3D Network of Sepia Melanin and N‐ and, S‐Doped Graphitic Carbon Quantum Dots for Sustainable Electrochemical Capacitors

Abstract

Access to clean and affordable energy for all is one of the 17 main sustainable development goals of the UN. [1] The increasing demand for energy due to worldwide population growth and changes in lifestyle, along with the depletion of natural resources at an alarming rate and concerns for the environment, are motivating the change from fossil fuels to intermittent renewable sources, such as wind and sun. The intermittency of energy received from the sun, however, highlights the need for efficient energy storage devices. Electrochemical capacitors (supercapacitors), despite having lower specific energy than batteries, are among the most prominent of these devices, owing to their high power density, fast charge−discharge (10 times faster than commercially available lithium-ion batteries), long cycle life (rated at 500 000 duty cycles) and high levels of reliability and operational safety. [2,3] They are attractive for applications such as autonomous sensors in planes, hybrid electric vehicles, portable devices, and memory backup systems that require high power levels. [2,4] Commercially available energy storage devices often utilize toxic electrode materials (e.g., lead, nickel, cadmium, cobalt and mercury in batteries) whose extraction can have dramatic social, environmental, and geopolitical impact. [5] Energy storage technologies that, aside from providing high energy/high power, are more affordable, sustainable, and environmentally friendly than conventional counterparts need to be developed. [6] The choice of a supercapacitive electrode material has been limited to electrical double layer (EDL) carbon-based materials [7] as well as pseudocapacitive materials such as metal oxides, [8] conducting polymers, [9,10] and, very recently, metal-organic frameworks (MOFs) [11] and MXenes. [10,12] Many studies in the literature have reported on the use of organic redox materials, to enhance the supercapacitive performance beyond that provided by EDLs. [13,14] Organic materials with redox active groups, such as phenol, carbonyl, quinones, carboxy, or amines [5] are becoming promising candidates for next-generation sustainable energy Organic electrode materials operating in aqueous electrolytes offer the opportunity to avoid toxic, critical, and expensive materials for electrochemical energy storage. When deposited on carbon current collectors, redox active organic materials add faradaic to electrostatic capacitance contribution to the electrodes. Here, a 3D network electrode material is reported upon, based on sepia melanin, a quinone macromolecule, and nitrogen-and sulfur-doped graphitic carbon quantum dots (N,S GCQDs) designed to achieve good electronic conductivity and electrolyte wettability. The effect of various undoped and doped carbon quantum dots is also investigated, synthesized from acetic acid and sucrose instead of graphite, on the electrochemical performance of sepia melanin. Sepia/N,S GCQD shows optimum areal capacitance (≈180 mF cm −2 ) that is about twice as high as sepia (≈77 mF cm −2 ) with lower...