APPLICATIONS – TRANSPORTATION | Electric Vehicles: Fuel Cells

Electric Vehicles: Fuel Cells C Hochgraf, General Motors Fuel Cell Activities, Honeoye Falls, NY, USA Published by Elsevier B.V.

Introduction – Why Fuel Cells? Hydrogen fuel cells are one of the most promising alternatives to internal combustion engine hybrids and pure battery electric power for propelling passenger vehicles. Compared to internal combustion engine hybrid vehicles burning hydrocarbon fuels, fuel cell vehicles offer three primary advantages. First, the fuel cell system produces no tank-to-wheel carbon dioxide emissions and no other harmful emissions such as oxides of nitrogen, carbon monoxide, or particulates. Second, the fuel cell system offers the potential for approximately 30% higher well-to-wheel energy efficiency. Third, the hydrogen fuel consumed by the fuel cell can be produced from a variety of renewable sources including carbon-free methods such as electrolysis of water. Compared to pure battery-run electric vehicles, the fuel cell vehicle offers three primary advantages. First, the fuel cell vehicle has more than twice the driving range of a vehicle using existing batteries. Second, it offers a much shorter refueling time, enabling brief refueling stops on long trips. Third, at cold temperatures, the fuel cell system can warm up much faster than a battery and therefore produce full power in a shorter period of time. A fuel cell vehicle can be refilled with compressed hydrogen at a rate of 2.0 kg hydrogen per minute. To recharge a battery electric vehicle at an equivalent rate would require the battery and charger to handle 2.5 MW of power. Such a charger would be 400 times larger than that typically used for battery electric vehicles. At –30 1C, many high-energy lithium battery chemistries cannot provide high power, that is, they cannot support discharge C-rates of 10 or more. To get full power capability, the battery would need to be warmed up. However, the time and energy required to accomplish this for a battery are significantly longer than for a welldesigned fuel cell system. Improved, but not full, power capability can be obtained at 30 1C by using lowerenergy-density chemistries such as those using nanoparticle lithium titanium oxide. The primary disadvantages of fuel cell systems, compared to gasoline hybrids, are the high present-day cost, shorter than required fuel cell stack life, poor energy density of fuel storage, and lack of a widespread hydrogen fueling infrastructure. Fuel cell system cost, while higher than a gasoline hybrid, is projected to be lower than that of an equivalent full-range electric

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vehicle with advanced batteries. Cost is being lowered and durability increased through engineering development efforts. The development of higher-energy-density hydrogen storage is an area of active research. Several studies have shown that the cost in the near term of producing, distributing, and dispensing hydrogen for use in fuel cell electric vehicles is in the range of US$2–3 per gallon of gasoline equivalent (on a cost-perkilometer basis, not including taxes). Hydrogen is produced in large quantities for industrial uses including oil refining and fertilizer production. The economics of hydrogen production by steam methane reforming are well understood. The US Department of Energy is targeting long-term costs of US$1.0–1.5 per gallon of gasoline equivalent. The primary challenges are the initial cost to deploy the fueling infrastructure and delays in getting approval to site dispensing stations due to the absence of uniform building codes and standards for hydrogen. Support of government policy is often cited as being essential to overcoming these challenges.

Requirements of an Automobile Fuel Cell Powertrain The requirements for an automobile propulsion system have evolved over many decades. Consumers’ expectations for on-road vehicle propulsion are guided by experience, which is almost entirely with internal combustion engines. Consumers expect vehicles to start instantly, accelerate quickly, drive for long periods without refueling, refuel in a few minutes, be cost effective, last for decades, and be safe to operate and service. These high-level requirements are quantified in Table 1. Achieving the desired driving range is particularly challenging for the fuel cell vehicle because the energy storage density of compressed hydrogen gas is less than 1 kW h L1 at 70 MPa compared to gasoline’s energy storage density of greater than 8 kW h L1. As a result, the requirement for fuel cell efficiency is significantly higher than that of a gasoline engine. Packaging the propulsion system to fit into a standard vehicle’s dimensions and shape drives compact and lightweight design solutions for the fuel cell system.

Powertrain Configuration A fuel cell powertrain consists of a fuel cell stack with balance of plant components for air supply, fuel control,

Applications – Transportation | Electric Vehicles: Fuel Cells

Table 1

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Key requirements of an automotive fuel cell system

Requirement

Value

Rationale

Peak power

80 kW

Durability

Cost

45000 h operation 410 years calendar life 435 000 starts/stops (keyon/off) o$50/kW

Needed for acceptable acceleration. Varies with vehicle Consumer expected usage

Range

4480 km

Refueling time Ambient temperature during operation Start time (ambient temperature above freezing) Start time (ambient temperature below freezing) Size and weight Safety codes and standards

o3 min  40 to þ 45 1C o2 s

Competitive with internal combustion engines. Includes hydrogen storage Consumer acceptability for long trips and infrequent fueling Consumer acceptability Global applicability of automobiles Consumer expectation from experience

o30 s

Consumer acceptability

100–250 kg, 200–400 L Society of Automotive Engineers Standards, Federal Motor Vehicle Safety Standards, regional standards 455% at 25% of rated power

Existing vehicle practice Existing vehicle practice

Efficiency

Needed to achieve acceptable range from limited amount of onboard fuel storage

Compressed hydrogen

Compressed hydrogen

Fuel cell system

Fuel cell system Motive power

Acceleration power

Motive power

Boost converter

Acceleration power High-voltage battery

High-voltage battery Braking power, load leveling

Inverter

Regenerative braking

Motor

dc/dc converter Braking power, load leveling

Motive power

Regenerative braking

Inverter

Motor

Motive power

Gearbox

Gearbox

Figure 1 Two common architectures for hybrid fuel cell powertrains. dc, direct current.

temperature control, and humidification. To these are added power-conditioning electronics, compressed hydrogen storage tanks, a motor drive unit consisting of inverter electronics, and a motor with gear reduction. A high-voltage battery may be optionally included.

