The influence of ruthenium substitution in LaCoO3towards bi-functional electrocatalytic activity for rechargeable Zn-air batteries

Article abstract of DOI:10.1039/d0ta06673g

The rechargeable zinc-air battery is a clean technology for energy storage applications but is impeded by the slow kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) during its cycling. Herein, a series of lanthanum cobaltate based perovskites are synthesised with the B-site cation deficiencies in the structure occupied by Ru substitution: LaCo1-xRuxO3-δ (x = 0, 0.1, 0.2, 0.3 and 0.5). These compositions were designed to enhance the OER/ORR activities, which are two vital reactions for rechargeable Zn-air batteries. Powder X-ray diffraction analysis revealed that increasing the Ru substitution >20% (x > 0.2) alters the LaCoO3 crystal structure from rhombohedral to orthorhombic. Photoelectron spectroscopy studies reveal that the surface oxygen vacancies increased in the Ru substituted catalyst, a property important for enhancing the OER/ORR efficiency. The LaCo0.8Ru0.2O3-δ (LCRO82) catalyst exhibits promising electrocatalytic activities in both the OER and the ORR in 0.1 M KOH solution. Furthermore, the LCRO82 catalyst was evaluated as a cathode for rechargeable Zn-air battery applications displaying a high power density of 136 mW cm-2 at a current density of 175 mA cm-2 and a stable charge-discharge voltage gap of 0.78 V after 1440 cycles, with excellent cycling stability over 240 h.

Full text of DOI:10.1039/d0ta06673g

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DOI: 10.1039/D0TA06673G
ARTICLE
Influence of Ruthenium Substitution in LaCoO towards Bi-
3
functional Electrocatalytic activity for Rechargeable Zn-Air Battery
Received 00th January 20xx,
Accepted 00th January 20xx
a,b,c
a,b
c
a,b
Shivaraju Guddehalli Chandrappa, Prabu Moni, Dehong Chen, Guruprakash Karkera,
a,b
c
a,b
Kunkanadu R Prakasha, Rachel A. Caruso and Annigere S. Prakash*
DOI: 10.1039/x0xx00000x
The rechargeable Zinc-air battery is a clean technology for energy storage applications but is impeded by the slow kinetics
of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) during its cycling. Herein, a series of
lanthanum cobaltate based perovskites are synthesised with the B-site cation deficiencies in the structure occupied by Ru
substitution: LaCo1-xRu
activities, which are two vital reactions for the rechargeable Zn-air batteries. The powder x-ray diffraction analysis
revealed that increasing the Ru substitution >20 % (x > 0.2) alters the LaCoO crystal structure from rhombohedral to
orthorhombic. The photoelectron spectroscopy studies reveals that the surface oxygen vacancies increased in the Ru
substituted catalyst, a property important for enhancing the OER/ORR efficiency. The LaCo0.8Ru0.2 3-δ (LCRO82) catalyst
x
O3-δ (x = 0, 0.1, 0.2, 0.3 and 0.5). These compositions were designed to enhance the OER/ORR
3
O
exhibits a promising electrocatalytic activities in both OER and ORR reactions in 0.1 M KOH solution. Further, the LCRO82
catalyst was evaluated as a cathode for rechargeable Zn-air battery application displaying high power density of 136 mW
cm−2 at a current density of 175 mA cm-2 and a stable charge-discharge voltage gap of 0.78 V after 1440 cycles, with
excellent cycling stability over 240 h.
Thus, a wide range of active and stable non-precious metal, bi-
1
1–15
functional electrocatalysts including transition-metal oxides,
Introduction
1
6,17
18,19
metal alloys,
and metal-free materials
have been studied for
application in rechargeable Zn-air batteries.
With the ever-increasing energy demand and depletion of fossil
fuels on the earth, the development of clean and sustainable
electrochemical energy systems that can store renewable energy
through efficient, environmental-friendly and cost-effective ways is
Perovskites have been widely explored owing to their remarkable
electrocatalytic activities, Zn-air battery performances and stability
1
3,20,21
1
–3
in the aqueous electrolytes.
chemical formula of ABO , where A is a rare earth metal or alkaline
earth element and B is a transition element (TM = Co, Ni, Mn, Fe). In
particular, LaCoO has been widely employed for electrocatalytic
Unfortunately, single electrocatalytic activity and
poor intrinsic property of LaCoO limit its application as an air-
Perovskite oxides have the
paramount.
