Water splitting is a generic term for chemical reactions in which water is separated into oxygen and hydrogen. Efficient and economical water splitting will be a key technology component of the hydrogen economy. Various water splitting techniques have been issued in water separation patents in the United States. In photosynthesis, the separation of water contributes electrons to the electron transport chain in Photosystem II.
Video Water splitting
Electrolysis
Water electrolysis is water decomposition (H 2 O) to oxygen (O 2 ) and hydrogen gas (H 2 ) because an electric current is passed through water. In chemistry and manufacturing, electrolysis is a method for separating chemically bound elements and compounds by passing the flow of electricity through them. One use of electrolysis of water or artificial photosynthesis (photoelectrolysis in photoelectrochemical cells) is to produce hydrogen. Recently, researchers have shown that water splitting can be broken down into two discrete steps using a hierarchical nano structure or polyoxometalate-based redox mediator.
Ruling for a gas production scheme, excess power or power from a peak created by a wind generator or solar panel is used to balance the load of the energy network by storing and then injecting hydrogen into the natural gas grid.
The production of hydrogen from water requires considerable amount of energy and is not competitive with its production from coal or natural gas. Potential electric energy supplies include hydro power, wind turbines, or photovoltaic cells. Typically, the electricity consumed is more valuable than the hydrogen produced so this method has not been widely used. Other potential energy supplies include heat from nuclear reactors and light from the sun. Hydrogen can also be used to store renewable electricity when it is not needed (like a wind blowing at night) and then used to meet the electricity needs during the day or for vehicle fuel. The quality of stored hydrogen helps make hydrogen a supporter of wider renewable energy use, and internal combustion engines. ( See hydrogen economy. )
High pressure electrolysis
When the water is pressurized and then electrolysis is carried out at high pressure, the resulting hydrogen gas is pre-compressed about 120-200 bar (1740-2900 psi). With pre-pressurizing hydrogen in electrolyser, energy is stored as the need for external hydrogen compressors is eliminated. The average energy consumption for internal compression is about 3%. The energy required to condense water is much less than it takes to compress hydrogen gas.
High temperature electrolysis
When the energy provided is in the form of heat (which comes from solar heat, or nuclear), the best path for hydrogen production is through high temperature electrolysis (HTE). In contrast to low temperature electrolysis, HTE water converts more of its initial heat energy into chemical energy (hydrogen), potentially doubling its efficiency to about 50%. Since some of the energy in HTE is provided in the form of heat, less energy must be converted twice (from heat to electricity, and then to chemical form), and so the process is more efficient.
The HTE process is generally only considered in combination with nuclear heat sources, as other non-chemical forms of high temperature heat (solar heat concentration) are not consistent enough to lower the cost of capital for HTE equipment. Research on HTE and high-temperature nuclear reactors can ultimately lead to a cost-competitive hydrogen supply with natural gas vapor reform. HTE has been demonstrated in the laboratory, but not on a commercial scale.
Maps Water splitting
photoelectric water splitting
Using electricity generated by photovoltaic systems potentially offers the cleanest way to produce hydrogen, other than nuclear, wind, geothermal, and hydroelectric. Again, water is broken down into hydrogen and oxygen by electrolysis, but electrical energy is obtained by photoelectrochemical cell processes (PEC). This system is also called artificial photosynthesis.
Photocatalytic water splitting
Converting solar energy into hydrogen through a process of water separation is one of the most attractive ways to achieve clean and renewable energy. However, if this process is assisted by a photocatalyst suspended directly in water rather than a photovoltaic or electrolytic system, the reaction proceeds in one step, making it more efficient.
Radiolysis
Nuclear radiation routinely cuts off water bonds, in the Mpeeng gold mine, South Africa, researchers found in high natural radiation zones, a community dominated by the new phylotype Desulfotomaculum , fed on H 2 . The use of nuclear fuel/"nuclear waste" is also seen as a potential source of hydrogen.
Photosbiological water splitting
Biological hydrogen can be produced in algal bioreactors. In the late 1990s it was found that if the algae lost sulfur, it would shift from oxygen production, that is normal photosynthesis, to the production of hydrogen. It seems that production is now economically feasible by exceeding 7-10 percent energy efficiency (converting sunlight into a hydrogen barrier). with a hydrogen production rate of 10-12 ml per liter of culture per hour.
