Electrostatic Force Microscopic Characterization of Early Stage

Letter pubs.acs.org/NanoLett

Electrostatic Force Microscopic Characterization of Early Stage Carbon Deposition on Nickel Anodes in Solid Oxide Fuel Cells Hyungmin Park,†,‡ Xiaxi Li,† Samson Y. Lai,† Dongchang Chen,† Kevin S. Blinn,† Mingfei Liu,† Sinho Choi,‡ Meilin Liu,*,† Soojin Park,*,‡ and Lawrence A. Bottomley*,§ †

Center for Innovative Fuel Cell and Battery Technologies, School of Materials Science and Engineering and §School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ‡ Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea Downloaded by UNIV OF NEW SOUTH WALES on August 31, 2015 | http://pubs.acs.org Publication Date (Web): August 28, 2015 | doi: 10.1021/acs.nanolett.5b02237

S Supporting Information *

ABSTRACT: Carbon deposition on nickel anodes degrades the performance of solid oxide fuel cells that utilize hydrocarbon fuels. Nickel anodes with BaO nanoclusters deposited on the surface exhibit improved performance by delaying carbon deposition (i.e., coking). The goal of this research was to visualize early stage deposition of carbon on nickel surface and to identify the role BaO nanoclusters play in coking resistance. Electrostatic force microscopy was employed to spatially map carbon deposition on nickel foils patterned with BaO nanoclusters. Image analysis reveals that upon propane exposure initial carbon deposition occurs on the Ni surface at a distance from the BaO features. With continued exposure, carbon deposits penetrate into the BaO-modified regions. After extended exposure, carbon accumulates on and covers BaO. The morphology and spatial distribution of deposited carbon was found to be sensitive to experimental conditions. KEYWORDS: Electrostatic force microscopy (EFM), solid oxide fuel cells (SOFCs), coking, nickel, barium oxide

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combined spectroscopic and microscopic investigation provides new insight into the nucleation and growth process of carbon deposition and the role of BaO in coking resistance. Results and Discussion. The morphology of the patterned BaO catalysts on nickel surface was analyzed by SEM and AFM and is shown in Figure 1. The detailed fabrication process can be found in Supporting Information (SI). Photolithography process created repeating lines 10 μm wide with 10 μm gaps between lines. The BaO nanoparticles were distributed in and confined to the gap regions. The morphology and distribution of BaO nanoparticles are defined by the feature size of block copolymers. Cross-sectional analysis of the AFM topographic image (Figure 1D) shows that the BaO covered region shared a same baseline with the uncovered region, confirming that the BaO nanoparticles are discretely distributed and that the nickel surface is exposed between particles. The square spaces in the BaO stripe are a characteristic defect of the BaO coating process, which was introduced unintentionally but helpful for identification of BaO covered regions. Model interfaces comprised of C on Ni and BaO on Ni, respectively, were first examined to test the capability of EFM to differentiate carbon and BaO from the nickel substrate.

ickel serves as a key component of the solid oxide fuel cell (SOFC) anode. By virtue of its ability to activate hydrocarbon bond cleavage, nickel-based SOFC anodes allow direct utilization of hydrocarbon fuels.1−3 However, as a side effect, nickel also catalyzes carbon deposition, or “coking”, on its surface that leads to loss of reaction sites and structure damage.4,5 Partial coating of the anode with BaO, CeO2, or barium cerate/zirconate has improved coking resistance by providing a pathway for coking removal.6−9 The interface between the modifiers and the nickel surface has been identified as critical venues for understanding coking resistance.6,8 We sought to gain insight into the role of surface-bond oxides in inhibiting carbon deposition from a microscopic examination of the BaO-Ni interfacial regions as a function of fuel exposure. Our hypothesis was that electrostatic force microscopy (EFM), a scanning probe technique based on the electrostatic interaction between the atomic force microscope (AFM) probe tip and surface phases on the electrode, could be used to distinguish species deposited on the nickel surface.10−15 In this study, a well-defined BaO-Ni interface was created by block copolymer patterning and deposition of 10−100 nm diameter BaO nanoclusters on the nickel surface.16−20 Early stage carbon deposition was simulated by treating the BaOmodified nickel anodes to propane-containing fuels for a short period of time. The distribution and morphology of early stage carbon deposition was revealed by EFM, confocal Raman spectroscopy, and scanning electron microscopy. This © XXXX American Chemical Society

