Studies on Electrochemical Performance of Flower-Like La4Mo7O27 Nanostructures towards Oxygen Evolution Reaction

Hydrogen production from water electrolysis demands modern and improved catalytic materials with increased electrocatalytic characteristics. A simple hydrothermal approach was used to prepare lanthanum molybdate (La4Mo7O27) nanoflowers. The X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques were used to characterise the physical structure and crystallinity of the prepared La4Mo7O27 nanoflowers. The SEM images reveal La4Mo7O27 was composed of nanoflowers. The prepared La4Mo7O27 nanoflowers were used as electrocatalyst to catalyse oxygen evolution reaction (OER) where it requires the overpotentials of 400 mV to generate hydrogen at the current density of 20 mA cm−2, which is less than that of commercially available bare nickel foam (450 mV).

The increased reliance on fossil fuels and their effect on environment have necessitated research into alternate renewable fuels. In this regard, effective use of clean and renewable energy resources can minimize serious environmental disaster. 1,2 In recent years, there has been a lot of interest in converting solar energy to chemical energy, particularly hydrogen. 3 Producing hydrogen from renewable sources, especially high abundant water, is a big technical problem. Water splitting, on the other hand, is an uphill operation, requiring 237 kJ mol −1 of energy. 4,5 Electricity generated from solar and wind technologies can be used to generate hydrogen from water by electrolysis. 6 In addition, the resultant hydrogen can be used as a fuel to generate electricity. 7 This pollution-free approach for hydrogen production is a good alternative to fossil fuels. Water electrolysis involves two essential steps: hydrogen evolution and oxygen evolution reactions (HER and OER) where the main challenge associated with these reactions is higher overpotential required to drive electrolysis. Therefore, it is important to design and develop efficient electrocatalysts. The nanomaterials are found to be promising in designing highly efficient devices for various applications from engineering, energy, electronics, environment and biomedical applications. [8][9][10][11][12][13][14][15] Similarly, for energy related applications such as battery, super capacitors, hydrogen fuel generation etc. various nanomaterials have been explored. A literature review discloses that transition metal oxides with formula ABO 4 (A=Co, Ni, Mn or La & B=Mo or W) have recently been investigated as electrocatalysts for OER, oxygen reduction reaction (ORR), supercapacitors and lithium-ion batteries. 16 Recently, V. K. V. P Srirapu et al. 17 have synthesised Fesubstituted manganese molybdates with compositional formula Fe x Mn 1-x MoO 4 (x = 0, 0.25, 0.50 and 0.75) by co-precipitation method and characterized by XRD, FTIR, XPS and TEM, electrochemical impedance and polarization technique. OER reveals that the Fe substitution from 0.25 to 0.75 for Mn increase the electrocatalytic activity of oxide showing E = 0.60 V vs Hg/HgO. Similarly R. N. Singh et al. 18 have synthesised Mo/Fe thin films with ratio 1.0, 1.5 and 3.0 by co-precipitation method and reports that the activity seems to depend upon the ratio of Fe and Mo. The Tafel slope of 35 mV at low potential is observed.
Different non-precious electrocatalysts including metal oxides, metal oxynitrides, metal hydroxides, metal carbides, metal sulfides, metal selenides, and metal nitrides have been explored. [19][20][21][22][23] Recently, rare Earth metal oxide-based electrocatalysts have received significant research attention to catalyze water electrolysis. Lanthanum oxide is considered a potential candidate because of its flexible redox behaviour (multiple oxidation states), high chemical inertness and low toxicity. 4,24 Nirmala Kumari et al., 25 prepared mixed molybdates with cobalt and nickel with different ratios by microwave co-precipitation method and examined as electrocatalyst for oxidation of water. The OER result was reported to be 0.56 V as overpotential to generate current density of 20 mA cm −2 . The electrochemical OER/HER/Sensor performance of bimetallic oxides such as Ir-Ni mixed oxide, 26 CoFe-LDH/CoFe 2 O 4 composite, 27 Fe x Co y O 4 -rGO, 28 3D urchin-like Mo-W 18 O 49 , 29 have proved their potential to be used as electrocatalyst. However, the interfacial designing the proper stochiometric bimetallic oxide towards improving the surface activity, selectivity, and stability of catalysts is very challenging. 30  The overpotential (η) was calculated in accordance with the following equation   Where D = Crystallite size, β = full width at half-max of the peak value, λ = wavelength of X-ray (1.54 KÅ), θ = Bragg's angle (in radian). The average crystallite size was found to be 35.50 nm.