Rechargeable metal-air (M = Li, Al and Zn, etc.)
batteries are deemed to be the most propitious candidates for
sustainable storage devices because of their higher specific energy
3
4
,5
3
densities than present lithium-ion batteries. Among the various
metal-air batteries, the rechargeable Zn-air system has attracted
intensive attention concerning electrochemical energy storage
owing to its reasonably high theoretical specific energy density
2
2–24
applications.
3
cathode in rechargeable Zn-air battery. Interestingly, the partial
A/B-site substitution of perovskite oxides with other transition
metal is an effective strategy to tailor the intrinsic properties like
−
1
−1
(
1086 Wh kg ), volumetric energy density (6136 Wh L ), the high
6
,7
abundance of zinc resources and relatively low cost.
Yet,
g
oxygen deficiency, a near unity (σ*-antibonding) e orbital filling in a
rechargeable Zn-air batteries are still suffering from low energy
efficiency, charge-discharge polarization and cycling instability on
account of the slow kinetics of the oxygen evolution reaction and
transition metal cation, closer Co 3d-O 2p-band center relative to
the Fermi level and higher covalency between transition metal and
oxygen bond, which are responsible for enhancing the bi-functional
8
–10
oxygen reduction reaction (ORR/OER) on the air electrode side.
2
5–29
(OER/ORR) activity of perovskite.
Also, the presence of mixed-
valence states of the B-site cation and oxygen vacancies provides
a. CSIR - Central Electrochemical Research Institute-Chennai Unit, CSIR Madras
2+
sites for migration of the charge carrier, electron or metal ion (Zn
Complex, Taramani, Chennai- 600113, India
+
12,30–
or Li ),during charging and discharging of the metal-air battery.
b.
Academy of Scientific and Innovative Research (AcSIR), Ghaziabad- 201002, India.
Applied Chemistry and Environmental Science, School of Science, RMIT University,
Melbourne, Victoria 3000, Australia.
3
2
Based on correlation between transition metal 3d-O 2p covalency
c.
g
and e electron filling on electrocatalytic activity of perovskites,
Email: prakash@cecri.res.in, prakash.as@gmail.com,
Electronic Supplementary Information (ESI) available: [Rietveld refined XRD data,
XPS binding energy data, Oxygen vacancy calculation, K-L plots, OER graph and
several investigations were proposed to enhance the
electrocatalytic activity in A/B-site substituted perovskites. For
3
3
example, Duan et al. reported Fe substituted in the B-site LaCoO
for OER application. They found that 10% Fe substituted LaCoO
3
Electrocatalytic
and
Zn-air
battery
comparison
table].
See
DOI: 10.1039/x0xx00000x
3
3
(LaCo0.9Fe0.1O ) owns increased overlap between Co 3d and O 2p
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Journal of Materials Chemistry A
Page 2 of 9
ARTICLE
Journal Name
states, which results in enhancement of covalency and optimal e
g
cathode for the rechargeable Zn-air battery, exhibVitieinwgArtaiclesOtanblinlee
charge-voltage gap even after 1440 cycles anDdOdI:i1s0c.h1a0r3g9e/Do0vTeAr02646073hG.
3
4
configuration and promoted OER activity. Similarly, Sun et al.
introduced Mn in the B-site of LaCoO perovskite lattice, which
helps to tailoring the Co 3d-O 2p covalency and the e electron
filling, promoting the OER reaction kinetics and Shen et al.
