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Abbreviation (ISO4): J Inorg Mat      Editor in chief: Lidong CHEN

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Journal of Inorganic Materials >

2024 , Vol. 39 >Issue 4: 367 - 373

DOI: https://doi.org/10.15541/jim20230353

RESEARCH ARTICLE

LaNi0.6Fe0.4O3 Cathode Contact Material: Electrical Conducting Property Manipulation and Its Effect on SOFC Electrochemical Performance

  • Kun ZHANG , 1 ,
  • Yu WANG 1 ,
  • Tenglong ZHU , 1 ,
  • Kaihua SUN 2 ,
  • Minfang HAN 3 ,
  • Qin ZHONG 1
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  • 1. School of Chemistry and Chemical Engineering, Nanjing University of Science & Technology, Nanjing 210094, China
  • 2. Xuzhou Huatsing Jingkun Energy Co., Ltd., Xuzhou 221005, China
  • 3. Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, China
ZHU Tenglong, associate professor. E-mail:

Received date: 2023-08-03

  Revised date: 2023-09-04

  Online published: 2024-04-25

Supported by

National Key R&D Program of China(2018YFB1502203)

Key R&D Program of Jiangsu Province(BE2022029)

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Abstract

In order to fulfil the requirement of low area specific resistance and highly stable cathode contact material in planar type solid oxide fuel cell (SOFC) stack assembling, this work investigated the electrical property evolution of LaNi0.6Fe0.4O3 (LNF) with manipulated particle size and its effect on SOFC electrochemical performance. The optimized pre-treatment strategies of LNF were obtained with decreasing ASR, improving SOFC single cell performance and thermal cycling stability. Results show that, the dry-pressed LNF-2 and the high-temperature sintering-pre-treated LNF-3 possess smaller area specific resistances of 0.074 and 0.076 Ω·cm², respectively, more stable particle sizes with shorter conditioning state and faster transfer into steady state after applying 1 A/cm2 current load at 750 ℃. Specifically, the single cell with LNF-2 shows improved peak power density of 0.94 W/cm2 compared to 0.66 W/cm2 of LNF without treatment at 750 ℃. However, it exhibits significant performance degradation during thermal cycling, decreasing by 20%. In contrast, the peak power density of LNF-3 single cell decreases by only 4% after 20 thermal cycles. This work is expected to provide guideline and valued reference for reliable SOFC stack assembling and stable operation.

Key words： solid oxide fuel cell (SOFC); cathode contact material; LaNi0.6Fe0.4O3; thermal cycling

Cite this article

Kun ZHANG , Yu WANG , Tenglong ZHU , Kaihua SUN , Minfang HAN , Qin ZHONG . LaNi0.6Fe0.4O3 Cathode Contact Material: Electrical Conducting Property Manipulation and Its Effect on SOFC Electrochemical Performance[J]. Journal of Inorganic Materials, 2024 , 39(4) : 367 -373 . DOI: 10.15541/jim20230353

在平板式固体氧化物燃料电池(SOFC)电堆的装配过程中, 陶瓷阴极和金属连接体的直接接触效果差、应力大, 容易产生较大的界面接触阻抗, 进而影响电堆的性能和稳定性[1-2]。因此, 在阴极和连接体之间通常需要添加阴极接触层来改善界面接触; 而且阴极接触层还具有电流收集和氧气扩散等作用[3], 如图1所示。研究表明, 增加10~100 μm厚的接触层可以明显改善阴极和连接体之间的接触, 大幅降低电堆的面电阻(ASR)[2,4 -5]。
图1 SOFC电堆中阴极与连接体接触界面示意图

