Tracking Electrical Fields at the Pt/H2O Interface during Hydrogen Catalysis

Article abstract of DOI:10.1021/jacs.9b05148

We quantify changes in the magnitude of the interfacial electric field under the conditions of H₂/H⁺ catalysis at a Pt surface. We track the product distribution of a local pH-sensitive, surface-catalyzed nonfaradaic reaction, H₂ addition to cis-2-butene-1,4-diol to form n-butanol and 1,4-butanediol, to quantify the concentration of solvated H⁺ at a Pt surface that is constantly held at the reversible hydrogen electrode potential. By tracking the surface H⁺ concentration across a wide range of pH and ionic strengths, we directly quantify the magnitude of the electrostatic potential drop at the Pt/solution interface and establish that it increases by 60 mV per unit increase in pH. These results provide direct insight into the electric field environment at the Pt surface and highlight the dramatically amplified field existent under alkaline vs acidic conditions.

Full text of DOI:10.1021/jacs.9b05148

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Article
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Tracking Electrical Fields at the Pt/HO Interface During Hydrogen Catalysis
Jaeyune Ryu, and Yogesh Surendranath
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b05148 • Publication Date (Web): 21 Aug 2019
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Tracking Electrical Fields at the Pt/H2O Interface During
Hydrogen Catalysis
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Jaeyune Ryu, Yogesh Surendranath*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United
States
ABSTRACT: We quantify changes in the magnitude of the interfacial electric field under the conditions of H2/H+ catalysis
at a Pt surface. We track the product distribution of a local pH-sensitive, surface-catalyzed non-faradaic reaction, H2
addition to cis-2-butene-1,4-diol to form n-butanol and 1,4-butanediol, to quantify the concentration of solvated H+ at a Pt
surface that is constantly held at the reversible hydrogen electrode potential. By tracking the surface H+ concentration
across a wide range of pH and ionic strengths, we directly quantify the magnitude of the electrostatic potential drop at the
Pt|solution interface and establish that it increases by ~60 mV per unit increase in pH. These results provide direct insight
into the electric field environment at the Pt surface and highlight the dramatically amplified field existent under alkaline vs
acidic conditions.
solid and is augmented upon adsorption of species from
solution, which can introduce additional dipole
contributions. The latter electrostatic potential (Volta
potential), 휑, determines the magnitude of the interfacial
electric field, resulting from the presence of solvated ions,
free charge, in the double layer.3 Through this manuscript,
we will refer to the “electrostatic potential” explicitly and all
other use of the word “potential” will take the common
meaning of electrochemical potential.
Because of this key distinction, quantifying the
magnitude of the interfacial field is challenging and
requires a technique that decouples the free charging
behavior of the interface from the surface adsorption
reactions that give rise to catalysis.19,20,21 This is
particularly true for catalytically active metals like Pt,
which strongly adsorb ions over a wide potential window.
This decoupling can be achieved by employed interface
specific spectroscopies such as sum frequency
generation (SFG)5,22,23,24,25,26 or ambient pressure X-ray
photoelectron spectroscopy (APXPS)27 in conjunction
with molecular reporters (Figure 1a, left). However, these
methods have, thus far, been predominantly limited to
■ INTRODUCTION
The efficient interconversion of electrical and chemical
energy requires control over inner-sphere bond activation
and electron transfer reactions taking place at electrode
surfaces. Unlike outer-sphere electron transfer reactions,
inner-sphere reactions require bonding between the
substrate and surface and, thus, the local environment
within molecular length-scales of the surface defines the
reaction profile. This local environment is radically
different from the environment in the bulk of electrolyte
because the polarization of the electrode surface
generates a sharp electrostatic potential gradient that
corresponds to electric fields up to 109 V m-1.1,2,3,4 At a
qualitative level, these fields are known to order the
solvent, orient dipolar species, and accumulate ions, all of
which serves to dramatically augment the free energy
landscape for inner-sphere electrocatalysis.5,6,7,8 As a
poignant example, it has long been recognized that Pt
electrodes are ~100 fold less active for H2 evolution
catalysis in alkaline than acidic media,9,10,11,12,13,14,15,16 and
a recent study attributed this to an increased interfacial
field strength in alkaline media that serves to slow proton
transfer to the surface.9,17,18 Clearly, a quantitative
understanding of the interfacial field environment under
reaction conditions is essential for understanding
reactivity trends, and for the rational design of new
catalysts and electrochemical transformations.
