A Novel Flexible Hybrid Battery-Supercapacitor Based on a Self-Assembled Vanadium-Graphene Hydrogel

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

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