HHO Technology
Equations
In the 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):
Cathode (reduction): 2H+(aq) + 2e− → H2(g)
At the positively charged anode, an oxidation reaction occurs, generating oxygen gas and giving electrons to the cathode to complete the circuit:
Anode (oxidation): 2H2O(l) → O2(g) + 4H+(aq) + 4e−
The same half reactions can also be balanced with base as listed below. Not all half reactions must be balanced with acid or base. Many do like the oxidation or reduction of water listed here. To add half reactions they must both be balanced with either acid or base.
Cathode (reduction): 2H2O(l) + 2e− → H2(g) + 2OH−(aq)
Anode (oxidation): 4OH−(aq) → O2(g) + 2H2O(l) + 4e−
Combining either half reaction pair yields the same overall decomposition of water into oxygen and hydrogen:
Overall reaction: 2H2O(l) → 2H2(g) + O2(g)
The number of hydrogen molecules produced is thus twice the number of oxygen molecules. Assuming equal temperature and pressure for both gases, the produced hydrogen gas has therefore twice the volume of the produced oxygen gas. The number of electrons pushed through the water is twice the number of generated hydrogen molecules and four times the number of generated oxygen molecules.
Thermodynamics of the process
Decomposition of pure water into hydrogen and oxygen at standard temperature and pressure is not favorable in thermodynamical terms. This is because, E(cell)=E(Oxidation) + E(Reduction). If E(cell) <>
Thus, the standard potential of the water electrolysis cell is 1.23 V at 25 °C.
The positive voltage indicates the Gibbs Free Energy for electrolysis of water is greater than zero for these reactions. This can be found using the Nernst Equation at equilibrium. The reaction cannot occur without adding necessary energy, usually supplied by an external electrical power source but also possible with thermal energy.
Hoffman voltameter connected to a direct current power source converter.
Electrolyte selection
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. These charged ions will repel the further flow of electricity until they have diffused away, a slow process. This is why pure water conducts electricity poorly and why electrolysis of pure water proceeds slowly.
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.[1]
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 (SO42-), as it is very difficult to oxidize, with the standard potential for oxidation of this ion to the peroxydisulfate ion being −0.22 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.
Techniques
Fundamental Demonstration
Two leads, running from the terminals of a battery, are placed in a cup of water with a quantity of electrolyte (not NaCl, anode creates clorine gas) added to establish conductivity. Hydrogen and oxygen gases will stream from the oppositely charged electrode. Oxygen will collect at the anode and hydrogen will collect at the cathode.
Match test used to detect the presence of hydrogen gas.
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 and gaseous hydrogen at the cathode. 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 purity hydrogen 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-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 is more efficient than 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.
Applications
About four 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.
2 NaCl + 2 H2O → Cl2 + H2 + 2 NaOH
The electrolysis of brine, a water sodium chloride mixture, is only half the electrolysis of water since the chloride ions are oxidized to chlorine rather than water being oxidized to oxygen. The hydrogen produced from this process is either burned, used for the production of specialty chemicals, or various other small scale applications.
The majority of hydrogen used industrially is derived from fossil fuels. One example is fossil fuel derived hydrogen used for the creation of ammonia for fertilizer via the Haber process and for converting heavy petroleum sources to lighter fractions via hydrocracking. The production of this hydrogen usually involves the formation of synthesis gas a mixture of H2 and CO. Synthesis gas can be hydrogen enriched through the water gas shift reaction. In this reaction the carbon monoxide is reacted with water to produce more H2 with CO2 byproduct.
There is some speculation about future development of hydrogen as an energy carrier in a hydrogen economy, although the rapid evolution of electric battery technology makes overall efficiency a major consideration.
Efficiency
Water electrolysis does not convert 100% of the electrical energy into the chemical energy of hydrogen. The process requires more extreme potentials than what would be expected based on the cell's total reversible reduction potentials. This excess potential accounts for various forms of overpotential by which the extra energy is eventually lost as heat. For a well designed cell the largest overpotential is the reaction overpotential for the four electron oxidation of water to oxygen at the anode. An effective electrocatalyst to facilitate this reaction has not been developed. Platinum alloys are the default state of the art for this oxidation. The reverse reaction, the reduction of oxygen to water, is responsible for the greatest loss of efficiency in fuel cells. Developing a cheap effective electrocatalyst for this reaction would be a great advance (see also[2]).
The simpler two-electron reaction to produce hydrogen at the cathode can be electrocatalyzed with almost no reaction overpotential by platinum or in theory a hydrogenase enzyme. If other, less effective, materials are used for the cathode then another large overpotential must be paid.
