Recent advances in various applications of nickel cobalt sulfide-based materials

Abstract

In recent years, nickel cobalt sulfides (NCSs) have received much attention as promising functional materials in various application fields, mainly due to their much lower price, abundant raw materials and considerable reactive activity with relatively higher electrical conductivity, weaker metal–sulfur bonds and better thermal stability compared to their oxide counterparts. In this review, we will briefly summarize the recent development of NCSs on the aspects of structural design, component optimization and composite preparation. Moreover, we will comprehensively review the recent applications of NCS nanomaterials in various fields (e.g., supercapacitors, batteries, electrocatalysis, photocatalysis, sensors and microwave absorption), and some representative examples will be highlighted for different applications of NCSs. Finally, we will outline the common problems and prospects of NCS nanomaterials used in various applications.

---

Gaofei Xue

Gaofei Xue received his B.S. in Material Physics from Fujian Normal University, China in 2020. He is now a graduate student in the College of Physical Science and Technology, Xiamen University, China. He joined Professor Meidan Ye's group in 2020. His research focuses on flexible materials and devices.

Tian Bai

Tian Bai received his B.S. from the Shaanxi University of Science & Technology, China in 2018. He is now a graduate student in the College of Physical Science and Technology, Xiamen University, China. He joined Professor Meidan Ye's group in 2019. His research focuses on 2D material-based flexible devices.

Weiguo Wang

Weiguo Wang received his B.S. from Fujian Agriculture and Forestry University, China in 2019. He is now a graduate student in the College of Physical Science and Technology, Xiamen University, China. He joined Professor Meidan Ye's group in 2019. His research focuses on flexible materials and devices.

Senjing Wang

Senjing Wang received his B.S. in Measurement Control Technology and Instruments from Xiamen University, China in 2021. He is now a graduate student in the College of Physical Science and Technology, Xiamen University, China. He joined Professor Meidan Ye's group in 2021. His research focuses on energy storage materials and devices.

Meidan Ye

Meidan Ye received her PhD from the College of Chemistry and Chemical Engineering at Xiamen University in 2014. She then joined the Research Institute for Soft Matter and Biomimetics, Department of Physics, College of Physical Science and Technology at Xiamen University as an associate Professor in 2014. She was promoted to a Professor in 2021. Her research interests are on multi-functional materials for flexible devices, i.e., electrochemical energy storage devices and wearable sensors.

---

1. Introduction

In recent years, transition metal chalcogenides have caught much research attention owing to their unique physicochemical characteristics, low cost, and broad application potential.^1–7 Among them, ternary sulfides have drawn rather extensive interest in the field of energy conversion and storage (e.g., solar cells, water splitting, hydrogen generation, supercapacitors, Li/Na-ion (Li–S, Li–O₂, and Zn–air) batteries, etc.) because of their outstanding electrochemical performance.^8,9 In particular, nickel cobalt sulfides (NCSs) have become one of the most concerned ternary sulfides, mainly due to their relatively high electrical conductivity, multiple valence states and rich crystallographic structures.^10,11 It is generally considered that nickel holds tetrahedral sites, while cobalt takes over both octahedral and tetrahedral sites within spinel NiCo₂S₄ (Fig. 1). The mixture of heterogeneous spin states from Ni and Co atoms will result in lattice distortion and subtle atomic arrangement due to the mismatch in the Jahn–Teller distortion degree (Co²⁺, t2g⁶eg¹, strong John–Teller effect; Ni²⁺, t2g⁶eg², no John–Teller effect), which would produce more exposed active octahedral edge sites of NCSs.¹² This is beneficial for more electrochemical reactions (e.g., pseudocapacitor or catalysis) and thus the synergistic effect between Ni and Co can effectively boost the electrochemical performance of NCSs with the coexistence of Ni²⁺, Ni³⁺, Co²⁺, and Co³⁺ ions. Compared to nickel cobalt oxides (Fig. 1), NCSs present relatively lower optical energy band gaps (NiCo₂S₄: 1.2 eV vs. NiCo₂O₄: 2.5 eV) and higher conductivity (∼100 times).^13,14 This enables the lowering of the charge-transfer resistance of NCSs and facilitates charge transport when they are applied in energy conversion and storage systems. Theoretical calculation results indicate that NiCo₂S₄ has a smaller band gap than NiCo₂O₄ (Fig. 1d and e), suggesting its higher electric conductivity. And NiCo₂S₄ possesses a lower diffusion energy than NiCo₂O₄ (Fig. 1c), revealing that ions can more easily diffused in NiCo₂S₄.¹⁵ The replacement of oxygen by sulfur for NCSs will create more flexible structures since the electronegativity of sulfur is lower than that of oxygen.¹⁶ This can alleviate the structural deformation originating from the intercalation of electrolyte ions and thus promote the reactions between electrolyte ions and NCSs, yielding much enhanced electrochemical performance.

Fig. 1 Schematic diagram of the crystal structures of (a) NiCo₂S₄ and (b) NiCo₂O₄. (c) The Li ion diffusion battier energies and (d and e) the density of states (DOS) of NiCo₂S₄ and NiCo₂O₄ calculated by using density functional theory (DFT) simulation.¹⁵ Reproduced from ref. 10 with permission from Elsevier.

In recent years, much effort has been devoted to the development of NCSs, which can be roughly divided into several aspects.

The first is the design of micro-/nano-structures (Fig. 2 and 3). In addition to conventional nanostructures (e.g., zero-dimensional (0D) nanoparticles,¹⁷ one-dimensional (1D) nanowires/nanorods/nanotubes,^18,19 and two-dimensional (2D) nanosheets²⁰), some special morphologies as well have been designed, such as nanotube-woven hexagonal microsheets,²¹ nanosheet-constructed hollow nanocages,²² nanoparticle-stacked microspheres,²³ hexagonal nanoplate-assembled microstructures,²⁴ and so on. Such hierarchical structures are beneficial to further amplify the active reaction sites, facilitate the charge transport and promote the ion diffusion, ultimately strengthening the performance of NCSs.^25–28 Although the hydrothermal/solvothermal method is still the most common way to synthesize the nanostructures of NCS materials,²⁹ multi-step strategies combining different processes (e.g., solution reactions (hydrothermal/solvothermal, sol–gel and chemical bath reactions), electrodeposition and high-temperature calcination) are becoming more and more popular to construct high-performance morphologies of NCSs.^30,31 More impressively, in situ growth of NCS nanostructured films directly onto conductive substrates (e.g., carbon cloth and Ni foam; Fig. 2 and 3d–f) is also well accepted mainly because it enables the simplification of the synthesis processes, removal of the use of binders, avoidance of the aggregation of nanomaterials, and reduction of the interfacial resistance between NCS films and substrates, therefore improving the performance of NCSs.^19,32

Fig. 2 Schematic diagrams of the crystal structures, nanostructures and applications of NCSs.

Fig. 3 Some representative NCS structures: transmission electron microscopy (TEM) images of (a) NiCo₂S₄ @ N-doped carbon hollow capsules,⁴⁸ (b) NiCo₂S₄/Co₉S₈ submicro-spindles,⁴⁹ and (c) NiCo₂S₄ double-shelled ball-in-ball hollow spheres;⁵⁰ SEM images of (d) NiCo₂S₄ nanosheets @ Ni foam,⁵¹ (e) caterpillar-like NiCo₂S₄ arrays @ carbon cloth,⁵² and (f) NiCo₂S₄ nanosheets @ Ni columns.⁵³ Reproduced from ref. 17, 18, 20 and 21 with permission from the Royal Society of Chemistry. Reproduced from ref. 19 and 22 with permission from American Chemical Society.

The second is the study of the stoichiometric effect. For ternary or quaternary semiconductors, the stoichiometric ratio of their elements is usually adjustable, which also plays an important role in their properties and device performance.^33,34 A variety of studies have been performed to investigate this stoichiometric effect on NCSs, and various NCSs with different element stoichiometry have been prepared, such as NiCo₂S₄,^19,35 CoNi₂S₄,^36,37 Ni_1.4Co_1.6S₄,³⁸ Ni_1.5Co_1.5S₄,³⁹ (Co_0.5Ni_0.5)₉S₈,⁴⁰ (Ni_0.33Co_0.67)₉S₈,⁴¹ Ni_1.77Co_1.23S₄,⁴² Ni_4.5Co_4.5S₈,⁴³ Ni_0.67Co_0.33S,⁴⁴etc. The comparison of the relative performance of NCSs with different element stoichiometry has also been discussed in depth (Fig. 4).⁴⁵ It is proposed that the high conductivity of NCSs is mainly due to the occupation of Ni in the tetrahedral sites of the spinel structure, the low bandgap and the synergistic coupling effect between Co and Ni with p-type (Co) and n-type (Ni) doping.⁴⁶ Then, the different ratios of Ni/Co contents in NCSs will influence their performance in applications. For example, three NCSs with varied Ni/Co ratios (Ni_0.8Co_2.2S₄, Ni_1.5Co_1.5S₄, and Ni_2.2Co_0.8S₄) were synthesized to study the composition effect on their electrocatalytic activity.⁴⁷ It was found that Ni_2.2Co_0.8S₄ had the highest current density and the lowest overpotential for the oxygen evolution reaction (OER) due to its largest content of Ni³⁺ with the conversion of Ni³⁺ to Ni⁴⁺ for producing more active sites to adsorb –OOH, while Ni_1.5Co_1.5S₄ exhibited the highest current density for the oxygen reduction reaction (ORR) owing to the larger content of Ni²⁺ and Co²⁺ with superior conductivity and the synergetic coupling between Ni and Co.⁴⁷

Fig. 4 SEM images (a–d) and electrochemical performance (e–g) of metal sulfides composed of different Ni/Co molar ratios: (a) pure Co, (b) pure Ni, (c) 1/2, and (d) 2/1.⁴⁵ Electrochemical performance (h–j) of NiCo₂S₄ films with different concentrations of surface sulfur vacancies.²⁰ Reproduced from ref. 60 with permission from American Chemical Society. Reproduced from ref. 61 with permission from the Royal Society of Chemistry.

