Carbon Deposition and Sulfur Poisoning in SrFe_0.75Mo_0.25O_3-δand SrFe_0.5Mn_0.25Mo_0.25O_3-δElectrode Materials for Symmetrical SOFCs

Abstract

Redox stable SrFe 0.75 Mo 0.25 O 3-δ and SrFe 0.5 Mn 0.25 Mo 0.25 O 3-δ electrode materials were studied in terms of their physicochemical properties and electrochemical performance in symmetrical SOFCs, with focus on their tolerance toward carbon deposition and sulfur poisoning. Both materials possess cubic, B-site cation-disordered Pm-3m structure in air up to high temperatures. SrFe 0.5 Mn 0.25 Mo 0.25 O 3-δ was found to transform to a B-site cation-ordered structure after reduction in dry 5 vol% H 2 /Ar at 1100 • C, while no structural changes were observed for SrFe 0.75 Mo 0.25 O 3-δ . Weight change upon annealing up to 850 • C in air and in dry 5 vol% H 2 /Ar indicated an increase in oxygen deficiency on the order of δ = 0.2. Seebeck coefficient and conductivity dependence on the oxygen partial pressure pO 2 showed that both oxides exhibit p-type conductivity in air and n-type conductivity under reducing conditions. Carbon deposition was found to depend on temperature, gas composition (CO or CH 4 ), and presence of Ce 0.8 Gd 0.2 O 1.9 electrolyte in the composite-type electrode. Stable fuel cell performance, without carbon deposition, was obtained for SrFe 0.75 Mo 0.25 O 3-δ -based SOFC in 10 vol% of CO in CO 2 . Also, SOFC operation with CH 4 as fuel was achieved without coking at temperatures ≤ 700 • C. However, both oxides suffer from sulfur poisoning-related effects in atmosphere with 800 ppm H 2 S. Compatibility of Solid Oxide Fuel Cells (SOFCs) with nonhydrogen fuels represents their important advantage over other types of fuel cells. Versatile fuel utilization can be an essential factor for future development and commercial adoption of SOFC technology in power generation, for instance, from biogas used as a renewable energy source.1,2 One of the main challenges with SOFCs running directly on carbon containing fuels, such as syngas, methane/biogas or other hydrocarbons, is carbon deposition occurring on the anode, which results in a significant, and sometimes detrimental decrease of the performance. In the case of well-established Ni-YSZ cermet anode, the performance degradation upon coking arises mainly from a decrease in length of the Triple Phase Boundary (TPB) on the anode, which limits the number of catalytic sites needed for fuel oxidation.1,2 Furthermore, the deposited carbon may expand and destroy mechanical integrity of the electrode. Also, the considered nonhydrogen fuels usually contain sulfur (in a form of H 2 S), which can irreversibly poison Ni-YSZ cermet anodes.1-3 It may be stated that construction of robust SOFCs with fuel versatility relies upon a development of the anode materials with improved tolerance towards carbon deposition and sulfur poisoning. Mo-containing perovskitetype oxides have shown promise in this respect.3,4 The considered family of materials possesses crystal structure that can be either B-site cation disordered, denoted as AM 1-x Mo x O 3-δ (A: Sr, Ba; M: 3d metals) or B-site cation-ordered one (so called double perovskite structure), A 2 M 2-x Mo x O 6-δ . 3,5 T...

Efficient reversible CO/CO2 conversion in solid oxide cells with a phase-transformed fuel electrode

“…2b shows the XRD patterns of the as-prepared SFMCo and reduced SFMCo powders. The SFMCo sample exhibits a cubic perovskite with the Pm m 3 space group like the isostructural P-SFM (ICSD-238948) [38], which implies that Co has been well doped into SFM perovskite without causing any structural change or formation of the secondary phase. After a reduction in H 2 at 800°C for 2 h, the pure perovskite phase is transformed into two different phases: an R-P phase and an alloy phase.…”

Enhanced coking resistance of Ni cermet anodes for solid oxide fuel cells based on methane on‐cell reforming by a redox‐stable double‐perovskite Sr 2 MoFeO 6‐δ

“…In recent years, oxygen‐deficient and mixed‐valent perovskite materials have been shown to have a potential application as SOFC anode materials because of their appropriate catalytic activity and electro‐conductivity. For example, molybdenum‐based double perovskites Sr 2 MMoO 6‐δ (M = Co, Ni, Fe, Mn, and Mg) have exhibited excellent catalytic activity towards the electro‐oxidation of H 2 and CH 4 . Among these oxides, Sr 2 NiMoO 6‐δ shows desirable electrochemical performance even in dry CH 4 , which is attributed to the increased catalytic activity for methane conversion because of in situ exsolution of Ni .…”

Rate-Determining Steps of Oxygen Surface Exchange Kinetics on Sr2Fe1.5Mo0.5O6−δ

“…where S is the surface area of the sample; N is the amount of oxygen in the gas phase; α is the 18 O fraction; y is the value equal to the difference between the concentration of 18 O 16 O molecules at time t from the equilibrium value; r is the sum of the rates of the three types of exchange. The first type of exchange r 0 occurs without the participation of oxygen oxide; one and two oxygen atoms from the oxide surface participate in the exchange of the second r 1 and third r 2 types, respectively.…”

“…Oxide materials with the like-perovskite structure have been recommended as cathode materials [1][2][3][4][5]. Recently, Sr 2 Fe 1.5 Mo 0.5 O 6−δ (SFM) oxide has been suggested as a promising material for electrochemical devices with symmetrical (i.e., with identical) electrodes [6,7] due to the high stability and electrical conductivity of SFM under oxidizing and reducing conditions [8][9][10][11][12][13][14][15][16][17][18][19]. The rates of hydrogen oxidation and oxygen reduction reactions are also quite high for these electrodes.…”

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The oxygen surface kinetics of Sr 2 Fe 1.5 Mo 0.5 O 6−δ was determined using the 16 O 2 / 18 O 2 isotope exchange method with gas phase analysis at 600-800 • C. The heterogeneous exchange rates (r H ) and the oxygen diffusion coefficients (D) were calculated by processing the concentration dependences of the 18 O fraction using Ezin's model. The rates of oxygen dissociative adsorption (r a ) and incorporation (r i ) were calculated based on a model using the three exchange type rates.

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