Role of sodium hydroxide for hydrogen gas production and storage
Sushant Kumar, Surendra K. SaxenaCenter for the Study of Matter at Extreme Conditions, College of Engineering and Computing, Florida International
University, Miami, Florida 33199, USA
The rapid depletion of fossil fuel, increase in pollution and related environmental hazards require us to discover newenergy sources. H2 is an efficient energy carrier with a very high specific energy content (~120MJ/kg) and energy density(10Wh/kg). It has been technically shown that hydrogen can be used for transportation, heating and power generation, andcould replace current fuels in all the present applications. Hydrogen can be produced using a variety of starting materials,derived from both renewable and non-renewable sources, through different process routes. Although sodium hydroxide iscorrosive in nature, it has a vast growing application as a starting ingredient in the field of hydrogen production andstorage.
Various current technologies include sodium hydroxide to lower the operating temperature, accelerate hydrogengeneration rate as well as sequester carbon dioxide during hydrogen production. Sodium hydroxide finds applications in allthe major H2 production methods such as steam methane reforming, coal gasification, biomass gasification, electrolysis,photochemical and thermochemical. Sodium hydroxide, being alkaline, acts as a catalyst, promoter or even a precursor.Different combinations of binary and complex hydrides when assisted with sodium hydroxide can efficientlyabsorb/desorb hydrogen at relatively mild conditions. Here, we discuss the technical and scientific role of sodiumhydroxide for hydrogen gas production and storage.
Hydrogen is the lightest and most abundant element on the earth. However, unlike oxygen, hydrogen is not found asfree in the nature at any significant concentration. Hydrogen is produced using both renewable and non-renewableresources, through various process routes. The available technologies for hydrogen production are reforming of naturalgas; gasification of coal and biomass; and the splitting of water by water-electrolysis, photo-electrolysis, photo-biological production, water splitting thermochemical cycle and high temperature decomposition. The principalmethods for the production of hydrogen involve water-electrolysis and natural gas reforming processes [1]. However,photo-electrolysis, photo-biological production and high temperature decomposition all are still in their early stage ofdevelopment. Thus, an extensive R&D is needed to mature these technologies for any future commercial applications.
In recent times, various researchers either proposed a modified version of the existing H2 production technologies orsuggested some innovative routes. Interestingly, a large number of the methods included sodium hydroxide as anessential ingredient. The use of sodium hydroxide for production of hydrogen is not new and was in application evenduring 19th century. In the following sections the significance of sodium hydroxide for the hydrogen production andstorage process will be discussed in details.
1.1 Overview of Sodium hydroxide (NaOH)
Previous technology for sodium hydroxide production included mixing of calcium hydroxide with sodium carbonate.This process was named as “causticizing”.
Ca (OH) 2 (aq) + Na2CO3 (s) = CaCO3 ↓ + 2 NaOH (aq) (1)
Currently, sodium hydroxide is produced by the electrolysis of brine (NaCl):
2NaCl + 2H2O = 2NaOH + Cl2 ↑ + H2 ↑ (2)
Besides H2 evolution, reaction (2) produces chlorine (a toxic gas) and sodium hydroxide. Moreover, the electrolysisof brine is also a high energy consuming process. Thus, the combined effect of high energy requirement and emission ofchlorine gas makes the production of sodium hydroxide using electrolysis of brine, an environmentally unsafe process.
Table 1 compares the three commercially available production methods for sodium hydroxide. It can be observedthat diaphragm cell process produces the lowest quality electrochemical caustic soda solutions (only 12wt. %).Therefore, evaporation is required to raise the concentration up to 50 wt. % solution as in mercury-cell process. Hence,the amount of steam consumption varies according to the strength of the produced electrochemical caustic sodasolutions.
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Energy consumption (kWh/MTCl2 ) at a current density (kA/m2) 2720(1.7) 3360(10) 2650(5)
Steam consumption (kWh/MTCl2 ) for concentration to 50%
NaOH610 0 180
% NaOH produced in USA 62 10 24
Figure 1 illustrates the membrane cell used for the electrolysis of brine.
Fig. 1 Membrane cell process schematic for production of sodium hydroxide.
