1 / 71

Galvanic Cells - INTRODUCTION

Galvanic Cells - INTRODUCTION. Energy sources How did the battery business start? History of batteries makes history of electric energy. Galvanic Cell. As ELECTROCHEMICAL DEVICE : Electrode reactions Thermodynamics and kinetics Properties of Materials. As ENERGY SOURCE :

kasi
Download Presentation

Galvanic Cells - INTRODUCTION

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Galvanic Cells - INTRODUCTION • Energy sources • How did the battery business start? • History of batteries makes history of electric energy Galvanic Cell As ELECTROCHEMICAL DEVICE : Electrode reactions Thermodynamics and kinetics Properties of Materials As ENERGY SOURCE : Position on energy market Power supply Technology & Economy WUT - MESC - Galvanic Cells II

  2. Electrical power generation • Fuel – combustion – heat effect – mechanical energy – generating electricity CHEMICAL ENERGY indirectly into ELECTRICAL • Renewable energy source ( wind, water, geothermal) – transformation of work to electric energy • Galvanic, fuel, fotovoltaic cells CHEMICAL ENERGY directly into ELECTRICAL WUT - MESC - Galvanic Cells II

  3. Chemical substances in electrodes Expressed as Q Electrode Reactions Expressed as U Energy = U . Q Stream of reagents Energy = U . Q DIFFERENT CELLS • galvanic cells – primary and secondary • Fuel cells Electrode Reactions Expressed as U ISOLATED PORTABLE/TRANSPORTABLE INDEPENDENT FORM ELECTROENERGETICAL NETWORK WUT - MESC - Galvanic Cells II

  4. Some milestones in history 1780 L. Galvani – „animal electricity” 1800 A. Volta – pile (battery of zinc and silver discs, separated by cloth wet with salty solution) 1866 G. Leclanche – zinc – MnO2 cathode battery 1859 G. Plante’ – lead acid accu made of Pb plates, 1881 – Faury et al – pasted plates instead of solid Pb WUT - MESC - Galvanic Cells II

  5. Transformation from isolated current sources to electrical network • Electromagnetic induction – discovered by Faraday about 1840 • Electromechanical generator – Siemens about 1857 • T. A . Edison : electric bulb 1879, lighting system in NY, Ni-Fe accumulator • DC contra AC – Edison contra Westinghouse, first big power plant in America – Niagara Falls – advantages of supplying energy with AC WUT - MESC - Galvanic Cells II

  6. Electrical circuits with batteries • Management of voltage and current – connecting the batteries • Ohm’s law in simple DC circuit : external resistance (load),internal resistance( ohmic drop on battery components), polarisation resistance (ohmic drop on reaction) E = I ( Rinter + Rpol + Rload) • Energy and power Energy = Q ∙U = I ∙ t ∙ U = (m / k) ∙U Power = energy produced/consumed in time unit WUT - MESC - Galvanic Cells II

  7. Electrode potential • φ= φo + RT/nF ln ( aMe / aMe(n+) ) • Standard potential at unit activity of particles - φo • + deviation from standard due to non-unit activity (concentration) • Can not be measured directly Electrode reaction • Transport of charge or charge and mass over phase boundary electrode – electrolyte • Phases : electrode = fragment of condensed phase electronically conductive electrolyte = ionically conducting „space” Observed effects of electrode reaction : • Change of oxidation grade of an atom in a molecule / ion in solution • Accompanying changes : creation / decomposition of a phase changes in phase structures WUT - MESC - Galvanic Cells II

  8. Potential φox Anodic reaction Ared →Box + ne- Potential φred Cathodic reaction Cox + n e- →Dred Overall cell reaction A + B = C + D With E = Δφ Electromotoric force E comes from change in free enthaply of the overall reaction, Also combining the ΔG with electrical equivalent of energy E = -ΔG /nF And defining Eo = ΔG o/nF for standard conditions we get Nernst equation : E = Eo – RT / nF ln K where K – equilibrium constant of reaction ABCD WUT - MESC - Galvanic Cells II

  9. Signs + / - in cells - convention More negative potential on left side : Zn = Zn2+ + 2e φ = - 0.76 V Less negative to the right : Cu = Cu2+ + 2e φ = 0.34 V formal scheme for the cell External connection / Zn / Zn SO4aq // CuSO4 aq / Cu / external connection Sign - // sign + But ..... WUT - MESC - Galvanic Cells II