Thermal water decomposition
Thermal decomposition, also called thermolysis, is defined as a chemical reaction in which chemicals are split into at least two chemicals when heated. At high temperatures, water molecules are divided into components of hydrogen and oxygen atoms. For example, at 2200 ° C about three percent of all H 2 O molecules are separated into various combinations of hydrogen and oxygen atoms, mostly H, H 2 , O, O 2 , and OH. Other reaction products such as H 2 O 2 or HO 2 remain small. At very high temperatures of 3000 ° C over half the water molecules decompose, but at ambient temperatures only one molecule in 100 trillion is separated by a heat effect.
The splitting of hot water has been investigated for hydrogen production since the 1960s. The high temperatures required to obtain sufficiently large amounts of hydrogen impose heavy requirements on the materials used in hot splitting devices. For industrial or commercial applications, material constraints have limited application success for hydrogen production from direct hot water splitting and with some exceptions to recent developments in the areas of catalytic and thermochemical cycles.
Nuclear-thermal
Several prototypes of Generation IV reactors, such as high temperature engineering test reactors, operate at 850 to 1000 degrees Celsius, much hotter than existing commercial nuclear power plants. General Atomics predicts that the hydrogen produced in High Temperature Gas Cooled Reactor (HTGR) will cost $ 1.53/kg. In 2003, natural gas vapor reform yielded hydrogen at $ 1.40/kg. At the 2005 gas price, the hydrogen price was $ 2.70/kg. Therefore, only in the United States, the savings of tens of billions of dollars per year are possible with nuclear power supplies. Much of this savings will be translated into reductions in oil and natural gas imports.
One side benefit of a nuclear reactor that produces electricity and hydrogen is that it can divert production between the two. For example, the plant may generate electricity during the day and hydrogen at night, matching its generation profile to daily demand variations. If hydrogen can be produced economically, this scheme will compete profitably with the existing grid energy storage scheme. Moreover, there is sufficient hydrogen demand in the United States that all the top generation daily can be handled by the plant.
Recent research on hybrid copper-chlorine thermoelectric cycles has focused on cogeneration systems using waste heat from nuclear reactors, particularly CANDU supercritical water reactors.
Solar-thermal
The high temperatures required for separating water can be achieved through the use of concentrated solar power. Hydrosol-2 is a 100 kilowatt pilot plant at Plataforma Solar de AlmerÃÆ'a in Spain that uses sunlight to get 800 to 1,200 ° C to separate water. Hydrosol II has been in operation since 2008. The design of this 100 kilowatt pilot plant is based on a modular concept. As a result, it is possible that this technology can be easily upgraded to a range of megawatts by multiplying available reactor units and by linking the plant to the heliostate field (solar tracking mirror field) of an appropriate size.
An interesting approach to solar hydrogen production is proposed by H2 Power Systems. Material resistance due to the required high temperatures above 2200 ° C is reduced by the design of the membrane reactor with simultaneous extraction of hydrogen and oxygen that exploits the specified thermal gradient and the rapid diffusion of hydrogen. With sunlight concentrated as a heat source and only water in the reaction chamber, the resulting gas is very clean with the only possible contaminant being water. A "Solar Water Cracker" with a concentrate of about 100 mÃ,² can produce nearly one kilogram of hydrogen per hour of sunlight.
Water Separation by Coordinate Compound
The enormous energy needs, future oil shortages and rapid increase in pollution are issues that need to be addressed with more effort in investigating clean and sustainable energy resources. In pursuit of these energy sources, efforts are placed in the light that breaks water into O 2 and H 2 in an attempt to convert solar energy into fuel. Water oxidation (2H 2 O -> 4H 4e - O 2 ) is the first important step providing electrons and protons which is required for the next step (proton reduction) in which the production of hydrogen takes place in the catalysis reaction by the proton reduction catalyst.
2H 2 O hv -> 2H 2 O 2 ? GÃ, Â ° = 4.92 eV (113 kcal mol - 1 ) Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â · 4H 4e - -> 2H 2 Â Â Â Â Â Â Â Â · Â · 2H 2 O -> 4H 4e - O 2
The water oxidation step has been regarded as an inhibitor of this process, so the design of a highly active and powerful water oxidation catalyst (WOCs) is an important step in the development of light-driven water separations. Water oxidation catalysts minimize overpotential and increase reaction rate. The ideal WOC is required to have overpotential, high stability, high activity/efficiency, low toxicity and low cost.
General steps in water oxidation catalysis
The first step in the oxidation water catalidation by WOC is the transfer of electrons from the catalyst to the oxidizing agent, which can be increased by introducing strong electrons to donate the ligands in the catalyst. The second step involves water oxidation where the redox potential of the catalyst must have high oxidation power at the highest oxidation state. The third step which is the formation of the OO bond has been investigated to proceed with two mechanisms: nucleophilic water (WNA) attack, in which water attacks the oxo metal complex group, yields two electron reductions of metal and O 2 formation; and the interaction of two M-O species (12M) that lead to the formation of peroxo-intermediate. The final step is the release of molecular oxygen which is a slow step.
