MoVN-coated MoNi4 -MoO2 nanorods as a bifunctional electrode for Electrochemical water splitting

doi:10.21203/rs.3.rs-3439121/v1

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Research Article

MoVN-coated MoNi4 -MoO2 nanorods as a bifunctional electrode for Electrochemical water splitting

https://doi.org/10.21203/rs.3.rs-3439121/v1

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published 09 Feb, 2024

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MoVN/MoNi₄-MoO₃ nanorods are synthesized using a two-step fabrication process consisting of the hydrothermal method and DC(V) and RF(Mo) magnetron co-sputtering technique. The resulting electrode exhibits a high surface area, that together with its intrinsic catalytic activity, achieves a synergistic effect, thereby improving the reaction kinetics of overall water splitting. The performance of the electrodes was tested in 1M KOH solution, to understand the reaction mechanisms and catalytic efficiency of both the half-cell reactions (Hydrogen Evolution Reaction and Oxygen Evolution Reaction). The electrodes performed exceptionally well with overpotentials of only 14 mV and 244 mV at 10 mA/cm² for HER and OER respectively compared to other electrode materials described previously in literature. Further, the stability of the electrodes was tested for 12 hours showing a negligible change in current density for both HER and OER reactions. Overall electrolysis was performed for 12 hours with the best electrode MoVN/MoNi₄-MoO₂ needing only 1.56 V to reach 10 mA/cm². Material characterization using X-ray Diffraction, Scanning Electron Microscopy, and Transmission Electron Microscopy were carried out for crystal structure and morphological analysis. X-ray photoelectron spectroscopy was performed to understand the nature of the surface species of the best-performing electrode and to study the degradation effects after electrolysis. Hence, this work uncovers the outstanding properties of the MoVN/MoNi₄-MoO₂ electrode material with relatively high electrochemical surface area, low overpotential, and negligible degradation which could provide a competitive path to the fabrication of low-cost and highly effective electrodes for application in commercial electrolyzers.

Overall water splitting

transition metals

electrocatalysis

magnetron sputtering

hydrogen production

Hydrogen has the potential to enable clean and efficient energy storage, essential for reducing carbon emissions, primarily due to its high gravimetric energy density and natural abundance of water. A variety of methods are available for producing hydrogen, including steam methane reforming, biomass gasification, and water electrolysis. However, the only method that does not generate carbon monoxide is electrochemical water splitting, which produces hydrogen with an efficiency of ~ 65% [1, 2] and an associated cost in the range from $2.7/kg to $6.8/kg, [1, 2]. Water electrolysis occurs via two half-cell reactions namely Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER). These half-cell reactions are relatively sluggish as their development is hindered by kinetic and thermodynamic barriers. The thermodynamic potential needed to initiate water electrolysis is 1.23 V at 25°C and 1 atm. In practice, extra potential is required to overcome the kinetic barriers. This overpotential (η) can be classified as (1) reaction overpotential – the overpotential due to the activation energy that must be achieved to break chemical bonds at both cathode and anode electrode surfaces, (2) charge transfer overpotential due to energy barriers that impede the electron transfer at the surface of the electrodes and (3) ohmic overpotential - due to the resistance to conduction of ions through the electrolyte, and to the resistance to the conduction of electrons through the elements of the circuit, [3]. To increase the efficiency of the electrolytic process, most of the recent research is targeting the reduction of these overpotentials. Hence, one of the challenges in applying water electrolysis at a large scale, is the high cost of noble catalyst materials used to activate the reaction with low overpotential. These noble catalyst materials such as Pt used for HER and Ir or Ru used for OER are among the rarest elements in the Earth’s crust and need to be replaced by abundantly available non-noble electrode materials. Most alternate catalyst materials proposed for HER show good catalytic activity but underperform in alkaline medium. However, industrial water splitting is conducted in an alkaline medium to overcome challenges such as corrosion associated with an acidic environment. Hence, catalyst electrode materials which perform well in alkaline medium are crucial for industrial applications, [4–7].

