Ni incorporation in MgFe₂O₄ for improved CO₂-splitting activity during solar fuel production

Abstract

Efficacy of the sol–gel derived Ni-doped Mg-ferrites for an enhanced CO₂ splitting activity is investigated. The results allied with the characterization indicate the formation of nominally phase pure Ni-doped Mg-ferrites with a coarser particle morphology. Ni-doped Mg-ferrites are further tested for multiple thermal reduction as well as CO₂ splitting steps by using a thermogravimetric analyzer. The results associated with the thermogravimetric analysis confirmed that most of the Ni-doped Mg-ferrites attained a steady TR aptitude after crossing the 5th or 6th cycle. Likewise, the CS capability of all the Ni-doped Mg-ferrites accomplished consistency after 4th cycle (except for Ni_0.11Mg_0.88Fe_2.01O_4.005). The Ni_0.90Mg_0.11Fe_2.04O_4.070 showed the highest amount of O₂ release (117.1 μmol/g cycle) and CO production (210.3 μmol/g cycle) in ten consecutive thermochemical cycles. Besides, Ni_0.29Mg_0.72Fe_1.98O_3.980 indicated better re-oxidation aptitude ($n_{\text{CO}} /n_{{{\text{O}}_{2} }}$ ratio = 1.89) when compared with other Ni-doped Mg-ferrites.

Introduction

Production of solar fuels, such as H₂ or syngas, via a metal oxide (MO) based two-step solar thermochemical H₂O/CO₂ cycle (STC) has emerged as one of the promising technologies [1,2,3]. H₂ can be used directly as a fuel [4], whereas the syngas can be converted into liquid fuels via a catalytic Fischer–Tropsch process [5]. With the help of solar fuel technology, the excessive utilization of fossil fuels and the continuous CO₂ emissions can be reduced. Besides, this technology can store the abundantly available solar energy in the form of liquid fuels such as gasoline, kerosene, and others.

In STCs, the MO plays a vital role in terms of the release and storage of the O₂. The O₂ gets released from the MOs during a thermal reduction (TR) step. On the other hand, the MOs can regain the O₂ during the H₂O/CO₂ (WS/CS) splitting reaction. In previous studies, a variety of MOs has been utilized for the thermochemical conversion of H₂O and CO₂, which mainly includes zinc oxide [6, 7], tin oxide [8, 9], iron oxide [10, 11], CeO₂ [12, 13], doped ceria materials [14, 15], ferrites [16,17,18,19,20], and La-based perovskites [21,22,23]. Among the listed MOs, CeO₂ is considered as a state-of-the-art material for the STCs due to its favorable reaction kinetics and high-temperature stability.

Recently, Takalkar et al. [24, 25] investigated the sol–gel derived Ni-ferrite and Mg-ferrite for the thermochemical splitting of CO₂. The reported results indicate that both Ni-ferrite (64.4 μmol of O₂/g cycle) and Mg-ferrite (58.7 μmol of O₂/g cycle) were capable of releasing higher levels of O₂ than CeO₂ (48.5 μmol of O₂/g cycle) [26] at 1400 °C. The amount of CO produced at 1000 °C by the Ni-ferrite (125.9 μmol of CO/g cycle) was higher and by the Mg-ferrite (79.6 μmol of CO/g cycle) was lower than CeO₂ (95.0 μmol of CO/g cycle). The obtained results show that both Ni-ferrite and Mg-ferrite possess a better TR aptitude than CeO₂. In contrast, work needs to be done to improve the CS ability of the Mg-ferrite.

