A high performance cathode for proton conducting solid oxide fuel cells

Abstract

Intermediate temperature solid-oxide fuel cells (IT-SOFCs) ), as one of the energy conversion devices, have attracted worldwide interest for their great fuel efficiency, low air pollution, much reduced cost and excellent longtime stability. In the intermediate temperature range (500-700 C), SOFCs based on proton conducting electrolytes (PSOFCs) display unique advantages over those based on oxygen ion conducting electrolytes. A key obstacle to the practical operation of past P-SOFCs is the poor stability of the traditionally used composite cathode materials in the steam-containing atmosphere and their low contribution to proton conduction. Here we report the identification of a new Ruddlesden-Popper-type oxide Sr 3 Fe 2 O 7Àd that meets the requirements for much improved long-term stability and shows a superior single-cell performance. With a Sr 3 Fe 2 O 7Àd -5 wt% BaZr 0.3 Ce 0.5 Y 0.2 O 3Àd cathode, the P-SOFC exhibits high power densities (683 and 583 mW cm À2 at 700 C and 650 C, respectively) when operated with humidified hydrogen as the fuel and air as the cathode gas. More importantly, no decay in discharging was observed within a 100 hour test. † Electronic supplementary information (ESI) available: Experimental details of conductivity, oxygen permeation and electrical conductivity relaxation tests; the results of electronic conductivity, oxygen permeation rate, surface exchange coefficients, BZCY content dependence of the ohmic resistance (R o ) and the polarization resistance (R p ), experimental and t Nyquist plots. See

Mixed‐Conducting Perovskites as Cathode Materials for Protonic Ceramic Fuel Cells: Understanding the Trends in Proton Uptake

“…For the layered GdBaCo 2 O 5.5 perovskite, a proton migration barrier of 0.63 eV has been extracted from pair potential molecular dynamics calculations . For the Sr 3 Fe 2 O 7 Ruddlesden–Popper phase, DFT calculations yield barriers in the range of 0.3–0.6 eV for long‐range diffusion paths . While more direct experimental measurements of proton conductivity and/or mobility are desirable, the theoretical results suggest that the proton mobility in cathode perovskites is roughly in the range of that in the barium zirconate electrolyte materials.…”

High-Performance La0.5Ba0.5Co1/3Mn1/3Fe1/3O3−δ-BaZr1−zYzO3−δ Cathode Composites via an Exsolution Mechanism for Protonic Ceramic Fuel Cells

“…The advantage of the exsolution method is the high degree of contiguity between the two phases [14]. All the materials prepared in this work fulfill the requirement of minimum electronic conductivity of~1 S/cm for a dense cathode [5,34], although they are close to the minimum value. The conductivity of the LB phase is expected to be 3-4 orders of magnitude higher than for the BZ phase, even with a minor amount of M transition metals on the B-site.…”

Oxygen Nonstoichiometry and Thermodynamic Explanation of Large Oxygen‐Deficient Ruddlesden–Popper Oxides LaxSr3−xFe2O7−δ

“…[1][2][3] Figure 1 shows the crystal structure of Sr 3 Fe 2 O 7Àd with tetragonal symmetry, where two perovskite SrFeO 3Àd and SrO rock salt structures are alternately stacked, resulting in three different anion positions: O1, O2, and O3 (oxygen vacancy concentration was expressed by the white color). [1][2][3] Figure 1 shows the crystal structure of Sr 3 Fe 2 O 7Àd with tetragonal symmetry, where two perovskite SrFeO 3Àd and SrO rock salt structures are alternately stacked, resulting in three different anion positions: O1, O2, and O3 (oxygen vacancy concentration was expressed by the white color).…”

“…T HE Ruddlesden-Popper Sr 3 Fe 2 O 7Àd -type oxides have been widely investigated as promising components for electrochemical devices because they can possess large oxygen storage capacity, mixed ionic and electronic conductivity, and excellent catalytic activity in the intermediate-to-lowtemperature range (773-1073 K). [1][2][3] Figure 1 shows the crystal structure of Sr 3 Fe 2 O 7Àd with tetragonal symmetry, where two perovskite SrFeO 3Àd and SrO rock salt structures are alternately stacked, resulting in three different anion positions: O1, O2, and O3 (oxygen vacancy concentration was expressed by the white color). Neutron diffraction measurements revealed that oxygen vacancies in Sr 3 Fe 2 O 7Àd -type oxides are preferentially located at the O1 sites between two FeO 6 octahedra along the c axis at low temperatures while oxygen vacancies at the O2 sites can be ignored.…”

“…Meanwhile, Sr 3 Fe 2 O 7Àd -type oxides own larger oxygen-ion diffusion coefficient, for example, 6.2 9 10 À6 cm 2 /s at 973 K, compared to simple cubic perovskite-type conductors. 2,7,10,11 To better meet severe conditions during the operation of electrochemical devices such as high temperatures, strongly reducing or oxidizing atmospheres, and a large chemical potential or temperature gradient, several strategies to address the challenge have been widely explored for Sr 3 Fe 2 O 7Àd -type oxides. One point is to increase oxygen deficiency and ionic conductivity under oxidizing conditions for components such as cathodes of SOFCs, catalysts and oxygen permeation membranes.…”