Molybdenum nitride made from molybdenum oxide shows promise as a supercapacitor electrode material. TEM image of MoO3 single crystal nanowires (top). TEM image of mesoporous, single crystal Mo3N2 (bottom). Credit: Lee et al., JACerS, Wiley.
Supercapacitors (also called electrochemical double-layer capacitors or ultracapacitors) are an interesting class of devices because their energy density is much higher than conventional dielectric capacitors, and they can deliver much more power density than batteries. (Typical storage capacity for dielectric capacitors is on the order of microfarads per gram of active material, whereas for supercapacitors, it is on the order of tens of farads, maybe more.)
Charge is stored in an electrical double layer where ions hug the surface of the electrode, which sets up a second—or double—layer of the opposite charge in the electrolyte. More surface means more charge storage capacity. Nanoporous materials have enormous specific surface areas and do not take up much space, which makes it possible to synthesis of thin film supercapacitors. Activated charcoal and other forms of porous carbon are the most common electrode material, but the search is on for other materials that can store more charge, be made thinner, or have other properties carbon lacks. (Although, research into improving carbon for supercapacitor electrodes remains a very active field.)
A new “Rapid Communication” in the January issue of The Journal of the American Ceramic Society reports on a candidate supercapacitor electrode material, molybdenum nitride (Mo3N2). According to the paper, molybdenum nitrides are interstitial compounds with superior chemical stability, attractive physical properties, and good electrical conductivity. However, they are dense compared to carbon and, in this work, the team of researchers from Soongsil University (Seoul, Korea) and the University of Washington, were interested in finding a better way to synthesize single crystal, mesoporous Mo3N2 nanowires. In the paper they say, “The crucial advantages for mesoporous structures are electrochemical active surface areas and controlled pore sizes in the nanometer range.”
Other researchers have made Mo3N2 by nitridation, for example by nitriding a “template” compound such as a zeolite. This group, instead, turned to a “topotactic reaction” to see whether higher specific surface area and better charge-discharge capabilities could be achieved.
Such topotactic reactions are not commonly found in ceramic synthesis methodology, but the concept is quite simple. According to the International Union of Pure and Applied Chemistry (known for their stewardship of the Periodic Table of the Elements), a topotactic reaction is “A reversible or irreversible reaction that involves the introduction of a guest species into a host structure and that results in significant structural modifications to the host, for example, the breakage of bonds.” They give the example of lithium insertion into Li[Mn2]O4 spinel with one crystal symmetry to make a layered structure with a different symmetry. These reactions are also called “insertion reactions.”
The topotactic reaction is surprisingly simple to execute. Researchers, Lee et al., started with MoO3 single crystal nanowires, loaded them into a quartz boat, and heated them to 700°C for three hours in flowing ammonia. They kept the ammonia flowing during the cool-down stage to prevent surface reoxidation.
Using standard electron microscopy, X-ray, and electrochemical characterization tools, they showed that the oxide did convert to Mo3N2 single crystal nanowire with a well-defined mesoporous nanostructure (average pore size was about 4.6 nanometers) and a very high specific surface area (about 45 square meters per gram). They conclude that the mesoporous structure most likely arises from the “rearrangement of the oxide structure into metal nitride, giving rise to the formation of pores in the framework of the molybdenum nitride.” These two structural features result in higher specific charge capacity than is seen in Mo3N2 synthesized by conventional nitridation.
Also, they report that the topotactically synthesized Mo3N2 has better charge-discharge properties than nitrided material. They suggest that this is because it is easy for the electrolyte to penetrate the uniform mesopore structure.
The paper is free to everyone through March 31. See “Single-Crystalline Mesoporous Molybdenum Nitride Nanowires with Improved Electrochemical Properties,” Kyung-Hoon Lee, Young-Woo Lee, A-Ra Ko, Guozhong Cao, Kyung-Won Park, JACerS. DOI: 10.1111/jace.12096.