E5 - Petrol must be 5% ethanol

B7 - Diesel must be 7% biodiesel

- Increasing fuel security in countries with limited petrol reserves (cause of a push in Brazil)

- Producing biofuels creates jobs and diversifies the economy

- Reliant on government policies, as production costs still greater than petrofuels.

Sugar cane - BE

Soya - BD

Rapeseed - BE

Wheat - BD

Agricultural Products

-Require significant areas of high-quality fertile land

- Vast amounts of fresh water required

- Competing with food crops brings up the "food vs. fuel" debate

Agricultural waste products

- Addresses arable land use

- Still the issue of water use, freshwater needed to sustain crops.

- Loss of biodiversity

1) Reduced waste, meaning more sustainability and reduced environmental impact.

2) Profitability is greater

3) More economically robust - more products means less vulnerability to market change

1) Well-defined chemicals, "speciality chemicals"

2) Transport Fuels

3) Feedstock chemicals - still require other processes

4) "Syngas" - Synthesis (not synthetic) gas, a mixture of H₂ and CO.

5) Heat & Power

Max. ethanol content is about 10%.

Distillation and subsequent processes necessary to increase this value.

Sugary - Sugar cane, sugar beet

Starchy - Maize, wheat

DDGS - Distiller Dried Grains -> Protein Animal Feed

CO₂ for the drinks/food industry

To prevent it from being drinkable

1) Safer

2) Not taxable

- Process takes 8-12 hours due to long reaction time

- Large vessels therefore needed

- This means high capital cost and areas of land taken up

- Get "beer" which is 7-10% ethanol

- Split into solids and liquids (separation by density) via centrifugation

- Solids are yeast cells that can be reused.

- Moves onto distillation

- Separation by boiling points is done

- For every litre of BE, there is 10 to 13 litres of stillage

- Stillage is other liquid and solid waste (goes to animal feed)

- Propanol and butanol are heavier with higher boiling points, also methanol.

What method is used for ethanol dehydration?

Pressure Swing Adsorption

- Separation by molecular size as H₂O is smaller than ethanol.

- Mixture pressurised to 600kPa, and fed into a 'bed'

- 3A Zeolites adsorb the water, well-defined pore structure means it can discriminate between the two compounds.

- Beds are switched intermittently to recharge pressure

- Output of 99.9% ethanol

Conversion of starches to sugars. 

Hydrolysis -> Water breaks up the molecules

Starch molecules need to be broken down into multiple individual sugar units

1) Acid-catalysed hydrolysis at high temperature

2) Enzyme-catalysed hydrolysis (preferred route)

1) High yield

2) Sugar not starch so less processing (no saccharification).

- Ethanol has lower calorific value, less than 70% of octane.

- Higher compression ratios can be used, and the required air needed for ethanol is less.

- Reduced emissions of carbon monoxide and hydrocarbons

- Oxygenated products like acetic acid are produced, but should be removed by the catalytic converter

- NO_x emissions are increased slightly due to higher combustion temperature and the oxygen in the fuel (unlike petroleum).

Used Cooking Oil.

Waste is repurposed. No land use change and feedstock production processes not considered.

- They are too viscous to flow through modern diesel engines or to atomise in the engine

- Leads to incomplete combustion causing problems like:

1) Formation of gums and lacquers on injection nozzles

2) Partial combustion products, such as acrolein.

- Transesterification

- Done to reduce molecule size, reducing viscosity.

- Reacted with methanol in the presence of a catalyst (usually an alkali)

- Fatty Acid Methyl Ester (FAME) or "Biodiesel" (flows and combusts better than triglycerides)

- Glycerol, a by-product

Sulphur (wt%) - PD: 0.05% and BD: 0%

- CO reduction of 48% with B100

 - 50% reduction in particulate matter 

- 67% reduction of un-burnt hydrocarbons

Negative:

- 10% increase in NOx emissions (due to higher temp combustion and oxygen availability)

BD has higher cetane number and greater oxygen content which means better combustion.

Less air required with BD combustion.

BD has higher cloud and pour points (°C) meaning B100 isn't viable in cold countries.

Sunflower and rapeseed oil :)

Palm oil :(

- Similar boiling point to BE so no inlet manifold redesign needed.

- BD acts as a rubber solvent, so some seals need replacing with resistant materials.

- Some high pressure engines limited to B5, but B7 widely used.

Palm Oil (by far the most productive)

Rapeseed

Sunflower 

Soya

Triglyceride + 3 Methanol ↔ 3 FAME + Glycerol

Twice as much methanol is used than required to push the reaction to the product side and achieve a 95% conversion (only 70% without)

1) The reaction is much slower, typically 8-10hrs, (otherwise <1), and requires a large excess of methanol (20:1)

2) More expensive materials needed to tolerate the acid.

