Uniquely Monodispersing NiFe Alloyed Nanoparticles in Three

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Uniquely Mono-dispersing NiFe Alloyed Nanoparticles in Three-dimensional Strongly Linked Sandwiched Graphitized Carbon Sheets for High-efficiency Oxygen Evolution Reaction Yangyang Feng, Huijuan Zhang, Ling Fang, Yanping Mu, and Yu Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00481 • Publication Date (Web): 02 Jun 2016 Downloaded from http://pubs.acs.org on June 4, 2016

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Uniquely Mono-dispersing NiFe Alloyed Nanoparticles in Three-dimensional Strongly Linked Sandwiched Graphitized Carbon Sheets for High-efficiency Oxygen Evolution Reaction Yangyang Feng, Huijuan Zhang, Ling Fang, Yanping Mu, Yu Wang* The State Key Laboratory of Mechanical Transmissions and the School of Chemistry and Chemical Engineering, Chongqing University, 174 Shazheng Street, Shapingba District, Chongqing City, P.R. China, 400044 E-mail: [email protected]; [email protected] Abstract Oxygen evolution reaction (OER) is known as a significant role in the renewable energy. Herein, we report a low-cost, highly active and superbly durable 3D sandwiched NiFe/C arrays grown on Ni foam by a general procedure. This special structure with both graphitized carbon and Ni foam possesses huge specific surface area, high electroconductivity and porous structure, effectively enhancing electrocatalytic activities for OER. Furthermore, the sandwiched structure with coupled graphitized carbon sheets encapsulating outside can hinder active materials from agglomeration and falling off during long-term operating, leading to outstanding durability, even among large temperature range. Key words: OER, Sandwich-like, Three-dimension, NiFe, Graphitized carbon Introduction With energy demand gradually growing, cumulative concerns about sustainable energy sources are stimulating great research attention.1,2 The challenges we meet in global energy demand mainly focus on exploiting abundant and renewable energy sources.3 Among the innovative solutions, the production and storage of hydrogen (H2) seem appealing and promising due to hydrogen’s high mass-specific energy density.4,5 There are several ways to produce H2, like fossil fuels and water splitting. When considered about purity and cost, the most effective method is water splitting by electricity or sunlight.6-8 To our knowledge, the water electrolysis to generate oxygen is a crucial half-reaction existing in water splitting.9 However, oxygen evolution reaction (OER) encounters kinetically sluggish process during electron transfers, which can largely increase the overpotential so as to impede the overall efficiency of the reaction.10-12 In the last decades, ruthenium dioxide (RuO2) and iridium dioxide (IrO2) are regarded as the state-of-art OER catalysts owing to their low Tafel slope and overpotential, particularly in basic solutions.13,14 However, they subject to mass production owing to the scarcity and ultrahigh cost.15 Currently, extensive endeavors have been taken to discover earth abundant, low-cost and highly active materials, mainly focus on non-noble transition-metal materials.4 Among the non-noble transition-metal OER electrocatalysts, nickel (Ni) is regarded as a most promising candidate in electro-catalytic as it is the earth-abundant first row transitional metal with low overpotential and high stability.16 In the previous researches, most of the Ni-based catalysts are combined with other transitional metal, such as Fe, Co and Mn, which seems to effectively enhance the OER activities on account of the detrimental effects in alkaline conditions.17-20 The effect of Fe on Ni-based composites discovered firstly by Edison accelerates more and further studies on NiFe-based catalysts. Recently, numerous advanced NiFe-based electrocatalysts have been designed, such as

