5 types of fuel cells and their application scenarios

Energy is the source of the development of human society, and the development of energy technology is one of the important indicators to measure the level of economic development and living standards of a country. Every progress in human society is inseparable from breakthroughs and innovations in energy technology. However, the existing energy structure and the unreasonable use of human beings have brought "heavy damage" to human society: environmental problems such as acid rain, greenhouse effect, global warming, air pollution, and ozone layer destruction are gradually "forcing" human beings to pay attention to the transformation of the energy structure. The search for efficient, clean and sustainable development of new energy technology is also a problem that all countries must face and pay attention to.

Fuel Cell (FC) is an electrochemical device that can effectively control the chemical reaction of fuel and oxidizer, and directly convert the chemical energy into electrical energy, is an energy converter that converts the chemical energy in fuel into electrical energy, and is known as the fourth power generation method after thermal power, hydropower and nuclear power. Although the fuel cell is called "battery", there is a difference between the actual and the battery, the battery belongs to the category of energy storage, and the fuel cell does not store energy, essentially just an energy converter, more like a "generator".

Compared with traditional power generation methods, fuel cells have a special feature: in the reaction process, the fuel cell energy conversion process has no open flame combustion activity, so the energy conversion efficiency is not limited by the "Carnot cycle". In addition, fuel cells also have the advantages of fuel diversification, low noise, cleaner exhaust, less environmental pollution, good maintainability and high reliability [1].

FIG. 1 shows the comparative data of fuel cell direct power generation and traditional indirect power generation (comparison of annual power consumption of a family of 4) obtained by Panasonic after market research. Compared with traditional indirect power generation, energy consumption of a single family through fuel cell power generation can save 3734 kW·h of electricity per year [2]. After years of development, fuel cells have been gradually used in civilian, transportation, military and other fields.

1. Development history of fuel cells

As a class 4 power generation technology, fuel cells have been listed by Time as the first high-tech in the 21st century and are considered to be the 21st century energy Star. Since the beginning of the 19th century, the concept of fuel cells has gradually "emerged" in the field of energy, and has experienced more than 200 years of development [3], its development source can be traced back to the discovery of electrochemical phenomena:

Volta was the first scientist to observe the phenomenon of electrochemistry, and he and Ritter are regarded as the founders of electrochemistry;

In 1839, British scientist Grove discovered the principle of fuel cells in the electrolysis process of water, and published the first article about fuel cells, introducing the fuel cell experiment;

In 1889, Mond and Langer improved Grove's invention by using a porous non-conductive material impregnated with electrolytes as a battery separator, using platinum black as a catalyst, and using a drilled platinum or gold sheet as a current collector to assemble a fuel cell using hydrogen and oxygen as fuel and oxidant.

In the 1950s, the US General Electric company invented the first proton exchange membrane fuel cell;

In 1959 Bacon produced the first working Bacon-type fuel cell (AFC), and Allis-Chalmers introduced the first farm tractor powered by a fuel cell.

In the 1960s, the United States Aeronautics and Space Administration (NASA) first used fuel cells as the main power source on the Apollo moon landing spacecraft, and fuel cells have thus made an "outstanding contribution" to the human moon landing. Since then, the research of fuel cell technology has attracted the attention of various countries and started to enter the stage of rapid development [4].

After the 1970s, under the dual pressure of environmental protection and energy demand, especially the outbreak of the 1973 oil crisis, countries around the world began to face up to the importance of energy, and more stimulated scientists' enthusiasm for the research and development of fuel cell technology, the first generation of fuel cells (phosphoric acid fuel cells fueled by purified reformed gas), PAFC), the 2nd generation fuel cell (molten carbonate fuel cell, MCFC, fueled by purified gas and natural gas), and the 3rd generation fuel cell (solid oxide electrolyte fuel cell, SOFC) have been developed successively.