The interfacial field is equal to the gradient of the
electrostatic potential at the interface. Importantly, the
electrostatic potential (Volta potential) of an electrode
results purely from the free charge separation existing at
the interface, and is fundamentally distinct from the
electrode (electrochemical) potential, E, which is readily
measured relative to a reference redox couple.1,2,3 Indeed,
E is the sum of the contributions from the intrinsic energy
of electrons in the solid (chemical and dipole potentials)
and the electrostatic contribution from charging the
surface. The former is related to the work function of the
Figure 1. (a) Previous spectroscopic (left) and electrometric
(right) approaches to investigate interfacial field strengths.
(b) This work investigates the field strength in operando
under the conditions of hydrogen catalysis using a surface-
specific non-faradaic reaction.
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studies of relatively inert metals such as Au or Ag.
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electrosorption steps, direct estimation of the field
strength under catalytic conditions requires a strategy for
decoupling faradaic electrosorption events from the free
charging behavior of the electrode that establishes the
strength of the interfacial field. We show that this
decoupling can be achieved by employing a non-faradaic
reaction probe, hydrogenation/hydrogenolysis of cis-2-
butene-1,4-diol, whose selectivity is uniquely sensitive to
the concentration of protons (solvated H+) at the interface.
We use this sensitivity to quantify the extent of H+
migration as a reporter of the magnitude of the interfacial
field strength. Using this approach, we track the interfacial
field strength as a function of pH under the conditions of
H2/H+ catalysis at a Pt surface. Conveniently, the
presence of H2 (1 atm) and the reversible catalytic activity
of Pt serve to pin the electrode to ~0 V vs the reversible
hydrogen electrode (RHE) even in the complete absence
of net faradaic charge flow. Combining these elements,
we show that the magnitude of the electrostatic potential
drop at the Pt interface increases by ~60 mV with each
unit increase in pH. This direct quantification establishes
the amplified field environment attendant to H2/H+
catalysis in alkaline vs acid media.
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Alternatively, as an electrometric approach, laser-induced
temperature-jump techniques can be used to determine
the potential at which the entropy of the solvent is
maximized (Figure 1a, right).28,29,30 This potential of
maximum entropy closely approximates the potential of
zero free charge, EPZFC, at which the interfacial
electrostatic potential is zero and the electric field
vanishes.19 If the population and orientation of surface
adsorbates remain unchanged across a given range of
potentials, the electrostatic potential, 휑, at the interface
can be accurately gauged by the difference between the
E and EPZFC. Unfortunately, for highly active materials
such as Pt, the adsorbate population under relevant
catalytic conditions is often dramatically different than that
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present under the inert conditions used to measure EPZFC
.
Consequently, measurements of EPZFC do not necessarily
provide direct insight into the electrostatic potential drop
and corresponding interfacial field under the conditions of
catalysis. This knowledge gap has impeded quantitative
understanding of the electric field environment under
which electrocatalytic reactions take place.
Herein, we provide quantitative insights into the
magnitude of the interfacial field under the conditions of
electrocatalysis. Since catalytic reactions entail
■ RESULTS AND DISCUSSION
Figure 2. Product fraction of n-butanol, B (in %), at approximately 30% total conversion for Pt/C catalyzed H2 addition to cis-2-
butene-1,4-diol as a function of electrolyte conditions. (a) pH dependence of B fraction under high ionic strength (I) conditions
in the presence of 280 (green, I=0.4 M), 640 (blue, I=0.8 M), and 780 mM (purple, I=1.0 M) of Na+. (b) pH dependence of B
fraction under low ionic strength conditions in the presence of 40 (orange, I=0.05 M) and 8 mM (red, I=0.01 M) of Na+. For
panels (a) and (b), data were collected in the presence of citrate (square) and phosphate (diamond) counter anions. (c) pH
dependence of B fraction with varying cation species, Na+ (orange), Li+ (red), and NH4+ (blue) with 280 (square, I=0.4 M) and 8
mM (diamond, I=0.01 M) of each cation. (d) pH dependence of B fraction with varying substrate (SM = starting material)
concentration, 0.25 (green), 0.15 (orange), and 0.04 M (red), under varying ionic strength electrolytes containing 280 (square,
I=0.4 M) and 8 mM (circle, I=0.01 M) of Na+. All data were collected at the open-circuit potential, ~0 V vs RHE.
