Bifunctional Electrocatalytic Activity of Nitrogen-Doped NiO

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Bifunctional Electrocatalytic Activity of Nitrogen-Doped NiO Nanosheets for Rechargeable Zn–Air Batteries Jinmei Qian, Xiaowan Bai, Shibo Xi, Wen Xiao, Daqiang Gao, and Jinlan Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08647 • Publication Date (Web): 05 Aug 2019 Downloaded from pubs.acs.org on August 5, 2019

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ACS Applied Materials & Interfaces

Bifunctional Electrocatalytic Activity of Nitrogen-Doped NiO Nanosheets for Rechargeable Zn–Air Batteries

Jinmei Qian‡, Xiaowan Bai‡, Shibo Xi, Wen Xiao, Daqiang Gao *, and Jinlan Wang*

Dr. J. M. Qian, Prof. D. Q. Gao Key Laboratory for Magnetism and Magnetic Materials of MOE, Key Laboratory of Special Function Materials and Structure Design of MOE, Lanzhou University, Lanzhou 730000, P. R. China E-mail: [email protected]

Dr. X. W. Bai, Prof. J. L. Wang School of Physics, Southeast University, Nanjing 211189, China. E-mail: [email protected]

Dr. S. B. Xi, Dr. W. Xiao Institute of Chemical and Engineering Sciences, A*STAR, 1 Pesek Road, Jurong Island, 627833, Singapore

ABSTRACT In order to improve the efficiencies and service lifetimes of rechargeable Zn-air batteries, it is necessary to develop highly efficient air electrocatalysts. In the present study, we prove that the bifunctional electrocatalytic activity in NiO nanosheets is

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effectively improved by the synergistic effects of N dopants and considerably porous structure. As an electrocatalyst for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), the as-prepared porous N doped NiO nanosheets exhibit good activities with the small overpotential and ideal half-wave potential, which is superior to Ir/C electrocatalyst. Besides, it is proved that the process of HO* is oxidized to O* is the OER potential rate-determining step, also the OER electrocatalytic performance of NiO can be markedly promote by the doping of N atoms using the density functional theory calculations. Furthermore, the fabricated Zn–air battery based on the porous N doped NiO nanosheets also exhibits superior activities, outperforming many reported NiO-based electrocatalyst materials. Two-series Zn-air cells with a voltage of 2.80 V can power a red light-emitting-diode, which shows their large potential for various application. Keywords: N-doping, NiO nanosheets, Oxygen evolution, Oxygen reduction, bifunctional electrocatalysis, Zn-air battery

1. INTRODUCTION Recently, Zn-air batteries are considered to be one of the optimal choices for nextgeneration energy conversion and storage technologies since their merits of zero pollution, high specific capacity, and operation without the need the fossil fuel.1-4 However, the mass production of Zn-air cells has been limited since the short actual service lift and relatively slow kinetics of OER/ORR process, which is primarily caused by the inefficient and unstable bifunctional electrocatalyst.5 In general, noble metal-


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based catalysts satisfy the requirements for conventional electrocatalysts to catalyze the ORR/OER, but their high prices and poor cycling characteristics hinder their use.6-7 Therefore, exploring and researching high performance, low cost and stable alternative bifunctional electrocatalyst to accelerate the ORR/OER reactions is a fundamental step toward extensive applications of the next-generation renewable electrochemical energy technologies.8-9 Transition-metal oxides (particularly cobalt, nickel and manganese oxides) are one of the most promising alternative catalysts since its abundant reserves, strong corrosion resistances. 10-13 Among them, porous nickel oxides (NiOx) nanosheets are considered the most suitable alternatives owning to the high superior catalytic activities, safety, low cost and easy preparation.14 However, the inherent low electronic conductivities, poor ion transport kinetics and cycling durability of nickel oxides restrict their extensive use.15-16 Recently, material design strategies demonstrated that the electrocatalytic activities of NiOx-based electrocatalysts can be efficiently increased by increasing the conductivity or inducing more active sites, e.g., by designing nanostructured NiO (nanoﬂakes or nanosheets), introducing foreign dopants (Fe doped NiO) or constructing heterostructures to accelerate the electron transfer (Co3O4@NiO, Ni@NiO).17-21 Among them, N doping has been widely used to improve the catalytic activities of the electrocatalysts since the electron-donor activities (N doped Ni2P4O12) or more available active surface area (N doped Co3O4 nanosheets, N-doped MoS2, or MoSe2 nanosheets).11, 22-24 In addition, porosity engineering is also an effective strategy to



