Scalable Dealloying Route to Mesoporous Ternary CoNiFe Layered

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Scalable Dealloying Route to Mesoporous Ternary CoNiFe Layered Double Hydroxides for Efficient Oxygen Evolution Chaoqun Dong, Lulu Han, Chi Zhang, and Zhonghua Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02656 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on October 31, 2018

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ACS Sustainable Chemistry & Engineering

Scalable Dealloying Route to Mesoporous Ternary CoNiFe Layered Double Hydroxides for Efficient Oxygen Evolution Chaoqun Dong,‡,§Lulu Han, † School

‡,§

Chi Zhang,† Zhonghua Zhang*,†,‡

of Applied Physics and Materials, Wuyi University, 22 Dongcheng Village, Jiangmen 529020, P.R.

China ‡ Key

Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education),

School of Materials Science and Engineering, Shandong University, Jingshi Road 17923, Jinan 250061, P.R. China §These

authors equally contribute to this work.

*Corresponding author. Email: [email protected].

Abstract The mass production of clean hydrogen fuels via (photo)electrochemical water splitting calls for highly-efficient, cost-effective and eco-friendly catalysts. Herein, a facile and scalable strategy, namely dealloying, is advanced to fabricate mesoporous ternary layered double hydroxides (LDHs) containing Co, Ni and Fe for highly-efficient oxygen evolution and overall water splitting, based upon elaborate design of precursors and accurate control of the dealloying process. The Co1Ni2Fe1-LDH exhibits remarkable catalytic properties towards oxygen evolution reaction (OER) in 1 M KOH, for instance low overpotentials (only requires 240.4 mV on glass carbon electrode, and 228.5 mV on Ni foam to drive 10 mA cm-2), a small Tafel slope (38.6 mV dec-1), as well as excellent stability (lasts 45 h for 10 mA cm-2 without deactivation). Surprisingly, a symmetric alkaline electrolyzer constructed with Co1Ni2Fe1-LDH serving as the catalyst for both cathode and anode, requires only 1.65 V to drive 10 mA cm-2. The distinguished features of the catalysts lie in the combined effects of the unique LDH structure with large interlayer spaces, the 3D porous structure, and the synergistic interplay of the metal species, concurrently contributing to the fully exposed active sites, accelerated electrolyte penetration and charge/ion transfer, as well as the well-promoted reaction kinetics. The consolidation of the electrocatalytic merits and the facile, economical fabrication route endows the ternary CoNiFe-LDHs as promising catalysts for the generation of renewable energy resources. 1 ACS Paragon Plus Environment

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Keywords: OER; LDH; dealloying; water splitting; mesoporous structure.

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Introduction Electrochemical water splitting into high-purity hydrogen and oxygen is broadly believed as a simple yet efficient technique to obtain a clean and sustainable fuel, which can not only satisfy the worldwide growing energy demands but also reduce the emission of carbon dioxide.1–4 However, large overpotentials are always indispensable due to the involvement of complex proton-coupled electron transfer with the formation of an oxygen-oxygen bond and the resultant sluggish reaction kinetics, which inevitably impair the energy efficiency of overall water splitting.5 Therefore, electrocatalysts are usually applied to modify electrodes, in order to lower the activation energy, boost the conversion rate and thus enhance the total cell efficiency. Among them, bifunctional water-splitting catalysts that are capable of enhancing both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are especially desirable, owing to the simplified structure and reduced fabrication cost of the electrolyzer systems, for which only one type of electrode and electrolyte are required.6 In practice, it is more tricky to design and prepare such bifunctional catalysts than the corresponding monofunctional ones. For instance, Pt is considered the state-of-the-art HER catalyst up to now, while RuO2 and IrO2 are much better towards OER. Beyond excellent catalytic activity, long-term catalytic stability is another key issue to assess catalysts. Many developed catalysts like metal phosphides, sulphides, nitrides