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pH and ionic strength dependence of the probe
reaction at the reversible hydrogen potential
the minimal shift of the OCP value from 0 V vs RHE (<10
mV) irrespective of substrate concentration. Together,
these data establish that the dependence of product
selectivity on bulk pH is affected solely by the ionic
strength of the electrolyte with negligible convolution from
other electrolyte variables.
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We exploited the Pt/C-catalyzed reaction of H2 with cis-2-
butene-1,4-diol as a sensitive non-faradaic reaction probe
of the of the environment at the Pt surface (See SI for
experimental details). Pt catalyzes hydrogenation and
hydrogenoloysis of this substrate to 1,4-butanediol (A)
and n-butanol (B), respectively. We first examined the
product selectivity at ~30% of total conversion as a
function of bulk pH in the presence of high ionic strength
electrolytes (Figure 2a). For these experiments, the
concentration of Na+ is fixed for each buffer employed and
the pH was varied by the addition of HClO4. For a Na+
concentration of 280 mM, corresponding to an ionic
strength of 0.4 M (Figure S1), irrespective of whether we
employed a citrate (Figure 2a, green square) or
phosphate buffer (Figure 2a, green diamond), we
observed overlaying data that evinces a systematic
increase in B fraction (B/(A+B) in %) as the pH is lowered.
We note that A and B are the only products detected in all
reaction conditions explored and that the B fraction (%) is
simply the 100-A fraction (%) by definition. Specifically,
the B fraction increases linearly from ~15 % to ~90 % as
the pH decreases from ~7 to ~1 and the dependence tails
towards 100 and 0 % at below pH 1 and above pH 7,
respectively (Figure 2a). The overlay of the data for
citrate and phosphate buffers indicates that this pH
dependence in product selectivity is negligibly convoluted
by specific interactions with the buffer proton donor or
corresponding conjugated base anion. Importantly,
increasing the electrolyte strength to an Na+ concentration
of 0.64 (I=~0.8 M) or 0.78 M (I=~1.0 M) with added
NaClO4 supporting electrolyte (Figure 2a, blue and
purple) leads to no change in the pH scaling in product
selectivity.31 Together these data indicate that (a) this
probe reaction’s selectivity is highly sensitive to the pH of
the electrolyte and that (b) this pH dependence is
insensitive to the ionic strength of the electrolyte beyond
0.4 M of ionic strength.
The pH-dependence of product selectivity is
dramatically altered as the ionic strength of the electrolyte
is lowered (Figure 2b). In the presence of a diluted
electrolyte with 40 mM of Na+ (I=0.05 M) (Figure 2b,
orange), the B fraction at any given pH is dramatically
enhanced relative to the corresponding value measured
in the presence of 280 mM of Na+ (Figure 2b, green).
This promotion of the B fraction grows as the electrolyte
strength is further decreased to 8 mM of Na+ (I=0.01 M)
(Figure 2b, red). The overall variation in product
selectivity is quite dramatic, changing, at pH 7, from a B
fraction of 16 % at 280 mM Na+ to 62 % at 8 mM Na+.
Across this series of decreasing electrolyte strength, we
also find that the pH-dependent selectivity profile is
entirely independent of the identity of the buffering anions
(Figure 2a and 2b) and its counter cations (Figure 2c).
