PEM Fuel Cells: Fundamentals, Advanced Technologies, and Practical Application

PEM Fuel Cells: Fundamentals, Advanced Technologies, and Practical Application provides a comprehensive introduction to the principles of PEM fuel cell, their working condition and application, and the latest breakthroughs and challenges for fuel cell technology. Each chapter follows a systematic and consistent structure with clear illustrations and diagrams for easy understanding.

The opening chapters address the basics of PEM technology; stacking and membrane electrode assembly for PEM, degradation mechanisms of electrocatalysts, platinum dissolution and redeposition, carbon-support corrosion, bipolar plates and carbon nanotubes for the PEM, and gas diffusion layers. Thermodynamics, operating conditions, and electrochemistry address fuel cell efficiency and the fundamental workings of the PEM. Instruments and techniques for testing and diagnosis are then presented alongside practical tests. Dedicated chapters explain how to use MATLAB and COMSOL to conduct simulation and modeling of catalysts, gas diffusion layers, assembly, and membrane. Degradation and failure modes are discussed in detail, providing strategies and protocols for mitigation. High-temperature PEMs are also examined, as are the fundamentals of EIS. Critically, the environmental impact and life cycle of the production and storage of hydrogen are addressed, as are the risk and durability issues of PEMFC technology. Dedicated chapters are presented on the economics and commercialization of PEMFCs, including discussion of installation costs, initial capital costs, and the regulatory frameworks; apart from this, there is a separate chapter on their application to the automotive industry. Finally, future challenges and applications are considered.

PEM Fuel Cells: Fundamentals, Advanced Technologies, and Practical Application provides an in-depth and comprehensive reference on every aspect of PEM fuel cells fundamentals, ideal for researchers, graduates, and students.

• Presents the fundamentals of PEM fuel cell technology, electrolytes, membranes, modeling, conductivity, recent trends, and future applications
• Addresses commercialization, public policy, and the environmental impacts of PEMFC in dedicated chapters
• Presents state-of-the-art PEMFC research alongside the underlying concepts

• Language: English
• Publisher: Elsevier Science
• Release date: Nov 16, 2021
• ISBN: 9780128237090

Book preview

Chapter 1

Proton exchange membrane fuel cells: fundamentals, advanced technologies, and practical applications

Piyush Sharma and O.P. Pandey,    School of Physics and Materials Science, Thapar Institute of Engineering & Technology, Patiala, India

Abstract

Proton exchange membrane (PEM) fuel cells emerged as promising substitute to fossil fuels. The potential to reduce overall energy consumption, zero carbon emission, and high energy density makes PEM fuel cells suitable for plethora of applications. In last few years, research interest in PEM fuel cell has been boosted with the growth in green energy requirements. PEM fuel cell technology is considered to most suitable for transportation sector and portable energy frameworks. Many vehicles and portable electronic devices have been commercialized in past decades. However, the commercialization of PEM fuel cells suffers from two major hurdles, that is, high cost and low durability. Several efforts have been made to overcome these two hurdles. It is crucial to understand the fundamentals related to each component of fuel cell before improving its performance. In this chapter, attempts have been made to provide insight understanding related to each component of PEM fuel cell. The technological status and practical application of PEM fuel cells are addressed. A summary related to need of futuristic research prior to the development of PEM fuel cell is presented.

Keywords

Green energy; PEM fuel cells; fundamentals; practical applications

1.1 Introduction

The modern technological ventures demand innovative energy frameworks for the progress of our society. The new energy infrastructure is the emergent need to improve energy access, economic growth, and to meet current environmental policies [1]. The most of the energy requirement is compensated with nonrenewable energy sources (coal, petroleum, and natural gas). The nonrenewable energy sources are finite and will be depleted soon if their usage is not restricted. In this scenario, the best alternative is to hunt novel pathways and develop energy frameworks based on the renewable sources such as solar, water, wind, geothermal, and biomass. These efforts not only resolve the problem of energy crisis but also help in improving the balance of our ecosystem [2]. In this regard, the electrochemical devices are promising to extract and store green energy from the renewable resources. These devices include supercapacitors, fuel cells, and batteries. Among these devices, fuel cell is identified as potential candidate for energy conversion due to its high energy density, salient operation, and offers similar performance for large as well as small devices [3]. In general, the fuel cells are commonly classified in six categories on the behalf of electrolytes. These include proton exchange membrane (PEM) fuel cell, alkaline fuel cell (AFC), direct methanol fuel cell (DMFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC), and solid oxide fuel cell (SOFC) [4,5]. The electrochemical reactions involved in these fuel cells and their unique features are listed in Table 1.1.

