However, it also can be done using solar energy to create electricity and that electricity at 9 to 12 volts or above is enough to create electrolysis of water which slowly over time would create (for free from the sun) as much hydrogen as you might need for your Hydrogen fuel cells. As long as water lasts on earth. (which might not be that long with 7 billion people or more doing this as a power source ongoing). And I'm not sure what this would do to the atmosphere etc. as all the H2O (H2) was converted to energy with a small amount of water to show for it. However, salt or acid would need to be added to the water for better electrical conduction while doing this electrically. See. simple artistic rendering below using a 9 to 12 volt battery 2 wires and (lead pencils?) in a simple science experiment.
The other thing you get from this is straight O2 (Oxygen) if you have a use for it as well.
Electrolysis - Wikipedia, the free encyclopedia
en.wikipedia.org/wiki/Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction.
Wikipedia
This was demonstrated by my 8th grade science teacher at Woodrow Wilson Junior High School in Glendale California around 1960 or so when he had a contraption made of glass with a battery and two wires put inside it. Then he filled the contraption with water and turned on the electricity and bubbles came up oxygen off of one direct current wire and hydrogen off of the other direct current wire. So, this could easily be done with any solar panel and a couple of wires coming off of it with even 12 volts of power. So basically anyone with an 8th grade science teacher and the right equipment can do this.
The reason I'm thinking about this is hydrogen fuel cells are starting to come into vogue through Hyundai and other car manufacturers in California. The output of this is water and electricity. nothing else. So, this likely (along with solar and wind) become the next big change here on earth away from petroleum and onto Solar, wind and Hydrogen Fuel Cells among other types of energy generation devices.
Oil will then mostly be for making plastics and other products which is very important too.
Simple setup for demonstration of electrolysis of water at home
Electrolysis of water is the decomposition of water (H2O) into oxygen (O2) and hydrogen gas (H2) due to an electric current being passed through the water.
Jan Rudolph Deiman and Adriaan Paets van Troostwijk used in 1789 an electrostatic machine to produce electricity which was discharged on gold electrodes in a Leyden jar with water.[1] In 1800 Alessandro Volta invented the voltaic pile, and a few weeks later William Nicholson and Anthony Carlisle used it for the electrolysis of water. When Zénobe Gramme invented the Gramme machine
in 1869 electrolysis of water became a cheap method for the production
of hydrogen. A method of industrial synthesis of hydrogen and oxygen
through electrolysis was developed by Dmitry Lachinov in 1888.[2]
Electrolysis of pure water requires excess energy in the form of overpotential to overcome various activation barriers. Without the excess energy the electrolysis of pure water occurs very slowly or not at all. This is in part due to the limited self-ionization of water. Pure water has an electrical conductivity about one millionth that of seawater. Many electrolytic cells may also lack the requisite electrocatalysts. The efficiency of electrolysis is increased through the addition of an electrolyte (such as a salt, an acid or a base) and the use of electrocatalysts.
Currently the electrolytic process is rarely used in industrial applications since hydrogen can currently be produced more affordably from fossil fuels.[4]
In pure water at the negatively charged cathode, a reduction reaction takes place, with electrons (e−) from the cathode being given to hydrogen cations to form hydrogen gas (the half reaction balanced with acid):
At the positively charged anode, an oxidation reaction occurs, generating oxygen gas and giving electrons to the anode to complete the circuit:
The negative voltage indicates the Gibbs free energy for electrolysis of water is greater than zero for these reactions. This can be found using the G = -nFE equation from chemical kinetics, where n is the moles of electrons and F is the Faraday constant. The reaction cannot occur without adding necessary energy, usually supplied by an external electrical power source.
If the above described processes occur in pure water, H+ cations will accumulate at the anode and OH− anions will accumulate at the cathode. This can be verified by adding a pH indicator
to the water: the water near the anode is acidic while the water near
the cathode is basic. The negative hydroxyl ions that approach the anode
mostly combine with the positive hydronium ions (H3O+)
to form water. The positive hydronium ions that approach the negative
cathode mostly combine with negative hydroxyl ions to form water.