Many chalcogenides based on transition metals such as MoS₂ have been reported as efficient electrocatalysts for HER, [8–11]. Transition metal hydroxides and oxides have also been reported as good OER catalysts, with a performance comparable to Ir and Ru. Incorporating two different catalysts for each half cell reaction would increase the product cost where potentially conflicting preparation and optimization procedures could be required to develop a hybrid with two mono-functional catalysts. One way to address this challenge would be to use bi-functional catalyst which could activate both HER and OER efficiently, [12–15]. Hence, to design an electrode that is efficient for both half-cell reactions, it is important to understand the parameters necessary to be evaluated to achieve high catalytic activity for each reaction. For a good electrocatalyst, the bond energies of the intermediates should be neither too high nor too low, [16–18]. For HER, the standard free energy of hydrogen adsorption on a material is expected to be close to zero as the standard redox potential for HER is zero. In the case of OER, the activity is limited by oxide and peroxide formation steps and the catalyst with lower binding energies to oxygen will stay closer to the theoretical potential of OER, [19]. Recent studies reveal that Mo-Ni alloy based electrocatalysts are promising candidates in activating HER due to the observed lowering of the activation energy barrier to 0.39 eV, [20]. This effect could be attributed to the upward shift in the d-band center with respect to Fermi energy due to the variation of adsorption energy from one transition metal surface to another. The observed synergistic effect normally exceeds the performance of the individual parent metals and approaches a reversible behavior within a wide range of current densities, [21].

On the other hand, transition metal nitrides exhibit abundant valence states, good chemical stability, and high electrical conductivity. The presence of nitrogen atoms in the host metal lattice expands the lattice parameter, contracting the d-band energy state and resulting in an increased density of states near the Fermi level. This redistribution of density of states (DOS) serves as a cause for electron-donating character in transition metal nitrides (TMNs) and increased catalytic activity, [22, 23]. Incorporation of vanadium in metal nitrides has been shown to improve the performance of HER and OER activity in previous studies, [24]. The improved performance has been attributed to the multi-valent states (III-V) of vanadium, that provide the required electronic configuration for a facile electron transfer. Further, embedding Co nanoparticles in vanadium nitride (VN) reported by Peng et.al has demonstrated good OER activity and excellent chemical stability over 2000 cycles at 200 mA/cm²,[25]. Wei et.al reported the synthesis of MoVN thin films and studied the performance of the alloy for both HER and OER in alkaline and acidic medium. The better electrocatalytic activity achieved for MoVN compared to individual nitrides Mo₂N and VN demonstrates the synergistic effect between the two metals,[26]. Moreover, the substrate on which the catalyst is synthesized or coated must also be highly conductive and exhibit catalytic activity such that fast electron transport is allowed. Three-dimensional transition metal-based porous foams open a new area of catalysis research where the porous channels increase the active surface area and generate more active sites for the effective adsorption and desorption of hydrogen and oxygen atoms, respectively. Among the transition metals, Ni atoms are known to act as excellent water dissociation centers, and Ni foam (NF) is used to study the potential benefits for various electrocatalysis processes. The major advantage of using porous NF as a substrate lies in the possibility of depositing nanostructures even inside the porous channels which could further enhance the specific surface area without affecting the dissipation of gas bubbles during the electrolytic process, [27–29].

In this work, we develop and investigate a novel architecture of an electrode fabricated in two steps. The first step includes the hydrothermal growth of MoNi₄ nanoparticles on MoO₂ nanorods with Ni foam as the substrate. During the second step, bi-metallic nitride (MoVN) nanoflakes are deposited on the nanorods using DC-RF magnetron co-sputtering technique. The sputtering of the bimetallic nitrides increases the stability and performance towards both half-cell reactions. The electrochemical surface area (ECSA) accountable for this increase in activity has been obtained through two different methods: cyclic voltammetry and electrochemical impedance spectroscopy. To determine structural and compositional characteristics, the electrodes under study were characterized by XRD, SEM, EDS and XPS. Overall, the fabricated electrode without any noble elements exhibits bi-functionality and stability and could be used as an efficient electrode towards overall water splitting.

Materials

As precursors for the synthesis of the bimetallic nitrides, ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄.4H₂O), nickel nitrate hexahydrate (Ni (NO₃)₂6H₂O), and potassium hydroxide (KOH) were purchased from Alfa Aesar. Nickel foam was purchased from MTI Corporation and Pt wire was used as the counter electrode.

