Ci a ion: Bei ia, J.; Ahedo, I.; Pa edes,
J.I.; Goikolea, E.; Ruiz de La amendi,
I. Explo ing Zinc-Doped Manganese
Hexacyano e a e as Ca hode o
Aqueous Zinc-Ion Ba e ies.
Nanoma e ials 2024,14, 1092. h ps://
doi.o g/10.3390/nano14131092
Academic Edi o : Ma ia G azia
Musolino
Recei ed: 25 May 2024
Re ised: 19 June 2024
Accep ed: 23 June 2024
Published: 25 June 2024
Copy igh : © 2024 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
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dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license (h ps://
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nanoma e ials
A icle
Explo ing Zinc-Doped Manganese Hexacyano e a e as Ca hode
o Aqueous Zinc-Ion Ba e ies
Julen Bei ia 1, Isabel Ahedo 1, Juan Ignacio Pa edes 2, Eide Goikolea 1and Idoia Ruiz de La amendi 1,*
1Depa amen o de Química O gánica e Ino gánica, Uni e sidad del País Vasco (UPV/EHU), Ba io Sa iena
s/n, 48940 Leioa, Spain; [email p o ec ed] (J.B.); [email p o ec ed] (I.A.); eide [email p o ec ed] (E.G.)
2
Ins i u o de Ciencia y Tecnología del Ca bono, INCAR-CSIC, C/F ancisco Pin ado Fe 26, 33011 O iedo, Spain;
pa edes@inca .csic.es
*Co espondence: idoia. [email p o ec ed]
Abs ac : Aqueous zinc-ion ba e ies (AZiBs) ha e eme ged as a p omising al e na i e o li hium-ion
ba e ies as ene gy s o age sys ems om enewable sou ces. Manganese hexacyano e a e (MnHCF)
is a P ussian Blue analogue ha exhibi s he abili y o inse di alen ions such as Zn
2+
. Howe e , in
an aqueous en i onmen , MnHCF p esen s weak s uc u al s abili y and su e s om manganese
dissolu ion. In his wo k, zinc doping is explo ed as a s a egy o p o ide he s uc u e wi h highe
s abili y. Thus, h ough a simple and easy- o-implemen app oach, i has been possible o imp o e
he s abili y and capaci y e en ion o he ca hode, al hough a he expense o educing he speci ic
capaci y o he sys em. By co ec ly balancing he amoun o zinc in oduced in o he MnHCF i
is possible o each a comp omise in which he loss o capaci y is no c i ical, while be e cycling
s abili y is ob ained.
Keywo ds: zinc; P ussian Blue analogue; ca hode; aqueous ba e y
1. In oduc ion
Nowadays, one o he mos c i ical challenges in ou socie y is o enhance he use o
na u al and enewable ene gy sou ces ha a e almos inexhaus ible and do no gene a e
any en i onmen al deg ada ion. The mos impo an enewable ene gies a e sola , wind,
ma ine, and geo he mal. As an ad an age, hese al e na i e ene gy o ms allow socie y o
ob ain subs an ial amoun s o ene gy wi hou pollu ing he en i onmen wi h g eenhouse
gas emissions among o he s. Howe e , hese sou ces depend on geog aphy, clima e, and
ime o day, so i is impo an o be able o s o e ene gy su pluses in an e icien way ha
allows hei use la e when he p oduc ion goes down [
1
–
3
]. To o e come he p oblem
ela ed o hei in e mi en na u e, esea ch e o s should also ocus on he de elopmen
o new ene gy s o age sys ems ha help managing he demand/supply dynamics.
Among he a ious ene gy s o age sys ems, ba e ies a e pa icula ly impo an . These
sys ems can s o e ene gy in he o m o a chemical eac ion and la e con e i back in o
elec ic cu en . Li hium ion ba e ies (LIBs) we e success ully ma ke ed by SONY in 1991,
and e e since hen, his ba e y echnology has s ood ou o i s ema kable g a ime ic
and olume ic ene gy densi ies and excellen cycle li e compa ed o o he echa geable
ba e ies. Howe e , his echnology has a numbe o signi ican d awbacks such as he
limi ed access o li hium ese es, he high cos o he componen s, and sa e y conce ns
ela ed o he lammable na u e o o ganic elec oly es [
4
]. Mo eo e , he massi e and
apid g ow h o he elec ic ehicle ma ke is incen i ising he de elopmen o al e na i e
echnologies o LIBs, no only o mee he needs o he po able ene gy s o age ma ke bu
also hose o s a iona y applica ions.
In ecen yea s esea ch has ocused on aqueous sys ems as an al e na i e o li hium-
based sys ems due o he need o en i onmen ally iendly, sa e, and cos -e ec i e de-
ices [
5
,
6
]. The e o e, ba e y sys ems ope a ing in an aqueous elec oly e a e p e e able o
Nanoma e ials 2024,14, 1092. h ps://doi.o g/10.3390/nano14131092 h ps://www.mdpi.com/jou nal/nanoma e ials
Nanoma e ials 2024,14, 1092 2 o 15
hose wo king in o ganic ones. Aqueous elec oly es exhibi in e es ing ad an ages such as
high ionic conduc i i y, in addi ion o being easie o assemble since he need o a con olled
a mosphe e is elimina ed. Among he di e en aqueous sys ems, aqueous zinc-ion ba e ies
(AZiBs) s and ou o hei unique p ope ies linked o he high abundance o me allic zinc,
eco- iendliness, in insic sa e y, and cos -e ec i eness, making hem a p omising choice
o la ge-scale enewable ene gy s o age applica ions [7,8].
AZiBs use me allic zinc in he o m o a oil o as powde o ming composi e elec ode,
which will oxidize h oughou he discha ge eleasing Zn
2+
ions. These ions will pass
h ough he nea -neu al (o mildly acidic) aqueous elec oly e eaching he ca hode whe e
he Zn
2+
ions will be e e sibly inse ed in o he ac i e ma e ial, hanks o he educ ion o
some elec oac i e elemen s ha make i up. Th oughou he cha ge, he e e se p ocess
occu s, wi h zinc being elec odeposi ed on he su ace o he anode. Du ing cycling, some
issues can occu which a e due o inhomogeneous deposi ion, which can o m me allic Zn
dend i es and sho -ci cui he sys em by he pe o a ion o he sepa a o o he elease
o hyd ogen in he anode (Hyd ogen E olu ion Reac ion, HER) which is o med in he
po en ials whe e Zn is elec odeposi ed. I should be no ed ha he o ma ion o his
gaseous hyd ogen is one o he mos impo an issues in he de elopmen o AZiBs and
needs o be s udied in mo e de ail. Recen s udies ha e shown ha he con ol o gas elease
is possible h ough con ol o he pH o he elec oly e o using addi i es in solu ion [
9
,
10
].
