High performance Sm substituted Ni-Zn catalysts for green hydrogen generation via Photo/Electro catalytic water splitting processes

2.1 Raw materials

A series of Ni_0.9Zn_0.1Sm_yFe_2-yO₄ (y = 0.00–0.03) nanomaterials are produced via using the AR grade raw materials of Rankem, Sigma Aldrich, and SimSon companies including the citric acid (99 %), nickel nitrate (98 %), zinc nitrate (96 %), samarium nitrate (SmN₃O₉, 99.9 %), and the ferric nitrate (98 %). Moreover, the ethylene glycol (99 %) and ammonia (25 %) are used as the gel and pH precursors, respectively.

2.2 Methodology for the creation of Sm doped Ni-Zn catalysts

The fabrication of Sm substituted Ni-Zn ferrite involves the utilization of inorganic SC method. The process began via preparing a metal nitrate solution in a 100 mL of distilled water. Simultaneously, C₆H₈O₇ solution is made in 100 mL of distilled water, with appropriate proportions based on specific requirements of the experiment. Both the solutions are prepared using hot plate magnetic stirrer. After the metal nitrate solution is entirely dissolved, then it is introduced to the citric acid solution, and the final solution is agitated for 2 h. To achieve neutral pH, the solution of ammonia is added to the mixture. The ethylene glycol is introduced in the solution in an appropriate mass ratio to activate the gelation and transforming the solution into a viscid substance. The gel is heated to 120 °C to initiate the self-ignition process. After the auto-combustion, a dark-coloured powdery substance is obtained. This powdery material is subsequently mashed by mortar pestle to obtain the fine powder. The crushed powder is then subjected to calcination (800 °C, 6 h), a process aimed at enhancing the stability and final formation of the product. Fig. 1 shows the SC technique for the creation of Sm doped and pure Ni-Zn nanomaterials.

Close

Fig. 1

SC scheme of Sm doped and pure Ni-Zn nanomaterials

Export to PPT

Herein, Ni_0.9Zn_0.1Fe₂O₄ (y = 0.00), Ni_0.9Zn_0.1Sm_0.01Fe_1.99O₄ (y = 0.01), Ni_0.9Zn_0.1Sm_0.02Fe_1.98O₄ (y = 0.02), and Ni_0.9Zn_0.1Sm_0.03Fe_1.97O₄ (y = 0.03) catalysts are named as Ni-Zn1, Ni-Zn2, Ni-Zn3, and Ni-Zn4, respectively.

2.3 Characterization techniques

The structural features of generated specimens, such as phase formation and crystal structure, are studied by a X-ray diffractometer (Panalytical's X'Pert Pro). Field emission scanning electron microscopy (JSM 6100 (JEOL)) examined the surface morphology of generated Ni-Zn nanoparticles. A vibrating sample magnetometer (Model MicroSense) is used at ambient temperature to determine the magnetic behavior of produced materials.

2.4 Procedure for the photocatalytic H₂ measurements

In the context of photocatalytic water splitting for the hydrogen generation under the ambient conditions and irradiation with UV–visible light spanning wavelengths from 200 to 2400 nm, the electron donor sites are facilitated by the sacrificial agents (SA), namely 0.128 and 0.079 M of Na₂S and Na₂SO₃ (500 mg) correspondingly. SA serve to deliver the electron donors during photocatalytic reaction. The study investigates the efficacy of pure and Sm-doped Ni-Zn photocatalysts in this process. In order to perform the hydrogen yield experiment, an airtight photoreactor is filled with 50 mL of double-distilled water, 20 mg of photocatalyst, and SA. This photoreactor is a quartz cell that is cylindrical in shape and has a flat base and a bottleneck diameter of 4 cm. The concoction underwent the stirring and nitrogen are purging for 30 min, to form an inert environment that abolish the gas impurities including the dissolved O₂. The concoction was poured into a quartz chamber with a cylindrical shape, fixed onto the agitator, and adjusted so that its flat side rested 7 cm from the lamp. Employed for illumination by using a HgXe arc lamp (200 W, Newport, MODEL: 66906-200HXF-R15), emitting a steady irradiance of 180 W, with the concoction undergoing the continuous stirring. The hydrogen emission is measured using a gas chromatograph (Perkin Elmer) with a TCD detector. Using a customized syringe, the samples are collected every one-hour interval. The photocatalysts are parted from the mixture using the centrifugation, and then dried out. After that, they are observed for their photostability, recyclability, photocatalytic response, and reusability representing highest H₂ generation over the 3–4 h rounds.

