Electrochemistry: Cells and Batteries

• This study material covers Electrochemical and Electrolytic cells, Electrode materials, Semiconductors, Lithium-ion batteries, Fuel cells, and Solar cells.

Electrochemical Cells

• Devices transforming spontaneous redox reaction energy into electricity, or using electricity to drive non-spontaneous reactions.
• Structure includes two electrodes (anode and cathode), an electrolyte (ionic conductor), and a metal wire (usually copper) linking electrodes.

Types of Electrochemical Cells

• Galvanic/Voltaic cells convert chemical reaction energy to electrical energy, with ΔG < 0.
• Electrolytic cells use electrical energy from an external source to drive non-spontaneous reactions, with ΔG > 0.

Galvanic/Voltaic Cells

• Voltaic cells, named after Alessandro Volta, use energy from spontaneous redox reactions (ΔG < 0).
• The anode is on the left (oxidation occurs), and the cathode is on the right (reduction occurs).
• The overall representation is M1 | M1n+ (C1) ║ M2n+ (C2) | M2.

Daniel Cell

• Invented by John Frederic Daniell.
• The Zinc Electrode is dipped in ZnSO4 solution, where oxidation (Zn → Zn2+ + 2 e-) occurs.
• The Copper Electrode is dipped in CuSO4 solution, where reduction (Cu2+ + 2 e- → Cu) occurs.
• Half cells are connected by a salt bridge (KCl or NH4Cl in gelatine), maintaining charge balance.
• Minimizes liquid junction potential, with a cell EMF of 1.1 V.

EMF of Electrochemical Cell

• Electromotive force (EMF) is the maximum potential difference between galvanic cell electrodes.
• Related to an element's tendency to gain/lose electrons and cell reaction's feasibility when Ecell is positive.
• Cell EMF is based on the Nernst Equation: Ecell = E0M1 - E0M2 + (0.059/n) log10 ([M1n+]/[M2n+]).
• For equimolar solutions, it simplifies to Ecell = E0M1 - E0M2.

Half Cell Reaction Example

• Problem: Cd|Cd2+ (0.01 M)║Cu2+ (0.5 M)|Cu, with standard reduction potentials of -0.40 V and 0.34 V.
• At the anode: Cd → Cd2+ + 2 e- (Standard reduction potential = -0.40 V)
• At the cathode: Cu2+ + 2 e- → Cu (Standard reduction potential = 0.34 V)
• Ecell = 0.34 – (-0.40) + (0.059/2) log10 (0.5/0.01) = 0.79 V

Battery Basics

• Battery: One or more electrochemical cells, in series/parallel, convert chemical energy to electrical energy.
• Components: reducing anode, oxidizing cathode, ionic conductor electrolyte.

Types of Cells/Batteries

• Primary batteries have irreversible reactions becoming dead when reactants turn to product, such as the Lechlanche, Alkaline and Lithium batteries.
• Secondary batteries reactions are reversible with electric current, such as Lead acid, Ni-Cd, Ni-Metal Hydride, & Lithium ion batteries.
• Flow batteries/fuel cells pass materials through the battery, e.g., Hydrogen-oxygen & Solid oxide fuel cells.

Lithium-Ion Batteries

• A secondary battery that does not have metallic lithium as the anode
• Lithium ions are key to charging and discharging.
• M Whittingham proposed the tech in the 1970s, using titanium sulphide for the cathode and lithium metal for the anode.
• The Nobel Prize in Chemistry 2019 acknowledged John B. Goodenough, M. Stanley Whittingham, & Akira Yoshino for Lithium-ion battery tech.

The Advantage of Lithium

• They are lightweight and use a very light element.
• The batteries have a high specific energy density.
• Because Lit has a very large negative standard reduction potential, it creates a high voltage per cell.
• The voltage is a max of 3.7 V, nearly three times higher than a 1.3 V per cell.
• Lithium-ion delivers more power in comparable sizes, for an energy density based on volume.

