Ming-Tsung Lee

Tightly connected MnO2–graphene with tunable energy density and power density for supercapacitor applications

MnO2 nanoparticles uniformly distributed and tightly anchored on graphene are prepared using an ethanol-assisted graphene-sacrifice reduction method, producing composite electrodes with tunable energy density (up to 12.6 Wh kg−1) and power density (up to 171 kW kg−1) via rational design of the oxide/graphene ratio.

Pseudocapacitive Mechanism of Manganese Oxide in 1-Ethyl-3-methylimidazolium Thiocyanate Ionic Liquid Electrolyte Studied Using X-ray Photoelectron Spectroscopy

The electrochemical behavior of anodically deposited manganese oxide was studied in pyrrolidinium formate (P-HCOO), 1-butyl-3-methylimidazolium hexafluorophosphate (BMI-PF6), and 1-ethyl-3-methylimidazolium thiocyanate (EMI-SCN) ionic liquids (ILs). The experimental data indicate that the Mn oxide electrode showed ideal pseudocapacitive performance in aprotic EMI-SCN IL. In a potential window of approximately 1.5 V, the oxide specific capacitance, evaluated using cyclic voltammetry and chronopotentiometry, was about 55 F/g. The electrochemical energy storage reaction was examined using X-ray photoelectron spectroscopy (XPS). It was confirmed that the SCN- anions, instead of the EMI+ cations, were the primary working species that can become incorporated into the oxide and thus compensate the Mn3+/Mn4+ valent state variation upon the charge-discharge process. According to the analytical results, a pseudocapacitive mechanism of Mn oxide in the SCN- based aprotic IL was proposed.

Improved supercapacitor performance of MnO2–graphene composites constructed using a supercritical fluid and wrapped with an ionic liquid

Supercritical CO2 (SCCO2) is used to synthesize MnO2. This process is found to be promising for producing oxide nanorods (∼5 nm in diameter) with little agglomeration. With suitable SCCO2 pressure and temperature, nanocrystalline α-MnO2 with an extremely high surface area of 245 m2 g−1 (versus 80 m2 g−1 for the oxide prepared in ambient air) was obtained. Introduction of graphene nanosheets and carbon nanotubes into MnO2 with and without the aid of SCCO2 is compared. SCCO2 can help debundle the graphene nanosheets and uniformly disperse the MnO2 nanorods in between, thus preventing graphene restacking. Since the oxide particles are sandwiched between the highly conductive sheets, a high electrochemical utilization is obtained. Accordingly, SCCO2-MnO2/graphene shows superior supercapacitor properties. The feasibility of using an ionic liquid (IL) as a conductive wrapping agent to improve the performance of the MnO2/graphene electrode is demonstrated. Because the intimate contact between the oxide and graphene can be ensured and the interfacial double-layer capacitance can be optimized, the electrode wrapped with an IL shows a higher capacitance, improved high-rate capability, and higher cyclic stability compared to those obtained without IL wrapping.

Pseudocapacitive behavior of Mn oxide in aprotic 1-ethyl-3-methylimidazolium–dicyanamide ionic liquid

The electrochemical behavior of anodically deposited Mn oxide was studied in three ionic liquids (ILs): 2-methylpyridine–trifluoroacetic acid (P–TFA), 1-ethyl-3-methylimidazolium–dicyanamide (EMI–DCA), and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI–BF4). In the aprotic and low-viscosity EMI–DCA IL, ideal pseudocapacitive behavior of the oxide electrode was observed; the specific capacitance, measured using cyclic voltammetry at a sweep rate of 5 mV/s, was 72 F/g. The operation potential window was as wide as 2 V, which is double that found in traditional aqueous electrolytes. Moreover, electrochemical stability of the Mn oxide electrode in EMI–DCA IL was excellent; after 600 redox cycles, the capacitance barely decayed. The charge storage mechanism of Mn oxide in the IL was examined using X-ray photoelectron spectroscopic (XPS) analyses. The results reveal that DCA−, instead of EMI+, is the primary working ion that penetrates into the oxide and compensates the Mn valent state variation. This is the first study that provides a detailed explanation of the pseudocapacitive properties of Mn oxide in IL.

Unique Pd/graphene nanocomposites constructed using supercritical fluid for superior electrochemical sensing performance

Supercritical CO2 fluid (scCO2) with gas-like diffusivity, low viscosity, and near-zero surface tension can help debundle graphene nanosheets and uniformly disperse Pd nanocrystals (approximately 3 nm) between them. The in situ synthesis protocol of Pd (from scCO2), which does not require pretreatment of the support, preserves the peculiar characteristics of graphene, such as the excellent electrical conductivity. The deposited Pd nanoparticles (NPs) can act as spacers to prevent restacking of the graphene nanosheets. Since the Pd catalyst particles are sandwiched between the highly conductive sheets, a superior electrochemical utilization can be achieved. The obtained Pd/graphene nanocomposite exhibited superior sensing performance (in terms of response current and potential resolution) toward ascorbic acid, dopamine, and uric acid, compared to that of Pd/graphene prepared using a conventional process. Promising detection sensitivity and selectivity of various scCO2-synthesized NPs/graphene heterostructures for various biological analytes can be expected.

Evaluation of Ionic Liquid Electrolytes for Use in Manganese Oxide Supercapacitors

Development of Mn oxide supercapacitors incorporating ionic liquid IL electrolytes was attempted. The experimental resultsindicate the possibility of achieving pseudocapacitive performance of Mn oxide in ILs without involving protons and alkalications, thus opening a route of developing electrolytes for oxide-based pseudocapacitors. In 1-ethyl-3-methylimidazolium-dicyanamide aprotic IL, Mn oxide can exhibit a specific capacitance of 72 F/g in a potential window of 2 V. Due to high ionicconductivity, large electrochemical windows, excellent thermal stability, and nonflammability of the IL, a high-voltage andlong-life energy storage system is successfully proposed.© 2008 The Electrochemical Society. DOI: 10.1149/1.3013028 All rights reserved.Manuscript submitted August 18, 2008; revised manuscript received October 3, 2008. Published November 6, 2008.