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Nickel centered metal-organic complex as an electro-catalyst for hydrogen evolution reaction at neutral and acidic conditions
international journal of hydrogen energy xxx (xxxx) xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Nickel centered metal-organic complex as an electro-catalyst for hydrogen evolution reaction at neutral and acidic conditions Madhumitha Rajakumar a, Matheswaran Manickam b, Nagarajan Nagendra Gandhi a, Karuppan Muthukumar b,* a
Department of Chemical Engineering, Alagappa College of Technology, Anna University, Chennai 600025, Tamil Nadu, India b Department of Chemical Engineering, National Institute of Technology, Tiruchirappalli 620015, Tamil Nadu, India
highlights Dihydropyrimidines substituted nickel electro-catalysts were synthesized and characterized. Catalyst showed excellent hydrogen evolution reaction (HER) activity. Low Tafel slope indicated that the reaction follows Volmer-Heyrovsky mechanism.
article info
abstract
Article history:
In this study, we report the development of dihydropyrimidines substituted nickel electro-
Received 22 July 2019
catalyst at mild conditions for hydrogen evolution reaction. The Biginelli type reaction
Received in revised form
was carried out to produce 4-(4-chlorophenyl)-3,4,5,6-tetrahydrobenzo[h]quinazoline-2(1H)-thi-
26 November 2019
one moiety. This was added with NiCl2.6H2O to synthesize Ni-quinazoline-2(1H)-thione cata-
Accepted 4 December 2019
lyst. The prepared catalyst was characterized using UVeVis, FT-IR, Mass, 1H and
Available online xxx
and SEM-EDX spectroscopy. It showed an excellent hydrogen evolution reaction (HER)
13
C NMR
activity. LSV polarization showed a low Tafel slope (34 mV/dec) nearly close to Pt/C (28 mV/ Keywords:
dec) in neutral medium whereas a Tafel slope of 100 mV/dec was observed with 0.5M H2SO4
Nickel complex
while Pt/C showed 50 mV/dec. Electrochemical impedance spectroscopy measured by
Biomolecule
applying 1.18 V in neutral medium showed two depressed semicircles whereas with 0.5M
Polarization measurements
H2SO4 and -1.45 V applied voltage single half was observed.
Overpotential
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
HER
Introduction Hydrogen is a promising alternate for carbon-based fuels and can be produced from cathodic hydrogen evolution reaction (HER). Hydrogen is an ideal energy carrier with no emission of
toxic pollutants during combustion [1]. The numerous studies pertaining to electrochemical hydrogen production uphold the importance of catalytic material used and its sustainability [2]. Metals such as Au, Co, Cu, Ni, Pd, Pt, Mo, Rh and Fe are extensively used as catalysts in HER. Among these metals,
* Corresponding author. E-mail address: [email protected] (K. Muthukumar). https://doi.org/10.1016/j.ijhydene.2019.12.031 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Rajakumar M et al., Nickel centered metal-organic complex as an electro-catalyst for hydrogen evolution reaction at neutral and acidic conditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.031
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Pt is a potential candidate for HER [3]. Though its overpotential is less, high cost hinders its application. The need for cost effective catalysts with high durability, low overpotential and excellent energy carrying capacity is inevitable. Nickel has been proposed instead of platinum as it is a non-precious transition metal catalyst [4]. The combination of Ni and other metals - binary and ternary alloys - such as Ni-Fe [5], NiMo [6], Ni-Se [7], Ni-Co [8], Ni-Cu [9], Ni-Mo-N [10], Ni-Fe-S [11], and Ni-Mn-S [12,13] were also used for HER. The main problem associated with these alloys is their electrochemical behavior control. On the other hand, metal particles embedded with stable biomolecules are new class of materials used for HER. Xulien et al. reported biomolecule-assisted one-pot synthesis procedure for the fabrication of novel CdS/MoS2/graphene hollow spheres. The composite prepared using cysteine (biomolecule) had a hollow like structure and biomolecule controlled the morphology of the composite. These hollow spheres were used for visible-light driven water-splitting and characterized using time-resolved fluorescence, electrochemical impedance and Mott Schottky measurements [14]. Caolong et al. prepared CdS and Bi2S3 hollow nano-spheres by simple “one-pot” biomolecule-assisted hydrothermal method using glutathione (GSH) as sulfur source and structuredirecting reagents. This showed a good absorbance under visible light irradiation (l 420 nm) in an aqueous solution by providing an electron donor for hydrogen evolution. Importantly, Pt was used as a co-catalyst in this study. Photocatalytic hydrogen evolution was performed in a topirradiation Pyrex cell [15]. Notable number of homogeneous catalysts have been reported in the recent past. But mostly they have been used in an organic media [16e19]. Transition metal complexes that works at acidic, basic and neutral conditions fulfilled the gap created by homogeneous catalysts. Zou et al. synthesized cobalt-embedded nitrogen rich molecular catalyst on carbon nanotubes for hydrogen evolution at neutral, acidic and basic conditions. The catalyst showed a Tafel slope of 69 mV/dec and current density of 10 mA/cm2 at an overpotential of 0.26 V in 0.5M H2SO4 [20]. Tang et al. reported metal organic framework (MOF) derived MoS2/CoS2 as electrocatalytic material in 0.5M H2SO.4 It showed a Tafel slope of 30 mV/dec and current density of 10 mA/cm2 at an overpotential of 90 mV. At an overpotential of 85 mV in 1 M KOH, 10 mA/cm2 of current density was arrived with 34 mV/dec of Tafel slope [21]. McCrory et al. prepared tetraazamacrocyclic based complexes and coated onto glassy carbon. NaClO4 was added as acid source to make the pH 2.2 and obtained an exchange current density of 0.51 mA/cm2 at an overpotential of 0.8 V [22]. Transition-metal phosphide/n-doped carbon frameworks was developed by Pu et al. to find its catalytic activity at 0.5M H2SO4 and reported a Tafel slope of 138 mV/dec [23]. An overpotential of 41mV was required to attain 10 mA/cm2 at a potential range of þ0.1 and 0.3VRHE [24]. Molybdenum phosphide doped on carbon nanotubes was prepared by Adam et al. and electrocatalytic studies carried out with and without oxalic acid gave Tafel slopes of 51.6 and 54.5mV/dec, respectively [25]. Kuo et al. found HER activity of plasmodic silver nanocubes (AgNCs) with (100) facet and silver nanooctahedra (AgNOs) on carbon fibre paper (CFP) in 0.5M H2SO4 and 1M ethanol with and without 808 nm laser irradiation using LSV
in a conventional three electrode system. Current density of 24.8, 18.9, 13.3 and 7.6 mA/cm2 at 0.4VRHE were obtained for AgNCs and AgNOs on CFP substrate without laser irradiation, respectively. In presence laser irradiation, current density of 0.430 mA/cm2, 0.271 mA/cm2 was obtained for AgNOs and AgNCs [26]. Cobalt-doped iron pyrite (P/Co-FeS2) catalysts surface modified with phosphide - was designed by Kuo et al. P/Co-FeS2 showed an overpotential of 60 mV at 20 mA/cm2 and a Tafel slope of 41mV/dec on carbon fiber paper [27]. The main aim of the present study was to synthesize a quinazoline thione based biomolecule embedded on nickel -serves as the central metal to form metal-organic complex that can help and promote cathodic reaction for enhanced hydrogen evolution by base catalysed three component onepot Biginelli type-reaction. Electrocatalytic activity of the synthesized nickel complex was studied in both neutral and acidic conditions and characterized using UVeVis, FTIR, SEMEDX, Mass, 1H and 13C analyses. Finally, the electrochemical studies were carried out using the complex prepared.
