Thermodynamic study on the solubility of NaBH4 and NaBO 2 in NaOH solutions

Transcription

1 Loughborough Unversty Insttutonal Repostory Thermodynamc study on the solublty of NaBH and NaBO n NaOH solutons Ths tem was submtted to Loughborough Unversty's Insttutonal Repostory by the/an author. Ctaton: SHANG, Y. and CHEN, R., 11. Thermodynamc study on the solublty of NaBH and NaBO n NaOH solutons. SAE Techncal Papers, , do:1.71/ Addtonal Informaton: Copyrght c 15 SAE Internatonal. Ths paper s posted on ths ste wth permsson from SAE Internatonal, and s for vewng only. Further use or dstrbuton of ths paper s not permtted wthout permsson from SAE. Metadata Record: Verson: Accepted for publcaton Publsher: c SAE Internatonal Rghts: Ths work s made avalable accordng to the condtons of the Creatve Commons Attrbuton-NonCommercal-NoDervatves. Internatonal (CC BY-NC-ND.) lcence. Full detals of ths lcence are avalable at: Please cte the publshed verson.

2 JSAE 1195 Thermodynamc Study on the Solublty of NaBH and NaBO n NaOH Solutons Y. Shang, R. Chen * Department of Aeronautcal and Automotve Engneerng, Loughborough Unversty, * Correspondng author, emal: r.chen@lboro.ac.uk, tel: + () , fax: + () Abstract Extensve research has been performed for on-board hydrogen generaton, such as pyrolyss of metal hydrdes (e.g. LH, MgH), hydrogen storages n adsorpton materals (e.g. carbon nanotubes and graphtes), compressed hydrogen tanks and the hydrolyss of chemcal hydrdes. Among these methods, the hydrolyss of NaBH has attracted great attenton due to the hgh stablty of ts alkalne soluton and the relatvely hgh energy densty, wth further advantages such as moderate temperature range (from 5oC to 1oC) requrement, non-flammable, no sde reactons or other volatle products, hgh purty H output. The H energy densty contaned by the system s fully depend on the solublty of the complcated soluton contans reactant, product and the soluton stablser. In ths work, an approach based on thermodynamc equlbrum was proposed to model the relatonshp between the solublty of an electrolyte and temperature, and the effect of another component on ts solublty. The relatonshp was then appled to NaBH and NaBO aqueous solutons, and the effect of ntroducton of NaOH on ther solublty after dervng ther solublty from phase dagrams. The data has been shown n good agreement wth the proposed model. Key words Sodum Borohydrde, Hydrogen Producton, Hydrogen Storage, Solublty, Hydrolyss 1

3 1. Introducton Wth the ncreasng concern about ar polluton and ol depleton, hydrogen, H, has been ntensvely studed as an alternatve energy source. The man problem wth hydrogen applcaton s that t s not readly transported n bulk. In order to use hydrogen wdely, especally for moble applcatons, a compact and safe method for storage s needed. Varous methods have been developed for H storage, such as hgh-pressure gas [1], lquefed hydrogen [, 3], adsorpton on materals wth hgh specfc surface area [], reformng of natural gas, alcohols and hydrocarbons[5, 6], catalytc reducton of water wth metals [7], and slush hydrogen [8] etc. Each of these technologes has ts nherted advantages as well as drawbacks, but the stll poor stored energy densty remans. One alternatve soluton whch has potental to store more H safely for moble applcatons s to utlze the catalytc reducton of water wth hydrdes [9-11]. There are many dfferent types of hydrdes whch has the potental to react wth water and produce hydrogen gas. To use such materals for H producton for moble applcatons, the energy densty and the system operaton safety are the major concerns. Table 1 lsted the energy release durng the hydrolyss reacton from a number of typcal hydrdes. It can be seen that most reactons between metal hydrdes and water are vgorous. The large amount of heat can be released durng the reacton whch may cause exploson. Table lsted the densty of these potental hydrdes. Apart from LH whch clearly shows the safety concern, the sodum borohydrde, NaBH, has the least weght densty. In comparson, t s clear that NaBH produces the least heat energy durng the hydrolyss reacton whle has a low weght densty. It has therefore the potental to be a successful canddate as an alternatve hydrogen storage technology for moble applcaton n partcular. Table 1 Heat released for 1gram hydrogen wth dfferent hydrdes [1] Hydrdes NaBH LH LAlH NaAlH CaH H (kjmol -1 ) Table Weght of reductants necessary for 1 gram of hydrogen Hydrdes NaBH LH LAlH NaAlH CaH Weght(g) The generaton of hydrogen from NaBH n aqueous soluton s shown n Eq.(1). It can be seen that one mole of NaBH n a water soluton reacts wth moles of the water and produces moles of H and one mole of sodum metaborate (NaBO ) as a by-product. Half of the produced H s extracted from the water. NaBH + + (1) HO H NaBO As a by-product, NaBO has to be removed durng the hydrolyss reacton to avod cloggng the catalyst whch wll sgnfcantly reduce the system reacton effcency [13, 1]. A practcal way to

