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An efficient and stable NiCo2O4/VN nanoparticle catalyst for electrochemical water oxidation Zhilin Zheng, Xuan Du, Yi Wang, Chang Ming Li, and Tao Qi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01530 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 4, 2018

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ACS Sustainable Chemistry & Engineering

An efficient and stable NiCo2O4/VN nanoparticle catalyst for electrochemical water oxidation Zhilin Zhenga#, Xuan Dua#, Yi Wanga,*, Chang Ming Lib, Tao Qia,* a

National Engineering Laboratory for Hydrometallurgical Cleaner Production

Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China b

Institute for Clean Energy & Advanced Materials, Southwest University, Chongqing

400715, China #These authors contributed equally. *

Corresponding authors: Y. Wang, Fax: (+86) 10 82544848-802, Tel: (+86) 10

82544967, E-mail: [email protected] ; T. Qi, E-mail: [email protected]

Abstract The oxygen evolution reaction (OER) is a critical step in water splitting to produce hydrogen. Therefore, the study of corresponding electrocatalysts for OER is of vital importance. However, the most used carbon-based supports suffer from corrosion which leads to substandard performance. In this research, a series of VN are synthesized as supports for NiCo2O4. Compared with the pristine NiCo2O4 and NiCo2O4 supported on carbon black (NiCo2O4/C), the NiCo2O4 supported on the VN prepared in 800 ℃ (NiCo2O4/VN800) as a catalyst exhibits the best OER catalytic activity and stability. According to X-ray photoelectron spectroscopy (XPS) and

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electrochemical characterizations,

the

good

electrocatalytic

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performance

of

NiCo2O4/VN800 is ascribed to the high conductivity and good corrosion resistance of VN as well as the higher contents of Co2+ and Ni3+ on the catalyst surface owing to the interaction of VN and NiCo2O4.

Keywords Oxygen evolution reaction; NiCo2O4; VN; Electrocatalysts; Support

Introduction Owing to the energy crisis and increasing awareness of environmental problems, more and more attention is paid to green and sustainable energy [1, 2]. Hydrogen is one of the most promising clean energy as an alternative to fossil energy for its high energy density and carbon-free emission [3-5]. To produce highly pure hydrogen, water electrolysis is an important approach with high efficiency, low cost, and environmental benignity [6]. The electrolysis process in alkaline medium consists of two parts: the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER) [7]. By comparison, the OER is inherently more complex and possesses more sluggish kinetics, which leads to a larger overpotential. The large overpotential is a major cause of the overall high energy consumption, thus making the OER a major bottleneck [8]. Extensive efforts have been devoted to searching for OER electrocatalysts with high quality and low cost. To date, the noble-metal electrocatalysts, such as IrO2 and

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RuO2 are considered as the benchmark OER catalysts [9-12]. However, the industrial applications are severely hampered by their low abundance and high price. Accordingly, much attention has been paid to alternative OER electrocatalysts made up of non-noble metal-based materials, such as Ni and Co, owing to their theoretically high catalytic activity, earth-abundance and low cost [13-19]. Due to containing the redox couples of Co3+/Co2+ and Ni3+/Ni2+, which are treated as active centers for OER, NiCo2O4 exhibits a higher catalytic activity than Co3O4 and NiO. Intensive studies have been mostly conducted on Ni-Co oxides by modulating the structures and morphology [20-23], as well as varying the compositions [17, 24, 25]. However, Ni-Co based oxides suffer from the intrinsically inferior electrical conductivity, which dramatically hinders the electron transfer process, leading to the limited overall OER efficiency. The conductive supports are then introduced to settle this problem. Frequently, carbon-supported nanostructures have been used as the electrocatalysts for the OER [15, 26-28]. Carbon black is the most widely used carbon material as substrates for various electrochemical applications by virtue of its relatively high specific surface area and conductivity, together with low density [29-32]. However, carbon-based materials would undergo severe corrosion at room temperature when the positive potentials is above +1.2 V vs RHE, which will accelerate secondary degradation process such as agglomeration and particles detachment [33, 34]. This shortcoming makes carbon-based materials rather unsuitable as a support for OER catalysts. Plenty

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of attempts have been made to improve the conductivity and stability of the electrocatalyst supports [35]. In our previous work [36-38], it was found that a Ti4O7 support can improve the catalytic activity and stability of the electrocatalyst for formic acid oxidation reaction and OER. Additionally, anchoring oxides on an Au support is another strategy to increase the activity and stability of OER catalysts. Liu et al [39] fabricated Au/NiCo2O4 nanoarrays by a facile hydrothermal method, which exhibited excellent OER activity as well as good stability.

