Accelerated oxygen evolution kinetics on NiFeAl-layered double hydroxide electrocatalysts with defect sites prepared by electrodeposition

international journal of hydrogen energy xxx (xxxx) xxx

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Accelerated oxygen evolution kinetics on NiFeAllayered double hydroxide electrocatalysts with defect sites prepared by electrodeposition Huixi Li a, Lingling Zhang a, Shengping Wang a,*, Jingxian Yu b a

Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan, 430074, China ARC Centre of Excellence for Nanoscale BioPhotonics (CNBP), School of Chemistry and Physics, The University of Adelaide, Adelaide, SA, 5005, Australia

b

highlights
Successfully preparing amorphous NiFeAl-LDHs electrocatalyst by electrodeposition.
Alkali etching treatment successfully introduces Ni2þ and Fe3þ defect sites.
The defect sites can improve the amount of Ni3þ, accelerating the adsorption of OH.
The mechanism of catalysts with defect sites in oxygen evolution reaction was given.

article info

abstract

Article history:

NiFe layered double hydroxides (LDHs) is considered to be one of the LDHs electrocatalyst

Received 16 July 2019

materials with the best electrocatalytic oxygen evolution properties. However, its poor

Received in revised form

conductivity and inherently poor electrocatalytic activity are considered to be the limiting

30 August 2019

factors inhibiting the electrocatalytic properties for oxygen evolution reaction (OER). The

Accepted 19 September 2019

amorphous NiFeAl-LDHs electrocatalysts were prepared by electrodeposition with nickel

Available online xxx

foam as the support, and the D-NiFeAl-LDHs electrocatalyst with defect sites was then obtained by alkali etching. The mechanism of catalysts with defect sites in OER was

Keywords:

analyzed. The ingenious defects can selectively accelerate the adsorption of OH, thus

Electrodeposition

enhancing the electrochemical activity. The D-NiFeAl-LDHs electrocatalyst had higher OER

Electrocatalyst

electrocatalytic activity than NiFe-LDHs electrocatalyst: its accelerated OER kinetics were

Layered double hydroxides

mainly due to the introduction of iron and nickel defects in NiFeAl-LDHs nanosheets,

Defect

which effectively adjusted the surface electronic structure and improved OER electro-

Oxygen evolution reaction

catalytic performance. There was only a low overpotential of 262 mV with the current density of 10 mA cm2, and the Tafel slope was as low as 41.67 mV dec1. The OER electrocatalytic performance of D-NiFeAl-LDHs was even better than those of most of the reported NiFe-LDHs electrocatalysts. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. E-mail address: [email protected] (S. Wang). https://doi.org/10.1016/j.ijhydene.2019.09.155 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Li H et al., Accelerated oxygen evolution kinetics on NiFeAl-layered double hydroxide electrocatalysts with defect sites prepared by electrodeposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.155

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Introduction As a unique class of layered anion exchange materials, layered double hydroxides (LDHs) is mainly composed of positively charged brucite-like layers and intercalated anions [1]. It has received extensive attention for application in catalysis [2e4], adsorbent [5,6], functional materials [7,8], and drug carriers [9]. In recent years, LDHs have been widely used in the study of CO2 adsorption and photocatalysis for its unique physical and chemical properties as well as excellent catalytic properties [3]. And the superior properties of structure endow cationic LDHs as a kind of adsorbent, with many advantages compared to the traditional porous materials [6]. Meanwhile, the LDHs also is a kind of water splitting electrocatalysis with excellent catalytic performance, especially for the oxygen evolution reaction. Electrochemically evolving hydrogen by splitting water is considered to be a convenient, nonpolluting and sustainable method [10e13]. However, because of the multiple steps with four-electron-transfer, oxygen evolution reaction (OER) is severely hindered by the sluggish kinetics and thermodynamics of water splitting [14e17]. Therefore, it is imperative to explore highly efficient electrocatalysts for OER. In recent years, considering the unique chemical and physical properties (e.g., tunable metal species and ratio; interlayer spacing) [18], good efficiency and anti-corrosion stability for OER in alkaline conditions and much lower cost than RuO2 or IrO2based electrocatalysts [19e21], transition metal LDHs have drawn researchers’ attention and have been intensively studied. Among various LDHs electrocatalysts, the NiFe-LDHs electrocatalyst exhibited the best OER electrocatalytic performance under alkaline conditions [22e24], and its turnover frequency is much higher than that of IrO2. However, low electronic conductivity and intrinsically poor electrocatalytic activity of reactive sites inhibit the practical application of NiFe-LDHs as electrode materials [21]. To overcome this shortcoming, many researchers have combined LDHs with highly conductive materials (such as nickel foam, copper mesh, graphene foil, etc.) to construct three-dimensional (3D) efficient OER electrocatalysts [4,14,25e27]. The conductive 3D structural material can provide a large specific surface to increase the contact area between the electrocatalyst and the electrolyte, accelerate the electron transport and increase the active sites of OER [28]. Previous literature has shown that structural defects and ion vacancies can promote the formation of low-coordinate Fe, Co and Ni as active sites in LDHs electrocatalysts [29,30]. Therefore, further increasing the ratio of low-coordinate Ni and Fe sites can improve the electrocatalytic activity of NiFe-LDHs. Elemental doping has been demonstrated to be an effective approach to adjust the coordination valence states and chemical environments of electrocatalysts [19,31e33]. In addition, by selectively etching/dissolving less stable substances [34e36], more defect sites can be created on the surface of the electrode and more surface metal atoms with low coordination number can be exposed, thereby further improving the OER performance of the electrocatalyst. Based on the above analysis, this study intends to prepare the defect-containing NiFeAl-LDHs electrocatalysts (named as

