Abstract

This paper presents a review of the hydrogen energy storage systems. Most developed countries have turned to search for other sources of renewable energy, especially solar energy, and hydrogen energy, because they are clean, environmentally friendly, and renewable energy. Therefore, many countries of the world began to accept the inevitability of shifting to an economy based on hydrogen as a future fuel. Among the difficult challenges in this transformation are the methods of storing electrical energy in fuel cells and storing hydrogen, as the race of large energy companies has begun to provide solutions to develop many types of fuel cells, given that they are the biggest challenge to energy generation and storage. This paper presents a review of fuel cells including Energy Storage Using Hydrogen Produced from Excess Renewable Electricity, as well as to cover the storage system includes three main components: electrolysis, fuel cell, and a hydrogen buffer tank. We will discuss the different types of fuel cells, such as: Alkaline Fuel Cells (AFC), Polymer Exchange Membrane Fuel Cell (PEMFC)...etc.) along with the different application of fuel cell hydrogen energy storage, we will also investigate some case studies of the hydrogen energy storage and main opportunities and challenges of the implementation.

1. Introduction

Energy is a necessity for everyone today because it powers all our personal, economic, transportation, and industrial activities ¹. Driving, lighting up a car, making a product, and keeping a building warm or cold all consume energy ². The need for energy is therefore enormous, and it is expected to grow even higher over the next two decades ³. Today, we rely on nonrenewable energy sources for most of our energy needs. Coal, natural gas, and oil are among the energy sources quickly diminishing because of massive usage ⁵. Furthermore, fossil fuels are a major contributor to global warming since the carbon dioxide (CO₂) gas they release into the atmosphere is extremely destructive to the ecosystem ⁶.

After extensive use, nonrenewable energy resources are depleted ⁷. These resources have been over-exploited and have caused a variety of issues, including as smog and pollution on land and in the air, increasing global warming, acid rain, and climate change ⁸. As a result, a clean renewable energy source is now essential ⁹. Solar, hydrogen, biomass, tidal, and geothermal energy are some examples of renewable energy sources. Because of the easy replenishment of these resources, there will never be a shortage. In the last few decades, the speed of renewable energy resources has increased, and many governments are making major investments to take use of these resources ¹⁰.

2. Historical Development of Hydrogen Fuel Cells

About 150 years ago, scientists first learned about fuel cells. They were first discovered in the 1800s and have since undergone extensive study in the latter half of the twentieth and the beginning of the twenty-first century. They are still being studied Hydrogen is a renewable, environmentally friendly, and abundant energy source. Hydrogen fuel cells have the potential to be the energy of the future, but the concept is not new. Hydrogen fuel cells were first conceived in 1839 by Welsh judge, inventor, and physicist Sir William Robert Grove. The amount of electricity produced by his invention, which combined hydrogen and oxygen in the presence of an electrolyte to produce electricity and water in a manner like today's fuel cells, was insufficient for practical use, and as a result, no further refinement was made ^{13, 14}.

A timeline of discoveries

• 1766 – Hydrogen is discovered as an element (over 250 years ago) ^{13, 14}

• 1801 – Humphry Daw demonstrates the principle of what became fuel cells ^{13, 14}

• 1806 – The development of the first combustion engine to be powered by hydrogen and oxygen ¹³

• 1839 – Sir William Grove invents the ‘gas battery’ later known as a fuel cell ¹³

• 1842 – Gas voltaic batteries prove hydrogen and oxygen can produce electric current ^{13, 14}

• 1889 – Charles Langer and Mond develop Grove’s invention and name it a fuel cell ¹³

• 1955 – The Proton Exchange Membrane (PEM) fuel cell is invented ¹³

• 1966 – NASA first uses fuel cells in space missions (over 50 years ago) [13-14] ¹³

• 1970s – The oil crises prompt the development of alternative energy technologies including Phosphoric acid fuel cells (PAFC) ¹³

• 1980s – The US Navy uses fuel cells in submarines ^{13, 14}

• 1990s – Large stationary fuel cells are developed for commercial and industrial locations (around 30 years ago) ¹³

• 2003 – The first fuel cell airplane takes off ¹³

• 2004 – Hydrogen fuel cell buses begin operating in London ^{13, 14}

• 2007 – Portable fuel cells begin to be sold commercially ¹³

• 2008 – The first fuel cell car is introduced ¹³

• 2009 – Portable fuel cell battery chargers are sold ¹³

• 2015 – The first hydrogen refuelling station opens in the UK ¹³

• 2016 – The first fuel cell passenger plane takes off ¹³

• 2017 – The world’s first hydrogen trams are launched in China ^{13, 14}

• 2018 – Hydrogen trains begin operating in Germany and the Metropolitan police in London add hydrogen fuel cell vehicles to their fleet ¹³

• 2019 – A hydrogen powered drone flies for over 1 hour carrying a load of 5kg ^{13, 14}

3. Concept of Hydrogen Fuel Cell

In recent years, hydrogen has been widely used as an energy carrier, particularly in fuel cells. Fuel cells essentially aid in the capture of hydrogen and the conversion of hydrogen power into useful energy. Interestingly, fuel cells have found their way into a broader range of applications, including backup power, remote power generation, distributed power generation, and electricity cogeneration. In this regard, it is widely anticipated that hydrogen will play a significant role soon in providing a clean, low-cost, and environmentally friendly alternative to fossil fuels ¹⁴. Because hydrogen requires a significant amount of energy to extract from water, it has made a significant contribution to the advancement of fuel cells. Hydrogen is regarded as a secondary source of energy, also known as an energy carrier. Energy carriers are used to transport, store, and deliver energy in a usable form. Many companies are working hard to develop technologies that will allow them to fully utilize the potential of hydrogen energy. A fuel cell is an electrochemical cell that converts chemical energy from a fuel into electricity via an electrochemical reaction between hydrogen-containing fuel and oxygen or another oxidizing agent ^{15, 16}.

