Electrochemical Dynamics of a Single Platinum Nanoparticle Collision Event for the Hydrogen Evolution Reaction

Article abstract of DOI:10.1002/anie.201712454

Chronoamperometry was used to study the dynamics of Pt nanoparticle (NP) collision with an inert ultramicroelectrode via electrocatalytic amplification (ECA) in the hydrogen evolution reaction. ECA and dynamic light scattering (DLS) results reveal that th

Full text of DOI:10.1002/anie.201712454

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Accepted Article
Title: Electrochemical Dynamics of Single Platinum Nanoparticle
Collision Event towards Hydrogen Evolution Reaction
Authors: Zhipeng Xiang, Hai-qiang Deng, Pekka Peljo, Zhiyong Fu,
Suli Wang, Daniel Mandler, Gong-quan Sun, and Zhenxing
Liang
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http://dx.doi.org/10.1002/ange.201712454

10.1002/anie.201712454
Angewandte Chemie International Edition
Electrochemical Dynamics of Single Platinum Nanoparticle
Collision Event towards Hydrogen Evolution Reaction
Zhi-peng Xiang,a,† Hai-qiang Deng,b,† Pekka Peljo,c Zhi-yong Fu,a Su-li Wang,d Daniel Mandler,b
Gong-quan Sun,d* Zhen-xing Lianga*
aKey Laboratory on Fuel Cell Technology of Guangdong Province, School of Chemistry and
Chemical Engineering, SouthChina University of Technology, Guangzhou 510641,P.R. China
bInstitute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel
cLaboratoire d’Electrochimie Physique et Analytique, École Polytechnique Fédérale de
Lausanne (EPFL Valais Wallis), Rue de I’Industrie, 17, 1951 Sion, Switzerland
dFuel Cell & Battery Division, Dalian National Laboratory for Clean Energy, Dalian Institute
of Chemical Physics, Chinese Academy of Sciences, Dalian116023, P.R. China
†Equal Contribution.
*Corresponding authors: Tel.: +86 20 87113584, E-mail: zliang@scut.edu.cn (Z.X. Liang);
Tel.: +86 411 84379063, E-mail: gqsun@dicp.ac.cn (G.Q. Sun).
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10.1002/anie.201712454
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Keywords. Nanoparticles, collisions, electrocatalytic amplification, HER, interfacial
dynamics.
Abstract. Chronoamperometry was used to study the dynamics of Pt nanoparticle (NP)
collision with an inert ultramicroelectrode via electrocatalytic amplification (ECA) towards
hydrogen evolution reaction. First, ECA and dynamic light scattering (DLS) results reveal
that the NP colloid remains stable only at low proton concentrations (1.0 mM) under helium
(He) atmosphere, ensuring that the collision events occur at genuinely single NP level.
Amperometry of single NP collisions under He atmosphere shows that each discrete current
profile of collision event evolves from spike to staircase at more negative potentials, while a
staircase response is observed at all of the applied potentials under hydrogen-containing
atmospheres. The particle size distribution estimated from the diffusion-controlled current in
He atmosphere agrees well with both the electron microscopy and DLS observations. The
work presented herein sheds lights on the interfacial dynamics of the single nanoparticle
collisionelectrochemistry.
Nanoparticles (NPs) have great impacts on many applications, such as energy,[1] catalysis (e.g.
photocatalysis and electrocatalysis),[2] sensors,[3] electrochemiluminescence[4] and
spectroscopy,[5] due to their fascinating physical and chemical properties. NP’s properties are
mainly dependent on its composition, size and shape.[6] Hence, many studies have been
conducted to elucidate the NP’s structure-activity relationship.[7] Traditional ensemble
measurements are limited to provide only averaged characteristics of a large number of NPs
with distribution of sizes and shapes in a typical sample. Hence, it is highly desirable to
develop techniques to characterize structure-activity relationship of a single NP as a means of
understanding and further tuning NP’s performance. Until now, spectroscopy (e.g. single-
molecule fluorescence microscopy,[8] infrared nanospectroscopy,[9] local surface plasmon
[10]
resonance, and tip-enhanced Raman spectroscopy[11]) has become the mainstream method
to study single molecules and nanoparticles. Nevertheless, spectroscopic methods are
generally limited by either chemical selectivity (e.g. limited by particular molecules) or
spatial resolution (e.g. diffraction limit). Electrochemical method[12] is emerging as an
indispensable alternative and/or a complement to study single NPs either by immobilizing a
NP onto a substrate electrode combined with scanning electrochemical microscopy (SECM)[13]
or scanning electrochemical cell microscopy (SECCM)[14] or by stochastic collision/impact of
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individual NPs onto an ultramicroelectrode (UME) without a tricky immobilization step.[12]
The second electrochemical method can directly characterize NPs in the colloidal solution.
The current signal induced by a NP collision can be either from bulk electrolysis (BE) of the
redox-active NP itself[15] or from electrocatalytic amplification (ECA) when a NP catalyzes an
inner-sphere-electron-transfer (ISET) reaction that is kinetically sluggish on the underlying
