The mechanism of surface modification of a MCFC anode

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Journal of Electroanalytical Chemistry 441 (1998) 65-68

The mechanism of surface modification of a MCFC anode Baizeng Fang *, Xinyu Liu, Xindong Wang, Shuzhen Duan Department of Physical Chemist~, University of Science and Technology, 30 Xueyuan Road, Beijing 100083, People's Republic of China Received 3 February 1997; received in revised form 20 March 1997

Abstract In this paper the anodic polarization performance of a pure nickel electrode and a nickel electrode modified by electrodeposition of niobium have been investigated. It is found that the anodic polarization characteristics of the nickel electrode is improved significantly by the deposition of niobium. The improvement is partially attributed to the increase in surface area of the nickel electrode and the increase in electrocatalytic activity, but is caused mainly by improvement in the wettability of the electrode. © 1998 Elsevier Science S.A. Keywords: Surface modification; Electrodeposition; MCFC; Electrode

1. Introduction The molten carbonate fuel cell (MCFC) is believed to be one of the most promising new energy conversion devices that converts chemical energy in fossil fuels into electricity. It is a highly efficient and environmentally clean source of power generation [1-5]. Many organizations worldwide are actively pursuing the development of this technology. Porous nickel has been employed almost exclusively as the anode and cathode materials in the recent development of the MCFC [6,7]. Its performance is relatively satisfactory, but still needs sc,me improvement. For example, its corrosion resistance, electrocatalytic activity, sintering resistance, creep resistance, etc. must be increased. Many efforts have been made to develop a new anode material that is more stable than porous nickel. To date, N i + C r , N i + C o , or N i + C u alloys have been considered as possible anode materials [8]. Nickel + niobium alloys have been shown to have the properties required of an insoluble anode [9] as well as excellent electrocatalytic activity [10]. In this study, electrocatalytic activity of nickel + niobium to oxidation of CO and the modification mechanism of the nickel electrode were investigated.

2. Experimental The preparation of a nickel + niobium surface alloy was carried out by electrochemical reducticn of niobium(IV)

* Corresponding author. 0022-0728/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 2 2 - 0 7 2 8 ( 9 7 ) 0 0 2 0 2 - 7

ions on a nickel cathode in molten fluorides. A eutectic mixture of 50 mol% NaF + 50 mol% LiF was used as a solvent for electrochemical alloying of nickel with niobium at 750°C. Melt purification was performed as following: the bath was initially dehydrated by heating under a vacuum (1 Pa) for 24 h at 500°C, then melted under an argon atmosphere at 750°C. Niobium(IV) ions were generated in situ by addition of potassium heptafluoroniobiate(V) and metallic niobium to the bath. The following reaction occurs 4Nb v + Nb = 5Nb w

(1)

When an excess of metallic niobium was added to the bath, the equilibrium was shifted towards the right and a valence less than 4.2 of niobium ions could be expected [11]. The performance tests of nickel and nickel + niobium alloy were carried out in a melt of (62 + 38) mol% (Li + K)CO 3, which is mostly used as the electrolyte for MCFC, at 650°C. The potential of the working electrode was controlled by a Wenking Model LB 81M potentiostat.

3. Results and discussion

3.1. Preparation of nickel + niobium surface alloy Niobium is more active than nickel in fluoride melt, therefore when the two metals were immersed in the bath provided with a electrical connection, a galvanic cell occurred in which the reaction at the nickel cathode is the

B. Fang et al. / Journal of Electroanalytic~l Chemistr3' 441 (1998) 65-68

66

formation of the alloy Ni~Nb. According to the X-ray microprobe analysis, the composition of the surface alloy layer corresponds to Ni3Nb compound. 3.2. Electrocatalytic activity of nickel + niobium surface alloy In MCFC technology electrocatalytic activity for a electrode material is as important as its corrosion resistance. CO and H~ have been employed almost exclusively as the fuel gas for MCFC at present, and at the negative electrode (anode), the fuel gas is oxidized, which may be written as H 2 + CO~-

~

H20

(2)

+ CO 2 + 2e-

C O + C O 2- ~ 2CO 2 + 2 e -

(3)

The anode functions as the catalyst for the electrode reaction (Eqs. (2) and (3)). In this study, CO was selected as fuel gas to examine the electrocatalytic activity of anodic materials. Fig. 1 shows anodic polarization curves obtained at different electrode materials. The initial sweep potential of the polarization curves was selected to be - 3 5 0 mV (with respect to a silver wire reference), which is the corrosion potential of nickel in (Lio.62, K0.38)2CO 3 melt at 650°C (experimental value). According to the standard electrode', potential with respect to the standard 0 2 / 0 2- potential [12], nickel is corroded to nickel oxide at a potential of - 0 . 8 0 2 V, and CO is oxidized at - 1.047 V, that is, the oxidation potential of CO is 0.245 V more negative r.han that of nickel. Therefore, the following information can be gained from Fig. 1. At a potential of before - 100 mV (with respect to the silver reference), the corrosion potential of the nickel + niobium alloy (experimental value), the anode polarization current observed at the nickel electrode consisted of two parts, the oxidation currents of CO and nickel. In the case of nickel-niobium alloy electrode the polarization current was due completely to the contribution of oxidation of CO. It is evident that the oxidation current of CO obtained at nickel + niobium was larger than that total polarization current at the nickel electrode. Thus, it can be

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3.3. The mechanism of surface modification of a MCFC anode

From the results above, it can be found that the anodic polarization performance of the electrode modified by niobium is improved significantly compared with that of the unmodified nickel electrode. The improvement may be attributed to: (1) The increase in electrode surface area caused by the deposition of niobium; (2) the increase in catalytic activity of the electrode; (3) the increase in electrode reaction area due to the improvement of the wettability of the electrode. These possibilities are discussed as following. 3.3.1. Increase in electrode surface area caused by the deposition of niobium Due to the electrodeposition of niobium, the surface of the nickel electrode increased. In order to simplify the calculation, two assumptions are made: The deposition of niobium produces semi-spherical grains with a radius of r and these grains are distributed over the entire surface of the electrode. Let the surface area of the electrode before and after the deposition be SNi and SNi Nb" Then SNi_Nb=(SNi--UTrr2) + U Z ~ r 2 = SNi + UTrr 2

- r 50

-50

50

E./mV Fig. 1. Polarization curves obtained at different materials in (Li0.62,Ko.3s)2CO 3 melt at 650°C under a CO atmosphere, sweeprate = 20 mV rain +l , (a) Ni, (b) N i + N b .

(4)

where, N is the number of grains. Since SNi ~_~N ' t r r 2

(5)

the following relation is obtained: SNi < SNi_Nb < 2SNi

(6)

Thus, it can be seen that the electrode surface area increases after the deposition of niobium. 3.3.2. Increase in catalytic activity of the electrode Nickel + niobium alloys have been shown to have the property of excellent electrocatalytic activity [10]. It is plausible that the modified nickel electrode possesses more active characteristics compared with the pure nickel electrode. For an electrode, the anodic current can be expressed as following I = knFaese-E/RT(e O- ~),F~/RT __ e-~,F~/RT)

5

-350

seen that as a catalyst for the oxidation of CO nickel + niobium alloy is more effective than nickel.

(7)

where, n, F, R, T and c~ have their usual significance, k, A, c.,, E and rl are the frequency factor, the surface area of the electrode, the concentration, the activation energy and anodic polarization potential, respectively [ 13]. Eq. (7) can also be written in the form: ln[ I / ( e "F~/RT - 1)] = In(knFac~) - ( cenFrl + E ) / R T

(8)

B. Fang e; al. / Journal of Electroanalytical Chemist~ 441 (1998) 65-68 -9

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Fig. 2, Relation between anodic polarization current and temperature for nickel and a nickel + niobium alloy electrode.

Fig. 4. Relation between the current and the height of the electrode above the electrolyte level.

Supposing that ot does not change even after deposition of niobium, the value of the activation energy, E, can be estimated from the slope -(o~nFr I + E ) / R of the I n [ I / (e nF~I/RT- 1 ) ] - l I T plot. Fig. 2 shows the relations between aaodic polarization current and temperature for different malerials. From Fig. 2 the slopes for the nickel electrode and nickel + niobium are - 4 × 10 3 and -'.2.67 × 10 3 K, respectively. From the slopes, and by assuming a = 0.5, the activation energy E is calculated to be 52.5 and 41.5 kJ tool-l, respectively. The activation energy, E, decreases slightly with the deposition of niobium. I¢: indicates that the deposition of niobium indeed caused tlTe increase of the catalytic activity of the electrode by a factor of about 1.27.

appreciably to the electrode reaction, for reaction gases diffuse through this thin electrolyte film easily and reach the electrode surface quickly. Thus, if the height of the electrode above the electrolyte surface level h is less than H, the total heights of the two menisci, the formation of such meniscuses will be disturbed, then the reaction area will decrease, and so will the Faradaic current. The anodic current I is a function of the height h of the electrode above the electrolyte surface level and the total heights H of the two menisci can be estimated from the breakpoint of the I - h plot. Fig. 4 shows the l - h plots of the different electrode materials. From Fig. 4, the heights of menisci of the modified nickel electrode and the unmodified one are 3 and 0 ram, respectively. When an electrode is completed immersed, i.e. H = 0, the reaction area is only that part of surface area which contacts directly with the bulk electrolyte. The anodic polarization current is proportional to this surface area. From Fig. 4, it can be found that the anodic polarization current for the modified nickel electrode is higher than that for pure nickel by a factor of 1.4. This means that the deposition of niobium caused the increase in surface area of the nickel electrode, and the increase factor is about 1.4. In the case of partial immersion, i.e., H > 0, the reaction area consists of two parts: one is the same part of the surface area which contacts directly with the bulk electrolyte (as described above), the other is the surface area which is above the electrolyte level covered by a thin electrolyte film due to the formation of the meniscus and the supermeniscus. The contribution of this part to the anodic polarization current will be much larger than that of surface area which contacts directly with the bulk electrolyte if these two surface areas are equal. This explains the fact why the increase in the reaction surface area due to the formation of the menisci of 3 mm is only about 1.5 times, but the increase in the anodic polarization current is about 2.1 times. Therefore, the most important factor for the increase in the anodic polarization current is the increase in the reaction surface area due to the formation of the meniscus and the supermeniscus, that is, the improvement in wettability of the electrode.

3.3.3. Increase in electrode reaction area When a wettable electrode is partially immersed in an electrolyte, a meniscus and supermeni:;cus film can be formed on the part of electrode above the electrolyte surface level [14,15]. The schematic of a wettable electrode immersed in the electrolyte is shown in Fig. 3. The pure nickel anode has a wetting angle of approximately 45 ° and therefore cannot be wetted well [7]. Its wettability can be improved by addition of proper alloying elements [16]. The electrode surface in contact with the meniscus and the supermeniscus if it exists, will contribute Ni wire Alumina tube

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B. Fang et al. / Joltrnal of Electroanalytical Chemistry 441 (1998) 65-68

4. Conclusions The anodic polarization performance: of the nickel electrode can be improved by electrodeposition of niobium. The improvement of anodic polarization characteristics is partially attributed to the increase in electrode surface area and the increase in catalytic activity of the electrode, but is mainly due to the improvement in the, wettability of the electrode.

Acknowledgements This work was supported by Natio~al Natural Science Foundation, and Corrosion-Erosion and Surface Technology Laboratory of the Ministry of Metallurgy Industry of China.

References [I] L. Blomen, M. Mugerwa (Eds,), Fuel Cell System, Plenum, New York, 1993.

[2] J. Appleby, F. Folkes. Fuel Cell Handbook, Van Nostrand-Reinhold, New York, 1990. [3] J.R. Selman, T.D. Claar (Eds.), Molten Carbonate Fuel Cell Technology, Pennington, New Jork, 1982. [4] K. Kinoshita, F.R. Mc Larnon, E.J. Cairns, Fuel Cell, A Handbook (DOE/METC-88/6096. 1988) Ch. 4. [5] K.I. Ota, B.T. Kim, H.Y. Take, N. Kamiya, High Temperature Corrosion of Metals with Molten Carbonate. Proc. of the Fifth China-Japan Bilateral Conf. on Molten Chem. and Technol., Kunruing, China, Sept. 28-Oct. 2 (1994) 42. [6] J.R. Selmam H.C. Maru, Adv. Molten Salt Chem. 4 (1983) 308. [7] J.R. Selman, L.G. Marianowski, in: D.G. Lovering (Eds.), Molten Salt Technology, Plenum Press, New York, 1982, p. 323. [8] H. Yabe, Y. lto, K. Ema, J. Oishi, J. Power Sources 24 (1988) 207. [9] T. Fujii, H. Baba, Boshoku Gijutsu 29 (1980) 457. [10] P. Taxil, J. Maheng, J. AppL Electrochem. 17 (1987) 261. [I 1] G.W. Mellors. S. Senderoff, J. Electrochem. Soc. 112 (1965) 266. [12] J.R. Selman, UG. Marianowski in: D G . Lovering (Eds.), Molten Salt Technology, Plenum Press. New York, 1982, p. 346. [13] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Fundamentals and Applications, Wiley, New York, 1980. [14] D.N. Bennion, C.W. Tobias, J. Electrochem. Soc. 113 (1966) 589. [15] M. Matsumura, J.R. Selman, J. Electrochem. Soc. 139 (1992) 1255. [16] J.M. Fisher, P.S. Bennett, J.F. Pignon. R.C. Makkus, R. Weewer, K. Hemrnes, J. Electrochem. Soc. 137 (1990) 1493.