Defect-Rich Ultrathin CobaltâIron Layered Double Hydroxide for

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Defect-Rich Ultrathin Cobalt-Iron Layered Double Hydroxide for Electrochemical Overall Water Splitting Peng Fei Liu, Shuang Yang, Bo Zhang, and Hua Gui Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12803 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on November 30, 2016

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ACS Applied Materials & Interfaces

Defect-Rich Ultrathin Cobalt-Iron Layered Double Hydroxide for Electrochemical Overall Water Splitting Peng Fei Liu,† Shuang Yang,† Bo Zhang‡ and Hua Gui Yang*† †

Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science

and Engineering, East China University of Science and Technology, Shanghai, 200237 (China) ‡

Department of Physics, East China University of Science and Technology, Shanghai, 200237

(China)

ABSTRACT: Efficient and durable electrocatalysts from earth-abundant elements play a vital role in the key renewable energy technologies including overall water splitting and hydrogen fuel cells. Here, generally-used CoFe based layered double hydroxides (LDHs) were firstly delaminated and exfoliated in the DMF-ethanol solvent (CoFe LDH-F), with enhancement both in oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The exfoliation process creates more coordinatively unsaturated metals and improves the intrinsic electronic conductivity, which is important in water electrolyzer reactions. In the basic solution, the CoFe LDH-F catalyst outperforms the commercial iridium dioxide (IrO2) electrocatalyst in activity and stability for OER and approaches the performance of platinum (Pt) for HER. The bifunctional electrocatalysts can be further used for overall water splitting, with a current density of ~10

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mA/cm2 at the applied voltage of 1.63 V for long-term electrolysis test, rivalling the performance of Pt and IrO2 combination as benchmarks. Our findings demonstrate the promising catalytic activity of LDHs for scale-up alkaline water splitting.

KEYWORDS: bifunctional electrocatalysts, layered double hydroxide, oxygen vacancies, overall water splitting, hydrogen evolution reaction, oxygen evolution reaction

1. Introduction Electrochemical water splitting as a well-established technique to obtain hydrogen (H2), has attracted increasing attention due to its promise to convert intermittent renewable energy sources to clean hydrogen fuel.1-5 However, the large overpotential (η) to motivate OER and HER process still hinders this practical use for mass hydrogen production. Currently, the applied potentials of commercial electrolyzers are around 1.8-2.2 V, which consume ~50% excess potential than the thermodynamic requirement of 1.23 V at room temperature. Therefore, tremendous efforts have been made to develop active electrocatalysts to make the process more energy-efficient, such as cobalt-phosphate,6 perovskite oxides7 and 3d transition metal based (oxy)hydroxides for OER,8 and transition metal dichalcogenides,9 nitrides10, phosphides11,12 and oxides13,14 for HER. Unfortunately, to build an overall water splitting device, electrocatalysts for HER and OER should remain their activity and stability in the same electrolyte, either strongly acidic or basic media, but most developed catalysts fail to stay highly efficient and stable over the wide range of pH.15 Thus, discovering bifunctional electrocatalysts with high activity towards HER and OER in the same electrolyte can simplify the system and lower the cost.16