and

selenides,

are

prone

to

be

oxidized

to

the

corresponding

metal

oxides/(oxy)hydroxides to some extent, especially at high oxidative potentials during the catalytic process.7 Currently, transition metal-based oxides and (oxy)hydroxides are recognized as promising electrocatalysts for water splitting, thanks to their variable valence states as well as the optimal interaction between metal ions and oxygen intermediates.8 The specific activity of a catalyst for a target reaction greatly relies on its chemical composition and electronic structure.9,10 For example, it has been found that a small amount of Fe doping leads to a dramatic effect on improving the OER catalytic performance of Ni oxides and hydroxides, which is due to the induced structural disorder and enhanced conductivity.11,12 Layered double hydroxides (LDHs), as typical 2D layered materials built from the alternative arrangement of positively charged brucite-like host layers and charge-balancing interlayered anions, have shown amazing performances towards water oxidation.13 This unique structure allows a wide tunability of diverse metal species and ratios in the intralayer, resulting in high 3 ACS Paragon Plus Environment


possibilities of regulating compositions and electronic structures of LDHs materials. The flexibility of incorporating metallic species along with the large interlayer spaces, endows LDHs with accelerated ion/electron transfer rates and fully exposed active sites, which are of great potentials towards highly-efficient water splitting.14 Up to now, various binary LDHs systems (e.g. NiFe,15 NiCo,16 CoFe,17 ZnCo18) have been rationally designed and employed as superior water-splitting electrocatalysts. Recently, the proper introduction of a third metal species into transition metal oxides or hydroxides was reported to play an active role in further promoting the catalytic activities. For instance, Stahl et al. performed combinatorial screening of almost 3500 trimetallic metal oxides for oxygen evolution and drew a conclusion that the most active materials are oxides composed of Ni, Fe and a third element under alkaline conditions.19 Sargent et al. developed an atomicallyhomogeneous gelled FeCoW oxyhydroxide, which exhibits superior catalytic activity towards OER than CoFe LDH. Moreover, they concluded that the modulation of W to the metal oxides provides near-optimal adsorption energies for OER intermediates based on density functional theory calculations.20 By realizing the abovementioned issues, attempts have been made to develop ternary LDHs with great activities toward water splitting. Despite these progresses, the fabrication of catalysts often requires several complex steps including pre-oxidation, electrodeposition, co-precipitation, hydrothermal process, calcination,21–23 making the preparation procedures slow, costly and difficult for mass production. Hence, it is technically required to propose a facile, cheap and scalable route to fabricate ternary LDHs for highly-effective water splitting. Dealloying is a common (electro)chemical corrosion process, where the more active element(s) is selectively corroded and the less active element(s) is left and reorganizes to assemble a nano/meso-porous structure. In the past years, this facile process has mainly been developing as one of the most important strategies to fabricate nano/meso-porous metals or metal oxides. Herein, dealloying is advanced to fabricate mesoporous ternary LDHs containing Co, Ni, and Fe, based upon exquisite devise of alloy precursors (taking into account the compositions, microalloying and phase constitution) and accurate control of dealloying conditions (like the dealloying media, temperature, duration and atmosphere). To our knowledge, this is the first trial to fabricate ternary LDHs materials via dealloying. The ternary CoNiFe-LDHs exhibit 4 ACS Paragon Plus Environment

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exceptional electrocatalytic activities and outstanding stability towards water oxidation and overall water splitting in alkaline media, which is comparable or even better than many state-ofthe-art electrocatalysts. These admirable qualities are attributed to the integration of well-defined LDHs structure with large interlayer spaces, the 3D macroporous and mesoporous microstructure, and the synergistic interplay among the metal species.

