Structure Effects of 2D Materials on Î±-Nickel Hydroxide for Oxygen

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Structure Effects of 2D Materials on #-Nickel Hydroxide for Oxygen Evolution Reaction Chenglong Luan, Guangli Liu, Yujie Liu, Lei Yu, Yao Wang, Yun Xiao, Hongyan Qiao, Xiaoping Dai, and Xin Zhang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b01296 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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Structure Effects of 2D Materials on α-Nickel Hydroxide for Oxygen Evolution Reaction Chenglong Luan1, Guangli Liu2, Yujie Liu1, Lei Yu1, Yao Wang1, Yun Xiao1, Hongyan Qiao1, Xiaoping Dai1, Xin Zhang*1

1

State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering,

China University of Petroleum, Beijing, 102249, China E-mail: [email protected] 2

Lanzhou Petrochemical Research Center, Petro China, Lanzhou, 730060, China

1

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Abstract: To engineer low-cost, high-efficiency and stable oxygen evolution reaction (OER) catalysts, structure effects should be primarily understood. Focusing on this, we systematically investigated the relationship between structures of materials and their OER performances by taking four 2D α-Ni(OH)2 as model materials, including layer stacked bud-like Ni(OH)2-NB, flower-like Ni(OH)2-NF and petal-like Ni(OH)2-NP as well as the ultra large sheet-like Ni(OH)2-NS. For the first three (layer-stacking) catalysts, with the decrease of stacked layers, their accessible surface areas, abilities to adsorb OH-, diffusion properties and the intrinsic activities of active sites are increasing, which accounts for their steadily enhanced activity. As expected, Ni(OH)2-NP shows the lowest overpotential (260 mV at 10 mA cm-2) and Tafel slope (78.6 mV dec-1) with a robust stability over 10 hours among the samples, which also outperforms the benchmark IrO2 (360 mV and 115.8 mV dec-1) catalyst. Interestingly, Ni(OH)2-NS relative to Ni(OH)2-NP exhibits even faster substance diffusion due to the sheet-like structure, but shows inferior OER activity, which is mainly because the Ni(OH)2-NP with a smaller size possesses more active boundary sites (higher reactivity of active sites) than Ni(OH)2-NS, considering the adsorption properties and accessible surface areas of the two samples are quite similar. By comparing the different structures and their OER behaviors of four α-Ni(OH)2 samples, our work may shed some light on the structure effect of 2D materials and accelerate the development of efficient OER catalysts.

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Key Words: 2D materials, α-nickel hydroxide, structure effects, electro-catalysis and oxygen evolution reaction

With a rising global population, increasing energy demands and impending climate change, major concerns have been raised on developing clean and sustainable energy conversion/storage technologies with high efficiency and low cost.1-4 Among the technologies, electricity-driven water splitting (2H2O→O2+2H2) to produce hydrogen fuel and oxygen is believed to be one of the most promising and appealing strategies to achieve the conversion of abundant water resource to hydrogen of high purity.5-12 However, the efficiency of electro-chemical water splitting has been, to a large extent, limited by the sluggish oxygen evolution reaction (OER) at anode, which involves the complex four-electron (4e-) transfer process.13, 14 Crucial to boosting this process is the development of improved catalysts with high activity and robust stability. Currently, the state-of-the-art catalysts for OER are mostly based on ruthenium (Ru) and iridium (Ir) oxides, though the scarcity and the high cost severely hinder their widespread applications.15-17 Fortunately, many transition-metal based especially Ni-containing materials show great potential as alternate catalysts for OER due to their good water oxidation performance and the earth-abundant nature.18-21 As shown in many literatures, the general strategy to fabricate Ni based OER materials is doping some heteroatoms (transition metal V, Fe, Co, Mn, etc.; nonmetal N, S, P, Se, etc.) and/or hybridizing some conductive substrates (carbon nanotube (CNT), Ni foam, graphene, carbon fiber paper, etc.) to achieve the synergistic effect (i.e. harness the 3


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internal electronic effect of components and the superior electrical conductivity of substrates to get better performance).22-44 These booming materials have been extensively investigated and some of them, such as NiCo layer double hydroxides (LDH),27 NiFe LDH/CNT30 and NiFeP microflowers/Ni foam,34 showed very good OER performances (the overpotentials at 10 mA cm-2 are 240-400 mV). It should be pointed out that these outstanding catalysts possess not only the appropriate compositions (mono- or multi-transition metal (hydr)oxides, sulfides and so on), but also the 2D and/or porous structures. Nowadays, computational and experimental studies working in concert can elucidate the composition effect in most cases from active sites and provide rational guidance towards the design of catalyst components.37, 40 While compared with the composition effect, fewer works have been reported about the structure effect of 2D OER materials due to the incapability of theoretical calculations in this filed (i.e. theoretical calculation can elucidate reaction from atomic level while hard to simulate the whole reaction process including diffusion, adsorption and so on) and the tremendous experimental difficulties to evolve different structures without changing their compositions. Therefore, the recognition of structure effect is superficial and often relies on fortuitous findings. To date, we just know the proper structures of OER catalysts can provide larger surface areas and more exposed active sites as well as faster mass diffusion in reaction,27 while do not understand how exactly the structure effect affects their OER performances, which needs to be imminently solved before engineering improved OER catalysts. 4


