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Fuel Cell Technology (FCT): An Overview

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Muhammad Sufaid Khan, Rozina Khattak and Abbas Khan

Submitted: 18 February 2024 Reviewed: 15 March 2024 Published: 12 April 2024

DOI: 10.5772/intechopen.1005102

Chemical Kinetics and Catalysis - Perspectives, Developments and Applications IntechOpen
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Chemical Kinetics and Catalysis - Perspectives, Developments and Applications [Working Title]

Rozina Khattak

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Abstract

The global need for energy is growing every day. In this situation, looking for alternative energy sources should be a priority. Alternative energy comes in a variety of forms. One of the most promising technologies to partially address the energy deficits is fuel cell technology, or FCT. Fuel cells can be classified according to their design and the electrolyte that was used to build them. The FCT is regarded as one of the most promising technologies for alternative energy sources since it has so many advantages over other forms of energy sources. The oxygen reduction reaction (ORR), which occurs on the fuel cell’s cathode, is the primary electrochemical process in fuel cell technology. Pt catalyst is used to increase ORR, which improves a fuel cell’s (FC’s) stability and performance. The use of platinum (Pt) metal is not without its problems, though; among them is the metal’s high cost and scarcity. Therefore, the challenge for researchers is to identify low-cost, easily accessible substitute electrocatalysts. These are some of the challenges or barriers that will need to be overcome in the future. Two major barriers to the commercialization of FCT are the stability of the catalytic materials and the availability of a substitute material for Pt. The FCT and technological research used to enhance it are summarized in this chapter.

Keywords

  • fuel cell technology
  • oxygen reduction reactions
  • energy
  • alloys
  • electrocatalysts

1. Introduction

These days, fossil fuels are a promising source of energy. However, fossil fuels come with a lot of drawbacks. For example, they are non-renewable and constantly pollute the air. In this case, the global community is calling for the development of an alternative fossil fuel contender with zero or very little pollution. The literature contains a variety of alternative energy sources, such as fuel cells (FCs). The most potential substitute for producing electricity and water in the process is fuel cell technology. The basic reaction that occurs on the corresponding cathode and anode of the fuel cell is an oxidation-reduction reaction, which transforms the chemical used as fuel into energy. Furthermore, the oxygen reduction reaction (ORR), which occurs on the fuel cell cathode, is the most significant reaction. Regarding the step that determines rate, the oxygen reduction reaction holds great significance [1, 2, 3]. The researchers have conducted extensive research on the fuel cell cathode due to the significance of ORR in electrochemical and storage devices. Furthermore, ORR plays a critical role in electrochemistry and other industrial processes [4, 5, 6].

Since the ORR identifies the rate of reaction as a rate-determining or slow step in the kinetics of fuel cells, the researchers focus on speeding up this reaction. To accelerate the OR reaction, electrocatalyst must be added to the fuel cell’s cathode. To catalyze the ORR on the FC cathode, Pt or Pt-based materials/compounds are employed [3].

Pt-based nanomaterials are most frequently utilized in FC. However, there are numerous issues with using Pt and Pt-based nanomaterials as electrocatalysts. For instance, comparatively costly materials, limited availability, and poor stability [7] are some of the issues facing fuel cells. These factors also drive up the cost of the technology and prevent it from being used by the average person in society [8, 9, 10]. Utilizing Pt-based materials as electrocatalysts to reduce the need for Pt to a certain degree is the focus of research on alternatives to Pt metal. The first benefit, though, is that Pt-based materials do, in fact, somewhat lessen the demand for Pt metal. Additionally, the Pt-based materials’ second benefit is that they accelerate the ORR on the fuel cell’s cathode. However, while Pt-based materials are utilized to catalyze ORR, their stability is relatively low. The issue for fuel cell technology to become commercially viable in this context is the replacement of Pt, either fully or partially. Researchers are now investigating substitutes for Pt and Pt-based materials that could be used as fuel cell cathode catalysts. Nevertheless, the Pt substitute material needs to be more stable and capable of increasing and accelerating ORR. In this approach, to lower the cost of FC and commercialize it, more affordable elements must take the place of more expensive ones, such as Pt. Fuel cell technology will become more commercially viable with this advancement [11, 12, 13].

