Applications of Nanoparticles in Fuel Cells

EE453 Project Report submitted by

Bruce Moore, , Fall 2008

The purpose of this paper will be to explore the applications of nanoparticles in fuel cell technology. The motivation for this topic is the desire for the American society to become less dependent on fossil fuels for its energy needs. To fully examine this topic, there will be an in depth examination of the technology of fuel cells, and a discussion about the advantages of using nanoparticles to advance this technology. Throughout this paper we will explore key components, applications, efficiency issues, and the future of fuel cells.

The first step in understanding what fuel cells are and how they operate is to understand some basic terms that are used in discussing this topic. First and foremost, let’s define fuel cell. A fuel cell is an electrochemical device that combines hydrogen and oxygen to produce electricity, with water and heat as its by-product. As long as fuel is supplied, the fuel cell will continue to generate power. Since the conversion of the fuel to energy takes place via an electrochemical process, not combustion, the process is clean, quiet and highly efficient – two to three times more efficient than fuel burning[1]. Nextwe will define a catalyst as a substance that speeds up a chemical reaction, but is not consumed by the reaction; hence the catalyst can be recovered unchanged at the end of the reaction it has been used to speed up, or catalyze [2]. Now, the terms anode and cathode need to be understood. These terms can be defined as, the electrode where the electrons are lost during the process, and the electrode where the electrons are gained during the process, respectively. Later in this discussion, we will also need to understand the meaning of a carbon nanotube.Carbon nanotubes are molecular-scale tubes of graphitic carbon with outstanding properties. They are among the stiffest and strongest fibers known, and have remarkable electronic properties and many other unique characteristics.[3]

With these formalities out of the way we can now discuss the technology of fuel cells. There are many different types of fuel cells. Some of which include Molten carbonate fuel cells (MCFC), Phosphoric acid fuel cells (PAFC), and the proton exchange membrane fuel cells (PEMFC).Molten Carbonate fuel cells (MCFC) use high-temperature compounds of salt (like sodium or magnesium) carbonates (chemically, CO3) as the electrolyte. Efficiency ranges from 60 to 80 percent, and operating temperature is about 650 degrees C (1,200 degrees F). Units with output up to 2 megawatts (MW) have been constructed, and designs exist for units up to 100 MW.Phosphoric Acid fuel cells (PAFC) use phosphoric acid as the electrolyte. Efficiency ranges from 40 to 80 percent, and operating temperature is between 150 to 200 degrees C (about 300 to 400 degrees F). Existing phosphoric acid cells have outputs up to 200 kW, and 11 MW units have been tested. PAFCs tolerate a carbon monoxide concentration of about 1.5 percent, which broadens the choice of fuels they can use. If gasoline is used, the sulfur must be removed. Platinum electrode-catalysts are needed, and internal parts must be able to withstand the corrosive acid.[4]For this discussion, we will focus most of our attention on the proton exchange membrane fuel cell (PEMFC).
Let’s start our discussion of PEMFC’s with a brief look into their history. PEM technology was invented at General Electric in the early 1960’s, through the work of Thomas Grubb and Leonard Niedrach.In 1955, W. Thomas Grubb, a chemist working for the General Electric Company (GE), further modified the original fuel cell design by using a sulphonated polystyrene ion-exchange membrane as the electrolyte. Three years later another GE chemist, Leonard Niedrach, devised a way of depositing platinum onto the membrane, which served as catalyst for the necessary hydrogen oxidation and oxygen reduction reactions. This became known as the 'Grubb-Niedrach fuel cell'. General Electric announced an initial success in mid-1960 when the company developed a small fuel cell for a program with the U.S. Navy's Bureau of Ships (Electronics Division) and the U.S. Army Signal Corps. General Electric continued working on PEM cells and in the mid-1970s developed PEM water electrolysis technology for undersea life support, leading to the US Navy Oxygen Generating Plant. The British Royal Navy adopted this technology in early 1980s for their submarine fleet. Other groups also began looking at PEM cells. In the late 1980s and early 1990s, Los Alamos National Lab and TexasA&MUniversity experimented with ways to reduce the amount of platinum required for PEM cells.[7] United Technology Corp.'s UTC Power subsidiary was the first company to manufacture and commercialize a large, stationary fuel cell system for use as a co-generation power plant in hospitals, universities and large office buildings. UTC Power continues to market this fuel cell as the PureCell 200, a 200 kW system.[12] UTC Power continues to be the sole supplier of fuel cells to NASA for use in space vehicles, having supplied the Apollo missions,[13] and currently the Space Shuttle program, and is developing fuel cells for automobiles, buses, and cell phone towers; the company has demonstrated the first fuel cell capable of starting under freezing conditions with its proton exchange membrane automotive fuel cell.[8]
The proton exchange membrane fuel cells have received the most attention from the public because of their promise to be a clean renewable energy source. The basic operation of a fuel cell is broken down into a few simple steps.Taking a look at figure 1 in appendix a before reading about the operation process of the fuel cell will enhance ones understanding of the subject. The process begins by feeding hydrogen into the anode. Here the anode has several tasks to complete.It conducts the electrons that are freed from the hydrogen molecules so that they can be used in an external circuit, and it has channels etched into it that disperse the hydrogen gas equally over the surface of the catalyst. The catalyst is usually made of platinum nanoparticles very thinly coated onto carbon paper or cloth. The catalyst is rough and porous so that the maximum surface area of the platinum can be exposed to the hydrogen or oxygen.Protons then move through the membrane to the cathode. This membrane is also referred to as the electrolyte. This specially treated material, which looks something like ordinary kitchen plastic wrap, only conducts positively charged ions. The membrane blocks electrons. For a PEMFC, the membrane must be hydrated in order to function and remain stable.[5]The cathode, which is on the opposite side of the electrolyte as the anode serves a few different purposes in this process. It has channels etched into it that distribute the oxygen to the surface of the catalyst. It also conducts the electrons back from the external circuit to the catalyst, where they can recombine with the hydrogen ions and oxygen to form water. The electrons from the anode side of the cells cannot pass through the membrane to the positively-charged cathode. They travel via the external electrical circuit to reach the other side of the cell. This produces the electrical current in the cell. The best thing about this type of cell is that the byproduct is simply water and heat [6]. A pictorial representation of this process can be seen in figure 2 of appendix A.
After understanding the basic operation of the PEMFC one can examine some of the applications for this type of technology. The fuel cell has the potential to serve a wide range of applications. Some of those applications include; stationary power installations for utilities, factories, emergency power for hospitals, communications facilities, credit card centers, police stations, banks and computer installations, diverse military applications, domestic power supplies for individual residences, and transportation. This list is only a small portion of what fuel cells can be used for. The most prominent application today is the application in the transportation field. The worldwide market for the hydrogen fuel cell is estimated to reach $11 billion by 2013, and the automotive engine application looks particularly promising.[7]