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Results and Discussion:

Scheme 1. The illustration for synthesizing α-Ni(OH)2 samples via the lamellar reverse micelles method. Herein, taking 2D α-Ni(OH)2 as model materials, we systematically investigated the relationship between structures and their OER performances. As well known, the hexagonal structured α-Ni(OH)2, consisting of positively charged Ni(OH)2-x sheets, intercalated anions (e.g., carbonate, nitrate, etc.) and water molecules, is widely used to catalyze OER and other electro-chemistry reactions. In our present work, 2D α-Ni(OH)2 samples were firstly synthesized via simple lamellar reverse micelles (LRM) method (see details in experimental section).43 Specifically, the precursor containing Ni2+ crystallizes in the confined LRM areas with proper amount of surfactant, then these crystallites stack/grow to layer-stacked/ultra-large α-Ni(OH)2 products, as shown in Scheme 1. Layer stacked samples are synthesized by delicately varying the water content, while the ultra-large one is fabricated with glycine instead of hexamethylenetetramine (HMTA) in the synthesis. The morphologies of these samples were subsequently captured by transmission electron microscope (TEM) and 5


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scanning electron microscope (SEM). According to their distinguishable bud-like, flower-like, petal-like and sheet-like nanostructures, these obtained α-Ni(OH)2 samples were denoted as Ni(OH)2-NB, Ni(OH)2-NF, Ni(OH)2-NP and Ni(OH)2-NS, respectively. Figure 1a, 1d and 1g show the representative TEM images of Ni(OH)2-NB, Ni(OH)2-NF and Ni(OH)2-NP, respectively. It can be observed that the three samples all exhibit the monodispersed feature and the diameter of a single α-Ni(OH)2 crystallite is about 150 nm (see details in Figure S1). Additionally, high resolution transmission electron microscope (HRTEM) was introduced to further reveal the structures of these Ni(OH)2 samples. The obtained HRTEM images of Ni(OH)2-NB, Ni(OH)2-NF and Ni(OH)2-NP samples were displayed in Figure 1b, 1e and 1h, respectively, which clearly show the layer-stacking nanostructures of these samples and simultaneously suggest these layers are ultrathin in thickness from their near transparency characteristics. Meanwhile, typical SEM images of Ni(OH)2-NB (Figure 1c), Ni(OH)2-NF (Figure 1f) and Ni(OH)2-NP (Figure 1i) also manifest the layer-stacking structures of these samples. Based on their TEM and SEM results, it can be identified that the layered Ni(OH)2 crystallites heavily stack for Ni(OH)2-NB, normally stack for Ni(OH)2-NF and slightly stack for Ni(OH)2-NP samples, which agrees well with their selected area electron diffraction (SAED) patterns (Figure S2). Besides, for better comparing the different structures of these samples, the ultra large sheet-like sample (Ni(OH)2-NS) was also tested. It can be observed that the diameter of a single Ni(OH)2-NS is more than a dozen microns with the thickness of 5 nm (aspect ratio ~2500), as verified by TEM images (Figure 1j and 1k) and atomic force 6


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microscope (AFM) measurement (Figure 1l).

Figure 1. Representative (a) TEM, (b) HRTEM and (c) SEM images of Ni(OH)2-NB; typical (d) TEM, (e) HRTEM and (f) SEM images of Ni(OH)2-NF; representative (g) TEM, (h) HRTEM and (i) SEM images of Ni(OH)2-NP; typical (j) TEM, (k) HRTEM and (l) AFM images of Ni(OH)2-NS samples. The insets of (b), (e), (h) and (k) are the corresponding models of different structures. Although the thicknesses of Ni(OH)2-NB, Ni(OH)2-NF and Ni(OH)2-NP samples are difficult to obtain from AFM due to their layer-stacking structures, they could be calculated by the established Scherrer Equation from powder X-ray diffraction (PXRD).44 In PXRD patterns (Figure 2a), the hexagonal crystal structure of these 7


