In the alkaline version of the dry cell battery, the sol i d NH 4 Cl i s replaced w i th KOH or NaOH. In th i s case the half-react i ons can be approx i mated as follows : The alkal i ne dry cell lasts longer ma i nly because the z i nc anode corrodes less rap i dly under bas i c cond i t i ons than under ac i d i c cond i t i ons. Other types of useful batter i es i nclude the silver cell, wh i ch has a Zn anode and a cathode that employs Ag 2 O as the ox i d i z i ng agent i n a bas i c env i ronment. Mercury cells, often used i n calculators, have a Zn anode and a cathode i nvolv i ng HgO as the ox i d i z i ng agent i n a bas i c med i um (see F i g. 17.15). An espec i ally i mportant type of battery i s the nickel–cadmium battery, i n wh i ch the electrode react i ons are As i n the lead storage battery, the products adhere to the electrodes. Therefore, a n i ckel– cadm i um battery can be recharged an i ndefin i te number of t i mes. Cathode react i on : N i O 2 2H 2 O 2e ¡ N i 1 OH 2 2 2OH Anode react i on : Cd 2OH ¡ Cd 1 OH 2 2 2e Cathode react i on : 2MnO 2 H 2 O 2e ¡ Mn 2 O 3 2OH Anode react i on : Zn 2OH ¡ ZnO H 2 O 2e Cathode react i on : 2NH 4 2MnO 2 2e ¡ Mn 2 O 3 2NH 3 H 2 O Anode react i on : Zn ¡ Zn 2 2e now wasted energy could be used to produce electr i c i ty, th i s would have tremendous fiscal i mpl i cat i ons. Another prom i s i ng appl i cat i on of TPV technology i s for cars w i th hybr i d energy sources. For example, an exper- i mental electr i c car bu i lt at Western Wash i ngton Un i vers i ty uses a 10-kW TPV generator to supplement the bat- ter i es that serve as the ma i n power source. Pro j ect i ons i nd i cate that TPV de- v i ces could account for $500 m i ll i on i n sales by 2005, ma i nly by subst i tut- i ng TPV generators for small d i esel- powered generators used on boats and by the m i l i tary i n the field. It appears that th i s technology has a hot future.
812 Chapter Seventeen Electrochemistry Fuel Cells A fuel cell i s a galvanic cell for which the reactants are continuously supplied. To i llus- trate the pr i nc i ples of fuel cells, let’s cons i der the exotherm i c redox react i on of methane w i th oxygen : CH 4 1 g 2 2O 2 1 g 2 ¡ CO 2 1 g 2 2H 2 O 1 g 2 energy Cathode (steel) Insulat i on Anode (z i nc conta i ner) Paste of HgO (ox i d i z i ng agent) i n a bas i c med i um of KOH and Zn(OH) 2 FIGURE 17.15 A mercury battery of the type used in calculators. CHEMICAL IMPACT Fuel Cells for Cars Y our next car may be powered by a fuel cell. Unt i l recently only affordable to NASA, fuel cells are now ready to become pract i cal power plants i n cars. Many car compan i es are test i ng veh i cles that should be commerc i ally ava i lable by 2004 or 2005. All of these veh i cles are powered by hy- drogen–oxygen fuel cells (see F i g. 17.16).
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