The EV technology has been regarded as one of the most promising solutions to resolve the problems of the environmental pollution and energy crisis (Chan and Chau, 2001). As the major component of the EV technology, the development of the electric motors has drawn many attentions (Chau, 2015). Nowadays, the electric motors can fulfil most of the requirements for the standard vehicular driving, such as high efficiency, high torque density, high power density, and maintenance-free operation. However, because of the bottle neck in the electrochemical energy sources, the EV models have not generally penetrated into the current market except in ancillaries (Chauet al., 1999; Burke, 2007).
The electrochemical energy sources are used to power the electric motor as well as the EV ancillaries. All the developed electrochemical energy sources can only provide either high specific energy or high specific power, but not both simultaneously (Khaligh and Li, 2010). Hence, the existing EV model cannot compete with the gasoline combustion engine vehicles at the moment (Chau, 2012).
To improve the situation, the development of electrochemical energy sources has become one of the hottest research topics recently (Whittingham, 2012). There are various electrochemical energy sources available in the existing market, and they
1Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong, China
2Department of Chemistry, The University of Hong Kong, Hong Kong, China
can be classified into three major groups, namely, the capacitor, the battery, and the fuel cell (Lukicet al., 2008).
2.1.1 Basic differences among electrochemical cells
The three major electrochemical energy sources can be further classified based on two different electrode processes, namely, the non-Faradaic and Faradaic pro- cesses. The non-Faradaic process means there is no chemical reaction associated with the cycling processes and the charges are distributed only by the physical mean without any formation of the chemical bonds. On the other hand, the Faradaic process means the chemical reactions, such as the oxidation–reduction reactions, are associated to transfer the charges. Typically, the capacitors (except the pseu- docapacitor) consist of the non-Faradaic process, while the batteries and fuel cells consist of the Faradaic processes. Since the capacitors that consist of the non- Faradaic process involve only the physical process of charges movement, its overall charging and discharging mechanisms are faster. Hence, these types of energy sources are more suitable for the power devices. On the other hand, the batteries and the fuel cells involve the Faradaic processes so that higher energy can be stored and released. The graphical comparisons among the representative energy sources are shown in Figure 2.1. Unfortunately, in the current situation, none of the single energy sources can fulfil the desired expectation for EV applications, so that the development of electrochemical energy sources has been a very hot research topic since the last century.
107
Capacitors
Ultracapacitors
Batteries Fuel cells 106
105 104
Specific power (W/kg)
103 102
10–2 10–1 1 102 103
Specific energy (Wh/kg) 10 10
0
Desired specification for EV applications
Figure 2.1 Comparison of the electrochemical energy sources
As the common candidates for EV applications, the batteries and fuel cells can be further distinguished by the types of chemical reactions. Fuel cells consist of combustion of a fuel as the overall reaction, and hence it involves oxygen (the most common and generally available oxidant) at the cathode (positive electrode).
Because of the difficulty of a three-phase reaction, i.e., phase of gas/solid/electro- lyte form, the fuel cell electrochemical reactions are relatively slower with poorer power density. Yet, it involves oxygen as the active material so that the energy density is higher.
A large variety of batteries provides the variety choices of active materials, voltage, current, electrolyte, reversibility, and safety. The variations stem from reactions at the negative and positive electrodes involving different reactants and products. Kinetics and reversibility are generally limited by mass and energy transfer across phase boundaries and the solid phase morphology change. Since a battery consists of a batch reaction, its capacity is limited by changes in solid phase of electrode. On the other hand, a fuel cell has continuous supply of active material (fuel) and therefore offering superior capacity.
2.1.2 Specific energy of electrochemical cells
The driving range has been one of the major considerations for EV applications, although it can be effectively determined by the specific energy of electrochemical cells. Theoretically, the specific energy can be calculated based on the following simple relationship asCell capacityCell voltage. Some examples are shown in Figure 2.2 which includes the nickel-metal hydride (Ni-MHx) battery, all-vanadium redox flow (VRF) battery, and hybridizing positive electrode of VRF with either
C+C– W = CV
C= C+ + C–
VRF
C = 156
C = 110
C = 164
1.83 V
1.80 V
1.25 V
1.26 V
mAh/g
mAh/g Ni(OH)2/
NiOOH NiMHx 1.00
0.45
Electrode potential (V) –0.26 –0.80 –0.83
V4+/V5+
H2/H2O V4+/V5+
V-MH V-H2
C+ = 165 mAh/g
C–= 68.5 mAh/g
C–= 340 mAh/g C–= 340 mAh/g C–= 26.9 Ah/g C+= 289 mAh/g
C+ = 165 mAh/g C+ = 165 mAh/g
mAh/g C = 48.4
V4+/V5+
V2+/V3+
mAh/g W = 60.5
W = 195
W = 200
Theoretical cell capacity (mAh/g)
W = 300
MHx/MHx–1
MHx/ MHx–1 Wh/kg
Wh/kg
Wh/kg Wh kg–1
Figure 2.2 Theoretical specific energies among various batteries Overview of electrochemical energy sources for electric vehicles 33
MH or hydrogen. In the VRF battery, VOSO4and V2(SO4)3are the active material for V4þ/V5þand V2þ/V3þredox couples, respectively.
The theoretical cell capacity is calculated byCẳCỵC/(CỵỵC), whereCỵ and Crepresent the theoretical capacity of positive electrode/electrolyte material (width of bar at the top) and negative electrode material (width of bar at the bottom), respectively (Liet al., 2009, 2011).CþandCare scaled by Faraday’s law to their individual active material, which states that capacity is proportional to the maximum number of charge transferred and inversely proportional to mass of the active material.
The theoretical cell voltage (or known as the electrochemical window), which serves as an important factor determining the key performances of the electro- chemical cells, can be described by the pairing of negative and positive electrode reactions and their corresponding standard potential, i.e., the position in the elec- trochemical activity series, as shown in Figure 2.3. As illustrated, the theoretical maximum cell voltage can be calculated by the difference between the individual voltages of the cathode and the anode.