Electrolysis Processes

Electrical energy can produce chemical changes and the processes based on it, called the Electrolytic Processes, are widely used

• For extraction of pure metals from their ores (e.g. Al, Zn, Cu etc…),

• For manufacturing of various chemicals (e.g. caustic soda, oxygen, hydrogen),

• For electro deposition of metals including electroplating, electrotyping, electroforming, building up of worn out parts in metallurgical, chemical and other industries.

What is Electrolysis?• Definition of Electrolysis:

→ A chemical process in which bonded elements and compounds are dissociated by the passage of an electric current.

• The electrolysis of water:→ 2H2O + energy = 2H2 + 2O2

A Basic Electrolyzer• Two electrodes:

– Cathode (negatively charged)– Anode (positively charged)

• An Electrolyte

• External circuit

• Diaphragm

FARADAY’S LAWS OF ELECTRO-DEPOSITION

• FARADAY’S FIRST LAW: According to this law the chemical deposition due to flow of current through an electrolyte is directly proportional to the quantity of electricity (Coulombs) passed through it.

i.e. mass of chemical deposition,M α QM α I tM = Z I t (A)Where, I is the steady current in amperes flowing through

the electrolyte for t seconds and Z is a constant of proportionality and is known as the electro chemical equivalent of the substances.

SI unit of Z is the kilogram per clulomb (kg C-1)

• FARADAY’S SECOND LAW: This law states that when the same quantity of electricity is passed through several electrolytes, the mass of the substances deposited are proportional to their respective chemical equivalents or equivalent weights.

The chemical equivalent or equivalent weight of a substance if defined as the weight of that substance which will combine with or displace unit weight of hydrogen.

The chemical equivalent of hydrogen is therefore 1. Since the valency of substance if equal to the no. of hydrogen atoms, which it can replace or with which it can combine, the chemical equivalent of a substance, therefore, may be defined as the ratio of its atomic weight to its valency.

Continued…..

Chemical Equivalent = Atomic weightValency

From this law it follows that the constant of proportionality Z in equation (A) is proportional to the chemical equivalent.

CURRENT EFFICIENCY:• Owing to impurities, which causes the secondary

reactions, the quantity of substance or substances liberated is slightly less than that calculated from Faraday’s laws.

• This is taken into account by employing a factor, called current efficiency.

• Current Efficiency is defined as the ratio of actual quantity of substance deposited or liberated to the theoretical quantity (from Faraday’s laws)

Current Efficiency = actual quantity of substance deposited or liberated

theoretical quantity of substance deposited or liberated

ENERGY EFFICIENCY:• On account of secondary effects and

reactions the actual substance deposited is less than that of theoretical value; Voltage required is also higher than that determined theoretically.

• Hence actual energy consumption is higher than that determined theoretically.

Energy Efficiency= Theoretical energy required

Actual energy required

EXTRACTION OF METALS:

• Extraction Of Metal is an electro-chemical process used for the production of metal with commercially acceptable purity.

• There are two methods of extraction of metals depending upon the physical state of the ore:

1) In one process the ore is treated with strong acid to obtain a salt and the solution of such a salt is electrolyzed to liberate the metal.

2) The second process is employed when the ore is available in molten state or can be fused and in this process the ore, which is in a molten state, is electrolyzed in a furnace.

EXTRACTION OF ALUMINIUM:Aluminium can be extracted from bauxite, Kaolinite or nepheline.

REFINING OF METALS:• Refining is the process whereby a highly

concentrated mixture of metals is subjected to electro-chemical treatment for recovering not only the principal metal in pure form, but also the precious metals like gold, silver, bismuth etc., which may be present in the form of minute trace.

Refining is done for following metals:1) Refining Of Copper,2) Refining Of Gold,3) Refining Of Silver,4) Refining Of Nickel,5) Refining Of Lead,6) Refining Of Zinc,

ELECTRO DEPOSITION:

• The process of depositing a coating of one metal over and other metal or non-metal electrically is called the Electro-deposition.

• It is used for protective, decorative and functional purposes and includes such processes as Electro-plating, Electro-forming, Electro-typing, Electro-facing, Electro-metallization, Electro-deposition of rubber and building up of worn out parts for repairs.

