1
Elec onic S uc u e Dis o ions in Ch omium Chela es
Impai Redox Kine ics in Flow Ba e ies
Jona han R. Thu s on1, Kei h P. Whi e2, Max Kudisch3, Luis Ki su Iglesias4, Julia Lo enze i5,6,
F ancesco Be nasconi5,7, Michael F. Toney2,4,8, Michael P. Ma shak9, Da id Rebe 5*
1) Depa men o Chemis y, Uni e si y o Colo ado – Boulde , Boulde , CO, 80309
2) Ma e ials Science and Enginee ing P og am, Uni e si y o Colo ado – Boulde , Boulde , CO, 80309
3) Chemis y & Nanoscience Depa men , Na ional Renewable Ene gy Labo a o y, Golden, CO, 80401
4) Depa men o Chemical and Biological Enginee ing, Uni e si y o Colo ado – Boulde , Boulde , CO,
80309
5) Empa, Swiss Fede al Labo a o ies o Ma e ials Science and Technology, 8600 Dübendo , Swi ze land
6) École Poly echnique Fédé ale de Lausanne, Ins i u des Ma é iaux, 1015 Lausanne, Swi ze land
7) ETH Zü ich, Depa men o Ma e ials, 8093 Zü ich, Swi ze land
8) Renewable and Sus ainable Ene gy Ins i u e, Uni e si y o Colo ado – Boulde , Boulde , CO, 80309
9) Depa men o Chemis y, Uni e si y o Wyoming, La amie, WY, 82072
*E-mail: da id. ebe @empa.ch
Keywo ds: chela e • molecula s uc u e • elec onic s uc u e • edox kine ics • low ba e y
Abs ac
Aminopolyca boxyla e chela es a e eme ging as a p omising class o elec oly e ma e ials o
aqueous edox low ba e ies, o e ing unable edox po en ials, solubili y, and pH s abili y h ough ca e ul
selec ion o ligands and ansi ion me al ions. Despi e hei po en ial, he impac o molecula s uc u e
modi ica ions on he elec onic and elec ochemical p ope ies o hese chela es emains unde explo ed.
He e, we examine how in oducing a hyd oxyl g oup, o en employed o i s solubilizing p ope ies, o he
backbone o C PDTA, a e e ence chela e ma e ial, signi ican ly changes he he modynamics and
kine ics o he chela e’s edox p ocess. We co ela e changes in molecula and elec onic s uc u es o
di e en elec ochemical esponses esul ing om he hyd oxyl addi ion and show ha he in oduc ion
o his unc ional g oup leads o a dis o ion in he oc ahed al coo dina ion o ch omium. Fu he mo e,
inc eased aniso opic spin densi y and non-in eg al oxida ion s a e changes in he C me al cen e esul
in a la ge ba ie o elec on ans e in C PDTA-OH. We demons a e ha p ese ing a hexacoo dina e
chela e s uc u e ac oss a b oad pH ange is c ucial o e icien low ba e y applica ion and emphasize
ha ligand modi ica ions mus a oid dis o ing he oc ahed al coo dina ion o he ansi ion me al.
In oduc ion
The chela ion o ansi ion me als wi h o ganic ligands can comple ely change he pH s abili y, edox
po en ials, kine ics, and solubili y o compounds ha a e o impo ance o ba e ies, homogeneous
ca alysis, was ewa e ea men , and biochemis y.[1,2] Aminopolyca boxyla es (APC) a e a class o
polyden a e ligands ha ha e eme ged as componen s o highly educing aqueous edox low ba e y
(RFB) nega i e elec oly e ma e ials due o hei abili y o inc ease s abili y o e a wide pH ange h ough
s ong mul i-coo dina e binding mo i s. I on o ch omium, o example, can be chela ed o imp o e he
solubili y and pH s abili y o hese cos -e ec i e aw ma e ials, enabling 2 V-class aqueous RFB
chemis ies.[3–7] Single-c ys al X- ay da a shows ha ch omium o ms a highly educing pen acoo dina e
chela e wi h e hylenediamine e aace a e (EDTA),[8,9] lea ing one ca boxyla e a m unbound in solu ion
This documen is he accep ed manusc ip e sion o he ollowing a icle:
Thu s on, J. R., Whi e, K. P., Kudisch, M., Iglesias, L. K., Lo enze i, J., Be nasconi,
F., … Rebe , D. (2025). Elec onic s uc u e dis o ions in ch omium chela es impai edox
kine ics in low ba e ies. Ba e ies and Supe caps, e202500250 (8 pp.).