D-NiFeAl-LDHs) on nickel foam (NF) by simple electrodeposition combined with alkali etching. Al is selected as the introduction element because the introduction of trivalent Al ions will change the coordination environment of Ni and Fe atoms in LDHs. Furthermore, the number of active sites will be greatly increased by the etching of Al species in strong alkaline solution, which is expected to be very beneficial to the OER performances in alkaline conditions. Electrodeposition method was used to successfully obtain catalysts with specific compositions and morphologies. How the catalyst with defect participated in the process of OER was analyzed. The defect of ingenious conception has the unique ability of accelerating the adsorption of OH, which enhances the electrochemical activity. The experimental results show that, when compared with NiFeAl-LDHs and NiFe-LDHs, the prepared D-NiFeAlLDHs electrocatalysts exhibit the most excellent OER electrocatalytic performance. Furthermore, the D-NiFeAl-LDHs electrocatalysts also have good OER performance, even compared with some recently reported nonprecious metalbased materials (Table S1).

Experimental section Electrocatalyst preparation Preparation of NiFe-LDHs/NF. NF (thickness: 1.6 mm, bulk density: 0.42 g cm3) was sonicated in 5 M HCl solution for 30 min to remove the NiOn layer on the surface, rinsed subsequently with water and ethanol three times, then dried in air. Electrodeposition was carried out in a standard three electrode electrochemical cell containing NF (1 cm  2 cm) as the working electrode, a parallel positioned platinum plate as the auxiliary electrode and a saturated calomel electrode as the reference electrode. The electrolyte bath contained 3 mM Ni(NO3)2$6H2O and 3 mM Fe(NO3)3$9H2O. The constant potential electrodeposition was carried out at 1.0 V (vs. saturated calomel electrode, SCE) at 25  C for 300 s. After the deposition, the nickel foam was removed from the electrolyte, washed three times with deionized water and absolute ethanol, and then dried in air. Preparation of NiFeAl-LDHs/NF. The electrolyte composition is shown in Table S2, and the same electrodeposition operation as described above was carried out to obtain NiFeAlLDHs/NF electrocatalysts. Preparation of D-NiFeAl-LDHs/NF. The obtained NiFeAlLDHs/NF electrocatalysts were immersed in a strong alkali solution of 6 M NaOH at 25  C (for 15, 30, 60, and 120 min) to obtain D-NiFeAl-LDHs/NF electrocatalysts.

Characterization The morphology and composition of these samples were examined by scanning electron microscopy (SEM, SU8010) and an energy dispersive X-ray spectroscope (EDX) attached to the SEM. Transmission electron microscopy (TEM) and electron diffraction (ED) measurements were carried out using a Philips CM12 system operating at 200 kV. The crystallinity of products was characterized by X-ray diffraction (XRD) on a Bruker/D8-FOCUS powder diffractometer using Cu Ka

Please cite this article as: Li H et al., Accelerated oxygen evolution kinetics on NiFeAl-layered double hydroxide electrocatalysts with defect sites prepared by electrodeposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.155

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(l ¼ 1.5418
A) radiation at a scan rate of 10 min1 between 10 and 90 . X-ray photoelectron spectra (XPS) were obtained using a K-Alpha 1063 X-ray photoelectron spectrometer with Al Ka X-ray radiation.

