Fuel Cell the Study Focuses

¶ … Fuel Cell

The study focuses on the hydrogen-oxygen fuel cell technology. The fuel cell is an energy delivered technology that combines hydrogen, oxygen and other oxidizing agents to produce energy. The paper discusses different types of fuel cell technologies, and their differences are based on their chemical components, their temperature output and differences in their level of power output. The fuel cell technologies have been used for different applications, which include space mission, transportation, back up power, and for the stationary plants. Despite the benefits derived from fuel cell technologies, the technologies are not still cost effective. The paper suggests that there is a need for further research to make the cell fuel systems to be more cost effective to be produced for commercial purpose.

Introduction

A fuel cell is a device that has ability to convert chemical energy from fuel into electricity using a chemical reaction combined with oxygen and other oxidizing agents. Fuel cell requires oxygen to supply constant supply of electricity and it directly converts the chemical energy from hydrogen into energy by combining with pure water and potentially useful heat as the only by-product. Hydrogen is a versatile gas that could be used to power nearly end-use energy needs.

"Fuel cells directly convert the chemical energy in hydrogen to electricity, with pure water and potentially useful heat as the only byproducts. Hydrogen-powered fuel cells are not only pollution-free, but also can have more than two times the efficiency of traditional combustion technologies." (U.S. Department of Energy, 2010).

Typically, the fuel cell is an energy conversion device that could efficiently use and capture the hydrogen power and it consists of electrolyte between two electrodes (an anode and a cathode). Typically, hydrogen ions generally moves through the cathode, electrode and combine with the electrons and oxygen to produce water. With this chemical process, fuel cells never run out.

Fuel cell could be used as back up power for distributed power generation, and remote location. More importantly, fuel cell has ability to power any portable application that uses batteries, portable generators and hand-held devises. Fuel cell is also a power energy device that could power transportation, which include trucks, personal vehicles, marine vessels and buses. Thus, hydrogen-oxygen fuel cell plays important roles in replacing the imported petroleum currently being used to power the cars and trucks.

One of the important benefits of hydrogen-Oxygen fuel cell is that they are pollution free and they are two or three times more efficient than traditional combustion technologies. A conventional combustion energy generates electricity between 33 and 35%. However, fuel cell systems generate electricity at the level reaching 60% efficiencies and higher with cogeneration. Under normal driving conditions, the gasoline engine in a conventional motor car is less than 20% efficient to convert chemical energy into gasoline power in order to move vehicles from one place to the other. On the other hand, hydrogen-oxygen fuel cell vehicles are much more energy efficient. They have ability to use between 40 and 60% of the fuel's energy leading to a 50% reduction in fuel consumption compared to the traditional convectional vehicle using a gasoline internal combustion engine. As being revealed in Table 1, fuel is more efficient than other power generation systems.

Table 1. Fuel Cell Comparison with other Power Generating Systems

Diesel: Reciprocating Engine

Turbine generator

Photovoltaic

Wind turbine

Fuel cells

Capacity range

500 kW -- 50 MW

500 kW -- 5 MW

1 kW -- 1 MW

10 kW -- 1 MW

200 kW -- 2 MW

Efficiency

35%

29 -- 42%

6 -- 19%

25%

40 -- 85%

Capital cost ($/kW)

200 -- 350

450 -- 870

1500 -- 3000

O & M. cost ($/kW)

0.005 -- 0.015

0.005 -- 0.0065

0.001 -- 0.004

0.01

0.0019 -- 0.0153

Typically, fuel cell is a dynamic and promising replacement of fossil fuels. It is effective to generate energy in places where there are no access to public grid or places requiring huge cost to generate electricity. Moreover, fuel cells have ability to deliver UPS (uninterrupted power supply) and are suited for variety of application because fuel cells have ability to move quietly and having fewer moving parts.

Objective this research is to investigate the basic technologies of Hydrogen-Oxygen fuel cell.

