Homogenous electrochemical water oxidation by a nickel(II) complex based on a macrocyclic N-heterocyclic carbene/pyridine hybrid ligand

Hossain M. Shahadatabc, Hussein A. Younus*bd, Nazir Ahmadbe, Md. Abdur Rahamanabf, Zafar A. K. Khattakab, Serge Zhuiykovh and Francis Verpoort*abgh
aState Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China
bSchool of Material Science and Engineering, Wuhan University of Technology, Wuhan 430070, China
cDepartment of Chemistry, Faculty of Science, Comilla University, Comilla 3506, Bangladesh
dChemistry Department, Faculty of Science, Fayoum University, Fayoum 63514, Egypt. E-mail: hay00@fayoum.edu.eg
eDepartment of Chemistry, GC University Lahore, Lahore 54000, Pakistan
fDepartment of Chemistry, Faculty of Science, Mawlana Bhashani Science and Technology University, Tangail 1902, Bangladesh
gNational Research Tomsk Polytechnic University, Lenin Avenue 30, 634050 Tomsk, Russian Federation
hGhent University, Global Campus Songdo, 119 Songdomunhwa-Ro, Yeonsu-Gu, Incheon 406-840, South Korea. E-mail: francis.verpoort@ghent.ac.kr

Received 26th July 2019 , Accepted 11th September 2019

First published on 11th September 2019


Water-soluble homogeneous nickel catalysts have been rarely investigated for catalytic water oxidation as compared to their heterogeneous counterparts. Herein, we report homogenous electrochemical water oxidation by a nickel(II) complex, ([NiL](PF6)2 (L = bis(2-pyridyl-methylimidazolylidene)methane), based on a macrocyclic N-heterocyclic carbene/pyridine hybrid ligand under neutral and alkaline conditions. The catalyst displayed the stable catalytic current of 0.65 mA cm−2 at the overpotential of 0.80 V (∼0.55 V at GCE for CV) with a ∼93% Faradaic efficiency at pH 9.0 for oxygen evolution in long-term bulk electrolysis. The CV, UV-vis, ESI-MS, SEM, and EDX results demonstrated that the catalyst was impressively stable even after long-term controlled potential electrolysis (CPE) (11 h) and homogeneous in nature. The synthesis of this catalyst is straightforward, and its complex is air and moisture stable. To the best of our knowledge, this is the first study on the investigation of a Ni–NHC complex for water oxidation under aqueous conditions (acetate/phosphate). According to the literature, the role of the phosphate ion in homogenous nickel-catalysed water oxidation was found to vary from catalyst poisoning to activation. Interestingly, the catalytic activity of our catalyst in phosphate buffer was significantly higher than that with acetate ions at the same pH value; this might indicate the key role of phosphate ions as proton acceptors, which boosted the catalyst activity via enhanced PCET during catalysis.


Introduction

Worldwide, scientists have been devoted to developing sustainable energy sources such as hydrogen by water splitting based on the concept of artificial photosynthesis.1–3 However, the process of water oxidation (WO) is identified as the bottleneck of water splitting as it is a stepwise multi-electron and multi-proton highly energy-demanding reaction. Thus, significant efforts have been made to design water oxidation catalysts (WOCs) using molecular systems and materials.4,5 In the last two decades, researchers have reported numerous robust homogeneous6–9 and heterogeneous10–14 WOCs for the catalytic oxidation of water. Homogeneous catalysts are more impressive as compared to heterogeneous catalysts due to their comprehensive mechanistic studies, tunable chemical structures, and controllable redox features;6,15–18 on the other hand, heterogeneous catalysts have the advantages of superior stability for the oxygen evolution reaction (OER) and easier immobilization on the electrode surface for practical application.19–22 Notable progress in molecular WOCs has been made based on Ru7,23–27 and Ir (ref. 28–31) complexes; however, the wide applications of these complexes are restricted due to their high cost and scarcity. Consequently, economically viable earth-abundant WOCs based on V,32 Mn,33–36 Fe,37–39 Co,40–44 Ni,6,45–49 and Cu (ref. 9, 50 and 51) have been extensively studied recently. Among them, Ni complexes are very interesting due to their structural aspects, diverse redox properties and strong oxidizing power of their high valence states.6,44–46,48,49,52 Moreover, NiOx and Ni(aq)2+ ions are highly efficient catalysts for WO, even at low concentrations and high pH.53–55 However, water-soluble homogeneous Ni catalysts have been rarely investigated for electrocatalytic WO6,47–49,56 as compared to heterogeneous WOCs.57–59