Water splitting in photosynthesis
In nature, solar energy is converted into chemical energy by the process of photosynthesis that produces energy essential for survival on earth. Photosynthesis works with two photo systems: Photosystems I (P700) and Photosystem II (P680). When my photosystem gets photo-fired, electron transfer reactions will start, resulting in a reduction of a series of electron acceptor, ultimately reducing NADP to NADPH and PS I oxidized. The photosystem oxidized I capture electrons from photosystem II through a series of steps involving agents such as plastoquinone, cytochrome and plastocyanine. The photosystem II then brings about the oxidation of water which results in the evolution of oxygen, a reaction catalyzed by CaMn 4 O 5 clusters embedded in complex protein environments; This complex is known as the oxygen-growing complex (OEC). Now the water oxidation procedure is clearly indicated by the Dolai diagram of the S. Ref states. Dolai, U. (2017). "Chemical Scheme of Air-Process Pitting during Photosynthesis by means of Experimental Analysis". Journal of IOSR Pharmaceutical and Biological Sciences. 12 (6): 65-67. doi: 10.9790/3008-1206026567. ISSN 2319-7676.
Water splitting by some metal complex
The separation of water by the coordination complex gets inspired by natural photosynthesis. This coordination complex is used as a catalyst in water splitting or water oxidation and is therefore called water oxidation catalyst. There are a number of synthetic metal complexes used for this purpose; a brief review is given below.
Water Sharing on Steel
In recent years, steel has received a striking resurgence as a water-separating catalyst. Above all oxidized Stainless Steel Nickel and oxidized Cobalt are found to have become highly active and highly volatile Oxidation Water Oxidation.
Water separation by Ruthenium complex
Among a number of WOCs we know, the ruthenium-based complex is known to show high rotation rates in catalytic activity. A number of Ru mononuclear catalysts have been reported as efficient catalysts, which have led us to believe that multicentre catalysts are not required for strong catalytic activity. Tanaka, Meyer, Llobet, and their colleagues have reported the synthesis of several ruthenium-aqua complexes that are active in water oxidation, and the catalytic mechanism has been studied extensively primarily based on the six-coordinate ruthenium model. Llobet and co-workers investigate the 2 ((where <3> (Âμ-bpp) terpyridine; bpp = 2,6-bis (pyridyl) pyrazolate) A number of non-aqua (trans- [Ru] complexes (pbn) (4-R-py) 2 )] 2 where (pbn = 2,2 '- (4 - (tert-butyl) pyridine-2,6-dizy) bis (1,8-naphthyridine); py = pyridine; R = CH 3 , CF 3 and N (CH 3 ) 2 ) has been reported by Thummel et al. . and proved to be very active in the process Duan et al. synthesize mononuclear [Ru III (bda) (pic) 2 ] complex 1 (pic = 4 -picoline) and isolate Ruiv dimeric intermediate 7-coordinated rare 2 (Ã,Âμ- (HOHOH) [Ru IV (bda) (pic) 2 > 2 (PF 6 ) 3 o2H 2 O) during the catalidation reaction a ir oxidation. found to have bridging ligands [HOHOH] - in addition to two water molecules, which appear in the H-bonded fo rm with the medium. The ability to contribute strong electrons from the carboxylic group in the complex causes more decreases in the oxidation potential of the complex 1 than the complex [RuII (L?) (Pic) 2 ] 2 (L? = 2,9-di (pyrid-2? -yl) -1,10-phenanthroline) reported by Thummel et al. . The chemical investigation of the catalytic process involves an enormous oxygen liberation when the complex is added to an aqueous aqueous acid solution with an IV IV oxidizer. Duan et al. . also reported the synthesis of another ruthenium mononuclear complex [Ru (bda) (isoq) 2 ] ( 3 ; H 2 bda = 2,2? -bipyridine-6,6? -acarboxylic acid, isoq = isoquinoline) which has increased the catalytic activity (TOF = 303 s -1 ) to water oxidation compared with other structurally similar mononuclear [Ru (bda )) (pic) 2 ] complex 1 . Very polarizable? isoquinolines in an axial position at 3 reduces the barrier for Ru-O rader dimerization because the interaction attracts non-covalent between them compared to 4-picoline in the 2 complex that makes it better catalyst. The catalytic activity is proportional to photosynthetic photosynthesis II, the driving force is Ce IV rather than visible light for the complex.
