Description of hydrogen and oxygen fuel cells

Fuel cell is a promising new power supply, generally hydrogen, carbon, methanol, borohydride, gas or natural gas as fuel, as a negative electrode, with oxygen in the air as a positive electrode. The main difference between the battery and the general battery is that the active substance of the general battery is pre-placed inside the battery, so the battery capacity depends on the amount of stored active substance; The active substances of the fuel cell (fuel and oxidizer) are continuously input at the same time of reaction, so this kind of battery is actually just an energy conversion device. This type of battery has the advantages of high conversion efficiency, large capacity, high specific energy, wide power range, no charging, but due to high cost, the system is more complex, limited to some special uses, such as spacecraft, submarines, military, TV transfer stations, lighthouses and buoys.

How it works:

Hydrogen and oxygen fuel cells use hydrogen as fuel as reducing agent, oxygen as oxidizing agent, through the combustion reaction of fuel, chemical energy into electric energy battery, the working principle is the same as that of galvanic cell.

When the hydrogen-oxygen fuel cell works, hydrogen is supplied to the hydrogen electrode and oxygen is supplied to the oxygen electrode. Hydrogen and oxygen, under the action of a catalyst on the electrode, form water through the electrolyte. At this time, the hydrogen electrode has an excess electron and is negatively charged, and the oxygen electrode is positively charged due to the lack of electrons. When the circuit is switched on, the reaction process, which is similar to combustion, can proceed continuously.

When working, fuel (hydrogen) is supplied to the negative electrode and oxidizer (oxygen) is supplied to the positive electrode. Hydrogen is decomposed into positive ions H+ and electrons e- under the action of a catalyst on the negative electrode. The hydrogen ions enter the electrolyte, while the electrons move along the external circuit towards the positive electrode. The load is connected to the external circuit. At the positive electrode, oxygen and hydrogen ions in the electrolyte absorb electrons arriving at the positive electrode to form water. This is the reverse process of the electrolytic reaction of water.

Hydrogen and oxygen fuel cells do not require a device that stores all reducing agents and oxidants in the battery.

The reactants of the hydrogen and oxygen fuel cell are all on the outside of the cell and it just provides a container for the reaction

Both hydrogen and oxygen can be supplied outside the battery.

A fuel cell is a chemical battery that uses the energy released during a chemical reaction to convert it directly into electricity. From this point of view, it is similar to other chemical batteries such as zinc-manganese dry batteries and lead-acid batteries. However, when it works, it needs to be continuously supplied with reactive substances - fuel and oxidant, which is not the same as other ordinary chemical batteries. Because it converts the energy released by the fuel through a chemical reaction into electrical energy output, it is called a fuel cell.

Specifically, fuel cells are "generators" that utilize the reverse reaction of electrolysis of water. It consists of a positive electrode, a negative electrode and an electrolyte plate sandwiched between the positive and negative electrodes. Initially, the electrolyte plate was formed by using electrolytes to penetrate porous plates, and in 2013, it is being developed to directly use solid electrolytes.

When working, fuel (hydrogen) is supplied to the negative electrode, and oxidizing agent (air, the active component is oxygen) is supplied to the positive electrode. Hydrogen splits into positive ions H+ and electrons e- at the negative electrode. When hydrogen ions enter the electrolyte, electrons move along the external circuit towards the positive electrode. The load is connected to the external circuit. At the positive electrode, oxygen in the air and hydrogen ions in the electrolyte absorb electrons arriving at the positive electrode to form water. This is the reverse process of the electrolytic reaction of water. The water in this process can be reused, and the power generation principle is similar to that of solar cells that can be used at night.

The electrode materials of fuel cells are generally inert electrodes, which have strong catalytic activity, such as platinum electrodes and activated carbon electrodes.

Using this principle, the fuel cell can continuously transmit electricity to the outside while working, so it can also be called a "generator".

In general, writing the chemical reaction equations of fuel cells requires a high degree of attention to the acidity and alkalinity of the electrolyte. The electrode reaction occurring on the positive and negative electrodes is not isolated, it is often closely related to the electrolyte solution. For example, hydrogen-oxygen fuel cells have two types of acid and basic:

If the electrolyte solution is alkali, salt solution

The negative reaction formula is:

The positive reaction formula is:

If the electrolyte solution is an acid solution

The negative reaction formula is:

The positive reaction formula is:

The total reaction equation is:

In alkali solution, the content of OHˉ is extremely high, so H+ generated at the negative electrode will directly combine with OHˉ to form water, while OHˉ generated at the positive electrode will exist in the form of ions due to the lack of H+ combination. The opposite is true for acidic solutions.