Lu and co-workers6 have reported the first Ni-based homogenous WO catalyst, [Ni(meso-L)](ClO4)2 (L = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane), which oxidizes water at neutral pH and moderate overpotential. The molecular nature of this catalytic system was concluded based on several electrochemical, spectroscopic and surface analyses. In addition, based on the density functional theory (DFT) calculations, its cis-conformation, which is formed by isomerization from its trans-isomer during WO, is beneficial for HO–OH coupling. Motivated by the remarkable robustness and easy synthesis of [Ni(meso-L)](ClO4)2, the same group further investigated a six-coordinated Ni(II) complex, [Ni(L)(H2O)2](ClO4)2 (ref. 47) (L = N,N′-dimethyl-N,N′-bis(pyridin-2-ylmethyl)-1,2-diaminoethane), with two cis-labile sites as a molecular catalyst for WO in an acetate solution, where acetate (OAc) ions were found to be essential in assisting the formation of catalytically active intermediates and proton transfer in the O–O bond-forming step. However, the catalyst did not show any WO catalytic activity in 0.2 M phosphate buffer at the same pH value; this was interpreted to be due to the occupancy of both labile sites in the catalyst structure by bidentate HPO42−. Subsequently, it has been reported that HPO42– plays a dual role in the electrocatalysis with the nickel complex [Ni(mcp)(H2O)2](ClO4)2 (mcp = (1R, 2R)-N1,N2-dimethyl-N1,N2-bis(pyridin-2-ylmethyl)cyclohexane). On the one hand, it can act as a proton acceptor to facilitate proton-coupled electron transfer (PCET) and the catalytic oxidation of water. On the other hand, it suppresses the catalyst activity by coordinating to the labile sites, causing a low initial current during electrolysis.16 These findings were further supported by the synthesis of several nickel(II) complexes with different numbers of methyl groups in the ligand motif, where the axially oriented methyl groups in the macrocyclic ligands with higher number of methyl groups (six and eight) could impose a steric effect on the axial position of the NiIII center; this suppressed the axial coordination of the phosphate anions with the NiIII center to achieve a better catalytic performance.60 Moreover, some other nickel complexes including nickel porphyrin complexes,49 nickel amine-pyridine complexes,47 nickel penta-pyridine complexes48 and polynuclear nickel-centered polyoxometalates56 have been documented as homogeneous WOCs. In addition, a recent study reported by Zhang et al.61 highlighted that the catalytic rate of electrochemical WO was accelerated by the addition of a base with stronger proton-accepting ability. For example, phosphate has a pKa (H2PO4) = 7.2, and thus, it has stronger proton-accepting ability than H2O with pKa (H3O+) = –1.74; therefore, the proton of H2O will transfer to phosphate, which is a proton acceptor, resulting in enhanced WO performance. However, all the reported nickel molecular catalysts for WO are based on poly nitrogen ligands, which are mostly tetradentate, whereas other types of ligands have been significantly less investigated. N-Heterocyclic carbene (NHC) ligands have become highly attractive in homogeneous catalysis due to their strong σ-donor power, high reactivity, good stability, strong solubility in water, and better acid tolerance.62 The introduction of NHCs into polydentate N-functionalized ligands is believed to further stabilize high-valent metal centers.63 However, to the best of our knowledge, Luo et al.62 investigated the first-NHC based nickel complexes similar to our complex for the catalytic proton reduction using an organic solvent (TBAP-DMF) that unfortunately displayed a high overpotential (1.28 V).

Consequently, we had a keen interest in investigating the WO activity using our synthesized complex in acetate (NaOAc)/phosphate (NaPi) buffer mostly under neutral/basic condition to investigate the effect of the NHC ligand on the structure and activity of catalysts and also clarify the effect of buffer anions on the catalyst activity. Thus, herein, we report homogenous electrochemical WO by a nickel(II) complex based on a macrocyclic NHC/pyridine hybrid ligand, ([NiL](PF6)2 (L = bis(2-pyridyl-methylimidazolylidene)methane), at pH 7.0 and 9.0 in both sodium acetate and phosphate electrolytes (Scheme 1). The Ni(II) complex (2) displayed impressive stability during electrochemical water oxidation under slightly basic conditions, even after long-term (11 h) controlled potential electrolysis (CPE), and the stable catalytic current of 0.65 mA cm−2 at the overpotential of 0.80 V (∼0.55 V at GCE for CV). The electrolyte-enhanced catalyst activity in the case of phosphate buffer and the complete loss of activity in the presence of acetate ions highlight the crucial role of anionic species in the solution.


image file: c9cy01485c-s1.tif
Scheme 1 Chemical structures of the tetradentate NHC/pyridine hybrid ligand [H2L](PF6)2 (1) and its Ni(II) complex, [NiL](PF6)2 (2).

