Nanocarbon Hybrids as Advanced

Dec 10, 2015 - ... based materials is a vital step toward a future hydrogen economy. .... diffraction peak of Ni0.9Fe0.1/NC indicated an average grain...
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Facile Synthesis of Nickel−Iron/Nanocarbon Hybrids as Advanced Electrocatalysts for Efficient Water Splitting Xing Zhang, Haomin Xu, Xiaoxiao Li, Yanyan Li, Tingbin Yang, and Yongye Liang* Department of Materials Science and Engineering, South University of Science and Technology of China, Shenzhen 518055, People’s Republic of China S Supporting Information *

ABSTRACT: Developing active, stable, and low-cost electrocatalysts which can promote the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in the same electrolyte is undoubtedly a vital progress toward a hydrogen economy. Herein, we report that such electrocatalysts can be easily prepared by pyrolyzing a precursor composed of nickel and iron salts with urea under inert atmospheres without any post-treatments. The obtained products are composed of metallic nickel− iron alloy nanoparticles either encapsulated in or dispersed on nitrogendoped bamboo-like carbon nanotubes (CNTs). This simple synthesis route could simultaneously realize nanostructuring, doping, and hybridizing with nanocarbon, which have been demonstrated as efficient strategies to optimize the catalytic activity of an electrocatalyst. The in situ formed hybrid catalysts exhibit good catalytic performances for both OER and HER under alkaline conditions, and the doping content of iron significantly affects the activities. When the best electrocatalyst is loaded on nickel foam with a loading of 2 mg cm−2, a symmetric two-electrode cell can execute overall water splitting at a current density of 10 mA cm−2 with only 1.58 V and shows negligible degradation after 24 h of operation. The excellent electrocatalytic activity and facile preparation method enable this hybrid electrocatalyst to be a promising candidate for future large-scale applications in water splitting. KEYWORDS: nickel−iron alloy nanoparticles, carbon nanotubes, hybrid electrocatalyst, oxygen evolution reaction, hydrogen evolution reaction

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(CNTs) which exhibited excellent electrocatalytic HER performance slightly inferior to Pt/C in acidic solutions.24 More notably, Chen et al. recently reported that a hierarchical nanoporous copper−titanium bimetallic electrocatalyst was able to produce hydrogen from water at more than twice the rate of the state-of-the-art Pt/C catalyst.25 As for the kinetically sluggish OER process, Dai et al. reported an electrocatalyst which could promote the OER process better than a commercial Ir/C catalyst through hybridizing ultrathin nickel−iron layered double hydroxide nanoplates with mildly oxidized multiwalled CNTs.18 The development of bifunctional electrocatalysts with high activities toward both the OER and HER in the same electrolyte could be a promising way to lower the cost of water-splitting devices, as it may simplify the requirement for diverse equipment and processes.2 Fortunately, there have been some exciting cases which have been demonstrated to reduce the overpotential in water-splitting devices with a bifunctional electrocatalyst.2,3,22,23 For example, Luo et al. reported that NiFe layered double hydroxide nanosheets deposited on nickel foam could achieve 10 mA

olecular hydrogen (H2) is extensively regarded as one of the most ideal energy vectors due to its zero-carbon emission, recyclability, and high energy conversion efficiency.1 Water splitting by electrolysis is an environmentally friendly way to generate H2.2,3 Especially, when it is coupled to a photovoltaic modulus, it will be a sustainable and promising energy system for future societies.4−8 Currently, the state-ofthe-art electrocatalysts for the HER and OER are Pt- and Irbased and Ru-based materials, respectively.9 However, the scarcity and high cost of these noble metal-based electrocatalysts limit their large-scale application. Therefore, developing active, stable, and low-cost electrocatalysts in place of precious metal based materials is a vital step toward a future hydrogen economy. Recently, transition metals (Mo, W, Ni, Co, Fe, Mn, Cu, etc.) and their derivatives (carbide, oxide, sulfide, phosphide, hydroxide, mixed-metal alloys, etc.) have been extensively investigated as either HER or OER and even bifunctional electrocatalysts.10−27 Excitingly, some of them exhibit electrocatalytic performances comparable to or even better than those of the state-of-the-art Pt/C for HER or Ir/C for OER in some specific cases. For example, Laasonen et al. reported an electrocatalyst composed of single-shell carbon encapsulated iron nanoparticles decorated on single-walled carbon nanotubes © 2015 American Chemical Society

Received: October 13, 2015 Revised: December 9, 2015 Published: December 10, 2015 580

