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NF nanowires promote hydrogen and oxygen production for overall water splitting in alkaline media
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Review Article
CoO/NF nanowires promote hydrogen and oxygen production for overall water splitting in alkaline media Shasha Zhu a, Jinglei Lei a,*, Lina Zhang a, Jianxin He b a b
College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 400044, China Southwest Technology and Engineering Research Institute Chongqing, 400039, China
highlights CoO/NF material is prepared using a hydrothermal-annealing method. The CoO/NF shows nanowires structure. The overpotential of CoO/NF at the current density of 10 mA cm2 is 307 mV for OER. The overpotential of CoO/NF at the current density of 10 mA cm2 is 224 mV for HER. CoO/NF surface has aerophobic property.
article info
abstract
Article history:
The exploration of catalysts with high activity and low cost for water splitting is still
Received 12 October 2019
necessary. Herein, a nanowire-like morphology CoO/NF electrode is synthesized using
Received in revised form
facile hydrothermal reaction and calcination treatment. The urea can regulate its
9 January 2020
morphology during the synthetic process of CoO/NF. Electrochemical studies reveal that
Accepted 13 January 2020
the as-obtained CoO/NF exhibits excellent electrocatalytic performance with overpotential
Available online xxx
of 307 mV at current density of 10 mA cm2 and Tafel slope of 72 mV dec1 for oxygen evolution reaction, and CoO/NF delivers current density of 10 mA cm2 at overpotential of
Keywords:
224 mV for hydrogen evolution reaction. The results of the oxygen evolution reaction
CoO/NF
stability show that the overpotential of CoO/NF electrode is only increased by 4 mV at
Electrocatalysts
current density of 10 mA cm2. The two-electrode water splitting with CoO/NF electrodes
Nanowire
as both anode and cathode needs a cell potential of 1.76 V to reach 10 mA cm2. Therefore,
Oxygen evolution reaction
this simple method to prepare CoO/NF electrode can enhance the properties of electro-
Hydrogen evolution reaction
catalysts, which makes CoO/NF a promising material to replace noble metal-based catalysts. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (J. Lei). https://doi.org/10.1016/j.ijhydene.2020.01.085 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Zhu S et al., CoO/NF nanowires promote hydrogen and oxygen production for overall water splitting in alkaline media, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.085
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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation of CoO/NF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation of IrO2/NF and Pt/C/NF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characterizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrochemical measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction To settle energy and environmental problems, hydrogen fuel has been deemed to be a significative alternative to fossil fuels. Electrochemical water splitting has been regarded as a promising strategy for the production of hydrogen, using electric energy coming from solar energy and wind energy source, and electrochemical water splitting process involves two half-reactions: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). However, water splitting is a thermodynamically ascent process, along with high overpotentials for both OER and HER, which requires efficient electrocatalysts to significantly decrease the overpotentials and accelerate the reaction rate [1e5]. Nowadays, commercial Ir- or Ru-based oxides and Pt-based materials are considered as excellent OER and HER catalysts, respectively. However, the high cost and scarcity seriously of noble metals are major barriers in their commercialization, except their weak stability under long-time operation in high concentration acidic or alkaline electrolyte [6e12]. Thus, to overcome these bottlenecks, great efforts have been absorbed in exploiting electrocatalysts with high efficiency, stability, cost-effectiveness and earth-abundant elements, which is highly desirable [13e18]. In recent years, a large number of transition-metal (Co, Ni, Fe, Mn, Cu and Mo, etc.) compounds with low price, abundant reserves, and high catalytic activity have drawn plenty of attention as promising electrocatalysts for both the HER and OER to replace noble-metal [19e23]. To prepare transitionmetal electrocatalysts with high catalytic activity, the electrocatalyst should possess intrinsically high activity elements (Co, Ni, Fe, Mn, Cu and Mo, etc.), have high specific surface area, exposure more active sites, have low mass transfer resistance and high electrocatalytic stability. Among them, cobalt-based oxides have been studied extensively, owing to the cobalt 3d electronic structure, simple preparation method and good catalytic activity [24e33]. As electrocatalysts, cobaltbased oxides still attract the most attention. As for an ideal electrocatalyst, promoted charge-transfer process, enriched
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active sites, and outstanding stability are extraordinarily desired. However, with cobalt oxides, a much higher overpotential is still needed to actuate the HER and OER processes due to inherent poor conductivity, which is limited charge transfer [34,35]. In this study, we prepared CoO single metal electrocatalyst with nanowire-like morphology directly grown on nickel foam (NF) substrate with three-dimensional porous structure to increase the number of reaction sites, improve electrolyte penetration and gas diffusion [36e41], which was prepared using simple hydrothermal reaction and calcination treatment involved urea as the morphological regulator [42]. The electrocatalyst has excellent catalytic performance and stability to corresponding control samples, which is promising for use in water splitting.
Experimental Materials Cobalt chloride hexahydrate (CoCl2$6H2O) and urea (CO(NH2)2) were provided by chemical Reagent factory of Chengdu Kelong. IrO2 and Pt/C were purchased from Aladdin Co., Ltd. (Shanghai, PR China). Ni foams (99.99% purity, 1.0 mm thickness, 110 PPI pore size) were purchased from Lifeixin Metal Co. Ltd. All the reagents were analytical grade and used without further purification.
