Facile Synthesis of 3d Transition-Metal-Doped α-Co(OH)2 Nanomaterials in Water-Methanol Mediated with Ammonia for Oxygen Evolution Reaction.

Layered cobalt hydroxides are cost-efficient electrocatalysts for oxygen evolution reaction (OER) in the field of energy conversion. Herein, we developed a facile synthesis method of 3d transition-metal-doped α-Co(OH)2 nanomaterials mediated with ammonia in water-methanol at room temperature. The doping of Cu2+ and Ni2+ leads to flower-like nanostructures similar to pure α-Co(OH)2, whereas the doping of Fe2+ produces nanoparticles with more than 2 times larger surface area in comparison with the Cu2+- and Ni2+-doped nanoflowers. The obtained dispersion with the addition of Nafion can be used directly as an electrocatalyst for OER with excellent catalytic activity, especially that the overpotential of Fe2+ doped is as low as 290 mV at 10 mA cm-2 and the turnover frequency is improved by 3 times as compared with that of α-Co(OH)2. Furthermore, the catalyst can be loaded onto foam nickel, which presents excellent durability with the current density unchanged under continuous chronoamperometry reaction for as long as 12 h and almost quantitative faradaic efficiency. The superior electrocatalytic properties combined with the simple synthesis without the tedious purification procedure is very promising for OER.

Introduction

Water splitting into oxygen and hydrogen is a promising strategy toward sustainable renewable energy resources.[1−4] There are various methods for water splitting including electrochemical, photocatalytic, photobiological, and thermal decomposition.[5−7] Among them, electrochemical approach is simple, cost-effective, and clean and provides great potential for water splitting.[8] Electrochemical water splitting involves water oxidation to oxygen at the anode and proton reduction to hydrogen at the cathode. Owing to the sluggish kinetics of oxygen evolution reaction (OER), the conversion efficiency of water to fuel is low.[9,10] Thus, development of efficient catalysts to reduce the kinetic barrier for OER is urgent in the field of water splitting.[11−14]

Layered cobalt hydroxides (LCHs) have gained increasing attention because of their promising electrochemical applications, especially for supercapacitors and electrochemical water splitting.[15−17] LCH can be crystallized in two polymorphs as the hydrotalcite-like α-Co(OH)2 and the brucite-like β-Co(OH)2. The β-Co(OH)2 is a thermodynamically stable phase as compared to the metastable α-Co(OH)2, whereas α-Co(OH)2 is theoretically expected to exhibit higher electrochemical activity than β-Co(OH)2.[18−20] Some research studies illustrated that amorphous Co(OH)2 presents high electrochemical activity as well.[21−24]

The application of Co(OH)2 in electrochemical water splitting requires the large surface area or more active sites on the surface of the catalysis.[25,26] Several methods for the synthesis of ultrathin nanostructural Co(OH)2 have been developed recently and used as electrocatalysis for OER. Three-dimensional flower-like α-Co(OH)2 hierarchical microspheres were fabricated via a solvothermal method with dodecyl benzene sulfate as a surfactant.[27] α-Co(OH)2 and β-Co(OH)2 nanosheets were selectively synthesized through an epoxide precipitation reaction under mediation of fluoride ions.[28] Unilamellar cobalt hydroxide nanosheets can be obtained by direct exfoliation of nanocones in formamide.[29,30] Ultrathin α-Co(OH)2 nanostructures were prepared through 2-methylimidazole-mediated methods with good to excellent electrocatalytic activity.[31−33] Transition-metal doping has also been proved as an efficient method to improve the electrocatalytic activity of Co(OH)2.[34−37] Copper- or iron-doped Co(OH)2 has been respectively reported to effectively catalyze water oxidation at the overpotential around 300 mV at 10 mA cm–2 current density with the former synthesized via NaBH4 reducing method and the latter precipitated with NaOH in the presence of NaNO3, NH4F, and sodium citrate under N2 atmosphere.[38,39] Cobalt iron hydroxides electrodeposited on nickel foam films have also been proved to exhibit excellent catalytic effect for OER.[40]