Fig. 1 Schematic diagram of the cathode and interconnector contact interface

贵金属如Ag、Au、Pd和Pt等在高温下具有良好的耐腐蚀抗氧化性、高电导率、良好的延展性等优势[6⇓⇓-9], 但成本较高。此外, 在高温工况下SOFC中的Ag易挥发流失, 导致电堆性能降低[10]。部分具有高电导率的尖晶石材料也表现出与其他电堆组件较合适的热膨胀匹配性和兼容性[11], 能够用于制备阴极接触层。除此之外, 最常见的阴极接触材料为钙钛矿结构阴极, 如(La,Sr)MnO3、(La,Sr)CoO3、(La,Sr)FeO3、(La,Sr)(Co,Fe)O3等[2-3,12]。相比于贵金属和尖晶石材料, 钙钛矿阴极材料一般具有电导率高、烧结容易、成本低等优势。但是, (La,Sr)MnO3在中低温的电导率较差; La0.6Sr0.4CoO3(LSC)和La0.6Sr0.4Co0.2Fe0.8O3−δ(LSCF)等Co基材料的热膨胀系数较大(22.0×10-6 K−1)。研究人员[13⇓⇓-16]系统评价了La(Ni,M)O3(M=Al, Cr, Mn, Fe, Co, Ga)材料体系, 发现LaNi0.6Fe0.4O3(LNF)在800 ℃的电导率达到580 S/cm, 热膨胀系数(11.4×10-6 K−1)与常用的SOFC电解质材料及连接体材料相近。此外, 研究还发现LNF阴极具有较好的抗Cr毒化的性能[17-18], 并表现出较好的长期稳定性[19-20]。
研究表明, 除单电池性能衰减及金属连接体氧化之外, 电堆性能衰减还与阴极/连接体之间的界面接触紧密相关[21]。在电堆装配过程中, 接触层的制备温度低, 导致其机械强度较低, 在电堆运行过程的多种应力作用下容易发生结构性破坏, 造成电堆性能衰减[22-23]。
根据电堆装配需求, 阴极接触组件要同时具备较小的面电阻、较低的空气扩散阻力以及较高的机械强度。在前期工作中, 本课题组采用LNF作为SOFC阴极接触和集电材料, 系统研究了材料制备、高温长期运行过程中的元素迁移及电导性能演变等问题[24-25]。同时还发现, 在0.3 A/cm2电流负载下, LNF接触组件的初期面电阻显著降低; 在超过3000 h长期运行后还出现了LNF颗粒尺寸增长的现象。然而, 材料微观结构过度变化容易损坏接触界面, 甚至导致组件粉化流失等问题。基于此, 本研究采用干压造粒和高温烧结两种方法, 制备了大颗粒LNF材料, 并研究在电流负载下的面电阻演变规律及其对SOFC单电池电化学性能的影响。

1 实验方法

1.1 材料制备

本研究使用的LNF粉体来自华清能源。将LNF粉体于行星式球磨机中球磨24 h, 干燥后用75 μm (200目)筛网筛分, 记为LNF-1; 将球磨后的LNF粉体在10 MPa的压力下干压5 min, 破碎后重复干压两次, 最后用147 μm(100目)和75 μm (200目)筛网筛分出75~147 μm的粉体, 记为LNF-2; 将球磨后的LNF粉体在1100 ℃煅烧5 h, 研磨后取75~147 μm 之间的粉体, 记为LNF-3。将上述不同粒径的LNF粉体和丝印胶以1 : 1.5的质量比在研钵中混合均匀后, 通过三辊机制备出LNF浆料。在厚度约400 μm的云母板中切割一个面积为1 cm2的圆孔, 采用干湿结合工艺在圆孔内涂覆LNF浆料, 表面采用银网取电, 上下采用氧化铝板固定并使用螺栓紧固[24]。通过直流四端子法(ITech 6412)测试LNF接触组件的电导率, 然后计算ASR。

1.2 单电池制备与性能测试

采用华清能源生产的NiO-YSZ|YSZ((Y2O3)0.08- (ZrO2)0.92)阳极支撑半电池, 通过丝网印刷并在1250 ℃煅烧制备Ce0.9Gd0.1O1.95 (GDC)隔离层; 然后丝印La0.6Sr0.4Co0.2Fe0.8O3−δ-Ce0.9Gd0.1O1.95(LSCF- GDC)阴极, 并将阴极在1075 ℃煅烧3 h获得单电池。在阴极表面分别丝印三种不同粒径的LNF接触浆料并在750 ℃煅烧; 最后在LNF表面丝印Ag网格作为电流收集层。
采用文献[24]报道的流程进行单电池密封及升温测试。将单电池阳极在750 ℃还原5 h、活化15 h且稳定1 h后进行电化学性能测试。单电池阳极燃料为室温加湿氢气(~3% H2O(体积分数), ~97% H2(体积分数), 50 mL/min), 阴极侧空气总流量为100 mL/min, 通过调节O2/N2比例使氧分压为1×105、2.1×104、1.4×104、7×103和3×103 Pa(图中分别以p1O2、p0.21O2、p0.14O2、p0.07O2和p0.03O2代替)。在0.8 V下测试单电池的电化学交流阻抗谱图(EIS, Princeton P4000A, 0.1~1 MHz, 50 mV), 然后采用5 mV/s的速率进行线性电压扫描。为研究热循环稳定性, 对LNF-2与LNF-3单电池进行热循环性能测试(400~750 ℃)。采用扫描电子显微镜(SEM, JEOL JSM-IT500HR)表征LNF接触材料微观形貌及单电池阴极与接触层的界面微观形貌。

