High efficiency and low cost transition metal compounds for (photo)electrochemical energy conversion

Abstract

As the world entered the high-tech industry, the demand for energy increased. The most widely used energy source is fossil fuels. However, fossil fuels are limited and cause severe air pollution and global warming. As a result, there is a growing interest in the infinite and nature-friendly renewable energy field. Among various clean energy sources, the solar industry is infinite and attractive. Solar energy can be used anywhere in the world by providing nearly unlimited raw materials. It can be used for various fields such as electricity generation, thermal energy, and water splitting through solar energy. Currently, the solar industry is mainly based on the silicon-based industry. However, due to the limitation of efficiency improvement(Shockley-Queisser limit, Si solar cell 29.4%), which is a characteristic of silicon, various methods of solar cell design are required. Therefore, the design of high-performance catalysts is necessary. Another clean energy source is the hydrogen industry, which is produced by breaking down water using electricity. Hydrogen energy can be applied directly to industry, and used hydrogen does not produce any impurities other than water. However, oxygen evolution reaction(OER) has a higher kinetic barrier than the hydrogen evolution reaction(HER) during water splitting, which makes it difficult to produce highly efficient hydrogen. As a result, research on high-performance OER catalyst, which is the core of water splitting reaction, is being actively conducted. In this paper, we designed a $CuBi_2O_4$ (CBO) thin film applicable to a silicon-thin tandem structure and a high-performance OER catalyst for water splitting. First, we studied CBO thin films with an ideal bandgap for application to silicon-thin tandem structures. CBOs with 1.6-1.8 eV bandgap can be used with silicon to deliver high-efficiency performance. We grew a dense CBO thin film on a fluorine-doped tin oxide(FTO) substrate using a simple method of spin coating. CBO thin film was dense and uniformly grown through the introduction of polyvinylpyrrolidone(PVP). The PVP introduced interacts with metal ions ($Bi^{3+}, Cu^{2+}$) to control the release and growth rate of metal ions. In order to grow the dense CBO thin film on the FTO substrate, the optimum conditions were investigated by controlling the precursor molarity and the calcination temperature step. The CBO thin film has a different density and contact with the substrate depending on the precursor molar concentration. In addition, the calcination temperature step for the decomposition of PVP-metal interaction was adjusted to improve the contact between the CBO thin film and the FTO substrate. The prepared dense CBO thin film was subjected to an oxygen reduction reaction(ORR) characterization to understand the electrochemical behavior. As a result of ORR evaluation, it was confirmed that the density of CBO thin film and the contact between the substrate and the thin film are very important. The fabricated CBO also yielded reasonable results in stability evaluation. In this way, we introduced a dense CBO thin film using a simple spin coating process and introducing PVP. Second, we studied catalysts to improve OER performance, the key to water decomposition. As a condition of good OER catalyst, it is required to select high performance, rich, inexpensive and environmentally friendly materials. We have studied $NiCo_2O_4$(NCO) materials as appropriate. The performance of the OER catalyst is directly related to the large active area, conductivity and stability. So, we designed a OER catalyst by introducing Prussian Blue Analogue(PBA) material, which is a metal-organic framework(MOF) structure, to have a large specific surface area. For high-performance OER performance, we searched for optimum conditions by adjusting hydrothermal synthesis time and temperature, PBA molar concentration, and calcining temperature. We doped K to increase conductivity and cause oxygen vacancies in the NCO catalyst. The doping concentration of K was controlled through the hydrothermal synthesis reaction time. The doping of K increased the $Ni^{3+}$ and $Co^{3+}$ states and induced oxygen vacancies through X-ray photoelectron spectroscopy(XPS) analysis. In addition, the conductivity improvement was presented through electrochemical impedance spectroscopy(EIS) analysis. The fabricated K doped NCO recorded a low overpotential of 0.369 V at 100 mA $cm^{-2}$ through OER evaluation. In addition, it showed reasonable stability of 12 hours even at high currents of 50 mA $cm^{-2}$ and 100 mA $cm^{-2}$. We showed K-doped high-performance NCO catalyst through a simple hydrothermal synthesis process of 100 $^\circ C$ for 1 hour and showed superiority as an OER catalyst. Finally, we conducted a study on metal sulfide OER catalysts. Metal sulfides are much researched due to their excellent electrochemical properties compared to metal oxides. We have introduced the Co-L-Cysteine complex by introducing inexpensive, abundant Co material and L-Cysteine to supply sulfur. The Co-L-Cysteine complex formed is heated and finally synthesized into $Co_3S_4$. We controlled the rate of degradation of the Co-L-Cysteine complex by introducing Ni ions to precisely control the conversion process from the Co-L-Cysteine complex to $Co_3S_4$. Synthesis Ni:Co-L-Cysteine complex and $Co_3S_4$ confirmed the shape and internal structure of particles through TEM analysis. As a result, the Ni:Co-L-Cysteine complex structure is semi-hollow, resulting in a more active specific surface area. The synthesized Ni:Co-L-Cysteine complex was characterized for OER with $Co_3S_4$. As a result, the Ni:Co-L-Cysteine complex showed a low overpotential of 0.364 V at 100 mA $cm^{-2}$. The Ni:Co-L-Cysteine complex also showed lower electrical resistance values in the EIS analysis. The Ni:Co-L-Cysteine complex and $Co_3S_4$ were subjected to stability evaluation at 50 mA $cm^{-2}$ for 30,000 seconds and showed that the Ni:Co-L-Cysteine complex is more stable. We introduced Ni:Co-L-Cysteine complex particles with a large specific surface area, good conductivity and improved stability with the simple addition of Ni ions. We synthesized and evaluated $CuBi_2O_4$, $NiCo_2O_4$, and Ni:Co-L-Cysteine complex materials for (photo)electrochemical energy conversion. Our new catalysts offer superior performance and easy synthesis. We believe that these high-performance catalysts are excellent candidates for a variety of (photo)electrochemical applications.