Synthesis The ligands and their metal complexes used for hydrogen evolution experiments were prepared using chemicals such as formic acid (HCOOH), acetonitrile (ACN), potassium bromide (KBr), tetrahydrofuran (THF), deuterated DMSO (DMSO-d6), deuterated chloroform (CHCl3-d), Nafion 117 solution, ethanol (C2H5OH), thiourea (CH4N2S), nickel chloride hexahydrate (NiCl2.6H2O) purchased from Merck, Sigma Aldrich and Alfa Aesar. They were used without any further purification. Sodium dihydrogen phosphate dihydrate (NaH2PO4.2H2O), and disodium hydrogen phosphate dihydrate (Na2HPO4.2H2O) were used for the preparation of phosphate buffer solution (PBS). Nitrogen gas was used to maintain inert atmosphere and double distilled was used for the synthesis. In the recent past, quinazoline-2(1H)-thione moiety based compounds have been synthesized by three component onepot Biginelli type-reaction arenas. Dihydropyrimidines are important class of biomolecules with anticancer, antibacterial, antiviral and calcium channel blockers. Generally, the methods followed for the synthesis of biomolecules employs microwave irradiation, ultrasound, Lewis acids, protic acids and ionic liquids. Irrespective of good yield, the above protocols utilize toxic reagents, require strong acidic media and are not cost-effective. Thus, an interest to produce these bioactive compounds via modified-Biginelli type-reaction is emerging [28,29]. Though acid based Biginelli type-reaction is on the focus, alternate base-mediated three component Biginelli reaction is also gaining importance. Herein we combined cyclic b-ketone 1, an aromatic aldehyde 2, and thiourea 3 under basic condition in the presence of NaOH (Fig. 1). The aim is to develop a Ni based complex that can reduce proton catalytically. The one-pot synthesized moiety was further treated with tetrahydrofuran and added with dropwise NiCl2.6H2O [29]. 3,4-dihydrophthalen-1(2H)-one 1 (3 mmol) was dissolved in ethanol (5 mL) and added to 4-chlorobenzaldehyde 2 (3 mmol) and thiourea 3 (3 mmol) in the presence of NaOH (3 mmol). This solution was refluxed at 70 C for 1 h 30 min. The
Please cite this article as: Rajakumar M et al., Nickel centered metal-organic complex as an electro-catalyst for hydrogen evolution reaction at neutral and acidic conditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.031
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S O
O
1
Step 1
+ Cl
H+ 2
S H2N
HN
NH
NaOH
NH2 EtOH, 70 oC, 1.5 h 3
Cl
4
Step 2 NiCl2.6H2O
THF
NH N S Cl
HN
S Ni
N
S
N N H Cl Cl
5 Fig. 1 e Scheme of three component one-pot Biginelli type-reaction followed by the crystallization of the metal complex by the addition of nickel chloride.
formation of yellow precipitate was observed in 15 min after the commencement of the reaction and that turned into a white solid later. The solid residue obtained by filtration was washed with cold ethanol (10 mL). The obtained moiety was washed several times to produce 3,4,5,6-tetrahydro-4substituted benzo[h]quinazoline-2(1H)-thione 4: Yield: 0.81 g, (83%); White solid; mp 252e254 C; IR (KBr): 3194, 2924, 1575, 1486, 1453, 1193, 1088, 1015, 779 cm1; 1HNMR (400 MHz, DMSO-d6): d 9.78 (s, 1H), 9.09 (s, 1H), 7.68 (m, 1H), 7.47 (m, 2H), 7.33 (d, 2H, J ¼ 8.46 Hz), 7.19 (m, 3H), 4.97 (s, 1H), 2.76 (m, 2H), 2.12 (m, 1H), 1.82 (m, 1H); 13C NMR (100 MHz, DMSO-d6): d 164.7, 142.2, 135.9, 132.9, 129.3, 129.1, 128.0, 127.3, 126.8, 122.1, 111.1, 58.1, 27.7, 23.9; MS (EI): m/z ¼ 328 [Mþ2]. This one-pot base mediated Biginelli type-reaction was carried out based on the procedure reported by Sivakumar et al. (2017) [28] (Figs. S1eS3). In a round-bottomed flask, compound 4 (3,4,5,6tetrahydro-4-substituted benzo[h]quinazoline-2(1H)-thione) (3 mmol) was placed in 20 mL tetrahydrofuran. The mixture was stirred at room temperature for 15 min. Then NiCl2.6H2O (1 mmol) dissolved in ethanol (5 mL) was added dropwise at equivalent temperature and the contents were stirred for 24 h. The progress of the reaction was monitored using thin layer
chromatography (TLC) and UVeVis spectrophotometer. After the completion of the reaction, the contents were filtered overnight. A pale yellow crystal of Ni based quinazoline-2(1H)thione moiety was obtained. The complex was characterized using UV, FT-IR, SEM and EDAX analyses.
Electrochemical experiments The electrochemical studies were conducted in a three electrode configuration on a Metrohm Autolab PGSTAT302N potentiostat with modified glassy carbon (GC) (2.5 mm) working electrode (WE), Ag/AgCl/NaCl (3.00 molL-1) and platinum wire (1 mm) as a reference and counter electrode (CE), respectively at neutral (phosphate buffer) and acidic conditions (0.5 M H2SO4). The synthesized nickel complex was coated on the geometric coating area (0.196 cm2) of GC. About 2.00 mg of the metal complex was pre-mixed with dichloromethane (DCM), which was impregnated on the surface of the GC and allowed to dry under air. About 2 mL of 0.5 wt% Nafion solution was added further to protect the casted GC and to bind the complex onto the surface of GC, which was dried at 50 C. All electrochemical tests were carried out at a potential
Please cite this article as: Rajakumar M et al., Nickel centered metal-organic complex as an electro-catalyst for hydrogen evolution reaction at neutral and acidic conditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.031
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range of 0.5 to 2.5 V vs Ag/AgCl. EIS was recorded at 1.45 V for acidic conditions (0.5 M H2SO4) and at 1.18 V for neutral conditions (overpotential at 10 mA/cm2).
Analytical methods JASCO FT-IR spectrophotometer model 400 plus and JASCO FT-IR 4700LE were used to record infrared (FT-IR) spectrogram. UVeVisible double beam spectrophotometer (Perkin Elmer) was used to record the spectrum of both ligand and Niquinazoline complex. The melting point of the samples was determined using melting point apparatus supplied by Guna Enterprise - to make sure the samples prepared were pure. The prepared ligand was confirmed using 1HNMR (400 MHz and 500 MHz) and 13C NMR (100 MHZ and 125 MHz) (BRUKERAVANCE III-FT NMR) spectrometer with TMS as an internal standard (chemical shift in d ppm) and DMSO-d6 as a solvent. In this study, chemical shifts are expressed in d downfield from the signal of internal TMS. SEM-EDX analysis was performed to find the morphology, chemical composition, crystalline structure and coordination of the material that make up the sample. It was conducted in FEI Quanta FEG 200-High Resolution Scanning Microscope (SEM). The image was scanned in a magnification range 10, 20, 50 and 100 mm.
