Electrochemical and magnetic properties of nanostructured CoMn₂O₄ and Co₂MnO₄

Abstract

In this study, we have focused on the synthesis of cobalt manganite nanostructures using a simplistic hydrothermal route. We have explored these spinels as alternative low-cost bifunctional electrocatalysts for oxygen reduction/evolution reactions (ORR/OER). Herein, we have developed energy-saving, facile and rapid synthetic methodologies for highly active spinel electrocatalysts. Two spinel phases, cubic Co₂MnO₄ and tetragonal CoMn₂O₄ have been successfully obtained by tuning the stoichiometric ratio of Co and Mn salts respectively. These CoMn₂O₄ and Co₂MnO₄ nanocubes have been used as bifunctional catalysts towards OER and ORR. Electrocatalytic experiments show that cubic Co₂MnO₄ nanocubes show five times higher activity towards ORR than tetragonal CoMn₂O₄ nanocubes while the tetragonal phase is a better electrocatalyst towards OER than the cubic Co₂MnO₄ phase. XPS studies revealed two types of oxygen (lattice O and surface adsorbed O species like OH⁻) and the efficiency of the catalyst could be related to the binding affinity of oxygen. This explains the better catalytic activity of cubic Co₂MnO₄ which has a large percentage of adsorbed oxygen species. The stability of the catalyst was confirmed by carrying out TEM studies on a sample after carrying out 25 cycles. Magnetization experiments reveal that both the tetragonal CoMn₂O₄ as well as cubic Co₂MnO₄ show hysteresis at 10 K and 100 K without reaching saturation, which confirms an existing ferrimagnetic order in the samples. Both the tetragonal and cubic phases show T_c ∼ 110 K and 150 K respectively.

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Introduction

Spinel compounds or mixed transition-metal oxides (MTMOs) (denoted as A_xB_3−xO₄; A, B = Co, Ni, Zn, Mn, Fe etc. X = chalcogen) have attracted great research interest due to their wide applications in magnetism, electronics and catalysis, including electrochemical energy storage and conversion devices.^1–3 These mixed transition metal oxides consist of the combination of two simple low-cost transition metal oxides (TMOs), or a TMO and a post-TMO, into spinel-like structures (A_xB_3−xO₄) with stoichiometric or even non-stoichiometric compositions but can be obtained as a single-phase ternary metal oxide with two different metal cations. In the spinel type compounds AB₂O₄, A and B as divalent and trivalent metal ions respectively, occupy part or all of the tetrahedral and octahedral sites correspondingly and oxygen anions are arranged in a face centred cubic (fcc) lattice.^4,5 Among various spinels, MMn₂O₄ (M = Co, Mn, Cu, Ni etc.) is one of the most interesting mixed transition metal oxides in which one of the metal ion is manganese which has low cost, high abundance, low toxicity, multiple valence and shows a prominent Jahn–Teller effect.⁶ Manganese-based spinels have numerous applications in lithium insertion electrodes, magnetism and catalysis.^7–9 Among all the manganites (MMn₂O₄), Co and Mn based spinel oxides (Co_xMn_3−xO₄) have been investigated extensively as promising electrode materials towards electrochemical energy and storage devices like in lithium ion batteries (LIBs), metal–O₂ batteries (MOBs, for example Zn–O₂ and Li–O₂ batteries), and fuel cells (FCs).^10–12 The complex chemical compositions of the MTMOs and their synergetic effects contribute to the high specific capacity/capacitance which results in exceptionally high electrochemical activity, than other electrode materials.¹³ In the spinels, the presence of multiple valences of the cations (Co and Mn ions) helps to obtain the desirable electrochemical behaviour towards oxygen reduction reaction (ORR) for high-performance metal-O₂ batteries and fuel cells through multiple donor–acceptor chemisorption sites, responsible for the reversible adsorption of oxygen.^14,15 Ternary transition metal oxides show higher electrical conductivity than simple transition metal oxides due to the relatively low activation energy for electron transfer between cations.^16–18