2. Hydrogen Production and storage process
2.1 Modified Industrial Hydrogen Production
Currently, steam methane reformation (SMR) is the most common and the least expensive industrial technology toproduce hydrogen [1]. Methane reacts at a high temperature 700-1100 ºC with steam to form syn gas (CO+ H2).
Syn gas can further react to form additional H2 at a lower temperature.
CO (g) + H2O (g) = CO2 (g) + H2 (g) ΔH= -242 kJ/mol (327ºC) (4)
The combined reaction isCH4 (g) + 2H2O (g) = CO2 (g) + 4H2 (g) ΔH= 431 kJ/mol (927ºC) (5)
The enthalpy change (ΔH) is given for the temperatures at which the reaction is generating maximum H2 levels. Asimple calculation can show that while producing 1 gram of H2 via SMR technique, about 10.5 grams of CO2 is emitted.Such an undesired vast emission of CO2 endangers the prolong use of conventional SMR technique to produce H2.Thus, any method which can produce H2 without or with reduced CO2 emission is required. In this regard, severalmethods have been proposed but most of them are either expensive compared to those using fossil fuels or are in thevery early stages of development [2-7].
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Reaction between NaOH and CO yielding sodium formate (HCOONa) was described by Berthelot in 1856. Whenheated above 250ºC, HCOONa transforms into oxalate with release of H2:
NaOH (s) + CO (g) = HCOONa (s) (6)
2HCOONa(s) = Na2C2O4(s) + H2 (g) (7)
In 1918 Boswell and Dickson demonstrated that when carbon monoxide is heated with excess of sodium hydroxideat temperatures at which formate is transformed into oxalate, oxidation almost quantitatively to CO2 occurs with theevolution of an equivalent amount of H2 [8]:
2 NaOH (s) + CO (g) = Na2CO3(s) + H2 (g) (8)
Similarly, Saxena proposed the inclusion of sodium hydroxide as an additional reactant to the conventional SMRsystem. The addition of sodium hydroxide serves the dual purpose of carbon sequestration and H2 production [9].
Figure 2 compares the standard SMR (5) and modified SMR (9). It can be observed from the phase equilibriumdiagram that the unlike modified SMR method, standard SMR technique produces a more complex composition of gas(CO, CO2, H2O,H2) and also requires comparatively more energy (431kJ/mol at 927ºC vs 244 kJ/mol at 427ºC).However, the modified SMR reaction cannot be considered as a global solution. As sodium hydroxide is produced usingelectrolysis of brine which itself is a very energy intensive process.
Fig. 2 Calculated equilibrium in the system (a) SMR and (b) modified SMR reactions.
Other methods such as coal-gasification and water- gas shift (WGS) reaction also produce H2 at a large scale. Coal-gasification needs coal as a reactant and is a very energy consuming process. However, the WGS method is anexothermic reaction and operates at a low temperature. WGS reaction is an integral step for SMR technique as itproduces additional H2. Therefore, any modifications to these conventional techniques that can significantly reduce CO2emission are highly welcome.
Thermodynamic calculation shows that the addition of sodium hydroxide to CH4, C and CO lowers the operatingtemperature and can also significantly reduce the CO2 emission. Moreover, the amount of coal needed to run theprocesses is also lowered. Table 2 summarizes the inclusion of sodium hydroxide to CH4, C and CO. Sodium hydroxidecaptures CO2 and forms sodium carbonate (Na2CO3), which finds huge application in different chemical sectors.A series of experimental work which establishes the optimum operation condition and illustrated the use of suitablecatalysts for these systems can be found elsewhere [10-12].
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A similar concept of sodium hydroxide inclusion for enhanced H2 production is already in use at industrial scale. Forinstance, the black liquor gasification process utilizes alkali hydroxide to serve the dual purpose of H2 production andcarbon sequestration. In a typical pulping process for paper production, approximately one-half of the raw materials areconverted to pulp and other half is dissolved in the black liquor. The black liquor solution consists of well- dispersedcarbonaceous material, steam and alkali metal which are burned to provide part of energy for the plant. Due to thepresence of carbonaceous material and water in the liquor, following carbon-water reaction predominates:
C (s) + H2O (g) = CO (g) + H2 (g) (10)
CO (g) + H2O (g) = CO2 (g) + H2 (g (11)
However, due to the thermodynamic limitations, reaction (11) never proceeds towards completion; therefore H2concentration does not exceed a certain limit. However, in the presence of NaOH, CO2 capture medium, the equilibriumcan be shifted to drive reaction (11) towards completion and therefore maximize H2 concentration. Consequently, COand CO2 concentration reduces significantly in the product gases.
