Amorphous Nickel Hydroxide Nanosheets with Ultrahigh Activity and

Article pubs.acs.org/crystal

Amorphous Nickel Hydroxide Nanosheets with Ultrahigh Activity and Super-Long-Term Cycle Stability as Advanced Water Oxidation Catalysts Yuqian Gao, Hongbo Li, and Guowei Yang* State Key Laboratory of Optoelectronic Materials and Technologies, Nanotechnology Research Center, School of Materials Science & Engineering, School of Physics & Engineering, Sun Yat-sen University, Guangzhou 510275, Guangdong, P. R. China S Supporting Information *

ABSTRACT: Good conductivity is conventionally considered as a typical reference standard in terms of selecting water electrolysis catalysts. Electrocatalyst research so far has focused on crystal rather than amorphous due to poor conductivity. Here, we demonstrate that the amorphous electrocatalyst made of 3D honeycomb-like amorphous nickel hydroxide (Ni(OH)2) nanosheets synthesized by a simple, facile, green, and low-cost electrochemistry technique possesses ultrahigh activity and super-long-term cycle stability in the oxygen evolution reaction (OER). The amorphous Ni(OH)2 affords a current density of 10 mA cm−2 at an overpotential of a mere 0.344 V and a small Tafel slope of 46 mV/dec, while no deactivation is detected in the CV cycles even up to 5000 times. We also establish that the short-range order, i.e., nanophase, of amorphous creates a lot of active sites for OER, which can greatly promote the electrochemical performance of amorphous catalysts. These findings show that the conventional understanding of selecting electrocatalysts with conductivity as a typical reference standard seems out of date for developing new catalysts at the nanometer, which opens a door ever closed to applications of amorphous nanomaterials as advanced catalysts for water oxidation.

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INTRODUCTION Electrochemical water splitting is a key enabling process for the storage of electricity from renewable, but intermittent, sources in the form of chemical fuels.1−3 One of the shortcomings to make these processes viable on an industrial scale is the inefficiency of available electrode catalysts for the oxygen evolution reaction (OER).2,4−6 Despite tremendous efforts, developing OER catalysts with high activity and long-term cycle stability at low-cost still remains a great challenge. Conventionally, conductivity is usually considered as a typical reference standard in terms of selecting water electrolysis catalyst, and the better the conductivity of catalysts is, the better the performance of catalysts is.7−9 Accordingly, electrolysis catalysts research so far has focused on crystal rather than amorphous because the crystalline phase generally exhibits good conductivity and the amorphous phase possesses poor conductivity.10 As a promising electrocatalyst for OER, crystalline Ni(OH)2 has nearly attracted all of the attention, while the amorphous phase is evaluated to be not suitable for OER catalysts.8,9,11,12 In fact, amorphous metal oxides have exhibited impressive electrocatalytic performance.13−15 Here, we synthesize 3D honeycomb-like amorphous Ni(OH)2 nanosheets by a simple, facile, green, and low-cost electrochemistry technique16 and then use the synthesized amorphous nanosheets as electrolysis catalysts for OER. Interestingly, our measurements show that, as an advanced OER catalyst, the amorphous © XXXX American Chemical Society

Ni(OH)2 nanosheets possess ultrahigh activity and super-longterm cycle stability. As a comparison, the integrated OER performances of the amorphous Ni(OH)2 nanosheets are much superior to that of crystalline β-Ni(OH)2 and commercial RuO2 crystals. Importantly, these results suggest that the conventional understanding of selecting water electrolysis catalysts with conductivity as a typical reference standard seems to no longer work for developing new OER catalysts, which really opens a door to applications of amorphous nanomaterials as advanced OER electrocatalysts.

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EXPERIMENTAL SECTION

Materials Synthesis. The amorphous Ni(OH)2 nanosheets are synthesized by a simple, facile, and green electrochemistry technique (Supporting Information, S1).16 Two parallel graphite electrodes are used as the cathode and anode electrodes immersed in a quartz chamber, and the distance between two electrodes is about 5 cm. A target of 99.99% pure nickel with an area of 25 × 25 mm2 is fixed at the center of a quartz chamber and immersed in highly pure deionized water (18.2 MΩ cm−1) without any chemical additives. The synthesis is carried out under a constant potential voltage of 90 V for 75 min, and the final products are deposited on the surface of the graphite Received: June 1, 2015 Revised: July 16, 2015