Water splitting by the Nickel complex
Nickel-based homogeneous catalysts are hard to find in the literature but they have been used as alkaline electrolytes on a commercial scale. A Ni-based oxide film has been synthesized that frees oxygen under quasi-neutral conditions overpotential ~ 425mV and shows long-lasting stability. X-ray spectroscopy shows the presence of μ-oxide bonds between Ni III /Ni IV but there is no evidence of a mono-Âμ-oxide bridge found between ions Similar structures can be found in Co-WOC film and Mn-WOC catalyst.
Water splitting by Cobalt complex
Cobalt salts can be used as WOCs, but their water oxidation capacity decreases over time due to salt deposition from homogeneous solutions, resulting in turbidity. Both Right and Nocera have shown the formation of dark thin films on the surface of indium tin oxide used electrodes, together with bubbles in solution. These bubbles are confirmed as oxygen. Co-WOC has proven to work very efficiently at pH 7.0. Heterogeneous cobalt oxide (Co 3 4 ) has been studied to work on the same pattern as other cobalt salts. No mechanisms have been reported so far for this WOC. A homogenous cobalt polyoxametalate complex [Co 4 (H 2 O) 2 (? - PW 9 O 34 ) 2 ] 10 - has been described very efficiently compared to the heterogeneous cobalt complex reported reported by Yin et al et al. prepare a stable WOC by absorb the Co II group as Co (OH) 2 on silica nanoparticles showing high water oxidation activity. WOC [Co (Py 5 ) is homogeneous (H 2 O)] (ClO 4 ) 2 reported by Waylenko and co-workers. This complex undergoes a proton-coupled electron transfer to form a stable [Co III - OH] 2 specie which in advanced oxidation forms an intermediate Co IV . Medium formed reacts with water to free O 2 .
Water split by iron complex
Lloret-Fillol, Costas and colleagues showed that the common iron complex catalyzes the water oxidation reaction to produce O 2 . Water soluble complex [Fe (OTf) 2 (Me 2 Pytacn)] (Pytacn = pyridine triazacyclononane; OTf = CF 3 SO < span> -
3 ) is reported as a highly efficient catalyst for water oxidation when chemical oxidant (CAN) is added. The concentration of catalyst and oxidant is found to greatly affect the oxidation process. The catalytic activity of the complex [FeCl 2 (mcp)], [Fe (OTf) 2 (mcp)], [Fe (OTf) 2 (bpbp)], [Fe (OTf) 2 (mep)], and [Fe (OTf) 2 (tpa)] (mcp = N, N? - dimethyl -N, N? -bis (2-pyridylmethyl) -cyclohexane-1,2-diamine; bpbp = N, N? -bis (2-pyridylmethyl) -2,2? -bipyrrolidine; mep = N, N? - dimethyl -N, N? -bis- (2-pyridylmethyl) -ethane-1,2-diamine; tpa = tris- (2-pyridylmethyl) amine) were studied that showed them to be active catalysts. All these complexes have two sites that can be accessed for coordination, cis of each other. Other complexes ([mcp]) 2 (Ã,Âμ-O) (Ã,Âμ-OH)] (OTf) 2 were found to show more catalytic activity low indicating a catalysis site. The complex is found to be degraded within hours of activity due to strong acid and oxidation environments. [Fe (OTf) 2 (tmc)] and [Fe (NCCH 3 ) (MePy 2 CH-tacn)] (OTf) < sub> 2 (tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane; tacn = 1,4,7-triazacyclononane) are found to be inactive, both of which have two trances and only one coordination site. So it can be called that the presence of cis reactive sites is a key factor for the oxidation water catalysis in the complex.
Water separation by iridium complex
Iridium oxide is a stable bulk WOC catalyst with low overpotential. McDaniel et al . Analog synthesized from [Ir (ppy) 2 (OH 2 ) 2 ] (ppy = 2-phenylpyridine ) with high turnover, but low catalytic level. Replacing the ppy complex with Cp * (C 5 Me 5 ) results in an increase in catalytic activity but decreases the number of changes. The water nucleophilic attack on Ir = O species was found to be responsible for the formation of O 2 .
So it can be concluded that a number of complexes of Iron, ruthenium, cobalt, iridium and many others can be used for water oxidation in artificial photosynthesis. The use of this complex is reliable in terms of high energy production and is more environmentally friendly than commonly used fuels. Some of these WOCs have good TONs and TOFs but unfortunately none reach the point of use on a large scale because they lose their activity after some time for some reason or because they are slow to react. In-depth research is needed to design an ideal WOC that can be used on a commercial scale.
