TiO2 nanotube arrays for enhanced electrocatalytic activity towards hydrogen evolution

Accepted Manuscript Fabrication of sandwich structured C/NiO/TiO2 nanotube arrays for enhanced electrocatalytic activity towards hydrogen evolution

Jun Yang, Lin Cheng, Lingling Wan, Jiabao Yan, Rongsheng Chen, Hongwei Ni PII: DOI: Reference:

S1388-2481(18)30271-6 doi:10.1016/j.elecom.2018.10.018 ELECOM 6322

To appear in:

Electrochemistry Communications

Received date: Revised date: Accepted date:

30 August 2018 10 October 2018 22 October 2018

Please cite this article as: Jun Yang, Lin Cheng, Lingling Wan, Jiabao Yan, Rongsheng Chen, Hongwei Ni , Fabrication of sandwich structured C/NiO/TiO2 nanotube arrays for enhanced electrocatalytic activity towards hydrogen evolution. Elecom (2018), doi:10.1016/j.elecom.2018.10.018

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ACCEPTED MANUSCRIPT

Fabrication of Sandwich Structured C/NiO/TiO2 Nanotube Arrays for

Enhanced

Electrocatalytic

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towards

Hydrogen

Evolution Jun Yang, Lin Cheng, Lingling Wan, Jiabao Yan, Rongsheng Chen,* and Hongwei Ni

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The State Key Laboratory of Refractories and Metallurgy, Institute of Advanced Materials and Nanotechnology, School of Chemistry and Chemical Engineering, Wuhan University of Science

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and Technology, Wuhan 430081, China. *Corresponding author.

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E-mail address: [email protected] (R.S. Chen)

ACCEPTED MANUSCRIPT Abstract A sandwich structured C/NiO/TiO2 nanotube arrays (C/NiO/TiO2 NTAs) were presented for highly efficient water splitting. The NiO nanoparticles were deposited on the inner wall of anodized TiO2 NTAs, followed by coating a carbon nanofilm via carbonization of a polydopamine membrane. The C/NiO/TiO2 NTAs exhibit a superior HER activity to most earth-abundant electrocatalysts, with an overpotential of 86 mV at 10 mA cm-2, a Tafel slope value of 67.1 mV dec-1 and amost no degradation of HER activity after successive operation for 24h.

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Keywords: Hydrogen evolution reaction; Electrocatalytic activity; Sandwich nanostructure; TiO2 nanotube arrays; Polydopamine; One-dimensional nanostructure

ACCEPTED MANUSCRIPT 1. Introduction Renewable energy from solar, wind, ocean tides, and other sustainable sources has been extensively exploited to gradually replace fossil fuels, which are in fast depletion and have produced severe environmental problems [1, 2]. However, most renewable energy tends to be intermittent and unpredictable. The investigation of efficient energy storage and conversion pathway has received intensive attention in the past few years [3-7]. Electrochemical water splitting is the most promising technology

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to make use of sustainable energy sources [8-11]. However, hydrogen evolution reaction (HER) usually shows high overpotential that will consume much more energy

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than thermodynamical value. Precious metal compounds made from Pt, Ir or Ru exhibit high electrocatalytic activity towards HER [12-14]. But their low abundance

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and high cost limit the large-scale applications. Various catalysts from earth-abundant metals such as Fe, Co, Ni, Mn, Mo and their hydroxides, selenides, sulfides,

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phosphides, oxides, mixed-metal alloys and so on [15-20], have been widely explored for HER.

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The electrode to conduct HER often suffers from poor adhesion of the nanostructured electrocatalysts to the supporting materials [21-23]. Moreover, many electrocatalysts made from earth-abundant elements are semiconductors that are

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naturally of high resistance for electron transfer [24-25]. These unfavourable phenomena will result in significant loss of catalytic activity and stability over time in

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practical applications [26-27]. Herein, we presented a sandwich structured C/NiO/TiO 2 nanotube arrays (C/NiO/TiO 2 NTAs) for high efficient water splitting. The NiO species were deposited on the inner wall of anodized TiO 2 NTAs by hydrothermal

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method, then coated by a carbon nanofilm from carbonization of the polydopamine membrane. This configuration provides not only a large specific area that are essential

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for electrocatalytic performance, but also a conductive 1D nanostructure to facilitate electron transfer. Furthermore, the NiO species wrapped between the inner wall of TiO2 NTAs and a carbon nanofilm are favourable for the stability of the working electrode in practical applications.