4 remove the NaBO from the catalytc reacton bed s to dssolve t nto the water left from the hydrolyss reacton and brng the soluton nto a exhaust system. Clearly, the water contaned n the NaBH hydrolyss reacton system has to not only cover the hydrolyss reacton but also to dssolve and remove the by-product. Too much water wll reduce the hydrogen generaton densty of the system [15, 16], whle nsuffcent water may results n catalyst cloggng and reduce the system reacton effcency. Ths necesstates the optmsaton of the NaBH concentraton. In order to dentfy the optmsed concentraton, a sem-emprcal smulaton method based on dssoluton equlbrum prncples has been developed and reported n ths paper. Theoretcal Solublty Model When a sold solute s left n contact wth a solvent, t dssolves untl the soluton s saturated,.e. an equlbrum between undssolved and dssolved solutes s reached. Ths dssoluton equlbrum can be expressed n a general term: K + - AB nho A + B + nho (a) where n s number of water crystallzed wth the solute AB, and K s the equlbrum constant. Due to the nteracton among the dssolved substances and the solvent, the performances of the dssolved substances n a real soluton dffer from that n the deal-dlute one. Such dfferences are represented by the actvtes of the substances n the soluton. Hence, the equlbrum constant of the dssoluton can be expressed as a + a A K = a B a n HO AB nh O (b) where a s the actvty, and the subscrpt denotes the substances. Actvty coeffcent of a substance s defned as the rato between actvty and deal-dlute concentraton, a γ = (3) m / m where m s the molarty of substance n the soluton, whch s the moles of substance contaned by 1g of solvent n the soluton, and m s the molarty of the substance at standard condtons. The actvty of a sold materal s unty. Snce the water s the bulk phase n the soluton, ts actvty s assumed to be a constant. Substtute Eq.(3) nto (b), the equlbrum constant can then be obtaned as K = n ah + + = + B O γ m m K m m A B A B γ A γ () 3

