Single Crystal (Mn,Co)3O4 Octahedra for Highly Efficient Oxygen Reduction Reactions

Electrochimica Acta 144 (2014) 31–41

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Single Crystal (Mn,Co)3O4 Octahedra for Highly Efficient Oxygen Reduction Reactions Huanying Liu a , Xuefeng Zhu a, *, Mingrun Li a , Qiwen Tang b , Gongquan Sun b , Weishen Yang a, * a b

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China Direct Alcohol Fuel Cell Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 13 June 2014 Received in revised form 30 July 2014 Accepted 11 August 2014 Available online 2 September 2014

Single-crystal (Mn,Co)3O4 octahedra were synthesized through a novel precipitation-aging method. Well-defined single-crystal octahedra can be formed by carefully controlling the precipitationdissolution equilibrium, oxygen concentration, and solution temperature during synthesis to match the rates of spinel nucleation and octahedra growth. The single-crystal (Mn,Co)3O4 octahedra expose {111} and {011} facets of cubic and tetragonal phases, respectively, depending on the Mn/Co ratio. However, only single-crystal octahedra of Mn2CoO4 and Mn2.5Co0.5O4 with exposed {011} facets show the highest selectivity towards 4e
and 2e
oxygen reduction reactions (ORR) in alkaline solution, respectively. Furthermore, the single-crystal Mn2CoO4 octahedra with exposed {011} facets shows 30 times higher area specific activity for ORR than that of nanoparticles with random/mixed facets. The high electrocatalytic activity and selectivity towards ORR are correlated with the octahedral shape, the exposed facet, and the ratio of cations on the exposed facets (especially the octahedrally coordinated Mn4 + cations). This facet dependent catalytic performance provides a new route for obtaining highly selective and active ORR electrocatalysts. ã 2014 Elsevier Ltd. All rights reserved.

Keywords: ORR selectivity facet-determined catalysis single-crystal octahedron (Mn,Co)3O4 spinel oxides

1. Introduction The catalytic activation of the oxygen reduction reaction (ORR) is at the heart of many clean-energy technologies (such as fuel cells and metal-air batteries) and many catalytic oxidation reactions.[1–4] An efficient ORR catalyst can reduce the overpotential[5,6] and enhance the chemical-to-electrical conversion efficiency of many electrochemical processes.[7,8] Pt-based noble-metal catalysts exhibit a highly selective 4e
ORR process for reducing oxygen molecules to hydroxide ions in alkaline solution,[9,10] while Pd/Au-based noblemetal catalysts show a highly selective 2e
ORR process for direct electrochemical synthesis of the important chemical H2O2 from oxygen.[11,12] However, the high prices and limited reserves of noble metals restrict their widespread use.[13,14] Therefore, extensive efforts have been focused on developing inexpensive alternative ORR electrocatalysts.[15–19] Spinel-type oxides (Mn,Co)3O4 from inexpensive and earthabundant elements have demonstrated impressive electrocatalytic

* Corresponding authors. Tel: +86-411-84379073, Fax: +86-411-84694447 E-mail addresses: [email protected] (X. Zhu), [email protected] (W. Yang). http://dx.doi.org/10.1016/j.electacta.2014.08.087 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.

activity as well as stability against corrosion for the ORR in alkaline media.[20–25] Most studies have focused on the synthesis and application of (Mn,Co)3O4 nanoparticles, while few have examined the electrocatalytic performance of shape-controlled (Mn,Co)3O4 nanostructures.[21–23] It is inevitable that spinel nanoparticles, generally lack preferentially exposed facets, show a mixed 2e
(equation 1) and 4e
(equation 2) processes.[20–25] O2 + H2O + 2e
fi OH
+ HO2


(1)

O2 + 2H2O + 4e
fi 4OH


(2)

Unfortunately, this mixed process leads to significantly reduced efficiency when such nanoparticles are used as cathodes in fuel cells [9,10] and batteries or as electrocatalysts for direct electrochemical synthesis of H2O2 from oxygen.[11,12] The activity of a catalyst is related to the exposed active centers on the facets. Different facets have different catalytic activities. Pt-based noble-metal catalysts with different exposed facets show significantly different catalytic activity towards ORR.[26,27] However, few studies have focused on the influence of exposed facets of (Mn,Co)3O4 catalysts on the ORR activity. Therefore, the controllable synthesis of (Mn,Co)3O4

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nanostructures and the attainment of a highly efficient ORR process for shaped (Mn,Co)3O4 with preferentially needed facets are still confronting great challenges. Previous studies examining Mn-based oxides have shown that a high Mn4+/Mn3+ ratio favors the 4e
ORR process, whereas a low ratio favors the 2e
process.[25,28–31] It was found that the presence of Co in (Mn,Co)3O4 tunes the Mn oxidation state through

an internal redox process,[32,33] therefore making it feasible to control the Mn4+/Mn3+ ratio by adjusting the Mn/Co ratio of (Mn, Co)3O4. Different surface facets in a single crystal naturally expose different cation arrangements, coordinations, and densities [34–40] Thus, it can be expected that single-crystal spinel preferentially presenting facets with high density and ratio of Mn4+/Mn3+ will show a highly efficient 4e
ORR process, whereas a

Figure 1. SEM images of Mn3-xCoxO4 octahedra prepared via precipitation in Ar followed by aging in 5% O2/Ar for 10 h at 55  C. Samples with cobalt content, x, of (a) 0, (b) 0.5, (c) 1.0, (d) 1.3, (e) 1.5, (f) 2.0, (g) 2.5 and (h) 3.0. The scale bar is 500 nm for all images.

