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. 2021 Nov 11;6(46):31339–31347. doi: 10.1021/acsomega.1c05209

Aerosol-Assisted Chemical Vapor Deposition Growth of NiMoO4 Nanoflowers on Nickel Foam as Effective Electrocatalysts toward Water Oxidation

Muhammad Ali Ehsan 1, Abuzar Khan 1,*
PMCID: PMC8613873  PMID: 34841177

Abstract

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The fabrication of active and durable catalysts derived from transition metals is highly desired for the realization of efficient water oxidation reactions. This is particularly important to address the slow oxygen evolution reaction (OER) kinetics and hence can contribute to the conversion and storage of sustainable energy. In this study, the deposition of crystalline flowerlike 2D nanosheets of nickel molybdate (NiMoO4) directly on nickel foam (NF) through an aerosol-assisted chemical vapor deposition process is reported. The NiMoO4 nanosheets were developed on NF by altering the deposition time for 60 and 120 min at a fixed temperature of 480 °C. The structural determination by XRD and XPS analyses revealed a highly crystalline single phase NiMoO4. The micrographs of NiMoO4 show that the surface consisted of vertically aligned 2D nanosheets assembled into flowerlike structures. The nanosheets produced after 60 min deposition time on a network of NF is found to perform better for OER as compared to the one developed for 120 min. A reference current density of 10 mA cm–2 was achieved at an overpotential (η) of 320 mV, which was better as compared to that reported for the benchmark OER catalyst in 1.0 M KOH. Moreover, a small Tafel value (75 mV dec–1) and good OER stability for >15 h were also observed.

1. Introduction

The International Energy Agency (IEA), in its 2020 report, predicted that the worldwide demand for natural gas is anticipated to rise ∼29% by 2040 accounting 25% of total energy and an increase of 7% in oil demand, which is ∼28% of total energy consumed globally, with a considerable carbon footprint.1,2 The use of hydrogen is expected to rise in the foreseeable future and hence there is a global race for transition to hydrogen-based economy.3,4 Most of the H2 produced to date is via energy-intensive steam reforming of methane (SMR) with greenhouse gas emission contributing to global warming.5 The increasing energy demand and the environmental problem associated with burning of fossil fuels,6 to meet energy needs, has sparked research interests toward blue and green hydrogen.7 Although hydrogen is not found in earth’s atmosphere due to its low density but exists in many chemical compounds, it can be generated via chemical reactions.8 Moreover, for hydrogen to play an important role in a carbon-free future, it is highly desired to produce it from renewable sources in a sustainable manner. In this direction, water electrolysis is a sustainable way to generate a carbon-free, high-purity hydrogen from renewable sources.9−11

Water splitting, an uphill reaction, requires ∼1.8–2.4 V, which is above the thermodynamic potential of 1.23 V.12 The high electricity consumption and the low efficiency of the electrocatalysts, other than noble metals, toward water splitting to produce H2 and O2 is a major bottleneck limiting practical application.13 The large overpotential in water splitting arise from the slow kinetics of the oxygen evolution reaction (OER) that requires multiproton-coupled electron-transfer steps.11 Therefore, to lower the energy consumption and to increase the efficiency of OER, it is crucial to develop electrocatalysts that can operate at an appropriate rate driven by a lower overpotential. The noble metal catalysts (IrO2 and RuO2) are the benchmark catalysts for OER.11,14 The noble metal catalysts are expensive and rare, as compared to transition metal-based catalysts, which severely impedes their practical applications. Tremendous efforts have been devoted to develop transition metal catalysts for OER, such as phosphides,15,16 phosphates,17,18 carbides,19 nitrides,19,20 selenides,21 sulfides,22−24 and so on. Despite the great progress in non-noble metal catalysts, alkaline water electrolysis at lower cell voltages still remains a great challenge.