4. Basic Operation of Fuel Cells

Instead of using combustion to create energy, a fuel cell uses an electrochemical reaction to do so. Hydrogen and oxygen are mixed in a fuel cell to produce electricity, heat, and water. Fuel cells are extremely efficient. Modern fuel cells have a wide range of uses, including powering homes and companies as well as hospitals, grocery shops and data centers. They are also used to move a wide range of automobiles, including passenger cars and buses, as well as forklifts and trains. Clean, efficient, reliable, and quiet power comes from fuel cell systems. Instead of needing to be recharged on a regular basis, fuel cells can keep producing energy as long as a fuel supply is available ¹⁷.

Electrolyte membranes and anodes are the main components of a fuel cell. In a conventional fuel cell, hydrogen and oxygen are exchanged at the anode and cathode, respectively. Catalysts break hydrogen molecules into electrons and protons at the anode point in the electrode. An electric current and surplus heat are generated when protons go through a porous electrolyte membrane and electrons are pushed to travel through a circuit. Water is made at the cathode when protons, electrons, and oxygen are combined. Fuel cells don't have any moving parts, thus they run quietly and reliably ^{17, 18}. Fuel cells are incredibly clean due to their chemistry. Hydrogen fuel cells produce just energy, heat, and water as waste products; hence they are fully carbon-free. Hydrocarbon fuels such as natural gas, biogas, methanol, and others can be used in some types of fuel cell systems. Traditional energy generation systems like steam turbines and combustion engines have lower efficiency than fuel cells because they use combustion to produce electricity instead of chemistry. A fuel cell can be connected to a combined heat and power system, which utilizes the waste heat from the cell to heat or cool the system. This will increase the efficiency even further ¹⁷.

The efficiency of fuel cells can be increased by using larger and larger fuel cells. As a result, stacks of fuel cells can be formed by connecting individual fuel cells together. These stacks can then be joined to create much more complex systems. Large-scale, multi-megawatt fuel cell facilities that provide electricity straight to the utility grid are as small as replacements for internal combustion engines in electric automobiles ¹⁷.

The goal of a fuel cell is to generate an electrical current that can be used to power an electric motor or illuminate a light bulb or an entire city outside of the cell. With the way electricity works, this current goes back and completes an electrical loop ¹⁷.

Fuel cells come in a variety of shapes and sizes, and each one works in a somewhat different way. However, in general, hydrogen atoms enter the anode of a fuel cell where they are stripped of their electrons via a chemical reaction. They have become "ionized," which means they have gained a positive electrical charge. Electrons, which are negatively charged, supply the current that flows through the wires to accomplish the work. If alternating current (AC) is required, the fuel cell's DC output must be connected to an inverter, which converts the voltage and current ¹⁸.

Oxygen enters the fuel cell at the cathode, where it mixes with electrons returning from the electrical circuit and hydrogen ions that have travelled through the electrolyte from the anode in some cell types (such as the one seen above); while in other kinds of cells, oxygen is oxidized by picking up electrons and travelling to the anode, where it joins with hydrogen ions to form water ¹⁸. The electrolyte is critical. Only the right ions should be able to flow between the anode and the cathode with this design. By disrupting the chemical reaction, free electrons or other molecules could pass through the electrolyte. Hydrogen and oxygen mix at either the anode or the cathode to generate water, which exits the cell. A fuel cell can create energy if it has access to hydrogen and oxygen ¹⁸.

Moreover, because fuel cells generate energy chemically rather than through combustion, they are not constrained by the same thermodynamic principles that govern traditional power plants. As a result, fuel cells are more effective in capturing energy from a fuel source than other technologies. Some cells' waste heat can be used to power the system even more efficiently by generating electricity from it ^{17, 18}.

5. Energy Storages Using Hydrogen Produced from Excess Renewable Electricity Power to Hydrogen

Underground gaseous hydrogen storage, liquid hydrogen storage in large containers, or chemical hydrogen storage, such as in liquid organic hydrogen carriers (LOHCs) or chemical components such as methanol or dimethyl ether, are the primary storage options for large quantities of hydrogen (DME). This ranking also reflects the storage systems' effectiveness. Active cooling to the point of liquefaction is most effective for very large installations when it comes to liquid hydrogen storage ¹⁹.

As a result, cooling is done passively by using evaporation heat from hydrogen boil off in all downstream shipping and storage steps up to the point of use. In this case, the efficiency is determined by whether the hydrogen can be effectively used elsewhere in the system or needs to be discarded, i.e., burned. Liquid hydrogen organic carriers that require heat to release the hydrogen have a similar case to make. Only about a third of the hydrogen's stored energy is released as heat. Off-heat from industrial processes or geothermal energy can help improve energy efficiency even if it isn't directly used in the process. When it comes to chemical components like methanol, the energy pathway considers the production of the chemical component as well as hydrogen production and storage as well as the supply of the carbon reaction partner from biomass or CO₂. Finally, the final use of hydrogen will have an impact on the storage option selected ¹⁹.

The only thing that has essentially remained is the know-how that came from these R&D activities since 1960 on PEM water electrolyzes and a handful of companies still working in this field as shown in Figure 5 ¹⁹.

Since its conception, low-temperature fuel cells (both PEM and alkaline) have been the subject of numerous scientific publications, projects, conference presentations, and more. An alternative to polluting fossil fuels, which are still used in both fixed and mobile applications, has commanded a great deal of attention due to the pressing need for such a source ¹⁹.

As a result, the fact that all these fuel cell hydrogen-based technologies rely on the same hydrogen, which currently has to be supplied from fossil-free energy sources, has been overlooked over the decades. We've always held out hope for the so-called "hydrogen economy," in which hydrogen could be produced using nuclear-based energy. Unfortunately, not enough resources (business and governmental) have been given to the R&D of water electrolyzes, which has resulted in a "knowledge gap" where there are not enough publications and know-how dedicated to both alkaline and PEM electrolyzes ¹⁹.

The "Evergreened" concept in Germany, which advocates storing gigawatts of intermittent renewable energy (wind and solar) in the form of water electrolyzes, has finally sparked a shift in our energy future. When it comes to capital or installation expenditures in the next few decades (up to 2050), the technology needs to be sustainable both in terms of its energy efficiency and its long-term viability. Only a few more years are left for us (researchers and business) to potentially transform both "old" technologies into "game changers" ¹⁹.

