Resea ch pape s
Mic os uc u al con ol by eeze-cas ing o CaO a chi ec u es o imp o ed
and s able he mochemical ene gy s o age pe o mance
Nabil Amgha
a
, Juan I o a-Ma inez
a,b
, An onio Pe ej´
on
a,c,*
, Do ian Hanao
d
,
Aleksande Gu lo
d
, Joaquín Ramí ez-Rico
a,e
, Luis A. P´
e ez-Maqueda
a,*
,
Ped o E. S´
anchez-Jimenez
a,*
a
Ins i u o de Ciencia de Ma e iales de Se illa (C.S.I.C.-Uni e sidad de Se illa), C. Am´
e ico Vespucio 49, Se illa 41092, Spain
b
Ins i u e o Ma e ials Technology (ITM), Uni e si a Poli `
ecnica de Val`
encia (UPV), Plaza Fe ´
andiz y Ca bonell 1, 03801 Alcoy, Alican e, Spain
c
Depa amen o de Química Ino g´
anica, Facul ad de Química, Uni e sidad de Se illa, 41012 Se illa, Spain
d
Technische Uni e si ¨
a Be lin, Facul y III - P ocess Sciences, Ins i u e o Ma e ial Science and Technology, Chai o Ad anced Ce amic Ma e ials, S aße des 17. Juni
135, 10623 Be lin, Ge many
e
Depa amen o Física de la Ma e ia Condensada, Facul ad de Física, Uni e sidad de Se illa, A da. Reina Me cedes SN, 41012 Se illa, Spain
ARTICLE INFO
Keywo ds:
F eeze-cas ing
Calcium Looping
The mochemical ene gy s o age
Po ous s uc u es
CaCO
3
ABSTRACT
This s udy in es iga es he de elopmen o po ous calcium-based monoli hs ia eeze-cas ing (FC) as a no el
app oach o he mochemical ene gy s o age, pa icula ly wi hin he Calcium Looping (CaL) p ocess. The eeze-
cas ing echnique enabled he ab ica ion o sca olds wi h con olled po osi y using poly inyl alcohol (PVA) as a
binde . Expe imen al esul s demons a ed ha eeze-cas monoli hs exhibi ed supe io mul icycle pe o mance
unde a ious ca bona ion and calcina ion condi ions. The FC-CaCO
3
monoli h achie ed he highes esidual
con e sion o 68.1 % unde mild acuum calcina ion condi ions (780 ◦C, 0.1 ba CO
2
), signi ican ly su passing
o he con igu a ions. Tes s conduc ed in an ine a mosphe e also yielded a o able esul s, wi h a con e sion o
56.1 %, ou pe o ming equi alen aw powde samples. The enhanced pe o mance is a ibu ed o imp o ed
CO
2
in e ac ion wi h he po ous s uc u e, mi iga ing sin e ing e ec s and p ese ing ac i e su ace a ea.
Mo phological obse a ions by X- ay omog aphy and SEM con i med limi ed pa icle sin e ing a e mul iple
cycles, main aining a eac i e su ace ha suppo ed consis en con e sion a es. The po e size dis ibu ion o he
ma e ial e ol es upon cycling esul ing in an inc eased mic opo osi y, while he po e ne wo k main ains a low
o uosi y (
τ
~ 1.5–2.0). The addi ion o dopan s such as Z O
2
and SiO
2
did no enhance pe o mance, as he
monoli hs' inhe en s uc u e p o ided su icien s abili y. These indings highligh eeze-cas ing as a p omising
me hod o c ea ing ad anced po ous ma e ials sui able o ene gy s o age applica ions.
1. In oduc ion
The Calcium Looping (CaL) p ocess has been ex ensi ely in es i-
ga ed as a p omising high- empe a u e he mochemical ene gy s o age
(TCES) me hod, compa ible wi h concen a ing sola powe (CSP) plan s
[1–3]. This p ocess elies on he e e sible eac ion be ween CO
2
and
CaO o o m CaCO
3
, as ep esen ed by Eq. (1):
CaCO3⇄CaO +CO2ΔH 0= ∓178 kJ/mol (1)
The CaL p ocess o e s signi ican ad an ages, including high ene gy
densi y and he abundan a ailabili y, low cos , and non- oxic na u e o
he aw ma e ials in ol ed, such as calcium-con aining mine als,
indus ial byp oduc s, and e en was e ma e ials [4–6].
In a ypical CSP-CaL ope a ion, sola adia ion d i es he endo-
he mic decomposi ion o CaCO
3
in o CaO and CO
2
[3,7,8]. These e-
ac ion p oduc s a e hen anspo ed o sepa a e s o age ese oi s,
whe e hey emain un il needed. When ene gy is equi ed, hey a e
ecombined in a ca bona o eac o , whe e he e e se exo he mic e-
ac ion eleases hea , which is subsequen ly ha nessed in a powe cycle,
ypically a CO
2
-closed B ay on cycle, o gene a e elec ici y [8–10]. Fo
e ec i e calcina ion, empe a u es o app oxima ely 750 o 950 ◦C,
con ingen on he calcina ion a mosphe e, a e essen ial [11]. While he
ca bona ion eac ion p oceeds apidly, i s ex en is in luenced by he
CO
2
pa ial p essu e and empe a u e [12,13]. Ca bona ion is gene ally
* Co esponding au ho s.
E-mail add esses: [email p o ec ed] (A. Pe ej´
on), [email p o ec ed] (L.A. P´
e ez-Maqueda), [email p o ec ed] (P.E. S´
anchez-Jimenez).
Con en s lis s a ailable a ScienceDi ec
Jou nal o Ene gy S o age
jou nal homepage: www.else ie .com/loca e/es
h ps://doi.o g/10.1016/j.es .2025.116681
Recei ed 22 Decembe 2024; Recei ed in e ised o m 27 Ma ch 2025; Accep ed 13 Ap il 2025
Jou nal o Ene gy S o age 125 (2025) 116681
A ailable online 8 May 2025
2352-152X/© 2025 The Au ho s. Published by Else ie L d. This is an open access a icle unde he CC BY-NC-ND license ( h p://c ea i ecommons.o g/licenses/by-
nc-nd/4.0/ ).
conduc ed a abou 800–850 ◦C o achie e maximum con e sion a es
and high he moelec ic e iciency [14,15].