Besides improved

conductivity, the additional synergies between the support and the active ingredient also contribute to the much better electrochemical performance. However, it is undoubtedly very expensive for a catalyst supported on gold. Recently, owing to a high electronic conductivity and the property of corrosion resistance, vanadium nitride (VN) has been explored as materials of supercapacitors and lithium-ion batteries and displayed excellent electrochemical performance [40, 41]. These advantageous properties of the VN make it a promising support for OER electrocatalysts. However, to the best of our knowledge, the corresponding research with VN as a support for OER catalyst is nearly unexplored. In this work, several NiCo2O4 catalysts supported on VN were prepared and their catalytic performance towards OER was explored.

Experimental Chemicals and materials

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The chemicals used in this work were KVO3, NH3, carbon black (Vulcan XC-72), Ni(NO3)2 •6H2O, Co(NO3)2 •6H2O, NH3 •H2O (25-28 wt%), Polyvinylpyrrolidone (PVP, 99%), KOH, Nafion solution. Synthesis of VN Ammonolysis of KVO3 was carried out in a 1 in. diameter stainless steel flow tube with an ammonia flow rate of 200 cm3.min-1 for 8 h; the reaction temperature was 700, 800 and 1000 ℃, respectively. The products were then cooled down to room temperature with an ammonia flow rate of 200 cm3.min-1. After washing the products in water and drying at 100 ℃ overnight, the final products were gained, which were denoted as VN700, VN800 and VN1000 according to the temperature of ammonolysis, respectively. Synthesis of NiCo2O4 on carbon black and VN, respectively Firstly, a certain amount of Co(NO3)2•6H2O and Ni(NO3)2•6H2O were dissolved in deionized water to ensure the molar ratio of Co/Ni as 2:1. Then, 0.35 g PVP was added. Secondly, 0.21 g of Vulcan XC-72 or as-prepared VN powder was uniformly dispersed in the aqueous solution with the aid of ultrasonication at 400 W for 0.5 h. A sufficient amount of NH3•H2O (5%) was added dropwise in the next step. About 8 hours later, precipitate was obtained through filtration, and washed with deionized water and ethanol. Finally, the product was obtained by annealing the precipitate in air at 200 ℃for 3 h. The catalysts obtained are labelled as NiCo2O4/C and NiCo2O4/VN. Physicochemical characterization

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X-ray powder diffraction (XRD, Smart lab (9 kW), Cu Kα) was used to study the crystal structure of the prepared catalysts. Field emission scanning electron microscopy (FE-SEM, Quanta 400) was used for the detection of surface morphology. The electrical conductivities were measured by a standard four-point-probe resistivity measurement system (RTS-9, Guangzhou, China). The chemical-state analysis of the samples was performed through X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). Three different sections of each sample are picked up to conduct the XPS characterization. Nitrogen sorption measurements were conducted at – 196 ℃ using a Micromeritics ASAP 2020 Analyzer. Prior to analysis, the sample was degassed under vacuum at 100 °C for 24 h. The specific surface area (SSA) was calculated using the multipoint Brunauer–Emmett–Teller (BET) method. Electrochemical measurements Electrochemical measurements were conducted at 298 K through an electrochemical workstation system (CHI760D, Chenhua, Shanghai). Pt foil was used as the counter electrode while saturated calomel electrode (SCE) was used as the reference electrode. The working electrode was prepared by dropping 5 µL of the electrocatalyst ink onto glassy carbon electrode (GCE), followed by the using of 1 µL Nafion solution to fix the electrocatalysts. Polarization curves (linear sweep voltammetry, LSV) were obtained by sweeping the potential from 0 to 0.6 V vs. SCE. Cyclic voltammetry (CV) was carried out at a scan rate of 5 mV/s in the range of 0 to 0.5 V vs. SCE. The working electrodes were scanned several times until the signals

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stabilized, and then the data were collected. Electrochemical impedance spectroscopy (EIS) was measured in the frequency range of 0.10 MHz to 0.01 Hz at 0.5 V. Chronoamperometry (CA) was carried out at 0.56 V for 25,000 seconds to test the stability. In this process, a rotating disk electrodes (RDE) (Pine Research Inc.) at a speed of 2400 rpm is used. The electrolyte was 1 M KOH solution. To measure the electrochemical surface area (ECSA), CV measurements were conducted in the range of 0.1-0.2 V vs. SCE at different scan rates: 0.025, 0.05, 0.1, 0.2, 0.4 and 0.8 V s−1. The working electrode was held at each potential vertex for 10 s before beginning the next sweep.