Composition structure;

A type of fuel cell using hydrogen as fuel and oxygen as oxidizer. Hydrogen and oxygen from the outside through the pipeline into the battery for electrochemical reaction and output electricity. The theoretical specific energy of the hydrogen - oxygen fuel cell is 3600 watt - hour/kg. The working voltage of a single battery is generally 0.8 ~ 0.97 volts, and in order to meet the working voltage required by the load, dozens of single batteries are often connected into a battery pack.

In order to maintain the normal operation of the battery, it is necessary to continue to supply hydrogen and oxygen, and timely exclusion of reaction products (water) and waste heat. The battery pack is composed of the following parts: ① Hydrogen and oxygen supply system: the hydrogen and oxygen carried by the spacecraft are stored in supercritical liquid, which can reduce the volume of the tank and solve the problem of the separation of gas and liquid under weightless conditions, but the storage tank is required to have good adiabatic performance, low temperature resistance and high pressure resistance (6 mpa for oxygen tank and 3 ~ 3.5 mpa for hydrogen tank). ② Drainage subsystem: there are two main ways of dynamic drainage and static drainage. The former circulates hydrogen with water vapor to the cooling device to condense the water vapor into water for separation; The latter relies on porous fiber braided materials (such as wicks) to adsorb the condensed water, also known as wick drainage. The water discharged from the battery pack can be purified for astronauts to drink or use as coolant. ③ Heat removal subsystem: The battery pack circulates through the coolant (such as glycol solution) to bring the waste heat to the radiator and discharge it outward to maintain the temperature range of the battery pack to work normally. (4) Automatic control subsystem: including the control and adjustment of the working pressure, temperature, drainage and exhaust, voltage, safety and coolant circulation of the battery pack. The measured parameters are transmitted to the display in the astronaut's cockpit or sent back to the ground by telemetry equipment. When the battery string is faulty, the system automatically switches to the backup battery string.

Battery classification:

According to the cell structure and working mode, hydrogen and oxygen fuel cells can be divided into three categories: ionic membrane, bacon type and asbestos membrane.

① Ionic membrane hydrogen oxygen fuel cell: acid fuel cell with cation exchange membrane as electrolyte, modern use of perfluorosulfonic acid membrane. When the battery is discharged, water is generated at the oxygen electrode and sucked out through the wick. This kind of battery works at room temperature, has compact structure and light weight, but the internal resistance of ion exchange film is large and the discharge current density is small.

② Bacon type fuel cell: alkaline battery. The hydrogen and oxygen electrodes are both double-layer porous nickel electrodes (the inner and outer layers have different pore sizes), and platinum is used as catalyst. The electrolyte is a caustic potassium solution of 80% to 85%, which is solid at room temperature and liquid at the operating temperature of the battery (204 to 260°C). This battery has a high energy utilization rate, but it consumes a lot of power, and it takes a long time to start and stop (24 hours to start, 17 hours to stop).

③ Asbestos film fuel cell: also an alkaline battery. The hydrogen electrode is made of porous nickel plate plus platinum and palladium catalyst, and the oxygen electrode is a porous silver electrode. The two electrodes are sandwiched with an asbestos film containing 35% caustic potassium solution, and then a fluted nickel plate is pressed on the two electrode plates as a flow collector to form a gas chamber and package into a single battery. Water is generated on one side of the hydrogen electrode during discharge, which can be discharged by circulating hydrogen or by static drainage. The battery has a start-up time of only 15 minutes and can be stopped instantaneously.

Main features:

1. The product is water, clean and environmentally friendly;

2. Easy to continue to pass hydrogen and oxygen, generating continuous current;

3. High energy conversion rate, more than 80%(ordinary combustion energy conversion rate of more than 30%);

4. Can be combined as a fuel cell power station, less waste emissions, low noise, green power station. [2]

Application field:

As a promising new power source, hydrogen and oxygen fuel cells have many applications:

1, large-scale power generation

2, portable mobile power supply

3. Emergency power supply

4. Home power supply

5. Aircraft, cars, warships [2]

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regenerative

A fuel cell that converts the battery reaction product (water) into a reactant (hydrogen and oxygen) through an electrolyzer and then re-uses it to produce electrical energy is composed of two parts: a fuel cell and an electrolyzer. It can be used as an energy storage device for the power supply system of high power solar cell array. When there is sunlight, the solar array provides power to the spacecraft load, and is also used to decompose water into hydrogen and oxygen, so that some of the electricity is stored. When the spacecraft enters the shadow area, the solar cells cannot produce electricity or the power supply is insufficient. Hydrogen and oxygen fuel cells are devices that do not require all reducing agents and oxidants to be stored in the battery.

Static drainage

On June 13, 2014, China Aerospace Academy 811 Institute developed a static drainage fuel cell module, which successfully achieved long-term stable discharge of high current density, and achieved no gas emissions for 8 hours for the first time in China