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In the alkaline electrolyte, the cobalt (Co) and nickel (Ni) based compounds (CoP,17 CoSe,18 Ni2P,19 Ni5P4,20 Ni3S221 and NiSe16) have been discovered as bifunctional electrocatalysts for overall water splitting. All these catalysts have already been demonstrated with high activity for HER and the really active species for OER are the in-situ generated CoOOH or NiOOH during the oxidation process.15-21 Another strategy to design bifunctional materials in the basic solution is to advance HER activity for the electrocatalysts which are efficient for OER. Following this idea, 3d transition metal based oxides and hydroxides have been designed for efficient and durable overall water splitting, for example, NiCo2O4 microcuboids,22 CoMn nanoparticle superlattices23 and NiFe layered double hydroxide (LDH) nanosheets stacked with graphene.24 Briefly, these reported electrocatalysts have been generally focused on nanostructuing or combining with carbon materials, which aims to increase the specific surface areas and improve the intrinsic electronic conductivity, thus affecting the electrocatalytic activity in alkaline water. Nontheless, new design strategies based on atomic or molecular level which could modulate the inherent catalytic activity should be further expanded. Recently, encouraging breakthroughs have revealed that oxygen vacancies can subtly tune the adsorption of OH intermediates and the electronic conductivity of spinel and perovskite oxides, which play a crucial role in both OER and HER in the basic medium.25-28 Furthermore, previous density-functional theory (DFT) calculations also prove that the unoccupied bonding t2g orbitals of MO6 center (MO6-x) could induce a high electron transfer conductivity and accelerate the adsorption reaction of OH- anions.8,29 Inspiring by this, we anticipate LDH based catalysts, abundant of edge-sharing octahedral MO6 layers, could also be ameliorated via generating oxygen vacancies. Therefore, we chose CoFe LDH nanosheets which are primarily not efficient enough for both OER and HER to explore bifunctional activity. Unexpectedly, the exfoliated


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CoFe LDH nanosheets rich in unsaturated metals are bifunctional electrocatalysts for overall water splitting. The resulting CoFe LDH nanosheets which are intercalated with formate ions (CoFe LDH-F) exhibit remarkable performance of both HER and OER, affording a current density (j) of 10 mA/cm2 at η of -166 mV for HER and 260 mV for OER, respectively. Moreover, the alkaline water electrolyzer based on CoFe LDH-F catalysts as both anode and cathode gives j of ~10 mA/cm2 and ~100 mA/cm2 at the applied voltage of 1.63 V and 1.74 V, respectively, with excellent stability for long-term electrolysis, which is superior to most reported systems employing nonprecious bifunctional electrocatalysts. 2. EXPERIMENTAL SECTION 2.1 Preparation of CoFe LDH Samples. To synthesize the CoFe LDH-C sample, CoCl2·6H2O, FeCl2·4H2O and HMT were dissolved in a flask, which consists of 100 mL mixed solution of ultrapure water (90 mL) and anhydrous ethanol (10 mL), with the final concentrations of 50, 25 and 450 mM, respectively. After that, the mixed homogenous solution was placed at the temperature of 90 oC under continuous stirring for 10 h. Resulting products were centrifuged at 6000 rpm for collection, and then washed with ultrapure water and anhydrous ethanol for several times. The final samples were dried at 25 oC. To fabricate the CoFe LDH-F sample, amounts of 100 mg of CoFe LDH-C samples were separately suspended in a flask, which consists of 100 mL mixture of DMF (50mL) /ethanol (50mL). The suspension was vigorously stirred under argon (Ar) flow for 24 h. Then the LDH-samples were isolated through centrifugation at 6000 rpm for 5 min, which were then washed with ultrapure water and anhydrous ethanol. The resulted products were dried at 25 oC.