Electrochemical measurements The electrochemical measurements were carried out at 25  C in a three electrode glass cell connected to an electrochemical workstation (CHI 660D). The nanosheet array electrocatalysts on the Ni-foam substrate (1 cm  1 cm) were used as working electrode. A platinum electrode and SCE were used as counter and reference electrode, respectively. Freshly prepared 1 M KOH aqueous solution was used as the electrolyte. All potentials measured were calibrated to reversible hydrogen electrode (RHE) using the following Eq. (1): ERHE ðVÞ ¼ ESCE þ 0:0591pH þ E0SCE ¼ ESCE þ 0:0591pH þ 0:231 (1) The electrochemical impedance spectroscopy (EIS) was performed by AC impedance technique in 1 M KOH solution at 1.58 V (h ¼ 350 mV) from 100 mHz to 100 KHz with an AC voltage of 5 mV. Polarization curves were obtained using LSV with a scan rate of 5 mV s1 between 1.2 V and 1.8 V. To obtain the apparent activation energy (Ea) of OER, the LSV curves at different temperatures (30e50  C) were also measured. The cyclic voltammetry (CV) curves were measured at the sweep rate of 10 mV s1 from 1.2 V to 1.6 V. The stability of the electrode was measured by applying a constant potential to achieve an initial current density of 10 mA cm2 for 18 h. CV for stability testing was conducted at a sweep rate of 100 mV s1 with the voltage range from 1.2 V to 1.6 V. Another non-Faradaic CV for evaluating the electrochemical active surface area (ECSA) was conducted from 1.20 V to 1.25 V in 1.0 M KOH with different sweep rates between 5 and 50 mV s1. All LSV polarization curves were recorded after activating by a chronopotentiometry scan with constant current density until reaching a stable state. All the polarization curves were corrected by eliminating iR drop with respect to the ohmic resistance of the solution. All CV cueves were recorded after the 10th cycle. We also calibrated with the SCE reference electrodes by measuring the RHE potential using a Pt electrode under a H2 atmosphere.

Results and discussion SEM and TEM The experimental section details the preparation process of different LDHs nanosheet electrocatalysts, as shown in Fig. 1a. The effect of alkali etching on the structure and morphology of the electrocatalyst was analyzed by TEM and SEM. Fig. 1bed are SEM images of NiFe-LDHs, NiFeAl-LDHs and D-NiFeAl-LDHs electrocatalysts, respectively. It can be seen that NiFe-LDHs and NiFeAl-LDHs electrocatalysts are typical two-dimensional nanosheet structures. D-NiFeAlLDHs obtained by alkali etching of NiFeAl-LDHs precursor electrocatalyst still retain obvious nanosheet structure. The TEM images of obtained LDHs products are shown in Figs.

3

S3aec, suggesting that ultrathin nanosheet structures with sizes smaller than 100 nm have been successfully synthesized. It is obvious to see from the TEM images that the nanosheets of D-NiFeAl-LDHs are smaller and thinner than those of NiFe-LDHs. This is because some Al3þ sites are removed by the surrounding OH binding with OH in the lye, causing the sheet to carry more positive charges to attract anions (such as NO 3 ) in the interlayer structure, resulting in a reduction in interlayer spacing. In addition, the EDX elemental mapping analysis (Fig. 1e) indicated that the Ni, Fe, and Al elements were homogeneously distributed across the entire sample of D-NiFeAl-LDHs. The XRD spectra of NiFe-LDHs, NiFeAl-LDHs and D-NiFeAl-LDHs electrocatalysts are shown in Fig. 1f, showing only the three diffraction peaks of the NF at 44.5 , 51.8 , and 76.4 , without any material peaks detected, and indicating that these LDHs electrocatalysts deposited on the nickel foam are in an amorphous state. The ED patterns (Figs. S3def) showed weak crystallinity, also indicating the amorphous state of the three LDHs electrocatalysts. The ED patterns were equivalent before and after alkaline etching, suggesting that the crystalline structure is not affected. However, the D-NiFeAl-LDHs electrocatalyst shows weaker diffraction intensity, probably owing to the creation of massive defects, which impede the long-range order of the LDHs crystals.