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nanorods,21 nanowires22, nanoparticles23, nanotubes24, dendritic 25 and flower-like structures 26. Nevertheless, the biggest problems existing in these composites are the low electrical conductivity, small specific surface area and poor stability when operated in basic solutions, which severely limits the practical applications. The most valid solution is to fabricate NiFe architectures on conductive substrates, graphene27, carbon fibers28 and Ni foam29 for instance. To our knowledge, carbon-related catalysts can remarkably improve the conductivity and surface area, leading to enhanced OER activities.30,31 To date, most of carbon-conjugated electrodes are directly depositing active materials onto the surface of the carbon, which may lead the samples easily peel off during electrochemical reactions.32,33 Among the various carbon-combined morphologies, sandwiched structure is considered as the promising and significant electrocatalyst when applied in OER reactions. This novel morphology with huge specific surface area and high electrical conductivity can offer larger active surface area as well as guarantee sufficient touch between electrolyte and electrode. Furthermore, it is efficient to prevent the active materials from peeling off and aggregation during long-time operations. In other aspect, growing active materials on Ni foam or other conductive substrates indeed improve the activities which possibly results from the increased electroactive surface area by constructing 3D architecture.34-36 Therefore, combining the conductive substrates can further enhance the OER activities due to the synergistic effect. Herein, we devise and fabricate 3D sandwiched NiFe/C arrays grown on Ni foam by a general method. Firstly, Ni(OH)2 nanosheets were grown on Ni foam by hydrothermal method. Using an ion-diffusion-exchange precess, we successfully synthesized Fe-doped Ni(OH)2 ((Fe)Ni(OH)2) nanosheets arrays. After the glucose was strictly covering (Fe)Ni(OH)2 by hydrothermal process, sandwiched NiFe/C arrays on Ni foam are prepared through high-temperature calcinations. This unique 3D structure combines the advantages of both carbon and Ni foam. On the one hand, the sandwiched structure can not only offer large active sites and guarantee the effective contact because of the high electroconductivity and specific surface area, but also keep the active materials from agglomeration and falling out during long term electrochemical process due to coupled graphitized carbon sheets encapsulating active electrode. Moreover, the nanopores on the outside carbon can efficiently accelerate OH- and electron fast transfer, leading to high catalytic activities. On the other hand, growing the active materials on Ni foam can further increase the conductivity and prevent the active materials from agglomeration because of the well-ordered grown on Ni foam. Therefore, it is no wonder that our novel sandwiched NiFe/C arrays immensely enhance the OER catalytic activities. Result and Discussion The whole processes are clearly illustrated in the Scheme 1. As described, Ni(OH)2 nanosheets are prepared on Ni foam under general and simple solvothermal conditions. Afterwards, (Fe) Ni(OH)2 nanosheet arrays is achieved via an ion-exchange method because of their near atom radius.37,38 Subsequently, the obtained (Fe) Ni(OH)2 are coated with glucose polymer via hydrothermal method, and finally changed into sandwiched NiFe/C arrays after high-temperature calcinations. The as-fabricated 3D sandwiched NiFe/C arrays on Ni foam can effectively improve the catalytic activities in OER operations. Fig. 1 describes the general characterizations of Ni(OH)2 nanosheet arrays which can be easily gotten in large quantities (Fig. 1a) by a common hydrothermal method through low-magnification scanning electron microscopy (SEM). As to the Fig. 1b, numerous sheet-like precursors are


