Supercapacitive Performances of Ternary CuCo2S4 Sulfides.

Currently, ternary CuCo2S4 sulfides are intensively investigated as electrode materials for electrochemical capacitors due to their low cost, high conductivity, and synergistic effect. The research of CuCo2S4 materials for energy storage has gradually grown from 2016. The supercapacitive performances of CuCo2S4 electrodes for electrochemical capacitors are briefly reviewed in this work. The structure, morphology, and particle size of CuCo2S4 are related to the synthesis conditions and electrochemical performances. The thin films of CuCo2S4 nanostructures deposited on conductive substrates and their composites both show better properties than single CuCo2S4. CuCo2S4 and its composites reveal large potential for asymmetric capacitors, delivering high energy densities. However, there is still much new space remaining for future research. The possible development directions, challenges, and opportunities for CuCo2S4 materials are also discussed.

Introduction

With the increasing demand for sustainable new power sources, energy storage devices with high energy densities are being intensively researched. In addition to lithium-ion batteries (LIBs) and full cells, electrochemical capacitors (ECs), or supercapacitors, have also gained much attention due to their advantages including large power density, long lifespan, fast charge–discharge rates, free maintenance, and low cost, etc. ECs are novel energy storage devices that bridge the gap between batteries and conventional capacitors. However, ECs still suffer from low-energy density before extensive application. The essential factor to primarily determine the performance of ECs is the electrode materials. Compared to carbon materials with high surface areas resulting in electric double layer capacitor (EDLC) attribution, the battery-like materials that can provide Faradaic redox reactions are deemed to be more prospective for actual application. Many transition metal compounds including sulfides,[1] oxides,[2] and hydroxides,[3] thus, have been studied for ECs.

Recently, transition metal sulfide has been considered as one of the most prominent materials for ECs and LIBs because of its complex valence and large crystal lattice values.[4] Metal sulfides usually have higher conductivity and activity, lower electronegativity, as well as lower band gap than corresponding oxides. Previous reports found that the presence of Cu in the host sulfides could improve the conductivity and played as a buffer matrix for the volume expansion.[4a] Thus, copper-containing sulfides have been highlighted because of their natural abundance, excellent stability, and low cost. At the same time, compared to binary sulfides, ternary copper sulfides usually exhibit richer redox reactions and higher conductivity, leading to improved electrochemical performances due to the combined contribution from each component and the synergistic effect between individual elements. Many Cu-based ternary sulfides have been developed for ECs, such as CuCo2S4, CuSbS2, Cu2MoS4, Cu2WS4, etc.[1,4a]

In this work, a short overview of the recent development of CuCo2S4 ternary sulfides toward EC electrodes is provided. To the best of our knowledge, little attention is paid to the current progress of CuCo2S4 for ECs. CuCo2S4 materials have been studied recently, and they have shown huge potential. The papers related to CuCo2S4 for ECs began to be published in 2016. Thus, CuCo2S4 materials and composites for ECs are summarized and discussed in this mini-review.

Pure CuCo2S4 Materials

CuCo2S4 is of a spinel phase with the space group of Fd3m, as schematically shown in Figure . Though CuCo2S4 is a very new material to ECs, it is the most widely researched electrode material among all the copper-containing ternary sulfides. The commonly reported approaches to synthesize CuCo2S4 electrodes are the anion exchange and solvothermal methods. The former involves multisteps and needs some hydroxides or oxides as a precursor to be exchanged by sulfur anions at moderate temperatures, even at room temperature.[5] The anion exchange process usually causes a hollow structure through the Kirkendall effect, leading to a high surface area. CuCo2S4 materials can also be directly synthesized by the solvothermal method.[6] It is usually carried out in polyols under a higher temperature than anion exchange.

Figure 1

Crystal structure of the spinel phase of CuCo2S4.

