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Cite this: Mater. Adv., 2022,
3, 8201

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Immobilization of molecular complexes ongraphitized carbon cloth as stable and hybrid
electrocatalysts for enhanced hydrogen evolution
reaction and oxygen evolution reaction†
Ram Murthyand Sundaresan Chittor Neelakantan*

The search for an environment friendly, sustainable and cost-eﬀective catalyst used in electrochemical
water splitting via the hydrogen evolution reaction (HER) has generated considerable interest in
renewable energy research worldwide. In the current study, we report cost-eﬀective and easily
synthesizable 2-thioureidobenzothiazole (BTT) metal complexes (M = Pd, Co, Ni) as dual and eﬃcientelectrocatalysts for HER and OER applications in acidic and alkaline conditions, respectively. The hybrid
molecular catalysts were synthesized by integrating transition metal complexes with graphitized carbon
cloth (GrCC) as the carbon support. The pristine Pd(II)BTT complex exhibited enhanced hydrogen
evolution activity with an applied overpotential value of 178 mV (vs. RHE) and a significantly lower Tafel
slope value of 73.74 mV per decade in 0.5 M H2SO4 solution. The high catalytic activity of the Pd(II)BTT
complex can be ascribed to the strong aﬃnity of the Pd metal ion towards H2. The Ni(II)BTT/GrCC
hybrid catalyst for the HER exhibited an overpotential of 335 mV (vs. RHE) to drive a current density of
10 mA cm2 and was found to be superior vis-a-vis the pristine nickel complex. The enhanced catalytic
activity of the hybrid catalyst is due to the conducting and porous nature of the graphitized carbon
Received 29th June 2022,
Accepted 29th August 2022
DOI: 10.1039/d2ma00764a

cloth. For OER activity, the pristine Ni(II)BTT complex exhibited a very low overpotential value of 250 mV
to drive a current density of 10 mA cm2 and gave a low Tafel slope value of 93 mV per decade. The
cost-eﬀective and robust nickel complex as a novel electrocatalyst was found to be very eﬀective forboth HER and OER applications. This work paves the way for a new avenue to combine molecular

rsc.li/materials-advances

catalysts with eﬀective carbon supports for energy storage applications.

Department of Chemistry, Sri Sathya Sai Institute of Higher Learning,
Brindavan Campus, Kadugodi, Bengaluru – 560067, India.
E-mail: cnsundaresan@sssihl.edu.in† Electronic supplementary information (ESI) available. See DOI: https://doi.org/
10.1039/d2ma00764a

significant and sustained eﬀort to develop inexpensive,
durable, and eﬃcient electrocatalysts for water splitting
reactions.
Ideal electrocatalysts would involve small overpotential andpossess fast reaction kinetics to produce hydrogen. To date,platinum (Pt)-based electrocatalysts are considered as the gold
standard for the HER, owing to their strength of Pt–H bond andfaster facile kinetics.3 Exorbitant cost and insufficient reservesof platinum have hindered industrial scalability. Palladium isemerging as a very promising alternative to platinum owing to
its availability, relatively lower cost and good catalytic activity.4
Currently, palladium metal is extensively utilized in the automotive industry to lower the toxicity of emissions from the
combustion engine.4 Many non-precious metals such as Co,5,6
Ni,7 and Fe are preferred for water electrolysis due to theiraffordable cost and abundance on the earth’s crust.8,9 Various
factors determine the efficiency of the electrocatalyst, i.e.,electrode modification, nature of the electroactive material,presence of a carbon support, the porosity of the material,
etc. Transition metals are preferred because of their ease of© 2022 The Author(s). Published by the Royal Society of Chemistry

Mater. Adv., 2022, 3, 8201–8210 | 8201

1 Introduction
Currently worldwide, there is an unprecedented awareness andthere are eﬀorts to reduce the carbon footprint by substituting
fossil fuels with a very sustainable and economically viablegreen renewable energy source. Though wind and solar energy
systems are evolving to provide viable and eﬃcient alternative
clean energy resources, they are beset with certain limitations.
In this context, hydrogen has tremendous potential as a promising future energy currency and is prognosticated as avery prominent next-generation renewable energy source.1
The hydrogen evolution reaction (HER) and oxygen evolution
reaction (OER) have been recognised as clean and sustainableways of generating green energy.2 Researchers have devoted