Results and discussion Figure 1(a) shows XRD patterns of VN700, VN800 and VN1000. Each sample exhibits a set of characteristic peaks of VN at 37.7, 43.8, 63.7, 76.4 and 80.5 ◦, which represent the (1 1 1), (200), (2 2 0), (3 1 1) and (2 2 2) planes of cubic phase VN (JCPDS card no. 01-073-2038), respectively. In addition, no other peaks are observed, which demonstrates that the samples are pure VN. The difference among the patterns of three samples is mainly reflected in the intensity of the diffraction peaks. With the increase of calcination temperature, the intensity of (1 1 1), (2 2 0), (3 1 1) and (2 2 2) lattice plane diffraction peaks increase, which implies that the calcination temperature can improve the crystallinity. The three VN samples were used as the supports of NiCo2O4 catalyst, and their catalytic activity for OER was evaluated by preliminary polarization curves shown in

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Figure 1(b). It can be seen that the onset potential follows the order that NiCo2O4/VN800 ＜ NiCo2O4/VN1000 ＜ NiCo2O4/VN700.

Meanwhile,

the

overpotential of NiCo2O4/VN800 is the lowest at the same current density. Thus, the catalyst with VN800 as support shows the best electrocatalytic activity. However, the SSAs of the three VN samples are 1.73, 2.32 and 2.38 m2/g for VN700, VN800 and VN1000, respectively, which are much smaller than that of carbon black (254 m2/g) and seem to not be beneficial for the improvement of catalytic performance. The conductivities of the support materials were measured in this work as well, and the values are 30.6, 346, 1250 and 676 S cm-1 for carbon black, VN700, VN800 and VN1000, respectively. VN800 shows the highest conductivity, which may explain the difference of catalytic activity. Therefore, in the remaining session, attention is focused on the NiCo2O4/VN800 catalyst for further research. The XRD patterns of NiCo2O4 catalysts without a support and with carbon black and VN800 as a support, respectively, are presented in Figure 1(c). The main diffraction peaks are in accordance with the characteristic peaks of the spinel NiCo2O4 phase (JCPDS card no. 20-0781). The other peaks of NiCo2O4/VN800 and NiCo2O4/C are in accordance with the standard peaks of the VN and C, respectively. In addition, the SEM images of NiCo2O4/C and NiCo2O4/VN800 are given in Figure 1(d) and (e). It is found that NiCo2O4/C is constructed by flakes while NiCo2O4/VN800 is an aggregate of particles.

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Figure 1 (a) XRD patterns of VN700, VN800 and VN1000, (b) polarization curves of NiCo2O4/VN700, NiCo2O4/VN800 and NiCo2O4/VN1000, (c) XRD patterns of NiCo2O4, NiCo2O4/C and NiCo2O4/VN800，(d, e) SEM images of NiCo2O4/C and NiCo2O4/ VN800. XPS measurements were performed to gain insight into the surface element valence states. As shown in Figure 2, the Co 2p and Ni 2p emission spectra of NiCo2O4/VN800 and NiCo2O4/C are best matched with two spin-orbit doublets and two shakeup satellites (identified as “Sat.”). Through the fitting data, the following information can be acquired: (1) The ratio of Co3+/Co2+ of NiCo2O4/VN800 and NiCo2O4/C equals 0.72 and 0.91, respectively. (2) While the ratio of Ni3+/Ni2+ of them 9



equals 1.36 and 1.25, respectively. To ensure the accuracy, three different sections of each sample are picked up to conduct the XPS characterization (the second and third test results are shown in Section 2 of Supporting Information). The average results are that the ratio of Co3+/Co2+ of NiCo2O4/VN800 and NiCo2O4/C equals 0.75 and 0.94, respectively, and the ratio of Ni3+/Ni2+ of them equals 1.35 and 1.21, respectively. The related ratios still obey the sequence.

Figure 2 XPS spectra for NiCo2O4/C and NiCo2O4/VN800.

