Strategies and experimental methods in photoelectrochemistry for the enhancement of light harvest performance and quantum conversion efficiency at the photo-sensitized semiconductor electrode are reviewed. Highlighted particularly in this review are the material solutions to the suppression of energy dissipation at sensitizer molecules and the semiconductor surface. Discussions are focused on sensitization of a thin n-type semiconductor, typically TiO2, with organic dyes and inorganic nanocrystals (quantum dots) that have high extinction coefficients. Based on the strategies proposed, current developments by our group of low-cost dye-sensitized solar cells by means of printable materials and technologies are also introduced.
In order to develop our society, human beings should consider the global environment where global cycles of materials are essentially important. Considering a global effect of an energy carrier such as hydrogen and hydrocarbons, the hydrogen cycle (water cycle) is compared with the carbon cycle in order to evaluate the effect of an energy carrier on natural cycles of materials. We introduced a new parameter named “Environmental impact factor”. The environmental impact factor was defined as a ratio of an annual quantity of materials produced by energy consumption of mankind to a natural movement on the earth. The environmental impact factor of water on the earth, 0.0001, is two orders of magnitude less than that of carbon/carbon dioxide, 0.036. This result means that the hydrogen/water cycle is superior to the carbon cycle considering the materials circulation for energy system of mankind on a global scale. In addition, we applied the environmental impact factor to cities and prefectures in Japan. The environmental impact factor of water was always smaller than that of carbon in all cities and prefectures in Japan. This result indicates that hydrogen energy is friendly to the environment.
This paper presents an experimental study on anode reactions in direct ethanol fuel cells with the effect of cell operating conditions on the generation ratio of anode reaction products (i.e. carbon dioxide, acetic acid, and methane). Decreasing the ethanol concentration in the fuel increased the generation ratio of carbon dioxide. Additionally, lowering the fuel flow rate produced more carbon dioxide. Thus, we conducted a detailed experiment using acetaldehyde which is an intermediate product of the anode reaction of ethanol. Similar to using ethanol as a fuel, more carbon dioxide was produced at a lower fuel concentration and at a slower flow rate. Additionally, increasing the oxygen concentration in the cathode gas significantly increased the carbon dioxide generation ratio, which was likely because the oxygen from the cathode side reaches at the anode via the electrolyte membrane to promote the carbon dioxide production reaction. The amount of carbon dioxide produced greatly exceeded the amount of oxygen permeated, and comparing the amount of generated current with the amounts of acetic acid and carbon dioxide suggests that another oxide is present at the anode. The methane generation ratio was not affected by the oxygen concentration. It is assumed that the decreasing fuel concentration at the electrode surface due to a lower fuel flow rate increases the carbon dioxide generation ratio. Isotope analysis on the generated carbon dioxide confirmed that the detected carbon dioxide was due solely to the oxidation of fuel and not due to the oxidation of electrode materials such as the catalyst support.
Two types of polyacrylonitrile (PAN) nano-fiber based nonwovens as separator for lithium ion battery, which consist of different fiber diameters, have been developed by an electrospinning technique. SEM observation exhibited that the electrospun PAN nano-fibers had homogeneous thicknesses, and their diameters were around 250 and 380 nm. The physical, electrochemical and thermal properties of the PAN nonwovens were characterized. The nonwovens possessed homogeneous pore size distribution with similar pore sizes to a polyolefin membrane separator. Moreover, they showed higher porosities, lower Gurley values than that of the polyolefin one. Cyclic voltammetry revealed that they were electrochemically stable in the voltage range of −0.5 to 4.5 V vs. Li0/Li+. They showed higher ionic conductivities than that of the polyolefin one in the temperature range of −10 to 60°C. Any internal short circuit was not observed for cells using them during charge-discharge tests. Cells using them showed better cycleabilities than that with the polyolefin one at −10 and 30°C. Besides, they showed better rate capabilities than a cell with the polyolefin one.