Unveiling the genesis of the high catalytic activity in nickel phthalocyanine for electrochemical ammonia synthesis

Article abstract of DOI:10.1039/d1ta00766a

Electrochemical ammonia synthesis by the nitrogen reduction reaction (NRR) using an economically efficient electrocatalyst can provide a substitute for the Haber-Bosch process. However, identification of active sites responsible for the origin of catalytic activity in transition metal phthalocyanine is a difficult task due to its complex structure. Herein, density functional theory (DFT) is applied to identify the probable active sites of nickel phthalocyanine (NiPc) for the NRR as well as the origin of catalytic activity which is associated with the d band center and density of states (DOS) of Ni in NiPc. Accordingly, NiPc nanorods (NRs), synthesized by a solvothermal method in large scale, exhibit an NH₃yield rate about 85 μg h^?1mg_cat^?1and a faradaic efficiency (FE) of 25% at ?0.3 Vvs.RHE. Moreover, the catalyst shows long term stability up to 30 hours while maintaining the NH₃yield and FE. The isotopic labelling experiment and other control investigation led to validation of the nitrogen source in NH₃formation. This study provides brand new insightful understanding of the active sites and the origin of the catalytic activity of NiPc for their NRR applications.

Full text of DOI:10.1039/d1ta00766a

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Paul, S. Kapse , R. Thapa, S. Chattopadhyay, A. N, S. N. Jha, D. B and U. K. Ghorai, J. Mater. Chem. A, 2021,
DOI: 10.1039/D1TA00766A.
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March 2018
Pages 4883-5230
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Unveiling the Genesis of the High Catalytic Activity in Nickel Phthalocyanine
for Electrochemical Ammonia Synthesis
1
1
2
2
3
Shyamal Murmu , Sourav Paul , Samadhan Kapse , Ranjit Thapa , Santanu Chattopadhyay ,
4
4
4
1
Abharana N , Shambhu N. Jha , Dibyendu Bhattacharyya and Uttam Kumar Ghorai *
1Department of Industrial Chemistry & Applied Chemistry, Swami Vivekananda Research
Centre, Ramakrishna Mission Vidyamandira, Belur Math, Howrah, 711202, India
2Department of Physics, SRM University – AP, Amaravati, Andhra Pradesh, 522502, India
3Rubber Technology Centre, Indian Institute of Technology, Kharagpur, 721302, India
4Atomic & Molecular Physics Division, Bhabha Atomic Research Centre, Mumbai, 400085,
India
Corresponding author:
*
Email: uttam.indchem@vidyamandira.ac.in
Abstract: Electrochemical ammonia synthesis by nitrogen reduction reaction (NRR) using
economically efficient electrocatalyst can provide a substitute of Haber-Bosch process. However,
identification of active sites responsible for the origination of catalytic activity in transition metal
pthalocyanine is difficult task due its complex structure. Herein, density functional theory (DFT)
is applied to identify the probable active sites of nickel phthalocyanine (NiPc) in NRR as well as
origin of catalytic activity which is associated with d band center and density of states (DOS) of
Ni in NiPc. Accordingly, the NiPc nanorods (NRs) were synthesized by solvothermal method in
large scale and the chemically prepared NiPc NRs exhibit the NH
3
yield rate about 85 µg h-1
mgcat and Faradaic efficiency (FE) of 25 % at -0.3 V versus RHE. Moreover, the catalyst shows
the long term stability till 30 hours while maintaining the NH yield and FE. The isotopic
labelling experiment and other control investigation led to validate the nitrogen source in NH
-1
3
3
formation. This manifold study provides brand new insightful understanding on the active sites
and origination of catalytic activity of NiPc for their NRR applications.
Keywords: Nickel Phthalocyanine, Nanorods, Nitrogen Reduction, Ammonia, d Band Center
1