Synthesis of MoNi₄/MoO₂ nanorods

For the hydrothermal growth of MoNi₄ nanoparticles on Ni foam, 0.04M of nickel nitrate hexahydrate, 0.01M of ammonium molybdate were added to 30 ml of deionized water and stirred continuously. The Ni foam was cleaned with 1M nitric acid and deionized water using sonication for 15 minutes. The solution prepared this way, was then transferred to a Teflon container together with the Ni foam substrate (3$\times$3 cm²) to be kept in an autoclave. The autoclave was maintained at 150°C for 24 hours. The NiMoO₄ structures deposited on the Ni foam were taken out of the autoclave after cooling down. The deposited NiMoO₄ material on Ni foam was annealed in Ar/H₂ atmosphere at 500°C for 180 minutes to form MoNi₄/MoO₂/NF.

Deposition of MoVN nanoflakes

MoVN layer depositions were carried on NiMoO₄/NF in a stainless-steel AJA International ATC Orion physical vapor deposition (PVD) reactor equipped with two confocal magnetron sputtering guns and rotating substrate. The sputtering targets were 3” in diameter disks of Molybdenum (99.9% purity) and Vanadium (99.99% purity) driven independently by a13.56 MHz rf and a DC power supplies, respectively. The reactor was evacuated using a combination of a mechanical pump and turbomolecular pump to a base pressure of ∼5 × 10^− 7 Torr. The MoVN layers were deposited on NiMoO4/NF by co-sputtering in Ar-N₂ atmosphere at the substrate temperature of 300°C which is kept constant throughout the depositions. The deposition pressure was maintained as 20 mTorr with nitrogen and argon flux of 7.4 and 20 sccm, respectively. The N₂:Ar gas flow was optimized in order to attain high metal nitride composition. The power supplied to the sputtering of the V target was 275W (DC) while for Mo target was 150W (RF). The deposited MoVN/NiMoO₄/NF was subsequently annealed at 500° in Ar/H₂ atmosphere for the formation of the final material MoVN/MoNi₄-MoO₂.

Structural characterizations

The resulted material crystallinity was characterized by X-ray diffraction (XRD) equipped with Cu Kα radiation source using a Bruker D8 Advance diffractometer. A Zeiss Leo 1550 scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) was used to investigate the morphology, microstructure, and the composition of the samples. X-ray photoelectron spectroscopy (XPS) with PHI Quantera II system was used for the analysis of the chemical state of the elements present at the surface and bonding configuration of the developed layers.

Electrochemical characterization

The hydrogen evolution reaction and oxygen evolution reaction activity measurement were performed on a three-electrode standard system controlled by PARSTAT 2273 potentiostat. A commercial Hg/HgO electrode was used as a reference electrode and a platinum wire was used as a counter electrode. The measurement of the HER and OER activity was performed through linear sweep voltammetry within the potential range − 0.5 V to -1.4 V and − 0.2 V to 1.2 V respectively vs Hg/HgO at scan rate 2 mV/s. The Tafel polarization analysis was performed through linear sweep voltammetry (LSV) with scan rate of 2 mV/sec. The potential of the reference electrode was determined with respect to the reversible hydrogen electrode (RHE) potential according to the Nernst Eq. (1).

E _RHE = E₀ + 0.098 + 0.059 *(pH) (1)

Structural Characterization

The crystallographic structure of the synthesized electrodes was investigated by X-ray diffraction. The diffractogram of the as-prepared NiMoO₄ nanorods on Ni foam is presented in Fig. 1(a). The pattern observed in the diffractogram matches well the reference pattern of NiMoO₄H₂O (JCPDS No: 13–0128). After annealing, XRD reveals peaks at 43.5° and 50.5° in Fig. 1(b)&(c) that correspond to the (121) and (310) planes of MoNi₄ and confirms the outward diffusion of Ni atoms from the foam substrate allowing the formation of MoNi₄ nanoparticles on the surface of the nanorods. Using Scherrer method, it was found that the average crystallite size of the MoNi₄ particles is 17.6 nm. The formation of MoO₂ after annealing is suggested by the strong peaks at 26.1°, 36.9° and 53.6° in Fig. 1(b) which correspond to ($\stackrel{-}{1}$11), ($\stackrel{-}{2}$11) and ($\stackrel{-}{3}$12) planes respectively (JCPDS No: 32–0671). The two intense peaks at 44.52° and 51.9° could be assigned to the (111) and (200) planes of Nickel foam substrate (JCPDS No: 65-2865). Further, MoVN flakes were synthesized on the Ni foam – MoNi₄-MoO₂ films, as well as on duplicate samples deposited at the same time on Si substrates. A series of experimental runs were performed to determine the deposition parameters of the magnetron sputtering to maximize the fraction of metal nitrides with respect to that of the metal oxides.

