Reversing the Tradeoff between Rate and Overpotential in Molecular Electrocatalysts for H2 Production

Article abstract of DOI:10.1021/acscatal.7b04379

A longstanding challenge in molecular electrocatalysis is to design catalysts that break away from the tradeoff between rate and overpotential arising from electronic scaling relationships. Here we report an inversion of the rate-overpotential correlation

Full text of DOI:10.1021/acscatal.7b04379

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Reversing the Tradeoff Between Rate and Overpotential
2
in Molecular Electrocatalysts for H Production
Christina M. Klug, Allan Cardenas, R. Morris Bullock, Molly O'Hagan, and Eric S. Wiedner
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04379 • Publication Date (Web): 07 Mar 2018
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Christina M. Klug, Allan Jay P. Cardenas,† R. Morris Bullock, Molly OʹHagan,* and Eric S.
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Wiedner*
Center for Molecular Electrocatalysis, Pacific Northwest National Laboratory, P.O. Box 999, K2-57, Richland, WA
USA, 99352
ABSTRACT: A long-standing challenge in molecular electrocatalysis is to design catalysts that break away from the tradeoff
between rate and overpotential arising from electronic scaling relationships. Here we report an inversion of the rate-over-
potential correlation through system-level design of [Ni(PR NRʹ )2]2+ electrocatalysts for the production of H2. The overpo-
tential is lowered by an electron-withdrawing ligand, while the turnover frequency is increased by controlling the catalyst
structural dynamics, using both ligand design and solvent viscosity. The cumulative effect of controlling each of these sys-
tem components is an electrocatalyst with a turnover frequency of 70,000 s–1 and an overpotential of 230 mV, corresponding
to a 100-fold rate enhancement and a 170 mV reduction in overpotential compared to the parent nickel catalyst. Molecular
Tafel plot analysis reveals that the new catalysts reported here are substantially more efficient than other leading molecular
electrocatalysts for production of H2.
2
2
Keywords: electrocatalysis, hydrogen, scaling relation, overpotential, nickel, proton relay, viscosity
event of the catalyst. For an ideal catalyst, the catalytic cur-
rent increases over a narrow potential range before reach-
There is a growing need for electrocatalysts that efficiently
ing a constant value due to steady-state catalytic turnover.
interconvert electrical and chemical energy due to the ex-
The potential-independent turnover frequency (TOF) of a
panded use of solar and wind energy.1-2 A key measure of
molecular catalyst is determined by the rate-limiting
efficiency for an electrocatalytic reaction is the overpoten-
chemical steps of catalysis,7-8 while the overpotential is fre-
tial, defined as the additional potential beyond the ther-
quently reported at the catalytic half-wave potential, Ecat/2
,
modynamic potential needed to drive a reaction at a cer-
tain rate. Balancing the tradeoff between increasing the re-
action rate (or current density) and decreasing the overpo-
tential (wasted electrical energy) is a pervasive challenge in
the search for improved electrocatalysts. In heterogeneous
electrocatalysis, scaling relationships between the catalytic
rate and the binding energies of catalytic intermediates
have emerged as an effective tool for understanding cata-
lytic activity and predicting new catalyst materials.3-5 Scal-
ing relationships can also be transformative in guiding the
design of molecular electrocatalysts. In this study, we
demonstrate that the rate-overpotential correlation of a
molecular electrocatalyst can be shattered if the chemistry
that governs its scaling relationships is well understood,
and therefore controllable.
which is a well-defined point in the voltammogram.9-12
Free-energy correlations between TOF and overpotential
are ubiquitous in molecular electrocatalysis, such that cat-
alysts with a larger TOF also display a higher overpotential.
This correlation has been observed in molecular electro-
catalysts for H+ reduction,13-16 CO2 reduction,17-19 and O2 re-
duction.20-21
Electronic scaling relationships lie at the root of this
tradeoff between rate and overpotential. Multiple studies
have demonstrated that the reduction potential of a cata-
lyst is correlated to the thermodynamics of substrate bind-
ing and activation.20, 22-26 As a result, lowering the overpo-
tential by changing the electronics of the catalyst also leads
to a decrease in the TOF. The pervasiveness of electronic
scaling relationships presents an obstacle to designing mo-
lecular electrocatalysts that display a high TOF and a low
overpotential. Catalysts that break away from the rate-
overpotential correlation are fairly limited and, similar to
enzymes, often contain functional groups in the second co-
ordination sphere to lower the activation barrier for sub-
strate binding to the metal center. Pendant proton relays,27
hydrogen bonding groups,28 electrostatic groups,18 and bi-
metallic catalysts29 have all been used to move away from
Scaling relationships are manifested differently in het-
erogeneous and molecular electrocatalysis due to differ-
ences in how their catalytic activity is measured and re-
ported. Heterogeneous electrocatalysts show an exponen-
tial increase in current at higher overpotential according to
the Tafel equation, and different catalysts are often com-
pared at either a defined overpotential or current density.6
Molecular electrocatalysis is initiated by a discrete redox
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the TOF-overpotential correlation. Alternatively, the rate- all of these system components, we can simultaneously
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overpotential correlation of a catalyst can be changed by
controlling the reaction medium, such as careful selection
of the organic acid30 or addition of water to an organic sol-
vent.31
lower the overpotential by ~200 mV and increase the TOF
by two orders of magnitude, relative to the parent catalyst.
This change in performance is an inversion of the typical
rate-overpotential correlation. This accomplishment re-
sults from a detailed mechanistic understanding leading to
precise control over the structure and function of the cat-
alyst. Analysis of the electronic scaling relationships of
these catalysts demonstrates the need for a system-level
approach to catalyst design, where the catalytic properties
are controlled all the way from the first coordination
sphere of the catalyst to the bulk properties of the reaction
medium.