5 If the molarty of substance A = m B AB + A equal to that of B and the solute AB n the soluton, m + = m then the equlbrum constant Eq.() can be further smplfed as γ m AB K = K (5) The equlbrum constant changes wth temperature. Ths can be expressed usng the van t Hoff equaton [17] d ln K dt o ΛH = or RT d ln K ΛH = d(1/ T ) R o (6) where T s temperature, H s the change of enthalpy of the dssoluton process whch equals to the molar heat of soluton and R s the unversal gas constant. Integraton of the van t Hoff equaton and substtute the equlbrum constant wth Eq.(6), gves o H AB ln m AB = + C (7) RT where C s an ntegraton constant, whch ncludes all the actvty coeffcents. Rearrange Eq.(7) then gves the molalty of the solute AB n the saturated soluton, m AB H AB / RT = Ae (8) For sodum borohydrde, NaBH, two potental crystallne states, NaBH H O and NaBH, may exst as the undssolved sold n ts saturated soluton dependng upon the temperature of the soluton as shown n Eq.(9a,b) [18]. When the temperature s lower than 39.K, the undssolved part s n the form of NaBH H O. Above ths temperature, the undssolved part s n the form of pure NaBH. K1 + NaBH HO Na + BH + HO (T < 39.K) NaBH - K + Na + BH (T > 39.K) - (9a) (9b) For sodum metaborate, NaBO, there are three crystallne states n the saturated sodum metaborate soluton NaBO H O, NaBO H O, and NaBO 1/H O, as shown n Eq.(1a,b,c). Agan, the solublty of each states depends on the temperature of the soluton [19]. + NaBO HO Na + B OH - ( ) + H O (73 < T 36.6K) < NaBO H O Na - ( ) (36.6 < T 378K) + B OH + < NaBO 1/ H O + 3/H O Na - ( ) (T 378K) + B OH + > (1a) (1b) (1c) Usng Eq.(7) and (8), the solublty of both NaBH and NaBO n terms of molalty can be obtaned as

6 o H NaBH ln m NaBH = + C RT (11) o H NaBO ln m NaBO = + C RT (1) whch gves m NaBH H / RT NaBH = Ae (13) m NaBO H NaBO / RT = A e (1) where, o H NaBH and o H NaBO s the standard enthalpy change of sodum metaborate soluton, whch equals to the molar heat of soluton. C s an ntegral constant, whch s related to the overall actvty coeffcent, and R s the unversal gas constant. From equaton (13) and (1), t can be seen that the solublty s related to the heat of soluton and the temperature at whch the dssolvng process takes place. When the dssolvng process s endothermc,.e. process s exothermc,.e. H >, a hgher temperature results n a larger solublty. When the dssolvng H <, a hgher temperature gves smaller solublty. 3 Sem-emprcal Solublty Model Equaton (11) and (1) shows that there s potentally a lnear relatonshp between ln m and 1/T. If such relatonshps can be dentfed, we may then be able to use these equatons to analyse the solublty of both reactant and by-product of the NaBH hydrolyss system and to develop a model to smulate and optmse the soluton for the NaBH hydrolyss system. Fgure 1 shows the measured solublty data of NaBH at varyng temperature [16, 18]. It can be seen that the solublty of sodum borohydrde ncreases as the temperature ncreases. Below 36. C (39.K), the crystallne state n the dssoluton equlbrum s NaBH H O, and above ths temperature the crystallne state n the dssolvng equlbrum s NaBH. At 36. C (39.K), two knds of crystallne, NaBH H O and NaBH, coexst n the saturated soluton, whch s regarded as the nvarant pont. 5

7 Fg.1 Measured solublty of NaBH [16, 18] Fgure shows the measured solublty of NaBO at varyng temperature [19]. There are two nvarable ponts at 53.6 C and 15 C, respectvely. These correspond to the transton temperatures of the sodum metaborate between ts three crystallne states, NaBO H O, NaBO H O, and NaBO 1/H O. Overall, ts solublty ncreases as the temperature ncreases up to the level of 15 C. Ths ndcates that the enthalpy change of the dssoluton s postve when the crystallne state s NaBO H O or NaBO H O. If the soluton temperature further ncreases, the solublty of sodum metaborate starts to declne. Fg. Measured solublty of NaBO [19] The solublty data cted n Fgure 1 and are n percentage by mass ( S parameters n the models, ths needs to be converted nto molalty defned as wt% ). In order to obtan the 1Swt% m = (15) (1 S ) M wt% 6