3
g
Experimental Section
3
5
3
+
2+
reported the partial substitution of La for Sr in LaFeO
3
increases
Materials
the OER activity. The authors attributed the enhanced activity to
4
+
presence of Fe -induced hole states in La1-xSr
x
FeO
3
and Lanthanum nitrate hexahydrate (La(NO ) ·6H O, 99.9% Alfa Aesar),
3
3
2
modification in the Fe 3d-O 2p covalency. On the other hand, many cobalt nitrate hexahydrate (Co(NO ) ·6H O, 97.7% Alfa Aesar)
3
2
2
studies were focused to tuning the B-site cation valance state and ruthenium trichloride trihydrate (RuCl .3H O, 30-40% Ru, SRL,
3
2
oxygen vacancies in the structure to boost the electrocatalytic India), ethylene glycol (AR, 99 %, SRL, India), citric acid anhydrous
3
6
3+
2+
activity. For instance, Wang et al. studied the redox pairs, Ni /Ni
and Mn /Mn and the oxygen vacancies in La0.8Sr0.2MnO
(AR, 99 %, SRL, India), KOH (AR, 99 %, SRL, India), Nafion ionomer
4
+
3+
3
by (Sigma Aldrich), ethanol, Milli-Q water, Pt-Ru/C with 40 wt. % Pt
substituting the Ni in the Mn site, which helps to modify the B site and 20 wt. % Ru on Vulcan XC-72R carbon (Quintech, Germany), Zn
M-O bond and the surface configuration and therefore promotes (99.95 %, Goodfellow GmbH, Germany), gas diffusion layer carbon
3
7
the bi-functional activity. Similarly, Vignesh et al. investigated the electrode (SIGRACET GDL 38 BC).
Ni substitution of the Co-site in the LaCo0.97 3−δ perovskites, which
resulted in the oxygen vacancy and redox pair Ni /Ni and Perovskite preparation
O
3
+
2+
2
+
3+
38
Co /Co , thereby enhancing OER activity. Abreu-Sepulveda et al.
3
9
and Malkhandi et al. introduced the various changes including the The LaCo1−xRu
electronic structure, oxygen vacancies as well as the oxidation state oxides denoted as LCO, LCRO91, LCRO82, LCRO73 and LCRO55
of Co in the La0.6Ca0.4CoO
structure by means of substituting the Fe respectively were synthesized through a modified citrate sol-gel
and Mn respectively in the Co site to achieve higher OER activity. method (Pechini method).
Liu et al. engineered abundant oxygen vacancies and also mixed La(NO
valence of Co /Co and Mn /Mn in LaMnO
x
O3-δ where x = 0, 0.1, 0.2, 0.3 and 0.5 perovskite
3
4
5
The stoichiometric amounts of
O (99.9 % Alfa Aesar), Co(NO ·6H O (97.7 % Alfa
perovskite by Aesar) and RuCl .3H O (30-40 % Ru, SRL), were dissolved in 50 mL of
4
0
3
)
3
·6H
2
3
)
2
2
2
+
3+
3+
4+
3
3
2
replacing Mn in part with Co to realize higher bi-functional activity. ethylene glycol together with citric acid (citric acid/metal = 1:2.5
These results show that the partial substitution of the A or B-site ratio). Then, the mixture was heated at 120 °C with continuous
cation in the perovskite with a transition metal atom could stirring for 12 h. The obtained gel was heated in a preheated muffle
favourably alter the electronic structure, bond strength, oxygen furnace at 350 °C for 2 h to decompose the nitrates. Finally, the
vacancies and synergistic redox couple interaction that induce obtained product was ground and calcined at 800 °C for 4 h.
higher bi-functional catalytic activities for rechargeable metal-air
battery applications.
Material characterization
The powder x-ray diffraction (XRD) data of the samples was
Furthermore, The OER activity as a function of oxygen binding
collected on a Bruker D8 Advance Da Vinci diffractometer using
energy resulted in Volcano type plot has been reported based on
CuKα (λ= 1.5418 Å) radiation. The morphological features were
observed on a Carl Zeiss 130 VP Field Emission Scanning Electron
the DFT calculations. These results show, Ru and Ir catalysts are the
closet to the top of the volcano. It also predicted that Ru and Ir
Microscope (FESEM) and a Technai-20 G2 transmission electron
based catalysts have optimal balance in the surface-oxygen
microscope (TEM). The oxidation state of the samples was
interaction energies as the activity-controlling parameter.