To utilize the TR capacity of both Ni-ferrite and Mg-ferrite together and to improve the CS ability of the Mg-ferrite, in this study, we have examined ternary Ni-doped Mg-ferrites (Ni_xMg_1−xFe₂O_4−δ) in multiple thermochemical cycles. It is believed that the incorporation of Ni in the Mg-ferrite crystal structure (partially replacing Mg⁺² by Ni⁺²) will significantly improve the CS aptitude. The redox reactions associated with the two-step Ni-doped Mg-ferrite (NMF) based CS cycle are as follows:

$$ Ni_{x} {\text{Mg}}_{1 - x} {\text{Fe}}_{2} {\text{O}}_{4 } \to {\text{Ni}}_{x} {\text{Mg}}_{1 - x} {\text{Fe}}_{2} {\text{O}}_{4 - \delta } + \delta /2{\text{O}}_{2} $$

(1)

$$ {\text{Ni}}_{x} {\text{Mg}}_{1 - x} {\text{Fe}}_{2} {\text{O}}_{4 - \delta } + {\text{CO}}_{2} \to {\text{Ni}}_{x} {\text{Mg}}_{1 - x} {\text{Fe}}_{2} {\text{O}}_{4 } + {\text{CO}} $$

(2)

Ni-doped Mg-ferrites were tested by performing ten thermochemical cycles in which the TR and CS steps were carried out in the temperature range of 1000–1400 °C. Obtained results indicate that most of the Ni-doped Mg-ferrites attained the redox stability after crossing the 4th or 5th thermochemical cycle. This paper reports a detailed analysis of the results obtained during synthesis, characterization, and thermochemical cycles.

Experimental section

Preparation of Ni-doped Mg-ferrites

Ni-doped Mg-ferrites were synthesized by using the sol–gel approach. The stoichiometric amounts of precursors, i.e., nickel nitrate, magnesium nitrate, and iron nitrate (procured from Sigma Aldrich), were first dissolved in ethanol at room temperature. Subsequently, a predetermined quantity of propylene oxide (purchased from Sigma Aldrich) was added into the mixture. The as-prepared solution was kept undisturbed for the formation of the Ni-doped Mg-ferrite gel. The as-synthesized gel was aged and then dried at 120 °C for 2 h. The dried Ni-doped Mg-ferrite was then crushed into a fine powder by using a mortar and pestle. Annealing of the powered Ni-doped Mg-ferrite up to 1000 °C for 4 h was performed by using a muffle furnace.

Characterization of Ni-doped Mg-ferrites

Crystallographic analysis of the Ni-doped Mg-ferrites was carried out by using a Powder X-ray Diffractometer (PXRD) with CuKα radiation, the irradiation time of 5 s, and Bragg angle from 25° to 70° (Panalytical XPert MPD/DY636, λ = 0.15,418 nm, steps = 0.05°). The microstructural morphology and elemental/chemical composition of the Ni-doped Mg-ferrites were scrutinized by using a scanning electron microscope (SEM, Nova Nano 450, FEI) equipped with an energy-dispersive X-ray spectroscopy (EDS).

Cyclic CO₂ splitting test

A CO₂ splitting cyclic study which comprised of multiple TR and CS steps was performed by using a TGA (procured from Setaram SETSYS Evolution, France, Fig. 1). The TGA setup was equipped with two mass flow controllers and a vacuum gauge. The temperature cooling and heating rates were normalized at 25 K/min. The mass variation observed during the TR and CS steps was recorded by using an inbuilt Calisto processing software supplied by the Setaram. During the TGA experiments, approximately 50 mg of the Ni-doped Mg-ferrite powder was loaded on an alumina crucible and placed inside the TGA. Multiple thermochemical cycles were performed by maintaining the operating conditions as follows: a) TR at 1400 °C for 60 min with an Ar flow rate at 100 ml/min and b) CS at 1000 °C for 30 min with an Ar/CO₂ (50:50) mixture flow rate at 100 ml/min. The mass variation recorded by the software was further converted into the amounts of O₂ released and CO produced by using the following equations. Additional details associated with the other essential parts of the TGA setup and the experimental procedure are listed in our previous contributions [24, 25].

$$ n_{{{\text{O}}_{2} }} = \frac{{\Delta m_{\text{loss}} }}{{\left( {M_{{{\text{O}}_{2} }} \times m_{\text{NMF}} } \right)}} $$

