Batteries and supercapacitors are considered as key technologies for portable and wearable electronics which require lightweight, highly efficient and often flexible energy storage systems. Batteries can store a high amount of specific energy but deliver electricity at rather low current densities due to their intrinsically low power-handling capabilities. In contrast, supercapacitors can provide high specific power with outstanding cyclic stability and efficiency but have a low energy content. Thus, a key challenge for the fabrication of wearable/portable power sources is to simultaneously increase the energy and power densities, along with high durability, rapid charging, and facile scalability. In this work, we introduce a novel hybrid energy storage system that employs an innovative self-assembled graphene hydrogel which encapsulates a (vanadium IV) redox species that can exist in more than two oxidation states. These vanadium-graphene hydrogels can be cut into slices and pressed on a carbon cloth to form a flexible thin-film electrode of around ~180 µm thickness and with a mass loading of 3.3 mg cm-3. To assemble the hybrid device, a cationic-exchange membrane is sandwiched between two identical hydrogel electrodes. During the initial charging of the device, the vanadium IV is oxidized to vanadium V at the positive electrode and reduced to vanadium (III) at the negative electrode. The high surface area of the graphene hydrogel matrix (> 1000 m2/g) enables the supercapacitor mechanism of the hybrid. The different oxidation state of the encapsulated vanadium electrolyte induces a cell potential (~0.9 V) which grants the battery mechanism to the hybrid device. The combination of both mechanisms results in an outstanding capacity of 225 mA/g and unique characteristics of this power source. The capacity is roughly 8 times higher than that of a comparable graphene hydrogel supercapacitor without vanadium content, but the charging time is only double demonstrating a fast charging ability. The device also shows a true hybrid behavior. When operated with high current densities, it works like a supercapacitor and loses only 5% of its capacitance over 1000 charge/discharge cycles. When operated with low current densities, it shows characteristics of a battery. Here, the capacity losses are rather 40% to 50% over 1000 cycles. However, these losses can be easily restored by simple electric measures and there is only a 7% capacity loss after 1000 cycles and a restoration cycle. Our investigations suggest that during self-discharge, capacitance losses are partially converted into capacity through vanadium redox reactions which mitigates the self discharge. Additionally, the self-discharge does not permanently damage the hybrid device. The reason for these outstanding features are related to the simple design. Both half-cells initially consist of the same vanadium graphene hydrogel. Although ion crossover lowers the efficiency and triggers self-discharge, a complete discharge of the device converts all species, including those that crossed over, back to their initial redox state and, thus, resets the device to its initial condition. The mechanical robustness and flexibility of the device are investigated at different bending conditions. The results show a capacity retention of 97% at a bending angle of 135°, indicating excellent integrity of electrode materials under mechanical stress. Our work demonstrates that the novel concept of utilizing a redox species which can exist in more than two redox states, along with a high surface area electrode, presents a facile, scalable and high-performance design for hybrid battery-supercapacitors while the fabrication is considerably simplified. Figure 1
The rapid development of portable and wearable electronics has been driving a demand for lightweight and flexible energy storage systems. In this regard, flexible supercapacitors are considered as promising since they possess long cycle life and high power rates. Among available materials for supercapacitor electrodes, graphene has gained considerable attention in recent years, due to its high specific surface area, high thermal and electronic conductivity as well as other favourable mechanical features. A key challenge for the development of graphene supercapacitors is associated with the scalability and simplicity of the fabrication process. Although printing technologies offer a rapid, precise, scalable, and cost-effective fabrication method for energy storage systems, printed graphene-based supercapacitors usually deliver a lower capacitance compared to those fabricated with other methods, which is mainly related to the re-stacking of the graphene sheets after printing. This work presents a straightforward and scalable leavening agent-assisted approach in order to enhance the capacitance of printed flexible graphene-based supercapacitors. Our method consists of increasing the surface area of the reduced graphene oxide electrodes (rGO) due to the addition of the leavening agent ammonium carbonate to the GO ink. The inks are printed with a micro-dispensing technique. During reduction of GO to rGO, the leavening agent decomposes and gases are released which enlarge the inter-flake gaps and therefore suppresses the restacking of the GO flakes. The lateral size is a key parameter that controls the self-alignment of the GO flakes and, thus, has an impact on the capacitance of rGO electrodes. To assess the effect of the lateral flake size, three stock dispersions with different size ranges were prepared. In detail, the processed dispersions contained flakes with lateral sizes in the nano- (< 100 nm), submicro- (0.1 - 1.0 µm) or the micro-meter (> 1.0 µm) range. Experiments were performed to investigate the relationships between GO lateral size, the concentration of leavening agent and the capacitance of the printed rGO electrodes. In absence of the leavening agent, the specific capacitance slightly decreases with an increase of the flake size. In contrast, for inks with leavening agent concentrations of 3.0 and 6.0 wt. %, the specific capacitance increases with increasing flake size. The experiments reveal that there is a maximum specific capacitance of 112.1 ± 6.5 F g-1 at a leavening concentration of 3.7 wt. %. This specific capacitance is significantly higher compared to that of the electrode without the leavening agent approach (62.4 ± 5.2 F g-1) indicating a successful process for increasing the gaps between rGO flakes. With respect to cycle life, it is observed that the risen electrodes maintain better long-term capacitance stability compared to agent-free electrodes. The electrodes with the leavening agent approach feature an 85% capacitance retention after 2,000 charge-discharge cycles at a current density of 1.0 A g-1 while the leavening agent-free electrodes show a gradual capacitance decay to 80 %. Figure 1
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