Raman spec oelec ochemis y o ope ando cha ac e iza ion o edox
low ba e ies
La a Lubian
a,b
, Rub´
en Rubio-P esa
a,b
, Vi ginia Ruiz
a,b
, Al a o Colina
b,*
,
Edga Ven osa
a,b,**
a
In e na ional Resea ch Cen e in C i ical Raw Ma e ials-ICCRAM, Uni e sidad de Bu gos, Plaza Misael Ba˜
nuelos s/n, E-09001, Bu gos, Spain
b
Depa amen o de Química, Facul ad de Ciencias, Uni e sidad de Bu gos, Pza. Misael Ba˜
nuelos s/n, E-09001, Bu gos, Spain
GRAPHICAL ABSTRACT
ARTICLE INFO
Keywo ds:
Redox low ba e ies
Raman spec oelec ochemis y
Ope ando cha ac e iza ion
Deg ada ion mechanism
ABSTRACT
Despi e he po en ial o Aqueous O ganic Redox Flow Ba e ies (AORFBs) o add ess in e mi en ene gy gen-
e a ion om enewable sou ces, hey mus imp o e some key pe o mance indica o s o become compe i i e, in
pa icula cycle s abili y. De elopmen o ad anced in-si u and ime- esol ed echniques plays a c i ical ole o
imp o e pe o mance o AORFBs by enabling elucida ion o he sou ces o ene gy s o age capaci y ading. The
de elopmen and implemen a ion o ope ando Raman spec oscopy is he ein epo ed o dihyd oxyan-
h aquinone– e ocyanide alkaline low ba e y. Valida ion o he echnique is ca ied ou using symme ical
cells, con i ming ha Raman spec oscopy can moni o in-si u he s a e o cha ge. In a ull ba e y, ime- esol ed
Raman spec oscopy is used o in es iga ing he Fa adaic imbalance p ocess, showing ha p esence o oxygen in
he anoly e leads o he p og essi e loss o a ailable e ocyanide. Raman spec oscopy also shows ha e i-
cyanide sel -discha ges when le a open ci cui . Finally, his echnique is used o ack in-si u he c osso e o
2,6-dihyd oxyan h aquinone. Su p isingly, he a e is ound o inc ease a ull s a e o cha ge, suppo ing ecen
* Co esponding au ho .
** Co esponding au ho . In e na ional Resea ch Cen e in C i ical Raw Ma e ials-ICCRAM, Uni e sidad de Bu gos, Plaza Misael Ba˜
nuelos s/n, E-09001, Bu gos,
Spain.
E-mail add esses: [email p o ec ed] (A. Colina), [email p o ec ed] (E. Ven osa).
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Jou nal o Powe Sou ces 646 (2025) 237272
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indings using NMR. This ope ando Raman spec oscopy is an icipa ed o p o ide unique insigh s in o c i ical
p ocesses in eme ging edox low ba e y chemis ies.
1. In oduc ion
Ene gy gene a ion om enewable sou ces has become c ucial o
achie e deca boniza ion and he ansi ion o a mo e sus ainable ene gy
p oduc ion [1]. I is expec ed ha ene gy om enewable sou ces will
accoun o app oxima ely 25 % o he global elec ici y supply by 2030
[2]. Howe e , he unp edic able and in e mi en na u e o enewable
ene gy sou ces such as sola o wind powe equi es e icien and
cos -e ec i e ene gy s o age sys ems (ESSs) o ma ch ene gy p oduc ion
and demand [3–5]. Redox low ba e ies (RFB) a e especially sui able o
la ge-scale s a iona y ene gy s o age, wi h compe i i e ad an ages o e
con en ional ba e ies including cos -e ec i eness, long cycle li e and
independen scalabili y o ene gy and powe [5,6]. The all- anadium
edox low ba e y (AVRFB) is cu en ly he s a e-o -a RFB ye i su -
e s om d awbacks such as he use o a c i ical aw ma e ial and highly
co osi e s ongly acidic solu ions [5]. This has igge ed in e es in
aqueous o ganic edox low ba e ies (AORFB), whe e ino ganic edox
species a e eplaced by mo e sus ainable and abundan o ganic
edox-ac i e compounds [5,7–10]. Se e al amilies o o ganic molecules
ha e been ecen ly esea ched as edox ac i es species in AORFB such as
an h aquinone [11–13], phenazine [14–16] o luo enone [17] de-
i a i es, which ha e p o en o deli e high pe o mance as anoly es in
alkaline media. In u n, iologen de i a i es exhibi p omising pe o -
mance in e ms o ene gy densi y and cycle s abili y a neu al pH [4].
While many o hose o ganic compounds ea u e sui able edox po en-
ials, as kine ics and high solubili y, hei cycle s abili y should be
u he imp o ed o la ge-scale deploymen [4,7].
The p og essi e decay in ene gy s o age capaci y in a edox low
ba e y is usually e e ed o as capaci y ading. The e a e h ee main
sou ces o capaci y ading in AORFB [18]. The i s one includes
deg ada ion o he ac i e species due o incompa ibili y among mole-
cules o in insic decomposi ion [4,19,20]. Ano he sou ce o capaci y
loss is ela ed o he cell design, such as memb ane deg ada ion elec-
oly e leakage o c osso e [21]. The hi d sou ce o capaci y ading is
cha ge ( a adaic) imbalance be ween bo h sides o he RFB due o
pa asi ic side eac ions such as hyd ogen and oxygen e olu ion o oxy-
gen educ ion [18]. Iden i ying he con ibu ion o each sou ce is c ucial
o de elop s a egies o p e en o minimize capaci y ading [18,22].
This equi es ad anced in si u cha ac e iza ion echniques o in es iga e
he pe o mance o edox low ba e ies ope ando condi ions, as ex-si u
echniques may lead o misin e p e a ions in case o sho -li ed edox
ac i e species o highly eac i e molecules [23]. So a , echniques such
as Ul a iole –Visible (UV–Vis) abso p ion [24–26], IR [27], elec on
pa amagne ic esonance (EPR) [28] and nuclea magne ic esonance
(NMR) [23,28,29] spec oscopies ha e been used o in si u cha ac e -
iza ion o RFB o in es iga e eac ion mechanisms and deg ada ion
p ocesses o o ganic edox species in AORFBs.
Among he analy ical echniques complemen a y o he ope ando
echniques es ablished so a , ime- esol ed Raman spec oscopy can
p o ide eal- ime in o ma ion abou he chemical composi ion o he
edox-ac i e molecules in he elec oly es, as Raman spec a a e like
inge p in s o he compounds. Raman spec oscopy ha has shown o
be a powe ul in si u echnique o non- low ba e y esea ch [30–32]
did no a ac much a en ion in he ield o RFBs. Indeed, he e a e only
ew examples employing Raman spec oscopy o edox low ba e y
esea ch. Fo example, Raman spec oscopy was used in a s a ic cell o
s udy anadyl-ion oxida ion mechanisms on ca bon pape elec odes
[33]. This echnique was also used o e alua e he e olu ion o ana-
dium elec oly e by aking samples a di e en s a e o cha ge [34,35].
