Nitrogen-Doped Sponge Ni Fibers as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction
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Freestanding N-doped sponge Ni micro/nanofibers exhibit a porous sponge structure.
An N-doping strategy is adopted to optimize the catalytic activity.
γ-NiOOH is identified as active phase by XPS and NEXAFS analyses.
KeywordsOxygen evolution reaction Electrocatalysis Nickel Sponge Structure Electrochemical energy conversion
Owing to the huge concerns associated with serious environmental pollution and rapid fossil energy consumption, the development of clean and renewable energy technologies has become a vital task [1, 2, 3, 4]. The oxygen evolution reaction (OER), as a key process in water splitting and rechargeable metal-air batteries, has attracted considerable attention for decades [5, 6, 7, 8, 9, 10, 11, 12, 13, 14]. However, the OER is a kinetically sluggish process with a high overpotential, which calls for efficient electrocatalysts that can reduce the overpotential and improve the reaction efficiency [15, 16, 17, 18, 19]. Currently, noble metal oxides such as iridium/ruthenium oxides (IrO2/RuO2) set the benchmark for OER electrocatalysts [20, 21, 22]. However, the scarcity, prohibitive cost, and poor long-term durability of these materials restrict their wide application [23, 24]. Therefore, it is highly desirable to develop high-performance and cost-effective OER catalysts.
In this context, transition metals are considered as potential alternatives and are attracting worldwide attention due to their reasonable cost, natural abundance, high conductivity, and outstanding stability [25, 26, 27, 28, 29]. Among these systems, nickel-based materials show a promising potential and have been extensively studied as efficient electrocatalysts for the water oxidation reaction. Nevertheless, nickel-based composites in bulk form are not competitive for electrocatalytic applications, owing to their low specific surface area and lack of exposed reactive sites. Hence, appropriate strategies, including compositing with conductive matrices or supports, rational structure design, and doping with heteroatoms, have been adopted in order to improve their electrocatalytic activity. For instance, Xu and coworkers designed nickel nanoparticles encapsulated in N-doped graphene (denoted as Ni@NC) by annealing a Ni-based metal–organic framework (MOF) and achieved an overpotential of 280 mV at the current density of 10 mA cm−2, with a small Tafel slope of 45 mV dec−1 . Liu et al.  fabricated nickel nanoparticles encapsulated in N-doped carbon nanotubes (Ni/N–CNTs) exhibiting an overpotential of 590 mV at 10 mA cm−2 and a Tafel slope of 138 mV dec−1, as well as high OER durability. Despite the enhanced electrochemical performance, the above materials still suffer from poor active site exposure and limited contact with electrolyte due to annealing-induced aggregation at high temperatures and structural collapse during rapid evolution of oxygen gas, resulting in significant performance degradation . At variance with powders and substrate-assisted materials, self-supported binder-free metal electrocatalysts can be directly used as electrodes with increased exposure of active sites and improved electrical conductivity; this avoids the use of binders and additives while enabling full utilization of the electrode–electrolyte interface, leading to remarkable catalytic performance. More recently, nano/microstructured sponge Ni with high electrical conductivity has emerged as a novel self-supported metal network, but no OER applications have been reported.
In the present work, we report for the first time N-doped sponge nickel (denoted as N-SN), composed of interconnected Ni micro/nanofibers, as a binder-free high-efficiency OER catalysts; the N-SN material was prepared by a hydrothermal method followed by annealing in NH3 and exhibits a unique 3D porous structure and high electronic conductivity. The as-prepared N-SN micro/nanofibers have an open porous framework and consist of secondary nanosheets, which can significantly increase the surface area accessible to the electrolyte and expose a higher number of active sites. Due to the N-doping strategy and favorable conductive sponge architecture, the N-SN catalyst displays a remarkable OER performance, with a relatively low overpotential of 365 mV (vs. reversible hydrogen electrode, RHE) at 100 mA cm−2, a Tafel slope of 33 mV dec−1, and high stability. Further analyses, including X-ray photoelectron spectroscopy (XPS) and near-edge X-ray adsorption fine structure (NEXAFS) tests, were used to investigate the electrocatalytic OER mechanism of N-SN. Our study can draw considerable attention on substrate-free metal materials as high-performance electrocatalysts.
