Abstract
With increasing demands on portable electronics, electric vehicles, hybrid electric vehicles and the quality life needs of modern society, novel energy storage/conversion systems with high energy or power delivery, such as fuel cells, lithium-ion batteries, dye-sensitized solar cells and supercapacitors, are highly important and attractive substituted energy. To fulfill the requirements of high performance energy storage/conversion systems, powerful and effective materials are necessary for advanced researches. In past decades, by the blooming developments and discoveries of nanotechnologies, fascinating and impressive nanomaterials showed up one after another. Owing to the mature nanotechnology, including synthesis and characterization approaches, numerous unique, unexpected and useful phenomena and physicochemical properties of nanomaterials were defined and discovered. These developments and discoveries provide convenience and are the fundamentals of modern sciences; especially, chemistry of materials plays extremely important roles in the way to better life and further demonstrations of natural law. In the trends of developing advanced applications, the main difficulty is to provide the detailed information for the reasons why the nanomaterials or nanotechnologies can achieve the excellent performance and functions in that way, but it is usually much easier to provide plenty of statements for the illness than benefits. Fortunately, based on the controllable nanotechnologies, we could distinguish the effects in a reasonable and reliable way. One of the issues, which we have demonstrated the phenomena as well as possible effects but the practical reasons, is the mass-transfer behaviors in porous materials. As everybody know and believe in, mass-transfer ability in mesoporous structures is far superior to that in micropores. However, it is always lack of an independent and systematic discussion in this field. Fortunately, nanotechnologies are helpful and useful tools to illustrate it with suitable structure design in detailed. In this study, we have developed porous materials with various porous structures, that is, the same configuration and chemical properties but different length scale in pores by the assistance of templating methods. Those materials, including ordered macroporous carbons, ordered mesoporous carbons and hierarchically ordered macro-/meso-porous carbons, were synthesized by introduction macroporous templates, polystyrene latex spheres, and mesoporous templates, triblock copolymer and silicate oligomers, to synthesis procedures. Consequently, we could obtain the desirable porous structures, also understand and manage the effects of mass-transfer in different length scale (pore size) under a specific electrochemical application. From the experimental results, hierarchically ordered porous materials are composed of ordered macroporous (~ 300 nm) and interconnected porous sturcutes (including ~5 nm ordered mesopores and micropores). The hierarchically porous structures could be served as ion-buffering reservoir of ions; in addition, the larger pores are beneficial on capacitance retention at high charge/discharge rate from impedance analysis. In summary, we found that the macropores can enhance the rate of mass-transfer toward smaller pores, and the mesopores provide sufficient pore volume and surface area as a platform of energy storage or chemical reactions. Besides, an interconnected mesopore shows their improvements in diffusion rate of each mesopores. At same time, by the advantage of hierarchically porous structures, which is a combination of different size pores, the performance of electrochemical applications (e.g. such as double layer capacitance, pseudocapacitance and electrochemical catalysis) is dramatically improvements due to the enhancements in mass-transfer rate upon porous materials. In conclusions, we successfully constructed specific porous structures and distinguish their roles in altering the mass-transfer behaviors and enhancing performance of energy storage/conversion systems by the assistance of templating fabrication strategies of porous materials.