6. Types of Fuel Cells

The type of electrolyte used in fuel cells determines the type of fuel cell it is. In addition to the types of electrochemical reactions that occur in the cell, this classification also affects the kind of catalysts needed, the temperature ranges in which cells run, the amount of fuel required, and other aspects of operation. These features have an impact on the types of uses for which these cells are most suited, as well. Fuel cells are currently being developed in a variety of forms, each with distinct advantages, disadvantages, and possible uses ²⁰.

Proton exchange membrane fuel cells, also known as polymer electrolyte membrane (PEM) fuel cells, have a high-power density and are lighter and smaller in volume than other types of fuel cells. As an electrolyte, PEM fuel cells utilize a solid polymer, while the carbon electrodes, which are porous, contain platinum or platinum alloy catalysts. Hydrogen, oxygen, and water are all they require to run. They're usually fueled by hydrogen that's been purified in reformers or storage tanks ^{21, 22, 23}.

Compared to other types of fuel cells, PEM fuel cells run at a low temperature of roughly 80°C (176°F). They can start faster (less warm-up time) with low-temperature operation and wear out system components less. A noble-metal catalyst (usually platinum) is needed to separate the hydrogen's electrons and protons, increasing the overall system cost. As a result of the platinum catalyst's sensitivity to carbon monoxide poisoning, if the hydrogen is obtained from a hydrocarbon fuel, a second reactor is required to minimize the amount of monoxide in it. It costs more to use this reactor. PEM fuel cells are mostly employed in transportation, though they can also be found in some stationary applications. For vehicles such as cars, buses, and heavy-duty trucks, PEM fuel cells are ideal ^{21, 22, 23, 24, 25}.

For the most part, hydrogen is used to power fuel cells. This hydrogen can be supplied directly to the fuel cell system or created within the fuel cell system by reforming hydrogen-rich fuels like ethanol or methanol and hydrocarbon fuels. But in direct methanol fuel cells (DMFCs), the anode is fed directly with methanol that has been combined with water ^{26, 27}.

Because methanol has a higher energy density than hydrogen—though less than gasoline or diesel fuel—direct methanol fuel cells do not have many of the fuel storage issues common to some fuel cell systems. Because methanol is a liquid, like gasoline, it's much easier to transport and distribute to the public using our current infrastructure. Portable fuel cell devices like cell phones and laptop computers commonly employ DMFCs to power them ^{26, 27}.

As one of the first fuel cell technologies, alkaline fuel cells (AFCs) were widely used by NASA's space program to generate electricity and purify water while in orbit. The electrolyte in these fuel cells is a potassium hydroxide solution in water, and the catalysts at the anode and cathode can be any non-precious metal. A new type of AFC has been created recently that utilizes a polymer membrane as the electrolyte. The alkaline membrane in these fuel cells differs from the acid membrane in conventional PEM fuel cells. AFCs' high efficiency is a result of the rapidity with which electro-chemical reactions occur within the cell. In space applications, they've shown efficiencies of over 60% ²⁹.

The vulnerability of this fuel cell type to carbon dioxide poisoning is a significant challenge (CO₂). A modest amount of CO₂ in the air can have a significant impact on cell function and longevity because of the production of carbonate. While alkaline cells with liquid electrolytes can be run in a recirculating mode to help prevent carbonate formation in the electrolyte, the recirculating mode brings problems with shunt currents as a side consequence of the regeneration ³⁰.

Additional issues with liquid electrolyte systems include increased wettability, corrosion, and problems with differential pressure handling. Liquid-electrolyte AFCs are more susceptible to CO₂ poisoning than AMFCs, which address these issues by using alkaline membrane fuel cells (AMFCs). Despite this, CO₂ has a negative impact on the AMFCs' performance and durability. In the W-to-kW scale, AMFCs are being studied for use in various applications. It is important for AMFCs to be able to tolerate carbon dioxide as well as conductivity and durability of membranes at higher temperatures ^{29, 30}.

As an electrolyte, Phosphoric acid fuel cells (PAFC) use a Teflon-bonded silicon carbide matrix and porous carbon electrodes containing a platinum catalyst. The acid is contained in the matrix. Modern fuel cells, including the PAFC, are referred to as the "first generation." As a mature cell type, it was the first commercially viable option. However, some PAFCs have been utilized to power large vehicles such as city buses, which are normally employed for stationary power generation with this fuel cell type ³².

While PEM fuel cells are easily "poisoned" by carbon monoxide due to carbon monoxide forming a bond with platinum catalyst at the anode, PAFCs may tolerate impurities in fossil fuels that have been reformed into hydrogen. PAFCs have an efficiency of over 85% when used in the cogeneration of electricity and heat, while they have a lower efficiency when used to generate electricity just (37%-42%). Combustion-based power plants, on average, run at about 33% efficiency. PAFCs are slightly more efficient. Given the same mass and volume, PAFCs are likewise less potent than other fuel cells. As a result, fuel cells of this size and weight are common. PAFCs also cost a lot of money. The cost of these fuel cells is higher since they require more expensive platinum catalyst loadings ^{32, 33}.

Natural gas and coal-based power plants, as well as industrial and military facilities, are all considering using molten carbonate fuel cells (MCFCs). When it comes to MCFCs, think of high-temperature fuel cells with an electrolyte made of molten salts suspended in a chemically inert ceramic matrix. Non-precious metals can be employed as catalysts at the anode and cathode since they work at high temperatures of 650°C (about 1,200°F) ^{34, 35, 36}.

Another benefit of MCFCs over phosphoric acid fuel cells is their increased efficiency. When used in conjunction with a turbine, molten carbonate fuel cells can achieve efficiencies as high as 65 percent, which is far superior to the 37 to 42 percent efficiency of a phosphoric acid fuel cell facility. Over 85% fuel efficiency can be achieved by capturing and using waste heat ^{34, 35, 36}.

