Multifunctional spinel MnCo₂O₄ based materials for energy storage and conversion: a review on emerging trends, recent developments and future perspectives

Abstract

The energy requirement of modern society increases every day. The depletion of the reserves of fossil fuel combined with the deleterious effects of CO₂ in the atmosphere is forcing all the world to search for alternative ways of generation and storing energy. Many scientists around the world are pursuing different forms to produce and store energy. Solar and wind sources are a reality for production of electricity, but are not continuous and require storage devices. The development of batteries and hybrid supercapacitors of high energy and power density is of great importance to complement this requirement of energy storage. Rechargeable metal–air batteries which utilize oxygen electrocatalysis seem to be an ideal choice, once the source of energy is not intermittent as solar and wind energy and is based on oxygen bifunctional electrocatalysis of both oxygen reduction and O₂ evolution reactions. In addition, water splitting allows the conversion and storage of solar/wind energy into chemical energy, generating fuels with high energy content. From this perspective, spinel MnCo₂O₄-based materials are promising structures for energy storage and conversion of energy. In this review, the use of low cost and abundant multifunctional materials for the development of supercapacitor devices and batteries was summarized. Completely, the design of electrocatalysts for water splitting and their capability to proportionate the tetra-electronic process of the oxygen reduction reaction are reviewed, including the main strategies in the preparation of these materials and considering their key multifunctional role in the way to a more sustainable society.

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Josué M. Gonçalves

Josué Martins Gonçalves is a postdoctoral researcher in the group headed by Prof. Lucio Angnes at the University of Sao Paulo (USP), Brazil, and honored with a prestigious research fellowship from FAPESP. He graduated in Chemistry from University Vale do Acaraú (UVA) in 2014 and received his Ph.D. degree from USP in 2019, under the supervision of Prof. Koiti Araki. His current research interests include applications of nanomaterials in sensors, electrocatalysis and energy conversion and storage devices.

Paulo R. Martins

Paulo Roberto Martins received his Ph.D. degree from the Institute of Chemistry of University of Sao Paulo in 2012, under the guidance of Professor Koiti Araki. Currently, he is Assistant Professor at the Federal University of Goiás, Brazil. His research interests are focused on the development of new materials based on layered double hydroxides for energy storage purposes.

Diego P. Rocha

Diego Pessoa Rocha graduated in Chemistry in 2013, and received his Master's degree in Chemistry (2015) and Ph.D. (2020) from Federal University of Uberlândia, Brazil, under the guidance of Rodrigo A. A. Munoz, who is currently Associate Professor of Chemistry at the same university. He graduated in Chemistry in 2002 and obtained his Ph.D. in 2006 from the University of Sao Paulo Brazil. His current research interests focus on the development of novel electrochemical devices for (bio)sensors and electrocatalysis. He is an affiliate member of the Brazilian Academy of Science. He is an affiliate member of the Brazilian Academy of Science.

Edson Nossol

Edson Nossol received his Ph.D. degree from the Department of Chemistry of Federal University of Paraná in 2013. Currently, he is Adjunct Professor at the Federal University of Uberlândia, Brazil. His research interests are focused on the preparation of hexacyanoferrates/carbon nanostructures for application in sensors and energy storage devices.

Lucio Angnes

Lucio Angnes is full professor at the Institute of Chemistry of University of Sao Paulo. His research interests include the construction of electrodes with new and alternative materials, development of modified electrodes, design of arrays of microelectrodes, design of different procedures of enzyme immobilization (on electrodes or inside microchannels) and association of the created devices with flowing systems. He has authorized 160 research papers, is a member of the scientific board of Biosensors & Bioelectronics, Electroanalysis, and the Journal of Pharmaceutical Research and serves as Coordinator of Innovation at FAPESP (São Paulo State Research Foundation). He is a member of the Sao Paulo State Academy of Science.

Chandra Sekhar Rout

Dr. Chandra Sekhar Rout is Associate Professor at Centre for Nano & Material Sciences, Jain University. Before joining CNMS, he was a DST-Ramanujan Fellow at I.I.T. Bhubaneswar, India (2013–2017). He received his B.Sc. (2001) and M.Sc. (2003) degrees from Utkal University and his Ph.D. from JNCASR, Bangalore (2008) under the supervision of Prof. C.N.R. Rao. He did his postdoctoral research at the National University of Singapore (2008–2009), Purdue University, USA (2010–2012) and UNIST, South Korea (2012–2013). His research interests include 2D materials for sensors, supercapacitors and energy storage devices, field emitters and electronic devices.

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1. Introduction

With the increasing demand for environmentally friendly energy sources, alternatives have accelerated research on various renewable energy technologies such as fuel cells, metal–air batteries, and water-splitting devices as alternative energy production and storage systems.¹

However, there are several scientific and technological challenges which require great efforts in the search for a more sustainable society. Among the main challenges, it is easy to identify the scientific race for low-cost and abundant materials for the command of the tetra-protonic and tetra-electronic reaction mechanism of the oxygen evolution reaction (OER),^2,3 a formidable challenge in the development of H₂ fuel cells. In addition, electrochemical oxygen reduction (ORR) and OER reactions are two key processes that limit the efficiency of important energy conversion devices such as metal–air batteries (MABs) and electrolytic cells.⁴ On the other hand, the quest for much higher power and energy density devices, especially hybrid supercapacitors (HSCs), as alternatives to lithium-ion batteries (LIBs), has been the main objective of several research groups, as they can combine the outstanding power density of supercapacitive materials with the high energy density of battery-type materials into a single device.⁵

In this sense, researchers in materials science have strived to develop advanced and multifunctional materials for modern energy technologies, aiming to overcome the main challenges of energy conversion and storage. In fact, among the several recently studied materials, transition metal oxides (TMOs) have garnered attention due their high electronegativity, rich redox reactions and abundant density of active sites, low cost, environmental friendliness, and excellent electrochemical performance.⁶ For instance, recently some review articles reported the use of Co₃O₄ and Co₃O₄-containing electrode materials for supercapacitors⁷ and batteries.⁸ In one of these recent studies, Hu et al.⁹ summarized the proposed strategies for improving specific capacitance, cycling stability, multifunctional capabilities of Co₃O₄ based materials and development prospects of Co₃O₄-based supercapacitor materials, providing a certain direction for application of Co₃O₄ in supercapacitors in the future. Complementarily, Shi and co-workers⁸ discussed the synthesis and application of pure Co₃O₄ and its composites (Co₃O₄/C, Co₃O₄/graphene, Co₃O₄/metal oxide) in the field of LIBs. On the other hand, in the field of energy conversion, M. R. N. S. Hamdani, R. N. Singh, & P. Chartier (2010)¹⁰ reviewed the performance of Co₃O₄ and Co-based spinel oxides as electrocatalysts for the OER or ORR.

Similar to Co₃O₄ and Co₃O₄-based materials, manganese-containing TMOs have also been intensively reported for applications in energy technologies,⁶ especially those based on Mn₃O₄. For example, Zhu et al.¹¹ reviewed the electrochemical properties and reaction principles of Mn₃O₄-based composites with carbon and other metal compounds for supercapacitor electrodes. In addition to the use in supercapacitors, Ubale and colleagues¹² reported the main advances in the deposition, characterization, and applications of nanostructured manganese oxide thin films (NMOTFs) in LIBs, highlighting the structural and morphological studies. On the other hand, Tian and co-workers¹³ reported the emerging applications of a series of MnO_x materials as highly efficient electrocatalysts for the OER, highlighting the reaction mechanisms, superiorities, and challenges of each type of MnO_x for future applications in the highly exciting energy-conversion-related areas.

As already mentioned in the studies cited above, spinel materials with a typical chemical formula of AB₂O₄ have been widely recognized and considered in the energy storage field,¹⁴ and also as electrocatalysts in energy conversion devices. In fact, special attention has been given to spinel materials with bimetallic oxide structure, as they can result in materials with higher electrochemical activity, electrical conductivity, and more abundant redox reactions compared with monometallic oxides of A and B.¹⁵ For example, some reviews reported the recent progress in the use of the NiCo₂O₄ spinel in supercapacitors,¹⁶ batteries¹⁷ and sensors.¹⁸ More recently, Zhao et al.¹⁴ summarized the main advances of 2D spinel structured Co-based MCo₂O₄ (M = Co, Ni, Zn, Cu, Fe, and Mn) materials as integrated electrodes for supercapacitor (SC) applications, detailing other different nanomaterials and 2D spinel structured Co-based materials for this application.

To our knowledge, more than five hundred articles report the preparation and/or use of the MnCo₂O₄ spinel for various applications, especially for energy conversion and storage. Thus, this compound has been widely recognized as a promising, versatile, and cost-efficiently bifunctional non-noble-metal electrocatalyst, due to its high redox stability, the complementation and synergy of both transition metals (manganese and cobalt), and efficient variable valence states.^19–22 As shown earlier, a few of the previous reviews have discussed the applications of Co₃O₄, Mn₃O₄ and NiCo₂O₄ spinels in energy storage, especially in supercapacitors and LIBs. However, as far as we know, there is no review work describing the promising results of MnCo₂O₄ in energy technologies. Therefore, in this review article we focus on the recent advances in MnCo₂O₄-based materials for energy applications and the main strategies used for the design of these materials (Scheme 1), including HSCs, LIBs and MABs, as well as the advancements achieved as electrocatalysts for water-splitting, more specifically for the hydrogen evolution reaction (HER) and OER. The pros and cons of using this spinel in the different devices are critically discussed. Finally, the evolving application of MnCo₂O₄ materials in the ORR is discussed, as well as the perspectives and future directions anticipated.

Scheme 1 Illustration of the strategies and applications of MnCo₂O₄ spinels. The atomic structure of the MnCo₂O₄ inverse spinel structure in the center of the scheme was reproduced with permission from ref. 23.

2. MnCo₂O₄ spinel: a supercapacitive or battery-type material?

The growing and fast demand for clean and sustainable energy storage devices has generated a scientific race for abundant and low-cost materials that can be used in high energy density devices, especially in high power density applications. However, this scientific race has also resulted in a great deal of confusion in the classification of supercapacitive and battery-like materials, especially in the distinction of “pseudocapacitive” and “battery” materials.²⁴ In this sense, several review articles have recently been published in order to alert the scientific community on these misunderstandings.

To clarify the confusion, Chodankar et al.²⁴ reported a review article that serves as a guide, providing the meanings and correct performance metrics of different electrode materials and using the electrochemical signatures and quantitative kinetics analysis as a method to distinguish battery-type and pseudocapacitive materials. For instance, electrical double-layer capacitors (EDLCs) that store energy purely in the double-layer on a high surface area conductor²⁵ show a typical electrochemical signature of a supercapacitive material (Fig. 1a), that is, a rectangular cyclic voltammogram (CV, Fig. 1b) and galvanostatic charge/discharge (GCD) profile in the form of a symmetrical triangle (Fig. 1c). Similarly, pseudocapacitive materials have quasi-rectangular CVs and quasi-triangular GCD curves, however the charge storage mechanisms involve either (a) redox reactions at or near the surface (intrinsic pseudocapacitors); or (b) intercalation-type reactions.²⁴

Fig. 1 The schematic illustration of the energy storage mechanisms with their corresponding electrochemical signatures (representative shapes of CV and CD curves): (a–c) electrical double layer capacitance, (d–f) surface redox capacitance, (g–i) intercalation capacitance, and (j–l) faradaic battery-type. Reproduced with permission from ref. 24. Copyright © 2020 Wiley-VCH GmbH.

Surface-redox pseudocapacitors, for example, are well represented by ruthenium (RuO₂) and manganese oxides (MnO₂). In fact, due to their fast proton and electron-conducting properties at the surface of the electrode (Fig. 1d), their electrochemical signatures resemble those of EDLCs, as shown in Fig. 1e and f. In contrast, some layered oxides, such as Nb₂O₅ and MoO₃, can store energy by faradaic processes through the intercalation of electrolyte ions into the layers (Fig. 1g), especially in a nonaqueous electrolyte system, but without crystallographic phase changes. These materials are of the type “intercalation pseudocapacitors”, but they should not be confused and called redox pseudocapacitors. According to Chodankar et al.,²⁴ a way to avoid this confusion is to carefully analyze the electrochemical features of intercalation pseudocapacitive materials and it was found that (i) they do not undergo phase transformations during intercalation, (ii) their peak potentials do not shift considerably with sweep rate, (iii) their current is linearly proportional to the sweep rate and (iv) their capacity does not vary significantly with charging time.

On the other hand, battery-type electrode materials have an electrochemical signature quite different from supercapacitive materials, since CVs display a couple of redox peaks (Fig. 1k) and plateau GCD profiles (Fig. 1l). This is due to the solid-state diffusion-controlled faradaic reactions characteristic of materials that present phase change of the electrode materials during the electrochemical process (Fig. 1j), such as oxides/hydroxides of Ni, Co, Cu, and Cd that react with hydroxide ions in alkaline media to store a charge.²⁴

Then, is the MnCo₂O₄ spinel a supercapacitive or battery-type material? In the literature it is possible to find some studies that classify MnCo₂O₄ as a supercapacitive material, while others as battery-type. For example, V. Sannasi & K. Subbian²⁶ reported the preparation of high-pseudocapacitance MnCo₂O₄ nanostructures, while S. G. Krishnan, M. H. A. Rahim & R. Jose²⁷ reported the synthesis and characterization of MnCo₂O₄ cuboidal microcrystals as intercalation pseudocapacitors, however, both studies presented the characteristic electrochemical signature of a battery-like material, such as CVs with a couple of redox peaks and plateau GCD profiles. In addition, peak potentials shifted considerably with sweep rate and capacity varied significantly with charging time. Thus, although many studies classify MnCo₂O₄ as a supercapacitive material, in this review work it was considered as a battery-type material, and in many cases the energy stored in the form of specific charge (C g⁻¹) was recalculated, since the average capacitance (F) was not constant throughout the potential window in the CVs.