In 1993, Canada's Ballard Power System launched the world's first vehicle powered by proton exchange membrane fuel cells, and fuel cells began to enter the civilian field. In the 1990s, fuel cells, as a clean, cheap and renewable energy use, gradually moved from the laboratory to the "homes of ordinary people" [4]. At present, many hospitals, shopping malls, schools and other public places in the world have installed fuel cell power supply, and automobile manufacturers in various countries have begun to develop a variety of new energy vehicles powered by fuel cells.

The United States, Japan, Canada, Europe and Australia in the field of fuel cell research and application in the forefront of the world, China from the 1950s, also opened the fuel cell research, in the 1970s, China's fuel cell research reached a climax, but later the enthusiasm was interrupted, in the 1990s, In the case of urgent international energy demand and domestic environmental deterioration, China's fuel cell development has once again become a hot field [5].

2. Fuel cell principle

After more than 200 years of development, fuel cells have gradually stepped out of the laboratory and integrated into human social life. Different types of fuel cells "shine" in different fields with their own performance advantages, but their principles are the same. A fuel cell consists of three parts: anode, cathode and electrolyte. The "power generation" process can be roughly decomposed into the following four steps (the working principle is shown in Figure 2 [6]) :

Under the action of anode catalyst, the fuel gas (hydrogen, methane, methanol, etc.) oxidizes to form cations and give free electrons.

Under the action of the cathode catalyst, the oxide (usually oxygen) undergoes a reduction reaction to obtain electrons and anions.

The cation produced by the anode reaction or the anion produced by the cathode is moved to the opposite electrode through the electrolyte, and the reaction product is generated and discharged outside the battery.

Driven by the potential difference, electrons move from the anode to the cathode through the external circuit, so that the whole reaction process reaches the material balance and charge balance, and the external electrical appliances obtain the electrical energy provided by the fuel cell.

Although the basic principles of different types of fuel cells are similar, usually due to different electrolytes, the carriers allowed to pass through are also different, so there are some differences in the corresponding battery reaction.

3. Fuel cell classification

According to the different electrolytes, fuel cells can be divided into five types: alkaline fuel cell (AFC), phosphor fuel cell (PAFC), fused carbonate fuel cell (MCFC), solid oxide fuel cell (SOFC) and proton exchange membrane fuel cell (PEMFC). Its performance comparison and power generation principle are shown in Table 1 [7-8].

There are currently five fuel cells in different stages of development, but there are also several special types of fuel cells: direct methanol fuel cells (DMFC), regenerative fuel cells (RFC), and direct carbon fuel cells (DCFC).

4. Fuel cell application

Fuel cell provides a new power generation mode to improve energy efficiency and reduce waste emissions, and its own unique advantages determine the important position of fuel cell in a new round of energy revolution.

According to the scale of fuel cells, fuel cells with different power levels have basically fixed application fields, as shown in Figure 3 [9]. According to the classification of fuel cells, different types of fuel cells also have their own application space due to different operating temperatures and fuel adaptability.

4.1 Proton Exchange Membrane Fuel Cell (PEMFC)

Proton Exchange Membrane Fuel Cell (PEMFC) is the closest fuel cell to commercialization and the most promising battery to power new energy vehicles in the future. PEMFC single battery is composed of anode, cathode and proton exchange membrane. Proton exchange membrane serves as the medium for transmitting H+ and only allows H+ to pass through. Its structure diagram is shown in 4 [10].

In order to provide a power source for the Gemini constellation, the United States General Company first developed PEMFC in 1960, but later due to poor performance, the battery scheme lost to alkaline fuel cell (AFC), and eventually did not be applied. In 1970, DuPont developed the perfluorinated sulfonic acid membrane (Nafion membrane), which is the best proton exchange membrane, laying the foundation for the rapid development of PEMFC in the later period. Nevertheless, there was a long dip in research as PEMFC lost its edge due to breakthroughs in other types of fuel cells during this historical period.