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synthesis, broad applicability, and functionality in neutral,
alkaline and acidic conditions.
2-Thioureidobenzheteroazoles (BTZs) undergo chelation
with metal atoms through thiazole sulphur and thioureido
sulphur atoms to give a stable metal complex. The synthesized
metal complexes with the general formula [M(C8H8N6S6)2Cl2]+2(M = Pd, Co, and Ni) have been used to evaluate HER & OER
activity. The Pd complex has square planar geometry, whileboth Co and Ni metal complexes have tetrahedral geometry.10
In order to further enhance the activity of the molecularcatalyst, the metal complexes were incorporated into a carbon
support.
Various carbon supports, such as carbon nanotubes11(MWCNTs, SWCNTs), fullerene,12 graphene,13 carbon fiber
cloth,14 and porous carbon,15 are favored for electrocatalytic
applications due to their excellent electrochemical stability,
conductivity16 and favorable mass-transport properties. These
carbon supports have shown remarkable electrochemicaleﬃciency in numerous batteries and supercapacitors.17 The
conventional strategy involves the modification of the electrodeby drop casting the catalyst on the surface of the electrode. This
strategy suﬀers from the problem of leaching due to the fragile
linkage between the molecular catalyst and electrode surface.
To overcome these limitations, molecular catalysts were integrated into carbon supports to enhance the HER activity (Fig. 2).
In this study, graphitized carbon cloth (CC) has been used asa carbon support owing to graphene’s excellent electronic
conducting nature and the porous nature of the carbon clothfor HER activity. The current study reports a method to synthesize low-cost transition metal complexes as dual electrocatalysts
for HER and OER applications. We demonstrate a simple
method of integrating transition metal complexes with graphitized carbon cloth as a carbon support for enhanced hydrogenand oxygen evolution in acidic and alkaline conditions, respectively. With this motive, Co, Ni, and Pd metal complexes of
2-thioureidobenzothiazoles were chosen as electroactive materials
for studying hydrogen evolution reactions and oxygen evolution
reactions.

Materials AdvancesFig. 1 (a) Synthetic route of the ligand 2-thioureidobenzothiazole and (b)general structure of the 2-thioureidobenzothiazole metal complex.

Pd, Co, and Ni coordination complexes were prepared underalkaline conditions (Fig. 1). The procedure detailing the synthesis
and characterization have been described in S2 and S3 (ESI†). The
metal complexes were immobilized on carbon cloth by a simple
drop-casting method and dried at room temperature (Fig. 2).
To confirm the successful immobilization of the complexes on
the graphitized carbon cloth, X-ray photoelectron spectroscopy(XPS) measurements were performed (shown in Fig. 3). The XPS
spectrum of the electrocatalyst immobilized graphitized carbon
cloth exhibits two characteristic peaks of 335.5 eV and 337.8 eV
corresponding to Pd 3d5/2 and Pd 3d3/2 metal (Table 1). The peakat 284.5 eV can be attributed to the sp2 hybridized carbon
atoms.18 In Fig. 3 and 4, the peak at 335.5  0.2 eV can be
attributed to Pd2+, which can be quickly bonded to an oxygen atom.18