In addition, it is also important to clarify that when assembling a battery-type electrode (ex.: MnCo₂O₄) with a supercapacitive-type electrode, a HSC is obtained, matching the advantages from both batteries and supercapacitors, and rendering them promising advanced energy storage devices for commercial applications.²⁸ In fact, experimental and theoretical studies²⁹ (shown below) demonstrated that MnCo₂O₄ has a superior electrical conductivity when compared to Co₃O₄, also showing greater storage capacity compared to other cobaltite spinels (MCo₂O₄; M = Ni,²⁸ Cu,²⁸ Zn³⁰ and Co^28,30) with good cycling lifespan.²⁹ These characteristics demonstrate the promising possibilities of using these materials in high performance HSCs, as discussed below.

3. MnCo₂O₄-based materials for energy storage applications

3.1. Supercapacitors

3.1.1. Pristine MnCo₂O₄. Supercapacitors have received great attention owing to their high energy and power densities. Supercapacitors are highly desirable since this type of device can deliver high power and reasonable energy densities concurrently. Carbonaceous materials, conducting polymers and transition metal oxides have displayed higher energy density compared to other materials.^31–33 In particular, binary metal oxides have been considered for supercapacitor electrodes due to their high electrical conductivity relative to single component oxides and advantages of achievable mixed valences.³⁴ MnCo₂O₄ has attracted considerable interest in supercapacitor application since cobalt has a high oxidation potential, whereas manganese can have multiple oxidation states and exhibit higher capacity.³⁵

In addition, it is important for a supercapacitor to have suitable fitting pore size distribution and large specific surface area, aiming to decrease the consumption of electrolyte by regulating the porous structure and morphology of the electrode, which determine the ion diffusion and conductivity, thereby affecting the capacitance of the supercapacitor. MnCo₂O₄ materials with different morphologies, such as spheres,³⁶ granules,³⁷ cuboidal microcrystals,²⁷ nanoneedles,³⁸ nanorods,^39,40 cubes,^26,41 nanosheets,^42–44 nanocages,⁴⁵ tunable porous structures,⁴⁶ hollow spheres,⁴⁷ and network-like porous structures,^48,49 can be prepared and tested for their usefulness as supercapacitor electrodes. For example, 1D MnCo₂O₄ nanowire arrays showed a specific capacitance of 349.8 F g⁻¹ at 1 A g⁻¹ and an energy density of 35.4 W h kg⁻¹ at a power density of 225 W kg⁻¹.⁵⁰ Similarly, specific capacitances of 1342 F g⁻¹ at 1 A g⁻¹ and 988 F g⁻¹ at 20 A g⁻¹ were observed for MnCo₂O₄ nanowires synthesized by Xu et al.⁵¹

Liu et al.²⁹ reported a MnCo₂O₄ mesoporous nanowire array grown on nickel foam (NF) with a high specific capacitance. From Fig. 2a it is possible to observe that the nanowire has a mesoporous characteristic being formed by MnCo₂O₄ nanoparticles with a size distribution of ∼20 nm (Fig. 2b) and a surface area of 98.5 m² g⁻¹ (determined from N₂ isotherms). In order to figure out the effect of Mn on the MnCo₂O₄ spinel the projected density of states and electronic band structures were determined and the results are shown in Fig. 2c and d, respectively. Those studies demonstrated that MnCo₂O₄ has a superior electrical conductivity when compared to Co₃O₄. In fact, MnCo₂O₄ presented a valence bond very near the Fermi level and a low bandgap of 0.35 eV at the G point, in contrast the Co₃O₄ presented a bandgap of 1.72 eV at the same point. Also, the electrochemical behavior of MnCo₂O₄ nanowires was superior to that of Co₃O₄. A specific capacitance of 2146 F g⁻¹ at a current density of 1 A g⁻¹ was observed for MnCo₂O₄ nanowires, while for Co₃O₄ the specific capacitance was 948 F g⁻¹,²⁹Fig. 2e. In addition, the MnCo₂O₄ nanowires presented an excellent capacitance retention of 92.1% even after 5000 cycles of charge–discharge process, as can be seen in Fig. 2f.

Fig. 2 TEM (a) and HRTEM (b) images of MnCo₂O₄ nanowires, (c and d) projected density of states and electronic band structures of MnCo₂O₄. (e) GCD curves of MnCo₂O₄ nanowires (black, at 1 A g⁻¹) and (f) cycling performance of MnCo₂O₄ nanowires (black, at 1 A g⁻¹). Reproduced with permission from ref. 29. Copyright © Marketplace™, Royal Society of Chemistry.

3.1.2. MnCo₂O₄–metal oxide composite. The recent development of hierarchical nanostructures obtained using metal oxides directly grown on an active material (current collector) showed promising results.⁵² These structures can be modulated in relation to their porosity and contact area between active materials and the electrolyte, providing more active sites in a given unit area without any auxiliary components, resulting in higher electrochemical properties. To link the performance gap between these materials, a variety of core–shell structure electrodes consisting of diverse compounds, such as Co₃O₄@MnCo₂O₄,^53,54 MnCo₂O₄@MnMoO₄,⁵⁵ MnCo₂O₄@NiMoO₄,^56,57 and MnCo₂O₄@CoMnO₄ have been constructed so far which remarkably improved the electrochemical properties of these materials compared with the individual components. In this way, Shrestha et al.⁵⁸ reported a sandwich-type architecture of MnCo₂O₄@M-C@MnO₂ as an electrode material exhibiting an excellent areal/gravimetric capacity of 0.75 mA h cm⁻²/312 mA h g⁻¹ at 3 mA cm⁻² with a capacity retention of 89.6% after 10 000 cycles. Besides, the hybrid supercapacitor presented an energy density of 68.2 W h kg⁻¹ at 749.2 W kg⁻¹ power density.

Liu et al.⁵⁹ reported the synthesis of a hierarchical MnCo₂O₄ nanowire@MnO₂ sheet core shell nanostructure showing an energy density of 85.7 W h kg⁻¹ at a power density of 800 W kg⁻¹. Zheng and coauthors⁶⁰ also reported a hierarchical MnCo₂O₄@MnO₂ core–shell nanowire array exhibiting an energy density of 135.6 W h kg⁻¹ at a power density of 513 W kg⁻¹.

Smarter integrated designs combined with different oxide materials are also reported, such as MnO₂,⁶¹ CoO,⁶² NiWO₄,⁶³ ZnO,⁶⁴ NiO,⁶⁵ CoCo₂O₄,⁶⁶ CuCo₂O₄,⁶⁷ NiCo₂O₄,⁶⁸ and CoMnO₄.⁶⁹ These types of structures possess many competitive advantages, including improvement of electrical conductivity, high electron aggregation efficiency, rich approachable electroactive sites, and even excellent synergetic effects or multifunctional properties of the nanostructure components.⁷⁰

3.1.3. MnCo₂O₄-conducting polymer composites. Conducting polymers can store energy through rapid faradaic charge transfer, since the electrochemical process occurs both on the surface and interface of the electrode material (between the electrode and electrolyte), and due to this, they have been combined with Mn₂Co₂O₄ in the form of composites to increase the capacitance.⁷¹ Conducting polymers with high electrical conductivity provide more active sites, which improve the maximum utilization of the MnCo₂O₄ electrode material. The MnCo₂O₄ nanoflakes@polypyrrole (PPy) nanowire electrode displayed a specific capacitance of 2933 F g⁻¹ at 20 A g⁻¹.³² Similarly, Wang et al.⁷² reported the preparation of MnCo₂O₄@PPy nanostructures on graphite foam (GNF), as described in Fig. 3a. The MnCo₂O₄@PPy/GNF was prepared in different concentrations of PPy, being denoted as MnCo₂O₄@PPy/GNF-n, where n was varied from 1 to 6 and the SEM images are shown in Fig. 3b–g. It is possible to observe that PPy grows vertically on MnCo₂O₄ as the concentration of PPy increases, thus forming interconnected network nanosheets. The MnCo₂O₄@PPy/GNF-5 exhibited a specific capacitance of 2364 F g⁻¹ and a rate capability of 55.2% from 1 to 50 A g⁻¹.

Fig. 3 Scheme of the synthesis process of PPy@MnCo₂O₄/GNF (a), SEM images of the PPy@MnCo₂O₄/GNF-n (n = 1–6) (b–g) and cycling stability performance of the hybrid supercapacitor PPy@MnCo₂O₄/GNF-5//a-MEGO (h). Reproduced with permission from ref. 72. Copyright © 2018 Elsevier B.V. All rights reserved. SEM images of nanographene@MECN at low (i) and high magnification (k) and B-n-MnCo₂O₄@MECN at low (j) and high magnification (l). Stability electrochemical performance of the hybrid supercapacitor B-n-MnCo₂O₄@MECN//AC@Ni foam (m). Reproduced with permission from ref. 77. Copyright © 2020 Elsevier B.V. All rights reserved.

Furthermore, a HSC was built with MnCo₂O₄@PPy/GNF-5 and activated microwave exfoliated graphite oxide (a-MEGO) as positive and negative electrodes, respectively. The HSC showed an energy density of 25.7 W h kg⁻¹ and a power density of 16.1 kW kg⁻¹, besides a capacitance retention of 85.5% after incredible 10 000 cycles (Fig. 3h). Hu et al.⁷³ also reported a composite based on PPy decorated by MnCo₂O₄ urchins showing an energy density of 0.785 mW h cm⁻¹ at a power density of 7.49 W cm⁻¹ as the positive electrode for supercapacitors.

3.1.4. MnCo₂O₄–carbon based composites. Materials based on carbon structures have a large specific surface area, good electrical conductivity and strong mechanical strength, thus becoming leading materials for the electrodes applied for supercapacitors. However, the specific capacitance of carbon is much lower than that of transition metal oxides, which limits their practical applications to a certain degree. Fortunately, carbon modifications can improve the electrical conductivity of MnCo₂O₄-based electrode materials.^74,75 Moreover, the combined effects of MnCo₂O₄ structure with different materials of conducting carbons, such as graphene,^31,33,76,77 activated carbon,^78,79 carbon nanofibers,⁸⁰ carbon aerogels,⁸¹ reduced graphene oxide (rGO),^82,83 carbon nanotubes (CNTs), and graphene quantum dots,⁸⁴ are very promising to improve the overall performance of the system. In this way, Saren et al.⁸⁵ reported the preparation of flower-like hybrid spinel MnCo₂O₄@graphene nanosheets and MnCo₂O₄@CNT nanocomposites by a hydrothermal method for supercapacitor application. A specific capacitance of 923.97 F g⁻¹, an energy density of 82.13 W h kg⁻¹ and a power density of 399.74 W kg⁻¹ at a current density of 1 A g⁻¹ were observed for MnCo₂O₄@graphene nanosheets, while a specific capacitance of 579.71 F g⁻¹ was achieved for MnCo₂O₄@CNT.

Wu and coworkers⁷⁷ developed a new bifunctional composite based on MnCo₂O₄/nanographene (B-n-MnCo₂O₄) prepared on a macroporous electrically conductive network (MECN) as an electrode material for supercapacitors and sodium ion batteries. The 3D structure of nanographene coating MECN and B-n-MnCo₂O₄@MECN can be seen in Fig. 3i–l. Fig. 3i and k show nanographene layers with a lateral size of 50–200 nm and when the B-n-MnCo₂O₄ was incorporated onto the MECN (Fig. 3j and l) the 3D nanostructure displayed a uniform diameter of ∼50 nm. This 3D interconnected morphology seems to be very interesting since it can improve the electronic conductivity, in addition to facilitating electrochemical reactions, because of its large surface area. The B-n-MnCo₂O₄@MECN presented a high specific capacitance of 7.02 F cm⁻² (2341 F g⁻¹) at 3 mA cm⁻². The specific capacitance of B-n-MnCo₂O₄@MECN is much larger than that of Mn₃O₄/nanographene@MECN (2.7 F cm⁻²) and Co₃O₄/nanographene@MECN (2.2 F cm⁻²) at the same current density, which highlights the presence of MnCo₂O₄ in the composite.

Also, a HSC was built with B-n-MnCo₂O₄@MECN and AC@Ni foam as positive and negative electrodes, respectively. The device provided an outstanding long lifetime and stability as well. Even after incredible 20 000 charge–discharge cycles the device showed a capacitance retention of 80%, as can be seen in Fig. 3m. The long lifetime can be attributed to good adhesion between all the components and good electrical contact between B-n-MnCo₂O₄ and MECN as the current collector.

3.1.5. Other MnCo₂O₄-based composite materials. MnCo₂O₄@CoS,⁸⁶ MnCo₂O₄/Ni,^87,88 MnCo₂O₄@nitrogen-doped carbon,⁸⁹ MnCo₂O₄/Ni/Cu,⁹⁰ MnCo₂O₄@Co(OH)₂,⁹¹ and MnCo₂O₄@Ni(OH)₂ belts⁹² are also introduced as positive electrodes for supercapacitor applications, which delivered capacitances comparable with those of the previously discussed composites. For example, Lv et al.⁹³ reported a novel self-supported MnCo₂O₄@Ni₃S₂ core–shell heterostructure, showing a specific capacitance of 2807 F g⁻¹ at 3 A g⁻¹. The same research group⁹⁴ also described a hierarchical MnCo₂O₄/NiMn composite deposited on Ni foam. The layer-by-layer architecture combined with the synergistic effect of both components of the composite provided a specific capacitance of 3063 F g⁻¹ at 3 A g⁻¹ and a cycle stability of 94.7% at 20 A g⁻¹ over 5000 cycles.

Most of the materials previously-mentioned were deposited on Ni foam as the conductive substrate,^27,57,60,65 which did not require the addition of polymer binders, and promoted rapid electron transport between the active material and current collector, thereby resulting in a substantially efficient substrate, due to the high electrical conductivity (1.43 × 10⁷ Ω⁻¹) and thermal conductivity (90.7 W mK⁻¹) of Ni.⁹⁰ Nevertheless, other conductive substrates and current collectors have also been employed, such as carbon aerogels,⁸¹ graphite foam,⁷² indium-doped tin oxide,⁴⁴ activated carbon,⁷⁸ microporous electrically conductive networks,⁷⁷ graphite paper,^59,67,95 carbon fiber paper⁹⁶ and copper foil.³⁵Table 1 presents the performance of different supercapacitor materials coupled with different types of electrodes based on MnCo₂O₄.

