doi.org/10.26434/chemrxiv.7716590.v1

Temperature-Dependent Electronic Structure of Bixbyite α-Mn2O3 andthe Importance of a Subtle Structural Change on Oxygen ElectrocatalysisJunais Mokkath, Maryam Jahan, Masahiko Tanaka, Satoshi Tominaka, Joel Henzie

Submitted date: 14/02/2019 • Posted date: 14/02/2019Licence: CC BY-NC-ND 4.0Citation information: Mokkath, Junais; Jahan, Maryam; Tanaka, Masahiko; Tominaka, Satoshi; Henzie, Joel(2019): Temperature-Dependent Electronic Structure of Bixbyite α-Mn2O3 and the Importance of a SubtleStructural Change on Oxygen Electrocatalysis. ChemRxiv. Preprint.

α-Mn2O3 is an inexpensive Earth-abundant mineral that is used as an electrode material in various kinds ofelectrochemical devices. The complex bixbyite structure of α-Mn2O3, and its subtle orthorhombic → cubicphase transformation near room temperature has made it challenging to accurately determine its electronicproper- ties. We used high-resolution X-ray diffraction to study the temperature-dependent structures ofphase-pure α-Mn2O3 prisms. Our measurements show a clear change in the crystal phase from orthorhombic→ cubic between 293K and 300K. We input the Rietveld refined high-resolution crystal structures collected atvarious temperatures (273, 293, 300, 330K) directly into density functional theory (DFT) calculations to modeltheir electronic properties. These calculations indicate that the orthorhombic phase α-Mn2O3 is a narrowbandgap semiconductor as expected. However, temper- atures higher than 300K transform the α-Mn2O3 intothe cubic phase, causing the molecular orbitals of the Mn 3d and O 2p bands to overlap and mix covalently,mak- ing the material behave as a semimetal. This subtle change in crystal structure will affect the bulkconductivity of the material as well as Mn-O-Mn bond distances that influence the quality of its catalytic activesites for oxygen electrochemistry. Elec- trochemical oxygen evolution (OER) and oxygen reduction reaction(ORR) experi- ments performed at various temperatures (∼ 288K to 323K) using the same prepared electrodeshow a marked enhancement in both OER and ORR performance that is attributed to the higher activity of thecubic phase.

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Temperature-dependent electronic structure of bixbyite α-Mn2O3

and the importance of a subtle structural change on oxygen

electrocatalysis

Junais Habeeb Mokkath1,2,‡, Maryam Jahan1,‡, Masahiko

Tanaka3, Satoshi Tominaka1,*, and Joel Henzie1∗

1. International Center for Materials Nanoarchitectonics (WPI-MANA),

National Institute for Materials Science,

(NIMS) 1-1 Namiki, Ibaraki 305-0044, Tsukuba, Japan

2. Department of Physics, Kuwait College of Science And Technology,

Doha Area, 7th Ring Road, P.O. Box 27235, Kuwait and

3. Synchrotron X-ray Station at SPring-8,

National Institute for Materials Science,

Kouto 1-1-1, Sayo, Hyogo 679-5148, Japan

(Dated: February 14, 2019)

1

Abstract

α-Mn2O3 is an inexpensive Earth-abundant mineral that is used as an electrode

material in various kinds of electrochemical devices. The complex bixbyite structure

of α-Mn2O3, and its subtle orthorhombic → cubic phase transformation near room

temperature has made it challenging to accurately determine its electronic proper-

ties. We used high-resolution X-ray diffraction to study the temperature-dependent

structures of phase-pure α-Mn2O3 prisms. Our measurements show a clear change in

the crystal phase from orthorhombic → cubic between 293K and 300K. We input the

Rietveld refined high-resolution crystal structures collected at various temperatures

(273, 293, 300, 330K) directly into density functional theory (DFT) calculations to

model their electronic properties. These calculations indicate that the orthorhombic

phase α-Mn2O3 is a narrow bandgap semiconductor as expected. However, temper-

atures higher than 300K transform the α-Mn2O3 into the cubic phase, causing the

molecular orbitals of the Mn 3d and O 2p bands to overlap and mix covalently, mak-

ing the material behave as a semimetal. This subtle change in crystal structure will

affect the bulk conductivity of the material as well as Mn-O-Mn bond distances that

influence the quality of its catalytic active sites for oxygen electrochemistry. Elec-

trochemical oxygen evolution (OER) and oxygen reduction reaction (ORR) experi-

ments performed at various temperatures (∼ 288K to 323K) using the same prepared

electrode show a marked enhancement in both OER and ORR performance that is

attributed to the higher activity of the cubic phase.