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2.2 Electrochemical Tests. The electrochemical tests for HER and OER were conducted in an H type cell using the three-electrode system connected to an electrochemical station (CHI 660E), with Ag/AgCl electrode (3.5 M KCl solution) as the reference electrode, a graphite rod (3 mm in diameter, spectral purity) as the counter electrode, and that a glassy carbon electrode (GCE) or a Ni foam as the working electrode. To prepare the catalysts deposited on the GCE, 5 mg of different samples and 80 µL of Nafion solution were dispersed in 1 ml dispersions consisting of 4:1 v/v water/ethanol, and then sonicated for at least 30 min to obtain a homogeneous ink solution. After that, about 3 µL of the ink solution was carefully deposited onto the a GCE (3 mm in diameter). The final loading for all samples on the GCE is about 0.20 mg/cm2. For the Ni foam substrate, about 20 mg of different samples and about 200 µL of Nafion solution were dispersed in 4 ml dispersions (4:1 v/v water/ethanol), and then sonicated for at least 30 min to obtain an ink solution. After that, about 200 µL of the ink solution was loaded onto the Ni foam (with the area of 1*1 cm2). The loading for all samples and Pt/C or IrO2 catalysts on the Ni foam substrate is about 1.00 mg/cm2. Before OER linear sweep voltammetry (LSV) test, the catalysts were cycled 20 times using cyclic voltammetry (CV) to fully activate the catalyst at OER condition. Before HER LSV test, the catalysts were biased at η of -600 mV for 30 min to stabilize the HER performance. All the LSV curves were obtained with a scan rate of 5 mV/s, which was carried out in 1 M KOH (pH ≈ 13.6). Before all tests, the electrolyte was purged with Ar for about 30 min. Additionally, all the potentials referenced to reversible hydrogen electrode (RHE) were calculated through the following equation: E RHE = E Ag/AgCl + 0.059 × pH + 0.205. All the LSV curves of CoFe LDH-F and controlled samples were iR-compensated if not mentioned. AC impedance tests were implemented biased at a certain η from 105 Hz to 1 Hz,


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with the AC voltage of 5 mV. The stability test was conducted at a constant η of -400 mV for HER and of 300 mV for OER. The Mott-Schottky plots were conducted based on the capacitances derived from the electrochemical impedance, and the donor density can be calculated. The capacitances derived from the electrochemical impedance were obtained at each potential with 3000 Hz frequency. To determine the capacitance value for evaluating the ECSAs, catalysts deposited onto the GCEs were first activated by 20 cycles. The CVs were carried out from 0.2 to 0.3 V (vs. Ag/AgCl/3.5 M KCl). The double layer capacitance (Cdl) was obtained by plotting the ∆j = (ja jc) at 0.25 V vs. Ag/AgCl/3.5 M KCl against different scan rates (40, 80, 120, 160 and 200 mV/s). 3. Results and Discussion CoFe LDH samples were firstly synthesized through a modified aqueous hydrothermal method, which contain CO32- as intercalated anions (CoFe LDH-C). Considering the high affinity carbonates for LDHs, decarbonation is important to expand the interlayer space for exposing more edge-sharing octahedral MO6 layers as active sites in the catalysis field of LDH chemistry.30 However, traditional decarbonation and anion-exchange methods are always conducted in the strongly acidic environment. Using these methods, appreciable weight loss process occurs and large amount of metal salts would be consumed. Therefore, we treated the CoFe LDHs in the N, N-dimethylformamide (DMF)-ethanol mixed solvent system, which provided a weakly acidic environment because of the solvent hydrolysis to generate formates, for continuous decarbonation and delamination. When stirred under Ar atmosphere, the DMF molecules would penetrate into the gallery of CoFe LDH-C firstly, breaking the integrated


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hydrogen bonding network and producing a loosely stacked and highly swollen phase.31-33 At the same time, the hydrolysis generated formates would possibly insert into the gallery. We anticipate that some hydrogen-bonded hydroxyl group would be broken and become metastable and then run off to form unsaturated metal centers (defect-rich structures), during the process of breaking hydrogen bonding network. Moreover, in the process of inverse interlayer anions to obtain ultrathin nanosheets, the surface defects would lead to the formation of coordinatively unsaturated metal centers, which was also demonstrated in the previous report.34,35 The consequent anion-exchanged and exfoliated CoFe-LDHs have a larger interlayer space and ultrathin thickness, creating more unsaturated metals as active sites and meanwhile improving the intrinsic electronic conductivity, which contributes to the enhancement of both HER and OER activity for overall water splitting (Figure 1a). We then collected the CoFe LDH-C and CoFe LDH-F samples to analyse their structures. The field emission scanning electron microscopy (FE-SEM) images of these two samples show a sheet-like morphology (Figure S1). After anion-exchanged process, the morphology and diameter of LDHs nearly remained. Nevertheless, CoFe LDH-F exhibits a thinner thickness than CoFe LDH-C. In addition, the energy dispersed X-ray spectrometry (EDX) characterizations show the atomic ratio of Co:Fe nearly stay unchanged after anion exchange (Figure S2). Lowmagnification transmission electron microscope (TEM) images clearly exhibit that the nanosheets are several hundred nanometers in size, and the measured thicknesses of both nanosheets further confirm that the CoFe LDH-C nanosheets were successfully exfoliated (Figure 1b, 1c and Figure 1f, 1g). In both samples, the freestanding nanosheets were decorated with well-dispersed small fragments. The thicknesses of the synthesized LDH nanosheets were also evaluated by atomic force microscopy (AFM, Figure 1d and 1h). In Figure 1e and 1i, the