Mn_xCo_3−xO₄ (0 ≤ x ≤ 3) is a good example of the relationship between solid state chemistry and electrocatalysis. From the earlier studies by Wickham and Croft¹⁹ and Naka et al.,²⁰ it is confirmed that a single stable phase of the cubic spinel exists at 1000 °C only for 0 ≤ x ≤ 1.3 while only tetragonal spinels exist for x > 1.9 and both the cubic and tetragonal spinels coexist in the range of 1.3 ≤ x ≤ 1.9. It has been shown that the distribution of Co and Mn ions in Co_3−xMn_xO₄ as Co²⁺[Co³⁺_2−xMn³⁺_x]O₄ with Curie temperature for T_c = 191 K (x ∼ 1.2) while decreases with further increase in x.¹⁹ In 1963, Blasse has reported that Co₂MnO₄ (x = 1) is inverse spinel with divalent cobalt and tetravalent manganese ions, Co²⁺[Co²⁺Mn⁴⁺]O₄.²¹ Lotgering et al.²² have reported the ferrimagnetic behaviour of the cubic spinel Co₂MnO₄. Joy et al. have reported ferrimagnetic behaviour with T_c ∼ 183 K.²³ The structural, magnetic and electronic properties of Mn_xCo_3−xO₄ have been reported over the whole range of x in the literature.^19,24 Since these compounds are extensively used in industry as thermistors, only the high-temperature resistivity studies have been reported.

The physicochemical properties of these mixed transition metal oxides are highly affected by their compositions, structures and oxidation state of metal ions, which depends on the synthesis procedure.^6,8 Traditional synthesis methods like solid state ceramic route involve high temperature and long heating time which will affect the electrochemical properties of the compounds.²⁵ Previously Co₂MnO₄ has been synthesized by solid state method at high temperature (1200 °C),²³ sol–gel process,¹¹ organic co-precipitation route,²⁶ flux method,²⁷ thermal decomposition of organometallic compounds²⁸ etc. But still rapid and rational synthesis of these manganites with controlled morphology at ambient condition is a challenge.

In our study, we have synthesized cobalt manganite nanostructures via a facile hydrothermal route without using any surfactant. We have stabilized both the cubic and tetragonal phases of Mn_xCo_3−xO₄ by tuning the reaction conditions. These cobalt manganite nanocubes have been used as bifunctional electrocatalyst towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Cubic Co₂MnO₄ phase shows higher electrocatalytic activity towards ORR than tetragonal CoMn₂O₄ while for the OER, catalytic activity is better for the tetragonal phase. We have also done the magnetic measurements for both the cubic and tetragonal phases and observed the saturation magnetization (M_s), coercivity (H_c) and remanent magnetization (M_r) of CoMn₂O₄ and Co₂MnO₄ nanocubes. At the low temperature ∼10 K, saturation magnetization (M_s at 6 T) for Co₂MnO₄ is ten times higher than tetragonal CoMn₂O₄ phase.

Results and discussion

Herein, we have synthesized CoMn₂O₄ and Co₂MnO₄ phases through a simple one pot approach using hydrothermal method without using any surfactant or any controlling agent. The PXRD patterns from Fig. 1(a) shows the prominent diffraction peaks corresponding to the reflections from (101), (112), (200), (103), (211), (202), (220), (204), (312), (321), (224), (400), which could be indexed on the basis of tetragonal space group I4₁/amd (no. 141) and concurred with the standard values of tetragonal CoMn₂O₄ (JCPDS card no. 77-471). No impurity peak was observed and the refined lattice parameters of body centered phase of tetragonal CoMn₂O₄ are a = 5.784(6) Å, and c = 9.091(2) Å. Similarly from Fig. 1(b), all the Bragg peaks (111), (220), (311), (222), (400), (422), (511), (440) were corresponding to the cubic structure of Co₂MnO₄ (space group of Fd3̄m with JCPDS card no. 840482). Pure face centred cubic phase of Co₂MnO₄ has been obtained without any impurity peak of the single metal oxide either cobalt oxide or manganese oxide. Refined lattice parameters for the cubic phase of Co₂MnO₄ was found to be a = 8.290(4) Å which is slightly higher than previously reported.¹⁵ Increase in the lattice cell parameters for CoMn₂O₄ as well as for Co₂MnO₄ confirms the nanocrystalline behaviour²⁹ of the particles obtained here.

Fig. 1 Powder X-ray diffraction of (a) CoMn₂O₄ and (b) Co₂MnO₄ nanostructures.

The FTIR spectra Fig. S1 (ESI†) are very similar for both the samples and reveal a finger print of the Co–Mn spinel oxides. From the Fig. S1(a),† it is confirmed that the bands at around 630 cm⁻¹ can be attributed to the vibration of the atoms in the tetrahedral oxygen environment (mainly cobalt, Co–O), while those around 525 cm⁻¹ can be related to the vibration of the atoms in the octahedral oxygen environment (mainly manganese, Mn–O). These values are quite similar to the previous literature values known for cobalt manganites.³⁰ The broader peaks of CoMn₂O₄ compared to Co₂MnO₄ indicates that the tetragonal spinel contains a distribution of metal (Co & Mn) cations over the tetrahedral and octahedral sites.³¹

The morphology of the cobalt manganite nanostructures have been investigated using transmission electron microscopy (TEM). From the Fig. 2(a) and (b), The TEM images of the Co₂MnO₄ and CoMn₂O₄ products show well-defined cubic morphology. The enlarged TEM images reveals that the edge length of CoMn₂O₄ cubes is ∼30–40 nm while for the Co₂MnO₄ nanocubes, edge length is ∼20 nm.

