Partial Surface Oxidation of Manganese Oxides as an Effective

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Partial Surface Oxidation of Manganese Oxides as an Effective Treatment to Improve Their Activity in Electrochemical Oxygen Reduction Reaction Shi He, Dong Ji, Peter Novello, Lucy Xueqian Li, and Jie Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04977 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 22, 2018

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The Journal of Physical Chemistry

Partial Surface Oxidation of Manganese Oxides as an Effective Treatment to Improve Their Activity in Electrochemical Oxygen Reduction Reaction Shi He, Dong Ji, Peter Novello, Xueqian Li and Jie Liu* Department of Chemistry, Duke University, Durham, North Carolina 27708, USA. *Email: [email protected]

ACS Paragon Plus Environment

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Abstract Enhancing the electrocatalytic activity of low-cost transition metal oxides for oxygen reduction reaction (ORR) is a crucial challenge for extensive application of fuel cells. A promising approach demonstrated previously is the formation of catalysts with mixed valent metal active sites. Since catalysis happens primarily on the surface of the catalyst, we hypothesis that creating such active sites only on the surface will be an effective strategy for improving the catalytic activities. Here, we present a partial oxidation approach that grows δ-MnO nanoflakes on the surface of octahedron 2

Mn O nanocrystals for increasing their ORR activity. The δ-MnO /Mn O nanocomposite exhibits 3

4

2

3

4

significantly improved ORR activity with a half-wave potential of 0.75 V vs. RHE, which is ~110 mV and ~90 mV lower than those of the Mn O nanocrystal and δ-MnO nanoflakes in their pure 3

4

2

forms, respectively. The electrochemical impedance spectroscopy reveals that the δ-MnO /Mn O 2

3

4

nanocomposite possesses a lower ORR charge transfer resistance than either component alone. We propose that the reason for such significant improvement in catalytic activities is due to the tuning of the position of δ-MnO nanoflake d-band center by Mn O nanocrystal which can effectively 2

3

4

facilitate the electron transfer between the active sites and adsorbed oxygen molecules. This work illustrates a facile pathway to improve catalytic activity of mixed valence metal oxides.


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1. Introduction Increasing energy demand in our society has dramatically stimulated the exploration of an efficient and clean alternative to traditional fossil fuel combustion. Fuel cells that directly 1-2

generating electricity by the electrochemical reaction of oxygen can provide an ideal solution as pollution-free power source In the electricity generation process of the alkaline fuel cells, Oxygen 3-4

reduction reaction (ORR) step plays a crucial role for the overall efficiency due to its slow kinetics of multistep electron transfer. Nowadays, platinum-based electrocatalysts have been exclusively used for ORR in proton exchange membrane hydrogen fuel cells. A critical issue for their large 5-6

scale practical application

is the high cost of Pt electrocatalyst, which in turn drives the

development of cheap, nontoxicity and earth-abundance catalyst like first-row transition metal oxides (TMO). However, the TMO electrocatalysts usually exhibit a lower ORR catalytic activity. 7

Their band structure has not met all of the prerequisites of ORR electrocatalysts, including the correct positioned d-band center relative to the Fermi level, easy adsorptions of O and reduction 8

2

of the kinetic barrier for activating intermediates. Therefore, finding a practical approach for 6

controlling the position of the d-band center by tuning composition of different chemical states on the catalyst surface is critical for boosting their ORR activity (Fig. 1c). In recent year, many researchers have been devoted to improving the ORR activity of transition metal oxides including Manganese, Iron, Cobalt, and Nickel oxides. In particular, manganese oxides are considered as promising candidates owing to their tunable position of d-band center near the Fermi level by incorporating different number of electrons filling in the e orbitals of Mn g

ions in the d-bands.

7, 9-10

Zhang et al. reported that α-MnO nanorods fabricated by a hydrothermal 11

2

synthesis procedure procured the ORR half-wave potential at 0.72 V vs. RHE. They claimed that the lower Mn–O bond strength of α-MnO phase was useful for cleaving the adsorbed oxygen. 2

Jaramillo et al. and Risch et al both showed that mixed-phase MnO thin film obtained by 12

13

x


3


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electrochemical deposition had a lower onset overpotential (~400 mV). They consider that the good ORR activity was due to the exposure of Mn O structure on the surface. Lei et al showed Ⅱ,Ⅲ

3

12-13

14

4

that a porous nanosphere of mixed-valent manganese oxide prepared by a chemical etching possessed an excellent ORR activity with a half wave potential of 0.73 V vs. RHE. They explained that the presence of multivalent Mn cations can stimulate fast charge transfer from the active site to the adsorbed oxygen. In addition, Stoerzinger et al. found the LaMnO thin film with mixed 14

