Ce 0.8-2x Sm 0.2 Zrx Mgx O2-d, {x= 0.05, 0.1 &0.15} Promising Electrolyte Compounds With Remarkable Conductivities for Intermediate Solid Oxide Fuel Cells (It-SOFCs) Applications

1. Introduction

The shift from dependence on fossil fuels to renewable energy resources is very necessary nowadays, especially with their zero-pollution impacts like in solar cells, fuel cells, and other types of renewables [1,2,3,4,5]. Solid oxide fuel cell (SOFC) devices recently proved their highly remarkable source of chemical clean energy conversions. The main structure of these devices is defined by two electrodes (Anode and Cathode) detached by highly dense electrolyte materials generating power through electrochemical reactions [1,2]. The used electrolyte can be a proton ion conductor, otherwise an oxygen ion conductor [2,3,4], and they should be relatively or closely compacted, durable, and chemically and thermally stable in a reducing and oxidizing environment [5,6,7].

The more important thing recently investigated is using an integrated system from biomass gasification and fuel cells together [8,9,10,11,12,13,14] to get higher performance and supply more power. And after developing new materials to be applied in SOFCs, there is a higher potential to apply it in biomass-gasification fuel cell systems investigated in many research works [8]. Since H₂ exists as a low-density gas, posing a significant storage challenge. Hydrogen storage in the case of solid state and liquid methods relying on metal hydrides outperforms traditional bulky gaseous in terms of protection, production costs, and density [15,16,17]. MgH₂ is one of the most effective types among light hydrides with reversible H₂ storage due to its capacity for high hydrogen storage, its reversibility, and H₂ storage abundance reserves on Earth [18,19,20].

In the last two decades, scientists and researchers were seeking high-performance techniques to investigate material properties and they were focusing on Magnesium based hydrogen [15,21] with remarkable activities, especially with doping of Mg-oxides [22]. The addition of metal oxides (Fe₂O₃, Nb₂O₅, CeO₂, etc.) [16,23,24] or (Ni, Co, Fe, etc.) transition metals [15,25,26] to magnesium were reported as an effective candidate due to their altered reaction pathway in optimized thermodynamics [27]. Hence, developing new electrolyte materials through rare earth elements in single or multi-dopants is essentially required when applied in SOFC devices. Thereby, Lanthanides doped cerium { Sm, Gd, Nd } based materials have shown preferred closely or relatively compacted electrolytes applied in SOFCs at intermediate or low-temperature levels (IT/LT- SOFCs) [28,29,30,31,32,33,34]. These based crystalline structures are dry and wet chemical synthesized and have introduced high oxygen ionic conductivities at temperature ranges from 500 to 800 °C [35,36]. The single or multi-dopant ceria-based Rare-earth and alkaline- elements have greatly enhanced the ionic conductivities at a reduced temperature range compared with common electrolyte ones YSZ [37,38,39]. But introducing Mg doped in these compounds will be expected to show remarkable results in obtained power and the whole performance of the designed system. Many trials were made to investigate the use of Mg in various applications through energy devices as introduced by V.A. Yartys et al. [40] and recent work done through fuel cell devices. A trial was made by Jianjiang Hu et al., [41] where a high-temperature PEM fuel cell stack was used for a prototype auxiliary power unit, through the application of Li-Mg-N-H, they succeeded to get the Max working temperature being 200 °C with a designed output of 1 kW. While, Jingfeng Xue et al., [42] used pure perovskite Mg co-doped compounds at intermediate temperature SOFCs. They have got power of 0.5 Wcm^-2 at 600 °C and max power of 0.8 Wcm^-2 at 850 °C.

Flores et al [43] have made recently an investigation of a multi-dopant including Mg co-doped compound as double perovskites derived material Sr₂MgMoO_6-d as anode with potential application in SOFCs and succeeded to introduce materials able to show acceptable electrical conductivity, power density, and thermal expansion coefficient, in redox environments synthesized through various techniques.