Experimental section Material synthesis ： The synthesis procedure of the CoNiFe LDHs can be described as the coupling of a facile single-roller melting technique and a dealloying treatment. First, Al-Co-NiFe alloys with nominal compositions of Al96Co1Ni2Fe1, Al95.5Co1.5Ni1.5Fe1.5, and Al96Co2Ni1Fe1 were obtained by co-melting the corresponding pure metal blocks (99.9 wt.% purity) in a vacuum arc furnace. The melts were cast into homogeneous alloy ingots in an iron chill mold and it took several minutes to cool down to ambient temperature. Then the ingots were remelted in a highfrequency induction heated quartz tube and melt-spun onto a spinning Cu roller (0.5 m in diameter) at a high speed of 1 krpm under Ar atmosphere. Then numerous 20-50 µm thick, 2-5 mm wide and several centimeters long alloy ribbons were obtained. As for the dealloying process, it was carried out in a 1 M NaOH aqueous solution at room temperature for around 2-3 h until no macroscopic bubbles appeared and then at 60 ˚C for about 0.5 h to further etch the residual Al (Figure S1). After the repeated rinse operation with ultrapure water (18.25 MΩ cm), the asdealloyed materials were exposed to an ambient environment for 24 h, followed by complete drying at 60 °C in vacuum. Then the CoNiFe LDHs were successfully obtained. For simplicity, the LDHs obtained from Al96Co1Ni2Fe1, Al95.5Co1.5Ni1.5Fe1.5, and Al96Co2Ni1Fe1 precursors were named as Co1Ni2Fe1, Co1.5Ni1.5Fe1.5, and Co2Ni1Fe1 respectively. For comparison, binary LDHs of Ni2Fe1, Co2Fe1, and Ni2Co1 were also fabricated in the same way. Microstructural and compositional characterization ： The morphology and nanostructure of the LDHs samples were first explored by means of a scanning electron microscope (SEM, FEI QUANTA FEG250) and a transmission electron microscope (TEM, JEM-2100). The crystalline nature of the samples was characterized by using selected-area electron diffraction (SAED). Xray diffraction (XRD, Beijing Purkinje General Instrument Co., Ltd, China) was carried out to identify the phase formation of the LDHs samples. Besides, X-ray photoelectron spectroscopy 5 ACS Paragon Plus Environment


(XPS, ThermoFisher Scientific USA, ESCALAB 250) was used to analyze the surface elemental characteristics. All reported binding energies were calibrated to the C 1s peak at 284.6 eV. To better study the physical specific surface area and pore distribution of the catalysts, nitrogen adsorption-desorption characterization was performed by using a V-Sorb 2800P Surface Area and Pore Size Analyzer. Electrochemical measurements ： The electrocatalytic performance of the catalysts towards OER and HER was first investigated in a standard three-electrode configuration connected to an electrochemical station (CHI 660E). A catalyst-modified glass carbon electrode (GCE, 5 mm in diameter) or a catalyst-decorated Ni foam was utilized as the working electrode, and an Ag/AgCl electrode was selected as the reference electrode. As for the counter electrodes, a platinum foil and a graphite plate were selected for OER and HER respectively. To prepare the catalyst suspension on GCEs, catalysts and XC-72 carbon powder with the mass ratio of 2:3 (7.84 and 11.76 mg respectively) were dispersed in a mixed solution composed of isopropanol (1.5 mL) and Nafion solution (0.5 wt.%, 0.5 mL), and then was sonicated for 30 min to obtain a homogenous ink solution. Then small amount of ink solution (about 10 µL) was carefully dropcast onto the clean GCEs, followed by drying at room temperature in vacuum. The final mass loading of all the catalysts is the same (0.2 mg cm-2). The Ni foam-supported Co1Ni2Fe1 electrodes were obtained by coating catalyst slurry onto Ni foams (1 cm × 1 cm) followed by drying at 80 °C overnight. The slurry was made by mixing catalysts, acetylene black (Super-P) and polyvinlidene fluoride (PVDF) binder with a mass ratio of 7.0:1.5:1.5 in N-methyl-2pyrrolidone (NMP) solvent. All the electrochemical measurements were executed in a 1 M KOH aqueous solution (pH=13.60), which was saturated with high-purity O2 (for OER) or N2 (for HER). The polarization curves of the catalysts on GCEs were recorded via linear sweep voltammetry (LSV) at 5 mV s-1 under a rotation speed of 1600 rpm. The electrochemical impedance spectroscopy (EIS) tests for OER were performed at 0.49 V vs. Ag/AgCl in a frequency range of 0.01-100k Hz with a 5 mV amplitude. The long-term stability of the Ni foam-supported catalyst was characterized by using chronopotentiometric method at 10 mA cm-2. The bifunctional electrocatalytic properties of the catalyst towards overall water splitting were assessed in a twoelectrode configuration with two pieces of Ni foam-supported Co1Ni2Fe1 electrodes as the 6 ACS Paragon Plus Environment

