Cathode materials

This reaction has a positive free energy of 422.2 kj (100.9 kcal) at 25°C and hence energy has to be suppHed in the form of d-c electricity to drive the reaction in a net forward direction. The amount of electrical energy required for the reaction depends on electrolytic cell parameters such as current density, voltage, anode and cathode material, and the cell design. 

Overvoltages for various types of chlor—alkali cells are given in Table 8. A typical example of the overvoltage effect is in the operation of a mercury cell where Hg is used as the cathode material. The overpotential of the H2 evolution reaction on Hg is high hence it is possible to form sodium amalgam without H2 generation, thereby eliminating the need for a separator in the cell. 

Uses. Silver fluoride has found many laboratory and special industrial appHcations. It is used as a soft (nHld) fluorinating agent for selective fluorination (7—17), as a cathode material in batteries (qv) (18), and as an antimicrobial agent (19). Silver fluoride is commercially available from Advance Research Chemicals, Inc., Aldrich Chemicals, Cerac Corp., Johnson/Matthey, PCR, Atochem, and other sources in the United States. The U.S. price of silver fluoride in 1993 was 1000— 1400/kg and the total U.S. consumption was less than 200 kg/yr. 

The cathode material is stainless steel. The lead produced by this method analyzes 99.99 + %. The overall power consumption is less than 1 kWh/kg of lead, so that the electrolytic process for treating spent batteries has much less of an environmental impact than the conventional pyrometaUurgical process. 

Batteries. Many batteries intended for household use contain mercury or mercury compounds. In the form of red mercuric oxide [21908-53-2] mercury is the cathode material in the mercury—cadmium, mercury—indium—bismuth, and mercury—zinc batteries. In all other mercury batteries, the mercury is amalgamated with the zinc [7440-66-6] anode to deter corrosion and inhibit hydrogen build-up that can cause cell mpture and fire. Discarded batteries represent a primary source of mercury for release into the environment. This industry has been under intense pressure to reduce the amounts of mercury in batteries. Although battery sales have increased greatly, the battery industry has aimounced that reduction in mercury content of batteries has been made and further reductions are expected (3). In fact, by 1992, the battery industry had lowered the mercury content of batteries to 0.025 wt % (3). Use of mercury in film pack batteries for instant cameras was reportedly discontinued in 1988 (3). 

Nickel—rhenium ahoys containing thoria or other additives have been developed for use as cathodes on electrovacuum devices. Rhenium is found to improve the strength properties substantiahy. At 1000°C the strength of a 90 wt % Ni—10% Re ahoy exceeds the strength of a Ni—V cathode material by 90%. Rigidity is exceeded by a factor of 150 to 200%. 

The principal use of titanium sulfides is as a cathode material ia high efficieacy batteries (11). In these appHcations, the titanium disulfide acts as a host material for various alkafl or alkaline-earth elements. 

Cylindrical alkaline cells are made in only a few standard si2es and have only one important chemistry. In contrast, miniature alkaline cells are made in a large number of different si2es, using many different chemical systems. Whereas the cylindrical alkaline batteries are multipurpose batteries, used for a wide variety of devices under a variety of discharge conditions, miniature alkaline batteries are highly speciali2ed, with the cathode material, separator type, and electrolyte all chosen to match the particular appHcation. 

Reserve batteries have been developed for appHcations that require a long inactive shelf period foUowed by intense discharge during which high energy and power, and sometimes operation at low ambient temperature, are required. These batteries are usually classified by the mechanism of activation which is employed. There are water-activated batteries that utilize fresh or seawater electrolyte-activated batteries, some using the complete electrolyte, some only the solvent gas-activated batteries where the gas is used as either an active cathode material or part of the electrolyte and heat-activated or thermal batteries which use a soHd salt electrolyte activated by melting on appHcation of heat. 

Ma.nga.neseDioxide. Graphite plates used as anodes in this process are coated with MnO during electrolysis. The anodes are removed from the solution periodically and the MnO is removed by mechanical methods. Graphite can also be used as the cathode material. Titanium is used as anode materials where high quaHty MnO is desired. 

Mild steel cathodes are used extensively in chlor-alkah and chlorate cells. Newer activated cathode materials have been developed that decrease cell voltages about 0.2 V below that for cells having mild steel cathodes. Some activated cathodes have operated in production membrane cells for three years with only minor increases in voltage (17). Activated cathodes can decrease the energy consumption for chlorine—caustic production by 5 to 6.5%. 

Mercury Cells. The cathode material ia mercury cells, mercury [7439-97-6] Hg, has a high hydrogen overvoltage. Hydrogen evolution is suppressed and sodium ion reduction produces sodium amalgam [11110-32-4J, HgNa. 

Selection. The widely used cathode materials iaclude Hg, Pb, Al, Zn, Ni, Fe, Cu, Sn, Cd, and C. Because of mechanical iaconvenience, mercury is not an attractive electrode material for large cells. The preferred material is lead or an amalgam. Because Pb is soft and has a tendency to deform, however, it presents some mechanical problems. The problems can be overcome by hot dip or electroplating on steel, copper, or other rigid base material. 

Product Reactants Company Cell type Anode material Cathode material... 

A simple electrochemical flow-through cell with powder carbon as cathodic material was used and optimized. The influence of the generation current, concentration of the catholyte, carrier stream, flow rate of the sample and interferences by other metals on the generation of hydrogen arsenide were studied. This system requires only a small sample volume and is very easily automatized. The electrochemical HG technique combined with AAS is a well-established method for achieving the required high sensitivity and low detection limits. 

In spite of the possibility of cathodic corrosion discussed in relation to Eq. (2-56), practical experience has shown that carbon steel is a suitable material for impressed current cathodes. Stress corrosion of the cathode material does not have to be considered because of the strong cathodic polarization as shown in Fig. 2-18. 

Fig. 5.24. The electrochemical properties of the galvanic cell shown have been studied under high pressure shock compression. The cell is composed of anode, electrolyte, and cathode materials studied in independent applications of thermal batteries.

The long-wavelength limit which depends on the cathode material is ca. 2 = 650 nm m the red region of the spectrum. If this does not suffice for the determination an antimony-alkaline metal alloy is employed as the cathode material [56—58]. 

Another application of the electrolysis of tantalum and niobium in fluoride melts is in the preparation of intermetalic compounds as a result of the interaction between the electrochemically precipitating metal and the cathode material. Based on an investigation of the electrochemical reduction of K2TaF7 or K2NbF7 in a LiF - NaF melt on nickel cathodes, Taxil and Qiao [565] determined the appropriate conditions for the formation of TaNi3 or NbNi3 in the form of stable phases in the bulk of the obtained layer. 

---