UME.[16] ECA is superior to BE in terms of the detectionsensitivityand temporal resolution.
The hydrogen evolution reaction (HER) is the simplest heterogeneous ISET reaction between
(solvated) protons and underlying electrified electrodes.[17] Accordingly, HER has served as a
model reaction in single Pt NP collision detection via ECA.[16a, 16b, 18] Pt NPs colloidal stability
and the explanation of the obtained electrochemical signal from HER on single Pt NPs are
still elusive.[19] The difficulty in interpreting the current profile in the ECA approach is due to
the variation of the interactions between NP and the substrate UME, resulting in either spikes
or staircases. In the case where a NP sticks onto the UME biased at the diffusion-limited
potentials, a current staircase is expected if no deactivation/poisoning of the NP catalyst
occurs,[20] while the catalyst deactivation can lead to a spiky response. In the present work, we
extensively investigate the mechanism underpinning the evolution of the current transient
profile of HER on single Pt NPs from spike to staircase with varying electrode potentials
under different gas atmospheres in the absence of oxygen (Scheme 1). It is found that the
interfacial dynamics plays a key role in determining the current shape. Specifically, it is the
drift in equilibrium potential and slower HER kinetics compared to mass-transfer of protons[21]
in aqueous to nanometer-sized Pt surface[16e, 22] that result in a spiky current transient in
helium-saturated solution at lower overpotentials. Nevertheless, staircase-shaped current
profile can be seen either in helium-saturated solution at higher overpotentials or in hydrogen-
containing solution even at lower overpotentials. The current shape is rationalized by the
analysis of thermodynamics, kinetics, and finite-element simulations of HER. Our findings
presented herein pave the way for electrocatalytic studies at a truly single nanoparticle
level.[23]
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Scheme 1. Current profile upon single Pt NP collision on the gold UME under either helium or
hydrogen atmosphere at varied applied potentials.
First, the issue of NP colloidal instability was addressed to guarantee single NPs
electrochemical detection. Figure S1 and Table S1 in the Supporting Information (SI) show
that Pt NPs have a modal size of 71 nm in diameter (scanning electron microscopy, SEM) and
a corresponding hydrodynamic diameter of 85 nm (dynamic light scattering, DLS). Uniform
current transients are observed in He-saturated 1.0 mM HClO4 solution (black trace in Figure
S3A, SI) at an applied overpotential of 0.10 V (Table S3, SI), indicating that individual
nanoparticles rather than aggregates collide with the UME surface. In comparison,
inhomogeneous current magnitudes, irregular time intervals and lower impact frequencies of
the ECA signals are observed in both 5.0 and 25 mM HClO4 solutions (red and blue traces in
Figure S3A, SI) at the same applied overpotential, indicating that NPs are unstable and
aggregate in the colloidal dispersion. It implies that proton concentration dictates the colloidal
stability via electrostatic interactions between NP’s double layer and H+. This observation was
corroborated by DLS kinetic measurements under both ambient and He atmospheres as shown
in Figures S3B and S5 in SI, respectively. The experimental NP/UME collision frequency of
0.05 Hz (Figure S4 in SI) in He-saturated 1.0 mM HClO4 is in reasonable agreement with the
calculated one of 0.09 Hz (Equation S5, SI). This deviation might originate from the
experimental errors, ineffective collision events,[24] Fermi-level equilibration of NP in close
contact with the electrode,[25] and the near-wall hindered diffusion[20b, 26] (more details seen in
pages S-9~10 in SI). We employed 1.0 mM HClO4 to carry out the ECA collision
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experiments throughout the work shown herein, although Pt NPs aggregate under H2
atmosphere even in 1.0 mM HClO4 (blue trace in Figure S5, SI).
ECA of HER observed upon NP collision should show a staircase current profile when NPs
colliding with the electrode stick to the surface at sufficient overpotentials where the
diffusion-limited regime of proton is reached. Instead, experimental results show a spiky
profile (Figure S3A, SI). Effects of hydrogen-under-potential-deposition (HUP D), capacitive
currents, or reduction of either oxygen or platinum oxide were excluded as the origin of the
current spikes (see Figures S6-7, SI). In addition, the elastic collision of the NP may account
for the spiky profile; however, the CVs (see Figure S8, SI) indicate that the collided Pt NPs
stick onto the Au UME after their landings, rather than elastically collide and leave the
electrode after impactingthe UME.
[18]
It was suggested by Stevenson et al. that ECA reaction, working electrode, and electrolyte
solution could affect the current transient shape. Herein, we found that the electrode potential
also has a substantial effect on the current transient shape of a collision event. Figure 1A
shows the amperometric curves of 0.58 pM Pt NPs colliding on a 25 µm Au UME in He-
saturated 1.0 mM HClO4. The current transient evolves from spike to staircase eventually