2.1 Material Synthesis
In a typical synthesis, 1 g nickel acetate (Ni(Ac)2) was first dissolved in 100 mL deionized water and then, 10 mL hydrazine hydrate was added dropwise to the resulting aqueous solution. After stirring for 30 min, the above solution was transferred into a Teflon-lined steel autoclave, which was kept at 150 °C for 12 h. After naturally cooling and drying overnight, sponge Ni was obtained successfully. Then, the as-prepared sponge Ni was annealed at 300 °C for 2 h under NH3 (50 sccm) atmosphere to obtain N-SN. For comparison, N-doped Ni foam (N-NF) was synthesized by following a similar procedure to that used to prepare N-SN, except that Ni foam (NF) was used as the skeleton. Commercial nickel foam (1.0 × 1.0 cm2) was ultrasonically cleaned before use in hydrochloric acid (1 mol L−1), ethanol, and deionized water.
2.2 Material Characterization
The morphologies and microstructures of the samples were characterized by field-emission scanning electron microscopy (FESEM, SU8010) and high-resolution transmission electron microscopy (HRTEM, JEM 2100F). X-ray diffraction (XRD) patterns were collected on a Rigaku D/Max-2550 instrument with Cu Kα radiation. XPS measurements were performed using an ESCALAB 250Xi spectrometer with an Al Kα source. Brunauer–Emmett–Teller (BET) surface area distributions were obtained with a pore size analyzer (JW-BK112). Ni L-edge NEXAFS spectra were measured at the photoemission end station of beamline BL10B of the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. A bending magnet was connected to the beamline, equipped with three gratings covering photon energies from 100 to 1000 eV. In this experiment, the samples were kept in the total electron yield mode under an ultrahigh vacuum at 5 × 10−10 mbar. The resolving power of the grating was typically E/∆E = 1000, and the photon flux was 1 × 10−10 photons s−1. Spectra were collected at energies from 831.4 to 884.6 eV in 0.2 eV energy steps.
2.3 Electrochemical Measurements
The OER performances of all samples were tested in a typical three-electrode configuration using an electrochemical workstation (CH Instrument 660D). The synthesized samples (1.0 × 1.0 cm2) were used as the working electrode, while a standard Hg/HgO electrode and a Pt foil were used as the reference and counter electrode, respectively. The electrolyte was a 1 M KOH aqueous solution. Potentials were referenced to the RHE by adding 0.9254 V. Twenty cyclic voltammetry (CV) cycles were performed to obtain a steady current. Then, linear sweep voltammetry (LSV) curves were obtained at the scan rate of 5 mV s−1 in the potential range from 0.2 to 1.2 V versus Hg/HgO electrode. Tafel slopes were derived from the LSV curves. Moreover, electrochemical impedance spectroscopy (EIS) measurements were performed at the same polarization voltage for each sample, with a current density of around 10 mA cm−2 and within the frequency range from 0.01 Hz to 100 kHz. In order to evaluate the stability of the samples, long-term chronopotentiometry measurements were continuously conducted for 24 h at a constant current density of 10 mA cm−2.
3 Results and Discussion
BET measurements were carried out to examine the porous nature and determine the surface area of the samples. The N2 adsorption/desorption isotherm curves are shown in Fig. S3. The as-prepared N-SN electrode exhibits a specific surface area of 44.4 m2 g−1, much larger than that of SN (23.7 m2 g−1), N-NF (13.6 m2 g−1), and NF (6.2 m2 g−1), indicating that the N doping and sponge structure result in larger surface areas and are beneficial for exposing a higher number of active sites, resulting in improved utilization of the active materials.
The electrochemical active surface areas (ECSAs) of N-SN, SN, N-NF, and NF were estimated from the electrochemical double-layer capacitance (Cdl) of each catalytic surface. The plots in Fig. 4d were obtained by measuring the non-Faradaic capacitive current associated with double-layer charging from the scan rate dependence of the cyclic voltammograms (Fig. S4). The ECSA is expected to be linearly proportional to the Cdl value, which is equal to half the slope of the plot . It should be pointed out that the Cdl value of N-SN is about 44.5 mF cm−2, about seven times higher than that of N-NF (6.2 mF cm−2), as well as higher than that of SN (19 mF cm−2) and NF (1.8 mF cm−2). These obvious differences indicate that the tailored design and fabrication of N-doped SN samples with micro/nanoscale structures lead to a marked increase in the number of active sites for OER, resulting in enhanced OER catalytic properties. On the other hand, the Cdl values of N-SN and N-NF are much higher than those of SN and NF, respectively, indicating that the N-doping strategy also plays an important role in increasing the electrochemical active surface area. It can be thus be concluded that the combination of N doping and micro/nanostructure design takes full advantage of the available electrochemical active sites, improving the catalytic performance of the samples. The turnover frequency (TOF) is regarded as the best parameter to compare the intrinsic activities of electrocatalysts at various loadings. Assuming that all metal atoms in the samples are active and accessible to the electrolyte, the TOF values can be obtained according to the equation TOF = iNA/(4FNatoms) where i, NA, F, and Natoms represent the current density at a specific overpotential, the Avogadro constant, the Faraday constant, and the number of atoms or active sites, respectively [43, 44]. The TOF of N-SN at the overpotential of 400 mV is calculated to be 1.190 s−1, which is much higher than that of SN (0.290 s−1), N-NF (0.137 s−1), and NF (0.065 s−1), indicating a better intrinsic activity of N-SN. In addition, the Faradaic efficiency was obtained by comparing the measured gas volume with the theoretical one. The Faradaic efficiency of N-SN is about 100%, because of the good agreement between the experimental and calculated volumes of evolved O2 (Fig. S5).
