chemistry
DESCRIPTION
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BATTERY TECHNOLOGY
Commercial Cells.
Galvanic cells used as source of electric energy for various consumer, industrial and military applications.
Classification:
Primary Cells. Eg.: Dry cell
Secondary Cells. Eg.: Lead acid cell.
Objectives: Describe the major features of commercial
cells.Know the two major types of batteries.Distinguish between primary & secondary
battery types.Know the various applications of Dry cell,
Nicad cell, Lead acid cell, H2-O2 fuel cell and CH3OH – O2 fuel cell.
Basic Requirements Of Primary Cell.
Compactness and lightweight. Fabricated from easily available raw materials. Economically priced. High energy density and constant voltage. Benign environmental properties Longer shelf life and discharge period. Leak proof containers and variety of design
options.
Basic Requirements Of Secondary Cell.
Long shelf-life and cycle life.
High power to weight ratio
Short time for recharging
Tolerance to service condition.
High voltage & high energy density.
Primary Cells.
Produce electricity from chemicals that are sealed into it.
Cannot be recharged as the cell reaction cannot be reversed efficiently by recharging. The cell must be discarded after discharging.
e.g. Zinc - manganese dioxide cell (Dry cell)
Mercuric oxide – Zinc cell.
Silver oxide – zinc cell.
Secondary Cells
Generation of electric energy, that can be restored to its original charged condition after its discharge by passing current flowing in the opposite direction. These cells have a large number of cycles of discharging and charging. They are known as rechargeable cells, storage cells, or accumulators.
e.g. Lead storage cell.Nickel- cadmium cell.Lithium- ion batteries.
Differences
Primary Batteries Secondary Batteries Cell reaction is irreversible Cell reaction is reversible.
Must be discarded after use. May be recharged Have relatively short shelf life Have long shelf life. Function only as galvanic Functions both galvanic cells . Cell & as electrolytic cell. They cannot be used as They can be used as energy
storage devices storage devices (e.g. solar/ thermal energy
converted to electrical energy) They cannot be recharged They can be recharged.
e.g. Dry cell. Li-MnO2battery.Lead acid, Ni-Cd battery.
DRY CELL(LECLANCHE CELL)
• Anode: Zinc metal container.
• Cathode: MnO2 + Carbon (powdered graphite)
• Electrolyte: Aqueous paste of NH4Cl and
ZnCl2
• Cell Scheme:
Zn(s)/ ZnCl2(aq),NH4Cl(aq),MnO2(s)/C
• O.C.V. = 1.5 V
Working.
Primary Electrode Reactions:
Anode: Zn(s)→Zn2+ (aq)+ 2e-
Cathode: 2MnO2(s)+H2O(l) + 2e- → Mn2O3(s) + 2OH-
(aq)
Net Reaction: Zn(s)+2MnO2(s)+ H2O(l) → Zn2+
(aq)+Mn2O3(s)+2OH-(aq)
Secondary Reactions:
NH4+
(aq)+OH-(aq) → NH3(g)+H2O(l)
Zn2+(aq)+2NH3(s)+2Cl - → [Zn(NH3)2 Cl2]
Zn + 2MnO2 + 2NH4Cl →[Zn(NH3)2Cl2]+ H2O+
Mn2O3
Applications:
In small portable appliances where
small amount of current is needed.
In consumer electronic devices-
quartz wall clocks, walkman etc.
Advantages.
Dry cell is cheap.
Normally works without leaking (leak proof cells).
Has a high energy density.
It is not toxic
It contains no liquid electrolytes.
Disadvantages.
Voltage drops due to build up of reaction products around the electrodes when current is drawn rapidly from it .It has limited shelf life because the zinc is corroded by the faintly acidic,ammonium chloride. The shelf life of dry cell is 6-8 months.They cannot be used once they get discharged.Its emf decreases during use as the material is consumed.
Lead-acid battery:
LEAD STORAGE BATTERY.
• Anode: Spongy lead on lead grid.
