Oxygen Reduction Response of La and Ce Co-Doped SrCoO_3−δ Perovskite Oxide Grown on Porous Ni-Foam Substrate

Abstract

Lately, ceramic fuel cells (CFCs) have held exceptional promise for joint small- and large-scale applications. However, the low-oxygen reduction response of cathode materials has hindered the low operating temperature of CFCs. Herein, we have developed a semiconductor based on La and Ce co-doped SrCoO₃ and embedded them in porous Ni-foam to study their electrochemical properties. The porous Ni-foam-pasted La_0.2Sr_0.8Co_0.8Ce_0.2O_3‒δ cathode displays small-area-specific resistance and excellent ORR (oxygen reduction reaction) activity at low operating temperatures (LT) of 450–500 °C. The proposed device has delivered an impressive fuel cell performance of 440 mW-cm⁻², using La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ embedded on porous Ni-foam substrate cathode operation at 550 °C with H₂ fuel and atmospheric air. It even can function well at a lower temperature of 450 °C. Moreover, La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ embedded on porous Ni-foam shows very good activation energy compared to individual SrCoO₃ and La_0.1Sr_0.9Co_0.9Ce_0.1O_3−δ embedded on porous Ni-foam, which help to promote ORR activity. Different characterization has been deployed, likewise: X-ray diffraction, photoelectron-spectroscopy, and electrochemical impedance spectroscopy for a better understanding of improved ORR electrocatalytic activity of prepared La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ embedded on porous Ni-foam substrate. These results can further help to develop functional cobalt-free electrocatalysts for LT-SOFCs.

1. Introduction

Addressing global energy demand and environmental pollution control requires an alteration to the current energy system by eliminating/removing the dependence on fossil fuels. Renewable energy, including solar, wind, and tidal, is gradually moving to mainstream power with a large capacity that exceeds the global energy consumption [1,2]. However, these energy forms’ inherent discontinuity and intermittency plague integration with electrical infrastructure. Ceramic fuel cells show increasing potential to overcome these obstacles, with a more efficient way to achieve electricity from hydrogen or hydrocarbon fuels to address complicated and various demands in current scenarios [3,4].

The excellent oxygen reduction reaction (ORR) electrocatalysts are vital for ceramic fuel cells, including SOFCs and PCFCs, as sluggish ORR kinetics limit the efficiency of these devices. The efficiency of SOFCs is especially highly targeted at low operating temperatures [2], while high operating temperatures (700–1000 °C) carry many drawbacks, making the technology more complex and limiting its widespread applications [3]. Therefore, the most important factor is to reduce the operating temperature into the range of 300–600 °C to advance SOFCs toward commercialization [4]. However, the high temperature is considered to be vital to achieve high enough ORR activity in currently used PCFC cathode materials [5,6,7]. Therefore, intensive efforts have been made to develop advanced PCFC cathode materials operating at LTs. However, many serious challenges are still present [7,8].

Generally, a suitable cation doping strategy to the A or B site is used in perovskite-oxides to induce mixed ionic–electronic (MIE) conduction to raise ORR activity. To date, Ba_1−xSr_xCo_1−yFe_yO₃ (BSCF), La_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ (LSCF), Pr_0.8Ba_0.2CoO₃, BaCo_xFe_1−xO_3−δ and BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3−δ perovskite have been developed with the most state-of-the-art cathodes for SOFCs [9,10,11,12,13,14,15]. Most state-of-the-art cathodes of SOFCs contain higher levels of Co, but their structural stability is challenging because of their large TECs (thermal expansion coefficients) [16]. Therefore, over the last decade, extensive efforts have been made toward Co-free cathodes, and Fe-based materials have gained substantial attention as an alternative because of their better structural stability [17].

In this work, apart from the conventional doping, we have developed a semiconductor cathode based on La-doping on the A-site, and Ce-doping on the B-site, of SrCoO₃ perovskite. Later, La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ particles were embedded on a porous Ni-foam substrate and their properties were studied as ORR electrocatalysts in an LT-SOFC cathode. The prepared La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ embedded on porous Ni-foam substrate shows excellent ORR activity and power output while operating at 450–550 °C. Various spectroscopic characterizations were used to understand the promoted electrocatalytic activity of La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ embedded on a porous Ni-foam substrate. However, we have found that this study would take a step forward in developing advanced ORR properties and some key heterostructure composites based on La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ embedded on porous Ni-foam structural features towards ORR mechanism.

