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MOLECULAR REDOX CARRIERS:LESSONS LEARNED

IN ROUTE TO NEW STRATEGIES

30 JANUARY, 2019WORKSHOP ON NON-AQUEOUS FLOW BATTERIES

CHRISTO S. SEVOVTHE OHIO STATE UNIVERSITY

Batteries are required to store the collected energy, and discharge upon demand.

Unconventional Battery – Unconventional Approach

Develop battery materials by utilizing principles fromphysical and synthetic organic chemistry.

catal.Substrate Product

APPROACH TO ENERGY STORAGE

C+

A–

CA cond

uctiv

e se

para

tor

A physical chemist’s perspective

Energy

0

A + C A– + C+

∆G° > 0

ELECTROCHEMICAL ENERGY STORAGE

potential (V)

A

+ e–

– e–

A–

– e–

CC+

+ e–

low-potential anolytes high-potential catholytes

chemical reversibility

Energy Density Cell Voltage x Solubility x #e– Transferred ∝

Electrolytes must be persistent at all redox states!

RFB ELECTROLYTES

“stability zone”

Py+

+ e–

TS‡dec

Py•

Chemist’s Goal: Maximize E1/2 and ∆G‡.

Py+

persistenceanolyte lifetime

∆G‡dec

stabilityanolyte energy

E1/2

A

A–

A–

ELECTROLYTE STABILITY VS PERSISTENCE

V3+V2+

Energy Density Cell Voltage x Solubility x #e– Transferred ∝

-1.8 potential (V vs. Ag/Ag+)

V(acac)3

0

O

O

OO

OO

V

A LOOK BACK: 1ST-GEN METAL COMPLEXES

Liu, Q.; Sleightholme, A. E. S.; Shinkle, A. A.; Li, Y.; Thompson, L. T. Electrochem. Commun. 2009, 11, 2312

-1.8 0 potential (V vs. Ag/Ag+)

Energy Density Cell Voltage x Solubility x #e– Transferred ∝These complexes, and most others, decompose during bulk cycling.

Cabrera, P. J.; Yang, X.; Suttil, J. A.; Hawthorne, K. L.; Brooner, R. E. M.; Sanford, M. S.; Thompson, L. T. J. Phys. Chem. C 2015, 119, 15882.

A LOOK BACK: REDOX-ACTIVE LIGANDS

L

L

L

M LL

LL

L

L

M LL

L

1. increase ligand denticity2. investigate metal

approach

N

N

N

N

N

M NN

NR

R

Sevov, C. S.; Fisher, S. L.; Thompson, L. T.; Sanford, M. S. JACS 2016, 138, 15378.

M = Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Zn2+

3RD-GEN COORDINATION COMPLEXES

Potential (V vs. Ag/Ag+)

MgL2

MnL2

FeL2

CoL2

NiL2

ZnL2

>200 stable cyclesthrough 2e–

stable for days infully-charged state

at high concentration

L

L

L

M LL

L + 2e–

– 2e–L

L

L

M LL

L

First examples of stable, multielectron charge-discharge cycling of NRFB electrolytes.

CYCLABLE COMPLEXES - NOT ALL

Sevov, C. S.; Fisher, S. L.; Thompson, L. T.; Sanford, M. S. JACS 2016, 138, 15378.

Helm, L.; Merbach, A. E. Chem. Rev. 2005, 105, 1923.

Substitution at octahedral Mn2+ is an

associative mechanism.

Lesson #1: Ligand shedding and its mechanism should be carefully considered.

PERSISTENCE OF M2+ COMPLEXES

Mn(MeCN)62+Ni(MeCN)62+

krel exchange = 10,000krel exchange = 1

Sevov, C. S.; Brooner, R. E. M.; Chénard, E.; Assary, R. S.; Moore, J. S.; Rodríguez-López, J.; Sanford, M. S. JACS 2015, 137, 14465.

Liquid NiL20.75 M maximum

N

N

N

N

N

Ni NN

N

R

R

N+

O Phbuild up

NR

break down

Lesson #2: Polydentate ligands decrease solubility.

NON-INNOCENT LIGANDS

Sevov, C. S.; Hickey, D. P.; Cook, M. E.; Robinson, S. G.; Barnett, S.; Minteer, S. D.; Sigman, M. S.; Sanford, M. S. JACS 2017, 139, 2924

start

N+

Ph O

1+N

Ph O–

1–Py– Py+N

OPh

1Py•

PYRIDINIUM ANOLYTES


[Py•](23 °C)

[Py•](70 °C)Con

cent

ratio

n of

[Py•

] / M

(Con

cent

ratio

n of

[Py•

])-1 /

M-1

time (s x 104)

The rate of decomposition fits a second-order plot, consistent with radical dimerization.Persistence (∆G‡) is the measured rate constant.

(12 h)

k(70 °C) = 6.6 x 10-4 M-1s-1

Py+

Py•

Py–

Anolyte can be isolated in all 3 redox states to characterize solubility and persistence.