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ACS Catalysis cm−2 for overall water splitting with a voltage of only 1.7 V.5 Soon after this, Hu et al. discovered that N2P nanoparticles could also be used as bifunctional electrocatalysts for water splitting and their symmetric two-electrode water-splitting electrolyzer (with catalyst deposited on nickel foam) could generate a current density of 10 mA cm−2 at 1.63 V.23 Wang et al. reported a one-pot synthesis of cobalt−cobalt oxide/Ndoped graphene material which could also simultaneously promote the HER and OER, but the active components (including Co, CoO, and Co3O4) are complex and are not welldefined.22 Despite this progress, the still unsatisfactory activity or instability and a poor understanding of the structures of active sites have stimulated further exploration of new electrocatalysts with higher activity, better stability, and lower cost.13 Several feasible strategies to optimize the catalytic performance of transition-metal-based materials have been proposed, such as (1) nanostructuring to increase the active surface area and selectively expose the active sites,3,10 (2) hybridizing with nanocarbons such as graphene, CNTs, and porous carbon, which could increase the conductivity of the catalyst, afford a large surface area to support catalytically active species, suppress the accumulation of active species due to the anchoring effect, and enhance the activity through a coupling synergistic effect between the nanocarbon substrates and catalytically active species,3,18−22,28−30 and (3) doping with heterogeneous elements to alter the electronic structure of the pristine active species for tunably optimizing its catalytic activity.21,31−36 However, it remains highly challenging to develop facile synthetic methods to combine all these strategies for advanced catalysts. Herein, we have designed and synthesized a series of high-performance bifunctional electrocatalysts which could simultaneously promote HER and OER processes in alkaline solution. The preparation method was simple and scalable, which was performed by pyrolyzing a precursor composed of nickel and iron salts with urea under inert argon atmospheres without any post-treatments. During the heating process, the metal ions (Ni2+ and Fe3+) were reduced to the metallic state by the reducing species released from the pyrolysis of urea and then the metallic nanoparticles served as catalysts to induce the growth of nitrogen-doped bamboo-like CNTs, affording Ni−Fe/nitrogen doped nanocarbon hybrids (NiFe/NC). All of these in situ formed hybrids exhibited good electrocatalytic performances toward both the OER and HER. The doping amount of Fe into the NiFe/NC was found to have a significant influence on both the HER and OER activities of the catalysts. For the OER, the best activity was achieved with 10 atom % Fe doping in NiFe/NC. However, for HER, the activity decreased with increased Fe content in NiFe/NC. When it was integrated into symmetric two-electrode water-splitting cells, the NiFe/NC with 10 atom % Fe doping (with a mass loading of 2 mg cm−2 on nickel foam for both the cathode and anode) could achieve a current density of 10 mA cm−2 with a voltage of only 1.58 V. This work is a successful practice in optimizing the activities of Ni-based bifunctional electrocatalysts used in alkaline electrolyzers by simultaneously implementing nanostructuring, hybridizing with nanocarbon, and doping with heterogeneous elements by a facile synthetic approach.

after pyrolyzing the inexpensive starting materials (Ni(CH3COO)2·4H2O, FeCl3·6H2O, and urea) with different molar ratios of Ni2+ to Fe3+ at 700 °C under an inert atmosphere. For convenience, the hybrid will be thereafter denoted as Ni/NC or Ni1−xFex/NC (with x = 0.1, 0.2, 0.3, 0.4, where x stands for the molar ratio of Fe to Fe + Ni) in the following descriptions. Scanning electron microscopy (SEM) images (Figure 1a,b and Figure S2 in the Supporting

Figure 1. (a) Low-magnification and (b) high-resolution SEM images of representative Ni0.9Fe0.1/NC. (c, d) TEM and (e) HRTEM images of the same sample. (f) HAADF-TEM image and (g−i) the corresponding EDS mappings of C, Ni, and Fe in the area marked in blue dotted rectangular frame in (f). (j) XRD patterns of Ni/NC and Ni0.9Fe0.1/NC and standard XRD pattern of face-centered cubic metallic Ni (JCPDS Card No. 89-7128).

Information) showed that nanoparticles with sizes ranging from 10 to 50 nm were well dispersed on or encapsulated in the bamboo-like nanotubes. No obvious morphology and size differences were observed among these samples with different Fe contents in the hybrid. The XRD patterns (Figure 1j and Figure S3 in the Supporting Information) of these Ni1−xFex/ NC samples were similar to each other, with three distinct diffraction peaks located around 44.3, 51.7, and 76.2°, which can be assigned to the (111), (200), and (220) crystal-plane reflections of a face-centered cubic nickel phase (JCPDS Card No. 89-7128). The incorporation of Fe into the Ni/NC did not result in any additional crystal diffraction peaks. However, a negative shift of 2θ angles with increasing iron concentrations could be observed, implying the substitutional incorporation of an Fe atom into the nickel cubic structure (Figure S3). Scherrer analysis of the broadening of (111) diffraction peak of Ni0.9Fe0.1/NC indicated an average grain size of 28 nm of Ni−Fe alloy nanoparticles along the [111] crystallographic axis direction, consistent with the results observed by SEM.