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Introduction
Ammonia is a crucial raw material widely used for various industrial sectors such as agricultural,
1
chemical products and medicine. Presently, Haber Bosch process is primarily used to
manufacture ammonia, by the reaction of nitrogen and hydrogen at high pressure (150-200 atm)
and high temperature (400-500ºC), resulting in an energy intensive process with prodigious
2
-3
amount of carbon dioxide gas emission. From the sustainability point of view, it is necessary to
develop energy efficient and eco-friendly process for ammonia production. Ammonia production
by electrochemical route using electrocatalyst through nitrogen reduction reaction (NRR) at
ambient condition is a budding alternative of industrial Haber-Bosch process.4-5 Yet, due to inert
nature of the N≡N covalent bond and clashing hydrogen evolution reaction (HER) during NRR
process, the evolution of efficient electrocatalyst with superior activity and selectivity is really
challenging for the electrochemical conversion of di-nitrogen into ammonia.6-8 Nowadays,
various potential electrocatalysts were developed by eminent research groups, including metals
and metalloids9-20, transition metal based sulfides, oxides, nitrides, carbides21-33 and metal free34-
3
6
catalyst. From the mentioned array of electrocatalyst, transition metals based catalysts are
contemplated as a propitious electrocatalyst for NRR due to their synergism, where unoccupied d
orbital metal center which can accept the lone-pair of electrons from nitrogen to boost the
inception of metal-nitrogen bond and the electrons present at occupied d orbital in metal donate
back to the antibonding orbital of nitrogen in order to weaken the N≡N bond.25-26
Simultaneously, there is a likelihood of the formation of metal-hydrogen bond by d orbital
electron in transition metal, as a consequence it reduces the catalytic efficiency by enhancing the
competitive HER. Contrarily non-metallic elements can be considered as nitrogen activation
center in NRR due to their weak hydrogen adsorption of nonmetallic elements and abundant
3
6
valence electrons. Combination of transition metallic component along with active nonmetallic
counterpart would be great interest and may aid in boosting the performance of NRR. Recently
Prof. Hu and coworkers37 reported iron phthalocyanine/carbon (FePc/C) as an active NRR
-1
-1
electrocatalyst where the yield rate of NH
3
to be 10.25 µg h mgcat at low potential of -0.3 V
and FE of 10.50%. In our recent work, we synthesized pristine CoPc nanotubes by solvothermal
-1
-1
38
39
method which showed the NH
3
yield 107.9 µg h mgcat and FE of 27.7%. Xu et al. studied
the electrocatalytic NRR activity of FePc/O-MWCNT (multiwall carbon nanotube) which
-1
-1
showed the NH
3
yield 36 µg h mgcat and FE of 9.73%. Their theoretical calculation proved
2

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4
0
that NRR preferred alternating pathway over distal pathway. Mukherjee et al. studied the NRR
property of atomically dispersed NiN site-rich catalyst showing an optimal NH yield 115 μg
3
3
−
2
−1
cm h and FE 21 ± 1.9% at neutral conditions. However, there is a lack in report on the NRR
property of NiPc nanostructure. As electrochemical property is an integral interplay of surface
activity and morphology, we firmly believe that superiority can be achieved through
morphological tailoring, initiated recently. 41 Nevertheless, the greatest challenge lies in precisely
determining the origination of catalytic NRR activity of the electrocatalyst and the potent
catalytic sites accountable for NRR are directly associated with precise selection of high
performance electrocatalyst. The d band center is the well-known electronic descriptor to define
the transition metals catalytic activity and it provides the information about the adsorption of
intermediates as well as over potential during NRR.42, 43 Unfortunately, there is very few work to
describe the nitrogen adsorption energy with d band center of the transition metal for NRR.
Through this present work, we demonstrate a theory guided design of NiPc as NRR
electrocatalyst. DFT study indicates that for NRR, the alternating pathway is preferred over
distal pathway. We have discerned the several active sites and estimated d band center to
correlate the charge transfer mechanism between N and Ni during NRR. To realize the NRR
2
catalytic activity of NiPc, we have prepared the NiPc nanorods by ethylene glycol assisted
solvothermal route in large scale and further studied the NRR activity. The chemically
synthesized NiPc nanorods showed the excellent NRR activities with NH
yield of 85 µg h-1
3
-1
mgcat at -0.3 V versus RHE and FE of 25%.
Results and discussion
DFT calculations were performed to trace the catalytic activity on pristine phthalocyanine
Pc) and nickel phthalocyanine (NiPc) towards NRR. According to reports, metal center is most
(
active site in the metal-nitrogen-carbon (M-N-C) materials due to high charge transfer property
and strong binding affinity towards adsorbed species.44-46 Therefore, our main focus is on Ni
center as an active site on NiPc, whereas other sites are also consider for comparison. In Figure
1
, we corroborated the free energy profile for NRR on Ni center via both distal and alternating
reaction pathways. Here it is seen that the second step (*N to *NNH) is the potential
determining step (PDS) for both the pathways. Afterward we observed that the reaction step of
2
*
NNH undergoes to *NHNH due to the downhill free energy change of 0.28 eV, whereas the
3

