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3D porous network heterostructure [email protected] electrocatalyst for efficient oxygen evolution reaction at large current densities
Applied Catalysis B: Environmental 260 (2020) 118199
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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
3D porous network heterostructure NiCe@NiFe electrocatalyst for efficient oxygen evolution reaction at large current densities ⁎⁎
Guang Liua,b, , Muheng Wanga, Yun Wua, Na Lia, Fei Zhaoa, Qiang Zhaoa, Jinping Lia,
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a Shanxi Key Laboratory of Gas Energy Efficient and Clean Utilization, Research Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, Shanxi, 030024, PR China b Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin, 300071, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrogen production Oxygen evolution reaction Electrocatalyst Heterostructure Large current densities
Three-dimensional (3D) self-supporting porous network NiCeOx@NiFeOx electrocatalyst on Ni foam (denotes as NiCe@NiFe/NF-N) is prepared by a simple two step electrodepositing process. NiCe@NiFe/NF-N attains 100 mA/cm2 at overpotential of 254 mV with Tafel slope of 59.9 mV/dec, which is much lower than that of NiCe@NiFe/NF-P, NiFe@NF, NiCe@NF and also commercial RuO2 electrocatalysts. Furthermore, the obtained NiCe@NiFe/NF-N electrocatalyst only needs overpotential of 359 mV to deliver high current densities of 1 A/ cm2 and demonstrates excellent stability at such large current densities for 20 h, which is also superior to that of NiCe@NiFe/NF-P electrocatalyst. Further study reveals that the enhanced water oxidation performance of NiCe@NiFe/NF-N electrocatalyst with good stability at high current densities could be attributed to the optimal surface and electronic structures, fast reaction kinetics and high intrinsic catalytic activity. Collectively, such electrocatalyst paves new opportunities for development of low-cost water oxidation electrocatalysts with high current densities for practical water splitting applications.
1. Introduction With the development of the world economy, the demand for resources in human society is also growing rapidly [1,2]. Traditional fossil energy sources not only have limited reserves, but also bring about increasingly prominent environmental problems. Hydrogen energy has been widely used as a new type of energy with high calorific value, clean and pollution-free [3,4]. Electrochemical water splitting plays a key role in the future hydrogen production and can be divided into two half reactions, cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) [5,6]. OER involves a complex multi-proton-couple electron transfer process with slow kinetics and high overpotentials, which is the rate-limiting step in the water-decomposition reaction [7,8]. Precious metal-based catalysts such as iridium and ruthenium oxide have high OER activity, but rare reserves and expensive prices limit their large-scale applications [9,10]. In recent years, extensive efforts have been devoted to designing and synthesizing efficient, durable, and low-cost alternatives based on earth-abundant 3d transition metals. especially nickel [11,12], iron [13], cobalt [14,15]. Furthermore, their corresponding oxides/
hydroxides [3], sulfides [4,16], phosphides [17,18], selenides [19,20] and nitrides [21,22] have also been widely studied. In spite of the great advances made recently in the development of transition metal-based OER electrocatalysts, there is still a big challenge in applying the newly developed catalysts in a viable water-splitting technology. At present, most OER catalysts work well only at small current densities (e.g. 10–100 mA/cm2). To be used for industrial applications, OER catalysts need to meet more harsh test environment such as delivery of large current densities (> 500 mA/cm2) at low overpotentials (< 300 mV), and mechanical robustness and prolonged durability during strong gas evolution [23,24]. In order to meet the above requirements, catalysts should fulfill multiple criteria simultaneously: (1) highly intrinsic OER performance, (2) surface rich catalytic active sites, (3) fast mass transport and charge transfer capability, (4) excellent stability, and (5) corrosion resistance [23,25–27]. Nevertheless, to date, there are few OER electrocatalysts have been developed to hit all of the above spots. This is challenging target for powdery OER electrocatalysts, which often need to be adhered to a current collector by using a polymer binder. Under such circumstances, long-term electrocatalytic performance with large current density is inacceptable
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Corresponding author. Corresponding author at: Shanxi Key Laboratory of Gas Energy Efficient and Clean Utilization, Research Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, Shanxi, 030024, PR China. E-mail addresses: [email protected] (G. Liu), [email protected] (J. Li). ⁎⁎
https://doi.org/10.1016/j.apcatb.2019.118199 Received 9 July 2019; Received in revised form 12 September 2019; Accepted 13 September 2019 Available online 14 September 2019 0926-3373/ © 2019 Elsevier B.V. All rights reserved.
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2.3. Synthesis of NiCe@NiFe/NF and NiCe/NF electrodes
because of the inferior interfacial resistance between the current collector and the catalyst as well as bubble evolving induced catalyst peeling during OER process. In pursuit of efficient electrocatalysts with high current densities performance, increasing mass loading is a viable pathway but might obstruct the mass transport and charge transfer. Other factors that influence the mass-diffusion process during OER at high current densities is the dissipation of gas bubbles formed on the electrodes, which would cause catalyst peeling off problem and large bubble overpotentials. In this regard, developing and synthesizing of self-supported electrocatalysts with excellent electrocatalytic performance, superior stability, favored electronic conductivity, and unique macroscopic structure are urgent needed, which could perform efficient water oxidation activities at high current densities with lower overpotential. Very recently some studies reported that cerium oxides has excellent electronic/ionic conductivity, good oxygen-storage capacity and reversible surface oxygen ion exchange owing to the flexible transition between the Ce3+ and Ce4+ oxidation states [28–31]. It has been found that CeOx-based materials can be served as efficient catalysts or catalyst-supports towards heterogeneous catalysis, e.g. CO oxidation [32], contaminants removal [33,34], hydrogenation catalysis [35]. These unique properties are expected to be applied to further reduce the electron transmission resistance and finally improve the performance of electrocatalysts towards water oxidation. Therefore, we select CeOx with Ni to form NiCeOx layers as the cocatalyst, which is expected to construct strongly coupled hetero-structure interface with NiFeOx that results in the changing of the local electronic structure for NiFeOx layers and exhibiting synergistic effects on the water oxidation performances, thus leading to improve the OER activities of NiFeOx at large current densities. Herein, we design a 3D self-supporting porous network NiCe@NiFe/NF electrocatalyst, which meets our target of performing efficient water oxidation activities at high current densities with lower overpotential. Such NiCe@NiFe/NF-N electrocatalyst can drive the current densities of 1 A/cm2 at an overpotential of 359 mV, and its OER activity at high current densities can be maintained for at least 20 h.
To synthesis of NiCe@NiFe/NF electrocatalysts, an electrodepositing method was applied to load nickel-cerium composites onto NiFe/NF electrodes. Typically, the electrodepositing process was carried out in a three-electrode system, in which NiFe/NF was served as the working electrode, platinum column as the counter electrode, Ag/ AgCl as the reference electrode. The potential was carried out at -1.0 V vs. Ag/AgCl and the deposition time was determined to be 30 min. The total amounts of Ni2+ and Ce3+ in the electrolyte were maintained at 10 mM and the Ni/Ce ratios was determined to be 7/3, and 3 mM NaCl and 6 mM (NH4)2C2O4 were also added into the electrolyte. The asprepared electrode was rinsed with ethanol and dried in air (denotes as NiCe@NiFe/NF-N). In addition, NiCe/NF electrode was synthesized by using bare Ni foam as the substrate, and NiCe@NiFe/NF with nanoparticles morphology (denotes as NiCe@NiFe/NF-P) was also prepared without adding (NH4)2C2O4. The loading density of these electrocatalysts was ∼3.5 mg/cm2 by calculating the weight difference between bare Ni foam and catalyst loaded Ni foam. 2.4. Structural characterizations X-ray diffraction (XRD) analysis was carried out using a Bruker (D8 Advance, Cu Kɑ radiation) diffractometer. Scanning electron microscopy (SEM) images were performed on a Hitachi SU8010 scanning electron microscope. Transmission electron microscopy (TEM) images were taken by JEOL 2010FEF high resolution transmission electron microscope with a field emission gun operated at 200 kV. Energy Dispersive X-ray spectrometry (EDS) analysis was analyzed by an IXRF SDD 2610 EDS system. Raman spectra was obtained using an InVia 1WU072 Raman Spectrometer (λ =532 nm). The surface composition and bonding configuration were investigated on a Thermo VG ESCALAB250 X-ray photoelectron spectroscopy (XPS). The metal content was determined by inductively coupled plasma (ICP) on an Avio 200 (PerkinElmer) after the sample was completely dissolved using diluted hydrochloric acid.