• Cathode: Porous PbO2.
• Electrolyte: H2SO4(aq)( 20 %)
(density 1.21-1.30g/ml)
• Cell Scheme:
Pb/PbSO4;H2SO4(aq);PbSO4;PbO2/Pb
O.C.V. = 2V (Pair of plates)
Reactions during discharging.
• Anode: Pb (s) → Pb2+ (aq) + 2e-
Pb2+(aq) + SO4
2-(aq) → PbSO4(s)
Pb(s)+ SO42-
(aq) → PbSO4(aq) + 2e-
• Cathode:PbO2(s)+ 4H+(aq)+2e- →Pb2+
(aq)+ 2H2O(l)
Pb2+(aq)+SO4
2-(aq)→PbSO4(s)
PbO2(s)+4H+(aq)+SO4
2-(aq)+2e- → PbSO4(s)+
2H2O(l)
• Overall: Pb (s)+PbO2 (s)+4H+(aq)+ 2SO4
2-(aq) →
2PbSO4 (s)+2H2O(l)
Charging the Lead-acid battery:
Charging reactions
• Cathode:
PbSO4(s)+2H2O(l)→PbO2(s)+ SO42-
(aq)+4H+(aq)
+2e-
• Anode :
PbSO4(s) + 2e- → Pb(s)+ SO42- (aq)
• Net:2PbSO4 (s)+ 2H2O(aq) → Pb(s)+ PbO2(s) +2H2SO4
Limitations.• Self discharge: They are subject to self discharge
with H2 evolution at negative plates and O2 evolution at positive plates.
Pb +H2SO4 PbSO4 + H2
PbO2 + H2SO4 PbSO4 +H2O +1/2 O2
Loss of Water: Due to evaporation, self discharge and electrolysis of water while charging. Hence water content must be regularly checked and distilled water must be added.
• Sulfation: If left in uncharged state, for a prolonged
period, or operated at too high temperatures or at too
high acid concentrations, transformation of porous
PbSO4 into dense and coarse grained form by re
crystallization.
* This results in passivation of negative plates
inhibiting their charge acceptance.
• Corrosion of Grid: Can occur due to
overcharging when grid metal gets exposed to
the electrolyte. This weakens the grid and
increases the internal resistance of the battery.
• Effectiveness of battery is reduced at low
temperature due to increase in the viscosity of
electrolyte.
• Recent years have seen the introduction of “maintenance – free batteries” without a gas – release vent. Here the gassing is controlled by careful choice of the composition of the lead alloys used i.e. by using a Pb-Ca (0.1 % ) as the anode which inhibits the electrolysis of water
• Alternatively, some modern batteries contain a catalyst (e.g. a mixture of 98% ceria (cerium oxide) & 2% platinum, heated to 1000o C) that combines the hydrogen and oxygen produced during discharge back into water. Thus the battery retains its potency and requires no maintenance. Such batteries are sealed as there is no need to add water and this sealing prevents leakage of cell materials.
Applications.
*Automative: For starting, lighting and
ignition of IC engine driven vehicles.
*Consumer Applications: Emergency
lighting, security alarm system.
*Heavy duty Application: Trains, lift
trucks, mining machines etc.
Advantages:
A lead storage battery is highly efficient. The voltage efficiency of the cell is defined as follows.
Voltage efficiency = average voltage during discharge
average voltage during charge
The voltage efficiency of the lead – acid cell is about 80 %.
The near reversibility is a consequence of the faster rate of the chemical reactions in the cell i.e. anode oxidizes easily and cathode reduces easily leading to an overall reaction with a high negative free energy change.
A lead – acid battery provides a good service for several years. Its larger versions can last 20 to 30 years, if carefully attended (i.e. longer design life)
It can be recharged. The number of recharges possible range from 300 to 1500, depending on the battery’s design and conditions. The sealed lead-acid batteries can withstand upto 2000 – rechargings. Generally the most costly, largest, heaviest cells are the longest–lived.
The battery’s own internal self – discharging is low.
