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Article

Supercilious Enhancement in Oxygen-Reduction Catalytic Functionalities of Cubic Perovskite Structured LaFeO3 by Co-Doping of Gd and Ce for LT-SOFCs

by
Jinpeng Li
1,
Naveed Mushtaq
2,
M.A.K. Yousaf Shah
2,
Yuzheng Lu
3 and
Shun Yan
3,*
1
Department of Electronic Engineering, Huainan Union University, Huainan 232000, China
2
Jiangsu Provincial Key Laboratory of Solar Energy Science and Technology/Energy Storage Joint Research Centre, School of Energy and Environment, Southeast University, No. 2 Si Pai Lou, Nanjing 210096, China
3
School of Electronic and Engineering, Nanjing Xiaozhuang University, Nanjing 211171, China
*
Author to whom correspondence should be addressed.
Crystals 2023, 13(2), 242; https://doi.org/10.3390/cryst13020242
Submission received: 26 October 2022 / Revised: 22 December 2022 / Accepted: 18 January 2023 / Published: 31 January 2023
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Abstract

:
Low-temperature solid fuel cells (LT-SOFCs) hold remarkable promise for the cooperative corporation of small- and large-scale applications. However, the meager oxygen-reduction retort of cathode materials mires the low operating temperature conditions of SOFCs. Herein, we have developed a perovskite structured LaFeO3 by the co-doping of Gd and Ce ions, and their electrochemical properties have been studied. The developed LaFe0.8Gd0.1Ce0.1O3-δ cathode exhibits very small-area-specific-resistance and good oxygen-reduction reaction (ORR) activity at low operating temperatures of 450–500 °C. We have demonstrated a high-power density of 0.419 W-cm−2 with a LaFe0.8Gd0.1Ce0.1O3-δ cathode operating at 550 °C with H2 and atmospheric air as fuels. Moreover, LaFe0.8Gd0.1Ce0.1O3-δ exhibits high activation energy as compared to individual LaFeO3, which helps to promote ORR activity. Various spectroscopic measurements such as X-ray diffraction, SEM, EIS, UV-visible, TGA, Ramana, and photoelectron spectroscopy were employed to understand the improved ORR electrocatalytic activity of Gd and Ce co-doped LaFeO3 cathode. The results can further help to develop functional cobalt-free electro-catalysts for LT-SOFCs.