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RESULTS AND DISCUSSION As described in the experimental section and Figure S1 in the Supporting Information, a black powdery product was obtained 581

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Ni0.9Fe0.1/NC sample. The high-resolution C 1s core-level spectrum (as shown in Figure 2b) could be deconvoluted into three peaks centered at ∼284.5, ∼285.3, and ∼286.4 eV, which was indexed to CC/C−C, CN, and C−O/C−N, respectively.37−40 The sharpest peak corresponding to CC/ C−C indicated that most of the carbon atoms were in the form of a conjugated honeycomb lattice.38 As can be seen in Figure 2c, the XPS N1s spectra for Ni0.9Fe0.1/NC sample can be deconvoluted into four different peaks located at ∼398.6, ∼ 400.2, ∼401.3, and ∼402.0 eV, which correspond to pyridinic, pyrrolic, graphitic, and oxidized pyridinic nitrogen, respectively.28 Pyridinic and pyrrolic nitrogens were the main nitrogen-containing species. This result indicated that nitrogen was successfully incorporated into the CNTs. It has been demonstrated that nitrogen doping into carbon materials could be a favorable factor to enhance chemical/thermal stability, i.e., oxidation resistance of carbons, and may cause some synergistic effects between catalytically active species and carbon supports.28−30 Two principal peaks in the Ni2p XPS spectrum (Figure 2d) centered at 853.4 and 870.9 eV confirmed the metallic state of Ni in Ni0.9Fe0.1/NC.4 A small peak located at 860.0 eV may be attributed to the satellite peak of Ni.41 The peaks located at 707.6 and 720.8 eV in the Fe2p XPS spectrum revealed the metallic state of Fe in Ni0.9Fe0.1/NC, and the peak located at 712.3 eV may be attributed to the satellite peak of Fe or the oxidized Fe ion on the surface of Ni−Fe alloy nanoparticles due to the exposure of the sample to air (Figure 2e).18,33 We first investigated the electrocatalytic OER activity of Ni1−xFex/NC on a glassy-carbon (GC) electrode in alkaline solutions (1 M KOH) in a standard three-electrode system (see details in Materials and Methods). During the measurements, the working electrode was continuously rotating at 1600 rpm to remove the generated oxygen bubbles. The linear sweep voltammetry (LSV) polarization curves suggested that Fe doping had a significant effect in altering the electrocatalytic OER performances (Figure 3a). In comparison with Ni/NC, the doping of 10 atom % Fe into the catalyst can remarkably reduce the overpotential from 371 to 330 mV for achieving a current density of 10 mA cm−2 (catalyst loadings were 0.2 mg cm−2). This high OER activity of the Ni0.9Fe0.1/NC catalyst is significantly superior to the literature reports on the “golden standards” of 20% Ir/C (380 mV) and 20% Ru/C (390 mV) measured under similar conditions.42 For comparison, commercial Ir/C catalyst (20 wt % Ir on Vulcan carbon black, Premetek Co.) with the same loading (0.2 mg cm−2) was also measured as a benchmark OER electrocatalyst under the same conditions. It can be seen from Figure S5 and Table S1 in the Supporting Information that the Ir/C catalyst showed a slightly larger overpotential (343 mV) than the Ni0.9Fe0.1/NC catalyst. However, further increasing the content of Fe in the Ni1−xFex/NC induced a reverse effect on the OER activities. These results here happened to be consistent with previously reported results that the optimum Fe-doping contents for OER activities were 10 atom % in the solution-casted Ni1−xFexO on Au/Ti substrate31 and the Fe-doped NiO nanocrystals.32 The Tafel slope first decreased from 54 mV/dec for Ni/NC to 45 mV/dec for Ni0.9Fe0.1/NC and then slowly increased to around 60 mV/dec with increased Fe content in Ni1−xFex/NC (Figure 3b). A small Tafel slope is beneficial for practical applications because it leads to a significant increase in the OER rate with slightly increased overpotential. The electrode kinetics of these catalysts in the OER process were also investigated by

Transmission electron microscopy (TEM) images (as shown in Figure 1c,d) showed that the tube walls of bamboo-like nanotubes were composed of several disorderedly stacked graphene layers. This could explain why only a very weak diffraction peak could be observed at around 26.1°, corresponding to the graphitic (002) crystal plane (see Figure 1j and Figure S3). The high-resolution TEM (HRTEM) image of the nanoparticles in Figure 1e presented 0.21 nm lattice fringes, which can be attributed to the (111) lattice plane of the Ni−Fe alloy, in line with the XRD results. The TEM images also showed that some Ni−Fe alloy nanoparticles were completely encapsulated by a few carbon layers around them. The high-angle angular dark field TEM (HAADF-TEM) image and the corresponding EDS mappings demonstrated that the bamboo-like nanotubes were mainly composed of carbon and the nanoparticles were composed of Ni and Fe (as shown in Figure 1f−i). The perfect superposition of the element distributions of Ni and Fe in their EDS mapping images further confirmed the alloy nature of the Ni−Fe nanoparticles in Ni0.9Fe0.1/NC. All of the characterization results above suggested that the pyrolyzed product was composed of metallic Ni−Fe alloy nanoparticles either encapsulated in or dispersed on bamboo-like CNTs. The specific surface area of a representative Ni0.9Fe0.1/NC catalyst was measured to be 153.7 m2 g−1 on the basis of Brunauer−Emmett−Teller (BET) surface area analysis, suggesting the hybrid has a loose interior structure which could benefit the infiltration and flowage of electrolyte. X-ray photoelectron spectroscopy (XPS) measurement was performed to investigate the specific surface composition and chemical environment of the representative Ni0.9Fe0.1/NC. The full spectra of the Ni0.9Fe0.1/NC revealed the presence of C, Ni, Fe, and slight N and O (Figure 2a). The XPS results revealed an N/C ratio of 3.4% and Ni/Fe ratio close to 9.0 in the