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step *NNH to *NNH shows uphill free energy change of 0.44 eV. It proposes that alternating
2
pathways is more favourable than distal for NRR. For alternating pathway, we have shown the
free-energy profile for negative applied potential (U = -1.22 V), where all the reaction steps are
exothermic in nature (Figure S1).
Figure 1 Demonstrate the free energy profile for nitrogen reduction reaction over NiPc considering
alternating and distal pathways. Ni, C, N and H atoms are denoted with pink, grey, blue, and green sphere
respectively. Intermediates are given in the inset. Dotted lines are used for indication purpose.
We have compared the free energy of N adsorption and formation of NNH intermediate
2
on Ni center of NiPc and carbon, pyrrolic-N3, pyrrolic-N2, and pyridinic-N1 of NiPc and Pc as
shown in Figure 2a and Figure S2. For easier understanding of active sites on NiPc and Pc
structures, we have provided Figure S4. It depicts that the weak binding of NNH intermediate on
the carbon, pyrrolic-N3, pyrrolic-N2, pyridinic-N1 sites of NiPc/Pc is responsible for the higher
free energy of NNH* and larger overpotential than the Ni center of NiPc (Figure 2a and Figure
S2). The overpotential towards NRR on different active sites are obtained in the following order:
Ni (1.22V) <pyrrolic-N3 (1.65V) <pyrrolic-N1 (1.77V) < pyridinic-N2 (1.789V) < carbon
(2.29V) of NiPc structure. On Pc structure the overpotential values are pyrrolic-N2 (1.69V) <
pyrrolic-N3 (1.70V) < pyridinic-N1 (1.78V) < carbon (2.19V). Furthermore, we corroborated the
4

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free energy profile of HER to confirm the dominant reaction in NiPc and Pc systems. We have
observed that the overpotentials for NRR on carbon/nitrogen sites of NiPc and Pc are higher than
the overpotentials for HER (Figure S3). The result depicts overpotential for NRR on Ni as an
active center is 1.22 V, which is in lower magnitude than hydrogen evolution overpotential (1.37
V). It demonstrates that the Ni center in NiPc is optimal active site to amplify the NRR by
suppressing HER activity.
In order to comprehend the activity of metal center on NRR, we have correlated the N
2
and NNH adsorption energy with d band center. The d band center is well defined as the
electronic descriptor for metal catalysts.42-43, 47-49 The d band center is estimated by using the
following equation,
0
∫
휌푑퐸ꢀ퐸
휌푑ꢀ퐸
∞
−
∞
C
d
=
(1)
0
∫
−
Here, ρ is the partial density of states of d orbital, E is the energy in unit of eV and the Fermi
d
level is set to zero. The calculated the value of d band center for Ni atom is -1.54 in the NiPc
systems using the equation (1). The value of d band center is responsible for the charge transfer
to the adsorbate and its adsorption energy. It exhibits that the d band center of metal atoms are
affecting to the adsorption of intermediates as well as overpotential during NRR.
Furthermore, the d orbital density of state (DOS) of the Ni active site in NiPc is
investigated to understand the adsorption of N
It exhibits that the d band center in DOS of Ni is shifted towards negative direction after the
adsorption of N (-1.63 eV) and NNH intermediate (-2.23 eV). Similarly, the d orbital electron
occupancy of Ni site is 8.52e decreased to 8.50e and 8.19e after the adsorption of N and NNH
respectively. It indicates that the charge is transferred from Ni active site to N and NNH and that
signify the adsorption of intermediates during NRR. In Figure 2e & 2f, the charge density
difference analysis of NiPc with the adsorbed N and NNH shows electron accumulation at the
2
and NNH intermediates in NRR (Figure 2b-2d).
2
2
2
2
N-Ni bond that helps in the reduction reaction of nitrogen molecule. Therefore, Ni center on
NiPc is a potential active site that should be accountable for the enhancement of NRR
performance in phthalocyanine systems.
5