History

A German scientist named Christian Friedrich in 1838 first discovered the fuel cells and his findings were published in one of the scientific magazines of the time. However, Sir William Robert Grove and a Welsh scientist demonstrated the first fuel cell in Philosophical Magazine and Journal of Science on February 1839 based on the discovery of Christian Friedrich. (Grove, 1842). In 1955, in the United States, a chemist named Thomas Grubb modified the original fuel cell for the GE (General Electric) company. The GE chemist used a "sulphonated polystyrene ion-exchange membrane as the electrolyte." (Toria, et al. 2008 P. 8). In 1958, Leonard Niedrach, another GE chemist, devised a method to deposit platinum into the membrane, which served as catalysts for the reaction of oxygen reduction and hydrogen oxidation reactions. The usefulness of fuel cell during the period made GE to develop the technology with NASA. (Toria, et al. 2008 ). In 1959, Harry Ihrig successfully built a 15 kW fuel cell tractor, which was demonstrated across the United States state fairs.

In the United Kingdom, Francis Thomas Bacon, a British engineer successfully developed a 5 kW stationary fuel cell in 1959. In the late 1959, Bacon and his team successfully demonstrated a practical five-kilowatt unit of fuel cell capable of powering a welding machine. In 1960s, Pratt and Whitney secured Bacon's U.S. patents and developed hydrogen and oxygen fuel cell that could be used for space program as well as supplying electricity and drinking water. In 1991, Roger Billings developed the first hydrogen fuel to power an automobile. For several years, UTC Power is the first company to manufacture and commercialize stationary fuel cell systems, which are generally used for power plants in hospitals and many large office buildings. Meanwhile, UTC continues to be a sole and major supplier of fuel cells to NASA for the space vehicles, Space Shuttle program and Apollo missions.

Basic Technology of Hydrogen-Oxygen Fuel Cell

All fuel cell technologies have the same configurations using combination of an electrolyte and two electrodes to generate power. Typically, the electrolyte determines the type of chemical reactions that would take place within the fuel cells. While there is different range of designs for the oxygen-hydrogen fuel cells, however, they all operate using the same basic principles. The major difference between various fuel cell designs is their chemical characteristics. Mekhilefa, et al. (2012) argues that hydrogen-oxygen fuel cell deliver heat and power via an electrochemical reaction which is actually a reversed electrolysis reaction. The electrochemical reaction happens between oxygen and hydrogen to form water.

The equation (1) reveals the electrochemical reaction shown the basic operating principle of a fuel cell.

Equation (1)

2H2 (g) + O2 (g) ? 2H2O + energy

Hydrogen + oxygen ? water + (electrical power +heat) (1)

As being revealed in Fig 1, a fuel cell consists of four main parts, which include cathode, anode, external circuit and electrolyte. At the node level, hydrogen is oxidized into electrons and protons. While at the cathode, the oxygen is reduced to oxide species and produces a reaction to form water.

"Depending on the type of electrolyte, either oxide ions or protons ions are transported through an ion-conductor electron-insulating electrolyte while electrons travel through an external circuit to deliver electric power. Nevertheless, fuel cells often produce only very small amount of current due to diminutive contact area between electrodes, electrolyte and the gas. Another problem to be considered is the distance between electrodes." (Manahana et al. 2011 P. 982).

Fig. 1: A Fuel Cell Operating Principle

However, there is a need to consider a thin layer of electrolyte with flat porous electrodes for electrolyte and the gas penetration. The reaction to generate power using hydrogen-oxygen cell fuels is different and the chemical reactions depend on the different types of fuel cells. Within an acid electrolyte fuel cell, protons and electrons (H+) are released from an ionizing hydrogen gas at the anode electrode.

"The generated electrons pass though an electrical circuit and travel to the cathode while protons are delivered via electrolyte. This exchange releases electrical energy. Simultaneously at the cathode side, the water is forming as a result of the reaction between electrons from electrode and protons from electrolyte." (Mekhilefa, Saidurb, Safari, 2012 P. 982).