Preparation of CoO/NF First, a piece of 1 1 cm nickel foam (NF) was sonicated in acetone, 3.0 M HCl solution and ethanol in turn. CoCl2$6H2O (0.5 mmol) and urea (1.5 mmol) were dissolved in 30 mL ultrapure water to form a solution. After stirring for 5 min, the solution and NF were transferred to a 50 mL Teflon-lined stainless steel autoclave, and the autoclave was heated at 120 C for 6 h in an electric oven. After the hydrothermal reaction, the NF was cleaned with distilled water and absolute ethanol sequentially, and dried it overnight in a vacuum oven.
Please cite this article as: Zhu S et al., CoO/NF nanowires promote hydrogen and oxygen production for overall water splitting in alkaline media, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.085
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The Co(CO3)0.5(OH)$H2O precursor can be obtained through the hydrothermal reaction process. Finally the precursor was annealed at 300 C for 1 h in a static argon environment, followed by being cooled naturally down to room temperature under the protection of argon. The as-prepared sample was denoted as CoO/NF, and the loading mass of CoO was about 0.9 mg cm2. Co(CO3)0.5(OH)$H2O precursor is a comparative sample of CoO/NF named CoO/NF-1.
Preparation of IrO2/NF and Pt/C/NF An IrO2/NF (Pt/C/NF) electrode was prepared as follows: 3 mg of IrO2 (Pt/C) was dissolved in 10 mL of 5 wt% Nafion solution in 1 mL of solvent containing deionized water and ethanol (deionized water:ethanol ¼ 800 mL:200 mL) and ultrasonicated for 30 min to form a homogeneous dispersion ink. An appropriate amount IrO2 (Pt/C) ink was then drop-cast onto the surface of NF several times, and the loading mass of IrO2 (Pt/C) was about 0.9 mg cm2.
Characterizations The morphology and elemental mapping distribution of the samples were investigated using a field-emission scanning electron microscope (FESEM) equipped with an energy dispersive spectrometer (EDS) (JEOL JSM-7800F, Japan). The phase composition of the samples was examined by using an X-ray diffractometer (XRD, X’pert PRO, PANalytical B.V., Holland) using Cu Ka radiation (0.15418 nm). The chemical composition of the sample was identified by X-ray photoelectron spectroscopy (XPS, Thermo electron ESCALAB250, USA) using Al Ka radiation. The sample surface superaerophobic property was evaluated by air (2 mL) contact angle measurements (Dataphysics OCA20, Germany) in the 1.0 M KOH under static condition.
Electrochemical measurements The measurement of electrochemical impedance spectroscopy was tested at 25 C in a standard three-electrode system connected to an autolab (PGSTAT302 N) electrochemical workstation, and other electrochemical measurements were performed with the standard three-electrode system using a CHI760B electrochemical workstation at 25 C. The standard three-electrode system includes a sample coated on Ni foam as the working electrode, a graphite plate as the counter electrode and an Hg/HgO electrode (0.1 M, 1.0 M and 30% KOH) or saturated calomel electrode (1.0 M PBS) as the reference electrode, respectively. Electrolyte aqueous solution was saturated with oxygen bubbles (OER) or nitrogen bubbles (HER) at least 30 min prior to experiment, and OER (HER) polarization curves were obtained by linear sweep voltammetry (LSV) with a scan rate of 5 mV s1. The resistance R was tested by electrochemical impedance spectroscopy (EIS). EIS measurements were carried out in the configuration from the frequency range of 105e0.1 Hz, and conducted under oxygen evolution voltage, which corresponds to the potential at 10 mA cm2. The stability test was performed by the chronoamperometry method at 10 mA cm2. The double-layer capacitances (Cdl) were tested by cyclic voltammetry at various scan rates
3
(2e10 mV s1) to evaluate the effective surface area of various catalysts [43,44].
Results and discussion Fig. 1 shows the schematic illustration for preparation of CoO/ NF. Firstly, the pretreated NF (Fig. 1a) is added to the solution of the cobalt salt and urea, then followed by hydrothermal reaction and calcination treatment for obtaining the CoO/NF with nanowire-like morphology (Fig. 1b). The crystal structure of the CoO/NF was investigated by XRD and the pattern was shown in Fig. 2a. The characteristic diffraction peaks at 36.5 ,42.4 ,61.5 ,73.7 and 77.5 index to (111), (200), (220), (311) and (222) lattice faces of CoO, respectively [45e49]. X-ray photoelectron spectroscopy (XPS) was performed to classify the elemental compositions and chemical valences of CoO/NF (Fig. 2b and c). The XPS spectrum is shown in Fig. S1 (Supporting Information, SI), indicating that the CoO/NF composite is composed of Co and O without any impurities. Fig .2b displays the Co 2p3/2, Co 2p1/2 and the characteristic peaks of Co2þ (780.2 and 795.6 eV) and satellite features (786.1 and 802.1 eV). Fig. 2c shows the high resolution O 1s spectrum, and the O 1s spectrum exhibits two peaks at 529.5 and 531.2 eV, which are due to the metal-oxygen bonds and the oxygen in the hydroxyl group on the surface of catalyst. The results of XRD and XPS confirm that the main component of the catalyst is CoO/NF. The CoO/NF exhibits nanowire-like morphology (Fig. 3a and b), and the nanowires uniformly grow on the NF substrate with the average diameter ca. 40 nm [50]. The FESEM images of CoO/NF-1 in Fig. S2 exhibits nanowire-like morphology, which means that the morphology does not change after calcination of the precursor. The elemental mapping images of CoO/NF are shown in Fig. 3c and d, which reveal that Co and O elements were homogeneously distributed on the NF substrate [51]. As shown in Fig. S3, the morphology of the CoO/NF is shown as nanosheet-like, which can be prepared without the addition of urea during the preparation process. From the
Fig. 1 e Schematic illustration for the synthesis route CoO/ NF.
