Three-dimensional thermal stress analysis of the re-oxidized Ni-YSZ anode functional layer in solid oxide fuel cells

Kim, Jun Woo,Bae, Kiho,Kim, Hyun Joong,Son, Ji-won,Kim, Namkeun,Stenfelt, Stefan,Prinz, Fritz B.,Shim, Joon Hyung Elsevier 2018 JOURNAL OF ALLOYS AND COMPOUNDS Vol.752 No.-

<P><B>Abstract</B></P> <P>Nickel-yttria-stabilized zirconia (Ni-YSZ) cermet is widely used as an anode material in solid oxide fuel cells (SOFCs); however, Ni re-oxidation causes critical problems due to volume expansion, which causes high thermal stress. We fabricated a Ni-YSZ anode functional layer (AFL), which is an essential component in high-performance SOFCs, and re-oxidized it to investigate the related three-dimensional (3D) microstructural and thermo-mechanical effects. A 3D model of the re-oxidized AFL was generated using focused ion beam-scanning electron microscope (FIB-SEM) tomography. Re-oxidation of the Ni phase caused significant volumetric expansion, which was confirmed via image analysis and calculation of the volume fraction, connectivity, and two-phase boundary density. Finite element analysis (FEA) with simulated heating to 500–900 °C confirmed that the thermal stress in re-oxidized Ni-YSZ is concentrated at the boundaries between YSZ and re-oxidized NiO (nickel oxide). NiO is subjected to more stress than YSZ. Stress exceeding the fracture stress of 8 mol% YSZ appears primarily at 800 °C or higher. The stress is also more severe near the electrolyte-anode boundary than in the Ni-YSZ cermet and the YSZ regions. This may be responsible for the electrolyte membrane delamination and fracture that are observed during high-temperature operation.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Re-oxidized NiO-YSZ was 3D-reconstructed via FIB-SEM tomography. </LI> <LI> Re-oxidation of Ni-YSZ removes pores via Ni volume expansion. </LI> <LI> The NiO-YSZ interface and the YSZ electrolyte exhibit high thermal stress. </LI> <LI> Thermal stresses greater than the fracture stress of YSZ appear at some interfaces. </LI> </UL> </P>

Thermal conductivity of ultrathin BaTiO₃ films grown by plasma-assisted atomic layer deposition

Cho, Jungwan,Park, Joonsuk,Prinz, Fritz B.,An, Jihwan Elsevier 2018 Scripta materialia Vol.154 No.-

<P><B>Abstract</B></P> <P>We report the first measurements of the room temperature thermal conductivity of ultrathin (12 and 24 nm) barium titanate (BaTiO<SUB>3</SUB>) films prepared by plasma-enhanced atomic layer deposition (PEALD). The measured thermal conductivities of as-deposited films are relatively low as compared to values reported previously. We further investigate the effects of a post-deposition remote oxygen plasma treatment on the crystallinity and thermal conductivity of our PEALD BaTiO<SUB>3</SUB> films. We find that the thermal conductivity slightly decreases with increasing duration of plasma treatment, which is most likely due to increased phonon scattering at interfaces between amorphous and crystalline phases within the films.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Evaluation of atomic layer deposited alumina as a protective layer for domestic silver articles: Anti-corrosion test in artificial sweat

Park, Suk Won,Han, Gwon Deok,Choi, Hyung Jong,Prinz, Fritz B.,Shim, Joon Hyung Elsevier 2018 APPLIED SURFACE SCIENCE - Vol.441 No.-

<P><B>Abstract</B></P> <P>This study evaluated the effectiveness of alumina fabricated by atomic layer deposition (ALD) as a protective coating for silver articles against the corrosion caused by body contact. An artificial sweat solution was used to simulate body contact. ALD alumina layers of varying thicknesses ranging from 20 to 80 nm were deposited on sputtered silver samples. The stability of the protective layer was evaluated by immersing the coated samples in the artificial sweat solution at 25 and 35 °C for 24 h. We confirmed that a sufficiently thick layer of ALD alumina is effective in protecting the shape and light reflectance of the underlying silver, whereas the uncoated bare silver is severely degraded by the artificial sweat solution. Inductively coupled plasma mass spectrometry and X-ray photoelectron spectroscopy were used for in-depth analyses of the chemical stability of the ALD-coated silver samples after immersion in the sweat solution.</P> <P><B>Highlights</B></P> <P> <UL> <LI> ALD Al<SUB>2</SUB>O<SUB>3</SUB> is effective in protecting the shape and light reflectance of Ag. </LI> <LI> An artificial sweat solution is used to evaluate the stability of ALD Al<SUB>2</SUB>O<SUB>3</SUB>-Ag. </LI> <LI> Uncoated bare Ag severely degraded by the artificial sweat solution. </LI> <LI> ICP-MS and XPS confirm no Ag dissolution or diffusion of Na/Cl ions through Al<SUB>2</SUB>O<SUB>3</SUB>. </LI> </UL> </P>