1. Introduction

Solid oxide fuel cells (SOFCs) are efficient and clean energy devices in power-generation systems and have been long-standing investigated as potential conversion tools. However, high operating temperature, low ionic conductivity, long-term startup, and sluggish oxygen-reduction reaction (ORR) kinetics act as stumbling blocks toward SOFCs commercialization. Cathode one of the key components readily hampers the electrochemical performance when it comes to operating fuel cells at low operating temperatures due to poor catalytic functionalities [1,2,3,4,5]. These challenges at low operating temperatures can be mitigated by using advanced fabrication technology or by developing high-performing cathodes via structural and surface modification [6]. These approaches lead to lowering the operating temperatures of SOFCs to 300–600 °C, which can significantly increase the number of applications of LT-SOFC technology and facilitate material selection with greater reliability [7].
High-performing cathode materials with enhanced mixed ionic and electronic conduction, along with low electrode polarization and fast ORR functionalities, are the utmost key requirement for the development of LT-SOFCs. In this regard, diverse strategies have been reported to improve electrochemical performance at low operating temperatures. Zhang et al. achieved polarization resistance of 0.394 Ω cm2 by introducing ionic conductor Gd0.1Ce0.9O2−δ into PrBaCo2O5+δ via surface modification [8,9]. Several other studies focus on material selection and found the perovskite cathode to be an efficient electrode with enhanced ORR kinetics at low temperatures <600 °C. Perovskite cathode Ba0.5Sr0.5Co0.8Fe0.2O3-δ delivers a peak power density of 454 mW cm−2 at 500 °C, higher than the conventional cathode La0.6Sr0.4Co0.2Fe0.8O3-δ and the Sm0.5Sr0.5CoO3-δ cathode. The reported result clearly shows that structure modification and material selection play a key role in enhancing ORR kinetic at low temperatures [7]. The reason for high catalytic in Perovskite is that enriched oxygen vacancies at lattice due to defects in crystal structure on doping leads to improved ORR catalytic properties. Co- and Fe-based perovskite has shown astonishing fast catalytic functionality at low operating temperatures. Particularly, LaxSr1-xCoyFe1-yO3-δ, Ba0.5Sr0.5Co1−xFexO3−δ, and BaCoxFe1-xO3−δ deliver exceptionally low polarization resistance in comparison to conventional cathode pure Fe-based cathode material [10,11]. Although, the presence of cobalt in the perovskite cathode has shown high performance, it undergoes poor structural stability and exhibits large thermal expansion coefficients and limiting ORR functionalities, particularly when the SOFC operate at low temperatures [12,13,14]. Therefore, the development of Co-free or replacing Cobalt with any other efficient element can gain considerable attention, due to their stability in an oxidizing atmosphere and improved electrocatalytic functionalities towards ORR [15].
From the reported literature, it can be perceived that either by surface modification or using structural changes, the performance and stability of the LaFeO3-δ cathode by Gd and Ce doping are improved. Therefore, in this work, we dope ionic conductors doping of Gd and Ce in LaFeO3 and use them as cathode materials in LT-SOFCs. LFGdCeO3 cathode materials are characterized and investigated in terms of a crystal structure along with the analysis of surface and interface properties, followed by being applied in SOFC to investigate I-V & I-P performance and EIS. It is found that LaFe0.8Gd0.1Ce0.1O3-δ delivers minimum electrode polarization resistance along with high power density of 0.419 Wcm−2 at the low temperature of 550 °C. Additionally, various spectroscopic analyses show that the configured SOFC cathode is a persistently good candidate for depicting fast catalytic functionality and chemical stability in an oxidizing atmosphere.

2. Materials and Methods

2.1. Synthesis Procedures

Gd and Ce-doped LaFeO3 perovskite structured residues were made via a sol-gel method using citric acid and ethylenediaminetetraacetic acid (EDTA) as complexing agents. In the first step, 0.1 moles of EDTA were dissolved in de-ionized H2O, and subsequently, ammonia solution was added into the solution to set the pH of the to 7.0 and make the solution transparent. Next, suitable amounts of La (NO3)2.6H2O, Fe (NO3)2.9H2O, Gd (NO3)2.9H2O, and Ce (NO3)2.9H2O bought from Alfa Aesar with 99.98% purity were detached into the solution. Afterward, 0.1 mole of citric acid was added in the above prepared solution. After that, the solution was continuously stirred at 240 rpm at 80 ℃ for 10 h, and as a result, a LaFe0.8Gd0.1Ce0.1O3-δ brownish gel was obtained from the solution. In the next step, the obtained brownish gel was dried at 140 °C in an oven. Finally, the dried gel was ground and calcinated at 950 °C for 6 h in air. The pure LaFeO3 and LaFe0.9Gd0.05Ce0.05O3-δ were prepared using same protocols for comparative study.

2.2. Characterizations Tools and Electrochemical Measurements

Bruker D8 with Cu-Kα radiation (λ = 1.5418 Å) was used to measure the X-ray diffraction pattern of LaFeO3, LaFe0.9Gd0.05Ce0.05O3-δ, and LaFe0.8Gd0.1Ce0.1O3-δ. Merlin compacts (Zeiss) were used to perform scanning electron microscopy (SEM). Surface analysis of the LaFeO3 and LaFe0.8Gd0.1Ce0.1O3-δ was performed using X-ray photoelectron spectroscopy (Physical Electronics Quantum 2000) with an Al Kα X-ray source at room temperature in ultra-high vacuum (UHV). XPS analysis was performed using Peak 41. The UV-visible absorption was measured using a UV-Vis 3600 spectrophotometer. The Gamry Reference 3000, USA workstation was used to measure electrochemical impedance spectroscopy (EIS) under the open-circuit voltage (OCV) with 10 mV of dc signal over the frequency range of 0.1 to 106 Hz. The recorded data were analyzed using ZSIMPWIN software to obtain the EIS data.