Results and discussion UV-spectra of the synthesized ligand 4 and Ni-complex 5 Fig. 2 illustrates the normalized UVeVis spectrum for both quinazoline-2(1H)-thione moiety and Ni-complex prepared. A
single distinct peak for the ligand (red in color) was observed at a shorter wavelength region (250e350 nm). This shorter wavelength shift is known as hypsochromic shift. Ni complex (black) shows an increase in the peak height at identical wavelength observed for ligand. The UVeVis structure is usually broad for polar samples since they do not have vibrational frequency. The complex synthesized by us is miscible only in non-polar solvents like THF, DCM and DCE. The side peak alongside of the ligand peak may be due to the presence of nickel in the complex prepared. For both ligand and complex, absorption was seen only in the UV region. This corresponds to pp* electron transition, which indicates that the complex has an ability to undergo electronic transition if excited by any external source.
FT-IR spectrum of ligand 4 and Ni-complex 5 Fig. 3a shows the FT-IR spectrum of ligand and Fig. 3b shows the spectrum of the complex. Fig. 3a shows covalent bonding range of all the atoms involved in the quinazoline based ligand. The CeH and CeC stretching absorption frequencies was around 2975-2850 cm1. This may be due to the presence of saturated carbon sites in the pyrimidine ring of quinazoline part of ligand. The symmetric C]C group absorption band occurred at 1680-1640 cm1. The aromatic heterocyclic structure of quinazoline includes a benzene ring along with a pyrimidine ring. The symmetric unsaturated absorption C]C corresponds to this site. The CH2 bending intensity had an absorption in the range 1480e1440 cm1. The C]CeH group stretching intensity was observed around 3100-3020 cm1. In C]CeH stretching, the CeH part shows the coordination with chlorobenzene and nitrogen of the pyrimidine ring. Two amine groups have stretching intensities with an absorption
Fig. 2 e UVeVis spectra for the quinazoline-2(1H)-thione moiety and Ni-quinazoline complex framework. Please cite this article as: Rajakumar M et al., Nickel centered metal-organic complex as an electro-catalyst for hydrogen evolution reaction at neutral and acidic conditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.031
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Fig. 3 e FT-IR spectroscopy (a) Ligand (b) Complex.
range of 3350e3300 cm1. The C]S group absorption ranged between 1045 and 1010 cm1. The aromatic region of absorption occurred around 2000-1650 cm1. Additional absorption at 800-600 cm1 was assigned to CeCl absorption. In Fig. 3b, the peak observed in the range of 2975e2850 cm1 indicates CeC stretching present in the quinazoline part of the complex. The FT-IR spectrum of Nicomplex showed a shift and this indicated the formation of Ni-based complex. There was a slight shift in the region of 3100e3020 cm1 due to the C]CeH group stretching. The coordination between the metal (Nickel) and amine group showed absorption at 3350-3300 cm1. This confirms the formation of dipolar covalent bond between metal and ligand. A lower absorption observed in the range 1045e1010 cm1 was tentatively assigned to sulfur group (CeS) attached to the central metal (Ni). The presence of CeCl bonds and aromatic bicyclic ring was confirmed by the characteristic peaks at 800600 cm1 and 2000-1650 cm1, respectively.
Scanning electron microscopy and energy dispersive X-ray spectroscopy Fig. 4 shows the SEM-EDX analysis of Ni complex prepared and the images obtained showed fine crystalline particles. These particles exhibited star-like structure with sharp edges and all images indicated same morphological behavior. The elemental analysis of the complex illustrates the atoms present in the crystallized Ni-quinazoline-2(1H)-thione complex. Sharp distinctive peaks for N, S, C, Cl and Ni can be clearly seen from Fig. 4.
Hydrogen evolution reaction 4-(4-chlorophenyl)-3,4,5,6-tetrahydrobenzo[h]quinazoline2(1H)-thione is a biomolecule that possess anticancer and antimicrobial activity. The complex is protected by chlorobenzene and protonation occurs only in heteroatoms that are attached to the central metal. Since this compound has biological activity, it possesses larger surface area that helps in the protonation. For electro-catalytic hydrogen evolution, initially the protonation occurs in the nitrogen
group of the quinazoline ring. After protonation, the central metal gets oxidized and releases electron. The heteroatom losses the proton obtained from the process of water splitting to produce hydrogen in the surface of the electrode (GC). The electro-catalytic HER activity of the Ni-quinazoline2(1H)-thione moiety was investigated by performing linear sweep voltammetry (LSV) in a potential range from 0.5 to 2.5 V at neutral condition and at a sweep rate of 100 mV/s with phosphate buffer as the solvating agent. The same condition was used in 0.5 M H2SO4 medium. The Tafel plot was obtained by fitting the Tafel equation: ɳ ¼ a þ blogj, where ɳ is the overpotential, ‘a’ is the intercept in mV, ‘b’ is the Tafel slope in mVdec1 and ‘j’ is the current density (Fig. 5). The different scans performed at various rates showed stable reduction from 0.5 to 2.5 V. Highest reduction was observed at a scan rate of 100 mV/s. Current-potential data corresponding to 100 mV/s were obtained and current density was determined. This current density was converted to logarithmic scale and plotted versus potential given. Exchange current density for the charge transfer processes should be high and in this study, we obtained a value of 1.12 mA/cm2 and -0.5 mA/cm2 for neutral and acidic conditions, respectively. The electro-catalytic reaction kinetics was formulated based on three principles: Hþ þ e
/ / / /
Had þ e þ Hþ
Had (Volmer reaction) H2 (Heyrovsky reaction)
Had þ Had
H2 (Tafel reaction)
2Hþ þ 2e
H2 (Overall reaction)
The hydrogen evolution kinetics was evaluated by two mechanisms: Either it takes place by fast discharge of protons called Volmer reaction followed by electrochemical recombination of slowly discharged proton with an additional proton known as Heyrovsky reaction or slowly discharged protons combined together to produce hydrogen at the electrode surface.
Please cite this article as: Rajakumar M et al., Nickel centered metal-organic complex as an electro-catalyst for hydrogen evolution reaction at neutral and acidic conditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.031
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Fig. 4 e SEM-EDX analysis of the Ni-quinazoline complex.
Fig. 5 e Tafel plots obtained from the LSV polarization curves for phosphate buffer (neutral) (a) and 0.5M H2SO4 (b).
Tafel parameter represents the proton discharge aspects and a slope value of 120 mV/dec indicates Volmer reaction as the rate determining step for HER. Tafel slope value of 30 or 40 mV/dec indicates either Tafel or Heyrovsky as the rate
determining step. Hydrogen reduction either follows VolmerTafel or Volmer-Heyrovsky mechanism. A Tafel slope of 34 mV/dec was calculated from the linear sweep voltammetry performed in the potential range from 0.5 V and 2.5 V. The
Please cite this article as: Rajakumar M et al., Nickel centered metal-organic complex as an electro-catalyst for hydrogen evolution reaction at neutral and acidic conditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.031
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low value of Tafel slope suggests the HER followed VolmerHeyrovsky reaction with Heyrovsky as the rate-determining step in neutral condition. It should be noted that lower the Tafel slope, higher will be the hydrogen evolution from the surface of the working electrode. Diffusion of the electron between the Ni complex coated on the surface of the glassy carbon and electrolytic solution used was determined by the exchange current density. This parameter also shows the kinetic response of a cathode material assembled in a three electrode system. It was obtained by extrapolating the Tafel plot to the zero current potential in the x-axis. In presence of 0.5 M H2SO4, a Tafel slope of 100 mV/dec was obtained, which indicates Tafel reaction as the rate determining step.