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DOI: 10.1021/acs.cgd.5b00752 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Figure 1. (a−c) SEM images of the as-synthesized amorphous Ni(OH)2 nanosheets on the graphite sheet. (d) The TEM bright-field image of the amorphous Ni(OH)2 sample. (e) The corresponding SAED pattern of (d). (f) STEM image and its corresponding (g) O, (h) Ni element mappings for the amorphous Ni(OH)2 sample. (i) The distribution histogram of the pores of amorphous Ni(OH)2 from SEM data and its Gaussian fitting curve show that the mean size of the pore is about 100 nm. (j) XRD patterns of the as-synthesized amorphous Ni(OH)2 sample on the graphite sheet and the bare graphite sheet. sheet (electrode). The detailed formation of the amorphous Ni(OH)2 is provided in the Supporting Information (S1). Materials Characterization. Scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and X-ray photoelectron spectroscopy (XPS) are employed to identify the morphology, structure, and composition of the assynthesized samples. In addition, Raman spectra of the product are obtained to confirm the results of XPS. Additionally, XRD and Raman spectra are employed to determine the intermediate of the amorphous Ni(OH)2 catalytic OER. Electrocatalytic Characterization. Electrochemical measurements are performed with a CH Instrument Workstation 760E potentiostat at room temperature. All measurements are carried out in 0.1 M KOH (aq) and conducted in a conventional three-electrode cell by using a Ag/AgCl (sat. KCl) electrode as the reference electrode, a Pt sheet as the counter electrode, and the sample on the graphite electrode as the working electrode. The potentials reported in our

work are referenced to the reversible hydrogen electrode (RHE) through RHE calibration17 and corrected for uncompensated resistance (Rμ). As a consequence, in 0.1 M KOH, E(RHE) = E(Ag/AgCl) + 0.96 V − iRu.8 The cyclic voltammetry (CV) curves are obtained by sweeping the potential from 0.96 to 1.96 V vs RHE at a sweep rate of 10 mV s−1. Electrochemical impedance spectroscopy (EIS) measurements are performed in the same configuration under open-circuit potential in an alternating current frequency range of 10 kHz to 10 mHz with an excitation signal of 5 mV. Steady-state j−E plots are acquired, in triplicate, through staircase voltammetry (10 mV steps, 20 s intervals). For the stability evaluations, the potential of the electrodes cycles between a potential range of 0.96−1.96 V vs RHE at a sweep rate of 50 mV s−1. All current densities are calculated using geometric surface area. The electrochemically active surface areas (ECSA) of the samples are estimated by double-layer capacitance measurements.18,19 To measure electrochemical capacitance, a series of CVs are performed B



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across ±50 mV of the open-circuit potential (OCP) at 50, 100, 200, 400, and 800 mV s−1 scan rates, respectively. The working electrode is held at each potential vertex for 10 s before beginning the next sweep. The anodic current densities at the OCP are plotted against the scan rates while the slope of this plot is recorded as the double-layer capacitance of the sample.

