Ultrafine NiFe clusters anchored on N-doped carbon as bifunctional electrocatalysts for efficient water and urea oxidation

Jingfang Zhang *a, Fei Xing a, Hongjuan Zhang a and Yi Huang *b
aDepartment of Chemistry, College of Science, Hebei Agricultural University, Baoding 071001, China. E-mail: zjf1991211@163.com
bDepartment of Chemistry, School of Science, Tianjin University, Tianjin 300072, China. E-mail: yihuang@tju.edu.cn; huangyibarry@126.com

Received 12th July 2020 , Accepted 10th August 2020

First published on 10th August 2020


Hydrogen production through electrocatalysis is crucial in renewable energy technologies but significantly impeded by sluggish anodic reactions. Developing bifunctional anode noble-metal-free electrocatalysts towards oxygen evolution reaction (OER) and urea oxidation reaction (UOR) to boost cathodic hydrogen evolution reaction (HER) is promising but challenging to meet different reaction media and multiple applications for simultaneous clean energy production and pollution treatment. Herein, a facile one-pot thermal treatment strategy is presented to anchor NiFe nanoclusters (with a size of about 2 nm) on N-doped carbon as bifunctional electrocatalysts for both OER and UOR. Such an electrocatalyst can deliver a current density of 20 mA cm−2 with a low overpotential of 260 mV and a small Tafel slope of 42 mV dec−1 for OER, superior to the state-of-the-art Ru-based materials. Besides, this electrocatalyst also shows excellent activity for UOR with the need for just 1.37 V (vs. RHE) to attain a current density of 100 mA cm−2. In a two-electrode electrolyzer for both cathodic HER and anodic UOR, only a cell voltage of 1.50 V is required to drive a current density of 10 mA cm−2, which is 140 mV lower than that of overall water splitting electrolysis (1.64 V). The excellent electrooxidative performance can be attributed to the improved conductivity, abundant active sites and fast charge transfer and transport benefiting from the ultrafine structure of NiFe clusters and their synergistic effect with N-doped carbon.


1. Introduction

The excessive depletion of fossil fuels and increasing environmental issues have stimulated considerable research on sustainable energy storage and conversion systems.1 Electrocatalytic water splitting is considered as one of the most promising technologies to generate clean energy carriers-hydrogen.2–4 However, the half reaction of water splitting, oxygen evolution reaction (OER), exhibits sluggish kinetics and thus becomes the bottleneck in water splitting technologies.5 Currently, intense efforts have been focused on two aspects to remove the obstacles: (1) breaking the scaling relation of HOO* and HO* intermediates to achieve better OER activity;6,7 (2) exploring alternative anodic reaction to replace OER.8–13 Urea oxidation reaction (UOR) has recently triggered great interest because of the lower equilibrium potential (0.37 V for UOR vs. 1.23 V for OER)14–16 as well as its wide applications in fuel cells,17 energy storage18 and wastewater treatment.19 Importantly, UOR can not only provide electrons for energy-saving H2 production from urea-rich wastewater but also complete de-ureation of wastewater process simultaneously.20–22 However, similar to OER, UOR also suffers from sluggish kinetics owing to a 6e transfer process.8,23 Hence, active electrocatalysts are desirable to accelerate OER/UOR reaction kinetics. To date, precious-metal-based catalysts are frequently used as the benchmark electrocatalysts for OER/UOR.20,24,25 However, their scarcity and high cost hinder the large-scale application. Therefore, developing earth-abundant and highly efficient OER/UOR bifunctional electrocatalysts is urgently demanded but remains a huge challenge.

Transition metal (Ni, Co, Fe, Mn, etc.) based nanomaterials hold for great promise for replacing precious-metal catalysts towards OER/UOR due to their low cost, abundance, and good stability.14,26–31 Among these candidates, NiFe-based compounds have attracted tremendous attention.32–38 Despite significant progress, NiFe-based bimetallic electrocatalysts are still facing the problems of poor conductivity and unsatisfactory activity in terms of both OER and UOR. Thus, optimization of NiFe bimetallic electrocatalysts to enhance the OER/UOR performance is a vital step. Several commonly used strategies are proposed to modify the metal-based electrocatalysts, such as (1) downsizing the electrocatalysts (below 5 nm in size) to significantly increase the electrochemically active surface area and the exposed active sites.32,39–41 However, small-sized bimetallic nanocrystals are prone to aggregate or cause phase segregation in the process of preparation and post-treatment owing to their large surface energy, making it hard for their synthesis and further use. (2) Hybridizing with heteroatom-doped carbon matrix to increase the conductivity, alter the electronic structure and inhibit the aggregation of ultrafine structure.42–46 In this regard, it is anticipated that the bimetallic electrocatalysts synthesized by combining the above strategies will generate remarkable electrocatalytic performance. Therefore, design and synthesis of small-sized NiFe anchored on heteroatom-doped carbon carriers is a challenging but meaningful task.