2.3. Complete Fabrication of Fuel Cells

Our aim was to study the practical applicability of the Gd, Ce co doping in LaFeO3, and LaFe0.8Gd0.05Ce0.05O3-δ and LaFe0.8Gd0.1Ce0.2O3-δ as air electrodes in SOFC over Gd0.1Ce0.9O2 (GDC) and Ni0.8Co0.15Al0.05LiO2-δ (NCAL-Ni-foam) anodes. The fuel cells were prepared by the dry pressing method. In first step, Ni0.8Co0.15Al0.05LiO2-δ was mixed with terpinol and painted on a porous Ni-foam substrate. The as-prepared LaFeO3, LaFe0.8Gd0.05Ce0.05O3-δ, and LaFe0.8Gd0.1Ce0.2O3-δ were used as air electrode and Ni-foam Ni0.8Co0.15Al0.05LiO2-δ (NCAL-Ni-foam) from Bamo Sci. and Tech. Joint Stock Ltd. as a fuel electrode (anode) prepared by the same method. In the next step, one piece of Ni-foam NCAL was placed in a steel mould followed by Sm0.2Ce0.8O2-δ (SDC) electrolyte powders (0.20 g) and LaFe0.8Gd0.1Ce0.2O3-δ powders and pressed at 220 MPa to obtain three-layer devices. All types of fuel cells were fabricated using the same protocol. Afterwards, the prepared fuel cell devices were heated at 600℃ for 4 h in Ar to obtain a dense electrolyte layer (≅ relative density of 92%) to avoid gas leakage. Furthermore, a symmetrical cell with LaFe0.8Gd0.1Ce0.2O3-δ electrodes over the SDC electrolyte was fabricated for ORR activity measurements. The active area of all the devices was 0.64 cm2. The performance demonstration of single cells was performed with 3% H2O humidified hydrogen fuel and air from the atmosphere as oxidants. The drift rates of H2/air were set to approximately 100–120 ± 5% mL/min.

3. Results

3.1. Structure and Composition Analysis

Figure 1a shows the XRD pattern LaFeO3, LaFe0.9Gd0.05Ce0.05O3-δ, and LaFe0.8Gd0.1Ce0.2O3-δ in the 2θ range from 10–90°. The central peaks of the LaFeO3 diffraction pattern are located at 23, 32, 39, 46, 57, 68, and 76, which can be indexed to the (100), (110), (111), (200), (211), (220), and (310) planes, respectively, of the cubic structure, with space group pm-3m (221) and a lattice of a = b = C = 3.94 and α = β = γ 90° [16]. Moreover, Figure 1b shows the comparative diffraction pattern of LaFeO3, LaFe0.9Gd0.05Ce0.05O3-δ, and LaFe0.8Gd0.1Ce0.2O3-δ in the 2θ range from 30–37° where the dominant diffraction peaks of La0.1Sr0.9Co0.9Ce0.1O3-δ and La0.2Sr0.8Co0.8Ce0.2O3-δ at 2θ of 32° shift towards a lower angle, possibly because of the high ionic radius of Gd3+ and Ce4+/3+ ion [17,18].
Figure 2a–f shows the microscopic SEM image of LaFe0.8Gd0.1Ce0.1O3-δ at a different scale. The morphology of LaFe0.9Gd0.05Ce0.05O3-δ is shown to be very fine and nano-structured. The LaFe0.9Gd0.05Ce0.05O3-δ shows the particle size of about 50 nm. Figure 2a–f shows the SEM image of LaFe0.9Gd0.05Ce0.05O3-δ, where a highly porous structure can be seen, while Figure 3b–f shows the EDS mapping image of LaFe0.8Gd0.1Ce0.1O3-δ of SEM image in Figure 3a. The EDS mapping image of the chemical distribution of each element in LaFe0.8Gd0.1Ce0.1O3-δ can clearly be seen. The elemental mapping image with mixed colors was used to reveal the chemical distribution by energy dispersive spectroscopy (EDS), where the homogenous chemical concentration of each element such as La, Fe, Gd, Ce, and O can be identified [19]. Moreover, the mapping of the chemical distribution of each element individually is shown in Figure 3c–f measured using SEM image in Figure 3a, which could help estimate the chemical distribution [20,21]. Moreover, Figure 3g shows the EDS spectrum of the LaFe0.8Gd0.1Ce0.1O3-δ sample, which could help one to understand the actual chemical composition of the powder.

