The development of sustainable and renewable energy storage systems is a promising approach toward steady and reliable energy supply. In this study, cellulosic palm loofah fibers were used as a precursor to produce amorphous carbon (Am-C) with retained crystalline cellulosic planes via a simple activation method. The Am-C exhibits a fairly high BET surface area of 2000 m2/g and a 3D-microporous structure with small mesopores. The symmetric Am-C//Am-C supercapacitor device tested in 1.0 M NaCl aqueous electrolyte showed specific capacitances of 201 F/g at 5 mV/s and 337 F/g at 1 A/g. The device exhibits a stable performance across a potential window of 1.8 V with ultrahigh energy and power densities of 51.4 Wh/kg at 4.5 kW/kg and 16.95 Wh/kg at 18 kW/kg. The device showed extraordinary increasing capacitive behavior upon cycling at 10 A/g for over 25000 cycles. The exceptional device performance could be ascribed to the electrochemical graphitization during long-term cycling together with the enhanced wettability as confirmed via Raman, Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), and contact angle measurements.

Transition-metal phosphides (TMPs) enjoy metalloid characteristics with good electrical conductivity, making them potential candidates for electrochemical supercapacitors. However, TMPs are difficult to synthesize by conventional methods, limiting their practical use in a plethora of applications. Herein, we demonstrate the successful fabrication of Ni–Cu binary phosphides (NCP) via a one-step, facile solvothermal method. More importantly, the correlation between the degree of phosphidation and the electrochemical behavior of the material is explored and discussed. The NCP electrode exhibited a battery-like behavior with an ultrahigh specific capacitance (C s) of 1573 F g–1 at 1 A g–1. Upon use as a positive electrode, it showed superior performance in a hybrid supercapacitor device with bioderived activated carbon (BAC) as the negative electrode (NCP//BAC), providing a high energy density of 40.5 W h kg–1 at 875 W kg–1 with exceptional capacity retention after 10,000 cycles. These values are 4 times higher than that of commercial supercapacitors (10–12 W h kg–1), suggesting the unique supercapacitance performance of the NCP//BAC device compared to the phosphide-based devices reported so far.

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