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Figure 2 (a) Free-energy diagrams for N
Pc (b) Shows the d orbital partial density of states of Ni center (c) Ni center after N
center after NNH adsorption of NiPc system. The Fermi level shown by dotted line at zero and the shaded
area describes the d orbital occupancy. Demonstrate the charge density difference for (e) N adsorbed
2
and NNH adsorption on Ni center of NiPc and various sites of
2
adsorption (d) Ni
2
NiPc and (f) NNH adsorbed NiPc. For easier understanding of the active sites (carbon, pyrrolic-N3,
pyrrolic-N2, and pyridinic-N1 atoms), the Pc and NiPc structures with indicators are shown in Figure S4.
6

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Figure 3 (a) Schematic formation process of NiPc 1D nanostructures and synthesis procedure; (b)
FESEM image of NiPc NRs at higher magnification; (c) FESEM images of NiPc NRs; (d) XRD spectrum
of NiPc NRs; (e)Normalized XANES spectra at Ni K-edge; (f) Experimental (R) vs.
R
data of NiPc
NRs measured at Ni K-edge.
The schematic representation of the synthesis procedure of nanorods is depicted in the
Figure 3a. Initially, small nanoclusters of NiPc are formed by the reaction of phthalonitrile,
nickel acetate and catalyst in ethylene glycol solvent. With the increasing reaction time, NiPc
nanoclusters are congregated through π-π interactions forming one dimensional rod like
structure. The optical image of the chemically derived NiPc crystal is also shown in the Figure
3
3
a. The FESEM images at different magnifications are shown in the Figure 3b & 3c. In Figure
b, the microstructure of the NiPc shows large scale rod like morphology. The length and
diameter of the chemically prepared NiPc are in the range of 2-12 µm and 50-250 nm
7

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respectively. In Figure 3d, shows the XRD diffraction pattern of NiPc nanorods and all the
5
0
peaks are well matched with the previous literature assuring the pure phase formation. The
major peaks appear at 6.72°, 7.38°, 9.60°, 10.31°, 15.88°, 24.92° values of 2θ, which
corresponds to the reflections from (100), (002),(102),(102), (202), (111) planes of NiPc
respectively. The chemical composition and electronic structures of NiPc NRs were investigated
by XPS. The survey scan of XPS spectrum demonstrates that major elements are Ni, N and C
(depicted in Figure S5). In Figure S6, the peaks at 855.1 eV and 872.3 eV are commensurate
5
0
with the electronic states of Ni 2p3/2 and 2p1/2 respectively. The existence of satellite peak at
63.6 eV ensures the Ni (II) state in NiPc.50, 51 The high resolution spectrum of N1s is depicted in
the Figure S7.
The synchrotron based X-ray absorption near-edge structure (XANES) and extended X-
8
ray absorption fine structure (EXAFS) analysis were conducted to investigate the valence state(s)
and local co-ordination environment of nickel in nickel pthalocyanine (NiPc) electrocatalyst
during ENRR process and prior to it. The X-ray absorption position of Ni K-edge spectra of
NiPc, before and after electrolysis was measured along with standard Ni foil and NiO, the X-ray
absorption edge position of NiPc lies in between Ni and NiO, hence the valence state of ‘Ni’ in
NiPc lies between 0 and +2 (Figure 3e). In XANES spectra, two distinct peaks at 8334 and 8339
5
2
eV are observed implying Ni-N
peak near 8334 eV commensurate with quadrupole allowed but dipole forbidden transition (1s to
d), implicitly suggests orbital alloying of 3d and 4p of the nickel atom in nickel
4
co-ordination in nickel pthalocyanine. The pre-edge small
3
5
3
pthalocyanine, whereas the shoulder peak at 8339 eV occurs for the 1s to 4p transition. The
fourier transforms of EXAFS was observed at -0.3V in post and pre-electrolysis condition. The
plot of EXAFS spectra in R co-ordinate space shows the following distinct peaks (Figure 3f):
5
3
The first peak at
.47 Å corresponds to the Ni-C co-ordination while the small extended peak at
due to superposition of peaks of Ni-N and Ni-C co-ordination.40, 54
The electrochemical NRR activity of NiPc supported on GCE has been investigated in
.1M HCl electrolyte using H-type electrochemical cell separated by a nafion 117 membrane
under ambient condition (shown in Figure S13 and detailed procedure in experimental section).
The Figure S14 demonstrates the linear sweep voltammetry (LSV) curves of NiPc NRs in N
99.999%, ultra-high purity grade) purged solution and argon (Ar) gas purged solution with in
̴
1.44 Å represents the first shell Ni-N co-ordination bonds, the second peak at
̴
2
̴
0.79 Å arises
0
2
(
8