Keywords: Bixbyite, α-Mn2O3, Electrode, OER, ORR, Fuel Cells

∗Electronic address: joelhenzie@protonmail.com

2

Electrochemical energy generating devices including fuel cells[1], water splitting

systems[2, 3], electrochemical capacitors[4], and rechargeable batteries[5, 6] require porous

electrodes to efficiently drive electrochemical reactions. Precious metal based electrocata-

lysts are currently favored in many of these technologies, but their high cost and scarcity

limits feasibility for large-scale adoption. Researchers have been searching for more Earth-

abundant transition metal catalysts that combine good performance with low economic cost.

Thus manganese oxide based minerals have emerged as an inexpensive alternative due to

the high abundance of rock-forming elements in the Earth’s crust[7, 8]. Manganese(III)

oxides such as α-Mn2O3 contain Mn3+ metal cations with surface Mn3+O6 octahedral sites

with one electron in the eg orbital and moderately conductive metal-oxygen bonds. Metal

oxide materials with these properties are predicted to support high catalytic activities for

the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR)[9–11]. It

has been reported that trivalent manganese oxides with Jahn-Teller distorted octahedra and

an anti-bonding eg1 electron have high activity for OER[12], enabling reversible interactions

with O2 molecules that contribute to structural flexibility during water oxidation[13, 14].

On the other hand, suppressing Jahn-Teller distortions in some manganese oxide systems

has been shown to increase OER catalytic activity[15]. α-Mn2O3 performs as a bifunctional

electrocatalyst for both OER and ORR in alkaline meda[16, 17]. In situ X-ray results indi-

cate that the Mn(III) active sites of the manganese oxide forms a mixed MnII/MnIII state

under OER conditions and a mixed MnIII/MnIV state under ORR conditions[18]. These

Mn-based redox systems enable rapidly reversible transitions between oxidation states de-

pending on surface hydration[19]. And numerical methods such as density functional theory

(DFT) enable researchers to add different atoms to the manganese oxide matrix to predict

how structure impacts electron distribution and oxygen bond strength in oxygen catalytic

cycles[20, 21]. But understanding how subtle changes in the bonding arrangement of binary

oxides provide a deeper understanding of the role of oxidation state and the dependence of

composition and structure on the quality of the active site in oxygen electrocatalysis.

α-Mn2O3 is a unique experimental system to study bonding in the MnIII active site

because it possesses a subtle orthorhombic → cubic phase change at 300K[22]. Small

impurities of Fe3+ cations can drive α-Mn2O3 into the cubic phase at lower temperatures,

so it is important to generate high purity crystals[23]. Previously, we reported a colloidal

method to generate α-Mn2O3 prisms bound by {100} facets by exploiting the cation bridging

3

effect between Mn Ions and sodium docusate (NaAOT)[24]. These α-Mn2O3 prisms are

extremely phase pure, and careful X-ray analysis showed that the as-prepared α-Mn2O3

prism electrodes adopted the orthorhombic phase. Thermally-treating the α-Mn2O3 powder

on a glassy carbon electrode (GCE) in air at 480◦C causes the crystals to adopt the cubic

phase, which it maintains even after the material is cooled to room temperature. The

cubic phase α-Mn2O3 prisms had equivalent or better bifunctional OER/ORR performance

compared to commercial Pt catalyst or α-Mn2O3 powders[24]. Surprisingly, the cubic phase

prisms were less catalytically active than the orthorhombic prisms. But it was possible

that this level of improvement could be due to different electrode preparation methods.

