광전기화학적 물 분해를 위한 천이금속산화물의 전기화학적 합성법에 관한 연구

Abstract

In the past few decades, the large amounts of fossil fuels are consumed indiscriminately, which causes the greenhouse effect. So, it is necessary to develop renewable and environment-friendly energy such as water, wind, solar power. Among them, because solar energy is almost infinite and huge, many researchers are studying. However, the electricity from solar energy by solar cell cannot be stored for a long time and it is difficult to produce electricity constantly due to the cloud or rain. Therefore, it is important to convert from electricity to chemicals, which can be stored for a long time. Hydrogen with high energy density and efficiency is promising as alternative energy of fossil fuel. Currently, hydrogen is mostly produced from natural gas by steam methane reforming, which yields a large amount of hydrogen as low cost. However, the production of hydrogen from natural gas is not environmentally friendly because a large amount of carbon dioxide is produced as well. Water splitting is very simple and able to produce high pure hydrogen, but the extra electricity is need for water splitting. So, the photoelectrochemical (PEC) splitting of water using sunlight is an environmentally friendly and economical method for producing hydrogen and oxygen without producing any undesired reactants and pollutants. A PEC cell consists of two semiconductor photoelectrodes in an aqueous electrolyte, and its operation is based on the electrochemical interaction between the photoelectrodes and the electrolyte. Controlling the facets exposed on the surfaces of the electrodes is important for enhancing the performance of the electrodes during PEC reactions. This is because the facets on the surface of an electrode and the electrolyte, the diffusion length of the minority carriers, and the optical absorption depth, among other parameters. Also, to enhance the performance of PEC cell, highly active electrocatalyst is need. In this study, we investigated the differences in photocurrent generated in Cu2O films with surfaces having different types of facets. The films were fabricated using electrochemical deposition methods. The type of facets exposed on the Cu2O films was controlled by controlling the pH of the electrolyte. As mentioned previously, the polarity of the exposed surface affects the interactions between the adsorbed electrolyte ions and the Cu2O electrode and, in turn, determines the overall photocatalytic activity of the electrode. In addition, the effects of three-dimensional (3D) Cu2O structures on the generation of photocurrent were studied. A 3D structure consisting of randomly exposed facets was synthesized using a template of polystyrene (PS) beads to increase the surface area and the light absorption. The growth mechanism of Cu2O microcrystals and nanocrystals synthesized by ED while using different surfactants was investigated. While positively charged hexamethylenetetramine (HMT) was adsorbed on negatively charged surfaces, negatively charged PVP was adsorbed on positively charged surfaces. The growth mechanism of the Cu2O crystals was affected by the differences in the adsorption behaviors of the two surfactants, with the resulting micron-sized Cu2O crystals exhibiting very different shapes. On the nanoscale, the shape of the Cu2O crystals could be controlled to change from cubes to octahedrons or from octahedrons to cubes by adding or removing a particular surfactant from the solution. 3D metal/Cu2O micro-pillar arrays were fabricated by electroless deposition of Ni on Si micro-pillars followed by the electrochemical deposition of Cu2O on the metal micro-pillars. The metal micro-pillars with heights of 16 μm and diameters of 1 to 1.5 μm showed low reflectance and conductance values similar to Cu2O plates, indicating that the 3D metal/Cu2O micro-pillars are good current collectors. The photocurrent of the 3D metal/Cu2O micro-pillar arrays was more than twice that of the Cu2O plates. This high photocurrent for the 3D metal/Cu2O micro-pillar results from its large surface area, long optical absorption depth (micro-pillar length) and short carrier diffusion length (thin Cu2O absorber). Also, the transition metal-based electrocatalysts such as Ni, Co and Fe were studied for alternating platinum or ruthenium catalysts with high cost and scarcity. The catalytic performance of nanostructured bimetallic hydroxide nanosheets (NSs) on the Ni foam was investigated by changing concentration ratios of Co/Ni. A facile but high-precision composition-controlled electrodeposition technique is employed to synthesize CoNi hydroxide nanosheets within tens of seconds. The CoNi hydroxide nanosheet with surface modification of Ni foam is a promisingly efficient oxygen evolution reaction (OER) catalyst with stability. The cobalt iron-phosphorus films with high activity were readily electrodeposited during short time as a bi-functional electrocatalyst. The cobalt iron-phosphorus films had cobalt iron metallic alloy in the bulk and phosphate phase in the surface. The electrodeposited cobalt iron-phosphorus films show outstanding electrocatalytic performance for both H2 and O2 evolution reaction (HER and OER, respectively).