Improving
the
Mg
Sacrificial
Anode
in
Tetrahydrofuran
for
Synthetic
Electrochemistry
by
Tailoring
Electrolyte
Composition
Wendy
Zhang,
Chaoxuan
Gu, Yi Wang,
Skyler
D. Ware,
Lingxiang
Lu, Song
Lin, Yue Qi,
and Kimberly
A. See
*
Cite
This:
JACS
Au
2023,
3, 2280−2290
Read
Online
ACCESS
Metrics
& More
Article
Recommendations
*
sı
Supporting
Information
ABSTRACT:
Mg
0
is commonly
used
as a sacrificial
anode
in
reductive
electrosynthesis.
While
numerous
methodologies
using
a
Mg
sacrificial
anode
have
been
successfully
developed,
the
optimization
of the
electrochemistry
at the
anode,
i.e.,
Mg
stripping,
remains
empirical.
In practice,
electrolytes
and
organic
substrates
often
passivate
the Mg electrode
surface,
which
leads
to
high
overall
cell
potential
causing
poor
energy
efficiency
and
limiting
reaction
scale-up.
In this study,
we seek
to understand
and
manipulate
the Mg metal
interfaces
for a more
effective
counter
electrode
in tetrahydrofuran.
Our
results
suggest
that
the ionic
interactions
between
the cation
and
the anion
of a supporting
electrolyte
can influence
the electrical
double
layer,
which
impacts
the Mg
stripping
efficiency.
We
find
halide
salt
additives
can
prevent
passivation
on the Mg electrode
by influencing
the composition
of the solid
electrolyte
interphase.
This
study
demonstrates
that,
by tailoring
the electrolyte
composition,
we can modify
the Mg stripping
process
and enable
a streamlined
optimization
process
for the development
of new
electrosynthetic
methodologies.
KEYWORDS:
sacrificial
anode, Mg passivation,
Mg stripping,
tetrahydrofuran-based
electrolyte,
reductive
electrosynthesis
■
INTRODUCTION
In recent
years,
electrochemistry
has received
renewed
interest
in the synthetic
community
as a tool
to prepare
useful
and
complex
organic
molecules.
1
−
4
Electrochemistry
offers
unique
advantages
over
traditional
synthetic
organic
methods
due to
its ability
to achieve
highly
selective
oxidative
and
reductive
transformations.
3
Using
electrons
as the reactants,
electro-
chemistry
avoids
the use of harsh
and often
toxic
traditional
oxidants/reductants,
giving
rise
to mild
reaction
conditions
along
with
high
atomic
efficiency.
5,6
Optimizing
an electro-
chemical
reaction
requires
careful
consideration
of the
reactions
that
occur
at both
the working
electrode
(WE)
and
the counter
electrode
(CE).
The
reaction
occurring
at the CE
is called
the counter
reaction.
For
organic
electrosynthesis,
efficient
oxidation/reduction
of a sacrificial
reagent
is typically
employed
as the counter
reaction.
7,8
The
simplest
counter
reaction
is metal
stripping,
in which
a sacrificial
metal
electrode
is simply
oxidized
to form
soluble
metal
cations
that
dissolve
into
the
reaction
mixture.
9,10
Magnesium
is commonly
employed
as a sacrificial
anode
due
to its low
oxidation
potential
(
−
2.37
V vs SHE),
high
Earth
abundance,
low
toxicity,
and apparent
ease
of handling
on the benchtop.
10
−
13
Although
Mg CEs
nominally
involve
metal
stripping,
side
reactions
can cause
issues
with
electrochemistry.
For example,
supporting
electrolyte
anions
such
as ClO
4
−
, PF
6
−
, BF
4
−
, triflate
(OTf
−
), and bis(trifluoromethanesulfonyl)imide
(TFSI
−
) react
with
Mg,
generating
high
impedance,
insoluble
interphases
(Figure
1).
12,14
In addition
to reacting
with
electrolyte
anions,
Mg can also react
with
organic
substrates,
especially
commonly
used
organohalides.
15
While
this reactivity
has proven
useful
for the formation
of Grignard
reagents,
it also causes
significant
changes
to the morphology
16,17
and
composition
of the Mg
electrode
surface.
Additionally,
the Mg stripping
process
occurs
to such
an extent
that
high
concentrations
of Mg
2+
salts
are
formed
in solution.
If the solubility
of the Mg
2+
salts
formed
is
low in the organic
solvent,
the salts
will precipitate
onto
the
electrode.
18
Supporting
electrolyte
anions
are necessary
for
electrolyte
conductivity,
organohalides
are
often
used
as
synthetic
building
blocks,
19
and
the formation
of Mg
2+
is
unavoidable;
thus,
it is challenging
to maintain
a stable
Mg
electrode
interface
during
electrolysis.