Figure 2. (a) Full XPS spectrum of the representative Ni0.9Fe0.1/NC sample. (b−e) High-resolution XPS spectra of C1s, N1s, Ni2p, and Fe2p, respectively. 582

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Figure 3. Electrochemical water-splitting activities of Ni1−xFex/NC on GC electrodes in 1 M KOH(aq) electrolyte: (a) LSV polarization curves for the OER; (b) corresponding Tafel plots for the OER; (c) LSV polarization curves for the HER; (d) corresponding Tafel plots for the HER; (e) Nyquist plots obtained by EIS at 340 mV overpotential for the OER (ηOER); (f) Nyquist plots obtained by EIS at 230 mV overpotential for HER (ηHER); (g) summary of the overpotentials for the HER and OER to achieve a current density of 10 mA cm−2 with different Fe contents in Ni1−xFex/ NC. The mass loadings of all catalysts were 0.2 mg/cm2.

dissociation of water molecules and subsequent desorption of atomic hydrogen.5,25,43 Here, we found that our Ni1−xFex/NC catalysts exhibited good activities to facilitate the HER in such a strong alkaline electrolyte. As can be seen in Figure 3c, the Ni/ NC catalysts exhibited the best HER activity, which only required an overpotential of 219 mV to reach a current density of 10 mA cm−2. This HER activity of our Ni/NC catalysts was much better than those of Ni1−xMnx alloy nanoparticles embedded in nitrogen-doped carbon sponge,21 smooth Ni wire,44 and NiOx electrodeposited on multiwalled CNTs,45 which had been reported in recent literature. This strongly suggested that nanostructuring of Ni and the in situ hybridization between Ni and N-doped CNTs are the main advantages of our Ni/NC catalysts in obtaining a superior electrocatalytic HER activity. In comparison with the benchmark Pt/C catalyst (20 wt % Pt on Vulcan carbon black, Premetek Co.) with a loading of 0.2 mg cm−2, Ni/NC needed an extra overpotential of 184 mV to achieve a current density of −10 mA cm−2 with the same loading under the same measurement conditions (see details in Figure S5 and Table S1 in the Supporting Information). In addition, we found that the Fe doping resulted in a negative effect on the HER activities of our Ni1−xFex/NC catalyst. Ni0.9Fe0.1/NC exhibited HER activities slightly worse than those of Ni/NC, and the activity showed a larger drop with a further increase in Fe doping content (Figure 3c). The Tafel slopes of these Ni1−xFex/NC catalysts are all in the range of 100−120 mV/dec, which

electrochemical impedance spectroscopy (EIS) measurements at 1.57 V vs RHE (i.e., ηOER = 340 mV). As shown in Figure 3e, the charge-transfer resistance, Rct, was determined from the semicircle registered at low frequencies (high Z′), which shows that the Rct values of these catalysts increased in the order Ni0.9Fe0.1/NC < Ni0.8Fe0.2/NC< Ni0.7Fe0.3/NC< Ni/NC< Ni0.6Fe0.4/NC. The variation tendency of the Rct was consistent with the LSV results, indicating that a smaller Rct value gives rise to faster electrode kinetics. We further checked the durability of Ni0.9Fe0.1/NC by continuously cycling the catalyst between 1.2 and 1.6 V vs RHE with an accelerated scan rate of 50 mV/s and recorded the LSV curve with a scan rate of 10 mV/s at every 5000 cycles after mechanical removal of all the possibly existing bubbles on the electrode (Figure S6a in the Supporting Information). We found that there were minor positive shifts of the onset potential after 10000 cyclic voltammogram (CV) cycles (perhaps caused by the detachment of catalysts on the surface of the glassy-carbon electrode during the durability test), confirming the excellent stability of this catalyst. We then performed the LSV measurements in a more negative potential region to check the electrocatalytic HER performances of these catalysts in 1 M KOH electrolyte. It is generally recognized that the HER in a basic environment requires higher overpotentials in comparison to those in an acidic environment, because multistep HER processes are associated with an alkaline environment, including the 583

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Figure 4. (a, c) Cyclic voltammograms (CVs) of bare Ni foam and Ni0.9Fe0.1/NC loaded on Ni foam (Ni0.9Fe0.1/NC-NF) in the OER and HER regions, respectively. The loading of the catalysts on Ni foam was 2 mg cm−2. (b, d) Chronopotentiometric curves of Ni0.9Fe0.1/NC-NF at different constant current densities. (e, f) Cyclic voltammogram and chronopotentiometric curve of a water-splitting cell composed of two symmetric Ni0.9Fe0.1/NC-NF electrodes. The inset in (f) shows the hydrogen and oxygen evolution during the constant-current electrocatalysis. The solid and dashed arrows in (a), (c), and (e) indicate the forward and reverse scan directions of the polarization curves, respectively.

