Strontium Manganese Oxide Getter for Capturing Airborne Cr

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Strontium Manganese Oxide Getter for Capturing Airborne Cr and S Contaminants in High-Temperature Electrochemical Systems Junsung Hong, Ashish Aphale, Su Jeong Heo, Boxun Hu, Michael Reisert, Seraphim Belko, and Prabhakar Singh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09677 • Publication Date (Web): 28 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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ACS Applied Materials & Interfaces

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Strontium Manganese Oxide Getter for Capturing

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Airborne Cr and S Contaminants in High-Temperature

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Electrochemical Systems

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Junsung Hong,† Ashish N. Aphale,† Su Jeong Heo,†,‡ Boxun Hu,† Michael Reisert,† Seraphim Belko,†

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and Prabhakar Singh,†

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†Department

of Materials Science and Engineering, University of Connecticut, Storrs, Connecticut 06269, United States

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‡Materials

Science Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States

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Keywords: getter, chromium capture, sulfur capture, strontium manganese oxide, stability, high-

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temperature electrochemical systems

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Abstract

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Traces (ppm to ppb) of airborne contaminants such as CrO2(OH)2 and SO2 irreversibly degrade the

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electrochemical activity of air electrodes in high-temperature electrochemical devices such as solid

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oxide fuel cells by retarding oxygen reduction reactions. The use of getter has been proposed as a cost-

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effective strategy to mitigate the electrode poisoning. However, owing to the harsh operating conditions

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(i.e., exposure to heat and moisture), the long-term durability of getter materials remains a considerable

Corresponding

author.

E-mail address: [email protected] (P. Singh) ACS Paragon Plus Environment

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challenge. In this study, we report our findings on strontium manganese oxide (SMO) as a robust getter

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material for co-capture of airborne Cr and S contaminants. The SMO getter with a 3D honeycomb

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architecture, fabricated via slurry dip coating, successfully maintains the electrochemical activity of

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solid oxide cells under the flow of gaseous Cr and S species, validating the getter’s capability of

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capturing traces of Cr and S contaminants. Investigations found that both Sr and Mn cations contribute

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to the absorption reaction and that the reaction processes are accompanied by morphological elongation

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in the form of SrSO4 nanorods and SrCrO4 whiskers, which favors continued absorption and reaction of

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incoming S and Cr contaminants. The SMO getter also displays robust stability at high temperatures and

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in humid environments without phase transformation and hydrolysis. These results demonstrate the

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feasibility of the use of SMO getter under operating conditions representative of high-temperature

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electrochemical systems.

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1. Introduction

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High-temperature electrochemical devices including solid oxide fuel cells (SOFC), solid

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oxide electrolysis cells (SOEC) and oxygen separation membranes (OSM) offer highly efficient

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and clean pathways for energy conversion, storage, and gas separation.1–4 These devices,

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however, remain susceptible to the electrocatalytic poisoning of the air electrode from the

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airborne trace contaminants, such as endogenous Cr, Si, and B vapors and exogenous SO2 and

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CO2 gases. The irreversible aspect of the electrode poisoning brings about long-term

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degradation of the electrode performance.5–9

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Although the concentration of SO2 is very low in air (75 ppb for hourly primary standard)10,

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the long-term exposure of the electrode to air flow leads to contaminant accumulation which

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degrades the cathodic activity for oxygen reduction.11,12 The degradation caused by SO2 is slow

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but continuous and severe in long term. Particularly, lanthanum strontium cobalt ferrite (LSCF)

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is considered vulnerable to sulfur poisoning as the surface-segregated SrO is prone to react with

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SO2 forming SrSO4, which is unlikely to dissociate once formed.8,11,13,14 Similarly, Cr vapor ACS Paragon Plus Environment

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species (CrO2(OH)2 and CrO3) have also been regarded as the most common and detrimental

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contaminants to air electrodes since they are continuously released from the chromia scales

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grown on metallic components within cell stacks (i.e., metallic interconnect, plenum, and

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manifolds) during the operating of high-temperature electrochemical systems.15,16 Balance-of-

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Plant (BoP) components also generate gaseous chromium species, contributing to the electrode

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poisoning.17

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Prior efforts to mitigate the degradation arising from Cr poisoning primarily focused on

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developing Cr-tolerant cathode materials (e.g., La(Ni,Fe)O3 and Nd2NiO4),18,19 cathode surface

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modification,20–22 oxidation-resistant alloys (e.g., ZMGTM232 series from Hitachi Metals),23 and

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surface coating of metallic components (e.g., MnxCo3-xO4 spinel).24 However, owing to the

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thermal expansion discrepancy and chemical incompatibility between adjacent materials, such

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methods still have issues under high-temperature operating condition. For instance,

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interdiffusion, crack propagation, and spallation impede the long-term efficacy of these

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materials.25,26 Since a trace amount of Cr accumulation can lead to drastic performance

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degradation, further development and improvement are required.