H. Liu et al. / Electrochimica Acta 144 (2014) 31–41

low ratio will favor the 2e
ORR process. (Mn,Co)3O4 spinel generally crystallizes into cubic and tetragonal structures.[21–23] For the cubic and tetragonal phases, the {111} and {011} facets respectively give the highest cation densities. An octahedron-like single crystal can ensure that all exposed facets are {111} and {011} for the cubic and tetragonal phases, respectively.[37,38] Thus, the ability to selectively synthesize single-crystal (Mn,Co)3O4 octahedra with a high density of specific facets possessing tunable cation composition and oxidation state would be of great significance to the development of a highly efficient electrocatalyst. Here, we present a novel and facile method to produce high purity, single-crystal Mn3
xCoxO4 octahedra with exposed {111} and {011} facets of the cubic and tetragonal phases, respectively,

33

and apply them as ORR electrocatalysts with high selectivity and activity in alkaline solution. A novel and facile precipitation-aging method is used for the repeatable synthesis of single-crystal Mn3
xCoxO4 octahedra. Well-defined octahedra can be formed only when the spinel nucleation rate is matched appropriately to the growth rate of the octahedra. Both rates are carefully controlled by the precipitation-dissolution equilibrium, oxygen concentration and solution temperature. The single-crystal Mn3
xCoxO4 octahedra show highly active and selective 4e
or 2e
ORR process to OH
or H2O2, respectively, controlled by the octahedral shape, the exposed facet and the Mn4+/B ratio on the exposed facets (Mn4+ is the number of Mn4+ ions in octahedral sites of each spinel unit, and B is the total number of cations in octahedral sites of each

Figure 2. Morphologies and structure of x = 1.5 samples prepared via precipitation under Ar followed by aging in 5% O2/Ar for different durations at 55  C. (a, b) 0.5 h, (c, d) 2 h, (e) 6 h, and (f) 10 h.

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H. Liu et al. / Electrochimica Acta 144 (2014) 31–41

spinel unit). To our knowledge, this is the first study to demonstrate the facet-dependent active and selective effects of Mn3
xCoxO4 electrocatalysts for ORR.[20–25,29,30] As a result of this study, new insights into the selection and synthesis of “designer” spinel electrocatalysts are proposed. These insights can guide the development of future highly active and selective electrocatalytic materials. 2. Experimental Section 2.1. Synthesis Single-crystal Mn3
xCoxO4 octahedra were synthesized through a novel and facile precipitation-aging method. 6.66 g polyvinylpyrrolidone (PVP k30, MW 58,000) was dissolved in a mixed metallic nitrates aqueous solution (0.20 M, 100 mL) of Mn (NO3)2 and Co(NO3)2. Under the protection of argon atmosphere, a NaOH aqueous solution (5.0 M, 10.0 mL) was added dropwise to the above solution to form a precipitate. The precipitate was aged for 2 h in Ar atmosphere and then for 10 h in 5% O2/Ar. The entire procedure was carried out under constant stirring and heating in a water bath at 55  C. The products were collected by centrifugation, washed with distilled water to remove sodium ions and polymer, and then dried at 40  C under vacuum for characterization. Other experimental conditions (oxygen concentration of the experiment atmosphere, solution temperature, and aging time) were systematically varied as indicated in Table S1 in order to gain insight into the synthesis mechanism and the effect of synthesis conditions. Mn2CoO4 nanoparticles (Mn2CoO4-NPs) were prepared through a similar procedure but in pure oxygen gas. A mixture of 6.66 g PVP, 4.76 g 50 wt.% Mn(NO3)2 aqueous solution, and 1.95 g Co(NO3)26H2O was dissolved in 100 mL deionized water under pure oxygen gas. NaOH solution (5.0 M, 10.0 mL) was added dropwise to the above solution to form a black precipitate. The product was then aged for 2 h, and then collected through the same centrifugation, washing, and drying procedures. 2.2. Electrochemical measurements The catalyst was a uniform suspension ink containing 50 wt. % asprepared single-crystal (Mn, Co)3O4 octahedra and 50 wt. % carbon powder (Vulcan XC-72R) in ethanol and Nafion solution (5 wt. %, DuPont, USA).[39] Rotating-disc electrode (RDE, F = 5 mm glassy carbon) measurements at room temperature were performed on a