MCFCs do not require an external reformer to convert fuels like natural gas and biogas to hydrogen, unlike alkaline, phosphoric acid, and PEM fuel cells. By using MCFCs at high temperatures where methane and other light hydrocarbons are transformed to hydrogen, the cost of manufacturing a fuel cell is reduced. The main problem with existing MCFC technology is that it wears out quickly. Component failure and corrosion are expedited in these cells due to the high temperatures and caustic electrolyte utilized. Corrosion-resistant materials for components and fuel cell designs are currently being investigated by scientists, who hope to quadruple the existing 40,000-hour (5-year) cell life without compromising performance ^{34, 35, 36}.

The electrolyte in solid oxide fuel cells (SOFCs) is a hard, non-porous ceramic compound. SOFCs are approximately 60% efficient at converting fuel to electricity. Overall fuel use efficiencies could exceed 85 percent in applications designed to capture and utilize the system's waste heat (co-generation). SOFCs operate at extremely high temperatures, up to 1,000°C (1,830°F). High-temperature operation eliminates the need for a precious-metal catalyst, lowering costs. It also enables SOFCs to reform fuels internally, allowing them to use a wider range of fuels and lowering the cost of adding a reformer to the system ^{37, 38, 39}.

SOFCs are also the most sulfur-resistant fuel cell type, able to withstand orders of magnitude more sulfur than other cell types. Furthermore, they are not harmed by carbon monoxide, which can even be used as a fuel. This property enables SOFCs to use natural gas, biogas, and coal-derived gases. There are drawbacks to high-temperature operation. It causes a slow startup and necessitates extensive thermal shielding to retain heat and protect personnel, which is fine for utility applications but not for transportation. Because of the high operating temperatures, materials must meet stringent durability requirements. The key technical challenge for this technology is the development of low-cost materials with high durability at cell operating temperatures ³⁸.

Scientists are currently investigating the possibility of developing lower-temperature SOFCs that operate at or below 700°C, have fewer durability issues, and are less expensive. Lower-temperature SOFCs, on the other hand, have not yet matched the performance of higher-temperature systems, and stack materials that will function in this lower temperature range are still being developed ³⁹.

Reversible fuel cells (RFC), like other fuel cells, generate electricity from hydrogen and oxygen while also producing heat and water as byproducts. Reversible fuel cell systems, on the other hand, can use electricity from solar, wind, or other sources to split water into oxygen and hydrogen fuel via a process known as electrolysis. Reversible fuel cells can provide power when it is required, but during times of high-power production from other technologies (for example, when high winds result in an excess of available wind power), reversible fuel cells can store the excess energy in the form of hydrogen. This energy storage capability has the potential to be a game changer for intermittent renewable energy technologies ⁴⁰.

An RFC is a fuel cell that can operate efficiently in both fuel cell and electrolysis modes to generate electricity and hydrogen/chemicals. An electrochemical cell is made up of two half-cells in general. Each half-cell is made up of two parts: an electrode and an electrolyte. The two half-cells could use the same electrolyte or different electrolytes. The electrolyte, electrodes, or an external substance may be involved in chemical reactions in the cell (as with fuel cells that may use hydrogen gas as a reactant) ^{40, 41}.

7. Advantages/Disadvantages of Hydrogen Fuel Cells

When compared to internal combustion engines, most fuel cells operate quietly ⁴³. As a result, they are ideal for use in buildings such as hospitals ⁴³. Fuel cells can eliminate pollution caused by the combustion of fossil fuels; in the case of hydrogen-powered fuel cells, the only by-product at the point of use is water ⁴³. If the hydrogen is produced through the electrolysis of water using renewable energy, then using fuel cells eliminates greenhouse gases throughout the entire cycle ⁴³. Because fuel cells do not require traditional fuels such as oil or gas, they can reduce economic dependence on oil-producing countries, resulting in greater energy security for the user nation ⁴³. Because hydrogen can be produced anywhere there is water and a source of power, fuel production can be distributed and is not grid-dependent ⁴³. The use of stationary fuel cells to generate power at the point of use enables a potentially more stable decentralized power grid ⁴³. Because of their low heat transmission, low temperature fuel cells (PEMFC, DMFC) are ideal for military applications ⁴³. Higher temperature fuel cells generate high-grade process heat in addition to electricity, making them ideal for co-generation applications (such as combined heat and power for residential use) ⁴³. Operating times are much longer than with batteries, because doubling the operating time requires only doubling the amount of fuel, not doubling the unit's capacity ⁴³. Unlike batteries, fuel cells do not have a "memory effect" when refueled ⁴³. Because there are few moving parts in the system, fuel cell maintenance is simple ⁴³.

Hydrogen is currently very expensive, not because it is scarce (it is the most abundant element in the universe), but because it is difficult to generate, handle, and store, necessitating bulky and heavy tanks like those used for compressed natural gas (CNG) or complex insulating bottles if stored as a cryogenic (super-cold) liquid like liquefied natural gas (LNG) ⁴⁴. It can also be stored at moderate temperatures and pressures in a tank with a metal-hydride absorber or a carbon absorber, though these are currently prohibitively expensive ⁴⁴.

8. Applications of Hydrogen Fuel Cells

The use of synthetic natural gas (SNG) instead of hydrogen could be an option due to the availability of natural gas combustion engines and the ease with which natural gas can be stored on board a vehicle. If methanation is 80 percent efficient, the entire energy pathway's efficiency drops to 13 percent, and the cost rises accordingly, assuming the CO₂ for synthesis is taken for granted or even more if it is paid for. For the purposes of this comparison, it is sensible to use hydrogen as the fuel and electrochemical conversion via a fuel cell rather than an internal combustion engine ⁴⁵.

Hyundai and Toyota are currently introducing fuel cell vehicles to the market, and numerous other automakers are working on the technology. It's a completely different story when it comes to trucks. Long-haul trucks are compared to passenger cars to make the distinction clearer. Long-haul trucks use a diesel engine that has a 40 percent higher efficiency than passenger cars, need about (200~300) kW of power, and can last more than 20,000 hours instead of 5,000 hours. For the most part, long-haul trucks operate at full throttle ⁴⁵.

