Novel Dithiolene Nickel Complex Catalysts for Electrochemical Hydrogen Evolution Reaction for Hydrogen Production in Nonaqueous and Aqueous Solutions

Three molecular catalysts based on mononuclear nickel(II) complexes with square planar geometries, [BzPy]2[Ni(mnt)2] (1), [BzPy]2[Ni(i-mnt)2] (2), and [BzPy]2[Ni(tdas)2] (3) (BzPy = benzyl pyridinium) are synthesized by the reaction of NiCl2∙6H2O, [BzPy]Br, and Na2(mnt)/Na2(i-mnt)/Na2(tdas) (mnt = 1,2-dicyanoethylene-1,2-dithiolate for (1), i-mnt = 2,2-dicyanoethylene-1,1-dithiolate for (2), and tdas = 1,2,5-thiadiazole-3,4-dithiolate for (3)), respectively. The structures and compositions of these three catalysts are characterized by XRD, elemental analysis, FT-IR, and ESI-MS. The electrochemical properties and the corresponding catalytic activities of these three catalysts are studied by cyclic voltammetry. The controlled-potential electrolysis with gas chromatography analysis confirms the hydrogen production with a turnover frequency (TOF) of 116.89, 165.51, and 189.16 moles of H2 per mole of catalyst per hour at a potential of − 0.99 V (versus SHE) in acetonitrile solutions containing the catalysts, respectively. In a neutral buffer solution, these three molecular catalysts exhibit a TOF of 411.85, 488.76, and 555.06 mol of H2 per mole of catalyst per hour at a potential of − 0.49 V (versus SHE), respectively, indicating that Complex 3 constitutes the better active catalyst than Complexes 1 and 2. For fundamental understanding, a catalytic HER mechanism is also proposed.


Introduction
Hydrogen has been considered to be one of the most promising energy carriers due to its high energy density, environmental-friendly, and renewable and abundant production sources [1][2][3]. In general, there are many renewable ways for hydrogen production, such as solardriven, electro-driven, and thermal-driven water splitting. Electro-driven electrochemical water splitting is envisaged to be one of the simplest ways to generate hydrogen with high purity and has attracted great interest. In a water electrolysis device, some electrocatalysts have to be applied to speed up the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode. Particularly, for HER electrocatalysts, the traditional ones are platinum-based materials, which have been regarded as the best electrocatalysts for hydrogen evolution. However, the high cost and the limited resources of such precious materials have limited their large-scale practical application. Therefore, developing low-/non-Pt catalysts for water electrolysis have been given great efforts in the last several decades [4][5][6][7][8][9][10]. In the search of such low-cost and efficient electrocatalysts [11,12], a number of new materials such as nickel [13][14][15][16], cobalt [17][18][19][20], and iron [21][22][23] complexes have been designed and synthesized. In recent years, a new class of dithiolene transition metal (nickel and copper) complexes with the crystal structures and magnetic properties have been explored for electrochemical energy applications [24][25][26][27][28]. In particular, the electrocatalytic activities of the nickel dithiolene complexes [29] and tetraphenylporphin cobalt complexes [30] have also been investigated in both the acetic acid and neutral buffer solution. Due to the special electronic property of dithiolene ligands, the electron density of these complexes are delocalized in metal-sulfur core, which benefits the electron storage [31]. The experiment results have demonstrated that such metal-dithiolene-based complexes could be applied as the efficient HER electrocatalysts for hydrogen production [32][33][34][35].

Syntheses of Complexes 1-3
Synthesis of Complex 1 A solution of Na 2 (mnt) (0.37 g, 2 mmol) in methanol (15 mL) was added to a stirred solution of NiC1 2 6H 2 O (0.24 g, 1 mmol) in methanol (15 mL) to form a brown solution, which was filtered off and then added to a solution of [BzPy]Br (0.51g, 2 mmol) in methanol (20 mL). After stirring for 30 min, a red precipitate was obtained, which was washed with methanol, filtered, and dried in a vacuum. The obtained red product was put in MeCN/i-PrOH (v/v = 3:1), and the solvent was slowly evaporated in two weeks to obtain a red square crystal with a yield of 80%. The obtained red crystal was analyzed with X-ray christallography with data analysis using the SHELXS program to obtain the molecular formula of C 32 H 24 N 6 NiS 4 with a structure as shown in Fig. 1a.
Complex 2 was synthesized following the steps of the preparation of Complex 1 by using Na 2 (i-mnt) instead of Na 2 (mnt), and the brown sheet crystal was obtained with a yield of 82%. The formula of C 32 H 24 N 6 NiS 4 with a structure shown in Fig. 1b.
Complex 3 was synthesized according to literature method [40], and the obtained formula is C 28 H 24 N 6 NiS 6 with a yield of 79%, and a structure as shown in Fig. 1c.