2. Experimental and methods
2.5. Electrochemical characterizations
2.1. Materials
Electrochemical measurements were performed with a Princeton VersaSTAT 3 electrochemical analyzer at 25 ℃. All electrochemical measurements were conducted in a typical three-electrode setup with an electrolyte solution of 1.0 M KOH (pH = 13.6), in which the obtained NiCe@NiFe/NF or NiFe/NF was served as the working electrode, platinum column as the counter electrode, and Hg/HgO (1 M KOH) electrode as the reference electrode and calibrated with regard to RHE. In 1 M KOH, all potentials measured were calibrated to reversible hydrogen electrode (RHE) using the following equation: ERHE =EHg/HgO+ 0.059*pH + 0.098. Polarization curves were recorded at a scan rate of 1 mV/s, corrected by the 95%-iR compensation. Electrochemical impedance spectroscopy (EIS) measurements were carried out at the overpotential of 460 mV by performing an applied voltage with the amplitude of 30 mV in a frequency range from 106 to 0.05 Hz (without iR compensation). Chronopotentiometric (J = 1 A/cm2) test was employed to investigate the long-term stability of the electrocatalyst. The electrochemical double-layer capacitance (Cdl) of the obtained electrodes was measured from its electrochemical capacitance in a nonfaradic region using a simple scanning-rate dependence cyclic voltammetry method. The cyclic voltammetry scan rate was set from 10 mV/s to 100 mV/s. The capacitive current is depended the scanning rate (v) and the capacitance, which is expected to be linearly proportional to the active surface area of electrode. By plotting the capacitive currents (Janodic-Jcathodic) versus the scanning rates, the capacitance can be estimated as half of the slope [36]. The specific capacitance can be converted into the electrochemical active surface area (ECSA) using the specific capacitance value (Cs, 60 μF/cm2) by the following equation of
Nickel nitrate hexahydrate (Ni(NO3)2.6H2O, AR), iron nitrate nonahydrate (Fe(NO3)3.9H2O, AR), ammonium oxalate monohydrate (C2H8N2O4.H2O, AR), sodium chloride (NaCl, AR), acetone, HCl, and ultrapure water (18.2 MΩ) were purchased from China National Medicines Corporation Ltd. Cerium nitrate hexahydrate (Ce (NO3)3.6H2O, 99.95%) and potassium hydroxide (KOH, 95%) were purchased from Aladdin. All chemicals were used without further purification.
2.2. Synthesis of NiFe/NF electrode Ni foam (NF, thickness: 2 mm, 1*1 cm) was first sonicated in acetone, HCl, water, ethanol several times to remove the surface impurities prior to the electrodepositing, then dried in air at 80℃ for 5 h. The electrodepositing of NiFe composites onto NF was undertaken in a three-electrode system containing equal molar (15 mM) of nickel (II) and iron (III) nitrates at room temperature, using NF as the working electrode, Pt as the counter electrode, Ag/AgCl (saturated KCl solution) as reference electrode. The deposition potential was −1.0 V vs. Ag/ AgCl. The best deposition time was determined to be 300 s. After deposition, the obtained NiFe electrocatalyst on NF electrode (denotes as NiFe@NF) was rinsed several times with ethanol, and dried in air overnight.
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ECSA = Cdl/Cs. And specific activity (SA, mA/ECSA2) can be calculated from the ECSA and measured current density J (mA/cm2) by the equation of SA = J/ECSA. The mass activity (Jm, A/g) was calculated from the catalyst loading m (3.5 mg/cm2) and the measured current density J (mA/cm2 at the overpotential of 360 mV) by the equation of Jm=J/m.
to the CeO2 (111), Fe3O4 (311), NiOOH (111) NiOOH (121), and Ni (OH)2 (111) structure, further confirming the coexistent of NiFeOx and NiCeOx phases and the successful deposition of NiCeOx on the surface of NiFe@NF electrode. In addition, the elemental compositions of these electrodes were also characterized by EDS analysis. As shown in Fig. 2a, it is found that the obtained NiCe@NiFe/NF-N is composed by Ni, Fe, Ce and O (the atomic ratio of these elements was determined to be 41.2%, 17.4%, 7.5%, and 33.9%, respectively), in which the atomic ratio of Ni and Fe is higher than that of NiFe/NF (Fig. S7) and similar with that of NiCe@NiFe/NF-P (Fig. S8), also indicating the NiCeOx layer was successfully deposited on the NiFe/NF. The content of Ce in NiCe@NiFe/NF-N is higher than that of NiCe@NiFe/NF-P may be due to the additive of (NH4)2C2O4 during depositing process [38]. Furthermore, the corresponding elemental mapping images of NiCe@NiFe/ NF-N also demonstrate obvious heterostructure of NiFeOx layer and NiCeOx network (Fig. 2b), and the elements of Ni, Fe, Ce and O are homogeneous distributed among the corresponding area of the NiCe@ NiFe/NF-N electrocatalyst (Fig. 2c–f). The successful deposition of NiFeOx and NiCeOx were further demonstrated by Raman spectroscopy. As shown in Fig. S9a, the fitting peak at around 450.6 cm−1 is determined to be the Fe-O bond, and the picks around 546 cm−1 and 715 cm−1 are determined to be the Ni-O band [39,40]. While after deposition of outer layer of NiCeOx, the corresponding peaks show a slight shift, and new peak around 462 cm−1 attributed to the F2g vibration mode of CeO2 and the peak at around 593 cm−1 associated with the defect-induced (D) modes of CeO2 are present (Fig. S9b). It can be seen that the peak intensity of NiCe@NiFe/NF-N is obvious higher than that of the NiCe@NiFe/NF-P, further implying the addition of (NH4)2C2O4 could promote the NiCeOx deposition. Detailed chemical structure and elemental valence state of the obtained samples were determined by X-ray photoelectron spectroscopy (XPS). The XPS spectrum of NiCe@NiFe/NF-N is shown in Fig. 3. For Ni 2p spectrum (Fig. 3a), the fitted peak at 856.2 eV and 873.8 eV are determined to Ni2+, while 861.9 eV and 881.7 eV can be attributed to Ni3+ [23,41]. For Fe 2p spectra (Fig. 3b), the peaks at 705.5 eV are corresponding to Fe (0), while 712.1 eV and 725 eV are indexed to Fe3+ [42,43]. In the XPS peaks for Ce 3d (Fig. 3c), the peaks located at 880.8 eV, 882.4 eV, 898.3 eV, 900.8 eV and 917 eV can be assigned to Ce4+, 885.7 eV and 904.4 eV are owned to Ce3+ [28,31]. In addition to the fitted peaks stated above, two additional shake-up satellite peaks can also be observed at 888.1 eV and 908.3 eV. Moreover, a high-resolution XPS spectrum of O 1s (Fig. 3d) provides further evidence for the existence of multiple oxidation states in the sample. Three fitted peaks located at 530.7 eV, 531.6 eV and 532.7 eV can be attributed to lattice oxygen, a substituted hydroxyl group and surface physic-& chemi-sorbed water [44,45]. For comparisons, XPS spectra of the relative elements for NiFe/NF (Fig. S10) and NiCe@NiFe/NF-P (Fig. S11) were also investigated. Compared with NiFe/NF sample, the zero-valence nickel is absent on the surface of both the NiCe@NiFe/NF-N and NiCe@NiFe/NF-P electrodes, suggests the successful decoration of NiCeOx on the surface of NiFeOx layer. Moreover, the coexistence of trivalent cerium (Ce3+) and tetravalent cerium (Ce4+) in the NiCeOx layer could accelerate the surface oxygen ion exchange, which could combine with the high oxygen storage capacity and further improve the electronic transmission capability, thus promoting the water oxidation activity. It is found that the binding energies for Ni 2p and Fe 2p are positively shift 0.9 eV and 0.8 eV in the NiCe@NiFe/NF-N electrode compared with those of NiFe/NF electrode (Fig. 3a and b). This, along with the change in the oxygen vacancy generation, confirms that there are strong electronic interactions between NiCeOx and NiFeOx layers, further indicates that deposited NiCeOx could change the local electronic structure and higher the oxidation state of NiFe species, thus significantly enhancing the catalytic activity for OER. Furthermore, it is found that the amount of Ce in NiCe@NiFe/NF-N is determined to be 5.06% and higher than 1.99% of NiCe@NiFe/NF-P, which is further consistent well with the EDS data (Table S1) and suggested that the