suggested that the first electron transfer process was the ratelimiting step (Figure 3d). The EIS results also showed an increase of Rct with increasing Fe content in Ni1−xFex/NC catalyst (Figure 3f). Accelerated durability tests confirmed that the Ni0.9Fe0.1/NC had an excellent catalytic stability for the HER under alkaline conditions (see Figure S6b in the Supporting Information). Figure 3g summarized the overpotentials (η) for HER and OER to achieve a current density of 10 mA cm−2 with different Fe contents in the Ni1−xFex/NC catalysts. In reference to the sum of ηHER and ηOER for 10 mA cm−2, Ni0.9Fe0.1/NC exhibited the smallest value, indicating that it is the most promising bifunctional catalyst on integration into a symmetric two-electrode water-splitting cell. In order to determine which composition in the hybrid was the main active species for the HER and OER, we further performed electrochemical characterizations on the Ni0.9Fe0.1/ NC catalyst after a thorough acid leaching process to remove the metal nanoparticles dispersed on the outer walls of the Ndoped CNTs (see details in Materials and Methods). A similar acid leaching method had been adopted by Bao et al. and Asefa et al. and was demonstrated to be efficient.46,47 After leaching, the solution turned from colorless to flavovirens, indicating that the metallic species in the hybrid were dissolved into the solution (see Figure S7 in the Supporting Information). However, the existing magnetism of the leached hybrid

suggested that some metallic nanoparticles remained in the hybrid. This is because some metallic Ni−Fe nanoparticles were completely encapsulated in the CNTs which could not be removed by acid leaching (see Figure S8 in the Supporting Information). The electrochemical test showed that the leached Ni0.9Fe0.1/NC catalyst exhibited a rather worse catalytic effect for the HER, which showed a much larger overpotential of 475 mV to achieve a current density of 10 mA cm−2 (see Figure S9a in the Supporting Information). The still observed HER activity here may be ascribed to the metallic Ni0.9Fe0.1 nanoparticles encapsulated in the N-doped CNTs which could create a synergistic effect with the N-dopants in the CNTs that could reduce the adsorption free energy of the H atom on CNTs and then facilitate the HER process.45 We also found that the OER activity of Ni0.9Fe0.1/NC catalyst dropped greatly after the acid leaching, which exhibited a large shift of onset potential (see Figure S9b in the Supporting Information). All these results revealed that the uncapsulated Ni0.9Fe0.1 nanoparticles could be the main active species for both the HER and OER, as the unleached Ni0.9Fe0.1/NC catalyst exhibited much better performance than the leached catalyst. The electrochemically active surface areas of Ni1−xFex/NC were roughly estimated from the electrochemical double-layer capacitance of the catalytic surface by measuring the non-Faradaic capacitive current associated with double-layer charging from the scan584

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ACS Catalysis rate-dependent CV.9 The determined capacitances of all five Ni1−xFex/NC catalysts were similar to each other (Figure S10 in the Supporting Information), suggesting that all Ni1−xFex/ NC catalysts had similar electrochemically active surface areas. In combination with the similar metal contents and similar size distributions in all of the Ni1−xFex/NC catalysts, it suggested that the different catalytic activities for the OER and HER of the Ni1−xFex/NC catalysts could be ascribed to the doping effect of Fe rather than the electrochemically active surface area. To determine the optimal metal content of Ni−Fe alloy nanoparticles on CNT backbones for achieving the best catalytic performance, we synthesized a series of Ni0.9Fe0.1/ NC catalysts with metal content ranging from 20 to 80 wt % by tuning the initial amount of metal precursors and urea. The SEM images of these samples (Figure S11a−c in the Supporting Information) showed that less metal content might result in thinner CNTs and sparser metal NP distribution on the CNT backbone. Further, the electrocatalytic activities of these Ni0.9Fe0.1/NC catalysts for the OER and HER were systematically measured and compared (Figure S11d,e in the Supporting Information). From the results we found that the OER activity gradually improved with increasing metal content from 20 to 80 wt %. However, the HER activity first increased with increasing metal content in Ni0.9Fe0.1/NC catalysts from 20 to 60 wt % and then significantly decreased for the 80 wt % sample. In terms of the bifunctional catalytic activities, it could be concluded that a metal content of 60 wt % was roughly the optimum for Ni0.9Fe0.1/NC catalyst to achieve the best catalytic activities. Hybrid materials with Ni0.9Fe0.1 alloy nanoparticles coated on commercially accessed multiwalled CNTs (denoted as Ni0.9Fe0.1-CNTs thereafter) were also prepared (see the synthetic details in the Supporting Information). The metal content in Ni0.9Fe0.1-CNTs was tuned to be same as in our Ni1−xFex/NC hybrids. The SEM and TEM images of Ni0.9Fe0.1CNTs showed that the metal nanoparticle sizes were similar to those in our Ni0.9Fe0.1/NC hybrids (see Figure S12a,b in the Supporting Information). The electrochemical characterizations of Ni0.9Fe0.1-CNTs also demonstrated good catalytic activities for both the OER and HER, but the activities were inferior to those of the Ni0.9Fe0.1/NC catalyst (see Figure S12d,e in the Supporting Information). These results also suggested that metal nanoparticles accounted for the main active components for catalytic OER and HER in the hybrids. We should point out that the synthesis of Ni0.9Fe0.1-CNTs involved multistep processes such as chemical vapor deposition of CNTs, mild oxidation of CNTs, solvothermal modification of metal oxide/ hydroxide on CNTs, and annealing the hybrids under vacuum, strengthening the advantage of the facile synthesis of our Ni1−xFex/NC catalyst. In consideration of the practical applications, we loaded our Ni0.9Fe0.1/NC catalysts onto a porous and current-conducting nickel foam substrate for achieving larger loading and evaluated its electrocatalytic HER and OER activities. With a larger loading of 2 mg cm−2 on nickel foam, Ni0.9Fe0.1/NC exhibited overpotentials of 270 and 85 mV for the OER and HER to reach a current density of 10 mA cm−2, respectively (see Figure 4a,c). It should be noted that the overpotentials determined here were obtained from the reverse scan rather than forward scan in the CV curves for the sake of avoiding the possible effects of capacitive/oxidation currents.2,5 The chronopotentiometric curves in Figure 4b,d demonstrated the steady potentials needed for the Ni0.9Fe0.1/NC-NF electrodes to maintain