2.5 Procedure for the electrocatalytic H₂ measurements

To evaluate the electrocatalytic activity of Ni-Zn materials, the electrochemical measurements are conducted using a three-electrode setup in a 0.5 M H₂SO₄ electrolyte solvent for hydrogen evolution reaction (HER). It is done at the ambient temperature via the Autolab PGSTAT204 equipment. Platinum and saturated calomel electrodes (SCE) are taken as counter and reference electrode. The working electrodes, composed of electrocatalysts, are prepared via coating catalyst dispersions onto ITO substrates (dimensions: 1 x 1 cm², sheet resistance < 10 O/sq, Vritra). Prior to the dispersion being drop-cast onto the substrates, the substrates are ultrasonically cleaned for ten minutes (using acetone, ethanol, and isopropanol). 6 mg of electrocatalysts are suspended in polymer binder consisting of isopropanol (300 μL) and Nafion (15 μL) to create the catalyst dispersions. The resultant concoctions underwent a half-hour sonication. After that, 150 μL of the dispersions are drop-cast onto the ITO substrate's conducting side, also known as the oxide layer side, and are vacuum-dried for a whole night at 60°C. Chronoamperometry (CA) data are acquired at −1 V for HER performance, and the linear sweep voltammetry (LSV) analysis at 100 mV/s. However, the cyclic voltammetry (CV) measurements are taken from 20-150 mV/s for the electrochemical investigation of Ni-Zn nanostructures.

3.1 XRD study

The structural characteristics of Ni_0.9Zn_0.1Sm_yFe_2-yO₄ nanomaterials are investigated within the 2theta range of 25-65⁰ through the utilization of XRD. Fig. 2(a) illustrates the XRD plots of Ni-Zn catalysts substituted with Sm, from which several parameters like hopping distances at interstitial sites (L_A & L_B), crystallite size (D), x-ray density (d_x), and lattice constant (a), are calculated and presented in Table 1. The specific diffraction peaks (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) observed in the XRD analysis show the creation of a singular spinel phase with Fd3m space geometry, excluding any other phases. All the diffraction results are in complete agreement with the JCPDS data: 019–0629 (Shahane et al., 2010). The “D” values of substituted and pure nanoferrites are computed by Debye-Scherrer formula (Table 1) as given below (Aisida et al., 2023).

Close

Fig. 2

(a) XRD (b) H-W graphs for the Ni_0.9Zn_0.1Sm_yFe_2-yO₄ nanomaterials

Export to PPT

The Hall-Williamson (H-W) analysis is used to determine the effect of both the crystallite size (D_H-W) and strain (ε) in the synthesised samples. The H-W study follow the following requirement as formulated below.