In designing new catalysts, it is recognized that the re-
lationship between TOF and overpotential can be gov-
erned by complex factors. For the well-known
ʹ
[Ni(PR NR )2]2+ electrocatalysts for production of H2, the
2
2
9
mechanism of catalysis has several branching points aris-
ing from multiple protonation sites, as well as competing
sequences of proton and electron transfer.32-34 Recently, we
demonstrated that incorporation of long alkyl chains onto
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ʹ
the pendant amines of [Ni(PR NR )2]2+ complexes increases
2
2
the TOF by three orders of magnitude with only a minimal
increase in overpotential (70 mV).35 This dramatic rate en-
hancement results from slowing the formation of inactive
“exo-pinch” isomers that arise from exo-protonation at a
single pendant amine, followed by chair-boat isomeriza-
tion of the ligand arms (Scheme 1). Incorporation of the al-
kyl chains slows the chair-boat ring flip that is necessary to
form the thermodynamically and kinetically stabilized
exo-pinch isomer. Importantly, this result demonstrated
that the correlation between rate and overpotential of
Synthesis and Characterization. Complexes 1, 1-C6,
ʹ
and 1-C14 are [Ni(PR NR )2]2+ complexes containing a phe-
2
2
nyl group on the phosphorus ligand and have been previ-
ously reported.35-36 The two new complexes, 2 and 2-C14,
contain electron-withdrawing CF3 groups in the para posi-
tion of the phenyl groups on the phosphorus atoms (Chart
1). Synthesis of complexes 2 and 2-C14 started with the
preparation of diethyl (4-(trifluoromethyl)phenyl)phos-
phonate using a copper-catalyzed variant of the Michaelis-
Arbuzov reaction.37 The phosphonate was converted into
ʹ
[Ni(PR NR )2]2+ complexes could be broken by modifying
2
2
ʹ
the new PR NR ligands using standard methods,15, 38 and
the outer coordination sphere of the catalyst.
2
2
subsequent metalation with [Ni(CH3CN)6](BF4)2 afforded 2
and 2-C14. A cyclic voltammogram of 2 displays two fully
reversible one-electron couples at –0.74 and –0.85 V versus
Cp2Fe+/0, assigned to the Ni(II/I) and Ni(I/0) oxidation
states. The electron-withdrawing CF3-groups of 2 shift the
Ni(II/I) and Ni(I/0) couples more positive than 1 by 70 mV
and 200 mV, respectively. The Ni(II/I) and Ni(I/0) couples
of 2-C14 are observed at –0.75 and –0.86 V, indicating the
alkyl chain on the pendant amine has only a minor effect
on the electronics of nickel.
Scheme 1. One of the branching points in the catalyst
cycle.a
Chart 1. Structures of the nickel complexes used in
this study.
a Each protonation step is a branch point leading to either an
active endo or an inactive exo pathway. The first protonation
step, protonation of Ni(I) (not shown), can also occur at the
exo site, resulting in additional branching that leads to a dou-
bly exo-pinched species.
Despite the tremendous gains in TOF, the
Effect of Solvent Viscosity on Structural Dynamics.
To examine the impact of solution viscosity on the
structural dynamics, we measured the boat-chair
isomerization rate of 1-C6 in mixed solvent systems
containing either acetonitrile or butyronitrile and a
second, more viscous, solvent or analyte. The nickel(II)
complexes reversibly bind acetonitrile to afford a trigonal
bipyramidal complex with two inequivalent phosphorus
ʹ
[Ni(PR NR )2]2+ catalysts continue to suffer from large over-
2
2
potentials (> 400 mV). In this paper, we demonstrate that
ʹ
the overpotential of [Ni(PR NR )2]2+ catalysts can be low-
2
2
ered while maintaining high TOF’s for H2 production. We
combine electron-withdrawing ligands and controlled lig-
and dynamics, achieved with long-alkyl chains on the pen-
dant amines and viscous dinitrile solvents. By controlling
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ACS Catalysis
ʹ
environments, which interconvert through dissociation of
of the catalyst in solution.44 As typical for [Ni(PR NR )2]2+
2
2
1
2
3
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7
8
the acetonitrile ligand, chair-boat isomerization of the
pendant amines, and re-coordination of acetonitrile.39 The
rate constant for this isomerization process was measured
for 1-C6 in the different solvent mixtures by lineshape anal-
electrocatalysts, each catalyst displayed an initial depend-
ence of the TOF on the concentration of both
[DMF(H)][OTf] and water, and became independent of
[DMF(H)][OTf] and water at higher concentrations (Fig-
ure 2b-2c). Therefore the maximum turnover frequency,
TOFmax, is defined at the acid and water concentration that
results in the highest TOF for each catalyst (Figure 2c). The
TOFmax values for each catalyst, as well as other catalytic
data, are given in Table 1.
31
ysis of variable temperature P{1H} NMR spectra as previ-
ously described.39 The microviscosity40-41 of each solvent
system was determined from the inverse of the
translational diffusion coefficient (1/D) of 1-C6, as
1
measured by H-DOSY NMR spectroscopy. Notably, the
9
rate of boat-chair isomerization of 1-C6 decreased as the
microviscosity of the solution was increased (Figure 1),
thereby confirming that solvent can be used to control the
structural dynamics of the catalyst.
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Figure 1. Plot of the chair-boat isomerization rate (298 K) of
1-C6 versus the microviscosity for different solvent mixtures.
In order of increasing microviscosity, the solvents are 20%
(v/v) CD3CN in CD2Cl2,35 190 mM tetrabutylammonium
tetrakis(pentafluorophenyl)borate in CH3CN, 50:45:5% (v/v)
butryonitrile:dibutylformamidium
bis(trifluoromethane-
sulfonyl)imide:H2O,35 20% (v/v) CD3CN in 2-propanol,35 and
20% (v/v) butyronitrile in 2,6-dimethyl-4-heptanol.
Electrocatalytic Measurements. Electrocatalytic ex-
periments were performed in organic solvent with 0.2-1.3
M H2O and the organic acid N,N-dimethylformamidium
triflate, [DMF(H)][OTf]. The addition of water has been
previously shown to enhance the catalytic rate by decreas-
ing the steric penalty of protonation and deprotonation.42-
43 Classical “S-shaped” catalytic waves were observed using
scan rates (υ) of 0.2-2.0 V s–1 and low catalyst concentra-
tions of 10-70 µM (Figure 2a). Appearance of this “S-
shaped” wave indicates that catalysis is occurring under
steady-state conditions in the electrochemical diffusion
layer. The catalytic turnover frequency, TOF, was meas-
ured using equation 1, where D is the diffusion coefficient
Figure 2. a) Cyclic voltamograms of 10 µM 1-C14 in hexanedi-
nitrile (0.2 M [Bu4N][BF4]) with increasing concentrations of
[DMF(H)][OTf], followed by increasing concentration of H2O.
b) Plot of TOF of 1-C14 versus concentration of
[DMF(H)][OTf]. c) Plot of TOF of 1-C14 versus concentration
of H2O. The dashed line indicates the maximum turnover fre-
quency, TOFmax
.