Two common configurations of fuel cell powertrain component systems have emerged from the high-level requirements in Table 1. They are shown in Figure 1. The fuel cell stacks use proton-exchange membranes (PEMs). Proton-exchange membrane fuel cells offer fast

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Applications – Transportation | Electric Vehicles: Fuel Cells

startup times, high dynamic response, and good power density. Most systems combine the fuel cell stack with a highvoltage battery in a hybrid arrangement to recover braking energy, increase driving range, and improve acceleration (Figure 2). The high-voltage battery is generally nickel–metal hydride or lithium ion. The power rating of the battery can be traded off versus the power rating of the fuel cell. A high-power battery, say rated at 70% of peak system power, can handle acceleration demands, allowing the fuel cell to be reduced to 30% of system power, sufficient to meet average power demand. An alternative is to use a small battery, say rated for 30% of peak system power, in combination with a larger fuel cell to handle acceleration. The smaller battery is sized to recover braking energy, which increases fuel economy. Non-hybrid vehicles have been built which demonstrate that PEM fuel cell systems can meet the fast dynamic power requirements of an automobile while avoiding the cost of the battery and its power conversion electronics. Power-conditioning electronics are required at the interface between the battery and the fuel cell to match the voltage characteristics of each source and control where power is drawn from. Typical battery packs have rated voltages that range from 150 to 350 V, while typical fuel cell voltages range from 200 to 400 V. The motor drive unit is smaller and more cost effective when the direct current (dc) input voltage is in the range of 400–650 V. High voltages keep the current ratings for conductors, connectors, and electronics lower, reducing weight and cost.

If the fuel cell voltage is very low relative to dc input voltage to the motor drive unit, a voltage-boosting converter with unidirectional power flow may be used. The full load efficiency of the boost converter is in the range of 97–98%. The part load losses can be reasonably low, making for an efficient system. If the fuel cell voltage is high, it may be connected directly to the dc input of the motor’s inverter electronics, avoiding power conversion losses for the main motive power. The battery is then interfaced through a bidirectional dc/dc converter. In this configuration, losses in the power conversion electronics occur both when energy is put into the battery and when energy is removed from the battery. The choice of which power conditioning configuration to use is influenced by the voltages of the battery and fuel cell and the relative power rating of the battery versus the fuel cell. If the fuel cell power rating is low compared to the battery, it may be more cost effective to put the power conversion electronics in the fuel cell electrical path. The dc electric power produced by the fuel cell and/ or battery is converted into mechanical power and coupled to the wheels by an inverter–motor–gearbox unit. Efficiency of this conversion can exceed 90%. Hydrogen for the fuel cell is stored as compressed gas. Compressed gas is preferred over liquid hydrogen tanks due to the high energy cost to liquefy hydrogen and the loss of range as the cryogenic liquid hydrogen boils off over several days. However, compressed gas storage tanks are very large for the relative amount of energy stored. Alternative hydrogen storage methods may offer higher storage density than compressed gas. Fundamental

Fuel cell system

12 V battery

Hydrogen storage system Rechargeable energy storage system (highvoltage battery)

Figure 2 Location of major fuel cell propulsion system components in a vehicle.

Applications – Transportation | Electric Vehicles: Fuel Cells

239

research is being conducted on hydrogen storage using metal hydrides, chemical hydrides, and sorbents. Onboard reformers to create hydrogen from other fuels are generally considered to be impractical due to the large amount of energy required for warming/starting the reformer and the additional cost and weight of the reformer. Standards for hydrogen fuel purity are under development by committees including ISO TC197/WG12 and ASTM D 03. In general, the hydrogen must be 99.99% pure and have less than 1 mg L1 of particulate materials, and individual particles must have diameters less than 10 mm. Tighter limits are specified for contaminants that are detrimental to the fuel cell, such as carbon monoxide and sulfur compounds.

streams, or sometimes both. Waste heat is removed by the cooling system. Control of a fuel cell system involves keeping the membrane humidified to a set value while feeding reactant gases (hydrogen and air) to the electrodes in sufficient quantity to produce the electrical current requested by the system. Coolant flow rate and temperature, hydrogen fuel pressure and humidity, and air inlet humidification and flow rate are all controlled in coordination with the load current drawn from the fuel cell. The subsystems of the fuel cell system may be realized in different ways. Optimization of configurations and components within the fuel cell system is an area of active engineering development. The top priorities of this development are:

Fuel Cell System Configuration

1. reducing cost; 2. improving freeze tolerance of components; 3. improving humidity transfer in humidification devices; 4. improving humidification control accuracy; 5. reducing packaged volume; 6. increasing efficiency by reducing parasitic power and pressure drop through components; and 7. improving cell voltage performance from the electrode and membrane.

The fuel cell system consists of four main subsystems: (1) fuel supply, (2) air supply, (3) cooling, and (4) the fuel cell stack. These subsystems are shown generically in Figure 3. The fuel cell stack receives hydrogen and air from the respective supply systems. It produces water, heat, and electricity. Water is recovered and used to humidify either the cathode (air side of fuel cell) or anode inlet

Hydrogen fuel Anode purge exhaust Injector with jet pump

Fuel cell system exhaust

Anode recirculation compressor

Exhaust pipe

Fuel cell stack Anode

Radiator and fan

+ dc electric power out

−

Stack voltage

Coolant Cathode Coolant pump

Humidity transfer device

Radiator bypass valve

Water and heat transfer Cathode exhaust

Air compressor Air

Figure 3 Illustration of generic fuel cell system configuration showing cathode air supply system, fuel supply system, and cooling system.

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Applications – Transportation | Electric Vehicles: Fuel Cells

Fuel Cell Stack

membrane decreases by 50%. For other materials the conductivity decrease can be smaller, B30%. Relative humidity describes the ratio of the pressure of water vapor present in a given gas over the maximum pressure of water vapor that can exist at a given temperature without condensation. The maximum pressure of water vapor (the saturation vapor pressure) increases rapidly with temperature. Consider that at a temperature of 100 1C, the vapor pressure of water becomes equal to atmospheric pressure (101 kPa) and water boils. To maintain sufficient RH in the fuel cell’s membrane, the pressure of water vapor in the cathode gas must increase directly with the increase in saturation vapor pressure as coolant temperature rises. This requires a higher total gas pressure on the cathode and is why many automotive fuel cells use a compressor to pressurize the cathode inlet air. If the cathode pressure cannot be raised to maintain constant RH, then the membrane must have high proton conductivity even when the RH is low. Figure 4 illustrates the trade-off between the coolant temperature, cathode pressure, and the ability of the membrane to operate at low RH. Low-RH-capable membranes allow either lower operating pressures for the same coolant temperature or higher coolant temperatures for the same pressure. Development of durable low-RH, high-temperature membranes is a key area of research.