3.2. Electrochemical Impedance and Electrical Conductivity

The cathodic polarization contributes the most to the entire impedance spectra of SOFCs. Hence it is necessary to understand and determine the cathodic polarization process and the rate-determining step involved in ORR activity and slow down it. To study the ORR properties of LaFe0.8Gd0.1Ce0.1O3-δ in a symmetrical cell over SDC electrolyte, EIS characterization was carried out using the electrochemical impedance spectroscopy as shown in Figure 4. The EIS was measured symmetrical cells in air at 400–550 °C, under OCV conditions. A comparison of the Nyquist plots of La0.2Sr0.8Co0.8Ce0.2O3-δ embedded on porous Ni-foam cells over an SDC electrolyte obtained from EIS data at different operating temperatures of 550–425 °C is shown in Figure 4a–f. The model circuit Ro − (Rg − CPE1) − (Rgb − CPE2) that fits the EIS results suggests a very low charge and mass transport resistance for the LaFe0.8Gd0.1Ce0.1O3-δ cathode at 550 °C, as shown in Figure 4a [22,23]. The fitted data describe two dominant polarization losses signified by the ASR at LF (low frequencies) and HF (high frequencies) [22,23,24,25]. Since ORR is described as a multistep process, such as (i) the surface adsorption/gas diffusion and separation of O2 from air; (ii) the diffusion of Oad; (iii) the conversion of absorbed O2 into O2-; and (iv) the transportation of O2− to the cathode/electrolyte interface. A step-by-step or parallel combination of these processes could be involved [26,27,28]. However, low ASR in the LaFe0.8Gd0.1Ce0.1O3-δ cathode could be the parallel combination of these steps. The large grain boundary in the single-phase cathode materials plays a significant role in slowing down the ORR process. The very-low-charge transport resistance of 0.04 Ωcm2 is trialed, whereas the resistance of mass transport is 0.32 Ω cm2. Most often, oxygen vacancies are virtually studied in simply doped oxide materials at low temperatures of interest for applications [29], while the co-doping of Gd and Ce to form LaFe0.8Gd0.1Ce0.1O3-δ can be applied to enhance ORR activity and TPBs for developing the SOFCs cathode. In addition, electrochemical impedance analysis indicates that the surface exchange process of oxygen is more dominant than the bulk diffusion process of oxygen in LaFe0.8Gd0.1Ce0.1O3-δ.