Close

Fig. 3

N-R plots of the Ni_0.9Zn_0.1Sm_yFe_2-yO₄ nanomaterials

Export to PPT

3.2 FESEM, EDX and elemental mapping studies

The surface of the Ni-Zn nanoferrites, pure and doped, are analyzed by FESEM analysis. Fig. 4(a-d) displays the grain size histograms and FESEM visuals of all the nanomaterials. The spherical and cubic shaped grains with well-defined boundaries are obtained by FESEM pictures of all prepared samples. Additionally, the size of grains is evaluated by the IMAGE J program. Based on the plots, it is clear that the generated Ni-Zn1, Ni-Zn2, Ni-Zn3, and Ni-Zn4 samples attains the grain size of 316, 200, 216, and 188 nm. The phenomena of crystallite aggregation is accountable for the occurrence of the larger grains (Hu et al., 2022; Kotwal et al., 2023c). Fig. 4(e) presents the EDX spectra and elemental mapping of Ni-Zn1 material. From EDX, the nickel, zinc, iron, and oxygen peaks are found. This observation provides the successful justification of successful preparation of the undoped Ni-Zn nanoferrite. Also, the elemental mapping of fabricated Ni-Zn1 sample, solidating the uniform distribution of Ni, Zn, Fe, and O across the synthesized nanomaterial.

Close

Fig. 4

FESEM images of (a) Ni-Zn1 (b) Ni-Zn2 (c) Ni-Zn3 (d) Ni-Zn4 samples and (e) EDX spectra and elemental mapping of Ni-Zn1 sample

Export to PPT

3.3 VSM study

The M−H study is done at a field of 20 KOe, as illustrated in Fig. 5(a). It is observed from the diagram that there are S-shaped loops indicating a soft ferromagnetic nature. Various factors like coercivity (H_c), remanence (M_r), squareness ratio (M_r/M_s), saturation magnetization (M_s), anisotropy constant (K), and magnetic moment (n_B) are determined based on the M−H data, as shown in Table 1. Further insights into the phenomenon of coercivity and remanence are presented in Fig. 5(b), whereas Fig. 5(c), shows the plots of M vs 1/H² for the manufactured samples. The M vs 1/H² plots are analyzed in order to determine the mean value of M_s. The change in M_s due to Sm doping can be elucidated by observing the exchange interactions and magnetic moment at the lattice sites. The Sm cations go to the site-A which increases the n_B at the site-B. This causing an increment in magnetization at the B-site. Hence, upto y = 0.02, the M_s of Ni-Zn nanomaterials are rised from 51.93 to 59.48 (Table 1). However, at high Sm doping (y = 0.03), the Sm ions prefer to go to the B-site and this will decrease the overall M_s of B-sublattice. This will happen due to lower n_B of Sm³⁺ (0 μ_B) ions in contrast to that of Fe³⁺ ions (5 μ_B). Therefore, the M_s of Ni-Zn4 sample goes on decreasing from 59.48 to 51.92 emu/g.

Close

Fig. 5

(a) M−H loop (b) Zoom view of “H_c” and “M_r” (c) Magnetization versus 1/coercivity² (d) Alternation of n_B, K, H_c, and M_s of Ni_0.9Zn_0.1Sm_yFe_2-yO₄ nanomaterials

Export to PPT

Fig. 5(d) illustrates the variation in H_c, M_s, n_B and K with the dopant content of Sm. The factors influencing the variation in “H_c” include the morphology and magneto-crystalline anisotropy. Brown's relation indicates that the M_s varies inversely with H_c. Consequently, this study similarly observes such trends in the produced samples (Table 1). With the increasing Sm doping, the values of “K” fall between 959.68 and 1846.11 erg/cm³ (Table 1). There are the direct relations among the K and H_c. Thus, the K exhibits a similar behavior to H_c. Moreover, the n_B and M_r/M_s are calculated using the respective relations (Kotwal et al., 2023a).

