High Specific Energy Electrochemical Capacitors

In the past yeas, the supercapacitors characterized by high power and long-life have attracted global attentions. However, the energy density of supercapacitors is still much less than that of recharge batteries. Thus, many researches on the supercapacitors aim to increase energy density of supercapacitors. According to the equation E=1/2 CV², two effective approaches can be used to improve the energy density of supercapacitors. One is to develop hybrid system with higher work voltage (V), the other is to develop nano-structure electrode with high capacitance (C). As mentioned above, my study mainly focused on the study about hybrid system and electrochemical capacitance performance of nano materials. Furthermore, I also make some study about fuel cells.1. Hybrid Aqueous Energy Storage Cells Using Activated Carbon and Lithium Intercalated Compounds: A new concept hybrid electrochemical surpercapacitor technology was invented. In this system, the activated carbon was used as a negative electrode and a lithium-ion intercalated compound LiMn₂O₄ as a positive electrode in a mild Li₂SO₄ aqueous electrolyte. The charge/discharge process is associated with the transfer of Li-ion between two electrodes. The hybrid cell exhibits a sloping voltage profile from 0.8 to 1.8 V, and delivers an estimated specific energy of ca. 35 Wh/kg based on the total weight of the active electrode materials. The cell exhibits excellent cycling performance with less than 5% capacity loss over 20,000 cycles at 10C charge/discharge rate. The new state-of-the-art hybrid aqueous supercapaictor technology overcomes the drawbacks of electrolyte depletion during charge process of conventional hybrid supercapcitors, and also solved the major problems of poor cycling life of lithium-ion battery electrode materials in aqueous electrolyte; The electrochemical profiles of three kinds of the Li-ion intercalated compounds, LiMn₂O₄, LiCo_1/3Ni_1/3Mn_1/3O₂, and LiCoO₂ used as the positive electrodes for the hybrid aqueous electrochemical supercapacitors in combination with activated carbon negative electrode were studied in a I M Li₂SO₄ solution. The effects of pH in the electrolyte solution on the stability of Li-ion intercalation reaction, the evolution potential of oxygen, and the specific capacity of both the Li-ion intercalated compound and AC electrode materials were intensively investigated. LiMn₂O₄ shows the stable Li-ion intercalation in the solution over pH 7, LiCo_1/3Ni_1/3Mn_1/3O₂ over pH 11, and LiCoO₂ over pH 9. Under the optimal conditions, the specific energy, rate capability and cycling performance of the hybrid cells based on the three kinds of positive electrodes and AC negative electrode were compared. There is no big difference in the specific energy among the three hybrid cells but the hybrid cell based on the LiMn₂O₄ shows good cycling life and rate capability; LiCoO₂ also has good rate capability but with poor cycling performance, and LiCo_1/3Ni_1/3Mn_1/3O₂ shows good cycling life but with poor rate capability; The electrochemical stability of LiCo_1/3Ni_1/3Mn_1/3O₂ in a Li⁺-containing aqueous electrolyte solution is critically dependent on the pH value of the electrolyte solution. It shows the capacity fading upon cycling in the electrolyte solution below pH 11. The mechanism responsible for the capacity fading has extensively investigated. It was found that LiCo_1/3Ni_1/3Mn_1/3O₂ and its partly charged product is chemical stable in the low pH electrolyte solution, while it is not electrochemical stable during discharge process in which the proton co-intercalation paralleled to the lithium-ion intercalation occurred, but the intercalated proton can not be reversibly extracted during charge process. The intercalated proton limits the intercalation of lithium-ion, thus results in the capacity fading; At last, I make some industry study about AC/LiMn₂O₄ hybrid system2. Alkaline Hybrid Supercapacitors Using CoAl double hydroxide and RuO₂/TiO₂ Nanotube Composite as Positive Electrode Materials: A nano-structured CoAl double hydroxide with an average particle size of 60-70 nm was prepared by a chemical co-precipitation. It was used as a positive electrode for the asymmetric hybrid supercapacior in combination with an activated carbon negative electrode in KOH electrolyte solution. A specific capacitance of 77 F/g with a specific energy density of 15.5 wh/kg was obtained for this hybrid supercapacitor within the voltage range of 0.9-1.5 V; We reported an asymmetric supercapacitor technology where RuO₂/TiO₂ nanotube composite was used