2.6. Calibration of the reference electrodes A standard three-electrode system was applied to perform the calibration of the reference electrodes. Ag/AgCl (saturated KCl solution) or Hg/HgO (1 M KOH solution) electrode as the reference electrodes, Pt foil (1*1 cm) as the working and counter electrodes. The calibrations of Ag/AgCl and Hg/HgO were carried out in HCl solution (1 M, bubbled with 99.999% H2, 25 ℃) and KOH solution (1 M, bubbled with 99.999% H2, 25 ℃), respectively. Then CV tests were conducted at the scan rate of 1 mV/s, and the average of the two potentials at which the current crossed zero was taken as the thermodynamic potential for the reversible hydrogen electrode. For Ag/AgCl in 1 M HCl solution, ERHE=EAg/AgCl+0.197 V. For Hg/HgO in 1 M KOH solution, ERHE=EHg/ HgO+0.9004 V. 3. Results and discussion The preparation of NiCe@NiFe/NF was achieved via a simple twostep electrodepositing process, as illustrated in Fig. 1a, nickel and iron ions were first deposited onto the nickel form to form a catalytic active phase of NiFeOx and then the outer NiCeOx layer was deposited on the surface of NiFeOx to form a heterostructure of NiCe@NiFe composite. It was proposed that the outer NiCeOx layer could regulate the electronic structure of the ferronickel surface and further enhance the electrocatalytic performance. Moreover, the additive of (NH4)2C2O4 during the deposition process could also produce a large amount of hydroxide ions, and then the metal ions combine with the generated hydroxide ions to form a 3D self-supporting porous network heterostructure on the Ni foam surface. XRD was applied to analyze the structural information of these as-prepared electrocatalysts (Fig. 1b), it is found that three main diffraction peaks at 2θ = 44.448°, 51.268°, and 76.358° can be well indexed to the diffraction peaks of nickel form (Ni, PDF no. 04-0850), indicated the low crystallinity of the obtained electrocatalysts. It should be pointed out that the electrocatalysts with amorphous features could further enhance the electrocatalytic performance [37]. Moreover, the surface morphology of these electrocatalyst was observed by SEM images. It can be observed that the bare NF shows a 3D porous structure (Fig. S1) and the NiFe/NF electrocatalyst exhibits a uniform, fine nanosheets covered on the stacked granular nanoparticles with the particles size of 150∼200 nm (Fig. S2). After depositing of NiCeOx layer, the NiCe@NiFe/NF-N electrode demonstrates a porous network morphology due to the addition of (NH4)2C2O4 (Fig. 1c). In contrast, the NiCe@NiFe/NF-P without using (NH4)2C2O4 shows a smaller graininess nanostructure with a diameter of 20–50 nm (Fig. S3). The microstructure of these electrocatalysts was further investigated by the transmission electron microscopy (TEM). As shown in Fig. S4a, the NiFeOx electrocatalyst demonstrates a crumpled layer nanostructure, which is consistent with the observation of SEM image. While after NiCeOx deposited, a heterostructure of network NiCeOx and layered NiFeOx can be well viewed (Fig. 1d). In contrast, the NiCe@NiFe/NF-P exhibits a particle and layer heterostructure morphology (Fig. S5a). The high-resolution transmission electron microscopy (HRTEM) images show that all the prepared samples have little lattice fringes, implying the poor crystallinity of these electrocatalysts (Figs. 1e, S4b and S5b), which are also good agreement with the XRD analysis (Fig. 1b). The partial lattice fringe space of 3.03 Å and 3.12 Å determined in the HRTEM image for NiCe@NiFe/NF-N are indexed to Ce2O3 (002) and CeO2 (111), and the Ni(OH)2 exists in an amorphous form. The selected-area electron diffraction (SAED, Fig. S6) pattern can be assigned 3
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Fig. 1. a) Schematic illustration of the synthesis of NiCe@NiFe/NF by two-step electrodepositing process. b) XRD patterns of NiFe/NF, NiCe@NiFe/NF and bare Ni foam. c) SEM, d) TEM images and e) HRTEM image of NiCe@NiFe/NF-N.
NiCe@NiFe/NF-N electrode exhibits the most excellent OER activity, with a current density of 100 mA/cm2 at the overpotential of 254 mV. This value is also superior to that of the state-of-the-art RuO2 electrocatalyst (overpotential is 470 mV at 100 mA/cm2) and is also comparable to the performances of many reported electrocatalysts for water oxidation [23,24,28,31,46–51] (Table S3). These results evidence that the decoration of NiCeOx layer on NiFe/NF results in a dramatically improved electrocatalytic activity for water oxidation. Furthermore, the NiCe@NiFe/NF-N electrode only needs an overpotential of 359 mV to efficiently deliver a large current density of 1A/cm2 in 1 M KOH solution, which is much smaller than that of 450 mV for NiCe@NiFe-P electrode to achieve the same current densities under the same conditions (Table 1), suggesting that the NiCe@NiFe/NF-N electrode meets the activity requirements and could be further used for practical wateralkali electrolyzer.
additive of (NH4)2C2O4 could make the NiCeOx deposited more uniformly on the 3D NiFeOx layer. The water oxidation performances of these electrocatalysts were evaluated by measuring the linear sweep voltammetry (LSV) in 1 M KOH alkaline electrolyte. The optimized conditions were investigated at first and it is found that the sample with the Ni/Ce feed ratio at 7/3 exhibits the best OER performance (Fig. S12) and it is illustrated that the accurately ratio of Ni/Ce is calculate to be 4.7 (Table S2), This can be also illuminated by the SEM images of these samples, which can be found that the NiCe@NiFe/NF-N electrocatalyst with the Ni/Ce feed ratio at 7/3 demonstrates the more uniformly network nanostructure than that of the other three samples (Fig. S13). The representative LSV curves of these samples demonstrate a decrease in water oxidation performance in the order NiCe@NiFe/NF-N, NiCe@NiFe-P, NiFe/NF, NiCe/NF, and nickel foam (Fig. 4a and Table 1). Particularly, the 4
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Fig. 2. a) EDS spectrum, b) EDS layer image and c) to f) the corresponding elemental mapping images of Ni, Fe, Ce, O for the NiCe@NiFe/NF-N.