Experimental

Materials and methods

All reactions were performed under an Ar atmosphere using the standard Schlenk techniques. The chemicals used herein were purchased from commercial suppliers (Aladdin Chemical Co., Ltd.) and used without further purification. 2-(Pyridyl-methyl)imidazole was prepared according to a literature procedure.24,64 The aqueous buffers NaOAc and NaPi at pH 7.0 and 9.0 were prepared in the laboratory using chemicals obtained from commercial suppliers. The pH value of the electrolyte was adjusted using 1.0 M CH3COOH, 1.0 M H3PO4, or and 1.0 M NaOH. The purity of used Ar was 99.999%. All solutions were prepared with ultrapure water (Millipore MilliQ® A10 gradient, 18.25 MΩ cm, 2–4 ppb total organic content) unless otherwise stated. Indium-doped tin oxide (ITO) glass (Rs = 6–7 Ω, 0.80 cm2) was purchased from Zhuhai Kaivo Electronic Components Co., Ltd. Elemental analyses for CHN in the prepared complexes were performed using Vario EL cube (Germany Elements Analysis System). The 1H and 13C NMR spectra of [H2L](PF6)2 (1) and [NiL](PF6)2 (2) were obtained via the Bruker 500 MHz NMR spectrometer using the DMSO-d6 solvent and TMS as an internal standard. Chemical shifts (δ) were determined in ppm relative to the residual protons of DMSO-d6 (1H: δ 2.50 and 13C: δ 39.50), whereas the coupling constants (J) were obtained in Hz. High-resolution electrospray ionization mass spectrometry (HR ESI-MS) measurements were conducted using the Bruker Daltonics microTOF-QII spectrometer with electrospray ionization and a time-of-flight system. FTIR spectroscopy was performed using the Bruker Vertex 80v FTIR spectrometer. UV-vis absorption spectra were obtained using the Shimadzu UV-1800 spectrometer at room temperature. All electrochemical measurements were performed using the CHI660E electrochemical workstation at room temperature with a normal three-electrode system, including glassy carbon (GC; 0.07 cm2) as the working electrode, Pt wire as the counter electrode and calomel electrode (3.50 M KCl, 0.25 V vs. NHE) as the reference electrode. Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy (EDX) spectra were obtained using a field-emission scanning electron microscope (FEI, Quanta 400).

Synthesis of [H2L](PF6)2 (1)

The compound (1) was synthesized according to previously reported procedures with some modifications.24,64,65 A solution of 2-(pyridylmethyl)imidazol (0.80 g, 5.0 mmol) in 5 mL CH2Br2 was refluxed overnight. The resulting residue was filtered and dissolved in 10 mL of water to produce an aqueous solution. The subsequent addition of NH4PF6 (1.0 g, 6.0 mmol) to the aqueous solution afforded an off-white precipitate, which was filtered and dried under vacuum. Yield: 1.08 g (70%). Anal. calcd for C19H20F12N6P2: C, 36.67; H, 3.24; N, 13.5 (%). Found: C, 36.37; H, 3.45; N, 13.88 (%). MP: 149–151 °C. 1H NMR (500 MHz, DMSO-d6, 25 °C): δ 9.58 (s, NCHN, 2H), 8.56 (d, J = 4.6 Hz, o-C5H4N, 2H), 8.02 (s, NCHCHN, 2H), 7.92 (m, NCHCHN + p-C5H4N, 4H), 7.54 (d, J = 7.8 Hz, m-C5H4N, 2H), 7.44 (2d, J = 4.3, 5.3 Hz, m-C5H4N, 2H), 6.71 (s, NCH2N, 2H), 5.64 (s, CCH2N, 4H) (Fig. S1). 13C NMR (125 MHz, DMSO-d6, 25 °C): δ 153.45; 150.08 (2 × o-C5H4N), 138.81(p-C5H4N), 138.08 (NCHN), 124.57; 124.31 (2 × m-C5H4N), 123.31; 122.65 (2 × NCHCHN), 59.02 (NCH2N), 53.89 (CCH2N) (Fig. S2).