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RESULTS AND DISCUSSIONS Structure and Morphology of Amorphous Ni(OH)2 Nanosheets. The morphology and microstructure of the assynthesized samples are investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Clearly, in Figure 1a−c, we can see a homogeneous surface constructed by many 3D honeycomb-like nanosheets, the large proportion of pores is in the range of 60−140 nm (Figure 1i), and the average thickness of sheets is about 10 nm. The representative TEM image (Figure 1d) along with the selected-area electron diffraction (SAED) pattern (Figure 1e), in which a broad and diffused halo ring can be observed, reveals its amorphous structure. Furthermore, we carry out scanning transmission electron microscopy (STEM) and elemental mapping analysis. As shown in Figure 1f−h, nickel and oxide elements are fully dispersed into the 3D honeycomb-like nanostructure while a small amount of oxygen in air adsorbed on the carbon film plated copper network, which indicates that nickel and oxide are distributed uniformly in the sample. The corresponding X-ray diffraction (XRD) measurement of the sample (Figure 1j) shows that there are no additional peaks compared with the bare graphite sheet, corresponding to the amorphous nature of the sample. Therefore, the SAED and XRD data demonstrate that the synthesized 3D honeycomblike nanosheets are amorphous. Additionally, the XRD pattern of crystalline β-Ni(OH)2 in our case is shown in the Supporting Information (Figure S2). The X-ray photoelectron spectroscopy (XPS) spectrum of the synthesized amorphous sample is used to clarify the binding energy, which can estimate different chemical states of bonded elements, and the results are shown in Figure 2. The Ni 2p XPS spectrum (Figure 2a) shows two major peaks with binding energies at 874.5 and 856.9 eV, which correspond to Ni 2p1/2 and Ni 2p3/2, respectively, with a spin-energy separation of 17.6 eV. These results thus are the characteristic of a Ni(OH)2 phase and in good agreement with previously reported data.11,20 There are some extra lines marked as satellite peaks near the expected Ni 2p1/2 and Ni 2p3/2 signals in the Ni 2p region, as shown in Figure 2a. In addition, Figure 2b shows that the O 1s spectrum with a strong peak at 531.6 eV represents the bound hydroxide groups (OH−).11,21 Thus, the XPS analysis indicates that the as-synthesized sample is Ni(OH)2. The Raman spectrum of the sample is shown in Figure 2c. It can be clearly seen that there are two broad bands at 335 and 465 cm−1, which are consistent with the Eg(T) mode of the Ni− OH lattice vibration and the A1g(T) mode due to υNi−OH, respectively.22,23 The third feature around 522 cm−1 is assigned to the presence of the structural defects20,23 and/or to an Eg(R) mode.24 The bands are blue-shifted compared with those of the crystalline phase, possibly due to defects and disorders within the amorphous phase.16 Accordingly, these measurements of morphology and microstructure of the synthesized products show that we synthesize 3D honeycomb-like amorphous Ni(OH)2 nanosheets by the developed electrochemical technique.

Figure 2. (a) Ni 2p spectrum of XPS. (b) O 1s spectrum of XPS. (c) Raman spectrum of the amorphous Ni(OH)2 sample.

Electrocatalytic Characterization of Amorphous Ni(OH)2 Nanosheets. The electrochemical performances of the as-synthesized amorphous Ni(OH)2 nanosheets, crystalline βNi(OH)2, and commercial RuO2 crystals are shown in Figure 3. First, the Ohmic potential drop (iR) losses from the solution resistance are corrected (Supporting Information, Figure S3). Figure 3a displays the CV curves of the amorphous Ni(OH)2 gained after different numbers of cycles. The quasi symmetric of the anodic and cathodic peaks in the first cycle suggests the fine reversibility of the amorphous Ni(OH)2. The onset potential of the catalytic wave is also observed at about 1.56 V. The oxidation/reduction waves can increase in amplitude with further cycling and reach to a stable value in 500 cycles, which shows the activation of the amorphous Ni(OH)2 as well as the increase of electroactive species on the electrode surface. This increase can bring in a negatively shifted OER catalytic wave which has a smaller onset potential of 1.54 V. The electrochemical features of the amorphous Ni(OH)2 has become stable after 500 cycles, while no deactivation is C



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Figure 3. (a−c) CV curves recorded at the 1st, 100th, 500th, and 1000th cycle (5000th cycle only for the amorphous Ni(OH)2 sample) for amorphous Ni(OH)2, β-Ni(OH)2, and RuO2 at a sweep rate of 50 mV s−1, respectively. (d) Comparison of CV curves recorded at the 500th cycle for amorphous Ni(OH)2, β-Ni(OH)2, RuO2, and the bare graphite at a sweep rate of 10 mV s−1 (CV curves are iR-compensated). (e) Tafel plots of amorphous Ni(OH)2, β-Ni(OH)2, and RuO2 after the 500th cycle (10 mV steps, 20 s intervals).

chemical degradation during the OER process.26 These results demonstrate that the amorphous Ni(OH)2 possessed considerably better stability to crystalline β-Ni(OH)2 and commercial RuO2.27−29 Take the occurrence of severe degradation at a larger number of CV cycles for all OER catalysts except the amorphous Ni(OH)2 under consideration, the OER data we choose for comparison are acquired after 500 CV cycles. These results show that the amorphous Ni(OH)2 exhibits greater current and earlier onset catalytic current than that of crystalline βNi(OH)2 and commercial RuO2 crystals, as illustrated in

detected in the following CV cycles even up to 5000 times. Another two similar measurements of crystalline β-Ni(OH)2 and commercial RuO2 catalysts are performed as comparisons. As for crystalline β-Ni(OH)2 (Figure 3b), the OER catalytic current increases initially to a maximum value after 100 cycles and then drops quickly to be indicative of poor catalyst stability. For RuO2 crystals, the catalyst displays a preeminent OER performance with an onset potential of 1.53 V in the first cycle (Figure 3c) as expected.25 However, after 500 cycles, the OER catalytic current decreased slightly, but continuously, according with the previous reports of RuO2 and IrO2 suffering from large D