Also, Raman spec oscopy has been applied o moni o he e olu ion o
ZnB concen a ion upon cycling in a con en ional zinc-b omine RFB
[36]. Despi e he in e es ing esul s, his powe ul cha ac e iza ion
echnique has no been applied o ime- esol ed in es iga ion unde
ope ando condi ions in a low ba e y o he eme ging ield o AORFBs.
In his epo , we use o he i s ime dynamic and ime- esol ed
Raman spec oscopy o cha ac e iza ion o an AORFB ope ando con-
di ions. A ailo ed op ical low cell o in si u Raman con ocal mic o-
scopy measu emen s was designed, 3D p in ed and i s pe o mance
alida ed wi h he widesp ead used po assium e ocyanide/ e icya-
nide edox couple. We illus a e he powe o ime- esol ed Raman
spec oelec ochemis y o in si u e alua ion o AORFB wi h an
an h aquinone de i a i e// e ocyanide alkaline edox low ba e y.
The echnique allowed de ec ing loss o e ocyanide due o he accu-
mula ion o cha ged species ( e icyanide) in he ca holy e caused by
oxygen-induced pa asi ic eac ions a he nega i e compa men ,
co obo a ing ha he capaci y ading obse ed in he AORFB was due
o Fa adaic imbalance be ween bo h hal -cells. Fu he mo e, in si u
Raman spec oscopy co obo a ed he mechanism o an elec ochemical
cha ge balancing s a egy implemen ed in an imbalanced AORFB o
e e ing he e ec s o he side eac ion and mi iga ing capaci y ading.
We belie e ha he p oposed examples illus a e he p omise o ime-
esol ed Raman spec oscopy o ope ando e alua ion o AORFB pe -
o mance, a echnique ha can con ibu e o a as e la ge-scale
deploymen o AORFB by p o iding be e unde s anding o deg ada-
ion p ocesses ha limi hei cycling s abili y.
2. Expe imen al sec ion
2.1. Ma e ials
Po assium e ocyanide (II) ihyd a e, po assium e ocyanide (III)
(>98 %, The moscien i ic) and KOH (VWR) we e used as ecei ed. The
syn he ic p ocedu e o 2,6-dihyd oxan aquinone (2,6-DHAQ) was
p e iously epo ed (Sec ion S1).
2.2. P epa a ion o elec oly es
In he symme ic K
3
Fe(CN)
6
//K
4
Fe(CN)
6
low cell, he ca holy e was
p epa ed by dissol ing po assium e ocyanide in 1 M KOH o a o d 13
mL o 0.3 M e ocyanide elec oly e. The anoly e was p epa ed by
dissol ing po assium e ocyanide and po assium e icyanide in 1 M
KOH o a o d 45 mL o 0.1 M e ocyanide and 0.2 M e icyanide
elec oly e (o e sized coun e -compa men ).
In he 2,6-DHAQ//K
4
Fe(CN)
6
low ba e ies, he ca holy es we e
p epa ed by dissol ing po assium e ocyanide in 1 M KOH o a o d 14
mL o 0.3 M e ocyanide elec oly e in all ull ba e ies. The 2,6-DHAQ
sample sys ema ically deli e ed 1.75 exchanged elec ons as shown in
Fig. S1, Sec ion S1 (la ge excess o ca holy e). This is a ibu ed o 12.5
w % wa e emaining a e he syn hesis. As o he anoly e, expe imen s
using wo di e en olumes we e employed: i) 10 mL o 0.2 M 2,6-
DHAQ in 1.4 M KOH elec oly e leading o a 20 % excess o po assium
e ocyanide ( o 1.75 exchanged elec ons o he 2,6-DHAQ), and ii)
12 mL o 0.2 M 2,6-DHAQ in 1.4 M KOH elec oly e leading o a sys em
wi h a balanced amoun o cha ge o ca holy e and anoly e. In ac , he
la e leads o 5 % excess o as i is e y di icul o ob ain a Coulombic
e iciency abo e 95 % in he i s cycles since oxygen adso bed on he
ca bon el and aces o oxygen in he elec oly e a e ex emely di icul
o be emo ed wi hou an A - illed glo ebox. Anoly e was pu ged wi h
a gon p io o use. All elec oly es we e p epa ed wi h deionized wa e .
L. Lubian e al.
Jou nal o Powe Sou ces 646 (2025) 237272
2
2.3. In si u Raman spec oelec ochemis y se up
2.3.1. Flow ba e y
A il e -p essed low cell using expanded g aphi e (SGL Ca bon),
g aphi e el (SGL Ca bon) and Na ion 212 (Ion Powe ) as cu en col-
lec o , elec ode and ion selec i e memb ane, espec i ely was used. The
p ojec ed a ea o he cell was 10 cm
2
. A pe is al ic pump (Mas e Flex L/
S) was used o p o ide a low a e o 24 mL min
−1
.
2.3.2. Raman low cell
The Raman low cell (Fig. 1) was designed using he Ske chUp so -
wa e and manu ac u ed using an Ul a iole (UV) Liquid C is al Display
(LCD) - based s e eoli hog aphy (SLA) 3D p in e (Pho on Mono SE,
Anycubic) and a comme cial clea esin (Anycubic). A e he cleaning
p ocedu e (wi h 70 % isop opanol solu ion), he p in ed pieces we e
assembled wi h a PET window sandwiched be ween wo Vi on® gaske s
(1 mm hick) and all he pieces we e held oge he wi h sc ews (sec ion
3.1, Fig. 1). Dimensions o he Raman low cell we e 10 x 8 ×3 cm
2
.
2.4. Spec oelec ochemical cha ac e iza ion
2.4.1. Elec ochemical cha ac e iza ion
Gal anos a ic cha ge-discha ge measu emen s we e conduc ed using
a po en ios a /gal anos a PGSTAT302 (AUTOLAB). The ba e ies we e
gal anos a ically cycled a ±20 mA cm
−2
wi h ol age limi s o ±0.3 V
o he K
3
Fe(CN)
6
//K
4
Fe(CN)
6
symme ic low cell and +1.6 V/+0.5 V
o he 2,6-DHAQ//K
4
Fe(CN)
6
low ba e y. Fo he elec ochemical
ebalancing p o ocol, he uppe ol age limi was inc eased o +2 V.