3+δ

valence Mn pairs can sufficiently reduce ORR onset overpotential to ~360 mV. They concluded Ⅲ/Ⅳ

15

that the remarkable improvements of ORR activity result from the Mn

Ⅲ/Ⅳ

cation pair as an efficient

redox mediator for accelerating exchange rate of hydroxy groups. These previous results had demonstrated that rational design of an active site for electrocatalysts ultimately determines the ORR activity. Hence, creating a high active site with the optimized structures on the surface of manganese oxides may further enhance their ORR activity. In this paper, in order to develop an efficient ORR electrocatalyst based on manganese oxides, we adopt a controllable synthetic strategy where the δ-MnO /Mn O nanocomposite was fabricated 2

3

4

through partially oxidizing the surface of lower valence octahedron Mn O nanocrystal. The 3

4

synthesis routine initially involves reduction of KMnO to octahedron Mn O nanocrystal template. 4

3

4

Subsequently, the surface of Mn O nanocrystal was oxidized by ammonia persulfate to grow thin 3

4

birnessite δ-MnO nanoflakes (as shown in Fig. 1a) The SEM, XRD and XPS studies unravel that 2

.

partial oxidation method can successfully create layered δ-MnO nanoflakes on the surface of 2

octahedron Mn O nanocrystals with rich Mn

Ⅲ/Ⅳ

3

4

cation oxidation sites. Significantly, the δ-

MnO /Mn O nanocomposite (as shown in Fig 1b) showed an improved ORR activity of a half2

3

4

wave potential of 0.75 V vs. RHE where the overpotential is ~110 and ~90 mV lower than those of Mn O nanocrystal and δ-MnO nanoflakes respectively. These results demonstrate that partial 3

4

2


4

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surface oxidation can be effectively used to decorate the surface of the Mn O nanocrystal for 3

4

enhancing the ORR activity. 2. Experimental Information 2.1 Synthesis. Mn O Octahedron were prepared by following the procedure reported by Lou et 3

4

al. In a typical hydrothermal synthesis, 0.694 g of KMnO4 and 1.0 mL of ethylene glycol were 16

added to 40 mL of deionized water under magnetic stirring to form the precursory suspension. After stirring for about 20 min, the suspension was transferred into a Teflon-lined stainless-steel autoclave with a capacity of 50 ml. The autoclave was then heated in an electric oven at different temperatures 200 C for 12 h. When the autoclave was cooled down naturally to room temperature, o

the precipitate was harvested by centrifugation and washed with deionized water for 3 times and acetone once before room temperature vacuum drying. Oxidation of Mn O nanocrystal (prepared by the previous procedure) were obtained by the 3

4

reaction of Mn O with (NH ) S O . Firstly, 50 ml of pH around 9 0.1 M (NH ) S O aqueous solution 3

4

4

2

2

8

4

2

2

8

was mixed with 100 mg Mn O in the above solution. After that, the mixture was stirred at room 3

4

temperature or 60-70 °C for different time. The final product was collected and washed with absolute ethanol and DI water several times. Finally, the products were dried in a vacuum oven at room temperature for 6h. The samples were labeled as Mn O -xh(x is the reaction time). 3

4

δ-MnO nanoflakes were synthesized by a standard procedure reported by Li et al. 0.79 g of 17

2

potassium permanganate (KMnO ) was first dissolved in 35 mL of deionized water under magnetic 4

stirring. The obtained mixture was transferred into a 50 mL Teflon-lined autoclave and heated at different temperatures 200 °C for 12 h. After cooled to room temperature, the products were washed with distilled water and ethanol for three times and then dried at 80 °C for 12 h.


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2.2 Characterization. Powder X-ray diffraction patterns were collected on a X’Pert PRO MRD HR XRD diffractometer with Cu Ka radiation. The X-ray diffraction pattern was refined by the HighScore Plus Rietveld refinement program. The SEM images were taken with a FEI XL30 SEMFEG microscope (operating voltage, 7 kV). Chemical composition was determined by FEI XL30 ESEM with Bruker XFlash 4010 EDS detector. The XPS was performed by Kratos Analytical Axis Ultra system. All the spectra were calibrated to the C 1s transition set at 284.8 eV. The Raman spectra were recorded by Horiba Jobin Yvon LabRam ARAMIS using a 633 nm laser excitation source and a ×100 objective lens. Calibration was performed using a silicon wafer (phonon line at about 520 cm ). The exposure time was limited to 5 s with 5 accumulations to minimize the beam −1

damage. Different spots across the sample were examined to confirm the consistency of the results. 2.3 Electrochemical test. ORR Electrode was fabricated from a mixture of 1.5 mg δMnO /Mn O nanocomposites and 3.5 mg carbon powder (Vulcan XC-72) was dispersed in solvent 2

