| With the increasing of pollution and energy crisis,the amount of renewable electricity generation has increased rapidly.But the renewable energy,such as wind and solar energy,are inherently intermittent and unstable,which impose great challenges to the conventional electric grid operations.To meet the requirement of balanced generation and utilization as well as stable electric power transmission and distribution,it is of great significance to develop large-scale energy storage systems.Among various large-scale energy storage techniques,organic flow batteries have gained immense attention owing to their advantages of low cost and highly modifiable properties.Organic redox couples usually involve only one type of counter-ion for ion exchange membranes.This feature avoids the problem of ion selectivity caused by the coexistence of a variety of similar charged counter-ions in traditional flow batteries,thus providing opportunities for the use of low-cost membranes in organic flow batteries.In this thesis,low-cost sulfonated poly?ether ether ketone??SPEEK?ion exchange membranes have been studied.The corresponding composite ion exchange membranes were prepared by doped modification of phosphoric silicate sol and nano-SiO2.The properties of the modified membranes and their performance in organic redox flow batteries were examined by various methods.In this study,we prepared sulfonated poly?ether ether ketone??SPEEK?membrane for application in acidic flow batteries of 1,2-dihydroxybenzene-3,5-disulfonic acid/anthraquinone-2-sulfonic acid.The proton conductivity of the prepared SPEEK membrane was 50%lower than that of the Nafion-212 membrane.However,the Coulombic efficiency of the SPEEK cell was similar to that of the Nafion cell,and the voltage efficiency was 0.99%–2.66%higher than that of Nafion cell at different current densities.The cell using the SPEEK membrane also exhibited long-term stability,retaining 96.93%reversible capacity after 100 charge/discharge cycles at the current density of 100 mA cm-2.The results of symmetric cell cycling indicated that the decay of reversible capacity in this organic flow battery system was mainly due to the instability of organic compounds in the positive electrolyte,which was also confirmed by the self-discharge measurements of the SPEEK-cell.The SPEEK/P-Si composite membrane was prepared by mechanical mixing method to combine SPEEK and phosphoric silicate sol with high proton conductivity.The composite membrane showed improved comprehensive properties owing to the combined advantages of polymer membranes and inorganic materials.The addition of phosphoric silicate can increase the proton conduction sites of the membrane.The proton conductivity of the SPEEK/P-Si composite membrane was 1.53 mS cm-1 higher than that of the pristine membrane,and the swelling ratio was 1.2%lower than that of the SPEEK membrane.The SPEEK/S7 composite membrane was prepared by incorporating nano-SiO2 particles into SPEEK.The swelling ratio of the SPEEK/S7 composite membrane decreased by 2.52%compared with the pristine membrane.Because of the weak acidity of SiO2 and its weak proton conductivity,the proton conductivity of SPEEK/S7 composite membrane decreased by 12.98%compared with the pure membrane.However,the Coulombic efficiencies at current density of 100 mA cm-2 with the cells assembled using SPEEK,SPEEK/P-Si,SPEEK/S7 membrane were almost equal to 99%,and the energy efficiencies were 51.65%,59%,51.46%,respectively.The porous composite membrane was prepared by mechanical stirring to mix PTFE emulsion and nano-SiO2 particles.For the first time,the resulting membrane was assembled in organic redox flow batteries for testing.At current density of 100 mA cm-2,the Coulombic efficiency tended to be 99%,but the energy efficiency was only 32.28%.The lower energy efficiency was caused by large pore sizes of the composite membrane.The preliminary results show that this new composite membrane has application potential in organic redox flow batteries. |
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