Synthesis of novel organic Ni(II) N-isonicotinoylhydrazine-carbothioamide complexes and their application in the oxygen evolution reactions†

Xiaopei Li^a, Xianxu Chu*^a, Kefan Ying^a, Xi Cheng^a, Peng Sun^a, Haiyun Xu^a, Lu Li^a, Jie Zhang*^b and Wenjuan Li*^a
aHenan Engineering Research Center of Green Synthesis for Pharmaceuticals, School of Chemistry and Chemical Engineering, Shangqiu Normal University, Shangqiu 176000, China
^bDepartment of Physics and Institute for Quantum Science and Technology, Shanghai University, Shanghai, 200444 China

First published on 18th April 2025

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Introduction

In the past half-century, the process of industrialization has undoubtedly provided a strong impetus to economic development, significantly improving the standards of living in human society. However, industrialization has also increased the demand for energy, leading to a continuous rise in global energy consumption.^1,2 Currently, the majority of this energy demand is fulfilled by traditional non-renewable sources such as coal, natural gas, and petroleum products, which have led to a range of environmental issues, including the greenhouse effect, ozone depletion, acid rain, and air pollution, due to their combustion.² Consequently, the development of renewable energy sources that are clean and sustainable has garnered widespread attention. Hydrogen, as a renewable and clean energy source, has garnered significant attention in recent years.³ The process of electrolyzing water to produce hydrogen is considered to be a sustainable, efficient, and environmentally friendly method.^4–6

The electrocatalytic hydrogen evolution reaction (HER), along with the oxygen evolution reaction (OER), constitutes the water electrolysis process. Both the HER and OER are dynamic and multi-step chemical processes that require additional energy to overcome activation barriers. The anodic OER (4OH⁻ → O₂ + 2H₂O + 4e⁻ in alkaline media) is limited by the high anodic overpotential⁷ (E° = +1.23 V vs. NHE at pH = 0) and slow kinetic processes, which in turn limits the generation of hydrogen at the cathode. Therefore, oxygen evolution materials with high catalytic activity are crucial for improving the performance of electrocatalysts. Noble-metal catalysts such as RuO₂, Pt and IrO₂ are considered benchmark electrocatalysts for the OER in alkaline media, exhibiting excellent efficiency in the oxygen evolution reaction and the hydrogen evolution reaction for water splitting.^8–12 However, their industrial application has been restricted due to their high cost and low natural abundance. Hence, designing economically viable and efficient electrocatalytic materials for the OER is of great significance.

A wide variety of Earth-abundant metals and their complexes, including transition metals such as cobalt,¹³ copper,^14–17 iron,^18,19 and manganese,²⁰ and nickel^21–33 have been used as electrocatalysts, demonstrating good electrolytic efficiency for water splitting. These transition metal complexes offer several advantages, including low cost, ease of synthesis, and the ability to control redox properties by modifying the ligands around the metal center. Nickel, in particular, has garnered significant attention in the field of electrocatalysis due to its promising activity in the oxygen evolution reaction (OER) at lower overpotentials and good catalytic stability. By altering the electronic and steric properties of the ligands in nickel complexes, the redox potential of electrocatalysts for the OER can be readily adjusted. Lu and co-workers have reported on electrocatalytic water oxidation at pH 7, where a low overpotential was achieved by using water-soluble macrocyclic Ni(II) complexes as electrocatalysts.^21,22 Subsequently, the Cao group and the Allen group also investigated water oxidation by utilizing nickel(II) porphyrin complexes and glycine complexes, respectively, and similarly achieved low overpotentials for electrohydrolysis.^23,24 Furthermore, a variety of nickel complexes have been developed to catalyze the electrochemical oxidation of H₂O to O₂ at low overpotentials.^25–33 Notably, the high-entropy Ni-based alloys developed by the Fang and Wang research groups demonstrate outstanding oxygen evolution reaction (OER) activity, achieving an overpotential of only 184 mV at 10 mA cm⁻², superior to many Ni-based complexes used as water electrolysis catalysts.^31–33

Although significant achievements have been made with various ligands that exhibit different overpotentials, previous reports have focused on inorganic, salt-based or high entropy alloy complexes, with fewer developments in organic complexes, specifically for a coordination complex containing two distinct organic ligands. Furthermore, the electronic effect of these two organic ligands on the complexes and their impact on the OER overpotential have not been fully elucidated to the best of our knowledge. The stability of mononuclear nickel complexes bearing organic ligands as the catalysts for the OER has been limited to approximately 7000 seconds (1.94 hours),³¹ which is insufficient for long-term continuous electrolytic reactions. Therefore, developing a stable electrocatalytic complex and gaining a detailed understanding of the electronic effects of ligands are crucial for designing Ni(II) complex materials for the OER.

