The massive consumption of fossil fuel is creating worldwide interest in searching for renewable energy sources [1, 2]. Splitting water into oxygen and hydrogen by sunlight is a promising approach to achieve this goal [3, 4]. However, water oxidation involves the release of 4e^- and 4H⁺, and it requires a high thermodynamic overpotential (2H₂O → O₂ + 4H⁺ + 4e^-, 1.23 V vs. the normal hydrogen electrode (NHE) at pH = 0) [5]. To efficiently drive this reaction, water oxidation catalysts (WOCs) with low overpotentials are crucial. Inspired by nature, great progress has been made in developing WOCs based on transition metals [6-14]. Although the activities of WOCs based on noble metals, such as ruthenium and iridium, are impressive, their practical application is hampered by their scarcity and high cost. Consequently, development of efficient WOCs based on earth-abundant elements is urgently required in the field of solar energy conversion.

Great effort has been made to develop WOCs derived from first row transition metals. In 2008, Nocera and co-workers reported that electrodeposition of cobalt phosphate (Co-Pi) on conductive substrates gave highly efficient electrocatalysts for water oxidation [15]. This method has been extended to other metals, such as iron and nickel [16-23]. For example, electrochemical deposition of an oxide film from molecular complexes is a promising approach to fabricate active Ni-based WOCs [17, 24, 25]. Recently, Spiccia and co-workers reported a facile protocol for electrodeposition of a NiO_x film from a nickel amine complex in borate buffer [24]. The film exhibited a steady current density of 1.8 mA/cm² at 1.3 V vs. NHE in borate buffer. More recently, Allen and co-workers reported that the combination of Ni²⁺ and glycine could act as a precursor for an efficient nickel oxide electrocatalyst in phosphate buffer at pH = 11 [25]. However, the activity and durability of these catalysts needs to be improved. Here, inspired by this progress, we report a simple [Ni(en)₃]Cl₂ (en = ethanediamine) complex for facile electrodeposition of a nickel oxide film in phosphate buffer solution. The resulting film is inert to corrosion and exhibits superior catalytic activity to other molecular complex-derived catalysts. A current density of 8.5 mA/cm² is maintained for at least 10 h at a constant applied potential of 1.3 V.

All of the chemicals were purchased from Aladdin Chemical Company and used without further purification. Deionized water (18.2 MΩ/cm) obtained from a Milli-Q system (Millipore, Direct-Q 3 UV) was used throughout. The phosphate buffer solution (0.25 mol/L, pH = 11) was prepared by dissolving appropriate amounts of Na₂HPO₄∙12H₂O and Na₃PO₄∙12H₂O in deionized water. The [Ni(en)₃]Cl₂ complex was prepared according to a reported procedure [26]. The fluorine-doped tin oxide (FTO) substrates were purchased from Dalian Heptachroma SolarTech Co., Ltd. (thickness ~2.2 mm, transmittance >90%, resistance ~8 mΩ). Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis were performed with a Nova NanoSEM 450 scanning electron microscopy. The SEM images and EDX spectra were obtained with acceleration voltages of 3 and 20 kV, respectively. The catalyst content on the surface of the working electrode (FTO) was determined by inductively coupled plasma mass spectrometry (ICP-MS, (Optima 2000DV, America PerkinElmer Co.). The sample used for ICP-MS was prepared by depositing the NiO_x film on FTO with an applied bias of 1.2 V (vs. NHE). The sample was then gently rinsed with deionized water and dissolved in concentrated HNO₃. The sample used for the measurement was diluted with water. X-ray photoelectron spectroscopy (XPS) was performed with a Thermo Scientific ESCALAB250 X-ray photoelectron spectrometer using 200 W K_α radiation. The electrochemical measurements were recorded with a CHI 630D electrochemical potentiostat. The counter electrode was platinum wire. The reference electrode was an aqueous Ag/AgCl (3 mol/L NaCl) electrode. A glassy carbon electrode (diameter 3 mm) or FTO film was used as the working electrode. All of the potentials were measured against a Ag/AgCl reference and converted to the NHE by addition of 0.197 V to the measured potentials.

The FTO substrates were ultrasonically cleaned in deionized water, ethanol, and acetone (30 min each), and then air-dried. The NiO_x films for the water oxidation experiments were deposited by constant potential electrolysis (CPE) at 1.2 V (vs. NHE) using a solution of [Ni(en)₃]Cl₂ (1 mmol/L) in phosphate buffer solution (0.25 mol/L). The films were then gently rinsed with deionized water and transferred to fresh 0.25 M phosphate buffer solution for anodization (1.3 V vs. NHE bias was applied for about 1.5 h).