The third is the fabrication of composites. NCS-based composites also have been widely fabricated to compensate for the shortcomings of NCSs (e.g., relatively low stability and slow mass transfer/reaction kinetics). NCSs can be rationally integrated with lots of other materials, for example, metal oxides (e.g., CeO₂,⁵⁴ CoMoO₄,⁵⁵ NiCo₂O₄,⁴³ Co₃O₄,⁵⁶etc.), sulfides (e.g., FeCo₂S₄,⁵⁷ Co₉S₈,⁵⁸ MgS,⁵⁹ MoS₂,⁶⁰ SnS₂,⁶¹etc.), selenides (e.g., Co_0.85Se,⁶² MoSe₂,⁶³etc.), phosphides (e.g., NiCoP,⁶⁴ NiFeP,⁶⁵etc.), carbon-based materials (e.g., carbon nanotubes (CNTs),⁶⁶ graphene,^67–69 carbon dots,⁷⁰ mesoporous carbon spheres,⁷¹etc.), polymers (e.g., polypyrrole,⁷² polyanilnie,⁷³etc.), layered double hydroxides (LDHs; NiMn-LDHs,⁷⁴ CoNo-LDHs,⁷⁵ NiFe-LDHs,⁷⁶etc.), two-dimensional titanium carbide (MXene),⁷⁷ and others (e.g., g-C₃N₄,⁷⁸ Ag,⁴⁴etc.). Besides, element doping (e.g., Mn,⁷⁹ and Cu⁸⁰) also have been widely used to further enhance the performance of NCSs.

The fourth is the development of applications (Fig. 2). NCSs are one type of the most popular electrode material used in supercapacitors due to their excellent electrochemical activity.^53,81 Besides, NCSs have been gradually applied to other electrochemical energy storage systems over the years, mainly including Li/Na-ion batteries,^18,82 Li–O₂ batteries,⁸³ Zn–air batteries,⁶⁹ and Li–S batteries.³⁵ NCSs are also able to act as effective counter electrodes in dye/quantum dot-sensitized solar cells (DSCs/QDSCs).^84,85 NCSs as well have great activity in photocatalytic reactions, such as the degradation of organic pollution and hydrogen evolution.^86,87 Moreover, NCSs can be efficient electrocatalysts for overall water splitting with bi-functions of the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER).^88,89 Particularly, NCSs can be used as bio-sensing materials due to their attractive electrochemical characteristics⁹⁰ and microwave absorbers because of their excellent magnetic loss properties.⁹¹

For NCSs, some articles have reviewed their applications in supercapacitors and batteries.^1,10,11 However, few reviews comprehensively summarize promising applications for NCSs in different fields. In this review, we focus on the recent applications of NSCs in a variety of fields in the last few years. We discuss some representative studies about the applications of NCSs in energy storage devices, electrocatalysis, photocatalysis, sensors and microwave absorption. We highlight the intercommunity of NCSs in different application fields. Finally, the general problems and future prospects of NCSs for various applications are analyzed and emphasized.

2. Applications of NCSs in energy storage devices

2.1. Supercapacitors

NCSs have been intensively applied in supercapacitors due to their relatively higher conductivity and more redox sites in comparison with their corresponding oxides and binary sulfides. It is commonly considered that the pseudocapacitive performance of NCSs is mainly due to the Faraday redox reactions between Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺. In an alkaline aqueous electrolyte, the reversible Faraday peaks can be associated with the reversible redox reactions from NiCo₂S₄ to NiSOH, CoSOH and CoSO species (Fig. 5). The theoretical capacitance of NiCo₂S₄ in supercapacitors is 2534 F g⁻¹.¹

Fig. 5 Schematic diagram and reactions of NCSs in supercapacitors and Li-ion batteries.

In order to amplify the charge storage ability of NCSs, a series of strategies have been developed (Table 1), including regulating the element ratio (e.g., NiCo₂S₄,^19,35 CoNi₂S₄^36,37 and NiCo₂S_4−x¹⁹), tuning the morphological structures in different dimensions (Fig. 6a and b),^45,92–94 doping other elements (e.g., Fe,⁹⁵ Mn,⁹⁶ P,²⁹ and V;⁹⁷Fig. 6d–f), and integrating with other materials (e.g., CNTs,⁹⁸ graphene,⁶³ Nb₂O₅,⁹⁹ Ni(OH)₂,¹⁰⁰ FeCo₂S₄,⁵⁷ MXenes,⁷⁷etc.). For example, the defect engineering strategy enables the production of rich sulfur vacancies on the surface of NCS crystals through various methods, such as low-temperature plasma induction (Fig. 4h–j),²⁰ Ar/H₂ gas-assisted annealing,²³ and NaBH₄ solution reduction.³⁰ The existence of sulfur vacancies will change the electronic structures of NCSs to increase their conductivity and reactive sites for enhanced electrochemical performance.²⁰ Besides, the 3D hierarchical structures of NCSs (Fig. 6) have been widely designed and fabricated for high-performance supercapacitors,^93,101–103 mainly because they have attractive properties to offer sufficient surface areas for more faradaic redox reactions, and build fast channels for effective charge transfer and ion diffusion.

Table 1 Performance of different NCS-based materials in supercapacitors

Material	Capacitance/capacity of electrodes	Capacitance/capacity of devices	Energy density/power density of devices	Cycling stability of devices	Ref.
Ni3S2/CoNi2S4 porous network	2435 F g^−1 @ 2 A g^−1	175 F g^−1 @ 1 A g^−1	40.0 W h kg^−1/17.3 kW kg^−1	92.8% capacitance retention after 6000 cycles @ 10 A g^−1	111
ZnS/NiCo2S4/Co9S8 nanotubes	1618.1 F g^−1 @ 1 A g^−1	—	66 W h kg^−1/0.75 kW kg^−1	62% capacitance retention after 5000 cycles @ 5 A g^−1	112
Ni–Co–S nanosheets @ graphene foam	2918.1 F g^−1 @ 1 A g^−1	209.82 F g^−1 @ 1 A g^−1	79.3 W h kg^−1/0.83 kW kg^−1	54.02% capacitance retention after 10 000 cycles @ 5 A g^−1	113
Zn-doped Ni–Co–S nanocomposite	2668 F g^−1 @ 1 A g^−1	150 F g^−1 @ 1 A g^−1	60.2 W h kg^−1/0.85 kW kg^−1	91.2% capacitance retention after 10 000 cycles @ 5 A g^−1	114
NiCo2S4/MXene	1028C g^−1 @ 1 A g^−1	171.2 F g^−1 @ 1 A g^−1	68.7 W h kg^−1/0.85 kW kg^−1	89.5% capacitance retention after 5000 cycles @ 5 A g^−1	77
NiFeP@NiCo2S4 nanosheets	874.4C g^−1 @ 1 A g^−1	197.6 F g^−1 @ 1 A g^−1	87.9 W h kg^−1/0.43 kW kg^−1	85.2% capacitance retention after 10 000 cycles @ 8 A g^−1	65
MoS2/NiCo2S4@C hollow microspheres	250 mA h g^−1 @ 2 A g^−1	120 F g^−1 @ 1 A g^−1	53.01 W h kg^−1/4.20 kW kg^−1	90.1% capacitance retention after 10 000 cycles @ 10 A g^−1	115
Carbon nanofiber/NiCo2S4 nanosheets @ polypyrrole	2961 F g^−1 @ 1 A g^−1	163.3 F g^−1 @ 1 A g^−1	44.45 W h kg^−1/0.70 kW kg^−1	85.1% capacitance retention after 5000 cycles @ 5 A g^−1	116
P-doped NiCo2S4−x nanotubes	1806.4C g^−1 @ 1 A g^−1	186.1 F g^−1 @ 1 A g^−1	68.2 W h kg^−1/0.80 kW kg^−1	97% capacitance retention after 10 000 cycles @ 30 A g^−1	19
NiMn-LDH@NiCo2S4 nanotubes	1018C g^−1 @ 1 A g^−1	193.0 F g^−1 @ 0.5 A g^−1	60.3 W h kg^−1/0.38 kW kg^−1	86.4% capacitance retention after 10 000 cycles @ 10 A g^−1	74
CoNi2S4 microspheres	1836.6 F g^−1 @ 1 A g^−1	100.7 F g^−1 @ 1 A g^−1	38.9 W h kg^−1/0.85 kW kg^−1	101.2% capacitance retention after 50 000 cycles @ 5 A g^−1	106
Ag@Ni0.67Co0.33S forest-like nanostructures	296.4 mA h g^−1 @ 2 mA cm^−2	1104.14 mF cm^−2 @ 5 mA cm^−2	0.36 mW h cm^−2/3.81 mW cm^−2	83.6% capacitance retention after 8000 cycles @ 20 mA cm^−2	44

---

Fig. 6 Schematic diagram (a) and electrochemical performance (b and c) of NiCo₂S₄ nanosheets @ Ni column arrays.⁵³ SEM image (d) and electrochemical performance (e and f) of P-doped Ni–Co–S@C hierarchical structures.¹⁰⁴ Reproduced from ref. 22 with permission from American Chemical Society. Reproduced from ref. 78 with permission from Elsevier.