The reactions occurring between anode and cathode are revealed in the following equations:

Equation (2)

Anode: 2H2 ? 4H+ + 4e

Equation (3)

Cathode: O2 + 4e? + 4H +? 2H2O

The proton exchange membranes are acid electrolytes and some polymers, which contain free H+ ions. They allow the H+ ions passing through it because they provide effective proton delivering functions. Thus, there is a lost of electrical current when electrons are delivered through the electrolytes. Evaluation of different fuel cell technologies provides a greater understanding of the benefits and challenges in fuel cell operations and applications.

Different Fuel Cell Technologies

U.S Department of Energy (2010) provides the description of different fuel cell technologies. The fuel cell technologies are differentiated according to their efficiency, operating temperatures, costs and application. The classifications are based on 6 major groups:

Alkaline fuel cell (AFC),

Phosphoric acid fuel cell (PAFC),

Solid oxide fuel cell (SOFC),

Molten carbonate fuel cell (MCFC),

Proton exchange membrane fuel cell (PEMFC);

Direct methanol fuel cell (DMFC).

Alkaline fuel cell (AFC)

The AFC generates electrical power using alkaline electrolyte KOH (potassium hydroxide) in water-based solution. The presence of hydroxyl ions within the electrolyte allows a circuit to extract electric energy. The illustration in Fig 2 reveals an alkaline fuel cell. As being revealed in Fig 2, two hydrogen gas molecules combines with 4 hydroxyl ions have a negative charge to release 4 electrons and 4 water molecules. The equation 4 reveals the reaction of oxidation that takes place. (Mark, 2003).

Equation (4)

(Oxidation) 2H2 + 4OH H2O + 4e?

Fig. 2. AFC (Alkaline fuel cell)

Source: (Andujar et al. 2009).

Typically, electrons are released in this reaction and reach the cathode and react with water to generate (OH?) ions. Moreover, 2 water molecule and oxygen combine with 4 electrons to form 4 negatively charged hydroxyl ions.

The equation 5 below reveals that reaction:

Equation (5)

(Reduction) O2 + 2H2O + 4e OH?

AFC generally performs better at a temperature between 60 and 90 "C. However, recent AFC design operates at temperature between 23 and 70 "C. Generally, AFC is a low cost catalyst, work at low temperature and the AFC electrical efficiency is approximately 60%, however its CHP efficiency is more than 80%, and has ability to generate electricity of up to 20kW.

NASA was the first organization that used AFC to generate electric power and supply drinking water during the space application. Based on the cost effectiveness of AFC, the AFC technology has now been used in boats, submarines, niche transportations, and forklift trucks applications. (Kordesch, 1999). Typically, AFC emits no green house gas and is very useful for space shuttle fleets and spacecrafts and operate with 70% efficiency. Despite the benefits derived from AFC, the technology could be easily poisoned with carbon dioxide. For example, when alkaline solution (KOH) in AFC electrolyte absorbs CO2, the chemical reaction will convert into potassium carbonate (K2CO3) which consequently poisons AFC. (Larminie & Dicks 2003). Typically, a small amount of CO2 could affect the cell operations. To make AFC more effective, there is a need to carry out the purification process. However, the purification process is very costly. AFC needs to be more cost effective to be effective used for commercial purpose.

Phosphoric acid fuel cell (PAFC)

PAFC uses H3PO4 (liquid phosphoric acid) electrolyte and carbon paper electrodes. The H3PO4 consist of:

3.09% H,

31.6% P,

65.3% O

Typically, H3PO4 is a clear colorless liquid used for food flavoring, detergents, fertilizers, and pharmaceuticals. The PAFC could operate at temperature ranging between 150 and 220 "C. The PAFC charge carrier is the hydrogen ion or proton.