(3)

$$ n_{\text{CO}} = \frac{{\Delta m_{\text{gain}} }}{{\left( {M_{\text{O}} \times m_{\text{NMF}} } \right)}} $$

(4)

where $n_{{{\text{O}}_{ 2} }}$ = moles of O₂ released (μmol/g); $n_{\text{CO}}$ = moles of CO produced (μmol/g); $\Delta m_{\text{loss}}$ = amount of loss in the mass (mg); $\Delta m_{\text{gain}}$ = amount of gain in the mass (mg); $M_{{{\text{O}}_{2} }}$ = molecular weight of O₂ (g/mol); $M_{\text{O}}$ = molecular weight of O (g/mol); $m_{\text{NMF}}$ = mass of the Ni-doped Mg-ferrites (mg)

Figure 1

A high-temperature TGA setup (Setaram SETSYS Evolution, France)

Results and discussion

The prime objective of this study is to identify the best combination of the Ni-doped Mg-ferrite, which shows better redox performance as compared to CeO₂, Ni-ferrite, and Mg-ferrite. Different combinations of the Ni-doped Mg-ferrites, which includes Ni_0.1Mg_0.9Fe₂O₄ (NMF19), Ni_0.3Mg_0.7Fe₂O₄ (NMF37), Ni_0.5Mg_0.5Fe₂O₄ (NMF55), Ni_0.7Mg_0.3Fe₂O₄ (NMF73), and Ni_0.9Mg_0.1Fe₂O₄ (NMF91), were synthesized by using the sol–gel method. The calcined Ni-doped Mg-ferrites were first analyzed by using PXRD, and the results obtained are reported in Fig. 2a, b. The PXRD peaks associated with each Ni-doped Mg-ferrite showed strong reflections of the cubic spinel structure with the non-existence of any metal or metal oxide impurities such as Ni, Mg, Fe, NiO, MgO, and Fe₃O₄. It was further noticed that the PXRD peaks were shifted toward a lower 2θ angle as the molar concentration of Ni was increased. As the crystal ionic radii of the Ni⁺² (83 pm) is smaller than the Mg⁺² (86 pm), the replacement of the Mg⁺² by Ni⁺² is the main reason for the shift in the PXRD peaks toward a lower 2θ angel. This result confirmed the substitution of Mg⁺² by Ni⁺² and the formation of a desired Ni-doped Mg-ferrite phase composition. An additional check was conducted by analyzing the Ni-doped Mg-ferrite samples by using an EDS apparatus. The obtained results (listed in Table 1) provide further assurance of the formation of nominally phase pure Ni-doped Mg-ferrites.

Figure 2

PXRD patterns of Ni-doped Mg-ferrites, a 2θ = 20° to 70°, and b 2θ = 34° to 38°

Table 1 Chemical composition of Ni-doped Mg-ferrites

In addition to the phase and chemical composition, the morphology of Ni-doped Mg-ferrites was scrutinized via SEM analysis. The SEM analysis established that the variation in the Ni⁺² and Mg⁺² molar concentrations has an insignificant influence on the morphology. Ni-doped Mg-ferrites showed coarser particle morphology with a diverse particle size in the range of ~ 100 nm (smallest particle) to ~ 1 micron (largest particle). The exemplified SEM images for the Ni_0.29Mg_0.72Fe_1.98O_3.980 and Ni_0.72Mg_0.31Fe_1.96O_3.970 are presented in Fig. 3.