It is worth noting that NCSs frequently show obvious battery-like charge storage behavior, that is, displaying typical redox peaks in cyclic voltammogram curves (CVs; vs. near-rectangular shapes or broadened peaks for capacitors; Fig. 6c) and clear discharge platforms in galvanostatic charge/discharge curves (GCDs; vs. quasi-triangular shapes for capacitors; Fig. 6e).¹⁰⁵ It means that the reaction kinetics of NCSs in the charge storage processes are partially determined by the diffusion-controlled redox behavior (i.e., , where I is the current and v is the scan rate in CVs), while the reaction kinetics of pseudocapacitance are dominated by the surface-controlled faradaic process (i.e., I ∝ v).⁵⁸ Thus, the corresponding devices based on battery-like NCS electrodes are named hybrid supercapacitors due to the capacity contribution both from the diffusion-controlled and surface redox processes (Fig. 6f).^{44,81,106–109} However, we find that the contribution ratio of these two kinds of electrochemical behavior for NCSs is significantly influenced by the micro-/nano-structures of NCSs. The electrochemical performance of four types of NiCo₂S₄ hierarchical structures assembled from similar nanorod building blocks was investigated,¹¹⁰ and it was indicated that the hat/flower-like structures with a compact packaging way showed unclear CV redox peaks and GCD discharge platforms, suggesting that the diffusion-controlled process was limited and the capacity was mainly from the surface electrochemical process. In contrast, sphere/brush-like structures with a loose and orderly assembly way presented more obvious CV redox peaks and GCD discharge platforms, revealing a higher utilization of active materials from the increased contribution of diffusion-controlled faradaic reactions in the inner part of the electrode, and thus they had larger capacity.¹¹⁰ Moreover, NCSs and their composites are commonly in situ grown on planar substrates (e.g., carbon cloth, Ni foam and metal meshes), and they usually show clear battery-like behavior,^58,81 but when the planar substrates are changed into ultrathin metal wires with a diameter on a scale of tens of micrometers and a high aspect ratio (Fig. 7a–d), clear CV redox peaks and GCD discharge platforms disappear (Fig. 7e and f), and most capacity is from the surface pseudocapacitance because of the relatively large surface area of ultrathin wires (Fig. 7g and h).⁵⁹ Therefore, it is possible to modify NCS electrodes on different aspects (e.g., components, morphologies, substrates and so on) to achieve high-performance electrochemical energy storage devices.

Fig. 7 SEM images (a, c and d), schematic diagram (b) and electrochemical performance (e–h) of NiCo₂S₄/MgS composites.⁵⁹ Reproduced from ref. 28 with permission from the Royal Society of Chemistry.

2.2. Batteries

In recent years, NCSs have also been employed as effective electrode materials in other electrochemical energy storage devices (Table 2), such as Li/Na/Zn-ion, Li–S, Li–O₂ and Zn–air batteries, similarly because of their intrinsic properties of high electrochemical activity stemming from their rich redox reactions with multi-valence states and relatively high electrical conductivity with the synergetic effect from Ni and Co atoms (Fig. 5). The theoretical capacity of NCSs in Li-ion batteries is 703 mA h g⁻¹.¹ For example, NiCo₂S₄ hollow nanowires (Fig. 8a) were uniformly grown on a 3D N-doped carbon nanosheet substrate derived from a bio-polymer of Chitin (poly(b-(1,4)-N-acetyl-D-glucosamine)),¹⁸ which exhibited attractive long-term cycling stability with a revisable capacity of 1198 mA h g⁻¹ at 500 mA g⁻¹ after 500 cycles (Fig. 8b). Besides, (Co_0.5Ni_0.5)₉S₈ hollow microspheres (Fig. 8c) were converted from the Ni–Co metal–organic framework (Ni–Co-MOF) and then modified with N-doped carbon (N–C).⁴⁰ When applied as the anode in Na-ion batteries with Na₃V₂(PO₄)₃@C as the cathode, the full cells delivered a capacity of 737.6 mA h g⁻¹ at 0.5 A g⁻¹ within 0.01–3.7 V after 50 cycles. It was found that the composite electrodes had a hybrid electrochemical charge storage behavior and the pseudocapacitive contribution was up to 88.8% at 1 mV s⁻¹ (Fig. 8d). In addition, NiCo₂S₄ porous nanoneedle arrays were in situ grown on N/S co-doped carbon cloth and displayed high sulfur loading capability due to the rich mesoporous structure and a great ability to absorb and convert polysulfide intermediates owing to the high electronic conductivity and catalytic activity.¹¹⁷ Therefore, NCS-based materials are as well promising for Li–S batteries.

Table 2 Performance of some representative NCS-based materials in different kinds of batteries

Materials	Specific capacity	Cycling stability	Remarks	Ref.
Li-ion batteries
NiCo2S4/SnS2 hollow spheres (anode)	1260 mA h g^−1 @ 0.1 A g^−1	627 mA h g^−1 @ 0.5 A g^−1 after 300 cycles	99 mA h g^−1 @ 0.1 A g^−1 within 2.0–5.0 V (full cells with NMC111 cathodes and NiCo2S4/SnS2 anodes)	61
NiCo2S4@C hollow microspheres (anode)	1946.7 mA h g^−1 @ 0.2 A g^−1	1242.8 mA h g^−1 @ 2 A g^−1 after 1000 cycles	Multi-shelled NiCo2S4 hollow microspheres were obtained by a multi-step method with solvothermal reactions, direct pyrolysis under air atmosphere and calcination in Ar/H2S gas	126
NiCo2S4 hollow nanowires (anode)	1437.9 mA h g^−1 @ 0.5 A g^−1	1198 mA h g^−1 @ 0.5 A g^−1 after 500 cycles	Chitin-derived N-doped carbon was used as a 3D substrate to support NiCo2S4 nanowires	18
Na-ion batteries
NiCo2S4 @ N-doped carbon nanoparticles (anode)	742.2 mA h g^−1 @ 0.2 A g^−1	395.6 mA h g^−1 @ 6 A g^−1 after 5000 cycles	Ether-based electrolyte: sodium trifluoromethanesulfonate (NaCF3SO3)/diethylene glycol dimethyl ether (DEGDME)	120
Flower-like NiCo2S4 structures (anode)	748 mA h g^−1 @ 0.1 A g^−1	376 mA h g^−1 @ 2 A g^−1 after 500 cycles	The capacitance contribution is up to 89% of total capacity @ 2 mV s^−1	82
(Co0.5Ni0.5)9S8 @ N-doped carbon hollow spheres (anode)	945.1 mA h g^−1 @ 0.1 A g^−1	723.7 mA h g^−1 @ 1 A g^−1 after 100 cycles	847.5 mA h g^−1 @ 0.5 A g^−1 within 0.01–3.7 V (full cells with Na3V2(PO4)3@C cathodes and (Co0.5Ni0.5)9S8/N–C anodes)	40
Zn-ion batteries
NiCoP/NiCo2S4 nanosheets (cathode)	251.1 mA h g^−1 @ 10 A g^−1; 190.3 mA h g^−1 @ 50 A g^−1	96.9% capacity retention @ 100 mV s^−1 after 5000 cycles for full cells	265.1 mA h g^−1 @ 5 A g^−1 at 1.7 V (full cells with NiCoP/NiCo2S4 cathodes and Zn anodes)	127
NiCo2S4−x nanotubes @ carbon cloth (cathode)	298.3 mA h g^−1 @ 0.5 A g^−1	144.4 mA h g^−1 @ 5.0 A g^−1 after 1000 cycles (85% retention)	146.3 mA h g^−1 @ 5 A g^−1 and 84.7% retention after 1500 cycles (flexible solid-state NiCo2S4−x@CC//Zn@CC with a sodium polyacrylate hydrogel electrolyte)	34
Li–S batteries
NiCo2S4 nanosheets @ carbon textile (CT) (cathode)	1600 mA h g^−1 @ 0.1C	836 mA h g^−1 @ 0.5C after 500 cycles (0.018% per cycle)	NiCo2S4 @ CT can be as a bifunctional interlayer for the S@MWCNT cathode with higher adsorptive ability for polysulfides	35
Hollow acicular-like NiCo2S4 microspheres (cathode)	1387 mA h g^−1 @ 0.2C	543 mA h g^−1 @ 2C after 500 cycles (0.06% per cycle)	A flexible Li–S battery with a sulfur loading of 8.9 mg cm^−2 has a capacity of 720 mA h g^−1 (6.52 mA h cm^−2) after 65 cycles at 0.1C	128
NiCo2S4−x hollow microspheres (cathode)	1304.5 mA h g^−1 @ 0.2C; 628.9 mA h g^−1 @ 5C	598.2 mA h g^−1 @ 1C after 500 cycles (0.0754% per cycle)	NiCo2S4−x has strong chemical adsorption ability toward LiPSs	23
Zn–air batteries
(Ni,Co)S2 nanosheets (cathode)	Specific capacity: 842 mA h gZn^−1 @ 5 mA cm^−2; charge/discharge voltage gap: 0.45 V @ 2 mA cm^−2; peak power density: 153.5 mW cm^−2	480 h @ 2 mA cm^−2	Cell charge/discharge voltage: 1.71/1.26 V @ 10 mA cm^−2	129
NiCo2S4 nanoparticles @ carbon nitrogen nanosheets (cathode)	Specific capacity: 801 mA h gZn^−1 @ 10 mA cm^−2; charge/discharge voltage gap: 0.69 V @ 10 mA cm^−2; energy density: 1025 W h kg^−2	180 h @ 10 mA cm^−2 (1000 cycles)	Cell charge/discharge voltage: 1.94/1.25 V @ 10 mA cm^−2	78
Li–O 2 batteries
NiCo2S4 nanorods @ carbon textile (cathode)	Specific capacity: 4506 mA h g^−1 @ 750 mA g^−1; charge/discharge voltage gap: 0.69 V @ 10 mA cm^−2; energy density: 1025 W h kg^−2	500 cycles with a cut-off capacity of 1000 mA h g^−1 @ 500 mA g^−1	Charge overpotential: 1.17 V	83
NiCo2S4 nanoflakes/S-doped carbon nanosheets (cathode)	Specific capacity: 14 173 mA h g^−1 @ 150 mA g^−1; charge/discharge voltage gap: 0.69 V @ 10 mA cm^−2; energy density: 1025 W h kg^−2	1704 h with a limited specific capacity of 1000 mA h g^−1 @ 150 mA g^−1	Initial overall overpotential: 1.27 V with 1.01 and 0.26 V at charge and discharge stages (1.01/0.26 V) with 68% round-trip efficiency	130

---

Fig. 8 SEM image (a) and cycling performance (b) of NiCo₂S₄/N-doped carbon.¹⁸ SEM image (c) and capacitive contribution (d and e) of (Co_0.5Ni_0.5)₉S₈/N-doped carbon.⁴⁰ Reproduced from ref. 49 and 12 with permission from the Royal Society of Chemistry.