Despi e ce ain issues ha s ill need o be sol ed, he comme cializa ion o hese
Zn-ion-based ba e ies becomes easible as a sui able, cheap, and en i onmen ally iendly
sys em ha ing high capaci y and s abili y in he long e m [
11
,
12
]. The selec ion o a ca hode
wi h a high wo king ol age and a la ge speci ic capaci y is a c i ical pa ame e o inc ease
he g a ime ic ene gy densi y o AZiBs and o be compe i i e wi h he LIBs [
13
]. In ac ,
he design and de elopmen o ca hode ma e ials wi h high s o age capaci y, high discha ge
po en ial, and a obus c ys alline s uc u e wi h easy inse ion and emo al pa hways
has been a g ea challenge in he de elopmen o high-pe o mance AZiBs. Among he
mos s udied ma e ials, oxides such as MnO
2
o V
2
O
5
and P ussian Blue analogues (PBAs)
a e ound. Conce ning he oxides, MnO
2
p esen s a good cycle li e, bu he low ionic and
elec onic conduc i i ies limi i s use, and in he case o anadium-based oxides, p omising
capaci ies and good cycle li e a e eached bu hei low ope a ing po en ial leads o low
ene gy densi y o he inal sys em [
14
–
16
]. The mixed- alence hexacyano e a e amily
(P ussian Blue and i s analogues) is ano he g oup o ma e ials ha is o special in e es as
ca hodes in AZiBs. Thei open s uc u e, combined wi h he wide a ie y o me als ha
can be pa o he s uc u e, allows he elec ochemical p ope ies o he ma e ial o be
uned [
17
,
18
]. These phases ha e high wo king po en ials, al hough hei speci ic capaci y
is limi ed and hey p esen poo cycling s abili y [13].
PBAs ha e a ac ed a en ion in he las ew yea s in he ield o ene gy s o age due
o hei easy and inexpensi e syn hesis p ocedu e by cop ecipi a ion, high speci ic capaci y
o he e e sible inse ion o me allic ions, hei high sa e y and non oxici y, and he
elec ochemical p ope ies ha can be uned h ough he a ia ion o he ma e ial composi-
ion [
19
,
20
]. These ma e ials came om he ancien disco e ed blue pigmen P ussian Blue
(PB; Fe
4
[Fe(CN)
6
]
3
) which is one o he oldes syn he ic coo dina ion polyme s. O e he
las cen u y, PB has been modi ied by doping he i on si es wi h o he ansi ion me als
such as Mn, Ni, Co, o Zn among o he s, o ob ain analogues (PBAs) and mee di e en
applica ion and esea ch equi emen s [
19
,
21
]. The gene al chemical o mula o PBAs is
A
a
TM
A
[TM
B
(CN)
6
]
n·
xH
2
O whe e A is he alkali me al ion ha can be sodium (Na
+
) o
po assium ions (K
+
), while TM
A
and TM
B
a e he ansi ion me al ions which eplace i on in
PB. In he s uc u e o PBAs (see Figu e 1), TM
A
is coo dina ed o he ni ogen a om o he
cyanide ion and TM
B
is su ounded by he ca bon a om o he cyanide ion. This coo dina-
ion o he cyanide g oup makes PBAs compounds exhibi a 3D open s uc u e wi h many
in e s i ial si es whe e zinc ions can be e e sely inse ed in he ba e y cycling p ocess.
Nanoma e ials 2024,14, 1092 3 o 15
Nanoma e ials 2024, 14, 1092 3 o 16
he cyanide ion and TMB is su ounded by he ca bon a om o he cyanide ion. This coo -
dina ion o he cyanide g oup makes PBAs compounds exhibi a 3D open s uc u e wi h
many in e s i ial si es whe e zinc ions can be e e sely inse ed in he ba e y cycling p o-
cess.
Figu e 1. S uc u e model o a P ussian Blue analogue (PBA).
Among he wide a ie y o possible PBAs, one o he mos s udied amilies is he
manganese-based hexacyano e a es (MnHCF) since i bene i s om he exis ence o wo
edox couples (Fe3+/Fe2+ and Mn3+/Mn2+) deli e ing la ge speci ic capaci ies [17,18,22]. The
cha ge s o age mechanism h ough Zn2+ inse ion/ex ac ion in AZiBs using MnHCF as
ca hodes ollows he eac ion (1):
x Zn2+ + MnHCF + 2x e− ⇌ ZnxMnHCF (1)
Du ing he discha ge, he Zn2+ ions p oduced in he oxida ion o he zinc anode a e
inse ed in o he MnHCF s uc u e h ough he educ ion o he wo ac i e edox couples.
Du ing cha ging, he e e se p ocess occu s, e u ning he Zn2+ ions ex ac ed om he
ca hode o he anode whe e hey will be educed o me allic zinc. The amoun o zinc ha
can be e e sibly inse ed depends on many ac o s such as wa e con en o pa icle size,
among o he s. The main p oblem ha MnHCF aces is ela ed o i s s uc u al ins abili y
in aqueous medium. Al hough in he i s cycles, MnHCF is capable o p o iding speci ic
capaci ies as high as 140 mAh g−1 (a cu en densi ies o 100 mA g−1) [23], he ma e ial
suffe s om s uc u al ins abili y upon cycling, o ming new zinc- ich phases and, ul i-
ma ely, dissol ing he ca hode ma e ial [24]. To a oid hese disad an ages, diffe en ap-
p oaches ha e been in es iga ed such as he p epa a ion o MnHCFs ancho ing MnO2
[25], he design o hyb id sys ems such as MnHCF/polyme [23,26], he nanos uc u ing
o he MnHCF h ough he use o su ac an s du ing he syn hesis [27], o adjus ing he
elec oly e o mula ion [28,29]. In his wo k, a diffe en s a egy is explo ed ha is based
on doping he MnHCF phase wi h zinc in he manganese posi ion. In his way, he aim is
o p o ide highe s uc u al s abili y o he ma e ial ha allows i s implemen a ion in
AZiBs. This app oach allows he p epa a ion o ma e ials h ough a simple and en i on-
men ally iendly syn he ic ou e.
