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KİMYA PROBLEMLƏRİ № 4 2015
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UDC 541.136
EFFECT OF DIFFERENT ADDITIVES ON THE ELECTROCHEMICAL BEHAVIOUR OF SODIUM BOROHYDRIDE ON Au ELECTRODE
Sadig Kuliyev
Vestel Defence Industry,
Universiteler Mah. Ihsan Dogramaci Bul.
Titanyum Blok 17/B Teknokent ODTU,
06800, Ankara, TurkeyVestel Defence Industry R&D Department,
Ankara, Turkey
Email:sadig. kuliyev@gmail. com
ABSTRACT
The paper presents results of the influence of different additives such as NaBO2, Na2B4O7 and ethylendiamine (EDA) to the oxidation of NaBH4. On the basis of this research, a method for analyzing the concentration of mechano-chemically synthesized NaBH4 from NaBO2 and Na2B4O7 compounds has been proposed. It has been shown that linear potentiodynamic (LPD) and cyclic voltamperometric (CV) methods are utilizable as easy, sensitive and accurate analyzing methods. It has been observed that EDA, NaBO2 and Na2B4O7 compounds do not contribute any obstacle for determining the NaBH4 quantity.
Keywords: Sodium borohydride, synthesis, determination, additives, cyclic voltammetry.
INTRODUCTION
Depletion of fossil fuels, especially petroleum and their trigger and effect of global warming make hydrogen an important clean energy source of today and the future. For this reason its production, storage and utilization (especially for fuel cell applications) are some of the most important problems to overcome. The storage of hydrogen is obtained by various methods such as by using pressurized tanks, by liquidification at low temperatures, by metal hydrides or with various chemicals. From these methods a safe and efficient method is storing hydrogen in chemicals such as sodium borohydride [1].
NaBH4 includes approximately 10.57% hydrogen of its weight. To release hydrogen, NaBH4 must be hydrolyzed with water. The following reaction takes place:
NaBH4 + 2H2O catalyst > 4H2 + NaBO2
As seen from the reaction, after the hydrolysis process, hydrogen from water is also captured, in other words NaBH4 releases 21% of hydrogen by its weight. Also with the catalysts used for this process such as Ruthenium (Ru) or supported Cobalt (Co), it is possible to create mechanisms that can produce hydrogen on demand [1,2]. Consequently, with the hydrolysis of 1 g of NaBH4, it is possible to produce approximately 2.4 liters of pure hydrogen. This is an important property which may be easily applied to vehicular [3] and many other applications related to fuel cells [4,5]. Other than its storage feature, NaBH4 is used in various industrial applications such as paper, textile, ceramic, porcelain, drugs, perfume, rocket fuels and etc. Due to these mentioned properties above NaBH4 is the most important of all commercially produced and utilized borohydrides. Recent studies on NaBH4 is concentrated on its production methods.
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1.1. Traditional Methods for NaBH4 Production
Due to the expensiveness of present production methods, researches are mainly concentrated around this area. One of the
production processes is done with the participation of NaH with Na2B4O7 at elevated hydrogen pressure and temperature [6].
Na2B4O7+ 16Na + 8H2 + 7SiO2 ^ 4NaBH4+ 7Na2SiO3
Another production method has multiple steps and includes the following reaction [7]:
4NaH+ B(OCH3)3 ^ NaBH 4 + 3NaOCH
Using NaBH4 as reducing agent makes both of these processes expensive. Investigations are done to keep the synthesis costs minimum. This can be done by mechano-chemical methods with cheap reducing metals
at elevated hydrogen pressures and temperatures [8,9]. This process includes NaBO2 and Na2B4O7 participation. Process is as follows:
NaBO2 + 2Mg + 2H2 ^ NaBH4 +2MgO
Process is carried out with continuous stirring of NaBO2 and powdered metal such as magnesium. Transformation of NaBH4 is effected by temperature, hydrogen pressure, dimensions of metal powder and the dimensions of used metals for stirring process.
Recent advances are synthesis at lower temperatures with decreased sodium or alkali metal requirements [10]. Today various companies, universities and research laboratories are working to put forward cheaper methods.
