Facile Synthesis of Perovskite-Type Sm1-xSrxMnO3 (0 ≤ x ≤ 0.8), a Non-Precious Metal Oxides and its Electrocatalytic Analysis Towards the Oxygen Evolution Reaction (OER)

Fossil fuels are becoming harder to come by, and people are becoming more aware of the environmental and geopolitical issues that come with their use, which has spurred significant work on the generation of innovative energy storage and conversion systems using materials that are affordable, abundant, and environmentally safe. Higher power and more energy-efficient storage devices, such as lowtemperature fuel cells and rechargeable metal-air batteries that runs on electrocatalysis, have become the centre of attraction in the field of renewable energy. Both, the oxygen reduction $\left(\mathrm{O}_2+4 \mathrm{H}^{+}+4 \mathrm{e}^{-} \rightarrow 2 \mathrm{H}_2 \mathrm{O}\right.$; oxygen reduction reaction; $\left.\mathrm{ORR}\right)$ and the water electrolysis $\left(2 \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{O}_2+4 \mathrm{H}^{+}+4 \mathrm{e}^{-}\right.$; oxygen evolution reaction; OER) are the fundamental techniques for generating green energy, but because of their sluggish kinetics, they require an appropriate electrocatalyst. Another challenge is that the efficient electrocatalysts for water splitting are economically unfavoured and comprised of rare noble metals, so not only the cost friendly but also more lucrative approaches are required for the encountering the efficient electrocatalysts based on earth abundant elements only.

The previous reports [6] also describes a technique for differential design of metal oxide as a electrocatalysts for the OER. It came up with the catalysts which was non-noble metals-based perovskite structure and that have a much better efficiency than the states of-the-art catalysts, iridium oxide. Furthermore, S. K. Tiwari et al. [7] synthesized Sr-substituted lanthanum cobaltates and reported that it has much more electrocatalytic efficiency than those previously prepared by conventional ceramic or thermal decomposition methods [8-12]. In addition to this they also reported the high electrochemically active surface area of the oxide (specially of $\mathrm{La}_{0.2} \mathrm{Sr}_{0.8} \mathrm{CoO}_3$) with the same oxide obtained by nitrate decomposition [7]. These results promoted us to test the electrocatalytic activity of such type of perovskite oxides prepared by a low-temperature sol-gel route.

This paper focuses the synthesis and physicochemical characterization of perovskite oxide $\left(\mathrm{Sm}_{1-\mathrm{x}} \mathrm{Sr}_{\mathrm{x}} \mathrm{MnO}_3\right)$ using malic acid sol get route as an active electrocatalyst for water electrolysis.

3.1 Physicochemical characterization

3.1.1 Scanning Electron Micrograph (SEM)

Figure 1 represents the SE-micrograph of pure and Sr-substituted Samarium Manganite powder sintered at $650^{\circ} \mathrm{C}$ for $5 \mathrm{hrs}$. Morphological structure of oxide powders appeared to be similar with each oxide. A crust like structure has been observed in the micrograph.

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Figure 1. SE Micrographs of oxide powder sintered at 650°C for 5 hrs

$D=\frac{0.9 \lambda}{\beta \operatorname{Cos} \theta}$

was used to calculate the crystallite size of the material, where $\lambda$ is the wavelength of $\mathrm{Cu}-\mathrm{K} \alpha$ radiation source, $\beta$ is the full width at half maximum of peak (FWHM) and $\theta$ is the diffraction angle and values are given in Table 1 . The refined plot and the crystal structure obtained by using vesta software is shown in Figure 3.

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Figure 2. XRD powder patterns Sintered at $650^{\circ} \mathrm{C}$ for $5 \mathrm{h}$

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Figure 3. The profile plot and the crystal structure of $\mathrm{Sm}_{0.6} \mathrm{Sr}_{0.4} \mathrm{MnO}_3$

3.1.2 X-ray diffraction

Crystal structure of Samarium Strontium Manganese oxides was investigated by X-ray diffraction (Figure 2) by Panalytical X-pert 3 powder diffractometer with step size $0.013^{\circ}$ and time per step $29 \mathrm{~s}$ using $\mathrm{Cu}-\mathrm{K}_{\mathrm{a}}=1.5405 Å$. The obtained XRD patterns are the same which are associated to those in COD ID 1006169. The results of powder XRD analysis of the materials showed that it has a Monoclinic crystal structure of space group P 21/a. The quality factors of the refinement are in acceptable range with $\chi^2$ value 2.4352 for $\mathrm{Sm}_{0.6} \mathrm{Sr}_{0.4} \mathrm{MnO}_3$. The $\mathrm{x}$-ray diffraction patterns of the samples were refined using Rietveld method to calculate accurate unit cell dimensions as given in Table 1. The Scherer's formula [15],