Recently, besides the ex situ measurement ways,¹¹⁸in situ characterization methods (e.g., in situ X-ray diffraction (XRD), photoluminescence (PL) and electrochemical impedance spectroscopy (EIS)) have been extensively developed to reveal more details about the charge storage processes for batteries. Thus, some studies have also reported the in situ characterization and analysis of the working mechanism of NCSs in Li/Na-ion batteries.^{41,66,119–122} For example, in situ XRD of the NiCo₂S₄@CNT electrodes in Li/Na-ion batteries showed that in the first discharge process with Li/Na ion intercalation, phase change occurred with the formation of Ni/Co particles, Li₂S and Na₂S, while in the charge process, NiS and CoS would generate due to the deintercalation of Li/Na ions (Fig. 9).⁶⁶ In addition, the in situ EIS analysis of the interfacial properties and charge transfer resistance of NiCo₂S₄@carbon electrodes also confirmed the phase change with the conversion of NiCo₂S₄ into NiS_x and CoS_x after 200 cycles.¹²⁰ The participation of carbon materials enabled the improvement of the electrical conductivity and ensured the high reversibility of structural and phase transition of NiCo₂S₄ during charge/discharge cycles, thus enhancing the stability of NCS-based batteries.^66,120 Therefore, it will be very beneficial to deeply elucidate the working mechanism and key processes in electrochemical energy storage devices by combining in situ and ex situ characterization methods, ultimately guiding the design of more high-performance devices.

Fig. 9 Schematic diagram of the charge storage mechanism of NiCo₂S₄@CNT electrodes in Li/Na-ion batteries.⁶⁶ Reproduced from ref. 53 with permission from Elsevier.

For the applications of Zn–air and Li–O₂ batteries, the electrocatalytic activity and durability in the oxygen reduction reactions (ORR) for the discharge process and the oxygen evolution reactions (OER) for the charge process are very essential for advanced cathode electrode materials. Many studies have confirmed that NCSs are feasible for metal–oxygen batteries as a class of bifunctional cathodes,^{69,78,123–125} still due to their excellent electrochemical activity and reversibility in redox reactions. For example, NiCo₂S₄ nanoparticles were integrated with carbon nitrogen nanosheets and demonstrated efficient bifunctional electrocatalytic performance (i.e., a positive half-wave potential of 0.83 V for the ORR and a low overpotential of 360 mV at 10 mA cm⁻² for the OER).⁷⁸ The corresponding Zn–air batteries showed a maximum power density of 92 mW cm⁻², a capacity of 801 mA h g⁻¹, an energy density of 1025 W h kg⁻¹, and outstanding cycling durability for 180 h with 1000 cycles at 10 mA cm⁻², which were comparable or superior to those (106 mW cm⁻², 759 mA h g⁻¹, 925 W h kg⁻¹, and 69 h) of the commercial Pt/C–RuO₂-based one.

3. Applications of NCSs in water splitting

Nowadays, electrochemical water splitting is widely recognized as one of the most prospective strategies to generate hydrogen (Fig. 10a), which is mostly accepted as an ideal energy choice to substitute for conversional fossil fuels. There are two important processes involved in the electrocatalysis of water, that is, the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), which require highly active electrocatalysts to overcome the large overpotential (1.6–2.0 V) of these reactions. To date, IrO₂ and Pt have been considered as the best catalysts for the OER and HER; nevertheless, the high price of noble metals restricts their large-scale applications. Thus, alternative electrocatalysts with low cost and high activity have attracted much attention. Because of the co-existence of Ni and Co with rich active sites from the redox couples (i.e., Co³⁺/Co²⁺ and Ni³⁺/Ni²⁺) in crystal structures, NCSs show higher conductivity and catalytic capability than binary sulfides (Fig. 10b–d), and then have been applied in electrocatalytic water splitting as a promising type of bifunctional catalyst (Fig. 10a and Table 3).^131–133

Table 3 Performance of different NCS-based catalysts for water splitting

Catalyst	Overpotential of the OER	Overpotential of the HER	Water splitting	Electrolyte	Stability	Ref.
Core–shell NiCo2S4 nanorods	200 mV vs. 330 mV (IrO2) @ 40 mA cm^2	190 mV @ 10 mA cm^−2	1.57 V vs. 1.54 V (IrO2/C–Pt/C) @ 10 mA cm^2	1.0 M KOH	12 h	141
NiCo2S4@Ni3V2O8	290 mV vs. 320 mV (IrO2) @ 35 mA cm^2	—	—	1.0 M KOH	10 h	132
NiCo2S4 nanorod arrays	220 mV vs. ∼320 mV (IrO2) @ 30 mA cm^2	222 mV vs. ∼30 mV (Pt/C) @ 30 mA cm^2	1.64 V @ 20 mA cm^−2	1.0 M KOH	18 h	142
NiCo2S4 nanoflakes	319 mV vs. ∼404 mV (NiCo2O4) @ 100 mA cm^2	169 mV vs. ∼238 mV (NiCo2O4) @ 10 mA cm^2	1.61 V @ 10 mA cm^−2	1.0 M KOH	70 h	88
Crossed NiCo2S4 nanowires	256 mV vs. 340 mV (IrO2) @ 40 mA cm^2	193 mV vs. ∼244 mV (Ni3S2) @ 10 mA cm^2	1.59 V @ 10 mA cm^−2	1.0 M KOH	12 h	134
NiCo2S4 nanosheets	247 mV vs. 264 mV (NiCo3S4) @ 10 mA cm^2	106 mV vs. ∼181 mV (NiCo3S4) @ 10 mA cm^2	1.62 V @ 10 mA cm^−2	1.0 M KOH	72 h	89
N-doped carbon–NiCo2S4 hollow nanotubes	280 mV vs. ∼290 mV (IrO2) @ 10 mA cm^2	183 mV vs. ∼214 mV (NiCo2S4) @ 10 mA cm^2	1.60 V @ 10 mA cm^−2	1.0 M KOH	15 h	135
Carbon dots/NiCo2S4/Ni3S2 nanorods	116 mV @ 10 mA cm^2	127 mV @ 10 mA cm^2	1.50 V @ 10 mA cm^−2	1.0 M KOH	12 h	131
NiCo2S4@N,S co-doped reduced graphene oxides	253 mV vs. 267.8 mV (RuO2) @ 10 mA cm^2	92.7 mV vs. 61.9 mV (Pt/C) @ 10 mA cm^2	1.58 V in 1 M KOH and 1.907 V in 1 M phosphate buffer solution @10 mA cm^−2	1.0 M KOH	10 h	136

---

Much effort has been devoted to the rational engineering of the components and nanostructures of NCSs,^{88,89,131,132,134,135} in order to further enlarge their reactive surface areas, improve their electrical conductivity and enhance their catalytic stability, ultimately synergistically strengthening their electrocatalytic performance. For example, nanorod assembled NiCo₂S₄ microspheres were in situ planted onto N,S co-doped reduced graphene oxides,¹³⁶ which exhibited outstanding bifunctional electrocatalytic ability in the HER and OER with a low cell voltage of 1.58 V in 1 M KOH for overall water splitting. More impressively, they also had electrocatalytic capability in a neutral electrolyte (1 M phosphate buffer solution) with a cell voltage of 1.91 V, which is very meaningful to avoid the corrosion of devices from alkaline electrolytes and reduce the operating cost for practical applications. Besides, NiCo₂O₄, NiCo₂S₄ and NiCo₂Se₄ nanosheets were separately grown on conductive carbon papers to compare their behavior in electrocatalytic water splitting.¹³⁷ For the HER at 10 mA cm⁻² , the overpotential value for NiCo₂Se₄ (56 mV) was relatively lower than those of NiCo₂S₄ (87 mV) and NiCo₂O₄ (141 mV), but still slightly higher than that of Pt/C (41 mV). However, for the OER at 25 mA cm⁻², NiCo₂S₄ possessed the lowest overpotential value of 251 mV as compared to other materials (i.e., NiCo₂O₄: 372 mV; NiCo₂Se₄: 321 mV; IrO₂/C: 358 mV). Accordingly, NiCo₂S₄ and NiCo₂Se₄ were employed as the anode and cathode for overall water splitting, respectively, yielding a low cell voltage of 1.51 V for driving the generation of H₂ and O₂ bubbles at 10 mA cm⁻². Theoretical calculation unraveled that the center position of the d band for electrodes was closely related to the electrocatalytic activity. It was found that NiCo₂S₄ and NiCo₂Se₄ had the d band center closer to the Fermi level, suggesting increased unoccupied orbitals due to the existence of S and Se, and the upward movement of the antibonding d state with strengthened adsorption interaction with reactive species (e.g., H₂O and H*) for enhanced electrocatalytic performance.¹³⁷

Fig. 10 (a) Schematic diagram of the water splitting process based on NCSs. (b) Nyquist plots, and (c) oxygen reduction reaction and oxygen evolution reaction curves of different catalysts. (d) Stability tests of the NCS catalyst.¹³³ Reproduced from ref. 133 with permission from American Chemical Society.

In addition, due to their excellent electrocatalytic activity, NCSs have also been proved to be effective electrocatalysts for the alkaline urea electrooxidation reaction in urea fuel cells, which are accepted as one promising strategy to use urea from wastewater to produce electricity and hydrogen.^52,138–140 For example, NiCo₂S₄ nanowires were in situ grown on a carbon sponge, which showed great electrocatalytic performance with a large electrochemically active surface area of 330.7 cm⁻² and a low activation energy of 15.3 kJ mol⁻¹ towards the urea oxidation reaction.¹⁴⁰ It is believed that the excellent electrocatalytic properties of NCSs will make them have good application prospects in other electrocatalysis fields.

4. Applications of NCSs in solar cells

NCSs are frequently used as counter electrode materials in dye-sensitized solar cells (DSCs) and quantum dot-sensitized solar cells (QDSCs).^84,143–146 In DSCs and QDSCs, counter electrodes (CEs) play an important role in collecting electrons from the external circuit to electro-catalyze the reduction of the electrolyte and thus realize continuous photoelectric conversion. For DSCs, Pt is an ideal choice as the CE due to its excellent electrocatalytic ability for the reduction of iodide/triiodide (I⁻/I₃⁻) redox electrolytes.¹⁴⁵ However, the high cost of Pt inspires people to develop cheaper alternatives. For QDSCs, Pt is inefficient for the reduction of polysulfide redox electrolytes because of the strong adsorption of sulfur atoms onto the Pt surface to reduce its conductivity and reduction ability.¹⁴³ Therefore, effective CE materials have been intensively developed, including carbon-based materials, metal sulfides and so on.

