Release of Hydrogen from Nanoconfined Hydrides by Application of Microwaves

1 Release o Hyd ogen om Nanocon ined Hyd ides by Applica ion o Mic owa es Luis Miguel Sanz-Mo al1, Alexande Na a e e1, Guido S u m2, Guido Link3, Mi iam Rueda1,Geo gios S e anidis2,4 and Ángel Ma ín1* 1 High P essu e P ocesses G oup, Depa men o Chemical Enginee ing and En i onmen al Technology, Uni e si y o Valladolid, Doc o Me gelina s/n,47011 Valladolid, Spain 2 P ocess & Ene gy Depa men , Mechanical, Ma i ime & Ma e ials Enginee ing Facul y, Del Uni e si y o Technology, Leeghwa e s aa 39, 2628 CB Del , The Ne he lands 3 Ins i u e o Pulsed Powe and Mic owa e Technology. Ka ls uhe Ins i u e o Technology (KIT). He mann- on-Helmhol z-Pla z-1. 76344 Eggens ein-Leopoldsha en, Ge many 4P ocess Enginee ing o Sus ainable Sys ems Sec ion, KU Leu en Celes ijnenlaan 200 – box 2424, 3001 Leu en, The Ne he lands Phone: +34 983184077, e-mail:[email p o ec ed] (Á. Ma ín) 2 Release o Hyd ogen om Nanocon ined Hyd ides by Applica ion o Mic owa es Luis Miguel Sanz-Mo al1, Alexande Na a e e1, Guido S u m2, Guido Link3, Geo gios S e anidis2,4, Mi iam Rueda1, and Ángel Ma ín1* 1 High P essu e P ocesses G oup, Depa men o Chemical Enginee ing and En i onmen al Technology, Uni e si y o Valladolid, Doc o Me gelina s/n,47011 Valladolid, Spain 2 P ocess & Ene gy Depa men , Mechanical, Ma i ime & Ma e ials Enginee ing Facul y, Del Uni e si y o Technology, Leeghwa e s aa 39, 2628 CB Del , The Ne he lands 3 Ins i u e o Pulsed Powe and Mic owa e Technology. Ka ls uhe Ins i u e o Technology (KIT). He mann- on-Helmhol z-Pla z-1. 76344 Eggens ein-Leopoldsha en, Ge many 4P ocess Enginee ing o Sus ainable Sys ems Sec ion, KU Leu en Celes ijnenlaan 200 – box 2424, 3001 Leu en, The Ne he lands Phone: +34 983184077, e-mail: [email p o ec ed] (Á. Ma ín) Abs ac The elease o hyd ogen om solid hyd ides by he molysis can be imp o ed by nanocon inemen o he hyd ide in a sui able mic o/mesopo ous suppo , bu he slow hea ans e by conduc ion h ough he suppo can be a limi a ion. In his wo k, a C/SiO2 mesopo ous ma e ial has been syn hesized and employed as ma ix o nanocon inemen o hyd ides. The ma ix showed high su ace a ea and po e olume (386 m²/g and 1.41 cm³/g), which enabled he con inemen o high concen a ions o hyd ide. Fu he mo e, by modi ica ion o he p opo ion be ween C and SiO2, he dielec ic p ope ies o he complex could be modi ied, making i suscep ible o mic owa e hea ing. As wi h his hea ing me hod he en i e sample is hea ed simul aneously, he hea ans e esis ances associa ed o conduc ion we e elimina ed. To demons a e his possibili y, e hane 1,2- diaminobo ane (EDAB) was embedded on he C/SiO2 ma ix a concen a ions anging om 11 o 31%w using a we imp egna ion me hod, and a de ice app op ia e o hyd ogen elease om his ma e ial by applica ion o mic owa es was designed wi h he aid o a nume ical simula ion. 3 Hyd ogen libe a ion es s by con en ional hea ing and mic owa es we e compa ed, showing ha by mic owa e hea ing hyd ogen elease can be ini ia ed and s opped in sho e imes. Keywo ds: hyd ogen s o age, hyd ide, mic owa e hea ing, ae ogel, supe c i ical CO2. 