Chemical production
Various materials react with water or acid to release hydrogen. Such a method is not sustainable. In terms of stoichiometry, this method resembles the process of steam reform. The big difference between such chemical methods and the steam reform (which is also a "chemical method"), is that the required reduced metal does not exist naturally and requires considerable energy for their production. For example, in a strong acid laboratory reacts with zinc metal in a Kipp tool.
In the presence of sodium hydroxide, the aluminum and alloys react with water to produce hydrogen gas. Unfortunately, due to its energy inefficiency, aluminum is expensive and can be used only for low volume hydrogen generation. Also a large amount of heat should be discarded.
Although other metals can perform the same reaction, aluminum is one of the most promising materials for future development as it is safer, cheaper and easier to transport than some other hydrogen storage materials such as sodium borohydride.
The initial reaction (1) consumes sodium hydroxide and produces hydrogen gas and aluminate byproducts. After reaching its saturation limit, the aluminate compounds decompose (2) into sodium hydroxide and aluminum hydroxide crystalline deposits. This process is similar to the reaction inside the aluminum battery.
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- (1) Al 3 H 2 O NaOH -> NaAl (OH) 4 1.5 H 2
- (2) NaAl (OH) 4 -> NaOH Al (OH) 3
Secara keseluruhan:
In this process, aluminum serves as a compact hydrogen storage material because 1 kg of aluminum can produce up to 0.111 kg of hydrogen (or 11.1%) of water. When used in fuel cells, the hydrogen can also generate electricity, recovering half of the water consumed beforehand. The US Department of Energy has outlined its purpose for a compact hydrogen storage device and researchers are trying many approaches, such as using a combination of aluminum and NaBH 4 , to achieve this goal.
Because the oxidation of aluminum is exothermic, this reaction can operate under light temperature and pressure, providing a stable and compact source of hydrogen. This chemical reduction process is particularly suitable for back-up, remote or marine applications. While aluminum passivation usually retards this reaction, its negative effects can be minimized by altering some experimental parameters such as temperature, alkali concentration, physical form of aluminum, and solution composition.
Research
Research is being done during photocatalysis, acceleration of photoreaction in the presence of a catalyst. His understanding has been possible since the invention of electrolysis of water using titanium dioxide. Artificial photosynthesis is a field of research that attempts to replicate the natural processes of photosynthesis, converting sunlight, water and carbon dioxide into carbohydrates and oxygen. Recently, it has successfully broken water into hydrogen and oxygen using an artificial compound called Nafion.
High temperature electrolysis (also HTE or steam electrolysis) is a method currently under investigation for the production of hydrogen from water with oxygen as a by-product. Other studies include thermolysis on a damaged carbon substrate, thus allowing the production of hydrogen at temperatures just below 1000 ° C.
The iron oxide cycle is a series of thermochemical processes used to produce hydrogen. The iron oxide cycle consists of two chemical reactions which are clean and water is clean and hydrogen and oxygen are clean products. All other chemicals are recycled. The process of iron oxide requires an efficient heat source.
The sulfur-iodine cycle (S-I cycle) is a series of thermochemical processes used to produce hydrogen. The S-I cycle consists of three chemical reactions which are clean and water is clean and hydrogen and oxygen are clean products. All other chemicals are recycled. The S-I process requires an efficient heat source.
More than 352 thermochemical cycles have been described for water splitting or thermolysis., This cycle promises to produce hydrogen oxygen from water and heat without using electricity. Since all of the input energy for the process is heat, they can be more efficient than high temperature electrolysis. This is because the efficiency of electricity production is inherently limited. The production of thermochemical hydrogen using chemical energy from coal or natural gas is generally not considered, since direct chemical pathways are more efficient.
Untuk semi process thermochemistry, recover ringkasannya adalah process decomposisi air:
All other reagents are recycled. None of the thermochemical hydrogen production processes have been demonstrated at the production level, although some have been demonstrated in the laboratory.
There is also research on the viability of nanoparticles and catalysts to lower temperatures when water is split.
Recently, the Metallic-Organic Framework-based material (MOF) has proven to be a very promising candidate for water splitting with a cheap first line transition metal.
Research is concentrated on the following cycles:
Patent
- Vion, AS. Patent 28.793 , "Method of increasing atmospheric electricity usage", June 1860.
See also
- Photocatalytic water splitting
- Substitution of the water gas reaction
References
External links
- JEAC
Source of the article : Wikipedia
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