Letter Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Enhanced Thermal Stability of Ultrathin Nanostructured Pt cathode by PdO: In Situ Nanodecoration for Low-Temperature Solid Oxide Fuel Cell Wonjong Yu,† Yeageun Lee,‡ Arunkumar Pandiyan,† Sanghoon Ji,§ Waqas Hassan Tanveer,∥ and Suk Won Cha*,†,⊥
ACS Appl. Energy Mater. Downloaded from pubs.acs.org by 188.72.126.99 on 10/07/18. For personal use only.
†
Department of Mechanical and Aerospace Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, South Korea ‡ Department of Mechanical Science and Engineering, University of Illinois at Urbana−Champaign, 1206 West Green Street, Urbana, Illinois 61801, United States § Future Strategy & Convergence Research Institute, Korea Institute of Civil Engineering and Building Technology, Goyangdae-ro, Ilsanseo-gu, Goyang-si, Gyeonggi 10223, Republic of Korea ∥ Department of Mechanical Engineering, School of Sciences and Technology (NUST), H-12, Islamabad 44000, Pakistan ⊥ Institute of Advanced Machines and Design, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea S Supporting Information *
ABSTRACT: Due to its high surface area and catalytic activity, the nanostructure of platinum significantly enhances the electrochemical reactions of electrodes. However, the inherently poor thermomechanical stability of the nanostructure has been a critical issue. This study suggests a simple method of using a Pd layer deposited by sputtering to enhance the stability of the 20 nm thick nanoporous Pt. The microstructural analysis reveals that the Pd films are partially oxidized at operating temperature to nano-polycrystalline Pd formed around Pt particles and inhibiting the mobility of the Pt grains. By preserving the Pt nanostructure, the degradation rate of the Pt−Pd cathode structure is 10 times lower than that of the Pt cathode. KEYWORDS: nanostructure, Pt cathode, thermal stability, palladium oxide, in situ nanodecoration, low-temperature solid oxide fuel cell
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providing more triple phase boundaries (TPBs).2,7,8 However, despite such strengths, a nanoporous Pt cathode suffers from inherently poor thermomechanical stability, which can change its microstructure to a great extent.9,10 It has often been reported that the morphological deformation (e.g., agglomeration and/or coarsening) of a nanoporous Pt cathode can diminish the effective TPB density at the cathode−electrolyte interface, which would cause an increase in the polarization resistance of SOFCs.11,12 This deformation of Pt could be explained by thermodynamic instability. Pt, which has high surface free energy, tends to aggregate and form larger particles to lower the surface energy at high temperatures. The agglomeration of Pt nanoparticles increases their size and consequently decreases the TPB density. This phenomenon
he use of platinum (Pt) as a cathode material is common in the SOFC research area because of its chemical stability in a strong oxidizing atmosphere and superior kinetics for oxygen reduction reaction (ORR) even in the lowtemperature range of 300−500 °C.1,2 Due to the high cost of Pt, several research studies have focused on the development of alternative cathode materials, mostly mixed ionic− electronic conductor (perovskite) materials such as LaxSr1−xCoyFe1−yO3−δ, BaxSr1−xCoyFe1−yO3−δ, SmxSr1−xCoO3−δ, and LaxSr1−xMnO3−δ.3,4 However, at a low operating temperature (below 600 °C), Pt is still the best material in terms of ORR activity and electronic conductivity compared with any others that have been developed so far. A technique is needed for fabricating a nanoporous Pt cathode.5,6 A nanoporous Pt cathode has nanosized grains at the cathode−electrolyte interface, providing a higher grain boundary density than a microscale Pt cathode structure. This higher density could lead to high reaction sites for ORR by © XXXX American Chemical Society
Received: August 29, 2018 Accepted: October 4, 2018 Published: October 4, 2018 A
DOI: 10.1021/acsaem.8b01450 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Energy Materials
Figure 1. (a) Schematic of cathode structure composed of ultrathin Pt catalyst structure and porous Pd layer with surface field emission scanning electron microscope (FESEM) and cross-sectional focused ion beam (FIB) images. Focused ion beam cross-sectional images of Pt and Pt−Pd cathode before (b, d) and after (c, e) cell operation.
