An advanced three-dimensionally ordered macroporous NiCo2O4spinel as a bifunctional electrocatalyst for rechargeable Zn-air batteries

Article abstract of DOI:10.1039/d0ta00874e

In this work, highly active NiCo₂O₄spinels with three-dimensionally ordered macroporous (3DOM) structures were obtained using poly(methyl methacrylate) (PMMA) spheres as a template, in which the high surface area of the 3DOM spinel, the presence of multiple valences of Co and Ni species and the high Ni content improve the activity for the oxygen reactions, presenting a current density value similar to that obtained for benchmark Pt/C. Also, the spinel needs only 10 mV more overpotential compared with benchmark IrO₂/C to achieve 10 mA cm^-2during the oxygen evolution reaction (OER). The 3DOM spinel shows superior stability for both reactions, retaining 80.5 (for the ORR) and 100% (for the OER) of the initial current after 10 h, respectively. This 3DOM spinel is used as an air electrode in a rechargeable Zn-air battery, enabling an open-circuit potential of 1.44 V, which is similar to that obtained for Pt/C (1.46 V), while also displaying a similar performance (109vs.101 mW cm^-2). Moreover, because of its excellent activity as a bifunctional electrocatalyst, the 3DOM spinel displays a superior charge-discharge capability after being operated for 21 h, while successfully supplying the energy requirements of a multi-sensor, which is operated for more than 2 h.

Full text of DOI:10.1039/d0ta00874e

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DOI: 10.1039/D0TA00874E
ARTICLE
An advanced three‒dimensionally ordered macroporous
NiCo2O4 spinel as bifunctional electrocatalyst for rechargeable
Zn‒Air batteries
Received 00th January 20xx,
Accepted 00th January 20xx
José Béjara, Lorena Álvarez-Contrerasb, Janet Ledesma-Garcíac, Noé Arjonaa* and Luis Gerardo Arriagaa**
DOI: 10.1039/x0xx00000x
In this work, highly active NiCo2O4 spinels with three-dimensionally ordered macroporous (3DOM) structures were obtained
using poly (methyl methacrylate) PMMA spheres as template, for which the high surface area of the 3DOM spinel, the
presence of multi‒valences of Co and Ni species and the high Ni content improve the activity for the oxygen reactions,
presenting a current density value similar to that obtained for benchmarked Pt/C. Also, the spinel shows only 10 mV of
overpotential compared with benchmarked IrO2/C for oxygen evolution reaction (OER). The 3DOM spinel shows superior
stability for both reactions, maintaining 80.5 (for ORR) and 100 % (for OER) of initial current after 10 h, respectively. This
3DOM spinel is used as an air electrode in a rechargeable Zn‒air battery, enabling an open-circuit potential of 1.44 V, which
is similar to that obtained for Pt/C (1.46 V), while also displaying a similar performance (109 vs. 101 mW cm−2). Moreover,
because of its excellent activity as a bifunctional electrocatalyst, the 3DOM spinel displays a superior charge‒discharge
capability after being operated for 21 h, while it provides successfully the energy requirements of a multi-sensor, which is
operated for more than 2 h.
in the performance and durability. Thereby, the development of
highly active and durable bifunctional materials is essential.
Nowadays, plenty bifunctional electrocatalysts based on noble
metals9,10, mixtures of transition metal oxides (MTMOs)11–14 and
Introduction
Metal‒air batteries are promising next-generation devices
with higher performances to that presented by benchmarked Li-
ion batteries. These electrochemical devices are powered by
metal oxidation (using metals like Li, Zn, Mg and Na) and
oxygen reduction1. Rechargeable Zn‒air batteries (RZABs) can
provide 5‒fold higher energy density than conventional Li‒ion
batteries2, and the lower cost of Zn metal make their fabrication
attractive for commercialization purposes. The abundance of Zn
metal in contrast with Li3, and the political/social issues recently
raised due to the concentration of this later metal in specific
geographic places make urgent to overcome the limitations of
RZABs components to obtain functional and durable energy
storage devices.
RZABs are constituted by the Zn anode, an air electrode,
separator/membrane, and electrolyte4. All of these components
have their unique challenges to overcome as is described in
several reviews5–8; however, the air electrode has the major
contribution in the overpotential losses. The sluggish kinetics of
oxygen reduction/evolution reactions (ORR/OER) plays a major
role during the charge/discharge processes in a RZAB and thus,
non-metallic materials15–17 have been reported. Among them,
MTMOs have displayed excellent electrocatalytic properties,
and they have the advantage of being abundant, making them
economically viable, as well as possessing an excellent stability
and durability. In addition, MTMOs can be composed of a mix
of different metals, and thus, tuneable electrocatalysts consisting
of materials with high activities towards the ORR and OER can
be easily synthesized to produce a synergistic effect18. One way
to develop MTMOs sharing the crystalline network is through
the spinel structure, which is described by the general formula
AB2X4 (A = Li, Mn, Zn, Cd, Co, Cu, Ni, Fe, etc.; B = Al, Cr,
Mn, Fe, Co, Ni, Ga, In, Mo, etc.; X = O, S, Se, Te, N, and
others)19. In this manner, nanomaterials with a spinel structure
based on nickel, cobalt, and manganese have been extensively
reported as bifunctional electrocatalysts for metal-air batteries20–
22, for which the NiCo2O4 spinel has displayed high functionality
as air electrode for RZABs due to its intrinsic high activity,
corrosion resistance, and easy availability19.
The electrocatalytic activity of NiCo2O4 spinels can be
modulated by tuning the crystallite size, Ni:Co atomic rate,
particle size and shape, and electrical conductivity, as well as
controlling metal valences, creating defects, and selecting the
proper conductive support, causing shifts in the atomic
arrangement and electronic properties19. In terms of atomic
arrangement, different NiCo2O4 and Co3O4 morphologies, as
a. Centro de Investigación y Desarrollo Tecnológico en Electroquímica S. C.,
Querétaro, C. P. 76703, México
b. Centro de Investigación en Materiales Avanzados S. C., Complejo Industrial
Chihuahua, Chihuahua, C. P. 31136, México
c. Facultad de Ingeniería, División de Investigación y Posgrado, Universidad
Autónoma de Querétaro, Querétaro, C. P. 76010, México
nanoparticles21–23
,
nanocubes24–26
,
nanowires27–29
,
and
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
nanoneedles30,31, among others, have been developed to improve
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the specific surface area (SSA) and thus, the electrocatalytic precursors were dissolved with stirring at 35 °C for 4 h, keeping
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activity. An uncommon method to improve the SSA consist of 1 mol L−1 total metal concentration.
