A Critical Review of Spinel Structured Iron Cobalt Oxides Based Materials for Electrochemical Energy Storage and Conversion

Abstract

Iron cobalt oxides, such as typical FeCo₂O₄ and CoFe₂O₄, are two spinel structured transitional metal oxide materials with excellent electrochemical performance. As the electrodes, they have been widely applied in the current energy storage and conversion processes such as supercapacitors, Lithium-ion batteries and fuel cells. Based on synthesis approaches and controlled conditions, these two materials exhibited broad morphologies and nanostructures and thus distinct electrochemical performance. Some of them have shown promising applications as electrodes in energy storage and conversion. The incorporation with other materials to form composites further improved their performance. This review briefly summarized the recent applications of FeCo₂O₄ and CoFe₂O₄ in energy storage and conversion, current understandings on mechanisms and especially the relevance of morphologies and structures and composites to electrochemical performance. Some recommendations were finally put forward addressing current issues and future prospects on electrodes of FeCo₂O₄ and CoFe₂O₄ based materials in energy storage and conversion, implying there was still space to further optimize their performance.

1. Introduction

The severe environmental issues relative to the use of fossil fuels calls for the urgently demanding development of highly-efficient energy conversion and storage devices, such as fuel cells, supercapacitors and Lithium-ion batteries (LIBs) [1,2,3]. Despite the energy storage and conversion mechanisms may be different among these three devices, they still share fundamentally electrochemical similarities. All energy storage and conversion processes occur at the interface of electrode and electrolyte and the electron and ion transport are separated [4]. A simplified Ragone plot (Figure 1) can demonstrate domains of these energy storage and conversion systems differentially from traditional power systems such as combustion engine, turbines and regular capacitors. Among of them, fuel cells belong to high-energy systems as the anode and cathode are used to transfer charge and fuels undergo the redox reactions without involving the process of combustion, whereas supercapacitors belong to high-power system since supercapacitors stores electrical charges both at the electrode surface and in the bulk near the surface of the solid electrode. Lithium-ion batteries have intermediate power and energy characteristics. Electrode materials are the most important components for energy storage and conversion systems, which usually include three groups of popular electrode materials such as carbon materials, conducting polymers and transition metal oxides. It is remarkable that transition metal oxides can be applied as electrode materials for these three major energy storage and conversion systems because of their relevance to redox reactions to some extent.

Transition metal oxides possess several oxidation states that are favorable for rapid redox reactions, leading to efficient and high-quality energy storage and conversion systems. However, the single metal oxides such as NiO [5,6], Co₃O₄ [7,8,9] and MnO₂ [10] generally suffer from low conductive properties and unfavorable stability, which constrain the performance of energy storage and conversion systems. Nanostructures and composite engineering of materials are promising approaches to address issues of single metal oxides [11]. This is why the recent rise of binary metal oxides which were paid much attention in energy storage and conversion applications by virtue of their better conductivity and electrochemical performance. Among all these binary metal oxides, spinel cobaltites—M_xCo_3-xO₄ (M = Ni, Mn, Zn, Cu and so on)—have exhibited better characteristics in fuel cells, Li-ion batteries and supercapacitors [12,13,14]. Not only can the conductivity and electrochemical activity be improved but the toxicity and cost can be reduced through partly substituting the Co-content with other less-toxic and highly-performed metals. Iron can be a promising substitutable metal due to its abundance, non-toxic and excellent substitution performance. For example, FeCo₂O₄ and CoFe₂O₄ have been two widely investigated promising binary metal oxides as electrode materials for supercapacitors, fuel cells and Li-ion batteries.

Since FeCo₂O₄ and CoFe₂O₄ based electrodes store charges and convert energy mainly on their surface and near-surface bulky structures, high specific surface areas are specially required. Nanostructured materials can achieve much high specific surfaces, providing the shorter transport and diffusion path lengths for ions and electrons as well as more electroactive sites for energy storage and conversion. On the other hand, the fabrication of iron cobalt oxides based composite materials also is another strategy to enhance the performance of supercapacitors, Li-ion batteries and fuel cells, in conductivity, mechanical strength and specific surface areas. These have been presented in a large quantity of the recent literatures but to date, no reviews has summarized these two binary metal oxides. In this paper, nanostructures and morphologies of FeCo₂O₄ and CoFe₂O₄ and their relative composites materials as well as their corresponding performance were summarized and mechanisms of FeCo₂O₄ and CoFe₂O₄ on energy storage and conversion was proposed, the applications of FeCo₂O₄ and CoFe₂O₄ based materials as energy storage and conversion electrodes were discussed.