3.3. Electrochemical Performance Measurements

The electrochemical performance of the prepared LaFeO3, LaFe0.9Gd0.05Ce0.05O3-δ, and LaFe0.8Gd0.1Ce0.1O3-δ as air electrodes was demonstrated in SOFC at 550 °C over the SDC electrolyte. Figure 5c displays a typical current (I)–voltage (V) and I-P characteristics curve of fabricated fuel cells. An OCV of over 1.1 V and the maximum power density (Pmax) of 0.419 W cm−2 using the LaFe0.8Gd0.1Ce0.1O3-δ cathode were compared to using LaFeO3, Pmax of 0.245 W cm−2 (Figure 5a) and Pmax of 0.338 W cm−2 (Figure 5b) for LaFe0.9Gd0.05Ce0.05O3-δ cathodes, respectively at 550 °C. The high electrochemical performance achieved by the LaFe0.8Gd0.1Ce0.1O3-δ cathode over individual LaFeO3 and LaFe0.9Gd0.05Ce0.05O3-δ samples suggests co-doping of Gd and Ce and plays an essential role in improving ORR electrocatalytic activity by leading lower barrier O2- transport and its migration energy [12,17] since the catalytic process at the cathode surface is a multistep process, and the LaFe0.9Gd0.05Ce0.05O3-δ could be enriched with oxygen vacancies, which would enhance the capability of capturing valance electrons to improve the electrical conductivity as well. Moreover, the cross-sectional SEM of the fuel cell using the LaFe0.9Gd0.05Ce0.05O3-δ cathode over the GDC cathode after fuel-cell testing is performed to the individual layer in the cell components, as shown in Figure 5d.

