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Article

Direct One-Step Growth of Bimetallic Ni2Mo3N on Ni Foam as an Efficient Oxygen Evolution Electrocatalyst

1
Department of Chemical Engineering, Interdisciplinary Program in Advanced Functional Materials and Devices Development, Kangwon National University, Chuncheon 24341, Korea
2
Department of Chemistry Education, Chonnam National University, Gwangju 61186, Korea
*
Authors to whom correspondence should be addressed.
Materials 2021, 14(16), 4768; https://doi.org/10.3390/ma14164768
Submission received: 28 June 2021 / Revised: 18 August 2021 / Accepted: 19 August 2021 / Published: 23 August 2021
(This article belongs to the Special Issue Advances in Nanostructured Catalysts)

Abstract

:
A simple and economical synthetic route for direct one-step growth of bimetallic Ni2Mo3N nanoparticles on Ni foam substrate (Ni2Mo3N/NF) and its catalytic performance during an oxygen evolution reaction (OER) are reported. The Ni2Mo3N/NF catalyst was obtained by annealing a mixture of a Mo precursor, Ni foam, and urea at 600 °C under N2 flow using one-pot synthesis. Moreover, the Ni2Mo3N/NF exhibited high OER activity with low overpotential values (336.38 mV at 50 mA cm−2 and 392.49 mV at 100 mA cm−2) and good stability for 5 h in Fe-purified alkaline electrolyte. The Ni2Mo3N nanoparticle surfaces converted into amorphous surface oxide species during the OER, which might be attributed to the OER activity.

1. Introduction

Hydrogen (H2) is a promising energy carrier due to its high mass-specific energy density (142 MJ kg−1), high utilization efficiency, and zero carbon emission when generated from renewable energy sources. Electrochemical water splitting from renewables such as solar or wind energy is considered a clean and efficient route for hydrogen production [1,2,3,4]. Water splitting consists of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The four electrons involved in OER (4OH → 2H2O + O2 + 4e) are kinetically sluggish relative to the two electrons involved in HER, requiring large overpotential values [5,6,7,8,9]. Ir- and Ru-based materials are typical catalysts for OER, but their high cost and scarcity restrict their widespread application [10,11,12]. Thus, developing alternative OER electrocatalysts based on low-cost and abundant materials is urgent for the large-scale proliferation of water-splitting systems.
Materials that include transition metals, such as transition metal oxides, transition metal nitrides (TMNs), and transition metal oxynitrides, demonstrated very promising OER activity [13,14]. Among them, various monometallic TMNs including Ni3N, Co4N, HfN, and Mn3N2 have been investigated as low-cost electrocatalysts [15,16,17,18]. TMNs possessing physical hardness, chemical stability, electrical conductivity, and unique electronic structure have been traditionally used as catalysts for chemical processes [19,20], and recently showed potential for energy applications [21,22]. However, these monometallic TMNs still exhibit limited OER performance. Designing bimetallic TMNs has proven to be an effective way to improve the OER performance of monometallic TMNs, which is expected to show the synergy between two distinct metal species [23,24]. In bimetallic TMNs, the presence of a second metal atom supplied more active sites and enhanced electronic conductivity, achieving higher OER activity compared to monometallic catalysts [25,26]. Among the various bimetallic TMNs, Ni-Mo nitrides have been extensively explored as OER electrocatalysts due to their high activity and stability. Although progress has been made, Ni-Mo nitrides are traditionally prepared by a complex method involving a two- or multistep annealing process. This typically involves hydrothermal Ni-Mo oxide formation and subsequent nitridation using NH3 gas for Ni-Mo nitride formation, making synthesis a challenge [27,28,29,30,31].
In this work, we report a simple and economical synthetic route for direct one-step growth of bimetallic Ni2Mo3N nanoparticles on Ni foam substrate (Ni2Mo3N/NF) for use as an OER catalyst. The Ni2Mo3N/NF catalyst was prepared by annealing Mo precursor, Ni foam, and urea at 600 °C under N2 flow in one pot. During annealing, inert N2 gas was used in exchange for toxic ammonia gas. In addition, no Ni precursor was added because Ni foam acted as the Ni source. Therefore, the suggested fabrication method is simple, economical, and eco-friendly. The resultant Ni2Mo3N/NF catalyst exhibited impressive OER catalytic performance with small overpotential values of 336.38 and 392.49 mV at current densities of 50 and 100 mA cm−2, respectively, and excellent stability over 5 h of operation at 50 mA cm−2. The high activity and stability with this simple synthetic method suggest that our Ni2Mo3N/NF catalyst could be a promising electrocatalyst for OER.

