UC Santa Cruz

Developing sustainable, high efficiency and clean energy technologies is one of the urgent missions for modern chemistry. Fuel cells with their high energy conversion efficiency, high reliability, low CO2 emission and extensive applications, show promising outlook among all new energy devices. However, the expensive electrocatalyst and their sluggish electrode reaction kinetics greatly hamper the commercialization of fuel cells. Thus, the advanced electrocatalyst should be designed and synthesized, meanwhile, the reaction mechanism should be better understood for material structural modification and activity enhancement. My thesis focused on design, synthesis of nano materials for advanced electrocatalysts and understanding and exploring the catalytic mechanism. Based on series of research projects, a systemic strategy framework for electrocatalyst preparation and understanding is established by combination of experimental and theory method. This framework contains a wide range of aspect of electrocatalyst studies. Specifically, Chapter 1 introduces the background of fuel cell, water electrolyzer and their related critical electrochemistry reactions. A systematic overview of electrocatalyst design, modification and reaction mechanism understanding is illustrated. Based on the current and previous results, the strategy loop and the central dogma of electrocatalyst design is proposed. Each of the work introduced in other chapters was carried out based on one or several aspects of the strategy loop, which including material synthesis, activity measurement, active site revealing, electronic structure understanding, modification of electronic structure. Chapter 2, chapter 3, chapter 4 introduces series work of carbon nanowires for electrocatalyst. These works include the aspects of material synthesis, activity understanding, active site identifying or understanding the source of activity. Specifically, chapter 2 introduces the design and synthesis of a novel nitrogen and iron-doped carbon nanowires material for great activity for alkaline oxygen reduction reaction, with the performance even surpass the commercial platinum carbon. Moreover, the active site was identified, and their activity was compared as the order of Stone-Wales FeN4 sites> Normal FeN4 sites> neighboring carbon atom of nitrogen atom in the carbon matrix. In addition, the source of activity of each important activity site were also revealed. The great activity of FeN4 comes from the d-orbital of Fe atom provide great chance for oxygenous intermediate species to absorb and detach. While, the neighboring carbon to nitrogen atom have much better ability for adsorption than normal carbon, due to the charge transfer from themselves to the nearby nitrogen atoms. Chapter 3 introduces the design and synthesis of novel ruthenium and nitrogen doped carbon nanowires for excellent activity for hydrogen evolution reaction in alkaline electrolyte, with the performance better than commercial platinum carbon and other noble metal-based material as reported previously. Moreover, it was identified that the activity is coming from a novel single atom ruthenium site, RuC2N2 structure embedded in the carbon, other than ruthenium nanoparticle or normal RuN4 sites as assumed previously. The ruthenium and its neighboring carbon atoms of RuC2N2 can provide efficient water dissociation process which is critical for overall reaction. Chapter 4 introduces a typical work for adopting theoretical calculation to reveal active sties and for guiding experimental electrocatalyst synthesis. In this work, a high throughput first principle calculation was first carried out and find that the platinum needs a minimum domain size (about 0.9 nm of diameter) to maintain activity for catalyzing oxygen reduction reaction. Any platinum species smaller than that limit will suffer great difficulty of adsorbing oxygen molecules due to the lack of states and electron near Fermi level. Moreover, it was further revealed cobalt atom doping can greatly assist in oxygen adsorption when the domain size of platinum is too small. The following experimental work confirmed the theoretical finding and successfully synthesized PtCo few atom clusters as high efficiency electrocatalyst for oxygen reduction reaction, which can have an ultrahigh mass activity as much as 48 times better than commercial platinum carbon. Chapter 5 and chapter 6 mainly focus on several individual works about interfacial charge transfer of nanomaterial and its induced electrocatalytic activity enhancement. For instance, Chapter 5 introduce several works about charge transfer induced electrochemical activity enhancement. In one work, it was observed oxygen vacancy doped TiO2 can transfer electrons to the palladium nanoparticles loaded on them. This charge transfer can significantly enhance the activity of ethanol oxidation reaction catalyzed by palladium nanoparticles. In another work, it was found that black phosphorus can donate significant number of electrons to the platinum, gold and silver nanoparticle load on it. This charge transfer can induce the activity decrease for platinum nanoparticles and increase for gold and silver nanoparticles towards oxygen reduction reaction. Chapter 6 introduces developing new methods for charge transfer. Specifically, TiO2 nanoparticles functionalizing with alkyne ligand can induce significant one-way charge transfer from ligand to surface of metal oxide through M-O-C≡C- core-ligand linkages. This linkage can also induce novel states inside of band gap. Moreover, this unique charge transfer can be universally observed on other metal oxide, exhibiting a powerful ability for increasing the charge density of metal oxide catalyst. Finally, all of finding are systemically summarized in chapter 7. In addition, a perspective of future works was put forward based on the previous works.