F om a ma e ials pe spec i e, a c i ical ba ie o he la ge-scale
deploymen o his echnology is he g adual deac i a ion o CaO, p i-
ma ily due o he loss o ac i e su ace a ea caused by sin e ing a he
ele a ed empe a u es equi ed by he p ocess [16,17]. Sin e ing is
u he accele a ed in CO
2
- ich en i onmen s [18,19]. Mul iple s a e-
gies ha e been employed o mi iga e sin e ing. One such app oach in-
ol es op imizing eac ion condi ions o lowe calcina ion empe a u es
by u ilizing an ine a mosphe e, educed p essu es, o s eam [20–22].
Ano he me hod ocuses on modi ying he s o age ma e ial by adding
s abilizing ine agen s such as me al oxides. These addi i es ac as
s uc u e-s abilizing agen s ha mi iga e he loss o eac i i y due o
sin e ing [23–25]. So ben enginee ing s a egies aim o enhance he
ma e ial's ex u al p ope ies and c ea e mac opo ous s uc u es ha
acili a e ca bona ion [25–28]. Techniques such as ace ic acid modi i-
ca ion o he use o sac i icial empla es can yield highly po ous s uc-
u es ha imp o e he beha io o limes one [21,29,30]. Finally, small
loads o alkali chlo ides and ca bona e sal s ha e been explo ed as ac i e
addi i es o enhance cyclic ene gy s o age densi ies [31–33].
F eeze cas ing, also known as ice- empla ing, is a e sa ile echnique
ha has been widely adop ed in ecen yea s o ab ica e po ous ma e-
ials om a a ie y o subs ances, including ce amics, me als, polyme s,
and biomac omolecules [34,35]. The p ocess yields s uc u es wi h
dis inc ex u al and mo phological p ope ies. Du ing eezing, he
sol en , ypically wa e o camphene, solidi ies o o m a c ys alline
s uc u e ha empla es he po es. Upon sublima ion o he ozen sol-
en , channels a e le h oughou he sca old, esul ing in a po ous
a chi ec u e. F eeze cas ing can mimic na u al cellula s uc u es,
impa ing speci ic po osi y o ma e ials. Consequen ly, i is a s a egic
app oach o designing syn he ic ma e ials inspi ed by na u e. By con-
olling he eezing pa ame e s, i is possible o manipula e he in e nal
s uc u e o achie e speci ic po e mo phologies, including cellula ,
dend i ic, and lamella o ms [35,36]. P e ious s udies ha e epo ed
ha di ec ional eezing echniques can signi ican ly imp o e he u ili-
za ion o enewable ene gy o elec ochemical ene gy s o age and
con e sion, u he enhancing he unc ionali y o ma e ials wi h mul i-
scale po ous s uc u es [37]. In di ec ional eeze cas ing, poly inyl
alcohol (PVA) is commonly used o enhance he dispe sibili y o ce amic
slu ies and imp o e he mechanical s eng h o he esul ing g een
ce amic bodies. PVA p e en s pa icle agglome a ion ha can hinde
c ys al g ow h and in e ac s wi h pa icles du ing eezing o supp ess
excessi e c ys al o ma ion. Adding PVA imp o es po e connec i i y
and open po osi y. As a soluble polyme , PVA enhances he mechanical
in eg i y o he ce amic body [38,39].
This s udy explo es o he i s ime da e a eeze cas ing app oach
o he syn hesis o po ous CaO so ben s. Using he inno a i e eeze-
cas ing echnique, highly po ous CaO a chi ec u es we e de eloped.
Mul icycle pe o mance we e e alua ed in a labo a o y eac o consid-
e ing a ious ope a ional condi ions ha ha e been p oposed o he -
mochemical ene gy s o age applica ions: i) calcina ion in ine
a mosphe e and ca bona ion in CO
2
. ii) closed CO
2
loop and iii) closed
CO
2
loop wi h calcina ion a educed CO
2
p essu e. The mo phology o
he p epa ed a chi ec u es was me iculously cha ac e ized h ough X-
ay omog aphy and scanning elec on mic oscopy (SEM). The key pa-
ame e s o he cas samples a e comp ehensi ely assessed o p o ide a
ho ough unde s anding o hei p ope ies and he impac o hese
mul iscale s uc u es on he eac ion kine ics, con e sion and ene gy
s o age pe o mance.
2. Expe imen al sec ion
2.1. Ma e ials
Limes one om he s anda d Eskal 500 se ies was ob ained om KSL
S aub echnik GmbH (Ge many). Samples ea u ing a well-de ined
pa icle size we e used: 5
μ
m (D (50) =4.71). Fig. 1 includes he pa -
icle size dis ibu ion and a digi al pho og aph o he a chi ec u es'
c oss-sec ion. Z O
2
(Tosoh, Re .TZ-0) and SiO
2
(99.5 % S em Chem-
icals, Re .931436) we e u ilized as s abilize s. The binde used in he
eeze-cas ing p ocess was p epa ed by dissol ing poly inyl alcohol
(PVA, Re .8148941001) in dis illed wa e .
2.2. Fab ica ion o po ous CaCO
3
a chi ec u es by eeze-cas ing
A solu ion con aining 40 w % solid loading was igo ously mixed
in o a 4 w % PVA solu ion p epa ed using dis illed wa e in a sonica o
ba h o 10 min. Fo he monoli hs inco po a ing addi i es, Z O
2
and
SiO
2
we e manually mixed wi h he CaCO
3
powde o 20 min, using an
aga e mo a , p io o he dispe sion in he sonica ion ba h. In bo h
cases, he addi i e amoun o 10 w % o he o al mass o he sample. I is
expec ed he sonica ion would p oduce a homogeneous mix u e o he
CaCO
3
and he addi i e.
The p epa ed mix u e was ans e ed in o an ac ylic glass mold wi h
an ex e nal cylind ical diame e o 25 mm, an in e nal diame e o 5
mm, and a leng h o 15 mm. This mold was placed on he uppe sec ion
o a coppe od imme sed in liquid ni ogen o ini ia e he cooling
p ocess. The eeze-cas ing sys em ensu ed unidi ec ional solidi ica ion
by cooling he coppe od om oom empe a u e o −50 ◦C h ough
imme sion in he liquid ni ogen ba h.