Synthesis of [NiL](PF6)2 (2)

The complex (2) was synthesized according to previously reported procedures with some modifications.64,65 A solution of 1 (250 mg, 0.4 mmol) and Ni(OAc)2·4H2O salt (100 mg, 0.4 mmol) in 5 mL of DMSO was heated at 50 °C under an Ar atmosphere for 4 h. During heating, the solution changed from dark brown to greenish-yellow. The solvent was then removed under vacuum evaporation. Then, the residue was washed with dry THF, filtered and dried to obtain the complex 2. Yield: 205 mg, (75%). Anal. calcd for C19H18F12N6NiP2: C, 33.61; H, 2.67; N, 12.38 (%). Found: C, 33.53; H, 2.65; N, 12.23 (%). MP: 239–241 °C. 1H NMR (500 MHz, DMSO-d6, 25 °C): δ 8.58 (d, J = 5.7 Hz, o-C5H4N, 2H), 8.22 (t, J = 7.7 Hz, p-C5H4N, 2H), 7.87–7.57 (m, m-C5H4N + NCHCHN, 8H), 6.41 (s, NCH2N, 2H), 5.80 (s, CCH2N, 4H) (Fig. S3). 13C NMR (125 MHz, DMSO-d6, 25 °C): δ 153.69; 152.86 (2 × o-C5H4N), 148.03 (Ni–C), 141.73 (p-C5H4N), 125.97; 125.92 (2 × m-C5H4N), 125.20; 122.46 (2 × NCHCHN), 62.55 (NCH2N), 53.03 (CCH2N) (Fig. S4). FT-IR (neat): 3635, 3410, 3180, 3145, 1614, 1573, 1490, 1430, 1334, 1309, 1164, 1068, 835, 777, 738, 690, and 557 cm–1 (Fig. S6). ESI-MS: m/z2+ = 194.046722 ([M-2PF6]2+), calcd: 194.05 (Fig. S7).

Results and discussion

Structural features of the imidazolium salt

Imidazolium bromide, [H2L]Br2, could be easily prepared via the condensation reaction of 2-(pyridyl-methyl)imidazole with CH2Br2. The subsequent addition of NH4PF6 to the aqueous solution of the resulting imidazolium bromide afforded the corresponding hexafluorophosphate (1) in high yield (Scheme S1). The imidazolium salt was characterized by 1H and 13C NMR spectroscopy and elemental analysis. The 1H NMR spectrum of 1 in DMSO-d6 showed a singlet at ca. 9.58 ppm, assignable to the acidic NCHN protons in the typical range of ca. 9–12 ppm of imidazolium salts (Fig. S1).64,65 Furthermore, its 13C NMR spectrum with peaks at 138.08 (NCHN) and 59.02 (NCH2N) ppm supported the proposed structure of 1 (Fig. S2). It should be noted that the quaternization reaction is highly selective without the formation of pyridinium salt as a side-product; this reflects the more nucleophilic nature of imidazole when compared with that of the pyridine functionality.64