Ni-based electrocatalysts have attracted considerable attention since nickel is the fifth abundant element in earth having good catalytic activity for OER, and its alloys offer excellent resistant to corrosion in alkaline media. Moreover, oxo-hydroxo species of nickel(III) such as NiOOH generated out of the anodic oxidation of Ni(OH)2 are highly active water oxidation electrocatalysts.25 Generally, the transition metal electrocatalysts show inherent slow transfer of electrons from the current collector to the catalyst, which affects their efficiency. The performance could be partly enhanced by alleviating this problem, i.e., by direct growth of electrocatalysts on a 3D current collector, such as carbon cloth (CC) and Ni foam (NF) and to form self-supporting electrodes. NF possesses a 3D network, which helps in dispersion of the deposited catalyst. This helps to minimize agglomeration and offers a huge number of active sites that help in faster transfer of electrons and hence boost the electrocatalytic OER performance. The performance of the electrocatalysts could also be improved by tuning the structure and morphology that reveals more active sites and realizes rapid mass diffusion.

Recently, nickel molybdates have been investigated as OER and HER catalysts owing to their versatile synthesis into different nanostructures.26−28 A number of publications listing the synthesis and application of nickel molybdate is available in the literature.26−33 Molybdenum leaching from NiMoO4 catalysts differing in structure and composition have been observed.29,34 It has also been observed that the NiMoO4 nanostructures having flakelike morphology are less prone to Mo leaching, i.e., they are more stable in the electrolyte as compared to the nanostructure bearing other morphology.29 The abundant edge sites and highly exposed active centers of nanosheets bestow unique physicochemical properties and promote the catalytic efficiency.30

Several techniques are employed for thin layer coating35 and deposition36 in various energy applications. Chemical vapor deposition (CVD) is a single step and rapid electrode fabrication process as compared to the hydrothermal method. Recently, Yedluri et al. reported the synthesis of NiMoO4 on the NF substrate using the hydrothermal procedure based on inorganic precursor reactants [Ni(NO3)2.6H2O] and [Na2MoO4.2H2O] in the presence of additives such as thiourea [CH4N2S] and ammonium fluoride [NH4F]. The hydrothermal reaction took 12 h of prolonged heating at 140 °C to complete. The collected NiMoO4/NF product was then further annealed at 200 °C for additional 5 h to obtain the electrode (ref (37)). In contrast, the aerosol-assisted chemical vapor deposition (AACVD) NiMoO4 thin film fabrication process is accomplished in 1–2 h and the resultant electrodes are readily available for characterization and evaluation.37 In this work, we have explored a rapid way of synthesizing flowerlike 2D nanosheets of nickel molybdate (NiMoO4) directly on the NF through an AACVD process. The nickel molybdate nanosheets have been grown on NF by varying the deposition time for 60 and 120 min at a fixed temperature of 480 °C, and their efficiency was investigated as OER catalysts in 1 M KOH electrolyte. The NiMoO4 nanosheets of NF obtained after 60 min of the AACVD process showed remarkable OER performance with the lowest overpotential of 320 mV to reach a standard current density of 10 mA cm–2. The catalyst continuously performed OER for 15 h, signifying its prominent stability under electrochemical conditions.

2. Results and Discussion

2.1. Characterization of the NiMoO4 Catalyst

Figure 1 shows the XRD patterns of NiMoO4 films fabricated through AACVD for 60 and 120 min deposition times. Both XRD patterns exhibited good crystallinity as evident from several small and long intensity peaks shown in Figure 1. The peaks are labeled with their corresponding reflections from which they are produced. The XRD fingerprints for NM1 and NM2 look similar in terms of peak positions and suggest the crystallographic analogy of the product synthesized in the two samples. The crystalline peaks at 2θ values of 23.6, 26.2, 26.7, 26.8, 27.4, 28.8, 32.5, 33.9, 37.2, 39.3, 40.6, 42.0, 44.2, 47.4, 52.3, 54.2, 60.7, 62.3, 64.8, and 78.4° are indexed to the reflections (02-1), (201), (002), (220), (11-2), (31-1), (112), (31-2), (400), (040), (330), (222), (33-2), (241), (20-4), (53-1), (42-4), (44-3), (333), and (82-2), respectively, and match with the crystallographic data of single phase nickel molybdenum oxide “NiMoO4” in the monoclinic crystal system (ICSD No. 00-045-0142). No other crystalline nickel or molybdenum oxide phases are identified from XRD patterns as impurities. In both XRD patterns, the dominant, 100% intensity peak is situated at 2θ 26.8°.

