Materials
and processes for energy: communicating current research and technological developments (A. Méndez-Vilas, Ed.)
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Role of sodium hydroxide for hydrogen gas production and storage
Sushant Kumar, Surendra K. Saxena
Center 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 new
energy 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, and
could 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 is
corrosive in nature, it has a vast growing application as a starting ingredient in the field of hydrogen production and
storage.
Various current technologies include sodium hydroxide to lower the operating temperature, accelerate hydrogen
generation rate as well as sequester carbon dioxide during hydrogen production. Sodium hydroxide finds applications in all
the 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 efficiently
absorb/desorb hydrogen at relatively mild conditions. Here, we discuss the technical and scientific role of sodium
hydroxide for hydrogen gas production and storage.
Keywords sequester; steam methane reforming; coal/biomass gasification; electrolysis; photochemical/thermochemical;
hydrides
1. Introduction
Hydrogen is the lightest and most abundant element on the earth. However, unlike oxygen, hydrogen is not found as
free in the nature at any significant concentration. Hydrogen is produced using both renewable and non-renewable
resources, through various process routes. The available technologies for hydrogen production are reforming of natural
gas; gasification of coal and biomass; and the splitting of water by water-electrolysis, photo-electrolysis, photobiological production, water splitting thermochemical cycle and high temperature decomposition. The principal
methods 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 of
development. 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 H 2 production technologies or
suggested some innovative routes. Interestingly, a large number of the methods included sodium hydroxide as an
essential ingredient. The use of sodium hydroxide for production of hydrogen is not new and was in application even
during 19th century. In the following sections the significance of sodium hydroxide for the hydrogen production and
storage 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 electrolysis
of brine is also a high energy consuming process. Thus, the combined effect of high energy requirement and emission of
chlorine 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 observed
that 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 soda
solutions.
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Table 1
Comparison of the commercially available production methods for NaOH
Factors
Use of mercury
Chlorine as a byproduct
Operating current density
(kA/m2)
Cell voltage (V)
NaOH strength (wt. %)
Energy consumption (kWh/MT
Cl2 ) at a current density (kA/m2)
Steam consumption (kWh/MT
Cl2 ) for concentration to 50%
NaOH
% NaOH produced in USA
Diaphragm
No
Yes
Mercury
Yes
No
Membrane
No
Yes
0.9-2.6
8-13
3-5
2.9-3.5
12
3.9-4.2
50
3.0-3.6
33-35
2720(1.7)
3360(10)
2650(5)
610
0
180
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 to
produce hydrogen [1]. Methane reacts at a high temperature 700-1100 ºC with steam to form syn gas (CO+ H2).
CH4 (g) + H2O (g) = CO (g) + 3H2 (g) ΔH= 397kJ/mol (1227ºC)
(3)
Syn gas can further react to form additional H2 at a lower temperature.
The combined reaction is
CO (g) + H2O (g) = CO2 (g) + H2 (g) ΔH= -242 kJ/mol (327ºC)
(4)
CH4 (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 H 2 levels. A
simple 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, several
methods have been proposed but most of them are either expensive compared to those using fossil fuels or are in the
very 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. When
heated 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 hydroxide
at temperatures at which formate is transformed into oxalate, oxidation almost quantitatively to CO 2 occurs with the
evolution 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 SMR
system. The addition of sodium hydroxide serves the dual purpose of carbon sequestration and H2 production [9].
2NaOH (s) + CH4 (g) + H2O (g) = Na2CO3(s) + 4H2 (g) ΔH= 244 kJ/mol (427ºC)
(9)
Figure 2 compares the standard SMR (5) and modified SMR (9). It can be observed from the phase equilibrium
diagram 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 using
electrolysis 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. Coalgasification needs coal as a reactant and is a very energy consuming process. However, the WGS method is an
exothermic reaction and operates at a low temperature. WGS reaction is an integral step for SMR technique as it
produces additional H2. Therefore, any modifications to these conventional techniques that can significantly reduce CO 2
emission are highly welcome.
Thermodynamic calculation shows that the addition of sodium hydroxide to CH4, C and CO lowers the operating
temperature and can also significantly reduce the CO 2 emission. Moreover, the amount of coal needed to run the
processes is also lowered. Table 2 summarizes the inclusion of sodium hydroxide to CH4, C and CO. Sodium hydroxide
captures 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 suitable
catalysts for these systems can be found elsewhere [10-12].