1. In oduc ion The inc easing sha e o sola and wind ene gy in he ene ge ic mix and he COP21 ag eemen pa e he way o a new amily o echnologies based on enewable ene gy [1]. Since hese enewable ene gy o ms a e luc ua ing by i s na u e, i is necessa y o de elop echnologies o s o age o ene gy in pe iods o excess supply, o use i du ing he pe iods o excess demand. Di ec use o hese ene gy o ms on onboa d applica ions is also no possible, and equi es an in e media e o m o ene gy s o age. S o age o his ene gy in o m o hyd ogen is one o he mos explo ed me hods. The challenge is being add essed using s o age sys ems based on comp essed, lique ied and ma e ials-bounded hyd ogen [2]. Compa ed o gas and liquid s o age anks, solid hyd ides can s o e high amoun s o hyd ogen in small olumes, which usually a e libe a ed by a e e sible chemical eac ion by he molysis. Howe e , cu en ly in es iga ed hyd ides a e limi ed by slow hyd ogen elease kine ics o by high he modynamic s abili ies which make i necessa y o apply high empe a u es o decompose hem [3]. The e o e, one o he echnological challenges which es ain he de elopmen o hyd ides o H2 s o age is he hea ans e inside he s o age anks [4]. G ea e o s ha e been made in o de o o e come his p oblem. Howe e , his issue can be ci cum en ed i hea ing ene gy is deli e ed di ec ly o he ma e ial, and no by hea ans e h ough he s o age ank. Applica ion o mic owa es o e s his possibili y. The decomposi ion o se e al me al hyd ides unde mic owa e i adia ion was s udied by Nakamo i e al. [5]. They concluded ha conduc i e loss and pa icle size we e he wo mos impo an pa ame e s con olling H2 elease. Sil a Dupim e al. [6] showed ha by pa icle size educ ion an 4 e ec i e hea ing o he pa icles was achie ed by app oxima ing he size o he pa icles o he pene a ion dep h o he elec omagne ic ield in he me allic powde . They ob ained his size educ ion by applying cold olling. Ano he al e na i e is modi ying he dielec ic p ope ies o he complex by mixing i wi h a hyd ide which ac ua es as a mic owa e abso be , as o example LiBH4 [7]. Zhang e al. p oposed he use o a honeycomb ce amic monoli h coa ed wi h 0.54 w % Ni (co esponding o a hin laye o Ni o 0.2 mic on) o hold he hyd ides, which allowed he apid hea ing o he complex [8]. An al e na i e o imp o e he kine ic decomposi ion o hyd ides is using a ca alys [9], bu his me hod adds weigh o he complex and some imes equi es he use o expensi e me als. Ano he al e na i e is he nanocon inemen o he hyd ide [10, 11]. By con ining he hyd ide inside he po ous hos , hyd ide pa icle size is es ained o he po e size, and pa icle agglome a ion and g ow h p ocess, which could ha e an ad e se e ec on he kine ic decomposi ion, a e a oided. Mo eo e , some po ous hos s, like ca bon o silica mesopo ous ma e ials, chemically in e ac wi h di e en hyd ides and des abilize i , u he imp o ing he decomposi ion kine ics [12]. By combina ion o hese e ec s, he decomposi ion empe a u e and elease kine ics can be signi ican ly imp o ed by nanocon inemen . On he nega i e side, he suppo ma e ials usually employed, such as mesopo ous silica, ha e e y low he mal conduc i i ies ha slow down he kine ics o hyd ogen elease i con en ional hea ing by conduc ion is applied, and he use o a suppo ma e ial also adds weigh o he compound. Howe e , he mesopo ous hos can be unc ionalized o p o ide addi ional p ope ies o he ma e ial [13]. In pa icula , i can be unc ionalized o enhance he abso p ion o mic owa e ene gy, and hus combine he main ad an ages o he wo app oaches: nanocon inemen and mic owa e hea ing. Indeed, a po ous ma e ial