An explanation of the structure and working principles of molten carbonate fuel cells, solid oxide fuel cells, and direct methanol fuel cells

  1. Molten carbonate fuel cell

Molten carbonate fuel cell is a fuel cell composed of porous ceramic cathode, porous ceramic electrolyte diaphragm and porous metal anode plate. The electrolyte is molten carbonate, and the electrocatalyst does not need to use precious metals, but mainly Raney nickel and nickel oxide. The cathode, anode and electrolyte layer form the so-called “sandwich” battery structure.

1.1. Structure of molten carbonate fuel cell

The single cell structure of molten carbonate fuel cell is composed of separator, fuel electrode (anode, Ni porous body), air electrode (cathode, NiO porous body) and electrolyte plate between two electrode plates (generally liaio2 porous ceramic plate impregnated with mixed carbonate of Li and K). The upper and lower parts of the monomer are diaphragm current collection plates, the middle part is electrolyte plate, both sides of the electrolyte plate are porous anode plate and cathode plate, and the electrolyte is molten carbonate.

Structure of molten carbonate fuel cell
Structure of molten carbonate fuel cell

1.2. Working principle of molten carbonate fuel cell

The working process of molten carbonate fuel cell is essentially the process of fuel oxidation and oxidant reduction. Fuel and oxidant gases flow through anode and cathode channels. O2 and CO2 in the oxidant react with electrons at the cathode to produce CO32 -, CO32 – in the electrolyte plate moves directly from the cathode to the anode, and H2 in the fuel gas reacts with CO32 – at the anode to produce CO2, H2O and electrons. The electrons are collected by the collector plate and then reach the diaphragm. The separator is located at the upper and lower parts of the fuel cell unit and is connected with the load equipment, thus forming a complete circuit part including electron transmission and ion movement.

1.3. Characteristics of molten carbonate fuel cell

Molten carbonate fuel cell is a high-temperature cell (600 ~ 700 ℃), which has many advantages, such as high efficiency (higher than 40%), low noise, pollution-free fuel diversification (hydrogen, gas, natural gas and biofuel), high waste heat utilization value and low cost of cell construction materials. It will be a green power station in the future.

  1. Solid oxide fuel cell

Solid oxide fuel cell belongs to the third generation fuel cell. It is an all solid-state chemical power generation device that directly converts the chemical energy stored in fuel and oxidant into electric energy efficiently and environmentally friendly at medium and high temperature. It is generally considered to be a fuel cell that will be widely used in the future as proton exchange membrane fuel cell.

2.1. Solid oxide fuel cell structure

Solid oxide fuel cell is mainly composed of electrolyte, anode, cathode, electrolyte and end plate. Solid electrolyte is the core component of solid oxide fuel cell, and its main function is to conduct oxygen ions. Its performance (including conductivity, stability, coefficient of thermal expansion, densification temperature, etc.) not only directly affects the working temperature and conversion efficiency of the battery, but also determines the selection of matching electrode materials and preparation technology. At present, the commonly used electrolyte material is cermet with Ni powder dispersed in oxidation fault. Its ionic conductivity does not change significantly when the oxygen partial pressure changes by more than ten orders of magnitude.

Solid oxide fuel cell structure
Solid oxide fuel cell structure

The electrode material itself is first of all a catalyst. The cathode needs to work in high temperature and oxidation for a long time to transfer electrons and diffuse oxygen. It should be a porous electron conductive film. The operating temperature of solid oxide fuel cell is high. Only precious metals or electronically conductive oxides are suitable for cathode materials. Because precious metals such as platinum and palladium are expensive, they are generally only used in the experimental range. Strontium doped lanthanum manganate is often used as cathode material of solid oxide fuel cell. At present, Ni / YSZ ceramic alloy has the lowest cost and is the preferred anode material in practical application.