thermally protective nanolayer for nanostructured Pt and electron pathways.19 This characteristic implies that the nanostructured Pt catalytic layer with ultralow loading (∼48.3 μg cm−2) could be sustained at the operating temperature of 500 °C by the simple sputtered protective layer. To evaluate the thermomechanical stability of the Pt−Pd cathode and the function of the Pd CCL, the nanostructure and electrochemical characteristics of the Pt−Pd cathode were investigated and compared with those of the Pt cathode. Schematic and electron micrographs of the Pt−Pd cathode nanostructure fabricated by sputtering methods on ScSZ substrates are shown in Figure 1a. As known from previous research, the nanoporous structure of a metal electrode is created with the optimal pressure of the sputtering chamber (90 mTorr).20 At this high working pressure, Pt grains randomly grow on the substrate due to the short mean free path. This random growth of the Pt grains produces a pore structure between the columns. As shown in the raw FESEM surface image, the pore structure is formed between the columns and the average width of the pore space is 7 ± 1.7 nm (ImageJ). As confirmed by the FIB-SEM cross-sectional micrograph, the Pt−Pd cathode is composed of a 20 nm thick nanoporous Pt layer and a 150 nm thick Pd CCL. Meanwhile, since the size of the grains becomes larger with increasing thickness of the film, the 20 nm thick Pt catalytic layer has smaller grains than the 150 nm thick Pt catalytic layer (Supporting Information Figure S1). Ryll et al. previously reported that the nanoscale platinum grain size could provide significant TPBs compared with the microscale Pt grain size due to the high density of grain boundaries.21 From this analysis and information, it could be assumed that the reduction of the catalytic layer thickness results in an increment of reactive sites. The cross-sectional micrographs of the Pt cathodes and the Pt−Pd cathodes before and after their electrochemical measurements are compared and described in Figure 1b−e. The electrochemical measurement was conducted at 500 °C for more than 3 h to investigate the nanoscopic structural change of the cathodes. The structural change of the Pt cathode, described in Figure 1b,c, is attributed to the agglomeration of Pt columns. This agglomeration increases the size of Pt particles, resulting in a reduction of the grain boundary density. As the Pt columns aggregate, the distance
has been observed at various temperatures ranging from 100 to 800 °C when Pt nanoparticles are used for cathode materials.13 As part of an effort to enhance the thermomechanical stability of a nanoporous Pt cathode, various methods such as alloying metals, core−shell structuring, and oxide coating have been developed.6,11,14 Among these methods, oxide coating has been recently used and has drawn a considerable amount of attention.15,16 Unlike other methods, the oxide coating method which uses thin film deposition techniques (sputtering, pulsed laser deposition, atomic layer deposition, and so on) is much simpler because it does not require additional heat treatments such as calcination or sintering steps. Furthermore, in contrast to alloying metals or core−shell structuring, the nanoscale oxide coating method can easily be applied to a large-area electrode, which is an essential technical point for commercialization.17,18 Recent reports showed that zirconia-based oxide materials were used to fabricate a few nanometers thick coating layer for a nanoporous Pt cathode. The long-term result clearly showed that the nanostructure of the Pt cathode with a 5 nm thick yttria-stabilized zirconia layer was intact for more than 4 times longer than that of the bare Pt cathode.15 To use oxide coating, a high loading of Pt (over 0.2 mg cm−2) is needed to create the moderate in-plane electrical conductivity of a nanoporous cathode, which leads to high manufacturing costs. Due to the high aspect ratio of a nanoporous structure, its inplane electrical resistance is exceedingly high, which hinders the supply of electrons to reaction sites.17 Consequently, even though the catalytic layer with a thickness of a few tens of nanometers could be formed, the majority of Pt cathodes applied to thin film SOFCs have a thickness of more than a few hundred nanometers.17 