounts of cobalt dispersed as individual atoms on nitrogen-doped graphene. This catalyst is robust and exceptionally active in aqueous media. A variety of analytical techniques and electrochemical measurements suggest that the catalytically active sites are associated with the metal centers coordinated to nitrogen.
electrochemical energy storage and conversion; lithium ion battery; water splitting; fuel cell