Structural features of the complex

Divalent metal complexes with a tetradentate NHC/pyridine hybrid ligand are typically prepared from imidazolium salt via the common metal acetate protocol.64–66 For the preparation of 2, the acetate protocol was useful because of the availability of nickel acetate and the subsequent formation of the stable divalent nickel complex of the NHC-type ligand. By heating a mixture of 1 with Ni(OAc)2·4H2O salt in DMSO at 50 °C under an Ar atmosphere for 4 h, 2 was obtained in a quantitative yield (75%) as a greenish-yellow solid (Scheme S2). This complex was air and moisture stable and mostly soluble in common organic solvents and an aqueous medium. The catalyst was characterized by elemental analysis, NMR and FTIR spectroscopy, and ESI-MS. The 1H NMR spectrum of 2 in DMSO-d6 consists of sharp signals at room temperature; however, nickel complexes are typically paramagnetic, leading to broad peaks. The absence of the signals of two acidic imidazolium protons (NCHN) at 9.58 ppm in the 1H NMR spectrum of 2 indicates the successful formation of the metal–carbene bond (Fig. S3). However, a multiplet peak at 7.87–7.57 ppm (m-C5H4N + NCHCHN, 8H) appeared for the pyridine and imidazolidine ring protons of 2, indicating that their structures in solution are symmetry-related.64 Moreover, the pyridine backbone presented a sharp triplet at 8.22 (p-C5H4N, 2H) and doublet at 8.58 (o-C5H4N, 2H) ppm because of its different environments. The methylene protons linking the two imidazolidene rings (H′) and pyridine moieties (H′′) display two sharp singlets (AB splitting pattern; intramolecular noncovalent interaction between H′ and H′′) at ca. 6.41 and 5.80 ppm, respectively, because of the conformational rigidity of the macrocyclic ring.64,67 In addition, the fast-exchange is also responsible for sharp proton signals in the spectrum at ambient temperature.64 In the 13C NMR spectrum of 2, the carbenic carbon (Ni–C) resonance signal appears as a singlet at 148.03 ppm, which also supports the proposed structure of 2 (Fig. S4). The H–H COSY NMR spectrum of 2 in DMSO also proves its structure (Fig. S5). The FTIR spectrum of 2 displays clear evidence of its metal–organic nature. It shows a broad band in the range of 3145–3635 cm–1 due to the aromatic CHsp2 stretching vibrations.41 Furthermore, the weak to strong peaks at 1614, 1430, 1309, 1164, and 1068 cm–1 can be assigned to the C[double bond, length as m-dash]Cstr, C–N, C[double bond, length as m-dash]N, and CH2bend features, respectively, whereas the peaks at 835 and 557 cm–1 are assigned to ν(PF6) (Fig. S6).24,41 The ESI-MS of the catalyst in acetonitrile displays a base peak at m/z2+ = 194.046722 (calcd: 194.05), corresponding to [NiL](PF6)2 ([M–2PF6]2+), which strongly supports the structure of the complex (Fig. S7). Its UV-vis absorption profile displays clear evidence of the metal-to-ligand charge transfer (MLCT) via a relatively weak absorbance band at 400 nm (Fig. S8). The oxidation state of Ni(II) is the usual oxidation state for nickel complexes with Schiff base ligands.68,69 In contrast, other oxidation states such as I, III and even IV have been reported using different ligands. In 2 and other similar Ni(II) complexes, the central Ni(II) ion is tetracoordinated by two pyridines and two NHC carbons in a square-planar geometry although a slight tetrahedral distortion is observed.64,65 Because of the geometric requirement of the substituents, the coordinated pyridine rings and imidazolylidene rings are not exactly coplanar with the central Ni(II) ion.65 Moreover, the tetradentate coordination around the Ni(II) ions in this case is energetically more stable.64

Cyclic voltammetry (CV)

For the investigation of true catalyst activity, the stability of the metal(II) complex in WO under the operating experimental conditions should be carefully examined. In the case of Ni(II) ions, it has been reported that an extremely low Ni loading on different electrodes leads to high activity for WO.68

Fig. 1 shows the cyclic voltammograms (CVs) of the complex 2 (1.0 mM) obtained on a GC electrode (0.07 cm2) in a 0.1 M aqueous solution (NaOAc/NaPi) at pH 9.0 (Fig. S9 shows the cyclic voltammograms for the case of pH 7.0). The potentials were measured vs. a calomel electrode (3.50 M KCl) and are reported vs. the normal hydrogen electrode (NHE). The CVs, linear sweep voltammograms (LSVs) and differential pulse voltammograms (DPVs) of the catalyst 2 (Fig. S10) reveal the redox properties of the complex. The potential was swept from 0 to 1.75 V vs. the NHE electrode in the positive direction. As presented in Fig. 1, the complex shows two irreversible waves at 0.93 and 1.40 V vs. NHE in NaPi buffer, which can be assigned to the NiIII/II and NiIV/II redox couples, respectively.6,70 These redox events could be observed using DPV at 0.87 and 1.18 V (Fig. S10b). However, when the complex was swept in the acetate electrolyte, some differences were observed as compared to the case of phosphate, as shown in Fig. 1 (see also Fig. S9 and S10 for a clear comparison). At first, the redox peaks are dependent on the buffer type rather than on the buffer pH value. Thus, the redox peaks shifted to higher potentials with the change from phosphate to acetate electrolyte although the pH value of the solution was fixed. However, no change in the potential value of the second redox peak (which was assigned to NiIV/II) was observed. This means that although phosphate ions can facilitate the oxidation of NiII to NiIII, which mostly occurs because the phosphate ions act as a base to receive protons in the PCET process, they cannot reduce the potential required to reach the higher oxidation state of NiIV, which is a prerequisite for WO to occur. This might also suggest a different role of phosphate ions in enhancing the catalytic current due to WO.


image file: c9cy01485c-f1.tif
Fig. 1 CVs of 1.0 mM 2 in 0.1 M aqueous buffer (NaOAc and NaPi) at pH 9.0 with GC (0.07 cm2) as the working electrode at the scan rate of 100 mV s−1. Background (black dotted line) is shown for comparison.