2 结果与讨论

2.1 LNF颗粒调控对面电阻演变规律的影响

在750 ℃模拟实际工况下测试LNF面电阻演变规律, 结果如图2所示。LNF-1的初始ASR最大, 经过干压造粒处理的LNF-2及高温烧结处理的LNF-3则初始ASR较小, 表明增大LNF颗粒尺寸可以降低接触组件的ASR。在施加1 A/cm2电流负载后, LNF-1和LNF-2的ASR均快速减小, 并在~10 h后进入稳定区间; LNF-3的ASR变化较小。结果表明, 增大LNF颗粒尺寸能够降低电流负载对接触组件面电阻的影响, 大颗粒接触组件在电堆运行过程中能较快进入稳态。约120 h后, LNF-1、LNF-2和LNF-3的ASR分别为0.093、0.074和0.076 Ω·cm²。
图2 在750 ℃和1 A/cm2工况下LNF的ASR随时间的变化规律

Fig. 2 ASR evolution of LNF versus time under the conditions of 750 ℃ and 1 A/cm2

图3为ASR测试前后LNF的微观形貌SEM照片, 图中标注为采用Image J软件统计获得的LNF平均粒径。LNF-1的平均粒径由(0.35±0.06) μm增加至(0.50±0.07) μm; LNF-2的平均粒径由(1.44±0.34) μm增加至(1.79±0.34) μm, 而且颗粒尺寸分布较宽; LNF-3的粒径变化较小且颗粒尺寸分布较均匀。结果表明：小粒径LNF在电流负载下的颗粒尺寸会显著增长; 经过干压造粒的LNF-2虽然具有较大颗粒尺寸, 但保留了较好的烧结活性, 因此在电流负载下也表现出较明显的烧结现象, 从而导致ASR减小。而经过高温烧结预处理的LNF-3则基本失去了烧结活性, 在电流作用下的颗粒尺寸变化较小, 因此其ASR保持稳定[26]。
图3 ASR测试前后LNF的SEM照片

Fig. 3 SEM images of LNF before and after ASR test

(a) Initial; (b) Post-test

2.2 LNF颗粒调控对单电池电化学性能的影响

图4和图5为阳极支撑单电池在750 ℃、0.8 V 和阴极侧不同氧分压下的EIS图谱及弛豫时间分布(DRT)拟合对比[27⇓-29]。结果表明, 具有较大颗粒尺寸的LNF-2和LNF-3单电池欧姆阻抗均小于LNF-1, 这与其较低的接触组件ASR和较好的阴极界面接触有关。随着阴极侧氧分压降低, 单电池的极化阻抗均逐渐增大, 且主要表现在P4(10~1 Hz)和P5(1~0.1 Hz)频段, 表明P4和P5响应主要代表氧在阴极侧的扩散和吸附过程。经过干压处理的LNF-2和经过高温烧结处理的LNF-3均具有更大的颗粒尺寸, 颗粒间孔隙较大, 能够降低氧气扩散阻力。因此, LNF-2和LNF-3单电池均表现出较小的极化阻抗, 表明增大LNF颗粒尺寸能够改善阴极侧空气中氧的传输扩散。其中, LNF-2单电池还表现出最小的欧姆阻抗和极化阻抗, 这可能与干压造粒的LNF-2具有一定的阴极氧还原催化活性有关。当阴极侧氧分压降低至小于7×103 Pa时, 所有单电池的极化阻抗均显著增大, 主要与LSCF-GDC阴极特性有关。
图4 单电池在750 ℃和不同氧气分压下的(a)EIS阻抗谱图和(b)DRT拟合图

Fig. 4 (a) EIS spectra and (b) DRT fitting plots for single cells tested at 750 ℃ and varied oxygen partial pressures

