A Metal-Organic Framework-Derived Porous Cobalt Manganese Oxide Bifunctional Electrocatalyst for Hybrid Na-Air/Seawater Batteries

Abstract

Spinel-structured transition metal oxides are promising non-precious metal electrocatalysts for oxygen electrocatalysis in rechargeable metal-air batteries. We applied porous cobalt manganese oxide (CMO) nanocubes as the cathode electrocatalyst in rechargeable seawater batteries, which are a hybrid-type Na-air battery with an open-structured cathode and a seawater catholyte. The porous CMO nanocubes were synthesized by the pyrolysis of a Prussian blue analogue, Mn3[Co(CN)6]2·nH2O, during air-annealing, which generated numerous pores between the final spinel-type CMO nanoparticles. The porous CMO electrocatalyst improved the redox reactions, such as the oxygen evolution/reduction reactions, at the cathode in the seawater batteries. The battery that used CMO displayed a relatively small voltage gap of ~0.53 V compared to the batteries employing commercial Pt/C (~0.64 V) and Ir/C (~0.73 V) nanoparticles and without any catalyst (~1.04 V) at the initial cycle. This improved performance was due to the large surface area (catalytically active sites) and the high oxidation states of the randomly distributed Co and Mn cations in the CMO. Using a hard carbon anode, the Na-metal-free seawater battery exhibited a good cycle performance with an average discharge voltage of ~2.7 V and a discharge capacity of ~190 mAh g⁻¹hard carbon during 100 cycles (energy efficiencies of 74–79%).

Full text

A Metal−Organic Framework Derived Porous Cobalt Manganese
Oxide Bifunctional Electrocatalyst for Hybrid Na−Air/Seawater
Batteries
Mari Abirami,
†
Soo Min Hwang,*
,†
Juchan Yang,
†
Sirugaloor Thangavel Senthilkumar,
†
Junsoo Kim,
†
Woo-Seok Go,
†
Baskar Senthilkumar,
†
Hyun-Kon Song,
†
and Youngsik Kim*
,†,‡
†
School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulsan
44919, Republic of Korea
‡
Energy Materials and Devices Lab, 4TOONE Corporation, UNIST-gil 50, Ulsan 44919, Republic of Korea
*
SSupporting Information
ABSTRACT: Spinel-structured transition metal oxides are promis-
ing non-precious-metal electrocatalysts for oxygen electrocatalysis
in rechargeable metal−air batteries. We applied porous cobalt
manganese oxide (CMO) nanocubes as the cathode electrocatalyst
in rechargeable seawater batteries, which are a hybrid-type Na−air
battery with an open-structured cathode and a seawater catholyte.
The porous CMO nanocubes were synthesized by the pyrolysis of a
Prussian blue analogue, Mn3[Co(CN)6]2·nH2O, during air-anneal-
ing, which generated numerous pores between the final spinel-type
CMO nanoparticles. The porous CMO electrocatalyst improved
the redox reactions, such as the oxygen evolution/reduction
reactions, at the cathode in the seawater batteries. The battery
that used CMO displayed a voltage gap of ∼0.53 V, relatively small
compared to that of the batteries employing commercial Pt/C
(∼0.64 V) and Ir/C (∼0.73 V) nanoparticles and without any catalyst (∼1.05 V) at the initial cycle. This improved performance
was due to the large surface area (catalytically active sites) and the high oxidation states of the randomly distributed Co and Mn
cations in the CMO. Using a hard carbon anode, the Na-metal-free seawater battery exhibited a good cycle performance with an
average discharge voltage of ∼2.7 V and a discharge capacity of ∼190 mAh g−1hard carbon during 100 cycles (energy efficiencies of
74−79%).
KEYWORDS: cobalt manganese oxide, electrocatalyst, metal organic framework, Na−air, Prussian blue analogue, seawater battery
■INTRODUCTION
Reduction of fossil fuel consumption and deployment of
renewable energy sources for climate change mitigation has
attracted worldwide attention, leading to the development of
various types of electrochemical energy storage (EES)
technologies.
1−3
Among the EES devices, Li-ion batteries
(LIBs) currently outperform others because of their high
energy density (250−300 Wh kg−1on a cell-level), long service
lifetimes (2−10 years), and reasonable energy cost ($250−400
per kWh).
3,4
Nevertheless, LIBs are still required to meet the
demand for an even higher energy density that enables long-
term use of portable electronic devices at a high-power
operation and sufficiently long driving distance for electric
vehicles.
3−5
However, existing LIBs based on intercalation
reactions are expected to provide limited energy densities, and
the steady increase in the price of lithium presents long-term
concerns. These prospects have led to a quest for new redox
chemistries for charge-carrier ion storage with a high energy
density in a safe and inexpensive way, the so-called “post-
LIBs.”
3−5
As one of the promising post-LIBs, metal−air (O2) batteries
have been investigated extensively because their theoretical
energy densities are several times higher than those of the
present LIBs.
5−7
Most of this research has focused on aprotic
(nonaqueous) metal−air batteries in which aprotic electrolytes
comprising organic solvents, such as ether-based solvents, with
metal−salts are used as the electrolyte and metal oxides (MOx)
are produced/decomposed as the discharge product at the air-
cathode side during cycling.
6−9
The battery performance is
basically dependent on the reversible formation/decomposition
of the solid-phase discharge product; however, it can clog the
air-electrode due to its insulated and insoluble characteristics,
consequently hindering the long-term operation.
7,8
Further-
more, the battery system requires an additional gas tank that
allows a pure O2supply and prevents the ingress of impurities,
such as H2O and CO2, into the aprotic electrolytes from
Received: August 11, 2016
Accepted: November 14, 2016
Published: November 14, 2016
Research Article
www.acsami.org
© XXXX American Chemical Society ADOI: 10.1021/acsami.6b10082
ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

ambient air, and thereby prevents corrosion of the metal
anode.
6,7,10
To circumvent these issues, our group and Hayashi’s group
have reported hybrid-type (which we simply refer to as
“aqueous”)Na−air batteries in which a Na anode in an organic
electrolyte and an air-cathode in an alkaline electrolyte,
NaOH(aq), are integrated by inserting a Na superionic
conductor (NASICON) ceramic electrolyte between
them.
11,12
The use of a NASICON separator eliminates the
need for a gas tank for pure O2. In this system, the aqueous
electrolyte (catholyte) takes part in the redox reactions at the
cathode as 4NaOH(aq) ↔4Na++O
2(g)+2H
2O(l) + 4e−,
making the charge/discharge reactions reversible because of the
formation of the water-soluble NaOH discharge product.
