Solid State Ionics 113–115 (1998) 253–258

Simultaneous generation of synthesis gas and electric power byinternal reforming fuel cells utilizing LaGaO based electrolytes3

*Takashi Yamada, Yoshiko Hiei, Taner Akbay, Tatsumi Ishihara , Yusaku TakitaDepartment of Applied Chemistry, Faculty of Engineering, Oita University, Dannoharu 700, Oita 870-11, Japan

Received 20 July 1998

Abstract

A novel solid oxide fuel cell (SOFC) utilizing partial oxidation of methane (CH 1 1/2O 5 CO 1 2H ) as internal4 2 2

reforming reaction was investigated in the present study. Large electric power as well as CO–H mixture with molar ratio of2

2 were obtained by applying LaGaO perovskite as electrolyte. Although the open circuit potential decreased, the maximum3

power density increased by doping a small amount of Co to the LaGaO electrolyte. In particular, the increase in the power3

density at 1073 K was significant. Increasing the amount of doped Co monotonically enhanced the hole conduction resultingin a decrease in the open circuit potential and an increase on the amount of leaked oxygen which results in diminishedelectric power. Consequently, it became clear that the optimized composition for this electrolyte wasLa Sr Ga Mg Co O considering the power density and the amount of oxygen leakage. Although the thickness0.9 0.1 0.8 0.115 0.085 3

of electrolyte with the above composition was as thick as 0.5 mm, a maximum power density and yield of synthesis gas were2obtained at 242 mW/cm and 16%, respectively at 1073 K. 1998 Published by Elsevier Science B.V. All rights reserved.

Keywords: Internal reforming fuel cells; LaGaO3

1. Introduction the electrochemical combustion of H . Inevitably,2

fuel gases will be composed of CO and H O. On2 2

Introduction of the reforming reaction to a fuel the other hand, an internally reforming fuel cellcell improves the energy utilization of the fuel [1]. utilizing partial oxidation is able to combust the fuelAt present, steam reforming reaction is considered as electrochemically to generate electric power fol-the only reforming reaction for fuel cells [2]. How- lowed by a reforming reaction of the unreacted fuelever, steam generation requires a large energy invest- with CO and H O to form CO and H . Therefore,2 2 2

ment. Furthermore, excessive amount of H O is the production of synthesis gas consisting of CO and2

needed for preventing the formation of carbon. The H which may readily be used for the synthesis of2

first step for a cell which employs steam reforming hydrocarbons or alcohol can also be used to simul-reaction is to reform fuel into H and CO and then taneously generate electric power. In particular,2 2

the following stage is to generate electrical energy by partial oxidation of CH expressed by the following4

reaction; CH 1 1/2O 5 CO 1 2H is attractive4 2 2

* since it yields a synthesis gas with a molar ratio of 2.Corresponding author. Tel.: 1 81-97-554-7895; fax: 1 81-97-554-7979; e-mail: isihara@cc.oita-u.ac.jp This is attractive because synthesis gas with molar

0167-2738/98/$ – see front matter 1998 Published by Elsevier Science B.V. All rights reserved.PI I : S0167-2738( 98 )00289-6

254 T. Yamada et al. / Solid State Ionics 113 –115 (1998) 253 –258

ratio of H to CO of 2 is the most useful for 1:4) was fed to the anode side of the reactor at a rate2

hydrocarbon synthesis and methanol. From this point of 50 ml /min while oxygen was supplied to theof view, internal reforming fuel cell with partial cathode side at 100 ml /min. The electrical poweroxidation of CH has the aspects of being both generation characteristics of the reactor was mea-4

power generation system and a chemical reactor. sured by four-probe method. The current across theEarlier, the authors discussed the feasibility of cells was controlled by a galvanostat (Hokuto

partial oxidation of CH certain solid oxide fuel cells Denko, HA-303) and the terminal voltage was4

