Facet-dependent catalytic activity of MnO electrocatalysts for oxygen reduction and oxygen evolution reactions

Article abstract of DOI:10.1039/c5cc01152c

This Communication highlights the facet-dependent electrocatalytic activity of MnO nanocrystals for OERs/ORRs. The MnO(100) facets with higher adsorption energy of O species can largely promote the electrocatalytic activity.

Full text of DOI:10.1039/c5cc01152c

View Article Online
View Journal
ChemComm
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: C. Kuo, I. Mosa,
S. Thanneeru, V. Sharma, L. Zhang, S. Biswas, M. Aindow, P. Alpay, J. F. Rusling, S. L. Suib and J. He,
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/chemcomm

View Article Online
DOI: 10.1039/C5CC01152C
ChemComm
Page 1 ofJ4ournal Name
►
ARTICLE TYPE
Facet-dependent catalytic activity of MnO electrocatalysts for oxygen
reduction and oxygen evolution reactions
Chung-Hao Kuo,a Islam M. Mosa,a Srinivas Thanneeru,a Vinit Sharma,b Lichun Zhang,b Sourav Biswas,a
Mark Aindow,b S. Pamir Alpay,b James F. Rusling,a Steven L. Suib,a,b,* Jie Hea,*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
that the catalytic activity of MnO nanocatalysts for both OER and
ORR was highly dependent on exposed crystal planes. Our study
highlights the correlation between OER/ORR activities and ex-
55 posed crystal planes of manganosite catalysts and it may provide
fundamental guidance for developing active and cost-effective
electrocatalysts.
This Communication highlights the facet-dependent electrocat-
alytic activity of MnO nanocrystals for OERs/ORRs. The MnO
10 (100) facets with higher adsorption energy of O species can
largely promote the electrocatalytic activity.
To meet the growing demands of renewable energy, various
electrochemical energy conversion processes between O2 and H2O,
e.g. oxygen evolution reaction (OER) and oxygen reduction reac-
15 tion (ORR) that are of great importance for solar water splitting
and fuel cells, have received much attention in the past decade.1 A
key technical barrier to achieve highly efficient OERs and ORRs is
the kinetically slow electron-transfer in which four electrons are
involved.2 The rational design of active electrocatalysts is central
20 to these energy conversion techniques. Traditionally, expensive Pt
and its bimetallic alloys have traditionally been the most active
catalysts for the ORR;3 and precious metal oxide IrO2 and RuO2
catalysts have proved to be very efficient for the OER.4 However,
the large-scale commercialization of these catalysts is limited by, i)
25 the cost effectiveness of the requisite raw materials, and ii) consid-
erable overpotential (η) required for both the OER and ORR
(0.3~0.8 V). The earth-abundant and inexpensive transition metal
oxides, e.g. manganese oxides (MnOx), have been explored as al-
ternative electrocatalysts for OERs and ORRs under alkaline con-
30 ditions.5 MnOx families have over 30 different crystal structures
and variable valence of Mn centers in different polymorphs.6 De-
spite numerous reports on the electrocatalytic performance of
MnOx for OERs and ORRs, insight into the effect of structural
parameters, e.g. topological structures, surface compositions and
35 energy, and valance of Mn centers on their catalytic activity, is still
lacking.5
In heterogeneous catalysts, control of complex anisotropic
structures with preferentially exposed crystal planes or surface
atoms is a key to achieving a high catalytic selectivity and activity.