2. Application for Supercapacitors

According to the energy storage mechanism, supercapacitors can be further divided into two categories: the first is electrochemical double-layer capacitor (EDLC) which stores energy by accumulating charge in the electrode/electrolyte interface and the second is the pseudo-capacitors based on the fast and reversible redox reactions at electrochemically active sites [15,16]. Spinel cobaltites and ferrites materials, such as FeCo₂O₄ and CoFe₂O₄, based materials can be used as the electrodes for pseudo-capacitors. They store charges both on the surface and in the bulk near the surface of the solid electrode where the Faradic reactions occur [17]. The energy storage mechanisms for pseudo-capacitors of FeCo₂O₄ and CoFe₂O₄ in alkaline solution can be described by the following equations [18]:

FeCo₂O₄ + OH⁻ + H₂O ↔ FeOOH + 2CoOOH + e⁻

(1)

CoFe₂O₄ + OH⁻ + H₂O ↔ 2FeOOH + CoOOH + e⁻

(2)

CoOOH + OH⁻ ↔ CoO₂ + H₂O + e⁻

(3)

FeOOH + H₂O ↔ Fe(OH)₃ ↔ FeO₄²⁻ + 3e⁻

(4)

Several morphologies of FeCo₂O₄ and CoFe₂O₄ have been reported, addressing nanoparticles, nanowires, nanosheets and nanoshelled-microspheres. Direct growth of nanostructured FeCo₂O₄ and CoFe₂O₄ electrode materials have been reported recently on highly conductive substrates such as the nickel foam and the stainless steel, avoiding the use of binder and additives to exposure almost all available surface involved in energy storage and conversion process. In addition, several other materials which also own the electrochemical capacitance properties such as carbon materials, metal oxides and conducting polymers have been combined with iron cobalt oxides to fabricate composite material electrodes. As expected, these composited materials displayed an enhanced performance and showed synergistic effects in supercapacitor applications. The currently available morphologies and composites of FeCo₂O₄ and CoFe₂O₄ were briefly summarized in

Table 1. Summary of iron cobalt oxides based materials for supercapacitor applications.

Materials	Synthetic Method	Potential Window	Specific Capacitance	Cycling Stability	Ref.
FeCo2O4 nanowires/NF	Hydrothermal	0 to 0.55 V (vs. Hg/HgO)	428 F g−1 (5 mV s−1)	142% retention after 2000 cycles	[18]
CoFe2O4 nanoparticles	Hard-templating	−1.0 to 0.5 V (vs. Hg/HgO)	142 F g−1 (2 mV s−1)	71.8% retention after 1000 cycles	[19]
CoFe2O4 hollow microspheres	Hydrothermal	0 to 0.4 V (vs. Ag/AgCl)	1790 F g−1 (2 A g−1)	98% retention after 500 cycles	[20]
CoFe2O4 nanoflakes/SS	Chemical bath deposition	−1.0 to −0.2 V (vs. SCE)	366 F g−1 (5 mV s−1)	90.6% retention after 1000 cycles	[21]
FeCo2O4 nanoflakes/NF	Hydrothermal	0 to 2.5 V (two-electrode systems)	433 F g−1 (0.1 A g−1)	62.5% retention after 2500 cycles	[22]
CoFe2O4 nanosheets/NF	Hydrothermal	0–0.4 V (vs. Ag/AgCl)	503 F g−1 (2 A g−1)	98% retention after 5000 cycles	[23]
CoFe2O4 nanomesh/NF	Hydrothermal	0 to 0.6 V (vs. SCE)	1426 F g−1 (1 A g−1)	92.6% retention after 3000 cycles	[24]
FeCo2O4 submicron-tube/NF	Chemical bath deposition	−0.2 to 0.6 V (vs. Ag/AgCl)	1254 F g−1 (2 mA cm−2)	91% retention after 5000 cycles	[25]
CoFe2O4 nanoparticles	Solution combustion	−1.1 to 0 V (vs. Hg/HgO)	195 F g−1 (1 mV s−1)	67% retention after 3000 cycles	[26]
CoFe2O4/graphene/PANI	Hydrothermal + polymerization	−0.8 to 0.2 V (vs. Hg/HgO)	1133.3 F g−1 (1 mV s−1)	96% retention after 5000 cycles	[27]
FeCo2O4-Tube/MnO2 nanosheets	Chemical bath deposition + Hydrothermal	0 to 0.6 V (vs. Ag/AgCl)	3.3 F cm−2 (1 mA cm−2)	94% retention after 2000 cycles	[28]
CoFe2O4/MnO2 nanosheet arrays	Hydrothermal	−0.2 to 0.4 V (vs. Ag/AgCl)	3.59 F cm−2 (2 mA cm−2)	91.5% retention after 2250 cycles	[29]
FeCo2O4/MnO2 nanosheet arrays	Hydrothermal	0 to 0.45 V (vs. SCE)	2491.8 F g−1 (4 mA cm−2)	87.2 retention after 5000 cycles	[30]
CoFe2O4/FeOOH	Hydrothermal	−0.2 to 0.3 V (vs. Hg/HgO)	332.4 F g−1 (0.5 A g−1)	91.3% retention after 1000 cycles	[31]
CoFe2O4/RGO/PANI	Solution combustion + polymerization	−0.174 to 0.926 V (vs. SHE)	239 F g−1 (1.5 A g−1)	100% retention after 1000 cycles	[32]

SS: stainless steel; NF: nickel foam; RGO: reduced graphene oxide; PANI: polyaniline.