XPS The OER activities of the NiFe electrocatalysts are highly dependent on the nature of the surface metal sites. Alkaline etching Al(OH) x of octahedra from the LDHs layer produces unsaturated metal sites and finally forms Ni2þ and Fe3þ defect sites. To balance the charge, the valence states of the metal atoms around the defect sites will change (e.g., Ni2þ/Ni3þ). XPS was employed to further study the chemical valence properties of Ni and Fe on the surface of the prepared electrocatalysts. As shown in Fig. S4, the survey spectra confirm that the D-NiFeAl-LDHs are mainly composed of Fe, Ni, and O, which agrees well with the EDS results. The Ni 2p XPS spectra is shown in Fig. 2 and Fig. S4b. The Ni 2p3/2 XPS spectrum of NiFe-LDHs nanosheets was split into four peaks: the peaks at 855.5 and 856.5 eV can be attributed to Ni2þ and Ni3þ coordinated to OH, respectively. The presence of Ni3þ in NiFe-LDHs (32.4%) can be caused by the small size of nanosheets exposing more unsaturated coordinate reactive sites. The peaks at 861.4 and 863.1 eV can be considered as the satellite peaks of Ni2þ and Ni3þ, respectively. For NiFeAl-LDHs and D-NiFeAl-LDHs electrocatalysts, the percentages of Ni3þ were 23.7% and 41.2%, respectively, indicating that the introduction of iron or nickel defect sites further changes the surface electronic structure of the NiFe-LDHs electrocatalyst and ultimately leads to the chemical valence further increasing from Ni2þ to Ni3þ. However, as shown in Fig. S4c, there was no obvious shift of the Fe 2p XPS spectrum in the NiFe-LDHs, NiFeAlLDHs, and D-NiFeAl-LDHs electrocatalysts, indicating that the valence state of Fe remains unchanged after etching of the Al(OH) x sites. And the Fe 2p XPS spectrum of the LDHs electrocatalysts was split into two peaks, the peaks at 712.5 and 725.4 eV were considered as Fe3þ coordinated to OH.

Please cite this article as: Li H et al., Accelerated oxygen evolution kinetics on NiFeAl-layered double hydroxide electrocatalysts with defect sites prepared by electrodeposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.155

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Fig. 1 e Schematic of the preparation of D-NiFeAl-LDHs (a), SEM images of NiFe-LDHs (b), NiFeAl-LDHs (c) and D-NiFeAl-LDHs (d), and EDX elemental maps of D-NiFeAl-LDHs electrocatalyst (e) and XRD spectrum (f) of the electrocatalysts.

CV (oxygen evolution process and double layer capacitor Cdl) Fig. 3a shows the CV curves of three LDHs electrocatalyst electrodes: NiFe-LDHs, NiFeAl-LDHs and D-NiFeAl-LDHs. The D-NiFeAl-LDHs electrocatalyst is prepared under the optimal conditions. The optimized experimental curves are shown in

Figs. S1 and S2, and the electrocatalyst prepared in the electrolyte with 1.0 mM Al3þ doping and 60 min of alkali etching exhibits the best OER performance. The cyclic voltammetry curves have a test range from 1.2 V to 1.6 V (vs. RHE), with a scan rate of 10 mV s1. It can be seen from Fig. 3a that the obvious redox peaks at approximately 1.40 V and 1.30 V during

Fig. 2 e Ni 2p2/3 XPS spectra of NiFe-LDHs (a), NiFeAl-LDHs (b) and D-NiFeAl-LDHs (c). Please cite this article as: Li H et al., Accelerated oxygen evolution kinetics on NiFeAl-layered double hydroxide electrocatalysts with defect sites prepared by electrodeposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.155

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Fig. 3 e CV curves of the electrocatalysts in the range of 1.2e1.6 V (a). CV curves of D-NiFeAl-LDH electrocatalyst at different scan rates from 5 to 50 mV s¡1 with 1.2e1.25 V in 1.0 M KOH (b), and the plots of current densities at 1.225 V against the scan rates (c).

the anodic and cathodic scans were the result of the transformation between Ni(OH)2 and NiOOH in the alkaline electrolyte NiOOH (Eq. (2)). It should be apparent from the shape and peak position of the cyclic voltammograms that the reaction of the electrocatalyst in the alkaline electrolyte is an irreversible process. The CV curve of D-NiFeAl-LDHs electrocatalyst has lower OER onset potential and higher peak current density, suggesting that the electrocatalyst exhibits better electrocatalytic performance for OER. The OER mechanism of the Ni-based LDHs electrocatalyst in alkaline solution can be expressed as follows [37]. The preconversion of Ni(OH)2 NiðOHÞ2 þ OH 4NiOOH þ H2 O þ e

(2)

NiOOH electrocatalyzes the specific reaction process of OER: NiOOH þ OH 4NiOðOHÞ2 þ e

(3)

NiOðOHÞ2 þ OH 4NiOOðOHÞ þ H2 O þ e

(4)

NiOOðOHÞ þ OH 4NiOOðOHÞ2 þ e

(5)

NiOOðOHÞ2 þ OH 4NiOO2 ðOHÞ þ H2 O

(6)

NiOO2 ðOHÞ 4 NiOOH þ O2 þ e

(7)