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distinctly observed, which is the necessary condition of the commercial application. In the section view of the precursor (Fig. 1c), the samples with the average size of ~2 micrometers can be successfully prepared, indicating a large loading of Ni(OH)2 nanosheets. From Fig. 1d, we clearly saw the precursor with the thickness of ~20 nm, revealing the ultrathin property of the precursor. After the ion diffusion-exchange process between Ni2+ and Fe2+ in 1 M ferrous chloride solution, (Fe) Ni(OH)2 is easily fabricated. The interrelated characterizations are described in Fig. S1. SEM image of (Fe) Ni(OH)2 displayed in Fig. S1a reveals that doping Fe can scarcely effect the sheet-like morphology, but indeed influence the color as presented in Fig. S1b. The XRD patterns in Fig. S2 refers to the crystal structures before and after Fe doping, in which we can see that all the peaks are virtually the same due to a relative small amount of Fe doping. To verify that Fe is indeed doped into the precursor, EDS Mapping is conducted. As Fig. S3 shows, element Fe is evenly distributed in the precursor, further suggesting that we have successfully achieved Fe-doped Ni(OH)2 through a low-energy consumption process. Under the suitable conditions through high-temperature calcinations, sandwiched NiFe/C arrays are fabricated in large production. As detected in Fig. 2a, a large number of sheet-like products on Ni foam are prepared, well inherited by (Fe) Ni(OH)2. From the amplified SEM image in fig. 1b, we can clearly see the sandwiched structure with numerous monodisperse NiFe nanoparticles strictly encapsulated inside and graphitized carbon layered outside. The enlarged SEM image (inset, Fig. 2b) further reveals the uniform nanoparticles tightly dotted inside even through the low scanning electron beam accelerated by 5 kV, which indicates that the carbon sheets are ultrathin. To our knowledge, graphitized carbon is considered as a promising electrocatalyst support due to its strong mechanical strength, excellent electronic and thermal conductivity as well as huge surface area.39-41 Moreover, the carbon layers wrapping outside can effectively stablize the sandwiched structure by avoiding the nanoparticles from agglomeration and dropping out during electrochemical operations, leading to great durability.42 After high-temperature annealing, porous structure can be undoubtedly obtained owing to a large amount of substances (H2O, CO, CO2) loss. To estimate the porous structure, nitrogen adsorption and desorption isotherms (BET) are provided. The specific surface area is ~ 266.025 m2/g (Fig. 2c), large enough to provide huge contact area and offer more active sites for electrocatalysis. In the inset of Fig. 2c of the Barrete−Joyner−Halenda (BJH) analysis, the pore size, varying from 1.8 to 4.5 nm, is mainly concentrated on 1.9 nm. To our knowledge, the porous structure can not only remit the volume expansion during continuous redox reactions, but also promote electron and OH- fast diffusion, leading to high OER activities. XRD is pretty important to evaluate the crystal structure. In order to accurately test the XRD, we also grow the composite on flat Ni plate and peeled the samples off from Ni plate. As shown in Fig. S4a, all peaks ranging from 5-90 degrees pointing to Ni (JCPDS Card No.87-0712), implying that a little Fe-doping scarcely influences the lattice parameter. Here, a weak peak at around 26.6o is observed, which refers to graphitized carbon (JCPDS Card No. 75-2078), suggesting that graphitized carbon is indeed obtained by glucose through suitable conditions. Corresponding EDS analysis (Fig. S4b) displays the coexistence of Ni and Fe with the molar ratio of 3：1. To discover the valence states of the major elements in sandwiched NiFe/C, X-ray photoelectron spectroscopy (XPS) is provided. In Fig. S5, both of the magnified Ni 2p and Fe 2p peaks are presented. As observed, the binding energy with 855.1 and 870.0 eV in Ni 2p peaks refer to the Ni2+ and Ni, respectively. For Fe 2p peaks, two peaks of 707.3 and 720.2 eV stand for Fe3+ and Fe, separatively.43,44 It is indicated that Ni2+ and Ni as well as Fe3+ and Fe are