1.2. Traditional Methods to Determine NaBH4 Concentrations
As all known, syntheses of NaBH4 with mechano-chemical means are generally produced with the addition of NaBO2 and Na2B4O7, with metals or metal hydrides at high hydrogen pressures and temperatures. After this process, NaBH4 is taken from the reaction products with the help of various organics or NH3. So, determination of NaBH4 at the presence of these additives is extremely important.
Traditional methods to determine the efficiency of synthesized NaBH4 are as follows:
- Titration: Analysis of NaBH4 with titration gives correct results only at a specific pH interval (4-7) which is not the suitable pH alkali ambient that sodium borohydride is resistant to. A method introduced by Lyttle et al. [11] is based on the reaction:
3BH4- + 4IO3- ^ 4I- + 3H2BO3- + 3H2O
This involves the addition of sample sodium borohydride to the solution which requires a large amount of iodate.
- Process introduced by Brown et al. [12] is based on the reaction of Ag+ ions with BH4-ions in 4% EDA and 40% NaOH solution forming Ag-EDA.
- The electrochemical oxidation of NaBH4 by linear and cyclic voltammetric methods was introduced by Mirkin et al. [13] and was observed it is possible to determine NaBH4 concentrations with the help of the oxidation of it on Au electrode.
These techniques have various disadvantages such as:
-Analysis can only be done with model concentrations of pure NaBH4 and NaBO2. -The effect of side reactants such as NaBO2, Na2B4O7, metal etc. could not be understood.
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-The effect of solvent which decomposes understood.
NaBH4 from reactant mixture could not be
1.3. Electrochemical Method to Determine NaBH4 Concentration
As all known, ethylendiamine is a suitable material for the extraction of NaBH4 from reactant products since it can dissolve NaBH4 effectively whereas has little or no role to dissolve NaBO2, Na2B4O7, catalyst and metal used in the synthesis process. For this reason, to obtain the transformation degree of produced NaBH4, it is necessary to obtain an accurate, fast and correct method to determine its concentration at the presence of EDA.
Literature researches have shown that electrochemical methods [14,15] can be utilizable as a fast and correct method for this process. But no fast and accurate analysis method at the presence of EDA has been put forward.
In this research, various electro-analytic methods have been used to analyze the mechano-chemically synthesized solid NaBH4 concentration taken out from reaction products designated in EDA.
2. EXPERIMENTAL
The electrochemical tests were accomplished with IVIUM Stat mark potentiostat. All tests were made with commonly used electrochemical cell containing three electrodes. The temperatures of cells were controlled by thermostat and it was 26 oC. The operating electrodes with smooth active surface radius approximately 1 mm were prior mechanically cleaned (polishing using special polishing paste) and washed with alcohol and distilled water. Prior to the electrochemical experiments, the electrodes were electro activated in a specific potential region. All solutions were prepared in bidistilled water. NaBH4 solutions which have standard concentrations were prepared using Merck mark chemicals. Used NaBO2
and dry Na2B4O7 were suitably prepared from the thermic treatment of NaBO2.4H2O and Na2B4O7.10H2O. Cyclic voltammetery and linear sweep techniques were used with gold (Au) and platinum (Pt) indicator electrodes for this purpose. It was noted that the most effective measurements were done on Au electrode [16] since catalytic decomposition of NaBH4 is possible at Pt electrode no such decomposition occurs on Au surface. Solutions of NaOH in bidistilled water were used as background electrolyte. Reference electrode was Ag/AgCl, KCl (saturated). To test the effect of additives, NaBO2 and Na2B4O7 quantities were kept close to NaBH4 amount whereas EDA quantity was kept dominant in the test samples.
3. RESULTS AND DISCUSSIONS
The periodic volt-ampere cycles of gold electrode used in experiments is shown in figure 1. At the potential range -1.0 / +0.8 V, oxide layer is formed at the surface in anodic half cycle and the desorption of oxygen from
the electrode surface occurs at +0.4 / -0.7 V potential interval in cathodic half cycles. Generally, all the cycles taken after cycle number 1 fall closely together.
