NiCo2O4 Nanocrystals Grown from the Skeleton of a 3D

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Mixed NiO/NiCo2O4 Nanocrystals Grown from Skeleton of 3D Porous Nickel Network as Efficient Electrocatalysts for Oxygen Evolution Reaction Chun Chang, Lei Zhang, Chan-Wei Hsu, Xui-Fang Chuah, and Shih-Yuan Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13127 • Publication Date (Web): 08 Dec 2017 Downloaded from http://pubs.acs.org on December 8, 2017

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ACS Applied Materials & Interfaces

Mixed NiO/NiCo2O4 Nanocrystals Grown from Skeleton of 3D Porous Nickel Network as Efficient Electrocatalysts for Oxygen Evolution Reaction Chun Chang,a,b Lei Zhanga,c, Chan-Wei Hsua, Xui-Fang Chuaha, and Shih-Yuan Lua*

a

. Department of Chemical Engineering, National Tsing Hua University, Hsinchu

30013, Taiwan, Republic of China. b

. College of Chemistry and Chemical Engineering, Bohai University, Jinzhou,

Liaoning 121013, P.R. China. c

. School of Materials Science and Engineering, Anhui University of Science and

Technology, Huainan, Anhui 232001, P. R. China

KEYWORDS: Mixed NiO/NiCo2O4 nanocrystals, 3D porous nickel network, oxygen evolution reaction, high current densities, NiO, NiCo2O4, Ni foam

ABSTRACT: Mixed NiO/NiCo2O4 nanocrystals grown in situ from skeleton of a 3D porous nickel network (3DPNN) were prepared with a simple hydrothermal method followed by a low temperature calcination, exhibiting outstanding electrocatalytic

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efficiencies toward oxygen evolution reactions (OER). The 3DPNN was prepared with a novel leaven dough method and served as both the nickel source for growth of the mixed NiO/NiCo2O4 nanocrystals and the charge transport highway to accelerate the sluggish kinetics of the OER. The mixed NiO/NiCo2O4 nanocrystals exhibited pronounced synergistic effects to achieve a high mass activity of 200 A g-1 at the catalyst mass loading of 0.5 mg cm-2, largely outperforming the corresponding single component

nanocrystal

systems,

NiO

(5.87)

and

NiCo2O4 (9.35).

The

NiO/NiCo2O4@3DPNN composite electrocatalyst achieved a low overpotential of 264 mV at the current density of 10 mA cm-2 and 389 mV at the practically high current density of 250 mA cm-2, which compete favorably among the top tier of previously reported OER electrocatalysts. Moreover, it exhibited good stability even at the high current density of 250 mA cm-2, showing only 9.40% increase in working applied potential after a continuous 12 h operation. The present work demonstrates a new design for highly efficient OER catalysts with in situ growth of mixed oxide nanocrystals of pronounced synergistic effects.

1. INTRODUCTION Global warming and fossil energy depletion remain the two most threatening and challenging issues of the world. Electrochemical water splitting is one of the promising technologies to provide renewable and clean fuel and basic chemical, ACS Paragon Plus Environment

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hydrogen, to tackle the global environmental and energy crises facing human being as a whole.1-4 The electrochemical water splitting process includes two half reactions: hydrogen evolution reaction (HER) at the cathode and oxygen evolution reaction (OER) at the anode. The development of efficient, cheap, and stable electrocatalysts for the OER however plays the decisive role for the success of the full electrochemical water splitting process because of the intrinsically sluggish kinetics of the OER, involving a thermodynamically un-favored reaction of four electron transfer.5-8 Efficient electrocatalysts for the OER are urgently needed to accelerate the reaction so that the working over-potential and thus the operational electric energy consumption can be reduced to make the process more competitive among the many existing hydrogen production technologies. Currently, precious metal oxides, for example, IrO2 and RuO2, are the most widely used and efficient OER catalysts under alkaline conditions, but their commercial applications are tremendously restricted by their limited availability, Earth-scarcity, high cost, and catalytic instability.9 To solve this problem, numerous efforts have been devoted to the development of highly efficient, durable, and low cost alternatives for the OER using non-noble metal based electrocatalysts

including

transition

hydroxides,10-14

sulfides,15-17

metal

phosphides,8,

oxides,3-4, 18-20

9

(layered

borates,21-22

double)

selenides,23-25

carbides,26-27 nitrides,28-29 fluorides,30 and metallic alloy,31-35 etc. To be practically