process of these electrodes, the Nyquist plots were also presented at the overpotential of 460 mV in 1 M KOH solution (Figs. 4c and S15). It is found that the radius of the semicircle of Nyquist plot for NiCe@NiFe/ NF-N is smaller than those of contrast electrodes (Table 1), indicating the low charge transfer resistance of the NiCe@NiFe/NF-N and further confirming the fast OER kinetics. It should also pointed out that, compared with those of NiFe/NF and NiCe/NF electrodes (insets in Fig. S15), the different equivalent circuit diagrams of NiCe@NiFe/NF-N and NiCe@NiFe/NF-P illustrate an additional interface resistance (inset in Fig. 4c), further implying the existence of heterostructure of NiCeOx on
The OER kinetics of these electrocatalysts was further studied by determining the corresponding Tafel slopes, as shown in Fig. 4b, Table 1 and Fig. S14. The Tafel slope for NiCe@NiFe/NF-N was determined to be 59.9 mV/dec, which is also smaller than those of NiCe@ NiFe-P (87.8 mV/dec), NiFe/NF (126.8 mV/dec), NiCe/NF (130.2 mV/ dec) as well as commercial RuO2 sample (151.8 mV/dec). The smaller Tafel slope for NiCe@NiFe/NF-N can be further attributed to the heterostructure of NiCeOx decorated NiFeOx, which could provide more better electrocatalytic activity and water oxidation kinetics for the NiCe@NiFe/NF-N. To gain further insight into the water oxidation 5
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Fig. 3. XPS spectra of NiCe@NiFe/NF-N for a) Ni 2p, b) Fe 2p, c) Ce 3d and d) O 1s regions.
of water oxidation catalysts often possess favored electrochemical active surface area (ECSA). Typically, the values of ECSA are positively related to their corresponding double-layer capacitances (Cdl), which can be obtained by performing scan-rate dependent cyclic voltammetry (CV) method (Fig. S16a–c). Based on this, the Cdl (Fig. S16d) and ECSA (Fig. 5) values for the NiCe@NiFe/NF-N is calculated to be 2.83 m F/
the surface of NiFeOx layer. Besides that, negligible Warburg diffusion resistances (ω) at low-frequencies can be found for NiCe@NiFe/NF-N and NiCe@NiFe/NF-P at large current densities, indicated the massdiffusion process at large current densities could be promoted by the distinct heterostructure between NiFeOx and NiCeOx. It is well accepted that the excellent electrocatalytic performances
Fig. 4. Electrochemical characterizations for various electrocatalysts in 1 M KOH solution of a) Linear sweep voltammetry curves, b) Tafel plots, and c) EIS Nyquist plots. d) Chronopotentiometric curve of NiCe@NiFe/NF-N in 1 M KOH with a current density of 1 A/cm2. 6
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ratio of Ce3+/Ce4+ for the post-OER sample is higher than the initial NiCe@NiFe/NF-N (Table S1). This phenomenon consistent well with the analysis results of Raman spectroscopy (Fig. S9b). It is found that the relative intensity ratio of ID/IF2g increase from 16.9% for NiCe@ NiFe/NF-N to 26.1% for the post-OER NiCe@NiFe/NF-N electrocatalyst, indicating that a large amount of oxygen vacancies generate on the surface of the electrocatalyst during OER process [52,53], thus providing more surface electrochemical active OER sites and further enhancing the water oxidation kinetics. Above all, the extraordinary OER activities of NiCe@NiFe/NF-N can be attributed to the following two factors: i) the intrinsic activity of NiCe@NiFe/NF-N caused by favored Ni/Ce composition, electronic structures and surface oxygen vacancies during the OER process, which could boost the water oxidation activities. ii) The heterostructure network of NiCeOx and NiFeOx layer with optimal Ni/Ce atomic ratio and morphology, which leads to the good synergistic effect between Ni2+ and Ce3+/4+ at atomic level and robust coupling between NiCeOx and NiFeOx layer at nanoscale level, thus resulting in the improved OER activities for NiCe@NiFe/NF-N electrode.
Table 1 Comparisons of OER properties for different electrocatalysts on Ni foam electrode. Sample
NiCe@NiFe/NF-N NiCe@NiFe/NF-P NiFe/NF NiCe/NF RuO2
η (mV) @100 mA/cm2
@1 A/cm2
254 264 282 380 470
359 450 N.A N.A N.A
Tafel slope (mV/dec)
Rct (Ω)
59.9 87.8 126.8 130.2 151.8
0.1542 0.1569 0.2881 1.756 2.643
Rct represents the charge transfer resistance. N.A denotes as not available.
4. Conclusion In conclusion, a three-dimensional self-supporting heterostructure network NiCe@NiFe/NF-N OER electrocatalytic electrode has been fabricated by a simple two step electrodepositing process. The electrocatalyst combines the structural and catalytic advantages of heterostructure network of NiCeOx and NiFeOx layer, delivering the current density of 100 mA/cm2 and 1 A/cm2 at the overpotentials of 254 mV and 359 mV, which is much superior to those of the contrast electrocatalysts in this work. Moreover, such heterostructure electrocatalyst also exhibits a superior long-term stability towards efficient water oxidation activities at 1 A/cm2 with 20 h. Detailed structural and electrochemical characterizations confirm that the excellent water oxidation performances of NiCe@NiFe/NF-N can be attributed to the optimal surface and electronic structures, that is the nano-sized heterostructure network of NiCeOx and NiFeOx layer with favored Ni/Ce composition and morphology, and the rest is the increased oxygen vacancies due to the ratio changes of Ce3+/Ce4+ promote the intrinsic activity of the catalyst during water oxidation process. These findings may provide ideas for designing high performance oxygen evolution catalysts, and encourage other researchers to explore energy-related catalysts with superior catalytic activity by using the similar synthetic approaches.
Fig. 5. The comparisons of ECSA, intrinsic activity (activity normalized by mass and ECSA) for NiFe/NF, NiCe@NiFe/NF-P and NiCe@NiFe/NF-N.
cm2 and 47.2 cm2, which is larger than those of NiCe@NiFe/NF-P (2.48 m F/cm2, 41.3 cm2) and NiFe/NF (1.77 m F/cm2, 29.5 cm2). It is found that the improvement of ECSA is really not significant, which may be ascribed to the intrinsic activity of the catalysts rather than ECSA is the distinct features for the enhanced OER performances. Therefore, the corresponding comparisons of intrinsic activity (activity normalized by mass and ECSA) are provided in the revised manuscript. As shown in Fig. 5, the specific activity (normalized by ECSA) is calculated to be 8.8, 10.3 and 21.9 mA/cm2 for NiFe/NF, NiCe@NiFe/NFP, NiCe@NiFe/NF-N, and respectively, implying that the intrinsic activity of NiCe@NiFe/NF-N is much higher than those of NiCe@NiFe/ NF-P and NiFe/NF. Moreover, the mass activity of NiCe@NiFe/NF-N is determined to be 299.1 A/g. The value is twice as much as that of NiCe@NiFe/NF-P and four times to that of NiFe/NF. Such enhanced intrinsic activities combined with the Tafel plots, EIS analysis as well as the structural characterizations of these electrocatalysts further indicated the excellent intrinsic catalytic activity originated from the 3D porous heterostructure of NiCe@NiFe/NF-N could be one of the distinct features contributed to the enhanced high OER performances at large current densities. Besides of superior water oxidation current densities, long-term OER durability at large current densities is also one of the crucial targets to evaluate the electrocatalytic performances of OER catalysts. As shown in Fig. 4d, a chronopotentiometric test was applied at 1A/cm2 in 1 M KOH solution for 20 h. It is found that NiCe@NiFe/NF-N could remain its electrocatalytic performance at such high current densities over 20 h with the applied potential has a slight decrease, suggesting an activation process may be existed during the long-term stability testing. Therefore, XPS measurement was carried out to analysis the NiCe@ NiFe/NF-N electrocatalyst after long-term OER stability test (Fig. S17). Although the morphology and microstructure of NiCe@NiFe/NF-N still keep unchanged (Fig. S18), it is concluded that some of the Ce on the surface corrodes into electrolyte after long-term stability test and the
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We appreciate the financial funding supported by the National Natural Science Foundation of China (No. 21878204). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2019.118199. References [1] S. Anantharaj, S.R. Ede, K. Sakthikumar, K. Karthick, S. Mishra, S. Kundu, ACS Catal. 6 (2016) 8069–8097. [2] K. Fan, Y. Ji, H. Zou, J. Zhang, B. Zhu, H. Chen, Q. Daniel, Y. Luo, J. Yu, L. Sun, Angew. Chem. Int. Ed. 56 (2017) 3289–3293.