wi h a high dielec ic los could con ine he hyd ide and change he global dielec ic p ope ies o he complex. A ma e ial ha ul ills his equi emen is ca bon, which can be manu ac u ed as mesopo ous ca bon o as ca bon ae ogel. Some esea che s ha e al eady con ined hyd ides like NaAlH4, LiBH4 [14] o Mg(BH4)2 in o ca bon sca olds [15, 16]. Howe e , hese 5 ca bon s uc u es p esen a limi a ion which is i s low po e olume ha es ic s he space a ailable o he con inemen o hyd ide inside he suppo . Due o his limi a ion, he maximum hyd ide loadings achie ed in ca bon ma ixes a e be ween 15 and 30% w ., which limi s he hyd ogen s o age capaci y below he equi emen s o p ac ical applica ions. An al e na i e could be a hyb id ma e ial wi h lea es mo e po e space o he con inemen o a hyd ide and p o ides app op ia e global dielec ic p ope ies o he complex, making i suscep ible o mic owa e hea ing. Silica ae ogel was al eady sugges ed as a e y p omising ma e ial o hyd ides con inemen due o i s ema kable su ace p ope ies, and in pa icula o i s high po e olumes ha allow con ining as much as a 50%w o hyd ide [17]. Ca bon/silica hyb id ae ogels can combine a o able dielec ic p ope ies wi h high po e olumes. Ca bon/silica po ous ma e ials can easily made by he ca boniza ion o eso cinol– o maldehyde/silica ae ogels, as desc ibed by Kong e al. [18], keeping a o able ex u al p ope ies o encapsula ion. In his wo k, a combined C/SiO2 po ous ma e ial suscep ible o be hea ed up by mic owa es has been syn hesized and imp egna ed wi h a hyd ide. E hane 1,2-diaminobo ane (EDAB) has been chosen as hyd ide because o i s p omising p ope ies o H2 s o age: high H2 con en (~10 w .%) and he absence o undesi able ola ile impu i ies in he eleased gas [19]. Ma e ial cha ac e iza ion and H2 libe a ion es s by con en ional hea ing and by applica ion o mic owa es ha e been pe o med in o de o e alua e he possible ad an ages o applica ion o mic owa e hea ing in nanocon ined ma e ials. In addi ion a nume ical simula ion o he de ice unde mic owa es has been pe o med o each be e unde s anding o he p ocess. 2. Ma e ials and me hods 2.1 Ma e ials Reso cinol (R, ≥99% pu i y om Digma-Ald ich), o maldehyde(F, 36.5-38 % in H2O om Digma- Ald ich), 3-(aminop opyl) ie hoxysilane (APTES, 99% pu i y om Digma-Ald ich ), e hanol(E OH, 99.5% om Pan eac) and echnical ca bon dioxide ( om Ca bu os Me álicos) we e 6 used o he ae ogels syn hesis. Technical ni ogen ( om Ca bu os Me álicos) was used o he ae ogels py olysis. Chlo o ime hylsilane (CTMS, ≥98% om Sigma-Ald ich) and me hanol (MeOH, 99.8% om Pan eac) was used o unc ionalize he C/SiO2 pa icles. EDAB (96%, om Sigma-Ald ich) and MeOH we e used o he we imp egna ion o samples. 2.2 Complex syn hesis The eso cinol- o maldehyde ae ogels we e syn hesized ollowing he me hodology o Kong e al. [18]. Cylind ical monoli hs we e made by using R:F:APTES:E OH, in a mola a io o 1:2:1:60. A e gela ion he alcogels we e d ied by using supe c i ical CO2 in o de o a oid capilla y s esses du ing sol en emo al which could damage de ae ogel s uc u e. The d ying ook place in a closed ci cui . The alcogels we e placed in a chambe which can be isola ed om he es o he ci cui . Then CO2 was pumped ill 10.5 MPa and hea ed ill 40°C. A e i he CO2 was eci cula ed ill he sol en was comple ely emo ed. Th ee loads o esh CO2 whe e needed o a comple e d ying o he gels. A