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© 2014 Woodhead Publishing Limited
685
22
Fuel-cell (hydrogen) electric hybrid vehicles
B. G. Pollet, University of the Western Cape, South Africa, 
I. StAffell, Imperial College london, UK, J. l. ShAnG, 
University of Birmingham, UK and V. MolKoV, 
University of Ulster, UK
DOI: 10.1533/9780857097422.3.685
Abstract: Decarbonising transport is proving to be one of today’s major 
challenges for the global automotive industry due to many factors such as 
the increase in greenhouse gas and particulate emissions not only affecting 
the climate but also humans, the increase in pollution, rapid oil depletion, 
issues with energy security and dependency from foreign sources and 
population growth. Major breakthroughs in low and ultra-low carbon 
technologies and vehicles are urgently required. this chapter highlights 
the current status of hybrid, battery and fuel cell electric vehicles from an 
electrochemical and market point of view.
Key words: automotive energy storage devices (eSD), hybrid electric 
vehicle (heV), battery electric vehicle (BeV), fuel cell electric vehicle 
(fCeV), hydrogen
22.1 Introduction
Decarbonising transport is proving to be one of the largest R&D projects 
of the early 21st century. there are around 1 billion automobiles in use 
worldwide, satisfying many needs for mobility in daily life [1]. the automotive 
industry is therefore one of the largest economic forces globally, employing 
nearly 10 million people and generating a value chain in excess of $3 trillion 
per year [2]. As a consequence of this colossal industry, the large number 
of automobiles in use has caused and continues to cause a series of major 
issues in our society:
∑ Greenhouse gas (GHG) emissions – the transportation sector contributes 
13.1% of GhG emissions worldwide (5 billion tonnes of Co2 per year). 
More than two thirds of transport-related GhG emissions originate from 
road transport [3]. Reducing the GhG emissions of automobiles has thus 
become a national and international priority. 
∑ Air pollution – tailpipe emissions are responsible for several debilitating 
respiratory conditions, in particular the particulate emissions from diesel 
vehicles. the increasing number of diesel vehicles on europe’s roads 
would further worsen air quality.
686 Alternative Fuels and Advanced Vehicle Technologies
∑ Oil depletion – oil reserves are projected to only last 40–50 years with 
current technology and usage. transport is already responsible for almost 
70% of the eU’s oil use and this share continues to increase [4].
∑ Energy security – europe’s dependence on foreign sources for more 
than 80% of its oil and reserves of conventional oil is increasingly 
concentrated in politically unstable regions [4]; dependency on fossil 
fuels for transportation therefore needs to be reduced.
∑ Population growth – It has recently been declared (31 october 2011) 
that there are 7 billion inhabitants in the world with an estimated figure 
of 9 billion in 40 years’ time. this will obviously have an important 
impact on climate change, food security and energy security.
the ever increasing demand for personal mobility and near total dependence 
on liquid hydrocarbons means that emission reductions from this sector will 
be particularly difficult.
 the development of alternative fuels to petrol and diesel has been on-
going since the 1970s, initially in response to the oil shocks and concerns 
over urban air pollution. efforts have gained momentum more recently 
as the volatility of oil prices and stability of supplies, not to mention the 
consequences of global climate change, have risen up political agendas the 
world over. low-carbon technologies are therefore rapidly advancing, with 
petrol and diesel hybrids, battery electric, hydrogen fuel cell, and hybrids 
of the two being developed by nearly every major manufacturer. Concerns 
about up-scaling production and the ‘true’ environmental and social costs 
of biofuels means that hydrogen and electricity are widely regarded as the 
sustainable transport fuels of the future.
 this chapter aims to highlight the current status of hybrid, battery and 
fuel cell electric vehicles from an electrochemical and market point of 
view. the chapter also discusses the advantages and disadvantages of using 
battery, hydrogen and fuel cell technologies in the automotive industry and 
the impact of these technologies on consumers.
22.2 Energy storage devices (ESDs) for the 
transport sector
energy storage devices are systems which store energy in various forms 
such as electrochemical, kinetic, pressure, potential, electromagnetic, 
chemical, and thermal; using, for example, fuel cells, batteries, capacitors, 
flywheels, compressed air, pumped hydro, super magnets, hydrogen, etc. 
The principal criteria of an ESD required for a specific application, in this 
case automotive, are:
∑ the amount of energy in terms of specific energy (in Wh.kg–1) and energy 
density (in Wh.kg–1 or Wh.l–1)
687Fuel-cell (hydrogen) electric hybrid vehicles
∑ the electrical power (in W.kg–1 or W.l–1), i.e. the electrical load 
required
∑ the volume and mass
∑ reliability
∑ durability
∑ safety
∑ cost
∑ recyclability
∑ environmental impact.
When choosing an eSD, the following characteristics should be 
considered: 
∑ specific power
∑ storage capacity
∑ specific energy
∑ response time
∑ efficiency
∑ self-discharge rate/charging cycles
∑ sensitivity to heat
∑ charge-discharge rate life time
∑ environmental effects
∑ capital/operating cost 
∑ maintenance.
for battery electric vehicles (BeVs), batteries with stored energies of 5–30 
kWh for small urban BeVs (up to 85 kWh, e.g. the recently launched tesla 
Model e) and up to 100 kWh for electric buses are required; whereas hybrid 
electric vehicles (heVs) hold 1–5 kWh of stored energy, and focus more 
exclusively on high power discharge. table 22.1 shows several types of 
electrochemical eSDs and their characteristics.
22.3 Batteries
A battery is an electrochemical cell (also known as a galvanic cell) that 
transforms chemical energy into electrical energy; it consists of an anode 
and a cathode, separated by an electrolyte (an ionic conductor which is also 
an electronically insulating medium). electrons are generated at the anode 
and flow towards the cathode through the external circuit while, at the same 
time, electroneutrality is ensured by ion transport across the electrolyte. table 
22.2 shows characteristics of various types of battery.
 the two main types of battery used in BeVs are nickel metal hydride 
(niMh) and lithium-ion (li-ion) batteries. niMh batteries are in most cases 
used as secondary energy sources in heVs (e.g. toyota Prius) where they 
are used in conjunction with an internal combustion engine (ICe), whereas 
688 Alternative Fuels and Advanced Vehicle Technologies
Table 22.1 Electrochemical ESD characteristics
Characteristics Supercapacitors Batteries Fuel cells
 (electrochemical 
 capacitors)
Charge/discharge time ms–s 1–12 hrs 1–300 hrs 
Operating temperature/°C –40 to + 85 –20 to + 65 + 25 to + 1,000
Operating cell potential 2.3–2.75 1.25–4.2 0.6–1.0 
 (DV)/V
Capacitance/F 0.1–2 – –
Life time 30,000 + hrs 150–1,500 cycles 1,500–10,000 hrs 
Weight/kg 0.001–2 0.001–10 0.02–10
Power density/kW.kg–1 10–100 0.005–0.4 0.001–0.1
Energy density/Wh.kg–1 1–5 5–600 300–3,000
Table 22.2 Characteristics of various types of battery [5]
Battery chemistry Type (primary Cell Theoretical Useful energy 
 (P)/secondary potential (practical) density/Wh.l–1
 (S)) (DV)/V specific
 energy/
 Wh.kg–1 
Alkaline zinc manganese P 1.5 358 (145) 400
 dioxide (Zn/MnO2)
Lithium iodine (Li/I2) P 2.8 560 (245) 900
Alkaline nickel S 1.3 244 (35) 100
 cadmium (NiCd)
Nickel metal hydride S 1.3 240 (75) 240
 (NiMH)
Lead acid (Pd/A) S 2.1 252 (35) 70
Sodium sulphur (Na/S) S 2.1 792 (170) 345
Sodium nickel chloride S 2.6 787 (115)kWh.kg–1 [15]. this is, however, an unfair and misleading comparison. 
While the fuel in a conventional vehicle weighs relatively little by itself, 
plenty of additional equipment is required to convert petrol into motion: an 
engine, radiator, transmission system, fuel delivery lines, an exhaust and 
catalytic converter. In contrast, the batteries of an electric vehicle comprise 
the majority of the power-train mass, as the motor, wiring and transmission 
are relatively small, light systems. fCeVs lie in between the two extremes, 
as the fuel cell stack and its ancillaries are a similar weight and volume to 
the high pressure storage tank.
 A second issue is that of conversion efficiency. The ‘value’ of hydrocarbons, 
hydrogen and electrons lies in how much of their energy can be converted into 
a useful form of energy, namely motion at the wheels, as this determines the 
range of the vehicle and cost per mile travelled. Modern combustion engines 
717Fuel-cell (hydrogen) electric hybrid vehicles
(without hybridisation) are approximately half as efficient as a practical fuel 
cell engine, and a quarter as efficient as battery charge-discharge cycles. Table 
22.6 demonstrates the impact that system weight and conversion efficiency 
have on the specific energy of petrol, hydrogen and lithium-ion batteries, 
using data from four modern vehicles. In all cases, the effective specific 
energy is substantially reduced from the values that are typically reported.
 In order to compete with conventional vehicles in terms of ‘usable energy 
density’ and thus offer a similar driving range, batteries require a five-fold 
increase in specific energy, while hydrogen only requires a 30% improvement. 
A better battery that could hold 666 Wh.kg–1 at the cell level, or a hydrogen 
tank that held 6 wt.% would therefore give comparable performance from 
the driver’s perspective. hydrogen has strong potential to meet this target, as 
‘type 4’ all-composite compressed hydrogen tanks are currently approaching 
5.2 wt.% at 700 bar [37]. Cryo-compressed hydrogen can presently attain 6–8 
wt.%, however it suffers from a significant energy cost of liquefaction. Several 
Table 22.6 Comparison of petrol, hydrogen and electrical storage systems in four 
leading vehicles
Conventional Hybrid Hydrogen Battery
Reference vehicle Volkswagen 
Golf VI
Toyota Prius 
III
Honda FCX 
Clarity
Nissan Leaf
Fuel weight (kg) 40.8 33.3 4.1 171a
Storage capacity (kWh) 500 409 137 24
Specific energy (Wh 
primary/kg fuel)
12,264 12,264 33,320 140a
Storage system weight 
(kg)
48 40 93 300b
Specific energy (Wh 
primary/kg of storage)
10,408 10,261 1,469 80
Net power (kW) 90 100 100 80
Power plant and auxiliary 
weight (kg)
233 253 222 100
Specific energy (Wh 
primary/kg total 
equipment)
1,782 1,398 315 60
Average conversion 
efficiency
21% 35% 60% 92%
Effective storage capacity 
(kWh useable)
105.0 143.1 82.0 22.1
Specific energy (Wh 
useable/kg total 
equipment)
374 489 260 55
Notes: a: bare laminated lithium-ion cells; b: Including battery management and 
cooling systems.
718 Alternative Fuels and Advanced Vehicle Technologies
forms of metal hydride storage are also on the horizon, with ammonia-borane 
(AB) complexes and metal-organic frameworks (Mof-5) also expected to 
exceed 6 wt.% [37]. toyota are targeting 8 wt.% storage by 2015 using a 
hybridisation of compressed gas and solid hydride storage tanks [38].
