From cell phones to automobiles, batteries are crucial to everyday life. Emerging technologies require batteries or fuel cells that are compact, yet efficient. To be effective at reasonable temperatures, these systems require catalysts that enable reactions to proceed quickly. Unfortunately, many of the catalytic materials with the properties that match the needs of such batteries are precious metals -- as the name indicates, they are quite valuable, and their supply is limited. As a result, researchers are challenged to find and design new materials that function as effective catalysts.
Scientists with the Center on Nanostructuring for Efficient Energy Conversion (CNEEC) have uncovered a new manganese oxide (Mn3O4) catalyst to convert oxygen to water via the oxygen reduction reaction, an important process in metal-air batteries and in fuel cells. Manganese oxide (MnO) catalysts provide an alternative to well-known and commonly employed platinum, which is significantly more expensive and rare. Manganese oxide, on the other hand, is relatively abundant, as well as environmentally friendly. The oxygen reduction reaction is a complicated reaction, the steps of which are still being studied; however, it consists of oxygen from the atmosphere being reduced (gaining electrons) to first form peroxide and then water. In a battery, this reaction occurs at one electrode, while, at the other, a material is oxidized (loses electrons), and the movement of electrons between electrodes results in electrical current.
The researchers of the CNEEC loaded tiny MnO particles (14 nanometers in diameter) onto glassy carbon supports. Glassy carbon is a common material for electrodes, with some ability to reduce oxygen on its own. Therefore, to accurately determine how effectively the MnO operates as a catalyst, the researchers performed measurements that would discriminate between the support's and the catalyst's activity.
To determine the activity levels, they heated the glassy carbon support with and without catalyst present and took a series of measurements. The scientists learned that upon heating, the MnO was converted to Mn3O4, and the glassy carbon developed pores in its otherwise smooth surface. The change in morphology (that is, the change from the smooth to porous surface) corresponded to a significant change in effectiveness.
The Mn3O4, when tested as a catalyst, held its own against the best comparable non-precious metal catalysts. The ability of the catalyst/support to facilitate oxygen reduction in the way it did is exciting, especially given the relative abundance of the materials involved. The work shows Mn3O4 as a potential means to create efficient metal-air batteries that are affordable to produce. Further study is needed to examine the particular features that make the catalyst so effective. Understanding them better will lead to improved electrode design in energy storage and battery technology.