develop high-efficiency electrocatalysts, which can expose and utilize more active sites and provide continuous charge transport pathways.25-26 In this paper, we report an excellent bifunctional electrocatalytic performances of porous N-doped NiO nanosheets (denoted as N-NiO) for both OER and ORR. The N dopants simultaneously effectively increase the electrical conductivity and reduce the potential rate-determining step of the Ni sites in NiO. Moreover, as a bifunctional electrocatalyst of Zn-air battery, N-NiO shows good rechargeable activity and stability with the high power density of 112.30 mW/cm2, making it promising for applications in future electric vehicles. 2. RESULTS AND DISCUSSION The N-NiO and NiO electrocatalysts were synthesized using two-step hydrothermal routes (see Experimental Section). Figure 1a illustrates the X-ray diffraction (XRD) crystal structure analysis that the N-NiO/NiO phase corresponds to the cubic NiO structure (JCPDS # 47-1049),which demonstrates the high purity of NNiO and NiO, where the peaks at 2θ of 37.1°, 43.1°, 62.6°, 75.0° and 79.0°can be well indexed to (111), (200), (220), (311), and (222) planes of the NiO. Explaining that the successful doping of N atoms does not result in other phase or precipitation were formed in NiO matrix.27 Figure S1 (Supporting Information) shows the morphologies of N-NiO/NiO that the two samples exhibit flower-like morphologies, composed of nanosheets with different orientations. Transmission electron microscopy (TEM) images reveal further details on the morphologies and microcell structures of the samples. As shown in Figure 1b and 1c, N-NiO exhibits a rough hexagonal nanosheet


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surface and multiporous structure, retaining the basic structure and morphology of pristine NiO (Figure S2, Supporting Information). Moreover, the lattice spacings of NNiO are consistent with the (111) (0.24 nm) and (200) (0.21 nm) interplanar distances of the typical NiO (Figure 1c).28 Elemental mapping analysis further illustrates that the N atoms uniformly doped into NiO matrix (Figure 1d). And the specific surface areas of N-NiO is larger than that of the pure NiO and the precursor (Ni(OH)2), demonstrating that the N doped NiO ensures the stability of the morphology structure of NiO while offering an effective conditions for the full reduction and adsorption of oxygen in ORR/OER process (Figure S3, Supporting Information). Moreover, the structural characteristics of the sample play a significant role in further exploring the electrocatalytic activity of ORR/OER. Hence the Ni K-edge XAFS spectra were measured by X-ray absorption fine-structure (XAFS) spectroscopy method. The absorption edge of N-NiO shifts to a lower binding energy position compared with pristine NiO (Figure S4, Supporting Information), owing to the covalent effect attributed to the N doping.29 Further, the small reduction in the amplitude for N-NiO was observed by Ni K-edge EXAFS spectroscopy k3χ (k) analysis, reflecting the change in the local atomic arrangement (Figure 1e). Figure 1f illustrates the Fouriertransformed EXAFS spectra, where the two main peaks consistent with the Ni-O, NiNi coordinations. The doping of non-metal N atoms and the unique porous morphology largely effect the difference in intensity.30-31



Figure 1. Structures and morphologies of NiO and N-NiO. (a) XRD profiles of samples. (b) Morphology and high-resolution TEM images of N-NiO. (d) Elemental mapping analysis of N-NiO. (e) EXAFS signal x(k) weighted by k. (f) Fourier- transformed EXAFS spectra of samples at Ni Kedge.

Besides, the surface characteristics are also important for electrocatalytic reaction progress, which were further analyzed by X-ray photoelectron spectroscopy (XPS) method. The typical N 1s peak at around 399.0 eV, revealing the Ni-N band was formed in N-NiO matrix after successfully doping of N (Figure 2a). The N concentration was estimated to be 5.1 at. %. As shown in Figure 2b, Ni 2P signal can be clearly observed, and there is a slightly shift to lower binding energies of Ni 2p in N-NiO than those in the pure NiO, which reveals extra charges in the Ni side. The specific positions of peaks are listed in Table S1 (Supporting Information).32-34 This is confirmed by the Bader charge analysis in Figure 2d, where the Ni atoms around the N dopants have larger charges in N-NiO than those in the pure NiO, which implies that the N dopants can induce the charge transfer in the NiO matrix. The O 1s spectra in Figure 2c show no


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considerable difference in oxygen structure between NiO and N-NiO.35 Therefore, these results show that we successfully synthesized porous N-doped NiO nanosheets.

Figure 2. (a) N 1s, (b) Ni 2p and (c) O 1s XPS spectra of samples. (d) Models of NiO and N-doped NiO (1N, 2N, 3N) at the (110) surface and calculated Bader charges for the surface Ni atoms.