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Table 2 Thermodynamic properties for different hydrogen production methods after inclusion of NaOH
CH4 + H2O
Temperature
700-1100
(0C)
Enthalpy
431(927ºC)
(ΔH, kJ/mol)
Mixture of
CO,CO2,H2
product gases
Coal / H2(g/g)
1.64
CO2/H2(g/g)
10.5
H.R.: Heat Recovery
NaOH+ CH4 +
H2O
C + H2O
600-800
800-1200
244 (427ºC)
95.73(327ºC)
H2
0.93
3.41
NaOH + C +
H2O
500-700
64.58(327ºC)
CO,CO2, H2
3.73
13.67
H2
3.49
1.80
CO + H2O
130
-242(327ºC)
CO2, H2
22(No H.R.)
NaOH+ CO
300-400
-119(327ºC)
H2
- (No H.R.)
A similar concept of sodium hydroxide inclusion for enhanced H2 production is already in use at industrial scale. For
instance, the black liquor gasification process utilizes alkali hydroxide to serve the dual purpose of H 2 production and
carbon sequestration. In a typical pulping process for paper production, approximately one-half of the raw materials are
converted to pulp and other half is dissolved in the black liquor. The black liquor solution consists of well- dispersed
carbonaceous material, steam and alkali metal which are burned to provide part of energy for the plant. Due to the
presence 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 H2
concentration does not exceed a certain limit. However, in the presence of NaOH, CO2 capture medium, the equilibrium
can be shifted to drive reaction (11) towards completion and therefore maximize H2 concentration. Consequently, CO
and 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 not
increase CO2 level in the atmosphere as it uptakes the same amount of carbon while growing as releases when burnt as a
fuel.
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 that
occurs during the gasification process. The undesirable tar may cause the formation of tar aerosol and a more complex
polymer structure, which are unfavorable for H2 production through steam reforming. The existing methods to minimize
tar 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 and
capture carbon [13]. Cellulose [C6H10O5], D-glucose [C6H12O6] and sucrose [C12H22O11] reacts with water vapor in the
presence of sodium hydroxide to form sodium carbonate and hydrogen. However, the product consists of hydrocarbons
such as CH4 and thus lowers the % hydrogen yield. Nickel catalysts supported on alumina can reduce the formation of
CH4 and increase the hydrogen yield to roughly 100% [14-17]. The mechanism of alkali promoted steam gasification
of biomass indicates that the dehydrogenation of cellulose in presence of Na + and OH- ions yields hydrogen. The
concentration 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 their
recycling 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 hydroxide
can also significantly decrease the pyrolysis temperature of biomass species [23]. Sodium ion, being small, can
penetrate 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 metals
form metal oxides and hydrogen during the reaction with sodium hydroxide [24]. Moreover, ferrosilicon when reacts
with 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 surface
passivation in neutral water occurs more easily and the metal activity with water is extremely low. Thus, improving the
aluminum activity in water is an important task. To solve the problem of surface passivation of Al, various solutions
have 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 favored
over other metal systems because of high H2 generation rate. When the reaction between Al and water is assisted by
alkali, 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
[6, 12]
2Al + 6H2O + NaOH → 2 NaAl(OH)4↓ + 3H2 ↑
(13)
NaAl(OH)4 → NaOH + Al(OH)3↓
(14)
Sodium hydroxide consumed for the H2 generation in exothermic reaction (13) will be regenerated through the
decomposition of NaAl (OH)4 via reaction (14). Reaction (14) also produces a crystalline precipitate of aluminum
hydroxide. The combination of above two reactions completes the cycle and shows that only water will be consumed in
the whole process if the process is properly monitored. Previous works reported kinetics of the reaction between Al and
H2O 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 H 2 generation behavior for
alkali assisted Al/H2O system. The parameters include temperature, alkali concentration, morphology, initial amount of
Al; and concentration of aluminate ions [4, 39-40]. Moreover, Soler et al. compared the H2 generation performance of
three different hydroxides: NaOH, KOH and Ca(OH)2 and found that NaOH solution consumes Al faster compared with
other two hydroxides [40]. Interestingly, S.S. Martinez et al. treated Al-can wastes with NaOH solution at room
temperature to generate highly pure hydrogen. The byproduct (NaAl (OH)4) was used to prepare a gel of Al (OH)3 to
treat drinking water contaminated with arsenic [41]. On the basis of the above mentioned reactions, several patents have
been 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 directly
split in H2 and O2 using this technology solution. But there is a series of chemical reactions which utilizes oxides and
thermal energy from renewable sources (such as solar energy) that can convert water into stoichiometric amounts of H 2
and O2. [50-52]The water splitting thermochemical cycle is demonstrated in Figure 3. The figure demonstrates a 3 step
water splitting process – (1) reduction of oxides (energy intensive process, 800-1000ºC) (2) reaction of reduced oxide
with 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 number
of 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 but
could 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 the
volatility of sodium hydroxide at temperature higher than 800ºC and incomplete Na+ extraction by water to recover
sodium hydroxide limits its application. Several research groups concluded that even though sodium or sodium
hydroxide assisted reaction has major advantages; their recovery could be a big challenge. Interestingly, Weimer et al
recommends membrane separation to recover sodium hydroxide [65]. Few researcher groups also pointed out the
possibility of using sodium carbonate rather than sodium hydroxide [66-68].