Comparing the XPS pattern of carbon cloth before and after the
HER reaction, Fig. 3(a and b) show the used Pd(II)BTT complex,
which is observed to show a decreased intensity at 343.88 eV(Table 1). In Fig. 4b, the Ni complex shows a characteristic peakat 853.7 eV, which can be attributed to Ni 2p3/2 (Ni2+) present
on the carbon cloth surface, and other peaks remain constant(Fig. S1, ESI†) (Table 1).
The FT-IR spectra of the metal complexes Pd(II)BTT,Co(II)BTT, and Ni(II)BTT are shown in Section S3 (ESI†). The
bands at 1585 cm1 and 755 cm1 are due to thioamide
stretching and bending vibrations, respectively. The band at
1629 cm1 corresponds to the n(N–H) bending of the ligand.
The bands at 1132 cm1, 1184 cm1, and 996 cm1 can be
attributed to n(CQS) for Pd(II), Co(II) and Ni(II) complexes,
respectively.10 The bands in the range of 740–755 cm1 are
attributed to n(C–S–C) of the thiazole ring in all threecomplexes.10 Complexation of the ligand with metals leads to ashift in the band to lower frequency, implying that the thiocarbonyl group is involved in complexing with the metal ion.
The UV-Vis absorbance spectra of 2-BTT and its complexes
are provided in Fig. S2 (ESI†). From Fig. S2 (ESI†), the ligand
shows a strong peak at 306 nm, but in the case of all the threecomplexes, there is a hypsochromic shift in the wavelength ofthe complex, revealing the complex formation. The electrontransition observed is p–p* for the complex. In the case of all
the three complexes, a hypsochromic shift from 306 nm to
290 nm is observed, depicted in Fig. S2 of the ESI.†The electronic spectrum of the Pd complex exhibited two
broad absorption bands at 28 089 cm1 and 34 482 cm1
corresponding to charge transfer bands. The peaks observedin the UV region reflect p–p* transition. The energy absorptionof 32 679 cm1 (306 nm) is due to the thiazole moiety of theligand. The energy absorption band moves to further higher
energy of 34 482 cm1 implying complexation. The crystal field
splitting energy (CFSE) values increase from 390 kJ mol18202 | Mater. Adv., 2022, 3, 8201–8210© 2022 The Author(s). Published by the Royal Society of Chemistry

2 Results and discussion

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Fig. 2

Schematic representation of immobilization of M(II)BTT complexes onto graphitized carbon cloth (GrCC) at 298 K.

Fig. 3

XPS spectra of the Pd(II)BTT complexes. (a) Pd(II)BTT/GrCC hybrid electrocatalyst before the LSV test. (b) PdBTT/GrCC after the test (LSV 6 h stability).

(306 nm) to 412.47 kJ mol1 (290 nm) reinforcing complex
formation.19

284.5

The thiocarbonyl moiety of the ligand undergoes strong
chelation with the metal atom. The occupied spx orbitals of
the ligand interact with dx2y2 orbitals of the metal to form a s
bond. Furthermore, the metal donates an electron back to theligand through p-back bonding. The back bonding is observed
between the dxz orbital of the metal and pz of the ligand.
This p-back bonding interaction results in increased mixingof the metal and ligand orbitals resulting in stable molecularcomplexes.20
2-Thioureidobenzothiazole metal complexes were subjected
to morphological analysis by scanning electron microscopy as© 2022 The Author(s). Published by the Royal Society of Chemistry

Mater. Adv., 2022, 3, 8201–8210 | 8203

Table 1 XPS peak positions obtained for the [Pd(BTT)/GrCC] catalyst
before and after the reaction and Ni(II)BTT/GrCC catalyst before the reaction

Complex

Pd 3d5/2 Pd 3d3/2 Ni 2p N 1s

[Pd (BTT)/GrCC] before 335.2
rxn[Pd (BTT)/GrCC] after 343.88
6h[Ni(BTT)/GrCC]
—

S 4s

C 1s

164

284.5

337.5

—

400

338.72—400.74 162.3 284.5—

853.7 400

164

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Materials AdvancesFig. 4 XPS fine spectra. (a) Pd 3d 3/2, 5/2 of the Pd(II)BTT complex on carbon cloth (CC). (b) XPS peak Ni 2p 3/2 and Ni 1p 1/2 of the Ni(II)BTT complex on
CC. (c) XPS peak of N 1s on GrCC. (d) XPS peak of C 1s on GrCC. (e) XPS peak of S 2p on GrCC.