Relatively few hydronium (hydroxyl) ions reach the cathode (anode). This
can cause a concentration overpotential at both electrodes.
Pure water is a fairly good insulator since it has a low autoionization, Kw = 1.0 x 10−14 at room temperature and thus pure water conducts current poorly, 0.055 µS·cm−1. Unless a very large potential is applied to cause an increase in the autoionization of water the electrolysis of pure water proceeds very slowly limited by the overall conductivity.
If a water-soluble electrolyte is added, the conductivity of the water rises considerably. The electrolyte disassociates into cations and anions; the anions rush towards the anode and neutralize the buildup of positively charged H+ there; similarly, the cations rush towards the cathode and neutralize the buildup of negatively charged OH− there. This allows the continued flow of electricity.[6]
Care must be taken in choosing an electrolyte, since an anion from the electrolyte is in competition with the hydroxide ions to give up an electron. An electrolyte anion with less standard electrode potential than hydroxide will be oxidized instead of the hydroxide, and no oxygen gas will be produced. A cation with a greater standard electrode potential than a hydrogen ion will be reduced in its stead, and no hydrogen gas will be produced.
The following cations have lower electrode potential than H+ and are therefore suitable for use as electrolyte cations: Li+, Rb+, K+, Cs+, Ba2+, Sr2+, Ca2+, Na+, and Mg2+. Sodium and lithium are frequently used, as they form inexpensive, soluble salts.
If an acid is used as the electrolyte, the cation is H+, and there is no competitor for the H+ created by disassociating water. The most commonly used anion is sulfate (SO2−
4), as it is very difficult to oxidize, with the standard potential for oxidation of this ion to the peroxodisulfate ion being −2.05 volts.
Strong acids such as sulfuric acid (H2SO4), and strong bases such as potassium hydroxide (KOH), and sodium hydroxide (NaOH) are frequently used as electrolytes due to their strong conducting abilities.
A solid polymer electrolyte can also be used such as Nafion and when applied with a special catalyst on each side of the membrane can efficiently split the water molecule with as little as 1.5 Volts. There are also a number of other solid electrolyte systems that have been trialled and developed with a number of electrolysis systems now available commercially that use solid electrolytes.[7]
Note that an aqueous solution of water with chloride ions will, when electrolysed, either result in either OH− if the concentration of Cl− is low, OR in chlorine gas being preferentially discharged if the concentration of Cl− is greater than 25% by mass in the solution.
Match test used to detect the presence of hydrogen gas
Water electrolysis is also used to generate oxygen for the International Space Station.[12][13]
Hydrogen may later be used in a fuel cell as a storage method of energy and water.[14]
Since each mole of water requires two moles of electrons, and given that the Faraday constant F represents the charge of a mole of electrons (96485 C/mol), it follows that the minimum voltage necessary for electrolysis is about 1.23 V.[15] If the HHV is used as a reference instead, the obtained voltage is 1.48 V, the thermoneutral voltage.
In the case of water electrolysis, Gibbs free energy represents the minimum work necessary for the reaction to proceed, and the reaction enthalpy is the amount of energy (both work and heat) that has to be provided so the reaction products are at the same temperature as the reactants (i.e. standard temperature for the values given above). This implies that an ideal, 100% efficient electrolyser would produce hydrogen and oxygen at a lower temperature than the provided water. An electrolyser operating at 1.48 V would be only 83% efficient, and would produce hydrogen and oxygen at the same temperature as provided water.
For a well designed cell the largest overpotential is the reaction overpotential for the four-electron oxidation of water to oxygen at the anode; electrocatalysts can facilitate this reaction, and platinum alloys are the state of the art for this oxidation. Developing a cheap, effective electrocatalyst for this reaction would be a great advance, and is a topic of current research. The simpler two-electron reaction to produce hydrogen at the cathode can be electrocatalyzed with almost no overpotential by platinum, or in theory a hydrogenase enzyme. If other, less effective, materials are used for the cathode (e.g. graphite), large overpotentials will appear.
