Materials and Molecules for Pollution Free Clean Energy

The combustion of fossil fuels produces a myriad of toxic air pollutants and carbon dioxide. These emissions have been the most significant threat to globally environmental problems, which influence the health and future of human society. Fossil fuels are formed from the remains of the living plants and animals that lived millions of years ago, therefore they are not renewable energy sources. It has been predicted that the global coal will run out in 115 years, however the oil and natural gas remaining will run out in ~50 years, respectively. In order to build a sustainable future for next generations, the human activities should become less dependent on the fossil fuels, and renewable and clean energy technologies should be the major energy provider for the next century. As one of the main renewable energy sources (wind, hydropower, solar, etc.), solar energy plays a very important role since sunlight is decentralized and inexhaustible in the world. In this thesis, I have focused on the exploration of new materials and molecules for pollution free clean energy based on the water splitting reactions that produces hydrogen fuel.

In the first part of the thesis, I have statistically analyzed density, size, morphology, adhesiveness and chemical composition of the airborne pollutant particles (mainly on PM_2.5 particles) through a series of nanoscale characterizations (such as AFM, SEM, TEM and EDS). The results indicate that there are mainly three kinds of PM_2.5 particles, including: i) soot aggregate, ii) elongated mineral, iii) spherical fly ash. Among them, the soot aggregate, which is rich in carbon from the incomplete combustion of hydrocarbons, is more toxic since it shows high adhesion and aggregation capabilities. This study helps to understand how the airbone pollutants influence the human health and give new insights for solving/improving the air pollution problem.

In the second and third parts of the thesis, I have studied the solar-driven water splitting devices for transferring solar energy to storable and transportable hydrogen, which are based on photoelectrochemical (PEC) and photovoltaic-electrolysis (PV-EC) configurations. The results show that in the PEC water splitting devices, copper, nickel and nickel/titanium metallic thin films can be deposited on the surface of silicon photoanodes, which can form CuO, NiO_X and NiO_X/TiO_X respectively and then serve as very active catalysts for water oxidation reaction to dioxygen and a protecting layer for silicon surface from corrosion. In PV-EC water splitting devices, a novel ruthenium molecular catalyst based anode has been used for the electrolyzer, which has been integrated with state-of-art perovskite solar cells and commercially available triple junction solar cells, respectively. The latter PV-EC water splitting device achieves the highest solar-to-hydrogen efficiency of 21.2 % at neutral pH. These results pave the way for the generation of large-scale solar energy converted hydrogen.