High Power Energy Storage: New Materials for Large Format Supercapacitors

Electrochemical double layer capacitors are well known for their high power output. However, pseudocapacitors have recently demonstrated the ability to compete here, with hybrid devices being a compromise between high power and high energy, by way of combined battery and supercapacitor technologies. Metal oxides have shown promising performance within battery and supercapacitor operation. However, cost and low conductivity have limited the performance of many of these materials and has led to the need for doping the material and refining particle shapes and sizes. 1 Molybdenum dioxide is a promising material, having many properties required for pseudocapacitance, including its electrical conductivity (atypical for a metal oxide.) 2,3 Further to this it has a theoretical specific capacity of 838 mAhg -1 when undergoing a four electron redox reaction. The various niobium oxides have multiple oxidation states, chemical stability and a high potential window making them suitable for energy storage. 4 As niobium pentoxide has a theoretical capacity of 200 mAhg -1 , which is higher than lithium titanate, (a common anode 5 ), these materials have been tested in batteries and supercapacitors. Yet there remains considerable room for investigation surrounding mixed and doped versions of these materials. Facile one-pot hydrothermal synthesis was used to make nanostructured pseudocapacitive materials. Molybdenum dioxides and niobium oxides were systematically synthesised with varied reaction conditions to gain shape and size control. Molybdenum dioxide nanoparticles were synthesised with the addition of a shape director to form spherical secondary particles. Upon doping the molybdenum dioxide with niobium, crystalline nanoparticles were formed with an average diameter of ~150 nm. Undoped niobium oxides form ‘flower-like’ particles, with the addition of a molybdenum precursor the reaction yields molybdenum dioxide/niobium oxide composite where molybdenum dioxide nanoparticles decorate the niobium oxide particles. All materials were manufactured into electrodes using conventional processing methods and activated carbon electrodes were made for asymmetric devices. The materials showed varying electrochemical properties directly influenced by characteristics gained through the synthesis conditions. Electrochemical testing was carried out on these materials vs. Li’Li + in half-cells and in asymmetric supercapacitor cells to assess their suitability for hybrid devices. Y. Gogotsi and R. M. Penner, ACS Nano , 12 , 2081–2083 (2018). Y. Liu, H. Zhang, P. Ouyang, and Z. Li, Electrochim. Acta , 102 , 429–435 (2013) http://dx.doi.org/10.1016/j.electacta.2013.03.195. X. Li et al., J. Power Sources , 237 , 80–83 (2013) http://dx.doi.org/10.1016/j.jpowsour.2013.03.020. P. Arunkumar et al., RSC Adv. , 5 , 59997–60004 (2015) http://xlink.rsc.org/?DOI=C5RA07895D. E. Lim et al., ACS Nano , 8 , 8968–8978 (2014). Figure 1