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coexisted in the surface of the composite as part of NiFe is oxidized. Transmission electron microscopy (TEM) is pretty vital for more detailed information. In the low-resolution TEM image of Fig. 2d, numerous nanoparticles are strictly anchored in the coupled carbon sheets. In order to affirm the sandwich-like morphology, TEM image of the cross-section is presented in Fig. S6. It observes 2-4 graphitized carbon layers on both top and bottom sides, with an interspace of ~5 nm, corresponding to the diameter of NiFe nanoparticles. It is indicated that NiFe nanoparticles are encapsulated in graphitized carbon sheets. Otherwise, carbon layers would not separate such a large gap if NiFe nanoparticles are deposited on the surface of graphitized carbon layers. By enlarged TEM image in Fig. 2e, the uniform nanoparticles with the size of 5 nm are apparently detected. Additionally, the interval space between each nanoparticles ranges from 2 to 20 nm, which efficiently remits volumetric expansion and protect active materials from agglomeration during electrochemical operations. From Fig. 2f, several graphitized carbon layers can be seen from the margin of the composite. To test the degree of graphitization, Raman spectroscopy (RS) is provided (Fig. S7). As observed, the intensity of G band (1600 cm-1, refers to graphitized carbon) is almost the same as D band (1360 cm-1, indicates amorphous carbon), indicating the well-graphitized carbon wrapping around NiFe nanoparticles.45,46 Carbon is pretty vital to improve the electroconductivity as well as anchor the active materials. Thus, it is essential to test the carbon content. As calculated, the carbon content in the composite (exclusive of the mass of Ni foam) is 6.1%, which hardly lowers the OER activities, but enhances the catalytic performance due to the excellent conductivity of the graphitized carbon. Fig. 2g describes the crystalline structure of NiFe nanoparticles, which shows the lattice space of 0.124 nm and 0.125 nm, in correspondence to the (220) and (02 2 ) plane. To further investigate the elemental distribution, elemental imaging of the selected area (Fig. 3) describes the even-distributed of Ni and Fe in each nanoparticles, as well as carbon uniformly wrapping around NiFe nanoparticles. NiFe-based compound is popular for its high electrocatalytic activities in OER. In order to evaluate the OER activities of sandwiched NiFe/C arrays, many related tests are carried out through a general three-electrode setup in alkaline solutions. Fig. 4 reveals the electocatalytic OER activity of our composite in different KOH solutions of 0.1 and 1 M. The polarization curves tested at low scan rate of 5 mV/s at different concentrations are displayed in Fig. 4a. As observed, the sandwiched composite demonstrates a sharp onset of ~1.47 V (vs. RHE, in correspondence with the overpotential of ~240 mV) in 0.1 M KOH, beyond which the current increases rapidly while the potential rises. In 1 M KOH, the electrocatalyst exhibits a lower onset potential of ~1.43 V, lower than other NiFe-based catalysts (Table S1)28,29,47-51 Furthermore, among the most of the state of art electrocatalysts, the sandwiched NiFe/C arrays exhibit excellent OER activities (Table S2)27,52-56. The superior performances are mostly ascribed to the strong association among NiFe nanoparticles, coupled graphitized carbon and Ni foam, which can accelerate charge transport and enhance catalyst activities. Tafel plots is an important measure to estimate the OER acivities. We fit the polarization curves through the Tafel equation (η= b log (j) + a, where b is the Tafel slope and j is the current density) at various pH values. Fig. 4b presents small Tafel slope of ~ 30 and 36 mV/dec in 1 M and 0.1 M KOH, which is much smaller than the reported Ir-based electrode.57 Noteworthily, the Tafel slope is also lower than most of other NiFe-based catalytic electrodes,28,47,49 further implying the structural superiorities. Long-time durability to OER is pretty critical for electrocatalysts. Therefore, the 4 days chronopotentiometry test has been conducted at various current densities of 20, 40, 60 mA/cm2.


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As displayed in Fig.4c, our sandwich-like composite shows almost unchanged operating potentials of 1.454, 1.484 and 1.505 V separately at the constant current densities of 20, 40, 60 mA/cm2 (less than 2%). The excellent durability demonstrates that our novel sandwiched arrays can undoubtedly enhance the stability in basic solutions owing to the structural advantages of protecting active materials from agglomeration and dropping out. To further measure its pH dependence, we also test its stability in 0.1 M KOH at a steady current density of 20 mA/cm2. As expected, our sandwiched NiFe/C arrays on Ni foam exhibits remarkable durability for 4 days with less than 2% increase in potential (Fig. 4d), indicating that our sandwiched structure can endure different pH values even for long-term operating. Solution resistance is one of the critical factors to estimate the OER activity. Therefore, the electrochemical impedance of sandwiched composite in 0.1 M and 1 M KOH is measured as shown in Fig. S8. As observed, the sandwiched electrode in 1 M KOH exhibits lower charge transfer resistance (Rct) (1.6 Ω vs 4.8 Ω), indicating the higher electrical conductivity in 1 M KOH, which is beneficial for better catalytic activities. To further disclose the durability of our sandwiched composite under continuous potential scanning conditions. Fig. 4e uncovers the well stability at 5 mV/s for 3000 cycles with unmeasurable loss. Furthermore, at high rate of 50 mV/s, our sandwiched catalyst also reveals a good durability with ignorable current loss even after 3000 cycles (Fig. 4f). These results indicate that our sandwiched NiFe/C arrays on Ni foam are desirable for stable OER catalyst in basic environments. To stand out the novel sandwiched structure, a sequence of contrast experiments are conducted. We compare our sandwiched NiFe/C arrays with NiFe nanoparticles and (Fe)Ni(OH)2 nanosheets. Fig. 5a and 5b are the low-resolution SEM images of (Fe)Ni(OH)2 and NiFe nanoparticles, respectively. Fig. 5c displays their polarization curves at 5 mV/s. It is demonstrated that our sandwiched catalyst exhibits the lowest onset potential of 1.43 V (vs RHE), whereas the (Fe)Ni(OH)2 and NiFe nanoparticles show the relative high onset potential of 1.48 and 1.57 V, separatively. Tafel slope refers to the effect of overpotential on steady-state current density, thus it is crucial to evaluate OER kinetics.58 Fig. 5d presents the Tafel plots stemmed from the corresponding polarization curves in Fig. 5c. As described, our sandwich-like composite demonstrates a lower Tafel slope of 30 mV/dec, while (Fe)Ni(OH)2 and NiFe nanoparticles display Tafel slope of 63 and 87 mV/dec. These high OER activities are attributed to the positive effect of our sandwiched arrays. In the subsequent tests, we further investigate the durability of (Fe) Ni(OH)2 and NiFe nanoparticles for comparison. From Fig. S9a, we can detect that (Fe)Ni(OH)2 disclose a phanic increase for 48 h in operating potential (~8%). The NiFe nanoparticles shows a dramatic rise (16%) in potential for 24 h. All the results mentioned above confirm that our unique sandwiched arrays can absolutely enhance the OER activities and stability because of the high electroconductivity and large specific surface area. As for electroconductivity, the electrochemical impedance spectrum (EIS) is provided. As observed in Fig. S9b, the sandwiched NiFe/C arrays on Ni foam possess the lowest Rct (~1.6 Ω) among the three NiFe-based composites (5.2 and 9.1 Ω for (Fe)Ni(OH)2 and NiFe nanoparticles, respectively). These results are attributed to our novel morphology. On one hand, the unique 3D architecture can offer huge specific surface area so as to facilitate active materials fully attaching to electrolyte. On the other hand, both the graphitized carbon and Ni foam can enhance the conductivity so as to improve the OER activities. Furthermore, the special sandwiched morphology, with carbon sheets coating outside and active materials encapsulated inside, can efficient avoid catalyst from