Colorful images are available on website

图5 单电池在2.1×104及3×103 Pa阴极氧分压下的(a)EIS阻抗谱图和(b)DRT拟合图; 拟合得到的(c)欧姆阻抗及(d)极化阻抗对比

Fig. 5 (a) EIS spectra and (b) DRT fitting plots of single cells under oxygen partial pressure of 2.1×104 and 3×103 Pa, and comparison of corresponding (c) Ohmic resistance and (d) polarization resistance

Colorful images are available on website

图6为单电池在750 ℃不同阴极氧分压下的电化学输出性能及对比。结果表明, 随着氧分压降低, 单电池开路电压、极限电流密度和峰值功率密度均有所降低。LNF-2单电池的性能输出最好, 在2.1× 104 Pa氧分压下的峰值功率密度达到0.94 W/cm2, 相比于LNF-1(0.66 W/cm2)提高了30%。从j-V-P曲线的演变规律可以看出, 在大电流工况下, LNF-2和LNF-3单电池的浓差极化现象相较于LNF-1单电池有一定改善[30], 表明大颗粒尺寸LNF接触材料有利于氧扩散, 从而提高单电池的输出性能。
图6 单电池在750 ℃和不同阴极侧氧分压下的(a~c)j-V-P曲线及(d)峰值功率密度对比

Fig. 6 (a-c) j-V-P curves and (d) peak power density comparison of single cells versus different oxygen partial pressure on cathode sides at 750 ℃

Colorful images are available on website

2.3 单电池热循环性能演变与稳定机制

图7为LNF-2和LNF-3单电池在2.1×104 Pa阴极氧分压和750~400 ℃热循环过程中的性能演变。从图中可以看出, 虽然LNF-2单电池的初始性能较好, 但在热循环过程中的欧姆阻抗和极化阻抗逐渐增大, 峰值功率密度也随之降低。循环20次后, 单电池峰值功率密度从0.94 W/cm2下降到0.75 W/cm2, 下降了20%。相比之下, LNF-3单电池的初始性能较低, 但在热循环过程中的变化较小, 循环20次后峰值功率密度从0.75 W/cm2减小到0.72 W/cm2, 降低了4%。
图7 单电池在热循环过程中的电化学性能演变

Fig. 7 Evolution of electrochemical performances for single cells under thermal cycling

(a) RΩ; (b) Rp; (c) Peak power density

Colorful images are available on website

结果表明, 虽然LNF-2的初始性能较好, 但由于其仅经过干压造粒, 材料自身仍然保留较好的烧结活性, 在长期高温运行和多次电化学性能测试过程中, 其颗粒更容易粗化, 从而在多次热循环作用下发生孔隙破坏和界面剥离等现象, 导致单电池性能明显衰减, 如图8所示。相比之下, 经过高温烧结预处理的LNF-3材料烧结活性较差, 在高温热循环过程中能够保持较好的结构稳定性。
图8 热循环前后单电池阴极接触界面变化示意图及微观形貌

Fig. 8 Schematic diagrams and SEM images for cathode contact interfaces of single cells before and after thermal cycling

(a) LNF-2; (b) LNF-3

3 结论

本研究通过调控LNF材料颗粒尺寸, 阐明了阴极接触组件ASR演变规律及其对SOFC单电池电化学性能及稳定性的影响机制, 主要结论如下:
1) 增大LNF颗粒尺寸可以降低阴极接触组件的ASR, 相比于干压造粒, 高温烧结预处理能够有效降低LNF的烧结活性, 在电流负载下接触组件的ASR可以快速达到稳态, 并在长期电流负载工况下保持结构稳定。
2) 大颗粒尺寸LNF接触材料能够优化阴极界面接触, 促进阴极侧的氧扩散运输, 提高单电池输出性能; 在750 ℃、2.1×104 Pa 阴极氧分压下, 相比于LNF-1单电池, 干压造粒LNF-2单电池的峰值功率密度从0.66 W/cm2增加到0.94 W/cm2。
3) 干压造粒的LNF材料仍保留一定的烧结活性, 导致热循环稳定性较差; 高温烧结预处理能够显著提升LNF阴极接触材料在热循环和放电过程中的结构稳定性, 20次热循环之后仍能保持较高的性能输出。

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