The fuel cell stack consists of individual fuel cells connected in series to produce a high-voltage, high-current dc electric power source. A single fuel cell has two catalyzed electrodes that are separated by a thin membrane. The membrane is permeable to hydrogen ions (protons), but blocks the flow of electrons. One electrode is referred to as the anode and is where hydrogen fuel is split into hydrogen ions (protons) and electrons. Electrons produced in the anode move through an external electrical circuit providing power and are returned to the other electrode which is referred to as the cathode. At the cathode, oxygen from air is combined with hydrogen protons and electrons to form water. Control of humidification is one of the most critical issues in a PEM fuel cell. Too much humidity will cause water to condense, potentially blocking reactant flows. Too little humidity and the membrane will not adequately conduct protons. The proton conductivity of the membrane varies strongly with how much water is present in the membrane. The amount of water is often expressed in terms of the relativity humidity of the gas at the membrane. As the relative humidity (RH) at the membrane decreases from 100% to 80%, the proton conductivity of a Nafions

Cathode outlet absolute pressure required and relative radiator performance required vs coolant temperature with relative humidity (RH) at membrane of 100% and 50% 700 Relative radiator performance required (~1/ITD) 2.5

600

Pressure for 100% RH Pressure for 50% RH

500 2 400 1.5 300 1 200 0.5

100

0 50

55

60

65

70

75

80

85

90

95

Stack cathode outlet absolute pressure (kPa)

Relative radiator performance required (~1/ITD)

3

0 100

Fuel cell coolant temperature (°C)

Figure 4 Illustration of the trade-off between the coolant temperature, cathode pressure, and the ability of the membrane to operate at low RH. Membranes capable of durable operation at low RH (e.g., 50%) allow either lower operating pressures for the same coolant temperature or higher coolant temperatures for the same pressure. If the membrane requires high RH (e.g., near 100%), then the cathode inlet pressure must be increased as the stack coolant temperature increases. ITD, inlet temperature difference.

Applications – Transportation | Electric Vehicles: Fuel Cells

Further detail on the operation and trade-offs of the fuel cell stacks can be found in other articles of this encyclopedia. However, in general, it is desirable that the stack has low pressure drops and stoichiometry ratios for its reactants. This will minimize parasitic losses in the coolant pump, air compressor, and fuel recirculation compressor.

241

performance and durability of the fuel cell stack. If the stack’s electrode and membrane are able to operate at low cathode outlet RH then the demands on the humidification device are minimized. Heat energy may also be transferred by the humidification device. In cold conditions, the exhaust air can warm the intake air, assisting in faster warm-up of the stack.

Cathode Air Supply System Proton-exchange membrane fuel cell stack performance can be increased by pressurizing and humidifying the cathode inlet air. Pressurized air helps to keep the membrane well humidified when the stack coolant temperature is high, for example, >60 1C. Higher oxygen partial pressure has a secondary benefit of increasing performance of the cathode electrode catalyst by reducing kinetic voltage losses in the oxygen reduction reaction. A typical air supply system uses a compressor to generate cathode pressure in the range of 1.1–3.5 bar absolute at cathode air flows with stoichiometry ratios of 1.4–2.0. Inefficiencies in the compressor may cause the discharge air temperature to be too hot for the humidifier or stack materials. A heat exchanger tied to the stack coolant loop may be used to cool the air. When the ambient air is much cooler than the stack coolant temperature, this same heat exchanger warms the air before it enters the humidification device. The compressor is often the largest consumer of ancillary power. In higher-pressure systems, an expander may be used to recover flow energy from the cathode exhaust. The compressor is also one of the highest-cost items in the fuel cell system, other than the fuel cell stack. Low pressure systems, near 1 bar absolute, are appealing from a cost perspective. However, they require special humidification schemes or they will be restricted to low stack operating temperatures, for example, o65 1C, which limits their ability to produce power in high ambient temperatures as seen in many regions of the globe. The upcoming section on heat rejection describes the issue in more detail. After pressurization, the air may be passed through a humidification device. The humidifier transfers a portion of the water in the cathode exhaust stream to the cathode inlet stream. The amount of humidity transfer can be controlled, if required, by valves that bypass air around the humidifier. Other means for control may be employed, depending on the technology used for the humidifier. Over-humidification at the stack inlet can result in liquid water droplets entering the stack. Depending upon the stack’s construction, the liquid water can cause localized blockage of reactant flows that may decrease

Anode Hydrogen Fuel Supply System In contrast to the cathode air supply system which has air continuously flowing through it to the exhaust, the anode operates as a closed system to maximize fuel economy. The fuel supply system provides pressurized hydrogen to the stack, which may or may not be humidified. During operation, small amounts of inert impurities in the fuel can accumulate over time as the main fuel constituent, hydrogen, is consumed by reaction in the fuel cell. Nitrogen in the air will slowly pass from the cathode through the fuel cell membrane to the anode and dilute the hydrogen fuel. The diluents can build up over several minutes causing localized hydrogen starvation that damages the fuel cell stack electrode. To avoid localized hydrogen starvation, the effective anode stoichiometry may be increased mixing the anode gas with a recirculation compressor or jet pump (ejector), or other means. While the fuel cell only requires an ideal anode stoichiometry of 1.0, the need for uniform distribution of the hydrogen often leads to requiring much higher flow rates through the anode. The flow rates for uniform mixing may be 1.2–3 times that of the flow rate required to supply the fuel consumed in reaction. Thus the apparent anode stoichiometry required may be 1.2–3. Architectures for the fuel supply system include recirculation systems, dead-ended operation, cascading of anode stages, and flow-shifting between two stacks. Recirculation systems appear to have the highest tolerance for diluents while keeping hydrogen emissions to a minimum. Recirculation can be achieved with a jet pump that draws gas from the anode outlet of the stack and mixes it with fresh hydrogen supplied by a fuel injector. This mixture is then fed to the anode gas inlet of the stack. The jet pump requires no additional accessory power but may not achieve sufficient mixing when the fuel cell power output is very low as there is very little fresh hydrogen being injected into the anode. An alternative is to use a recirculation compressor, either by itself or in combination with a jet pump. The recirculation compressor keeps the fuel cell anode gas well mixed even when the fuel cell is running at very low power. Design of the compressor is challenging. It must operate with a mixture of water vapor and liquid water droplets, yet not be damaged or made inoperable in