This is because thermal treatment can increase the surface area of the catalyst or improve

contact between the α-Mn2O3 and GCE. On the other hand, the increase in performance

could be due to a subtle change in bonding arrangement of the Mn(III) active site affects

OER/ORR activity. In this manuscript, we use temperature-dependent high-resolution X-

ray diffraction (XRD) measurements on the phase-pure α-Mn2O3 prisms to obtain precise

orthorhombic and cubic crystal structures at various temperatures. These structures were

then used as models in DFT simulations to examine how electronic band structure changes

in the orthorhombic → cubic phase transition. We discovered that the phase change causes

α-Mn2O3 to transform from a semiconductor to a semimetal due to the suppression of some

Jahn-Teller distortions. Then we performed OER/ORR experiments in a temperature-

controlled electrochemical setup to examine how the α-Mn2O3 orthorhombic→ cubic phase

transition affects the exchange current density and activation energy in OER/ORR.

Figure 1 shows an XRD pattern of α-Mn2O3 prisms collected at 293K (19.85◦C) at the

at the SPring-8 BL15XU beamline[25]. The CeO2 NIST standard was used as a reference

to calibrate the wavelength (λxrd = 0.653144 A). XRD patterns were also obtained at 100K

(-173.15◦C), 200K (-73.15◦C), 270K (-3.15◦C), 300K (26.85◦C) and 330K (56.85◦C) (Fig.

S2-S8). The data was analyzed with the Rietveld refinement method using the bixbyite

structures in the orthorhombic cell for the lower temperatures and in the cubic cell for

the higher temperatures that we define here using the abbreviation: Temperature[Crystal

System] (e.g. 270K[Ortho] denotes the 270K data refined using the orthorhombic crystal

system of α-Mn2O3). The simulated curves closely match the high-resolution experimental

patterns with low Rw values and the goodness-of-fit (GOF) parameters (Table S1). Rietveld

analysis shows that the cubic crystal system provides the best numerical match for the α-

4

2Theta (degrees)

15.7 16.116.015.915.8 16.2

330 K300 K 293 K 270 K

400α-Mn2O3 (293 K)Rietveld refinement, orthorhombicdifference

10 403020 50

orthorhombiccubic

orthorhombic

cubic

Temperature (K)0 15010050 200 300250 350

Latti

ce P

aram

eter (

Å)

9.36

9.46

9.42

9.40

9.38

9.44

9.50

9.48

Temperature (K)0 15010050 200 300250 350

Volu

me (

Å3)

832.0

834.5

833.5

833.0

832.5

834.0

835.5

835.0

B

A

C

Inten

sity

FIG. 1: (a) High-resolution X-ray diffraction pattern for α-Mn2O3 performed at t = 293 K (i.e.

293K[Ortho] showing both experimental data and the simulated curve obtained by Rietveld re-

finement using the orthorhombic crystal system (λxrd = 0.653144 A). Similar measurements were

performed at t = 100, 200, 273, 293, 300 and 330 K. The inset graph is a zoom-in of the experi-

mental XRD data showing 400 reflection at various temperatures near the orthorhombic to cubic

phase transition(t = 273, 293, 300 and 330 K).

Mn2O3 prisms at 300K. The orthorhombic→ cubic transition can be easily visualized in the

inset of Figure 1 by observing the 400, 040 and 004 reflection peaks of the orthorhombic

phase (Pcab symmetry, space group 61) as they progressively merge into a single degenerate

5

peak at higher temperatures in the cubic phase. Detailed crystallographic information files

are included in the Supporting Information.

These structures show that α-Mn2O3 has Mn3+ ions octahedrally coordinated to six O2-

ions. Each oxygen ion has four Mn neighbors. Geller previously explained that the or-

thorhombic Pcab phase of α-Mn2O3 emerges at low temperatures, caused by Jahn-Teller

instabilities that distort the coordination of all 32 Mn atoms in the unit cell[22]. Heat-

ing α-Mn2O3 above the phase transition temperature suppresses or relieves the distortions

on 8 out of 32 Mn atoms and the material adopts the cubic phase. Comparisons of our

temperature-dependent lattice parameters and unit cell volumes with Geller[22] and recent

neutron diffraction data by Cockayne, et al [26] are included in Figure 1C,D. This data

shows that lattice parameters are relatively continuous up to the phase change >293K.

There appears to be a small discontinuity in the volume change between 293K[Ortho] →

300K[Cubic] which might indicate phase transition is higher than first order as predicted

earlier[22]. Regardless, difference in surface area between the 293K[Ortho] and 300K[Cubic]

unit cell is <0.005%, thus we can rule out its impact on oxygen electrocatalysis.