2. Materials and Methods

2.1. Synthesis Procedures

La- and Ce-doped SrCoO₃ perovskite-structured residues were made using the sol–gel technique. In the sol–gel method, citric acid and ethylenediaminetetraacetic acid (EDTA) are used as complexing agents. In the first step, 0.1 moles of EDTA were dissolved in de-ionized H₂O. Subsequently, an ammonia solution was added to the solution to set the pH to 7.0 and make the solution transparent. After a while, suitable amounts of Sr (NO₃)₂.6H₂O, La (NO₃)₂.6H₂O, Co (NO₃)₂.9H₂O, and Ce (NO₃)₂.9H₂O bought from Alfa Aesar with 99.98% purity were detached into the solution. Afterward, 0.1 moles of citric acid were added to the above-prepared solution. Then, later, the solution was continuously stirred at 240 rpm at 80 °C for 8 h, resulting in the brownish gel of La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ. Subsequently, the brownish gel was dried at 150 °C in an oven. In the end, the dried gel was fired at 1000 °C for 8 h in the air. The pure SrCoO₃ and La_0.1Sr_0.9Co_0.9Ce_0.1O_3−δ were prepared using the same protocols as stated earlier for the comparative study.

2.2. Characterizations Tools and Electrochemical Measurements

A Bruker D8 X-ray diffractometer with Cu-Kα radiation (λ = 1.5418 Å) was employed to evaluate the X-ray diffraction pattern of prepared powders of SrCoO₃, La_0.1Sr_0.9Co_0.9Ce_0.1O_3−δ, and La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ. Merlin compacts (Zeiss, Goeschwitzer Strasse 51-5207745 Jena, Germany) were used to perform scanning electron microscopy (SEM) to study the morphology of proposed materials. Surface properties and chemical oxidation of the SrCoO₃, La_0.1Sr_0.9Co_0.9Ce_0.1O_3−δ, and La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ were evaluated via employing X-ray photoelectron spectroscopy (Physical Electronics Quantum 2000) with an Al Kα X-ray source at room temperature in an ultra-high vacuum (UHV). XPS peaks were fitted using Peak 41. The Gamry Reference 3000, USA workstation was set to study the EIS (electrochemical impedance spectroscopy) spectra under the condition of OCV (open-circuit voltage) 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 be obtained as EIS data.

2.3. Complete Fabrication of Fuel Cells

To check and evaluate the feasibility of the prepared materials of SrCoO₃, La_0.1Sr_0.9Co_0.9Ce_0.1O_3−δ, and La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ as an air electrode in SOFC, the fuel cell pellets were prepared by dry pressing. In the first step, SrCoO₃, La_0.1Sr_0.9Co_0.9Ce_0.1O_3−δ, and La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ were mixed with the appropriate amount of terpinol. Then, the solution was mixed and painted on porous Ni-foam. The as-prepared SrCoO₃, La_0.1Sr_0.9Co_0.9Ce_0.1O_3−δ, and La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ embedded on porous Ni-foam substrate were used as an air electrode, and Ni-foam Ni_0.8Co_0.15Al_0.05LiO_2−δ (NCAL-Ni-foam) from Tianjin Bamo Sci. & Tech. Joint Stock Ltd Tianjin China powders as fuel electrodes (anode) prepared by the same method used. In the next step, one piece was placed in a steel mold, followed by Gd_0.1Ce_0.9O_2−δ electrolyte powders (0.20 g) and NCAL-Ni-foam, and pressed at 220 MPa to obtain three-layer devices. All types of fuel cells were fabricated using the same protocol. Afterward, the prepared fuel cell devices were heated at 600 °C for 4 h in Ar to obtain a dense electrolyte layer (≅relative density of 90%) to avoid gas leakage. Furthermore, a symmetrical cell with SrCoO₃, La_0.1Sr_0.9Co_0.9Ce_0.1O_3−δ, and La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ electrodes over the GDC electrolyte was made for the measurements of ORR activity. The active area of all prepared pellets was 0.64 cm². Demonstrated power density and EIS of single cells were performed, with 3% H₂O humidified hydrogen fuel and air from the atmosphere as oxidants. The H₂/air gases were supplied with a drift rate of approximately 100–120 ± 5% mL/min.

3. Results

3.1. Structure and Composition Analysis

Figure 1a illustrates the XRD pattern of SrCoO₃, La_0.1Sr_0.9Co_0.9Ce_0.1O_3−δ, and La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ in the 2θ range from 10° to 90°. The attained peaks of the SrCoO₃ diffraction pattern reside at different angles of 23, 32, 39, 46, 57, 68, and 76. The attained peaks are further correlated to the different planes of (100), (110), (111), (200), (211), (220), and (310), respectively, revealing the cubic structure with space group pm-3m (221), and lattice of a = b = C = 3.9 and α = β = γ 90° [18]. Moreover, Figure 1b depicts the comparative diffraction pattern of SrCoO₃, La_0.1Sr_0.9Co_0.9Ce_0.1O_3−δ, and La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ in the 2θ range from 20 to 45°, where dominant diffraction peaks of La_0.1Sr_0.9Co_0.9Ce_0.1O_3−δ and La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ located at 2θ of 32° seem to be shifted towards a lower angle, which might be because of the high ionic radius of La³⁺ and Ce^4+/3+ ion [19].