By comparison, internal combustion engines have an efficiency of 47% at their most efficient ⁴⁶ and fuel cells have particularly benign part-load characteristics and efficiency decreases with increasing load ⁴⁶, so the advantage of fuel cells for this application is diminished. This can be offset by using a larger fuel cell, but doing so necessitates a higher initial investment, leading to consideration of a hydrogen-powered internal combustion engine as an alternative. Fossil fuel engines produce soot, but electric motors don't, which is why electric vehicles need NO_x cleanup.

Another option would be to go the methanation route and use a combustion engine and CH₄ as fuel. Gaseous H₂ with a fuel cell would have an efficiency of 32~38 percent for trucks, combustion engine efficiency of 25 percent, and compressed natural gas (CNG) efficiency of 20 percent. Because the reforming process, which has an efficiency of 80%, would further reduce the total efficiency to 16%, combining the use of SNG and a fuel cell is likely to be referred to niche markets. Further research and demonstration are needed, as the picture is not yet clear for long-haul trucks. Additionally, barriers that have been overcome for passenger cars are still an issue due to the much higher standards ^{45, 46}.

Unlike in rural areas, where hybridization is less effective due to rapidly depleting batteries, it works well in urban areas, allowing for regenerative braking and powerful acceleration with small engines, whether they are internal combustion engines or electric motors. As a result, fuel cell buses are an option for city transportation. In this use case, fuel cells outperform internal combustion engines like those found in passenger cars in terms of efficiency; however, combustion engines can be hybridized as well ^{45, 46}.

Most urban fuel cell buses are equipped with fuel cell systems derived from passenger cars, sometimes using the same hardware, and this is especially true for delivery trucks and buses. Long-haul trucks, on the other hand, have characteristics that are similar to those of coaches. Due to its high energy density in a passenger car, liquid hydrogen can be considered an alternative to gaseous hydrogen. Liquefaction uses about a third of the hydrogen's energy content, resulting in a passenger car's fuel cell efficiency being only 26%, compared to a gaseous hydrogen vehicle's efficiency being 38%. The fuel would be gasified and compressed for storage in a car anyway to avoid the problem of the necessary hydrogen evaporation for cooling a liquid tank, which increases fuel consumption when the car is not in use for an extended period and prevents access to public parking garages for the sake of convenience for the consumer ^{45, 46}.

If the cost of imported hydrogen is considered, the picture of efficiency changes. At sea, hydrogen will be carried as a liquid or in the form of chemical substances like methanol rather than compressed gas. Because liquefaction was required for overseas transport anyway to deliver liquid hydrogen to a port, the efficiency argument previously made against it is now moot. Instead, liquid hydrogen's easier distribution can be used to its full advantage ^{45, 46, 47, 48}.

9. Differences between Fuel Cells and Batteries

Global energy systems are undergoing a fundamental transformation. As our reliance on fossil fuels continues to decline, the world is demanding more diverse power solutions for our transportation needs. Batteries and fuel cells technologies are well-positioned to make a significant impact on the transportation market, but the interaction of batteries and fuel cells is frequently misunderstood ⁴⁹.

In a battery, the chemical reactions that produce the electrical current are produced from materials that are already in the battery itself, whereas in a fuel cell, the reactants, almost always Hydrogen and Oxygen are fed to the cell like an external fuel. For example, the very common Carbon-Zinc battery uses the Zinc case as the anode. The reaction there is: Zn = Zn(2+) + 2e(-). The cathode is a carbon rod with MnO2 at its surface. The electrolyte solution, which is more of a paste, is a mixture of NH4Cl and ZnCl2. The reaction there is ⁴⁹:

Where the + and - signs indicate charges, aq means aqueous, i.e., dissolved in the paste, and l means liquid. The reactions don’t take place until an external electrical load is hooked up. Then reaction takes place and the electrons flow through that circuit ⁴⁹.

In a fuel cell compressed Hydrogen enters and a catalyst strips it of its electrons, which become the electrical current. The resulting hydrogen ions pass through a proton exchange membrane after which they encounter another catalyst that combines them with incoming oxygen to form water at that electrode where electrons are picked up. The water is then discharged ⁴⁹.

The advantage of batteries is that they are self-contained. On the other hand, what are called primary batteries eventually discharge and must be discarded. However, there are secondary batteries, such as the Lead-Acid batteries in most cars and the various varieties available for ordinary use, which can be re-charged. But at best, batteries are energy storage devices ⁴⁹.

A fuel cell on the other hand takes a true fuel (hydrogen) and by combining it with atmospheric oxygen, produces an unending supply of electrical energy if the fuel is supplied. Its big advantage is that compared to other ways to make electricity it doesn’t produce any pollution only water. However, the hydrogen itself must be produced somehow and its source would have to be some hydrocarbon. Thus, to produce it, CO₂ would be put into the atmosphere. So, although fuel cells are called sources of clean energy that neglects the problem of how it is produced which is polluting ⁴⁹. Fuel cell power systems are intended to supplement and expand on the battery and electric drive platforms that are gaining popularity around the world. While each system has advantages of its own, combining the emerging technologies of batteries and fuel cells can help us reduce overall carbon and increase adoption of sustainable power without sacrificing performance ⁴⁹.

Fuel cells are like batteries in that they store energy, but unlike batteries, they do not need to be recharged. If the fuel is available, they can generate both electricity and heat. A fuel cell has two electrodes sandwiched together around an electrolyte: a negative electrode also known as an anode) and a positive electrode (also known as a cathode). The anode receives a fuel such as hydrogen, while the cathode receives air. Protons and electrons in hydrogen are separated in a fuel cell by an anode catalyst, and then travel in distinct directions to the cathode. An electric current is created when electrons travel through an external circuit. While moving through the electrolyte, protons meet up with oxygen and electrons at the cathode, where they combine to form water and heat ⁴⁹.

Comparing a battery and a fuel cell can be perplexing because both can be used as power sources, but in different ways. Batteries in battery electric vehicles store and deliver energy to the powertrain. Types of batteries are Lithium-ion batteries; Nickel-metal hydride batteries and Lead-acid batteries ^{45, 46, 47, 48, 49}.