Current-potential data were obtained by performing controlled potential electrolysis in 0.25 mol/L phosphate buffer solution at pH = 11 with a variety of applied potentials. A FTO electrode (1 cm²) coated with a NiO_x film (after anodization) was used as the working electrode. Ag/AgCl and Pt wire were used as the reference and counter electrodes, respectively. Before data collection, the solution resistance (35 Ω) was measured with a clean FTO working electrode using the potential loss for resistance (iR) test function to correct the Tafel plot for the iR drop. A catalyst film (1 cm²) was prepared by electrodeposition. Preconditioning the film by subjecting it to bulk electrolysis for several hours is necessary to obtain a reproducible Tafel slope value. A current density of 1.1 mA/cm² was applied for ~6 h before collecting data for the Tafel plot. The steady-state currents were measured at a variety of applied potentials while the solution was stirred. The Tafel plot measurements were performed in 10 mV steps between 0.88 and 0.97 V. In a typical experiment, the current reached a steady state at a particular potential in 3 min and the current values were recorded after 5 min. All of the measurements were performed twice. The obtained current values ranged from 22 μA/cm² to 1.84 mA/cm² in the applied potential range. The variation in the steady-state current of two runs at a particular potential was < 5%. According to the current-potential data, the Tafel slope is 43 mV/decade.

The current-pH data were collected by performing electrolysis at a fixed applied potential of 0.91 V (vs. NHE) in 0.25 mol/L phosphate buffer solution with a variety of pH values. A FTO electrode (1 cm²) coated with a NiO_x film (after anodization) was used as the working electrode. Ag/AgCl and Pt wire were used as the reference and counter electrodes, respectively. The solution had an initial pH value of 10.4. The pH value gradually increased by adding small amounts of NaOH solution, and the current density was recorded after 5 min electrolysis at each pH value point. All of the data were collected with iR correction using a solution resistance value measured before each electrolysis. The pH values of the solution ranged from 10.4 to 11.6 and the measured currents ranged from 14 μA/cm² to 1.56 mA/cm².

The faradaic efficiency measurements for oxygen evolution were performed in a gas-tight electrochemical cell. The cell was equipped with a FTO electrode coated with a NiO_x film. Ag/AgCl and Pt wire were used as the reference and counter electrodes, respectively. Before the measurements, the solution was degassed by bubbling Ar for 2 h with vigorous stirring. Electrolysis was initiated at 1.3 V without iR correction. During bulk electrolysis, the amount of evolved oxygen in the headspace was determined by gas chromatography.

Fig. 1 shows the cyclic voltammograms (CVs) of a glassy carbon electrode immersed in 0.25 mol/L phosphate buffer solution (pH = 11) containing 1 mmol/L [Ni(en)₃]Cl₂. In the first anodic scan, an anodic peak occurs at 0.89 V (all of the reported potentials are versus NHE), which is followed by a steep catalytic current arising from water oxidation. In the cathodic return scan, there is a broad peak centered at 0.85 V, which is attributed to reduction of the surface deposit formed in the initial anodic sweep. Repeated CV scans show the onset potential of water oxidation cathodically shifts while the cathodic peak anodically shifts. The catalytic current and cathodic peak increase in amplitude with scanning, suggesting growth of the material deposited on the surface. The intensity of the anodic peak decreases for the first three cycles and then remains stable. When Ni²⁺ is used instead of [Ni(en)₃]Cl₂, the CV trace is relatively featureless and the current is below 10 μA, essentially overlapping the background. This is probably because of the low solubility of Ni²⁺ in the phosphate electrolyte solution inhibiting surface deposition.

Based on the CV experiments, a thin catalyst film was fabricated by anodic deposition on a FTO substrate at a constant potential of 1.2 V in 0.25 mol/L phosphate buffer solution (pH = 11) containing 1 mmol/L [Ni(en)₃](Cl)₂. With 0.5 C/cm² charge, about 0.84 μmol/cm² of Ni was deposited on the surface of FTO, as determined by ICP-MS measurement. The as-prepared film was transferred into a fresh phosphate buffer solution. Upon applying a constant potential of 1.3 V, the current density further increased until reaching a plateau. According to previous studies, this phenomenon might be associated with an anodization process [17, 27, 28].