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α-Ni(OH)2 samples is firstly confirmed and no peaks of impurity can be detected. The characteristic peaks of samples at 2θ=9.9o, 17.8o, 33.9o and 60.4o can be assigned to the (003), (006), (101) and (110) planes, respectively, of pure α-Ni(OH)2 phase (JCPDS: 38-0715). The negatively shifted diffraction angles of the α-Ni(OH)2 (003) and (006) planes suggest an increase in the d spacing along the c-axis, presumably due to the existence of intercalated anions in Ni(OH)2 lattice. Similar negative shifts were also reported for anions intercalated NiCo LDH nanoplate and α-Co(OH)2 nanocones.27, 45 The asymmetric feature of the reflections at 2θ=33.9° indicate the formation of turbostratic α-Ni(OH)2 phase, which can also be observed in other α-Ni(OH)2 materials.46-48 Then, from the full width at half maximum (FWHM) of these (003) reflections, the average crystallite sizes of samples along the c-axis were calculated to be 2-3 nm, which means that the number of sheets per crystallite of the hydroxides is 2-3 since the interplanar space is 8-9 Å (Table S1). Although the thickness of Ni(OH)2-NS calculated from PXRD is thinner than that measured from AFM (5 nm, Figure 1l) due to the incomplete crystallization, the similar thickness of the as-synthesized four α-Ni(OH)2 is confirmed, which is important to guarantee the single variate to investigate the structure effect on their OER performances in following studies. Besides, the Tyndall effect can be clearly observed when irradiating the transparent sample suspensions (the concentration is 0.1 mg ml-1) with a laser beam (Figure 2b), demonstrating the colloidal nature of the suspensions and the ultrathin sheet structures of these α-Ni(OH)2 samples.27, 49, 50

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Figure 2. (a) Powder X-ray diffraction (PXRD) patterns of the four samples; (b) optical image of sample colloidal solutions (the concentration is 0.1 mg mL-1 and the Tyndall effect are visible when irradiating the solution with a laser beam); (c) N2 adsorption/desorption isotherms and (d) pore-size distributions for the as-synthesized Ni(OH)2 samples. To assess the geometrical surface areas of these ultrathin Ni(OH)2 samples, the N2 adsorption-desorption isotherms were conducted. As shown in Figure 2c, it is found that all the as-synthesized Ni(OH)2 samples exhibit type IV isotherm with H3-type hysteresis loop, characteristics of narrow mesoporous size distributions. The specific surface areas measured are of the same magnitude with 150.3, 165.5, 179.5 and 178.6 m2 g-1 for Ni(OH)2-NB, Ni(OH)2-NF, Ni(OH)2-NP and Ni(OH)2-NS samples, respectively. And the typical slit pores resulting from their layer-stacking 9


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nature show the similar pore-size distribution and the most probable pore diameter of 3.80 nm (Figure 2d). Collectively, these α-Ni(OH)2 samples with distinguishable layer-stacking structures possess the similar specific surface areas and the pore properties, providing us a perfect single variate platform to investigate the relationship between structures (stacked layers) and their OER performances.

Figure 3. (a) CV, (b) LSV curves and (c) Tafel slopes of Ni(OH)2-NB, Ni(OH)2-NF, Ni(OH)2-NP, Ni(OH)2-NS and the benchmark IrO2 catalyst; (d) the specific activity and mass activity for the as-prepared Ni(OH)2 samples. All measurements were performed in O2-purged 1 M KOH (pH ∼14) at 2500 rpm with rotating disk electrode (RDE), the sweep rate was 10 mV s−1. The catalyst loading was ∼0.2 mg cm−2. The current density (jgeo), specific activity (jBET) and mass activity (jmass) were normalized to their electrode geometric area, BET surface area and the mass loading, respectively. To explore the OER catalytic capabilities of these samples, cyclic voltammetry 10


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(CV) curves were firstly recorded with a three-electrode system after 40 cycles to stabilize their performances. From the CV curves in Figure 3a, the quasi reversible oxidation and reduction peaks due to the oxidation of α-Ni(OH)2 to γ-NiOOH (at ~1.35 V) and the successive reduction back to α-Ni(OH)2 (at ~1.25 V) can be observed.48,