Very often electrolyser vendors provide efficiencies based on enthalpy (LHV or HHV). These, being larger than the corresponding Gibbs free energy, make the calculated efficiency values appear significantly better than the actual values. To assess the claimed efficiency of an electrolyser it is therefore important to establish how it was defined by the vendor.
There are two main technologies available on the market, alkaline and PEM electrolysers. Alkaline electrolysers are cheaper in terms of investment, but less efficient; PEM electrolysers, conversely, are more expensive but also more efficient, and can be cheaper if the hydrogen production is large enough. Reported efficiencies are in the range 50–80%.[17][18][19]
This page was last modified on 17 April 2014 at 16:18.
Electrolysis of water
From Wikipedia, the free encyclopedia
Contents
History
Principle
An electrical power source is connected to two electrodes, or two plates (typically made from some inert metal such as platinum, stainless steel or iridium) which are placed in the water. Hydrogen will appear at the cathode (the negatively charged electrode, where electrons enter the water), and oxygen will appear at the anode (the positively charged electrode). Assuming ideal faradaic efficiency, the amount of hydrogen generated is twice the number of moles of oxygen, and both are proportional to the total electrical charge conducted by the solution.[3] However, in many cells competing side reactions dominate, resulting in different products and less than ideal faradaic efficiency.Electrolysis of pure water requires excess energy in the form of overpotential to overcome various activation barriers. Without the excess energy the electrolysis of pure water occurs very slowly or not at all. This is in part due to the limited self-ionization of water. Pure water has an electrical conductivity about one millionth that of seawater. Many electrolytic cells may also lack the requisite electrocatalysts. The efficiency of electrolysis is increased through the addition of an electrolyte (such as a salt, an acid or a base) and the use of electrocatalysts.
Currently the electrolytic process is rarely used in industrial applications since hydrogen can currently be produced more affordably from fossil fuels.[4]
Equations
At the positively charged anode, an oxidation reaction occurs, generating oxygen gas and giving electrons to the anode to complete the circuit:
- Oxidation at anode: 2 H2O(l) → O2(g) + 4 H+(aq) + 4e−
- Cathode (reduction): 2 H2O(l) + 2e− → H2(g) + 2 OH-(aq)
- Anode (oxidation): 4 OH- (aq) → O2(g) + 2 H2O(l) + 4 e−
- Overall reaction: 2 H2O(l) → 2 H2(g) + O2(g)
Thermodynamics of the process
Decomposition of pure water into hydrogen and oxygen at standard temperature and pressure is not favorable in thermodynamic terms.- Anode (oxidation): 2 H2O(l) → O2(g) + 4 H+(aq) + 4e− Eo
ox = -1.23 V (Eo
red = 1.23 )[5]) - Cathode (reduction): 2 H+(aq) + 2e− → H2(g) Eo
red = 0.00 V
The negative voltage indicates the Gibbs free energy for electrolysis of water is greater than zero for these reactions. This can be found using the G = -nFE equation from chemical kinetics, where n is the moles of electrons and F is the Faraday constant. The reaction cannot occur without adding necessary energy, usually supplied by an external electrical power source.
Electrolyte selection
Pure water is a fairly good insulator since it has a low autoionization, Kw = 1.0 x 10−14 at room temperature and thus pure water conducts current poorly, 0.055 µS·cm−1. Unless a very large potential is applied to cause an increase in the autoionization of water the electrolysis of pure water proceeds very slowly limited by the overall conductivity.
If a water-soluble electrolyte is added, the conductivity of the water rises considerably. The electrolyte disassociates into cations and anions; the anions rush towards the anode and neutralize the buildup of positively charged H+ there; similarly, the cations rush towards the cathode and neutralize the buildup of negatively charged OH− there. This allows the continued flow of electricity.[6]
Care must be taken in choosing an electrolyte, since an anion from the electrolyte is in competition with the hydroxide ions to give up an electron. An electrolyte anion with less standard electrode potential than hydroxide will be oxidized instead of the hydroxide, and no oxygen gas will be produced. A cation with a greater standard electrode potential than a hydrogen ion will be reduced in its stead, and no hydrogen gas will be produced.