2.2 Biomass
Biomass is a renewable energy resource obtained from solar energy, carbon dioxide and water. Biomass does notincrease CO2 level in the atmosphere as it uptakes the same amount of carbon while growing as releases when burnt as afuel.
Biomass + heat + steam → H2 + CO + CO2 + CH4 + Light/ Heavy hydrocarbons + Char (12)
One of the major issues other than high carbon emission in biomass gasification is to deal with tar formation thatoccurs during the gasification process. The undesirable tar may cause the formation of tar aerosol and a more complexpolymer structure, which are unfavorable for H2 production through steam reforming. The existing methods to minimizetar formation are: (a) proper designing of gasifier (b) proper control and operation and (c) use of additives or catalysts.Sodium hydroxide-promoted biomass gasification to generate H2 without CO or CO2 formation generates H2 andcapture carbon [13]. Cellulose [C6H10O5], D-glucose [C6H12O6] and sucrose [C12H22O11] reacts with water vapor in thepresence of sodium hydroxide to form sodium carbonate and hydrogen. However, the product consists of hydrocarbonssuch as CH4 and thus lowers the % hydrogen yield. Nickel catalysts supported on alumina can reduce the formation ofCH4 and increase the hydrogen yield to roughly 100% [14-17]. The mechanism of alkali promoted steam gasificationof biomass indicates that the dehydrogenation of cellulose in presence of Na+ and OH- ions yields hydrogen. Theconcentration of Na+ and OH- ions strongly influences the dehydrogenation of cellulose [18-19].
Despite that the sodium hydroxide-promoted reaction provides many advantages; the alkali metal costs and theirrecycling are major concerns [3,20] Su et al used a new catalyst derived from sodium aluminum oxide (Al2O3.Na2O),Al2O3.Na2O.xH2O/NaOH/Al(OH)3, to increase the hydrogen content in the product after steam gasification of cellulose.The gasification temperature was kept below 500ºC to prevent any tar formation [21-22]. Moreover, sodium hydroxidecan also significantly decrease the pyrolysis temperature of biomass species [23]. Sodium ion, being small, canpenetrate into the biomass texture and break the hydrogen bridges. Consequently, devolatilization occurs rapidly. Thus,it can be seen that sodium hydroxide does play a significant role in biomass gasification.
2.3 Metals
Metals can react in the presence or absence of water and sodium hydroxide to produce hydrogen. Transition metalsform metal oxides and hydrogen during the reaction with sodium hydroxide [24]. Moreover, ferrosilicon when reactswith sodium hydroxide produces sodium silicate and hydrogen [25]. Here, we focus on the Al-NaOH-H2O system.
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Hydrogen gas is generated from the chemical reaction between Al and water (3.7wt% H2, theoretical yield)[26].Al/H2O system is indeed a safe method to generate H2. But the system has kinetic limitations as the metal surfacepassivation in neutral water occurs more easily and the metal activity with water is extremely low. Thus, improving thealuminum activity in water is an important task. To solve the problem of surface passivation of Al, various solutionshave been suggested so far. The solutions either include the addition of hydroxides [27-28] metal oxides [29-30]selected salts [31-32] or alloying Al with low melting point metal [33-36]. Alkali-promoted Al/H2O system is favoredover other metal systems because of high H2 generation rate. When the reaction between Al and water is assisted byalkali, OH- ions are able to destroy the protective oxide layer on the aluminum surface forming AlO2
-.The reaction between Al and H2O with sodium hydroxide solution produces H2, which can be expressed as follows
Sodium hydroxide consumed for the H2 generation in exothermic reaction (13) will be regenerated through thedecomposition of NaAl (OH)4 via reaction (14). Reaction (14) also produces a crystalline precipitate of aluminumhydroxide. The combination of above two reactions completes the cycle and shows that only water will be consumed inthe whole process if the process is properly monitored. Previous works reported kinetics of the reaction between Al andH2O with sodium hydroxide solution and calculated the activation energy in the range of 42.5-68.4 kJ/mol [37-38].