2. Materials and Methods

2.1. Materials

Molybdenum chloride (MoCl5) was purchased from Alfa Aesar. Urea (CH4N2O), ethanol (C2H5OH), and a 1.0 M potassium hydroxide (KOH) solution were purchased from Samchun. Notably, an Fe-free 1.0 M KOH electrolyte was prepared by following a previously reported method to avoid incidental Fe incorporation and consequent OER activity enhancement during the electrochemical tests [32]. For preparation of an Fe-free 1.0 M KOH solution, Ni(NO3)2·6H2O was dissolved in ultrapure water and 1.0 M KOH was added to precipitate high-purity Ni(OH)2. After three centrifugation and washing cycles, the high-purity Ni(OH)2 solid was mechanically stirred in 1.0 M KOH for at least 10 min and rested for 3 h. The mixture was centrifuged, and the purified KOH supernatant was transferred to a clean bottle and used as an Fe-free electrolyte. The Ni foam was purchased from Goodfellow (Ni003852), having a pore size of ca. 450 μm and a strut diameter of ca. 70 μm. Commercial IrO2 catalysts were purchased from Alfa Aesar (A17849).

2.2. Synthesis of Ni2Mo3N/NF

A measure of 3.66 mmol MoCl5 was dissolved in 2.53 mL ethanol, then 5.49 mmol urea (molar ratio of urea/Mo = 1.5) was added to the solution, which was stirred for 1 h until the urea was completely dissolved. The solution was transferred to an alumina boat with pieces of Ni foam and annealed at 600 °C (ramping at 3.3 °C min−1) for 3 h under flowing N2 gas (100 sccm) to fabricate the Ni2Mo3N/NF electrocatalyst.

2.3. Characterizations

A scanning electron microscope ((SEM, JEOL JSM-7900F) (Jeol, Peabody, MA, USA)) with an energy dispersive X-ray spectrometer (EDS) and a high-resolution transmission electron microscope ((HRTEM, JEM-2100F, JEOL (Acc. Voltage: 200 kV) (Jeol, Peabody, MA, USA)) were used to reveal detailed structural information. Crystalline structures of the prepared catalysts were investigated by X-ray diffraction (XRD, Miniflex 600, Rigaku, Tokyo, Japan) using Cu-Kα (wavelength = 1.5406 Å) radiation at 40 kV and 15 mA. Surface chemical states were analyzed using X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, K-Alpha, Waltham, MA, USA) with an X-ray source of Al-Kα1. The recorded binding energies were calibrated using the adventitious carbon C 1s peak at 284.8 eV. In addition, the XPS spectra for Ni 2p and Mo 3d were deconvoluted to have area ratios of 1:2 (2p1/2:2p3/2) and 2:3 (3d3/2:3d5/2), respectively.

2.4. Electrochemical Tests

Electrochemical characterizations were carried out in a three-electrode cell system using a potentiostat ((PAR, VersaSTAT 4) (Ametek, Berwyn, PA, USA)) under an O2-purged Fe-free 1.0 M KOH solution. The Ni2Mo3N/NF (1 × 1 cm2) was directly used as a working electrode. The Ag/AgCl (3 M NaCl) and Pt wire were used as a reference and counter electrode, respectively. All potentials were converted to the reversible hydrogen electrode (RHE) using the equation (ERHE = EAg/AgCl + 0.059 pH + E°Ag/AgCl). Linear sweep voltammetry (LSV) polarization curves were obtained using iR compensation at a scan rate of 5 mV s−1. The long-term stability test was carried out using the chronopotentiometric method. Electrochemical impedance spectroscopy (EIS) was performed from 105 to 10−1 Hz with a modulation amplitude of 20 mV at 400 mV overpotential, and EIS plots were fitted with Z-view software.