As a result, modern researchers are interested in finding alternative electrocatalysts that may provide improved catalytic performance for ORR at FC cathode. Researchers are working to develop Pt-free materials with improved stability and catalytic activity in this respect. Nonetheless, additional enhancements are required to provide superior ORR performance in comparison to materials based on Pt. Additionally, studies have been conducted to determine Pt’s alternatives that have alloyed with other noble metals to create catalytic materials with the best catalytic behavior for ORR and long-term stability. A solid mixture with various atom types dispersed randomly throughout the lattice is called an alloy. An alloy is normally composed of one, two, or three components at most. Utilizing inexpensive, earthly components can increase the electrocatalytic performance of a precious-metal-based alloy while also bringing it down in price. The atoms’ bonding energies and diffusion kinetics are intimately related to the activation energy, which is a significant component influencing the atomic order. Inter-metallic structures provide inherent thermodynamic stability because their constituent elements are precisely arranged in an ordered manner. Nevertheless, previous studies have largely focused on conventional disordered alloy nanocatalysts. Such a disordered structure shows just a slight increase in activity and poor stability in a corrosive electrochemical environment. On the other hand, organized inter-metallic nanocrystals’ high electron interaction and variations in bond length and structure may greatly increase catalytic activity.

Various research teams are working on the Pt replacement. In terms of ORR and stability, the majority of study groups have come to the conclusion that palladium (Pd) would be a useful choice material for Pt [14, 15, 16, 17]. Pd is becoming more popular as a substitute catalytic material for a number of reasons. A few of the advantages of Pd over Pt are listed as follows: (I) Both Pt and Pd are found in the same group of elements of the periodic table. As a result, it is possible that both have comparable chemical behaviors. (II) Pt is comparatively less expensive than Pd, but Pd reserves are more abundant than Pt reservoirs on the Earth [18]. (III) Certain materials based on Pd exhibit stability and comparatively superior catalytic activity for the ORR. (IV) Additionally, the scientists noted that the Pt had higher mass activity than the Pd. Also, the addition of other metals increases the mass activity.

Various researchers studied Pd in various combinations with other metals. For example, copper has excellent catalytic behavior and has the ability to create alloys with Pt. Given that Pt and Pd have comparable characteristics, alloys containing Pd and Cu are likewise capable of catalyzing ORR in the fuel cell cathode, and some of them exhibit extended stability [19]. Certain elements have been studied that, when added to Pd as an alloy, increased both the stability and the catalytic activity for ORR. Examples of elements that could be mixed with Pd include cobalt (Co), copper (Cu), and nickel (Ni). Pd-based materials exhibit stability for the fuel cell cathode and carry increased catalytic activity for ORR. When compared to pure Pd, these are thought to be the most promising alternative electrocatalysts [14, 20]. Comparing PdCo, PdAg, PdNi, and PdCu to pure Pd in acidic and alkaline conditions, for instance, reveals higher catalytic activity for ORR and longer-term stability.

