In-Situ Observation of Temperature Distribution on a Planar Type SOEC During Start-Stop Cycle Operation
Authors: Okamoto Takeaki , Yasutake Masahiro , Tachikawa Yuya , Sasaki Kazunari 
Published: September 2023

Introduction

It is important to establish various analytical methods for solid oxide electrolysis cells (SOECs) under practical operating conditions in order to improve their performance and to analyze their degradation phenomena. In-situ observation of temperature distribution on a planar type SOEC enables to reveal the distributions of electrochemical reactions and the cell degradation. While there have been several reports on the internal visualization of solid oxide fuel cells (SOFCs)1, 2, there have been only a few reports on that of SOECs. Since the endothermic electrolysis reaction occurs in SOECs, comparing with SOFCs, a different temperature distribution will appear. Therefore, it should be necessary to evaluate the phenomena on the planar SOECs for analyzing the temperature distribution taking gas flow and current distribution into consideration.
The purpose of this study is to evaluate the effect of cell degradation caused by the start-stop cycle operation on the temperature distribution using a 5 cm × 5 cm planar cell observed by infrared camera.

Experimental

In this study, a planar cell was fabricated using a scandia-stabilized zirconia electrolyte (ScSZ, 200 µm thick, 5 cm × 5 cm in area) as a substrate. LSM ((La0.8Sr0.2)0.98MnO3) and LSM-ScSZ composite materials were used for the air electrode, and nickel and ScSZ cermet were used for the fuel electrode. The electrode area was 4 cm × 4 cm (16 cm2). Each electrode material was screen-printed and sintered. A platinum mesh was used as the current collector.
In this test, the cell was operated at 800 °C and 50%-humidified hydrogen (200 ml min-1) was supplied to the fuel electrode. The durability test was conducted under the accelerated degradation conditions of a start-stop cycle repeated in every 1 hour with 0.2 A cm-2 for 500 cycles (1000 hours). Electrochemical characteristics were measured by connecting Pt wires, spot-welded every 1 mm on one side of both electrode surfaces, to the voltage terminals of the electrochemical measurement setup and to the current terminals of the external power supply unit.
Figure 1 shows a schematic view of the experimental setup and temperature change compared with the initial condition. The temperature distribution was observed by infrared camera (FLIR SC2500 - NIR, uncertainty ±0.02 oC) from the air electrode side open to the electric furnace. The difference between the temperatures before and during current loading was derived as the distribution of temperature changes.

Results and discussion

First, the electrochemical characteristics of the planar type SOECs during the start-stop cycle test were obtained. The cell voltage at 0.2 A cm-2 increased from 1.19 V to 1.29 V during the test up to 500 cycles. The cell voltage was 1.27 V even after the initial 50 cycles. This suggests that SOEC electrode degradation3,4 occurred in the early stage of the start-stop cycling.
The temperature changes after 50 cycles showed that each temperature change was relatively small because the cell was operated near the thermoneutral potential (around 1.29 V at 800 °C). It was confirmed that the endothermic electrolysis reaction was predominant near the fuel inlet. In addition, heat generation due to Joule heating was also appeared near the current collecting Pt wire connected to the power supply. Therefore, in SOEC operation, it is suggested that the temperature distribution is affected by the shape of current collector, gas supply condition, and electrochemical reaction distribution.
The detailed analysis on the SOEC electrochemical properties and temperature distribution observed by in-situ measurements during cycling tests is also presented.

Acknowledgements

A part of this research was supported by New Energy and Industrial Technology Development Organization (NEDO), project No. JPNP14026.

References

(1) S. Shinichi and H. Iwai, J. Power Sources, 482, 229070 (2021).
(2) Y. Shiratori, T. Ogura, H. Nakajima, M. Sakamoto, Y. Takahashi, Y. Wakita, T. Kitaoka, and K. Sasaki, Int. J. Hydrogen Energy, 38 (25), 10542 (2013).
(3) M. Keane, M. K. Mahapatra, A. Verma, and P. Singh, Int. J. Hydrogen Energy, 37 (22), 16776 (2012).
(4) M. Hubert, J. Laurencin, P. Cloetens, B. Morel, D. Montinaro, and F. Lefebvre, J. Power Sources, 397, 240 (2018).

Figure 1