3. Results and Discussion

Figure S1 shows digital photographs for the synthetic procedure. MoCl5 was dissolved in ethanol to form a dark-greenish solution (Figure S1a). At this step, MoCl5 reacts with ethanol vigorously, generating molybdenum orthoester and releasing HCl gas [33,34]. The addition of urea to the solution yielded a viscous Mo-urea complex (Figure S1b) [35,36]. Ni foams and Mo-urea complex were transferred to an alumina boat and annealed at 600 °C for 3 h under N2 flow (Figure S1c,d). Consequently, Ni2Mo3N nanoparticles grown directly on nickel foam were fabricated (Figure S1e,f). During the synthetic procedure, nitrogen was supplied from urea; thus, toxic ammonia gas was not employed for nitridation. Even more rewarding was that no Ni precursor was added for Ni2Mo3N generation because the Ni foam support acted as a Ni source through thermal diffusion [37,38,39,40]. These alterations in the technique enabled our synthetic method to be economical and straightforward.
Before further experiments, the annealing temperatures of Ni2Mo3N/NF samples were optimized. Figure S2 displays the XRD patterns of prepared samples at various annealing temperatures from 550 to 650 °C. A pure crystalline Ni2Mo3N phase was obtained only at an annealing temperature of 600 °C. At 650 °C, mixed phases of Ni2Mo3N and Mo2N were detected in XRD patterns, while at 550 °C, an oxide phase was observed with low peak intensities of Ni2Mo3N. Thus, the annealing temperature of 600 °C was employed as the optimum temperature condition to form Ni2Mo3N/NF.
Figure 1a shows SEM images of Ni2Mo3N/NF. A three-dimensional porous structure stems from the pristine Ni foam, and a rough surface originates from the growth of the Ni2Mo3N particles on the Ni foam. The elemental mapping image of Ni is consistent with Mo and N, indicating that the Ni2Mo3N nanoparticles are uniformly dispersed on the Ni foam substrate. Figure 1b and Figure S3 show the TEM image and the corresponding particle size distribution graph of Ni2Mo3N/NF, where 7.2 ± 1.1 nm Ni2Mo3N nanoparticles are observed without heavy aggregation. The observed lattice fringe of 2.21 Å in the high-resolution TEM (HRTEM) image (Figure 1c) corresponds to the Ni2Mo3N (221) plane.
Figure 2a shows X-ray diffraction (XRD) patterns of Ni2Mo3N/NF. The intense peaks at 45, 52, and 76° can be indexed to metallic Ni (JCPDS no. 00-004-0850) from Ni foam. The other diffraction peaks observed at 40.7, 43.1, 45.3, 72.6, and 77.4° correspond to (221), (310), (311), (510), and (520) planes of reference in cubic Ni2Mo3N patterns (JCPDS no. 01-089-4564). No other phases such as MoO3 or Mo2N were detected; hence, phase-pure Ni2Mo3N was grown on the Ni foam. The Ni2Mo3N possesses a filled β-manganese structure composed of corner-sharing Mo6N octahedra and interpenetrated net-like Ni atoms [41,42].
The chemical states of Ni2Mo3N/NF were analyzed by X-ray photoelectron spectroscopy (XPS). Figure 2b shows the Ni 2p XPS spectra of Ni2Mo3N/NF. The peaks shown at 852.9 and 870.4 eV are ascribed to 2p3/2 and 2p1/2 of metallic Ni (Ni0), while the peaks centered at 856.4 and 873.9 eV are due to Ni2+ 2p3/2 and 2p1/2, respectively [27,33,43,44]. The high-resolution Mo 3d XPS spectra (Figure 2c) can be deconvoluted into three pairs with binding energies of 228.5/231.8, 229.4/232.6, and 233.4/235.7 eV corresponding to Mo0, Mo3+, and Mo6+, respectively [33,43,45]. The Mo0 and Mo3+ valence states originated from Ni2Mo3N, and the presence of Mo6+ is due to surface oxide formation [44,45,46,47]. In the N 1s XPS spectra (Figure 2d), the two deconvoluted peaks at 397.9 and 399.5 eV are attributed to metal-N and N-H groups, respectively. The N-H groups are likely associated with surface-adsorbed NHx species due to reaction with moisture from air exposure [48]. In addition, the peak at 394.7 eV originated from partially overlapped Mo 3p [27,30,31].
Figure 3a exhibits the polarization curves for the OER with Ni2Mo3N/NF in an Fe-free 1.0 M KOH solution along with commercial IrO2 and pure Ni foam for comparison. The observed peak around 1.4 V for Ni2Mo3N/NF is ascribed to the oxidation of Ni(II)/Ni(III or IV) [49,50]. The Ni2Mo3N/NF exhibited a much higher current density over the whole potential region than the others. The overpotential values of Ni2Mo3N/NF at 50 mA cm−2 and 100 mA cm−2 were 336.38 mV (η50) and 392.49 mV (η100), respectively. The η50 value of Ni2Mo3N/NF was even smaller than 450.55 mV for commercial IrO2. The pure Ni foam did not reach 50 mA cm−2 in the measured potential range (Figure 3b) and showed poor OER activity with an η10 value of 358.91 mV, suggesting that the loaded Ni2Mo3N phase was mainly responsible for the OER activity. In addition, at an overpotential value of 400 mV, the current density of the Ni2Mo3N/NF reached 111.18 mA cm−2, which is 4.5 and 21.6 times higher than commercial IrO2 and Ni foam, respectively (Figure 3b). The Ni2Mo3N/NF recorded one of the best OER catalytic performances among reported TMN-based electrocatalysts (Table S1).
