Ce-directed Double-Layered Nanosheets Architecture of NiFe-based

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Ce-directed Double-Layered Nanosheets Architecture of NiFebased Hydroxide as Highly Efficient Water Oxidation Electrocatalyst Qiong Zhang, Shihao Zhang, Yang Tian, and Sihui Zhan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03852 • Publication Date (Web): 12 Oct 2018 Downloaded from http://pubs.acs.org on October 13, 2018

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ACS Sustainable Chemistry & Engineering

Ce-directed Double-Layered Nanosheets Architecture of NiFe-based Hydroxide as Highly Efficient Water Oxidation Electrocatalyst Qiong Zhang,1 Shihao Zhang,1 Yang Tian1* and Sihui Zhan2*

1Department

of Chemistry, Beijing Key Laboratory for Optical Materials and

Photonic Devices, Capital Normal University, 105 North Road of Western 3rd Ring, Haidian District, Beijing 100048, China 2College

of Environmental Science and Engineering, Nankai University, 38 Tongyan

Road, Jinnan District, Tianjin 300071, PR China *Corresponding author: Yang Tian; Sihui Zhan Email: [email protected]; [email protected] Tel: +86-10-68903033

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Abstract: The development of highly available low-cost water-oxidation electrocatalysts is meaningful for sustainable energy generation. Herein, the unique architecture of double-layered

FeNi-based

nanosheet-stack

grown

on

Ni

foam

(Fe:Ni(OH)2/FeCe:Ni(OH)2/NF) was induced by Ce ions in the reaction solution. The as-synthesized double-layered catalyst on the substrate of Ni foam exhibited excellent electrochemical behavior in the catalysis of water oxidation. At the current density of 10 mA cm−2, the overpotential was so pretty low as 201 mV in alkaline electrolyte. The Tafel slope was only 42.4 mV dec−1. The high OER electrocatalysis performance of the double-layered nanosheets was related intensely with the introduction of Ce ions, which not only induced the unique architecture of the double-layered vertical nanosheets but also provided more effective charge and mass transfer for electrochemical water oxidation kinetics. These results indicated the promise of expanding the field of lanthanide-doped NiFe-based nanomaterials for applications in energy conversion and storage.

Keywords: electrocatalysis; oxygen evolution reaction; nanosheet; Ce species.


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Introduction Water splitting into hydrogen and oxygen is a sustainable and efficient method to acquire hydrogen energy with zero environmental pollution.1-2 In the water splitting reaction, the oxygen evolution reaction (OER) is kinetically sluggish because it is a multistep process of charge transfer that demands large potential, presenting a bottleneck in the transformation of water into O2 and H2.3-5 The most efficient OER catalysts widely used are noble metal-based catalysts, including Ir, Ru and their oxides, but the extortionate cost and rarity block their large-scale applications.6-7 Currently, considerable efforts have devoted to noble- metal-free electrocatalysts for the OER, especially in the transition-metal-based electrocatalysts because of their earth abundance and environmental friendliness.8-12 However, there are still challenges to the synthesis of profitable catalytic electrode materials for the OER. Recently, Ni-based catalysts are the subject of deep investigation because of their attractive electrochemical behaviors for water oxidation.13-15 Further research suggested that catalysts incorporating both Ni and Fe together could effectively enhance OER catalytic activity in alkaline conditions.16-22 The understanding of electrochemically active species of NiFe-based materials under OER condition is still controversial. Some researchers proposed that the presence of Fe decrease the affinity to oxygen of the Ni surface sites and promotes the formation of the high oxidation state of Ni (e.g., Ni3+ or Ni4+),23-24 while others suggest that Fe sites with oxidation 3 ACS Paragon Plus Environment


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states might be the active sites.25 It is believed that the high oxidation State-modified electronic structures of Ni-based materials can improve the catalytic capability towards the OER.26 Although the FeNi-based OER electrocatalysts have appealed to researchers, further improvement of their catalytic activity derived little antecedent attention. Two strategies are reported generally to reform the reaction activity of an electrocatalyst system.27 One strategy is increasing the quantity of catalytically active sites on a given electrode. For example, the morphology and structure of catalysts were controlled to expose more active sites. Especially, the two-dimensional (2D) nanostructures have received explosive attention recently owing to their anisotropic morphology with more activity sites for the catalysis of water oxidation.