2
1
푖cat
[
Butanedinitrile (succinonitrile) and hexanedinitrile (ad-
iponitrile) were used as the solvent in electrocatalytic stud-
ies to increase the solution viscosity relative to acetonitrile
without substantially changing the other bulk properties.45
Butanedinitrile is a solid at room temperature and was
TOF =
(
)
(1)
]
푇
퐷 2퐹퐴 Cat
of the catalyst, icat is the potential-independent catalytic
current, F is Faraday’s constant, A is the geometric surface
area of the electrode, and [Cat]T is the total concentration
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Table 1. Turnover frequencies and overpotentials for H2 production by [Ni(PR NR 2)2]2+ catalysts.
Page 4 of 12
′
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1
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Catalyst
Solvent a
Microviscosity, 1/D
TOFmax
(s-1)
E°ʹBH+/H2
(V vs Fc+/0
-0.43 d
-0.43 d
-0.43 d
-0.43 d
-0.43 d
-0.39
Ecat/2
(V vs Fc+/0
-0.83
η at Ecat/2
(mV)
400
(×109 s m-2)
)
)
b
1
acetonitrile c
acetonitrile e
acetonitrile f
1.0
720
1-C6
1-C14
2
1.3
740
-0.79
360
1.6
98,000
160
-0.83
400
acetonitrile
0.8
-0.74
310
9
2-C14
1-C6
1-C14
1-C6
1-C14
2-C14
acetonitrile
1.7
11,000
7,700
-0.69
-0.69
-0.72
260
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80% butanedinitrile
80% butanedinitrile
hexanedinitrile
hexanedinitrile
hexanedinitrile
12.5
300
13.3
400,000
4,800
1,500,000
70,000
-0.39
330
26.3
30.3
25.6
-0.49
-0.77
280
-0.49
-0.79
300
-0.49
-0.72
230
a Conditions: 0.2 M [Bu4N][BF4], 0.2-0.6 M [DMF(H)][OTf], 0.2-1.3 M H2O, 298 K. b Open-circuit potential of 1:1 DMF:[DMF(H)]OTf
with 0.2-1.3 M H2O and 1 atm H2. c Reference 31. d Reference 46. e Reference 47. f Reference 35.
used as a mixed acetonitrile/butanedinitrile solvent sys-
tem. Hexanedinitrile is a liquid at room temperature and
was used as a neat solvent. To report overpotentials in
these dinitrile solvents, the equilibrium potential of 1:1
DMF:[DMF(H)][OTf] was measured in the presence of 0.2-
1.3 M H2O and 1 atm of H2 using a previously reported open-
circuit potential method.46
point of the solvent mixture which does not afford the tem-
perature range necessary to measure the dynamic process.
However, it can be inferred from the studies above, with
data summarized in Figure 1, that the rate of chair-boat
isomerization of 1-C6 slows by an order of magnitude upon
moving from pure acetonitrile to 80 mol% butanedinitrile,
based upon the change in the microviscosity of 1-C6 in
those solvents.
Using 1-C6 as the catalyst in acetonitrile/butanedinitrile
solvent, little change in TOFmax is observed at low bu-
tanedinitrile concentrations (<25 mol%) compared to pure
acetonitrile (TOFmax = 740 s–1).47 A marked increase in TOF-
Catalyst 1-C14 was also studied in the 80 mol% bu-
tanedinitrile solution using [DMF(H)][OTf] as the acid. A
TOFmax of 400,000 s–1 was measured at an initial water con-
centration of 0.28 M, corresponding to a 4-fold increase in
the catalytic rate compared to acetonitrile/water under
similar acid concentrations. TOFmax decreased at water
concentrations above 500 mM, possibly resulting from a
decrease in the solubility of 1-C14 due the hydrophobic al-
kyl chains. Therefore hexanedinitrile, a less polar dinitrile,
was investigated to improve the solubility of 1-C14 at high
water concentrations. In hexanedinitrile, the TOF of 1-C14
increases with increasing water concentration (0.27 M –
1.34 M), resulting in a TOFmax of 1,500,000 s–1 (Figure 2a-b).
This data represents a 15-fold enhancement in catalytic rate
compared to acetonitrile. Similarly, 2-C14 displayed a TOF-
is observed at butanedinitrile concentrations greater
max
than 25 mol%. TOFmax was determined to be 4100 s–1 with
50 mol% of butanedinitrile and 7700 s–1 at 80 mol% bu-
tanedinitrile (Figure 3). This equates to a 10-fold rate en-
hancement in 80 mol% butanedinitrile compared to ace-
tonitrile under similar conditions. We were unable to
quantify the change in structural dynamics of 1-C6 in ace-
tonitrile/butanedinitrile due to the relatively high melting
of 70,000 s–1 in hexanedinitrile, corresponding to a 7-
max
fold increase in rate over acetonitrile.
2-C14 was found to be substantially less stable in the
presence of acid than 1-C14. Cyclic voltammetry experi-
ments of 2-C14 were kept under one hour in length due to
slow decomposition of 2-C14 in the bulk solution. In con-
trolled potential electrolysis experiments, the stability of
2-C14 appeared to be electrode-dependent. With an un-
treated carbon foam electrode, a rapid decrease in current
was observed over 10 minutes during electrolysis of 2-C14
in acetonitrile with [DMF(H)]+ and H2O, resulting in an av-
erage turnover number (TONavg) of 6 and a Faradaic effi-
ciency of 75 ± 5% for production of H2. The electrolysis
Figure 3. Plot of TOFmax for 1-C6 versus microviscosity (298 K)
in acetonitrile / butanedinitrile solvent mixtures. In order of
increasing microviscosity, the solvents are 0%, 25%, 50%, and
80% butanedinitrile in acetonitrile.
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Figure 4. Plot of log(TOFmax) versus overpotential at Ecat/2 showing the different scaling relationships that dictate catalytic perfor-
mance. (A) describes catalysts that differ in the identity of the phosphorus substituent, R. (B) describes catalysts that differ by the
length of the alkyl chain on the pendant amine. (C) describes the electronic scaling relationship of 1-C14 and 2-C14 in acetonitrile.