de ailed desc ip ion o he se up can be ound in a p e ious wo k [20]. Subsequen ly he RF/SiO2 ae ogels we e py olysed in a homemade ubula o en. The samples we e hea ed ill 800°C we e he T was held o 3 hou s. All his p ocess was made unde ine a mosphe e (~10 NmLN2/h). The py olysed monoli hs we e hen milled in a plane a y ball mill (PM100 om Re sch) a 100 pm du ing 3 h. The esul an powde was imme sed in a CTMS MeOH mix u e o e nigh , in o de o minimize he hyd oxyl and ca boxyl con en o he ma ix su ace which could des abilize he imp egna ed hyd ide. The es o he unc ionaliza ion agen was washed wi h excess o MeOH. Finally he powde s we e loaded wi h EDAB by we imp egna ion, using MeOH as sol en o he hyd ide (28 mg/mL). 2.3 Ma e ial Cha ac e iza ion 7 The chemical s uc u e o he complex was s udied by Fou ie T ans o m In a ed Spec oscopy (FT IR model TENSOR om BRUKER). The c ys allini y o he imp egna ed EDAB was analyzed by X- ay di ac ion (Disco e D8- B uke ) Di e en ial scan calo ime y (DSC) assays we e pe o med wi h a Me le -Toledo 822e de ice. The hea ing a e was 5°C/min om 30 o 250°C unde a N2 cons an low o 60 mL/min. The ex u al p ope ies o he complex we e de e mined by ni ogen iso he mal adso p ion- deso p ion. A Su ace A ea and Po osi y Analyze (ASAP2020 om Mic ime ics) was used. The speci ic su ace a ea was calcula ed by he BET (B unaue –Emme –Telle ) me hod. The speci ic po e olume was de e mined by he single poin adso p ion me hod. The a e age po e diame e was calcula ed wi h he deso p ion iso he m o he Ba e -Joynes-Halenda (BJH) me hod. The de e mina ion o he complex dielec ic p ope ies was ca ied ou by means o a ca i y pe u ba ion me hod desc ibed by Ramopoulos e al. [21]. Namely, he powde ed sample was in oduced in a qua z ube (inne diame e 7.8mm) wi hou compac ion. 0.49cm3 o ma e ial was es ed o samples wi h ~11%w EDAB. Then, he sample was in oduced a he cen e o a TE-111 mode ca i y wi h a esonance equency close o 2.45GHz. 2.4 Nume ical simula ions. Design o hyd ogen libe a ion cell The design o he cell used in hyd ogen libe a ion expe imen s by applica ion o mic owa es was suppo ed on a nume ical simula ion. Fo his, he p ocess o mic owa e hea ing and elease o he hyd ogen om he ma e ial is ep esen ed in his wo k by h ee basic s ages [22, 23]: 1) Gene a ion o he mic owa es in he magne on. 2) P opaga ion o he elec omagne ic wa es. 3) Hea gene a ion and ans e in he ma e ial. 8 The model combines all ele an physics. I simula es he elec omagne ic in e ac ions in he mic owa e ci cui , including he o wa d and backwa d in e ac ion be ween he applica o sec ion o he ci cui and he magne on mic owa e sou ce. This was done ia mic owa e ne wo k analysis. The ime ha monic s a iona y mic owa e ield was simula ed o e he applica o sec ion o he mic owa e ci cui including he load, while he cha ac e is ics o he mic owa e sou ce we e ep esen ed by an idealized lumped magne on model. The model cha ac e is ics we e desc ibed in mo e de ail in a p e ious wo k [22]. Mo eo e , he model also included conduc i e hea ans e . Using he in e ac ion be ween elec omagne ic wa es and he ma e ial in o ma ion, he empe a u e and hea gene a ion we e p edic ed. The medium pa ame e s ele an in he elec omagne ics model a e he ela i e pe mi i i y o he ma e ials p esen in he mic owa e ci cui . The dielec ic p ope ies o he composi e, measu ed as desc ibed ea lie , we e in oduced in he model. All he simula ions we e ca ied ou using Comsol Mul iphysics® 3.5 [24]. 