Anion‐Exchange Formation of Hollow NiCo₂S₄ Nanoboxes from Mesocrystalline Nickel Cobalt Carbonate Nanocubes towards Enhanced Pseudocapacitive Properties

Abstract

An efficient anion‐exchange protocol was investigated for the controllable fabrication of hollow NiCo2S4 nanoboxes (NBs) from mesocrystalline nickel cobalt carbonate nanocubes as promising pseudocapacitive materials for electrochemical capacitors. The underlying processes of the formation of the hollow architecture were systematically investigated. Originating from the unique structural and compositional advantages, the resultant hollow NiCo2S4 NB electrode with a loading of 5 mg cm2 delivered a large specific capacitance of 777 F g−1 at a high rate of 4 A g−1 in a three‐electrode configuration with 6 m KOH as electrolyte. Furthermore, an asymmetric device constructed with the hollow NBs and activated carbon (AC) as positive and negative electrodes, respectively, showed extraordinary supercapacitance within an electrochemically operating voltage window from 0.0 to 1.5 V. The unique AC//NiCo2S4 NB hybrid capacitor exhibited a large specific energy density (active mass normalized) of approximately 17.1 W h kg−1 at a high power density of 2250 W kg−1, and desirable cycling durability with approximately 75 % specific capacitance retention after 5000 consecutive cycles at a current rate of 2 A g−1. These electrochemical investigations strongly indicated that the as‐fabricated hollow NiCo2S4 NBs can be elegantly utilized as powerful candidates for advanced electrode platforms.

Uniform Hollow Mesoporous Nickel Cobalt Sulfide Microdumbbells: A Competitive Electrode with Exceptional Gravimetric/Volumetric Pseudocapacitance for High‐Energy‐Density Hybrid Superapacitors

“…The distinct peaks at binding energy (BE) of ≈782.4 eV for Co 2p 3/2 and ≈798.5 eV for Co 2p 1/2 are evident, suggesting the co-existence of Co(III) and Co(II) in the NCS-MDs. [4,5,19] The highcontent trivalent Ni/Co observed here is unambiguously propitious to enhancing the electric conductivity of the resultant NCS-MDs electrode itself for even better electrochemical capacitances, especially at high rates. [4,7,18,19] The S 2p spectrum ( Figure S1, Supporting Information) illustrates two fitted peaks at BEs at ≈163.5 and ≈162.3 eV, corresponding to S 2p 1/2 and S 2p 3/2 .…”

“…The Co 2p and Ni 2p XPS spectra were fitted well by using Gaussian fitting method, considering two spin-orbit doublets and two shakeup satellites (Sat.). [4,7,18,19] The S 2p spectrum ( Figure S1, Supporting Information) illustrates two fitted peaks at BEs at ≈163.5 and ≈162.3 eV, corresponding to S 2p 1/2 and S 2p 3/2 . The distinct peaks at binding energy (BE) of ≈782.4 eV for Co 2p 3/2 and ≈798.5 eV for Co 2p 1/2 are evident, suggesting the co-existence of Co(III) and Co(II) in the NCS-MDs.…”

“…Figure 2c depicts the high-resolution Co 2p spectrum and corresponding fitted curves. [18][19][20] According to the intergrated area under the fitted profiles, the atomic percentage of Co(III) and Ni(III) can be estimated as high as ≈61.5 and ≈59.2 at%, respectively. [4,18,19] Figure 2d shows the Ni 2p XPS spectrum with two peaks at ≈857.6 eV for Ni 2p 3/2 and ≈875.3 eV for Ni 2p 1/2 , which implies the concomitance of both Ni 2+ and Ni 3+ .…”

“…[18,[22][23][24] As for our case here, typical ionexchange reaction of the NCC with the S 2− ions happens during the hydrothermal ion-exchange process, in which the outward transport of Co/Ni ions is even faster than the inward diffusion of the S 2− ions. [18,19,24] In virtue of distinctive discrepancy in diffusion rate between elemental Ni/Co and S 2− ions, the inner cavity is gradually created with reaction proceeding. As a result, the hollow MDs were formed accordingly.…”

Mixed Metal Sulfides for Electrochemical Energy Storage and Conversion

“…In general, a self-template method involves the synthesis of template precursors and subsequent sulfidation treatment process through ion-exchange reactions and/or Kirkendall effect. Up to present, numerous intriguing templates have been developed, including mixed metal oxide, [72,73] mixed metal hydroxide, [74,75] layered double hydroxide (LDH), [76,77] mixed metal carbonate, [78,79] mixed metal glycerate, [28,80,81] mixed metal carbonate hydroxide, [27,82] mixed metal acetate hydroxide, [83] mixed metal silicate, [84,85] and metal-organic frameworks (MOFs). [86][87][88] These self-engaged templates could directly determine the shape, size, and composition of the resultant MMSs.…”