The length of time that is generally required for re-charging process is less i.e. recharge time is 2-8 hours depending on the status of battery.
Low environmental impact of constituent materials is an added advantage
It has sensitivity to rough handling and good safety characteristics.
Ease of servicing as indicated by several local battery service points.
It is a low- cost battery with facilities for manufacture throughout the world using cheap materials.
NICKLE- CADMIUM CELL
Anode: Porous cadmium powder
compressed to cylindrical pellets.
Cathode: Ni(OH)3 or NiO(OH) mixed with 20%
graphite powder
Electrolyte: 20-28% Aq. KOH jelled with a
jelling agent.
Cell Scheme:
Cd/Cd(OH)2,KOH,Ni(OH)2, Ni(OH)3/Ni
O.C.V. = 1.25V
Reactions during discharging.
Anode:
Cd(s)+2OH-(aq)→Cd(OH)2(s)+ 2e-
Cathode:
2Ni(OH)3(s)+2e- → 2Ni(OH)2(s)+2OH-(aq)
• Net Reaction:
Cd(s)+2Ni(OH)3(s)→ 2Ni(OH)2(s)+ Cd(OH)2(s)
Charging reactions:Anode:Cd(OH)2(s)+2e-→ Cd(s) +2OH-
(aq)
Cathode: 2Ni(OH)2(s) +2OH-
(aq)→2Ni(OH)3(s)+2e-
Net:
2Ni(OH)2(s)+Cd(OH)2(s)→2Ni(OH)3(s)+ Cd(s)
Discharging reaction:
Anode: Cd(s)+2OH-(aq) → Cd(OH)2(s) +
2e-
Cathode: 2NiO (OH) (s) + 2 H2O + 2 e- →
2Ni (OH)2(s) + 2OH-(aq)
Net Reaction: Cd(s) + 2NiO (OH) (s) + 2H2O → 2 Ni(OH)2 (s) + Cd(OH)2(s)
Charging reactions:
-ve pole: Cd(OH)2 (s) + 2e-→ Cd(s) + 2OH-
(aq)+ve pole: 2 Ni(OH)2(s) + 2OH-(aq) → 2
NiO(OH) (s) + 2H2O+2e-
Overall reaction: 2 Ni(OH)2 (s) + Cd(OH)2(s) →
2 NiO(OH) (s) + Cd(s) +2H2O(l)
Applications.
In flash lights, photoflash units and portable electronic equipments.
In emergency lighting systems, alarm systems.
In air crafts and space satellite power systems.
For starting large diesel engines and gas turbines etc.,
Advantages.
Can be recharged many times.They maintain nearly constant voltage level through out their discharge. There is no change in the electrolyte composition during the operation.It can be left unused for long periods of time at any state of charge without any appreciable damage (i.e. long shelf life).It can be encased as a sealed unit like the dry cell because gassing will not occur during nominal discharging or recharging.They exhibit good performance ability at low temperatures.
They can be used to produce large instantaneous
currents as high as 1000-8000 A for one second.
It is a compact rechargeable cell available in three
basic configurations – button, cylindrical and
rectangular.
They have low internal resistance.
Disadvantages.
It poses an environmental pollution hazard due to higher toxicity of metallic cadmium than lead.
Cadmium is a heavy metal and its use increases the weight of batteries, particularly in larger versions.
Cost of cadmium metal is high and hence the cost of construction of NiCad batteries is high.
The KOH electrolyte used is a corrosive hazardous chemical.
Lithium cells/ batteries
Lithium is a theoretically active material for negative electrode of the electrochemical cells owing to its least noble nature and low specific gravity.
1. Primary cells with metallic lithium electrodes and non-aqueous electrolytes were successfully introduced into the market with outstanding features like high voltage, high energy density, low self-discharging rate and wide range of operation etc.
2. The secondary lithium negative electrodes have attracted much attention with high energy density however they are facing many more practical problems such as poor cycle life, need for long charging time, poor safety characteristics etc.,
Primary lithium cells
Lithium primary cells can be classified into several categories, based on the type of electrolyte and cathode material that is used.