The key to making a fuel cell work is a catalyst, which is a metal that facilitates the reaction of hydrogen and oxygen. The most common catalyst is platinum, but it is expensive. Currently, the amount of platinum catalyst required per kilowatt to power a fuel cell engine is about 0.5 to 0.8 grams, or .018 to .028 ounces. At a cost of about $1,500 per ounce, the platinum catalyst alone would cost between $2,300 to $3,700 to operate a small, 100-kilowatt mid-sized, light duty vehicle - a significant cost given that an entire 100-kilowatt gasoline combustion engine costs about $2,000. To make the transition to fuel cell-powered vehicles possible, the automobile industry needs something better and less expensive.An alternative catalyst solution that demonstrates great promise is using lower cost metals at the nanoscale to replace platinum. Palladium is a first example, as it resembles platinum chemically, is extracted from copper-nickel ore, and is already used as catalyst material in the catalytic converters of automobiles. It is 75% less expensive than platinum, and when used at the nano scale in direct methanol fuel cells, palladium has demonstrated an increased power density of 45%. This power enhancement is due to the improved selectivity of the palladium catalyst and the additional surface area in nano scale materials--more particles are on the surface that can chemically interact, translating to a dramatic efficiency improvement of the catalytic reaction. Thus, using nano scale palladium is both less expensive and leads to better performance [7]

Another leading issue with the implementation of fuel cells is safety. The issue of safety is that hydrogen is a very combustible element and improper storage could lead to major problems.In their attempt to store hydrogen, researchers bombarded a film of carbon nanotubes with a hydrogen beam. Then they studied the film with different x-ray spectroscopy techniques to see if any hydrogen atoms had formed chemical bonds with the carbon. To their delight, they found that about 65 percent of the carbon atoms had bonded to hydrogen atoms. Single-walled carbon nanotubes are essentially a one-atom-thick layer of carbon rolled into a tube. All the carbon atoms are on the surface, allowing easy access for bonding. The carbon atoms have double bonds with each other. The incoming hydrogen’s break the double bonds, allowing a hydrogen to attach to a carbon while the carbon atoms renew their grip on each other with single bonds. The carbon nanotubes offer safe storage because the hydrogen atoms are bonded to other atoms, rather than freely floating as a potentially explosive gas.[7] Carbon nanotubes are unique nanostructures with remarkable electronic and mechanical properties. Interest from the research community first focused on their exotic electronic properties, since nanotubes can be considered as prototypes for a one-dimensional quantum wire. As other useful properties have been discovered, particularly strength, interest has grown in potential applications.[7] A graphical representation of a carbon nanotube can be seen in figure 3 of appendix A. This picture shows a very long carbon nanotube that was grown at the NevadaNanotechnologyCenter.