51, 52

The second oxidation waves with onset potentials of ∼1.45 V

attributed to water oxidation are also observed. And with the increase of applied voltage, the current densities resulting from OER are varying more obviously especially when the voltage reaches 1.6 V, the current densities are 27.4, 47.9, 147.7, 66.7 and 11.6 mA cm-2 for Ni(OH)2-NB, Ni(OH)2-NF, Ni(OH)2-NP, Ni(OH)2-NS and the commercial IrO2, respectively, which preliminary shows the different activities of these samples. Then the linear sweep voltammetry (LSV) measurements also exhibit the similar trend that the overpotentials required to drive the current density of 10 mA cm-2 are 320, 310, 260, 300 and 360 mV for Ni(OH)2-NB, Ni(OH)2-NF, Ni(OH)2-NP, Ni(OH)2-NS and IrO2 (Figure 3b), respectively. The lower overpotential (260 mV) of Ni(OH)2-NP indicates its higher OER activity that the lower voltage (1.49 V vs. RHE) is needed to deliver the benchmark current density (10 mA cm-2). Further assessment of these samples was extracted from the Tafel slopes, which were calculated to be 92.4, 89.3, 78.6, 77.4 and 115.8 mV dec-1 for Ni(OH)2-NB, Ni(OH)2-NF, Ni(OH)2-NP, Ni(OH)2-NS and IrO2 samples (Figure 3c), respectively. According to Tatsuya’s report,53 a Tafel slope of 120 mV dec-1 can be observed when the surface species formed in the step just before the rate-determining step is predominant, and when the surface adsorbed species produced in the early stage of the 11


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OER remains predominant, the Tafel slope decreases. In our present work, the tapered Tafel slopes (from 92.4 mV dec-1 of Ni(OH)2-NB, 89.3 mV dec-1 of Ni(OH)2-NF, 78.6 mV dec-1 of Ni(OH)2-NP to 77.4 mV dec-1 of Ni(OH)2-NS) indicate the adsorption of reactant (OH-) becomes easier (more favourable kinetics) with the decrease of stacked layers.54, 55 Interestingly, the equivalent Tafel slopes of Ni(OH)2-NP and Ni(OH)2-NS suggests that the adsorption (kinetics) properties of petal-like and sheet-like structures are quite similar. As mentioned, these nuances in Tafel slopes clearly demonstrate the subtle relationship between structures and their kinetics properties. The lower Tafel slopes of Ni(OH)2-NP and Ni(OH)2-NS suggest that the petal-like and sheet-like nanostructures possess more favourable kinetics properties to boost OER process for higher activities. Table 1. Comparison of OER activity data calculated from different samples. η at Samples

Mass activity -2

TOF at

Specific activity Tafel slope

j=10 mA cm

at η=350 mV

at η=350 mV

(mV)

(A g-1)

(A m-2)

Ni(OH)2-NB

320

90.1

0.60

92.4

0.027

Ni(OH)2-NF

310

154.4

0.94

89.3

0.047

Ni(OH)2-NP

260

530.3

2.95

78.6

0.16

Ni(OH)2-NS

300

207.6

1.16

77.4

0.063

(mV dec-1)

η=350 mV (s-1)

For further comparing their activities of the as-prepared α-Ni(OH)2 samples, the specific activities (jBET) and the mass activities (jmass) of the samples were also calculated, which were normalized to their Brunner−Emmet−Teller (BET)-measured surface areas and mass loadings (Figure 3d), respectively. From these curves, the trend of OER activity: Ni(OH)2-NP >Ni(OH)2-NS >Ni(OH)2-NF >Ni(OH)2-NB can 12


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be easily obtained, which is in good agreement with their current densities (jgeo) and Tafel slopes (Figure 3b and 3c). Moreover, the turnover frequency (TOF), mass activity and specific activity of the samples at the overpotential of 350 mV were summarized to get insight into their intrinsic activities. As shown in Table 1 and Supporting Information, the calculated TOF values are 0.027, 0.047, 0.16 and 0.063 s-1 for Ni(OH)2-NB, Ni(OH)2-NF, Ni(OH)2-NP and Ni(OH)2-NS, respectively, demonstrating the structures not only affect their kinetics properties but also result in different intrinsic activities. Less layer stacked Ni(OH)2-NP shows the highest TOF among the samples (about 5.9-, 3.4- and 2.6-folds than Ni(OH)2-NB, Ni(OH)2-NF and Ni(OH)2-NS, respectively), illustrating the unique structure (petal-like structure in present work) can significantly improve the intrinsic activity.

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Figure 4. (a) Current density measured in non-faradaic potential from 1.10 V to 1.20 V at different scan rates: 20, 50, 100, 150 and 200 mV s-1 for Ni(OH)2-NP sample; (b) the capacitive current as a function of scan rates for different catalysts; (c) CV curves for Ni(OH)2-NP with different scan rates; (d) the liner relationship of peak potentials and scan rates (ip-V1/2) for different samples during the oxidation of α-Ni(OH)2 to γ-NiOOH; (e) the non-liner relationship of ip-V; (f) EIS for different α-Ni(OH)2 samples, the solution resistance (1 Ω cm2) is consistent for all the measurements. 14