 It is unlikely that Li-ion batteries will attain a specific energy of 666 
Wh.kg–1, as the optimisation of current chemistries has reached practical 
limits [8]. Several alternative chemistries are under development which could 
reach this goal in time; with lithium metal, lithium-sulphur, lithium-air, and 
non-lithium intercalation chemistries all offering the potential of systems-
level specific energies of at least 500 Wh.kg–1 [39].
 there are no near- or long-term developments in capacitor technology 
that are expected to deliver such an improvement in specific energy. Novel 
graphene electrodes have demonstrated specific energies up to 86 Wh kg–1 
of bare electrode, implying around 20 Wh.kg–1 for a complete system [40]. 
these developments are likely to spill over into battery electrodes, with 
graphene anodes offering the potential to double energy density in lithium-
ion batteries [46].
22.9.4 Efficiency
A high charge/discharge or conversion efficiency is also important as it 
impacts on the overall energy efficiency, running costs and life-cycle CO2 
emissions of the vehicle. Maximising efficiency must be done at the expense 
of other goals, notably power density and charge/discharge rates, which in 
turn impact on system size, weight and cost. Capacitors currently hold the 
advantage, with cycle efficiencies in the range of 90–98%, leaving little 
room for improvement. Battery efficiency has also improved substantially 
with the transition from lead acid (75–85%) and niMh (65–85%) to lithium 
chemistries. Under ideal conditions, cycle efficiencies of lithium-ion batteries 
now rival those of capacitors at 90–94% in automotive use [47–49].
 Battery system efficiency is closely related to operating conditions, 
decreasing with both current and temperature. the desire of consumers to 
move to rapid charging systems is therefore cause for concern. When charged 
with a 3 kW household charger the charging efficiency of the Mitsubishi 
i-MieV is around 90%; however, the 50 kW quick-charge option can be 
as little as 60% efficient, due to the additional energy required to cool the 
battery [47]. Inductive charging is another promising option, due to its speed 
and inherent safety; however, it reduces efficiency by a further 10% [50].
 the US Department of energy (Doe) and Japan’s neDo hydrogen 
energy roadmap both set a target of 60% efficiency for FCEVs circa 2015. 
It appears from the latest field trials in California that this target is close 
to being attained, with fuel cell vehicles from five major manufacturers 
averaging 52%, and the top manufacturer achieving 57% [36].
719Fuel-cell (hydrogen) electric hybrid vehicles
 A vehicle’s well-to-wheel (WtW) pathway traces from the original 
energy source through conversion, distribution and storage, to the wheels of 
the car. table 22.7 summarises the typical well-to-tank (Wtt) and WtW 
efficiencies for conventional and electrochemical vehicles. The efficiency of 
these electrochemical systems only plays a part in determining the amount 
of energy required by the vehicle, and thus its fuel economy and maximum 
driving range. the energy required to move a vehicle scales linearly with its 
mass [51, 52], and so doubling the vehicle’s efficiency will have a negligible 
impact if its weight also doubles. this is clearly evident with the range of 
fCeVs produced to date. Despite 40 years of technical development, the 
honda fCX Clarity has virtually the same fuel economy as the Austin A40 
converted to run on an alkaline fuel cell developed by Professor Karl Kordesch 
in the 1970s [53–56]. figure 22.7 compares these vehicles with 15 other 
models, demonstrating the relationship between mass and fuel economy.
22.10 Improving the safety of hydrogen-powered 
vehicles
22.10.1 Introduction
high priority research directions for the hydrogen economy include safety as 
a technological, a psychological and sociological issue [57]. here we consider 
some technological safety issues relevant to hydrogen-fuelled vehicles. 
hydrogen is not more dangerous nor safer compared to other fuels [58]. Safety 
of hydrogen and fuel cell systems and infrastructure fully depends on how 
professionally it is handled at the design stage and afterwards. hydrogen-
powered vehicles are one of the main applications of hydrogen and fuel cell 
technologies. hazards and associated risks for hydrogen-fuelled cars should 
be understood and interpreted in a professional way with full comprehension 
of accidentconsequences by all stakeholders starting from system designers 
Table 22.7 Typical well-to-tank, tank-to-wheel and well-to-wheel efficiencies for 
each vehicle technology
Vehicle 
type
Well-to-
tank
Tank-to-wheel Well-to-
wheel
BEV 32%–
100%
Charger 
90%
Battery 
92%
Inverter 
96%
Motor 
91%
Mechanical 
92%
21.3%–
66.5%
H2 FCEV 75%–
100%
Fuel cell 51.8% Inverter 
96%
Motor 
91%
Mechanical 
92%
31.2%–
41.6%
Hybrid 82.2% 30.2% 24.8%
Diesel 88.6% 17.8% 15.8%
Petrol 82.2% 15.1% 12.4%
720 Alternative Fuels and Advanced Vehicle Technologies
through regulators to users. Today the first series of hydrogen-fuelled buses 
and cars are already on the road and refuelling stations are operating in 
different countries around the world. But the question still remains…how 
safe are hydrogen-powered vehicles?
22.10.2 Current safety issues 
Compressed gaseous hydrogen is currently the main stream for on-board 
storage. Maximum storage pressure varies from 35 to 70 MPa. tanks of 
Type 3 (aluminium liner) and Type 4 (polymer liner) made of carbon fibre-
reinforced resin are in use due to their weight. Unfortunately, fire resistance 
of these tanks is low. for example, reported time to catastrophic failure of 
type 4 tanks is from 3.5 to 6.5 minutes [59]. the current ‘solution’ to prevent 
catastrophic failure of these tanks in fire conditions is to release hydrogen 
quickly through a thermal pressure relief device (tPRD) of comparatively 
large diameter of about 5 mm (before the catastrophic failure). however, this 
‘solution’, which is probably acceptable in the open atmosphere, generates 
serious problems for life safety and property protection when a vehicle is 
in confined space like garage, tunnel, etc.
Fuel cell vehicles Linear fit LTI London 
Taxi TX4
Microcab H4 
(original)
Microcab H4 
(upgraded)
Austin A40
Riversimple
Honda FCX Clarity
0 500 1,000 1,500 2,000 2,500 3,000
Vehicle mass (kg)
F
u
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l 
c
o
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s
u
m
p
ti
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 (
M
J
k
m
–
1
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F
u
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U
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 m
p
g
 g
a
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li
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q
u
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t)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
30
40
50
75
100
150
200
22.7 Energy consumption (MJ km–1) measured from ‘real-world’ 
testing of 23 hydrogen fuel cell vehicles plotted against vehicle mass 
(kg) [51]. Source: I. Staffell and K. Kendall, International Journal of 
Low Carbon Vehicles, 7(2012) 10–15, copyright © Oxford University 
Press.
721Fuel-cell (hydrogen) electric hybrid vehicles
22.10.3 Some large-scale experimental results
A hfCV was equipped with a tPRD with a vent pipe internal diameter of 
4.2 mm. A small storage vessel of 36 litres volume at pressure 70 MPa was 
used. The fire spread from a gasoline vehicle to a HFCV was investigated 
to address scenarios when different types of vehicles are catching fire in a 
car collision or natural disaster like an earthquake. the experiment revealed 
that when the TPRD of the HFCV is activated by a gasoline fire a fireball 
of more than 10 m diameter is formed.
 In another test by tamura et al. [60], two vehicles were parked approximately 
0.85 m apart and the spread of fire from the HFCV to the gasoline vehicle 
was investigated. It can be concluded that self-evacuation from the car or 
safeguarding by first responders with such design of hydrogen release system 
is impossible. tamura et al. [60] concluded that in car carrier ships and other 
similar situations with closely parked hfCVs, the test results point to the 
possibilities of a fire in an HFCV to activate its TPRD and thereby to generate 
hydrogen flames which in turn may cause the under floor TPRD activation 
in an adjoining HFCV. To minimize damage by HFCV fire, therefore, the 
authors suggested that it is important to detect early and extinguish the 
fire before TPRD activation. It is known that hydrogen fire is difficult if 
not impossible to extinguish in many practical situations. hopefully, car 
manufacturers will develop appropriate safety engineering solutions, including 
reduction of flame length from hydrogen-powered vehicle in a mishap; thus 
excluding the ‘domino’ effect in accident development and assisting first 
responders to control such fires and successfully perform rescue operations. 
experiments by tamura et al. [60] have clearly demonstrated that hydrogen-
powered vehicle fire consequences can be very ‘challenging’ both from life 
safety and property loss points of view.
 The modern definition of risk is provided by ISO/IEC Guide 73:2002 
[61] that states that it is the ‘combination of the probability of an event and 
its consequence’ while safety is defined as the ‘freedom from unacceptable 
risk’. this means that safety is a societal category and cannot be numerically 
defined while risk is a technical measure that can be calculated [62]. Society, 
in consequence, establishes acceptable levels of risk or risk acceptance 
criteria.
 the general requirement is that the risk associated with hydrogen-fuelled 
vehicles should be the same or below the risk for today’s vehicles using 
fossil fuels. Currently this requirement is not yet achieved throughout the 
whole range of hydrogen and fuel cell systems. Indeed, the consequences 
of a hydrogen-powered car fire to life safety and property loss, e.g. in 
confined spaces such as garages and tunnels, are more ‘costly’ compared to 
the consequences of a fossil fuel vehicle fire given the current level of fire 
resistance of hydrogen on-board storage (from 1 to 6.5 minutes only) and 
722 Alternative Fuels and Advanced Vehicle Technologies
the design of pressure relief devices. In fact, the probability of external to 
vehicle fire, e.g. at home garages and general vehicle parking garages will 
be the same independent of the vehicle type. 
 The garage fire statistics from National Fire Protection Association (NFPA) 
are as follows. During the four-year period of 2003–2006 an estimated average 
of 8,120 fires per year started in the vehicle storage areas, garages, or carports 
of one or two-family homes [63]. Vehicles were involved in the ignition of 
only about 13% of structure fires. This statistic makes it clear that safety 
strategies and solutions, including those developed by car manufacturers, 
have yet to be improved to rely on a firm engineering design rather than a 
general risk assessment of which uncertainties are impossible to define for 
emerging technologies.
22.10.4 Car in a garage scenario
the Commission Regulation (eU) no. 406/2010 [64] requires on-board 
hydrogen storage to be equipped with pressure relief devices (PRDs). the 
PRD (TPRD) is fitted to the fuel tank and starts to release hydrogen when 
high temperature of about 110 °C is reached, e.g. in fire conditions. The 
PRD can provide rapid release of the hydrogen, if a large orifice diameter 
is used, thus minimising the possibility of tank catastrophic failure during 
long exposure to fire. However, the hazards resulting from a rapid release 
indoors were studied only recently.
 let us consider a release from a typical on-board hydrogen storage tank 
at pressure of 35 MPa, through a 5.08 mm diameter ‘typical’ PRD [65]. the 
release is assumed to occur in a garage of size LxWxH = 4.5 ¥ 2.6 ¥ 2.6 m 
(SAe J2579, [66]) and volume of 30.4 m3 with a single vent equivalent in 
area to a typical brick LxH = 25 ¥ 5 cm located flash with the ceiling. A 
conservative approach is taken, i.e. a constant mass flow rate of 0.39 kg.s–1 is 
applied (ignoring a pressure drop in the storage tank) after the PRD opening. 
the transient pressure load in the vented enclosure according to the selected 
scenario is given in fig. 22.8.
 figure 22.8 illustrates how the overpressure within the enclosure resulting 
from the injection of hydrogen reaches a level above 10 kPa capable of 
rupturing the garage [67] within only 1 s. evacuation of people in such 
circumstances is impossible and this life safety issue has yet to be addressed 
by carmanufacturers by reconsidering requirements to fire resistance of on-
board storage and PRD use. there seems only one engineering solution to 
this problem, i.e. to reduce the mass flow rate from on-board storage during 
fire. This is turn will require a higher fire resistance level of on-board storage 
tanks compared to current 3.5–6.5 minutes for type 4 vessels [59].