However, the continued consumption of the aqueous electro-
lyte (H2O) during cell operation, which limits the specific
capacity and cycle performance, remains unsolved.
6
We recently published several reports on a “seawater
battery,”a novel type of aqueous Na−air battery using naturally
abundant seawater, an aqueous electrolyte containing ∼0.47 M
Na+ions, as the catholyte, which is constantly fed through a
flow-type configuration.
13−15
The seawater battery operation
involves the evolution/reduction reactions of gaseous O2and
(marginally) Cl2at the cathode and simultaneous redox
reactions of Na+ions at the anode upon charging and
discharging.
13,14
Considering the Na+ion concentration in
seawater (∼0.47 M), the O2partial pressure in 100% air-
saturated seawater (∼0.20 atm), and the pH of seawater (∼8),
the overall reactions during charge/discharge processes and the
theoretical cell voltage (Ecell) can be expressed as follows:
++
=
XYoooooooo
E
4Na(s) 2H O(l) O (g) 4NaOH(aq)
3.48 V
22
charge
discharge
cell (1)
+
=
XYoooooooo
E
Cl (g) 2Na(s) 2NaCl(aq)
4.07 V
2charge
discharge
ocell (2)
The charge/discharge processes would involve reaction 2,
depending on the kinetics of reaction 1 at the cathode. Reaction
2may be followed by chemical reactions of Cl2(g) + H2O→
HOCl + H++Cl
−and HOCl →H++ OCl−, which are highly
dependent on processing conditions, such as applied current,
pH, and local concentration of Cl−near the air-cathode. The
charge and discharge processes ordinarily accompany large
Figure 1. TEM images (a−f) of as-prepared Mn3[Co(CN)6]2nanocubes (a−c) and porous CMO nanocubes (d−f), XRD pattern (g), and N2
adsorption/desorption isotherm (h) of porous CMO. The dotted lines in (a) and (d) highlight the sharp edges of the cube-shape, and the phase of
the region marked by a square in (f) was identified using the corresponding fast Fourier transform (FFT) pattern (refer to Supporting Information).
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.6b10082
ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
B

overpotentials because of the sluggish kinetics of the oxygen
evolution/reduction reactions (OER/ORR in reaction 2). This
causes large voltage gaps between the charge and discharge
graphs (low voltage efficiency), leading to poor cycling stability
and low energy densities.
11,12,16
A typical way to resolve the
issue is to employ electrocatalysts promoting the OER and/or
ORR, such as noble metals (Pt, Pd, Ir, Ru, and Au), in the air-
cathode; however, their high costs and scarcity impede their
widespread use in large-scale applications.
11,16−18
Therefore,
most studies of electrocatalysts have focused on developing
nonprecious metal oxides or heteroatom(s)-incorporated
carbonaceous materials.
16,19−24
In particular, spinel-type
mixed transition metal oxides based on Ni, Co, and/or Mn
have been recognized as promising bifunctional electrocatalysts
toward both the OER and ORR.
22−25
In this work, we chose spinel-type cobalt manganese oxide
(CoxMn3−xO4, CMO) as a noble-metal-free electrocatalyst in
the air-cathode of seawater batteries. We prepared porous
CMO nanoparticles using a well-known Prussian blue analogue
(PBA), Mn3[Co(CN)6]2·nH2O, as the precursor and a self-
sacrificing template for the final porous morphology because
the large surface area (i.e., large number of active sites) of the
electrocatalyst is essential for ensuring high catalytic activity in
the air-cathode. Mn3[Co(CN)6]2is a metal−organic framework
consisting of Mn2+ and Co3+ ions bridged by cyanide ligands
(−CN),
26−28
which can be easily synthesized at room
temperature. We are not aware of any reports on porous
spinel oxide nanostructures as the cathode catalyst for aqueous
Na−air batteries.
28
The porous CMO nanoparticles showed a
good bifunctional electrocatalytic activity toward the OER/
OER, resulting in a stable cycling performance with a high
round-trip efficiency (∼85% at 0.01 mA cm−2). Moreover, by
employing a hard carbon electrode as the anode and the CMO
catalyst, we demonstrated an excellent cycling performance of a
Na-metal-free seawater battery with an average discharge
voltage (∼2.7 V), high Coulombic efficiencies (>96%), and
high energy efficiencies (74−79%) over 100 cycles.
■RESULTS AND DISCUSSION
Phase and Structural Characteristics. Porous CMO
nanoparticles with a hollow structure were derived from a
Mn3[Co(CN)6]2·nH2O precursor by thermal treatment in air.
Mn3[Co(CN)6]2·nH2O was prepared based on a previous
report,
27
and the morphology and microstructure of the
Mn3[Co(CN)6]2precursor were characterized by X-ray
diffraction (XRD) and electron microscopy. As shown in the
transmission electron microscopy (TEM) images (Figure 1a,b),
the as-synthesized Mn3[Co(CN)6]2·nH2O particles had a cubic
morphology with sharp edges and a smooth surface but a rather
large particle size distribution of 100−750 nm (Figure S1a),
which was probably due to insufficient amounts of poly-
vinylpyrrolidone (PVP) acting as the capping agent for a
uniform morphology and size.
27,29
The XRD pattern (Figure
S1b) exhibited a typical cubic Mn3[Co(CN)6]2·nH2O phase
(Powder Diffraction File (PDF) No. 51−1898, Joint
Committee on Powder Diffraction Standards (JCPDS),
[year]) and high crystallinity; however, we failed to gain static
TEM-selected area electron diffraction (SAED) patterns of the
Mn3[Co(CN)6]2·nH2O precursor because of its instability
under the electron beam irradiation, which resulted in
interparticle necking (particle growth). Likewise, we could
not find any noticeable trace of long-range order (lattice
fringes) in the high-resolution TEM images (Figure 1c).
We performed thermogravimetric analysis (TGA) on
Mn3[Co(CN)6]2·nH2O to probe the thermal decomposition
behavior of the Mn3[Co(CN)6]2·nH2O precursor nanocubes in
ambient air. As illustrated in Figure S1c, the TGA curve
displayed two distinct steps accompanying the large weight
losses. The former (30−140 °C) can be attributed to
dehydration of the precursor particles, and the latter (310−
330 °C) corresponded to pyrolysis of the cyanide ligands.