The electrochemical active surface area (ECSA), the area of the electrode materials that is accessible to the electrolyte for electrochemical reaction, was estimated by the double layer capacitance (Cdl) method (Figure S4). The measured Cdl value for Ni2Mo3N/NF is 347.24 mF cm−2, whereas pure Ni foam and commercial IrO2 recorded small Cdl values of 0.68 and 0.55 mF cm−2, respectively. The high ECSA of Ni2Mo3N/NF suggests that the enhanced contact area between the catalyst and electrolyte is fruitful for improving the electrochemical activity of Ni2Mo3N/NF.
Electrochemical impedance spectroscopy (EIS) was conducted to characterize the prepared catalysts further, and the resulting Nyquist plots are presented in Figure 3c. A semicircle in the Nyquist plot represents the charge transfer resistance (Rct) and corresponding capacitance, describing the charge-transfer process at the catalyst/electrolyte interface. Generally, the Rct value is inversely proportional to electrochemical activity. The Ni2Mo3N/NF catalyst exhibited a smaller Rct value (1.861 Ω) than pure Ni foam (4.742 Ω), indicating enhanced OER catalytic activity due to the synergy between the Ni2Mo3N phase with high activity and the Ni foam providing a large surface area and high conductivity.
Chronopotentiometry tests were carried out to characterize the long-term stability of the OER, as it is an essential parameter for electrocatalysts. At 50 mA cm−2, the activity of Ni2Mo3N/NF was generally maintained for 5 h with a marginal overpotential increase, shown in Figure 3d. Therefore, the Ni2Mo3N/NF catalyst has excellent electrochemical activity and durability for the OER.
Further characterizations, including XRD, XPS, SEM, and TEM measurements, were conducted to monitor the structural changes of Ni2Mo3N/NF after the durability test. Figure 4a shows the XRD patterns of the Ni2Mo3N/NF catalyst after the 5 h durability test. The Ni2Mo3N peaks disappeared, and only metallic Ni peaks were observed, suggesting the transformation of crystalline Ni2Mo3N into an amorphous phase during the durability test. In the Ni 2p spectra of Ni2Mo3N/NF (Figure 4b), Ni0 peaks disappeared, and Ni2+ and Ni3+ peaks intensified, possibly due to the formation of NiOx or NiOOH species. The peaks can be deconvoluted into three pairs with binding energies of 855.2/872.5, 856.4/874.2, and 861.3/879.4 eV corresponding to the 2p3/2/2p1/2 doublets of Ni2+, Ni3+, and satellites [27,44,45,51]. In Ni-containing catalysts, surface NiOx species are generated in a low potential range below 1.35 V, which are further oxidized to NiOOH at ca. 1.4 V [52,53]. These NiOx and NiOOH phases are indicated as a significant contributor to the OER performance [27,54]. The Mo 3d XPS spectra in Figure 4c show two pairs with binding energies of 231.7/234.9 and 233.5/235.9 eV, originating from Mo5+ and Mo6+ [43,55,56]. Additionally, the intensity of the N 1s spectra was significantly decreased after the durability test (Figure 4d). These results indicate the formation of amorphous surface oxide species from crystalline nitride species. However, in the Ar-sputtered Mo 3d and N 1s spectra of Ni2Mo3N/NF after the durability test (Figure S5), the nitride-related peaks appeared again [44,51]. In Figure S1a, Mo0 and Mo3+ peaks showed up as in the fresh Ni2Mo3N/NF sample, and the Mo 3d spectra showed four Mo oxidation states: Mo0 (228.3/231.9 eV), Mo3+ (229.5/232.6 eV), Mo5+ (232.0/235.0), and Mo6+ (233.5/235.8 eV) [43,44,55,56]. In Figure S1b, the intensity of the N 1s spectra increased where metal-N, N-H, and Mo 3p peaks were observed at 397.8, 399.5, and 394.8 eV, respectively [28,31]. These results lead us to conclude that the amorphous surface oxide species were formed after the OER tests, and the metal nitride species remained in the bulk.
SEM and TEM measurements were conducted to inspect the morphologies of the Ni2Mo3N/NF after the durability tests (Figure 5). The SEM image in Figure 5a shows that Ni2Mo3N remains on the Ni foam similar to fresh Ni2Mo3N/NF. SEM-EDS elemental mapping images indicate that the Ni, Mo, and N elements are still uniformly distributed over the Ni foam with a clear O presence due to the formation of amorphous surface oxide species. Nanoparticles less than 10 nm can be observed in the TEM images (Figure 5c,d) without noticeable aggregation. Additionally, the lattice structure of the particles was not observed, indicating the conversion of crystalline Ni2Mo3N/NF into amorphous phases.