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the potential range of -0.2 V to -0.6 V versus RHE and the enhancement of current density in N
2
solution which is starkly different than Ar solution within the potential range indicating the
2
2
prominent NRR activity. Figure 4a shows the time dependent chronoamperometric curves of
NiPc NRs in N saturated solution at -0.2 V to -0.6 V versus RHE and corresponding UV–vis
spectra with prominent absorption at 655 nm confirming the NRR activity is shown in Figure
b. After NRR electrolysis for 6000 sec at different potential, the indophenol blue method and
Watt & Chrisp method were employed to calculate the produced NH (SI 2 and Figure S15-S16)
and N respectively (SI 3 and Figure S17-S18). No by-product hydrazine (depicted in Figure
S19) is formed during electrolysis, confirming the good selectivity of NiPc catalyst towards N
to NH . The calculated NH yield and FE at various potential are presented in Figure 4c. The
maximum NH
2
4
3
2
H
4
2
3
3
-1
-1
3
yield and FE of NiPc NRs at -0.3 V vs. RHE are 85 µg h mgcat and 25 %
1
respectively. The quantification of NH
3
yield was also carried out using H NMR technique and
the calibration plot shown in the supporting information (Figure S20-S22 and SI4). The NiPc
-1
-1
NRs results in NH yield and FE of 85.4 µg h mgcat and 25.1% which are in unison with the
3
result as obtained from UV-visible method.
Several control experiments were performed to verify the detected NH was originated
3
from NRR process or other source. First, pristine Glassy Carbon Electrode (GCE) shows the
negligible amount of electrochemical NRR activity (Figure S23). Second, No NH
for NiPc NRs in N
Figure S23) at -0.3 V. Third, isotopic labelling experiment was performed with N
3
is detected
2
saturated open circuit potential (Figure S23) and Ar saturated electrolyte
1
5
(
2
as feeding
source; as
1
5
+, hence it validates that ammonium is produced solely from N
gas to produce NH
shown in Figure 4e. After electrolysis using 14N
showed triplet and doublet peaks due to formation of NH
4
2
and 15
N
2
as feeding gas, H NMR result
1
2
1
4
+
4
15
+
and NH
4
ions, whereas no such
5
5
peaks were observed when Ar was used as feeding gas. The reusability and long term stability
are the important parameters of an electrocatalyst for practical application. To reconnoitre the
durability of NiPc catalyst, cycling test was executed in N
.3 V vs RHE. The NH yield and corresponding FE was almost constant even after 5
consecutive cycles, implying the stable NNR electrocatalytic behaviour (Figure 4d). The long
term electrocatalytic stability test of NiPc catalyst was performed at -0.3 V in N saturated 0.1 M
2
saturated 0.1 M HCl electrolyte at -
0
3
2
HCl electrolyte at different (6, 12, 18, 24 and 30 (hrs.)) testing periods. The catalyst
demonstrated the steady and stable catalytic performance without significant change in current
9

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density (shown in Figure S25). The obtained NH
loss in compare with the initial cycles (Figure 4f).
3
yield and corresponding FE had negligible
Figure 4:(a) Time reliant current density (J) curves for NiPc NRs at various potential;(b) UV-vis
absorption spectra of the electrolytes stained with an indophenol indicator at different potential after 6000
sec NRR electrolysis; (c) Histogram of average NH
3
yield rate and corresponding Faradaic efficiency of
NiPc NRs at different potential; (d) Recycling stability tests of NiPc NRs for NRR at -0.3 V vs RHE for
1
14
+
15
+
five times; (e) H NMR spectra of NH
14N and 15
as the N source; f) Histogram of average NH
efficiency of NiPc NRs at different time duration.
4
and NH
4
produced from ENRR (at -0.3 V vs RHE) using
2
N
2
2
3
yield rate and corresponding Faradaic
1
0