Table 1.1

Recently, PEM fuel cell has attracted scientific society due to its broad applicability in different fields such as transportation, portable electronic devices, and distributed generation systems [8–10]. Several electric vehicles (EVs) have been developed based on PEM fuel cell technology. This includes Honda Clarity, Toyota Mirai, and Hyundai Nexo [11]. The distributed generation system is mainly focused to develop small energy frameworks for households, where load is <10 kW [12]. The PEM fuel cell technology can also provide power to portable devices such as laptops, portable power banks, wearable electronic devices, and military radio/communication devices [13]. The research activities have been exponentially increased for the development of PEM fuel cell. The commercialization of PEM fuel cell turned into reality for some applications. However, higher cost and low durability restricted the implementation for wide-range of applications. These two hurdles are interrelated, and it is challenging to improve durability by reducing the cost [14]. These challenges will be overwhelmed with a boom in the commercialization of the PEM fuel cells. Therefore scientific community has thought of to improve design and production schemes for the implementation of fuel cells.

Once the mass production of fuel cells started then the manufacturing processes, durability, design, and efficiency will be improved. The mass production of fuel cell will also cut down the overall manufacturing cost of the PEM fuel cells. Several government and private agencies around the globe are promoting research programs for the development of fuel cells to produce green energy [15]. The prime motto is to reduce the dependency on PEM fuel cell based on finite nonrenewable resources. These efforts have brought motivation toward utilization of green energy and development of new energy frameworks. Such growth in research activity is favorable to develop advanced energy framework based on PEM fuel cells. This chapter deals with the fundamentals including detailed description related to materials, design, and operational controls of PEM fuel cells. The efforts are made to address the current state-of-art for PEM fuel cells technology. The major challenges are also outlined associated with the practical implementation of the PEM fuel cells.

1.2 Proton exchange membrane fuel cells

A PEM fuel cell is a simple device that electrochemically transforms the chemical energy of a fuel directly into electrical energy. The proper functioning of a PEM fuel cell requires uninterrupted supply of fuel. The by-products of the transformation also include heat energy and water. It is theoretically predicted that the maximum efficiency can be achieved up to 90% in a PEM fuel cell. This technology is best-in-class owing to its high energy density at lower temperatures (~70°C–90°C). A single PEM fuel cell produces an output voltage ~0.5–0.7 V [16]. The schematic representation of PEM fuel cell and its stacked components is presented in Fig. 1.1.

Figure 1.1 Schematic representation of PEM fuel cells [17].

The PEM fuel cells are stacked in series with the help of highly conductive bipolar plates (BPs), as shown in Fig. 1.2A–C. The overall efficiency of a PEM fuel cell significantly depends on the output of a single cell. In a PEM fuel cell, hydrogen (H2) gas passes though anode resulting into hydrogen oxidation reaction (HOR) and generation of protons and electrons [19,20]. The generated electrons produce electricity at external circuit when connected to the load, while the protons are drifted with the aid of PEM. The activation energy required for HOR is ~65 kJ/mol. The barrier of activation energy can be reduced with the choice of catalyst. In case of platinum (Pt) catalyst, the required activation energy for HOR drops down to ~18 kJ/mol. When the H2 interacts with the surface of the anode catalyst then dissociation adsorption of H2 occurs. Afterwards, electrons flow from H2 to catalyst and lastly release of adsorbed H2 occurs (Fig. 1.3A). At cathode, protons recombine with electrons and oxygen to form water and OOH* atoms or H2O2 via oxygen reduction reaction (ORR) [22].

Figure 1.2 (A) Stacked enclosure and rubber paddings of commercial PEM fuel cell, (B) stacked fuel cell, and (C) components of stacked fuel cells [18].

Figure 1.3 (A) HOR reaction mechanism at the surface anode electrode (Pt), (B) O2 adsorption pathway at the cathode surface (Pt). ORR reaction mechanism with (C) four electrons and protons, and (D) two electrons and protons reaction pathways. The given values of 0.68, −0.13, and 1.23 V correspond to potential of the ORR reaction versus normal hydrogen electrode [21].