Because the electrochemical performances of CuCo2S4 are influenced by the morphology, structure, and particle size and store charge only in the first few nanometers from the surface, CuCo2S4 materials with abundant mesopores and small particle sizes are favored. Researchers have found that the reaction medium had a large impact on the structure of CuCo2S4 materials.[7,8] Tang and coworkers[7] compared CuCo2S4 materials synthesized in water and polyols, and the material prepared in polyols showed higher surface area and larger specific capacitance than that synthesized in water, even up to 5030 F g–1. Zhu et al.[8a] reported that the solvent and mole content of chemicals determined the microstructure and electrochemical performances of CuCo2S4 materials. Zequine et al.[8b] also reported the morphology dependence of CuCo2S4 on the volume ratio of water to ethanol, and the optimized electrode showed a much higher specific capacitance of 3190.8 F g–1 than its oxide precursor. Other reaction conditions were also investigated. For example, microwave irradiation could accelerate the reaction process to prepare CuCo2S4 nanoparticles in several minutes.[9] Guo et al.[5] found that the reaction temperature influenced the crystallinity and electrochemical properties of CuCo2S4. The partial substitution of Co by Ni can improve the electrochemical performances of CuCo2–NiS4. However, a high content of Ni will produce a multiphased composite.[10] The reaction conditions and chemical composition will greatly affect CuCo2S4 materials and their electrochemical performance.[10,11]

Hollow CuCo2S4 structures are especially expected to show high performances due to the available inner space. Hollow CuCo2S4 microspheres could be prepared by a self-templated method[12] or bubble-supported solvothermal method.[6a]Figure (a) shows the ionic transport process of hollow CuCo2S4 microspheres and the Faradaic reaction equations.[12] The hollow morphology could improve the structural and cycling stability by shortening diffusion pathways for ions. The CuCo2S4 materials with hollow cores demonstrated a specific capacitance of 1137.5 F g–1 at 2 A g–1.[6a]

Figure 2

(a) Illustration of ionic transport process in the CuCo2S4 electrode. Reprinted with permission from ref (12). Copyright 2019, American Chemical Society. (b) The fabrication process of CuCo2S4 hollow nanoneedles on Ni foam. Reproduced with permission from ref (18). Copyright 2016, Royal Society of Chemistry.

Apart from electrode materials, the electrode structure is also an important factor to determine the final performances of ECs. The growth of nanostructured CuCo2S4 materials on current collectors is a highly preferred strategy for EC electrodes due to the reduced resistance, large conductivity, and high utilization efficiency of the materials. A variety of CuCo2S4 structures deposited on different conductive substrates as current collectors have been synthesized and used as work electrodes directly, such as CuCo2S4 nanorod arrays grown on carbon textile,[13] CuCo2S4 microspheres on carbon cloth,[14] CuCo2S4 nanosheets,[15] flower-like CuCo2S4,[16] CuCo2S4 nanowires,[17] CuCo2S4 hollow nanoneedles,[18] and oriented CuCo2S4 nanograss arrays deposited on Ni foam.[19] The synthetic procedure of CuCo2S4 hollow nanoneedles on Ni foam is illustrated in Figure (b), where the hollow structure is formed in the second hydrothermal step through the Kirkendall effect.[18] This strategy can avoid painting electrode materials, without conductive additive and polymer binders. However, it is hard to achieve a large mass loading on given-size conductive substrates. Too high mass loading will cause a low conductivity and weak adhesion between the substrates and thick CuCo2S4 films. Thus, a reasonable mass loading on each electrode should be optimized.

Table shows the electrochemical properties of pure CuCo2S4 materials for ECs. Powder CuCo2S4 samples with various morphologies have different specific capacitance values. However, the thin films of CuCo2S4 grown on conductive substrates usually exhibit higher specific capacitance as compared to powder materials.