Table 1 Performances of some reported MnCo₂O₄-based supercapacitors^a

Type	Material	Specific capacitance (F g^−1)	Specific capacity (C g^−1)	Potential window (V vs.)	Retention (%), rate capability (A g^−1)	Cycling stability	Highest energy density (W h kg^−1)	Highest power density: (kW kg^−1)	Negative electrode material	Ref.
a AC = activated carbon; a-MEGO = activated microwave exfoliated graphite oxide; CFs = carbon fibres; rGO = reduced graphene oxide; NF = nickel foam; N-CNTs = N-doped carbon nanotubes; PPy = polypyrrole; rMnCo2O4@rMnO2 = reduced core–shell structured MnCo2O4@MnO2. * By cyclic voltammetry.
Pristine	Flake-like MnCo2O4	1487 F g^−1 at 1 A g^−1	594.8 C g^−1	0.0–0.4 V (vs. SCE)	—	93.3% (2000 cycles)	—	—	—	35
	MnCo2O4	405 F g^−1 at 1 mA cm^−2	—	0–0.4 V (vs. SCE)	67.9%, (5–40 mA cm^−2)	95.1% (1000 cycles)	—	—	—	37
	MnCo2O4 nanorods	845 F g^−1 at 1 A g^−1	422.5 C g^−1	0.0–0.5 V (vs. Hg/HgO)	51.6%, (1–20 A g^−1)	90.2% (2000 cycles)	53.7	8	rGO/NF	39
	MnCo2O4 nanorods	718.75 F g^−1 at 0.5 A g^−1	287.5 C g^−1	0.0–0.4 V (vs. Ag/AgCl)	—	100% (1000 cycles)	—	—	—	40
	MnCo2O4 hollow spheres	0.8 F cm^−2 at 2 mA cm^−2	0.32 C cm^−2	0.0–0.4 V (vs. SCE)	—	99% (2000 cycles)	0.052 mW h cm^−3	320 mW cm^−3	AC/CFs	47
	Porous sphere MnCO2O4	1044 F g^−1 at 0.5 A g^−1	574.2 C g^−1	0.0–0.55 V (vs. Hg/HgO)	68.9%, (1–20 A g^−1)	133.3% (10 000 cycles)	42.27	0.4	Starch-derived carbon foam	36
	Network-like porous MnCo2O4	647.42 F g^−1 at 1 A g^−1	323.71 C g^−1	0.1–0.6 V (vs. Hg/HgO)	70.67%, (1–10 A g^−1)	93.68% (3000 cycles)	—	—	—	48
	MnCO2O4 nanowires	2146 F g^−1 at 1 A g^−1	858.4 C g^−1	0.0–0.4 V (vs. SCE)	—	92.1% (5000 cycles)	29.3	8	AC	29
	MnCO2O4 nanowires	405 F g^−1 at 1 A g^−1	202.5 C g^−1	0.0–0.5 V (vs. Hg/HgO)	51.4%, (1–10 A g^−1)	91% (4000 cycles)	—	—	—	97
	MnCO2O4 nanowires	1342 F g^−1 at 1 A g^−1	738.1 C g^−1	0.0–0.55 V (vs. Hg/HgO)	73.6%, (1–20 A g^−1)	—	—	—	—	51
	MnCO2O4 nanowires	349.8 F g^−1 at 1 mV s^−1	157.4 C g^−1	0.0–0.45 V (vs. Ag/AgCl)	94%, (1–20 A g^−1)	94% (4000 cycles)	33.3	4.5	—	50
	MnCo2O4 nanoparticles	1068.5 F g^−1 at 1 A g^−1	427.2 C g^−1	0.0–0.4 V (vs. SCE)	50%, (1–8 A g^−1)	90% (2000 cycles)	—	—	—	26
	Chestnut-like MnCo2O4 nanoneedles	1535 F g^−1 at 1 A g^−1	921 C g^−1	0.0–0.6 V (vs. SCE)	61.8%, (1–10 A g^−1)	94.3% (12 000 cycles)	∼60.4	∼0.375	Graphene/NF	38
	MnCo2O4 nanosheets	2000 F g^−1 at 0.5 A g^−1	1000 C g^−1	0.0–0.5 V (vs. Hg/HgO)	57.5%, (0.5–20 A g^−1)	92.3% (5000 cycles)	73.95	15	rGO	42
	Nanocage MnCo2O4	1763 F g^−1 at 1 A g^−1	969.65 C g^−1	0.0–0.55 V (vs. Hg/HgO)	—	95% (4500 cycles)	54.15	0.324	Nanocage MnCo2O4	45
	Flower-like MnCo2O4	249.3 F g^−1 at 0.5 A g^−1	174.5 C g^−1	−0.3–0.4 V (vs. Hg/HgO)	78.8%, (0.5–5 A g^−1)	93.6% (2000 cycles)	—	—	—	98
	Flower-like MnCo2O4	571 F g^−1 at 0.5 A g^−1	285.5 C g^−1	−0.1–0.4 V (vs. Hg/HgO)	87.7%, (0.5–5 A g^−1)	96.1% (2000 cycles)	—	—	—	99
	MnCo2O4 cubes	480.5 F g^−1 at 1 A g^−1	264.3 C g^−1	0.0–0.55 V (vs. Hg/HgO)	75.7%, (1–25 A g^−1)	96.6% (3000 cycles)	—	—	—	41
	MnCo2O4 cuboidal microcrystals	600 F g^−1 at 0.5 A g^−1	300 C g^−1	0.0–0.5 V (vs. Ag/AgCl)	55.7%, (0.5–5 A g^−1)	132% (3000 cycles)	—	—	—	27
	MnCo2O4	270 F g^−1 at 10 mV s^−1	—	−1.2–1.5 V (vs. Ag/AgCl)	—	92.4% (1000 cycles)	14.85	0.495	—	49
	Porous MnCo2O4	151 F g^−1 at 5 mV s^−1	—	0–0.5 V (vs. SCE)	83.6%, (0.1–5 A g^−1)	—	—	—	—	95
	MnCo2O4 nanosheets	—	282 C g^−1 at 1 A g^−1	0–0.55 V (vs. SCE)	25.2%, (1–10 A g^−1)	84% (1500 cycles)	2.55	37.57	FeMn2O4	43
	MnCo2O4 nanorods	187.0 F g^−1 at 0.25 A g^−1	51.9 C g^−1	0.0–0.4 V (vs. Ag/AgCl)	8%, (0.25–30 A g^−1)	90% (2000 cycles)	12.77	2.520	AC	100
	MnCo2O4 nanosheets	250 F g^−1 at 0.25 A g^−1	100 C g^−1	0.0–0.4 V (vs. Ag/AgCl)	10.8%, (0.25–10 A g^−1)	95% (1000 cycles)	10.04	5.2	—	44
MnCo2O4-metal oxide	Co3O4@MnCo2O4	736.5 F g^−1 at 1 mA cm^−2	—	0–0.5 V (vs. SCE)	—	79.95% (3000 cycles)	—	—	—	53
	MnCo2O4@Co3O4	—	1440 C cm^−2 at 1 mA cm^−2	0.0–0.45 V (vs. SCE)	—	101.23% (8000 cycles)	31	0.208	AC	54
	MnCo2O4@CoMnO4	2115.38 F g^−1 at 1.1 A g^−1	1100 C g^−1	0.0–0.52 V (vs. Hg/HgO)	47.4%, (1.1–6.6 A g^−1)	119% (5000 cycles)	37.5	0.527	AC	69
	MnCo2O4@CoCo2O4	614 F g^−1 at 1 A g^−1	276.3 C g^−1	0.0–0.45 V (vs. SCE)	76.9%, (1–10 A g^−1)	77.5% (5000 cycles)	—	—	—	66
	MnCo2O4@NiMoO4	1718 F g^−1 at 1 A g^−1	859 C g^−1	0.0–0.5 V (vs. Hg/HgO)	69.8%, (1–8 A g^−1)	84% (6000 cycles)	42.3	0.797	AC	56
	CuCo2O4/MnCo2O4	1434 F g^−1 at 0.5 A g^−1	860.4 C g^−1	0.0–0.6 V (vs. SCE)	56.6%, (0.5–15 A g^−1)	81.4% (5000 cycles)	42.1	0.4	Graphene/NF	67
	MnCo2O4@NiMoO4	1244 F g^−1 at 1 A g^−1	746.4 C g^−1	0.0–0.6 V (vs. SCE)	91%, (1–10 A g^−1)	81% (2500 cycles)	42	0.8	AC	57
	NiCo2O4–MnCo2O4	1152 F g^−1 at 1 A g^−1	576 C g^−1	0.0–0.5 V (vs. Ag/AgCl)	72.2%, (1–10 A g^−1)	95.4% (3000 cycles)	40	0.2	—	68
	MnO2/MnCO2O4	497 F g^−1 at 0.5 A g^−1	248.5 C g^−1	0.0–0.5 V (vs. Hg/HgO)	62.8%, (0.5–10 A g^−1)	60% (5000 cycles)	—	—	—	61
	MnCo2O4@NiO	508.3 F g^−1 at 2 A g^−1	279.6 C g^−1	0.0–0.55 V	67.7%, (0.5–6 A g^−1)	89.7% (2000 cycles)	—	—	—	65
	MnCo2O4@NiWO4	5.09 F cm^−2 at 1 mA cm^−2	2.54 C cm^−2	0.0–0.5 V (vs. Hg/HgO)	26.1%, (1–20 mA cm^−2)	96% (5000 cycles)	0.23 mW h cm^−2	2.66 mW cm^−2	AC	63
	CoO/MnO2/MnCO2O4 composite nanowires	1650 F g^−1 at 1 A g^−1	660 C g^−1	0.0–0.4 V (vs. Ag/AgCl)	60.0%, (1–15 A g^−1)	—	90	10	CoO/MnO2/MnCO2O4 composite nanowires	62
	rMnCo2O4@rMnO2	3.39 F cm^−2 at 3 mA cm^−2	0.4 mA h cm^−2	0.0–0.4 V (vs. SCE)	48.9%, (3–60 mA cm^−2)	92.5% (3000 cycles)	32.4	4.524	AC	101
	MnCo2O4@MnO2 core–shell arrays	2262 F g^−1 at 1 A g^−1	904.8 C g^−1	0.0–0.4 V (vs. SCE)	48.8%, (1–20 A g^−1)	87.1% (5000 cycles)	34.7	24	Graphene/NF	59
	ZnO@MnCo2O4	631.2 F g^−1 at 1 A g^−1	504.9 C g^−1	−0.2–0.6 V (vs. SCE)	31.7%, (1–10 A g^−1)	92.3% (1000 cycles)	56.10	0.4	—	64
	NiCo2O4–MnCo2O4 rose-like composite	1100 F g^−1 at 1 A g^−1	440 C g^−1	0.0–0.4 V (vs. SCE)	69.1%, (1–10 A g^−1)	—	—	—	—	102
	MnCo2O4@MnMoO4 core–shell	—	885 C g^−1 at 3 A g^−1	0.0–0.4 V (vs. SCE)	71%, (3–30 A g^−1)	95% (5000 cycles)	49.4	0.815	AC	55
MnCo2O4-conducting polymer	MnCo2O4@PPy/GNF	2933 F g^−1 at 2 A g^−1	1466.5 C g^−1	0.0–0.5 V (vs. Ag/AgCl)	55.3%, (2–20 A g^−1)	86.7% (5000 cycles)	78.2	1.121	AC	32
	MnCo2O4@PPy	—	2.862 mA h cm^−2 at 1 mA cm^−2	0.0–0.5 V (vs. Hg/HgO)	84.5%, (1–10 mA cm^−2)	88% (2000 cycles)	0.785 mW h cm^−3	7.49 mW cm^−3	AC	73
	Polypyrrole@MnCo2O4/graphite foam	2364 F g^−1 at 1 A g^−1	1182 C g^−1	−0.1–0.4 V (vs. Ag/AgCl)	55.2%, (1 to 50 A g^−1)	85.3% (1000 cycles)	25.7	16.1	a-MEGO	72
MnCo2O4-carbon based composites	MnCo2O4/graphene	503 F g^−1 at 1 A g^−1	201.2 C g^−1	0.0–0.4 V (vs. SCE)	65.5%, (1–20 A g^−1)	97.4% (5000 cycles)	—	—	—	76
	Flower-like MnCo2O4/rGO	923.97 F g^−1 at 1 A g^−1	739.2 C g^−1	0.0–0.8 V (vs. Ag/AgCl)	—	111% (5000 cycles)	82.13	0.399	Carbon black	85
	MnCo2O4@nitrogen doped graphene@ MnO2	1170 F g^−1 at 1 A g^−1	585 C g^−1	0.0–0.5 V (vs. Ag/AgCl)	69.9%, (1–20 A g^−1)	87.6% (5000 cycles)	48.5	∼0.808	Nitrogen doped graphene hydrogel	33
	C@MnCo2O4	728.4 F g^−1 at 1 A g^−1	364.2 C g^−1	0.0–0.5 V (vs. Ag/AgCl)	71.3%, (1–10 A g^−1)	—	25.5	0.856	—	79
	MnCo2O4@activated carbon	—	443.5 C g^−1 at 0.5 A g^−1	−1.0–0.4 V (vs. SCE)	—	66.95% (5000 cycles)	—	—	—	78
	MnCo2O4 graphene quantum dots	1625 F g^−1 at 1 A g^−1	812.5 C g^−1	0.0–0.5 V (vs. Hg/HgO)	80%, (0.5–10 A g^−1)	80% (5000 cycles)	46	0.066	rGO	84
	B-n-MnCo2O4@MECN	7.02 F cm^−2 at 3 mA cm^−2	—	−0.1–0.45 V (vs. Hg/HgO)	31%, (3–20 mA cm^−2)	98% (5000 cycles)	50.5	6.4	AC/NF	77
Other MnCo2O4-based composite materials	MnCo2O4@Co(OH)2		1185 C g^−1 at 1 A g^−1	0–0.5 V (vs. SCE)	78%, (1–20 A g^−1)	96% (5000 cycles)	67.2	0.8	N-CNTs@rGO	91
	MnCo2O4@Ni(OH)2	2154 F g^−1 at 5 A g^−1	969.3 C g^−1	0.0–0.45 V (vs. SCE)	32.6%, (5–20 A g^−1)	90% (2500 cycles)	48	14.9	MnCo2O4@Ni(OH)2	92
	MnCo2O4@Ni3S2 core–shell heterostructures	2807 F g^−1 at 3 A g^−1	1122.8 C g^−1	0.0–0.4 V (vs. SCE)	69.0%, (3–30 A g^−1)	92% (5000 cycles)	—	—	—	93
	MnCo2O4@NiMn	3063 F g^−1 at 3 A g^−1	1378.3 C g^−1	0.0–0.45 V (vs. SCE)	75.6%, (3–30 A g^−1)	94.7% (5000 cycles)	51.9	0.8	AC	94
	Dual-MnCo2O4/Ni	2265 F g^−1 at 1.7 A g^−1	283 mA h g^−1	0–0.45 V (vs. Ag/AgCl)	70%, (2–10 mA cm^−2)	85% (2000 cycles)	—	—	—	88
	MnCo2O4@CoS	1607.4 F g^−1 at 0.8 A g^−1	723.3 C g^−1	0.0–0.45 V (vs. SCE)	—	91.5% (5000 cycles)	55.1	0.477	AC	86