Ammonia as a weak and evaporable alkaline has been broadly used to synthesize transition-metal-layered hydroxide materials.[41−43] It is different from NaOH or KOH to only afford [OH]− and ammonia can also act as a ligand to resolve Co(OH)2 through forming Co(NH3)62+.[44−46] Xu and Zeng systematically examined the interconversion of α-Co(OH)2 and β-Co(OH)2 mediated with liquid ammonia. They concluded that the addition time of Co2+, aging time, and preparative atmosphere play key roles in controlling the phase of Co(OH)2.[47] Rajamathi et al. successfully synthesized α-Co(OH)2 mediated with ammonia solution containing an excess of sodium salt.[48] Although α-Co(OH)2 can be synthesized easily via ammonia-mediated method, to the best of our best knowledge, there is still no report about their electrocatalytic application probably because of their limited active sites on the surface. In this manuscript, we present a facile method for the synthesis of α-Co(OH)2 and 3d transition-metal-doped α-Co(OH)2 nanomaterials. The synthesis was carried out by simply reacting ammonia with cobalt salt with or without the addition of the 3d transition-metal chloride using water–methanol as a solvent at room temperature. The obtained α-Co(OH)2 nanoflowers can be directly used as a catalyst, which shows moderate electrocatalytic activity for OER. Interestingly, the electrocatalytic activity can be improved via incorporating the 3d transition metal into α-Co(OH)2, especially that the Fe doped α-Co(OH)2 has an overpotential as low as 290 mV at 10 mA cm–2 current density. The resulted doped α-Co(OH)2 can be loaded onto nickel foam as an electrode, which demonstrated long durability.

Results and Discussion

Aggregated α-Co(OH)2 was produced via precipitating Co(NO3)2 with ammonia using water as a solvent (Figure S1). However, by replacing water with water–methanol solution, homogeneous green dispersion can be obtained within 30 min, which presented the typical UV–vis absorption peaks of α-Co(OH)2 at 586 and 642 nm (the inset of Figure a).[31] The characteristic X-ray diffraction (XRD) peaks of α-Co(OH)2 at 9.6°, 19.3°, 33.4°, and 59.6° correspond to the lattice distances of (003), (006), (012), and (110) planes of α-Co(OH)2 (JCPDS no. 46-0605) (Figure a).[49,50] As shown in the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images in Figure b,c, the products have the flower-like nanostructure with two-dimensional petals. The high-resolution TEM (HRTEM) images show the clear lattice fringes of 0.26 nm, which belong to the (012) planes of α-Co(OH)2 (Figure d). The Brunauer–Emmett–Teller (BET) surface area of the product is 42 m2/g, which exhibits the type IV adsorption (Figure S2a). The solvent-induced effects have been demonstrated to play an important role in nanostructure growth processes. Especially, the dual-solvent system of alcohol and water can lead to the formation of ultrathin α-Co(OH)2 using 2-methylimidazole as the alkali source and the surface modifier.[51,52] The advantage of the present synthesis method is that it does not need expensive organic ligands and complicated purification process.

Figure 1

(a) XRD pattern of α-Co(OH)2 (inset: UV–vis absorption and optical image of α-Co(OH)2); (b) SEM image of α-Co(OH)2; (c) TEM image of α-Co(OH)2; and (d) HRTEM image of α-Co(OH)2.

In the next step, 3d transition metals were incorporated into α-Co(OH)2 via adding various amounts of FeCl2, CuCl2, or NiCl2 to the precursor solution. As shown in the XRD patterns in Figure , the diffraction peaks of α-Co(OH)2 were kept unchanged with three extra peaks at 16°, 31°, and 39° appeared for Cu2+- and Ni2+-doped samples, which can be assigned to Cu(OH)2 and Ni(OH)2, respectively. In contrast, the XRD pattern of Fe-doped sample exhibits an obvious broadening of the diffraction peaks and shifts to higher angles (∼0.4°) relative to α-Co(OH)2, which probably indicates the partial substitution of Co2+ by Fe2+ to form CoFe(1–(OH)2.[24] The morphologies of the synthesized 3d transition-metal-doped α-Co(OH)2 were investigated with SEM and TEM (Figure ). Copper and nickel doping results in the flower-like nanostructures similar to pure α-Co(OH)2 (Figure a–f). Unexpectedly, iron doping leads to particle-like morphology. Elemental mappings of the doped α-Co(OH)2 indicate the uniform distribution of 3d transition metal in the products. The BET surface area of the products was 35, 40, and 110 m2/g for Cu-Co(OH)2, Ni-Co(OH)2, and Fe-Co(OH)2, respectively (Figure S2b–d). Especially, Fe-Co(OH)2 has an obvious mesoporous hole with a width of 3–4 nm. This illustrates that the doping of Cu2+ and Ni2+ does not change the morphology of Co(OH)2 significantly. In contrast, the Fe doping leads to nanoparticles with more than 2 times larger surface area in comparison with the nanoflowers.