Quality Of ELECTRO DEPOSITION Depends Upon Following Factors:1) Nature of electrolyte, (smooth deposition)2) Current density, (rate of crystal growth)3) Temperature, (formation of small of large crystals)4) Conductivity, (economy in power consumption and

reduce tendency to form rough deposition)5) Electrolytic concentration, (high Current density can be

obtain)6) Additional agents, (reduces resistance)7) Throwing power, (it is the ability of electrolyte to produce

uniform deposit on an article of irregular shape)8) Polarization, (it affects the rate of deposition when current

density limit reaches)

ELECTRO-PLATING:

• Electro-plating is an art of depositing a superior or a more noble metal on an inferior or a base metal of electrolysis of an aqueous solution of a suitable electrolyte.

• Preparation For Plating:1) Removal of oil, grease or other organic

material.2) Removal of rust, oxide or other inorganic

coatings adhering to the metal.3) Mechanical preparation of surface of metal to

receive the deposited metal, by polishing.

Alkaline Electrolyzers• Similar to PEM electrolyzers, except that they use

an alkaline solution as an electrolyte. • Usually this solution is sodium hydroxide or

potassium hydroxide.

• This type of electrolyzer has been in use commercially for several decades

Solid Oxide Electrolyzers

• A solid ceramic material is used as the electrolyte. • At the cathode, water combines with electrons from the

external circuit to produce hydrogen gas and negatively charged oxygen ions.

• The oxygen ions move through the solid oxide membrane and release electrons to the external circuit.

• In order for this type of electolyzer to function properly, the solid oxide membrane must be between 500 – 800 degrees Celsius, which is much higher than the temperatures required by the other electrolyzers

Energy Balance and Efficiency of Electrolysis

The electricity needed for hydrogenproduction by electrolysis can currently begenerated by a variety of sources,including:

• fossil fuels• wind power • photovoltaic cells• hydropower

Necessary Water Inputs For Electrolysis

• Amount of water needed to meet average US person’s energy demand though electrolysis: 3,000 liters of water per year

• Amount of water currently used by an average US person for indoor residential purposes: 138,770 liters a year

Electrolysis Efficiency Basics

• Although hydrogen is a promising alternative fuel, hydrogen production by electrolysis is not extremely efficient.

• The primary energy inputs to be considered are the energy requirements for building and running an electrical generating facility.

Energy Balance - Part I

• An input of 1.4 billion kW per hour of electricity is required to produce 1 billion kW per hour of hydrogen by electrolysis.

• Energy balance = (Useful Energy Output)/(Energy Input)

• = (1 kW/hr electricity)/(1.4 kW/hr hydrogen energy)

• = 0.71, or 71% efficiency for the initial electrolysis process.

Energy Balance - Part II

• The other main process to consider in production of hydrogen gas is the necessary cooling of hydrogen to about minus 253 degrees Celsius.

• This process demands considerable energy, resulting in a loss of approximately 30 percent of the hydrogen energy.

• As a result of each stage of the hydrogen production process, the total production efficiency is approximately 30 %.

High Temperature Electrolysis

• Process which could increase hydrogen efficiency to the range of 45 to 50 %

• The DOE is currently examining the use of high temperature electrolysis powered by fossil fuel, renewable, and even nuclear technologies.

• High temperature electrolysis utilizes the solid oxide electrolyzer described earlier.

High Temperature Electrolysis

• The efficiency increase is achieved because high temperature electrolysis utilizes a significant amount of heat, for example from a nuclear reactor.

• The added heat decreases the amount of electricity required to separate the water into hydrogen and oxygen.

Photoelectrolysis

Photoelectrolysis: Clean and renewable means ofderiving hydrogen Also known as ‘Water Splitting’(2 processes):1) Conversion of solar radiation to electricity in

photovoltaic cells 2) Electrolysis of water in a separate cell

Conversion efficiency = 3% - 32%

Photoelectrolysis

However, the 2 processes can be combined ina single nanoscale process: Photon absorption creates a local electron-hole pair that electrochemically splits a neighboring water molecule. In theory, rather than 2 sequential process, the combination can allow for greater overall efficiency,

Photoelectrolysis

Challenges: Finding a robust semiconductor to satisfy the competing requirements of nature. Solar photons are primarily visible light, a wavelength that requires semiconductors that require small bandgaps < 1.7 eV - for efficient absorption.

Photoelectrolysis

Possible solution: Oxide based conductors -Titanium oxide

• Advantage – robust in aqueous environments but have

• Disadvantage - wide bandgaps ~ 3.0 eV

Photoelectrolysis

Dye-sensitized photocells:accumulate energy from multiple low-energy

photons to inject higher-energy electrons into the semiconductor – a promising direction for matching the solar spectrum.

Other Applications of Electrolysis:

• pH meters -

Other Applications of Electrolysis:

• Electroplating

Other Applications of Electrolysis:

• Anionic polymerization