As with any other technology, fuel cells face the issue of being as efficient as possible. If a technology is developed but the efficiency is very poor, the general public will quickly turn against that technology. The efficiency of a fuel cell is dependent on the amount of power drawn from it. Drawing more power means drawing more current, this increases the losses in the fuel cell. As a general rule, the more power (current) drawn, the lower the efficiency. Most losses manifest themselves as a voltage drop in the cell, so the efficiency of a cell is almost proportional to its voltage. For this reason, it is common to show graphs of voltage versus current for fuel cells. If the fuel cell is powered with pure hydrogen, it has the potential to be up to 80-percent efficient. That is, it converts 80 percent of the energy content of the hydrogen into electrical energy. However, we still need to convert the electrical energy into mechanical work. This is accomplished by the electric motor and inverter. A reasonable number for the efficiency of the motor/inverter is about 80 percent. So we have 80-percent efficiency in generating electricity, and 80-percent efficiency converting it to mechanical power. That gives an overall efficiency of about 64 percent.A typical cell running at 0.7 V has an efficiency of about 50%, meaning that 50% of the energy content of the hydrogen is converted into electrical energy; the remaining 50% will be converted into heat.[8]

As we can see, fuel cell technology has many promising applications in our lives. Cutting edge research is exploring the improvements that can be made to the cells using nanotechnology. If the application of the carbon nanotubes is perfected as a way to harness the hydrogen gas more efficiently, and the manufacturing costs are reduced, fuel cells can quickly become a mainstay in society. So, the impending question is will we ever see the full scale implementation of the fuel cell technology in our everyday lives? Many advances still need to be made in this field to accurately answer this question. Hypothetically, let’s say that everything worked out in the favor of the fuel cells. Researchers were able to maximize the efficiency of the cells, while minimizing the manufacturing costs and the three major U.S. automakers agreed to implement fuel cells across the board of their vehicle lineups. Everything sounds great, but then the government would have to invest billions (maybe even trillions) of dollars into new infrastructure to support the hydrogen filling stations needed to keep the fuel cells operating. In today’s society, many people are creatures of habit, and live by the saying “if its not broken then don’t fix it”. What most people do not realize is that our system is broken. We let other nations and big oil companies dictate what we do in our everyday lives. As things stand now in our society, the price of a gallon of gas can go upwards of five dollars and there is nothing that we can do about it. We have to make sacrifices in other areas of our lives while the big oil companies sit back and enjoy record profits every quarter. Also, our government cannot fully act without the consent of the people and if we can’t come together as American’s and agree that a major change in our energy supply is eminent, that too may spell doom for fuel cells. There are many scenarios that can play out to influence the outcome of this technology. It seems as though fuel cells have many hurdles to jump in order to become a mainstay in our society.

Appendix A

Figure 1. The parts of a PEM fuel cell

Figure 2

Figure 3

A very long carbon nanotube grown at NNC

, Las Vegas

References

[1]Fuel Cells 2000. The Online Fuel Cell Information Resource. Retrieved on 11-25-08, from

[2] ChemiCool. Definition of Catalyst. Retrieved on 11-24-08, from

[3]A Carbon Nanotube Page. Carbon Nanotube Science and Technology. Retrieved on 11-25-08, from

[4]Fuel Cells. Fuel Cell Basics. Retrieved on 11-26-08, from

[5]How Stuff Works. How Fuel Cells Work. Retrieved on 11-24-08, from

[6]HyFleet Cute. H2 Fuel Cell Technology. Retrieved on 11-26-08, from

[7]Quantum Sphere. Fuel Cells. Retrieved on 11-25-08, from

[8]Physorg. Carbon Nanotubes Store Hydrogen in Step Toward Hydrogen Vehicles. Retrieved on 11-26-08, from

[9] Wikipedia:The Free Encylopedia. Fuel Cell. Retrieved on 11-24-08, from