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The reason for long term stable NRR activity of NiPc NRs occurs mainly due to the crystalline
nature of NiPc NRs electrocatalyst. XPS survey scan profile of NiPc NRs after ENRR, concludes
that the main constituents elements such as C, Ni and N are present in the catalyst (Figure S27).
The additional peaks of Fluorine (F) arise because of the incorporation of Nafion binder in the
NiPc prior to electrochemical testing. TEM image (Figure S28) of the post ENRR catalyst
sample show that the one dimensional nanorod like morphology almost remains same after 30
hrs electrochemical test.
Conclusion: In summary, we have identified the several active sites of NiPc catalyst with the
aid of DFT calculation which are responsible for N
calculated the d band center and DOS of Ni to decipher the genesis of the catalytic activity
together with charge transfer mechanism of Ni site to N * and NNH*. Accordingly, NiPc NRs
have been prepared by low temperature solvothermal method in large scale and studied for
electrocatalytic NRR. The chemically synthesized NiPc NRs show the high NH yield and FE
due to its presence of actives sites and specific-selectivity. The long term stability and reusability
of the NiPc NRs catalyst along with maintaining the NH yield rate and FE make it demanding
due to the practical applicability for electrochemical route to synthesize NH .These results help
2
adsorption and activation. Then, we have
2
3
3
3
to develop the theory guided judicious approach of the NRR electrocatalyst to produce NH .
3
Experimental Section:
Computational details: All Geometry optimization calculations were performed using density
functional theory (DFT) as implemented in the Vienna Ab initio Simulation Package (VASP).56
The generalized gradient approximations (GGA) with Perdew–Burke–Ernzerhof (PBE)
5
7
functional was employed to describe the exchange and correlation effect. The electron-electron
interaction was described by the projected augmented wave (PAW) method.58 The plane-wave
cutoff energy set to 450 eV for simulations of all systems and Brillouin zone sampled using a
single point k (located at the gamma point). The optimizations were followed until the net energy
-6
less than 10 eV per atom and force converged to 0.01 eV/Å. To save calculation time, we
considered frozen structures of NiPc and Pc (except active site). The vacuum in all directions of
1
5 Å is ready to avoid interaction between repeating images.
1
1

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Theoretically, the overpotential (η) calculated through free energy profiles are utilized to
estimate the catalyst activity towards nitrogen reduction reaction (NRR). The Gibbs free energy
(G) for free energy profile is obtained by the accompanying equation, G = E +ZPE–TS-neU,
where E is the energy calculated from DFT, ZPE is the zero-point energy, TS is the entropic
5
9
term, n is number of electrons transferred and U is applied potential at the electrode. However,
the TS and ZPE of the adsorbed atoms/ions are negligible in comparison of gaseous phase at
room temperature and pressure. In this case, the reported values of entropies and zero-point
energies from chemical database (https://janaf.nist.gov.) are taken for the free molecules and
considered a situation at U = 0 V in the free energy profile.
Materials synthesis and characterization: All analytical grade reagents were used. For the
preparation of NiPc, 8.6 mmol of phthalonitrile, 2.2 mmol of nickel acetate and specific amount
of ammonium molybdate as catalyst were mixed with ethylene glycol solvent (Sigma Aldrich-
9
9.8% pure) and stirred continuously for 20 minutes. Then the solution mixture was transferred
to 90 ml Teflon lined autoclave and then placed in a heating oven for 4 hours at 180°C. After
natural cooling to ambient temperature, the obtained material was washed with 1.0 N HCl,
ethanol and mild hot water for several times to wash away the residual reagents. Eventually, the
obtained blue colored precipitate was dried in an oven for 12 hours at 60°C. The phase purity and
crystallinity of the synthesized NiPc NRs were confirmed by XRD technique using Bruker D-8
advanced Eco X-ray powder diffractometer (radiation Cu-Kα with λ=0.15404 nm, current 25
mA, voltage 40 kV). The different chemical bonding in the NiPc nanostructures were examined
by Shimadzu IRAffinity-1S FTIR spectrometer. Chemical compositions of NiPc crystals were
examined by XPS using an OMICRON-0571 system. The electronic transitions of NiPc NRs
were performed by UV-vis spectroscopy (Shimadzu UV-3600 Plus). The morphological
structures of the chemically derived NiPc nanorods were observed by FESEM (Zeiss-Germany)
and transmission electron microscope (FEG-TEM, JEOL-JEM 2100F).
Electrochemical measurements and working electrode preparation: The ENRR tests were
carried out in dual compartment H-type cell in 0.1 M HCl with Nafion 117 membrane (Sigma
Aldrich Chemical Reagent Co.) as a separating membrane. The amount of electrolyte in cathode
and anode chamber was 20 ml respectively. In detail, Nafion 117 membrane was pre-treated in
o
the following way, at first boiling in water at 80 C for an hour, then boiling in H
2
2
O aqueous
1
2