Construction of Lithium-Ion Batteries

• Cathode: Layers of lithium-metal oxide (like LiCoO2, LiNiO2, LiMn2O4, LiNiMnCoO2) or polyanionic materials (like LiFePO4, LiMnPO4, LiFeSO4F).
• Anode: Made from graphite, usually with composition Li0.5C6.
• Electrolyte: A mixture of organic carbonates like ethylene and diethyl carbonate.
• Separator: Prevents contact, absorbs electrolyte, and enables ion passage.

Charging Reaction

• Cobalt ions are oxidized and release electrons.
• Simultaneously, Li+ ions migrate from LiCoO2 into the graphite.
• Electrons flow from the positive to negative electrode
• Electrons/Li+ ions combine at the negative electrode

Discharging Reaction

• Li+ ions move from anode and migrate through the electrolyte.
• There they entire the spaces between the cobalt oxide layers.
• Simultaneously, electrons flow through the external circuit.
• Electrons reduce cobalt ions and the positive electrode to regenerate LiCoO2.

Reactions in Lithium Battery

• Charging, Anode: LiCoO2 → Li1-nCoO2 + n Li+ + n e-
• Charging, Cathode: C6 + n e- + n Li+ → LinC6
• Overall charing: LiCoO2 + C6 → Li1-nCoO2 + LinC6
• Discharging, Anode: LinC6 → C6 + n e- + n Li+
• Discharging, Cathode: Li1-nCoO2 + n Li+ + n e- → LiCoO2
• Overall discharging: Li1-nCoO2 + C6Lin → LiCoO2 + C6

Lithium Ion Battery Variants

• Lithium Cobalt (LiCoO2, LCO): High capacity, used in cell phones, laptops, and cameras.
• Lithium Manganese Oxide (LiMn2O4, LMO): Lower capacity, used in power tools and medical devices.
• Lithium Iron Phosphate (LiFePO4, LFP): Lower capacity, used in power tools and medical devices.
• Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2, NMC): Lower capacity, used in power tools and medical devices.
• Lithium Nickel Cobalt Aluminium Oxide (LiNiCoAlO2, NCA): Used in electric vehicles and grid storage.
• Lithium polymer batteries: Feature an output of 2.8 V and moderately high energy density.

Lithium-Ion Battery Applications

• Portable power in cell phones, laptops, etc.
• Back-up power during outages.
• Power source with good charging capacity for electric vehicles.
• Alternative to gasoline/lead-acid for work boats and yachts.
• Power for wheelchairs, bikes, and scooters
• For storing energy from solar panels and wind turbines with fast recharge

Advantages of Lithium Ion Batteries

• High energy density allows lithium ion batteries for gadgets and vehicles
• The batteries' low self-discharge better that Ni-Cad and NiMH forms.
• The batteries' low maintenance, and require no upkeep.
• Cell voltage is consistent at 3.6 volts per cell so less cells are needed in applications.
• Wide variety of lithium ion is available to align with application needs.

Disadvantages of Lithium Ion Batteries

• Must be protected from over/under charging.
• The batteries age, with a limited 500-1000 charge cycles.
• The batteries cost more than nickel cadmium.
• There are bad designs, manufacturing defects, where electrodes short from expanding batteries
• Overcharging leads to oxygen release, and overheating.
• Overheating causes Dimethyl carbonate to decompose with pressure buildup.

Fuel Cells

• A device that converts stored chemical potential energy of molecular bonds into electrical energy
• Combines hydrogen with oxygen, without combustion, to produce water and heat.
• Provide great electrical efficiency (≥ 40%) than conventional power systems.

Types of Fuel Cells

• PEMFCs (Proton Exchange Membrane Fuel Cells)
• AFCs (Alkaline Fuel Cells)
• PAFCs (Phosphoric Acid Fuel Cells)
• MCFCs (Molten Carbonate Fuel Cells)
• SOFCs (Solid Oxide Fuel Cells)
• DMFCs (Direct Methanol Fuel Cells)
• DAFCs (Direct Ammonia Fuel Cells)
• DCFCs (Direct Carbon Fuel Cells)
• Apart from DAFCs, DMFCs, & DCFCs, other fuel cells are fed hydrogen.