CHI 760D electrochemical workstation system with a conventional three-electrode cell. A glassy carbon disc electrode as the working electrode was coated with 0.25 mg cm
2 of the catalyst. A Pt wire and an Hg/HgO/OH–(MMO) electrode served as the counter and reference electrodes, respectively. Measurements were calibrated with respect to the reversible hydrogen electrode (RHE). The calibration was performed in a high-purity hydrogen-saturated electrolyte with a Pt wire as the working electrode. In 1.0 M NaOH, E (RHE) = E (MMO) + 0.932 V. The activity tests towards the ORR for these catalysts were carried out in an O2-saturated NaOH solution (1.0 M). The working electrode was scanned at a rate of 10 mV s
1 with varying rotation speed from 400 to 2500 rpm. Koutecky-Levich plots were analyzed at various electrode potentials. The slopes of the best linear fits were used to calculate the electron transfer number (n) in the Koutecky-Levich equation: i ¼ 0:62nFC0 D2=3 n
1=6 v1=2 where i is the mass-diffusion limiting current, n is the electron transfer number per oxygen molecule, F is the Faraday constant (96 500C mol
1), C0 is the oxygen concentration (0.843  10
6 mol cm
3 in 1.0 M NaOH), D is the O2 diffusion coefficient (1.43  10
5 cm2 s
1 in 1.0 M NaOH), n is the kinetic viscosity (0.01128 cm2 s
1 in 1.0 M NaOH), and v is the rotational rate in radians.[39,40] 2.3. Characterization Powder XRD patterns were collected at room temperature on a Rigaku D/Max-2500 diffractometer using Cu Ka radiation. FT-IR spectra of samples diluted with KBr powder were recorded on a Nicolet Impact 410 spectrophotometer. The morphologies of the as-synthesized samples were observed under a field emission scanning electron microscope (FESEM, FEI Quanta 200 F). X-ray photoelectron spectroscopy (XPS) patterns were obtained on a Thermo ESCALAB 250Xi XPS spectrometer with Al Ka radiation (hn = 1486.6 eV). XPS fitting conditions are that the function type is 20% Lorentzian - 80% Gaussian, and the method of background removal in addition to the peak location stated is Shirley. The surface areas were obtained through the Brunauer-Emmett-Teller (BET) method by using an NDVA 4200e Micromeritics automatic analyser. TEM images were observed under an FEI Tecnai F30 microscope and a FEI Tecnai G2 microscope operated at an accelerating voltage of 300 and 120 kV, respectively. An ultrathin carbon film supported on a copper grid was used to hold the TEM samples.

Scheme 1. The synthesis mechanism of Mn3-xCoxO4 octahedra. In process (1), the precipitation reaction occurs in Ar atmosphere at 55  C. The precipitation-dissolution equilibrium occurs in process (2) under Ar atmosphere. In process (3), the aging process is carried out in 5% O2/Ar at 55  C. In another process (4), both precipitation reaction and aging processes take place in pure oxygen atmosphere at 55  C.

H. Liu et al. / Electrochimica Acta 144 (2014) 31–41

3. Results and discussion All synthesis conditions and sample information are summarized in Table S1. Figure 1 shows SEM images of Mn3-xCoxO4 samples with varying cobalt content (0  x  3.0) prepared via precipitation from solution under Ar followed by aging under 5% O2/Ar for 10 h at 55  C. Particles of the x = 0 (Mn only) sample show a mixture of octahedral and irregular shapes (Figure 1a); while for the x = 3.0 (Co only) sample, uniform nanoflakes are formed with diameter of 400 nm and thickness of  20 nm (Figure 1h). XRD (Figure S1) and FT-IR spectra (Figure S2) reveal that the x = 0 sample consists of Mn3O4 (JCPDS No. 24–0734) and MnOy&903;H2O, whereas the

35

x = 3.0 sample is indexed to a hexagonal structure of Co(OH)2 with space group of P-3m1. For 0.5  x  2.0, the samples consist of welldeveloped octahedra with edges length of about 300 nm and an opposite-vertical-apex length of about 500 nm. Octahedra samples are obtained with high purity (Figure S3). All the octahedronshaped samples have spinel structure, and their phase structures changes from tetragonal (for x = 0.5 and 1.0) to cubic (for x = 2.0 and 2.5), whereas a mixed phase of cubic and tetragonal is found for 1.3 < x < 2.0. The refined cell parameters of the pure phase samples are listed in Table S2. Compared with the Mn-only and Co-only samples, successful preparation of well-defined octahedra from mixed cation reactants is because the ability of cobalt ions can prevent the deep

Figure 3. (a, b) Low-magnification TEM image of the octahedra. The top insets show the schematic model of an ideal octahedron enclosed with {011} (a) and {111} (b) facets projected along the [100] and [001] direction respectively. (c, d) Corresponding SAED patterns along the [100] (a) and [001] (b) directions, respectively. (e, f) HRTEM images taken from the white square area in (a) and (b), respectively. Images in (a), (c) and (e) depict Mn2CoO4 octahedra; those in (b), (d) and (f) depict MnCo2O4 octahedra.