Fig. 5 SEM images depicting the morphology of the M(II)BTT complexes.
(a) Pd(II)BTT, (b) Co(II)BTT, (c) Ni(II)BTT, and (d) 2-thioureidobenzothiazole
ligand.

complex compared to the 2-thioureidobenzothiazole ligand aiding
in better adsorption and evolution of hydrogen in the process.
The thermal stability of the Pd, Co, and Ni(II) complexes ofthe 2-thioureidobenzothiazoles was investigated by thermogravimetric (TGA) analysis. Three prominent stages of decomposition were observed for each of the complexes (Fig. S5, ESI†). At
temperatures ranging from 170 1C to 320 1C, the loss of HCl wasobserved.10 Subsequently, the loss of the thioureido moiety wasobserved at a slightly higher temperature between 280 1C and
620 1C. A significant mass loss of the complex was observed attemperatures above 350 1C, and exo-eﬀects with maxima at
370–390 1C were observed for the above decomposition. In thethird stage of degradation, the ligand completely decomposesat the temperature range of 760–870 1C; the endo effect isobserved, and no further weight loss was observed with metaloxide (MOx) as the final product10 (Fig. S5, ESI†).
The magnetic properties of the 2-thioureidobenzothiazole
metal complexes (Pd, Co, and Ni) were investigated using thevibrating sample magnetometer (VSM) technique. From Fig. S4(ESI†), it was evident that the Pd(II)BTT and Ni(II)BTT complexes
behave as diamagnetic due to the eﬀective bonding of theunpaired electrons with the sulfur atoms of the ligand. In thecase of the Co(II)BTT complex, ferromagnetic behavior isreflected in Fig. S4 (ESI†).

8204 | Mater. Adv., 2022, 3, 8201–8210© 2022 The Author(s). Published by the Royal Society of Chemistry

shown in Fig. 5(a–d). The SEM images revealed the crystalline
nature of the complexes. The palladium complex possesses a rodlike morphology while the nickel and cobalt complexes showed
flaky morphology. The SEM images show increased space in theView Article Online

Materials AdvancesFig. 6 (a) Linear sweep voltammetry curves of pristine M(II)BTT and M(II)BTT/
GrCC complexes in the acid electrolyte at a scan rate of 50 mV s1. (b)Comparison plot of the linear sweep voltammetry curve of 10% Pt/C and
M(II)BTT, M(II)BTT/GrCC at 10 mA cm2 in acidic conditions. (c) Tafel slopes ofthe linear polarization curves of pristine M(II) complexes and Pt/C. (d) Tafel
slopes of the linear polarization curves of the M(II)BTT/GrCC complexes.