Wa e emo al om he samples was achie ed using a eeze-d ye
(Ch is Gamma 2–20, Ma in Ch is Ge ie ocknungsanlagen GmbH,
Ge many) equipped wi h a acuum pump. This p ocess was conduc ed
a −30 ◦C unde a acuum p essu e o 0.03 mba o 24 h. A e subli-
ma ion, he g een bodies we e ca e ully ex ac ed om he ac ylic glass
mold.
As shown in Fig. 2, he p ocess o cons uc ing he a chi ec u es in-
ol es s i ing he solu ion o ensu e homogenei y, ollowed by
con olled eezing o p omo e channel o ma ion. Du ing eeze d ying,
wa e is emo ed, c ea ing a po ous s uc u e ha enhances gas in e -
ac ion wi h he limes one. This channel s uc u e is p ese ed
h oughou he p ocess, esul ing in a highly po ous a chi ec u e. The
samples p epa ed by his me hod a e named FC-CaCO
3
, FC-CaZ 10 and
FC-CaSi10 depending on he addi i e included.
P io o he mul icycle es s, he binde was emo ed by g adually
hea ing he samples in an ai a mosphe e a a a e o 1 ◦C/min un il
eaching 585 ◦C.
2.3. Cha ac e iza ion me hods
The mo phological cha ac e is ics o he samples we e analyzed
using a scanning elec on mic oscope (SEM) Hi achi S4800. P io o
imaging, he samples we e spu e -coa ed wi h a hin gold laye using an
Emi ech K550 Tels a spu e -coa ing sys em.
The 3D mic os uc u e o he samples was analyzed using high-
esolu ion ansmission X- ay compu ed omog aphy (XCT) wi h an
X adia Ve sa 610 XRM sys em (Zeiss), capable o achie ing sub-
mic ome e esolu ion. A olume o app oxima ely 300
μ
m ×300
μ
m ×
400
μ
m aligned so ha he z-axis ma ched he eezing di ec ion was
econs uc ed using a CCD cama a wi h a 20×mic oscope objec i e
coupled o he scin illa o a a oxel size o 460 nm. Imaging was con-
duc ed a an accele a ion ol age o 140 kV and a cu en o 150
μ
A,
ensu ing adequa e con as . Image segmen a ion o sepa a e he solid
and po e phases was ca ied ou using a machine lea ning pixel classi ie
[40], and his og ams o solid and po e hicknesses (as a p oxy o he po e
size dis ibu ion) we e calcula ed using he BoneJ plugin in FIJI [41,42].
The aniso opic o uosi y o he po e space was de e mined nume ically
by sol ing he di usion equa ion using a ini e-di e ence scheme [43],
wi h calcula ions un on he He cules supe compu e a he Andalusian
Scien i ic Compu ing Cen e (CICA) – mo e de ails a e gi en in he
Supplemen a y in o ma ion.
The pa icle size dis ibu ion was de e mined h ough lase
N. Amgha e al.
Jou nal o Ene gy S o age 125 (2025) 116681
2
di ac ion wi h a Mas e size 2000 (Mal e n). To p e en agg ega ion,
he samples we e sonica ed o 30 min and dispe sed in dis illed wa e .
2.4. Mul icycle expe imen s
Mul icycle expe imen s we e conduc ed using a cus om-buil he -
mog a ime ic eac o designed o ope a e ac oss a b oad p essu e ange
(0.01 o 5 ba ). The eac o included a mic obalance and a Wa low
u nace wi h a sealed, non-po ous mulli e ube u ilizing O- ings. A la
alumina c ucible, suspended om he mic obalance wi e, ensu ed
e ec i e con ac be ween he sample and he gas. Fu he de ails o he
expe imen al se up can be ound in [21,44].
The pe o mance o he samples in mul icycle expe imen s was
e alua ed unde a ious ope a ing condi ions, as de ailed in Table 2.
A e binde emo al, he empe a u e was inc eased o he a ge alue
a a a e o 1 ◦C/min, ollowing he speci ic expe imen al condi ions
ou lined in Table 1. Upon eaching he desi ed empe a u e, ca bona ion
s ages we e main ained o 15 min, while calcina ion s ages las ed 25
min. Unde T1 condi ions, gas changes we e pe o med manually. Fo
T2 condi ions, he cycles we e con olled by al e ing he empe a u e a
a hea ing and cooling a e o 10 ◦C/min. In T3 condi ions, he ca bon-
a ion and calcina ion s ages we e egula ed by adjus ing he chambe
p essu e using a acuum pump.
2.5. E ec i e con e sion and esidual e ec i e con e sion
To e alua e he mul icycle pe o mance o he samples, he e ec i e
con e sion (Eq. (2)) was employed [11]. The e ec i e con e sion is
de ined as he mass a io o calcium oxide con e ed du ing he
ca bona ion s age a each N h cycle o he o al mass o he sample
be o e ca bona ion (m), including solids ine o ca bona ion. The e o e,
he e m m
ca b
−m ep esen s he CO
2
up ake in each cycle. The FC-
CaZ 10 and FC-CaSi10 composi es including 10 w % o ine addi i e
will exhibi a maximum a ainable e ec i e con e sion o 0.9.
500 µm
0.1 1 10 100
0
1
2
3
4
5
6
7
8
9
Volume ic ac ion (%)
Pa icle diame e (µm)
Fig. 1. Pa icle size dis ibu ion and a pho og aph o he ini ial a chi ec u e.
Con olled eezing p ocess F eeze d ying
Slu y p epa a ion by s i ing
PVA chains
CaCO3 powde
Des illed wa e
Liquid N2 con aine
Me hac yla e box
Coppe od
Ac ilic glass mold
Slu y
Fig. 2. Schema ic diag am illus a ing he p epa a ion p ocess o calcium-de i ed eeze-cas ma e ials.
Table 1
Ope a ing condi ions o calcina ion/ca bona ion cycles used in his wo k.
