of 12
Research
article
Sacrificial
MOF-derived
MnNi
hydroxide
for high
energy
storage
supercapacitor
electrodes
via DFT-based
quantum
capacitance
study
Elahe
Torabi
a
, Amir
Kazemi
a
,
b
, Mohsen
Tamtaji
c
, Faranak
Manteghi
a
,
*
,
Sohrab
Rohani
b
, William
A. Goddard
d
a
Research
Laboratory
of Inorganic
Chemistry
and Environment,
Department
of Chemistry,
Iran University
of Science
and Technology,
16846-13114,
Tehran,
Iran
b
Department
of Chemical
and Biochemical
Engineering,
Western
University,
London,
ON, N6A
5B9 Canada
c
Department
of Chemical
and Petroleum
Engineering,
Sharif
University
of Technology,
Tehran,
Iran
d
Materials
and Process
Simulation
Center,
California
Institute
of Technology,
Pasadena,
CA, 91125,
USA
ARTICLE
INFO
Keywords:
MOF-74
Bimetallic
hydroxide
Supercapacitor
DFT calculations
ABSTRACT
Electrochemical
energy
storage
plays
a critical
role in the transition
to clean
energy.
With
the
growing
demand
for efficient
and sustainable
energy
solutions,
supercapacitors
have
gained
significant
attention
due to their
high
specific
capacitance,
rapid
charge/discharge
capabilities,
long
lifespan,
safe operation
across
various
temperatures,
and minimal
maintenance
needs.
This
study
introduces
a novel
approach
for the synthesis
of high-performance
supercapacitor
elec
-
trodes
by using
MnNi-MOF-74
as a precursor.
Bimetallic
Mn(OH)
/Ni(OH)
hydroxides
(MnNi-x,
where
x
=
2, 6, 12) with
tailored
morphologies
were
successfully
fabricated
by treating
MnNi-
MOF-74
anchored
on nickel
foam
with
different
concentrations
of KOH.
Among
the various
synthesized
samples,
MnNi-6
exhibited
the best performance,
with
a remarkable
specific
capac
-
itance
of 4031.51
mF cm
2
at 2 mA cm
2
, attributed
to its high
surface
area
of 186 m
2
/g, opti
-
mized
particle
size,
and abundant
micropores.
Furthermore,
MnNi-6
demonstrated
exceptional
thermal
stability,
positioning
it as a promising
candidate
for high-temperature
supercapacitors.
It
also exhibited
excellent
cycling
stability,
retaining
86.34
% of its capacity
after
10,000
cycles
at
10 mA cm
2
, highlighting
its remarkable
durability.
Density
functional
theory
(DFT)
calculations
were
conducted
to explore
the quantum
capacitance
of the bimetallic
hydroxide.
The DFT results
revealed
electron
density
near
the Fermi
level,
which
directly
contributes
to the high
quantum
capacitance
of Mn(OH)
/Ni(OH)
with
a Mn:Ni
molar
ratio
of 3:1. This
work
underscores
the
potential
of MOF-derived
materials
as a promising
route
for the development
of high-performance
supercapacitor
electrodes,
paving
the way for future
advances
in electrochemical
energy
storage
technologies.
*
Corresponding
author.
E-mail
addresses:
elahetorabi1377@gmail.com
(E. Torabi),
Kazemistudy@gmail.com
(A. Kazemi),
Mtamtaji@connect.ust.hk
(M. Tamtaji),
f_
manteghi@iust.ac.ir
(F. Manteghi),
srohani@uwo.ca
(S. Rohani),
wagoddard3@gmail.com
(W.A.
Goddard).
Contents
lists available
at
ScienceDirect
Heliyon
journal
homepag
e:
www.cell.co
m/heliyon
https://doi.org/10.1016/j.heliyon.2024.e41261
Received
19 September
2024;
Received
in revised
form
19 November
2024;
Accepted
13 December
2024
Heliyon
11
(2025)
e41261
Available
online
18
December
2024
2405-8440/©
2024
Published
by
Elsevier
Ltd.
This
is
an
open
access
article
under
the
CC
BY-NC-ND
license
(
http://creativecommons.org/licenses/by-nc-nd/4.0/
).
