Quantification of Interfacial pH Variation at Molecular Length Scales Using a Concurrent Non-Faradaic Reaction

Article abstract of DOI:10.1002/anie.201802756

We quantified changes in interfacial pH local to the electrochemical double layer during electrocatalysis by using a concurrent non-faradaic probe reaction. In the absence of electrocatalysis, nanostructured Pt/C surfaces mediate the reaction of H₂ with cis-2-butene-1,4-diol to form a mixture of 1,4-butanediol and n-butanol with selectivity that is linearly dependent on the bulk solution pH value. We show that kinetic branching occurs from a common surface-bound intermediate, ensuring that this probe reaction is uniquely sensitive to the interfacial pH value within molecular length scales of the surface. We used the pH-dependent selectivity of this reaction to track changes in interfacial pH during concurrent hydrogen oxidation electrocatalysis and found that the local pH value can vary dramatically (>3 units) relative to the bulk value even at modest current densities in well-buffered electrolytes. This study highlights the key role of interfacial pH variation in modulating inner-sphere electrocatalysis.

Full text of DOI:10.1002/anie.201802756

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Accepted Article
Title: Quantifying Interfacial pH Variation at Molecular Length Scales
Using a Concurrent Non-Faradaic Reaction
Authors: Jaeyune Ryu, Anna Wuttig, and Yogesh Surendranath
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201802756
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Quantifying Interfacial pH Variation at Molecular Length Scales
Using a Concurrent Non-Faradaic Reaction
Jaeyune Ryu, Anna Wuttig, and Yogesh Surendranath*
Abstract: We quantify changes in the interfacial pH local to the
electrochemical double layer during electrocatalysis, using
inner-sphere mechanisms that require the binding of small-
molecule substrates to the electrode surface. Thus, the pH within
molecular length scales of the surface, rather than the value
remote from the electrode in the reaction diffusion layer, is critical
for defining the free-energy landscape for catalysis. Despite its
central importance, direct measurements of the pH variation in
this narrow region near the electrochemical double layer (< 1 nm
from the electrode surface) are, to the best of our knowledge,
unprecedented.
Herein, we establish the first localized measurements of pH
variation within the critical double layer region during
electrocatalysis by utilizing a concurrent, surface-catalyzed, non-
faradaic probe reaction that displays pH-dependent selectivity.
Specifically, using the hydrogen oxidation reaction (HOR) on Pt
as a model system, we exploit the pH-dependent bifurcation
between hydrogenation and hydrogenolysis of a cis-2-butene-1,4-
diol probe molecule to measure the pH variation <1 nm from the
catalyst surface over a diverse range of current densities and
buffer compositions (Scheme 1).
a
concurrent non-faradaic probe reaction. In the absence of
electrocatalysis, nanostructured Pt/C surfaces mediate the reaction of
H2 with cis-2-butene-1,4-diol to form a mixture of 1,4-butanediol and
n-butanol with a selectivity that is linearly dependent on the bulk
solution pH. We show that kinetic branching occurs from a common
surface-bound intermediate, ensuring that this probe reaction is
uniquely sensitive to the interfacial pH within molecular length scales
of the surface. We use the pH-dependent selectivity of this reaction to
track changes in interfacial pH during concurrent hydrogen oxidation
electrocatalysis and find that the local pH can vary dramatically, > 3
units, relative to the bulk value even at modest current densities in
well-buffered electrolytes. This work highlights the key role that
interfacial pH variation plays in modulating inner-sphere
electrocatalysis.