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height of CoFe LDH-F is approximately 4.5 nm, which is about 5~6 times the thickness of 0.8 nm for a single layer of LDH. Therefore, all the results above demonstrate the successful exfoliation process. The X-ray diffraction (XRD) patterns reveal both samples possess a highly (00n) preferred orientation of LDHs (Figure 2a), which was also verified by HRTEM image. The interlayer distance of (003) increases from 7.5 to 8.0 Å, suggesting the successful anion exchange from CoFe LDH-C to CoFe LDH-F. Meanwhile, the intensities of the (00n) peaks decreased, indicating the accompanying exfoliation process.36 Furthermore, Fourier-Transform Infrared (FTIR) spectra of these two samples also evidenced the intercalation of formate (Figure 2b). The appearance of a new band at 1584 cm-1 which can be ascribed to ʋas (COO-) mode reveals that the presence of HCOO- ion in CoFe LDH-F.37 The X-ray photoelectron spectroscopy (XPS) was used to study the chemical valence states of elements in the CoFe LDHs samples. CoFe LDH-F contains Co, Fe, C and O as the main components (Figure S3a), without any impurities. The Co 2p XPS spectra of both samples show the coexistence of Co3+ (779.1 eV) and Co2+ (781.0 eV) (with their shakeup satellites denoted as “sat”, Figure 2c).38 The spin-orbit splitting value of the Co 2p3/2 and Co 2p1/2 is over 15 eV, also revealing the existence of Co3+ and Co2+ composition.39 The relative atomic ratio of Co2+/Co3+ on the surface of the CoFe LDHs could be obtained, by comparing the area that fitted curve covered. The atomic ratio of Co2+/Co3+ (1.22) on the CoFe LDH-F surface is higher than that (1.02) on CoFe LDH-C, indicating that more Co2+ species than Co3+ present on CoFe LDH-F, that is, coordination-unsaturated MO6-x octahedrons were generated by exfoliation process.8,29 In the Fe 2p spectra, two main peaks at 711.2 and 724.7 eV are associated with the Fe 2p3/2 and 2p1/2 electronic configurations, respectively (Figure S3b).40 An weak satellite peaks of the Fe


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2p3/2 main line in CoFe LDH-C is observed at approximately 719.3 eV, indicating the main components of Fe3+ in CoFe LDH-C. For comparison, the satellite peaks centered at 717.2 and 719.3 eV indicate the coexistence of Fe2+ and Fe3+ species, which further confirms existence of low-coordinated metal-oxygen structure. However, for comparing the difference of XPS spectra of these two LDH samples for Co and Fe, we’d like to conclude that the oxygen vacancies mainly result in the unsaturated Co centers. In the O 1s spectra (Figure 2d), the peaks at 528.9 and 529.8 eV are attributed to typical metal-oxygen bonds and oxygen in hydroxyl groups, respectively.41 The third peak with a higher binding energy at 531.1 eV was assigned to a high defect sites with a low oxygen coordination in the sample of CoFe LDH-F.25 It is mentionable that these defect sites (oxygen vacancies) could decrease the barrier for the adsorption of OHanions because of the low-coordination sites of MO6 structures. Consequently, a much smaller HER and OER overpotential can be achieved.8,25-29 The electron spin resonance (ESR) spectroscopy could provide fingerprinting information about trapped electrons and surface vacancies. ESR data were recorded for CoFe LDH-C and CoFe LDH-F shown in Figure S4. All these two CoFe LDH samples exhibit an ESR signal at g ≈ 2.24, which is in agreement with ESR data previously reported for cobalt complex by Enise Ayyildiz.42 The signal intensity illustrates that the CoFe LDH-F possesses relatively higher defect concentration compared with CoFe LDH-C, which agrees well with the XPS results.. The electrochemical tests were estimated in 1 M KOH media purged with Ar for at least 30 min. All the electrocatalysts were electrochemical pre-treated before the evaluation (see details in Experimental Section) to get a stable state. The LSV curves in Figure 3a show j versus potential (vs. RHE) for commercial Pt/C, CoFe LDH-C and CoFe LDH-F, respectively. The sample of CoFe LDH-F produced j of 10 mA/cm2 at η of -255 mV (η10 = -255 mV). In contrast,