Table 1. Lattice parameters for oxide powder sintered at 650°C for 5h

3.2 Electrochemical characterization

3.2.1 Cyclic voltammetry (CV)

The cyclic voltammogram of each oxide film electrode on $\mathrm{Ni}$ was recorded in the potential $0.0-0.7 \mathrm{~V}$ at the scan rate of $20 \mathrm{mV} \mathrm{sec}{ }^{-1}$ in $1 \mathrm{M} \mathrm{KOH}$ at $25^{\circ} \mathrm{C}$ to know their redox behaviour Figure 4 (A & B). Each voltammogram was observed to similar in nature. Each voltammogram consists of pair of peaks i.e. anodic (up) and cathodic (down), just prior to the oxygen evolution. The value of redox peaks is almost similar to the redox couple of the $\mathrm{Ni}^{16}$. Thus, in the case of oxide film electrode, these peaks were originated due to the redox reaction at Ni-support in contact with electrolyte ${ }^{14}$ through cracks, pores, intercrystalline gaps formed in the catalytic film. Also, the oxides prepared at low temperatures have a hydrophilic character and quickly hydrate in aqueous solutions, soaking the entire film thickness. ${ }^{16}$. However, the stability of the film electrode is not affected from this behaviour of the catalyst. From each voltammogram, the values of cyclic voltammetric parameters such as the anodic and cathodic peak potential ($\mathrm{E}_{\mathrm{Pa}_{\mathrm{a}}}$ and $\mathrm{E}_{\mathrm{pc}}$ respectively), the peak separation potential $(\Delta \mathrm{E})$ and the formal redox potential $\left(\mathrm{E}^{\circ}\right)$ were calculated for the surface redox reaction and are given in Table 2. From the data obtained it can be clearly observed that on increasing the scan rates the anodic and cathodic peak shifted towards higher current values. Apart from this the ratio of anodic and cathodic peak current was found to be 2 . This indicates that redox process is irreversible$^{17,18}$.

Table 2. Values of the cyclic voltammetric parameters on Sm_1-xSr_xMnO₃ (0 ≤ x ≤ 0.8) in 1 M KOH at 25℃ (scan rate = 20 mVsec^-1)

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Figure 4. (A) Cyclic voltammograms of $\mathrm{SmMnO}_3$ film electrode at different $\mathrm{pH}$ on $\mathrm{Ni}$ at $20 \mathrm{mV} / \mathrm{sec}$ scan rate in $1 \mathrm{M}$ $\mathrm{KOH}\left(25^{\circ} \mathrm{C}\right)$. (B) Cyclic voltammogram of pure and Sr-substituted manganite $(\mathrm{pH}=3.5)$ film electrode on $\mathrm{Ni}$ at 20 $\mathrm{mV} / \mathrm{sec}$ scan rate in $1 \mathrm{M} \mathrm{KOH}\left(25^{\circ} \mathrm{C}\right)$

The variation of anodic current, cathodic current and voltammetric charge has also been studied with each oxide electrode. The values of $|j p|$ vs square root of scan rate was plotted for each oxide electrode and are shown in Figure 5 (A). From the figure, it is clear that the anodic and cathodic peak current density is linearity varied with square root of scan. The plot of voltammetric charge (q) vs scan rate^-1/2 was also constructed for each oxide electrode and shown in Figure 5 (B). The straight line obtained indicates that the surface redox behaviour is diffusion controlled¹⁶.

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Figure 5. (A) Plot of $\left|j_p\right|$ vs (scan rate) $)^{1 / 2}$ for the oxide film electrodes on $\mathrm{Ni}$ in $1 \mathrm{M} \mathrm{KOH}$ at $\left(25^{\circ} \mathrm{C}\right)$

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Figure 5. (B) Plot of q vs (scan rate) $)^{-1 / 2}$ for the oxide film electrodes on $\mathrm{Ni}$ in $1 \mathrm{M} \mathrm{KOH}$ at $\left(25^{\circ} \mathrm{C}\right)$

3.2.2 Electrocatalytic activity

The electrocatalytic activity of the oxide film electrode towards OER was determined by recording the Tafel curves ($\mathrm{E}$ vs $\log~ \mathrm{j}$) at $0.2 \mathrm{mVsec}^{-1}$ (a slow scan rate) in 1 M KOH at $25^{\circ} \mathrm{C}$. The Tafel polarization curves, so obtained, are shown in the Figure 6 (A). Nature of the polarization curve with each oxide electrode was similar irrespective of the preparation $\mathrm{pH}$ and metal ion substitution in the base oxide. The values of Tafel slopes (b), current density (j) at different potentials as well as potential at constant current densities were estimated from the polarization curve and values are given in the Table 3.