The transformations at the surface of the material are revealed by the scanning electron microscopic images shown in Fig. 2. The aspect of the Ni foam after the deposition can be seen in Fig. 2(a).

The formation of smooth NiMoO₄ nanorods on the nickel foam after the hydrothermal deposition can be seen in Fig. 2(b), while in Fig. 2(c) the formation of MoNi₄ nanoparticles on MoO₂ nanorods can be observed. The composition of Mo:Ni:O alloy is estimated from energy dispersive X-ray spectroscopy measurement and presented in the supplemental information section in Figure S4(b) approximately matches with the stoichiometry of the material MoNi₄/MoO₂ as 1:2:1 confirming the presence of these elements. When the annealing time increased, we could observe the growth of tetragonal MoNi₄ crystals on MoO₂ nanorods as shown in Fig. 2(d).

2This demonstrates the nucleation and growth of MoNi₄ crystals that occurs because of longer annealing time. The particle size analysis from SEM, shows that the particles are of radius 16.88 nm which agrees well with the crystallite size measured from XRD, with an error of $\pm$0.72 nm. Figures 2(e) &(f) show the deposition of MoVN nanoflakes on the nanorods. EDS mapping was performed on the sputtered MoVN/MoNi₄-MoO₂ depicting the presence of major transition elements of the electrode material synthesized including Mo, Ni, V in Figure S3. The EDS spectrum and compositional analysis of MoVN/MoNi₄-MoO₂ in Figure S3 shows the atomic concentration of these elements on the surface. As we can see there is very little contribution of vanadium as it acts as the surface enhancer and higher Molybdenum and Nickel concentration shows that the existing alloy of these elements contribute to the high catalytic activity of the electrode.

The optimized MoVN film on Si substrate sample was also analyzed by XRD and the diffractogram is presented in the supplemental information section in Figure S1. The left shift of the standard peak 37.9° to 37.8° indicates substitution of V in Mo₂N of (111) plane (JCPDS No: 24–0768) and confirms the presence of MoVN phase. The peak at 44° and 64.12° can be ascribed to the (200) and (220) planes of cubic-VN respectively (JCPDS No:25-1252). EDS analysis and the elemental distribution together with the corresponding atomic concentrations of the same sample is shown in Figure S2. Transmission electron microscopy was performed on the MoNi₄/MoO₂ nanorods presented in Fig. 4(g) was performed to determine the crystallographic details at the surface and it can be seen that small crystal particles cover the surface of the nanorods. The orientation of these crystal particles was determined through the analysis of the selected area electron diffraction (SAED) pattern as shown in Fig. 3(h). The analysis confirms that the (002) plane of tetragonal MoNi₄ nanoparticles is parallel to the surface plane of the MoO₂ nanorods.