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oxidation of manganese ions, and the manganese ions can promote the oxidation of cobalt ions. Therefore, manganese and cobalt ions work synergistically to form spinel octahedra. The Mn1.5Co1.5O4 sample shows very well-defined octahedral morphology (Figure 1e) even though it is a mixture of tetragonal and cubic-phase crystals at a weight ratio of about 1:1 (Figure S4). This morphology implies that the phase structure does not influence the growth of octahedral crystals; thus, Mn1.5Co1.5O4 was used to further investigate the synthesis mechanism of the octahedra so that the formation of both the cubic- and tetragonal-phase crystals could be simultaneously evaluated. After precipitation under the protection of Ar atmosphere, the influence of preparation conditions during the aging step on the formation of octahedra were investigated, including aging time, solution temperature, oxidizing atmosphere and oxygen concentration. As oxygen-contained gases were continuously bubbled into the solution, (Mn,Co)(OH)2 dissolved gradually (Figure 2 and S5) accompanied by the formation of spinel nuclei. Since the concentration of metallic ions is very low, only a few nuclei are generated. When the oxygen concentration is too high (e.g. pure oxygen or air is supplied), both the spinel nucleation and the octahedra growth are too fast; as a result, twin crystals are produced (Figure S6). Under a suitably low oxygen concentration, such as 5% O2/Ar, the spinel nucleation matches with the octahedra growth; as a result, high-quality octahedra are produced (Figure S3d). During 10 h aging under 5% O2/Ar, (Mn,Co)(OH)2 gradually dissolves and crystallizes into spinel oxides with octahedral outline (Figure 2). When the solution temperature is decreased from 55  C to 35  C (Figure S7a), the degree of crystallinity is low because of the slow growth kinetics. The growth kinetics also become slowly when the synthesis temperature is increased from 55  C to 75  C (Figure S7b) because the concentration of the dissolved oxygen in aqueous solution decreases markedly at elevated temperature. On the other hand, when the initial precipitation reaction is carried out in an oxidizing rather than an inert environment, the as-prepared product is ill-defined spinel oxide nanoparticles (Figure S8), because a large number of spinel nuclei are immediately produced, which inhibit the

growth of the nuclei to octahedra. Thus, controlling the metallic cation concentrations through the precipitation-dissolution equilibrium is critical to tune the number of crystal nuclei formed in the oxidation step. The subsequent growth of the nuclei into the desired octahedra can be controlled by changing the oxygen concentration in the supplied gas and maintaining the proper solution temperature during the aging treatment. The well-defined octahedra can be formed only when rates of spinel nucleation and octahedra growth are appropriately matched. Both rates are controlled by the precipitation-dissolution equilibrium, oxygen concentration and solution temperature. These experiments enabled us to establish a synthesis mechanism for the spinel octahedra as shown in Scheme 1. After the precipitation reaction in process (1), the concentration of free metallic ions in the solution is very low because of the precipitation-dissolution equilibrium, that is, process (2). When oxygen-contained gases are subsequently bubbled into the solution in process (3), the metallic cations in solution are oxidized to insoluble nuclei of spinel oxide crystals. Consequently, the concentrations of the metallic cations are decreased, which causes the precipitation-dissolution equilibrium (process (2)) shifts to the right. Because of the limited number of crystal nuclei produced in the initial oxidation step and the proper crystal growth rate achieved only under 5% O2/Ar atmosphere, the growth of twin crystals is inhibited. If the precipitation reaction is conducted under an atmosphere of pure oxygen (process 4), a large number of crystal nuclei immediately form, inhibiting the growth of nuclei to octahedra. Thus, the well-defined Mn3
xCoxO4 octahedra can be synthesized only by carefully controlling the precipitation atmosphere (Ar), aging atmosphere (5% O2/Ar), aging time (10 h), and aging temperature (55  C) to match spinel nucleation with octahedra growth. Through the precipitationaging method, Mn3
xCoxO4 octahedra with high purity can therefore be synthesized with high reproducibility. Detailed structural information on Mn3-xCoxO4 octahedra is provided by high-resolution transmission electron microscopy

Figure 4. High-resolution XPS spectra and spectral fits of Mn2p (a) and Co2p (b) of the as-synthesized single-crystal Mn3-xCoxO4 octahedra and Mn2CoO4-NPs.

