Elsevier

Journal of Power Sources

Volume 450, 29 February 2020, 227682
Journal of Power Sources

Microextrusion printing for increasing electrode–electrolyte interface in anode-supported solid oxide fuel cells

https://doi.org/10.1016/j.jpowsour.2019.227682Get rights and content

Highlights

  • Microextrusion printing is used to increase the electrode–electrolyte interface.

  • Mathematical model of the microextrusion printing is developed.

  • SOFC performance is improved by increasing the interfacial area.

  • Performance improvement is greatest at a low operating temperature.

Abstract

Microextrusion printing is employed for increasing the interfacial area between the electrode and electrolyte of anode-supported solid oxide fuel cells (SOFCs) to enhance their electrochemical performance. A homogeneous anode paste is prepared as an extrusion material and its rheological properties are measured. Then, the printing resolution (line width of the printed paste) is evaluated by extruding the paste under various printing conditions to determine the appropriate stage speed and flow rate for cell fabrication. A mathematical model is also developed to predict the printing resolution from the printing parameters, and the experimental data of the line width agree well with the predicted values, particularly when the contact angle between the paste and substrate is considered. By varying a paste coverage area, two types of anode-supported SOFCs having different interfacial area enlargement ratios are prepared by extruding the anode paste onto the flat anode substrates. The electrochemical performance of the cell is improved by increasing the interfacial area; the current density at the terminal voltage of 0.7 V at 600 °C is 0.17 A cm−2 in a flat cell, whereas 0.32 A cm−2 in a patterned cell whose enlargement factor is 1.14 compared with that of the flat cell.

Introduction

Inside the porous electrodes of solid oxide fuel cells (SOFCs), the electrochemical reaction of hydrogen and oxygen in an anode and a cathode takes place at reaction sites called triple-phase boundaries (TPBs) and double-phase boundaries (DPBs). Thus, the TPBs and DPBs play an important role in determining the power generation performance of SOFCs. However, it has been confirmed that the reaction mainly occurs not at all reaction sites inside the electrodes, but at those in a very thin region (about 20 μm) in the vicinity of the electrode–electrolyte interface [[1], [2], [3], [4]]. Hence, it is considered effective to increase the size of the electrochemical reaction region near the interface to enhance cell performance [5].

Recently, various studies on expanding the reaction region on the electrode–electrolyte interface to attain higher performance of SOFCs have been conducted. For instance, Konno et al. [6] machined mesoscale (10–100 μm) grooved structures on a flat electrolyte made of yttria-stabilized zirconia (YSZ), which was followed by sintering a NiO–YSZ anode to prepare a cell having a corrugated interface structure. By evaluating the electrochemical performance of the cell, they clarified that the cell had a higher performance than a flat cell. Specifically, the current density of the cell where the anode–electrolyte interfacial area was increased by 60% compared with that of the flat cell was 25% higher than that of the flat cell at the terminal voltage of 0.5 V at 800 °C with pure hydrogen. Similarly, it has also been reported that grooves on an electrolyte substrate can be patterned by using other processes, e.g., micropowder imprinting [[7], [8], [9]], and laser machining [[10], [11], [12], [13], [14]]. Both of these approaches resulted in the enhanced electrochemical performance of the cells because of the increased electrode–electrolyte interfacial area [8,10,[13], [14], [15]]. Nevertheless, it is difficult to control the grooved structures precisely and to prevent breakage and cracks during such mechanical processing of electrolyte surfaces [7,11,13]. Thus, it is necessary to find alternative fabrication methods for increasing the interfacial area between the electrode and the electrolyte in order to overcome these critical problems.

