Nitrogen-doped Ni2P/Ni12P5/Ni3S2 three-phase heterostructure arrays with ultrahigh areal capacitance for high-performance asymmetric supercapacitor

Interface engineering of hierarchical flower-like N, P, O-doped NixPy self-supported electrodes for highly efficient water-to-hydrogen fuel/oxygen conversion

Rational construction of efficient bifunctional catalysts with robust catalytic activity and durability is significant for overall water splitting (conversion between water and hydrogen fuel/oxygen) using non-precious metal systems. In this work, the hierarchically porous N, P, O-doped transition metal phosphate in the Ni foam (NF) electrode (hollow flower-like NPO/NixPy@NF) was prepared through facile hydrothermal method coupled with phosphorization treatment. The hierarchical hollow flower-like NPO/NixPy@NF electrodes exhibited high bifunctional activity and stability for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline solutions. The optimized electrode showed low overpotentials of 76 and 240 mV for HER and OER to reach a current density of 10 mA cm-2, respectively. Notably, the NPO/NixPy@NF electrode only required a low voltage of 1.99 V to reach the current densities of 100 mA cm-2 with long-term stability for overall water splitting using the NPO/NixPy@NF|| NPO/NixPy@NF cell, surpassing that of the Pt/C-RuO2 (2.24 V@ 100 mA cm-2). The good catalytic and battery performance should be attributed to i) the open hierarchical structure that enhanced the mass transfer; ii) a highly conductive substrate that accelerated the electron transfer; iii) the rich heterojunction and strong synergy between Ni2P and Ni5P4 that improved the catalytic kinetic; iv) the proper-thickness amorphous phosphorus oxide nitride (PON) shell that realized the stability. This work demonstrates a promising methodology for designing bifunctional transition metal phosphides with high performance for efficient water splitting.

Stable Hierarchical Porous Heterostructure Ni2P/NC@CoNi2S4 Fabricated via the NiCo-LDH Template Strategy for High-Performance Supercapacitors

Transition metal phosphide/sulfide (TMP/TMS) heterostructures are attractive supercapacitor electrode materials due to their rapid redox reaction kinetics. However, the limited active sites and weak interfacial interactions result in undesirable electrochemical performance. Herein, based on constructing the NiCo-LDH template on Ni-MOF-derived Ni2P/NC, Ni2P/NC@CoNi2S4 with a porous heterostructure is fabricated by sulfurizing the intermediate and is used for supercapacitors. The exposed Ni sites in the phosphating-obtained Ni2P/NC coordinate with OH- to in situ form an intimate-connected Ni2P/NC@NiCo-LDH, and the CoNi2S4 nanosheets retaining the original cross-linked structure of NiCo-LDH integrate the porous carbon skeleton of Ni2P/NC to yield a hierarchical pore structure with rich electroactive sites. The conducting carbon backbone and the intense electronic interactions at the interface accelerate electron transfer, and the hierarchical pores offer sufficient ion diffusion paths to accelerate redox reactions. These confer Ni2P/NC@CoNi2S4 with a high specific capacitance of 2499 F·g-1 at 1 A·g-1. The NiCo-LDH template producing a tight interfacial connection, significantly enhances the stability of the heterostructure, leading to a 91.89% capacitance retention after 10,000 cycles. Moreover, the fabricated Ni2P/NC@CoNi2S4//NC asymmetric supercapacitor exhibits an excellent energy density of 73.68 Wh kg-1 at a power density of 700 W kg-1, superior to most reported composites of TMPs or TMSs.

Electrostatic interaction-controlled dispersion of carbon nanotubes in a ternary composite for high-performance supercapacitors

Effective dispersion of carbon nanotubes (CNTs) is of great importance to achieve their intrinsic performance. Normally, it is believed that CNT dispersion is decided by interactions between CNTs and their dispersants, while other interactions are often neglected. Herein, three ionic surfactants, sodium dodecyl sulfate (SDS), dodecyl dimethyl betaine (BS-12) and cetyltrimethylammonium bromide (CTAB), are used to disperse CNTs in a ternary composite, i.e., poly(p-phenylenediamine)-phosphomolybdic acid@reduced graphene oxide (DMoG), respectively, leading to three different DMoGC composites. It has been found that the CNT dispersion in DMoGC was mainly controlled by electrostatic interactions between the surfactants and DMoG, which further exerted vital influences on the constitution, content, morphology, porous structure and supercapacitive performance of the DMoGC composites. Among the three surfactants, cationic CTAB showed the best CNT dispersion, while amphoteric BS-12 could hardly disperse CNTs in DMoGC, leading to DMoGC-CTAB with a 2 times larger specific surface area (152.3 m² g^-1) and 1.5 times higher specific capacitance (422 F g^-1) than those of DMoGC-(BS-12). Our study can provide valuable guidelines for selecting/designing effective dispersants to prepare multi-component composites containing uniformly dispersed CNTs.

The Preparation and Electrochemical Pseudocapacitive Performance of Mutual Nickel Phosphide Heterostructures

Transition metal phosphide composite materials have become an excellent choice for use in supercapacitor electrodes due to their excellent conductivity and good catalytic activity. In our study, a series of nickel phosphide heterostructure composites was prepared using a temperature-programmed phosphating method, and their electrochemical performance was tested in 2 mol L−1 KOH electrolyte. Because the interface effect can increase the catalytic active sites and improve the ion transmission, the prepared Ni2P/Ni3P/Ni (Ni/P = 7:3) had a specific capacity of 321 mAh g−1 under 1 A g−1 and the prepared Ni2P/Ni5P4 (Ni/P = 5:4) had a specific capacity of 218 mAh g−1 under 1 A g−1. After the current density was increased from 0.5 A g−1 to 5 A g−1, 76% of the specific capacity was maintained. After 7000 cycles, the capacity retention rate was above 82%. Due to the phase recombination effect, the electrochemical performance of Ni2P/Ni3P/Ni and Ni2P/Ni5P4 was much better than that of single-phase N2P. After assembling the prepared composite and activated carbon into a supercapacitor, the Ni2P/Ni3P/Ni//AC had an energy density of 22 W h kg−1 and a power density of 800 W kg−1 and the Ni2P/Ni5P4//AC had an energy density of 27 W h kg−1 and a power density of 800 W kg−1.