Fig. 2 Transmission electron micrographs of CoMn₂O₄ and Co₂MnO₄ nanostructures.

High resolution TEM (HRTEM) image of Fig. 3(a) displays, distinct lattice fringes with an interplanar distance of 0.23 nm, corresponding to the reflection of (211) planes of CoMn₂O₄ crystals and the HRTEM image (Fig. 3(b)) show lattice spacing of 0.25 nm, resulting from (311) planes of tetragonal Co₂MnO₄.

Fig. 3 HRTEM images of (a) CoMn₂O₄ and (b) Co₂MnO₄ nanostructures.

Energy-dispersive X-ray spectrometry (EDS) mapping analysis (Fig. 4(a) and (b)) shows that the Co and Mn are uniformly distributed in CoMn₂O₄ as well as in Co₂MnO₄ nanocubes.

Fig. 4 Elemental EDX mapping of (a) CoMn₂O₄ and (b) Co₂MnO₄ nanostructures.

Furthermore, from the energy-dispersive spectrum (EDS), quantitative analysis has been done. Fig. S2(a) (ESI†) confirms the ratio of Co : Mn is 1 : 2 in CoMn₂O₄ while from Fig. S2(b) (ESI†), it is confirmed that Co : Mn ratio is 2 : 1 in Co₂MnO₄ nanostructures.

To obtain more detailed information about elemental composition and oxidation state of the CoMn₂O₄ and Co₂MnO₄ nanocubes, we have carried out the X-ray photoelectron spectroscopy (XPS) analysis. The survey scan (Fig. 5(a) and (b)) represents the presence of cobalt, manganese, oxygen and carbon in both the samples.

Fig. 5 Survey spectrums for both the (a) CoMn₂O₄ and (b) Co₂MnO₄ nanocubes.

The presence of carbon and additional oxygen is associated with the impurities incorporated in the sample during sample mounting and loading. The Co 2P_3/2 peak was deconvoluted into two major components at 779.4 eV and 780.5 eV which are attributed to +2 and +3 states respectively along with shakeup satellite (Fig. 6(a) and (c)).^32,33

Fig. 6 XPS spectra of CoMn₂O₄ (a) Co2p core level, (b) Mn 2p core level, and (e) O 1s core level and of Co₂MnO₄ (c) Co 2p core level, (d) Mn 2p core level, and (f) O 1s core level respectively.

The XPS results (Table 1) suggest that for the oxides (Co_3−xMn_x O₄), the Co²⁺/Co³⁺ ratio increases with x³⁴ as Co²⁺ oxidation state dominates in CoMn₂O₄ while Co³⁺ state dominates in Co₂MnO₄. The deconvoluted Mn 2P_3/2 core level depicts two identified peaks at 641.5 eV and 643.1 eV associated with +2 and +3 state.³⁵

Table 1 Percentage analysis of the Co and Mn ions in CoMn₂O₄ and Co₂MnO₄ nanostructures

Compounds	Co^2+	Co^3+	Mn^3+	Mn^4+
CoMn2O4	52.5	26.5	52.7	47.3
Co2MnO4	38.5	43.8	45.46	54.53

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The specific surface area of cobalt-manganese oxide nanocubes were obtained by Brunauer–Emmett–Teller (BET) nitrogen adsorption/desorption measurements. The isothermal plots of N₂ adsorption/desorption for the Co₂MnO₄ and CoMn₂O₄ nanocubes with hysteresis loop between adsorption and desorption are shown in Fig. S3 (a) and (b) (ESI†). On the basis of the BET equation, the specific surface area of the CoMn₂O₄ and Co₂MnO₄ are ∼35 m² g⁻¹ and ∼40 m² g⁻¹, respectively. The difference in surface area of the two nanostructures is not significant as the particle size and dimensions of the nanocubes in both the compounds like CoMn₂O₄ and Co₂MnO₄ is almost equal, as confirmed by TEM study.

