Charge and Mass Transfer on a La2O3 Added Li/Na or Li/K Molten Carbonate Meniscus Electrode of LaNiO3 Coated Au Ring for Oxygen Reduction Reaction

Introduction

The LaNiO₃ is an alternative cathode material of NiO for molten carbonate fuel cells (MCFCs) because of extremely higher stability than NiO in molten carbonates.¹ In our previous study, the activity of the LaNiO₃ was not less than that of NiO for the oxygen reduction reaction (ORR) on a meniscus electrode.²However, the activity should separate charge transfer and mass transfer enough.

In this study, the charge and mass transfer of the ORR on LaNiO₃ and NiO have been evaluated by the potential step method (PSM) and the steady method (SM) on a meniscus electrode to discuss the ORR activity and mechanism of the LaNiO₃for MCFC application.

Theoretical background

The apparent exchange current density: i₀, and the cathodic symmetry factor: a_c were determined by mass transfer free polarization curves with the Allen-Hickling plot of following the Butler-Volmer equation. The dependence of i₀on gas composition is as follows

i₀ = i₀⁰p_O2^ap_CO2^b                                                                      (1)

where i₀⁰, a, and b were the standard exchange current density and the apparent reaction orders for O₂ and CO₂, respectively.

The limiting current density: i_lc, was determined with the polarization curves with mass transfer resistance and the i₀from the Butler-Volmer equation as follows.

h = {1/(Rf i₀)+1/i_lc}iRT/(nF)                                             (2)

where Rf and i were roughness factor and current density per geometric area. The dependence of i_lcon gas composition is as follows

i_lc = i_lc⁰p_O2^a’p_CO2^b’                                                                (3)

where i_lc⁰, a’, b’ were the standard limiting current density, and the apparent limiting current order for O₂ and CO₂, respectively.

Experimental

Li/Na (=52/48 mol%) or Li/K (=62/38 mol%) molten carbonate saturated La₂O₃ (1.5mol%) were used as an electrolyte. The working electrode was the LaNiO₃or NiO coated Au ring whose height was controlled by a micrometer.  Reference electrode was a reversible oxygen electrode (ROE), and counter electrode was a gold coil electrode.

The LaNiO₃ powder was prepared by the thermal decomposition of the mixture of lanthanum nitrate hexahydrate and nickel acetate tetrahydrate at 1073 K for 12 h in air.³ The LaNiO₃ powder was sintered on a gold ring at 1273 K for 48 h while NiO (wako, 99.9%) was sintered at 1073 K for 12h.  Specific surface area of the electrode was evaluated with the BET specific surface area of the LaNiO₃or NiO powder after sintering at each condition.

The ORR was determined in ambient pressure with oxygen partial pressure (p_O2) range from 0.02 to 0.9 atm, and carbon dioxide pressure (p_CO2) range from 0.001 to 0.9 atm in an argon-balance.  The temperature was at 923 K. The chronoamperometry (CA) from 10 to -10 mV vs. ROE was performed for 600s. The meniscus height was h=2mm of the PSM for mass transfer free polarization. The initial current density was evaluated by extrapolation to t=0 with the linear relation between current and the square root of time. The meniscus height was h=4mm of the SM with mass transfer resistance. The current density was evaluated by the average value for 500~600 s of CA. 

Results and discussion

The i₀ and as a function of the p_O2 or p_CO2 on LaNiO₃ or NiO electrode in Li/Na or Li/K molten carbonate were shown in Fig. 1.  The i₀ on LaNiO₃ electrode was not less than that on NiO electrode in both electrolytes. The a, and b were 0.39 ± 0.08 and -0.18 ± 0.04 for both the electrolytes and the electrodes, respectively. Since the percabonate path mechanism (PCP) whose a and b, were 0.5 and 0 while those of the peroxide path mechanism (POP) were 0.5 and -1.0, the reaction on LaNiO₃for the ORR might be mixed mechanism of POP and PCP.

 The i_lc and as a function of the p_O2 or p_CO2 on LaNiO₃ or NiO electrode in Li/Na or Li/K molten carbonate were shown in Fig. 2. The i_lc in Li/K molten carbonate was higher than that in Li/Na molten carbonate. O₂ solubility in Li/K molten carbonate was twice as much as that in Li/Na molten carbonate.⁴ Therefore, the i_lcwhich is flux that passed through the meniscus for Li/K would be larger than that for Li/Na.

Reference

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

2.  K. Matsuzawa, Y. Esaki, Y. Takeuchi, K. Watanabe, Y. Kohno, K. Ota and S. Mitsushima, Proc. 4^th Asian Conf. Molten Salt Chem. Tech., p.285 (2012).

3.  T. Noda, K. Komaki, and T. Kawasaki, Panasonic Tech. J., 55, 108 (2009) [in Japanese].

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