Mechanism of improved electrochemical performance of anode-supported solid oxide fuel cells by mesostructural modification of electrode–electrolyte interface
Introduction
Solid oxide fuel cells (SOFCs) are attracting widespread attention owing to their high energy conversion and fuel diversity. To minimize the size of SOFC systems and thereby reduce their manufacturing costs and heat capacity, it is necessary to increase the volumetric power density of individual cells. For this purpose, researchers have been focusing on the design of a cell structure on the mesoscale order (10–100 μm) to enlarge the electrode–electrolyte interfacial area [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]]. This is because the electrochemical reactions in porous electrodes actively occur at the reaction sites distributed in a very thin region with a thickness of around 5–20 μm near the electrode–electrolyte interface [[11], [12], [13], [14], [15]].
Owing to the recent advances in manufacturing technology, the mesostructural modification of SOFCs has been realized, resulting in markedly enhanced cell performance [1,[3], [4], [5], [6], [7], [8], [9], [10],16]. For instance, we achieved an increase of 14% in the electrode–electrolyte interfacial area of an anode-supported SOFC relative to a flat cell by forming anode ridge structures on a surface of a flat anode disk using a microextrusion printing technique [7]. Through electrochemical impedance spectroscopy, it was confirmed that the relative decreases in the ohmic and activation resistances were 42–45 and 54–59%, respectively, under the open-circuit voltage condition at 600–700 °C. More recently, Shin et al. [8] have fabricated an anode-supported SOFC where the electrode–electrolyte interfacial area was enlarged by about 23% compared with that of a flat cell by micropatterning through polymer-to-ceramic transformation. They reported that the ohmic and polarization resistances in the cell obtained at a terminal voltage of 0.75 V at 500–600 °C were 29–52 and 15–23% lower than those in the flat cell, respectively. Although it has been demonstrated that mesostructural modification improves cell performance, there is no consistency in the relationships between the relative increase in interfacial area and the relative decrease in cell resistance among the existing experimental results [1,[3], [4], [5], [6], [7], [8], [9], [10],16]. This is because the electrochemical performance of SOFCs is affected by multiple factors such as the cell structure, operating conditions, and materials for the cell components. Accordingly, numerical simulation in which these factors are appropriately considered is expected to provide important clues for understanding the effect of mesostructural modification.
Thus far, various numerical models have been developed to investigate the effect of the mesostructural modification of SOFCs on improving the cell performance [1,6,[17], [18], [19], [20], [21], [22], [23]]. For example, Konno et al. [1,18] analyzed the distributions of physicochemical quantities within mesoscale-modified cells during power generation and thereby found that introducing mesoscale corrugated structures into the electrode–electrolyte interface reduced both ohmic and activation overpotentials. Moreover, they clarified that its effect became more prominent with increasing electrode–electrolyte interfacial area, particularly in a cell having a thin electrolyte such as an anode-supported SOFC. It has also been reported that the electrode microstructure [6,18,22] and the geometrical shape of the electrode–electrolyte interface [17,[19], [20], [21],23] determine the effect of mesostructural modification. Despite these key findings, a detailed mechanism whereby mesostructural modification improves cell performance has not yet been clarified. In addition, a lack of understanding regarding the improvement in cell performance by mesostructural modification in quantitative terms still remains [1,20]. For example, according to the literature [1], the current density at 0.5 V at 800 °C increased from 42.2 to 52.8 mA cm−2 in the measurement, whereas it increased from 259 to 287 mA cm−2 in the simulation. These quantitative discrepancies between the electrochemical characteristics of each cell obtained from the experiment and simulation led to a mismatch of the increases in the electrochemical performance of a mesoscale-modified cell relative to a flat cell. This is because the cell structures at multiple scales—the electrode microstructure, cell component thickness, and geometric shape of the electrode–electrolyte interface—used in the electrochemical testing were not properly considered in the numerical model. Thus, a numerical model with high validity must first be constructed to clarify the aforementioned unresolved issues.