8 where M s the molecular weght of NaBH (equals to g/mol) or NaBO (equals to 65.8 g/mol). Fgure 3 and shows the rearranged solublty data cted n Fgure 1 and by convertng the mass solublty nto molalty n the form of ln m vs. 1/T for NaBH. It can be seen that a reasonable lnearty exsts at each temperature range. There are some dfferences between the measured data and the lnear ft. Ths s probably manly due to the fact that the water actvty n the soluton has been assumed to be constant at varous NaBH concentraton and temperature. Further work n the area s undertakng. Fg.3 Temperature effect on NaBH solublty Fg. Temperature effect on NaBO solublty Comparng the lnearty wth the dssoluton equlbrum theory, Eq.(11) and (1), both H and constant C can be obtaned. These are lsted n Table 3. The postve value of the heat of soluton suggests that the dssolvng process s endothermc. Increasng temperature s favourable for the 7

9 dssoluton. On the other hand, the negatve heat value ndcates that the dssoluton process s exothermc and ncrease n temperature wll decrease the solublty of the solute. Table 3 Sem-emprcal parameters H and C Parameters Speces H (kj/mol) Pre-exponental factor (Mol/kg water) NaBH NaBH.H O (<39.K) NaBH ( 39.K) NaBO NaBO.H O (<36.6K) NaBO.H O ( K) NaBO.1/H O ( 378K) Substtute the heat and the constant nto Eq. (15) and (16), the solublty of NaBH and NaBO n the form of percentage by mass at varous temperature can then be obtaned as S F / T 1Ee = (16) F / T 1 + Ee where E and F are sem-emprcal parameters lsted n Table. Table Sem-emprcal parameters E and F Speces Parameters E F NaBH NaBH.H O (<39.K) NaBH ( 39.K) NaBO NaBO.H O (<36.6K) NaBO.H O ( K) NaBO.1/H O ( 378K) Fgure 5 and 6 show the comparson between calculated solublty usng Eq.(18) and the measured value. It can be seen that a good agreements are obtaned at all temperature ranges. 8

10 Fg.5 Comparson of calculated and measured solublty of NaBH Fg.6 Comparson of calculated and measured solublty of NaBO. System Optmsaton In order to obtan the maxmum possble hydrogen producton densty, the water contaned n the NaBH hydrolyss system needs to be optmsed. There are three parts of water nvolve n the hydrolyss reactons: water used to produce a saturated NaBH soluton, w 1, water consumed by the hydrolyss reacton, w, and the water needed to dssolve and remove the by-product, w 3. Table 5 lsted the mnmum amount of water requred by each part of the requrement of the hydrolyss system at varyng temperature ranges as shown. 9

11 Temperature range (K) Table 5 Water needed for NaBH hydrolyss system Water n Water Water requred saturated NaBH requred for for dssolvng soluton hydrolyss, NaBO, w 1 (g) w (g) w 3 (g) Water requred by the system (w +w 3 ) (g) > It can be seen that the amount of water needed to react wth NaBH and to solve the NaBO s sgnfcantly larger than the amount of water contaned n the saturated NaBH soluton. In other words, t s the water requred to hydrolyss the NaBH and dssolve the by-product NaBO decdes the optmsed water content n the hydrolyss system. The optmsed concentraton of the system can thus be calculated by C NaBH ( wt %) = 1wNaBH w + w + w (17) 3 NaBH where wnabh s the weght of NaBH. Fgure 7 shows both calculated maxmum optmsed NaBH concentraton n the hydrolyss system and the concentraton of saturated NaBH soluton at varous temperatures. Two nterestng phenomenon need to be addressed. Frst, the optmsed concentraton of NaBH for the hydrolyss system s about half the level of saturated soluton of NaBH. By smply lookng at the concentraton of the NaBH to desgn the hydrolyss reacton system s clearly nsuffcent. Second, the optmsed concentraton ncreases as the soluton temperature ncreases. Ths clearly ncreases the hydrogen producton densty. However, such beneft only exsts when the soluton temperature s lower than 378K. Further ncrease n temperature would decrease the optmsed concentraton, so reduces the hydrogen producton densty of the system. Ths s due to the fact that the dssoluton of NaBO at temperatures above 378K becomes exothermc and hgh temperature wll reduce ts solublty. 1