A key operating difference between batteries and fuel cells is that batteries have a charge and discharge cycle, while fuel cells produce electricity continuously, if their fuel is available ^{45, 46, 47, 48, 49}. Batteries are useful in applications where they can be recharged. This can be accomplished with the power grid supply or with a renewable generation system. When combined with renewable generation, batteries enable a power supply that is independent from external sources ^{45, 46, 47, 48, 49}. Fuel cells are a better option when there is no way to recharge batteries. For a given amount of energy, a fuel cell and its respective tank are much more compact and lighter than an array of fully charged batteries ^{45, 46, 47, 48, 49}. Note that a fuel cell can be refueled while operational, since the tank is separate. On the other hand, a battery cannot be charged and discharged simultaneously. The only exception is flow batteries, which have properties of both conventional batteries and fuel cells: Energy is stored as a charge difference between two electrolyte tanks ^{45, 46, 47, 48, 49}. Electricity is produced when these electrolytes interact through a membrane in the flow battery ^{45, 46, 47, 48, 49}. Most electric vehicles are powered by lithium-ion batteries. They have a high power-to-weight ratio, are energy efficient, perform well at high temperatures, and have a low self-discharge rate. Nickel-metal hydride batteries, which have a much longer life cycle than lead-acid batteries, are widely used in hybrid-electric vehicles. However, nickel-metal hydride batteries are quite expensive, have a high self-discharge rate, and generate a lot of heat in hot temperatures. Lead-acid batteries can be designed to be powerful while remaining affordable, safe, and dependable. However, these batteries may perform poorly in cold temperatures and have a shorter life cycle than other batteries ⁴⁹. A hydrogen-powered fuel cell electric vehicle generates electricity while also supplying energy to the powertrain. The fuel cell can also be used to charge the battery. Like a battery, hydrogen acts as an energy carrier and storage device. However, unlike batteries, most fuel cell configurations have a limited ability to manage the powertrain energy demand in a dynamic manner. The battery system is responsible for providing the quick response required to match the load demand from the powertrain ^{45, 46, 47, 48, 49}. Fuel cells continue to be a necessary enhancement to close many of the performance and operational gaps found in battery electric vehicles. Furthermore, fuel cells have the potential to improve the large-scale utilization of renewable energy and accelerate the adoption of sustainable power sources ^{45, 46, 47, 48, 49}. The most significant difference between electric fuel cell and battery technology in application is their suitability for medium to heavy duty transport. Maximum allowable axle weights limit the number of battery packs that can be installed before compromising road weight limits and payload capacity in battery electric vehicles ^{45, 46, 47, 48, 49}.

Long distances and heavy payloads necessitate larger and heavier batteries, and larger and heavier batteries result in decreased performance and efficiency. In some cases, where routes and payloads are not limiting factors, battery electric vehicles can effectively serve operational requirements and patterns before they reach the level of diminishing efficiency, such as package delivery vehicles with shorter city routes and frequent stops ^{45, 46, 47, 48, 49}.

Finally, it is not a matter of which technology is superior, but of which is more appropriate to a customer's circumstances and needs. Fuel cell electric vehicles, on the other hand, can travel longer distances and carry more weight than battery electric vehicles, making them more suitable for longer hauls and heavier loads. Fuel cell electric vehicles have a much higher energy density per weight than battery electric vehicles, allowing them to overcome the range and weight challenges that battery electric vehicles face. Hydrogen tanks are also smaller and lighter than a collection of fully charged batteries. Furthermore, adding more hydrogen tanks is typically less expensive than adding more batteries ^{45, 46, 47, 48, 49}.

Another operational difference between the technologies is the impact on vehicle downtime and overall utilization. Electric vehicles powered by fuel cells can be refueled in minutes. This results in significantly less downtime than other alternative power solutions, allowing fuel cell vehicles to be on the road in the same way that conventional vehicles powered by an internal combustion engine are ^{45, 46, 47, 48, 49}.

Battery electric vehicles are more useful when they can be easily recharged and have usage patterns that allow for recharging downtime, such as a daily route that starts and ends at the same location, as well as a designated charging depot ^{45, 46, 47, 48, 49}.

The ongoing cost evolution of infrastructure will also play a significant role in selecting the best powertrain. High power charging solutions, such as megawatt level charging, have the potential to reduce not only charging time from hours to minutes, but also the amount of on-board battery storage required to carry out a mission effectively. Real-time or dynamic inductive charging may also help to reduce the amount of onboard battery storage required. Vehicles powered by batteries would be able to recharge themselves while on the road ^{45, 46, 47, 48, 49}.

Furthermore, continued growth in the availability and distribution of hydrogen will alleviate the challenges associated with hydrogen fuel costs while also ensuring the long-term availability of Lithium and Platinum, critical elements in batteries and hydrogen fuel cells, respectively. It is also critical to recycle and develop second life opportunities for these systems ^{45, 46, 47, 48, 49}.

10. Case Study of Hydrogen Production Projects in Saudi Arabia and in the World

With the European Union alone investing $500 billion to scale up its infrastructure, hydrogen is evolving from a niche power source used in zeppelins, rockets, and nuclear weapons to a big business. Skeptics point to Saudi Arabia's poor track record in capitalizing on what should be a competitive advantage in the renewables business, especially solar, where there are many plans but few operational projects, as major obstacles to gas becoming a major part of the energy transition. However, countries are vying for dominance in a future global hydrogen market, and the kingdom is one to keep an eye on ^{44, 45}.

China is deploying fuel-cell buses and commercial vehicles, and Japan plans to use natural gas in steel production. The United Kingdom is hosting 10 projects to use natural gas to heat buildings. Kerry, the U.S. president's climate envoy, encouraged domestic oil and gas producers to seize "huge opportunities" presented by hydrogen. As a result, the company Helios Green Fuels should have no trouble finding customers ^{44, 45}.