h ps://doi.o g/10.1002/ba .202500250
2
and allowing wa e o coo dina e.[10] Howe e , his pen acoo dina e s uc u e ende s he ma e ial
unsui able as a nega i e elec oly e, as he coo dina ion o wa e o ch omium in i s educed s a e is
hypo hesized o ca alyze he hyd ogen e olu ion eac ion (HER).[3] Adding a me hylene g oup o he
diamine backbone allows o he ligand, 1,3-diaminop opane-N,N,N’,N’- e aace a e (PDTA), o w ap
a ound he me al cen e mo e e ec i ely, educing s ain in he equa o ial plane enough o he ou h
ca boxyla e a m o coo dina e o he me al cen e .[3,11,12] The esul ing C PDTA chela e is s able in bo h
i s educed and oxidized s a es, and eme ges as one o he mos compe i i e nega i e elec oly e
ma e ials o nea -neu al pH aqueous RFBs.[13] The excellen s abili y o C PDTA in RFB cycling is linked
o he hexacoo dina e s uc u e emaining also in solu ion, which excludes wa e om he inne
coo dina ion shell, p e en ing pa icipa ion o he chela e in ca alyzing wa e educ ion. Unde s anding
he impac o ligand modi ica ions on he ac i e ma e ial is hus c ucial o designing be e me al chela e
RFB elec oly es. O e he pas decade, many o ganic molecules ha e been conside ed o aqueous
o ganic RFBs leading o iden i ica ion o design ends; he addi ion o hyd ophilic moie ies o iologens,
quinones and o he o ganic amewo ks, o example, was shown o d as ically inc ease solubili ies.[14,15]
Func ional g oups like –OH, –NH2, –SO3H and –PO3H2 a e pa icula ly well sui ed o imp o e o ganic
molecule aqueous solubili y due o hei hyd ogen bonding abili y, acidi y, and pola i y.[16–18] In conjuga ed
o ganic molecules, he addi ion o such g oups is, howe e , associa ed wi h a shi in he molecule’s
educ ion po en ial.[19,20] Subs i u ing a oma ic ings wi h elec on-dona ing g oups such as –OH o –NH2
dec ease he elec on a ini y o he molecule and shi he edox po en ial o mo e nega i e alues, while
elec on-wi hd awing g oup subs i u ions (–COOH, –PO3H2, –CF3, –NO2) show an opposi e e ec .[16,19,21]
S uc u al modi ica ions hus equi e ca e ul conside a ion o s ike a balance be ween solubili y and
educ ion po en ial, as hese me ics di ec ly a ec he ba e ies' ene gy and powe densi ies.
The e o e, ex ending hese s a egies o me al-o ganic chela es o e s a p omising app oach o
imp o ing neu al pH RFBs. He e, we syn hesize and cha ac e ize a no el ch omium chela e, C PDTA-
OH, in oducing a hyd oxyl subs i uen wi h induc i e elec on-wi hd awing p ope ies o he cen al
ca bon a om o he alkyl backbone o PDTA.[22] We s udy he impac o his modi ica ion on educ ion
po en ial and eac ion kine ics, alongside a de ailed analysis o he chela es molecula and elec onic
s uc u e, complemen ing X- ay pho oelec on spec oscopy (XPS) wi h elec on pa amagne ic
esonance spec oscopy (EPR). Ou indings e eal ha he in oduc ion o he OH-g oup induces
signi ican changes in elec ochemical beha io , which a e d i en by al e a ions in he C me al cen e 's
elec onic p ope ies s emming om di e ences in bonding wi h chela ing a oms, i.e., dis o ions o he
oc ahed al coo dina ion sphe e. We hen conduc RFB cycling expe imen s and di ec ly link ba e y
Coulombic ine iciencies o pa asi ic wa e educ ion h ough in-si u gas ch oma og aphic hyd ogen
analysis. Ou esul s show how me al-ligand binding mo i s may supp ess he HER and p o ide guidance
o de eloping new chela ing ligands o applica ion in RFBs.
Resul s and Discussion
Syn hesis and elec ochemical cha ac e iza ion
KC PDTA-OH was syn hesized based on li e a u e me hods[3,23] and pu i ied ia ec ys alliza ion,
yielding da k ed c ys als. KC PDTA-OH exhibi s a UV-Vis spec um cha ac e is ic o hexacoo dina e
ch omium chela es (Figu e 1a).[24] A e e i ying he epo ed mola abso p i i y o he 4A2g 4T2g
ansi ion o 116 M−1 cm−1 a 506 nm using a known concen a ion o KC PDTA-OH, he maximum
solubili y o KC PDTA-OH was de e mined as 0.70 M, compa ed o he p e iously de e mined solubili y
o 1.3 M o KC PDTA.[24,25] This dec ease in solubili y con adic s con en ional assump ions in which he
addi ion o hyd ophilic g oups o o ganic molecules enhances aqueous solubili y, highligh ing he ac ha
me al-o ganic chela es canno be ea ed he same.