Fig. 7 shows the zero-field-cooled (ZFC) and field cooled (FC) magnetization curves at 500 Oe. The magnetization during FC for both samples increase with decreasing temperature. The Curie temperature of ∼110 K was found for CoMn₂O₄, which is almost equal to that reported by Zhang et al.³⁶

Fig. 7 The temperature dependence of magnetization at applied field of 500 Oe.

From Fig. 7, it is confirmed that T_c for Co₂MnO₄ nanocubes is ∼150 K which is lower than bulk Co₂MnO₄ (∼183 K).²³ Thus as expected the T_c decreases with decreasing particle size. Magnetization with respect to magnetic field up to 7 T of CoMn₂O₄ and Co₂MnO₄ nanocubes was recorded at 200 K, 100 K and 10 K.

It can be noted from Fig. 8(a) and (b), the M vs. H curves of both CoMn₂O₄ and Co₂MnO₄ recorded at 10 K and 100 K show hysteresis without reaching saturation, which confirms an existing ferrimagnetic order in the present samples. The magnetization curve at 200 K is linear, which is consistent with the paramagnetic state as inferred from the M (T) curve (Fig. 7). The observed saturation magnetization (M_s), coercivity (H_c) and remanent magnetization (M_r) values of CoMn₂O₄ and Co₂MnO₄ nanocubes are given in Table 2.

Fig. 8 M vs. H plot of (a) CoMn₂O₄ and (b) Co₂MnO₄ nanocubes at 7 T.

Table 2 Details of the magnetic properties of CoMn₂O₄ and Co₂MnO₄ nanostructures

Sample	Temperature (K)	Saturation magnetization (Ms at 6 T) (emu g^−1)	Remanent magnetization (Mr) (emu g^−1)	Coercive field (Hc) (Oe)
CoMn2O4	10 K	1.18	0.37	2846
	100 K	0.37	0.05	1722
	200 K	Paramagnetic behavior
Co2MnO4	10 K	11	3.9	2750
	100 K	5.3	0.5	1810
	200 K	Paramagnetic behavior

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Fig. 9 shows the ORR characteristics recorded on glassy carbon electrodes loaded with the CoMn₂O₄ and Co₂MnO₄ nanostructures. Fig. 9(a) shows the linear sweep voltammogram of the ORR, measured on rotating disk electrodes (RDEs) which exhibited similar profiles for both the Co–Mn oxide nanostructures, with two regions of potential–current response. It is confirmed that as the potential is scanned cathodically, the detected currents increase rapidly in the mixed kinetic–diffusion control region (approximately −0.1 to −0.45 V) and then in the presence of diffusion-limiting currents (I_d). The measured current decreases as observed for other nanostructured transition-metal oxides MnO₂, C-supported Mn oxides etc.^37,38 From the LSV (Linear Sweep Voltammetry), clearly the nanocrystalline cubic phase of Co₂MnO₄ outperformed the corresponding tetragonal phase in terms of having a higher positive onset potential and larger value of current. The ORR onset potential and peak potential of Co₂MnO₄ were −0.2 V and −0.45 V with current density of ∼2.5 mA cm⁻² which is almost five times higher than that obtained for the tetragonal CoMn₂O₄ nanostructures (onset potential ∼ −0.28 V and peak potential ∼ −0.47 V with current density ∼ −0.5 mA cm⁻²).

Fig. 9 Electrochemical application of nanocrystalline CoMn₂O₄ and Co₂MnO₄ as ORR electrocatalyst (a) linear sweep voltammogram (LSV) of the ORR using catalyst-modified RDEs in O₂-saturated alkaline electrolyte at 400 rpm (b) rotation-speed dependent current–potential curves recorded on Co₂MnO₄ electrodes in O₂-saturated 1 M KOH solution and (c) the corresponding K–L curve (d) chronoamperometry curves of the ORR for Co₂MnO₄ nanocubes obtained at −0.45 V in oxygen-saturated 0.1 M KOH under constant rotating (400 rpm).

The electrocatalytic activity of Co₂MnO₄ towards ORR was analyzed by six sets of voltammetry curves which were recorded on RDEs at different speeds of rotation (400, 800, 1200, 1600, 2000 and 2400 rpm). Fig. 9(b) demonstrates that the current increased with rising rotational rates (ω) as a result of the faster oxygen flux to the electrode surface. Theoretically, the observed rotation speed (ω) dependent current (I) can be expressed by the Koutechky–Levich (K–L) eqn (1),^31,39 for analyzing ORR kinetics (Fig. 9(c)) by calculating the transferred electron number per oxygen molecule (n) during ORR.³²

B = 0.62nFC_oD_o^2/3v^−1/6 (2)

J_K = nFkC_o (3)

where J is the measured current density, J_K and J_L are the kinetic and diffusion-limiting current densities, ω is the angular velocity, n is transferred electron number, F is the Faraday constant, C_o is the bulk concentration of O₂, v is the kinematic viscosity of the electrolyte, and k is the electron-transfer rate constant.

