Kim Doyeub, Park Jin Wan, Yun Byung-Hyun, Park Jeong Hwa, Lee Kang Taek
Department of Energy Science and Engineering , DGIST , Daegu 42988 , Republic of Korea.
ACS Appl Mater Interfaces. 2019 Sep 4;11(35):31786-31792. doi: 10.1021/acsami.9b06569. Epub 2019 Aug 23.
The Sr segregation at the surface of a perovskite LaSrCoFeO (LSCF) oxygen electrode is detrimental to the electrochemical performance and durability of energy conversion devices such as solid oxide fuel cells. However, a quantitative correlation of degradation of the oxygen surface exchange kinetics with Sr precipitation formation at the LSCF surface is not clearly understood yet. Herein, the correlation of the time-dependent degradation mechanisms of the LSCF catalysts with respect to Sr segregation phenomenon at the surface were investigated at 800 °C for a prolonged annealing time (∼800 h) by combining in situ electrochemical measurements, and ex situ chemical and structural analyses at the multiscale. The in situ monitored surface exchange coefficient () was found to drastically drop by ∼86% over the 800 h, and it was accompanied by the formation of Sr-containing secondary phases on the bulk LSCF surface, as expected. However, the estimated coverage of Sr segregation on the LSCF surface was only ∼15%, even after 800 h of aging time, showing significant deviation from the degradation rate (∼86%). The surface chemistry evolution at the clean surface area, which is believed to be electrochemically active, was further analyzed on the nanoscale. The quantified results showed that the Sr elemental fraction of the A-site at the outermost surface of the LSCF samples was becoming deficient from ∼4.0 at 0 h to ∼0.27 at 800 h annealing. Interestingly, the time-dependent behavioral tendencies between degradation and surface Sr fractional changes were highly analogous. Thus, our results suggest that this Sr deficiency at the clean surface region more dominantly impacts the degradation process rather than an electrochemical activity passivation by the SrO precipitates, which has been shown to be a major degradation mechanism of LSCF performance.
钙钛矿型LaSrCoFeO（LSCF）氧电极表面的Sr偏析不利于诸如固体氧化物燃料电池等能量转换装置的电化学性能和耐久性。然而，氧表面交换动力学的降解与LSCF表面Sr沉淀形成之间的定量相关性尚未完全清楚。在此，通过结合原位电化学测量以及多尺度的非原位化学和结构分析，研究了LSCF催化剂随时间变化的降解机制与800℃下长时间退火（约800小时）时表面Sr偏析现象之间的相关性。原位监测的表面交换系数（）在800小时内急剧下降约86%，并且如预期的那样，伴随着块状LSCF表面含Sr次生相的形成。然而，即使经过800小时的老化时间，LSCF表面Sr偏析的估计覆盖率仅约为15%，这与降解率（约86%）存在显著偏差。在纳米尺度上进一步分析了被认为具有电化学活性的清洁表面区域的表面化学演变。定量结果表明，LSCF样品最外表面A位的Sr元素分数从0小时时的约4.0逐渐降低至800小时退火后的约0.27。有趣的是，表面交换系数降解与表面Sr分数变化之间随时间变化的行为趋势高度相似。因此，我们的结果表明，清洁表面区域的这种Sr缺乏对降解过程的影响更为显著，而不是像SrO沉淀导致的电化学活性钝化那样，后者已被证明是LSCF性能的主要降解机制。