Figure 1.

Figure 1

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XRD patterns of single phase NiMoO4 samples prepared for 60 and 120 min via AACVD.

The surface morphology of the fabricated films, developed after 60 and 120 min deposition time on NF, were discerned by field-emission scanning electron microscopy (FESEM) analysis and the observed micrographs are displayed in Figure 2. Low resolution images shown in Figure 2a,b indicate that the NF strut is wreathed with a layer of crystallites. The enlarged images shown in Figure 2a1,b1 display several intertwined close-packed spherical objects, which flourished into blooming flowerlike patterns. Further insight into these structures reveal that the intertwined patterns comprised a large number of vertically arranged cross-linked nanosheet-like petals with clear grain boundaries in the sample NM1 (Figure 2a11). These petal-like features were wilted when the deposition time was increased to 120 min (NM2) due to the extended sintering process (Figure 2b11). These hierarchical flowerlike structures comprising 2D nanosheets play a significant role in various electrochemical-based devices such as supercapacitors, dye sensitized solar cells, hydrogen storage, and water splitting studies. The hierarchical petal networks on 3D NF present a high surface area and provide effective contact between the catalyst material and electrolyte ions. Therefore, it is expected that the NiMoO4 petals obtained after 60 min of deposition time (NM1) comprised a greater number of interfacial and electrocatalytic active sites that could lead to enhanced OER performance.

Figure 2.

Figure 2

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FESEM images of NM1 (a) and NM2 (b). Low resolution images (a and b); high resolution (10 Kx) images (a1 and b1); and (50 Kx) images (a11 and b11).

The atomic composition in NiMoO4 films was determined by energy-dispersive X-ray (EDX) analysis. As the films were grown on NF substrates, it was expected to have higher Ni concentration due to the contribution from NF substrates; therefore, the atomic concentration was measured from the analogous film samples grown on plane glass substrates. Figure 3 displays the EDX patterns of NiMoO4 film samples with measured % atomic concentration of Ni and Mo atoms, which empirically exist in a 1-to-1 mole ratio. The elemental map analysis (Figure 4) shows the atomic dispersion of Ni and Mo depicting a uniform and even distribution of both elements on the film surface.

Figure 3.

Figure 3

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EDX spectra of NiMoO4 films NM1 (a) and NM2 (b).

Figure 4.

Figure 4

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EDX elemental map analysis of NM1.

The chemical behavior and oxidation state of the elements involved in the NiMoO4 film (NM1) were investigated by X-ray photoelectron spectroscopy (XPS). Figure 5 indicates the high resolution XPS spectrum of constituent Ni, Mo, and O elements. The Ni 2p spectrum involves two major peaks related to Ni 2p3/2 and Ni 2p1/2 spin orbitals at a binding energy value of 858 and 876 eV, respectively.38 The corresponding satellite peaks appeared at 863 and 883 eV. This electronic structure signifies the existence of Ni2+ oxidation in NiMoO4. The Mo 5d spectrum exhibits a doublet peak of binding energies located at 232.5 eV and 235.6 attributed with Mo3d5/2 and Mo 3d3/2, respectively.39 The difference in binding energy (ΔE) is ∼3.1 eV, which is typical for the Mo6+ oxidation state in NiMoO4.40 The symmetrical O 1s spectrum shows two peaks at binding energies of 531 and 531.7 eV, characteristic of metal oxygen bonds in NiMoO4.41 The XPS observations are comparable with the previous reports on NiMoO4 materials. The XPS results confirm that oxidation states of Ni, Mo, and O elements are +2, +6, and −2, respectively, which agrees well with the chemical formula NiMoO4 identified from XRD data.

Figure 5.

Figure 5

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High resolution XPS spectrum of NM1. (a) Ni 2p, (b) Mo 3d, and (c) O 1s.