28-30

The

other the effective strategy is increasing the intrinsic activity of each site (e.g., through tuned electronic structures or produced surface deficiencies). Although increasing the intrinsic activity plays more dominated roles than the quantity of catalytically active sites in the improve catalysis, these two strategies are not repellent each other and can admirably be addressed concurrently, producing the considerable enhancement in activity.27 So, it is desirable to develop facile strategies for increasing the quantity of catalytically active sites and intrinsic activity of electrocatalysts, and accordingly to enhance their electrocatalytic performance. Herein, we designed and fabricated a double-layered NiFe-based nanosheet-stack 4 ACS Paragon Plus Environment

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structure via metallic Ce4+ directing in the preparation system. This electrocatalyst simultaneously increased the quantity of catalytically active sites and intrinsic activity for improving the OER performance of water splitting. The role of Ce in the reaction system and the hierarchical structure are of the key and summarized as follows: (i) increasing quantity of catalytically active sites via directing the vertically aligned a new second 2D Fe:Ni(OH)2 nanosheet layer; (ii) increasing the intrinsic activity via doping into the first layer of nanosheets. Good electronic/ionic conductivity of Ce ions improved the OER kinetics and mass-transfer ability of NiFe-hydroxide and accordingly

this

would

enhance

the

catalytic

performance;31-32

the

high

oxygen-storage capacity could beneficial to more NiIII/IV active species due to the flexible transition between the CeIII and CeIV oxidation states.33-34 As expected, the as-synthesized double-layered nanosheets exhibited high OER performance. At the current density of 10 mA cm−2, the overpotential was so ideally low as 201 mV in alkaline electrolyte. The Tafel slope was only 42.4 mV dec−1.

These results promise

the double-layered NiFe-based nanosheet-stack structure as a new type of electrocatalyst for practical water splitting.

Experimental section Chemicals Cerium(III) nitrate hexahydrate (Ce(NO3)3·6H2O, 99.99% mass fraction) was obtained from SANN Chemical Technology (Shanghai) Co., Ltd. Iron(III) nitrate 5 ACS Paragon Plus Environment


nonahydrate (Fe(NO3)3·9H2O, 99% mass fraction) was purchased from Beijing Shuanghuan Reagent Co., Ltd.. Dimethylglyoxime (DMG) and ethanol (Analytical Reagent, 99.9%) were provided by J&K and Tianjin chemical factory, respectively. Nickel foam (1.0 mm thickness; 0.45 g/cm3) was obtained from Beijing San Yuan Sheng Xing Trading Co., Ltd. All the reagents were used directly with no depuration.

Synthesis of the double-layered nanosheets of Fe:Ni(OH)2 on FeCe: Ni(OH)2 supported by NF Typically, Fe(NO3)3·9H2O (0.12 mmol) and Ce(NO3)3·6H2O (0.24 mmol) were weighed into a 25 mL Teflon-lined stainless-steel autoclave. Subsequently, 8 mL DI water and 8 mL DMG ethanol solution (0.05mol L-1) were also added under agitation for 1 h to achieve a homogeneous solution. Nickel foams (NF, 2 cm × 3 cm) were carefully sonicated to remove surface oxide in HCl aqueous-solution, ethanol, and DI water for 6 h. After that, the autoclaves were cooled down naturally. The obtained catalysts were rinsed with DI water for several times, and then dried at room-temperature. The double-layered nanosheet-stack structure on Ni foam (Fe:Ni(OH)2/FeCe:Ni(OH)2/NF) was formed. Synthesis in control of the Ce-free catalysts on NF The Ce-free electrocatalysts were prepared similarly to the method described above without Ce(NO3)3·6H2O in the reaction solution. Characterizations The products were characterized by X-ray diffraction (Rigaku D/Max 2200 with Cu- target). Scanning electron microscopy (SEM) was employed to observe microstructure. The model of the SEM is Hitachi SU8010. An energy-dispersive X-ray spectroscopy (EDX) analyzer was used to analyze to elemental composition 6 ACS Paragon Plus Environment