29
Based on the TGA data, we calcined the precursor nanocubes
at 430 °C for 2 h in ambient air to form the final CMO
nanoparticles (Figure S1d). Figure 1d−f displays the TEM
images of the CMO nanoparticles. The CMO nanoparticles
were shaped as rather distorted cubes (inherited from the
precursor), and the individual nanocubes featured a hierarchical
porous structure with hollow interiors and numerous
subconstituent nanoparticles (8−23 nm). The SAED pattern
of the CMO nanocubes (Figure S2a) showed polycrystalline
characteristics with Debye rings, which were assigned to the
(111), (311), (400), (511), and (404) planes of tetragonal
spinel (Co,Mn)(CoMn)2O4(I41/amdS). In addition, the fast
Fourier transformed (FFT) pattern of the lattice fringe region
(marked by the square in Figure 1f) confirmed the presence of
the spinel CMO phase (refer to Figure S2b).
The CMO nanocubes exhibited XRD reflections of the
tetragonal spinel (Co,Mn)(CoMn)2O4(PDF No. 18−0408,
JCPDS, [year]), as shown in Figure 1g. The (Co,Mn)-
(CoMn)2O4phase had random distributions of Co and Mn
cations in the tetrahedral and octahedral sites because of their
similar chemical states and ionic radii and hence miscibility with
each other.
28
This phase is an intermediate phase between the
normal spinel structure (AB2O4) and inverse spinel structure
(B[AB]O4), where A and B are metals occupying some or all of
the octahedral and tetrahedral sites in the lattice. The relative
molar ratio of Co/Mn was estimated to be 1:1.4 from the
inductively coupled plasma optical emission spectrometry
(ICP-OES) of the CMO nanocubes, which was close to the
nominal ratio in the Mn3[Co(CN)6]2precursor.
We examined the porosity and specific surface area of the
CMO nanocubes and Mn3[Co(CN)6]2·nH2O precursor using
N2sorption isotherms. Figure 1h shows the N2adsorption−
desorption isotherm of the CMO nanoparticles and the pore-
size distribution (inset) calculated by the Barrett−Joyner−
Halenda (BJH) method. The CMO nanocubes showed a type
IV isotherm with a broad pore size distribution (average pore
size, Dpore of ∼16.8 nm). The broad pore size distribution was
due to the presence of pores between the subconstituent
nanoparticles (mesopores) and the internal void within the
individual CMO nanocubes (macropores). The Brunauer−
Emmett−Teller (BET) specific surface area (SBET) and total
pore volume (Vpore) were evaluated as ∼64.5 m2/g and ∼0.27
cm3/g, respectively. In contrast, the Mn3[Co(CN)6]2·nH2O
precursor showed type I isotherm behavior (microporosity)
with Dpore of ∼1.7 nm; the SBET and Vpore values were ∼817 m2/
g and ∼0.34 cm3/g, respectively (Figure S3). Based on the
above results, we considered that the hollow, porous structure
of the CMO nanocubes formed by the pyrolysis of the
microporous Mn3[Co(CN)6]2·nH2O precursor and oxidation
of Co and Mn accompanying the Kirkendall effect (inducing
the hollow structure) during air annealing.
29
In particular, the
out-diffusion of internal gaseous byproducts generated during
pyrolysis of the cyanides (including possibly remnant PVP)
could leave a number of mesopores within the final CMO
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.6b10082
ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
C

nanocubes (Mn3[Co(CN)6]2·nH2O+O
2⎯→⎯⎯⎯⎯⎯⎯⎯⎯
ΔH(in air)
Co1.3Mn1.7O4±δ+CO
x(g)+NO
y(g) + zH2O(g)). Although
the significantly reduced SBET compared to that of the
precursor, the porous structure featuring mesopores and
macropores of the CMO would provide abundant active sites
as the electrocatalyst for the OER/ORR.
24,25
Surface Chemistry. The surface chemical state and
composition of the CMO nanocubes were investigated by X-
ray photoelectron spectroscopy (XPS). The recorded spectra
were deconvoluted by a Gaussian−Lorentzian fitting.
30−33
Figure 2a−c displays the narrow-scan Co 2p, Mn 2p, and O 1s
spectra. The Co 2p spectrum (Figure 2a) showed two major
peaks at ∼779.7 eV (2p3/2) and ∼794.9 eV (2p1/2) with a spin−
orbit splitting (ΔE)of∼15.2 eV, with other minor satellite
peaks. The deconvoluted Co 2p3/2 peak revealed the
coexistence of Co2+ (781.0 eV) and Co3+ (779.5 eV) oxidation
states, which were also corroborated by the presence of the
satellites marked by asterisks.
32
In the Mn 2p spectrum (Figure
2b), two main peaks were found at ∼641.5 eV (2p3/2) and
∼652.9 eV (2p1/2), with a ΔEof ∼11.4 eV. The Mn 2p3/2 peak
was deconvoluted into three components: Mn2+ (640.8 eV),
Mn3+ (642.2 eV), and Mn4+ (644.3 eV) states.
30
The existence
of Mn4+ could be the result of the evolution of Co2+−Mn4+
pairs by internal redox reactions of the Co3+−Mn3+ pairs in the
octahedral sites of the CMO spinel.
31
The deconvoluted O 1s
spectrum (Figure 2c) had three minor peaks: OI(529.5 eV),
OII (531.6 eV), and OIII (533.1 eV), owing to the lattice oxygen
species, chemisorbed oxygen species (such as hydroxyl groups),
and adsorbed moisture, respectively.
The electronic structure of the metal ions of transition metal
oxide electrocatalysts exerts a significant effect on their oxygen
adsorption ability on the surface and hence oxygen electro-
catalysis (OER/ORR).
31,34−36
The multivalent 3d metals in the
metal oxides would contribute positively to the electrocatalytic
performance, owing to the enhanced electron transport (by
hopping) and redox reaction-mediated charge transfer.
35
In
particular, transition metals with a single electron in the eg
orbital (such as t3e1for Mn3+ and t5e1for Co3+) at the
octahedral sites would catalytically provide more active sites for
oxygen electrocatalysis.
34−36
In this work, the surface of the
tetragonal spinel CMO nanocubes had average oxidation states
of +2.46 for Co and +2.69 for Mn, calculated using the
deconvoluted XPS data. The randomly distributed config-
urations of the Co and Mn cations in the tetrahedral and
octahedral sites, which arose from the (Co,Mn)(CoMn)2O4
phase, should be responsible for such high oxidation states of
both the Co and Mn ions and play a positive role in ensuring
the bifunctional catalytic property by balancing the OER and
ORR activities.
30
Interestingly, the molar ratio of Co/Mn near
the surface was estimated to be 1:2.1 from the XPS qualitative
measurements, which was distinct from that of the bulk
composition (1:1.4) measured by ICP-OES. The high
concentration of Mn (i.e., the segregation of Mn cations near
the surface of the CMO nanoparticles) can likely be attributed
to the higher (outward) diffusion rate of Mn cations than Co
cations during the phase formation by the heat treatment,
forming the Mn-rich surface structure.