determined from X-ray Photoelectron Spectroscopy (XPS) using a
Therefore, Ru and Ir based catalysts are currently best OER catalysts
Thermo Fischer K-Alpha™ Spectrometer with non-monochromatic
to provide enhanced catalytic activity. To balance the cost and the
Al Kα radiation (1486.7 eV). The C 1s peak of the adventitious
activity, Ru is the least expensive platinum group metal and appears
to be an ideal choice 41–44 and in the current scenario, the most
trend setting state of the art catalysts Pt-Ru/C with 40 wt. % Pt and
carbon (284.8 eV) was used as the reference binding energy for
calibration. The oxygen non-stoichiometry (휹) values were obtained
from iodometric titration.
2
0 wt. % Ru on Vulcan XC-72R carbon was used as a bi-functional
catalyst owing to their renowned ORR-OER activity; though, the
scarcity pricing, derisory reserve and destitute electrochemical
stability of the Pt-Ru/C catalyst have considerably limited its
substantial applications. Nevertheless, new type of catalyst is
essential to improve the catalytic activity along with cost
Electrochemical activity tests
Rotating-disc electrode experiments were carried out in a PINE RDE
setup with glassy carbon as the working electrode, a Pt wire as the
effectiveness and stability issues. One among them is to reduce the counter electrode and an Ag/AgCl (sat. with KCl) electrode with a
Pt, Ru loading and use highly active catalyst support without
affecting the performance.
3
KNO saturated salt bridge as the reference electrode as the
working electrode. The catalyst ink was prepared by mixing 5 mg
oxide catalyst and 1 mg Valcan-XC72 carbon in a mixture of 100 μL
of Nafion ionomer, 600 μL of ethanol and 300 μL mQ water. The 10
μL dispersed catalyst ink was coated on the glassy carbon (loading =
In this context, substitution of Ru into LaCoO
3
x
(LaCo1-xRu O3-δ, x = 0,
0
.1, 0.2, 0.3 and 0.5) perovskite is reported here. Among these, the
LaCo0.8Ru0.2O3-δ catalyst shows superior bi-functional activity in an
-2
aqueous alkaline electrolyte medium, which can be attributed to 0.204 mg cm ). The bi-functional activity (ORR-OER) studies of the
2
+
3+
3+
4+
the mixed oxidation state of B-site cations (Co /Co and Ru /Ru )
and oxygen vacancies present in the structure. In addition, the
2 2
catalysts were carried out in both N and O saturated aqueous
3 δ
LaCo0.8Ru0.2O - catalyst has been demonstrated to function as a
2
| J. Name., 2012, 00, 1-3
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Fig. 1 Rietveld refined powder XRD patterns of the a) LCO, b) LCRO82, c) LCRO73 and d) LCRO55 phases. Experimental data
points (red), calculated patterns (black), their differences (green) and Bragg diffraction positions (blue ticks for R/3c and
#
purple sticks for Pbnm space group) are presented. (The diffraction peaks marked with ‘*’, ‘ ’ corresponds to RuO
2
[JCPDS
No.: 01-070-2662] and La
2 3
O [JCPDS No.:00-022-0369] respectively).
0
.1 M KOH solutions. The potentials recorded and referred to phase with an R/3c space group. The progressive substitution of Co
by Ru results in a broadening of the Bragg peaks, which indicates a
decrease in crystal size and substitution is inferred by a shift to
lower angles in the peaks. The LCRO73 diffraction pattern has been
refined with two phases having a rhombohedral structure (space
group: R/3c) and orthorhombic structure (space group: Pbnm) with
the ratio between the phases being 55:45. The LCRO55 diffraction
pattern could be refined using an orthorhombic structure with
Pbnm space group. Thus, with increasing Ru substitution, the crystal
structure alters from rhombohedral to orthorhombic. These results
are consistent with other reported B-site substituted perovskite
Ag/AgCl in each experiment were calculated using the formula
E
RHE = EAg/AgCl + 0.059 pH + 0.198 V
Where, ERHE is the potential vs. reversible hydrogen electrode (RHE),
Ag/AgCl is the potential vs. Ag/AgCl electrode, and pH is the pH value
E
of electrolyte.
