Brief Introduction on Fuel Cells Types Currently in the Market

A fuel cell is a biological as well as electrochemical system that is classified based on the electrolyte used and the manufacturing process. These fuel cells produce various forms of energy through an electrochemical process or are used to generate heat-based energy or electricity without the need for processes such as combustion or gasification. There are several fuel cell technologies currently in the prototype development or research stages, with some of the prominent ones differing in the electrolytes used, the chemical reactions that occur, the catalysts involved, operating temperatures and the type of fuel used as feedstock. The following will give you a brief introduction to the currently known fuel cell classifications and characteristics. The figure below briefly presents the fuels used in various types of fuel cells, as well as simple structural explanations and operating temperatures; the following table presents the electrolytes, operating temperatures, fuels, and advantages and disadvantages required for each type of fuel cells.

I. Polymer Electrolyte Membrane Fuel Cells (PEMFC - Polymer Electrolyte Membrane Fuel Cells)

       The high power density and light weight of polymer electrolyte membrane fuel cells (PEMFC) make it a more popular choice than other types of fuel cells. It is also the most common and widely promoted technology route for commercial applications. This name is used interchangeably with proton exchange membrane fuel cells, and here it also specifically refers to low-temperature fuel cells (LT-PEM). Solid polymer electrolytes and carbon electrodes with platinum or platinum alloy catalysts allow these fuel cells to use only hydrogen and oxygen (air). Typically, PEMFCs use pure hydrogen (99.99%+) from storage tanks or reformers as fuel. These fuel cells operate at a lower temperature, around 80°C (176°F), resulting in faster start-up times and less wear and tear on system components, which means better durability. However, using precious metal catalysts such as platinum increases system costs. While PEMFCs are primarily used for transportation purposes, they can also be used in stationary power generation combined heat and power applications. They are particularly suitable for vehicle applications such as buses, cars and heavy trucks. There are many big players, such as Japan's Toyota, South Korea's Hyundai, Ballard, etc., and domestic companies are too numerous to mention.

II. Direct Methanol Fuel Cells (DMFC - Direct Methanol Fuel Cells)

Most fuel cells are powered by hydrogen, which can be supplied directly to the system or produced by reforming hydrogen-rich fuels such as methanol, ethanol or hydrocarbons. However, DMFC (Direct Methanol Fuel Cell) operation is based entirely on 100% methanol, which is usually mixed with water before being injected directly into the fuel cell anode. DMFCs do not suffer from the fuel storage difficulties common with other fuel cell systems because methanol has a higher energy density than hydrogen (although lower than gasoline or diesel fuels). Additionally, methanol is a liquid, much like gasoline, which makes it easier to transport and distribute through existing infrastructure. DMFCs are commonly used in portable fuel cell applications such as cell phones and laptops. Currently, some companies are doing commercial promotion, especially in scenarios such as military industry and base stations. The structural principle is shown in the figure:

III. Alkaline Fuel Cells (AFC - Alkaline Fuel Cells)

Alkaline electrolyzed water (AEC) is well known to everyone, but alkaline fuel cells (AFC) may not be, though AFC is one of the earliest fuel cell technologies invented and is widely used in the U.S. space program to produce electricity and water on spacecrafts. A solution of potassium hydroxide and water is used as the electrolyte, which in turn gives the freedom to use large amounts of non-noble metal catalysts at the anode and cathode. Recently, new AFCs using polymer membranes as electrolytes have emerged. These fuel cells are similar to traditional PEM fuel cells but use an alkaline membrane instead of an acidic membrane. One of the major obstacles to alkaline fuel cells (AFCs) is their vulnerability to carbon dioxide (CO2) poisoning. Even trace amounts of carbon dioxide in the air can create carbonate deposits that can severely impact battery efficiency and lifespan. While AFCs with liquid electrolytes can be run in recirculation mode to regenerate the electrolyte and reduce carbonate formation, this mode introduces shunt current and other issues such as wettability, corrosion, and differential pressure handling. Alkaline membrane fuel cells (AMFCs) solve these problems and are less susceptible to carbon dioxide poisoning than AFCs that use liquid electrolytes. However, carbon dioxide still affects their performance, and they currently lag behind proton exchange membrane fuel cells (PEMFCs) in both performance and durability. AMFCs are currently being studied for their potential use in the multi-watt to multi-kilowatt range. However, in practical applications, some challenges need to be addressed, such as tolerance to carbon dioxide, membrane conductivity, and durability. Commercial promotion is weak and well-known companies are few.