2.2. Working principle of circular oxide fuel cell

When the solid oxide fuel cell works, electrons flow from the anode to the cathode through the external circuit, and oxygen ions flow from the cathode to the anode through the electrolyte. The reduction reaction of oxidant occurs at the cathode, that is, the electrons obtained by oxygen molecules are reduced to oxygen ions. Under the driving force of potential difference and concentration difference on both sides of the electrolyte diaphragm, the oxygen ion O2 – makes a directional transition to the anode side through the oxygen holes in the electrolyte diaphragm. The oxidation reaction of the fuel occurs at the anode, that is, the fuel reacts with the oxygen ions transmitted through the electrolyte to generate water. At the same time, the electrons are released to the external circuit, and the electrons reach the cathode through the external circuit to form direct current.

2.3. Characteristics of solid oxide fuel cell

Solid oxide fuel cell not only has the common characteristics of fuel cell efficiency, cleanliness and environmental friendliness, but also has the following advantages.

(1) Solid oxide fuel cell is an all solid-state battery structure, which has no electrolyte leakage problem, avoids the problems of corrosion and electrolyte loss caused by the use of liquid electrolyte, and can realize long-life operation without configuring electrolyte management system.

(2) It has strong adaptability to fuel and can directly use natural gas, coal gas and other hydrocarbons as fuel.

(3) Solid oxide fuel cells directly convert chemical energy into electrical energy without thermal engine process, so they are not limited by Carnot cycle. High power generation efficiency, high energy density and high energy conversion efficiency.

(4) The working temperature is high, the electrode reaction speed is fast, and there is no need to use precious metals as electrocatalysts.

(5) High temperature can be used for internal fuel reforming to optimize the system.

(6) Ceramic electrolyte requires medium and high temperature operation (600 ~ 1000 ℃), which speeds up the reaction process of the battery, realizes the internal reduction of various hydrocarbon fuel gases, and simplifies the equipment.

Solid oxide fuel cells also have the following disadvantages:

(1) The oxide electrolyte material is ceramic material, which is brittle and easy to crack, so it is difficult to assemble the battery stack.

(2) High temperature thermal stress will cause battery cracking, so the thermal expansion rate of main components should be strictly matched.

(3) There is a loss of free energy.

(4) With high working temperature and long preheating time, it is not suitable for non fixed places that need to be started frequently.

  1. Direct methanol fuel cell

Direct methanol fuel cell directly uses methanol aqueous solution or steam methanol as the fuel supply source, without the need to produce hydrogen through the reforming of methanol, gasoline and natural gas for power generation. Compared with proton exchange membrane fuel cell, direct methanol fuel cell has the characteristics of low-temperature rapid start, clean fuel and simple structure, which makes direct methanol fuel cell possible to become the mainstream of portable electronic products in the future.

3.1. Structure and principle of direct methanol fuel cell

Direct methanol fuel cell is mainly composed of anode, solid electrolyte membrane and cathode. The anode and cathode are respectively composed of diffusion layer and catalyst layer with porous structure. Carbon black and polytetrafluoroethylene with different hydrophobicity and hydrophilicity are usually used as anode and cathode materials of direct methanol fuel cell. Methanol and water in the anode oxidize to generate CO2 and release electrons and protons. The oxygen in the cathode reacts with the proton generated by the anode to generate water, and the electrons are transferred from the anode to the cathode through the external circuit to form direct current.

Structure and principle of direct methanol fuel cell
Structure and principle of direct methanol fuel cell

3.2. Characteristics of direct methanol fuel cell

(1) Methanol has abundant sources, low price and convenient storage and carrying.

(2) Compared with hydrogen oxygen proton exchange membrane fuel cell, it has simpler structure and more convenient operation.

(3) Compared with proton exchange membrane fuel cell, the volume energy density is higher.

(4) Compared with reforming methanol fuel cell, it has no methanol reforming unit, lighter weight, smaller volume and faster response time.

(5) When methanol is converted to hydrogen and carbon dioxide at low temperature, more catalysts are needed than proton exchange membrane fuel cells.

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