3

4

containing 0.5 ml water, 0.5 ml isopropyl alcohol and 17.5 µl neutralized Nafion solution (5 wt%, Sigma-Aldrich). After thorough sonication, 10 µl of the formed catalyst ink was pipetted on the glassy carbon electrode, which was air-dried to afford a mass loading of 0.071 mg oxide/cm . 2

The Cyclic voltammetry (CV) was performed in the same three electrode configuration as mentioned above. The cell was purged with O2 (UHP, Airgas) for 30 min to saturate the electrolyte followed by an initial scan to record the electrochemical oxygen reduction activities of the materials at a scan rate of 50 mV/s at 900 rpm. Then, the ORR activities were obtained by conducting more scans until a reproducible scan was obtained, and O2 was kept flowing over the surface of the electrolyte during the test process. The background currents were also collected by purging the cell with N2 for 30 min and then measuring the CVs under the same conditions as was used in O2. The N2 flow was maintained during the background collection.


6

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Linear sweep voltammograms (LSV) were measured on rotating disk electrodes (RDEs) with the aid of rotators (Pine Instruments). The voltammogram was measured by rotating the RDEs at various rotating rates (400, 700, 1000, 1300 and 1600 rpm) with a scan rate of 5 mV s−1 starting from 0.1 to −0.6 V (versus 4M KCl Ag/AgCl). The cell used the same configuration as mentioned above. The reference potential is then converted to the potential versus Reversible Hydrogen Electrode (RHE) based on the formula in the literature.1 Like the CV experiments, O2 was purged in the electrolyte for 30 min before each test and was kept flowing over the electrolyte during the tests. A chronopotentiometry test was done in the same three-electrode cell using O -saturated 0.1 M 2

KOH as the electrolyte with continuous O flowing over the electrolyte during the test, and the 2

catalyst coated electrodes were rotated at 900 rpm. A continuous current density of 5 mA/cm was 2

used for the anodic reaction. The potential window was set -0.3 V (versus Ag/AgCl), which is in the OER active potential window.

12

The Electrochemical Impedance Spectroscopy (EIS) test was conducted using a Biologic SP300 electrochemical workstation. The measurements were carried out in O -saturated 0.1 M KOH 2

solution on the manganese oxide coated PG carbon electrodes at a cathodic polarization potential of -0.3 V vs Ag/AgCl (～0.7 V vs RHE). The spectra were collected in a frequency range of 0.1 Hz−100 kHz with an amplitude of 5 mV. The protocol of measuring Electrochemical Surface Area (ECSA) is modified from a literature protocol. In a typical experiment, powder-based inks for each catalyst were made using 2 mL 218

propanol, 40 μL of 5% Nafion solution, and 30 mg of the oxide powder without any supported conductive media such as carbon black. The inks were sonicated for 30 minutes, and then 4 μL of the inks were drop-casted onto mirror-polished 0.07 cm glassy carbon electrode using a micro2


7


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pipette. The double layer capacitance of the electrode was measured by the CV techniques with scan rates of 5, 20, 50, 100 and 200 mv/s in 1M NaOH solutions. The ECSA of the system is then calculated by dividing the double-layer capacitance by a general specific capacitance of 1M NaOH solution which is 0.040 mF/cm .

2 18

Figure 1. (a) Schematic representations of synthesis route and (b) surface structure (top view) of the δ-MnO2-Mn3O4 nanocomposite. (White: Hydrogen, Red: Oxygen, Yellow: MnⅢ, Brown: MnⅣ) (c) electronic structure of the manganese cations on the surface relative to the adsorbate oxygen σ bonding.