The following cations have lower electrode potential than H+ and are therefore suitable for use as electrolyte cations: Li+, Rb+, K+, Cs+, Ba2+, Sr2+, Ca2+, Na+, and Mg2+. Sodium and lithium are frequently used, as they form inexpensive, soluble salts.
If an acid is used as the electrolyte, the cation is H+, and there is no competitor for the H+ created by disassociating water. The most commonly used anion is sulfate (SO2−
4), as it is very difficult to oxidize, with the standard potential for oxidation of this ion to the peroxodisulfate ion being −2.05 volts.
Strong acids such as sulfuric acid (H2SO4), and strong bases such as potassium hydroxide (KOH), and sodium hydroxide (NaOH) are frequently used as electrolytes due to their strong conducting abilities.
A solid polymer electrolyte can also be used such as Nafion and when applied with a special catalyst on each side of the membrane can efficiently split the water molecule with as little as 1.5 Volts. There are also a number of other solid electrolyte systems that have been trialled and developed with a number of electrolysis systems now available commercially that use solid electrolytes.[7]
Techniques
Main article: Water splitting
Fundamental demonstration
Two leads, running from the terminals of a battery, are placed in a cup of water with a quantity of electrolyte to establish conductivity in the solution. Using NaCl (table salt) in an electrolyte solution results in chlorine gas rather than oxygen due to a competing half-reaction.With the correct electrodes and correct electrolyte, such as baking soda, hydrogen and oxygen gases will stream from the oppositely charged electrodes. Oxygen will collect at the positively-charged electrode (anode) and hydrogen will collect at the negatively-charged electrode (cathode). Note that hydrogen is positively charged in the H2O molecule, so it is "pulled out" at the negative electrode. (And vice versa for oxygen.)Note that an aqueous solution of water with chloride ions will, when electrolysed, either result in either OH− if the concentration of Cl− is low, OR in chlorine gas being preferentially discharged if the concentration of Cl− is greater than 25% by mass in the solution.
Hofmann voltameter
Main article: Hofmann voltameter
The Hofmann voltameter is often used as a small-scale electrolytic
cell. It consists of three joined upright cylinders. The inner cylinder
is open at the top to allow the addition of water and the electrolyte. A
platinum
electrode is placed at the bottom of each of the two side cylinders,
connected to the positive and negative terminals of a source of electricity. When current is run through the Hofmann voltameter, gaseous oxygen forms at the anode (positive) and gaseous hydrogen at the cathode(negative). Each gas displaces water and collects at the top of the two outer tubes, where it can be drawn off with a stopcock.Industrial electrolysis
Many industrial electrolysis cells are very similar to Hofmann voltameters, with complex platinum plates or honeycombs as electrodes. Generally the only time hydrogen is intentionally produced from electrolysis is for specific point of use application such as is the case with oxyhydrogen torches or when extremely high hydrogen purity or oxygen is desired. The vast majority of hydrogen is produced from hydrocarbons and as a result contains trace amounts of carbon monoxide among other impurities. The carbon monoxide impurity can be detrimental to various systems including many fuel cells.High pressure electrolysis
Main article: High pressure electrolysis
High pressure electrolysis is the electrolysis of water with a compressed hydrogen output around 120-200 Bar (1740-2900 psi).[8] By pressurising the hydrogen in the electrolyser the need for an external hydrogen compressor is eliminated, the average energy consumption for internal compression is around 3%.[9]High-temperature electrolysis
Main article: High-temperature electrolysis
High-temperature electrolysis (also HTE or steam electrolysis) is a
method currently being investigated for water electrolysis with a heat engine.