Table 1

Summary of Electrochemical Performances of CuCo2S4 Materials and the Nanostructures on Conductive Substrates

shape and structure	electrolyte	specific capacitance	cycling stability	ref
3D nanorods	2 M KOH	515 F g–1 at 1 A g–1	93.3% after 10000 cycles	(5)
hollow spheres	6 M KOH	1137.5 F g–1 at 2 A g–1	94.9% after 6000 cycles	(6a)
mesoporous particles	2 M KOH	752 F g–1 at 2 A g–1	98.1% after 5000 cycles	(6b)
nanoparticles	polysulfide	5030 F g–1 at 20 A g–1	79.5% after 2000 cycles	(7)
nanoparticles	2 M KOH	652 F g–1 at 20 A g–1	95.6% after 5000 cycles	(8a)
nanoparticles	2 M KOH	580 F g–1 at 1 A g–1	99.5% after 6000 cycles	(9)
microspheres	3 M KOH	1566 F g–1 at 2 A g–1	95.7% after 5000 cycles	(12)
nanorod arrays on carbon cloth	3 M KOH	1536.9 F g–1 at 1 A g–1	94.9% after 10000 cycles	(13)
microspheres on carbon cloth	6 M KOH	1200 F g–1 at 1 A g–1	91.2% after 3000 cycles	(14)
nanosheets on Ni foam	3 M KOH	3132.7 F g–1 at 1 A g–1	–	(15a)
nanosheets on Ni foam	2 M KOH	908.9 F g–1 at 4.5 A g–1	91.1% after 2000 cycles	(16)
nanowires on Ni foam	6 M KOH	2446.6 F g–1 at 1 A g–1	82% after 10000 cycles	(17)
nanorods on Ni foam	3 M KOH	2163 F g–1 at 2 A g–1	96.3% after 6000 cycles	(18)
nanorods on Ni foam	2 M KOH	1852 F g–1 at 2 A g–1	96% after 4000 cycles	(19)

CuCo2S4 Composites

Though various CuCo2S4 nanostructures and thin films on conductive substrates have demonstrated promising potential for ECs, there are still some problems. The electrical conductivity and the accessible surface area should be further increased in order to obtain much better performances. To homogeneously improve the conductivity, an effective method is designing a composite consisting of CuCo2S4 and highly conductive materials such as graphite, carbon nanotubes (CNTs), graphene, graphene quantum dots, etc. Carbon materials are ideal choices to integrate with active CuCo2S4, and they can also deliver EDLC. Meanwhile, the presence of guest carbon materials can form 3D architectures with large surface area and reduced crystal size of CuCo2S4.

The composite of CuCo2S4@graphite exhibited a high specific capacitance, two times higher than CuCo2S4, as reported by Chen et al.[20] Nanocrystallites of CuCo2S4 deposited on CNTs were synthesized by the solvothermal method, and the composite showed excellent performances for both LIBs and ECs.[21] As exhibited in Figure (a,b), small CuCo2S4 nanoparticles with size of about 5–20 nm are anchored on the surface of CNTs, and C, N, Cu, Co, and S uniformly distribute throughout the whole structure.[21] Hierarchical CuCo2S4 nanosheets decorated on CNTs were synthesized by a two-step process, and the composite exhibited a specific capacitance of 1690.3 F g–1 at 1 A g–1.[22]

Figure 3

(a) TEM image of single CNTs@CuCo2S4 and (b) corresponding EDS mapping images, which demonstrate that C (red), N (green), Cu (orange), Co (blue), and S (yellow) are homogeneously distributed. Reprinted with permission from ref (21). Copyright 2018, Elsevier. (c) SEM image of CuCo2S4/graphene and the inset showing the SEM image of CuCo2S4. (d) TEM image of CuCo2S4/graphene, and the inset is the size distribution of CuCo2S4. Reprinted with permission from ref (23). Copyright 2019, Elsevier.

(a) TEM image of single CNTs@n class="Chemical">CuCo2S4 and (b) corresponding EDS mapping images, which demonstrate that C (red), N (green), Cu (orange), Co (blue), and S (yellow) are homogeneously distributed. Reprinted with permission from ref (21). Copyright 2018, Elsevier. (c) SEM image of CuCo2S4/graphene and the inset showing the SEM image of CuCo2S4. (d) TEM image of CuCo2S4/graphene, and the inset is the size distribution of CuCo2S4. Reprinted with permission from ref (23). Copyright 2019, Elsevier.