(D) describes the electronic scaling relationship of 1-C14 and 2-C14 in hexanedinitrile.
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performance was significantly improved by pretreating the
carbon foam electrode at 850 °C under a N2 atmosphere to
reduce the quantity of oxygen functionalities on the carbon
surface.48 Using the pretreated electrode, 2-C14 afforded a
sustained electrolysis current over 1 hour with TONavg = 36
and Faradaic efficiency = 93 ± 5%. These results suggest
that oxygen functionalities on the electrode surface play a
role in decomposition of 2-C14 under cathodic conditions.
tion of the catalytically incompetent exo-protonated spe-
cies. In the following discussion, each scaling relationship
is analyzed within the context of these two rate-limiting
processes.
Scaling Relationships. Rate-overpotential correlations
can be circumvented through mechanistic understanding
that guides system-level control. This result is clearly illus-
trated by the evolution of TOFmax and overpotential for sev-
eral catalysts under different conditions, which can be dis-
sected into four different scaling relationships (Figure 4).
In order of increasing level of control over the catalyst
structure and dynamics, these scaling relationships are: (A)
changing the electronics of nickel through variation of the
phosphorus substituent, (B) slowing the structural dynam-
ics using long alkyl chains on the pendant amines, (C)
combining electronics and structural dynamics by chang-
ing the phosphorus group while using a C14-alkyl chain, and
(D) increasing the solvent viscosity while using the com-
bined electronics and structural dynamics. Qualitative in-
sight into the origin of these relationships can be gained
MeCN
Figure 5. Plot of the pKa
for the protonated pendant
amine versus E1/2 of the Ni(II/I) couple for two intermediates
of different [Ni(PR NPh2)2]2+ complexes. The pKaMeCN for values
2
were determined using previously reported thermodynamic
correlations derived from computations.24
ʹ
from prior computational studies of [Ni(PR NR )2]2+ cata-
2
2
Incorporation of the electron-withdrawing group on
phosphorus for 2 leads to a 90 mV decrease in the overpo-
tential relative to 1. This decrease in overpotential is ac-
companied by a decrease in the TOFmax from 720 s–1 for 1 to
160 s–1 for 2. These findings are consistent with other
lysts. In a microkinetic modeling study of 1, the main cata-
lytic intermediates in the electrochemical diffusion layer
were found be the nickel hydride complex,
[HNiII(PPh2NPh2)2]+, and two different exo-protonated spe-
cies (Scheme 1 and Figure 5).33 TOFmax could be increased
by either accelerating the rate of protonation of the nickel
hydride complex, or by increasing the rate of deprotona-
[Ni(PR NPh2)2]2+ catalysts in which the Ni(II/I) couples are
2
controlled by the identity of the phosphorus substituent. A
very shallow slope of mA = 1/200mV is obtained when
log(TOFmax) is plotted versus the overpotential for various
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catalysts in acetonitrile (Figure 4, black circles), and this By using the solvent viscosity to further slow the struc-
1
2
3
4
5
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7
8
correlation can be considered as the “parent” electronic
tural dynamics of complexes with long alkyl chains, the im-
pact of exo-protonation on the rate-overpotential correla-
tion is further diminished. The electronic scaling relation
of 1-C14 and 2-C14 in hexanedinitrile displays a slope of mD
= 1/50mV (Figure 4, green diamonds), which is steeper than
for the same catalysts in acetonitrile (mC = 1/120mV). As a
result, TOFmax increases more rapidly in hexanedinitrile
than in acetonitrile as the overpotential is increased. Rais-
ing TOFmax by an order of magnitude in hexanedinitrile
only requires an extra 50 mV (2.3 kcal mol–1) of driving
force for the net conversion of 2H+/2e– to H2.
ʹ
scaling relationship of [Ni(PR NR )2]2+ catalysts.
2
2
In addition to increasing the overpotential, shifting the
Ni(II/I) couple to more negative potentials also leads to an
increase in the pendant amine basicity due to inductive ef-
fects.24 This is illustrated in Figure 5 for two key catalytic
intermediates, the active endo-protonated nickel hydride
and the inactive exo-exo intermediate. Because of the in-
ductive effect, complexes that have a higher overpotential
will have a greater thermodynamic driving force and lower
kinetic barrier for protonation of the nickel hydride, lead-
ing to an increase in the rate of H2 formation from this in-
termediate. The effect is counter-balanced by slower recov-
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ʹ
Controlling the system components of [Ni(PR NR )2]2+
2
2
has an extraordinary impact on the catalytic performance.
ʹ
ery from the inactive exo-pinch species at higher overpo-
Complex 1, the first [Ni(PR NR )2]2+ complex reported as an
2
2
electrocatalyst for production of H2,36 displays a TOFmax of
720 s–1 and an overpotential of 400 mV in acetonitrile using
[DMF(H)][OTf] as the acid.31 In complex 2-C14, electron
withdrawing groups were added to phosphorus to lower
the overpotential, and TOFmax was increased by adding
long alkyl chains to the pendant amines and using a vis-
cous dinitrile solvent, both of which slow the chair-boat
flip that leads to the inactive exo-pinch isomers. These sys-
tem-level changes result in a TOFmax of 70,000 s–1 at an
overpotential of 230 mV, corresponding to a 100-fold in-
crease in TOFmax and a 170 mV decrease in the overpoten-
tial compared to 1. This inversion of the rate-overpotential
correlation was only possible by understanding the influ-
ence of the inactive exo-pinch intermediates on the elec-
MeCN
tential, resulting from an increased pKa
of the exo-
pinched intermediates. The shallow slope for the scaling
relation (mA =1/200mV) results from the tradeoff between
endo-protonation and exo-deprotonation as the catalyst
electronics are varied.
The second method used to control the rate-overpoten-
tial correlation is slowing the structural dynamics of the
pendant amines by adding long alkyl chains into the outer
coordination sphere of the catalyst. As described previ-
ously, this modification increases TOFmax by orders of mag-
nitude with only a small increase in the overpotential (Fig-
ure 4, red squares).35 The alkyl chains slow the chair-boat
flip that leads to the thermodynamically stabilized exo-
pinch isomers, thereby allowing the acidic non-pinched
exo-isomers to be deprotonated (Scheme 1). This is purely
a kinetic effect that does not change the electronics of the
catalyst, resulting in a very steep slope of mB = 1/20mV in
the plot of log(TOFmax) versus overpotential. In other
words, it only costs 20 mV (0.9 kcal mol–1) in extra driving
force to increase TOFmax by an order of magnitude.