At a more positive potential of approximately 1.40 V (1.18 V by DPV, Fig. S10b), another irreversible oxidation current peak (Ep1) appeared, with a large current above the background, suggesting the catalysis of WO, which was confirmed by the detection of produced oxygen via CPE. In both cases, the oxidation waves of NiII/III and NiII/IV are pH-dependent, suggesting that the oxidations proceed through PCET; interestingly, when the CV of 2 is scanned at a potential above 1.40 V vs. NHE,6,72 it appears that the initial NiIV species favor the two-electron reduction to NiII instead of making it proceed via the NiIII intermediate.6,71 To further ensure the PCET process, a Pourbaix diagram (Fig. 2) for 2 was also constructed via the DPV measurements (Fig. S11) in the pH range of 4.0 < pH < 11.0. The Pourbaix diagram demonstrates that the two redox processes of NiII are pH-dependent. The first redox couple NiIII/II appears through the PCET from the slope of ca. −58 mV pH−1 unit with an increase in pH, which is consistent with a 1e/1H+ coupled transition; the second redox couple has a slope of ca. −28 mV pH−1, which is consistent with a 2e/1H+ PCET process for NiIV/II.


image file: c9cy01485c-f2.tif
Fig. 2 Pourbaix diagrams of 2 in the pH range of 4.0 < pH < 11.0. Experimental values were obtained from the DPV measurements of 2 (1.0 mM) in 0.1 M NaPi buffer in the pH range of 4.0–11.0.

Furthermore, the catalytic current density at 1.40 V increased linearly with an increase in the concentration of 2 in NaPi buffer (Fig. S12), demonstrating single-site nickel catalysis.6 In addition, the current density increases with an increase in scan rate (Fig. S13), indicating that O–O bond formation is the rate-limiting step.6,51,73 Based on the type of electrolyte, we predict that more efficient proton acceptors (such as HPO42–)17,73–75 can again easily facilitate the NiIV/II reduction through the PCET. Moreover, the concentration of electrolyte may greatly affect the catalytic efficiency of 2; thus, to investigate the effect of the concentration of phosphate buffer as a base on the catalytic performance of 2, CV has been conducted at different buffer concentrations ([NaPi] = 0–1.0 M; Fig. S14a). The catalytic currents due to water oxidation increased linearly with an increase in the concentration of the NaPi buffer up to 0.1 M; however, the increase in the current density was lower at higher electrolyte concentrations as compared to the case of lower concentrations (Fig. S14b).17,48 In addition, an acceleration in the rate of Ni(IIIII) oxidation was observed with an increase in the concentration of phosphate ions as a proton acceptor base in the solution. This observation indicates the base-assisted oxidation of NiII → NiIII, as reported by other studies.76 This result suggests that phosphate ions can act as a proton acceptor during the rate-determining step, i.e. O–O bond formation, and thus enhance the rate of the OER.16,17,75,77 Therefore, the catalytic rate was obviously accelerated by the added base (phosphate) with stronger proton-accepting ability. For example, phosphate has a stronger proton-accepting ability [pKa (H2PO4 = 7.20] than H2O [pKa (H3O+ = −1.74], and hence, the proton of H2O will transfer to phosphate as a proton acceptor, resulting in enhanced water oxidation performance.17,61 This observation is similar to that obtained for most homogeneous WOCs working in the NaPi system.47,49,73,74 Measurements in different aqueous buffer solutions demonstrated that the sharp NiII/III wave did not appear until the NaPi buffer was utilized rather than NaOAc buffer (Fig. 1 and S9). Generally, the buffer capacity of NaPi [pKa(H2PO4/HPO42–) = 7.20] is significantly higher than that of NaOAc [pKa(HOAc/OAc) = 4.76] at neutral pH,16 and even high pH of the reaction solution is thermodynamically favorable for the oxidation of H2O to O2.40,78 The decrease in the catalytic current at pH 7.0 as compared to that at pH 9.0 (Fig. S9) is a result of the lower activity of the catalyst as the pH decreases.79,80 The order of the catalytic efficiency in different buffers is as follows: pH 9.0 NaPi > pH 7.0 NaPi > pH 7.0 NaOAc > pH 7.0 NaOAc at relatively moderate overpotentials. The onset potential (Ep,o) for catalytic WO in NaPi buffer appears at approximately 1.25 V vs. NHE, with a moderate overpotential of ∼0.55 V, which is comparable to that of most of the reported homogeneous earth-abundant metal-based WOCs (300–600 mV).6,47,51,73,74,81