• Soluble cathode cells:- Use liquid or gaseous cathode materials, such as SO2,
SO2Cl2 (sulfuryl dichloride) , SOCl2 etc that dissolve in the electrolyte or in the electrolyte solvent. Results in the formation of protective thin film on the lithium anode
• Solid cathode materials:- Uses solid for the cathode. Ex. Li/MnO2 cell
Ex: 1) LITHIUM/COPPER SULFIDE (Li/CuS) 2) LITHIUM/COPPER OXIDE (Li/CuO) CELLS:-• Solid Electrolyte cells:- Extremely long storage life
LITHIUM/COPPER SULFIDE (Li/CuS)
Construction
• Anode: Lithium
• Cathode: Copper sulphide.
• Electrolyte: mixture of 1,2-dimethoxyethane, 1,3-dioxolane
and 3,5-dimethylisoxazole as a stabilizer with LiClO4 solute.
OR
Tetrahydrofuran (THF) and dimethoxyethane (DME) binary
solvent with LiClO4 for the solute
O.C.V. = 1.7 V
Cell reaction
• Anode reaction: x Li x Li+ + x e-
• Cathode reaction: CuS + xLi+ + xe- LixCuS
On continued discharge, the second step of the cathode discharge occurs:
2LixCuS + 4e- 2LixCu + 2S2-
An electrolyte reservoir is placed between the cathode and can to obtain the most efficient use of the cathode. This reservoir retains electrolyte at the cathode surface next to the can and improves higher discharge rate performance.
disadvantages
• Cells can withstand short-circuit but should not be forced-discharged or exposed to temperatures as high as 180oC,the melting point of lithium.
• New designs, replacing the LiClO4 which is very reactive, appear effective in Improving the safety of the cell under abusive conditions.
Lithium ion cells:-
The essential feature of the Lithium ion battery is that at no stage in the charge-discharge cycle should there be any Lithium metal present.
Rather, Lithium ions are intercalated into the positive electrode in the discharged state and into the negative electrode in the charged state and move from one to the other across the electrolyte.
Lithium-ion batteries thus operate based on what is sometimes called the "rocking chair" or "swing" effect. This involves the transfer of lithium ions back and forth between the two electrodes. Hence called lithium rocking chair or swing batteries.
Construction Cathode: a layered graphite crystal into which
lithiated metal oxides are inserted.
Anode: made up of graphite, coated on copper foil 14.
Separators: polyolefin’s using 3-8- μm layers with 50% porosity.
Electrolyte: The electrolyte is usually a 1-molar solution of a lithium salt in an organic solvent.
Ex: 1)Lithium hexafluorophosphate (LiPF6) in the solvent
propylene carbonate 2)Lithium tetrafluoroborate(LiBF4 ) in the solvent
ethylene carbonate
Discharge reaction:- The main principle is based on the movement of
lithium ions between anode and cathode through the electrolyte occurs during charge and discharge process.
Anode reaction: Li ( C ) Li+ + e-
Cathode: Li+ + e- +CoO2 LiCoO2
The overall cell reaction is as follows:
Overall: CoO2 + Li ( C ) LiCoO2
Applications: Lithium-ion batteries are most commonly used in mobile telephones and mobile computing devices, where the battery needs to be a particular shape, laptops, cellular phones, camcorders.
Cathodes:-
Cathode material Practical Theoretical Capacity
LiCoO2 140 275
LiNiO2 (or mixed) 190-200 274
LiMn2O4 120 148
Advantages:- (b) Smaller, lighter and provide more energy. (c) Operated in a wide temperature range.
Disadvantages: (a) Poor charge retention. (b) High self discharge rate. (c) Highly expensive.