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The use of getters, which capture gaseous impurities, is a cost-effective way to mitigate the

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electrode poisoning without the need to replace the established component materials. The getter

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materials for high-temperature electrochemical systems require exceptional stability and

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reliability, as well as the gettering capability. Singh and co-workers have suggested strontium

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nickel oxide (SNO) as a Cr capturing material, which can conserve cathodes from Cr

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poisoning.27,28 Although its performance for absorbing Cr vapors has been demonstrated, further

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studies revealed the structural instability accompanied by phase separation at high temperatures

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(≥950 °C) and in humid environments.29,30 Such limitation brings difficulty in fabricating

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substrate-supported SNO getters and restricts the application of the getter.

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Herein, an advanced getter composed of strontium manganese oxide (SMO) is presented and

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discussed. The prototypical design for the SMO getter to capture the Cr and S contaminants in ACS Paragon Plus Environment

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the airstream is illustrated in Figure 1. In selecting the Sr and Mn mixed oxide, not only the

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absorption ability and affinity to Cr and S species but also the stability of the mixed oxides and

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the reaction products between SMO and Cr and S contaminants were taken into consideration

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(see Figures S1–S3 and Table S1 in the Supporting information). The honeycomb getter covered

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with SMO nanoparticles was fabricated by dip-coating. The capability of capturing both Cr and

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S gaseous species is demonstrated by employing electrochemical impedance spectroscopy (EIS).

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Post-test characterization of the getter elucidates the absorption process and morphological

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transformation. The SMO phase presents excellent durability at high temperatures and in humid

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environments. Therefore, the SMO-based getter has broad application prospects in high-

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temperature catalysts and technologies.

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2. Results and discussion

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2.1. Fabrication of SMO getter. The strontium manganese oxide getter was fabricated by dip-

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coating of cordierite honeycomb substrates with horizontal channels of 400 cells per square inch (cpsi)

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in an aqueous precursor slurry comprised of SrMnO3, MnCO3, Sr(NO3)2 and polyvinylpyrrolidone,

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followed by heat-treatment at 1000 °C, as shown in Figure 2a. X-ray diffraction (XRD) identified that

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the as-fabricated SMO getter is mainly composed of SrMnO3 (Figure 2b). Scanning electron microscopy

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(SEM) shows the honeycomb getter where granular SMO particles (tens-to-hundreds of nm in size) is

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uniformly coated on the substrate with a thickness of ~5 μm (Figure 2c). The honeycomb architecture is

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intended to permit steady, incompressible and laminar flow of the airstream for SOFC systems, and the

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structural design can be modified depending on the system size and operating conditions as shown by

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earlier simulation work.31

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2.2. Cr and S capture by SMO getter. The absorption capability of SMO getter for airborne Cr and

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S contaminants was evaluated using electrochemical impedance spectroscopy (EIS) of solid oxide cells

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(LSM|YSZ|Pt). The changes in the ohmic and non-ohmic polarization of the electrode were examined ACS Paragon Plus Environment

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over time. The test setup is shown in Figure 3d, and the test conditions are tabulated in Table 1. The

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electrochemical cell served as a Cr/S sensor since the change in the electrode impedance is sensitive to

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contamination of the air electrode.32,33 The electrochemical performance was monitored over time (~230

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h) under the flow of humidified air containing 4 ppm SO2 and ~1 ppb CrO2(OH)2 (evolved from Cr2O3)

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toward the LSM cathode through the SMO getter. This performance was compared with those recorded

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under the flow of either the SO2 or CrO2(OH)2 gases in the absence of the getter.

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Figure 3 shows changes in the current density of the cell under three different exposure conditions

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(Table 1). The current density remained stable under the flow of humidified air (~3% H2O by water-

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bubbling) (Curve (a) in Figure 3: 0–110 h). Immediately after the injection of 4 ppm SO2 gas, the

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current density gradually decreased (Curve (a) in Figure 3: 110–150 h). Similarly, under the generation

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of Cr vapor, the current density rapidly dropped (Curve (b) in Figure 3). The degradation in the current

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density for both cases is due to the increase in polarization resistance,34,35 corresponding to the

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increasingly enlarged Nyquist arcs over time (Figures 4a and b). The degradation mechanism is inferred

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from the Bode plots in Figures 4a and b, given that the impedance at low (≤ 30 Hz) and high (≤ 3 kHz)

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frequencies are responses to the transfer of O2- ions along the surface and across the bulk,

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respectively.18,36–38 The electrode performance degradation by Cr-poisoning is due to preferential Cr

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deposition at the triple phase boundary (TPB) between air, electrode, and electrolyte via the following

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electrochemical reduction.39–41

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2CrO2(OH)2 (g) + 6e- → Cr2O3 + 2H2O + 3O2-

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In contrast, the degradation by S-poisoning is due to the chemisorption of SO2 on the LSM surface,

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resulting in SrSO4 precipitation and non-stoichiometric, Sr-depleted LSM.14,42,43

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In the presence of SMO getter, the current density remained stable for 230 h (Curve (c) in Figure 3)

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despite the injection of the same levels of Cr vapor and SO2 gas (i.e., ~1 ppb CrO2(OH)2 for the initial