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of the products. The crystalline structure was observed under a high-resolution transmission electron microscopy (TEM, Tecnai FEI F20). The corresponding crystalline lattice was also studied by selected area electron diffraction (SAED). The elemental distribution was characterized by high-angle annular dark field and scanning TEM (HAADF-STEM). The surface states of the prepared samples were analyzed by a X-ray photoelectron spectroscopy (XPS), whose model is PHI-5300 ESCA (Perkin−Elmer). The exciting source is Al. The achieved XPS spectra were analyzed by the standard peak of carbon, which locates at 284.7 eV. Electrochemical measurements Electrochemical tests were conducted on a workstation, whose model is CHI-660D. In a standard three-electrode system, the prepared materials on the NF with 1 cm2 worked as anode electrode, a Pt foil was used as the counter electrode, and Hg/HgO (1.0 M KOH) electrode as the reference electrode. All measurements were performed in 1.0 M KOH (pH=13.6) at room temperature. All potentials measured were changed to RHE via the equation (1). 𝐸𝑅𝐻𝐸 = 𝐸𝐻𝑔/𝐻𝑔𝑂 + 0.098 + 0.059𝑝𝐻

(1)

The 𝐸𝑅𝐻𝐸 represents the potential versus RHE. The 𝐸𝐻𝑔/𝐻𝑔𝑂 is the tested potential versus the Hg/HgO reference. Polarization curves were conducted using linear sweep voltammetry (LSV) with a scan rate of 0.5 mV s−1. The durability tests were performed

by

chronoamperometry

and

chronopotentiometry

methods.

The

electrochemical impedance spectroscopy (EIS) were tested between 1000 kHz and 0.01 Hz during the water oxidation process. The tested potential was1.50 V referenced RHE with 5 mV amplitude. It needs to be noted that all the LSV curves were recorded after about 10 CV cycles in the alkaline electrolyte until the curves were stable and corrected by the following equation (2): 𝐸 corrected = 𝐸 measured – iR

(2) 7

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where R is the ohmic potential drop losses from the solution resistance. The loading mass of the Fe:Ni(OH)2/FeCe:Ni(OH)2 hybrid on nickel foam is about 1.4 mg cm-2. The electrode loaded with commercial RuO2 powders was prepared based on the reference.35 An online gas chromatograph (GC 2060, Shanghai) was used to monitor O2 production mass with Ar carrier gas.

Results and discussion

Scheme 1. The model of various products at different reaction conditions in this work. As illustrated in Scheme 1, with the presence of Ce ions in the reaction solution, the NF was covered by growing a layer of Ce-doped Fe:Ni(OH)2 nanosheets at initial 15 min, which was named as ML1. When the reaction lasted for 6 hours, the second 8 ACS Paragon Plus Environment

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layer growing with larger nanosheets of Fe:Ni(OH)2 was produced on the bottom layer, and the sample was named as DL1. As a control, we performed the same experiments free of Ce ions in the reaction solution. As shown in Scheme 1, the initial product was Fe:Ni(OH)2 nanosheets growing on the NF, and we named it as ML2. After the reaction lasted for 6 hours, the second layer of Fe:Ni(OH)2 nanospheres covered on the first Fe:Ni(OH)2 nanosheet layer. This sample was named as DL2.