40 Moreover, the surface interactions between O species and catalytic
centers towards the formation of O-O bonds in -OOH species on
catalytic centers, dominate the reaction activity of both the OER
and the ORR. To this end, for MnOx families, a fundamental un-
derstanding of correlations between the catalytic performance for
45 OER and ORR and surface properties is urgently needed. As a
prototype system, we herein choose to concentrate on a simple
halite MnO with a face-centered cubic (fcc) structure to evaluate
the effect of surface energy on catalytic activity for both OER and
ORR. 3-D complex MnO nanoflowers and polypods with selective-
50 ly oriented MnO crystals were prepared using a limited ligand
protection method. Electrocatalytic measurements demonstrated
60 Fig 1. (a,b) TEM images of MnO nanoflowers. The inset in (a) is a zoom-in
view of a single nanoflower. (c) The powder XRD pattern of MnO
nanoflowers. (d) Mn 2p XPS spectrum of MnO nanoflowers. (e-g) TEM
images of MnO nanocrystals obtained at different Mn(oleate)2/OA molar
ratios: (e) 1:0.09; (f) 1:0.27; (g) 1:0.9.
65
MnO nanoflowers and polypods were synthesized by using
thermal decomposition of Mn(oleate)2 in a non-coordinating, apo-
lar hydrocarbon solvent.7 Briefly, 1.24 g of Mn(oleate)2 was dis-
solved in 10 g of octadecene in a 25 mL three-neck round-bottom
70 flask. The mixture was first degassed at 80oC under vacuum for 30
o
o
min and then heated to 320 C at a rate of 10 C/min under argon.
After stirring for another 30 min, the reaction mixture was cooled
and then precipitated with an excess of acetone. The obtained na-
noparticles (NPs) were first characterized by transmission electron
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

View Article Online
DOI: 10.1039/C5CC01152C
ChemComm
Page 2 of 4
microscopy (TEM). As shown in the low-magnification TEM im-
age (Fig 1a and more images in Fig S1), the nanoflowers are com-
posed of a spherical core decorated by densely packed well-defined
MnO nanorods. The as-synthesized MnO nanoflowers are quasi-
spherical and have an average diameter of 126.5±17.6 nm (Fig
S2). These nanorods are nearly single crystalline MnO (see below)
and have a length of 40~50 nm and a diameter of ~15 nm. High-
resolution TEM analysis showed the manganosite crystalline struc-
ture of MnO with d-spacings of 0.22 nm and 0.32 nm that corre-
zone axis of the MnO fcc structure. Only (111) planes are exposed
for octahedral NPs. This is similar to previously reported FeO and
CuO octahedral NPs.8
5
10 spond to (200) and (110) planes (Fig 1b), in good agreement with
the powder X-ray diffraction (XRD) pattern in Fig 1c. The decon-
volution of X-ray photoelectron spectroscopy (XPS) data (Fig 1d)
shows the binding energy of Mn 2p1/2 (653.4 eV) and Mn 2p3/2
(641.6 eV) falls in the Mn 2p region.5a The co-existence of a satel-
15 lite peak at 646 eV suggests the presence of Mn2+ species on the
surface of the nanoflowers. X-ray absorption near-edge spectra also
confirmed the average Mn oxidation state of 2.02 (Fig S8). No
significant amount of Mn species with higher oxidation state was
detected for months.
70
Fig 2. (a,b) The high-resolution TEM image of a single rod of MnO poly-
pods (a) and the corresponded SAED pattern (b). (c) The schematic illustra-
tion indicating the growth direction and exposed planes of MnO nanorods
on MnO polypods. (d,e) The high-resolution TEM image of a single MnO
20
The oriented attachment of 1D nanorods to nanoflowers, e.g.
the number and length of 1D nanorods, appeared to be highly con-
trollable using oleic acid (OA) as a free ligand. These results are
summarized in Figs 1e-g (see more TEM images in Figs S3-5). At
a low concentration of OA (Mn(oleate)2/OA=1:0.09, mol/mol,
75 octahedral NP (d) and the corresponding SAED pattern (e). (f) The sche-
matic illustration indicating the growth direction of MnO octahedral NPs. O
is red and Mn is yellow.