Fig. 9(c) shows the constructed K–L curves, which plot I⁻¹ versus ω^−1/2 at −0.45 V, 0.4 V and −0.35 V, where the transferred electron number per oxygen molecule (n) in the ORR can be calculated from the slopes of the fitted linear line. For Co₂MnO₄, two approximate linear regions for each potential are seen, with the slopes changing from a value corresponding to n ∼ 3 at the lower overpotential, as 2e⁻ process dominates over 4e⁻ process and lower rotation rates to a value for n = 2 for the higher overpotential and higher rotation rates.

These changes in the number of electrons of the Koutecky–Levich slopes were reported by Vago and Calvo et al.⁴⁰ for O₂ reduction on iron oxides. According to these results, it is proposed that the ORR based on Co₂MnO₄ electrocatalyst follows the 2-electron mechanism. The current was observed due to the catalytic oxygen reduction and the ORR mechanism of OH⁻ production on Co₂MnO₄ may be described by partial reduction with two step 2 electron process yielding HO₂⁻ ions as recorded for manganese oxide⁴¹

O₂ + H₂O + 2e⁻ → HO₂⁻ + OH⁻, E° = −0.065 V vs. NHE for pOH = 0 (4)

Followed by either the 2e⁻ reduction of HO₂⁻

HO₂⁻ (ads) + H₂O + 2e⁻ → 3OH⁻, E° = 0.867 V vs. NHE for pOH = 0 (5)

Or by the purely chemical disproportionation reaction of HO₂⁻ in solution

2HO₂⁻ (ads) → O₂ + 2OH⁻ (6)

In agreement to previous studies,⁴² at low rotation rates or low overpotential, this reaction may be followed by a slow disproportionation reaction of HO₂⁻ (eqn (6)), and when HO₂⁻ is completely disproportionate, it involves overall 4⁻ electron process per O₂ molecule.⁴³ The prevalence of the 2e⁻ process at high overpotential can be attributed to the fact that under this condition the oxidation reaction of the HO₂⁻ ion (eqn (6)) goes in backward direction. Hence, the overall process of ORR involving four electrons may occur at a large extent near the equilibrium potential for the HO₂⁻ reduction step as follows

O₂ + 2H₂O + 4e⁻ → 4OH⁻, E° = 0.401 V vs. NHE for pOH = 0 (7)

For the nanocrystalline Co₂MnO₄ spinels, the steady currents observed in the continuous polarization period of 6000 s (Fig. 9(d)), indicates the stability of the catalyst in an alkaline medium. Also in the inset of Fig. 9(d), TEM image of Co₂MnO₄ taken after the electrocatalytic measurements shows that there is no variation in the morphology of the Co₂MnO₄ nanocubes, which confirms the morphological stability of Co₂MnO₄ catalyst. The good ORR stability of the Co₂MnO₄ nanocubes could be attributed to the structural and chemical stability of the spinel phase in the alkaline medium.⁴⁴ The ORR performance of the Co₂MnO₄ nanocubes (regarding to onset potential and current density) was compared with the carbon-supported platinum nanoparticles (Pt/C),¹⁵ which is nearly comparable. The relatively lower onset potential for the spinels could be improved by cation doping or metal decorating to change the surface electronic properties or enhance conductivity.⁴⁵ Thus, it is clear that nanocrystalline cobalt manganite spinels (Co₂MnO₄ nanocubes) should find a practical application in metal–air batteries and fuel cell applications.

Mixed transition metal oxide based spinel structures have been shown to be a good water oxidation catalyst for oxygen evolution reaction.⁴⁶ In our study we have explored the cobalt manganites as a bifunctional catalyst towards electrochemical reactions (both ORR and OER). So, in our next step, we have measured the catalytic behavior of CoMn₂O₄ and Co₂MnO₄ using cyclic voltammetry in 1 M KOH in the potential range of 0.0–1.0 V (Fig. 10(a)). It is evident that the tetragonal CoMn₂O₄ nanostructures exhibited higher activity than the corresponding cubic Co₂MnO₄ phase. CoMn₂O₄ nanocubes generate an OER current nearly five times higher than that of the Co₂MnO₄ nanocubes, which is in contrast with the results for ORR. The tetragonal phase (CoMn₂O₄) shows the current density of ∼45 mA cm⁻² (at 1 V) and overpotential of ∼0.38 V while on the other hand, for cubic Co₂MnO₄, the current density and overpotential was observed to be ∼8 mA cm⁻² and ∼0.4 V respectively (Fig. 10(a)).