2. Experimental All chemicals were of analytical grade and were used without further purification. Titanium foil (Alfa Aesar, 0.89 mm in thickness, 99.7%) was sliced into pieces with an area of 10×10 mm2. After mechanical polishing, the Ti foil was ultrasonically rinsed in acetone, ethanol and distilled water sequentially. The foil was then immersed in a solution containing

ACCEPTED MANUSCRIPT H2O, HF, and HNO3 with a volume ratio of 5:1:4 for 5 min to remove the surface native oxide layer. Anodization was carried out at 25 ºC with a direct current power supply (IT6152, ITECH, Nanjing, China). A graphite foil and the Ti foil served as the cathode and anode, respectively. Anodization was performed at 60 V for 30 min in ethylene glycol solution containing 0.5 wt% NH4F, 5 vol% CH3OH and 5 vol% distilled water. After anodization, the foil was rinsed with distilled water and was annealed in a tube furnace at 450 °C for 3h in air. The annealed foil was immersed in nickel acetate solution and placed in an oil bath at 90 °C

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with continuous stirring. The foil was then put in a 1 mg/mL dopamine hydrochloride (99%, Alfa Aesar) solution prepared by 10 mM Tris-HCl buffer at pH 8.5. The product was rinsed

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by distilled water and annealed at 550 °C in a tube furnace for 3h under nitrogen flow. The morphology of the product was examined by a field emission scanning electron

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microscope (SEM, Nova 400 NanoSEM, FEI, USA) at an accelerating voltage of 20 kV and a transmission electron microscope (TEM, JEM-2100 UHR STEM/EDS, JEOL, Japan) at an

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accelerating voltage of 200 kV. X-ray diffraction (XRD) pattern was recorded with a XRD-7000S diffractometer (Shimadzu). Raman spectra were obtained from a LabRam HR

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raman spectrometer (Horiba JobinYvon, France) with an excitation wavelength of 514 nm. X-ray photoelectron spectra (XPS) were collected by an ESCALAB 250xi Spectroscopy (ThermoFisher, USA). Thermogravimetric analysis (TGA) was conducted by a STA 449c

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analyzer (NETZSCH, Germany) with a temperature variation range of 30-700 °C in nitrogen. An inductively coupled plasma emission spectrometer (ICP-AES, IRIS Advantage ER/S,

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Thermo Elemental, USA) was employed to determine the concentration of dissolved Ni species in the solution. Electrochemical test was conducted with a CHI 660E potentiostat (CH Instruments, Shanghai, China) with a conventional three-electrode system, which contained

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the as-prepared product as the working electrode, an Ag/AgCl electrode as the reference electrode and a platinum foil as the counter electrode. The commercial product of 20 wt %

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Pt-C was prepared for benchmark control. The reference electrode was calibrated to the reversible hydrogen electrode (RHE) scale in all measurements as follows: E(RHE) = E(Ag/AgCl) + 0.059pH + 0.21V in 0.50 M H2SO4.

3. Results and discussion From the top view (Fig. 1A) and side view (inset of Fig. 1A) SEM images, highly ordered TiO2 NTAs are uniformly produced on the surface of Ti foil after electrochemical anodization. The nanotubes show an inner diameter of about 80 nm and length about 5 μm. After hydrothermal treatment in nickel acetate solution, NiO nanoparticles with diameters of ~20 nm decorated on the open ends and the inner walls of the nanotubes are observed (Fig.