Received: June 7, 2015 Revised: August 18, 2015

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DOI: 10.1021/acs.nanolett.5b02237 Nano Lett. XXXX, XXX, XXX−XXX

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Downloaded by UNIV OF NEW SOUTH WALES on August 31, 2015 | http://pubs.acs.org Publication Date (Web): August 28, 2015 | doi: 10.1021/acs.nanolett.5b02237

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Figure 2C. The Δϕ−U curves on these two regions show distinctive separation, enabling discrimination of the composition of image features.13 The EFM and AFM analysis of the BaO-Ni interface are shown in Figure 2B,D. BaO covered region showed less vibration phase lag than the blank nickel region under either positive or negative bias, and there is no separation of the Δϕ−U curves. The absence of difference in observed surface potentials is due to the poor conductivity of BaO, resulting in little contribution to the electrostatic response. However, the presence of BaO clusters attenuated the interaction between the nickel substrate and the conductive AFM tip, providing contrast in the EFM images. Coking resistance of the patterned BaO catalyst was tested by treating the sample with unhumidified propane mixture (20% C3H8, 3% H2 and 77% Ar) at 600 °C. Exposure to such gas mixture at a constant flow rate of 75 sccm for 4 min translated to an equivalent propane dosage of 60 mL (with total gas mixture dosage of 300 mL). This dosage was chosen to limit the amount of coking thereby simulating the early stages of carbon deposition. The morphology and distribution of carbon deposition on nickel surface after exposure to propane in the presence of patterned BaO catalyst is inspected with combined AFM and EFM. As shown in Figure 3 (A), the AFM

Figure 1. Barium catalysts on nickel surface through a combined photolithography and block copolymer patterning method. (A, B) SEM images of the nickel surface with patterned BaO catalyst. (C) AFM topographic image on the interface between BaO and Ni regions. (D) The topography profile along the line indicated in the AFM image.

Figure 3. EFM analysis showing the coking resistance of BaO patterns. (A,D) Topographic images, (B,E) corresponding EFM images with tip biased at −1 V, and (C,F) EFM images with tip biased at +3 V. A,B,C are centered on the gap between the BaO patterned clusters, and D,E,F are centered on top of a stripe of the BaO clusters.

Figure 2. EFM analysis of model C−Ni and BaO−Ni interfaces. Topographic image and EFM images collected with tip with biases of different signs, (A) on C−Ni interface and (B) BaO−Ni interface. (C) Change of oscillation phase angle (Δϕ) as functions of tip bias, collected on the carbon-covered region and nickel-covered region, and (D) collected on the nickel-covered region and BaO covered region.

topographic image identifies the nickel surfaces covered by Ba catalyst, which has higher topography level over the bare nickel surface and square-shaped defects. EFM images are collected from the same region, as shown in Figure 3 (B, C), with negative and positive tip bias, respectively. While AFM topographic image shows only roughening of surface across the sample, clear contrast is observed in EFM images, indicating the presence of carbon deposition. In particular, when the tip bias was switched from −1 V to 3 V, the contrast of the patches inversed, corroborating that observed on modeled carbon films. Combined AFM and EFM analysis was also conducted on a region focusing on a line of BaO nanoclusters, as shown in Figure 3D,E,F. Comparing the EFM images with the topographic image, carbon deposition is registered with the region of significant roughening. Coking patches formed mainly

Topography and EFM images were collected simultaneously across the edge of evaporated carbon film as a model C−Ni interface (Figure 2A). The topographic image showed a clear 80 nm step on the interface, indicating the thickness of the evaporated carbon film deposit. The Δϕ image collected by EFM manifested clear contrast between the carbon covered region and the bare nickel surface. When the tip bias direction was inverted, the contrast between Ni and C reversed, indicating different surface potentials across the interface. The Δφ values on different regions (blank Ni surface and C covered Ni surface) are plotted as functions of tip bias, as shown in B