Several researchers examined the effects of other crucial parameters which control the H2 generation behavior foralkali assisted Al/H2O system. The parameters include temperature, alkali concentration, morphology, initial amount ofAl; and concentration of aluminate ions [4, 39-40]. Moreover, Soler et al. compared the H2 generation performance ofthree different hydroxides: NaOH, KOH and Ca(OH)2 and found that NaOH solution consumes Al faster compared withother two hydroxides [40]. Interestingly, S.S. Martinez et al. treated Al-can wastes with NaOH solution at roomtemperature to generate highly pure hydrogen. The byproduct (NaAl (OH)4) was used to prepare a gel of Al (OH)3 totreat drinking water contaminated with arsenic [41]. On the basis of the above mentioned reactions, several patents havebeen filed in last decade [42-49].
2.4 Water Splitting Thermochemical Cycle
Solar energy is used to produce H2 via 2 or 3 steps water splitting process. It should be noted that water is not directlysplit in H2 and O2 using this technology solution. But there is a series of chemical reactions which utilizes oxides andthermal energy from renewable sources (such as solar energy) that can convert water into stoichiometric amounts of H2and O2. [50-52]The water splitting thermochemical cycle is demonstrated in Figure 3. The figure demonstrates a 3 stepwater splitting process – (1) reduction of oxides (energy intensive process, 800-1000ºC) (2) reaction of reduced oxidewith sodium hydroxide (hydrogen generation step) and (3) hydrolysis reaction (sodium hydroxide recovery step).
Any thermodynamically favorable oxide can be selected and use to generate hydrogen. Thus, so far, a large numberof oxides have been considered. The water splitting thermochemical cycle reactions can be mainly classified as (1)2- step water splitting [53-57] (2) iodine-sulfur process [58-60] and (3) calcium-bromine process [61-63].
Fig. 3 Schematic for Water Splitting Thermochemical Cycle (MO= metal oxide)
Here, we focus only on the alkali metal assisted water splitting thermochemical cycles and table 3 summarizes them.The presence of alkali hydroxide (sodium hydroxide) is able to reduce the H2 generation reaction temperature. Recently,Miyoka et al considered sodium redox reaction and conducted several experiments in a non-equilibrium condition butcould not achieve a 100% conversion [64]. It was attributed to the slow kinetics of both the hydrogen generation
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reaction and sodium recovery. Moreover, sodium hydroxide facilitates oxidation in the water splitting step. But thevolatility of sodium hydroxide at temperature higher than 800ºC and incomplete Na+ extraction by water to recoversodium hydroxide limits its application. Several research groups concluded that even though sodium or sodiumhydroxide assisted reaction has major advantages; their recovery could be a big challenge. Interestingly, Weimer et alrecommends membrane separation to recover sodium hydroxide [65]. Few researcher groups also pointed out thepossibility of using sodium carbonate rather than sodium hydroxide [66-68].
Table 3 Alkali metal assisted water splitting thermochemical cycle
Besides sodium hydroxide recovery, there are other limitations too. For instance, the reduction of oxides requires avery high temperature. If such a high temperature will be provided by the solar energy, a large scale solar heat plant isneeded. Thus at present, the construction of thermochemical hydrogen production plants is restricted by the location,cost and safety issues. Therefore, the major challenge is to lower the operating temperature of water splitting process. Alow temperature water splitting process will allow the utilization of small-scale solar heat systems or even exhaust heatfrom industries. As sodium hydroxide can significantly reduce the operating temperature of the water splitting process,hence sodium hydroxide has a major role to play.
2.5 Organic Compounds
2.5.1 Formic acid (HCOOH)
Formic acid and its solution are industrial hazards. Any use of such chemical waste will be of a great advantage. Formicacid is considered for both H2 production and storage. Basically, formic acid can produce hydrogen using two methods:(1) Thermo catalytic decomposition and (2) Electrolysis in presence of sodium hydroxide.