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Table 1. Comparison of Catalytic Parameters of Amorphous Ni(OH)2 and β-Ni(OH)2 catalyst

loading (mg cm−2)

η at j = 10 mA cm−2 (V)

Tafel slope (mV dec−1)

mass activity at η = 0.35 V (A g−1)

specific activity at η = 0.35 V (mA cm−2)

RF

amorphous Ni(OH)2 β-Ni(OH)2

0.147 0.200

0.344 0.553

46 110

76.9 11.6

0.07 0.04

16.2 5.4

Figure 3d. The current density of 10 mA cm−2, which is a metric relevant to solar fuel synthesis,30 can be achieved at a small overpotential (η) of 0.344 V for the amorphous Ni(OH)2 nanosheets, which is close to the performance of the best reported crystalline Ni(OH)2 (0.331 V),8 and smaller than that of β-Ni(OH)2 (0.553 V) and RuO2 (0.497 V) (Table 1). Of note, the bare graphite electrode fails to reach such a current density, which indicates that the bare graphite electrode does not affect the OER activity. At η = 0.35 V, the mass activity and specific activity8 (based on ECSA) for the amorphous Ni(OH)2 catalyst are found to be 76.9 A g−1 and 0.07 mA cm−2, respectively. For β-Ni(OH)2, the mass activity is 11.6 A g−1 and the specific activity is 0.04 mA cm−2 (Table 1). The Tafel plots (log j−η) of the catalysts above are obtained to get the related OER kinetics information (Figure 2e). In detail, the Tafel slope of the amorphous Ni(OH)2 is a mere 46 mV dec−1, which is much smaller than that of β-Ni(OH)2 (110 mV dec−1) and RuO2 (86 mV dec−1) (Table 1). Note here that the Tafel slope of the amorphous Ni(OH)2 catalyst is comparable to that of the best reported Ni(OH)2 (42 mV dec−1).8 Therefore, these results demonstrate that the amorphous Ni(OH)2 possesses more efficient kinetics of water oxidation than that of βNi(OH)2 and RuO2 in our case. On the basis of the electrochemical characterizations above, we can conclude that, as an electrolysis catalyst for OER, 3D honeycomb-like amorphous Ni(OH)2 nanosheets exhibit ultrahigh activity and super-long-term cycle stability and the integrated OER performances of the amorphous Ni(OH)2 nanosheets are much superior to those of crystalline βNi(OH)2 and commercial RuO2 crystals. Characterization of Amorphous Ni(OH)2 after OER. The XRD measurement of the amorphous Ni(OH)2 after OER (Figure 4a) shows that there are no additional peaks compared with the bare graphite sheet, corresponding to the amorphous nature of the intermediate. The Raman spectrum of the sample is shown in Figure 4b. The two broad bands at 471 and 528 cm−1 are consistent with the Eg Ni(3+) = O or Ni(3+)−O bending and A1g Ni(3+)−O stretching, respectively.31 The third feature around 500 cm−1 is assigned to the presence of the structural defects20,23 and/or to an Eg(R) mode.24 The bands are red-shifted compared with those of the crystalline phase, possibly due to the low-energy excitation wavelength and the defects and disorders within the amorphous phase.16 Thus, these results prove the generation of the amorphous NiOOH in the process of OER. Mechanism of Amorphous OER Catalysts. Understanding the mechanism of OER can help in identifying the elementary processes contributing to the efficiency of OER. The following pathway is generally accepted for the OER process at Ni(OH)2 in alkaline media11,32 Ni(OH)2 + OH− ↔ NiOOH + H 2O + e−

(1)

NiOOH + OH− ↔ NiO(OH)2 + e−

(2)

NiO(OH)2 + 2OH− ↔ NiOO2 + 2H 2O + 2e−

(3)

Figure 4. (a) XRD patterns of the amorphous Ni(OH)2 after OER and the bare graphite sheet. (b) Raman spectrum of the amorphous Ni(OH)2 after OER.