In this study, five novel organic nickel complexes based on N-(aryl/alkyl)-2-isonicotinoylhydrazine-1-carbothioamides bearing two kinds of different organic ligands for the corresponding coordination complex (Ni-1 to Ni-5, Scheme 1) with varying electron-rich and electron-deficient groups were synthesized and utilized for the oxygen evolution reaction (OER) in alkaline aqueous media. The formation of the Ni complexes was characterized using infrared and Raman spectroscopy, and the electronic effects of the mononuclear nickel complexes bearing two different organic ligands on the OER overpotential were determined for the first time by designing five different substituent groups of the Ni-complex ligand. The Ni-1 complex, which contains an electron-rich group, demonstrated the best electrochemical performance, with the lowest overpotential of 454 mV at 10 mA cm⁻². This outcome can be attributed to the electron-rich group's ability to activate the ligand and enhance the catalytic activity for the water splitting reaction. Additionally, better catalytic stability was observed, surpassing previous reports.³¹

Results and discussion

Characterization of the Ni-1 complex

Firstly, the Ni-1 complex was characterized by IR spectroscopy, which showed characteristic signals at 602 and 503 cm⁻¹, corresponding to the vibrations of ν(Ni–O) and ν(Ni–N), respectively (Fig. 1a).³⁴ Furthermore, a new energy band was observed at 1578 cm⁻¹ (Fig. 1a), attributed to the stretching vibration of O–CN, while the ν(N–H) band peak of N-cyclohexyl-2-isonicotinoylhydrazine-1-carbothioamide ligand-1 at 3142 cm⁻¹ disappeared (Fig. S1†). This change indicates the loss of two hydrazine hydrogens and the CO group undergoing an enolization reaction, resulting in the formation of a new enolization structure. Then, a new chemical bond was formed through the enolization of an oxygen atom and a hydrazine nitrogen atom. The Ni-1 complex was not only detected by infrared spectroscopy but also by Raman spectroscopy to gain a more comprehensive understanding of the structural characteristics of the sample (Fig. 1b). An absorption peak was observed at 1055 cm⁻¹, which is attributed to the stretching vibration of the C–N bond. It is worth noting that a peak was observed at 425 cm⁻¹, which is referred to as the characteristic peak of the Ni–N coordination bond.³⁵ The existence of this characteristic peak not only confirms the chemical bonding relationship between Ni and N atoms but also provides important information about the metal coordination environment in the complex.

Oxygen evolution reaction performance of Ni-complexes

Due to the multi-step electron and proton transfer involved in electrochemical water oxidation, the kinetic process is slow. Ni-based complexes are crucial for improving catalytic activity and achieving the mechanism of O–O bond formation.³⁶ The synthesized Ni-1 complex was immobilized on a glassy carbon electrode using a Nafion solution, and its electrochemical oxygen evolution performance was tested in an alkaline medium (1 M KOH solution). The OER activity was tested using Ag/AgCl as the reference electrode and linear sweep voltammetry at a scanning rate of 5 mV s⁻¹. According to the corresponding formula, the electrode potential (RHE) of the Ni-1 complex is 1.684 V (Table 1, run 1). Next, the OER activity of the Ni-2 complex, which has a relative electron deficiency compared to the Ni-1 complex, was tested under the same conditions, and the electrode potential (RHE) of the Ni-2 complex was 1.692 V, slightly higher than that of the Ni-1 complex (Table 1, run 2). When the electron-deficient Ni-5 was used, the RHE value increased to 2.063 V (Table 1, run 5). The potential and current density of the Ni complexes were plotted, and the plot showed that the electron-donating effects of the ligand groups of the complex increase with the increasing current density (Fig. 2a).

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Next, the overpotentials of Ni-1 to Ni-5 complexes were obtained by linear sweep voltammetry at a current density of 10 mA cm⁻², providing direct evidence of their performance in electrochemical reactions (Table 1, runs 1 to 5). The Ni-1 complex shows the lowest overpotential among the others, indicating the importance of the ligand group for the electrocatalyst (Fig. 2b). It is worth noting that although these complexes provide similar coordination environments for the metal centers, the different substituents of the ligand group on the thioformamide exhibit significantly different electrochemical properties. Therefore, the electronic properties of the substituents reveal very important factors affecting the oxidation of Ni(III)/Ni(II) in the OER process. This result not only provides important clues for us to deeply understand the electrochemical behavior of these complexes but also offers valuable guidance for the subsequent optimization and design of electrocatalysts with higher activity. To further evaluate the intrinsic activity of the catalyst, cyclic voltammetry (CV) tests were conducted at different scan rates within the non-faradaic region (0.2–0.3 V vs. Ag/AgCl) to obtain double-layer capacitance (C_dl). As shown in Fig. 2c, the C_dl of Ni-1 with an electron-donating configuration is 0.355 mF cm⁻², much larger than other Ni complexes (Table 1). Due to the direct proportionality between the electrochemically specific surface area (ECSA) and the C_dl value, the corresponding ECSA of Ni-1 is the largest compared with others. This means that Ni-1 has the most active sites, the fastest charge transfer rate, and the highest electrocatalytic activity, which also proves the importance of the electric effect of the ligand for water electrolysis catalysts.