Crystal Structure Determination and Power X-ray Diffraction
The single crystals of Complexes 1-3 were mounted on a Smart APEX CCD area detector, and data was collected using a graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) by φ-ω scan mode at room temperature. Data were treated using a SAINT software program [41]. The structures were solved by a direct method using the SHELXS program [42,43] and refined by the full-matrix leastsquares techniques (SHELXL) on F 2 [44]. The details of the crystallographic data, structure refinement, and data collection are summarized in Table S2, and selected bond lengths and angles are listed in Tables S3-S4. PXRD measurement was collected on a Rigaku-Ultima IV X-ray diffractometer using Cu Kα

Electrochemical Methods
The cyclic voltammograms (CVs) were performed using a CHI 660E electrochemical analyzer under argon in a onecompartment three-electrode cell in which a glassy carbon was as the working electrode (Φ = 1mm), a platinum as the counter electrode, a Ag/AgNO 3

Controlled-Potential Electrolysis
Bulk electrolysis was carried out in a two-compartment, gas-tight, H-type electrolysis cell, and the geometric surface area of the working electrode was 0.41 cm 2 . Homogeneous bulk electrolysis was performed the potential at − 0.99 V (versus SHE), with an solvent of MeCN (20 mL) containing

Crystal Structures
The reaction of nickel chloride, sodium dithiolene (Na 2 mnt, Na 2 i-mnt, or Na 2 tdas) and benzyl pyridinium bromide in methanol at ambient conditions can result in three molecular catalysts (Complexes 1-3). The crystals of such three complexes can obtained by slow evaporation technique at room temperature in MeCN or MeCN/i-PrOH, and the obtained products can be characterized by elemental analysis (Table S1), FT-IR (Figs. S1, S2 and S3), ESI-MS (Figs. S4, S5, S6 and S7), and X-ray single-crystal diffraction. These complexes can crystallize in the monoclinic space group C2/c (Table S2)  PXRD was investigated to confirm the high purity of the crystals of Complexes 1-3 for further experiment. As shown in Figs. S8, S9 and S10, for Complex 1, the simulated XRD patterns with the diffraction peaks are located  2 2) crystal planes, respectively, which are corresponding to the experimental XRD patterns. Therefore, the experimental XRD patterns for Complexes 1-3 are consistent with the simulated XRD patterns on the basis of the single-crystal structure, confirming the high purity of the as prepared complexes.

Cyclic Voltammetry Studies
The electrochemical behavior of the three Ni(ΙΙ) complexes and the corresponding cations were examined by cyclic voltammetry in MeCN with [(n-Bu) 4 N]ClO 4 as the supporting electrolyte (Fig. 3a-d). The cyclic  Table 1). The first reduction wave could be assigned to the Ni ΙΙ / Ni Ι process and the second one could be Ni Ι /Ni 0 process. Furthermore, as observed in Fig. 4a-f, the plots of peak current (i p ) of the three complexes versus the square root of scan rate show the linear correlations, indicating that the diffusion-controlled process with the electrochemically active species freely diffusing in the solution and possible usage as the electrocatalysts of these three complexes for hydrogen evolution.
Furthermore, we have explored the catalytic behavior of the three complexes in phosphate buffer solution (pH 5.0). As shown in Figs. S11, S12 and S13, the electrode without catalyst in the solution can only give a weak current at − 1.10 V, but with the addition of the catalyst, the peak current is increased obviously. In particular, the Complex 3 can give the highest catalytic current (20.39 μA) in the same conditions.

Effect of Acid Concentrations on Redox Potentials
In order to investigate the impact of proton on the catalytic HER activity by the three complexes, we examined the change of CVs with increasing the concentration of acetic acid (AcOH, pK a = 22.3 in MeCN) [45,46]. As shown in Fig. 5a-c, the HER currents catalyzed by the three complexes are increased fast when the concentration of acetic acid is increased from 0 up to 0.17 M. The maximum peak current (i cat ) can be used to calculate the catalytic rate constant (k obs ) [47][48][49] and to value the electro-catalytic activity according to Eq. 1 [24,26] where i cat is the catalytic current, i p is the peak current without adding AcOH, n is the number of electrons in the reaction, R is the ideal gas constant, T is the temperature, F is the Faraday's constant, and v is the scan rate.
At the highest acid concentration (0.17 M), values of i cat / i p for Complexes 1, 2, and 3 are 134.9, 132.9 and 520.1 calculated from Eq. 1, which are coincided with the acidindependent turnover frequencies of 3529.3 s −1 , 3425.6 s −1 , and 52476.0 s −1 with an overpotential of 388.1 mV, 344.1 mV, and 520.1 mV, respectively (Tables S5, S6 and S7). In addition, it can be found that the i cat /i p values of the three complexes are increased linearly with increasing the concentration of AcOH (Fig. 6). Even at high acid concentrations, the values of i cat /i p vs [AcOH] are still linearly changed with AcOH concentration, showing that the three complex catalysts can keep stable electrocatalytic activity in higher concentration of acid [14]. In particular, the turnover frequencies of Complex 3 is 15 times larger than those of Complexes 1 and 2, indicating the ligand of tdas 2− possesses an excellent electron storage ability [31].
Normally, the overpotential (η) can reflect the activity of electro-catalyst; low η value and high TOF (the turnover frequency) can result in better electrocatalytic activity [5,50], the details calculation of η and TOF were displayed in supplementary information (Fig. S18). The η values of the three complex catalysts are decreased distinctly with increasing the concentration of AcOH (Tables S5, S6 and S7), indicating that the catalysts have high protonreduction abilities toward hydrogen even in weak acid conditions.

