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Nanoionics and Nanocatalysts: Conformal Mesoporous Surface Scaffold for Cathode of Solid Oxide Fuel Cells

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ABSTRACT

Nanoionics has become increasingly important in devices and systems related to energy conversion and storage. Nevertheless, nanoionics and nanostructured electrodes development has been challenging for solid oxide fuel cells (SOFCs) owing to many reasons including poor stability of the nanocrystals during fabrication of SOFCs at elevated temperatures. In this study, a conformal mesoporous ZrO2 nanoionic network was formed on the surface of La1−xSrxMnO3/yttria-stabilized zirconia (LSM/YSZ) cathode backbone using Atomic Layer Deposition (ALD) and thermal treatment. The surface layer nanoionic network possesses open mesopores for gas penetration, and features a high density of grain boundaries for enhanced ion-transport. The mesoporous nanoionic network is remarkably stable and retains the same morphology after electrochemical operation at high temperatures of 650–800 °C for 400 hours. The stable mesoporous ZrO2 nanoionic network is further utilized to anchor catalytic Pt nanocrystals and create a nanocomposite that is stable at elevated temperatures. The power density of the ALD modified and inherently functional commercial cells exhibited enhancement by a factor of 1.5–1.7 operated at 0.8 V at 750 °C.

No MeSH data available.


Electrochemical impedance spectroscopy of cells at 750 °C.(a) Nyquist plot of cell #1 (the baseline cell, in black squares), cell #5 (~10 nm Pt, in cyan triangles), cell #6 (Pt + 40 nm ZrO2, in yellow triangles) and cell #7 (40 nm ZrO2 + Pt in olive triangles). (b) Bode plot of cell #1, cell #5, cell #6 and cell #7 showing the trend of the intermediate-frequency arc in the range of 1–100 Hz.
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f5: Electrochemical impedance spectroscopy of cells at 750 °C.(a) Nyquist plot of cell #1 (the baseline cell, in black squares), cell #5 (~10 nm Pt, in cyan triangles), cell #6 (Pt + 40 nm ZrO2, in yellow triangles) and cell #7 (40 nm ZrO2 + Pt in olive triangles). (b) Bode plot of cell #1, cell #5, cell #6 and cell #7 showing the trend of the intermediate-frequency arc in the range of 1–100 Hz.

Mentions: An alternative engineered structure is created by applying Pt nano-particles into the mesopore region of a ZrO2 conformal surface layer using ALD processing. In this approach, cell #7 with 40 nm coated ZrOx layers were subjected to one thermal treatment and subsequent ALD processing for Pt deposition. Figure 4 (from cell #7) depicts a subjacent ZrO2 coating layer 40 nm thick in which the original ZrO2 mesopore regions were decorated by ~3 nm Pt crystallites. After thermal treatment, the Pt particles possess diameter of ~10 nm, and disperse on the ZrO2 surface and inside the original pore regions (depicted in Supplementary Figure 4). Such surface structural engineering prevents evolution of Pt particle diameter from 10 nm to 100 nm, and preserves electro-catalytic activity. For commercial inherently functional full cell, a large performance enhancement factor of 1.7, significantly higher than performance enhancement factor of ~1.3 achieved using solution based infiltration343536, is also listed in Table 1 and clearly illustrates the realizable advantages. To understand such large performance enhancement induced by various surface scaffold, it is critical to consider the evidence of the impedance analyses that were completed for each cell after operation at approximately 48 hours as seen in Figs 1b,c and 5a,b, and Table 1. Two peaks are discernable in all of the impedance spectroscopy traces derived from samples containing ZrO2, though the exhibited peak frequencies are broadly distinguishable based on the bulk structure. For the baseline cell and cells with only ZrO2, the two peaks are convoluted and the peak frequency of the larger arc is approximately 1 × 101 Hz at 0.3 A/cm2. When Pt is introduced alone, the peak frequency shifts to 3 × 101 Hz. When Pt and ZrO2 are both introduced, the peaks separate, equalize in magnitude, and separate to 2 to 4 × 100 Hz and 5 to 10 × 101 Hz. As discussed earlier, tetragonal ZrO2 on LSM would increase triple phase boundary length (therefore decreasing Rp). Nevertheless, an increase in triple phase boundary length will not result in significant shifting of the impedance peak frequency unless the mechanism of transport has also been altered. It is possible that introduction of the ZrO2 + Pt phase has both the effect of increasing TPB length and influencing the global reaction mechanism, which does not preclude the introduction of new ionic transport pathways through the ZrO2 crystallite interfaces.


