Bifunctional electro-catalytic performances of NiMoO₄-NRs@RGO nanocomposites for oxygen evolution and oxygen reduction reactions

1 Introduction

The development of the clean and sustainable energy resources is the current challenge of the researchers to overcome energy crisis globally. Splitting of water by electrochemical process to produce clean and sustainable energy is one of the alternative approaches. Electrochemical splitting of water could consist of three reactions i.e. oxygen evolution reactions (OER, anodic region), oxygen reduction reactions (ORR, cathodic region) and hydrogen evolution reactions (HER, cathodic region). In the last one decade, noble metal (e.g. Ir, Ru, Pt) based nanostructured materials were used frequently as active and highly efficient electro-catalysts for water splitting reactions (Ahmed and Mao, 2016; Fang et al., 2019; Reier et al., 2012). High cost and non-commercial benefits could be main disadvantages of these electro-catalysts. To explore bifunctional/trifunctional electro-catalysts with low cost and high efficiency are highly desirable in order to develop the clean and sustainable energy resources and to control the energy crisis worldwide. Therefore, present work is focused on bifunctional electro-catalytic activities of low cost electrode materials in alkaline medium. In the recent years, nickel based molybdate materials have been widely used as electro-catalysts in water splitting for HER and OER at cathode and anode respectively (Ahmed et al., 2019b; Geng et al., 2019; Kasturi et al., 2020; Lu et al., 2020; Wang et al., 2018; Zhang et al., 2019, 2018). The water oxidation reaction suffers from slow reaction kinetics in electron-transfer process (4e⁻) due to high energy loss (Shaner et al., 2016). To overcome these issues and to enhance the efficiency of the electro-catalysts in energy conversion, this is necessary to develop highly efficient electro-catalysts by lowering the activation energy of water oxidation reactions. Recently, carbon-based nickel molybdate nanocomposites are reported as efficient electro-catalysts to support the enhancement in the electrolysis of water because of high electrical conductivity, chemical stability and surface area (An et al., 2020). Hydrothermally synthesized CoP₃/NiMoO₄ hierarchical hetero-structured materials showed efficient electro-catalytic activity in water splitting with the over-potential of 347 mV to OER using 1.0 M KOH (Wang et al., 2019). NiMoO₄/rGO (Jinlong et al., 2017), NiMoO₄/C (Jiang et al., 2019; Tong et al., 2019) and NiMoO₄/N-doped graphene (Feng et al., 2020) nanocomposites are recently reported as high performance energy storage materials. Moreover, NiMoO₄ nanostructured materials have also shown importance in variety of applications including supercapacitors (Abbas et al., 2020; Anil Kumar et al., 2020; Cai et al., 2014; Moosavifard et al., 2015), sensing (Karami and Taher, 2019), photo-catalysis (Ray et al., 2018), nitro-phenol reduction (Oudghiri-Hassani and Al Wadaani, 2018) and electro-oxidation of urea (Yang et al., 2019). Previously, we have designed metal tungstate (Ahmed et al., 2018; Alshehri et al., 2018; AlShehri et al., 2017) and metal molybdate (Ahmed et al., 2019a, 2019b) nanostructured materials for multifunctional catalytic activities including energy conversion and storage. Herein, we report the hydrothermal synthesis of NiMoO₄-NRs@RGO nanocomposites as efficient electro-catalysts in water splitting for OER and ORR. The synthesized NiMoO₄-NRs@RGO nanocomposites were characterized in details using various analytical techniques including XRD, Raman, FTIR, FESEM, HRTEM, and XPS. The electrochemical performances of NiMoO₄-NRs@RGO nanocomposites were also investigated in details for OER/ORR activities in alkaline medium.

2 Experimental

The following reagents, as received chemicals, have been used in the preparation of NiMoO₄-NRs@RGO nanocomposites: Ni(NO₃)₂·6H₂O (Sigma Aldrich, 99.90%), Na₂MoO₄·2H₂O (Merck, 99.50%), NaNO₃ (Alfa Aesar, 99.00%), KNO₃ (Alfa Aesar, 99.00%), ethylene glycol (Sigma Aldrich) and reduced graphene oxide (RGO, Sigma Aldrich). NiMoO₄-NRs were produced using the molten salts method as also reported earlier (Ahmed et al., 2019b). 100 mg NiMoO₄-NRs and 10 mg RGO were mixed together with 2 ml ethylene glycol and 20 ml de-ionized water followed by the sonication for 15 min to make the suspension. The pH of the suspension was maintained at ∼8.5 by the addition of an appropriate amount of potassium hydroxide. The prepared suspension was transported to the hydrothermal reactor and heated at 140 °C for 24 h. The dark grey colored product was obtained, followed washing by de-ionized water for several times. The resulting powder samples were then dried at 65 °C for 5 h.