Formic acid thermally decomposes to produce H2 and CO2 (ΔGº = -32.9 kJ/mol, ΔHº = 31.2 kJ/mol), which isactually the reversible reaction of CO2 hydrogenation [71-79]. Electrolysis of formic acid solutions in the presence ofsodium hydroxide requires theoretically much lower energy than water [80]. Hence, use of formic acid solution togenerate hydrogen will have double benefits of tackling pollution and generating clean energy. The electrochemicalreaction for the electrolysis of formic acid solutions is as follows [81]:
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Figure 4 demonstrates the scheme of electricity generation via the combined use of alkaline hydroxide (sodiumhydroxide) for the electrolysis of formic acid (HCOOH) and fuel cell. The separation of H2 and CO2 is desired prior toinjection in the fuel cell.
Fig. 4 Electricity generation using alkaline hydroxide (NaOH) for the electrolysis of HCOOH
2.5.2 Formaldehyde (HCHO)
An aqueous solution of formaldehyde when mixed with sodium hydroxide produces very small amount of hydrogen[82]. Thus, it is obvious that H2 evolution competes with the disproportionation of formaldehyde to correspondingalcohol and acid [83-84]. Further, Ashby et al proposed a mechanistic explanation of hydrogen evolution fromformaldehyde in the presence of sodium hydroxide [85].The mechanism indicates that one hydrogen atom originatesfrom the water and the other from the organic moiety. The experimental study exhibits that when a dilute solution offormaldehyde (4x10-4M) reacts with concentrated sodium hydroxide (19M) at room temperature, hydrogen is producedin a significant amount. However, concentrated solution of formaldehyde when interacts with dilute sodium hydroxidesolution produces only a trace amount of hydrogen.
When solution of hydrogen peroxide is mixed with formaldehyde in presence of sodium hydroxide, hydrogen isgenerated [86]. Hydrogen peroxide oxidizes formaldehyde to formic acid and sodium hydroxide further neutralizes theacid.
H2O2 + 2HCHO + 2NaOH = 2HCOONa + H2 + 2H2O (16)
However, no trace of hydrogen is observed in the absence of sodium hydroxide [87-88]. The reaction (16) is limitedby slow kinetics and requires a large excess of alkali hydroxide. When hydrogen peroxide is replaced by cuprous oxide,H2 is generated in a quantitative amount.
2.6 Hydrides
Hydride rapidly reacts with water to produce hydrogen. For instance, the binary hydride (LiH) reacts spontaneouslywith water while the complex hydride (NaBH4) reacts slowly unless catalyzed [89-92] However, for both hydrides, it isdifficult to achieve the stoichiometric amount of hdyrogen.
2.6.1 Binary Hydrides
(a) NaH/ LiH- NaOHReaction of LiH with water produces LiOH. LiOH is hygroscopic in the nature and therefore binds excess water [93]. Alarge amount of water requires extra heat to regenerate anhydrous LiOH. To avoid such situation, LiH is mechanicallymixed with NaOH in a molar ratio of 1:1 and 2:1 to generate H2 [94].
It can be observed that as the molar ratio of LiH increases, the reaction between NaOH and LiH tends to be anexothermic reaction and thus expected to occur at a lower temperature. The following mixtures with the molar ratio of1:1 and 2:1 for LiH and NaOH, heated at 250ºC, produces 1.92 and 3.20 wt. % H2 respectively. These yields are about50% and 66% of the total hydrogen content present in the mixtures. After heating the mixtures at 250ºC, besides H2,product also contains Li2O, NaH and NaOH. There was no sign of Na2O and LiOH formation. The reabsorption of H2by the product mixture was not reported so far, thus the reversibility of the presented scheme still needs to beinvestigated.
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When LiH is replaced by NaH, only H2 and Na2O are the products. The combination of NaH and NaOH canrelease/uptake ~ 3 wt. % H2 reversibly at temperature below 300ºC [95]. It should be noted that the reaction betweenNaH and NaOH has a large endothermic enthalpy (ΔHº = 64.3 kJ/mol-H2) and ΔGº is negative only above 400ºC. Na2Ois unstable in H2 atmosphere at temperatures below 400ºC.