Given the issues of a nanoporous Pt cathode, a design strategy for the cathode should consider both the thermal stability of the nanostructure and the amount of Pt loading used. In this regard, we propose fabricating a current collecting layer (CCL) composed of nanoporous palladium (Pd)/ palladium oxide materials to enhance the thermomechanical stability of nanoporous Pt and simultaneously reduce the amount of Pt. In this study, we describe a Pt−Pd cathode with a nanoporous Pd CCL on a 20 nm thick Pt catalytic layer, both of which are deposited by sputtering technique. Our approach is based on the fact that part of the Pd metal is transformed to nano-polycrystalline Pd oxide at 500 °C which functions as a B
DOI: 10.1021/acsaem.8b01450 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Energy Materials
Figure 2. (a) HR-TEM cross-sectional images of Pt−Pd cathode after long-term operation. Fast Fourier transform (FFT) analysis of Pt (1) and Pd layers (2) obtained from the magnified inset image of the Pt/Pd interface. (b) Scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX) of Pt−Pd cathode.
suggests that the Pd oxide nanoparticle can function as a protective layer against the thermal agglomeration of the Pt nanostructure. The role of oxide coating in the thermal agglomeration of Pt has been previously reported in several studies.23,24 To utilize the stabilized Pt nanostructure, the CCL should be simultaneously electrically conductive and thermally stable. The characteristics of Pd oxide satisfy these two conditions, which makes Pd a suitable material for the CCL.25 The current density (J)−voltage (V) behaviors of the Pt and Pt−Pd cathodes were measured to evaluate their timedependent performance and compared with each other in Figure 3a,b. The measurement was conducted once and then again after about 3 h. The open circuit voltages (OCVs) of both the Pt and Pt−Pd cathode cells are 1.02−1.07 V, which is close to the theoretical voltage (1.15 V) at 500 °C. It is a concern that the variation in the OCV could be attributed to the quality of ScSZ pellets prepared during the manufacturing process. Nevertheless, the difference in the OCV is too small (less than 5%) to affect the peak power density of the cells and thus does not significantly affect the performance. The peak power density of the Pt cathode is higher than that of the Pt− Pd cathode at the first measurement. After 3 h of continuous operation, however, dramatic performance degradation is observed in the Pt cathode and the peak power density of the Pt−Pd cathode becomes comparable to that of the Pt cathode. It is widely known that the degradation of a Pt-based electrode cell is greatly affected by the agglomeration of Pt grains located on the surface of the ceramic electrolyte. In this regard, the lower performance of the Pt−Pd cathode could suggest that the Pd CCL plays a significant role in stabilizing the Pt catalyst layer. In addition, the performance of the Pd cathode (only Pd used) was the lowest among the cells. As this result clearly shows, the performance of the Pd cathode is much lower than that of the Pt cathode even though Pd has catalytic activity for ORR. Thus, it can be speculated that the main effect of applying Pd as a CCL for the electrochemical performance is stabilizing the nanostructure of the Pt catalytic layer. To investigate the specific factors affecting the performance change, an EIS analysis on the cells was conducted. The electrochemical analyses conducted at 500 °C are described in Figure 3c−f, the representative Nyquist plots of the Pt cathode and the Pt−Pd cathode cell, respectively. In EIS analysis, it is well-known that an intercept with the X-axis at