In general, four electrons and protons are required for an ORR reaction. It is supposed that O2 molecule is initially adsorbed over the surface of the cathode (Fig. 1.3B). Firstly, O–O bond breaks resulting in adsorption of oxygen atoms 2O*. These atoms could react with protons, which are drifted from the anode to cathode through the membrane [23]. The combination of protons and electrons at cathode leads to formation of OOH* atoms. Later, the formation of water occurs when surface bonded OOH* atoms combine with proton (H+) and two electrons. The water formed at cathode leaves the surface, as shown in Fig. 1.3C. However, two electrons and protons may also solve the purpose for an ORR reaction depending on the performance of the catalyst. In this reaction pathway, O2 directly combines with proton and is reduced to produce hydrogen peroxide (H2O2). H2O2 may react with the cathode electrode or adsorb at the cathode surface (Fig. 1.3D). The formation of H2O2 adversely affects the durability of membrane by oxidizing it. Therefore two electrons reaction pathway is not favorable. To restrict the formation of H2O2, it is important that O2 adsorption must occur at the surface of the cathode electrode. The prime reason behind formation of H2O2 is crossover and low hydration level [24]. To gain more insight related to the functioning of PEM fuel cell, it is crucial to understand the role of each component of fuel cell. The fundamental aspects and functioning of various PEM fuel cell components are addressed in the later section.

1.3 Components of PEM fuel cells

The primary components of a PEM fuel cell include membrane, electrodes, BPs, current collector, and other components, as shown in Fig. 1.1. The electrode at anode and cathode of the PEM fuel cell are comprised of catalyst layer (CL), gas diffusion medium (GDM), and microporous layer (MPL). A detailed description related to the working of each component is presented here.

1.3.1 Membrane

The membrane in a PEM fuel cell is placed in between anode and cathode catalyst layers. The foremost role of the membrane is to act as a separator, provide drift to protons from anode to cathode, and barricade electron transfer. The material used for the membrane must be a good insulator, offers high ionic conductivity, restrict crossover of gasses, prevents electron exchange, and possess superior mechanical and chemical stability [25]. The polystyrene sulfonic acid was the first membrane used in the PEM fuel cell in 1960s. This membrane was found to be flimsy and can be operatable up to 500 hours. In 1967 nafion membrane developed by DuPont was used in the PEM fuel cell [26]. The nafion being perfluorosulfonic acid (PFSA) membrane is a copolymer comprises of polytetrafluroethylene (PTFE), sulfonic acid group ( glyph SO3H), and perfluorinated as main, end, and side chains, respectively [25,27]. The PTFE polymer is the backbone of the membrane. The sulfonic acid group is ionically bonded due to which SO3− ion is present at the end of the side chain. The structure of PTFE polymer makes nafion membrane hydrophobic in nature. While, the sulfonic acid group makes membrane hydrophilic and responsible for the conductivity of protons, so, the final structure is named as an ionomer. The reason behind good conductivity of sulfonic acid group is associated with the bond strength. The C–F bonds are much stronger than C–H bonds and C–C bonds are well safeguarded by F bonds. The presence of F-atoms makes sulfonic acid further acidic in nature due to their higher electron affinity. Hence, the proton conductivity improves with the increase in acidic nature of sulfonic acid [28,29]. In addition, a perfluorinated polymer side chain offers higher mechanical and chemical stability. The length of the side chain significantly affects the mechanical stability of the membrane [30]. The shorter length side chains are more crystalline in comparison to longer length side chains. Therefore shorter side chains offer good mechanical properties that are favorable for the development of efficient membrane [31–34]. Fig. 1.4A demonstrated the general chemical formula of PFSA ionomers prepared by different industries such as Dow, Solvay, and Asahi chemical companies. The phase separation in the membrane (Fig. 1.4B) is the important parameter that governs the chemistry of the ionomer. The influence of EW and side-chain is also presented in Fig. 1.4C.

Figure 1.4 (A) General structure of PFSA ionomers developed by different companies. (B) Phase-separation with hydration in a membrane. (C) The role of EW and side-chain chemistry membrane of the different ionomers [35]. Credit : From New Insights into Perfluorinated Sulfonic-Acid Ionomers Chem. Rev. 2017, 117, 3, 987–1104, https://pubs.acs.org/doi/10.1021/acs.chemrev.6b00159.

Several reports suggested that the performance of membrane significantly depends on the thickness and hydration [10,36–41]. Decreasing thickness of the membrane increases water adsorption and offers ease to hydration of the membrane. The ionic nature of the membrane promotes high water adsorption resulting in rapid humidification. The conductivity of protons remarkably improves when the membrane is perfectly humidified. This significantly improves the overall efficiency of membrane. Drop in thickness also reduce the cost of the membrane. The level of hydration directly affects the proton conductivity of the membrane. If the hydration level is low then water molecules adsorb on the hydrophilic walls of the pore. Consequently, a very thin layer of water is available to transfer proton due to which proton conductivity decreases significantly.

The flow of protons in a membrane occurred by three means: surface diffusion, Grotthuss hopping, and vehicular diffusion [42,43]. The surface diffusion mechanism happens when the hydration level is low or near to pore wall. In this mechanism, the protons hop from one sulfonic site to another. At higher hydration levels, the Grotthuss hopping and vehicular diffusion mechanisms are responsible for proton transfer. In the former mechanism, the protons jump from one hydrolyzed site to another. While, the vehicular diffusion mechanism involves diffusion of hydrated proton with the aid of electroosmotic drag [44]. Fig. 1.5 presents possible proton diffusion mechanism in a nafion membrane.