1. Introduction
The extensive
reliance
on fossil
fuels
such
as oil, gas, and coal has led to environmental
challenges,
due to the emission
of CO
2
and
the resulting
greenhouse
effect
[
1
,
2
]. Therefore,
the development
of clean
and sustainable
energy
sources
is crucial
to address
environmental
challenges
and meet
the growing
global
energy
demands
while
minimizing
the impact
on the planet.
This
shift
is
essential
for ensuring
a sustainable
future
and reducing
reliance
on traditional,
non-renewable
energy
sources
[
3
,
4
]. As a result,
the
focus
has shifted
toward
advanced
energy
storage
technologies,
which
play
a crucial
role in transitioning
to sustainable
energy
so
-
lutions.
Various
renewable
and sustainable
electrochemical
energy
storage
devices,
including
batteries
and supercapacitors,
are now
available
[
5
,
6
]. Supercapacitors
(SCs)
have
several
advantages
over
traditional
batteries
and capacitors,
including
a longer
lifespan,
high
specific
capacitance,
and high
power
density.
They
offer
exceptional
cycling
stability,
high
power
output,
and operate
safely
with
a lower
risk of failure
[
7
9
]. Energy
storage
in supercapacitors
is based
on electrochemical
conversion
reactions,
which
allow
them
to
store
and release
energy
efficiently
[
10
12
]. Energy
storage
in supercapacitors
(SCs)
involves
three
main
mechanisms
that work
together
to ensure
efficient
energy
storage
and rapid
discharge.
The first mechanism
is ion adsorption
and release
at the interface
between
the electrode
and the electrolyte.
The second
involves
chemical
reactions
near
the electrode
surface,
where
electron
transfer
plays
a crucial
role.
Lastly,
controlled
electron
movement
through
diffusion
within
the electrode
material
further
contributes
to energy
storage.
These
processes
enable
SCs to deliver
high
power
output
and excellent
cycling
stability
[
13
16
]. However,
achieving
high-performance
energy
storage
capacitors
heavily
depends
on the structure
and composition
of the electrode
materials.
Various
materials
have
been
explored
for this purpose,
including
metal
oxides
[
17
,
18
], perovskite
oxides
[
19
,
20
], layer
double
hydroxides
(LDHs)
[
21
,
22
], metal
chalcogenides
[
23
,
24
], MXenes
[
25
,
26
], and metal-organic
frameworks
(MOFs)
[
27
,
28
]. The field
of energy
storage
has seen
a significant
increase
in the exploration
of various
materials,
including
MOFs,
to meet
the rising
demand
for sus
-
tainable
and high-performance
energy
solutions.
These
materials
offer
promising
potential
for improving
energy
storage
systems
by
enhancing
efficiency,
capacity,
and environmental
friendliness
[
29
,
30
]. Among
the vast array
of MOFs
available,
MOF-74
has emerged
as a prominent
candidate
due to its exceptional
structural
stability
and high surface
area [
31
,
32
]. The unique
and adjustable
structures
of MOFs
enable
precise
control
over
the secondary
building
units
(SBUs)
and organic
ligands.
This
capability
allows
researchers
to
optimize
the size,
shape,
and properties
of the pores,
tailoring
them
to meet
the performance
specifications
needed
for specific
ap
-
plications
[
33
,
34
] (see
Scheme
1
).
MOF-74
is one of the unique
metal
organic
frameworks,
which
is synthesized
by using
divalent
metal
salts,
including
transition
metals
such
as Mn
2
+
, Fe
2
+
, Co
2
+
, Ni
2
+
, Cu
2
+
, Zn
2
+
, and the organic
ligand
2,5-dihydroxy
1,4-benzene
dicarboxylate
[
35
,
36
]. In
MOF-74,
the metals
are coordinated
with
five oxygen
atoms
of the H
4
DOT
ligand
and one solvent
molecule
and form
a hexagonal
structure
with
a one-dimensional
channel
with
a honeycomb
topology.
In addition,
MOF-74
has a microporous
structure
with
a pore
size of 12 Å and a specific
surface
area
of 1218
m
2
g
1
[
37
,
38
].