In his wo k, elec ochemical beha iou in Zn chemis y o K(Mn1-xZnx)[Fe(CN)6] (x
= 0, 0.25, 0.5, 0.75, and 1) PBAs is s udied. This new amily o zinc-doped manganese-
based PBAs was ob ained by cop ecipi a ion h ough con olling he d opping speed and
he eac an s’ concen a ion. The s uc u e and mo phology o he ob ained ma e ials we e
s udied in o de o examine he effec o zinc doping. Also, one o he mos impo an
Figu e 1. S uc u e model o a P ussian Blue analogue (PBA).
Among he wide a ie y o possible PBAs, one o he mos s udied amilies is he
manganese-based hexacyano e a es (MnHCF) since i bene i s om he exis ence o wo
edox couples (Fe
3+
/Fe
2+
and Mn
3+
/Mn
2+
) deli e ing la ge speci ic capaci ies [
17
,
18
,
22
].
The cha ge s o age mechanism h ough Zn
2+
inse ion/ex ac ion in AZiBs using MnHCF
as ca hodes ollows he eac ion (1):
x Zn2+ + MnHCF + 2x e−⇌ZnxMnHCF (1)
Du ing he discha ge, he Zn
2+
ions p oduced in he oxida ion o he zinc anode a e
inse ed in o he MnHCF s uc u e h ough he educ ion o he wo ac i e edox couples.
Du ing cha ging, he e e se p ocess occu s, e u ning he Zn
2+
ions ex ac ed om he
ca hode o he anode whe e hey will be educed o me allic zinc. The amoun o zinc ha
can be e e sibly inse ed depends on many ac o s such as wa e con en o pa icle size,
among o he s. The main p oblem ha MnHCF aces is ela ed o i s s uc u al ins abili y
in aqueous medium. Al hough in he i s cycles, MnHCF is capable o p o iding speci ic
capaci ies as high as 140 mAh g
−1
(a cu en densi ies o 100 mA g
−1
) [
23
], he ma e ial su -
e s om s uc u al ins abili y upon cycling, o ming new zinc- ich phases and, ul ima ely,
dissol ing he ca hode ma e ial [
24
]. To a oid hese disad an ages, di e en app oaches
ha e been in es iga ed such as he p epa a ion o MnHCFs ancho ing MnO
2
[
25
], he
design o hyb id sys ems such as MnHCF/polyme [
23
,
26
], he nanos uc u ing o he
MnHCF h ough he use o su ac an s du ing he syn hesis [
27
], o adjus ing he elec-
oly e o mula ion [
28
,
29
]. In his wo k, a di e en s a egy is explo ed ha is based on
doping he MnHCF phase wi h zinc in he manganese posi ion. In his way, he aim is o
p o ide highe s uc u al s abili y o he ma e ial ha allows i s implemen a ion in AZiBs.
This app oach allows he p epa a ion o ma e ials h ough a simple and en i onmen ally
iendly syn he ic ou e.
In his wo k, elec ochemical beha iou in Zn chemis y o K(Mn
1−x
Zn
x
)[Fe(CN)
6
]
(x = 0, 0.25, 0.5, 0.75, and 1) PBAs is s udied. This new amily o zinc-doped manganese-
based PBAs was ob ained by cop ecipi a ion h ough con olling he d opping speed and
he eac an s’ concen a ion. The s uc u e and mo phology o he ob ained ma e ials we e
s udied in o de o examine he e ec o zinc doping. Also, one o he mos impo an
goals o his wo k is o be able o op imize he pe o mance o each o he sys ems ha a e
used, and es ing and inding he mos sui able ma e ial o achie e g ea e elec ochemical
pe o mance. The new amily o zinc-doped manganese-based PBAs will be ob ained
by he cop ecipi a ion me hod h ough con olling he d opping speed and he eac an s’
concen a ion. The s uc u e and mo phology o he ob ained ma e ials will be s udied in
o de o examine he e ec o zinc doping. Finally, he elec ochemical pe o mance o he
Nanoma e ials 2024,14, 1092 4 o 15
ma e ials will be examined del ing in o he ole o zinc on he beha io o MnHCF and i s
s uc u al s abili y.
2. Ma e ials and Me hods
2.1. Syn hesis o he PBA Compounds
The syn hesis o K(Mn
1−x
Zn
x
)[Fe(CN)
6
] (x = 0, 0.25, 0.5, 0.75, and 1), o Mn
1−x
Zn
x
HCF,
compounds was ca ied ou by cop ecipi a ion me hod. Fo he syn hesis, s oichiome ic
quan i ies o he eagen s we e mixed as de ailed in Table 1. In his ega d, ZnSO
4
(Sigma-
Ald ich, S . Louis, MO, USA) and Mn(NO
3
)
2
(Sigma-Ald ich, S . Louis, MO, USA) we e
sepa a ely dissol ed in dis illed wa e (100 mL) a oom empe a u e. On he o he hand,
ano he solu ion was simul aneously p epa ed dissol ing K
3
Fe(CN)
6
(0.002 mol, Sigma-
Ald ich, S . Louis, MO, USA) in dis illed wa e (100 mL) a oom empe a u e oo. Once all
he eagen s we e comple ely dissol ed, PBAs we e syn hesized by mixing d opwise he
solu ions (Figu e S1), which ga e a whi e-colou ed solid in suspension in a solu ion wi h a
pH o 5–6.
Table 1. Syn heses ha we e ca ied ou in he wo k.
Sample ZnSO4(mol)
Mn(NO
3
)
2
(mol)
K3Fe(CN)6(mol) Theo e ical Ra io (Zn/Mn)
Zn100 0.002 - 0.002 100:0
Zn75Mn25 0.0015 0.0005 0.002 75:25
Zn50Mn50 0.001 0.001 0.002 50:50
Zn25Mn75 0.0005 0.0015 0.002 25:75
Mn100 - 0.002 0.002 0:100
2.2. Physico-Chemical Cha ac e iza ion o he PBAs
The iden i ica ion and he s uc u al cha ac e iza ion o he compounds we e ca ied
ou by X- ay di ac ion (XRD, Panaly ical X’Pe PRO) be ween 5 and 70
◦
(2
θ
) using Cu
adia ion (Cu-K
αaλ
= 0.15418 nm). Also, a enua ed o al e lec ion in a ed spec oscopy
(FTIR) was pe o med using Shimadzu FTIR-8400S (Kyo o, Japan) equipmen o analyse
di ec ly he p e-cycling ma e ials wi hou any p e- ea men . The mo phology o he ini ial
samples was s udied by scanning elec on mic oscopy (SEM, Hi achi S-4800, Tokyo, Japan).
2.3. P epa a ion o Elec odes
The posi i e elec ode composi ion was 70 w .% PBA, 20 w .% conduc i e Ke jen
black ca bon, and 10 w .% poly e a luo oe hylene; PTFE, om a 60 w .% dispe sion in
H
2
O (Sigma-Ald ich, S . Louis, MO, USA). Nega i e elec ode composi ion was 99.9 w .%
ace zinc ( hickness 0.25 mm, Sigma-Ald ich, S . Louis, MO, USA). This mix u e was
d enched in e hanol and blended un il he plas ici y was good enough o ob ain a hin black
ilm (150
µ
m) on ca bon pape (H23C6; Quin ech B enns o zellen Technolgie, Göppingen,
Ge many). This hin ilm was le o d y in a acuum o en a 80
◦
C o 24 h, and a e
d ying he lamina es we e cu in o 11 mm diame e ci cula elec odes and inally, weighed
and labelled.
2.4. Elec ochemical Cha ac e iza ion
The di e en samples we e elec ochemically es ed using a mul ichannel po en io-
s a /gal anos a VMP3 (BioLogic, Seyssine -Pa ise , F ance) pe o ming cyclic ol am-
me y measu emen s a scan a es o 0.1, 0.2, 0.3, 0.5, 0.8, and 1 mV s
−1
. Gal anos a ic
cha ge/discha ge measu emen s using di e en cu en densi ies o 0.1, 0.2, 0.3, 0.5, 1, and
2Ag−1we e pe o med be ween 0.005–2.0 V s. Zn2+/Zn.
In he case o he cyclic ol amme y measu emen s, he assembly o he elec ochemi-
cal cells was ca ied ou in h ee-elec ode Swagelok
®
cell sys ems (Swagelok
®
, Solon, OH,
USA) using s ainless s eel cu en collec o s, Mn
1−x
Zn
x
HCF-based elec odes as wo king
elec odes, me allic zinc disc as he coun e , a Ag/AgCl in sa u a ed KCl as he e e ence
Nanoma e ials 2024,14, 1092 5 o 15
elec ode, and a po ous glass ib e (Wha man GF/A, Maids one, UK) memb ane as he
sepa a o wi h a 3 M zinc i luo ome hanesul ona e (Zn(OT )
2
) (Sigma-Ald ich, S . Louis,
MO, USA) in dis illed wa e as he elec oly e.
Fo gal anos a ic measu emen s, Mn
1−x
Zn
x
HCF compounds we e es ed in wo-
elec ode Swagelok
®
cell sys ems (Swagelok
®
, Solon, OH, USA) using s ainless s eel cu en
collec o s, a me allic zinc disc as bo h he coun e and he e e ence elec ode and a po ous
glass ib e (Wha man GF/A, Maids one, UK) memb ane as he sepa a o . In all cases, 3 M
zinc i luo ome hanesul ona e (Zn(OT )
2
) (Sigma-Ald ich, S . Louis, MO, USA) dissol ed
in dis illed wa e was used as he elec oly e.
3. Resul s and Discussion
A new amily o PBAs wi h di e en Zn and Mn con en s, Mn
1−x
Zn
x
HCF (x = 0,
0.25, 0.5, 0.75, and 1), has been syn hesised and cha ac e ised. PBAs can c ys allise in
di e en c ys al s uc u es depending on ac o s such as he syn hesis p ocess, he ype
o ansi ion me al, o he deg ee o zinc inse ion. Among he mos common s uc u es
in he li e a u e cubic, o ho hombic, hombohed al, and monoclinic a e ound [
30
–
32
].
Figu e 2a shows he di ac og ams ob ained o he syn hesised samples oge he wi h he
heo e ical posi ions o he main di ac ion peaks o he cubic (space g oup: Fm-3m) and
hombohed al (space g oup: R-3c) s uc u es. Using he cop ecipi a ion syn hesis ou e, in
he case o he manganese sample (Mn100), he di ac ion peaks a e well-de ined and sha p
and he di ac ion pa e n ma ches wi h he cubic s uc u e, in good ag eemen wi h he
s uc u e epo ed by o he au ho s in he li e a u e [33–37]. The di ac ion pa e n o he
Zn100 sample i s a hombohed al/hexagonal s uc u e (R-3c), as has been desc ibed in he
bibliog aphy [
35
,
38
,
39
], and no impu i ies a e obse able. The samples doped wi h Zn in
he Mn si e p esen a di ac ion pa e n co esponding o a cubic s uc u e, al hough as he
Zn con en inc eases, he p esence o a small peak a ound 16.2
◦
(2
θ
), accompanied by o he
weak signals a highe angles can be de ec ed. This new peak appea s a an angle e y close
o he 100% in ensi y peak o he hombohed al phase co esponding o he Zn100 sample.
The p esence o his peak can be explained based on he p esence o a small concen a ion
o a seconda y phase wi h a hombohed al s uc u e oge he wi h a p e e en ial phase
wi h a cubic s uc u e.
Nanoma e ials 2024, 14, 1092 6 o 16
su ely on he su ace and e en in he open si es o he s uc u e o he compounds. In he
cen al pa o he in a ed spec um, be ween 2300 and 1200 cm
−1
, ou signals can be
dis inguished. The peaks a ound 2100 cm
−1
co espond o he cyanide g oup (C≡N) and
he peak a ound 1600 cm
−1
is he H-O-H bond bending ib a ions o wa e molecules [43].
Finally, in he egion be ween 1400 and 1000 cm
−1
, he bands ela ed o ino ganic com-
pounds a e obse ed, and in some cases, in he zone be ween 1000 and 500 cm
−1
, he bands
co esponding o unc ional g oups bond o he me al can be de ec ed. In his case, wo
signals ha co espond o he bonding o he me al wi h he cyanide g oup a e obse ed.