ʹ
tronic scaling relations of [Ni(PR NR )2]2+ catalysts, as well
2
2
as understanding the detailed mechanism of exo-pinch for-
mation.
Molecular Tafel Plots. Molecular Tafel plots can be
used to readily compare 2-C14 to previously reported elec-
trocatalysts (Chart 2, Figure 6).49 The potential-dependent
turnover frequency (TOFE) of an electrocatalyst can be de-
termined (equation 2) through application of the Nernst
Complex 2-C14 combines electronic effects and a long
alkyl chain in a single catalyst. As a result, 2-C14 does not
fall within either of the individual rate-overpotential cor-
relations for electronics or structural dynamics in acetoni-
trile. Instead, 2-C14 should be considered as part of a new
scaling relation in which the size of the alkyl chain is held
constant and the electronics of the group on phosphorus
are varied (Figure 4, blue squares). Strikingly, the new elec-
tronic scaling relation of 1-C14 and 2-C14 has a slope of mC
= 1/120mV, which is a steeper tradeoff between TOFmax and
overpotential than the parent electronic scaling relation
(mA = 1/200mV). Thus, by slowing the formation of the exo-
pinch, the new electronic scaling relation is less influenced
by the rate of exo-deprotonation than the parent relation.
As a result of the steeper scaling relation, the alkyl-chain
effect is much less pronounced for catalysts having both a
lower overpotential and a diminished basicity of the pen-
dant amines. Catalyst 1-C14 features a 130-fold rate en-
hancement compared to 1, while the electron-deficient cat-
alyst 2-C14 only displays a 70-fold rate enhancement com-
pared to 2.
TOF
TOF퐸
=
(2)
퐹
1 + exp [ (퐸퐻
푅푇
+
− 휂 − 퐸푐푎푡/2)]
equation, where TOF is the potential-independent TOF
measured from icat, F is Faraday’s constant, R is the gas con-
stant, T is the temperature, EH+ is the thermodynamic po-
tential for reduction of the acid, Ecat/2 is the catalytic half-
wave potential, and η is the overpotential.7 The TOF at
zero-overpotential (TOFo) is obtained by extrapolation of
the Tafel plot to η = 0 V. Similar to the exchange current
density in heterogeneous electrocatalysis, TOFo corre-
sponds to the forward rate of proton reduction at η = 0 V,
but not the rate of the reverse reaction. As a result, there is
zero net current at η = 0 V even for large values of TOFo.
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Co(dmgH)2(py), was previously reported by Artero and
Chart 2. Complexes used in Tafel plot analysis.
Savéant.49 It is worth noting that the TOFo value of
Co(dmgH)2(py) was obtained by extrapolating kinetic data
measured at low acid concentrations to 1M acid, which as-
sumes that catalysis remains first-order in acid over this
large range of acid concentrations. Dempsey and co-work-
ers observed a switch in the acid dependence for the BF2-
bridged variant, Co(dmgBF2)2, leading to measurement of
an acid-independent TOF.56 A molecular Tafel plot can be
constructed for Co(dmgBF2)2 using the literature data (see
Supporting Information for details). Interestingly, both co-
baloximes give a similar TOFo despite the difference in re-
action conditions (Figure 6). The low overpotential at Ecat/2
is likely related to the low Co-H bond dissociation free en-
ergies of the cobaloxime hydride intermediates,57-58 such
that making and breaking the Co-H bond during catalysis
does not come with a large thermodynamic penalty. De-
spite their high efficiency, the cobaloxime catalysts are
outperformed by 2-C14 by more than 2 orders of magnitude
in TOFo. Artero has proposed that the ligand oxygen atoms
in cobaloximes function as a proton relay during electro-
catalytic production of H2.53, 59-60 The superior performance
of 2-C14 indicates that precise control over the positioning
of the proton relay is critical.
[FeFe]-hydrogenase61 is a primary inspiration for the use
of proton relays in molecular electrocatalysis. Accurate ki-
netic measurements of the enzyme are complicated by the
need to determine the surface coverage of electroactive en-
zyme molecules that display good interfacial electron
transfer with the electrode.62 In a single-molecule imaging
study of CaHydA [FeFe]-hydrogenase, the TOF for H2 pro-
duction was estimated to be 21,000 s–1 at an overpotential
of 90 mV.63 This value can be used to estimate a TOFo of
630 s–1 by assuming the same Nernstian Tafel slope as found
with molecular electrocatalysts. Comparison of [FeFe]-hy-
drogenase to 2-C14 in this manner reveals that the enzyme
is more efficient by nearly 2 orders of magnitude in TOFo.
Thus, while 2-C14 represents an extraordinary advance-
ment in the design of energy-efficient molecular electro-
catalysts, it does not reach the enzymatic level of perfor-
mance.
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Figure 6. Molecular Tafel plots for selected electrocatalysts
for H2 production. [FeFe]-H2ase at pH 7.0 in H2O. 2-C14:
[DMF(H)]+ in hexanedinitrile/water. Co(dmgH)2py: [HNEt3]+
in DMF, extrapolated to 1M acid. Co(dmgBF2)2: 4-trifluoro-
methoxyanilinium in acetonitrile. 1: [DMF(H)]+ in acetoni-
trile/water. [Ni(7PPh2NPh)2]2+: [DMF(H)]+ in acetonitrile/wa-
ter.