ACCEPTED MANUSCRIPT 1B). Polydopamine coating is a versatile carbon precursor to produce highly electroactive carbon film with precisely controlled thickness. As illustrated in Fig. 1C, the highly ordered NTAs morphology is still retained, leaving the NiO nanoparticles wrapped between the carbon nanofilm and TiO2 NTAs. Microstructure of the C/NiO/TiO2 NTAs prepared at the optimized condition was further investigated. TEM image of an individual C/NiO/TiO2 nanotube is shown in Fig. 1D. The elemental mapping in Fig. 1E suggests that Ti, O, Ni and C are homogeneously distributed in the nanotube. The selected mapping area was the same as in

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Figure 1D. The HR-TEM image in Fig. 1F shows two typical interlayer distance, 0.242 nm corresponding to the (111) plane of NiO and 0.357 nm corresponding to the (101) plane of

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TiO2. The NiO crystal is partially wrapped by the amorphous C species formed at 550 °C. To estimate the amount of carbon in C/NiO/TiO2 materials, the carbonization process of

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the polydopamine coated NiO/TiO2 nanotube arrays was examined by TGA in nitrogen (Fig. 2A). The weight loss is proportional to the amount of polydopamine coating and the remained

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carbon species after carbonization. Therefore, the amount of carbon increases accordingly with the increase of immersion time of NiO/TiO2 NTAs in dopamine solution. Microstructure

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of the prepared materials was examined by XRD and Raman spectra. The main peaks in XRD patterns from TiO2-NTAs (Fig. 2B) should be indexed to anatase TiO2 and titanium metal substrate [28]. The diffraction peaks associated with nickel oxide are observed from NiO/TiO2

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NTAs and C/NiO/TiO2 NTAs, suggesting the decoration of NiO species onto TiO2 NTAs after hydrothermal reaction [29]. Two shifts at 1358 and 1601 cm−1 arised from Raman spectra of

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the C/NiO/TiO2 NTAs should be ascribed to the D and G bands of carbon (Fig. 2C). The other shifts at 148, 392, 522, and 640 cm−1 are indexed to the anatase TiO2, indicating the formation of the nanocomposite C/NiO/TiO2 NTAs. The XPS spectra further confirm the existence of

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the Ti, O, Ni and C species in C/NiO/TiO2 NTAs (Fig. 2D). To demonstrate the stability of the C/NiO/TiO2 NTAs in HER, continuous linear sweep voltammetry (LSV) at a scan rate of 1

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mV s−1 for 1000 cycles was conducted. No significant changes can be observed between the original C/NiO/TiO2 NTAs and the 1000 cycles LSV treated C/NiO/TiO2 NTAs, indicating the high robustness and stability of the prepared catalyst. The HER activity of the prepared products were evaluated by LSV curves at a scan rate of 1 mV/s with current density normalized by geometric surface area. For comparison, commercial Pt/C (20 wt % Pt/XC-72) deposited on Ti foil were also examined. The products immersed in dopamine solution for 1h, 3h, and 6h are referred as C-1/NiO/TiO2 NTAs, C-3/NiO/TiO2 NTAs, and C-6/NiO/TiO2 NTAs, respectively. And the products immersed in nickel acetate solution with concentration of 0.1 M, 0.2 M and 0.4 M are referred as C/NiO-1/TiO2 NTAs, C/NiO-2/TiO2 NTAs, and C/NiO-4/TiO2 NTAs, respectively. As

ACCEPTED MANUSCRIPT illustrated in Fig. 3A-B, with a thin layer of carbon coating, the C-1/NiO/TiO2 NTAs show a lower overpotential of 376 mV at a current density of 10 mA cm−2 and a smaller Tafel slope of 84.6 mV dec-1 than those of NiO/TiO2 NTAs. The C-3/NiO/TiO2 NTAs show much improved HER activity than that of C-1/NiO/TiO2 NTAs. The HER activity begins to fall with further increase of the carbon coating thickness, as displayed from the curves of C-6/NiO/TiO2 NTAs. The variations in HER activity should be attributed to the inherent properties of the carbon nanofilm made from polydopamine precursor, which not only exhibits excellent

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electrochemical activities that resemble multi-layered graphene, but also is permeatable for mass transfer procedures in HER reaction between the beneath NiO nanoparticles and the

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bulk solution. The formation of the carbon nanofilm is favorable for electron transfer of NiO species to enhance HER activity, while the increased thickness in carbon nanofilm is adverse

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to mass transfer that will reduce the HER activity. From Fig. 3C-D, the product made from 0.2 M nickel acetate solution shows the highest HER activity, with an overpotential of 86 mV