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cathode and anode respectively. For comparing, the polarization curve of pure Ni foam without any decoration was also recorded. Afterwards, the stability for bifunctional water splitting was evaluated by performing chronopotentiometric test at 10 mA cm-2. All the potentials were calibrated to reversible hydrogen electrode (RHE) according to the following relation: ERHE = EAg/AgCl + 0.197 + 0.059 * pH. And all the LSV results of the catalysts were iR-compensated. Results and discussion For dealloying, the design of precursors is of great significance. Generally, Al-based alloys are preferable thanks to the selective removal of Al in NaOH aqueous solutions, contributing to surface diffusion and reorganization of the other element(s).24,25 According to the common criteria of dealloying process, the content of the less active element (here is Al) in precursors should be higher than its parting limit. Therefore, Al-Co-Ni-Fe alloys with different atomic ratios of Al96Co1Ni2Fe1, Al95.5Co1.5Ni1.5Fe1.5, and Al96Co2Ni1Fe1 were reasonably prepared. The dealloying process was executed in 1 M NaOH at room temperature by considering that a lower temperature can slower the surface diffusion of Co, Ni and Fe to obtain a reduced dimensional scale of ligaments/channels. XRD was firstly performed to identify the phase constitution of the as-dealloyed catalysts (Figure 1a). Overall, the three materials show quite similar diffraction peaks. The sharp peaks at 11.6˚, 23.4˚ and 34.1˚ are three landmark diffraction peaks of LDHs materials, corresponding to the (003), (006), and (012) facets of the well-known binary LDH structure (JCPDF 50-0235).26 In particular, the position of (003) peak is attributed to the interlayer separation caused by the occupation of water molecules and CO32- ions, indicating the existence of all the three metallic species in the intralayer of the LDH structure. SEM and TEM were further utilized to study the morphology and microstructure of the as-dealloyed materials (Figure 1b-f, Figures S2 and S3). The low-magnification SEM images (inset of Figure 1b, Figures S2a and S3a) reveal rough surfaces formed mainly by some particles. Meanwhile, careful observation can find that numerous nanosheets are uniformly interconnected to form 3D nanostructures on the surfaces (Figure 1b, Figures S2b and S3b). Notably, the arrangement of the tens of nanometers thick nanosheets forms an obvious porous nanostructure, which is favorable for fast electrolyte diffusion, accelerated charge transfer, and easy elimination of gases during catalysis. TEM images (Figure 1c-e) further demonstrate a typical 3D porous nanostructure constructed by highly-interconnected nanosheets. SAED pattern in the inset of Figure 1e reveals 7 ACS Paragon Plus Environment


nanocrystalline nature of the sample. In addition, HRTEM image in Figure 1f presents a visible lattice spacing of 7.7Å, corresponding to the (003) planes of the LDH structure. Besides, nanopores (or mesopores) with 10-15 nm in dimension could be also well-identified. Furthermore, the elemental compositions of the samples were explored via energy-dispersive X-ray (EDX) analysis, as shown in Figure S4. The results indicate that there was still minor residual Al in the final products, which is inevitable according to the dealloying mechanism.27 The N2-adsorption/desorption isotherms (Figure 2a, Figures S5a and S6a) of the CoNiFe-LDHs are typical IV isotherms with a hysteresis loop, illustrating the mesoporous structure of the materials. The BET surface areas are 51.68, 80.99 and 61.76 m2 g-1 for Co1Ni2Fe1, Co1.5Ni1.5Fe1.5, and Co2Ni1Fe1 respectively. Moreover, the mesoporous characteristic of the samples could be further verified by the pore size distribution results (Figure 2b, Figures S5b and S6b). To further explore the chemical states of the as-dealloyed Co1Ni2Fe1 material, XPS measurement was executed (Figure 2c-f, Figure S7). The Co 2p spectrum could be fitted with one spin-orbit doublet and two shakeup satellites (Figure 2c). The two main peaks of Co 2p1/2 and Co 2p3/2 are located at 797.26 and 781.80 eV respectively, indicating that Co exists in the form of Co2+.28 In Figure 2d, the two main peaks at 856.35 and 873.90 eV correspond to Ni 2p3/2 and Ni 2p1/2, accompanied by two satellites at 862.20 and 879.96 eV respectively. The energy separation of 17.55 eV between the Ni 2p3/2 and Ni 2p1/2 peaks implies the existence of Ni2+.28 The two main peaks at 712.65 and 724.63 eV in the Fe 2p spectrum (Figure 2e) correspond to Fe 2p3/2 and Fe 2p1/2 respectively, indicating the presence of Fe3+.23,29 It has been demonstrated that the peaks of Fe2+ are located at between 708 and 711.5 eV,30 while this does not appear in this spectrum, demonstrating the domination of Fe3+ in Co1Ni2Fe1. For the O 1s spectrum (Figure 2f), it is complicated due to the complexity of the bonds between various metal cations and O in the LDH structure. It can be divided into four peaks: 532.7 eV for Co-O-H, 531.48 eV for Ni-O-H, 531.07 eV for Fe-O-H, and 533.6 eV for physisorbed or chemisorbed water on the sample surface.28,31 Consequently, the above XPS results indicate the successful fabrication of CoNiFe-LDH in this work. The electrocatalytic activity of the catalysts towards the OER was studied in a three-electrode system in the O2-saturated 1 M KOH solution. The iR-compensated OER polarization curves (Figure 3a) were collected at the slow rate of 5 mVs-1 to minimize the effect of capacitive current. Before data collection, tens of cyclic voltammetric scans were performed to reach a stable 8 ACS Paragon Plus Environment