Figure 2

XRD pattern of 3d transition-metal-doped Co(OH)2.

Figure 3

Morphologies of 3d transition-metal-doped α-Co(OH)2. Cu-Co(OH)2: (a) SEM image; (b) TEM image; and (c) EDX mapping. Ni-Co(OH)2: (d) SEM image; (e) TEM image; and (f) EDX mapping. Fe-Co(OH)2: (g) SEM image; (h) TEM image; and (i) EDX mapping.

The produced Co(OH)2 dispersion with the addition of Nafion was loaded directly onto the glassy carbon working electrode for the electrochemical measurement at 1 M KOH alkaline solution. To obtain reliable data, multiple cyclic voltammetry (CV) tests were performed before the liner sweep voltammetry (LSV) experiment was carried out. Figure a shows the LSV curves of all the electrodes with the scan rate of 10 mV s–1. α-Co(OH)2 obtained in water–methanol solution exhibits moderate OER performance with the onset potential of 1.40 V and the overpotential of 340 mV at 10 mA cm–2. In comparison, α-Co(OH)2 obtained in pure water displays much poor OER performance. The doping of Ni2+ does not improve the performance. In contrast, the incorporation of Cu2+ or Fe2+ reduces the overpotential significantly (Figure a). After carefully optimizing the ratio of 3d transition metal to Co2+, the samples containing 30% Cu2+ or 20% Fe2+ in mole gave rise to the lowest overpotential of 295 and 290 mV at 10 mA cm–2, respectively (Figure S3). For comparison, LSV experiments of the 3d metal doping Co(OH)2 synthesized at pure water were undertook to give rise to the negative results similar to Co(OH)2@H2O (Figure S3). This indicates that the using solvent has an important effect on the reactivity for the same element content catalysis, which the pure water leads to the aggregated morphology (Figure S1).

Figure 4

OER performances of Co(OH)2. (a) LSV polarization curves; (b) LSV curves before and after the 1000th CV test; (c) Tafel plots at 5 mV s–1 in 1 M KOH; (d) EIS spectra were recorded at 1.64 V vs RHE in 0.01 Hz to 100 kHz with an ac amplitude of 5 mV, the inset is the equivalent circuit model; and (e) current density vs scan rate plot for estimation of ECSA.

Encouraged by these results, we then investigated the stability of α-Co(OH)2, Cu-Co(OH)2, and Fe-Co(OH)2 via performing the continuous CV between 0.0 and 0.6 V (vs Ag/AgCl) at a scan rate of 100 mV s–1. The LSV curve of Fe-Cu(OH)2 has no obvious change even after 1000 continuous cycles, whereas the curves of α-Co(OH)2 and Cu-Co(OH)2 change slightly (Figure b). These indicate that Fe-Co(OH)2 not only has the lowest overpotential but also is of excellently stability. Additionally, the turnover frequencies (TOFs) were measured. The TOF values of α-Co(OH)2, Cu-Co(OH)2, and Fe-Co(OH)2 are 1.24 × 10–3, 3.24 × 10–3, and 4.00 × 10–3 s–1, respectively. The value of Fe-Co(OH)2 is the highest and 3 times higher than that of α-Co(OH)2.

To better understand the OER process, the electrocatalytic kinetics of the OER process were evaluated using Tafel plots (Figure c). Fe-Co(OH)2 has the lowest Tafel slope of 69 mV dec–1 compared to 81 and 95 mV dec–1 for α-Co(OH)2 and Cu-Co(OH)2, indicating its quicker electrocatalytic kinetics and higher OER electrochemical catalytic activity. The large Tafel slope of Cu-Co(OH)2 is probably due to the raised LSV curve before 1.5 V caused by the copper oxidation peak. Furthermore, electrochemical impedance spectroscopy (EIS) was conducted to evaluate the electron transport ability and further study the OER kinetics. Equivalent circuit model is shown in the inset in Figure d. The solution resistance (Rs) values of the samples are the same as 12 Ω. The electron-transfer resistance (Rct) values of α-Co(OH)2, Cu-Co(OH)2, and Fe-Co(OH)2 is 29, 17, and 15 Ω, respectively (Figure d). The smaller Rct values of Fe-Co(OH)2 and Cu-Co(OH)2 indicate their more efficient charge transport during the electrochemical OER process than α-Co(OH)2. Furthermore, the electrochemically active surface areas (ECSAs) were estimated from the electrochemical double-layer capacitance (Cdl) in the non-faradaic region in the scan rate range of 1–20 mV s–1 (Figure S4). The Cdl are 41.17, 60.75, and 109.11 mF cm–2 for α-Co(OH)2, Cu-Co(OH)2, and Fe-Co(OH)2, respectively (Figure e). The improved catalytic activity of Cu-Co(OH)2 and Fe-Co(OH)2 can be attributed to the lower charge-transfer resistance and the larger electrochemical surface area than Co(OH)2 because of Cu or Fe doping.