To create mesoscale structures in SOFCs without structural failures, additive manufacturing processes including inkjet printing [[16], [17], [18]], stereolithography [19,20], and microextrusion printing (also called as direct writing) [[21], [22], [23], [24]] are considered as candidates. Among these candidates, it is important to find the most desirable additive manufacturing process which has a sufficiently high printing resolution of a printing technique, and high shape retainability and hardenability of printing material. Young et al. [16] demonstrated the applicability of inkjet printing for the fabrication of NiO–YSZ anode functional layers and YSZ electrolyte layers with thicknesses of about 6–18 and 6–12 μm, respectively, by multilayer printing. However, since the ink for use in inkjet printing has relatively lower viscosity (below 20 mPa s) and surface tension (28–350 mN m−1) [25], it is difficult to keep its shape after being printed. Hernández-Rodríguez et al. [20] used stereolithography to fabricate a self-supported YSZ electrolyte with a thickness of about 100 μm by multilayer printing. However, the suspension used in stereolithography needs not only to be cured by photochemical treatment but also to be eventually sintered by thermal treatment to obtain a stiff layer. In microextrusion printing, a highly viscoelastic paste composed of metal and/or ceramic powders is extruded through a micronozzle onto an object, whose relative position is precisely controlled. The microextrusion printing process allows the construction of 3D structures in the range from about 1 to 250 μm [26,27], since it enables production with high speed, positional accuracy, and good reproducibility because of robotic control [27]. Also, the paste can easily retain its shape after printing because of its relatively higher viscosity (1–105 Pa s [28]). Moreover, the printed paste can form a stiff layer by only thermal treatment, so that the hardening process is relatively simple. Therefore, microextrusion printing can be an alternative fabrication method for increasing the electrode–electrolyte interface.

The feasibility of microextrusion printing for fabricating SOFCs having complex structures and patterns has been demonstrated. For instance, Kim et al. [22] produced a planar SOFC cell consisting of two serially integrated cells by extruding paste materials of all cell components through a micronozzle of 210 μm diameter. They achieved a cell having line-shaped electrodes and an electrolyte whose width and thickness were over 500 μm and up to 30 μm, respectively. Also, Khun et al. [23,24] used microextrusion printing to manufacture single-chamber SOFCs having patterned electrodes on electrolyte plates. However, the aforementioned studies only focused on the fabrication of SOFC electrodes and electrolytes using microextrusion printing.

In this study, the applicability of microextrusion printing as an alternative fabrication method for increasing the electrode–electrolyte interfacial area is verified and then the effect of its increase on the electrochemical performance of SOFCs is investigated. Anode-supported SOFCs are used unlike electrolyte-supported SOFCs in the previous studies [[6], [7], [8], [9], [10], [11], [12], [13], [14]], since the increase in the interfacial area is most effective when the electrolyte is thinner [6,29]. First, a homogeneous NiO–YSZ anode paste is prepared as an extrusion material and its rheological properties are evaluated. Then, printing resolution defined as line width in this study is evaluated to determine printing parameters for cell fabrication, by comparing with a developed mathematical model. By varying one of the parameters for the microextrusion printing process, two types of anode-supported SOFCs having different interfacial area enlargement ratios are prepared by extruding the anode paste onto anode substrates. The cross-sectional structure of the cells is observed by scanning electron microscopy (SEM) and their electrochemical performance is evaluated. Finally, the effect of increasing the interfacial area on the cell performance under several operating temperatures and hydrogen partial pressures is discussed.

Section snippets

Preparation of anode paste

The anode paste material used in this study was a mixture of NiO and YSZ powders. 66 wt% NiO powder (FUJIFILM Wako Pure Chemical Corp., Japan, average particle size = ca. 1–2 μm) and 34 wt% YSZ powder (TZ-8Y, Tosoh Corp., Japan, crystallite size = 22 nm) were mixed; then an appropriate amount of isopropanol was added to the mixed powder with zirconia balls (φ 4.0 mm), then the mixture was ball-milled at 450 rpm for 1 h to disperse the particles. After that, the isopropanol was evaporated using

Rheological properties

Fig. 2 shows the shear stress and apparent viscosity curves of the anode paste as a function of the shear rate. A solid line in Fig. 2 represents the fitted result of the measured data (scatter plot) to the Herschel-Bulkley fluid model, a generalized model of non-Newtonian fluid, which combines yield stress and shear-thinning or shear-thickening behavior in a fluid, described asτ=τ0+K(ur)n,where τ is the shear stress, τ0 is the yield stress correlated to the viscoelasticity of the fluid, K

Conclusions

For the first time, the additive manufacturing process was used to increase the electrode–electrolyte interfacial area of anode-supported SOFCs to improve the electrochemical performance. To predict the printed width in microextrusion printing, a printing resolution study was conducted experimentally, and the results were compared with the mathematical model. The experimental data agreed well with the mathematical model, particularly when the contact angle between the paste and the substrate

Declaration of competing interests

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.

Acknowledgement

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 supported by JSPS KAKENHI Grant Number 19K04216 and by JSPS joint Research Project with Poland, “Development of mesoscale electrode–electrolyte interfaces in SOFCs using 3D printing for efficient energy conversion”. We also acknowledge to Dr. G. Brus

References (32)

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