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condition. For comparison, the same tests were carried out on commercial IrO2, XC-72 carbon powder as well as binary LDHs of Ni2Fe1, Co2Fe1, and Ni2Co1 (the XRD patterns are shown in Figure S8). It is obvious that XC-72 carbon shows very poor OER activity. In contrast, the current densities for LDHs first maintain zero before activation, and then burst with a swift rise after the onset potential with the applied potential (Figure 3a and Figure S9). Figure 3b exhibits the overpotential required to drive 10 mA cm-2, which approximately corresponds to 10 % efficient solar water-splitting units and is commonly adopted as a vital parameter to evaluate the OER activities of catalysts. Remarkably, Co1Ni2Fe1 could achieve such a current density at a low overpotential of merely 240.4 mV, which is more negative than that of Co1.5Ni1.5Fe1.5 (246.5 mV), Co2Ni1Fe1 (258.5 mV) and IrO2 (338.9 mV). The overpotentials for Ni2Fe1, Co2Fe1, and Ni2Co1 are 274 mV, 287 mV and 364 mV respectively, all of which are inferior to the CoNiFe LDHs, further indicating the promoted catalytic performance attributed to the synergistic effect of Co, Ni and Fe. To obtain more information about the OER kinetics of the catalysts, Tafel plots extracted from the polarization curves were also assessed. Normally, a relatively low Tafel slope is preferable because it indicates more advantageous kinetics and higher catalytic activity. The Tafel slopes for Co1Ni2Fe1, Co1.5Ni1.5Fe1.5, and Co2Ni1Fe1 are 38.6, 38.8 and 41.6 mV dec-1 respectively, all of which are significantly lower than that of IrO2 (80 mV dec-1), indicating the superior kinetics of the CoNiFe-LDH materials (Figure 3c). Generally, a lower Tafel slope indicates that the rate-determining step is at the later part of a consecutive reaction. In this work, all of the three CoNiFe-LDH catalysts display low Tafel slopes of around 40 mV dec-1 in the lower overpotential domain, suggesting that the second step is the rate-determining step for all the cases.32 Table S1 compares the electrochemical performance of Co1Ni2Fe1 with some recently-reported transition metal-based OER electrocatalysts,33–54 which clearly validates the superiority of Co1Ni2Fe1 towards OER. EIS could provide an opportunity to understand the underlying electrochemical behaviour associated with the interface between electrocatalysts and electrolyte. Briefly, a smaller semicircular diameter in Nyquist diagrams implies a smaller contact and charge transfer resistance property during electrocatalysis. As shown in Figure S10, the Co1Ni2Fe1 shows a minimum semicircular diameter, suggesting its excellent charge transport kinetics. A suitable electrical equivalent circuit model (Figure S11) specifically for metal oxides/hydroxides based OER catalysts was utilized to simulate the obtained Nyquist plots.55 In the model, Rs reflected in the 9 ACS Paragon Plus Environment