Electrocatalytic Hydrogen Production in Acetonitrile and Water
To further validate the catalytic activity of the as-prepared complex materials, electrocatalytic hydrogen production was estimated by bulk electrolysis. Processes of hydrogen production in homogeneous bulk electrolysis for three complex catalysts in MeCN are shown in Fig. 7a (Table S8), respectively. However, only 121.4 µmol of hydrogen can be formed in bulk electrolysis without a catalyst at 4 h of reaction time.
The processes of hydrogen production in heterogeneous bulk electrolysis for complex catalyst-containing three phosphate buffer solutions are shown in Fig. 8a (Table 2), respectively. While at the same condition without catalyst only 3.1 μmol of hydrogen can be obtained.
No matter in homogeneous bulk electrolysis and heterogeneous bulk electrolysis, among three complex catalysts for HER, Complex 3 is the best one for hydrogen production, confirming that the ligand of tdas 2has the positive free energy for HER [16].
In addition, to further explore the activity and stabilization of three complex catalysts, 48 h of electrolysis were performed in three phosphate solutions. To maintain the liquid level in two-compartment electrolysis cell, the amount of produced hydrogen at the first hour and the last hour during electrolysis were measured (Fig. S14). The results show that 395.3, 409.1, and 466.0 Coulomb of chargers are passed for Complex 1, Complex 2, and Complex 3 containing electrolytes, respectively. After 48 h of electrolysis, the catalysts still keep a commendable hydrogen production activity, showing that the complex catalysts can maintain a certain stability under a long electrocatalysis and can perform a good hydrogen-evolving reaction.

Effect of Proton on the Redox Potentials
In order to study the effect of proton on three complex catalysts, the change of the UV-vis spectra in MeCN with increasing the concentration of acetic acid were investigated, as shown in Figs. S15, S16 and S17. It can be seen that when AcOH concentration is added from 0 up to 50 L (0.17 M), no obvious change of UV-vis spectra of the three Ni(II) complexes, showing that there is no protonation appearance on the complexes under the condition. It can also be seen from the CVs of three complexes in the solution with various concentrations of AcOH, the first reduction Wave 1 and Wave 3 keep the same, the values of E cat/2 from the second reduction wave of the three complexes positively shift quickly with increasing AcOH concentration, suggesting that it might be the protoncoupled electron transfer steps, in which the electrochemical reduction coupled with the proton adoption appears. The phosphate buffer solution with various pH (5.0-7.0) values were also used in bulk electro-catalysis. As shown in Fig. 9a- These results indicate that the amount of H 2 is decreased with increasing pH value. Therefore, the proton concentration can strongly impact the HER through directly affect the reaction rate.

Mechanism of the Electrocatalytic H 2 Evolution
Based on literature precedents [13,51,52] and our results, the proposed catalytic cycle for hydrogen generation from aqueous solution mediated by three complex catalysts is shown in

Conclusions
In conclusion, three earth-abundant Ni-dithiolene complexes are synthesized and studied for catalysts of hydrogen evolution reaction (HER) for hydrogen production. These three complex catalysts possess good electro-catalytic activity and long-term electrocatalytic stability in acetonitrile and phosphate buffer solutions. The present work reveals that the dithiolene ligands play a vital role in catalytic HER and provides important an insight into new design and synthesis of high efficient metal molecular electrocatalysts for hydrogen production.

Declarations
Accession Codes CCDC-1915576, CCDC-1915577, and CCDC-698890 contain the supplementary crystallographic data for 1-3. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc. cam. ac. uk/ data_ reque st/ cif.

Conflict of Interest
The authors declare no competing interests.
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Fig. 10
Proposed mechanism of electrocatalytic hydrogen production with Complex 1, Complex 2, and Complex 3