Effects of Fe doping on enhancing electrochemical properties of NiCo2S4 supercapacitor electrode

Internal Polarization Field Induced Hydroxyl Spillover Effect for Industrial Water Splitting Electrolyzers

The formation of multiple oxygen intermediates supporting efficient oxygen evolution reaction (OER) are affinitive with hydroxyl adsorption. However, ability of the catalyst to capture hydroxyl and maintain the continuous supply at active sits remains a tremendous challenge. Herein, an affordable Ni2P/FeP2 heterostructure is presented to form the internal polarization field (IPF), arising hydroxyl spillover (HOSo) during OER. Facilitated by IPF, the oriented HOSo from FeP2 to Ni2P can activate the Ni site with a new hydroxyl transmission channel and build the optimized reaction path of oxygen intermediates for lower adsorption energy, boosting the OER activity (242 mV vs. RHE at 100 mA cm-2) for least 100 h. More interestingly, for the anion exchange membrane water electrolyzer (AEMWE) with low concentration electrolyte, the advantage of HOSo effect is significantly amplified, delivering 1 A cm-2 at a low cell voltage of 1.88 V with excellent stability for over 50 h.

NiCo2S4 combined 3D hierarchical porous carbon derived from lignin for high-performance supercapacitors

Metal sulfides with the nature of low electronegativity and high electrochemical activity are potentially considered effective electrode materials for supercapacitors. Meanwhile, hierarchical porous carbon (HPC) materials derived from eco-friendly enzymatic hydrolysis lignin are the ideal matrix for holding nanoparticles (NP) that allows the overall NP/HPC composite to achieve outstanding electrochemical performance. In this study, NiCo₂S₄ nanoparticles were in-situ synthesized on the inner surface of 3D HPC that derived from enzymatic hydrolysis lignin with a simple one-step solvothermal method, thus forming a high-performance composite electrode material for supercapacitor applications. As a result, the NiCo₂S₄/HPC composite yields an outstanding specific capacity of 1264.2 F g^-1 at 1 A g^-1 and also exhibits remarkable rate performance. Such remarkable property is attributed to the effective combination of NiCo₂S₄ plus HPC and their strong chemical bonds, which enable excellent electronic conductivity and abundant exposed electroactive sites. The asymmetric supercapacitor assembled by utilizing NiCo₂S₄/HPC and active carbon as the positive and negative electrodes, respectively, provide an excellent energy density of 32.05 Wh kg^-1 at a power density of 193.9 W kg^-1. This work puts forward a practical optimization strategy for metal sulfides used in electrochemical energy storage devices.

Dual modulation of the morphology and electric conductivity of NiCoP on nickel foam by Fe doping as a superior stability electrode for high energy supercapacitors

Nickel-cobalt bimetallic phosphide (NiCoP) is a potential electrode material for supercapacitors on account of its high theoretical specific capacitance. However, its practical application is restricted because of its relatively poor cycling stability and rate performance. Herein, we constructed self-standing NiCoP nanowires and Fe doped NiCoP nanoarrays with different iron ion concentrations on nickel foam (Fe-NiCoP/NF-x%, x = 4, 6.25, 12.5, 25) as a positive electrode for asymmetric supercapacitors (ASCs). The morphological result reveals that the nanostructure of the material evolves from nanowires to nanosheets with the iron doping concentration, and the Fe-NiCoP/NF-12.5% nanosheets possess a more stable structure than NiCoP/NF nanowires. The density functional theory analysis implies that the conductivity of the material enhances after Fe doping because of the increased charge density and electron states. The combination of multicomponents and structural advantages endows the optimal Fe-NiCoP/NF-12.5% electrode with an ultrahigh areal capacitance of 9.93 F cm^-2 (2758.34 F cm^-3) under 1 mA cm^-2, excellent rate capability (82.58% from 1 mA cm^-2 to 50 mA cm^-2) and superior cycling stability (95.72% retention over 5000 cycles under 20 mA cm^-2), and the areal capacitance of Fe-NiCoP/NF-12.5% is 2.27 times higher than that of the pristine NiCoP/NF electrode at 1 mA cm^-2. Moreover, the assembled Fe-NiCoP/NF-12.5%//activated carbon ASC device delivers a high energy density of 0.327 mW h cm^-2 (60.43 mW h cm^-3) at 1.10 mW cm^-2 (202.54 mW cm^-3). Therefore, this strategy may provide a novel route for the application of NiCoP with its intrinsic advantages in the energy storage field.