 If the garage does not rupture first, then the pressure within the garage 
for this typical scenario reaches a maximum level in excess of 50 kPa. this 
723Fuel-cell (hydrogen) electric hybrid vehicles
maximum pressure then drops off and tends towards a steady state value; 
considerably lower, and equal to that predicted by the simple steady state 
orifice equations for pure hydrogen release from the garage. The maximum 
pressure is reached in less than 10 s. Within this time the entire garage would 
be destroyed with missiles flying around and creating more damage. These are 
consequences of pressure build up without even considering the combustion 
of the released hydrogen. Pressure peaking phenomenon of hydrogen release 
in a vented enclosure is a new aspect of life safety provision that hydrogen 
safety engineers and manufacturers of hydrogen and fuel cell systems have 
to address [65].
 figure 22.8 illustrates the predicted overpressure in the garage versus the 
time for three fuels with the same mass flow rate of 0.39 kg.s–1: hydrogen, 
methane, and propane with the molecular mass of 0.002, 0.016 and 0.044 
kg.kmol–1, respectively. It is clearly seen that the maximum overpressure 
drops with the increase of molecular mass. there is a very small pressure 
peak for methane release as its molecular mass is below that of air. the 
pressure peaking phenomenon is absent for propane altogether as its molecular 
mass is higher than air. this should be taken into account when designing 
PRDs for use with different gases for indoor systems. the same PRD used 
for compressed natural gas (CNG) or liquefied petroleum gas (LPG) should 
not be assumed to behave in the same way for hydrogen. 
Hydrogen
Methane
Propane
0 10 20 30 40 50 60
Time (s)
O
v
e
rp
re
s
s
u
re
 (
k
P
a
)
60
50
40
30
20
10
0
22.8 Pressure peaking phenomenon for release of hydrogen with 
mass flow rate 0.39 kg/s into the enclosure of 30.4 m3 with a vent 
of typical brick size 25 ¥ 5 cm compared with pressure dynamics for 
releases of methane and propane at the same conditions.
724 Alternative Fuels and Advanced Vehicle Technologies
 the pressure peaking phenomenon, i.e. existence of the maximum in 
pressure transient that is above the steady state overpressure value, during a 
release of lighter than air gas into vented enclosure, should be accounted for 
when performing safety engineering for indoor use of hydrogen or helium 
systems. overall the conclusion is drawn that PRDs currently available 
for hydrogen-powered vehicles should be redesigned, along with essential 
increase of fire resistance rating of on-board hydrogen storage tanks, and 
regulations, codes and standards (RCS) updated if the vehicle is intended 
for garage parking.
22.10.5 Hydrogen jet flame length
Current engineering solutions for hydrogen-powered vehicles imply very long 
flames up to 15 metres. Hydrogen safety engineers have to be equipped with 
tools for calculation of jet flame length as a function of storage pressure and 
PRD diameter. figure 22.9 shows the nomogram for graphical calculation 
of hydrogen jet flame length [68]. the nomogram is derived from the best 
fit line of the dimensional flame length correlation [68]. The use of the 
nomogram for calculation of flame length is demonstrated in Fig. 22.9 by 
thick lines with arrows. firstly, an actual nozzle diameter of a leak, e.g. 3 mm 
in this example, and a storage pressure, e.g. 35 MPa, are chosen. Secondly, 
a horizontal line is drawn from the diameter axis at point 3 mm to the right 
until it intersects with the pressure line denoted as 35 MPa. thirdly, a vertical 
line is drawn from the intersection point upward until the second intersection 
with the only line available in coordinates (Lf, (m.D)1/2). finally, to obtain 
a flame length, a horizontal line is draw from the second intersection to the 
left until the final intersection with the axis denoted ‘Hydrogen flame length 
Lf, m’. In the example considered, the leak of hydrogen at pressure 35 MPa 
through the orifice of 3 mm diameter will result in the flame length of 5 m. 
The conservative estimate of the flame length is 50% longer, i.e. 7.5 m. 
 the nomogram incorporates a special feature based on the results by Mogi 
et al. [69] and others, i.e. no stable flame was observed for nozzle diameters 
0.1–0.2 mm as the flame blew off although the spouting pressures were as 
high as 40 MPa (denoted as ‘No flame area’ in Fig. 22.9). For example, a 
stable jet flame cannot exist at pressures equal or below 35 MPa if the leak 
orifice diameter is below 0.3 mm. It should be noted that the nomogram does 
not account for conditions when flow losses in a leakage pathway cannot 
be ignored. In such cases a straight forward use of the nomogram gives a 
conservative result.
22.10.6 Separation distances
hydrogen releases from high pressure on-board storage or equipment will 
create highly under-expanded jets. this could lead to formation of a large 
725Fuel-cell (hydrogen) electric hybrid vehicles
flammable hydrogen-air envelope (unignited release, e.g. due to failure 
of the PRD) or long jet flame. The size of the flammable envelope is the 
deterministic separation distance from the release source. Indeed, if the 
flammable envelope (hydrogen concentration in air equal to the lower 
flammability limit of 4% by volume) reaches a location of air intake into 
high-rise buildings, then consequences for occupants and building structure 
can be catastrophic. here we do not consider separation due to pressure 
effects of hydrogen deflagrations or detonations. It is widely accepted that 
H
y
d
ro
g
e
n
 fl
a
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 l
e
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g
th
, 
L
F
 (
m
)
A
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n
o
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 d
ia
m
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te
r,
 D
 (
m
m
)
10
1
0.1
0.01
0.1
1
10
100
No flame area
1E-005 0.0001 0.001 0.01 0.1 (m.D)1/2, (kg/m/s)1/2
70 MPa
35 MPa
10 MPa
1 MPa
0.2 MPa
22.9 The nomogram for hydrogen jet flame length [68].
726 Alternative Fuels and Advanced Vehicle Technologies
separation distance is the minimum separation between a hazard source and 
an object (human, equipment or environment) which will mitigate the effect 
of a likely foreseeable incident and prevent a minor incident escalating into 
a larger incident.
Separation from unignited release
the nomogram for graphical calculation of hydrogen concentration decay by 
the similarity law and the under-expanded jet theory without losses is shown 
in fig. 22.10. the nomogram includes four core graphs (‘Volumetric to mass 
fraction’, ‘the similarity law’, ‘Choose leak diameter’ and ‘Choose density 
in the nozzle exit’) and one additional graph ‘Calculate density in the nozzle 
exit by storage tank pressure and temperature’. let us demonstrate how to use 
the nomogram to calculate the distance from the nozzle of 1 mm diameter to 
concentrations of hydrogen in air 4% (dashed line, corresponds to the lower 
flammability limit of hydrogen in air) and 11% (solid line, corresponds to 
rN:
1 kg/m3
3 kg/m3
10 kg/m3
20 kg/m3
50 kg/m3
D:
15 mm
10 mm
5 mm
3 mm
2 mm
1 mm
0.5 mm
0.1 mm
100
10–1
10–2
10–3
10–4
H
y
d
ro
g
e
n
 m
a
s
s
 f
ra
c
ti
o
n
10–3 10–2 10–1 100
Hydrogen volumetric fraction
Volumetric to 
mass fraction
4% v/v
11% v/v
The similarity law
Choose density in 
the nozzle exit
23 kg/m3
23 kg/m3
 102
 101
 100 
10–1
10–2
10–3
10–1 100 101 102 103
Distance to concentration of interest, x (m)
100 101 102 103 104
x/[5.4·D·(rN/rs)
1/2]
400 K
Calculate density in the nozzle exit by storage tank pressure and temperature
300K
20.3 K
80 K
 0.10.2 0.3 0.5 1 2 3 5 10 20 30 50
Density in the nozzle exit, rN(kg/m3)
S
to
ra
g
e
 p
re
s
s
u
re
 (
M
P
a
)
102
30
101
3
100
0.3
10–1
22.10 The nomogram for concentration decay calculation in unignited 
jets.
727Fuel-cell (hydrogen) electric hybrid vehicles
the average location of jet flame tip; scattering of flame tip location is from 
8% to 16% by volume) by volume along the axis of a release from a storage 
tank at pressure 70 MPa and temperature 300 K.
 firstly, the line from the axis ‘hydrogen volumetric fraction’ is drawn 
downward from the chosen concentration of interest (4% and 11% by volume 
in our example) until the intersection with the line of the graph ‘Volumetric 
to mass fraction’. Secondly, the horizontal line from this intersection point 
is drawn to the intersection with the similarity law line at the right top graph 
‘the similarity law’. thirdly, the vertical line is drawn downward until 
intersection with line ‘1 mm’ at the graph ‘Choose leak diameter’. there 
are eight lines on the ‘Choose leak diameter’ graph which correspond to 
the following leak diameters (from top to bottom): 15 mm, 10 mm, 5 mm, 
3 mm, 2 mm, 1 mm, 0.5 mm, 0.1 mm). these labels are shown on the right 
hand side of the graph.
 the fourth stage is to calculate the density using the additional graph 
‘Calculate density in the nozzle exit by storage tank pressure and temperature’ 
at the bottom of the nomogram using given pressure (70 MPa) on the ordinate 
axis and a line corresponding the chosen temperature (300 K). this is shown 
by two thick grey lines on the ‘Calculate density in the nozzle exit by storage 
tank pressure and temperature’ graph. Calculated graphically, density at the 
nozzle exit for 70 MPa and 300 K is about 23 kg.m–3.
 the calculated value of density of 23 kg.m–3 is applied at the fifth stage 
of calculations to draw the horizontal line from the intersection with the ‘1 
mm’ line at the ‘Choose leak diameter’ graph to the left until the intersection 
with an imaginary line corresponding to 23 kg.m–3 at the ‘Choose density in 
the nozzle exit’ graph (between the two shown on the graph lines 20 kg.m–3 
and 50 kg.m–3). There are five lines at the graph corresponding to densities 
1 kg.m–3, 3 kg.m–3, 10 kg.m–3, 20 kg.m–3, and 50 kg.m–3 from top to bottom 
respectively. these labels are shown at the left-hand side of the graph.
 The final stage is to draw a vertical line downward from the intersection 
with the line 23 kg.m–3 to the intersection with abscissa axis of the ‘Choose 
density in the nozzle exit’ graph. thus, the calculated graphically distance 
from the nozzle exit to hydrogen concentration of 4% by volume is about 
7.7 m, and to concentration of 11% by volume is about 2.7 m. the error 
of graphical calculations is estimated to be at the acceptable level of below 
10%.
 this nomogram is a performance-based tool for calculation of separation 
distance for unignited releases as the alternative to prescriptive codes. for 
instance, the International fire Code (IfC) [70] presents a table of prescribed 
separation distances between a hydrogen system and various potential targets, 
including air intake to a building. however, this code does not provide or 
require any information either on the hydrogen storage parameters (pressure 
and temperature) and leak size from the system. this IfC code is an example 
728 Alternative Fuels and Advanced Vehicle Technologies
of an archaic prescriptive approach that contradicts modern requirements for 
codes and standards to be performance-based.
Three separation distances from a jet fire
Figure 22.11 shows measured axial temperature of a hydrogen flame [71,72] 
as a function of distance from the nozzle, x, normalised by the flame 
length, Lf. three accepted criteria for three different separation distances 
are presented by horizontal lines: 70 °C – ‘no harm’ separation distance; 
T
(K
)
2,200
2,000
1,800
1,600
1,400
1,200
1,000
800
600
400
200
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
x /LF
[74] 1 mm, 1.2 MPa
[74] 1 mm, 2.0 MPa
[74] 1 mm, 3.5 MPa
[74] 2 mm, 0.3 MPa
[74] 2 mm, 0.5 MPa
[74] 3 mm, 0.4 MPa
[74] 3 mm, 0.7 MPa
[74] 3 mm, 1.0 MPa
[70] 1996
[71] 2010, 5 s
[71] 2010, 20 s
[71] 2010, 50 s
[71] 2010, 60 s
[71] 2010, 70 s
300 °C (third degree burn, 20 s)
115 °C (pain limit, 5 min)
70 °C (“no harm” limit)
22.11 Measured axial temperature as a function of distance 
expressed in flame calibres [71, 72, 74] and three criteria for jet fire 
effects (lines).
729Fuel-cell (hydrogen) electric hybrid vehicles
115 °C – pain limit for 5 min exposure; 309 °C – third degree burns for 
20 s (‘death’ limit). Comparison between the axial temperature profile and 
named criteria provides three separation distances: x = 3.5Lf for ‘no harm’ 
separation (70 °C), x = 3Lf for pain limit (115 °C, 5 min), x = 2Lf for third 
degree burns (309 °C, 20 s).
 Recently an ‘unexpected’ conclusion has been drawn [66, 73, 74] from 
the performed analysis that in the conservative case all three separation 
distances for jet fire are longer or equal to the separation distance from 
unignited release from the same source based on the lower flammability limit 
(lfl). In particular, the separation distance from a hydrogen leak source to a 
location with axial concentration equal to the lfl, e.g. to prevent ingress of 
flammable mixture into a ventilation system of building, is practically equal 
to the ‘death’ separation distance for jet fire. Two other separation distances 
for jet fires (‘no harm’ and ‘pain’ limits) are longer than the separation 
distance to lfl in the case of unignited release.
22.10.7 Concluding remarks
numerous knowledge gaps have been closed by the international hydrogen 
safety community (www.hysafe.org). however, more issues appear as we 
learn more. the key issue at the moment that is directly related to hydrogen-
powered vehicles is low fire resistance of on-board storage. The increase of 
fire resistance from the current 3.5–6.5 minutes for Type 4 tanks to above 30 
minutes could drastically change public perception of hydrogen technologies. 
Indeed, flame length from TPRDs could be reduced from current 10–15 m 
to about 1 m. Destruction of civil structures due to the pressure peaking 
phenomenon during accidental release of hydrogen would be excluded. 
Moreover, small length hydrogen jet flames could be considered not to be 
a hazard but a source of extinguishing agent, i.e. water, to control fire in an 
enclosure like a garage. High fire resistance would allow essential mass flow 
rate reduction from TPRDs and exclude formation of large flammable clouds 
in tunnels and bring deflagration pressure below harmful levels. A higher 
fire resistance rating on hydrogen storage tanks and shorter flame length 
would allow self-evacuation of civilians from the accident scene, efficient 
intervention of first responders to control the accident, etc.
 Thus, the way forward with hydrogen safety is to increase the fire resistance 
of on-board storage and reduce the mass flow rate from the TPRD. The 
issue of fire resistance of on-board storage has yet to be addressed by car 
manufacturers to allow longer time for blow-down of tanks. Unfortunately, 
self-evacuation of passengers from currently designed cars or safeguarding 
by first responders with such design of hydrogen storage and PRD systems 
is impossible and car manufacturers in close cooperation with experts in 
hydrogen safety have yet to address this customer safety issue.
730 Alternative Fuels and Advanced Vehicle Technologies
22.11 Conclusions
there has been a lot of progress in electrochemical storage devices during the 
past decade, and the near future holds prospects for many further advances. 
Cost, durability and energy density are the main areas where improvements 
are required to compete with conventional fossil fuels.
 Costs have been falling exceptionally rapidly, and are expectedto continue 
doing so for the next 5–10 years. for example, the cost of manufacturing 
fCeVs has decreased by a staggering 90% since 2005, and the publicly 
acceptable goal of $50,000 for a luxury sedan is now within reach. BeVs 
have already reached such a price-point – with current vehicles retailing at 
around $40,000 – with $20,000–30,000 attainable within 10 years.
 the durability of battery and capacitor technologies is already expected to 
be sufficient for automotive use, giving 10 year’s calendar life and 150,000 
miles. fuel cell stacks appear to still be falling short of the US Doe’s 2009 
target of 2,000 hours operation, corresponding to approximately 25,000 
miles before a 10% drop in power output. new MeA designs with improved 
catalysts and reduced crossover currents are expected to improve this situation 
in the future.
 energy density is still the Achilles heel of batteries and capacitor 
technologies. It is a sad fact that even the latest nanotechnology-based 
ultracapacitors are not expected to offer close to the energy density of battery 
technologies, and so they are unlikely to ever be used for more than peak 
power provision. However, the high efficiency and relatively light ancillary 
systems required by electric power-trains mean that batteries need only offer 
666 Wh.kg–1 at the cell level to offer the same driving range and consumer 
acceptability as current petrol vehicles. the next generation of lithium-based 
chemistries are expected to approach this value, and so the perennial problem 
of ‘range anxiety’ may soon be overcome. hydrogen storage will, however, 
be the first technology to exceed petrol in terms of the range offered, as only 
6 wt.% hydrogen is required, and several forms of metal-hydride storage are 
on the horizon which are expected to beat this.
 Japanese automakers announced at the 2010 fuel Cell expo in tokyo, 
a programme designed to deploy 2 million fCeVs in Japan by 2025 – a 
point at which the industry estimates fCeVs would be fully competitive. It 
has also become apparent that several (but not all) oeMs had redeployed 
their technical staff from fuel cell R&D to focus on li-ion batteries. In 
addition, some of these automotive companies also stated that technology 
investment in the fuel cell supply chain is in serious decline. thus, these 
two factors combined reduce the likelihood of the very fast progress in a 
hydrogen economy that was expected a few years ago. It is unlikely that 
global market forces alone will be able to achieve the Intergovernmental 
Panel on Climate Change (IPCC) 80% carbon reduction goal by 2050, the 
731Fuel-cell (hydrogen) electric hybrid vehicles
policy makers are therefore required to give an aggressive push towards this 
objective.
22.12 Acknowledgements
the authors would like to thank Advantage West Midlands (AWM), ePSRC 
(Contract no: eP/e034888/1) and the RCUK CDt in hydrogen, fuel cells 
and their applications for their kind financial support. JLS would also like to 
thank the Birmingham Centre for hydrogen and fuel Cell Research, College 
of ePS and the University of Birmingham for its generous support.
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22.14 Appendix: abbreviations
AB ammonia-borane
AfC alkaline fuel cell
BeRR Business enterprise and Regulatory Reform
BeV battery electric vehicle
BMS battery management system
Cl catalyst layer
CnG compressed natural gas
Cnt carbon nanotube
DoD depth of discharge
Doe US Department of energy
DMfC direct methanol fuel cell
eC electrochemical capacitor
eC–DMC ethylene carbonate–dimethyl carbonate
eDlC electrochemical double layer capacitor
735Fuel-cell (hydrogen) electric hybrid vehicles
elhP european Integrated hydrogen Project
eSD energy storage device
fCeV fuel cell electric vehicle
FF flow field
GDl gas diffusion layer
GhG greenhouse gas
heV hybrid electric vehicle 
hoR hydrogen oxidation reaction
hfCV hydrogen fuel cell vehicle 
IFC international fire code
ICe internal combustion engine
IPCC Intergovernmental Panel on Climate Change
LFL lower flammability limit
li-ion lithium-ion battery
LPG liquified petroleum gas
MCfC molten carbonate fuel cell
MCMB mesocarbon microbead
MeA membrane electrode assembly
MfC microbial fuel cell
Mof metal-organic framework
nfPA national fire Protection Association
oRR oxygen reduction reaction
PAfC phosphoric acid fuel cell 
PeM proton exchange membrane
PeMfC proton exchange membrane fuel cell
PGM platinum group metal
PPS peak power source
PRD pressure relief device
RCS regulations, codes and standards
RtIl room temperature ionic liquid
She standard hydrogen electrode
SoC state of charge
SofC solid oxide fuel cell
SRP standard reduction potential
StP standard temperature and pressure
tPRD thermal pressure relief device
Wtt well-to-tank
WtW well-to-wheel190
 (NiMH)
Lithium-ion (Li-ion) S 4.1 410 (180) 400
Source: M.R. Palacin, Chemical Society Reviews, 38 (2009) 2565–2567, copyright © 
Royal Society of Chemistry.
li-ion batteries are used as primary energy sources in BeVs such as the 
nissan leaf and Mitsubishi iMiev (fig. 22.1). the $35,000 5–door nissan 
leaf BeV is powered by 12 ¥ 4 cells (48 modules) providing a capacity of 
24 kWh and taking up to 8 hours to fully charge from a standard domestic 
outlet from zero state-of-charge (SoC), or 30 minutes from a three-phase 
AC socket.
22.3.1 Nickel metal hydride (NiMH) batteries
niMh batteries are used in over 95% of all heVs, and major manufacturers 
have so far invested substantially in the last 10 years. the major advantage 
689Fuel-cell (hydrogen) electric hybrid vehicles
from a manufacturing point of view is the safety of niMh batteries compared 
to li-ion batteries, and, so far, no incidents have been reported in the press. 
furthermore, niMh batteries are preferred in industrial and consumer 
applications due to their design flexibility (e.g. ranging from 30 mAh to 250 
Ah), environmental acceptability, low maintenance, high power and energy 
densities, cost and most importantly safety (in charge and discharge modes, 
especially at high voltages). niMh batteries are currently priced at $250 to 
$1,500 per kWh, hence the total price of the battery pack for a hybrid (e.g. 
toyota Prius, although the newer models use 5.2 kWh li-ion battery packs) 
varies anywhere between $600 and $3,000 per vehicle.
 the niMh battery was patented in 1986 by Stanford ovshinsky, founder 
of ovonics, when researching hydrogen storage materials. ovshinsky also 
described the niMh battery as the ‘hydrogen ion’ or ‘protonic’ battery by 
analogy with lithium-ion batteries as the niMh electrochemical reaction 
involves the transfer and ‘insertion’ of h+. the components of niMh batteries 
include an anode of hydrogen absorbing alloys (Mh), a cathode of nickel 
hydroxide (ni(oh)2) and a potassium hydroxide (Koh) electrolyte (fig. 
22.2).
 the general electrochemical reactions are as follows:
22.1 Mitsubishi iMiev. 
690 Alternative Fuels and Advanced Vehicle Technologies
Anode (–) M + e– + h2o Æ Mh + oh– [22.1]
Cathode (+) ni(oh)2 + oh– Æ nio(oh) + h2o + e– [22.2]
overall reaction M + ni(oh)2 Æ Mh + nio(oh) [22.3]
where ‘M’ is an intermetallic alloy capable of forming a metal hydride 
phase.