Figure 3(a) displays the CV curves of NiCo2O4, NiCo2O4/C and NiCo2O4/VN800. The area sizes covered by redox couples of CV curves obey the order that NiCo2O4/VN800 ≈ NiCo2O4/C >> NiCo2O4. As reported by Chen et al. [42], the increased peak current density as well as enclosed area indicate larger electrochemically active surface area. To get more details, precise experiments have

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been conducted according to Mccrory’s work [43]. From Figure S1, the ECSAs of the three samples (NiCo2O4, NiCo2O4/C and NiCo2O4/VN800) are 9.75, 17.63 and 15.00 cm2, respectively. Meanwhile, because the OER catalytic activity of the supports such as carbon black and VN can be negligible, the contribution of the supports to the ECSAs should be excluded. It is obtained from Figure S1 that the ECSAs of C and VN800 are 3.75 and 1.38 cm2, respectively. Thus, the ECSAs of NiCo2O4/C and NiCo2O4/VN800 eliminating the contribution of the supports are 13.88 and 13.62 cm2, respectively, which are very close to each other. This also indicates that there are some positive factors that are beneficial to the electrocatalytic process of NiCo2O4/VN800 although the SSA of VN800 is much smaller than that of carbon black. One pair of broad and strong redox peaks can be observed in the CV curves, which mainly originates from the charge transfer processes of solid state redox couples of Co3+/Co2+ (Co3O4/CoOOH) and Ni3+/Ni2+ (NiO/NiOOH) [44, 45]. Although two redox couples exist on the surface of the NiCo2O4 according to the XPS results, only one pair of redox peaks is observed, which could be ascribed to the very close redox potentials of nickel and cobalt ions. Besides, different from the mechanical mixing of nickel oxide and cobalt oxide, nickel and cobalt in the NiCo2O4 are mixed in atom scale. This makes it easily to overlay and difficult to separate in the CV curves. Meanwhile, NiCo2O4/VN800 exhibits the largest redox couples with anodic peak shifted in the negative direction, which may result from the different

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coefficient between the oxidation of Co2+ /Co3+ and Ni2+/ Ni3+ on the surface [46]. XPS result verifies that NiCo2O4/VN800 and NiCo2O4/C have the different surface Co cations and Ni cations distribution. In other words, there are more Co2+ and Ni3+ on the surface of the former catalyst. Furthermore, the content of cobalt is more than that of nickel in the NiCo2O4. Thus, more contents of low valence cations result in the negative shift of the peak. As shown in Figure 3(b), it can be concluded that the VN as a support can notably lower the onset potential and the NiCo2O4/VN800 catalyst possesses the lowest onset potential which is about 290 mV. Hence, NiCo2O4/VN800 has the easiest access to arise the OER process. In addition, the overpotential for NiCo2O4, NiCo2O4/C and NiCo2O4/VN800 to reach the current density of 10 mA/cm2 is 426, 418 and 385 mV, respectively, which indicates that NiCo2O4/VN800 possesses the best catalytic activity among these catalysts. To gain more insight into the OER activity, Tafel plots were derived from LSV. It was obtained that Tafel slope of NiCo2O4/VN800, NiCo2O4/C and NiCo2O4 is 75.7, 69.4 and 77.7 mV/dec, respectively. Nyquist plots of the EIS obtained at 0.5 V are shown in Figure 3(c). All the three samples showed almost the same solution resistance (Ru), which can be neglected while compared with the large charge transfer resistance (Rct). Besides, it can be found that the kinetic processes of the electrodes were under the control of electrochemical reaction. This ensures the focus on the promoting ability of the

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catalysts for charge transfer. Furthermore, it is well known that the smaller semicircle at the low frequency region reflects the faster charge transfer process in electrodes, namely the smaller Rct. Therefore, NiCo2O4/VN800 displays much smaller Rct than NiCo2O4/C and NiCo2O4, which may be partly ascribed to the higher conductivity of VN800. Thus, the growth of NiCo2O4 on the VN800 substrate promotes the charge transfer process, leading to a superior electrocatalytic performance.

Figure 3 (a) CV curves, (b) polarization curves, (c) Nyquist plots of NiCo2O4, NiCo2O4/C and NiCo2O4/VN800.

In addition, it is showed in the XPS results and confirmed by the CV analyses that the addition of the VN800 as a support could lead to higher contents of Co2+ and Ni3+ on the surface of the catalyst. Wang et al [47] synthesized reduced Co3O4 nanowires (NWs) in which a part of Co3+ was reduced to Co2+. Compared with pristine Co3O4

