고체산화물 전해전지의 산소극 열화기구에 대한 연구

Abstract

고체 산화물 전해전지(Solid Oxide Electrolysis Cell, SOEC)를 기반으로 한 고온수증기분해법은 신재생에너지원의 잉여 전력 및 열을 활용하여 수소를 경제적이고 효율적으로 제조할 수 있는 차세대 대용량 에너지 저장기술로 최근 주목받고 있다. SOEC는 발전/전해 양방향으로 운전이 가능하며 셀 구조와 구성소재는 일반적으로 SOFC기술을 기반으로 한다. 하지만 SOFC와 SOEC 간에는 기체와 전류의 방향, 외부 전압 인가 여부, 열적 특성 등에 있어 근본적인 차이점이 존재하기 때문에 SOEC 기술의 성공적인 개발과 보급을 위해서는 이러한 특성들이 성능과 안정성에 미치는 영향에 대한 근본적인 이해가 필요하다. 현재 SOFC에서 가장 널리 사용되고 있는 LSM-YSZ 산소극은 SOFC 모드에서는 안정하지만 SOEC 모드에서 운전할 경우 산소극/전해질 계면의 박리나 구조 변화로 인하여 성능이 열화되고 수명이 단축되는 것으로 알려져 있다. 따라서 SOEC 기술의 상업적 개발을 위해서는 성능 열화 메커니즘에 대한 명확한 이해를 바탕으로 한 장기안정성 향상이 반드시 필요하다. 본 연구에서는 LSM-YSZ 산소극의 열화 메커니즘을 규명하기 위하여 반전지를 제조하고 화학적, 구조적, 전기화학적 평가를 다각도로 수행하였다. LSM-YSZ의 임피던스를 구성하는 성분들의 rate lilimiti process를 규명하였다. 중주파수대의 임피던스는 표면반응에 의하여 결정되고 삼상계면의 상태에 민감하게 반응하며, 저주파수대의 임피던스는 mass transport에 의하여 결정되며 전극의 기공구조에 의존했다. 장기운전시의 열화거동은 초기단계에서의 중주파수(10~100Hz) 영역 아크 증가, 작동 중 오믹저항의 점진적 증가, 마지막 단계에서 저주파수(1~10Hz) 영역 아크의 급격한 증가 뒤 치명적인 전지 손상(파괴)으로 나타났다 . 초기단계 열화는 산소 공공 농도의 감소 또는 산화전류 하에서 LSM 표면에서의 이차상 분리에 의한 LSM의 비활성화에 기인하였고 YSZ 전해질의 입계를 따라 발생한 입계파괴로 인하여 오믹저항이 점진적으로 증가하였다. 마지막 단계에서 저주파수 영역 아크의 증가는 산소극의 치밀화에 의해 야기되었으며 결국 과도한 기체 압력증가에 의한 산소극의 박리가 셀 파괴의 주된 원인으로 파악되었다. 이러한 여러가지 열화의 공통적인 원인은 산소 잉여 비화학양론(oxygen excess nonstoichiometry)에 의한 양이온 공공의 생성과 외부에서 인가된 전기장에 의해 촉진된 양이온 이동으로 밝혀졌으며 따라서 양이온 이동현상을 억제하기 위하여 oxygen excess nonstoichiometry를 갖지 않는 혼합 전자이온 전도체인 LSCF-GDC 산소극을 대칭반전지에 적용하여 장기안정성 평가를 하였다. 고분극 조건 운전시 LSM-YSZ 반전지는 ~120시간 운전 후 셀이 파괴된 반면, LSCF-GDC 반전지는 200시간동안 9%이하의 성능 감소율(열화율)을 보였으며 셀 파괴는 발생하지 않았다. Ni-YSZ 수소극, YSZ 전해질, GDC 완충층, LSCF 산소극으로 구성된 수소극 지지형 단전지 를 평가하였으며, 300시간 동안 3%이하의 성능 감소율(열화율)로 우수한 장기 안정성을 확인하였다.|In recent years, solid oxide electrolysis cells (SOEC) has attracted increasing attention for development of a highly-efficient energy conversion and storage system especially when using electricity and heat supplied from renewable sources. It is known that the most widely used (La,Sr)MnO3 (LSM) – Yttria-stabilized Zirconia (YSZ) air electrode is stable in SOFC mode, however, it often delaminates from the electrolyte and structural change, leading to performance degradation and life-shorthening in SOEC mode. Thus, it is important to understand the performance degradation mechanism and improve long-term stability for commercial development of SOEC technology. In this study, in order to investigate degradation mechanism of LSM-YSZ air electrode, we fabricated symmetric half cell and perform electrochemical, structural, and microstructural analysis. The rate limiting process of component composed of LSM-YSZ is investigated. Mid frequency arc(RMF) are strongly depends on the properties of triple phase boundaries(TPB), and Low frequency acr(RLF) is dependent on pore structure of the electrode. The degradation behavior in long-term operation showed mid-frequency arc at the initial stage, gradual increase of ohmic resistance throughout the operation, and sharp rise of low frequency resistance at the final stage, followed by catastrophic cell failure. Initial stage degradation is attributed to deactivation of LSM, resulting from reduction of oxygen vacancy concentration and/or segregation of passivation species on LSM surface under anodic current passage. Intergranular fracture, which occurs along the grain boundaries of the YSZ electrolyte, is responsible for gradual increase of ohmic resistance. Increase of low frequency arc at the final stage is caused by densification of the air electrode, leading to excessive pressure build-up and delamination