Solid oxide fuel cells and molten carbonate fuel cells

Solid oxide fuel cell

Solid Oxide Fuel Cell (SOFC) is an all-solid-state chemical power generation device that directly converts chemical energy stored in fuel and oxidant into electrical energy with high efficiency at high temperature. Solid yttrium oxide and zirconium oxide electrolytes have strong ion conduction function at high temperature, can conduct O2, and play the role of transferring O2 and separating air and fuel in the battery. The working principle of the solid oxide fuel cell is shown in Figure 1.

Solid oxide fuel cells and molten carbonate fuel cells
Figure 1 – The working principle of a solid oxide fuel cell

On the cathode (air electrode), oxygen molecules gain electrons and are reduced to oxygen ions O2:

O2c+4e→2Oe2-

In the formula: the subscripts c and e represent the state in the cathode and the state in the electrolyte, respectively.

It is then transported through the electrolyte to the anode, where it reacts with hydrogen (or carbon monoxide). Produces water (or carbon dioxide) and electrons.

2Oe2-+2H2a→2H2Oa+4e

In the formula: the subscript a represents the state in the anode.

Solid oxide fuel cells usually use a flat plate type and a tube type of structure. The tubular structure can avoid stress concentration, and at the same time greatly ease the difficulty of air tightness and thin-layer fabrication of solid oxide fuel cells. Tubular solid oxide fuel cells are currently the closest commercialized solid oxide fuel cell power generation technology.

Compared with the first generation (phosphoric acid fuel cell) and the second generation fuel cell (molten salt carbonate fuel cell), the solid oxide fuel cell has the following advantages:

(1) Higher current density and power density.

(2) The anodic and cathodic polarizations are negligible, and the losses are concentrated in the internal resistance drop of the electrolyte.

(3) Hydrogen, hydrocarbons (methane), methanol, etc. can be directly used as fuels without using precious metals as catalysts.

(4) The corrosion and sealing problems of acid-base electrolyte or molten salt electrolyte of medium and low temperature fuel cells are avoided.

(5) It can provide high-quality waste heat, realize cogeneration, high fuel utilization rate, and energy utilization rate as high as about 80%. It is one of the efficient and clean energy systems.

(6) Ceramic materials are widely used as electrolyte, cathode and anode, with all-solid-state structure.

(7) The ceramic electrolyte requires medium and high temperature operation (600 ℃ ~ 1000 ℃ C), which speeds up the reaction of the battery, and can also realize the internal reduction of various hydrocarbon fuel gases, which simplifies the equipment.

Molten carbonate fuel cells

Molten Carbonate Fuel Cell (MCFC) uses molten carbonate of alkali metals (Li, Na, K) as electrolyte, hydrogen-rich fuel gas (transformed from natural gas, methane, coal gas, etc.) as fuel, and O2 (air) plus CO2 as oxidant. The working temperature is about 650°C, the waste heat utilization value is high, the electrocatalyst is mainly nickel, and there is no need to use precious metals, and the power generation efficiency is high.

The reaction principle of the molten carbonate fuel cell is shown in Figure 2.

Solid oxide fuel cells and molten carbonate fuel cells
Figure 2 – The working principle of molten carbonate fuel cells

The single cell of molten carbonate fuel cell is composed of cathode, electrolyte, electrolyte separator and anode. If a battery stack is formed, components such as bipolar plates, current collectors, and bubble screens are also required. Among them, the diaphragm is the core component of the molten carbonate fuel cell. It must have high strength, high temperature molten salt corrosion resistance, can block the passage of gas after being immersed in the molten salt electrolyte, and has good ionic conductivity (the conductive ion of molten carbonate fuel cell is CO3). Through the screening of various materials and years of research, lithium metaaluminate has been widely used to prepare the separator of molten carbonate fuel cells.

Solid oxide fuel cells and molten carbonate fuel cells

The high temperatures at which such cells operate can internally reform hydrocarbons such as natural gas and oil, producing hydrogen within the fuel cell structure. At these high temperatures, although sulfur is still a problem, CO pollution is not, and the platinum catalyst can be replaced by a cheap type of nickel metal, and its excess heat can also be used by a combined heat and power plant. The molten carbonate fuel cell has high working temperature and high utilization value of waste heat, and can be combined with coal gasification combined cycle to form an efficient clean coal power generation technology. The efficiency of such fuel cells can be as high as 60%. Its potential efficiency can be as high as 80% if the heat it wastes can be used.

However, the high temperature can also bring some problems. Such batteries take a long time to reach operating temperature, so they cannot be used for transportation, and the temperature and corrosion properties of their electrolytes make them less safe for home power generation. However, its higher power generation efficiency is attractive for large-scale industrial processing and power generation turbines.