H. Liu et al. / Electrochimica Acta 144 (2014) 31–41

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Table 1 Mn2p and Co2p XPS fitting data for the as-synthesized single crystal Mn3-xCoxO4 octahedra and Mn2CoO4-NPs. Sample

Mn2p

Co2p

Mn2p 3/2 x = 0.5 x = 1.0 x = 1.3 x = 1.5 x = 2.0 Mn2CoO4-NPs a

641.6 641.3 641.2 641.3 641.3 641.3

Mn2p 1/2 640.5 642.6 642.6 642.5 642.5 642.5

653.1 653.0 653.0 653.0 653.0 653.1

651.6 654.3 654.3 654.2 654.2 654.0

a

Co2p 3/2

S1

780.3 780.2 780.2 780.1 780.0 780.4

786.4 786.6 786.5 786.0 786.2 786.8

S2

a

790.3 789.7 789.5 790.2

a

Co2p 1/2

S3

795.8 795.4 795.2 795.0 795.0 795.4

802.3 802.5 802.1 802.2 802.1 802.5

795.3 795.3 795.3 795.6

S4

a

804.2 804.5 804.2 804.3

S1, S2, S3 and S4 are the satellite peaks of Co2p.

(HRTEM) (Figure 3). In agreement with the SEM findings, the as-prepared Mn2CoO4 (Figure 3a) showed well-defined octahedral morphology with clean edges. A low-magnification image of an octahedral particle viewed along the [100] direction and its corresponding selected-area electron diffraction (SAED) pattern are shown in Figure 3c. The SAED pattern can be indexed to the [100] zone axis of the Mn2CoO4 tetragonal structure (space group I41/amd), which implies that the as-prepared Mn2CoO4 octahedra grains are single crystals. The octahedral grain exposes the {011} facets, and so presents a diamond-like outline. These structural features agree well with the model of single-crystal Mn2CoO4 octahedra enclosed by {011} facets projected along the [100] direction (Figure 3a, top inset). A magnified HRTEM image (Figure 3e) shows interplanar distances of 3.02 Å correspond-ing to the lattice fringes of 112 and 112. This suggests cell parameters of a = 5.79 Å and c = 9.04 Å, which are coincident with the refined cell parameters from XRD (Table S2). The same Mn2CoO4 octahedron was rotated to the [110,301] and [210] directions from the [100] zone axis to further confirm the exposed surfaces as {011} facets (Figure S9). The Mn2.5Co0.5O4 and Mn1.7Co1.3O4 samples similarly consist of single-crystal octahedra enclosed by {011} facets. Figure 3b shows a low-magnification image of a MnCo2O4 octahedron enclosed by {111} facets. The SAED pattern (Figure 3d) along the [001] direction confirms that it is a single crystal and consistent with the schematic model of an ideal cubic-octahedral crystal. To further confirm the identity of the exposed surfaces of the octahedral MnCo2O4, the same particle was rotated to the [101], ½112, and ½114 directions (Figure S10). This operation verified that the exposed surfaces consist entirely of {111} facets. The relevant HRTEM image (Figure 3f) indicates a highly crystalline character with (220) interplanar spacing of 2.91 Å, which is in good agreement with the value obtained from XRD (Table S2). XPS was used to characterize the oxidation state of the surface cations of the as-synthesized single-crystal Mn3-xCoxO4 octahedra. A detailed analysis of the oxidation states of cobalt and manganese on the surface of the as-prepared single-crystal octahedra is shown in Figure 4. The binding energies of the Mn2p 3/2 peak for Mn2+, Mn3+ and Mn4+ are 640.4 0.2, 641.4 0.2 and 642 0.2 eV, respectively.[32] As shown in Figure 4a, Table 1 and S3, the binding energies of the peaks of Mn2p 3/2 and Mn2p 1/2 for Mn2+ and Mn3+ are 640.5 and 651.6 eV, and 641.6 and 653.1 eV, respectively. The fitting results of the Mn2p peaks for Mn2CoO4 show the presence of Mn3+ and Mn4+ at a ratio of 0.70/0.30, as shown in Table 2.

