Batteries (Cells With Spontaneous Redox Reactions)

Batteries (cells with spontaneous redox reactions) Dry Cell X Zn2+(aq) + 2e- anode: Zn(s) cathode: 2MnO2(s) + 2NH4+(aq) + 2e- Ecell = 1.5 V Problems: X Mn2O3(s) + 2NH3(aq) + H2O(l) (Indep. of cell size. Current output depends on cell size. As cell vol. increases, Rint decreases and Icell increases; V=IR) Can’t recharge because Zn2+ diffuses away. Acidic NH4+ corrodes Zn, leading to poor shelf life. Alkaline Battery uses KOH instead of NH4+ and ZnCl2 X ZnO(s) + + H2O(l) + 2e- anode: Zn(s) + 2OH-(aq) cathode: 2MnO2(s) + H2O(l) + 2e- X Mn2O3(s) + 2OH-(aq) Ecell = 1.54 V lasts longer, but slightly more expensive because of the construction Mercury Battery X ZnO(s) + H2O(l) + 2e- anode: Zn(s) + 2OH-(aq) cathode: HgO(s) + H2O(l) + 2e- X Hg(l) + 2OH-(aq) Ecell = 1.34 V More current for a given size (used for calculators, watches, hearing aids, cameras). More expensive because of the Hg. Ecell constant during discharge because cell reaction doesn’t change cell electrolyte composition. Not rechargeable. Disposal of Hg is also a problem. Nickel-Cadmium (nicad) & Nickel-Metal Hydride (NiMH) Batteries anode (nicad): Cd(s) + 2OH-(aq) X Cd(OH)2(s) + 2e- (NiMH) MH(s) + OH-(aq) X M(s) + H2O(l) + e- cathode: NiO(OH)(s) + H2O(l) + e- X Ni(OH)2(l) + OH-(aq) Ecell = 1.2-1.4 V (fairly constant during discharge) Products adhere to electrodes - reactions can be reversed by charging. Disposal of cadmium a problem (nicad). Used in calculators, cordless appliances and portable power tools. Lithium Ion Battery X Li+ (in polymer) + e- anode: Li(in polymer) cathode: Li+ (in CoO2) + e- + CoO2 X LiCoO2 Rechargeable, long life, higher voltage (Ecell = 3.4 V) . Used in portable computers, cellular phones, cameras. Lead Storage Battery X PbSO4(s) + H+ + 2e- anode (porous Pb): Pb(s) + HSO4-(aq) cathode (PbO2): PbO2(s) + 3H+(aq) + HSO4-(aq) + 2e- Ecell = 2.0 V Car battery has six cells in series to provide 12 V. Rechargeable, since PbSO4 sticks to electrodes. X PbSO4(s) + 2H2O(l) Fuel Cells Involve the direct conversion of chemical energy Y electrical energy (~70% efficiency) (otherwise, chemical Y heat Y electrical energy is only ~ 30% efficient) Reactants are continuously supplied. Example: H2 fuel X 2H2O(l) + 2e- anode: H2(g) + 2OH-(aq) cathode: O2(g) + 2H2O(l) + 4e- X 4OH- E0cell = 1.23 V (Ecell ~ 0.7 V) The Direct Fuel Cell installation on campus works in the following way. Electrolysis - reactant-favored reactions H2O (acidic solution) X O2(g) + 4H+(aq) + 4e- E0ox = -1.23 V anode: 2H2O(l) cathode: 2H+(aq) + 2e- X H2(g) ____________________________ 2H2O(l) X O2(g) + 2H2(g) overall: E0red = 0.00 V E0 = -1.23 V H2O (basic/neutral solution) X O2(g) + 2H2O(l) + 4e- anode: 4OH- cathode: 2H2O(l) + 2e- X H2(g) + OH-(aq) E0red = -0.83 V ____________________________ 2H2O X O2(g) + 2H2(g) E0 = -1.23 V overall: E0ox = -0.40 V Must have mobile ions in solution so that charge balance can be maintained. Electrolysis of 1.0 M CuSO4 at pH = 5.0, PH2 =1 atm, PO2 = 1 atm possible cathode reactions Cu2+(aq) + 2e2H+(aq) + 2e- X Cu(s) X H2(g) Ered = 0.34 V - (0.0592/2)log1/[Cu2+] = 0.34 V Ered = 0.00 V - (0.0592/2)logPH2/[H+]2 = -0.296 V Cu2+ is reduced (Cu will plate out on cathode) because it has the more favorable Ered. possible anode reactions 2SO422H2O(l) X S2O82-(aq) + 2e- E0ox = -2.00 V X O2(g) + 4H+(aq) + 4e- Eox = -1.23 V - (0.0592/4)logPO2[H+]4 = -0.93 V O2 is produced at the anode because it has the more favorable Eox. Electrolysis of 1.0 M ZnSO4 at pH = 5.0, PH2 =1 atm, PO2 = 1 atm possible cathode reactions Zn2+(aq) + 2e- X Zn(s) Ered = -0.76 V - (0.0592/2)log1/[Zn2+] = -0.76 V 2H+(aq) + 2e- X H2(g) Ered = 0.00 V - (0.0592/2)logPH2/[H+]2 = -0.296 V H2 should form (it has the more favorable Ered), but Zn plates out due to overvoltage. For Zn: Ered = -0.76 V - overvoltage(Zn) For H2: Ered = -0.296 V - overvoltage(H2) Overvoltage is a kinetic effect. Electron transfer is inhibited at the electrode. It is usually small for the deposition of metals, but often large (~1 V) for the evolution of gases. This explains why you can plate out Cr3+ and Ni2+, and produce Cl2(g) in the electrolysis of NaCl(aq), even though they have unfavorable half-cell potentials.