2.4.2. Raman spec oscopy cha ac e iza ion
Raman spec a we e measu ed using a con ocal Raman mic oscope
Voyage (BWTEK). A 20 ×objec i e was used, wi h an exci a ion line a
532 nm and a powe o 5 mW. Raman spec a we e collec ed by a CCD
a ay wi h a spec al esolu ion o 3.8 cm
−1
. Sampling in e al was 30 s
o cycling he K
3
Fe(CN)
6
//K
4
Fe(CN)
6
symme ic low cell and o he
2,6-DHAQ//K
4
Fe(CN)
6
low ba e y. The in eg a ion ime o each
spec um was 3000 ms. Ma lab was used o analyze, plo and co ec he
baseline o measu ed Raman spec a. Raman spec ome e is
synch onized wi h he po en ios a using a pulse igge in o de o
ob ain concomi an ly he op ical and elec ical esponse.
3. Resul s and discussion
3.1. Design and alida ion o he in si u Raman spec oelec ochemical
se up
As indica ed in he in oduc ion, o he bes o ou knowledge, ime-
esol ed Raman spec oscopy had no been p e iously conduc ed in si u
du ing elec ochemical cycling o AORFB o eal- ime moni o ing o
ac i e species o deg ada ion p oduc s ope ando. Awa e o he g ea
p omise o his echnique o explo ing deg ada ion p ocesses causing
capaci y ading in AORFB, we ha e de eloped a Raman low cell and
in eg a ed i in he low sys em o a K
3
Fe(CN)
6
//K
4
Fe(CN)
6
symme ic
low cell and a 2,6-DHAQ//K
4
Fe(CN)
6
low ba e y o ollow he e o-
lu ion o ac i e species du ing cell/ba e y ope a ion. The expe imen al
se up o in si u Raman spec oelec ochemical measu emen s wi h
AORFB is shown schema ically in Fig. 1A. The Raman low cell was
moun ed on he con ocal Raman mic oscope holde a he inle o he
elec ochemical eac o in he ca holy e side o in es iga e he s a e o
cha ge o he elec oly e in he ank. The de ailed con igu a ion o he
3D p in ed Raman low cell is shown in Fig. 1B. I essen ially consis s o
wo p in ed pa s made o esin (one piece ha ing a low- h ough and
he o he ha ing an opening), a ec angula piece o PET as op ical
window and wo Vi on® gaske s (1 mm hick) o seal he cell. Di-
mensions o each pa a e gi en in Fig. S2 (plans). The p in ed compo-
nen s we e ca e ully designed o ensu e p ope elec oly e low, a oid
solu ion leakage and maximize measu ed Raman in ensi y. On he one
hand, he ailo ed shape o he low channel (2.5 mL) in he bo om low-
h ough piece, widening a he inle and na owing a he ou le (see
c oss-sec ion, Fig. S2) allows o al illing and emp ying o he chambe ,
hus p e en ing e en ion o ac i e species inside i ha would esul in
capaci y ading o he ba e y. On he o he hand, he op op ical piece
was p in ed wi h a conical ca i y o he igh size (see c oss-sec ion,
Fig. S2) o inse he Raman mic oscope objec i e and allow mo ing i in
he z-axis o ocus he lase beam on di e en planes o he low channel
o op imal signal moni o ing. The ca i y dep h in he op ical piece is
Fig. 1. (A) Schema ic illus a ion o he expe imen al se up o in si u Raman spec oelec ochemical measu emen s a he ca holy e side o an AORFB, and (B) a
scheme showing a b eakdown o he Raman cell.
L. Lubian e al.
Jou nal o Powe Sou ces 646 (2025) 237272
3
la ge enough o p e en collision o he objec i e wi h he PET window.
The PET op ical window is sandwiched be ween wo ec angula Vi on®
gaske s wi h he same size as he p in ed op and bo om cell pa s. An
opening in he bo om gaske (Vi on A) wi h he same shape and size as
he uppe mos plane o he bo om p in ed low piece allows he solu ion
eaching he op ical window. The lase beam passes h ough a ci cula
cen al hole in he op gaske (Vi on B). All cell componen s excep he
PET window ha e holes a hei edges o he sc ews o keep all he
pieces joined and he cell igh ly sealed. A p e iously calib a ed o que
w ench was used o ensu e applying a homogeneous p essu e o all he
sc ews.
An ini ial alida ion o he Raman low cell and he whole expe i-
men al se up o in si u Raman spec oelec ochemical cha ac e iza ion
o RFB was done du ing gal anos a ic cycling o a symme ic po assium
e ocyanide/ e icyanide alkaline edox low cell. This model edox
sys em was selec ed because i is one o he mos widely used aqueous
ca holy es in AORFB and also o explo e he s abili y o he de eloped
low cell unde alkaline condi ions. Fi s , he Raman low cell was
calib a ed using aqueous solu ions wi h di e en concen a ion a ios o
po assium e ocyanide/ e icyanide in 1 M KOH (Sec ion S3). Bo h
species o he edox couple ha e e y dis inc Raman ea u es. While he
Raman spec um o po assium e ocyanide (Fig. S4) exhibi s wo bands
cen e ed a 2052 and 2090 cm
−1
co esponding o he s e ching i-
b a ion o cyanide bonds
ν
(C ≡N), he spec um o po assium e icy-
anide (Fig. S4) shows only one cyanide ib a ional band a 2124 cm
−1
[37]. Calib a ion cu es ob ained o he in ensi y o cha ac e is ic
Raman bands o bo h po assium e ocyanide and e icyanide (Fig. S5)
e eal high linea i y o he op ical signal wi h analy e concen a ion o
all bands.
The olume o he anoly e compa men (45 mL, 0.2 M po assium
e icyanide) o he e ocyanide/ e icyanide symme ic low cell and,
hus, he amoun o ac i e species and capaci y was signi ican ly
inc eased so ha he ca holy e compa men (13 mL, 0.2 M po assium
e ocyanide), whe e Raman spec a we e moni o ed, became he
limi ing side. Thus, he elec ochemical da a can be a ibu ed o he
ca holy e limi ing compa men , which becomes he “wo king
compa men ”, while he la ge anoly e compa men ac s as “coun e -
compa men ”. Mo eo e , using a la ge excess o ac i e species in he
Fig. 2. In si u Raman spec oelec ochemical cha ac e iza ion o a symme ic e ocyanide/ e icyanide alkaline edox low cell ope ando du ing gal anos a ic
cycling a ±20 mA cm
−2
wi h ol age limi s o ±0.3 V. (A) Time e olu ion o cell ol age and Raman in ensi y a 2090 cm
−1
du ing he i s cha ge/discha ge cycle.
(B) Raman spec a o he ca holy e moni o ed a he cycle imes indica ed in (A). (C) Cha ge/discha ge capaci y and Raman in ensi y a 2090 cm
−1
measu ed in si u
du ing cycling. (D) E olu ion o he no malized capaci y e en ion and he no malized Raman in ensi y a 2090 cm
−1
. Ca holy e: 13 mL o 0.3 M po assium
e ocyanide in 1 M KOH, anoly e: 45 mL o 0.1 M e ocyanide and 0.2 M e icyanide in 1 M KOH.