Necessary for "low-grade" oils such as UCO. "Yellow/brown grease" rather than pure, virgin oils. Required due to water and breakdown products in the oil, especially free fatty acids (FFAs).

- Dedicated energy crops (wood, shrubs)

- Agricultural residues (wheat stalks and leaves)

- Wood residues (sawdust)

- Waste paper

Bottom 3 are all waste materials

Biochemical - 2.6

Thermochemical - 5.0

Biochemical has more processing steps due to pretreatment and separation.

 What is cellulose?

Percentage of wood?

- Polysaccharide (starch polymer)

- Made up of C6 sugar units (such as glucose).

Typically about 40-50% of lignocellulosic material.

What is hemi-cellulose, downsides?

Percentage of lignocellulosic materials?

- Polymer made up of C5 sugar units ("xylose" normally). 

- Amorphous, and about 25-30% of lignocellulose

- As it is C5 not C6 it cannot be fermented into bioethanol.

Lignin provides strength to the plant by bonding with hemicellulose. 

Transports water through plant, typically about 15-20% of lignocellulosic material.

Made of phenols and aromatic alcohols. It isn't linear, and is highly branched. 

Difficult to make specific products from due to complex chemistry.

1) Combustion - Heat & Power

2) Gasification - Hydrogen, Alcohol, Olefins, Gasoline, Diesel

3) Liquefaction (Hydrothermal is a subset) - Hydrogen, Methane, Oils

4) Pyrolysis - Hydrogen, Olefins, Oils, Speciality chemicals

1) Break up the woody structure

2) Increase surface area for hydrolysis

More energy required for woody biomass

Conventional: Hot water, dilute acid or alkalis

Others:

a) Ammonia fibre explosion - contact with liquid ammonia at high pressure then release the pressure.

b) Steam explosion with catalyst - high pressure and temp (then release)

a) Feedstock composition (low/high lignin content)

b) Production of toxic compounds

c) Energy costs

d) Requires large-scale usually meaning significant capital investment

Used to convert somplex sugars to simple sugars:

- Cellulose to glucose

- Hemicellulose to xylose

Used in thermochemical conversion of lignocellulosics at demonstrator scale. 

Biomass is converted to syngas (CO + H₂) before synthesis.

During synthesis get an output of hydrocarbon fuels (and water).

1) Algae growth

2) Harvest and dewatering

3) Drying

4) Fuel production

Per ton:

- 1.82 tonnes of CO₂ absorbed

- 0.43 tonnes of water 

- 0.32 tonnes of nutrients

- 1.54 tonnes of O₂ produced (photosynthesis)

- 0.048 tonnes of calcium hydroxide produced

Open pond - 0.2 to 1g per litre (dry biomass)

Closed photo-bioreactor - 1 to 10g per litre

1) Mechanically capture wet algae via filtration

2) Heat until dry

3) Solvent extract the oil

4) React to make biodiesel

Issues:

- Drying is so energy-intensive that the NER is less than 1

- Hexane used for solvent extraction, large carbon footprint

- Biodiesel reaction is not water tolerant, need less than 0.5% water

1) In-situ transesterification (fast and needs excess methanol) -> biodiesel

2) Hydrothermal processing (requires much separation) -> produce "bio-oil"

- Production rates are potentially massive per sq. ha

- Can use a wide range of water sources: Wastewater and Seawater

- Can be used to sequestrate CO₂

- Produce lipids up to 77% of their total weight

- Water removal too energy intensive, requires many steps

- Not all lipids produced are easily transesterifiable; you want neutral lipids like triglycerides, but environmental conditions can cause complex growth

- Contamination by protozoa or bacteria can decrease production yields.

- Way too expensive to produce atm, also NER is shit.

1) Plate tectonics

2) Volcanic eruptions and dust storms

3) Solar variations

4) Orbital variations

- Continental shift can influence climate at specific locations

- Can also influence global temperature by redistributing the collection of solar radiation and/or providing land masses on which continental glaciers can form 

Both release particles that can reflect radiation.

Volcanoes emit CO₂, H₂O and SO₂ that can get trapped in the upper parts of the atmosphere. These can react to create sulphates and aerosols. Aerosols lower surface temperature.

In lesser amounts emit CO, H₂S, CS₂, HCl, etc.

1) Eccentricity

2) Precession of the Equinox

3) Obliquity

Eccentricity is the extent to which an object's orbit around another body deviates from a perfect circle.

Earth's orbit changes with a period of 100k years, at the moment it is fairly circular but in 50k years it will be more eccentric.

Further from the sun = Colder

Axial precession is the gradual shift of Earth's axis of rotation in a cycle of approx. 26,000 years

Changes the time of year that seasons occur.