Herein, we demonstrated a facile one-pot thermal treatment strategy to synthesize NiFe nanoclusters (with a size of about 2 nm) decorated on N-doped carbon hybrids (NiFe/N–C) as efficient electrocatalysts towards both OER and UOR. The NiFe/N–C was achieved by employing citrate sodium as a chelating agent for anchoring nickel and iron ions on commercial carbon (Ketjenblack EC-600JD), and melamine with a high nitrogen content of 67 wt% as a nitrogen source to produce carbon nitrogen species in subsequent pyrolysis for doping nitrogen into carbon matrix. As expected, the NiFe/N–C catalysts show enhanced activity with a low overpotential of 260 mV at 20 mA cm−2 and a small Tafel slope of 42 mV dec−1 for OER. Meanwhile, the NiFe/N–C catalysts exhibit excellent electrocatalytic UOR activity with a current density of 100 mA cm−2 at 1.37 V vs. RHE. The enhanced OER and UOR performance is ascribed to improved conductivity, abundant active sites, and accelerated electron-transfer rate originating from the ultrafine nanostructure of NiFe clusters and their synergistic electronic effect with N-doped carbon.

2. Experimental

2.1 Material preparation

2.1.1 Synthesis of the NiFe/N–C. In a typical procedure, 60 mg of Ketjenblack (EC-600JD) were mixed with 44 mg of Ni(NO3)2·6H2O, 42 mg of FeSO4·7H2O and 221 mg of sodium citrate in 10 mL of ultrapure water. The molar ratio of Fe/Ni is close to 1. The solution was kept stirring for 3 hours and then rotary evaporated at 75 °C to remove the water. The obtained dry gel was well mixed with melamine with a mass ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]4. The obtained mixture was transferred to a quartz tube and calcined at 900 °C for 2 hours with a heating rate of 5 °C min−1 in argon atmosphere. Various NiFe/N–C samples with different mole ratios of Fe and Ni were synthesized, recorded as NiFe/N–C-x (here x stands for the mole ratio of Fe/Ni). The synthesis of NiFe/N–C-x (x = 0.5 and 1.5) is similar to that of NiFe/N–C, except with different amounts of FeSO4·7H2O and Ni(NO3)2·6H2O (28 mg and 58 mg for NiFe/N–C-0.5, respectively; 50 mg and 35 mg for NiFe/N–C-1.5, respectively).
2.1.2 Synthesis of the NiFe nanoparticles decorated on carbon (NiFe/C). The NiFe/C was prepared with the same procedure as that of NiFe/N–C except without adding melamine.
2.1.3 Synthesis of the N-doped carbon (N–C). The Ketjenblack (EC-600JD) was well mixed with melamine with a mass ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]4. The obtained mixture was transferred to a quartz tube and calcined at 900 °C for 2 hours with a heating rate of 5 °C min−1 in argon atmosphere.

2.2 Material characterization

The X-ray diffraction (XRD) patterns of samples were recorded on a Rigaku Miniflex-600 with Cu Kα radiation (λ = 0.15406 nm). Scanning electron microscope (SEM) images are recorded by a JEOL 7600F microscope coupled with energy-dispersive X-ray spectroscopy (EDS). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were determined by using a FEI Tecnai G2 F20 system. X-ray photoelectron spectroscopy (XPS) was performed on a photoelectron spectrometer using Al Kα radiation as the excitation source (PHI 5000 VersaProbe). All the peaks were calibrated with C 1s spectrum at binding energy of 284.8 eV. Inductively coupled plasma mass spectrometry (ICP-MS) was carried out on an Agilent 7700× gas chromatograph equipped with an Auto sampler Injector.