All the aforementioned
reactions
result
in the
formation
of a high
impedance
interphase
at the CE. The
high-impedance
interphase
increases
Received:
June
12, 2023
Revised:
July
7, 2023
Accepted:
July
11, 2023
Published:
July
28,
2023
Article
pubs.acs.org/jacsau
© 2023
The Authors.
Published
by
American
Chemical
Society
2280
https://doi.org/10.1021/jacsau.3c00305
JACS
Au
2023,
3, 2280
−
2290
This article is licensed under CC-BY-NC-ND 4.0
the cell voltage
and
lowers
the efficiency
of the reaction.
In
extreme
cases,
the
cell
voltage
increases
beyond
the
compliance
limits
of the potentiostat,
and the reaction
at the
WE can no longer
proceed.
To achieve
effective
Mg oxidation
at the CE, it is therefore
important
to control
the Mg
metal
interface
to avoid
the
formation
of passivating
interphases.
To date,
there
has been
limited
effort
to address
the
nature
of passivation
or
modification
of reaction
conditions
to control
the Mg interface
during
organic
electrosynthesis.
The
electron
transfer
events
at
the sacrificial
anode
occur
in a heterogeneous
environment,
and thus
their
study
requires
the use of research
techniques
not
traditionally
used
by the
organic
synthetic
community.
10
Traditional
optimization
of electrochemical
reactions
involves
screening
solvents,
supporting
electrolytes,
and
sacrificial
anodes
to achieve
high
yields
of the
desired
product.
20
However,
this
approach
lacks
any
understanding
of the
individual
processes
occurring
at the electrode
interfaces
and
thus
is often
met with
issues
of high
cell voltage,
which
may
or
may
not be due to the CE.
Mg sacrificial
anode
passivation
can not only
hamper
the
optimization
of a new
organic
electrochemical
reaction
but also
make
reaction
scale-up
challenging
due
to the resulting
high
cell
voltage.
18,21
−
28
For
instance,
Lin,
See,
and
co-workers
developed
an electrochemically
driven
cross-electrophile
coupling
of alkyl
halides.
18
Attempts
to perform
the electro-
chemical
reaction
on gram
scale
were
thwarted
by high
anodic
potential
at the Mg CE due to a high
impedance
interphase.
The
interphase
was
composed
of MgBr
2
and
Mg(ClO
4
)
2
as
determined
by various
surface
characterization
techniques.
The
addition
of dimethoxyethane
(DME),
thought
to facilitate
Mg
2+
salt solvation,
resulted
in a decrease
of Mg electrode
passivation,
leading
to a successful
scale-up
of the reaction.
This
study
demonstrates
the
practicality
of tailoring
the
electrolyte
by intentionally
leveraging
an understanding
of the
side
reactions
at the Mg CE.
However,
changing
the solvent
composition
can
dramatically
affect
the
efficiency
and
selectivity
of an organic
reaction.
While
DME
addition
effectively
limits
the Mg
anode
passivation
in the electro-
chemically
driven
cross-electrophile
coupling
reaction,
it may
not be a suitable
solution
to all reactions
that
require
a Mg
sacrificial
anode.
Therefore,
we would
like to study
the Mg
electrode
interfaces
under
common
organic
electrosynthesis
conditions
to gain
more
insight
into
sacrificial
anode
behavior
and
provide
promising
alternative
solutions
to resolve
the
issues
caused
by passivation.
Here,
we investigate
the effect
of supporting
electrolytes
on
Mg stripping
with
the aim of improving
Mg sacrificial
anode
performance
in tetrahydrofuran
(THF)-based
electrolyte.
Currently,
the most
commonly
employed
solvent
for systems
using
Mg sacrificial
anodes
is dimethylformamide
(DMF).
29
However,
the evident
solvent
limitation
could
pose
challenges
when
attempting
to broaden
the
application
of reductive
electrosynthesis
to different
types
of organic
transformations.
Additionally,
due to the toxicity
of DMF
and the restrictions
imposed
by the European
Commission
on its use,
30
finding
alternative
solvents
is of great
interest.
Recently,
researchers
have
attempted
to use
THF
as the
optimal
solvent
in
combination
with
Mg sacrificial
anodes
but have
encountered
anode
passivation
issues.
18,28
By studying
the effects
of the
supporting
electrolyte
on Mg stripping
in THF,
we hope
to
provide
insights
into
the fundamental
factors
affecting
the Mg
sacrificial
anode
performance
and
pave
the
way
for the
discovery
of more
cathodic
reduction
transformations
that
are
achievable
only
in ethereal
electrolytes.