Figure 1.5 A schematic representation of proton diffusion mechanism in a nafion membrane [21].

Swelling in the membrane also occurred at this point and dimensions of the membrane changed remarkably during hydration and dehydration [45]. This could result in detachment of GDM from the membrane and adversely affect the life-span of the membrane. The possible remedy to restrict planar dimensional change in membrane is to incorporate a thin porous sheet, that is, expanded PTFE. The expanded PTFE (e-PTFE) is nonconducting and it is thin (≤25 μm) with 95% porosity to develop PEM with good proton conductivity. Such membranes are also known as reinforced membrane. Moreover, the performance of the membrane can be improved by reducing the equivalent weight (EW) of the membrane. The EW is defined as the mass of the repeating polymer per mole of the sulfonic acid groups [46]. A completely hydrated nafion membrane with EW 1100 possesses ~22 water molecules. However, reducing EW (<700 g equiv−1) adversely affect the mechanical strength of the membrane. Hence, the working span of the membrane reduces. It is recommended to develop a membrane with lower EW (>850 g equiv−1) ionomer with short-ended chain for good proton conductivity and mechanical properties. It is challenging to improve overall performance of the membrane without compromising mechanical properties. The only way out is to optimize the morphology of the ionomer to develop membrane with improved performance as well as mechanical stability.

1.3.2 Anode and cathode electrodes

To achieve high HOR and ORR rates in PEM fuel cell, the electrochemical reaction interface must be carefully designed. This will lead to increase in surface area of the catalyst and improves the overall performance of the cell [47]. A schematic representation of electrode assemble is presented in Fig. 1.6. An electrode assemble in a PEM fuel cell consists of CL, gas diffusion backing layer (GDBL), and MPL. Design of foremost CL comprises of three-dimensional layers to enhance the surface are of the catalyst. A CL must be porous enough to aid to-and-fro drift of reactants and products. The efficiency of CL significantly depends on the catalyst material used. The best catalyst to prepare CL is nanosized Pt and Pt-based alloys [49,50]. Pt offers outstanding catalytic properties but it is rare, expensive, and prone of carbon monoxide poising. Consequently, research in this area primarily focused to reduce the loading of Pt or to hunt alternative catalyst with superior catalytic activity. There are certain criteria that must be followed to fabricate a supported nanosized Pt-based catalyst. The utilization of catalyst significantly improves when a support material with higher electron and proton conductivity is employed. It is crucial that support material must withstand in every condition such as highly oxidative atmosphere at cathode. A highly active Pt nanoparticles also promotes the oxidation of the support material. Most of the metals cannot resist oxidative atmosphere and thus are not suitable as support material [51].

Figure 1.6 A schematic representation of electrode assemble [48].

In this regard, carbon is a suitable support material [52]. The nanosized Pt (~2–3 nm) supported with carbon black is suitable for both anode and cathode. Fig. 1.7 presents the high-resolution transmission electron micrograph of   Pt particle supported on carbon. Gontard and co-workers [53] reported the loading of Pt is ~40–60 wt.%. However, carbon also oxidize at lower potential (0.21 V), but possess slower oxidation kinetics. In open circuit voltage (OCV), carbon is more prone to corrosion with faster rate kinetics. This is the prominent reason behind failure of cathode in a fuel cell at the OCV. Whenever the formation of H2—air boundary occurs at anode then cathode experiences 1.6 V potential during start or stop of the system. At this moment, carbon oxidized rapidly and thickness of the CL reduces [54]. This results into failure of CL in a PEM fuel cell. There are some carbon-based materials that can overwhelmed the shortcomings of carbon [55]. These materials include boron/nitrogen/alloy doped carbon as support materials. The doped carbon-based materials improve the durability as well as performance of the catalyst [56]. The problem associated with CO poisoning of the Pt catalyst can also be avoided by using CO tolerant support materials such as Ru, Mo, etc. [57]. Another way out to improve the durability and activity of the catalyst is to fabricate core shell catalyst. In a core shell catalyst, Pt act as shell and non-Pt material act as the core. Several layers of Pt shell on the core improves the catalytic activity due to interactions between Pt shells. Moreover, core material increases the vacancies in Pt resulting in improvement of electron transaction between O2 and Pt. The only drawback of the core shell is leaching out of core material that adversely affect the efficiency of the catalyst [58].