L. Lubian e al.
Jou nal o Powe Sou ces 646 (2025) 237272
4
coun e -compa men educes capaci y ading due o occu ence o he
OER, which canno be neglec ed in alkaline media [22]. Time e olu ion
o cell ol age and Raman in ensi y a 2090 cm
−1
(cha ac e is ic o
po assium e ocyanide) du ing he i s gal anos a ic cha ge/discha ge
cycle a e plo ed oge he in Fig. 2A. As can be seen, elec ochemical and
spec oscopic signals a e o ally co ela ed. A he s a o he cycle, he
Raman in ensi y a his wa enumbe (Fig. 2A, poin 1) con i med he
p esence o po assium e ocyanide in he ca holy e (which con ained
0.2 M po assium e ocyanide). The ull spec um measu ed (Fig. 2B–1)
co esponds o he s a ing uncha ged po assium e ocyanide. Upon
cha ging, cha ged po assium e icyanide s a s o be p oduced a he
ca holy e side, gi ing ise o a dec ease o he Raman in ensi y (Fig. 2A,
poin 2) ha eaches a minimum alue a he end o he cha ging s ep
(Fig. 2A, poin 3) when all po assium e ocyanide has been oxidized o
e icyanide. Full Raman spec um measu ed a hal s a e o cha ge
(Fig. 2B-2) exhibi s h ee bands, e ealing he p esence o bo h e o-
cyanide (bands a 2052 and 2090 cm
−1
) and e icyanide (band a 2124
cm
−1
). Consump ion o ini ial e ocyanide upon cha ging is e idenced
by he lowe in ensi y o i s cha ac e is ic bands while eme ging o he
new band a 2124 cm
−1
p o es concomi an e icyanide gene a ion.
The spec um moni o ed a ull s a e o cha ge (Fig. 2B–3) co esponds
o e icyanide alone, as he cha ac e is ic bands o e ocyanide ha e
comple ely disappea ed.
Upon discha ging, e icyanide is educed back o e ocyanide and
hence, he in ensi y o i s cha ac e is ic Raman band inc eases un il
eaching he ini ial alue a he beginning o he cycle (Fig. 2A, poin 4).
The ull spec um a he end o he cycle (Fig. 2B–4) o e laps wi h he
ini ial one (Fig. 2B–1), co obo a ing ha he low cell ca holy e is ully
discha ged. As expec ed, Raman in ensi y o he e icyanide band
ollow exac ly he opposi e ends du ing he cha ge/discha ge cycle
(Fig. S6C), inc easing in he cha ge s ep and e u ning o he ini ial alue
a he end o he discha ge s ep, con i ming ull in e con e sion be ween
bo h ac i e species o he edox pai .
A e alida ing he Raman spec oelec ochemis y se up o ollow
in si u he s a e o cha ge in he ca holy e o a symme ic e ocyanide/
e icyanide alkaline edox low cell in only one cycle, he echnique was
applied o assess he cycling s abili y o his low cell. E olu ion o
cha ge/discha ge capaci y and Raman in ensi y a 2090 cm
−1
( e o-
cyanide band) measu ed in si u du ing cycling (Fig. 2C) e eals a
ema kable co ela ion be ween elec ochemical and op ical esponses.
Thus, he 14 % d op in capaci y no ed a e he i s cycle was accom-
panied by a compa able dec ease (16 %) o Raman in ensi y ampli ude
o he cha ge. Bo h capaci y and Raman in ensi y emained oughly
s able o he es o he cycles due o he high s abili y o he e ocy-
anide/ e icyanide edox pai in alkaline media unde he ope a ion
condi ions. This co ela ion be ween elec ochemical and op ical e-
sponses can be easily obse ed in Fig. 2D by compa ing he no malized
capaci y e en ion (no malized by he capaci y in he i s cycle) wi h he
Raman in ensi y e en ion (no malized by he in ensi y in he i s
cycle). The excellen co ela ion be ween op ical and elec ochemical
signals o e ocyanide in a simple symme ical cell con i ms ha his
signal (2090 cm
−1
) can be used in mo e complex sys ems o moni o he
s a e o cha ge o he ca holy e. I should be no ed ha simila in o -
ma ion can be in e ed om he addi ional Raman signals o e ocya-
nide (2052 cm
−1
) and e icyanide (2124 cm
−1
) ha a e shown in
Figs. S6 and S7. The ollowing sec ions will ocus on he Raman signal a
2090 cm
−1
(po assium e ocyanide) o simpli y he explana ions.
3.2. P obing capaci y imbalance o a 2,6-DHAQ//K
4
Fe(CN)
6
alkaline
RFB by in si u Raman spec oscopy
A e demons a ing he capabili y o he in-si u Raman spec-
oelec ochemis y se up o ope ando cha ac e iza ion o a symme ic
edox low cell, a e y ele an use case was selec ed o explo e appli-
cabili y o he echnique o AORFB es ing. Speci ically, he se up was
used o p obe capaci y ading due o Fa adaic imbalance du ing cycling
s abili y assessmen o a s a e-o -a alkaline low ba e y chemis y,
namely 2,6-DHAQ//K
4
Fe(CN)
6
. Fa adaic imbalance e e s o he une en
Fig. 3. Schema ic illus a ion o : (A) a edox low ba e y su e ing o pa asi ic side eac ion ela ed o he p esence o oxygen in he nega i e compa men , (B) he
concep o ba e y imbalance du ing he i s cha ge and (C) discha ge cycle in a low ba e y by oxygen-induced pa asi ic eac ions o hyd ogen e olu ion in he
nega i e elec ode.
L. Lubian e al.
Jou nal o Powe Sou ces 646 (2025) 237272
5
s a e o cha ge o ca holy e and anoly e igge ed by pa asi ic side e-
ac ions ha i e e sibly consume cha ges unde he ope a ing condi-
ions such as he eac ion o oxygen wi h educed ac i e species in he
nega i e elec ode [18]. I should be no ed ha c osso e o ac i e
species h ough he memb ane also leads o he imbalance o a ailable
cha ge be ween anoly e and ca holy e, bu his p ocess does no lead o
an accumula ion o cha ged species in one o he sides which is he main
p ocess o be moni o ed in his case s udy. Thus, c osso e is no u he
discussed in his explana ion ha is ocused on Fa adaic imbalance
leading o accumula ion o cha ged species. In o de o p e en hese
oxygen-induced pa asi ic eac ions and he associa ed i e e sible ca-
paci y losses in long- e m cycling, AORFB a e usually ope a ed in
a gon- illed glo eboxes o academic esea ch. Howe e , his app oach
is no p ac ical o an e en ual la ge-scale implemen a ion o AORFB,
which u ges o seek ope ando analy ical ools o p o iding insigh s in o
side eac ions o deg ada ion p ocesses leading o ba e y cha ge ca-
paci y ading [18]. Ine i able hyd ogen e olu ion eac ion and oxygen
e olu ion eac ion o ac i e species ha ing edox po en ials beyond he
s abili y window o he elec oly e ha e he same consequences on he
Fa adaic imbalance.