2.3 Electrochemical measurements

Electrochemical measurements were carried out in a typical three-electrode cell consisting of a working electrode, a glassy carbon counter electrode, and a Hg/HgO (1 M KOH) reference electrode using an electrochemical workstation (CHI 760E, Chenhua). For a typical procedure for fabricating the working electrode, 2 mg of catalyst was dispersed in 1 mL of a mixture solvent (the volume ratio of ethanol: 5% Nafion is 1[thin space (1/6-em)]:[thin space (1/6-em)]0.020), and the mixture was sonicated for 1 h to get a homogeneous ink. Then 152 μL of the ink (containing 304 μg of catalyst) was dropped onto the clean Ni foam and dried in flowing argon. The loading mass for each catalyst was same (about 0.304 mg cm−2). All the potentials in the text, if not specified, were recorded relative to the reversible hydrogen electrode (vs. RHE) and the current density was normalized to the geometrical surface area. OER and UOR measurements were carried out in 1 M KOH without or with 1 M urea as electrolyte. The scan rate of polarization curves is determined as 5 mV s−1 with 95% iR-compensation. Chronopotentiometric measurements were obtained under the same experimental setup without compensating iR drop. The electrochemical impedance spectroscopy (EIS) measurements were carried out in the same configuration at 1.52 V (vs. RHE) from 100 kHz to 0.01 Hz. The faradaic efficiency was calculated by comparing the amount of gas theoretically calculated and experimentally measured. The gas experimentally generated from the water splitting was collected by water-gas displacing method. The theoretical amount of O2 was calculated by applying the Faraday law.

2.4 Calculation of electrochemically active surface area (ECSA)

The ECSA was measured by cyclic voltammetry (CV) at no apparent faradaic potential with different scan rates of 20, 40, 60, 80, 100 and 120 mV s−1. By plotting the current density Δj ((jajc)/2) at 1.06 V against the scan rate, the linear slope is the double layer capacitance (Cdl). The ECSA was calculated by dividing Cdl by the specific capacitance value for a flat standard with 1 cm2 of real surface area. The specific capacitance for a flat surface is normally taken to be in the range of 20–60 μF cm−2. In this study, we assume 40 μF cm−2 for the calculation of ECSA.

2.5 Calculation of turnover frequency (TOF)

The TOF was calculated according to the following equation:
image file: d0dt02459g-t1.tif
where j is the measured geometrical current density at a given overpotential of 300 mV, A is the surface area of the electrode, the number 4 represents four electron transfer for per mole of O2, F is the Faraday constant, and n is the number of moles of the Ni and Fe atom on the electrode. The amount of Ni and Fe was obtained from ICP-MS results.

3. Results and discussion

The as-prepared NiFe/N–C hybrid electrocatalyst was first characterized by SEM and TEM. As shown in Fig. 1a and b, the NiFe/N–C has a morphology of cross-linked chain networks, which maintains the structure of commercial Ketjenblack (Fig. S1), and no obvious nanoparticles are observed in the hybrid. HRTEM images in Fig. 1c and d show that nanoclusters (marked by red dashed circle) with sizes of about 2 nm are decorated on the nanochains. The measured lattice spacings of 0.21 nm and an interlayer spacing of 0.34 nm correspond to the (111) facet of NiFe alloy and the (002) plane of graphitic carbon, respectively.47,48 EDS spectrum was conducted to analyze the composition of the hybrid. Fig. S2 shows the coexistence of Ni, Fe, N and C in the hybrid, and the Ni/Fe atomic ratio is 47.04[thin space (1/6-em)]:[thin space (1/6-em)]52.96, which is in consistence with the stoichiometric value of 1[thin space (1/6-em)]:[thin space (1/6-em)]1. The high-angle angular dark field TEM (HAADF-TEM) and the corresponding EDS mapping images in Fig. 1e reveal that Ni and Fe signals are well matched, suggesting the nanoclusters are composed of Ni and Fe. Meanwhile, N are homogeneously distributed on the whole networks. The broad peak between 20° and 30° in the XRD pattern (Fig. 1f) of NiFe/N–C is corresponded to the (002) plane of graphitic carbon, which is similar with that of N–C catalyst prepared without the addition of Ni and Fe sources, indicating an amorphous structure of carbon matrix.49 The three distinct diffraction peaks located around 43.6, 50.8, and 74.7° in NiFe/N–C can be assigned to the (111), (200), and (220) crystal-plane reflections of a face-centered cubic NiFe alloy phase (JCPDS Card no. 47-1405), respectively.
image file: d0dt02459g-f1.tif
Fig. 1 Morphology and structure characterizations. (a) SEM, (b) TEM and (c and d) HRTEM images of NiFe/N–C. The red dashed circles in (c and d) indicate the NiFe clusters. (e) HAADF-TEM image of NiFe/N–C and the corresponding EDS mappings of C, N, Ni, and Fe. (f) XRD patterns of NiFe/N–C and N–C samples.