density of 10 mA cm2. The Pd/C material has an overpotentialof 91 mV to achieve a current of current density of 10 mA cm2.21
The activity of pristine Pd(II)BTT is better in comparison to Pd/C
in 0.5 M H2SO4. LSV plots for the hybrid catalyst showed a strong
indication of hydrogen evolution with high electrocatalytic current with increased bubble formation at the electrode/electrolyte
interface. Electrocatalysis is intimately related to reducing
Pd(II) to Pd(0) in the aqueous acidic solution. The decreased
particle size of the Pd metal complex compared to the Co and
Ni complexes significantly contributed to the enhanced HER
activity.22 The Pd complex in pristine form has better electrocatalytic activity due to the aﬃnity of the palladium ion towardshydrogen and the eﬀective ligand to metal charge transfer
reaction.
The Pd(II)BTT/GrCC, Co(II)BTT/GrCC and Ni(II)BTT/GrCC
showed a maximum current density of 44 mA cm2, 63 mA cm2
and 110 mA cm2 at 720 mV, respectively (Fig. 6a). In the case of thepristine complexes, Pd(II)BTT showed the maximum current densityof 247 mA cm2, and the Co(II) and Ni(II) complexes of BTT showed acumulative current density of 18 mA cm2, and 56 mA cm2 at thepotential of 720 mV (Fig. 6a). In the case of Ni and Co complexes,graphitized carbon cloth enhances the catalytic performance due tothe carbon cloth’s conducting and porous nature. The porosity
determines the ease of accessibility (pore dimensions) of the active
sites and aﬀects the mass transport.23 Current density ( j) is ameasure of rate of the reaction, which in turn is proportional to
the number of active sites on the electrode surface.24 The porousnessof the hybrid electrocatalyst on carbon cloth has a pore diameter of
37  5 mm, while bare carbon cloth has a pore diameter of 42 
5 mm. The results implied that the porous electrode shows muchhigher ( j) compared to planar systems of similar dimensions.25 The
enhanced electrocatalytic activity exhibited by the hybrid catalyst
complex can be ascribed to the synergistic eﬀect of the M(II)BTT
complex and graphitized carbon cloth. The weaved textile-like
property with graphite exposes a large fraction of active surface area,
in turn enlarging the contact area with the electrolyte accelerating
the diﬀusion. These factors favor excellent electrocatalytic activity for
the HER.
Tafel slopes were obtained from linear sweep voltammetry
measurements by plotting potential as a function of log jusing the relation Z = a + b log j.26 The Tafel slope values reveal
the kinetics of the HER reaction. The Tafel slope value of
73.74 mV dec1 of Pd(II)BTT is comparable to the Tafel slope
value of Pt/C (47.34 mV dec1) (Fig. 6c). The slope values revealthat the HER process involves the Volmer–Tafel mechanism,27,28
where hydrogen on the electrode surface recombines with ahydronium ion to produce H2. The Tafel mechanism ofPd(II)BTT/GrCC, Co(II)BTT/GrCC, and Ni(II)BTT/GrCC exhibitedhigh Tafel slope values of 144 mV dec1, 410 mV dec1, and
165 mV dec1, respectively (Fig. 6d) indicating sluggish reaction
kinetics and obey Volmer–Heyrovsky as the possible mechanistic
pathway.29,30 To understand the high catalytic activity of thePd, Co, and Ni complexes, the double-layer capacitance wasmeasured. Cyclic voltammetry was performed at different scanrates ranging from 20 to 200 mV and the plot of Dj( ja  jc)
against the scan rate yielded the slope value (Cdl). The hybrid© 2022 The Author(s). Published by the Royal Society of Chemistry

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Electrochemical reaction

The 2-thioureidobenzothiazole metal complexes were evaluatedfor the hydrogen evolution reaction. The complexes weredirectly immobilized on graphitized carbon cloth (GrCC) by asimple drop-casting method and were dried at room temperature. The catalyst loading of 0.04 mg cm2 and metal loading of
0.011 mg cm2 were achieved on the carbon cloth. In thecase of the pristine metal complexes, a catalyst loading of
0.282 mg cm2 was attained on the electrode surface. The
catalytic activity of the hybrid catalyst (M(II)BTT/GrCC, M =Pd, Co, and Ni) was compared to that of the pristine metal
complexes (M(II)BTT) in 0.5 M H2SO4. The electrochemicalmeasurements were obtained using a three-electrode system
configuration with a glassy carbon electrode (GCE) as theworking electrode, graphite rod as a counter electrode to avoid
possible contamination by redeposition of Pt, and a saturated
calomel electrode (SCE) as the reference electrode under continuous N2 flow during the experiment. Graphitized carbon
cloth (0.5  1 cm) of area 0.5 cm2 was pretreated with deionizedwater and ethanol for 10 min sequentially. The pretreated GrCC
was used to obtain a hybrid electrocatalyst for the HER.
2.1.1 Hydrogen evolution reaction. Linear sweep voltammetry measurements were obtained between 0 and 1 V at ascan rate of 50 mV s1, as shown in Fig. 6(a). The Pd(II)BTT
complex exhibited an extremely low overpotential (Z) of
178 mV (vs. RHE) compared to Pd(II)BTT/GrCC, which showed
an overpotential (Z) of 419 mV to drive a current density ( j) of
10 mA cm2 (Fig. 6a). The pristine Co(II)BTT and Ni(II)BTT
complexes showed poor HER activity with overpotential valuesof 720 mV and 565 mV, respectively, to drive a current densityof 10 mA cm2. In contrast, Co(II)BTT and Ni(II)BTT complexes
on graphitized carbon cloth showed an improved activity withan Z of 335 mV and 472 mV, respectively, to achieve a current