These observations allow us to exclude substantial
contributions from specific adsorption of anions as well as
cations at the Pt surface. Additionally, we find that the
results are independent to the concentration of probe
molecules (Figure 2d), suggesting that substrate
adsorption is not substantially influencing the observed
changes in selectivity. The negligible effect of the
substrate surface interactions is further substantiated by
Surface sensitivity and kinetics of the probe reaction
Kinetic analysis establishes that the probe reaction is
highly sensitive to the local proton concentration (Figure
3, Figure S2-S4). With high ionic strength electrolytes
(280 mM of Na+, Figure 3), we found that, as the pH is
increased, the rate (kobs) of formation of A (blue squares)
increases, whereas the rate of formation of B (red
squares) declines. Despite these changes in the rate of
formation of each product, the aggregate rate of product
generation (grey squares) remained constant over the
entire pH range examined (~0 to 9). At low ionic strength
(8 mM of Na+), we found that the rate of formation of A
(green circles) is suppressed at each pH relative to the
high ionic strength condition (blue squares). In contrast,
the rate of formation of B is enhanced at low (orange
circles) vs high (red squares) ionic strength. These
correlated changes in the observed rate for A and B
formation give rise to the selectivity enhancement for B at
low ionic strength (Figure 2b). Importantly, despite the
changes in the rate of formation of A and B, the total
conversion rate (black circles) remained unchanged over
the entire pH range examined (~2 to 11) and is the same
as the total conversion rate at high ionic strength (grey
squares). Indeed, regardless of the electrolyte strength,
composition, or pH, the aggregate rate of the non-faradaic
reaction to produce A and B remains constant (Figure
S5). Only the selectivity for B relative to A changes as the
pH or electrolyte strength is altered. In addition, our
previous mechanistic analysis,31 confirmed that the Pt
surface does not catalyze the conversion of A to B. Taken
together, these results indicate that the reaction proceeds
through the pH-independent rate-limiting formation of a
common surface-bound intermediate, I*, followed by pH-
dependent kinetic branching to yield A or B (Figure 4a).
In line with previous literature,32 we propose that I* is a
partially hydrogenated Ptalkyl species. Bronsted and
Lewis acids are known to interact with oxygen moieties in
the C-O bonds of many organic substrates, thereby
lowering the activation barrier for C-O cleavage.33 We
expect that, in a similar way, the proton concentration
buildup near the surface assists or promotes the
hydrogenolysis (C-O bond cleavage) of I* that leads to the
formation of B. Regardless of the precise structure of this
intermediate, the fact that it is a surface-bound species
ensures that this reaction is a highly localized probe of the
proton population within molecular length scales of the Pt
surface. Therefore, changes in product selectivity provide
a measure of the variation in proton excess within the
double layer.
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produced by concurrent HOR catalysis augment the local
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pH and lead to an increased B% (Figure S6c) while
maintaining the same overall conversion rate. This further
establishes that the probe reaction is sensitive to the local
proton concentration but is otherwise orthogonal to the
H2/H+ equilibration reactions.
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The local pH change estimates the electrostatic
potential at the reaction plane inside the double layer
Taking into account that (a) the probe reaction is sensitive
to proton concentration near the OHP and (b) that the
potential is pinned to 0 V vs RHE irrespective of varying
electrolyte compositions, the observations in Figure 2a
and 2b can be rationalized based on field-induced
migration of H+ to the surface (Figure 4). For any metal-
solution interface, electronic charge excess at the
electrode will be counter-balanced by an opposite ionic
charge excess in solution. For electrode potentials
negative of EPZFC, the solution charge excess will be
comprised predominantly of cations, which, in this case,
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Figure 3. Log kobs versus pH for the rate of total conversion,
1,4-butanediol (A) formation and n-butanol formation (B).
Squares correspond to data collected with high ionic strength
electrolytes (I=0.4 M) containing 280 mM of Na+ (grey, blue,
and red correspond to total conversion and A and B
formation, respectively). Circle correspond to data collected
with low ionic strength electrolytes (I=0.01 M) containing 8
mM of Na+ (black, green, and orange correspond to total
conversion and A and B formation, respectively).
+
consist of Na+ (or Li+/NH4 ) and H+, the only two cations in
the medium. At high Na+ concentrations beyond 280 mM
(I=0.4 M), the electrode charge excess is fully
compensated by Na+, and thus, we sample the pH
dependence of reaction selectivity in the absence of
proton migration (Figure 2a and Figure 4b). Importantly,
upon changing the ionic strength, the potential of the
electrode remains pinned at 0 vs RHE in this system and
thus the charge excess in the Pt at given pH should also
remain constant. As the Na+ concentration is decreased,
the charge excess in the metal must be balanced, to a
greater extent, by H+ ions that migrate to the surface.
Therefore, the resulting proton excess leads to an
increase in B fraction at a given pH as the Na+
concentration is lowered (Figure 2b and Figure 4c).