Among them, NCS-based materials are expected to be promising CE alternatives because they provide rich redox reactions and relatively high conductivity from the synergetic effect of Ni and Co atoms. For instance, NiCo₂S₄ quantum dots (QDs) were decorated onto the surface of nitrogen-doped carbon nanotubes (N-CNTs) as CEs for DSCs (Fig. 11a).¹⁴⁷ It was found that such CEs showed comparable electrocatalytic performance to Pt (Fig. 11b and c; photoelectric conversion efficiency of DSCs: 7.65% for NiCo₂S₄@N-CNTs vs. 7.39% for Pt). This was mainly because of the uniform distribution of NiCo₂S₄ QDs on CNTs for abundant reaction sites, the special 1D channels of CNTs for rapid charge transport and ion diffusion, and the strong metal–nitrogen bonding between NiCo₂S₄ QDs and CNTs due to the nitrogen doping.

Fig. 11 Schematic diagram of the synthesis process and TEM image for the NiCo₂S₄ quantum dots @ CNTs (a), and CV curves (b) and J–V curves of DSCs (c) for different counter electrodes.¹⁴⁷ Reproduced from ref. 110 with permission from American Chemical Society.

5. Applications of NCSs in photocatalysis

Recently, metal sulfides have been widely applied in photocatalysis,^148–150 and NCSs are also introduced into the photocatalytic system mainly due to their great photoelectric conductivity and p-type semiconductor characteristics (E_g = ∼2.5 eV). For example, NCS@MXene composites were prepared as a photocatalyst and exhibited great photocatalytic activity and stability in the degradation of rhodamine B under visible light conditions.⁸⁶ Besides, NiCo₂S₄/CdS (Fig. 12),⁸⁷ NiCo₂S₄@Zn_0.5Cd_0.5S,¹⁵¹ and NiCo₂S₄/g-C₃N₄ (ref. 152) co-catalysts have been fabricated and proved to be efficient in visible-light-irradiation photocatalytic hydrogen generation because the participation of NCSs is able to significantly enhance the carrier separation and migration due to the heterojunction formation in the co-catalysts.

Fig. 12 Light absorption plots and real pictures (a), and H₂ production (c) of different photocatalysis (a), and schematic diagram of the working process (b) and cycling performance (d) for photocatalytic H₂ generation by NiCo₂S₄/CdS (b).⁸⁷ Reproduced from ref. 55 with permission from the Royal Society of Chemistry.

Moreover, NiCo₂S₄ micro-particles were synthesized via a solvothermal method and then applied as noble-metal-free catalysts in the CO₂ photoreduction system.¹²² Photocatalytic CO₂ reduction is considered as a promising strategy to realize the direct conversion of solar energy into special chemicals (e.g., CO) with the advantages of using renewable solar energy and alleviating the climate change from CO₂ emission. Here, the NiCo₂S₄ photocatalyst showed considerable activity for visible-light CO₂ reduction, yielding a production rate of 43.5 μmol mg⁻¹ h⁻¹ for CO. In situ steady-state photoluminescence and transient photocurrent tests indicated that the NiCo₂S₄ catalyst enabled the reduction of the recombination and the acceleration of the transport of photo-generated charges during the CO₂ reduction process.¹²²

6. Applications of NCSs in sensors

The accurate detection of glucose concentration in various sources (e.g., human blood, foods and pharmaceuticals) is essential and necessary for personal healthcare, diagnosis and therapy of diseases (e.g., diabetes mellitus), drug analysis, food monitoring and so on.¹⁵³ It requires that glucose-monitoring devices should be efficient, reliable, low-cost, and simple. Enzyme-immobilized electrodes are widely used as glucose sensors due to their high sensitivity. However, such glucose sensors demand complex operating conditions for the attachment of enzymes onto the electrode surface, and the biomass enzymes are usually unstable under high temperature, humidity or acidity and alkalinity conditions.⁹⁰

To address these problems, enzyme-free glucose sensors based on various inorganic materials (e.g., metal nanoparticles, metal oxides, sulfides, etc.) have been developed recently. NCSs and their composites with different structures (e.g., NiCo₂S₄ nanosheets/nanowires/hollow spheres,^90,154,155 and NiS/CoS/NiCo₂S₄ micro-flowers¹⁵⁶) are also considered as an optimal choice for non-enzymatic glucose sensors based on their good electrocatalytic activity toward glucose oxidation. Moreover, they have several advantages, such as facile synthesis, high sensitivity, great stability and low cost. For example, flower-like NiCo₂S₄ structures constructed from many interlaced nanosheets were simply electrodeposited onto Ni-coated cellulose filter paper (Fig. 13a),¹⁵³ which showed satisfactory sensing performance for glucose detection (Fig. 13b–e), including a wide linear range (0.5 μM to 6 mM; Fig. 13d) for the glucose content, considerable sensitivity (283 μA mM⁻¹ cm⁻²), a low detection limit (50 nM), great selectivity (Fig. 13e), excellent repeatability and electrochemical stability.

Fig. 13 Schematic diagrams of the preparation process (a) and sensing process (b), and glucose sensing performance: (c) CV curves in 2 mM glucose and 0.1 M NaOH at 20 mV s⁻¹, (d) linear properties and (e) selectivity with interfering species (AA, UA, DA, U, AP, CA, KCl, NaCl, etc.) and glucose (Glu) for NiCo₂S₄/Ni/cellulose filter paper.¹⁵³ Reproduced from ref. 113 with permission from American Chemical Society.

Similarly, the excellent electrocatalytic capability of NCSs also endows them with great sensing performance for the detection of other substances (e.g., sulfadimethoxine (SDM)⁵⁴ and pyrimethanil¹⁵⁷). It was reported that a NiCo₂S₄@N/S-doped CeO₂ composite was able to be an effective signal amplification label in a DNA aptasensor for detecting SDM, which is widely used for the veterinary treatment of coccidiosis and other bacterial infections but harmful to organisms even at a low concentration. This was because the composite had great electrocatalytic performance for the oxygen reduction reaction, and thus excellent electrocatalytic amplification for SDM detection with high sensitivity and selection.⁵⁴ In addition, NCS-based materials (e.g., NiCo₂S₄/reduced graphene oxide nanosheets) also can act as gas sensors for ethanol monitoring by recording the resistance change of sensing materials. This is mainly due to the adsorption of O₂ molecules on the electrode surface and the reaction between the ethanol molecules and the adsorbed O₂ molecules will change the electrode resistance.¹⁵⁸

7. Applications of NCSs in microwave absorption

With the improvement of science and technology, more and more electronic products come into our daily life, while the accompanying electromagnetic radiation and interference will affect the human health, influence the normal operation of electronic devices, and even threaten the national defense safety. Thus, a variety of microwave absorption materials have been extensively studied in order to effectively absorb and then transform electromagnetic microwaves into other types of energy (e.g., thermal energy; Fig. 14a).¹⁵⁹ Ideal microwave absorbers should have a high absorption capacity, a wide absorption frequency range, low cost and light weight. Carbon-based materials (e.g., carbon nanotubes, graphene and porous carbon materials) have been widely used to absorb microwaves primarily due to their tunable dielectric properties, large specific surface areas, light weight and high chemical stability.¹⁶⁰

Fig. 14 Proposed microwave absorption mechanism of NCS-based materials (a) and comparison of microwave absorption ability among different materials (b).¹⁶³ Reproduced from ref. 121 with permission from Elsevier.

Recently, NCSs have also been considered to be a potential microwave absorption material owing to their great magnetic loss performance.^161,162 For example, NiCo₂S₄ nanosheets or microspheres were combined with carbon materials (e.g., biomass derived carbon mesoporous^91,163 and hollow carbon microspheres¹⁶⁴) to achieve excellent microwave absorption capability (Fig. 14b), such as a minimum reflection loss value of −64.74 dB and a wide absorption range from 9.22 to 14.48 GHz.¹⁶³ It was found that the assistance of NiCo₂S₄ in the biomass-derived carbon composite was able to improve the impedance matching and the attenuation constant for strong microwave absorption.⁹¹ Therefore, it is promising to develop highly efficient and low-cost microwave absorbers based on NCS materials.

8. Conclusion and prospects

In summary, NCS-based materials have been extensively studied in recent years. They possess great performances in different applications due to their intrinsic characteristics of highly reactive activity from the multivalent states of crystals, and relatively high electronic conductivity from the cooperation of Ni and Co atoms. This review summarizes the recent development of NCSs on various aspects of micro/nano-structure design, element modulation and dopants, heterojunction building and composite integration. It emphasizes the applications of NCSs in several fields, including electrochemical energy storage devices (e.g., supercapacitors, Li/Na/Zn-ion batteries, and Li–S/Zn–air/Li–O₂ batteries), water splitting, DSCs/QDSCs, photocatalysis, sensors and microwave absorption. It is indicated that these important applications are all mainly based on the attractive properties of NCSs, such as relatively high conductivity, rich active sites, and tunable components.

Although much progress has been achieved on the development of NCS-based materials for different devices, these materials are still unsatisfactory to meet the requirement for practical applications. For example, the specific capacity, energy density, and stability of the corresponding energy storage devices aren't quite high. Although the energy density of NCS-based supercapacitors is higher than that of the carbon-based ones, their power density and cycling stability are reduced concurrently. Compared to other typical batteries with high energy density, NCS-based batteries are still unsatisfactory. Besides, the high self-discharge rate (10–40%/day) of NCS-based supercapacitors should be addressed for their practical applications. Similarly, the catalytic activity and cycling stability of NCSs for the applications of electrocatalysis, photocatalysis, and sensing devices also need further enhancement. In addition, the manufacturing cost, environmental compatibility, and industrial-scale preparation should be especially concerned in future. Moreover, the relationship between the microscopic characteristics of NCS materials and the macroscopic performance should be understood more for the further rational design of NCS materials.