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3.2. Batteries

3.2.1. Lithium-ion batteries. Rechargeable Li-ion batteries (LIBs) were shown as the most efficient energy storage devices, since the small size of lithium ions makes their diffusion more favorable in a variety of structures.¹⁰³ Spinel binary transition-metal oxide materials, such as MnCo₂O₄, have been widely investigated as anode electrodes for LIBs, due to their lower cost and better electronic conductivity than single-metal oxides.^104,105 A large variety of nanoscale building blocks of MnCo₂O₄ have been widely investigated, especially due to their good electrochemical performance.¹⁰⁶

In order to improve the electrochemical performance of MnCo₂O₄ structures as anodic materials in LIBs, morphological modifications and synthesis methods have been the focus of investigations.^107–113 One of these strategies has been focused on the preparation of porous MnCo₂O₄ materials^114,115 and on the development of diverse substrates as current collectors (carbon cloth,^116–118 Ni foam¹¹³ and copper foil^115,119) with robust adhesion to obtain a binder-free anode material. Li et al.¹²⁰ first developed a two-step method to prepare uniform hollow MnCo₂O₄ submicrospheres with multilevel interiors (mesoporous, hollow, yolk–shell, shell-in-shell, and yolk-in-double-shell spheres). The yolk–shell morphology (Fig. 4a) showed the best performance among these multilevel interior structures, with an initial discharge capacity of 1425 mA h g⁻¹ at a current density of 400 mA g⁻¹ (Fig. 4b). Huang et al.¹²¹ also fabricated spherical yolk–shell MnCo₂O₄ powders (Fig. 4c) by a hydrothermal method followed by a thermal treatment, with an initial discharge capacity of 1445 mA h g⁻¹ at 0.2 A g⁻¹ and capacity retention of ∼860.0 mA h g⁻¹ after 40 cycles at 0.2 A g⁻¹ (Fig. 4d). The literature reports the preparation of different MnCo₂O₄ porous based structures for anode electrodes, such as spheres,^122,123 yolk–shell microspheres¹²⁴ (Fig. 4e and f), microflowers,¹²⁵ hydrangea-like structures,¹²⁶ and dumbbell-shaped structures.¹²⁷

Fig. 4 Scanning electron microscopy images (SEM) and galvanostatic charge–discharge profiles (GCD) of yolk–shell MnCo₂O₄. SEM images of (a) hollow MnCo₂O₄ submicrospheres. (c) Hierarchical porous MnCo₂O₄ yolk–shell microspheres and (e) spherical yolk–shell MnCo₂O₄ powders. GCD curves of (b) hollow MnCo₂O₄ submicrospheres, (d) hierarchical porous MnCo₂O₄ yolk–shell microspheres and (f) spherical yolk–shell MnCo₂O₄ powders. (a) and (b) adapted with permission from ref. 120. Copyright © 2014, American Chemical Society. (c and d) adapted with permission from ref. 121. Copyright © Marketplace™, Royal Society of Chemistry. (e and f) adapted with permission from ref. 124. Copyright © Marketplace™, Royal Society of Chemistry.

Fu et al.¹⁰⁶ reported the preparation of microspheres of MnCo₂O₄ by a calcination-free method; in this work, two kinds of MnCo₂O₄ crystals with different exposed facets of (1̄1̄0) and (11̄2̄) were synthesized, presenting two different morphologies, particle-assembled and sheet-assembled microspheres, respectively. The anode was evaluated, and the microspheres delivered a capacity of 722 mA h g⁻¹ after 25 cycles at a current density of 200 mA g⁻¹, and capacities up to 553 and 320 mA h g⁻¹ after 200 cycles at a current density of 400 and 900 mA g⁻¹, respectively.

Huang and colleagues¹²⁸ proposed a novel core–shell ellipsoidal MnCo₂O₄ powder with a desired micro-/nano-structure and unique concentration gradient. The battery tests demonstrated excellent values of initial discharge capacities (1433.3 mA h g⁻¹ at 0.1 A g⁻¹ and 1248.4 mA h g⁻¹ at 0.4 A g⁻¹), capacity retention (∼900.0 mA h g⁻¹ after 60 cycles at 0.1 A g⁻¹) and rate performance (∼620.0 mA h g⁻¹ after 50 cycles at 0.4 A g⁻¹).

Developing composites of MnCo₂O₄ with other materials is an important strategy to improve the performance of LIBs.^129,130 A 3D sandwich-shape graphene based nanocomposite intercalated with double-shelled hollow MnCo₂O₄ spheres as an anode material for LIBs has been synthesized, showing a rate capability of 538 mA h g⁻¹ at a current density of 1000 mA g⁻¹ and outstanding cycle performance, with a capability of 703 mA h g⁻¹ after 100 cycles at 200 mA g⁻¹.¹³¹ The literature also reports the synthesis of MnCo₂O₄ containing nickel,^132,133 Co₃O₄,¹¹⁹ CoO,¹¹⁸ MnO₂,¹³⁴ TiO₂,¹³⁵ and NiCo₂O₄.¹⁰² Huang and coauthors¹³⁶ designed a MnCo₂O₄@N-doped carbon@MnO₂ three layered core shell octahedron as an anode material for Li-ion storage, which displayed a discharge capacity of 894 mA h g⁻¹ at a current density of 500 mA g⁻¹ after 120 cycles. Even at a high current density of 1000 mA g⁻¹, the discharge capacity remained at 839 mA h g⁻¹ after 600 cycles.

Some conductive substrates have also served as current collectors to further improve the electrochemical performance of electrode materials, including carbon materials and conductive polymers, which have been mixed with MnCo₂O₄ structures,^137,138 such as graphene,¹³⁹ CNTs,¹⁴⁰ carbon cloth^116,117 and PPy.¹⁴¹ Hence, the construction of composites with the combination of two or more different materials has been proved as a promising strategy to boost the electrochemical performance of MnCo₂O₄. Due to the electronic conductivity and the specific surface area, rGO has been considered for the formation of composites with MnCo₂O₄. Fan et al.¹⁴² reported the synthesis of MnCo₂O₄/rGO composites with an initial discharge capacity of 1657 mA h g⁻¹ at a current density of 0.1 A g⁻¹, and a reversible capacity of 791 mA h g⁻¹ at 0.2 A g⁻¹ for 100 cycles. A MnCo₂O₄@PANi-rGO composite was also synthesized by Huang et al.,¹⁴³ with a discharge capacity of 745 mA h g⁻¹ and a coulombic efficiency of 100% after 1050 cycles at a current density of 500 mA g⁻¹.

3.2.2. Sodium ion batteries. Operating with similar chemistry to Li-ion electrodes, sodium-ion batteries (SIBs) are under intense investigation to overtake LIBs with the advantages of low-cost and safety. Wu and coworkers¹⁴⁴ reported mesoporous Ni-doped MnCo₂O₄ hollow nanotubes (MCNO-HNTs) as an anode in SIBs, with a remarkable capacity retention of 81% at 1 A g⁻¹ even after 11 000 cycles. Flower-like MnCo₂O₄ synthesized by a co-precipitation method exhibited a discharge capacity of 244 mA h g⁻¹ after 40 cycles at 50 mA h g⁻¹, which corresponds to 77.1% compared with the second discharge capacity cycle.¹⁴⁵Table 2 summarizes the performance of different cells of metal ion batteries coupled with different types of electrodes based on MnCo₂O₄.

Table 2 Performances reported for MnCo₂O₄-based metal ion batteries^a

Battery type	Material type	Material	Initial discharge (mA h g^−1)	Potential window (V vs.)	Reversible capacity (Nth) mA h g^−1	Stability (cycle numbers)	Ref.
a rGO = reduced graphene oxide, MWCNT = multi wall carbon nanotube, PANi = polyaniline, and NC = N-doped carbon.
Lithium ion batteries (LIBs)	Pristine MnCo2O4	MnCo2O4 yolk–shell microspheres	1035.8 at 0.1C	0.01–3 Li/Li^+	691.3 (500) at 1C	—	124
		Core–shell ellipsoidal MnCo2O4	1433.3 at 0.1 A g^−1	0.01–3 Li/Li^+	750 (70) at 100 mA g^−1	—	128
		Micro-octahedral MnCo2O4	1438 at 300 mA g^−1	0.01–3 Li/Li^+	720.4 (200) at 300 mA g^−1	88% (200 cycles)	110
		MnCo2O4 nanoflakes	1795 at 50 mA g^−1	0.01–3 Li/Li^+	925 (50) at 100 mA g^−1	—	109
		MnCo2O4 microspheres	1473 at 60 mA g^−1	0.01–3 Li/Li^+	533 (200) at 400 mA g^−1	—	106
		MnCo2O4 cubic microcrystals	1443 at 100 mA g^−1	0.01–3 Li/Li^+	—	28% (100 cycles)	104
		MnCo2O4 needle-shaped	1326 at 60 mA g^−1	0.01–3 Li/Li^+	368 (50) at 60 mA g^−1	55% (50 cycles)	107
		MnCo2O4 hollow spheres	1561 at 200 mA g^−1	0.01–3 Li/Li^+	1023 (200) at 60 mA g^−1	—	122
		MnCo2O4 hollow submicrospheres	1119 at 400 mA g^−1	0.01–3 Li/Li^+	800 (100) at 100 mA g^−1	—	120
		MnCo2O4 quasi-hollow microspheres	1473 at 200 mA g^−1	0.01–3 Li/Li^+	610 (100) at 400 mA g^−1	—	123
		MnCo2O4	900 at 60 mA g^−1	0.005–3 Li/Li^+	816 (50) 60 mA g^−1	—	111
		MnCo2O4 microspheres	1425.8 at 400 mA g^−1	0.005–3 Li/Li^+	1033.3 (200)/400 mA g^−1	74.2% (200 cycles)	105
		Dumbbell-shaped porous MnCo2O4	2073 at 200 mA g^−1	0.01–3 Li/Li^+	955(180) at 200 mA g^−1	46% (180 cycles)	127
		Erythrocyte like MnCo2O4	1538 at 200 mA g^−1	0.01–3 Li/Li^+	960 (100) at 200 mA g^−1	77.6% (100 cycles)	112
		Mesoporous MnCo2O4 microflowers	1465.1 at 100 mA g^−1	0.01–3 Li/Li^+	732 (50) at 100 mA g^−1	—	125
		MnCo2O4 nanospheres	1184.8 at 400 mA g^−1	0.01–3 Li/Li^+	749.1 (50) at 200 mA g^−1	89.8% (50 cycles)	108
		Porous MnCo2O4	1750.0 at 400 mA g^−1	0.01–3 Li/Li^+	690.1 (100) at 0.5C	—	114
		Flake-like MnCo2O4	1460 at 100 mA g^−1	0.01–3 Li/Li^+	952 (100) at 100 mA g^−1	89% (100 cycles)	35
		MnCo2O4 nanosheets	3.9 mA h cm^−2 at 800 μA cm^−2	0.01–3 Li/Li^+	3.0 mA h cm^−2 (60) at 800 μA cm^−2	—	116
		Porous MnCo2O4 nanosheets	1044 at 0.2 A g^−1	0.01–3 Li/Li^+	848 (200) at 0.2 A g^−1	81% (200 cycles)	113
		Porous MnCo2O4 microspheres	1034 at 1000 mA g^−1	0.01–3 Li/Li^+	740 (1000) at 1000 mA g^−1	—	115
		Yolk–shell MnCo2O4 microspheres	1445.1 at 0.2 A g^−1	0.01–3 Li/Li^+	860 (40) at 0.2 A g^−1	—	121
		MnCo2O4	1220 at 0.1C	0.1–3 Li/Li^+	907 (50) at 0.5C	90% (50 cycles)	146
		MnCo2O4 nanotubes	1211.9 at 0.5 A g^−1	0.01–3 Li/Li^+	701.4 (320) at 500 mA g^−1	—	147
		Porous hydrangea-like MnCo2O4	1232 at 0.1 A g^−1	0.01–3 Li/Li^+	930 (100) at 0.1 A g^−1	87% (100 cycles)	126
	MnCo2O4–metal oxide composite	CoO/MnCo2O4.5 nanorods	1183 at 200 mA g^−1	0.01–3 Li/Li^+	1030 (120) at 200 mA g^−1	87% (120 cycles)	118
		MnCo2O4@NC@MnO2	1380 at 500 mA g^−1	0.01–3 Li/Li^+	894 (120) at 500 mA g^−1	—	136
		NiO–MnCo2O4–Ni6MnO8	1284 at 30 mA g^−1	0.01–3 Li/Li^+	779 (120) at 1C	—	129
		Co3O4–MnCo2O4 powder, 20–50 nm	1781 at 50 mA g^−1	0.01–3 Li/Li^+	1250 (200) at 1000 mA g^−1	68.2% (200 cycles)	119
		MnCo2O4–TiO2 microspheres	1396.9 at 100 mA g^−1	0.01–3 Li/Li^+	1271 (200) at 100 mA g^−1	91% (200 cycles)	135
		Porous MnCo2O4@MnO2	1927.8 at 100 mA g^−1	0.0–3 Li/Li^+	1162.8 (200) at 1 A g^−1	96% (200 cycles)	134
		NiO–MnCo2O4 microspheres	1206 at 200 mA g^−1	0.01–3 Li/Li^+	846 (50) at 200 mA g^−1	—	130
	MnCo2O4–carbon based composites	3D sandwich-shaped graphene-based MnCo2O4 hollow spheres	1244 at 200 mA g^−1	0.01–3 Li/Li^+	703 (100) at 200 mA g^−1	80% (100 cycles)	131
		Graphene-like 2D spinel MnCo2O4	1157.7 at 0.2 A g^−1	0.01–3 Li/Li^+	780 (200) at 0.2 A g^−1	—	139
		MnCo2O4 nanoparticles embedded in graphene sheets	1350 at 100 mA g^−1	0.01–3 Li/Li^+	584.3 (250) at 2000 mA g^−1	92.3% (250 cycles)	138
		MnCo2O4@carbon cloth	1886.2 at 1 A g^−1	0.01–3 Li/Li^+	1289 (200) at 1 A g^−1	—	117
		MnCo2O4/rGO composite	1657 at 0.1 A g^−1	0.01–3 Li/Li^+	791 (100) at 0.2 A g^−1	74.1% (100 cycles)	142
		MnCo2O4/C	1284.5 at 1 A g^−1	0.01–3 Li/Li^+	978 (800) at 1 A g^−1	—	148
		MWCNT composite	1471 at 946 mA g^−1	0.005–3 Li/Li^+	871 (30) at 60 mA g^−1	—	140
	MnCo2O4-conducting polymer composites	MnCo2O4/polypyrrole	1398 at 200 mA g^−1	0.01–3 Li/Li^+	910 (100)/200 mA g^−1	—	141
		Flower-like MnCo2O4@PANi–rGO	—	0.01–3 Li/Li^+	745 (1050)/500 mA g^−1	—	143
	Other MnCo2O4-based composite materials	Mn0.4Ni0.6Co2O4 nanowires	1054 at 0.1 A g^−1	0.01–3 Li/Li^+	706 (200) at 500 mA g^−1	98% (200 cycles)	132
		MnCo2O4 porous nanospheres@N-doped carbon	1099.9 at 1 A g^−1	0.01–3 Li/Li^+	883.3 (500) at 1 A g^−1	—	137
		Ni-doped MnCo2O4 submicron-spheres	1849 at 0.2 A g^−1	0.01–3 Li/Li^+	174.7 (2000) at 5 A g^−1	—	133
Sodium ion batteries	—	Mesoporous Ni-doped MnCo2O4 hollow nanotubes	340 at 0.1 A g^−1	0.01–3 Na/Na^+	109 (11 000) at 1 A g^−1	81% (11 000 cycles)	144
	—	Flower-like MnCo2O4	697 at 25 mA g^−1	0.01–3 Na/Na^+	244 (40) at 50 A g-^1	77.1% (40 cycles)	145
	MnCo2O4-carbon based composites	MnCo2O4/nanographene	1120 at 0.05 A g^−1	0.01–3 Na/Na^+	541.2 (200) at 0.05 A g^−1	—	77