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100 h, and then both ~1 ppb CrO2(OH)2 and 4 ppm SO2 for another 120 h). Corresponding impedance ACS Paragon Plus Environment

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spectra (Nyquist and Bode plots) in Figure 4c remained largely unchanged during the experiment,

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indicating that the electrode had been free of contaminants, which supports the efficacy of the SMO

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getter for capturing Cr and S contaminants. The cross-sectional analysis of the LSM/YSZ interface of

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the three cells also shows good agreement with the above results. To be specific, the cross-section of the

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LSM tested in the presence of the SMO getter under the flow of Cr and S gases shows a clean surface

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morphology (Figure 4c) whereas the sulfur-contaminated LSM shows a modified surface with nano

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protrusions of SrSO4 precipitates (Figure 4a) (for more SEM images, see Figure S6 in the Supporting

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information).33,42

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The SMO getter sample, exposed to Cr vapor and SO2 gas during the electrochemical test, was

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structurally and chemically analyzed using SEM and Raman spectroscopy. Figure 5 shows the

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concentration of Cr and S along the air flow path in the inner channel of the getter. Elemental analysis

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of the getter shows the highest concentration of S and Cr to be at the getter inlet. The concentration then

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decreases along the air flow path in the getter channel towards the outlet, indicating SMO’s ability to

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capture both S and Cr gaseous species in air. In addition, the morphological observation (Figure 6a)

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shows the transformation of the microstructure along the channel length from oblong (0–2 mm) and

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angular (2–4 mm) to spherical (at a distance of > 4 mm) shapes, corresponding to the Cr and S

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concentrated areas as seen in Figure 5. This indicates that the granular SMO particles become elongated

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by absorbing SO2 gas. The smooth surface, as well as the angular particles, also incorporates sulfur, as

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can be seen in the EDS spectra (Figure 6a: inlet). The elongated particles, or nanorods, were identified

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as SrSO4 by electron diffraction spectroscopy (EDS) and Raman spectroscopy44 (Figures 6b and c).

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The absence of Cr compounds at the surface is considered to be caused by the predominant formation

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of SrSO4 over SrCrO4 under the relatively high concentration of SO2 (4 ppm). During the test conducted

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in the presence of only Cr vapor, the similar morphological transformation and the growth of SrCrO4

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nanowhiskers were observed (see Figures S7–S10 in the Supporting information). The vertical growth

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of reaction products during the capturing process would be followed by the consumption of Cr and S

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saturated near the surface, thus allowing for continued absorption of incoming S and Cr contaminants.

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2.3. Thermal and hydrolytic stability. Although SMO getter possesses excellent capture ability for

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the Cr and S gaseous species, as indicated above, the material must also keep structural stability under

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high-temperature and humid conditions to ensure long-term use in high-temperature electrochemical

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systems. This aspect cannot be neglected, particularly for Sr-containing perovskites because the large

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size of Sr2+ ions (1.44 Å in coordination number of 12) and its high affinity with H2O tend to promote

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Sr segregation and precipitation.30,45–47 For instance, Sr–Ni–O and Sr–Fe–O systems are prone to react

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with moisture forming hydroxide compounds accompanied by volume expansion, eventually resulting

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in deformation and pulverization (see Ref. 30 and Figure S2 in the Supporting information).

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The structure of SrMnO3 phase under high temperatures was investigated using in situ high-

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temperature (HT) XRD to observe any evidence of Sr segregation, such as the formation of SrO and

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SrCO3, as well as phase separation, which often happens above 500 °C in Sr-containing perovskites.45,46

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Figure 7 shows the in situ HT–XRD patterns recorded at ~25, 500, 700, and 900 °C. The major five

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peaks at 2θ = 27, 33, 35, 43 and 49° in the initial XRD pattern (RT) were in good agreement with those

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of 4H perovskite SrMnO3.48 At elevated temperatures (500, 700, and 900 °C), no new peaks relating to

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SrCO3 and SrO phases (typically at around 2θ = 25, 30, and 35°) arose, despite collective peak shift

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caused by thermal expansion (8.6 • 10-6 K-1 by calculation), verifying the high thermal stability of

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SrMnO3 without Sr-segregation. This result corresponds to the phase diagram of SrMnO3, where the

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4H–SrMnO3 phase is regarded to be stable up to 1447 °C,48,49 and to the literature50, which shows the

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stability of Sr-rich SrxLa1-xMnO3 (x = 0.4–0.8) up to 1000 °C. Further analysis was conducted using X-

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ray photoelectron spectroscopy (XPS) on the surface of the SrMnO3 pellet that was heat-treated at

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700 °C for 100 h in ambient air. Figures 8a–e shows the XPS depth profile and montage of survey

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spectra for Mn, Sr, O, and C. The concentration of Sr depth profile is parallel to that of Mn (Figure 8a), ACS Paragon Plus Environment

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thus implying the absence of internal stresses around Sr-ions which are considered a driving force for

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segregation of Sr from the bulk to the surface in many Sr-containing perovskites.13,30,45,46

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The stability of SrMnO3 in humid atmosphere was also evaluated by comparing the structure of