F om he analysis o he spec a, wo impo an ea u es a e highligh ed. On he one hand,
he exis ence o signals clea ly assignable o wa e molecules makes clea he p esence o
wa e in he compounds. On he o he hand, he change in he in ensi y a io o he bands
a ibu ed o he s e ching ib a ion peak o he b idging cyano g oup (–CN), sugges s a
change in he coo dina ion en i onmen o his g oup depending on he Zn/Mn a io. In
he Zn100 sample, a single peak can be seen ha , as he amoun o Mn inc eases, loses
in ensi y, and a new peak appea s a sligh ly highe wa enumbe alues (blue shi ). The
posi ions o he bands co esponding o he cyano g oup assigned o he Fe
III
—CN—M
II
and Fe
II
—CN—M
II
chains a e a ound 2160 and 2080 cm
−1
, espec i ely, o he samples
ha ha e manganese in hei composi ion. In he case o he Zn100 phase, a single band
ela i e o he Fe
II
-C≡N-M
III
bonds is obse ed, loca ed a 2100 cm
−1
[45,46]. The shi ap-
p ecia ed in he bands is due o he change in he elec onic s a e o he ca ions linked o
he N, which unde go a ansi ion om high-spin (MII) o low-spin (MIII) [47].
(a) (b)
Figu e 2. (a) XRD pa e ns and (b) in a ed spec a o Mn
1-x
Zn
x
HCF (x = 0, 0.25, 0.5, 0.75, and 1)
samples.
The effec o Zn doping a he Mn si e on he mo phology o he ma e ials was ana-
lyzed by SEM and he eco ded images a e shown in Figu e 3. The Mn100 sample exhibi s
a cubic shape wi h pa icles o diffe en sizes, be ween 50 nm and 3 μ m, as has been
widely epo ed in he li e a u e [26,33,48,49]. The solubili y cons an o he
K
2
Mn[Fe(CN)
6
] phase (K
sp
= 10
−12.1
) is e y low [50], which causes his compound o p e-
cipi a e e y easily and quickly in solu ion, gi ing ise o pa icles wi hou a de ined
shape. When he p ecipi a ion p ocess is con olled by adding, o example, chela ing
agen s o slowing down he o ma ion o he p oduc , i is possible o ob ain cubic-shaped
pa icles wi h ew de ec s [51]. In he syn hesis p ocess o he PBAs p esen ed in his wo k,
Figu e 2. (a) XRD pa e ns and (b) in a ed spec a o Mn
1−x
Zn
x
HCF (x = 0, 0.25, 0.5, 0.75, and 1) samples.
Nanoma e ials 2024,14, 1092 6 o 15
In his way, he e ec o zinc is e iden , dis o ing he cubic s uc u e ha hese
ypes o compounds usually exhibi , al hough zinc-based PBAs wi h cubic s uc u e ha e
also been epo ed [
40
]. The ype o s uc u e exhibi ed by hese ypes o compounds is
highly dependen on he syn hesis condi ions [
40
,
41
]. A sligh change in he p epa a ion
empe a u e o he pH o he medium can easily cause a change in he s uc u e. In o de
o a oid his ype o e ec , all he samples in his wo k ha e been syn hesised unde he
same condi ions, ying o a oid p oducing s uc u al o mo phological changes beyond
hose imposed by he change in phase composi ion.
Going on wi h he s uc u al s udy, he analysis and in e p e a ion o he FTIR spec a
p esen ed in Figu e 2b ha e been ca ied ou [
42
–
44
]. The eco ded spec a show one
majo signal in he 3700–3000 cm
−1
ange ela ed o he H
2
O bands (H-O ensile ib a ion
peaks), ypical o hese compounds [
34
]. This indica es he p esence o wa e molecules,
su ely on he su ace and e en in he open si es o he s uc u e o he compounds. In he
cen al pa o he in a ed spec um, be ween 2300 and 1200 cm
−1
, ou signals can be
dis inguished. The peaks a ound 2100 cm
−1
co espond o he cyanide g oup (C
≡
N) and
he peak a ound 1600 cm
−1
is he H-O-H bond bending ib a ions o wa e molecules [
43
].
Finally, in he egion be ween 1400 and 1000 cm
−1
, he bands ela ed o ino ganic com-
pounds a e obse ed, and in some cases, in he zone be ween 1000 and 500 cm
−1
, he bands
co esponding o unc ional g oups bond o he me al can be de ec ed. In his case, wo
signals ha co espond o he bonding o he me al wi h he cyanide g oup a e obse ed.
F om he analysis o he spec a, wo impo an ea u es a e highligh ed. On he one hand,
he exis ence o signals clea ly assignable o wa e molecules makes clea he p esence o
wa e in he compounds. On he o he hand, he change in he in ensi y a io o he bands
a ibu ed o he s e ching ib a ion peak o he b idging cyano g oup (–CN), sugges s
a change in he coo dina ion en i onmen o his g oup depending on he Zn/Mn a io.
In he Zn100 sample, a single peak can be seen ha , as he amoun o Mn inc eases, loses
in ensi y, and a new peak appea s a sligh ly highe wa enumbe alues (blue shi ). The
posi ions o he bands co esponding o he cyano g oup assigned o he Fe
III
—CN—M
II
and Fe
II
—CN—M
II
chains a e a ound 2160 and 2080 cm
−1
, espec i ely, o he samples
ha ha e manganese in hei composi ion. In he case o he Zn100 phase, a single band
ela i e o he Fe
II
-C
≡
N-M
III
bonds is obse ed, loca ed a 2100 cm
−1
[
45
,
46
]. The shi
app ecia ed in he bands is due o he change in he elec onic s a e o he ca ions linked o
he N, which unde go a ansi ion om high-spin (MII) o low-spin (MIII) [47].