between the columns becomes larger. This structural change seems to increase the porosity of the cathode and thus improve the performance of SOFCs. However, according to the electrochemical measurement, the performance of the agglomerated cathode is severely degraded by 52%, which implies that the enlarged Pt columns decrease reaction sites for ORR. The structural change of the Pt−Pd cathode before and after the 3 h measurement is shown in Figure 1d,e, respectively. Before the measurement, nanopore structures in a straight-line shape are formed in the cathode. The nanopore structures change to a dotted-line combined shape after the measurement. This change is attributed to the partial oxidation of Pd metal in the CCL. The XPS results in Figure S3 show that the atomic percent of oxygen in the Pd CCL dramatically increases after the measurement. As the Pd metal phase is exposed to an oxidative atmosphere during the measurement, the Pd nanoparticles surrounding the Pt catalyst layer also transform to nano-polycrystalline Pd oxide. The crystallinity of the transformed Pd CCL was analyzed from TEM crosssectional images of the Pt−Pd cathode. For a detailed structural analysis of the Pt−Pd cathode after the measurement, TEM images of the Pt−Pd cathode and magnified images of the Pt and Pd/PdO interface were collected. Figure 2a shows the 20 nm thick Pt catalyst layer (dark black region) and the Pd/PdO nanoporous CCL (dark gray region). The HR-TEM images and fast Fourier transform (FFT) of the interface between Pt and Pd/PdO are analyzed. From the FFT, the nano-polycrystalline Pd surrounding the Pt nanoparticles is confirmed. Due to sputtering by Ar ion bombardment of the target, the sputtered Pd layer is composed of a number of small grains with different crystal orientations, which results in its nano-polycrystalline structure.22 Scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX) were applied to investigate the distribution of the elements Pt, Pd, O, and Zr. The bright regions of the STEM images correspond to the dark regions of the HR-TEM images. The EDX mapping result confirms that the bright regions in the STEM images represent Pt particles; the result also shows that the Pd and O elements are distributed over the CCL and on the Pt particle. From this distribution, it can be concluded that the Pd oxide nanoparticle is formed around the Pt nanoparticle. The oxidation of the Pd nanoparticle surrounding the Pt catalyst structure after prolonged measurement of electrochemical performance C
DOI: 10.1021/acsaem.8b01450 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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ACS Applied Energy Materials
Figure 3. Current density (J)−voltage (V) behaviors of the Pt and Pt−Pd cathodes (a) before aging and (b) 3 h later. The representative electrochemical impedance spectroscopy (EIS) plots of the Pt cathode and the Pt−Pd cathode cell measured at 500 °C (c) before and (d) after 3 h operation. Time-dependent (e) ohmic and (f) polarization resistance of Pt, Pt−Pd, and Pd cathode cells.
resistance is readily included in the Rohmic of the EIS analysis. Therefore, for easy interpretation, electrode reaction and charge transfer resistances are separated from the raw data (Figure S2). The time-dependent resistances of the Pt, Pt−Pd, and Pd cathodes are described in Figure 3e,f. An increase in the ohmic resistance of the Pt cathode is only about 0.24 Ω·cm2 (3.78 − 4.02 Ω·cm2) after 3 h of measurement. This insignificant increase could be attributed to the loss of in-plane connection of the Pt nanostructure, which was due to the agglomeration, and frequently confirmed by surface SEM images. Compared with that, the ohmic resistance of the Pt−Pd cathode increases by about 4.36 Ω·cm2 (6.36 to 10.72 Ω·cm2). Because the
high-frequency range represents ohmic resistance (denoted as Rohmic) and the size of an arc indicates the polarization resistance (denoted as Relectrode) of SOFCs.26 Based on this knowledge, the EIS plot was fitted to the equivalent circuit shown in Figure S2. The total thicknesses of the cathodes are set to be almost equal to each other for minimizing the difference in electrical resistance of cathode current collection. However, as Pd metal is transformed into Pd oxide, the density of the CCL decreases by about 30% (volume change), which results in an inevitable difference in total thickness. Even though the thickness of the cathodes slightly varies, the phase transition of the Pd metal electrode contributes much more to the change in resistance. This transition effect on electrical D
