Alternative Ni-Impregnated Mixed Ionic-Electronic Conducting Anode for SOFC Operation at High Fuel Utilization

Abstract

Redox-stable anodes are developed for zirconia-based electrolyte-supported SOFCs in order to improve the durability against fuel supply interruption and for higher fuel utilization, as an alternative to the conventional Ni-YSZ cermet. GDC (Ce 0.9 Gd 0.1 O 2 ) is utilized as a mixed ionic-electronic conductor (MIEC), and combined with LST (Sr 0.9 La 0.1 TiO 3 ) as an electronic conductor. Ni catalyst nanoparticles are incorporated via impregnation. The electrochemical characteristics of SOFC single cells using these anode materials are investigated in humidified H 2 at 800 • C. The stability against redox cycling and under high fuel utilization is analyzed and discussed. Ni-impregnated anodes with dispersed Ni catalyst nanoparticles on conducting oxide LST-GDC backbones exhibit lower anode non-ohmic overvoltage, and improve I-V performance. These anodes also show better redox stability compared to conventional anodes because of the isolation of Ni catalysts, preventing their agglomeration. Moreover, the co-impregnation of Ni catalysts and GDC nanoparticles further improves electrochemical characteristics due to a decrease in anode ohmic (IR) loss and non-ohmic overvoltage. This anode shows comparable I-V performance to conventional anodes for typical humidified hydrogen fuels, and is a promising redox-stable alternative for application at high fuel utilization. Solid oxide fuel cells (SOFCs) are promising electrochemical energy conversion systems that can directly produce electricity from chemical fuels, without combustion. The operating temperature is generally around 800• C, leading to several advantages including high electric conversion efficiency, fuel flexibility, and noble-metal-free fabrication. For example, in Japan, the commercialization of SOFCs as residential co-generation systems started in 2011. The electric efficiency reached a lower heating value (LHV) of 52% in 2016. Development of these systems for industrial applications is also in progress, and combining them with micro gas turbines and/or steam turbines can achieve even higher electric efficiency. 1The porous Ni yttrium-stabilized-zirconia (YSZ) cermet has been used as a conventional SOFC anode material for decades. However, the electron conducting pathways through the Ni metal phase (which also acts as the electrocatalyst) can be easily destroyed by redox reactions during potential cycling, where Ni redox reactions result in significant changes in volume. This leads to crack formation in the ionic conducting YSZ phase, and deterioration of the electrochemical performance, especially during e.g. system shutdown, fuel supply interruption, and at high fuel utilization.2-8 Therefore, extra hydrogen (or hydrogen-containing gas) is often required during system shutdown in order to maintain reducing conditions on the anode side of the device at all times. The fuel utilization in practical systems must also be kept low for the same reason. Higher efficiency is required for wide-spread commercialization of SOFCs in the future, and therefore tolerance...

Influence of Surface State on the Electrochemical Performance of Nickel-Based Cermet Electrodes during Steam Electrolysis

“…This kind of modifications are most of the time irreversible and can be displayed by post mortem microscopy images of the cell. , In addition, changes of the chemical composition and the oxidation state of the cathode electrode have been also reported to induce deactivation . Oxidation of nickel to NiO or Ni­(OH) 2 may occur when the cell is operating under low H 2 production and high steam concentration conditions, by air leaks in the fuel circulation system, or even after long-term operation. Extended nickel oxidation or bulk NiO formation has an evident macroscopic influence on the cell (e.g., micro cracks, coarsening, etc.…”

Study of Operating Conditions for High Efficiency and Anode Safety of Industrial-Size Solid Oxide Fuel Cell

“…在 SOFC 的相关研究中, 有很大一部分采用纽扣电池进行实验, 由于电池面积很 小(0.45～1 cm 2 ), 燃料流量往往是过量的, 因此燃料利 用率一般在 10%以下. 这部分的工作往往聚焦于电池结 构、电极材料和制备工艺的改进 [12] , 主要关注的是电池 的峰值功率密度、欧姆电阻及极化电阻等电化学特 性 [13] . 然而, 在 SOFC 的实际应用中, 经济成本是不得 不考虑的一个问题.…”

Identification of carbon deposition and its removal in solid oxide fuel cells by applying a non-conventional diagnostic tool

“…15 One possible way to increase the cell stability when operating under high fuel utilization conditions is to use alternative materials or anodes impregnated with noble metal catalysts. 16,17 Their applicability has already been approved for small button cells, but has not yet been approved for the industrial-sized cells. For this reason, it is necessary to make a compromise between achieving high fuel utilization or high power.…”