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3.2.3. Li–O₂ batteries. Lithium–oxygen batteries have a specific potential energy density of approximately 1700 W h kg⁻¹, which is 5-fold higher than that of conventional current LIBs. This type of device has also significant advantage in their gravimetric energy densities.¹⁴⁹ Transition metal oxides, such as MnCo₂O₄, have been investigated as cathodes for Li–O₂ batteries,^150,151 especially due to their low-cost catalyst, good stability, high activity, and simple preparation.¹⁵² Wu et al.¹⁵³ reported hierarchical porous 3D MnCo₂O₄ nanowire bundles as a cathode for Li–O₂ cell application (Fig. 5a), which exhibited specific capacities of 500 and 1000 mA h g⁻¹ over 300 and 144 cycles, respectively, and a discharge capacity of 12 919 mA h g⁻¹ at 0.1 mA cm⁻². More importantly, after two months of cycling, the microstructure of the cathode was maintained and a recyclability of over 200 cycles was achieved. Other structures with high electrochemical performance can be obtained, such as nanotubes¹⁵⁴ (Fig. 5b) and spheres^155,156 (Fig. 5c). Composites are also explored to maximize the electrochemical performance of Li–O₂ batteries, particularly highlighting materials containing porous carbon,¹⁵⁷ Ti₄O₇ ¹⁵⁸ and MoO₂/Ni ¹⁵⁹.

Fig. 5 Scanning electron microscopy images (SEM) of different morphologies of MnCo₂O₄. SEM images of (a) MnCo₂O₄ nanowires, (b) single-wall MnCo₂O₄ nanotubes and (c) MnCo₂O₄ nanospheres. Fig. 1a adapted with permission from ref. 53. Copyright © 2017, American Chemical Society. Fig. 1b, adapted with permission from ref. 154. Copyright © Marketplace™, Royal Society of Chemistry. Fig. 1c adapted with permission from ref. 155. Copyright © Marketplace™, Royal Society of Chemistry.

Large surface areas can provide a promising electrocatalytic activity for the ORR and OER in Li–O₂ batteries. In this way, to enhance the charge transfer rate, composites between MnCo₂O₄ and carbon materials have been reported.^{152,160–163} A peanut shaped MnCo₂O₄ which is encapsulated by multi-walled carbon nanotubes (MCO/MWCNTs) was synthesized through a solvothermal method. The batteries exhibited a discharge capacity of 8849 mA h g⁻¹ with a restricted voltage of 2 V at 100 mA g⁻¹ and a cycle life of 120 times at 100 mA g⁻¹ with a limited capacity of 500 mA h g⁻¹.¹⁶⁴

3.2.4. Other metal–air batteries. MnCo₂O₄ electrodes can be prepared as active materials for different metal–air batteries. Ishihara et al.¹⁶⁵ studied ORR/OER on mesoporous spinels for Zn–air rechargeable batteries. The MnCo₂O₄ spinel showed a surface area of 108 m² g⁻¹ and an average pore size of 2 nm, providing a decrease of overpotential for the ORR/OER in Zn–air batteries, which showed a stable discharge potential and capacity at 1.05 V and 700 mA h g⁻¹, respectively.

Carbonaceous materials and heteroatom doped-carbon materials were also mixed with MnCo₂O₄ due to their intrinsic advantages as ORR catalysts, such as higher surface area and electrochemical stability. Chandrappa et al.¹⁶⁶ reported a composite formed by combining MnCo₂O₄ nanospheres with graphene sheets (MCO/GS) as a bifunctional cathode catalyst for Zn–air batteries. The electrochemical measurements revealed a unique small charge–discharge overpotential, cycling stability and higher rate capability than a bare MCO catalyst. Carbon coated MnCo₂O₄ nanowires (MnCo₂O₄@C) were also used as a bifunctional oxygen catalyst for rechargeable Zn–air batteries. The authors recorded an excellent electrochemical performance and improved cycling stability, with an onset potential of 0.92 V and current retention rate of 99% within 10 000 s at 0.80 V vs. RHE.¹⁶⁷ Besides Zn–air batteries,^166–169 Na–air devices have also been reported in the literature.^170,171Table 3 summarizes the performance of different cells coupled with different metal–air batteries of electrodes based on MnCo₂O₄.

Table 3 Performances reported for MnCo₂O₄-based metal–air batteries

Type	Cathodes	Initial discharge (mA h g^−1)	Discharge voltage (V)	Charge voltage (V)	Overpotential^a (V)	Stability (cycle numbers)	Ref.
a The overpotential was calculated based on the difference of discharge–charge voltage plateaus. rGO = reduced graphene oxide, MWCNT = multi wall carbon nanotube, and C = carbon.
Na air battery	Co3O4@MnCo2O4.5 nanocubes	8400 at 500 mA g^−1	2.3	2.75	0.45	(135 cycles)	171
	dp-MnCo2O4/N-rGO	—	2.75	3.14	0.39	(25 cycles)	170
	Ni-doped hollow nanotubes	340.5 at 0.1 A g^−1	—	—	—	81% (11 000 cycles)	144
Zn air battery	MnCo2O4 nanofibers	125 at 10 mA cm^−2	—	—	1.23	(500 cycles)	168
	MnCo2O4 nanoparticles embedded in nitrogen-doped macroporous carbon nanofiber arrays	—	—	—	0.55	(100 cycles)	169
	MnCo2O4@C nanowires	—	—	—	0.89	—	167
Li air battery	MnCo2O4@Ni	10 520 at 100 mA g^−1	2.79	—	0.65	1000 mA h g^−1 (119 cycles)	150
	Nanowires	12 919 at 0.1 mA cm^−2	2.92	3.46	0.54	1000 mA h g^−1 (144 cycles)	153
	Microspheres	2809.1 at 500 mA g^−1	2.7	3.9	—	1000 mA h g^−1 (50 cycles)	156
	P-Doped hierarchical porous carbon	13 150 at 200 mA g^−1	2.75	4.0	—	1000 mA h g^−1 (200 cycles)	157
	Ti4O7/MnCo2O4	5400 at 100 mA g^−1	2.85	3.6	0.75	500 mA h g^−1 (100 cycles)	158
	MnCo2O4 nanospheres	8518 at 100 mA g^−1	2.86	—	0.85	1000 mA h g^−1 (20 cycles)	155
	MnCo2O4 nanorods	1334 at 0.1 mA cm^−2	2.61	4.10	—	500 mA h g^−1 (40 cycles)	151
	MnCo2O4/graphene	3784 at 100 mA g^−1	2.95	3.75	∼0.8	1000 mA h g^−1 (40 cycles)	162
	Peanut shaped MnCo2O4/MWCNTs	8849 at 100 mA g^−1	—	—	—	500 mA h g^−1 (120 cycles)	164
	MnCo2O4/graphene	10 092 at 100 mA g^−1	2.9	3.7	0.8	1000 mA h g^−1 (250 cycles)	161
	MnCo2O4@carbon cloth	7238 at 200 mA g^−1	—	—	1.46	500 mA h g^−1 (108 cycles)	160
	MnCo2O4–graphene	11 092.1 at 200 mA g^−1	—	—	—	1000 mA h g^−1 (35 cycles)	163
	Double-wall MnCo2O4 nanotubes	8100 at 100 mA g^−1	2.77	4.14	1.37	1000 mA h g^−1 (278 cycles)	154
	MnCo2O4/MoO2@Ni nanosheets	4210 at 200 mA g^−1	—	—	0.75	(400 cycles)	159
	MnCo2O4 nanowires	8364 at 200 mA g^−1	—	—	0.823	500 mA h g^−1 (167 cycles)	152

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4. MnCo₂O₄-based electrocatalysts for energy conversion and storage

4.1. ORR catalysts in energy storage

As already highlighted in the previous topics, the development of high-performance electrocatalysts is essential for achieving high-performance energy devices, especially catalysts for oxygen reactions (OER and ORR) due to the sluggish reaction kinetics, which often requires a large overpotential to sustain a reasonable rate of electrode reactions.¹⁷² In fact, the design and optimization of catalysts for the ORR/OER is of fundamental importance for the development of more efficient and competitive energy storage devices, such as for metal–air batteries¹⁷³ and proton exchange membrane fuel cells (PEMFCs).¹⁷⁴

Currently, Ir and/or Ru based oxides and Pt-based materials are the most widely used catalysts for the OER and ORR, respectively. However, the high cost, scarcity, and poor bifunctional activity of precious metals greatly hinder their industrial application on a large scale.¹⁷⁵ To solve these disadvantages, intensive efforts have been devoted to development of noble metal-free oxygen reaction catalysts with low cost and high activity in the past decades,¹⁷⁶ especially for ORR catalysts. Among these reported noble metal-free ORR catalysts or bifunctional oxygen electrocatalysts, MnCo₂O₄ and MnCo₂O₄-derived composites show great potential as electrocatalysts because of their high intrinsic activity, and the corresponding activities can be further tuned through their phase and composition.¹⁷⁷

In fact, based on Table 4, it is possible to perceive important and well-known strategies that have been employed in the design of electrocatalysts containing MnCo₂O₄ for the ORR, as for example: (1) active site engineering, obtained through the control of size, morphology and defects, as well as the crystalline phase, in order to maximize the density of active sites,^3,178 and (2) conductivity optimization, obtained especially by doping with hetero-atoms and/or by formation of composites with conducting materials.^3,178,179

Table 4 Catalytic activity parameters of recently reported ORR MnCo₂O₄-based electrocatalysts^a

