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Concept 06 / 07Electrochemistry

Batteries and Fuel Cells: Dry Cell, Mercury Cell, Lead Storage Battery, Ni-Cd and H2-O2 Fuel Cell

Primary and secondary batteries with their electrode reactions: Leclanche dry cell, mercury cell, lead storage battery (discharge and recharge), nickel-cadmium cell and lithium-ion cell, and fuel cells with the hydrogen-oxygen cell, its efficiency and advantages.

Figure 1
A battery is one or more galvanic cells used as a portable source of electrical energy. A useful battery should be light, compact, and give a nearly constant voltage over its life. Primary batteries (not rechargeable; the reaction occurs once). Leclanche (dry) cell: anode a zinc container; cathode a carbon (graphite) rod surrounded by powdered MnO2\mathrm{MnO_{2}} and carbon; electrolyte a moist paste of NH4Cl\mathrm{NH_{4}Cl} and ZnCl2\mathrm{ZnCl_{2}}. Anode: Zn→Zn2++2e−\mathrm{Zn \rightarrow Zn^{2+} + 2e^{-}}. Cathode: MnO2+NH4++e−→MnO(OH)+NH3\mathrm{MnO_{2} + NH_{4}^{+} + e^{-} \rightarrow MnO(OH) + NH_{3}} (Mn from +4+4 to +3+3). Ammonia complexes with Zn2+\mathrm{Zn^{2+}} as [Zn(NH3)4]2+\mathrm{[Zn(NH_{3})_{4}]^{2+}}. Voltage about 1.5 V1.5\,\mathrm{V}. Mercury cell (hearing aids, watches): anode zinc-mercury amalgam, cathode a paste of HgO and carbon, electrolyte KOH and ZnO paste. Anode: Zn(Hg)+2OH−→ZnO(s)+H2O+2e−\mathrm{Zn(Hg) + 2OH^{-} \rightarrow ZnO(s) + H_{2}O + 2e^{-}}. Cathode: HgO+H2O+2e−→Hg(l)+2OH−\mathrm{HgO + H_{2}O + 2e^{-} \rightarrow Hg(l) + 2OH^{-}}. Overall: Zn(Hg)+HgO→ZnO+Hg\mathrm{Zn(Hg) + HgO \rightarrow ZnO + Hg}; constant voltage of about 1.35 V1.35\,\mathrm{V} throughout its life, since no ion concentration appears in the overall reaction. Secondary batteries (rechargeable; reversing the current regenerates the reactants). Lead storage battery (vehicles, inverters): anode lead, cathode a grid of lead packed with PbO2\mathrm{PbO_{2}}, electrolyte about 38%38\% H2SO4\mathrm{H_{2}SO_{4}} (density 1.294 g mL−11.294\,\mathrm{g\,mL^{-1}}). Discharge, anode: Pb+SO42−→PbSO4+2e−\mathrm{Pb + SO_{4}^{2-} \rightarrow PbSO_{4} + 2e^{-}}. Discharge, cathode: PbO2+SO42−+4H++2e−→PbSO4+2H2O\mathrm{PbO_{2} + SO_{4}^{2-} + 4H^{+} + 2e^{-} \rightarrow PbSO_{4} + 2H_{2}O}. Overall: Pb+PbO2+2H2SO4→2PbSO4+2H2O\mathrm{Pb + PbO_{2} + 2H_{2}SO_{4} \rightarrow 2PbSO_{4} + 2H_{2}O}. Each cell gives about 2 V2\,\mathrm{V}; six in series give 12 V12\,\mathrm{V}. As it discharges, sulfuric acid is consumed and the density of the electrolyte falls, which indicates the state of charge. Charging reverses the reactions. Nickel-cadmium cell: Cd+2Ni(OH)3→CdO+2Ni(OH)2+H2O\mathrm{Cd + 2Ni(OH)_{3} \rightarrow CdO + 2Ni(OH)_{2} + H_{2}O}; longer life than lead batteries but more expensive. Lithium-ion batteries (phones, laptops, electric vehicles) shuttle Li+\mathrm{Li^{+}} between a graphite anode and a metal oxide cathode. Fuel cells. Galvanic