Simple Preparation of Porous FeCo₂O₄ Microspheres and Nanosheets for Advanced Asymmetric Supercapacitors

Abstract

Transition metal oxides (TMOs) can serve as advanced electrode materials and have attracted extensive attention in the past decade. Herein, porous FeCo 2 O 4 microspheres (FeCo 2 O 4 MSs) and FeCo 2 O 4 nanosheets (FeCo 2 O 4 NSs) have been successfully synthesized in different solvent systems, accompanied by a corresponding annealing treatment of precursors. The FeCo 2 O 4MSs and NSs possessed a mesoporous structure, and the BET surface areas reached 40.9 and 38.5 m 2 g −1 , respectively. The electrochemical studies were performed in a KOH aqueous electrolyte, and such FeCo 2 O 4 MSs delivered 231.5 C g −1 at 1 A g −1 and exhibited a good long-term cycling performance during 5000 cycles with 71.3% capacity retention. In contrast, the FeCo 2 O 4 NSs delivered a higher capacity of 339.0 C g −1 at 1 A g −1 and exhibited a better rate capability with 73% capacity retention at 13 A g −1 . Furthermore, the assembled FeCo 2 O 4 NSs//AC ASC could preserve 100.5% specific capacity after 5000 cycles and exhibited a high energy density of 37.1 W h kg −1 at 928.0 W kg −1 , while the FeCo 2 O 4 MSs//AC ASC possessed a relatively low energy density with 26.2 W h kg −1 at 965.5 W kg −1 . The outstanding electrochemical behavior of FeCo 2 O 4 NSs and MSs makes them promising electrode materials in electrochemical storage devices. Additionally, this current synthetic strategy possesses some advantages such as simple handling, low maintenance cost, and environmental friendliness; thus, it can be employed for synthesizing other TMOs with superior electrochemical properties.

Advances in Electrochemical and Catalytic Performance of Nanostructured FeCo2O4 and Its Composites

“…Thereafter, Li et al [14] fabricated porous FeCo 2 O 4 microspheres (MSs) and FeCo 2 O 4 nanosheets (NSs) (see inset in Figure 3-b) via a facile solvothermal method (for synthesis details, see Entry 3 and 4, Table 1). The as-synthesised FeCo 2 O 4 -NSs exhibited better capacitance (339.0 C g À 1 at 1 Ag À 1 ) and rate performance (73% capacity retention after 5000 cycles) than FeCo 2 O 4 -MSs (Specific Capacitance = 231.5 C g À 1 at 1 A g À 1 and 71.3% retention of capacity) as shown in Figure 3-b.…”

“…However, although this [48] (b) Cyclic performance as well as coulombic efficiency of FeCo2O4 NSs obtained during 5000 cycles at 5 A g À 1 , with its FE-SEM image as inset. [14] [52] Adapted with permissions from Ref. [48].…”

“…The mixed transition metal oxides (MTMOs), especially cobaltite (MCo 2 O 4 : M = Mn, Fe, Ni, Cu, Zn), are known to offer higher catalytic and electrochemical activity in comparison to STMOs, owing to their unique redox reaction stability and better conductivity. The STMO of Co 3 O 4 has been extensively explored for various electrochemical and catalytic applications due to its very high stability, superior theoretical capacitance (3560 F g À 1 for supercapacitors and 890 mA h g À 1 for batteries), remarkable cycling durability (over 105 cycles) and excellent water oxidation potential of + 1.23 V. [14][15][16] However, the experimentally determined capacitance as well as cycling performance of Co 3 O 4 is disappointingly lower than the predicted value, owing to its very low intrinsic conductivity (10 À 3 to 10 À 4 S cm À 1 ) and slow rate of ion diffusion. [14,17] This semiconductor material also suffers from high rates of charge recombination and limited surface active sites which lowers its catalytic activity.…”

“…The synergistic effect between Co and M atoms along with the multiple valence states of MTMOs and the partially filled 3d orbitals of transition metals is anticipated to display excellent electrochemical and catalytic activities owing to the better electron transfer rate and greater number of surface-active sites. [14,19,20] The spinel structure (A x B 3-x O 4 ) of MTMOs is also equally responsible for their better redox potentials and electrical conductivities in comparison to the STMOs. [21] Moreover, as will be illustrated further in the subsequent sections, among the MTMOs, Fe and Co based spinel oxides are widely being researched due to the presence of multiple oxidation states (Fe 2 + /Fe 3 + ; Co 2 + /Co 3 + ), very high conductivity of Fe metal along with its greater abundance, its ease of substitution, and its non-toxic nature in comparison to other metal ion candidates.…”

“…The synergistic effect between Co and M atoms along with the multiple valence states of MTMOs and the partially filled 3d orbitals of transition metals is anticipated to display excellent electrochemical and catalytic activities owing to the better electron transfer rate and greater number of surface‐active sites [14,19,20] . The spinel structure (A x B 3‐x O 4 ) of MTMOs is also equally responsible for their better redox potentials and electrical conductivities in comparison to the STMOs [21] .…”