S.T. Senthilkumar

Flexible and wearable fiber shaped high voltage supercapacitors based on copper hexacyanoferrate and porous carbon coated carbon fiber electrodes

In this work, we report the fabrication of a new high voltage hybrid fiber supercapacitor (HFSC) with porous carbon coated carbon fibers (PC@CFs) as the negative electrode and copper hexacyanoferrate coated carbon fibers (CuHCF@CFs) as the positive electrode. Carbon fibers (CFs) were used as both the substrate and the current collector due to their good conductivity, high flexibility, good mechanical strength, and light weight. The as-fabricated HFSC can be cycled reversibly in the range of 0–2 V and exhibits excellent electrochemical performance with a specific capacitance of 19.2 F g−1 (68.2 mF cm−2 or 3.1 F cm−3) and an energy density of 10.6 W h kg−1 (180.85 μW h cm−2 or 8.11 mW h cm−3), better than those reported in the previous literature. Additionally, the HFSCs have retained their original electrochemical performance even after bending, suggesting good flexibility of the device. The promising results show great potential in developing HFSCs with CuHCF@CFs and PC@CFs electrodes for practical wearable devices.

Seawater battery performance enhancement enabled by a defect/edge-rich, oxygen self-doped porous carbon electrocatalyst

Low-cost oxygen evolution/reduction reaction (OER/ORR) catalysts are critically important for energy conversion/storage systems. Here, a porous carbon (PC) catalyst is prepared as a low-cost catalyst from grapefruit peel biowaste using a facile hydrothermal carbonization combined with a chemical activation process. When examined as an OER/ORR catalyst in seawater, the PC exhibits an unexpected, highly efficient catalytic activity. From density functional theory (DFT) calculations, we found that sp3-bonded carbon atoms, zigzag edges with or without –COOH and –OH, and armchair edges with –O– are responsible for its superior OER/ORR activity. The suitability of the PC as a catalyst is tested in the half-cell of a Na/seawater battery, which exhibits a decreased voltage gap of ∼0.47 V between charge and discharge voltage curves. This value is even lower than those of Pt/C (∼0.68 V), IrO2 (∼0.66 V) and MnO2 (∼0.73). Also, a full cell of the metal-free seawater battery is assembled using hard carbon and PC as an anode and a catalyst, respectively. The full cell shows a lower voltage gap (∼0.65 V) with the voltage efficiency of ∼83–84% and excellent cycle life over 100 cycles. Our results confirm PC derived from grapefruit peels as an alternative to expensive Pt/C and IrO2 catalysts for the OER/ORR activities in seawater batteries.

Enhancing Capacity Performance by Utilizing the Redox Chemistry of the Electrolyte in a Dual-Electrolyte Sodium-Ion Battery

A strategy is described to increase charge storage in a dual electrolyte Na-ion battery (DESIB) by combining the redox chemistry of the electrolyte with a Na+ ion de-insertion/insertion cathode. Conventional electrolytes do not contribute to charge storage in battery systems, but redox-active electrolytes augment this property via charge transfer reactions at the electrode-electrolyte interface. The capacity of the cathode combined with that provided by the electrolyte redox reaction thus increases overall charge storage. An aqueous sodium hexacyanoferrate (Na4 Fe(CN)6 ) solution is employed as the redox-active electrolyte (Na-FC) and sodium nickel Prussian blue (Nax -NiBP) as the Na+ ion insertion/de-insertion cathode. The capacity of DESIB with Na-FC electrolyte is twice that of a battery using a conventional (Na2 SO4 ) electrolyte. The use of redox-active electrolytes in batteries of any kind is an efficient and scalable approach to develop advanced high-energy-density storage systems.