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SrMnO3 before and after its exposure to moisture. Figure 9a shows a typical XRD pattern of SrMnO3

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(P63/mmc). In contact with moisture and water, Sr-ions in SrMnO3 might precipitate as hydrates such as

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Sr(OH)2•8H2O and Sr(OH)2•H2O accompanied by volume expansion (molar volumes: 142.27 and 35.15

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cm3 mol-1 for Sr(OH)2•8H2O and SrMnO3, respectively) if hygroscopic or hydrolytic, as observed in Sr–

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Ni–O30 and Sr–Fe–O systems (Figure S2, Supporting information). Figures 9b and c show XRD

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patterns of the SrMnO3 exposed to moisture and soaked in water, respectively. It appears that the XRD

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pattern of SrMnO3 remained unchanged without the appearance of peaks corresponding to Sr(OH)2,

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Sr(OH)2•H2O, Sr(OH)2•8H2O and SrCO3. The pellet sample also remained dimensionally stable with no

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deformation (Figure 9d). These results demonstrate the resistance of SrMnO3 to water attack.

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2.4. Cr and S capture mechanism. To understand the Cr and S absorption process and reaction

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mechanism, the SMO getter exposed to Cr vapor and SO2 gas during the EIS test was analyzed. Results

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from scanning transmission electron microscopy (STEM) have elucidated the chemical and

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morphological changes associated with the surface reaction. Figure 10 displays TEM and elemental

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mapping images of the cross-section of the inlet side of the getter prepared by focused ion beam (FIB)

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(refer to the sampling procedure in Figure S11, Supporting information). The SMO layer appears to be

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composed of two phases (white, and white-gray spotted) by the contrast of the HAADF TEM images in

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Figure 10a. The elemental mapping images (Sr, Mn, S, and Cr) of the surface nanorod (white) in Figure

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10b show the Sr and S enrichment of the nanorod, corresponding to that of SrSO4 as identified by the

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Raman spectrum (Figure 6c). Figure 10c shows the elemental images of the cross-section of a small

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particle protruding from the top surface of the getter. It reveals that the particle has a core-shell structure

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composed of a shell of Sr and S and a porous core of Mn and Cr. FFT patterns of high resolution (HR)

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TEM images of the surface area (Figure 10c: bottom) identify SrSO4 (Pnma) and SrMnO3 (P63/mmc) ACS Paragon Plus Environment

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phases on the surface, thus confirming the sulfur-capturing capability of the SMO getter. Further

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analysis focused on the porous area. Figure 10d shows STEM images (HAADF and elemental maps) of

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the interface area between the shell and the porous core and electron diffraction patterns (SAED and

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FFT) of the porous area. The elemental maps reveal that the absorbed Cr resides in the Mn-enriched

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porous core, whereas Sr and S are homogeneously distributed in the surrounding shell (Figures 10c and

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d: mapping images). These results indicate that the process for the absorption of S and Cr species is

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accompanied by the outward growth of SrSO4 and SrCrO4 (with Sr diffusion toward the external

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surface), leaving the Mn-rich phase and voids at the core. Subsequently, the absorbed Cr in the shell

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migrates into the Mn-enriched core, resulting in the core-shell architecture as shown in Figure 10c.

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It is noted that Cr compounds were not detected by TEM-FFT analysis although the element Cr

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resides in the Mn-enriched core. The structure of porous core was identified as nanocrystalline

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manganese oxides from halo rings and spots in the selected area electron diffraction (SAED) pattern

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(Figure 10d: bottom left), where Mn2O3 (space group R-3:H) is the most dominant phase among Mn

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oxides based on the FFT pattern (Figure 10d: bottom right) and the phase diagram.51,52 It is considered

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that the Mn2O3 phase can accommodate Cr atoms as a form of (Mn,Cr)2O3 solid solution because of the

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very similar size of Cr and Mn (VICr3+ and VIMn3+ radii are 0.615 Å and 0.580–0.645 Å, respectively)53,

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which is also supported by the phase diagram54 (displayed in Figure S12, Supporting information).

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The dominant product of the capturing reaction was investigated from the thermodynamic point of

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view. As discussed above, the interaction of the S and Cr contaminants with the SMO getter leads to the

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formation of SrSO4, SrCrO4, and Mn2O3, as per the reaction: 2SrMnO3 + CrO2(OH)2

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SrCrO4 + SrSO4 + Mn2O3 + H2O (g). In the early stages of the getter operation, both SrSO4 and SrCrO4

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phases form and coexist. Once Cr and S are saturated, however, phase transformation occurs depending

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on temperature and partial pressures of CrO2(OH)2

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developed as functions of the partial pressures of CrO2(OH)2 (g) and SO2 (g) at 500–900 °C using the

(g)

and SO2

(g).

(g)

+ SO2

(g)

=

A phase stability diagram was


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database in HSC chemistry 6 (see Figure S13 in the Supporting information). The major reaction

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associated with the phase transformation under the operating conditions are found as follows:

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SrCrO4 + SO2 (g) = SrSO4 + ½Cr2O3 + ¼O2 (g).