DOI: 10.1039/D0TA00874E
developing materials employing polymer templates that can help
The impregnation of the PMMA hard template was carried
to obtain a defined morphology32. Among these nanostructures, out under vacuum. Similar methodology was followed for the
three‒dimensionally ordered macroporous morphologies three spinels (Co3O4, Ni0.5Co2.5O4 and NiCo2O4). For this
(3DOMs) have allowed novel electrocatalysts to emerge. These purpose, a piece of CCT (Fig. S1, ~2 g in weight) was placed
electrocatalysts are promising because the periodic pore inside of a Buchner funnel in a vacuum filtration system. Then,
arrangement enables a high SSA as well as a better interaction 6 layers of 3 ml precursor solution (each one) were deposited,
between active sites and reactants.33,34
applying vacuum for 5 min between each layer. Finally, the
Most of the reported 3DOM materials are based on carbon or material was submitted to vacuum for 2 h to eliminate excesses.
transition metals and have been prepared specifically for soot The obtained precursor/template mixture was dried in air at 50
combustion35–39, where 3DOM materials with above 50 nm °C for 24 h. Then, the sample was calcined in a tube furnace to
average pore diameters36 allow soot particles (>25 nm) to enter remove the PMMA hard template. The thermal treatment process
into the inner pores to react. Regarding oxygen reactions, few was carried out in two steps: (1) the dried solid was calcined
works have dealt with 3DOM materials as bifunctional under 90 mL min−1 N2 flow and 1 °C min−1 of temperature rate
electrocatalysts, with one of them based on 3DOM perovskites40 until 310 °C, and this temperature was maintained for 3 h. (2)
and the other based on a 3DOM Co3O4 spinel.41 To our The treated solid was then kept at 310 °C under 90 mL min−1 air
knowledge, there are no reports concerning bimetallic spinels in flow for 1 h, and then, the temperature was increased until 550°C
a 3DOM morphology for the oxygen reduction and evolution at 1 °C min−1 under air atmosphere and held for 4 h.
reactions. Moreover, in most of the mentioned works, the 3DOM
structures were constructed using a polymer template with Physicochemical characterization
particle sizes larger than 300 nm, because their application in
soot combustion; however, for oxygen reactions, particle sizes PMMA CCT was characterized by scanning electron microscopy
smaller than 300 nm can be beneficial to attain higher surface (SEM) using a field‒emission scanning electron microscope
areas.
(JEOL JSM‒7401F) to confirm the formation of the CCT
In this work, we report for the first time, the synthesis of three assembly. The structure of the 3DOM materials was also
spinels materials Co3O4, Ni0.5Co2.5O4 and NiCo2O4 with a analysed by SEM using the JEOL JSM-7401F microscope, while
3DOM structure. These materials are evaluated as bifunctional elemental mapping was performed via energy‒dispersive X‒ray
electrocatalysts for the oxygen reduction and evolution reactions spectroscopy (EDS) using a Bruker EDX detector coupled to a
using benchmarked Pt/C and IrO2/C, and their mixture as Hitachi H7700 apparatus. Further morphological analysis was
reference electrocatalysts. The effect of Ni incorporation in the performed by transmission electron microscopy (TEM, Hitachi
cobalt oxide structure to form bimetallic spinels (Ni0.5Co2.5O4 H7700), where the particle sizes and d‒spacings were calculated
and NiCo2O4) is studied in terms of changes in surface area, using the ImageJ® software.45 In addition, selected area electron
electrical conductivity and electrocatalytic activity. Moreover, diffraction (SAED) patterns obtained using the Hitachi TEM
the activity of 3DOM NiCo2O4 is tested in a Zn‒air battery, apparatus were also analysed by the ImageJ® software to
where the higher performance achieved by this material, in determine the crystalline planes in the spinels. BET surface area
contrast with that of Pt/C and IrO2/C, is attributed to a greater analysis was performed using N2 adsorption/desorption
activity and stability.
isotherms employing a Quantachrome® S‒BET Autosorb iQ2,
and the Barret‒Joyner‒Halenda (BJH) method was used to report
surface areas. These materials were further characterized by
X‒ray diffraction (XRD) and X‒ray photoelectron spectroscopy
(XPS) using a Bruker D8 Advance diffractometer operated at 30
kV and 30 mA and a Thermo Scientific Escalab 250 Xi XPS
apparatus, respectively. The photoelectrons were generated with
a monochromatic Al Ka (1486.68 eV) X‒ray source. Survey
spectra of the samples were collected over 1100 eV range at 1
eV per step, 100 ms and a 150 eV pass energy. High‒resolution
spectra were collected for the chemical species of interest at 0.1
eV per step and 20 eV pass energy. All measurements were done
under ultra‒high vacuum conditions (10-10 torr). The
deconvolved XPS spectra were normalized using the O 1s peak,
which was adjusted using the reported value (531.0 eV).46
EXPERIMENTAL
Synthesis of 3DOM electrocatalysts
The 3DOM materials were synthesized using a poly (methyl
methacrylate) (PMMA) colloidal crystal template (CCT). The
detailed procedure for the polymerization and CCT assembly is
described in the supplementary information (SI, Fig. S1).