- 51% absorbed by land and oceans

- 19% absorbed by atmosphere

- 20% reflected by clouds

- 6% reflected by the atmosphere

- 4% reflected by the Earth's surface

- 55% radiated to space from the atmosphere

- 20% becomes latent heat as water vapour

- 13% of radiation is absorbed by the atmosphere

- 6% is conducted or becomes rising air

- 5% is radiation from Earth to space

Albedo is the percentage of incoming radiation that is reflected back into space.

The value is 31% for Earth

Nearly half of sunlight is in the visible light spectrum, the rest is in the ultraviolet and infrared ranges.

The radiation emitted by the Earth is infrared.

The change in net (down minus up) irradiance (in W/m²) due to the change in the amount of an agent, with surface temperature assumed constant.

Positive radiative forcing tends to warm the Earth's surface and lower atmosphere. Negative radiative forcing tends to cool them.

E.g. increase in greenhouse gases leads to positive forcing.

A measure of the future impact of emitting unit mass of a particular greenhouse gas today. It is normally measured relative to CO₂ and is calculated for a specific time horizon

GWP = time-integrated radiative forcing for the gas ÷ time-integrated radiative forcing for CO₂

It is assumed gas is released instantaneously and decays gradually.

CO₂ - 1, 1.5W/m²

CH₄ - 36, 0.48W/m²

N₂O - 300, 0.15W/m²

Refrigerants (CFC-12) - 10600, 0.17W/m²

- Found via bubble plume distribution analysis, in the

Cascadia Margin (in Pacific off the west coast of NA).

- Observed increased activity of methane emission 150-250m deep.

These reservoirs potentially causing warming of North Pacific, also pH increase and tsunami activity.

Perfluorocarbons (PFCs) and SF₆, only present in low amounts.

- Manufacture of aluminium and magnesium

- Semiconductor processing

Effects are complex and not fully understood and they

may decrease warming. Sources of aerosols include:

- Anthropogenic: smoke from burning fossil fuels, sulphates

- Natural: volcanoes, dust-storms

Pre 1750: 280ppm

1999: 367ppm

2020: 418ppm

Land sinks: Photosynthesis, respiration/decay, fossilisation over a long time schedule

Ocean sinks: Plankton, sedimentation of CaCO₃, becoming rock

- Large-scale ocean circulation driven by global density gradients (cold and more saline water is more dense)

- Created by surface heat and freshwater fluxes

- Has an important role in supplying heat to polar regions and thus in regulating sea ice.

- Could be changed due to large shifts in radiative forcing

- Likelihood is uncertain but impacts on climate could be catastrophic

1) Sea level rise (0.1-0.8m), destruction of habitat.

2) Mass extinctions, potentially 1/3rd of species

3) More intense hurricanes

4) Atlantic conveyor shut down

1) Time delays: Greenhouse gases have lifetimes of many years and the ocean reacts to atmospheric changes over decades

2) Improvements in energy efficiency and cleaner energy sources will be offset by population growth and industrialisation in developing countries.

The capture of carbon dioxide and storage.

- Removed from flue gases such as at power stations, before storage in underground reservoirs.

Biological sequestration

Geological sequestration

Marine sequestration

Mineral sequestration

- Reforestation

- Alternative agricultural methods

- Plants can provide biofuels :)

- Requires significant land use :(

- Bury CO₂ in rocks or aquifers

- Can be used to capture CO₂ before it reaches the atmosphere :)

- May leak out over time :(

- Seed oceans with chemicals that increase CO₂ intake.

- Could be an effective an easy fix :)

- Science not fully established, side effects on ecosystem?

:(

The formation of carbonates using captured carbon dioxide.

Example is the potential use of sea urchins which can naturally complete this process.

Urchins are pollutant sensitive, reproducible

Liquefied petroleum gas (LPG) is propane which can work to replace CFCs to reduc damage to the ozone layer. It burns with no soot and low sulphur emission.

Provides electricity and heating. 

• 1. The Sun

• 2. The motion and gravitational potential of the Sun, Moon and Earth

• 3. Geothermal energy from cooling, chemical reactions and radioactive decay in the Earth

• 4. Human induced nuclear reactions

• 5. Chemical reactions from mineral sources (batteries, dry cells etc.)

1) Stand-alone system

2) Grid-connected system

The dilution of solar radiation when it touches the top of the atmosphere.

[image]

Any object when heated above absolute zero emits energy in the form of electromagnetic radiation.

The rate at which energy is emitted increases with temperature, according to the formula for a grey body:

S_b = ε*σ*T⁴ (W/m²)

Where ε is emissivity (between 0 and 1), depends on material and surface finish.

Stefan-Boltzmann Constant: σ = 5.67x10^-8 Wm^-2K^-4

A black body absorbs all light that falls on it (absorbance, α = 1)

Kirchoff's Law states that α = ε.

Meaning that it also emits the maximum possible amount of radiation.