The order of OER with respect to $\mathrm{OH}^{-}$ion concentration was determined by recording Tafel polarization curves at different $\mathrm{KOH}$ concentrations $(0.25-1.5 \mathrm{M} \mathrm{KOH})$. The ionic strength of the medium was maintained at 1.5 by using an inert electrolyte $\mathrm{KNO}_3$. Polarization curve for $\mathrm{SmMnO}_3(\mathrm{pH}=3.5)$ $\& \mathrm{Sm}_{0.6} \mathrm{Sr}_{0.4} \mathrm{MnO}_3(\mathrm{pH}=3.5)$ oxide electrodes are shown in Figure 6 (B) & (C), respectively.

With a constant applied voltage, the value of current density was measured for each oxide catalyst at each concentration with the help of following relation:

$\eta=a \pm b \log j$                (3.1)

where $\eta$ is overpotential and $j$ is current density. ' $a$ ' is constant and equals to $\pm \mathrm{b} \log  ~\mathrm{j}_{\mathrm{o}}$. ' $\mathrm{b}$ ' is another constant term known as Tafel slope given by relation:

$\mathrm{b}=\frac{d \eta}{d(\log j)_{T, P, \mu}}$               (3.2)

here $\mathrm{T}, \mathrm{P}$ and $\mu$ are absolute temperature, pressure and ionic strength. The positive and negative sign shows the anodic and cathodic reaction, respectively. Because the second Tafel region was less defined due to increased polarisation at higher potential, the first linear Tafel region of the polarisation curve was typically taken into consideration for this reason. From the data, plot of $\log ~\mathrm{j}$ vs. $\log \left[\mathrm{OH}^{-}\right]$was constructed at a constant potential $\mathrm{E}=700 \mathrm{mV}$ and is shown in the Figure 6 (D). The value of reaction order (p) was estimated measuring the slope of plot of $\log \mathrm{j}$ vs $\log \left[\mathrm{OH}^{-}\right]$and given in the Table 3.

3.2.3 Thermodynamic parameters

On each oxide electrode, the impact of temperature on OER has also been investigated. Anodic polarisation curves in $1 \mathrm{M}$ $\mathrm{KOH}$ at various temperatures were acquired for the purpose. The polarization curves, so obtained are shown in Figure 7 (A) & (B). The reference electrode's temperature was kept constant throughout the experiment.

Table 3. Electrode kinetic parameters for oxygen evolution reaction on $\mathrm{Sm}_{1-\mathrm{x}} \mathrm{Sr}_{\mathrm{x}} \mathrm{MnO}_3(0 \leq \mathrm{x} \leq 0.8)$ in 1 M KOH at $25^{\circ} \mathrm{C}$

The slope of the Arrhenius plot, $\log \mathrm{j}$ versus $1 / \mathrm{T}$ [Figure 7 (C)] determined at a specific potential $(E=700 \mathrm{mV})$, was used to estimate the standard apparent enthalpy of activation $\left(\Delta \mathrm{H}_{e l}^{\circ}\right)$. The value of $\Delta \mathrm{H}_{e l}^{\circ \#}$ varies with potential and found that the value decreases with increasing the potential [Figure 7 (D)]. The standard enthalpy of activation $\left(\Delta H^{\circ \#}\right)$ and standard entropy of activation $\left(\Delta S^{\circ \#}\right)$ were calculated by using following relations (4.1) and (4.2), respectively ${ }^{19,20}$;

$\Delta \mathrm{H}_{e l}^{\circ \#}=\Delta \mathrm{H}^{\circ \#}-\alpha \mathrm{F} \eta$              (4.1)

$\Delta S^{\circ \#}=2.3 R\left[\log j+\Delta \mathrm{H}_{e l}^{\circ \#} / 2.3 R T-\log \left(n F {\omega _{\mathrm{COH}}}^{-}\right)\right]$                (4.2)