Graphene is also an ideal skeleton to support CuCo2S4 nanoparticles, as shown in Figure (c,d).[23] The CuCo2S4 nanoparticles on graphene are not aggregated, and the sizes of them are uniform. On the contrary, the synthesized CuCo2S4 without graphene (the inset in Figure c) appears to undergo severe aggregation. The TEM image of CuCo2S4/graphene in Figure (d) shows the CuCo2S4 nanoparticles and thin graphene layers. The histogram inserted in Figure (d) reveals that the nanoparticles have an average diameter of 21 nm. N-doped graphene could provide a large surface area to reduce the aggregation and size of CuCo2S4 nanoparticles, too. The composite of CuCo2S4/N-doped graphene showed a specific capacitance of 1005 F g–1 at 1 A g–1 and good rate capability.[24] Graphene quantum dots/CuCo2S4 electrodes demonstrated a specific capacitance of 1725 F g–1 at 0.5 A g–1.[25] The reported electrochemical performances of the composites comprised of CuCo2S4 and carbon materials are summarized in Table . From the above reports, the composites of CuCo2S4 and conductive carbon nanomaterials have a better ability to store energy than pure CuCo2S4.

Table 2

Electrochemical Performances of the Composites of CuCo2S4 and Carbon Materials

composites	electrolyte	specific capacitance	cycling stability	ref
CuCo2S4@graphite on Ni foam	6 M KOH	1244 F g–1 at 50 A g–1	85.3% after 2000 cycles	(20)
CNTs@NC@CuCo2S4	6 M KOH	1064 F g–1 at 1 A g–1	93.6% after 2000 cycles	(21)
CNTs@CuCo2S4	1 M KOH	1690.3 F g–1 at 1 A g–1	95.5% after 10000 cycles	(22)
CuCo2S4 on graphene	3 M KOH	688 F g–1 at 1 A g–1	84.5% after 8000 cycles	(23)
CuCo2S4@N-doped graphene	6 M KOH	1005 F g–1 at 1 A g–1	96.3% after 5000 cycles	(24)
graphene quantum dots@CuCo2S4	3 M KOH	1725 F g–1 at 0.5 A g–1	90% after 10000 cycles	(25)

Another kind of composite is combing CuCo2S4 with other battery-like electrode materials including transition metal hydroxides, sulfides, oxides, etc. Very recently, the composites of CuCo2S4 and battery-like materials begin to draw increasing attention because each component can have redox reactions to storage energy. Ma et al.[26a] reported that CuCo2S4–NiCo2S4 core–shell nanostructures on Ni foam had a large surface area and rapid diffusion of electrolyte ions by numerous channels and that the composite presented excellent electrochemical performances owing to the synergistic effect. A core–shell CuCo2S4-NiCo(OH)2 electrode had a higher capacitance of 2340 F g–1 at 1 A g–1 than any individual component due to both contributions in the core–shell hybrid.[26b] Similarly, Lin et al.[26c] reported that the CuCo2S4@NiMn(OH)2 electrode had a higher specific capacitance than the single component. The structure of CuCo2S4@NiMn(OH)2 on Ni foam is exhibited in Figure (a). A typical TEM image in Figure (b) shows than a CuCo2S4 nanotube as the core is uniformly covered by thin NiMn(OH)2 nanosheets as the shell.[26c] CuCo2S4/CuCo2O4 heterostructures with different S contents were prepared, and they exhibited a high surface area, large conductivity, and rapid electron and ion transport rates.[26d] This kind of composite was just reported in the last two years. It is urgent to develop and research new composites with unique structures and compositions.