Therefore for a black body:

S_bb = σ*T⁴

E = hf

Where h is Planck's constant (6.63*10^-34)

and f is frequency

E can be measured in joules but is more commonly measured in eV, the energy needed to raise 1 electron through a potential of 1 volt.

1eV = 1.60*10^-19 J

λ_maxT = γ

Where γ = 2.8977729x10^-3 mK

The rate at which radiant energy arrives at a specific area of surface during a specific time interval.

Typically W/m²

Dilution factor = (r/r₀)²

r = Distance from centre of emitter (Sun) to the measured surface (Earth's atmosphere).

r₀ = Emitter radius (Sun radius)

Value for Earth is 46500

S is the irradiance just outside the Earth's atmosphere on a plane facing the Sun.

It is not a true constant as ellipticity of Earth's orbit leads to variation of ±3.5%. The 11 year sunspot cycle of the Sun also leads to variation of about 0.1%

G₀ = S*cosθ

Where: 

S = Solar Constant

θ = Zenith Angle

It is the angle between the sun and the vertical (perpendicular to the Earth's radius at a particular point).

[image]

cosθ = (sinδ*sinφ)+(cosδ*cosφ*cosω)

Where θ = Zenith angle

δ = Angle of declination: sinδ = 0.4sin(2πn/365)

φ = Latitude

n = Day number counted from spring equinox (21st March), when declination is zero

ω = Solar hour angle, propertional to time of day. ω=0 at solar noon and advances 15° every hour.

Global irradiance on a surface, G = B + D + R

Where R = Albedo

B = Direct radiation

D = Diffuse radiation

Albedo, R, is a measure of the reflectivity of the Earth's surface at a certain point.

Albedo irradiance can be neglected for most photovoltaic applications because it is only worth factoring in on polar caps or icy mountains.

Direct component:

B_h = B*cosθ

Where θ is Zenith angle

Direct component, B:

B_s = B*cosβ

Where β is the angle between the Sun and the perpendicular of the tilted surface.

Therefore:

B_s = (B_h*cosβ)/cosθ

Diffuse component, D:

D_s = ½(1+cosγ)D_h

Where γ is the angle between the horizontal and the tilted surface.

[image]

A phenomenon where light is scattered by particles that are larger than or comparable in size to the wavelength of light.

Occurs with aerosols (clouds) and dust.

A greater value for air mass (AM) leads to a decrease in irradiance on the ground. 

Remember that air mass is proportional to Zenith angle.

1) Direct measurements:

- Only available in some locations and may not go back enough years. 

-Usually only gives G_h

2) Number of hours of sunshine measurement on a given location

3) Stochastic modelling: Markov Chain

4) From satellite observations

- The global radiation flux on a horizontal surface on Earth, divided by the extra-terrestrial irradiance on the horizontal.

K = G_h/G₀

In general can't be predicted due to dependence on atmospheric conditions. 

Must be in the range 0-1.

Used to represent a ratio of total radiative energies received over a period of time (e.g. day/month period).

How can sunshine hours be correlated to clearness index?

How is sunshine hours measured?

K = 0.18 + 0.62(N_S/N_D)

Where K = Clearness Index

N_S = Number of hours of sunshine per day

N_D = Number of hours in the day (sunrise to sunset)

Would sum over months of interest

Hours of sunshine per day is measured using a Campbell-Stokes recorder

Diffuse fraction - expresses diffuse radiation as a fraction of global radiation on the horizontal.

K_D = D_h / G_h

Where D_h is diffuse radiation on the horizontal

and

G_h is total radiation on the horizontal.

A process whose future probabilities are determined by recent values.

Needed as sometimes short-term variations are needed but due to storage requirements, this data is often not available, and so random chains need to be generated.

[image]

Database of historical time series of irradiation, temp, humidity etc.

Has hourly data since 2010.

Uses weather station data. Uses interpolation and a stochastic model (along with hourly values of D_h) to resolve direct, diffuse and albedo.

Donor elements have one extra electron than the silicon, acceptor's have one less.

Donor:

- P, As, Sb 

- The additional electron is now free to move through the lattice

Acceptor:

- B, Al, In

-One less bond means neighbouring Si left with an empty state.

Majority carrier: The carrier that exists in higher population

Minority carrier: The carrier that exists in lower population

Valence band - the lower filled band

Conduction band - The upper band (partially filled or empty), free to move and can contribute to electrical conductivity.

For donor atoms in the valence band, the energy level is higher, making it easier for an electron to leave to the conduction band.

In the conduction band, acceptor atoms have a lower energy level, meaning only a small amount of energy is required for an electron from the valence band to move to their energy level.

Diffusion occurs towards the material where that particular donor is the minority carrier (e.g. electron diffusion towards the p-type material)

Carrier drift occurs in the opposite direction. 

Charged particle motion in response to an electric field.

Holes move in the direction of the electric field (+ to -).

Electrons move against the field (- to +).

Average net motion is described by drift velocity, v_d with units cm/second. Net motion gives rise to current.

J_n,drift = q*n*v_d 

Where:

J = Electron drift current density

q = Charge

n = Concentration of electrons

v_d = Drift Velocity

When electric field value is low:

J_n,drift = q*n*μ_n*E

Where:

μ is "mobility" of the semiconductor material.

Max value for v_d is v_sat (saturation vel.)

Within intermediate values of electric field strength there will be a peak value of drift velocty that can be achieved. Want to ensure the system works near this value for faster operation.

[image]

Ohm's Law states: J=σE=E/ρ

Integrating equations for electron & hole drift currents at low electric fields:

σ = q(μ_nn+μ_pp) and ρ = 1/[q(μ_nn+μ_pp)]

However minority carriers changes very little and so can become:

[image]

What is Fick's Law?

Rewritten for electrons and holes in semiconductors?

A description of diffusion:

F = -D∇η

Where η is the concentration and D is the diffusion coefficient.

J_n|diffusion = -qD_n∇n

or 

J_p|diffusion = -qD_p∇p

Equilibrium means there can be no net current flow due to caused by drift and diffusion. They MUST balance each other.

Get the following equations as holes and electrons operate independently:

J_n|diffusion + J_n|drift = 0

J_p|diffusion + J_p|drift = 0

Cylindrical silicon rod produced through slow rotation as it is pulled out of a silicon melt (similar process to making a candle).

Ingot is cut into smaller square blocks before being cut into wafers.

Fine contact fingers made from Ag (silver) paste are screen-printed on the front of the silicon. These are connected to 2-3 bus bars in the perpendicular direction. 

The bus bars provide electrical connection from the top face of the diode.

What is the string-ribbon process for silicon cell production? Advantages?

What is EFG

The liquid silicon spans like a skin between two parallel wires drawn through a melt.  

This method reduces material loss.

EFG is Edge-defined film-fed growth.

Silicon solidifies on an octagonal die upon being drawn out of the melt, these sides become the later wafers

Used to form pyramid-like structure on the surface to improve light capturing. Etching angle depends on crystal orientation.

Has reduced reflection loss. If the inner back surface is made reflective, optical capture also improves.

During charging where does oxidation and reduction occur?

Vs discharging?

Charging

Positive terminal: Anode, oxidation

Negative terminal: Cathode, reduction

Discharging

Positive terminal: Cathode, reduction

Negative terminal: Anode, oxidation

Remember current flows from + to - terminal

ΔG = -nFE

Where ΔG = the Gibbs Free Energy change in kJ/mol

n = Number of electrons in the reaction

F = Faradays Constant = 96485 C/mol

E = The potential in volts

- Increases kinetics of reaction, meaning enhanced current output (until extreme temps)

- Also means new reactions can occur, potentially corrosion and self-discharge.

ΔG = ΔH – TΔS, so cell voltage changes as T changes

- Max and min operation temps need to be considered, temp drop may cause increased electrolyte resistance (even freezing??)

- Large surface area electrodes

- Shorter distance between electrodes

- Increasing operation temp (below extreme)

- Useable voltage range

- Duty cycle/current output/discharge rate

- Shelf life/service life

- Environmental conditions - temp. range

- Physical restrictions - size & weight - energy density

Amount of active material

Units = Ampere-hours

Capacity is decreased by:

- Voltage drop below required supply level

- Extent of utilisation of active material

- Self-discharge reactions consuming active material

Max. estimated using Faraday's Law

Used to estimate cell capacity.

Q = nFm

Where

Q = Charge in coulombs (Amp seconds)

n = Number of electrons in reaction

F = Faraday Constant (96485 C/mol)

m = Molar mass of active material (Mass / Atomic Mass)

96485 C/mol

Means that one mole of electrons is equivalent to 96485 coulombs of electric charge.

A device which stores chemical energy which on

demand can be converted to electrical energy.

- Primary vs Secondary, disposable vs rechargeable

- Consumer vs industrial, small single-cell or large multi-cell

Impurities i.e. iron, nickel and copper, can cause galvanic corrosion, leading to zinc corroding away.

Can be controlled through the use of a corrosion inhibitor or by using cleaner materials during manufacture.

Where dissimilar metals are in contact and exposed to an electrolyte.

Extent of attack is influenced by position in the galvanic series.

Significant internally and at terminals.

Negative terminal (anode):

Zn -> Zn²⁺ + 2e^-

Positive terminal (cathode):

Mn⁴⁺ + e^- = Mn³⁺

Electrons flow from anode to cathode, powering circuit.

- Manganese dioxide cathode powder

- Zinc anode powder

- Alkaline paste of potassium hydroxide

Control venting required.

Commercialisation was therefore delayed until polymer seals and sleeves became available.

- Have lower internal resistance vs. zinc/carbon

- Current collectors are more corrosion resistant, with even finer carbon powder.

- More utilisation of active materials.

- Have a good shelf life

- 4 times higher capacity vs. zinc/carbon.

- Good low temperature operation

Anode material in Li primary cells? Equation?

Cathode materials? Electrolyte requirements?

- Anode material is always lithium

- Li(s) -> Li⁺ + e^-

- Common cathodes : manganese dioxide or

thionyl chloride, SOCl2 , giving 3V and 3.5 V,

respectively 

- Electrolyte has to be non-aqueous, i.e. polar organic solvent/conducting polymer or conducting ceramic.

- Addition of inhibitors to electrode materials to decrease gassing

- In lead/acid cells can remove impurities from

lead alloy grids which promote gassing

- Life ends when output is not adequate to drive load or when capacity is too small

Caused by:

- Corrosion of grids/busbars/terminals

- Excessive charge or discharge rates - heating

- Overcharging - loss of water

- Dendrite growth leading to internal shorting

- Leaving discharged for a long time - crystallisation

Lead and lead oxide

Issues with lead-acid battery?

Prevention?

Lead sulphate has low conductivity and can build up on electrode surfaces during discharge (passivation).

- Deep discharge may lead to insoluble sulphate crystals depositing.

- Aim to maximise electrode surface area and

prevent passivation – by adding barium

sulfate to solution and carbon to electrode

surfaces

- Also do not leave the cells discharged for any

long periods of time.

Occurs in cells with aqueous electrolytes. 

At positive electrode - Oxygen produced

At negative electrode - Hydrogen produced

Summarised equation:

H₂O → ½O₂ + H₂

From ΔG, E = 1.23V

Can occur in cells with a greater voltage.

In electrolyte, concentrated acid can sink during charging as concentration is related to density. This can cause sulphate formation in the cell base.

Electrolyte dilutes and rises during recharge cycles.

- Active materials are stable with low self-discharge

- Reactions easily reversible

- High electrolyte conductivity

- Reasonable cost/can be recycled

- Sulphate conductivity and solubility are low

- Plate material losses, shedding then shorting between electrodes at cell base.

- Stratification of electrolyte.

Checked via specific gravity.

Cell voltage is greater when acid concentration, and hence density are greater.

Charging usually constant voltage DC source. Overpotential required to drive electrons, approx. 0.3V per cell. 

12V battery is 6 2V  cells:

0.3*6 = 1.8V

Therefore 13.8V charging voltage.

- Start with high initial current flow and low voltage.

- Sudden current drop once voltage reaches "float" voltage, stays like this for the rest of the charge time.

Used in lead/acid cells. For 12V battery float voltage set to 13.8V due to need for overpotential.

 [image]

- Cheap charging system, no voltage regulator.

- Cannot leave on as current does not fall to near zero with time. Taper gradient relates to internal resistance of the cell and charger unit.

[image]

- Constant current - used in test labs for Q=It data

- Combination voltage/current control - more expensive/complex chargers which improve battery life by careful operation e.g. pulse charging and switch mode charging

- Battery Monitoring Systems (BMS) - Advanced battery charging with feedback and control.

a) Cast alloy grid.

b) Expanded alloy grid.

c) Current balancing grid where conductor section increases towards connector.

d) Tubular Design - less shedding and more vibration resistant.

 [image][image]

- Used to be bitumen, now injection moulded polymers that can be recycled.

- Allows space for debris/shed material to collect at base without shorting.

- Valve regulated batteries, can be used in any orientation.

- Controlled venting

- Gel electrolytes - decrease acid leakages

- Little fluid loss

- Seal electrolyte away from circuitry

- Decrease terminal corrosion

Advantages:

Excellent temperature performance: -20 to 40 degrees min.

Reliable and low maintenance

Disadvantages:

Cadmium an environmental issue

In sealed cells memory effect is an issue

If cells partially discharged and re-charged the cell capacity decreases cycle-by-cycle.

Must discharge fully and then fully charge to recondition.

Jelly roll design has to be built under inert gas to prevent Li catching fire. Means it is expensive to make.

However it is done for high surface area and low internal resistance.

Range of operation is -20° to 150°C.

At -20, performance drops to 30%.

Replacing the electrolyte with lower temperature fluids can be a fix.

Pros: 

Cheaper, more stable lithium-iron phosphate cathode with a graphite anode.

No issues with cobalt raw material supply for cathode.

Cons:

Slightly lower voltage than other lithium cells.

Some conductivity issues and poor low temperature operation.

As well as the standard electrostatic charge, an ionic electrolyte also holds charge in a double layer.

Store 10 to 100x more energy per unit volume than standard capacitors.

- Batteries with integral charging and monitoring, improving efficiency.

- Integrated circuit optimises charging of individual cells.

- Re-calibration to battery required as cells age to prevent over-charging, and cost 25% more to produce.

Inputs: 

Fuel and Oxidants (O₂)

Outputs:

Products, electrical energy (IVt), heat.

1) Separate fuel and oxidant

2) Facilitate ion transport between anolyte and catholyte

3) Prevents electrical short circuit between anode and cathode.

Can be liquid, solid or polymeric.

Equations at anode and cathode in typical fuel cell?

Oxidation or reduction?

Anode:

H₂ → 2H⁺ + 2e^-

Electrons are removed from the fuel, therefore oxidation

Cathode:

½O₂ + 2H⁺ + 2e^- → H₂O

Electrons used to reduce the oxidant (reduction)

Electrons pass from anode to cathode via external circuit, and so do electrical work (electrical circuit).

Protons pass through the electrolyte from anode to cathode, where the e^- are used to reduce O₂ to water.

H₂ + ½O₂ → H₂O.

Two electrons transferred per mole H₂ 

Combines entropy and enthalpy into a single value. The charge in free energy, ΔG is equal to the sum of enthalpy plus the product of entropy and temperature in the system.

G = H - TS

η, an indication of deviation of cell voltage/potential from that predicted by thermodynamics.

E_cell = E_therm - η_total

Where:

η_total = η_act + IR + η_conc

act -> activation losses

conc -> mass transport (concentration) losses

Refers to the active material breaking away from the plates and falling to the bottom of the battery.

Process: Lead sulphate forms on electrode surface causing expansion, during discharge this gets released back into the active material causing plate contraction.

Repeated expansion and contraction weakens the bond between active material and plate grids.

Ends up forming conductive layer at bottom of battery.

The mass of a substance (m) altered at an electrode during electrolysis is directly proportional to the quantity of electricity transferred at that electrode. Quantity of electricity refers to quantity of charge.

m_O = MQ/zF = MIt/zF

Where M is molecular mass

PV = nRT

P = Pressure (Pa not atm)

V = Volume

n = Number of moles

R = 8.314

T = Temp (Kelvin)

To maximise current, I, the surface area, A, must be maximised, or current density, J, can be increased. J is an electrode property but can be increased through the use of a catalyst.

Connecting multiple individual cells within the fuel cell casing, typically in series to give the required voltage. Known as a fuel cell 'stack'.

Common arrangement employs bipolar plates with the anode of one cell pressed against one side and the cathode of the next cell on the other.

Simply a conducting block, usually graphite with channels cut into one face to conduct the liquid/gas fuel and in the other face for air or oxygen.

Voltage: Dependent on the number of layers

Current: Dependent on the surface area of the electrodes

AFC - Alkaline Fuel Cell

PEMFC - Polymer Electrolyte Membrane Fuel Cell

DMFC - Direct Methanol Fuel Cell

MCFC - Molten Carbonate Fuel Cell

PAFC - Phosphoric Acid Fuel Cell

SOFC - Solid Oxide Fuel Cell

Potassium Hydroxide (KOH)

Has high surface tension and substrate pores can be flooded. Therefore a thin layer of PTFE is placed over the surface of the gas diffusion layer (GDL) control porosity and prevent electrolyte passing through the electrode.

Why is the electrolyte in AFCs normally pumped?

Purpose of evaporator?

Allows for the straightforward replacement of 'spent' electrolyte, called 'mobile electrolyte'.

Movement of the electrolyte prevents crystallisation of the KOH at the cathode.

Evaporator allows excess heat produced in the FC to be removed.

- Mass

- Additional equipment required

- Surface tension of KOH means additional care to prevent leaks.

- Often needs to be used at a particular orientation

- CANNOT use bipolar plates due to need for a PTFE layer

- Not practical to have each cell in a stack pumped individually.

- All cells being in ionic contact can lead to a 'leak current' and short-circuiting.

To solve this:

1) Flow path between cells are made as narrow as possible.

2) Cells are connected by a combination of series and parallel connections.

Orientation problems, as such a static electrolyte has to be used.

This means electrolyte replacement is not possible, and as such H₂ and O₂ used must be pure.

PEM Nafion Fuel Cells were developed.

Liquid electrolytes are expensive in energy and cost to handle.

Hydroxide ions attack the Alkaline Anion Exchange

Membrane

Nafion had better ionic conductivity.

- Carbon-fluorine bonds are very strong and resistant to chemical attack. 

- Mechanically strong so can be used as very thin sheets

- Sulphonic acid groups cluster in hydrated regions, and attract water

- Nafion absorbs up to 50% of its dry weight in water.

- Conductivity increases when hydrated (actually needed!)

- Highly conductive to H⁺ when hydrated

CO in fuel must be below 100ppm - cover the Pt catalyst.