high frequency region signifies the solution resistance, and R1-Q1 refer to the resistive and dielectric properties of the catalysts layer, while Rct-Qct are connected to the kinetics of the charge transfer process at the catalyst/electrolyte interface, which is the most important element to reflect the properties of the overall reaction. The fitted Rct values of Co1Ni2Fe1, Co1.5Fe1.5Ni1.5, and Co2Ni1Fe1 are 5.428, 6.185, and 8.644 Ω respectively, which are commonly believed to play an important role in their superior catalytic activities. In general, the overall OER reaction can be divided into three sub-steps, including the adsorption of H2O molecules onto the electrode surface, the oxygen evolution at the active sites, and the final elimination of O2. Both the first two steps benefit significantly from the large interlayer spaces of the unique LDH structure and the inherent 3D porous structure. This structural design can offer much larger surface area and further expose more active sites than the corresponding bulk products, which is expected to be advantageous to both the H2O adsorption and principal reaction steps. Furthermore, the 3D porous structure offers plenty of open channels for the last O2 release step, which could avoid the problem of charge transfer blockage induced by the adhesion of gaseous product on the electrode and further facilitate the charge transport process. In addition to these factors, the incorporation of a third proper metal species to LDHs could modify the electronic structure and further optimize the binding conditions between the intermediates (oxo, peroxide and superoxide) and reactants during OER. These elements concurrently contribute to the low charge transfer resistance of the CoNiFe LDHs catalysts and the promoted reaction kinetics. Additionally, the excellent long-term stability towards OER is also a pivotal property for efficient OER electrocatalysts. A chronopotentiometric test was performed for a Ni foam-supported Co1Ni2Fe1 electrode at 10 mA cm-2. The initial overpotential is around 228.5 mV and there is almost no significant change for a long time period of 45 h (Figure 3d). The slight increase of the overpotential is attributed to the minor exfoliation of the catalyst from the Ni substrate under the impact effect of produced vigorous bubbles or the slight deactivation of the Co1Ni2Fe1 catalyst. The exploration of bifunctional electrocatalysts towards both OER and HER in the same electrolyte is preferred, which provides a new avenue for simplifying the electrolysis device and reducing the expense of overall water splitting. By catering to this practical requirement, catalytic properties of the CoNiFe-LDH materials towards HER were also evaluated in a N2-saturated 1 M KOH solution. Figure S12a shows the iR-compensated HER polarization curves of the CoNiFeLDHs and Pt/C on GCEs. The required overpotentials for the CoNiFe-LDHs to drive 10 mA cm-2 10 ACS Paragon Plus Environment

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are obviously higher than that of the Pt/C electrode, which is regarded as the best catalyst for HER. However, given the sluggish kinetics of Pt/C towards OER, it has no distinct advantage for bifunctional water splitting. In alkaline media, HER initially proceeds with a proton discharge step (Vomer reaction), followed by two possible steps: either by a hydrogen recombination (Tafel reaction) or an electro-desorption (Heyrovsky reaction) step.56 The HER Tafel slope is often estimated to determine its rate-determining step. Figure S12b shows the Tafel slopes of the catalysts obtained from polarization curves in Figure S12a. The Tafel slope of Pt/C at low overpotential range is 30 mV dec-1, demonstrating that the HER undergoes the Volmer-Tafel pathway, and that the hydrogen recombination reaction is the rate-determining step. The Tafel slopes of Co1Ni2Fe1, Co1.5Ni1.5Fe1.5, and Co2Ni1Fe1 are 78, 105, and 75 mV dec-1, manifesting that the HER proceeds predominantly via the Volmer-Heyrovsky pathway and the electrodesorption reaction is the rate-determining step.57 Inspired by excellent bifunctional electrocatalytic properties of the CoNiFe-LDHs for both half reactions of water splitting, a two-electrode alkaline electrolyzer was considered to be assembled. Taking into account the overall catalytic activities towards both OER and HER, there is no significant distinction among the three CoNiFe-LDH materials. Therefore, Co1Ni2Fe1 was determined to be a typical example for further study of their overall water splitting performance. For this purpose, Ni foam-supported Co1Ni2Fe1 electrodes were fabricated and utilized to construct a symmetric alkaline electrolyzer. Strikingly, the electrolyzer displays distinguished performance with a low cell voltage of 1.65 V to drive the current density of 10 mA cm-2 (Figure 4a). For reference, another electrolyzer integrated by using two Ni foams as electrodes demands a much higher voltage of 1.96 V to afford the same water-splitting current. The stability of the Ni foam-supported Co1Ni2Fe1 electrolyzer was evaluated through a chronopotentiometric test at 10 mA cm-2. The cell voltage increases only a bit after a continuous test for 12 h (Figure 4b), demonstrating the robust stability of the Co1Ni2Fe1 catalyst. TEM was implemented to check the morphology of the Co1Ni2Fe1 catalyst after chronopotentiometric tests on the Ni foam substrate. The results are shown in Figure S13, where the nanoparticles and nanoplates tend to accumulate and overlap with each other due to the effect of PVDF binder. Nevertheless, it is obvious that the nanostructure is still similar with that before the measurements (Figure 1), albeit the enlarged dimensions to some extent.