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agglomeration, pulverization and dropping out so that it can keep the structural stability, leading to well durability in alkaline conditions. To further affirm the sandwiched structural advantages, the active surface area is also tested by measuring the double layer capacitance (Cdl) through cyclic voltammetry. Fig. 6a and 6c show the CV plots of the sandwiched NiFe/C and NiFe nanoparticles at the scan rates from 10 to 100 mV/s. The electrochemically active surface area is calculated by the slop through the linear relationship of the scan rate and current density (Fig. 6b and 6d). It is observed that the Cdl of the sandwiched NiFe/C arrays is ~25 mF/cm2, while the Cdl of the NiFe nanoparticles is ~8 mF/cm2. It is indicated that our sandwiched composite possesses larger electrochemically active surface area, which significantly verifies the excellent catalytic activity of the novel structure. In order to confirm the structural integrity of our sandwiched arrays, we revisit the SEM images among sandwiched NiFe/C arrays, (Fe)Ni(OH)2 and NiFe nanoparticles (Fig. S10) after OER operation. It reveals that the sandwiched NiFe/C can be well-maintained even cycling for long time. But (Fe)Ni(OH)2 and NiFe nanoparticles are seriously aggregated. It is no doubt that our sandwiched catalyst demonstrates outstanding durability. Additionally, we also conduct a train of measurements at different temperatures from 0 to 25, 50 oC. Fig. 7a shows the polarization data of our sandwiched composite measured in 1 M KOH at various temperatures. It is observed that our sandwiched catalyst exhibits slightly increase from 1.42 V, 1.43 to 1.47 V as the temperature varies from 50, 25 to 0 oC, separatively. It is indicated that our sandwiched composite can endure a large temperature range and exhibits high OER activities. The durability is also measured in Fig. 7b. As observed, there is negligible potential increase at the invariable current density of 20 mA/cm2 at 25 and 50 oC for 48 h (~1%). When tested at 0 oC, it shows a slightly increase (~2%) for 48 h, implying the excellent stability of our sandwiched structure even during a large temperature range. According to the results and discussions presented above, the novel sandwiched NiFe/C arrays are more suitable for OER operation and may realize the commercial applications. Conclusion The novel 3D sandwiched NiFe/C arrays on Ni foam are synthesized via a common wet-chemical method followed by high-temperature calcinations. The special 3D structure has plentiful outstanding properties, such as large specific surface area to offer larger contact area between active materials and electrolyte, high electroconductivity to accelerate fast electron and ion diffusion and porous structure to facilitate electron transfer. Based on the structural superiorities, our sandwiched NiFe/C arrays on Ni foam are regarded as the promising catalytic electrode. When applied in OER operations, it exhibits low onset potential of 1.43 V (η=200 mV) and excellent durability in alkaline solutions (less than 2 % for 4 days). Importantly, the sandwiched catalyst displays superb stability during large temperature range. It is no doubt that our 3D sandwiched NiFe/C will provide a new idea to develop renewable energy sources.