Nanoionics and Nanocatalysts: Conformal Mesoporous Surface Scaffold for Cathode of Solid Oxide Fuel Cells
Electrochemical impedance spectroscopy of cells at 750 °C.(a) Nyquist plot of cell #1 (the baseline cell, in black squares), cell #5 (~10 nm Pt, in cyan triangles), cell #6 (Pt + 40 nm ZrO2, in yellow triangles) and cell #7 (40 nm ZrO2 + Pt in olive triangles). (b) Bode plot of cell #1, cell #5, cell #6 and cell #7 showing the trend of the intermediate-frequency arc in the range of 1–100 Hz.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5015090&req=5

f5: Electrochemical impedance spectroscopy of cells at 750 °C.(a) Nyquist plot of cell #1 (the baseline cell, in black squares), cell #5 (~10 nm Pt, in cyan triangles), cell #6 (Pt + 40 nm ZrO2, in yellow triangles) and cell #7 (40 nm ZrO2 + Pt in olive triangles). (b) Bode plot of cell #1, cell #5, cell #6 and cell #7 showing the trend of the intermediate-frequency arc in the range of 1–100 Hz.
Mentions: An alternative engineered structure is created by applying Pt nano-particles into the mesopore region of a ZrO2 conformal surface layer using ALD processing. In this approach, cell #7 with 40 nm coated ZrOx layers were subjected to one thermal treatment and subsequent ALD processing for Pt deposition. Figure 4 (from cell #7) depicts a subjacent ZrO2 coating layer 40 nm thick in which the original ZrO2 mesopore regions were decorated by ~3 nm Pt crystallites. After thermal treatment, the Pt particles possess diameter of ~10 nm, and disperse on the ZrO2 surface and inside the original pore regions (depicted in Supplementary Figure 4). Such surface structural engineering prevents evolution of Pt particle diameter from 10 nm to 100 nm, and preserves electro-catalytic activity. For commercial inherently functional full cell, a large performance enhancement factor of 1.7, significantly higher than performance enhancement factor of ~1.3 achieved using solution based infiltration343536, is also listed in Table 1 and clearly illustrates the realizable advantages. To understand such large performance enhancement induced by various surface scaffold, it is critical to consider the evidence of the impedance analyses that were completed for each cell after operation at approximately 48 hours as seen in Figs 1b,c and 5a,b, and Table 1. Two peaks are discernable in all of the impedance spectroscopy traces derived from samples containing ZrO2, though the exhibited peak frequencies are broadly distinguishable based on the bulk structure. For the baseline cell and cells with only ZrO2, the two peaks are convoluted and the peak frequency of the larger arc is approximately 1 × 101 Hz at 0.3 A/cm2. When Pt is introduced alone, the peak frequency shifts to 3 × 101 Hz. When Pt and ZrO2 are both introduced, the peaks separate, equalize in magnitude, and separate to 2 to 4 × 100 Hz and 5 to 10 × 101 Hz. As discussed earlier, tetragonal ZrO2 on LSM would increase triple phase boundary length (therefore decreasing Rp). Nevertheless, an increase in triple phase boundary length will not result in significant shifting of the impedance peak frequency unless the mechanism of transport has also been altered. It is possible that introduction of the ZrO2 + Pt phase has both the effect of increasing TPB length and influencing the global reaction mechanism, which does not preclude the introduction of new ionic transport pathways through the ZrO2 crystallite interfaces.

View Article: PubMed Central - PubMed

ABSTRACT

Nanoionics has become increasingly important in devices and systems related to energy conversion and storage. Nevertheless, nanoionics and nanostructured electrodes development has been challenging for solid oxide fuel cells (SOFCs) owing to many reasons including poor stability of the nanocrystals during fabrication of SOFCs at elevated temperatures. In this study, a conformal mesoporous ZrO2 nanoionic network was formed on the surface of La1−xSrxMnO3/yttria-stabilized zirconia (LSM/YSZ) cathode backbone using Atomic Layer Deposition (ALD) and thermal treatment. The surface layer nanoionic network possesses open mesopores for gas penetration, and features a high density of grain boundaries for enhanced ion-transport. The mesoporous nanoionic network is remarkably stable and retains the same morphology after electrochemical operation at high temperatures of 650–800 °C for 400 hours. The stable mesoporous ZrO2 nanoionic network is further utilized to anchor catalytic Pt nanocrystals and create a nanocomposite that is stable at elevated temperatures. The power density of the ALD modified and inherently functional commercial cells exhibited enhancement by a factor of 1.5–1.7 operated at 0.8 V at 750 °C.

No MeSH data available.