Original Article
Modifying defect structures at interfaces for high-performance solid oxide fuel cells

https://doi.org/10.1016/j.jeurceramsoc.2020.03.009Get rights and content

Abstract

Oxygen vacancies are important charged defects in oxide ion conductors for oxygen reduction reactions (ORR). They are important reaction sites because of their high oxygen exchange, incorporation ability, and ionic transport reactivity. Herein, to control the defect concentration on the Gd0.1Ce0.9O2-δ (GDC) scaffold, the interface between the electrolyte and the electrode was engineered by a wet-chemical-based infiltration technique which constructed the conformal thin-film-like GDC interlayer (10 nm thickness) with smaller grain size than that of the GDC scaffold. X-ray photoelectron spectroscopy revealed the enriched oxygen vacancies in the infiltrated GDC interlayer with a reduced valence state of Ce. ORR kinetics was substantially improved with the infiltrated GDC interlayer, exhibiting a ∼2-fold decrease in polarization resistance and ∼1.41-fold increase in peak power density (0.072 Ωcm2 and ∼780 mW/cm2, respectively) at 650 °C. The defect structures at the interfaces and enhanced performance remained unchanged for 200 h at 650 °C.

Introduction

Solid oxide fuel cells (SOFC) are one of the most promising energy conversion systems; they have high energy conversion efficiency, high power output, and flexibility in the type of fuel they use, without forming harmful secondary products [1]. Generally, SOFCs require high operating temperatures (>700 °C) to achieve reasonable ionic conductivity of ceramic electrolytes such as Y0.08Zr0.92O2-δ (YSZ). This high operating temperature causes issues such as high system cost, severe performance degradation, slow start-up and shutdown cycles, hindering the rapid commercialization of SOFCs [2,3]. Extensive research has focused on reducing the operating temperature of SOFCs down to an intermediate temperature (IT) (500−700 °C), where the reduction of balance of plant costs and the wider materials selection for interconnects and seals are possible [1,4,5].

The electrode polarization losses impact the entire cell performance as the operating temperature is reduced to the IT regime [6,7]. In particular, the oxygen reduction reaction (ORR) in the cathode become the rate determining step (RDS), which significantly dominates the overall performance of the SOFC system. Extending reaction sites and enhancing the reactivity at the cathode surface as well as the interfaces between the cathode and the electrolyte are crucial to the ORR kinetics at IT regime [[8], [9], [10], [11]]. In addition to the structural aspect, defect structures, especially anion defects such as interstitial oxygen atoms, oxygen vacancies, impurity atoms, and line defects of configured structures have significant effects on the material properties such as electronic structure, ionic/electronic conductivities and thus ORR kinetics [12,13]. Especially, it has been reported that oxygen vacancies have substantial responsibility on the performance of IT-SOFCs because an oxygen vacancy is deeply involved in the ORR processes such as oxygen exchange, oxygen incorporation, and oxygen transport. A number of high resolution characterizations have been carried out revealing that the enrichment of the oxygen vacancies at the grain boundaries provides the extended reaction sites for ORR with lowered activation energy for ion incorporation and charge transfer at triple phase boundaries (TPBs). [[14], [15], [16], [17]] According to density functional theory (DFT) calculations, oxygen vacancies crucially contribute to the adsorption and incorporation of dissociated oxygen ions into the lattice by lowering their activation energies for each reaction, boosting ORR processes at the surface and/or interface of the cathode [18,19]. Therefore, engineering the concentration and distribution of oxygen vacancies at the surface/interface of the cathode is a reasonable strategy to enhance the ORR kinetics at the IT regime.

There are several techniques used to control the oxygen vacancy concentration at the surface/interface of the cathode such as atomic layer deposition (ALD) [20], pulsed laser deposition (PLD) [16,21], and sputtering [14,22]. Vacuum deposition techniques can be used for the various oxygen stoichiometry by controlling pO2 conditions and the deposition cycle during the process [23,24]. For example, ALD can control the oxygen vacancy concentration at the interface by varying the super-cycle of Y and Zr to form a different stoichiometric balance of Y1-xZrxO2-δ [25]. However, such vacuum deposition techniques involve practical difficulties such as cost, scalability, and compatibility with other fabrication processes, and are difficult to apply to real SOFC systems.