Zn-air battery fabrication and testing
The Zn-air battery studies of the prepared lanthanum-based
3
3,46
materials.
The obtained crystallographic parameters are
summarized in Table S1.
catalysts were carried out in
a homemade cell with an
FESEM images of the prepared catalysts are displayed in Fig. 2(a-d).
It is seen that there is a significant change in the morphology and
grain size in the bare and Ru substituted catalyst. The particles are
smaller for LCRO82, LCRO73 could be slightly bigger than LCO and
LCRO55 are significantly bigger. The elemental mapping of the
LCRO82 sample, confirming the uniform distribution of ruthenium
on the surface (Fig. 2). The corresponding Energy Dispersive X-ray
Analysis (EDAX) profile is also given in Fig. 2(e). The TEM
micrographs of LCO and LCRO82 are presented in Fig. S1. Both LCO
and LCRO82 samples exhibit an interconnected agglomerated
structure. It should be noted that the LCO sample shows larger
particle size about (80-100 nm) than the LCRO82 sample (40-60
nm), these observations are consistent with FESEM results. The
relatively smaller crystallite size for the 20% Ru substituted LaCoO3
samples (LCRO82), could be due to the fact that up to 20 % Ru, it
retained the rhombohedral phase whereas beyond 20 % it changed
to orthorhombic phase. These structural changes are found to be
influencing the particle size too. Lower the particle size higher will
electrochemical workstation (Biologic, VSP/VMP 3B-20, France) at
room temperature by galvanostatic and galvanodynamic
techniques. The polished zinc foil, 6 M KOH solution, Whatman
glass microfiber separator and the prepared catalyst coated on the
gas diffusion layer were used as anode, electrolyte, separator and
cathode, respectively. The air-breathing cathode was prepared by
coating the slurry mixture of 1 mg of catalyst (LCO or LCRO82) in
ethanol, 1 mL of Milli-Q water and 0.11 mg of 5 wt% Nafion
ionomer solution on gas diffusion layer (GDL) carbon electrode and
then the coated electrode was dried in a vacuum oven overnight at
TM
8
0 °C.
Results and discussion
x
The Rietveld refined XRD patterns of the LaCo1-xRu O3-δ (x = 0.0, 0.2,
0
.3 and 0.5) powder samples are presented in Fig. 1. The LCO and
LCRO82 diffraction patterns were refined using a rhombohedral
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DOI: 10.1039/D0TA06673G
Fig. 2(a) FESEM images of (a) LCO, (b) LCRO82, (c) LCRO73, (d) LCR055 (e) EDAX profile of LCRO82 and elemental mapping of
LCRO82 for Mix, La, Co and Ru, respectively.
be the activity due to increased surface area and oxygen vacancies. The O 1s XPS spectra of the LCO and LCRO82 catalysts are
2
9,47
49–51
2-
Further details of oxygen vacancies and oxidation states of the deconvoluted into four components,
lattice oxygen species (O
/O), hydroxyl groups or the
) and adsorbed molecular water
2
-
materials were obtained with X-ray photoelectron spectroscopy ), highly oxidative oxygen species (O
2
-
(XPS) studies. Fig. 3(a-b) shows the survey spectra of LCO and surface adsorbed oxygen (OH /O
2
LCRO82, respectively. Both display the presence of La 3d, Co 2p, O (H
2
O) at 528.8 eV, 530.2 eV, 531.3 eV and 533.3 eV respectively
1
s, and C 1s, and LCRO82 has additional Ru 3p and Ru 3d peaks. The (Fig. 3(e-f)). The relative area values of all deconvoluted peaks are
2
-
-
corresponding binding energy values are listed in Table S3, given in Table S5. The oxidative oxygen species (O
2
/O ) correlate
confirming that Ru substitution was successful. Fig. 3(c-d) reveals to surface oxygen vacancies, which are favourable for catalysing
5
2,53
the Co 2p spectra of LCO and LCRO82, which split into two spin- OER/ORR activity.