The efficient interconversion of electrical and chemical energy
requires functional interfaces capable of catalyzing complex multi-
proton coupled electron transfer (PCET) reactions of diverse
small molecule substrates.[1],[2],[3],[4],[5] Consequently, interfacial
electrocatalysis is strongly dependent on the proton activity at the
electrode surface. Indeed, there is a growing appreciation that the
interfacial pH and proton donor environment play a central role in
determining the kinetics, thermodynamics, and mechanism of
elementary PCET steps,[3],[4] thereby defining the selectivity and
efficiency of key interfacial reactions ranging from CO2 and O2
reduction to the oxidation of small molecule fuels such as MeOH
and HCO2H.[6],[7],[8],[9],[10],[11],[12] More broadly, the interfacial pH has
been shown to impact a wide variety of electrochemical processes
including metal finishing, corrosion chemistry, electrodeposition,
and electro-organic synthesis.[13],[14] Clearly, knowledge of the pH
at the electrode surface and its variation under electrochemical
polarization is an essential prerequisite for achieving molecular-
level understanding of interfacial reactivity and catalysis.
Scheme 1. Highly localized measurements of interfacial pH near the double
layer region during HOR electrocatalysis (left) are enabled by a concurrent pH-
sensitive non-faradaic reaction (right): Pt catalysed H2 addition to cis-2-butene-
1,4-diol to produce 1,4-butanediol and n-butanol.
The aqueous pH dependence of the Pt-catalyzed reaction of
H2 with cis-2-butene-1,4-diol was measured in the absence of
current flow in order to determine the inherent pH sensitivity of the
non-faradaic probe reaction. Using commercially available fuel
cell electrodes (Pt/C GDE), which contains Pt nanoparticles
embedded in microporous carbon layers; we examined the
reaction at the open circuit potential (OCP) under a variety of bulk
pH and electrolyte conditions (Figure 1) (See the supporting
information for detailed reaction conditions). In all cases, the
reaction proceeds cleanly to generate a mixture of 1,4-butanediol
(A) and n-butanol (B). The ratio of these two products is strongly
dependent on the bulk pH. In the presence of 100 mM phosphate
electrolyte (Figure 1a), the percent of the hydrogenolysis product,
B, decreased linearly with increasing pH, ranging from ~80% at
pH 2.3, to ~18% at pH 6.8. Nearly identical pH scaling in the
product distribution is observed at a lower buffer strength, 50 mM
phosphate (Figure 1b), as well as in the presence of alternative
buffers, 50 or 100 mM citrate (Figure 1c and Figure 1d).
Consistent with the charge neutrality of both the hydrogenolysis
Electrochemical PCET catalysis invariably generates an influx
or efflux of protons that gives rise to a pH gradient, which can
perturb the thermodynamics and kinetics of interfacial catalysis.
Depending on the reaction conditions and electrode morphology,
this pH gradient can extend 1-10 µm from the electrode surface
into the reaction diffusion zone.[15],[16] Variations in the pH in the
reaction diffusion zone have been quantified using a variety of
spectroscopic and electrometric methods.[13],[14],[17],[18],[19],[20]
However, heterogeneous electrocatalytic reactions proceed via
[*]
J. Ryu, A. Wuttig, Prof. Dr. Y. Surendranath
Department of Chemistry
Massachusetts Institute of Technology
77 Massachusetts Ave., Cambridge, MA 02139-4307 (USA)
E-mail: yogi@mit.edu
Supporting information for this article is given via a link at the end of
the document
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10.1002/anie.201802756
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and hydrogenation pathways, the bulk pH was unchanged over
the course of the reaction for all electrolyte and pH conditions
examined. Importantly, for these calibration experiments, the
electrode was held at the OCP (close to RHE in all cases),
ensuring the complete absence of net current flow and, thus, no
proton influx/efflux from the surface. Furthermore, plots of the
percent yield of B vs pH overlay for all the buffer conditions
examined (Figure 1e), indicating that the reaction is uniquely
responsive to the pH rather than buffering environment at the
interface.
from the hydrogenation of the double bond, B is formed via both
hydrogenation and hydrogenolysis of the reactant. Even at the low
end of the pH range explored, we observe no reactivity of 1,4-
butanediol (A), indicating that A is not an intermediate en route to
the formation of B. To further probe the mechanism of bifurcation,
we monitored the rate of product formation up until a total
conversion of ~40-50%. Over this range, we observe linear,
pseudo-zero-order kinetic traces for both reactant consumption
and product formation (Figure S1-S4), indicative of a typical
heterogeneous surface-catalyzed process. The slopes of these
traces were used to extract the observed rate constants (Figure
2a and Figure S5).