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Experimental Materials: The materials and chemicals in this experiment were used directly without any purification: nickel nitrate (Ni(NO3)2, Aldrich, 99.9%), ammonium fluoride (NH4F, Aldrich, 99.9%), urea (Aldrich, 99.9%), nickel foam (Alfa Aesar) and glucose (Cica-Reagent). Synthesis of Ni(OH)2 nanosheets on Ni foam: Firstly, Ni foam was cut into 2 cm * 3 cm and cleaned by ~37 wt% hydrochloric solution in order to get rid of the NiO layer. In this process, Ni(NO3)2 1 mmol were added in 30 mL deionized water to develop a uniform solution. Additionally, 0.08 g NH4F and 0.30 g urea were than added. Afterwards, the mixture was diverted in an autoclave of 45 mL. Then put autoclave in the electric oven of 150 oC for 1 h. After naturally cooling down in air, the Ni foam was got out and cleaned several times and dried at 60 oC. Synthesis of Fe-doped Ni(OH)2 nanosheets on Ni foam: Firstly, the as-prepared Ni(OH)2 nanosheet arrays were added in 1 M FeCl2 aqueous solution (30 mL). After standing stirring for 3 h at 25 oC, the color of Ni(OH)2 nanosheets became yellow. At last, the composite on Ni foam were cleaned and dried in electric dry oven at 60 oC. Synthesis of sandwiched NiFe/C arrays on Ni foam: One piece of Ni foam with Fe-doped Ni(OH)2 nanosheet arrays included was added into 5 mL glucose solution and 25 mL D.I. water. The mixed solution and Ni foam with samples were added in autoclave (50 mL) and heated in the oven (180 oC, 4 h). Afterwards, the Ni foam was washed and dried at 60 oC. Then put Ni foam in the tube furnace and anneal it under Ar atmosphere at 700 oC for 200 min. Synthesis of Fe-doped Ni(OH)2 nanosheets and NiFe nanoparticles: Firstly, to prepare Ni(OH)2 nanosheets, 10 mL ethylene glycol, 12 mL NH3•H2O, 4mL Na2CO3 (1M), and 4 mL Ni(NO3)2 (1M) were added one by one. And then the mixture was stirred for 10 min. Afterwards, the mixture was passed on to autoclave of 45 mL. Ni(OH)2 nanosheets can be obtained after reacted at 170 oC for 16 h in electric oven. Subsequently, to synthesize Fe doped Ni(OH)2 nanosheets, 20 mL 1M FeCl2 aqueous solution was mixed with 100 mg synthesized Ni(OH)2 nanosheets under ultrasound to obtain a uniform solution. Then, the solution stands for 3 h at 25 o C and the color was gradually changed into yellow. In this way, (Fe)Ni(OH)2 nanosheets are fabricated. Monodispersed NiFe nanoparticles are prepared from (Fe)Ni(OH)2 nanosheets through high-temperature calcinations under H2 atmosphere. Carbon content of sandwiched NiFe/C arrays on Ni foam: In order to evaluate the electrochemical properties, we also fabricated the sandwiched arrays on the flat Ni sheet so as to easily scrape the composite off by tweezers. The NiFe/C (0.2 g) was added in 10 M HNO3 solution. After vigorous stirring, the mix solution was stood for ~2 days. After the active materials were dissolved, the samples were washed. Then the sample was dried for 1 day in order to get rid of the residual ethanol and water. Through calculation using the formula, the carbon content of the composite was as follows: C%=M(C)/M(NiFe/C)×100% Where M(C) and M (NiFe/C) were separatively the mass of graphitized carbon and sandwiched NiFe/C arrays. Material characterization: The as-prepared materials are characterized through many instruments: Scanning electron microscope (SEM, JSM-7800F, 5kV); Energy dispersive spectrometer (EDS) analyzer; X-ray diffractometer (XRD, Bruker D8 Advance, Cu Kα); Surface-area and pore-size analyzer (BET, Quantachrome Autosorb-6B); Transmission electron