In addition to pure MnO nanocrystals, other transition metal
80 cations, e.g. Co2+, can be doped into the complex nanostructure of
MnO by mixing metal oleate precursors. TEM images of Co-doped
MnO nanoflowers with Co/Mn molar ratios of 5% and 10% were
given in Fig S9. The 3-D topological architectures of nanoflowers
did not significantly change with the doping amount of Co cations.
85 With 5% of Co, the diameter of Mn-Co nanoflowers was 95.8±
14.2 nm (Figs S9c) and nanorods appeared to pack more densely
on the surface; while further increase of the Co/Mn molar ratio to
10% would lead to the increase of their average size to 186.2±
18.1 nm. Energy-dispersive X-ray elemental mapping confirmed
90 the uniform distribution of Mn and Co elements in nanoflowers
(Figs S9d~g). No change in the MnO crystalline structures was
observed from XRD after doping (Fig S12).
25 M/O ratio hereafter), the formation of MnO polypods was observed
(Fig 1e). These polypods essentially retained the 3-D complex
architectures of nanoflowers but had far fewer branches. Of them,
MnO tetrapods composed of four nanorod branches were the dom-
inant product. Compared to MnO nanoflowers, the length and di-
30 ameter of nanorods in MnO polypods did not change significantly.
With increase of M/O ratio to 1:0.27 (mol/mol), irregular polyhe-
dra with surface protuberances were obtained where MnO nano-
rods were significantly shortened (Fig 1f). The average size of
MnO polyhedra decreased to ~32 nm. By further increasing the
35 M/O ratio to 1:0.9 (mol/mol), the complex surface structures dis-
appeared and nearly mono-disperse MnO octahedral nanocrystals
were obtained with a diameter of ~19 nm (Fig 1g).
The growth of MnO 3-D complex nanocrystals was likely due
to the limited ligand supply.7a Without the presence of OA free
40 ligands, the initially formed MnO seeds were not covered by OA
ligands; thus, the growth of MnO nanocrystals was nucleated on
the exposed surface of MnO seeds. Limited ligand protection
would lead to the formation of nanoflowers with oriented nanorods.
In the presence of a low concentration of OA ligands (<20 mol%),
45 the OA would partially cover the surface of MnO seeds. The less
exposed surface resulted in the formation of MnO polypods with
much less branched nanostructures. With the addition of 1: 0.9
equivalence of OA ligands, the (111) planes of MnO seeds were
preferentially bonded with OA ligands thus leading to the for-
50 mation of octahedral MnO nanocrystals.
To gain more insight into the crystallographic structure and
growth mechanism of MnO nanocrystals, high resolution TEM
characterization with selected area electron diffraction (SAED)
studies were further performed. The results of surface planes of
55 MnO nanocrystals are intriguing. The TEM image of a single nano-
rod on MnO polypods (Fig 2a) shows the (200) and (110) planes
with an interfacial angle of 45o. The SAED pattern of nanorods can
be indexed to the manganosite fcc structure with the <100> zone
axis parallel to (001) and (010) planes (Fig 2b). MnO polypods
60 exposed (100), (001), and (010) planes on their branches (Fig 2c).
These results further support the growth mechanism as aforemen-
tioned. The free OA ligands have a favorable interaction with (111)
planes with a slightly larger surface energy, resulting in the aniso-
tropic growth of MnO nanocrystals. For MnO octahedral NPs, the
65 corresponding SAED pattern (Fig 2e) can be indexed as the <110>
95 Fig 3. (a) Linear sweep voltammetry (LSV) curves of MnO nanocatalysts
for electrochemical OERs at a scan rate of 5 mV s-1 in 0.1 M of KOH. C-
MnO is commercialized MnO. (b) The corresponded Tafel plots of MnO
nanocatalysts in (a). (c) The Nyquist plots obtained from the electrochemi-
cal impedance spectroscopy measurements at 1.76 V vs. RHE and a fre-
100 quency range of 0.1 to 105 Hz. (d) Linear sweep voltammetry (LSV) curves
for ORRs in 0.1 M of KOH solution.