DOI: 10.1021/acsaem.8b01450 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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ACS Applied Energy Materials electrolyte thickness is fixed for all the cells, this increase in the Rohmic is due to the phase transition of the Pd CCL and is the main cause of the degradation of the Pt−Pd cathode performance. However, the Relectrode of the Pt cathode increases more than 3-fold (13.24 Ω·cm2) while the Relectrode of the Pt− Pd cathode increases to 1.09 Ω·cm2. Also, the first measured Relectrode of the Pt cathode (6.47 Ω·cm2) is 60% higher than that of the Pt−Pd cathode (4.01 Ω·cm2). This observation suggests that the Pt cathode structure is much more vulnerable to annealing even at a relatively low temperature (500 °C). Furthermore, these results are consistent with the FIB-SEM micrographs that show the agglomerated columnar structure of the Pt cathode after 3 h of testing. Last, current densities of both the Pt−Pd and Pt cathode cells were measured at a constant voltage of 0.5 V. This constant voltage measurement was conducted 24 h after the initial measurement. In constant voltage measurement, Figure 4, the noise detected in the constant voltage measurement of
promising method for improving the catalytic activity or stability of SOFCs. However, in this report, we demonstrated the enhanced thermal stability of an ultrathin Pt catalyst nanostructure with a Pd protective layer deposited by sputtering. A partially oxidized Pd layer of the Pt−Pd cathode acted as a protective structure for the ultrathin Pt cathode which is vulnerable to aggregation at high operating temperatures. The time-dependent performance measurements of the Pt−Pd cathode cell suggest that Pd oxide nanoparticles prevent the Pt nanostructure from changing, which sustains the reaction sites for ORR during the electrochemical measurement. Furthermore, only a simple physical vapor deposition method was used and without a thermal annealing process for the fabrication of the Pt nanostructure with enhanced thermal stability. This simple method of using the metal oxide CCL for protecting catalytic nanostructures from thermal agglomeration could be applied to any material with an electrically conductive oxide.
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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/acsaem.8b01450. Basic information used in study, SEM images, EIS analysis, XPS analysis, XRD patterns, and power density (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Suk Won Cha: 0000-0002-4044-2079 Notes
The authors declare no competing financial interest.
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Figure 4. Time-dependent current densities of Pt−Pd and Pt cathode cells at constant voltage 0.5 V.
ACKNOWLEDGMENTS This research was supported by the Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (Grant 2017M3D1A1040688). This work was supported by the “New & Renewable Energy Core Technology Program” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20153030040930; KIST 2E26590).
Pt−Pd cathode cell could result from transformation of Pd metal to Pd oxide. However, it is distinct that the degradation rate of the Pt−Pd cathode cell is far lower than that of the Pt cathode cell. Pt cathode is degraded 81% within 2 h measurement (40.0% degradation rate per hour) while Pt− Pd cathode sustains for 24 h with much lower degradation rate (2.1% degradation rate per hour), which is about 20 times lower than the Pt cathode. Although thermal agglomeration of nanoporous Pt anode is attributed to performance degradation in constant voltage measurement, the main cause of difference in performance degradation could be considered as a cathode nanostructure since both Pt and Pt−Pd cells have same anode structure. These results demonstrate that enhanced thermal stability of an ultrathin Pt nanostructure could be achieved by applying a Pd CCL using a simple deposition method. To the best of our knowledge, this is the first study to improve the thermal stability of a 20 nm thick Pt catalytic nanostructure of an SOFC cathode using a protective Pd CCL and a simple physical vapor deposition method. In this study, a Pd CCL was used to enhance the thermomechanical stability of the Pt nanostructure with ultralow loading (∼48.3 μg cm−2). Since Pd is easily oxidized in a high-temperature oxidizing environment, the utilization of Pd as a cathode material has not been considered as a
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DOI: 10.1021/acsaem.8b01450 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsaem.8b01450 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
