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.
13. Nickel–metal hydride battery.
Nickel-metal hydride batteries (Ni-MH) – secondary galvanic cell in which the anode is a metal hydride hydrogen electrode (usually a nickel hydride or nickel-lanthanum-lithium), an electrolyte – potassium hydroxide cathode – nickel oxide. New metal hydride compound sufficiently stable for use in batteries, developed in 1980 year. Since the late 1980s, the NiMH batteries are constantly improved, mainly on the stored energy density. Battery Parameters: theoretical energy capacity of 300 Wh/kg, the specific power consumption of about 60-72 Wh/kg, the specific energy density of about 150 Wh/l, emf 1.25 V, a life of about 300-500 charge/discharge cycles , self-discharge up to 100% per year (the old battery types). In the nickel-metal hydride battery "Krona" type, as a rule – the initial voltage of 8.4 V, the voltage gradually reduced to 7.2 V, and then, when the battery power is exhausted, the voltage decreases rapidly. This type of battery is designed to replace the nickel-cadmium batteries. Nickel-metal hydride batteries are approximately 20% more capacity with the same dimensions, but a shorter life of 200 to 300 charge/discharge cycles. Self-discharge is about 1.5-2 times higher than that of nickel-cadmium batteries.
Nickel-metal hydride batteries with a low self-discharge (the low self-discharge nickel-metal hydride battery, LSD NiMH), were introduced by Sanyo in November 2005 under the brand Eneloop brand. This type of battery has a reduced self-discharge, and therefore has a longer shelf life as compared to conventional NiMH. Batteries are sold as "ready to use" or "pre-charged" and positioned as a replacement for alkaline batteries. Nickel-metal hydride batteries with a low self-discharge typically have much lower internal resistance than conventional batteries, the NiMH. It affects very positively in devices with high current consumption: a more stable voltage, reduced heat dissipation, especially in the rapid charge/discharge mode, higher efficiency, the ability to high pulsed current output (example: camera flash is charging is faster), the possibility of continuous operation in devices low power consumption (example: remote controls, watches). Applications: replacement of the standard electrochemical cell electric vehicles, defibrillators, rocket and space technology, independent power supply system, radio equipment, lighting equipment. Ni-MH-battery design. Positive electrode: nickel oxide NiOOH. Negative electrode: metal alloy (M), which can reversibly absorb hydrogen (forming hydride MH) and desorb it (examples of alloys: LaNi5; TiFe; Mg2Ni). Electrolyte: 26-31% aqueous KOH soluion. Electrochemical system: (-) MH | KOH | NiOOH (+). Electrochemical processes.The electrode reaction at the positive nickel oxide: Ni(OH)2 + OH- NiOOH + H2O + e- (charge), NiOOH + H2O + e- Ni(OH)2 + OH- (discharge) (E0 = 0.49 B ). The electrode reaction at the negative electrode metal having absorbed hydrogen is converted into metal hydride: M + H2O + e- MH + OH- (charge), MH + OH- M + H2O + e- (discharge) (E0»-0.9 B). The general reaction in Ni-MH battery is recorded as follows: Ni(OH)2 + M NiOOH + MH (charge), NiOOH + MH Ni(OH)2 + M (discharge). Scheme of Ni-MH-battery below:
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