According to certain researchers’ theoretical studies, the PdCu alloy has a strong potential to outperform catalytic activity for oxygen reduction reaction. Additionally, they investigated their various forms, including mesoporous and nanotubular, which produced improved outcomes for the catalytic activities of the ORR. Such materials even exhibit greater activity than Pt metal sold commercially, which is utilized as a catalyst [18]. Another study that looked at PdCu supported carbon with varying atomic compositions—1:1 and 1:3—and temperature treatment is documented in the literature. In contrast to pure Pd supported on carbon, they found that temperature treatment increased the catalytic activity of the ORR in the acidic solution [21]. Furthermore, studies on Pd-based materials with a propensity for ORR have been conducted, yielding results for the four-electron mechanism [22, 23]. In summary, it has been noted that, in comparison to the pure metal of Pt, the catalytic activity and stability of the pure Pd metal for the ORR in the acidic medium are notably lower. Since Pd metal loses stability in acidic environments, it is very challenging to use it to catalyze ORR on the fuel cell cathode. The generation of hydrogen peroxide and the process continuing with two electrons were proven to be the cause of lower activity and stability. Because the active catalytic sites were blocked, the catalytic activity was reduced consequently, leading to low stability [23, 24, 25].

Nonetheless, the literature has shown that Pd exhibits superior ORR catalytic activity as well as superior durability in the alkaline medium. It could even be preferable in pure Pt material [25]. With reference to this, the ORR activities in the acidic medium cause the materials to corrode and then move into the solution, influencing the materials’ catalytic activities. As a result, research in the past decade has tended to focus on alkaline media, where Pd exhibits stronger activity than Pt [21]. Additionally, the ORR was impacted by the Pd and Cu atomic composition differences during the alloy production [21, 26]. This is due to the fact that various oxygen ions and species, such as OH−, O2−, HO2−, and H2O2, are formed during the ORR mechanism when the entire process occurs on the surface of the electrode of the fuel cell’s cathode [27].

However, further study was carried out using the Pd and Ni alloy. Different compositions of Pd and Ni atoms were synthesized and maintained on carbon support. Modified polyol method was used to prepare the PdNi/C nanoparticles. The results showed that PdNi/C produced in this way had better long-term stability in the alkaline medium and higher catalytic activity for ORR [28]. This indicates that the FCT is commercially viable when taking into account ORR catalysis by an appropriate electrocatalyst in the alkaline medium, which is based on the alloy with the optimal composition of selected metals. These alloys have excellent catalytic performance for ORR on the fuel cell cathode and are both stable and reasonably priced.

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2. What is a fuel cell?

Generally speaking, a fuel cell is an electrochemical system that can transform chemical energy into electrical energy while producing a byproduct that is safe for the environment, such as water. The primary distinction between batteries and fuel cell technology is that batteries are able to store chemicals and transform them into electrical energy, while fuel cell technology produces electrical energy from the chemical that is accessible to it. After the chemical has run out, the battery will shut off. Conversely, the fuel cell can potentially operate constantly as long as fuel and oxygen are available to it.

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3. Brief historical background of fuel cell technology

It has been reported that Sir William Robert Groove invented the first fuel cell in 1839 [29]. He conducted numerous experiments on water splitting in a device known as a gas voltaic battery in the beginning. The two Pt electrodes that were placed in the sulfuric acid medium formed the basis of the device’s construction. In the water splitting experiment, he noticed that water splits into H2 and O2 when an electric current is passed through it. This division may also be followed by the potential initiation of the opposite reaction. Following some time for the science to advance in fuel cells, a professor at Cambridge University in England created the alkaline fuel cell, also referred to as the Bacon cell, in 1932. This name was given after the name of Professor, Frances Thomas Bacon. It was a potentially useful device with a 5–6 kW power capability. However, because of the high pressure gas employed, the cell’s mass was extremely high [28]. Pratt and Whitney, a manufacturer of engines and aircraft, held a license to use the Bacon cell in 1960. These were also used in the Apollo spacecraft following the modification to the Bacon cell. Reducing the Bacon cell’s size and using a concentrated alkaline solution was the primary goal of the time. Following the fuel cell’s reduction in size by Thomas Grubb and Leonard Niedrich in the 1960s, the Gemini spacecraft used it to generate power [30]. Numerous researchers have periodically made these enormous efforts to bring the fuel cell to this point of commercialization. These days, a variety of fuel cell types are available, and power plants of different kinds are built using fuel cell technology. Additionally, compared to other alternative energy sources, the operational power of FC-based technology is superior [30].