22.3.2 Lithium-ion (Li-ion) batteries
lithium-ion batteries are light, compact and operate with a cell voltage of 
4V with a specific energy in the range of 100–180 Wh.kg−1. In these type of 
battery both the anode (graphite, e.g. mesocarbon microbeads – MCMB) and 
cathode (lithium metal oxide – lMo2, e.g. liCoo2) are materials into which, 
and from which, lithium (as li+) migrates through the electrolyte (typically 
H+
H+
MH electrode
M+H2O+e–
ÆMH+OH–
MH electrode
MH+OH–
ÆM+H2O+e–
Ni electrode
Ni(OH)2+OH+
ÆNiOOH+H2O+e–
Ni electrode
NiOOH+H2O+e–
ÆNi(OH)2+OH–
OH+
OH+
H2O
H2O
Ni(OH)2
Ni(OH)2
NiOOH
NiOOH
Charge
Discharge
22.2 Schematic diagram of the electrochemical reaction processes 
of a NiMH battery. For the charge process, the hydrogen atom 
dissociates from Ni(OH)2 and is absorbed by the MH alloy. For the 
discharge process, the hydrogen atom dissociates from the MH alloy 
and joins with NiOH to form Ni(OH)2 [6]. Source: F. Feng, M. Geng 
and D.O. Northwood, International Journal of Hydrogen Energy, 26 
(2001) 725–734, copyright © Elsevier.
691Fuel-cell (hydrogen) electric hybrid vehicles
a lithium salt, e.g. lithium hexafluorophosphate (LiF6) in an organic solvent, 
e.g. ethylene carbonate–dimethyl carbonate (eC–DMC) in a separator felt), 
then is inserted (intercalation process) or extracted (deintercalation process) 
into the electrodes (Plate XIX between pages 392 and 393).
 thus when a lithium-ion battery is discharging, li is extracted from the 
anode (–) and inserted into the cathode (+) and when it is charging, the 
reverse process occurs according to the following reactions:
 yC + liMo2 Æ lixCy +li(1−x)Mo2
where x 0.5, y = 6, and Vcell = 3.7 V [22.4]
for the case of liCoo2:
 liCoo2 Æ li1–xCoo2 + xli+ + xe– [22.5]
 xli+ + xe– + 6C Æ lixC6 [22.6]
the overall reaction is:
 xli+ + xe- + liCoo2 Æ li2o + Coo [22.7]
During recharging, li+ ions are removed and the oxidation of Co3+ to Co4+ 
occurs. the Co3+/Co4+ couple supplies a cell voltage of about 4.0 V vs. 
metallic li.
 li-ion batteries store more energy than niMh; however, they suffer from 
major issues such as cost ($1,000/kWh), wide operational temperature ranges, 
materials availability (e.g. li), environmental impact and safety. for example, 
liCoo2 batteries are unsafe as they are thermodynamically unstable although 
they are kinetically stable in practice. It is often observed that these batteries 
suffer from electrolyte decomposition leading to the formation of oxide films 
on the anode, thus blocking extraction sites of lithium and severe oxidative 
processes at the cathode due to overcharging, in turn causing dissolution 
of protective films on the cathode and excess and continuous oxidation of 
the electrolyte (gas evolution). table 22.3 gives a summary of the main 
secondary batteries and their characteristics and Plate XX (between pages 
392 and 393) shows a diagram comparing the various battery technologies 
in terms of volumetric and gravimetric energy density [9].
22.3.3 Current issues
lithium-ion ‘chemistry’ for batteries has not progressed much since their 
introduction to the market in the early 1990s by Sony and Asahi Kasei 
following the pioneering work from Whittingham, tarascon, Armand and 
Scrosati [9]. therefore, breakthroughs in lithium-ion battery technology 
are urgently required, with innovative, performing and durable material 
chemistries for both the electrodes and the electrolyte sub-components. the 
Table 22.3 Various secondary batteries and their reactions [10]
Battery system Anode (–) Electrolyte Anodic (A)/cathodic (C)/overall (O) reactions Cathode (+)
Lead/acid Pb H2SO4 A: Pb + SO4
2– i PbSO4 + 2e– PbO2
 C: PbO2 + 4H+ + SO4
2– + 2e– i PbSO4 + 2H2O
 O: PbO2 + 2H2SO4 + Pb i 2PbSO4 + 2H2O
NiCd Cd KOH A: Cd + 2OH– i Cd(OH)2 + 2e– NiOOH
 C: 2NiOOH + 2H2O + 2e– i 2Ni(OH)2 + 2OH–
 O: 2NiOOH + Cd + 2H2O i Ni(OH)2 + Cd(OH)2
NiMH Hydrogen KOH A: M + e– + H2O i MH + OH– NiOOH
 adsorbed C: Ni(OH)2 + OH– i NiO(OH) + H2O + e–
 alloy O: M + Ni(OH)2 i MH + NiO(OH)
Lithium-ion Li + C Organic A: Li(C) i Li+ + e– CoO2
 electrolyte + C: Li+ + e– + CoO2 i LiCoO2
 Li salt O: Li(C) + CoO2 i LiCoO2
Source: M Wakihara, Materials Science and Engineering: R: Reports, 33 (2001) 109–134, copyright© Elsevier.
693Fuel-cell (hydrogen) electric hybrid vehicles
principal objective is to identify materials exhibiting higher performance and 
durability than those currently offered. Currently worldwide R&D efforts 
focus upon the replacement of (i) graphite and liCoo2 with alternative 
high capacity and low cost materials and (ii) ethylene carbonate-dimethyl 
carbonate with other electrolytes which do not suffer from decomposition 
under oxidative regimes.
 According to a report from the Strategy Consultancy Roland Berger 
[11], the supply of li-ion batteries will exceed demand by more than 100% 
by 2015, in other words the market will grow from circa $1.5 billion in 
2011 to over $9 billion in 2015. It is speculated that the market for li-ion 
batteries in the automotive sector will reach over $50 billion by 2020 [11]. 
At present li-ion batteries are expensive but it is anticipated that the price 
will decline rapidly and that they will be the cheapest rechargeable batteries 
in 10 year’s time. furthermore, the USA are supporting BeV technology 
with approximately $10 billion, while China are spending $15 billion on 
R&D for BeVs [12].
 Scarcity of lithium was once thought of as a looming concern for theelectrification of vehicle fleets. However, it should be noted that only around 
1% of a lithium-ion battery is li by weight, implying around 0.08 kg of li per 
kWh of storage capacity (approximately 1–2 kg per BeV) [13]. At present, 
this lithium is not recycled due to excessive cost and energy requirements; 
however, if future supply shortages lead to increasing material prices, the 
recycling of these batteries will become standard.
22.4 Hydrogen and fuel cells
22.4.1 Hydrogen energy
Hydrogen facts, production, storage and usage
Worldwide, 50 million tonnes of hydrogen is produced, mainly through 
reformation of fossil fuels, with 100,000 tonnes of h2 produced in the UK. 
Recent worldwide hydrogen production totals show that 48% of hydrogen 
is produced from natural gas, 30% from oil, 18% from coal and only 4% 
from renewable sources [14]. nowadays, hydrogen is used in chemical 
processing, the petroleum industry, fats and oils, metals, electronics, space 
flights, utilities, glass manufacturing, etc.
 hydrogen storage on board vehicles is the key factor for achieving market 
success for fuel cell electric vehicles (fCeVs). to be competitive with 
ICe vehicles, hydrogen fuel cell vehicles (hfCVs) should have a similar 
driving range. As the volumetric energy density of hydrogen is very low, 
storing enough hydrogen on board remains a challenge in terms of weight, 
volume, kinetics, safety and cost. However, the efficiency of a hydrogen 
ICe is circa 25% and that of a hfCV is 60%; this is three times better than 
694 Alternative Fuels and Advanced Vehicle Technologies
today’s petrol-fuelled engines (18–20% for a petrol ICe reaching 40% at 
peak efficiency). The low volumetric energy density can only be increased 
by storing the hydrogen either under increased pressure, at extremely low 
temperatures as a liquid or in metal hydride systems.
 Indeed, there are various methods for storing hydrogen in vehicles:
∑ Liquid hydrogen – the energy density of liquid hydrogen is high, but to 
store hydrogen in a liquid state, it is necessary to maintain it at –253°C 
and at ambient pressure. therefore a highly insulated liquid hydrogen tank 
is required. furthermore, a quarter of the chemical energy of hydrogen 
itself is consumed in the liquefaction process.
∑ Compressed hydrogen – the most popular method chosen by leading 
fCeV manufacturers. honda and nissan use 350 bar, whereas toyota 
prefers 700 bar. the energy density is relatively low and the energy is 
highly consumed in compression. 
∑ Metal hydrides – these are the safest method, but weight is a major issue 
for transportation applications (although not for maritime applications), 
in addition a lot of time is required to store the hydrogen (i.e. long 
refuelling time), and it has an insufficient release rate.
∑ Hydrogen absorbed onto carbon nanotubes (CNT) and metal organic 
frameworks (MOF) – still at developmental stages.
As stated previously, hydrogen has a the highest energy content of any fuel 
by weight (33,320 Wh.kg–1) – i.e. about three and seven times more than 
gasoline (12,700 Wh.kg–1 or 8,760 Wh.l–1), natural gas (13,900 Wh.kg–1 
or 5,800 Wh.l–1) and coal (7,200 Wh.kg–1), but it has a very low energy 
content by volume (about 3,000 times less than gasoline at StP) [15]. this 
makes storage and distribution to the point of use costly. 5 kg of hydrogen is 
equivalent to 5 gallons (or 22 litres) of petrol, but to store it under ambient 
conditions would require a 5 metre diameter vessel which is impractical! 
table 22.4 compares the density of hydrogen to conventional fuels under 
different conditions.
Table 22.4 Comparison of density and energy densities of various fuels
 Density Energy Density (LHV)
Fuel kg.L–1 kg.m–3 MJ.kg–1 MJ.L–1 kWh.kg–1 kWh.m–3
H2 at 20 °C, 1 atm 0.0000899 0.0899 120 0.01006 33.3 2.79
H2 at 20 °C, 350 bar 0.025 25 120 2.8 33.3 775.86
H2 at 20 °C, 700 bar 0.039 39 120 4.4 33.3 1,210.34
H2 liquid at boiling point,
1 atm 0.0708 70.8 120 7.92 33.3 2,197.24
Petrol 0.702 702 42.7 31.2 11.86 8,666.67
Diesel 0.855 855 41.9 36.5 11.64 10,138.88
695Fuel-cell (hydrogen) electric hybrid vehicles
 So far, compressed hydrogen is the simplest and most cost effective 
method at present, and it is seen as the most viable solution for automobile 
use; it only requires a compressor and a pressure tank. Because of the low 
storage density under standard temperature and pressure (StP), it has to be 
compressed up to 10,000 psi. nevertheless, few leading fuel cell vehicle 
manufacturers have been successfully demonstrated in several prototype 
models, for instance, honda and nissan opted for a 350 bar (5,000 psi) 
pressurised tank, whereas toyota uses 700 bar (10,000 psi). All pressurised 
tanks are made of aluminium and carbon-fibre-reinforced composite materials 
(e.g. Dynetek – www.dynetek.com); however, the extremely high pressure 
causes public concern about their safety, although the 10,000 psi composite 
tanks have been demonstrated to be very safe (a 2.35 safety factor, i.e. a 23,500 
psi burst pressure) as required by the european Integrated hydrogen Project 
(EIHP) specifications. In addition, the cost to manufacture such containers is 
still high and the tank dimensions require more space than conventional petrol 
tanks. Currently, both 350 bar and 700 bar tanks are commercially available 
at low production volumes but at high costs. In addition, compressing the 
hydrogen to 350 bar and 700 bar requires an additional 7%–15% energy, 
although this can be lower if the hydrogen is produced at high pressure (350 
bar = 10% wt)
 Hydrogen can be liquefied for on-board vehicle storage. The density of 
liquid hydrogen is 70.8 kg.m–3, hence the energy density of liquid hydrogen is 
quite high, but to store hydrogen in a liquid state, it is necessary to maintain 
it at –253°C at ambient pressure. therefore a highly insulated liquid hydrogen 
tank is needed. furthermore, a quarter of the chemical energy of hydrogen 
itself is used through a number of steps of the liquefaction process; hence 
the cost is much higher compared to compressed hydrogen, although the 
transportation costs are much lower. to avoid hydrogen ‘boil-off’, a well-
insulated container has to be used, which adds more cost. A few decades 
ago (1980s), BMW developed and tested liquid hydrogen ICe vehicles 
whereby 8 kg of liquid hydrogen was stored on-board enabling the vehicle 
to reach a distance of more than 200 kms (125 miles), with a fill up time of 
the hydrogen tank of less than 8 mins. however, after more than 2 million 
accumulative miles testing, BMW recently suspended the project.