12 Fg.7 Calculated NaBH solublty and ts maxmum concentraton n the hydrolyss system 5. Conclusons Based on the van Hoff s equaton, a thermodynamc dssoluton equlbrum model has been developed. Usng exstng measured solublty data, a group of sem-emprcal parameters requred by the thermodynamc dssoluton equlbrum model for NaBH and NaBO were obtaned. Usng these sem-emprcal parameters, the solublty of both NaBH and NaBO was predcted by the thermodynamc dssoluton equlbrum model agrees well wth the measured data. The calculated results showed that the optmum concentraton of the NaBH soluton used for the hydrolyss reacton s about half the level of ts saturated soluton. It ncreases as the soluton temperature ncreases but only up to 378K. further ncrease n temperature wll results n decrease n optmsed concentraton. References: [1] Loubeyre P, et. al. Couplng statc and dynamc compressons: Frst measurements n dense hydrogen. Hgh Pressure Research, ; (1): 5. [] Zuettel A. Hydrogen storage methods. Naturwssenschaften, ; 91(): 157. [3] Amann CA. The passenger car and the greenhouse effect. Internatonal Journal of Vehcle Desgn, 199; 13(): 35. [] Zhou L and Zhou Y. Determnaton of compressblty factor and fugacty coeffcent of hydrogen n studes of adsorptve storage. Internatonal Journal of Hydrogen Energy, 1; 6(6): 597. [5] Dudfeld C, Chen R, and Adcock P. Evaluaton and modellng of a CO selectve oxdaton reactor for sold polymer fuel cell automotve applcatons. Journal of Power Sources, ; 85():

13 [6] Dudfeld C, Chen R, and Adcock P. Compact CO selectve oxdaton reactor for sold polymer fuel cell powered vehcle applcaton. Journal of Power Sources, ; 86(1-): 1. [7] John W, Hydrogen-powered automoble wth n stu hydrogen generaton. H Power Corporaton (Bellevlle, NJ): USP 5699, [8] DeWtt RL, Hardy TL, Whalen MV, and Rchter GP. Slush hydrogen (SLH) technology development for applcaton to the Natonal Aerospace Plane (NASP). Advances n Cryogenc Engneerng, 199; 35: 171. [9] Geres R, Calcum Hydrde Gas Generator. Amerca as represented by the Secretary of the Navy, Washngtom, D.C.: US , 197. [1] Stedman, JK and Troccola, JC. Method of and arrangement for replenshng hydrogen consumed by a fuel cell devce. Internatonal Fuel Cells Corporaton (South Wndsor, CT): US 5195, [11] Schulz R, Huot J, Lang G, and Boly S, Method for producng gaseous hydrogen by chemcal reacton of metals or metal hydrdes subjected to ntense mechancal deformatons. Hydro- Quebec (Montreal, CA): US , 3. [1] Dean JA, Lange's Handbook of Chemstry. Ffth Edton ed [13] Davs RE, Bromels E, and Kbby CL. Boron Hydrdes. III. Hydrolyss of Sodum Borohydrde n Aqueous Soluton. Journal of Amercan Chemcal socety, 196; 8: 885. [1] Davs RE and Swan CG. General Acd Catalyss of the Hydrolyss of Sodum Borohydrde. Journal of Amercan Chemcal socety, 196; 8: 599. [15] Amendola SC, et. al. A safe, portable, hydrogen gas generator usng aqueous borohydrde soluton and Ru catalyst. Internatonal Journal of Hydrogen Energy, ; 5: 969. [16] Kojma Y, et. al. Development of 1 kw-scale hydrogen generator usng chemcal hydrde. Journal of Power Sources, ; 15(1):. [17] Atkns PW, Physcal Chemstry. ed. Oxford: Oxford Unversty Press. 199 p.169. [18] Mellor JW, Mellor's comprehensve treatse on norganc and theoretcal chemstry. Vol.5. Part BI Boron-hydrogen compounds. London [19] Mellor JW, Mellor's comprehensve treatse on norganc and theoretcal chemstry. Vol.5. Part A Boron-oxygen compounds. London