Hydrogen–Oxygen Fuel Cells (HOFC)

• Basic type of fuel cell, similar to galvanic cells, with two half cells.
• Porous graphite electrode with platinum, silver, or a metal oxide catalyst.
• Electrodes are placed in NaOH/KOH (alkaline) or H2SO4 (acidic) electrolyte.
• Hydrogen/oxygen are supplied at 50 atm & diffuse at electrodes.
• Overall reaction: conversion of hydrogen by oxygen: 2 H2 + O2 → 2 H2O.

Proton Exchange Membrane Fuel Cell (PEMFC)

• Anode H2 (g) → 2 H+ + 2 e-
• Cathode: O2 (g) + 4 H+ + 4 e- → 2 H2O
• Overall: O2 (g) + 2 H2 (g) → 2 H2O (I), ECell= 1.23 V.

Membrane details

• Utilize water-based, acidic Nafion membranes for proton (ion) exchange.
• Membranes have polytetrafluoroethylene (PTFE) backbones with ether & sulfonic acid units.
• Operate at low temperatures (< 80°C) & require pure hydrogen.

Solid Oxide Fuel Cell (SOFC)

• High-temperature fuel cell utilizes solid ceramic oxide, e.g., Yttria-stabilized Zirconia (YSZ), which may also be called ceramic FC.
• Using hydrogen & carbon monoxide fuels.
• Operates at 800-1,000°C.
• Efficiency: over 60% with fuel-to-electricity conversion, resistant to sulphur, usable with coal gas.

Structure of SOFC

• Anode/Fuel Electrode: Nickel/YSZ cermet (ceramic & metal mix), porous for fuel flow to electrolyte.
• Cathode/Air Electrode: Mixed ion-conducting & electronically conducting ceramic, thin porous layer for oxygen reduction, e.g., strontium-doped lanthanum manganite (LSM).
• Electrolyte: Oxide ion (O2-) conducting ceramic, bilayer composite YSZ+gadolinium CeO2 or ZrO: CaO mix.

Reactions in Solid Oxide Fuel Cell

• At the anode (oxidation): H2 (g) + CO (g) + 2 O2- → H2O (g) + CO2 (g) + 4e-
• At the cathode (reduction): O2 (g) + 4e- → 2 O2-
• The net reaction: H2 (g) + CO (g) + O2 (g) → H2O (g) + CO2 (g)

Advantages of SOFCs

• Benefits from solid materials, high operating temperature - no electrolyte loss or electrode corrosion.
• Tolerates impurities, high efficiency (high-quality waste heat) with low emissions so considered cleanest fuel cells.

Disadvantages of SOFCs

• High operating temperature leads to start-up, mechanical, /compatibility issues with high costs.
• Can be applied in trains, ships, vehicles, stationary power generation, cogeneration for efficiency as well as auxiliary power

Solar Energy Potential

• Theoretical solar energy is 1.2x105 TW (1.76 x105 TW striking Earth, 0.30 global mean albedo)
• Practical solar energy is 600 TW (50-1500 TW depending on land; WEA 2000).
• There is 60 TW onshore generation with 10% conversion efficiency and 90 TW photosynthesis.

Types of Solar Energy Conversion Cells

• Photovoltaic Cells
• Dye-sensitized solar cells

Photovoltaic Cells

• A device converts sunlight directly to electricity via the photovoltaic effect.
• The photovoltaic effect creates voltage/electric current from electromagnetic radiation.
• These effect are related but different than the photoelectric effect.
• Use semiconductors to generate current from photons, usually doped silicon

The Advantage of Silicon

• It is a good energy conversion material because it is the second most abundant element (~ 28% by mass)
• It can be readily synthesized from sand
• The reaction formula is SiO2 + C → Si + CO2
• Also because it has an optimum band gap of 1.23 eV at 300 K, good cost, and easy doping

Types of Solar Cell

• Crystal structure, or atomic arrangement play crucial role in material properties for photovoltaic cells.
• With single-crystal silicon, the material has an ordered array of repetitive dispursed atoms, and is is 15–18% efficient but expensive to make.
• With polycrystalline silicon, each crystalline sub-section which is 12–16% efficient and cheap
• Amorphous silicon has no atomic regularity, but is is 4–8% Efficient,the cheapest, known as "thin film" for surface use