37,38
Electrocatalytic Activity toward the ORR and OER. As a
potential oxygen catalyst in the air-cathode of seawater
batteries, the electrocatalytic activity toward the ORR and
OER of the porous CMO nanocubes was evaluated by rotating
ring-disk electrode (RRDE) measurements. The CMO sample
was prepared by mixing conductive carbon nanoparticles (20 wt
%) and coating them onto a disk electrode. The catalytic
activity for the ORR and OER was compared to that of
Figure 2. XPS spectra of Co 2p (a), Mn 2p (b), and O 1s (c), and a
schematic illustration showing the morphological evolution of porous
CMO nanocubes by air-annealing (d). The orange and violet asterisks
in (a) indicate the corresponding satellites of Co3+ and Co2+,
respectively.
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.6b10082
ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
D

commercial Pt/C (50 wt %) and Ir/C (20 wt %) nanoparticles,
respectively. Figure 3a shows the cathodic polarization graphs
for the ORR measured in an O2-saturated 0.1 M NaOH
solution at a rotation speed of 1600 rpm for the CMO and Pt/
C samples. Although the CMO sample showed a relatively low
onset potential (Eonset ≈0.86 V vs RHE) and a low half-wave
potential (E1/2 ≈0.74 V vs RHE) compared to those of the 50
wt % Pt/C (Eonset ≈0.98 V; E1/2 ≈0.86 V vs RHE), similar
diffusion-limiting currents (∼6.3 mA cm−2) were found at 0.3 V
vs RHE. On the basis of the currents measured from the ring
and disk electrodes, the number of electrons (n) transferred
during the ORR for the CMO was calculated to be ∼3.9, which
is comparable to that of the Pt/C (∼4.0) and suggests that the
ORR on the CMO catalyst follows a four-electron transfer
pathway.
35,39
In the case of the ORR in seawater (Figure 3b),
both the Pt/C and CMO samples displayed rather poor
catalytic behaviors, compared to those in the NaOH(aq): the
Eonset was ∼0.80 and ∼0.67 V vs RHE for the Pt/C and CMO,
respectively. Nevertheless, both the Pt/C and CMO exhibited
the ORR activity via a four-electron transfer pathway (the inset
in Figure 3b).
The OER activity of the CMO catalyst was examined from
the anodic polarization curves, which were measured in N2-
saturated 0.1 M NaOH and seawater at 1600 rpm (Figure 3c).
Although the OER started at a slightly higher potential, the
CMO sample showed almost similar potentials to those of the
Ir/C at 10 mA cm−2(1.69 and 1.65 V vs RHE in NaOH; 1.86
and 1.85 V vs RHE in seawater). As compared with other
cobalt−manganese oxide-based catalysts in previous reports
(refer to Table S1),
23,30,31,35,39−42
the porous CMO nanocubes
showed moderately good bifunctional catalytic performance
toward the ORR and OER. We ascribed this result to the large
surface area, resulting from the porous structure, and to the
unique surface structure with high oxidation states for both the
Co and Mn cations (possibly due to their random distributions
throughout the tetrahedral and octahedral sites in the
tetragonal (Co,Mn)(CoMn)2O4phase). The inferior catalytic
activities in seawater, compared to that of alkaline, could be
attributed to competitive reactions on the catalyst surface by
other dissolved ions, such as Cl−,SO
42−, and Mg2+, as well as to
the neutral pH at which H+or OH−ions are deficient,
43,44
which requires in-depth studies for enhancing the catalytic
activity toward the OER and ORR in seawater.
Electrochemical Properties of the Seawater Battery.
The porous CMO nanocubes were employed as the electro-
catalyst in the air-cathode for seawater batteries, and the
electrochemical properties of the batteries were investigated by
galvanostatic charge−discharge processes. The seawater bat-
Figure 3. Cathodic polarization curves for the ORR measured in O2-saturated 0.1 M NaOH (a) and seawater (b) of the CMO catalyst compared to
50 wt % Pt/C, and anodic polarization curves for the OER measured in 0.1 M NaOH (solid) and seawater (dash) of the CMO catalyst compared to
20 wt % Ir/C (c). All polarization data were recorded at a rotating speed of 1600 rpm and a scan rate of 10 mV s−1. The insets in (a) and (b) depict
the calculated electron transfer numbers (n) against the electrode potential (E).
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.6b10082
ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
E

teries were assembled using different air-electrodes, such as Pt/
C-, Ir/C-, and CMO-coated carbon papers or bare carbon
paper. The assembly procedure is detailed in the Experimental
Section. First, we compared the initial charge−discharge voltage
profiles of the cells containing different electrocatalysts at a
current density of 0.01 mA cm−2for 20 h at each step (Figure
4a). Although all the cells showed charge and discharge curves
that departed from the theoretical voltage value (∼3.48 V), the
cell using the CMO-loaded carbon paper exhibited the best
electrochemical behavior compared that of to the others. The
cell without any electrocatalyst (carbon paper sample) showed
charge and discharge voltages of 3.85 and 2.80 V, respectively,
resulting in the largest voltage gap (ΔV) between the charge
and discharge voltage (∼1.05 V). In contrast, the cell using the
CMO electrocatalyst displayed a slightly lower discharge
voltage (3.13 V) than that of the cell using the Pt/C (3.15
V), but a considerably reduced charge voltage (3.71 V) relative
to that of the carbon paper sample, leading to the narrowest ΔV
of ∼0.58 V; the ΔVvalues for the Pt/C and Ir/C cells were
∼0.64 and ∼0.73 V, respectively. During the charge process, the
Na+ions were transported from the seawater into the anode
compartment through the NASICON separator and reduced to
metallic Na. The OER simultaneously occurs at the air-cathode.
On discharging, the Na metal at the anode side is oxidized and
moves into the seawater together with the concurrent ORR at
the cathode side. The cathode reactions largely induce large
values for ΔVbecause of the sluggish kinetics of the OER/
ORR.
13,24,35
The comparison of the CMO cell with the Pt/C
and Ir/C cells reveals excellent bifunctional electrocatalytic
activity of the porous CMO nanocubes toward both the OER
and ORR. We also examined the charge−discharge voltage
profiles of the CMO cell at different current rates of 0.01−0.1
mA cm−2(Figure S4). Increasing the current rates increased the
ΔVvalue between the charge and discharge curves: the cell
showed ΔVvalues of ∼0.96 and ∼1.16 V at current rates of
0.025 and 0.1 mA cm−2, respectively.
