Different types of fuel cells: which one will dominate the future?

Fuel cells are classified based on the electrolyte material they use – i) alkaline fuel cell (AFC), ii) phosphoric acid fuel cell (PAFC), iii) polymer electrolyte fuel cell (PEFC), iv) molten carbonate fuel cell (MCFC), and v) solid oxide fuel cell (SOFC). Besides, there are a couple of fuel cells which take their names from the fuel used, e.g. direct alcohol/methanol fuel cell (DAFC/DMFC) and direct carbon fuel cell (DCFC). In fact, DAFCs/DMFCs represent a special subcategory of PEFCs whereas DCFCs refer to the conceptual fuel cells (usually MCFCs or SOFCs) which make use of solid carbon fuel such as coal and biomass directly without an intermediate gasification step.

Here is an overview of the different types of fuel cells:

Alkaline fuel cell (AFC)



Electrolyte: Potassium hydroxide (KOH) solution retained in an asbestos matrix
Electrodes: Transition metals loaded with platinum or other electro-catalysts
Fuel: Hydrogen
Operating temperature: 65-220 deg. C
Electrical efficiency: ~60%
Applications: Military, space

Advantages
- Superior cathode reaction kinetics
- Quick start-up due to low temperature operation
- Low weight and volume

Disadvantages
- Extremely intolerant to carbon dioxide (as a result pure oxygen or the air free of carbon dioxide should be used as the oxidant)
- Electrolyte handling problems
- Relatively short lifetime

AFCs are widely known for space applications since their first use in NASA’s Apollo program in 1965. However, terrestrial applications of AFCs seem too costly due to the strict requirement of CO2-free fuel and oxidant. As a result, R&D work on AFCs has gone down in recent years.

Phosphoric acid fuel cell (PAFC)


Electrolyte: Liquid phosphoric acid soaked in a silicon carbide (SiC) matrix
Electrodes: Carbon loaded with platinum
Fuel: Hydrogen
Operating temperature: 150-220 deg. C
Electrical efficiency: ~40%
Applications: Distributed power generation

Advantages
- Less sensitive to CO poisoning than PEFC and AFC
- Waste heat can be utilized in combined heat and power (CHP) applications/bottoming Rankine (steam turbine) cycle

Disadvantages
- Corrosive nature of electrolyte which necessitates the use of expensive materials in the stack
- Poor operating reliability in the long term

PAFC represents the first fuel cell technology that was developed commercially and is still the only commercially available one. Over 250 units of PC-25, a 200 kW PAFC system developed by UTC Fuel Cells, are reported to have been sold and installed in different countries around the world since the early 1990s. Despite the success PAFCs achieved, interest in them started declining in the late 1990s mainly owing to their high cost coupled with insufficient operating reliability in the long-term.

Polymer electrolyte fuel cell (PEFC)
(Also known as proton exchange membrane/polymer electrolyte membrane (PEM) fuel cell (PEMFC))

Electrolyte: Fluorinated sulfonic acid polymer (commonly Nafion)
Electrodes: Carbon loaded with platinum
Fuel: Pure hydrogen
Operating temperature: 40-80 deg. C
Electrical efficiency: 40-60%
Applications: Automotive systems, portable applications, small scale distributed power

Advantages
- No corrosion and electrolyte management problems
- Quick start-up due to low temperature operation
- High power density (over 2 kW/l and 2 W/cm2)

Disadvantages
- Highly sensitive to impurities of hydrogen (does not tolerate >50 ppm of CO and has a low tolerance to sulfur particles and ammonia)
- Difficulty in water management ensuring sufficient hydration of the electrolyte membrane against flooding

With the availability of more stable proton exchange membranes such as Nafion (1960s) and the development of modern membrane electrode assembly (MEA) with reduced catalyst loadings (mid-1980s), PEFC technology has become potentially attractive especially for automotive applications. However, further reduction in system cost (such as the development of non-platinum catalysts and cheaper membranes) as well as improvement in long-term performance (mainly CO tolerance) is imperative in order to penetrate the markets.

Molten carbonate fuel cell (MCFC)


Electrolyte: Mixture of molten carbonate salts (lithium carbonate + potassium carbonate/sodium carbonate) retained in a ceramic matrix (LiAlO2)
Electrodes: Nickel (anode) and nickel oxide (cathode)
Fuel: Hydrogen, CO, methane, etc
Operating temperature: 600-700 deg. C
Electrical efficiency: ~60%
Applications: Electric utility, large scale distributed power

Advantages
- No need of expensive electro-catalysts
- Fuel flexibility (spontaneous internal reforming of hydrocarbon fuels)
- High grade waste heat (suitable for CHP applications/bottoming cycles)

Disadvantages
- Very corrosive nature of the electrolyte
- Material problems due to high temperature operation
- High intolerance to sulfur (1.5 ppm max.)
- Slow start-up

MCFCs, one of the earliest fuel cells used in practical applications, are preferred for natural gas and coal-based power plants. However, poor long-term reliability has been the major hindrance to their commercialization, which is associated with the issues such as gradual dissolution of nickel oxide from the cathode, anode creep and corrosion of metal parts. Consequently, research and engineering work on MCFCs has gradually shrunk since the early 2000s and the interest of developers has shifted to another high temperature fuel cell, SOFC.

Solid oxide fuel cell (SOFC)


Electrolyte: Ceramics (mainly yttria stabilized zirconia (YSZ))
Electrodes: Composite of ceramics and metal (mainly Nickel-YSZ cermet) as anode and perovskite ceramics (mainly strontium doped lanthanum manganite (LSM)) as cathode
Fuel: Hydrogen, CO, methane, etc
Operating temperature: 600-1000 deg. C
Electrical efficiency: ~60%
Applications: Electric utility, large scale distributed power, auxiliary power units (APUs)

Advantages
- No corrosion and electrolyte management problems
- No need of expensive electro-catalysts
- Fuel flexibility (spontaneous internal reforming of hydrocarbon fuels)
- High grade waste heat (suitable for CHP applications/bottoming cycles)

Disadvantages
- Material problems due to high temperature operation
- Slow start-up

Elimination of electrolyte management problems, fuel flexibility and high electrical efficiencies have all made SOFC an attractive emerging technology for future power generation, especially in stationary applications. Though most of the benefits of SOFCs result from their high temperature operation, cost and durability issues associated with such high temperatures are yet to be solved. Therefore, current research on SOFCs in academia, industry and governmental laboratories is primarily focused on developing intermediate temperature SOFCs (IT-SOFCs) (operating at < 700 deg. C) which allow the use of low cost materials with improved durability and offer the same advantages as the conventional SOFCs do.

Direct methanol fuel cell (DMFC)

Electrolyte: Fluorinated sulfonic acid polymer
Electrodes: Carbon loaded with platinum
Fuel: Methanol
Operating temperature: 50-130 deg. C
Electrical efficiency: ~40%
Applications: consumer electronics (as a replacement of batteries)

Advantages
- Direct use of liquid fuel (can be recharged like batteries by simply changing the cartridge of fuel)

Disadvantages
- Lower efficiency due to methanol crossover problem
- Higher cost due to increased loading of noble metal at anode

Despite their low efficiency, DMFCs are expected to find their applications in consumer electronic products such as mobile phones, digital cameras and laptop computers in which energy density, compactness in design and convenience of refilling the fuel are of prime concern. To make DMFCs competitive with state-of-the-art lithium-ion batteries in consumer electronics, current research is focused on finding suitable electrolyte materials so as to minimize the fuel crossover problem and developing more active anode catalysts to enhance methanol oxidation.

TREND OF RESEARCH PUBLICATIONS

Following bar graph shows the trend of research publications on various fuel cell technologies from 1970 until the current year based on the data obtained from Web of Science.


As there might be some alterations depending on the search keywords, it should be noted that for the above data, search keywords used are:

AFC – “Alkaline fuel cell”
PAFC – “Phosphoric acid fuel cell”
PEMFC – “PEM fuel cell”
MCFC – “Molten carbonate fuel cell”
SOFC – “Solid oxide fuel cell”
DMFC – “Direct methanol fuel cell”

TREND OF FUEL CELL VEHICLES AND POWER PLANTS

Light duty vehicles

Following graph from Fuel Cell Today illustrates the annual growth of new units of fuel cell light duty vehicles from 1997 to 2007.


Large stationary power plants (> 10 kW)

Annual number of units and MW installed:


Annual technology mix, by percentage adopted:


Further surveys on fuel cell and hydrogen markets can be searched by topic here.

To sum up, there has been a considerable increase in the work on fuel cells over the past few decades indicating the inevitability of transition to fuel cells from the existing power generation technologies. During the course of technological development, some fuel cell technologies have gained more attention while others have lost their charm. Currently PEFCs and SOFCs seem to be the most promising candidates for mobile and stationary applications, respectively. Thus, it seems that these two fuel cell technologies are going to dominate not only the fuel cell industry but also the entire energy sector of the future. Nevertheless, other kinds of fuel cells such as AFCs may still be preferable for some niche applications.

Note: The first four illustrative figures are taken from FCTec.