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The corresponding phase diagram as functions of temperature and partial pressure of SO2 (g) is illustrated

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in Figure 11. It appears that, below 704 °C, SrCrO4 is not very stable and is likely to react with SO2 (g)

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(75 ppb) in ambient air, forming SrSO4 in the end; simultaneously, segregated Cr can migrate to the

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Mn2O3 pocket. This thermodynamic prediction is consistent with the experimental results (Figure 10).

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Furthermore, the Mn 2p core-level XPS spectra of SrMnO3 helps give a better understanding of the

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absorption mechanism. As shown in Figure 8b, the peak for Mn 2p3/2 (637–643 cm-1) shifts to lower

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binding energies with increasing depth, indicating the chemical state change of Mn ions from 4+

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valance to +4/+3 mixed valance (i.e., from ideal SrMnO3 to oxygen-deficient SrMnO3-δ).

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Correspondingly, the oxygen concentration decreases over depth (Figure 8a). Additionally, the Raman

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spectrum of the SrMnO3 pellet in Figure 8f shows the signals for E1g and A1g modes (434 and 634 cm-1,

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respectively)55 of Mn2O9 moiety (= two of MnO6 octahedra), confirming the existence of Mn2O9 dimers,

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as well as Sr, on the SMO surface. According to the literature regarding the catalytic activity of SMO,

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the surface presenting Mn2O9 tends to adsorb O2 in air, and the resulting Mn-superoxo species play an

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important role as a catalyst in oxidation and dehydrogenation.56 It is thus suggested that the Mn2O9

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dimers on the surface have also functioned to facilitate the capturing of gaseous contaminants,

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especially for SO2 as per the reaction:

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Sr surface + O2 adsorbed + SO2 (g) → SrSO4 absorbed (as illustrated in Figure 12b).

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The capturing process is summarized in Figure 12. Under the flow of gaseous S and Cr species, the

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Sr-terminated surface absorbs Cr vapor and SO2 gas while the Mn2O9 dimers on the surface, along with

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Sr, assist the absorption reaction (Figures 12b and c). The reaction of SMO with the absorbed S and Cr

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species results in the formation of SrSO4 and SrCrO4, respectively. The continued absorption occurs, ACS Paragon Plus Environment

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accompanied by Sr diffusion toward the external surface, thus, leading to the vertical growth of SrSO4

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and SrCrO4, and leaving voids and Mn-oxides at the core (where segregated Cr can reside).

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The use of getter has demonstrated the successful capture of gaseous Cr and S contaminants under

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SOFC operating conditions. The application of the getter, fabricated from low-priced precursors via

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simple dip-coating process, provides a cost-effective solution to prevent the electrode poisoning without

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the need to replace the state-of-the-art component materials, thus making it suitable for large scale

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applications. For practical application, the modular design of the getter is considered useful since it can

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be replaced and disposed after its predetermined life. Modeling and simulation based on computational

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fluid dynamics (CFD) further help to design and optimize getter architectures depending on the scale

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and operating conditions of the system for prolonged lifetime.31

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3. Conclusions

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The SrMnO3 is proposed as a promising, robust getter material for absorbing gaseous S and Cr

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contaminants in high-temperature electrochemical systems. The honeycomb getter covered with SMO

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nanoparticles has been fabricated using dip coating. The getter’s ability to capture contaminants has

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been demonstrated by electrochemical testing using EIS. Post-test characterization of the getter revealed

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that the absorption of S and Cr contaminants leads to elongation of granular SMO particles forming

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SrSO4 nanorods and SrCrO4 whiskers, leaving Mn oxides underneath in which inward-migrating Cr can

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reside. The vertical growth of reaction products during the capturing process favors continued

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absorption of incoming S and Cr contaminants. The Mn2O9 dimers existing on the surface also

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contribute to absorbing S and Cr impurities, along with Sr-terminated surface. SMO shows excellent

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thermal and hydrolytic stability with no phase transformation and hydrolysis during exposure to high-

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temperatures and humid environments, fulfilling the requirement for the operation in high-temperature

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electrochemical systems. The SMO-based getter, therefore, has potential applications in other high-


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temperature catalysts and technologies, such as selective catalytic reduction (SCR) of NOx and

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autothermal reforming (ATR).

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4. Experimental section

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Materials. The following materials were used as-received — strontium hydroxide octahydrate

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(Sr(OH)2•8H2O; Sigma-Aldrich, USA), manganese(IV) oxide (MnO2; Alfa Aesar, USA), strontium

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nitrate

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polyvinylpyrrolidone (PVP, average mol weight of 360,000; Sigma-aldrich, USA), cordierite

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honeycomb substrate (400 cpsi; Corning Inc., USA), lanthanum strontium manganite ink

9

((La0.80Sr0.20)0.95MnO3-x; Fuelcellmaterials, USA), Ink vehicle (terpineol-based; Fuelcellmaterials, USA),

(Sr(NO3)2;

Sigma-Aldrich),

manganese

carbonate

(MnCO3;

Alfa

Aesar,

USA),

10

yttria-stabilized zirconia button electrolyte (YSZ; φ = 25 mm; Fuelcellmaterials, USA), platinum (Pt)

11

gauze (52 mesh woven from wire with 0.1 mm in diameter; Alfa Aesar, USA), Pt paste (ESL

12

ElectroScience, USA), alumina paste (Zircar Ceramics Inc., USA), and chromium(III) oxide (Cr2O3;

13

Alfa Aesar, USA).