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NWs, the reduced Co3O4 NWs exhibited better electrocatalytic performance for OER. Therefore, the higher amount of Co2+ in the NiCo2O4/VN800 could lead to the better electrocatalytic activity [48]. Xia et al [49] prepared an OER catalyst by using three-dimensional graphene as a conductive substrate for NiCo2O4, followed by the addition of a small amount of Au. The addition of Au is conducive to the formation of Ni3+, thereby enhancing the activity of the catalyst. Hence, it is suggested that the better catalytic activity of the NiCo2O4/VN800 also results from the higher amount of Ni3+. In short, the higher amount of Co2+ and Ni3+ on the surface of NiCo2O4/VN800 could also accelerate the charge transfer process and lead to the excellent catalytic activity. The different element valence states of NiCo2O4/VN800 on the surface are ascribed to the electrostatic interaction between NiCo2O4 and VN. Because the valence electron configuration of V is 3d34s2 and the valence state of V in VN is +3, V in VN has the tendency to lose electron. Meanwhile, nickel and cobalt cations in the pristine NiCo2O4 possess unoccupied orbits. The possible mechanism is explained as follows: (1) Electrons of VN may have a trend to transfer to the surface of NiCo2O4. (2) Co3+ may be much easier affected than Ni2+ by this trend because of the stronger oxidizing ability of Co3+, resulting in the increase of Co2+ content. (3) To maintain the balance of charge, the content of Ni3+ rises accordingly. The possible mechanism schematic diagram is presented in Figure 4.

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Figure 4 The possible mechanism schematic diagram for the OER catalyzed by NiCo2O4/VN800.

CA detects the electrochemical stability of the catalyst. As shown in Figure 5, after testing for 25,000 seconds, NiCo2O4/VN800 exhibits a slight current attenuation of 37%, whereas NiCo2O4/C and pure NiCo2O4 display a larger current attenuation of 67% and 83%, respectively. The good stability of NiCo2O4/VN800 may be the result of high stability of VN in this environment, which would prevent the particles of NiCo2O4 from being aggregated. Therefore, it can be concluded that NiCo2O4/VN800 owns not only an excellent catalytic activity but also better electrochemical stability. The better stability may be attributed to the strong corrosion resistance of VN.

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Figure 5 CA data of NiCo2O4, NiCo2O4/C and NiCo2O4/VN800

Conclusions A series of VN are attained through ammonolysis of KVO3 at different annealing temperatures, and then these materials are used as supports for NiCo2O4. According to the catalytic activity measurement results for OER, it was found that the NiCo2O4 supported on the VN prepared in 800 ℃ (NiCo2O4/VN800) possesses the best catalytic activity. Afterwards, the catalytic performance of NiCo2O4/VN800 for OER was compared with that of NiCo2O4/C and NiCo2O4, and NiCo2O4/VN800 still exhibits the best catalytic activity and stability. The superior catalytic performance can be attributed to the following factors: (1) the highest conductivity of the VN800 support benefits the charge transfer process, which is implied by the EIS analyses; (2) the higher contents of Co2+ and Ni3+ caused by the interaction between the VN

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support and NiCo2O4 on the surface, which is demonstrated by the XPS analyses, also accelerate the charge transfer process; (3) the strong corrosion resistance of VN. In addition, the poor physical properties of VN, such as large density, imporosity, low surface area, leave room for further improvement.

Supporting Information. Method used to measure electrochemical active surface area is displayed in detail. XPS spectra in two different sections of NiCo2O4/C and NiCo2O4/VN800 are also included.

Acknowledgments The authors are grateful for the financial support by Key Research Program of Frontier Sciences of Chinese Academy of Sciences (Grant No. QYZDJ-SSW-JSC021), Chinese National Programs for High Technology Research and Development (2014AA06A513), as well as by the 973 Program (Grant No. 2015CB251303).

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Figure 1 (a) XRD patterns of VN700, VN800 and VN1000, (b) polarization curves of NiCo2O4/VN700, NiCo2O4/VN800 and NiCo2O4/VN1000, (c) XRD patterns of NiCo2O4, NiCo2O4/C and NiCo2O4/VN800，(d, e) SEM images of NiCo2O4/C and NiCo2O4/ VN800. 538x466mm (144 x 144 DPI)


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Figure 2 XPS spectra for NiCo2O4/C and NiCo2O4/VN800. 1041x788mm (144 x 144 DPI)



Figure 3 (a) CV curves, (b) polarization curves, (c) Nyquist plots of NiCo2O4, NiCo2O4/C and NiCo2O4/VN800. 249x181mm (144 x 144 DPI)


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Figure 4 The possible mechanism schematic diagram for the OER catalyzed by NiCo2O4/VN800. 186x125mm (150 x 150 DPI)



Figure 5 CA data of NiCo2O4, NiCo2O4/C and NiCo2O4/VN800 223x168mm (300 x 300 DPI)


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TABLE OF CONTENTS (TOC) GRAPHIC

Synopsis: VN as support facilitates charge transfer as well as modulates the surface properties of NiCo2O4, improving the water splitting performance for sustainable hydrogen production.


VN nanoparticle catalyst for

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