Thermally stable current-collecting silver grid coated with ceramic-capping layer for low-temperature solid oxide fuel cells

Hong, Soonwook,Lim, Yonghyun,Prinz, Fritz B.,Kim, Young-Beom Elsevier 2018 CERAMICS INTERNATIONAL Vol.44 No.18

<P><B>Abstract</B></P> <P>In an effort to decrease the operating temperature of solid oxide fuel cells (SOFCs), nano-porous thin Pt layers have been used as a cathode material with catalytic activity. Because of porous and thin characteristic of the Pt cathode, however, a large cathode area results in a significant performance deterioration because of the increased sheet resistance of the Pt cathode. In this study, we developed a Ag-patterned grid as a current-collecting layer and a samaria-doped ceria (SDC) oxide-capping layer on the porous Pt cathode to decrease sheet resistance and enhance electrochemical performance. Enhanced electron transportation and thermo-stable behavior of fabricated fuel cells indicated three-fold enhanced peak power density and more than two-fold thermomechanical stability, as per scanning electron microscopy (SEM) and electrochemical analysis results.</P>

MEMS-based thin-film solid-oxide fuel cells

An, Jihwan,Shim, Joon Hyung,Kim, Young-Beom,Park, Joong Sun,Lee, Wonyoung,Gü,r, Turgut M.,Prinz, Fritz B. Cambridge University Press (Materials Research Soc 2014 MRS bulletin Vol.39 No.9

<▼1><B>Abstract</B><P/></▼1><▼2><P>Thin-film solid-oxide fuel cells (TF-SOFCs) fabricated using microelectromechanical systems (MEMS) processing techniques not only help lower the cell operating temperature but also provide a convenient platform for studying cathodic losses. Utilizing these platforms, cathode kinetics can be enhanced dramatically by engineering the microstructure of the cathode/electrolyte interface by increasing the surface grain-boundary density. Nanoscale secondary ion mass spectrometry and high-resolution transmission electron microscopy studies have shown that oxygen exchange at electrolyte surface grain boundaries is facilitated by a high population of oxide-ion vacancies segregating preferentially to the grain boundaries. Furthermore, three-dimensional structuring of TF-SOFCs enabled by various lithography methods also helps increase the active surface area and enhance the surface exchange reaction. Although their practical prospects are yet to be verified, MEMS-based TF-SOFC platforms hold the potential to provide high-performance for low-temperature SOFC applications.</P></▼2>

Localized charge transfer reactions near the Pt-YSZ interfaces using Kelvin probe microscopy

Lee, Wonyoung,Prinz, Fritz B. Springer-Verlag 2014 International Journal of Precision Engineering and Vol.1 No.3