3.4. Spectroscopic Analysis

Additionally, a wide range of spectroscopic methods, including UV–visible, TGA, Raman, and X-ray photoelectron spectroscopy, were used to investigate additional structural characteristics of LaFeO3 and Gd/Ce co-doped LaFeO3 powders. Figure 6a shows the absorbance spectra of pure LaFeO3 and LaFe0.8Gd0.1Ce0.1O3-δ measured by UV–visible spectroscopy. It is possible to see a substantial difference in the absorbance spectra of the as-synthesized LaFeO3 and LaFe0.8Gd0.1Ce0.1O3-δ powders. The variations in the absorbance spectra indicate that the LaFeO3-δ energy band gap is lowered with the doping of Gd and Ce; this is only possible because of the creation of oxygen vacancies. This is widely reported when a large number of oxygen vacancies are produced, which also reduce the metal oxides’ energy band gaps [30,31].
Moreover, the TGA was conducted in air at 30 to 800 °C to further investigate the thermal effects of the Gd and Ce doping on the LaFeO3 particles, as demonstrated in Figure 6b. The evaporation of the absorbed water is indicated by the rapid weight loss in both samples around 100 °C. The rapid weight changes begin at around 300 °C for the LaFe0.8Gd0.1Ce0.1O3-δ powders, while the other rapid weight changes start at 500–600 °C to release lattice oxygen in the form of oxygen vacancies. The LaFe0.8Gd0.1Ce0.1O3-δ sample exhibits the highest mass change, which is fully evident and supports our hypotheses. The large mass change in LaFe0.8Gd0.1Ce0.1O3-δ could be released easily because of the softening of the O bands when Gd and Ce are doped in LaFeO3.
Figure 7a shows the Raman spectroscopy of LaFe0.8Gd0.1Ce0.1O3-δ in comparison with LaFeO3. The specific Raman bands of LaFeO3 and LaFe0.8Gd0.1Ce0.1O3-δ are centered at 220, 463, and 1310 cm−1, respectively. However, a down-shift in the bands was observed for the LaFe0.8Gd0.1Ce0.1O3-δ sample. Typically, the down-shift in the LaFe0.8Gd0.1Ce0.1O3-δ peak causes a decrease in energy when exciting the vibrations along gradually looser bonds in the presence of LaFe0.8Gd0.1Ce0.1O3-δ. It confirms that the high ORR activity could result after loose oxygen bonding at the interface of LaFe0.8Gd0.1Ce0.1O3-δ, hence resulting in high densities of active surface-oxygen species to enhance ORR electrocatalytic activity.
Figure 7b–f shows the high-resolution XPS spectra for the individual elements in LaFe0.8Gd0.1Ce0.1O3-δ before and after doping, where the change in oxidation state of each element such as Fe, Ce, and O1s can clearly be observed after dispersing doping of Gd and Ce. After subtracting Shirley’s background, high-resolution XPS spectra were fitted by a mixture of Lorentzian and Gaussian function. Our focus was to observe the chemical and electronic state configuration changes of Fe-2p (Figure 7b,c), Ce-3d (Figure 7d,e), and O 1s (Figure 7f) spectra, respectively. The change in the chemical state of each element can be observed after Gd and Ce doping to pristine LaFeO3 materials. The Fe4+-2p(3/2, 1/2) and Fe3+-2p (3/2, 1/2) peaks in LaFeO3 appear at 713.2/723.6 eV and 711.85/724.63 ± 0.02 eV, whereas in LaFe0.8Gd0.1Ce0.1O3-δ they are at 712.25/723.73 and 709.58/724.93 ± 0.02 eV, showing a binding energy up shift of 0.8 ± 0.02 eV [32,33], signifying a down-shift of 0.5 ± 0.05 eV to lower binding energy (B.E). Figure 7d, e shows the XPS spectra of Ce-3d of the LaFe0.9Gd0.05Ce0.05O3-δ and LaFe0.8Gd0.1Ce0.1O3-δ samples. The Ce4+ and Ce3+ peaks of the LaFe0.8Gd0.1Ce0.1O3-δ sample appear at 881.82/900.65 eV and 898.45/915.72 eV, whereas for pure the LaFe0.9Gd0.05Ce0.05O3-δ sample they are at 882.152/901.65 and 899.32/916.45 eV [16], showing a significant change in a binding energy shift up to about 0.6 ± 0.05 eV for the LaFe0.8Gd0.1Ce0.1O3-δ sample [34,35]. However, Fe-2p (3/2, 1/2) shows a down-shift in B.E., while the Ce-3d spectra shifts up to exhibit more mixed valence states, which help to create extra oxygen vacancies by maintaining charge-neutrality due to a difference in the electro negativity after dispersing the LaFe0.8Gd0.1Ce0.1O3-δ sample at porous Ni-foam [36,37,38].
The O1s spectrum of the La0.2Sr0.8Co0.8Ce0.2O3-δ sample contains lattice oxygen (lattice O2-) and oxygen vacancy V o ∘∘ peaks. The O1s spectra of the LaFe0.8Gd0.1Ce0.1O3-δ sample cathode material display two partially superimposed peaks (Figure 7f). There are two major excitations: the first includes O1s of the LaFe0.8Gd0.1Ce0.1O3-δ sample bands ranging from 528 to 533.5 eV. The low BE peak at 529.2 can be ascribed to the lattice oxygen (O Lattice), and the higher one at 531.4- to extra V o ∘∘ . The high area percentage ratio of Olat/Ovac of the LaFe0.8Gd0.1Ce0.1O3-δ sample after the Gd and Ce doping cathode indicates its high oxygen-vacancy concentration and good oxygen-adsorption capability, which play an important role in high ORR activity [36,38]. The different steps involved in the ORR mechanism in the LaFe0.8Gd0.1Ce0.1O3-δ sample with the Ni-foam cathode are shown in Figure 8.

4. Conclusions

In conclusion, in this work we have developed a semiconductor LaFeO3 doped by both the A-site and B-site using Gd and ceria, respectively. Later, the developed Gd- and Ce-doped LaFeO3 particles were used as air electrodes and their electrochemical properties were studied. The prepared LaFe0.9Gd0.05Ce0.05O3-δ and LaFe0.8Gd0.01Ce0.1O3-δ samples exhibit good ORR electrochemical performance at LTs, e.g., the maximum power density of 0.419 W cm−2 using a cathode over the SDC electrolyte at 550 °C. The excellent ORR electrocatalytic activity LaFe0.8Gd0.01Ce0.1O3-δ can be attributed to the porous structure obtained by the Gd and Ce doping into LaFeO3. The mechanism for high electrochemical performance of LaFe0.8Gd0.01Ce0.1O3-δ is discussed in detail by different experimental approaches. The obtained results show that this approach could not be only useful in developing efficient ORR electrocatalysts but could also be applied to other relevant applications.

