Developing sodium-doped ceria-based electrolytes for low-temperature solid oxide fuel cells

Abstract

Na-doped CeO2 (NDC) electrolytes with 0.05, 0.10, 0.15, and 0.20 molar ratios of Na ions (0.05NDC, 0.1NDC, 0.15NDC, and 0.2NDC) were synthesized and systematically evaluated for low-temperature solid oxide fuel cell (SOFC) applications. Density functional theory (DFT) calculations reveal that Na doping lowers the oxygen-vacancy formation energy. Structural analysis confirms progressive lattice expansion in NDCs and a maximum oxygen-vacancy concentration in 0.15NDC, while incomplete incorporation of Na in 0.2NDC yields residual Na2CO3. Conductivity studies demonstrate negligible electronic conductivity and a peak ionic conductivity in 0.15NDC, suggesting that moderate Na doping enhances ionic transport, whereas excessive dopant is detrimental. Two 0.15NDC-based SOFCs are fabricated by ceramic and dry-pressing methods, and their maximum power densities at 550°C are 208 and 778 mW·cm−2, respectively, indicating the rapid ionic transport of the 0.15NDC electrolyte. These results demonstrate that Na doping is an effective route for developing advanced low-temperature SOFC electrolytes.

1. Introduction

In the pursuit of efficient and sustainable energy conversion technologies, solid oxide fuel cells (SOFCs) have emerged as promising candidates for future energy systems due to their high efficiency, low emissions, and versatile fuel adaptability [1]. The electrolyte, a core component of SOFCs, conducts oxide ions and maintains electrical insulation between the electrodes, thereby playing a crucial role in determining the overall performance and efficiency of the SOFCs [2]. Developing an electrolyte with high ionic conductivity, low electronic conductivity, and good chemical stability is essential for advancing SOFC technology [3]. At present, Y2O3-stabilized ZrO2 (YSZ) is the most widely used electrolyte because of its high ionic conductivity, excellent stability at high temperatures, and extremely low electronic conductivity under reducing conditions. However, YSZ electrolyte requires high temperatures above 750°C to achieve sufficient ionic conductivity for high-power SOFC operation [4–5], which adversely affect the sealing processes and durability of SOFCs, thereby hindering the commercialization of SOFCs.

To address this challenge, research in the past decades has focused on developing alternative electrolytes based on CeO2, Bi2O3, LaGaO3, BaCeO3, and BaZrO3 [6–9]. Among these oxides, CeO2 has attracted the most attention because its ionic conductivity surpasses that of YSZ in the intermediate-temperature range (550–750°C) [10–11]. Various approaches have been proposed to reduce the ohmic resistance of CeO2-based electrolytes, such as thin-film, doping, and compositing methods [12–14]. Among these, cation doping is particularly effective because it introduces oxygen defects in the fluorite structure of CeO2, increasing the concentration of oxygen vacancies for faster oxygen ion transport [15–17]. A series of doped-CeO2 electrolytes have been developed using Sm3+, Gd3+, La3+, Y3+, Yb3+, Nd3+, Pr3+, and Eu3+ dopants [17–19]. Subsequent studies explored co-doping, such as Sm3+/Y3+ and Sm3+/Nd3+, to promote oxygen-vacancy generation in CeO2 [20–21], while tri-doping has also been studied to tailor the structural, electrical, and thermal properties of CeO2 for improved stability and reduced electronic mobility [22–25].

These studies reveal a clear preference for using trivalent cations (Ln3+) to substitute Ce4+ in CeO2 when developing new electrolytes, rather than divalent or monovalent cations [15–24,26]. This is primarily because substituting Ce4+ with divalent cations (such as Ca2+ and Sr2+) and monovalent cations (Na+ and K+) tends to cause defect association in the CeO2 lattice. Strong coulombic interaction between divalent/monovalent ions and oxygen vacancies can immobilize these vacancies, thereby reducing ionic mobility [27–28]. In contrast, the substitution of a Ce4+ with a Ln3+ introduces 0.5 oxygen vacancy in accordance with the charge balance mechanism, which is conducive to forming lower defect association energy in CeO2 [29]. Moreover, the introduction of lower-valence cations may promote the reduction from Ce4+ to Ce3+, which increases the electronic conductivity of CeO2-based electrolytes and thus leads to current leakage of SOFCs [30]. In contrast, Ln3+ doping can inhibit the transition from Ce4+ to Ce3+ under reducing atmosphere, suppressing the n-type electronic conduction [31]. Consequently, divalent and monovalent cations are generally considered less suitable for CeO2-based electrolyte design.