Synthesis of a Molybdate-Chelated Biodegradable Gel Electrolyte for High Energy Density Supercapacitors

The recent advancement in the field of energy storage devices requires the use of environmentally friendly and abundantly available materials to ensure the commercialization of a safe and economical device without compromising on the efficiency. For this reason, we report the fabrication of an activated carbon-based supercapacitor using an alginate-chelated biodegradable gel electrolyte. The supercapacitor device energized by a potassium alginate (K-Alg) gel electrolyte performed very well in a 1 V potential window and demonstrated a specific capacitance of 65.7 F g-1 along with a specific energy value of 9-1 W h kg-1. The performance of the device was further enhanced by optimizing the concentration of KOH in the polymer matrix. The specific concentration of KOH (i.e., 5% w/v KOH to K-Alg or simply KOH5) demonstrated a specific capacitance value of 105.2 F g-1, which is 60% enhancement to that of the K-Alg device capacitance. Further, the KOH5 gel electrolyte was doped with ammonium molybdate as a redox-active specie in different percentages and denoted as MoX, where (X = 1, 3, 5, 7, and 10% w/v). The results revealed that the addition of MoX to the KOH5 electrolyte significantly enhanced the electrochemical properties of the carbon supercapacitors demonstrating successful chelation between Mo ions and the alginate matrix. For the specific concentration of Mo ions to the gel matrix (i.e., 3% Mo to KOH5 or simply Mo3), the supercapacitor device demonstrated a large specific capacitance value of 229.5 F g-1, which is ∼250% enhancement to K-Alg and ∼118% enhancement to the KOH5 device capacitance value, respectively. In addition, the supercapacitor device composed of an Mo3 electrolyte achieved an ultra-high specific energy of 95-1 W h kg-1 along with the maximum specific power of 15.0 kW kg-1. The device also demonstrated prolonged cycling stability by retaining 86% of its initial capacitance value even after 10,000 charge-discharge cycles.