Figure 2a,b shows the microscopic SEM image of La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ at a different scale. The morphology of La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ appears very fine and nanostructured. The La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ shows a particle size in the range of ≤50 nm. Figure 3a shows the SEM image of La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ dispersed on porous Ni-foam, where a porous structure with high porosity can be noticed, while Figure 3b–h shows the EDS mapping image of La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ after embedding on Ni-foam. The EDS mapping image of each element in La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ on porous Ni-foam confirms the presence of all elements. The basic mapping image of different features with mixed colors was used to reveal the chemical distribution by energy dispersive spectroscopy (EDS), where the homogenous chemical concentration of each element was confirmed, including Ni, La, Sr, Co, Ce, and O. Moreover, the mapping of the chemical distribution of each component individually is shown in Figure 4b, measured using an SEM image in Figure 4a, which could help estimate the chemical distribution [20,21].

3.2. Electrochemical Impedance and Electrical Conductivity

Cathodic polarization plays the largest contributing part in 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 it down. To study the ORR properties of different La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ embedded on porous Ni-foam, EIS characterization was carried out using electrochemical impedance spectroscopy, as shown in Figure 5. The EIS measured symmetrical cells in the air at 400–550 °C, under OCV conditions. The comparison with Nyquist plots of La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ embedded on porous Ni-foam cells over GDC electrolytes obtained from EIS data at different operating temperatures is shown in Figure 5. The fitted model circuit ${\mathrm{R}}_{\mathrm{o}}-\left({\mathrm{R}}_{\mathrm{g}}-{\mathrm{CPE}}_1\right)-\left({\mathrm{R}}_{\mathrm{gb}}-{\mathrm{CPE}}_2\right)$ of EIS results using Z-simpwin software suggests very low grain ${\mathrm{R}}_{\mathrm{g}}$, and the grain boundary (${\mathrm{R}}_{\mathrm{gb}}$) prepared electrode La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ embedded on porous Ni-foam cathode is very low 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) [24–26]. ORR is described as a multistep process; (i) surface adsorption/gas diffusion and separation of O₂ from the air, (ii) diffusion of O_ad, (iii) conversion of absorbed O₂ into O₂₋ and (iv) transportation of O²⁻ to the cathode/electrolyte interface. Step-by-step or parallel combinations of these processes could be involved [24,25]. However, low ASR in La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ embedded on porous Ni-foam cathode could be the parallel combination of these steps. The large grain boundary in single-phase cathode materials plays a significant role in slowing down the ORR process. While very low grain resistance (R_g) of 0.08 is trialed, the resistance refers to grain boundary resistance (${\mathrm{R}}_{\mathrm{gb}}$) at 0.38 Ω cm². Most often, oxygen vacancies are virtually studied in simply doped oxide materials at low temperatures of interest for applications [26], while the La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ embedded on porous Ni-foam approach can be applied to enhance ORR activity and TPBs for developing SOFCs cathode. In addition, electrochemical impedance analysis signifies that oxygen’s surface exchange process is much more dominant than the bulk diffusion process of oxygen in La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ embedded on porous Ni-foam.

3.3. Electrochemical Performance Measurements

The electrochemical performance of the prepared SrCoO₃, La_0.1Sr_0.9Co_0.9Ce_0.1O_3−δ, and La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ embedded on porous Ni-foam as an air electrode was demonstrated in SOFC at 550 °C over a GDC electrolyte. Figure 6a displays a typical current (I)–voltage (V) and I–P characteristics curve of fabricated fuel cells. An OCV of 1.06 V and the maximum power density (Pmax) of 440 mW cm⁻² using La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ embedded on porous Ni-foam cathode were compared to using SrCoO₃, Pmax of 218 mW cm⁻² (Figure 6a) and Pmax of 260 mW cm⁻² for La_0.1Sr_0.9Co_0.9Ce_0.O_3−δ cathodes, respectively, at 550 °C. Moreover, the La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ embedded on porous Ni-foam cathode also displays decent performance at low operating temperatures such as at 500 and 450 °C, as shown in Figure 6b, where the peak power densities of 182 and 373 mW-cm⁻² were achieved. The high electrochemical performance of La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ embedded on porous Ni-foam over individual SrCoO₃ and La_0.1Sr_0.9Co_0.9Ce_0.1O_3−δ embedded on porous Ni-foam samples suggests bi-metal doping. This is essential in improving ORR electrocatalytic activity via lowering the energy-barrier for O²⁻ transport, and migration energy [12,17], since it is known that, at the cathode surface, the catalytic process includes a multi-step process, and the La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ embedded on porous Ni-foam is enriched with oxygen vacancies, which would enhance the capability of capturing valance electrons to improve the electrical conductivity as well. Moreover, cross-sectional SEM of the fuel cell using La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ embedded on porous Ni-foam cathode after fuel cell testing is performed on individual layers in the cell components, as shown in Figure 5c.

