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PORTABLE FUEL CELLS – FUNDAMENTALS, TECHNOLOGIES

AND APPLICATIONS

C. O. COLPAN1, I. DINCER2*, AND F. HAMDULLAHPUR1 1Mechanical and Aerospace Engineering Department, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, Canada K1S 5B6 2Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario, Canada L1H 7L7

1. Introduction

Unlike transportation, small and large scale stationary applications, portable applications are generally considered to have power requirement less than 1 kW. PFCs are used in this kind of devices which are small and lightweight. Portable devices requiring low power have progressed from primary (dis-posable) and secondary (rechargeable) batteries to portable fuel cells. It is foreseen that PFCs will soon start replacing with Li-based or other rechargeable batteries since these battery systems are not suitable for high- power and long-lifespan portable devices due to their limited specific energy and operational time. In the case of high-power applications, PFCs will be preferred to internal combustion engines since they are more effi-cient, quiet and environmentally friendly. Some possible application areas that PFCS may be selected instead of internal combustion engines include, but not limited to, buildings and film sets.

Recently, many fuel cell companies have started paying the highest attention to portable fuel cells (PFCs) for micro applications and faster commercialization. Such fuel cells are especially crucial for the devices where high power density and long operation time are needed. Their ap-plication areas include, but not limited to, laptops, battery chargers, external power units and military applications. Proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) are considered the most feasible fuel cell types for niche applications.

*Ibrahim Dincer, Faculty of Engineering and Applied Science, UOIT, 2000 Simcoe Street North,

Oshawa, Ontario, Canada L1H 7L7, e-mail: Ibrahim.Dincer@uoit.ca

© Springer Science+Business Media B.V. 2008 S. Kakaç, A. Pramuanjaroenkij and L. Vasiliev (eds.), Mini-Micro Fuel Cells.

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On the other hand, Solid Oxide Fuel Cell (SOFC) is a very promising type that may penetrate the market in the future, especially for military applications. This study is intended to discuss the PFCs, current techno-logies and challenges, potential applications. Some illustrative examples are presented to highlight the importance of these PFCs.

2. Fundamentals

Many companies and research groups work on developing different types of PFC technologies to make them more advantageous than their competing technologies, i.e. batteries. However, battery manufacturers always seek solutions to increase the battery energy densities. Current Li-ion batteries are achieving 400 Wh/l.1 But, it is very risky to increase battery capacity too much because a high capacity battery means a large amount of energy is packed into a small space which may dangerous like a bomb. On the other hand, in addition to the high power density and lifetime benefits of portable fuel cells, life cycle costs of these fuel cells are expected to be lower.

2.1. COMPARISON OF PORTABLE FUEL CELLS AND BATTERIES

Many properties and their associated variables should be taken into account in comparing different power sources for a specific application.2, 3 Among them, energy, size, weight, operation time, transient behavior and cost are the key properties; and volumetric energy density, gravimetric energy den-sity, change of power density with time, specific cost and life cycle cost are their associated key variables. The linkages between these key variables and properties are illustrated in Figure 1. Most important ones of them are discussed in detail in the following subsections.

Figure 1. Key variables and their associated properties for portable fuel cells.

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PORTABLE FUEL CELLS 89

2.1.1. System Size

An active portable fuel cell system consists of three parts: fuel storage, stack, and balance of plant (peripherals); whereas the battery has just one part as shown in Figure 2.

Figure 2. Schematic of components of a portable fuel cell and a battery.

The power requirement of the device determines the size of the stack and balance of plant; whereas the operation time is related to the size of the storage. So, the volume of the fuel cell system, VFCS, is written as

)t(VV)t(V fPSFCS += + (1)

where VS+P is the total volume occupied by the stack and balance of plant and Vf is the volume of the stored fuel.

The power required by the device, P, is defined as

BOPffBOPBOP EnUFzEIPEIP ηη ⋅⋅⋅⋅⋅=⋅⋅=−⋅= & (2)

where z is the number of electrons produced for each mole of fuel, F is the Faraday constant, UF is the fuel utilization ratio, fn is the molar flow rate of fuel entering the fuel cell, E is the operating cell voltage, ηBOP is the efficiency of balance of plant accounting for the parasitic loads. Here, E depends on the fuel cell parameters such as geometry of the fuel cell and materials, type of the fuel and other operation variables such as temperature and pressure.

The volumetric discharge flow rate of the stored fuel is given as

BOPff

ff EUFz

PMV

ηρ ⋅⋅⋅⋅⋅

⋅=& (3)

where Mf is the molar weight of the fuel and ρf is the density of the fuel.