3. Results and Discussion The rational synthesis procedures of δ-MnO /Mn O nanocomposites are schematically shown in 2

3

4

Fig. 1a. Firstly, the octahedron Mn O nanocrystals are obtained by hydrothermally treating a 3

4

mixture of ethylene glycol and KMnO aqueous solution. The XRD pattern in Fig. 2a reveals that 4

the as-synthesized manganese oxide sample is well indexed to standard data of the hausmannite Mn O (ICCD No.00-024-0734) phase with the characteristic peaks of (211), (103) and (224) 3

4

crystalline planes. The characteristic Raman band of Mn O octahedron nanocrystal at 652 cm in 16

−1

3

4


8

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Fig. 2b is in good agreement with the breathing vibration of Mn –O in tetrahedral coordination of 2+

the Mn O compound.19-20 From the SEM image in Fig. 2b, the morphology of Mn O nanocrystals 3

4

3

4

can be further observed as a defined octahedron shape with an edge length in the range of 300– 400 nm and an apex-to-apex height in the region from 600 to 800 nm. Secondly, the as-synthesized octahedron Mn O nanocrystals are partially oxidized by a strong oxidant (NH ) S O in aqueous 3

4

4

2

2

8

solution at 60-70 °C (see Experimental section). Samples with the reaction time of less than 5h display similar XRD profiles to that of Mn O nanocrystal as shown in Fig. 2a and Figure S1a, 3

4

indicating the essential preservation of their crystalline structures upon initial chemical oxidation. Also, a new Raman band appears at around 632 cm for the oxidized Mn O -1h in Fig. 2b and −1

3

4

Figure S1b. This band is consistent to the characteristic peak of symmetric stretching vibration Mn–O of the MnO groups in birnessite δ-MnO . The corresponding SEM images of the oxidized 21

6

2

Mn O -1h and oxidized Mn O -3h in Fig. 2d and Figure S2 show that a small number of δ-MnO 3

4

3

4

2

nanoflakes start forming on the surface of the Mn O nanocrystals. When the reaction time reaches 3

4

5h, a significant amount of δ-MnO nanoflakes stacking on the surface of the Mn O nanocrystals 2

3

4

is observed in the SEM image of the oxidized Mn O -5h (Fig. 2e). A very board and weak vibration 3

4

band in the range from 620 to 670 cm can be found in Fig. 2b for the oxidized Mn O -5h, and a -1

3

4

shoulder vibration band at 568 cm is referred to the Mn–O stretching band at in the basal plane −1

of δ-MnO .

22-23

2

This broad and weak vibration band indicates the mixing results of Mn O and δ3

4

MnO Mn–O bonding structure on the surface of oxidized Mn O -5h. After reacting with 24

2

3

4

(NH ) S O for 17h, a weak and broad peak with 2θ at 37 that can be attributed to the (111) planes 0

4

2

2

8

of birnessite δ-MnO (ICCD No.00-080-1098) appears in the XRD pattern of the oxidized Mn O 25

2

3

4

17h in Fig. 2a. The vibration bands at 632 cm and 568 cm are evidently rising for the oxidized -1

−1

Mn O -17h while the vibration band at 652 cm is decreased. The SEM images of Fig. 2f reveals −1

3

4


9


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that a large amount of nanoflakes have covered the surface of Mn O nanocrystals for the highly 3

4

oxidized Mn O -17h. With the increase of the oxidation time, the XRD pattern of the oxidized 3

4

Mn O -72h in Fig. 2a shows the continuous rising for the peaks of (111), (112) and (113) planes of 3

4

δ-MnO while the main peaks of Mn O nanocrystals (211) and (103) planes nearly fade out. As 2

3

4

for the Raman spectrum of the oxidized Mn O -72h, two sharp characteristic bands can be observed 3

4

at 632 cm and 568 cm . In contrast, the vibration band at the 652 cm was nearly diminished, -1

−1

-1

which indicates only the δ-MnO phase on the surface of oxidized Mn O -72h. 2

3

4

26-27

In compared with

the SEM image of δ-MnO nanoflakes shown in Fig. 2h, the oxidized Mn O -72h in Fig. 2g has the 2

3

4

mixed phases of the Mn O and δ-MnO Combining with the large area SEM image (Figure S3), 3

4

2.

these results evidently confirm that the δ-MnO /Mn O nanocomposites are successfully 2

3

4

synthesized by partial oxidation with the strong oxidant (NH ) S O . 4

2


2

8

10

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Figure 2. (a) X-ray diffraction patterns of as-synthesized Mn O nanocrystal and δ-MnO /Mn O nanocomposites. The XRD patterns show that the products are transformed from hausmanniteMn O (denote as orange squares ICCD No.00-024-0734) to the mixture of Mn O and birnessiteMnO (denote as brown circles ICCD No.00-080-1098). (b) Raman spectrum of as-synthesized Mn O nanocrystal and δ-MnO /Mn O nanocomposites. (c-h) SEM images of (c) without oxidation treatment Mn O octahedron and the products after oxidizing by ammonia persulfate for 1h (d), 5h (e), 17h (f), 72h (g) and δ-MnO nanoflakes(h). 3

3

4

4

2

3

3

4

4

2

3

4

2

3

3

4

4

2

An qualitative analysis of Mn valence change can be determined by Raman spectroscopy using the relative intensity of corresponding vibration band.10, 22 The Raman band located at 652 cm−1 is indexed to the characteristics vibration of Mn2+ in the tetrahedral coordination of Mn3O4,22, 28 and


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the intensity of vibration band at 632 can be an indicator of the Mn4+ in the δ-MnO2. It can be observed that the vibration band intensity located at 632 cm-1 reaches the same level as the band at 652 cm−1 in Mn3O4-5h and exceeds its intensity in the Mn3O4-17h and Mn3O4-72h. Therefore, these results suggest that some of Mn2+ and Mn3+ in the Mn3O4 are partially oxidized to the Mn4+ on the surface of the δ-MnO2/Mn3O4 nanocomposite. To quantify the evolution of oxidation states by partial oxidation, the Mn O nanocrystals and the 3

4

δ-MnO /Mn O nanocomposites are analyzed by X-ray photoelectron spectroscopy (XPS). The Mn 2

3

4

2p spectra in Fig. 3a display that Mn 2p and Mn 2p peaks of octahedron Mn O nanocrystals are 1/2

3/2

3

4

at 652.6 and 641.1 eV respectively, which are consistent with previously reported ones for the hausmannite Mn O XPS spectra. After the treatment with (NH ) S O , the binding energy of Mn 29

3

4

4

2

2

8

2p peak of the oxidized Mn O -5h shifts to the higher value at around 642.1 eV. In comparison 3/2

3

4

with the standard characteristic peak of Mn and Mn located at 642.5 eV and 641.3 eV, the XPS Ⅳ

Ⅲ

29

results clearly indicate that the Mn oxidation state of δ-MnO /Mn O nanocomposite are increased, 2

3

4

The splitting of Mn 3s peak in Fig. 3b can provide quantitatively


12

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Figure 3. The XPS spectra of the Mn O nanocrystal and δ-MnO /Mn O nanocomposites (a) Mn 2p , (b) Mn 3s, and (c) O 3

3/2

4

2

3

4

1s

information to determine the accurate Mn oxidation state of the Mn O nanocrystals and the δ3

MnO /Mn O nanocomposite.

29-30

2

3

4

4

The energy separations of Mn 3s for Mn O , Mn O -1h Mn O -3h, 3

4

3

4

3

4

Mn O -5h, and Mn O -17h are 5.6, 5.2, 5.0, 4.8, and 4.6 eV, corresponding to the Mn oxidation 3

4

3

4

states of 2.7,3.3, 3.5, 3.7, and 3.9, respectively. The XPS analysis of Mn 3s spectrum verifies the mixed valences of Mn and Mn residing in the δ-MnO /Mn O nanocomposites. Seen from XPS 3+

4+

2

3

4

and SEM results, we consider that a thin layer of δ-MnO formed on the surface of Mn O 2

3

4

nanocrystal. Moreover, as shown in Fig. 3c, it can be observed that the relative peak intensity and area of O 1s corresponding to surface adsorbed oxygen is about 10% increase for the δMnO /Mn O nanocomposite than that for the Mn O nanocrystal.

31

2

3

4

3

4


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The ORR activity of Mn O nanocrystals, the δ-MnO /Mn O nanocomposites and the δ-MnO 3

4

2

3

4

2

nanoflakes are measured in 0.1 M KOH by using rotating disk electrode (RDE) technique. The RDE can reduce the length of diffusion layer and mass transport losses of oxygen. Fig. 4a shows the linear scan voltammograms (LSV) of all the samples recorded at a scanning rate of 5 mV/s with a rotation speed of 900 rpm. The half-wave potential of the Mn O nanocrystal is 0.64 V vs 3

4

RHE, which is almost the same as the result (0.63 V vs RHE) reported by Su et al..

32

Figure 4: (a) Linear Scan voltammetry of as-synthesized Mn O nanocrystal and δ-MnO /Mn O nanocomposites in O saturated 0.1M KOH solution at a rotation speed of 900 rpm. (b) Kinetic current density(E ) of as-synthesized Mn O nanocrystal and δ-MnO /Mn O nanocomposites at 0.7V vs RHE. (c) Chronoamperometric profiles of oxidized Mn O -5h and Pt/C. (d) The electrochemical surface area (ECSA) of each catalyst which is estimated from measurements of the double-layer charge process data shown in Figure S6. 3

4

2

3

4

2

k

3

4

2

3

3

4

4

After the oxidation with (NH ) S O for 3h, the half wave potential decreases about 30 meV for the 4

2

2

8

oxidized Mn O -3h Notably, when the oxidation time is 5h, the onset potential significantly shifts 3

4


14

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to a lower value of 0.87 V vs RHE and a half-wave potential of 0.75 V vs RHE, which is about 110 mV lower and 90 mV lower than the Mn O nanocrystal and δ-MnO nanoflakes, respectively 3

4

2

(as shown in the enlarged Figure S5). This finding supports the design that the partial oxidation by (NH ) S O can promote the ORR electrocatalytic activity for the manganese oxides. The LSV curve 4

2

2

8

of the oxidized Mn O -5h then remained stable at the appearance of diffusion-limiting currents (I ) 3

4

d

of 3.34 mA/cm . With further increasing the oxidation time to 17h, the half wave potential 2

decreases to 0.67 V vs RHE for the oxidized Mn O -17h. The cyclic voltammograms (CVs) of the 3

4

δ-MnO /Mn O nanocomposites collected at a scanning rate of 20 mV/s are demonstrated in Figure 2

3

4

S4. A stronger cathodic current is in O saturated solution where no apparent peaks is observed in 2

the N saturated solution. These results confirm that the δ-MnO /Mn O nanocomposites can have 2

2

3

4

excellent reduction activity of O in the potential range of 0.2 to 0.8 V vs RHE. The effect of partial 2

oxidation is also distinguishable by the kinetic current density shown in the Fig. 4b. It is worth noting that the kinetic current density of oxidized Mn O -5h is around 15 mA/cm and surpasses 2

3

4

that of the other δ-MnO /Mn O nanocomposites. 2

3

4

Catalyst

E1/2vs.RHE (V)

Limiting current(mA)

Mn3O4

0.64

0.62

Mn3O4-1h

0.66

0.68

Mn3O4-3h

0.67

0.63

Mn3O4-5h

0.75

0.82

Mn3O4-9h

0.72

0.72

Mn3O4-17h

0.65

0.56

δ-MnO2

0.66

0.55

Table 1. Summary of half-wave potential and limiting current from δ-MnO /Mn O nanocomposites LSV data 2


3

4

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Page 16 of 29

The Long-term stability is important for the practical electrocatalysts. As shown in Fig. 4c, the durability of the oxidized Mn O -5h obtained from chronoamperometric measurements can 3

4

maintain high current retention of above 80% in a continuous operation of 15h at 0.70 V vs RHE. It is higher than that (74%) of the reported Pt/C electrode tested under the identical conditions. In 9

addition to the overpotential and kinetic current density, the electrochemical surface area (ECSA) of δ-MnO /Mn O nanocomposites were estimated from the data shown in Figure S6. The 2

3

4

calculation results of each catalyst ECSA are displayed in Fig. 4d. The oxidized Mn O -5h has the 3

4

intermediate ECSA of 0.96 m /g. In contrast, the lower ORR activity oxidized Mn O -17h has the 2

3

4

highest ECSA (6.3 m /g) which indicates the improved ORR activity does not only result from 2

increasing surface area and number of active sites. The oxidized Mn O -5h was evaluated based on 3

4

XPS measurements after electrolysis at 0.7 V versus RHE for 3 hours. In the Figure S7, the binding energy of Mn 2p3/2 peak centered at 641.9 eV is almost no change for the oxidized Mn O -5h after 3

4

ORR electrolysis, and it is identical with that of the oxidized Mn O -5h before ORR electrolysis. 3

4

Hence, this result indicates that the Mn valence states of the oxidized Mn O sample are stable 3

4

during the ORR electrolysis. Moreover, the position of O 1s XPS spectra for the oxidized Mn O 3

4

5h (before and after) can be deconvoluted as peaks centered at 529.8, 531.2, 532.7 and 533.6 eV, which corresponds to the lattice oxygen, oxygen species adsorbed on the surface, adsorbed molecular H O, and oxygen from Nafion binder, respectively.

33-35

2

The peak positions of lattice

oxygen and oxygen species adsorbed on the surface remain the same in the samples of oxidized Mn3O4-5h before and after ORR electrolysis. Only slightly change of relative intensity of the H O 2

adsorbed on surface and oxygen from Nafion binder happens in the sample after ORR electrolysis. These results show that the surface of δ-MnO /Mn O nanocomposites can maintain the initial Mn 2

3

4

oxidation states under the ORR electrolysis condition.