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useful, the electrocatalysts need to deliver high current densities, say 250 mA cm-2, at low over-potentials and remain stable, both electrocatalytically and mechanically, at the high current density, at which the strong bubble evolution exerts large shear stresses to the electrode and may cause severe detachment and thus loss of electrocatalysts. This critical issue however is generally overlooked in most of the past works of the area. It is to be stressed that over-potentials and stability investigated at a typically low current density of 10 mA cm-2 are not enough for practical applications. It is crucial to investigate the overpotentials and stability of the electrodes at high current densities to assess the practical merits of the electrocatalysts. Recently, spinel structured oxides have drawn a great deal of research attention and have been intensively and extensively studied because of their Earth-abundance and excellent corrosion resistance toward OER.5,

36-44

Nevertheless, their

electrocatalytic activities and thus widespread applications have been limited because of the intrinsically insulating nature of oxides.38 One way to get around this obstacle is to deposit functional catalysts on some conductive supports to accelerate the overall charge transport. One convenient candidate for such supports is commercial nickel foam. Commercial nickel foam with 3D porous architecture has been a popular choice to accommodate functional catalysts for the OER application.45 The combination of


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spinel structured oxides and commercial nickel foam accelerates the involved charge transfer and transport, which takes full advantages of the functional catalyst and thus significantly increases the OER activity.46 For instance, Co3O4 nanowire,36 NiCo2O4 nanograss,40 NiCo2O4 nanosheet,42 and chestnut-like MnCo2O441 incorporated on the nickel foam have been investigated as the OER catalyst. Among them, NiCo2O4 is of particular interest to the OER application. It can offer a rich variety of redox reactions because of the combined contributions from both nickel and cobalt ions and also possess outstanding electric conductivities among oxides, both contributing positively to the electrocatalytic activity toward the OER.47-51 Nevertheless, the commercial nickel form, with typical pore sizes of several hundred micronmeters and skeleton thicknesses of several tens micronmeters, cannot offer high enough surface areas to accommodate desirable high mass loading of the catalyst. We recently developed a novel leaven dough method to prepare 3D porous nickel network (3DPNN) that is with typical pore sizes and skeleton thicknesses one order of magnitude smaller than those of the commercial nickel foam and offers specific surface areas at least one order of magnitude higher than that of the commercial nickel foam, 11 vs. < 1 m2 g-1.46 Furthermore, it has been demonstrated that mixed components may induce synergistic effects to boost the targeted electrocatalytic activities.52-53 Based on the above, in this work, we develop a simple



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hydrothermal reaction followed by low temperature calcination to fabricate a novel composite OER electrocatalyst, composed of mixed NiO/NiCo2O4 nanocrystals grown in situ from the skeleton of the 3DPNN. The NiO/NiCo2O4 nanocrystals were grown in situ from the skeleton of the 3DPNN by taking 3DPNN as the nickel source. Consequently, the adhesion and contact between the NiO/NiCo2O4 nanocrystals and the skeleton of the 3DPNN are strong, good for charge transport across the interface and resisting shear stresses generated from strong bubble evolution. This design is particularly critical for the successful application of the catalyst in commercial electrolyzers, in which the water splitting is operated at high current densities, say 250 mA cm-2, for long periods of time. At high current densities, the charge transport must be fast enough to support the water oxidation reaction and the catalyst structure must be strong enough to resist the large shear stresses induced from intense bubble evolution. Only a few works have demonstrated the high current density capability of the electrocatalysts, including amorphous mesoporous Ni-Fe composite nanosheets,54 and NiFe2O4-NiOOH nanosheet arrays.55

The present work adds one additional

example, NiO/NiCo2O4@3DPNN, to the list. The

as-prepared

electrocatalyst

exhibited

outstanding

electrocatalytic

performances not just at the typical low current density of 10 mA cm-2, but more importantly at the high current density of 250 mA cm-2, with low over-potentials of


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263 and 389 mV achieved at 10 and 250 mA cm-2, respectively. The synergistic effect of the mixed NiO/NiCo2O4 nanocrystals is quite pronounced as revealed with a high mass activity of 200 A g-1 in comparison to those of single component samples, 5.87 for NiO and 9.35 for NiCo2O4. The long-term stability at high current densities was also excellent, exhibiting a moderate 9.40% increase in working potential after a continuous 12 h operation. The developed electrocatalyst holds a high potential for practical large scale applications, and the material design and synthesis strategy developed herein can be readily extended to other material systems.