According to the followed methodology, PMMA microspheres
with a diameter close to 160 nm were synthesized via emulsion
polymerization 42,43(Fig. S2). The 3DOM spinels were prepared
by making modifications to a previously reported CCT method.44
Briefly, 1 g citric acid (Sigma‒Aldrich, ≥99.5 %), as complexing
agent, was added to 20 mL methanol (Sigma‒Aldrich, ≥99.9 %)
and stirred for 1 h to obtain a uniform precursor solution. Then,
suitable amounts of Co(NO3)2·6H2O (Sigma‒Aldrich, ≥98.0 %)
and Ni(NO3)2·6H2O (Sigma‒Aldrich, ≥97.0 %) metal nitrate
Electrocatalytic evaluation
The electrochemical and electrocatalytic characterization of
materials was performed using a BioLogic® Potentiostat/
2 | J. Name., 2012, 00, 1-3
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Galvanostat. For this purpose, a three‒electrode glass‒jacketed The Zn‒air battery was constructed using two PMMA plates and
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DOI: 10.1039/D0TA00874E
electrochemical cell was used. The electrochemical a polydimethylsiloxane (PDMS) separator / electrolyte chamber
configuration consisted of using Hg/HgO 1M KOH electrode (Fig. 8c, inset). The battery dimensions were 35×25×13 mm in
and a graphite rod as the reference and counter electrode, length, width and depth respectively, while the upper PMMA
respectively. Rotating disk electrode with glassy carbon tips plate (5 mm in depth) had an air window of 15×3 mm length and
(Pine®, 0.07 cm2) was used as the working electrode for the ORR width (patterned using CNC machining). The PDMS film used
and with gold tips for the OER (the influence of gold was as the separator / chamber had the same shape and dimensions as
discarded through an electrochemical test determining the those of the PMMA plates; however, the thickness of this film
number of catalyst layers needed to overlap its signal Fig. S3). was 3 mm, and the chamber is 15×6 mm (length and width,
Additionally, 0.1 M KOH was employed as the electrolyte for all which is depicted as a green film in Fig. 8c, inset). The electrode
experiments. 3DOM spinels and benchmarked electrocatalysts dimensions were patterned into the PMMA plates to serve as
(Pt/C 20 wt.%, IrO2/C 20 wt.% from E‒TEK, and their mixture guides when placing the electrodes. The anode consisted in a
in a 1:1 mass ratio) were deposited onto working electrodes polished high‒purity Zn foil (25×3 mm length and width, and 0.5
through catalytic inks. The ink was prepared using isopropyl mm in depth; Belong Store® China) with 15 mm effective length,
alcohol as dispersant and Nafion® 117 (Sigma‒Aldrich, 5 wt.% while 0.2 M ZnCl2 (Granular, J. T. Baker) dissolved in 4 M KOH
in isopropyl alcohol) as the binder. Two kinds of catalytic inks (87 %, Sigma‒Aldrich) was used as the electrolyte.
were prepared: one for 3DOM spinels, where 6 mg catalyst, 270 SIGRACET® 39B slides (25×3 mm length and width, and 0.4
μL isopropyl alcohol and 45 μL Nafion were mixed and mm in depth) impregnated with the catalytic ink were used as the
sonicated for 30 min, and then, 5 layers of 1 μL solution were air electrodes. The catalyst loading was 1 mg cm−2 for Pt/C,
transferred to the electrode, maintaining 0.27 mg cm−2 of total IrO2/C and their mixture, and 2 mg cm−2 for the NiCo2O4. A
catalyst mass. Catalytic inks for Pt/C and IrO2/C were prepared BioLogic® Potentiostat/Galvanostat was used to analyse the
by the above procedure using catalyst loadings close to0.135 mg battery charge/ discharge performance and stability. The stability
cm−2 of total catalyst mass (only 2‒fold lower catalyst mass to determination consisted of an accelerated charge and discharge
that used for 3DOM materials).
test performed at 10 mA cm−2 during 150 cycles with a time of
Cyclic voltammetry was performed to obtain the 500 seconds per cycle. Moreover, the current was normalized to
electrochemical profiles in alkaline media. The cyclic the Zn geometric area (0.45 cm2).
voltammograms were recorded in a potential range between 0.04
and 1.59 V vs. RHE (reversible hydrogen electrode) for spinels- RESULTS AND DISCUSSION
based materials and between 0 and 1.3 V vs. RHE for Pt/C. The
scan rate used in the tests was 20 mV s−1, and 50 cycles were Physicochemical results
recorded under an inert atmosphere using nitrogen gas (Praxair,
5.0 for research purposes). The electrocatalytic activity toward XRD patterns for Co3O4 (Fig. 1a-I) and Ni0.5Co2.5O4 (Fig. 1a-II)
ORR and OER was evaluated using the rotating disk electrode present nine characteristic diffraction peaks, where according to
(RDE) technique by varying the rotatory speed. For the ORR, their positions (Table S1), they can be indexed to the (111),
linear sweep voltammograms (LSVs) were acquired using a (220), (311), (222), (400), (422), (511), (440) and (533) planes
potential range from 0.14 to 1.04 V vs. RHE at a scan rate of 5 of the cubic structure of Co3O4 and NiCo2O4 spinels (PDFS
mV s−1 (For 3DOM spinels and IrO2/C) and a range from 0.26 to number 03-065-3103 and 00-020-0781, respectively).47 NiCo2O4
1.14 V vs. RHE for Pt/C at the same scan rate. In addition, the material (Fig. 1a-III) presented shifts in diffraction peaks with
experiments for the OER were performed in a nitrogen‒rich respect to those of Co3O4 (Table S1), suggesting a substitutional
solution using 1600 rpm as the rotatory speed for a potential incorporation of Ni into the Co3O4 structure. The absence of
window from 0.94 to 1.94 V vs. RHE at 5 mV s−1. The stability additional peaks also suggested that there was no secondary
tests were performed by chronoamperometry technique. The phase (or impurity) in any of these materials. In addition, the
ORR stability was determined at the half‒wave potential of each elemental composition analyzed by EDS (Table S2) and XPS
material while maintaining a rotation speed of 1600 rpm and (Table S3) together with the theoretical and experimental
constant oxygen bubbling for 10 h. Meanwhile, the OER stability positions and d‒spacings (Table S1) corroborated the formation
was evaluated at the potential found at 10 mA cm−2 for each of these spinels. Moreover, the crystallite sizes were determined
material while maintaining a rotation speed of 1600 rpm and by the Debye–Scherrer equation.48 The calculated crystallite
constant nitrogen bubbling for 10 h. Additionally, sizes were 12.3, 9.4 and 9.3 nm for Co3O4, Ni0.5Co2.5O4 and
electrochemical impedance spectroscopy (EIS) was performed NiCo2O4, respectively.
