Principal Research Results

Development of Lifetime Estimation Method for MCFC

Background
Cell voltage of molten carbonate fuel cell (MCFC) decreases with operating time since the electrolyte in the cell is consumed by mainly corrosion of cell components. In Japanese project of MCFC development, the target lifetime of MCFC is 40,000
hours. Cell voltage has to be higher than 90% of initial cell voltage after 40,000 hours operation. Five years is required to approve thistarget. However, if a lifetime estimation method that can predict cell voltage change with time based on initial cell voltage is developed, improvement of material or design for MCFC will be achieved efficiently. The estimation of initial cell voltage can use the reaction resistance model developed by CRIEPI. Reaction resistance model is related to reaction area and physical properties. If dependence of reaction area on operating time is confirmed, cell voltage change with time can be estimated. And Cell voltage after 40,000
hours operation can be estimated by this method

ObjectiveTo estimate cell voltage after 40,000 hours operation, a new equation, which is able to estimate electrolyte content in the cellat a certain operating time, is proposed. The relationship between electrolyte content and reaction area is clarified. Using these results,
the lifetime estimation method is developed.

Principal Results(1) Reaction area seemed to depend on electrolyte content. At first, a method for quantitative estimation of electrolyte content fromobserved internal resistance (IR) was developed. An equation for calculating electrolyte content with time was derived based onelectrolyte loss by the corrosion. Parameters of the equation in Table 1-① were determined using observed IR. Electrolyte content
based on IR agrees with the equation as shown in Fig. 1.
(2) A method for determining reaction area from reaction resistance model and observed data which were measured at various conditions such as electrolyte composition and temperature, was developed. Subsequently, the relationship between reaction area and estimated electrolyte content was investigated. Relative reaction area (for example SH2), which is divided by initial reaction area, wasproportional to electrolyte content as shown in Fig. 2. As a result, estimation of cell voltage at a certain operating time wasachieved.
(3) Cell voltage was estimated based on electrolyte content change with time according to the procedure described in Table 1.
Estimated cell voltage agreed with observed voltage within ± 5 mV. Cell voltage estimation after 40,000 hours operation wasachieved. (Fig. 3a,b).

Future Developments
This method is a strong tool for estimating 40000 hours lifetime of MCFC and will be applied to contract research with NEDO. This method will be improved for application to actual stack in which electrolyte loss behavior is different from that of single
cell.
Main Researcher: Yoshihiro Mugikura, Ph. D.,
Senior Research Scientist, Advanced Power Engineering Section, Energy Engineering Research Laboratory
Reference
Y. Mugikura and H. Morita, 2003, “MCFC Cathode Performance Estimation Method Using Electrolyte Properties Based on Reaction
Resistance Model”, The 11th FCDIC Fuel Cell Symposium Proceedings, pp. 133-136, (in Japanese)946. Fossil Fuel Power Generation - Improving the efficiency of thermal power generation

Table 1 Lifetime estimation method based on reaction resistance model①Electrolyte content at certain time (e)− 10170
e = 3.1− (0.01124t 0.5 + 0.08233) exp()(for Li/K cell)
1 / T− 1 / 923②Reaction area ( SH 2 , S H 2 g )S H 2 = 0.318 e, S H 2 g = 1.75 S H 2③Parameters (A1 , A 2 )A 1 = RTδH 2 / F 2 n 2 K H 2 D H 2 S H 2 , A 2 = R 2 T 2 / F 2 n 2 K H R S H 2 g
2④Reaction resistance (Ra )R a = (A 1 + A 2 P 0.5 ) PH- 02 .5
V = E −ηNE − j( R a − R c − R ir )⑤Cell voltage (V)t：Time, T：Temperature, R：Gas constant, K：Henry constant, F：Faraday constant,n：Electron number, P：Pressure, D：Diffusion constant, δ：Diffusion distance,
S：Reaction area, E：Open circuit voltage, j：Current density, ηNE：Nernst loss,
Rc：Cathode reaction resistance, Rir：Internal resistance, g：Gas phase①Calculation of e, ②Calculation of S from e, ③Calculation of A with S and physical properties,④Calculation of R from A and P, ⑤Calculation of V from E, η, j and R
1.4

4

Li/K600℃
Li/K650℃
Li/K700℃
Li/Na650℃
Li/Na600℃

1.2

Li/K 600℃
Li/K 650℃
Li/K 700℃

3Relavive reaction area

Estimaetd content

Li/K 600℃
Li/K 650℃
Li/K 700℃

21Electrolyte content agrees withthe equation in Table 1−①.

0.8
0.60.4

Reaction area is proportional
to electrolyte content.

0.20005000
10000
Operating time (h)0Fig.1 Comparison between measured andestimated electrolyte content.
400

800

Li/K cell
P：0.１MPａ、T：
６００℃
current density：
１５０ｍＡ/ｃｍ２
Fuel：Ｈ２/ＣＯ２/Ｈ２Ｏ＝６４/１６/２０％
Gas utilization ６０％
Oxidant：Ａｉｒ/ＣＯ２＝７０/３０％
Gas utilization ４０％700600500
01000020000
30000
Operatng time (h)300200100900

Cell voltage (mV)Measured cell voltage
Estimaed cell voltage
Measured IR
Estimated IR4400

Measured cell voltage
Estimated cell voltage
Measured IR
Estimated IR

800

300

Li/Na cell
P: 0.3MPa、T: 600oC
Current density: 200mA/cm2
Fuel: H2/CO2/H2O=56/14/30% Uf=60%
Oxidant: Air/CO2=90/10% UO2/UCO2=11/40%
H2O: 10% added

700

200Electrolyte addition

600

040000

Fig.3-a Result of lifetime estimation for Li/K cell

123
Estimated electrolyte content (g)Fig.2 Relationship between electrolyte
content and reaction area.

Voltage drop by IR (mV)

900

Cell voltage (mV)

1100500
01000020000
30000
Opearating time (h)Voltage drop by IR (mV)Electrolyte content (g)Values based on IR

040000

Fig.3-b Result of life time estimation for Li/Na cell

Cell voltage and IR based on Table 1 agree with measured values even if electrolyte composition
and temperature are different.

95