Korean Chemical Engineering Research

The Korean Institute of Chemical Engineers (한국화학공학회)
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Optimization of anode and electrolyte microstructure for Solid Oxide Fuel Cells

고체산화물 연료전지 연료극 및 전해질 미세구조 최적화

  • Noh, Jong Hyeok (Department of materials science and engineering, Incheon National University) ;
  • Myung, Jae-ha (Department of materials science and engineering, Incheon National University)
  • 노종혁 (인천대학교 신소재공학과) ;
  • 명재하 (인천대학교 신소재공학과)
  • Received : 2019.02.28
  • Accepted : 2019.04.10
  • Published : 2019.08.01
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Abstract

The performance and stability of solid oxide fuel cells (SOFCs) depend on the microstructure of the electrode and electrolyte. In anode, porosity and pore distribution affect the active site and fuel gas transfer. In an electrolyte, density and thickness determine the ohmic resistance. To optimizing these conditions, using costly method cannot be a suitable research plan for aiming at commercialization. To solve these drawbacks, we made high performance unit cells with low cost and highly efficient ceramic processes. We selected the NiO-YSZ cermet that is a commercial anode material and used facile methods like die pressing and dip coating process. The porosity of anode was controlled by the amount of carbon black (CB) pore former from 10 wt% to 20 wt% and final sintering temperature from $1350^{\circ}C$ to $1450^{\circ}C$. To achieve a dense thin film electrolyte, the thickness and microstructure of electrolyte were controlled by changing the YSZ loading (vol%) of the slurry from 1 vol% to 5 vol. From results, we achieved the 40% porosity that is well known as an optimum value in Ni-YSZ anode, by adding 15wt% of CB and sintering at $1350^{\circ}C$. YSZ electrolyte thickness was controllable from $2{\mu}m$ to $28{\mu}m$ and dense microstructure is formed at 3vol% of YSZ loading via dip coating process. Finally, a unit cell composed of Ni-YSZ anode with 40% porosity, YSZ electrolyte with a $22{\mu}m$ thickness and LSM-YSZ cathode had a maximum power density of $1.426Wcm^{-2}$ at $800^{\circ}C$.

고체산화물 연료전지의 성능과 안정성은 전극의 기공률, 기공 분포와 전해질의 치밀도, 두께에 따라 결정 된다. 연료극의 기공률과 기공 분포는 활성면적와 연료 흐름에 영향을 주고, 전해질의 치밀한 미세구조와 두께는 단위전지의 Ohmic 저항에 영향을 준다. 하지만 이를 위해 값 비싼 공정 장비를 이용하거나 여러 단계의 제작 공정이 추가 될 경우 단위전지 제작비가 증가하므로 상업화를 목표로 하는 연구에는 적합하지 않다. 본 연구에서는 위와 같은 문제점들을 해결하기 위하여 상용 소재 기반의 NiO-YSZ 연료극을 선정 후 간단한 혼합 방법 및 일축가압 성형법과 담금코팅(dip coating) 공정을 사용하여 저비용 고효율의 세라믹 공정 기반의 고성능 단위전지를 제작하였다. 연료극의 기공률은 기공형성제로서 사용되는 카본 블랙(CB, carbon black)의 첨가량(10~20 wt%)과 최종 소결온도($1350{\sim}1450^{\circ}C$)를 변경하며 제어하였고, YSZ 전해질의 두께와 미세구조는 담금코팅 슬러리의 고상 분말량(YSZ, 1~5 vol%)을 제어하여 치밀한 박막의 전해질을 구현하고자 하였다. 그 결과 Ni-YSZ 연료극에서 최적의 값으로 잘 알려진 40%의 기공률은 카본 블랙을 15 wt% 첨가하고최종소결온도를 $1350^{\circ}C$로설정함으로써얻을수있었다. 담금코팅을통한 YSZ 두께는 $2{\sim}28{\mu}m$까지 제어가 가능하였고, 3 vol%의 고상분말량에서 치밀한 전해질 미세구조가 형성되었다. 최종적으로 40%의 기공률을 갖는 Ni-YSZ 연료극, $20{\mu}m$ 두께의 치밀한 YSZ전해질, LSM-YSZ 공기극으로 구성된 단위전지는 $800^{\circ}C$에서 $1.426Wcm^{-2}$의 우수한 성능을 얻을 수 있었다.

Keywords

HHGHHL_2019_v57n4_525_f0001.png 이미지

Fig. 1. Shrinkage and porosity changes of NiO-YSZ anode supported cells depending on carbon black contents and sintering temperature : (a) shrinkage change before reduction, porosity changes (b) before and (c) after reduction. Microstructure SEM images of reduced Ni-YSZ with 15 wt% CB by diverse sintering temperatures (d) 1350 ℃ (e) 1400 ℃, (f) 1450 ℃.

HHGHHL_2019_v57n4_525_f0002.png 이미지

Fig. 2. Electrolyte thickness changes depending on amount of YSZ loading (vol%): SEM images (a) YSZ 1, (b) YSZ 2, (c) YSZ 3, (d) YSZ 4, (e) YSZ 5 and (f) thickness change graph.

HHGHHL_2019_v57n4_525_f0003.png 이미지

Fig. 3. SEM images of electrolyte surface (a) YSZ 3-no defect (b) YSZ 2-pin hole (c) YSZ 5-crack and cross section defect (d) YSZ 1, (e) YSZ 4, (f) YSZ 5.

HHGHHL_2019_v57n4_525_f0004.png 이미지

Fig. 4. (a, c) I-V curves and EIS spectra of unit cells coated with YSZ 1, YSZ 3, YSZ 4 electrolyte, and (b, d) I-V curves and EIS spectra of unit cell coated with YSZ 3 electrolyte at 700 to 900 ℃.

Table 1. YSZ electrolyte slurries with different YSZ loading (vol%)

HHGHHL_2019_v57n4_525_t0001.png 이미지

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