ORR catalysts	Incorporated or doping atom	Substrate	E ORR onset potential (V vs. RHE)	Half wave potential (V vs. RHE)	E ORR at −3 mA cm^−1 (V vs. RHE)	E OER at 10 mA cm^−1 (V vs. RHE)	ΔE EOER–EORR (V vs. RHE)	Current density (mA cm^−1)	Tafel slope (mV dec^−1)	Average electron transfer number (n)	Stability (h)	pH condition	Ref.
a a = obtained by cyclic voltammetry, b = vs. Ag/AgCl, — = information not found in the paper, c = 1 M KOH, d = ΔE = |Ej10 | − |E1/2|, MCO/NS-MCS = cobalt spinel oxides supported on nitrogen and sulfur co-doped mesoporous carbon spheres, N-doped MWCNT = nitrogen-doped multi-walled carbon nanotube, N,S-CNT = N,S-doped carbon nanotubes, 3D-G = three-dimensional graphene, N–C = N-doped carbon, pNGr = N-doped porous graphene, D-AC = AC-based defective carbon, t-CMO CBs = tetragonal CoMn2O4 cubes, tetragonal CoMn2O4 microspheres, and c-CMO NRs = cubic Co2MnO4 nanorods.
MCO-700	—	GCE	—	—	—	—	—	6.69 mA cm^−1	—	3.2–3.5	−16.66	0.1 M KOH	189
CMO-3.9/CNT	CNT	GCE	—	0.86	—	1.61	—	2.90 mA cm^−1 at 0.7 V	65–126	3.98	94% 8.33	0.1 M KOH	190
MnCo2O4	—	—	0.88	0.77	—	—	—	—	—	—	87% 5.55	0.1 M KOH	167
MnCo2O4@C	Carbon	—	0.92	0.80	—	1.66	0.89^d	—	—	3.61	99% 5.55	0.1 M KOH	167
MnCo2O4/C	Vulcan carbon	GCE	—	0.76	—	∼1.74	—	—	∼61	3.51	−8.33	0.1 M KOH	184
MnCo2O4/C nanosheets	Porous C nanosheets	GCE	0.945	0.767	—	—	—	—	—	3.82	72.5% 2.77	0.1 M KOH	183
MnCo2O4	—	GCE	∼0.83^a, 1.12	0.66	—	—	—	−2.30 mA cm^−1 at 0.36 V	−142.2	—	−2.22	0.1 M KOH	20
MnCo2O4-rGO	rGO	GCE	∼0.89^a, 1.11	0.77	—	—	—	−3.33 mA cm^−1 at 0.36 V	−150.1	3.8	−2.22	0.1 M KOH	20
MCO	—	GCE	−0.165	−0.225	−0.647	0.840	1.487	—	145	n > 3	—	0.1 M KOH	21
MCO + NS-MCS	NS-MCS	GCE	−0.112	−0.186	−0.221	0.817	1.038	−4.60 mA cm^−1 at −1.0 V	131	n > 3	—	0.1 M KOH	21
MCO/NS-MCS	NS-MCS	GCE	−0.079	−0.160	−0.186	0.774	0.960	−5.03 mA cm^−1 at −1.0 V	124	3.64–3.88	82.3% 5.55	0.1 M KOH	21
MnCo2O4 at 500 °C	—	GCE	0.81^a	0.58	—	—	—	—	—	—		0.1 M KOH	22
N-MWCNT-MnCo2O4 at 500 °C	N-MWCNT	GCE	0.83^a, 0.86	0.75	—	—	—	—	—	3.9	−20	0.1 M KOH	22
N-MWCNT-MnCo2O4 at 500 °C	N-MWCNT	GCE	0.86^a	0.60	—	—	—	—	—	—	—	0.1 M KOH	22
MCO	—	GCE	—	—	<0.1	>1.9	>2	—	—	—	—	0.1 M KOH	177
MCO + NCNTs	NCNTs	GCE	—	—	0.70	1.74	1.04	—	—	—	—	0.1 M KOH	177
MCO@NCNTs	NCNTs	GCE	—	—	0.76	1.70	0.94	—	−96 and −125	3.9	—	0.1 M KOH	177
MnCo2O4/CNT	CNT	GCE	—	—	—	—	—	—	—	3.75	—	0.1 M KOH	191
MnCo2O4/N,S-CNT	N,S-CNT	GCE	—	—	—	—	—	—	—	3.83	72%, 5	0.1 M KOH	191
MnCo2O4/rGO	rGO	GCE	0.94	0.78	—	—	—	—	75.4	3.90	—	0.1 M KOH	185
MnCo2O4/3D-G	3D-G	GCE	0.98	0.81	—	—	—	—	68.5	3.96	79.84%, 60	0.1 M KOH	185
MnCo2O4/CNTs	CNTs	GCE	0.93	0.74	—	—	—	—	84.6	3.81	—	0.1 M KOH	185
MnCo2O4/C	C	GCE	0.92	0.72	—	—	—	—	87.4	3.76	—	0.1 M KOH	185
MnCo2O4	—	—	0.865	0.552	—	—	—	−3.26 mA cm^−1 at 0.2 V	—	—	—	0.1 M KOH	186
MnCo2O4 + N–C	N–C	—	0.918	0.780	—	—	—	−5.28 mA cm^−1 at 0.2 V	—	—		0.1 M KOH	186
MnCo2O4/N–C	N–C	—	0.943	0.795	—	—	—	−5.78 mA cm^−1 at 0.2 V	86	3.50–3.83	89.68% 2.77	0.1 M KOH	186
CoMn/pNGr (2 : 1)	pNGr	GCE	0.94^a, 0.9^a	0.791	—	—	—	—	74	3.98	−5000 cycles	0.1 M KOH	192
CoMn/pNGr (1 : 1)	pNGr	GCE	0.9^a	0.726	—	—	—	—	—	3.66	—	0.1 M KOH	192
CoMn/pNGr (1 : 2)	pNGr	GCE	0.92^a	0.734	—	—	—	—	—	3.46	—, —	0.1 M KOH	192
MnCo2O4/NGr	NGr	GCE	0.86^a	0.648	—	—	—	—	—	—	—, —	0.1 M KOH	192
MnCo2O4/N-rmGO	N-rmGO	GCE	0.95^a, c	—	—	—	—	—	36^c	∼3.9^c	96.5% 5.55	1 M KOH	188
MnCo2O4 + N-rmGO	N-rmGO mixture	GCE	0.91^a, c	—	—	—	—	—	—	∼3.7^c	∼75% 5.55	1 M KOH	188
Mesoporous MnCo2O4	—	GCE	0.95^a	—	0.80	1.63	0.83	—	—	3.94	85% 10	0.1 M KOH	182
m-MnCo2O4	—	GCE	−0.04	—	—	—	—	—	−95	3.96	87% 95	0.1 M KOH	181
MnCo2O4/C	C	GCE	0.93^c	0.76^c	—	1.52^c	0.59^c	—	68/207^c	3.92^c	−24	1 M KOH	193
dp-MnCo2O4/CNT	CNT	GCE	−0.11^b	—	—	—	—	−5.33 mA cm^−1 at −0.8 V^b	106	∼4.0	—, —	0.1 M KOH	194
dp-MnCo2O4/N-rGO	N-rGO	GCE	−0.09^b	—	—	—	—	−5.71 mA cm^−1 at −0.8 V^b	65	∼4.0	—, —	0.1 M KOH	194
MnCo2O4	—	GCE	0.84	0.59	0.55	1.77	1.22	—	78.8	2.5–3.7	—	0.1 M KOH	187
CMO/20N-rGO	N-rGO	GCE	0.93	0.79	0.77	1.68	0.91	—	50.8	3.9–4	86.3% 8	0.1 M KOH	187
NCNTs	NCNTs	GCE	—	—	—		—	—	—	3.4	—, —	0.1 M KOH	195
NCNT-500	NCNTs	GCE	—	—	—	1.495	—	—	—	3.8	—, —	0.1 M KOH	195
MnCo2O3/C	C	GCE	—	0.86^c	—	—	—	—	45^c	—	—, 10 000 cycles	1 M KOH	196
MnCo2O4/C	C	GCE	—	0.84^c	—	—	—	—	50^c	—	—, 10 000 cycles	1 M KOH	196
c-CMO NRs	—	GCE	0.9	0.72	—	—	—	5.9 mA cm^−1	—	3.9	95%, 2.77	0.1 M KOH	180
t-CMO MSs	—	GCE	0.89	0.70	—	—	—	5.5 mA cm^−1	—	∼3.8	95%, 2.77	0.1 M KOH	180
t-CMO CBs	—	GCE	0.89	0.65	—	—	—	5.51 mA cm^−1	—	3.6	89%, 2.77	0.1 M KOH	180
MnCo2O3/CNF	CNF	GCE	−0.08^b	−0.21^b	—	—	1.04	—	—	∼3.96	89%, 8.33	0.1 M KOH	197
D-AC@2Mn–4Co	D-AC	GCE	883	803	—	—	—	4.72 mA cm^−1 at 0.2 V	37.5	3.83	92.7%, 20 000	0.1 M KOH	198
Co2MnO4	—	—	—	—	0.59	1.92	1.33	—	3.89	—	—, —	0.1 M KOH	199
Co3O4 + Co2MnO4	—	—	—	—	0.55	1.86	1.31	—	3.85	—	—, —	0.1 M KOH	199
Co3O4/Co2MnO4	—	—	—	—	0.68	1.77	1.09	—	3.97	—	∼94%, 2.77	0.1 M KOH	199

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As an excellent example of the active site engineering strategy, Yang et al.¹⁸⁰ successfully reported a facile precursor pyrolysis method to prepare porous spinel cobalt manganese oxides with tunable size, shape, chemical composition and crystalline structure via a facile precursor pyrolysis method (Fig. 6a). The capping agent and reaction temperature in the reaction were found to be crucial in the formation of porous spinel cobalt manganese oxides from cubic Co₂MnO₄ nanorods (c-CMO NRs) to tetragonal CoMn₂O₄ microspheres (t-CMO MSs) and tetragonal CoMn₂O₄ cubes (t-CMO CBs).

Fig. 6 (a) The synthetic process of different spinel Co_xMn_3−xO₄. ORR data of the prepared CMOs in O₂versus Ar-saturated 0.1 M KOH with a catalyst mass loading of 0.21 mg cm⁻². (b) Cyclic voltammetry curves of electrocatalysts in O₂versus Ar-saturated 0.1 M KOH. (c) Linear sweep voltammograms of the electrocatalysts in 0.1 M KOH at 1600 rpm. (d) Electron transfer number n at different potentials. (e) Chronoamperometric responses (percentage of current retained versus operation time) of the different spinel CMOs and 20% Pt/C kept at 0.50 V vs. RHE in O₂-saturated 0.1 M KOH. Reproduced with permission from ref. 180. Copyright © 2016, Tsinghua University Press and Springer-Verlag Berlin Heidelberg.

As illustrated in CVs (Fig. 6b) and polarization curves (Fig. 6c) of porous CMOs and 20% Pt/C, all the prepared spinel CMOs exhibited good ORR electrocatalytic activities, of which the c-CMO NRs showed a much more positive onset potential of 0.9 V and a half-wave potential of 0.72 V, which are very close values to those obtained by a commercial Pt/C. Furthermore, the n value of the c-CMO NRs in ORR electrocatalysis was calculated to be about 3.9 in the range of 0.45 and 0.60 V (Fig. 6d, which is in good agreement with a 4-electron oxygen reduction process) and demonstrated a desired durability with negligible degradation of their electrocatalytic activity after a continuous operation time of 10 000 seconds (Fig. 6e), which is much better than that of the commercial Pt/C electrocatalyst.¹⁸⁰

The active site engineering strategy has also been used in the development of a mesoporous MnCo₂O₄ electrode material.¹⁸¹ In one of these studies, Wang and co-workers¹⁸² reported a mesoporous bifunctional oxygen MnCo₂O₄ electrocatalyst synthesized through a spray-pyrolysis route (Fig. 7a), with Mn^IV in the surface and Mn^III in the bulk while Co^II was present both in the surface and bulk, as confirmed by X-ray near-edge structure (XANES) and XPS investigation. As a result, the MnCo₂O₄ exhibited both Co₃O₄-like activity for the OER (Fig. 7b) and Mn₂O₃-like performance for the ORR (Fig. 7c), with a potential difference between the ORR and OER of 0.83 V. According to the Koutecky–Levich (K–L) equation, the electron transfer number (n) of MnCo₂O₄ was calculated to be 3.94 and after 10 h, the loss of current density for MnCo₂O₄ was only 15%, indicating higher stability of MnCo₂O₄ than Pt/C (Fig. 7e). Another advantage is that the electrode material can be obtained on a large-scale at a relatively low temperature with precise chemical control of the components. The prominent bifunctional activity shows that MnCo₂O₄ could be used in metal–air batteries and/or other energy devices, as confirmed by the home-build Zn–air battery used to study the bifunctional stability of mesoporous MnCo₂O₄ (Fig. 7e).¹⁸²

Fig. 7 (a) SEM of MnCo₂O₄; (b) OER polarization curves of catalysts at 1600 rpm; (c) ORR polarization curves of catalysts at 1600 rpm; (d) percentage of peroxide and electron numbers (n) of Co₃O₄, Mn₂O₃, MnCo₂O₄, Pt/C, and the physical mixture of Mn₂O₃ and Co₃O₄. (e) Chronoamperometric measurements of MnCo₂O₄ and Pt/C at −0.3 V (V vs. Ag/AgCl) in O₂-saturated 0.1 M KOH at 1600 rpm. (f) A green light emitting diode (LED) panel powered by six Zn–air batteries (containing MnCo₂O₄). Reproduced with permission from ref. 182. Copyright © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim University Press and Springer-Verlag Berlin Heidelberg.

On the other hand, conductivity optimization by the formation of composites with conducting carbon materials has been the main strategy for preparing excellent electrocatalysts for the ORR. In fact, pure MnCo₂O₄ nanoparticles displayed certain ORR catalytic activity, but with a poor onset potential and peak potential.¹⁸³ In this context, among the most used carbon materials for this application, it is possible to highlight Vulcan carbons,¹⁸⁴ CNTs²² and graphene derivatives.²⁰

In one of these studies, Zhang et al.¹⁸⁵ designed a strategy to prepare MnCo₂O₄ on three-dimensional graphene (3D-G), as shown in Fig. 8. Typically, 3D-G (with multilayered structure of graphene) was synthesized using a coal tar pitch as the carbon source and nano MgO as the template (Fig. 8a). Then, spinel MnCo₂O₄ nanoparticles were in situ prepared and deposited on the inner walls of pores in the 3D-G by a facile hydrothermal method, resulting in the MnCo₂O₄/3D-G composite.¹⁸⁵ Surprisingly, the MnCo₂O₄/3D-G catalyst showed an onset potential of 0.98 V (vs. RHE) and the half-wave potential was 0.81 V (vs. RHE) in a solution of 0.1 M KOH (Fig. 8b), which was clearly superior to those of 20 wt% Pt/C (0.97 V, 0.80 V), MnCo₂O₄/rGO (0.94 V, 0.78 V), MnCo₂O₄/CNTs (0.93 V, 0.74 V), and MnCo₂O₄/C (0.92 V, 0.72 V).¹⁸⁵ In addition, the electron transfer number was 3.96 at 0.4 V (vs. RHE), and its catalyzed ORR mainly follows a four-electron process (Fig. 8c), indicating that the MnCo₂O₄/3D-G catalyst possesses superior selectivity for the ORR process. Besides that, MnCo₂O₄/3D-G showed the lowest Tafel slope of 68.5 mV dec⁻¹ compared to those of Pt/C (70.2 mV dec⁻¹ of 20 wt%), MnCo₂O₄/rGO (75.4 mV dec⁻¹), MnCo₂O₄/CNTs (84.6 mV dec⁻¹) and MnCo₂O₄/C (87.4 mV dec⁻¹) (Fig. 8d), and the durability test demonstrated that the MnCo₂O₄/3D-G catalyst has a much better durability than commercial Pt/C.¹⁸⁵ This work shows that the preparation of composites with carbonaceous materials is really an inspiring strategy to prepare high performance electrocatalysts for the development of fuel cells.