"The hydrogen ions pass from the anode to the cathode through the electrolyte and the expelled electrons return to the cathode through the external circuit and generate the electrical current. At the cathode side, water is forming as the result of the reaction between electrons, protons and oxygen with presence of platinum catalyst to speed up the reactions." (Mekhilef et al. 2012 P. 983).

Illustration in Fig 3 reveals the hydrogen that expels at the anode splits into 4 electrons and 4 protons. At cathode, 4 electrons and 4 protons combine to form water as being revealed in equation 6 and equation 7.

Fig. 3. Operating Principle of Phosphoric Acid Fuel Cell

Equation (6)

(Oxidation) 2H2 ? 4H+ + 4e?

Equation (7)

(Reduction) O2 + 4H+ + 4e H2O

When protons and electrons pass through the electrolyte and the external circuit respectively, the reaction generates heat and electrical current. The heat could be used for heating water, or for steam generation. The PAFC is considered the first generation of fuel cell, and is one of the most mature fuel cells. Moreover, PAFC is the first fuel cell to be used commercially, and it is being used for stationary power generation. PAFC has also been used to power large vehicles.

However, steam reactions within the PAFC produce carbon monoxide (CO), which may poison the fuel cell and reduce the PAFC performance. However, solution to decline the CO absorption is to increase the tolerance of anode temperature. PAFC run on air and could be easily operate with reformed fossil fuels. Moreover, PAFC is very expensive, electrical efficiency of PAFC is between 40 and 50%, and its CHP efficiency is about 85%.

Solid Oxide Fuel Cell (SOFC)

Contrary to other fuel cell technologies, SOFCs high temperature fuel cells containing metallic oxide solid ceramic electrolyte. As being revealed in Fig 4, SOFC uses mixture of carbon monoxide and hydrogen to form air and hydrocarbon fuel. The oxidation process in SOFC is that the oxygen is oxidized and reacts with the cathode at 1000 "C. On the other hand, the fuel oxidation occurs at anode as being revealed in the following equation:

Equation (8)

"(Oxidation) (1/2)O2 (g) + 2e

O2?(s)"

Equation (9)

"(Reduction) O2? (S) + H2 (g) ? H2O (g) + 2e?"

Fig 4: Solid -- oxide Fuel Cell

SOFC has ability to generate large-scale power systems reaching the capacity of hundreds of MWs. The byproduct of the heat is used to generate electricity. The SOFC is considered advantageous because it can generate power at areas having no access to public grids. Moreover, the SOFC could be maintained with low costs and deliver noise free operation. The challenges of SOFC are that it produces high temperature corrosion.

Molten carbonate fuel cell (MCFC)

Molten carbonate fuel cells are generally high temperature fuel cell and use molten carbonate salt mixed with electrolyte and suspended in a porous solid electrolyte. The Illustration in Fig 5 presents a MCFC.

Fig. 5: Molten Carbonate Fuel Cell

Illustration in Fig 5 reveals that hydrogen fuel and carbonate ion react to water, carbon dioxide, and electrons. At anode level, reaction of methane Chapter 4 with water produce carbon dioxide (CO2), hydrogen (H2), and carbon monoxide (CO) as being illustrated in the following equation:

Equation (10)

CH4 + H2O ? CO + 3H2

Equation (11)

CO + H2O ? CO2 + H2

Moreover, the oxidation reaction is presented in following equation

Equation (12)

H2 + CO32

H2O + CO2 + 2e?

Equation (13)

(Oxidation 2) CO + CO CO2 + 2e?

However, the reduction occurs at cathode and expels new carbonate ions from carbon dioxide (CO2) and oxygen (O2). Thus, cell voltage and electric current could be collected at electrodes as being revealed in the following equation:

Equation (13)

(1/2)O2 + CO2 + 2e

CO32?