Numerous studies have shown that the apparent electrochemical active area is proportional to the number of electrocatalytic activity sites of OER, and the electrochemical double layer capacitance (Cdl) can be used to evaluate the apparent electrochemical active area [38e40]. To further study the influence of the generation of defect sites on the number of active sites on the electrode surface for OER, a narrow potential range of the double capacitance potential region was selected, and CV was performed with different sweep rates. As shown in Fig. 3a, a purely capacitive potential region can be selected from 1.2 V to 1.25 V, in which only the non-Faraday charging process occurs. All voltammetric curves were recorded between 1.2 V and 1.25 V as a function of scan rate, v (5  v  100 mV s1), and of the current measured, iE, at constant potential (E ¼ 1.225 V). The Cdl was obtained from the slope of iE vs. v graphics. Fig. 3b shows the voltammetric

curves of the D-NiFeAl-LDHs electrocatalyst, which were recorded in the capacitive potential region (between 1.2 V and 1.25 V) at different scan rates. The voltammetric curves of the NiFe-LDHs and the NiFeAl-LDHs electrocatalysts under the same conditions are shown in Figs. S6aeb, respectively. The iE vs. v graphics of the three electrocatalysts are shown in Fig. 3c. The Cdl values of the three LDHs catalysts are shown in Table S3. The Cdl value of NiFeAl-LDHs electrocatalyst before alkali etching is 5.15 mF cm2; however, the Cdl value of DNiFeAl-LDHs obtained after proper alkali etching increases to 6.49 mF cm2, even larger than that of NiFe-LDHs. This indicates that the D-NiFeAl-LDHs catalyst obtained after alkali etching has more active sites for OER and better OER catalytic activity. This is attributed to the D-NiFeAl-LDHs catalyst having a micro-nanopore structure and cation vacancy on the surface after strong alkali etching, increasing both the surface area of the catalyst and the number of active sites.

Polarization curve (overpotential h10, exchange current density j0, Tafel slope b, activation energy Ea) To measure the OER electrocatalytic performance of the electrocatalysts, the anodic polarization curve was adopted. Fig. 4a shows the anodic polarization curves of different electrocatalysts in 1 M KOH electrolyte after iRs correction (i is Faraday current and Rs is uncompensated electrolyte resistance). The iRs-corrected Tafel lines (h-log i) of the LDHs electrocatalysts are shown in Fig. 4b, where the slope b values and potential intercepts of each curve were calculated and analyzed by origin 8.6, and the results are shown in Table 1. The overpotential h was identified as one of the most important criteria for OER; the overpotential h10 calculated from the potential corresponding to the current density of 10 mA cm2 was used to compare the OER electrocatalytic performance of each electrocatalyst. The overpotential h10 of the D-NiFeAl-LDHs electrocatalyst obtained after alkali etching is only 262 mV, which is smaller than that of the NiFe-LDHs electrocatalyst of 283 mV, even better than the commercial IrO2 and RuO2 electrocatalysts [41e43] and obviously superior to the RuO2 shown in Fig. S7, suggesting that the introduction of defects by alkali etching can improve the OER electrocatalytic performance of LDHs electrocatalyst.

Please cite this article as: Li H et al., Accelerated oxygen evolution kinetics on NiFeAl-layered double hydroxide electrocatalysts with defect sites prepared by electrodeposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.155

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Fig. 4 e Polarization curves (a) and Tafel fitting curves (b) of the electrocatalysts after iR correction.

According to the electrochemical theory, the exchange current density j0 can characterize the polarization of the electrode and the reversibility of the electrode reaction. It is generally believed that the high exchange current density indicates that the electrode is difficult to be polarized, exhibits good reversibility and that the electrode reaction is more likely to occur [44]. The exchange current density of NiFeAl-LDHs electrocatalyst after introduction of Al3þ is lower than that of NiFe-LDHs electrocatalyst. The exchange current density of D-NiFeAl-LDHs obtained by alkali etching is 1.68  103 A cm2 better than that of NiFe-LDHs, indicating its good OER electrocatalytic activity. This is because the electrocatalytically inactive Al3þ will occupy some Ni2þ and Fe3þ sites in NiFe-LDHs after the introduction of Al3þ in NiFe-LDHs, resulting in a decrease in electrocatalyst activity. After NiFeAl-LDHs electrocatalyst was treated with appropriate al2þ kali etching, most of the Al(OH) x dissolved to produce Ni 3þ and Fe defects and exposed more active sites, increasing the OER activity of D-NiFeAl-LDHs electrocatalyst. Fig. 4b shows the Tafel fitting curves of the LDHs electrocatalysts. The Tafel slope b can characterize the speed of the electrocatalytic reaction overpotential varying with the current density. The smaller the slope b, the slower the overpotential changes with the current, and the better the electrocatalytic performance of the electrocatalyst. At the same time, the rate determining step during OER can also be judged according to the b value [24]. The Tafel slope of D-NiFeAl-LDHs electrocatalyst is only 41.67 mV dec1, superior to that of NiFeAl-LDHs, NiFe-LDHs and most OER electrocatalysts, exhibiting good OER electrocatalytic performance.

