CaNbO₂N Photoanode:Synthesis And Photoelectrochemical Activity For Water Splitting

Today in the 21st century, energy crisis and environmental pollution are the most serious problems which are closely related to humanity’s survival and development all over the world. The great demands for energy in modern society lead to rapid depletion of fossil energy because fossil fuel itself is not sustainable. Besides, the burning of fossil fuels leads to a large number of emissions of greenhouse gases and pollutants which can cause serious ecological and environmental problems. So it is necessary to develop clean and non-polluting renewable energy sources to eliminate the modern society’s dependence on fossil energy, which is the only way to keep the sustainable development of human society. Solar energy is an inexhaustible source of energy and one of the major sources of future social energy consumption. However, the solar energy has the low energy density and it is not easy to storage. Through the use of photoelectrochemical cells, converting solar energy into hydrogen to storage is a main way of the effective use of solar energy. In order to carry out the industrial application of the photoelectrochemical cells, the semiconductor photoelectrode which is the most important part in photoelectrochemical cells should have high solar-to-hydrogen efficiency, strong photoelectrochemical stability and it must be low-cost and easy to get.This thesis mainly explores a new type of semiconductor photoelectrode-calcium niobium oxynitride（CaNbO₂N）. It belongs to a member of the metal oxynitrides which have the perovskite structure. With a suitable band gap, CaNbO₂N has excellent visible light absorption in the visible region. On top of this, for CaNbO₂N, the cost is relatively low and the synthetic method is relatively simple. So it has great potential in research and development in theory, some forward-looking exploration is of great value and significance.The research and experiment about CaNbO₂N semiconductor photoelectrode mainly focuses on three aspects:the synthesis of samples, the preparation of photoelectrode and photoelectrochemical activity test for overall water splitting.（1） The synthesis of samples:first of all, the exploratory experiments with the method of solid-state sintering reaction to synthesize the pure phase precursor were proved failed. So we give up solid-state sintering reaction and use the polymerized complex method （PC） to synthesize the pure phase precursor-Ca₂Nb₂O₇. And then through the way of controlling the nitridation temperature and the nitridation time, pure phase CaNbO₂N was synthesized by the method of high temperature nitridation in ammonia atmosphere using the precursor-Ca₂Nb₂O₇.（2） The preparation of photoelectrode:CaNbO₂N photoelectrodes were prepared by electrophoretic deposition. And then TiCl4 methanol solution was used to improve the electrical connection of the photoelectrodes. Through the analysis about scanning electron microscope photograph and photoelectrochemical activity test, it can be known that the technology of electrical connection is of great significance to the photoelectrodes, and it can improve the photoelectrochemical properties of CaNbO₂N photoelectrodes.（3） Photoelectrochemical activity test:in three electrode test system, the photocurrent for CaNbO₂N photoelectrodes which were prepared by the polymerized complex method was ca. 100μA/cm2 at 1.3 VRHE in 1M NaOH aqueous solutions（pH=13.6） under AM 1.5G simulated sunlight irradiation（100mW/cm2）. In order to confirm the photocurrent which is related to CaNbOxN photoelectrodes, we exclude the impact of the complex phase-NbOxNy with the analysis about NbOxNy by the method of X-ray diffraction and photoelectrochemical activity test. At the same time we test the photoelectrochemical performance about CaNbO₂N photoelectrodes by the Xenon lamp which was using 420nm filter in a three electrode system and confirm the photoelectrochemical activity of the CaNbO₂N under visible light irradiation.