242

Applications – Transportation | Electric Vehicles: Fuel Cells

freezing temperatures. The efficiency of the compressor is generally low due to the gas properties of hydrogen. The compressor may draw several hundred watts of electric power, reducing overall efficiency of the fuel cell system, particularly at low load conditions. To reduce the effect of diluents, the anode of the fuel cell may be periodically purged or have a small continuous bleed flow to remove diluents. Along with the diluents, hydrogen fuel is lost during the purge. This decreases fuel economy and also creates hydrogen emissions. What little hydrogen is emitted may be combined with the cathode outlet gas of the air supply system to ensure the mixture is not flammable when it leaves the exhaust pipe. Continuous mixing of the anode gas will reduce the risk of local starvation and can decrease the amount of fuel wasted by purges or bleeding. Anode purges can also be used, if needed, to remove excess water. Liquid water in the anode of the stack may block reactant flows resulting in electrode damage from hydrogen starvation. Systems with recirculation compressors have an advantage in that they can increase the anode gas flow, independent of load current, to clear or prevent blockages without having to purge the anode. Avoiding purges reduces the amount of fuel exhausted, increasing efficiency. Cooling System The cooling system consists of a pump, a bypass or threeway valve, radiator, and fan. The valve controls the amount of coolant that bypasses the radiator, so that the fuel cell temperature can be maintained at a desired set point. The pump flow rate may be varied with power level to control the temperature profile across the fuel cell stack and achieve the desired humidity profile from inlet to outlet in the fuel cell stack. Despite the apparent simplicity of the coolant system, it has many challenging requirements to meet. These include material compatibility for corrosion resistance, maintaining low coolant conductivity, removing large amounts of low temperature heat, and quickly and accurately regulating temperature to control membrane humidification. The coolant system also plays a critical role during startup of the fuel cell in freezing conditions. The coolant pump is a parasitic load on the fuel cell system and reduces overall efficiency of the system. The need for low coolant conductivity is discussed later in the section on isolation resistance.

Heat Rejection Cooling of an automotive fuel cell is more challenging than an internal combustion engine due to (1) the

relatively low operating temperature of the fuel cell and (2) the relatively higher amount of heat energy put into the cooling system. For equivalent performance in high ambient temperatures, the fuel cell requires a larger radiator and fan. Alternatively, power production in the fuel cell must be reduced at high ambient temperatures. To illustrate the cooling challenge, consider the difference in the amount of heat coupled into the coolant for an internal combustion engine compared to a fuel cell. At high power, a gasoline internal combustion engine produces roughly equal thirds of mechanical power, exhaust gas enthalpy, and heat to the coolant. Therefore, the ratio of heat power rejected to the coolant over the useful mechanical power delivered is about 1.0. At high power in a fuel cell, the amount of heat rejected to the coolant is about the same as the gross stack electrical power. An ideal 100% efficient fuel cell would produce a cell voltage of 1.23 V, as computed from the electrochemistry of the cell. However, in operation, the cell voltage is typically around 0.615 V. From this, the inefficiency of the fuel cell can be calculated to be about 50% of the gross fuel power (1.23–0.615 V)/1.23 V ¼ 50%. A low percentage of the heat is removed by the cathode exhaust stream under high power conditions. When auxiliary loads and aging of the stack are considered, the net electrical power decreases to 40% and the heat rejected to coolant increases to 60%. As a result, the ratio of heat power rejected to the coolant over the useful mechanical power delivered is about 1.5 at full load. The major factor adversely affecting fuel cell cooling is the relatively low temperature of the fuel cell coolant, typically around 80 1C, compared to an internal combustion engine at 120 1C. The amount of heat rejected (kW) is approximately linearly proportional to the frontal area of a welldesigned radiator–fan system and is inversely proportional to the initial inlet temperature difference (ITD) between the two fluid streams it combines (DT), for example, air and coolant. With an ambient air temperature of 40 1C and a fuel cell coolant temperature of 80 1C, the ITD is 40 1C. In the internal combustion engine with ambient air at 40 1C and coolant at 120 1C, the ITD is 80 1C. Thus for the same heat rejected, the fuel cell radiator must be twice the size. The relative increase in radiator size with coolant temperature is illustrated in Figure 4. If the stack is aged and auxiliary loads are high, the heat rejection performance of the fuel cell radiator may need to be 3 times that of an internal combustion engine. To address the heat rejection issue, fuel cell efficiency must be kept high by running at high cell voltage and with low auxiliary load losses. Additional radiator frontal area can be added through the use of wheelhouse radiators located in the front of the wheels, or at other

Applications – Transportation | Electric Vehicles: Fuel Cells

Cold Weather Starting and Operation Water is produced by the fuel cell and may be used to humidify the air and hydrogen reactants fed to it. If water at critical locations freezes during operation, the fuel cell performance may be degraded or it may be unable to start or continue running after startup. Ice can block reactant flows in the humidification device, flow-field channels, gas diffusion layer, and catalyst layer. Ice can prevent operation of control valves, injectors, and recirculation pumps. Sensors for temperature, pressure, and humidity can produce inaccurate readings or be damaged. Cold Weather Starting Careful insulation of the fuel cell system and its balance of plant components can prevent the system from freezing for a period of 8–24 h or more. Eventually, if no heat is added through running, battery discharge, or burning of fuel, the system will freeze and must be able to restart. Successful starting of the fuel cell depends on conditioning the fuel cell properly before it freezes. A shutdown purge of the cathode with air can remove water blockages in the valves, humidification devices, flow fields, gas diffusion media, and membrane–electrode assembly (MEA). Purging or dry operation of the anode prior to shutdown may be needed, though the noise, time, and energy consumption associated with this preparation must be considered. Care must be taken to not overly dry the MEA or the fuel cell may have difficulty restarting. Even after purging, some amounts of water and water vapor are likely present in the fuel cell system. As the fuel cell system cools, condensation can form and drops of water may block reactant paths or affect valves and sensors. Each component’s location, orientation, and ability to tolerate ice must be carefully engineered.