To explain he p ocess, a hypo he ical case is discussed in which he
alues a e a bi a ily aken. Fig. 3 illus a es schema ically he concep
o ba e y imbalance du ing he i s cha ge/discha ge cycle in a low
ba e y due o oxygen-induced pa asi ic eac ions in he nega i e elec-
ode, The anoly e (N, 2,6-DHAQ in ou case) su e ing oxygen-induced
pa asi ic eac ions will no be ully cha ged (e.g. 90 %) as some cha ges
(e.g. 10 %) a e i e e sibly consumed by he pa asi ic eac ion (HER o
ORR), while he ca holy e (P, K
4
Fe(CN)
6
in ou case) will be able o be
ully cha ged (e.g. 100 %). The occu ing o pa asi ic eac ions (HER and
ORR) in he anoly e i e e sibly consumed cha ge leading o a Fa adaic
imbalance in he s a e o cha ge o anoly e and ca holy e a he end o
he cha ge s ep (Fig. 3B). Du ing he discha ge p ocess, he discha ge
capaci y will be limi ed by he s a e o cha ge achie ed in he anoly e (e.
g. 90 %), hence ull discha ging will no be achie ed in he ca holy e,
which will emain pa ially cha ged a he end o he discha ge s ep (e.g.
10 %), as illus a ed in Fig. 3C. This will educe he capaci y o he
ba e y in he ollowing cha ge s ep as now only 90 % o he ca holy e
species can be cha ged. Mo eo e , he anoly e will no be ully cha ged
again due o oxygen-induced pa asi ic eac ions, hus esul ing in an
inc easingly la ge misma ch be ween he s a es o cha ge o anoly e and
ca holy e wi h he numbe o cycles as long as oxygen is p esen , as
illus a ed schema ically o he i s h ee cha ge/discha ge cycles in
Fig. S8. I should be no ed ha he occu ence o hyd ogen e olu ion
eac ion would ha e he same e ec .
In ou s udy case o he 2,6-DHAQ//K
4
Fe(CN)
6
alkaline RFB, an
o e sized ank o he ca holy e wi h 20 % excess o po assium
e ocyanide was used, o 1.75 exchanged elec ons (as explained in he
expe imen al sec ion) o he 2,6-DHAQ (94 mAh). Acco ding o he
esul s o he i s 2 cycles (Fig. 4A), he capaci y (94.7 mAh) was indeed
limi ed by he anoly e ( heo e ical capaci y o 94 mAh and 112.5 mAh
o anoly e and ca holy e, espec i ely), while he excess o e ocyanide
enabled main aining a s able capaci y o he i s cycles. Howe e , he
excess o e ocyanide is quickly consumed by he emaining amoun o
oxygen in he elec oly e, adso bed oxygen on he g aphi e el s and/o
leakage o small amoun o oxygen in he nega i e compa men leading
o a poo e Coulombic e iciency in he i s cycles. In o he wo ds,
e ocyanide oxida ion is accompanied by pa asi ic oxygen educ ion
a he han 2,6-DHAQ educ ion, leading o a dec ease in Coulombic
e iciency. A e 3 cycles, bo h cha ge and discha ge capaci y s a ed o
dec ease s eadily wi h he numbe o cycles (Fig. 4A) leading o only
60.1 % capaci y e en ion a e 25 cycles. Based on he Coulombic e -
iciency o ca. 97 %, a leakage o small amoun o oxygen in he nega i e
compa men is likely he o igin o he capaci y ading. Al hough his
ading mechanism is plausible, an in-si u echnique is needed be p o ide
e idence and suppo such a hypo hesis.
The e o e, accumula ion o cha ged ac i e species and he conse-
quen ly inc easing de ici o e ocyanide a he beginning o each
cha ge in he ca holy e du ing ba e y ope a ion was explo ed ope ando
by Raman spec oscopy. E olu ion o Raman in ensi y a 2090 cm
−1
(cha ac e is ic o po assium e ocyanide) measu ed in si u in he
ca holy e du ing he i s gal anos a ic cha ge/discha ge cycle is plo ed
in Fig. 4B oge he wi h he ol age p o ile. Full Raman spec a o he
ca holy e measu ed a he beginning and end o he cha ge s ep (poin s 1
and 2, espec i ely) and a he end o he discha ge s ep (poin 3) a e
shown in Fig. 4C. A he s a o he cycle, he measu ed spec um
(Fig. 4C–1) co esponded o uncha ged po assium e ocyanide, bands
a 2052 and 2090 cm
−1
(Fig. 4B, poin 1), hence he null Raman in-
ensi y a 2124 cm
−1
(Fig. 4C, poin 1). Upon ba e y cha ging, po as-
sium e icyanide s a ed o be p oduced a he ca holy e side and
Raman in ensi y a he cha ac e is ic wa enumbe o e ocyanide
dec eased un il eaching a minimum alue a he end o he cha ging
s ep (Fig. 4B, poin 2). Spec um measu ed a his poin (Fig. 4C–2)
displayed no only he band o cha ged po assium e icyanide a 2124
cm
−1
bu also small-in ensi y bands a 2052 and 2090 cm
−1
, e ealing
he p esence o a small amoun o uncha ged po assium e ocyanide as
we used an o e sized ank o he ca holy e wi h 20 % excess o e o-
cyanide o compensa e cha ge consump ion by oxygen-induced pa asi ic
eac ions a he anoly e. No e he di e ence wi h he spec um moni-
o ed a he end o he cha ge s ep in he case o he symme ic e o-
cyanide/ e icyanide low cell wi h o e sized coun e -compa men
(Fig. 2B–3), whe e he cha ac e is ic bands o e ocyanide dis-
appea ed comple ely as po assium e ocyanide was ully oxidized o
Fig. 4. In si u Raman spec oelec ochemical measu emen o a 2,6-DHAQ//K
4
Fe(CN)
6
alkaline edox low ba e y du ing gal anos a ic cycling a ±20 mA cm
−2
wi h ol age limi s o +1.6/+0.5 V. (A) Discha ge capaci y and Coulombic e iciency e sus numbe o cycles. (B) Time e olu ion o cell ol age and Raman in ensi y
a 2090 cm
−1
measu ed in si u in he ca holy e du ing he i s cha ge/discha ge cycle. (C) Raman spec a o he ca holy e co esponding o he cycle imes indica ed
in (B). Ca holy e: 14 mL o 0.3 M e ocyanide in 1 M KOH; anoly e: 10 mL o 0.2 M 2,6-DHAQ in 1.4 M KOH.
L. Lubian e al.
Jou nal o Powe Sou ces 646 (2025) 237272
6
e icyanide.