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Then the electro-chemical surface areas (ECSA) of the catalysts were evaluated to further recognize the structure effect of 2D α-Ni(OH)2 samples. As we know, the ECSA can be reflected by its electrochemical double layer capacitance (Cdl) due to their proportional relationship.56, 57 Based on CV curves recorded at different scan rates from 20 to 200 mV s-1 (Figure 4a and S3), the plots of scan rates against current densities are shown in Figure 4b. According to their linear slopes, the Cdl values can be calculated (the slopes divided by 2) to be 2.36, 3.16, 3.74 and 3.30 mF cm-2 for Ni(OH)2-NB, Ni(OH)2-NF, Ni(OH)2-NP and Ni(OH)2-NS (Figure 4b), respectively. By using the equation of ECSA =

(1)

in which specific capacitance Cs = 0.040 mF cm-2 is adopted in 1M NaOH (details please see Supporting Information),17 ECSA values of these samples are then obtained to be 59.0, 79.0, 93.5 and 82.5 cm-2 for Ni(OH)2-NB, Ni(OH)2-NF, Ni(OH)2-NP and Ni(OH)2-NS, respectively. Further, the ECSA-evaluated geometric surface areas of samples can be easily calculated from ECSAs and catalyst loadings (0.04 mg) to be 147.5, 197.5, 233.8 and 206.3 m2 g-1 for Ni(OH)2-NB, Ni(OH)2-NF, Ni(OH)2-NP and Ni(OH)2-NS, respectively. It can be observed that BET and ECSA-measured surface areas have the same trend and their values are mostly similar with some deviations in reasonable limits, which is possibly due to experimental errors and the inappropriate Cs adopted in equation (1). Despite these interferons, the higher ECSA and BET-derived surface areas of Ni(OH)2-NP consistently indicate this sample possesses larger accessible surface areas (more exposed active sites) to take part in OER and 15


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hence to achieve better activity. On the basis of in situ X-ray absorption near-edge structure spectroscopy (XANES) studies, Daniel and his co-workers have pointed out that the γ-NiOOH phase is vital for high OER activity.58, 59 Therefore, estimating the proton diffusion coefficients of these nickel hydroxide samples during the oxidation from α-Ni(OH)2 to γ-NiOOH should be helpful to better understand their OER behaviors. Figure 4c and S4 depict the typical CV curves of four α-Ni(OH)2 samples with different scan rates. As can be observed, the peak potential due to the oxidation of Ni(OH)2 shifts to a more positive direction as the scan rate increases while the current density resulting from OER is unchanged. In semi-infinite diffusion controlled cyclic voltammetry of liquid electrolytes, ip (ip: peak current) vs. V1/2 (V: scan rate) gives a linear relationship regardless of scan rate for a kinetically uncomplicated redox reaction; for an adsorption process, ip vs. V is expected to be linear at different rates.51, 60, 61 In the present study, the linear relationship of ip-V1/2 (Figure 4d) and the non-linear relationship of ip-V (Figure 4e) clearly elucidate that the oxidation of nickel hydroxide is diffusion limited. Then according to the classical Sevick equation of ip = 2.69×105 n3/2 A D01/2 V1/2 C0

(2)

where ip is the peak current, A; n is the number of electrons transferred, usually to be 1; A is the apparent surface area of the electrode, 0.196 cm-2; D0 is the diffusion coefficient of the rate limiting species (i.e., proton), cm2 s-1; V is the scan tate, V s-1; C0 is the proton concentration, mol cm-3 (see Supporting Information),51 the proton diffusion coefficients (D0) from α-Ni(OH)2 to γ-NiOOH were calculated to be 2.48× 16


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10-10, 2.97×10-10, 6.72×10-10 and 1.02×10-9 cm2 s-1 for Ni(OH)2-NB, Ni(OH)2-NF, Ni(OH)2-NP and Ni(OH)2-NS samples (Figure 4d), respectively, which values are very close to some reported literatures.51, 62 The bigger proton diffusion coefficients of Ni(OH)2-NS (1.02×10-9 cm2 s-1) and Ni(OH)2-NP (6.72×10-10 cm2 s-1) indicate their faster transformation from α-Ni(OH)2 to active γ-NiOOH phase and then more enhanced OER activity is obtained. Simultaneously, it also suggests that the sheet-like and petal-like nanostructures are favourable for substance diffusion, which coincides well with the subjective assumptions from their morphologies (Figure 1). The superiority of sheet-like and petal-like structure can also be seen from their electro-chemical impedance spectroscopies (EIS, Figure 4f). Compared to the other two samples, the Ni(OH)2-NP and Ni(OH)2-NS exhibit the smaller semicircular diameters indicating these two samples possess lower charge-transfer resistance, which is mainly due to synergistic effects of the larger interfacial contact areas with the electrolyte and shorter ion diffusion paths resulting from their unique nanostructures.63, 64 Notably, the impedance of Ni(OH)2-NP is slightly smaller than that of Ni(OH)2-NS, which may be because the Ni(OH)2-NP show larger accessible surface areas (Cdl) than Ni(OH)2-NS (Figure 4b). Table 2. The OER data summarized from as-synthesized α-Ni(OH)2 samples. η at j=10 mA cm-2