to analyse the electrical properties of the 3DOM materials,
BET surface areas were obtained from nitrogen
specifically the resistance to charge transfer (Rct). The ads/desorption isotherms (Fig. 1b). The ads/desorption
measurements were taken at 0.82 V vs. RHE with the frequency behaviour indicated a mixture of types 2 and 4 isotherms and a
ranging from 100 kHz to 0.01 Hz.
type H3 hysteresis loop. The latter description reveals that the
isotherms did not show any limiting adsorption at high P/Po in
the range of 0.7‒1, which is characteristic of mesoporous
materials41,49,50. BET surface areas of 39.6, 46.4 and 46.8 m2 g−1
were obtained for Co3O4 (Fig. 1b-I), Ni0.5Co2.5O4 (Fig. 1b-II) and
Zn‒Air battery device
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NiCo2O4 (Fig. 1b-III), respectively, with pore volumes of 0.286, demonstrate well‒defined periodically macroporVoiuewsArstticrluecOtnulirnee
0.289 and 0.286 m3 g−1. These results were attributed to the and interconnected network of these materials (Figs. 2a, 2c and
DOI: 10.1039/D0TA00874E
mesopores observed in the results of the pore size distributions 2e), being the bimetallic spinels the materials with better
(Fig. 1b insets). Concerning the effect of the nickel addition to periodicity (Figs. 2c, and 2d). Two main pore sizes were found,
the cobalt spinel, is possible to conclude that nickel had a regardless of the material; the first type was large macropores,
positive effect because it increasing the surface area by 8 m2 g−1 which formed upon the removal of the PMMA spheres, and they
(approximately 20%), while a non‒notable difference was are called “skeletons”. The diameter of these skeletons was
observed for the amounts of nickel in the Ni0.5Co2.5O4 and found to be close to 130-140 nm (Figs. 2b, 2d, and 2f). The
NiCo2O4 materials. Finally, the use of the template with spheres second type of pores comprised the so-called “window pores”,
approximately 160 nm in size benefited the surface area, being these pores generated by contact points between PMMA
obtaining values higher than similar materials with spheres of spheres. For this case, window pore sizes close to 50 nm were
250 nm and above.40,41
found. Additionally, the wall thickness was found to be 16 nm.
The obtained skeleton sizes were lower than the diameter of the
PMMA microspheres (160 nm), and this size reduction was
attributed to the shrinkage of PMMA microspheres during the
calcination of the metal nitrates, because PMMA may experience
a contraction at glass transition temperature. Previous works
have also observed a substantial shrinkage when preparing
3DOM materials.44,51,52
a-III)
NiCo2O4
Ni0.5Co2.5O4
Co3O4
a)
a-II)
a-I)
PDF No. 00-020-0781
NiCo2O4
PDF No. 03-065-3103
Co O4
3
20
30
40
50
60
70
80
90
2
Co3O4
b)171
Des
0.10
0.08
0.06
0.04
0.02
0.00
114
57
0
Ads
0
20
40
60
80
nm
100
120
Pore Diameter
/
Ni0.5Co2.5O4
171
114
57
0
Des
0.10
0.08
0.06
0.04
0.02
0.00
Ads
0
10
20
30
40
50
60
nm
70
80
Pore Diameter
/
NiCo2O4
171
114
57
0
Des
0.12
0.10
0.08
0.06
0.04
0.02
0.00
Figure 2. SEM images at two different magnifications for a & b) Co3O4; c & d)
Ni0.5Co2.5O4; and e & f) NiCo2O4. Figs. a, c and e display the 3DOM periodicity and Figs.
b, d and f the open window sizes and wall thickness.
Ads
0
20
40
60
nm
80
100
Pore Diameter
/
TEM micrographs of Co3O4 (Fig. 3a), Ni0.5Co2.5O4 (Fig. 3b)
and NiCo2O4 (Fig. 3c) corroborated the existence of 3DOM
structures. Sizes close to the 50 nm of the window pores can be
observed in Figs. 3a‒I, 3b‒I, and 3c‒I. In addition, the small
particle size of the spinels can be observed in Figs. 3a‒II, 3b‒II,
and 3c‒II. Histograms of the particle distributions are insets in
these figures; as shown, the Co3O4 particles displayed a size of
11.9 ± 0.9 nm, while the Ni0.5Co2.5O4 and NiCo2O4 particles had
sizes of 8.6 ± 0.9 nm and 9.6 ± 1.0 nm, respectively.
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/Po)
Figure 1. a) XRD patterns of the 3DOM electrocatalysts Co3O4 (a-I), Ni0.5Co2.5O4 (a-II)
and NiCo2O4 (a-III) and b) Nitrogen adsorption-desorption isotherms and pore-size
distribution plots (inset) for Co3O4 (b-I), Ni0.5Co2.5O4 (b-II) and NiCo2O4 (b-III).
The morphology and microstructure of the 3DOM materials
were analysed by SEM and TEM. SEM micrographics
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.
Figure 3. TEM images and SAED patterns of aI-IV) Co3O4, bI-IV) Ni0.5Co2.5O4 and cI-IV) NiCo2O4.
These particle sizes agreed with the crystallite sizes. high surface area exposing more active sites needed for the
Furthermore, TEM micrographs at higher magnifications were oxygen electrochemical reactions (ORR / OER) as was observed
taken to analyze the lattice fringes (Figs. 3a‒III, 3b‒III, and by BET results.
3c‒III). Magnifications of selected areas are shown in the insets
of these figures to reveal the lattice fringes used for calculating
d‒spacings with the ImageJ® software. The d‒spacing patterns,
as a function of distance between lattice fringes (yellow line),
indicated that the obtained d‒spacings were enclosed to that of
the (111) plane. Moreover, the ring distances in the SAED
patterns (Figs. 3a‒IV to 3c‒IV) corroborated the formation of
spinel structures because the indexed crystalline planes agreed
with those reported in the above mentioned XRD results (Table
S1).
a) Ni0.5Co2.5O4
Ni
O
100 nm
100 nm
Co
EDS mappings of the bimetallic spinels (Figs. 4a and 4b for
Ni0.5Co2.5O4 and NiCo2O4, respectively) confirmed the
homogeneous distributions of Ni and Co in the material.