Prevent hydrogen reaching the catalyst, called CO poisoning.

In DMFCs, methanol is oxidised directly at the anode. 

In IMFCs, methanol is reformed into H₂.

- Methanol is comparatively easy to transport vs hydrogen.

- Lower market entry barriers (less explosive)

- Cheap and easy to manufacture

- Allows for low temperature operation

- High energy density

- Poor oxygen reduction kinetics. Much lower power density than H₂/O₂

- Poisoning an issue due to adsorbed CO. Ru co-catalyst needed.

- Methanol permeates through the membrane (membrane cross-over), reducing cell voltage.

- Agglomeration of Pt

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- Enhanced cathode kinetics, lower activation voltage losses, noble metal catalyst often not needed.

- Heat recovery, combined heat and power (CHP)

- Simpler operation

- Enhanced CO tolerance

- Temperature matching with hydrogen supply system means smaller, lighter, more efficient systems.

Above 150°C, phosphoric acid has high conductivity, solubility for oxygen and low chemisorption of Pt.

Only stable inorganic acid, with a low volatility at these temperatures.

Below 150°C the volatility is too high. Tends to decompose above 220°C

- At higher temperature, CO tolerance is improved (1-2%)

- Stationary Supply

- Large vehicles

- Limited by large mass due to need for external equipment.

Although there is good CO tolerance, H₂S in reformates can poison the anode catalyst.

Therefore a desulphuriser is needed

- H₂S poisoning anode catalyst

- Requires a lot of external equipment, increasing weight. 

- Reliability, lifetime and maintenance costs.

- Corrosive electrolyte

- Low power density

Anode:

CO₃^2- + H₂ → H₂O + CO₂ + 2e^-

Cathode: 

CO₂ + ½O₂ → CO₃^2-

CO₂ would cancel out in overall reaction but can be shown as:

H₂ + ½O₂ + CO_2(Cathode) → H₂O + CO_2(Anode)

Anode:

Ni/Cr/Al Alloy

Cathode:

NiO

Both porous

Need to establish a stable electrolyte/gas interface in the porous electrodes. This requires balancing of capillary pressures to establish electrolyte interfacial boundaries.

Ensures complete filling of the electrolyte matrix with molten carbonate, whilst the porous electrodes are only partially filled.

- Noble metal catalysts no longer necessary

- At the anode can employ CO as a fuel

- Wider range of useable feedstocks

- Indirect internal reforming (IIR) - reforming occurs in separate compartment to electrochemical reactions.

Anode:

H₂ + O^2- → H₂O + 2e^-

CO + O^2- → CO₂ + 2e^-

Cathode:

O₂ + 4e^- → 2O^2-

Overall:

H₂ + CO + O₂ → H₂O + CO₂

(4 electrons transferred)

- Simpler concept than any other fuel cells 

- CO is a fuel rather than a poison.

- SOFC most sulphur-resistant fuel cell, although generally removed before reformation.

- Precious metal catalysts not needed

- Internal reforming is possible

- A wide range of fuels can be employed

- Reduced ohmic losses through electrode and electrolyte material.

Zirconia, ZrO₂, stabilised by Yttria (YSZ), Y₂O₃.

Relatively stable at high temps and can be made thin to minimise ohmic losses.

Incorporation of internal reforming is seen as necessary, due to reduced complexity and the cost of an external reformer.

Problems:

The endothermic reforming reaction leads to coking.

Can be caused by both methane and carbon monoxide reacting and producing carbon.

Oxidation of Ni to NiO.

- Normally nickel oxide mixed with YSZ to form Cermet (CERamic METal)

- Catalysts limited to Ni, Co, Pt and Au

- Active with respect to oxygen reduction

- Stable

- Highly electronically conducting

- Porous to O₂

- Unreactive in respect to other components

LaSrMnO₃ most commonly used

Tubular. Joined together into stacks, consisting of 'bundles'.

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- Link anodes and cathodes of adjacent cells

- Must be stable in high temperatures in both oxidising and reducing environments

- Must not react with electrode materials.

- Must be impermeable to the fuel and air.

- Must be a thermal match with YSZ electrolyte.

- High ohmic loss/low power densities

- Coatings must be made via electrochemical vapour deposition.

- LaCrO₃ interconnects hard to process

- Utilise significantly shorter conduction paths, hence higher power densities.

- Interconnects bipolar plates providing both electronic connection and fuel/oxidant distribution

- Sealing is hard

- Glass ceramics are the only major sealing option, very brittle and prone to thermal stress failure.

- Glass seals also prone to failure under the tension necessary for sealing.

- Difficult to manufacture thin enough

- Difficult to manufacture large areas.

η_max,th = E_r/E_h = ΔG_r/ΔH_r = 1 - (TΔS_r/ΔH_r)

This is at zero current