orbit doublets around 779 eV (2p3/2) and 795 eV (2p1/2), indicating The oxygen non-stoichiometry 휹 values obtained from iodometric
2
+
3+
the presence of mixed oxidation states of Co and Co in both the titration of LCO and LCRO82 were 0.23 and 0.31 respectively (Table
2
+
3+
catalysts. Table S4 presents the calculated Co /Co ratios obtained S2), which further confirms more oxygen vacancies in the LCRO82
2
+
3+
54
2-
-
from the area of fitted peaks, which shows that the Co /Co ratio sample. The LCRO82 exhibits a higher O
is higher for LCRO82 (1.50) than that of LCO (1.01).
2
/O species ratio than
4
6,48
30,55,56
LCO which implies surface oxygen vacancies.
Fig. 3 XPS spectra of LCO and LCRO82. (a) Survey scan of LCO, (b) survey scan of LCRO82, (c-d) Co 2p region, (e-f) O1s region of
LCO and LCRO82, respectively and (g) C 1s for LCO, and (h) for Ru 3d and C 1s region of LCRO82 catalyst.
4
| J. Name., 2012, 00, 1-3
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Fig. 4 Bi-functional ORR/OER activities of LaCo1−xRu
x
O
3-δ (x=0, 0.2, 0.3 and 0.5) in O
mV s , (b) LSV curves of LCO catalyst at different rotation rpm (400 -2200 rpm), (c) LSV curves of LCRO82 catalyst at different rotation
rpm (400-2200 rpm), (d) comparison of bi-functional activities of LaCo1−xRu 3-δ (x=0, 0.1, 0.2, 0.3 and 0.5) and Pt, Ru/C catalysts, (e)
2
-saturated 0.1 M KOH electrolyte (a) CV curves at 50
-1
x
O
OER and ORR overpotential of LaCo1−xRu
IR corrected).
x
O
3-δ (x=0.1, 0.2, 0.3 and 0.5) and Pt, Ru/C catalysts, and (f)OER Tafel slopes of all the catalysts
(
The oxygen evolution reaction (OER) activities (Fig. S2(b)) of
x
LaCo1−xRu O3-δ (x=0, 0.1, 0.2, 0.3 and 0.5) and commercial Pt, Ru/C
From these results it is concluded that Ru substitution (LCRO82)
creates oxygen vacancies in the structure, these observations
catalysts were further examined by LSV experiments in 0.1 M KOH
solution at 1600 rpm. The LCRO82 catalyst exhibits superior OER
2
+
3+
correlate with the higher Co /Co ratio and smaller particle size
for LCRO82, which may have a great impact on the electrocatalytic
activity and are consistent with other reported B-site cation
-2
activity (1.69 V vs RHE at a current density of 10 mA cm ) than that
of LCO, LCRO73, LCRO55 and commercial Pt, Ru/C catalysts. Fig.
4
0
x
(d) shows the bi-functional activity of LaCo1−xRu O3-δ (x= 0, 0.1, 0.2,
.3 and 0.5) and commercial Pt,Ru/C catalysts.
5
7,58
substituted perovskites.
The electrocatalytic activities of the prepared LaCo1−xRu
.1, 0.2, 0.3 and 0.5) catalysts towards oxygen redox were assessed
by cyclic voltammetry (CV) and rotating disc electrode (RDE)
measurements in O and N -saturated 0.1 M KOH solution. Fig. 4(a)
shows typical cyclic voltammograms of LCO and LCRO82 measured
at a scan rate of 50 mV s . The LCRO82 catalyst has an ORR onset
potential of 0.73 V vs. the reversible hydrogen electrode (RHE),
which is more positive than that of LCO, 0.63 V. Fig. 4(b-c) illustrate
x
O3-δ (x=0,
Table. 1. Summary of the half-cell testing results for the
LaCo (x =0, 0.1, 0.2, 0.3 and 0.5) and Pt, Ru/C.