Meanwhile, the elemental analysis showed that the
stoichiometries of these materials were close to the calculated
compositions (Table S2). In addition, the porous structure was
highlighted by the elemental distributions since the elements
only appeared on the skeletons. However, it should be noted that
although the materials were dispersed by sonication for the EDS
analysis, both maintained their well‒connected walls, which was
important for the further electrochemical characterization/
evaluation. It was observed in the SEM and TEM images that
hierarchically ordered pores resulted in a honeycomb-like
mesoporous structure, where the open structure could provide a
100 nm
b) NiCo2O4
Ni
O
90 nm
80 nm
Co
80 nm
80 nm
Figure 4. EDS elemental mapping for a) Ni0.5Co2.5O4 and b) NiCo2O4.
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XPS was used to analyze the metal species in surface layers observed at 854.12 eV and 873.33 eV for Ni2+ andVaietw8A5rt5ic.le9O5nelinVe
of the catalysts, composition (Table S3) and the changes in the and 873.53 eV for Ni3+ species.54–56 To understand the structural
DOI: 10.1039/D0TA00874E
structure of Co3O4 in the presence of nickel (Fig. 5). Figures 5a changes generated by the addition of nickel, it was necessary the
to 5c show the high‒resolution XPS spectra of the Co 2p core introduction of a more suitable format for a spinel, given by
level. Co3O4 (Fig. 5a) displayed peaks at 779.74 eV and 795 eV A1‑xBx(AxB2‑x)O4, from which three assumptions can be made:
that were attributed to Co3+, while the peaks at 781.19 eV and x=0 corresponds to a normal spinel (Co3O4), x=1 is attributed to
796.84 eV were assigned to Co2+.53 Ni0.5Co2.5O4 (Fig. 5b) an inverse spinel, and 0<x<1 refers to a complex spinel.19 For
presented the same peaks at 779.58 eV and 794.83 eV for Co3+ example, when 0.5 nickel is added, the material is in the regime
and at 781.12 eV and 796.75 eV for Co2+, while NiCo2O4 (Fig. of a complex spinel; on the other hand, when x=1 is reached, it
5c) presented the peaks related to Co3+ at 779.59 eV and 794.91 should be an inverse spinel. However, from XPS results it can
eV and those related to Co2+ at 781.14 eV and 797.02 eV. observe that NiCo2O4 exhibited the four species with a clear
High‒resolution XPS spectra of the Ni 2p core level are decrease in Co2+, and from previous reports, the conclusion is
displayed in Figs. 5d and 5e, where both spectra showed two that
a
more exact formula for NiCo2O4 is Co2+
1-
spin‒orbit doublets corresponding to Ni2+ and Ni3+ species. xCo3+x[Co3+Ni2+xNi3+1-x]O4 (0≤x ≤1)53 (cations within the square
Ni0.5Co2.5O4 (Fig. 5d) presented peaks assigned to Ni2+ at 854.39 bracket are in the octahedral sites, and cations outside the
eV and 871.76 eV and to Ni3+ species at 856.06 eV and 873.41 brackets occupy the tetrahedral sites).
eV. In the case of NiCo2O4 (Fig. 5e), similar peaks were
2p3/2
Co 2p
Co3O4
Co 2p
2p3/2
a)
b)
Ni0.5Co2.5O4
Co3+
Co2+
Co2+
Co3+
Sat
2p1/2
Co3+
Co2+
Sat
2p1/2
Co3+
Sat
Co2+
Sat
780
790
800
810
780
790
800
810
Binding energy / eV
Binding energy / eV
Ni 2p
Ni0.5Co2.5O4
c)
Co 2p
2p3/2
d)
2p3/2
NiCo2O4
Ni3+
Co3+
Ni2+
Co2+
2p1/2
2p1/2
Ni3+
Sat
Co3+
Sat
Co2+
Ni2+
Sat
Sat
780
790
800
810
850
860
870
880
Binding energy / eV
Binding energy / eV
Ni 2p
e)
2p3/2
NiCo2O4
Ni3+
Ni2+
2p1/2
Sat
Ni2+
Ni3+
Sat
850
860
870
880
Binding energy / eV
Figure 5. High-resolution XPS spectra of Co 2p a) Co3O4, b) Ni0.5Co2.5O4, and c) NiCo2O4 and Ni 2p spectra for d) Ni0.5Co2.5O4 and e) NiCo2O4
6 | J. Name., 2012, 00, 1-3
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Electrochemical evaluation
activity performed at 1600 rpm as a reference (Fig. 6b), indicated
that the lowest performance was achieved by IrO2/C, which
could be attributed to the followed oxygen reduction pathway.
This comparison also showed that the bimetallic 3DOM
materials had excellent performance towards the ORR, reaching
current densities like that obtained when Pt/C (e.g., 5.11 vs. 5.25
mA cm−2) is used (Table S4). In terms of onset potential (Eonset),
the NiCo2O4 electrocatalyst had the most similar value to that
obtained with Pt/C (η= 80 mV) as catalyst, while the lowest
current density and Eonset were found for IrO2/C (Fig. 6b, Table
S3). In addition, is worth mentioning that reports have described
MTMOs as bifunctional catalysts that presented Eonset and
half‒wave potential (E1/2) values closed to those obtained by
using Pt/C. Nonetheless, these values were achieved using
specific supports, such as graphene or carbon nanotubes. In this
work, Vulcan carbon was used as the support, and despite this,
the herein presented 3DOM materials performed similar
electrocatalytic activities to those of the already reported
materials. Moreover, the 3DOM materials also presented
activities superior to those of other materials that were evaluated
using a Vulcan carbon or carbon black support (Table S5).