Consequently, X-ray photoelectron spectroscopy (XPS) measurements were carried out to examine the electronic structure of α-Co(OH)2, Cu-Co(OH)2, and Fe-Co(OH)2. The obtained spectra show distinctive peaks of Co, Cu, Fe, and O (Figure S5), indicating the successful doping of Cu2+ and Fe2+ in α-Co(OH)2. The high-resolution Co 2p spectra of α-Co(OH)2, Cu-Co(OH)2, and Fe-Co(OH)2 are shown in Figure a. There is a slight shift of 0.24 and 0.41 eV for Co 2p3/2 and 0.15 and 0.2 eV for Co 2p1/2 in Cu-Co(OH)2 and Fe-Co(OH)2 toward lower binding energy as compared with that of α-Co(OH)2. The shift of the binding energy in Cu- and Fe-doped α-Co(OH)2 reveals the strong coupling between Cu, Fe, and Co for the doped α-Co(OH)2.[53,54] In addition, we can observe that the content of Co2+ increases and the content of Co3+ decreases after doping. This means that doping facilitates the formation of hydroperoxy (OOH) species,[23,55,56] which was regarded as the active phase for many Co-based OER catalysts.[53] The O 1s XPS spectra for Cu-Co(OH)2 and Fe-Co(OH)2 also present the obvious shift toward lower binding energy side (Figure b). These indicate the strong electronic interaction between Cu and Fe with α-Co(OH)2.[57] The high-resolution Cu XPS curve of Cu-Co(OH)2 can be ascribed as Cu 2p3/2 (935.2 eV) and Cu 2p1/2 (955.1 eV) with two shake satellite peaks (943.1 and 962.9 eV). For the high-resolution Fe XPS spectra of Fe-Co(OH)2, two main peaks of Fe 2p3/2 (712.6 eV) and Fe 2p1/2 (725.8 eV) with the satellite peak at 717.6 eV are observed, demonstrating the presence of Fe3+.[57] Fe3+ is usually considered as a redox-active species that can cooperate with Co to participate in the redox-hopping-type charge transfer, resulting in better OER activity.[26]

Figure 5

(a) High-resolution XPS spectra of Co 2p; (b) O 1s for α-Co(OH)2, Cu-Co(OH)2, and Fe-Co(OH)2; (c) high-resolution Cu XPS spectrum of Cu-Co(OH)2; and (d) high-resolution Fe XPS spectrum of Fe-Co(OH)2.

As can be seen from the above results, Fe-Co(OH)2 has the smallest Tafel slope and electron-transfer resistance with the largest electrochemical double-layer capacitance, which presents the best catalytic activity among the tested catalysts. To extend its application in water split, Fe-Co(OH)2 was loaded onto a nickel foam (1 × 1 cm) and the chronoamperometry (CP) experiment was performed on the resulted electrode. The current density remains unchanged at 10 mA cm–2 under continuous CP reaction for as long as 12 h at the overpotential of 270 mV (Figure a), and the LSV curve barely changes after 12 h test (Figure b). This demonstrates the long durability of the electrode. Furthermore, the volume of the released mixed gas of H2 and O2 was measured with a gas burette. 31.2 mL of gas was produced within 60 min, and the faradaic efficiency is calculated as 99% (Figure c).[15,29]

Figure 6

Performances of the electrode of Fe-Co(OH)2/nickel foam. (a) CP curve at a constant current of 10 mA cm–2 for 12 h; (b) LSV curves before and after the 12 h CP test; (c) theoretical and experimental values of oxygen release at a constant current 10 mA cm–2 in 1 M KOH.