Figure 2. Log kobs vs pH (a) for the rate of total conversion (grey), n-butanol
formation (red), and 1,4-butanediol formation (blue). Data recorded at the open-
circuit potential in 50 mM citrate, 0.5 M NaClO4. Rate constant fraction for the
formation of B vs pH in the presence of various electrolytes (b).
The kinetic data provide further information about the
mechanism of product bifurcation. The rate of 1,4-butanediol
formation increases as the pH rises, whereas the rate of n-butanol
formation increases as the pH declines. Importantly, over the
entire pH range explored, we observe that the total conversion
rate remains constant, despite large variations in the product
selectivity (Figure 2a). Additionally, varying the H2 partial pressure
from 0.2 to 1.0 atm leads to an increase in the rate of overall
product generation, with a reaction order of ~0.6, but leads to no
substantial change in product selectivity (Figure S6). Taken
together, the data lead us to postulate the following simplified
mechanistic sequence (Scheme 2). First, we posit that the
substrate adsorbs to the surface in major equilibrium, explaining
its zero-order dependence. Second, we invoke concurrent non-
Langmurian H2 dissociative chemisorption in intermediate
equilibrium in order to explain its fractional positive reaction order.
Finally, we propose that, following these adsorption events, rate-
limiting surface recombination of H and the substrate generates a
common surface-bound intermediate I*, that bifurcates to A and B
via pH dependent pathways (Scheme 2).
Figure 1. Product fraction of n-butanol, B (in %), at ~30% total conversion for
Pt/C catalyzed H2 addition to cis-2-butene-1,4-diol as a function of electrolyte
bulk pH. Data collected at the open-circuit potential in 0.5 M NaClO4 electrolytes
containing (a) 100 mM phosphate (b) 50 mM phosphate, (c) 100 mM citrate,
and (d) 50 mM citrate. Overlay (e) of data from (a)-(d). Data points are the
average and standard deviation of three independent experiments.
Even in the absence of current flow, the proton activity at the
interface will depend on both the interfacial electrostatic potential
profile (electric field) and the bulk proton activity, as dictated by
the Boltzmann relation.[15],[16] Thus, the electrostatic potential
contribution may serve to shift the interfacial proton activity
relative to the bulk values plotted in Figure 1. In order to minimize
this effect, all experiments were conducted in the presence of
excess inert screening electrolyte, > 0.5 M NaClO4. Additionally,
irrespective of any field induced shift in absolute interfacial pH,
Figure 1 provides a useful calibration for tracking variations in
interfacial pH resulting from electrocatalysis (see below).
Scheme 2. Simplified proposed mechanism for Pt/C catalyzed formation of 1,4-
butanediol (A) and n-butanol (B) containing pH-independent (left) and pH-
sensing (right) reaction steps.
Interestingly, pH-dependent kinetic branching is found to be
independent of the buffer type or concentration and displays a
surprisingly weak reaction order in pH of ~0.1 (Figure 2). These
The origin of this strong linear pH dependence was probed
through a series of mechanistic investigations. Whereas A results
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observations indicate that (1) proton transfer events that lead to
the formation of n-butanol are in equilibrium with H3O+ in solution
and (2) the isotherm for protonating key surface intermediates can
be non-Langmurian near the electrical double layer.[21],[22] Using
the kinetic model provided in Scheme 2, detailed product forming
rate expressions are derived in the supporting information and
these expressions are able to closely simulate the observed
product selectivity (Figure S7) and product forming rates (Figure
S8) as a function of pH. Irrespective of the specific mechanism by
which the interfacial pH directs product bifurcation, the
mechanistic data establish that pH-dependent kinetic branching
occurs following the formation of a common surface bound
species, I*. This ensures that the reaction probe is uniquely
sensitive to the local pH environment within 1-2 C–C/C–O bond
lengths of the Pt surface.