Electrocatalysis by CPE

To investigate the catalytic performance of 2, long-term CPE (11 h) was performed with the catalyst and blank (without catalyst) at 1.50 V vs. NHE at the ITO (0.80 cm2, 6–7 Ω resistivity) electrode (Fig. 3). The experiment was performed in a cell with 1.0 mM 2 in 0.1 M aqueous buffer (NaPi/NaOAc) under an Ar atmosphere. It displayed high activity for WO in the pH 9.0 NaPi buffer as compared to that in the case of pH 7.0, but its activity was negligibly low in NaOAc at both the pH values of 7.0 and 9.0. During CPE at 1.50 V, the background current in the absence of the catalyst was negligibly small; on the other hand, the catalyst displayed an increase in current in the NaPi buffer solution within the initial 1 h and then reached a relatively stable current density of approximately 0.65 mA cm−2 (at pH 9.0) at the overpotential of 0.80 V upon further electrolysis (Fig. 3), which was related to continuous WO.17 To measure the Faradaic efficiency,6,82,83 an air-tight electrochemical cell was used, and the O2 formed in the headspace was monitored using gas chromatography using a thermal conductivity detector (GC-TCD)41 during CPE at different intervals. After 2 h bulk electrolysis, ca. 12.2 μmol of O2 was detected at pH 9.0 (0.1 M NaPi), with the Faradaic efficiency of 93% (Fig. S15). The abovementioned observations suggest that the catalysis may include some active species that are formed in situ around the ITO electrode. Herein, the active species is unlikely to be a nickel oxide (NiOx) film or particles as active heterogeneous WOCs,16,55 as determined by subsequent experiments. A more reasonable assignment of the active species may be a soluble, molecular intermediate, which accumulates around the ITO electrode.6,16
image file: c9cy01485c-f3.tif
Fig. 3 Long-term (11 h) CPE by the ITO electrode (0.80 cm2, 6–7 Ω resistivity) at 1.50 V vs. NHE containing 1.0 mM 2 in 0.1 M aqueous buffer (NaOAc and NaPi) at pH 7.0 and 9.0. Background (black dotted line) is shown for comparison.

Catalyst stability and homogeneous catalysis

A relationship exists between the molecular structure and catalytic stability of Ni-based homogeneous WOCs. From the structure of the complex, coordinated nitrogen (N) atoms rather than oxygen (O) atoms benefit the overall stability of Ni(II)-based homogeneous WOCs.17 In 2, the central Ni(II) ion is tetracoordinated by two nitrogen-containing pyridines and two NHC carbons in a square-planar geometry. Since this is the first Ni–NHC complex considered for water oxidation under aqueous conditions (neutral/alkaline), we postulate that the presence of two N-heterocyclic carbene ligands with known strong σ-donor properties, which will stabilize the high oxidation states of nickel, may offer high stability for the complex 2 under the operating conditions. Indeed, stable metal–carbene bonds provide high thermal stability and hydrolytic durability.64 After long–term (11 h) CPE, the CV, ESI-MS and UV-Vis spectral analyses of the resulting solution confirm the stability and homogeneous character of 2.

The electronic absorption spectra of complex 2 and the tetradentate NHC/pyridine hybrid ligand 1 were obtained in both an organic solvent (acetonitrile) and aqueous buffer solutions (Fig. S8). The ligand precursor 1 showed sharp and intensive absorbance at 225–285 nm, ascribed to the π–π* transitions of the pyridyl groups.62,84 Compound 2 readily dissolved in acetonitrile and in an aqueous medium, resulting in a clear greenish-yellow solution within a very short time (1 min). It displayed intense ligand-centered (LC) π–π* transitions of the pyridyl groups at 225–350 nm. In the visible region, it exhibited a broad and relatively weak absorbance band at 400 nm, which was assigned to the MLCT transition. To check the stability of 2 during electrochemical WO, we carried out UV-vis absorption measurements over 72 h (Fig. S16–S18), and its spectrum was obtained after long-term CPE (11 h) at 1.50 V. The UV-vis analysis discloses that the absorbance spectrum of 2 after 72 h aging in a buffer solution of pH 9.0 completely overlaps with the original spectrum; this suggests that the complex is stable in the buffer solutions for at least for 72 h regardless of the buffer type NaPi/NaOAc. The spectrum of 2 in acetonitrile also matched with that obtained in aqueous buffer, excluding the coordination of a coordinating solvent such as acetonitrile, which did not change with time. Furthermore, its spectrum did not exhibit any change even after 11 h CPE of bulk electrolysis at 1.50 V vs. NHE (Fig. S17 and S18); this indicated the high stability and homogeneous character of the complex under operating conditions.6,16–18