Concerning the Tafel plots (Fig. 6c), the NiCo2O4 spinel showed
a similar behavior to that obtained when Pt/C was used as
catalyst, presenting values of 97.3 and 96.6 mV Dec−1, which
indicated similar electrode kinetics. Moreover, the NiCo2O4
spinel yielded lower values than that obtained by using Co3O4
and Ni0.5Co2.5O4 materials. This improvement could be
attributed to the effect of the Ni3+ species in the NiCo2O4 spinel,
enabling a higher electronic conductivity and thus, a lower
resistance to charge transfer (Rct). Rct values were measured by
electrochemical impedance spectroscopy for 3DOM materials at
0.82 V during the ORR (Fig. S6), obtaining values of 2743
(Co3O4), 931 (Ni0.5Co2.5O4) and 413 (NiCo2O4) Ω cm2. The Rct
value obtained by NiCo2O4 was 4.8 and 2.2 times lower to that
obtained for Co3O4 and Ni0.5Co2.5O4, respectively. As can be
observed, the higher Ni content in the NiCo2O4 spinel enabled a
lower Rct, which could be contributing to the activity towards the
ORR. Finally, the stability of these materials was tested by
chronoamperometric tests for 10 h in the presence of constant O2
atmosphere (Fig. 6d). 3DOM materials showed a superior
stability compared with that obtained when was used Pt/C,
maintaining 80.5 % (NiCo2O4), 76.8 % (Ni0.5Co2.5O4) and 69.6
% (Co3O4) of their initial current, while Pt/C only retained 50.3
%. The improved stability of the 3DOM spinels could be related
to the 3DOM structure with well‒defined ordered pores and low
crystallite sizes.
The electrochemical profile of Co3O4 (Fig. S4, red line)
displayed three anodic peaks at 1.05 (labelled as I), 1.28 (II) and
1.54 V(III) vs. RHE. According to the literature, these peaks
could be attributed to the formation of several cobalt species such
as Co(OH)2, Co3O4, CoOOH and CoO2, which are stable in
alkaline solutions. During the backward scan, two peaks at 1.47
(IV) and 1.20 V vs. RHE(V) are observed and attributed to the
reduction of the as-formed oxide/hydroxides species. This
behavior could be related to the redox reaction between
Co2+/Co3+ and Co3+/Co4+ species,57,58 while peak VI is associated
with the ORR. The electrochemical profiles of Ni0.5Co2.5O4 (blue
line) and NiCo2O4 (green line) were similar, displaying two pairs
of oxidation/reduction peaks labelled as peaks I to IV. Peaks I
and II generated during the cathodic scan correspond to the
oxidation of Co and Ni species. On the other hand, the peak
generated during the anodic sweep (III) is attributed to the
reduction of the species generated during the cathodic sweep.
Finally, peak IV is associated with oxygen reduction.59–61
ORR/OER in half-cell. Concerning the electrocatalytic
evaluation of synthesized materials toward oxygen reduction,
Fig. 6a displays the linear sweep voltammetry curves at different
rotatory speeds of NiCo2O4, as representative electrocatalytic
material, while the electrocatalytic responses of the Co3O4 and
Ni0.5Co2.5O4 3DOM materials and benchmarked Pt/C and IrO2/C
are presented in Fig. S5. These five materials displayed increases
in current density with the rotatory speed, reaching stationary
currents during each speed. These materials presented
differences in onset potentials (Eonset) and current densities;
however, prior to this discussion, the number of transferred
electrons was obtained for determining the material effect on the
oxygen reduction pathway. For this purpose, Koutecky‒Levich
plots were obtained at three potentials and are shown as insets in
Figs. 6a and S5.
Koutecky‒Levich plots displayed in Figs. 6a and S5 also
include the theoretical descriptions of the 2e− and 4e− pathways.
NiCo2O4 electrocatalyst (Fig. 6a, inset) clearly displayed a 4e−
pathway, presenting an electron transfer number (n) of 3.9 e−. In
the case of the other 3DOM materials, Co3O4 yielded n=3.6 e−,
and Ni0.5Co2.5O4 showed n=3.7 e−. Pt/C exhibited the typical 4 e−
pathway, with an electron transfer number closed to 4e−;
however, IrO2/C had a tendency for both pathways evidenced by
a n=3.2 e−, suggesting that the formation of hydrogen peroxide
from the 2e− pathway could compromise the activity and stability
of this catalyst during the ORR. In fact, the comparison of the
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0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.35V
0.40V
0.45V
n= 4e-
n= 2e-
b)
a) 0
-1
Co3O4
0
-1
-2
-3
-4
-5
-6
Ni0.5Co2.5O4
NiCo2O4
Pt/C
IrO2/C
-2
0.06
0.07
0.08
0.09
0.10
0.11
0.12
0.13
 s1/2/rad1/2
)
-3
600 rpm
900 rpm
-4
1200 rpm
1600 rpm
2000 rpm
2500 rpm
-5
= 80 mV
=100 mV
=110 mV
=160 mV
-6
-7
0.2
0.4
0.6
0.8
1.0
0.2
0.4
0.6
0.8
1.0
1.2
Potential / V vs RHE
Potential / V vs RHE
Co3O4
d)100
90
c)1.05
1.00
Ni0.5Co2.5O4
NiCo2O4
80
Pt/C
0.95
IrO2/C
70
96.6 mV Dec-1
114.0 mV Dec-1
60
0.90
50
97.3 mV Dec-1
0.85
40
102.4 mV Dec-1
Co3O4
30
0.80
Ni0.5Co2.5O4
NiCo2O4
Pt/C
20
165.6 mV Dec-1
0.75
10
0.70
0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.0
5.0k
10.0k 15.0k 20.0k 25.0k 30.0k 35.0k
Log(Jk / mAcm-2)
Time / s
Figure 6. a) Linear voltammograms of NiCo2O4 at different rotation speeds and the K-L plot (inset). b) Linear voltammograms of the 3DOM spinels, Pt/C and IrO2/C for the ORR at
1600 rpm rotating speed. c) Tafel plots determined from the corresponding linear voltammetry data. d) Chronoamperometric curves for the ORR stability under O2-saturated 0.1 M
KOH for the 3DOM spinels and Pt/C.