Proton-Exchange Membrane Fuel Cell Characteristics During Frozen Starts With the supply pathways for air and fuel clear and ready for gas flow, the fuel cell startup can begin. Below 10 1C, the PEM fuel cell’s power output characteristics are significantly different than at the normal operating temperature of 80 1C. The fuel cell’s polarization curve (voltage vs current characteristic) changes with temperature as shown in Figure 5. The fuel cell cannot produce full power when it is cold and therefore must be warmed up. For a given current, the fuel cell stack’s voltage is lower than normal, resulting in inefficient operation of the stack. The extra heat produced assists in a rapid warm-up of the stack. At –30 1C, the fuel cell’s maximum current is quite limited. As the fuel cell stack warms up, it is able to produce more power and thereby produce more waste heat. As soon as current is drawn from the fuel cell, water is produced in the electrode. This water will rapidly form ice on the flow field, diffusion media, and electrode, all of which are starting at temperatures below the freezing point of water. The ice can eventually block access of the reactants to the electrode and prevent the fuel cell from starting. To prevent the fuel cell from filling with ice, the fuel cell must be rapidly brought to a temperature >0 1C. The energy required to warm the fuel cell is dominated by the heat capacity of the fuel cell system rather than the amount of heat loss to the environment. Reducing the heat capacity of the fuel cell is critical to fast and successful starts. For a fuel cell with a heat capacity of 100 kJ K1, an average of 100 kW of heat power would be required to warm it from –30 to 0 1C in 30 s (3 MJ of energy). Typically, the waste heat power from the fuel cell alone is

Opencircuit voltage

Stack voltage

locations. A larger fan can be used to push more air through the radiator, but the increase in parasitic power to drive the fan reduces system efficiency with only a small gain in total heat rejection. The difficulty in rejecting heat from a PEM fuel cell is one of the primary drivers for research and development of membranes that can operate at higher temperatures and with lower RH in the membrane. One difficulty is that at above 80 1C maintaining humidification of the membrane and electrode becomes increasingly challenging. If cooling is insufficient, and coolant temperature is allowed to rise, the membrane will quickly dry out and stack power production will be reduced. It should be noted that heat rejection limits are usually only encountered when traveling at high speeds or up steep grades on hot days.

243

80 °C 0 °C −10 °C −30 °C −20 °C

Stack current

Limiting current

Figure 5 The polarization curve (i.e., fuel cell voltage vs current curve) of a proton-exchange membrane (PEM) fuel cell changes strongly with temperature below 0 1C. Initial power production of the fuel cell is limited below 0 1C, necessitating a rapid warm-up.

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Applications – Transportation | Electric Vehicles: Fuel Cells

much less than this and is insufficient to warm the fuel cell in 30 s. Additional heat can be created by using electric heaters in the stack coolant loop. Power for the heater can come from the fuel cell stack electric output or from an electric energy storage device such as a battery or an ultracapacitor. Catalytic reaction of hydrogen fuel with air can also be used to directly create heat. Cold Weather Operation Keeping the fuel cell system from freezing during operation is challenging. Relatively little heat is produced by the fuel cell at light loads due to its inherently high efficiency. As little as 650 W of waste heat may be produced by a fuel cell vehicle idling while producing 600 W of net electric load. The amount of heat lost to the ambient environment at –30 1C can greatly exceed the amount of heat produced at idle. Coolant leaking around the thermostat to the radiator at a rate of less than 100 cc per minute can result in hundreds of watts of heat loss. If the fuel cell is not able to keep itself warm, performance will suffer or worse the system may freeze, causing it to shut down. Maintaining the desired temperature of the fuel cell involves balancing the heat generation and loss. Insulation may be added around the fuel cell stack and balance of plant components to reduce heat loss. Valves in the coolant path to the radiator will need to have very low leakage rates. The effect of heat gain or loss from gas reactant and product flows must also be considered. Generating more heat in the stack than the driver demand dictates, or using supplemental heaters will adversely affect fuel economy in cold weather, but may be required under certain operating conditions to prevent components from becoming impaired by frozen product water. Cabin Heating The automobile requires heat energy of 4 kW or more for defrosting the windshield and for comfort heating of the passenger cabin. In an internal combustion vehicle, ample waste heat is available for cabin heating due to the overall inefficiency of the engine. This is not the case in a fuel cell vehicle. Consider for example a fuel cell vehicle whose average power required during a city driving cycle is on the order of 5 kW. Only 2.8 kW of heat energy is available, assuming the stack efficiency is 64% at this light load. The heat required can be provided by adding electric heating elements that directly warm the cabin or defrost air or heat the fuel cell coolant, which is coupled to the cabin air through a heat exchanger. Warming the fuel cell coolant has an added benefit of allowing one component to both provide cabin heating and reduce the time it takes

to start in freezing conditions. In either case, electric energy required for cabin heating reduces cold operation fuel economy.

Hybridization A fuel cell system may be combined with a bidirectional energy storage device to create a hybrid powertrain, benefiting both the vehicle and fuel cell. The vehicle benefits from faster dynamic power response, increased driving range from capturing braking energy, and increased efficiency through optimizing the loading profile of the fuel cell. The fuel cell benefits from hybridization by reducing the severity of events that cause electrode degradation. These severe events include cycling of the fuel cell voltage from high to low as the load on the fuel cell varies up and down, and extended operation with the fuel cell at high voltage, for example, when idling with no load on the fuel cell stack. Vehicle Benefits The response of the fuel cell is usually limited by the dynamics of the air compressor system but may be limited by the fuel system if high levels of diluents, such as nitrogen, are present in the anode (hydrogen fuel side of the fuel cell stack). A PEM fuel cell system can achieve a dynamic response o1 s from 10% to 90% power, which is adequate to meet most consumer expectations, but a typical bidirectional energy storage device, such as a nickel–metal hydride battery, lithium-ion battery, or ultracapacitor, can respond even faster. Energy that is normally lost in braking can be captured by the energy storage device to be used later. An increase in driving range of 5–20% is possible. Overall efficiency of the vehicle can be increased by operating the fuel cell near its peak efficiency point and avoiding lower-efficiency regimes, such as idle and maximum power. The fuel cell provides the average power demands by continually charging the energy storage device, while the energy storage device provides rapid power bursts when needed. Efficiency can be further improved by temporarily shutting off the fuel cell when very little power is demanded of it, such as when the vehicle is stopped at a traffic light. Energy normally used to run the air compressor or other support systems is saved. The fuel cell can be rapidly restarted when power is demanded, using energy from the energy storage device. Overall efficiency can be improved by up to a few percentage points. The main requirements are that the energy storage device has a round-trip energy efficiency that is sufficiently high, say >90%, and that the energy storage device is able to supply the peak power demands of restarting accessories while initially propelling the vehicle.