  10. Cond. matrix Redox active electrolyte Structure and functions of electrodes A/ metallic reactive electrodes (deposition-dissolution, formation of compounds on the surface) Reagent and current collector(two-in-one) Charge and mass transport – on the surface B/ inert electrodes metalls, graphite, semiconductors Current collector, not a redox reagent Charge and mass transport – on the surface C/ multi-function, multi-component electrodes electroactive component (often insulator) electronically conducting matrix other additives with special functions Charge and mass transport – on triple-contact sites WUT - MESC - Galvanic Cells II

  11. Various types of batteries WUT - MESC - Galvanic Cells II

  12. Specyfic energy - Energy density WUT - MESC - Galvanic Cells II

  13. Typical battery application WUT - MESC - Galvanic Cells II

  14. WUT - MESC - Galvanic Cells II

  15. Zn/MnO2 Cells • Leclanche type – electrolytes lightly acidic or neutral: anodic reaction – product: Zn salts soluble in the electrolyte ( NH4Cl, NH4OH, ZnCl2 → complexes of Zn with OH- and Cl- • Alkaline – electrolyte: concentrated KOH: anodic reaction – product: solid ZnO – the composition ot the electrolyte does not change • Different anodic mechanism → different yields of the cells : in alkaline cells the maximum current density is higher WUT - MESC - Galvanic Cells II

  16. Zn electrode and redox cycling • Solid Zn anode : Zn – 2e-→ Zn2+ in solution + 2e- → Zn as powder, needles (→ due to specyiic features of electrocrystallization of metals) Volumen of anode ↑ electrical contact within the anode ↓ • Powder Zn anode : Zn – 2e-→ ZnO ( in OH-solution) + 2e- → Zn as powder discharge (work) charge • Zn metallurgical foil - 100% material as energy • complex structure (Zn + conducting matrix + glue) - part of electrode „useless” as energy source WUT - MESC - Galvanic Cells II

  17. MnO2 cathode • MnIVO2 + H2O ↔ MnIIIO(OH) + OH- (other compounds of MnIII possible) • OH- ion takes part in the anodic reaction – formation of Zn complexes • At higher load (high current density) possible limitation of anode kinetics due to low concentration of comlexing ions • Valid for Leclanche type ( Zn complex salts soluble in the electrolyte) WUT - MESC - Galvanic Cells II

  18. Cells with Zn anode WUT - MESC - Galvanic Cells II

  19. Zn - air • A : Zn → Zn2+ (as ZnO) + 4e- • C : O2 + 2 H2O + 4e- → 4 OH- EMF = 1.65 V • Cathode reaction on inert catalytic electrode ( graphite + catalyst + binder) • Oxygen supply forced by underpressure in cathode space • Slow kinetics of oxygen electrode – main limitation for current value • Parasitic processes : Zn + O2 OH-+ CO2 loss of water WUT - MESC - Galvanic Cells II

  20. Electric vehicles • „zero-emission” buses and vans on tests in USA and Germany • Repleceable anodic casette of Zn with KOH (gelled) • Ca. 200 Wh/kg and 90 W/kg at 80% d.o.c. • Supercapacitor in hybrid system to boost accelaration • External regeneration of anodes WUT - MESC - Galvanic Cells II

  21. Zn/MnO2 cells WUT - MESC - Galvanic Cells II

  22. Zn/MnO2 cells WUT - MESC - Galvanic Cells II

  23. How to get „more” from a single cell? • Redox potential for Me – Men+ couples • Apply special conditions of discharge • Eliminate water from cells Reserve cells one-time discharge non-aqueous solutions synthesis in inert atmosphere WUT - MESC - Galvanic Cells II

  24. Reserve (activated) cells inactive electrolyte : -closed in a vessel -solid salt to be molten dry electrodes • Separated elements – • Signal to make contact electrolyte – electrodes : closing the circuit inside the cell • Activation on signal (decision) or by event (water flow, emergency) • No or poor activity if energy demand intermittent • Very long storage time (no parasitic reactions and self-discharge) • Energy supply – short time, but high current densities WUT - MESC - Galvanic Cells II

  25. Reserve cells - examples • Mg anode reactions Mg + 2 H2O Mg(OH)2 + 2H+ + 2e Mg(OH)2 + H2 (Mg covered with MgO Mg open to water, layer, proton recombinates no contribution to current with OH from cathode space) drawned from the cell • Both reactions take place, H2 evolution wastes part of electrode, but • Gas bubbling → intensive stirring →quick transport →high current WUT - MESC - Galvanic Cells II