[3–5]. It was found that the application of LaGaO measured by a digital voltmeter (Advantest TR6845).3

based perovskite type oxide ion conductor as elec- The flue gas was analyzed with gas chromatographtrolyte is effective to obtain a high electrical power (Shimadzu GC-8ATP) after a cold trap. The internalas well as high yield of synthesis gas. In the previous resistance of the cell was measured with the currentstudy, a mixture of CH and O was fed as a interrupt method. A constant current pulse was4 2

reactant. This was due to the insufficient amount of formed by a current pulse generator (Hokuto Denkooxygen permeation through relatively thick elec- HC111) and applied to anode or cathode against atrolyte that was used. However, it is obvious that reference electrode attached to the end of the cathodepure CH alone as a fuel would be more effective to site of electrolyte disk. The transient potential4

generate electric power. In this study, we investi- change was monitored with a spectrum analyzergated a novel internal reforming fuel cell to which (Takedariken TR-9404).CH (diluted with N ) is fed as fuel. Furthermore,4 2

Co doped La Sr Ga Mg O was chosen as the0.9 0.1 0.8 0.2 3

electrolyte, since it was clear that doping small 3. Results and discussionamount of Co was effective in increasing the oxideion conductivity, in particular, at low temperatures. 3.1. Power generation characteristics of SOFC in

which N diluted CH fuel is used2 4

2. Experimental procedure Fig. 1 shows the electrical power generationcharacteristics of the internal reforming fuel cell in

The fuel cell reactor is the same as that was which CH diluted with N is employed as fuel.4 2

previously reported except the fact that no catalystbed is mounted under the anode in this study [3].The reactor consists of a single planar type solidoxide fuel cell which had a diameter of 17 mm. Thefully dense disks with thicknesses of 0.5 mm werealways used as electrolytes.

LaGaO oxide was prepared by a conventional3

solid state reaction with powder mixtures composedof La O (99.99% Kishida), SrCO (99.5% pure,2 3 3

Wako), MgO (99.5% Wako), Ga O (99.99%,2 3

Kishida), and CoO (99.5%, Wako). Powders instoichiometric ratio were mixed with ball mill mixer.The mixture was precalcined at 1273 K for 6 h andthen pressed into a disk shape by cold isostatic pressat 274.6 MPa. Then the disks were sintered at 1773K for 6 h. La Sr CoO and Ni (obtained by0.6 0.4 3

reducing a commercial NiO powder (99.5% Wako))were used for cathode and anode, respectively. The Fig. 1. Electrical power generation characteristics of the solid2effective electrode area was set to 0.72 cm . oxide fuel cell at 1273 K when N diluted CH fuel was fed to2 4

The gaseous mixture of CH and N (CH :N 5 anode. (s) power density; (d) terminal voltage.4 2 4 2

T. Yamada et al. / Solid State Ionics 113 –115 (1998) 253 –258 255

Comparing the power generation characteristics feed- conductivity of LaGaO based oxide. It is expected3

ing a gaseous mixture of CH with O (CH /O 5 that the power density would be increased by using4 2 4 2

4) as fuel, the open circuit voltage of the cell slightly La Sr Ga Mg O doped with Co for Ga sites0.9 0.1 0.8 0.2 3

increased from 1.29 to 1.34 V which is slightly (denoted as LSGMC in this paper).smaller than the theoretical potential (1.42 V).Speculatively, this may be related with the complete 3.2. Co doped La Sr Ga Mg O as the0.9 0.1 0.8 0.2 3

oxidation of CH for which theoretical open circuit electrolyte for the new internal reforming fuel cells4

potential is 1.02 V. On the other hand, maximumpower density was greatly improved and it reached Fig. 3 shows the temperature dependence of

22526 mWcm , while yield of synthesis gas was maximum power density and CH conversion on the4

sharply decreased from 72 to 27%. However, the cells in which La Sr Ga Mg O (LSGM) or0.9 0.1 0.8 0.2 3

molar ratio of CO and H remained 2 and the La Sr Ga Mg Co O (LSGMC) was2 0.9 0.1 0.8 0.115 0.085 3

amount of CO formation was negligible. Further- used as the electrolyte. Obviously, both power2

more, no apparent formation of carbon on the surface density and CH conversion were improved by using4

of the anode was observed visually. Therefore, it LSGMC for electrolytes. In particular, the increase inseems most likely that partial oxidation of CH power density at low temperature was significant. At4

