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
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
[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
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
•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.