나노기공성 기판을 사용한 산화물박막의 제조

박용일,Park, Yong-Il,Prinz, Fritz B. 한국세라믹학회 2004 한국세라믹학회지 Vol.41 No.12

현재까지 개발되어 온 고체산화물 연료전지는 전해질로 사용되는 산소이온전도성 산화물의 저온에서의 낮은 전도도로 인해 그 사용영역이 제한되어 왔으며, 기판재료가 연료가스 확산층으로 사용되어야 한다는 점 때문에 저온작동을 위한 박막화 역시 명확한 한계를 가지고 있다. 이러한 문제점은 고도의 평활도를 갖는 균일한 나노기공성 기판재를 도입함으로써 해결될 수 있으며, 본 연구에서는 나노기공성 기판에 비정질 금속박막을 증착/산화하는 방안을 제시한다. 초박막형 성공정으로서, 산화 후 산소이온전도성 산화물을 구성하는 합금 타겟을 장착한 DC-magnetron sputter를 사용하여 $20{\sim}200nm$의 기공크기를 갖는 나노기공성 양극산화 알루미나 기판에 비정질 금속합금막을 형성하여 산화/열처리 과정을 거쳐 초박막 산화물 전해질의 제조공정을 실현하였다. 얻어진 박막의 가스투과특성, 입자/입계의 관찰, 상전이에 따른 결정구조/미세구조변화를 관찰하여 초박막 증착 및 전해질의 나노구조제어에 필요한 제반 기본물성데이터를 확보하였다. Solid oxide fuel cells have a limitation in their low-temperature application due to the low ionic conductivity of electrolyte materials and difficulties in thin film formation on porous gas diffusion layer. These problems can be solved by improvement of ionic conductivity through controlled nanostructure of electrolyte and adopting nanoporous electrodes as substrates which have homogeneous submicron pore size and highly flattened surface. In this study, ultra-thin oxide films having submicron thickness without gas leakage are deposited on nanoporous substrates. By oxidation of metal thin films deposited onto nanoporous anodic alumina substrates with pore size of $20nm{\sim}200nm$ using dc-magnetron sputtering at room temperature, ultra-thin and dense ionic conducting oxide films with submicron thickness are realized. The specific material properties of the thin films including gas permeation, grain/gran boundaries formation, change of crystalline structure/microstructure by phase transition are investigated for optimization of ultra thin film deposition process.

Grain-Controlled Gadolinia-Doped Ceria (GDC) Functional Layer for Interface Reaction Enhanced Low-Temperature Solid Oxide Fuel Cells

Hong, Soonwook,Yang, Hwichul,Lim, Yonghyun,Prinz, Fritz B.,Kim, Young-Beom American Chemical Society 2019 ACS APPLIED MATERIALS & INTERFACES Vol.11 No.44

<P>In this Research Article, gadolinia-doped ceria (GDC), which is a highly catalyzed oxide ionic conductor, was explored to further improve oxygen surface reaction rates using a grain-controlled layer (GCL) concept. Typically, GDC materials have been used as a cathode functional layer by coating the GDC between the electrode and electrolyte to accelerate the oxygen reduction reaction (ORR). To further improve the oxygen surface kinetics of the GDC cathodic layer, we modified the grain boundary density and crystallinity developed in the GDC layer by adjusting RF power conditions during the sputtering process. This approach revealed that engineered nanograins of GDC thin films directly affected ORR kinetics by catalyzing the oxygen surface reaction rate, significantly enhancing the fuel cell performance. Using this innovative concept, the fuel cells fabricated with a GDC GCL demonstrated a peak power density of 240 mW/cm<SUP>2</SUP> at 450 °C.</P> [FIG OMISSION]</BR>

원자층 증착법을 이용한 백금 전극 제작 및 고체산화물연료전지에의 적용

안지환(J. An),김영범(Y.-B. Kim),Fritz B. Prinz 한국정밀공학회 2015 한국정밀공학회 학술발표대회 논문집 Vol.2015 No.12월

Nanostructuring Methods for Enhancing Light Absorption Rate of Si-Based Photovoltaic Devices: A Review

김영범,홍순욱,배지웅,구봉준,장익황,조구영,차석원,Fritz B. Prinz 한국정밀공학회 2014 International Journal of Precision Engineering and Vol.1 No.1

In recent years, there has been growing consideration of renewable energy especially photovoltaic devices. A silicon (Si) based solar cell is the most popularly and frequently considered among the photovoltaic devices, but its bulk thickness issue lowers the performance and hinders widespread application due to the material cost. Also, this thick nature causes difference in length between minority carrier diffusion and sufficient light absorption. To mitigate the issues there have been many recent studies on Si photovoltaic devices adopting nanostructuring strategies to enhance the performance. Therefore, we report two different approaches on recent nanostructuring techniques for photovoltaic devices; bottom-up and top-down processes, which are composed of vapor-liquid-solid, solution-liquid-solid, reactive ion etching with Langmuir Blodgett and metal assisted chemical etching. Those fabrication processes enable the fabrication of nanostructures with a highly ordered and alignment structures leading to enhance the light absorption and have an appropriate thickness of Si substrate regressing Auger recombination. The fabricated nanowire and nanocone array structures outperform existing results with light absorption exceeding 90%.