Please cite this article as: Zhu S et al., CoO/NF nanowires promote hydrogen and oxygen production for overall water splitting in alkaline media, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.085
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Fig. 2 e (a) X-ray diffraction (XRD) pattern of CoO/NF; High-resolution XPS spectra of CoO/NF in the (b) Co 2p, (c) O 1s regions.
characterization results, it can be concluded that urea plays a vital role in regulating the morphology during the preparation process of CoO/NF. Therefore, the carbonate and hydroxyl anions provided by the hydrolysis of urea have an important effect on the crystal grown of CoO/NF [16]. The reactions involve in the preparation of the CoO/NF could be as follows: COðNH2 Þ2 þ H2 O/2NH3 þ CO2 [ NH3 þ H2 O/NHþ 4 þ OH þ CO2 þ H2 O/CO2 3 þ 2H
Co2þ þ OH þ 0:5 CO2 3 þ H2 O / CoðCO3 Þ0:5 ðOHÞ,H2 O CoðCO3 Þ0:5 ðOHÞ , H2 O / CoO þ H2 O þ CO2 [ The OER performances of the CoO/NF and CoO/NF-1 were evaluated under 1.0 M KOH. The polarization curves (Fig. 4a) show that CoO/NF and CoO/NF-1 display OER activities with overpotentials of 307 (415) and 310 (441) mV at 10 (100) mA cm2, respectively (Fig. 4b). When the current density is less than 100 mA cm2, the OER performance of IrO2 is superior to
CoO/NF. On the contrary, the OER catalytic performance of CoO/NF is better than that of IrO2 when the current density is greater than 100 mA cm2. The OER kinetics of CoO/NF and CoO/NF-1 catalysts were studied through Tafel plots (Fig. 4c). A smaller Tafel slope indicates an accelerated reaction [22,37,52e58]. The Tafel slope values of CoO/NF and CoO/NF-1 are 72 and 87 mV dec1. The Tafel slope value of CoO/NF is lower than that of CoO/NF-1, confirming that CoO/NF has a more favourable kinetics. The OER electrocatalytic performance of CoO/NF is further investigated in 1.0 M PBS, 0.1 M and 30% KOH electrolyte. The catalytic performance of CoO/ NF in 1.0 M KOH electrolyte is better than that of 0.1 M KOH and 1.0 M PBS electrolytes, and weaker than that of 30% KOH electrolyte (Fig. S4) [59e64]. The activity of CoO/NF catalyst is more positive than other non-precious metal OER catalysts (Table S1). The HER performances of the CoO/NF and CoO/NF-1 were also evaluated under 1.0 M KOH (Fig. 4d). The Pt/C/NF only requires 22 (143) mV overpotential to afford 10 (100) mA cm2, and this value is smaller than that of CoO/NF and CoO/NF-1 with the overpotential of 224 (365) and 316 (501) mV at the current densities (j) of 10 (100) mA cm2 (Fig. 4e). The Tafel slope is also an important indicator in the HER for the kinetic
Please cite this article as: Zhu S et al., CoO/NF nanowires promote hydrogen and oxygen production for overall water splitting in alkaline media, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.085
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Fig. 3 e (a), (b) FESEM images of the CoO/NF at different magnifications. (c), (d) Elemental mapping images of CoO/NF.
Fig. 4 e (a) OER polarization curves of CoO/NF and CoO/NF-1 in 1.0 M KOH. (b) Corresponding overpotential of 10, 50 and 100 mA cm¡2 of CoO/NF and CoO/NF-1 for OER. (c) Corresponding Tafel plots of CoO/NF and CoO/NF-1 in 1.0 M KOH for OER. (d) HER polarization curves of CoO/NF and CoO/NF-1 in 1.0 M KOH. (e) Corresponding overpotential of 10, 50 and 100 mA cm¡2 of CoO/NF and CoO/NF-1 for HER. (f) Corresponding Tafel plots of CoO/NF and CoO/NF-1 in 1.0 M KOH for HER.