Figure 4

(a) SEM image of CuCo2S4@NiMn(OH)2 on Ni foam and (b) TEM image of CuCo2S4@NiMn(OH)2. Reprinted with permission from ref (26c). Copyright 2018, Elsevier. (c) Schematic diagram and (d) a typical photo of CuCo2S4/PAN film. Reprinted with permission from ref (27). Copyright 2018, Elsevier.

(a) SEM image of CuCo2S4@n class="Chemical">NiMn(OH)2 on Ni foam and (b) TEM image of CuCo2S4@NiMn(OH)2. Reprinted with permission from ref (26c). Copyright 2018, Elsevier. (c) Schematic diagram and (d) a typical photo of CuCo2S4/PAN film. Reprinted with permission from ref (27). Copyright 2018, Elsevier.

In addition to carbon and battery-like electrode materials combined with CuCo2S4, polyacrylonitrile (PAN) was also reported. CuCo2S4/PAN flexible film electrodes were prepared by curing the ink directly and investigated for ECs, as reported by Chen et al.[27]Figure (c,d) illustrates the schematic diagram of the preparation of CuCo2S4/PAN film and its photograph. The ink of CuCo2S4/PAN showed excellent heat stability with a specific capacitance of 385 F g–1 at 1 A g–1.

CuCo2S4 Electrodes for Asymmetric ECs

The energy density equation for ECs is E = CV2/2. It can also be increased by enlarging the voltage window. In order to obtain a wide voltage window, building asymmetric ECs (AECs) is an effective way. Generally, AECs can be classified into two kinds, EDLC//redox[5,13,22,15,17,20,22,24] and redox//redox.[15b]

For AECs of CuCo2S4 electrodes, carbon negative electrodes are popularly used, typically activated carbon (AC). The voltage range of AECs is large, usually higher than 1.5 V, revealing practical application. CuCo2S4 and AC electrodes were assembled on an AEC device that delivered an energy density of 50.56 Wh kg–1 within 1.6 V.[5]Figure (a) clearly shows the bunched structure of CuCo2S4 nanorods on carbon textiles. The AEC of CuCo2S4 nanorod arrays//AC had a high energy density of 56.96 Wh kg–1 at 1.6 V, and two AECs connected in series could light up 15 light-emitting diodes (LED), as shown in Figure (b).[13] With CuCo2S4/CNTs as positive electrode and CNTs as negative electrode, an AEC showed a large energy density of 37.32 Wh kg–1 at 800.7 W kg–1 within 1.6 V.[22] A hybrid capacitor of CuCo2S4@NiCo2S4//AC delivered an energy density of 23.4 Wh kg–1 with a voltage window of 1.6 V.[26a] Xu et al.[26d] prepared 3D mesoporous flower-like CuCo2S4/CuCo2O4 composites with compatible interfaces and tunable composition. The AEC devices are fabricated in parallel-plate geometry, as illustrated in Figure (c). The measured specific capacitance and corresponding capacitance retention of the ASC device based on the discharge curves at different current densities are plotted in Figure (d). The AECs of CuCo2S4/CuCo2O4//graphene provided an energy density of 33.2 Wh kg–1 at a power density of 800 W kg–1 in 1.6 V.[26d] The energy densities and power density of the reported CuCo2S4 electrodes in AECs are summarized in Table . As compared to other AEC devices,[2,3] the energy density of CuCo2S4 electrodes is higher. From the above results, both CuCo2S4 and its composites have shown excellent performances as positive electrodes of AECs.

Figure 5

(a) SEM image of CuCo2S4 nanorod arrays on carbon textiles and (b) two AEC devices connected in series could light up 15 LEDs.[13] (c) Schematic illustration of an ASC device and (d) specific capacitance and corresponding capacitance retention of the ASC device at different current densities. Reprinted with permission from ref (26d). Copyright 2018, Elsevier.