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microscope (TEM, Philips, Tecnai, F30), Raman Microscope (voltage 100-240V, Power 150W). Electrochemical testing: OER was tested through a three-electrode setup by electrochemical workstation (CHI660E). In the three-electrode setup, the working electrode is NiFe/C on Ni foam, while the reference electrode and counter electrode are saturated calomel electrode (SCE) and Pt foil, separatively. The mass loading of NiFe/C is 1.0-1.3 mg/cm2. Linear sweep voltammetry (LSV) was tested under the potential from 0 to 0.7 V in basic solution (1.0 M and 0.1 M KOH) at 5 and 50 mV/s. EIS measurements were performed in potentiostatic state from 105-0.1Hz. The potentials were present by the reversible hydrogen electrode (RHE) (E(RHE) = E(SCE) + 0.241 V + 0.059 pH). The overpotential (η) is calculated by the equation: η = E(RHE) – 1.23 V. Supporting Information More SEM, TEM, XRD, EDS and electrochemical data are available in the supporting information. Acknowledgements Financial support provided by Thousand Young Talents Program of the Chinese Central Government (Grant No.0220002102003), the National Natural Science Foundation of China (NSFC, Grant No. 21373280 ， 21403019), Fundamental Research Funds for the Central Universities (0301005202017), Hundred Talents Program at Chongqing University (Grant No. 0903005203205) and Beijing National Laboratory for Molecular Sciences (BNLMS).


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Figures and Captions

Scheme 1. Schematic illustration to introduce the whole fabrication route from Ni(OH)2 nanosheets on Ni foam to Fe-doped Ni(OH)2 nanosheets on Ni foam and sandwiched NiFe/C on Ni foam.


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ACS Catalysis

Figure 1. General characterizations of Ni(OH)2 nanosheets grown on Ni foam. a), b), c) and d) are the SEM images at different resolutions.


ACS Catalysis

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Figure 2. The typical characterizations of 3D sandwiched NiFe/C arrays. a) and b) are the SEM images in different resolutions. c) BET isotherm. d), e) and f) are the TEM images at various resolutions. g) HRTEM image.


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ACS Catalysis

Figure 3. Elemental images via EDS of 3D sandwiched NiFe/C arrays. a) TEM image, b), c) and d) are the distribution of C, Ni and Fe, respectively.


ACS Catalysis

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Figure 4. Electrochemical performances of 3D sandwiched NiFe/C catalyst. a) Polarization curves at a scan rate of 5 mV/s in 1 M and 0.1 M KOH. b) Tafel curves obtained from the polarization curves. c) Chronopotentiometry curves at different current density of 20, 40, 60 mA/cm2 in 1 M KOH. d) Chronopotentiometry curves at 20 mA/cm2 in 0.1 M KOH. e) and f) are the durability tests for 3000 cycles in 1 M KOH at the rate of 5 mV/s and 50 mV/s, respectively.


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ACS Catalysis

Figure 5. SEM images of a) Fe-doped Ni(OH)2 nanosheets and b) NiFe nanoparticles. c) polarization curves and d) Tafel curves of 3D sandwiched NiFe/C arrays, Fe-doped Ni(OH)2 nanosheets and NiFe nanoparticles for comparison.


ACS Catalysis

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Figure 6 CV curves of a) sandwiched NiFe/C arrays and b) NiFe nanoparticles tested at various scan rates from 10-100 mV/s. Scan rate dependence of the current densities of c) sandwiched NiFe/C arrays and d) NiFe nanoparticles at 1.0 V vs RHE.


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ACS Catalysis

Figure 7. Temperature effect of 3D sandwiched NiFe/C arrays. a) Polarization curves tested at 0, 25 and 50 oC at a scan rate of 5 mV/s in 1 M KOH. b) Durability test at 0, 25 and 50 oC at a constant current density of 20 mA/cm2.


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Table of Content

We report novel and unique 3D sandwiched NiFe/C arrays on Ni foam, which exhibit high-efficiency catalytic activities in OER.


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Uniquely Monodispersing NiFe Alloyed Nanoparticles in Three

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