A standard three-electrode system was performed to measure the OER performances in 1 M KOH. Figure 3a and 3b illustrate that the small overpotential of 270 mV (vs. reversible hydrogen electrode (RHE) at 10 mA/cm2 and corresponding Tafel slope (83 mV/dec) are achieved for N-NiO electrocatalyst, lower than those of the pristine NiO electrocatalyst (470 mV, 115 mV/dec), affirming its good electrocatalytic kinetics achieved by the N doping. Further, the OER reaction kinetics was analysed by electrochemical impedance spectroscopy (EIS) measurement.36 The charge transfer resistance (Rct) value of N-NiO catalyst is 136 Ω (Figure 3c), which is significant lower than pure NiO catalyst (191Ω), implying that N-NiO exhibits a higher conductivity and faster electron transfer in the OER.37-38 Besides, ECSA is one of the



vital criteria for determing the electrocatalytic activity of catalyst, which is linearly related to the double layer capacitance (Cdl) value (Figure S5, Table S2, Supporting Information). And the Cdl of N-NiO is 28.4 mF cm-2 (18.6 mF cm-2 for NiO), corresponding to the ECSA of 236.7 m2 g-1 (155.0 m2 g-1 for NiO), evidencing that N atoms doping can effectively promote the OER performance of pristine NiO.39-40 In addition, the ORR activities of N-NiO, NiO nanosheets, and Pt/C were evaluated through an electrocatalytic test. It can be seen from Figure 3d that the small onset potential (0.90 V) and ideal half-wave potential (E1/2, 0.69 V) were expected for N-NiO electrocatalyst. The ORR kinetics was further evaluated by the Koutecky−Levich (K−L) plot of the N-NiO electrocatalyst, with the desired electron transfer number (n) was obtained for N-NiO (n ~ 3.8) as shown in Figure 3e, larger than the estimated n ~ 3.0 for NiO (Figure S6, Supporting Information), which ensures an adequate reduction of oxygen through an almost four-electron pathway of N-NiO, demonstrating the excellent electrocatalytic ORR activity of the N-NiO electrocatalyst. 41

In addition, the overall oxygen activities of the electrocatalysts were assessed based

on the analysis of potential gap (∆𝐸 = 𝐸j=10 − 𝐸1⁄2 ); a smaller value corresponds to a higher bifunctional electrocatalytic performance.42-43 Figure 3f shows that ΔE of NNiO is 0.83 V, whereas the value is 1.12 V for pristine NiO, confirming that the excellent bifunctional catalyst activity of the porous N-NiO nanosheets.


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Figure 3. Electrochemical information of samples. (a) LSV curves and (b) Tafel plots obtained from (a) of OER. (c) EIS analysis of N-NiO and pure NiO catalyst. (d) LSV curves for ORR electrocatalytic activities at 1600 rpm. (e) ORR LSV curves of N-NiO and the corresponding K–L plots. (f) Bifunctional electrocatalytic activities of NiO and N-NiO toward both ORR and OER.

Furthermore, for the NiO (110) with xN (x=1, 2, and 3) doping and without doping, the DFT method was used to calculate electronic structures and the Gibbs free energies of the four-step OER process. First, N doping creates new states in the gap with the increase of doping content (Figure 4a-4b, Figure S7, Supporting Information), which is make for the generation of higher electrical conductivity and larger charge transfer (Figure 4c) due to the doping of N atoms into NiO matrix, thereby improving the catalytic performance of the pure NiO. Then, it can be seen from Figure 4d-4f, Figure S8 (Supporting Information) that lower Gibbs free energies of the rate-determining step is acquired for the N doped NiO electrocatalyst, thus reducing the theoretical overpotential (). For instance, the overpotential of 3N-doped NiO is calculated to be 0.53 V, smaller than that of the pristine NiO nanosheet of 1.11 V (Figure 4e).



Meanwhile, Figure S9 (Supporting Information) further illustrates that the (111) and (100) planes of the NiO show the same trend, These theoretical analyses again suggest that the OER performance of the electrode could be increased by the N doping to improve the electric conductivity and significantly increase the OER activity, consistent with the experimental results.

Figure 4. Density-of-states diagrams of NiO (a) and N-NiO (b) at the (110) surface. (c) Charge density of the 3N-doped NiO at the (110) surface. (d) Free energy diagram at U=0 V for the OER on the NiO (110) surface with 3N doping and without doping. (e) Overpotential changes with the increase in the N doping concentration at the NiO (110) surface. (f) Calculated model of the potential limiting step for 3N-NiO at the (110) surface.