To examine the nature of the surface species and composition, the best performing electrode was investigated using X-ray photoelectron spectroscopy. In Fig. 4(a), Mo 3d spectra is deconvoluted giving rise to three different oxidation states for Mo namely Mo^δ+, Mo⁴⁺, Mo⁶⁺ found at 228.2 eV, 230.8 eV and 232.6 eV that corresponds to MoN, MoO₂ and MoO₃, respectively Mo 3d_5/2 region. The peak positioned at 229.5 eV indicates the MoOOH species present in 3d_5/2 region. The two peaks at 233.6 eV and 235.8 eV correlate with the MoO₂ and MoO₃ species in Mo 3d_3/2. Similarly, the binding energies for N 1s and Mo 3p^δ+ 3p_3/2 were obtained from the de-convolution of a broad peak at 397eV. With 393 eV and 398 eV being the reference energies of Mo 3p and N 1s, the Mo 3p peak tends to shift to higher energies while the N 1s peak is shifted to lower energies. This shift is due to the increased electron density of nitrogen which destabilizes the N 1s core energy level, [30]. From Fig. 4(d), we can see that the Mo 3p_3/2 and Mo 3p_1/2 levels are split due to spin-orbit effects. The peak at 395.3 eV is assigned to Mo⁴⁺ (4d³) corresponds to the MoO₂ species dominant in the electrode and the peak at 398.8 eV is assigned to Mo³⁺ corresponds to MoO₃ phase. Further, the peaks at 415 eV and 413 eV in Mo 3p_1/2 region, are due to Mo⁴⁺ and Mo³⁺ ions, respectively. It is also assumed that the metal-like behavior is seen in the MoO₂ compound when Mo³⁺ content is reduced, [31]. The peak at 397.6 eV corresponds to the VN species in the N 1s region, [22]. The deconvoluted Ni 2p spectra is shown in Fig. 4(b), in which the prominent peaks having binding energies 852.9 eV, corresponds to Ni metal suggesting that the MoNi₄ alloy formed on the surface after annealing. The deconvolution of the broad peak positioned at 856 eV allows the identification of various Nickel oxides. Thus, the first peak at 856.2 corresponds to NiMoO₄ and the peak at 857.2 is identified as Ni₂O₃ in the Ni 2p_3/2 states with a satellite peak at 862 eV. This suggests that the higher oxidation states of Ni are present in the sample analyzed. Both V 2p region and O1s region can be identified in Fig. 4(c). The O1s region is found around 530.6 eV and can be related to oxygen atoms linked to single metal or metal hydroxide present at the extreme surface, therefore named “O1s (O-M)”. The distinct peaks at 516.5 eV and 523.5 eV show that the V 2p_3/2 (4+) and V 2p_1/2(4+) respectively, are also present. However, in this state vanadium is unstable and dismutate gradually to higher and lower oxidation states like V2p_3/2 (5+) or V2p3/2 (3+), [32]. The analysis presented confirms the presence of various metal oxides and nitrides deposited on each step of the process. Similarly, MoNi₄-MoO₂ and MoVN deposited on silicon was also investigated by XPS for elemental atomic surface concentrations and the data for the electrodes analyzed is tabulated in Table S1.