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Hence, the volume of the fuel cell system results in

opBOPff

fPSFCS t

EUFzPM

VV ⋅⋅⋅⋅⋅⋅

⋅+= + ηρ

(4)

where top is the operation time of the device. In the case of batteries, the size is directly proportional to the energy

density. The volume of the battery, VB, is written as

opB tEDPV ⋅= (5)

where ED is the energy density of the battery. Here we conduct a comparison study on the system size required for

both batteries and fuel cells as shown in Figure 3. As clearly seen, the sizes of both battery and fuel cell vary linearly as a function of the operational time. This shows that the fuel cells appear to be more advantageous than batteries for the operational time of the device greater than tmin since the system size becomes smaller for fuel cell after this time.

Figure 3. Comparison of battery and fuel cell in terms of system size.

Here, tmin may be calculated by equating VFCS and VB.

⎟⎟⎠

⎞⎜⎜⎝

⎛

⋅⋅⋅⋅⋅−⋅

= +

BOPff

f

PSmin

EUFzM

ED1P

Vt

ηρ

(6)

Illustrative Example-I: Let’s consider that we want to choose between Li-ion battery and DMFC to be used in a laptop requiring an average capacity of 13 W. For comparison, size of the system is considered as the design criterion. The input parameters that are assumed for the battery and the fuel cell are shown in Table 1.

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PORTABLE FUEL CELLS 91

TABLE 1. Input parameters for the Illustrative Example-I.

DMFC Li-ion battery Volume of stack and BOPa 520 cm3 Energy density 340 Wh/l Fuel utilization 0.85 Operating cell voltage 0.35 V Efficiency of Balance of Plant 0.90

a The size of the DMFC is approximated as P/0.05 4 and BOP is assumed to have same volume with the DMFC

Using Eq. (6), we find that the minimum operation time required to

consider DMFC instead of Li-ion battery is 20 hours. For this time, the volume of the system is equal to 765 cm3. For this example, we also see that the volume of the fuel cartridge of the DMFC system is 12.25 cm3/hour.

Using efficient components for balance of plant plays a significant role in the preference of portable fuel cells instead of batteries. Its effect on the minimum time, tmin, is shown in Figure 4. As one can see from this figure, tmin decreases from 48 to 20 hours as the balance of plant efficiency increases from 40% to 90%, respectively.

Figure 4. Effect of balance of plant efficiency on the minimum time to consider portable fuel cells instead of batteries.

2.1.2. Recharging

It can take several hours to recharge a battery depending on type of the battery. Unless you are carrying extra batteries, you have to find electricity outlet to recharge them. In the case of PFCs, there is no need for recharging. Instead, you have to change the fuel cartridge. Moreover, most fuel cells can run one or two minutes during cartridge change which is a sustainable operation due to fuel recirculation.

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2.1.3. Weight

Weight is another important criterion in the selection of power source, especially for laptops, wearable devices and power devices carried by soldiers. PFCs should be preferred over batteries when the device is in-tended to be used for long operation time. Illustrative Example-II: As an example, let’s consider that for a laptop we replace the power source from Li-ion battery to a DMFC cartridge and compare their weights for the same operation time. The input data for both battery and DMFC as assumed are given in Table 2.

TABLE 2. Input parameters for the example.

DMFC Li-ion battery Weight of stack and BOP 1.1 kg Weight 0.143 kg/hour Weight of fuel cartridge 0.02 kg/hour

Currently, PFCs are not manufactured to fit inside the laptop like Li-ion

batteries since they will not be advantageous in terms of operation time. In this example, we consider that both DMFC and Li-ion battery are put outside the laptop, so we do not have size restrictions. We find that the minimum operation time needed to consider DMFC instead of Li-ion battery for a lighter device is 8.94 hours If we take a 20 hours of operation time, the weight becomes 1.5 kg for DMFC and 2.86 kg for Li-ion battery; which shows us that PFCs are more advantageous and should be preferred for longer operation time.

2.1.4. Life Cycle Cost

The economical benefits of PFCs become clear when the life cycle cost of them are considered. Compared to batteries, PFCs are expected to have lower life cycle costs, which is illustrated with the following example. Illustrative Example-III: Let’s consider that a professional video camera manufacturer thinks of making a system operating with a PFC instead of a battery. The existing system works with a set of three batteries connected in series. As the running time capacity will decrease as the camera is used, the manufacturer suggests to change the batteries one by one at certain years, such as first battery after 1.6 years, second battery after 2.8 years and third battery after

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the battery in this system. If we use a PFC instead, there will be only initial cost and fuel cost. A comparison depending on a manufacturer’s data5 is illustrated in Table 3. As it can be seen here, the life cycle cost for a PFC system is lower, and hence becomes more cost effective.