Determining how the orthorhombic→ cubic phase transformation impacts the electronic

band structure and electrochemical activity of α-Mn2O3 is the ultimate purpose of these high

resolution temperature-resolved XRD measurements. The Rietveld refined crystal structures

were input directly into the Vienna Ab initio Simulation Package (VASP)[27, 28] implemen-

tation of DFT[29] without any geometry optimization and modeled using the PBE0 hybrid

functional which is used to reproduce the electronic and magnetic properties of a wide class

of materials[30, 31]. Previous work showed that hybrid functionals such as PBE0 are able

to accurately predict some of the properties of α-Mn2O3 and show the material is a narrow

band gap semiconductor. But these pioneering results are based on low-resolution crystal

structures, and electronic properties were calculated using a structure optimized with the

HSE functional[32]. We used DFT to calculate the spin and orbital resolved density of

states (DOS) of α-Mn2O3 at t = 270K, 293K, 300K and 330K. The spins of the atoms are

dealt with using the fixed spin moment (FSM) method which is a standard technique in

DFT to work with magnetic systems. Figure 2A,B; bottom shows the DOS plots of the

293K[Ortho] and 300K[Cubic] structures, with isosurfaces illustrating the charge densities

between -4 to -6 eV, and -2 to 0 eV. The DOS of the 293K[Ortho]-derived structure is a

narrow band gap semiconductor. Although the 300K[Cubic] structure had a similar net spin

6

Dens

ity of

Stat

es (1

/eV)

0 642 8-10 -6 -4 -2-8Energy (eV)

-60

-40

-20

60

40

20

0

0 642 8-10 -6 -4 -2-8Energy (eV)

Spin Up

Spin Down

TotalMn(d)

O(p)t2geg

A B

TotalMn(d)

O(p)t2geg

FIG. 2: The calculated spin and orbital resolved DOS of 293K[Ortho] (A) and 300K[Cubic] (B). A

Lorentzian of width 0.2 eV has been used to broaden the discrete energy levels. The Fermi energy

is represented by a vertical dotted line located at 0 eV. In the top region the iso-surfaces (iso-value

of 0.4 e/A3) of partial density of states coming from two DOS peaks between -4 to -6 eV, 0 and

-2. The atomic bonds are also shown having green (red) spheres representing Mn (O) atoms.

polarization as the 293K[Ortho] structure, the band gap almost vanishes due to the creation

of new electronic states straddling the Fermi level (Ef). DFT calculations were performed

on the 270K[Ortho] and 330K[Cubic] structures and they show the same semiconductor to

semimetal transition (Figure S8).

The large difference in electronegativity between Mn (1.5) and O (3.5) is expected to

generate strong ionic bonding. This is demonstrated by plotting the difference charge density

iso-surfaces of the 293K[Ortho] and 300K[Cubic] structures derived from DFT (Figure S9).

However, due to the translational symmetry of the 300K[Cubic] structure, the molecular

orbitals are able to transform into Mn 3d and O 2p bands and mix covalently due to

their spatial overlap and energetic similarity[21]. This is shown by plotting the difference

7

charge density iso-surfaces (Figure 2A,B; Top). These plots reveal the nature of electronic

states involved in bonding, for example, in the energy range from -4 to -6 eV there is

an overlap of t2g and eg derived states and the corresponding partial charge density iso-

surface reflects the coexistence of strongly directional eg (dz2) states and non-directional

t2g states. Whereas in the energy interval between 0 and -2 eV the t2g-derived states are

predominant and corresponding partial charge density iso-surface well reflects it. There

are also differences in the partial charge density iso-surfaces of O atoms, indicating different

overlap with neighboring atoms. No significant contributions from 3d states in the minority-

spin VB close to the EF, indicating strong spin polarization in the system. The O 2s states

are well localized around -25 eV (not shown in Figure 2A) and therefore do not contribute

significantly to chemical bonding.

The DFT results indicate that the transformation from orthorhombic to cubic phase re-

lieved Jahn-Teller distortions that un-tilted the Mn3+O6 octahedra and leads to improved Mn

3d and O 2p covalent bonding and bandwidth. The observed semiconductor-to-semimetal

transition can be explained using well known Hubbard model[33] considering the fact that

PBE0 functional contains 25% of full range Hartree-Fock exchange. In the Hubbard model,

electron motion among the atomic sites is controlled by the ratio of intra-atomic Coulomb

repulsion strength (U) and 3d bandwidth (W). A large U/W ratio yields electron localiza-

tion and hence a semiconductor or insulating state, while a small value enhances electron

delocalization and a metallic nature. Figure 2 shows that the W value in the cubic phase

is nearly 0.5 eV greater compared to the orthorhombic phase, thus interpreting the results

using a simplified Hubbard model is relatively convincing.