For the performance improvement, the exploitation and construction of highly effective NCS materials concerning various aspects (e.g., modulation of the element ratio, formation of efficient nanostructures, and integration of other components) are definitely required to expose abundant active surface sites, strength electronic conductivity, build fast channels for charge transfer and mass diffusion, and enhance the cycling durability of NCS-based systems. Besides, deep understand of the material behavior and working mechanisms of NCSs in different application systems is very important for their performance improvement. The combination of ex situ and in situ characterization techniques is very useful to disclose the underlying mechanisms (e.g., component conversion, structural evolution, reaction pathways, and interfacial interaction) of NCS-based applications.^{41,66,119–121,165} Besides, the optimization of mathematical modeling and theoretical simulation with higher rationality and reliability is also very helpful to promote the practical applications of NCSs.^{15,69,117,121,166} It is noteworthy that machine learning has been rapidly developed and widely applied in many fields, which is possibly a powerful way for the investigation of NCSs with superior performance in various applications by the purposeful design and simple, large-scale, high efficiency and low-cost preparation strategies.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the financial support from the Shenzhen Science and Technology Program (No. JCYJ20190809161407424), the National Natural Science Foundation of China (No. 22075237), the Natural Science Foundation of Fujian Province of China (No. 2020J01007), and the Fundamental Research Funds for the Central Universities of China (No. 20720210029).