Conclusions

In summary, we demonstrate the facile synthesis of 3d transition-metal-doped α-Co(OH)2 nanostructures in H2O/MeOH cosolvent mediated with ammonia at room temperature. The doping of Cu2+ and Ni2+ gives rise to the nanoflower structures similar to pure α-Co(OH)2; however, the doping of Fe2+ leads to particle morphology with more than 2 times larger surface area in comparison with the nanoflowers. The obtained dispersion together with Nafion can be used directly as electrocatalysis for OER with good to excellent catalytic activity, avoiding time-consuming purification procedure. The incorporation of Cu2+ and Fe2+ improves the OER performance significantly, especially that the overpotential of the Fe2+-doped sample is as low as 290 mV at 10 mA cm–2 and the TOF is improved 3 times as compared with that of α-Co(OH)2. The electrode of Fe-Co(OH)2/nickel foam presents excellent durability and almost quantitative faradaic efficiency. The ammonia-mediated synthesis route proposed in this work is simple and of low cost and up-scale synthesis can be performed easily. The produced products are very promising catalysts for electrochemical water splitting.

Experimental Section

Materials and Characterization

Co(NO3)2·6H2O (99%), CuCl2·2H2O (99%), FeCl2·4H2O (99%), NiCl2·2H2O (99%), and NH3·H2O (25–28%) were purchased from Aladdin Industrial Corporation. Nafion perfluorinated resin solution was received from Sigma-Aldrich. All chemicals were used without further purifications.

Morphology characterization was performed using a scanning electron microscope (JSM-6700F) and a transmission electron microscope (JEM-2100F) equipped with EDX (X-Max 80T, Oxford). Powder XRD patterns were recorded on a powder diffractometer (SmartLab9, Rigaku). The absorption spectra were recorded using a TU-1901 UV–visible spectrometer. XPS was recorded in a RBD-upgraded PHI-5000C ESCA system (PerkinElmer) with Al Kα radiation (hν = 1486.6 eV). Nitrogen adsorption–desorption isotherms were operated on an automated surface area pore size analyzer (QUADRASORB evo).

Synthesis of the α-Co(OH)2 Nanoflowers

Co(NO3)2·6H2O (582 mg, 2 mmol) was dissolved in 15 mL of methanol/H2O (3/1, v/v). NH3·H2O (300 μL) diluted in 5 mL of methanol/H2O (3/1, v/v) was then added slowly under magnetic stirring. The reaction continued for 30 min at room temperature to give a homogeneous green colloid solution. Finally, the green α-Co(OH)2 product was collected via centrifugation and dried under air. The synthesis of 3d transition-metal-doped Co(OH)2 was similar to this procedure except with the addition of various amounts of CuCl2·2H2O, FeCl2·4H2O, or NiCl2·2H2O, respectively.

Electrochemical Measurement

The glassy carbon electrode (GCE) with a diameter of 3 mm was selected as the working electrode. For the preparation of catalysis ink, 40 μL of ethanol solution of Nafion (5 wt %) was added to 1 mL of Co(OH)2 reaction mixture and sonicated for 30 min. Subsequently, 6 μL of catalyst ink was dropped on the GCE and then dried overnight at room temperature. The electrochemical tests were carried out on a standard three-electrode system with an Autolab Nova III electrochemical workstation. An Ag/AgCl electrode with 3 M KCl solution and a platinum foil were used as the reference and counter electrodes, respectively. KOH solution (1.0 M, pH = 13.9) was used as the electrolyte. All potentials measured were calibrated to the reversible hydrogen electrode (RHE) using the following equation

LSV curves were recorded at a scan rate of 10 mV s–1. The impedance spectra were recorded under 0.602 V (vs Ag/AgCl) in the frequency range from 105 to 0.1 Hz with a sinusoidal 5 mV amplitude, possibly.

The TOF values were calculated from equationwhere J is the current density at a fixed overpotential (e.g. η = 300 mV), A is the geometrical surface area of the electrode, F is the Faraday constant (a value of 96 485 C mol–1), and M is the mole of metal on the electrode. All the metal atoms are assumed to be the catalytically active sites, which indicates that the minimum values of TOF were calculated.where F is the Faraday constant, nO (mol) is the total amount of the produced O2, the four electrons are needed to produce one O2 molecule, and It means under a constant oxidation current (I) within a certain time (t).