The energy efficiency of water electrolysis varies widely with the numbers cited below on the optimistic side. Some report 50–80%[3].[4] These values refer only to the efficiency of converting electrical energy into hydrogen's chemical energy. The energy lost in generating the electricity is not included. For instance, when considering a power plant that converts the heat of nuclear reactions into hydrogen via electrolysis, the total efficiency may be closer to 30–45%.[5]
Chemical energy content of water
Spontaneous chemical reactions do not create energy; they release it by converting unstable bonds into more stable bonds and/or by increasing entropy. The burning of conventional fuels such as petrol (gasoline), wood, and coal converts the fuel into substances with less energy, mostly water and carbon dioxide. In the combustion of fossil fuels water is a waste product, and the overall reaction can be represented with the following chemical equation:
CnHm + (n + m/4) O2 → n CO2 + m/2 H2O
Water is such an abundant chemical compound in part because it has very stable bonds that resist most reactions. In order for water to participate in a reaction that produces energy, high energy compounds must be added. For example, it is possible to generate the combustible fuel acetylene by adding calcium carbide to water. However, the calcium carbide, a high energy material, is the 'fuel,' not water. Under conditions common on Earth, chemical energy cannot be extracted from water alone.[5][4] (It is theoretically possible to extract nuclear energy from water by fusion, but fusion power plants of any scale remain impractical, and no allegedly water-fuelled cars are claimed to be powered by fusion.)
Electrolysis
Many alleged water-fuelled cars obtain hydrogen or a mixture of hydrogen and oxygen (sometimes called "oxyhydrogen", "HHO", or "Brown's Gas") by the electrolysis of water, a process that must be powered electrically. The hydrogen or oxyhydrogen is then burned, supposedly powering the car and also providing the energy to electrolyse more water. The overall process can be represented by the following chemical equations:
2H2O → 2H2 + O2 [Electrolysis step]
2H2 + O2 → 2H2O [Combustion step]
Since the combustion step is the exact reverse of the electrolysis step, the energy released in combustion exactly equals the energy consumed in the electrolysis step, and—even assuming 100% efficiency—there would be no energy left over to power the car. In other words, such systems start and end in the same thermodynamic state, and are therefore perpetual motion machines, violating the first law of thermodynamics. And under actual conditions in which hydrogen is burned, efficiency is limited by the second law of thermodynamics and is likely to be around 20%.[7][8] More energy is therefore required to drive the electrolysis cell than can be extracted from burning the resulting hydrogen-oxygen mixture.
Claims of functioning water-fuelled cars
Genepax Water Energy System
In June 2008, Japanese company Genepax unveiled a car which it claims runs on only water and air,[9] and many news outlets dubbed the vehicle a "water-fuel car".[10] The company says it "cannot [reveal] the core part of this invention,” yet,[11] but it has disclosed that the system uses an onboard energy generator (a "membrane electrode assembly") to extract the hydrogen using a "mechanism which is similar to the method in which hydrogen is produced by a reaction of metal hydride and water".[12] The hydrogen is then used to generate energy to run the car. This has led to speculation that the metal hydride is consumed in the process and is the ultimate source of the car's energy, making the car a hydride-fuelled "hydrogen on demand" vehicle, rather than water-fuelled as claimed.[13][14][15] On the company's website the energy source is explained only with the words "Chemical reaction".[16] The science and technology magazine Popular Mechanics has described Genepax's claims as "Rubbish."[17]
Stanley
Stanley Meyer claimed that he ran a dune buggy on water instead of petrol. He replaced the spark plugs with "injectors" to spray a fine mist of water into the engine cylinders, which he claimed were subjected to an electrical resonance. The "fuel cell" would split the water mist into hydrogen and oxygen gas, which would then be combusted back into water vapour in a conventional internal combustion engine to produce net energy. Meyer's claims were never independently verified, and in 1996 he was found guilty of fraud in an
Garrett electrolytic carburetor
Charles H. Garrett allegedly demonstrated a water-fuelled car "for several minutes", which was reported on
Thushara Priyamal Edirisinghe
Also in 2008, Sri Lankan news sources reported that Thushara Priyamal Edirisinghe claimed to drive a water-fuelled a car about 300 kilometers[22] on three liters of water.[23][24][25][26] Like other alleged water-fuelled cars described above, energy for the car is supposedly produced by splitting water into hydrogen and oxygen using electrolysis, and then burning the gases in the engine. Thushara showed the technology to Prime Minister Ratnasiri Wickramanayaka, who extended the Government’s full support to his efforts to introduce the water-powered car to the Sri Lankan market.[23]
Aquygen
The firm Hydrogen Technology Applications has also patented an electrolyser design[27] and has trademarked the term "Aquygen" to refer to the hydrogen oxygen gas mixture produced by the device.[28][29][30] Originally developed as an alternative to oxyacetylene welding, the company also claims to be able to run a vehicle exclusively on "Aquygen" and invoke an unproven state of matter called "magnegases" and a discredited theory about magnecules to explain their results.[31][32] Company founder Dennis Klein claims to be in negotiations with a major
Hydrogen as a supplement
In addition to claims of cars that run exclusively on water, there have also been claims that burning hydrogen or oxyhydrogen in addition to petrol or diesel fuel increases mileage. Around 1970, Yull Brown developed technology which allegedly allows cars to burn fuel more efficiently while improving emissions. In Brown's design, a hydrogen oxygen mixture (so-called "Brown's Gas") is generated by the electrolysis of water, and then fed into the engine through the air intake system. Whether the system actually improves emissions or fuel efficiency is debated.[35] Similarly, Hydrogen Technology Applications claims to be able increase fuel efficiency by bubbling "Aquyen" into the fuel tank.
