Effect of Sm co-doping on structural, mechanical and electrical properties of Gd doped ceria solid electrolytes for intermediate temperature solid oxide fuel cells

Highlights

• Effect of Sm co-doping in 10 mol% Gd doped ceria solid solutions has been studied.

• Ce_0.8Sm_0.Gd_0.1O_1.90 found to be the optimized electrolyte based on ionic conductivity.

• Co-doping of Sm³⁺ and Gd³⁺ is found to be a ‘synergetic effect’.

• A mechanism of vacancy association for conductivity enhancement has been proposed.

Abstract

Two series of samarium co-doped in 10 mol% Gd doped ceria Ce_0.90Gd_0.1O_1.95(GDC10) formulated as Ce_0.9-xSm_xGd_0.1O_2-δ(CS) and Ce_0.9Gd_0.1-xSm_xO_2-δ(GS) (x = 0.0, 0.03, 0.05 0.10) were synthesized by glycine nitrate auto combustion method. The structural effects of Sm substitution in the GDC10 solid solution have been studied by X-ray diffraction (XRD) and Raman spectroscopy techniques. An increased lattice constant (5.407 Å to 5.414 Å) with decreasing crystallite size (28.6 nm 22.5 nm) was observed in both the Ce substituted by Sm (CS) and Gd substituted by Sm (GS) in GDC10 crystal system. The highest oxygen vacancy concentration ($V_O^{\cdot \cdot }$ = 2.65 × 10²¹ cm⁻³) was found for 10 mol % Sm and 10 mol % Gd doped ceria (Ce_0.8 Sm_0.1Gd_0.1O_1.90) composition (CS10) evaluated by spatial correlation model from Raman spectroscopy. The microhardness of the sintered pellets was investigated by Vickers hardness measurement. The highest fracture toughness was found to be 1.85 ± 0.27 MPa m^1/2 for Ce substituted by 3 mol% Sm composition (CS3) among the overall compositions. The surface morphology and elemental composition of the CS and GS compositions were analyzed by FESEM. The morphology and composition of optimized electrolyte (CS10) was further analyzed by HRTEM and XPS respectively. AC impedance spectroscopy revealed that CS10 composition has an improved ionic conductivity (σ₈₀₀ = 0.147 × 10⁻³ S‧cm⁻¹) with significantly reduced activation energy (0.85 eV) in the temperature range of 673–1073 K under air atmosphere. A mechanism for conductivity enhancement by oxygen vacancy for CS compositions has been proposed. The effect of Sm as a secondary co-dopant in the GDC10 electrolyte was studied in detail to establish a candidate electrolyte material for operating solid oxide fuel cells in the intermediate temperature range (673–1073 K).

Introduction

Solid oxide fuel cell (SOFC) technology has become one of the notable electric power generating technologies among the electrochemical devices [1,2]. The researchers have been showing great interest in the development of efficient electrolyte material as the operating temperature of SOFC has been governed by the performance of electrolytes [3,4]. Doped cerium oxides are well known material for their excellent ionic conductivity in the intermediate temperature range (673–1073 K) [5]. The doping process plays a crucial role in modifying the microstructure and electrical properties. A small amount of dopants such as Li₂O [6] Cu [7], Zn [8] are helpful to increase the grain boundary conductivities by trapping (scavenging effect) the impurity at the grain boundaries [9]. The rare earth doped ceria has been widely studied and used as solid electrolyte towards SOFC application [10,11]. The type of the dopant determines the electrical properties of the ceria. Anirban et al. [12] recently investigated the effect of ionic radii of various rare earth dopants (Er, Ho, Gd, Sm, Eu, Nd, La) and found that Gd³⁺ doped ceria shows the highest conductivity due to the minimum strain among the other dopants. Similarly Arabaci et al. [13] studied the effect of Gd or Sm on structural and electrical properties of ceria. They found that Sm doped ceria has higher sinterability compared to that of the Gd-doped sample at low sintering temperature (1573 K) due to smaller grain volume. Ruston et al. [14] studied the impact of lattice strain and doping on oxygen diffusion in rare earth doped ceria (Gd, Sm, Ho, Dy, Yb, Er, Nd, La) using dynamic atomic scale simulation techniques. They reported that the Nd doped ceria has the lowest activation energy of migration (Ea = 0.48 eV). However Sm doped ceria exhibits minimum strain (4%) and activation energy increases beyond this strain level.