Currently, MCFCs are being employed at coal-based and natural gas power plant and converted to electrical utility. The shortcoming of MCFC is that it is being operated with high operating temperature. However, MCFC does not require infrastructure development to install.

"PEMFC (Proton Exchange Membrane Fuel Cell)"

PEMFCs activate hydrogen by catalyst to form proton ion as well as ejecting electron at the anode. Typically, the protons are able to pass through the membrane and the electrons are forced to the external circuit to generate power. In PEMFC, electron interacts with proton ion and oxygen to form water. Fig 6 illustrates the chemical reactions occurring at PEMFC.

Fig 6: "PEMFC (Proton Exchange Membrane Fuel Cell)"

Equation (15)

Anode: H2 (g) ? 2H+ + 2e?

Equation (16)

Cathode: (1/2)O2 (g) + 2H+ + 2e

H2O (l)

Equation (17)

H2 (g) + (1/2)O2 (g) ? H2O (l) (Overall reaction).

The benefits of PEMFC are that it is low temperature fuel cells and operating at temperature between 60 and 100°C. Typically, PEMFC is a lightweight compact system that is easy and cheaper to manufacture. From the efficient point of few, higher efficiency can be gained from using PEMFC. Typically, PEMFC's electrical efficiency is between 40 and 50% and its output can be as high as 250 kW. Moreover, PEMFC requires minimum maintenance

DMFC (Direct methanol fuel cell)

DMFC is suitable source of power that could be used for portable energy purposes due to its ability to generate low temperature. As being revealed in Fig 7, DMFC is a clean renewable energy from oxygen available in air. Equation 18 reveals that methanol is formed from carbon dioxide (CO2) and cathode steam is formed from oxygen in the air.

Fig. 7: DMFC (Direct methanol fuel cell)

Equation 18

CH3OH + H2O ? CO2 + 6H+ + 6e? (Anode)

Equation (19)

(3/2)O2 + 6e? + 6H+ ? 3H2O (Cathode)

The benefit of DMFC is that "it does not have many of the fuel storage problems typical of some fuel cells because methanol has a higher energy density than hydrogen -- though less than gasoline or diesel fuel. Methanol is also easier to transport and supply to the public using our current infrastructure because it is a liquid, like gasoline." (U.S. Department of Energy, 2012 P. 8).

However, DFMC is relatively new compared to other fuel cell technologies, and it has not being used for various applications.

Comparative Analysis of Fuel Cell Technologies

Comparatively, different form of fuel cell technologies has different application chemical reactions and operation specification of each fuel cell differs. Table 2 presents summary of operation specifications of different fuel cells technologies and Table 3 present characteristics of different fuel cell technologies.

Table 2: Operational Specifications of Different Fuel Cell Technologies

Fuel Cell Type

AFC

PAFC

SOFC

MCFC

PEMFC

DMFC

Common Electrolyte

Its Aqueous solution of potassium hydroxide is soaked in a matrix

Liquid phosphoric acid is soaked in a matrix

Yttria stabilized zirconie

Liquid solution of sodium, lithium, and potassium. Carbonates are soaked in a matrix

Solid organic polymer poly-perfluorosulfonic acid

Solid polymer membrane

Anode reaction

2H2 + 4OH H2O + 4e?

2H2 ? 4 H+ + 4e?

O2? (S) + H2(g) ? H2O (g) +2e?

H2O + ? H2O + CO2 + 2e?

H2 (g) ? 2H++2e?

CH3OH+ H2O ? CO2+ 6H+ + 6e?

Cathode reaction

O2 + 2H2O + 4e OH?

O2 + 4 H+ + 4e

H2O

1/2 O2(g) + 2e

O2?(s)

1/2 O2 + CO2 + 2e

1/2O2(g) + 2H++2e

H2O

3/2O2 + 6e? + 6H+ ? 3H2O

Charge carrier

OH?

H+

O?

CO3?