Figure 4b presents the cycle performance of the CMO cell at
a current density of 0.01 mA cm−2for 10 h at each step. The
cell cycled without any notable degradation (ΔV= 0.53−0.58
V) over 30 cycles. The changes in the terminal charge and
discharge voltage values and the voltage efficiency of the CMO
cell at each cycle are plotted in Figure 4c. The average charge
and discharge voltages were ∼3.58 and ∼3.05 V during 30
cycles, respectively, yielding a high voltage efficiency of ∼85%.
We attribute the cycling stability mainly to the good
electrocatalytic activity of the porous CMO nanocubes.
We used a hard carbon anode to fabricate Na-metal-free
seawater batteries to ensure the safety and long-term cyclability.
First, we evaluated the electrochemical properties of a hard
carbon electrode using a 2032 coin-type half-cell; the
galvanostatic charge−discharge curves during 20 cycles are
shown in Figure S5. The large irreversibility at the initial cycle is
attributed to the formation of a solid electrolyte interface (SEI)
layer on the hard carbon surface during the discharge process.
45
The hard carbon anode delivered a reversible capacity of ∼295
mAh g−1during 20 cycles. By applying a hard carbon electrode
as the anode, we fabricated the full-cells (hard carbon|seawater)
with and without the CMO catalyst. The full-cells were cycled
with a capacity-limited charging (200 mAh g−1hard carbon) and a
voltage cutoffdischarging (0.5 V) at a current rate of 0.01 mA
cm−2.Figure 5a shows the fifth charge−discharge voltage
profiles of the two full-cells. The full-cell employing the CMO
catalyst exhibited a significantly reduced ΔVbetween the
charge and discharge curves compared to that of the
counterpart containing only carbon paper. The CMO cell
had average charge and discharge voltages of ∼3.42 and ∼2.69
V, respectively, which was due to the high catalytic activity of
the CMO nanocubes at the cathode during the charge and
Figure 4. Initial charge−discharge voltage profiles of a seawater battery
(Na|seawater) with different electrocatalysts at a current density of
0.01 mA cm−2(a), charge−discharge curves (cycle performance) of a
seawater battery with the CMO catalyst during 30 cycles (b), and the
terminal voltages and voltage efficiency during cycling (c).
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.6b10082
ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
F

discharge processes. With the increasing cycle number, the
discharge capacity of the CMO cell steadily increased and was
saturated at ∼195 mAh g−1hard carbon after 10 cycles (Figure
5b,c). During 100 cycles, the full-cell showed a stable cycling
performance with high Coulombic efficiencies (>96%) and high
energy efficiencies (74−79%). These results are attributed to
the reversible sodiation/desodiation reactions of the hard
carbon anode from the seawater and the high electrocatalytic
performance of the porous CMO nanocubes toward the OER/
ORR during cycling. To investigate the structural stability of
the CMO electrocatalyst, we disassembled the CMO full-cell
after 100 cycles, and the CMO electrocatalyst was analyzed by
XRD and XPS. No prominent changes were found for the
phase and surface electronic states, except that the relative ratio
of the Co3+/Co2+ and Mn2+/Mn3+(Mn4+) components in the
XPS spectra were slightly decreased (Figure S6),
39
which
reveals the structural durability of seawater batteries during
cycling.
■CONCLUSIONS
We fabricated porous CMO nanocubes (using a PBA of
Mn3[Co(CN)6]2·nH2O as the precursor) and employed them
as the cathode catalyst in seawater batteries. By annealing in
ambient air, the Mn3[Co(CN)6]2·nH2O precursor was
pyrolyzed and converted to tetragonal spinel-type CMO
nanocubes with a large surface area because of the presence
of numerous pores between the subconstituent nanoparticles
(mesopores) and internal hollow space (macropores) within
the individual nanocubes. The porous CMO catalyst enhanced
the kinetics of the cathode reactions (OER/ORR) in seawater
batteries because of the large surface area (active sites) and high
oxidation states of both the Co (+2.46) and Mn (+2.69)
cations. The cell containing the CMO showed a significantly
reduced ΔVof ∼0.53 V (voltage efficiency of ∼85%) compared
to that of the cells containing Pt/C (∼0.64 V) or Ir/C (∼0.73
V) or without any catalyst (∼1.05 V) at 0.01 mA cm−2.
Furthermore, when a hard carbon electrode was applied as a
Na-metal-free anode, the seawater batteries displayed a good
cycling stability with an average discharge voltage of ∼2.7 V and
a discharge capacity of ∼190 mAh g−1hard carbon during 100
cycles, which resulted in high energy efficiencies of 74−79%.
The phase and surface states of the CMO catalyst remained
almost intact after cycling the seawater battery. These results
are attributed to the bifunctional electrocatalytic activity of the
porous CMO nanocubes in the cathode of the seawater
batteries, which would enhance the applicability of eco-friendly
seawater batteries and aqueous Na−air batteries in various EES
areas.
■EXPERIMENTAL SECTION
Sample Preparation. Porous CMO nanocubes were synthesized
by the heat-treatment of Mn3[Co(CN)6]2·nH2O in ambient air. The
Mn3[Co(CN)6]2·nH2O precursor was prepared by a coprecipitation
method with some modifications of the chemical compositions at
room temperature.
27
First, 3 mmol of Mn(CH3COO)2·4H2O (Alfa
Aesar) and 1 g of polyvinylpyrrolidone (PVP, MW≈80 000, Alfa
Aesar) were dissolved in a mixed solvent (120 mL) of distilled water
and ethanol (1:2) with vigorous stirring to obtain a clear solution
(solution A). Subsequently, 60 mL of aqueous solution B containing 2
mmol K3[Co(CN)6] (Alfa Aesar) was added dropwise to solution A
with vigorous stirring for 1 h and then kept at room temperature for
24 h without any perturbation. The white precipitates (Mn3[Co-
(CN)6]2·nH2O) were collected by washing with distilled water and
absolute ethanol using centrifugation and dried at 80 °C for 24 h in a
convection oven. Finally, the Mn3[Co(CN)6]2·nH2O powder was
calcined to form porous CMO nanoparticles according to the
following heat-treatment in ambient air: ramping at 2 °C min−1to
430 °C, maintaining at 430 °C for 2 h, and then furnace cooling to
room temperature.
Sample Characterizations. The morphology and microstructure
of the Mn3[Co(CN)6]2·nH2O and CMO powders were observed by
Figure 5. Charge−discharge voltage profiles of the full-cells (hard
carbon|seawater) with/without the CMO catalyst at the fifth cycle at a
current rate of 0.01 mA cm−2(a) and the full-cell with respect to cycle
number at 0.01 mA cm−2(b). Variations of the discharge capacity and
Coulombic efficiency of the full-cell with cycle number to 100 cycles
(c).
