Reversible hydrogen sorption in the composite made of magnesium borohydride and silica aerogel

1 REVERSIBLE HYDROGEN SORPTION IN THE COMPOSITE MADE OF MAGNESIUM BOROHYDRIDE AND SILICA AEROGEL Mi iam Rueda a*, Luis Miguel Sanz-Mo al a, I an Saldan b, Alessand o Gi ella c, Chia a Milanese c, Ángel Ma ín a a Depa men o Chemical Enginee ing and En i onmen al Technology - Uni e si y o Valladolid , c/ Doc o Me gelina s/n 47011 Valladolid b Depa men o Physical and Colloid Chemis y, I an F anko Na ional Uni e si y o L i ,6 Ky yla and Me odia S ., 79005, L i , Uk ain c Pa ia H2 Lab, C.S.G.I & Depa men o Chemis y, Physical Chemis y Di ision, Viale Ta amelli 16, 27100 Uni e si y o Pa ia (I aly) 2 REVERSIBLE HYDROGEN SORPTION IN THE COMPOSITE MADE OF MAGNESIUM BOROHYDRIDE AND SILICA AEROGEL Mi iam Rueda a*, Luis Miguel Sanz-Mo al a, I an Saldan b, Alessand o Gi ella c, Chia a Milanese c, Ángel Ma ín a a Depa men o Chemical Enginee ing and En i onmen al Technology - Uni e si y o c/ Doc o Me gelina s/n 47011 Valladolid b Depa men o Physical and Colloid Chemis y, I an F anko Na ional Uni e si y o L i ,6 Ky yla and Me odia S ., 79005, L i , Uk ain c Pa ia H2 Lab, C.S.G.I & Depa men o Chemis y, Physical Chemis y Di ision, Viale Ta amelli 16, 27100 Uni e si y o Pa ia (I aly) Abs ac Magnesium bo ohyd ide Mg(BH4)2 is a p omising hyd ogen s o age ma e ial as i eleases high hyd ogen s o age capaci y a mild deso p ion empe a u es, bu i is s ill limi ed by slow hyd ogen elease kine ics and by he ha sh condi ions equi ed o e-hyd ogena e his compound. In his wo k, composi es made o comme cial Mg(BH4)2 and syn hesized silica ae ogel mic opa icles we e p epa ed by he mal ea men in hyd ogen unde 120 ba and 200ºC. As a esul , he so p ion p ope ies o he hyd ide a e imp o ed: calo ime ic measu emen s show ha decomposi ion empe a u e is educed by 60ºC, and he ypical 3-s ep decomposi ion mechanism o Mg(BH4)2 changes o a single-s ep mechanism in ange o 220- 400°C. The kine ics o he i s dehyd ogena ion a 300ºC was wo imes as e in Mg(BH4)2- SiO2 composi es han in he case o bulk γ-Mg(BH4)2. Addi ionally, he e-hyd ogena ion o his ma e ial a compa a i ely mode a e condi ions o 390ºC and 110 ba is p esen ed o he i s ime, achie ing cyclabili y wi h a e e sible elease o hyd ogen up o 6w %. Di e en amoun s o hyd ogen we e exchanged depending on he empe a u e o deso p ion (300ºC o 400ºC) and he p esence o absence o silica ae ogel. This esul indica es ha silica ae ogel chemically in e ac s wi h Mg(BH4)2, ac ing as an addi i e, which can esul in di e en hyd ogena ion- dehyd ogena ion ou es in which di e en amoun s and ypes o in e media es a e o med, in luencing he kine ics and he cyclabili y. *Co esponding au ho : Tel: +34 699142912, e-mail: mi iam. ueda.no ieg[email p o ec ed] (M.Rueda) 3 Keywo ds: hyd ogen s o age, addi i e, kine ic, in e media e, e e sibili y 1. In oduc ion Hyd ogen socie y, which is ela ed o he use o hyd ogen as an ene gy ec o , could be a solu ion o cu en en i onmen al and economic p oblems associa ed o he use o ossil uels [1][2]. Due o he low densi y o hyd ogen gas, an impo an challenge o onboa d applica ions is he sea ch o an e ec i e echnology o e e sibly s o e hyd ogen in a compac sys em [3]. Among cu en ly s udied solid s a e hyd ogen s o age ma e ials, complex me al hyd ides, such as LiBH4 [4] o NaAlH4 [5], a e e y a ac i e due o hei high hyd ogen s o age capaci y, bu hey a e limi ed by hei kine ics and he modynamics. In his wo k, magnesium bo