Fig. 10 (a) Cyclic voltammogram of the OER in 1 M KOH electrolyte (b) stability curve of CoMn₂O₄ nanostructures for 25 cycles.

Stability of an electrode is also one of the main factors which affect their usability in electrochemical applications.⁴⁷ The studies on CoMn₂O₄ nanostructures (Fig. 10(b)) for 25 consecutive cycles show negligible variation in activity indicating its stability in alkaline medium. We have also done the TEM analysis of CoMn₂O₄ after the electrocatalytic experiments towards OER and we have observed the nanocrystalline morphology of CoMn₂O₄ nanocubes which describes the morphological stability of the catalyst. From the above results, it is confirmed that among both the phases, catalytic activity of cubic Co₂MnO₄ is higher than tetragonal CoMn₂O₄ towards ORR while for OER, the reactivity order is reverse as tetragonal CoMn₂O₄ > cubic Co₂MnO₄. Since the catalytic process for oxygen reduction involves the reduction and oxidation of surface metal oxide species, the number and activity of these redox centres would be important factors to define the catalytic performance, also ORR activity is associated with the absorption affinity for oxygen.^48–50 To explain the differences in the intrinsic activities of the cubic (Co₂MnO₄) and tetragonal (CoMn₂O₄) spinels, we have performed X-ray photoelectron spectroscopy (XPS) (Fig. 6). Compositional analysis of the elements and the percentage of the oxidation states of both the Co and Mn elements are given in Table 1. From the above results it is confirmed that in CoMn₂O₄, Co(II) dominates over Co(III) and Mn(III) dominates over Mn(IV), while in the cubic Co₂MnO₄, we have observed that Co(III) extent is higher than Co(II) and also Mn(IV) is higher than Mn(III). From the Table 1, it is confirmed that the relative intensity and percentage of Mn(IV) is higher in cubic spinel (Co₂MnO₄) rather than in tetragonal phase CoMn₂O₄.

Oxidation state dependent electrocatalytic activity of the transition metal oxides towards ORR reaction is attributed to the following sequence of reactions, which involves the change of Mn(IV) to Mn(III) and involves overall 2-electron 2-step process during ORR:⁵¹

Mn⁴⁺ + e⁻ → Mn³⁺ (fast) (8)

O₂ → O_2,ads (fast) (9)

Mn³⁺ + O_2,ads → Mn⁴⁺ + O_2,ads⁻ (slow) (10)

O_2,ads⁻ + H₂O + e⁻ → HO_2,ads⁻ + OH⁻ (fast) (11)

where the rate determining step (eqn (10)) is related to the electron transfer of Mn(III) to the adsorbed oxygen without O₂ splitting. At low rotation rates and low overpotential, this reaction sequence is followed by the disproportionation of HO₂⁻ (Reaction (6)), recycling O₂ and, eventually leading to the participation of 4e⁻ per O₂ molecule. The large reactivity of Co₂MnO₄ with higher extent of Mn(IV) was ascribed to the occurrence of a mediation processes (according to eqn (8)) involving the reduction of Mn(IV) to Mn(III), followed by the electron transfer of Mn(III) to oxygen. Koutecky–Levich analyses have shown that the electrode reaction goes though the 2-electron mechanism to produce HO₂⁻ followed by a disproportion reaction of HO₂⁻ into O₂ and OH⁻.⁴³ Higher activity of cubic phase towards ORR is also described by stronger oxygen binding ability of Co₂MnO₄ than the tetragonal phase CoMn₂O₄.

Fig. 8(e) and (f) shows the asymmetric O 1 s signal can be deconvoluted into two distinguishable peaks centred at 530.0 and 531.4 eV, which correspond to lattice oxygen and surface adsorbed oxygen-containing species (possibly hydroxide or water).^36,52 The multistep process of electrocatalytic oxygen reduction involves the formation of HO₂⁻ species followed by its further decomposition/reduction to OH⁻ ions.¹⁵ The whole electrocatalytic oxygen reduction process is dependent on available active sites on catalyst surface and the absorption affinity of oxygen.^46,48,53 The relative intensity and peak areas corresponding to the adsorbed oxygen species (OH⁻) in deconvoluted O (1s) Fig. 8(e) and (f) spectra was observed to be higher for the cubic spinel phase. This specifies a stronger oxygen affinity for the cubic phase compared to tetragonal spinel phase. Note that the relative intensity and peak areas corresponding to the adsorbed oxygen species are higher for the cubic spinel than for the tetragonal spinel, which indicates the stronger oxygen affinity of the cubic phase (Table 3).