2.2. Electrochemical Water Oxidation on NiMoO4 Nanosheets

The well-formed crystalline, hierarchical, and porous nanostructures on NF can maximize the effective mass transport and boost the electrocatalytic performance.42 The NM1 and NM2 containing NiMoO4 nanosheets were investigated for their electrocatalytic efficiency by studying OER in alkaline media. The surface of the electrodes deposited with NiMoO4 nanosheets were electrochemically activated employing cyclic voltammetry (CV). The electrodes were scanned for 50 consecutive cycles at a scan rate of 50 mV s–1. The peaks indicative of strong reversible redox reactions between Ni2+/Ni3+ and NiO/NiOOH were observed in both the samples. The oxidative peaks for Ni oxidation (Ni+2 to Ni+3) appeared between 1.3 and 1.5 V vs RHE implying a favorable electrochemical activity for oxidation reactions.43 The voltammograms showed that the intensity of redox peaks increased with increasing the number of cyclic scans. It could be inferred that a greater number of NiOOH active sites were generated due to increased Ni+2 oxidation with each cycle. The increase in catalytic sites resulted in improved overpotential and the current density of the catalyst.

The linear sweep voltammetry (LSV) was performed under static conditions to measure the OER activity of NiMoO4 nanosheets. The LSV curves are displayed in Figure 6. Both the electrodes, i.e., NM1 and NM2 demonstrated a higher current density and lower overpotentials as compared to the bare NF (Figure 6a). A final current density of ∼178 mA cm–2 was attained at a potential of ∼2.0 V (vs RHE) with NM1, whereas NM2 showed a current density of ∼158 mA cm–2. Moreover, the electrode NM1 showed an improved overpotential as compared to that obtained after 120 min. To deliver a current density of 10 mA cm–2, the overpotential required was 320 mV by NM1, whereas NM2 displayed a relatively high onset potential and required 360 mV to achieve a current density of 10 mA cm–2 (Figure 6b). This indicates that a better OER could be achieved by employing a deposition time of 60 min. The decrease in performance of NM2 could be attributed to the crumbling of the petal-like microstructure (Figure 2b11), which might have lost the catalytic active sites to decrease its OER performance.

Figure 6.

Figure 6

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Electrochemical characterization of NiMoO4 electrocatalysts compared to the NF substrate. (a) Polarization curves (LSVs) obtained at a scan rate of 10 mV s–1. (b) Comparison of overpotential values of the NiMoO4 catalyst to reach benchmark current densities of 10 mA cm–2. (c) Polarization curves of best OER activity of the NiMoO4 catalyst recorded at different scan rates of 1, 5, 10, 25, and 50 mV s–1. (d) Tafel plot indicating the OER kinetics for NiMoO4 catalysts.

The effect of the scan rate on the electrocatalytic efficiency of NM1 for OER was investigated by varying the scan rates (5, 10, 25, 50, and 100 mV s–1) in LSV as shown in Figure 6c. It is clear from the results that the oxidation peak shifted toward a higher potential with an increase in the scan rate suggesting a diffusion-controlled charge-transfer mechanism44 with no obvious change in the final current density. The internal diffusion resistance within the active material increased leading to a shift in redox peaks with increasing scan rates.31 Moreover, the intensities of the redox peaks changed and an increase in the current density of the redox peak was observed upon increasing the scan rate. This is indicative of the surface-controlled electrochemical process, i.e., the kinetics of the interface faradic redox reactions.31

To gain further insights into the OER kinetics, Tafel slopes were derived by fitting the linear part of the polarization curves for all the electrodes. Generally, a lower value of the Tafel slope signifies superior catalytic activity.45 The Tafel slope of NM1 was estimated to be 75 mV dec–1, much lower than that of NM2 (92 mV dec–1) and the bare NF (230 mV dec–1), confirming a faster OER kinetics on NM1. The improved electrocatalytic performance of NM1 could be attributed to the high surface area and the effective interfacial contact between the electrocatalyst and the electrolyte. The homogeneous mesoporous structure formed due to the growth of 2D nanosheet petals on the NF provided a greater number of active sites, leading to the enhanced OER performance.46

The electrocatalytic OER performance of the synthesized NiMoO4 nanosheets on NF by AACVD is compared with benchmark OER catalysts. The OER performance at a current density of 10 mA cm–2 overpotential along with stability of the catalysts is presented in in Table 1. The OER performance of the NiMoO4 catalyst prepared by AACVD is better than benchmark OER catalysts mentioned in Table 1.