Fo compa ison, C EDTA, whe e a wa e molecule displaces one ligand a m, exhibi s abso p ion
peaks a much lowe ene gies, as e iden om i s pu ple colo .[24] The wa e molecule in his
pen acoo dina e complex is dep o ona ed in alkaline en i onmen s (pKa = 7.39), d as ically changing he
p ope ies o he, now blue, chela e.[10] In con as , C PDTA and C PDTA-OH show no di e ences in hei
abso p ion spec a be ween pH 6 and 8, sugges ing ha hei hexacoo dina e s uc u e emains in ac ,
3
which is assumed o be c i ical o RFB in eg a ion (Figu e 1a).[3] These binding mo i s do no allow di ec
compa ison o C EDTA and C PDTA/C PDTA-OH in he pH ange o in e es , and o unde s and he
e ec o hyd oxyl subs i u ion on chela e p ope ies we ocus on he la e in he ollowing discussion.
Figu e 1. Cha ac e iza ion o chela es. a) UV-Vis abso p ion spec a o aqueous KC PDTA, KC PDTA-
OH, and KC EDTA solu ions a pH 6 and 8, no malized o he 4A2g 4T2g ansi ion. b) Cyclic
ol ammog ams o 5 mM KC PDTA and KC PDTA-OH in 1 M KCl, 0.1 M bo a e bu e (pH 9) using
bismu h pla ed glassy ca bon elec odes wi h a scan a e o 100 mV s–1. The Lewis s uc u e o he
chela es is shown in he inse , R = H in C PDTA and R = OH in C PDTA-OH.
The e ec o subs i u ion on chela e elec ochemical esponse was examined using cyclic
ol amme y (CV).[3,26] The edox eac ion was no obse able o KC PDTA-OH using a s anda d glassy
ca bon elec ode, as wa e educ ion occu ed be o e educing he ac i e ma e ial (Figu e S1). The
elec ode was hus pla ed wi h bismu h, an elec oca alys known o educe he cha ge ans e
esis ance.[27–29] The edox eac ions o bo h KC PDTA and KC PDTA-OH can be clea ly obse ed on
he modi ied elec ode wi h educ ion po en ials o −1.31 V and −1.27 V s. Ag/AgCl, espec i ely
(Figu e 1b). P e ious s udies on a oma ic o ganic molecules show ha he addi ion o elec on-
wi hd awing g oups shi s he educ ion po en ial o mo e posi i e alues, o example he addi ion o a
4
sul onic acid g oup (–SO3H) o hyd oquinone inc eased he edox po en ial by ca. 150 mV.[20,30,31] The
shi obse ed o C PDTA-OH can po en ially be explained by he induc i e elec on-wi hd awing e ec
exhibi ed by he hyd oxyl g oup and is in es iga ed below. CV measu emen s eco ded a di e en scan
a es p o ide di usion coe icien s o 2.51 x 10−5 and 1.63 x 10−5 cm2 s−1, and educ ion a e cons an s o
6.67 x 10−9 and 1.36 x 10−11 cm s−1, o C PDTA and C PDTA-OH, espec i ely (Figu e S2). C PDTA-
OH has a sligh ly lowe di usion coe icien , which can be explained by he molecule's la ge size and an
inc ease in hyd ogen bonding in e ac ions be ween he hyd oxyl g oup and su ounding wa e molecules.
These ac o s con ibu e o a la ge sol a ed adius, which is e idenced by highe iscosi y and lowe
ionic conduc i i y (Tables S1 and S2). Mo e in e es ingly, C PDTA-OH exhibi s a educ ion a e cons an
wo o de s o magni ude lowe han ha o C PDTA sugges ing ine icien elec on ans e because o
changes in he chela e's elec onic s uc u e. No e ha hese measu emen s a e o quali a i e
compa ison o he wo chela es only, as he su ace a ea o he pla ed elec odes is no known.
Chela e s uc u al cha ac e iza ion
To unde s and he s a k di e ences in elec ochemical ac i i y, single-c ys al X- ay di ac ion (XRD) was
used o compa e he s uc u es o KC PDTA o KC PDTA-OH. The molecula s uc u e ob ained om
XRD o he hyd oxyla ed de i a i e is shown in Figu e 2a, exhibi ing a hexacoo dina e, pseudo-
oc ahed al s uc u e analogous o C PDTA (Figu e S3) in which he wo amino g oups and ou
ca boxyla e a ms a e bound o he C me al cen e . The equa o ial ligand a oms a e de ined in he
ho izon al plane and consis o N1, N2, O2, and O6, while he axial ligand a oms consis o O4 and O8.