Figure 3

SEM images of a Ni_0.29Mg_0.72Fe_1.98O_3.980, b Ni_0.72Mg_0.31Fe_1.96O_3.970

After synthesizing and analyzing the physical properties, the redox reactivity of each Ni-doped Mg-ferrite toward the thermochemical splitting of CO₂ was investigated by using a TGA setup. Firstly, the TR and CS ability of each Ni-doped Mg-ferrite was explored by performing a single thermochemical cycle, and the TGA profiles obtained are reported in Fig. 4. As expected, Ni-doped Mg-ferrites showed a decrease in the mass during the TR step (due to the release of O₂ and formation of O₂ vacancies) and an increase in the weight during the CS step (via re-oxidation reaction). These mass variations were converted into the $n_{{{\text{O}}_{ 2} }}$ (by using Eq. 3) and $n_{\text{CO}}$ (by using Eq. 4) by each Ni-doped Mg-ferrite during 1st cycle (Fig. 5). As per the findings, $n_{{{\text{O}}_{2} }}$ by Ni_0.90Mg_0.11Fe_2.04O_4.070 (327.7 μmol/g) and Ni_0.51Mg_0.50Fe_2.03O_4.055 (317.9 μmol/g) was considerably higher than the other three Ni-doped Mg-ferrites. Likewise, $n_{\text{CO}}$ by Ni_0.90Mg_0.11Fe_2.04O_4.070 (262.8 μmol/g) was the upmost when compared with other Ni-doped Mg-ferrites. Interesting to note that all the Ni-doped Mg-ferrites showed a higher $n_{{{\text{O}}_{2} }}$ as compared to $n_{\text{CO}}$.

Figure 4

TGA profiles obtained for Ni-doped Mg-ferrites during 1st thermochemical cycle

Figure 5

$n_{{{\text{O}}_{2} }}$ and $n_{\text{CO}}$ by Ni-doped Mg-ferrites during 1st thermochemical cycle

It is believed that during the high-temperature TR step, the chemicals left over from the synthesis of Ni-doped Mg-ferrites were burned. Due to this burning, an additional drop in the mass was recorded which contributed to the false indication of a higher $n_{{{\text{O}}_{2} }}$. In addition to the role of unburnt chemicals, there is a chance that the re-oxidation capacity of Ni-doped Mg-ferrites was not sizable enough and hence $n_{\text{CO}}$ was considerably lower than $n_{{{\text{O}}_{2} }}$. For further exploration, the Ni-doped Mg-ferrites were tested for an additional two thermochemical cycles. TGA profiles obtained for all three cycles are presented in Fig. 6. Besides, a comparison between the three cycles based on $n_{{{\text{O}}_{ 2} }}$ and $n_{\text{CO}}$ by Ni-doped Mg-ferrites is presented in Table 2.

Figure 6

TGA profiles obtained during three consecutive thermochemical cycles

Table 2 $n_{{{\text{O}}_{2} }}$ and $n_{\text{CO}}$ by Ni-doped Mg-ferrites during three consecutive thermochemical cycles

Table 2 clearly shows that $n_{{{\text{O}}_{2} }}$ by Ni-doped Mg-ferrites was decreased in the 2nd cycle as compared to the 1st cycle. In terms of numbers, $n_{{{\text{O}}_{2} }}$ by Ni_0.11Mg_0.88Fe_2.01O_4.005, Ni_0.29Mg_0.72Fe_1.98O_3.980, Ni_0.51Mg_0.50Fe_2.03O_4.055, Ni_0.72Mg_0.31Fe_1.96O_3.970, and Ni_0.90Mg_0.11Fe_2.04O_4.070 was reduced by 59.3%, 67.7%, 66.3%, 58.7%, and 59.0%, respectively, in cycle 2 as compared to cycle 1. The above-mentioned results confirmed that the higher levels of $n_{{{\text{O}}_{2} }}$ in cycle 1 was due to the release of the volatile matter and burning of the residual chemicals leftover from the synthesis. A further fall in $n_{{{\text{O}}_{2} }}$ which was recorded in cycle 3 was considerably lower than the reduction reported in cycle 2. For instance, a decline of about 11.0%, 1.9%, 4.1%, 6.9%, and 4.0% in $n_{{{\text{O}}_{2} }}$ by Ni_0.11Mg_0.88Fe_2.01O_4.005, Ni_0.29Mg_0.72Fe_1.98O_3.980, Ni_0.51Mg_0.50Fe_2.03O_4.055, Ni_0.72Mg_0.31Fe_1.96O_3.970, and Ni_0.90Mg_0.11Fe_2.04O_4.070 was noticed in cycle 3 when compared with the results obtained in cycle 2.