.

According to the date listed in Table 1, showed 3D-structured iron cobalt oxides seemed to have higher specific capacitance than 1D and 2D structures. Remarkably, Wang et al. [20], successfully fabricated multi-shelled CoFe₂O₄ hollow microspheres with a controllable number of layers from 1 to 4 via a simple facile one-step hydrothermal method using cyclodextrin as a template. The representative figures multi-shelled CoFe₂O₄ hollow microspheres are shown in Figure 2. The specific capacitance of triple-shelled CoFe₂O₄ hollow microspheres reached 1790 F g⁻¹ at a current density of 2 A g⁻¹, which was the highest capacitance among CoFe₂O₄ electrodes. FeCo₂O₄ submicron-tube arrays grown on nickel foam served as binder/additive-free electrodes for supercapacitor [25]. The optimized specific capacitance of FeCo₂O₄-tube electrode reached 1254 F g⁻¹ at 2 mA cm⁻². Both of these two 3D-structured materials can maintain high capacitances at 98% of their original specific capacitances after 500 cycles and at 91% after 5000 cycles, respectively. Unlike other binary metal oxides such as NiCo₂O₄ [33,34], CuCo₂O₄ [35] and ZnCo₂O₄ [36], the 3D micro-structured and submicron-structured iron cobalt oxides even exhibited relatively high specific capacitances and cycling-stability compared to their 1D and 2D nano-structured counterparts such as nanowires or nanosheets. The excellent performance may contribute to unique morphology and high porosity of these materials which can provide a large electrode/electrolyte interface. The 3D-structured frames likely constructed a cross-linking electron transport path and also an ion reservoir for electrolyte ion accumulation. Furthermore, 3D-structured frames had better structural mechanical stability, realizing a better cycling-stability. Pendasheh et al. [18], fabricated FeCo₂O₄ porous wires with an average edge length about 200 nm which was in submicron scale supported on the nickel foam and achieved 407 F g⁻¹ at 10 mV s⁻¹. Interestingly, cycling for 2000 cycles seemed to electro-activate the material and to achieve the pore opening (Figure 3), leading a subsequent increase in the capacitance up to 610 F g⁻¹, which was a significant characteristic of the electrode materials for supercapacitor. In addition, we noticed that most of iron cobalt oxides based materials grown on highly conductive substrates, such as the nickel foam, exhibited the better cycling stability (more than 90% retention after 2000 cycles) than those on powder materials. This should be attributed to that the electrical connection of the electrode materials to the Ni foam without any binder or conductive agents can enhance the long-term performance of the hybrid electrodes. Moreover, the enhanced electrical conductivity of bimetallic iron cobalt oxides has been confirmed by previous works and ours. Our group directly fabricated ultrathin CoFe₂O₄ nanosheets on the excellent-conductive nickel foam free of binders and exhibited better electrical conductivity than those of both its corresponding monometallic oxides. This approach clearly facilitated both the better charge transfer and ions diffusion [23]. The as-prepared FeCo₂O₄ nanowires also exhibited similar results [18] (Figure 4).

Combing different materials to form composites is also an important approach to improve the performance of supercapacitors. Some literatures reported FeCo₂O₄ and CoFe₂O₄ based composite materials served as supercapacitor electrodes. Zhu et al. [28], reported the MnO₂-nanosheet covered sub micrometer FeCo₂O₄-tube forest (Figure 5) as the high energy density supercapacitor electrode. The deposition of MnO₂ nanosheets increased the specific capacitance to 3.3 F cm⁻² at 1 mA cm⁻², which doubled that of its bare FeCo₂O₄-tube electrode. Conducting polymers were promising as both excellent conductive substrates to junction to FeCo₂O₄ and CoFe₂O₄ and self-active pseudo-capacitance materials. For example, CoFe₂O₄/graphene/polyaniline nanocomposite was fabricated by the hydrothermal and in-situ polymerization processes. This ternary hybrid electrode exhibited a high capacitance of 1133.3 F g⁻¹ at a scan rate of 1 mV s⁻¹ and a 96% retention of the initial capacitance after 5000 cycles [27]. There were other reports on the enhancement of the specific capacitance and the cycling-stability of NiCo₂O₄ electrodes by combining carbon aerogel [37], polypyrrole [38] and transition metal oxides such as NiO [39]. This easily inspired the search for more alternative conductive juncture materials_, such as rarely reported carbon nanotubes, graphene, polypyrrole and other transition metal oxides, to design and fabricate new novel FeCo₂O₄ and CoFe₂O₄ based composite electrodes.