 Many metals and alloys are capable of repeatedly absorbing and releasing 
large amounts of hydrogen and store it at solid state under low pressure. 
the process chemically ‘bonds’ the hydrogen in the interatomic lattice of 
the metal, absorb hydrogen into the lattice through cooling and releases 
hydrogen through heating. It is probably the safest method, but it is very 
heavy and has a low energy density. Metal hydride storage systems are 
unlikely to successfully achieve the US Doe target of 9%wt by 2015. In 
addition, the refuelling time is quite long, and the release kinetics are too 
slow for automobile applications.
696 Alternative Fuels and Advanced Vehicle Technologies
 thus, for transportation applications the challenge is how to store the 
necessary amount of hydrogen required for considerable driving range (>300 
miles), within the constraints of weight, volume, durability (>1,500 cycles), 
efficiency and cost. A hydrogen fuelled vehicle may need to carry 5–13kg 
hydrogen on-board in order to match the current conventional vehicles’ 
performance.
 Another possibility is to produce hydrogen on-board the vehicle. for 
example, hydrogen can be produced from hydrogen-rich resources such as 
methanol, ethanol, natural gas, and petrol/diesel fuels [15]. While hydrogendistribution infrastructure and on-board storage still pose some difficulties 
for automotive purposes, generating hydrogen on-board via a reformer is 
considered to be a potential ‘solution’ for automotive applications [15]; 
obviously a successfully integrated on-board reformer would have many 
benefits, such as reducing the complications and initial capital investments. 
But the main disadvantages for on-board reforming systems are cost, size, 
weight, response time, efficiency and durability. Generally, on-board vehicle 
reformers would be required to produce sufficient hydrogen for its use in the 
vehicle. Although tremendous progress has been made towards achieving 
technical targets for on-board fuel processing, it is unlikely that it will improve 
sufficiently to support the transition to a hydrogen economy in the short 
term. for example in 2004, the US Doe terminated the on-board hydrogen 
reforming programme; the focus then shifted towards the development of 
lightweight hydrogen storage materials and tanks as well as a hydrogen 
infrastructure.
Current hydrogen distribution methods
hydrogen is currently transported/stored in gaseous form using tube trailers 
or cylinders and in liquid form in cryogenic liquid hydrogen tankers and 
to a very limited extent via pipeline. liquid hydrogen, cooled to –253°C, 
is transported by road in super-insulated cryogenic tankers with capacities 
of up to 60,000 litres. Pipelines present the most cost effective means of 
transporting large quantities of gaseous hydrogen over long distances. 
Currently the hydrogen infrastructure in the world is limited. the US Doe 
target for hydrogen distribution is(SofC) and
6. microbial fuel cell (MfC).
 PeMfC, AfC, PAfC, and MfC operate at low temperatures in the range 
of 50–200°C and MCfC and SofC at high temperatures in the range of 
650–1,000°C (fig. 22.3).
 the heart of a fuel cell consists of a non-conductive electrolyte material 
sandwiched between two electrodes – the anode and cathode [14]. the fuel 
and the oxidant are fed continuously to the anode and the cathode sides, 
respectively. At the anode side the fuel is decomposed into ions and electrons. 
The insulator electrolyte material allows only ions to flow from both the 
anode and cathode sides. The free electrons generated at the anode flow to 
the cathode side through an external electrical circuit. the recombination 
of the ions with the oxidant occurs at the cathode to form ‘pure’ water. 
In contrast to water electrolysis, the polarity of a fuel cell on the anode is 
negative and on the cathode is positive [14].
PEMFCs
PeMfC is a technology which was initially developed for military and 
spacecraft applications at Ge (General electric, USA) in the 1960s but was 
 AFC – 100 °C
 MFC –that the 0.2 grams will work well in a 100 kW system 
(fig. 22.4). furthermore, unless the Pt can be economically recycled to the 
purity needed, additional Pt will have to be mined to replace the Pt in the 
refurbished fuel cells in the existing fleet.
 however, from an ‘electrochemical point of view’, Pt is still the best 
electrocatalyst for PeMfC as it is stable, ‘durable’ and very active towards 
the hoR and the oRR. there have been many attempts at developing non-
precious electrocatalysts to replace Pt [23–25]. Although promising in the 
laboratories, the new nanomaterials have shown poor durability and stability 
in acidic and aggressive environments and cell voltage cycling, i.e. under 
‘real’ operating conditions (although it has been shown that iron-based 
nanostructures on nitrogen functionalised mesoporous carbons have exhibited 
durability and stability comparable to Pt alone).
Y
e
a
r
2015
2010
2009
2008
2007
2006
0 0.2 0.4 0.6 0.8 1.0 1.2
Platinum loadings (gPt kW–1 or mgPt cm–2)
US$30 kW–1 (US DOE target)
1 W cm–2
US$51 kW–1
833 mW cm–2
US$61 kW–1
833 mW cm–2
US$73 kW–1
715 mW cm–2
US$94 kW–1
583 mW cm–2
US$108 kW–1
700 mW cm–2
Key
Power density (W cm–2)
Estimated balance of plant
(US$ kW–1) (including assembly 
and testing)
Platinum loading (mgPt cm–2) 
used in automotive PEMFC 
stacks at a cell voltage of 0.676 V
Platinum loading (gPt kW–1) 
used in automotive PEMFC 
stacks
22.4 Evolution of total platinum loadings and power densities in a 
PEMFC stack.
703Fuel-cell (hydrogen) electric hybrid vehicles
 In addition to the high material cost, the precious-metal Pt catalyst is 
extremely sensitive to poisoning by Co, h2S, nh3, organic sulphur-carbon 
and carbon-hydrogen compounds in the h2 stream, and nox and Sox in air. 
Pt is also prone to dissolution and/or agglomeration resulting in performance 
degradation. More significantly, Pt is rare and is mined in a limited number 
of countries worldwide, with >80% mined in South Africa. Pt deposits are 
likely to become scarce in the coming years (~40 years reserves at the present 
rate). However, other minerals have historically seen a ‘falsification’ of 
resource scarcity, as the quantity of known reserves tends to increase in line 
with, or in excess of, increases in the rate of consumption due to economic 
factors [26]. 
 In the last five years, pt utilisation in automotive stacks has increased 
five-fold to over 5 kW.g–1 of platinum group metal (PGM). General Motors, 
for example, have reduced Pt usage in their newest generation of fuel cell 
stack from 80 g down to 30 g, and believe they can achieve Pt loadings 
of 10 g by 2020 [12]. this compares more favourably with Pt usage in 
conventional ICe vehicles of around 4–5 g (1 g for petrol, 8 g for diesel) for 
the catalytic converter [27]. Around 70 tonnes of pt are used by the eU auto 
industry each year; however, loadings are decreasing due to soaring metal 
costs, and the risk of opportunistic theft of catalytic converters. Uptake of 
fuel cell electric vehicles will certainly increase demand for Pt, but not by 
an unfeasible amount.
22.4.3 Hydrogen and fuel cell challenges
Hydrogen and fuel cell technologies have been identified as priorities for 
direct investment by many governments such as those of the UK and other 
eU countries. these technologies will contribute to tackling the UK/eU 
climate change targets (80% reduction in Co2/GhGs by 2050) and energy 
security, whilst providing significant market opportunities to a strong UK/
eU capability base. Many governments have set targets for fuel cell and 
hydrogen technologies, e.g. increasing durability and performance levels 
up to 8,000 hours for transport applications and 40,000 hours for stationary 
applications with target costs of longer term sustainable hydrogen below 
€5/kg ($6.80/kg). the expected impact will be through a breakthrough in 
materials bringing costs towards commercial cost and performance targets of 
€45/kW ($62/kW) (mobile applications) and €1,500/kW ($2,000/kW) (small 
scale stationary applications) with power density above 1W.cm–2 (2015 US 
Doe target (see fig. 22.4).
 there are a few challenges related to hydrogen generation, storage and 
utilisation and governments, together with academic and industry researchers, 
are currently looking to tackle them. these include:
704 Alternative Fuels and Advanced Vehicle Technologies
∑	 the design and development of low-cost and efficient hydrogen production 
systems using novel technologies, with a particular emphasis on the 
production of ‘green’ hydrogen from renewable sources (biomass, 
biowaste, electrolysers powered by photovoltaic and wind energy systems, 
etc.)
∑	 novel technologies associated with low carbon emission hydrogen 
production and utilisation from fossil fuels (including hydrogen separation 
technologies) and distributed hydrogen technologies (in terms of transport, 
this would include innovative on-site vehicle refuelling systems) and
∑	 the development of novel systems, materials and solutions for hydrogen 
storage and transportation, with low costs and high energy efficiency 
(with a focus on systems offering storage solutions suitable for integration 
with hfC vehicles).
22.5 Electrochemical capacitors (ECs)
electrochemical capacitors (eC), also called supercapacitors or ultracapacitors 
are high-energy-density devices. there are two energy storage mechanisms 
for eCs:
1. electrochemical double layer capacitors (eDlC), i.e. double-layer 
capacitance arising from the charge separation at the electrode/electrolyte 
interfaces – they consist of activated carbon with high specific area as 
electrodes and an organic electrolyte able to reach a specific capacitance 
in excess of 7,000 f.kg–1 (or 9,000 f.l–1) [28] and
2. pseudo-capacitors, i.e. pseudo-capacitance arising from fast, reversible 
faradaic reactions occurring at or near solid electrode surfaces – they 
contain transition metal oxides, nitrides and polymers possessing relatively 
high surface areas (1,500–2,400 m2.g–1), such as hydrous Ruo2, fe3o4, 
porous niox, Coox, Mno2 and Sno2 as electrode materials [29].
 the use of Ruo2 for pseudo-capacitors has been extensively investigated 
as it is highly conductive, possessing three oxidation states 600 F.g–1. Due to the high cost of 
Ru-based eCs and their limitations, i.e. a maximum cell voltage of 1.0 V 
(ideal for small electronic devices), eCs based on Mno2 have been studied 
as they offer higher specific capacities (~150 F.g–1) and are cheaper [29].