The bond leng hs in C PDTA-OH do no a y signi ican ly om hose o C PDTA. The de ia ion o he
equa o ial oxygens bond angle (O2–C –O6) om 90° is used as a measu e o s ain o APC chela es.[32]
C PDTA-OH exhibi s a la ge equa o ial oxygen angle han C PDTA, 100.5° s 99.4°, espec i ely,
indica ing a mo e dis o ed oc ahed on. Addi ionally, he bond angle be ween he axial oxygens (O2–C –
O8), de ia es u he om 180°, 173.2° o C PDTA-OH s 176.2° o C PDTA. The la ge de ia ions in
bo h angles o C PDTA-OH indica e ha he ca boxyla e a ms canno w ap a ound he C cen e o bond
as e ec i ely, i.e., wi h less o bi al o e lap, o achie e an oc ahed al geome y. Addi ional bond leng hs
and angles a e gi en in Table S3 and e inemen de ails a e shown in Table S4.
Figu e 2. Chela e s uc u al cha ac e iza ion. a) C ys al s uc u e o KC PDTA-OH showing 50%
p obabili y ellipsoids, C a om in magen a, K a om in pu ple, O a oms in ed, N a oms in blue, C a oms in
g ey. The C me al cen e , K ca ion, and coo dina ing a oms a e labelled. One wa e o hyd a ion and
hyd ogen a oms a e omi ed o cla i y. b) Weigh ed adial dis ibu ion unc ion, R( ), spec a o he powde
and 0.5 M solu ion (pu e wa e spec a sub ac ed) o KC PDTA and KC PDTA-OH. c) FTIR spec a o
5
0.5 M KC PDTA and KC PDTA-OH wi h he wa e backg ound sub ac ed in he ange o he COO−
bands.
The chela e s uc u es in solu ion we e s udied ia X- ay o al sca e ing and Fou ie - ans o m
in a ed spec oscopy (FTIR). X- ay o al sca e ing p o ides s uc u al in o ma ion om he sca e ing o
in e a omic pai s h ough pai dis ibu ion unc ion (PDF) analysis, p o iding in o ma ion on he sho - and
long- ange s uc u e o ma e ials. This allows o he compa ison o solid s a e and solu ion s uc u e.[33,34]
The G( ) spec a (whe e is he in e a omic dis ance), de i ed om he Fou ie - ans o m o he co ec ed
and no malized X- ay o al sca e ing da a, show dis ances be ween neighbo ing a oms. The R( ) spec a
ep esen he weigh ed adial dis ibu ion unc ion which allows di ec quan i ica ion o coo dina ion
numbe s by in eg a ing he a ea unde he peak.[35] Fo bo h ma e ials, PDF we e measu ed o he solid
ma e ial and o 0.5 M solu ions o e alua e he coo dina ion o ch omium in solid and in solu ion. The G( )
spec a (Figu e S4) show peaks a ca. 1.95 Å associa ed wi h he C –O and C –N bonds in he ch omium
coo dina ion sphe e o bo h chela es. To e i y he ch omium coo dina ion, he PDF o he solids was
i ed using he XRD a omic pa ame e s as he ini ial con igu a ion and cons aining he i wi h he
symme y exhibi ed in he c ys al s uc u e, o ho hombic and monoclinic, espec i ely, o KC PDTA and
KC PDTA-OH. Full i ing de ails a e desc ibed in he Suppo ing In o ma ion (Me hods, and Tables S5
and S6). As expec ed, and co obo a ing he XRD s uc u e, he ch omium is hexacoo dina e in he solid
s a e o bo h ma e ials (Figu e S5). The R( ) spec a (Figu e 2b) we e hen used o compa e
coo dina ion numbe s in he solid and solu ion (wa e -sub ac ed) phases. By in eg a ing he a ea o he
peak associa ed wi h he C –O and C –N bonds a ca. 1.95 Å, coo dina ion numbe s o 6 we e ob ained
o bo h KC PDTA and KC PDTA-OH in solu ion, in ag eemen wi h ou UV-Vis analysis. We conclude
ha coo dina ion o ch omium in bo h chela es emains hexacoo dina e in solu ion, an impo an
conside a ion o RFB nega i e elec oly es o p e en wa e spli ing.[3,10,36]
To u he in es iga e he ch omium bonding en i onmen , FTIR spec a o 0.5 M chela e solu ions
we e eco ded. Me al-APC chela ion in ol es co alen bonding h ough elec on sha ing be ween he
elec on dense ca boxyla e and elec on de icien me al cen e .[37,38] Fo FTIR, i has been shown ha
chela ion o he ca boxyla e o he me al cen e esul s in a la ge elec on localiza ion on he ca bonyl
g oup by dis u bing ca boxyla e esonance, esul ing in mo e single bond cha ac e o he g oup, and a
edshi o he ligand ca bonyl band o lowe equencies wi h s onge chela ion.[37] In he FTIR spec a
o KC PDTA and KC PDTA-OH (Figu es 2c and S6), one ca bonyl band is p esen a ca. 1625 cm−1 o
each chela e, indica ing he ou ca boxyla e a ms emain chela ed in solu ion.[26,38] A blueshi o ca. 4
cm−1 o highe equencies is obse ed o he ca bonyl band o C PDTA-OH indica ing weake chela ion
compa ed o ha o C PDTA. This implies ha he oc ahed al dis o ion obse ed in he XRD s uc u e
pe sis s in solu ion and sugges s ha he slowe edox kine ics obse ed in elec ochemical es s a e
ela ed o less e icien C –O o bi al o e lap esul ing om inc eased oc ahed al dis o ion, hinde ing
elec on ans e .