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Figure 1. Periodic voltamperometric cycles on Au electrode in 1M NaOH solution (Potential
region= -1.0/ +1.0 V, Scan rate: 0.050 V/s).
Mainly, the following reaction of NaBH4 oxidation takes place at the Au electrode in alkali medium:
BH4" + 8 OH- ^ BO2" + 6 H2O + 8 e-
Influence of scan rate to the maximum of oxidation of NaBH4 is presented in fig.2
Figure 2. Linear potentiodynamic sweeps of various scan rate values at 2 M NaOH + 10-3 M NaBH solutions
As is shown in figure 2, with the increase of scan rate, the height of peak maximum (ip) is increased and shifting of the
peak potential to more positive values is observed. The effect of EDA to the NaBH4 concentration is shown in figure 3.
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Figure 3. Periodic voltamperometric cycles of (1)10 -3 M NaBH4, (2) 10-3M NaBH4 + 0,29 M
EDA in 2 M NaOH. (Scan Rate: 0.05 V/s)
Table 1. Numerical values of figure 3.
Cycle No Vpeak (V) Current (A) Charge (C)
1 0.13 3.19 x10-5 2.0 x 10-4
2 0.21 3.02 x 10-5 1.85 x 10-4
From table 1, we can see that measured current and charge values slightly change after addition of EDA. The line of log(I) versus log(c) of various NaBH4 concentrations is shown at figure 4. The line plotted with
addition of ethylendiamine practically does not change. As seen from figure, with the increase of NaBH4, linearly increase is observed at log scale.
Figure 4. The line shows current values of NaBH4 oxidations to the corresponding NaBH4 concentrations. (Log I /Logc) in 1 M NaOH solution ( Scan Rate: 0.05 V/s).
The linear potentiodynamic sweeps of sodium borohydride, NaBO2, and EDA bearing 1 M NaOH solution is shown in figure 5.
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Figure 5. Potentiodynamic sweeps of Au electrode in (1) 10"3 M NaBH4, (2) 10"3 M NaBH4 + 10-3 M NaBO2 and (3) 10-3 M NaBH4 + 10-3 M NaBÜ2+ 0.29 M EDA in 1M NaOH (Scan Rate:
0.050 V/s)
Table 2. Numerical values of figure 5.
Cycle No Vpeak (V) Current (A) Charge (C)
1 0.076 2.72x10-5 1.63x10"4
2 0.196 2.40x10"5 1.50x10"4
3 0.288 1.27x10"5 0.278x10-4
From the values of table 2, we can see that NaBO2 addition to the NaBH4 sample has a slightly decreasing effect on current and charge values.
Also with the addition of Na2B4O7 we can see that the oxidation potential of NaBH4 has a slight increase (fig 6). Their values are given in table 3.
Figure 6. Linear potentiodynamic sweeps of (1) 10"3 M NaBH4, (2) + 10"3 M Na2B4O7, (3) + 0.29 M EDA in 2 M NaOH. (Scan Rate v:0.05 V/s
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Table 3. Numerical values of figure 6.
Cycle No Vpeak (V) Current (A) Charge (C)
1 0.036 3.76 x10-5 2.2 x10-4
2 0.092 3.93 x10-5 2.87 x10-4
3 0.112 3.45 x 10-5 2.14 x10-4
4. CONCLUSIONS
Proceeding from our measurements, we can conclude the followings:
Au electrode gives repeatable results on the oxidation of NaBH4;
Presence of NaBO2 and Na2B4O7 compounds have small effect on the electro-oxidation of sodium borohydride on Au electrode but this does not create an obstacle to determine NaBH4 concentrations even at very low values such as 10-4 M.;
The difference in the molarity of the background electrolyte particularly has a small effect on the Vpeak values but does not contribute any obstacle to observe the effect of additives and consequently the concentration of NaBH4;
This method is effective when ethlendiamine is used as solvent to remove NaBH4 from reaction products.