cells that convert the energy of combustion of fuels (hydrogen, methane, methanol) directly into electricity, with reactants supplied continuously (see figure). In the hydrogen-oxygen fuel cell, H2\mathrm{H_{2}} and O2\mathrm{O_{2}} bubble through porous carbon electrodes (with finely divided Pt or Pd catalyst) into concentrated aqueous NaOH or KOH: Cathode: O2+2H2O+4e−→4OH−\mathrm{O_{2} + 2H_{2}O + 4e^{-} \rightarrow 4OH^{-}}. Anode: 2H2+4OH−→4H2O+4e−\mathrm{2H_{2} + 4OH^{-} \rightarrow 4H_{2}O + 4e^{-}}. Overall: 2H2+O2→2H2O\mathrm{2H_{2} + O_{2} \rightarrow 2H_{2}O}. Efficiency about 70%70\%, compared with about 40%40\% for thermal power plants, because the Gibbs energy is converted directly rather than through heat. They are pollution-free (the only product is water) and were used in the Apollo space programme, where the water was used for drinking. Key Takeaways: - Dry cell: Zn anode, MnO2\mathrm{MnO_{2}} cathode, NH4Cl\mathrm{NH_{4}Cl} paste, 1.5 V1.5\,\mathrm{V}. - Mercury cell: constant 1.35 V1.35\,\mathrm{V} (no ions in the overall reaction). - Lead storage: Pb+PbO2+2H2SO4→2PbSO4+2H2O\mathrm{Pb + PbO_{2} + 2H_{2}SO_{4} \rightarrow 2PbSO_{4} + 2H_{2}O}; 2 V2\,\mathrm{V} per cell; acid density falls on discharge. - H2\mathrm{H_{2}}-O2\mathrm{O_{2}} fuel cell: continuous supply, about 70%70\% efficient, water as product. - Pitfall: in the lead battery, PbSO4\mathrm{PbSO_{4}} forms at both electrodes during discharge. Brain-Teaser 1: How many moles of H2SO4\mathrm{H_{2}SO_{4}} are consumed when a lead storage battery delivers 2 F2\,\mathrm{F} of charge? Solution: The overall reaction transfers 22 electrons and consumes 2 H2SO42\,\mathrm{H_{2}SO_{4}}. So 2 F2\,\mathrm{F} consume 2 mol2\,\mathrm{mol} H2SO4\mathrm{H_{2}SO_{4}} (196 g196\,\mathrm{g}) and form 2 mol2\,\mathrm{mol} PbSO4\mathrm{PbSO_{4}}. Brain-Teaser 2: Why does the mercury cell give a constant voltage while the dry cell's voltage drops during use? Solution: In the mercury cell, the overall reaction Zn(Hg)+HgO→ZnO+Hg\mathrm{Zn(Hg) + HgO \rightarrow ZnO + Hg} involves only solids and a liquid, so QQ stays constant and so does EE (Nernst). In the dry cell, ions such as NH4+\mathrm{NH_{4}^{+}} and Zn2+\mathrm{Zn^{2+}} change concentration, and products accumulate at the electrodes, so the voltage falls. Brain-Teaser 3: The standard Gibbs energy of formation of liquid water is −237.2 kJ mol−1-237.2\,\mathrm{kJ\,mol^{-1}}. Find the standard emf of the H2\mathrm{H_{2}}-O2\mathrm{O_{2}} fuel cell. Solution: H2+12O2→H2O\mathrm{H_{2} + \frac{1}{2}O_{2} \rightarrow H_{2}O} transfers n=2n = 2 electrons. E∘=−ΔG∘nF=2372002×96487=1.23 VE^{\circ} = -\frac{\Delta G^{\circ}}{nF} = \frac{237200}{2 \times 96487} = 1.23\,\mathrm{V}.

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Batteries and Fuel Cells: Dry Cell, Mercury Cell, Lead Storage Battery, Ni-Cd and H2-O2 Fuel Cell — DPP

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