Table 1 e Kinetic parameters of polarization curves for the electrocatalysts.

NiFe-LDHs NiFeAl-LDHs D-NiFeAl-LDHs

b (mV dec1)

h10 (mV)

j0 (A cm2)

47.76 70.43 41.67

283 307 262

1.33  103 4.5  104 1.68  103

The correlation of Tafel slope and the rate determining step based on microkinetic analysis was discussed by Takanabe et al. [45]. Theoretically, a Tafel slope of 120 mV dec1 will be observed when the surface species formed in the step just before the rate determining step is predominant, and in other cases, the Tafel slope should be lower than 120 mV dec1. When Eqs. (4) or (6) determine the overall rate, a Tafel slope of 30 mV dec1 should be observed with high coverage of the empty sites. If the rate determining step is Eq. (5), a Tafel slope near 40 mV dec1 can appear. Therefore, in this research, Eq. (5) should be the rate determining step for NiFeAl-LDHs, since its Tafel slope is 70.43 mV dec1; however, Eq. (7) should be the rate determining step for D-NiFeAl-LDHs and NiFe-LDHs, since their Tafel slopes are 41.67 and 47.76 mV dec1, respectively. It is known that whether the reaction occurs easily or not can be characterized by activation energy (Ea). Lowering the activation energy is the common feature of an electrocatalytic reaction. The smaller the activation energy is, the better the electrocatalytic performance of the electrocatalyst. By plotting the anodic polarization curve at different temperatures (T), the activation energy can be determined using the Arrhenius law by Eq. (8) [46]: lni ¼ lnA 

Еa RT

(8)

where A is the pre-exponential factor, R is the gas constant (8.314 mol1 K1), T is the absolute temperature, and Ea is the molar activation energy. The anodic polarization curves at different temperatures (30e50  C) of the D-NiFeAl-LDHs electrocatalyst are shown in Fig. 5a, and Figs. S8aeb show the polarization curves of NiFe-LDHs and NiFeAl-LDHs electrocatalysts under the same conditions, respectively. In the oxygen evolution zone, the current density corresponding to the oxygen evolution potential of 1.58 V (h ¼ 350 mV) was taken as i, and lni-1/T was linearly fitted (as shown in the inset of Fig. 5a); thus, the activation energy Ea can be calculated by the slope value.

Please cite this article as: Li H et al., Accelerated oxygen evolution kinetics on NiFeAl-layered double hydroxide electrocatalysts with defect sites prepared by electrodeposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.155

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Fig. 5 e Polarization curves with different temperature and lni-1/T fitting curves of D-NiFeAl-LDHs (a) and lni-1/T fitting curves for the electrocatalysts (b). Nyquist diagrams of the electrocatalysts and the corresponding equivalent circuit diagram (c), the original data curves and the fitting curves are indicated by dotted and solid lines, respectively.

There is a clear relationship between activation energy and OER. Low activation energy is more conducive to the occurrence of anodic OER. The Arrhenius linear curve lni-1/T of different electrocatalysts is shown in Fig. 5b. The different slopes correspond to the oxygen evolution reactivity of different electrocatalysts in this electrochemical process. The corresponding activation energy data are listed in Table S3. The OER activation energy of NiFeAl-LDHs electrocatalyst is 33.01 kJ mol1 at the potential of 1.58 V, while the Ea value of the D-NiFeAl-LDHs electrocatalyst obtained by alkali etching is greatly reduced to 25.57 kJ mol1, which is less than the 29.81 kJ mol1 of NiFe-LDHs electrocatalyst, indicating that the OER electrocatalytic performance of NiFeAl-LDHs electrocatalyst is obviously improved after alkali etching treatment, which is compatible with the conclusion of the LSV polarization curve analysis. This is mainly because the Al(OH) x in the surface layer structure of the NiFeAl-LDHs electrocatalyst reacts with the OH combination in the strong alkali solution after alkali etching treatment to dissolve in the solution to form metal cation vacancies, and the existence of cation vacancies changed the electronic structure in the nanosheet structure of LDHs, promoting the conversion of Ni2þ to Ni3þ, which is more electrocatalytically active: that is, the conversion of Ni(OH)2 / NiOOH also promotes the conversion of Fe(OH)3 to FeOOH, resulting in reduced energy barrier of OER. At the same time, the Arrhenius diagram of the D-NiFeAlLDHs electrocatalyst was constructed according to the current density recorded for different potentials (Fig. S9). As the potential increased from 1.57 V to 1.59 V (vs. RHE), the activation energy decreased from 27.77 kJ mol1 to 23.75 kJ mol1, indicating that the potential is an important factor for OER performance of our electrocatalyst.