The fitting patterns of Mn1.7Co1.3O4 are close to those of Mn2CoO4, suggesting a Mn3+/Mn4+ ratio similar to that in Mn2CoO4. This ratio is also confirmed by the fact that they show the same values of DEMn3s (Figure S11 and Table S4). For x larger than 1.3, the small increase in DEMn3s reveals that the average valence decrease. The corresponding Mn2p fitting patterns reveal that the percentage of Mn4+ drops. For example, the Mn3+/Mn4+ ratio for Mn1.5Co1.5O4 and MnCo2O4 is 0.78/0.22 and 0.81/0.19, respectively. The detailed ratios of manganese cations of the octahedral samples are listed in Table 2. The Co2p pattern for the single-crystal Mn2.5Co0.5O4 octahedra (Figure 4b) possesses a sharp Co2p 3/2 satellite peak at a binding energy of 786.4 eV, suggesting the oxidation state of the Co ions is +2. This is in accordance with other fitting patterns and with prior results for Co-based spinel.[41] The presence of Mn in (Mn, Co)3O4 spinel results in the reduction of Co, possibly through an internal redox reaction.[32,33,40] This change suggests that cobalt ions of the Co-poor spinel preferred a valance of +2. The satellite peak of Co2p 3/2 of Mn2CoO4 octahedra is sharp and similar to that of Mn2.5Co0.5O4, also suggesting the oxidation state of Co is +2. This oxidation state is also confirmed by the very close values of DECo2p for both samples—i.e, 15.45 eV for Mn2.5Co0.5O4 and 15.40 eV for Mn2CoO4. Gautier and co-workers reported a DECo2p value of 15.50 eV for (Mn, Co)3O4 spinel and suggested a Co valence of +2. [25,30,42,43] Wei also found a DECo2p value of 15.50 eV, which is attributed to Co2+.[44,45] The Co2p 3/2 satellite peak of Mn1.7Co1.3O4 manifests a shape that is different from those of Mn2.5Co0.5O4 and Mn2CoO4. The satellite peak at 790.3 eV indicates the presence of Co3+. With increasing x, the satellite peak at 790.0 0.3 eV intensifies, indicating an increase in the Co3+ percentage. The Co2+/Co3+ ratio varies from 0.82/0.18 (for Mn1.7Co1.3O4) to 0.58/0.42 (for MnCo2O4). As reported by Joy the oxidation state of cobalt ions in Co-poor (Mn,Co)3O4 spinel is normally +2.[33] The detailed ratios of cobalt cations of the octahedral samples are listed in Table 2. Co- and Mn-based spinel oxides have the structure of normal spinel ((A)4a[B2]8dO4). [25,29,30,37,38] Joy found that the change of Mn3+ $ Mn4+ is associated with Co3+ $ Co2+ in (Mn,Co)3O4 spinel. [33] According to the ion distribution of (Mn,Co)3O4 spinel reported by researchers, [25,29–47] Co2+ has a larger preference in tetrahedral sites than Mn ions, whereas Co3+ generally prefers the octahedral position and in a low spin state (CoIII). Based on the XPS data and the careful fitting patterns, the Co2+/3+ and Mn2+/3+/4+ oxidation-state distributions of the as-synthesized single-crystal

Table 2 The structure patterns of single-crystal Mn3-xCoxO4 octahedra and Mn2CoO4-NPs from XPS spectra.

a

Sample

Co2+/Co3+

Mn2+/Mn3+/Mn4+

Formula

x = 0.5 x = 1.0 x = 1.3 x = 1.5 x = 2.0 Mn2CoO4-NPs

1.0/0 1.0/0 0.82/0.18 0.74/0.26 0.58/0.42 0.77/0.23

0.19/0.81/0 0/0.70/0.30 0/0.71/0.29 0/0.78/0.22 0/0.81/0.19 0/0.48/0.52

(Co2+0.5Mn2+0.47Mn3+0.03)[Mn3+]2O4 (Co2+)[Mn3+1.40Mn4+0.60] O4 (Co2+)[Co2+0.07CoIII0.23Mn3+1.20Mn4+0.50] O4 (Co2+)[Co2+0.11CoIII0.39Mn3+1.17Mn4+0.33] O4 (Co2+)[Co2+0.17CoIII0.83Mn3+0.81Mn4+0.19] O4 (Co2+0.77 Mn3+0.23)[CoIII0.23 Mn3+0.73Mn4+1.04] O4

Co3+ in (Mn,Co)3O4 is generally in low spin state (CoIII). [41–47]

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Mn3-xCoxO4 octahedra are estimated as shown in Table 2. The cation distributions of Mn and Co in this work are coincident with those reported by Bordeneuve.[46] A unit cell with tetragonal or cubic structure is considered here. As shown in Figure 5a and c, the A-site cations are in tetrahedral coordination, the B-site cations are in octahedral coordination and the oxygen anions form the tetrahedral and octahedral interstices. Many researchers reported that the octahedrally coordinated cations are typically preferentially exposed in Mn/Co-based spinel oxides.[48–52] Therefore, the terminal cations of the as-synthesized single-crystal (Mn,Co)3O4 octahedra are hypothesized to initially be B-site cations. As shown in Figure 5b and d, the tetragonal {011} and cubic {111} facets show closely similar cationic densities and arrangements. For clarity, the oxygen anions are ignored in these schematics. As the exposed cations are B-site cations on the topmost layer, it is the cationic valence state of the B-site ions that is most directly relevant to the ORR activity and selectivity of these octahedra. Single crystal Mn3-xCoxO4 (x = 0.5, 1.0, 1.5, 1.3 and 2.0) octahedra were tested to assess their ORR catalytic activity in O2-saturated 1.0 M NaOH aqueous solution at room temperature by rotating disk electrodes. Similar potentiodynamic ORR profiles for all samples were observed. These profiles have two distinct regions of potential–current response, as shown in Figure 6a. Scanning the potential cathodically, the detected currents increased rapidly in the mixed kinetic–diffusion control region (approximately 0.77–