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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
3D porous network heterostructure NiCe@NiFe electrocatalyst for efficient oxygen evolution reaction at large current densities ⁎⁎
Guang Liua,b, , Muheng Wanga, Yun Wua, Na Lia, Fei Zhaoa, Qiang Zhaoa, Jinping Lia,
T
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a Shanxi Key Laboratory of Gas Energy Efficient and Clean Utilization, Research Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, Shanxi, 030024, PR China b Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin, 300071, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrogen production Oxygen evolution reaction Electrocatalyst Heterostructure Large current densities
Three-dimensional (3D) self-supporting porous network NiCeOx@NiFeOx electrocatalyst on Ni foam (denotes as NiCe@NiFe/NF-N) is prepared by a simple two step electrodepositing process. NiCe@NiFe/NF-N attains 100 mA/cm2 at overpotential of 254 mV with Tafel slope of 59.9 mV/dec, which is much lower than that of NiCe@NiFe/NF-P, NiFe@NF, NiCe@NF and also commercial RuO2 electrocatalysts. Furthermore, the obtained NiCe@NiFe/NF-N electrocatalyst only needs overpotential of 359 mV to deliver high current densities of 1 A/ cm2 and demonstrates excellent stability at such large current densities for 20 h, which is also superior to that of NiCe@NiFe/NF-P electrocatalyst. Further study reveals that the enhanced water oxidation performance of NiCe@NiFe/NF-N electrocatalyst with good stability at high current densities could be attributed to the optimal surface and electronic structures, fast reaction kinetics and high intrinsic catalytic activity. Collectively, such electrocatalyst paves new opportunities for development of low-cost water oxidation electrocatalysts with high current densities for practical water splitting applications.
1. Introduction With the development of the world economy, the demand for resources in human society is also growing rapidly [1,2]. Traditional fossil energy sources not only have limited reserves, but also bring about increasingly prominent environmental problems. Hydrogen energy has been widely used as a new type of energy with high calorific value, clean and pollution-free [3,4]. Electrochemical water splitting plays a key role in the future hydrogen production and can be divided into two half reactions, cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) [5,6]. OER involves a complex multi-proton-couple electron transfer process with slow kinetics and high overpotentials, which is the rate-limiting step in the water-decomposition reaction [7,8]. Precious metal-based catalysts such as iridium and ruthenium oxide have high OER activity, but rare reserves and expensive prices limit their large-scale applications [9,10]. In recent years, extensive efforts have been devoted to designing and synthesizing efficient, durable, and low-cost alternatives based on earth-abundant 3d transition metals. especially nickel [11,12], iron [13], cobalt [14,15]. Furthermore, their corresponding oxides/
hydroxides [3], sulfides [4,16], phosphides [17,18], selenides [19,20] and nitrides [21,22] have also been widely studied. In spite of the great advances made recently in the development of transition metal-based OER electrocatalysts, there is still a big challenge in applying the newly developed catalysts in a viable water-splitting technology. At present, most OER catalysts work well only at small current densities (e.g. 10–100 mA/cm2). To be used for industrial applications, OER catalysts need to meet more harsh test environment such as delivery of large current densities (> 500 mA/cm2) at low overpotentials (< 300 mV), and mechanical robustness and prolonged durability during strong gas evolution [23,24]. In order to meet the above requirements, catalysts should fulfill multiple criteria simultaneously: (1) highly intrinsic OER performance, (2) surface rich catalytic active sites, (3) fast mass transport and charge transfer capability, (4) excellent stability, and (5) corrosion resistance [23,25–27]. Nevertheless, to date, there are few OER electrocatalysts have been developed to hit all of the above spots. This is challenging target for powdery OER electrocatalysts, which often need to be adhered to a current collector by using a polymer binder. Under such circumstances, long-term electrocatalytic performance with large current density is inacceptable
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Corresponding author. Corresponding author at: Shanxi Key Laboratory of Gas Energy Efficient and Clean Utilization, Research Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, Shanxi, 030024, PR China. E-mail addresses: [email protected] (G. Liu), [email protected] (J. Li). ⁎⁎
https://doi.org/10.1016/j.apcatb.2019.118199 Received 9 July 2019; Received in revised form 12 September 2019; Accepted 13 September 2019 Available online 14 September 2019 0926-3373/ © 2019 Elsevier B.V. All rights reserved.
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2.3. Synthesis of NiCe@NiFe/NF and NiCe/NF electrodes
because of the inferior interfacial resistance between the current collector and the catalyst as well as bubble evolving induced catalyst peeling during OER process. In pursuit of efficient electrocatalysts with high current densities performance, increasing mass loading is a viable pathway but might obstruct the mass transport and charge transfer. Other factors that influence the mass-diffusion process during OER at high current densities is the dissipation of gas bubbles formed on the electrodes, which would cause catalyst peeling off problem and large bubble overpotentials. In this regard, developing and synthesizing of self-supported electrocatalysts with excellent electrocatalytic performance, superior stability, favored electronic conductivity, and unique macroscopic structure are urgent needed, which could perform efficient water oxidation activities at high current densities with lower overpotential. Very recently some studies reported that cerium oxides has excellent electronic/ionic conductivity, good oxygen-storage capacity and reversible surface oxygen ion exchange owing to the flexible transition between the Ce3+ and Ce4+ oxidation states [28–31]. It has been found that CeOx-based materials can be served as efficient catalysts or catalyst-supports towards heterogeneous catalysis, e.g. CO oxidation [32], contaminants removal [33,34], hydrogenation catalysis [35]. These unique properties are expected to be applied to further reduce the electron transmission resistance and finally improve the performance of electrocatalysts towards water oxidation. Therefore, we select CeOx with Ni to form NiCeOx layers as the cocatalyst, which is expected to construct strongly coupled hetero-structure interface with NiFeOx that results in the changing of the local electronic structure for NiFeOx layers and exhibiting synergistic effects on the water oxidation performances, thus leading to improve the OER activities of NiFeOx at large current densities. Herein, we design a 3D self-supporting porous network NiCe@NiFe/NF electrocatalyst, which meets our target of performing efficient water oxidation activities at high current densities with lower overpotential. Such NiCe@NiFe/NF-N electrocatalyst can drive the current densities of 1 A/cm2 at an overpotential of 359 mV, and its OER activity at high current densities can be maintained for at least 20 h.
To synthesis of NiCe@NiFe/NF electrocatalysts, an electrodepositing method was applied to load nickel-cerium composites onto NiFe/NF electrodes. Typically, the electrodepositing process was carried out in a three-electrode system, in which NiFe/NF was served as the working electrode, platinum column as the counter electrode, Ag/ AgCl as the reference electrode. The potential was carried out at -1.0 V vs. Ag/AgCl and the deposition time was determined to be 30 min. The total amounts of Ni2+ and Ce3+ in the electrolyte were maintained at 10 mM and the Ni/Ce ratios was determined to be 7/3, and 3 mM NaCl and 6 mM (NH4)2C2O4 were also added into the electrolyte. The asprepared electrode was rinsed with ethanol and dried in air (denotes as NiCe@NiFe/NF-N). In addition, NiCe/NF electrode was synthesized by using bare Ni foam as the substrate, and NiCe@NiFe/NF with nanoparticles morphology (denotes as NiCe@NiFe/NF-P) was also prepared without adding (NH4)2C2O4. The loading density of these electrocatalysts was ∼3.5 mg/cm2 by calculating the weight difference between bare Ni foam and catalyst loaded Ni foam. 2.4. Structural characterizations X-ray diffraction (XRD) analysis was carried out using a Bruker (D8 Advance, Cu Kɑ radiation) diffractometer. Scanning electron microscopy (SEM) images were performed on a Hitachi SU8010 scanning electron microscope. Transmission electron microscopy (TEM) images were taken by JEOL 2010FEF high resolution transmission electron microscope with a field emission gun operated at 200 kV. Energy Dispersive X-ray spectrometry (EDS) analysis was analyzed by an IXRF SDD 2610 EDS system. Raman spectra was obtained using an InVia 1WU072 Raman Spectrometer (λ =532 nm). The surface composition and bonding configuration were investigated on a Thermo VG ESCALAB250 X-ray photoelectron spectroscopy (XPS). The metal content was determined by inductively coupled plasma (ICP) on an Avio 200 (PerkinElmer) after the sample was completely dissolved using diluted hydrochloric acid.