3.4. Spectroscopic Analysis

Figure 7 shows the high-resolution XPS spectra for individual elements in La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ, and after dispersing on the Ni-foam, where a change in the oxidation state of each element, such as Co, Ce, and O1s, can be observed after dispersing La_0.2Sr_0.8Co_0.8Ce_0.2O_{3−δ at} Ni-foam substate. After subtracting Shirley’s background, high-resolution XPS spectra were fitted by a mixture of Lorentzian and Gaussian functions. The main task and aim are to study the chemical and electronic state configuration changes of Co-2p (Figure 7a), Ce-3d (Figure 7b), and O 1s (Figure 7c) spectra, respectively. Each element’s chemical state change can be observed after dispersing on Ni-foam, compared to pristine La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ materials. The Co³⁺ and Co²⁺ peaks of Ni-foam-dispersed La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ appear at 778.820/793.765 eV and 779.827/794.896 eV, whereas for La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ powders, they are at 779.420/794.465 eV and 780.827/795.796 eV, as shown in Figure 7b [27], signifying a downshift of 0.5 ± 0.05 eV to lower binding energy (B.E). The Ni-foam-embedded La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ sample exhibits more Co²⁺ states than that of La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ. Figure 7b shows the XPS spectra of Ce-3d of the La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ and La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ samples after depositing on Ni-foam. The Ce⁴⁺ and Ce³⁺ peaks of the La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ sample after depositing on Ni-foam appear at 881.82/900.65 eV and 898.45/915.72 eV, whereas for the pure La_0.2Sr_0.8Co_0.8Ce_0.2O_3−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 La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ sample, to Ce-3d of the only La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ sample. However, Co-2p (3/2, 1/2) shows a downshift in B.E while Ce-3d spectra shift up to exhibit more mixed valence states, which help to create additional oxygen vacancies by maintaining charge-neutrality due to the difference in the electronegativity after dispersing the La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ sample on porous Ni-foam [27,28,29,30,31,32,33,34,35,36,37,38]. The O1s spectrum of the La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ sample contains lattice oxygen (lattice O²⁻) and oxygen vacancy $\left({\mathrm{V}}_{\mathrm{o}}^{°°}\right)$ peaks. The O1s spectra of the La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ sample after dispersing on Ni-foam cathode material display two partially superimposed peaks (Figure 7c). There are two major excitations: the first includes O1s of La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ sample bands, and the second a La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ sample, with Ni-foam bands with binding energy (BE) 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 ${\mathrm{V}}_{\mathrm{o}}^{°°}.$ The high area percentage ratio of Olat/Ovac in the La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ sample after dispersing on Ni-foam cathode indicates its high oxygen vacancy concentration and good oxygen adsorption capability, which plays an important role in high ORR activity [36,38]. However, different steps for the ORR mechanism in the La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ sample with Ni-foam cathode are shown in Figure 8. All studies revealed that the prepared cathode is suitable in order to generate better ORR activity for energy application [39].

4. Conclusions

In conclusion, in this work, we have developed a semiconductor, SrCoO₃, doped by both A-site and B-site using lanthanide and ceria, respectively. Later, La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ particles were embedded on porous Ni-foam, and their electrochemical properties were studied. The prepared La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ embedded on porous Ni-foam sample exhibits good ORR electrochemical performance at LTs, e.g., the maximum power density of 440 mW cm⁻² achieved using cathode over GDC electrolyte at 550 °C. The excellent ORR electrocatalytic activity of La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ embedded on porous Ni-foam can be attributed to a porous structure obtained by porous Ni-foam and the A- and B-site deficiency produced by La- and Ce-doping into SrCoO₃.The mechanism for the high electrochemical performance of La_0.2Sr_0.8Co_0.8Ce_0.2O_3−δ embedded on porous Ni-foam substrate is discussed in detail by different experimental approaches. The obtained results show that this approach could be useful not only in developing efficient ORR electrocatalysts, but could also be applied to other relevant applications.