In view of the good bifunctional elecrtocatalytic property, a simple Zn-air battery was further assembled to explore rechargeable and stability performance, where N-NiO as the air electrode (Figure 5a). A higher power density (112.30 mW/cm2) was achieved for the assembled aqueous Zn-air battery (Figure 5b). Besides, this Zn-air battery has a small charging-discharging voltage difference of 1.97 and 1.23 V at 5 and 10 mA/cm2,


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respectively (Figure 5c), and a large open-circuit potential of 1.44V (Figure 5d). Cycling stability tests at 5 mA/cm2 reveal that the initial voltage gap of the N-NiObased Zn–air battery can reach the value of 0.90 V. The cycling was stable for 160 h (240 cycles) without obvious change (the voltage was approximately 1.13 V), which demonstrates its better performance than that of the NiO-based battery (Figure 5e). In addition, after removing the mass of used Zn plate, the specific capacity is approximately 819 mAh/gZn at 5 mA/cm2, better than the pure NiO (Figure S10-S11, Supporting Information). These results show that this N-NiO-based rechargeable battery exhibits superior performances and recycling durability, outperforming the pristine-NiO-based and most reported bifunctional-electrocatalyst-based Zn–air battery (Table S3-S2, Supporting Information). In addition, two series Zn–air batteries generate a sufficiently high and stable open-circuit potential (2.80 V), which is easy to make a red LED light shine , as shown in Figure 5f, further confirming large potential of the Zn–air batteries.



Figure 5. Electrochemical activities of Zn-air cell. (a) A simple schematic conﬁguration of Zn–air cell, where the air electrode is N-NiO. (b) Polarization/power density and (c) Charge-discharge curves. (d) Open circuit plots of N-NiO and NiO based Zn-air batteries. (e) and (g) Charging and dischargeing cycling curves of Zn-air cell at 5 mA/cm2. (f) A red LED light glowing Photograph.

3. CONCLUSION In summary, the fabricated porous N-doped NiO material has good bifunctional electrocatalytic performances for both OER and ORR, because of the good conductivity and effective active surface area caused mainly by the doping of N atoms and porous structure. Further, the primary porous N-doped NiO-based Zn–air cell shows a high specific capacity (819 mAh/gzn), and long service lifetimes without significant decrease even after more than 160 h (960 cycles). These results can guide further studies on Zn– air batteries based on NiO-based bifunctional electrocatalysts.


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4. EXPERIMENTAL SECTION 4.1 Preparation of materials Preparation of porous NiO nanosheets: First, 0.005 mol NiCl2·6H2O was mixed with 0.01 mol of NaOH in a beaker with 60 ml of deionized water, followed by stirring for 2 h using a magneton. The hydrothermal reaction was then maintained for 12 h at a temperature of 130 °C. Subsequently, the obtained products were washed three times with ultrapure water and anhydrous ethanol. Finally, the precursor of Ni(OH)2 was calcined at 400 °C to obtain the porous NiO nanosheets. Preparation of porous N-NiO nanosheets: First, 0.2 g porous NiO was uniformly dispersed in 40 mL of ultrapure water; the obtained liquid was ultrasonicated after addition of 20 mL of ammonium hydroxide. This suspension liquid underwent another hydrothermal reaction for 24 h at a temperature of 180 ℃. Physical characterization: In order to characterize the morphology, structures, crystalline phases, state of surface elements of the samples, we carried out TEM (Tecnai ™ G2 F30), SEM (Hitachi S-4800), XRD (X’pert Pro Philips), XPS (Kratos AXIS UltraDLD), and Micrometrics ASAP 2020 V403 conventional means analyses. In addition, we measured transmission XANES and EXAFS spectra at room temperature at the XAFCA beamline of the Singapore Synchrotron Light Source; a nickle foil was used to calibrate the energy. The default window function was utilized to complete the Fourier transform 𝑥(𝑘), weighted by 𝑘 3 (e.g. Hanning function).44-45 4.2 Electrochemical Measurements



All of the electrocatalytic performances were measured using a standard threeelectrode system at a CHI760e work station, the detailed process can be seen in the Supporting Information. ASSOCIATED CONTENT Supporting Information Electrochemical measurements of OER/ORR and Zn-air battery, calculation methods, SEM images of NiO and N-NiO, TEM image of NiO, BET results, XANES spectra, Cdl curves of NiO and N-NiO, ORR polarization and corresponding Koutecky–Levich plots of NiO, DOS and free energy diagram of N-NiO and 2N-NiO at the (110) surface, free energy diagram NiO and 3N-NiO for (111) and (100) planes, discharge curves and the corresponding specific capacities of Zn-air battery on N-NiO and NiO, polarization and power density curves of NiO, table comparison results of XPS, Cdl and performances. The Supporting Information is available free of charge on the ACS Publications website at DOI: AUTHOR INFORMATION Corresponding Author [email protected] (D. Q. Gao), [email protected] (J. L. Wang)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS


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This work is supported by the National Natural Science Foundation of China (Grant No. 11474137, 11674143), lzujbky-2019-cd02, and Program for Changjiang Scholars and Innovative Research Team in University (IRT 16R35). The authors would like to acknowledge the XAFCA beamline for the XAFS measurements.

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Graphical abstract


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Bifunctional Electrocatalytic Activity of Nitrogen-Doped NiO

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