EIS analysis (charge transfer resistor Rct and electric double layer capacitor Q dl) Employing EIS to study the gas evolution reaction on the surface of the solid electrode is very informative. This paper uses this method to study the effect of introducing cation vacancies on the electrocatalytic oxygen evolution of NiFeLDHs electrocatalysts. Fig. 5c demonstrates the Nyquist plot

of the three different LDHs electrocatalysts at a test potential of 1.58 V (h ¼ 350 mV) in 1 M KOH electrolyte. It can be clearly seen that two capacitive reactance arcs appear in the entire frequency region of the Nyquist diagram, corresponding to two time constants. The capacitive reactance arc corresponding to the medium-high frequency region on the Nyquist complex diagram is related to the physical response of the pore structure on the surface of the electrocatalyst, and the half-capacitance arc in the low-frequency region characterizes the electrochemical reaction process between the electrocatalyst hydroxide coating and the solution interface. The equivalent circuit can be represented by Rs(RfCf)(RctCdl), as shown in the inset of Fig. 5c. In some earlier literature, this equivalent circuit was commonly applied to fit the impedance data of the electrocatalyst during OER [47,48]. In the equivalent circuit, Rs is the uncompensated solution resistance, Rf stands for the interfacial impedance formed by the migration of reactive ions in the solution to the interfacial layer of the electrocatalyst, and Cf is the corresponding capacitance of the interface layer. The (RfCf) combination in parallel characterizes the physical response of the electrocatalyst interface in the mid-high frequency region. Rct is the charge transfer resistance for the OER; meanwhile, Cdl is the corresponding capacitance. The (RctCdl) combination is definitely related to the process of OER in the low frequency domain. A constant phase element (CPE) is employed to replace the capacitive element for considering the nonhomogeneity of the electrode surface. The impedance of the CPE can be represented with Eq. (9): ZCPE ¼

1 n QðjWÞ

(9)

where Q is the CPE constant, n is the CPE power and w is the angular frequency. The chi-squared values of the fitting parameters obtained from the above equivalent circuit are all less than 103, suggesting that the selected equivalent circuit can accurately reflect OER behavior of the electrocatalyst in the 1 M KOH electrolyte. The fitted EIS data for all electrical parameters are listed in Table 2. It is obvious that the values of the uncompensated solution resistance Rs change little, showing the good stability of the electrolysis system. Several

Please cite this article as: Li H et al., Accelerated oxygen evolution kinetics on NiFeAl-layered double hydroxide electrocatalysts with defect sites prepared by electrodeposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.155

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Table 2 e Equivalent circuit fitting parameters of the electrocatalysts.

NiFe-LDHs NiFeAl-LDHs D-NiFeAl-LDHs

Rs (U cm2)

Cf (F cm2)

Rf (U cm2)

Qdl (S sn cm2)

n

Rct (U cm2)

0.7015 0.7112 0.7064

0.0779 0.1244 0.1311

0.1217 0.1462 0.09399

0.05744 0.05442 0.06753

0.8136 0.7603 0.8329

0.1527 0.2823 0.1458

studies have shown that the electric double layer capacitance (Qdl) and the charge transfer resistance Rct can be applied to evaluate and explain the electrocatalytic activity of the oxide/ hydroxide electrocatalyst [47,49]. The larger Qdl and smaller Rct values are considered to be more conducive to the occurrence of oxygen evolution electrocatalytic reactions, suggesting the better electrocatalytic activity of the electrocatalyst. The charge transfer resistance Rct in the low frequency region of the D-NiFeAl-LDHs electrocatalyst has a minimum value, and the corresponding Qdl has a maximum value, indicating that the electrocatalyst exhibits excellent OER performance. It can be clearly seen from the data in Table 2 that the value of charge transfer resistance increases and the corresponding capacitance Qdl value decreases when NiFe-LDHs electrocatalyst is doped with Al, indicating that the electrocatalytic performance of the NiFe-LDHs electrocatalyst decreases due to Al3þ doping; owing to the incorporation of Al3þ, which occupied a portion of the Ni2þ and Fe3þ sites in the NiFe-LDHs electrocatalyst, the number of Ni2þ and Fe3þ sites having OER activity was reduced. The OER electrocatalytic performance of the D-NiFeAl-LDHs electrocatalyst after alkali etching is better than that of NiFe-LDHs, because the cation vacancies generated by alkali etching boost the conversion of Ni2þ to Ni3þ, which has higher electrocatalytic activity, thus promoting the electrocatalytic performance of the electrocatalyst by increasing the number of Ni3þ sites.