Linear
sweep
voltammetry
(LSV)
demonstrates
that
the supporting
electro-
lyte
choice
has
a significant
impact
on the
stripping
overpotential
and current
density.
Molecular
dynamics
(MD)
simulations
reveal
the
influence
of the
ionic
interaction
between
the cation
and
anion
of the supporting
electrolyte
on the composition
of the electrical
double
layer
(EDL),
which
we correlate
to the
Mg
stripping
current
density.
X-ray
photoelectron
spectroscopy
(XPS)
of the Mg anode
surface
after
anodic
polarization
reveals
the formation
of insulating
interphases
upon
contact
with
ClO
4
−
, PF
6
−
, BF
4
−
, and
tosylate
(OTs
−
) anions.
Inspired
by Mg battery
research,
we use halide
salts
as co-supporting
electrolytes
to inhibit
the formation
of
insulating
interphases
on the Mg electrode.
XPS
reveals
that
bromide
salt addition
results
in a thinner
interphase
that
is
MgBr
2
-enriched.
The
addition
of Br
−
salts
improves
the
efficiency
of Mg stripping
in various
electrolytes
and effectively
prevents
organohalides
from
corroding
the Mg electrode
under
electrolysis
conditions.
■
RESULTS
AND
DISCUSSION
Effect
of
the
Supporting
Electrolyte
Cation
on
Mg
Stripping
To understand
the effects
of electrolyte
composition
on Mg
sacrificial
anode
performance,
we probe
the Mg
stripping
behavior
in THF
with
supporting
electrolytes
commonly
employed
for organic
electrosynthesis
using
LSV.
The
LSV
experiments
are conducted
in three-electrode
cells
with
a Mg
plate
WE,
graphite
CE,
and
Pt
|
Fc/Fc
+
reference
electrode
(RE)
(Figure
2a). All potentials
referenced
hereafter
are vs the
Pt
|
Fc/Fc
+
RE unless
otherwise
noted.
First,
we sweep
the
voltage
positive
from
the open-circuit
voltage
(OCV)
to 0.3 V
at 5 mV s
−
1
. At this point,
the electrode
has been
anodically
polished
to expose
fresh
Mg metal.
Following
the oxidation,
the cell rests
at OCV
for 10 min,
allowing
the freshly
exposed
Mg metal
to chemically
react
with
the electrolyte.
The
LSV
−
OCV
protocol
is repeated
5 times
and the resulting
5th LSV
is
Figure
1.
Mg sacrificial
anodes
are common
CEs
used
in reductive
organic
electrosynthesis.
(a) Ideally,
Mg CEs
undergo
extensive
Mg
oxidation
to Mg
2+
(Mg
stripping)
without
impediment.
(b) In reality,
Mg
0
reacts
with
the electrolyte,
generating
high
impedance
surface
films
that
inhibit
Mg stripping.
JACS
Au
pubs.acs.org/jacsau
Article
https://doi.org/10.1021/jacsau.3c00305
JACS
Au
2023,
3, 2280
−
2290
2281
shown
in Figure
2. The
prior
LSVs
are
shown
in the
Supporting
Information.
All onset
potentials
and
current
densities
at 0.2 V of the LSV
experiments
are tabulated
in
Table
1.
To understand
how
the cations
of the supporting
electrolyte
affect
Mg
stripping,
we compare
the
LSVs
obtained
in
electrolytes
with
tetrabutylammonium
(TBA
+
) and Li
+
cations.
TBA
+
and
Li
+
salts
with
weakly
coordinating
anions
are
popular
supporting
electrolytes
for organic
electrosynthesis
due
to their
high
solubility
in polar
aprotic
solvents
and
minimal
interference
with
organic
reactions.
1
Figure
2 shows
the LSVs
of Mg stripping
in TBA
+
/Li
+
electrolytes
with
TFSI
−
,
OTf
−
, and
ClO
4
−
anions.
Interestingly,
the Li
+
electrolytes
consistently
yield
lower
current
densities
for Mg
stripping
compared
to the TBA
+
electrolytes
with
the same
anions.
The
current
density
additionally
depends
on the anion.
While
the
current
densities
for Mg stripping
are comparable
in LiTFSI
and
TBATFSI
electrolytes,
TBA
+
electrolytes
support
much
higher
current
densities
with
OTf
−
and ClO
4
−
anions
compared
to their
Li
+
counterparts.
The
low conductivities
of LiOTf
and
LiClO
4
electrolytes
(16.5
and 62.6
μ
S/cm,
respectively)
could
be responsible
for the poor
Mg stripping
behavior.
However,
LSVs
with
iR
compensation
show
that
the Li
+
electrolytes
afford
much
lower
current
densities
for Mg
stripping
(see
Supporting
Information),
indicating
that
low
electrolyte
conductivity
does
not explain
the observed
cation
effect.
