Fine-tuning of electrocatalytic surfaces of multi-metallic hydroxides for energy conversion

Abstract

With the development of science and technology, human beings face various problems such as energy and resource depletion, climate change and natural disasters, food and poverty problems, and these problems are further accelerated and pose a direct threat to humanity's public prosperity. In particular, the use of fossil fuels produced a large amount of carbon dioxide, and the concentration of carbon dioxide in the atmosphere increased to 300 ppm in the 1950s, approaching 410 ppm in 2019. Climate change has increased the global average temperature by nearly 1 $^\circ C$ compared to 100 years ago, and it has aroused many side effects, such as rising global sea levels, decreasing ecosystem diversity, and decreasing the productivity of crops. Countries around the world are stepping up their efforts to enter a sustainable clean energy society through technological cooperation, as well as policy coordination through international cooperation for strategic responses. At present, the world's annual energy consumption is about 600 EJ (6x$10^{20}$ joules), and the demand for energy and resources is increasing rapidly due to the rapid economic growth of China, India and other emerging countries. Therefore, one of the most critical issues we face now is to discover new renewable energy resources and secure stable energy sources through continuous research and development. Solar energy is an infinite source of energy and has been studied for a long time. Solar energy reaching the Earth is about 4 million EJ, which is about 7,000 times the energy of humans annually. Therefore, the solar cell technology using a semiconductor having a photoelectric effect can be said to be a typical renewable energy technology. However, photovoltaic power generation has the variability that the power generation is available only during the day and is not affected by the weather and environment. Also, the generated electricity has difficulty in storing, supplying, and transporting. Therefore, the method of storing the produced electrical energy as chemical energy, such as converting the produced electrical energy into valuable chemicals or fuels by using a catalyst can solve the problem in the storage and utilization of energy. Photochemical energy conversion, which converts solar energy directly into chemical energy, can be found in nature. Plants on Earth use solar energy to produce chemical energy from carbon dioxide and water in the atmosphere through photosynthesis. Through learning from nature, artificial photosynthesis has been studied for the past decades, which converts solar energy directly into chemical energy using semiconducting materials. Also, electrochemical energy conversion technology that produces electricity through solar cells and converts electrical energy into chemical energy secondarily is actively studied. Electrochemical energy conversion technology can produce hydrogen from water and produce high value-added carbon compounds or fuels using carbon dioxide in the atmosphere as part of carbon dioxide capture and utilization technologies. However, high activation energy is required in order for this electrochemical energy conversion to occur, so that, it is essential to develop an electrochemical catalyst for controlling the thermodynamics and the reaction rate of the reaction. This dissertation aims to develop a material to reflect and solve the following factors as an electrochemical energy conversion reaction and an electrochemical catalyst required for such a reaction. (1) Development of low-cost materials using the Earth-abundant and environmentally friendly metals that could replace the existing catalysts using expensive rare Earth metals. (2) Development of efficient catalysts with high selectivity on the specific product and high stability in the reaction condition through the fine-tuning of the electrocatalytic surfaces. Therefore, in order to satisfy the above factors, this dissertation introduces multi-metallic hydroxides materials consisting of more than one metals. Also, the electrocatalytic surfaces were precisely processed by applying plasma technology to control the activity on the specific reactions such as (1) oxygen evolution reaction and (2) ammonia production reaction through nitrogen reduction in two different reactions. Through the study on the methodology to finely tune the electrocatalytic surfaces and its application on the electrocatalysis, I believe that this work could contribute to providing a perspective not only for the catalysis but also for the development of various energy researches.