The SEM image in Fig. 2(a) show that the obtained film consists of compact packed nodules with sizes in the range 200-400 nm. XRD showed no characteristic diffraction peaks belonged to nickel species, indicating the amorphous character of the catalyst. EDX analysis was directly performed on the film after thorough rinsing with distilled water to remove any surface adsorbed components. The results showed that the film contained Ni, C, O, N, and P, as well as Sn and Si from the FTO substrate. The surface composition on the catalyst film was further investigated by XPS. The survey spectrum indicated the presence of Ni 2p, P 2p, and O 1s. In the Ni 2p spectrum (Fig. 3(a)), the peaks at 855.4 and 873.2 eV and the two satellite peaks at 860.7 and 879.1 eV are ascribed to typical Ni(Ⅱ) species. In the P 2p spectrum (Fig. 3(b)), the characteristic peak of P at 132.7 eV indicates the presence of a very small amount of PO₄^3-. In the O 1s spectrum (Fig. 3(c)), the peak at 531.2 eV can be attributed to oxide and hydroxide on the surface of the film. According to the XPS results, the main component of the catalytic film is NiO_x.

Long-term electrolysis was performed at a constant potential of 1.3 V. Fig. 4 shows that a steady current of 8.5 mA/cm² is maintained for at least 10 h, confirming the robustness of the catalyst film for water oxidation. The SEM image of the electrode shows no morphology change after electrolysis (Fig. 2(b)). During this process, the faradaic efficiency was determined to be 98%. The high activity and long-term stability show the potential of using Ni complexes as precursors for preparation of efficient WOCs.

The Tafel plot of log (j) against the overpotential (η) has a slope of 46 mV/decade for current densities ranging from 22 μA/cm² to 1.8 mA/cm² (Fig. 5(a)). It should be noted that preconditioning the film by bulk electrolysis for several hours is necessary to obtain a reproducible Tafel slope value. The slope of the Tafel plot of [Ni(en)₃]²⁺-derived NiO_x is comparable with that of a glycine-nickel derived WOC [25] and significantly lower than those of other Ni oxides reported in the literature [15, 22, 27, 28], indicating favorable kinetics for water oxidation. According to the Tafel plot, water oxidation starts at η = 330 mV (reaching a current density of 10^-4 A/cm²) and an overpotential of 375 mV is required to reach a current density of 1 mA/cm². These values are much lower than those of the glycine-nickel WOC under the same working conditions [25]. The electrochemical properties of [Ni(en)₃]²⁺-derived NiO_x are summarized in Table 1, and they compare favorably with those of other NiO_x oxygen evolution reaction catalysts.

The pH-dependent O₂ evolution activity of NiO_x was evaluated in Ni-free phosphate buffer solution. As shown in Fig. 5b, there is a linear relationship between log (j) and the pH at a bias of 0.91 V, corresponding to a slope of 1.94 decade/pH. Using the equation , the dependence of the overpotential on the pH is -87 mV/pH unit, suggesting that the loss of 1 e^- is accompanied by transfer of ~1.5 protons. For the nickel-glycine derived catalyst, the slope is 130 mV/pH, which is consistent with a 2e^-/1H⁺ process, and a mechanism involving proton-coupled oxidation of Ni(Ⅱ) to Ni(Ⅳ) was proposed [25]. Compared with the nickel-glycine catalyst, our catalyst film clearly has a different catalytic mechanism. In our system, a 2e^-/3H⁺ step may be operative. This is similar to a previously reported iridium oxide catalyst derived from dimerization of an Ir(Ⅳ) hydro-hydroxo species, which has a slope of -89 mV/pH unit [31, 32].

In this study, we used a nickel amine complex as a precursor to prepare a NiO_x water oxidation catalyst. The molecular-derived NiO_x film exhibits favorable catalytic properties for electrocatalytic oxygen evolution at pH = 11. A low overpotential of 375 mV with an activity of 1 mA/cm² is stable for at least 10 h, and a current density of 8.5 mA/cm² can be achieved at a constant applied potential of 1.3 V. The low Tafel slope of 46 mV/decade also indicates the favorable kinetic characteristics for electrocatalytic water oxidation. These results will be important for development of heterogeneous WOCs using inexpensive molecular precursors. The high activity and outstanding stability under benign conditions suggest more active WOCs may be accessible by varying the structure of the molecular precursor.

There are no conflicts to declare.

Acknowledgments

The acknowledgements come at the end of an article after the conclusions and before the notes and references. This research was supported by the National Basic Research Program of China (973 program, 2014CB239402), the National Natural Science Foundation of China (21476043), the Swedish Energy Agency and K & A Wallenberg Foundation.