Figure 1.7 (A) HRTEM micrograph and (B) simulated image of a 6 nm Pt particle supported on carbon [59].

In addition, the size of the supported Pt catalyst also affects the efficiency of the fuel cell [60]. If the size of catalyst is <3 nm then the coalesce probability of Pt particle increases. This results in decrease in the catalytic activity [61]. In most of the cases, the size of supported Pt catalyst is kept in the range of 2–4 nm. The content of Pt on the support material also alters catalytic activity. A higher Pt content is found to be inappropriate and leads to the coalesce of Pt particles. The supported nanosized Pt-based catalyst impregnated over the surface of the ionomer is preferred. The ionomer thin film acts as a binder and promotes proton conductivity at anode and adsorb oxygen at cathode surface for ORR. The content of ionomer must be optimized to achieve good response from a PEM fuel cell. An excessive amount of ionomer restricts the access of oxygen to the catalyst and decreases the gas diffusion route. For the proper functioning of CL, 30% nafion loading is reported to be optimum. It is also crucial that CL should be porous, offers ease of reactant flow, and possess higher proton and electron conductivity. Higher thickness of CL increases the catalytic surface area per geometric area and improves the performance. However, resistance also increases with increase in thickness of CL due to which overall improvement in the performance is negligible. Therefore the thickness of the CL must be kept around few microns to 20 microns. The appropriate thickness of the CL can be determined through transport properties and kinetics involved during an electrochemical reaction. It is critically important that protons, electrons, and oxygen/hydrogen gas must approach the catalytic site for HOR and ORR reactions. In general, Pt-based CLs are 1–10 µm thick and demonstrate suitable kinetics for an electrochemical reaction. Whereas nonprecious metal CL is thicker (30–100 µm) but offers comparable kinetics. It is theoretically predicted that electron and oxygen transport resistance is small when the thickness of CL at cathode is 10 µm. A CL with low thickness and higher catalytic surface area is highly recommended to design an efficient PEM fuel cell [62,63].

Furthermore, the CL is connected to a GDBL. The GDBL promotes the transfer of reactants through flow field channels from BPs to CL [64]. It also allows the transfer of products from CL to BP. So, it is important that GDBL must be highly porous (<80%) and possess pore size ~10 µm. The GDBL not only provides the mechanical strength to the membrane but also protects CL from corrosion or erosion. The dimensions of GDBL significantly depend on the species involved such as electron, proton, oxygen, water, and heat. A thinner GDBL offers higher thermal resistance to heat removal process through a gas flow channel and result into excessive heating. While thicker GDBL possess lower thermal resistance and provide better mechanical stability to the membrane and corrosion or erosion protection to the CL. The commercially available GDBL has thickness ~100 µm [65]. The carbon fiber-based materials are commonly used to fabricate GDBL. However, the fabrication carbon fiber-based GDBL is complex and demonstrates lower electrical and thermal conductivities in comparison to metals. In this context, metallic GDBL emerged as promising candidate to replace carbon fiber-based GDBLs. The metallic GDBL offers ease of machining that makes designing of GDBL flexible. However, metallic GDBL are prone to corrosion in fuel cell environment. The possible way out is to coat metallic GDBL in order to improve its corrosion resistance. Recently, 3D printed technique is employed to fabricate GDBL by using polyamide and titanium (Ti) [66]. In addition, silicon-based GDBL are proposed for PEM fuel cells [67]. There are variety of commercial GDBLs including SGL 10BA, P75 Ballard, SGL 24BA, SGL 34BA, 090 Toray, and E-Tek Cloth A [68]. The microstructure of these GDBLs is shown in Fig. 1.8.

Figure 1.8 Micrographs of different GDBLs: (A) SGL 10BA, (B) P75 Ballard, (C) SGL 24BA, (D) SGL 34BA, (E) 090 Toray, (F) E-Tek Cloth A [68].

The MPL is sandwiched in between GDBL and CL. It is a thin layer made of carbon and PTFE mixture. The usage of MPL becomes more helpful to maintain water balance in the case of CL bonded with nafion. This type of CL is highly susceptible to be flooded. An MPL must be properly hydrophobic to uphold the water balance. Microstructure of carbon fiber-based GDBL and MPL is shown in Fig. 1.9. The figure clearly confirmed rough irregular mesh of carbon fiber-based GDBL although microstructure of MPLs demonstrated fine layers. There are four main function of MPL in a PEM fuel cell: (1) Helpful to aid water management in the entire PEM fuel cell. (2) Water can be transferred from one more hydrophobic MPL to other side low hydrophobic MPL or no MPL. This improves the humidification of the membrane and thus improves the performance of fuel cell. (3) Provides good adhesion between GDBL and CL. (4) Protect CL from coarse and rough gas diffusion layer (GDL) fibers and provide good resistance to contacts at interfaces [70]. The thickness of MPL is ~50 µm. A thicker MPL offers higher water transport resistance and may cause flooding at CL. Therefore it is critically important to optimize the content of PTFE, thickness, and type of material to design MPL.