Tes Calcina ion Ca bona ion
Tempe a u e,
(◦C)
Gas Absolu e
CO
2
p essu e
(ba )
Tempe a u e,
(◦C)
Gas Absolu e
CO
2
p essu e
(ba )
T1 800 N
2
–800 CO
2
1
T2 950 CO
2
1 850 CO
2
1
T3 780 CO
2
0.1 780 CO
2
1
N. Amgha e al.
Jou nal o Ene gy S o age 125 (2025) 116681
3
Xe =mca b −m
m⋅WCaO
WCO2
=mCO2( )
m
WCaO
WCO2
(2)
In his equa ion, m
ca b
is he sample mass a e ca bona ion a he N h
cycle, m is he mass o he calcined sample a he i s cycle, and W
CaO
(56 g/mol) and W
CO2
(44 g/mol) a e he mola masses o CaO and CO
2
,
espec i ely. To compa e he pe o mance o di e en samples, he
mul icycle con e sion da a can gene ally be well i ed using he semi-
empi ical equa ion [45]:
Xe ,N=X +Xe ,1
k(N−1) + (1−X /Xe ,1)−1(3)
In his exp ession, X
e ,1
is he e ec i e con e sion a he i s cycle, N
ep esen s he cycle numbe , k is he deac i a ion a e cons an , and X
is
he esidual con e sion owa ds which he con e sion con e ges a e a
la ge numbe o cycles.
3. Resul s and discussion
3.1. Mul icycle pe o mance o he eeze-cas a chi ec u es
The mul icycle pe o mance o he p epa ed pelle s was s udied ac-
co ding o h ee di e en ope a ional condi ions, as de ailed in Table 1.
Condi ion T1 in ol es calcina ion in N
2
and ca bona ion in CO
2
. This
scheme is p obably he mos commonly used in wo ks dealing wi h
Calcium Looping o TCES applica ions [24,27,46]. In ine gas, he
calcina ion empe a u e is main ained a abou 750 o 800 ◦C. The
ca bona ion eac ion is ca ied ou a abou 800–850 ◦C in CO
2
o ensu e
a as eac ion a e [12,46]. Howe e , his app oach equi es he
implemen a ion o a complex and cos ly gas sepa a ion s age. To add ess
his issue, closed CO
2
loop ope a ional schemes we e de ised
[11,14,27]. The main d awback o such schemes is ha he e ec i e
calcina ion empe a u e inc eases up o 950 ◦C, wha leads o in ense
deac i a ion o he CaO due o sin e ing [11,17]. These ope a ional
condi ions a e ep esen ed by T2. Recen ly, he concep o closed CO
2
loop was ex ended o con empla e calcina ion a educed p essu es
[21,47,48]. This adjus men no ably lowe ed he e ec i e calcina ion
empe a u e and imp o ed he esidual con e sion exhibi ed by he
CaO. These ope a ional concep is simula ed by T3 condi ions.
Fig. 3 p esen s he empe a u e and mass ime e olu ions measu ed
du ing a es ca ied ou unde T1 condi ions o he FC-CaCO
3
sample,
along de ails o he second and en h cycles. Be o e s a ing he cycle
p ocess, he binde used in he manu ac u ing o he monoli hs was
debinded in ai a 585 ◦C. Due o his he mal ea men , PVA chains a e
ola ilized, p o iding a po ous s uc u e and p omo ing an ini ial mass
loss due o he deg ada ion o he binde . In he li e a u e, i is epo ed
ha PVA is ully decomposed a empe a u es below 500 ◦C wi h esi-
dues close o ze o [49]. In his s udy, app oxima ely 2.4 % o PVA was
inco po a ed in o he monoli hs o acili a e gel o ma ion and ensu e
p ope dispe sion o CaCO
3
. This pe cen age co ela es well wi h he
mass loss obse ed du ing he debinding s age.
Following he binde deg ada ion, he Calcium Looping (CaL) cycles
0 250 500 750 1000 1250 1500 1750
0
100
200
300
400
500
600
700
800
Time (min)
Tempe a u e (ºC)
55
60
65
70
75
80
85
90
95
100
Mass (%)
Ai debinding N2/CO2 calcium looping
a)
CO2N2CO2N2
2410 2420 2430 2440 2450 2460
Time (min)
60
70
80
90
100
Mass (%)
1170 1180 1190 1200 1210
Time (min)
60
70
80
90
100
Mass (%)
b) c)
Fig. 3. Time e olu ion o empe a u e and sample mass o he limes one-de i ed eeze-cas ma e ials du ing mul icycle es s unde T1 condi ions: a) 20 cycles, b)
de ail o he second cycle and c) de ail o he en h cycle.
N. Amgha e al.
Jou nal o Ene gy S o age 125 (2025) 116681
4
commence. As illus a ed in Fig. 3b and c, ca bona ion p oceeds apidly
as soon as CO
2
en e s he chambe [12]. Unde he expe imen al con-
di ions used, he cycle exhibi s a cha ac e is ic p o ile, including a slow-
di usion s age ha ollows he ini ial as ca bona ion. In powde o m,
ca bona ion occu s swi ly a he su ace o he CaO pa icles h ough a
eac ion-con olled mechanism [50,51]. Howe e , he o ma ion o a
CaCO
3
laye on he pa icle su ace du ing his s age obs uc s CO
2
di usion o he un eac ed CaO co e. Consequen ly, he eac ion p o-
g esses ia solid-s a e di usion h ough he CaCO
3
blocking laye
[52–54]. In con as , he calcina ion s age is ini ia ed wi h a delay a e
he in oduc ion o ni ogen, as ce ain ime is equi ed o pu ge esidual
CO
2
om he chambe o alues below he equilib ium p essu e a he
eac ion empe a u e. This p ocess is inhe en ly slowe , wi h pa ame e s
such as he gas low a e, CO
2
di usi i y in he pu ge gas, eac o design
and pa icle sizes signi ican ly in luencing he eac ion kine ics. Lowe
pu ge low a es ex end he ime equi ed o CO
2
o di use ou side he
pa icles, pa icula ly in he case o la ge pa icles o whe e in e nal
mass ans e esis ances become a limi ing ac o [55,56].
