Surface Amorphization: A Simple and Effective Strategy toward

Aug 23, 2017 - In summary, surface introduction of amorphous Ni-Bi layer on NiO has been proven as an effective strategy toward greatly boosted water ...
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Surface Amorphization: A Simple and Effective Strategy toward Boosting the Electrocatalytic Activity for Alkaline Water Oxidation Rong Zhang,† Zao Wang,† Shuai Hao,† Ruixiang Ge,† Xiang Ren,† Fengli Qu,‡ Gu Du,§ Abdullah M. Asiri,∥ Baozhan Zheng,*,† and Xuping Sun*,† †

College of Chemistry, Sichuan University, Chengdu 610064, China College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, China § Chengdu Institute of Geology and Mineral Resources, Chengdu 610064, China ∥ Chemistry Department, King Abdulaziz University, Jeddah 21589, Saudi Arabia ‡

S Supporting Information *

ABSTRACT: It is urgent but still remains challenging to boost the alkaline water oxidation activity of transition metal oxide electrocatalysts for applications. In this work, we report our recent finding that surface introduction of an amorphous nickel−borate (Ni-Bi) layer on a nickel oxide (NiO) nanosheet array on carbon cloth (NiO/CC) can greatly enhance its electrochemical water oxidation activity under alkaline conditions. In a 1.0 M KOH solution, the resulting core−shell NiO@Ni-Bi/CC shows superior electrochemical catalytic activity of only 290 mV to drive 10 mA cm−2, 100 mV less than that for NiO/ CC. Meanwhile, this catalyst electrode shows great long-term durability for at least 25 h. The high activity can be ascribed to the Ni-Bi layer on NiO promoting NiOOH generation as the active species. This study provides us an abundant water oxidation catalyst in an alkaline water electrolysis device for highperformance, durable electrolytic hydrogen production. KEYWORDS: OER catalyst, Electrocatalytic activity, Alkaline water oxidation, Carbon cloth, Nickel−borate

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INTRODUCTION The increasing depletion of fossil fuels and growing environmental pressure have prompted the search for clean alternatives as energy carriers.1,2 Hydrogen is considered as a candidate because of its high efficient and zero-carbon emissions.3,4 Water splitting in alkaline solution is regarded as a promissing way for hydrogen generation.5 Water oxidation at the anode, where a four-electron transfer process is involved for covalent O−O bond formation, however, is sluggish and maintains a key bottleneck for efficient overall water splitting.6,7 Efficient water oxidation catalysts (WOCs) must be used to lower the large overpotential.8 As the most active WOCs, RuO2 and IrO2 suffer from low abundance and high price.9 As such, design and fabrication of earth-abundant alternatives is urgent for highperformance water oxidation electrocatalysis. Nickel (Ni) is a noteworthy transition element due to its electrocatalytic ability of water oxidation, and great progress is achieved in developing Ni-based electrocatalysts with high catalytic activity for the oxygen evolution reaction (OER).10−16 Nickel oxyhydroxide (NiOOH) is an active and stable electrocatalyst for alkaline water oxidation, and NiOOH deposition has been used to enhance the performance of WOCs like Ni-Co oxide, NiPS, and NiSe.17−19 Ni oxides20−23 and hydroxides24−27 have been widely studied as active WOCs for alkaline water oxidation thanks to their low toxicity, natural abundance, and good reaction potential, and amorphous © XXXX American Chemical Society

NiOOH is in situ electrochemically derived on the surface of these catalysts as the active species. We thus anticipate that an increase in thickness of the NiOOH layer could be effective to enhance the catalytic ability of such WOCs, which, however, remains unexplored so far. In this work, we demonstrate that introducing an amorphous nickel−borate (Ni-Bi) layer on a nickel oxide (NiO) nanosheet array on carbon cloth (NiO/CC) can greatly boost its electrocatalytic activity toward alkaline water oxidation. Here, carbon cloth was chosen as the substrate to integrate catalysts on account of its 3D, highly conductive, and flexible characteristics for application in technological devices.28 In a 1.0 M KOH solution, NiO@Ni-Bi/CC outperforms most reported Ni oxides and hydroxides catalysts, capable of driving 10 mA cm−2 at a low overpotential of 290 mV, which is 100 mV less than that of NiO/CC. Meanwhile, it shows great long-term durability for at least 25 h. This work would point out a new direction to developing superior transition metal oxides-based flexible catalysts toward electrochemical oxidation of water and other small molecules (such as alcohol,29 urea,30 and hydrazine31) for applications. Received: June 15, 2017 Revised: August 19, 2017 Published: August 23, 2017 A

DOI: 10.1021/acssuschemeng.7b01952 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION Figure 1a shows the X-ray diffraction (XRD) patterns. NiO/CC shows four peaks with 2θ values of 37.25°, 43.27°, 62.87°, and