Ch omium elec onic s uc u e cha ac e iza ion
Ou da a indica es ha he obse ed a ia ions in elec ochemical esponse s em om al e a ions in
he ch omium elec onic s uc u e due o modi ica ions o he chela e geome y. To be e unde s and
how he hyd oxyl addi ion a ec s he C me al cen e elec onic s uc u e, we used XPS and EPR o
in es iga e he dispa i ies in he elec ochemical esponse.
T adi ionally, XPS has been applied o p obe he su ace chemis y o mo e ex ensi e molecula
s uc u es while s udies o he in ica e in e ac ions wi hin smalle molecula compounds a e less
common.[39–41] XPS spec a o ansi ion me als, oxides, and hyd oxides a e well eco ded in he li e a u e
and he C 2p spec a is mos commonly used o he ch omium co e le el analysis due o i s sensi i i y
o changes in oxida ion s a e and la ge pho oioniza ion c oss-sec ion a he Al Kα X- ay ene gy.[42–45] In
he pho oemission p ocess, he ou e alence con igu a ion o he emi ing elemen (i.e., C ) sc eens
pho oelec on co e-hole in e ac ions o pho o-ejec ed elec ons, educing he measu ed kine ic ene gy o
hese elec ons. The s eng h o hese sc eening in e ac ions go e ns he measu ed binding ene gy
6
posi ion o co e le el elec ons.[46] As such, XPS o he C 2p co e elec ons p o ides a p oxy o he
elec on densi y su ounding ch omium.
Ch omium is no able in ha i exhibi s complex mul iple spli ing beha io in XPS spec a. This
spli ing a ises om he in e ac ion be ween unpai ed d-elec ons o he C (III) ion and he co e hole
o med du ing he pho oemission p ocess. The esul ing mul iple, nea ly degene a e, inal s a es lead o
a cha ac e is ic dis ibu ion o peaks in he spec al en elope, e lec ing bo h spin-o bi coupling and
exchange in e ac ions. Shi s in he magni ude o mul iple peak in ensi ies and posi ions can hus be
indica i e o changes in he e ec i e elec on densi y (i.e., oxida ion s a e) on he emissi e elemen due
o di e en coo dina ing ligand en i onmen s.[44] XPS da a was eco ded o bo h compounds, including
a wide spec um showing no elemen al impu i ies (Figu e S7), and high- esolu ion spec a o he C 2p,
C 1s and K 2p, N 1s, and O 1s co e le els. In he C 2p spec a (Figu es 3a and S8), i is e iden ha
he C 2p1/2 and 2p3/2 peaks o C PDTA-OH a e shi ed o highe binding ene gies by ca. 0.4 eV compa ed
o C PDTA, indica ing a mo e elec on-de icien C me al cen e in C PDTA-OH. Fo con ex , a shi in
binding ene gy o 0.4 eV co esponds o he di e ence obse ed be ween C (III) peaks in C (OH)3 and
C 2O3.[44] This shi esul s om less co alen cha ac e in he chela e bonds, suppo ing ou hypo hesis
o educed elec on sha ing om he ligand a oms o he C me al cen e shown in he FTIR da a and
explains he posi i ely shi ed edox po en ial o C PDTA-OH compa ed o C PDTA. Fo ull i ing de ails
and ligand XPS spec a/analyses, see Suppo ing In o ma ion Me hods and Figu es S9-S11.
Figu e 3. Elec onic s uc u e cha ac e iza ion. a) Fi ed C 2p XPS spec a o KC PDTA and KC PDTA-
OH powde samples, wi h dashed lines in bo h spec a indica ing he binding ene gy o KC PDTA-OH
componen peak cen e s. b) Expe imen al X-band EPR spec a o ozen aqueous solu ions o KC PDTA
and KC PDTA-OH.