4. Conclusions

We prepared Ni2Mo3N nanoparticles directly grown on Ni foam using one step, by annealing Ni foam, MoCl5, and urea in one pot. The resultant Ni2Mo3N/NF shows impressive electrocatalytic performance for OER in an Fe-purified alkaline electrolyte, with small overpotential values of 336.38 (η50) and 392.49 mV (η100) and good durability for 5 h. The OER tests revealed that the surface of Ni2Mo3N converted to amorphous surface oxide species, which might be responsible for its exceptional catalytic activity. Our work offers a facile and economical route for bimetallic nitrides and provides a new avenue for designing highly efficient electrocatalysts.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/ma14164768/s1, Figure S1: Digital photographs for the synthetic procedure. Figure S2: XRD patterns of prepared samples with various annealing temperatures. Figure S3: Histogram showing the particle size distribution of Ni2Mo3N nanoparticles from the TEM images. Figure S4: Cyclic voltammograms of (a) Ni2Mo3N/NF, (b) pristine Ni foam and (c) IrO2 at different scan rates in 1.0 M KOH solution. (d–f) The corresponding current density versus scan rate plots showing Cdl values for Ni2Mo3N/NF, pristine Ni foam and IrO2. Figure S5: XPS spectra of Ni2Mo3N/NF after Ar-sputtering in the (a) Mo 3d and (b) N 1s, respectively. Table S1: Comparison of OER performances in alkaline media with reported TMN-based catalysts.