as positive electrode and the activated carbon as negative electrode in 1mol/L KOH electrolyte solution. This asymmetric supercapacitor has electrochemical capacitance performance within potential range 0～1.4 V. A power density 1207 W/kg was obtained with an energy density of 5.7 Wh/kg.3. Application of Nano-materials in Supercapacitors: The Ni(OH)₂/multi-walled carbon nanotubes (MWNTs) nanocomposites were synthesized by in-situ loading Ni(OH)₂ on the carbon nanotubes in an alkaline solution. The effects of the added carbon nanotubes on the morphology and electrochemical capacitance of Ni(OH)₂ were investigated in various loaded amounts of Ni(OH)₂. The MWNTs substrates can reduce the aggregation of Ni(OH)₂ nanoparticles, inducing a well distribution of the nano-sized Ni(OH)₂ particles on the cross-linked netlike structure MWNTs. As a result, it improved greatly the rate capability and utilization of Ni(OH)₂, as well as reduced the composite electrode resistance. The hybrid supercapacitor based on such Ni(OH)₂/MWNTs composite positive electrode and activated carbon negative electrode delivered a specific energy of 32wh/kg at a specific power of 1500 W/kg based on the total weight of the active electrode materials. It also exhibited good cycling performance and kept 90% of initial capacity over 2000 cycles; NiO with ordered mesoporous structure was synthesized by replicating template SBA-15 and its electrochemical capacitance characterization was for the first time studied in 2M KOH electrolyte solution. The Electrochemical tests results indicated the ordered mesoporous structure can greatly increase the utilization of NiO, which is attribute to its structure allows the active material to be readily accessible for electrochemical reactions. The capacitance of NiO with order mesoporous structure was about 120 F/g, about four times larger than that of NiO prepared by direct calcining Ni(NO₃). 6H₂O at 550℃. On the other hand, the mesoporous NiO showed a good rate capability, which was due to that the ordered mesopores did not limit the ion motion within the pores; We reported an ordered whisker-like polyaniline grown on the surface of mesoporous carbon by a novel synthesis process. The loosely packed whisker-like structure can create the electrochemical accessibility of electrolyte through the PANI phase, which is fundamental for materials showing characters of supercapacitors. Furthermore, the nano-size reduces the distance within the PANI phase over which electrolyte must transport ions. Thus, this kind of material has good electrochemical capacitance performance; A facile one-step method for the preparation of the polyaniline (PANI) intercalated layered manganese oxides was introduced. In this preparation process, the PANI was in-situ intercalated into the layered manganese oxide at an aqueous/organic interface in which the aniline was dissolved in an organic solvent (CCl₄) and the oxidant, potassium permanganate, was dissolved in an aqueous solution. The prepared polymer intercalated manganese oxide has several novel characteristics—swelled layered structure, uniform meso-porous structure and typical nano-size. The electrochemical capacitance performance of the prepared compound was also studied. 4. Application of MnO_x/carbon Electrochemical Catalysts in Fuel Cells: The Mn₃O₄ nano particles (ca. 10 nm) were artificially loaded the on the outer surface of CMK-3, rather than forming within the pores of CMK-3, by utilizing the hydrophobic property of CMK-3 itself and its narrow pore size. The electrocatalytic performance for the oxygen reduction of the prepared Mn₃O₄/CMK-3 composite was extensively studied as an air diffusion electrode material in comparison with the composite catalysts based on the other carbons. The results of electrochemical tests indicate that the Mn₃O₄/CMK-3 composite can provide sufficient effective three-phase interface area due to the unique structures, which plays the important role in the complex three-phase interface electrocatalytic reaction. As a result, the composite electrode behaves much higher electrocatalytic performance for oxygen reduction than that of the composite catalysis based on the other carbons; A simple direct borohydride fuel cell (DBFC) technology in which a MnO₂ was used as cathode catalyst and a hydrogen storage alloy used as anode catalyst containing a hydrogen-releasing agent borohydride alkaline electrolyte solution was introduced. The new type alkaline fuel cell overcome the problem of the conventional fuel cell in which both a high cost noble metal catalysts and expensive ion exchange membrane was used;...