To unde s and he di e ences in edox kine ics we complemen ou XPS analysis wi h EPR
measu emen s. EPR is used o p o ide addi ional elec onic s uc u e in o ma ion on pa amagne ic
sys ems such as C (III) (d3, S = 3/2) by p obing he local en i onmen and in e ac ions o unpai ed
elec ons. EPR was pe o med on ozen aqueous solu ions o each chela e. Spec a we e simula ed o
elucida e he magni ude o he axial ze o ield spli ing pa ame e , |D|, and he associa ed hombici y,
λ = E/D (whe e E is he hombic ze o ield spli ing pa ame e ), which is a measu e o he asymme y in
he elec on densi y dis ibu ion a ound a pa amagne ic cen e . Simula ed spec a we e compu ed using
a model inco po a ing a dis ibu ion o λ alues as has been p e iously done in a s udy on C (III) chela es
o ca alysis.[47] In S = 3/2 sys ems whe e |D| is la ge han he mic owa e equency used o EPR
measu emen s, he spec al line shape and e ec i e g- alues a e de e mined p edomina ely by λ which
is e y sensi i e o small changes in he coo dina ion en i onmen .[48]
7
The EPR spec a (Figu e 3b) con ain wo sha p ea u es o bo h KC PDTA and KC PDTA-OH a
compa able ield magni udes o 118 mT and 135 mT ha a e assigned ia simula ion as he Ms = ±3/2 (B
pa allel o z-axis) and Ms = ±½ (B pa allel o y-axis) ansi ions, espec i ely. Two addi ional Ms = ±½
ansi ions a e obse ed, he i s a ~200–350 mT (B pa allel o x-axis), and he second a ~400–450 mT
(B pa allel o z-axis) and ex ending beyond he expe imen al window. Bo h ansi ions occu a highe
ields o KC PDTA-OH compa ed o KC PDTA, consis en wi h inc eased hombici y in KC PDTA-OH.
As such, he spec a ag ee well wi h simula ions compu ed using models (Figu e S12 and Suppo ing
In o ma ion Me hods) con aining iden ical pa ame e s excep o an inc eased hombici y (KC PDTA,
λ = 0.200; KC PDTA-OH λ = 0.212) and a la ge ull wid h hal maximum (FWHM, p opo ional o λ). The
inc eased hombici y in KC PDTA-OH indica es a di ec ional asymme y in elec on densi y, consis en
wi h he XRD and FTIR da a showing dec eased molecula symme y, compa ed o KC PDTA. I was
p e iously epo ed ha an inc ease in hombici y in S = 1/2 Cu(II) ac i e si es o p o eins co ela es wi h
an inc ease in he ac i a ion ba ie o elec on ans e , e a ding he edox kine ics.[49] A simila
co ela ion be ween hombici y and edox kine ics is obse ed he ein, whe e KC PDTA-OH exhibi s
highe hombici y and slowe edox kine ics ela i e o KC PDTA. Thus, he inc ease in hombici y may
be associa ed wi h an inc eased ba ie o elec on ans e , po en ially a ising om he s uc u al
dis o ions away om ideal oc ahed al geome y desc ibed abo e.
Fu he mo e, he la ge FWHM o KC PDTA-OH sugges s inc eased luc ua ion in he solu ion s a e
coo dina ion geome y, consis en wi h he hyd ogen bonding capabili y in aqueous solu ion in oduced
by he p esence o he hyd oxyl g oup. In S = 5/2 Fe(III) complexes, an inc ease in hombici y was
obse ed wi h dec easing empe a u e ha was asc ibed o pe u ba ions o hyd ogen bonding
in e ac ions in solid s a e, suppo ing ou in e p e a ion ha sup amolecula in e ac ions in luence he
hombici y o he chela e.[50] As such, he EPR da a is consis en wi h he iscosi y and ionic conduc i i y
da a which indica es ha inc eased hyd ogen-bonding con ibu es o he slowe di usion obse ed upon
educ ion/oxida ion o KC PDTA-OH.
