A cobalt-free SrFe_0.9Sb_0.1O_3−δ cathode material for proton-conducting solid oxide fuel cells with stable BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ electrolyte

Abstract

A cobalt-free cubic perovskite oxide SrFe_0.9Sb_0.1O_3−δ (SFSb) is investigated as a novel cathode for proton-conducting solid oxide fuel cells (H-SOFCs). XRD results show that SFSb cathode is chemically compatible with the electrolyte BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb) for temperatures up to 1000 °C. Thin proton-conducting BZCYYb electrolyte and NiO–BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (NiO–BZCYYb) anode functional layer are prepared over porous anode substrates composed of NiO–BZCYYb by a one-step dry-pressing/co-firing process. Laboratory-sized quad-layer cells of NiO–BZCYYb/NiO–BZCYYb/BZCYYb/SFSb are operated from 550 to 700 °C with humidified hydrogen (∼3% H₂O) as fuel and the static air as oxidant. An open-circuit potential of 0.996 V, maximum power density of 428 mW cm⁻², and a low electrode polarization resistance of 0.154 Ω cm² are achieved at 700 °C. The experimental results indicate that the cobalt-free SFSb is a promising candidate for cathode material for H-SOFCs.

Introduction

Currently, proton-conducting solid oxide fuel cells (H-SOFC), of which the water is formed at the cathode, have attracted tremendous attention because of some advantages over those cells with oxygen-ion conducting solid oxide fuel cells (O-SOFC) such as simpler fuel-recycling instruments [1], [2] and a lower active energy of proton transport which ensures that the H-SOFCs operate at an intermediate-to-low-temperature range of 400–650 °C. In order to develop new electrolyte for insufficient conductivity at such low operating temperatures, several proton-conducting oxides were found to have attractive ionic conductivity at reduced temperature [1], [3], [5]. At temperatures below 750 °C, BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb) showed the highest ionic conductivity, at 500 °C, for example, the ionic conductivity of BZCYYb was about 1.2 × 10⁻² S cm⁻¹, that of BaZr_0.1Ce_0.7Y_0.2O_3−δ was 9 × 10⁻³ S cm⁻¹, and the ionic conductivity of GDC was 5 × 10⁻³ S cm⁻¹. Besides, BZCYYb possesses rapid transport of both protons and oxide ion vacancies and resists deactivation by sulfur and coking for sulfur oxidation and hydrocarbon cracking and reforming [4].

Accordingly, the development of alternative cathode material for proton-conductor IT-SOFCs in order to reduce cathode–electrolyte interface polarization remains a challenge. Efforts have been devoted to lowering the cathode polarization resistance for H-SOFCs by introducing novel cathode materials to meet the demand for commercialization [5], [6], [7], [8], [9]. Recently, Aguadero et al. [10] have reported that a tetragonal perovskite phase by doping the SrCoO_3−δ system with 10% of Sb was stabilized in the cobalt position obtaining good properties as a mixed conductor to be used as a cathode material in IT-SOFCs, which can be explained because the oxide with the cubic phase showed the highest electronic and oxygen ionic conductivity among the various phase structures of SrCoO_3–δ with a maximum reported total electrical conductivity of ∼160 S cm⁻¹ at ∼950 °C [11]. However, to our knowledge, cobalt-based cathodes often encountered some problems like high thermal expansion coefficient and poor stability [12], [13]. Therefore, it is desirable to develop the cobalt-free cathodes with good electrocatalytic activity for proton-conductor IT-SOFCs. To the best of our knowledge, the performance of a novel cobalt-free SrFe_0.9Sb_0.1O_3−δ (SFSb) as potential cathode has not been reported to date. In this work, the ceramic material of SrFe_0.9Sb_0.1O_3−δ (SFSb) synthesized by auto-ignition process was examined as a new cobalt-free cathode for a protonic SOFC with a BZCYYb electrolyte.