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4. Working principle of the fuel cell

A fuel cell is an electrochemical device made up of two electrodes in the middle of the membranes, called the cathode and anode. Figure 1 provides a detailed illustration of a fuel cell [31]. In the electrochemical reaction, oxygen enters the fuel cell through the cathode from the air, and hydrogen enters the anode from the splitting of water. Oxygen is reduced on the cathode, and hydrogen oxidized on the fuel cell’s anode during the coupled redox reaction, i.e., oxidation and reduction process, resulting in the production of water as the end product.

Figure 1.

Representative diagram for fuel cell [31].

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5. Types of fuel cell

There are several fuel cell types that can be categorized in a number of ways. For example, fuel cells can be categorized according to the electrolytic materials they utilize to function, or according to the materials they are composed of that are used as fuels. The fuel cell can be categorized as follows using the categories mentioned above: (1) solid oxide fuel cell (SOFC); (2) phosphoric acid fuel cell (PAFC); (3) alkaline fuel cell (AFC); (4) molten carbonate fuel cell (MCFC); and (5) proton exchange membrane fuel cell (PEMFC) [1, 7, 8]. Aside from the previously mentioned classification, the direct methanol fuel cell (DMFC) is a special kind of fuel cell technology in which methanol is employed as the anode’s fuel source.

Proton exchange membranes (PEMs) are electrolyte polymers that are composed of fluorinated sulfonic acid polymer or other similar polymer, which accelerates the flow of protons through the membrane. This polymeric membrane effectively promotes protons in a water environment. The free proton is produced by the acid’s breakdown in the aqueous environment. Operational water is the most important requirement overall. Water should be retained in the membrane since it keeps it moist, but excessive amounts of water can also be quite hazardous. As a result, the membrane’s water content should remain balanced [9]. The typical temperature range for PEMFC operation is 80 to 120°C.

PEMFCs are primarily viewed as a promising alternative energy source that may be applied to a variety of power-related tasks, such as stationary and mobile applications. Comparably, the PEMFC exhibits long-term durability throughout operations for a variety of functions, a high rate of conversion, and an appropriate operating temperature [32]. It also causes no pollution. Furthermore, the literature claims that PEMFCs have power capacities comparable to those of batteries, power grids, and certain combustion engines [33]. PEMFC is a more powerful energy source and is also less expensive than other options. Nevertheless, there are certain difficulties in using this technology. However, these challenges ought to be resolved quickly [26].

Phosphoric acid fuel cells (PAFCs) are a different kind of fuel cell. As the name suggests, the material injected into the process is phosphoric acid, which generates protons in a manner akin to that of a proton exchange membrane. Phosphoric acid is an inorganic substance that is utilized as an electrolyte in PAFCs. Phosphoric acid has various benefits when used as an electrolyte. For example, it has a high thermal value, great resistance to stability, and strong chemical bonding, which results in a very low vaporization value. Phosphoric acid fuel cells operate at temperatures between 150 and 220°C [10].

Alkaline fuel cells (AFCs) are the other kind of fuel cell. As the name suggests, a strong alkaline medium is the fuel cell’s supplied material. The alkaline medium’s concentration likewise rises with warmth. As an illustration, the concentration of KOH is around 85% at 250°C and drops to 50% at 120°C. Without a doubt, this technology is excellent for producing alternative energy sources. However, it has certain drawbacks as well, namely the formation of carbonates that alter the alkaline electrolyte’s capabilities [11]. AFC is also a promising fuel cell in terms of its potential applications for power production and commercialization; the space program is one of its most well-known usages. The space program previously used this kind of fuel cell in the 1960s. Compared to other fuel cell types, this one has multiple advantages, particularly when it comes to PEMFCs. However, the quick kinetics during the ORR is one of the biggest advantages of the AFC over the PEMFC. The non-metals can be used as electrocatalysts for the ORR because of this AFC characteristic [12].