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.6b10082
ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
G

SEM (Verios 460, FEI Company, 10 kV) and TEM (JEM-2100F,
JEOL, 200 kV). The phase identification was performed by XRD (D/
Max, Rigaku apparatus) equipped with a Cu KαX-ray source and
TEM-SAED and FFT. The specific surface area and the pore structure
were characterized by N2adoption-desorption isotherms at 77 K
(Micromeritics ASAP 2020) and the BET and BJH methods. The
chemical bonding state and composition were examined by XPS (K-
alpha, Thermo Fisher, UK) with a Mg KαX-ray source. ICP-OES
(Varian, 700-ES) was also performed for analyzing the chemical
composition of the final CMO nanoparticles. TGA (TA, Q500) was
performed by varying the temperature from 30 to 800 °C (the
temperature was increased by 10 °C min−1) under an air-flowing
atmosphere.
Electrochemical Testing. The electrocatalytic activity of the
CMO catalyst for the ORR and OER was investigated using RRDE
voltammetry based on a three-electrode electrochemical cell, where a
Pt wire and Hg/HgO (for alkaline) and Ag/AgCl (1 M KCl, for
seawater) electrodes were used as the counter electrode and reference
electrodes, respectively. RRDE linear sweep voltammetry (LSV)
measurements were performed at 1600 rpm and a scan rate of 10 mV
s−1in 0.1 M NaOH and seawater (Sigma). The catalyst ink was
prepared by uniformly mixing the CMO nanoparticles (16 mg) and
conductive carbon nanoparticles (Ketjenblack, 4 mg) in a solvent
mixture of ethanol (0.9 mL) and Nafion solution (0.1 mL, 5 wt % in
lower aliphatic alcohols and H2O) and coated onto a polished glassy
carbon disk electrode (4 mm in diameter). For comparison,
commercially available Pt/C (50 wt % Pt, Alfa Aesar) and Ir/C (20
wt % Ir, Premetek) nanopowders were used as representative
electrocatalysts toward the ORR and OER, respectively. The loading
levels of Pt and Ir were ∼0.48 and ∼0.19 mg cm−2, respectively. The
electron transfer number (n) during the ORR was calculated by the
following equation:
46
=
+
n
I
IIN
4
/
D
DR (3)
where IDis the disk current, IRis the ring current, and Nis the current
collection efficiency at the Pt ring.
The air-cathodes for seawater batteries were prepared as follows.
First, a slurry was fabricated by mixing CMO nanoparticles (80 wt %),
carbon black Super-P (CB, 10 wt %, TIMCAL), and poly(vinylidene
fluoride) (PVDF, 10 wt %, Sigma-Aldrich) binder in N-methyl-2-
pyrrolidone (NMP, Sigma-Aldrich). The slurry was uniformly coated
on one side of the carbon paper (Fuel Cell Store) with an area of 15
mm ×15 mm and then dried in an oven at 80 °C. The loading amount
of the CMO/CB catalyst was 4 mg cm−2. For comparative studies, the
Pt/C and Ir/C catalyst-coated carbon papers were prepared with the
same loading level. Coin-type NASICON ceramic electrolytes
(Na1+xZr2SixP3−xO12,x= 2) with a diameter of 16 mm were fabricated
according to the procedure described elsewhere
11,13,14
and used as the
Na+ion conductive membrane to separate the anode from the
cathode.
Next, pouch-type seawater battery cells were assembled in the
following manner. For the anode compartment, the NASICON
ceramic was mounted onto the open-structured top part of the anode
and then sealed with the anode bottom part, which consisted of an
organic electrolyte of 1 M NaCF3SO3(Sigma-Aldrich) in tetraethylene
glycol dimethyl ether (Sigma-Aldrich) and an anode of Na metal
(99.9%, Sigma-Aldrich) or hard carbon electrode attached to Ni taps
(anodic current collector, Solbrain LTK). The assembly process was
performed in a glovebox under a high-purity Ar atmosphere (O2and
H2O < 1 ppm). The assembled cells containing the anode
compartment and air-electrode were immersed in seawater electrolyte
(Sigma) and electrically connected to a measurement system (WBCS
3000 battery cycler, WonATech).
The hard carbon electrode was prepared by making a slurry
containing hard carbon (MeadWestvaco Corporation), carbon black
Super-P (TIMCAL), and PVDF at a weight ratio (wt %) of 80:10:10,
coating the slurry onto the copper foil (14 μm thick) with a doctor
blade, and then drying at 80 °C in a vacuum oven for 12 h. The
electrode was roll-pressed and dried in a vacuum oven. The loading
level of hard carbon was around 2.92 mg cm−2. The electrochemical
properties of the batteries were tested by the battery cycler at room
temperature. The cells were galvanostatically charged and discharged
at current rates of 0.01−0.1 mA cm−2. The hard carbon anode was
examined using a 2032 coin-type half-cell at a current rate of 20 mA
g−1in the voltage window of 0−2 V vs Na+/Na.
The full-cells were tested at a current rate of 0.01 mA cm−2with a
capacity cutoffof 200 mAh g−1hard carbon upon charging and a voltage
cutoffof 0.5 V upon discharging. To check the structural stability of
the CMO catalyst after cycling, the cells were disassembled and the
catalyst powders were acquired by cleaning the catalyst-coated carbon
papers in DI water and dissolving the PVDF binder from the carbon
papers using a NMP solvent.
■ASSOCIATED CONTENT
*
SSupporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsami.6b10082.
Additional experimental data on the phase, morpho-
logical, and thermal characterizations of the Mn3[Co-
(CN)6]2·nH2O nanocubes, phase indexing for the SAED
and FFT patterns of the CMO, N2sorption isotherms
and size distribution graph of the Mn3[Co(CN)6]2·
nH2nanocubes, comparison of the electrocatalytic
properties with literatures, the initial charge/discharge
voltage profiles of seawater batteries with the CMO
catalyst at different current densities, galvanostatic
charge/discharge voltage profiles of the hard carbon
half-cell, and changes in the phase and surface structure
of the CMO catalyst after cycling. (PDF)
■AUTHOR INFORMATION
Corresponding Authors
*E-mail: smhwang@unist.ac.kr.
*E-mail: ykim@unist.ac.kr.
ORCID
Youngsik Kim: 0000-0001-7076-9489
Author Contributions
M.A. and S.M.H. contributed equally to this work.
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
This work was supported by the 2016 Research Fund
(1.160004.01) of UNIST (Ulsan National Institute of Science
and Technology).