Importantly, all of these measurements were conducted
with the electrode held at the open circuit potential (OCP).
Despite the absence of external potential control, we find
that the high reversibility of Pt for the H2/H+ couple serves
±
to pin the OCP at 0 10 mV vs RHE across all of the
reaction conditions examined. Consequently, this method
provides a direct probe of the local proton concentration
under the conditions of HER/HOR catalysis. Because of
the complete absence of net current flow, we are able to
uniquely probe the proton excess without convolution
from net H+ production or consumption. We stress that
since Pt is a reversible catalyst for the H2/H+ couple, H2
evolution and H2 oxidation are simultaneously taking
place at equal and opposite rates under all conditions of
our study.
The kinetic orthogonality between the probe reaction
and the faradaic hydrogen catalysis
We stress that the probe hydrogenation/hydrogenolysis
reactions at the Pt surface are non-faradaic and kinetically
orthogonal to the concurrent potential-dependent faradaic
process
involving
H2/H+
equilibration.
Several
experiments (see SI for details) establish this
orthogonality. First, at a fixed pH in the presence of
excess supporting electrolyte, varying the applied
potential from 0 to 0.25 V vs RHE led to no change in the
total conversion rate of the probe reaction (Figure S6a &
S6b). Second, in the presence of a very high buffer
strength, 500 mM citrate, to suppress local pH change
induced by concurrent HOR catalysis,31 varying the
applied potential from 0 to 0.25 V vs RHE led to no
appreciable change in product selectivity (Figure S6c).
These experiments establish that the production of A and
B is not coupled directly to any electrochemical surface
reaction and that the formation of both is a purely non-
faradaic process. We postulate that the kinetic
orthogonality arises from the known differential reactivity
of electrochemically generated H bound in hollow sites
and the atop H formed from dissociative adsorption of
H2.34 Upon decreasing the buffer strength, the protons
Figure 4. (a) Putative mechanism for Pt/C-catalyzed
formation of 1,4-butanediol (A, blue) and n-butanol (B, red)
(RLS stands for “rate-limiting step”). Simplified schemes
depicting the H+ concentration profiles from the reaction
plane (blue) to the bulk solution (grey) in the absence (b) and
presence (c) of migration effects. Grey boxes indicate diffuse
double layer regions. For simplicity, other free ion species in
solutions are omitted in these pictures and the
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nanoparticulate Pt surface is depicted as locally planar Pt in
(b) and (c).
bulk pH. Data were computed using the pH and Eq. (2). (c)
Scheme of the interfacial electrostatic potential profile at very
high, >280 mM (black), intermediate, 40 mM (blue), and low,
8 mM (red), Na+ concentration at fixed pH.
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The above qualitative picture can also be elaborated
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mathematically. For this treatment, we define the local
푐푅퐻푃
concentration of H+ as
at the reaction plane, RP,
휑푅푃
+
Although the discussion above highlights that
is
strongly dependent on the ionic strength of the solution,
where pH-dependent product branching occurs (Figure
4a). This local H+ concentration is related to the H+
휑
Eq. (2) does not imply an explicit dependence of
on
푅푃
푐푏푢푙푘
휑
+
pH. If the
were pH-independent, the second term in
푅푃
concentration in the bulk solution (
relation,2
) by the Boltzmann
퐻
Eq. (2) would be constant at any given ionic strength and
this would simply lead to a lateral shift of the B fraction
plot as the ionic strength is lowered, but no change in the
slope of the pH-dependence of product selectivity. This is
clearly at odds with what we observe experimentally – the
slope of the plot of B fraction vs pH is much shallower at
low ion strength than at high ionic strength (Figure 2b).
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― 푒휑푅푃
푘푇
푐푅퐻푃 = 푐푏퐻푢푙푘 ∗ 푒푥푝
(ퟏ)
+
+
(
)
휑푅푃
where
is the electrostatic potential at RP, k is the
Boltzmann constant, and T is the absolute temperature.