The electronic states are clearly affected by the phase transformation, but the impact of

this change on oxygen electrocatalysis is an open question. Based on a purely ionic point

of view, the eg orbitals point directly towards the O atoms and t2g orbitals lie between the

O atoms. As a result electrons in the t2g (eg) orbitals will be more localized (itinerant)

[34]. Figure 2 clearly shows that the phase transformation causes significant changes in the

regions close to the Fermi energy (Ef). The 293K[Ortho] phase structure has a majority band

gap at 2.2 eV and a minority band gap at 6.1 eV. We mainly consider the majority states

since the minority states are far away from Ef. Note that Ef is shifted to 0 eV to simplify

the description. The majority-spin VB runs from near -1 to -10 eV and consists of Mn 3d

and O 2p states with a dominant Mn 3d character. Moreover, 3d states (predominantly t2g

8

states) and 2p states overlap in the energy range from -1 to -3 eV and -7 to -9 eV, whereas

the Mn 3d states dominate the -4 to -6 eV energy range.

The majority-spin conduction band (CB) spans from 1.5 to 4 eV and consists of 3d

(predominantly t2g states) and 2p states. The overlap of 3d and 2p states give rise to distinct

partial charge density iso-surfaces (spin up plus spin down) as shown in the top region of

Figure 2A. These plots reveal the nature of electronic states involved in bonding, for

example, in the energy range from -4 to -6 eV there is an overlap of t2g and eg derived states

and the corresponding partial charge density iso-surface reflects the coexistence of strongly

directional eg (dz2) states and non-directional t2g states. Whereas in the energy interval

between 0 and -2 eV the t2g-derived states are predominant and corresponding partial charge

density iso-surface well reflects it. There are also differences in the partial charge density

iso-surfaces of O atoms, indicating different overlap with neighboring atoms. No significant

contributions from 3d states in the minority-spin VB close to the EF, indicating strong spin

polarization in the system. The O 2s states are well localized around -25 eV (not shown in

Figure 2A) and therefore do not contribute significantly to chemical bonding.

In the 300K[Cubic] phase structure in Figure 2B, states close to Ef are dominated by

majority states with strongly overlapping Mn 3d and O 2p states. Again, minority states

appear far away from Ef, indicating a strong spin polarization in the system similar to

293K[Ortho]. The majority band shows metallic behavior whereas the minority bandgap

amounts to 6.5 eV. The majority-spin VB near Ef is predominantly an admixture of Mn

t2g and O 2p states with a minor contribution from Mn eg states. The Mn 3d and O 2p

contributions show similarities in shape close to Ef revealing a high degree of covalency in

bonding. The O 2s states are much lower in energy around -25 eV (not shown in the Figure

2B) and thus do not participate in the chemical bonding. The overlapping of Mn 3d and

O 2p states give raise to distinct charge density iso-surfaces as shown in the top region of

Figure 2B. The partial charge density iso-surface in the energy range from -4 to -6 eV

depicts a mixed t2g and eg character while predominant t2g character in the energy intervals

between 0 to -2 eV and 0 to 2 eV.

In general, improving the bulk conductivity of the electrode improves electrocatalytic

performance, but this does not directly address any improvement in the performance of the

catalytic active site. Our X-ray diffraction results show that the Mn-O-Mn bond distances

become more equivalent in the cubic phase, and DFT simulations suggest this small change

9

Temperature (°C)

Exch

ange

Cur

rent

Den

sity

(×10

-9 A

/cm

2 )

15

30

20

10

050454035302520

50

40

ORRA

Temperature (°C)15

0.2

0 50454035302520

1.0

1.4

1.6

0.4

0.6

0.8

1.2

OERB

FIG. 3: Exchange current density for α-Mn2O3 electrodes at different temperatures obtained using

the Tafel plots of the (A) ORR and (B) OER regions. The arrows on each graph show the location

of the orthorhombic → cubic phase transition, indicating there is some lag in alkaline media.

in bonding arrangement opens up distinct electronic features close to the Ef that have

primarily t2g and some eg character. The PBE0 functional we used is known to overestimate

band gaps in DFT[31], so there may be more of these low-lying electronic states that provide

a lower energy barrier to enable the coexistence of Mn oxidation states that is critical for

catalytic activity.