  26. Reserve cells – examples cont. • Cathodes in Mg cells : • 2 AgCl + 2e → Ag + 2 Cl- • 2 CuCl + 2e → Cu + 2 Cl- • other simple salts : PbCl2 , CuSCN, Cu2I2 • Overall reaction : Mg + PbCl2 = MgCl2 + Pb • Electrolytes : sea-water, simple salts specific for best cathode rate • construction: composite cathodes, mechanical separation of electrodes, soakable separators for electrolyte WUT - MESC - Galvanic Cells II

  27. Water and gas activated batteries - applications • Air-sea rescue systems • Sono and other buoys • Lifeboat equipment • Diverse signals and alarms • Oceanographic and meteo eq. • And many others, including military WUT - MESC - Galvanic Cells II

  28. Molten salts and thermal batteries Main parts of a thermal battery Anodes : Li alloys : Li(20)Al, Li(40)Si (melt higher than Li – 181 and 600/7090C resp.) Cathodes : Ca, K, Pb chromates, Cu, Fe, Co sulfides, V2O5, WO3 Electrolyte: molten LiCl-KCl eutectic 3520C Combination with bromides Thermal dissociation KCl = K+ + Cl-, high conductivities, simple reaction mechanism WUT - MESC - Galvanic Cells II

  29. Thermal batteries – applications • Pyrotechnic heat source – squib, burned serves as inter-cell conductor • Insulation – ceramic, glass, polymers – depends on time of discharge (salt must be kept molten !) • Voltages – single OCV : 1.6 V (Li/FeS2) , to ca. 3 V (Ca/K2Cr2O7) • Activated life-time : minutes, in special constructions hours • Energy density : 2 – 35 Wh/kg • High currents possible • Applications – mainly military WUT - MESC - Galvanic Cells II

  30. Solid electrolyte cell Na-S Anode Na → Na+ Cathode xS → Sx2- , x 3~5 Overall 2Na + xS → Na2Sx OCV = 2.07 V Temperature 310 – 350oC sulphur Tmelt = 118, boil= 444oC β-alumina Na2O∙11Al2O3 , conducts Na ions σ300 C ca 0.5-0.1 S/cm WUT - MESC - Galvanic Cells II

  31. Insulated enclosure Cooling system Electrical networking Heat distribution heaters Solid electrolyte cell Na-S • Can be used as rechargeable cell • Applications : stationary energy storage, motive power • Working with high-temperature cells: warm-up on start keep warm at intervals in operation manage excessive heat during operation (ohmic and reaction) • Construction of stacks : electrical and heat management WUT - MESC - Galvanic Cells II

  32. Stationary energy storage Na-S system WUT - MESC - Galvanic Cells II

  33. Lithium – iodine solid electrolyte cell • Anode : Li → Li+ + 2e • Cathode : nI2∙P2VP + 2e → (n-1)I2P2VP + 2 I- (poly-2-vinylpyridine) • Overall : 2Li + I2 → 2 LiI • LiI thin layer on contact between Li and cathode, ionically conducting • OCV ca 2.8 V • Discharge rates 1 – 2 μA/cm2 (very low) WUT - MESC - Galvanic Cells II

  34. Primary and secondary cells - basic WUT - MESC - Galvanic Cells II

  35. Secondary cells - basic • Energy density from < 20 (Pb) , 35 (NiCd), 75 (NiMeH) to 150 Wh/kg (Li-ion) • Cycling life 220-700 (Pb) 500 – 2000 (Ni-Cd) • Voltage 2 V (Pb) 1.2 V (Ni-Cd) • Flat discharge profiles • Poor charge retention (shelf life of Ni-Cd – fully discharged, Pb must be kept charged because of sulfation of plates) • Vented constructions – evolution of H2 / O2 • Tight closure of cells – oxygen recombination ( at end of charge oxygen developing in anodic process diffuses to cathode and oxidates surplus of cathode material – no overpressure : • Valve-Regulated-Lead-Acid sealed Ni-Cd WUT - MESC - Galvanic Cells II