Fig. 8 (a) The synthesis route to the MnCo₂O₄/3D-G catalyst. (b) LSV curves of the different catalysts. (c) Electron transfer number per oxygen molecule of the different catalysts at different potentials. (d) Tafel slopes of the electrode assemblies fabricated with different catalysts. Reproduced with permission from ref. 185. Copyright © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

It is important to highlight that results as promising as those obtained in the work by Zhang et al.¹⁸⁵ were obtained by forming composites (conductivity optimization strategy) with conductive carbonaceous materials, applying the active site engineering strategy to the catalytic nanocarbon sites of these composites. In this combination of strategies, N and S-doped carbonaceous materials have been intensively studied,²¹ for example, Fu and co-workers¹⁸⁶ successfully prepared composed of N-doped carbon (N–C) and MnCo₂O₄ NPs for the ORR (MnCo₂O₄/N–C), with an ORR onset potential of 0.943 V, ORR half-wave potential of 0.795 V, synthesized by pyrolyzing the mesoporous-silica-protected zeolitic imidazolate framework-8 (ZIF-8) and etching, followed by a facile hydrothermal procedure (Fig. 9a). The superior performance of MnCo₂O₄/N–C was attributed to its porous structure and large surface area, N-doping effect, small size MnCo₂O₄ NPs and synergistic effects between the doped active species.¹⁸⁶ Based on the values of onset and half-wave potentials shown in Table 1, it is possible to infer that the MnCo₂O₄/N–C reported by Fu et al.¹⁸⁶ presented slightly better performance than other composites containing N-doped carbonaceous materials, such as N-MWCNT-MnCo₂O₄ (ORR onset potential of 0.86 V, ORR half-wave potential of 0.75 V),²² and CMO/20N-rGO (ORR onset potential of 0.93 V, ORR half-wave potential of 0.79 V).¹⁸⁷

Fig. 9 Schematic illustration of preparation of (a) MnCo₂O₄/N–C nanocomposites. Reproduced with permission from ref. 186. Copyright © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Schematic illustration of preparation of (b) the MnCo₂O₄/N-rmGO hybrid. (c) SEM image and (d) TEM image with an inset of the electron diffraction pattern of the MnCo₂O₄/N-rmGO hybrid, respectively. (e) A high-magnification TEM image of the MnCo₂O₄/N-rmGO hybrid. (f) CV curves of the MnCo₂O₄/N-rmGO hybrid, MnCo₂O₄ + N-rmGO mixture, Co₃O₄/N-rmGO hybrid, and N-rmGO on glassy carbon electrodes in O₂-saturated (solid line) or N₂-saturated (dashed line) 1 M KOH. The peak position of Pt/C was shown as a dashed line for comparison. (g) Rotating-disk electrode voltammograms of the MnCo₂O₄/N-rmGO hybrid, MnCo₂O₄ + N-rmGO mixture, Co₃O₄/N-rmGO hybrid, N-rmGO, and Pt/C in O₂-saturated 1 M KOH at a sweep rate of 5 mV s⁻¹ at 1600 rpm. Reproduced with permission from ref. 188. Copyright © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Using a slightly different approach, Liang et al.¹⁸⁸ developed hybrid composites through direct NP nucleation and growth on nitrogen doped-reduced graphene oxide (N-rmGO) sheets and Mn substitution of spinel Co₃O₄ NPs (average size of ∼5 nm) for the ORR under alkaline conditions (Fig. 9b), as confirmed by the SEM (Fig. 9c) and TEM (Fig. 9d) images, as well as by the HRTEM images, showing the lattice fringes of the nanocrystals, consistent with the MnCo₂O₄ crystal structure (Fig. 9e). This method results in covalent coupling between oxide NPs and N-rmGO sheets, yielding higher activity and stronger durability than the physical mixture of NPs and N-rmGO.¹⁸⁸

Interestingly, the C–O and C–N bonds in the N-rmGO sheet were strongly perturbed, suggesting the formation of C–O–metal and C–N–metal bonds between N-rmGO and spinel oxide NPs, as confirmed by electrochemical and XANES investigations. As a result, the composite showed a more positive onset (0.95 V vs. RHE, Fig. 9f–g) and a greater electron transfer number (∼3.9) than the corresponding physical mixture of MnCo₂O₄ NPs and N-rmGO (0.91 V vs. RHE and the electron transfer number = ∼3.7).¹⁸⁸ Based on the above, it is feasible to mention that the combination of conductivity optimization and active site engineering strategies should benefit the design of advanced ORR electrocatalysts for energy conversion and storage.

4.2. Water-splitting electrocatalysts for energy conversion (OER and HER)

Over the years, the energy demand has increased significantly, and this consumption has intensified year after year, which has led to the depletion of non-renewable energy sources (fossil fuels), as well as an increase in environmental pollution.²⁰⁰ Therefore, the development of new technologies, in which energy is obtained safely, cheaply, and without harming the environment is essential for the preservation of our society.

Among the energy conversion systems, electrochemical water splitting has proved to be very efficient when it comes to obtaining clean and high purity fuels.²⁰¹ In fact, from the electrochemical water splitting it is possible to obtain O₂ and H₂ through OER and HER that occur at the anode and cathode electrodes, respectively.^2,202

The HER can be expressed depending on electrolyte pH, according to eqn (1) and (2):

Acidic medium: 2H⁺ + 2e⁻ → H₂ (1)

Neutral/alkaline medium: 4H₂O + 4e⁻ → 2H₂ + 4OH⁻ (2)

In addition, the HER can be divided into two steps called Volmer and Heyrovsky or Tafel pathways, where H_ads acts as an intermediate species and plays a crucial role in the mechanism. The first step can proceed in acidic (eqn (3)) and neutral/alkaline solutions (eqn (4)), as indicated by the equations below:

H⁺ + e⁻ → H_ads (3)

H₂O + e⁻ → H_ads + OH⁻ (4)

Depending on the coverage ratio of H_ads (θ_H), the second step can occur through Heyrovsky or Tafel pathways. Whereas the Heyrovsky pathway occurs due to low θ_H (eqn (5) in acidic medium and eqn (6) in neutral/alkaline medium), the Tafel pathway occurs in consequence of high θ_H, regardless of the pH value (eqn (7)).

H_ads + H⁺ + e⁻ → H₂ (5)

H₂O + e⁻ + H_ads → H₂ + OH⁻ (6)

H_ads + H_ads → H₂ (7)

However, the obtaining of H₂ is limited by the sluggish reaction kinetics of the OER, because of the four-step electron transfer process (4OH⁻ → H₂O + O₂ + 4e⁻) in neutral or alkaline medium.

Currently, catalysts formed by using noble metals, such as RuO₂, IrO₂, and Pt, have been used in electrochemical water splitting in order to overcome the slow reaction kinetics of the OER.²⁰³ However, these catalysts are scarce and expensive, and their use in industry is not feasible. Therefore, noble metal catalysts have been replaced by alternative ones such as layered double hydroxides,^2,204 oxides,^205,206 nitrides and sulfides,^207,208 and spinel structures.^209,210 Among these catalyst groups, spinel oxides with AB₂O₄ (A and B transition metals) in special MnCo₂O₄ have stood out as promising electrode materials for water splitting due to the ease of preparation, variable valence states and high redox stability in alkaline medium.¹⁹⁴

The main MnCo₂O₄-based catalysts for the OER and HER are shown in Table 5. Most of the studies reported in the literature are related to the electrochemical performance of MnCo₂O₄ concerning the OER, and very few studies were found in the literature using MnCo₂O₄ as an electrocatalyst for the HER, suggesting that there is a vast unexplored field that deserves attention.

Table 5 Catalytic activity parameters of recently reported HER/OER MnCo₂O₄-based electrocatalysts: overpotential at 10 mA cm⁻², onset potential, Tafel slope, stability, and pH condition^a

	Catalysts	Substrate	Overpotential at 10 mA cm^−2 (mV vs. RHE)	Onset potential (V vs. RHE)	Tafel slope (mV dec^−1)	Stability (h)	pH condition	Ref.
a NW = nanowire; NF = nickel foam; NS = nanosheet; MCO = MnCo2O4; NS-MCS = nitrogen and sulfur co-doped mesoporous carbon spheres; rGO = reduced graphene oxide; GCE = glassy carbon electrode; YSM = yolk–shell; N-rmGO = N-doped reduced graphene oxide; NC = nitrogen doped carbon; CMO/20N-rGO = Co3O4–MnCo2O4/N-doped reduced graphene oxide with a mass ratio of NrGO/(Co + Mn) of ca. 20 wt%; NFF = Ni–Fe foam.
OER catalysts	MnCo2O4@CoS NW	NF	280 at 20 mA cm^−2	1.51	139.19	10	1.0 M KOH	217
	MnCo2O4@CoS NS	NF	270 at 20 mA cm^−2	1.50	131.81	10	1.0 M KOH	217
	MCO/NS-MCS	NS-MCS	508	1.738	124	5.5	0.1 M KOH	21
	MnCo2O4	—	290	1.52	97	24	1.0 M KOH	193
	MnCo2O4@Mn–Co–P	Ti	269	1.50	102	100	1.0 M KOH	224
	Mn1−xNixCo2O4/rGO	CGE	250	1.48	78	2.8	1.0 M KOH	211
	Mn1−xZnxCo2O4/rGO	GCE	320	1.48	80.6	2.8	1.0 M KOH	212
	YSM-MCO	NF	360	1.59	65.6	80	1.0 M KOH	225
	MnCo2O4@Ni3S2	NF	200 at 40 mA cm^−2	—	43.9	15	1.0 M KOH	226
	MnCo2O4/N-rmGO	GCE	330	1.56	—	—	1.0 M KOH	188
	MnCo2O4-rGO	GCE	530	1.56	106.9	—	0.1 M KOH	20
	MnxCo3−xO4	NF	327	—	79	25	1.0 M KOH	227
	MnCo2O4@Ni2P	NF	240	—	114	30	1.0 M KOH	221
	MnCo2O4@NC	GCE	287	1.46	55	20	0.1 M KOH	218
	Ce–MnCo2O4	GCE	390	—	125	11.1	1.0 M KOH	213
	CMO/20N-rGO	GCE	450	1.68	80.2	8	0.1 M KOH	187
	Ce–MnCo2O4	GCE	389	—	96	12	1.0 M KOH	213
HER catalysts	MnCo2O4@Ni2P	NF	57	—	89	20	1.0 M KOH	221
	NiFe–MnCo2O4/NFF	NFF	98	—	80.78	48	1.0 M KOH	19
	MnCo2O4@Ni3S2	NF	110	—	212.15	15	1.0 M KOH	226

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Despite advantages mentioned above, the catalytic activity of MnCo₂O₄ is limited by its low electrical conductivity. However, strategies have been explored in order to improve the electrical conductivity such as introduction of hetero-atoms, combining MnCo₂O₄ with conducting materials forming composites, incorporation of oxygen vacancies and nanoparticle size control.

In this sense, Rebekah and co-authors^211,212 showed that the catalytic activity of MnCo₂O₄ has been improved by the introduction of hetero-atoms (Ni and Zn), as well as by the combination of spinel oxide with rGO. Both Ni and Zn substituted MnCo₂O₄ on the rGO surface were synthesized through a hydrothermal method. The electrochemical behavior of Mn_1−xNi_xCo₂O₄/rGO and Mn_1−xZn_xCo₂O₄/rGO electrodes towards the OER was verified by Linear Sweep Voltammetry (LSV). Better results were achieved for the following compositions, Mn_0.4Ni_0.6Co₂O₄/rGO²¹¹ (overpotential of 250 mV at 10 mA cm⁻² and a Tafel slope of 78 mV dec⁻¹) and Mn_0.8Zn_0.2Co₂O₄/rGO²¹² (overpotential of 320 mV at 10 mA cm⁻² and a Tafel slope of 80.6 mV dec⁻¹). In summary, good electrochemical performance achieved by Mn_1−xNi_xCo₂O₄/rGO and Mn_1−xZn_xCo₂O₄/rGO electrodes towards the OER can be explained by the faster electron transport due to the more exposed active sites in consequence of the high surface area of rGO and of the reduction of metal ion aggregation owing to stacking between the sheets. In addition, the incorporation of another metallic ion resulted in a material with excellent electrochemical behavior and high conductivity.

The strategy of doping MnCo₂O₄ was also used by Huang et al.²¹³ in order to obtain an efficient electrocatalyst for both OER and HER. Indeed, the authors doped MnCo₂O₄ with Ce, being named as Ce–MnCo₂O₄. The OER and HER performances were evaluated by LSV and the material reached an overpotential of 390 and 379 mV, respectively. Those values when compared to MnCo₂O₄ without Ce doping are much superior. In fact, MnCo₂O₄ without Ce presented an overpotential at 10 mA cm⁻² of 560 and 477 mV, respectively for the OER and HER. The OER results can be attributed to the introduction of Ce into MnCo₂O₄ that facilitates oxygen transfer through adsorption, dissociation and release of atomic O for the OER, besides the introduction of oxygen vacancies to dissociate water.^214–216

Also, the electrical conductivity of MnCo₂O₄ can be improved through design of hierarchical 3D core@shell structures. Indeed, Du and co-authors²¹⁷ reported the synthesis of two materials based on MnCo₂O₄@CoS with different morphologies, using a hydrothermal method followed by the electrodeposition technique. As a matter of fact, MnCo₂O₄@CoS was synthesized in the nanowire and nanosheet shapes, as can be seen in TEM images displayed in Fig. 10a and b, respectively.