The e ec o Zn doping a he Mn si e on he mo phology o he ma e ials was analyzed
by SEM and he eco ded images a e shown in Figu e 3. The Mn100 sample exhibi s a
cubic shape wi h pa icles o di e en sizes, be ween 50 nm and 3 µm, as has been widely
epo ed in he li e a u e [
26
,
33
,
48
,
49
]. The solubili y cons an o he K
2
Mn[Fe(CN)
6
]
phase (K
sp
= 10
−12.1
) is e y low [
50
], which causes his compound o p ecipi a e e y
easily and quickly in solu ion, gi ing ise o pa icles wi hou a de ined shape. When he
p ecipi a ion p ocess is con olled by adding, o example, chela ing agen s o slowing
down he o ma ion o he p oduc , i is possible o ob ain cubic-shaped pa icles wi h
ew de ec s [
51
]. In he syn hesis p ocess o he PBAs p esen ed in his wo k, special
ca e has been aken o con ol he eac ion ime, adding he eagen s simul aneously and
slowly, d op by d op, in o de o con ol he nuclea ion and g ow h o he pa icles in
an e icien and simple way. In his case, i is c i ical o con ol he syn hesis condi ions
since hey ha e a g ea impac bo h on he s uc u e o he compound and on he size
and mo phology o he pa icles. Lee and Huh al eady demons a ed ha by inc easing
he concen a ion o HNO
3
in he syn hesis o KFe
III
[Fe
II
(CN)
6
, he pa icle mo phology
e ol ed om cubes o s a -like hexapods [
41
]. This mo phological change allowed hem
o de e mine ha he oxida ion p ocesses began in he co ne s o he cubes, which ac as
ac i e si es. Zhang e al. ca ied ou a simila s udy on zinc hexacyano e a e, and by
adjus ing he concen a ion o eagen s and con olling he d opping speed, pa icles wi h
di e en polyhed al shapes we e p epa ed: cuboc ahed ons, unca ed oc ahed on, and
oc ahed on zinc hexacyano e a es [
40
]. In ac , i has been epo ed ha in he case o
Nanoma e ials 2024,14, 1092 7 o 15
ce ain zinc-based PBAs, he cubic s uc u e is no s able, since wa e molecules can lea e
he s uc u e ela i ely easily, gi ing ise o a ans o ma ion owa ds a hombohed al
phase [
52
,
53
]. In ou case, Zn100 pa icles show a cuboc ahed al shape, e y di e en
om ha o Mn100 pa icles. Fu he mo e, he pa icles appea o be co e ed wi h small
agmen s, p esen ing ough su aces, unlike he Mn100 sample in which he su aces o he
sides o he cubes a e qui e smoo h. As he zinc a io o he samples inc eases, he shape o
he pa icles e ol es om cubes o cuboc ahed ons. As de e mined in he s uc u al s udy,
he samples end o main ain he s uc u e and shape imposed by manganese in he PBAs,
bu a high Zn con en s, he compound is o ced o eadap , aking on mo e impo ance he
cha ac e is ics o he Zn100 compound. Thus, while he Zn25Mn75 sample is made up o
cubic pa icles wi h an edge o a ound 100 nm, he pa icles o he Zn75Mn25 sample show
a clea ly cuboc ahed al shape, wi h smoo h and clean su aces.
Nanoma e ials 2024, 14, 1092 7 o 16
special ca e has been aken o con ol he eac ion ime, adding he eagen s simul ane-
ously and slowly, d op by d op, in o de o con ol he nuclea ion and g ow h o he pa -
icles in an efficien and simple way. In his case, i is c i ical o con ol he syn hesis con-
di ions since hey ha e a g ea impac bo h on he s uc u e o he compound and on he
size and mo phology o he pa icles. Lee and Huh al eady demons a ed ha by inc eas-
ing he concen a ion o HNO
3
in he syn hesis o KFe
III
[Fe
II
(CN)
6
, he pa icle mo phology
e ol ed om cubes o s a -like hexapods [41]. This mo phological change allowed hem
o de e mine ha he oxida ion p ocesses began in he co ne s o he cubes, which ac as
ac i e si es. Zhang e al. ca ied ou a simila s udy on zinc hexacyano e a e, and by ad-
jus ing he concen a ion o eagen s and con olling he d opping speed, pa icles wi h
diffe en polyhed al shapes we e p epa ed: cuboc ahed ons, unca ed oc ahed on, and
oc ahed on zinc hexacyano e a es [40]. In ac , i has been epo ed ha in he case o
ce ain zinc-based PBAs, he cubic s uc u e is no s able, since wa e molecules can lea e
he s uc u e ela i ely easily, gi ing ise o a ans o ma ion owa ds a hombohed al
phase [52,53]. In ou case, Zn100 pa icles show a cuboc ahed al shape, e y diffe en om
ha o Mn100 pa icles. Fu he mo e, he pa icles appea o be co e ed wi h small ag-
men s, p esen ing ough su aces, unlike he Mn100 sample in which he su aces o he
sides o he cubes a e qui e smoo h. As he zinc a io o he samples inc eases, he shape
o he pa icles e ol es om cubes o cuboc ahed ons. As de e mined in he s uc u al
s udy, he samples end o main ain he s uc u e and shape imposed by manganese in
he PBAs, bu a high Zn con en s, he compound is o ced o eadap , aking on mo e
impo ance he cha ac e is ics o he Zn100 compound. Thus, while he Zn25Mn75 sample
is made up o cubic pa icles wi h an edge o a ound 100 nm, he pa icles o he
Zn75Mn25 sample show a clea ly cuboc ahed al shape, wi h smoo h and clean su aces.
Figu e 3. SEM images o Mn
1-x
Zn
x
HCF (x = 0, 0.25, 0.5, 0.75 and 1) samples.
Indeed, he composi ion o he phases is i al o de e mine he elec ochemical p op-
e ies o he ma e ial, bu i has been shown ha he mo phology o he pa icles also has
an impo an impac on he pe o mance o he ca hode. Speci ically, he cuboc ahed al
shape o he Zn-based PBAs p o ides la ge discha ge capaci ies a high a es, in addi ion
o exhibi ing g ea e s abili y in cycling [40]. This ac has been ela ed o he high su ace
a ea ha his shape o he pa icles p esen s. In he case o he compounds in his s udy, a
change in he mo phology o he pa icles is obse ed, al hough he e does no seem o be
an effec on he pa icle size wi h he zinc con en .
Figu e 3. SEM images o Mn1−xZnxHCF (x = 0, 0.25, 0.5, 0.75 and 1) samples.