3. Application for Lithium-Ion Batteries

Compared with supercapacitors, Lithium-ion battery possess a higher energy storage and high cell voltage but a lower power output. Its energy densities and power densities are twice and five times greater than those of the Pb-acid and the Ni-Cd batteries, respectively [40]. The further development of LIB is largely dependent on performance of its anode electrode materials. Graphite is a conventional commercial electrode material, only possessing a theoretical specific capacity at about 372 mA h g⁻¹ [41]. Transition metal oxides have been intensively explored as new promising anode materials because of their higher theoretical specific capacitances. For example, CoFe₂O₄ was reported to possess a high theoretical specific capacity at 914 mA h g⁻¹ for LIBs which was two times higher than that of graphite [42]. Considering the abundance on the earth and eco-friendliness of iron, iron cobalt oxides have been widely investigated as anode materials of LIBs. Previous literature reported that the electrochemical mechanisms of the Li-ions’ storage of FeCo₂O₄ and CoFe₂O₄ obeyed the displaced redox reactions, demonstrated by the following equations.

FeCo₂O₄ + 8Li⁺ + 8e⁻ → Fe + 2Co + 4Li₂O

(5)

CoFe₂O₄ + 8Li⁺ + 8e⁻ → 2Fe + Co + 4Li₂O

(6)

Fe + Li₂O ↔ 2Li⁺ + FeO + 2e⁻

(7)

Co + Li₂O ↔ 2Li⁺ + CoO + 2e⁻

(8)

CoO + 1/3Li₂O ↔ 1/3 Co₃O₄ + 2/3 Li⁺ +2/3e⁻

(9)

FeO + 1/2Li₂O ↔ 1/2Fe₂O₃ + Li⁺ + e⁻

(10)

Binary metal oxides electrode stored Li-ion mainly on both its interface to the electrolyte and its near-surface bulky structure. Figure 6 shows a schematic diagram of the LIB’s charging-discharging process, in which, the electrode involves a reversible insertion and extraction of Li ions as described by above equations. The charging process is accomplished with Li ions extracted from the cathode and inserted into the anode and the discharging process is accomplished with Li ions deintercalated from the anode and intercalated into the cathode. The energy storage and release are realized by the intercalation and deintercalation of Li ions between the anode and cathode. However, there typically existed an irreversible capacity loss of FeCo₂O₄ and CoFe₂O₄ LIB starting from its first cycle due to the electrolyte-reduction induced incomplete conversion reaction and the formation of solid electrolyte interface (SEI) layer at the electrode/electrolyte interface [43]. The Li-ion insertion/extraction process also induced the large volume change, the severe aggregation and the low conductivity, resulting in the capacity degradation and the poor cycling stability [44,45].

Efforts have been devoted to remedy the aforementioned disadvantages of FeCo₂O₄ and CoFe₂O₄ electrodes in LIBs. This correlated their various morphologies and structures to different performance, as shown in

Table 2. Summary of previous bare iron cobalt oxides for Lithium-ion batteries applications.

Materials	Synthetic Method	Reversible Capacity (mA h g−1)	Rate Capability	Ref.
FeCo2O4 nanoflakes/NF	Hydrothermal	905 at 200 mA g−1 after 170 cycles	1222 mA h g−1 at 800 mA g−1	[22]
3D ordered macroporous CoFe2O4	Templating	702 at 0.2 mA cm−2 after 30 cycles	816 mA h g−1 at 5 mA cm-2	[42]
Hollow CoFe2O4 nanocubes	Metal-organic frameworks	1115 at 1 C after 200 cycles	815 mA h g−1 at 20 C	[51]
FeCo2O4 nanoneedles/NF	Hydrothermal	1129 at 100 mA g−1 after 350 cycles	875 mA h g−1 at 2 A g−1	[52]
Hollow FeCo2O4 nanospheres	Soft-templating	1060 at 100 mA g−1 after 50 cycles	823 mA h g−1 at 1 A g−1	[53]
FeCo2O4 octahedra	Evaporation-induced self-assembly	1101 at 1000 mA g−1 after 200 cycles	518 mA h g−1 at 10 A g−1	[54]
CoFe2O4 nanosheets	Thermal decomposition	806 at 1 A g−1 after 200 cycles	303 mA h g−1 at 10 A g−1	[55]
CoFe2O4 nanowires/FCF	Hydrothermal	954.3 at 200 mA g−1 after 150 cycles	595.3 mA h g−1 at 3.2 A g−1	[56]
CoFe2O4 nanotubes	Self-templating	988 at 100 mA g−1 after 100 cycles	654 mA h g−1 at 5 A g−1	[57]
Cubic aggregated CoFe2O4 nanoparticles	Hydrothermal	1133.5 at 100 mA g−1 after 120 cycles	679 mA h g−1 at 3.2 A g−1	[58]
Layer-stacked CoFe2O4 platelets	Co-precipitation	580 at 5 A g−1 after 2000 cycles	654 mA h g−1 at 10 A g−1	[59]
CoFe2O4 nanorods	Templating	800 at 1 A g−1 after 300 cycles	840 mA h g−1 at 1 A g−1	[60]
CoFe2O4 octahedra	Sol-gel	992 at 100 mA g−1 after 200 cycles	366 mA h g−1 at 5 A g−1	[61]
Hollow CoFe2O4 nanospheres	Hydrothermal	1185 at 90 mA g−1 after 50 cycles	1000 mA h g−1 at 900 mA g−1	[62]
Flower-like CoFe2O4 microspheres	Hydrothermal	733.5 at 200 mA g−1 after 50 cycles	717 mA h g−1 at 1 A g−1	[63]

NF: nickel foam; FCF: flexible carbon fabric.