Lithium –ion battery characteristics
Type Secondary Chemical reaction Varies, depending on electrolyte Operating temperature 4oF to 140oF (-20oC to 60oC) Recommended for Cellular telephones, mobile
computing devices. Initial voltage 3.6 & 7.2 Capacity Varies (generally up to twice the
capacity of a Ni-Cd cellular battery) Discharge rate Flat Recharge life 300-400 cycles Charging temperature 32oF to 140oF (0oC to 60oC) Storage life Losses less than 0.1% per month Storage temperature -4oF to 140oF (-20oC to 60oC)
Discharge-charge cycle
Serial no Cobalt Manganese
Energy density
(Wh/kg)
140 120
Safety On overcharge, the cobalt electrode
provides extra lithium, which can form
into metallic lithium, causing a
potential safety risk if not protected by
a safety circuit.
On overcharge, the manganese electrode
runs out of lithium, causing the cell only
to get warm. Safety circuits can be
eliminated for small one- and two-cell
packs
Temperature Wide temperature range Capacity loss above 40 degrees C
Aging Short-term storage is possible,
impedance increases with age newer
versions offer longer storage
Slightly less aging than cobalt, impedance
changes little over the life of the cell, and
due to continuous improvements, the
storage time is difficult to predict.
Life expectancy Minimum 300, 50 percent at 500 cycles May be shorter than cobalt
Cost Raw materials is relatively high,
protection circuits adds to costs
Raw materials are 30 percent lower than
cobalt. Cost advantage on less circuitry
LiMn2O4:-
(1) Lower charge-discharge efficiency at the first cycle than LiCoO2.(2) Highest stability among the three candidates at high temperatures.(3) Low cost.(4) Abundance of manganese resources.(5) Poor cyclability.(6) A little higher operating voltage than LiCoO2.
Fuel Cells.
A fuel cell is a galvanic cell in which chemical energy of a fuel – oxidant system is converted directly into electrical energy in a continuous electrochemical process.
• Cell Schematic Representation:
Fuel;electrode/electrolyte/electrode/oxidant.
e.g. H2-O2; CH3OH-O2
• The reactants (i.e. fuel + oxidant) are constantly supplied
from outside and the products are removed at the same
rate as they are formed.
• Anode:
Fuel+ oxygen → Oxidation products+ ne-
• Cathode:
Oxidant + ne- → Reduction products.
Advantages:High fuel to electricity conversion efficiency of 70-75% is observed in fuel cells. Typically, a thermal power plant converts only 35-40% of the chemical energy of coal to electricity. Therefore, the efficiency of a fuel cell is about twice that of a conventional thermal power plant, thus saving natural fuel resources.Fuel cell producers do not cause pollution problems such as noise pollution, chemical pollution & thermal pollution normally associated with conventional power plants.
A fuel cell will produce a steady electric current as long as fresh reactants are available.
A small instrument contain fuel cell could be used to detect ‘drinking & driving’by a roadside test by police.
Fuel cells operate at near constant efficiency independent of size. So fuel cell power plants can be configured in a wide range of electrical levels from a few watts to hundreds of MW.
High reliability and low maintenance
The amount of CO2 released into the atmosphere is less per MW of electricity than other electricity generating processes, which is very important for environmental reasons.
Disadvantages
Degradation or malfunction of components limits the practical operating life of working fuel cells on a large scale.
They are sensitive to fuel contaminants such as CO,H2S, NH3 & halides, depending on the type of fuel cell. These contaminants must be minimized in the fuels to enhance the cells’ efficiency
High initial cost because of the expensive noble metals required in the construction of certain fuel cells. At the moment, cells with relatively inexpensive fuels require expensive catalysts, whereas those with relatively inexpensive catalysts require expensive fuels.
Classification of fuel cells:-
Alkaline fuel cell
Phosphoric acid fuel cell
Proton exchange membrane fuel cell
Direct methanol fuel cell
Molten carbonate fuel cell
Solid oxide fuel cell
Alkaline fuel cell
One of the first fuel cell technologies developed
First type widely used in the U.S. space program to produce electrical energy and water onboard spacecraft
• Anode: Porous graphite electrode/ porous nickel
electrode impregnated with finely divided Pt/Pd.
• Cathode: Porous graphite electrode/porous nickel
electrode impregnated with finely divided Pt/Pd.
• Electrolyte: 35-50% KOH held in asbestos matrix.
• Operating Temperature: 90oC.
• O.C.V. =1.20V
• Anode :
2H2(g) +4OH- (aq)→ 4H2O(l)+4e-
• Cathode:
O2(g)+2H2O(l)+4e- →4OH (aq)
• Net Reaction:
2H2(g)+O2(g)→2H2O(l).
*Water should be removed from the cell.
*O2should be free from impurities.
Applications.
Used as energy source in space shuttles e.g. Apollo spacecraft.
Used in small- scale applications in submarines and other military vehicles.
Suitable in places where, environmental pollution and noise are objectionable.
Limitations and contemporary advancements
• Method of preparation of the electrodes.
• Costs of the electrode, stacks and fuel cell systems.
• Life time of the electrode.
• Diaphragm made of asbestos.
• CO2 – contaminated fuel gases (carbonating of electrolyte and electrodes).
Phosphoric acid fuel cell (PAFC)
• Construction:-• Anode: - PTFE-bonded Pt/C Vulcan XC-72 0.10 mg
Pt/cm2
• Cathode: - PTFE-bonded Pt black Vulcan XC-72 0.25 mg/cm2
• Electrode support: - Carbon paper• Electrolyte support:- PTFE-bonded SIC (ball milling)
• Electrolyte:- Liquid H3PO4 99% (wt) with additives like imidazole and 1-methyl imidazole
• Interconnect:- Graphite• Catalyst:- Platinum• Operating temperature:-150- 205oC. • Charge carrier:- H+
first generation” of modern fuel cells.
typically used for stationary power generation
Cell reactions:- At anode:- 2H2 4H+ + 4e- (E0 = 0.0 V Vs. SHE)
At cathode:-
O2 + 2H2 2H2O (Ecell = 1.23 V)
O2 + 4H+ + 4e- 2H2O (E0 = 1.23 Vs. SHE)
Net reaction:-
Advantages
• PAFCs are much less sensitive to CO than PEFCs and AFCs.
• The operating temperature is low enough to allow use of common construction materials, the operating temperature also provides considerable design flexibility for thermal management.
• PAFCs have demonstrated system efficiencies of 37 to 42 percent.
• The expelled water can be converted to steam for space and water heating. In this combined heat and power application, overall efficiencies can approach 80
Disadvantages
• Cathode-side oxygen reduction is slower than in AFC, and requires the use of a platinum catalyst.
• PAFCs still require extensive fuel processing, including typically a water gas shift reactor to achieve good performance .
• The highly corrosive nature of phosphoric acid requires the use of expensive materials in the stack.
Applications
1. Extensive use in stationary power industry where on site, high quality and reliable power is needed
2. Used in hotels, hospitals, office buildings and large vehicles( buses) in USA , Japan etc.
Proton exchange membrane cell
Construction• Electrolyte:- Ion exchange polymeric membranes. • Electrodes:- Typical gas diffusion electrodes,
made up of porous C impregnated with Pt catalyst.
• Fuel:- Hydrogen• Oxidant:- Air• Catalyst:- Platinum• Interconnect:- Carbon or metal• Operating temperature:- 40 – 80oC. • Charge-carrier:- H+
Perflourinated membrane by DuPont.
C
F
F F
F
C C
F
F
C
F
F
C
F
C
F
F
C
F
F
C
F
F
C
F
FO
C FF
F FC
O
F FC
F FC
O=S=O
O-
H+
PERFLUOROSULFONIC ACID MEMBRANE
Single cell structure of representative PEFC
Membrane-electrode assembly (MEA)
• PEMFC electrode • Anode reaction:
• H2 2H+ + 2e-
• Cathode reaction:
• O2 + 2H+ + 2e- H2O
• Overall reaction:-
• H2 + O2 H2O
Advantages
• Solid electrolyte provides excellent resistance to gas crossover.