References

1. X. Chen, Q. Liu, T. Bai, W. Wang, F. He and M. Ye, Chem. Eng. J., 2021, 409, 127237.
2. B. Yan, X. Li, W. Xiao, J. Hu, L. Zhang and X. Yang, J. Mater. Chem. A, 2020, 8, 17848–17882.
3. X. Zhao, F. Gong, Y. Zhao, B. Huang, D. Qian, H.-E. Wang, W. Zhang and Z. Yang, Chem. Eng. J., 2020, 392, 123675.
4. C. Miao, C. Zhou, H.-E. Wang, K. Zhu, K. Ye, Q. Wang, J. Yan, D. Cao, N. Li and G. Wang, J. Power Sources, 2021, 490, 229532.
5. J. Ren, M. Shen, Z. Li, C. Yang, Y. Liang, H.-E. Wang, J. Li, N. Li and D. Qian, J. Power Sources, 2021, 501, 230003.
6. J. Zhao, G. Wang, K. Cheng, K. Ye, K. Zhu, J. Yan, D. Cao and H.-E. Wang, J. Power Sources, 2020, 451, 227737.
7. T. Zhu, Z. He, Y. Ren, W. Zeng, J. Mao and L. Zhu, Sol. RRL, 2021, 5, 2100021.
8. G. Fu and J.-M. Lee, J. Mater. Chem. A, 2019, 7, 9386–9405.
9. P. Kulkarni, S. K. Nataraj, R. G. Balakrishna, D. H. Nagaraju and M. V. Reddy, J. Mater. Chem. A, 2017, 5, 22040–22094.
10. T. Chen, S. Wei and Z. Wang, ChemPlusChem, 2020, 85, 43–56.
11. T.-F. Yi, J.-J. Pan, T.-T. Wei, Y. Li and G. Cao, Nano Today, 2020, 33, 100894.
12. X. Xu, H. Liang, F. Ming, Z. Qi, Y. Xie and Z. Wang, ACS Catal., 2017, 7, 6394–6399.
13. X. Wang, X. Xu, J. Chen and Q. Wang, ACS Sustainable Chem. Eng., 2019, 7, 12331–12339.
14. S. Hyun and S. Shanmugam, ACS Omega, 2018, 3, 8621–8630.
15. H. Zhang, J. Liu, X. Lin, T. Han, M. Cheng, J. Long and J. Li, J. Alloys Compd., 2020, 817, 153293.
16. Q. Hu, W. Ma, G. Liang, H. Nan, X. Zheng and X. Zhang, RSC Adv., 2015, 5, 84974–84979.
17. K. Guo, Y. Ding, J. Luo and Z. Yu, ACS Appl. Mater. Interfaces, 2018, 10, 19673–19681.
18. X. Wu, S. Li, B. Wang, J. Liu and M. Yu, Chem. Commun., 2021, 57, 1002–1005.
19. L. Kang, M. Zhang, J. Zhang, S. Liu, N. Zhang, W. Yao, Y. Ye, C. Luo, Z. Gong, C. Wang, X. Zhou, X. Wu and S. C. Jun, J. Mater. Chem. A, 2020, 8, 24053–24064.
20. X. Wang, R. Zhou, C. Zhang, S. Xi, M. W. M. Jones, T. Tesfamichael, A. Du, K. Gui, K. Ostrikov and H. Wang, J. Mater. Chem. A, 2020, 8, 9278–9291.
21. J. Wu, X. Shi, W. Song, H. Ren, C. Tan, S. Tang and X. Meng, Nano Energy, 2018, 45, 439–447.
22. V. Ganesan, P. Ramasamy and J. Kim, Int. J. Hydrogen Energy, 2017, 42, 5985–5992.
23. W. Wang, J. Li, Q. Jin, Y. Liu, Y. Zhang, Y. Zhao, X. Wang, A. Nurpeissova and Z. Bakenov, ACS Appl. Energy Mater., 2021, 4, 1687–1695.
24. F. Lu, M. Zhou, K. Su, T. Ye, Y. Yang, T. D. Lam, Y. Bando and X. Wang, ACS Appl. Mater. Interfaces, 2019, 11, 2082–2092.
25. L. Hou, H. Hua, R. Bao, Z. Chen, C. Yang, S. Zhu, G. Pang, L. Tong, C. Yuan and X. Zhang, ChemPlusChem, 2016, 81, 557–563.
26. L. Hou, R. Bao, Z. Chen, M. Rehan, L. Tong, G. Pang and C. Yuan, Electrochim. Acta, 2016, 214, 76–84.
27. T. Zhu, J. Wang and G. W. Ho, Nano Energy, 2015, 18, 273–282.
28. Z. Chen, X. Wang, H. Wang, H. Wang, T. Bai, F. Ren, P. Ren, H. Yan, K. Xiao and Z. Shen, J. Alloys Compd., 2022, 891, 161988.
29. J. Lin, Y. Wang, X. Zheng, H. Liang, H. Jia, J. Qi, J. Cao, J. Tu, W. Fei and J. Feng, Dalton Trans., 2018, 47, 8771–8778.
30. Y. Liu, Y. Wen, Y. Zhang, X. Wu, H. Li, H. Chen, J. Huang, G. Liu and S. Peng, Sci. China Mater., 2020, 63, 1216–1226.
31. G. Xiang, Y. Meng, G. Qu, J. Yin, B. Teng, Q. Wei and X. Xu, Sci. Bull., 2020, 65, 443–451.
32. Y. Chen, T. Liu, L. Zhang and J. Yu, ACS Sustainable Chem. Eng., 2019, 7, 11157–11165.
33. M. Chuai, K. Zhang, X. Chen, Y. Tong, H. Zhang and M. Zhang, Chem. Eng. J., 2020, 381, 122682.
34. C. Han, T. Zhang, J. Li, B. Li and Z. Lin, Nano Energy, 2020, 77, 105165.
35. B. Liu, S. Huang, D. Kong, J. Hu and H. Y. Yang, J. Mater. Chem. A, 2019, 7, 7604–7613.
36. N. Kurra, C. Xia, M. N. Hedhili and H. N. Alshareef, Chem. Commun., 2015, 51, 10494–10497.
37. T. Wang, B. Zhao, H. Jiang, H.-P. Yang, K. Zhang, M. M. F. Yuen, X.-Z. Fu, R. Sun and C.-P. Wong, J. Mater. Chem. A, 2015, 3, 23035–23041.
38. X. Zhang, Y. Zheng, J. Zhou, W. Zheng and D. Chen, RSC Adv., 2017, 7, 13406–13415.
39. H. Chen, J. Jiang, Y. Zhao, L. Zhang, D. Guo and D. Xia, J. Mater. Chem. A, 2015, 3, 428–437.
40. D. Cao, W. Kang, S. Wang, Y. Wang, K. Sun, L. Yang, X. Zhou, D. Sun and Y. Cao, J. Mater. Chem. A, 2019, 7, 8268–8276.
41. S. Li, C. Li, W. K. Pang, Z. Zhao, J. Zhang, Z. Liu and D. Li, ACS Appl. Mater. Interfaces, 2019, 11, 27805–27812.
42. Y. Tang, S. Chen, S. Mu, T. Chen, Y. Qiao, S. Yu and F. Gao, ACS Appl. Mater. Interfaces, 2016, 8, 9721–9732.
43. N. Zhao, H. Fan, J. Ma, M. Zhang, C. Wang, H. Li, X. Jiang and X. Cao, J. Power Sources, 2019, 439, 227097.
44. G. Nagaraju, S. C. Sekhar, B. Ramulu and J. S. Yu, Small, 2019, 15, 1805418.
45. Y. Wen, Y. Liu, T. Wang, Z. Wang, Y. Zhang, X. Wu, X. Chen, S. Peng and D. He, ACS Appl. Energy Mater., 2021, 4, 6531–6541.
46. X. Li, Q. Li, Y. Wu, M. Rui and H. Zeng, ACS Appl. Mater. Interfaces, 2015, 7, 19316–19323.
47. Y. Xu, A. Sumboja, Y. Zong and J. A. Darr, Catal. Sci. Technol., 2020, 10, 2173–2182.
48. B. Guo, T. Yang, W. Du, Q. Ma, L.-z. Zhang, S.-J. Bao, X. Li, Y. Chen and M. Xu, J. Mater. Chem. A, 2019, 7, 12276–12282.
49. L. Hou, Y. Shi, S. Zhu, M. Rehan, G. Pang, X. Zhang and C. Yuan, J. Mater. Chem. A, 2017, 5, 133–144.
50. Y. Jiang, X. Qian, C. Zhu, H. Liu and L. Hou, ACS Appl. Mater. Interfaces, 2018, 10, 9379–9389.
51. P. Liu, Y. Liu, J. Li, M. Wang and H. Cui, Nanoscale, 2020, 12, 22330–22339.
52. W. Song, M. Xu, X. Teng, Y. Niu, S. Gong, X. Liu, X. He and Z. Chen, Nanoscale, 2021, 13, 1680–1688.
53. Z. Hao, X. He, H. Li, D. Trefilov, Y. Song, Y. Li, X. Fu, Y. Cui, S. Tang, H. Ge and Y. Chen, ACS Nano, 2020, 14, 12719–12731.
54. L. Li, L. Yang, S. Zhang, Y. Sun, F. Li, T. Qin, X. Liu, Y. Zhou and S. Alwarappan, J. Mater. Chem. C, 2020, 8, 14723–14731.
55. C. Wang, Z. Guan, Y. Shen, S. Yu, X.-Z. Fu, R. Sun and C.-P. Wong, Chem. Eng. J., 2018, 346, 193–202.
56. Y. Ouyang, H. Ye, X. Xia, X. Jiao, G. Li, S. Mutahir, L. Wang, D. Mandler, W. Lei and Q. Hao, J. Mater. Chem. A, 2019, 7, 3228–3237.
57. J. Zhu, S. Tang, J. Wu, X. Shi, B. Zhu and X. Meng, Adv. Energy Mater., 2017, 7, 1601234.
58. Q. Liu, X. Hong, X. You, X. Zhang, X. Zhao, X. Chen, M. Ye and X. Liu, Energy Storage Materials, 2020, 24, 541–549.
59. X. Zhang, X. Chen, T. Bai, J. Chai, X. Zhao, M. Ye, Z. Lin and X. Liu, J. Mater. Chem. A, 2020, 8, 11589–11597.
60. X.-Z. Song, F.-F. Sun, Y.-L. Meng, Z.-W. Wang, Q.-F. Su and Z. Tan, New J. Chem., 2019, 43, 3601–3608.
61. H. Zhang, A. Hao, Z. Sun, X. Ning, J. Guo, Y. Lv and D. Jia, J. Alloys Compd., 2020, 847, 156505.
62. C. Zhang, M. Hou, X. Cai, J. Lin, X. Liu, R. Wang, L. Zhou, J. Gao, B. Li and L. Lai, J. Mater. Chem. A, 2018, 6, 15630–15639.
63. J. Shen, J. Wu, L. Pei, M.-T. F. Rodrigues, Z. Zhang, F. Zhang, X. Zhang, P. M. Ajayan and M. Ye, Adv. Energy Mater., 2016, 6, 1600341.
64. X. Chang, W. Li, Y. Liu, M. He, X. Zheng, J. Bai and Z. Ren, J. Colloid Interface Sci., 2018, 538, 34–44.
65. L. Wan, C. He, D. Chen, J. Liu, Y. Zhang, C. Du, M. Xie and J. Chen, Chem. Eng. J., 2020, 399, 125778.
66. S. Fan, H. Liu, S. Bi, C. Gao, X. Meng and Y. Wang, Electrochim. Acta, 2021, 388, 138618.
67. S.-x. Yan, S.-h. Luo, J. Feng, P.-w. Li, R. Guo, Q. Wang, Y.-h. Zhang, Y.-g. Liu and S. Bao, Chem. Eng. J., 2020, 381, 122659.
68. H. S. Lee, J. Pan, G. S. Gund and H. S. Park, Adv. Mater. Interfaces, 2020, 7, 2000138.
69. W. Liu, B. Ren, W. Zhang, M. Zhang, G. Li, M. Xiao, J. Zhu, A. Yu, L. Ricardez-Sandoval and Z. Chen, Small, 2019, 15, 1903610.
70. Y. Zhu, J. Li, X. Yun, G. Zhao, P. Ge, G. Zou, Y. Liu, H. Hou and X. Ji, Nano-Micro Lett., 2020, 12, 16.
71. Y. Liu, G. Jiang, Z. Huang, Q. Lu, B. Yu, U. Evariste and P. Ma, ACS Appl. Energy Mater., 2019, 2, 8079–8089.
72. X. Zhao, Q. Ma, K. Tao and L. Han, ACS Appl. Energy Mater., 2021, 4, 4199–4207.
73. X. He, Q. Liu, J. Liu, R. Li, H. Zhang, R. Chen and J. Wang, Chem. Eng. J., 2017, 325, 134–143.
74. S. Chen, C. Lu, L. Liu, M. Xu, J. Wang, Q. Deng, Z. Zeng and S. Deng, Nanoscale, 2020, 12, 1852–1863.
75. Y. Zhu, S. An, X. Sun, D. Lan, J. Cui, Y. Zhang and W. He, Chem. Eng. J., 2020, 383, 123206.
76. J. Liu, J. Wang, B. Zhang, Y. Ruan, L. Lv, X. Ji, K. Xu, L. Miao and J. Jiang, ACS Appl. Mater. Interfaces, 2017, 9, 15364–15372.
77. J. Fu, L. Li, J. M. Yun, D. Lee, B. K. Ryu and K. H. Kim, Chem. Eng. J., 2019, 375, 121939.
78. J.-Z. He, W.-J. Niu, Y.-P. Wang, Q.-Q. Sun, M.-J. Liu, K. Wang, W.-W. Liu, M.-C. Liu, F.-C. Yu and Y.-L. Chueh, Electrochim. Acta, 2020, 362, 136968.
79. Z. Peng, C. Yang, Y. Hu, F. Bai, W. Chen, R. Liu, S. Jiang and H.-C. Chen, Appl. Surf. Sci., 2022, 573, 151561.
80. M. Zhang, H. Zheng, H. Zhu, M. Zhang, R. Liu, X. Zhu, X. Li and H. Cui, J. Alloys Compd., 2022, 901, 163633.
81. G. Shao, R. Yu, X. Zhang, X. Chen, F. He, X. Zhao, N. Chen, M. Ye and X. Y. Liu, Adv. Funct. Mater., 2020, 30, 2003153.
82. Y. Miao, X. Zhao, X. Wang, C. Ma, L. Cheng, G. Chen, H. Yue, L. Wang and D. Zhang, Nano Res., 2020, 13, 3041–3047.