Electro-catalytic Activities

Hydrogen Evolution Reaction

The electrode materials in this work fabricated were tested in 1M KOH solution in a 3-electrode system, with Hg/HgO as the reference electrode for hydrogen evolution reaction (HER). Initially, cyclic voltammetry was conducted such that the electrode materials could achieve stable surface chemistry and to obtain a steady-state reaction environment. Then, linear sweep voltammetry was conducted with a small scan step of 2 mV/dec. The iR-corrected polarization curves in Fig. 5(a) show that the MoVN coated MoNi₄/MoO₂ nanorods exhibit best performance with a very small overpotential of 14 mV at 10 mA/cm². The other polarization curves presented for comparison show that other electrodes fabricated such as MoNi₄/MoO₂ and bare Ni foam present larger overpotentials, such as 68 mV and 169 mV at 10 mA/cm² respectively. The reaction mechanism of HER is evaluated using the Tafel slope that describes the catalytic reaction mechanism in terms of electron-transfer kinetics. A smaller Tafel slope reflects a significant current density increase due to a decrease of the overpotential. The Tafel slope values calculated for MoVN/MoNi₄/MoO₂, MoNi₄/MoO₂, and Ni foam are 59 mV/dec, 107 mV/dec and 165 mV/dec respectively shown in Fig. 5(b). These slopes suggest that the HER reaction kinetics at the surface of the electrode MoVN/MoNi₄/MoO₂ exhibits a fast Tafel reaction with 59 mV/dec slope relative to other electrodes obeying Volmer-Heyrovsky reaction mechanism. This superior performance can be attributed to faster electron transfer ability of the MoVN composite of MoNi₄ when compared to MoNi₄-MoO₂, [33, 34]. Further, the catalytic activities of similar materials in the literature have been compared in Table S6. According to the data presented, it is evident that the composite electrode presented in this work has exhibited good performance with low overpotential that is comparable to that of the best electrode materials and low Tafel slope values. Cyclic voltammetry was performed in the non-faradic region of each electrode at different scan rates for the calculation of Double layer capacitance (C_DL) as shown in Figure S5(a). Figure 5(c) shows the electrochemical impedance spectroscopy analysis using Nyquist plots. The EIS is conducted with AC amplitude of 10 mV with a frequency range of 100 kHz to 10 MHz at the onset potential of each electrode. The obtained Nyquist plots were fitted by equivalent circuit (Rs(Q(Rct)(QRp)) to find the solution resistance R_s, charge transfer resistance R_ct, and Polarization resistance (Rp) for the fabricated electrode. This equivalent circuit is chosen because the performed test includes a simple electrochemical interface (electrode-electrolyte) with heterogenous electrode material. The results of the EIS analysis are presented in Table S2 and the very low R_ct value 0.028 Ω of final electrode (MoVN/MoNi₄/MoO₂) when compared to 0.25 Ω, 1.53 Ω R_ct values of MoNi₄/MoO₂ and NF proves the improved charge transfer efficiency associated with the deposition of the transition metal nitride on MoNi₄/MoO₂ nanorods. Apart from the improved activity of the electrodes with respect to the HER, the stability over time of the activity plays an important role in the perspective that such electrode materials could be commercialized. Therefore, in this work, we perform static prolonged electrolysis using the fabricated electrodes and chronoamperometry for 12 hours at the overpotential of 14 mV vs RHE needed to achieve a current density of 10 mA/cm². The stability of the HER activity is noticeable in Fig. 5(d), which shows that the current density is maintained virtually unchanged, suggesting excellent mass transport properties and physical/chemical stability of the electrode-electrolyte interface. In addition to the three-electrode configuration testing, the synthesized electrode was tested using a flow electrode setup (Figure S6) where the electrolyte is constantly circulated. This reduces the bubbles that cover the surface of the catalyst once H₂ starts evolving. Further, the produced gas is collected and subjected to gas chromatography-mass spectrometry (GC-MS). Through this technique, the quality of the gas evolved is obtained by comparing it with the H₂ standard. Finally, the hydrogen production rate was also calculated which is normalized using reactor volume and time (Table S4).

Oxygen Evolution Reaction

The oxygen evolution reaction, which is the anodic half-cell reaction, is more favorable in alkaline medium when transition metals like Co, Fe, Ni and Mo are used as catalyst materials, [39]. Under alkaline conditions, the catalytic activity leads to the formation of hydroxyl groups (OH⁻) as an intermediary step. The reaction is assumed to proceed through four intermediate steps [18] and the hydroxyl groups are ultimately oxidized to H₂O and O₂ with the release of an electron. Unlike HER, the OER mechanism cannot be determined by the Tafel slope value as it includes more than two steps and the activation of each step will add up to the overall overpotential. Here, we also evaluate the OER activity of the fabricated electrodes in alkaline medium (1M KOH). As previously done (HER), we start by stabilizing the electrode activity subjecting it to several cycles of oxidation and reduction. Following the initialization step, linear sweep voltammetry was performed in the potential range − 100 mV to 600 mV vs. the reversible hydrogen electrode (RHE) using a 2 mV/sec sweep rate.