2. Experimental and methods
2.5. Electrochemical characterizations
2.1. Materials
Electrochemical measurements were performed with a Princeton VersaSTAT 3 electrochemical analyzer at 25 ℃. All electrochemical measurements were conducted in a typical three-electrode setup with an electrolyte solution of 1.0 M KOH (pH = 13.6), in which the obtained NiCe@NiFe/NF or NiFe/NF was served as the working electrode, platinum column as the counter electrode, and Hg/HgO (1 M KOH) electrode as the reference electrode and calibrated with regard to RHE. In 1 M KOH, all potentials measured were calibrated to reversible hydrogen electrode (RHE) using the following equation: ERHE =EHg/HgO+ 0.059*pH + 0.098. Polarization curves were recorded at a scan rate of 1 mV/s, corrected by the 95%-iR compensation. Electrochemical impedance spectroscopy (EIS) measurements were carried out at the overpotential of 460 mV by performing an applied voltage with the amplitude of 30 mV in a frequency range from 106 to 0.05 Hz (without iR compensation). Chronopotentiometric (J = 1 A/cm2) test was employed to investigate the long-term stability of the electrocatalyst. The electrochemical double-layer capacitance (Cdl) of the obtained electrodes was measured from its electrochemical capacitance in a nonfaradic region using a simple scanning-rate dependence cyclic voltammetry method. The cyclic voltammetry scan rate was set from 10 mV/s to 100 mV/s. The capacitive current is depended the scanning rate (v) and the capacitance, which is expected to be linearly proportional to the active surface area of electrode. By plotting the capacitive currents (Janodic-Jcathodic) versus the scanning rates, the capacitance can be estimated as half of the slope [36]. The specific capacitance can be converted into the electrochemical active surface area (ECSA) using the specific capacitance value (Cs, 60 μF/cm2) by the following equation of
Nickel nitrate hexahydrate (Ni(NO3)2.6H2O, AR), iron nitrate nonahydrate (Fe(NO3)3.9H2O, AR), ammonium oxalate monohydrate (C2H8N2O4.H2O, AR), sodium chloride (NaCl, AR), acetone, HCl, and ultrapure water (18.2 MΩ) were purchased from China National Medicines Corporation Ltd. Cerium nitrate hexahydrate (Ce (NO3)3.6H2O, 99.95%) and potassium hydroxide (KOH, 95%) were purchased from Aladdin. All chemicals were used without further purification.
2.2. Synthesis of NiFe/NF electrode Ni foam (NF, thickness: 2 mm, 1*1 cm) was first sonicated in acetone, HCl, water, ethanol several times to remove the surface impurities prior to the electrodepositing, then dried in air at 80℃ for 5 h. The electrodepositing of NiFe composites onto NF was undertaken in a three-electrode system containing equal molar (15 mM) of nickel (II) and iron (III) nitrates at room temperature, using NF as the working electrode, Pt as the counter electrode, Ag/AgCl (saturated KCl solution) as reference electrode. The deposition potential was −1.0 V vs. Ag/ AgCl. The best deposition time was determined to be 300 s. After deposition, the obtained NiFe electrocatalyst on NF electrode (denotes as NiFe@NF) was rinsed several times with ethanol, and dried in air overnight.
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ECSA = Cdl/Cs. And specific activity (SA, mA/ECSA2) can be calculated from the ECSA and measured current density J (mA/cm2) by the equation of SA = J/ECSA. The mass activity (Jm, A/g) was calculated from the catalyst loading m (3.5 mg/cm2) and the measured current density J (mA/cm2 at the overpotential of 360 mV) by the equation of Jm=J/m.
to the CeO2 (111), Fe3O4 (311), NiOOH (111) NiOOH (121), and Ni (OH)2 (111) structure, further confirming the coexistent of NiFeOx and NiCeOx phases and the successful deposition of NiCeOx on the surface of NiFe@NF electrode. In addition, the elemental compositions of these electrodes were also characterized by EDS analysis. As shown in Fig. 2a, it is found that the obtained NiCe@NiFe/NF-N is composed by Ni, Fe, Ce and O (the atomic ratio of these elements was determined to be 41.2%, 17.4%, 7.5%, and 33.9%, respectively), in which the atomic ratio of Ni and Fe is higher than that of NiFe/NF (Fig. S7) and similar with that of NiCe@NiFe/NF-P (Fig. S8), also indicating the NiCeOx layer was successfully deposited on the NiFe/NF. The content of Ce in NiCe@NiFe/NF-N is higher than that of NiCe@NiFe/NF-P may be due to the additive of (NH4)2C2O4 during depositing process [38]. Furthermore, the corresponding elemental mapping images of NiCe@NiFe/ NF-N also demonstrate obvious heterostructure of NiFeOx layer and NiCeOx network (Fig. 2b), and the elements of Ni, Fe, Ce and O are homogeneous distributed among the corresponding area of the NiCe@ NiFe/NF-N electrocatalyst (Fig. 2c–f). The successful deposition of NiFeOx and NiCeOx were further demonstrated by Raman spectroscopy. As shown in Fig. S9a, the fitting peak at around 450.6 cm−1 is determined to be the Fe-O bond, and the picks around 546 cm−1 and 715 cm−1 are determined to be the Ni-O band [39,40]. While after deposition of outer layer of NiCeOx, the corresponding peaks show a slight shift, and new peak around 462 cm−1 attributed to the F2g vibration mode of CeO2 and the peak at around 593 cm−1 associated with the defect-induced (D) modes of CeO2 are present (Fig. S9b). It can be seen that the peak intensity of NiCe@NiFe/NF-N is obvious higher than that of the NiCe@NiFe/NF-P, further implying the addition of (NH4)2C2O4 could promote the NiCeOx deposition. Detailed chemical structure and elemental valence state of the obtained samples were determined by X-ray photoelectron spectroscopy (XPS). The XPS spectrum of NiCe@NiFe/NF-N is shown in Fig. 3. For Ni 2p spectrum (Fig. 3a), the fitted peak at 856.2 eV and 873.8 eV are determined to Ni2+, while 861.9 eV and 881.7 eV can be attributed to Ni3+ [23,41]. For Fe 2p spectra (Fig. 3b), the peaks at 705.5 eV are corresponding to Fe (0), while 712.1 eV and 725 eV are indexed to Fe3+ [42,43]. In the XPS peaks for Ce 3d (Fig. 3c), the peaks located at 880.8 eV, 882.4 eV, 898.3 eV, 900.8 eV and 917 eV can be assigned to Ce4+, 885.7 eV and 904.4 eV are owned to Ce3+ [28,31]. In addition to the fitted peaks stated above, two additional shake-up satellite peaks can also be observed at 888.1 eV and 908.3 eV. Moreover, a high-resolution XPS spectrum of O 1s (Fig. 3d) provides further evidence for the existence of multiple oxidation states in the sample. Three fitted peaks located at 530.7 eV, 531.6 eV and 532.7 eV can be attributed to lattice oxygen, a substituted hydroxyl group and surface physic-& chemi-sorbed water [44,45]. For comparisons, XPS spectra of the relative elements for NiFe/NF (Fig. S10) and NiCe@NiFe/NF-P (Fig. S11) were also investigated. Compared with NiFe/NF sample, the zero-valence nickel is absent on the surface of both the NiCe@NiFe/NF-N and NiCe@NiFe/NF-P electrodes, suggests the successful decoration of NiCeOx on the surface of NiFeOx layer. Moreover, the coexistence of trivalent cerium (Ce3+) and tetravalent cerium (Ce4+) in the NiCeOx layer could accelerate the surface oxygen ion exchange, which could combine with the high oxygen storage capacity and further improve the electronic transmission capability, thus promoting the water oxidation activity. It is found that the binding energies for Ni 2p and Fe 2p are positively shift 0.9 eV and 0.8 eV in the NiCe@NiFe/NF-N electrode compared with those of NiFe/NF electrode (Fig. 3a and b). This, along with the change in the oxygen vacancy generation, confirms that there are strong electronic interactions between NiCeOx and NiFeOx layers, further indicates that deposited NiCeOx could change the local electronic structure and higher the oxidation state of NiFe species, thus significantly enhancing the catalytic activity for OER. Furthermore, it is found that the amount of Ce in NiCe@NiFe/NF-N is determined to be 5.06% and higher than 1.99% of NiCe@NiFe/NF-P, which is further consistent well with the EDS data (Table S1) and suggested that the