Author Contributions

The conceptualization of this work was completed by J.L. and N.M.; the methodology was completed by Y.L.; and formal analysis and investigation were completed by Y.L. and S.Y. The resources and data-curation facilities were provided by Y.L. and J.L. Original draft preparation was done by J.L. The review and editing were carried out by M.Y.S. and Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported the industry-University-Research Cooperation Project of Jiangsu in China (Grant No. BY2021057), and the Qing Lan Project of Jiangsu Province. This work is also partly supported by the Jiangsu Province Higher Vocational College Young Teachers Enterprise Practice Training Funding Project (Grant No. 2021QYSJ048).

Data Availability Statement

The data that support the findings of this study are available from the corresponding authors upon reasonable request.

Acknowledgments

The authors would like to acknowledge the industry-University-Research Cooperation Project of Jiangsu Province in China (Grant No. BY2021057) for providing the funding to complete this work.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) measured X-ray diffraction pattern of LaFeO3, LaFe0.9Gd0.05Ce0.05O3-δ, and LaFe0.8Gd0.1Ce0.2O3-δ, respectively. (b) Comparison of peak shifting of LaFeO3 with Gd and Ce doping.
Figure 1. (a) measured X-ray diffraction pattern of LaFeO3, LaFe0.9Gd0.05Ce0.05O3-δ, and LaFe0.8Gd0.1Ce0.2O3-δ, respectively. (b) Comparison of peak shifting of LaFeO3 with Gd and Ce doping.
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Figure 2. (a–f) The surface morphology studied by SEM for LaFe0.8Gd0.1Ce0.1O3-δ powder at different scale of 10–500 nm.
Figure 2. (a–f) The surface morphology studied by SEM for LaFe0.8Gd0.1Ce0.1O3-δ powder at different scale of 10–500 nm.
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Figure 3. (a) The surface morphology studied by SEM for LaFe0.8Gd0.1Ce0.1O3-δ powders; (b–f) combined and individual EDS mapping image of La, Fe, Gd, Ce, and O in LaFe0.9Gd0.05Ce0.05O3-δ of SEM image in (a); and (g) EDS spectrum of LaFe0.8Gd0.1Ce0.1O3-δ.
Figure 3. (a) The surface morphology studied by SEM for LaFe0.8Gd0.1Ce0.1O3-δ powders; (b–f) combined and individual EDS mapping image of La, Fe, Gd, Ce, and O in LaFe0.9Gd0.05Ce0.05O3-δ of SEM image in (a); and (g) EDS spectrum of LaFe0.8Gd0.1Ce0.1O3-δ.
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Figure 4. (a–f) Nyquist plot of impedance spectra for symmetrical LaFe0.8Gd0.1Ce0.1O3-δ electrode cell cathode over SDC electrolyte different operating temperatures of 550 °C, 525 °C, 500 °C, 475 °C, 450 °C, and 425 °C, respectively.
Figure 4. (a–f) Nyquist plot of impedance spectra for symmetrical LaFe0.8Gd0.1Ce0.1O3-δ electrode cell cathode over SDC electrolyte different operating temperatures of 550 °C, 525 °C, 500 °C, 475 °C, 450 °C, and 425 °C, respectively.
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Figure 5. (a–c) Electrochemical performances: characteristic curves of utilizing our LaFe0.8Gd0.1Ce0.1O3-δ when comparing LaFeO3 and LaFe0.9Gd0.05Ce0.05O3-δ cathodes in fuel cells over SDC electrolyte and NCAL-Ni-foam anode operated at 550 °C (d) cross-sectional SEM images of tri-layer electrolyte supported fuel cell along with prepared LaFe0.8Gd0.1Ce0.1O3-δ cathode and SDC electrolyte examined after the electrochemical test.