The electrostatic potential of the uncharged bulk solution,
At high ionic strength (Na+
280 mM) the linear region
≥
휑
푏푢푙푘, is zero. Taking the logarithm of Eq. (1) yields:
of the B fraction plot displays a slope value of 14 (Figure
2b, green), whereas, this slope decreases in magnitude
to 10 (Figure 2b, orange) and 7 (Figure 2b, red) at 40
mM and 8 mM of Na+, respectively. This can only be
푒휑푅푃
푘푇
푝퐻푅푃 = 푝퐻푏푢푙푘 + 0.43 ∗
(ퟐ)
Consistent with Eq. (1) and (2), the non-zero value of
푅푃 inside the double layer provides the driving force for
migration of charged free ions to the interface. We find
that increasing the Na+ concentration beyond 280 mM
leads to no further change in the reaction selectivity
across all reaction conditions. Thus, by Eq. (2), we
휑
explained by an explicit pH dependence of 푅푃. Indeed,
휑
the change in the slope of the B fraction plot indicates that
the magnitude of the electrostatic potential inside the
double layer is increasing (more negative) as the pH
increases at a given ionic strength.
Using the high ionic strength data as a calibration for
the intrinsic pH-dependence of product selectively, we
conclude that at sufficiently high electrolyte ionic strength,
+
휑
beyond 280 mM of Na ,
approximates to zero and,
푅푃
휑푅푃
estimate pHRP and ultimately
at reduced ionic
consequently, pHRP is equal to pHbulk. Under these high
ionic strength conditions, the electrostatic potential is
effectively screened by the supporting electrolyte ions. In
contrast, at low ionic strength, we do see a systematic
increase in B fraction, indicating that pHRP is lower than
strength. Indeed, the B fraction observed under a reduced
ionic strength is also observed at a high ionic strength, but
at a lower pH. Matching the B fraction difference between
the low and high ionic strength curves in Figure 2a & 2b,
therefore, provides a direct estimate of the pH at the RP
and the corresponding ΔpH relative to the bulk value
(Figure 5a). Consistent with the change in the slope of
the B fraction plot with decreasing ionic strength, we see
these ΔpH values increase (surface is more acidic than
bulk) as the bulk pH increases. Using Eq. (2), we
converted each ΔpH to a corresponding electrostatic
휑
pHbulk as a result of an increase in 푅푃. This mathematical
treatment quantifies the relationship between changes in
the pHRP and the interfacial electrostatic potential and
highlights that interfacial field-induced migration of H+ is
the cause of changes in selectivity of the probe reaction
with decreasing ionic strength.
휑
potential at the reaction plane, 푅푃, as a function of the
bulk pH (Figure 5b). We find that the electrostatic
potential of the reaction plane shifts between 40 mV at
pH 2 to 110 mV at pH 7 with the electrolytes containing
40 mM of Na+ (Figure 5b, blue) and between 60 mV at
pH 2 to 190 mV at pH 7 with the electrolytes containing
8 mM of Na+ (Figure 5b, red). Over the entire pH range
휑푅푃
examined, the estimated
is more negative at lower
ionic strength. Since the metal’s electrochemical
potential, E, is pinned at the same value for a given pH,
irrespective of ionic strength, the corresponding
휑
electrostatic potential at the metal surface, 푀, also
remains constant. Thus, our data are consistent with
reduced electrostatic screening at the reaction plane
under lower ionic strength conditions (Figure 5c). To the
best of our knowledge, these values constitute the first
direct measurements of the electrostatic potential inside
the double layer, and its pH dependence, under the
conditions of H2/H+ catalysis at a Pt electrode.
Figure 5. (a) The pH (= pHRP – pHbulk) as a function of bulk
pH for data collected at 40 (blue) and 8 (red) mM of Na+.
Values were calculated from the data in Figure 2a and 2b.
Estimation of the electrostatic potential at the Pt
surface and its relationship with bulk pH
휑
(b) The electrostatic potential at the reaction plane ( 푅푃) vs
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Whereas the probe reaction provides direct insight into provide a richer picture of the electrostatic potential of the
Page 6 of 9
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the electrostatic potential at the reaction plane, the
HER/HOR reactions on Pt electrodes take place at the
surface itself. To provide insight into the electrostatic
surface.
휑
휑
potential at the metal surface ( 푀), we translate 푅푃 into
휑
푀 by applying Gouy-Chapman-Stern (GCS) theory to the
data obtained under the most dilute electrolyte conditions
(8 mM of Na+, I=0.01 M). The electric field profile
described by GCS theory is shown diagrammatically for
two pH values in Figure 6a. This diagram illustrates the
information extracted from Figure 5b and shows the shift
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휑푅푃
in
to more negative values as the pH is increased.