dominantly proceeds in the reactor even though only 1073 K, the maximum power density reaches 2502CH is fed as fuels. Fig. 2 shows the product mW/cm , which is three times larger than that of cell4

distribution as a function of current density. Clearly, utilizing LSGM electrolyte. This agrees well with theconversion of CH increases with an increase in the results of conductivity measurements too. Namely,4

current density, since the amount of permeated doping Co decreases the activation energy for theoxygen is increased. This suggests that the rate oxide ion conduction which in turn results in thedetermining step of this reactor is the permeation of increase in the oxide ion conductivity at low tem-oxygen through the solid electrolyte. Consequently, perature. However, hole conduction also increases byit is expected that the increased oxide ion con- doping Co. Therefore, open circuit potential slightlyductivity enables further increase in the power decreased when LSGMC is used for electrolytes atdensity together with the yield of synthesis gas. In 1073 K.our recent work, it was found that doping smallamount of Co is effective to increase the oxide ion

Fig. 3. Temperature dependence of maximum power density andCH conversion of cells utilizing LSGMC and LSGM as elec-4

trolyte. LSGM cell (d) maximum power density; (j) CH4

Fig. 2. Product distribution as a function of current density of the conversion. LSGMC cell (s) maximum power density; (h) CH4

fuel cell when N diluted CH was fed to anode. conversion.2 4

256 T. Yamada et al. / Solid State Ionics 113 –115 (1998) 253 –258

Fig. 5. Maximum power density, CH conversion, and yield ofFig. 4. Conversion of CH , yield of CO and H , and molar ratio 44 2

CO and H at 1073 K as a function of X values inCO/H at 1273 K as a function of current density. LSGM cell (d) 22

La Sr Ga Mg Co O . (d) Maximum power density; (s)CH conversion; (j) Yield of CO; (m) Yield of H ; (g) H /CO 0.9 0.1 0.8 0.22X X 34 2 2

CH conversion; (h) Yield of CO; (n) Yield of H .ratio. LSGMC cell (s) CH conversion; (h) Yield of CO; (n) 4 24

Yield of H ; (y) H /CO ratio.2 2

Fig. 4 shows the conversion of CH , yield of CO4

or H and their molar ratio as a function of current2

density for the cell utilizing LSGMC. Since theamount of oxygen permeation increases, CH con-4

version and yield of CO and H become higher on2

LSGMC than on LSGM. However, on both cells,yield of CO and H is almost the same as CH2 4

conversion within an experimental error and molarratio of H and CO is almost 2 for all current density2

and did not largely deviate from this value. Conse-quently, partial oxidation of CH is also dominant in4

the fuel cell utilizing LSGMC. Although the com-plete oxidation of methane becomes dominant at lowtemperature, reaction products are still only H and Fig. 6. Open circuit potential and leaked oxygen amount at 10732

K as a function of X values in La Sr Ga Mg Co O . (s)CO with a molecular ratio of 2. Therefore, it is 0.9 0.1 0.8 0.22X X 3

Open circuit potential; (n) Leaked oxygen.obvious that the dominant reaction occurring at 1073K is still partial oxidation of CH . Consequently, it4

can be concluded that LSGMC is an attractive creasing amount of Co, and in particular, the de-electrolyte for CH partial oxidation. crease in the transport number becomes significant4

Effects of Co doping for the Ga site of LSGM on when the amount of doped Co becomes higher thanthe yield of synthesis gas and power density at 1073 10 mol%. This is because the hole conductionK are shown in Fig. 5. The open circuit potential and becomes dominant highly Co doped samples. Inthe amount of leaked oxygen which is estimated by good agreement with the results of electrical con-measuring the current and the amount of oxygen in ductivity, the open circuit potential decreases andproducts are also shown in Fig. 6. Although the total amount of leaked oxygen increases with the increaseelectrical conductivity monotonically increases, in Co concentration. At 15 mol%, fairly largetransport number of oxide ion decreases with in- amount of leaked oxygen is observed. On the other

T. Yamada et al. / Solid State Ionics 113 –115 (1998) 253 –258 257

hand, power density increases with increasing density would be further improved. Considering theamount of dopant and reaches a maximum at 10 overpotential of electrode, cathodic overpotentialmol%. For this composition, the maximum power was not significant even at 1073 K on both cells. In