Concerning the oxygen evolution reaction OER, the LSV curves kinetics. In addition, a higher activity was accompanied by the
(Fig. 7a) showed that the activity was greatly influenced by presence of Co3+ and Ni3+ species, because these species promote
nature/composition of the electrocatalyst. As expected, IrO2/C, the adsorption of OH− ions, generating species such as CoOOH
as the best catalyst for the reaction, which required a low and NiOOH that are responsible for the OER. 62–64
potential to achieve 10 mA cm−2 (1.63 V vs. RHE). However, The stability tests (Fig. 7c) were performed at the potential
NiCo2O4 3DOM spinel displayed a similar activity, needing only required to reach 10 mA cm−2 each material. The results showed
10 mV more than that overpotential performed by IrO2/C to that NiCo2O4 proved to be the most stable material for the OER,
achieve 10 mA cm−2. The bimetallic 3DOM spinel with a low Ni maintaining 100% of initial current. IrO2/C catalyst, despite
content (Ni0.5Co2.5O4) utilized 1.71 V to achieve this current, being considered as one of the most active catalysts, only
while the monometallic 3DOM spinel (Co3O4) and Pt/C required maintained 30.9% of initial current. Ni0.5Co2.5O4 catalyst kept
1.75 V and 1.82 V, respectively. Then, the activity trend for the 90% of initial current after 10 h, while Co3O4 kept 32 %. The
OER assumed: IrO2/C ≥ NiCo2O4 > Ni0.5Co2.5O4 > Co3O4 > Pt/C. higher stability of NiCo2O4 could be related not only the
The Tafel plots (Fig. 7b) were obtained to explain the activity morphology that allowed a better mass transport, also to the
changes. In this way, the Tafel slope for the NiCo2O4 3DOM presence of Ni3+ and Ni2+ species, which has been reported to
spinel was less (65 mV Dec−1) than that for the rest of the improve the conductivity and corrosion resistance64. ORR and
materials (Fig. 7b, inset values), indicating a faster reaction OER thermodynamically occurs at 1.23 V vs. RHE. However,
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98 mV Dec-1
Co3O4
a)70
b)1.75
Co3O4
120 mV Dec-1
135 mV Dec-1
Ni0.5Co2.5O4
NiCo2O4
Ni0.5Co2.5O4
1.70
1.65
1.60
1.55
1.50
1.45
1.40
60
50
40
30
20
10
0
66 mV Dec-1
100 mV Dec-1
NiCo2O4
Pt/C
IrO2/C
Pt/C
IrO2/C
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
1.0
1.2
1.4
1.6
1.8
2.0
Log (J / mA cm-2)
Potential / V vs RHE
14
12
10
8
c)
Co3O4
d)
Ni0.5Co2.5O4
NiCo2O4
100
80
60
40
20
0
Pt/C
IrO2/C
6
4
2
1.23 V
0
Co3O4
-2
-4
-6
Ni0.5Co2.5O4
NiCo2O4
IrO2/C
0.0
5.0k
10.0k 15.0k 20.0k 25.0k 30.0k 35.0k
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Potential / V vs RHE
Time / s
Figure 7. a) Electrocatalytic OER activity of the synthesized 3DOM spinels and their Pt/C and IrO2 counter-parts. b) Tafel plots determined from the corresponding OER linear
voltammetry. c) Chronoamperometric curves for OER stability in N2-saturated 0.1 M KOH for the 3DOM spinels and IrO2/C. d) Global polarization curves for the 3DOM spinels, Pt/C
and IrO2/C.
polarization losses are presented during the ORR and OER, and This decrease can be attributed to the presence of Ni3+ which is
because of this, the ΔE determination is employed to evaluate the known for its good electrical conductivity. The activity increase
viability of materials to be used as bifunctional electrocatalysts. during OER can be related to the presence of Ni2+ species, which
In this sense, ΔE can be calculated from the onset potential, or as are known to be responsible of the improvement in other reported
other authors have suggested, from the difference between the works, providing also electrochemical stability. The presence of
potential to achieve 10 mA cm−2 during the OER and the these species was corroborated by XPS results (Fig. 5), where
half‒wave potential during the ORR (Fig. 7d). A ΔE of 0.87 V NiCo2O4 showed a higher content of Ni2+ and Ni3+ than
was found for NiCo2O4, which was lower than that found for Ni0.5Co2.5O4. Thus, the improvement of activity and stability can
IrO2/C (0.93 V) and Pt/C (0.97 V). Moreover, this potential be attributed to these aspects.
difference was lower than that reported for many other highly Zn‒air battery evaluation. The excellent bifunctional
active bifunctional materials (Table S5). In summary, the behaviour of NiCo2O4 was the motivation to investigate its
incorporation of Ni species into the Co3O4 spinel produced performance in a secondary Zn‒air battery composed of 0.2 M
morphological changes, the BET surface area increased from ZnCl2 + 4 M KOH electrolytic solution, a Zn foil anode and the
39.6 m2 g−1 for Co3O4 to 46.8 m2 g−1 for NiCo2O4 (20 % higher). bifunctional air electrode (Fig. 8a), 4M KOH was used because
Additionally, the Ni incorporation also resulted in electronic at this concentration the highest current density was obtained
changes like the decrease of the resistance to charge transfer (Rct) (Fig. S7). The discharge plot (Fig. 8b) indicated that open-circuit
during ORR evaluation (Fig. S6). The Rct decreased from 2743 potentials of 1.44, 1.46 and 1.30 V were achieved by NiCo2O4,
(Co3O4) to 931 (Ni0.5Co2.5O4) and then to 413 Ω cm2 (NiCo2O4). Pt/C and IrO2/C, respectively. Pt/C displayed lower ohmic losses
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Journal Name
than the other materials, achieving a higher current density. The both reactions) was undertaken to study the catalyst durability
View Article Online
DOI: 10.1039/D0TA00874E
lower activity of IrO2/C to perform the ORR resulted in a (Fig. 8d, and S8b). For both processes, NiCo2O4 displayed 2 V
decrement of the maximum current density compared with that of initial charge potential and 1.2 V of discharge potential,
value obtained when were used the other materials. In this way, whereas after 150 cycles (approximately 21 h), the charge
the use of Pt/C resulted in a maximum power density of 109 mW potential increased to 2.1 V, while the discharge potential
cm−2, while the use of NiCo2O4 produced 101 mW cm−2 , which remained practically constant (1.14 V). In the cases of Pt/C only
was similar to the obtained by benchmarked Pt/C, and 24 mW operated for 21 cycles, IrO2/C operated for 130 cycles, and
cm−2 higher than that obtained by IrO2/C. The charge‒discharge mixture operated for 21 cycles attributed to the negative effect of
plot (Fig. 8c) indicated that the NiCo2O4 3DOM material Pt/C (Fig. S8b). According to this data, the inability of Pt/C to
possessed good reversibility at both low and high current perform the OER efficiently was attributed to low stability, while
densities. Considering 100 mA cm−2 as reference, NiCo2O4 the need for more energy to achieve 10 mA cm−2 during the ORR
exhibited 1.43 V of potential gap, while Pt/C presented a was the cause of the stability loss in IrO2/C. From these results,
potential gap of 1.5 V. Similar tests were made for the mixture the round-trip energy efficiency was calculated for NiCo2O4,
of benchmarked Pt/C + IrO2/C displaying good reversibility (Fig. displaying at the first cycle 57.2 % of efficiency, and it decreased
S8a). An accelerated charge‒discharge test at 10 mA cm−2 (for to 53.8 % at the final cycle.