The XPS was conducted to analyze the chemical composition and elemental states of NiFe/N–C sample. The XPS results show the weight ratio of Ni/Fe is 1.07, in accordance with ICP-MS results (1.04). In the C 1s region (Fig. 2a), the three peaks centered at ∼284.5, ∼285.3, and ∼286.4 eV correspond to C–C, C–N, and C–O, respectively.42 The N 1s spectrum in Fig. 2b was deconvoluted into three types of nitrogen species located at 398.3, 400.1, and 401.8 eV, corresponding to pyridinic-N, pyrrolic-N, and graphitic-N, respectively.42 The presence of N 1s signals indicates the successful incorporation of N into carbon matrix, in line with the previous work.50 Note that pyridinic-N dopants can facilitate the adsorption of OER intermediates (OH, OOH) and further accelerate the formation of OHads due to their excellent electron-accepting ability, which are helpful to improve the OER activity.51 Furthermore, the N dopant is important for the synthesis of ultrafine NiFe clusters. Large NiFe nanoparticles with a size ranging from 50–300 nm were observed in NiFe/C prepared without the adding of N dopant (Fig. S3 and S4). For the Ni 2p3/2 spectrum (Fig. 2c), the peak centered at 852.1 eV was attributed to the metallic state of Ni. The peaks located at 855.5 and 861.5 eV can be ascribed to Ni2+, which may be caused by the surface oxidation.52–54 For the Fe 2p3/2 spectrum (Fig. 2d), the peaks located at 707.8 eV revealed the metallic state of Fe in NiFe/N–C, and the peaks located at 710.8 and 712.5 eV were attributed to Fe2+ and Fe3+, respectively, which are probably due to the oxidation of the sample in the ambient environment.55 All in all, according to the above results, we can conclude that NiFe/N–C hybrid catalysts were successfully synthesized. Moreover, such a facile strategy can be easily applied to gram–scale synthesis of the NiFe/N–C catalysts (Fig. S5).


image file: d0dt02459g-f2.tif
Fig. 2 XPS characterization. Deconvoluted (a) C 1s, (b) N 1s, (c) Ni 2p3/2, and (d) Fe 2p3/2 high-resolution XPS spectra for the NiFe/N–C.