Ba e y in eg a ion
To co obo a e ou spec oscopic in es iga ion unde p ac ical condi ions, he h ee chela es s udied
he e we e used as nega i e elec oly es in ull-cell RFBs using ligands as bu e s (Figu es 4a, b and
S13a).[29] The e o/ e icyanide edox couple, a common posi i e elec oly e,[3,20,51,52] was used in excess
o ensu e all a ia ions be ween he cells esul ed om he capaci y limi ing ch omium nega i e
elec oly es. The cells we e cycled a 100 mA cm−2 wi h lowe and uppe ol age cu -o s o 0.7 V and
2 V, espec i ely. C PDTA-OH exhibi s a signi ican ly lowe a e age Coulombic e iciency o 98.2%
compa ed o 99.3% o C PDTA. The addi ion o he hyd oxyl g oup and la ge sol a ed adius, as
e idenced by dec eased di usi i y, may anspo mo e wa e o he elec ode su ace upon pola iza ion,
making wa e mo e eadily a ailable o be educed a he elec ode in a s a ic sys em. Howe e , gi en
he o ced con ec ion in he low cell se up, mass anspo limi a ion o wa e is unlikely o domina e
hyd ogen e olu ion a es, and we hypo hesize ha he slowe edox kine ics o he ac i e ma e ial esul
in mo e elec ons being a ailable o side eac ions, such as HER, pe uni ime. This hypo hesis was
es ed by pe o ming linea sweep ol amme y on esh cells while sampling he headspace o he
nega i e elec oly e ese oi ia gas ch oma og aphy o hyd ogen. This allows p ecise de e mina ion o
he HER onse po en ial as well as he cell ol age dependen hyd ogen gene a ion a e. Vol age sweeps
we e pe o med a 0.05 mV s−1 om 1.2 o 1.7 V, and he gas was sampled in i e-minu e in e als (Figu e
4c). The onse o hyd ogen gene a ion is de ec ed a 1.35 and 1.38 V wi h C PDTA-OH and C PDTA,
espec i ely. While simila o e all cu en s a e obse ed (Figu e S13b), he hyd ogen gene a ion a e is
an o de o magni ude highe wi h C PDTA-OH, consis en wi h i s lowe Coulombic e iciency in RFB
es s. The compa able hyd ogen gene a ion onse po en ials bu ma kedly di e en hyd ogen gene a ion
a es sugges ha ewe elec ons a e aken up by C PDTA-OH pe uni ime compa ed o C PDTA. This
p o ides a clea demons a ion o he slowe edox kine ics o C PDTA-OH, caused by s uc u al
dis o ions, leading o dec eased Coulombic e iciency in high- ol age RFBs.
8
The C PDTA-OH cell also exhibi s p onounced capaci y ading (Figu es 4a and S13a). pH-
dependen CV and UV-Vis expe imen s e eal ha bo h C PDTA and C PDTA-OH lose hei edox
ac i i y abo e pH 12, likely due o hyd oxide ions g adually displacing he ligand a ms (Figu e S14). This
chemical deg ada ion is accompanied by a colo change om ed o blue a ound pH 12, esembling he
beha io o pen acoo dina e C EDTA nea pH 8 (Figu e 1a), and he e en ual o ma ion o g een
p ecipi a es abo e pH 13, consis en wi h he o ma ion o C (OH)3. The UV-Vis spec um o C PDTA-OH
a e ba e y cycling co obo a es pH-induced deg ada ion, showing a shi o longe wa eleng hs
cha ac e is ic o pen acoo dina e ligand binding while he spec um o he C PDTA elec oly e emains
unchanged (Figu es S13c and 1a). No ably, e en hough he hyd ogen gene a ion a e is much highe
wi h C PDTA-OH, he bulk pH o he C PDTA-OH elec oly e a e ba e y cycling was lowe (inc eased
om 9.6 o 9.8) han o C PDTA (inc eased om 9.5 o 9.9), sugges ing ha hyd oxyl g oups de i ed
om wa e educ ion a e consumed in he C PDTA-OH solu ion, mos likely by binding o ch omium as
sugges ed by ou spec oscopic analysis. The apid ise in local pH d i en by wa e educ ion hus causes
capaci y ading as hyd oxides displace he ligand a ms.
Figu e 4. Compa ison in edox low ba e ies. a) Vol age p o iles o RFBs con aining 0.5 M KC PDTA o
KC PDTA-OH nega i e elec oly e, espec i ely, cycled a 100 mA cm–2, and b) he co esponding
Coulombic e iciencies. c) Hyd ogen gene a ion du ing linea ol age sweeps in ull cells.
These challenges a e e en mo e p onounced wi h C EDTA elec oly es. Wi h his pen acoo dina e
chela e ha has a hyd oxyl bound o ch omium a pH 9.5, we obse e an a e age Coulombic e iciency
o only 96.9% and much as e capaci y ading as homogeneous wa e educ ion is ca alyzed by he
ch omium cen e al eady wi h he i s cha ge cycle (Figu e S15). This in e p e a ion is suppo ed by a
signi ican ly lowe hyd ogen gene a ion onse po en ial o 1.28 V and p onounced wa e educ ion. The
hyd ogen gene a ion is so ex ensi e e en a ela i ely low cell ol ages ha he cu en and hyd ogen
low a e peak a 1.5 V, likely due o elec ode su ace a ea being blocked by gas bubbles (Figu e S15d,
e). This highligh s he c i ical impo ance o main aining hexacoo dina e chela ion ha excludes wa e
om he coo dina ion sphe e and a mo e egula chela ion geome y o e icien elec on ans e as key
design p inciples o de eloping me al-o ganic chela es as nega i e elec oly es in aqueous edox low
ba e ies.