REFERENCES
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2. Ye W., Zhang H., Xu D., Ma L., Yi B.: “Hydrogen generation utilizing alkaline sodium borohydride solution and supported cobalt catalyst”, J. Power Sources, 164 (2007) 544-548.
3. Wu Y.: “Hydrogen Storage via Sodium Borohydride” , Presented at Stanford University, 2003.
4. Mohring R.: “Hydrogen Battery
Technologies for Portable Applications”, Small Fuel Cells for Portable Applications Conference 2006, Washington D.C, 2007.
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(2005) 46-49.
6. Broja G., Shlabacker W.: “Process for the production of Alkali Metal Borohydrides” DE Patent No: 1108670, 1959.
7. Fedor W., Banus M., Ingalls D.: “Potassium Borohydride Manifacture”,
Ind. & Eng. Chemistry, vol.49 No10,1957, pp.1666-1672.
8. Kojima Y., Haga T.: “Hydrogen Storage and Generation via Sodium Borohydride”, Int. J. Hydrogen Energy, 28 (2003) 989993
9. Li Z., Liu B., Aria K., Morigazaki N,
Suda S: “Preparation of sodium
borohydride by the reaction of MgH2 with dehydrated borax through ball milling at room temperature”, Journal of Alloys and Compounds, Vol. 349 (2003) 232-236.
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11. Lyttle D., Jensen E., Struck W.: “A Simple Volumetric Assay for Sodium Borohydride”, Anal. Chemistry,Vol. 24 1952, pp.1843-1844.
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14. Gyenge E.: “Electrooxidation of
borohydride on platinum and gold electrodes: Implications for direct
borohydride fuel cells”, Electrochimica Acta, Vol.49, 2004, 965-968.
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Borohydride Using a Gold Electrode” Turk J Chemistry, 29 (2005), 519-524.
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ВЛИЯНИЕ РАЗЛИЧНЫХ ДОБАВОК НА ЭЛЕКТРОХИМИЧЕСКОЕ ПОВЕДЕНИЕ БОРОГИДРИДА НАТРИЯ НА ЗОЛОТОМ ЭЛЕКТРОДЕ
С.Кулиев
Департамент оборонной промышленности гр. Vestel 06800, Анкара, унив.кв., бульвар Ихсан Дограмачи; e-mail:[email protected]
В настоящей статье представлены результаты исследования влияния различных добавок, таких как NaBO2, Na2B4O7 и этилендиамин (EDA) на окисление NaBH4. По результатам проведенного исследования предлагается метод определения концентрации механохимически синтезированного NaBH4. Показано, что линейный
потенциодинамический (LPD) и циклический вольт-амперометрический (CV) методы можно применять в качестве простых, чувствительных и точных методов анализа. Отмечается, что соединения EDA, NaBO2 и Na2B4O7 не мешают количественному определению NaBH4.
Ключевые слова: борогидрид натрия, синтез, определение, добавки, циклическая вольтамперометрия.
NATRİUMBOROHİDRİDİN QIZIL ELEKTROD ÜZƏRİNDƏ ELEKTROKİMYƏVİ ÇEVRİLMƏSİNƏ MÜXTƏLİF ƏLAVƏLƏRİN TƏSİRİ
S. Quliyev
Vestel Müdafiyə Sənayesi
Türkiyə 06800, Ankara, e-mail:[email protected]
Natrium borohidridin qızıl elektrod üzərində elektrokimyəvi oksidləşməsinə NaBO2, Na2B4O7 və etilendiamin kimi əlavələrin təsiri öyrənilmişdir. Tədqiqatın nəticələrinə əsaslanaraq göstərilib ki, NaBH4 miqdarını xətti potensiodinamik və tsiklik voltampeometrik metodları ilə təyin etmək olar və bu zaman NaBO2, Na2B4O7 və etilendiamin natrium borohidridin təyininə mane olmur.
Açar sözlər: natrium borohidrid, sintez, əlavələr, tsiklik voltampeometrik metodu.
Redaksiyaya daxil olub 23.05.2015.
KİMYA PROBLEMLƏRİ № 4 2015
