Energy Storage and Materials Quiz 1 Concepts

Anode

Electrode in an electrochemical cell on which the oxidation reaction occurs.

Cathode

Electrode in an electrochemical cell on which the reduction reaction occurs

Electrochemistry

A field of chemistry that focuses on the interchange between electrical and chemical energy

Electricity

Flow of electrons over a wire that is affected by the presence and flow of electric charge.

Electrolysis

The decomposition of a substance by means of electric current. This method pushes a redox reaction toward the non-spontaneous side

Electrolytic Cell

Electrochemical cell that is being pushed toward the non-spontaneous direction by electrolysis

Electromotive Force (EMF, cell potential)

Difference of potential energy of electrons between the two electrodes.

Oxidation Number

Charge on an atom if shared electrons where assigned to the more electronegative atom.

Oxidation

Lose of electrons, can occur only in combination with reduction

Reduction

Gain of electrons, can occur only in combination with oxidation.

Redox Reaction

Reduction-Oxidation reaction

Voltaic/Galvanic Cell

An electrochemical cell that uses redox reaction to produce electricity spontaneously

Driving force for a chemical reaction? A + B = AB

Gibbs Free Energy

Function of electrolyte

Act as a filter for the passage of ionic (not electronic) species

Chemical driving force upon cell is due to…

Difference in chemical potentials of electrodes, the standard GFE

Balance between chemical and electrical forces upon the ions is expressed as…

Charging of a system

Electronic Path > Voltage

Causes current to flow in opposite direction

Reconstitution Reaction

Microstructure of one or more of the electrode materials gets reconstituted

Formation Reaction

A+B=AB

New phase AB formed in one of electrodes

Displacement Reaction

A + BX = AX + B

LCO Displacement Reaction

Li + C2₂O = Li₂O + Cu

Insertion Reaction

Insertion of guest species into normally unoccupied interstitial sites on a host crystal

Is the host crystal layered for insertion reactions?

No

Intercalation Reaction

Act of guest species going into a LAYERED crystal structure

Example of intercalation reaction

xALi+ TiS₂ = Li_xTiS₂

Specific energy

Energy available per unit weight

Specific power

Power available per weight

Energy density

Energy per unit volume

Power density

Power per unit volume

Cycle life

Number of times a device can be recharged before its performance becomes degraded

Energy density units

Wh/liter

Specific energy units

Wh/kg

Is high temperature or low temperature heat more useful?

High temperature

Power, Voltage, and Resistance Relationship

P= E² / R

When is high voltage required?

Hybrid/all-electric vehicles

Energy integral

Integral of voltage multiplied by charge capacity

Maximum capacity

maximum amount of charge that can be theoretically stored in a battery

Discharge and Charge Curves

Cell voltage plotted as function of the state of charge

Coulombic Efficiency

Fraction of the prior charge capacity that is available during the following discharge

Self-discharge

Decrease in available capacity with time, even without energy being taken from the cell

Electrochemistry - Big Idea

Chemical energy is converted to electrical energy through oxidation and reduction reactions

Secondary battery

Rechargeable

Primary Battery

Non-rechargeable battery

Half-Cell reactions

Anodic and Cathodic reactions taking place

Electron is generated at the _____ and must be consumed at the _____ so net charge is conserved in overall process

anode; cathode

Salt bridge purpose

Completes cell circuit so that charge remains balanced, otherwise there would be a charge buildup

Voltage definition

Potential difference between anode and cathode

Capacity

Amount of charge stored (usually given per unit mass or volume)

In the process of oxidation and reduction, energy is converted from _______ to __________

chemical; electrical

Fuel Cells

Collect electrons from reaction of fuels supplied by holding tanks (or the air)

Example of fuel cells

H₂/O₂, Alkaline, Solid Oxide, Direct Methanol

Capacity Definition

Ah (Coulombs)

Current integrated over time

Specific capacity units

Ah/kg

Capacity Density Units

Ah/kg

Energy Units

Wh = Capacity (Ah) * Potential (V)