Table 3 Percentage calculation of adsorbed oxygen in CoMn₂O₄ and Co₂MnO₄ nanostructures

Sample peak position	Lattice O	Adsorbed O
CoMn2O4 (530.2)	530.1 (65%)	531.4 (35%)
Co2MnO4 (530.0)	530.0 (52%)	531.4 (48%)

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Theoretical calculations (DFT) of binding energies (E_b) of an oxygen molecule on Co/Mn defect sites show that cubic surface generates much more stable molecular oxygen adducts than the tetragonal surface.¹⁵ Furthermore, the surfaces of both phases (cubic as well as tetragonal) contain moreover the same value of surface area or the same number of catalytic sites per surface unit cell, whereas the area of the cubic Co₂MnO₄ unit cell is smaller. Therefore, for a given surface area, the number of available active sites of the cubic Co₂MnO₄ surface surpasses that on the tetragonal CoMn₂O₄ surface. All these factors are responsible for the enhanced ORR catalytic activity of cubic spinel nanocubes of CoMn₂O₄.

Experimental details

Materials and methods

Co(CH₃COO)₂·4H₂O (CDH, 99%), Mn(CH₃COO)₂·4H₂O (CDH, 99%), NH₄OH (Fisher Scientific, 25% NH₃), ethanol (Merck, 96%) and KOH (Fisher Scientific, 85%) were used in the synthesis of CoMn₂O₄ and Co₂MnO₄ nanostructures.

Synthesis of CoMn₂O₄ and Co₂MnO₄ nanostructures. All the chemical reagents were of analytical grade and used without further purification. In a typical synthesis of CoMn₂O₄, mixture of Co(CH₃COO)₂·4H₂O (0.1 M), Mn(CH₃COO)₂·4H₂O (0.1 M) (Co : Mn: 1 : 2), and NH₄OH (0.1 M) were loaded into a 100 mL Teflon-lined autoclave. After 1 h of magnetic stirring, the autoclave was sealed and maintained at 180 °C for 24 h and then allowed to cool at room temperature. The product was collected by centrifugation, washed several times with absolute ethanol and distilled water, and finally dried at room temperature.

For the synthesis of Co₂MnO₄, only the ratio of Co(CH₃COO)₂·4H₂O (0.1 M) and Mn(CH₃COO)₂·4H₂O (0.05 M) is different (Co : Mn: 2 : 1) otherwise all the other conditions for the preparation of Co₂MnO₄ is kept same as for the synthesis of CoMn₂O₄ nanostructures.

Characterization

Powder X-ray diffraction data was collected using a Bruker D8 Advance diffractometer with Ni-filtered Cu Kα radiation (λ = 1.5418 Å) source. For the X-ray measurements, 2-theta range was set from 10° to 70° at a scanning speed of 0.02° s⁻¹ while tube voltage and current were fixed at 40 kV and 30 mA respectively. Raw data were exposed to background correction and Kα₂ lines were removed. Refinement of the lattice parameters of the observed ‘d’ values was done by least squares fit of the observed d-values using Powder Cell software.⁵⁴ FTIR studies of CoMn₂O₄ and Co₂MnO₄ nanostructures were carried out on a Nicolet Protege 460 spectrometer with KBr disk in the range of 500–4000 cm⁻¹. Structure and composition investigation of the CoMn₂O₄ and Co₂MnO₄ nanostructures were carried out by transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDX) studies on a FEI Technai G² 20 electron microscope operated at 200 kV. TEM specimens were prepared by dispersing the sample in ethanol by ultrasonic treatment and pouring a drop of the dispersion on a porous carbon film supported copper grid which was then dried in air at room temperature. To investigate the homogeneity and distribution of the metal ions in both the CoMn₂O₄ and Co₂MnO₄, elemental EDX mapping has been done using FESEM-EDX (FESEM, JEOL and JSM-6700 F). X-ray photoelectron spectroscopy measurements have been carried out in ultrahigh vacuum based Omicron Multiprobe Surface Analysis System operating at a base pressure of 5 × 10⁻¹¹ Torr. For XPS analysis, monochromatic Al-K_α radiation source (1486.7 eV) was employed. An OMICRON EA125 hemispherical analyser (operating at pass energy of 20 eV) equipped with a 7 channeltron parallel detection unit was used to collect the XPS spectra. The calibration of binding energy in photoemission spectra was done referring to Au 4f_7/2 emission line.⁵⁵ The core level (CL) fitting has been done using Shirley background and Voigt line shape after necessary carbon correction.