Fig. 4 compa es he mul icycle pe o mance o a ious samples in
e ms o CaO con e sion, as de e mined h ough he mog a ime ic
expe imen s and calcula ed using Eq. (2). Table 2 p esen s a summa y o
bo h he ini ial and he esidual con e sion alues de i ed om ma h-
ema ical i ings o Eq. (3). I is well es ablished ha he eac i i y o
CaO p og essi ely declines ac oss successi e ca bona ion and calcina-
ion cycles. This decline in CaO con e sion is p ima ily due o ex ensi e
sin e ing caused by he high empe a u es equi ed o he p ocess,
which leads o a subs an ial loss o su ace a ea [16,57,58]. Fu he -
mo e, unde he ypical ope a ion condi ions o TCES -which in ol e
calcina ion in an ine gas en i onmen ollowed by high- empe a u e
ca bona ion in CO
2
- po e plugging becomes a limi ing ac o [58,59].
Po e plugging occu s when he su ace po osi y o he CaO pa icles is
obs uc ed by he apidly o ming CaCO
3
laye . This laye hinde s CO
2
di usion, he eby es ic ing he maximum con e sion ha can be
achie ed du ing each ca bona ion phase. Addi ionally, la ge pa icles
a e mo e p one o po e-plugging phenomena [58,60]. I is an icipa ed
ha he eeze-cas s uc u es cons uc ed using small pa icles can
enhance mul icycle pe o mance by mi iga ing bo h sin e ing-induced
deac i a ion and po e blockage issues. This hypo hesis is suppo ed by
he mul icycle expe imen s conduc ed in his s udy. Fo samples es ed
unde N
2
/CO
2
a mosphe es (Fig. 4a), he ac i i y o FC-CaCO
3
was
obse ed o dec ease om 0.72 in he i s cycle o a esidual alue o
0.56. In con as , he mul icycle pe o mance o CaCO
3
powde es ed
unde simila condi ions exhibi much smalle esidual con e sion: 0.3.
This cons i u es a signi ican imp o emen . The conside able a iabili y
in ope a ing condi ions, pa icle sizes and equipmen used in he li e -
a u e complica es di ec compa ison wi h p e ious esul s. Ne e heless,
he obse ed esidual con e sion alue is compa able o, o e en ex-
ceeds, hose epo ed o CaO p epa ed by we chemical me hods, which
ypically p oduce pa icle sizes smalle han hose employed in his wo k
[23,24,61]. E en mo e no able is he s abiliza ion o he con e sion
om he en h cycle onwa d. P e ious s udies wi h small CaO pa icles
ypically epo con inuously dec easing ends due o he high sin e -
abili y o such small pa icles [24,29,62,63]. This sugges s ha he
eeze-cas s uc u es a e less p one o sin e ing-induced deac i a ion,
p obably due o limi ed in e -pa icle con ac s compa ed o loose
powde .
The T1 condi ions we e also employed o assess he pe o mance o
FC-CaZ 10 and FC-CaSi10 samples, which we e p epa ed h ough cas -
ing wi h he addi ion o Z O
2
and SiO
2
, wo widely employed s uc u al
s abilize s. Unde ypical CaL ope a ing condi ions, SiO
2
eadily o ms
calcium silica es ha ac as a suppo ing skele on ha inhibi he sin-
e ing o CaO pa icles [25,64]. Z O
2
se es a simila pu pose; howe e ,
i s highe he mal s abili y makes he calcium zi cona es o appea in a
longe ime ame and equi e highe empe a u es [11,23,63,65]. X- ay
di ac ion pa e ns o he cycled specimens a e p o ided in Fig. S1.
In e es ingly, he inco po a ion o hese addi i es o he eeze-cas
s uc u es did no enhance he mul icycle beha io o CaO. In ac , he
FC-CaCO
3
samples demons a ed signi ican ly be e pe o mance han
FC-CaZ 10 and FC-CaSi10. The no able educ ion in con e sion shown
by FC-CaSi10 samples, wi h an es ima ed esidual con e sion o 0.34,
can be a ibu ed o he emo al o ac i e CaO due o he o ma ion o
calcium silica es. Al e na i ely, no signi ican amoun o calcium zi c-
ona es a e expec ed unde T1 condi ions. Thus, he esidual con e sion
obse ed in FC-CaZ 10 is 0.51, only sligh ly smalle han FC-CaCO
3
. The
limi ed educ ion in con e sion can be a ibu ed o he p esence o ine
Z O
2
, wha educes he maximum e ec i e con e sion heo e ically
010203040
0.0
0.2
0.4
0.6
0.8
1.0
010203040
0.0
0.2
0.4
0.6
0.8
1.0
Raw CaCO3-T1
FC-CaCO3-T1
FC-CaZ 10-T1
FC-CaSi10-T1
Xe
N
a) b) FC-CaCO3-T1
FC-CaCO3-T2
FC-CaCO3-T3
Xe
N
Fig. 4. Compa ison o he e olu ion o he mul icycle con e sion ob ained o : a) samples subjec ed o T1 condi ions, and b) FC-CaCO
3
samples subjec ed o T1, T2
and T3 ope a ing condi ions.
Table 2
Main esul s ob ained om i ing o he con e sion esul s. E ec i e con e sion
a cycle 1 (X
e 1
), esidual con e sion (X
), deac i a ion cons an (k) and R-
squa ed (R
2
).
Sample X
e ,1
X
k R
2
Raw CaCO
3
-T1 0.77 0.30 1.360 0.832
FC-CaCO
3
-T1 0.72 0.56 3.471 0.875
FC-CaZ 10-T1 0.65 0.51 4.071 0.867
FC-CaSi10-T1 0.59 0.34 0.637 0.991
FC-CaCO
3
-T2 0.83 0.16 0.472 0.993
FC-CaCO
3
-T3 0.94 0.69 1.183 0.985
N. Amgha e al.
Jou nal o Ene gy S o age 125 (2025) 116681
5
a ainable. This sugges s ha he po ous a chi ec u e o he eeze-cas
ma e ials esul s in inhe en ly s able a chi ec u es ha ende s he use
o s abilizing addi i es unnecessa y, as i s con ibu ion as sin e ing in-
hibi o s is played by he open eeze-cas a chi ec u e. Simila s able
beha io s ha e been epo ed in hollow mic osphe es o co e-shell
mo phologies ab ica ed h ough empla e syn hesis, whe e he a ail-
able ee space accommoda es olume changes and mi iga es sin e ing-
induced deac i a ion [28,66,67]. Howe e , in hose cases, me al oxides
emain essen ial o p ese e he s uc u al in eg i y o he po ous
s uc u e. In con as , eeze cas a chi ec u es do no seem o equi e
he inclusion o s uc u al s abilizing addi i es.