In the current study, Ce_0.8-2xSm_0.2Zr_xMg_xO_2-d series of materials were synthesized {x = 0.05, 0.1 &0.15} as novel electrolyte compounds working in both intermediate and high-temperature ranges applied in SOFC devices which can be also used in an integrated system of biomass gasification with SOFC. Mg doping has shown a very significant and good impact on hydrogen storage capacity and as a result, we are trying to confirm this investigation by doping it in some materials that can be applied in fuel cell applications. The obtained results have shown the potential of applying Mg as electrolyte materials and it can also be used in electrode materials as well in SOFCs devices. Physical and electrochemical characterization for the materials were made and the obtained results showed remarkable results in both physical characterizations and the output power.

2. Experimental

2.1. CSZM materials synthesis

Series of Mg_x doped Ce_0.8-2xSm_0.2Zr_xO_2-d, {x= 0.05, 0.1 &0.15} were synthesized using solid-state chemistry method in the air at 1400 °C for 12 hours inside a muffle furnace. The chemicals of CeO₂, ZrO₂, Sm₂O₃, and MgO in precise stoichiometric ratios were used from Sigma Aldrich (purity of more than 99.95%). Collected powders were preheated at 350 °C for 8 hours in a muffle furnace for humidity and re-carbonation avoidance followed by quick weight on an accurate digital microbalance. All compounds with a little amount of acetone CH₃CoCH₃ were hand mixed in an agate mortar pestle for almost 2 hours. The ball milling technique was followed in the previous step to get a homogenous mixture of the compounds with an appropriate amount of ethanol C₂H₅OH for 48 hours. Afterward, a mixture of ball-milled compounds was collected and dried in a fume hood overnight. Finally, the collected powders were heated on a magnetic hotplate at 70 °C for 4 hours of each composition. The calcination process at 800 °C for 12 hours was achieved for the collected powders at a rate of 5 °C/min (heating and cooling). Again, samples were ground for 45 Minutes with a small amount of binder of terpineol before being compressed to form pellets of 2 grams 20 mm in diameter using the hydraulic press of 100 MPa. The formed pellets were fired at 1200 °C in the air for 12 hours in a carbolite furnace. Following that the X-Ray diffraction (XRD) for the compounds was checked to confirm the phase purity and reaction. Then again after regrinding like the earlier stage, CSZM pellets again sintered at a final temperature of 1400 °C for 12 hours to achieve a single phase and complete reaction of all synthesized series. Figure 1 summarizes the preparation process until getting the final pure phase.

2.2. Characterizations of the samples

The thermal response and behavior of the formed materials were investigated for the uncalcined powder through thermogravimetric analysis (TGA) PerkinElmer, STA 6000, USA in a range of 50 to 1000 °C and rate of 5 °C/min with flowing nitrogen 50 ml/min where samples were kept isothermally for 30 min at the final temperature. After that, the XRD patterns of the synthesized samples for Mg_x doped Ce_0.8-2xSm_0.2Zr_xO_2-d, {x= 0.05, 0.1 &0.15} where D8 Advanced Bruker XRD system with CuKα1 radiation of λ = 1.5406 Å and using 2θ = 10° to 90° angular range with 0.02° step size in RT. After XRD data were collected then WinXpow and TREOR90 [44] were used for indexing. Following the previous step, Rietveld refinements analysis of XRD data was done carefully using the FullProf suite [45]. Scanning electron microscope (SEM) of JEOL 5600 was used for studying the microstructure and crystal features for the 3 synthesized compounds of 5%,10%, and 15%. In addition investigation of Energy-dispersive X-ray Spectroscopy (EDX) analysis for the tested samples for element percentage in the final formed compound. For impedance measurements and analysis, a Solartron EIS spectrometer that has a frequency response analyzer was utilized. Where, a symmetrical cell was prepared to have a size of 13 mm Diameter ~1.1 mm thickness, where on both sides of the tested pellet platinum paste was printed with an effective area of 1cm². Afterward, the tested pellet was inserted in the furnace for drying at 700 °C for 1 h. In the impedance measurement, stage 50 mA current amplitude was applied in a range of temperature 300 to 800 °C with a stepped temperature of 50 °C and of 3 °C /min heating rate in a wet 5 %H₂/Ar with a range of frequency from1 MHz to 10 MHz.