We
next
hypothesize
that
the observed
Mg
stripping
behavior
stems
from
the ionic
interaction
between
TBA
+
/Li
+
and
the
anions
in the electrolytes.
Compared
to TBA
+
, Li
+
presumably
forms
stronger
ionic
bonds
with
the anions
in the electrolyte
due to its greater
charge
density.
31
The
strength
of the ionic
interactions
can change
the composition
of the EDL
at the Mg
electrode
surface,
which
may
affect
the Mg stripping
process.
To experimentally
probe
the effects
of cation
identity
on
ionic
interactions
in the bulk
electrolyte,
we measure
the
Raman
spectra
of the solutions.
Figure
3a,b
shows
the Raman
spectra
of TBATFSI
and
LiTFSI
in THF.
We measure
the
electrolytes
at the
concentration
that
is used
for
the
electrochemistry,
0.1, and 0.5 M to observe
greater
signal
to
noise.
In all cases,
the speciation
does
not shift
significantly
between
the 0.1 and
the 0.5 M solutions.
The
TBATFSI
solution
has only
one mode
at 742 cm
−
1
(mode
a). Mode
a can
be assigned
to the symmetric
bending
mode,
δ
s
, of the CF
3
in
free
(i.e.,
uncoordinated)
TFSI
−
with
minimal
interactions
with
the cation.
31
However,
the LiTFSI
solution
has
two
modes,
including
mode
a and a new
mode
at 747 cm
−
1
(mode
b). Mode
b is the same
δ
s
CF
3
mode
in the TFSI
−
, but it is
shifted
due
to coordination
with
the Li
+
.
31
The
TBATFSI
Figure
2.
(a) Schematic
of the three-electrode
cell with
a Mg WE,
graphite
CE, Pt
|
Fc/Fc
+
RE, and 0.1 M supporting
electrolyte
in 7 mL
of THF.
Linear
sweep
voltammograms
of Mg stripping
in Li
+
/TBA
+
electrolytes
with
(b) TFSI
−
, (c) OTf
−
, and
(d) ClO
4
−
anions.
All
voltammograms
are collected
at a scan
rate
of 5 mV s
−
1
. Generally,
the TBA
+
electrolytes
yield
higher
anodic
current
densities.
Table
1. Conductivity
(
σ
), Onset
Potential
(
E
on
), and Current
Density
(
j
) of Mg Stripping
in THF
with
Various
Supporting
Electrolytes
supporting
electrolyte
a
σ
(
μ
S/cm)
b
E
on
(V vs Fc/Fc
+
)
c
j
(mA/cm
2
)
d
Figure
refs.
LiTFSI
869.0
−
0.65
1.69
Figure
2b
TBATFSI
846.6
−
0.65
1.90
Figure
2b
LiOTf
16.5
Figure
2c
TBAOTf
327.0
−
1.70
1.19
Figure
2c
LiClO
4
62.6
−
0.90
0.13
Figure
2d
TBAClO
4
289.6
−
0.69
0.30
Figure
5a
TBAClO
4
+ LiBr
157.8
−
2.37
1.21
Figure
5a
TBAClO
4
+ TBABr
212.7
−
2.45
1.60
Figure
5a
TBAOTs
109.0
Figure
5b
TBAOTs
+ LiBr
52.6
−
1.46
0.26
Figure
5b
TBAOTs
+ TBABr
110.5
−
1.42
0.60
Figure
5b
TBAPF
6
506.3
Figure
5c
TBAPF
6
+ LiBr
308.4
−
2.36
1.72
Figure
5c
TBAPF
6
+ TBABr
346.3
−
2.47
1.57
Figure
5c
TBAPF
6
+ LiCl
254.2
−
0.91
0.92
Figure
S3
TBAPF
6
+ LiI
272.4
−
2.25
0.64
Figure
S3
TBABF
4
288.5
Figure
5d
TBABF
4
+ LiBr
118.0
−
1.41
0.23
Figure
5d
TBABF
4
+ TBABr
219.2
−
1.57
1.08
Figure
5d
a
The
electrolyte
is THF
with
0.1 M supporting
electrolyte
of interest
or 0.05
M supporting
electrolyte
+ 0.05
M halide
salt additive.
b
Conductivity
is measured
at 22.0
±
1.0
°
C.
c
E
on
is defined
as the potential
at which
d
j
/d
E
exceeds
0.3.
d
j
at 0.2 V is reported.
JACS
Au
pubs.acs.org/jacsau
Article
https://doi.org/10.1021/jacsau.3c00305
JACS
Au
2023,
3, 2280
−
2290
2282