Electrochemistry, rechargeable batteries and fuel cells Energy topic 6 Electrochemistry, rechargeable batteries and fuel cells

Metals Normally move randomly but in an electric circuit they move orderly

Batteries Batteries also have some internal resistance as ions pass through the electrolyte The electromotive force (E) is the total energy that is made available by the chemical reactions in the cell per unit charge. As some of this energy is lost this value will be less that V

Concentration and Ecell Consider the following redox reaction: Zn(s) + 2H+ (aq) Zn2+(aq) + H2(g) E°cell = 0.76 V DG°= -nFE°cell < 0 (spontaneous) What if [H+] = 2 M? Expect driving force for product formation to increase. Therefore DG decreases, and Ecell increases How does Ecell depend on concentration?

Concentration and Ecell (cont.) Recall, in general: DG = DG° + RTln(Q) However: DG = -nFEcell -nFEcell = -nFE°cell + RTln(Q) Ecell = E°cell - (RT/nF)ln(Q) Ecell = E°cell - (0.0591/n)log(Q) The Nernst Equation

Concentration and Ecell (cont.) With the Nernst Eq., we can determine the effect of concentration on cell potentials. Ecell = E°cell - (0.0591/n)log(Q) Example. Calculate the cell potential for the following: Fe(s) + Cu2+(aq) Fe2+(aq) + Cu(s) Where [Cu2+] = 0.3 M and [Fe2+] = 0.1 M

Concentration and Ecell (cont.) • First, need to identify the 1/2 cells Fe(s) + Cu2+(aq) Fe2+(aq) + Cu(s) Cu2+(aq) + 2e- Cu(s) E°1/2 = 0.34 V Fe2+(aq) + 2e- Fe(s) E°1/2 = -0.44 V Fe(s) Fe 2+(aq) + 2e- E°1/2 = +0.44 V Fe(s) + Cu2+(aq) Fe2+(aq) + Cu(s) E°cell = +0.78 V

Concentration and Ecell (cont.) • Now, calculate Ecell Fe(s) + Cu2+(aq) Fe2+(aq) + Cu(s) E°cell = +0.78 V Ecell = E°cell - (0.0591/n)log(Q) Ecell = 0.78 V - (0.0591 /2)log(0.33) Ecell = 0.78 V - (-0.014 V) = 0.794 V

Concentration and Ecell (cont.) • If [Cu2+] = 0.3 M, what [Fe2+] is needed so that Ecell = 0.76 V? Fe(s) + Cu2+(aq) Fe2+(aq) + Cu(s) E°cell = +0.78 V Ecell = E°cell - (0.0591/n)log(Q) 0.76 V = 0.78 V - (0.0591/2)log(Q) 0.02 V = (0.0591/2)log(Q) 0.676 = log(Q) 4.7 = Q

Concentration and Ecell (cont.) Fe(s) + Cu2+(aq) Fe2+(aq) + Cu(s) 4.7 = Q [Fe2+] = 1.4 M

Concentration Cells Consider the cell presented on the left. The 1/2 cell reactions are the same, it is just the concentrations that differ. Will there be electron flow?

Concentration Cells (cont.) Ag+ + e- Ag E°1/2 = 0.80 V • What if both sides had 1 M concentrations of Ag+? • E°1/2 would be the same; therefore, E°cell = 0.

Concentration Cells (cont.) Anode: Ag Ag+ + e- E1/2 = ? V Cathode: Ag+ + e- Ag E1/2 = 0.80 V Ecell = E°cell - (0.0591/n)log(Q) 0 V 1 Ecell = - (0.0591)log(0.1) = 0.0591 V

Concentration Cells (cont.) Another Example: What is Ecell?

Concentration Cells (cont.) Ecell = E°cell - (0.0591/n)log(Q) e- 2 Fe2+ + 2e- Fe 2 e- transferred…n = 2 anode cathode Ecell = -(0.0296)log(.1) = 0.0296 V

Summary DG = DG° + RTln(Q) DG = -nFEcell Ecell = E°cell - (0.0591/n)log(Q) • None of these ideas is separate. They are all connected, and are all derived directly from thermodynamics.

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Applications using Batteries

Battery Convert stored chemical energy into electrical energy Reaction between chemicals take place Consisting of electrochemical cells Contains Electrodes Electrolyte

Electrodes and Electrolytes Cathode Positive terminal Chemical reduction occurs (gain electrons) Anode Negative terminal Chemical oxidation occurs (lose electrons) Electrolytes allow: Separation of ionic transport and electrical transport Ions to move between electrodes and terminals Current to flow out of the battery to perform work Think of questions to ask the class Different kinds of batteries, chemistry behind them Why one type over another in various applications Biggest restrictions, thing that need improving

Battery Overview Battery has metal or plastic case Inside case are cathode, anode, electrolytes Separator creates barrier between cathode and anode Current collector brass pin in middle of cell conducts electricity to outside circuit

Primary Cell One use (non- rechargeable/disposable) Chemical reaction used, can not be reversed Used when long periods of storage are required Lower discharge rate than secondary batteries  Use: smoke detectors, flashlights, remote controls Electrochemical reactions are non-reversible materials in the electrodes are utilized, therefore cannot regenerate electricity

Secondary Cells Rechargeable batteries Reaction can be readily reversed Similar to primary cells except redox reaction can be reversed Recharging: Electrodes undergo the opposite process than discharging Cathode is oxidized and produces electrons Electrons absorbed by anode

Nickel-Cadmium Battery Anode: Cadmium hydroxide, Cd(OH)2 Cathode: Nickel hydroxide, Ni(OH)2 Electrolyte: Potassium hydroxide, KOH The half-reactions are discharging: Cd+2OH- → Cd(OH)2+2e- 2NiO(OH)+Cd+2e- →2Ni(OH)2+2OH- Overall reaction discharged: 2NiO(OH) + Cd+2H2O→2Ni(OH)2+Cd(OH)2