MnO nanocrystals were further evaluated as bifunctional cata-
lysts for the OER and ORR using rotating-disk electrode tech-
105 niques. Fig 3a presents the typical linear sweep voltammograms

View Article Online
DOI: 10.1039/C5CC01152C
Page 3 of 4
ChemComm
(LSVs) of MnO nanocrystals for OERs in KOH solution (0.1 M)
exposed lattice facets of MnO nanocrystals. The MnO (100) planes
with higher adsorption energy of O species could largely promote
the electrocatalytic activity for the OER and the ORR. Our results
may illustrate a new paradigm for developing highly active and
with a sweep rate of 5 mV s-1. MnO polypods with exposed (100)
planes showed a superior activity with an overpotential (η) of 0.58
V at a current density (j) of 10 mA cm-2, much lower than that of
5
other MnO nanocrystals. At η=0.35 V, the mass activity for MnO 70 cost-effective electrocatalysts.
polypods catalyst was found to be 17.75 A g-1. The turnover fre-
quency (TOF) of MnO polypods reached 4.19×10-4 s-1 (see Table
Acknowledgements
S1), comparable with the reported best values of MnOx poly-
JH thanks the financial support of startup funds from the Universi-
ty of Connecticut. SLS acknowledges support of the US Depart-
75 ment of Energy, Office of Basic Energy Sciences, Division of
Chemical, Biological and Geological Sciences under grant DE-
FG02-86ER13622.A000. This work was partially supported by the
Green Emulsions Micelles and Surfactants (GEMS) Center.
morphs.5a,f,8a,9 MnO octahedra with (111) dominated surface exhib-
10 ited a much lower activity and current densities, nearly overlapped
with that of commercial MnO catalysts. MnO nanoflowers with
multi-exposed planes displayed a moderate catalytic activity with a
TOF of 1.32×10-4 s-1. Tafel plots derived from LSV curves are
given in Fig 3b. Tafel slopes of MnO polypods and nanoflowers
15 are 149 and 169 mV/dec, respectively. The lower Tafel slopes
indicate that the OER of polypods and nanoflowers is kinetically
more favorable compared to that of octahedra. Fig 3c shows the
Nyquist plot of the electrochemical impedance spectra (EIS) of
MnO nanocrystals. The charge-transfer resistance value is inverse-
20 ly proportional to the electron transfer rate. MnO polypods and
nanoflowers have Rct of 256 and 275 Ω, respectively, lower than
that of MnO octahedra and commercial MnO. This result is in good
agreement with their OER activities.
80 Notes and references
a
Department of Chemistry, b Department of Materials Science and Engi-
neering and Institute of Materials Science, University of Connecticut,
Storrs, CT 06269 USA.
E-mail: steven.suib@uconn.edu (SS); jie.he@uconn.edu (JH)
85
† Electronic Supplementary Information (ESI) available: Experimental
details, characterization, computational details, and additional results for
electrochemical studies. See DOI: 10.1039/b000000x/
ORR activity of MnO nanocrystals was also examined by cy-
25 clic voltammetry (CV) in 0.1 M of KOH solution. In argon-
saturated solution, the voltammogram without an obvious peak was
observed (Fig S14); while, a well-defined cathodic oxygen reduc-
tion peak with a high current density appeared when the electrolyte
was saturated with O2, indicating MnO nanocrystals are electrocat-
30 alytically active for the ORR. The ORR catalytic results of MnO
nanocrystals are given in Fig 3d. MnO polypods, again, displayed a
superior activity with a half-wave potential of 0.77 V, only 80 mV
lower than that of Pt/C catalysts.5f The diffusion-limiting current of
MnO polypods reaches ~5.0 mA cm-2, higher than MnO nanoflow-
35 ers with 3.8 mA cm-2 and octahedra with 3.1 mA cm-2, indicating
that the preferentially exposed (100) planes indeed improved the
ORR activity as well. As compared to other reported MnOx cata-
lysts for ORRs, e.g. MnOx film reported by Gorlin et al. with a
potential of 0.73 V5f and α-MnO2 reported by Meng et al. with a
40 potential of 0.76 V at J=-3 mA cm-2,5a MnO polypods have some
of the lowest overpotentials for the ORR. The stability of MnO
ploypods for the ORR was also studied by current-time (i-t) chron-
oamperometric response at a potential of 0.66 V (vs. RHE) (Figure
S17). MnO ploypods maintained 80% of its original activity after
45 an hour. Furthermore, the overall oxygen electrode activity
(ΔE=EOER-EORR) is as low as 1.02 V, smaller than that of Ir/C and
Pt/C,3,4,5f suggesting MnO polypods as a superior bifunctional elec-
trocatalyst for OER/ORR.