According to Bloomberg NEF, Saudi Arabia wants to be the world's largest hydrogen supplier by 2050, a market worth up to $700 billion. A broader range of energy exports helps a country be more resilient. Regardless of how fast or when the energy transition occurs, it's protected by diversification. Although plans are being developed and strategies announced, the industry is still in its infancy. Hydrogen is difficult to store, expensive to produce, and emits greenhouse gases when burned. Instead of burning fossil fuels to create hydrogen, green energy sources are used. According to the International Renewable Energy Agency, the cost to produce one kilo is currently $5 ⁵⁰.

Perpetual sunshine and wind give Saudi Arabia a competitive advantage, as do large swaths of unused land. According to BNEF, Helios's costs will be among the lowest in the world and could reach $1.50 per kilogram in 2030. That's less expensive than today's nonrenewable hydrogen alternatives ^{44, 45}.

European renewable energy production is more expensive, and the continent's anticipated demand for the Green Deal will likely outstrip the continent's ability to supply it. These trillion-dollar spending plans aim to make the continent carbon-neutral by the year 2050. They won't be able to make all the hydrogen by themselves, by any means. Offshore wind simply does not have access to enough North Sea or usable water ⁴⁴.

Prince Mohammed bin Salman, Saudi Arabia's 35-year-old de facto ruler, sees Neom as a zero-emissions exemplar that can help revolutionize society and the economy. This vision includes a hydrogen plant. Neom's $500 billion price tag raises doubts about its viability, but the hydrogen project is unaffected by Neom's overall success. Other issues to consider include Even though the country is responsible for producing one-eighth of the world's oil supply, its operational renewables capacity is modest by regional standards ⁵⁰.

When it comes to building the green hydrogen plant, the government is teaming up with Saudi Arabia's sovereign wealth fund-owned Acwa Power and Allentown, Pennsylvania-based Air Products and Chemicals Inc., a $58 billion conglomerate. Helios will use 4 gigawatts of solar and wind power, with the three parties sharing the costs. This plant will be the first of its kind and will be able to help spur on new technology development. To begin with, Helios will electrolyze 650 tons of hydrogen every day, which can be converted to 1.2 million tons of green ammonia annually via conversion. Rather than shipping liquid hydrogen or gas, Air Products will purchase all the ammonia and convert it back to its original form before delivering it to customers ⁵⁰.

About 20,000 city buses' worth of green hydrogen will be produced each year. Air Products wants to be a mainstay in hydrogen depots, which operate about 3 million buses worldwide. Saudi Arabia isn't going to wait until 2025 to start thinking about expanding its nuclear power capacity. According to BNEF, fuel-cell vehicles could account for up to 30% of global bus fleet volume by 2050, with most of the growth coming from China and the EU ^{44, 45}.

At first, hydrogen will be more expensive than other polluting options, but governments and businesses alike are under pressure to meet strict carbon reduction targets, according to Moore. According to BNEF, thirteen countries have hydrogen strategies in place, and another eleven are working on them. Our research shows that even at current hydrogen prices, investors are interested in investing in Saudi Arabia ⁵⁰.

While doing all of this, the federal government is also trying to increase its own meager use of renewable energy sources Currently, only about 700 megawatts of Spain's total installed capacity is being used. Renewable energy will provide half of the country's electricity needs by 2030, and several projects are currently under construction or about to get underway ⁴⁴.

There are only a few countries that regularly burn crude to generate electricity, including Saudi Arabia. Environmentalists say the Neom plant's use of electricity should be diverted to the national grid instead of into the atmosphere, which peaked in August of last year at a four-year high. The emphasis is still on exports, however. As a result of climate change goals, oil-producing countries like Saudi Arabia stand to lose up to $13 trillion by 2040 ⁵⁰.

The hydrogen plant can only produce 15,000 barrels of oil equivalent per day, which is insufficient to meet Saudi Arabia's daily crude oil consumption of 9 million barrels of crude. It's still important to find a way to corner the clean-fuels market, though ⁵⁰.

11. Challenges and Opportunities of Hydrogen Production

A good source of clean energy to help combat climate change and poor air quality is low-carbon hydrogen. Saudi Arabia can decarbonize its transportation, shipping, global energy markets, and industrial sectors with a hydrogen-based economy if it uses it effectively. Some hurdles remain in the way of a hydrogen-based economy being realized. These include the high cost of hydrogen production on a large scale and the associated infrastructure investments as well as the need for large-scale bulk storage and transportation and distribution as well as safety considerations ⁵².

A sufficiently large renewable energy source is needed to power the production of clean fuels at an industrial scale that is relevant around the world. As an example of a country with enormous potential for clean hydrogen, Saudi Arabia is a shining example of a production process that avoids, mitigates, or captures and stores carbon emissions in the atmosphere. With net-zero carbon dioxide (CO₂) emissions becoming a requirement and/or condition for survival, using clean hydrogen to sustainably power the global economy gives the Kingdom a competitive edge. As a result, this commentary examines Saudi Arabia's opportunities and challenges as it seeks to lead the emerging global clean hydrogen market ^{50, 52}.

12. Latest Developments in Hydrogen Fuel Cells

In addition to its traditional use as an industrial feedstock to produce ammonia, methanol, and petroleum refining, hydrogen has recently emerged as a new energy vector. Hydrogen and hydrogen carriers such as ammonia and methylcyclohexane appear to be solutions to address these long-term energy supply difficulties in energy-scarce developed countries like Japan and Korea, in addition to environmental sustainability issues. As China strives to satisfy its environmental goals and energy needs, the country is creating renewable energy and hydrogen infrastructure at a rapid pace. Hydrogen electrification through proton-exchange membrane fuel cells (PEMFCs) is the subject of this review, particularly for fleets of cabs, buses, and logistics vehicles, which require minimal hydrogen infrastructure assistance to operate ⁵³.

For fuel cell commercial applications, the FCV is the most important component, and a complete system is needed to make it feasible to manufacture. Global FCV developers are still focusing on durability, safety, hydrogen storage and refueling, system integration, and pricing. Thanks to national governments' interest in the FCV, substantial investment has been made in this area. Some difficulties, including as hydrogen and oxygen regeneration, transient performance, fuel usage, and power conversion efficiency, need to be addressed to improve the system's overall performance ⁵³.