Figure 5. (a–c) Electrochemical performances: characteristic curves of utilizing our LaFe0.8Gd0.1Ce0.1O3-δ when comparing LaFeO3 and LaFe0.9Gd0.05Ce0.05O3-δ cathodes in fuel cells over SDC electrolyte and NCAL-Ni-foam anode operated at 550 °C (d) cross-sectional SEM images of tri-layer electrolyte supported fuel cell along with prepared LaFe0.8Gd0.1Ce0.1O3-δ cathode and SDC electrolyte examined after the electrochemical test.
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Figure 6. (a) The UV-visible spectra of LaFeO3 and LaFe0.8Gd0.1Ce0.1O3-δ and (b) thermogravimetric analysis of LaFeO3 and LaFe0.8Gd0.1Ce0.1O3-δ cathode, respectively.
Figure 6. (a) The UV-visible spectra of LaFeO3 and LaFe0.8Gd0.1Ce0.1O3-δ and (b) thermogravimetric analysis of LaFeO3 and LaFe0.8Gd0.1Ce0.1O3-δ cathode, respectively.
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Figure 7. (a) Raman spectra of LaFeO3 and LaFe0.8Gd0.1Ce0.1O3-δ (b,c) Fe-2p X-ray photoelectron spectra of LaFeO3-δ individual powders and after doping of Gd and Ce to form LaFe0.8Gd0.1Ce0.1O3-δ. (d,e) Ce-3d, XPS spectra of LaFe0.9Gd0.05Ce0.05O3-δ and LaFe0.8Gd0.01Ce0.1O3-δ sample and (f) O1s spectra, respectively.
Figure 7. (a) Raman spectra of LaFeO3 and LaFe0.8Gd0.1Ce0.1O3-δ (b,c) Fe-2p X-ray photoelectron spectra of LaFeO3-δ individual powders and after doping of Gd and Ce to form LaFe0.8Gd0.1Ce0.1O3-δ. (d,e) Ce-3d, XPS spectra of LaFe0.9Gd0.05Ce0.05O3-δ and LaFe0.8Gd0.01Ce0.1O3-δ sample and (f) O1s spectra, respectively.
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Figure 8. Schematic diagram of the different steps for electrochemical processes from fuel converting the fuel energy into electricity of Gd/Ce-doped LaFeO3-based cathode fuel cell over SDC electrolyte.
Figure 8. Schematic diagram of the different steps for electrochemical processes from fuel converting the fuel energy into electricity of Gd/Ce-doped LaFeO3-based cathode fuel cell over SDC electrolyte.
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Li, J.; Mushtaq, N.; Yousaf Shah, M.A.K.; Lu, Y.; Yan, S. Supercilious Enhancement in Oxygen-Reduction Catalytic Functionalities of Cubic Perovskite Structured LaFeO3 by Co-Doping of Gd and Ce for LT-SOFCs. Crystals 2023, 13, 242. https://doi.org/10.3390/cryst13020242

AMA Style

Li J, Mushtaq N, Yousaf Shah MAK, Lu Y, Yan S. Supercilious Enhancement in Oxygen-Reduction Catalytic Functionalities of Cubic Perovskite Structured LaFeO3 by Co-Doping of Gd and Ce for LT-SOFCs. Crystals. 2023; 13(2):242. https://doi.org/10.3390/cryst13020242

Chicago/Turabian Style

Li, Jinpeng, Naveed Mushtaq, M.A.K. Yousaf Shah, Yuzheng Lu, and Shun Yan. 2023. "Supercilious Enhancement in Oxygen-Reduction Catalytic Functionalities of Cubic Perovskite Structured LaFeO3 by Co-Doping of Gd and Ce for LT-SOFCs" Crystals 13, no. 2: 242. https://doi.org/10.3390/cryst13020242

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