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Electrochemical Impedance Spectroscopy Fig. 6 shows the Electrochemical Impedance Spectroscopy (EIS) of the complex and electrolytic solution. The kinetics of charge transfer and performance of catalyst can be determined using EIS. A resistance developed in the three compartment system in a potentiostat with the reaction conditions of the study material was studied using Nyquist plot. Impedance spectra can determine one or more electrochemical reactions taking place in a single reaction system. The charge transfer resistance is indicated by the diameters of Nyquist plot. In this plot, there are two depressed capacitive semicircles that show the behavior of
Fig. 6 e Electrochemical impedance spectroscopy of Ni-complex in neutral (a) and 0.5M H2SO4 (b).
Please cite this article as: Rajakumar M et al., Nickel centered metal-organic complex as an electro-catalyst for hydrogen evolution reaction at neutral and acidic conditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.031
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Table 1 e Electrochemical impedance spectroscopy parameters of nickel complex at neutral condition. Parameters
Ni complex
R1(U) R2(kU.cm2) R3(kU.cm2) C2(F) C3(F)
57.4 0.67 5.62 3.53*109 0.911*108
Table 2 e Electrochemical impedance spectroscopy parameters of nickel complex at acidic condition (0.5M H2SO4). Parameters R1 (U) C2 (F) R2 (U)
Ni complex in acidic condition 1*103 0.1*106 100
Fig. 7 e Cyclic voltammetry polarization of Ni complex at 100 mV/s at neutral condition.
the surface where hydrogen bubbles evolved. The polarization resistance of the solution (Rs ¼ R1 þ R2) illustrated in the plot suggest that the larger semicircle corresponds to the solution resistance and the smaller circle indicates the catalyst resistance. Smaller degree of resistance implies the electro-catalyst resistance since the electrode surface had faster hydrogen abstraction sites and electrochemically active surface area. The resistance is inversely proportional to the current density. The increased catalytic sites near the electrode surface for hydrogen adsorption may facilitate faster charge transfer and thus reducing the charge transfer resistance. The increase in solution resistance is due to the ineffective electron transport and uneven rushing of hydrogen bubbles onto the electrode surface. Hence, larger depressed semicircle was formed. A potential of 1.18 V was applied for the impedance study since 10 mA/cm2 (current density) was obtained at this potential at neutral condition. In presence of 0.5M H2SO4, a potential
Fig. 8 e Cyclic voltammetry polarization of Ni complex at different scan rates in 0.5M H2SO4.
Please cite this article as: Rajakumar M et al., Nickel centered metal-organic complex as an electro-catalyst for hydrogen evolution reaction at neutral and acidic conditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.031
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Fig. 9 e (A) SEM analysis of Ni-complex taken before the electrochemical study; (B) after the HER activity in neutral medium; (C) after HER activity in 0.5M H2SO4.
of 1.45 V was applied to perform the impedance study. The equivalent electrical circuit diagram for Niquinazoline-2(1H)-thione moiety in phosphate buffer solution is shown in Fig. 6a. The formula obtained from the circuit diagram is given below and the relevant values are shown in Table 1. Rs ¼ R1 þ C2/R2 þ C3/R3 Fig. 6b shows the half wave semicircle along with an equivalent circuit diagram (Rs ¼ R1þC1/R2) for Ni-quinazoline2(1H)-thione moiety in 0.5M H2SO4 and the relevant values are given in Table 2.
SEM analysis before and after the reduction reaction Fig. 9A shows the SEM image of the complex before HER and it shows crystalline, rough and uneven pattern of Ni catalyst formation. After the potentiostatic analysis with three electrode system, SEM image of the surface of the glassy carbon in neutral media shows scratched surface (where the catalyst was dip coated) (Fig. 9B). Fig. 9C shows the image of the electrode in acidic medium. In both the images, fine crystalline molecules were broken and uneven clustered particles were observed. This confirms the occurrence of reduction during the reaction.
Stability tests and TGA Electrochemical stability A simple potential-current polarization curve was recorded at a scan rate of 100 mV with glassy carbon as a working electrode, platinum wire as a counter electrode and Ag/AgCl as the reference electrode at pH 7 (Fig. 7a, b) and with 0.5M H2SO4 (Fig. 8a, b). The starting potential was fixed as 0.5 V and the voltammetric studies indicated the occurrence of reduction at an overpotential of 1.45 V. The reduction sweeps through the negative region extending till 2.5 V confirms the stability of Ni-quinazoline complex. Different scan rates (50e200 mV) were set to the same system of fixed 0.5 V to 2.5 V potential at neutral pH. Catalytic material coated on the surface of glassy carbon was taken as the working electrode. Reduction started at almost same overpotential and gave different reduction rates. Among all, scan rate of 100 mV showed higher reduction than other scan rate. This scan rate was fixed for the HER study.
Cyclic voltammetry polarization was carried out at a scan rate of 100 mV/s for 10 cycles. Fig. 10A, B show the CV curves of nickel complex in neutral and 0.5M H2SO4 medium. The catalyst was found to be stable and minor changes only were observed. Fig. 11 shows thermos-gravimetric analysis of the synthesized nickel complex and it shows three zones. First two portions are predominant in weight loss and the third part is with minimum loss (8%). The zone 1 was attributed to nickel nitrate. Nickel nitrate salt usually starts to decompose just above the room temperature and here the decomposition starts at 54.15 C. Moreover, its melting point is 136.7 C. In zone 1, 16.41% weight loss was observed. The elemental analysis showed the amount of Ni present as 17%. Thus this loss is due to the fragmentation of Ni(NO3)2.6H2O. The zone2 exhibited a major weight loss of 61.64% due to the decomposition of ligand present in the complex. The remaining residual (0.00699 mg) is only negligible indicating
Please cite this article as: Rajakumar M et al., Nickel centered metal-organic complex as an electro-catalyst for hydrogen evolution reaction at neutral and acidic conditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.031
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Conclusion This study demonstrated the usage of Ni-quinazoline complex framework as a promising electrocatalyst for HER. The base catalysed three component one-pot Biginelli reaction was carried out and the obtained 4-(4-chlorophenyl)-3,4,5,6tetrahydrobenzo[h]quinazoline-2(1H)-thione moiety was successfully characterized. The further synthesized Ni-quinazoline-2(1H)-thione was characterized and subjected to the electrochemical hydrogen activity in neutral and acidic medium. In phosphate buffer electrolyte, the metal complex followed Volmer-Heyrovsky mechanism with a very good catalytic reduction capacity. In presence of 0.5M H2SO4, the metal complex followed Volmer-Tafel mechanism. Stability of the Ni catalyst on the surface of the glassy carbon was found by performing current-potential experiment in a range of 0.5 to 2.5 V. The reduction started at around 1.45 V and increased for different scan rate. This confirmed the better stability of Ni-quinazoline complex. The EIS indicated smaller degree of resistance for the Ni-quinazoline catalyst. The SEM analysis performed using Ni-quinazoline catalyst remaining in the glassy carbon depicted the breakdown of the fine crystalline Ni-complex. We conclude that this dihydropyrimidines based Ni complex reported for HER activity in this study was found to be an effective electrocatalyst.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.12.031.
references Fig. 10 e Cyclic voltammetry polarization curves for 10 cycles in neutral (A) and 0.5M H2SO4 (B). complete decomposition of the molecules. Majidi et al. reported similar pattern for decomposition for nickel nitrate [30].