The Tafel slope is an important parameter for determining the reaction kinetics and can be calculated via the LSV polarization curve of the complex (eqn (1)).

η = blog(j/jo)	(1)

where η is the overpotential, b is the Tafel slope, j is the current density, and j_o is the exchange current density.

The slopes of the Tafel plots for Ni-1 to Ni-5 complexes are 111.9, 132.9, 144.3, 177.6, and 585.5 mV dec⁻¹, respectively (Table 1, runs 1–5, Fig. 3a). The Ni-1 complex has the lowest Tafel slope value, which directly reflects its excellent electrocatalytic activity and also indicates a more rapid water oxidation kinetics process. Electrochemical impedance spectroscopy (EIS) was performed on the complexes to further investigate the electronic effects of the complexes affecting the OER performance (Fig. 3b). In addition to not interfering with the interface between the catalyst and the solution, this testing method also helps in accurately analyzing the obstacles that the catalyst needs to overcome to promote charge transfer in water separation reactions, making the electrochemical results more reliable.³⁶ Generally speaking, smaller semicircles often represent smaller resistance and faster interface charge transfer efficiency in EIS spectra.^37,38 The impedance test results clearly show that the diameter of the half arc (DIA) of the Ni-1 complex is much smaller than that of other electrocatalysts. The result strongly demonstrates that the Ni-1 complex has a faster rate of electron transfer, resulting in more efficient reaction kinetics, which is consistent with the previously observed high OER activity. The EIS result also suggests that the electronic effect of the ligand greatly affects the charge transfer properties of the complex.

Finally, the chronoamperometric method was employed to comprehensively evaluate the stability of Ni-1 during long-term electrolysis. Small fluctuations in voltage within a certain range were observed due to the continuous generation of bubbles throughout the 40-hour electrolysis process. However, it is worth noting that at a current density of 10 mA cm⁻², the overpotential decrease of the complex is only about 0.05 V, which is well within the normal range of variation. Overall, the metal complexes demonstrated satisfactory and persistent OER activity in electrochemical stability tests over a period of 40 hours. X-ray photoelectron spectroscopy (XPS) measurements were performed on Ni-1 before and after the OER test. The results indicate no changes in the XPS survey spectra: C 1s, O 1s, and Ni 2p spectra. Specifically, the Ni 2p spectra show that nickel maintains a +2 oxidation state (Ni²⁺) in the complex both before and after the OER test (Fig. 3d and Fig. S3†). Additionally, Fourier-transform infrared (FT-IR) spectroscopy was conducted before and after the stability test of Ni-1, and the results confirm that its structure remains unchanged (Fig. S4†). These findings further demonstrate the structural stability of the Ni-1 complex.

Conclusions

In this work, five novel organic Ni-1 to Ni-5 complexes with different electronic substituents were successfully synthesized and designed for use as electrocatalysts for the oxygen evolution reaction (OER) in alkaline media. The synthetic method was strictly adhered to, ensuring the precise synthesis of the complexes, and their structures were identified using infrared and Raman spectroscopy. The overpotential and Tafel slope of Ni-1 were as low as 454 mV and 111.90 mV dec⁻¹, respectively, indicating excellent electrochemical performance. During the 40-hour electrolysis process, Ni-1 exhibited minimal voltage fluctuation and demonstrated good electrochemical stability. This study clarified the electronic effect of mononuclear nickel complexes bearing two different organic ligands on water splitting. The overpotential and Tafel slope increase with the decrease in the electron density of the substituents. These research results are of great significance for the design of OER catalysts.

Experimental

Materials

Isonicotinic hydrazide, cyclohexyl isothiocyanate, ethyl isothiocyanate, meta-toluene isothiocyanate, benzyl thiocyanate, 3-chlorophenyl isothiocyanate, 1,10-phenanthroline, and nickel acetate tetrahydrate were supplied by Aladdin Reagents Ltd. Ultra-pure water (18.2 MΩ cm⁻¹) was purified throughout the work with a Mill-Q water purification system (Millipore, Bedford, France). Unless otherwise noted, chemicals obtained from the suppliers were used as received.

Electrochemical testing

The CHI 660E electrochemical workstation, equipped with a three-electrode system, was utilized to investigate the electrochemical properties of the prepared nickel complexes. A glassy carbon electrode (GC) with a diameter of 4 mm and 5 mm was used as the working electrode. The reference electrode and counter electrode were, respectively, an Ag/AgCl electrode and a Pt wire electrode.³⁹ Before the experiment, the working electrode was pre-cleaned, and then the material was coated onto the surface of the glassy carbon electrode using a drop coating method.