Fig. 4b shows he mul icycle pe o mance ob ained in es s con-
duc ed unde CO
2
closed-loop condi ions. This ope a ional scheme helps
o a oid he high cos and added complexi y associa ed wi h gas sepa-
a ion memb anes [14,68]. Howe e , ope a ing a a mosphe ic p essu e
unde CO
2
(T2 condi ions) equi es highe calcina ion and ca bona ion
empe a u es, which leads o accele a ed sin e ing and apid deac i a-
ion o he limes one [11,17]. As a esul , FC-CaCO
3
samples es ed
unde T2 condi ions exhibi a much lowe esidual con e sion o 0.16.
Howe e , his esidual con e sion is s ill g ea e han he alues a ained
o CaCO
3
powde unde simila condi ions in he same appa a us [17].
Recen s udies ha e demons a ed ha ope a ing unde mild acuum
condi ions can mi iga e he deac i a ion caused by sin e ing [47].
Wo king a low p essu e shi s he he modynamic equilib ium o he
CaCO
3
/CaO sys em o a lowe empe a u e, he eby acili a ing calci-
na ion a educed empe a u es. Thus, T3 condi ions in ol e iso he mal
calcina ion a 780 ◦C unde 0.1 ba absolu e p essu e o CO
2
and
ca bona ion a 1 ba o CO
2
. The FC-CaCO
3
sample subjec ed o T3
condi ions demons a es he bes pe o mance, wi h a e y high esidual
con e sion o 0.69. Again, such esidual alue cons i u es a signi ican
imp o emen o e he alue ob ained in a p e ious wo k o CaCO
3
powde unde simila condi ions in he same ins umen [47]. Ne e -
heless, he compa ison should be aken wi h ca e as he pa icle size
used in he p e ious wo k was much la ge . These esul s again
demons a e he imp o ed ca bona ion pe o mance a ained by CaO
ma e ials unde educed p essu e condi ions. While ope a ion unde
acuum implies an addi ional in es men cos and ex a powe con-
sump ion, low p essu e ( acuum) acili ies a e usual a indus y le el.
Fo ins ance, in s eam powe plan s, in e ilize s indus y and in
pe oleum- ela ed ac i i ies. Such expe ience could be ex apola ed o
acili ies o ene gy s o age wi h Calcium Looping.
3.2. Ca bona ion and calcina ion kine ics in he eeze-cas CaO
a chi ec u es
Fig. 5 illus a es he kine ic beha io o he calcina ion and ca bon-
a ion p ocesses o he mos ep esen a i e samples analyzed in his
s udy (FC-CaCO
3
-T1 and FC-CaCO
3
-T3), ocusing on he second and
en h cycles as benchma ks o pe o mance. As discussed in he p e ious
sec ion, CaO ca bona ion obeys a wo-phase p ocess: an ini ial apid
eac ion phase ollowed by a signi ican ly slowe di usion-con olled
s age [19,51]. The con as be ween bo h s ages is e idenced in Fig. 5.
Du ing he eac ion-con olled phase, ca bona ion occu s on he exposed
su ace o CaO, leading o he o ma ion o dispe sed CaCO
3
islands ha
e en ually coalesce in o a con inuous CaCO
3
laye on he su ace o he
CaO pa icles [69,70]. Thus, he ex ension o he ca bona ion du ing his
s age depends on he ex e nal di usion o gas molecules o he so ben 's
su ace and hen he in apa icle and in e pa icle di usion h ough he
po osi y o he so ben . As a esul , i depends on he eac ion empe -
a u e [13], pa ial p essu e o CO
2
[12], pa icle size [60] and po osi y
[27]. Once he su ace CaCO
3
o ms, he eac ion p oceeds h ough a
much slowe solid-s a e di usion mechanism [52]. Highe di usi e
FC-CaCO3-T1
FC-CaCO3-T3
a) b)
d)c)
0 2 4 6 8 1012141618202224
-0.22
-0.20
-0.18
-0.16
-0.14
-0.12
-0.10
-0.08
-0.06
-0.04
-0.02
0.00
dXe /d ime
Time (min)
012345678910
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Xe
Time (min)
FC-CaCO3-T1
FC-CaCO3-T3
FC-CaCO3-T1
FC-CaCO3-T3
012345678910
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
dXe /d ime
Time (min)
024681012141618202224
Xe
Time (min)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Fig. 5. Time e olu ion o CaL p ocess: a) con e sion e olu ion o he ca bona ion p ocess b) con e sion e olu ion o he calcina ion p ocess c) i s de i a i e o he
e ec i e con e sion o he ca bona ion p ocess and d) i s de i a i e o he e ec i e con e sion o he calcina ion p ocess. Con inuous line indica es esul s om
he second cycle and he dashed line om he en h cycle.
N. Amgha e al.
Jou nal o Ene gy S o age 125 (2025) 116681
6
ca bona ion a es a e obse ed in milled samples [70], ma e ials wi h
la ge po es [62], small pa icles [60], s eam [22] and addi i es ha
p omo e he di usion o CO
3
2−
such as mol en ca bona es [71], o sin-
e ing inhibi o addi i es [11].
FC-CaCO
3
-T3 demons a es g ea e eac i i y in his phase han FC-
CaCO
3
-T1, bo h in he second and en h cycle. The di usion con olled
s age o bo h FC-CaCO
3
-T3 and FC-CaCO
3
-T1 exhibi he seemingly la
p o ile ypical o eac ion schemes comp ising calcina ion in ine a -
mosphe e and ca bona ion a high empe a u es in CO
2
[60]. Howe e ,
FC-CaCO
3
-T3 achie es sligh ly as e ca bona ion a e, e idenced by he
slope, wha can p oduce ele an di e ences a longe ca bona ion
imes. Since he s a ing mo phology o he sample is simila , he
enhanced pe o mance can only be a ibu ed o he milde calcina ion
condi ions used in T3, which help mi iga e deac i a ion by sin e ing.