■REFERENCES
(1) Yang, Z.; Zhang, J.; Kintner-Meyer, M. C. W.; Lu, X.; Choi, D.;
Lemmon, J. P.; Liu, J. Electrochemical Energy Storage for Green Grid.
Chem. Rev. 2011,111, 3577−3613.
(2) Dunn, B.; Kamath, H.; Tarascon, J.-M. Tarascon, Electrical
Energy Storage for the Grid: A Battery of Choices. Science 2011,334,
928−935.
(3) Larcher, D.; Tarascon, J.-M. Towards Greener and More
Sustainable Batteries for Electrical Energy Storage. Nat. Chem. 2014,
7,19−29.
(4) McCloskey, B. D. Expanding the Ragone Plot: Pushing the Limits
of Energy Storage. J. Phys. Chem. Lett. 2015,6, 3592−3593.
(5) Choi, J. W.; Aurbach, D. Promise and Reality of Post-Lithium-Ion
Batteries with High Energy Densities. Nat. Rev. Mater. 2016,1, 16013.
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.6b10082
ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
H

(6) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M.
Li−O2and Li−S Batteries with High Energy Storage. Nat. Mater.
2011,11,19−29.
(7) Lu, J.; Li, L.; Park, J.-B.; Sun, Y.-K.; Wu, F.; Amine, K. Aprotic
and Aqueous Li−O2Batteries. Chem. Rev. 2014,114, 5611−5640.
(8) Hartmann, P.; Bender, C. L.; Vrăcar, M.; Dürr, A. K.; Garsuch, A.;
Janek, J.; Adelhelm, P. A Rechargeable Room-Temperature Sodium
Superoxide (NaO2) Battery. Nat. Mater. 2012,12, 228−232.
(9) Ortiz-Vitoriano, N.; Batcho, T. P.; Kwabi, D. G.; Han, B.; Pour,
N.; Yao, K. P. C.; Thompson, C. V.; Shao-Horn, Y. Rate-Dependent
Nucleation and Growth of NaO2in Na−O2Batteries. J. Phys. Chem.
Lett. 2015,6, 2636−2643.
(10) Cho, M. H.; Trottier, J.; Gagnon, C.; Hovington, P.; Clément,
D.; Vijh, A.; Kim, C.-S.; Guerfi, A.; Black, R.; Nazar, L.; Zaghib, K. The
Effects of Moisture Contamination in the Li-O2Battery. J. Power
Sources 2014,268, 565−574.
(11) Sahgong, S. H.; Senthilkumar, S. T.; Kim, K.; Hwang, S. M.;
Kim, Y. Rechargeable Aqueous Na−Air Batteries: Highly Improved
Voltage Efficiency by Use of Catalysts. Electrochem. Commun. 2015,61,
53−56.
(12) Hayashi, K.; Shima, K.; Sugiyama, F. A Mixed Aqueous/Aprotic
Sodium/Air Cell using a NASICON Ceramic Separator. J. Electrochem.
Soc. 2013,160, A1467−A1472.
(13) Kim, J.-K.; Mueller, F.; Kim, H.; Bresser, D.; Park, J.-S.; Lim, D.-
H.; Kim, G.-T.; Passerini, S.; Kim, Y. Rechargeable-Hybrid-Seawater
Fuel Cell. NPG Asia Mater. 2014,6, e144.
(14) Kim, H.; Park, J.-S.; Sahgong, S. H.; Park, S.; Kim, J.-K.; Kim, Y.
Metal-Free Hybrid Seawater Fuel Cell with an Ether-Based Electrolyte.
J. Mater. Chem. A 2014,2, 19584−19588.
(15) Kim, J.-K.; Mueller, F.; Kim, H.; Jeong, S.; Park, J.-S.; Passerini,
S.; Kim, Y. Eco-friendly Energy Storage System: Seawater and Ionic
Liquid Electrolyte. ChemSusChem 2016,9,42−49.
(16) He, P.; Zhang, T.; Jiang, J.; Zhou, H. Lithium−Air Batteries with
Hybrid Electrolytes. J. Phys. Chem. Lett. 2016,7, 1267−1280.
(17) Hashimoto, T.; Hayashi, K. Aqueous and Nonaqueous Sodium-
Air Cells with Nanoporous Gold Cathode. Electrochim. Acta 2015,182,
809−814.
(18) Dong, S.; Chen, X.; Wang, S.; Gu, L.; Zhang, L.; Wang, X.;
Zhou, X.; Liu, Z.; Han, P.; Duan, Y.; Xu, H.; Yao, J.; Zhang, C.; Zhang,
K.; Cui, G.; Chen, L. 1D Coaxial Platinum/Titanium Nitride
Nanotube Arrays with Enhanced Electrocatalytic Activity for the
Oxygen Reduction Reaction: Towards Li−Air Batteries. ChemSusChem
2012,5, 1712−1715.
(19) Yoo, E.; Zhou, H. Li-Air Rechargeable Battery Based on Metal-
free Graphene Nanosheet Catalysts. ACS Nano 2011,5, 3020−3026.
(20) He, P.; Wang, Y.; Zhou, H. Titanium Nitride Catalyst Cathode
in a Li−Air Fuel Cell with an Acidic Aqueous Solution. Chem.
Commun. 2011,47, 10701−10703.
(21) Li, L.; Manthiram, A. O- and N-Doped Carbon Nanowebs as
Metal-Free Catalysts for Hybrid Li-Air Batteries. Adv. Energy Mater.
2014,4, 1301795.
(22) Li, L.; Chai, S.-H.; Dai, S.; Manthiram, A. Advanced Hybrid Li−
Air Batteries with High Performance Mesoporous Nanocatalysts.
Energy Environ. Sci. 2014,7, 2630−2636.
(23) Wang, L.; Zhao, X.; Lu, Y.; Xu, M.; Zhang, D.; Ruoff, R. S.;
Stevenson, K. J.; Goodenough, J. B. CoMn2O4Spinel Nanoparticles
Grown on Graphene as Bifunctional Catalyst for Lithium-Air Batteries.
J. Electrochem. Soc. 2011,158, A1379−A1382.
(24) Ge, X.; Sumboja, A.; Wuu, D.; An, T.; Li, B.; Goh, F. W. T.;
Hor, T. S. A.; Zong, Y.; Liu, Z. Oxygen Reduction in Alkaline Media:
From Mechanisms to Recent Advances of Catalysts. ACS Catal. 2015,
5, 4643−4667.