Figure 1. (a) XRD patterns and (b) EDX spectra of the DL1 (black line) and ML1 (red line) products. The bottom shows the corresponding standard XRD patterns; (c) SEM image of ML1; (d) Low magnification SEM image of the DL1; (e) partially magnified area of the double-layered DL1 supported on NF in SEM image (the red frame), where the red arrow shows the bottom layer and the green arrow shows the second 9 ACS Paragon Plus Environment


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layer.

The obtained samples, DL1 and ML1, were analyzed by powder X-ray diffraction (XRD). The red line in Figure 1a represents the pattern of the sample ML1. The two clearly diffractions located at 44.5° and 51.8° are due to the Ni foam substrate (JCPDS: 04-0850). No other clear diffraction peaks were detected in the XRD

pattern,

indicating

the

amorphous

nature

of

the

ML1.

However,

energy-dispersive X-ray (EDX) analysis indicated the ML1 had a composition of Ni, Ce, Fe and O elements (Figure 1b, the red line), demonstrating the compound of ML1 could be amorphous Fe- and Ce-doped Ni(OH)2 (FeCe:Ni(OH)2). The mole ratio of Ce : Fe : Ni in ML1 measured by EDX analysis is 2.0 : 2.10 : 39.03. When the solvothermal reaction time was extended to 6 h, the diffraction peaks of the (001), (100), (101), (110), and (111) lattice planes of the sample DL1 (the black line in Figure 1a) matched well the hexagonal phase Ni(OH)2 (JCPDS: 14-0117). The element composition of DL1 was further confirmed by EDX in Figure 1b, indicating Fe doped Ni(OH)2 (Fe:Ni(OH)2) was formed in the DL1 sample. The scanning electron microscopy (SEM) image in Figure 1c depicted that the NF surface was rapidly grown with small-sized and vertically grown FeCe:Ni(OH)2 nanosheet layers (ML1) during the initial hydrothermal process of 15 minutes. The size of the first-layered FeCe:Ni(OH)2 nanosheet was at 200 nm approximately. The hydroxide layer covered the NF surface and served as a platform for the further growth of the second-layered nanosheets. When the solvothermal reaction time was prolonged to 6 h, uniformly distributed and vertically aligned Fe:Ni(OH)2 nanosheet in ∼50 nm thickness and 0.5-2 μm

extensiveness growing on the first layer, as

shown in Figure 1d and 1e. The SEM image in Figure S1 (see SI) clearly demonstrated the side-view of the DL1 structure. Such unique double-layered nanosheet-stack structure can offer abundant interconnected electrolyte channels for 10 ACS Paragon Plus Environment

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the surface of catalysts, which provide short ion diffusion/exchange paths and large surface area for electrochemical reactions.36

Figure 2. High-resolution TEM image (a), SAED pattern (b), TEM image (c), HAADF image (d), and corresponding elemental maps (e-g) of the Fe:Ni(OH)2 nanosheet obtained from the top layer of the sample DL1.

The top-layered Fe:Ni(OH)2 nanosheets with larger size obtained from the DL1 has been characterized in detail. As seen in Figure 2a, high-resolution transmission electron microscopy (HRTEM) image of the Fe:Ni(OH)2 nanosheets suggested the crystal lattice spacing was 0.233 nm, corresponding to the (101) crystallographic 11 ACS Paragon Plus Environment


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planes of hexagonal Ni(OH)2 phase. Figure 2b is the selected area electron diffraction (SAED) pattern, demonstrating that the Fe:Ni(OH)2 nanosheets had an excellent crystalline structure. Figure 2c−2g showed the high-angle annular dark field (HAADF) image and the associated scanning TEM-energy dispersive spectroscopy (STEM-EDS) elemental mapping images. They revealed the uniform spatial distribution of Ni, Fe, and O elements, indicating that the Fe element was successfully doped into the top Ni(OH)2 nanosheets. The atomic ratio of Fe and Ni elements was estimated to be 1% approximately in the top layered nanosheet by the EDX result.