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Materials Advances

catalyst showed a Cdl value of 12.06 mF cm2, 26.19 mF cm2,and 8.94 mF cm2 for the Pd, Co, and Ni complexes on GrCC,
respectively (Fig. 7b). The electrochemical active surface area
(ESCA) is a good descriptor of the electrochemical activity toward
the hydrogen evolution reaction. The ECSA was determinedfrom eqn (2), and the double-layer capacitance current is directly
proportional to the electrochemical active surface area (ECSA).31
The linearity obtained from the plot of geometrical current
density and scan rate revealed a diffusion-controlled electrocatalytic process. The facile electron flow in the process is due todiffusion.
Cdl ¼

Dj
v

(1)

Cdl
Cs

(2)

ECSA ¼

where Dj is current density (mA cm2), and V is potential. Cs is
the specific capacitance (Cs = 0.04 mF cm2).32 Pd(II)BTT/GrCC,
Co(II)BTT/GrCC and Ni(II)BTT/GrCC hybrid complexes involve an
ECSA value of 3.015 m2 g1, 2.235 m2 g1 and 6.547 m2 g1,
respectively. The high surface area of the electrocatalysts is dueto the carbon cloth’s porous and woven textile framework. In
order to evaluate the electrode kinetics of the HER process,electrochemical impedance spectroscopy (EIS) was performed
within the frequency range of 100 kHz to 10 mHz. The Nyquist
plots for all the complexes were obtained with a signal amplitude
perturbation of 10 mV. As can be seen from Fig. 7d, both hybrid
and pristine complexes exhibited a single capacitative loop
indicating the charge transfer process as the rate-determining

step (RDS). The diameter of the semicircle directly correlateswith the charge transfer resistance (Rct). From Fig. 7d, Pd(II)BTT
showed the least charge transfer resistance of 90 ohms suggestingsuperior kinetics over the rest of the complexes. However, thePd(II)BTT/GrCC, Co(II)BTT/GrCC and Ni(II)BTT/GrCC complexes
exhibited relatively high charge transfer resistance of 190, 480
and 830 ohms, respectively. The hybrid catalyst of Co(II) and Ni(I)showed favourable kinetics towards hydrogen evolution comparedto the pristine complexes. The details of the proposed mechanism
for the hydrogen evolution reaction have been described in
Section S8 (ESI†). The general hydrogen evolution reaction follows
the proton coupled electron transfer mechanism (PCET) shown in
Fig. 8a.20
Carbon supports have been extensively used to enhance thedurability of electrocatalysts.33 The metal complexes were subjected to linear sweep voltammetry in N2 saturated 0.5 M H2SO4
before and after 6 h of chronoamperometry (Fig. 7c). The
durability of the electrodes during the HER process was investigated using chronoamperometry. A constant potential wasapplied to the electrode for 6 h. As can be seen from Fig. 9(a–c), the hybrid catalysts showed good durability with the current
density decreasing after 4 h. From Fig. 9a, the Pd complex (Z =0.410 V) showed excellent stability for 6 h with 85% current
density retention activity compared to the Co and Ni complex,
which showed a drop in the current density value after 4 h(Fig. 9b and c). In contrast, the Co(II) and Ni(II) hybrid catalyst
activity gradually diminished after 4 h (Fig. 8b and c). The
durability has been one of the prominent limitations withrespect to molecular complexes. The durability of the complex
can be improved further with better carbon support materials.34

Fig. 7 (a) Overpotentials to produce a current density of 10 mA cm2 under acidic conditions. (b) Evaluation of the double layer capacitance (Cdl) of the
M(II)BTT complexes in 0.5 M H2SO4. (c) Stability study through linear polarization curves for 6 h in acid electrolyte. (d) Nyquist plot of M(II)BTT and
M(II)BTT/GrCC complexes obtained at the corresponding onset potentials.

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Fig. 8 (a) Durability test performed for M(II)BTT by the chronoamperometry technique at a constant potential: (a) Pd(II)BTT/GrCC 0.410 V, (b) Co(II)BTT/
GrCC 0.472 V and (c) Ni(II)BTT 0.335 V. (d) Digital photograph depicting the hydrogen evolution reaction from the experimental set up in 0.5 M H2SO4.