Nitrogen adsorption–desorption isotherms were recorded at liquid nitrogen temperature (77 K) using a Nova 2000e (Quantachrome Corp.) equipment and the specific area was determined by the Brunauer–Emmett–Teller (BET) method. The samples were degassed at 150 °C for 4 h prior to the surface area measurements. Temperature dependent magnetization was carried out with a SQUID (superconducting quantum interference device) under field-cooled (FC) and zero-field-cooled (ZFC) conditions in the temperature range of 5–300 K in presence of an applied magnetic field of 500 Oe. Magnetic hysteresis measurements have been carried out at 10 K, 100 K and 200 K up to 7 T. Electrocatalytic experiments were performed with a computer-controlled electrochemical work-station (Metrohm Autolab 302/PGSTAT). The three-electrode cell consists of reference electrode as Ag/AgCl, Pt wire as counter electrode and glassy carbon as working electrode. Glassy carbon electrode (GCE) was polished using (0.05 μm) alumina paste, ultrasonication treatment was done in distilled water and in ethanol for the electrocatalytic studies (both oxygen evolution reaction (OER) as well as oxygen reduction reaction (ORR)).

Fabrication of the electrode for electrocatalysis measurement

A mixture of 5 mg of the catalyst with 5 μl of isopropyl alcohol was prepared by ultrasonication for 30 min and 5 μl of Nafion was added as a binder. From the above solution, 2 drops were taken and placed on the glassy carbon electrode and air-dried for 1 h. 1 M KOH was used as an electrolyte and the glassy carbon electrode was placed in the cell containing 40 mL of 1 M KOH solution. For each experiment freshly prepared solutions of KOH was used. Cyclic voltammetry (CV) experiment was carried out at room temperature with a scan rate of 0.01 V s⁻¹ with a peak window between 0 and 0.9 V vs. Ag/AgCl electrode. All potentials were referred to the reference electrode. All electrochemical measurements were performed at 25 °C.

Rotating disk electrode (RDE) measurement

The catalyst modified working electrode was prepared by the same method as for CV. The working electrode was scanned cathodically at a rate of 5 mV s⁻¹ with varying rotating speed from 400 to 2400 rpm. Koutecky–Levich plots (J⁻¹ vs. ω^−1/2) were analysed at various electrode potentials. The slopes of their best linear fit lines were used to calculate the number of electrons transferred (n) on the basis of the Koutecky–Levich equation.⁵⁶

Conclusions

Nanocrystalline Mn_xCo_3−xO₄ spinels with x = 1 (Co₂MnO₄) and x = 2 (CoMn₂O₄) have been synthesized as bifunctional catalysts via a facile hydrothermal route. Two distinct pure phases (cubic and tetragonal) of cobalt manganites have been obtained by tuning the reaction parameters and the stoichiometric ratio of Co and Mn. The cubic phase Co₂MnO₄ outperforms the tetragonal phase towards ORR activity while the tetragonal phase (CoMn₂O₄) shows five times higher activity towards OER than the cubic spinel (Co₂MnO₄). The intrinsic electrocatalytic activity of cubic and tetragonal Co–Mn–O spinels correlates with the oxygen binding ability on the catalyst surface. The presence of larger amount of Mn(IV) and larger binding affinity for oxygen enhances the activity of Co₂MnO₄ towards ORR. These results reveal that mixed transition metal manganites possess long-term stability under anodic conditions, good electrical conductivity and high electrocatalytic performances towards oxygen evolution reaction as well as towards oxygen reduction reaction that may be of significant applications in metal–air batteries and fuel cells.

Acknowledgements

A. K. G. thanks CSIR, DST and DIT Government of India for financial support. NG is thankful to UGC, Government of India for fellowship. We thank Dr Kanaga Raj for their prolific discussions regarding magnetic experiments.

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Footnote

† Electronic supplementary information (ESI) available: Details of FTIR spectra of CoMn₂O₄ and Co₂MnO₄ nanostructures, TEM-EDX of (a) CoMn₂O₄ and (b) Co₂MnO₄ nanostructures, nitrogen adsorption and desorption isotherms measured at 77 K for (a) CoMn₂O₄ and (b) Co₂MnO₄. See DOI: 10.1039/c5ra16937b

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