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DOI: 10.1039/D1TA00766A
o
o
solution at 80 C for an hour, again water treatment at 80 C for an hour, then 3 hours treated with
o
o
0
.5 M H
triple distilled water, the membrane was used.
mg NiPc catalyst and 5 μL of Nafion solution (5 wt%) were dispersed in absolute
2
SO
4
at 80 C, finally treated in water at 80 C for 6 hours. After rinsing it few times with
1
isopropanol (100 μL) (Merck Co. Ltd.) with the help of sonication for a minute, followed by
mixing for 5 minutes to form a homogeneous catalytic ink. Then, the 8 μL dispersion of
homogeneous catalytic ink was loaded onto glassy carbon electrode (GCE) with effective area of
2
0
.07 cm and dried under ambient condition. The electrochemical experiments were performed
with an electrochemical workstation (CHI760E) using three-electrode configuration, where
catalyst coated on GCE acts as working electrode, Pt wire as counter electrode and Ag/AgCl
(
saturated in 3.5 M KCl) as reference electrode. All potentials values were converted to the
reversible hydrogen electrode (RHE). The ENRR activity was checked by linear sweep
voltammogram (LSV), measured in Ar or N (99.999% ultra-high purity grade) saturated
2
-1
electrolyte (scan rate 50 mV s ), the provided LSV curves were the steady state curves obtained
after several cycles.
Conflicts of interest
There are no conflicts of interest to declare.
ACKNOWLEDGEMENTS
UKG acknowledges the central DST-FIST programme (SR/FST/College-287/2015) for
providing financial support. UKG thanks to DBT star college scheme (BT/HRD/11/036/2019)
for funding. UKG acknowledges the Teachers Associateship for Research Excellence (TARE)
scheme (TAR/2018/000763) of SERB, Govt. of India for research grant and fellowship. UKG
also acknowledges Science & Technology and Biotechnology Department, Govt. of West Bengal
for providing the financial support [199 (Sanc.)/ST/P/S&T/6G-12/2018]. R.T. thanks Board of
Research in Nuclear Sciences (BRNS), India, for the financial support (Grant Nos.
3
7(2)/20/14/2018-BRNS/37144). RT thanks High Performance Computing Center, SRM IST for
providing the computational facility. The authors wish to thank Ashadul Adalder and Dr.
Subhajit Saha for their technical assistance.
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AUTHOR CONTRIBUTIONS:
UKG proposed the idea and designed the experiments. SM and UKG synthesized the materials
and performed XRD, FTIR, UV-vis. SP, SM and UKG carried out all the electrochemical
measurements and analyzed the results. SK and RT performed the DFT calculation. RT, SK, SM
and UKG analyzed the DFT results. SC and SM did the morphological characterization. SP, SM
and UKG analyzed the NMR data. AN, SJ and DB performed the XANES and EXAFS
experiment. DB, AN, SP, SM and UKG analyzed XANES and EXAFS results. UKG wrote the
original manuscript. SP, RT, AN, SM and SC co-wrote the manuscript. All authors approved the
final version of the manuscript. UKG supervised the entire project.
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