14

Fabrication of SMO honeycomb getter. The SMO honeycomb getter was fabricated by dip-

15

coating with an aqueous slurry (Figure 2a). First, Strontium manganese oxide (SrMnO3) powder was

16

synthesized by conventional solid-state reaction of Sr(OH)2•8H2O and MnO2 at 1300–1350 ˚C in

17

ambient air followed by grinding using a planetary ball mill with alumina milling media (Across

18

International LLC, USA). Second, an aqueous SMO slurry was prepared by blending as-synthesized

19

SrMnO3 powder (3.3 M) in an aqueous solution of 2.7 M Sr(NO3)2 and 3.3 M MnCO3 with 33.3 g L-1

20

polyvinylpyrrolidone to improve the adhesion between the coating layer and the substrate. Third, a

21

cordierite honeycomb substrate with horizontal channels of 400 cells per square inch (cpsi), cut into a

22

cylindrical shape of 20 mm in diameter and 40 mm in height, was submerged in the slurry followed by

23

vacuum impregnation. The slurry clogging cell channels was blown out by pressurized air. Forth, the

24

honeycomb substrate coated with the SMO slurry was dried at 60 °C for one day and then heat-treated

25

at 1000 °C for 3 h in ambient air. The morphology and thickness of SMO layers on the getter were ACS Paragon Plus Environment

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Page 13 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

observed using scanning electron spectroscopy (SEM; Quanta 250 FEG, FEI, USA). The chemical

2

structure was analyzed using the X-ray diffractometer (XRD; D8 Advance, Bruker, Germany) with Cu

3

kα radiation (λ = 0.1542 nm).

4

In operando electrochemical tests with SMO getter. The capture of gaseous S and Cr species

5

over the SMO getter resulting in electrochemical performance stability of the cathode was investigated

6

using electrochemical impedance spectroscopy (EIS) technique. For the preparation of LSM│YSZ│Pt

7

cells, the lanthanum strontium manganite ink blended with the ink vehicle was painted (φ = 10 mm)

8

onto the YSZ button electrolyte using a screen printer (Model 810, Systematic Automation Inc., USA),

9

followed by sintering at 1200 ˚C for 2 h in ambient air.57 Pt paste was painted on the opposite side as an

10

anode. Pt gauzes and wires were subsequently attached to both sides of the LSM│YSZ│Pt cell as

11

current collectors, followed by heat-treatment at 850 ˚C for 1 h in ambient air. The as-prepared

12

LSM│YSZ│Pt cell was sealed onto an alumina tube using an alumina paste. The electrolyte and both

13

electrodes were electrically connected to a potentiostat (VMP3, Bio-Logic, France). The EIS tests of the

14

cell were conducted at 650–700 ˚C under the three different conditions as follows (refer to Table 1): (a)

15

the flow of SO2 gas without the getter, (b) the flow of Cr vapor without the getter, and (c) the flow of

16

both Cr vapors and SO2 gas with the getter. The spectra were recorded under a bias of 0.5 V in the

17

frequency range from 0.5 Hz to 200 kHz using a sinus amplitude of 10 mV at an interval of 1 h. During

18

the measurement for the Test (a), 4 ppm SO2 gas, mixed with humidified air (~3% H2O by water-

19

bubbling), was injected to the LSM cathode (120 SCCM) for 100 h (110–210 h) after the electrode

20

activation for initial 110 h under the flow of humidified air, in which the concentration of SO2 gas (air

21

balance) was precisely controlled using a gas-mixing system (Series 4000, Environics, USA). During

22

the measurement for the Test (b), Cr vapor (i.e., ~1 ppb CrO2(OH)2 (g)) flowed (120 SCCM) to the LSM

23

cathode for 100 h, in which pieces of Cr2O3 pellet wrapped with silver gauze were placed in the tube,

24

and the humid air flowed through the Cr2O3 pieces to generate the Cr vapor.58 For the Test (c), the SMO

25

honeycomb getter was inserted in the alumina tube one-inch far from the cathode, as illustrated in

26

Figure S5. For the initial 110 h, the Cr vapor flowed to the LSM cathode, and then, both Cr vapor (~1 ACS Paragon Plus Environment

13


Page 14 of 29

1

ppb) and SO2 gas (4 ppm) flowed for another 120 h (110–230 h). After the EIS tests, the cross-section

2

of the LSM electrodes was observed using SEM equipped with energy dispersive spectroscopy (EDS).

