Highly Electrocatalytic Non-Precious Materials for Lithium Oxygen Batteries

The increased concern for energy storage technology leads many scientists to have interests in the development of new conceptual batteries. Among many storage devices, lithium air batteries are attractive because they are compact, light weight and cost-effective and, more to the point, employ a lighter air cathode operating on environmentally abundant oxygen. Despite the high energy density of lithium air batteries, current lithium air battery system presents many challenges, like limited electrical efficiency which is due to the overpotential or polarization losses at the cathode during discharge and charge. Current Li–air batteries can only be discharged–charged at a current density of 0.1–0.5 mA cm² and the voltage gap between the charge and discharge is larger than 1.0 V, which results in a low voltage efficiency of <70% . It is generally believed that oxygen electrocatalysts are critical to improving the power density, cycling capability, and round-trip energy efficiency of Li−air batteries. Platinum and gold have been successfully employed as cathode catalyst in the lithium air batteries. But these are very expensive metals of low abundance. Therefore it is need to develop cheap and abundant catalytic materials.

 Recently we have investigated improved electrocatalytic activity of carbon materials by nitrogen doping. After nitrogen doping all the carbon materials exhibit increase in surface area, which is basic requirement for cathode materials of lithium oxygen batteries. All the nitrogen doped carbons show the positive peak potential than undoped carbon materials and as well as the electron transfer number calculated for the nitrogen doped carbons are closed to the commercial Pt/C which attributed to the improved performance of carbon materials by nitrogen doping[1]. On the other hand Manganese oxides (MnOx) have been widely studied as an alternative to Pt-based catalysts because of their many advantages, such as abundance, low cost, environmental friendliness, and acceptable activity. We have investigated the performance of α and δ-MnO2 with various different nanostructures as cathode catalyst in lithium air batteries. α-MnO₂ shows the best capacity retention and 100% efficiency in comparison with δ-MnO₂. [2] Although α-MnO₂ is believed to be a promising bi-functional catalyst for ORR and OER.  Nevertheless, it is evident that the bifunctional activity of α-MnO₂ still falls below those of Pt-based catalysts. Therefore, in order to enhance the catalytic activity of α-MnO₂ nanostructures, silver nanoparticles have been deposited over the α-MnO₂ nanorods by a simple hydrothermal assisted Polyol method. Silver exhibits similar electrocatalytic activity as that of platinum which is also comparatively cheaper and abundant. We have optimized the deposited ratio of silver nanoparticles on α-MnO2 nanorods. The silver deposited MnO₂ materials exhibit improved cycleability with reduced voltage gap than bare MnO₂ nanorods throughout the cycle life of lithium air battery.  In order to improve further catalytic activity of MnO₂ nanomaterials, currently we are working on different morphologies of α-MnO₂-RuO₂ composites as well as MnO₂/N-doped Carbon composites. Addition of ruthenium in the α-MnO₂ has significantly improved the OER activity of the composite which results in the reduction of charge potential. On the other hand α-MnO₂/N-doped carbon composite shows the best capacity retention and stable cycling even after 50 cycles. Other physical and electrocatalytic studies are under investigation.  In this presentation, we will mainly introduce some of promising results from α-MnO₂-RuO₂ and MnO₂/N-doped Carbon composites works. Our findings add to our understanding of non-precious catalysts and further development of low-cost and high-efficiency alkaline polymer electrolyte fuel cells and metal-air batteries.

[1] Awan Zahoor, Ho Saeng Jang, Jeong Sook Jeong, Maria Christy, Yun Ju Hwang, Kee Suk Nahm, Applied Catalysis B: Environmental, 147, 633-641 (2014).

[2] Awan Zahoor, Ho Saeng Jang, Jeong Sook Jeong, Maria Christy, Yun Ju Hwang,  Kee Suk Nahm, RSC Adv., 2014, 4, 8973.