The Tafel plots further highlight the net gain in electro-
catalytic efficiency resulting from the system-level design
approach. TOFo of 2-C14 in hexanedinitrile is nearly 5 or-
ders of magnitude larger than for 1 in acetonitrile. This dif-
ference is much larger than the difference in TOFmax for
these compounds (2 orders of magnitude), and reflects the
positive shift in the electrocatalytic wave of 2-C14 relative
to 1. Comparison of 2-C14 to [Ni(7PPh2NPh)2]2+, the land-
mark catalyst from Helm and co-workers,50 reveals a very
large difference of more than 7 orders of magnitude in
TOFo. The 7PPh2NPh ligand was an early attempt to improve
ʹ
the [Ni(PR NR )2]2+ catalysts by eliminating the possibility
Discovery of efficient electrocatalysts that store electrical
energy in chemical bonds is critical for expanding the use
of renewable energy. The structure of a molecular catalyst
is highly customizable, affording opportunities for precise
control over the catalyst performance. The traditional ap-
proach for designing a molecular electrocatalysts is to
modify the first coordination sphere of ligands bound to
the metal. However, this strategy leads to an undesirable
tradeoff between rate and overpotential due to electronic
scaling relationships of the catalyst. We employed a sys-
tem-level design approach for controlling each catalyst
component from the first coordination sphere all the way
to the bulk solvent. In addition to tuning the first and sec-
ond coordination spheres, we use the outer coordination
2
2
of exo-pinch formation. However, the ligand had two un-
intended consequences for catalysis. First, the smaller bite
angle of the 7PPh2NPh ligand leads to a large negative shift
in the Ni(II/I) couple, resulting in much larger overpoten-
tials. Second, with only a single pendant amine on each lig-
and, the probability of the amine being properly positioned
relative to the nickel center is substantially decreased.51-52
Conformations with the pendant amine pointed away from
nickel do not allow the amine to serve as a proton relay,
leading to lower TOF values than desired.
Cobaloximes are one of the most studied families of mo-
lecular electrocatalysts for H2 production.53-55 The molecu-
lar Tafel plot of the proton-bridged variant,
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sphere and the solvent viscosity to control the dynamics of
Page 8 of 12
to determine the kinetics of the structural dynamics ex-
change process.39
1
2
3
4
5
6
7
8
the pendant amines in the second coordination sphere.
The resulting catalyst system shows an enhanced turnover
frequency at a significantly reduced overpotential, thus cir-
cumventing the rate-overpotential correlation of parent
catalyst.
Electrochemical measurements were recorded using a
CH Instruments 620D potentiostat equipped with a stand-
ard three-electrode cell. The working electrode was a 1 mm
PEEK-encased glassy carbon disc, the counter electrode
was a glassy carbon rod, and the pseudo-reference elec-
trode was a silver wire suspended in a 0.2 M [Bu4N][BF4]
solution of the solvent mixture and separated from the an-
alyte solution by a Vycor frit. Prior to the collection of each
voltammogram, the working electrode was polished using
0.25 µm diamond paste on a polishing pad lubricated with
ethylene glycol, then rinsed with acetonitrile. Ferrocene
was used as an internal reference and all potentials are ref-
erenced to the ferrocenium/ferrocene couple at 0 V. Open
circuit potential measurements were performed as previ-
ously described.46
This study illustrates that efficient proton delivery de-
mands a high level of cooperativity beyond the first and
second coordination spheres. In the synthetic systems of
this study, as in [FeFe]-hydrogenase, the positioning of the
pendant amine needs to be precisely controlled for effi-
cient proton transfer. It is a formidable task to control the
positioning of the relay through ligand design alone with-
out adversely affecting the catalyst thermodynamics or
blocking protonation altogether. Designing the catalyst
beyond the ligand set at a system level parallels the intri-
cate macromolecular structures of hydrogenases and other
enzymes. Notably, the catalyst system presented here
comes closer to matching the efficiency of [FeFe]-hydro-
genase than any other synthetic catalyst reported to date.
These findings suggest that system-level control over the
catalyst structure and function will be required to develop
next generation electrocatalysts for energy transduction
between electricity and chemical bonds.
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Synthesis
(4-(trifluoromethyl)phenyl)diethylphosphonate. In a
modification of a previously described procedure,37 a 100
mL Schlenk flask was charged with Cs2CO3 (24.3 g, 75
mmol) and 40 mL of toluene. To remove residual water
from the Cs2CO3, a Dean-Stark trap and a condenser were
added to the flask under a stream of dinitrogen, and the
mixture was brought to reflux. The mixture was cooled to
room temperature after 1 hour of refluxing, and the liquid
contained in the Dean-Stark trap was discarded. CuI (0.353
g, 3.9 mmol), triethylphosphite (3.2 mL, 3.1 g, 19 mmol),
and 4-trifluoromethyliodobenzene (2.65 mL, 4.90 g, 18.4
mmol) were added and the reaction was brought to reflux.
After five days, the reaction was cooled to room tempera-
ture and filtered. The resulting filtrate was washed with 100
mL of water and extracted with dichloromethane (3 × 100
mL). The organic layers were combined, dried over MgSO4,
and the solvent was removed under vacuum. The resulting
yellow oil was purified by column chromatography (silica,
2:1 hexanes:ethyl acetate, Rf = 0.25). The desired fractions
were combined and the solvent removed under reduced
pressure, resulting in a light yellow oil (3.35 g, 11.8 mmol,
66% yield). 1H NMR (CDCl3): δ 7.9 (dd, 2 H, J= 7.9, 13 Hz),
7.73 (dd, 2 H, J = 3.4, 8.0 Hz), 4.14 (m, 4 H), 1.33 (t, 6 H, J =
7.0 Hz). 31P{1H} NMR (CDCl3): δ 15.6. 19F NMR (CDCl3): δ –
63.3.
General Procedures.
All manipulations were performed under a dinitrogen
atmosphere using standard Schlenk techniques or in a Vac-
uum Atmospheres glovebox. Protio solvents were deoxy-
genated by sparging with nitrogen and dried by passage
through neutral alumina columns in an Innovative Tech-
nology, Inc., PureSolv solvent purification system. Acetoni-
trile-d3 (CD3CN) was dried over P2O5 and purified by vac-
uum transfer. Butanedinitrile was sublimed prior to use.
Dichloromethane-d2 (CD2Cl2) was dried over 3 Å molecular
sieves and purified by vacuum transfer. Hexanedinitrile
was deoxygenated by sparging with nitrogen and passed
through a neutral alumina column. All other reagents from
commercial sources were used as received. H, P{1H} and
19F NMR spectra were recorded on a 500 MHz 1H frequency
Agilent spectrometer. 31P{1H} NMR spectra were referenced
to an external phosphoric acid standard. F NMR spectra
were referenced to an external α,α,α-Trifluorotoluene
1
31
19
1
standard at –63.72 ppm. All H chemical shifts were refer-
(4-(trifluoromethyl)phenyl)phosphine.
