Yuki Takeuchi, Koichi Matsuzawa, Yuji Kohno, and Shigenori Mitsushima

Transcription

1 .49/644.65ecst The Electrochemical Society Charge and Mass Transfer on a La O Added Li/Na or Li/K Molten Carbonate Meniscus Electrode of LaNiO Coated Au Ring for Oxygen Reduction Reaction. Yuki Takeuchi, Koichi Matsuzawa, Yuji Kohno, and Shigenori Mitsushima Green Hydrogen Research Center, Yokohama National University 79-5, Tokiwadai, Hodogaya-ku, Yokohama 4-85, Japan LaNiO is an alternative cathode material of NiO for molten carbonate fuel cells (MCFCs) because of extremely higher stability than NiO in La saturated molten carbonates. In this study, the charge transfer on LaNiO and mass transfer resistance through La saturated molten carbonates for the ORR have been evaluated by the combination of a potential step method and steady state method on a meniscus electrode to discuss the ORR activity and mechanism of the LaNiO in Li/Na and Li/K carbonate eutectic with saturated La. The activity of ORR on LaNiO electrode was not less than that on NiO electrode and the rate-determination step would be competition of peroxide (POP) and percarbonate path (PCP) at meniscus state, and mass transfer resistance controlled over all reaction rate for steady state. In addition, Li/K system had higher ORR rate than Li/Na system because of large solubility of O under ambient pressure. Introduction Molten carbonate fuel cell (MCFC) is one of the most commercialized fuel cells as a dispersed power system in all over the world (, ). The MCFC system can use various fuels without precious metal, and has higher energy conversion efficiency compared with room temperature fuel cells such as polymer electrolyte fuel cell (PEFC) systems (). Pressurized MCFC system has higher efficiency than ambient pressure one, but it has a serious problem in lifetime caused by dissolution of NiO cathode into the electrolyte and the internal short-circuit of Ni (Ni shorting) (4). 65

2 In our previous study, the LaNiO had extremely higher stability than NiO in molten carbonates (5, 6). Though we had also studied fundamental research of LaNiO for the oxygen reduction reaction (ORR) (7, 8), its activity and mechanism for the ORR are not still clarified in detail. In addition, our previous analysis for the ORR would not separate charge transfer and mass transfer enough in both Li/Na and Li/K carbonate eutectic. In this study, the charge transfer and mass transfer of the ORR on LaNiO and NiO have been evaluated by a potential step method (PS) and a steady state method (SS) on a meniscus electrode to discuss the kinetics of ORR and mass transfer of oxygen for the LaNiO cathode. Theoretical background The apparent exchange current density: i was determined by mass transfer free polarization curves of Butler-Volmer equation. i / R f = i [exp{(- c )nf / (RT )}- exp(- c nf /RT )] [] where i, R f, n, F,, R, and T were current density per geometric area, roughness factor, number of electron transfer of apparent rate determining step, Faraday s constant, overpotential, gas constant, and temperature, respectively. The cathodic symmetry factor: c was determined by the Allen-Hickling plot. The dependence of i on gas composition is as follows (9): i = i p O a p CO b [] where i, a, and b were the standard exchange current density and the apparent reaction orders for O and CO, respectively. The reaction orders after accounting for the Nernst effect were calculated by the following expression (9). { log (i ) / log (p O )} T,E = a+ c /4 [] { log (i ) / log (p CO )} T,E = b+ c / [4] The limiting current density: i lc, was determined by the polarization curves with mass transfer resistance and the i from the Butler-Volmer equation as follows: 66

3 i / R f = i [exp{(- c )nf / (RT )}-(-i /i lc )exp(- c nf / RT )] [5] The dependence of i lc on gas composition is as follows: i lc = i lc p O a p CO b [6] where i lc, a, b were the standard limiting current density, and the apparent limiting current order for O and CO, respectively. Experimental Preparation of LaNiO and NiO electrodes The LaNiO powder was prepared by the thermal decomposition method with : of the molar ratio of lanthanum nitrate hexahydrate (99.9%, Wako Co.,) and nickel acetate tetrahydrate (98.%, Junsei Chemical Co.,) (). A lanthanum precursor was dried at 4 K for h, then dissolved in -methoxyethanol (99.%, Wako Co.,). A nickel precursor was dried at 47 K for h in air, then dissolved in the mixture of -methoxyethanol (99.%,ibid) and -aminoethanol (99.%,ibid). Both precursor solutions were stirred at 4 K for h, respectively, then, they were mixed and stirred at 4 K for h. The solution was dried to obtain precursor, and it was sintered at 7 K for h in air. Finally, the LaNiO powder was formed. The LaNiO ink, which was a mixture of LaNiO powder, polyvinyl alcohol (96. mol%, Wako Co.,), and deionized water, was sintered on a gold ring under two conditions at 7 K for 48 h for LaNiO (), or at 7 K for h for LaNiO () in air. On the other hand, NiO/Au ring electrode was prepared with the same procedure using NiO powder (99.97%, high purity chemicals) and the sintering condition at 7 K for h in air. Specific surface area of the electrode was evaluated with the BET specific surface area of the sintered LaNiO or NiO powder simulated the sintering condition on the gold ring. Electrochemical measurements Li/Na (=5/48 mol%) and Li/K (=6/8 mol%) molten carbonates saturated La O (.5mol%) were used as electrolytes. The working electrode was the LaNiO or NiO coated Au ring whose height was controlled by a micrometer. Reference electrode was a 67

4 reversible oxygen electrode (ROE), and counter electrode was a gold coil electrode. The ORR was determined in ambient pressure with oxygen partial pressure (p O ) in the range of. to.9 atm, and carbon dioxide pressure (p CO ) in the range of. to.9 atm with an argon-balance. The temperature was varied from 8 to K. The chronoamperometry (CA) was performed for 6 s at to - mv vs. ROE. The PS with the mm of meniscus height: h was for determination of the mass transfer free polarization. The initial current density was evaluated by extrapolation to t = with the linear relation between current and the square root of time. The SS with the 4 mm of h was for determination of mass transfer resistance. The current density was used as the average value in 5~6 s of CA. Results and discussion Material Characterization before tests Figure shows the XRD patterns of the LaNiO () and the LaNiO (). Both of them were rhombohedral LaNiO. Figure shows the SEM images of the samples. The NiO and LaNiO () were almost same particle size while LaNiO () was almost half of them. 68

5 Table shows the BET specific surface area and roughness factor of the samples. The NiO and the LaNiO () were almost the same BET specific surface area and roughness factor while LaNiO () was almost as twice as the others. These results correspond to the SEM images. TABLE I. BET specific surface area and roughness factor of the samples. Samples NiO LaNiO () LaNiO () BET specific surface area / m g Roughness factor / Electrochemical behavior Figure and 4 show polarization curves of LaNiO () and LaNiO () electrodes in Li/Na molten carbonate at 9 K under p O / p CO =. /. atm by PS and SS. The PS and SS respect real and geometrical surface area, respectively. Therefore, the current with the PS was almost times with the SS in the same surface area. The current density per real surface area of LaNiO () was almost as same as that of LaNiO (). This result with the PS should indicate the current was kinetic control which was free from mass transfer effect. On the other hand, as shown in Figure 4, current density per geometric area of LaNiO () was almost as same as that of LaNiO (). The mass transfer through the thin film of molten carbonate should controlled the steady state current for the SS. Figure 5 shows Arrhenius plots of the i of LaNiO and NiO electrodes in Li/Na and Li/K molten carbonates. Table shows the apparent activation energy: E app form the slope of Figure 5. The E app of LaNiO () was slightly larger than that of NiO in both Li/Na and Li/K molten carbonates, and the E app s of two electrodes were almost as same as in both Li/Na and Li/K molten carbonates. 69

6 i / A cm - -real i / A cm - -geo LaNiO () LaNiO () / mv Figure. polarization curves of each LaNiO electrode in Li/Na molten carbonate at 9 K under p O /p CO =./.atm by PS LaNiO () LaNiO () / mv Figure 4. polarization curves of each LaNiO electrode in Li/Na molten carbonate at 9 K under p O /p CO =./.atm by SS. Figure 6 shows the Allen-Hickling plot of LaNiO and NiO electrodes in Li/Na and Li/K molten carbonates at 9 K. The cathodic symmetry factor: c s which were calculated from the slopes, were ca..4 for all experiments. 64

7 T / K 9 8 log(i / A cm - -real) LaNiO ()_Li/Na TABLE Ⅱ T - / K - Figure 5. Arrhenius plots of i under p O / p CO =./. atm by PS. Apparent activation energy: Eapp of ORR of each sample in each electrolyte. Samples_Electrolytes LaNiO ()_Li/Na E app / kj mol log [i {exp(nf R - T - )-} - ] / A cm - -real 4 LaNiO ()_Li/Na / V Figure 6. Allen-Hickling plots at 9 K under p O /p CO =./.atm by PS. The i s as a function of the p O and p CO on LaNiO and NiO electrodes in Li/Na and Li/K molten carbonates were shown in Figure 7(a) and 7(b). The i on LaNiO electrode was not less than that on NiO electrode in both electrolytes. The slopes for 64

8 p O and p CO were.9.8 and in both electrolytes on both electrodes, respectively. log (i / A cm - -real) The ORR mechanisms in molten carbonates have been proposed mainly in three paths that are peroxide path (POP), superoxide path (SOP), and percarbonate path (PCP) mechanisms. Each mechanism of POP, SOP, or PCP was expressed as follows (9): LaNiO ()_Li/Na - - log(p O / atm) (a) Dependence of i on p O at p CO of. atm. log (i / A cm - -real) LaNiO ()_Li/Na log(p / atm) CO (b) Dependence of i on p CO at p O of. atm. Figure 7. Dependence of i on p O or p CO on LaNiO and NiO electrodes in Li/Na and Li/K molten carbonates at p CO or p O of. atm at 9 K. Peroxide path mechanism (POP): - /O + CO O - + CO [7a] O - + e - O - + O - (r.d.s) [7b] O - + CO + e - - CO [7c] O CO CO [7d] Superoxide path mechanism (SOP): - /4O + /CO O - + /CO [8a] O - + e - O - (r.d.s) [8b] O - + e - O - + O [8c] O - + CO + e - - CO [8d] O CO CO [8e] Percarbonate path mechanism (PCP): 64

9 - - /O + CO CO 4 CO e - CO - + O - (r.d.s) O - + e - O - O CO CO [9a] [9b] [9c] [9d] If the cathodic symmetry factor: c is.5, the apparent exchange current density: i of POP, SOP and PCP were expected from eqs. [], [] and [4] as follows: POP: i (POP) = i (POP) p O.75 p CO -.5 SOP: i (SOP) = i (SOP) p O.65 p CO -.75 PCP: i (PCP) = i (PCP) p O.75 p CO -.5 [] [] [] The value of c evaluated from Allen-Hickling plots was.4, the a and b of reaction order in the eqs. [4] and [5] were..8 and -.5.4, respectively. So the reaction on LaNiO for the ORR might be competed mechanism of POP and PCP. The i lc as a function of the p O and p CO on LaNiO and NiO electrodes in Li/Na and Li/K molten carbonates were shown in Figure 8(a) and 8(b). The i lc on LaNiO and NiO electrodes were almost same in the same molten carbonate while that in Li/K molten carbonate was larger than that in Li/Na molten carbonate at any p O or p CO. The i lc depended on the property of molten salts, and was independent from electrode materials. O solubility in Li/K molten carbonate is larger than that in Li/Na molten carbonate (). Therefore, the i lc which is flux that passed through the meniscus for Li/K carbonate would be larger than that for Li/Na carbonate. log (i lc / A cm - -geo) LaNiO ()_Li/Na - - log(p O / atm) (a) Dependence of i lc on p O at p CO of. atm. log (i lc / A cm - -geo) LaNiO ()_Li/Na log(p / atm) CO (b) Dependence of i lc on p CO at p O of. atm. Figure 8. Dependence of i lc on p O or p CO on LaNiO and NiO electrodes in Li/Na and Li/K molten carbonates at p CO or p O of. atm at 9 K. 64

10 Conclusions In order to develop the alternative cathode material for MCFC, we focused on the ORR activity of LaNiO, which is very stable material in La saturated alkaline molten carbonates. The activity of ORR on LaNiO electrode was not less than that on NiO electrode in both electrolytes. The rate-determination step of the ORR would be completion of the POP and the PCP at meniscus state, and mass transfer resistance controlled overall reaction rate for steady state. In addition, Li/K system had higher ORR rate than Li/Na system because of large solubility of O under ambient pressure operation. Therefore, the LaNiO should have potential for new alternative cathode material for MCFC. References. A. Kulkarni and S. Giddey, J. Solid State Electrochem., 6, ().. The Operating Agents and the Secretariat of the Executive Committee, Annual Report of IEA Advanced Fuel Cells, pp.5 ().. A. J. Appleby, J. Power Sources, 58, 5 (996). 4. Y. Mugikura, T. Abe, S. Yoshioka and H. Urushibata, J. Electrochem. Soc., 4, 97 (995). 5. K. Matsuzawa, Y. Akinaga, S. Mitsushima and K. Ota, J. Power Sources, 96, 57 (). 6. K. Matsuzawa, S. Zhai, K. Ota and S. Mitsushima, Chem. Lett., 4, 87 (). 7. K. Matsuzawa, K. Watanabe, S. Mitsushima and K. Ota, ECS Trans., (7), 449 (). 8. K. Matsuzawa, Y. Esaki, Y. Takeuchi, K. Watanabe, Y. Kohno, K. Ota and S. Mitsushima, Proc. 4th Asian Conf. Molten Salt Chem. Tech., p.85 (). 9. S.H. Lu and J.R. Selman, J. Electroanal. Chem.,, 57 (99).. T. Noda, K. Komaki and T. Kawasaki, Panasonic Tech. J., 55, 8 (9) [in Japanese].. T. Nishina, Y. Matsuda and I. Uchida, Molten Salt Chemistry and Technology 99 (Eds. M-L. Saboungi and H. Kojima),PV 9-9, p.44, The Electrochemical Society Proceedings Series, Pennington, NJ (994). 644