3

Evaluation of the hydrolytic and thermal stability. To examine the moisture resistance, a

4

SMO pellet (φ = 13 mm) was placed in a humid environment (2.7% H2O, measured by a ThermoPro

5

hygrometer), and the shape and color were visually observed for 12 days, similar to what has been

6

reported in the literature.30 The SrMnO3 powers, exposed to the humid environment for 14 days and

7

soaked in distilled (DI) water for a day, were analyzed using XRD. The thermal stability of SrMnO3 was

8

also evaluated employing in situ high temperature (HT) XRD (D8 Advance, Bruker, Germany)

9

equipped with an Anton Paar HTK-1200 chamber (Anton-Paar GmbH, Graz, Austria). During the

10

analysis, the SMO powder, placed on an alumina holder of the HT-XRD, was heated up to 900 °C at a

11

ramp rate of 5 °C min-1. The XRD patterns were recorded at ~25, 500, 700, 900 °C, and room

12

temperature (after cooling down). In addition, the XPS depth profiling was carried out using a Quantum

13

2000 scanning ESCA microprobe (Physical Electronics, USA) on the SMO pellet obtained by heat-

14

treatment at 700 °C for 100 h and immediate cool-down at 10 °C min-1. A 4 keV Ar ion beam was used

15

for sputtering (raster area of 3 mm  3 mm, and 60 sec sputter time per cycle). The Mn 2p spectrum was

16

fitted with Gaussian-Lorentzian function and Shirley background subtraction (see Figure S14,

17

Supporting information). The pellet surface was also analyzed by Raman spectroscopy (Ramanscope

18

2000, Renishaw, Gloucestershire, UK) with a laser of 514.5 nm wavelength.

19

Post-test Characterization of SMO getter. The SMO getter, exposed to SO2 gas and Cr vapor

20

for 230 h during the EIS test, was dissected for characterization. The Cr and S concentrations and the

21

morphology along the central channel were analyzed using SEM equipped with EDS. The chemical

22

structure of the inlet side of the getter was analyzed by Raman spectroscopy. The inlet section of the

23

getter was sliced using a focused ion beam (FIB; Helios Nanolab 460F1, FEI, USA), and the specimen

24

was analyzed using scanning transmission electron microscopy (STEM; Talos F200X S/TEM; FEI,

25

USA) at 200 kV. The thermodynamic calculations for reaction products of SMO with SO2 and

26

CrO2(OH)2 gases were carried out from the database in HSC Chemistry 6.0 (Outotec, Finland). ACS Paragon Plus Environment

14

Page 15 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 2

ASSOCIATED CONTENT

3

Supporting Information

4

The Supporting Information is available free of charge on the ACS publication website at DOI: xxx.

5

Thermodynamic consideration for selecting getter materials, XRD patterns of the strontium ferrite and

6

strontium manganite exposed to humid conditions, SEM observation of the LSM electrodes exposed to

7

SO2 gas, Characterization of the SMO getter that was exposed to Cr vapor for 500 h using SEM-EDS

8

and Raman spectroscopy, FIB sample preparation process for TEM, Phase diagrams for Mn−Cr−O

9

(core) and Sr(s)−S(g)−Cr(g)−O (shell) systems, and Peak fitting of the XPS spectrum of SMO.

10 11

ASSOCIATED CONTENT

12

Corresponding Author

13

*E-mail: [email protected]

14

ORCID

15

Junsung Hong: 0000-0003-2238-532X

16

Ashish Aphale: 0000-0002-4200-4996

17

Su Jeong Heo: 0000-0002-7933-9714

18

Boxun Hu: 0000-0002-0823-4632

19

Prabhakar Singh: 0000-0002-5000-4902

20

Notes

21

The authors declare no competing financial interests.

22 23

ACKNOWLEDGEMENTS

24

This work was supported by the US Department of Energy and National Energy Technological

25

Laboratory (Grant No. DE-FE 0023385). The authors gratefully acknowledge Dr. Bill Willis and Prof. ACS Paragon Plus Environment

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Page 16 of 29

1

Steven L. Suib in the Department of Chemistry at the University of Connecticut for XPS measurements.

2

We also appreciate the assistance of Dr. Lichun Zhang from Fisher Scientific Center for Advanced

3

Microscopy and Materials Analysis (CAMMA) at the University of Connecticut for TEM analysis.

4 5

6

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

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7 8 9

Figure caption

10

Figure 1 Schematic illustration of the SMO honeycomb getter to purify the Cr and S contaminated

11

airstream.

12

Figure 2 (a) Fabrication process of the SMO getter by dip-coating of a cordierite honeycomb substrate

13

(400 cpsi) in an aqueous precursor slurry, followed by heat-treatment, (b) XRD patterns of a bare

14

cordierite substrate and the SMO getter (indexed on the basis of JCPDS data in Figure S4), and (c) SEM

15

images of the cross-section and surface of the SMO coating layer on the substrate.

16

Figure 3 Current density of the LSM|YSZ|Pt cells over time under the flow of either (a) 4 ppm SO2 gas

17

(injected after 110 h) or (b) Cr vapor (~1 ppb CrO2(OH)2) toward the LSM cathode in the absence of

18

SMO getter, and (c) that recorded under the flow of both Cr vapor and 4 ppm SO2 gas (injected after

19

110 h) toward the LSM cathode through the SMO getter. (d) Configuration for the EIS measurement

20

with the insertion of the SMO getter (Figure S5). The corresponding test conditions are tabulated in

21

Table 1.