2. EXPERIMENTAL 2.1 Materials. Nickel nitrate hexahydrate (Ni(NO3)2•6(H2O)) and cobalt nitrate hexahydrate (Co(NO3)2•6(H2O)) were purchased from Acros. (EO)106(PO)70(EO)106 triblock copolymer (Pluronic F127) was purchased from sigma-Aldrich. All reagents were of analytical grade and used as received without further purification. 2.2 Preparation of electrocatalysts. The preparation process of the electrocatalysts is illustrated in Scheme 1. The 3DPNN was synthesized as previously reported.46 Briefly, 1.45 g of Ni(NO3)2•6(H2O) and 0.5 g of F127 were mixed together and ground in a mortar for 20 min, and was transferred into a petri dish and placed in an oven set at 80 °C for 12 h. The resultant leaven dough-like intermediate was heated in a muffle furnace under air atmosphere at a heating rate of 5 °C min-1 up to 300 °C and held at ACS Paragon Plus Environment


300 oC for 1 h to afford the 3DPNN. The electrocatalysts were synthesized with a hydrothermal method. A desired amount of Co(NO3)2•6(H2O), 0.15, 0.6, 1.5, 3.0, or 6.0 mmol, was dissolved in 40 mL nitric solution of corresponding concentrations of 7.5, 30, 75, 75, or 75 mmol/L, respectively. An amount of 1.5 mmol 3DPNN was added into the above solution (making the molar ratios of Ni/Co = 20:2, 20:8, 20:20, 20:40, 20:80), and the mixture was transferred to a 50 mL Teflon-lined autoclave. The autoclave was sealed in a stainless steel tank and heated at 120 °C for 6 h. The autoclave was then let cool to room temperature naturally. The resultant powders were calcined in an air atmosphere at 350 °C for 2 h, with a heating rate of 5 °C min-1 to reach 350 °C, to afford the electrocatalysts. For referring convenience, the electrocatalysts were termed 20/2, 20/8, 20/20, 20/40, 20/80 according to the molar ratio of Ni/Co used in the preparation.

Scheme 1 Schematic for preparation of electrocatalysts.


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2.3 Characterizations. The morphologies and microstructure of the samples were characterized with a high resolution transmission electron microscope (HRTEM, JEOL, JEM-2010, Japan) and a field emission scanning electron microscope (FESEM, Hitachi S-4800, Japan). The crystalline structure of the samples was examined with an X-ray diffractometer (XRD, Shimadzu XRD-6000, Japan). An X-ray photoelectron spectrometer (XPS, Thermo ESCALAB 250XI, America) with monochromatized Al Kα X-ray as the excitation source was employed to study the chemical state of the elements contained in the sample. 2.4 Electrochemical measurements. The electrocatalytic performances of the samples were investigated on a CHI6275D electrochemical workstation with a three electrode system in 1 M KOH (pH = 13.7) aqueous solution, using the as-prepared electrocatalysts as the working electrode, platinum plate (1×1 cm2) as the counter electrode, and saturated calomel electrode (SCE) as the reference electrode. The working electrode was fabricated by using the electrocatalyst powders (85% wt) as the active material and polyvinylidene fluoride (15% wt) as the binder. They were mixed in N-methylpyrrolidone (NMP) to form slurry, which was sonicated for 30 min before use. The working electrode was fabricated by drop casting the above slurry onto a graphite electrode (1 cm × 1 cm), and dried at 80 °C in an oven for subsequent electrochemical measurements. The mass loading of the electrocatalyst in the working



electrode was controlled to be around 0.5 mg cm-2. Before the electrochemical measurement, the working electrode was cycled at least 30 times between 0 to 0.8 V

vs. Hg/HgO at a scan rate of 100 mV s-1 to condition the electrode for stable current densities. After the conditioning, the polarization curves of the electrode were recorded by sweeping the potential from 0 to 0.8 V (vs. Hg/HgO) at a scan rate of 10 mV s-1. All potential values in this study were reported referring to the reversible hydrogen electrode (RHE) using the Nernst equation ERHE = EHg/HgO + 0.118 + 0.059 × pH, where EHg/HgO is the experimentally measured potential against the Hg/HgO reference electrode. The overpotential (η) was calculated using the equation: η = ERHE – 1.23. The mass activity was calculated from the catalyst mass loading m (0.50 mg cm-2) and the measured current density j (mA cm-2): jm = j/m. The current densities were iR corrected at a compensation level of 90% according to a criterion we developed (see the Supporting Information and Figure S1 for details). The electrochemical impedance spectroscopy (EIS) was conducted by applying an AC voltage with 5 mV amplitude in the frequency range of 105 to 0.01 Hz.

