Image courtesy of Meilin Liu
Scanning electron microscopy of conductive carbon fibers coated with metal oxide nanowires (left) and close-ups of the cobalt oxide (Co3O4) nanowires (top right) and the nanowire surface (bottom right). These materials are being developed to improve the storage capacity of a type of supercapacitor known as a psuedocapacitor.
Researchers have discovered that controlling the nanostructured architecture of metal oxides coated on carbon fibers can lead to an unusually high capacity to store electrical charge in a special type of supercapacitor known as a pseudocapacitor.
Scientific understanding of the key factors that lead to a material’s ability to store a greater amount of electrical charge enables a rational design process for inexpensive supercapacitors that have an optimal combination of power and energy storage. These results suggest that metal oxide pseudocapacitors can achieve high energy densities, close to those of existing lithium-ion batteries, while maintaining the high discharge rate capabilities and excellent cycle life advantages of conventional capacitors.
Pseudocapacitors are supercapacitors that store electrical charge both like a capacitor (with the normal electrostatic double layer of charges at the electrode-electrolyte interface) and like a battery (with multiple chemical reaction mechanisms involving charge transport across the electrode-electrolyte interface). Metal oxides such as cobalt or manganese oxide (Co3O4 or MnO2) store pseudocapacitive charge via metal ions which change oxidation state (e.g., Mn+3 Û Mn+4) as a result of the charge transfer. Researchers at the Energy Frontier Research Center on Heterogeneous Functional Materials, the “HeteroFoaM Center,” have discovered how the relative sizes, shapes, atomic arrangements and interfaces of the materials in psuedocapacitors control the amount of charge that can be stored and even the mechanisms of charge storage. In fact, the electrochemical storage properties are not limited by the properties of the materials and can be radically different if the “heterogeneity” of the composite material is understood and optimized. For example, as-deposited MnO2 on conductive carbon fiber showed high specific capacitance (333 F/g) due to psuedocapacitance of the manganese ions, but conversion of the material through heat treatment to a different heterogeneous arrangement – a mixed-valence, nano-porous MnOx coating – dramatically enhanced storage capacity, achieving very high specific capacitance (~2,500 F/g) while maintaining excellent power density (~98 kW/kg at ~122.7 A/g).
Georgia Institute of Technology
Director, Heterogeneous Functional Materials (HeteroFoaM) EFRC
DOE Office of Science, Basic Energy Sciences (BES), Energy Frontier Research Centers (EFRC) Program and DOE Office of Energy Efficiency and Renewable Energy (EERE), Vehicle Technology Program (through the High Temperature Materials Laboratory user program and authors K.W.N and X.Q.Y).
M.-K. Song, S. Cheng, H. Chen, W. Qin, K.-W. Nam, S. Xu, X.-Q. Yang, A. Bongiorno, J. Lee, J. Bai, T. A. Tyson, J. Cho, and M. Liu, Anomalous Pseudocapacitive Behavior of a Nanostructured, Mixed-Valent Manganese Oxide Film for Electrical Energy Storage, Nano Lett., 12, 3483 (2012). [DOI: 10.1021/nl300984y]
L. Yang, S. Cheng , Y. Ding, X. Zhu, Z. L. Wang, and M. Liu, Hierarchical Network Architectures of Carbon Fiber Paper Supported Cobalt Oxide Nanonet for High-Capacity Pseudocapacitors, Nano Lett., 12, 321 (2012). [DOI: 10.1021/nl203600x]
Heterogeneous Functional Materials (HeteroFoaM) EFRC
University, DOE Laboratory, SC User Facilities, BES User Facilities, NSLS, ShaRE