The synthesized NiMoO₄-NRs@RGO nanocomposites were initially characterized using XRD (Bruker D-8 Advanced diffractometer), FESM (JEOL-JSM-7600F) and HRTEM (JEOL-JSM-2100F) for phase identification and surface morphology. Energy dispersive studies (EDS) of NiMoO₄-NRs@RGO nanocomposites were carried out for elemental analysis. FTIR (Bruker TENSOR-27) and Raman spectroscopic (Renishaw instrument) studies were used to investigate the functional groups of the materials. BET (Brunauer–Emmett–Teller) surface area of NiMoO₄-NRs@RGO nanocomposites were analyzed using physi-sorption instrument (Micromeritics ASAP-2020) by nitrogen adsorption–desorption isotherm. Ultra-high vacuum XPS (Kratos Axis-Ultra X-ray photoelectron spectrometer) was also used to investigate the oxidation states of the elements present in NiMoO₄-NRs@RGO nanocomposites.

Electro-catalytic performances of the NiMoO₄-NRs@RGO nanocomposites for OER/ORR activities were investigated on potentiostat–galvanostat electrochemical workstation (CHI 660E) using 0.5 M KOH electrolyte. The working electrode was prepared by preparing the slurry containing of 2.5 mg NiMoO₄-NRs@RGO nanocomposites as electro-catalysts, 0.50 ml isopropanol and 0.10 ml Nafion. One drop of prepared slurry has been placed on to the glassy carbon (GC, 0.07 cm²) which is act as working electrode. The prepared working electrodes were dried at 60 °C. The loaded amount of NiMoO₄-NRs@RGO catalysts was maintained of ∼0.22 mg/cm². Pt-wire and Ag/AgCl electrodes work as counter and reference electrodes, respectively. Electro-catalytic measurements including CV, LSV, Tafel and CA were carried out for OER/ORR. CV and LSV measurements were performed at various scan rates (i.e. 10, 25 and 50 mV/s) versus Ag/AgCl for OER and ORR in 0.5 M KOH. Rotating disc electrode experiments were also performed by LSV in O₂-saturated 0.5 M KOH versus Ag/AgCl at various rotation speeds from 500 to 3000 rpm. Koutecky-Levich (K–L) equation was employed to calculate the number of electrons involved in OER/ORR reactions (Wu et al., 2012; Zhou et al., 2016). CA experiments (current density vs time) were conducted at 0.55, 0.65 and 0.75 V in 0.5 M KOH versus Ag/AgCl for stability check.

3 Results and discussion

Phase identification and morphology of the prepared NiMoO₄-NRs@RGO nanocomposites were analyzed by XRD and FESEM studies. XRD patterns confirm the presence of NiMoO₄ and RGO in the prepared nanocomposite. XRD reflections of 〈1 1 0〉, 〈 −1 1 1〉, 〈 −2 0 1〉, 〈1 1 1〉, 〈0 2 1〉, 〈 −1 1 2〉, 〈2 2 0〉, 〈−2 2 2〉, 〈2 0 2〉, 〈0 4 0〉, 〈3 3 0〉, 〈2 4 0〉 and 〈 −2 0 4〉 correspond to monoclinic phase of NiMoO₄ material (JCPDS#31-0902) while presence of 〈0 0 2〉 reflection at two theta value of 25.88° belongs to RGO in nanocomposite (Fig. 1a). FESEM study of NiMoO₄-NRs@RGO nanocomposites reveals that NiMoO₄ nanorods are supported by reduced graphene oxide sheets as expected (Fig. 1b). FTIR spectrum of NiMoO₄-NRs@RGO nanocomposites shows two strong bands at ∼970 and ∼650 cm⁻¹ of NiMoO₄ (Klissurski et al., 2006; Wan et al., 2013) while FTIR bands at ∼1210 and ∼1555 cm⁻¹ confirm the presence of RGO in the nanocomposites (Fig. 1c). Fig. 1d represents Raman spectrum of NiMoO₄-NRs@RGO nanocomposites. The appeared bands at ∼704, ∼815, ∼910 and ∼958 cm⁻¹ belong to NiMoO₄ (Tian et al., 2018) whereas the acted bands at ∼1375 and ∼1575 cm⁻¹ symbolize to RGO (Kumar et al., 2017). BET surface area of NiMoO₄-NRs@RGO nanocomposites was found to be ∼40 m²/g from the N₂ adsorption and desorption studies. Fig. 2a shows TEM micrograph of RGO sheets as received. TEM and HERTEM studies of NiMoO₄-NRs@RGO nanocomposites are shown in Fig. 2b and c, respectively. Inset of Fig. 2c shows a clear image of the formation of lattice fringes with d-spacing value of ∼3.08 Å correspond to 〈2 2 0〉 plane of NiMoO₄. EDS study of NiMoO₄-NRs@RGO nanocomposites also confirms the presence of Ni, Mo and C elements (Fig. 2d). Note that EDS is equipped with TEM machine and the presence of Cu element in EDS is due to Cu based TEM grid.