TABLE 3. Life cycle cost comparison of a battery system and PFC for a video camera.

Battery PFC Component Cost Component Cost

Battery pack 3 × $525 = $1,575 Year: 0 (initial cost) Charger $1,595

Fuel cell system

$4,000

Year: 1.6 New battery $525 Fuel cost $269 Year: 2.8 New battery $525 Fuel cost $202 Year: 3.7 New battery $525 Fuel cost $151 Year: 4 – – Fuel cost $50 Total cost of electricity

$175 –

Total life cycle cost

~$4,920 ~$4,672

2.2. FUEL CHOICE

Depending on the technology and type employed, hydrogen, methanol, ethanol and hydrocarbons generally appear to be potential fuels for PFCs. Among them, hydrogen and methanol are considered the most promising ones. Safety, storage and distribution issues are compared below for these fuels.

2.2.1. Safety

In the case of an accident such as leak or fire, the severity of the situation depends upon the physical properties of the fuel such as flammability.

Also, the risk depends on the physical conditions, i.e. well-ventilated space or totally enclosed space. The flammability range is higher for hy-drogen and it involves a higher risk than methanol in enclosed spaces, whereas its risk is lower than methanol in well ventilated space. Another problem with methanol is its toxicity. If it mixes with any drink, it may cause severe health problems. It may be concluded that as long as all the safety precautions are taken and safety standards are applied, fuel cells may be designed safely.

3.7 years. The lifetime without losing its performance significantly is considered as 4 years. There is also an additional electricity cost to charge

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2.2.2. Storage and Distribution

Hydrogen storage methods may be classified into two general groups: direct storage of hydrogen for use and generation of hydrogen through chemical methods (e.g. reforming) on demand. The first one may include storage in compressed cylinders, metal hydrides and carbon nanotubes. The latter one includes reacting chemicals to liberate hydrogen and reforming liquid fuels. The substances used in this method include methanol, alkali metal hydrides, sodium borohydride and ammonia. For example, among these methods, compressed cylinders seem the cheapest and straightforward method; however the container of hydrogen is thick and heavy. Metal hydride is suitable for small systems, however the performance is low. In general, che-

to physical storage. Reforming offers even higher energy density and it is more economic compared to other options.

Methanol is more advantageous than hydrogen in terms of storage since it is a liquid. The main problem with methanol storage is its ability to mix with water, which may cause a corrosive environment eventually. Due to this, rather than ordinary steel, stainless steel or glass should be used as the material of the container. On the other hand, methanol can only be shipped by plane in the checked baggage compartment subject to dangerous goods regulations; but its existence in passenger compartment is expected to be allowed very soon.

3. Technologies

There are various types of fuel cells that may be used for portable applica-tions. Except SOFCs, all of them may be regarded as low temperature fuel cells. These fuel cell types are discussed below.

3.1. PEMFC

This type of fuel cell also known as the polymer electrolyte membrane fuel cell, consists of a proton conducting membrane, such as Nafion, which is chemically highly resistant, mechanically strong, acidic, good proton conductor and water absorbent. The reactions occurring at anode, cathode and overall reaction are given in Eqs. (7)–(9), respectively.

−+ +→ eHH 222 (7)

OHe2H2O5.0 22 →++ −+ (8)

OHO5.0H 222 →+ (9)

mical reaction based subsystems provide higher energy storage compared

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PORTABLE FUEL CELLS 95

Some main advantages of this type of fuel cell are:

• Fast startup capability since it works at low temperatures • Compactness since thin MEAs can be made • Elimination of corrosion problem since the only liquid present in the

cell is water.

The main disadvantage of this type of fuel cell is the need for expensive catalysts as promoters for the electrochemical reaction. Additionally, carbon monoxide can not be used as a fuel since it poisons the cell. On the other hand, the main challenge for PEMFC is the water management which may be summarized as follows: The proton conductivity of the electrolyte is directly proportional to the water content and high enough water content is necessary to avoid membrane dehydration. Contrarily, low enough water should be present in the electrolyte to avoid flooding the electrodes. Hence, a balance between the production of water by oxidation of the hydrogen and its evaporation has to be controlled.

3.2. DMFC

This type of fuel cell also uses a proton conducting membrane like PEMFCs. Its main difference is the direct feeding of methanol to the fuel cell instead of reforming it before feeding. The electrochemical reactions occurring at this fuel cell are as follows: The reactions occurring at anode, cathode and overall reaction are given in Eqs. (10)–(12), respectively.