Figure 1.9 Microstructure of the GDL and MPL layers [69].

1.3.3 Bipolar plates

The BPs play a vital role for the practical application of the PEM fuel cell [71]. BPs are the major component of PEM fuel cells that contribute 40%–50% in overall cost and 60%–80% by weight, as shown in Fig. 1.10. Choice of suitable BP materials, optimizing fabrication processes, and an appropriate design of flow fields are crucial for the commercialization of PEM fuel cell. BPs act as separator between two-unit cells and provide mechanical strength to the stack of PEM fuel cell. The plates are also responsible for the transportation of reactants, products, electrons, and heat released among the unit cells. The characteristics of suitable BPs for PEM fuel cell includes high thermal and electrical conductivity, good corrosion resistance, impermeable to H2/O2, and superior mechanical properties. Graphite is commonly used to prepare BPs due to its high thermal and electrical conductivity, good corrosion resistance, and large gas absorbency. However, brittle nature of graphite makes it less durable and problematic for bulk production. There are several other materials that have been proposed for the preparation of BPs including carbon-based, metals, and alloys. The carbon-based BPs comprises of polymer binder and carbon-based filler [72,73]. The polymer binder provides mechanical strength and gas impermeability, while carbon-based filler is responsible of transportation in BPs. The electrical and thermal conductivity of carbon-based BPs depend on the constituents and amount of the carbon-based filler. Multiple constituents in carbon-based filler such as carbon black, graphite, carbon fiber, and carbon nanotubes increase the conductivity of BPs. Moreover, increasing content of filler reduces the mechanical strength of the BPs. The properties of carbon fillers also depend on the morphology of the fillers used.

Figure 1.10 Mass fraction of PEM fuel cell components.

The metallic BPs offers several advantages over graphite BPs such as superior electrical and thermal conductivity, high gas impermeability, and better mechanical stability. In the current scenario, the metallic BPs has gained significant attention from the scientific community due to their unique properties. Several metals and alloys such as aluminum, titanium, stainless steel, carbon steels, copper alloys, nickel alloys, and aluminum alloys emerge as promising BL materials [74]. Among these materials, stainless steels are studied extensively due to low cost and availability. Only lacuna in this type of BPs is low corrosion resistance. A protective coating solves the purpose and improves corrosion resistance. Coating material must be conductive, adhere and possess nearly same thermal expansion coefficient as that of base metal. Typically, carbon-based and metal-based coatings are done on metallic BPs. Moreover, coated stainless steel (316 L) is also investigated for the fabrication of BPs [75]. A gold-coated stainless-steel BPs demonstrate comparable performance to graphite BPs. Moreover, the flow field design plays vital role to improve the overall performance of PEM fuel cell. An appropriate design of flow field leads to 50% increase in the energy density. The flow field design is also responsible for water balance, uniform reactant distribution, and increases reactant transport. Till date, there are various flow field configurations that have been developed such as pin-type, series-parallel, serpentine, integrated, interdigitated, and metal sheets-based flow fields, as shown in Fig. 1.11.

Figure 1.11 Various types of flow field configurations [21].

It is observed that pin-type and conventional straight of parallel flow field are inadequate in water balance and promote formation of stagnant areas. Serpentine and interdigitated flow fields demonstrate good water removing capability. However, serpentine design suffers from extreme pressure drop that may consequence short circuiting of the reactants. Whereas, parasitic power requirement for air compression in interdigitated flow fields restricts its implementation in small PEM fuel cells. Table 1.2 presents the advantages and disadvantages of various flow field configurations. It is worthwhile to note that improvement in flow field design increases the efficiency and reduces the cost of fuel cell by 50% [76].

Table 1.2

1.3.4 Other components

Other major components of PEM fuel cell involve a current collector, sealing material, and end plate material. A current collector plate is attached with BP in a PEM fuel cell. The chief role of current collector is to gather current generated in the fuel cell. The BPs may also act as current collectors in a unit cell or stacked PEM fuel cell [77]. The electron produced at anode must flow toward cathode via an external circuit with the aid of current collector and electrode. To design an efficient current collector, a material must be light in weight, highly conductive, and possess higher electrochemical and mechanical stability. The most used materials for current collector are metals and alloys such as titanium, aluminum, copper, and stainless steel. These materials are coated with other metals to further enhance their conductivity. Among these materials, copper and stainless steel are abundant, low in cost, and offers ease of machining. The only drawback of utilizing copper and stainless steel as current collector is their density, which adversely affect the specific power density of a PEM fuel cell. Recently, exfoliated graphite is proposed as current collector. The exfoliated graphite is highly conductive and low in weight in comparison to above considered materials. The overall performance improves with the use of exfoliated graphite. However, exfoliated graphite offers low mechanical stability and required protection. So, it is crucial to protect exfoliated graphite with some other material without compromising its performance. This will also improve the mechanical strength of the current collector.