H+

H+

Fuel

Pure H2

Pure H2

H2, CO, Chapter 4, other

H2, CO, Chapter 4, other

Pure H2

CH3OH

Oxidant

O2 in air

O2 in air

O2 in air

O2 in air

O2 in air

O2 in air

Cogeneration

No

Yes

Yes

Yes

No

No

Reformer is required

Yes

Yes

Yes

Cell voltage

1.0

1.1

0.8 -- 1.0

0.7 -- 1.0

1.1

Table 3. Technical characteristics of Different Fuel Cell Technologies

Fuel cell type

Operating Temperature ("C)

System Output (kW)

Electrical Efficiency (%)

Combines Heat and Power (CHP) Efficiency

Applications

Advantages

Alkaline (AFC)

90 -- 100

10 -- 100

60

>80

Military

Space

Can use a variety of catalysts

Cathode reaction is faster in alkaline electrolyte, leads to higher performance

Phosphoric Acid (PAFC)

150 -- 200

50 -- 1000

>40

>85

Distributed generation

Overall higher efficiency with CHP

Increased tolerance to impurities in hydrogen

Solid Oxide (SOFC)

600 -- 1000

35 -- 43

Electric utility

Large distributed generation

High efficiency

Fuel flexibility

Can use a variety of catalysts

Solid electrolyte reduces electrolye management problems

Suitable for CHP

Hybrid/GT cycle

Molten Carbonate (MCFC)

600 -- 700

45 -- 47

>80

Electric utility

Large distributed generation

High efficiency

Fuel flexibility

Can use a variety of catalysts

Suitable for CHP

Polymer Electrolyte Membrane (PEM)

50 -- 100

53 -- 58

70 -- 90

Backup power

Portable power

Small distributed generation

Specialty vehicle

Transportation

Solid electrolyte reduces electrolyte and corrosion

Electrolyte management problems

Low temperature

Quick start-up

Direct methanol fuel cell (DMFC)

60 -- 200

0.001 -- 100

40

80

Replace batteries in mobiles;

computers and other portable devices

Reduced cost due to absence of fuel reformer

As being revealed in Fig 8, the operating temperature output power of SOFC is the highest with maximum operating temperature reaching 1000 OC and maximum output reaching 3000 kW. On the other hand, PEMFC delivers the lowest operating temperature of 250OC and output power of 250 kW.

Fig. 8. Comparison of Fuel Cell Output Power vs. Maximum Operating Temperature

However, Fig 9 demonstrates efficiencies of different types of fuel cells. Based on the illustration, SOCF has the highest efficient in heat and power, while AFC delivers the highest electrical efficiencies. Overall records reveals that SOFC delivers overall highest efficient level compared to other fuel cell technologies.

Fig. 9. Different Fuel Cell Types Efficiency

Discussion

Analysis of different type fuel cell systems reveals that the fuel cells vary with operating temperature level, electrical efficiency, and output power. Based on their different level of operating temperature and their different electrical efficiencies, the fuel cell technologies provide different applications.

"Depending on the application, a fuel cell stack may contain only a few or as many as hundreds of individual cells layered together. This "scalability" makes fuel cells ideal for a wide variety of applica-tions, from laptop computers (20-50 W) to homes (1-5 kW), vehicles (50-125 kW), and central power generation (1- 200 MW or more)." (U.S. Department of Energy, 2010 P2) .

Alkaline fuel cell is vastly applicable for military and space mission and the AFC is being preferred for the military application and space mission because of its high performances and its general low costs of components. The AFC generally demonstrated 60% efficiency in space application. A report by Department of Energy (2012) shows that Alkaline fuel cell was one of the first technologies developed and was the first fuel technologies that the U.S. government widely use for space program.

On the other hand, PEM fuel cells are primary used for some stationary application and transportation application. The PEM fuel cells are primarily suitable for the use of passenger vehicles such as busses and cars. Moreover, PEM is also suitable for the following application: