Electrochemistry for Engineers 0581.5271 Electrochemistry for Engineers LECTURE 10 Lecturer: Dr. Brian Rosen Office: 128 Wolfson Office Hours: Sun 16:00

HW #5 Clarifications ENTROPY IN UNITS OF J / (mol*K)

Batteries

Recall Important Definitions Coulombs – Unit of CHARGE Amperes – Unit of CURRENT [Coulombs per second] Volts – Unit of POTENTIAL [Joules per Coulomb] Watt – Unit of POWER [Joules per second] Joule – Unit of ENERGY [Watts x seconds] Watt-hour (Wh) is also a unit of energy [Watts x hours]

Recall Important Definitions Pt 2 POWER DENSITY – Rate of Energy Transfer per unit volume or mass [kW/m3 or kW/kG]   ENERGY DENISTY – The amount of energy stored in a given system [kJ/m3 or kJ/kg]

Basic Operating Principle A  AO Oxidation at Anode (-) BO  B Reduction at Cathode (+) Charged State Discharged State The chemical energy within the bonds of the “charged” state is greater than that of the discharged state

Primary Batteries Batteries can be classifieds as two types as primary batteries and secondary batteries. Primary batteries In primary batteries, the electrochemical reaction is not reversible. During discharging the chemical compounds are permanently changed and electrical energy is released until the original compounds are completely exhausted. Thus the cells can be used only once.

Secondary Batteries Secondary batteries In secondary batteries, the electrochemical reaction is reversible and the original chemical compounds can be reconstituted by the application of an electrical potential between the electrodes injecting energy into the cell. Such cells can be discharged and recharged many times.

For Example Leclanché Battery (Primary) Nickel-Cadmium Battery (Secondary) The zinc + Manganese (II) oxide system has a greater enthalpy than the zinc oxide and Mn (III) oxide

Energy Density

Inside A Battery

Lead-Acid Battery

Pb-Acid Battery: The Anode

Pb-Acid Battery: The Anode

Pb-Acid Battery: The Cathode

Pb-Acid Battery: The Cathode

Pb-Acid Battery: Discharging

Pb-Acid Battery: Charging

Nernst Equation for Pb-Acid Battery

Self Discharge (Leakage Current) Electrochemical reaction, permitted by thermodynamics, can occur on the electrode Surface and must be balanced by the discharge of the electrode (since the cell is at open circuit) (+) (-) Since the potential of the (+) terminal is very high, side reactions can occur. If the potential of the (+) terminal is above the reduction potential for the side reaction the electrons produced by the side reaction will be consumed by discharging the (+) terminal

Self Discharge of (-) Terminal

Self Discharge of (+) Terminal

Store Batteries in the Fridge!

Why is Lead Advantageous for Storing Chemical Energy?

Battery Polarizability activation overpotential Why is the charging curve above the discharge curve? Depletion Charge-Discharge Curve at Constant Current

Mechanisms Affecting Voltage Resistive drops at electrodes (lead sulfate is a poor conductor) Electrolyte gradient near the electrode surface (depletion) Resistance of ionic movement through electrolyte (ohmic losses) Activation overpotentials

Battery Capacity, C and Cp

Effect of Discharge Rate on C

Example Data (U. Colorado)

Theoretical Specific Capacity M = molecular weight in kg/mol F = faraday constant n = number of electrons q = specific capacity

Practical Specific Capacity W = weight of catalyst in g A = electrochemical area area in cm2 j = current density in mA/cm2 q,prac = practical specific capacity Why is the utilization generally below 100%?

Theoretical Specific Energy V = voltage (function of time) i = current (held constant) T,cutoff = cutoff time W = catalyst weight

Theoretical Specific Power V = voltage (function of time) i = current (held constant) T,cutoff = cutoff time W = catalyst weight

Battery Efficiency Typical coulomb efficiency = 90% Approximate voltage efficiency =(2V/2.3V) = 87% Energy efficiency = (90%)(87%) = 78%

Charging Management

Solubility of Discharge Products Initial Discharge Recharge Soluble discharge product Insoluble discharge product

Zn dendrite formation and inhibition by polyethylene glycol Particularly susceptible when using Li or Zn electrodes “Short Circuit” Zn dendrite formation and inhibition by polyethylene glycol

Cycle Testing

Need for Porous Electrode Materials Lead electrodes need to have high surface area for high energy density Without high porosity, surface would passivate quickly