ohyd ide Mg(BH4)2 is also conside ed as e ec i e s o age ma e ial due o i s high g a ime ic (14.8%w H2) and olume ic capaci y (0.112 kg/L) [6], which exceeds he ul ima e a ge s om Depa men o Ene gy (DoE) o 2020. Because o hese ou s anding p ope ies, i could be also o in e es in ba e ies applica ions, since Mg me al holds be e olume ic capaci y and is mo e abundan han Li. Ba e y- ela ed applica ions o his complex hyd ide has been ecen ly e iewed in [7]. Mo eo e , Mg(BH4)2 s a s he decomposi ion a 270ºC, and al hough his decomposi ion empe a u e s ill is oo high, i is lowe han he decomposi ion empe a u e o simila al e na i e compounds. Howe e , due o he o ma ion o s able in e media es [6], ehyd ogena ion s ill emains as he main challenge o use o Mg(BH4)2 o hei composi es. High condi ions o p essu e and empe a u e a e necessa y in o de o ob ain a high con e sion g ade o Mg(BH4)2 om decomposed p oduc s. Re e sibili y o 11w % o hyd ogen [9] was expe imen ally shown s a ing om MgB2 o Mg(BH4)2 a 950 ba H2 and 400°C du ing 108 hou s. In ano he wo k o Newhouse e al. [10], MgB2 ob ained a e he mal decomposi ion a 600°C was pa ially ehyd ogena ed, ge ing 9.7 w % o hyd ogen by ea men a simila condi ions o p essu e and empe a u e (900 ba H2 and 390°C) o 3 days. Li e al. [11][12] epo ed 6.1w % hyd ogen abso bed a e ehyd ogena ion a 400 ba H2 and 270°C o 48 h h ough he o ma ion o he s able MgB12H12 in e media e. Howe e , when mild p essu e and empe a u e condi ions (134 ba H2 and ~300°C) we e used du ing ehyd ogena ion, only 2.7-3.1 w % [6] o 3.5w % H2 [13] was achie ed which mos p obably co esponded o he o ma ion o MgH2. Also in [14] a ibo ane Mg(B3H8)2 in addi ion o MgH2 was expe imen ally ound a e dehyd ogena ion o Mg(BH4)2 a 200ºC o 5 weeks and a ound 2.5w % H2 was eabso bed a 250ºC and 120 ba o 48 hou s. In a ecen wo k [15], he ehyd ogena ion was pe o med a simila condi ions o 280 °C and 120 ba hyd ogen o 4 14hou s, eabso bing only 2.5 w % sugges ing he o ma ion o wo Mg-B-H phases du ing deso p ion, only one o which being e e sible and could explain he limi ed abso p ion. The o he in e media e compound could be oo s able and p e en he comple e ehyd ogena ion. Di e en s a egies ha e been es ed in o de o imp o e no only he e e sibili y bu also he kine ics and he empe a u e o he deso p ion eac ions [16] and ecen ly e iewed in [8]. The addi ion o di e en addi i es o ca alys s such as NbF5, TiO2, CoCl2, CoF2 o TiCl3 among o he s [14-15], he con inemen in po ous ma e ials (mainly in ca bon ma e ials) [19– 21], hei combina ions [23] o he o ma ion o composi es [24] a e some o he p oposals ha ha e been s udied. Fi chne e el. [20], success ully con ined a ound 44w % o he hyd ide in ac i a ed ca bon (BET a ea o 860 m2/g and po e olume o 0.61 g/cm3) using we imp egna ion as me hod and die hyl e he as sol en . As a esul , a composi e wi h lowe decomposi ion empe a u e and lowe ac i a ion ene gy was ob ained. Wahab e al. [23] in il a ed 45 w % Mg(BH4)2 in o de ed mesopo ous ca bon CMK-3 sca old (BET a ea o 1499m2/g and a olume o po es o 1.63cm3/g) wi h addi ion o 5 w % o Ni. In his case, a lowe ing on 200ºC in he decomposi ion empe a u e was ob ained and kine ics o hyd ogen elease we e 10 imes as e compa ed o he bulk Mg(BH4)2 due o he syne gic e ec o nanocon inemen and o he addi ion o he Ni ca alys . Since decomposi ion ends o occu be o e mel ing poin , mel in il a ion is no a sui able app oach o nanocon inemen in his case [19][25]. The only possibili