Figure 2. (a) LSV curves for bare CC, RuO2/CC, NiO/CC, and NiO@Ni-Bi/CC. (b) Tafel plots of RuO2/CC, NiO/CC, and NiO@ Ni-Bi/CC. (c) Nyquist plots of NiO/CC and NiO@Ni-Bi/CC with an equivalent circuit (inset). (d) Durability test of NiO@Ni-Bi/CC using 2000 cyclic voltammetry cycles and long-term bulk electrolysis at 320 mV. Electrolyte: 1.0 M KOH solution. Figure 1. (a) XRD patterns for bare CC and catalysts on CC. (b,c) Typical SEM images of NiO/CC and NiO@Ni-Bi/CC, respectively. (d) Corresponding elemental mapping images for NiO@Ni-Bi/CC. HRTEM analysis of (e) NiO and (f) NiO@Ni-Bi. (g−i) XPS analyses for NiO@Ni-Bi.

catalyst electrode is also efficient in a 0.1 M KOH solution, with the demand of 360 mV to deliver 10 mA cm−2 (Figure S2). These overpotentials for NiO@Ni-Bi/CC are smaller than all the reported overpotentials for Ni oxides and hydroxides electrocatalysts except the α-Ni(OH)2 hollow sphere (Table S1).24 The reaction kinetics was further studied by Tafel plots. As shown in Figure 2b, a Tafel slope for RuO2/CC is as low as 48 mV dec−1. Notably, NiO@Ni-Bi/CC exhibits a Tafel value of only 71 mV dec−1. This value is much smaller compared with that of NiO/CC (102 mV dec−1), suggesting a faster reaction rate on NiO@Ni-Bi/CC.36,37 Electrochemical impedance spectroscopy (EIS) analyses are further presented in Figure 2c. The inset image is an equivalent circuit model, where Rs is the series resistance, Rct represents the charge transfer resistance, and CPE is the constant phase elements. As observed, compared with NiO/CC, NiO@Ni-Bi/CC has a much smaller semicircular diameter. The Rct (9.1 Ohm) of NiO@Ni-Bi/CC is significantly smaller compared with that for NiO/CC (28.5 Ohm), implying faster charge transfer on NiO@Ni-Bi/CC.38 The stability is of vital importance for the practicability of electrocatalysts. Therefore, we recorded the LSV curves of NiO@Ni-Bi/CC before and after continuous 2000 cyclic voltammetry scanning in the potential range of 1.4−1.8 V using a fast sweep rate of 100 mV s−1. The LSV curves before and after 2000 cycles overlay almost completely (Figure 2d), suggesting superior stability of NiO@Ni-Bi/CC. Furthermore, when operating the OER at a fixed overpotential of 320 mV, our catalyst electrode demonstrates strong longterm electrochemical stability for at least 25 h (Figure 2d). Moreover, NiO@Ni-Bi/CC still maintains its morphological integrity after the test (Figure S3), further demonstrating its robust nature. XPS analysis of NiO@Ni-Bi/CC after a 25 h stability test was also presented. The XPS spectrum in the B 1s region concludes the absence of the B element (Figure S4). As shown in Figure 3a, Ni 2p1/2 and Ni 2p3/2 regions show peaks at 874.1 and 856.4 eV with two satellites, respectively, implying the existence of Ni3+.39 Two strong peaks are apparent in the O 1s region. One peak at 529.4 eV suggests the existence of O2−, and another

75.41°, which corrrespond to cubic NiO (PDF #47-1049). NiO@Ni-Bi/CC also presents peaks characteristic of NiO. The morphology of catalysts was surveyed by scanning electron microscopy (SEM). As shown in Figure 1b, numerous NiO nanosheet arrays are uniformly covered on CC. Figure S1 further indicates that such nanosheets are about 1 μm in thickness. Note that NiO@Ni-Bi/CC keeps its morphology (Figure 1c). The corresponding energy-dispersive X-ray (EDX) analysis for NiO@Ni-Bi/CC is shown in Figure 1d, revealing that Ni, B, and O elements are evenly dispersed throughout the carbon fiber. Figure 1e presents the high-resolution transmission electron microscopy (HRTEM) image of NiO. Clearly, the sample gives lattice fringes with a lattice distance of 0.241 nm, which is assigned to the NiO(111) facet (Figure 1e). It is very interesting to have found that an ultrathin amorphous shell about 2 nm in thickness was formed on the NiO surface (Figure 1f). Figure 1g−i shows the X-ray photoelectron spectroscopy (XPS) analyses for NiO@Ni-Bi. The Ni 2p1/2 shows one peak at 872.9 eV, while the Ni 2p3/2 region shows a peak at 855.7 eV, implying the existence of Ni2+ or Ni3+ in NiO.32,33 An additional two peaks are assigned to shake-up satellites, which are arising from Ni2+ or Ni3+. In the O 1s region (Figure 1h), two peaks are apparent at 531.2 and 529.5 eV, along with a peak in the B 1s region at 192.0 eV (Figure 1i). The peaks at 192.0 and 531.2 eV correspond to the BEs for central oxygen and boron in borate species, respectively.34 An additional peak can be assigned to the O2− in NiO.35 To evaluate the electrocatalytic activity of catalysts for water oxidation, a three-electrode system was utilized in a 1.0 M KOH solution. Figure 2a presents the polarization curves from linear sweep voltammetry (LSV) tests. RuO2/CC has excellent water oxidation activity, while bare CC has poor performance. Of note, NiO@Ni-Bi/CC is also highly active for water oxidation with only 290 mV to afford 10 mA cm−2. This overpotential is 100 mV less than that needed by NiO/CC. Our B