3. RESULTS AND DISCUSSION 3.1. Characterization of electrocatalysts. The XRD patterns of the electrocatalysts prepared at increasing Co source addition together with that of bare 3DPNN are shown in Figure 1 for comparison. An enlarged version of Figure 1 is presented in ACS Paragon Plus Environment

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Figure S2 for pattern details. For the bare 3DPNN, three major diffraction peaks located at the 2θ values of 44.5, 51.8, and 76.4°, corresponding to the (111), (200), and (220) crystalline planes of cubic Ni [JCPDS No. 87-0712], respectively, can be identified. Once treated hydrothermally with the addition of HNO3 and Co source, the electrocatalysts acquired extra diffraction peaks. Diffraction peaks located at the 2θ values of 37.2, 43.3, 62.8, and 75.3° emerged with increasing HNO3 and Co source addition. These diffraction peaks can be indexed to the (111), (200), (220), and (311) crystalline planes of cubic NiO [JCPDS No. 73-1532], suggesting that cubic NiO nanocrystals were formed. As the Co source concentration was increased to 20, an additional set of diffraction peaks appeared, with the major diffraction peaks consistent with those of NiCo2O4 [JCPDS No. 73-1702], indicating the formation of mixed NiO/NiCo2O4 nanocrystals. With further increase in Co source addition, the characteristic diffraction peaks of NiCo2O4 turned strong, whereas those corresponding to NiO weakened and eventually disappeared for the 20/80 sample. Evidently, as the concentration of the Co precursor increases, the Ni of the 3DPNN is consumed more toward the formation of NiCo2O4 instead of NiO. The characteristic diffraction peaks located at the 2θ values of 18.9, 31.2, 36.7, 38.4, 44.6, 55.4, 59.1, 65.0 observed for NiCo2O4 can be assigned to the crystalline planes of (111), (220), (311), (222), (400), (422), (511), and (440), respectively.



♦NiCo2O4 JCPDS # 73-1702

♦ ♦

♦♦ ♦

20/20

♦ ♦♦ • ♥ ♦• •

20/8

•

•

•

20/2

•

• ♥

•

♦

Intensity

(440) (511) (422)

20/40

♦

(400)

♦

(222) (311)

20/80

(220)

(111)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(111)

3DPNN

JCPDS # 73-1532 NiO •

10

20

(111)

30

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♦ ♦ •♦ ♥

♥

♥

♦ •♦

(200)

(220) ♥

(200) (220)

40

50

60

(311)

70

80

2θ θ (°) Figure 1. XRD patterns of 3DPNN and samples 20/2, 20/8, 20/20, 20/40, and 20/80. Figure 2 shows the SEM images of the bare 3DPNN and sample 20/20. The morphologies of the different electrocatalysts are quite similar. Here, only sample 20/20 was examined as a representative case since sample 20/20 was the best electrocatalyst among all samples investigated here. It is evident from Figure 2a that the 3DPNN is composed of a 3D network of chain-like skeletons, and these chain-like skeletons possess smooth surfaces with discrete pin holes. As for sample 20/20, it is evident that the original smoothness of the skeleton surface disappears and the mixed NiO/NiCo2O4 nanocrystals grown from the skeleton surface significantly roughens the skeleton.


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Figure 2. SEM images of bare 3DPNN at 2500× (a1) and at 13000× (a2), sample 20/20 at 500× (b1) and 2500× (b2). Sample 20/20 was fragmented for TEM imaging to further examine its detailed morphology and component structure. It appears that the in situ grown nanocrystals covers the surface of the Ni skeleton as shown in Figure 3a, giving the rough appearance of the sample presented in Figure 2b. The associated selected area electron diffraction (SAED) pattern, shown in the inset of Figure 3a, can be identified the presence of the (111) and (200) planes of cubic Ni, the (111), (200), and (220) planes of cubic NiO, and the (220), (311), (511), and (440) planes of the spinel structured NiCo2O4, in consistent with the XRD analysis.46,

56

The lattice fringes of the

component crystals were further observed with HRTEM images as shown in Figure 3b. Five different lattice fringes can be identified, one with an inter-layer distance of



0.20 nm corresponding to the (111) planes of cubic Ni, one with inter-layer distances of 0.21 and 0.24 nm in good agreement with the d-spacing of the (200) and (111) planes of cubic NiO, and the last one with inter-layer distances of 0.25 and 0.47 nm attributable to the (311) and (111) planes of cubic NiCo2O4.57 It is to be noted that because of the limit of the depth of field of the HRTEM and the rough morphology of the sample, some parts of Figure 3b are not in focus and appear blurred. The inset located at the upper left corner shows the in focus image of the marked region at the correct depth of field, showing the clear lattice fringes for layer distance determination.


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Figure 3. (a) Low and (b) high-resolution TEM images of sample 20/20. Inset of Figure 3a: corresponding SAED pattern. Inset of Figure 3b: in focus image of marked region. To further investigate the chemical state of NiO and NiCo2O4 grown on the skeleton of the 3DPNN, X-ray photoelectron spectroscopy (XPS) was conducted and the results for samples 20/8 (NiO@3DPNN) and 20/20 (NiO/NiCo2O4@3DPNN) were presented in Figure S3 for comparison. First, the full survey spectra of both



samples were shown and compared in Figure S3a. Both spectra exhibit signals from O 1s and Ni 2p, whereas that of Co 2p appears only for sample 20/20 as expected. The details on the chemical states of Co 2p, Ni 2p, and O 1s were discussed using the high resolution XPS spectra shown in Figures S3b, S3c1, S3c2, S3d1, and S3d2. All binding energy peaks were de-convoluted with the Lorentzian-Gaussian fitting method to reveal the detailed information. The Co 2p spectrum (Figure S3b) of sample 20/20 is composed of two spin-orbit doublets, 773.3 and 788.3 eV for Co3+ and 779.7 and 795.4 eV for Co2+, and two shake-up satellites, 782.3 and 802.8 eV, in good agreement with literature.42 The Ni 2p spectra of samples 20/8 and 20/20 are presented in Figures S3c1 and S3c2, respectively for comparison. For sample 20/8, peaks at 853.9 and 871.3 eV are contributed by Ni2+, and those at 855.7 and 873.0 eV are indexed to Ni3+.58 The two peaks located at around 861.2 and 879.2 eV are associated satellites for Ni. As for sample 20/20, the two Ni2+ peak positions slightly higher-shift to 854.0 and 871.6 eV, respectively, and similarly the two Ni3+ peak positions higher-shift to 855.8 and 873.3 eV, respectively. The co-existence of divalent and trivalent states for Co and Ni implies that both Ni and Co were partially oxidized and reduced, respectively to balance the formation of oxygen vacancies.4 The O 1s spectra for samples 20/8 and 20/20 are compared with Figures S3d1 and S3d2. For sample 20/8, three peaks were identified, with the one at 529.3 eV


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attributed to the metal-oxygen bonds, the one at 531.0 eV associated with the hydroxyl groups and under-coordinated lattice oxygen, and the one at 532.2 eV contributed by the presence of other oxygen-containing species such as H2O and O2 adsorbed on the sample surface.59-60 The corresponding binding energy peaks of sample 20/20 lower-shift to 529.2, 529.8, and 531.2 eV, respectively. 3.2 Electrochemical performances of electrocatalysts. The OER performance of the supporting template, 3DPNN, was first investigated against that of commercial Ni foam with linear sweep voltammetry (LSV) measurements in 1 M KOH at a scan rate of 10 mV s-1 and a mass loading of 0.5 mg/cm2 as presented in Figure S4. Evidently, the current densities achieved by the 3DPNN electrode are significantly higher than those of the commercial Ni foam. This is expected since the 3D porous Ni network prepared with the present leaven dough method can offer much larger electrocatalyst/electrolyte contact surface areas, achieved with much smaller pores and much thinner backbones as shown in Figure S5, for the OER to proceed. This demonstrates the advantage of the present 3DPNN as a superior supporting template to host active materials. The OER performances of the electrocatalysts, including samples 20/2, 20/8, 20/20, 20/40, and 20/80 together with bare 3DPNN for comparison, were next investigated. The resulting polarization curves are presented and compared in Figure 4. The



polarization curves generally show a broad oxidation peak at around 1.37 V (vs. RHE), as evident from the inset of Figure 4, before the sharp increase in current density caused by the water oxidation reaction for oxygen evolution, 4OH- → O2 + 2H2O + 4e-.61 The broad oxidation peak was contributed by the formation of the active species NiOOH and CoOOH from NiO and NiCo2O4 through oxidation of Ni2+ and accompanying formation of CoOOH in the alkaline medium, NiO + OH− ↔ NiOOH + e- and NiCo2O4 + OH− + H2O ↔ NiOOH + 2CoOOH + e-, respectively.46, 62

NiOOH and CoOOH have been identified as the active species to catalyze the

OER.63-64 Consequently, the intensity of this characteristic peak, or more precisely the area enclosed by the peak, signifies the amount of NiOOH and CoOOH formed in the system and correlates positively to the OER efficiency of the electrocatalyst. Accordingly, one would expect the OER efficiency of these electrocatalysts to be in the increasing order of bare 3DPNN, 20/2, 20/8, 20/80, 20/40, and 20/20. To quantify the OER efficiency of an electrocatalyst, one common judging parameter is the overpotential needed to achieve the current density of 10 mA/cm2, η10, which is the smaller the better. The η10 read from Figure 4 were summarized in Table 1. It is evident that the η10 of the bare 3DPNN is the largest and decreases with increasing NiO content in the electrocatalyst, sample 20/2 then sample 20/8. The η10 reaches the lowest level for sample 20/20, in which both NiO and NiCo2O4 are