Nickel-Cadmium Battery Advantages: This chemistry is reliable Operate in a range of temperatures Tolerates abuse well and performs well after long periods of storage Used in electronics and toys Disadvantages: It is three to five times more expensive than lead-acid Its materials are toxic and the recycling infrastructure for larger nickel-cadmium batteries is very limited

Lead-Acid Battery Anode: Porous lead Cathode: Lead-dioxide Electrolyte: Sulfuric acid, 6 molar H2SO4 Discharging anode:PbO2(s)+4H+(aq)+SO42-+2e- → PbSO4(s)+ 2H2O  cathode: Pb(s) + SO42-(aq) → PbSO4(s) + 2e-  During charging  anode: PbSO4(s) + 2H2O(l) → PbO2(s) + 4H+(aq) + SO42-(aq) + 2e-  cathode: PbSO4(s) + 2e- → Pb(s) + SO42-(aq)

Lead-Acid Battery The lead-acid cells in automobile batteries are wet cells Deliver short burst of high power, to start the engine Battery supplies power to the starter and ignition system to start the engine Battery acts as a voltage stabilizer in the electrical system Supplies the extra power necessary when the vehicle's electrical load exceeds the supply from the charging system

Lead-Acid Battery Advantages: Disadvantages: Batteries of all shapes and sizes, available in Maintenance-free products and mass-produced Best value for power and energy per kilowatt-hour Have the longest life cycle and a large environmental advantage Ninety-seven percent of the lead is recycled and reused in new batteries Disadvantages: Lead is heavier compared to alternative elements Certain efficiencies in current conductors and other advances continue to improve on the power density of a lead-acid battery's design

Lithium-Ion Battery Anode: Graphite Cathode: Lithium manganese dioxide Electrolyte: mixture of lithium salts Lithium ion battery half cell reactions CoO2 + Li+ + e- ↔ LiCoO2 Eº = 1V Li+ + C6+ e- ↔ LiC6 Eº ~ -3V Overall reaction during discharge CoO2 + LiC6 ↔ LiCoO2 + C6 Eoc = E+ - E- = 1 - (-3.01) = 4V

Lithium-Ion Battery Ideal material Low density, lithium is light High reduction potential Largest energy density for weight Li-based cells are most compact ways of storing electrical energy Lower in energy density than lithium metal, lithium-ion is safe Energy density is twice of the standard nickel-cadmium  No memory and no scheduled cycling is required to prolong battery life  http://batteryuniversity.com/learn/article/is_lithium_ion_the_ideal_battery

Lithium-Ion Battery Advantages: Disadvantages: It has a high specific energy (number of hours of operation for a given weight) Huge success for mobile applications such as phones and notebook computers Disadvantages: Cost differential Not as apparent with small batteries (phones and computers) Automotive batteries are larger, cost becomes more significant Cell temperature is monitored to prevent temperature extremes No established system for recycling large lithium-ion batteries

What is a hydrogen fuel cell? Hydrogen fuel cells (HFCs) are a type of electrochemical cell. HFCs generate electricity by reduction and oxidation reactions within the cell. They use three main components, a fuel, an oxidant and an electrolyte. HFCs operate like batteries, although they require external fuel. HFCs are a thermodynamically open system. HFCs use hydrogen as a fuel, oxygen as an oxidant, a proton exchange membrane as an electrolyte, and emit only water as waste.

How do they work? Fuel (H2) is first transported to the anode of the cell Fuel undergoes the anode reaction Anode reaction splits the fuel into H+ (a proton) and e- Protons pass through the electrolyte to the cathode Electrons can not pass through the electrolyte, and must travel through an external circuit which creates a usable electric current Protons and electrons reach the cathode, and undergo the cathode reaction

Cathode half- reaction: Acidic Oxidation At the anode of the cell, a catalyst (platinum powder) is used to separate the proton from the electron in the hydrogen fuel. Anode half-reaction: 2H2  4H+ + 4e- Eo = 0.00V Reduction At the cathode of the cell, a second catalyst (nickel) is used to recombine the protons, electrons, and oxygen atoms to form water. Cathode half- reaction: 4H+ + O2 + 4e-  2H2O Eo = 0.68V In electrochemistry, the Eocell value (energy) of a fuel cell is equal to the Eo of the cathode half-reaction minus the Eo of the anode half-reaction. For a hydrogen fuel cell, the two half reactions are shown above. So to calculate the energy of one fuel cell, we need to subtract the anode energy from the cathode energy. For a HFC, the Eocell = 0.68V – 0.00V which equals 0.68V

Alkaline

Uses of hydrogen fuel cells There are many different uses of fuel cells being utilized right now. Some of these uses are… Power sources for vehicles such as cars, trucks, buses and even boats and submarines Power sources for spacecraft, remote weather stations and military technology Batteries for electronics such as laptops and smart phones Sources for uninterruptable power supplies.

Problems regarding hydrogen fuel cells Lack of hydrogen infrastructure Need for refueling stations Lack of consumer distribution system Cost of hydrogen fuel cells 2009 Department of Energy estimated $61/kw Honda FCX Clarity costs about half a million dollars to make Carbon cost of producing hydrogen Problems with HFC cars Short range (~260 miles) Warm up time (~5 minutes)

Methanol fuel cell

Microbial fuel cell MFCs are bioelectrical devices that harness the natural metabolisms of microbes to produce electrical power directly from organic material

MFC Basics Oxygen Poor Oxygen Rich

MFC

MFC