A number of websites exist promoting the use of oxyhydrogen (often called "HHO"), selling plans for do-it-yourself electrolysers or entire kits with the promise of large improvements in fuel efficiency. According to a spokesman for the American Automobile Association, "All of these devices look like they could probably work for you, but let me tell you they don't."[36]
The gasoline pill and related additives
Related to the water-fuelled car hoax are claims that additives, often a pill, convert the water into usable fuel, similar to a carbide lamp, in which a high-energy additive produces the combustible fuel. This "gasoline pill" has been allegedly demonstrated on a full-sized vehicle, as reported in 1980 in Mother Earth News. Once again, water itself cannot contribute any energy to the process, the additive or the pill is the fuel.
Hydrogen on demand technologies
A hydrogen on demand vehicle uses some kind of chemical reaction to produce hydrogen from water. The hydrogen is then burned in an internal combustion engine or used in a fuel cell to generate electricity which powers the vehicle. While these may seem at first sight to be 'water-fuelled cars', they actually take their energy from the chemical that reacts with water, and vehicles of this type are not precluded by the laws of nature. Aluminium, magnesium, and sodium borohydride are substances that react with water to generate hydrogen, and all have been used in hydrogen on demand prototypes. Eventually, the chemical runs out and has to be replenished.[37][38][39] In all cases the energy required to produce such compounds exceeds the energy obtained from their reaction with water.[40]
One example of a hydrogen on demand device, created by scientists from the University of Minnesota and the Weizmann Institute of Science, uses boron to generate hydrogen from water. An article in New Scientist in July 2006 described the power source under the headline "A fuel tank full of water,"[40] and they quote Abu-Hamed as saying:
| “ | The aim is to produce the hydrogen on-board at a rate matching the demand of the car engine. We want to use the boron to save transporting and storing the hydrogen. | ” |
A vehicle powered by the device would take on water and boron instead of petrol, and generate boron trioxide. The chemical reactions describing the energy generation are:
4B + 6H2O → 2B2O3 + 6H2 [Hydrogen Generation Step]
6H2 + 3O2 → 6H2O [Combustion step]
The balanced chemical equation representing the overall process (hydrogen generation and combustion) is:
As shown above, boron trioxide is the only net byproduct, and it could be removed from the car and turned back into boron and reused. Electricity input is required to complete this process which Al-Hamed suggests could come from solar panels. [40]
Production
A pure stoichiometric mixture is most easily obtained by water electrolysis, which uses an electric current to dissociate the water molecules:
electrolysis: 2 H2O → 2 H2 + O2
combustion: 2 H2 + O2 → 2 H2O
William Nicholson was the first to decompose water in this manner in 1800. The energy required to generate the oxyhydrogen always exceeds the energy released by combusting it. (See Electrolysis of water#Efficiency).
Water torch
A water torch is a kind of oxyhydrogen torch, that is fed by oxygen and hydrogen generated on demand by water electrolysis. The device avoids the need for bottled oxygen and hydrogen, but requires electricity and distilled water. Some models of water torches mix the two gases immediately after production rather than at the torch tip, allegedly making the gas mixture more accurate.[10] This electrolyzer design is referred to as "common-ducted",[7] and the first was invented by William A. Rhodes in 1966.[11] Water torches must be designed to mitigate flashback by strengthening the electrolytic chamber. Use of an intermediary water bubbler eliminates potential electrolyzer damage from flashback, with a dry flashback arrestor being ineffective due to flame velocity. The bubbler is connected directly in series with the output gas. A water bubbler is sometimes referred to as a wet flashback arrestor, and effectively captures any remaining electrolyte in the output gas. Suitable electrolytes include sodium or potassium hydroxide, and other salts that ionize well.[7] Also "the electrolyzer system must be of high enough pressure to keep the gas velocity at the nozzle above the combustion velocity of the flame, or the system will backfire".[7]
Yull Brown's design
Oxyhydrogen gas produced in a common-ducted electrolyzer has been referred to as "Brown's gas"[12] after Yull Brown, a Bulgarian inventor naturalised in
Tidak ada komentar:
Posting Komentar