High temperature electrolysis may be preferable to traditional
room-temperature electrolysis because some of the energy is supplied as
heat, which is cheaper than electricity, and because the electrolysis
reaction is more efficient at higher temperatures.[10][11]Alkaline water electrolysis
Main article: Alkaline water electrolysis
Polymer electrolyte membrane electrolysis
Main article: Polymer electrolyte membrane electrolysis
Applications
About five percent of hydrogen gas produced worldwide is created by electrolysis. The majority of this hydrogen produced through electrolysis is a side product in the production of chlorine. This is a prime example of a competing side reaction.- 2NaCl + 2H2O → Cl2 + H2 + 2NaOH
Water electrolysis is also used to generate oxygen for the International Space Station.[12][13]
Hydrogen may later be used in a fuel cell as a storage method of energy and water.[14]
Efficiency
Thermodynamics
The electrolysis of water in standard conditions requires a theoretical minimum of 237 kJ of electrical energy input to dissociate each mole of water, which is the standard Gibbs free energy of formation of water. Very often, although incorrectly, the standard enthalpy of formation of liquid water is used as a reference (286 kJ/mol), or possibly that of water vapour (242 kJ/mol). These are also often indicated as the higher heating value (HHV) and lower heating value (LHV) of hydrogen.Since each mole of water requires two moles of electrons, and given that the Faraday constant F represents the charge of a mole of electrons (96485 C/mol), it follows that the minimum voltage necessary for electrolysis is about 1.23 V.[15] If the HHV is used as a reference instead, the obtained voltage is 1.48 V, the thermoneutral voltage.
In the case of water electrolysis, Gibbs free energy represents the minimum work necessary for the reaction to proceed, and the reaction enthalpy is the amount of energy (both work and heat) that has to be provided so the reaction products are at the same temperature as the reactants (i.e. standard temperature for the values given above). This implies that an ideal, 100% efficient electrolyser would produce hydrogen and oxygen at a lower temperature than the provided water. An electrolyser operating at 1.48 V would be only 83% efficient, and would produce hydrogen and oxygen at the same temperature as provided water.
Overpotential
Real water electrolysers require higher voltages for the reaction to proceed. The part that exceeds 1.23 V[16] is called overpotential or overvoltage, and represents any kind of loss and nonideality in the electrochemical process.For a well designed cell the largest overpotential is the reaction overpotential for the four-electron oxidation of water to oxygen at the anode; electrocatalysts can facilitate this reaction, and platinum alloys are the state of the art for this oxidation. Developing a cheap, effective electrocatalyst for this reaction would be a great advance, and is a topic of current research. The simpler two-electron reaction to produce hydrogen at the cathode can be electrocatalyzed with almost no overpotential by platinum, or in theory a hydrogenase enzyme. If other, less effective, materials are used for the cathode (e.g. graphite), large overpotentials will appear.
Industrial State of the Art
Efficiency of modern hydrogen generators is measured by Power consumed per standard volume of hydrogen (MJ/m³ or kWh/m³), assuming standard temperature and pressure of the H2. A 100%-efficient electrolyser would consume then 10.6 MJ/m³ (2.94 kWh/m³); the lower the actual, the higher efficiency.Very often electrolyser vendors provide efficiencies based on enthalpy (LHV or HHV). These, being larger than the corresponding Gibbs free energy, make the calculated efficiency values appear significantly better than the actual values. To assess the claimed efficiency of an electrolyser it is therefore important to establish how it was defined by the vendor.
There are two main technologies available on the market, alkaline and PEM electrolysers. Alkaline electrolysers are cheaper in terms of investment, but less efficient; PEM electrolysers, conversely, are more expensive but also more efficient, and can be cheaper if the hydrogen production is large enough. Reported efficiencies are in the range 50–80%.[17][18][19]
See also
References
- Levie, R. de (October 1999). "The electrolysis of water". Journal of Electroanalytical Chemistry 476 (1): 92–93. doi:10.1016/S0022-0728(99)00365-4. Retrieved 2009-12-30.
- Lachinov Dmitry Aleksandrovich at Great Cyrill and Methodius Encyclopedia (Russian)
- Carmo, M; Fritz D, Mergel J, Stolten D (2013). "A comprehensive review on PEM water electrolysis". Journal of Hydrogen Energy. doi:10.1016/j.ijhydene.2013.01.151.