  36. Lead-acid accumulator WUT - MESC - Galvanic Cells II

  37. WUT - MESC - Galvanic Cells II

  38. Phenomena in discharge cycle • CH2SO4 • PbSO4 – insulator ( ca. 1010Ώcm) • Vmol PbSO4 > Vmol Pb, PbO2 worse porosity diffusion of the electrolyte into the structure impaired R int What happens with: current density at U = const ? Voltage at I = const. ? WUT - MESC - Galvanic Cells II

  39. WUT - MESC - Galvanic Cells II

  40. WUT - MESC - Galvanic Cells II

  41. Alkaline accumulators • Ni –Cd , Ni – Fe, Ni – MeH ( 1.2V) Ag – Zn ( 1.5V) Ni – Zn (1.6V) • Cathode Ni NiIII OOH + H20 + e- Ni(OH)2 + OH- • Anode Cd Cd + 2(OH-) Cd(OH)2 + 2e- • Ag-Zn : Ag2O + H2O + 2e 2Ag + 2 OH- Zn + 2(OH-) Zn(OH)2 + 2e WUT - MESC - Galvanic Cells II

  42. Ni-Cd accumulator WUT - MESC - Galvanic Cells II

  43. (further electrolysis after charging effects in evolution of O2) ((further electrolysis after charging effects in evolution of H2) WUT - MESC - Galvanic Cells II

  44. Oxygen and hydrogen formation in cells • Reactions possible in water solution • Equilibrium potentials : E (H+/H2) = 0V , E (OH-/O2) = 0.4 V • BUT – overpotentials due to phenomena at gas-solid electrode phase boundary make true potentials higher • For different metals the hydrogen evolution potential grows from: Pt - Ni - Ag - Zn - Cd - Pb (and compounds) • Still, at the end of charge/discharge cycle co-evolution of gases in cells occurs • In effect: overpressure inside the cell, - H2 i O2 • „oxygen recombination” – electrodes not equivalent in charge, ex. QCd > QNi WUT - MESC - Galvanic Cells II

  45. Basic secondary cells WUT - MESC - Galvanic Cells II

  46. Compresed powder NiSO4→Ni(OH)2 CdSO4 →Cd(OH)2 Encapsulated in steel/Ni pocket Sintered plate Porous Ni plate Impregnated with Ni , Cd salts Transformed to hydroxides „in situ” Technology of electrode masses in Ni-Cd • Electrodes prepared in discharged state : Ni(OH)2 and Cd(OH)2as • Additives: graphite ,”-” mass – Fe+ Ni (→ Cd crystallization) • Formation of plates : several charge-discharge cycles • Assembly and hermetic closure • Separators – ionic conductivity and oxygen diffusion (thickness ca0.2 mm) • For O2 recombination higher capacity of „-” mass (Cd) – fully charged Ni mass – O2evolution – diffusion – Cd oxidised to CdO, no possiblity of H2 formation WUT - MESC - Galvanic Cells II

  47. Nickel/Metal Hydride • Anode : 2 NiO(OH) + 2 H2O + 2e → Ni(OH)2 + 2 OH- • Cathode : H2 + 2 (OH-) → 2 H2O + 2e • Hydrogen stored as hydride in metallic phase, • Capacity of metal hydride electrode c. 0.4 Ah/g -- comparable with Cd and Ni sintered plates 0.3-0.5 Ah/g WUT - MESC - Galvanic Cells II

  48. Scheme for reaction mechanism at Me electrode overcharge charge discharge H2O O2 H2O OH- OH- H2O Hads Hads Hads H2 Me-H Reversibility of electrode reaction, catalytic for H adsorption and H-O2 recombination WUT - MESC - Galvanic Cells II

  49. Hydrogen absorbing alloys • A – metal forming stable hydrides • B – weak hydrides, catalyst, resistance to corrosion, control Hads pressure • Nickel - catalyst for H2 dissociation,, regulator for Zr, Ti, V hydride formation, WUT - MESC - Galvanic Cells II

  50. Some details on production of alloys • Ni mass – traditional, new technologies for MeH electrode powder • Ovonic alloy – example : main components : Zr-Ti-V-Ni + Cr, Mn, Co, Fe... • Preparative technics: electric arc or inductive oven, Ar atmosphere • Production of powder : hydrogenation of cast alloy (volume expansion = crushing of a piece), followed by mechanical pulverisation • Sintered plates : MeH powder + Ni, Ni(CO)5 + resin → pressing and sintering under vacuum WUT - MESC - Galvanic Cells II

More Related