Principal Research Results

BackgroundA molten carbonate fuel cell (MCFC) is an energy-producing electrochemical system which is expected to become

commercial in the near future. However, the achievement of the aimed for lifetime of 40,000 hours of pressurized operation remains a

challenge. The most important causes of limited lifetime is dissolution of the cathode (nickel-shorting) 1). We have clarified that the

nickel-shorting phenomenon is accelerated with high CO2 partial pressure in the cathode gas. However, a cell using Li/Na carbonate

has a higher performance at high pressure and a lower nickel deposition rate in the matrix than that of a Li/K cell. This suggests that a

Li/Na cell can be used for long operation and that tests at high pressure over 10 atm can accelerate dissolution and are useful to

elucidate the NiO dissolution phenomenon in a short time. As a highly efficient MCFC plant system combined with a gas turbine of 

more than 10 atm is proposed, clarification of the performance of the MCFC at high pressure of more than 10 atm is extremely

important.

ObjectivesThe performance of a cell using lithium/potassium and lithium/sodium carbonate as an electrolyte and which are expected to

achieve high performances at a high pressure upto 45 atm are measured and analyzed. The performance estimation formulation is

derived from the measured performance at high pressure. In addition, factors which produce a higher cell performance at a high

pressure are clarified based on the formulation.

Principal ResultsThe cell performance using Li/K and Li/Na carbonates as an electrolyte at a high pressure of more than 10 atm was clarified.

(1) The pressurized performance of the Li/Na and Li/K cell over 10 atm has not been measured and investigated in the past. In this

study, the performance under various gas conditions was systematically analyzed and a model of the polarization was investigated

at pressures upto 45 atm. The results indicate that anode and cathode polarization can be treated as the resistance at pressures of 

higher than 10 atm, that the anode reaction resistance is nearly 0 m( over 15 atm (Equation 1) and that the formulation which has

been developed under 10 atm (Equation 2) (Fig. 1) can be applied to the cathode reaction resistance.

(2) Analysis using this formulation for cathode reaction resistance indicates that the coefficient (C) of a Li/Na cell is smaller than that

of a Li/K cell (Table 1). This means that the small cathode polarization dependence on the CO2 concentration and the small IR

drops of a Li/Na cell enable a higher performance than that of a Li/K cell (Fig. 2). Based on these results, a Li/Na cell can achieve

a power density of 3.3 KW/m2 and a high cell voltage (0.82 V) can be obtained at 400 mA/cm2 at 16 atm (compared to the past

target of 1.2 KW/m2) (Fig. 3). This result shows the feasibility of a high efficiency MCFC plant system combined with a gas

turbine and a new application prospect for the MCFC.

Future DevelopmentsTo develop a preventive method for nickel-shorting using a Li/Na cell and new cathode material.

Main Researcher:Masahiro Yoshikawa

Senior Researcher, Chemical Engineering Department, Yokosuka Research Laboratory

Reference

Behavior of MCFCs Using Li/K and Li/Na as the Electrolyte at High Pressure, CRIEPI Report No. W97009, April, 1998

(Japanese only)

Behavior of Molten Carbonate Fuel Cell (MCFC)

at High Pressure (Over 10 atm)

 

2. Fossil Fuel Power Gene ration - High Efficincy Techno logy

Coefficient

Cell type

B

(Dependent

on O2 )

1 .9E-4

1.8E-4

C

(Dependent

on CO2 )

6 .3E-5

3.2E-5

Li / K ce l l

Li/ Na cell

Table 1 Summary of parameters for Eq.(2)

Fig. 1 Order analysis of cathode reaction

resistance assuming mixed diffusion of

O2 and CO2 (assumed mCO2)

Fig. 2 Dependence of the cell voltage on pressure

(Li/Kcell, Li/Nacell)

Fig. 3 Power density vs. Current density

(Li/Na cell)

Pressure1,16,45atm

Temperature650 C

Current density150mA/cm2

FuelH2/CO2/H2O=32/8/60%

Fuel utilization =40%

Current density=150mA/cm2

Temperature=650 C

Anode:H2/CO2/H2O=32/8/60

Caode:Air/CO2=70/30

Temperature650 C

An:H2/CO2/H2O=32/8/60%,Uf=60%

Ca:Air/CO2=70/30%,Uox=40%

In the case of cathode reaction resistance

corresponding to the formulation which assumes

mixed diffusion of O2- and CO2, PCO2-0.75 PCO 20.5 m CO2 "

vs. Ra+Rc)mCO2 " plots should exhibit a linear

relation

Li/Na cell achieved 3.3kW/m2 in power

density at 400mA/cm2 under 16atm