In he discha ge p ocess o he 2,6-DHAQ//K
4
Fe(CN)
6
alkaline RFB,
he Raman inge p in o e ocyanide inc eased du ing he whole s ep
wi hou eaching he ini ial alue a he s a o he cycle (Fig. 4B, poin
3). The ull spec um (Fig. 4C–3) con i med he p esence o accumula ed
cha ged e icyanide in he ca holy e a he end o he discha ge s ep
(and hence less e ocyanide han ini ially), p o iding he spec oscopic
e idence o ba e y Fa adaic imbalance om he i s cycle due o i e-
e sible cha ge consump ion a he anoly e ha p e en ed his “capaci y
limi ing side” eaching i s ull s a e o cha ge. Indeed, he Raman signal
indica es a 12 % accumula ion o cha ge in he i s cycles, which is in
good ag eemen wi h he Coulombic e iciency. Impo an ly, he Raman
signal con i ms ha Fa adaic imbalance is igge ed by side eac ions in
he nega i e elec ode.
O e all, ope ando Raman spec oelec ochemical cha ac e iza ion o
ca holy e ac i e species du ing cycling s abili y es ing o he 2,6-
DHAQ//K
4
Fe(CN)
6
alkaline RFB showed ema kable co ela ion be-
ween elec ochemical and op ical esponses (Fig. S9). Vol age p o iles
a e plo ed oge he wi h he Raman in ensi y o he e ocyanide band
(2090 cm
−1
) measu ed e sus he numbe o cha ge/discha ge cycles in
Fig. 5. While capaci y g adually aded and cycles became sho e , he
spec oscopic signal e ealed p og essi e loss o po assium e ocyanide
a e each discha ge cycle, as Raman in ensi y did no e u n o i s ini ial
alue (g een ci cles in Fig. 5). Fu he mo e, he excess o e ocyanide is
consumed by cycle 3 (pu ple squa e in Fig. 5)., as he band o e ocy-
anide disappea s a he end o he 3 d cha ge s ep (Fig. S10), which is
consis en wi h he beginning o he capaci y ading (Fig. 4A). Simila
in o ma ion can be in e ed om he addi ional Raman signals o
e ocyanide (2052 cm
−1
) and e icyanide (2124 cm
−1
) ha a e shown
in Fig. S11.
3.3. P obing elec ochemical cha ge balancing o a 2,6-DHAQ//K
4
Fe
(CN)
6
alkaline RFB by ope ando Raman spec oscopy
Once demons a ed by ope ando Raman spec oscopy ha Fa adaic
imbalance in a 2,6-DHAQ//K
4
Fe(CN)
6
alkaline RFB leads o he loss o
po assium e ocyanide in he ca holy e by p og essi e accumula ion o
e icyanide, he echnique was used o shed ligh on he undamen s o
e e ing he ba e y Fa adaic imbalance by applying a simple elec o-
chemical cha ge balancing s a egy ecen ly p oposed [18]. In a p e i-
ous wo k, ou g oup p o ed ha he e ec s o he side eac ion could be
e e ed by an elec ochemical ebalancing p ocedu e, hus mi iga ing
he capaci y ading and p olonging he cycling pe o mance o AORFB
( ema kable 20- old educ ion o capaci y ading). While de ailed dis-
cussion on he implica ion o he me hod (capaci y ading o e
long- e m, co osion, ene gy e iciency, e c) can be ound elsewhe e
[18], he undamen s o he p ocesses in ol ed we e no un a eled due
o he lack o sui able ope ando echniques. The e o e, he imple-
men a ion o ope ando Raman spec oscopy on he p e iously epo ed
elec ochemical balancing me hods is conside ed a sui able case s udy o
showcase he po en ial o he echnique o p o ide unambiguous esul s
o un a el key p ocesses o ully unde s and mechanism. Rega ding he
me hods, simply explained, elec ochemical “ ebalance” o an unbal-
anced ba e y consis s o in en ionally p omo ing an i e e sible side
eac ion in he “unbalanced compa men ” (whe e cha ged species a e
accumula ed) o d i e also an accumula ion o “cha ged species” a he
opposi e side, hus “ ebalancing” he s a e o cha ge o bo h sides. In he
case o ou 2,6-DHAQ//K
4
Fe(CN)
6
alkaline RFB, whe e oxygen-induced
pa asi ic eac ions a he anoly e (O
2
+2H
2
O +4e
−
→ 4OH
−
) is
esponsible o he Fa adaic imbalance, p omo ion o he oxygen e o-
lu ion eac ion (OER) by wa e spli ing (4OH
−
→ O
2
+2H
2
O +4e
−
) a
he ca holy e will compensa e he cha ges i e e sibly consumed by he
ORR, leading o a mo e balanced s a e o cha ge o bo h elec oly es.
Since he edox po en ial o he OER dec eases wi h inc easing pH, he
OER is acili a ed by he ba e y alkaline media and hence can be p o-
mo ed by inc easing he uppe cu -o ol age in he cha ging s ep o a
highe alue (+2.0 V) han ha p e iously applied in he s anda d
cycling p o ocol epo ed in Sec ion 3.2 (+1.6 V). To simpli y he
explana ion, discussion is ocused on he i s cycle ha usually deli e s
a lowe Coulombic e iciency due o he p esence o small amoun o
oxygen (e en a e pu ging wi h A ) and oxygen abso bed on he
g aphi e el s. The excess o e ocyanide was emo ed ( o bo h Fig. 6A
and B), so ha he e is no bu e o e ocyanide o compensa e he ORR
(and a oid conduc ing mul iple cycles o consume his bu e ). Fo a
cu -o ol age o +1.6 V, Fig. 6A shows ha he e ocyanide signal is
no es o ed o i s ini ial alue since some e ocyanide was i e e sible
oxidized o compensa e he cha ge consumed by oxygen- ela ed e-
ac ions in he anoly e. Raman signal o m Fig. 6A con i ms ha an
accumula ion o e icyanide ook place, implying ha he cha ge s ep
was limi ed by he ca holy e ( he e a e no enough cha ged species in he
anoly e o ully discha ge he ca holy e). When he cu -o ol age is
li ed o +2.0 V, a second pla eau appea ed a highe ol age (Fig. 6B)
o a balanced ba e y (113 mAh o posi i e and nega i e sides). This
could be a ibu ed o he occu ence o oxygen e olu ion eac ion in he
posi i e elec ode, which compensa es he oxygen eac ion in he ano-
ly e and enables eaching ull s a e o cha ge o he anoly e. The Raman
Fig. 5. (A) Time e olu ion o cell ol age and Raman in ensi y a 2090 cm
−1
measu ed in si u in he ca holy e du ing gal anos a ic cycling o a 2,6-DHAQ//K
4
Fe
(CN)
6
alkaline RFB a ±20 mA cm
−2
wi h ol age limi s o +1.6/+0.5 V. Ci cles deno e po assium e icyanide accumula ion a he end o each cycle). Ca holy e: 14
mL o 0.3 M e ocyanide in 1 M KOH; anoly e: 10 mL o 0.2 M 2,6-DHAQ in 1.4 M KOH.