Fig. 11 e TGA-DTA curve of the nickel complex.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Nickel centered metal-organic complex as an electro-catalyst for hydrogen evolution reaction at neutral and acidic conditions Madhumitha Rajakumar a, Matheswaran Manickam b, Nagarajan Nagendra Gandhi a, Karuppan Muthukumar b,* a
Department of Chemical Engineering, Alagappa College of Technology, Anna University, Chennai 600025, Tamil Nadu, India b Department of Chemical Engineering, National Institute of Technology, Tiruchirappalli 620015, Tamil Nadu, India
highlights Dihydropyrimidines substituted nickel electro-catalysts were synthesized and characterized. Catalyst showed excellent hydrogen evolution reaction (HER) activity. Low Tafel slope indicated that the reaction follows Volmer-Heyrovsky mechanism.
article info
abstract
Article history:
In this study, we report the development of dihydropyrimidines substituted nickel electro-
Received 22 July 2019
catalyst at mild conditions for hydrogen evolution reaction. The Biginelli type reaction
Received in revised form
was carried out to produce 4-(4-chlorophenyl)-3,4,5,6-tetrahydrobenzo[h]quinazoline-2(1H)-thi-
26 November 2019
one moiety. This was added with NiCl2.6H2O to synthesize Ni-quinazoline-2(1H)-thione cata-
Accepted 4 December 2019
lyst. The prepared catalyst was characterized using UVeVis, FT-IR, Mass, 1H and
Available online xxx
and SEM-EDX spectroscopy. It showed an excellent hydrogen evolution reaction (HER)
13
C NMR
activity. LSV polarization showed a low Tafel slope (34 mV/dec) nearly close to Pt/C (28 mV/ Keywords:
dec) in neutral medium whereas a Tafel slope of 100 mV/dec was observed with 0.5M H2SO4
Nickel complex
while Pt/C showed 50 mV/dec. Electrochemical impedance spectroscopy measured by
Biomolecule
applying 1.18 V in neutral medium showed two depressed semicircles whereas with 0.5M
Polarization measurements
H2SO4 and -1.45 V applied voltage single half was observed.
Overpotential
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
HER
Introduction Hydrogen is a promising alternate for carbon-based fuels and can be produced from cathodic hydrogen evolution reaction (HER). Hydrogen is an ideal energy carrier with no emission of
toxic pollutants during combustion [1]. The numerous studies pertaining to electrochemical hydrogen production uphold the importance of catalytic material used and its sustainability [2]. Metals such as Au, Co, Cu, Ni, Pd, Pt, Mo, Rh and Fe are extensively used as catalysts in HER. Among these metals,
* Corresponding author. E-mail address: [email protected] (K. Muthukumar). https://doi.org/10.1016/j.ijhydene.2019.12.031 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Rajakumar M et al., Nickel centered metal-organic complex as an electro-catalyst for hydrogen evolution reaction at neutral and acidic conditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.031
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Pt is a potential candidate for HER [3]. Though its overpotential is less, high cost hinders its application. The need for cost effective catalysts with high durability, low overpotential and excellent energy carrying capacity is inevitable. Nickel has been proposed instead of platinum as it is a non-precious transition metal catalyst [4]. The combination of Ni and other metals - binary and ternary alloys - such as Ni-Fe [5], NiMo [6], Ni-Se [7], Ni-Co [8], Ni-Cu [9], Ni-Mo-N [10], Ni-Fe-S [11], and Ni-Mn-S [12,13] were also used for HER. The main problem associated with these alloys is their electrochemical behavior control. On the other hand, metal particles embedded with stable biomolecules are new class of materials used for HER. Xulien et al. reported biomolecule-assisted one-pot synthesis procedure for the fabrication of novel CdS/MoS2/graphene hollow spheres. The composite prepared using cysteine (biomolecule) had a hollow like structure and biomolecule controlled the morphology of the composite. These hollow spheres were used for visible-light driven water-splitting and characterized using time-resolved fluorescence, electrochemical impedance and Mott Schottky measurements [14]. Caolong et al. prepared CdS and Bi2S3 hollow nano-spheres by simple “one-pot” biomolecule-assisted hydrothermal method using glutathione (GSH) as sulfur source and structuredirecting reagents. This showed a good absorbance under visible light irradiation (l 420 nm) in an aqueous solution by providing an electron donor for hydrogen evolution. Importantly, Pt was used as a co-catalyst in this study. Photocatalytic hydrogen evolution was performed in a topirradiation Pyrex cell [15]. Notable number of homogeneous catalysts have been reported in the recent past. But mostly they have been used in an organic media [16e19]. Transition metal complexes that works at acidic, basic and neutral conditions fulfilled the gap created by homogeneous catalysts. Zou et al. synthesized cobalt-embedded nitrogen rich molecular catalyst on carbon nanotubes for hydrogen evolution at neutral, acidic and basic conditions. The catalyst showed a Tafel slope of 69 mV/dec and current density of 10 mA/cm2 at an overpotential of 0.26 V in 0.5M H2SO4 [20]. Tang et al. reported metal organic framework (MOF) derived MoS2/CoS2 as electrocatalytic material in 0.5M H2SO.4 It showed a Tafel slope of 30 mV/dec and current density of 10 mA/cm2 at an overpotential of 90 mV. At an overpotential of 85 mV in 1 M KOH, 10 mA/cm2 of current density was arrived with 34 mV/dec of Tafel slope [21]. McCrory et al. prepared tetraazamacrocyclic based complexes and coated onto glassy carbon. NaClO4 was added as acid source to make the pH 2.2 and obtained an exchange current density of 0.51 mA/cm2 at an overpotential of 0.8 V [22]. Transition-metal phosphide/n-doped carbon frameworks was developed by Pu et al. to find its catalytic activity at 0.5M H2SO4 and reported a Tafel slope of 138 mV/dec [23]. An overpotential of 41mV was required to attain 10 mA/cm2 at a potential range of þ0.1 and 0.3VRHE [24]. Molybdenum phosphide doped on carbon nanotubes was prepared by Adam et al. and electrocatalytic studies carried out with and without oxalic acid gave Tafel slopes of 51.6 and 54.5mV/dec, respectively [25]. Kuo et al. found HER activity of plasmodic silver nanocubes (AgNCs) with (100) facet and silver nanooctahedra (AgNOs) on carbon fibre paper (CFP) in 0.5M H2SO4 and 1M ethanol with and without 808 nm laser irradiation using LSV
in a conventional three electrode system. Current density of 24.8, 18.9, 13.3 and 7.6 mA/cm2 at 0.4VRHE were obtained for AgNCs and AgNOs on CFP substrate without laser irradiation, respectively. In presence laser irradiation, current density of 0.430 mA/cm2, 0.271 mA/cm2 was obtained for AgNOs and AgNCs [26]. Cobalt-doped iron pyrite (P/Co-FeS2) catalysts surface modified with phosphide - was designed by Kuo et al. P/Co-FeS2 showed an overpotential of 60 mV at 20 mA/cm2 and a Tafel slope of 41mV/dec on carbon fiber paper [27]. The main aim of the present study was to synthesize a quinazoline thione based biomolecule embedded on nickel -serves as the central metal to form metal-organic complex that can help and promote cathodic reaction for enhanced hydrogen evolution by base catalysed three component onepot Biginelli type-reaction. Electrocatalytic activity of the synthesized nickel complex was studied in both neutral and acidic conditions and characterized using UVeVis, FTIR, SEMEDX, Mass, 1H and 13C analyses. Finally, the electrochemical studies were carried out using the complex prepared.