Author Contributions

Conceptualization, investigation, and writing—original draft, S.H.P.; conceptualization, writing—original draft, and editing, D.H.Y. and S.H.K.; funding acquisition and supervision, D.H.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the R&D Program for Forest Science Technology (FTIS2020216B10-2022-AC01) provided by the Korea Forest Service (Korea Forestry Promotion Institute) and the National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Education) (2019R1I1A3A01052741). The central laboratory of Kangwon National University and Korea Basic Science Institute (Chuncheon) provided significant assistance with TEM analyses.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available upon request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) SEM image of Ni2Mo3N/NF and SEM-EDS elemental mapping images (scale bar = 300 μm). (b,c) TEM images of Ni2Mo3N/NF.
Figure 1. (a) SEM image of Ni2Mo3N/NF and SEM-EDS elemental mapping images (scale bar = 300 μm). (b,c) TEM images of Ni2Mo3N/NF.
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Figure 2. (a) XRD patterns of Ni2Mo3N/NF. XPS spectra of Ni2Mo3N/NF for (b) Ni 2p, (c) Mo 3d, and (d) N 1 s.
Figure 2. (a) XRD patterns of Ni2Mo3N/NF. XPS spectra of Ni2Mo3N/NF for (b) Ni 2p, (c) Mo 3d, and (d) N 1 s.
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Figure 3. Electrochemical characterization of the prepared catalysts. (a) Polarization curves (1.0 M KOH solution), (b) bar graphs showing overpotentials at 50 mA cm−2 and current densities at an overpotential of 400 mV, (c) Nyquist plots, and (d) durability measurement.
Figure 3. Electrochemical characterization of the prepared catalysts. (a) Polarization curves (1.0 M KOH solution), (b) bar graphs showing overpotentials at 50 mA cm−2 and current densities at an overpotential of 400 mV, (c) Nyquist plots, and (d) durability measurement.
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Figure 4. (a) XRD patterns of Ni2Mo3N/NF (fresh and after the durability test). XPS spectra of Ni2Mo3N/NF after the durability test. (b) Ni 2p, (c) Mo 3d, and (d) N 1s.
Figure 4. (a) XRD patterns of Ni2Mo3N/NF (fresh and after the durability test). XPS spectra of Ni2Mo3N/NF after the durability test. (b) Ni 2p, (c) Mo 3d, and (d) N 1s.
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Figure 5. Ni2Mo3N/NF characterization results after the durability tests: (a) SEM images; (b) SEM-EDS elemental mapping images (scale bar = 300 μm); (c,d) TEM images.
Figure 5. Ni2Mo3N/NF characterization results after the durability tests: (a) SEM images; (b) SEM-EDS elemental mapping images (scale bar = 300 μm); (c,d) TEM images.
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Park, S.H.; Kang, S.H.; Youn, D.H. Direct One-Step Growth of Bimetallic Ni2Mo3N on Ni Foam as an Efficient Oxygen Evolution Electrocatalyst. Materials 2021, 14, 4768. https://doi.org/10.3390/ma14164768

AMA Style

Park SH, Kang SH, Youn DH. Direct One-Step Growth of Bimetallic Ni2Mo3N on Ni Foam as an Efficient Oxygen Evolution Electrocatalyst. Materials. 2021; 14(16):4768. https://doi.org/10.3390/ma14164768

Chicago/Turabian Style

Park, Sang Heon, Soon Hyung Kang, and Duck Hyun Youn. 2021. "Direct One-Step Growth of Bimetallic Ni2Mo3N on Ni Foam as an Efficient Oxygen Evolution Electrocatalyst" Materials 14, no. 16: 4768. https://doi.org/10.3390/ma14164768

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