DOI: 10.1021/acssuschemeng.7b01952 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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electrodes are presented in Figure S6c. Figure 4b shows the plots of TOF vs overpotential of post-OER NiO/CC and postOER NiO@Ni-Bi/CC, suggesting much higher TOFs for postOER NiO@Ni-Bi/CC when the overpotential is higher than 0.3 V.

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CONCLUSIONS In summary, surface introduction of amorphous Ni-Bi layer on NiO has been proven as an effective strategy toward greatly boosted water oxidation performance in basic media. The core−shell NiO@Ni-Bi nanoarray only requires overpotential as low as 290 mV to drive 10 mA cm−2. This overpotential is 100 mV less than that of NiO nanoarray. The NiO@Ni-Bi/CC catalyst electrode also shows great long-term durability in 1.0 M KOH solution. The enhanced water oxidation activity can be explained that the effective promotion of active species generation by amorphous Ni-Bi layer. This study would offer us a promising earth-abundant water oxidation electrocatalyst toward alkaline water oxidation. Moreover, it points out a new direction to developing superior transition metal oxides-based catalysts for applications.

Figure 3. (a,b) XPS analysis of NiO@Ni-Bi after stability tests. (c) Raman spectrum for NiO@Ni-Bi after OER electrolysis. HRTEM analysis of (d) NiO and (e) NiO@Ni-Bi after OER electrolysis.

peak at 531.1 eV corresponds to OH− (Figure 3b).40,41 The Raman spectrum shows two characteristic peaks of NiOOH at 475 and 554 cm−1 (Figure 3c), further confirming that NiOOH was generated as the active phase for OER.42 The HRTEM image for NiO after the OER test indicates that the generated NiOOH shell is amorphous and about 2 nm, as shown in Figure 3d. It is worthwhile mentioning that the NiOOH layer derived from NiO@Ni-Bi, however, possesses a much larger thickness of 4−5 nm (Figure 3e). All these observations strongly support that the introduction of the Ni-Bi layer on NiO favors the production of more active phases for superior OER electrocatalysis in basic media. To further understand the superior catalytic activity of NiO@Ni-Bi/CC after OER electrolysis, the electric doublelayer capacitances of both electrodes were tested, which can be utilized to estimate the active surface areas.43 The cyclic voltammograms (CVs) were obtained in the potential window from 0.175 to 0.275 V (Figure S5). The capacitances are 38.8 and 54.8 mF cm−2 for post-OER NiO/CC and post-OER NiO@Ni-Bi/CC (Figure 4a), respectively, suggesting that post-

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01952. Experimental section, LSV curve, SEM image, XPS spectrum, CVs, oxidation peak current vs scan rate plots, and Table S1. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.Z.). *E-mail: [email protected] (X.S.). ORCID

Abdullah M. Asiri: 0000-0001-7905-3209 Xuping Sun: 0000-0001-5034-1135 Author Contributions

R.Z. and Z.W. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21575137). We also appreciate Hui Wang from the Analytical & Testing Center of Sichuan University for her help with SEM characterization.

Figure 4. (a) Electric double-layer capacitances of NiO/CC and NiO@Ni-Bi/CC after OER tests. (b) Calculated TOF of NiO/CC and NiO@Ni-Bi/CC after OER tests. Electrolyte: 1.0 M KOH solution.

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OER NiO@Ni-Bi/CC owns higher surface roughness and thereby more exposed active sites.44 The assessment of TOFs makes it possible to compare the intrinsic activities of postOER NiO@Ni-Bi/CC with post-OER NiO/CC, and we quantified the surface concentration of active sites by electrochemistry (see SI for details).45 Figures S6a and b show the CVs for NiO/CC and NiO@Ni-Bi/CC after OER electrolysis, respectively, and the corresponding line relationships of the oxidation peak current and scan rate for both C

DOI: 10.1021/acssuschemeng.7b01952 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acssuschemeng.7b01952 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX