Surface modification of nickel electrodes for molten carbonate fuel cells

Journal of Power Sources, 24 (1988) 207 - 214

207

SURFACE MODIFICATION OF NICKEL ELECTRODES FOR MOLTEN CARBONATE FUEL CELLS HJDEKI YABE, YASUHIKO ITO*, KEIKO EMA and JU N OISHI

Department of Nuclear Engineering, Faculty of Engineering, Kyoto Unwers~ty, Sakyo-ku, Kyoto 606 (Japan) (Received May 26, 1988)

Summary The surface of a porous nmkel electrode is modified by electrodeposltlon of either tungsten (from an L1C1-KC1 eutectm melt containing K2WO4) or tungsten and tungsten carbide (from a mtxture of K2WO4 and K2CO3 at 973 K) Polarlzatmn curves show the performance of the tungstenmodffmd electrode to be supermr to that of its bare counterpart. However, the bare nmkel electrode operates well as an anode for molten carbonate fuel cell systems when a small a m o u n t of K2WO4 is added to the electrolyte. This behawour is considered to be the result of m s~tu modffmatmn of the electrode surface

Introduction

The molten carbonate fuel cell (MCFC) constitutes the second-generation of fuel cell technology and has m a n y advantages in terms of fuel flexibility and system design [1 - 4] The technology m d t r e c t e d towards largescale power generahon t h a t can utilize a wide varmty of fosml fuels (mcludmg gasffmd coal, natural gas and alcohol) with high effmmncy and low poUutlon. In order to develop MCFC systems, m a n y technologmal problems have to be solved through conducting strategm research programmes. The development of statable electrode materials is vital, present materials severely hmlt both the hfetnne and the performance of the cell As a consequence, m a n y investigations have been devoted to this area of endeavour ( e . g , refs. 4, 5) In order to develop an appropriate anode material for MCFCs, it is necessary to conmder m a n y factors, e . g , electrocatalytm achvlty, corrosion resistance, smtermg resistance, creep resistance, etc To date, N1-Cr, N1-Co or N1-Cu alloys have been considered as possible anode materials.

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© Elsevmr Sequma/Prmted m The Netherlands

208 A d i f f e r e n t a p p r o a c h , d e s c r i b e d here, is t o m o d i f y t h e surface o f t h e n m k e l a n o d e b y e l e c t r o d e p o s l t m n o f t u n g s t e n or t u n g s t e n carbide f r o m m o l t e n hahde

Expernnental T h e e x p e r n n e n t a l a p p a r a t u s used to e f f e c t t h e e l e c t r o d e p o s l t l o n is s h o w n m Fig. 1. A gastlght vessel, m a d e o f stainless steel, c o n t a m e d a high p u r i t y a l u m i n a or n m k e l crucible t h a t c o u l d be raised or l o w e r e d . By this a r r a n g e m e n t , e l e c t r o d e s t h a t w e r e fLxed on t h e u p p e r flange c o u l d be imm e r s e d m t h e m o l t e n e l e c t r o l y t e c o n t a i n e d m t h e crucible T h e e l e c t r o l y t e c o n s i s t e d o f an L1C1-KC1 e u t e c t m m e l t ( 5 9 m o l % LIC1, 41 m o l % KC1) t h a t was p r e p a r e d u n d e r an a r g o n a t m o s p h e r e All c h e m m a l s w e r e o f r e a g e n t grade ( W a k o C h e m m a l s Co , L t d ) a n d w e r e d r m d u n d e r v a c u u m f o r several d a y s at a t e m p e r a t u r e j u s t b e l o w t h e m e l t m g p o i n t o f the mLxture A n A g / A g + e l e c t r o d e o r a p l a t i n u m wire was used as a r e f e r e n c e e l e c t r o d e T h e f o r m e r e l e c t r o d e c o m p r i s e d a silver wire and silver chloride t h a t w e r e b o t h c o n t a i n e d m a p y r e x glass t u b e w h m h h a d a v e r y free tip. T h e p o t e n t i a l o f t h e e l e c t r o d e was c a h b r a t e d against an a l k a h m e t a l e l e c t r o d e , L1/L1 +, t h a t was e l e c t r o d e p o s l t e d o n a n m k e l wire [6] T h e e l e c t r o d e t h u s p r e p a r e d e x h i b i t e d g o o d r e p r o d u c l b l h t y Unless o t h e r w i s e stated, all p o t e n t m l s are r e p o r t e d w i t h r e s p e c t t o t h e L1/L1 + r e f e r e n c e e l e c t r o d e T h e a n o d e was m a d e f r o m glassy c a r b o n S m t e r e d n m k e l - p o w d e r or m c k e l - h b e r ( p o r o s i t y 60 - 80%, m e a n p o r e size 8 p m ) , p l a c e d reside an a l u m i n a t u b e , was e m p l o y e d as t h e c a t h o d e

Fig 1 Experimental apparatus for electrodeposlhon studms A thermocouple, B mrcoma electrode, C Ll/Ll + electrode, D counter electrode (anode), E working e l e c t r o d e (cathode), F reference electrode, G gas inlet, H gas outlet

209 After melting the salt, vacuum-dried potassium tungstate or a mkxture o f potassium tungstate and potassium carbonate was added at 0 0 2 - 0 1 tool% The electrodeposltlon was c o n d u c t e d on the cat hode at a temperature o f 973 K and at a potential of 0 . 1 - 0.5 V or a current density of 1 0 0 - 300 mA cm -2. After the electrolysis, 2 - 100 mg cm -2 of tungsten or tungsten carbide was deposited on the electrode surface. T he electrodeposited samples were removed f r om the vessel after several hours, washed with distilled water, dned, and ke pt m a desiccator. T he samples were subjected to analysis using SEM, EPMA, EDX, ESCA, RBS and X-ray diffraction techniques Th e expernnental apparatus used for polarization studies is shown m Fig. 2. Th e electrolyte comprised an L12CO3-K2CO3 eutectlc melt (62 mol% L12CO3, 38 mol% KsCO3) cont m ne d m a high-purity alumina crucible. The electrolyte was dried under vacuum at just under the melting p o i n t for a few days. It was t he n melted u n d e r an argon atmosphere and maintained at the t e m p e r a t u r e of the experiment. An " o x y g e n e l e c t r o d e " [7] (Os COs -- 1"2) or p l a t m u m wire was e m p l o y e d as a reference electrode The c o u n t e r electrode was also an " o x y g e n electrode". The working electrode (anode) was made f r o m nickel (see above) and was housed in an alumina tube. Fuel gas (H: COs = 4 1, after passing through water at 323 K) was introduced through the alumina tube. In order to effect m s z t u surface modification o f the wo r k m g electrode, KsWO 4 was added to the m ol t en carbonate electrolyte.

:IFr_ Fig 2 Experimental apparatus for polarization studms m molten carbonate A thermocouple, B zlrconla electrode, C counter electrode ("oxygen electrode"), D platinum

wire, E mckel wire, F reference electrode ("oxygen electrode"), G working electrode (MCFC anode), H gas inlet, I gas outlet

210 Results and discussmn Figure 3 shows an electron mmrograph of a typmal tungsten-coated electrode obtained at 0.1 V m an L1C1-KC1 melt containing 0 I mol% o f K~WO4 at 973 K Analysis showed that 13 2 mg cm -2 of tungsten was deposited o n the surface An EPMA investigation revealed t hat small amounts o f tungsten were uni f or m l y deposited on the mckel substrate (Fig 4) Note, the deposit was :dentffmd as tungsten by X-ray dfffractmn analysis

(a) (b) Fig 3 Electron mmrograph of electrodeposlted tungsten on smtered mckel powder substrate

F:gure 5 presents an electron mmrograph of a typmal tungsten carbide sample obtained at 0 2 V m an L1C1-KC1 melt containing 0 1 tool% of K2WO4 and 0 1 mol% o f K2CO3 at 973 K A c o a t m g o f 2 6 m g c m -2was f o u n d on th e surface EPMA studms showed (Fig 6) small am ount s o f tungsten and carbon to be u m f o r m l y dispersed across the surface Using X-ray dlffrachon, it was f o u n d t hat the deposit was composed of W2C, WC and W The results f r om EDX and RBS furt her c o n h r m e d that bot h tungsten and tungsten carbide were electro-depos:ted from the chloride melt o n t o the porous nmkel substrate The results of polarization measurements c o n d u c t e d on an MCFC anode, modlfmd by tungsten and tungsten carbide and immersed m m ol t en carbonate, are presented m F:g 7 In this Figure, the apparent current density is p lo tted along the absc:ssa and the potential of the MCFC anode (vs " o x y g e n ele ct r ode" ) on the ordinate Curve (a) was obtained on a bare, smtered nmkel-powder substrate (porosity 60%), curve (b) on a smtered mckel-powder substrate (porosity 60%) coated with tungsten, and curve (c) on a smtered mckel-hbre substrate (porosity 80%) coated with tungsten. It can be seen that when modlfmd by tungsten the MCFC exh:blts good p e r f o r m a n c e

211

N

Fig 4 E P M A

of electrodepomted tungsten sample shown m Fig 3

The polarization characteristics of a bare, slntered mckel-powder substrate (porosity" 60%) m molten carbonate containing 0.1 mol% tungstate 1on are given by curve (d) in Fig. 7. This system was examined m order to study the effects of m s z t u surface modlflcatmn in the MCFC electrolyte. The data show that this very simple treatment enhances the performance of the electrode. Curve (e) represents the polarization characteristics of the MCFC anode when modlfmd by tungsten and tungsten carbide by adding 1.0 mol% of

212

(a)

(b)

Fig 5 Electron mmrograph of electrodeposlted tungsten carbide on smtered mckel powder substrate

m Fig 6 EPMA of tungsten carbide sample shown m Fig 5

tungstate 1on to the m ol t en carbonate This anode was o b t a m e d at 0 2 V m an L1C1-KC1 melt contmnmg 0 1 mol% K2WO4 and 0 02 mol% K2CO 3 at 973 K After the electrolysis, about 10 mg cm -2 o f tungsten plus tungsten carbide were deposited on the surface This anode shows good electrmal

213 -06

(a)

ILl -0~ E

(d)

/~.~ /

(~)

.,'

-(~ -lC v

-~ -12

f t 00 Current

i

200 Density

( mA/cm

i

300 2 )

Fig 7 Polarization curves for electrodes m L12CO3-K2CO3 eutectm, 923 K (a) Smtered mckel powder, (b) slntered nmkel powder modlfmd by tungsten (Fig 3), (c) smtered mckel fiber rnodffmd by tungsten, (d) smtered mckel powder, (e) smtered mckel powder modffmd by tungsten and tungsten carbide

performance, greater durabthty, and enhanced resistance to corrosmn. Other studms demonstrated that electrodeposltmn of tungsten carbide alone increased electrode durability but did not improve the polarlzatmn characterlstms

Conclusions The principal results obtamed from the above experiments are summarlzed as follows (1) Electrodeposltlon of tungsten and tungsten carbide from molten chloride on porous mckel substrates has been achmved. (11) Polarization measurements show t h a t the performance m molten carbonate electrolyte of an MCFC anode modlfmd by tungsten is superior to t h a t of its bare, porous nmkel counterpart. However, stmdar improvem e n t was n o t observed when the anode was modlfmd by tungsten carbide. (lil) The porous mckel electrode itself gwes excellent performance as an MCFC anode when K2WO4 is added to the electrolyte. This may be due to tungsten exerting a catalytm effect. The above electrodes when tested with tile electrolyte showed similar improvement m polarization and durability over more than several hundred hours [8].

Acknowledgements The authors are mdebted to Mr Selp Terada of Kawasakl Heavy Industins, Ltd for valuable discussions. The work was supported by a Grant-m-Aid from the Japanese Ministry of Education, Scmnce and Culture

214

References 1 J R Selman and L G Marlanowskl, in D G Lovermg (ed), Molten Salt Technology, Plenum, New York, 1982 2 J R Selman and H C Maru, m G Mamantov and J Braunstem (eds), Advances in Molten Salt Chemzstry, Vol 4, Plenum, New York, 1981 3 Molten Salt Committee of The Electrochemmal Socmty of Japan (ed), Proc 1st Syrup on Molten Carbonate Fuel Cells, Kyoto, Japan, 23 - 24 Aprd, 1985 4 Molten Salt Committee of The Electrochemmal Socmty of Japan (ed), Proc 2nd Syrup on Molten Carbonate Fuel Cells, Kyoto, Japan, February, 1988 5 P A Lessmg, G R Miller and H Yamada, J Electrochem S o c , 133 (1986) 1537 6 T Takenaka, Y Ito and J Olshl, Denk~ Kagaku, 53 (1985) 476 7 A Borucka and C M Suglyama, Electrochzm Acta, 13 (1963) 1887 8 S Terada, C Nagal and H Ito, Abstract, Spring Meeting o f the Electrochemical S o c w t y o f Japan, Tokyo, Japan, 5 - 7 Aprd, 1988