Within the framework of GCS theory, the closest point of
approach of solvated ions corresponds to the OHP and
our observation of a strong dependence on B fraction on
solvated H+ and Na+ indicates that the RP cannot be
closer to the surface than the OHP. If we assume that the
RP is equal to the OHP, then 푀 can be simply calculated
from the
equation,2
휑
휑
values in Figure 5b using for the following
푅푃
1/2
8푘푇푁퐴퐶 ∗
휀휀표
푒휑푅푃
2푘푇
휑푀 = 휑푅푃
+
∗ 푠푖푛ℎ
∗ 푥푂퐻푃 (ퟑ)
(
)
(
)
Figure 6. (a) Scheme of the interfacial electrostatic potential
profile under low ionic strength conditions at low (red) and
high (blue) pH. (b) The electrostatic potential at the Pt surface
휀표 is the absolute permittivity,
휀
is the dielectric
Where
constant of aqueous solution under the dilute electrolyte
strengths,
휑
휑
(
푀) vs bulk pH, computed using the
in Figure 5b red,
푅푃
푥
Eq. (3), and varying 푂퐻푃 distances of 0.5 (red), 0.6 (orange),
and 0.7 (green) nm. (c) Potential vs bulk pH Pourbaix
diagram showing the pH dependence of the reversible
hydrogen couple (blue, dotted line) and the EPZFC values
푁퐴
is the Avogadro’s number. While this
equation applies rigorously to a 1:1 symmetric electrolyte
퐶 ∗
with
a
concentration of
, computational analysis
indicates that this equation can closely approximate the
푥
estimated from our analysis assuming 푂퐻푃 of 0.5 (red), 0.6
behavior for the dilute asymmetric electrolytes used
(orange), and 0.7 (green) nm.
here.35,36 . The
term is the distance between the
,37
푥
푂퐻푃
surface and OHP (Figure 6a), which is generally
estimated to be 0.50.7 nm based on the sum of the
Stokes radii of the charge compensating cations (H+ or
Na+) and the thickness (diameter) of the water layer that
is adsorbed to the surface.1,2 Encouragingly, this OHP
distance range matches the sum of PtC and one or two
CC/CO bond lengths (0.50.7 nm) (see Figure 4a)
suggesting that this is a reasonable estimate of the
position of the reaction plane (RP). Figure 6a qualitatively
illustrates a relationship between
described by GCS theory.
Using Eq. (3), we calculated
range examined in this study (Figure 6b). While the
magnitude of the electrostatic potential is small at pH 2,
~-0.1 V, it systematical grows in magnitude as the pH
increases, reaching a value of ~-0.4 V at pH 7.
Remarkably, the 푀 values exhibit a Nernstian shift (~60
mV pH-1) over the entire pH range examined (Figure 6b).
휑
To analyze the data further, we can subtract 푀 from E
to arrive at an effective EPZFC. This value, which could
never be measured directly, corresponds to the
hypothetical potential of an uncharged electrode with the
adsorbate population and surface structure existent under
catalytic conditions. Thus, this effective EPZFC needs not
match the EPZFC measured independently in the absence
of catalysis. Indeed, we find an effective EPZFC of ~0 V vs
SHE (Figure 6c), which is ~0.3 V more negative than the
EPZFC measured for a Pt(111) surface in the absence of
concurrent H2/H+ catalysis.9,38 While some of this shift may
result from the polycrystalline surface structure employed
here,29 the replacement of the water adlayer with a
saturated adlayer of adsorbed H present under the
condition of catalysis is expected to contribute as well.
Furthermore, we find that our effective EPZFC values are
roughly constant across the pH range (Figure 6c),
suggesting that, at least between pH 2 and 7, the
adsorbate population and surface structure of the Pt
under HER/HOR conditions remain unchanged. Our
results provide the first direct indication of a Nernstian pH-
dependence in the magnitude of the electrostatic potential
under the conditions of HER/HOR. These results are in
line with contemporary PZFC theory9,17,20,38 and highlights
that for a Pt electrode over this pH range, the electrostatic
(Volta) potential change fully accounts for the aggregate
Nernstian shift in the electrode potential.