As shown in Table 2, $n_{\text{CO}}$ by Ni-doped Mg-ferrites follows the trend similar to the one observed in the case of $n_{{{\text{O}}_{2} }}$. $n_{\text{CO}}$ by Ni_0.11Mg_0.88Fe_2.01O_4.005, Ni_0.29Mg_0.72Fe_1.98O_3.980, Ni_0.51Mg_0.50Fe_2.03O_4.055, Ni_0.72Mg_0.31Fe_1.96O_3.970, and Ni_0.90Mg_0.11Fe_2.04O_4.070 was declined by 10.9%, 8.6%, 4.9%, 15.6%, and 11.8%, respectively, in cycle 2 as compared to cycle 1. In cycle 3, however, the percentage decrease in $n_{\text{CO}}$ for all Ni-doped Mg-ferrites was extremely lower when compared to cycle 2. The overall results obtained indicate that Ni_0.90Mg_0.11Fe_2.04O_4.070 possesses better redox reactivity toward the TR and CS reactions than the other Ni-doped Mg-ferrites. The CO production aptitude of all Ni-doped Mg-ferrites was observed to be steady in cycle 2 and cycle 3. However, it was also recorded that the O₂ releasing ability of Ni-doped Mg-ferrites has not attained stability in the first three cycles.

A set of successive ten thermochemical cycles was carried out to understand the long-term redox reactivity and stability of Ni-doped Mg-ferrites. The experimental conditions were kept unchanged, and the TGA profiles obtained for each Ni-doped Mg-ferrite are reported in Fig. 7. From the results obtained until now it is clear that $n_{{{\text{O}}_{2} }}$ by all Ni-doped Mg-ferrites during cycle 1 is not precise. Hence, to avoid publishing any misleading information, the data obtained during the 1st cycle were disregarded, and all the analysis was focused on the remaining nine cycles.

Figure 7

TGA profiles obtained during ten consecutive thermochemical cycles

Figure 7 shows the TGA profiles obtained during ten consecutive thermochemical cycles.

The TGA profiles reported in Fig. 7 provide an initial indication that the Ni-doped Mg-ferrites have attained a stable TR and CS ability after crossing cycle 5 or cycle 6. However, it was essential to verify this visual confirmation with the help of numbers. Hence, the mass variations recorded in the TGA profiles were converted into $n_{{{\text{O}}_{2} }}$ and $n_{\text{CO}}$ with the help of Eqs. (3) and (4).

$n_{{{\text{O}}_{2} }}$ by each Ni-doped Mg-ferrite (from cycle 2 to cycle 10) is presented in Fig. 8. The trends reported supported the visual confirmation provided by Fig. 7 and clearly shows that the Ni-doped Mg-ferrites have attained a steady TR aptitude after surpassing cycle 5 or cycle 6. To be precise, Ni_0.11Mg_0.88Fe_2.01O_4.005, Ni_0.29Mg_0.72Fe_1.98O_3.980, Ni_0.51Mg_0.50Fe_2.03O_4.055, Ni_0.72Mg_0.31Fe_1.96O_3.970, and Ni_0.90Mg_0.11Fe_2.04O_4.070 have indicated a stable $n_{{{\text{O}}_{2} }}$ once they cross over cycle 6, cycle 5, cycle 6, cycle 4, and cycle 5, respectively. In terms of the average TR ability from cycle 2 to cycle 10, Ni-doped Mg-ferrites can be arranged as follows: Ni_0.90Mg_0.11Fe_2.04O_4.070 > Ni_0.51Mg_0.50Fe_2.03O_4.055 > Ni_0.29Mg_0.72Fe_1.98O_3.980 > Ni_0.72Mg_0.31Fe_1.96O_3.970 > Ni_0.11Mg_0.88Fe_2.01O_4.005. The improvement in the TR ability of doped MO depends on the interaction between the metal cations incorporated in the crystal structure. At this moment, we are not in a position to provide any specific reason for the trends reported in Fig. 8 as we have not studied the molecular interaction between the metal cations associated with the Ni-doped Mg-ferrites.