A
50 mL
enced to the residual protio solvent, and all J values are
given in Hz. [Bu4N][B(C6F5)4],64 [DMF(H)][OTf],65
[Ni(CH3CN)6](BF4)2,66 1-C6,47 and 1-C1435 were synthesized
as previously described.
Schlenk flask was charged with 20 mL of THF, prechilled
to -35 °C, then LiAlH4 (270 mg, 7 mmol, 3.8 eq) was added
and cooled to -35 °C for 1 hour. Me3SiCl (0.9 mL, 7 mmol,
3.8 eq) was added to the flask and the reaction was further
cooled to -78 °C. A solution of (4-(trifluoromethyl)phe-
nyl)diethylphosphonate (0.53 g, 1.9 mmol) in 5 ml of THF
was slowly added to the reaction over 10 minutes. The mix-
ture was allowed to stir at -78 °C for an additional 10
minutes, then at room temperature for 45 minutes. The
mixture was cooled to 0 °C, then 8 mL of deoxygenated wa-
1
A 300 MHz H frequency Agilent VNMRS system
equipped with a direct detect dual band probe was used for
diffusion experiments. The standard VNMRJ 4.0 H Diffu-
1
sion Ordered Spectroscopy (DOSY) pulse sequence was
1
used.67 A 500 MHz H frequency Agilent VNMRS system
was used to record variable temperature 31P{1H} NMR spec-
tra, and lineshape analysis of the resulting spectra was used
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ter was added and allowed to stir for 5 minutes. The reac- reduced pressure. The resulting solid was dissolved in ace-
1
2
3
4
5
6
7
8
tion was filtered with an inline Schlenk frit and washed
with 15 mL of THF. The solution of phosphine was used in
subsequent reactions without isolation. 31P NMR (THF): δ
–126.0 (t, JPH = 203.7 Hz).
tonitrile (10 mL), filtered, and the solvent removed under
reduced pressure. This solid was dissolved in toluene (10
mL), filtered and the solvent removed under reduced pres-
sure. The solid was triturated with hexanes (20 mL) to ob-
tain a free flowing brick-red solid. Yield = 494 mg (0.22
Bis(hydroxymethyl)(4-(trifluoromethyl)phenyl)phos-
phine. In a 50 mL Schlenk flask, paraformaldehyde (220
mg, 7.3 mmol) was added to a solution of (4-(trifluorome-
thyl)phenyl)phosphine (334 mg, 1.9 mmol) in 20 mL of
THF. The reaction was heated at 60 °C for 18 hours. The
solution was filtered and dried over MgSO4 for 10 minutes.
The solution was decanted and the solvent removed under
reduced pressure. The resulting yellow oil was used in sub-
sequent reactions without further purification. 31P{1H}
NMR (THF): δ –19.6. 19F NMR (THF): δ –66.0.
PArCF32NPh2. In a 100 mL Schlenk flask, bis(hydroxyme-
thyl)(4-(trifluoromethyl)phenyl)phosphine (373 mg, 1.6
mmol) and aniline (102 mg, 0.1 mL, 1.1 mmol) were com-
bined and dissolved into 30 mL of absolute ethanol. The
reaction was stirred under dinitrogen at 70 °C for 16 hours,
during which time a white precipitate formed. The solid
was isolated by vacuum filtration, washed with 10 mL of
cold absolute ethanol, and dried under vacuum for 1 hour.
Yield = 78 mg (0.13 mmol, 24% yield). 31P{1H} NMR
(CD2Cl2): δ –50.3. 19F NMR (CD2Cl2): δ –64.8.
mmol, 91 % yield). The product contained ~5%
[Ni(PArCF32NArC142)2Cl]+ as indicated by additional reso-
31
nances centered at 23.2 and –10.3 ppm in the P{1H} NMR
spectrum (Supporting Information, Figures S7 and S8).
The source of Cl– is unknown; chloride sources, such as
chlorinated solvents, were not used during the synthesis
and workup. We were unable to purify 2-C14 further due
the high solubility of the two complexes. Control experi-
ments demonstrate that the catalytic current decreases in
the presence of added [Bu4N][Cl] (Supporting Information,
Figure S9), indicating that the [Ni(PArCF32NArC142)2Cl]+ impu-
rity is not responsible for the catalytic current of 2-C14. 1H
NMR (CD3CN): δ 7.48 (bs, 8 H), 7.44 (d, 8 H, J = 8.1 Hz),
7.21 (d, 8 H, J = 8.4 Hz), 7.16 (d, 8 H, J = 8.4 Hz), 4.26 (d, 8
H, J = 14.0 Hz), 3.93 (d, 8 H, J = 13.8 Hz), 2.59 (t, 8 H, J = 7.5
Hz), 1.59 (t, 8 H, J = 6.6 Hz), 1.29 (s, 16 H), 1.22 (s, 72 H),
0.86 (t, 12 H, J = 6.9 Hz). 31P{1H} NMR (CD3CN): δ 5.95. 19F
NMR (CD3CN): δ – 64.48, –151.65 (10BF4), –151.70 (11BF4).
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PArCF32NArC142. In a 100 mL Schlenk flask, bis(hydroxyme-
thyl)(4-(trifluoromethyl)phenyl)phosphine (447 mg, 1.9
mmol) and 4-tetradecylaniline (543 mg, 1.9 mmol) were
combined and dissolved into 30 mL of absolute ethanol.
The reaction was stirred at 70 °C for 16 hours, during which
time a white precipitate formed. The solid was isolated by
vacuum filtration, washed with 20 mL of cold absolute eth-
anol, and dried under vacuum for 1 hour. Yield = 480 mg
(0.49 mmol, 52% yield). 31P{1H} NMR (THF): δ –51.5. 19F
NMR (THF): δ –64.2.
Determination of Diffusion Coefficients.
A 300 MHz 1H frequency Agilent VNMRS system
equipped with a direct detect dual band probe was used for
1
diffusion experiments. The standard VNMRJ 4.0 H Diffu-
sion Ordered Spectroscopy (DOSY) pulse sequence was
used.67 Experimental parameters included 4-32 scans, 2.0-
4.0 ms gradient pulse lengths, and 50-400 ms diffusion de-
lays. The diffusion coefficient (D) for 1-C6 in electrocata-
lytic conditions was determined using similar electrolyte,
acid, and water concentrations as the electrocatalytic ex-
periments. Due to the limited solubility and spectral over-
lap of acid resonances and aryl resonances for 1-C14 and 2-
C14, the diffusion coefficients were determined in neat sol-
vents for those complexes. Based on differences in D for 1-
C6 in neat hexanedinitrile (D = 0.39 ± 0.01 × 10–10 m2 s–1) and
with the addition of electrolyte, acid, and water (D = 0.36
± 0.02 × 10–10 m2 s–1), these additives are expected to have
little influence on D for 1-C14 and 2-C14.