NiOO2 + OH− ↔ NiOOH + O2 + e−

(4)

Our case is compatible with this mechanism above. It is noteworthy that β-Ni(OH)2 is often oxidized into β-NiOOH while α-Ni(OH)2 is converted to γ-NiOOH in the step (1).8,33 As the right type of oxide for the catalysis of OER, β- or γNiOOH plays an important role in promoting the efficiency of water electrolysis.8,34 Thus, we can conclude that more NiOOH transformed in the reaction, namely, more Ni(OH)2 transforming into NiOOH, conducive to the smooth start of the subsequent reaction, which results in a good catalysis effect. Now, we turn to the position to discuss the mechanism of amorphous OER catalysts. Conventionally, the better the conductivity of the catalysts is, the better the performance of the catalysts is.7−9 It is known that the long-range disorder of amorphous would lead to poor conductivity. Nevertheless, the amorphous Ni(OH)2 has excellent electrocatalytic performance even better than crystalline β-Ni(OH)2 and commercial RuO2 crystals, which indicates that good conductivity as a selection criteria of electrocatalysts is not suitable for amorphous. Therefore, we establish that the short-range order of amorphous creates a lot of active positions for OER, which can greatly promote the electrochemical performance of amorphous catalysts, which explains why amorphous has comparable electrocatalytic properties with crystal. This mechanism schematic diagram is shown in Figure 5. In the E



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In the case of the OER process of the amorphous Ni(OH)2, attributed to the short-range order of amorphous, there are many localized order dots on the surface; i.e., many crystalline Ni(OH)2 dots are embedded into the amorphous Ni(OH)2 medium, as shown in Figure 5b. During the OER process, these crystalline dots on the surface of the amorphous Ni(OH)2 are converted into the localized amorphous dots of β- or γ-NiOOH (Figure 5b). Therefore, these crystalline dots induced by the short-range order of amorphous are the active positions for OER. However, the number of the active sites on the surface of crystals is limited,37 while amorphous has the advantage of the short−long order, which creates abundant active sites on the surface. Additionally, in our case, both large specific surface area and lots of defects all can result in rich active sites for the electrochemical process.16,38 Therefore, amorphous is capable of catalyzing water splitting as good as crystals. Electrochemical impedance spectroscopy of amorphous electrocatalyst supports our proposed mechanism above, as shown in Figure 5c, which displays the Nyquist plots of amorphous Ni(OH)2 and crystalline β-Ni(OH)2. In the corresponding equivalent circuit of the impedance analysis (the inset of Figure 5c), the CPE element models the nonideal double-layer capacitance and Rs represents the electrolyte resistance, while a Warburg element (Zw) takes into account the proton diffusion in the electrolyte. Most notably, the electron transfer resistance (Rt) controls the kinetics of the interfacial charge transfer reaction.39 In other words, in the impedance analysis, Rt represents all chemical reactions on the surface in the OER process, and the smaller Rt is, the more efficient the OER reactions are. A comparison on the equivalent circuit parameters of amorphous Ni(OH)2 and crystalline βNi(OH)2 is listed in Table 2. Clearly, we can see that the Rt of amorphous Ni(OH)2 is 10.92 Ω, which is much smaller than that of crystalline β-Ni(OH)2 (202.6 Ω). Therefore, this result shows that more efficient charge transport takes place on the surface of amorphous rather than a crystal in our case, which is consistent with the electrocatalysis measurements above. The electrochemically active surface area for each system is estimated from the electrochemical double-layer capacitance of the catalytic surface.18,19 CVs of ±50 mV across the OCP are scanned at 50, 100, 200, 400, and 800 mV s−1, respectively, as shown in Figure 6a (the amorphous Ni(OH)2) and Figure 6b (β-Ni(OH)2), and the anodic current densities at OCP for each scan is plotted against the scan rates, as shown in Figure 6c. The slopes from such a plot provide the double-layer capacitances of the amorphous Ni(OH)2 (0.486 mF cm−2) and β-Ni(OH)2 (0.162 mF cm−2), respectively. The ECSA is then calculated from the double-layer capacitance according to37