Figure 8

$n_{{{\text{O}}_{2} }}$ by Ni-doped Mg-ferrites from cycle 2 to cycle 10

According to Fig. 9, the CS capability of all the Ni-doped Mg-ferrites has attained consistency after crossing cycle 4 except for Ni_0.90Mg_0.11Fe_2.04O_4.070 for which a steady $n_{\text{CO}}$ was recorded from cycle 5 to cycle 10. In terms of the average $n_{\text{CO}}$ from cycle 2 to cycle 10, Ni-doped Mg-ferrites can be ranked as: Ni_0.90Mg_0.11Fe_2.04O_4.070 > Ni_0.29Mg_0.72Fe_1.98O_3.980 > Ni_0.51Mg_0.50Fe_2.03O_4.055 > Ni_0.11Mg_0.88Fe_2.01O_4.005 > Ni_0.72Mg_0.31Fe_1.96O_3.970. It is noteworthy that the ranks achieved by Ni-doped Mg-ferrites in the case of CS capability are not identical to the ranks attained in case of the TR aptitude. For instance, Ni_0.51Mg_0.50Fe_2.03O_4.055 and Ni_0.29Mg_0.72Fe_1.98O_3.980 have achieved a position of 2nd and 3rd in terms of $n_{{{\text{O}}_{2} }}$. However, based on $n_{\text{CO}}$, Ni_0.51Mg_0.50Fe_2.03O_4.055 ranked 3rd whereas Ni_0.29Mg_0.72Fe_1.98O_3.980 has attained the 2nd rank. A similar observation was noticed in the case of Ni_0.72Mg_0.31Fe_1.96O_3.970 and Ni_0.11Mg_0.88Fe_2.01O_4.005. In contrast, Ni_0.90Mg_0.11Fe_2.04O_4.070 was observed to be the best candidate and attained the 1st position among all Ni-doped Mg-ferrites in terms of $n_{{{\text{O}}_{ 2} }}$ and $n_{\text{CO}}$.

Figure 9

$n_{\text{CO}}$ by Ni-doped Mg-ferrites from cycle 2 to cycle 10

As shown in Fig. 10, the average $n_{{{\text{O}}_{2} }}$ by Ni-doped Mg-ferrites from cycle 2 to cycle 10 was higher than CeO₂ (48.5 μmol of O₂/g cycle), NiFe₂O₄ (64.4 μmol of O₂/g cycle), and MgFe₂O₄ (58.7 μmol of O₂/g cycle). In terms of the maximum yield, the TR aptitude of Ni_0.90Mg_0.11Fe_2.04O_4.070 was recorded to be greater than the CeO₂, NiFe₂O₄, and MgFe₂O₄ by 68.6 μmol of O₂/g cycle, 52.7 μmol of O₂/g cycle, and 58.4 μmol of O₂/g cycle, respectively. Similar to the TR capacity, the CS ability of Ni-doped Mg-ferrites was superior to CeO₂, NiFe₂O₄, and MgFe₂O₄ (Fig. 10). The comparison shows that $n_{\text{CO}}$ by Ni_0.90Mg_0.11Fe_2.04O_4.070 was higher than CeO₂, NiFe₂O₄, and MgFe₂O₄ by a factor of 2.21, 1.67, and 2.64, respectively.