83. A. Hu, J. Long, C. Shu, C. Xu, T. Yang, R. Liang and J. Li, ChemElectroChem, 2019, 6, 349–358.
84. S. A. Ansari, S. Goumri-Said, H. M. Yadav, M. Belarbi, A. Aljaafari and M. B. Kanoun, Sol. Energy Mater. Sol. Cells, 2021, 225, 111064.
85. H.-J. Kim and V. T.-V. Chebrolu, New J. Chem., 2018, 42, 18824–18836.
86. S. Vigneshwaran, C. M. Park and S. Meenakshi, Sep. Purif. Technol., 2021, 258, 118003.
87. J. Peng, J. Xu, Z. Wang, Z. Ding and S. Wang, Phys. Chem. Chem. Phys., 2017, 19, 25919–25926.
88. J. Yu, C. Lv, L. Zhao, L. Zhang, Z. Wang and Q. Liu, Adv. Mater. Interfaces, 2018, 5, 1701396.
89. Z. Kang, H. Guo, J. Wu, X. Sun, Z. Zhang, Q. Liao, S. Zhang, H. Si, P. Wu, L. Wang and Y. Zhang, Adv. Funct. Mater., 2019, 29, 1807031.
90. P. K. Kannan, C. Hu, H. Morgan and C. S. Rout, Chem.–Asian J., 2016, 11, 1837–1841.
91. P. Hu, S. Dong, X. Li, J. Chen and P. Hu, ACS Sustainable Chem. Eng., 2020, 8, 10230–10241.
92. Y. Xu, X. Gao, W. Chu, Q. Li, T. Li, C. Liang and Z. Lin, J. Mater. Chem. A, 2016, 4, 10248–10253.
93. Y. Xiao, J. Huang, Y. Xu, H. Zhu, K. Yuan and Y. Chen, J. Mater. Chem. A, 2018, 6, 9161–9171.
94. L. Hou, R. Bao, M. Rehan, L. Tong, G. Pang, X. Zhang and C. Yuan, Adv. Electron. Mater., 2017, 3, 1600322.
95. P. Phonsuksawang, P. Khajondetchairit, T. Butburee, S. Sattayaporn, N. Chanlek, P. Hirunsit, S. Suthirakun and T. Siritanon, Electrochim. Acta, 2020, 340, 135939.
96. P. Phonsuksawang, P. Khajondetchairit, K. Ngamchuea, T. Butburee, S. Sattayaporn, N. Chanlek, S. Suthirakun and T. Siritanon, Electrochim. Acta, 2021, 368, 137634.
97. S. Abureden, F. M. Hassan, G. Lui, S. Sy, R. Batmaz, W. Ahn, A. Yu and Z. Chen, J. Mater. Chem. A, 2017, 5, 7523–7532.
98. T. Peng, H. Yi, P. Sun, Y. Jing, R. Wang, H. Wang and X. Wang, J. Mater. Chem. A, 2016, 4, 8888–8897.
99. M. Zhang, H. Liu, Z. Song, T. Ma and J. Xie, Chem. Eng. J., 2020, 392, 123669.
100. M. Liang, M. Zhao, H. Wang, J. Shen and X. Song, J. Mater. Chem. A, 2018, 6, 2482–2493.
101. Y. Cui, J. Zhang, C. Jin, Y. Liu, W. Luo and W. Zheng, Small, 2018, 15, e1804318.
102. X. Chen, D. Chen, X. Guo, R. Wang and H. Zhang, ACS Appl. Mater. Interfaces, 2017, 9, 18774–18781.
103. L. Lin, J. Liu, T. Liu, J. Hao, K. Ji, R. Sun, W. Zeng and Z. Wang, J. Mater. Chem. A, 2015, 3, 17652–17658.
104. C. Liu, X. Wu and B. Wang, Chem. Eng. J., 2020, 392, 123651.
105. M. L. Aparna, T. Thomas and G. R. Rao, J. Electrochem. Soc., 2022, 169, 020515.
106. L. Yang, X. Lu, S. Wang, J. Wang, X. Guan, X. Guan and G. Wang, Nanoscale, 2020, 12, 1921–1938.
107. Y. Yan, A. Li, C. Lu, T. Zhai, S. Lu, W. Li and W. Zhou, Chem. Eng. J., 2020, 396, 125316.
108. X. Song, C. Huang, Y. Qin, H. Li and H. C. Chen, J. Mater. Chem. A, 2018, 6, 16205–16212.
109. W. Chen, X. Zhang, L.-E. Mo, Y. Zhang, S. Chen, X. Zhang and L. Hu, Chem. Eng. J., 2020, 388, 124109.
110. X. Chen, C. He, W. Wang, T. Bai, G. Xue and M. Ye, Phys. Rev. Appl., 2021, 15, 064042.
111. W. He, C. Wang, H. Li, X. Deng, X. Xu and T. Zhai, Adv. Energy Mater., 2017, 7, 1700983.
112. Y. Sui, Y. Zhang, H. Hu, Q. Xu, F. Yang and Z. Li, Adv. Mater. Interfaces, 2018, 5, 1800018.
113. C. Zhang, X. Cai, Y. Qian, H. Jiang, L. Zhou, B. Li, L. Lai, Z. Shen and W. Huang, Adv. Sci., 2018, 5, 1700375.
114. Z. Peng, L. Gong, J. Huang, Y. Wang, L. Tan and Y. Chen, Carbon, 2019, 153, 531–538.
115. Q. Li, W. Lu, Z. Li, J. Ning, Y. Zhong and Y. Hu, Chem. Eng. J., 2020, 380, 122544.
116. J. Huang, J. Wei, Y. Xu, Y. Xiao and Y. Chen, J. Mater. Chem. A, 2017, 5, 23349–23360.
117. T. Sun, C. Huang, H. Shu, L. Luo, Q. Liang, M. Chen, J. Su and X. Wang, ACS Appl. Mater. Interfaces, 2020, 12, 57975–57986.
118. Y. Tang, X. Li, H. Lv, D. Xie, W. Wang, C. Zhi and H. Li, Adv. Energy Mater., 2020, 10, 2000892.
119. J. Zhang, K. Song, L. Mi, C. Liu, X. Feng, J. Zhang, W. Chen and C. Shen, J. Phys. Chem. Lett., 2020, 11, 1435–1442.
120. S. Li, P. Ge, F. Jiang, H. Shuai, W. Xu, Y. Jiang, Y. Zhang, J. Hu, H. Hou and X. Ji, Energy Storage Materials, 2019, 16, 267–280.
121. Y. V. Lim, S. Huang, J. Hu, D. Kong, Y. Wang, T. Xu, L. K. Ang and H. Y. Yang, Small Methods, 2019, 3, 1900112.
122. Z. Xiong, L. Huang, J. Peng, Y. Hou, Z. Ding and S. Wang, ChemCatChem, 2019, 11, 5513–5518.
123. B. He, J.-J. Song, X.-Y. Li, C.-Y. Xu, Y.-B. Li, Y.-W. Tang, Q.-L. Hao, H.-K. Liu and Z. Su, Nanoscale, 2021, 13, 810–818.
124. F. Wang, G. Li, X. Meng, S. Xu and W. Ma, J. Power Sources, 2020, 462, 228162.
125. W. Liu, J. Zhang, Z. Bai, G. Jiang, M. Li, K. Feng, L. Yang, Y. Ding, T. Yu, Z. Chen and A. Yu, Adv. Funct. Mater., 2018, 28, 1706675.
126. X. Zuo, Y. Song and M. Zhen, Appl. Surf. Sci., 2020, 500, 144000.
127. F. Yang, Y. Shen, Z. Cen, J. Wan, S. Li, G. He, J. Hu and K. Xu, Sci. China Mater., 2021, 65, 356–363.
128. S. Li, P. Xu, M. K. Aslam, C. Chen, A. Rashid, G. Wang, L. Zhang and B. Mao, Energy Storage Materials, 2020, 27, 51–60.
129. J. Zhang, X. Bai, T. Wang, W. Xiao, P. Xi, J. Wang, D. Gao and J. Wang, Nano-Micro Lett., 2019, 11, 2.
130. S. Hyun, B. Son, H. Kim, J. Sanetuntikul and S. Shanmugam, Appl. Catal., B, 2020, 263, 118283.
131. X. Zhao, H. Liu, Y. Rao, X. Li, J. Wang, G. Xia and M. Wu, ACS Sustainable Chem. Eng., 2019, 7, 2610–2618.
132. X. Du, X. Zhang, Y. Li and M. Zhao, Int. J. Hydrogen Energy, 2018, 43, 19955–19964.
133. X. Feng, Q. Jiao, H. Cui, M. Yin, Q. Li, Y. Zhao, H. Li, W. Zhou and C. Feng, ACS Appl. Mater. Interfaces, 2018, 10, 29521–29531.
134. Y. Gong, Y. Lin, Z. Yang, J. Wang, H. Pan, Z. Xu and Y. Liu, ChemistrySelect, 2019, 4, 1180–1187.
135. F. Li, R. Xu, Y. Li, F. Liang, D. Zhang, W.-F. Fu and X.-J. Lv, Carbon, 2019, 145, 521–528.
136. H. Li, L. Chen, P. Jin, Y. Li, J. Pang, J. Hou, S. Peng, G. Wang and Y. Shi, Nano Res., 2021, 15, 950–958.
137. J. Zhou, Y. Dou, T. He, A. Zhou, X.-J. Kong, X.-Q. Wu, T. Liu and J.-R. Li, Nano Res., 2021, 14, 4548–4555.
138. L. Sha, K. Ye, G. Wang, J. Shao, K. Zhu, K. Cheng, J. Yan, G. Wang and D. Cao, Chem. Eng. J., 2019, 359, 1652–1658.
139. D. Khalafallah, Q. Zou, M. Zhi and Z. Hong, Electrochim. Acta, 2020, 350, 136399.
140. B. Li, C. Song, J. Rong, J. Zhao, H.-E. Wang, P. Yang, K. Ye, K. Cheng, K. Zhu, J. Yan, D. Cao and G. Wang, J. Energy Chem., 2020, 50, 195–205.
141. X. Du, W. Lian and X. Zhang, Int. J. Hydrogen Energy, 2018, 43, 20627–20635.
142. X.-X. Li, X.-T. Wang, K. Xiao, T. Ouyang, N. Li and Z.-Q. Liu, J. Power Sources, 2018, 402, 116–123.
143. S. S. Rao, I. K. Durga, N. Kundakarla, D. Punnoose, C. V. V. M. Gopi, A. E. Reddy, M. Jagadeesh and H.-J. Kim, New J. Chem., 2017, 41, 10037–10047.
144. L. Li, X. Zhang, S. a. Liu, B. Liang, Y. Zhang and W. Zhang, Sol. Energy, 2020, 202, 358–364.
145. K. S. Anuratha, S. Mohan and S. K. Panda, New J. Chem., 2015, 40, 1785–1791.
146. J. Qiu, D. He, H. Wang, W. Li, B. Sun, Y. Ma, X. Lu and C. Wang, Electrochim. Acta, 2021, 367, 137451.
147. P. Su, Q. Jiao, H. Li, Y. Li, X. Liu, Q. Wu, D. Shi, Y. Zhao, T. Wang and W. Wang, ACS Appl. Energy Mater., 2021, 4, 4344–4354.
148. L. Nie and Q. Zhang, Inorg. Chem. Front., 2017, 4, 1953–1962.
149. X.-Q. Qiao, Z.-W. Zhang, Q.-H. Li, D. Hou, Q. Zhang, J. Zhang, D.-S. Li, P. Feng and X. Bu, J. Mater. Chem. A, 2018, 6, 22580–22589.
150. X.-Q. Qiao, Z.-W. Zhang, F.-Y. Tian, D.-F. Hou, Z.-F. Tian, D.-S. Li and Q. Zhang, Cryst. Growth Des., 2017, 17, 3538–3547.
151. S. Zhao, J. Xu, M. Mao, L. Li and X. Li, Appl. Surf. Sci., 2020, 528, 147016.
152. K. Jiang, W. Iqbal, B. Yang, M. Rauf, I. Ali, X. Lu and Y. Mao, J. Alloys Compd., 2021, 853, 157284.
153. K. J. Babu, T. R. Kumar, D. J. Yoo, S.-M. Phang and G. G. Kumar, ACS Sustainable Chem. Eng., 2018, 6, 16982–16989.
154. Q. Guo, T. Wu, L. Liu, Y. He, D. Liu and T. You, J. Alloys Compd., 2020, 819, 153376.
155. D. Chen, H. Wang and M. Yang, Anal. Methods, 2017, 9, 4718–4725.
156. D. Li, X. Zhang, L. Pei, C. Dong, J. Shi and Y. Xu, Inorg. Chem. Commun., 2019, 110, 107581.
157. Y. He, T. Wu, J. Wang, J. Ye, C. Xu, F. Li and Q. Guo, Talanta, 2020, 219, 121277.
158. B. Li, J. Xia, J. Liu, Q. Liu, G. Huang, H. Zhang, X. Jing, R. Li and J. Wang, Chem. Phys. Lett., 2018, 703, 80–85.
159. M. Zhou, W. Gu, G. Wang, J. Zheng, C. Pei, F. Fan and G. Ji, J. Mater. Chem. A, 2020, 8, 24267–24283.
160. M. Green and X. Chen, J. Materiomics, 2019, 5, 503–541.
161. R. Peymanfar and S. Ghorbanian-Gezaforodi, Nanotechnology, 2020, 31, 495202.
162. J. Liu, Z. Yang, L. Yang, Y. Zhu, T. Xue and G. Xu, J. Alloys Compd., 2021, 853, 157403.
163. S. Dong, P. Hu, X. Li, C. Hong, X. Zhang and J. Han, Chem. Eng. J., 2020, 398, 125588.
164. P. Hu, S. Dong, F. Yuan, X. Li and C. Hong, Adv. Compos. Hybrid Mater., 2021 DOI:10.1007/s42114-021-00318-w.
165. Z. Sun, C. Zhao, X. Cao, K. Zeng, Z. Ma, Y. Hu, J.-H. Tian and R. Yang, Electrochim. Acta, 2020, 338, 135900.
166. L. Xing, X. Zheng, M. Schroeder, J. Alvarado, A. von Wald Cresce, K. Xu, Q. Li and W. Li, Acc. Chem. Res., 2018, 51, 282–289.

---