Electrocatalytic stability test To evaluate the OER electrocatalytic stability of the electrocatalyst, the chronopotentiometric curves of NiFe-LDHs, NiFeAl-LDHs and D-NiFeAl-LDHs electrocatalysts were measured at 10 mA cm2 current density for 18 h (Fig. 6a). As

seen, the potentials rose rapidly at first and then remained stable after reaching a potential plateau. The increase of potential is mainly due to an activation process in which the surface layers of Ni(OH)2 and Fe(OH)3 were converted into electrocatalytically active substances NiOOH and FeOOH, respectively. The 18 h stability test curve showed a stable overpotential platform, indicating that each LDHs electrocatalyst exhibits relatively stable electrocatalytic performance in alkaline solution. The D-NiFeAl-LDHs electrocatalyst showed the lowest overpotential platform, indicating that the electrocatalyst has the best OER electrocatalytic performance. To measure the stability of the D-NiFeAl-LDHs electrocatalyst after long cycles, 2500 cycles of voltammetry were performed in the potential range of 1.2e1.6 V to compare the changes of polarization curves before and after the cycles. It can be seen from Fig. 6b that the polarization curves before and after the cyclic test exhibited almost no significant change, and the electrocatalyst still retained the intact nanosheet structure after the cycle (as shown in the inset of Fig. 6b), indicating the good electrocatalytic stability of D-NiFeAl-LDHs electrocatalyst. The D-NiFeAl-LDHs electrocatalysts exhibit excellent OER electrocatalytic performance, which should be attributed to the following points. 1) Depositing D-NiFe-LDHs nanosheets directly on NF and tightly joining these nanosheets to form a network maintains good structural stability while promoting efficient electron transport. 2) The Ni2þ and Fe3þ defect sites are formed by the alkali etching treatment of NiFeAl-LDHs, changing the surrounding electronic structures of partial Ni2þ and Fe3þ sites, thereby exposing and generating more electrocatalytic active sites. 3) It is well known that the amorphous nature of D-NiFeAl-LDHs and other amorphous materials enables superior electrocatalytic

Fig. 6 e Chronopotentiometric curves of the electrocatalysts at 10 mA cm¡2 for 18 h (a), and polarization curves of D-NiFeAlLDHs electrocatalyst in the range of 1.2e1.6 V before and after 2500 cycles of voltammetric scanning (b); the inserted image is the SEM image of the electrocatalyst surface after cyclic stability testing. Please cite this article as: Li H et al., Accelerated oxygen evolution kinetics on NiFeAl-layered double hydroxide electrocatalysts with defect sites prepared by electrodeposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.155

international journal of hydrogen energy xxx (xxxx) xxx

activity versus crystalline materials due to the concentration of unsaturated coordination sites. 4) The electrodeposition method is simple, and the prepared electrocatalyst has the advantages of good mechanical adhesion and lack of binder.

[8]

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Conclusions [10]

In summary, we synthesized the NiFeAl-LDHs electrocatalyst by a simple electrodeposition method and obtained the defect-containing D-NiFeAl-LDHs electrocatalyst by appropriate alkali etching treatment, which showed high electrocatalytic activity and excellent durability performance in 1 M KOH electrolyte. Reaching the current density of 10 mA cm2, the D-NiFeAl-LDHs electrocatalyst requires only a low overpotential of 262 mV, and its OER Tafel slope is as low as 41.67 mV dec1, possessing large exchange current density, electric double layer capacitance, and a small OER activation energy, and the electrocatalyst also exhibits good stability after 18 h of chronopotentiometric curve testing. The simple electrodeposition method combined with alkali etching treatment successfully introduces Ni2þ and Fe3þ defect sites, which further improves the electrocatalytic performance of NiFe-LDHs electrocatalyst. This is a simple and feasible method for producing excellent LDHs oxygen evolution electrocatalysts with defect sites and for providing new insights for the development of effective OER electrocatalysts.

Appendix A. Supplementary data

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Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.09.155. [18]

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Please cite this article as: Li H et al., Accelerated oxygen evolution kinetics on NiFeAl-layered double hydroxide electrocatalysts with defect sites prepared by electrodeposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.155