The Ni complex used as a catalyst (20 mg) was dispersed in a mixture of methanol (1 mL) and Nafion (20 μL) using an ultrasonic method to obtain a catalytic solution. Then, the catalytic solution (10 μL) was applied as a working electrode onto a glassy carbon (GC) electrode (5 mm in length, 4 mm in diameter). The electrochemical testing was initiated after the sample had dried. All potentials were referenced to the reversible hydrogen electrode (RHE) in 1 M KOH, using the following formula:

ERHE = EAg/AgCl + 0.059 × pH + 0.197 V	(2)

Cyclic voltammetry (CV) tests were conducted at a scanning rate of 30 mV s⁻¹ in a 1 M KOH solution before OER activity testing to remove impurities on the electrode surface and obtain a stable current density. The overpotential was measured using linear sweep voltammetry (LSV) with a scanning rate of 5 mV s⁻¹ at voltages ranging from 0 to 0.8 V vs. Ag/AgCl. Electrochemical impedance spectroscopy (EIS) was conducted in the frequency range of 10 mHz to 10 kHz. The electrochemical data used in this work were not corrected for iR drop because of the high conductivity of the 1 M KOH electrolyte and the negligible contribution of this solution resistance.^40,41

Cyclic voltammetry curves were plotted in the non-Faraday zone from 0.2 to 0.3 V vs. Ag/AgCl. The resulting double-layer capacitance (C_dl) and the corresponding electrochemically active surface area (ECSA) were thus obtained.

The chemical states of the elements were determined by X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250Xi) (excitation source: Al Kα, beat spot: 500 μm, hν: 1486.6 eV, 150 W). The positions of all peaks have been corrected by the setting C 1s peak situated at 284.8 eV.

Synthesis of N-cyclohexyl-2-isonicotinoylhydrazine-1-carbothioamide (Scheme 2, ligand-1)

(2,4-Diazanaphth-1-yl)(pyridin-4-yl) methanone (1.38 g, 0.01 mol) was dissolved in ethanol (25 mL) at room temperature, and then isothiocyanatoethane (1.41 g, 0.01 mol) was added dropwise to the solution. The reaction mixture was stirred at 80 °C for 5 hours. The crude product was precipitated and washed with ethanol. After removing the solvent, the pure product ligand-1 was obtained as a white solid. (2.49 g, 89% yield). ¹H NMR (400 MHz, DMSO-d₆) δ 10.57 (s, 1H), 9.30 (s, 1H), 8.92–8.65 (m, 2H), 7.83 (d, J = 11.2 Hz, 2H), 7.80 (s, 1H), 4.13 (s, 1H), 1.93–1.51 (m, 5H), 1.37–0.97 (m, 5H); ¹³C NMR (DMSO-d₆, 100 MHz) δ 180.51, 164.35, 150.24, 139.65, 121.71, 53.23, 31.90, 25.24, 25.04.

General procedure for the synthesis of Ni-based isonicotinoylhydrazine thiocarboxamide complexes (Scheme 3, Ni-1 to Ni-5)

N-(Aryl/alkyl)-2-isoniazide thioformamides (1 mmol) were dissolved in ethanol, and Ni(OAc)₂·4H₂O in methanol (1 mmol, 20 mL) was slowly added with continuous stirring. The resulting mixture in solution was stirred at room temperature for 3 hours. The crude product was precipitated and washed with methanol giving the solid product after removing the solvent.

The above solid product was added in a 1:1 volume ratio of CH₃OH–CHCl₃ mixture solution and stirred evenly. Then the 1,10-phenanthroline (2 mmol) methanol solution was dropwise added and stirred continuously to obtain a transparent solution. The oily product was obtained after evaporating the solvent and was mixed with petroleum. The mixture was stirred for 12 hours, and the Ni-based isonicotinoylhydrazine thiocarboxamide complex was obtained after precipitating and drying.

Author contributions

Xiaopei Li: data curation and writing – original draft. Xianxu Chu: supervision. Kefan Ying: validation. Xi Cheng: formal analysis. Peng Sun: formal analysis. Haiyun Xu: project administration. Lu Li: visualization. Jie Zhang: supervision. Wenjuan Li: methodology and writing – review and editing.

Data availability

Data will be made available on request.

Conflicts of interest

The authors declare that there are no competing financial interests that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was performed using the equipment of Shangqiu Normal University and was funded in part by the Key Research and Development and Promotion Projects in Henan Province (No. 222102520032, 242102310422 and 252102231056) and the Key Scientific Research Projects of Colleges and Universities in Henan Province (No. 23B150008).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5qi00033e

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