L. Lubian e al.
Jou nal o Powe Sou ces 646 (2025) 237272
7
signal is essen ial o assess his hypo hesis. Fig. 6B shows ha he ini ial
concen a ion o e ocyanide is eco e ed a he end o he i s
discha ge p ocess, in con as o he measu emen ca ied ou using a
cu -o ol age o +1.6 V (Fig. 6A). Acco ding o he Coulombic e i-
ciency in he i s cycle (92 %), he OER should accoun o 8 % o he
e e sible capaci y in he i s cycle o enable eco e y o ini ial con-
cen a ion o e ocyanide. In addi ion, he synch oniza ion o he
elec ochemical and spec oscopy signals enables us o conclude ha he
Raman in ensi y o e ocyanide dec eased cons an ly du ing he i s
ol age pla eau, bu i s opped dec easing du ing he second pla eau
(Fig. S12). Thus, he oxida ion o e ocyanide is no he main eac ion
occu ing du ing he second pla eau, bu he OER is. Thus, Fig. 6 clea ly
illus a e he e ec o i e e sible side eac ions in he anoly e on he
accumula ion o cha ged species in he ca holy e., and how he OER can
p o ide he necessa y cha ges o comple e he cha ge s ep in he anoly e.
3.4. P obing balanced pa asi ic eac ions in he posi i e and nega i e
elec odes
In e es ingly, he ela i ely low Coulombic e iciency in his pa ic-
ula measu emen (88 % o 1s cycle and 97 % o subsequen cycles)
made us e iew he A -lines and a small leakage was ound. A e ixing
he leaking, he Coulombic e iciencies (>99.5 %) we e compa able o
hose ob ained in an A - illed glo ebox. The o ma ion o 2,6-DHAQ
dime s (e idenced by he appea ance o a lowe ol age pla eau as
discussed below, Fig. S13) con i med ha he a e o oxygen leaking was
e y small o he new measu emen s (bea ing in mind ha comple e
p e en ion is no possible wi hou an A - illed glo ebox). Fig. 7A shows
he e olu ion o he capaci y and Coulombic e iciency wi h cycles. The
capaci y ading a e emained cons an o he i s 25 cycles. Acco ding
o he Coulombic e iciency, he small excess o ca holy e should ha e
been consumed by cycle 7. Howe e , a change in he capaci y ading a e
was no obse ed, which indica es ha he pa asi ic side eac ions ha
a e esponsible o he Coulombic ine iciency do no esul in a sho -
e m Fa adaic imbalance. Indeed, he Raman signal con i med ha he
ca holy e is no limi ing he capaci y since he in ensi y o e ocyanide
signal ne e app oaches alues close o 0, as he bands o e ocyanide
a e obse ed a he end o he cha ge s ep e en o he las cycle
(Fig. S14). Tha is, e ocyanide was ne e deple ed a he end o he
cha ge s ep, as i would ha e been expec ed when he ca holy e was he
limi ing side as i occu ed in he p e ious expe imen (Fig. 5). The e-
o e, Raman spec oscopy con i ms ha he chemical deg ada ion o
anoly e is esponsible o he capaci y ading, which is suppo ed by he
appea ance o a lowe ol age pla eau in he discha ge p ocess (Fig. S13)
ha is a ibu ed o he o ma ion o 2,6-DHAQ dime s [38].
Thus, a measu emen was designed o shed ligh in o he occu ence
o i e e sible side eac ions wi hou he appa en impac on he ca-
paci y e en ion. Speci ically, he 2,6-DHAQ//K
4
Fe(CN)
6
alkaline RFB
was ope a ed o 10 cycles. Then, he ba e y was ully cha ged and le
a open ci cui ol age o 12 h. Con inuous low o elec oly e was
main ained du ing he open ci cui . A e ha , he ba e y was dis-
cha ged and con inued cycling. The capaci y- ading a e was cons an
be o e and a e he applica ion o he open ci cui ol age: 4.2 %⋅day
−1
.
Du ing he open ci cui ol age, he capaci y ading a e inc eased (5.4
Fig. 6. In si u Raman spec oelec ochemical measu emen s o 2,6-DHAQ//K
4
Fe(CN)
6
alkaline RFBs. Time e olu ion o cell ol age and Raman in ensi y a 2090
cm
−1
measu ed in si u a he ca holy e du ing he i s cha ge/discha ge cycle a ±20 mA cm
−2
(A) ba e y wi h ol age limi s o +1.6/+0.5 V (s anda d condi ions)
using an o e sized ank o he ca holy e (ca holy e: 14 mL o 0.3 M e ocyanide in 1 M KOH; anoly e: 10 mL o 0.2 M 2,6-DHAQ in 1.4 M KOH), and (B) ba e y wi h
ol age limi s o +2.0/+0.5 V (elec ochemical ebalancing condi ions) using a balanced amoun o cha ge o ca holy e and anoly e (ca holy e: 14 mL o 0.3 M
e ocyanide in 1 M KOH; anoly e: 12 mL o 0.2 M 2,6-DHAQ in 1.4 M KOH).
Fig. 7. In si u Raman spec oelec ochemical measu emen o a 2,6-DHAQ//K
4
Fe(CN)
6
alkaline edox low ba e y du ing gal anos a ic cycling a ±20 mA cm
−2
wi h ol age limi s o +1.6/+0.5 V. (A) E olu ion o he discha ge capaci y and Coulombic e iciency upon cycling. (B) Time e olu ion o cell ol age and Raman
in ensi y a 2090 cm
−1
measu ed in si u in he ca holy e du ing he i s 25 cha ge/discha ge cycles. Ca holy e: 14 mL o 0.3 M e ocyanide in 1 M KOH; anoly e: 10
mL o 0.2 M 2,6-DHAQ in 1.4 M KOH.