Synthesis The ligands and their metal complexes used for hydrogen evolution experiments were prepared using chemicals such as formic acid (HCOOH), acetonitrile (ACN), potassium bromide (KBr), tetrahydrofuran (THF), deuterated DMSO (DMSO-d6), deuterated chloroform (CHCl3-d), Nafion 117 solution, ethanol (C2H5OH), thiourea (CH4N2S), nickel chloride hexahydrate (NiCl2.6H2O) purchased from Merck, Sigma Aldrich and Alfa Aesar. They were used without any further purification. Sodium dihydrogen phosphate dihydrate (NaH2PO4.2H2O), and disodium hydrogen phosphate dihydrate (Na2HPO4.2H2O) were used for the preparation of phosphate buffer solution (PBS). Nitrogen gas was used to maintain inert atmosphere and double distilled was used for the synthesis. In the recent past, quinazoline-2(1H)-thione moiety based compounds have been synthesized by three component onepot Biginelli type-reaction arenas. Dihydropyrimidines are important class of biomolecules with anticancer, antibacterial, antiviral and calcium channel blockers. Generally, the methods followed for the synthesis of biomolecules employs microwave irradiation, ultrasound, Lewis acids, protic acids and ionic liquids. Irrespective of good yield, the above protocols utilize toxic reagents, require strong acidic media and are not cost-effective. Thus, an interest to produce these bioactive compounds via modified-Biginelli type-reaction is emerging [28,29]. Though acid based Biginelli type-reaction is on the focus, alternate base-mediated three component Biginelli reaction is also gaining importance. Herein we combined cyclic b-ketone 1, an aromatic aldehyde 2, and thiourea 3 under basic condition in the presence of NaOH (Fig. 1). The aim is to develop a Ni based complex that can reduce proton catalytically. The one-pot synthesized moiety was further treated with tetrahydrofuran and added with dropwise NiCl2.6H2O [29]. 3,4-dihydrophthalen-1(2H)-one 1 (3 mmol) was dissolved in ethanol (5 mL) and added to 4-chlorobenzaldehyde 2 (3 mmol) and thiourea 3 (3 mmol) in the presence of NaOH (3 mmol). This solution was refluxed at 70 C for 1 h 30 min. The
Please cite this article as: Rajakumar M et al., Nickel centered metal-organic complex as an electro-catalyst for hydrogen evolution reaction at neutral and acidic conditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.031
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S O
O
1
Step 1
+ Cl
H+ 2
S H2N
HN
NH
NaOH
NH2 EtOH, 70 oC, 1.5 h 3
Cl
4
Step 2 NiCl2.6H2O
THF
NH N S Cl
HN
S Ni
N
S
N N H Cl Cl
5 Fig. 1 e Scheme of three component one-pot Biginelli type-reaction followed by the crystallization of the metal complex by the addition of nickel chloride.
formation of yellow precipitate was observed in 15 min after the commencement of the reaction and that turned into a white solid later. The solid residue obtained by filtration was washed with cold ethanol (10 mL). The obtained moiety was washed several times to produce 3,4,5,6-tetrahydro-4substituted benzo[h]quinazoline-2(1H)-thione 4: Yield: 0.81 g, (83%); White solid; mp 252e254 C; IR (KBr): 3194, 2924, 1575, 1486, 1453, 1193, 1088, 1015, 779 cm1; 1HNMR (400 MHz, DMSO-d6): d 9.78 (s, 1H), 9.09 (s, 1H), 7.68 (m, 1H), 7.47 (m, 2H), 7.33 (d, 2H, J ¼ 8.46 Hz), 7.19 (m, 3H), 4.97 (s, 1H), 2.76 (m, 2H), 2.12 (m, 1H), 1.82 (m, 1H); 13C NMR (100 MHz, DMSO-d6): d 164.7, 142.2, 135.9, 132.9, 129.3, 129.1, 128.0, 127.3, 126.8, 122.1, 111.1, 58.1, 27.7, 23.9; MS (EI): m/z ¼ 328 [Mþ2]. This one-pot base mediated Biginelli type-reaction was carried out based on the procedure reported by Sivakumar et al. (2017) [28] (Figs. S1eS3). In a round-bottomed flask, compound 4 (3,4,5,6tetrahydro-4-substituted benzo[h]quinazoline-2(1H)-thione) (3 mmol) was placed in 20 mL tetrahydrofuran. The mixture was stirred at room temperature for 15 min. Then NiCl2.6H2O (1 mmol) dissolved in ethanol (5 mL) was added dropwise at equivalent temperature and the contents were stirred for 24 h. The progress of the reaction was monitored using thin layer
chromatography (TLC) and UVeVis spectrophotometer. After the completion of the reaction, the contents were filtered overnight. A pale yellow crystal of Ni based quinazoline-2(1H)thione moiety was obtained. The complex was characterized using UV, FT-IR, SEM and EDAX analyses.
Electrochemical experiments The electrochemical studies were conducted in a three electrode configuration on a Metrohm Autolab PGSTAT302N potentiostat with modified glassy carbon (GC) (2.5 mm) working electrode (WE), Ag/AgCl/NaCl (3.00 molL-1) and platinum wire (1 mm) as a reference and counter electrode (CE), respectively at neutral (phosphate buffer) and acidic conditions (0.5 M H2SO4). The synthesized nickel complex was coated on the geometric coating area (0.196 cm2) of GC. About 2.00 mg of the metal complex was pre-mixed with dichloromethane (DCM), which was impregnated on the surface of the GC and allowed to dry under air. About 2 mL of 0.5 wt% Nafion solution was added further to protect the casted GC and to bind the complex onto the surface of GC, which was dried at 50 C. All electrochemical tests were carried out at a potential
Please cite this article as: Rajakumar M et al., Nickel centered metal-organic complex as an electro-catalyst for hydrogen evolution reaction at neutral and acidic conditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.031
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range of 0.5 to 2.5 V vs Ag/AgCl. EIS was recorded at 1.45 V for acidic conditions (0.5 M H2SO4) and at 1.18 V for neutral conditions (overpotential at 10 mA/cm2).
Analytical methods JASCO FT-IR spectrophotometer model 400 plus and JASCO FT-IR 4700LE were used to record infrared (FT-IR) spectrogram. UVeVisible double beam spectrophotometer (Perkin Elmer) was used to record the spectrum of both ligand and Niquinazoline complex. The melting point of the samples was determined using melting point apparatus supplied by Guna Enterprise - to make sure the samples prepared were pure. The prepared ligand was confirmed using 1HNMR (400 MHz and 500 MHz) and 13C NMR (100 MHZ and 125 MHz) (BRUKERAVANCE III-FT NMR) spectrometer with TMS as an internal standard (chemical shift in d ppm) and DMSO-d6 as a solvent. In this study, chemical shifts are expressed in d downfield from the signal of internal TMS. SEM-EDX analysis was performed to find the morphology, chemical composition, crystalline structure and coordination of the material that make up the sample. It was conducted in FEI Quanta FEG 200-High Resolution Scanning Microscope (SEM). The image was scanned in a magnification range 10, 20, 50 and 100 mm.