y is o con ine MgH2 in a suppo ia mel in il a ion and hen ea he composi e wi h B2H6 in o de o ob ain Mg(BH4)2 in il a ed in he hos [21]. In he case ha composi es a e o med [24], 3.6w % o hyd ogen was e e sible in 1h a 90ba hyd ogen, a e 12h o decomposi ion p ocess a 180 ᵒC o 20 cycles due o he o ma ion o polybo ane in e media es which could ans o m o [BH4]-, and make he whole sys em e e sible unde mild condi ions. In his wo k, we epo he expe imen al ial o con ine Mg(BH4)2 in mic opa icles o silica ae ogel by empe a u e ea men in hyd ogen a mosphe e. Mic opa icles o silica ae ogel, which we e used o he i s ime wi h his complex hyd ide, ha e been p oduced by supe c i ical CO2 d ying, a echnique ha enables o p oduce a ma e ial wi h ou s anding su ace p ope ies. The d ying me hod employed is a key aspec ha de e mines he ex u al p ope ies o his po ous hos . I he sol en is emo ed by e apo a ion o lyophiliza ion, he capilla y s esses associa ed o he o ma ion o apo -liquid in e aces inside he po es o he suppo a e esponsible o pa ial collapses o he po e s uc u e o he ma e ial. In case o SiO2 ma ixes, 5 he ma e ials ob ained by hese d ying me hods usually show po e olumes below 0.5 – 1.0 cm3/g, such as SBA-15 o MCM-41 mesopo ous silica. In con as , i p essu ized o supe c i ical ca bon dioxide is used o ex ac he sol en , he collapse o he po e s uc u e is a oided o minimized, because unde hese condi ions ca bon dioxide is comple ely miscible wi h he o ganic sol en , and he e o e he ex ac ion p oceeds wi hou o ma ion o gas-liquid in e aces[26] and wi hou capilla y s esses, hus esul ing in po e olumes up o 4 cm3/g [27]. The p epa ed Mg(BH4)2–SiO2 composi es we e cha ac e ized by SEM, XRD and coupled calo ime ic-manome ic echnics. Thei kine ic cu es we e ob ained du ing se e al hyd ogen deso p ion-abso p ion cycles and compa ed o hose o he bulk γ-Mg(BH4)2. 2. Expe imen al me hods 2.1 Reac an s The comme cial ϒ- Mg(BH4)2 powde (Sigma Ald ich, 95%), he ea e e e ed o as MBH, was cons i u ed by p isma ic pa icles o a ound 100 µm (see SEM images epo ed in Sec ion 3). Te ame hylo hosilica e (TMOS, 98.0% pu i y) and ammonium hyd oxide (NH4OH, 28.0-30.0% ammonia) we e supplied by Sigma-Ald ich. Me hanol (MeOH; 99.8%) and n- hexane (95%) we e pu chased om Pan eac. Gaseous Ca bon dioxide (CO2, 99.95%) was supplied by Ca bu os Me álicos S.A. These eac an s a e necessa y o ob ain mic opa icles o silica ae ogel. Mic opa icles o silica ae ogel we e syn hesized using a ba ch supe c i ical equipmen as d ying me hod as epo ed in a p e ious wo k [28]. They we e p epa ed by a sol-gel eac ion, using TMOS as p ecu so and me hanol as sol en . While wi h his p ocedu e i is common o p epa e la ge gel monoli hs, in his wo k he gel was syn he ized as mic opa icles, in o de o educe he possible hea and mass ans e esis ances ha could be caused by la ge ae ogel monoli hs. To do his, he sol-gel eac ion media we e dispe sed in hexane unde mechanical s i ing, in o de o ob ain small d ople s o TMOS in me hanol dispe sed wi hin he hexane con inuous phase. A e 10 minu es o mechanical s i ing o his mix u e, an aqueous solu ion o NH4OH was added as condensa ion ca alys , which induced he gela ion o TMOS. The mola a io used was he ollowing: 1 mol TMOS: 4.4 mol MeOH: 3.3 mol H2O: 4.5 mol hexane: 0.08 mol NH4OH. Gel pa icles we e hen kep in a closed essel imme sed in me hanol o an aging pe iod o a leas 4 days. Du ing his ime, he me hanol was enewed 2-3 imes in o de o emo e he wa