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present in the electrocatalyst. Its value however bounces back up for sample 20/40, in which the amount of NiO decreases and that of NiCo2O4 increases, and increases further for sample 20/80, in which only NiCo2O4 remains in the electrocatalyst. In summary, the η10 values of the electrocatalysts are in the decreasing order of bare 3DPNN, 20/2, 20/8, 20/80, 20/40, and 20/20, in good agreement with the trend of enclosed area of the characteristic oxidation peak of the polarization curves described above. 300 1.23 V j=250 mA cm-2

250

Current/mA cm-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3DPNN 20/2 20/8 20/20 20/40 20/80

200 150

12 8 4 0 1.25 1.30 1.35 1.40 1.45 1.50

100

j=100 mA cm-2

50 j=10 mA cm-2 0 1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

Potential / V vs. RHE Figure 4. Polarization curves of 3DPNN and samples 20/2, 20/8, 20/20, 20/40, and 20/80 recorded in 1 M KOH at scan rate of 10 mV s-1. Although η10 has been commonly used to quantify the electrocatalyst efficiency, it is inappropriate for commercial applications of the electrocatalyst. For commercial applications, the electrolyzer has to be operated at high current densities, say 250 mA cm-2, to be economically useful and competitive. Therefore, the overpotentials needed



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to achieve high current densities, 100 and 250 mA cm-2, are also determined from Figure 4 and summarized in Table 1. Note that only η100 of samples 20/20, 20/40, and 20/80 and η250 of samples 20/20 and 20/40 are determined and presented, with those of other samples being too large to be useful for comparison. The trend of the η100 and η250 follows that of η10, with sample 20/20 achieving low η100 and η250 of 350 and 389 mV, respectively.

Table 1. Summary of OER performances of electrocatalysts. Samples

Components

η10/mV

η100/mV

η250/mV

jm/A g-1 at η=350

Tafel slope/mV

mV

dec-1

3DPNN

Ni

460

—

—

1.01

96.7

20/2

NiO@Ni

431

—

—

1.87

91.2

20/8

NiO@Ni

385

—

—

5.87

89.5

20/20

NiO/NiCo2O4@Ni

264

350

389

200

79.3

20/40

NiO/NiCo2O4@Ni

327

430

477

34.2

99.8

20/80

NiCo2O4@Ni

374

473

—

9.35

87.5

Although overpotential is a convenient performance index for the OER catalyst, it is in fact a parameter whose value depends on the mass loading of the catalyst on the electrode. To eliminate the influence of mass loading, mass activity (jm), defined as the ratio of the current density achieved at a specific overpotential to the catalyst mass used, serves as another useful performance index to quantify the efficiency of the electrocatalyst. Here, the current density achieved at the overpotential of 350 mV was used to determine the mass activities of the present electrocatalysts as summarized in Table 1 for comparison. Evidently, the trend is in good agreement with that of the


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overpotentials, η10, η100, and η250, with sample 20/20 largely outperforming all other samples with a high mass activity of 200 A g-1. Mass activity appears to be a more sensitive index for the OER efficiency, giving much pronounced differences between samples. Furthermore, the electrochemical active surface area (ECSA) was also determined to confirm the high catalytic activity of sample 20/20. It is well known that ECSA is directly proportional to the amount of active sites of the electrocatalyst, and the ECSA can be determined from the electrochemical double-layer capacitance (Cdl).19,

65

The Cdl values of the samples were determined based on the cyclic

voltammograms recorded at a non-Faradaic potential window (0.9322-1.0722 V vs. RHE) at increasing scan rates (10-50 mV s-1) as shown in Figure S6(a-e). The Cdl was obtained as the slope of the capacitive current density (at 1.0022 V vs. RHE) vs. scan rate curve, as presented in Figure S6(f). The Cdl values of samples 20/2, 20/4, 20/20, 20/40, and 20/80 are 0.16, 0.38, 13.55, 6.40, and 4.85 mF cm-2, respectively. The Cdl value of sample 20/20 is almost 85-fold of that of sample 20/2, and 2.8-fold of that of sample 20/80. These results confirm that sample 20/20 possesses a much larger number of active sites than all other samples, leading to its outstanding OER activity. If one combines the results of XRD with those of overpotentials, mass activities, or ECSA, one can easily correlate the component contained in the sample to the OER efficiency of the sample. From the XRD results, one concludes that with increasing