subject to parasitic side reactions under positive potential bias
and concurrent HOR over the range of conditions explored. These
observations establish that concurrent HOR serves to increase
the selectivity towards B in all cases and that the magnitude of
this change is positively correlated to the current density and is
also dependent on the buffer type and concentration. Although a
modest potential shift (~40 mV) is observed in the presence of the
probe reaction, the HOR kinetic traces are similar to those in the
absence of the non-faradaic reaction, with nearly identical Tafel
slopes (~80 mV dec-1) (Figure S9). This observation highlights the
kinetic orthogonality of the faradaic and non-faradaic reactions
(see below).
The observed changes in selectivity with concurrent HOR are
best attributed to changes in the interfacial pH as a result of the
H+ generated via the faradaic reaction. To exclude other possible
sources of the change in selectivity, we performed a series of
control experiments. First, the change in selectivity could be
attributed to a drift in bulk pH (Figure 1) but we exclude this
possibility because the bulk pH was unaltered over the course of
the experiment in all cases. Second, the change in selectivity
could be attributed to an irreversible restructuring of the surface
upon polarization to the potentials necessary for HOR. We
exclude this possibility because the original product selectivity
under OCP conditions was recovered, within error, after
termination of the positive potential bias in all cases (Figure 3,
grey). Additionally, regardless of the magnitude of the anodic
current passed or the electrolyte conditions, the total conversion
rates of the reaction did not vary from the rates recorded at OCPs
(Figure S10 and S11) – only the product selectivity varied. These
observations, taken together with the mechanistic model
developed earlier, lead us to conclude that concurrent HOR does
not dramatically alter the pH-independent processes (Scheme 2,
grey) that determine the aggregate reaction velocity. This
observed orthogonality between the faradaic and non-faradaic
reactions may arise from distinct adsorbed H intermediates for
HOR vs substrate conversion[23] or from a minimal perturbation in
the surface concentration of a common adsorbed H intermediate
under the HOR current densities explored here. Nevertheless,
these control experiments establish that the transient decrease in
the interfacial pH generated via HOR catalysis drives changes in
the rates of the pH-dependent selectivity-determining steps of the
reaction (Scheme 2, yellow) leading to enhanced yields of B.
Under conditions of concurrent electrocatalysis, the proton
activity at the interface will depend on both the interfacial
electrostatic potential profile (electric field)[15],[16] and proton flux
generated via HOR. Several pieces of evidence suggest that the
latter is the main contributor to changes in selectivity under these
conditions. First, we find that each current density probed
corresponds to a similar applied potential vs RHE (~70 mV for 2.0
mA cm-2, ~100 mV for 3.3 mA cm-2, and ~150 mV for 4.6 mA cm-
2) but still generates B with a yield which is inversely correlated
with buffer strength (Figure 3). Indeed, at a very high buffer
strength, 500 mM citrate (Figure S12), we observe a negligible
change in the product selectivity relative to the OCP conditions
indicating minimal convolution from interfacial field effects under
these conditions.
Figure 3. Percent n-butanol, B, generated from Pt/C catalyzed H2 addition to
cis-2-bututene-1,4-diol as a function of anodic H2 oxidation current (green, blue,
orange, and red) and upon returning to the open-circuit potential (grey). Data
collected in the presence of (a) 50 mM phosphate (pH 6.8), (b) 100 mM
phosphate (pH 6.8), (c) 50 mM citrate (pH 5.8), and (d) 100 mM citrate (pH 5.8)
electrolyte. Data points are the average and standard deviation of three
independent experiments.