Tafel slope

Samples

TOF at η=350

-1

-1

Cdl

D0 -2

(mV)

(mV dec )

mV (s )

(mF cm )

(cm s )×10-9

Ni(OH)2-NB

320

92.4

0.027

2.36

2.48

Ni(OH)2-NF

310

89.3

0.047

3.16

2.97

Ni(OH)2-NP

260

78.6

0.16

3.74

6.72

Ni(OH)2-NS

300

77.4

0.063

3.30

10.20

17


2 -1

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By far, the monodispersed α-type Ni(OH)2-NB, Ni(OH)2-NF and Ni(OH)2-NP and the ultra-thin/ultra-large Ni(OH)2-NS samples were successfully synthesized and fully investigated as model materials for OER. Based on the obtained results, the proposed structure effects on OER can be summarized to 4 points: (I) the ability of adsorbing OH-/kinetics property (Tafel slope); (II) the accessible surface area/the number of exposed active site (Cdl); (III) the rate of substance diffusion/proton diffusion coefficient (D0) and (IV) the reactivity of active site/intrinsic activity (TOF). These parameters of the samples are displayed in Table 2. For the layer-stacked Ni(OH)2-NB and Ni(OH)2-NF samples, the slightly enhanced activity of Ni(OH)2-NF is mainly due to its higher reactivity of active sites (about 1.7-folds TOF than Ni(OH)2-NB) since the Cdl, Tafel slopes and D0 values of the two samples are similar. While the significantly increased activity from Ni(OH)2-NF to Ni(OH)2-NP can be attributed to the synergistic effect of higher reactivity of active site (3.4-folds TOF than Ni(OH)2-NF), more favourable kinetics (10.7 mV dec-1 smaller than Ni(OH)2-NF) and better diffusion property (2.3-folds D0 than Ni(OH)2-NF) of Ni(OH)2-NP. As shown, for layer stacked samples, the proper petal-like nanostructure can reach higher OER activity by concurrently improving the intrinsic activity of active sites, kinetics and diffusion properties compared with bud-like and flower-like structures. It is worth mentioning, Ni(OH)2-NS relative to Ni(OH)2-NP exhibits even faster substance diffusion property (larger D0), but presents inferior catalytic activity. This is because the reactivity of boundary sites is higher than that of plan sites in OER,27 and smaller sized Ni(OH)2-NP possess more 18


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boundary sites than Ni(OH)2-NS. Thus the active sites of Ni(OH)2-NP show much higher reactivity (TOF value) than that of Ni(OH)2-NS, causing the higher catalytic activity of Ni(OH)2-NP. For the different EIS in this work, it can be thought of as a combined effect of the accessible surface areas (Cdl) and the substance diffusion (D0). As overall the fewer layers stacked petal-like structure (Ni(OH)2-NP), with favourable substance adsorption/diffusion properties, large accessible surface area (more exposed active sites) and high reactivity of active sites (intrinsic activity), is more desirable structure of 2D materials to catalyze OER effectively.

Figure 5. (a) Stability tests of Ni(OH)2-NP and IrO2 for 5,000 cycles CV; (b) chronoamperometry test at 1.49 V (10 mA cm-2) for 10 hours, insets are the TEM images of Ni(OH)2-NP before and after the test. It is widely accepted that the stability of catalysts is equally vital to their activity. To assess the durability of outstanding Ni(OH)2-NP sample, CV curves were repeatedly recorded during the 5,000 cycles test. For comparison, the stability of IrO2 was also tested. As can be observed, the comparison between the negligible decay of Ni(OH)2-NP and the sharp decrease of IrO2 after 5,000 CV cycles (Figure 5a) clearly demonstrates the robust cycle stability of Ni(OH)2-NP. Furthermore, the 19


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chronoamperometry at current density of 1.49 V (10 mA cm-2) was also performed to exam the long-term durability. It can be seen that the current density is slightly fluctuant due to the interference of O2 bubbles generated and it keeps 96 % of initial current density after 10 hours test (Figure 5b). This shows the long-term stability of Ni(OH)2-NP during the continuous oxygen evolution reaction. Then, by comparing the TEM images before and after the long-term test (Figure 5b inset), we found the petal-like structure is mostly maintained and no layer aggregation is observed, suggesting the robust structure stability of the sample.