Fu he mo e, s udies ha e shown ha calcina ion a educed p essu e
p oduces CaO wi h e y small c ys alli e size and a la ge su ace a ea
[21,47]. The ac ha he ca bona ion a e emains la gely unchanged
be ween he 2nd o he 10 h ca bona ion cycles sugges s ha he
a o able po e s uc u e achie ed h ough he eeze-cas p ocess is
be e p ese ed in T3. In con as , unde T1 condi ions, he ca bona ion
a e a he 10 h cycle is sligh ly slowe han a he 2nd cycle, indica ing a
g adual e olu ion in he po e s uc u e.
The calcina ion eac ion is he slowes p ocess, equi ing 8–12 min
o comple ion, depending on he condi ions. The calcina ion a e o he
s uc u es is also as e unde T3 condi ions.
3.3. Mo phology s udy o he eeze-cas ing monoli hs
The s udy o mo phology is c i ical o unde s anding he beha io o
he sample in e ms o con e sion ex ension and eac ion kine ics p o-
cess, as e icien gas low wi hin he sample and po e blockage ha e
signi ican in luence. In his con ex , he e olu ion o he con e sion
along he cycles as shown in Fig. 4 a e consequence o mo phological
changes. Fig. 6a and b display SEM images o he FC-CaCO
3
-T1 a cycle
1. The ma e ial appea s composed o pa icles loosely bonded oge he .
Pa icle size is in ag eemen wi h he dis ibu ion shown in Fig. 1. The
con ou s o he indi idual pa icles a e e iden and he deg ee o sin-
e ing is s ill limi ed. The obse ed po osi y is a ibu ed o he eeze-
cas ing ab ica ion me hod and binde decomposi ion du ing he
ini ial p ocessing s age. A e 40 cycles (Fig. 6c and d), no iceable
changes a e e iden in he high-magni ica ion images, e ealing a sig-
ni ican inc ease in pa icles size. This change is a consequence o he
sin e ing p ocess, which leads o a dec ease o su ace nano-sized
po osi y and an inc ease o in e pa icle po e olume [72]. Despi e he
la ge numbe o cycles, indi idual pa icles emain dis inguishable,
ensu ing good con e sion a es and cycle s abili y h oughou he
cycling p ocess. Acco ding o he li e a u e, limes one subjec ed o
20–30 he mal cycles ypically exhibi s a mo e ex ensi e sin e ing, wi h
clea me ging o neighbo ing g ains [24].
XCT obse a ions o he samples subjec ed o one and 40 ca bona ion
cycles con i m he SEM obse a ions and p o ide insigh in o he
mic os uc u al e olu ion du ing cycling. Fig. 7 summa izes hese ob-
se a ions using wo-dimensional sec ions o ease o isualiza ion
( h ee-dimensional olumes a e shown in he Supplemen a y ideo).
Fig. 7a shows he o iginal mic os uc u e o eeze-cas samples,
exhibi ing aniso opic po osi y wi h elonga ed channels along he
eezing di ec ion. O e he cycles, his geome y changes, esul ing in
an iso opic dis ibu ion o he po es along he obse ed su ace, as
shown in Fig. 7d o he sample wi h 40 cycles. These esul s a e
consis en wi h he o uosi y measu emen s p esen ed in he suppo ing
in o ma ion, whe e he alue ob ained in he z-di ec ion dec eases o e
he CaL cycles. The changes obse ed be ween cycles 1 and 40 could be
ela ed o he p e e en ial sh inkage o eeze-cas samples in he eeze
cas ing di ec ion, which educes po e o ien a ion [73].
The po e size and pa icle size dis ibu ions we e quan i ied using a
hickness algo i hm. This algo i hm calcula es, o each poin in he po e
o solid space, he diame e o he la ges sphe e ha i s he po e o
solid and con ains he poin , se ing as a p oxy o de e mining he po e
and pa icle size dis ibu ions. Resul s o hese calcula ions a e shown in
Fig. 7b and c o he po es and solid pa icles, espec i ely, o he
sample subjec ed o 1 cycle. A e 40 cycles, he sample exhibi s a clea
inc ease in po e size, likely due o he sin e ing p ocess occu ing du ing
he cycles a high empe a u es. Calcula ed po e and pa icle size dis-
ibu ions a e p esen ed in Fig. 8, which we e i ed o log-no mal
unc ions o de e mine he a e age po e and pa icle sizes. Compa i-
son o cycle 1 o cycle 40 shows an opening o po osi y, mos likely due
o local sin e ing e ec s in he solid walls. This esul s in inc eased po e
olume ac ion ( om 55 % ol o 63 % ol) and a e age po e size ( om
5.0
μ
m o 9.0
μ
m), as well as a b oadening o he po e size dis ibu ion.
The mean pa icle size also inc eases sligh ly upon cycling, om 4.7
μ
m
10 µm
b)
10 µm
d)
a)
2 µm
c)
2 µm
Fig. 6. Mo phology o he ca bona ed FC-CaCO
3
-T1 monoli h a ×5000 and ×1000 magni ica ion: a) cycle 1 a high magni ica ion, b) cycle 1 a low magni ica ion,
c) cycle 40 a high magni ica ion and d) cycle 40 a low magni ica ion.
N. Amgha e al.
Jou nal o Ene gy S o age 125 (2025) 116681
7
o 6.0
μ
m, which is consis en wi h he e ec o local sin e ing. An in-
c ease in po e size due o sin e ing p ocesses has been epo ed by o he
au ho s in s uc u es p epa ed by eeze cas ing in ce amic ma e ials
[74]. Changes in pa icle size we e also obse ed using XCT in ce amic
samples p epa ed by eeze cas ing [75]. This mo phological e olu ion
may explain he changes in he ca bona ion a e be ween he 2nd and
10 h cycles, as obse ed in Fig. 5c.
I is impo an o no e ha due o he limi ed esolu ion o XCT
compa ed o SEM, wha we e e o as pa icle size in his con ex is no
he p ima y pa icle o c ys alli e size, bu a he pa icle agglome a es
o a ew
μ
m and la ge , such as hose shown in Fig. 6, which indeed
shows quali a i ely he same agglome a e enla gemen upon cycling.
