Ohmic Resistance Response of a Practical Protonic Ceramic Fuel Cell upon Hydration/Dehydration
Authors: Murakami Takeru , Li Kunpeng , Araki Takuto , Mori Masashi , Kobayashi Shun 
Published: December 2024

Protonic Ceramic Fuel Cells (PCFCs) can show high performance, power density and efficiency at an intermediate temperature range from 400°C to 600°C. Proton-conducting ceramics is reported to have multiple mobile defects such as proton, hole, and oxygen vacancy. Conductivity of those defect species depends on the concentration of the atmosphere gas such as hydrogen, oxygen and water vapor. To date, the effect of hydration/dehydration on the proton conductivity as a single electrolyte material has been studied1,2. However, as a whole cell assembled with electrolyte and electrode, there are few reports on that effect. When using such a whole cell to measure the hydration/dehydration effect, practical gas for a real operation can be supplied. This study focuses on the transition characteristics with hydration/dehydration in practical gas condition by using a BaZrO3-based perovskite cell.
Firstly, to assess the time delay of hydration/dehydration reactions at the electrolyte-electrode interface, experiments were conducted using the cells with different electrolyte thicknesses, 6μm, 13μm and 24μm at open-circuit voltage. Figure1(a) shows the results. Time constant increased with thicker electrolyte. Despite the difference in electrolyte thicknesses, almost the same Fourier numbers were calculated by using each of the response time point crossing 63% from one steady state to another. This result suggests that the reaction rate at the interface was fast enough compared to the transport rate within the bulk of electrolyte.
Secondly, transport within the electrolyte was considered. Incorporation of water vapor into the electrolyte is considered to follow next equilibrium:
H2O+Vo
··+OO
×⇔OHO
· (1)
When the water vapor concentration in supplied gas is switched, a pair of proton and oxygen, described as OHO
· in equation (1), is generated into the electrolyte. Protons detached from OHO
· can be transported by hopping to the neighboring oxygen atom1. Protons could be also transported via vehicle mechanism, as a pair of proton and oxygen. Diffusion coefficient for the measured curve was determined from the solid curve in Figure1(a), which was fitted by using Fick's law. Fitted diffusion coefficient was 4.5×10-13 m2/s at 600 °C. This diffusion coefficient is significantly smaller than reported diffusion coefficients of proton and hole3. In this cited paper, those diffusion coefficients are determined through conductivity measurement. Protons could mainly be transported via hopping mechanism in a fixed atmosphere gas, suggesting that the reported value for proton was related to hopping mechanism. Holes are considered to be similar with electrons, suggesting high diffusivity. Therefore, the fitted value in our research could be for the transport via vehicle mechanism, potentially influencing the determination of the relaxation time by limiting the proton transport. For further investigation of the relaxation characteristics, experiments will be conducted at different external current densities providing varying potential gradients within the electrolyte.

Acknowledgements

This presentation is the result of the research and development of ultra-efficient proton-conducting ceramic fuel cell devices, JPNP20003, funded by the New Energy and Industrial Technology Development Organization (NEDO), Japan. We would like to express our gratitude to all parties involved.

Reference

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Kwati L., et al., J Mater Chem A, 6 (39) (2018)

Figure 1