Applications – Transportation | Electric Vehicles: Fuel Cells

Fuel Cell Benefits

2.0

−10

1.6

−20

1.2

−30

0.8

−40

0.4

−50

0.0 0.82 0.84

0.86

0.88

0.90

0.92

0.94

ECA relative to initial (%)

Voltage decay rate (µV cycle−1)

Reducing the dynamic response requirement on the fuel cell may allow decreased power ratings for some accessory components, such as the compressor inverter, and therefore lower cost. The power rating of the entire fuel cell system can also be reduced. The amount of benefit varies with system design. Life of the fuel cell can also be extended by hybridization. Platinum–carbon electrodes have been shown to lose electrochemical area when their voltage is repeatedly cycled from 0.6 (full load) to 0.94 V (no load). Figure 6 shows that the degradation rate due to voltage cycling is lower when the highest potential that a fuel cell sees is reduced from 0.94 to 0.85 V. The highest cell potential in hybrid fuel cell powertrain can be reduced by keeping the fuel cell loaded to a predetermined minimum power value at all times. A similar loss in electrochemical area over time occurs when platinum–carbon electrodes are held near open circuit voltage, such as when the fuel cell system is idling for an extended period. Keeping the fuel cell operating at a current density greater than a set minimum value is effective in reducing this voltage level and reducing the rate of electrode degradation. Any excess power produced by the fuel cell is used to slowly charge the battery. Degradation of the fuel cell electrode during startup and shutdown is a major contributor to shortened fuel cell life. A normal usage profile for an engine can include 35 000 start and stop events. Hybridization can help the fuel cell during startup and shutdown by having power available to rapidly establish air flow, purge or dilute gases, and condition the fuel cell humidity for shutdown. Finally, in freezing conditions, the fuel cell may be able to start faster using stored energy to warm up the

−60 0.96

Upper limit voltage (V)

Figure 6 Effect on fuel cell durability of using hybridization to control the highest cell voltage seen by the fuel cell during operation. Reducing the highest voltage seen by the fuel cell results in lower rate of cell voltage decay and less loss of electrochemical surface area (ECA) over time. Reproduced with permission from SAE publication 2008-01-0423.

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fuel cell rapidly, thaw small components if they are unexpectedly frozen, and run accessories while the fuel cell’s power output capability is limited.

Packaging A fuel cell propulsion system does not have some of the packaging constraints of an internal combustion engine system. The fuel cell system produces less structural vibration, it may not have to carry reaction forces from the driveline, and it does not produce the high exhaust temperatures (>500 1C) of the internal combustion engine. Components of the system, such as the powerconditioning electronics and the final motor drive unit, can be located remotely from the fuel cell. This has given car designers freedom to come up with new vehicle structural and styling concepts. The fuel cell can be placed in the underbody structure, rather than under the hood, allowing a clear view forward. It can also allow a variety of body styles to be fit to the same foundational powertrain and chassis. Fuel cells do have their own set of new constraints. An insulation layer may be needed to control heat loss in cold environments. This reduces the space available for the fuel cell and also places a constraint on how components are distributed throughout the vehicle. Pipes, plumbing, and compressors for the fuel cell may require insulation to prevent heat loss, freeze-up, or undesired condensation. The volumetric power density of a state-of-the-art fuel cell system, including power conversion electronics and radiator, is lower than that of a gasoline engine. The fuel cell vehicle requires a hydrogen storage system that, for equivalent vehicle range, has a volume that is several times larger than that of a gasoline vehicle. Unlike a gasoline tank, the high-pressure hydrogen tank vessel is not conformable to the shape of the vehicle. Packaging of a fuel cell is strongly influenced by crash requirements. The larger volume of the fuel cell system may make it more challenging to achieve the required crush zone clearance. Components mounted on an automobile are subject to g-forces of 3–11 gs in normal operation from going over potholes, hitting curbs, and low-speed (8 km h1) bumper impacts. If the fuel cell is coupled to the drive unit, such as may be done when the fuel cell is packaged under the hood, then additional clearance may be left to account for roll of the fuel cell when reaction torque is transmitted through it from the drive motor unit. As components are distributed across the vehicle, care must be taken to ensure electromagnetic interference is not created due to the electromagnetic energy produced by the high-frequency switching of the power electronics. A variety of design approaches can be used such as

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Applications – Transportation | Electric Vehicles: Fuel Cells

keeping wire lengths short, shielding conductors, or slowing the rise times of switching waveforms.

Design for Safety Crash Safety One relatively unique set of requirements for automobile fuel cell applications are those related to crash safety for occupants and emergency first responders. The vehicle is designed and the fuel cell system packaged to minimize the risk of hazards in the event of a crash. Crash requirements for vehicles and components in the United States are defined in Federal Motor Vehicle Safety Standards (FMVSS); for example, FMVSS 301 addresses fuel system integrity, FMVSS 303 addresses fuel system integrity of compressed gas, FMVSS 304 addresses compressed gas fuel container integrity, and FMVSS 305 addresses electric-powered vehicles. Updates to these standards are under development to include aspects specific to hydrogen fuel cell systems. For example, the use of aqueous coolant in the fuel cell creates a slightly conductive path from the high voltage bus to chassis. The normal range of isolation resistance for fuel cells is therefore different from a battery. The crash events to which the vehicle is subjected to are also defined. For example, SAE J1766 (SAE, Society of Automotive Engineers) specifies that fuel cell vehicles shall meet the performance criteria when tested to the crash procedures of FMVSS 208 frontal impact (including the full frontal and 301 front angle impacts), FMVSS 214 side impact, and FMVSS 301 rear impact. The standards identify hazards to be avoided including high-voltage shock, excess hydrogen fuel release, occupant space excessively intruded upon, and excessive spillage of battery electrolyte outside of the passenger compartment or inside the passenger compartment. Careful design, placement, and robust testing of the storage tank, valves, lines, and supporting structure ensure integrity of the hydrogen storage system. Compressed hydrogen tanks are subjected to extreme abuse testing to ensure they do not fail. Testing includes overpressurization, pressure cycling, impact testing, drop testing, gun fire testing, and engulfing in a bonfire for 20 min. The hydrogen fuel system is designed with safety features such as redundant shutoff valves, thermally activated pressure relief devices, and heat shields. If a crash is detected, valves shut off the flow of fuel at the tank. The tanks, valves, and lines are protected by careful integration into the vehicles, allowing deformation of the vehicle in a crash and minimizing the risk of leaking from the tanks. Crash simulations and testing are used to verify that components survive and meet requirements.