Please cite this article as: Zhu S et al., CoO/NF nanowires promote hydrogen and oxygen production for overall water splitting in alkaline media, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.085
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analysis. The Tafel slope values of CoO/NF and CoO/NF-1 are 115 and 126 mV dec1 (Fig. 4f), so CoO/NF has a more favourable kinetics than CoO/NF-1. The HER catalytic activity of CoO/ NF in 1.0 M KOH electrolyte is better than that of 0.1 M KOH and 1.0 M PBS electrolytes, and worse than that of 30% KOH electrolyte (Fig. S5). The activity of CoO/NF catalyst is more positive than other non-precious metal HER catalysts (Table S2). EIS was used to elucidate the electrode/electrolyte interface of the CoO/NF. It can also explore CoO/NF and CoO/NF-1 catalytic activities (Fig. 5a). The Rs is solution resistance and the Rct is the charge transfer resistance during the process of electrocatalysis. The Rct values are estimated as 1.6 and 2.7 U for CoO/NF and CoO/NF-1, respectively, implying that CoO/NF has a faster reaction rate [65]. Furthermore, a study of the intrinsic activity of the catalyst, the electrocatalytic active surface area (ECSA) was calculated by testing the double-layer capacitances (Cdl). The Cdl of CoO/NF and CoO/NF-1 were also measured by cyclic voltammetry method at different scan rates (Fig. S6). The Cdl values of CoO/NF and CoO/NF-1 are 63.2 and 1.2 mF cm2 (Fig. 5b) [66]. The ECSAs of CoO/NF and CoO/ NF-1 are about 1580 cm2 and 30 cm2, respectively. It can
be seen that CoO/NF has larger electrocatalytic active surface area. In addition, to in-depth knowledge activity of the catalyst, the turn over frequency (TOF) is used to characterize the intrinsic catalytic efficiency of the catalyst, which is calculated by the number of conversion per active site per unit time. It shows the reaction rate of the catalyst in the catalytic reaction. The TOF value of CoO/NF is about 0.0017 s1. The stability of CoO/NF and CoO/NF-1 were tested at 10 mA cm2 current density in 1.0 M KOH solution for 20 h (Fig. 5c and d), and the overpotentials of CoO/NF and CoO/NF-1 increased by 4 and 3 mV at a current density of 10 mA cm2 and by 8 and 26 mV at a current density of 100 mA cm2, respectively. The investigation of stability confirmed that the stability of CoO/NF is better than that of CoO/NF-1 for OER. Figs. S7 and S8 show the test results of stability at different duration and different current densities, confirming that CoO/ NF has excellent stability. Therefore, CoO/NF shows outstanding stability due to the retention of morphology during the OER process (Figs. S9eS11), and the surface of the catalyst was converted to CoOOH after stability test (Fig. S12). The possible mechanistic of CoO/NF for OER in 1.0 M KOH can be explained as follows [67]:
Fig. 5 e (a) Nyquist plot representations of electrochemical impedance spectra of CoO/NF and CoO/NF-1. (b) Linear fitting of capacitive current versus scan rate obtained from cyclic voltammetry tests for CoO/NF and CoO/NF-1. Chronoamperometry curves of CoO/NF (c) and CoO/NF-1 (d) obtained at 10 mA cm¡2 current density in 1.0 M KOH solution. Please cite this article as: Zhu S et al., CoO/NF nanowires promote hydrogen and oxygen production for overall water splitting in alkaline media, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.085
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Fig. 6 e (a) Polarization curves of CoO/NF catalyst for HER and OER. (b) Polarization curves for overall water splitting of CoO/ NF catalyst in a two-electrode configuration in 1.0 M KOH.
II
III
Co þ 3OH 4Co OOH þ H2 O þ 3e
CoIII OOH þ OH 4CoIV OðOHÞ2 þ e CoIV OðOHÞ2 þ 2OH 4CoIV OO2 þ 2H2 O þ 2e CoIV OO2 þ OH /CoIII OOH þ O2 þ e Total OER：4OH / O2 þ 2H2 O þ 4e Since CoO/NF has excellent OER and HER catalytic, we built homemade two-electrode using the CoO/NF electrodes as both anode and cathode for water splitting. The CoO/NF requires a cell voltage of 1.91 V to afford the current density of 50 mA cm2 (Fig. 6b). For comparison, the polarization curves of CoO/NF for OER and HER are also shown in Fig. 6a. The voltage difference (DV) between the anode reaction and the
cathode reaction is 1.91 V, and this result is in agreement with the result of Fig. 6b. Moreover, the stability test was performed through chronoamperometry method (Fig. S13). The chronoamperometry curve at 1.76 V reveals that the current density has hardly changed after 20 h test, implying remarkable stability. The bubble contact angle of CoO/NF is 147 (Fig. 7), and the CoO/NF electrode surface is close to super-aerophobic state, which can improve the catalytic activity by attenuating the interaction between the bubbles and the electrode surface, thus favoring rapid release of bubbles [68e73]. The superior OER and HER performance of CoO/NF may be characterized as follows: (i) the nanowire-like morphology of CoO/NF can expose rich edge active sites; (ii) the binder-free catalyst and beneficial contacts between CoO and NF can provide low resistance pathways for electrons; (iii) the aerophobic property of CoO/NF facilitates bubble release can decrease the overpotential and benefit the electron transfer.
Conclusions
Fig. 7 e Bubble contact angle of CoO/NF.
In summary, we have successively synthesized the CoO/NF electrode using cobalt chloride hexahydrate, urea and NF substrate by hydrothermal reaction and calcination treatment. The morphology of the catalyst CoO on NF substrate is nanowire-like, and the average diameter of the wire is about 40 nm. Electrochemical tests show that CoO/NF has excellent catalytic activity with overpotential of 307 mV at current density of 10 mA cm2 for OER, and CoO/NF catalyst delivered current density of 10 mA cm2 at overpotential of 224 mV for HER. CoO/NF has outstanding stability. After 20 h of chronoamperometry, the overpotential almost unchanged for OER. Water splitting with two CoO/NF electrodes needs a cell potential of 1.76 V to reach 10 mA cm2. As a result, the CoO/ NF catalyst showed improved performance due to the decreased charge transfer resistance and increased number of active sites to the corresponding control samples. These results make CoO/NF a promising material to replace noble metal-based catalysts.
Please cite this article as: Zhu S et al., CoO/NF nanowires promote hydrogen and oxygen production for overall water splitting in alkaline media, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.085
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Acknowledgements This study was supported by the Project No. CDJXS12222254 Supported by the Fundamental Research Funds for the Central Universities, and the sharing fund of Chongqing University’s Large-scale Equipment.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.01.085.