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degree of coking at a moderate level. BaO are critical sites for dissociative water adsorption that provides −OH groups for coking suppression.8 In this study, water was not introduced in the simulated coking process. BaO may have already been hydrated before the testing and therefore provides coking resistance for a limited period of time. Conclusions. In conclusion, the morphology and distribution of early stage carbon deposition was studied with electrostatic force microscopy (EFM), a technique capable of phase identification at nanometer scale. Patterned barium catalysts were fabricated onto the polished nickel surfaces and showed resistance against carbon deposition. While carbon deposition develops in the form of patches on the blank nickel surface, the presence of BaO nanoparticles inhibited the growth of carbon deposition. Growth of the carbon patches can penetrate into the barium covered region, and also lead to the growth of smaller carbon nuclei in the barium-covered regions, and after extended exposure, carbon accumulate on and cover BaO. The combination of nanopatterned catalyst features and the nanoscale phase characterization provides a unique approach to study the resistance of coking by surface modifications. A future study may systematically vary the type of surface modifications (e.g., CeO2, barium zirconium cerates, etc.), testing atmosphere compositions, and electrochemical environments to gain further insight into the mechanisms of coking resistance.


in the region devoid BaO crystal deposits, suggesting that BaO is effective in resisting carbon deposition under the experimental conditions used. The effectiveness of coking resistance of the patterned BaO was also validated with confocal Raman mapping (Figure S1) and SEM analysis (Figure S2), which are presented in Supporting Information. While BaO may partially convert to BaCO3 upon propane exposure,21 it does not impact its coking resistance at an early stage. To gain insight into the coking resistance of BaO, a detailed topographical analysis and phase identification was conducted on carbon patches formed at different regions, as shown in Figure 4. In most locations, carbon deposits were seen at a

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b02237.

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Details of BaO nanocluster fabrication, additional characterization by Raman spectroscopy and SEM, and supporting figures (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected].

Figure 4. Carbon deposition on different locations with respect to BaO modification. AFM, EFM images, and schematic showing carbon deposition (A) at a certain distance from BaO nanoclusters, (B) at the boundary of BaO nanoclusters, and (C) in a region protected by BaO nanoclusters.

Author Contributions

H.M.P. and S.H.C. designed and fabricated the block copolymer patterned nano catalysts; X.X.L., and M.F.L. set up the instrument and simulated coking on the modified nickel surface; X.X.L. and L.A.B. performed the electrostatic force microscopy analysis of carbon distribution; S.Y.L., D.C.C., and K.S.B. performed SEM and Raman spectroscopy analysis of the samples; H.M.P., X.X.L., M.L.L., S.J.P., and L.A.B. conceived the research and planned the experiments. All authors have given approval to the final version of the manuscript. H.P. and X.L. contributed equally.

distance from the BaO nanoclusters (Figure 4A). In regions where the carbon patches grows larger, the resistance of BaO to carbon nucleation is overcome as shown in Figure 4B. In rare cases, carbon also formed within the BaO covered regions, as shown in Figure 4C. The intrusion and nucleation of carbon patches in the BaO modified region is also corroborated by SEM analysis, as shown in Figure S3. The proximity of carbon to the BaO nanoclusters provides insight into the process of coking. Carbon deposits preferentially on the nickel surface. On the uncovered nickel surface, large patches of coking can be identified. Penetration of carbon into the BaO-covered region indicates that the coking resistance of BaO can be compromised under prolonged propane exposure (Figure S4). Under SOFC operating conditions, additional coking removal mechanisms, such as introduction of water steam or oxygen ions, can keep the

Funding

This work was supported by the HeteroFoaM Center, an Energy Frontier Research Center funded by the U.S. DOE, Office of Science, Office of Basic Energy Sciences (BES) under Award Number DE-SC0001061. Notes

The authors declare no competing financial interest. C


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Electrostatic Force Microscopic Characterization of Early Stage

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