2.2

Oxygen evolution reaction

The three complexes were also evaluated for their oxygen evolution reaction (OER) activity using a standard three-electrode cellwith a rotating disk electrode (RDE) as the working electrode,graphite rod as an auxiliary electrode, and mercury/mercury
oxide as the reference electrode. The OER reaction was carriedout in 0.1 M KOH solution with a rotation rate of 1600 rpm inoxygen saturated environments. The LSV curves obtained were

Fig. 9 (a) Linear sweep voltammetry curves of Pd(II)BTT, Co(II)BTT, and Ni(II)BTT complexes in 0.1 M KOH. (b) Overpotentials at a current density of
10 mA cm2. (c) Corresponding Tafel slopes of the linear polarization curves of the M(II)BTT complexes (Pd, Co, and Ni(II)) in 0.1 M KOH. (d) Nyquist plots
depicting the charge transfer resistance of the M(II)BTT complexes (Pd, Co, and Ni(II)) BTT in acidic conditions.

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Materials AdvancesFig. 10 (a) Determination of the double-layer capacitance value of the M(II)BTT complexes. (b) Stability study of the M(II)BTT complexes by linear
polarization curves for 6 h in 0.1 M KOH.

all iR corrected with a scan rate of 50 mV s1 (Fig. 10a). Fig. 10a
displays the linear sweep voltammetry polarization curves. The
Ni(II)BTT complex exhibited remarkably high OER performancewith an overpotential of 250 mV to achieve a current density of
10 mA cm2 at 1.48 V (Fig. 10a). The Co(II)BTT and Pd(II)BTT
complexes exhibited an overpotential of 773 and 285 mV,
respectively, to drive a current density of 10 mA cm2
(Fig. 10b). The Co(II)BTT and Pd(II)BTT complexes exhibited amaximum current density of 17.12 mA cm2 and 63 mA cm2 at
1.98 V and 1.92 V, respectively. The Ni(II)BTT complex exhibited amaximum current density of 163 mA cm2 at 1.92 V (Fig. 10a).
The observed catalytic activity is due to the charge transfer fromligand to metal atoms (LMCT). From Fig. 8b, Ni(II) contains
oxygen bound to a high valent metal center leading to (Ni(II)–O).

(Ni(II)–O) is a key intermediate in the oxygen evolution reaction
process, which in turn reacts with H2O through a water nucleophilic attack mechanism to give O–O bond formation, and theintermediate formation is the rate determining step. The reaction is aided by a hydroxide ion coupled electron transfer
reaction resulting in an oxygen evolution reaction (Fig. 8b).
The Tafel slopes for all the three complexes are shown in
Fig. 9c, where Ni(II)BTT showed a slope value of 93.82 mV
dec1, Co(II)BTT showed 421.21 mV dec1, and the Pd(II)BTT
complex showed 182.6 mV dec1 (Fig. 9c). The Tafel slope
values are the lowest compared to many nanocomposites1
and other transition metal nanosheets.35 The metal complex
on the modified glassy carbon surface undergoes oxidationfrom Ni(0) to Ni(II), leading to reduction of the hydroxide ion to

Fig. 11 Durability test performed for the M(II)BTT complexes by the chronoamperometric technique at constant potential: (a) Pd(II)BTT (1.48 V vs. RHE),(b) Co(II)BTT (2.0 V vs. RHE) and (c) Ni(II)BTT (1.52 V vs. RHE). (d) Oxygen evolution reaction image from the experimental set up in 0.1 M KOH.

