Nanocarbon Hybrids as Advanced

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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 ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02291 • Publication Date (Web): 10 Dec 2015 Downloaded from http://pubs.acs.org on December 13, 2015

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ACS Catalysis

Facile Synthesis of Nickel-Iron/Nanocarbon Hybrids as Advanced Electrocatalysts for Efficient Water Splitting

Xing Zhang, Haomin Xu, Xiaoxiao Li, Yanyan Li, Tingbin Yang, Yongye Liang* Department of Materials Science and Engineering, South University of Science and Technology of China, Shenzhen, 518055, China *To whom correspondence should be addressed. E-mail: [email protected]

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Abstract Developing active, stable and low-cost electrocatalysts which can promote oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in the same electrolyte is undoubtedly a vital progress to 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 nitrogen-doped bamboolike carbon nanotubes (CNTs). This simple synthesis route could simultaneously realize nanostructuring, doping, and hybridizing with nanocarbon, which had been demonstrated as efficient strategies to optimize catalytic activity of electrocatalyst. The in-situ formed hybrid catalysts exhibit good catalytic performances for both OER and HER in alkaline condition, 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 hours’ operation. The excellent electrocatalytic activity and facile preparation method enable this hybrid electrocatalyst to be a promising candidate for future large-scale application in water splitting.

Keywords: nickel-iron alloy nanoparticles, carbon nanotubes, hybrid electrocatalyst, oxygen evolution reaction, hydrogen evolution reaction


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Molecular 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 environmental-friendly way to generate H2.2, 3 Especially, when coupling to photovoltaic modulus, it will be a sustainable and promising energy system for future society.4-8 Currently, the state-of-the-art electrocatalysts for HER and OER are Pt-based and Ir-based or Rubased 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 lowcost electrocatalysts in place of precious metal-based materials is a vital step towards future hydrogen economy. Recently, transition metal (Mo, W, Ni, Co, Fe, Mn, Cu etc.) and their derivatives (carbide, oxide, sulfide, phosphide, hydroxide and mixed-metal alloy etc.) have been extensively investigated as either HER or OER and even bifunctional electrocatalysts.10-27 Excitingly, some of them exhibited comparable or even better electrocatalytic performances than the state-of-theart 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 (CNTs) which exhibited an excellent electrocatalytic HER performances slightly inferior to Pt/C in acidic solutions.24 More noticeably, 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 multi-walled CNTs.18 Developing bifunctional electrocatalysts with high activities towards both OER and HER in the


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same electrolyte could be a promising way to lower the cost of water splitting devices as it may simplify the requirement on diverse equipment and process.2 Fortunately, there were some exciting cases which had been demonstrated in reducing the overpotential in water-splitting devices with a bifunctional electrocatalyst.2,

3, 22, 23

For example, Luo et al. reported a NiFe

layered-double-hydroxides nanosheets deposited on nickel foam could achieve a 10 mA 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 a 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/N-doped graphene material which could also simultaneously promote the HER and OER, but the active components (including Co, CoO and Co3O4) is complex and not well-defined.22 Despite these progresses, the still unsatisfactory activity or instability, as well as poor understanding on the structures of active site stimulate 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 nanocarbon such as graphene, CNTs, and porous carbon, which could increase the conductivity of the catalyst, afford large surface area to support catalytic active species, suppress the accumulation of active species due to the anchoring effect, and enhance the activity through the coupling synergistic effect between the nanocarbon substrates and catalytic active species;3,

18-22, 28-30

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


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methods to combine all these strategies for advanced catalysts. Herein, we 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 metallic state by the reducing species released from the pyrolysis of urea and then the metallic nanoparticles serve as catalysts to induce the growth of nitrogen-doped bamboo-like CNTs, affording Ni-Fe/nitrogen doped nanocarbon hybrids (NiFe/NC). All these in-situ formed hybrids exhibited good electrocatalytic performances toward both OER and HER. The doping amount of Fe into the NiFe/NC was found to have significant influences on both the HER and OER activities of the catalysts. For OER, the best activity was achieved with 10 at% Fe doping in NiFe/NC. While for HER, the activity decreased with increased Fe content in NiFe/NC. When integrated into a symmetric twoelectrode water splitting cells, the NiFe/NC with 10 at% Fe doping (with 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 to optimize the activities of Ni-based bifunctional electrocatalyst used in alkaline electrolyzers by simultaneously implementing nanostructuring, hybridizing with nanocarbon and doping with heterogeneous elements by a facile synthetic approach. Results and discussion As described in the experimental section and Figure S1 in the supporting information (SI), a black powder product was obtained after pyrolyzing the inexpensive starting materials (Ni(CH3COO)2·4H2O, FeCl3·6H2O and urea) with a different molar ratio of Ni2+ to Fe3+ at 700


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°C in inert atmosphere. For convenience, the hybrid will be thereafter denoted as Ni/NC or Ni1xFex/NC

(with x = 0.1, 0.2, 0.3 and 0.4, where x standing 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 SI) 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 content in the hybrid. The XRD patterns (Figure 1j and Figure S3 in SI) 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 (222) 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 Fe atom into the nickel cubic structure (Figure S3 in SI). 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. Transmission electron microscopy (TEM) images (as shown in Figure 1c and 1d) showed that the tube walls of bamboo-like nanotubes were composed of several disorderedly stacked graphene layers. This can 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 In SI). 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


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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 1(f~i)). The perfect superposition of the element distribution 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 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 based on the Brunauer−Emmett−Teller (BET) surface area analysis, suggesting the hybrid own 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 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 eV, ~285.3 eV 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 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 And pyridinic and pyrrolic nitrogen were the main nitrogen-containing species. This result indicated that nitrogen was successfully incorporated into the CNTs. It had been demonstrated that


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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 principle peaks in 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 firstly investigated the electrocatalytic OER activity of Ni1-xFex/NC on glassy carbon (GC) electrode in alkaline solutions (1 M KOH) in a standard three-electrode system (see details in the experimental section). 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 to alter the electrocatalytic OER performances (Figure 3a). Compared with Ni/NC, the doping of 10 at% Fe into the catalyst can remarkably reduce the overpotential from 371 mV to 330 mV for achieving a current density of 10 mA cm-2 (catalysts’ loading 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 in the similar condition.42 For comparison, commercial Ir/C catalyst (20 wt% Ir on Vulcan carbon black, Premetek Co.) with same loading (0.2 mg cm-2) was also measured as a benchmark OER electrocatalyst under the same condition. It can be seen from Figure S5 and Table S1 in SI 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 were happened to be consistent with the previously reported results that the


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optimum Fe-doping contents for OER activities were 10 at% in the solution-casted Ni1-xFexO on Au/Ti substrate31 and the Fe-doped NiO nanocrystals32. The Tafel slope firstly decreased from 54 mV/dec for Ni/NC to 45 mV/dec for Ni0.9Fe0.1/NC and then slowly raised to around 60 mV/dec with increased Fe content in Ni1-xFex/NC (Figure 3b). A small Tafel slope is beneficial for practical application because it leads to a significant increase of OER rate with slightly increased overpotential. The electrode kinetics of these catalysts under OER process were also investigated by 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 of these catalyst increased in the order Ni0.9Fe0.1/NC

Nanocarbon Hybrids as Advanced

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