0
x
3 δ
Ru1-xO -
2
2
Catalyst
OER performance
ORR performance
-
1
-2
imax (mA cm-2)
@ E = 0.3 V
E (V)
@ i = 10
mA cm
imax (mA cm )
@ E = 1.9 V
vs. RHE
E (V)
@ i = -3
mAcm
-2
-2
vs. RHE
2
the linear sweep voltammograms (LSV) of LCO and LCRO82 in O -
saturated 0.1 M KOH solution at different rotation speeds. With the
increasing rotation speed, the current densities of both the
catalysts gradually increase, due to the faster oxygen flux and
X=0
-
3.0
16.8
24.0
15.2
13.7
-
0.16
0.53
0.63
0.41
0.32
0.72
-2.32
-3.8
5
9
shorter diffusion length on the electrode surface. Fig. S2(a) shows
the Koutecky-Levich (K-L) plot of LCRO82 at a potential range of 0.3
to 0.4 V. The plot has characteristic linearity and parallelism,
X=0.1
X=0.2
X=0.3
X=0.5
Pt, Ru/C
1.80
1.69
1.81
1.83
1.75
-4.75
-4.32
-3.11
-3.7
4
7
suggesting first-order kinetics with respect to dissolved oxygen.
From the slope obtained from the K-L plot, the number of electrons
transferred for overall ORR reaction is found to be ~ 4, indicating
reduction to water.
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DOI: 10.1039/D0TA06673G
Fig. 5 Zn–air battery performance of LCO, LCRO82 and Pt, Ru/C catalysts. (a), (b) The deep discharge curve and specific capacity
2
of the primary Zn-air battery studies at current density of 5 mA/cm , (c) discharge polarization and power density curves, (d) rate
2
capability test, (e) rechargeable Zn-air battery cycling data at a current density 5 mA/cm in short interval (10 min per cycle), (f)
Nyquist plots (inset show EIS fitted equivalent circuit) and (g) galvanostatic charge-discharge cyclic stability test .
The bi-functional activities are calculated by taking the potential (x=0, 0.1, 0.2, 0.3 and 0.5) and commercial Pt, Ru/C electrocatalysts
difference between OER/ORR activity at benchmark current density during OER activity, the Tafel slopes of these samples were
values (ORR= -3 mA cm-2 and OER = 10 mA cm ). The calculated compared (Fig 4f). The LCRO82 catalyst displays the Tafel slope of
-2
60
−1
bi-functional activities of Ru substituted catalysts are in the about 43 mV dec , which is lower than that of LCRO91 (61 mV
−
1
−1
−1
following order LCRO82 (1.06 V) >LCRO91 (1.27 V) > LCRO73 (1.4 V) dec ) LCRO73 (66 mV dec ) LCRO55 (68 mV dec ) Pt,Ru/C (128
>
−1
−1
LCRO55 (1.51 V) as shown in Fig 4(e). The summary of bi- mV dec ) and LCO (167 mV dec ). These results reveal that the
functional half-cell data of these samples is shown in Table 1. The faster OER reaction kinetics as well as the enhanced OER behavior
LCRO82 catalyst also shows a substantial oxygen electrode activity of LCRO82 catalyst.
and is compared to other reported perovskites in Table S7.
Furthermore, to understand the reaction kinetics of LaCo1−xRu
x
O
3-δ
x 3-δ
Based on the electrocatalytic studies of the LaCo1−xRu O
compositions, LCRO82 has shown superior bi-functional activity
6
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compared to the other Ru substituted catalysts. Thus, LCO and LaCo0.8Ru0.2O3-δ demonstrates itself as a promising candidate for the
View Article Online
LCRO82 have been chosen as cathode candidates for Zn-air battery rechargeable Zn-air batteries.