The polarization curves presented in Fig. 6(a) were determined for each of the three electrode materials and are iR-corrected for potential drop. Ni oxidation peak which takes place below 1.4 V, prior to the onset of OER, is omitted for clarity. It is evident that the OER performance is better when MoVN/MoNi₄/MoO₂ electrode material is used, as evidenced by the lower overpotential required to attain a current density of 10 mA/cm², suggesting that the addition of the MoVN lowers the energy barrier necessary for the transfer of the electrons from Ni in high valence states (Ni³⁺. Ni⁴⁺) that are believed to participate in OER, [19]. According to the data measured during OER experiments, MoVN/MoNi₄/MoO₂ electrode material exhibited a low overpotential of 244 mV compared to other electrode materials MoNi₄/MoO₂ and NF for which the measured overpotential was 252 mV and 310 mV respectively. Thus, the current vs. potential measurements show that the best electrode for HER (MoVN/MoNi₄/MoO₂₎ also performed well for OER indicating that the material has significant bifunctional potential for water splitting applications. Overpotentials of various other electrocatalyst are compared with the electrodes in this work and presented in the supplementary information section in Table S5. The decreased Tafel slope values in OER can be attributed to the improved reaction kinetics and again the electrode with low overpotential (MoVN/MoNi₄/MoO₂) has lowest Tafel slope value of 28 mV/dec followed by MoNi₄/MoO₂ and NF with 49 mV/dec and 118 mV/dec respectively as shown in Fig. 6(b).

Cyclic voltammetry was performed in non-faradic regions at different scan rates to find C_DL for the determination of ECSA shown in Figure S5(b). EIS was performed with a frequency range of 100 kHz to 10 MHz at the onset potential of each electrode. The AC amplitude used is 10 mV for all the measurements and Nyquist plots obtained were fitted to an equivalent circuit (Rs(Q(Rct)(QRp)), in Fig. 6(c). The results of the EIS analysis are tabulated in Table S3. The measured R_ct values of the three electrodes MoVN/MoNi₄/MoO₂, MoNi₄/MoO₂ and NF are 0.45 Ω, 1.98 Ω and 5.37 Ω respectively, [40]. Here, one can observe that there is a significant decrease in the charge transfer resistance of Nickel foam compared to MoVN/MoNi₄/MoO₂, demonstrating that transition metals are good catalytic materials for OER in alkaline conditions.

Further, the stability of OER activity was also checked by performing chronoamperometry for 12 hours at the overpotential value of 1.47V used for OER. In Fig. 6(d), there is small change with 0.75% reduction in the current density after 12 hours showing that the electrode has stable activity for a long period. This decrease in the current density can be attributed to a decrease of the active sites due to the formation of oxygen bubbles at the electrode surface. Despite this reduction in catalytic activity with time, the fabricated electrode proved to be stable and X-ray photoelectron spectroscopy (XPS) analysis has found the surface free of chemical degradation.

Determination of Electrochemical Surface Area

The determination of electrochemical surface area is important to find the specific activity of the catalysts under study as ECSA-normalized specific activity at any current density will stay constant regardless of the loading of the catalyst. The main challenge with this method is to determine the exact ECSA of a given catalyst. The ECSA is calculated for both half-cell reactions by using the double-layer capacitance method which is universally applicable to all electrocatalysts and results from the adsorption and desorption of ions in the electrolyte on the surface of the electrode, [41]. Hence, ECSA was obtained for each electrode by using the C_DL calculated from EIS results. This was compared to the ECSA obtained from cyclic voltammetry as shown in Fig. 7. It was found that the ECSA of the composite electrode is higher compared to MoNi₄-MoO₂ and Ni foam demonstrating that the increased active surface area that contributes to the better catalytic activity of MoVN/MoNi₄-MoO₂. The smaller values of the ESCA obtained from the EIS, when compared to those obtained from CV, can be attributed to the fact that EIS considers only the actual surface on which the specific reaction (HER/OER) occurs and excludes all electrochemically inactive surfaces. Also, another contributing factor may be the reduction of surface area due to the formation of gas bubbles on the surface at the onset potential in EIS measurements.

Overall electrolysis

Given the demonstrated activity of the synthesized electrode towards both HER and OER, the overall water splitting was carried out by fabricating both the anode and cathode electrodes of the same material and testing them in 1M KOH solution. As one can observe in Fig. 8(a), electrolysis was performed under stable conditions for 12 hours and it needed only 1.56V to achieve a current density of 10 mA/cm², Fig. 8(b). The negligible change in the current density proves the stability and bifunctionality of the synthesized electrode for water splitting. The combined overpotential required for both HER and OER is only 330 mV and is comparable to the electrode performance used in the commercial electrolyzer systems.

