Development of fuel cells

Fuel cell is an energy conversion device that continuously converts chemical energy into electrical energy through oxidation-reduction reaction of anode and cathode. In recent 20 years, fuel cells have experienced several types of development stages, such as alkaline, phosphoric acid, molten carbonate and solid electrolyte. The United States, Japan and other countries have successively established some carbonate fuel cell power plants, molten carbonate fuel cell power plants and proton exchange membrane fuel cell power plants. The structure of a fuel cell is basically the same as that of a common battery, with an anode and a cathode separated by an electrolyte. Different from ordinary batteries, fuel cells are an open system. It needs a continuous supply of chemical reactants to ensure continuous power supply. Its working principle: the fuel cell consists of anode, cathode and ion conductive electrolyte. Fuel is oxidized at the anode, oxidant is reduced at the cathode, and electrons flow from the anode to the cathode through the load, forming a circuit and generating current.

Fuel cell has the advantages of not being limited by Carnot cycle, high energy conversion efficiency, the output power of fuel cell is determined by the performance of single cell, electrode area and the number of single cells, less environmental problems, fast load response and high operation quality. At that time, the first development peak appeared, focusing on the development of alkaline FC. Although it has not been used in vehicles and large power plants as expected, the success of FC in space flight proves its outstanding advantages. In the early 1970s, due to the decrease of investment, FC research entered a low tide. At the end of 1970s, due to the progress of materials science and the worldwide energy shortage, developing new power generation technology and improving the utilization rate of fossil fuels such as oil, natural gas and coal became a topic of great concern and far-reaching significance, so FC research reached its second climax, and the focus was on it at this time.

Develop phosphate fuel cells (PAFC), molten carbonate fuel cells (MCFC) and solid oxide fuel cells (SOFC). Now, as the fourth generation of energy after water power, fire power and atomic energy, fuel cells have attracted the attention of the world.

Molten carbonate fuel cell is mainly composed of anode, cathode, electrolyte substrate and current collecting plate or bipolar plate.

(1) anode

MCFC first used silver and platinum as anode catalysts, and later used nickel with good conductivity and electrocatalytic performance in order to reduce costs. However, it is found that nickel will sinter and creep under the action of working temperature and battery assembly force, so MCFC uses nickel chromium or nickel aluminum alloy as anode electrocatalyst. The purpose of adding 2%~ 10%Cr is to prevent sintering, but the nickel-chromium anode is prone to creep. In addition, chromium can be lithiated by electrolyte and consume carbonate. The decrease of chromium content will reduce the loss of electrolyte, but the creep will increase. In contrast, the creep of Ni-Al anode is small, and the electrolyte loss is small, and the creep reduction is due to the formation in the alloy.

(2) Cathode

Nickel oxide is widely used as cathode catalyst for molten carbonate fuel cells. The typical preparation method is porous nickel electrode.

In-situ oxidation will occur during the heating process of the battery, but the NiO electrode prepared in this way will expand and squeeze the battery shell outward, destroying the wet seal between the shell and the electrolyte matrix. There are several ways to improve this defect:

The long Ni electrode is oxidized outside the battery, and then doped with Li in the battery. Or both oxidation and Li doping are carried out outside the battery;

2. Sintering directly with NiO powder, and doping Li before sintering, or doping Li:

3. Sintering metallic nickel powder in air, so that sintering and oxidation can be completed at the same time;

(3) electrolyte substrate

Electrolyte substrate is an important part of MCFC, and its use is also one of the characteristics of MCFC. The electrolyte matrix consists of a carrier and carbonate, wherein the electrolyte is fixed in the carrier. The substrate is not only an ionic conductor, but also a cathode and anode separator. It must have high strength, high temperature resistance and molten salt corrosion resistance, can block the passage of gas after being immersed in molten salt electrolyte, and has good ionic conductivity. Its plasticity can be used for gas sealing of batteries to prevent gas leakage, which is called "wet sealing".

(4) Current collecting plate (bipolar plate)

Bipolar plate can not only separate oxidant and reductant, provide gas flow channel, but also collect current and conduct electricity, so it is also called collector plate or isolation plate. In the working environment of the battery, the surface of stainless steel on the cathode side is formed, and there is chromium oxide in its inner layer, both of which play the role of passivation film and slow down the corrosion rate of stainless steel. In order to slow down the corrosion rate of anode side of bipolar plate, nickel plating was adopted. MCFC is sealed with metaaluminate embedded diaphragm immersed in molten salt, which is called wet seal. In order to prevent galvanic corrosion at the wet seal, aluminum coating is usually used to protect the wet seal of bipolar plate. Under the working conditions of the battery, the coating will form a dense insulating layer of metaaluminate.

To sum up, fuel cells are developing rapidly and will reach a considerable scale. Its research and development focuses on: catalyst membrane for fuel conversion, cost reduction, safety facilities, hydrogen storage technology and so on.