The Lifetime of MCFCs

In view of their ability to work with different types of fuel, molten-carbonate fuel cells are of great interest. The electrical and operating properties of these fuel cells are sufficient for building economically justified stationary power plants with a relatively large power output. The only problem so far is an insufficiently long period of trouble-free operation. The minimum length of time a large (and expensive) power plant should work until replacement is 40,000 hours (4.5 to 5 years). 

The first laboratory models of MCFCs built in the 1960s were in the best case operative for only a few months. At present, intense research and engineering efforts have made it possible to build individual units that have worked several hundreds and thousands of hours (Bischoff et al., 2002). Yet the road to a guaranteed five-year period of operation is still long. Many causes lead to a gradual decline in the performance of such power plants, or even premature failure. The three most important reasons associated with the fuel cells themselves (rather than with extraneous issues rooted in ancillary equipment or operating errors) are described below. 

Gradual Dissolution of Nickel Oxide from the Oxygen Electrode 

Nickel oxide (NiO) dissolves in the carbonate melt according to the equation 

Due to this dissolution process, the weight and thickness of the cathode will decrease by about 3% after 1000 operating hours of the fuel cell. The concentration of ions in the melt can attain values of 10 to 15 ppm. These ions spread by diffusion all across the electrolyte-filled matrix. The nickel sites are reduced to metallic nickel by diffusing hydrogen  

First laboratory prototypes of molten carbonate fuel cells, built in the 1960s, were in the best case operative for just a few months. At present, intense research and 

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The NiO solubility increases with increasing temperature, pCOj, and cation fraction Li" ". The dissolved NP+ ions diffuse through the electrolyte toward the anode, where it is reduced and precipitated as metallic nickel. This causes a sink for the Ni ions, which facilitates further NiO dissolution. The lifetime of MCFC is significantly reduced by the slow dissolution of nickel oxide cathode and precipitation of Ni ions in the matrix. 

Another factor limiting the lifetime of MCFCs is corrosion of various structural metal parts of the fuel cells. Usually, nickel-plated stainless steel is used to make... 

Endurance of the cell stack is a critical issue in the commercialization of MCFCs. Adequate cell performance must be maintained over the desired length of service, quoted by one MCFC developer as being an average potential degradation no greater than 2mV/1000 hours over a cell stack lifetime of 40,000 hours (42). Current state-of-the-art MCFCs (55,64,66,87,88) depict an average degradation over time of... 

Another point to be mentioned for the dissemination of MCFCs is extending their lifetime. Electrolyte management is strongly related to the problem. The molten carbonate electrolyte is depleted mostly by corrosion with metals and weakening of electrolyte holding in the matrices. Cell design and surface treatment of metal should be considered. It is also necessary to search for appropriate material for the matrix. 

In recent decades, research has intensified to develop commercially viable fuel cells as a cleaner, more efficient source of energy, due to the global shortage of fossil fuels. The challenge is to achieve a cell lifetime suitable for transportation and stationary applications. Among the possible fuel cell types, it is generally believed that PEM fuel cells hold the most promise for these uses [10, 11], In order to improve fuel cell performance and lifetime, a suitable technique is needed to examine PEM fuel cell operation. EIS has also proven to be a powerful technique for studying the fundamental components and processes in fuel cells [12], and is now widely applied to the study of PEM fuel cells as well as direct methanol fuel cells (DMFCs), solid oxide fuel cell (SOFCs), and molten carbonate fuel cells (MCFCs). 

Austenitic stainless steels like 31 OS, 316, or 316L are typically used for the construction of cathode and anode current collectors and bipolar separator plates. Corrosion of these steel components is a major lifetime-limiting factor in MCFC. The corrosion behavior of stainless steel components in molten carbonate conditions has been studied extensively during the past decade. Research is being aimed at increasing the corrosion resistance of these components by altering the alloy composition or by surface modification techniques. ... 

Development work at MTU resulted in a 250 kW system with molten carbonate fuel cells (MCFCs) operated at Ruhrgas in Dorsten in 1997. The system had a short lifetime, but provided valuable information for improvements. In total, 35 HotModul systems had been dehvered (together with FCE) by the end of 2006. Several systems had a Hfetime of > 30000 b until the stack had to be replaced. The overall electrical efficiency is between 45 and 46% at 50-100% of the rated power [158]. 

Moreover, the sorption-enhanced steam methane reforming (SE-SMR) process has been proposed to reduce CO2 and CO to a low level and produce almost pure hydrogen [21], which is used as a fuel feed to an MCFC anode. The novel electricity-generation system proposed, which can operate at lower energy consumption and with an almost pure hydrogen feed, is helpful for the performance and lifetime of the MCFC. 

A molten carbonate fud cell (MCFC) costs approximately 1679 per kilowatt to build. By assuming the fuel cell has a serviceable lifetime of five years, what is the hardware cost per kilowatt-hour for this form of electricity generation What other costs are involved in actually generating the electricity ... 

Another fuel cell design is the molten carbonate fuel cell (MCFC) (Yuh, 1995), which operates in the temperature range 620-660°C with an efficiency of >50%. FuelCell Energy, Inc. (Danbury, CT) produces MCEC units. These units are designed as back-up generators for intermittent use. The operational lifetimes of fuel cell systems need to be extended. In order to do so, it is necessary to limit component corrosion. 

All PAFC stacks are fitted with manifolds that are nsnally attached to the ontside of the stacks external manifolds) We shall see later that an alternative internal manifold arrangement is preferred by some MCFC developers. Inlet and outlet manifolds simply enable fnel gas and oxidant to be fed to each cell of a particnlar stack. Carefnl design of inlet fnel manifold enables the fuel gas to be supplied uniformly to each cell. This is beneficial in minimising temperature variations within the stack thereby ensuring long lifetimes. Often a stack is made of several sub-stacks arranged with the plates horizontal. 

MCFCs can operate on many different hydrocarbon fuels such as natural gas, gasified coal, and even biomass however, for reliable operation, fuel supply quality must be regulated. Also, adequate heat transfer and control are essential to the performance and lifetime of the fuel cell system. These requirements can be met using a basic set of equipment as described below. 

Durability The lifetime durability of MCFCs has been greatly improved dnring the most recent period of development. The MCFC system still suffers from a variety of durability... 

There are of course other degradation modes, but they tend to be more minor. There has been a resurgent interst in these systems, and progress has reduced the decay rate to nearly acceptable levels. Measured voltage decay rates in MCFC stacks manufactured by several companies show losses approaching the lifetime goal of 0.25% voltage loss per 1000 h, or 4% over the 40,000-h lifetime between overhaul for stationary applications [32]. 

MCFC components are limited by several technical problems (30), particularly those described in Section 6.1.1. A review of the literature from 1994 to the present shows that research efforts described in a previous issue of this handbook (31) essentially continue. It should be noted that MCFC component designs and operational approaches exist on an individual basis that would result in operation for a 40,000-hour lifetime at atmospheric pressure and with natural gas fuel. 

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