To investigate how the temperature-dependent phase transition affects oxygen electro-

catalysis, we performed OER and ORR catalysis experiments in alkaline media at different

temperatures. The α-Mn2O3 prisms were heated in air at 480◦C on the GCE. The details

of the electrode preparation method are provided in the Supporting Information. Linear

sweep voltammetry (LSV) measurements were performed in O2-saturated 0.1 M KOH and

scanned using a rotating disc electrode (RDE) at 1600 rpm in a three-electrode electrochemi-

cal cell with temperature controlled via a recirculating water bath setup (EYELA NTT-20S;

±0.05C). Initially we compared the α-Mn2O3 prism electrodes that had been heat-treated

versus electrodes where the step had been omitted, performing LSVs at a temperature of

25◦C to replicate our earlier work performed at room temperature(Figure S11)[24]. Again,

the heat treated prisms exhibited superior performance for both OER/ORR at 25◦C, and

even better performance at 50◦C. Table S3 summarizes the performance of these electrodes

10

versus commercial α-Mn2O3 and commercial Pt/C. Next, the electrochemical properties of

the heat-treated α-Mn2O3 prisms were measured stepwise at temperatures from t = 15 to

50◦C (i.e. 288K to 323K). LSV curves and Tafel plots for the α-Mn2O3 prisms were col-

lected and plotted in (Figure S11). The OER/ORR Tafel slopes in addition to the ORR

potentials at -3 mA·cm-2 and OER potentials at 10 mA·cm-2 are summarized in (Table

S3). These results show that the α-Mn2O3 prisms have higher activity at higher tempera-

ture. Obviously, higher temperatures will typically increase the kinetics of bond cleavages

and rearrangements, thus a closer look for change around the orthorhombic → cubic phase

transition temperature is necessary.

The exchange current density (i0) reflects the intrinsic rate of electron transfer versus

area in addition to the number of active sites and their quality. Our XRD results show that

any change in the surface area of α-Mn2O3 with temperature is negligible. Thus if the same

electrode is used in the temperature-controlled electrochemical experiments over a modest

range, we reasoned that the surface area of the electrode should be relatively constant,

so i0 will provide information on the quality of the active site. Figure 3 shows i0 versus

temperature in the ORR (A) and OER (B) regions of the Tafel plots. In both ORR and OER

we observed a marked increase and inflection in i0 near the phase transition of α-Mn2O3,

indicating the cubic phase has better active sites. The inflection in i0 is (∼35◦C), slightly

higher that the phase transition temperature, but this could be due the high concentration of

O2 or alkalinity of the electrolyte that increases the orthorhombic → cubic phase transition

temperature.

In conclusion, we used phase pure α-Mn2O3 prisms to obtain the highest resolution XRD

patterns to date for various temperatures around the orthorhombic→ cubic phase transition.

This phase change causes a subtle rearrangement in the bonding of α-Mn2O3 and suppression

of Jahn-Teller distortions. These crystal structures were input directly into DFT, showing

that the orthorhombic → cubic phase transition caused the material to transform from

semiconducting to semi-metallic and opened up states around the Ef in addition to an

improvement in the covalency of the Mn-O-Mn bonds. α-Mn2O3 prisms were prepared as

electrodes and their OER and ORR performance was examined in a temperature-controlled

electrocatalytic setup. These results show that the activity of the catalyst improves at

high temperature. A closer examination of the i0 shows the electrode has lower activation

polarization losses and better performance above the phase transition temperature. The

11

combined structural, theoretical and electrochemical measurements here demonstrate how a

tiny change in crystal structure may change the electronic properties of materials and their

catalytic properties. Suppressing Jahn-Teller distortions in phase pure α-Mn2O3 improves

OER/ORR activity, and the origin of this phenomenon might be due to a combination of the

semimetallic behavior of cubic phase α-Mn2O3 and improvement in the active sites. Overall

it suggests that there is much more to learn about the electronic and catalytic properties of

binary and mixed metal oxides by exploring their phase behavior.