3.4 Photo/Electro catalytic water splitting study

3.4.1

3.4.1 Photocatalytic H₂ generation study

The process of water splitting study is performed to examine the catalytic activity of Ni-Zn nanoparticles for the photocatalytic H₂ yield. Fig. 6(a) illustrates a recording of photo hydrogen evolution over a period of time. In Fig. 6(b), the total apparent H₂ yield for the Ni-Zn1, Ni-Zn2, Ni-Zn3, and Ni-Zn4 photocatalysts after 4 h is 16.17,15.02, 23.47 and 24.99 mmol ${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$ (Table 2). The order of photocatalyst activity for the H₂ evolution is as follows: Ni-Zn4 > Ni-Zn3 > Ni-Zn2 > Ni-Zn1, with respective rates of 3.78, 3.22, 6.21, and 7.22 mmolg^-1_cath^-1 (Table 2). Among these compositions, the Ni-Zn4 demonstrated the highest photocatalytic activity, reaching 24.99 mmolg^-1_cath^-1. The repeatability and stability of the photocatalysts were assessed over multiple 4-hour reaction cycles under identical conditions, utilizing a gas chromatography analyzer for precise hydrogen evolution rate measurements. Fig. 6(d) presents the analysis of three consecutive cycles for the optimal photocatalyst, examining its H₂ emission rate, stability and repeatability. Notably, the 3rd cycle exhibited a slight decrement in H₂ emission to 23.35 mmolg_cat^-1 related to the initial cycle.

Close

Fig. 6

(a) Evolution curves of photocatalytic hydrogen exposed to UV–visible light (b) A comparative analysis of photocatalytic activity (c) average H₂ emission of fabricated photocatalysts per hour (d) Ni-Zn4 stability test for a maximum of three cycles

Export to PPT

Table 2 Photo/electro catalytic activity of Ni_0.9Zn_0.1Sm_yFe_2-yO₄ catalysts.

Catalysts	Photocatalytic Performance		Electrocatalytic Performance (HER)
	H2/8h (mmol ${\mathbf{g}}_{\mathbf{c}\mathbf{a}\mathbf{t}}^{-1}$ )	Average H2 Evolution (mmol ${\mathbf{g}}_{\mathbf{c}\mathbf{a}\mathbf{t}}^{-1}$ h−1)	Onset potential (V)	Overpotential (V) to attain 10 mA/cm2	Tafel slope (mV/dec)
Ni-Zn1	16.17	3.78	−0.88	−1.09	52.55
Ni-Zn2	15.02	3.22	−0.8	−1.06	48.67
Ni-Zn3	23.47	6.21	−0.8	−1.04	40.85
Ni-Zn4	24.99	7.22	−0.8	−1.03	39.18

3.4.2

3.4.2 Electrocatalytic H₂ generation study

The electrocatalytic activity of Ni-Zn materials as electrocatalysts are determined towards the electrocatalytic HER performance (Fig. 7). LSV and CV are used to investigate the hydrogen evolution reaction (HER), as illustrated in Fig. 7(a, b). The increase in Sm concentration within the synthesized Ni-Zn materials corresponded to an increase in overpotential observed at a cathode current density of 10 mAcm⁻², indicating the enhanced electrocatalytic HER activity of the Ni-Zn4 sample. Specifically, the overpotential values for the Ni-Zn1, Ni-Zn2, Ni-Zn3, and Ni-Zn4 electrocatalysts at 10 mAcm⁻² are recorded as −1.09, −1.06, −1.04, and −1.03 V, respectively. Chronoamperometric (CA) measurements are conducted to assess the durability and stability of Ni-Zn4 electrocatalyst, operated at a voltage of −1V for up to 12,000 s, revealing a small increment in current density (Fig. 8). Tafel polarization plots for all the electrocatalysts in water electrocatalysis are presented in Fig. 7(c), with the corresponding electrochemical parameters for HER as summarized in Table 2. Furthermore, the decrement in resistance throughout the applied potential window is detected at different scan rates ranging from 20 to 150 mV. Finally, as shown in Fig. 7(d), the CV analysis of the HER for the Ni-Zn4 electrocatalyst is carried out.

Close

Fig. 7

(a) LSV graph of HER (b) CV graph of HER (c) HER Tafel graph of the prepared electrocatalysts (d) CV plots of Ni-Zn4 at different scan rates from 20-150 mV towards HER

Export to PPT

Close

Fig. 8

CA study at −1 V for Ni-Zn4 sample

Export to PPT