In equation 4.1 , ' $\alpha$ ' is the transfer coefficient given by 2.303RT/bF, where R, F and T are the gas constant, Faraday constant and absolute temperature, respectively. ' $b$ ' is the Tafel slope (in $\mathrm{mV}$ decade $^{-1}$) determined from the polarization curve recorded at different temperature. $\eta$ is the overpotential $\left(\eta=\mathrm{E}-\mathrm{E}_{\mathrm{O}_2 / \mathrm{OH}^{-}}\right.$, where $\mathrm{E}$ and $\mathrm{E}_{\mathrm{O}_2 / \mathrm{OH}^{-}}(=0.303 \mathrm{~V}$ vs. $\mathrm{Hg} / \mathrm{HgO})$ [21] are the applied potential across the catalyst/ $1 ~\mathrm{M} ~\mathrm{KOH}$ interface and the theoretical equilibrium Nernst potential in 1 $\mathrm{M} \mathrm{KOH}$ at $25^{\circ} \mathrm{C}$, respectively). The terms in equation (4.2) have their usual meaning comprising the frequency term $\omega$ (= $\mathrm{k}_{\mathrm{B}} \mathrm{T} / \mathrm{h}$) where, $\mathrm{k}_{\mathrm{B}}$ and $\mathrm{h}$ are the Boltzmann constant and Planck's constant, respectively. All the calculated thermodynamic parameters are given in the Table 4.1. The most active electrode, $\mathrm{Sm}_{0.8} \mathrm{Sr}_{0.2} \mathrm{MnO}_3(\mathrm{pH}=3.5)$ was found to have lowest $\Delta \mathrm{H}_{e l}^{\circ \#}$ value $\left(24.1 \mathrm{~kJ} \mathrm{~mol}{ }^{-1}\right)$. While, it found maximum with $\mathrm{SmMnO}_3(\mathrm{pH}=4.0)$.

From the Table 4.2 , the decrease in standard electrochemical energy of activation $\left(\Delta \mathrm{H}_{e l}^{\circ \#}\right)$ can be observed clearly with rise in applied potential. The decrease in $\Delta \mathrm{H}_{e l}^{\circ \#}$ is also predicted as per equation (4.1) and previous literature^20,21 .

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Figure 6. (A) Anodic polarization curve for the pure and $\mathrm{Sr}$ substituted samarium manganite film electrodes on $\mathrm{Ni}$ in $1 \mathrm{M}$ $\mathrm{KOH}\left(25^{\circ} \mathrm{C}\right)$; scan rate: $0.2 \mathrm{mVsec}^{-1}$ (B) Anodic polarization curve for the $\mathrm{SmMnO}_3(\mathrm{pH}=3.5)$ oxide film electrodes on $\mathrm{Ni}$ at different $\mathrm{KOH}$ concentrations $(\mu=1.5)$ at $25^{\circ} \mathrm{C}$ (C) Anodic polarization curve for $\mathrm{Sm}_{0.6} \mathrm{Sr}_{0.4} \mathrm{MnO}_3(\mathrm{pH}=3.5)$ oxide film electrodes on $\mathrm{Ni}$ at different $\mathrm{KOH}$ concentrations $(\mu=1.5)$ at $25^{\circ} \mathrm{C}$ (D) Plot of $\log \mathrm{j}$ vs $\log \left[\mathrm{OH}^{-}\right]$for $\mathrm{Ni} / \mathrm{Sm}_{1-}$ ${ }_{\mathrm{x}} \mathrm{Sr}_{\mathrm{x}} \mathrm{MnO}_3(0 \leq \mathrm{x} \leq 0.8)$ electrodes at $700 \mathrm{mV}$

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Figure 7. (A) Anodic polarization curve of the $\mathrm{SmMnO}_3$ $(\mathrm{pH}=3.5)$ film electrode on $\mathrm{Ni}$ at different temperature. (B) Anodic polarization curve of the $\mathrm{Sm}_{0.6} \mathrm{Sr}_{0.4} \mathrm{MnO}_3(\mathrm{pH}=3.5)$ film electrode on $\mathrm{Ni}$ at different temperature. (C) The Arrhenius plot of $\mathrm{Sm}_{1-\mathrm{x}} \mathrm{Sr}_{\mathrm{x}} \mathrm{MnO}_3(0 \leq \mathrm{x} \leq 0.8)$ in $1 \mathrm{M} \mathrm{KOH}$ at a constant applied potential $(\mathrm{E}=700 \mathrm{mV})$ (D) The Arrhenius plot of $\mathrm{Sm}_{0.6} \mathrm{Sr}_{0.4} \mathrm{MnO}_3$ in $1 \mathrm{M} \mathrm{KOH}$ at different applied potentials

The value of $\Delta \mathrm{S}^{\circ \#}$ was found to highly negative which suggests the adsorption phenomena in the oxygen evolution reaction.

Table 4.1 Thermodynamic parameters for $\mathrm{O}_2$ evolution on $\mathrm{Sm}_{1-\mathrm{x}} \mathrm{Sr}_{\mathrm{x}} \mathrm{MnO}_3(0 \leq \mathrm{x} \leq 0.8)$ in $1 \mathrm{M} \mathrm{KOH}($ at $\mathrm{E}=700 \mathrm{mV})$

Table 4.2 Thermodynamic parameters for $\mathrm{O}_2$ evolution on $\mathrm{Sm}_{0.6} \mathrm{Sr}_{0.4} \mathrm{MnO}_3$ in $1 \mathrm{M} \mathrm{KOH}$ at different potentials $(\alpha=0.86)$