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Review Article
CoO/NF nanowires promote hydrogen and oxygen production for overall water splitting in alkaline media Shasha Zhu a, Jinglei Lei a,*, Lina Zhang a, Jianxin He b a b
College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 400044, China Southwest Technology and Engineering Research Institute Chongqing, 400039, China
highlights CoO/NF material is prepared using a hydrothermal-annealing method. The CoO/NF shows nanowires structure. The overpotential of CoO/NF at the current density of 10 mA cm2 is 307 mV for OER. The overpotential of CoO/NF at the current density of 10 mA cm2 is 224 mV for HER. CoO/NF surface has aerophobic property.
article info
abstract
Article history:
The exploration of catalysts with high activity and low cost for water splitting is still
Received 12 October 2019
necessary. Herein, a nanowire-like morphology CoO/NF electrode is synthesized using
Received in revised form
facile hydrothermal reaction and calcination treatment. The urea can regulate its
9 January 2020
morphology during the synthetic process of CoO/NF. Electrochemical studies reveal that
Accepted 13 January 2020
the as-obtained CoO/NF exhibits excellent electrocatalytic performance with overpotential
Available online xxx
of 307 mV at current density of 10 mA cm2 and Tafel slope of 72 mV dec1 for oxygen evolution reaction, and CoO/NF delivers current density of 10 mA cm2 at overpotential of
Keywords:
224 mV for hydrogen evolution reaction. The results of the oxygen evolution reaction
CoO/NF
stability show that the overpotential of CoO/NF electrode is only increased by 4 mV at
Electrocatalysts
current density of 10 mA cm2. The two-electrode water splitting with CoO/NF electrodes
Nanowire
as both anode and cathode needs a cell potential of 1.76 V to reach 10 mA cm2. Therefore,
Oxygen evolution reaction
this simple method to prepare CoO/NF electrode can enhance the properties of electro-
Hydrogen evolution reaction
catalysts, which makes CoO/NF a promising material to replace noble metal-based catalysts. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (J. Lei). https://doi.org/10.1016/j.ijhydene.2020.01.085 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Zhu S et al., CoO/NF nanowires promote hydrogen and oxygen production for overall water splitting in alkaline media, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.085
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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation of CoO/NF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation of IrO2/NF and Pt/C/NF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characterizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrochemical measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction To settle energy and environmental problems, hydrogen fuel has been deemed to be a significative alternative to fossil fuels. Electrochemical water splitting has been regarded as a promising strategy for the production of hydrogen, using electric energy coming from solar energy and wind energy source, and electrochemical water splitting process involves two half-reactions: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). However, water splitting is a thermodynamically ascent process, along with high overpotentials for both OER and HER, which requires efficient electrocatalysts to significantly decrease the overpotentials and accelerate the reaction rate [1e5]. Nowadays, commercial Ir- or Ru-based oxides and Pt-based materials are considered as excellent OER and HER catalysts, respectively. However, the high cost and scarcity seriously of noble metals are major barriers in their commercialization, except their weak stability under long-time operation in high concentration acidic or alkaline electrolyte [6e12]. Thus, to overcome these bottlenecks, great efforts have been absorbed in exploiting electrocatalysts with high efficiency, stability, cost-effectiveness and earth-abundant elements, which is highly desirable [13e18]. In recent years, a large number of transition-metal (Co, Ni, Fe, Mn, Cu and Mo, etc.) compounds with low price, abundant reserves, and high catalytic activity have drawn plenty of attention as promising electrocatalysts for both the HER and OER to replace noble-metal [19e23]. To prepare transitionmetal electrocatalysts with high catalytic activity, the electrocatalyst should possess intrinsically high activity elements (Co, Ni, Fe, Mn, Cu and Mo, etc.), have high specific surface area, exposure more active sites, have low mass transfer resistance and high electrocatalytic stability. Among them, cobalt-based oxides have been studied extensively, owing to the cobalt 3d electronic structure, simple preparation method and good catalytic activity [24e33]. As electrocatalysts, cobaltbased oxides still attract the most attention. As for an ideal electrocatalyst, promoted charge-transfer process, enriched
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active sites, and outstanding stability are extraordinarily desired. However, with cobalt oxides, a much higher overpotential is still needed to actuate the HER and OER processes due to inherent poor conductivity, which is limited charge transfer [34,35]. In this study, we prepared CoO single metal electrocatalyst with nanowire-like morphology directly grown on nickel foam (NF) substrate with three-dimensional porous structure to increase the number of reaction sites, improve electrolyte penetration and gas diffusion [36e41], which was prepared using simple hydrothermal reaction and calcination treatment involved urea as the morphological regulator [42]. The electrocatalyst has excellent catalytic performance and stability to corresponding control samples, which is promising for use in water splitting.
Experimental Materials Cobalt chloride hexahydrate (CoCl2$6H2O) and urea (CO(NH2)2) were provided by chemical Reagent factory of Chengdu Kelong. IrO2 and Pt/C were purchased from Aladdin Co., Ltd. (Shanghai, PR China). Ni foams (99.99% purity, 1.0 mm thickness, 110 PPI pore size) were purchased from Lifeixin Metal Co. Ltd. All the reagents were analytical grade and used without further purification.
Preparation of CoO/NF First, a piece of 1 1 cm nickel foam (NF) was sonicated in acetone, 3.0 M HCl solution and ethanol in turn. CoCl2$6H2O (0.5 mmol) and urea (1.5 mmol) were dissolved in 30 mL ultrapure water to form a solution. After stirring for 5 min, the solution and NF were transferred to a 50 mL Teflon-lined stainless steel autoclave, and the autoclave was heated at 120 C for 6 h in an electric oven. After the hydrothermal reaction, the NF was cleaned with distilled water and absolute ethanol sequentially, and dried it overnight in a vacuum oven.
Please cite this article as: Zhu S et al., CoO/NF nanowires promote hydrogen and oxygen production for overall water splitting in alkaline media, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.085
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The Co(CO3)0.5(OH)$H2O precursor can be obtained through the hydrothermal reaction process. Finally the precursor was annealed at 300 C for 1 h in a static argon environment, followed by being cooled naturally down to room temperature under the protection of argon. The as-prepared sample was denoted as CoO/NF, and the loading mass of CoO was about 0.9 mg cm2. Co(CO3)0.5(OH)$H2O precursor is a comparative sample of CoO/NF named CoO/NF-1.
Preparation of IrO2/NF and Pt/C/NF An IrO2/NF (Pt/C/NF) electrode was prepared as follows: 3 mg of IrO2 (Pt/C) was dissolved in 10 mL of 5 wt% Nafion solution in 1 mL of solvent containing deionized water and ethanol (deionized water:ethanol ¼ 800 mL:200 mL) and ultrasonicated for 30 min to form a homogeneous dispersion ink. An appropriate amount IrO2 (Pt/C) ink was then drop-cast onto the surface of NF several times, and the loading mass of IrO2 (Pt/C) was about 0.9 mg cm2.
Characterizations The morphology and elemental mapping distribution of the samples were investigated using a field-emission scanning electron microscope (FESEM) equipped with an energy dispersive spectrometer (EDS) (JEOL JSM-7800F, Japan). The phase composition of the samples was examined by using an X-ray diffractometer (XRD, X’pert PRO, PANalytical B.V., Holland) using Cu Ka radiation (0.15418 nm). The chemical composition of the sample was identified by X-ray photoelectron spectroscopy (XPS, Thermo electron ESCALAB250, USA) using Al Ka radiation. The sample surface superaerophobic property was evaluated by air (2 mL) contact angle measurements (Dataphysics OCA20, Germany) in the 1.0 M KOH under static condition.
Electrochemical measurements The measurement of electrochemical impedance spectroscopy was tested at 25 C in a standard three-electrode system connected to an autolab (PGSTAT302 N) electrochemical workstation, and other electrochemical measurements were performed with the standard three-electrode system using a CHI760B electrochemical workstation at 25 C. The standard three-electrode system includes a sample coated on Ni foam as the working electrode, a graphite plate as the counter electrode and an Hg/HgO electrode (0.1 M, 1.0 M and 30% KOH) or saturated calomel electrode (1.0 M PBS) as the reference electrode, respectively. Electrolyte aqueous solution was saturated with oxygen bubbles (OER) or nitrogen bubbles (HER) at least 30 min prior to experiment, and OER (HER) polarization curves were obtained by linear sweep voltammetry (LSV) with a scan rate of 5 mV s1. The resistance R was tested by electrochemical impedance spectroscopy (EIS). EIS measurements were carried out in the configuration from the frequency range of 105e0.1 Hz, and conducted under oxygen evolution voltage, which corresponds to the potential at 10 mA cm2. The stability test was performed by the chronoamperometry method at 10 mA cm2. The double-layer capacitances (Cdl) were tested by cyclic voltammetry at various scan rates
3
(2e10 mV s1) to evaluate the effective surface area of various catalysts [43,44].
Results and discussion Fig. 1 shows the schematic illustration for preparation of CoO/ NF. Firstly, the pretreated NF (Fig. 1a) is added to the solution of the cobalt salt and urea, then followed by hydrothermal reaction and calcination treatment for obtaining the CoO/NF with nanowire-like morphology (Fig. 1b). The crystal structure of the CoO/NF was investigated by XRD and the pattern was shown in Fig. 2a. The characteristic diffraction peaks at 36.5 ,42.4 ,61.5 ,73.7 and 77.5 index to (111), (200), (220), (311) and (222) lattice faces of CoO, respectively [45e49]. X-ray photoelectron spectroscopy (XPS) was performed to classify the elemental compositions and chemical valences of CoO/NF (Fig. 2b and c). The XPS spectrum is shown in Fig. S1 (Supporting Information, SI), indicating that the CoO/NF composite is composed of Co and O without any impurities. Fig .2b displays the Co 2p3/2, Co 2p1/2 and the characteristic peaks of Co2þ (780.2 and 795.6 eV) and satellite features (786.1 and 802.1 eV). Fig. 2c shows the high resolution O 1s spectrum, and the O 1s spectrum exhibits two peaks at 529.5 and 531.2 eV, which are due to the metal-oxygen bonds and the oxygen in the hydroxyl group on the surface of catalyst. The results of XRD and XPS confirm that the main component of the catalyst is CoO/NF. The CoO/NF exhibits nanowire-like morphology (Fig. 3a and b), and the nanowires uniformly grow on the NF substrate with the average diameter ca. 40 nm [50]. The FESEM images of CoO/NF-1 in Fig. S2 exhibits nanowire-like morphology, which means that the morphology does not change after calcination of the precursor. The elemental mapping images of CoO/NF are shown in Fig. 3c and d, which reveal that Co and O elements were homogeneously distributed on the NF substrate [51]. As shown in Fig. S3, the morphology of the CoO/NF is shown as nanosheet-like, which can be prepared without the addition of urea during the preparation process. From the
Fig. 1 e Schematic illustration for the synthesis route CoO/ NF.
Please cite this article as: Zhu S et al., CoO/NF nanowires promote hydrogen and oxygen production for overall water splitting in alkaline media, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.085
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Fig. 2 e (a) X-ray diffraction (XRD) pattern of CoO/NF; High-resolution XPS spectra of CoO/NF in the (b) Co 2p, (c) O 1s regions.
characterization results, it can be concluded that urea plays a vital role in regulating the morphology during the preparation process of CoO/NF. Therefore, the carbonate and hydroxyl anions provided by the hydrolysis of urea have an important effect on the crystal grown of CoO/NF [16]. The reactions involve in the preparation of the CoO/NF could be as follows: COðNH2 Þ2 þ H2 O/2NH3 þ CO2 [ NH3 þ H2 O/NHþ 4 þ OH þ CO2 þ H2 O/CO2 3 þ 2H
Co2þ þ OH þ 0:5 CO2 3 þ H2 O / CoðCO3 Þ0:5 ðOHÞ,H2 O CoðCO3 Þ0:5 ðOHÞ , H2 O / CoO þ H2 O þ CO2 [ The OER performances of the CoO/NF and CoO/NF-1 were evaluated under 1.0 M KOH. The polarization curves (Fig. 4a) show that CoO/NF and CoO/NF-1 display OER activities with overpotentials of 307 (415) and 310 (441) mV at 10 (100) mA cm2, respectively (Fig. 4b). When the current density is less than 100 mA cm2, the OER performance of IrO2 is superior to
CoO/NF. On the contrary, the OER catalytic performance of CoO/NF is better than that of IrO2 when the current density is greater than 100 mA cm2. The OER kinetics of CoO/NF and CoO/NF-1 catalysts were studied through Tafel plots (Fig. 4c). A smaller Tafel slope indicates an accelerated reaction [22,37,52e58]. The Tafel slope values of CoO/NF and CoO/NF-1 are 72 and 87 mV dec1. The Tafel slope value of CoO/NF is lower than that of CoO/NF-1, confirming that CoO/NF has a more favourable kinetics. The OER electrocatalytic performance of CoO/NF is further investigated in 1.0 M PBS, 0.1 M and 30% KOH electrolyte. The catalytic performance of CoO/ NF in 1.0 M KOH electrolyte is better than that of 0.1 M KOH and 1.0 M PBS electrolytes, and weaker than that of 30% KOH electrolyte (Fig. S4) [59e64]. The activity of CoO/NF catalyst is more positive than other non-precious metal OER catalysts (Table S1). The HER performances of the CoO/NF and CoO/NF-1 were also evaluated under 1.0 M KOH (Fig. 4d). The Pt/C/NF only requires 22 (143) mV overpotential to afford 10 (100) mA cm2, and this value is smaller than that of CoO/NF and CoO/NF-1 with the overpotential of 224 (365) and 316 (501) mV at the current densities (j) of 10 (100) mA cm2 (Fig. 4e). The Tafel slope is also an important indicator in the HER for the kinetic
Please cite this article as: Zhu S et al., CoO/NF nanowires promote hydrogen and oxygen production for overall water splitting in alkaline media, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.085
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Fig. 3 e (a), (b) FESEM images of the CoO/NF at different magnifications. (c), (d) Elemental mapping images of CoO/NF.
Fig. 4 e (a) OER polarization curves of CoO/NF and CoO/NF-1 in 1.0 M KOH. (b) Corresponding overpotential of 10, 50 and 100 mA cm¡2 of CoO/NF and CoO/NF-1 for OER. (c) Corresponding Tafel plots of CoO/NF and CoO/NF-1 in 1.0 M KOH for OER. (d) HER polarization curves of CoO/NF and CoO/NF-1 in 1.0 M KOH. (e) Corresponding overpotential of 10, 50 and 100 mA cm¡2 of CoO/NF and CoO/NF-1 for HER. (f) Corresponding Tafel plots of CoO/NF and CoO/NF-1 in 1.0 M KOH for HER.
Please cite this article as: Zhu S et al., CoO/NF nanowires promote hydrogen and oxygen production for overall water splitting in alkaline media, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.085
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analysis. The Tafel slope values of CoO/NF and CoO/NF-1 are 115 and 126 mV dec1 (Fig. 4f), so CoO/NF has a more favourable kinetics than CoO/NF-1. The HER catalytic activity of CoO/ NF in 1.0 M KOH electrolyte is better than that of 0.1 M KOH and 1.0 M PBS electrolytes, and worse than that of 30% KOH electrolyte (Fig. S5). The activity of CoO/NF catalyst is more positive than other non-precious metal HER catalysts (Table S2). EIS was used to elucidate the electrode/electrolyte interface of the CoO/NF. It can also explore CoO/NF and CoO/NF-1 catalytic activities (Fig. 5a). The Rs is solution resistance and the Rct is the charge transfer resistance during the process of electrocatalysis. The Rct values are estimated as 1.6 and 2.7 U for CoO/NF and CoO/NF-1, respectively, implying that CoO/NF has a faster reaction rate [65]. Furthermore, a study of the intrinsic activity of the catalyst, the electrocatalytic active surface area (ECSA) was calculated by testing the double-layer capacitances (Cdl). The Cdl of CoO/NF and CoO/NF-1 were also measured by cyclic voltammetry method at different scan rates (Fig. S6). The Cdl values of CoO/NF and CoO/NF-1 are 63.2 and 1.2 mF cm2 (Fig. 5b) [66]. The ECSAs of CoO/NF and CoO/ NF-1 are about 1580 cm2 and 30 cm2, respectively. It can
be seen that CoO/NF has larger electrocatalytic active surface area. In addition, to in-depth knowledge activity of the catalyst, the turn over frequency (TOF) is used to characterize the intrinsic catalytic efficiency of the catalyst, which is calculated by the number of conversion per active site per unit time. It shows the reaction rate of the catalyst in the catalytic reaction. The TOF value of CoO/NF is about 0.0017 s1. The stability of CoO/NF and CoO/NF-1 were tested at 10 mA cm2 current density in 1.0 M KOH solution for 20 h (Fig. 5c and d), and the overpotentials of CoO/NF and CoO/NF-1 increased by 4 and 3 mV at a current density of 10 mA cm2 and by 8 and 26 mV at a current density of 100 mA cm2, respectively. The investigation of stability confirmed that the stability of CoO/NF is better than that of CoO/NF-1 for OER. Figs. S7 and S8 show the test results of stability at different duration and different current densities, confirming that CoO/ NF has excellent stability. Therefore, CoO/NF shows outstanding stability due to the retention of morphology during the OER process (Figs. S9eS11), and the surface of the catalyst was converted to CoOOH after stability test (Fig. S12). The possible mechanistic of CoO/NF for OER in 1.0 M KOH can be explained as follows [67]:
Fig. 5 e (a) Nyquist plot representations of electrochemical impedance spectra of CoO/NF and CoO/NF-1. (b) Linear fitting of capacitive current versus scan rate obtained from cyclic voltammetry tests for CoO/NF and CoO/NF-1. Chronoamperometry curves of CoO/NF (c) and CoO/NF-1 (d) obtained at 10 mA cm¡2 current density in 1.0 M KOH solution. Please cite this article as: Zhu S et al., CoO/NF nanowires promote hydrogen and oxygen production for overall water splitting in alkaline media, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.085
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Fig. 6 e (a) Polarization curves of CoO/NF catalyst for HER and OER. (b) Polarization curves for overall water splitting of CoO/ NF catalyst in a two-electrode configuration in 1.0 M KOH.
II
III
Co þ 3OH 4Co OOH þ H2 O þ 3e
CoIII OOH þ OH 4CoIV OðOHÞ2 þ e CoIV OðOHÞ2 þ 2OH 4CoIV OO2 þ 2H2 O þ 2e CoIV OO2 þ OH /CoIII OOH þ O2 þ e Total OER：4OH / O2 þ 2H2 O þ 4e Since CoO/NF has excellent OER and HER catalytic, we built homemade two-electrode using the CoO/NF electrodes as both anode and cathode for water splitting. The CoO/NF requires a cell voltage of 1.91 V to afford the current density of 50 mA cm2 (Fig. 6b). For comparison, the polarization curves of CoO/NF for OER and HER are also shown in Fig. 6a. The voltage difference (DV) between the anode reaction and the
cathode reaction is 1.91 V, and this result is in agreement with the result of Fig. 6b. Moreover, the stability test was performed through chronoamperometry method (Fig. S13). The chronoamperometry curve at 1.76 V reveals that the current density has hardly changed after 20 h test, implying remarkable stability. The bubble contact angle of CoO/NF is 147 (Fig. 7), and the CoO/NF electrode surface is close to super-aerophobic state, which can improve the catalytic activity by attenuating the interaction between the bubbles and the electrode surface, thus favoring rapid release of bubbles [68e73]. The superior OER and HER performance of CoO/NF may be characterized as follows: (i) the nanowire-like morphology of CoO/NF can expose rich edge active sites; (ii) the binder-free catalyst and beneficial contacts between CoO and NF can provide low resistance pathways for electrons; (iii) the aerophobic property of CoO/NF facilitates bubble release can decrease the overpotential and benefit the electron transfer.
Conclusions
Fig. 7 e Bubble contact angle of CoO/NF.
In summary, we have successively synthesized the CoO/NF electrode using cobalt chloride hexahydrate, urea and NF substrate by hydrothermal reaction and calcination treatment. The morphology of the catalyst CoO on NF substrate is nanowire-like, and the average diameter of the wire is about 40 nm. Electrochemical tests show that CoO/NF has excellent catalytic activity with overpotential of 307 mV at current density of 10 mA cm2 for OER, and CoO/NF catalyst delivered current density of 10 mA cm2 at overpotential of 224 mV for HER. CoO/NF has outstanding stability. After 20 h of chronoamperometry, the overpotential almost unchanged for OER. Water splitting with two CoO/NF electrodes needs a cell potential of 1.76 V to reach 10 mA cm2. As a result, the CoO/ NF catalyst showed improved performance due to the decreased charge transfer resistance and increased number of active sites to the corresponding control samples. These results make CoO/NF a promising material to replace noble metal-based catalysts.
Please cite this article as: Zhu S et al., CoO/NF nanowires promote hydrogen and oxygen production for overall water splitting in alkaline media, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.085
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Acknowledgements This study was supported by the Project No. CDJXS12222254 Supported by the Fundamental Research Funds for the Central Universities, and the sharing fund of Chongqing University’s Large-scale Equipment.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.01.085.
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Please cite this article as: Zhu S et al., CoO/NF nanowires promote hydrogen and oxygen production for overall water splitting in alkaline media, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.085
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Please cite this article as: Zhu S et al., CoO/NF nanowires promote hydrogen and oxygen production for overall water splitting in alkaline media, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.085