The calcula ed o uosi y along he eezing di ec ion is
τ
1
=2.0 o
cycle 1 and
τ
2
=1.5 o cycle 40, which can be a ionalized in e ms o an
a) b) c)
d) e) )
50 µm 50 µm 50 µm
50 µm 50 µm 50 µm
25 µm
20 µm
15 µm
10 µm
5 µm
0 µm
25 µm
20 µm
15 µm
10 µm
5 µm
0 µm
25 µm
20 µm
15 µm
10 µm
5 µm
0 µm
25 µm
20 µm
15 µm
10 µm
5 µm
0 µm
Fig. 7. Mic os uc u al e alua ion o FC-CaCO
3
-T1 monoli hs by high esolu ion XCT: a) abso p ion con as sec ion a e 1 cycle, b) po e size quan i ica ion a e 1
cycle, c) solid phase quan i ica ion a e 1 cycle, d) abso p ion con as sec ion a e 40 cycles, e) po e size quan i ica ion a e 40 cycles, c) solid phase quan i ica ion
a e 40 cycles. Each oxel is colo ed acco ding o he size o he la ges sphe e ha i s he po e space and con ains he oxel. In all images, he eezing on
di ec ion is e ical in he page.
1 cycle 40 cycles
0 5 10 15 20 25
0
5
10
15
a) b)
Volume ac ion (%)
Po e diame e (µm)
55% po e olume
mean po e d= (5.0 ± 1.7) µm
Po e diame e (µm)
0 5 10 15 20 25
0
5
10
15
Volume ac ion (%)
63% po e olume
mean po e d= (9 ± 4) µm
0 5 10 15 20 25
0
5
10
15
45% solid olume
mean pa icle d= (4.7 ± 1.2) µm
Volume ac ion (%)
Pa icle diame e (µm) Pa icle diame e (µm)
0 5 10 15 20 25
0
5
10
15
37% solid olume
mean pa icle d= (6.0 ± 1.2) µm
Volume ac ion (%)
c) d)
Fig. 8. Dis ibu ion o he po e and solid ac ion o he FC-CaCO
3
-T1 monoli hs: a) po e size and dis ibu ions a e 1 cycle, b) po e size and dis ibu ions a e 40
cycles, c) solid pa icle size and dis ibu ions a e 1 cycle and d) solid pa icle size and dis ibu ions a e 40 cycles.
N. Amgha e al.
Jou nal o Ene gy S o age 125 (2025) 116681
8
inc ease o mic opo osi y and opening o he po e space, which acili-
a es gas low inside he po ous monoli hs. Bo h alues a e qui e low o
mac opo ous s uc u es, con i ming ha he special mic os uc u e ob-
ained by eeze-cas ing con ibu es o as gas di usion and helps
explain he excellen obse ed kine ics.
4. Conclusions
This s udy alida es he easibili y and e ec i eness o eeze-cas ing
o ab ica ing po ous calcium-based monoli hs ailo ed o CaL in he -
mochemical ene gy s o age. By employing poly inyl alcohol as a binde ,
he me hod yielded highly po ous sca olds h ough con olled eezing
in cylind ical molds and subsequen eeze-d ying. Mul i-cycle es s
demons a ed ou s anding pe o mance, wi h he FC-CaCO
3
-T3 mono-
li h achie ing a no able esidual con e sion o 0.69 unde mode a e
acuum calcina ion, su passing con en ional powde -based sys ems.
Monoli hs calcined in ine a mosphe es also a ained a esidual con-
e sion o 0.56, unde sco ing he ad an ages o ope a ing a educed
CO
2
p essu e, which in ol es lowe calcina ion empe a u es, educed
deac i a ion, and sho e calcina ion/ca bona ion imes. SEM and XCT
analyses con i med he o ma ion o a obus po ous ne wo k ha
main ained eac i e su aces and mi iga ed sin e ing o e mul iple cy-
cles. Al hough addi i es we e in oduced, hey did no enhance pe o -
mance, a i ming he inhe en s uc u al s abili y o he monoli hs.
While high- empe a u e exposu e induced modes inc eases in po e
olume and pa icle size, hese changes did no comp omise o e all e -
icacy. Collec i ely, hese indings posi ion eeze-cas ing as a e sa ile,
scalable app oach o de eloping ad anced ma e ials capable o mee ing
indus ial demands o du able, high- empe a u e ene gy s o age.
Supplemen a y da a o his a icle can be ound online a h ps://doi.
o g/10.1016/j.es .2025.116681.
CRediT au ho ship con ibu ion s a emen
Nabil Amgha : In es iga ion, Fo mal analysis. Juan I o a-Ma i-
nez: W i ing – o iginal d a , Fo mal analysis, Da a cu a ion. An onio
Pe ej´
on: Supe ision, Me hodology, Concep ualiza ion. Do ian
Hanao : Supe ision, Me hodology. Aleksande Gu lo: Resou ces.
Joaquín Ramí ez-Rico: Valida ion, Me hodology, Fo mal analysis.
Luis A. P´
e ez-Maqueda: Visualiza ion, Resou ces, P ojec adminis a-
ion, Me hodology, Concep ualiza ion. Ped o E. S´
anchez-Jimenez:
W i ing – e iew & edi ing, W i ing – o iginal d a , Supe ision, Re-
sou ces, P ojec adminis a ion.
Decla a ion o compe ing in e es
The au ho s decla e ha hey ha e no known compe ing inancial
in e es s o pe sonal ela ionships ha could ha e appea ed o in luence
he wo k epo ed in his pape .
Acknowledgmen s
Financial suppo is acknowledged om g an s TED2021-131839B-
C22 and PDC2021-121552-C21 (MCIN/AEI/10.13039/501100011033
and Eu opean Union Nex Gene a ion EU/PRTR) and by Eu opean
Union Nex Gene a ionEU/PRTR, and he g an PID2022-140815OB-
C22 unded by MCIN/AEI/10.13039/501100011033 and ERDF A way
o making Eu ope. PhD Fellowship g an ed o N. Amgha (PRE2018-
085866) om he Spanish Go e nmen Agency Minis e io de Ciencia,
Inno aci´
on y Uni e sidades. J. I o a-Ma inez hanks Gene ali a
Valenciana - GVA o unding a pos doc posi ion h ough he CIAPOS
p og am co- unded by ESF In es ing in you u u e, g an numbe
CIAPOS/2023/362. XCT measu emen s we e pe o med a he X- ay
labo a o y o he Resea ch, Technology and Inno a ion Cen e (CITIUS)
o he Uni e si y o Se ille.
Da a a ailabili y
Da a will be made a ailable on eques .
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