of the air electrode. Cation migration, which is facilitated by oxygen excess nonstoichiometry of LSM and externally applied electric field, is considered to be the main cause of permanent damages. Thus, in order to suppress cation migration, LSCF-gadolinia-doped ceria (GDC) air electrode, which don’t have oxygen excess nonsntoichiometry was applied to symmetric half cell and performed in long-term stability. In high polarization condition, half cell applied to LSM-YSZ run for 120h and failed, on the other hand, half cell applied to LSCF-GDC run for 200h with ~9% degradation rate and appears no cell failure. SOEC which are composed of Ni-YSZ hydrogen electrode, YSZ electrolyte, GDC interlayer, LSCF-GDC air electrode was performed and showed excellent long-term stability with ~3% degradation rate for 300h.; In recent years, solid oxide electrolysis cells (SOEC) has attracted increasing attention for development of a highly-efficient energy conversion and storage system especially when using electricity and heat supplied from renewable sources. It is known that the most widely used (La,Sr)MnO3 (LSM) – Yttria-stabilized Zirconia (YSZ) air electrode is stable in SOFC mode, however, it often delaminates from the electrolyte and structural change, leading to performance degradation and life-shorthening in SOEC mode. Thus, it is important to understand the performance degradation mechanism and improve long-term stability for commercial development of SOEC technology. In this study, in order to investigate degradation mechanism of LSM-YSZ air electrode, we fabricated symmetric half cell and perform electrochemical, structural, and microstructural analysis. The rate limiting process of component composed of LSM-YSZ is investigated. Mid frequency arc(RMF) are strongly depends on the properties of triple phase boundaries(TPB), and Low frequency acr(RLF) is dependent on pore structure of the electrode. The degradation behavior in long-term operation showed mid-frequency arc at the initial stage, gradual increase of ohmic resistance throughout the operation, and sharp rise of low frequency resistance at the final stage, followed by catastrophic cell failure. Initial stage degradation is attributed to deactivation of LSM, resulting from reduction of oxygen vacancy concentration and/or segregation of passivation species on LSM surface under anodic current passage. Intergranular fracture, which occurs along the grain boundaries of the YSZ electrolyte, is responsible for gradual increase of ohmic resistance. Increase of low frequency arc at the final stage is caused by densification of the air electrode, leading to excessive pressure build-up and delamination of the air electrode. Cation migration, which is facilitated by oxygen excess nonstoichiometry of LSM and externally applied electric field, is considered to be the main cause of permanent damages. Thus, in order to suppress cation migration, LSCF-gadolinia-doped ceria (GDC) air electrode, which don’t have oxygen excess nonsntoichiometry was applied to symmetric half cell and performed in long-term stability. In high polarization condition, half cell applied to LSM-YSZ run for 120h and failed, on the other hand, half cell applied to LSCF-GDC run for 200h with ~9% degradation rate and appears no cell failure. SOEC which are composed of Ni-YSZ hydrogen electrode, YSZ electrolyte, GDC interlayer, LSCF-GDC air electrode was performed and showed excellent long-term stability with ~3% degradation rate for 300h.