−+ ++→+ e6H6COOHOHCH 223 (10)

OH3e6H6O5.1 22 →++ −+ (11)

OH2COO5.1OHCH 2223 +→+ (12)

The main advantages of this kind of fuel cell are as follows:

• Methanol as fuel is a readily available and less expensive • High energy density of methanol • Simple to use and very quick to refill.

The main disadvantage of this type of fuel cell is the slow reaction kinetics of the methanol oxidation, which results in a lower power for a given size. The second major problem is the fuel crossover which is summarized as follows: Polymer membrane of DMFC is permeable to methanol which means it may diffuse from the anode through the electrolyte to the cathode. Hence, migrated fuel is wasted which will decrease the amount of electron produced. It also reduces the cell voltage, hence the cell performance. The current approach to minimizing the methanol permeation rate is to limit the

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methanol concentration to approximately 5 wt%6 despite the loss in per-formance.

There are two types of DMFC, active type and passive type and their schematics are shown in Figure 5. In the active type, fuel and air flows are controlled to get high performance. In the passive type, the air is introduced into the cell by natural flow, i.e. self breathing, and the fuel is infiltrated into the cells. There is less control over the variables of fuel and air stoi-chiometry in this type. The passive one is much simpler compared to the active type, but the performance is not that high. Active type is good for high power products, such as laptops, LCD-TVs, and digital cameras. Passive type is good for small and low power products such as fuel cell powered mp3 player.

(a) (b)

Figure 5. Different types of DMFC: (a) passive type, (b) active type.

3.3. AFC

These have become popular, particularly for space vehicles, but the success in other low-temperature fuel cells has declined the interest in this type of fuel cell. The main reasons for this were the issues with cost, reliability and ease of use. However, there is one type of AFC which still takes attention and is a candidate to be used in portable applications. It is the Direct Borohydride Fuel Cell (DBFC) which uses a solution of sodium borohy-dride as fuel. The reactions occurring at anode, cathode and overall reaction are given in Eqs. (13)–(15), respectively.

−− ++→+ e8OH6NaBOOH8NaBH 224 (13)

−− →++ OH8OH4e8O2 22 (14)

OH2NaBOO2NaBH 2224 +→+ (15)

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The main advantages of DBFC are as follows:

• Eight electrons are formed from one mole of fuel • Highly alkaline fuel and waste borax prevent the fuel cell from CO2

poisoning • It is very simple to make it as the electrolyte and the fuel are mixed

The main disadvantage of DBFC is the side reaction called hydroly- sis reaction at which hydrogen is produced as NaBH4 reacts with water. However, with modern techniques, hydrogen can be oxidized immedia- tely giving eight electrons provided that hydrolysis reaction is not pro-ceeding too quickly.

3.4. SOFC

SOFCs are mostly used for stationary power generation applications; however they may also be applied to portable applications. Unlike the other fuel cell types mentioned, they are high temperature fuel cells which may operate between 500°C and 1,000°C. The most common material used for electrolyte is ytrria stabilized zirconia. The reactions occurring at anode, cathode and overall reaction are given in Eqs. (16)–(18), respectively when H2 is used as fuel.

−− +→+ e2OHOH 22

2 (16)

22 Oe2O5.0 −− →+ (17)

OHOH 222 5.0 →+ (18)

If CO is used as fuel, the reactions occurring at anode, cathode and overall reaction become as shown in Eqs. (19)–(21):

−− +→+ e2COOCO 22 (19)

22 Oe2O5.0 −− →+ (20)

22 COO5.0CO →+ (21)

The main advantages of this fuel cell are as follows:

• Fuel flexibility (methane, propane, butane, JP-8 may be used as fuel) • Direct reforming at the anode catalyst • Elimination of precious metal electrocatalysts.

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especially severe for planar design due to the sealing problem. In tubular design, the cells may expand and contract without any constraints. The second major problem is the deposition of carbon particles at the anode. This may be solved by sending sufficient amount of external water or recir-culating the depleted fuel at the fuel channel exit to the inlet.

Another important consideration is the balance of plant which may include air pump, valves, sensors, piping, tank, recuperator, fan, etc. The main criteria for selecting these components are lightweight, efficient, low power consumption and low cost. Heat interaction of components between each other should be well designed to obtain a highly efficient system.

The energy and exergy efficiencies of a SOFC system may be defined as shown in Eqs. (22) and (23), respectively.