Close

Fig. 1

(a) XRD, (b) FESEM, (c) FTIR, and (d) Raman studies of NiMoO₄-NRs@RGO nanocomposites.

Export to PPT

Close

Fig. 2

TEM images of (a) RGO sheets as received, and (b) NiMoO₄-NRs@RGO nanocomposites. (c) HRTEM, and (d) EDS studies of NiMoO₄-NRs@RGO nanocomposites. Inset of Fig. 2c shows lattice fringes from HRTEM.

Export to PPT

XPS measurement was carried out for the study of surface composition of the NiMoO₄-NRs@RGO nanocomposites as shown in Fig. 3. The appearance of four peaks of C 1s, Mo 3d, O 1s and Ni 2p at ∼285, ∼233, ∼535 and ∼856 eV respectively indicate that NiMoO₄- NRs are supported by RGO sheets. High resolution XPS spectrum of C 1s is associated with three de-convoluted peaks of C-O, C—C, and C⚌C (Fig. 3a). High resolution XPS spectrum of Mo 3d shows bands at ∼233 and ∼238 eV of Mo 3d_5/2 and Mo 3d_3/2, respectively, belong to Mo⁶⁺ oxidation state (Fig. 3b). Fig. 3c shows high resolution XPS spectrum of O 1s, indicated two de-convoluted peaks at ∼532 and ∼535 eV for oxygen contributions with metal oxygen bonds and RGO. High resolution XPS spectrum of Ni 2p shows two main bands at ∼856 and ∼875 eV of Ni 2p_3/2 and Ni 2p_1/2, respectively, with two satellite bands at ∼862 and ∼880 eV (Fig. 3d). The resulting XPS data is consistent with previous reports (Xu et al., 2018).

Close

Fig. 3

High resolution XPS spectra of (a) C 1s, (b) Mo 3d, (c) O 1s, and (d) Ni 2p of NiMoO₄-NRs@RGO nanocomposites.