application and compared with the state-of-the-art Pt, Ru/C
DOI: 10.1039/D0TA06673G
catalyst. Fig. 5(a-b) presents primary Zn-air battery studies for LCO,
LCRO82 and Pt, Ru/C. The open-circuit voltage of LCO and LCRO82
Conclusions
(
1.28 V) are lower than that of Pt, Ru/C (1.38 V). LCRO82 exhibited a
-1
,
higher specific capacity and stability (433 mAh g discharged over
3
The B-site substitution of LaCoO
.2, 0.3 and 0.5) provides a highly efficient non-precious metal bi-
functional catalyst for rechargeable Zn-air battery application in
alkaline media. The LaCo0.8Ru0.2 (LCRO82) catalyst exhibits
3 x
with Ru (LaCo1-xRu O3-δ x = 0, 0.1,
3 h) than LCO (304 mAh g-1
discharged over 23 h), whereas
,
0
commercial Pt, Ru/C discharged over 43 h and delivered the highest
-1
discharge capacity of 551 mAh g . Fig. 5(c) displays the discharge
polarization and power density curves of Zn-air battery using LCO,
LCRO82 and Pt, Ru/C cathode catalyst. The LCRO82 catalyst delivers
3 δ
O -
promising electrocatalytic activities in both OER and ORR reactions
in 0.1 M KOH solution with smaller OER Tafel slope of 43 mV/dec
and 4 electron transfer of reduction to water. The rechargeable Zn-
air battery studies using the LCRO82 catalyst reveal superior charge
and discharge potential and improved round trip efficiency than the
LCO catalyst and comparable to state-of-art Pt,Ru/C catalyst.
Moreover, the mechanically rechargeable Zn-air battery containing
the LCRO82 catalyst shows a stable charge-discharge voltage gap
even after 1400 cycles with excellent cycling stability for 240 h.
−2
a peak power density of 136 mW cm at the current density of 175
-2
mA cm , which is much better than that of the LCO catalyst (20 mW
−
2
-2
cm at a current density of 105 mA cm ) and is closer to the Pt,
−2
-2
Ru/C catalyst (178 mW cm at current density of 200 mA cm )
respectively. Fig. 5(d) shows current step down and step up
discharge rate capability test for the LCO, LCRO82 and Pt, Ru/C
catalysts with different current densities (1-30 mA cm ) at 5 min
discharge. Fig. 5(e) presents typical galvanostatic charge-discharge
cyclic stability tests for the secondary Zn-air battery at a current
density of 5 mA cm . The charge and discharge cycling stability of
the LCO and LCRO82 shows a comparable voltage gap, higher than
that of Pt, Ru/C for the initial few cycles. After 180 cycles, LCRO82
shows a lower charge-discharge gap of 0.78 V with a higher round-
trip efficiency of 60 %, which is a lower than that of LCO (0.82 V,
-2
These results strongly suggest that LaCo0.8Ru0.2O3-δ (LCRO82) can be
used as an alternative efficient non-precious metal bi-functional
catalyst for Zn-air battery applications.
-2
Conflicts of interest
round-trip efficiency 59 %) and comparable to Pt, Ru/C (0.78 V, There are no conflicts to declare
round-trip efficiency 60 %). Thus, Ru substituted perovskite is a
highly promising catalyst for rechargeable Zn-air battery
applications, showing higher cycling stability with prominent round-
trip efficiency. To further understand the internal resistance of the
constructed Zn-air battery containing LCO, LCRO82 and Pt, Ru/C,
electrochemical impedance spectroscopy (EIS) studies were
conducted (Fig. 5(f)). The impedance data are fitted using an
Acknowledgements
Shivaraju G. C. acknowledges the CSIR-UGC-SRF for granting a
Fellowship, Dr. Prabu Moni grateful to the Department of Science
and Technology (DST), New Delhi, India for awarding INSPIRE
equivalent circuit and details are given in supporting information Faculty Award (DST/INSPIRE/04/2016/000530), the authors thank
Fig S3 and table S6). The R(QR)(QR)W equivalent circuit is used for the Central Instrumentation Facility, CSIR-CECRI Karaikudi, for
(
fitting of impedance data, where R implies the resistance
contributions of different components of cell: solution resistance
providing characterization facilities.
(
Rs), solid-electrolyte interface resistance (R
resistance (R )) and Q is the constant phase elements (CPE).
Warburg diffusion models cannot be related to M-air batteries as
concentrations tend to vary point to point within cell
environments. Thus, W is attributed to the mass transfer of oxygen
1
), charge transfer
2
Notes and references
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2
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