The electrocatalytic properties of the NiFe/N–C catalysts for OER were first evaluated in 1 M KOH electrolyte by using a standard three-electrode system. Fig. 3a shows linear sweep voltammetry (LSV) curves (with iR correction) of NiFe/N–C, NiFe/C, N–C, RuO2, and bare Ni foam towards OER (Fig. S6 without iR correction). The oxidation peak around 1.36 V for NiFe/N–C and NiFe/C could be attributed to the conversion of Ni2+ to Ni3+/4+ species.56,57 The NiFe/N–C exhibited an excellent OER performance with much smaller overpotentials of 260 and 287 mV to reach 20 and 50 mA cm−2, respectively, compared with NiFe/C (270 and 311 mV), N–C (390 and 450 mV), state-of-the-art commercial RuO2 (310 and 370 mV), and bare Ni foam (398 and 460 mV) (Fig. 3b). Specially, the current density of NiFe/N–C driven at 1.55 V is 189.90 mA cm−2, 3.17 times, 39.8 times and 8.1 times than that of NiFe/C (59.95 mA cm−2), N–C (4.77 mA cm−2) and RuO2 (23.32 mA cm−2), respectively, highlighting the important role of ultrafine NiFe nanoclusters and their synergistic effect with N-doped carbon. The outstanding catalytic activity of NiFe/N–C was also reflected by TOFs and Tafel slopes. The NiFe/N–C shows a high TOF value (0.263 s−1) at an overpotential of 300 mV, which is 2.5 times higher than that of NiFe/C (0.105 s−1), indicating the high intrinsic activities of NiFe/N–C. The Tafel slope of 42 mV dec−1 was observed for NiFe/N–C, much smaller than NiFe/C (80 mV dec−1), N–C (151 mV dec−1), and commercial RuO2 (113 mV dec−1), respectively, indicating a more favorable and faster OER kinetics for NiFe/N–C (Fig. 3c). As expected, NiFe/N–C exhibits a mass activity of 500 A g−1 with an overpotential of 315 mV towards OER, superior to that of other control catalysts (Fig. S7). These results verify that NiFe/N–C is among the most active non-noble-metal/carbon hybrid electrocatalysts for OER (Fig. 3d, Fig. S8, Table S1).3,42,46,48,58–62 The multi-step chronopotentiometric curve for NiFe/N–C in Fig. 3e was conducted with the current density being increased from 50 to 250 mA cm−2 (50 mA cm−2 per 500 s). The potentials at each step remained constant, implying the strong durability of NiFe/N–C. Moreover, the immediate response time (within 0.1 s) of potentials to current ramps indicated the excellent mass transportation ability of NiFe/N–C. In order to investigate the long-term stability and durability of NiFe/N–C, continuous CV cycling and chronoamperometry measurements were carried out. The polarization curves before and after 1000 cycles are almost overlapping, indicating its high stability during the catalytic process (Fig. 3f). Additionally, the chronoamperometry curve exhibited a negligible loss for ∼12 h, demonstrating its excellent durability (inset in Fig. 3f). The faradaic efficiency was calculated to be ∼100% (Fig. S9), indicating the high utilization efficiency of the active sites on NiFe/N–C during the OER process. To investigate the effect of Fe/Ni ratio on OER activity, the NiFe/N–C with different mole ratios of Fe/Ni were synthesized. The LSV curves show that the NiFe/N–C with the Ni/Fe mole ratio of 1 exhibits the highest activity (Fig. S10).


image file: d0dt02459g-f3.tif
Fig. 3 Electrocatalytic measurements of catalysts for OER. (a) Polarization curves for NiFe/N–C, NiFe/C, N–C, RuO2, and bare Ni foam. (b) The overpotentials for NiFe/N–C, NiFe/C, N–C, and RuO2 at the specific current densities. (c) Tafel plots for NiFe/N–C, NiFe/C, RuO2, and N–C. (d) Comparison on the overpotentials at 10 mA cm−2 and Tafel slopes of recently reported non-noble-metal/carbon hybrid electrocatalysts (FeNi@NC,3 NiFe/N-CNT,42 HCM@Ni-N,46 FeNi@NC-NG,48 NiFe/CNx,58 FeCoNi@G,59 NiFe@C,60 Co-N–C,61 Co@C62). (e) Multi-current process of NiFe/N–C at several different current densities (started at 50 mA cm−2 and ended at 250 mA cm−2, with an increment of 50 mA cm−2 per 500 s without iR correction) (inset: partially enlarged curve). (f) Polarization curves of NiFe/N–C before and after 1000 CV cycles. Inset: chronopotentiometric curve of NiFe/N–C with a constant current density of 25 mA cm−2 for about 12 h.

To further analyze the origins of superior activity of NiFe/N–C, ECSA and EIS were carried out. Based on the CV measurements performed at various scan rates in the non-faradaic region, electrochemical Cdl can be calculated (Fig. 4).63 The Cdl of NiFe/N–C (4.61 mF cm−2) exhibit 1.6 and 2.3 times than that of NiFe/C (2.95 mF cm−2) and N–C (2.00 mF cm−2), respectively. The ECSAs of NiFe/N–C, NiFe/C and N–C are calculated to be 115.25 cm2, 73.75 cm2, and 50.00 cm2, respectively. The NiFe/N–C possessed larger ECSA, indicate its abundant catalytically active sites towards OER. Nyquist plots results (Fig. S11) demonstrated that the NiFe/N–C exhibited a smaller charge-transfer resistance than NiFe/C and N–C catalyst, further justifying its faster charge transfer between the catalyst and the electrolyte. We conclude that the NiFe/N–C with the characteristics of good conductivity, high ECSA and great stability/durability render an effective OER electrocatalyst.


image file: d0dt02459g-f4.tif
Fig. 4 ECSA measurements of catalysts. The CV curves of (a) NiFe/N–C, (b) NiFe/C, and (c) N–C with different scan rates from 20 to 120 mV s−1. (d) Δj of NiFe/N–C, NiFe/C and N–C catalysts plotted against scan rate at the potential of 1.06 V vs. RHE. The linear slope, equivalent to twice the Cdl, was used to represent the ECSA.