Herein, the mechanism whereby mesostructural modification improves the electrochemical performance of anode-supported SOFCs is elucidated. After preparing two types of anode-supported SOFC having different electrode–electrolyte interfacial areas, we carry out their structural analyses and electrochemical characterization. Next, we develop a two-dimensional (2D) numerical model in which the structures of the tested cells are implemented and then verify its validity by comparing experimental and simulation results. Subsequently, changes in the overpotential components of the mesoscale-modified cell relative to the flat cell are quantitatively evaluated. Through analyzing the distributions of physicochemical quantities that contribute to the electrochemical reactions inside the cells, the mechanism of the improved cell performance by mesostructural modification is discussed on the basis of the energy losses attributed to such quantities. Finally, the effect of mesostructural modification at various current densities is investigated.
Section snippets
Cell preparation
Two types of 20-mm-diameter anode-supported button SOFC having different electrode–electrolyte interfacial areas, a flat cell (FLAT) as a reference and a mesoscale-modified cell (MESO), were prepared. An anode disk and anode ridge structures were prepared by tape casting and microextrusion printing, respectively, using an identically prepared homogeneous NiO (FUJIFILM Wako Pure Chemical Corp., Japan)–(Y2O3)0.08(ZrO2)0.92 (YSZ) (TZ-8Y, Tosoh Corp., Japan) anode slurry (NiO:YSZ = 60:40 wt%). The
Numerical model
We developed a steady-state 2D numerical model in which the structures of the cells after the electrochemical testing were implemented. Fig. 1(a) shows cross-sectional SEM images of the tested cells [24]. The green, yellow, white, and gray areas in the SEM images correspond to the Ni–YSZ anode, YSZ electrolyte, GDC barrier layer, and LSCF cathode; their average thicknesses in the flat cell were measured to be about 450, 8.0, 6.0, and 36 μm, respectively. On the other hand, the thicknesses of
Model validation
Fig. 2 shows the i–V and current–power (i–P) curves of the cells obtained from the experiment and numerical simulation at 700 °C. Note that the average current density was defined as the total current divided by the apparent electrode area. It was found that the electrochemical performance of the mesoscale-modified cell is improved compared with that of the flat cell under the same operating conditions in both the experimental and numerical results. This indicates that the cell overpotential is
Conclusions
We clarified the mechanism of the improved electrochemical performance of an anode-supported SOFC by the mesostructural modification of the electrode–electrolyte interface. After preparing two types of anode-supported SOFC having different electrode–electrolyte interfacial areas, we developed a 2D numerical model in which cell structures with a wide range of length scales were implemented. It was verified that simulation results well reproduced experimental results in terms of electrochemical
CRediT authorship contribution statement
Haewon Seo: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing – original draft, Writing – review & editing, Visualization. Masashi Kishimoto: Software, Formal analysis, Investigation, Writing – review & editing, Supervision. Taishi Nakagawa: Methodology, Software, Validation, Investigation, Writing – original draft, Visualization. Hiroshi Iwai: Writing – review & editing, Supervision, Project administration, Funding acquisition. Hideo Yoshida: Writing – review
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work was partially supported by Japan Science and Technology Agency (JST) under Collaborative Research Based on Adaptable and Seamless Technology Transfer Program through Target-driven R&D (A-STEP) Grant Number JPMJTS1613. Also, this work was partially supported by the Kyoto University Nanotechnology Hub in the “Nanotechnology Platform Project”, sponsored by the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT). Moreover, this work was partially supported by JSPS
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2023, International Journal of Hydrogen EnergyCitation Excerpt :Therefore, LSCF can be mixed with GDC to improve its thermal expansion performance and electrochemical activity. The mass transmission of SOFCs is of great interest to researchers because mass transfer will lead to severe fuel cell performance losses and the concentration of reactants and products is determined inside the catalytic layer distributed in a very thin region with a thickness of around 5–20 μm near the electrode–electrolyte interface rather than in the flow field of the fuel cell [21–23]. Currently, most researchers have optimised the structure of hydrogen electrodes and electrolytes.