(25) Park, M.-S.; Kim, J.; Kim, K. J.; Lee, J.-W.; Kim, J. H.; Yamauchi,
Y. Porous Nanoarchitectures of Spinel-Type Transition Metal Oxides
for Electrochemical Energy Storage Systems. Phys. Chem. Chem. Phys.
2015,17, 30963−30977.
(26) Natesakhawat, S.; Culp, J. T.; Matranga, C.; Bockrath, B.
Adsorption Properties of Hydrogen and Carbon Dioxide in Prussian
Blue Analogues M3[Co(CN)6]2, M = Co, Zn. J. Phys. Chem. C 2007,
111, 1055−1060.
(27) Hu, L.; Zhang, P.; Chen, Q.-w.; Yan, N.; Mei, J.-y. Prussian Blue
Analogue Mn3[Co(CN)6]2·nH2O Porous Nanocubes: Large-Scale
Synthesis and Their CO2 Storage Properties. Dalton Trans. 2011,40,
5557−5562.
(28) Zhang, J.; Wang, L.; Xu, L.; Ge, X.; Zhao, X.; Lai, M.; Liu, Z.;
Chen, W. Porous Cobalt−Manganese Oxide Nanocubes Derived from
Metal Organic Frameworks as a Cathode Catalyst for Rechargeable
Li−O2 Batteries. Nanoscale 2015,7, 720−726.
(29) Hu, L.; Yan, N.; Chen, Q.; Zhang, P.; Zhong, H.; Zheng, X.; Li,
Y.; Hu, X. Fabrication Based on the Kirkendall Effect of Co3O4
Porous Nanocages with Extraordinarily High Capacity for Lithium
Storage. Chem. - Eur. J. 2012,18, 8971−8977.
(30) Ge, X.; Liu, Y.; Goh, F. W. T.; Hor, T. S. A.; Zong, Y.; Xiao, P.;
Zhang, Z.; Lim, S. H.; Li, B.; Wang, X.; Liu, Z. Dual-Phase Spinel
MnCo2O4 and Spinel MnCo2O4/Nanocarbon Hybrids for Electro-
catalytic Oxygen Reduction and Evolution. ACS Appl. Mater. Interfaces
2014,6, 12684−12691.
(31) Lee, E.; Jang, J.-H.; Kwon, Y.-U. Composition Effects of Spinel
MnxCo3−xO4 Nanoparticles on Their Electrocatalytic Properties in
Oxygen Reduction Reaction in Alkaline Media. J. Power Sources 2015,
273, 735−741.
(32) Tan, B. J.; Klabunde, K. J.; Sherwood, P. M. A. XPS Studies of
Solvated Metal Atom Dispersed (SMAD) Catalysts. Evidence for
Layered Cobalt-Manganese Particles on Alumina and Silica. J. Am.
Chem. Soc. 1991,113, 855−861.
(33) Restovic, A.; Ríos, E.; Barbato, S.; Ortiz, J.; Gautier, J. L. Oxygen
Reduction in Alkaline Medium at Thin MnxCo3‑xO4(0 ≤x≤1)
Spinel Films Prepared by Spray Pyrolysis. Effect of Oxide Cation
Composition on the Reaction Kinetics. J. Electroanal. Chem. 2002,522,
141−151.
(34) 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. 2011,3, 546−550.
(35) Li, C.; Han, X.; Cheng, F.; Hu, Y.; Chen, C.; Chen, J. Phase and
Composition Controllable Synthesis of Cobalt Manganese Spinel
Nanoparticles towards Efficient Oxygen Electrocatalysis. Nat.
Commun. 2015,6, 7345.
(36) 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. 2016,6, 2019−2023.
(37) Wei, W.; Chen, W.; Ivey, D. G. Rock Salt−Spinel Structural
Transformation in Anodically Electrodeposited Mn−Co−O Nano-
crystals. Chem. Mater. 2008,20, 1941−1947.
(38) Hwang, S. M.; Kim, S. Y.; Kim, J.-G.; Kim, K. J.; Lee, J.-W.; Park,
M.-S.; Kim, Y.-J.; Shahabuddin, M.; Yamauchi, Y.; Kim, J. H.
Electrospun Manganese−Cobalt Oxide Hollow Nanofibres Synthe-
sized via Combustion Reactions and Their Lithium Storage Perform-
ance. Nanoscale 2015,7, 8351−8355.
(39) Jung, K.-N.; Hwang, S. M.; Park, M.-S.; Kim, K. J.; Kim, J.-G.;
Dou, S. X.; Kim, J. H.; Lee, J.-W. One-Dimensional Manganese-Cobalt
Oxide Nanofibres as Bi-Functional Cathode Catalysts for Rechargeable
Metal-Air Batteries. Sci. Rep. 2015,5, 7665.
(40) Liu, X.; Park, M.; Kim, M. G.; Gupta, S.; Wang, X.; Wu, G.;
Cho, J. High-performance non-spinel cobalt−manganese mixedoxide-
based bifunctional electrocatalysts for rechargeable zinc−air batteries.
Nano Energy 2016,20, 315−325.
(41) Liang, Y. Y.; Wang, H. L.; Zhou, J. G.; Li, Y. G.; Wang, J.;
Regier, T.; Dai, H. Covalent Hybrid of Spinel Manganese-Cobalt
Oxide and Graphene as Advanced Oxygen Reduction Electrocatalysts.
J. Am. Chem. Soc. 2012,134, 3517−3523.
(42) Oh, D.; Qi, J.; Han, B.; Zhang, G.; Carney, T. J.; Ohmura, J.;
Zhang, Y.; Shao-Horn, Y.; Belcher, A. M. M13 Virus-Directed
Synthesis of Nanostructured Metal Oxides for Lithium−Oxygen
Batteries. Nano Lett. 2014,14, 4837−4845.
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.6b10082
ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
I

(43) Tarasevich, M. R.; Korchagin, O. V. Electrocatalysis and pH (a
Review). Russ. J. Electrochem. 2013,49, 600−618.
(44) Cheng, F.; Shen, J.; Peng, B.; Pan, Y.; Tao, Z.; Chen, J. Rapid
Room-Temperature Synthesis of Nanocrystalline Spinels as Oxygen
Reduction and Evolution Electrocatalyst. Nat. Chem. 2011,3,79−84.
(45) Irisarri, E.; Ponrouch, A.; Palacin, M. R. ReviewHard Carbon
Negative Electrode Materials for Sodium-Ion Batteries. J. Electrochem.
Soc. 2015,162, A2476−A2482.
(46) Xing, W.; Yin, G.; Zhang, J. Rotating Electrode Method and
Oxygen Reduction Electrocatalysts; Elsevier: Amsterdam, The Nether-
lands, 2014.
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.6b10082
ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
J