22

Figure 4 Impedance spectra (Nyquist and Bode plots) of the LSM|YSZ|Pt cell under the conditions 1–3

23

(Table 1), which are of the plots in Figure 3, and cross-sectional SEM images at the LSM/YSZ interface

24

exposed to (a) SO2 gas, (b) Cr vapor, and (c) both Cr vapor and SO2 gas in the presence of the SMO

25

getter, respectively.


19


Page 20 of 29

1

Figure 5 Distribution of S and Cr along the inner channel from the inlet to the outlet of the SMO getter

2

that was exposed to 4 ppm SO2 (120 h) and ~1 ppb Cr vapor (230 h) at 700 °C during the EIS test,

3

analyzed using EDS.

4

Figure 6 (a) SEM images of the inner channel surface of the SMO getter that was exposed to 4 ppm

5

SO2 (120 h) and ~1 ppb Cr vapor (230 h) at 700 °C during the EIS test: at a distance of 0, 1, 2, 3, 4, 5

6

and 40 mm from the inlet side. (b) EDS spectra of the nanorods. (c) Raman spectrum of the nanorod.

7

Figure 7 High-temperature XRD patterns of SrMnO3 powder: room temperature (RT), 500 °C, 700 °C,

8

900 °C, and RT after cooling down.

9

Figure 8 (a) XPS depth profile of a SrMnO3 pellet that was heat-treated at 700 °C for 100 h in ambient

10

air, (b–e) Montage of depth profile survey spectra for Mn, Sr, O, and C, and (f) Raman spectrum of the

11

SrMnO3 surface.

12

Figure 9 XRD patterns of (a) as-synthesized SrMnO3 powder, (b) the power placed in a humid

13

environment (2.7% H2O) for 2 weeks, and (c) the powder soaked in water for a day; and (d)

14

Photographs of SrMnO3 pellets before and after exposure to the humid environment for 12 days.

15

Figure 10 (a) High-angle annular dark-field (HAADF) STEM images of the cross-section of the sample

16

taken from the inlet area of the SMO getter that was exposed to SO2 gas and Cr vapor for 230 h at

17

700 °C during the EIS test (Figure 3c). The sample was prepared using FIB (see Figure S11, Supporting

18

information). (b) Elemental mapping images (Sr, Mn, S, and Cr) of the nanorod grown on the surface by

19

STEM-EDS. (c) Elemental mapping images of a particle protruding from the top surface of the getter

20

and Fast-Fourier-Transform (FTT) patterns of high resolution (HR) TEM images of the particle. (d)

21

Elemental mapping images and electron diffraction patterns (SAED and FFT) of the interface between

22

the core (black-and-gray dotted area, bottom right in the HAADF image) and the shell (white-colored

23

area, top left in the HAADF image) beneath the surface.


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Figure 11 Phase diagram as functions of the partial pressure of SO2 (g) and temperature for the reaction between SrCrO4 and SO2 (g). Figure 12 Schematic representation of the S/Cr capturing mechanism of SMO getter: (a) Capturing process accompanied by the vertical growth of reaction compounds and the formation of the core-shell structure, and the proposed reaction mechanisms for the absorption of (b) SO2 and (c) CrO2(OH)2.

Figure 1


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Figure 2

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Figure 3


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Figure 4

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Figure 5 ACS Paragon Plus Environment

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Figure 6

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4 ACS Paragon Plus Environment

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Figure 7

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Figure 8

4 ACS Paragon Plus Environment

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Figure 9

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Figure 10

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Figure 11

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Figure 12

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Table 1. Conditions for the electrochemical tests: (a) sulfur poisoning, (b) chromium poisoning, and (c)

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SMO getter a. SO2 gas Temperature 700 °C Cell LSM | YSZ | Pt composition Getter ×

b. Cr vapor 650 °C

c. Getter 700 °C

LSM | YSZ | Pt

LSM | YSZ | Pt

×

○ (i) Air + 3%H2O + Cr vapor* (0–110 h) (ii) Air + 3%H2O + Cr vapor* + 4 ppm SO2 (110–230 h)

Cathode (i) Air + 3%H2O (0–110 h) Air + 3%H2O Atmosphere (ii) Air + 3%H2O + Cr vapor* (120 SCCM) + 4 ppm SO2 (110–230 h)

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Anode Atmosphere Dry air Dry air Dry air (50 SCCM) Applied bias 0.5 V 0.5 V 0.5 V Duration 230 h 100 h 230 h *The Cr vapor is mainly composed of ~1 ppb CrO2(OH)2 (g), calculated using HSC Chemistry 6.0 (Outotec, Finland), and the partial pressure of the Cr vapor over temperatures at 500–900 °C was plotted in Figure S1a.

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Table of Contents Graphic

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Strontium Manganese Oxide Getter for Capturing Airborne Cr

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