Nickel-Metal Hydride (NiMH) Battery Negative electrode: Metal Hydride such as AB2 (A=titanium and/or vanadium, B= zirconium or nickel, modified with chromium, cobalt, iron, and/or manganese) or AB5 (A=rare earth mixture of lanthanum, cerium, neodymium, praseodymium, B=nickel, cobalt, manganese, and/or aluminum) Positive electrode: nickel oxyhydroxide (NiO(OH)) Electrolyte: Potassium hydroxide (KOH) Cobasys batteries

Nickel-Metal Hydride (NiMH) Battery Redox occurs in the lattice The negative electrode material must be an alloy capable of large amount of hydrogen adsorption LaNi5 TiN2 ZrNi Ti2Ni Typical electrodes can adsorb up to 2wt% hydrogen when charged

Nickel-Metal Hydride (NiMH) Battery It is not advisable to charge Ni-MH batteries with a constant-voltage method. Ni-MH batteries do not accept well a high initial charging current. Float voltage is about 1.4 V (voltage of full capacity, compensating for self discharge) Minimum voltage is about 1 V. Cobasys Nigen battery Saft NHE module battery

Nickel-Metal Hydride (NiMH) Battery Effects of temperature: Saft NHE module battery http://www.panasonic.com/industrial/battery/oem/images/pdf/panasonic_nimh_overview.pdf

NiMH Over-charge and Over-discharge

Nickel-Metal Hydride (NiMH) Battery Advantages: Less sensitive to high temperatures than Li-ion and Lead-acid Handle abuse (overcharge or over-discharge better than Li-ion bat Disadvantages: More cells in series are need to achieve some given voltage. Cost

Li-Ion Battery Positive electrode: Lithiated form of a transition metal oxide (lithium cobalt oxide-LiCoO2 or lithium manganese oxide LiMn2O4) Negative electrode: Carbon (C), usually graphite (C6) Electrolyte: solid lithium-salt electrolytes (LiPF6, LiBF4, or LiClO4) and organic solvents (ether) discharge http://www.fer.hr/_download/repository/Li-ION.pdf

Li-Ion Battery Cathode Anode Overall Cathode more positive redox potential Discharge: Li+ intercalates the positive materials -> provide outer electron flow Charge: Li+ deintercalates from cathode and intercalates the anode. Li-ion shuttles b/w cathode and anode during cycling -> conversion & storage of electrochemical energy within the Cells Energy density: storage of large amount of Li Power density: fast ionic/electronic transfer 48

Li-Ion Battery A typical Li-ion battery can store 150 watt-hours of electricity in 1 kilogram of battery as compared to lead acid batteries can sore only 25 watt-hours of electricity in one kilogram All rechargeable batteries suffer from self-discharge when stored or not in use. Normally, there will be a three to five percent of self- discharge in lithium ion batteries for 30 days of storage.

Li-Ion Battery Charging Contrary to lead-acid batteries, Li-ion batteries do not accept well a high initial charging current. In addition, cells in a battery stack needs to be equalized to avoid falling below the minimum cell voltage of about 2.85 V/cell. Thus, Li-ion batteries need to be charged at least initially with a constant-current profile. Hence they need a charger Typical float voltage is above 4 V (typically 4.2 V).

Li-Ion Battery Temperature Effects Effects of temperature:

Li-Ion Battery Equalization Controlled charging has 2 purposes: Limiting the current Equalizing cells “Increased Performance of Battery Packs by Active Equalization” Jonathan W. Kimball, Brian T. Kuhn and Philip T. Krein “Advanced Lithium Ion Battery Charger” V.L. Teofilo, L.V. Merritt and R.P. Hollandsworth Saft Intensium 3 Li-ion battery

Li-Ion Battery Factors affecting life: Charging voltage. Temperature Age (time since manufacturing) Degradation process: oxidation

Li-Ion Battery Advantages with respect to lead-acid batteries: Less sensitive to high temperatures (specially with solid electrolytes) Lighter (compare Li and C with Pb) They do not have deposits every charge/discharge cycle (that’s why the efficiency is 99%) Less cells in series are need to achieve some given voltage. Disadvantages: Cost

Li-Air Batteries

Replacing the IC Engine?

GDL of a Li-Air Battery

Challenges with Li-Air Batteries Poor efficiency (> 70%, ORR kinetics) Low reaction rate (0.01 – 0.1 mA/cm2) Low cycle life (10-100 cycles) Engineering challenges No moisture exposure Instability of Li Dendrite formation

Testing ORR Materials for Li-Air

ORR on Li-Air : A Comparison

Translate to REAL materials