705Fuel-cell (hydrogen) electric hybrid vehicles
 In all cases, eCs rely upon the separation of chemically charged species 
at an electrified interface between a solid electrode and an electrolyte. The 
electrolyte between the anode and cathode is ionic (usually a salt in an 
appropriate solvent). the operating cell voltage is controlled by the breakdown 
voltages of the solvents (i.e. the decomposition cell voltages) with aqueous 
(1.1 V) and organic electrolytes (2.5 to 3 V). 
 eCs are currently being proposed by many automotive manufacturers 
(VW, honda, toyota, nissan, hyundai, etc.) for heVs, BeVs and fCeVs 
to (i) load level energy demand, (ii) quickly inject or absorb power to help 
minimise voltage fluctuations in the electronic systems, and (iii) provide 
pulse power well over 1,000 W.kg–1 with a cycle life reaching more than 
500,000 cycles. When eCs are fully charged or discharged in seconds their 
energy density may reach up to ~5 Wh.kg–1,and higher power delivery or 
uptake, up to 10 kW.kg–1, can be achieved in a few seconds. In contrast to 
batteries and fuel cells, the lifetime of ECs is virtually ‘indefinite’ and in 
some cases their energy efficiency rarely falls below 90% provided they are 
kept within their design limits. only eCs can provide a combination of high 
power density and relatively high energy density. however, unlike batteries 
and fuel cells, almost all of this energy is available in a quasi-reversible 
process. Figure 22.5 shows the specific power (W kg−1) against specific 
energy (Wh kg−1), also called a Ragone plot, for various electrical energy 
storage devices.
 finally, it is speculated that the next generations of eCs are expected to 
come close to li-ion battery technologies in energy density while maintaining 
their high power density. this will only be possible if (i) room temperature 
ionic liquids (RtIl) with a voltage window of more than 4 V are used and 
(ii) novel nanomaterials combining both double-layer and pseudo-capacitance 
are discovered [29].
22.6 Current status of low-carbon vehicle 
technologies
Many automotive manufacturers have now shifted their research effort to 
focus on high energy efficiency and renewable energy vehicles due to the 
problems caused by conventional vehicles. there are numerous potential 
solutions to eliminating these problems, such as hybrid vehicles, bio fuel 
vehicles, battery electric vehicles and fuel cell electric vehicles. Currently, 
there is no clear answer as to which one could dominate the future low carbon 
vehicle market. Many automobile companies have an equal interest in all four 
power-trains and scholars broadly agree that all currently viable technologies 
are likely to play a part in a future sustainable transport system [30].
706 Alternative Fuels and Advanced Vehicle Technologies
22.6.1 Conventional internal combustion engine (ICE) 
vehicles
the ICe powered vehicle is one of the greatest achievements in human 
history; the design has been perfected over 150 years and become the most 
popular power plant for vehicles. this is because the ICe vehicle had one 
dominant feature; it used petrol/diesel as a fuel, and so had a range that was 
far superior to other vehicles, for instance, battery vehicles. this can be 
explained by the energy density of petrol/diesel fuel compared to batteries 
– see comparison chart Ragone plot (Plate XX). the ICe dominated and 
will continue to dominate automobile technology for many years, even in 
today’s most advanced hybrid vehicles the internal combustion engine is still 
Capacitors
3.6 ms
0.36 s
3.6 s
36 s
1 h
Li-ion
Ni/MH
Li-primary
10 h
Pb02 
Pb
E
le
ctro
ch
e
m
ica
l ca
p
a
citio
rs
10–2 10–1 1 10 102 103
Specific energy (Wh kg–1)
S
p
e
c
ifi
c
 p
o
w
e
r 
(W
 k
g
–
1
)
105
104
103
102
10
1
22.5 Specific power (W kg–1) against specific energy (Wh kg–1), also 
called a Ragone plot, for various electrical energy storage devices. 
If a supercapacitor is used in an electric vehicle, the specific power 
shows how fast it can go, and the specific energy shows how far it 
can go on a single charge. Times shown are the time constants of 
the devices obtained by dividing the energy density by the power 
[29]. Source: P. Simon and Y. Gogotsi, Nature Materials, 7 (2008) 
845–854, copyright © Nature Publishing Group.
707Fuel-cell (hydrogen) electric hybrid vehicles
the first choice as the main power supply. However the hybrid vehicle’s ICE 
is different from the conventional vehicle’s ICe: the engine in the hybrid 
vehicle is normally smaller and runs at its high efficiency point for longer 
periods of time, hence achieving better fuel economy [31].
 Continuing to improve the efficiency of ICE is still the primary task for 
automotive engineers; some popular methods of improvement such as engine 
downsizing have doubled engine efficiency over the last 20 years. However, 
most engines are still not equipped with the most efficient technologies such 
as turbo chargers or variable valve timing, mainly because of their increased 
cost. 
22.6.2 Advantages of HEVs
Broadly speaking, a hybrid vehicle power-train can combine any two power 
sources. Possible combinations include, but are not limited to, diesel/petrol 
ICE with a battery, capacitor or flywheel, or fuel cell with a battery or 
capacitor. typically one component is for storage and the other is for the 
conversion of a fuel into useable energy. Conventional vehicles with ICe 
provide good performance and long operating range by combusting liquid 
fuels, with the advantage of high energy density. however, conventional ICe 
vehicles have the disadvantages of poor fuel economy and environmental 
pollution. the main reasons for their poor fuel economy are:
∑	 mismatch of engine fuel efficiency characteristics with real-world driving 
conditions
∑	 waste of vehicle kinetic energy while braking, especially when driving 
in urban conditions
∑	 energy wastage during engine idling and standby
∑	 low efficiency of hydraulic transmission (automatic) in current vehicles 
under stop-and-go driving conditions.
BeVs on the other hand possess some advantages over conventional ICe 
vehicles, such as high energy efficiency and zero tailpipe emissions. However, 
the limited range and long time charging makes them far less competitive than 
ICe vehicles, due to the much lower energy density of the batteries compared 
to liquid fuels. heVs, which combine the best features of the two power 
sources, gain the advantages of both ICe vehicles and BeVs, and overcome 
their individual disadvantages. heVs are more expensive than conventional 
vehicles because of the extra components and complexity required, but less 
expensive than BeVs due to the high cost of batteries.
708 Alternative Fuels and Advanced Vehicle Technologies
22.7 Battery electric vehicles (BEVs)
22.7.1 History
The first BEV was built by Thomas Davenport in 1834; even a few decades 
earlier than the first ICE vehicle. The first vehicle to surpass the 100 km/h 
barrier was also a battery vehicle, namely the ‘Jamais Contente’ which was 
driven by Camille Jenatzy in 1899 [32]. In comparison with ICe vehicles, 
BeVs were comfortable, quiet and clean. however, due to the limited 
energy storage capacity of the battery, the range was very limited, and at 
the same time, the ICe was improving dramatically. As a consequence, the 
BeV almost vanished by the 1930s [33]. But, due to the energy crisis and 
oil shortage in the 1970s, automakers and policy makers started to re-think 
the BEV, as it offered high energy efficiency and allowed the diversification 
of energy resources, as well as having zero local emissions and helping to 
improve urban air quality.
22.7.2 Current status
Battery vehicles use an electric motor for traction instead of an ICe, and use 
batteries for their energy source instead of liquid fuels. BeVs have many 
advantages over conventional ICe vehicles, such as no tailpipe emissions, 
high efficiency and potential for independence from fossil fuels, and quiet 
and smooth operation. the characteristics of the BeV and heV battery packs 
are very different. The BEV battery pack has high specific energy while 
the HEV battery pack has high specific power. Since the motor in a power-
assist (grid-independent) heV is used intermittently and must be capable 
of producing high power for short periods of time (e.g. during maximum 
acceleration), its battery pack should be optimised for high power. 
 the battery vehicle drive train consists of three major sub-systems:
1.	 Electric motor propulsion system – vehicle controller, power electronic 
converter, the electric motor, and transmission.
2.	 Battery system – batteries, battery management system (BMS), and 
charging unit
3.	 Auxiliary system – heating/cooling, electronic pumps, and other electronic 
auxiliaries.
the principle of the BeV is very straightforward, based on the control 
inputs from the accelerator and brake pedals, the vehicle controller provides 
proper control signals to the electronic power convertor, which functions to 
regulate the power flow between the electric motor and battery. The motor 
can also play the role of a generator, converting the braking energy to 
electrons and charging the battery. the energy management unit cooperates 
with the vehicle controller to control the regenerative braking and its energy 
709Fuel-cell (hydrogen) electric hybrid vehicles
recovery. the electric motors produce a great amount of torque from rest to 
give amazing performance. In terms of acceleration and power, BeVs are 
superior to ICe vehicles.
22.7.3 Future developments
While significant progress has been made in developing automotive batteries, 
major challenges remain, as follows:
∑ Reducing cost – currently a li-ion battery with 35 kWh storage capacity 
costs around $30,000 to manufacture, while a few organisations (Anl, 
IeA, ePRI, and CARB) project future prices around one third of this. 
Reducing the cost of battery packs is therefore the key challenge for 
BeV development [34].
∑ Improving safety – current nickel and cobalt-based oxide li-ion cathode 
materials have potential issues with overcharging, clearly; and voltage 
control at cell, module and battery level is critical to prevent overcharging 
of automotive li-ion batteries, but these are all factors that will inevitably 
increase li-ion battery cost further. lithium iron phosphate cathodes offer 
a promising future but with lower specific energy and power density.
∑ Prolonging the life-span – as an automotive battery, it should last at 
least 10 years or 150,000 miles under a variety of conditions, whereas 
for example the current average life of vehicles registered in the UK is 
14 years.
∑ Shortening the charging time to a matter of minutes, and providing better 
charging facilities.
∑ Reducing the size and weight of the battery pack. 
Compared with the problems posed by batteries, the challenges for the motor/
controller for electric vehicles may seem relatively small, with remaining 
developments aimed at improving the efficiency and reliability.
22.8 Fuel cell electric vehicles (FCEVs)
22.8.1 Introduction
fCeVs share many of the same components as BeVs, such as electric motors 
and power controllers or inverters; however, the major difference is the main 
energy source. While BeVs use energy stored in the battery, fCeVs use fuel 
cells as they are superior to batteries in many ways. the major advantages 
are that fuel cells are lighter and smaller and can produce electricity as 
long as the fuel is supplied. Due to the clear similarities between batteries 
and fuel cells, both these technologies will coexist in the future, while the 
BeV is suitable for short range and small vehicles, the fCeV is suitable for 
medium-large and long range vehicles.
710 Alternative Fuels and Advanced Vehicle Technologies
 the PeMfC is the best choice for automobile use. the principle of how 
fCeVs work is simple; they use low temperature fuel cells to generate 
electricity from hydrogen; the electricity is then either used to drive the vehicle 
or stored in an energy storage device, such as batteries or ultra capacitors. 
Since fuel cells generate electricity from chemical reactions, they do not 
combust fuel and therefore do not produce pollutants and produce much 
less heat than an ICe. the by-product of a hydrogen fuel cell is water. fuel 
cells have no moving parts or irregular shapes, so they have the potential for 
high reliability and low manufacturing cost. Although fCeVs possess many 
advantages, they also have certain limitations; these relate to the fuel cell 
stack itself and its fuel, hydrogen production, transportation and storage.