Table 1. Comparison of OER Parameters of NiMoO4 Catalysts with Benchmark OER Catalysts.

electrocatalyst synthesis method overpotential (mV) @10 mA cm–2 electrolyte reference
IrO2 commercial 301 1 M KOH (47)
RuO2 commercial 322 1 M KOH (48)
NiMoO4 hydrothermal 340 1 M KOH (49)
NiMoO4 thermal route 410 1 M KOH (50)
NiMoO4 AACVD 320 1 M KOH this work
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For any practical application, it is imperative to test the electrochemical stability of the developed electrodes. The long-term electrochemical stability of the developed electrode NM1 was investigated by employing chronoamperometry in 1 M KOH solution. The chronoamperometric test was performed by applying a potential of 1.6 V (vs RHE), and the current density was monitored for a period of 15 h as shown in Figure 7a. The catalyst displayed a constant and steady current for the investigated period, and the measured current density after 15 h was 9.7 mA cm–2, i.e., a drop of only 3% was observed. After the long-term stability test, the polarization curve for used NM1 was recorded and compared with its fresh form as indicated in Figure 7b. Both polarization curves are almost identical suggesting the stable OER performance of the NM1 electrode even after long-term electrochemical studies.

Figure 7.

Figure 7

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Chronoamperometric response measured at an applied fixed potential of 1.6 V (a), LSV polarization curves before and after stability tests (b), and EIS plot for the NM1 electrode (c).

Furthermore, electrochemical impedance spectroscopy (EIS) was performed to measure the internal resistance and charge-transfer resistance at the electrode/electrolyte interface. The Nyquist plots of the synthesized materials are shown in Figure 7c. The charge-transfer resistance (Rct) is closely related to the electrocatalytic kinetics and can be determined by measuring the diameter of the semicircle. From the Nyquist plots, it is clear that the diameter of the semicircle for NM1 is smaller as compared to NM2 and bare NF. The smaller diameter indicates that NM1 has the minimum charge-transfer resistance and faster electrode dynamics as compared to the NM2 sample. The Rct value of NM1 was found to be lower than that of NM2 and bare NF.

The surface of NM1 was analyzed after the long-term chronopotentiometry stability test and images are shown in Figure 8. The FESEM images at low and high magnification reveled a flakelike structure, and the footprints of these flakes show some resemblance with the initial flowerlike morphology of the unused NM1 electrode (Figure 2c). This confirms that the thin film electrode after prolong stability has retained its structure with slight deterioration.

Figure 8.

Figure 8

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FESEM images of the NM1 thin film electrode after long-term electrochemical OER study. Low resolution images (a and b). High resolution (50 Kx) image (c).

Figure 9 shows the corresponding EDX spectrum of NM1. Both the key elements Ni and Mo are present on the surface of used NM1, while K atoms are included from the KOH electrode. It is difficult to predict the exact concentration of Ni atoms due to the contributions from the NF substrate. However, the % atomicity of Mo atoms is found to be 6.65%, which is almost half of the initial % atomic concentration of Mo (13.49%) found in the NM1 electrode before electrochemical investigation. The decrease in Mo atomic concentration suggests that Mo atoms are leached out during the OER process, which is a common phenomenon reported in previous studies.29,34

Figure 9.

Figure 9

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EDX analysis of the NM1 electrode after the long-term OER stability test.