90 (1) (a) Voiry, D.; Yamaguchi, H.; Li, J.; Silva, R.; Alves, D. C. B.; Fujita,
T.; Chen, M.; Asefa, T.; Shenoy, V. B.; Eda, G.; Chhowalla, M. Nat. Mater.
2013, 12, 850. (b) Ji, X.; Lee, K. T.; Holden, R.; Zhang, L.; Zhang, J.;
Botton, G. A.; Couillard, M.; Nazar, L. F. Nat. Chem. 2010, 2, 286. (c)
Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Nat.
95 Mater. 2011, 10, 780.
(2) (a) Bing, Y.; Liu, H.; Zhang, L.; Ghosh, D.; Zhang, J. Chem. Soc. Rev.
2010, 39, 2184. (b) Zhang, H.; Jin, M.; Xia, Y. Chem. Soc. Rev. 2012, 41,
8035. (c) Guo, S.; Zhang, S.; Sun, S. Angew. Chem. Int. Ed. 2013, 52, 8526.
(d) Guo, S.; Zhang, X.; Zhu, W.; He, K.; Su, D.; Mendoza-Garcia, A.; Ho,
100 S. F.; Lu, G.; Sun, S. J. Am. Chem. Soc. 2014, 136, 15026.
(3) (a) Koenigsmann, C.; Santulli, A. C.; Gong, K.; Vukmirovic, M. B.;
Zhou, W.-p.; Sutter, E.; Wong, S. S.; Adzic, R. R. J. Am. Chem. Soc. 2011,
133, 9783. (b) Zhang, S.; Zhang, X.; Jiang, G.; Zhu, H.; Guo, S.; Su, D.;
Lu, G.; Sun, S. J. Am. Chem. Soc. 2014, 136, 7734. (c) Strasser, P.; Koh, S.;
105 Anniyev, T.; Greeley, J.; More, K.; Yu, C.; Liu, Z.; Kaya, S.; Nordlund, D.;
Ogasawara, H.; Toney, M. F.; Nilsson, A. Nat. Chem. 2010, 2, 454.
(4) Reier, T.; Oezaslan, M.; Strasser, P. ACS Catal. 2012, 2, 1765.
(5) (a) Meng, Y.; Song, W.; Huang, H.; Ren, Z.; Chen, S.-Y.; Suib, S. L. J.
Am. Chem. Soc. 2014, 136, 11452. (b) Indra, A.; Menezes, P. W.;
110 Zaharieva, I.; Baktash, E.; Pfrommer, J.; Schwarze, M.; Dau, H.; Driess, M.
Angew. Chem. Int. Ed. 2013, 52, 13206. (c) Najafpour, M. M.; Ehrenberg,
T.; Wiechen, M.; Kurz, P. Angew. Chem. Int. Ed. 2010, 49, 2233. (d)
Boppana, V. B. R.; Jiao, F. Chem. Commun. 2011, 47, 8973. (e) Iyer, A.;