Power units

IV = W

Watt-hour meaning

Energy required to power a 1 Watt bulb for 1 hour

Specific Energy units

Wh/kg

Energy density

Wh/I

Fuel Cells

Collect electrons from reactions of fuels supplied by holding tanks (or the air)

All fuel cells have chemical reactions at both electrodes at the _____ molar rate

Same

ΔG_cell =

ΔG_anode + ΔG_cathode

ΔG

Useful free energy available to do work outside the cell

ΔH

Enthalpy (total internal energy embodied by the chemical reactions)

ΔS

Thermodynamic penalty representing energy lost inherently to reaction kinetics and structural changes, this loss manifests as action overpotential and dissipates as heat

Do you want an over- or under-potential for the kinetics of a fuel cell reaction?

Lower overpotential (energy penalty)

Li-Ion Anode

Graphitic Carbon

Li-Ion Cathode

LiMO₂, where M = Co, Ni, Mn

Why is lithium so ubiquitous in batteries?

It is highly electropositive (-3.04 V)

Very good reducing agent

Concentration Polarization

Depletion or accumulation of reactant ions near electrode surface due to mass transport limitations

Severe performance voltage drops

ΔG_Cell =

Wh = integral (V_cell I_cell dt)

I_cell means

number of electrons that you move between electrodes

Li + TiS2 ←→ LiTiS2

What is the anode?

Li —> Li+ + e-

Li + TiS2 ←→ LiTiS2

What is the cathode?

Li+ + e- + TiS2 -→ LiTiS2

When does the Nernst Equation break down?

When compound created during the reaction cannot form a solid solution with the pre-reaction compound it shares an electrode with

Electrode State

Battery electrodes are self-contained collections of solid or liquid matters with fixed internal mass

How do fuel cells perform redox reactions?

With a supply of gas and fluid reactants

Cell Voltage Formula

V_cell = V_{positive electrode} - V_{negative electrode}

Universal Baseline Cell

Hydrogen Electrode, 0V

Anode reaction of Hydrogen-Oxygen Fuel Cell

H_² —> 2H⁺ + 2e-

Cathode reaction of Hydrogen-Oxygen Fuel Cell

2H+ + 2e- + ½ O₂ —> H₂O

Mechanism of Hydrogen-Oxygen Fuel Cell

• Catalytic surfaces split H₂ into Protons and Electrons

• Protons cross a proton exchange membrane, and electrodes do external work

• Both reunite to form water

Electrolysis

Running fuel cell in reverse requires electricity and water to generate stored H and O

Types of Fuel Cells

• Alkaline Fuel Cells

• Solid Oxide Fuel Cells

• Direct Methanol Fuel Cells

Alkaline Fuel Cells

Relies on OH- (Hydroxide) to exchange in basic environments

Solid Oxide Fuel Cells

High-temperature systems that conduct oxygen ions through ceramic membrane

Direct Methanol Fuel Cells

Use room-temperature liquid methanol on one side, and air/oxygen on the other

Require expensive catalysts

Difference between a battery and a fuel cell?

Battery stores electric energy which it uses

Fuel cell generates electric energy by converting available fuel

Capacity Units

Amp-hours

Capacity Definition

Integral of current over time

Measures stored coulombs of electrons not total energy

Specific Capacity Units

Ah/kg

Specific Capacity definition

Amount of charge held per unit mass of active material or fuel cell

Is Specific Capacity an intrinsic or extrinsic property?

Intrinsic

Capacity Density Units

Ah/L

Capacity density definition

Amount of charge held per unit volume

What is capacity density highly dependent on?

Tap packing or tap density

Which term represents the actual capacity to do real physical work?

Energy (watt-hour)

What can reduce specific energy?

Higher discharge rates, due to resistive losses

Resistivity

Measures how strongly a material can resist an electric current

Electrical Conductivity

A material’s ability to conduct an electrical current