QUANTIFYING ANOLYTE PERSISTENCE (∆G‡)

0

1

10

100

1000

-1.25 -1.20 -1.15 -1.10 -1.05 -1.00 -0.95 -0.90 -0.85 0

1

10

100

1000

-1.25 -1.20 -1.15 -1.10 -1.05 -1.00 -0.95 -0.90 -0.85 0

1

10

100

1000

-1.25 -1.20 -1.15 -1.10 -1.05 -1.00 -0.95 -0.90 -0.85

half-

life

at 7

0 °C

(h, 0

.5 M

)

redox potential (V vs. Fc/Fc+)

N

O Ph

N

O Ph

N

PhO

high energylow persistence

low energylow persistence

high energyhigh persistence

low energyhigh persistence

N

O

OCH3

N

O

N

O

Cl

N

O Ph

CF3

PHYSICAL PROPERTIES VS. DEGRADATION


•N

PhO

SH

0.49E1/2 + 1.17SH + 0.07 = ∆G‡dec

Lesson #3: Persistence can be controlled independently of E1/2 by tuning steric properties.

SOLID-STATE ANALYSIS

Persistent radical cations: derivatives of Wurster’s blue.

NR2

NR2

aromatic

Can this architecture be exploited to identify persistent, high potential catholytes?

NR2

NR2R2N

••

E1/2 > 0.2 V?

N

N

N

N

•+– e–

+0.2 V

STERIC HINDRANCE - CATHOLYTES

NR2

NR2

R2N

PF6– - e–

NR2

NR2

R2N

2PF6–

The radical dication is isolable as a pure solid.

N

N

N

Sevov, C. S.; Samaroo, S. K.; Sanford, M. S. Adv. Energy Mater. 2016, 1602027.

CHARGED, ISOLABLE CYCLOPROPENIUM

+0.83 V

Membrane selection in combination with molecular design is key for long-term cycling.

capacity-limiting crossover

NEt2

NEt2

Et2NN+

O Ph

with Dr. Koen Hendriks

PF6–

PF6–

N

RO

•

+ e–

PF6– NEt2

Et2N NEt2

+•+

– e–

PF6–

necessary charge balancingse

para

tor

FULL-CELL RFB TESTING

Pair a microporous separator with a redox-active oligomer to prevent crossover

oligomers

Doris, S. E.; Ward, A. L.; Baskin, A.; Frischmann, P. D.; Gavvalapalli, N.; Chénard, E.; Sevov, C. S.; Prendergast, D.; Moore, J. S.; Helms, B. A. ACIE 2017, 56, 1595

Lesson #4: Crossover is a critical limitation, especially at high concentration.

PIM SEPARATORS FOR OLIGOMERIC ANALOGS

NN

NN

NR2R2N

R2N NR2

R2N NR2

R2N NR2

Hendriks, K. H.; Robinson, S. G.; Braten, M. N.; Sevov, C. S.; Helms, B. A.; Sigman, M. S.; Minteer, S. D.; Sanford, M. S. ACS Central Science 2018, 4, 189

Py(•)Py(+) Py(–)-1.2 V -1.6 V

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0electrons per molecule

Lesson #5: Electron transfer between redox states of multielectron species leads to low voltaic efficiency.

0.5 e-/mol at -1.6 V

1 e-/mol at -1.6 V

Py(•)Py(–)

Py(–) is behaving as a redox carrier to charge the Py(+).

Multielectron electrolytes can reduce MW/e– of electrolytes (target ≤150).

2

Py(+)

+Py(•) Py(–)

ELECTRON EXCHANGE

Hendriks, K. H.; Sevov, C. S.; Cook, M. E.; Sanford, M. S. ACS Energy Letters 2017, 2, 2430

1.1 Å2.0 Å

-2.0 -1.5 -1.0 -0.5 0.0

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

Cur

rent

(mA)

E vs. Fc/Fc+ (V)

TBAPF6

KPF6

NaPF6

LiPF6

Salt 1e E1/2 (V)2e E1/2 (V)TBAPF6 -1.18 -1.80KPF6 -1.16 -1.66NaPF6 -1.15 -1.60LiPF6 -1.14 -1.44

start

+ e– •

N

Ph O

N

OPh

N+

Ph O

N

OPh

vs.+ e–

Potassium salts support more negative anolyte potentials than lithium salts.

undesirable anolyte stabilization (lose 260 mV = 6 kcal/mol)

E21/2 = –1.40 VE21/2 = –1.66 V

Li+K+

ELECTROLYTE INTERACTIONS

Hendriks, K. H.; Sevov, C. S.; Cook, M. E.; Sanford, M. S. ACS Energy Letters 2017, 2, 2430

LESSONS IN ELECTROLYTE DESIGN

1. Ligand shedding and its mechanism should be carefully considered.The metal, its oxidation state, and the ligands should be carefully considered.

2. Polydentate ligands generally decrease solubility.Breaking symmetry and polar functional groups increase solubility in polar aprotic solvents.

3. Persistence can be controlled independently of E1/2 by tuning steric properties.Electronic tuning increases lifetime, but generally reduces cell voltage.

Tuning of steric parameters decouples these two features.

4. Crossover is a critical limitation, especially at high concentration.Macromolecules or oligomers paired with inexpensive separators are potential solutions.

5. Multielectron electrolytes often suffer from comproportionation events that reduce voltaic efficiency.

Simultaneous multielectron transfer is preferred over two, single-electron transfer events.

6. The electrolyte can dramatically impact redox potentials because stabilizing interactions.Potassium salts are preferable over lithium salts for anolyte chemistries.