specific constant current densities for 1200 s. The overpotential needed for achieving a specific steady current density determined from these chronopotentiometric curves is consistent with the results determined from reverse scans of the CV curves. This indicated that there were little contributions of capacitive/oxidation currents to the observed current values in these reversely scanned LSV curves. Furthermore, we integrated two Ni0.9Fe0.1/NC-NF electrodes into a symmetric two-electrode cell and studied the performance of this bifunctional catalyst for overall electrochemical water splitting. It can be seen from Figure 4e that a remarkable electrocatalytic activity was achieved with Ni0.9Fe0.1/NC-NF electrodes. The voltage needed to support a current density of 10 mA cm−2 was about 1.58 V, determined from the reverse scan of the CV curve. The achieved electrocatalytic performance of our Ni0.9Fe0.1/NC catalysts here is comparable or even superior to that of some advanced asymmetric and symmetric bifunctional water-splitting electrocatalysts reported recently.2−5,21−23,48−51 In addition, the two-electrode watersplitting cell exhibited a negligible performance degradation with only about a 30 mV raise of applied voltage over 24 h of continuous operation at a current density of 10 mA cm−2. A slight increase in potential over time as shown in Figure 4f may be caused by gas accumulation on the electrodes or detachment of the catalysts. As commercial Ni foam itself has little intrinsic HER activity and derived OER activity, we further used more inert carbon fiber papers (CFPs) as the catalyst support.5,48,49 The CFP itself exhibited negligible electrocatalytic activities in water splitting (see Figure S13 in the Supporting Information). It can be seen from Figure S14 and S15 in the Supporting Information that Ni0.9Fe0.1/NC supported on CFPs (referred as Ni0.9Fe0.1/NC-CFPs) with a loading of 2 mg cm−2 also exhibited striking catalytic activities for both the HER and OER. However, the voltage needed to achieve a current density of 10 mA cm−2 for overall water splitting of the Ni0.9Fe0.1/NCCFPs is 1.64 V, slightly larger than that for Ni0.9Fe0.1/NC-NF (see Figure S15). This may due to the less porous and thinner structure characteristic of CFPs with respect to nickel foam. In addition, the water-splitting cell composed of two symmetric Ni0.9Fe0.1/NC-CFP electrodes exhibited a negligible performance degradation after 3 h of continuous operation at a current density of 10 mA cm−2 (see Figure S15) and at a constant voltage of 1.7 V (see Figure S16 in the Supporting Information). After the stability tests, we performed SEM, TEM, XRD, and XPS characterizations to check the morphology and structure changes of the Ni0.9Fe0.1/NC catalysts supported on CFPs. As can be seen in Figure S17 in the Supporting Information, no obvious morphology changes occurred on CNTs and Ni0.9Fe0.1 nanoparticles after long-term operation in both the HER and OER, suggesting an excellent structure stability of the catalysts. Furthermore, no additional XRD diffraction peaks can be observed in addition to the diffraction peaks of graphitic carbon and metallic Ni0.9Fe0.1 (see the XRD patterns in Figure S18 in the Supporting Information), indicating that there were no significant composition or structure changes after the stability test for both HER and OER. However, the high-resolution Ni2p XPS spectrum (as shown in Figure S19a in the Supporting Information) of Ni0.9Fe0.1/NC after the OER stability test showed a positive shift of the binding energy in comparison with metallic Ni, which is close to either the Ni(OH)2 or NiOOH phase (both of which have Ni 2p3/2 binding energies 585

