Alternative SOFC Anode Materials with Ion– and Electron–Conducting Backbones for Higher Fuel Utilization
Authors: Futamura Shotaro , Tachikawa Yuya , Matsuda Junko , Lyth Stephen Matthew , Shiratori Yusuke , Taniguchi Shunsuke , Sasaki Kazunari 
Published: February 2020

Introduction

 Solid oxide fuel cells (SOFC) have a great potential to generate electricity with a high efficiency. Currently, Ni-YSZ cermet is widely used as the SOFC anode material. As anode Ni can be oxidized under a high fuel utilization, however, SOFC fuel cannot be fully used [1]. In this study, we analyze alternative anode materials composed of ionic conducting GDC (Ce0.9Gd0.1O2) and electronic conducting LST (Sr0.9La0.1TiO3), both of which act as stable ion- and electron-conducting frameworks, respectively, and Ni acts only as an electrocatalyst, as described in Figure 1. In addition, Rh, despite Ni, is also impregnated for higher fuel utilization.

Experimental

 Electrolyte-supported cells with ScSZ (10 mol% Sc2O3 - 1 mol% CeO2 - 89 mol% ZrO2) plate (20 mmφ×0.2 mmt) were used in this study. Mixture of LST and GDC (50:50 in volume) was used for the anode and was sintered at 1300oC for 3h. Mixture of (La0.8Sr0.2)0.98MnO3 (LSM) and ScSZ with a weight ratio of 50:50 was used for the cathode. In order to further improve the anodic performance, Ni and GDC were impregnated into the porous LST-GDC composite anode. Electrode area was 8×8 mm2 and Pt mesh was used as the current collector. Cell performance such as I-V characteristics, anodic overvoltage, and anode-side ohmic loss were measured at 800oC by feeding humidified fuel. This Ni-GDC co-impregnated anode was evaluated through the I-V measurements, FESEM analysis, and Red-Ox cycling tests [2]. Moreover, in order to investigate the stability for the operation under high fuel utilization in the downstream of practical SOFC systems, the long-term durability test (fuel: 95%-humidified H2, constant current density: 0.2 A cm-2) was conducted. In addition, the Rh-GDC co-impregnated anode performance was investigated for higher fuel utilization.

Results and discussion The Ni-GDC co-impregnated anode (Ni:GDC = 1:5 in volume) showed comparable performance to the conventional Ni-ScSZ anode, as shown in Figure 2. The Ni loading was 0.167 mg-Ni/cm2 and the I-V performances were investigated using 3%-humidified H2 fuel. This anode was more stable against redox cycling than the conventional anode. The FIB-SEM micrograph of the Ni-GDC co-impregnated anode is shown in Figure 3, showing that catalytic Ni nanoparticles were supported on the LST-GDC composite frameworks in a highly dispersed manner, which could prevent Ni agglomeration. Although the Ni-GDC co-impregnated anode was stable against the redox cycling and its performance was comparable to the conventional one, it deteriorated in the durability test at high fuel utilization. Ni particles were oxidized to lose their catalytic activity because the initial anode voltage of the co-impregnated anodes was lower (< 0.701 V [3]) under the heavily-humidified hydrogen fuel. On the other hand, Rh-GDC co-impregnated (0.232 mg-Rh/cm2, Rh:GDC = 1:5 in volume) anode was stable against this durability test because Rh is a stable catalyst even under high oxygen partial pressure. Consequently, these anodes could be promising alternative anodes for high fuel utilization operation of SOFCs.

References

[1] M. H. Pihlatie, A. Kaiser, M. Mogensen, and M. Chen, Solid State Ionics, 189, 82-90 (2011).

[2] M. Hanasaki, C. Uryu, T. Daio, T. Kawabata, Y. Tachikawa, S. M. Lyth, Y. Shiratori, S. Taniguchi, and K. Sasaki, J. Eletrochem. Soc,.
161(9), F850-860 (2014).

[3] T. Kawasaki, J. Sugimoto, Y. Tachikawa, Y. Shiratori, S. Taniguchi, and K. Sasaki, ECS Trans., 8(1), 1345-1352 (2015).

Figure 1