Conclusion
We explo ed he impac o aminopolyca boxyla e ligand modi ica ions on he molecula and elec onic
s uc u e, as well as he elec ochemical beha io , o KC PDTA and i s hyd oxyl-modi ied de i a i e
KC PDTA-OH. C ys allog aphic and X- ay o al sca e ing analyses e ealed ha KC PDTA-OH exhibi s
a mo e dis o ed pseudo-oc ahed al geome y compa ed o KC PDTA, which pe sis s in solu ion. FTIR,
XPS, and EPR analyses u he showed ha he hyd oxyl g oup in oduces elec on de iciency a he
ch omium me al cen e , aising he edox po en ial and hinde ing elec on ans e h ough weake ligand-
9
me al o bi al o e lap. These s uc u al dis o ions esul in sluggish edox kine ics and an inc eased
p opensi y o hyd ogen e olu ion du ing low ba e y ope a ion. Ou indings highligh he c i ical ole o
main aining a hexacoo dina e s uc u e and minimizing oc ahed al dis o ions o op imize ligand-me al
in e ac ions o high-pe o mance edox low ba e y nega i e elec oly es, as a dis o ed coo dina ion
geome y esul s in dec eased me al-ligand o bi al o e lap, which in u n leads o slowe edox kine ics
and mo e p onounced wa e educ ion, causing pH-induced ma e ial deg ada ion. Fu u e s udies should
explo e he ole o elec on-dona ing o -wi hd awing subs i uen s and longe ca boxyla e a ms o
disen angle he e ec s o oc ahed al dis o ion and elec onic modi ica ions on edox beha io and
s abili y.
Addi ional In o ma ion
The au ho s ha e ci ed addi ional e e ences wi hin he Suppo ing In o ma ion.[53–61]
Deposi ion Numbe (s) <u l
h e ="h ps://www.ccdc.cam.ac.uk/se ices/s uc u es?id=doi:10.1002/ba .202500250"> 2391652 ( o
KC PDTA), 2391651 ( o KC PDTA-OH)</u l> con ain(s) he supplemen a y c ys allog aphic da a o
his pape . These da a a e p o ided ee o cha ge by he join Camb idge C ys allog aphic Da a Cen e
and Fachin o ma ionszen um Ka ls uhe <u l h e ="h p://www.ccdc.cam.ac.uk/s uc u es">Access
S uc u es se ice</u l>.
Acknowledgemen s
The PDF esea ch used beamline 28-ID-1 (PDF) o he Na ional Synch o on Ligh Sou ce II, a
US Depa men o Ene gy (DOE) O ice o Science Use Facili y ope a ed o he DOE O ice o Science
by B ookha en Na ional Labo a o y unde Con ac No. DE-SC0012704. The au ho s hank Keenan
Wya o his assis ance in collec ing he PDF da a. The XPS da a was collec ed a he COSINC acili y
a CU Boulde . EPR measu emen s and analysis we e pe o med a NREL using he Ad anced Spin
Resonance Facili y wi h unding p o ided by he US Depa men o Ene gy, O ice o Science, as pa o
BioLEC EFRC unde g an No. DE-SC0019370. This wo k was unded in pa by he Na ional Science
Founda ion unde G an No. 2155227. J.R.T. was unded in pa by he King Fellowship o his wo k.
D.R. acknowledges unding om he Swiss Na ional Science Founda ion (SNSF) Ambizione Fellowship
Z00P2_209078. D.R. also hanks Co sin Ba aglia and Empa's Labo a o y Ma e ials o Ene gy
Con e sion o hos ing his esea ch g oup.
This wo k was au ho ed in pa by he Na ional Renewable Ene gy Labo a o y, ope a ed by
Alliance o Sus ainable Ene gy, LLC, o he U.S. Depa men o Ene gy (DOE) unde Con ac No. DE-
AC36-08GO28308. The iews exp essed in he a icle do no necessa ily ep esen he iews o he DOE
o he U.S. Go e nmen . The U.S. Go e nmen e ains and he publishe , by accep ing he a icle o
publica ion, acknowledges ha he U.S. Go e nmen e ains a nonexclusi e, paid-up, i e ocable,
wo ldwide license o publish o ep oduce he published o m o his wo k, o allow o he s o do so, o
U.S. Go e nmen pu poses.