BOPutilfcfpsys ηηηηη ×××= (22)

BOPutilfcfpsys εεεεε ×××= (23)

Here, the subscripts fp, fc, util and BOP stands for fuel processor, fuel cell, fuel utilization and balance of plant, respectively. Illustrative Example-IV: As an example, let’s look at the exergy efficiency of the system for a practical representation for two cases. Considering the best case, we take each term in Eq. (23) as 90%, the system exergy efficiency will become ~66%. On the other hand, for the worst case (by taking each term as 50%), the system exergy efficiency will result in 6%. So, one may expect that the system exergy efficiency vary between 6% and 66% for a portable SOFC system.

3.5. OTHER FUEL CELL TYPES

Direct Formic Acid Fuel Cell (DFAFC), Direct Ethanol Fuel Cell (DEFC) and biofuel cell (BFC) may be used in some of the portable applications. The first two uses a PEM where formic acid and ethanol are fed directly. DFAFC is advantageous due to its high catalytic activity, easier water management and minimal balance of plant. However, performance of the cell strongly depends on the feed concentration of formic acid due to mass

this issue also depends on the design type. For example, this problem is

The main disadvantage of this fuel cell may be given as the challen- ges for construction and durability due to its high temperature. However,

transport limitations. Generally, high feed concentrations are needed. DEFC

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PORTABLE FUEL CELLS 99

crossover is a problem. BFC may be used in very low power applications. Mainly, there are two classes of BFC which are Microbial Fuel Cell and Enzymatic Fuel Cell. The first one has higher efficiency and complete oxidation of fuel; but lower power density. Hence, it is more applicable for larger scale applications such as generating power on the seafloor. The latter one has high power density but lower efficiency and incomplete oxidation of fuel. It may be used in small scale application such as im-plantable devices.

4. Applications

Niche applications are now becoming the main market area for PFCs, which include laptops, mobile phones, camcorders, digital cameras, portable generators for camping and other recreational activities, battery chargers, etc. In all of these applications, the consumer prefers small, lightweight and long operated devices, which may be provided by portable fuel cells. Additionally, batteries might not be able to supply the power needed for the new devices with a greater amount of functions. In this aspect, portable fuel cells should be preferred since they have a higher power density. For ex-ample, fuel cells can enable the universal connectivity of wireless devices, such as laptop computers and 3G phones. Currently, there are several companies developing portable fuel cells on DMFC technology.

The military defense research plays an important role in the develop-ment of PFCs since there is a big funding in this area. These fuel cells are important for military because the future soldiers are intended to have equipment needing high power such as night vision devices, global posi-tioning systems, target designators, climate controlled body suits and digital communication systems. These should be light enough for soldiers to carry. They should also be able to operate for a long time. It is obvious that batteries cannot provide these energy needs at an acceptable weight. Therefore, PFCs are expected to be a must for the military for their future purposes. Another important point for military is the type of fuel used in these fuel cells since they prefer fuel that is available in the battle area in any part of the world such as diesel and JP-8. Hence, portable SOFC is the best option for the purposes where the fuel availability is the main criteria. However, PEMFC and DMFC may also be preferred depending on the size and purpose of a military application.

may be preferable due to the advantages of ethanol such as high energy density, safer to use and easy to store. However, in the electrochemical reactions a lot of acetaldehyde is produced which is a very flammable and harmful liquid. Additionally, reaction kinetics is very slow and ethanol

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TABLE 4. Application examples and their rank of appearance in the market for PFCs.

Rank (1-highest, 7-lowest)

Fuel cell type Application examples

1 DMFC Laptop, cell phone, mp3 player, battery charger 2 PEMFC Professional video camera, flashlight, bicycle light 3 SOFC Camping devices, military applications 4 AFC Portable charger, cell phone, PDA, digital camera 5 BFC Microelectronics and biomedical applications 6 DFAFC Cell phone 7 DEFC Wearable military power packs

5. Conclusions

In this book contribution the portable fuel cells have been discussed as potential alternatives to replace batteries for portable applications where high power density, long operation time and lightweight are of great importance. There are various technologies available with each of them having advantages and disadvantages and operating with different fuel. Non-technical considerations such as safety, storage and distribution should be taken into account in choosing the fuel. The current trend shows that DMFCs are the leading fuel cell type for niche applications, whereas portable SOFCs are the most promising type for military applications where the fuel availability is the major concern.

Acknowledgements

The support of an Ontario Premier’s Research Excellence Award and the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.

Some application examples of various PFCs as discussed in this paper are listed in Table 4 and ranked in regards to their market potential. It is obvious that portable DMFCs are ranked as the highest for market.

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