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.

Nakoso IGCC plant

On the last day of the 10th China-Japan Symposium on Fluidization, today I got an opportunity to visit the Nakoso IGCC (Integrated coal Gasification Combined Cycle) demonstration power plant located in Fukushima Prefecture of Japan. It was a great experience to have direct acquaintance with a clean coal power generation technology which is still under R&D stage worldwide.



Coal is a widely distributed and abundantly reserved fossil fuel that can ensure stable supply for future power generation. However, conventional coal combustion steam-electric plants make use of heat produced from burning coal, which results in low efficiency and high emission of pollutants including carbon dioxide. Therefore, clean coal technologies such as IGCC and IGFC (Integrated coal Gasification Fuel Cell) have been the focus of current R&D in this field. In IGCC technology, coal is converted into gaseous products, known as syngas (composed of hydrogen and carbon monoxide). Before the syngas is burned in a gas turbine to produce electricity, impurities can be removed from the fuel and turned into re-usable byproducts. This results in lower emission of pollutants such as particulates, sulfur dioxide and mercury. This capacity for pre-combustion clean-up of pollutants is one of the technology’s primary advantages over conventional coal combustion approaches. Moreover, to improve overall process efficiency, heat is recovered from both the gasification process and also the gas turbine exhaust in waste heat boilers producing steam. This steam is then used to produce additional electric power.

Several IGCC demonstration plants are now under construction or undergoing demonstration operation across different parts of the world. In Japan, as a national R&D project, a 200 ton/day entrained bed coal gasification combined cycle pilot plant (25 MW) was constructed and operated for 4770 h from 1991 to 1996 by the IGC Research Association under the sponsorship of NEDO (New Energy and Industrial Technology Development Organization), and was successfully completed in March 1996. Based on this experience, Clean Coal Power R&D Co. Ltd. was founded in 2001 in collaboration between Japanese government and Japanese electric power companies to construct a 250 MW IGCC demonstration plant. It is the same demonstration plant which is now commonly known as “Nakoso” from the name of the place where it is located. It has come into operation from September 2007.

Process flow diagram of the Nakoso IGCC plant

Details of the Nakoso IGCC plant as well as some basics of IGCC technology can be found here.

Fuel cells in action

A recent report entitled "The Business Case for Fuel Cells: Why Top Companies Are Purchasing Fuel Cells Today" by Fuel Cells 2000 features 38 US companies, including 11 Fortune 500 ones, that are making substantial benefits by using fuel cells in their facilities. The applications of fuel cells in these companies range from motive power like vans and forklifts to combined heat and power (CHP) to backup power for telecom equipment.

Here is an interesting video that shows the application of hydrogen-powered lift trucks at a distribution center of the United Natural Foods, Inc. (UNFI). With 65 lift trucks powered by hydrogen fuel cells, the company expects to cut carbon emissions by approximately 132 metric tons annually along with annual energy savings of approximately 640,000 kWhs.

History of fuel cell development at a glance

The history of fuel cells dates back to 1839 when a Welsh judge and physical scientist, Sir William Grove, conducted the first known demonstration of the fuel cell by reversing a water electrolysis reaction. Below is a quick summary of the landmarks in the history of fuel cell technologies:

1839 – Demonstration of the first fuel cell (called “gas voltaic battery” at the time) by William Grove

Sketch of Grove's original fuel cell (with four cells in series and a water electrolyzer as the load). Fuel cell technology remained just as a scientific curiosity without much success in practical applications for almost a century after this invention.

1889 – Coining of the term “fuel cell” by Ludwig Mond and his assistant Carl Langer who attempted to use air and coal gas (also referred to as “fuel gas”) to generate electricity

1894 – Theoretical explanation of the operation of fuel cells and manifestation of the advantages of producing electricity directly via fuel cells instead of conventional steam engines by Wilhelm Ostwald

1899 – Demonstration of the yttria-stabilized zirconia (YSZ) solid-state ionic conductor by Walther Nernst, which is still used as the electrolyte of solid oxide fuel cells (SOFCs)

1921 – Development of the first molten carbonate fuel cell (MCFC) by Emil Baur of Switzerland

1932 – Research on alkali electrolyte fuel cells (AFCs) started by Francis T. Bacon

Late 1930s – Experimentation with solid oxide electrolytes using materials such as zirconium, lanthanum and yttrium by Emil Baur and H. Peris

1955 – Invention of the polymer electrolyte fuel cell (PEFC) by William Grub at General Electric

1960 – Successful operation of an MCFC continuously for 6 months reported by Dutch scientists G. H. J. Broers and J. A. A. Ketelaar

1961 – New promise in phosphoric acid electrolytes revealed by G. V. Elmore and H. A. Tanner of Kettering Foundation (USA) leading to the development of phosphoric acid fuel cells (PAFCs)

1962 – Demonstration of the first practical SOFC by the scientists at Westinghouse Electric Corporation (now Siemens Westinghouse)

1965 – First commercial use of PEFCs in NASA’s Gemini space program

1966 – First application of AFCs, manufactured by Pratt and Whitney based on Bacon’s patents, in NASA’s Apollo space program

1966 – First hydrogen fuel cell car, Electrovan, built by the General Motors by employing PEFCs supplied by the Union Carbide

1970s – Invention of the electrochemical vapor decomposition (EVD) process by Arnold Isenberg at Westinghouse thereby opening the door to the development of thin film electrolytes for SOFCs

1983 – PAFC pilot plant of 4.8 MW built in New York City by UTC Power

Mid 1980s – Development of electrode assembly techniques in PEFCs resulting in reduced catalyst loading at Los Alamos National Laboratory

1990 – Invention of the direct methanol fuel cell (DMFC), a sub-category of PEFCs, by Surya Prakash and George A. Olah of the University of Southern California’s Loker Hydrocarbon Research Institute

1993 – Demonstration of fuel cell-powered buses by Ballard Power Systems

1993 – Demonstration of the first passenger car running on PEFCs by Energy Partners

1996-97 – Operation of a 2 MW MCFC demonstration plant in Santa Clara, California by Energy Research Corp. (now Fuel Cell Energy Inc.)

2000 – Proof-of-concept demonstration of the first SOFC/gas turbine hybrid system with a design output of 220 kW at the University of California, Irvine’s National Fuel Cell Research Center

2007 – Announcement of the first mass production of fuel cell cars (FCX Clarity) by Honda

-------------------------------------------------------------------------------------

Details on the history of fuel cells can be found in:

J. M. Andujar and F. Segura, Fuel cells: History and updating. A walk along two centuries, Renewable and Sustaible Energy Reviews 2009, 13, 2309–2322

Leo J. M. J. Blomen and Michael N. Mugerwa, Fuel Cell Systems, New York: Plenum Press, 1993

M. L. Perry and T. F. Fuller, A historical perspective of fuel cell technology in the 20th century, Journal of The Electrochemical Society 2002, 149, S59-S67

http://americanhistory.si.edu/fuelcells/index.htm

Enhanced electrocatalysis of the oxygen reduction reaction in phosphoric acid fuel cells

Research group led by Nenad M. Markovic of Argonne National Laboratory (Illinois, USA) has succeeded in developing new concepts for designing oxygen reduction reaction (ORR) catalysts with potential use in phosphoric acid fuel cells (PAFCs) or any environments containing strongly adsorbing tetrahedral anions. The findings have been published online on 15 August 2010 in Nature Chemistry.

Reduction of the considerably high ORR overpotential on cathode side is still remaining as one of the major hindrances for quick penetration of hydrogen-based fuel cells into the market. Development of suitable catalysts for ORR reactions is challenging as the catalysts need to be both noble (to minimize adsorption of poisonous “spectator” species) and catalytically active (to break oxygen molecule at potentials close to the ORR reversible potential, i.e. 1.23 V). The approaches of i) fine-tuning of the electronic properties of metal surface atoms and ii) systematic alteration of the components and structure of the electrochemical double layer have recently demonstrated some success in polymer electrolyte fuel cells (PEFCs) and alkaline fuel cells (AFCs), respectively. However, these approaches cannot be effective for designing catalysts for PAFCs due to the fact that ORR in PAFCs is governed by the surface coverage of spectator phosphoric acid anions.

Therefore, this research has come up with an approach based on molecular patterning of platinum surfaces with cyanide adsorbates forming Pt(111)-CNad. Modification of platinum by adsorbed cyanide adlayer helps in blocking the sites for adsorption of covalently bonded spectator anions while providing a sufficient number of free metal sites to chemisorb the oxygen molecule and break the O-O bond.

The research shows the enhancement of ORR activity of Pt(111)-CNad by 25 times in the presence of sulfuric acid while by 10 times in the presence of phosphoric acid.

Hydrogen economy, electricity and fuel cells

First coined in 1970 by John Bockris, a Texas A&M professor, “hydrogen economy” is an ideal scenario in which all energy needs of the humankind are met by hydrogen. If we look at the history of energy use, there has always been a gradual switch toward the energy sources with lower carbon content. Thus, many researchers and scientists believe that it is only a matter of time before widespread adoption of hydrogen fuel takes place making realization of the hydrogen economy.

Though hydrogen is being widely accepted as the energy carrier of the future, there are various factors, ranging from technological to political, which will decide the fate of hydrogen in near term. Development of cost-competitive, efficient and safe technologies for the production, storage, distribution as well as end use of hydrogen obviously plays the crucial role. Besides, the importance of policy support in national and international level cannot be ignored. In fact, move toward the ideal hydrogen economy seems possible only when there is sufficient investment in infrastructure build-up and incentives are provided in terms of targets, regulations, standards, grants, etc.

Hydrogen vs. electricity

It is important to evaluate the future of hydrogen with respect to its competitors. Though currently there are many competing options, in the long term hydrogen will have to compete only with electricity because electricity potentially offers the same benefits with respect to greenhouse gas reduction, energy security and reduction of local air pollution. But it is argued that hydrogen and electricity should not be viewed as competitors, rather they are complementary to each other to satisfy all energy needs of modern civilization.

Hydrogen and electricity are closely interacting energy carriers. On the one hand, hydrogen can be produced from electricity via electrolyzers and on the other hand, reverse process can be employed to produce electricity from hydrogen with the help of fuel cells. This interchangeability is promising particularly to use hydrogen as a storage medium for electricity from intermittent renewable energy sources such as solar and wind. Moreover, hydrogen could be produced from off-peak electricity and stored for later use, e.g. as a transportation fuel. This results in effective utilization of existing electrical capacity where there are significant diurnal and/or seasonal fluctuations in supply to demand ratio.

Hydrogen and fuel cells

Fuel cell technology is often associated with hydrogen or hydrogen economy. Of course, fuel cells are one of the key elements in the hydrogen economy but the beauty of fuel cell technology is that not all fuel cells require pure hydrogen as fuel. Fuel cells like SOFC (solid oxide fuel cell) and MCFC (molten carbonate fuel cell) can run with straight hydrocarbon fuels such as natural gas offering a number of benefits over conventional energy conversion systems (as discussed in my first post). Thus, it is not hydrogen economy that will boost fuel cell technology but the vice versa is true.

Also, fuel cells are generally perceived as only the replacement of IC engines in automobiles. Though R&D in some fuel cells, especially PEFCs (polymer electrolyte fuel cells), is primarily focused on vehicular applications, utility of fuel cells ranges from large scale stationary power generation to portable electronic devices.

Commercial use of fuel cells in automobiles, stationary power plants as well as domestic CHP systems has already started. Despite the fact that fuel cell systems are currently not cost-competitive with conventional systems, R&D efforts are continually being made to reduce their cost and it would not be an optimistic illusion to expect fully cost-competitive fuel cell systems in the market within next few years!

"Scaling Up Alternative Energy"

Science magazine has published a special section on "Scaling Up Alternative Energy" in its 13 August 2010 issue. Though the write-ups in this special section are not exhaustive in terms of alternative energy technologies, they provide useful insights into the trend and future of ongoing research in this field. The podcast featuring interviews with authors of the special section is particularly interesting.

Enjoy reading/listening :)

Welcome!

Dear visitors,

Welcome to the archive of my random collections and discussions about energy technologies and issues of interest to the research and scientific community.

In recent years, there has been a lot of talk about both "energy" and "sustainability". Though there still remain different debates, it is visible that human activities, especially burning of conventional fossil fuels, are responsible for global warming and climate change. Moreover, the reserve of fossil fuels itself is depleting in an unprecedented rate, making the issue of energy security more severe. Thus, move for alternatives to the conventional fossil fuels is indispensable.

A variety of alternative fuels have been proposed so far. They range from primary energy sources like biofuels to the secondary ones like electricity and hydrogen. As an energy carrier, hydrogen has received special attention because of its potential environmental and energy-supply benefits. Though almost all hydrogen currently being produced is from the fossil sources, hydrogen produced from renewable energy sources (like solar, wind, hydropower and geothermal) is expected to ultimately satisfy the needs of modern civilization.

Though hydrogen can also be used as a fuel in internal combustion (IC) engines and conventional combustion turbines, the preferable long-term approach is to employ hydrogen to run fuel cells. Fuel cells are electrochemical energy conversion devices that generate electricity by using hydrogen (or a hydrogen-rich fuel) and oxygen. Various advantages of fuel cells over conventional power systems can be listed as follows:

• Because there is no intermediate conversion of chemical energy into thermal energy and mechanical work, fuel is converted to electricity more efficiently than any other existing electricity generating technology.
• As combustion is avoided, fuel cells produce power with zero or very low emissions, depending on the fuel used.
• Fuel cells have minimum moving parts and thus require minimal maintenance, reducing life cycle costs for energy production. Moreover, their operation is quieter.
• Fuel cells operate efficiently at partial load. This also suggests their suitability for application in motor vehicles, which are usually operated at partial load, e.g. during urban driving.
• Fuel cells are modular in design, offering flexibility in size and efficiencies in manufacturing.
• Fuel cells can be utilized for combined heat and power (CHP) applications, further increasing the efficiency of energy conversion.
• Unlike batteries that must be disposed of once their chemicals are used up, fuel cells provide continuous electricity as their reactions do not degrade over time (at least theoretically!).

Well, this is just an overview of the subject matter I will be covering in my future posts.

Please keep visiting and yes, don't forget to leave your invaluable comments/suggestions.

Cheers!!