0.90 V vs. RHE), and then reached a plateau corresponding to the mass-diffusion limiting currents. Single-crystal Mn2CoO4 and Mn1.7Co1.3O4 octahedra yielded an ORR half-wave potential of 0.83 V vs. RHE and a limiting current density as high as 2.95 mA cm
2 at 1600 rpm (Figure S12). To further analyze the ORR kinetics, the observed rotation-speed-dependent currents were fitted with the Koutecky–Levich equation in order to determine the electron transfer number of the ORR at 0.6 V vs. RHE (Figure 6b). Samples for which x = 0.5, 1.0, 1.3, 1.5 and 2.0 yield electron transfer numbers of 2.3, 4.0, 3.9, 3.5 and 3.2, respectively. These results reveal that Mn2.5Co0.5O4 octahedra catalyst shows the highest selectivity toward 2e
ORR process for hydrogen peroxide production, and Mn2CoO4 octahedra catalyst shows the highest selectivity toward 4e
ORR process for hydroxyl ions production. In contrast, the other octahedral catalysts show a mixed 2e
and 4e
ORR process. Importantly, if the surface cations of the single-crystal Mn3xCoxO4 octahedra were the A-site cations, then the samples (x = 1.0, 1.3, 1.5 and 2.0) should have similar electron transfer numbers because they have the same surface cations (Table 2). However, they show very different performances towards ORR, which implies that the terminal cations are not A-site cations. The transition from 2e
to 4e
process does not occur monotonically with the increase in cobalt composition (x) (Figure S13a). Instead, there is an abrupt switch from the pure 2e
process to the pure 4e
process as x increases from 0.5 to 1.0, followed by a gradual transition from the 4e
process to a mixed 2e
/4e
process with

Figure 5. Unit cell of normal spinel (A)4a[B2]8dO4 with tetragonal (a) and cubic (c) crystal structures. The A-site cations are tetrahedrally coordinated, the B-site cations are octahedrally coordinated, and the oxygen anions form the tetrahedral and octahedral interstices. A, B, and O ions are blue, green, and red, respectively. The B-site terminated surface ionic configuration (4  4) of the {011} (b) and {111} (d) planes (O2
anions are not shown here).

H. Liu et al. / Electrochimica Acta 144 (2014) 31–41

further increase in x. As shown in Figure 6c and S13b, the dominance of the 2e
or 4e
process is determined by the relative concentration of Mn4+ surface cations (i.e., Mn4+/B ratio, Mn4+ is the number of Mn4+ ions in octahedral sites of each spinel unit, and B is the total number of cations in octahedral sites of each spinel unit) rather than the absolute Mn/Co ratio. For the indirect 4e
ORR in alkaline solution, HO2
forms as an intermediate in the first step, and then converts to hydroxide ions or undergoes disproportionation in the second step. Mn3+ is found to be active center in HO2
formation, whereas Mn4+ ions are active in the disproportionation of HO2
.[25,28–31,53] As the exposed {011} facets of Mn2.5Co0.5O4

Figure 6. (a) Potentiodynamic curves of single-crystal Mn3-xCoxO4 octahedra in O2saturated 1.0 M NaOH solution at 1600 rpm at a scan rate of 10 mV s
1. (b) KouteckyLevich plots of single-crystal Mn3-xCoxO4 octahedra at 0.6 V vs. RHE; the dashed lines correspond to the ideal 4e
(n = 4) and 2e
(n = 2) processes. (c) The dependence of the electron transfer number (n) of ORR on the Mn4+/B ratio of single-crystal Mn3-xCoxO4 octahedra. B stands for the total number of cations in the B-site of (A)4a[B2]8dO4.

39

octahedra contain only Mn3+, the correspondingly smallest electron transfer number of 2.3 is found. This material is therefore a highly selective electrocatalyst for the direct synthesis of H2O2 from oxygen in alkaline solution. Its half-wave potential is 0.81 V vs. RHE (Figure 6a), which is more positive than those of most nonprecious metallic electrocatalysts.[54–56] In a word, the best electrocatalyst towards highly selective 2e
and 4e
ORR processes are the single-crystal octahedra with exposed facet of {011}, Mn4 + /B ratio of 0 and 0.3, respectively. In order to prove that the surface-facet control is critical to the ORR mechanism, activity and selectivity, Mn2CoO4 nanoparticles (Mn2CoO4-NPs) with random/mixed facets were synthesized under pure oxygen atmosphere. The XPS study shows the Mn oxidation states and Mn4+/B ratio in Mn2CoO4-NPs are higher than that of the single-crystal Mn2CoO4 octahedra (Table 1, 2, and S3 and Figure S11). The as-synthesized Mn2CoO4-NPs, which also possess the tetragonal spinel structure, are about 30 nm in diameter (Figure S14). Mn2CoO4-NPs yielded a half-wave potential of 0.81 V vs. RHE (Figure 7a). The Mn2CoO4-NPs has a BET surface area of 91 m2 g
1, which is much higher than that of single-crystal Mn2CoO4 octahedra (7 m2 g
1). However, the Mn2CoO4-NPs yield a 20 mV negative shift of the half-wave potential relative to that of single-crystal Mn2CoO4 octahedra. Therefore, the area-specific activity[57,58] of single-crystal Mn2CoO4 octahedra with exposed {011} facets is 30 times higher than that of Mn2CoO4-NPs with random/mixed facets. The electron transfer number of the Mn2CoO4-NPs was calculated to be 3.3 (Figure 7b). This value implies a mixed 2e
and 4e
process even though the nanoparticles has a higher Mn4+/B ratio compared with that of Mn2CoO4