NaH + NaOH ↔ Na2O + H2 (19)
Q. Xu et al. reported complete conversion of Na2O to NaH and NaOH under H2 atmosphere (100 bars) at 150ºC. Theaddition of NaOH to NaH can also significantly decrease the dehydrogenation temperature of NaH. However, neitherLiH-NaOH nor NaH-NaOH release/ uptake H2 in a significant amount and therefore lags behind the target set up by USDepartment of Energy. Moreover, the operating temperature of absorption and desorption does not meet the goal. Thecurrent price of Li or Na hydride is too much and hence is unfit to deliver economical H2.
2.6.2 Complex Hydrides
(a) Mg (NH2)2-2LiH-NaOHMg (NH2)2 -LiH system undergoes reaction as shown below to produce H2 reversibly.The desorption enthalpy changefor the first step is 38.9kJ/mol-H2 [96] which is much lower than LiNH2-LiH mixture (66kJ/mol-H2). The reducedenthalpy change permits the H2 to be absorbed /desorbed at moderate temperature and pressure.
Mg (NH2)2 + 2LiH ↔ ½ Li2Mg2N3H3 + ½ LiNH2 + ½ LiH + 3/2 H2 ↔ Li2Mg (NH) 2 + 2H2 (20)
Several efforts have been devoted to lower the operating temperatures (<200ºC) and enhancing thehydrogenation/dehydrogenation rate of Mg (NH2)2-2LiH system [97-101]. Liang et al introduced NaOH to the Mg(NH2)2-2LiH system as a catalyst precursor and investigated its effect on the operating temperatures forhydrogenation/dehydrogenation rate [102].They achieved ~36ºC reduction in the dehydrogenation peak temperature forMg (NH2)2-2LiH-0.5NaOH. During ball milling, NaOH reacts with Mg (NH2)2 and LiH to convert to NaH, LiNH2 andMgO. The hydrogen desorption temperature may be reduced by the addition of NaOH to the system. However, the useof NaOH has an adverse effect on the hydrogen capacity of the hydride sample. The total hydrogen capacity decreasedfrom 5.28 wt. % [pristine sample] to ~3.61 wt. % [Mg (NH2)2-2LiH-0.5NaOH sample] due to the dilution of NaOH.
(b) NaBH4-NaOHA NaBH4 solution with alkaline stabilizer, sodium hydroxide, reacts with water in the presence of catalysts to produceH2 and sodium metaborate (NaBO2) [103-109].The catalytic reaction for NaBH4-NaOH system is heterogeneous,irreversible and exothermic in nature. Hung et al has tabulated the kinetic models for different catalysts, concentrationof NaBH4, temperature, time and activation energy for NaBH4-NaOH system [110]. The reaction is generally performedin the temperature range of 10-60 ºC for various times and the activation energy varies from 40-55 kJ/mol. Based onthese conditions and various catalysts behavior, different reaction kinetic models such as zero- order, first-order andLangmuir- Hinshelwood have been suggested for NaBH4-NaOH system.
NaBH4 + 2H2O → 4H2 + NaBO2 ΔHº = -210 kJ/mol (21)
A study for hydrolysis kinetics of NaBH4-NaOH system over Ru/G (Ruthenium –Graphite) catalysts is recentlyreported. The result shows that H2 generates at a rate of 32.3 L min-1g-1 Ru in a 10 wt. % NaBH4 + 5 wt. % NaOHsolution [111].The Ru/G catalyst with high activity and durability demonstrates a potential for the portable fuelgenerators.
3. Conclusion
The use of sodium hydroxide for hydrogen production and storage is justified by the high reaction rate, lower operatingtemperature and overall reduction in CO2 emission. Depending on the reaction and its conditions, sodium can produceor store hydrogen. Moreover, several situations illustrate how sodium hydroxide binds with CO2 and forms valuablechemical compound, Na2CO3.
However, a major limitation for the inclusion of sodium hydroxide as one of the primary ingredients for obtaining thehydrogen is the production route of sodium hydroxide itself. At present, high energy intensive process (electrolysis ofbrine) is the prime method to produce sodium hydroxide. It will be of a great interest to invent or modify a method inwhich sodium hydroxide can be produced using the renewable resources such as solar, water and wind. Any suchmethod would certainly mitigate the CO2 emission by a huge amount and eventually increase the role of sodiumhydroxide for hydrogen production and storage purpose.
Catalyst
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Acknowledgement: S. Kumar would like to acknowledge the financial support from Florida International University graduate school, dissertation year fellowship (DYF).
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