Co source addition, first the NiO is introduced to the bare 3DPNN as in sample 20/2, and the amount of NiO increases as in sample 20/8. Further increase in Co source addition leads to formation of mixed oxide nanocrystals, NiO and NiCo2O4, on the skeleton of the 3DPNN as in sample 20/20. Even higher amount of Co source addition decreases the NiO but increases the NiCo2O4 amount in the product as in sample 20/40. Eventually, NiO disappears from the product with only NiCo2O4 remaining as in sample 20/80. The trend of the overpotential (or mass activity and ECSA) of the samples in decreasing (increasing) order is: 3DPNN, 20/2, 20/8, 20/80, 20/40, 20/20. Several points can be made from the above observation. First, the presence of NiO or NiCo2O4 enhances the OER efficiency. Second, the presence of NiCo2O4 enhances more of the OER efficiency than that of NiO. Third, the co-presence of NiO and NiCo2O4 achieves even more enhancements for the OER efficiency. Finally, the co-presence of comparable amounts of NiO and NiCo2O4 is most effective in boosting the OER efficiency. This phenomenon can be attributed to the strong synergistic effect between NiO and NiCo2O4. It has been demonstrated that the close contact of mixed functional components may accelerate the charge transfer and achieve enhanced OER efficiencies.53 The synergistic effect can also be understood based on the bifunctional effect of the strong metal-support interaction theory, in which the reaction intermediates from the surfaces of the mixed metal oxides, here NiO and NiCo2O4,


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migrate to the intrinsically more active contact perimeters to complete the reaction.66 Moreover, the integration of two or more metals in a heterogeneous catalyst may enhance the multiple valence states of the cations with more complex electronic structure and acquire higher electrical conductivities, thus contributing to much enhanced electrocatalytic performances.67 The use of the highly conductive 3DPNN as the substrate to accommodate the active materials, NiO and NiCo2O4, also plays an important role for the pronounced enhancement in the OER efficiency. The 3DPNN not only facilitates the critical charge transfer and transport during the OER occurring at the surface of the active materials, but also disperses the active materials to enhance their degree of utilization for the OER. It is worth mentioning that turn over frequency (TOF) is another characteristic parameter for catalyst performances. Nevertheless, its determination requires number of moles of the active materials. For the present catalysts, the number of moles of the active material is difficult, if not impossible, to determine. Thus, the catalyst efficiency was only studied by η10, η100, η250, and ηm. In addition, the catalytic kinetics of the OER catalysts are often investigated and quantified by Tafel slopes, which can be determined from the polarization curves by re-casting the data into the Tafel equation, η = b log|j| + a, with b being the Tafel slope. Tafel slope gives the measure on how effectively the electrocatalyst can boost the



current density at the expense of increasing the overpotential. Smaller Tafel slopes are desired so that high current densities can be achieved at relatively low overpotentials, thus less electric power consumption. The results are shown in Figure 5 and summarized in the last column of Table 1 for comparison. Sample 20/20 again significantly outperforms all other samples with a Tafel slope of 79.3 mV dec-1. The three important parameters for efficiency quantification of the OER catalyst, η10 (also η at higher current densities if available), mass activity, and Tafel slope, of sample 20/20 are compared with those of Ni/Co based OER catalysts reported in recent years in Table S1. Evidently, sample 20/20 competes quite favorably among the top tier of these Ni/Co based OER catalysts.

Overpotential / V vs. RHE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3DPNN 20/2 20/8 20/20 20/40 20/80

0.45 96

.7

0.40

.5 89 .2 91

.5 87

0.35

99

.8 -1

.3 79

0.30 0.0

0.5

1.0

m

e Vd

1.5

c

2.0

Log current density / mA cm-2

Figure 5. Overpotential vs. log j and determined Tafel slopes of 3DPNN and samples 20/2, 20/8, 20/20, 20/40, and 20/80.


Page 25 of 36

To further investigate the electrocatalytic kinetic characteristics of the samples, electrochemical impedance spectroscopy was conducted. The applied potential was set at 1.5322 V vs. RHE to ensure occurrence of the OER for all samples. The resulting Nyquist plots are presented in Figure 6. Semi-arcs, formed at high frequencies, are generated because of the OER occurring at the electrode/electrolyte interface and the diameter of the arc is a measure of the charge transfer resistance involved. Rapid charge transfer at the electrode/electrolyte interface is required for fast OERs. Smaller semi-arcs mean smaller charge transfer resistances and thus faster OERs. The trend of the arc diameter is in good agreement with those of η10 and jm, with again that of sample 20/20 being markedly smaller than those of all other samples. 20/80 20/40 20/20 20/8 20/2 3DPNN

-8

Z imaginary (ohms)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-6 -4 -2 0 2 0

5

10

15

20

25

Z real (ohms) Figure 6. Nyquist plots of all samples at applied potential of 1.5322 V vs. RHE.