Table 3 Alkali metal assisted water splitting thermochemical cycle
System
Reactions
Conditions
H2 Yield
Remarks
Ref
Mn2O3= 2MnO + 0.5O2 (<1600ºC)
2MnO + 2NaOH = H2 + 2α-NaMnO2(~700ºC)
2α-NaMnO2 + H2O = Mn2O3 + 2NaOH (<100ºC)
H2O = H2 + 0.5O2
H2 generation
at 750ºC
100% conversion
under vacuum(0.5h)
and under N2 purge
(3h)
NaOH recovery improved
from 10% to 35% in
(MnO + Fe) mixture
Difficult Mn2O3- NaOH
separation
MnO
[69]
Ce2Ti2O7,
Ce2Si2O7,
CeFeO3,
CeVO4,
CeNbO4
MO(ox) = MO(red) + 0.5 O2
MO(red) + 2 M'OH = M'2O.MO(ox) + H2
M'2O.MO(ox) + H2O = MO (ox) + 2M'OH
Mixed oxide
synthesis
around
15000C
H2 generation
T range
500-800ºC
(1.5-1.94) mmol/g
oxide
At 530ºC, Ce2Si2O7
(highest reaction
efficiency)
H2 generation infeasible
up to 1000ºC
[70]
Zn-Mn-O
Zn0.66Mn2O3.66 = 2Zn0.33MnO1.33 + 0.5O2(~1600ºC)
2Zn0.33MnO1.33+ 2NaOH = H2 +
Na2Zn0.66Mn2O4.66(>650ºC)
Na2Zn0.66Mn2O4.66 + H2O = Zn0.66Mn2O3.66 + 2NaOH
(<100ºC)
H2 generation
above 650ºC
80-90% conversion
rate under low pressure
and residence time of
0.5h
NaOH
2NaOH + Na = Na2O + H2 (Teq = 32ºC)
2Na2O = Na2O2 + 2Na (Teq = 1870ºC)
Na2O2 + H2O= 2NaOH + 0.5O2 (100ºC)
Nonequilibrium
technique for
NaOH may be recovered
using membrane process
[65]
Yield of H2 generation
and Na separation
>80% at 350ºC
H2
production,
below 400ºC
<100% , kinetic
limitation, suitable
catalysts needs to be
investigated
[64]
Besides sodium hydroxide recovery, there are other limitations too. For instance, the reduction of oxides requires a
very high temperature. If such a high temperature will be provided by the solar energy, a large scale solar heat plant is
needed. 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. A
low temperature water splitting process will allow the utilization of small-scale solar heat systems or even exhaust heat
from 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. Formic
acid 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 is
actually the reversible reaction of CO 2 hydrogenation [71-79]. Electrolysis of formic acid solutions in the presence of
sodium hydroxide requires theoretically much lower energy than water [80]. Hence, use of formic acid solution to
generate hydrogen will have double benefits of tackling pollution and generating clean energy. The electrochemical
reaction for the electrolysis of formic acid solutions is as follows [81]:
Anode:
Cathode:
Overall reaction:
HCOOH + OH- →
2H2O + 2eHCOOH
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CO2 + H2O + 2e-
→ H2 + 2OH→
H2 + CO2
(15)
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Figure 4 demonstrates the scheme of electricity generation via the combined use of alkaline hydroxide (sodium
hydroxide) for the electrolysis of formic acid (HCOOH) and fuel cell. The separation of H2 and CO2 is desired prior to
injection 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 corresponding
alcohol and acid [83-84]. Further, Ashby et al proposed a mechanistic explanation of hydrogen evolution from
formaldehyde in the presence of sodium hydroxide [85].The mechanism indicates that one hydrogen atom originates
from the water and the other from the organic moiety. The experimental study exhibits that when a dilute solution of
formaldehyde (4x10-4M) reacts with concentrated sodium hydroxide (19M) at room temperature, hydrogen is produced
in a significant amount. However, concentrated solution of formaldehyde when interacts with dilute sodium hydroxide
solution produces only a trace amount of hydrogen.
When solution of hydrogen peroxide is mixed with formaldehyde in presence of sodium hydroxide, hydrogen is
generated [86]. Hydrogen peroxide oxidizes formaldehyde to formic acid and sodium hydroxide further neutralizes the
acid.
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 limited
by 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 spontaneously
with water while the complex hydride (NaBH4) reacts slowly unless catalyzed [89-92] However, for both hydrides, it is
difficult to achieve the stoichiometric amount of hdyrogen.