Fig. 10 TEM images of (a) MnCo₂O₄@CoS nanowires and (b) MnCo₂O₄@CoS nanosheets. Reproduced with permission from ref. 217. Copyright © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. (c, d) TEM and (e) HRTEM images of MnCo₂O₄@NC. (f) LSV curves of MnCo₂O₄@NC, RuO₂ and MnCo₂O₄ and (g) LSV initial curve and after 4000 curves for MnCo₂O₄@NC. Reproduced with permission from ref. 218. Copyright © 2016, Tsinghua University Press and Springer-Verlag Berlin Heidelberg.

The electrocatalytic activity performances of MnCo₂O₄@CoS with different morphologies towards the OER are very similar, reaching 280 mV and 270 mV at 20 mA cm⁻² for MnCo₂O₄@CoS nanowires and MnCo₂O₄@CoS nanosheets, respectively. Also, the surface areas of the materials were very similar as well. For MnCo₂O₄@CoS nanowires and MnCo₂O₄@CoS nanosheets the BET surface areas were 68.46 and 69.38 m² g⁻¹, respectively. Therefore, the electrochemical results cannot be attributed to the materials' surface area, but it can be related to the synergistic effect between MnCo₂O₄ and CoS, since the poor electrical conductivity of MnCo₂O₄ is compensated by the conductive CoS, and by the abundant oxygen vacancies. In fact, the existence of a large amount of oxygen vacancies can be estimated through XPS studies, where due to the co-existence of Co²⁺ and Co³⁺ ions the molar ratio of Co²⁺/Co³⁺ is a good parameter to evaluate the oxygen vacancies. The molar ratio found for the MnCo₂O₄@CoS nanowires and MnCo₂O₄@CoS nanosheets was 0.88 and 0.93, respectively, indicating a large amount of oxygen vacancies.

Over the years, several studies have been reported in the literature where it is demonstrated that the size control of nanoparticles plays a key role in improving the properties of the materials. Using this strategy, Su et al.²¹⁸ encapsulated MnCo₂O₄ nanoparticles using nitrogen-doped carbon (NC), since the NC can not only act as an encapsulating agent controlling the growth of the nanoparticles but also improve the catalytic performance in water splitting.²¹⁹ The MnCo₂O₄ was encapsulated in NC, here denoted as MnCo₂O₄@NC from a metal–organic complex as a precursor, using a hydrothermal method. The TEM and HRTEM images of the as-prepared material are shown in Fig. 10c–e. It is possible to observe that the MnCo₂O₄@NC nanowires were made of several well-dispersed nanoparticles with less than 10 nm of average diameter (Fig. 10c). Besides, in Fig. 10e, the HRTEM image of MnCo₂O₄@NC is displayed, where it is clearly possible to observe that MnCo₂O₄ was encapsulated by NC, forming a core@shell structure. However, the MnCo₂O₄ nanowires without NC presented nanoparticles with an average diameter of 100 nm, demonstrating that the growth of MnCo₂O₄ nanoparticles was limited by the NC.

The results presented above can directly influence the catalytic activity of the material. The MnCo₂O₄@NC presented an electrochemical performance (overpotential of 287 mV at 10 mV cm⁻² and Tafel slope of 55 mV dec⁻¹) far superior to that of MnCo₂O₄ nanowires (overpotential of ∼420 mV at 10 mV cm⁻² and Tafel slope of 101 mV dec⁻¹), Fig. 10f. Also, the MnCo₂O₄@NC did not show any change in the electrochemical profile after 4000 cycles, as can be seen in Fig. 10g. These results demonstrated that the size control of MnCo₂O₄ nanoparticles (less than 10 nm) using NP, in order to limit the growth of the nanoparticles, provided several active sites are exposed to oxygen adsorption and desorption. Besides, the nanoporous core@shell structure provided easy access of electrolyte ions and improved the electron transfer rate.

Recent studies have pointed out that one of the causes of the low catalytic activity and stability of Co-based compounds for water oxidation is the Jahn–Teller distortion. In this sense, Hirai et al.²²⁰ performed a systematic study for tetragonal spinel oxides Mn_3−xCo_xO₄ (0 ≤ x < 1 and 1 < x ≤ 1.5) in order to evaluate the relation between the Jahn–Teller distortion and catalytic activity for the OER. They figured out that for Mn_3−xCo_xO₄ (0 ≤ x < 1) the catalytic activity was improved with the increase of Co concentration, due to the Jahn–Teller distortion suppression. However, for Mn_3−xCo_xO₄ (1 < x ≤ 1.5) the OER activity decreased with Co concentration above 1 up to 1.5, when compared to Mn_3−xCo_xO₄ (0 ≤ x < 1), as can be seen through LSV curves in Fig. 11a. Although Mn³⁺ still remains occupying the octahedral sites on the Mn_3−xCo_xO₄ (0 ≤ x < 1), when cobalt is added it will occupy the tetrahedral sites, and thus the Jahn–Teller distortion is suppressed, represented by an indicator c/√2a (Fig. 11b) and consequently there is an increase in catalytic activity, due to the strong interaction between the antibonding electron e_g and oxygen species adsorbed (O₂²⁻ and O²⁻). Nevertheless, when the cobalt concentration is above >1, the octahedral sites are occupied by a mixture of Mn³⁺, Co³⁺, Mn⁴⁺, and Co²⁺ ions and then the e_g orbital can be occupied by more than 1 electron or even not be occupied thus decreasing the catalytic activity.

Fig. 11 LSV curves of different samples of Mn_3−xCo_xO₄ (0 < x ≤ 1.5) (a) and schematic orbital energy diagram for Mn³⁺ 3d at octahedral sites and Mn²⁺ and Co²⁺ at tetrahedral sites (b). Reproduced with permission from ref. 220. Copyright © Marketplace™, Royal Society of Chemistry. TEM image of MnCo₂O₄@Ni₂P (c), LSV curves for the electrocatalytic performance of the OER (d) and HER (e). Reproduced with permission from ref. 221. Copyright © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Similarly, Ge et al.,²²¹ using an interface engineering strategy to suppress the Jahn–Teller distortion in MnCo₂O₄, grew Ni₂P nanosheets on the MnCo₂O₄ surface obtaining a bifunctional catalyst for the OER and HER. The MnCo₂O₄@Ni₂P was obtained in four steps. The precursor MnCo-LDH was obtained through the hydrothermal method, and then the as-precursor MnCo-LDH was annealed in order to obtain MnCo₂O₄. A second hydrothermal method was used to deposit Ni(OH)₂ nanosheets on MnCo₂O_4. After that, the MnCo₂O₄@Ni(OH)₂ was calcined in the presence of NaH₂PO₂ in order to obtain MnCo₂O₄@Ni₂P. In Fig. 11c the TEM image of MnCo₂O₄@Ni₂P is displayed, where it is possible to observe that the MnCo₂O₄ nanoneedles are coated by a large number of Ni₂P nanosheets.

The electrochemical performance of MnCo₂O₄@Ni₂P towards the OER and HER was evaluated by LSV, and the curves are shown in Fig. 11d and e. The hierarchical MnCo₂O₄@Ni₂P structure exhibited an excellent overpotential for the OER (240 mV at 10 mA cm⁻²) and a Tafel slope of 114 mV dec⁻¹. Also, the MnCo₂O₄@Ni₂P showed an outstanding HER performance with an overpotential of 10 mA cm⁻² (57 mV) and a Tafel slope of 89 mV dec⁻¹. This HER performance can be explained by the presence of several Ni⁰ and Ni²⁺ species, as determined by XPS after phosphorization, once those species can provide energy in order to stabilize the H_ads through the weakening of the O–H bond of adsorbed water.²²²

The electronic interactions between MnCo₂O₄ and Ni₂P were clarified through differential charge density. The high charge density is placed on the Ni₂P side, and thus the electrons migrate from Ni₂P to MnCo₂O₄ due to the strong interfacial polarization.²²³ Thus, the strong interaction between MnCo₂O₄@Ni₂P reduces the Jahn–Teller distortion and the Ni₂P with metallic properties increases the electronic conductivity and charge transfer rate of MnCo₂O₄@Ni₂P.

5. Conclusions and future directions

Lately spinel MnCo₂O₄-based materials have stood out in the energy conversion and storage technologies, especially due to their low cost, simple preparation and chemical composition versatility obtained through different strategies that enabled the rational design of these materials, thus allowing tuning of their electronic properties. In addition, reducing Co ions in the structure of Co₃O₄ by replacing them with Mn ions has been an excellent strategy to increase the conductivity and improve the electrochemical performance of the electrode materials. In fact, studies have shown that the conductivity of MnCo₂O₄ is greater than that of Co₃O₄, and its electrochemical performance is superior since Co provides a higher oxidation potential than Mn and Mn brings a higher capacity than Co owing to its efficient electron transport.¹¹⁵ Thus, the recent advances have been summarized in this review and special emphasis was directed to spinel MnCo₂O₄ based materials, which are highly promising for the construction of supercapacitors and batteries, and thus the development of arrangements for water-splitting for energy conversion. It is necessary to highlight that the application of MnCo₂O₄ as a multifunctional material still needs fine control of synthesis conditions, which will impact the phase, morphology, cation distribution, and especially the electrical properties, and it is an important requirement for energy applications.

The construction of supercapacitors using MnCo₂O₄ in the pristine form or as composites based on metal oxides/hydroxides, polymers or carbon-based composites or other materials was described as an important strategy to increase the capacitance and rate capability of hybrid devices, resulting in supercapacitors with higher energy and power density.

Additionally, LIBs, SIBs and metal–O₂ batteries are also covered. In order to improve the electrochemical performance in LIBs, in addition to developing composites of MnCo₂O₄ with other materials, morphological modifications and synthesis methods have been the focus of researchers, particularly on the preparation of porous materials (mesoporous, hollow, yolk–shell, shell-in-shell, and yolk-in-double-shell spheres). Mesoporous transition metal-doped MnCo₂O₄ has also been reported for application in SIBs, while heteroatoms doped-carbon materials were mixed with MnCo₂O₄ due to intrinsic advantages as ORR catalysts, such as high surface area and electrochemical stability in metal–O₂ batteries. The results obtained with MnCo₂O₄ for the construction of supercapacitors, metal-ion batteries, and metal–air batteries were tabulated.

Conversely, MnCo₂O₄ based materials showed great potential as electrocatalysts in energy storage devices. For instance, conductivity optimization by the formation of composites with conducting carbon materials has been the main strategy for preparing excellent electrocatalysts for the ORR (or bifunctional electrocatalysts for the OER/ORR). However, porous spinels with tunable size, shape, chemical composition and crystalline structure were also reported via a facile precursor pyrolysis method.

MnCo₂O₄ was also recently used as an electrocatalyst for energy conversion, mainly in water splitting. Most of the studies reported in the literature are related to the electrochemical performance of MnCo₂O₄ concerning the OER, and very few studies were found in the literature using MnCo₂O₄ as an electrocatalyst for the HER, suggesting that there is a vast unexplored field that deserves attention. Thereunto, strategies have been explored in order to improve the electrical conductivity and electrocatalytic activity, such as introduction of hetero-atoms (or doping), combination with conducting materials forming composites, incorporation of oxygen vacancies and nanoparticle size control.

As demonstrated in this article, the utilization of spinel MnCo₂O₄-based materials for energy storage and conversion is a promising new concept in energy technologies. It is also important to highlight that future research should continue to enrich spinel MnCo₂O₄-based materials, focusing attention on the electrochemical performance and reasonable architectural design of those materials for practical application in energy technologies and expanding the fields of application. In fact, it is possible to visualize a field still unexplored and of great potential for application, for instance, the rational design of different composite materials composed of MnCo₂O₄ and/or heterojunctions containing the recent advanced 2D materials (such as MXene, transition metal chalcogenides, black phosphorus, etc.), aiming at future applications of these supercapacitor/battery materials for flexible/wearable devices, self-charged energy storage devices, and microsupercapacitors. The future applications of flexible/wearable energy devices depend on suitable flexible substrates that can stably and efficiently incorporate MnCo₂O₄ or its composites. In this direction, novel current collectors have been investigated and carbon cloth has shown promising results due to its excellent flexibility and conductivity. Additionally, different synthesis strategies urge investigation, and in this sense, electrochemical growth on flexible conductive substrates is promising (without binders and using a simplified rotocol). Considering large-scale and reproducible production, additive manufacturing (or three-dimensional printing) protocols offer great promise in this area.

Moreover, bifunctional and multifunctional catalysts represent promising directions,¹⁵ especially for electrocatalysts for water splitting (HER/OER) and metal–air batteries (ORR/OER). Other applications, such as the use of MnCo₂O₄ catalysts to convert greenhouse gases (CO₂) and toxic gas (CO) into chemical fuels, are also feasible. From this perspective, MnCo₂O₄-based materials have a key role in the way to a more sustainable society and industrial applications.

Conflicts of interest

The authors declare that there is no conflict of interest.

Acknowledgements

This work was supported by the Sao Paulo Research Foundation (FAPESP processes 2018/16896-7, 2017/13137-5 and 2020/00325-0), the National Council for Scientific and Technological Development (CNPq Processes 307271/2017-0 and 311847/2018-8), Coordination for the Improvement of Higher Education Personnel (CAPES) Financial code 001 and to Brazilian Institute of Science and Technology in Bioanalytics (INCTBio) (CNPq grant no. 465389/2014-7 and FAPESP grant no. 2014/50867-3) and Brazilian Institute of Science and Technology (INCT) in Carbon Nanomaterials. CSR acknowledges the Department of Science and Technology (DST)-SERB Early Career Research project (Grant No. ECR/2017/001850), DST-Nanomission (DST/NM/NT/2019/205(G)) and Karnataka Science and Technology Promotion Society (KSTePS/VGST-RGS-F/2018-19/GRD NO. 829/315).

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