Wang
and Co-worker
reported
the synthesis
of Co-MOF-74
micro
flower
with
good
performance
for hybrid
SCs specific
capacitance
of 164.2
F/g at 0.5 A g
1
in a three-electrode
system
[
39
]. Aghazadeh
and Co-worker
reported
the synthesis
of a novel
tri-metallic
MOF
grown
onto
Ni foam
as an electrode
for SCs. At a current
density
of 1 A g, the TM-MOF
electrode
had a high specific
capacity
of 412 C/g,
and at 15 Ag
1
had a high-rate
capability
of 52.9 %, and exceptional
cycling
stability
of 94.2 % after
6000
cycles
[
40
]. Manganese
(Mn)
plays
a crucial
role among
metals
in supercapacitors
due to its high theoretical
capacitance
and structural
stability,
offering
advantages
such
as environmental
friendliness
and long-term
performance.
However,
challenges
like poor
conductivity
and easy ion accumulation
result
in a practical
capacitance
much
lower
than
its theoretical
value.
To overcome
these
issues,
structural
adjustments
and controlled
strategies
are needed.
MOFs,
with
their
high
specific
surface
area and tunable
structure,
hold
potential
to improve
the electrochemical
properties
of Mn-based
materials
for energy
storage
applications
[
41
]. Wang
and coworkers
synthesized
a manganese-based
super
-
capacitor
by embedding
mixed
cobalt
and manganese
oxides
into
a nanoporous
carbon
framework
(M/MO@C).
This
material
maintained
the original
morphology
of CoMn-MOF-74
and showed
improved
conductivity
due to the uniform
dispersion
of Co, MnO,
and CoO nanoparticles.
The supercapacitor
exhibited
a high
specific
capacitance
of 800 F g
1 at 1 A g
1 and demonstrated
excellent
cycling
stability,
with
85 % capacitance
retention
after
1000
cycles
[
42
]. Despite
their
exceptional
properties,
MOFs
face limitations
in
energy
storage
performance
due to factors
such
as low electrical
conductivity
and restricted
electrolyte
accessibility
[
43
,
44
]. These
limitations
hinder
their
ability
to efficiently
store
and release
electrical
energy
[
45
47
]. To address
these
challenges,
researchers
have
Scheme
1.
Schematic
of nickel
foam
activation.
Nickel
foam
activation
refers
to a process
used
to enhance
the performance
and properties
of nickel
foam,
a porous
material
made
of interconnected
nickel
fibers.
The activation
process
involves
treating
the nickel
foam
in a specific
manner
to modify
its surface
characteristics
and improve
its functionality
for various
applications.
E. Torabi
et al.
Heliyon
11
(2025)
e41261
2
devised
various
strategies
to enhance
the energy
storage
efficiency
of MOFs
in supercapacitors
[
48
,
49
]. These
strategies
aim to
improve
key performance
metrics
of MOFs
in supercapacitors,
including
specific
capacitance,
rate capability,
and cycling
stability
(performance
over
multiple
charge
discharge
cycles)
[
50
,
51
]. In other
study,
Zhang
and coworker
reported
the synthesis
of
NMC-LDH/ZnO@CC
nanocomposites
by growing
layered
double
hydroxide
nanosheets
in-situ
on ZIF-67
precursor
covering
vertical
arrays
of ZnO nanorods.
The resulting
binder-free
electrode
showed
a high
capacitance
of 9258
mF cm
2
at 2 mA cm
2
with
87.5
%
retention
after
5000
cycles.
They
also assembled
a solid-state
asymmetric
supercapacitor
with
an energy
density
of 24.6 W h kg
1
and a
power
density
of 170 W kg
1
. Additionally,
a single
ultracapacitor
powered
a versatile
instrument
for monitoring
environmental
conditions
for over
30 min.
This
work
offers
a novel
approach
for electrochemical
energy
storage
[
52
]. Significant
strides
can be
achieved
in unlocking
the complete
energy
storage
potential
of MOFs
for supercapacitors
by effectively
addressing
these
inherent
limitations
and implementing
cutting-edge
approaches.
Among
metals,
nickel
stands
out for its high
electrical
conductivity
and
exceptional
electrochemical
activity,
making
it ideal
for enhancing
charge
transfer
processes.
Nickel
foam,
with
its 3D porous
structure,
further
amplifies
these
properties
by providing
a large
surface
area and excellent
electrical
pathways,
making
it a preferred
substrate
for supercapacitor
electrodes
[
53
,
54
].
This study
introduces
an innovative
approach
to fabricating
high-performance
supercapacitor
electrodes
by utilizing
MnNi-MOF-
74 as a sacrificial
material.