Del-Pilar, J.; King’ondu, C. K.; Kissel, E.; Garces, H. F.; Huang, H.; El-
115 Sawy, A. M.; Dutta, P. K.; Suib, S. L. J. Phys. Chem. C 2012, 116, 6474.
(f) Gorlin, Y.; Jaramillo, T. F. J. Am. Chem. Soc. 2010, 132, 13612. (g)
Kuo, C. H.; Li, W.; Pahalagedara, L.; El‐Sawy, A. M.; Kriz, D.; Genz, N.;
Guild, C.; Ressler, T.; Suib, S. L.; He, J. Angew. Chem. Int. Ed. 2015, 54,
2345. (h) Huynh, M.; Bediako, D. K.; Nocera, D. G. J. Am. Chem. Soc.
120 2014, 136, 6002. (i) Jiao, F.; Frei, H. Chem. Commun. 2010, 46, 2920.
(6) Suib, S. L. Acc. Chem. Res. 2008, 41, 479.
For MnO (100) and (111) crystal planes, the (100) planes with
50 mixed Mn2+ and O2 ions have a lower surface energy; while, the
−
(111) planes are polar surfaces consisting of alternating layers of
Mn2+ and O2 ions generating an electrostatic dipole field perpen-
−
(7) (a) Narayanaswamy, A.; Xu, H.; Pradhan, N.; Peng, X. Angew. Chem.
Int. Ed. 2006, 45, 5361. (b) Ould-Ely, T.; Prieto-Centurion, D.; Kumar, A.;
Guo, W.; Knowles, W. V.; Asokan, S.; Wong, M. S.; Rusakova, I.; Lüttge,
125 A.; Whitmire, K. H. Chem. Mater. 2006, 18, 1821. (c) Zitoun, D.; Pinna,
N.; Frolet, N.; Belin, C. J. Am. Chem. Soc. 2005, 127, 15034.
(8) (a) Kim, D.; Lee, N.; Park, M.; Kim, B. H.; An, K.; Hyeon, T. J. Am.
Chem. Soc. 2008, 131, 454. (b) Cheon, J.; Kang, N.-J.; Lee, S.-M.; Lee, J.-
H.; Yoon, J.-H.; Oh, S. J. J. Am. Chem. Soc. 2004, 126, 1950. (c) Liang, X.;
130 Gao, L.; Yang, S.; Sun, J. Adv. Mater. 2009, 21, 2068.
(9) Zaharieva, I.; Chernev, P.; Risch, M.; Klingan, K.; Kohlhoff, M.;
Fischer, A.; Dau, H. Energy Environ. Sci. 2012, 5, 7081.
(10)(a) Lima, F. H. B.; Calegaro, M. L.; Ticianelli, E. A. Electrochim. Acta
2007, 52, 3732. (b) Mao, L.; Zhang, D.; Sotomura, T.; Nakatsu, K.;
135 Koshiba, N.; Ohsaka, T. Electrochim. Acta 2003, 48, 1015.
dicular to the surface. To gain further insight into the correlation
between OER/ORR activities and the exposed crystal planes of the
55 MnO nanocrystals, adsorption energies of O species, e.g. OH- and
O2 have been estimated for different MnO crystal planes (Table S2)
using density functional theory. The adsorption of O species is
known as a rate-determining step for ORRs/OERs.10 In comparison
with (111) planes, the MnO (100) planes are highly exothermic for
60 OH- and O2 adsorption. The large exothermic interaction may also
be understood in terms of the unsaturated coordination on MnO
(100) planes.
In summary, 3-D complex anisotropic MnO nanocrystals were
demonstrated as superior bifunctional electrocatalysts. Their elec-
65 trocatalytic activity for OERs/ORRs was strongly correlated with

View Article Online
DOI: 10.1039/C5CC01152C
ChemComm
Page 4 of 4
►
Journal Name
ARTICLE TYPE
Table of Content
The MnO (100) facets with higher adsorption energy of O species can largely promote the electrocatalytic activity.
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 4