**4. Why should energy storage unit be integrated with fuel cell for the FCEV application?**

Although there are a lot of merits, PEMFCs are still not close to perfect. The major concern is relevant to performance and lifetime of the fuel cell. The performance of PEMFC is influenced by many internal and external factors, for instance fuel cell design and assembly, degradation of materials, operational conditions, and impurities [53]. Hybridization is one of solutions to reduce the problems. The hybridization in the energy sector can be categorized into four systems as follows:


In terms of transportation applications, PEMFC material suffers from closely random power load cycling such as the frequent start-up and shut-down [55]. An interesting approach to improve the efficiency and increase the lifetime of the PEMFC is to incorporate the new emerged energy storage, named Supercapacitor, in the system. A system that include several sources and/or energy storage devices is also known as hybrid system.

The concept of fuel cell-supercapacitor hybridization is quite new compared to the fuel cell-battery hybridization. The advantages of supercapacitors over the batteries are the higher number of charge/discharge cycles and the higher current rating. Once the supercapacitor is connected to the system, the stress due to the transient current were handled by the supercapacitor. There are many topologies of fuel cell-supercapacitor hybridization but most of them are connected together via various types of DC/DC converter. The very new concept of fuel cell/supercapacitor hybridization is to connect them directly together, this method is call

direct-hybridization [56–59]. This concept is very interesting because it is able to increase efficiency, lifetime and reduce the system cost due to the absence of the DC/DC converter, which has significant impact on system design. **Table 1** presents the crucial characteristics of a supercapacitor applied to the energy storage hybridization. The supercapacitor generates higher power than a battery does, and either charging or discharging time is faster than the ability of a battery. The fast charging and discharging characters can diminish materials degradation and PEMFC lifespan. Also, a supercapacitor can offer transient power to meet load demand in a short time. According to this advantage feature chemical kinetic energy can be recovered during regenerative braking occurring while the automobile is slowing down or stopping. The supercapacitor can also save energy and protect materials components inside PEMFCs from deterioration [60].

Supercapacitors in a FCEV operate with two features; charging electricity by PEMFC and discharging electricity to PEMFC. Once electrical current is charged into a supercapacitor, the positive charges of electrolyte move to a negative electrode using electrostatic force, and the negative charges of electrolyte transfers to the positive electrode. Supercapacitors are normally composed of three main structures; electrolytes, electrodes, and separators. Typically, the electrode of the supercapacitor is made from carbon particles due to they have high surface areas and high porosity required for collecting the charge. During charging duration [61], the charges transfer through the pores, and then they are stacked layer by layer as shown in **Figure 9**. On the other hand, a discharging process is an inversion operation of the charging step. Supercapacitors discharge electricity to load based on charge volume and voltage change over time leading to speedy response to load.

The powertrain configuration of the FCEV is usually comprised of supercapacitors connected with PEMFC. The supercapacitor and convertor can be directly connected with PEMFC. The supercapacitor can also be connected in parallel with PEMFC through energy converters. A converter is an electromechanical device


*the cell as a result of its internal impedance.*

#### **Table 1.**

*Characteristics of battery and supercapacitor for hybridization.*

#### *Hydrogen Fuel Cell Implementation for the Transportation Sector DOI: http://dx.doi.org/10.5772/intechopen.95291*

transforming a source of direct current (DC) from one voltage level to another. The energy converter functions as an energy collector storing generated energy. If the system quickly requires energy, the generated energy will be supplied by the converter. This connection feature makes the system more complex and expensive. A directly connected structure that a supercapacitor directly connects in parallel with PEMFC plays a role in self-energy management, therefore, the system requires an energy management design. This scenario can directly protect against rapid power variations that can increase the dynamics of the hybridization system. It is worth noting that a directly connected structure between PEMFC and supercapacitor, a supercapacitor is a necessity to be pre-charged before utilization to limit inrush current [62]. A hybrid system requires PEMFC as the main power source and supercapacitors as an auxiliary energy source. The supercapacitors assist the system to reduce voltage fluctuations at an unstable demand. The supercapacitor also stores electrical energy from PEMFC when there is excess energy. In contrast to this, the supercapacitor will supply power to PEMFC once the load demand is high. This system acquires less equipment, less sophistication, and provides higher effectiveness [63].

The noticeable data from investigation of supercapacitor effect on PEMFCsupercapacitor direct hybridization performance related to a driving behavior

#### **Figure 9.**

*The operations of a supercapacitor.*

#### **Figure 10.**

*The polarization curves of PEMFC and PEMFC-SC direct hybridization (A) non-charging supercapacitor (B) pre-charging supercapacitor at 0.90 V.*

protocol are shown **Figure 10** [64]. Throughout the testing period, the PEMFC generates electricity to load demand and supercapacitor for charging process until the voltage level between PEMFC and supercapacitor are equivalent.

The curves in **Figure 10** can be separated into three transitions. In the first step of the test using a non-charging supercapacitor, the PEMFC charge electricity to load as indicated in low current density range (yellow area) until the electrical power of the PEMFC and supercapacitor is in the same level. Both of electrical providers supply electrical power to load demand in the second step. Due to supercapacitor properties, fast charging and discharging, the voltage of supercapacitor dramatically decreases observed in the third step. At this situation the voltage level of supercapacitor is lower than the one belongs to the PEMFC, thus, the PEMFC charges electricity to charge supercapacitor again. In pre-charging point of view, the voltage of PEMFC-SC direct hybridization is higher than PEMFC in all transition. It implies that the PEMFC and supercapacitor jointly supply electrical power to load demand. The supercapacitor assists the system to reduce voltage loss at high current density. The major voltage loss occurs from the mass transport [65].