IV. Phosphoric Acid Fuel Cells (PAFCs——Phosphoric Acid Fuel Cells)

Phosphoric acid fuel cells (PAFCs) use liquid phosphoric acid as the electrolyte, which is wrapped in a silicon carbide matrix and bonded with polytetrafluoroethylene (PTFE). The electrode consists of porous carbon containing a platinum catalyst. PAFCs are often referred to as the "first generation" modern fuel cells and are considered one of the most mature fuel cell types and have been used commercially for some time. They are mainly used for stationary power generation, but there are also examples of PAFCs powering large vehicles such as city buses. Compared to proton exchange membrane (PEM) cells, PAFCs have advantages in tolerating impurities in fossil fuels that are converted into hydrogen. PEM cells are easily "poisoned" by carbon monoxide, which binds to the platinum catalyst in the anode, reducing the efficiency of the fuel cell. In contrast, PAFCs have a certain tolerance for impurities and are more than 85% efficient when combined with heat and power. However, they are less efficient when generating electricity alone, with efficiencies ranging from 37%-42%. This efficiency level is only slightly better than that of combustion-based power plants, which are typically around 33% efficient. Additionally, PAFCs are less powerful and larger than other fuel cells of the same weight and volume, making them more expensive to produce. Manufacturing these fuel cells requires large amounts of expensive platinum catalysts, which results in higher costs. Representative companies such as South Korea's Doosan (DOOSAN). PAFCs are mainly used for stationary power generation and drones.

V. Molten Carbonate Fuel Cells (MCFC-Molten Carbonate Fuel Cells)

Development of molten carbonate fuel cells (MCFCs) is currently underway with the aim of using them in natural gas and coal-based power plants for a variety of applications in the electric utility, industrial and military sectors. These fuel cells are designed to operate at temperatures around 650°C (1200°F) and are considered high-temperature fuel cells. The electrolyte used in MCFC is a molten carbonate salt suspended in a porous inert ceramic lithium aluminum oxide matrix. One benefit of operating at high temperatures is that non-noble metals can be used as catalysts for both the anode and cathode, which helps reduce costs. MCFCs can significantly reduce costs due to increased efficiency. When combined with turbines, these fuel cells can achieve efficiencies of up to 65%, far exceeding the 37-42% efficiency of phosphoric acid fuel cells. Additionally, when waste heat is captured and utilized, overall fuel efficiency can exceed 85%. Unlike alkaline, phosphoric acid and PEM fuel cells, MCFCs do not require an external reformer to convert fuels such as natural gas and biogas into hydrogen. Instead, the fuel cell itself converts methane and other light hydrocarbons into hydrogen through internal reforming, thereby reducing costs. With current technology, MCFCs face a major shortcoming in durability. Harsh operating conditions at high temperatures and the use of corrosive electrolytes accelerate the corrosion and decomposition of components, ultimately reducing lifetime. Researchers are actively investigating the use of corrosion-resistant materials and developing fuel cell designs that could double the current service life of 40,000 hours (about five years) without sacrificing performance. Representative companies such as Fuel Cell Energy in the United States.

VI: High-temperature solid oxide batteries (SOFC-Solid Oxide Fuel Cells)

Solid oxide fuel cells (SOFCs) utilize a hard, non-porous ceramic compound as an electrolyte and are highly efficient at converting fuel into electricity, around 60%. In combined heat and power applications, the overall fuel efficiency can exceed 84% by capturing and utilizing the system's waste heat. These fuel cells operate at very high temperatures, up to 1830°F.

This high-temperature operation does not require precious metal catalysts, reducing costs. Additionally, SOFCs can reform the fuel internally, allowing the use of a variety of fuels and reducing the expense associated with adding a converter to the system. Solid oxide fuel cells (SOFC) have distinct advantages over other types of fuel cells. For example, they can tolerate more sulfur than other types, and they are not sensitive to carbon monoxide poisoning, meaning they can run on a variety of fuels such as natural gas, biogas and coal-derived gases. Additionally, high-temperature operation eliminates the need for precious metal catalysts, making it cheaper. However, high-temperature operation poses challenges, including slow start-up and heat shielding requirements, which make them unsuitable for transportation. Additionally, durability is a critical issue due to harsh conditions at high temperatures. Therefore, it is crucial to develop low-cost materials that can withstand these conditions. One promising approach is to develop cooler-temperature SOFCs, which have fewer durability issues and lower operating costs. While these low-temperature SOFCs have not yet achieved the same performance as high-temperature systems, scientists are currently developing stacks of materials that can operate in this lower temperature range. At present, there are many related companies at home and abroad. The specific working and structural principles are shown in the figure.

VII. Reversible Fuel Cells (RFC - Reversible Fuel Cells)

Strictly speaking, reversible fuel cells do not refer to a specific technical route. Currently, the common ones are alkaline reversible, PEM reversible, and SOC reversible (referring to reversible electrolysis of water and fuel cells). Their working principles are similar to other fuel cells because They generate electricity and heat from hydrogen and oxygen and water as by-products. However, reversible fuel cell systems have the additional ability to perform electrolysis and produce hydrogen and oxygen fuels using electricity from renewable sources such as solar and wind power. This means that a reversible fuel cell not only provides electricity when needed, but it can also store excess energy in the form of hydrogen when renewable energy technologies produce more energy than is currently needed. This energy storage potential is critical to making intermittent renewable energy more efficient and reliable.