The sealing material is also one of the crucial components in a PEM fuel cell. It is sandwiched in between membrane and BPs to restricts the leakage of reactants, products, and coolants. The material for sealing must exhibit good gas impermeability, inexpensive, ease to machine, higher thermal, electrochemical, and chemical stability [78]. PTFE, PTFE-based, and silicon elastomers are mainly used for the fabrication of sealing material. PTFE and PTFE-based materials suffer from compressibility issue. Consequently, a high compression blade is needed to install PTFE and PTFE-based sealing materials in a fuel cell. In case of silicon elastomer, the major apprehension is related to its decomposed during long working of fuel cells. The decomposed products of silicon elastomer stick on to CLs and adversely effect the overall performance of fuel cell. Furthermore, end plate is one of the important materials to develop an efficient fuel cell. These plates are positioned at both ends of the fuel cell, that is, anode end and cathode end. These plates may serve the purpose of field flow when metal flow channels are employed. Though, a separate plate is frequently used as end plate when graphite flow channels are used. To decrease overall cost and weight of the fuel cell, aluminum and polymer-based materials are used to design end plates.

1.4 Practical applications of PEM fuel cells

The technological status of PEM fuel cell is mainly growing in two sectors, that is, transportation and portable power systems. Current research focus is to design a PEM fuel cell in such a way that overall cost, dimensions, and weight must be reduced to meet emergent energy needs [79]. The power requirements to build portable power system, electric vehicles (EVs), and smaller aircrafts ranges from 5 to 50 W, 20 to 250 kW, and 100 W to few kWs, respectively.

1.4.1 Portable power systems

In the age of technology, innovative portable electronic devices are growing swiftly. Some of these devices are essential part of our everyday living. These devices include phones, laptops, tablets, wearable electronics, nonautomotive devices, and other portable appliances. The ability of these devices to integrate and interact with mankind have brought ease of comfort. These electronic devices are depended on portable power systems such as batteries and fuel cells. The fuel cells offer high energy density in comparison to batteries. Therefore utilization of fuel cell in portable power system is aggressively progressing in past few years. Recently, Intelligent Energy commercialized 2.4 kW PEM fuel cell for unmanned aerial vehicle. Moreover, Swedish myFC launched a power bank JAQ Hybrid and revealed a thin power system based on PEM fuel cell for mobile phones [51]. It is expected that demand of fuel cell-based compact power systems demand will grow exponentially in the electronic market. In the field of auxiliary power systems, Aquafairy, a Japanese company, developed a water-activated small charger for emergency power systems based on PEM fuel cells [80]. Danish Serenergy offers compact PEM fuel cells with the help of reformed methanol. This type of fuel cell could be applicable for both liquid and solid portable cooling systems. Likewise, Horizon Fuel Cell Technologies are also involved in the development of portable power system for smartphones [81].

PEM fuel cell-based small power system offers high energy density, robust, rapid charging, and noiseless operation. All these unique features make PEM fuel cell highly suitable for military applications. Few industries are also providing power solutions to military based on PEM fuel cell. Ballard Power Systems and Ardica Technologies have collaborated to develop wearable electronic devices for soldiers [82–84]. In this field, HES Energy Systems have commercialized portable fuel cells with constant power capacity ~30 W. Recently, Californian UltraCell has developed portable methanol reformate systems for US army [85].

1.4.2 Transportation

PEM fuel cells have emerged as most suitable for transportation sector. Several automotive industries are working to develop EVs based on PEM fuel cells [14]. The prime objective is to develop low cost and durable PEM technology capable enough to start at lower temperatures. Approximately, 5000 fuel cell vehicles (FCVs) and 30 power infrastructures based on hydrogen gas are working in the United States. Moreover, at Los Angeles International Airport, 39th retail hydrogen gas station is in working. In Germany, there are ~100 hydrogen gas power stations and it is expected that more will open in near future. Furthermore, hybrid FCVs have also attracted the scientific community to couple fuel cell along with other energy conversion and storage technologies such as battery [86,87]. This will bring new opportunities in transportation sector. Fig. 1.12 presents comparison of energy density versus power density and specific energy with respect to specific power for various energy sources.