Indeed, he composi ion o he phases is i al o de e mine he elec ochemical p op-
e ies o he ma e ial, bu i has been shown ha he mo phology o he pa icles also has
an impo an impac on he pe o mance o he ca hode. Speci ically, he cuboc ahed al
shape o he Zn-based PBAs p o ides la ge discha ge capaci ies a high a es, in addi ion
o exhibi ing g ea e s abili y in cycling [
40
]. This ac has been ela ed o he high su ace
a ea ha his shape o he pa icles p esen s. In he case o he compounds in his s udy, a
change in he mo phology o he pa icles is obse ed, al hough he e does no seem o be
an e ec on he pa icle size wi h he zinc con en .
Be o e in eg a ing he p epa ed ma e ials in o AZiBs, he eac ion kine ics o each
compound we e s udied by means o cyclic ol amme y (CV) measu emen s a di e en
scan a es om 0.1 o 1.0 mV s
−1
. I should be no ed ha in he case o Zn100 sample,
he CV ob ained does no show any peak (Figu e S2) and he e o e i s elec ochemical
ac i i y is p ac ically ze o, in good ag eemen wi h esul s epo ed by o he au ho s [
54
,
55
].
Also, as can be seen in Figu e 4, in he case samples wi h high amoun s o Mn (Mn100
and Zn25Mn75), CV cu es show wo majo peaks, which main ain he shape du ing he
expe imen a di e en a es. The peaks ha appea a 0.95/0.98 V s. Ag/AgCl du ing
oxida ion and a ca. 0.90 V du ing educ ion co espond o he Mn
3+
/Mn
2+
edox pai and,
as expec ed, as he scan a e inc eases he in ensi y o he peak inc eases, pe cei ing a sligh
shi owa ds highe po en ials in he oxida ion b anch and owa ds lowe alues in he
educ ion b anch. By inc easing he zinc con en (and he e o e dec easing he amoun o
Mn in he sample), he appea ance o ano he signal a 0.9/0.87 V s. Ag/AgCl (oxida ion)
Nanoma e ials 2024,14, 1092 8 o 15
and ca. 0.75 V ( educ ion) is dis inguished o he Zn50Mn50 phase ha is assigned o he
Fe
3+
/Fe
2+
edox pai [
39
,
56
]. The in ensi y a io o he edox pai s co esponding o Mn
and Fe is u he balanced when examining he sample Zn75Mn25. Taking in o accoun
he maximum in ensi ies eached by he di e en samples, he supe io i y in e ms o
elec ochemical ac i i y o he Mn100 sample is clea .
Nanoma e ials 2024, 14, 1092 8 o 16
Be o e in eg a ing he p epa ed ma e ials in o AZiBs, he eac ion kine ics o each
compound we e s udied by means o cyclic ol amme y (CV) measu emen s a diffe en
scan a es om 0.1 o 1.0 mV s
−1
. I should be no ed ha in he case o Zn100 sample, he
CV ob ained does no show any peak (Figu e S2) and he e o e i s elec ochemical ac i i y
is p ac ically ze o, in good ag eemen wi h esul s epo ed by o he au ho s [54,55]. Also,
as can be seen in Figu e 4, in he case samples wi h high amoun s o Mn (Mn100 and
Zn25Mn75), CV cu es show wo majo peaks, which main ain he shape du ing he ex-
pe imen a diffe en a es. The peaks ha appea a 0.95/0.98 V s. Ag/AgCl du ing oxi-
da ion and a ca. 0.90 V du ing educ ion co espond o he Mn
3+
/Mn
2+
edox pai and, as
expec ed, as he scan a e inc eases he in ensi y o he peak inc eases, pe cei ing a sligh
shi owa ds highe po en ials in he oxida ion b anch and owa ds lowe alues in he
educ ion b anch. By inc easing he zinc con en (and he e o e dec easing he amoun o
Mn in he sample), he appea ance o ano he signal a 0.9/0.87 V s. Ag/AgCl (oxida ion)
and ca. 0.75 V ( educ ion) is dis inguished o he Zn50Mn50 phase ha is assigned o he
Fe
3+
/Fe
2+
edox pai [39,56]. The in ensi y a io o he edox pai s co esponding o Mn and
Fe is u he balanced when examining he sample Zn75Mn25. Taking in o accoun he
maximum in ensi ies eached by he diffe en samples, he supe io i y in e ms o elec o-
chemical ac i i y o he Mn100 sample is clea .
Figu e 4. CV cu es o he Mn
1-x
Zn
x
HCF (x = 0, 0.25, 0.5 and 0.75) samples a diffe en scan a es: 0.1,
0.2, 0.3, 0.5, 0.8 and 1 mV s
−1
.
In Figu e S3 he ela ionship be ween he peak cu en (I
p
; mA g
−1
) and he squa e
oo o he used scan a es (√V; √mV s) is ep esen ed [57]. This ela ionship is almos
linea in all he samples, which indica es ha all sys ems a e compa ible wi h and ideal
diffusion-con olled a adaic p ocess. Fu he mo e, he anodic and ca hodic lines p esen
g ea symme y, which is indica i e o he high e e sibili y o he edox p ocess. Mo eo-
e , he peak sepa a ion a 0.1 mV s
−1
is abou 20 mV and inc eases jus up o 35 mV a 1
mV s
−1
, which is also indica i e o he e e sibili y o he eac ion.
Figu e 4. CV cu es o he Mn
1−x
Zn
x
HCF (x = 0, 0.25, 0.5 and 0.75) samples a di e en scan a es:
0.1, 0.2, 0.3, 0.5, 0.8 and 1 mV s−1.
In Figu e S3 he ela ionship be ween he peak cu en (I
p
; mA g
−1
) and he squa e
oo o he used scan a es (
√V
;
√mVs−1
) is ep esen ed [
57
]. This ela ionship is almos
linea in all he samples, which indica es ha all sys ems a e compa ible wi h and ideal
di usion-con olled a adaic p ocess. Fu he mo e, he anodic and ca hodic lines p esen
g ea symme y, which is indica i e o he high e e sibili y o he edox p ocess. Mo eo e ,
he peak sepa a ion a 0.1 mV s
−1
is abou 20 mV and inc eases jus up o 35 mV a 1 mV s
−1
,
which is also indica i e o he e e sibili y o he eac ion.