After a crash event, the high-voltage system of the fuel cell vehicle is designed to not present a hazard to emergency first responders, passengers, or others. Highvoltage cables and harnesses are visually identified with an orange covering material. The high-voltage system is continuously monitored to verify that it is isolated from chassis. If the isolation is lost in a crash event, for example by damage to the fuel cell stack, high-voltage components, or insulated wiring, the high-voltage bus is de-energized. In a severe crash that would cause activation of an airbag, the high-voltage bus may be automatically de-energized by means such as opening contactors to the battery and fuel cell. Training materials for emergency first responders illustrate how to de-energize the high-voltage system before attempting to assist passengers. The system may be de-energized by turning off the ignition, cutting the 12 V negative battery cable, and waiting for 10 s. Isolation Resistance The risk of electrical shock in an electric vehicle is minimized by insulating all high-voltage conductors and by isolating both the positive and negative sides of the fuel cell from the vehicle chassis. If the insulation on one side of the high-voltage bus fails and a person comes in contact with that side of the high-voltage bus and chassis, there is no shock hazard because the circuit is not completed. Isolation ensures that the other side of the high-voltage bus has no path for current flow to chassis. SAE J2578 Recommended Practice for General Fuel Cell Vehicle Safety specifies that the isolation resistance be at least 125 O V1 from either terminal of the highvoltage bus to chassis, in consideration of safety and the constraints on fuel cell system coolant conductivity and coolant path geometry. For example, in a system with a nominal voltage of 400 V, the measured isolation resistance would need to be greater than or equal to 50 kO. For battery electric vehicles, FMVSS 305 specifies that the isolation resistance from either terminal of high voltage to chassis is at least 500 O V1. For example, in a system with a nominal voltage of 400 V, the measured isolation resistance would need to be at least 200 kO. In a fuel cell vehicle, the aqueous coolant that passes through a fuel cell stack may become more conductive over time, creating an electrical path between the cells within the stack and the coolant pump, coolant piping, and radiator. If one of these components outside the stack is grounded to chassis, the isolation resistance between high-voltage bus and chassis is reduced. The effective isolation resistance depends on the coolant conductivity, path length, and path geometry. A long, narrow coolant path from the stack to the first chassis-grounded component will increase the isolation resistance, but at the expense of coolant pressure drop.

Applications – Transportation | Electric Vehicles: Fuel Cells

Maintaining low coolant conductivity over time requires that contamination and leaching is minimized. One design solution is the use of deionization cartridges, which may require periodic replacement. Hydrogen Emissions SAE J2578 Recommended Practice for General Fuel Cell Vehicle Safety specifies limits on the concentration of hydrogen emissions from the fuel cell system. Emission limits are specified as a percentage of the lower flammability limit for hydrogen. The lower flammability limit of hydrogen is 4% hydrogen (by volume) in air. Therefore, an emission limit of 25% of the lower flammability limit is equivalent to 1% hydrogen by volume in air. For normal operation, which includes startup and shutdown purges, the concentration outside the vehicle should be kept below 50% of the lower flammability limit at the point of discharge and below 25% of the lower flammability limit in the surrounding area. Inside the vehicle, the concentration of hydrogen should be kept below 25% of the lower flammability limit. The emissions from the vehicle must also be low enough so that when a vehicle is parked in a garage or enclosure without forced ventilation, the concentration of hydrogen does not exceed 25% of the lower flammability limit. A natural air exchange rate of 0.18 air changes per hour is assumed. The total amount of hydrogen exhausted during operation, startup, and shutdown must also be kept relatively small in order to meet the requirements for high efficiency and long range. Hydrogen emission limits can be successfully met using a variety of methods such as compartment barriers, forced convection, catalytic reactors, dilution with cathode exhaust, and other means.

Conclusions As of 2008, most major manufacturers have hydrogen vehicle demonstration fleets on the road and additional fleets planned. The fleets range in size from a dozen to over a hundred vehicles. These fleets have shown good driving performance and cold-weather start ability. Durability of the fuel cell system is rapidly improving, but does not yet meet the target of greater than 5000 operating hours. Manufacturing costs are high due to the relatively low production volumes. Catalyst cost remains a significant, but not insurmountable, issue. Despite the challenges, the strong benefits of fuel cells are driving rapid technological development. Researchers are working to reduce the amount of catalyst required and looking for lower-cost, higher-performance catalysts. A great amount of progress is being made in membranes with new materials showing better mechanical strength,

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chemical resistance, and higher conductivity at low RH. Electrodes are being developed that are more resistant to voltage cycling and start/stop degradation. Through the concerted effort of manufacturers, governments, and academia, tremendous progress is being made toward the widespread use of clean and efficient hydrogen fuel cells.

Nomenclature Symbols DT

temperature difference

Abbreviations dc ECA FMVSS ITD MEA PEM RH SAE

direct current electrochemical surface area Federal Motor Vehicle Safety Standards inlet temperature difference membrane-electrode assembly proton-exchange membrane relative humidity Society of Automotive Engineers

See also: Applications – Transportation: Hybrid Electric Vehicles: Batteries; Light Traction: Fuel Cells; Fuel Cells – Overview: Modeling; Fuel Cells – Proton-Exchange Membrane Fuel Cells: Catalysts: Life-Limiting Considerations; Dynamic Operational Conditions; Freeze Operational Conditions; High Temperature PEMFCs; Impurities in Fuels and Air; Life-Limiting Considerations; Membrane–Electrode Assemblies; Membranes: Design and Characterization; Membranes: Elevated Temperature; Stacks; Systems; Water Management; Fuels – Hydrogen Storage: Compressed; Liquid; Measurement Methods: Structural Properties: Neutron and Synchrotron Imaging, In-Situ for Water Visualization; Safety: High Voltage.