e con en o he solu ion. A e his ime, mic opa icles o alcogel imme sed in me hanol we e d ied using supe c i ical CO2 a 110 ba and 40ºC, employing he supe c i ical d ying appa a us desc ibed in a p e ious wo k [28]. Fo his, he sys em was 6 slowly p essu ized and dep essu ized a a a e o 3 ba /min in o de o a oid he c acking o he alcogel/ae ogel and mechanical s esses ha could damage he s uc u al p ope ies o he inal p oduc . Sa u a ed CO2 in he sys em was enewed ou imes in o de o ob ain comple ely d ied pa icles. D ied silica ae ogel mic opa icles had a po ous s uc u e wi h su ace a ea o 723 m2/g and a po e olume o 1.35 cm3/g (mean po e size =7.5nm) [29]. Ob ained SiO2 ae ogel pa icles we e collec ed and s o ed in a closed ial a oom empe a u e un il used o MBH– SiO2 composi e p epa a ion. 2.2 The MBH-SiO2 p epa a ion and cha ac e iza ion All handling and p epa a ion o he samples ook place in a MBRAUN Unilab glo e box which con inuously pu i ied a gon a mosphe e whe e oxygen and mois u e alues we e kep below 1ppm. Comme cial γ-MBH and silica ae ogel pa icles we e ea ed ia a p essu e- empe a u e me hod. Fi s , SiO2 ae ogel we e hea ed a 5°C/min in a alumina boa placed in a ubula o en a 110°C in he A glo e-box o 2 hou s in o de o emo e wa e aces adso bed by he ae ogel om he ai . Subsequen ly, MBH and silica ae ogel pa icles we e mixed in a mo a wi h a mass a io o 1:1. A ound 200 mg o he mix u e we e in oduced in a high p essu e s ainless s eel sample holde unde A a mosphe e in he glo e box and hen ans e ed o he Sie e ype appa a us (PCT-P o 2000, Se a am and Hy-Ene gy), whose expe imen al se -up is schema ically shown in igu e 1. The sample was ea ed a 120 ba o hyd ogen and 200°C o 3 hou in o de o ob ain a composi e in which a e phase ans o ma ion [6] he i e e sible β- phase o MBH, can be coa ed by SiO2. (FIGURE 1) Kine ic measu emen s and he s udy o e e sibili y we e done in he same Sie e ype appa a us. The p epa ed MBH–SiO2 composi e was dehyd ogena ed and ehyd ogena ed o 3 cycles in he same high p essu e cell. Hyd ogen deso p ion was es ed a 300 and 400 ºC bo h in s a ic acuum while ehyd ogena ion was made a 390 ºC unde 110 ba H2. The same condi ions we e used o bulk γ-MBH in o de o compa e i s e e sibili y and cyclabili y wi h hose o he p epa ed composi es. The gases e ol ed om he sample du ing dehyd ogena ion we e analyzed in a esidual gas analyze (RGA P o, Se a am & Hy-Ene gy) connec ed o he manome ic ins umen . Coupled manome ic-calo ime ic measu emen s we e done by connec ing he Sie e appa a us o a high p essu e di e en ial scanning calo ime e (DSC, Sensys Se a am) in o de o s udy he in luence o silica ae ogel in he composi e sys em. The calo ime e cell was loaded wi h a ound 19 mg o he sample and hea ed up o 460ºC a a a e o 5ºC/min in s a ic acuum. A gon was 7 used as ca ie gas a 10 mL/min. Calis o was used as so wa e o da a acquisi ion and p ocessing. C ys allini y o he di e en samples was examined using an ex-si u X- ay powde di ac ome e (model D5005 B uke ). The measu ing condi ions we e CuKα adia ion, λ=1.54060 Å, 2θ angle anging om 5º o 90º wi h a scan a e o 10 s/s ep and a s ep size o 0.020º. A B uke -dome was used in o de o a oid he con ac wi h ai and pe o m measu emen s unde A gon (gas in he glo e-box) and oom empe a u e. Pa icle mo phology was obse ed by Scanning Elec on Mic oscopy (SEM) using EVO-MA10-HR (Zeiss, Ge many) wi h ene gy-dispe si e mic op obe analyze INCA Ene gy 350X Max om Ox o d ins umen s. A special home-made sample holde was used in o de o a oid he con ac o he powde s wi h a mosphe ic oxygen and humidi y: in pa icula , he samples we e ixed on Al s ubs by C- apes in he glo e box and closed in he sample holde . Rough acuum was c ea ed in i by a acuum pump be o e i s ex ac ion om he glo e box. Only a e eaching high acuum in he SEM, he sample holde was open and hanks o he 3D mo emen s o he SEM s age and he samples could be analyzed. 3. Resul s and discussion 3.1 XRD and SEM analyses XRD p o iles showed ha c ys alline ϒ-MBH and amo phous SiO2 (Fig. 2c) we e con e ed in o an amo phous mix u e a e in il a ion ea men (Fig. 2b). The same amo phous spec um was ob ained a e se e al cycles. This ac sugges s ha in he p epa ed composi es MBH was amo phous o in il a ed in o he po es o SiO2 ae ogel. (FIGURE 2) Fo he hand-made mix u e o MBH and SiO2 ae ogel Rie eld e inemen was pe o med simila o he me hod p oposed in [31]. Ob ained esul s sugges ha c ys alline powde has pa icle size a ound 138 nm and a cell pa ame e o cubic s uc u e was es ima ed o 1.5 nm. In Fig. 3, he SEM image o he p epa ed MBH–SiO2 composi e is shown. SEM-EDX analysis sugges ed ha MBH and silica we e mixed uni o mly and hei homogenei y and con ac o bo h componen s was highe compa ed o ha o hei hand-made mix u e. A hese condi ions o p essu e and empe a u e, i was di icul o conclude i MBH was in il a ed in o he po es o he silica ae ogel. Mo e p essu e would be necessa y in o de o achie e his, o al e na i ely ano he me hod o in il a ion such as we imp egna ion should be used. A simila mo phology o he composi e was obse ed a e 3 d deso p ion, as i is obse ed in Fig. 3b, which means ha silica coa ing a oided he agg ega ion o MBH du ing he pe o mance o 8 cycles. In compa ison, he powde o bulk MBH was sin e ed in o a single agg ega e a e h ee dehyd ogena ion-hyd ogena ion cycles (Fig. 4). In Fig. 4b some single g ains o he compac ed pelle ob ained a e cycling we e obse ed. The expe imen al esul s sugges ha used SiO2 ae ogel wo ked as nanosca old a oiding agg ega ion o MBH du ing hyd ogen cycling. (FIGURE 3 AND 4) 3.2 Coupled calo ime ic-manome ic measu emen s. DSC esul s ob ained wi h bulk γ-MBH a e shown in Fig. 5 (sepa a ely in Fig. S1). Ob ained esul s a e in good ag eemen wi h hose p e iously epo ed in [6][13][32][8]. Small empe a u e di e ences be ween he esul s ob ained in his wo k and li e a u e esul s can be due o di e en applied condi ions o p essu e and hea ing a e du ing he measu emen [32]. Acco ding o Solo eichik e al. [6] , Li e al. [11] and Saldan [8], he i s wo endo he mic peaks (162 and 194ºC) a e ela ed o he phase ans o ma ion o MBH. Then, in he ange o 300-450ºC, MBH was decomposed in 3 di e en s eps (D1-D3). Du ing he i s wo s eps (T=307ºC and T=361ºC, espec i ely), MBH was decomposed in o MgH2 (c ys allized du ing E1) and amo phous bo on. A e his wo-s ep eac ion, MgH2 was decomposed in o Mg in he hi d s ep (T= 384ºC) [33][13]. Since MBH–SiO2 composi es p epa a ion was done a 200 ºC, he used MBH should be con e ed o i s β modi ica ion. Coupled DSC-manome ic measu emen s we e ca ied ou o bo h bulk γ-MBH and he p epa ed MBH–SiO2 composi e (Fig. 5). Onse o decomposi ion o he composi e was shi ed o lowe empe a u e on app oxima ely 60°C compa e o bulk MBH. A simila esul was ob ained in a p e ious wo k [34], in which TiO2 was used as addi i e. In his case, he dehyd iding empe a u e was lowe ed o 50ºC. The e o e, in his wo k silica could also ac as addi i e imp o ing he b eakage o B-H bond om MBH which is he limi a ion o he decomposi ion p ocess [21], simila ly o hose epo ed in [29]. Mo eo e , Fig. 5 showed ha silica changes he mechanism o he eac ion. The h ee decomposi ion s eps om bulk MBH we e con e ed in o only one b oad endo he mic peak and one highe empe a u e shoulde o he p epa ed MBH-SiO2 composi e, a ypical beha io o con ined hyd ides [20]. This ac could be ela ed o he in imal con ac be ween silica and MBH. Simila esul was obse ed in he case o MBH-0.5LiH composi e [24], only one b oad peak loca ed a 210ᵒC, in ol ing complica ed in e ac ions which may lead o o ma ion o me as able phases. Also in [35] hyd ogen was e ol ed in only one hyd ogen s ep a e he o ma ion o MBH-e hylendiamine composi e. Since mass o MBH in he p epa ed composi e was 50%, he inal hyd ogen elease was ~6.6 w % H2 ha was wo imes lowe han o he bulk MBH (almos 12.0 w % H2). The small di e ence in he inal amoun ha is eleased ill 450 ºC could be due o a small ins umen al 9 e o du ing he p epa a ion o he MBH–SiO2 sample o su ace oxida ion o MBH be o e he measu emen . O he wise i could be due o an e ec o he SiO2 ma ix. This would mean ha silica no only lowe ed he decomposi ion empe a u e bu also i s p esence makes MBH o elease mo e hyd ogen. As i is known, MBH has a heo e ical con en o hyd ogen o 14.8 w %, so highe condi ions o empe a u e (>500ºC) a e necessa y in o de o elease all o i . (FIGURE 5) 3.3 Hyd ogen deso p ion a 300ºC and 400ºC Figu e 6 shows he kine ics cu es o hyd ogen deso p ion o he i s deso p ion cycle a 300ºC and 400ºC o bulk γ-MBH and he p epa ed MBH–SiO2 composi e. Hyd ogen elease kine ics a 300°C was imp o ed in he sample in which silica was p esen . Di e en esul s we e ob ained espec o he wo k o Al-Kukhun e al. [17]. In his wo k, no changes in hyd ogen elease kine ics om MBH a 300ºC we e obse ed a e addi ion o SiO2. This ac con i med ou SEM obse a ion ha a be e con ac be ween MBH and SiO2 was ound in he p epa ed composi e, which was no ob ained wi h he simple mixing employed by Al-Kukhun e al. [17], he e o e he used ae ogel migh be conside ed as nanosca old o MBH Al e na i ely, i could be also due o some di e en p ope ies o he silica used in his wo k and in he wo k o Al-Kukhun e al. [17], such as acidi y, which could be esponsible o he weakening o B-H bond and he kine ic imp o emen du ing dehyd ogena ion [18]. Rega ding he kine ics a he lowes empe a u e, i ook less han 4 hou s o comple e deso p ion om MBH in he composi e (~5 w % H2), whe eas a ound 8 hou s we e necessa y o elease he same no malized amoun o hyd ogen (~10 w % H2) om bulk MBH. This means ha dehyd ogena ion ime was educed by a ac o o 2. Simila esul s we e obse ed in [10] using TiF3 and ScCl3 as addi i es ball milled wi h MBH. A 400ºC, he elease kine ics was simila in bo h samples. In he case o he composi e, he kine ic was shi ed o he le because bo ohyd ide was decomposed in a sho e ime, as obse ed in he calo ime ic measu emen s discussed in he p e ious sec ion. Mo eo e , a di e en pa hway was ollowed du ing he dehyd ogena ion as i was also epo ed in DSC measu emen s. Th ee di e en s eps we e dis inguished in bulk MBH whe eas only one was seen in he p epa ed composi e. The di e en mass o hyd ogen ha was eleased aking in o accoun ha MBH:SiO2 sample was p epa ed in a a io 50:50 can be explained by he same easons epo ed in couple DSC-manome ic measu emen s: a di e en deg ee o oxida ion, o small e o s du ing he p epa a ion o samples.