To further illustrate the OER mechanism of N-SN, XPS and NEXAFS were used to examine the samples after 100 cycles and identify the actual active materials on the surface of N-SN (Fig. 5). According to the XPS analysis, the active peaks before and after the OER cycles change considerably, indicating that pure N-SN is not the active surface materials for OER and new products are produced during the OER process. The Ni 2p spectra (Fig. 5a, b) show a marked decrease in the intensity of the peak at 852.8 eV, characteristic of Ni metal, while the previous peak at 870.1 eV disappears. At the same time, the existence of Ni3+ species is evidenced by the peaks at 857.2 and 872.7 eV, assigned to γ-NiOOH originating from the surface oxidation of N-SN, which proceeds as follows: Ni2+ + 3OH− − 3e− → NiOOH + H2O . γ-NiOOH is probably the active species actually contributing to the OER process. The binding energy at 856.4 eV corresponds to Ni(OH)2, because γ-NiOOH is not stable and will transform into Ni(OH)2 as follows: NiOOH + e− + H2O → Ni(OH)2 + OH− . Moreover, the peaks at 855.4 and 873.7 eV, attributed to NiO, remain almost unchanged because of the surface oxidation of N-NS. Figure 5c shows peaks at 399.0 and 400.5 eV corresponding to N–Ni and N–O bonds, respectively. The separation and shift of the N 1s region reflect a change in chemical conditions after 100 cycles. A clear shift in the main O 1s peaks is observed in Fig. 5d. The peak located at 530 eV is characteristic of γ-NiOOH, which is the typical species consistent with lattice oxygen . This clearly demonstrates the existence of γ-NiOOH, consistent with the Ni 2p spectra discussed above. The two remaining peaks at 531.4 and 532.3 eV correspond to surface hydroxyl groups and adsorbed water, respectively. As shown in Fig. 5e, f, the L-edge NEXAFS results show the local structural variation around Ni sites in N-SN. The L-edge NEXAFS technique is one of the best tools to investigate the electronic structure of first-row transition metals. The initial N-SN shows a sharp L3 maximum near 854.7 eV and a relatively broad L2 edge, while a primary L3 peak near 855.0 eV and a relatively similar L2 region are observed after 100 cycles. Generally, the average L3 absorption centroid shifts to higher energies as Ni is oxidized from NiI to NiII, NiIII, and NiIV. In other words, the L3 peak of N-SN shifts ~ 0.3 eV higher after 100 cycles, which indicates that Ni is oxidized to higher valence states during the OER process. In addition, as shown in the SEM images of N-SN after 100 cycles (Figs. S6, S7), the morphology of N-SN changes significantly and new small nanosheets are formed on the pristine secondary nanosheets due to the growth of γ-NiOOH and Ni(OH)2, in agreement with the XPS and NEXAFS analyses discussed above.
We have rationally designed and fabricated N-doped sponge nickel as a novel and efficient OER electrocatalyst. Using a hydrothermal method combined with a thermal treatment in NH3 atmosphere, a self-supported N-SN consisting of micro/nanostructured fibers was successfully synthesized. The new material exhibits a 3D porous sponge skeleton with increased accessible surface area. The N-SN electrode shows high conductivity, large surface area, and abundant active sites, which result in excellent electrocatalytic performance, with low overpotential and high cycling stability. XPS and NEXAFS measurements were used to study the OER mechanism of N-SN; γ-NiOOH, originating from the oxidation of N-SN in alkaline solution, is identified as the actual active material for OER. In this work, we have not only demonstrated the potential of N-SN as a novel electrocatalyst, but also provided insights into how the sponge structure and N-doping strategy can enhance the electrocatalytic performance.
This work is supported by National Natural Science Foundation of China (Nos. 51728204 and 51772272), Fundamental Research Funds for the Central Universities (2018QNA4011), Qianjiang Talents Plan D (QJD1602029), and Startup Foundation for Hundred-Talent Program of Zhejiang University. The authors acknowledge the SEM/TEM support from Qiaohong He, Xiaokun Ding, and Fang Chen from Department of Chemistry, Zhejiang University.
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