The research
involves
the synthesis
of MnNi-MOF-74
via a bimetallic
solvothermal
method
and its sub
-
sequent
conversion
to Mn(OH)
2
/Ni(OH)
2
with
diverse
microstructures
in an alkaline
solution
(KOH).
One of the drawbacks
of MOFs
is
their
limited
conductivity
for energy
storage.
To address
this issue,
MOF-74
was grown
on nickel
foam
(NF)
to enhance
conductivity.
The designed
electrodes
were
evaluated
to investigate
their
electrochemical
performance
for energy
storage
in a three-electrode
system.
This study
highlights
the utilization
of MOF-74
as a sacrificial
material,
showcasing
its potential
as a promising
pathway
to
fabricate
high-performance
supercapacitor
electrodes.
By employing
MOF-74
in this manner,
it opens
up exciting
possibilities
for
advancements
in the field
of electrochemical
energy
storage
devices.
2. Experimental
section
2.1.
Material
All chemicals
of reagent
grade
were
commercially
available
and used
without
further
purification.
2,5-Dihydroxyterephthalic
acid
(H
4
DOT,
98.0
%), Manganese(II)
nitrate
tetrahydrate
(Mn(NO
3
)
2
4H
2
O, 99.0
%), were
purchased
from
Sigma
Aldrich
Co. Ltd. N,N-
dimethylformamide
(DMF),
ethanol,
HCl (37.5
%), methanol,
acetone,
and potassium
hydroxide
(KOH,
95 %) were
analytical
grade
(AR)
and obtained
from
Merck.
Nickel
foam
(NF)
(thickness:
0.3 mm;
pore
density:
130 ppi) were
supplied
by American
elements
used
as the substrate.
Milli-Q
water
(
>
18.2
M
Ω
cm) was used
throughout
the whole
experiment.
2.2.
Material
characterization
PXRD
measurements
were
carried
out with
a Philips
PW1730,
X-ray
diffractometer
using
Cu K
α
1
radiation
to validate
the phase
purity.
The morphology
and microstructure
of the materials
were
recorded
by field
emission
scanning
electron
microscopy
(FEI,
Quantum
2000).
Transmission
electron
microscopy
(TEM)
images
were
measured
on a Philips
EM208S
transmission
electron
mi
-
croscope.
Fourier
Transform
Infrared
Spectrometer
spectra
were
recorded
by PerkinElmer
Spectrum
1 instrument.
N
2
adsorp
-
tion
desorption
experiments
were
determined
using
an (BET
BELSORP
Mini
II) instrument.
Thermal
Gravimetric_
Differential
Thermal
Analysis
were
determined
using
an (STA504)
instrument.
2.3.
Material
preparation
Synthesis
of bimetallic
MnNi-MOF-
74/NF
.
The synthesis
of MOF-74
was carried
out using
a procedure
that was slightly
modified
from
that previously
reported.
In brief,
121.4
mg (0.48
mmol)
Mn(NO)
3
4H
2
O and 29.7
mg (0.15
mmol)
2,5-dihydroxyterephthalic
acid (H
4
DOBDC)
were
dissolved
in a mixture
solution
which
consisted
of 0.6 mL distilled
water,
0.6 mL ethanol
and 10 mL DMF,
according
to
Scheme
1
. Then,
the resulting
solution
was transferred
into 20 mL screw-capped
vials
and added
a clean
Ni foam
(NF, 1
×
1 cm
2
). Finally,
the screw-capped
vials
were
maintained
at 120
C for 24 h. The electrocatalysts
obtained
after
cooling
to room
temperature
were
washed
three
times
with
DMF
solvent
and three
times
with
fresh
methanol
to remove
any impurities.
After
the
process
was completed,
the obtained
products
were
activated
to remove
all solvents
by heating
them
stepwise
under
vacuum
(50
100
mTorr)
at a heating
rate of 4
C/min
to 150
C for 24 h. The mass
loading
for MOF-74
was determined
by careful
measurement
to be
~11.3
mg cm
2
.
Synthesis
Mn (OH)
2
/Ni (OH)
2
on Ni foam
. The obtained
NFs coated
with
MnNi-MOF-74
were
immersed
in 10 mL of 2 M KOH,
6 M
KOH
and 12 M KOH
solutions
at room
temperature
for 24 h, respectively.