3. Results and discussion

3.1. Structural and phase analysis of XRD data

The collected data of XRD spectra for the 3 synthesized series of Mg_x doped Ce_0.8-2xSm_0.2Zr_xO_2-d, x= 0.05, 0.1 &0.15} were analyzed by Rietveld analysis at RT after the final sintering temperature of 1400 °C is appearing in Figure 2 below with peak indexing showing the single-phase purity. The Rietveld analyses of the XRD patterns of all materials confirm a stable single-phase cubic structure in the F m-3m fluorite type in a space group with nearly similar cell parameter a= 5.4 ⁰A for all compounds.

Due to the indexed peaks with Winx POW and TREOR90 software the existence of sharp peaks like in (111), (200), (220) and (311) is because of well crystalline after the calcination process. Considering that values of observed minus calculated (obs-calc) represents indexed materials in a precise symmetry (cubic) and space group (F m-3m). following that the refinement parameters of all compounds are similar in their lattice in addition to the calculated theoretical density is almost very close in values when calculated using the following equation [46]:

$${\rho }_{Th}=\frac{4M{W}_{CSZM}}{N_{A}a}$$

(1)

Where MW_CSZM is the molecular weight of CSZM electrolytes and NA is Avogadro’s number, a representing lattice parameter as listed in Table 1 below. Rietveld refinement parameters of the CSZM materials showing similar values of lattice parameters as the samarium (Sm) amount is the same in the whole synthesized compounds and due to its ionic radius (0.958 Å, coordination number (CN) = 6) compared to cerium (Ce) (0.87 Å with the same CN) however density increased with increasing Mg doped amount from 5% to 15% due to the lattice parameter increase which directly affects the volume of the synthesized sample. The confirmed structure of the synthesized material after getting XRD patterns is obeying Vegard’s law. It is clear also that the change that happened in the peak shift as shown in Fig 1a was due to the small change of lattice parameters and due to the increase of Mg % dopant results in an increase of tensile strength that mamakes this small shift of the plane towards the lower angle. And this also can be clearly observed from the lattice parameters and volume of the indexed XRD patterns from the given data in Table 1 and confirms this with previous work in the literature [47].

However, the atomic position of the multi-dopant is almost the same in the 3 compounds and that was due to the stability of the structure with higher densification of Mg doping and when the percentage increased from 5% to 15% porosity increased with grain growth increase at higher sintering temperature of 1400 °C.

• Surface morphology of synthesized samples using SEM & EDX

Surface morphology and microstructure of the 3 synthesized compounds of electrolyte materials were carefully checked through SEM to show the level of densification at the 5%,10%, and 15% levels. Figure 3 shows clear view of the microstructures for CSZM compounds and images are showing that with higher doping of Mg resulted in higher porosity however, the final sintering temperature was 1400 °C. Thereby, the Mg doping has shown abnormal grain growth due to the thermomechanical treatments that have adirect effect and significant role in the investigation of the synthesized samples owing to the close dependence of properties upon grain size [48].