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at a current density of 10 mA cm−2 and a Tafel slope of 67.1 mV dec-1, demonstrating a superior HER activity to most electrocatalysts made from earth-abundant elements [30-32]. Stability is one of the key factors in the long-term operation of HER. Continuous LSV at a

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scan rate of 1 mV s−1 for 1000 cycles is illustrated (Fig. 3E), suggesting much improved stability of the C/NiO/TiO2 NTAs over NiO/TiO2 NTAs. Elemental analysis by ICP-AES

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shows that the dissolved Ni content in the solutions after 1000 cycles HER treatment by NiO/TiO2 NTAs and C/NiO/TiO2 NTAs is 0.62 mg/L and 0.12 mg/L, respectively. The

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chronopotentiometric curves were further applied at η=−329 mV with a current density of 100 mA cm−2 (Fig. 3F). Almost no degradation of HER is observed after 24 h test. The excellent stability of the C/NiO/TiO2 NTAs should be attributed to the sandwich configuration of the

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electrocatalyst, in which the carbon nanofilm serves as a strong shield to prevent the loss of

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NiO species in long-term operation as well as a direct pathway for electron transfer.

4. Conclusions

In summary, we present a sandwich structured C/NiO/TiO2 NTAs for high performance HER activity. The 1D TiO2 NTAs fabricated on the Ti foil surface by electrochemical anodization serve as the template. The intermediate layer of the sandwich structure is NiO nanoparticles deposited on the inner wall of TiO2 NTAs. The external layer is made from a carbon nanofilm by carbonization of the polydopamine membrane coated on the NiO species. The sandwich configuration exhibits a superior HER activity to most earth-abundant electrocatalysts. The high performance should be ascribed to the 1D nanostructure that is easily accessible to the molecules in bulk solution, the carbon nanofilm which is a naturally

ACCEPTED MANUSCRIPT protective layer to prevent the loss of electrocatalyst in long-term operation and a conductive highway for electron transfer.

Acknowledgements This work was supported by National Natural Science Foundation of China (No. 51471122) and Science and Technology Innovation Special Major Project of Hubei Province

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(No. 2017ACA179).

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ACCEPTED MANUSCRIPT Figure Legends Fig. 1 (A-C) FE-SEM images of the top-surface and cross-section (inset) morphology of TiO2 NTAs, NiO/TiO2 NTAs, and C/NiO/TiO2 NTAs, respectively; TEM images (D) and corresponding EDS mappings (E) of an individual C/NiO/TiO2 nanotube; (F) HR-TEM image of a selected area on the inner wall of an individual C/NiO/TiO2 nanotube.

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Fig. 2 (A) TGA curves of polydopamine coated NiO/TiO2 NTAs; (B) XRD and (C) Raman spectra of TiO2 NTAs, NiO/TiO2 NTAs, and C/NiO/TiO2 NTAs; (D) XPS spectra of C/NiO/TiO2 NTAs before and after LSV treatment at a scan rate of 1 mV s−1 from 0 to 1.0 V for 1000 cycles in 0.50 M H2SO4 solution.

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Fig. 3 LSV curves (A) and Tafel plots (B) of NiO/TiO2 NTAs, C-1/NiO/TiO2 NTAs, C-3/NiO/TiO2 NTAs, C-6/NiO/TiO2 NTAs and Pt/C on Ti foil in 0.50 M H2SO4 solution; LSV curves (C) and Tafel plots (D) of C/NiO-1/TiO2 NTAs, C/NiO-2/TiO2 NTAs, C/NiO-4/TiO2 NTAs and Pt/C on Ti foil in 0.50 M H2SO4 solution; (E) LSV curves of NiO/TiO2 NTAs and C/NiO/TiO2 NTAs before and after 1000 potential cycles; (F) The j-t curves of C/NiO/TiO2 NTAs under −0.329 V vs. RHE.

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Graphical abstract

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Highlights  Sandwich structured C/NiO/TiO2 nanotube arrays are presented for water splitting.  The carbon nanofilm serves as a protective layer and a highly conductive pathway.  The sandwich configuration exhibits a much improved electroactivity and stability.

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