Acknowledgments

This work was supported by the World Premier International Research Center Initiative

on Materials Nanoarchitectonics (WPI-MANA) from MEXT, Japan, the Japan Society for

the Promotion of Science (JSPS) Bilateral Joint Research Program, and JSPS KAKENHI

(Grant Number 18K05192). The synchrotron radiation experiment at BL15XU of SPring-8

was performed with the approval of the Japan Synchrotron Radiation Research Institute

(JASRI) (2015B4503).

[1] Dresselhaus, M. S.; Thomas, I. L. Alternative energy technologies, Nature 414, 332-337 (2001).

[2] Gray, H. B. Powering the planet with solar fuel, Nat. Chem. 1, 7 (2009).

[3] Kanan, M. W.; Nocera, D. G. In situ formation of an oxygen-evolving catalyst in neutral

water containing phosphate and Co2+, Science 321, 1072-1075 (2008).

[4] Li, W.; Shao, J.; Liu, Q.; Liu, X.; Zhao, X.; Hu, J. Facile synthesis of porous Mn2O3 nanocubics

for high-rate supercapacitors, Electrochim. Acta 157, 108-114 (2015).

[5] Armand, M.; Tarascon, J.-M. Building better batteries. Nature 451, 652-657 (2008).

[6] Henzie, J.; Etacheri, V.; Jahan, M.; Rong, H.; Hong, C. N.; Pol, V. Biomineralization-inspired

crystallization of monodisperse α-Mn2O3 octahedra and assembly of high-capacity lithium-ion

battery anodes. J. Mater. Chem. A 5, 6079-6089 (2017).

[7] U.S. Geological Survey. USGC Fact Sheet 087-02. (2002). doi:10.3133/fs08702

[8] Stoerzinger, K. A.; Risch, M.; Han, B.; Shao-Horn, Y. Recent Insights into Manganese Oxides

in Catalyzing Oxygen Reduction Kinetics. ACS Catal. 5, 6021-6031 (2015).

12

[9] Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-horn, Y. A Perovskite

Oxide Optimized for Molecular Orbital Principles. Science 334, 1383-1385 (2011).

[10] Suntivich, J.; Gasteiger, H. A.; Yabuuchi, N.; Nakanishi, H.; Goodenough, J. B.; Shao-Horn,

Y. Design principles for oxygen-reduction activity on perovskite oxide catalysts for fuel cells

and metal-air batteries. Nat. Chem. 3, 546-550 (2011).

[11] Fabbri, E.; Habereder, A.; Waltar, K.; Ktz, R.; Schmidt, T. J. Developments and perspectives

of oxide-based catalysts for the oxygen evolution reaction. Catal. Sci. Technol. 4, 3800-3821

(2014).

[12] Maitra, U.; Naidu, B. S.; Govindaraj, A.; Rao, C. N. R. Importance of trivalency and the

eg1 configuration in the photocatalytic oxidation of water by Mn and Co oxides. Proc. Natl.

Acad. Sci. U. S. A. 110, 11704-11707 (2013).

[13] Robinson, D. M.; Go, Y.; Mui, M.; Gardner, G.; Zhang, Z.; Mastrogiovanni, D.; Garfunkel,

E.; Li, J.; Greenblatt, M.; Dismukes, G. C. Photochemical water oxidation by crystalline

polymorphs of manganese oxides: Structural requirements for catalysis. J. Am. Chem. Soc.

135, 3494-3501 (2013).

[14] Ramrez, A.; Hillebrand, P.; Stellmach, D.; May, M. M.; Bogdanoff, P.; Fiechter, S.; Evaluation

of MnOx, Mn2O3, and Mn3O4 Electrodeposited Films for the Oxygen Evolution Reaction of

Water. J. Phys. Chem. C 118, 14073-14081 (2014).

[15] Hirai, S.; Yagi, S.; Seno, A.; Fujioka, M.; Ohno, T.; Matsuda, T. Enhancement of the oxygen

evolution reaction in Mn3+ based electrocatalysts: correlation between Jahn-Teller distortion

and catalytic activity. RSC Adv. 6, 2019-2023 (2016).