. P. Lavela et al. [46], synthesized CoFe₂O₄ by way of the sol-gel process and Y. Sharma et al. [47], synthesized FeCo₂O₄ by way of the urea combustion method, both confirming that the LIB performance of CoFe₂O₄ and FeCo₂O₄ were better than that of NiFe₂O₄ and MgCo₂O₄, respectively. Different synthesis methods generated a broad and different dimensional morphologies of CoFe₂O₄ and FeCo₂O₄ materials. It has been found that the nanostructured iron cobalt oxides generally had higher reversible capacities and a more excellent rate capability than those of corresponding microstructured and bulky materials. The hollow-typed morphologies performed better than solid-typed morphologies in LIBs. Nanostructures provided higher surface areas which were favorable to reduce electron and lithium ions diffusion paths and increase active sites for lithium ion insertion/extraction [48,49]. Hollow-typed morphologies and structures can afford adequate spaces to relax the large volume changes during constant charge/discharge processes [50]. Both lead their excellent performance in LIBs. As typical examples, H. Guo et al. [51], fabricated hollow porous CoFe₂O₄ nanocubes via metal-organic frameworks and exhibited its excellent rate performance, as shown in Figure 7.

Its surface area was 102.7 m² g⁻¹ and its reversible capacity remained stable as high as 1115 mA h g⁻¹ after 200 cycles at a constant current density of 1 C. Its capacity could reach 815 mA h g⁻¹ even at a high current density of 20 C. Xiong et al. [63], developed an ascorbic acid-assisted hydrothermal method to prepare porous CoFe₂O₄ flower-like microspheres, and particles. The flower-like CoFe₂O₄ microsphere with the highest surface area of 51.0 m² g⁻¹ showed a higher specific capacity of 733.5 mA h g⁻¹ at 200 mA g⁻¹ after 50 cycles, contrasting to its microsphere counterpart of 616.7 mA h g⁻¹ with a surface area of 32.3 m² g⁻¹ and its powder counterpart of 427.5 mA h g⁻¹ with a surface area of 15.1 m² g⁻¹. Another recent binder/additive-free strategy was to directly grow CoFe₂O₄ and FeCo₂O₄ nanostructured materials on conductive substrates. Advantages of this approach were to enhance the electronic conductivity of electrodes, to provide a frame to organize nanostructured materials in order and to prevent the aggregation and pulverization of the electrode materials. Liu et al. [52], successfully synthesized a porous FeCo₂O₄ nanoneedles array on nickel foam (Figure 8) and confirmed the validity of this strategy. As shown in Figure 9a, the as-synthesized FeCo₂O₄ nanoneedles array exhibited a high rate capability of 875 mA h g⁻¹ at 2 A g⁻¹, contrasting to bulky CoFe₂O₄ and FeCo₂O₄ nanoneedles array with 526 and 323 mA h g⁻¹ at 2 A g⁻¹, respectively. It’s found that the discharge capacity of FeCo₂O₄ nanoneedles array maintained at 1129 mA h g⁻¹ with a capacity retention of 57.5%, while CoFe₂O₄ nanoneedles array dropped to 36.8% and bulky FeCo₂O₄ was only 19.3% (Figure 9b). Interestingly, we noticed that some reported FeCo₂O₄/CoFe₂O₄ based electrodes present much higher discharge capacity that their theoretical specific capacity. This higher discharge capacity should be attributed to an interfacial-storage mechanism, which was originated from the reversible formation–dissolution of an organic polymeric gel-like layer via electrolyte decomposition, inducing an extra capacity in the electrode material by way of the pseudocapacitive behavior [64].

Alternatively, the CoFe₂O₄ and FeCo₂O₄ based composite materials could also incorporate carbon materials, transition metal oxides and many other materials, dependent on if the incorporated material can enhance electrochemical performance of CoFe₂O₄ and FeCo₂O₄ in LIBs. It’s found the proper morphologies and optimized ratio of incorporated components in composites were vital to well-performed electrodes via overcoming drawbacks of individual components and taking advantage of all constituents.

Table 3. Summary of previous iron cobalt oxides based nanocomposites for Lithium-ion batteries applications.