• Low operating temperature allows rapid start-up
• capable of high current densities
Disadvantages:-• The low and narrow operating
temperature range makes thermal management difficult
• Dehydration of the membrane reduces the proton conductivity and excess water can lead to the flooding of the electrolyte. Both the conditions leading to poor performance.
• Perflourinated membranes have high cost
• quite sensitive to poisoning by trace levels of contaminants including CO, sulfur species and ammonia.
Direct methanol fuel cell
Construction• Electrodes: porous nickel plates impregnated with
finally- divided platinum.
• Fuel: Methyl alcohol
• Oxidant: Pure oxygen / air
• Electrolyte: Concentrated phosphoric acid
• Temperature: 150-200oC
Working
Working:-
Anode:- CH3OH + H2O CO2 + 6H+ +6e-
Cathode:- O2 + 6H+ + 6e- 3 H2O
Net reaction:- CH3OH + O2 CO2 + 2H2O
Advantages
• MeOH can be easily transported stored & dispensed
• The fuel is very cheap and available in large quantities.
• only products of combustion are CO2 and H2O which can be removed easily.
• no production of NOx gases as the operating temperature is about 150oC.
• Methanol is stable in contact with the acidic membrane.
Disadvantages:-• anode reaction has poor
electrode kinetics, particularly at lower temperatures.
• The reduction of oxygen at the cathode is slow.
• permeability of the current perfluorosulfonic acid membranes to methanol, allowing considerable crossover of fuel.
• Methanol is toxic to humans,
Molten carbonate fuel cell
• Developed for natural gas and coal-based power plants for electrical utility, industrial and military applications.
Construction
• Electrolyte: Molten carbonate salt mixture. The composition of electrolyte varies, but usually consists of lithium carbonate and potassium carbonate.
• Electrodes:- Anode is porous sintered Ni powder, alloyed with Cr The cathode is a porous nickel oxide material doped with Li.
• Fuel: Hydrogen or CO• Oxidant: Oxygen• Catalyst:- Electrode material• Interconnect:- Stainless steel or nickel• Operating temperature:- 650oC
• Charge-carrier:- CO32-
Cell reaction
• Anode reaction:
H2 + CO32- H2O +CO2 + 2e-
• Cathode reaction: ½ O2 + CO2 + 2e- CO3
2-
• Overall reaction:
H2 + 1/2O2 H2O
Advantages:-
• Relatively high operating temperature.• Noble metal catalysts are not required. • Very high efficiency.• An external reformer to convert more energy-dense
fuels to hydrogen not required.• Not prone to carbon monoxide or carbon dioxide
"poisoning.“• More power is available at higher fuel efficiency
from MCFC.• Produce excess heat at a temperature, which is high
enough to yield high-pressure steam, which may be fed to a turbine to generate additional electricity.
• It could operate directly on gaseous hydrocarbon fuels.
Disadvantages
• Very corrosive and mobile electrolyte, which requires use of nickel and high-grade stainless steel as the cell hardware.
• Higher temperatures promote material problems, impacting mechanical stability and stack life.
• source of CO2 is required at the cathode.
• High contact resistances and cathode resistance limit power densities to around 100 – 200 mW/cm2 at practical operating voltages.
Solid Oxide Fuel Cells
Construction• Electrolyte:- Perovskites (Ceramics)• Electrodes:- Anode constructed from an
electronically conducting Co-ZrO2 or Ni-ZrO2 (ceria/Nickel cement) and cathode is Sr doped LaMnO3 (La, Sr, Co, Fe oxides)
• Catalyst:- Electrode material• Interconnect:- Nickel, ceramic or steel• Operating temperature:- 600-1000oC• Charge-carrier:- O2-
• Fuel: Hydrogen, CO• Oxidant: oxygen
Cell reaction:-
Anode reaction: H2(g) + O2- H2O(g) + 2e-
CO(g) + O2- CO2(g) + 2e-
Cathode reaction: O2 + 4e- 2O2-
Overall reaction: H2 + O2 + CO H2O + CO2
Limitations Of Fuel Cells.