Figure 3. (a) Survey XPS spectra of the ML1 and DL1 samples; (b)

High-resolution

XPS patterns of Ce 3d of the ML1 (S0 – CeⅢ3d5/2, S1 – CeⅣ3d5/2, S2 – CeⅣ3d5/2, S0’– CeⅢ3d3/2, S1’– CeⅣ3d3/2, S2’– CeⅣ3d3/2); (c) High-resolution XPS patterns of Ni 2p in the ML1 and DL1; (d) High-resolution XPS patterns of Fe 2p in the ML1 and DL1 12 ACS Paragon Plus Environment

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samples.

The surface composition and elemental valence of the DL1 and ML1 were characterized by X-ray photoelectron spectroscopy (XPS). Compared with the survey XPS spectrum of DL1, the ML1 XPS survey (Figure 3a) revealed the existence of both Fe, Ni, and Ce in the bottom layer of the double-layered nanosheet-stack structure supported on Ni foam, which was in accordance with the EDX results (Figure 1b). Figure 3b is the high-resolution XPS pattern of Ce 3d of ML1, which can be resolved into five 3d5/2 (labeled as S) and 3d3/2 (labeled as S’) spin-orbit doublet peaks related to CeIII and CeIV species along with satellites. As shown in Figure 3b, three doublet peaks labeled as Ce

IV3d

5/2–Ce

IV3d

3/2

(882.7 and 901.2 eV), S1–S1’

(888.6 and 906.9 eV) and S2–S2’ (898.1 and 916.5 eV) with spin-orbit separation of 18.5, 18.3 and 18.4 eV, respectively, are assigned for the Ce IV species, whereas peaks labeled as CeIII3d5/2–CeIII3d3/2 (885.5 and 903.9 eV) and S0–S0’ (881.6 and 899.9 eV) are associated with CeIII species.37 The S2’ peak separation is relatively far with respect to the rest of the spectrum and is the characteristic of the presence of tetravalent Ce (Ce IV) in Ce compounds.33 These results indicated the coexistence of Ce IV and CeIII in the ML1 product. Figure 3c showed the high-resolution Ni 2p XPS spectra of the ML1 and DL1. The Ni 2p spectrum exhibited two pairs of dominant peaks: 855.5 and 873.1 eV in the DL1 and 856.0 and 873.6 eV in the ML1, which can be assigned to the spin−orbit splitting of the Ni 2p3/2 and 2p1/2 with splitting energy of 17.6 eV, characteristic of NiII.38 The fitting Ni 2p3/2 peaks at 855.4, 855.6 and 856.8 eV indicated the Ni oxide and Ni hydroxide species.38-41 Peak around 862 eV is attributed to the shake-up satellite of NiII 2p3/2,39 and the NiII 2p1/2 satellite peak cannot be resolved due to the presence of CeIII 3d5/2 satellite peak in the same region as NiII 2p1/2. Furthermore, Figure 3d displayed the Fe 2p high-resolution spectra for the DL1 and ML1 samples. The binding energies of Fe 2p of the DL1 at 711.8 and 13 ACS Paragon Plus Environment


723.8 eV correspond to the Fe 2p3/2 and 2p1/2 spin orbits, respectively.42 The binding energies of the Fe 2p3/2 and Fe 2p1/2 spin orbits of the ML1 sample are the same as the DL1, respectively. These XPS results demonstrated that the element Fe mainly exists as FeIII oxidation state in both the DL1 and ML1 samples.