Our electrochemical data suggest that the CoNiFe-LDHs materials are novel electrocatalysts with exceptional water-splitting activity and stability in alkali. The intrinsically high activity of the CoNiFe-LDH would derive from the following aspects. First, the characteristic 2D nanosheetshaped configuration could accelerate the ion transfer in nanoscale dimension and the large interlayer space between adjacent CoNiFe-LDH sheets acts as “ion-buffering reservoirs” of electrolyte to ensure sufficient electrolyte supply for efficient catalytic reaction on the active sites.58 Second, the 3D open porous structure has a multiple effect on the as-dealloyed CoNiFeLDH catalysts, including efficient exposure of the electrocatalytic active sites, the fast electrolyte penetration and charge/ion transfer, as well as the easier release of produced gases during water electrolysis. Furthermore, the synergistic effect of the metal species may also play a vital role in improving the electronic conductivity and altering the adsorption energies for reaction intermediates.59

Conclusions In conclusion, we demonstrate the traditional dealloying approach as a facile, fast, controllable and economic method to fabricate CoNiFe-LDHs materials for highly-efficient OER and overall water splitting. The resultant Co1Ni2Fe1-LDH shows quite low overpotentials to achieve 10 mA cm-2 (240.4 mV on GCE, and 228.5 mV on Ni foam), low Tafel slope (38.6 mV dec-1), and impressive long-term stability for OER (lasts for 45 h without noticeable degradation), which surpasses the commercial IrO2 and many other excellent OER catalysts. The alkaline electrolyzer constructed with two Ni foam-supported Co1Ni2Fe1 electrodes can split water efficiently with a low voltage of 1.65 V to afford the current density of 10 mA cm-2. These results pinpoint the great capacity of the CoNiFe-LDHs materials for advanced OER and overall water splitting implements. Moreover, dealloying is a cost- and time-effective synthesis method, which holds considerable promise for large-scale production (Figure S14), and is expected to show potential applications in renewable and clean energy resources. Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Photographs, SEM images, EDX spectra, N2 adsorption-desorption isotherms and pore size


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distributions, XPS spectrum, XRD patterns, polarization curves, Nyquist diagrams, electrical equivalent circuit model, TEM images, OER performance comparison. Conflicts of interest There are no conflicts to declare. Acknowledgements The authors gratefully acknowledge financial support by National Natural Science Foundation of China (51671115) and Young Tip-top Talent Support Project (Department of Science & Technology of Shandong Province).



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Figures

Figure 1. (a) XRD patterns of the CoNiFe-LDHs materials. (b) High-magnification and (b, inset) low-magnification SEM images of the Co1Ni2Fe1 catalyst. (c-e) TEM images and (f) HRTEM image of the Co1Ni2Fe1 catalyst. (e, inset) Corresponding SAED pattern.



Figure 2. (a) N2 adsorption-desorption isotherm and (b) pore size distribution of the Co1Ni2Fe1 catalyst. (c-f) XPS spectra of the Co1Ni2Fe1 catalyst: (c) Co 2p, (d) Ni 2p, (e) Fe 2p and (f) O1s.


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Figure 3. (a) OER polarization curves of different CoNiFe-LDH catalysts loaded on GCEs in 1 M KOH. (b) The overpotentials required to drive the current of 10 mA cm-2 for different catalysts. (c) Tafel plots of the catalysts. (d) Chronopotentiometric curve of a Ni foam-supported Co1Ni2Fe1 electrode for OER at 10 mA cm-2 for 45 h.



Figure 4. (a) Polarization curves of Ni foam and Ni-supported Co1Ni2Fe1 catalyst for overall water splitting. (b) Chronopotentiometric curve at 10 mA cm-2 of the two-electrode electrolyzer assembled with two Ni foam-supported Co1Ni2Fe1 electrodes.


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TOC A scalable dealloying process is advanced to fabricate mesoporous ternary CoNiFe layered double hydroxides for highly-efficient electrocatalytic oxygen evolution.


Scalable Dealloying Route to Mesoporous Ternary CoNiFe Layered

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