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CoFe LDH-C required η10 of -415 mV. To reach a large j of 100 mA/cm2, CoFe LDH-F only needed η of -352 mV, which is smaller than Pt/C (η100 = -381 mV). Moreover, the LSV curves of CoFe LDH-F and Pt/C before and after iR-correction were shown in Figure S5. The HER kinetics for all the samples were evaluated by the corresponding Tafel plots (Figure 3b). The Tafel slope of CoFe LDH-F is 95 mV/dec, which is smaller than that of Pt/C (166 mV/dec) and CoFe LDH-C (116 mV/dec), demonstrating CoFe LDH-F has a more efficient kinetics of HER. To evaluate the electron transport ability of CoFe LDHs, the electrical impedance spectroscopy (EIS; Figure 3c) was then conducted. When operated at η of -400 mV, the CoFe LDH-F sample showed a decreased transport resistance, which can accelerate the exchange of carriers in the nanosheets relative to CoFe LDH-C. To further evaluate the stability of CoFe LDH-F in the basic electrolyte, the catalysts decorated electrodes were biased at a static η to continuously generate hydrogen (Figure 3d). When operated at a constant η of -400 mV, the CoFe LDH-F sample could continuously generate molecular H2 for more than 10 h. At the same time, the activation process was also observed (Figure 3d, Figure S6 and S7). We attribute the significant HER performance improvement to the cathodization process, which creates more defect sites for the CoFe LDH-F sample. The XPS spectra of O 1s region for the sample before and after HER test were also characterized to verify the population of defect sites (Figure S8). Previously, the cathodization treatment has also been widely used to generate defect sites to modulate the catalytic activity.43,44 Additionally, the TEM images after HER stability test (Figure S9) reveal that the thicknesses of CoFe LDH-F somewhat become larger, further demonstrating that the defect sites play a more important role in the improvement of HER performance. In sharp contrast, the Pt/C sample showed a poor stability in the basic solution, with j declining from -70 mA/cm2 to -15 mA/cm2 in the first 100 s and gradually leveling at ~ -5 mA/cm2 for the consecutive electrolysis (Figure


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S10). The OER performance was also conducted in the same configuration. In Figure 3e, CoFe LDH-F exhibits an earlier onset of catalytic current than CoFe LDH-C. The η10 of CoFe LDH-F is about 300 mV, which is less than that of CoFe LDH-C (η10 = 345 mV) and IrO2 benchmarks (η10 = 315 mV). As shown in Figure 3f, the Tafel slope for CoFe LDH-F is only 40 mV/dec, which is smaller than that of CoFe LDH-C (47 mV/dec) and benchmark IrO2 (52 mV/dec), implying a more rapid OER rate for CoFe LDH-F. As shown in Figure 3g, the EIS spectra again demonstrate the improved charge transfer ability, which is critical for the surface catalytic reaction. It is mentionable that the decreased transport resistance of CoFe LDH-F in both OER and HER test could also elucidate the abundance of coordinatively unsaturated metals (Figure 3c and 3g). The stability of the CoFe LDH-F was estimated at η of 300 mV for more than 12 h, and nearly no sign of decay was observed (Figure 3h). For comparison, IrO2 benchmarks showed poor stability in the basic solution (j declined from 14 mA/cm2 to 3 mA/cm2 during electrolysis at η of 300 mV for 3h). The Mott-Schottky plots were carried out based on the capacitances derived from the electrochemical impedance (Figure S11), from which the donor density can be calculated. The donor density can be calculated according to the following equation (Equation 1): Nd = (2 / eoεεo)[d(1 / C2) / dV]-1