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Figure 4. SEM images of the ML2 (a) and the DL2 (b-d)

Similar characterizations were performed on the samples of ML2 and DL2 (shown in Scheme 1), which were produced by the free of Ce ions in the reaction solutions. As shown in the SEM image of Figure 4a, the ML2 exhibits small nanosheets growing on the NF with the size of ~200 nm, which is close to that of the ML1 sample. Its XRD pattern (Figure S2, see SI) and EDX spectrum (Figure S3, see SI) demonstrated the composition of the nanosheet in the ML2 was amorphous Fe:Ni(OH)2. The DL2 sample produced by the reaction of 6 hours was also observed by SEM as shown in Figure 4b-4d with various magnifications. It illustrated that lots of nanospheres of 200-400 nm grew as the second layer on the first-layered amorphous Fe:Ni(OH)2 nanosheets. Furthermore, these nanospheres would be assembled to form connected films and thus cover the first layer of small nanosheets almost completely. The composition of the nanospheres was characterized to be crystalline Fe:Ni(OH)2 by XRD pattern (Figure S2, see SI) and EDX analysis (Figure 15 ACS Paragon Plus Environment


S4, see SI). Based on the above results, we infer that the presence of Ce in the solution can induce the formation of double-layered vertical nanosheets, which should be related with the local electric field between the space between the first-layered neighboring nanosheets by Ce-doping. These prepared double-layered vertical nanosheets are expected to provide the catalyst larger surface area and higher reaction activity, beneficial for the excellent OER electrocatalysis.29

Figure 5. (a) Polarization curves of DL1, ML1, DL2, Ni foam and the benchmark RuO2/NF electrocatalyst for comparison; (b) the corresponding Tafel curves for the catalysts derived from (a); (c) their corresponding EIS Nyquist plots; (d) scan rate dependence of the current densities for the samples of DL1, DL2, and ML1 at 1.05 V (vs RHE) in 1M KOH.

The water oxidation reaction was applied to study the prepared unique 16 ACS Paragon Plus Environment

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double-layered nanosheet structure on the electrocatalytic activity. The OER performance of the double-layered nanosheets sample DL1 was evaluated using a three-electrode system, where a Hg/HgO electrode was used as the reference electrode, and a Pt foil was used as the counter electrode in the alkaline electrolytes (1.0 M KOH). For comparison, the ML1, DL2, Ni foam and commercial RuO2/NF were also measured under the same conditions. The overpotential required to reach a current density of 10 mA cm−2 (η) is generally regarded as a critical parameter for the OER.43 As shown in the polarization curves in Figure 5a, the DL1 sample possessed a much lower overpotential of 201 mV to achieve a geometrical catalytic current density of 10 mA cm-2. The overpotential is lower than those of the DL2 (224 mV), ML1 (234 mV) and Ni foam (416 mV), ever lower than the commercial OER catalyst RuO2/NF (287 mV). Furthermore, when the current density is 50 mA cm-2, the overpotential is 232 mV; the current density can reach 100 mA cm-2 at an overpotential of 245 mV for the prepared DL1 sample. Apparently, the catalytic current density of the DL1 surpassed all the contrast catalysts as the applied potential increasing. The results indicated that the prepared double-layers nanosheets DL1 had higher intrinsic activity and subsequently was more favorable for the OER catalytic activity. To investigate the OER kinetics, the Tafel plots derived from the corresponding polarization curves of the catalysts were drawn in Figure 5b. The Tafel slope of the DL1 was 42.4 mV dec-1, and it was smaller than those of the DL2 (49.2 mV dec-1), ML1 (55.3 mV dec-1), Ni foam (126.7 mV dec-1), and RuO2/NF (72.7 mV dec-1). With the

comparison of the Tafel slopes, it was demonstrated that the catalysts of bilayer

nanosheet architecture with Ce incorporation could facilitate its OER kinetics.44-45 To investigate the transport properties of the catalysts, electrochemical impedance spectroscopy (EIS) measurements were performed at an applied potential of 1.5 V versus RHE from 1000 kHz to 0.01 Hz in 1.0 M KOH (Figure 5c). The 17 ACS Paragon Plus Environment


diameter of the semicircle of the Nyquist plot is ascribed to the charge transfer resistance (Rct) at the catalyst/electrolyte interfaces during the OER.46 The lowest Rct for the DL1 indicates a faster electron transfer capability and thus better OER electrocatalytic performance. Furthermore, electrochemically active surface area (ECSA) was studied by the electrochemical double-layer capacitance (Cdl) between 1.0−1.1 V vs. RHE (Figure S5, see SI). As shown in Figure 5d, the DL1 electrode has a much larger Cdl of 5.61 mFcm-2 than those of DL2 (2.38 mF cm-2) and ML1 (1.38 mF cm-2), indicating the more accessible active sites at the DL1 surface.47 The OER performance of all the samples are listed in Table S1 (Table S1, see SI).

