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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We demonstrate the fabrication of binder-free electrospun nickel− manganese oxides embedded into carbon-shell fibrous electrodes. The morphological and structural properties of the assembled electrode materials were elucidated by high-resolution transmission electron microscopy (HR-TEM), field-emission scanning electron microscopy, and glancing-angle X-ray diffraction. The fibrous structure of the electrodes was retained even after annealing at high temperatures. The X-ray photoelectron spectroscopy and HR-TEM analyses revealed the formation of nickel and manganese oxides in multiple oxidation states (Ni 2+ , Ni 3+ , Mn 2+ , Mn 3+ , and Mn 4+ ) embedded in the carbon shell. The embedded nickel−manganese oxides into the carbon matrix fibrous electrodes exhibit an excellent capacitance (1082 F/g) in 1 M K 2 SO 4 at 1 A/g and possess a high rate capability of 73% at 5 A/g. The high rate capability and capacitance can be attributed to the presence of carbon crosslinked channels, the binder-free nature of the electrodes, and various oxidation states of the Ni−Mn oxides. The asymmetric supercapacitor device constructed of the asfabricated nanofibers and the bio-derived microporous carbon as the positive and negative electrodes, respectively, sustains up to 1.9 V with a high specific capacitance at 1.5 A/g of 108 F/g. The nanofibrous//bio-derived device exhibits an outstanding specific energy of 54.2 W h/kg with a high specific power of 1425 W/kg. Interestingly, the tested device maintains a high capacitive retention of 92% upon cycling over 10,000 charging/discharging cycles.

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The direct growth of sub-100 nm thin-film metal oxides has witnessed a sustained interest as a superlative approach for the fabrication of smart energy storage platforms. Herein, sub-100 nm Zr-doped orthorhombic Nb 2 O 5 nanotube films are synthesized directly on the Nb-Zr substrate and tested as negative supercapacitor electrode materials. To boost the pseudocapacitive performance of the fabricated films, supplement Nb 4+ active sites (defects) are subtly induced into the metal oxide lattice, resulting in 13% improvement in the diffusion current at 100 m V/s over that of the defect-free counterpart. The defective sub-100 nm film (H-NbZr) exhibits areal and volumetric capacitances of 6.8 mF/cm 2 and 758.3 F/cm 3 , respectively. The presence of oxygen-deficient states enhances the intrinsic conductivity of the thin film, resulting in a reduction in the band gap energy from 3.25 to 2.5 eV. The assembled supercapacitor device made of nitrogen-doped activated carbon (N-AC) and H-NbZr (N-AC//H-NbZr) is able to retain 93, 83, 78, and 66% of its first cycle capacitance after 1000, 2000, 3000, and 4500 successive charge/discharge cycles, respectively. An eminent energy record of approximately 0.77 μW h/cm 2 at a power of 0.9 mW/cm 2 is achieved at 1 mA/cm 2 with superb capability.

Electrochemical energy storage (EES) technologies are playing a leading role in the global effort to address the energy challenges. Current EES systems are limited by their energy density, capacity, and cycling stability. Some of those limitations arise from nanoscale phenomena, which are not fully understood or accounted for. Electrochemical activation (ECA), an often-overlooked process, creates more active sites on the electrode material and boosts the activity of the system to achieve a higher storage capacity. Herein, the ECA of bimetallic Ni–Co oxyphosphides is investigated via a plethora of spectroscopic techniques, including transmission electron microscopy enhanced by multivariate statistical analysis as a tool to better analyze the obtained spectra. Interestingly, ECA induces an in situ reconstruction of the pre-electrode via phosphorus leaching, together with accelerated surface segregation of the reconstructed Ni and Co species. The electrodes with reconstructed composition showed 110% higher supercapacitive performance than their pre-electrode counterparts. Thanks to the electrochemical optimization approach, a hybrid device has been assembled with a superb performance. The device exhibits energy density values comparable with batteries: 89 W h kg–1 at a power density of 848 W kg–1 with an excellent stability over 10,000 galvanic charge–discharge cycles as manifested by the steady capacitive retention (94.2–100.9%) even during the last 1000 cycles.