Taking advantage of the unique surface-sensitivity of this
reaction, we examined the dependence of product selectivity on
concurrent HOR electrocatalysis as a function of the electrolyte
environment (Figure 3). In 50 mM phosphate electrolyte, pH 6.8,
(Figure 3a) varying the HOR current densities from 0.0 to 4.6 mA
cm-2, gave rise to a dramatic increase in selectivity for B from 21
to 63%. When the buffer strength was increased to 100 mM
phosphate, the same range of HOR current densities led to an
attenuated increase in selectivity for B from 20 to 38%.
Exchanging the electrolyte for citrate also attenuated the change
in product selectivity at a given current density. In 50 mM citrate
electrolyte, the same range of current densities gave rise to a
change in selectivity for B from 37% to 50% (Figure 3b), whereas
in 100 mM citrate, the selectivity change was further attenuated
with a variation from 30% to 41% over the same current range.
Importantly, NMR analysis confirms that the substrates were not
Based on the analysis above, the product selectivity for the
non-faradaic reaction provides a measure of the interfacial pH
near electrochemical double layer under concurrent HOR
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catalysis. With the pH dependence of product selectivity
measured at OCP (Figure 1), we used the product selectivities
measured during concurrent HOR (Figure 3) to estimate the
change in interfacial pH relative to the bulk value (ΔpH) (Figure
4a). Remarkably, at a modest current density of 4.6 mA cm-2 in 50
mM phosphate electrolyte, we estimate an interfacial pH of 3.6,
3.2 units lower than the bulk value of 6.8. Unsurprisingly, ΔpH is
negative in all cases because HOR generates H+ at the interface
and, as expected, the ΔpH becomes more negative as the HOR
current density increases. Interestingly, the 50 and 100 mM
phosphate buffers (Figure 4a, green and blue) display a more
negative ΔpH than 50 or 100 mM citrate buffers (Figure 4a, red
and orange). These trends are consistent with the enhanced
buffer capacity[24] of citrate relative to phosphate below pH ~6.25
(Figure 4b) resulting from citrate’s closely-spaced pKa values
(Figure 4c) and are qualitatively reproduced in stochastic
simulations (See SI and Figure S13-S15). The rather dramatic pH
variations measured by this technique under even modest current
densities highlights the central role that the local, rather than bulk,
pH plays in defining electrocatalytic reaction pathways.
variation under diverse electrolyte and current density conditions
and find that even modest current densities < 5 mA cm-2 can lead
to relatively large interfacial pH variations of > 3 units relative to
the bulk value. The work stresses that the bulk pH cannot simply
be assumed to be equivalent to the interfacial pH and that careful
consideration of the latter is key for understanding and designing
improved interfacial electrocatalysts.
Acknowledgements
We thank Cyrille Costentin, Megan Jackson, and Sterling Chu for
helpful discussions. This research was supported by the Air Force
Office of Scientific Research under AFOSR Award No. FA9550-
15-1-0135 and Samsung Scholarship (J.R). Y.S. acknowledges
the Sloan Foundation and the Research Corporation for Science
Advancement (Cottrell Award).
Keywords: interfacial pH • electrocatalysis • proton-coupled
electron transfer • electrochemical double layer • hydrogenation
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dependent
bifurcation
between
hydrogenation
and
hydrogenolysis reaction pathways. Kinetic investigations
establish that pH-dependent kinetic branching proceeds from a
surface bound intermediate ensuring that the probe is uniquely
sensitive to the pH near the double layer region. Using Pt-
mediated HOR as a model reaction, we measure interfacial pH
[23]
[24]
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Table of Contents
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J. Ryu, A. Wuttig, Y. Surendranath*
Page No. – Page No.
Quantifying Interfacial pH Variation at
Molecular Length Scales Using a
Concurrent Non-Faradaic Reaction
We quantify changes in the interfacial pH local to the electrochemical double layer during electrocatalysis, using a concurrent
non-faradaic probe reaction. The reaction displays pH-dependent bifurcation between hydrogenation and hydrogenolysis via a
common surface-bound intermediate, enabling quantification of interfacial pH variation within molecular length scales of the
surface.
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