2density exceeds 300 mW/cm . In contrast, CH contrast, overpotential of anode is fairly large on4

conversion monotonically increases. Considering 0.5 both cells comparing the cathodic overpotential.mm thickness of electrolyte and operating tempera- Therefore, anodic reaction seems to be the rateture of 1073 K, this power density is exceptionally limiting step for the internal reforming SOFC inhigh. Hole conduction becomes significant for dop- which partial oxidation of CH proceeds. Since the4

ant concentrations of 15 mol% and the cell becomes electrical conductivity of LSGM is improved byinternally short circuited. Therefore, open circuit doping Co, the IR drop could be decreased bypotential as well as maximum power density de- applying LSGMC as the electrolyte. However, thecreases sharply at 15 mol% LSGMC while CH overpotential of anode also decreases. At present, the4

conversion monotonically increases. Although the reason for the decrease in anodic overpotential is notmaximum power density was obtained at 10 mol% clear, however, improved hole conduction by dopingCo doped LSGM, it can be concluded that the Co might be related to this phenomenon. In any way,optimized composition as an electrolyte for internal the improved power density can be attributed toreforming SOFC is 8.5 mol%, since the amount of decreases in IR loss and anodic overpotential.oxygen leakage and the open circuit potential dropare still significant at 10 mol% Co doped LSGM.

Fig. 7 shows the details of internal resistance of 4. ConclusionSOFC utilizing La Sr Ga Mg Co O and0.9 0.1 0.8 0.115 0.085 3

La Sr Ga Mg O for electrolyte at 1073 K. Because of fairly large power densities and pro-0.9 0.1 0.8 0.2 3

Clearly, main reason for potential drop was electrical duction of synthesis gases with molar ratio of 2,resistance (IR) loss, which almost corresponds to the internal reforming SOFC with partial oxidation ofvalue estimated by electrical conductivity. Therefore, methane is highly attractive for the simultaneousif one chooses a thinner electrolyte, the power generation of useful compounds and electric powder.

Fig. 7. Internal resistances of LSGM and LSGMC cells at 1073 K. (h) IR drop; (s) Overpotential of cathode; (n) Overpotential of anode.

258 T. Yamada et al. / Solid State Ionics 113 –115 (1998) 253 –258

This study reveals that the partial oxidation of CH standpoint of a separation of air and upgrading of4

stoichiometrically occurs in SOFC with no signifi- natural gas.cant carbon formation even though CH diluted with4

N is used as fuel. In addition, it was found that2

doping of small amount of Co for electrolyte is Acknowledgementseffective in increasing the power density and theyield of CO and H . In particular, the power density A part of this study was financially supported by2

at 1073 K is greatly improved by using LSGMC as Kajima Research Foundation and Proposal-Basedthe electrolyte. Since the excess amount of Co only R&D Program of New Energy and Industrial Tech-enhances the hole conduction which decreases the nology Development Organization (NEDO).power density, the optimized composition for theelectrolyte is La Sr Ga Mg Co O . In a0.9 0.1 0.8 0.115 0.085 3

cell utilizing this electrolyte, power density and yield References2of CO and H were 242 mW/cm and 13% at 10732

K. Considering that the thickness of electrolyte was [1] N.Q. Minh, J. Am. Ceram. Soc. 76 (1993) 563.[2] K. Kanemura, S. Yoshioka, Z. Takehara, J. Electrochem. Soc.0.5 mm and small Gibbs free energy change for the

138 (1991) 2165.partial oxidation of CH into CO and H , power4 2 [3] T. Ishihara, Y. Hiei, Y. Takita, Sol. State Ion. 79 (1995) 371.generation characteristics of the present cell was [4] Y. Hiei, T. Ishihara, Y. Takita, Sol. State Ion. 86–88 (1996)superior. It is also noted that the decrease in the 1267.power density is not significant when air is used as [5] T. Ishihara, Y. Hiei, Y. Takita, Proc. 2nd Inter. Fuel Cell

Conf. (1996) 303.oxidant instead of oxygen. Consequently, presentinternal reforming fuel cell is also attractive from the