Periodic Reporting for period 1 - SGHES (Second-Generation Hybrid Electrolyte Supercapacitor)
Période du rapport: 2019-11-01 au 2021-02-28
The problem of fossil energy exhaustion and environment pollution results in the urgent need for the sustainable development, in which the high-performance electrochemical energy storage system such as rechargeable batteries and supercapacitors is the key technology.
Although many advanced achievements have been obtained in the field of supercapacitors, there is still an urgent need to improve their energy density without sacrificing the power density.
There are basically three approaches to improve the energy density: designing advanced electrode materials with enhanced charge storage capacitance, adjusting the configuration of the positive and negative electrodes, and optimizing electrolyte to increase the cell voltage.
The cell voltage achieved with the first-generation electrolyte was 2.4 V. This voltage was comparable with commercial supercapacitors because of the increased capacitance and conductivity that is a characteristic of the hybrid electrolyte. But the objective of the project is to increase the cell voltage up to the 2.8 V by using second-generation hybrid electrolyte. Such voltage increase would result in a 36 % energy increase which combined with the other characteristics of the hybrid electrolyte like low cost, environment friendliness, etc. would give a revolutionary supercapacitor that results in an expansion of the market because of increased competitiveness with other technologies. The main problem in this way is the water decomposition at higher voltages. Using metal oxide at positive electrode is one approach that can help to overcome this problem.
Although many advanced achievements have been obtained in the field of supercapacitors, there is still an urgent need to improve their energy density without sacrificing the power density.
There are basically three approaches to improve the energy density: designing advanced electrode materials with enhanced charge storage capacitance, adjusting the configuration of the positive and negative electrodes, and optimizing electrolyte to increase the cell voltage.
The cell voltage achieved with the first-generation electrolyte was 2.4 V. This voltage was comparable with commercial supercapacitors because of the increased capacitance and conductivity that is a characteristic of the hybrid electrolyte. But the objective of the project is to increase the cell voltage up to the 2.8 V by using second-generation hybrid electrolyte. Such voltage increase would result in a 36 % energy increase which combined with the other characteristics of the hybrid electrolyte like low cost, environment friendliness, etc. would give a revolutionary supercapacitor that results in an expansion of the market because of increased competitiveness with other technologies. The main problem in this way is the water decomposition at higher voltages. Using metal oxide at positive electrode is one approach that can help to overcome this problem.
Main focus of the innovation associate in the past year was on the finding ways of extending the potential window of the hybrid electrolyte as well as finding proper gelling agents for both hybrid and organic electrolytes.
Among the reported transition metal oxides, manganese oxide is an attractive supercapacitor electrode material due to its large earth abundancy, low cost, environment benignity, well-established synthesis methods, and its OER overpotential is higher in neutral pH medium. Based on the literature survey, Na0.5MnO2 is a material that can help to improve the operating potential window of the supercapacitor in the positive range. This material can be synthesized by chemical, hydrothermal and electrochemical methods. Making a composite of this metal oxide with carbon material is helpful in order to increase the conductance of the electrode material.
In this project, after trying different methods for preparing Na0.5MnO2 we used hydrothermal method as most effective, one-step way. By compositing the Na0.5MnO2 with pistachio shell-derived carbon (PC) as positive electrode (1:1 mass ratio) and using PC as negative electrode, two-electrode supercapacitor with hybrid electrolyte showed an increase of 0.2 V in operating voltage and the cell voltage increased from 2.3 to 2.5 V. At the next step, by modifying the negative electrode material with sodium, with the purpose of pushing the hydrogen evolution reaction to the lower potentials, the cell potential increased up to 2.8 V at hybrid electrolyte. This increase in the cell voltage, resulted in about 50% increase in the energy density of the designed Na0.5MnO2-PC//Na+ doped-PC supercapacitor in comparison with the PC//PC symmetric supercapacitor.
Among the reported transition metal oxides, manganese oxide is an attractive supercapacitor electrode material due to its large earth abundancy, low cost, environment benignity, well-established synthesis methods, and its OER overpotential is higher in neutral pH medium. Based on the literature survey, Na0.5MnO2 is a material that can help to improve the operating potential window of the supercapacitor in the positive range. This material can be synthesized by chemical, hydrothermal and electrochemical methods. Making a composite of this metal oxide with carbon material is helpful in order to increase the conductance of the electrode material.
In this project, after trying different methods for preparing Na0.5MnO2 we used hydrothermal method as most effective, one-step way. By compositing the Na0.5MnO2 with pistachio shell-derived carbon (PC) as positive electrode (1:1 mass ratio) and using PC as negative electrode, two-electrode supercapacitor with hybrid electrolyte showed an increase of 0.2 V in operating voltage and the cell voltage increased from 2.3 to 2.5 V. At the next step, by modifying the negative electrode material with sodium, with the purpose of pushing the hydrogen evolution reaction to the lower potentials, the cell potential increased up to 2.8 V at hybrid electrolyte. This increase in the cell voltage, resulted in about 50% increase in the energy density of the designed Na0.5MnO2-PC//Na+ doped-PC supercapacitor in comparison with the PC//PC symmetric supercapacitor.
We expected to improve the potential window of the hybrid supercapacitor up to 2.8 V. Obvious approaches for material design involved widening the upper limit of positive electrodes and the lower limit of negative electrodes for increased potential window to pursue high energy density.
There are some reports about the high voltage supercapacitors with aqueous electrolytes using lithium-based salts that can operate up to 2.8 V. LiTFSI salt is an expensive one (more than 10 times higher than NaClO4 salt that we use in our electrolyte) with fluorine groups that are environmentally harmful. In comparison with these supercapacitors, our designed supercapacitor is low-cost and environmental-friendly.
There are some reports about the high voltage supercapacitors with aqueous electrolytes using lithium-based salts that can operate up to 2.8 V. LiTFSI salt is an expensive one (more than 10 times higher than NaClO4 salt that we use in our electrolyte) with fluorine groups that are environmentally harmful. In comparison with these supercapacitors, our designed supercapacitor is low-cost and environmental-friendly.