Figure 5. (a, b) The mechanism schematic diagrams of crystalline βNi(OH)2 and amorphous Ni(OH)2 in the OER process. (a) The plane on the left represents the ordered surface of crystalline β-Ni(OH)2; the light green dots on the right plane represent the active sites of βNi(OH)2 transformed into the localized amorphous dots of βNiOOH, which can promote OER. (b) The left plane reflects the crystalline dots induced by the short-range order of amorphous Ni(OH)2, and these ordered dots might be crystalline Ni(OH)2 embedded in the amorphous Ni(OH)2 medium. The green points on the right plane represent the localized amorphous sites of β- or γNiOOH transformed from crystalline Ni(OH)2, which are the active sites of amorphous Ni(OH)2 for OER. Note that β- or γ-NiOOH is a preeminent catalyst for mediating OER. (c) Nyquist plots of amorphous Ni(OH)2 and crystalline β-Ni(OH)2. The inset in the upper right corner shows the corresponding Nyquist plot of amorphous Ni(OH)2 at the high-frequency range, and the inset in the middle shows the accordingly equivalent circuit.

case of the OER process of crystalline β-Ni(OH)2, some specific active positions on the surface of Ni(OH)2 are oxidized to generate the localized disordered β-NiOOH (Figure 4a),23,35,36 which drive the catalytic reaction. Meanwhile, the active positions of the Ni(OH)2 surface are transformed into the localized amorphous dots of β-NiOOH, as shown in Figure 5a. Therefore, these converted dots are the so-called “active sites” in the OER process. Generally, the efficiency of water catalysis depends on the number of the surface active sites; i.e., the more active positions on catalysts there are, the better the performance of catalysts is.

ECSA = Cdl /Cs

(5)

where Cs is the capacitance of an atomically smooth planar surface of the material per unit area under identical electrolyte conditions. An average value of Cs = 0.030 mF cm−240 is used in this work. The roughness factor (RF) is then calculated by dividing the estimated ECSA by the geometric area of the

Table 2. Equivalent Circuit Parameters Obtained from Fitting of EIS Experimental Data electrode

Rs (Ω)

Rt (Ω)

CPE-T(F)

CPE-P(F)

W-R (Ω)

W-T (Ω)

W-P (Ω)

amorphous Ni(OH)2 β-Ni(OH)2

10.40 13.73

10.92 202.6

11.17 × 10−2 9.00 × 10−5

0.38817 0.91700

0.73321 0.02542

8.89 × 10−3 9.59 × 10−15

0.45256 0.11436

F



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60 min samples have poor catalytic performance may be due to the large numbers of gaps between the materials. On the other hand, the 90 and 105 min samples not do well probably because of agglomeration of their surfaces. The 75 min sample has excellent electrocatalytic performance perhaps on account of its good surface with large specific surface area that provides substantial active sites for electrocatalytic. These results indicate that the surface of the material rather than the conductivity itself influences the electrocatalytic efficiency, which supports the mechanism we presented above. It is noticed that the commercial KOH is not so pure and contains Fe.41 Thus, we carefully carried out the experimental study to check the influence of Fe of the commercial KOH in our case on the OER performance of the amorphous Ni(OH)2. We used the purification method41 to get the purified KOH, and then we redid the electrochemical test in 0.1 M purified KOH. The results are presented in Figure 7. Clearly, we can see that the electrochemical performances of the amorphous Ni(OH)2 are good and stable in the purified KOH (Figure 7a), unlike the increase of the OER performance in the Fe impurities KOH with the increase in circulating (Figure 7b), which is consistent with the version that the absorption of Fe impurities is responsible for the dramatic increase in activity of Ni(OH)2.41 Figure 7c shows the comparison of CV curves recorded after 1000 cycles for the amorphous Ni(OH)2 in the Fe free KOH and the Fe impurities KOH at a sweep rate of 10 mV s−1, respectively. These results demonstrate nearly the same onset potential and potential at the current density of 10 mA cm−1. Therefore, our amorphous Ni(OH)2 without Fe impurities can exhibit the electrocatalytic efficiency as good as that of the Fe impurities Ni(OH)2 (before 1000 cycles even better than the Fe impurities Ni(OH)2). Accordingly, in our case, Fe impurities seem to have no effect on the OER performance of the amorphous Ni(OH)2. Figure 7d shows the XPS Fe 2p spectra for the amorphous Ni(OH)2 after 500 CV cycles in the purified KOH and 500 CV cycles in the Fe impurities KOH, respectively. We can see that there is no detectable Fe 2p XPS signal in the amorphous Ni(OH)2 after 500 CV cycles in the purified KOH. However, the Fe 2p spectrum of the amorphous Ni(OH)2 after 500 CV cycles in the Fe impurities KOH affirms the incorporation of Fe in the process of electrocatalysis.41 Note that, Trotochaud et al. reported that Fe impurities can promote the OER performance of the Ni(OH)2 crystals by enhancing electron transfer in the electrochemical process.41 However, in our case, the water electrolysis catalyst is the amorphous Ni(OH)2, not crystalline. There may be different mechanisms of electron transfer working in the electrochemical processes of an amorphous structure and a crystalline structure, respectively. Although Fe impurities can enhance the OER performance of the Ni(OH)2 crystal, the amorphous Ni(OH)2 is not basically affected by Fe impurities. Importantly, our experimental measurements have confirmed these deductions above.