Energy plans and public policies with a strong focus on disrupting the old energy system and requiring low-carbon technological innovation are essential for successful applications. Considering the disparity across countries, a regional coordinating agency should devise appropriate policies and assist countries in hydrogen policymaking. Hydrogen and fuel cell technology innovation and development require a well-established legislative framework that can provide the stability and vigor necessary to meet realistic performance and cost targets ⁵³.

Full and extensive plans have been drawn out in Japan for hydrogen generation and use in stationary fuel cells, FCVs, and commercial use from 2020 to 2030. To achieve their environmental aims and the expanding energy demand, several developing countries, such as China, are actively constructing renewable energy and hydrogen infrastructure, such as FCVs and hydrogen refueling stations (HRS) ⁵³.

DuPont's Nafion membranes are used in most fuel cells. A membrane's production process is difficult, and the current market pricing (about 2000 $ m²) is expensive. The development of PEMs necessitates a reduction in production costs and an improvement in chemical and mechanical stability. Nafion membranes have been steadily improving in recent years, and Ballard has created a PFSA membrane with similar qualities to Nafion membranes. The manufacturing method of a PFSA membrane is extremely straightforward, and the processing costs are minimal. The cost of the membrane could be greatly decreased by mass production if the market demand continues to grow ⁵³. For the PEMFC, Sutradhar et al. ⁵⁴ created sulfonated poly phenylene benzo phenone membrane (SPPBP) with excellent thermal and chemical stabilities and high proton conductivity (up to 92.9 mS cm^-1) using carbon–carbon coupling polymerization, which they describe as "a promising membrane." SPPBP had an ion exchange capacity of 1.18–2.30 meq g^-1 and a water absorption rate of 34.2-78.3 percent, according to the results of this study.

With high porosity and superior specific surface area, activated carbon extracted from coconut shell (ACGS) might improve proton exchange and cut the cost to about 45 $ m^-2. Thus, despite its organic nature, an ACGS/Clay membrane fuel cell outperformed a Nafion membrane fuel cell in terms of proton coefficient (36 10^-6 cm² s^-1), operating voltage (1.5 times higher) and power density (two times greater), according to a study by Neethu et al. ⁵⁵.

Haragirimana et al. ⁵⁶ fabricated a series of four sulfonated poly aryl ether sulfones (SPAES) copolymer blend PEMs with different sulfonation degrees, leading to better thermal and mechanical stabilities, better hydrolytic and oxidative stabilities, and lower methanol permeability than the control membrane.

Low temperature and high efficiency polymer electrolyte membrane fuel cells can be used in automobiles to minimize fossil fuel use and CO₂ emissions. Fuel cells powered by hydrogen can achieve a practical efficiency of up to 66%, with just water as a waste. Most major automakers are working on fuel cell electric vehicles (FCEVs). Both Toyota's Mirai and Hyundai's NEXO have a range of 312 to 402 miles and a cold-start capability from -30°C, which have been released to the public recently. In California, there are about 50 fuel cell electric buses (FCEBs), and most of them have met the 25,000-hour durability requirement in real-world driving conditions. More than 8,573 FCEVs have been sold or leased in the United States as of September 2020. As of September 2019, over 3,500 FCEVs and 22 FCEBs have been sold or leased in Japan. The widespread use of FCEVs and FCEBs necessitates a large hydrogen station network. More than 44 hydrogen fueling stations (HFSs) are currently in operation across the United States, with most of them based in California. Over 139 HFSs are currently operational in Europe, with plans to add another 1500 stations by 2025 ⁵⁷.

FCEV and FCEB technologies, as well as HFS development, require further R&D resources and assistance. High-temperature PEMs and low-cost catalyst layers (CLs), such as those with minimal Pt loading or PGM-free materials, are still active research topics for PEMs. The fuel cell cathode side of Mirai uses porous media flow fields that have also seen a lot of R&D attention. Two-phase flow in GFCs can cause considerable flow maldistribution and complicate the GFC and cooling system designs at the stack level. While a new cooling technique has been investigated for reducing stack volume by serving more than one cell per plate, further thermal study is required to reduce temperature variance among individual cells. The cooling of fuel cell engines and heat rejection to the outside environment are far more difficult tasks in automotive fuel cells than in ICE engines. To increase total power density and dependability, the balance of plants (BOPs) might be further lowered. Fuel cells and their materials must be designed to allow for an unassisted cold start to achieve the DOE's cold start target in order to have a cold start capability that can be evaluated using established basics. A battery stack is a standard feature on all FCEVs and FCEBs, allowing for better power control and fuel efficiency. System-level modelling and evaluation and optimization are crucial to fuel cell automobile applications. Fuel cell stack and system control and optimization can benefit from machine learning and artificial intelligence ⁵⁷.

The HFS sector is still in its infancy, and new HFS equipment is being developed all the time. Research and development efforts are concentrated on enhancing component and station operation efficiency while also reducing costs. High-pressure storage and wider distribution, as well as storage of liquid hydrogen, are two areas that are now under development. It is also a focus of current research to improve the customer's fueling experience by managing pressure and state of charge in on-site hydrogen storage tanks through the use standards and codes. In addition to the development of HFS equipment, a great deal of work is being put forth to overcome the technology's high cost and widespread skepticism. Cost-cutting is predicted because of economies of scale; however, supporting government programs and policies are still crucial to drive growth in the early market penetration of FCE and FCEB ⁵⁷.

13. Conclusions

With the expectation of the near depletion of fossil fuel resources within the next twenty years, and in accordance with the recommendations of the World Climate Conference to reduce carbon dioxide, most developed countries have turned to search for other sources of renewable energy, especially solar energy, and hydrogen energy, because they are clean, environmentally friendly, and renewable energy. Therefore, many countries of the world began to accept the inevitability of shifting to an economy based on hydrogen as a future fuel. Among the difficult challenges in this transformation are the methods of storing electrical energy in fuel cells and storing hydrogen, as the race of large energy companies has begun to provide solutions to develop many types of fuel cells, given that they are the biggest challenge to energy generation and storage. This paper presented a review of fuel cells including Energy Storage Using Hydrogen Produced from Excess Renewable Electricity.

References