Figure 6. CV curves of the ML1 and ML2 samples that were carried out in 1.0 M KOH. To further study the effect of Ce element doped in the Fe:Ni(OH)2 nanosheets for 18 ACS Paragon Plus Environment

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the electrocatalysis of OER, the cyclic voltammetry (CV) curves of ML1 and ML2 were compared in 1.0 M KOH electrolyte as shown in Figure 6. An apparent oxidation peak ranging from 1.2 to 1.5 V is observed in their CV curves, which can be attributed to the transformation of Ni(II) to Ni (III or IV) species.48-49 In contrast to the ML2, the oxidation potential of the ML1 decreased significantly from 1.43 V to 1.37 V, indicating that ML1 had higher transfer efficiency from NiII to NiIII/IV than ML2. Furthermore, the extent of the NiII/NiIII/IV transformation of ML1 showed about 1.4-fold compared with the ML2 sample by integrated oxidation peak areas. These indicated that Ce doping provided more active species NiIII/IV, which was reported due to the high oxygen-storage capacity of Ce ions induced by the flexible oxidation states transition between CeIII and CeIV.31, 50 Considering the highly oxidative NiIII/IV cations are the active species for the water oxidation in alkaline solution, Ce doping in the bottom layer of the Fe:Ni(OH)2 nanosheets on the NF was beneficial for the enhancement of the OER electrocatalysis activity. Moreover, in addition to the flexible oxidation states transition, the Ce incorporation were also reported to change the electronic properties of the active sites, inducing an OER descriptor value lower in magnitude because of the oxophilicity of Ce.51-52



Figure 7. Time dependence of the current density of the DL1 sample under a 1.50 V vs. RHE and 1.55 vs. RHE without iR correction. Inset: Long-term stability test measured at a fixed current density of 10 mA cm−2.

The catalytic durability of the DL1 electrode was also examined by chronoamperometry and chronopotentiometry methods to evaluate the OER performance. Firstly, two constant potentials of 1.5 V vs. RHE and 1.55 V vs. RHE were applied to investigate the current density change related to the OER time, respectively. As shown in Figure 7, a slight decay of the current density can be observed in the first about 10 hours at both 1.5 V and 1.55 V vs. RHE, which might be attributed to the transformation of NiII/III active species.53-54 Since then the DL1 electrode retained good stability for a long time between the 10 hours and 50 hours. Meanwhile, a chronopotentiometry method was measured at a constant current density of 10 mA cm−2 to further evaluate the OER durability. The inset of Figure 7 showed that the DL1 electrode exhibited 1.47 V vs. RHE after 60 h of continuous 20 ACS Paragon Plus Environment

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water oxidation reaction. Overall, these results indicate that the prepared unique double-layered nanosheets electrode has long-term durability and excellent OER performance in alkaline media.

Figure 8. (a) The amount of Oxygen theoretically calculated and experimentally(left) and the corresponding Faradaic efficiency (right) measured versus time at a constant potential of 1.50 V vs. RHE without iR-correction; (b) polarization curve of overall water splitting in a two-electrode configuration; (c) a photograph of the electrolyzer set-up powered by an AA battery (1.5 V) for overall water splitting, which used the DL1 anode and platinum cathode; (d) the enlarged photograph of the DL1 anode and platinum cathode with bubbles at work.

