Microstructural evolution of lamellar Fe-25Ni foams during steam-hydrogen redox cycling
Microstructural evolution of lamellar Fe-25Ni foams during steam-hydrogen redox cycling
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DOI：
10.1016/j.actamat.2022.118148
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发表时间：
2022-07
期刊：
影响因子：
9.4
通讯作者：
Jacob B. Mack;Samuel M. Pennell;D. Dunand
中科院分区：
文献类型：
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作者：
Jacob B. Mack;Samuel M. Pennell;D. Dunand
与铁-空气电池相关的循环水蒸气氧化和氢气还原是在冷冻铸造的Fe-25Ni(at.%)泡沫上进行的，这些泡沫由多个平行的片层组成，由通道隔开，两个∼都厚20微米，长度为毫米。这种结构的设计是为了适应与铁的循环氧化和还原相关的体积变化。在不同氧化还原阶段进行的金相成像，以及氧化还原循环过程中X-射线衍射的时间分辨，详细描述了泡沫的反应动力学和相演变，其片层结构的演变，以及其内部结构的最终退化。由于铁优先于镍的氧化，每个片层形成一个外部的氧化铁膜，金属镍被排斥到片层的核心，这形成了一个相互连接的铁氧化物矿脉网络。富镍金属核限制了Kirkendall气孔的积累，并提供了对氧化铁层的粘附力，从而防止了在非合金泡沫铁中观察到的层状断裂。虽然镍的存在减缓了氧化速度，但还原速度加快了，因为镍起到了催化剂的作用，氧化的纹理网络迅速还原并成为开放的微通道，从而提供了快速的氢通道(驱动还原)和蒸汽出口到片层内部。在完全还原后，富Fe的外壳和富Ni的核心相互扩散和均匀，这有助于消除片层中的Kirkendall气孔和微通道。此外，韧性的富镍核限制了层状屈曲(另一种在泡沫铁中活跃的致密化机制)。结构的变化也会影响对内部损伤的抵抗力，较小的片状菌落表现出更好的抗屈曲能力。镍合金化提供的这些综合效应使Fe-25Ni泡沫塑料在10次氧化还原循环后仍能保持较高的沟道孔隙率(-gt；40%孔隙率)，从而保持较高的活性表面积，而非合金化的Fe泡沫塑料几乎完全丧失了开放沟道孔隙率。
Cyclical steam oxidation and hydrogen reduction, relevant to iron-air batteries, is performed on freeze-cast Fe-25Ni (at.%) foams consisting of colonies of parallel lamellae separated by channels, both ∼20 µm thick and millimeters in length. This structure is designed to accommodate volumetric changes associated with the cyclical oxidation and reduction of Fe. Metallographic imaging performed at various redox stages, together with time-resolvedin situX-ray diffraction during redox cycling, detail the reaction kinetics and phase evolution of the foam, the evolution of its lamellar microstructure, and the eventual degradation of its internal architecture. As Fe preferentially oxidizes over Ni, each lamella develops an outer Fe-oxide scale, with metallic Ni rejected to the cores of the lamellae which develops an interconnected network of Fe-oxide veins. The Ni-rich metallic core limits the accumulation of Kirkendall pores and provides adhesion to the Fe-oxide scale, thus preventing lamellar fracture observed in unalloyed Fe foams. While the oxidation rate is slowed by the presence of Ni, the reduction rate is accelerated, as Ni acts as a catalyst and as the network of oxidized veins reduces quickly and become open microchannels, thereby providing rapid hydrogen access (driving reduction) and steam egress to the lamellar interior. After complete reduction, the Fe-rich shell and the Ni-rich core interdiffuse and homogenize, which helps eliminate both Kirkendall pores and microchannels from the lamellae. Furthermore, the ductile Ni-rich core limits lamellar buckling (another densification mechanism active in Fe foams). Architectural changes also affect resistance to internal damage, with smaller lamellar colonies exhibiting better resistance to buckling. These combined effects provided by Ni alloying allow the Fe-25Ni foams to maintain a high channel porosity (>40% porous), and thus high active surface area, after 10 redox cycles, as compared to a near complete loss of open channel porosity reported in unalloyed Fe foams.