Ni-Doped Cobalt Phosphite, Co₁₁(HPO₃)₈(OH)₆, with Different Morphologies Grown on Ni Foam Hydro(solvo)thermally for High-Performance Supercapacitor

Abstract

Ni-doped Co(HPO)(OH) with different morphologies was directly grown on Ni foam hydro(solvo)thermally under different synthetic conditions. The optimum condition is solvothermal reaction for 6 h in an ethanol/water (EW) mixed solution, the molar ratio of NaHPO/Co(NO) being 0.5:0.1, and the obtained S0.5-6 h-EW shows three-dimensional (3D) porous nanowire bundles. Whereas in the water-only solution, microrods are obtained, suggesting that the nanowires in bundles are aggregated together via the lateral (400) direction. Long reaction time and low molar ratio of reactants are all beneficial for the lateral growth of the nanowires, and the possible formation mechanism is proposed. All the obtained Ni-doped Co(HPO)(OH)/Ni foam samples are directly used as supercapacitor electrodes, and S0.5-6 h-EW shows the best electrochemical performance with a specific capacity of 159 mAh g at 0.5 A g, which is close to the theoretical value of 212 mAh g for Co(HPO)(OH), and it is the largest reported value so far. The excellent capacitive behavior of S0.5-6 h-EW is ascribed to the 3D porous nanowire bundles directly grown on a Ni foam collector without an additive and a binder, as well as to the doping of Ni into the cobalt phosphite. The S0.5-6 h-EW//activated carbon asymmetrical supercapacitor shows a maximum energy density of 58.7 Wh kg at a power density of 532 W kg and good cycling stability with the capacity retention of 90.5% after 10 000 charging-discharging cycles at 5.5 A g.

Nickel Cobalt Phosphite Nanorods Decorated with Carbon Nanotubes as Bifunctional Electrocatalysts in Alkaline Medium with a High Yield of Hydrogen Peroxide

“…The nanorods have an average diameter of 150–200 nm with shorter rods having an average length of 1–2 μm and longer ones having 4–5 μm length. Such superstructures of nanorods forming flower‐like structures have been observed in the case of Ni doped Co phosphites by H. Dan et al . and Y. Ni et al .…”

“…The O1s spectra can be deconvoluted to include three peaks; that at 530.3 and 530.8 eV correspond to the lattice oxygen (O 2− ) and hydroxyl group (OH), respectively, which is in agreement with the composition of (Ni/Co) 11 (HPO 3 ) 8 (OH) 6 . The peak at 532.1 eV may be related to the surface adsorbed water molecules resulting from the synthesis (Figure (d)) . XPS results confirm the presence of NiCo‐phosphite synthesized by hydrothermal method.…”

“…Ni 2p spectra contain two strong peaks at 854.7 (Ni 2p 3/2 ) and 873 eV (Ni 2p 1/2 ) and are attributed to Ni 2+ in Ni 11 (HPO 3 ) 8 (OH) 6 along with two satellite peaks at 860 and 878.7 eV (Figure (a)) . The peaks in Co 2p spectra seen at 779.6 eV (Co 2p 3/2 ) and 795.8 eV (Co 2p 1/2 ) correspond to Co 2+ in Co 11 (HPO 3 ) 8 (OH) 6 along with two satellites that appear at 783.5 and 801.5 eV, which are in accordance with the previous reports . The intensity of Co peaks are much stronger than that of Ni indicating a higher Co/Ni ratio, which concurs with the doping amount of nickel along with cobalt taken in the reaction mixture.…”

“…Sodium cobalt phosphates (NaCo(PO 3 ) 3 , NaCoPO 4 and Na 2 CoP 2 O 7 ) and other alkali transition metal phosphates (e. g. NaFePO 4 , Mn 3 (PO 4 ) 2 ) have been explored as electrocatalysts for oxygen evolution, oxygen reduction or both . Lately, there has been an interest in metal phosphites of the formula M 11 (HPO 3 ) 8 (OH) 6 . Transition metal phosphites also possess microporous crystal structure similar to phosphates, however, phosphites contain PO 3 H tetrahedral units and MO 6 octahedra (M‐metal) instead of PO 4 units in phosphates.…”

“…While Ni 2+ centers are more active for OER, phosphite anions supported by Ni 2+ favor HER . Doping cobalt phosphite with Ni and vice versa has been undertaken to improve the efficiency of the electrocatalyst for overall water splitting . J. Wu et al.…”