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ACS Catalysis close to 855.6 eV).4,33 In addition, the diconvoluted highresolution Fe2p XPS spectrum exhibited two extra peaks located at 710.1 and 723.2 eV in addition to another three peaks located at 706.8, 719.6, and 714.2 eV, which were ascribed to Fe2p core-level peaks and the satellite peak of metallic Fe species (see Figure S19b in the Supporting Information and Figure 2e).18,33 This result indicated that some Fe3+ species were generated in Ni0.9Fe0.1/NC catalysts after extensive OER tests. The EDS mapping of the Ni0.9Fe0.1/NC sample after the OER exhibited an enhanced O content in the superposition region of Ni0.9Fe0.1 nanoparticles, indicating that some oxidation occurred on the surface of the Ni0.9Fe0.1 nanoparticles (see Figure S20c in the Supporting Information). This was consistent with the previously reported result that a NiOx shell (with thickness less than 1 nm) formed on an ultrathin nickel film deposited on a silicon photoanode after a long-term photoelectrochemcial oxygen evolution reation.4 Such an ultrathin and amorphous NiFeOx or NiFe(OH)x shells around the metallic NiFe core are the main active species for OER, but they were hard to detect by XRD or TEM here (see Figures S18 and S20).4,28−36 Above all, the stability in morphology, composition, and electrochemical performances strongly suggested that our Ni0.9Fe0.1/NC catalyst is especially stable in practical alkaline water electrolyzers. The experimental results herein demonstrated that Fe doping had a significant influence on OER activities of Ni1−xFex/NC catalysts. However, detailed mechanisms related to the Fe doping effect on the OER activities of nickel-based materials are still a matter of debate. Some plausible explanations for the effect of Fe doping on OER activities had been suggested to be as follows: (1) Fe doping can increase the conductivity of the Ni/Fe-based electrocatalyst; (2) Fe-induced partial-charge transfer can decrease the average oxidation state of Ni sites which have stronger oxidizing power and thus possibly faster OER kinetics; and (3) Fe doping can induce structural transformations of Ni-based catalysts to more active species.33−36 Recently, in combination with operando X-ray absorption spectroscopy and theoretical computation, Bell et al. demonstrated that Fe3+ occupying the octahedral sites in Ni1−xFexOOH was the possible active site for the observed high OER activity of Ni−Fe compounds.35 Therefore, increasing Fe contents in Ni−Fe compounds might increase the OER active sites. However, due to the limited solubility of Fe in Ni(OH)2/ NiOOH, a large incorporation amount of Fe would generate inactive FeOOH species, which might screen the OER active sites and result in worse OER activity.32,33,35 In our work, we also observed that Fe3+ species existed in Ni0.9Fe0.1/NC catalyst after OER measurements from the high-resolution F2p XPS spectrum (Figure S19b in the Supporting Information); thus, Fe3+ species in Ni1−xFexOOH shell may account for the observed excellent OER activity. However, accurate identification of the contribution of all the possible mechanisms mentioned above is beyond the scope of this work. The results in this work showed that the alloying of Ni and Fe might not be an effective approach to increase the HER activity of Ni-based materials such as the alloying of Ni and Mn.21 For HER, the activities of different catalytic surfaces can be correlated with their hydrogen-binding energy (HBE). A catalyst with either too small or too large HBE away from the optimal HBE would be detrimental for hydrogen adsorption or hydrogen desorption, respectively, and would lead to inferior electrocatalytic HER activity.11,25 Fe possesses a larger positive deviation from the optimum HBE than Ni, which may afford

lower electrocatalytic HER activity with Fe doping to Ni due to the inefficient hydrogen desorption on the surface of Fe.25 This could explain why the electrocatalytic HER activity of the Ni1−xFex/NC catalyst decreased with increasing Fe incorporation content. In addition, the variation tendency of exchange current density determined from the Tafel plots seems to support this deduction (see Table S1 in the Supporting Information).

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CONCLUSION By pyrolyzing a mixture of metal salts and urea, we have developed a series of Ni1−xFex/NC hybrid electrocatalysts which exhibit excellent performances to catalyze both the HER and OER in alkaline electrolyte. The influences of Fe doping content on the electrocatalytic activities of Ni1−xFex/NC electrocatalysts have also been systematically investigated, and it is found that Ni0.9Fe0.1/NC catalyst can achieve the best performance as a bifunctional water splitting electrocatalyst. An efficient electrolyzer composed of two symmetric Ni0.9Fe0.1/NC loaded nickel foam electrodes can achieve a current density of 10 mA cm−2 for overall water splitting at a voltage of 1.58 V, which is among the best performance for a symmetric twoelectrode water electrolyzer reported so far. The excellent bifunctional activities, superior stability, and low-cost characteristics of our in situ formed hybrid catalyst could pave a new way to advanced electrocatalysts for large-scale water splitting.

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MATERIALS AND METHODS Synthesis of Ni1−xFex/NC Electrocatalysts. The metal salt precursors (nickel acetate tetrahydrate, Ni(CH3COO)2· 4H2O, and iron(III) chloride hexahydrate, FeCl3·6H2O) were completely dissolved in 2 mL of ethanol in the desired molar ratio while maintaining a total metal ion content of 0.5 mmol. Then, 2 g of urea was added to the solution following a sonication treatment for 5 min. The mixture was subsequently dried at 70 °C in an oil bath upon stirring for 6 h to remove the ethanol. The dried mixture was thoroughly ground into a homogeneous fine powder and then transferred into a ceramic crucible equipped with a cover. Next, the covered ceramic crucible was placed at the center of a tubular furnace and the temperature was raised from 25 to 550 °C at a programming rate of 0.5 °C/min and then maintained at 550 °C for 3 h. After that, the temperature was further raised to 700 °C in 1 h and maintained for 2 h and finally cooled naturally. The whole heating process proceeded under an inert Ar/N2 atmosphere with a gas flow rate of 50 sccm. The overall content of metal elements in the obtained product was determined to be about 60 wt % according to a microbalance analysis. It should be noted that no product could be obtained if there was no metal salt (Ni or Fe) added to the precursor. This indicated that the metal species were indispensable for facilitating the formation of N-doped carbon materials in the final products. Materials Characterizations. Scanning electron microscopy (SEM) samples were prepared by drop-drying the suspension of catalyst dispersed in ethanol onto the silicon substrate, and SEM analysis was carried out with a ZEISSMerlin scanning electron microscope. Transmission electron microscopy (TEM) samples were prepared on copper grids by drop-drying their ethanol suspensions. TEM characterizations and energy dispersive X-ray spectroscopy (EDS) analysis were carried out on an FEI Tecnai G2 F30 transmission electron microscope. X-ray photoelectron spectroscopy (XPS) measure586