Among the several alternative deposition techniques used to overcome the abovementioned issues, a precursor-based infiltration technique is one of the most promising candidates, because of its cost-effectiveness, scalability to large areas with jetting processes, compatibility to other fabrication processes, and applicability to other structures in different scales [[26], [27], [28]]. Moreover, infiltrated nanoparticles generally form the nanocrystalline phase at lower temperatures (<900 °C) than those by the conventional powder synthesizing process (950−1100 °C). The smaller grain size or higher grain boundary density of infiltrated layers induces substantially different structural and chemical characteristics compared to those by the conventional powder synthesizing process, resulting in different electrochemical behaviors [29,30]. Increased grain boundary density of infiltrated layers can be beneficial because the lattice oxygen at the grain boundaries has lower coordination numbers and the oxygen-to-metal bond is easily loosened, resulting in enriched oxygen vacancies compared to those in the grain bulk [14,22].

In this study, we engineered the electrode/electrolyte interface to control over the defect concentration, especially the oxygen vacancy using the infiltration technique. The infiltrated conformal GDC interlayer, 10 nm thick, increase oxygen vacancy concentration between a porous Gd0.1Ce0.9O2-δ (GDC) scaffold and infiltrated Sm0.5Sr0.5CoO3-δ (SSC) layer which occurs cathodic reaction. X-ray photoelectron spectroscopy (XPS) revealed the enriched oxygen vacancies in the infiltrated GDC interlayer which affect to the valence state of Ce. ORR kinetics was substantially improved with the infiltrated GDC interlayer, exhibiting ∼2-fold decrease in polarization resistance and a 1.41-fold increase in peak power density of 0.072 Ωcm2 and 780 mW/cm2 at 650 °C, respectively. Our results provide new insights into defect engineering at the interface between the cathode and the electrolyte for achieving high-performance IT-SOFCs.

Section snippets

Infiltration solution preparation

The GDC precursor solution with a concentration of 0.1 M was prepared by dissolving Gd(NO3)3·6H2O (99.9 %, Sigma-Aldrich) and Ce(NO3)3·6H2O (99 %, Sigma-Aldrich) in a solution of deionized water and ethanol with a volume ratio of 3:2. Glycine was added to the solution as a chelating agent, with a glycine/nitrate ratio of 0.5, to form the desired fluorite phase. The SSC precursor solution with a concentration of 0.2 M was prepared by dissolving Sm(NO3)6H2O (99.9 %, Sigma-Aldrich), Sr(NO3)2

Results and discussion

Fig. 1 shows the schematic illustration of the fabrication process of the infiltrated cathode configuration. Details of the processes were described in the experimental details section. A porous GDC scaffold was screen printed on a dense GDC pellet of ∼6 μm thickness for the precursor solution injection. For SSC-infiltrated cells, the SSC precursor was infiltrated into the GDC scaffold to prepare the scaffold layer for cathodic reactions, as shown in Fig. 1(a). For GDC/SSC-infiltrated cell, GDC

Conclusion

We investigated the significantly improved ORR kinetics by modifying the interface between the electrolyte and the electrode by improving its oxygen vacancy concentration using infiltration technique. A thin film-like GDC interlayer, 9−10 nm thick, was conformally deposited on the porous GDC scaffold. The new interlayer had smaller grain size and higher concentration of oxygen vacancies than those of the non-infiltrated GDC scaffold. The enriched oxygen vacancies of the electrolyte/electrode

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was supported by the Global Frontier R&D Program at the Center for Multiscale Energy System (Grant No. NRF-2014M3A6A7074784), the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (Grant No.2019R1A2C4070158) and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20173010032170).

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    Both authors equally contributed to this work.

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