Figure 10

Comparison between Ni-doped Mg-ferrites and CeO₂, NiFe₂O₄, and MgFe₂O₄ based on the average $n_{{{\text{O}}_{ 2} }}$ and $n_{\text{CO}}$

Figure 11 shows the variations in the average $n_{\text{CO}} /n_{{{\text{O}}_{2} }}$ ratio for each Ni-doped Mg-ferrite. As per the data reported, the re-oxidation ability of Ni_0.29Mg_0.72Fe_1.98O_3.980 was observed to be maximum when compared with other Ni-doped Mg-ferrites. The average $n_{\text{CO}} /n_{{{\text{O}}_{2} }}$ ratios for the Ni_0.11Mg_0.88Fe_2.01O_4.005 and Ni_0.90Mg_0.11Fe_2.04O_4.070 were comparable, whereas Ni_0.51Mg_0.50Fe_2.03O_4.055 and Ni_0.72Mg_0.31Fe_1.96O_3.970 indicated the lowest re-oxidation capacity. It was further understood that the average $n_{\text{CO}} /n_{{{\text{O}}_{2} }}$ ratio of each Ni-doped Mg-ferrite was lower than CeO₂ (average $n_{\text{CO}} /n_{{{\text{O}}_{2} }} = 1.96$) and the NiFe₂O₄ (average $n_{\text{CO}} /n_{{{\text{O}}_{2} }} = 1.96$). In contrast, the comparison with MgFe₂O₄ (average $n_{\text{CO}} /n_{{{\text{O}}_{2} }} = 1.35$) indicates that the inclusion of Ni⁺² in the Mg-ferrite crystal structure has improved the average $n_{\text{CO}} /n_{{{\text{O}}_{2} }}$ ratio of Ni-doped Mg-ferrites.

Figure 11

Comparison between Ni-doped Mg-ferrites based on the average $n_{\text{CO}} /n_{{{\text{O}}_{2} }}$ ratio

Summary and conclusions

By applying the sol–gel method, Ni-doped Mg-ferrites were synthesized. The PXRD and EDS analysis confirmed the formation of Ni_0.11Mg_0.88Fe_2.01O_4.005, Ni_0.29Mg_0.72Fe_1.98O_3.98, Ni_0.51Mg_0.50Fe_2.03O_4.055, Ni_0.72Mg_0.31Fe_1.96O_3.97, and Ni_0.90Mg_0.11Fe_2.04O_4.07. The SEM analysis indicates the formation of coarser particles with sizes in the rage of ~ 100 nm (smallest particle size) to ~ 1 micron (largest particle size). Ni-doped Mg-ferrites were tested in multiple thermochemical cycles by using a TGA (TR at 1400 °C and CS at 1000 °C). Most of the Ni-doped Mg-ferrites indicated stable $n_{{{\text{O}}_{2} }}$ and $n_{\text{CO}}$ from cycle 5 to cycle 10. In terms of $n_{{{\text{O}}_{2} }}$, Ni-doped Mg-ferrites can be arranged in the following order: Ni_0.90Mg_0.11Fe_2.04O_4.07 > Ni_0.51Mg_0.50Fe_2.03O_4.055 > Ni_0.29Mg_0.72Fe_1.98O_3.98 > Ni_0.72Mg_0.31Fe_1.96O_3.97 > Ni_0.11Mg_0.88Fe_2.01O_4.005. Based on their CO production capacity, Ni-doped Mg-ferrites can be ranked as: Ni_0.90Mg_0.11Fe_2.04O_4.07 > Ni_0.29Mg_0.72Fe_1.98O_3.98 > Ni_0.51Mg_0.50Fe_2.03O_4.055 > Ni_0.11Mg_0.88Fe_2.01O_4.005 > Ni_0.72Mg_0.31Fe_1.96O_3.97. Ni-doped Mg-ferrites showed higher $n_{{{\text{O}}_{2} }}$ and $n_{\text{CO}}$ as compared to CeO₂, NiFe₂O₄, and MgFe₂O₄. In contrast, the average re-oxidation ability of Ni-doped Mg-ferrites was recorded to be lower than CeO₂ and NiFe₂O₄, and higher than MgFe₂O₄. Our research team is currently exploring the redox ability of Ni-doped Mg-ferrites for more than 100 thermochemical cycles.