• Cost of power is high as a result of the cost of electrodes.
• Fuels in the form of gases and O2 need to be stored in tanks under high pressure.
• Power output is moderate.
• They are sensitive to fuel contaminants such as CO,H2S, NH3 & halides, depending on the type of fuel cell.
Differences.• Fuel Cell Galvanic Cell
*Do not store chemical Stores chemical energy energy *Reactants are fed from The reactants form an
outside continuously. integral part of it. *Need expensive noble These conditions are metal catalysts. not required*No need of charging Get-discharged when stored – up energy is exhausted.
*Never become dead Limited life span in use *Useful for long-term Useful as portable power services
electricity generation.
CH3OH-O2 Fuel Cell
• Both electrodes: Made of porous nickel plates impregnated with finely- divided
Platinum.
• Fuel: Methyl alcohol.
• Oxidant: Pure oxygen / air.
• Electrolyte: Conc.Phosphoric acid/Aq.KOH
• Operating Temperature: 150-200oC.
• At anode:
CH3OH + 6OH- →CO2 + 5H2O + 6e-
• At cathode:
3/2 O2 +3H2O + 6e →6OH-
Net Reaction:
CH3OH +3/2O2 →CO2 + 2H2O.
It is used in military applications and in large scale
power production. It has been used to power
television relay stations.
If Acid Electrolyte used:
• Anode:CH3OH + H2O → CO2(g) + 6H+(aq) + 6e-
• Cathode:3/2O2(g) + 6H+(aq) + 6e- →
3 H2O(l)• Net reaction: CH3OH + 3/2 O2(g)
→CO2(g) + 2H2O(l)
Advantages:
1) MeOH can be easily Transported stored & dispensed within the current fuel network because it is a liquid fuel.
2) The fuel is very cheap and available in large quantities.
3) The only products of combustion are CO2 & H2O which can be removed easily.
4) There is no production of NOx gases as the operating temperature is about 150o C.
5) Methanol is stable in contact with the acidic membrane.
Disadvantages:
1) The anode reaction has poor electrode kinetics, particularly at lower temperatures.
2) The reduction of oxygen at the cathode is also slow.
3) Methanol is toxic to humans, causing blindness
in low doses and death in larger amounts. So there are concerns about their proper handling.
• The emf of the cell is 1.20 V at 25oC.
• MeOH is one of the most electro active organic fuels in
the low temperature range as
*It has a low carbon content
*It posseses a readily oxidizable OH group
*It is miscible in all proportions in
aqueous electrolytes.
Numerical Problems Q. 1) What standard cell potential can be expected
from a fuel cell that consumer’s hydrogen and
oxygen gases and produces water vapor? (ΔGo =
-237.2KJ/ml)
Solution: The overall reaction is
H2(g) + ½ O2(g) → H2O(l)
ΔGo = - ήFEo
E o = - ΔG / ήF (+ 237.2 KJmol -1) (1000 J/KJ /
2(96,500 C/mol) = 1.14V.
2) Calculate the standard emf of the H2 – O2 fuel cell if the Eo values are – 0.40V & 0.83 V for the hydrogen and oxygen half cells respectively.
Eo cell = Eoc – EoA = 0.83 – (-0.40) =
+1.23 V.
3) The standard free energy change ΔGo for the net reaction in the lead acid cell at room temperature is – 393.9 KJ mol -1. Calculate the thermodynamic cell voltage.
ΔGo = - ήEoF
Eo cell = - ΔGo/ήF
= 393.9 x 1000/ 2 x 96500 = 1.9 V
4.Calculate the standard cell potential from the standard reduction potentials of -0.356V & 1.685 V for the anode and cathode half sections of a lead –acid cell Eocell = EoC – EoA = Eo PbO2/PbSO4 –
Eo PbSO4/Pb
= 1.685 –(-0.356) = 2.041 V