Results and Discussion
Structural characterization.- Figure 2 presents the SEM images of lanthanum molybdate nanoflowers with different magnification. The lanthanum molybdenum oxides prepared by hydrothermal process are composed of nano flowers. It is predicted that the during the hydrothermal conditions at very high temperature and pressure, initially the La and Mo ions forms crystal seeds. The further crystallisation growth of the formed crystal seeds propagates in a most favourable and thermodynamically stable crystal plane of La and Mo oxide, which results into formation of flower shaped morphology at nanoscale. Further, the presence of only lanthanum, molybdenum and oxygen in EDX (Fig. 3)  The difference in theoretical and experimental error is due to the fact that the EDS analysis was carried out on one spot, in which the orientation of crystallites and as well as atoms are different. However, the ratio would be close enough if the EDS measured at different spots and taken average. It is expected that the flower morphology provides an enhanced active site for OER.
The OER performance of the as-prepared lanthanum molybdate nanoflowers has been evaluated in an electrolyte 1.0 M KOH (Figs. 3a and 3b). An overpotential required to acquire 20, 50 and 100 mA cm −2 has been used to evaluate OER performance.   Figure 3b shows the LSV curve of lanthanum molybdate nanoflowers where it requires overpotential of 400, 410 and 420 to acquire 20, 50 and 100 mA cm −2 respectively. The OER performance at 20 mA cm −2 is to difficult measure in case of RuO 2 as we have an oxidation peak in the LSV curve. Therefore, the OER performance of RuO 2 is measured at 40 mA cm −2 and it is observed that the RuO 2 coated nickel foam exhibits the current density of 40 mA cm −2 at an overpotential of 380 mV, indicating inferior catalytic activity La 4 Mo 7 O 27 compared to the RuO 2. However, the OER performance of La 4 Mo 7 O 27 modified nickel foam is better than the bare nickel foam (450 mV). The improved OER performance of La 4 Mo 7 O 27 is may be due to the flower like morphology, which provides higher surface roughness and greater active sites for enhanced OER. The higher surface roughness and greater active sites result in the maximum water adsorption capability and expands the active site by easing the exposure of active material to electrolyte, which helps in the enhancing of intrinsic electrocatalytic activity. 31 The observed OER performance has been compared with the reported molybdenum based OER electrocatalysts and the results of which are presented in Table I.
To further understand the kinetic behaviour of the electrocatalysts, electrochemical impedance studies were carried out and the observed results were presented in Fig. 3c. The charge transfer resistance (R ct ) strongly associated with the electrocatalytic kinetics where lower R ct value indicates faster reaction rate and high exchange current density. The lanthanum molybdate nanoflowers exhibit lowest R ct value 8.8 Ω. The Tafel slope value is another descriptor for evaluating the kinetics of OER. It provides crucial information about the kinetics and mechanism of OER. As shown in the Fig. 3d, the sample lanthanum molybdate nanoflowers exhibits Tafel slope of 33 mV dec −1 indicating the faster catalytic kinetics.
The stability of an electrode is an important parameter to assess the electrocatalytic performance. The stability test is conducted using chronoamperometic technique and the observed result is shown in the Fig. 4, where the La 4 Mo 7 O 27 modified electrode exhibits the current density of 10 mA cm −2 even after 35 h, indicating good stability of La 4 Mo 7 O 27 in alkaline electrolyte.

Conclusions
In summary, lanthanum molybdate nanoflowers were successfully prepared through one pot hydrothermal method and used as an electrocatalyst for oxygen evolution reaction. The XRD result shows the successful formation of lanthanum molybdate with orthorhombic phase. The SEM images show that lanthanum molybdenum oxides prepared by hydrothermal process are composed of nano flowers. Further, the presence of only lanthanum, molybdenum and oxide in EDX confirms the purity of the lanthanum molybdate. The OER study reveals that the lanthanum molybdate nanoflowers requires overpotential 400 mV to acquire the current density of 20 mA cm −2 .