Semiconductors

• They can conduct or not under conditions, great for electrical current, filled valence band, empty conduction band, narrow energy gap (1 eV).
• Silicon/Germanium: band gaps are1.0/0.7 eV.
• They become insulators at 0 K (electrons freeze), but conductivity improves with increased temp.
• Semiconductors come in these two versions

Types of semiconductors

• Intrinsic: pure form, and uses Ge, Si (ne = nh = ni).
• Extrinsic: doped with impurities, N-type (pentavalent impurities like P,As,Sb/ND donar) or P-type (trivalent impurities like Ga, B, IN, Al/ND donar)

Intrinsic Semiconductor

• It is chemically pure
• Has equal amounts of positive and negative carriers (holes and electrons)

Extrinsic Semiconductor

• A small amount of impurities (doping), alters properties & raises conductivity.
• Doping, two groups will emerge: positive (p-type) and negative charge (n-type)
• The most common elemental choices are silicon and germanium or InSb, InAs, GaP, GaSb, GaAs, SiC, GaN,

Types of Semiconductors: n-type

• Pentavalent impurities such as, P, As, Sb, etc doping for intrinsic
• A small amount of phosphorus (P) added, where valence electrons are free for surplus.

Types of Semiconductors: p-type

• It has trivalent impurities such as, B, Al, In, etc with intrinsic doping.
• Doping with boron (B), bond silicon/boron may result in holes.

Semiconductor Preparation

• Includes distillation with separation based on boiling points (GeGeCl4/Si SiHCl3),
• Removing As in GeCl4, & obtaining GeCl4 through chlorine distillation
• Water is used on a cooled GeCl4 to get germanium oxide, then reduction GeCl4 gives pure Ge

Doping Techniques: Epitaxy

• It happens with continuous crystal growth for thin crystal deposition.
• A graphite boat houses silcon/germanium placed in a long quartz with RF coil.
• Doping occurs as gases mix with calculated with appropriate silicon/germanium based compounds.
• Silicon uses SiCl4, H2, N2 with phosphine (PH3) for p-type doping→ and Diborane (B2H6) for n-type.

Doping Techniques: Diffusion Technique

• Semiconductor modification technique through solid or gaseous atom deposition in crystal without material change (P-type dopant heated).
• The impurity depth is controlled to a few micrometers.

Dye Sensitization – Grätzel Cell:

1. Sunlight (photon) passes through TiO2 layer. Electrons are released due to the extra energy.
2. Releases extra electrons accumulate at -ve plates.
3. Free electrons start to flow creating electric current.
4. Free electrons oxide the iodide and the cycle repeats

Features of the Cell

• Uses Titamium Dioxide as a conductive with a Graphite plate and iodide electrolyte
• A dye is made which is then adsorped onto the TiO2 layer

Transparent and Conductive Substrate

• It acts a a semiconductor, with a catalyst, and a collection material.
• Substrate qualities:
• High More than 80% transparence
• High electrical conductivity .
• Common examples are FTO/ITO.
• 8.5/18 ohms/cm2 resistance.

Working Electrode

• WE is made through a deposit a thin oxide semiconducting layer
• Commonly uses as TiO2 (due to it non toxicity) on tranparent glass.
• Highy porous structure for light molecules adhesion/absorption

Photosensitizer or Dye

• The photo sensitizers' job is light max. Due to this job, good electro-physical qualities.
• The dye should.
• Best if luminescent.
• Be absorbent for many types of light (UV) and hydrophobic for enhanced stability

Electrolyte

• 1 is key component, and can contain iodine or other cations
• Redox coupling, additives and ionic liquids, make them efficient, should also offer - chemical, thermal as well as electrochemical stability without corrosion

Counter Electrode

• The CE consists of elemental composition with metal and conductive, Pt, CoS

Grätzel cell

• Organic dyes are used like Ruthenium-Polypyridine, and Indoline
• Can be extracted from simple foods like berries and hibiscus
• It is constructed of Glass Plates layered with TiO2. and the dyes

Working Principle

• Involves light with 4 steps: absorption injection transport and collection
• The sun's particles transfer electrons. These electrons are absorbed,
• The absorption increases energy which generates oxidized dyes