Materials	Synthetic Method	Reversible Capacity (mA h g−1)	Rate Capability	Ref.
CoFe2O4 nanoclusters/graphene aerogels	Solvothermal	966 at 500 mA g−1 after 300 cycles	221 mA h g−1 at 8 A g−1	[68]
CoFe2O4 nanoparticles/graphene	Carbon dioxide-induced deposition	1114 at 100 mA g−1 after 100 cycles	636 mA h g−1 at 3 A g−1	[69]
Flower-like CoFe2O4/MWCNTs	Hydrothermal	823 at 45 mA g−1 after 50 cycles	359 mA h g−1 at 1.8 A g−1	[70]
CoFe2O4/carbon nanofibers	Pyrolysis-oxidation	705 at 100 mA g−1 after 250 cycles	-	[71]
CoFe2O4 nanoparticles/Vulcan XC-72	Thermal decomposition	766 at 100 mA g−1 after 25 cycles	478 mA h g−1 at 1 C	[72]
C/CoFe2O4 fiber-in-tube nanostructure	Electro-spinning	740 at 200 mA g−1 after 200 cycles	488 mA h g−1 at 1.6 A g−1	[73]
Core/shell structured CoFe2O4/onion-like C	Arc discharge	914.2 at 91.6 mA g−1 after 500 cycles	617.1 mA h g−1 at 916 mA g−1	[74]
CoFe2O4 nanoclusters/RGO	Solvothermal	1040 at 91.4 mA g−1 after 30 cycles	380 mA h g−1 at 18.28 A g−1	[75]
CoFe2O4/graphene	Liquid-solid-solution assembly	1102 at 200 mA g−1 after 100 cycles	410 mA h g−1 at 6.4 A g−1	[76]
CoFe2O4/graphene nanocomposite	Hydrothermal	910 at 100 mA g−1 after 50 cycles	406 mA h g−1 at 2 A g−1	[77]
CoFe2O4/C twin elliptical frustums	One-pot refluxing reaction	875 at 500 mA g−1 after 600 cycles	631 mA h g−1 at 4 A g−1	[78]
CoFe2O4 nanosheets/RGO	Solvent method	835.6 at 400 mA g−1 after 200 cycles	848.6 mA h g−1 at 1 A g−1	[79]
CoFe2O4/graphene sandwich	Solvothermal	1047 at 200 mA g−1 after 160 cycles	440 mA h g−1 at 1.6 A g−1	[80]
Co3O4/ CoFe2O4 nanocomposites	Auto combustion	896.4 at 64.1 mA g−1 after 60 cycles	328.1 mA h g−1 at 6 A g−1	[81]
CoO/CoFe2O4 nanocomposites	Separate nucleation and aging	1040 at 100 mA g−1 after 30 cycles	490 mA h g−1 at 6.4 A g−1	[82]
CoO/CoFe2O4/N-doped graphene	Hydrothermal	1172 at 500 mA g−1 after 100 cycles	680 mA h g−1 at 2 A g−1	[83]
CoFe2O4/MnO2/C nanotubes	Electrospinningand hydrothermal	713.6 at 100 mA g−1 after 250 cycles	310.6 mA h g−1 at 1 A g−1	[84]

RGO: reduced graphene oxide; MWCNT: multiwalled carbon nanotube.

presents current iron cobalt oxides based composites for LIB applications. Carbon materials, such as carbon fibers, carbon nanotubes and graphene were typically used for synthesizing CoFe₂O₄ and FeCo₂O₄ based composite electrodes by virtue of their outstanding electrical conductivity, high surface areas and excellent mechanical and chemical stability [65]. Better electrical conductivity and higher surface area of iron cobalt oxides based materials could be further enhanced after their incorporation into porous carbon materials which could weaken the disadvantageous influence of polarization during the repeated discharge-charge process. Graphene was more favorable as an ideal component candidate toward the composite fabrication for LIBs, because of its conductive and accessible carbon open network to provide with sufficient growth sites for active materials and also prevent aggregation [66,67]. Wang et al. [68], immobilized CoFe₂O₄ nanoclusters on the aerogel-type graphene (Figure 10) via a facile one-step solvothermal method. The aerogel-type graphene clearly possessed an interconnected 3D framework, resulting in a high specific area, fast mass and electron transport rates and a unique nanostructure holding a large capacity of CoFe₂O₄ nanoclusters. As a result, the as-synthesized CoFe₂O₄ nanoclusters/graphene aerogels electrode exhibited 1070 mA h g⁻¹ at a current density of 100 mA g⁻¹ after 100 cycles, contrasting to only 473 mA h g⁻¹ of the bare CoFe₂O₄ electrode. It still possessed 221 mA h g⁻¹ at a high current density of 8 A g⁻¹. L. Wang et al. [69], also developed a green method to synthesize CoFe₂O₄ and graphene nanocomposite via a supercritical carbon dioxide. The uniform CoFe₂O₄ nanoparticles/graphene composite electrode exhibited a high reversible capacity of 1114 mA h g⁻¹ at a current density of 100 mA g⁻¹ after 100 cycles and an excellent rate capability of 636 mA h g⁻¹ at 3 A g⁻¹. Several reports on other components to be incorporated in CoFe₂O₄ addressed multiwalled carbon nanotubes [70], carbon nanofibers [71] and carbon nanoparticles [72] but were not competitive with graphene. This implied well-structured graphene was an optimal carbon material to be incorporated in iron cobalt oxides for the performance enhancement as electrode in LIBs.