It was evident
from
the change
in color
of the NF that the
metal
hydroxide
was incorporated
into the engineered
NF. Bimetallic
Mn(OH)
2
/Ni(OH)
2
hydroxides
(MnNi-x,
x
=
2, 6, 12) were
produced
with
tailored
morphologies
through
a successful
synthesis
process.
The samples
were
named
MnNi-2,
MnNi-6
and MnNi-12
respectively.
In the end, the electrocatalysts
were
washed
with
deionized
water
and ethanol
to remove
KOH
and dried
at 80
C for 12 h
before
use.
Preparation
of NF
. In order
to clean
and activate
the Ni foam
surface,
1
×
1 cm
2
pieces
were
first prepared
and then
treated
with
ultrasound
for 20 min in dilute
HCl (1 M), acetone
and ethanol,
respectively,
then dried
at room
temperature.
Following
the treatment,
the NF samples
were
dried
in an oven
at 60
C for 30 min in order
to remove
any solvent
residue.
E. Torabi
et al.
Heliyon
11
(2025)
e41261
3
2.4.
Electrochemical
measurements
Electrochemical
analyzes
were
carried
out by using
the Autolab
PGSTAT204
compact
and modular
potentiostat/galvanostat
device
controlled
by Nova
2.1 software).
A three-electrode
system
including
platinum
electrode
as counter
electrode,
active
materials
based
on MnNi
as working
electrode,
calomel
electrode
(Hg/HgO(as
reference
electrode
was used
in (2M KOH)
solution
electrolyte.
Cyclic
voltammetry
curves
were
performed
with
different
scan
rates
of 5
50 mV s
1
and in the potential
range
of 0
0.6 V. Galvanostatic
charge-discharge
(GCD)
curves
were
measured
at different
current
densities
with
a potential
window
from
0 to 0.4 V. The electro
-
chemical
impedance
spectroscopy
(EIS)
measurements
were
conducted
at the open
circuit
potential,
covering
a frequency
range
of
0.01
100 kHz.
The specific
capacity
was obtained
according
to the charge-discharge
curve
and Equation
(1)
.
C
=
I
Δ
t
S
Δ
V
(1)
In the above
equation,
C (mF cm
1
) represents
the areal
capacitance,
S (cm
2
) denotes
the electrode
s surface
area,
I (A) represent
the
discharge
current,
Δ
t (s) represents
the total
discharge
time,
and
Δ
V (V) represents
the potential
range
during
the discharge
process.
3. Results
and discussion
The process
of forming
MnNi-MOF-74
derived
composites
of Mn(OH)
2
/Ni(OH)
2
is shown
in
Scheme
S1
. The process
of forming
MnNi-MOF-74
derived
composites
of Mn(OH)
2
/Ni(OH)
2
is described
as follows.
According
to a solvothermal
reaction,
bimetallic
MOF-
74 was grown
in situ on NF by a facile
one-pot
solvothermal
strategy
with
metal
ions and 2,5-dihydroxyterephthalic
acid.
NF has
provided
as the source
of the third
metal.
In fact, through
the in-situ
method
and chemical
interaction
between
the linker
and NF, Ni
2
+
ions are released,
and as a result,
bimetallic
MOF-74
grows
on the activated
substrate.
The electrocatalysts
are treated
with
2M, 6M,
and 12 M alkaline
solutions,
respectively.
Then,
the electrocatalysts
are treated
with
2, 6, and 12 M alkaline
solutions,
respectively,
and
their
color
change
from
light
brown
to dark
indicates
the conversion
process
on the NF surface.
Equations
(2)
(5
) are related
to the
reaction
of metal-ligand
bond
formation
and breaking
and metal-OH
bond
formation.
2
Mn
2
+
+
H
4
DOBD
Mn
2
DOBDC
+
4
H
+
(2)
2
Ni
2
+
+
H
4
DOBDC
Ni
2
DOBDC
+
4
H
+
(3)
Mn
2
DOBDC
+
4
OH
(
aq
)
2
Mn
(
OH
)
2
(
s
)+
DOBDC
4
(
aq
)
(4)
Ni
2
DOBDC
+
4
OH
(
aq
)
2
Ni
(
OH
)
2
(
s
)+
DOBDC
4
(
aq
)
(5)
The FTIR
spectrum
recorded
for MnNi-MOF-74
is shown
in black
in
Fig. 1
. The characteristic
peaks
above
800 cm
1
are related
to
the vibration
of organic
ligands
in the structure
[
55
]. The peak
appearing
at 1380
cm
1
is related
to the C
O stretching
vibration,
which
indicates
the presence
of dihydroxy
terephthalic
acid ligand
in the MOF.