2.6.1 Binary Hydrides
(a) NaH/ LiH- NaOH
Reaction of LiH with water produces LiOH. LiOH is hygroscopic in the nature and therefore binds excess water [93]. A
large amount of water requires extra heat to regenerate anhydrous LiOH. To avoid such situation, LiH is mechanically
mixed with NaOH in a molar ratio of 1:1 and 2:1 to generate H2 [94].
NaOH + 2LiH → Li2O + NaH + H2
NaOH + LiH → 0.5Li2O + 0.5NaH + H2
ΔHº = -48.1 kJ/mol-H2
(17)
ΔHº = 8.1 kJ/mol-H2
(18)
It can be observed that as the molar ratio of LiH increases, the reaction between NaOH and LiH tends to be an
exothermic reaction and thus expected to occur at a lower temperature. The following mixtures with the molar ratio of
1:1 and 2:1 for LiH and NaOH, heated at 250ºC, produces 1.92 and 3.20 wt. % H2 respectively. These yields are about
50% 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 H2
by the product mixture was not reported so far, thus the reversibility of the presented scheme still needs to be
investigated.
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When LiH is replaced by NaH, only H2 and Na2O are the products. The combination of NaH and NaOH can
release/uptake ~ 3 wt. % H2 reversibly at temperature below 300ºC [95]. It should be noted that the reaction between
NaH and NaOH has a large endothermic enthalpy (ΔHº = 64.3 kJ/mol-H2) and ΔGº is negative only above 400ºC. Na2O
is 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. The
addition of NaOH to NaH can also significantly decrease the dehydrogenation temperature of NaH. However, neither
LiH-NaOH nor NaH-NaOH release/ uptake H2 in a significant amount and therefore lags behind the target set up by US
Department of Energy. Moreover, the operating temperature of absorption and desorption does not meet the goal. The
current 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-NaOH
Mg (NH2)2 -LiH system undergoes reaction as shown below to produce H2 reversibly.The desorption enthalpy change
for the first step is 38.9kJ/mol-H2 [96] which is much lower than LiNH2-LiH mixture (66kJ/mol-H2). The reduced
enthalpy 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 (<200 ºC) and enhancing the
hydrogenation/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 for
hydrogenation/dehydrogenation rate [102].They achieved ~36ºC reduction in the dehydrogenation peak temperature for
Mg (NH2)2-2LiH-0.5NaOH. During ball milling, NaOH reacts with Mg (NH2)2 and LiH to convert to NaH, LiNH2 and
MgO. The hydrogen desorption temperature may be reduced by the addition of NaOH to the system. However, the use
of NaOH has an adverse effect on the hydrogen capacity of the hydride sample. The total hydrogen capacity decreased
from 5.28 wt. % [pristine sample] to ~3.61 wt. % [Mg (NH2)2-2LiH-0.5NaOH sample] due to the dilution of NaOH.
(b) NaBH4-NaOH
A NaBH4 solution with alkaline stabilizer, sodium hydroxide, reacts with water in the presence of catalysts to produce
H2 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, concentration
of NaBH4, temperature, time and activation energy for NaBH4-NaOH system [110]. The reaction is generally performed
in the temperature range of 10-60 ºC for various times and the activation energy varies from 40-55 kJ/mol. Based on
these conditions and various catalysts behavior, different reaction kinetic models such as zero- order, first-order and
Langmuir- Hinshelwood have been suggested for NaBH4-NaOH system.
Catalyst
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 recently
reported. The result shows that H2 generates at a rate of 32.3 L min-1g-1 Ru in a 10 wt. % NaBH4 + 5 wt. % NaOH
solution [111].The Ru/G catalyst with high activity and durability demonstrates a potential for the portable fuel
generators.
3. Conclusion
The use of sodium hydroxide for hydrogen production and storage is justified by the high reaction rate, lower operating
temperature and overall reduction in CO2 emission. Depending on the reaction and its conditions, sodium can produce
or store hydrogen. Moreover, several situations illustrate how sodium hydroxide binds with CO2 and forms valuable
chemical compound, Na2CO3.
However, a major limitation for the inclusion of sodium hydroxide as one of the primary ingredients for obtaining the
hydrogen is the production route of sodium hydroxide itself. At present, high energy intensive process (electrolysis of
brine) is the prime method to produce sodium hydroxide. It will be of a great interest to invent or modify a method in
which sodium hydroxide can be produced using the renewable resources such as solar, water and wind. Any such
method would certainly mitigate the CO2 emission by a huge amount and eventually increase the role of sodium
hydroxide for hydrogen production and storage purpose.
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and processes for energy: communicating current research and technological developments (A. Méndez-Vilas, Ed.)
____________________________________________________________________________________________________
Acknowledgement: S. Kumar would like to acknowledge the financial support from Florida International University
graduate school, dissertation year fellowship (DYF).
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