Gal anos a ic cha ge/discha ge measu emen s we e pe o med be ween 0.005–2.0 V
s. Zn
2+
/Zn using a cu en densi y o 0.1 A g
−1
. Figu e S4 shows he e olu ion o he
cha ge/discha ge cu es o e 10 cycles o each sample. Again, i is possible o dis inguish
wo di e en beha iou s depending on he Zn con en in he sample. On he one hand, he
samples ich in manganese, Mn100 and Zn25Mn75, show 2–3 pla eaus, being mo e de ined
in he case o he Mn100 sample. These small pla eaus ha e been also epo ed in p e ious
wo ks using PBAs, speci ically in wo ks in which manganese has been used in his ype
o s uc u e [
58
–
60
]. In he i s discha ge cu es o he wo samples wi h he highes Mn
con en , a small pla eau is dis inguished a ound 1.8 V s. Zn2+/Zn ha p og essi ely dis-
appea s, being impe cep ible om he se en h cycle onwa ds. Likewise, wo o he pla eaus
a e dis inguished be ween 0.8 and 1.5 V, he one wi h he highes po en ial being assigned
o he Mn
3+/2+
edox couple, while he one a lowe po en ials co esponds o he Fe
3+/2+
edox couple, as was es ablished in he analysis o CVs. The con ibu ion o he capaci y
o bo h p ocesses dec eases upon cycling, wi h he pla eau due o Fe
3+/2+
disappea ing
Nanoma e ials 2024,14, 1092 9 o 15
in he las cycles. This change in he p o iles, which ul ima ely become sloppy, may be
ela ed o s uc u al changes ha a e usually ela ed o he dissolu ion o Mn, leading o
he o ma ion o low c ys alline MnO2[61], and changes in he coo dina ion en i onmen
o he ansi ion me als in he s uc u e [
60
]. When a highe amoun o Zn is in oduced
in o he s uc u e (Zn50Mn50 and Zn75Mn25 samples), i is ba ely possible o dis inguish
he p esence o well-de ined pla eaus; ins ead, a p og essi e d op is pe cei ed. This ype
o p o ile di e s om hose epo ed by o he au ho s analyzing zinc hexacyano e a es
in which only i on ac s as an elec ochemically ac i e ansi ion me al, dis inguishing a
pla eau co esponding o he ans e ence o one elec on in sodium-based ba e ies [
35
].
He e, he p esence o i on accompanied by manganese, which also p esen s elec ochemical
ac i i y, p oduces a clea change in he p o ile. In any case, simila discha ge p o iles ha e
been epo ed o zinc-doped hexacyano e a es [
26
]. The p o iles in- he- o m slope in
AZiBs ha e been jus i ied based on he (in)s abili y o he PBAs, especially in he i s cycles
in which a s uc u al eadjus men occu s due o he dein e cala ion o he K
+
ca ions [
56
].
Kim e al. analyzed he e ec o he in oduc ion o Zn in cobal hexacyano e a es, obse -
ing a simila ansi ion om a cubic s uc u e o he Co phase o a hombohed al one as he
Zn con en inc eased [
62
]. The coexis ence o bo h phases wi h di e en s uc u es in he
samples doped wi h Zn (mix u e o cubic and hombohed al phases obse ed in he s udy
using XRD), oge he wi h he dec ease in he con en o elec ochemically ac i e me als
causes a dec ease in capaci y, leading o a loss o ac i i y obse ed in he case o he Zn100
phase. The p esence o he hombohed al phase in he samples wi h Zn acili a es highe
s abili y in hese compounds since he s uc u al changes due o he (de)in e cala ion o
Zn
2+
ions in he cyclabili y a e less se e e [
62
]. This way, he s abili y o he ma e ial is
highe , allowing o supe io capaci y e en ion.
In e ms o elec ochemical p ope ies, he highe he Mn con en is, he highe he
capaci y alues a e, as shown in Figu e 5. In con as , a high Mn con en s, capaci y alues
dec ease signi ican ly by he en h cycle. Fo ins ance, o Mn100 sample capaci y e en ion
is 39% (130 mA h g
−1
s. 51 mA h g
−1
) while o Zn25Mn75 sample his e en ion is 53%
(98 mA h g
−1
s. 52 mA h g
−1
). Fo Zn-en iched samples, ins ead, he capaci y e en ion is
much highe : 61% o Zn50Mn50 (64 mA h g
−1
s. 39 mA h g
−1
) and 94.5% o Zn75Mn25
(73 mA h g
−1
s. 69 mA h g
−1
). A low cu en in ensi ies, he ac i e ma e ial has enough
ime o ully cha ge and discha ge, i.e., Zn
2+
ions can in e cala e and dein e cala e in o
he PBA s uc u e. As he cu en inc eases, di usion p ocesses and e en cha ge ans e
p ocesses a e hinde ed and he eac ion yield is much lowe . On he o he hand, he
dec ease in capaci y wi h inc easing zinc con en is ela ed o he lack o elec oac i i y
o Zn. The highe he concen a ion o Zn in he PBA s uc u e, he lowe he amoun o
Mn3+/2+ edox couple, which has a di ec impac on he capaci y o he compound.
Nanoma e ials 2024, 14, 1092 10 o 16
Figu e 5. Capaci y alues a diffe en cu en s o Mn
1-x
Zn
x
HCF (x = 0, 0.25, 0.5, and 0.75) samples.
Gal anos a ic discha ge and cha ge p o iles a diffe en cu en densi ies, i.e., 0.1, 0.2,
0.3, 0.5, 1.0, and 2.0 A g
−1
a e shown in Figu e S5. As has been poin ed ou , a high cu en
densi ies he capaci y o he sys em decays, due o he limi a ions imposed in diffusion
and cha ge ans e p ocesses [63]. These limi a ions also cause he pola iza ion o g ow.
This inc ease in pola iza ion is mo e e iden in samples wi h highe Mn con en , which
can be a ibu ed o a highe difficul y o he cubic s uc u e o acili a e he diffusion o
Zn
2+
ions. The compa ison o he a e pe o mance o he diffe en ma e ials analyzed in
his wo k is shown in Figu e 6.
Figu e 6. Ra e pe o mance and Coulombic efficiencies (open do s) o Mn
1-x
Zn
x
HCF (x = 0, 0.25, 0.5
and 0.75) samples.
Figu e 5. Capaci y alues a di e en cu en s o Mn
1−x
Zn
x
HCF (x = 0, 0.25, 0.5, and 0.75) samples.