[16] Gorlin, Y.; Jaramillo, T. F. A bifunctional nonprecious metal catalyst for oxygen reduction

and water oxidation. J. Am. Chem. Soc. 132, 13612-13614 (2010).

[17] Gorlin, Y.; Jaramillo, T. F. Investigation of Surface Oxidation Processes on Manganese Ox-

ide Electrocatalysts Using Electrochemical Methods and Ex Situ X-ray Photoelectron Spec-

troscopy. J. Electrochem. Soc. 159, H782-H786 (2012).

[18] Gorlin, Y.; Benedikt, L.-K.; Benck, J. D.; Gul, S.; Webb, S. M.; Yachandra, V. K.; Yano,

J.; Jaramillo, T. F. In situ X-ray absorption spectroscopy investigation of a bifunctional

manganese oxide catalyst with high activity for electrochemical water oxidation and oxygen

reduction. J. Am. Chem. Soc. 135, 8525-8534 (2013).

[19] Birkner, N.; Navrotsky, A. Rapidly reversible redox transformation in nanophase manganese

13

oxides at room temperature triggered by changes in hydration. Proc. Natl. Acad. Sci. 111,

6209-6214 (2014).

[20] Rugg, G.; Genest, A.; Rsch, N. DFT Variants for Mixed-Metal Oxides. Benchmarks Using

Multi-Center Cluster Models. J. Phys. Chem. A 122, 7042-7050 (2018).

[21] Hong, W. T.; Risch, M.; Stoerzinger, K. A.; Grimaud, A.; Suntivich, J.; Shao-Horn, Y.;

Toward the rational design of non-precious transition metal oxides for oxygen electrocatalysis.

Energy Environ. Sci. 8, 1404-1427 (2015).

[22] Geller, S. Structure of α-Mn2O3, (Mn0.983Fe0.017)2O3 and (Mn0.37Fe0.63)2O3 and relation to

magnetic ordering. Acta Crystallogr. Sect. B Struct. Crystallogr. Cryst. Chem. 27, 821-828

(1971).

[23] Grant, R. W.; Geller, S.; Cape, J. A.; Espinosa, G. P. Magnetic and Crystallographic Transi-

tions in the Mn2O3-Fe2O3 System. Phys. Rev. 175, 686-695 (1968).

[24] Jahan, M.; Tominaka, S.; Henzie, J. Phase pure α-Mn2O3 prisms and their bifunctional

electrocatalytic activity in oxygen evolution and reduction reactions. Dalt. Trans. 45, 18494-

18501 (2016).

[25] Tanaka, M.; Katsuya, Y.; Matsushita, Y.; Sakata, O. Development of a synchrotron powder

diffractometer with a one-dimensional X-ray detector for analysis of advanced materials. J.

Ceram. Soc. Japan 121, 287-290 (2013).

[26] Cockayne, E.; Levin, I.; Wu, H.; Llobet, A. Magnetic structure of bixbyite α-Mn2O3: A

combined DFT+U and neutron diffraction study. Phys. Rev. B 87, 1-11 (2013).

[27] Kresse, G. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave

basis set. Phys. Rev. B 54, 11169-11186 (1996).

[28] Kresse, G. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys.

Rev. B 59, 1758-1775 (1999).

[29] Hohenberg, P. Inhomogeneous Electron Gas. Phys. Rev. 136, B864-B871 (1964).

[30] Perdew, J. P.; Ernzerhof, M.; Burke, K. Rationale for mixing exact exchange with density

functional approximations. J. Chem. Phys. 105, 9982-9985 (1996).

[31] Garza, A. J.; Scuseria, G. E. Predicting Band Gaps with Hybrid Density Functionals. J. Phys.

Chem. Lett. 7, 4165-4170 (2016).

[32] Franchini, C.; Podloucky, R.; Paier, J.; Marsman, M.; Kresse, G. Ground-state properties of

multivalent manganese oxides: Density functional and hybrid density functional calculations.

14

Phys. Rev. B - Condens. Matter Mater. Phys. 75, 1-11 (2007).

[33] Hubbard, J. Electron Correlations in Narrow Energy Bands. Proc. R. Soc. A Math. Phys.

Eng. Sci. 276, 238-257 (1963).

[34] Tokura, Y. Correlated-electron physics in transition-metal oxides. Phys. Today 56, 50-55

(2003).

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