16

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Figure 5. (a) Linear Scan voltammetry of Mn O -5h in O saturated 0.1M KOH solution at different rotation speeds. (b) Koutecky-Levich (K-L) plots of the Mn O -5h where the recorded potential is relative to RHE. (c) Electron transfer number calculated from (K-L) plots shown in the Figure S8. (d) Inverse of the charge transfer resistance as a function of post oxidized time interval, proving that the activity of ORR changes at different oxidation time. The R was obtained from the fitting parameters for the impedance spectrum in Figure S9 which collects at the 0.7V vs RHE. 3

4

2

3

4

CT

The electrocatalytic kinetics of the δ-MnO /Mn O nanocomposites are studied by measuring the 2

3

4

polarization curves at different electrode rotation speeds. The voltammogram of the oxidized Mn O -5h in Fig. 5a shows that about 20% limiting current density is increased at all electrode 3

4

rotation speed compared with the oxidized Mn O -3h and Mn O -17h as shown in Figure S8. To 3

4

3

4

analyze the reason of this finding, Fig. 5b is the constructed Koutecky−Levich (K−L) curves that plot the inverse of current (I ) versus the inverse square root of rotation speeds (ω ) from 0.6 to -1

-1/2

0.4 V vs RHE. The oxidized Mn O -5h has linear relationship between the I and the ω with near -1

3

-1/2

4

parallelism in the K-L plot curves. This finding indicates that the oxidized Mn O -5h adopts a first3


4

17


Page 18 of 29

order reaction kinetic in the ORR process which is favor for the fast ORR reaction rates observed in contemporary literature. The ORR electron transfer number (n) per oxygen molecule of the δ36

MnO /Mn O nanocomposites obtained from fitting the slopes of K−L curves in Figure S8 and the 2

3

4

results are presented in Fig. 5c. The average electron transfer numbers of the oxidized Mn O -3h, 3

4

Mn O -5h, and Mn O -9h are approximate 4. The results of electron transfer numbers suggest the 3

4

3

4

reaction pathway via a dominant formation of hydroxide ions for their ORR process. In contrast, the average electron transfer numbers of the Mn O nanocrystal is about 2.6, suggesting the 3

4

preference of forming peroxide as the ORR product. Together these kinetics results provide an important insight that the Mn O nanocrystals oxidized partially by (NH ) S O can realize a high 3

4

4

2

2

8

ORR electrocatalytic efficiency by facilitating the product decomposition of peroxide to hydroxide ions. To understand the charge transport rates of the samples tuned by the partial oxidation, the electrochemical impedances of the δ-MnO /Mn O nanocomposites are collected at 0.7 V vs RHE 2

3

4

under ORR mixing diffusion and kinetics region. The accurate R in Fig. 5d are obtained from 12

ct

fitting equivalent circuit of Nyquist plots in Figure S9. It can be found that the relationship between the inverse of R and oxidation time follows a rough volcano shape. All the δ-MnO /Mn O ct

2

3

4

nanocomposites show charge transfer resistance (R ) in the range from 100 to 160 Ω. The oxidized ct

Mn O -3h and Mn O -9h have an intermediate R at about 130Ω, and the oxidized Mn O -17h 3

4

3

4

ct

3

4

exhibits the largest R of 157Ω. Interestingly, the oxidized Mn O -5h presents the lowest R of 106 ct

3

4

ct

Ω while the Mn O nanocrystal has an extremely high R value of 213 Ω. The smaller R in the 3

4

ct

ct

Nyquist plots indicates the increase of charge transfer rate that is important to enhance the ORR activity.

6, 37


18

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The electrochemical characteristics reveal that the oxidized Mn O -5h possesses highest activity 3

4

and selectivity among the partially oxidized manganese oxides. The enhancement of ORR activity after partial oxidized by (NH ) S O is possibly ascribed to two aspects: the change of Mn valence 4

2

2

8

state and the increasing of active sites after chemical treatment. In alkaline media, the ORR process on transition metal oxide is usually limited by the rates of oxygen molecule adsorption to replace the OH adsorbed on the Mn site. The rate of this process is determined by the energy level of Mn -

8

ion d-band center. The XPS results verify that the partial oxidation method can oxidize some of 5, 7

Mn cations with two electrons filling in the e orbitals to Mn and Mn cations with one and zero II

III

IV

g

electron filling in the e orbitals, respectively. This transformation optimizes the energy level of dg

band center by forming different ratios of mixed-valence Mn

III/IV

cation pairs to regulate oxygen

molecule adsorption ability and the exchange kinetics of OH by oxygen molecules. As a result, -

the ORR activity of δ-MnO /Mn O nanocomposites exceeds the individual component of Mn O 2