(1)

Where Nd is the dopant density, eo is the electron charge, ε is the dielectric constant of LDH material (with an ε value of 2.2 for CoFe LDH), εo is the permittivity of vacuum, and C is the capacitance derived from the electrochemical impedance obtained at each potential with 3000 Hz frequency. As the result, the Nd of CoFe LDH-F was calculated to be 2.83 × 1018, which is larger than that of CoFe LDH-C (7.91 × 1017), further verifying the more concentrated defects in CoFe LDH-F material.


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To compare the electrochemical surface areas (ECSAs) of the CoFe LDH catalysts, the Cdl of the CoFe LDH samples, which are linearly proportional to the ECSAs, were tested by using simple CV methods. As shown in Figure S12, the plots of ∆j = (ja - jc) at 0.25 V against different scan rates were recorded. Notably, from CoFe LDH-C to CoFe LDH-F, the slope, which is equivalent to the twice of Cdl, increased by nearly 6-fold, revealing that CoFe LDH-F has a larger active surface area. Overall, considering the excellent activity and stability for both HER and OER in the basic solution, CoFe LDH-F is a promising alternative for Pt and Ir based noble metals in alkaline water splitting. To obtain a highly efficient catalytic electrode for overall water splitting, we increased the conductivity and surface area of the substrate by loading the catalysts on the Ni foam, simultaneously increasing the mass loading of catalyst samples to 1 mg/cm2. In Figure 4, the absolute performances of each sample for HER and OER are significantly improved. As shown in Figure 4a and 4c, we can clearly see that CoFe LDH-F needs η of -166 mV and -255 mV to reach j of 10 mA/cm2 and 100 mA/cm2 for HER, and η of 260 mV and 310 mV to reach j of 10 mA/cm2 and 100 mA/cm2 for OER. The corresponding Tafel slopes of CoFe LDH-F are 92 mV/dec for HER and 47 mV/dec for OER, which are smaller than that of Pt/C for HER and IrO2 for OER (Figure 4b and 4d). The EIS spectra of CoFe LDH-F and controlled benchmark samples were shown in Figure S13. Compared with Pt/C for HER, the CoFe LDH-F sample showed the similar contact and charge transfer impedance. While for OER, CoFe LDH-F possessed a smaller charge transfer impedance, revealing its better charge transport ability. Based on these performances tested aforementioned, we anticipate that CoFe LDH-F could act as a bifunctional electrocatalyst for electrochemical overall water splitting. Thus, a two-electrode configuration was applied. When the CoFe LDH-F was used as both anode and cathode catalysts