L. Lubian e al.
Jou nal o Powe Sou ces 646 (2025) 237272
8
%⋅day
−1
) because he 2,6-DHAQ is o ally educed, which a o s he
disp opo ion leading o dime iza ion [38]. The appea ance o a second
ol age pla eau a a lowe ol age (ca. 0.7–0.6 V) indica es ha dime s
we e s ill p esen a e lea ing he cell a open ol age o 12 h
(Fig. S15). Indeed, he leng h o he lowe ol age pla eau (dime s)
inc eased a e he open ci cui as shown in Fig. S15, which is a ibu ed
o he accele a ed dime iza ion occu ing a ully cha ged s a e. The
discha ge capaci y a e he applica ion o he open ci cui ol age
dec eased by 14.5 %, and he mos o i (11.8 %) was eco e ed in he
subsequen cycle (Fig. 8A). While elec ochemical da a alone is no
enough o elabo a e u he on undamen als o he key p ocesses, he
addi ion o he Raman signal p o ided in si u in o ma ion ha allowed
us o p opose a easonable explana ion. One impo an poin o his
discussion is ha Raman spec oscopy con i med ha e ocyanide was
no limi ing he capaci y a e he open ci cui ol age (nei he be o e),
as shown in Fig. S16 and discussed la e . A e uling ou Fa adaic
imbalance, he main sou ces o i e e sible capaci y ading can be
a ibu ed o c osso e and deg ada ion o 2,6-DHAQ. Raman spec os-
copy enabled an es ima ion o he c osso e o 2,6-DHAQ. The 2,
6-DHAQ has a dis inc signal a 1773 cm
−1
. Indeed, he in ensi y a
1773 cm
−1
inc eased lineally wi h 2,6-DHAQ concen a ion o a cali-
b a ion ca ied ou in he p esence o e icyanide (Fig. S17). Du ing he
open ci cui , he signal om 2,6-DHAQ measu ed in he ca holy e
sligh ly inc eased (Fig. S18). Howe e , he amoun was de e mined o be
below 0.5 mM (<0.25 % o ini ial concen a ion). Thus, he c osso e
accoun ed only o a capaci y ading o 0.5 %⋅day
−1
(0.25 % in 12 h) ou
o he 5.4 %⋅day
−1
(i e e sible capaci y ading) obse ed du ing he
open ci cui . In e es ingly, in si u moni o ing o c osso e du ing he
open ci cui ol age s ep sugges s ha c osso e o 2,6-DHAQ inc eased
a highe s a e o cha ge. This is a he su p ising since 2,6-DHAQ be-
comes mo e nega i ely cha ged a ull s a e o cha ge, which should
dis a o c osso e when using a ca ion-exchange memb ane. Ne e -
heless, ecen ly, in-si u NMR measu emen s also epo ed his su p is-
ing inc ease in c osso e a highe s a e o cha ge [21], This wo k
concluded ha he c osso e is mainly d i en by he mig a ion o K
+
om he nega i e compa men o he posi i e compa men . This hy-
po hesis was p oposed based on he obse a ion ha he c osso e a e
inc eased wi h inc easing cu en densi y. Howe e , ou indings we e
ob ained a open ci cui po en ial whe e mig a ion o K
+
h ough he
memb ane does no occu . Consequen ly, ou inding indica es ha
mig a ion o K
+
h ough he memb ane is no he only ac o . Since ou
esul s we e ob ained a equilib ium, he o igin is no likely o be
elec ochemically d i en. Conside ing he la ge di e ences in elec o-
ly e iscosi y o ully cha ged and ully discha ged s a e, his p ope y
may be ela ed o he enhanced c osso e . In pa icula , he anoly e
inc eases signi ican ly i s iscosi y while he ca holy e unde goes mino
changes. Since he sys em was ope a ed a cons an low a e, p essu e in
he nega i e hal -cell is expec ed o inc ease wi h inc easing s a e o
cha ge, while p essu e in he posi i e hal -cell almos does no change.
The p essu e di e ences in he wo hal -cells should each a maximum a
ull s a e o cha ge, which is specula ed o p omo e he c osso e .
Ne e heless, a comple e s udy should be ca ied ou o ob ain unam-
biguous esul s o cla i y his unexpec ed obse a ion in he c osso e
a e. In any case, ou in-si u Raman measu emen s a e highly ele an o
suppo such an unexpec ed beha io . Re u ning o he capaci y ading,
compa ed wi h he alue epo ed in li e a u e (4.6 %⋅day
−1
, [39]), he
sligh ly highe capaci y ading o 5.4 %⋅day
−1
obse ed du ing he open
ci cui a ull s a e o cha ge is now a ibu ed no only o he accele a ed
deg ada ion o 2,6-DHAQ bu also o he sligh ly highe c osso e o
cha ged 2,6-DHAQ.
The mos in iguing aspec o his measu emen occu ed du ing he
open ci cui ol age. The discha ge capaci y dec eases wi h espec o
he p e ious cha ge capaci y. We e e i o as o al capaci y loss (14.5
%). The capaci y o he subsequen cycle inc eased almos o he alue
so he p e ious cycles. Howe e , a loss o 2.7 % wi h espec o he
capaci y o he cycle be o e he open ci cui was obse ed, which is
e e ed o as i e e sible capaci y loss. Thus, a la ge loss o capaci y
was eco e ed, which is e e ed o as he e e sible capaci y loss and
accoun ed o 11.8 % (14.5 % o al – 2.7 % i e e sible) du ing he open
ci cui ol age (Fig. 8A). The e e sible capaci y loss is a ibu ed o he
sel -discha ge p ocesses, namely oxygen e olu ion in he posi i e elec-
oly e which con e s e icyanide o e ocyanide, and hyd ogen
e olu ion o oxygen educ ion in he nega i e elec oly e o oxidize
educed 2,6-DHAQ. I his capaci y ading was due o oxygen-induced
pa asi ic eac ions, a Fa adaic imbalance would occu leading o pa -
ial i e e sible losses. In addi ion, i should be conside ed ha 2,6-
DHAQ can ope a e ou side he he modynamic s abili y window o he
elec oly e (0.142 V s RHE) [13] a high s a e o cha ge (Ne ns ian shi
o he po en ials wi h he s a e o cha ge), so ha hyd ogen e olu ion
eac ion may be expec ed as well, which would ha e he same impac as
oxygen-induced pa asi ic eac ions in he Fa adaic imbalance. Al hough
he kine ics o he hyd ogen e olu ion eac ion (HER) a e expec ed o be
sluggish, a comp ehensi e assessmen o i s long- e m con ibu ion o
Fa adaic imbalance necessi a es u he analysis, which is beyond he
Fig. 8. In si u Raman spec oscopy p obing balanced pa asi ic eac ions in he posi i e and nega i e elec odes o a 2,6-DHAQ//K
4
Fe(CN)
6
alkaline RFB. (A)
Discha ge capaci y and Coulombic e iciency e sus numbe o cycles. (B) Vol age p o iles and Raman in ensi y measu ed in si u du ing gal anos a ic cycling o a 2,6-
DHAQ//K
4
Fe(CN)
6
alkaline edox low ba e y a he ca holy e side a wa enumbe s cha ac e is ic o e ocyanide: 2090 cm
−1
. Cu en densi y: ±20 mA cm
−2
,
ol age limi s o +2.0/+0.5 V. Ca holy e: 14 mL o 0.3 M e ocyanide in 1 M KOH; anoly e: 10 mL o 0.2 M 2,6-DHAQ in 1.4 M KOH.
L. Lubian e al.
Jou nal o Powe Sou ces 646 (2025) 237272
9
