Determination of Charge and Mass Transfer for Molten Carbonate Fuel Cell Cathode Using NiO Coated Ring Electrode

Introduction

Molten carbonate fuel cell (MCFC) systems have very high energy conversion for distributed generation systems, and the largest electric power generation capacity in stationary fuel cell systems. In order to improve energy conversion efficiency and durability, cathode and electrolyte materials are key factors. We found that the solubility of LaNiO₃ in the La₂O₃ saturated molten alkaline metal carbonates is significantly smaller than that of NiO in the La₂O₃ free molten carbonates¹⁾, and should know the kinetics on the LaNiO₃ in the carbonates for oxygen reduction reaction (ORR). However, even the kinetics of the ORR on the NiO is not clarified enough²⁾. Furthermore, the performance of gas diffusion electrode for practical system should be affected by both charge and mass transfer.

In this study, charge transfer on a sintered NiO electrode and mass transfer through a meniscus on the electrode have been investigated with chronoamperometry using a meniscus ring electrode to establish electrochemical method for the evaluation of charge and mass transfer separately.

Experimental

Li/Na (=52/48 mol %) or Li/K (=62/38 mol %) molten carbonate were used as an electrolyte. The working electrode was a sintered NiO on a gold ring electrode. The height of the working electrode from surface of molten carbonate was controlled by a micrometer. Reference electrode was a reversible oxygen electrode (ROE), and counter electrode was a gold coil electrode. The surface area of the sintered NiO was BET method.

The oxygen reduction reaction current was determined with chronoamperometry (CA) from 10 to -10 mV vs. ROE. The initial current density was evaluated by extrapolation to t = 0 with the linear relation between current and the square root of time to determine mass transfer free polarization at 2mm of the height. The steady state current density was evaluated for 100 s after 500 s of CA at 4mm of the height with mass transfer resistance of the meniscus.

Symmetry factor: a was determined with Allen-Hickling equation for the mass transfer free polarization curve, and the exchange current density: i₀ was determined with mass transfer free Butler-Volmer equation, the a and polarization curve. The limiting current density: i_l was determined with Butler-Volmer equation with the i_l, i₀, a, the steady state polarization curve.

Results and discussion

Figure 1 shows the i₀ (Fig. 1-a)) and the i_lc (Fig. 1-b)) as a function of CO₂ or O₂ partial pressure in CO₂-O₂-Ar. The CO₂ or O₂ partial pressure was fixed at 0.1 atm for p_O2 or p_O2 function, respectively. The i₀ the i_lc were determined at 2 mm height to minimize the influence of ohmic drop of the electrolyte and at 4 mm height to minimize mass transfer resistance of reaction gas through the meniscus. Here, the i₀ and the i_lc are normalized the BET surface area and geometrical surface area, respectively. The BET surface area of the sintered NiO was 2.5 m²g^-1. Therefore, the apparent exchange current density per geometrical surface area was 1000 times larger than the limiting current density. The dependence of the i₀s on the partial pressures were smaller than that of the i_lcs. All slopes could not explained by a simple reaction mechanism model of peroxide, superoxide, or parcarbonate path²⁾. The polarization of practical electrode depends on liquid – gas interface surface area rather than real surface area of the electrode³⁾. Therefore, performance of the practical electrode is controlled by the combination of the charge and mass transfer, and the i₀ and the i_lc, which is a function of gas solubility, diffusion coefficient, and morphology of molten carbonate film, should be an effective index to determine activity of electrode and property of electrolyte.

References

1)    K. Matsuzawa, Y. Akinaga, S. Mitsushima, and K. Ota, J. Power Sources, 196, 5007 (2011).

2)    T. Nishina, Y. Masuda and I. Uchida, Molten Salt Chem., 93, 424 (1993).

S. Kuroe, M. Takeuchi, S. Nishimura, K. Ohtsuka, DENKIKAGAKU, 58, 928 (1990).