Export to PPT

The electrochemical properties of NiMoO₄-NRs@RGO nanocomposites for bifunctional catalytic activities (OER/ORR) were investigated in 0.5 M KOH. Initially, CV measurements were carried out to check bifunctional electro-catalytic behavior of NiMoO₄-NRs@RGO nanocomposites at different scan rates (i.e. 10, 25 and 50 mV/s) under the applied potential range from −1 to +1 V vs Ag/AgCl as shown in Fig. 4a. Thereafter, LSV polarization experiments were conducted to confirm the bifunctional OER and ORR activities of NiMoO₄-NRs@RGO nanocomposites. LSV polarization curves of NiMoO₄-NRs@RGO nanocomposite for OER were collected in anodic region under the applied potential window from 0.0 to 1.0 V at the scan rates of 10, 25 and 50 mV/s as displayed in Fig. 4b. We observed that the NiMoO₄-NRs@RGO nanocomposites show efficient OER activity with the onset over-potential of ∼185 mV, which is even better than that of previously reported OER electro-catalysts. Fig. 4c shows LSV polarization curves of NiMoO₄-NRs@RGO nanocomposites for ORR in cathodic region from 0.0 to −0.7 V at 10, 25 and 50 mV/s. The NiMoO₄-NRs@RGO nanocomposites exhibit excellent ORR performance with low onset over-potential of ∼75 mV, which is much better than other reported ORR electro-catalysts. LSV polarization studies of NiMoO₄-NRs@RGO nanocomposites were also carried out by applying cathodic potential window range using rotating disc electrode at various rotations from 500 to 3000 rpm at 25 mV/s vs Ag/AgCl in O₂-saturated 0.5 M KOH electrolyte for ORR (Fig. 4d). This is noteworthy that the diffusion current densities of NiMoO₄-NRs@RGO nanocomposite increase significantly with rotations as expected because of oxygen diffusion distance. The number of electrons involved in water oxidation reactions (4e⁻) was calculated using the LSV polarization data at various rotations followed by Koutecky–Levich (K–L) equation. Details of K–L equation were also reported elsewhere (Wu et al., 2012; Zhou et al., 2016). The standard reaction for ORR in alkaline medium can be defined as follows: O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻. Tafel plots of NiMoO₄-NRs@RGO nanocomposites for OER and ORR at 10 mV/s are shown in Fig. 4e. Tafel studies explain the kinetics of water splitting reactions to OER/ORR depending on the nature and surface area of the materials. Tafel slope values of NiMoO₄-NRs@RGO nanocomposites were found to be ∼247 and ∼54 mV/decade for OER and ORR, respectively, in O₂-saturated 0.5 M KOH vs Ag/AgCl. Note that the experimental error in Tafel slopes to be ±5. Tafel slopes of noble metals oxides (IrO₂, RuO₂) were reported in the range from 45 to 215 mV/decade for OER/ORR (Cruz et al., 2011; Fang and Liu, 2010; Kadakia et al., 2014; Ouattara et al., 2009). Low onset over-potential and low Tafel slopes sustain low energy loss during electro-chemical water splitting reactions. Therefore, based on present results, the NiMoO₄-NRs@RGO nanocomposites exhibit excellent electro-catalytic performance in OER/ORR activities. Stability of NiMoO₄-NRs@RGO electrode materials was also investigated using potentio-static CA experiments. CA experimentations were conducted at various fixed potentials of 0.55, 0.65 and 0.75 V for 1000 s using the same electrolyte concentration (Fig. 4f). We have observed from CA measurements that the obtained current densities are quite constant at fixed potential with time, which indicates the stable nature of the electrode materials. Moreover, the stability of the NiMoO₄-NRs@RGO nanocomposites was also tested from XRD after electro-catalytic measurements as shown in Fig. 5. The sample for XRD was prepared on carbon paper followed by the LSV polarization studies under the similar conditions for OER/ORR. XRD patterns show that the electro-catalyst is quite stable with a small amount of impurity phase detected at two theta of 28.30°. Our results confirmed that NiMoO₄-NRs@RGO nanocomposites are quite stable and could be used as an alternate of noble metal oxides electro-catalysts for energy conversion as future electro-catalysts.

Close

Fig. 4

Electrochemical OER/ORR properties of NiMoO₄-NRs@RGO nanocomposites in 0.5 M KOH. (a) CV for OER/ORR. (b) LSV polarization curves for OER. LSV polarization curves (c) at various scan rates, and (d) at various rotations for ORR. (e) Tafel plots for OER/ORR. (f) CA plots for stability of NiMoO₄-NRs@RGO nanocomposites.

Export to PPT

Close

Fig. 5

XRD patterns of NiMoO₄-NRs@RGO nanocomposites after electro-catalysis.

Export to PPT

4 Conclusions

We conclude present work that NiMoO₄-NRs@RGO nanocomposites were synthesized by hydrothermal process. The prepared NiMoO₄-NRs@RGO nanocomposites show excellent electrochemical performance in alkaline medium (0.5 M KOH). The NiMoO₄-NRs@RGO nanocomposites exhibit low onset over-potentials (∼75 mV for ORR, ∼185 mV for OER) and low Tafel slopes (∼247 mV/dec for ORR and ∼54 mV/dec for OER) with 4e⁻ system in redox reactions. Therefore, low cost NiMoO₄-NRs@RGO nanocomposites show admirable electro-catalytic performance with low energy loss and could be an alternate of expensive electro-catalysts.