Table 3

Summarized Electrochemical Properties of CuCo2S4-Based AECs

positive electrode	negative electrode	electrolyte	voltage	energy density	energy density	ref
CuCo2S4	AC	2 M KOH	1.6 V	50.56 Wh kg–1	4600 W kg–1	(5)
CuCo2S4 on carbon textile	AC	PVA/KOH	1.6 V	56.96 Wh kg–1	320 W kg–1	(13)
CuCo2S4 on carbon textile	AC	PVA/KOH	1.6 V	17.12 Wh kg–1	194.4 W kg–1	(14)
CuCo2S4 on Ni foam	AC	3 M KOH	1.6 V	46.1 Wh kg–1	991.6 W kg–1	(15a)
CuCo2S4 on Ni foam	Fe2O3/graphene	PVA/KOH	1.6 V	89.6 Wh kg–1	663 W kg–1	(15b)
CuCo2S4 on Ni foam	AC	6 M KOH	1.5 V	33.4 Wh kg–1	751.5 W kg–1	(17)
CuCo2S4/graphite	reduced graphene	6 M KOH	1.6 V	58.4 Wh kg–1	797 W kg–1	(20)
CuCo2S4/CNTs	CNTs	1 M KOH	1.6 V	37.32 Wh kg–1	800.7 W kg–1	(22)
CuCo2S4/graphene	graphene	6 M KOH	1.6 V	53.3 Wh kg–1	795 W kg–1	(24)
CuCo2S4@NiMn(OH)2	AC	6 M KOH	1.5 V	45.8 Wh kg–1	1499 W kg–1	(26c)
CuCo2S4/CuCo2O4	graphene	2 M KOH	1.6 V	33.2 Wh kg–1	800 W kg–1	(26d)

In addition to AECs, symmetric capacitors were also researched using CuCo2S4 electrodes. Wang et al.[19] applied CuCo2S4 nanograss@Ni foam electrodes to assemble symmetric ECs. The symmetric device showed an energy density of 31.88 Wh kg–1 at 3.03 kW kg–1 within 1.5 V. More recently, core–shell like nanoparticles of CuCo2S4@NiCo(OH)2 on Ni foam were used as two electrodes to build a symmetric capacitor in 3 M KOH.[26b] It exhibited the power density of 32 Wh kg–1 at 750 W kg–1 within 1.5 V. The energy densities of the two reports are comparable.

Conclusions and Outlooks

CuCo2S4 has multiple valence and high electrical conductivities that endow it superior photocatalyst and electrocatalyst activity. The research on CuCo2S4 materials for energy storage began to boost in 2016, and most reports were recently published. It is a new research focus, and there still is a lot of room for researchers to deeply investigate it in the future.

Some basic features of CuCo2S4 materials such as morphology, particle size, structure, and crystallinity have been proved to be influenced by the synthesis conditions. Thus, they can be finely controlled by the reaction conditions. New methods are still expected to be developed to prepare CuCo2S4 with excellent electrochemical performances.

Designing new composites has been revealed to be an effective method to further improve the electrochemical performance of CuCo2S4. The composites consisting of CuCo2S4 and carbon nanomaterials or metal compounds can exhibit better electrochemical behavior than single CuCo2S4. However, only limited pseudocapacitive materials have been reported to integrate with CuCo2S4 up to now.

The reported electrochemical performances, such as specific capacitance and energy density, for CuCo2S4 and its composites are diverse in a wide range. In addition to various electrolytes and measurement systems, it can be attributed to the difference in both materials including structure, surface area, particle size, etc., and the structure of electrodes, such as mass loading, binder involving or not. Meanwhile, CuCo2S4 and its composites are also potential electrodes for AECs due to their large specific capacitance and high energy densities. Assembling asymmetric and symmetric capacitor devices using CuCo2S4 electrodes are two promising directions.

Very recently, Gao et al.[10] found that the surface of Ni-substituted n class="Chemical">CuCo2S4 was partially oxidized by air to form metal sulfate, and this caused the gradual dissolution of electrode materials in the aqueous electrolyte, leading to an unsatisfied cycling stability. Thus, the possibility of surface oxidization for CuCo2S4 materials is an important issue.