Scheme 2. The illustration of Ni(OH)2-NP catalyze OER in alkaline conditions. The model of Ni(OH)2-NP shows the advantages of petal-like structure in OER. Collectively, Ni(OH)2-NP not only possesses sufficient exposed active sites and OER-favored substance adsorption/diffusion properties as above mentioned, at the same time, it can avoid the aggregation issue of 2D materials through its stable and mutual support layer nanostructures, as shown in Scheme 2. Benefiting from these advantages, Ni(OH)2-NP shows the highest OER activity (260 mV) with excellent stability among reported mono-metal hydroxide materials to our knowledge. It also 20


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outperforms most bimetal hydroxide OER catalysts including NiV LDH nanosheet (318 mV),66 NiCo LDH/carbon paper (367 mV)67 and NiCo LDH nanosheet/Ni foam (420 mV)70 as well as other typical 2D hydroxide OER catalysts, as summerised in Table 3. These results not only demonstrate the superiority of our Ni(OH)2-NP, more importantly, show a good example and the great potential to construct efficient OER catalysts from structure design. While compared with the most advanced Ni-Fe system OER catalysts, for example Dai et al. reported NiFe LDH nanoplate/CNT,30 although the activity of Ni(OH)2-NP is not as good as NiFe LDH nanoplate/CNT (240 mV) due to its unique synergies, the stability of Ni(OH)2-NP is much better. The activity of NiFe LDH nanoplate/CNT increases in first 10 minutes then declines in following 50 minutes, causing a 7 mV (from 1.483 to 1.490 V) increase of overpotential to maintain 10 mA cm-2 current after 60 minutes test. While our steady Ni(OH)2-NP can afford 10 mA cm-2 current density consistently at 1.49 V for even 5,000 CV cycles (Figure 5a) and continues catalysing OER over 10 hours with negligible decay (Figure 5b), which may benefit from its robust structure durability. Considering α-Ni(OH)2 is a fundamental material and thus the catalytic performance of Ni(OH)2-NP may be further enhanced when tuning its components, our following work mainly on derivatives of Ni(OH)2-NP is currently underway and will be present in near future.

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Table 3. Comparison of OER performances of the most typical 2D hydroxides catalysts in alkaline electrolytes. Materials

Electrolytes

Mass loading

η10

Tafel slope

(mg cm−2)

(mV)

(mV dec−1)

Stability

Reference

α-Ni(OH)2-NP/C

1.0 M KOH

0.20

260

79

excellent

This work

γ-CoOOH nanosheet

1.0 M KOH

0.15

300

38

excellent

8

1.0 M KOH

0.20

240

31

good

30

0.1 M KOH

0.20

330

42

excellent

46

α-Ni(OH)2 nanosheet

1.0 M KOH

0.16

350

144

not good

54

CoMn LDH nanoplate

1.0 M KOH

0.14

324

43

excellent

65

NiV LDH nanosheet

1.0 M KOH

0.14

318

50

good

66

1.0 M KOH

0.23

367

40

good

67

0.1 M KOH

0.25

430

73

excellent

68

0.1 M KOH

0.28

474

87

excellent

69

0.1 M KOH

──

420

113

excellent

70

NiFe LDH nanoplate/CNT α-Ni(OH)2 hollow spheres

NiCo LDH/carbon paper ZnCo LDH/RGO β-Ni(OH)2 nanoplate/MCNT NiCo LDH nanosheet/Ni foam

η10: The overpotential η10 was measured at the current density of 10 mA cm−2.

22


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Conclusion: In summary, four 2D α-Ni(OH)2 samples with different bud-like, flower-like, petal-like and sheet-like nanostructures were successfully synthesized via simple LRM method. By deliberately tuning the synthesis conditions, the as-synthesized Ni(OH)2-NB, -NF, -NP, and -NS have different stacked layers, while endowing the same magnitude of BET surface areas, average pore sizes as well as the thickness of layers, providing us a perfect single variate platform to investigate their structure-performance relationship of OER. The systematic experiments and analyses indicate that the less layers stacked petal-like structure (Ni(OH)2-NP) is more desirable to catalyze OER effectively since it possess not only favourable substance adsorption/diffusion properties, large accessible surface area (more exposed active sites) and high reactivity of active sites to boost OER process but also stable and mutual support layer nanostructures to guarantee the durability. These unique advantages enable Ni(OH)2-NP to be one of the most promising candidate catalysts for OER, showing the avenue to fabricate improved catalysts from structure design. By comparing the different structures and their OER behaviors of four α-Ni(OH)2 samples, our work may shed some light on the structure effect of 2D materials and accelerate the development of efficient OER catalysts. Further, our findings here can also be extended to other reactions and systems since α-Ni(OH)2 is the widely used inorganic material.