Transition metal oxides have also been favorable candidates to be incorporated in iron cobalt oxides for the performance enhancement in of LIBs. Cobalt oxides, such as CoO and Co₃O₄, were two widely-investigated. The CoO [85] and Co₃O₄ [86] themselves were anode materials of LIBs with higher theoretical capacities at about 744 mA h g⁻¹ [87] and 890 mA h g⁻¹ [88], respectively. Previous report showed the hybridized bi-component transition metal oxides such as Co₃O₄ nanowire/MnO₂ core/shell nanocomposite had an improved energy storage performance over each individual component [89]. Some researchers further fabricated transition metal oxides/iron cobalt oxides composites as electrodes in LIBs. Rai et al. [81], reported Co₃O₄/CoFe₂O₄ nanocomposite with their ratio at 76.3:23.6% and found this electrode exhibited a higher reversible capacity about 1172 mA h g⁻¹ at 500 mA g⁻¹ after 100 cycles and also maintained a high rate capability of 328.1 mA h g⁻¹ at a current density of 6 A g⁻¹. Li et al. [82], fabricated CoO/CoFe₂O₄ nanocomposites via scalably prepared single-resource precursors of CoFe-layered double hydroxides. This CoO/CoFe₂O₄ composite showed the currently best performance with a capacity of 603 mA h g⁻¹ at a rate of 1000 mA g⁻¹ after 100 cycles and 490 mA h g⁻¹ at a high current density of 6.4 A g⁻¹. Its performances were much better than those of single CoO, CoFe₂O₄ and physical mixture of CoO and CoFe₂O₄. Results from composite studies revealed the synergistic effects of incorporation of transition metal oxides in iron cobalt oxides for LIB applications. There were many reports on other transition metal oxides, such as NiO [90], ZnFe₂O₄ [91], MnO₂ [92], and SnO₂ [93], which exhibited their excellent performance in LIBs and implied their promising functionalities to be incorporated in iron cobalt oxides.

4. Application for Fuel Cells

Figure 1 has shown fuel cells can be electrochemical energy conversion devices with high energy density. Unlike supercapacitor and LIBs, fuel cells directly convert chemical energy of fuels into electricity in a highly efficiency. They involve combustion resulting in a highly efficient utilization of fuels. Different applied electrolytes categories fuel cells into five types, including Phosphoric Acid Fuel Cell (PAFC), Proton Exchange Membrane Fuel Cell (PEMFC), Alkaline Fuel Cell (AFC), Molten Carbonate Fuel Cell (MCFC) and Solid Oxide Fuel Cell (SOFC) [94]. Recent AFCs using potential non-noble catalysts have aroused more attention for achieving more efficient kinetics in both the oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) in alkaline system [95]. In alkaline electrolyte of AFC, oxygen can be reduced through either a four-electron process (O₂ + 2H₂O + 4e⁻ → 4OH⁻) or two two-electron processes (O₂ + H₂O + 2e⁻ → HO₂⁻ + OH⁻; HO₂⁻ + H₂O + 2e⁻ → 3 OH⁻) [96]. Noble metal materials such as Pt, [97] Pd, [98] Ru, [99] and Pt-based alloys [100] have long been considered as the most efficient catalysts for the ORR reaction in the past decades but its high cost restrains the commercialization of AFCs. Transitional metal oxide catalysts were such new group electrode materials for AFC to substitute the traditional noble metals in AFC applications.

Remarkably, CoFe₂O₄ and FeCo₂O₄ show excellent electrocatalytical activity for the ORR reaction in AFCs.

Table 4. Summary of previous cobalt iron oxides based materials for oxygen reduction reaction (ORR).

Materials	Synthetic Method	Onset Potential	Tafel Slope	Durability	Ref.
3D CoFe2O4 hollow nanospheres	Hydrothermal	0.78 V (vs. RHE)	-	34% decay after 43,200 s	[101]
Ni-doped CoFe2O4 hollow nanospheres	Hydrothermal	−0.15 V (vs. Ag/AgCl)	-	-	[104]
FeCo2O4/hollow graphene spheres	Electrostatically induced assembly method	−0.09 V (vs. Ag/AgCl)	56 mV/dec	7.9% decay after 86,400 s	[103]
CoFe2O4 nanospheres/graphene	Solvothermal	−0.11 V (vs. Ag/AgCl)	61 mV/dec	6% decay after 72,000 s	[105]
CoFe2O4/biocarbon nanocomposites	Biotemplate and chemical precipitation	−0.14 V (vs. Ag/AgCl)	-	15.1% decay after 43,000 s	[105]
CoFe2O4/rod-like carbon	Hydrothermal	−0.1 V (vs. Ag/AgCl)	99 mV/dec	9.5% decay after 20,000 s	[106]
CoFe2O4/Carbon nanotubes	Solvothermal	−0.124 V (vs. Ag/AgCl)	-	-	[107]
CoFe2O4/graphene	Solvothermal	−0.136 V (vs. Ag/AgCl)	67 mV/dec	5.5% decay after 43,200 s	[108]
N,S dual-doped 3D RGO/CoFe2O4	Hydrothermal	−0.14 V (vs. Ag/AgCl)	70 mV/dec	6.7% decay after 43,200 s	[109]
Ag/CoFe2O4/C	Solvothermal	E1/2 at −0.13 V (vs. Hg/HgO)	-	5.2% decay after 5000 s	[110]
Co/CoO/CoFe2O4/graphene nanocomposites	Separate nucleation and aging	E1/2 at −0.25 V (vs. Ag/AgCl)	-	20% decay after 20,000 s	[111]
MnO2-CoFe2O4/C	Solvothermal	0.85 V (vs. RHE)	-	7.3% decay after 10,000 s	[112]