[Ni(PArCF32NPh2)2(CH3CN)](BF4)2 (2).
A
solution of
[Ni(CH3CN)6](BF4)2 (29 mg, 0.06 mmol) in 5 mL of acetoni-
trile was combined with a solution of PArCF32NPh (78 mg,
2
0.13 mmol) in 5 mL of THF. The reaction was stirred at
room temperature for 18 hours. The solution was filtered
through a Celite plug and the solvent removed under re-
duced pressure to obtain a red solid. Crystals were grown
from an acetonitrile/toluene solvent mixture. Yield = 73 mg
1
(0.05 mmol, 84% yield). H NMR (CD3CN): δ 7.65 (d, 8 H,
NMR Line Shape and Kinetic Analysis.
The gNMR program68 was used to analyze the exchange
J= 7.6 Hz), 7.48 (t, 4 H, J = 7.5 Hz), 7.33-7.21 (m, 24 H), 4.31
(d, 8 H, J = 4.3 Hz), 4.05 (br s, 8H). 31P{1H} NMR (CD3CN):
δ 1.8. 19F NMR (CD3CN): δ –62.39, –151.65 (10BF4), –151.70
(11BF4).
31
process observed in the variable temperature P{1H} spec-
tra. The spectra were processed in gNMR using a Lo-
rentzian function with line broadening up to 6 Hz. A two-
site exchange model was used to determine the rates of nu-
clei exchange by iteration of the simulated line widths, us-
ing both manual and gNMR algorithm methods. Eyring pa-
rameters were determined using the equation:
[Ni(PArCF32NArC142)2(CH3CN)](BF4)2 (2-C14). A solution of
[Ni(CH3CN)6](BF4)2 (123 mg, 2.5 mmol) in acetonitrile (10
mL) was added to a suspension of PArCF32NArC14 (480 mg,
2
4.9 mmol) in toluene (10 mL). The solution was allowed to
stir at room temperature for 2 days, during which time the
solution became dark red in color. The solution was fil-
tered through a Celite plug and the solvent removed under
‡
‡
∆푆
⁄
푘 = 푇 푒∆퐻
푒
푘
퐵
⁄
푅푇
푅
ℎ
Error bars of 15% were determined for each data point
based on visual inspection of the overlaid experimental
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and simulated spectrum from gNMR. Nonlinear regression
Page 10 of 12
1
2
3
4
5
6
7
8
was done using Profit software with the error applied to the
individual data points. The sample temperature in the
NMR spectrometer was calibrated using a methanol stand-
ard sample with an estimated error of 0.5 K. The error in
the activation parameters were determined by the error
propagation method described by Girolami et al.69
† Department of Chemistry and Biochemistry, State Univer-
sity of New York at Fredonia, Fredonia, NY, USA 14063
This research was supported as part of the Center for Molecu-
lar Electrocatalysis, an Energy Frontier Research Center
funded by the U.S. Department of Energy, Office of Science,
Office of Basic Energy Sciences. The authors thank Dr. Abhi-
jeet Karkamkar for thermal activation of the carbon foam elec-
trodes. Pacific Northwest National Laboratory is operated by
Battelle for DOE.
Electrocatalytic Experiments.
Aliquots of a [DMF(H)][OTf] stock solution were added
to an electrolyte solution of the catalyst until a constant
value of icat was achieved. A similar procedure was followed
for water concentration by adding known volumes of water
to the electrochemical cell containing the electrolyte, acid,
and catalyst. Upon achievement of acid concentration and
water concentration independence, the independence of
the scan rate on the catalytic current was determined. The
performance under optimal conditions was cross-checked
by adding the catalyst to a fresh solution of the electrolyte,
acid, and water. Experiments using 2-C14 were kept under
one hour in length due to catalyst decomposition over
time.
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1. Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.;
Nørskov, J. K.; Jaramillo, T. F. Combining Theory and Experiment
in Electrocatalysis: Insights into Materials Design. Science 2017,
355, eaad4998.
2. Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath, Y.;
Teets, T. S.; Nocera, D. G. Solar Energy Supply and Storage for the
Legacy and Nonlegacy Worlds. Chem. Rev. 2010, 110, 6474-6502.
3. Nørskov, J. K.; Bligaard, T.; Rossmeisl, J.; Christensen, C. H.
Towards the Computational Design of Solid Catalysts. Nat. Chem.
2009, 1, 37-46.
Controlled Potential Electrolysis.
Electrocatalytic hydrogen production for 2-C14 was con-
firmed by controlled potential electrolysis in acetonitrile.
A 30 mL three compartment bulk electrolysis cell was
charged with 8.3 mL of a solution containing 0.2 M
[Bu4N][BF4], 0.1 M [DMF(H)][OTf], 0.8 M water, 50 μM
2-C14, and ferrocene. The working electrode was a cylinder
of reticulated vitreous carbon fixed to a copper wire using
conductive silver epoxy. In some experiments, the carbon
electrode was pretreated by heating to 850 °C under a flow
of N2 for 4 hours. The pseudo-reference electrode was a sil-
ver wire suspended in a 0.2 M [Bu4N][BF4] solution of the
solvent mixture and separated from the analyte solution by
a Vycor frit. The counter electrode was a nichrome wire
suspended in a 0.2 M [Bu4N][BF4] acetonitrile solution sep-
arated by an ultrafine frit. Experiments were performed at
–1 V vs Cp2Fe+/0. Samples of the headspace were removed
via gastight syringe upon completion of the experiment
and analyzed by gas chromatography. Gas analysis for hy-
drogen and nitrogen were performed using an Agilent 6850
gas chromatograph fitted with a 10 ft long Supelco 1/8” Car-
bosieve 100/120 column equipped with a thermal conduc-
tivity detector. The percentage of hydrogen and nitrogen
in the headspace were determined by calibration using gas
standards of known composition.
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