High-Performance Flexible Solid-State Asymmetric Supercapacitors Based on Bimetallic Transition Metal Phosphide Nanocrystals

Abstract

Transition metal phosphides (TMPs) have recently emerged as an important type of electrode material for use in supercapacitors thanks to their intrinsically outstanding specific capacity and high electrical conductivity. Herein, we report the synthesis of bimetallic Co x Ni 1−x P ultrafine nanocrystals supported on carbon nanofibers (Co x Ni 1−x P/CNF) and explore their use as positive electrode materials of asymmetric supercapacitors. We find that the Co:Ni ratio has a significant impact on the specific capacitance/capacity of Co x Ni 1−x P/ CNF, and Co x Ni 1−x P/CNF with an optimal Co:Ni ratio exhibits an extraordinary specific capacitance/capacity of 3514 F g −1 /1405.6 C g −1 at a charge/discharge current density of 5 A g −1 , which is the highest value for TMP-based electrode materials reported by far. Our density functional theory calculations demonstrate that the significant capacitance/capacity enhancement in Co x Ni 1−x P/CNF, compared to the monometallic NiP/CNF and CoP/CNF, originates from the enriched density of states near the Fermi level. We further fabricate a flexible solid-state asymmetric supercapacitor using Co x Ni 1−x P/CNF as positive electrode material, activated carbon as negative electrode material, and a polymer gel as the electrolyte. The supercapacitor shows a specific capacitance/capacity of 118.7 F g −1 /166.2 C g −1 at 20 mV s −1 , delivers an energy density of 32.2 Wh kg −1 at 3.5 kW kg −1 , and demonstrates good capacity retention after 10000 charge/discharge cycles, holding substantial promise for applications in flexible electronic devices.

True Meaning of Pseudocapacitors and Their Performance Metrics: Asymmetric versus Hybrid Supercapacitors

“…Furthermore, binary TMPs have also been reported in the literature as a positive electrode for the HSSCs to overcome the limitations of the unitary TMPs. For instance, bimetallic Co x Ni 1− x P ultrafine nanocrystals supported on carbon nanofibers (Co x Ni 1− x P/CNF) have been synthesized by Zhang et al [ 245 ] for the highly efficient HSSC application. They find that the Co:Ni ratio has a significant impact on the specific capacitance/capacity of Co x Ni 1− x P/CNF, and Co x Ni 1− x P/CNF with an optimal Co:Ni ratio exhibits an extraordinary specific capacitance/capacity of 3514 F g −1 /1405.6 C g −1 at a charge/discharge current density of 5 A g −1 , which is the highest value for TMP‐based electrode materials reported by far.…”

Cost‐Effective Synthesis of Electrodeposited NiCo2O4 Nanosheets with Induced Oxygen Vacancies: A Highly Efficient Electrode Material for Hybrid Supercapacitors

“…The cycle stability of NCO‐0.5 M//JC ASC tested at 2 A g −1 shows stability for 10,000 cycles with exceptional retention of 90.6 % of the initial capacitance indicating excellent cycle stability and suitability for the supercapacitor applications (Figure 8a). The obtained Nyquist plot from Figure 8b shows that the assembled ASC has the lowest series resistance of 0.26 Ω cm 2 in comparison to the other reported ASC's like “Ni 2 P and rP@rGO”, [46] “Co 0.1 Ni 0.9 P/CNF/CC||AC/CC” [47] and “NiCo 2 O 4 nanoneedle on Ni foam” [48] indicating the excellent diffusion of ions for the charge storage mechanism. Eqs.…”

Design and Fabrication of Hierarchical NiCoP–MOF Heterostructure with Enhanced Pseudocapacitive Properties

“…The derived energy and power densities are plotted in Figure 6d. The maximum energy density of 50.3 Wh kg −1 is delivered at a power density of 1011.2 W kg −1 , which is superior to some other HSCs in literatures, including CoNi–MOF//AC, [ 24 ] NiCoP nanoplates//graphene, [ 37 ] Co 0.1 Ni 0.9 P/CNF/CC||AC/CC, [ 44 ] NiCoP/NiCo‐OH30//PC, [ 54 ] ZnNiCoPNF//AC, [ 60 ] NC@SS//Fe 2 O 3 @SS, [ 61 ] P(Ni,Co)Se 2 //ZC, [ 62 ] NiCo 2 S 4 /GA//AC, [ 63 ] ZnNiPS//AC, [ 64 ] NiCoPCoP//PNGF, [ 65 ] and β‐FeOOH//NiCo hydroxides/Cu(OH) 2 /CF. [ 66 ] This device also displays a power density as high as 10 550.3 W kg −1 at 29.7 Wh kg −1 within 11 s, showing a high‐power nature.…”

“…[43] As expected, only NiCoP-MOF shows the P 2p peaks for the phosphate (PO, 134.1 eV) and metal phosphide (PM, 129.6 eV). [44,45] The P-O species could protect NiCoP crystals from serious damage during the charge/discharge process, thereby boosting the structural stability of the NiCoP-MOF. [46] All the aforementioned results confirm the generation of phosphorization-induced NiCoP-MOF heterostructure.…”