Results and discussion UV-spectra of the synthesized ligand 4 and Ni-complex 5 Fig. 2 illustrates the normalized UVeVis spectrum for both quinazoline-2(1H)-thione moiety and Ni-complex prepared. A
single distinct peak for the ligand (red in color) was observed at a shorter wavelength region (250e350 nm). This shorter wavelength shift is known as hypsochromic shift. Ni complex (black) shows an increase in the peak height at identical wavelength observed for ligand. The UVeVis structure is usually broad for polar samples since they do not have vibrational frequency. The complex synthesized by us is miscible only in non-polar solvents like THF, DCM and DCE. The side peak alongside of the ligand peak may be due to the presence of nickel in the complex prepared. For both ligand and complex, absorption was seen only in the UV region. This corresponds to pp* electron transition, which indicates that the complex has an ability to undergo electronic transition if excited by any external source.
FT-IR spectrum of ligand 4 and Ni-complex 5 Fig. 3a shows the FT-IR spectrum of ligand and Fig. 3b shows the spectrum of the complex. Fig. 3a shows covalent bonding range of all the atoms involved in the quinazoline based ligand. The CeH and CeC stretching absorption frequencies was around 2975-2850 cm1. This may be due to the presence of saturated carbon sites in the pyrimidine ring of quinazoline part of ligand. The symmetric C]C group absorption band occurred at 1680-1640 cm1. The aromatic heterocyclic structure of quinazoline includes a benzene ring along with a pyrimidine ring. The symmetric unsaturated absorption C]C corresponds to this site. The CH2 bending intensity had an absorption in the range 1480e1440 cm1. The C]CeH group stretching intensity was observed around 3100-3020 cm1. In C]CeH stretching, the CeH part shows the coordination with chlorobenzene and nitrogen of the pyrimidine ring. Two amine groups have stretching intensities with an absorption
Fig. 2 e UVeVis spectra for the quinazoline-2(1H)-thione moiety and Ni-quinazoline complex framework. Please cite this article as: Rajakumar M et al., Nickel centered metal-organic complex as an electro-catalyst for hydrogen evolution reaction at neutral and acidic conditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.031
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Fig. 3 e FT-IR spectroscopy (a) Ligand (b) Complex.
range of 3350e3300 cm1. The C]S group absorption ranged between 1045 and 1010 cm1. The aromatic region of absorption occurred around 2000-1650 cm1. Additional absorption at 800-600 cm1 was assigned to CeCl absorption. In Fig. 3b, the peak observed in the range of 2975e2850 cm1 indicates CeC stretching present in the quinazoline part of the complex. The FT-IR spectrum of Nicomplex showed a shift and this indicated the formation of Ni-based complex. There was a slight shift in the region of 3100e3020 cm1 due to the C]CeH group stretching. The coordination between the metal (Nickel) and amine group showed absorption at 3350-3300 cm1. This confirms the formation of dipolar covalent bond between metal and ligand. A lower absorption observed in the range 1045e1010 cm1 was tentatively assigned to sulfur group (CeS) attached to the central metal (Ni). The presence of CeCl bonds and aromatic bicyclic ring was confirmed by the characteristic peaks at 800600 cm1 and 2000-1650 cm1, respectively.
Scanning electron microscopy and energy dispersive X-ray spectroscopy Fig. 4 shows the SEM-EDX analysis of Ni complex prepared and the images obtained showed fine crystalline particles. These particles exhibited star-like structure with sharp edges and all images indicated same morphological behavior. The elemental analysis of the complex illustrates the atoms present in the crystallized Ni-quinazoline-2(1H)-thione complex. Sharp distinctive peaks for N, S, C, Cl and Ni can be clearly seen from Fig. 4.
Hydrogen evolution reaction 4-(4-chlorophenyl)-3,4,5,6-tetrahydrobenzo[h]quinazoline2(1H)-thione is a biomolecule that possess anticancer and antimicrobial activity. The complex is protected by chlorobenzene and protonation occurs only in heteroatoms that are attached to the central metal. Since this compound has biological activity, it possesses larger surface area that helps in the protonation. For electro-catalytic hydrogen evolution, initially the protonation occurs in the nitrogen
group of the quinazoline ring. After protonation, the central metal gets oxidized and releases electron. The heteroatom losses the proton obtained from the process of water splitting to produce hydrogen in the surface of the electrode (GC). The electro-catalytic HER activity of the Ni-quinazoline2(1H)-thione moiety was investigated by performing linear sweep voltammetry (LSV) in a potential range from 0.5 to 2.5 V at neutral condition and at a sweep rate of 100 mV/s with phosphate buffer as the solvating agent. The same condition was used in 0.5 M H2SO4 medium. The Tafel plot was obtained by fitting the Tafel equation: ɳ ¼ a þ blogj, where ɳ is the overpotential, ‘a’ is the intercept in mV, ‘b’ is the Tafel slope in mVdec1 and ‘j’ is the current density (Fig. 5). The different scans performed at various rates showed stable reduction from 0.5 to 2.5 V. Highest reduction was observed at a scan rate of 100 mV/s. Current-potential data corresponding to 100 mV/s were obtained and current density was determined. This current density was converted to logarithmic scale and plotted versus potential given. Exchange current density for the charge transfer processes should be high and in this study, we obtained a value of 1.12 mA/cm2 and -0.5 mA/cm2 for neutral and acidic conditions, respectively. The electro-catalytic reaction kinetics was formulated based on three principles: Hþ þ e
/ / / /
Had þ e þ Hþ
Had (Volmer reaction) H2 (Heyrovsky reaction)
Had þ Had
H2 (Tafel reaction)
2Hþ þ 2e
H2 (Overall reaction)
The hydrogen evolution kinetics was evaluated by two mechanisms: Either it takes place by fast discharge of protons called Volmer reaction followed by electrochemical recombination of slowly discharged proton with an additional proton known as Heyrovsky reaction or slowly discharged protons combined together to produce hydrogen at the electrode surface.
Please cite this article as: Rajakumar M et al., Nickel centered metal-organic complex as an electro-catalyst for hydrogen evolution reaction at neutral and acidic conditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.031
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Fig. 4 e SEM-EDX analysis of the Ni-quinazoline complex.
Fig. 5 e Tafel plots obtained from the LSV polarization curves for phosphate buffer (neutral) (a) and 0.5M H2SO4 (b).
Tafel parameter represents the proton discharge aspects and a slope value of 120 mV/dec indicates Volmer reaction as the rate determining step for HER. Tafel slope value of 30 or 40 mV/dec indicates either Tafel or Heyrovsky as the rate
determining step. Hydrogen reduction either follows VolmerTafel or Volmer-Heyrovsky mechanism. A Tafel slope of 34 mV/dec was calculated from the linear sweep voltammetry performed in the potential range from 0.5 V and 2.5 V. The
Please cite this article as: Rajakumar M et al., Nickel centered metal-organic complex as an electro-catalyst for hydrogen evolution reaction at neutral and acidic conditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.031
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low value of Tafel slope suggests the HER followed VolmerHeyrovsky reaction with Heyrovsky as the rate-determining step in neutral condition. It should be noted that lower the Tafel slope, higher will be the hydrogen evolution from the surface of the working electrode. Diffusion of the electron between the Ni complex coated on the surface of the glassy carbon and electrolytic solution used was determined by the exchange current density. This parameter also shows the kinetic response of a cathode material assembled in a three electrode system. It was obtained by extrapolating the Tafel plot to the zero current potential in the x-axis. In presence of 0.5 M H2SO4, a Tafel slope of 100 mV/dec was obtained, which indicates Tafel reaction as the rate determining step.