3

4

3

4

and δ-MnO . Additionally, the formation of mixed-valence Mn cations can perform as the 2

electrochemical mediator for promoting charge transfer between metal cations and oxygen molecules in the ORR process. Notably, the relationship of inverse charge transfer resistance of 38

oxidized δ-MnO /Mn O nanocomposites and post-oxidation time follows a rough volcano shape, 2

3

4

with a peak value at 5h where the oxidized Mn O -5h has around 30% of Mn and 70% of Mn on 3+

3

4+

4

the surface (as shown in Fig. 5d). This finding explains the reason that the oxidized Mn O -5h 3

4

sample has lower charge transfer resistance, which in turn results in facile interfacial charge transfer between the Mn cation and adsorbed oxygen molecules. On the other hand, the phase transition from the spinel Mn O to the layer structure δ-MnO by oxidation process increases their 3

4

2

specific surface areas as shown by the SEM images, thus favoring more exposed active sites. Moreover, a negligible H O yield at potentials of interest for avoiding the chemical attacks of 2

2


19


Page 20 of 29

electrocatalyst support. As shown in the Fig. 5c and Figure S8, the K-L analysis displays that the 39

average electron transfer numbers of the oxidized Mn3O4-3h and Mn3O4-5h are approximate 4 while those of the Mn3O4 and δ-MnO2 compounds are close to the 2. Our XPS and Raman results in Fig. 3 and Fig. 2b demonstrate that the oxidized Mn3O4-3h and Mn3O4-5h nanocomposites have mixed valence states of Mn3+ and Mn4+. Hence, we consider that the unpaired electron in the eg orbital of Mn3+ can perform as an active site for the O2 adsorption on the surface, and the Mn4+ can increase the hybridization of the Mn-O to boost rates of electron transfer to the O−O π*.40-41 Thus the mixed valence of Mn3+ and Mn4+ on the surface of δ-MnO2/Mn3O4 nanocomposites can synergistically accelerate the rate of electron transfer to the O−O π*. The fast rate of electron transfer to the O−O π* can also help the cleavage of O-O bond in the hydroperoxide (HO2-) intermediate that is the determined reaction factor to produce OH- (4e- pathway).42 In comparison to Mn2+/Mn3+ or Mn4+ in Mn3O4 or δ-MnO2, the mixed Mn3+/Mn4+ valence states are beneficial to the decomposition of peroxide.43 Therefore, these two factors in the oxidized Mn3O4-3h and Mn3O4-5h samples can effectively remove the amount of peroxide to a negligible level and strongly improve the electron transfer number of ORR. Therefore, the partial oxidation of the δMnO /Mn O nanocomposite can enhances the selectivity of ORR for the alkaline fuel cell. 2

3

4

4. Conclusion In summary, this study presents a partial surface oxidation method to create mixed-valent manganese oxide active sites, enabling a systematic investigation of the relationship between surface Mn and Mn oxidation states and their ORR catalytic performance. The obtained δⅢ

Ⅳ

MnO /Mn O nanocomposite can exhibit the high activity and excellent stability. These results 2

3

4

indicate that partial chemical oxidation can provide an efficient strategy to enhance the ORR


20

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electrocatalytic performance of transition metal oxides, which promote their applications in alkaline electrochemical fuel cells.

Associated Contented The Supporting Information is available with the component: Additional X-ray diffraction pattern of δ-MnO /Mn O nanocomposites, additional SEM images of 2

3

4

δ-MnO /Mn O nanocomposites, large area SEM images of δ-MnO /Mn O nanocomposites, Cyclic 2

3

4

2

3

4

voltammetry of δ-MnO /Mn O nanocomposites, Linear Scan voltammetry of δ-MnO /Mn O

4

nanocomposites compared with the commercial catalyst, Cyclic voltammetry of δ-MnO /Mn O

4

2

3

4

2

2

3

3

nanocomposites with potential window of 0.1V centered at the open circuit voltage, KouteckyLevich plots of δ-MnO /Mn O nanocomposites and Electrochemical impedance spectra of δ2

3

4

MnO /Mn O nanocomposites. 2

3

4

Author Information Corresponding Authors *Email: [email protected]

Notes The authors declare no competing financial interest.

Acknowledgments


21


Page 22 of 29

This work was supported in part by a grant from NSF (CHE-1565657) and Duke University through a Foundation of Nanoscience and Nanoengineering Fellowship. The authors also acknowledge the support by the Duke University Shared Materials Instrumentation Facility (SMIF), a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), which is supported by the National Science Foundation (Grant ECCS-1542015) as part of the National Nanotechnology Coordinated Infrastructure (NNCI).

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TOC Figure


29

Partial Surface Oxidation of Manganese Oxides as an Effective

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