11. Various classifications of electrochemical cells. Give example.

Primary batteries, or primary cells, can produce current immediately on assembly. These are most commonly used in portable devices that have low current drain, are used only intermittently, or are used well away from an alternative power source, such as in alarm and communication circuits where other electric power is only intermittently available. Disposable primary cells cannot be reliably recharged, since the chemical reactions are not easily reversible and active materials may not return to their original forms. Common types of disposable batteries include zinc–carbon batteries and alkaline batteries. Reaction on anode: Zn (solid) = Zn²⁺ (aqueous) + 2e. Reaction on cathode: 2NH₄⁺ (aqueous) + 2MnO₂ (solid) + 2e = Mn₂O₃ (solid) + 2NH₃ (aqueous) + H₂O (liquid). When using ammonium chloride general equation: 2MnO₂ + 2NH₄Cl + Zn → ZnCl₂· 2NH₃ + H₂O + Mn₂O₃. When using zinc chloride equation: 8MnO₂ + 4Zn + 2ZnCl₂ + 9H₂O → 8MnOOH + ZnCl₂ · 4ZnO · 5H₂O. Secondary batteries, also known as secondary cells, or rechargeable batteries, must be charged before first use; they are usually assembled with active materials in the discharged state. Rechargeable batteries are (re)charged by applying electric current, which reverses the chemical reactions that occur during discharge/use. Devices to supply the appropriate current are called chargers.

The oldest form of rechargeable battery is the lead–acid battery. Reaction on cathode:  (+) PbO₂ + 3H⁺ + HSO₄^- +2e = PbSO₄ + 2H₂O. Reaction on anode: (–) Pb + HSO₄^- = PbSO₄ + H⁺ +2e. Reaction of discharge: PbO₂ + Pb + H₂SO₄ = 2 PbSO₄ + 2H₂O, E = 2.047 V at 25⁰С. The fuel cell represents the fourth category of batteries. Fuel cells are similar to batteries except for the fact that that all active materials are not an integral part of the device (as in a battery). In fuel cells, active materials are fed into batteries from an outside source. The fuel cell differs from a battery in that it possesses the capability to produce electrical energy as long as active materials are fed to the electrodes, but stop operating in the absence of such materials. A well-known application of fuel cells has been in cryogenic fuels used in space vehicles.

12. Give the description of fuel elements.

The chemical current source in which electrical power is generated by chemical reaction between the reducing agent and oxidizing agent, continuously and separately supplied to the fuel cell electrodes from outside. The reaction products are continuously output from the fuel cell. Anode reaction: H2 - 2e → 2H⁺. Cathodic reaction: ½ O2 + 2H + + 2e → H2O. Current-producing reaction: H2 + ½ O2 → H2O. The first fuel cells were invented in 1838. There are many types of fuel cells, but they all consist of an anode, a cathode, and an electrolyte that allows positively charged hydrogen ions (or protons) to move between the two sides of the fuel cell. The anode and cathode contain catalysts that cause the fuel to undergo oxidation reactions that generate positively charged hydrogen ions and electrons. The hydrogen ions are drawn through the electrolyte after the reaction. At the same time, electrons are drawn from the anode to the cathode through an external circuit, producing direct current electricity. At the cathode, hydrogen ions, electrons, and oxygen react to form water. As the main difference among fuel cell types is the electrolyte, fuel cells are classified by the type of electrolyte they use and by the difference in startup time ranging from 1 second for proton exchange membrane fuel cells (PEM fuel cells, or PEMFC) to 10 minutes for solid oxide fuel cells (SOFC). In addition to electricity, fuel cells produce water, heat and, depending on the fuel source, very small amounts of nitrogen dioxide and other emissions. The energy efficiency of a fuel cell is generally between 40-60%.

Requirements to the electrodes: providing conditions for the high speed current-producing chemical reaction in the fuel cell: porous; catalytically active; versatile material - platinum Pt; durable; resistant to corrosion and electrolyte components. Low-temperature alkaline fuel cells: Electrolyte - liquid alkaline solution; Electrode Material - nickel (stable in alkaline solutions); The catalyst – platinum; Application - space and military programs; The commercial use is limited because of the use of platinum and pure hydrogen and oxygen. Low-temperature acid fuel cells: Electrolyte - liquid acid; The oxidant may be oxygen, air, since the air component chemically react with the acid electrolyte; Material of the electrodes - graphite (stable in acidic solutions); The catalyst - platinum and its alloys; Application - stationary generating devices in buildings, hotels, hospitals, airports and power plants; The commercial use is limited because of the use of pure hydrogen and platinum.

Membrane electrolyte. The polymeric membrane Nafion, used in the solid polymer fuel cell. Fuel cells with solid polymer electrolyte: Electrolyte - a solid polymer ion exchange membrane; simplified sealing element; decreases corrosion; increases service life; The material of the electrodes – Graphite; The catalyst - platinum and its alloys. The reducing agent (fuel) can be used methanol, which is previously converted to hydrogen by reacting CH3OH + H2O → CO2 + 3H2. Either electric oxidizied directly at the anode: CH3OH + H2O - 6e- →CO2 + 6H⁺. Application - transport and fixed installations of small size: the commercial use is limited because of the use of platinum and the high cost of the ion exchange membranes. Biofuel element: Principle - the use of natural catalysts; Enzymes dehydrogenase responsible for oxidation and the formation of hydrogen, are unique effective non-platinum catalysts for these processes. Disadvantages: short service life and low power.