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Electrochemical Impedance Spectroscopy Fig. 6 shows the Electrochemical Impedance Spectroscopy (EIS) of the complex and electrolytic solution. The kinetics of charge transfer and performance of catalyst can be determined using EIS. A resistance developed in the three compartment system in a potentiostat with the reaction conditions of the study material was studied using Nyquist plot. Impedance spectra can determine one or more electrochemical reactions taking place in a single reaction system. The charge transfer resistance is indicated by the diameters of Nyquist plot. In this plot, there are two depressed capacitive semicircles that show the behavior of
Fig. 6 e Electrochemical impedance spectroscopy of Ni-complex in neutral (a) and 0.5M H2SO4 (b).
Please cite this article as: Rajakumar M et al., Nickel centered metal-organic complex as an electro-catalyst for hydrogen evolution reaction at neutral and acidic conditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.031
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Table 1 e Electrochemical impedance spectroscopy parameters of nickel complex at neutral condition. Parameters
Ni complex
R1(U) R2(kU.cm2) R3(kU.cm2) C2(F) C3(F)
57.4 0.67 5.62 3.53*109 0.911*108
Table 2 e Electrochemical impedance spectroscopy parameters of nickel complex at acidic condition (0.5M H2SO4). Parameters R1 (U) C2 (F) R2 (U)
Ni complex in acidic condition 1*103 0.1*106 100
Fig. 7 e Cyclic voltammetry polarization of Ni complex at 100 mV/s at neutral condition.
the surface where hydrogen bubbles evolved. The polarization resistance of the solution (Rs ¼ R1 þ R2) illustrated in the plot suggest that the larger semicircle corresponds to the solution resistance and the smaller circle indicates the catalyst resistance. Smaller degree of resistance implies the electro-catalyst resistance since the electrode surface had faster hydrogen abstraction sites and electrochemically active surface area. The resistance is inversely proportional to the current density. The increased catalytic sites near the electrode surface for hydrogen adsorption may facilitate faster charge transfer and thus reducing the charge transfer resistance. The increase in solution resistance is due to the ineffective electron transport and uneven rushing of hydrogen bubbles onto the electrode surface. Hence, larger depressed semicircle was formed. A potential of 1.18 V was applied for the impedance study since 10 mA/cm2 (current density) was obtained at this potential at neutral condition. In presence of 0.5M H2SO4, a potential
Fig. 8 e Cyclic voltammetry polarization of Ni complex at different scan rates in 0.5M H2SO4.
Please cite this article as: Rajakumar M et al., Nickel centered metal-organic complex as an electro-catalyst for hydrogen evolution reaction at neutral and acidic conditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.031
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Fig. 9 e (A) SEM analysis of Ni-complex taken before the electrochemical study; (B) after the HER activity in neutral medium; (C) after HER activity in 0.5M H2SO4.
of 1.45 V was applied to perform the impedance study. The equivalent electrical circuit diagram for Niquinazoline-2(1H)-thione moiety in phosphate buffer solution is shown in Fig. 6a. The formula obtained from the circuit diagram is given below and the relevant values are shown in Table 1. Rs ¼ R1 þ C2/R2 þ C3/R3 Fig. 6b shows the half wave semicircle along with an equivalent circuit diagram (Rs ¼ R1þC1/R2) for Ni-quinazoline2(1H)-thione moiety in 0.5M H2SO4 and the relevant values are given in Table 2.
SEM analysis before and after the reduction reaction Fig. 9A shows the SEM image of the complex before HER and it shows crystalline, rough and uneven pattern of Ni catalyst formation. After the potentiostatic analysis with three electrode system, SEM image of the surface of the glassy carbon in neutral media shows scratched surface (where the catalyst was dip coated) (Fig. 9B). Fig. 9C shows the image of the electrode in acidic medium. In both the images, fine crystalline molecules were broken and uneven clustered particles were observed. This confirms the occurrence of reduction during the reaction.
Stability tests and TGA Electrochemical stability A simple potential-current polarization curve was recorded at a scan rate of 100 mV with glassy carbon as a working electrode, platinum wire as a counter electrode and Ag/AgCl as the reference electrode at pH 7 (Fig. 7a, b) and with 0.5M H2SO4 (Fig. 8a, b). The starting potential was fixed as 0.5 V and the voltammetric studies indicated the occurrence of reduction at an overpotential of 1.45 V. The reduction sweeps through the negative region extending till 2.5 V confirms the stability of Ni-quinazoline complex. Different scan rates (50e200 mV) were set to the same system of fixed 0.5 V to 2.5 V potential at neutral pH. Catalytic material coated on the surface of glassy carbon was taken as the working electrode. Reduction started at almost same overpotential and gave different reduction rates. Among all, scan rate of 100 mV showed higher reduction than other scan rate. This scan rate was fixed for the HER study.
Cyclic voltammetry polarization was carried out at a scan rate of 100 mV/s for 10 cycles. Fig. 10A, B show the CV curves of nickel complex in neutral and 0.5M H2SO4 medium. The catalyst was found to be stable and minor changes only were observed. Fig. 11 shows thermos-gravimetric analysis of the synthesized nickel complex and it shows three zones. First two portions are predominant in weight loss and the third part is with minimum loss (8%). The zone 1 was attributed to nickel nitrate. Nickel nitrate salt usually starts to decompose just above the room temperature and here the decomposition starts at 54.15 C. Moreover, its melting point is 136.7 C. In zone 1, 16.41% weight loss was observed. The elemental analysis showed the amount of Ni present as 17%. Thus this loss is due to the fragmentation of Ni(NO3)2.6H2O. The zone2 exhibited a major weight loss of 61.64% due to the decomposition of ligand present in the complex. The remaining residual (0.00699 mg) is only negligible indicating
Please cite this article as: Rajakumar M et al., Nickel centered metal-organic complex as an electro-catalyst for hydrogen evolution reaction at neutral and acidic conditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.031
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Conclusion This study demonstrated the usage of Ni-quinazoline complex framework as a promising electrocatalyst for HER. The base catalysed three component one-pot Biginelli reaction was carried out and the obtained 4-(4-chlorophenyl)-3,4,5,6tetrahydrobenzo[h]quinazoline-2(1H)-thione moiety was successfully characterized. The further synthesized Ni-quinazoline-2(1H)-thione was characterized and subjected to the electrochemical hydrogen activity in neutral and acidic medium. In phosphate buffer electrolyte, the metal complex followed Volmer-Heyrovsky mechanism with a very good catalytic reduction capacity. In presence of 0.5M H2SO4, the metal complex followed Volmer-Tafel mechanism. Stability of the Ni catalyst on the surface of the glassy carbon was found by performing current-potential experiment in a range of 0.5 to 2.5 V. The reduction started at around 1.45 V and increased for different scan rate. This confirmed the better stability of Ni-quinazoline complex. The EIS indicated smaller degree of resistance for the Ni-quinazoline catalyst. The SEM analysis performed using Ni-quinazoline catalyst remaining in the glassy carbon depicted the breakdown of the fine crystalline Ni-complex. We conclude that this dihydropyrimidines based Ni complex reported for HER activity in this study was found to be an effective electrocatalyst.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.12.031.
references Fig. 10 e Cyclic voltammetry polarization curves for 10 cycles in neutral (A) and 0.5M H2SO4 (B). complete decomposition of the molecules. Majidi et al. reported similar pattern for decomposition for nickel nitrate [30].
Fig. 11 e TGA-DTA curve of the nickel complex.
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