Furthermore, the stability of the catalyst during the course of bulk electrolysis was investigated by ESI-MS using 1.0 mM 2 in 0.1 M NaPi buffer (pH 7.0 and 9.0); an aliquot of the catalyst solution was analyzed before and after the long-term CPE (11 h) at room temperature. The same species were observed in the mass spectrum obtained before (Fig. S19a and S20a) and after the CPE (Fig. S19b and S20b), confirming the impressive stability and homogeneous behaviour of the catalyst under the operating conditions. Furthermore, the ITO electrode was subjected to CV before and after 11 h CPE in the NaPi buffer (Fig. 4 and S21), which displayed almost the same current density, excluding any deposition of active NiOx species at the electrode during the bulk electrolysis experiment and indicating that the catalyst was homogenous in nature.6,16


image file: c9cy01485c-f4.tif
Fig. 4 CVs of 1.0 mM 2 obtained before (red) and after (blue) 11 h CPE in 0.1 M NaPi buffer (pH = 9.0) with an ITO electrode (0.80 cm2, 6–7 Ω resistivity) at a scan rate 100 mV s−1. Background (black) is shown for comparison.

Moreover, consecutive scanning of a 1.0 mM solution of 2 in aqueous NaPi buffer at a GC electrode was performed, where the catalytic current decreased during multiple scans and became almost constant/stable after about 5 scan cycles (Fig. S22).6,16 This finding is contrary to that of the reported heterogeneous WOCs, where the currents are continuously enhanced with repeated scanning owing to the electrodeposition of NiOx as the active catalysts.55,85 Thus, to further confirm this result, the GC electrode was rinsed after multiple scans (50 scans), but not polished, then cycled in catalyst-free electrolyte (0.1 M NaPi buffer) at pH 7.0 and 9.0. No remarkable catalytic currents were observed relative to a freshly polished GC electrode (Fig. S23a and S24a). A similar result was observed at the ITO electrode after 11 h of CPE (Fig. S23b and S24b), confirming that the electrocatalysis with 2 is homogeneous.16,86 SEM measurements of 2 revealed no Ni-based particles on the ITO electrode after 11 h CPE (Fig. S25 and S26), and no elemental signal of nickel or phosphorus was found on the ITO surface by EDX after CPE (Fig. S27 and S28), demonstrating again the homogeneous nature of 2 in catalytic WO.6,16

Finally, the CV, UV-Vis, ESI-MS, SEM, and EDX results of catalyst 2 after 11 h of CPE in aqueous buffer (pH = 7.0 and 9.0) solutions were almost identical to that before electrolysis (Fig. 4 and Fig. S15–S28), indicating that catalyst 2 did not decompose during electrolysis. It is interesting to note that the resulting solution after electrolysis evaporated slowly at room temperature, which quantitatively provided a greenish-yellow colored catalyst 2. Hence, it is confirmed that 2 is a homogeneous water oxidation electrocatalyst, which is highly stable in neutral and slightly alkaline medium.

Conclusion

In conclusion, the Ni(II) complex based on a tetradentate NHC/pyridine hybrid ligand (2) is highly stable under aqueous conditions. It acts as a homogeneous electrocatalyst for WO at the moderate overpotential of ∼0.55 V (0.80 V at ITO for 11 h CPE with the Faradaic efficiency of 93%), which is comparable to that of most of the reported homogeneous earth-abundant metal WOCs (300–600 mV). Several experiments were performed to confirm the molecular nature of the catalyst and exclude that the possible formation of active NiOx was responsible for its activity. The CV, UV-Vis, ESI-MS, SEM, and EDX analyses demonstrate that 2 is impressively stable after long-term bulk electrolysis, and it is homogeneous in nature. Interestingly, the electrolyte-enhanced catalyst activity in the case of phosphate buffer and the complete loss of activity in the acetate solution highlight the key role of phosphate ions as a proton acceptor that boosts the catalyst activity via enhanced PCET during catalysis; this agrees with the recent studies on nickel catalysts for WO. Thus, based on the ease of synthesis and overall stability, this type of molecular catalyst may be competitive with the other common poly N-dentate complexes not only for nickel but also for other earth-abundant metals.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors express their deep appreciation to the State Key Lab of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology, China) for financial support. H. M. S. acknowledges the China Scholarship Council (CSC) for his PhD study grant 2016GF034 and to the Comilla University, Bangladesh, for the PhD study leave.

Notes and references

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Footnote

Electronic supplementary information (ESI) available: NMR, FT-IR, ESI-MS, UV-vis, CV, SEM and EDX figures. See DOI: 10.1039/c9cy01485c

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