The peak
appearing
at 560 cm
1
is attributed
to the
metal-oxygen
bond
vibration.
The peak
corresponding
to C
=
O appears
at 1633
cm
1
, in Mn-MOF-74,
the corresponding
peak
shifts
from
1633
cm
1
1547
cm
1
, which
shows
that after
the addition
of metals
to the ligand,
all groups
of carboxylic
acid are deproto
-
nated,
and there
is no unreacted
dihydroxy
terephthalic
acid on the surface
or pores
of MOF-74,
and it can be concluded
that MOF-74
has been
successfully
synthesized
[
56
]. The broad
peak
located
at 3420
cm
1
is related
to the OH functional
group
of coordinated
water
in the structure
of MnNi-MOF-74
and hydroxide
samples.
The absorption
peak
at 1622
cm
1
is related
to the bending
vibration
of the water
molecule
adsorbed
on MnNi-2,
MnNi-6,
and
MnNi-12
by hydrogen
bonding
[
57
]. The
characteristic
peaks
of
MnNi-MOF-74
almost
disappeared
in the FTIR
spectra
of MnNi-2,
MnNi-6,
and MnNi-12
samples,
which
indicates
its etching
and
transformation
[
58
]. The peak
of organic
groups
is present
even
after
alkalization,
which
may
be related
to the residues
of
Fig. 1.
FT-IR
spectra
of MnNi-MOF-74,
MnNi-2,
MnNi-6
and MnNi-12
samples.
E. Torabi
et al.
Heliyon
11
(2025)
e41261
4
MnNi-MOF-74
in the oxidized
samples.
In addition,
the extra
absorption
peak
at 1000-459
cm
1
may
be assigned
to Mn
O
H and
Ni-O
H stretching
vibration
[
57
].
The X-ray
diffraction
pattern
of MOF-74
can be seen
in
Fig. 2
. There
are two strong
characteristic
peaks
at 6.8
and 11.8
, which
correspond
to (2 1 0) and (3 0 0) crystal
planes,
respectively,
indicating
that MOF-74
has been
successfully
synthesized
[
59
]. All
characteristic
peaks
of MOF-74
disappeared
upon
alkalinization
[
57
]. The characteristic
peaks
of manganese
hydroxide
have
appeared
at 18.9
and 37
, and the peak
intensity
of MnNi-12
sample
has increased
due to the increase
in KOH
concentration.
Also,
the
characteristic
peak
of nickel
hydroxide
has appeared
at 18.9
[
60
,
61
].
The SEM
images
of MnNi-MOF-74,
MnNi-2,
MnNi-6
and MnNi-12
are shown
in
Fig. 3
a
d. During
the synthesis
of MnNi-MOF-74,
the surface
of the NF undergoes
etching,
leading
to the release
of Ni
2
+
ions into the solvent
mixture.
This etching
process
is facilitated
by H4DOT.
Subsequently,
the released
Ni
2
+
ions coordinate
with
DOT
4
to form
the MnNi-MOF-74
structure.
As shown
in
Fig. 3
a,
MnNi-MOF-74
particles
grew
irregularly
with
sharp
edges
and with
polydisperse
size on the substrate
[
58
]. After
immersion
in 2 M, 6
M and 12 M KOH
solution,
solutions
were
maintained
for 24 h at room
temperature,
the surface
of the MnNi-MOF-74
microstructure
became
smaller,
and softer
particles
were
obtained
with
a more
regular
appearance.
As a result
of the etching
process,
the metal
centers
(Mn
2
+
and Ni
2
+
) are separated
from
DOT
4
and are released
into the solution
and bind
with
OH
at the same
time.
In the TEM
image
shown
in
Fig. 3
e, the particles
have
grown
in a dense
spherical
shape
with
a particle
diameter
of about
200 nm. According
to the EDS
mapping
diagram
(
Fig. S1
), the samples
consist
of elements
Ni, Mn, C, and O, indicating
that the Ni foam
plays
a dual
role as a
conductive
substrate
and a source
of Ni2
+
during
the chemical
reaction.