Besides the catalytic efficiency, the stability of the electrocatalysts is another major concern for an OER catalyst. For large scale applications, the electrolyzer must be operated at high current densities, say 250 mA cm-2, to be economically useful and competitive and thus the stability of the electrodes should be investigated at high current densities, instead of at the typical low current density of 10 mA cm-2, to explore the true merits of the electrocatalysts for practical applications.68 At high current densities, the OER proceeds strongly to impose severe stability challenges to the electrocatalyst. In this regard, the stability of sample 20/20 was tested by chronoamperometric measurements at 1.619 V vs. RHE and by chronopotentiometric measurements at 250 mA cm-2, both for 12 h. Here, 1.619 V is the potential, corresponding to an overpotential of 0.389 V, needed to achieve a current density of 250 mA cm-2. The results are shown in Figure 7. The current density achieved at the applied potential of 1.619 V drops by 24.0% after a continuous operation of 12 h, and the potential needed to drive the current density of 250 mA cm-2 increases by 9.40% after a continuous operation of 12 h. These results show a reasonable stability of sample 20/20 considering the high current density imposed. The morphology of sample 20/20 after the OER operation was shown and observed in Figure S7. The characteristic 3D porous nickel network structure was maintained, implying reasonable electrocatalytic stability of the product. A video, Video S1, for the oxygen


Page 26 of 36

Page 27 of 36

evolution experiment is available in the SI to further show the fast oxygen evolution achieved by the present NiO/NiCo2O4@3DPNN electrode at 250 mA cm-2. The loss of catalyst from the electrode because of the detachment caused by the vigorous bubble evolution, observed experimentally in this study, is the main cause for the moderate decay in generated current density and increase in applied potential.69-70 6

750

a

500

Potential (V vs. RHE)

Current density (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


250 0 -250

b

4 2 0 -2

-500 0

10000

20000

30000

40000

0

10000

20000

30000

40000

Time (s)

Time (s)

Figure 7. (a) Chronoamperometric and (b) chronopotentiometric stability tests of sample 20/20 conducted in 1 M KOH at 1.619 V (vs. RHE) and 250 mA cm-2, respectively.

4. CONCLUSION In conclusion, a highly efficient composite OER catalyst was designed and fabricated. The composite OER catalyst was composed of mixed metal oxides grown in situ from the skeleton of a 3D porous nickel network, which was prepared with a novel leaven dough method. The NiO/NiCo2O4@3DPNN catalyst exhibited outstanding OER performances, with low η10 of 263 mV and η250 of 389 mV and high mass activity of



Page 28 of 36

200 A g-1, largely outperforming the corresponding single component catalysts, NiO@3DPNN and NiCo2O4@3DPNN, clearly demonstrating the pronounced synergistic effect between NiO and NiCo2O4. The strategy developed in this work for structure design and fabrication of the electrocatalyst proves successful and can be readily extended to other material systems, not only for the applications of OER catalyst development but also development for many other relevant electrochemical devices,

including

hydrogen

evolution

reaction,

supercapacitors,

sensor,

electrochemical conversion, etc.

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on ACS publications website at DOI: development of criterion for iR compensation; polarization curves of sample 20/20 at increasing compensation levels; XRD patterns of 3DPNN and samples 20/2, 20/8, 20/20, 20/40, and 20/80; XPS spectra of samples 20/8 and 20/20; polarization curves of commercial nickel foam and 3DPNN; SEM images of commercial Ni foam and as-prepared 3D porous nickel network (3DPNN); cyclic voltammograms for samples 20/2, 20/4, 20/20, 20/40, and 20/80, and linear fitting of capacitive current density vs. scan rate curves; SEM images of NiO/NiCo2O4@3DPNN electrocatalyst after 12 h OER operation; comparison of OER performances: present work vs. ACS Paragon Plus Environment

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literature; and a video showing oxygen evolution at 250 mA/cm2 (MP4).

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] ORCID: Shih-Yuan Lu: 0000-0003-3217-8199 Notes

The authors declare no competing financial interest.

ACKNOWLEDGEMENTS Authors Chun Chang, Lei Zhang, Chan-Wei Hsu, Xui-Fang Chuah, and Shih-Yuan Lu received funding from the Ministry of Science and Technology of Taiwan under grant MOST 103-2221-E-007-119-MY2 (SYL). Author Chun Chang received funding from the National Natural Science Foundation of China under grant No. 51508026 (CC).

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265x185mm (150 x 150 DPI)


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NiCo2O4 Nanocrystals Grown from the Skeleton of a 3D

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