휑푀
휑푅푃
as
and
휑푀
over the entire pH
휑
푥
Changing 푂퐻푃 from 0.5 or 0.7 nm leads to small changes
in the slope from 53 to 64 mV pH-1, but preserves the
overall trend. These
휑푀
values correspond to the true
electrostatic potential drop at the interface under
HER/HOR conditions. While we acknowledge that the RP
can extend into the diffuse doubly layer beyond the OHP,
we find that this possibility leads to very small changes in
the calculated electrostatic potential at the Pt surface
(Figure S7) relative to the above assumption that the RP
is at the OHP. Although GCS theory provides a
reasonable estimate of the electrostatic potential at the
metal surface, we also acknowledge that more complex
theories of the double layer structure could be used to
■ CONCLUDING REMARKS
In conclusion, we establish a rigorous framework for
understanding the electric field environment experienced
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by a Pt surface while it catalyzes the interconversion of H+
and H2. We use the H2/H+ couple to pin the Pt electrode
at the reversible potential for this reaction across a wide
pH range and then introduce a non-faradaic reaction
probe to sense the local pH at the Pt surface. We show
that the migration of protons to the surfaces lowers the
local pH relative to the bulk value at low ionic strengths,
causing a change in the selectivity of the probe reaction.
By quantifying the selectivity as a function of ionic strength
and pH, we extract the electrostatic potential inside the
double layer. Consequently, our probe reaction provides
the first in situ measurements of the magnitude of the
interfacial electric field under the conditions of reversible
H2/H+ catalysis. We show that the magnitude of the
electrostatic potential at the Pt surface increases by 60
mV for each unit increase in the bulk pH over the range
explored in this study. Thus, our results show that Pt
surfaces would experience a negligible interfacial field
when catalyzing H2/H+ conversion at ~pH 1, but
experience an appreciable field of ~108 V m-1 at pH 7.
Linear extrapolation of our data to pH 14 would imply an
even stronger field in alkaline media, although we
acknowledge that binding of *OHx species39,40 could
attenuate the field strength under strongly basic
conditions. Nonetheless, the dramatic difference we
uncover in the magnitude of the interfacial field at RHE
should influence the rate of solvent reorganization and
may contribute to the strong pH-dependence of the
kinetics of H2/H+ conversion.9 Together, this work
highlights that differences in the interfacial electric field
strength must be considered when comparing the kinetics
of electrocatalysis across the pH range.
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Fried, S. D.; Boxer, S. G. Measuring Electric Fields and
Noncovalent Interactions Using the Vibrational Stark Effect.
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Leading to an Inverse Free-Energy Relationship for N−N
Bond Formation by Mn VI Nitrides. Angew. Chemie Int. Ed.
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Climent, V.; Feliu, J. M. Effect of the Interfacial Water
Structure on the Hydrogen Evolution Reaction on Pt(111)
Modified with Different Nickel Hydroxide Coverages in
Alkaline Media. ACS Appl. Mater. Interfaces 2019, 11 (1),
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Liu, E.; Huang, Y.; Jia, Q.; Zhao, Z.; Li, J.; Mukerjee, S.; Liu,
Z.; Doan, H. T. T.; Abraham, K. M.; Jiao, L. Unifying the
Hydrogen Evolution and Oxidation Reactions Kinetics in
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Climent, V.; Coles, B. A.; Compton, R. G. Coulostatic
Potential Transients Induced by Laser Heating of a Pt(111)
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ASSOCIATED CONTENT
Supporting Information.
Full experimental and analytic details, raw reaction profiles,
and additional kinetic data. The Supporting Information is
available free of charge on the ACS Publications website.
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AUTHOR INFORMATION
Corresponding Author
*yogi@mit.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank Cyrille Costentin, Michael Pegis, and Corey
Kaminsky for helpful discussions. This work is supported by
the Air Force Office of Scientific Research (AFOSR) under
award number FA9550-18-1-0420. J.R. acknowledges
support from a Samsung Scholarship. Y.S. acknowledges
the Sloan Foundation, Research Corporation for Science
Advancement (Cottrell Scholar), and the Canadian Institute
for Advanced Research (CIFAR Azrieli Global Scholar).
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