presents performance of current CoFe₂O₄ and FeCo₂O₄ based materials for the ORR reaction. Yang’s group designed and fabricated several iron cobalt oxide nanostructures and nanocomposites, among which solid and hollow CoFe₂O₄ nanospheres were successfully fabricated [101]. Compared with the most efficient Pt/C, the hollow CoFe₂O₄ nanospheres showed a negative shift of 180 mV at the half-wave potential, which is 40 mV more positive shift than the solid one, indicating that improved electrocatalytic activities were expected and also depended on the structures and morphologies of materials. When CoFe₂O₄ nanospheres were combined with graphene via a one-pot solvothermal method [102], their nanocomposite showed a low onset potential at about −0.11 V (vs. Ag/AgCl) which was higher than −0.17 for CoFe₂O₄ nanospheres only. The stability of CoFe₂O₄ nanospheres/graphene was also excellent in the ORR reaction, the ORR current density was still maintained at 94% after 72,000 s of continuous operation. The enhanced performance of CoFe₂O₄ nanospheres/graphene should be attributed to the suppressed agglomeration CoFe₂O₄ nanospheres and restacking of graphene as well as synergistic electrocatalytic effects of CoFe₂O₄ nanospheres and graphene in ORR reaction. The FeCo₂O₄/hollow graphene spheres [103] (Figure 11) was the currently best performed CoFe₂O₄ and FeCo₂O₄ based nanocomposites, with an onset potential at about −0.09 V (vs. Ag/AgCl) which was very close to −0.01 V of the Pt/C electrode. Alternatively, FeCo₂O₄/hollow graphene spheres exhibited high durability with 92.1% retention in the ORR current density over 86,400 s, contrasting to only 41.1% of the commercial Pt/C. The FeCo₂O₄/hollow graphene spheres could be a substitutable low-cost and efficient material for the ORR process. Overall, the critical composite fabrication strategy is to select proper guest materials and optimize stoichiometric ratios and also control morphologies. This will call for novelties in both optimal morphology designs and proper material selections toward enhancing the electrocatalytic performance in the ORR reaction.

5. Summary and Outlook

In this review, we summarized recent advances in CoFe₂O₄ and FeCo₂O₄ based materials for energy storage and conversion applications, covering supercapacitors, Lithium-ion batteries and fuel cells. In order to realize better electrochemical performance, researchers have developed novel morphologies, structures and composites of CoFe₂O₄ and FeCo₂O₄ based materials. However, there was still some space to optimize performance of CoFe₂O₄ and FeCo₂O₄ based materials. Several recommendations were summarized as follows:

• Previous studies identified the critical roles of morphologies and structures of electrode materials in enhancement of electrochemical performance. Novel structures were desired to be synthesized for further improving their electrochemical activities. It is noteworthy to mention that nanostructured materials should focused considering their high specific areas and more active sites, 3D micron-structured or submicron-structured materials should also be addressed because of their excellent electrochemical performance.
• More work focused on electrode materials, contrasting few works emphasized on electrolytes, separators and counter electrodes. The future works should oversee all involved components for the establishment of optimal systems to realize better performance.
• More works emphasized on fabricating novel morphologies and structures, while few clearly addressed the essential mechanisms on charging-discharging process in energy storage and conversion. This missing correlation between morphologies and structures of CoFe₂O₄ and FeCo₂O₄ based materials has hampered their further optimization. The mechanical studies using DFT method was also highly recommended.
• Current applied approaches to fabricate CoFe₂O₄ and FeCo₂O₄ based materials were limited to some classical methods such as hydrothermal, solvothermal and sol-gel. Those methods likely introduced impurities into electrode materials which later affected their performance to some extent. There was actually some space to refine the current methods but the development of new synthesis strategies of CoFe₂O₄ and FeCo₂O₄ based materials were highly demanding.
• The scale-up preparation of CoFe₂O₄ and FeCo₂O₄ based materials was still challenging for their successful commercial applications. This was also true for other transition metal oxide electrode materials.
• Many works only focused on the improvement of the basic electrochemical performance but missed the relevance toward their practical applications, such as adaptability to the load change and durability under severe working conditions. The current gap implied a long way ahead toward the commercialization.