This
implies
that during
the reaction
process,
MnNi-6
is
formed
on the substrate,
as shown
in
Fig. 3
f.
As shown
in
Fig. 4
a and b, the NF skeleton
is coated
with
MnNiMOF-74,
which
indicates
that the MnNi-MOF-74
nanostructure
is
successfully
immobilized
on the conductive
NF substrate.
In accordance
with
the isotherms
obtained
from
samples
MnNi-MOF-74,
MnNi-2,
MnNi-6,
and MnNi-12
in
Fig. 5
a, the absorption
diagrams
show
the materials
mesoporous
nature
[
62
]. Analyzing
the information
obtained
from
BET analysis,
the specific
surface
area
of sample
MnNi-MOF-74
was 490 m
2
g
1
, sample
MnNi-2
was 140 m
2
g
1
, sample
MnNi-6
was 186 m
2
g
1
and sample
MnNi-12
was
84 m
2
g
1
. The results
show
that after
the etching
process,
the surface
area of the hydroxides
is reduced
compared
to MnNi-MOF-74,
and MnNi-6
has a higher
surface
area than
that of samples
MnNi-2
and MnNi-12.
The average
pore
size of all samples
was compared
using
the Barrett-Joyner-Halenda
(BJH)
model,
and shown
in
Fig. 5
b. Also,
BET surface
area
(single
and multi-point)
and pore
size
analysis
(BJH
method)
for the MnNi-MOF-74,
MnNi-2,
MnNi-6,
and MnNi-12
are shown
in
Fig. S2
.
The thermogravimetric
analysis
(TGA)
of the synthesized
MnNi-6
was performed
to determine
its exact
decomposition
temperature
Fig. 6
a illustrates
the obtained
thermogram
of the synthesized
product
within
the temperature
range
of 20
700
C. According
to the
TGA
curve,
the synthesized
MnNi-6
loses
weight
at two different
temperatures.
In the first temperature
range
less than
200
C
(
<
200
C), minor
weight
loss occurs
due to the elimination
of physically
adsorbate
water.
The second
temperature
range
extends
up to
430
C, where
major
weight
loss is caused
by the decomposition
of Mn(OH)
2
into Mn
3
O
4
. The weight
loss resulting
from
the removal
of
physically
and chemically
attached
water
can be expressed
by Equation
6 and,
7
.
Mn
(
OH
)
2
nH
2
O
Mn
(
OH
)
2
+
Mn
(
OH
)
2
+
nH
2
O
(6)
Mn
(
OH
)
2
4
Mn
3
O
4
+
8
H
2
O
+
O
2
(7)
Moreover,
Fig. 6
b illustrates
the difference
between
the thermal
decomposition
of MnNiMOF-74
and MnNi-6.
The MnNiMOF-74
loses
about
54.8
% of its weight
after
being
exposed
to a temperature
range
up to 700
C, whereas
MnNi-6
loses
20.3
%.
3.1.
Electrochemical
characterization
The electrochemical
performance
of MnNi-MOF-74,
MnNi-2,
MnNi-6,
and MnNi-12
in the three-electrode
system
and 2 M KOH
solution
was investigated
and compared
with
CoNi-6,
which
was previously
reported.
Cyclic
voltammetry
(CV)
curves
in a scan rate of
Fig. 2.
PXRD
patterns
of the MnNi-MOF-74,
MnNi-2,
MnNi-6
and MnNi-12
samples.
E. Torabi
et al.
Heliyon
11
(2025)
e41261
5
Fig. 3.
FESEM
images
of (a) MnMOF-74,
(b) MnNi-2,
(c) MnNi-6,
(d) MnNi-12.
(e) TEM
and (f) EDX
image
of MnNi-6.
Fig. 4.
FESEM
micrograph
of nickel
foam
before
(a) and after
(b) coating
by MnMOF-74.
Fig. 5.
(a) N
2
adsorption
desorption
isotherms,
and (b)Pore
size distribution
of MnNi-MOF-74,
MnNi-2,
MnNi-6
and MnNi-12.
E. Torabi
et al.
Heliyon
11
(2025)
e41261
6