1. Field of the Invention
The invention is related to ultracapacitors also known as supercapacitors. The applications of the ultracapacitors may include hybrid/electric vehicles, spacecrafts, uninterrupted power supplies and memory backup power supplies. Also, the invention is related to transition metal nitrides and their synthesis methods. The forms that the transition metal nitrides may take thin film layers, micrometer-sized particles, and nanometer-sized particles. The applications may include a thin film as a wear protective layer, particles for ultracapacitors, particles of catalysts, particles as an additive of a wear-resistant coating, and magnets.
2. Description of the Existing Technology and some of its Problems
Ultracapacitors store energy using either ion adsorption (electric double layer capacitors, EDLCs) or fast surface redox (reduction-oxidation) reactions (pseudo-capacitors). The ultracapacitors can store several magnitudes of higher electric charges than the conventional capacitors. Compared with the conventional batteries, the ultracapacitors can be charged-discharged much faster because it does not involve chemical redox reactions. In addition, due to extremely low internal resistance, it can supply much higher current than the conventional batteries. Although the total energy density is about one order of magnitude lower than that of the conventional batteries, the unique characteristics of the ultracapacitors found their applications in memory back-ups, booster power supplies for hybrid/electric vehicles, and temporary power supplies for short power outage. For weight-sensitive applications such as hybrid/electric vehicles, aircrafts, and spacecrafts, ultracapacitors with higher energy density is required. Ultracapacitors having comparable energy density with the conventional batteries are strongly demanded recently.
The ultracapacitors are conventionally constructed with carbon-based electrodes; however, due to low specific capacitance of carbon-based electrodes, the energy density is more than one order of magnitude lower than that of the conventional batteries. The specific energy density is proportional to the specific capacitance, therefore, it is very important to increase the specific capacitance of the electrodes. Similar to the conventional capacitors, the specific capacitance is proportional to the specific surface area and the dielectric constant, thus many improvements have been made to improve the electrodes.
For example, the US patent (US 2008 0180881 A1)[1] discloses extremely large specific surface area (1500 m2/g) by utilizing nano-sized porous carbon structure. However, the specific capacitance is as low as the order of 100 F/g. In the US patent (US 2011-0149473 A1)[2], the specific capacitance is increased by adding nano-sized particle having high dielectric constant onto the carbon-based electrodes. Although this approach is effective to increase the specific capacitance, the carbon-based electrode has a disadvantage of inherently low specific capacitance. Alternative material such as metal oxide is explored, and ruthenium oxide (RuO2) is reported to have high specific capacitance of 600 F/g [3]. However, due to extremely high cost of Ru, this is not preferable for a practical use. In contrast, metal nitride is reported to have high specific capacitance. Vanadium nitride recorded the specific capacitance as high as 1340 F/g [4]. Also, US patent (US 2010-0019207 A1)[5] disclosed high specific capacitance of ternary transition metal nitride, i.e. mixture of transition metal nitrides.
Although transition metal nitride has a great potential to improve the performance of ultracapacitors, its synthesis is challenging due to its chemical stability. The Ref. 4 and 5 discloses a synthesis method of transition metal nitride using chloride precursors. However, the method may leave halide impurities, which potentially causes corrosion of the support metals for electrodes, the electric terminals or the housing. To achieve reliable ultracapacitors with transition metal nitride, a synthesis method which does not involve a halide element is preferred.
Historically, transition metal nitride has been used as wear-resistant coatings and thermal barriers, due to its strong mechanical and thermal property. Along with the development of structural and mechanical engineering, the coatings and barriers are required to cover complicated and fine structures. In other words, the surface area relative to its volume is becoming larger in recent years.
It is also reported that transition metal nitride is useful as a functional material for ultracapacitors, catalysts, and magnet. To use transition metal nitride as ultracapacitor, catalysts, or magnets, it is important to increase the surface area of the material. Nanotechnology using nano-sized particles has a potential of obtaining superior characteristics due to extremely large surface area relative to its weight. Functional materials requiring large surface area such as ultracapacitors and catalyst receives tremendous benefit from nanotechnology.
With increasing demand of covering small-sized material with large surface area, the existing synthesis method is facing several challenges. Since transition metal is more readily oxidized than nitridized, synthesis of transition metal nitride requires elimination of oxygen and moisture. Synthesis methods typically involve vapor phase reaction in vacuum/air tight reactors. To form a nitride layer on transition metal parts, physical vapor deposition or plasma deposition are used. However, these methods are unable to coat complicated structure having deep blind holes because vapor phase reactant does not reach the bottom surface of the deep holes.
In the case of particle synthesis, vapor phase method is even less efficient because of extremely high surface area to cover. When the particles have size less than 10 nm or specific surface area larger than 10 m2/g, it becomes challenging for the gaseous agents to cover the entire surface. For example, vanadium nitride nanoparticles are synthesized using VCl4 as precursor. The VCl4 is dissolved and stirred in anhydrous chloroform inside a glovebox. The solution is then transferred to an Ar-filled glove bag, where the dissolved chloride is reacted with anhydrous ammonia gas over solution for 8 hours. The as-prepared powder is collected by evaporating the solvent at 100° C. under continuous NH3 gas flow. Final heat treatment for nitridization is conducted under an anhydrous ammonia atmosphere with a heating and cooling rate of 5° C./min. The temperature for heat treatment is 400° C. [4]. As shown in this example, the final heat treatment involves vapor phase reaction with constant ammonia flow at high temperature. A high temperature such as this can cause sintering, resulting in larger particle size than what would be achieved using a lower-temperature process. Also, this process may leave chlorine impurities in the synthesized transition metal nitride.
The challenges in the existing synthesis methods of transition metal nitride are summarized as follows: (1) the existing methods use vapor phase reaction which is unable to cover surfaces of complicated structures or small particles; (2) the existing methods require constant flow of source gas such as ammonia or nitrogen; (3) some existing methods use metal halide precursors, which leaves halogen impurities unfavorable to ultracapacitor applications; (4) some existing methods requires multiple steps to obtain transition metal nitride; (5) some existing methods requires high temperature which causes larger particle size.