22.8.2 Proton exchange membrane (PEM) fuel cell stack
the PeM fuel cell stack is like the ‘engine’ in a conventional vehicle, which 
is the most important part. PeM fuel cell technologies have been developing 
at a good pace recently, however, two key limitations still remain: cost and 
durability. the cost of an ICe power plant is about $25–35 per kW, but 
current fuel cell systems are estimated to be about 5 times more expensive, 
even taking into account cost savings for high-volume manufacturing [35]. 
this is because materials and manufacturing costs for catalysts, bipolar 
plates, membranes, and gas diffusion layers are currently too high. on the 
other hand, the fuel cell stack as an ‘automotive engine’ is expected to be as 
durable and reliable as current automotive engines, i.e. 5,000 hours lifespan 
or 150,000 miles equivalent under a range of operating conditions, including 
different temperatures and climates.
22.8.3 Hydrogen as fuel for fuel cell hybrids
fuel cells prefer to operate in a consistent condition and achieve their 
maximum efficiency at partial load. However, a vehicle requires a variety of 
power outputs according to the road and traffic conditions. Hybridisation of 
a fuel cell with peak power sources (PPS) could solve these problems. for 
instance, by using batteries or ultra capacitors when power demand is high, 
such as with higher loads or acceleration, PPS could help the fuel cell to 
provide the boost while power demand is low, such as during deceleration 
or when travelling downhill. the extra energy can be replenished to PPS; 
this allows the fuel cell system to be operated more efficiently. On the other 
hand, a smaller fuel cell can be used, for instance, to maintain a vehicle at 
70 mph continuously requires 15 kW at the wheels, so a 20 kW fuel cell 
stack should able to cope with this. 
 So a hybrid fuel cell with a PPS could offer a viable solution for electric 
vehicles. Such a configuration will offer the following advantages compared 
711Fuel-cell (hydrogen) electric hybrid vehicles
to full fuel cell vehicles:
∑ smaller fuel cell means lower cost
∑ fuel cell could operate at optimum efficient point most of time under 
appropriate control strategy
∑ fuel cell life will be extended
∑ the fuel cell designer will be able to optimise the cells for power instead 
of cycle life
∑ deep discharging from the battery will be eliminated; therefore improving 
the batteries’ life
∑ allows fast start-up of the fuel cell
∑ allows capture of regeneration energy.
the disadvantages of hybridisation are the complexity of the vehicle system, 
weight increase, complexity of the control system, and extra battery cost.
22.8.4 Example of hybrid battery FCEV
In 2006 the Birmingham Centre for hydrogen and fuel Cell Research (UK) 
received support from Advantage West Midlands to buy a prototype plus 
four improved vehicles which could then be trialled using the university 
campus as a test track (fig. 22.6(a)). the Air Products refueller was opened 
at the School of Chemical Engineering in April 2008 and the five cars 
were then used to test design calculations over a two-year test programme 
undertaken by a consortium of West Midlands’ suppliers part-funded by the 
UK government department Business enterprise and Regulatory Reform 
(BeRR). the results [36–38] showed that the initial campus design gave the 
desired acceleration, with a top speed of 20 mph and range of 60 miles on 
a campus drive cycle, but could not achieve the eCe15 urban cycle. this 
was not surprising because the fuel cell was only the 1.2 kW Ballard nexa 
PeMfC stack (less than 1 g of Pt is used compared with 60g for a 100 kW 
system), with a hydrogen tank of 0.6 kg at 350 bar and a 2 kWh lead acid 
battery. The main problem was the battery capacity which was insufficient 
to give regenerative braking and plug-in range. other issues such as the need 
for a lightweight aluminium chassis, the space requirements for fleet vehicles, 
the performance of lightweight body panels and better heat management for 
window and cab temperature control wereaddressed in 2010 when the next 
batch of eight cars was assembled. 
 So far, Microcab Industries, in collaboration with the University of 
Birmingham have made very good progress on the new hydrogen hybrid 
fuel cell vehicle (fig. 22.6(b)) [39], for example:
∑ the range of the vehicle has been extended to 200 miles (a current state-
of-the-art lithium-ion battery vehicle can only do 60 miles)
712 Alternative Fuels and Advanced Vehicle Technologies
(a) (i)
(a) (ii)
(b) (i)
22.6 (a) Microcab hydrogen hybrid fuel cell vehicle 2nd generation 
and (b) new Microcab generation.
713Fuel-cell (hydrogen) electric hybrid vehicles
∑ a short refuelling time of three minutes (battery vehicles have a domestic 
charging time of 5–8 hours). 
∑ the lifetime of the fuel cell stack has been increased to 5,000 hours with 
a high temperature PeMfC. 
economically, if 500,000 hydrogen hybrid vehicles were produced per 
annum, less than 0.5 tonnes of Pt would be required assuming a Pt loading 
of 0.6 g/kW. of course, the key issues are cost, lifetime and reliability in 
both start-up and duty cycles, which still require some attention. Based on 
a PeMfC stack production cost of $225/kW (mainly attributed to Pt and 
membrane cost) with a Doe target of $30/kW by 2015, fuel cell-electric 
hybrids become a very attractive option for commercial deployment.
 of course, another option is to convert a conventional petrol combustion 
hybrid car to run on hydrogen in order to test the filling station infrastructure 
and the performance/costs of the new systems. In 2006, Statoil did this in 
Norway by opening the first of its HyNor fuelling stations in Stavanger with 
15 modified Toyota Prius hybrids running on hydrogen gas. By 2009 there 
were seven hydrogen stations connecting the 600 km from oslo to Stavanger 
and the number of hydrogen vehicles had increased to 50 with manufacturers 
Mazda, think and toyota [40]. the advantages of demonstrating this 
technology in norway were substantial:
H2 tank
Fuel cell
FC 
controller
24V–12V 
converter
12V–24V 
converter
24V–48V 
converter
+ –
+ – 12V
48V
Auxiliaries
Electrical
Mechanical
Motor 
controller
M
(b) (ii)
22.6 (Continued)
714 Alternative Fuels and Advanced Vehicle Technologies
∑ car taxes are high and can be reduced to incentivise consumers
∑ hydrogen cars can drive in bus lanes and are exempt from road tolls 
∑ hydrogen is readily available from the Statoil plants
∑ hydrogen can be produced from renewable sources such as hydropower 
to give green energy.
22.8.5 Future developments
the following issues need to be addressed:
∑ Reducing the cost of hydrogen – the cost of hydrogen, which includes the 
cost of production and delivery, has to be competitive with conventional 
fuels 
∑ improving hydrogen storage technology – the low volumetric energy 
density of hydrogen makes storage a challenge; the storage needs to 
meet vehicle packaging, cost, and performance requirements
∑ reducing fuel cell cost and improving durability – the cost of fuel cell 
power systems has to be reduced and durability must be improved for 
fuel cells to compete with conventional technologies. 
22.8.6 Comparisons with BEVs
table 22.5 compares the different types of vehicles in terms of cost, 
performance, and Co2 emissions; here the models chosen are representative 
vehicles within their class, and represent the most advanced technologies 
to date.
22.9 Technical prospects and barriers
Many of the technical advantages and limitations of batteries, fuel cells and 
capacitors are rooted in the fundamental electrochemical mechanisms they 
employ. It is unlikely that the performance of these three technologies will 
ever converge, and so their suitability for particular applications will always 
remain different [41].
22.9.1 Cost
Upfront cost is a key factor for public acceptance and uptake, and is proving 
to be one of the steepest challenges for electrochemical energy systems. these 
alternative technologies must overthrow a century of refinement and extreme 
economies of scale that ICEs benefit from. The advantage these nascent 
technologies possess is that, while still in their early years, technological 
progress has been impressive and enabled costs to fall rapidly. toyota reports 
715Fuel-cell (hydrogen) electric hybrid vehicles
to have cut the cost of making its fuel cell vehicles by 90% since 2005, from 
$1 million per vehicle down to around $100,000. they expect to halve their 
current costs before retail sales begin in 2015 [42].
 Major organisations share a common view on the pace of fuel cell cost 
reductions – centred around $75,000 at the start of commercialisation in 
2015, under $50,000 after 5 years, and a floor level of around $30,000 by 
2025 [12]. Battery prices are expected to fall less rapidly, by 50% within 10 
years compared to 75% for fuel cells. By 2020, a 25 kWh battery pack is 
expected to cost $6–10 thousand, giving a similar premium over conventional 
vehicles as fCeVs [12]. these costs are still a long way off $1,000 for the 
ICe they replace; however, they represent only a modest increase in the cost 
of a complete vehicle, and are widely thought to be low enough to encourage 
the development of a substantial vehicle market.
Table 22.5 Comparison of vehicle specifications from a consumer’s perspective
ICE (VW GOLF 
1.4TSI)
Hybrid (Toyota 
Prius III)
BEV (Nissan 
Leaf)
FCEV (Honda FCX 
Clarity)
Power supply IC engine ICE, electric 
motor
Battery and 
electric motor
PEM fuel cells and 
electric motor
Fuel Petrol, diesel 
and alternative 
fuel
Petrol/diesel as 
main fuel
‘Electricity’ Hydrogen
Top speed, 
(mph)
124 112 94 100
Acceleration, 
(s)
9.5 10.4 7 10
Range (miles) 552 716 73–109 240
Purchase 
price
$29,400 $33,400 $41,250 
(including 
$8,000 
government 
incentive)
$80,000 
(estimated)
Running 
fuel price 
(per mile)
$0.22 $0.14 from $0.02 From $0.07
Fuel economy 
(mpg or mpg 
equivalent)
45.6 72.4 99 81
Tailpipe CO2, 
emission 
(g/km)
144 89 0 0
716 Alternative Fuels and Advanced Vehicle Technologies
22.9.2 Durability and degradation
the prospects for capacitors are excellent in this respect, with calendar 
lifetimes measured in decades and charge/discharge cycles in the millions. 
the calendar life of lithium-ion batteries is still a problem, as the rate of 
capacity loss has not improved in seven years, at approximately 5% per 
year [43, 44].
 lifetimes for lithium-ion chemistries are in the order of 2,000 cycles to 
80% depth of discharge (DoD) before 20% of power is lost. the number of 
cycles is approximately reciprocal with the DoD, meaning that ~4,000 cycles 
to 40% DoD can be expected. With the 75 mile range of current BeVs, this 
suggests that the battery should last for at least 100,000 miles, comparable 
to an internal combustion engine. this level of durability has not yet been 
verified for lithium-ion chemistries, but has been obtained in real-world 
usage of niMh battery packs produced some 14 years ago [45].
 fuel cells lifetimes are assessed by the number of hours until 10% power 
is lost. the latest generation of fCeVs from ford, Daimler and GM are 
projected to last 800–1,100 hours, falling short of the Doe’s 2009 target of 
2,000 hours [36]. Based on these vehicles’ driving patterns around California, 
this corresponds to 21,000–28,000 miles, and so a five-fold improvement in 
durability is required to compete with battery technologies. the reliability 
of current fCeVs must also be improved, as some vehicles are seen to lose 
10–25% performance within the first 300 hours of driving (~8,000 miles) 
[36].
22.9.3 Energy and power density
energy density is often cited as the largest problem for electrochemical storage 
devices. The specific energy and energy density of batteries and capacitors 
are unlikely to ever compete with liquid hydrocarbons holding around 12