But the grain growth clearly occurred on 5% doping of Mg during the densification [49] compared to other percentages. 1 μm average grain size for the pellet sintered at 1200 °C, whereas for 1300 1.6 to 2.3 μm and at the final temperature of 1400 °C, the grain size ranged from 2.5 to 3.7 μm, From Figure 3a,c of the compound Ce_0.7Sm_0.2Zr_0.05Mg_0.05O_1.9. showing some inhomogeneity in grain sizes due to higher sintering temperatures, but the most interesting is getting well compact and dense electrolyte materials compared to the other 2 compounds. Some pores started to occur and with Mg doping increase, abnormal grain growth resulted in some pores being trapped inside the microstructure even with higher sintering temperatures it is difficult to erase [50,51,52]. The EDX elemental analysis of CSZM was investigated as shown in Figure 4 and it shows from the selected spectrum the existence of all chemicals Ce, Sm, Zr, and Mg with a total percentage of 100% without any other impurities and proof of a full and complete solid-state reaction occurred for all compound showing a pure phase fluorite type. What was interesting, is the homogeneity of the surface and there is no existence of any impurities or agglomerations [52] and it is clearly observed in the selected spectrum, considering this case for all synthesized compounds with the different doping percentages of Mg 5%, 10% and 15%.

• Thermogravimetric (TGA) of the synthesized series of Mg_x/2 doped Ce_0.8-xSm_0.2Zr_x/2O_2-d

Thermogravimetric data was collected on uncalcined samples of Ce_0.7Sm_0.2Zr_0.05Mg_0.05O_1.9 (5%Mg), Ce_0.6Sm_0.2Zr_0.1Mg_0.1O_1.9 (10 %Mg) and Ce_0.5Sm_0.2Zr_0.15Mg_0.15O_1.9 (15%Mg) to know the sintering behavior of the samples. The weight loss started around 60 °C, and lost about 6% until 275 °C which was due to the evaporation of the absorbed water molecule. From 275 °C to 730 °C, another big loss (about 30%) was observed due to the decomposition of some oxides and the organic fuel into different gases, such as nitrogen, carbon dioxide, and water vapors [53]. The final weight loss was observed until 900 °C which was about 15%. Figure 5 illustrates the TGA profile of all 3 samples. There are small differences between the samples which can be neglected. The high-temperature loss is normally related to the chemical reaction [54,55], oxygen vacancy formation, and valence change of the constituting cations. This change confirms the full crystallization and phase formation above 900 °C. All gaseous products generated by volatilization and chemical reaction are transferred from the furnace chamber with the help of N2 gas which guarantees that there is no interference with the sample during thermal treatment. The 50% total weight loss is normal, as we heated the samples under a nitrogen atmosphere to avoid the presence of H₂ or O₂. The high-temperature region of TGA in the nitrogen atmosphere indicates the oxygen loss from the crystallite.

• Electrochemical Impedance spectroscopy (EIS) of the prepared samples under 5 % H2/Ar.

The solid electrolyte electrical properties were investigated and studied using (EIS) measurements and based on the given Nyquist plots in the three synthesized compounds total ionic conductivities were evaluated accordingly according to the given data in Table 2, where symmetrical cells testing of the pellets were prepared and EIS measurements were achieved in the wet condition of 5% H₂/Ar.

The EIS developed measurements were used in measuring the electrical properties of CSZM compounds and the related electrical properties of the synthesized electrolyte material in a symmetrical cell testing based on anode-supported materials as can be observed from Table 2 and the resultant ionic conductivity was accordingly calculated. Through Nyquist plots and with the fitting results it was easy to identify the needed information about the separate contribution of grain resistance, grain boundary resistance, and electrode process. The EIS measurements were achieved and analyzed using a Solartron 1255 (Schlumberger) frequency response analyzer (0.01 Hz~1 MHz). Results from Nyquist patterns analysis followed the included circuit range of 400 °C to, 700 °C with a step of 50 °C in wet 5% H₂/Ar. Figure 6a,b,c show the fitted impedance plots obtained for CSZM oxides within the range of 400 °C to 700 °C. Resulted in plots were characterized by the start of inductance L1 and followed by semicircles of two regions indicating the polarization resistance Rp from the electrode as grain boundaries [56] along the whole tested ranges