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Experimental Section Reagents and Chemicals. Nickel acetate with four water (Ni(Ac)2·4H2O), PEO-PPO-PEO (P123), glycine and hexamethylenetetramine (HMTA) are analysis reagent (A.R.) and purchased from Sinopharm Chemical Reagent Co. Ltd. Ethanol and ethylene glycol are A.R. and purchased from Beijing Chemical Reagent Company. All reagents were used as received without further purification. Deionized water is used for the synthesis of α-Ni(OH)2 samples. Syntheses of 2D layer-stacked Ni(OH)2 nanostructures. 0.2 g P123 was added into 13 g ethanol with 0.5 g H2O to form a clear solution with stirring for 15 minutes, and then 0.125 g Ni(Ac)2·4H2O and 0.07 g HMTA were added into the ethanol solution. After stirring for around 15 minutes, a cyan solution was obtained. Then, 13 ml ethylene glycol was added into the ethanol solution. After stirring for 30 minutes, a transparent solution was obtained. Then transferred it into a 40 ml autoclave and heated at 170 oC for 2 hours. The product of the solvothermal reaction (Ni(OH)2-NB) was washed with deionized water and ethanol for three times. The cyan powders were collected after washing/centrifugation and dehydration at 80 oC for 24 hours. Ni(OH)2-NF and Ni(OH)2-NP could be synthesized by increasing the water to 1.0 g and 2.0 g, respectively with other conditions unchanged. Syntheses of ultrathin Ni(OH)2 nanosheet. 0.2 g P123 was added into 13 g ethanol with 2.0 g H2O to form a clear solution with stirring for 15 minutes, and then 0.125 g Ni(Ac)2·4H2O and 0.035 g glycine were added into the ethanol solution. After stirring for around 15 minutes, a cyan solution was obtained. Then, 13 ml ethylene glycol was 24


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added into the ethanol solution. After stirring for 30 minutes, a transparent solution was obtained. The obtained solution was statically aged for 24 hours then transferred into a 40 ml autoclave and heated at 170 oC for 2 hours. The product of the solvothermal reaction (Ni(OH)2-NS) was washed with deionized water and ethanol for three times. The cyan powders were collected after washing/centrifugation and dehydration at 80 oC for 24 hours. Characterization. The morphologies of the samples were captured by JEM 2100 transmission electron microscope (TEM), Tecnai G2 F20 S-Twin high-resolution transmission electron microscope (HRTEM) and Gemini Sigma 300/VP scanning electron microscope (SEM). Powder X-ray diffraction (PXRD) patterns of samples were recorded on a Bruker D8-advance X-ray powder diffractometer operated at voltage of 40 kV and current of 40 mA with CuK radiation (λ = 1.5406 Å). N2 adsorption/desorption isotherms were obtained by a Kubo X10000 static volumetric gas adsorption at -196 oC. Before measurements, the samples were degassed at 200 oC for 30 minutes in vacuum. The specific surface area was calculated from the adsorption branches in the relative pressure range of 0.05-0.20 by the Brunauer-Emmett-Teller (BET) method. Electrochemical measurements. Experiments of OER were carried out using CHI 760E

electrochemical

analyzer

(CHI

Instrument,

CHN).

A

conventional

three-electrode cell was used, including a saturated mercuric oxide electrode (SME) as reference electrode, a Pt wire as counter electrode, and a rotating disk electrode (RDE, 5 mm in diameter) as working electrode. To make the electrode, the 25


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as-prepared catalyst (2 mg) and carbon black (1 mg) were evenly dispersed in the mixture of deionized water/ethanol (1.0 mL, 4:1, v/v) and Nafion solution (40 µL, 5 wt %, Alfa Aesar). Afterward, 20 µL of the homogeneous ink was spread out on the surface of GC electrode, with the mass loading 0.2 mg cm-2 for OER. The electrolyte is 1 M KOH solution saturated with O2 for 30 minutes before cyclic voltammetry test. To compare the influence of the structures and the OER activities, the electro-catalytic activities of other catalysts were measured under the similar conditions, and the corresponding electro-catalytic results are included in the main text.

Acknowledgements: The authors thank the understanding and support from families and the harmonious relationship between workmates. The authors acknowledge the financial supports from the NSFC (Nos. 21573286, 21173269, 91127040, 21576288 and U1662104) and Ministry of Science and Technology of China (No. 2011BAK15B05), Specialized Research Fund for the Doctoral Program of Higher Education (20130007110003), Science Foundation of China University of Petroleum, Beijing (No. 2462015YQ0304), and the open fund of the State Key Laboratory of Chemical Resource Engineering, Beijing.

Supporting Information: Supporting Information is available free of charge via the Internet at http://pubs.acs.org. 26


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Table of content graphic Different structures of 2D α-Ni(OH) will result in very different properties in accessible surface area (number of exposed active site), OH- adsorption, substance diffusion and the reactivity of active site, which will in return to affect their overall oxygen evolution reaction (OER) performances.

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Structure Effects of 2D Materials on Î±-Nickel Hydroxide for Oxygen

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