Further Reading Abraham DP, Heaton JR, Kang SH, Dees DW, and Jansen AN (2008) Investigating the low-temperature impedance increase of lithium-ion cells. Journal of the Electrochemical Society 155(1): A41--A47. Ahluwalia RK and Wang X (2008) Fuel cell systems for transportation: Status and trends. Journal of Power Sources 177(1): 167--176. Al Hallaj S and Selman JR (2000) A novel thermal management system for electric vehicle batteries using phase-change material. Journal of the Electrochemical Society 147(9): 3231--3236. Aoyama T, Iiyama A, Shinohara K, Kamegaya S, Yamamoto S, and Ban Y (2008) Status of FCV Development at Nissan and Future Issues, SAE (SP-2167). Fuel Cell Vehicle Applications. Warrendale, PA: Society of Automotive Engineers International. Ariyoshi K, Yamatoa R, Makimura Y, Amazutsumi T, Maeda Y, and Ohzukua T (2008) Three-volt lithium-ion battery consisting of Li[Ni1/2Mn3/2]O4 and Li[Li1/3Ti5/3]O4: Improvement of positive-

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electrode material for long-life medium-power applications. Electrochemistry 76(1): 46--54. Belt JR, Ho CD, Miller TJ, Habib MA, and Duong TQ (2005) The effect of temperature on capacity and power in cycled lithium ion batteries. Journal of Power Sources 142(1–2): 354--360. Brinkman N, Wang M, Weber T, and Darlington T (2005) Well-to-Wheels Analysis of Advanced Fuel/Vehicle Systems – A North American Study of Energy Use, Greenhouse Gas Emissions, and Criteria Pollutant Emissions. Technical Reports, Argonne National Laboratory, May. Bristow AL, Tight M, Pridmore A, and May AD (2008) Developing pathways to low carbon land-based passenger transport in Great Britain by 2050. Energy Policy 36(9): 3427--3435. Chalk SG and Miller JE (2005) Key challenges and recent progress in batteries, fuel cells, and hydrogen storage for clean energy systems. Proceedings of the 3rd International Conference on Materials for Advanced Technologies (ICMAT-2005)/9th International Conference on Advanced Materials (ICAM 2005), pp. 73–80. Singapore: Elsevier Science. Chalk SG, Miller JF, and Wagner FW (1999) Challenges for fuel cells in transport applications. Proceedings of the 6th Grove Fuel Cell Symposium Fuel Cells – The Competitive Option for Sustainable Energy Supply, pp. 40–51. London: Elsevier Science. Chapman L (2007) Transport and climate change: A review. Journal of Transport Geography 15(5): 354--367. De Francesco M and Arato E (2002) Start-up analysis for automotive PEM fuel cell systems. Journal of Power Sources 108(1–2): 41--52. Fronk MH, Wetter DL, Masten DA, and Bosco A (2000) PEM Fuel Cell System Solutions for Transportation SAE (SP–1505). Fuel Cell Power for Transportation. Warrendale, PA: Society of Automotive Engineers International. Gasteiger HA and Mathias MF (2002) Fundamental research and development challenges in polymer electrolyte fuel cell technology. In: Murthy M (ed.) Proceedings of the 3rd Symposium on Proton Conducting Membrane Fuel Cells, pp. 1--24. Salt Lake City, UT: Electrochemical Society. Gasteiger HA, Panels JE, and Yan SG (2002) Dependence of PEM fuel cell performance on catalyst loading. 8th Ulm Electro-Chemical Talks on Electrochemical Energy Conversion and Storage, pp. 162–171. Ulm, Germany: Elsevier Science. Kang KS, Meng YS, Breger J, Grey CP, and Ceder G (2006) Electrodes with high power and high capacity for rechargeable lithium batteries. Science 311(5763): 977--980.

Lasher S (2005) Fuel choice for FCVs: Hydrogen infrastructure costs. Presented at the 2005 Hydrogen Merit Review Meeting. Crystal City, VA. Lipman TE and DeLucchi MA (1996) Hydrogen-fuelled vehicles. International Journal of Vehicle Design 17(5–6): 562--589. Meng H (2008) Numerical analyses of non-isothermal self-start behaviors of PEM fuel cells from subfreezing startup temperatures. International Journal of Hydrogen Energy 33(20): 5738--5747. Moriarty P and Honnery D (2008) The prospects for global green car mobility. Journal of Cleaner Production 16(16): 1717--1726. National Research Council (2008) Review of the Research Program of the FreedomCAR and Fuel Partnership Second Report. Washington, DC: National Academy of Sciences. Ogden JM, Williams RH, and Larson ED (2004) Societal lifecycle costs of cars with alternative fuels/engines. Energy Policy 32(1): 7--27. Oszcipok M, Riemann D, Kronenwett U, Kreideweis M, and Zedda A (2004) Statistic analysis of operational influences on the cold start behaviour of PEM fuel cells. Meeting on Fuel Cells Science and Technology, pp. 407–415. Munich, Germany: Elsevier Science. Rousseau A, Sharer P, and Ahluwalia R (2004) Energy storage requirements for fuel cell vehicle. SAE Paper No. 2004-01-1302. Warrendale, PA: Society of Automotive Engineers International. Shukla AK and Kumar TP (2008) Materials for next-generation lithium batteries. Current Science 94(3): 314--331. Thompson EL, Jorne J, Gu WB, and Gasteiger HA (2008) PEM fuel cell operation at –20 degrees C: II. Ice formation dynamics, current distribution, and voltage losses within electrodes. Journal of the Electrochemical Society 155(9): B887--B896. Vergragt PJ and Brown HS (2007) Sustainable mobility: From technological innovation to societal learning. Journal of Cleaner Production 15(11–12): 1104--1115. von Helmolt R and Eberle U (2007) Fuel cell vehicles: Status 2007. Journal of Power Sources 165(2): 833--843. Wagner U, Eckl R, and Tzscheutschler P (2006) Energetic life cycle assessment of fuel cell powertrain systems and alternative fuels in Germany. Energy 31(14): 3062--3075. Wu JF, Yuan XZ, Martin JJ, et al. (2008) A review of PEM fuel cell durability: Degradation mechanisms and mitigation strategies. Journal of Power Sources 184(1): 104--119.