From the analysis and fitting of the EIS plots, the capacitance was calculated for CSZM compounds [56] in a range from 4.45 x 10^-3 F to 1.16 x 10^-10 F in a temperature from 400 to 700 °C for 5% doping with a narrow range of polarization resistance values from 3.35 x 10^-3 Ω to 3.77 x 10⁺¹Ω [49]. For 10% CSZM the values of calculated capacitance were 8.54 x 10^-3 F to 5.2 x 10^-11 in the same range of temperature with a range of polarization resistance from 1.55 x 10^-3 Ω to 1.74 x 10⁺² Ω. While in the last composition, 15% CSZM the calculated values of capacitance are 8.54 x 10^-3 F to 5.34 x 10^-11 with 1.55 x 10^-2 Ω to 1.34 x 10⁺². The total conductivity of the symmetrical cell (σ), the total capacitance (C), and the area-specific resistance (ASR) in addition to the polarization resistance (Ω) values are listed in the following Table 3, Table 4 and Table 5.

It was clear from the given results listed in the above tables that the compounds of CSZM electrolyte materials are showing remarkable values in the measured conductivities however there is a small gap in the total conductivities between the 5% CSZM and 15% CSZM but they are still higher than some reported electrolyte materials [57]. The Arrhenius plots in Figure 7 demonstrated that Ce_0.7Sm_0.2Zr_0.05 Mg_0.05O_1.9 is showing a higher value of 1.0461 x 10⁺¹ S/cm at 700 °C while the minimum value was 2.7329 x 10^-2 S/cm at 400 °C and the total activation energy was found E_a 0.6865 eV. The CSZM 5% was the highest in conductivity values compared to both 10% and 15% CSZM and that returns to the highly dense electrolyte materials as some pores occurred in them. This was noticed from the SEM images. Meanwhile, the occurrence of this porosity still gives higher values of conductivities compared to the previously investigated electrolyte materials [58] and this can be observed from plotted Arrhenius plots in Figure 7 under 5% H₂/Argon wet atmosphere.

The CSZM electrolyte conductivity at 700 °C with 5% dopant has shown a higher value and it was higher than the reported ones in multi-dopant ceria electrolytes [59,60,61,62] and this because of the ordering of oxygen vacancies with small ionic radii and mobility makes these materials very conductive. However, the other 10% and 15% showed lower values due to the occurrence of voids and pores which can be overcome with doping of some materials in B sight like lithium oxide, [63] or Zin oxides [64] and it can be achieved at lower range of temperature and less than 1400 °C. The most essential finding of Mg doping for giving the higher conductivity values other than porosity and density in all synthesized series is due to the Mg doping altered the electronic structure [65] of the host compound by altering the ion bond and crystal layer gap, and the chemical bond length between the doped and O atoms is proportional to the atomic radius of the doped cations.

4. Conclusion and Recommendations

Due to the investigated materials with different percentage dopants of Mg and Zr with 5, 10, and 15% in the CSZM electrolyte materials compounds. The obtained results showed remarkable findings when the compounds were characterized physically and electrochemically with higher stability and conductivity in symmetrical cell testing. Structural analyses of the synthesized compounds were achieved accurately after careful analysis and refinement, showing the materials crystallized in a pure phase fluorite cubic structure with the goodness of the fitting in the (S.G. Fm3m) with a cell parameter value of 5.401742 $°A$ after the final sintering temperature of 1400°C. Highly dense materials appeared in the SEM images, especially in 5% dopant and some pores existed in both 10 and 15%. However, there were some pores in the 10 and 15% dopant but still showed higher values in conductivities compared to similar materials reported before. EIS analysis indicates that the 5% CSZM under investigation of 5% H₂/Ar atmosphere shower highest value of conductivity 2.347 S/cm at 700 °C higher than the previously similar reported electrolyte materials. The doping of 5% Mg showed higher conductivity a bit higher than investigated compounds from literature when comparing it with the microstructure features in terms of densification and porosity. It can be observed clearly from SEM images that 5% have higher densification over the other two percentages that in 10 and 15 % give higher porosity. This important notice can increase more densification with an increase of oxide-ion conductivity by adding zinc oxide or lanthanum oxides with an expectation of higher performance and less sintering temperature.