2.6. Calibration of the reference electrodes A standard three-electrode system was applied to perform the calibration of the reference electrodes. Ag/AgCl (saturated KCl solution) or Hg/HgO (1 M KOH solution) electrode as the reference electrodes, Pt foil (1*1 cm) as the working and counter electrodes. The calibrations of Ag/AgCl and Hg/HgO were carried out in HCl solution (1 M, bubbled with 99.999% H2, 25 ℃) and KOH solution (1 M, bubbled with 99.999% H2, 25 ℃), respectively. Then CV tests were conducted at the scan rate of 1 mV/s, and the average of the two potentials at which the current crossed zero was taken as the thermodynamic potential for the reversible hydrogen electrode. For Ag/AgCl in 1 M HCl solution, ERHE=EAg/AgCl+0.197 V. For Hg/HgO in 1 M KOH solution, ERHE=EHg/ HgO+0.9004 V. 3. Results and discussion The preparation of NiCe@NiFe/NF was achieved via a simple twostep electrodepositing process, as illustrated in Fig. 1a, nickel and iron ions were first deposited onto the nickel form to form a catalytic active phase of NiFeOx and then the outer NiCeOx layer was deposited on the surface of NiFeOx to form a heterostructure of NiCe@NiFe composite. It was proposed that the outer NiCeOx layer could regulate the electronic structure of the ferronickel surface and further enhance the electrocatalytic performance. Moreover, the additive of (NH4)2C2O4 during the deposition process could also produce a large amount of hydroxide ions, and then the metal ions combine with the generated hydroxide ions to form a 3D self-supporting porous network heterostructure on the Ni foam surface. XRD was applied to analyze the structural information of these as-prepared electrocatalysts (Fig. 1b), it is found that three main diffraction peaks at 2θ = 44.448°, 51.268°, and 76.358° can be well indexed to the diffraction peaks of nickel form (Ni, PDF no. 04-0850), indicated the low crystallinity of the obtained electrocatalysts. It should be pointed out that the electrocatalysts with amorphous features could further enhance the electrocatalytic performance [37]. Moreover, the surface morphology of these electrocatalyst was observed by SEM images. It can be observed that the bare NF shows a 3D porous structure (Fig. S1) and the NiFe/NF electrocatalyst exhibits a uniform, fine nanosheets covered on the stacked granular nanoparticles with the particles size of 150∼200 nm (Fig. S2). After depositing of NiCeOx layer, the NiCe@NiFe/NF-N electrode demonstrates a porous network morphology due to the addition of (NH4)2C2O4 (Fig. 1c). In contrast, the NiCe@NiFe/NF-P without using (NH4)2C2O4 shows a smaller graininess nanostructure with a diameter of 20–50 nm (Fig. S3). The microstructure of these electrocatalysts was further investigated by the transmission electron microscopy (TEM). As shown in Fig. S4a, the NiFeOx electrocatalyst demonstrates a crumpled layer nanostructure, which is consistent with the observation of SEM image. While after NiCeOx deposited, a heterostructure of network NiCeOx and layered NiFeOx can be well viewed (Fig. 1d). In contrast, the NiCe@NiFe/NF-P exhibits a particle and layer heterostructure morphology (Fig. S5a). The high-resolution transmission electron microscopy (HRTEM) images show that all the prepared samples have little lattice fringes, implying the poor crystallinity of these electrocatalysts (Figs. 1e, S4b and S5b), which are also good agreement with the XRD analysis (Fig. 1b). The partial lattice fringe space of 3.03 Å and 3.12 Å determined in the HRTEM image for NiCe@NiFe/NF-N are indexed to Ce2O3 (002) and CeO2 (111), and the Ni(OH)2 exists in an amorphous form. The selected-area electron diffraction (SAED, Fig. S6) pattern can be assigned 3
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Fig. 1. a) Schematic illustration of the synthesis of NiCe@NiFe/NF by two-step electrodepositing process. b) XRD patterns of NiFe/NF, NiCe@NiFe/NF and bare Ni foam. c) SEM, d) TEM images and e) HRTEM image of NiCe@NiFe/NF-N.
NiCe@NiFe/NF-N electrode exhibits the most excellent OER activity, with a current density of 100 mA/cm2 at the overpotential of 254 mV. This value is also superior to that of the state-of-the-art RuO2 electrocatalyst (overpotential is 470 mV at 100 mA/cm2) and is also comparable to the performances of many reported electrocatalysts for water oxidation [23,24,28,31,46–51] (Table S3). These results evidence that the decoration of NiCeOx layer on NiFe/NF results in a dramatically improved electrocatalytic activity for water oxidation. Furthermore, the NiCe@NiFe/NF-N electrode only needs an overpotential of 359 mV to efficiently deliver a large current density of 1A/cm2 in 1 M KOH solution, which is much smaller than that of 450 mV for NiCe@NiFe-P electrode to achieve the same current densities under the same conditions (Table 1), suggesting that the NiCe@NiFe/NF-N electrode meets the activity requirements and could be further used for practical wateralkali electrolyzer.
additive of (NH4)2C2O4 could make the NiCeOx deposited more uniformly on the 3D NiFeOx layer. The water oxidation performances of these electrocatalysts were evaluated by measuring the linear sweep voltammetry (LSV) in 1 M KOH alkaline electrolyte. The optimized conditions were investigated at first and it is found that the sample with the Ni/Ce feed ratio at 7/3 exhibits the best OER performance (Fig. S12) and it is illustrated that the accurately ratio of Ni/Ce is calculate to be 4.7 (Table S2), This can be also illuminated by the SEM images of these samples, which can be found that the NiCe@NiFe/NF-N electrocatalyst with the Ni/Ce feed ratio at 7/3 demonstrates the more uniformly network nanostructure than that of the other three samples (Fig. S13). The representative LSV curves of these samples demonstrate a decrease in water oxidation performance in the order NiCe@NiFe/NF-N, NiCe@NiFe-P, NiFe/NF, NiCe/NF, and nickel foam (Fig. 4a and Table 1). Particularly, the 4
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Fig. 2. a) EDS spectrum, b) EDS layer image and c) to f) the corresponding elemental mapping images of Ni, Fe, Ce, O for the NiCe@NiFe/NF-N.
process of these electrodes, the Nyquist plots were also presented at the overpotential of 460 mV in 1 M KOH solution (Figs. 4c and S15). It is found that the radius of the semicircle of Nyquist plot for NiCe@NiFe/ NF-N is smaller than those of contrast electrodes (Table 1), indicating the low charge transfer resistance of the NiCe@NiFe/NF-N and further confirming the fast OER kinetics. It should also pointed out that, compared with those of NiFe/NF and NiCe/NF electrodes (insets in Fig. S15), the different equivalent circuit diagrams of NiCe@NiFe/NF-N and NiCe@NiFe/NF-P illustrate an additional interface resistance (inset in Fig. 4c), further implying the existence of heterostructure of NiCeOx on
The OER kinetics of these electrocatalysts was further studied by determining the corresponding Tafel slopes, as shown in Fig. 4b, Table 1 and Fig. S14. The Tafel slope for NiCe@NiFe/NF-N was determined to be 59.9 mV/dec, which is also smaller than those of NiCe@ NiFe-P (87.8 mV/dec), NiFe/NF (126.8 mV/dec), NiCe/NF (130.2 mV/ dec) as well as commercial RuO2 sample (151.8 mV/dec). The smaller Tafel slope for NiCe@NiFe/NF-N can be further attributed to the heterostructure of NiCeOx decorated NiFeOx, which could provide more better electrocatalytic activity and water oxidation kinetics for the NiCe@NiFe/NF-N. To gain further insight into the water oxidation 5
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Fig. 3. XPS spectra of NiCe@NiFe/NF-N for a) Ni 2p, b) Fe 2p, c) Ce 3d and d) O 1s regions.
of water oxidation catalysts often possess favored electrochemical active surface area (ECSA). Typically, the values of ECSA are positively related to their corresponding double-layer capacitances (Cdl), which can be obtained by performing scan-rate dependent cyclic voltammetry (CV) method (Fig. S16a–c). Based on this, the Cdl (Fig. S16d) and ECSA (Fig. 5) values for the NiCe@NiFe/NF-N is calculated to be 2.83 m F/
the surface of NiFeOx layer. Besides that, negligible Warburg diffusion resistances (ω) at low-frequencies can be found for NiCe@NiFe/NF-N and NiCe@NiFe/NF-P at large current densities, indicated the massdiffusion process at large current densities could be promoted by the distinct heterostructure between NiFeOx and NiCeOx. It is well accepted that the excellent electrocatalytic performances
Fig. 4. Electrochemical characterizations for various electrocatalysts in 1 M KOH solution of a) Linear sweep voltammetry curves, b) Tafel plots, and c) EIS Nyquist plots. d) Chronopotentiometric curve of NiCe@NiFe/NF-N in 1 M KOH with a current density of 1 A/cm2. 6
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ratio of Ce3+/Ce4+ for the post-OER sample is higher than the initial NiCe@NiFe/NF-N (Table S1). This phenomenon consistent well with the analysis results of Raman spectroscopy (Fig. S9b). It is found that the relative intensity ratio of ID/IF2g increase from 16.9% for NiCe@ NiFe/NF-N to 26.1% for the post-OER NiCe@NiFe/NF-N electrocatalyst, indicating that a large amount of oxygen vacancies generate on the surface of the electrocatalyst during OER process [52,53], thus providing more surface electrochemical active OER sites and further enhancing the water oxidation kinetics. Above all, the extraordinary OER activities of NiCe@NiFe/NF-N can be attributed to the following two factors: i) the intrinsic activity of NiCe@NiFe/NF-N caused by favored Ni/Ce composition, electronic structures and surface oxygen vacancies during the OER process, which could boost the water oxidation activities. ii) The heterostructure network of NiCeOx and NiFeOx layer with optimal Ni/Ce atomic ratio and morphology, which leads to the good synergistic effect between Ni2+ and Ce3+/4+ at atomic level and robust coupling between NiCeOx and NiFeOx layer at nanoscale level, thus resulting in the improved OER activities for NiCe@NiFe/NF-N electrode.
Table 1 Comparisons of OER properties for different electrocatalysts on Ni foam electrode. Sample
NiCe@NiFe/NF-N NiCe@NiFe/NF-P NiFe/NF NiCe/NF RuO2
η (mV) @100 mA/cm2
@1 A/cm2
254 264 282 380 470
359 450 N.A N.A N.A
Tafel slope (mV/dec)
Rct (Ω)
59.9 87.8 126.8 130.2 151.8
0.1542 0.1569 0.2881 1.756 2.643
Rct represents the charge transfer resistance. N.A denotes as not available.
4. Conclusion In conclusion, a three-dimensional self-supporting heterostructure network NiCe@NiFe/NF-N OER electrocatalytic electrode has been fabricated by a simple two step electrodepositing process. The electrocatalyst combines the structural and catalytic advantages of heterostructure network of NiCeOx and NiFeOx layer, delivering the current density of 100 mA/cm2 and 1 A/cm2 at the overpotentials of 254 mV and 359 mV, which is much superior to those of the contrast electrocatalysts in this work. Moreover, such heterostructure electrocatalyst also exhibits a superior long-term stability towards efficient water oxidation activities at 1 A/cm2 with 20 h. Detailed structural and electrochemical characterizations confirm that the excellent water oxidation performances of NiCe@NiFe/NF-N can be attributed to the optimal surface and electronic structures, that is the nano-sized heterostructure network of NiCeOx and NiFeOx layer with favored Ni/Ce composition and morphology, and the rest is the increased oxygen vacancies due to the ratio changes of Ce3+/Ce4+ promote the intrinsic activity of the catalyst during water oxidation process. These findings may provide ideas for designing high performance oxygen evolution catalysts, and encourage other researchers to explore energy-related catalysts with superior catalytic activity by using the similar synthetic approaches.
Fig. 5. The comparisons of ECSA, intrinsic activity (activity normalized by mass and ECSA) for NiFe/NF, NiCe@NiFe/NF-P and NiCe@NiFe/NF-N.
cm2 and 47.2 cm2, which is larger than those of NiCe@NiFe/NF-P (2.48 m F/cm2, 41.3 cm2) and NiFe/NF (1.77 m F/cm2, 29.5 cm2). It is found that the improvement of ECSA is really not significant, which may be ascribed to the intrinsic activity of the catalysts rather than ECSA is the distinct features for the enhanced OER performances. Therefore, the corresponding comparisons of intrinsic activity (activity normalized by mass and ECSA) are provided in the revised manuscript. As shown in Fig. 5, the specific activity (normalized by ECSA) is calculated to be 8.8, 10.3 and 21.9 mA/cm2 for NiFe/NF, NiCe@NiFe/NFP, NiCe@NiFe/NF-N, and respectively, implying that the intrinsic activity of NiCe@NiFe/NF-N is much higher than those of NiCe@NiFe/ NF-P and NiFe/NF. Moreover, the mass activity of NiCe@NiFe/NF-N is determined to be 299.1 A/g. The value is twice as much as that of NiCe@NiFe/NF-P and four times to that of NiFe/NF. Such enhanced intrinsic activities combined with the Tafel plots, EIS analysis as well as the structural characterizations of these electrocatalysts further indicated the excellent intrinsic catalytic activity originated from the 3D porous heterostructure of NiCe@NiFe/NF-N could be one of the distinct features contributed to the enhanced high OER performances at large current densities. Besides of superior water oxidation current densities, long-term OER durability at large current densities is also one of the crucial targets to evaluate the electrocatalytic performances of OER catalysts. As shown in Fig. 4d, a chronopotentiometric test was applied at 1A/cm2 in 1 M KOH solution for 20 h. It is found that NiCe@NiFe/NF-N could remain its electrocatalytic performance at such high current densities over 20 h with the applied potential has a slight decrease, suggesting an activation process may be existed during the long-term stability testing. Therefore, XPS measurement was carried out to analysis the NiCe@ NiFe/NF-N electrocatalyst after long-term OER stability test (Fig. S17). Although the morphology and microstructure of NiCe@NiFe/NF-N still keep unchanged (Fig. S18), it is concluded that some of the Ce on the surface corrodes into electrolyte after long-term stability test and the
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