Figure 6. Double-layer capacitance measurements for determining electrochemically active surface area for the amorphous Ni(OH)2 and β-Ni(OH)2. (a, b) CV curves performed across ±50 mV of the opencircuit potential (OCP) at 50, 100, 200, 400, and 800 mV s−1 scan rates for the amorphous Ni(OH)2 and β-Ni(OH)2, respectively. (c) The slope of current density at OCP vs scan rate.

electrode.19 Accordingly, the RF of the amorphous Ni(OH)2 is 16.2, which is 3 times larger than that of β-Ni(OH)2 (5.4) (Table 1). Therefore, this result is a good illustration of the prediction of the abundant active sites in the amorphous Ni(OH)2 and agrees with the EIS analysis. To further demonstrate our proposed mechanism, we study the effects of the sample’s thickness on the electrocatalytic efficiency. It is accepted that, with the extension of the deposition time, our sample will be thicker. CV curves at the 500th cycle for the amorphous Ni(OH)2 samples synthesized under a constant potential voltage of 90 V for 45, 60, 75, 90, and 105 min, respectively, are shown in the Supporting Information (Figure S4). At the current density of 10 mA cm−2, the overpotentials of the amorphous Ni(OH)2 produced with the electrodeposition times of 45, 60, 75, 90, and 105 min are 0.53, 0.49, 0.34, 0.37, and 0.47 V, respectively. That the 45 and

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CONCLUSION In summary, we have demonstrated that 3D honeycomb-like amorphous Ni(OH)2 nanosheets synthesized by a simple, facile, and green electrochemistry are advanced water electrolysis catalysts with ultrahigh activity and super-longterm cycle stability, and we have also established that, although the long disorder of amorphous results in poor conductivity, the short-range order (nanophase) of amorphous creates G



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Figure 7. (a, b) CV curves recorded at the first, 100th, 200th, 300th, 400th, 500th, and 1000th cycle for the amorphous Ni(OH)2 in the Fe free KOH and the Fe impurities KOH at a sweep rate of 50 mV s−1, respectively. (c) Comparison of CV curves recorded after 1000 cycles for the amorphous Ni(OH)2 in the Fe free KOH and the Fe impurities KOH at a sweep rate of 10 mV s−1 (CV curves are iR-compensated). (d) XPS Fe 2p spectra for the amorphous Ni(OH)2 after 500 CV cycles in the purified KOH and 500 CV cycles in the Fe impurities KOH, respectively.

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ACKNOWLEDGMENTS The National Natural Science Foundation of China (91233203) and the State Key Laboratory of Optoelectronic Materials and Technologies supported this work.

abundant active sites on the surface, which greatly promote electrocatalysis performance. Furthermore, the irregular surfaces and high structural disorder of the amorphous materials also lead to excellent electrocatalytic activity. These findings indicated that the conventional understanding of selecting water electrolysis catalysts with conductivity as a typical reference standard seems to no longer work for developing new OER catalysts, which provided opportunities for amorphous nanomaterials as advanced electrocatalysts for water oxidation.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed description of the synthesis of amorphous Ni(OH)2, a schematic illustration of the experimental setup, formation mechanism of amorphous Ni(OH)2, XRD pattern of crystalline β-Ni(OH)2, CVs recorded at 500th cycle with and without iR correction, and CVs recorded at 500th cycle for amorphous Ni(OH)2 synthesized under a constant potential voltage of 90 V (PDF). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.cgd.5b00752.

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