Effect of added Ni on defect structure and proton transport properties of indium-doped barium zirconate

https://doi.org/10.1016/j.ijhydene.2019.11.128Get rights and content

Highlights

  • A tubular cell is fabricated with the use of BaZr0.8In0.2O3−δ.

  • The average dissolution of Ni in the electrolyte N is measured by SIMS to be 0.015.

  • The electrical conductivity decreases as the NiO content increases.

  • The density decreases as the NiO content increases.

Abstract

We investigated the influence of Ni on protonic ceramic fuel cells based on indium-doped barium zirconate. A tubular fuel cell was fabricated and evaluated with BaZr0.8In0.2O3−δ as an electrolyte. The maximum power density was 0.143 W cm−2 and the ohmic resistance of the electrolyte was 0.91 Ω cm2 at 873 K. We used secondary ion mass spectrometry to measure the dissolution of Ni in the electrolyte N to be 0.015. To clarify the effect of Ni on proton transport properties of BaZr0.8In0.2O3−δ, electrical conductivity and proton concentration were measured by AC impedance analysis and thermogravimetric analysis. Electrical conductivity decreased as the NiO content increased. Conversely, proton concentration was independent of the NiO content and proton diffusivity decreased. The sample density also depended on the NiO content. The density decreased as NiO content increased. These results were consistent with the density calculated based on a model describing formation of oxygen vacancies.

Introduction

In recent years, protonic ceramic fuel cells (PCFCs) have been widely studied and have shown great potential as intermediate temperature solid oxide fuel cells (SOFCs). PCFCs are expected to be next-generation energy conversion devices because of their high-power generation efficiency [[1], [2], [3], [4]] and low production costs [5]. There have been many studies of fuel cells based on proton conductors as electrolytes [[1], [2], [3], [4], [5], [6], [7], [8]]. In particular, barium zirconate is known to have a higher chemical stability to CO2 than that of barium cerate [[9], [10]], and acceptor-doped barium zirconate has a high proton conductivity [[11], [12], [13], [14]]. The proton conductivity of yttrium-doped barium zirconate is 1 × 10−2 Scm−1 at 873 K, making it attractive for use as an electrolyte for fuel cells and water vapor electrolysis cells.

Generally, Ni is used as the anode of PCFCs because Ni tends to show a lower anodic overpotential than those of other metals. It is also known that Ni works as a sintering aid for acceptor-doped barium zirconate and barium cerate [15]. On the other hand, Ni dissolves in proton conducting oxides and might increase the ohmic resistance of the electrolyte [16].

Yttrium-doped barium zirconate is the proton conductor with the highest reported conductivity [17]. Therefore, especially in recent years, many studies on PCFC using Y-doped barium zirconate have been reported. Uthayakumar et al. reported that yttrium-doped barium zirconate were prepared by hydrothermal assisted coprecipitation method [18]. Huang et al. reported the fabrication methods by multilayer tape-casting and solid-state reactive sintering [19]. Onish et al. reported the appropriate value of NiO initial composition of the anode [20].

However, BaY2NiO5 is formed when sintering the anode and the electrolyte [17] and this material can act as a proton blocking layer and cause cracking of the cells. For practical use, fuel cells should have a durability better than 90,000 h and reliability of several hundred start/stop cycles. The formation of a complex oxides by the reaction of the electrode and the electrolyte is expected to contribute to losses of durability and reliability. Conversely, indium-doped barium zirconate has better sintering behavior than that of yttrium-doped barium zirconate [21] and is stable under wet 3% CO2 [21]. Similar reaction products to those of yttrium-doped barium zirconate have not been observed in BaIn2NiO5 [21]. However, the proton conductivity of indium-doped barium zirconate is lower than that of yttrium- or scandium-doped barium zirconate [16]. Sun et al. reported that the maximum power density of PCFCs based on indium-doped barium zirconate electrolytes was 0.034 Wcm−2 at 600 °C [21]. Moreover, they showed that the maximum power density was 0.221 Wcm−2 based on indium and yttrium co-doped barium zirconate [22]. Ling et al. reported that the maximum power was 0.18 Wcm−2 at 600 °C, with the use of indium and bismuth co-doped barium zirconate [23]. Thus, the proton conductivity was improved by adding a dopant to indium, and the maximum output was increased. No problematic interactions have been previously reported between Ni and yttrium-doped barium zirconate. However, interactions of indium-doped barium zirconate and Ni have yet to be investigated in detail. In fact, the open circuit voltage (OCV) of such PCFCs is considerably lower than the theoretical value assuming that the proton transport number is unity. These results suggest that the proton transport number is decreased by Ni dissolution.

In this study, we aimed to investigate the influence of Ni dissolution on the conductivity and the lattice expansion required for the electrolyte. We focused on indium-doped barium zirconate as a PCFC electrolyte material because no reaction products such as BaIn2NiO5 have been observed. To clarify the influence of Ni on the performances of the PCFC, anode supported tubular cells were fabricated from indium-doped barium zirconate as the anode and electrolyte. We used electrochemical impedance spectroscopy measurements to characterize the total resistance of the cell as components from the ohmic and the polarization resistance. We also measured the Ni concentration in the electrolyte layer by secondary ion mass spectrometry (SIMS).

To clarify the important factors determining the low proton conductivity of the indium doped barium zirconate containing Ni, we measured proton conductivity and proton concentration by impedance analysis and thermogravimetric analysis. The solubility limit of the NiO and the effects of added Ni on the proton transport properties and defect structure were examined for indium doped barium zirconate.

Section snippets

Material preparation

The BaZr0.8In0.2O3−δ (BZI20) was prepared by a glycine-nitrate combustion synthesis [11] to obtain a dense pellet. First, Ba(NO3)2 (99.9%), In(NO3)3·2.8H2O (99.9%), ZrO(NO3)2·2.3H2O (99.9%), distilled water, citric acid, and EDTA were mixed, and the pH was adjusted to 10 by addition of aqueous ammonia. The mixture was heated to 573 K. The obtained precursor was then calcined in air for 10 h at 1173 K. The powder was milled and pressed into a pellet at 250 MPa by cold isostatic pressing (CIP).

Results and discussion

Fig. 1(a) shows the I–V and I–P characteristics and impedance spectra for the tubular cells using BZI20 at 600 °C. The maximum power density was 0.143 Wcm−2 and the OCV was 1.01 V. The power density and OCV were higher than the values reported by Sun et al. [20]. Fig. 1(b) shows the impedance spectra under open circuit condition. As shown in this impedance spectra, the ohmic resistance RΩ was 1.26 Ω cm2 and the polarization resistance was 0.49 Ω cm2. The ohmic resistance includes the resistive

Conclusions

We fabricated a tubular cell with BaZr0.8In0.2O3−δ as an electrolyte and evaluated its fuel cell characteristics. The maximum power density of the cell was 0.143 Wcm−2 and the ohmic resistance of the electrolyte was 0.91 Ωcm2. The estimated conductivity was 1.1 × 10−3 Scm−1. The average dissolution of NiO content N in the electrolyte was determined by SIMS to be N = 0.015. Conversely, a sample in which NiO was intentionally dissolved in BZI20 had a conductivity of approximately 1.3 × 10−3 S cm−1

Acknowledgements

A part of the results in this paper was obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

References (30)

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