The Faradaic efficiency of the DL1 electrocatalyst was detected by online gas 21 ACS Paragon Plus Environment


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chromatography (GC) in a sealed three-electrode system. Chronoamperometry was performed at a constant potential of 1.50 V vs. RHE without iR-correction. The Faradaic efficiency was estimated by comparing the tested gas amount with the theoretically calculated value according equation (3): Faradaic efficiency = 2nF/Q

(3)

where Q is the charge passing through the electrodes, F is the Faraday constant (96,500 C mol-1), number 2 refers to the 2 moles of electrons are transferred per 1 mole of H2 generated, and n is the number of moles of H2 produced. Figure 8a showed that the amount of hydrogen evolved compared with the theoretical data corresponds to a Faradic yield of >93% with the OER duration of 300 minutes. Moreover, we assembled a water electrolyzer using the DL1 anode and a platinum cathode in 1.0 M KOH solution. The polarization curve with iR-correction was recorded by linear sweep voltammetry at 2 mV s−1. As shown in Figure 8b, the onset potential of the DL1 electrode was 1.39 V to achieve overall water splitting catalysis. The required catalytic potential was 1.62 V to reach the current density of 10 mA cm−2, which exhibited an excellent water splitting activity. Encouraged by the high efficiency and activity of the prepared double-layered nanosheet electrode, we assembled a two-electrode configuration powered by an AA battery with a nominal voltage of 1.5 V at room temperature for overall water splitting (Figure 8c). Clearly, hydrogen and oxygen bubbles could be observed on the surface of electrodes, suggesting that the DL1 sample proficiently acted as a high-efficient anode for generating oxygen from water, as shown in Figure 8d. These results provide a preliminary assessment of the feasibility of using the prepared double-layered nanosheets in operational water electrolysis systems.

Conclusion In summary, Ce ions induced growth, for the fabrication of double-layered NiFe-based nanosheet-stack structure grown on Ni foam was developed and 22 ACS Paragon Plus Environment

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demonstrated. As confirmed by the SEM results, the top nanosheet layers were formed in the role of Ce incorporation. Otherwise, nanosphere layers were formed. The designed double-layered electrode DL1 exhibited excellent OER catalytic activity for the water oxidation with good durability in alkaline medium solution. The excellent OER electrocatalysis could be contributed by the larger surface-area and higher intrinsic activity induced by Ce ions. Compared with the Ce-free sample DL2, the overpotential of DL1 decreased from 224 to 201 mV, and the Tafel slope reduced from 49.2 to 42.4 mV dec-1, much better than the benchmark RuO2/NF. Furthermore, when used for overall water splitting, the DL1 electrode catalyzed the water splitting at an onset potential as low as 1.39 V and needs a cell voltage of 1.62 V to achieve a current density of 10 mA cm−2. The role of the Ce in our product are considered as: (1) the Ce directing the unique double-layer architechure and therefore increasing the quantity of catalytically active sites; (2) the flexible oxidation states transition between CeIII and CeIV providing more active species; (3) the Ce incorporation changing the electronic properties of the active sites and inducing an OER descriptor value lower in magnitude. This work provides an inspiring pathway for a range of self-supported electrocatalysts composed of nonnoble-metal elements develop OER activity.

Supporting Information SEM images, element mapping, EDX, XRD patterns, XPS spectra, electrochemically active surface area, LSV curves, and exchange current density, as well as electrochemical information, and the electrochemical behaviors compared with the state-of -the-art electrocatalysis materials. Notes The authors declare no competing financial interest.



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Acknowledgment We thank Beijing Nature Science Foundation (2182013), High-level Teachers in Beijing Municipal Universities–China in the Period of 13th Five–year Plan (Grant No.IDHT20180517), Capacity Building for Sci-Tech Innovation Fundamental Scientific Research Funds (025185305000/195, 123, and 208), and Youth Innovative Research Team of Capital Normal University – China.

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Double-Layered Nanosheets directed by the existence Ce ions show high performance of OER electrocatalysis in water splitting for the promising sustainable energy H2.


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Ce-directed Double-Layered Nanosheets Architecture of NiFe-based

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