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(CoFe LDH-F / CoFe LDH-F), j of 10 mA/cm2 could be observed at the applied potential of 1.63 V with a Tafel slope of 126 mV/dec, which is comparable for Pt/C / IrO2 and better than Pt/C / Pt/C, IrO2 / IrO2 and blank Ni foam / Ni foam (Figure 5a). Unfortunately, the Tafel slope of Pt/C / IrO2 is 241 mV/dec, which is much larger than CoFe LDH-F / CoFe LDH-F (Figure S14). Therefore, CoFe LDH-F surpasses the Pt/C / IrO2 at the applied potential of 1.63 V to reach a high j for catalyzing overall water splitting. A more detailed comparison of overall water splitting performance of CoFe LDH-F and other reported bifunctional nonprecious catalysts is included in Table S1 in the Supporting Information. Moreover, different potentials were also applied to drive overall water splitting (Movie S1). In addition, CoFe LDH-F / CoFe LDH-F can maintain the high activity at the applied potential of 1.63 V for more than 35 h for overall water splitting (Figure 5b). 4. Conclusion In conclusion, a bifunctional, non-noble-metal electrocatalyst based on CoFe LDHs have been successfully synthesized. This ultrathin CoFe LDH-F material is shown to efficiently catalyze both HER and OER. The significantly better catalytic activity of CoFe LDH-F compared with CoFe LDH-C was attributed to unsaturated metals as active sites and subsequently improved electronic conductivity. We have also verified that an efficient water electrolyzer achieving j of 10 mA/cm2 at the applied voltage of 1.63 V with excellent stability. These performance and costeffectiveness of CoFe LDH-F make it a promising material to replace noble-metal-based catalysts for large-scale water splitting.


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Figure 1. (a) Schematic representation of materials’ structures. After being vigorously stirred in the DMF-ethanol solution, the high-affinity CO32- interlayers of the CoFe LDHs have been dissolved and exchanged to HCOO- ions. The consequent exfoliated process exposes more oxygen vacancies as active sites and improves the electronic conductivity. Note: d1 and d2 are the interlayer distances, d2 > d1. TEM images of (b, c) CoFe LDH-C and (f, g) CoFe LDH-F. AFM images of (d) CoFe LDH-C and (h) CoFe LDH-F. Height profiles of (e) CoFe LDH-C and (i) CoFe LDH-F.


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Figure 2. (a) XRD patterns of CoFe LDH-C and CoFe LDH-F. The left shift of the diffraction peaks indicates the expansion of the interlayer space. (b) FT-IR spectra for CoFe LDH-C and CoFe LDH-F, suggesting the decarbonation process and introduction of HCOO- ions. XPS spectra for (c) Co 2p and (d) O 1s of the CoFe LDH-C and CoFe LDH-F samples, respectively.


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Figure 3. Bifunctional electrochemical performance of CoFe LDHs and controlled samples on the GCE substrates with three electrode system in 1 M KOH aqueous electrolyte. (a) The polarization curves and (b) corresponding Tafel slopes for HER. (c) EIS spectra of CoFe LDHs at η of -400 mV. (d) Chronoamperometric electrolysis of CoFe LDH-F and Pt/C for HER at η of -400 mV (not iR-corrected). (e) The polarization curves and (f) corresponding Tafel slopes for OER. (g) EIS spectra of CoFe LDHs at η of 350 mV (h) Chronoamperometric electrolysis of CoFe LDH-F and IrO2 for OER at η of 300 mV (not iR-corrected).


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Figure 4. The bifunctional electrochemical behavior of the CoFe LDH-F and controlled samples on the Ni foam substrates. The polarization curves for (a) HER and (c) OER. The corresponding Tafel slopes for (b) HER and (d) OER.


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Figure 5. (a) Polarization curves for overall water splitting at the scan rate of 5 mV/s. (b) Chronoamperometric measurement of the overall water splitting at the applied potential of 1.63 V. Inset of (b): photograph of the system showing the hydrogen (left) and oxygen (right) generation during water electrolysis.


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ASSOCIATED CONTENT Supporting Information. Figures show SEM images, EDX data, XPS spectra and electrochemical data; table for comparison of bifunctional water splitting activity. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ACKNOWLEDGMENT This work was financially supported by National Natural Science Foundation of China (21573068, 21503079), SRF for ROCS, SEM, SRFDP, Program of Shanghai Subject Chief Scientist (15XD1501300), Shanghai Municipal Natural Science Foundation (14ZR1410200), Fundamental Research Funds for the Central Universities (WD1313009) and 111 Project (B14018).


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Table of Contents Graphic


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