To evaluate the UOR performance of NiFe/N–C, LSV was tested in 1 M KOH with the presence of 1 M urea. Fig. 5a shows the LSV curves of NiFe/N–C in KOH with and without urea (corresponding to UOR and OER, respectively). It can be observed that NiFe/N–C exhibits much higher current densities after adding urea, demonstrating its high catalytic response activity for UOR. Specifically, NiFe/N–C demands a lower potential (1.37 V vs. RHE) to attain a current density of 100 mA cm−2 towards UOR than that of OER (1.53 V vs. RHE). The NiFe/N–C exhibits enhanced UOR activity, which can be confirmed by higher current density compared with NiFe/C, N–C, and bare Ni foam (Fig. 5b). The UOR activity is superior to those of most reported Ni(Fe)-based electrocatalysts (Table S2).14,22,34,53 Furthermore, as shown in Fig. 5c, the Tafel slope of NiFe/N–C can be calculated about 15 mV dec−1, which is much lower than that of NiFe/C (21 mV dec−1) and N–C (56 mV dec−1). The durability is also an important criterion for judging electrocatalysts. After a continuous potential cycling test (1000 cycles), negligible changes in current densities were observed, suggesting the robust durability of NiFe/N–C towards UOR (Fig. 5d). Then, an overall urea electrolyser with a two-electrode system using NiFe/N–C as the anode and Pt foil as the cathode was constructed. For comparison, an overall water-splitting cell was also constructed with the same electrode configuration. As shown in Fig. 5e, NiFe/N–C||Pt couple system required only a cell voltage of 1.50 V to drive current density of 10 mA cm−2 for urea electrolysis, which is 140 mV lower than that for overall water splitting electrolysis (1.64 V), revealing the high activity of NiFe/N–C as the electrode of urea electrolyser. The stability of overall urea electrolysis system was examined by chronopotentiometric measurement in 1 M KOH with 1 M urea. As shown in Fig. 5f, the cell voltage of electrolysis system can maintain constant for over 14 h. In addition, the SEM, TEM and XPS characterization of NiFe/N–C after electrocatalytic measurements were performed. The SEM and TEM images (Fig. S12 and S13 and inset of Fig. 5d) collected after a series of experiments show that the morphology of the cross-linked chain networks and the structure of ultrafine clusters are still preserved. The XPS after electrochemical test suggests the formation of oxidized Ni and Fe species (Fig. S14), which may act as the actual active site for electrooxidation reactions.12,13


image file: d0dt02459g-f5.tif
Fig. 5 Electrocatalytic measurements of catalysts for UOR. (a) LSV curves of NiFe/N–C in 1 M KOH with and without 1 M urea. (b) LSV curves and (c) Tafel slopes of NiFe/N–C, NiFe/C, N–C, and bare Ni foam in 1 M KOH with 1 M urea. (d) LSV curves of NiFe/N–C before and after 1000 cycles for UOR. (e) LSV curves for NiFe/N–C||Pt couple in 1 M KOH with and without 1 M urea. Inset: schematic illustration of a two-electrode electrolyzer. (f) The long-term stability test of urea electrolysis performed in a two-electrode configuration with a constant current density of 10 mA cm−2. Inset: the HRTEM image of NiFe/N–C after stability test.

4. Conclusions

In summary, we have demonstrated an effective approach to synthesize NiFe nanoclusters with size of about 2 nm anchored on N-doped carbon as efficient and stable electrocatalysts towards both OER and UOR. Their outstanding activities are mainly attributed to the improved conductivity, abundant active sites as well as enhanced electron transfer and transport, highlighting the ultrafine nanoscale NiFe and their synergistic effect with N-doped carbon. More importantly, this work may pave a new way for the development of other novel carbon materials with the decoration of ultrafine active metal nanoclusters for diverse applications in renewable energy storage and conversion.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Youth Program of National Natural Science Foundation of China (21805069), Natural Science Foundation for Excellent Young Scholars of Hebei Province (B2019204269), and Foundation of Hebei Agricultural University (ZD201716 and PT2018001).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d0dt02459g

This journal is © The Royal Society of Chemistry 2020