3. Conclusions

In this study, the fabrication of 2D nanoflowers of NiMoO4 on NF by an AACVD approach has been successfully demonstrated. The NiMoO4 nanosheets grown into flowerlike structures with each sheets representing petals of individual flowers. The AACVD process duration has a profound effect on particle morphology as well as electrocatalytic activity for OER. The film grown for 60 min showed a hierarchical, crystalline, ultrafine, and highly porous structure with well-defined grain boundaries. However, an increased AACVD duration, i.e., 120 min had a detrimental effect on morphology of the NiMoO4 structures and hence the electrocatalytic performance. The NiMoO4 catalyst obtained after 60 min of AACVD deposition exhibited a relatively small overpotential value to attain a current density of 10 mA cm–2. The overpotential required to this current density was 320 and 360 mV for the films obtained after 60 and 120 min, respectively. The Tafel plot value also supported a higher OER kinetics, which was corroborated by the low charge resistance shown by EIS measurement. In addition, the catalyst was durable and sustainable enough to continuously catalyze OER for 15 h. Our findings suggests that transition metal-based electrocatalysts could be fabricated by a simple AACVD approach that could be a potential replacement for the noble metal-based electrocatalysts for OER.

4. Experimental Section

4.1. Materials and Methods

Nickel(II) acetylacetonate ((Ni(acac)2), molybdenum diacetylacetonate dioxide MoO2(acac)2, and methanol were obtained from Sigma-Aldrich. The NF substrate of thickness 0.9 mm and porosity 93% was obtained from Goodfellow, a global supplier for materials.

4.2. Nickel Molybdenum Oxide Thin Film Fabrication

Solid solution NiMoO4 thin films were fabricated on NF substrates using AACVD as shown in Scheme 1. The feedstock solution was prepared prior to the AACVD process. The details of the setup and working principle of AACVD is also elaborated in our recent work.51 Briefly, the precursors, 100 mg (0.4 mmol) of Ni(acac)2 and 126 mg (0.4 mmol) of MoO2(acac)2, were dissolved in 20 mL of methanol at room temperature and the solution was stirred continuously. The yellowish green solution obtained after 30 min of stirring acted as a feedstock for thin film deposition. Films were prepared by altering the deposition time for 60 and 120 min at a fixed temperature of 480 °C. The liquefied precursor was then converted into a gaseous stream with the help of an ultrasonic generator and delivered to a reactor tube, fitted in a horizontal tube furnace, with the aid of carrier gas (N2, 99.99% purity). The temperature of the tube furnace was set at 480 °C, and the NF substrate was positioned in such a way that the precursor mist directly landed on its surface. The precursor cloud decomposed on the heated NF surface to form thin films. The AACVD process for different time periods was carried out to obtain samples with different microstructures. After completing the deposition process, samples were cooled under the flow of N2 gas until the furnace temperature reached 50 °C. The gray films obtained after 60 and 120 min of deposition were designated as NM1 and NM2, respectively.

Scheme 1. AACVD Process for Synthesis of NiMoO4 Films on the NF Substrate.

Scheme 1

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4.3. Thin Film Characterization

The crystallinity and phase structure of the NiMoO4 films were revealed by powder XRD analysis recorded on a benchtop X-ray diffractometer (Rigaku MiniFlex X-ray diffractometer, Japan) using Cu Kα1 radiation (α = 0.15416 nm). The surface morphology of the films was analyzed on a field-emission scanning electron microscope (TESCAN LYRA). The elemental compositions were determined with EDX analysis (Oxford Instruments). The chemical behavior and oxidation state of the elements involved in the NiMoO4 film were investigated by XPS.

4.4. Electrochemical OER Studies

The prepared electrodes were evaluated on an Autolab potentiostat supported by NOVA 2.0 software. The electrochemical measurements were conducted in a three-electrode cell using 1 M KOH electrolyte. Platinum (Pt) and saturated calomel electrodes (SCE) served as the counter and reference electrodes, respectively. The measured potential against the reference electrode was converted to the RHE scale using eq 1.

4.4. 1

CV was performed at a scan rate of 50 mV s–1. The OER performance was measured by LSV at a 10 mV s–1 scan rate. Chronoamperometric data were recorded at a constant applied potential of 1.57 V vs RHE for 15 h for the durability test.

Acknowledgments

The authors Muhammad Ali Ehsan and Abuzar Khan greatly acknowledge the Deanship of Research Oversight and Coordination (DROC), the KFUPM for research funding, and the Interdisciplinary Research Center for Hydrogen and Energy Storage (IRC-HES) at King Fahd University of Petroleum and Minerals (KFUPM) for support.

The authors declare no competing financial interest.

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