Increased electricity storage and faster delivery

Electrochemical electrode material with combined supercapacitor and battery behaviors

The advancements of mobile electronic devices have increased the demand for energy storage devices with not only high storage capacity but also fast power performance. Supercapacitors are capable of delivering fast power with both high coulombic and energy efficiencies; however, the charge-storage capacities of supercapacitors are substantially lower than those of batteries. The conventional strategy to combine supercapacitors and batteries involves making electronic hybrid systems that fulfill the energy and power requirements of modern high-technology devices. One of the recent focuses in the Energy Materials Laboratory (EML) led by Professor Nae-Lih Wu in the Department of Chemical Engineering is the exploration of novel bifunctional electrode materials that simultaneously exhibit supercapacitor and battery behaviors and enable the advantages of supercapacitors and batteries to be achieved in a single electrochemical charge-storage cell.

One example of a bifunctional electrode material is a tailored SiO2_MnO2 nanocomposite synthesized via a facile hydrothermal process. MnO2 is an attractive pseudocapacitive electrode material because of its low cost, natural abundance, and environmental friendliness. The use of aqueous MnO2-based pseudocapacitors has been limited to a small operating voltage window of approximately 1 V, and the electrode exhibits purely capacitive behavior with a limited charge-storage capacity. The operating voltage window is mainly limited by the structural instability of the oxide caused by an irreversible phase transformation and loss of the active material via dissolution toward the lower voltage limit.

Dr. Yu-Ting Weng of the EML derived a new material design strategy to extend the electrochemical stability of the MnO2 electrode over a substantially widened operating voltage window. This strategy was based on the concept of forming space-confined MnO2 nanodomains of which the surfaces are stabilized using an electrochemically inactive foreign oxide material, such as SiO2, that can restrain the phase change of the MnO2 nanodomains and suppress interface erosion. Spherical mesoporous SiO2 nanobeads were used as hosts for the deposition of MnO2 via a hydrothermal process, facilitating the formation of Mn–O–Si composite interfaces.

Figure 1. Scanning (a) and transmission (b) electron micrographs revealing the microstructures of the derived nanoporous MnO2_SiO2 spheres.

The electrochemical behavior of the resulting nanocomposite material, which exhibits a 2 V-operating window (-1.0~1.0 V versus Ag/AgCl reference), is schematically described in Figure 2. The pseudocapacitance originating from Mn(IV)-Mn(III) charge transfer, denoted as PC (IV)-(III), occurs between 0 and 1 V. Pseudocapacitance from the Mn(III)-Mn(II) redox reaction, denoted as PC (III)-(II), commences below 0 V. The battery behavior involving the bulk reduction of Mn(IV) to Mn(III) and then to Mn(II), denoted as B (IV)®(III) and B (III)®(II), respectively, occurs primarily between 0 and -1 V. However, the battery behavior involving oxidation from Mn(II) to Mn(III) and then to Mn(IV), i.e., B (II)®(III) and B(III)®(IV), respectively, concurrently occurs with PC (IV)-(III) from -0.3 to 1 V. The figure also illustrates that the battery behavior provides a substantial amount of charge-storage capacity in addition to pseudocapacitance at low scan rates. This study suggests a new strategy to design and develop new electrochemical electrode materials for more efficient energy storage and wider applications.

Figure 2. Schematic of the various charge-storage mechanisms as a function of potential; B: battery behavior; PC: pseudocapacitance; (IV), (III) and (II): Mn ions with a valence of 4, 3 and 2, respectively. The inset compares the charge-storage capacities between the bifunctional MnO2_SiO2 composite electrode (red) and a conventional MnO2 electrode (blue).

Yu-Ting Weng, Hsiao-An Pan, Rung-Chuan Lee, Tzu-Yang Huang, Yun Chu, Jyh-Fu Lee, Hwo-Shuenn Sheu and Nae-Lih Wu. (2015). Spatially Confined MnO2 Nanostructure Enabling Consecutive Reversible Charge Transfer from Mn(IV) to Mn(II) in Mixed Pseudocapacitor-Battery Electrode. Advanced Energy Materials, 5, 1500772. DOI: 10.1002/aenm.201500772.

Professor Nae-Lih Wu
Department of Chemical Engineering