- Staff (2007). "Hydrogen Basics — Production". Florida Solar Energy Center. Retrieved 2008-02-05.
- standard electrode potential (data page)
- Linus Pauling, General Chemistry, Section 15-2. San Francisco, 1970
- Badwal, SPS; Giddey S, Munnings C (2012). "Hydrogen production via solid electrolytic routes". WIRES Energy and Environment. doi:10.1002/wene.50.
- 2001-High pressure electrolysis - The key technology for efficient H.2
- 2003-PHOEBUS-Pag.9
- Hi2h2 - High temperature electrolysis using SOEC
- WELTEMP-Water electrolysis at elevated temperatures
- "Making Space Safer with Electrolysis". ASME. Retrieved 2012-05-26.
- "Breathing Easy on the Space Station". NASA Science. Retrieved 2012-05-26.
- "Solar Hydrogen Fuel Cell Water Heater (Educational Stand)". Scribd.
- Hyman D. Gesser (2002). Applied Chemistry. Springer. pp. 16–. ISBN 978-0-306-46700-4. Retrieved 18 December 2011.
- 1.23 V is the standard potential; in non-standard conditions it may be different, in particular it decreases with temperature.
- Werner Zittel; Reinhold Wurster (1996-07-08). "Chapter 3: Production of Hydrogen. Part 4: Production from electricity by means of electrolysis". HyWeb: Knowledge – Hydrogen in the Energy Sector. Ludwig-Bölkow-Systemtechnik GmbH. Archived from the original on 2007-02-07.
- Bjørnar Kruse; Sondre Grinna, Cato Buch (2002-02-13). "Hydrogen—Status and Possibilities" (pdf). The Bellona Foundation. Archived from the original on 2002-02-13. "Efficiency factors for PEM electrolysers up to 94% are predicted."
- high-rate and high efficiency 3D water electrolysis
External links
 |
Wikimedia Commons has media related to Water electrolysis. |
- "Electrolysis of Water". Experiments on Electrochemistry. Retrieved November 20, 2005.
- "Electrolysis of Water". Do Chem 044. Retrieved November 20, 2005.
- EERE 2008 - 100 kgH2/day Trade Study
- NREL 2006 - Electrolysis technical report
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end quote from:
Electrolysis of water
If you remember the big dirigible or blimp called Hindenburg it was filled with Hydrogen rather than Helium. the Hindenburg after flying over to the U.S. I believe from Europe caught fire and Blew up. This ended pretty much building Hydrogen dirigibles in Europe and the U.S. Though Hydrogen has more lifting power than Helium does in the air, often when balloons were sold at sporting events filled with Hydrogen in Mexico and South America sometimes they exploded like bombs at sporting events when combusted by accident (or on purpose by cigarettes). I'm not sure these are still legal or not in Mexico and South America to be sold anymore.
Electrolysis of water
If you remember the big dirigible or blimp called Hindenburg it was filled with Hydrogen rather than Helium. the Hindenburg after flying over to the U.S. I believe from Europe caught fire and Blew up. This ended pretty much building Hydrogen dirigibles in Europe and the U.S. Though Hydrogen has more lifting power than Helium does in the air, often when balloons were sold at sporting events filled with Hydrogen in Mexico and South America sometimes they exploded like bombs at sporting events when combusted by accident (or on purpose by cigarettes). I'm not sure these are still legal or not in Mexico and South America to be sold anymore.
Airship - Wikipedia, the free encyclopedia
en.wikipedia.org/wiki/Airship
An airship or dirigible
is a type of aerostat or lighter-than-air aircraft that is propelled
through the air by engine power. An aerostat stays aloft by having a
large ...
Wikipedia
Dirigibles, Zeppelins, and Blimps: The Differences Explained
www.airships.net/dirigible
What is the difference between a blimp and a zeppelin? What is the definition of a dirigible? The basic types of Lighter-than-Air aircraft are explained.
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