In the case of electrical properties, conductivity enhancement of ceria has been achieved mainly due to the defect formation by doping process. In general, when ceria is partially doped (substituted) with tri or divalent cations (aliovalent), it results in the production of oxygen vacancy in the crystal system. The oxygen vacancy is responsible for the oxide ion conductivity through vacancy migration (hopping) mechanism [15]. The vacancy concentration increases with increasing dopant concentration. Although doping generates structural defects (oxygen vacancies) in the ceria lattice, the fluorite structures are capable of accepting certain limit, but above an optimum dopant level, defect association (RE-${\text{V}}_{\text{O}}^{\cdot \cdot }$) takes place between the dopant cation and vacancy which tends to decrease the ionic conductivity. Minimization of the defect association with preferred dopant at optimized level is still challenging for improving the ionic conductivity for few decades. In this aspect, recent reports have suggested that co-doping strategy as an effective methodology for tailoring electrical conductivity of ceria solid solution [[16], [17], [18]]. The method is recommended to reduce the defect association, and grain refinement for the enhancement of grain-boundary conduction [19]. The choice of the dopant is very important in minimizing defect association to reduce the activation energy for oxygen diffusion reactions. It is well known that the 10 mol% gadolinia-doped ceria (GDC10) is a high performing conductive electrolyte among the singly doped ceria [20]. However, a great amount of interest is also shown in search of a suitable co-dopant for Gd to establish a potential electrolyte with improved electrical characteristics. Many of the recent reports reveal that the influence of the co-dopant on densification and conductivity. An improved microstructure, sintered density and electrochemical properties (σ₆₀₀ = 3.5✕10⁻² S cm⁻¹) were achieved by the addition of 3 mol% Li in GDC10 [21]. Similarly a small amount of cobalt (0.5 mol% of Co²⁺) addition to GDC enhances the densification and superior conductivity (σ₆₀₀ = 4.5✕10⁻² S cm⁻¹) as reported by Accardo et al. [22]. The same research group also investigated the effect on addition of Bi in GDC10 and reported an extremely high density (ρ = 99.7%, T = 1200 °C) for 5 mol% bismuth co-doped GDC. Arabaci et al. [23] have reported an improved density and ionic conductivity by co-doping of Pr in to GDC20. Spiridigliozzi et al. [24] have recently reported a remarkable effect of Pr on sintering behavior of GDC pellets and achieved the highest electrical conductivity (σ₈₀₀ = 1.25 S cm⁻¹) for 6 mol% of co-doped GDC synthesized by co-precipitation method. However, utilizing Pr precusours as co-dopant is considered to be an expensive approach in the commercialization of the electrolyte in bulk quantities. The synthesis parameters can also influence the electrical and micro structural properties of GDC electrolytes. Dell″Agli et al. [25] reported an improved sintering behavior of various di-valent and trivalent co-doped (Ca, Sr, Er, Pr) samarium co-doped ceria (SDC20) using ammonium carbonate precipitant in different carbonate environments. Anwar et al. [26] have investigated Er as a potential co-dopant for SDC20 system. They reported improved electrical properties (σ₆₀₀ = 1.3✕10⁻² S cm⁻¹, Ea = 0.58 eV) for the 10 mol% of Er co-doped composition. They also found a negative effect on the electrical conductivity by co-doping Sr into erbium doped ceria (EDC) due to the formation SrCeO₃ secondary phase with EDC [27]. A significantly improved conductivity was found by the addition of ternary carbonate salt compositions (Li₂CO₃, Na₂CO_3, K₂CO₃) to Sr single doped ceria (Ce_0.95Sr_0.05O_2-δ) due to the mixed (H⁺/O²⁻) conduction phenomenon reported by Anwar et al. [28]. Interestingly, some of the recent investigation showed encouraging structural properties of Sm and Gd co-doping in ceria [11,29,30]. For instance, 5 mol% Sm co-doping in GDC10 exhibited an improved conductivity (σ₇₅₀ = 4.2✕10⁻² S cm⁻¹) as reported by Accardo et al. [31]. However, some of the molecular dynamics stimulation studies showed conflicting evidence that co-doping strategy is an average effect of the conductivities of the parent singly doped compositions [32]. However, the key reason for the conductivity enhancement in the nanocrystalline co-doped ceria electrolytes is not clearly understood so far. Therefore Gd³⁺ and Sm³⁺ co-doping in ceria would be of a great interest in order to understand the co-doping effect in the ceria.

In this study, we prepared two series of compositions comprising Sm co-doped in 10 mol.% Gd singly doped ceria (GDC10) formulated to be Ce substituted by samarium Ce_0.9-xSm_xGd_0.1O_2-δ(CS) and Gd substituted by Sm in Ce_0.9Gd_0.1-xSm_xO_2-δ(GS) (x = 0.0, 0.03, 0.05, 0.10) to understand the compositional effects on their structural, mechanical and electrical properties.