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Fig. 12 (a) Mechanistic pathway of the hydrogen evolution reaction of the M(II)BTT complexes (Pd, Co and Ni). (b) Mechanism of the oxygen evolution
reaction of the M(II)BTT complexes (Pd, Co and Ni).

an oxygen molecule. Hence, the Ni(II) complex showed better
OER catalytic activity compared to Co(II) and Pd(II). However,in the case of the HER, the Pd(II) complex exhibited enhancedhydrogen evolution reaction activity due to the aﬃnity of
palladium towards hydrogen. The Ni and Pd complexes exhibit
promising activity in terms of overpotential towards both thehydrogen and oxygen evolution reaction.
The kinetics of the oxygen evolution reaction were evaluatedby impedance analysis under alkaline conditions. All threecomplexes’ onset potential were used to obtain impedanceplots. The lower the semicircle diameter, the better the electrocatalytic eﬃciency of the catalyst. The Ni(II) complex showed
good catalytic activity with a charge transfer resistance value of
246 ohms (Fig. 9d). Pd(II) and Co(II) exhibited a charge transfer
resistance of 1600 ohms and 2700 ohms, respectively, as shownin Fig. 9d. The Ni(II)BTT complex showed the least chargetransfer resistance compared to the Co and Pd complexes. The
catalysts showed a Cdl value of 20.04 mF cm2, 14.9 mF cm2,
and 43.2 mF cm2 for the Pd, Co, and Ni complexes, respectively(Fig. 11a), thus reinforcing the Ni complex as an excellent
electrocatalyst towards the oxygen evolution reaction.
The durability of the complexes was tested using chronoamperometric i–t curves (Fig. 12). From Fig. 12a–c, in the case of
the Ni(II)BTT complex, it can be seen that the current density
gradually decreases at 1.48 V (vs. RHE) after 4 h demonstrating
good durability for all the three complexes. Linear sweep
voltammetry was performed before and after 6 h of chronoamperometry, and the catalysts showed 85% of current density
retention capability (Fig. 11b).

density of 10 mA cm2. The enhanced catalytic activity is due tothe conducting and porous nature of the graphitized carbon
cloth. Furthermore, the pristine Ni(II) complex exhibited an
overpotential value of 250 mV to achieve 10 mA cm2 for OER
activity. The Ni(II) metal has strong aﬃnity for O2 molecules,
leading to better oxygen evolution activity the other two metal
complexes. The Ni(II)BTT complex is preferred over the other two
metal complexes due to its low cost and abundance on theearth’s crust. The Ni(II)BTT complex exhibited good durability
with 85% current density retention capability. This work paves a
new avenue to design eﬃcient electrocatalysts in combination
with more eﬀective carbon supports for clean energy storage andconversion.

Author contributions
Ram Murthy – original manuscript writing, data curation,
analysis, data interpretation revision. C. N. Sundaresan – conceptualization, ideation, review and editing, supervision.

Conflicts of interest
The authors declare there is no conflict of interest to report.

Acknowledgements

In conclusion, M(II)BTT/GrCC hybrid electrocatalysts weresynthesized using 2-thioureidobenzheteroazole metal complexes
and graphitized carbon cloth. The hybrid catalysts were utilized
for the HER and the pristine metal complexes were used for the
OER. The Pd(II)BTT complex showed an outstanding overpotential value of 178 mV to achieve a current density of 10 mA cm2
in 0.5 M H2SO4. In contrast, the Ni(II)BTT/GrCC hybrid catalyst
showed an overpotential value of 335 mV to drive a current

The authors express their deep sense of gratitude to theirfounder, chancellor Bhagawan Sri Sathya Sai Baba, for being
a constant source of inspiration and motivation for the work.
RM is grateful to Prof. Noriyoshi Matsumi and Dr Rajashekar
Badam at ‘Japan Advanced Institute of Science Technology’(JAIST) and Sakura Science Exchange Program for providing
hands on training with the instruments. The authors thank Dr
Raman Vedarajan from ARCI for his insightful suggestions anddiscussions. RM is thankful to Dr B. V. V. S. Pavan Kumar, IIT
Roorkee, for the XPS characterization facility. The authors
acknowledge Central Research Instrumentation Facility (CRIF)
for the characterization facility. The authors thank SSSIHL for
the financial support for carrying out the work.

© 2022 The Author(s). Published by the Royal Society of Chemistry

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8210 | Mater. Adv., 2022, 3, 8201–8210© 2022 The Author(s). Published by the Royal Society of Chemistry