The other form of fuel cell is called a molten carbonate fuel cell (MCFC), which requires a high temperature of 600 to 700°C to operate. The electrolytes in these fuel cells are typically carbonates of potassium and lithium (Li2CO3 and K2CO3). The high temperature causes these metallic carbonates to melt and produce carbonate ions for conduction. Compared to other forms of fuel cells that operate at lower temperatures, this type of fuel cell has the benefit of operating at a high temperature, which prevents CO and CO2 poisoning [34].

Zirconia (ZrO2) serves as the electrolyte in certain fuel cell types, such as solid oxide fuel cells (SOFCs). It could be stabilized, though, by using yttria (Y2O3). An 800–1000°C operating temperature is extremely high for a solid oxide fuel cell. This temperature causes the employed electrolyte zirconia to melt, resulting in conductor materials. Because solid materials are used as electrolytes rather than liquid ones, which helps lessen leakage issues, solid oxide fuel cells offer an advantage over carbonate fuel cells [13].

The fuel cell may also be categorized according to its operating temperature. Fuel cells can be classified as high-temperature fuel cells or low-temperature fuel cells (between 30 and 220°C). Technologies for low-temperature fuel cells include PEMFC, PAFC, and AFC. The remaining variants are taking into account fuel cell technology operating at high temperatures (500–1000°C). The temperature base classification concludes that although higher-temperature fuel cells produce carbonate and other compounds, lower-temperature fuel cells produce protons or hydroxyl ions. The parameters of the classification are summarized in Table 1 [14] for easier comprehension.

TypeWorking temperatureElectrolytic materialConduction ionsApplication in daily life
Proton exchange membrane fuel cell (PEMFC)30 to 100°CPoly perfluorosulf-onic acid
(PFSA)		H+Transportation, energy storage systems, space, military
Phosphoric acid fuel cell (PAFC)220°CLiquid phosphoric acid soaked in matrixH+
Alkaline fuel cell (AFC)50 to 200°CAqueous solution of potassium hydroxide soaked in a matrixOH−
Molten carbonate fuel cell (MCFC)~650°CLiquid solution of lithium, sodium and/or potassium carbonates, soaked in a matrixCO32−Combined heat and power for stationary decentralized systems and for transportation (trains, boats)
Solid oxide fuel cell (SOFC)500 to 1000°CSolid zirconium oxide to which a small amount of yttria is addedO2−

Table 1.

Classification of various fuel cell technologies.

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6. Conclusion

One significant substitute option for an additional energy source is fuel cell technology. Several fuel cell types are described in the literature. Based on the materials used or possibly the operating temperature, this classification is made. It is far superior to other energy sources in a plethora of ways. It is really effective and a green technique. Fuel cell technology has progressed through several stages of development to reach its current state. The fuel cell’s cathode may experience the primary reaction, or ORR. Its slow kinetics has an impact on the fuel cell’s performance. The primary cause is the electrocatalyst(s) utilized in the fuel cell’s cathode. The performance of the fuel cell can be improved by raising the ORR’s rate, which requires the proper electrocatalyst. The fuel cell also requires additional catalysts, which are less expensive and more commonly available. The commercialization of fuel cells and the difficulties they encounter are still being researched.

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Acknowledgments

I (principal author) would like to express gratitude to Assoc. Prof. Dr. Rozina Khattak for her assistance and inspiration in writing this chapter. Additionally, her step-by-step instructions made it easier to finish this chapter.

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Conflict of interest

The authors claim no conflict of interest.

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Notes/thanks/other declarations

I (principal author) want to thank Dr. Rozina Khattak for all of her help and advice in getting this chapter finished.

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Written By

Muhammad Sufaid Khan, Rozina Khattak and Abbas Khan

Submitted: 18 February 2024 Reviewed: 15 March 2024 Published: 12 April 2024

© The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.