The composite electrode MoVN/MoNi₄-MoO₂ which was used for overall water electrolysis was investigated for degradation by X-ray photoelectron spectroscopy. XPS spectra of the electrode before and after electrolysis is shown in Fig. 9. It was found that there is difference in the surface concentration of various species including Ni (OH)₂, between the anode and the cathode. The presence of Ni (OH)₂ at 855 eV indicates that new phases of the material are seen because of continuous oxidation and reduction at both ends. This also suggests that during OER Ni electrocatalyst transitions to Ni (OH)₂ and is expected to oxidize to NiOOH prior to onset of the reaction, [42]. A large amount of potassium is observed at 294 eV which could arise due to the intercalation of the potassium from the KOH electrolyte used. Recent findings establish that the incorporation of trace quantities of Fe impurity from commonly used KOH electrolyte may contribute to the increased OER activity, [43]. The cathode and anode subjected to electrolysis with KOH electrolyte for 12 hours showed an increase in the Fe concentration of 1.2% and 0.81%, respectively. The complete results of the XPS analysis are presented in the supplemental information section, Table S7. It is also noticeable that the metal nitride content is reduced at the cathode and this decrease could be attributed to its chemical instability in reduction reaction, [44]. The metal oxide species content ~ 530 eV is seen to increase significantly due to the rapid oxidation of the electrode surface at the anode. The subsistence of Ni metal signifies that the MoNi₄ alloy on the surface of the electrode and that the electrode is devoid of any chemical separation between Mo and Ni under electrochemical potential. Overall, the slight changes in surface chemistry of the electrode after the application of the bias suggest that electrode had not undergone any significant degradation for the 12 hours of continued electrolysis and support the assertion that MoVN/MoNi₄-MoO₂ is an efficient bifunctional electrode for water splitting.

In this work, a facile method was used to prepare MoNi₄ particles on MoO₂ nanorods using hydrothermal synthesis. Further, coating transition metal nitride (MoVN) on synthesized nanorods using DC and RF co-sputtering technique showed significant increase in the catalytic activity towards both HER and OER with measured overpotentials of 14 mV and 244 mV at 10 mA/cm², respectively. Electrochemical impedance spectroscopic results also show the decreased charge transfer resistance of the synthesized electrode for both half-cell reactions due to high electrochemical surface area. The stability tests of the electrode were also performed for both reactions under consideration, have shown that the electrode can deliver the expected current density for 12 hours without degradation. Furthermore, the low potential of 1.56V (@ 10 mA/cm²) required for overall water splitting without major chemical degradation suggests that the reported electrode material could have applicability in commercial H₂ production technologies.

CONFLICT OF INTERESTS

All authors declare that they have no conflicts of interest.

FUNDING

This work was supported by the Center of Excellence in Nanoelectronics and Nanotechnology awarded by NYSTAR Division of Empire State Development.

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You are reading this latest preprint version

MoVN-coated MoNi4 -MoO2 nanorods as a bifunctional electrode for Electrochemical water splitting

Status:

Journal Publication

Version 1

Abstract

Figures

INTRODUCTION

MATERIALS AND METHODS

Materials

Synthesis of MoNi₄/MoO₂ nanorods

Deposition of MoVN nanoflakes

Structural characterizations

Electrochemical characterization

RESULTS AND DISCUSSIONS

Structural Characterization

Electro-catalytic Activities

Hydrogen Evolution Reaction

Oxygen Evolution Reaction

Determination of Electrochemical Surface Area

Overall electrolysis

CONCLUSION

Declarations

CONFLICT OF INTERESTS

FUNDING

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1

MoVN-coated MoNi4 -MoO2 nanorods as a bifunctional electrode for Electrochemical water splitting

Status:

Journal Publication

Version 1

Abstract

Figures

INTRODUCTION

MATERIALS AND METHODS

Materials

Synthesis of MoNi4/MoO2 nanorods

Deposition of MoVN nanoflakes

Structural characterizations

Electrochemical characterization

RESULTS AND DISCUSSIONS

Structural Characterization

Electro-catalytic Activities

Hydrogen Evolution Reaction

Oxygen Evolution Reaction

Determination of Electrochemical Surface Area

Overall electrolysis

CONCLUSION

Declarations

CONFLICT OF INTERESTS

FUNDING

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1

Synthesis of MoNi₄/MoO₂ nanorods