Figure 7. (a) Potentiodynamic curves of Mn2CoO4-NPs at different rotation rates in O2-saturated 1.0 M NaOH solution at a scan rate of 10 mV s
1. (b) Koutecky-Levich plots of Mn2CoO4-NPs at different potentials vs. RHE.

40

H. Liu et al. / Electrochimica Acta 144 (2014) 31–41

octahedra. The electrocatalytic performance of Mn2CoO4-NPs is consistent with the results published by Cheng [21] and Liang,[22] who obtained electron transfer numbers of 2.9  3.4 for Mn2CoO4NPs. These results highlight the strong and direct dependence of the ORR electrocatalytic selectivity on the exposed facets of the spinel oxide. Basing on the above discussion, one can find that both the ORR activity and selectivity of the Mn2CoO4 catalyst depend on the exposed {011} facets. Two influencing factors, namely, exposed facets and Mn4+/B ratio, are fundamentally important to the 2e
and 4e
ORR processes in alkaline solution. Summarily, the precipitation atmosphere of Ar and aging in 5% O2/Ar for 10 h at 55  C are controlled to obtain single-crystal octahedra, which have exposed facet of {011}, and Mn4+/B ratio of 0 and 0.3 for highly selective 2e
and 4e
ORR processes, respectively. 4. Conclusions A strategy for the facile synthesis of single-crystal Mn3-xCoxO4 octahedra was developed by using the novel precipitation-aging method. The optimum synthesis parameters are precipitation atmosphere of Ar and aging in 5% O2/Ar for 10 h at 55  C to obtain well-defined octahedra. The exposed facets of the single-crystal octahedra show high electrocatalytic activity and facet-dependent selectivity. Highly efficient ORR catalyst has exposed facet of {011}, Mn4+/B ratio of 0 for highly selective 2e
ORR processes and the ratio of 0.3 for 4e
. This facet-dependent catalytic performance provides a new route for using inexpensive and corrosion-resistant spinel materials as highly efficient ORR electrocatalysts, and potentially introduces a new general strategy towards spinel oxides with novel catalytic properties for use in a variety of fields. Acknowledgements This research work was financially supported by the National Natural Science Foundation of China (21271169), the research fund of the State Key Laboratory of Catalysis (R201107). We also thank Prof. Ryan O'Hayre for his suggestion and comments, and the Chinese Academy of Sciences Visiting Professorships (2012T1G0015). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta.2014.08.087. References [1] J.S. Spendelow, A. Wieckowski, Electrocatalysis of oxygen reduction and small alcohol oxidation in alkaline media, Phys. Chem. Chem. Phys. 9 (2007) 2654. [2] J. Greeley, I.E.L. Stephens, A.S. Bondarenko, T.P. Johansson, H.A. Hansen, T.F. Jaramillo, J. Rossmeisl, I. Chorkendorff, J.K. Nørskov, Alloys of platinum and early transition metals as oxygen reduction electrocatalysts, Nat. Chem. 1 (2009) 552. [3] D. Wang, H.L. Xin, R. Hovden, H. Wang, Y. Yu, D.A. Muller, F.J. DiSalvo, H.D. Abruña, Structurally ordered intermetallic platinum–cobalt core–shell nanoparticles with enhanced activity and stability as oxygen reduction electrocatalysts, Nat. Mater. 12 (2013) 81. [4] Z.-S. Wu, S. Yang, Y. Sun, K. Parvez, X. Feng, K. Mullen, 3D nitrogen-doped graphene aerogel-supported Fe3O4 nanoparticles as efficient electrocatalysts for the oxygen reduction reaction, J. Am. Chem. Soc. 134 (2012) 9082. [5] J.-J. Lv, J.-X. Feng, S.-S. Li, Y.-Y. Wang, A.-J. Wang, Q.-L. Zhang, J.-R. Chen, J.-J. Feng, Ionic liquid crystal-assisted synthesis of PtAg nanoflowers on reduced graphene oxide and their enhanced electrocatalytic activity toward oxygen reduction reaction, Electrochim. Acta 133 (2014) 407. [6] C. Shi, G.-L. Zang, Z. Zhang, G.-P. Sheng, Y.-X. Huang, G.-X. Zhao, X.-K. Wang, H.-Q. Yu, Synthesis of layered MnO2 nanosheets for enhanced oxygen reduction reaction catalytic activity, Electrochim. Acta 132 (2014) 239. [7] Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier, H. Dai, Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction, Nat. Mater. 10 (2011) 780.

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