Figure 1.12 (A) Specific energy density versus power density and (B) specific energy with respect to specific power for various energy sources [88].

Several car makers including Toyota, Honda, Mercedes, General motor, and Hyundai have also started commercializing FCVs based on PEM fuel cells. Toyota’s first commercialized FCV is Mirai that possess 3.1 kW/L power density and 114 kW (153 HP) fuel cell stack. In this vehicle, Toyota has reduced the weight and volume of PEM fuel cells by replacing the humidifiers with thin membrane [89]. General Motors Heritage Center has also accomplished a target of Driveway project. The aim of the project was to cover 3 million miles through 119 Equinox electric vehicle by 5000 customers. General Motors are also developing Chevrolet Colorado ZH2 for US military by utilizing 94 kW fuel cell. The Zh2 offers a unique feature to produce 2 gallons of water during electrochemical reaction. Honda, Hyundai, and Mercedes have launched fuel cell-based EVs, that is, Clarity, Tucson, and GLC-F-CELL, respectively. GLC-F-CELL is basically a hybrid electric vehicle based on battery as well as fuel cell [90]. In Europe, a project named CUTE (Clean Urban Transport for Europe) was started [91]. Under this project, Daimler produced Citaro, an innovative fuel cell-based bus (FCB). This FCB possesses 50% efficacy and capability to travel 155 miles. The durability and efficiency of the FCBs has been improved in the past few years. In 2015 FC durability target was achieved about 18,000 hours and it was further improved in 2016 to reach 23,000 hours, though the cost of the FCBs is very high in comparison to conventional buses based on diesel engines. It is estimated that a FCB cost around 1.8 million USD [92,93].

Moreover, PEM fuel cells are also applicable in aircraft and marine sectors. The chief application of PEM fuel cells in aircraft includes power system for unmanned operations and auxiliary power units. Numerous efforts have been made to develop fuel cell-based manned aircraft, nevertheless, only few experimental results are published [94,95]. It is important to develop a fuel cell at small scale, with high durability and long range. In 2003, AeroVironment tested first aircraft developed by using PEM fuel cells [96]. In this flight, a liquid hydrogen tank was also installed and its range was increased by employing hybrid fueling technology provided by Millennium Cell Inc. The Naval Research Lab also completed its first examination of using PEM fuel cell-based flight. A 550 W PEM fuel cell was installed with which 48 hours working range was achieved in a flight [97,98]. Moreover, a commercial fuel cell-based unmanned aerial vehicle has been developed by the Blue Bird Aero system [99]. Inha University in South Korea is also involved to develop unmanned aerial vehicles (UAV). They made a UAV with 200 W fuel cell stack and testified the flight duration of 14 minutes. H3 Dynamics developed a drone, that is HYWINGS based of PEM fuel cell. This drone is capable enough to travel 310 miles or 10 hours flight [100,101]. Recently, Protonex started sales of fuel cell-based UAVs. In addition, Energy or Technologies developed a highly durable UAV named as FAUCON H2 aircraft with flight range of 10 hours [102,103]. Boeing, a Spain company, tested a two-seater aircraft powered by PEM fuel cell, which can fly for about 20 minutes at 60 miles/h. Recently, a plane maker company (Pipistrel), a fuel cell making company (Hydrogenics), University of Ulm, and German Aerospace Center collaborated to develop HY4 flight [104,105].

1.5 Summary

PEM fuel cell directly transforms the chemical energy into electrical energy and possesses unique features such as zero carbon emission and high energy density. The PEM fuel cell exhibit invincible combined advantages of batteries as well as internal combustion engines. It emerged as a critical technological advancement, which is capable to uphold the growing energy demand. Instant start even at ambient temperature, suitable for backup and portability of PEM fuel cell make them appropriate candidate for transportation and portable power system applications. The two main factors restrict the broad implementation of PEM fuel cell, that is, high cost and low durability. To improve the durability and reducing the cost, it is highly essential to understand the functioning of each component PEM fuel cell. The key components of PEM fuel cell include membrane, CL, GDL, MPL, BPs, and current collector. BPs are one of the most heavy and bulky part of the PEM fuel cell. These plates contribute 70%–90% volume of a fuel cell. The dimensions of BPs can be reduced with the use of gas flow channel (GFC). An optimum design of GFC may lead to increase in ~50% efficiency of a PEM fuel cell. Till date, several commercial fuels cell-based vehicles are developed. Numerous government agencies and industries are involved in the development of energy frameworks based on PEM fuel cells. Tremendous advancements to develop PEM fuel cell have been made in last few years, nevertheless, challenges still exist. It is important to conduct more studies related to optimization of each component of fuel cell by reducing the cost and improving