DOI: 10.1021/acscatal.5b02291 ACS Catal. 2016, 6, 580−588

Research Article

ACS Catalysis

two symmetric catalyst electrodes as both the anode and cathode.

ment was performed on a PerkinElmer Model PHI 5600 XPS system with a resolution of 0.3−0.5 eV from a monochromatic aluminum anode X-ray source. X-ray diffraction (XRD) samples were prepared by compressing the sample powders onto glass slides to form thick films, and XRD measurement was performed on a Shimadzu thin film diffractometer equipped with Cu Kα radiation (λ = 1.540598 Å). The Brunauer− Emmett−Teller (BET) surface areas of the samples were measured by using a Micromeritics ASAP 2020 HD88 instrument. Sample Preparation for Electrochemical Characterizations. For measurements on an RDE, 5 mg of catalyst was dispersed in 0.75 mL of water, 0.25 mL of ethanol, and 20 μL of 5 wt % Nafion solution by at least 60 min sonication to form a homogeneous ink. Then 8 μL of the catalyst ink (containing 40 μg of catalyst) was loaded onto a glassy-carbon electrode 5 mm in diameter (corresponding to a mass loading of 0.2 mg cm−2). For the preparation on nickel foam or carbon fiber paper (CFP) electrodes (purchased from Fuel Cell Store), 20 mg of catalyst was dispersed in 0.75 mL of water, 0.25 mL of ethanol, and 100 μL of 5 wt % Nafion solution by at least 60 min sonication to form a homogeneous ink. After the sonication, 100 μL of the catalyst ink was drop-dried onto nickel foam or carbon fiber paper with a catalyst covered area of about 1 cm2 to achieve a catalyst loading of 2 mg cm−2. For the leaching process, 20 mg of Ni0.9Fe0.1/NC catalyst was dispersed into 20 mL of 30 wt % HCl solution by ultrasonication for 1 h and then kept at 80 °C for 1 week. After that the solid product was washed three times with ultrapure water, collected by centrifugation, and dried at 80 °C for 24 h. Through this acid leaching process, the loading amounts of metal on the CNT backbone of all Ni1−xFex/NC species were determined to be in the range of 40 ± 2 wt % by microbalance analysis of the mass difference between the pristine and acid-leached samples. Electrochemical Characterizations. Electrochemical tests were carried out on a CHI 760D electrochemistry workstation. Catalyst ink cast on the rotating-disk electrode (RDE) or nickel foam or carbon fiber paper was used as the working electrode, a graphite rod and a saturated calomel electrode were used as the counter electrode and reference electrode, respectively. The reference was calibrated against and converted to the reversible hydrogen electrode (RHE). All measurements were performed in 1 M KOH aqueous solution which had been first degassed by bubbling argon for at least 30 min. For all of the HER or OER measurements, the corresponding electrolytes were saturated with H2 or O2 by continuous purging with high-purity H2 (99.999%) or O2 (99.999%), respectively, during the entire measurement processes. The catalyst electrodes were continuously scanned by CV until a stable CV curve could be repeated before measuring polarization curves. All of the potentials in the linear sweep voltammetry (LSV) polarization curves were iR corrected unless specifically indicated. Experiments involving RDE were conducted with the working electrode continuously rotating at 1600 rpm to get rid of the generated bubbles. The scan rates for all the LSV and CV measurements were set to be 10 mV/s to minimize the capacitive current. Electrochemical impedance spectroscopy (EIS) studies were performed when the working electrode was biased at certain potentials while sweeping the frequency from 10 kHz to 10 mHz with a 5 mV ac amplitude. The impedance data were fit to a simplified Randles circuit to extract the series and charge transfer resistances. Overall water splitting was performed in a two-electrode system which was composed of

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b02291. Additional experimental details, SEM, TEM, EDX, XRD, XPS, and electrochemical characterization data, and extensive comparisons of the catalytic performance of Ni0.9Fe0.1/NC with those of other reported catalysts (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for Y.L.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the South University of Science and Technology of China, “The Recruitment Program of Global Youth Experts of China”, the Shenzhen fundamental research programs (No. JCYJ20130401144532128), the Shenzhen Key Lab funding (ZDSYS201505291525382), and the Shenzhen peacock program (No. KQTD20140630160825828).

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