2.5 Hyd ogen libe a ion expe imen s. To decompose he hyd ide and libe a e hyd ogen, he sample was hea ed by wo di e en me hods: - By con en ional hea ans e , in oducing he essel in a gas ch oma og aphy o en (Agilen 7890). - Wi h a mic owa e o en (CEM Disco e ), by placing he essel e ically, cen e ed and a a high o 3cm om he bo om o he ca i y. Tempe a u e was ollowed by placing a ibe -op ic empe a u e senso TS2 (OPTOCOM) inside he in e nal capilla y o he glass essel. The H2 low gene a ed by he he molysis was measu ed by a olume ic me hod by using a ully open mass low con olle s (EL-Flow F-200 om B onkho s ) wi h anges om 0.02 o 1 mL min-1. In hese measu emen s, i was conside ed ha all he eleased gas was hyd ogen. This supposi ion ag ees wi h a ailable expe imen al da a, which indica es ha he gas p oduced a he ope a ing empe a u es conside ed in his wo k is nea ly pu e hyd ogen [25]. While he p esence o decomposi ion by-p oduc s in he gas canno be disca ded, he p esence o 9 such compounds a mino concen a ions would ha e a negligible in luence on hyd ogen low a e calcula ions, which a e he main ocus o his wo k. On he o he hand, he p esence o hese compounds would ha e a s ong impac on he ope a ion as hey can damage uel cells e en a low concen a ions, and he e o e hei p esence should be e alua ed in u u e wo ks. 3. Resul s 3.1 Ni ogen iso he mal adso p ion-deso p ion iso he ms Table 1 shows he main ex u al p ope ies o he non-imp egna ed ae ogel and he imp egna ed ones. Ca bon ae ogels ob ained om he py olysis o eso cinol o maldehyde a e mic opo ous ma e ials wi h po e olume unde 1 cm3/g [26]. By con as , silica ae ogels a e mesopo ous ma e ials wi h high po e olume (abo e 2 cm3/g) [27]. By mixing silica and ca bon i is in ended o main ain he high he po e olume o silica ae ogels, which is impo an in o de o ha e enough po e space o con ine he hyd ide. Resul s p esen ed in Table 1 demons a e ha he ma e ials ob ained combining silica and ca bon ha e po e olumes abo e 1 cm3/g. In he po e diame e dis ibu ion, p esen ed in Figu e 1, wo sepa a e egions o mic opo es and mesopo es a ibu ed o he ca bon and silica egions a e dis inguished, wi h a b oad mesopo ous size dis ibu ion. As could be expec ed, by imp egna ing hyd ide a signi ican dec ease o he su ace a ea and he po e olume is obse ed, illing and blocking he smalle po es a his concen a ion o hyd ide (~11%). Conce ning he iso he ms showed on Figu e 1, he sca olds p esen ype IV iso he ms, ypical o mesopo ous ma e ials. In addi ion, adso p ion and deso p ion b anches a e almos e ical and nea ly pa allel o e an app eciable ange o gas up ake, co esponding o a beha io classi ied as hys e esis loop H1. This kind o po ous ma e ial consis s o well- aligned, sphe es and b ique es [28]. Table 1.Tex u al p ope ies o he samples. BET Su ace A ea (m2/g) -Plo Mic opo e A ea (m2/g) -Plo Ex e nal Su ace A ea Po e Volume (cm3/g) Po e diame e (Ø) (nm) 16 and he empe a u e s a ed o d op as soon as he mic owa e was s opped. A he end o he es he powe was hold ill he o al amoun o hyd ogen con ained in he sample was exhaus ed. The esul s o his es a e shown in Figu e 7. Figu e 7. H2 low and sample empe a u e du ing a libe a ion es wi h mic owa e pulses o 25W Finally, i mus me no ed ha he e e sibili y o he hyd ogen s o age-libe a ion p ocess is dependen on he hyd ide compound employed o s o e hyd ogen. As a he momen he e is no any p ocedu e a ailable o make he decomposi ion o EDAB e e sible, he p oposed compound is no e e sible. Howe e , he same me hodology p oposed in his wo k could be used wi h o he hyd ides ha a e e e sible, such as me allic hyd ides. 3.6 IR Spec oscopy Figu e 8 shows he IR spec a o he complex be o e and a e H2 elease es s. The py olysed ae ogel shows in ense silicon–oxygen co alen bond ib a ions, appea ing mainly in he 1200–1000 cm−1. The e y in ense and b oad band appea ing a 1050 cm−1 and he shoulde a a ound 1200 17 cm−1 a e espec i ely assigned o he ans e sal op ical and longi udinal op ical modes o he Si-O- Si asymme ic s e ching ib a ions. On he o he hand, he symme ic s e ching ib a ions o Si-O- Si appea a 800 cm−1 [31]. The s ong band a 1569 cm-1 s ems om he a oma ic C=C s e ching ib a ions o he ca bons [32]. The b oad band cen e ed a a ound 3100–3600 cm−1 co esponds o he o e lapping o he O-H s e ching bands o su ace silanols and ca boxylic g oups. Fu he mo e, he Si-O in-plane s e ching ib a ions o he silanol Si-OH g oups appea a a ound 960 cm−1. The su ace unc ionaliza ion wi h CTMS has educed he in ensi y o he wo OH a eas. In addi ion he band a 1705 cm-1 o he un ea ed ca bons ela ed o he ca bonyl and he one a 1359cm-1 (OCO) [33] ha e disappea ed because o he ea men wi h CTMS. Ins ead, CH peak appea s a 740 cm−1, 1249 cm−1 and 2965 cm-1 (CH3) and he Si-C s e ching ib a ion a 841 cm−1. These wo peaks con i ms he eplacemen o some hyd oxyl g oups by alkyl g oups. This unc ionaliza ion causes a dec ease o he oxygen g oups on he suppo su ace, a oiding he chemical in e ac ions be ween EDAB and he suppo which could des abilize EDAB. The spec a o he imp egna ed ae ogel shows he ypical peaks o EDAB: 702 cm−1 o he BN bond; 1039 cm−1 o CN bond; 1162 cm−1 and 1189 cm−1 o BH bond and 1357 cm−1, 1581 cm−1,3221 cm−1 and 3257 o NH bond. A e decomposi ion, BH and NH peaks disappea , and wo new bands a 1326 cm−1 and 1362 cm−1 g ow s onge . These bands a e cha ac e is ic o B=N s e ching, hus e idencing he o ma ion o double bonds be ween B and N [25]. The only sample which keeps he BH and NH signals a e he hea ing is he one pe o med a 10W in he mic owa e. As i was desc ibed abo e, his powe was no high enough o each he decomposi ion empe a u e, which jus i ies he p esence o hyd ogen bonds in he sample a e hea ing. Fu he mo e, his esul p o es he s abili y o he composi e a empe a u es below i s decomposi ion empe a u e. 18 Figu e 8. In a ed spec a o EDAB and he imp egna ed complex be o e and a e he H2 eleased. 4. Conclusions A hyb id C/SiO2 ae ogel has been syn hesized and unc ionalized keeping high po e olume in o de o hold EDAB inside i s po es. The con inemen o he hyd ide has allowed educing he esponse ime du ing he decomposi ion and has minimized he second decomposi ion s ep cha ac e is ic o pu e EDAB. 19 The addi ion o ca bon o he ma ix has modi ied he dielec ic p ope ies o he ma ix and has allowed hea ing up he sys em by using mic owa es, imp o ing he hea ing a e. In addi ion his hea ing a e could be easily con olled by uning he powe o he elec omagne ic ield. Wha is mo e, he sys em jus hea s up he sample, and no he con aine . 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