NiCo2O4
b)1.75
a)
e-
e-
Pt/C
IrO2/C
100
80
60
40
20
0
1.50
1.25
1.00
0.75
0.50
0.25
0.00
Air
0
50
100
150
200
250
Current density / mA cm-2
NiCo2O4
NiCo2O4
d)
c)
2.5
Pt/C
IrO2/C
IrO2/C
Pt/C
2.5
2.0
1.5
1.0
0.5
0.0
2.0
1.5
1.0
0.5
0.0
10
20
30
40
50
60
70
80
90 100
0
25
50
75
100
125
150
Current density / mA cm-2
Cycle number
Figure 8. a) A schematic configuration of Zn-air battery. b) Polarization curves and their corresponding power densities for NiCo2O4, Pt/C and IrO2/C. c) Charging and discharging
polarization curves for the Zn-air battery using NiCo2O4, Pt/C and IrO2/C cathodes. d) Charge and discharge cycling curves of rechargeable Zn-air battery based on NiCo2O4, Pt/C and
IrO2/ cathodes at 10 mA cm–2 with a duration of 500 seconds per cycle and e) System configuration used to power the multi-sensor using the Zn-Air battery.
10 | J. Name., 2012, 00, 1-3
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Finally, the capability of the NiCo2O4 3DOM spinel to supply OER. These differences in stabilities were related to the
the energy requirements of a device was evaluated coupling this detachment of the catalytic ink instead of activity loss by
metal-air battery to a multi-sensor (Texas Instruments® model deactivation of active sites. During the Zn‒air battery tests, the
CC2650SensorTag, Fig. 8e). This device incorporates ten NiCo2O4 3DOM spinel enabled 1.44 V of open-circuit potential,
different low power sensors such as temperature, ambient light, which was like that obtained by using Pt/C (1.46 V). Moreover,
gyroscope and humidity, which are accessible from an Android because of its excellent activity as a bifunctional electrocatalyst,
system (as cellphone) through the simplelink SensorTag® app. the NiCo2O4 3DOM spinel displayed
a
superior
The battery was connected to the device multisensory through a charge‒discharge capability after 21 h, while Pt/C operated only
homemade voltage amplifier (Fig. 8e), which increase the input for 3 h. In summary, this work enlightens the possibility of using
voltage to an output voltage of 2.70 V. The initial voltage of the 3DOM structures in electrocatalysis, a field where this kind of
battery was 1.455V (Fig. 8e); when the device/Sensor is material has not been used exhaustively, especially for the ORR
connected to the metal-air battery, the voltage decreased to 1.279 and OER, and finally, confirms that 3DOM materials can be used
V due to energy demand of 10 mA of the sensor, finally the in practical applications, such as Zn-Air batteries.
system was connected via Bluetooth to the cell phone, with a
final voltage of 1.028 V. The system was operated continuously
by 2 h with high stability of the battery showing 1.405 V of final
Conflicts of interest
voltage, like the initial value voltage. These results demonstrated
that metal-air batteries based on 3DOM spinels electrocatalyst
materials can be used in practical application with high
performance and low cost.
There are no conflicts to declare.
Acknowledgements
The authors thank the Mexican Council for Science and
Technology (CONACYT) for their financial support through
projects CB-2015-01 Grant 256749, SENER 246079,
Laboratorios Nacionales 2019-299058. Also, the authors
acknowledge the TEM support provided by Cesar Cutberto
Leyva Porras from CIMAV-Nanotech.
Conclusions
This work demonstrated the feasibility to obtain different
3DOM spinels using a template of PMMA spheres with
diameters close to 160 nm, which allowed to obtain surface areas
from 39.6 m2 g−1 for Co3O4 to 46.6 ± 0.2 m2 g−1 for bimetallic
Ni0.5Co2.5O4 and NiCo2O4 3DOM spinels. The increase in
surface area displayed by the bimetallic spinels was attributed to
the decrease in particle size from 11.9 nm to 8.6 / 9.6 nm. The
multi‒valences of the Co and Ni species in the NiCo2O4 3DOM
spinel and a higher Ni content than that in the Ni0.5Co2.5O4
3DOM spinel improved the activity towards the oxygen
reduction / evolution reactions. NiCo2O4 3DOM spinel presented
a similar current density to that obtained when benchmarked
Pt/C was used as catalyst in the ORR (5.11 vs. 5.25 mA cm−2),
and only displayed 80 mV of overpotential. According to the
Tafel plots, this similarity in activities was related to the
electrode kinetics, as both materials followed a 4 e− pathway.
Moreover, the improved activity of NiCo2O4 in comparison with
that presented by the other 3DOM spinels was related to the
presence and higher content of Ni providing high electrical
conductivity to the material, enabling a decrease of the resistance
to charge transfer from 2743 Ω cm−2 for Co3O4 to 413 Ω cm−2
for NiCo2O4 3DOM spinel. In a similar way, the NiCo2O4
3DOM spinel required only 10 mV more energy than a
benchmarked IrO2/C electrocatalyst to achieve 10 mAcm−2
during the OER. Furthermore, the 3DOM spinel displayed a
superior stability for both the ORR and OER, maintaining 80.5
% of initial current after 10 h of the ORR and 100 % during the
Notes and references
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Page 13 of 13
Journal of Materials Chemistry A
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DOI: 10.1039/D0TA00874E
Table of Contents Entry
A rechargeable Zn-air battery was assembled using NiCo2O4 three-dimensionally ordered
macroporous (3DOM) spinel displaying an open circuit potential of 1.44 V, a power density of
101 mW cm−2, and a high durability during charge/discharge cycling curves (being operated for
21 h). In addition, the resulting device can supply the energy demand of a remote multi-sensor.