while polarizing the electrode to more negative potentials. One possible explanation would be
that the interfacial dynamics dictates the current transient shape: (1) drift in the equilibrium
potential, and (2) slower HER kinetics compared to mass-transfer rate of H+ to nanometer-
sizedPt surface.[16e, 22]
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Figure 1. Amperometric i−t curves of 0.58 pM Pt NPs colliding with a 25 µm Au UME biased from
−0.203 to −0.403 V vs. SHE. (A) He-saturated 1.0 mM HClO4 solution, and (B) H2-saturated 1.0 mM
HClO4 solution.
To gain further information, amperometric i−t curves were collected under the same
conditions except using H2-saturated solution (Figure 1B) as well as He-H2 mixture with
different partial pressures of H2 (Figure S10, SI). In all the cases, the current signal shows a
staircase for each collision event independent of the applied potential. Figure 2A shows the
average current of the collision events as a function of the applied potentials plotted from
Figure 1. The current first increases with decreasing the potential, and then levels off at more
negative potentials ≤ −0.293 V in helium-saturated solution (trace “a” in Figure 2A). At less
negative potentials, the HER is at least partially controlled by kinetics; therefore, the current
gradually increases with biasing the UME to more negative potentials. When the potential is
sufficiently negative, the HER changes from the kinetics control to the diffusion control,
which explains the subsequent presence of a current plateau. It is also interesting to note that
the current magnitudes remain the same despite the considerable change in the partial pressure
of hydrogen from 0.2 to 1 atm (Figure 2B). However, the current transient magnitude in He
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atmosphere is higher than that in H2 containing atmospheres, which is unexpected as the
current should depend only on the diffusion of protons. The blocking effect of hydrogen
bubble formation on the NP surface[27] was also excluded as a possible origin for the spike
response under He atmosphere (see page S-18 in SI for further details).
To further address this concern, the HER was simulated with finite element method using
COMSOL Multiphysics (see pages S-24~25 in SI for more details), assuming the Volmer-
Tafel mechanism (H+ + e− + Pt ↔ H-Pt and 2H-Pt ↔ H2 + 2Pt) and with the parameters
reported by Compton et al.,[28] with Pt NP radius of 35 nm and 0.63 mM of protons in the bulk
solution. The initial bulk hydrogen concentration, which is experimentally controlled by the
partial pressure in the mixture, used in simulation varies from 0 to 0.79 mM.[28a] The
simulation results are tabulated in Table S5 in SI. The c urrents obtained from the simulation
(trace “c” in Figure 2A) fit well with the experimental data in He-saturated solutions. In
comparison, Figure 2A-B reveals that the current response under H2-containing atmospheres
is always at least five times lower (e.g. experimental currents at −0.343 Vin Figure 2A).
Figure 2. (A) The average current transient magnitudes of the collision events from Figure 1 as a
function of applied potentials in (a) helium-saturated 1.0 mM HClO4 solution, (b) hydrogen-saturated
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1.0 mM HClO4 solution, and (c) from simulation under 100% helium atmosphere. (B) The average
current magnitudes of the collision events (Figure S10, SI) biased from −0.203 to −0.403 V in 1.0 mM
HClO4 solution saturated with He-H2 mixture.
A possible explanation is that the dissolved hydrogen molecule spontaneously undergoes
chemical dissociation into H atoms and adsorb/absorb onto/inside the Pt NPs. As a
consequence, the HER electrocatalysis can be passivated by the hydrogen absorption into the
crystal lattice of Pt NPs,[29] as suggested earlier by Bard et al.[16a, 16b] In this process, the
change in the Gibbs energy is sufficiently negative such that the process can proceed
completely, and is independent of the partial pressure of hydrogen. It is inferred that the Pt
NPs will get passivated to a similar degree even though the partial pressure of hydrogen
remarkably varies from 1 to 0.2 atm. As such, the current signal of NP’s collision is
essentially the same in hydrogen-containing solution considering the same proton
concentration. Interestingly, Compton and co-workers observed similar passivation effect for
the hydrogen oxidation reaction (HOR) on single Pt NPs.[28b] In addition, the instability of the
Pt NPs colloid revealed by DLS (Figure S5, SI) can be seen as a collateral evidence for the
hydrogen effect. In such solutions, the Fermi level of the Pt NP is equilibrated with the H+/H2
redox couple.[30] At these potentials, the citrate may be replaced by the dissociated H atoms on
the NP surface, thus degrading the colloidal stability.
Furthermore, the NP’s size distribution is calculated by using the steady-state current under
100% helium atmosphere (Figure 1A) and Equation S13 in SI. The modal size is found to be
ca. 66 nm (Figure S11 in SI), which coincides well with the SEM result of ~ 71 nm (Figure
S1 and Table S1, SI).
In summary, we investigated the effect of the interfacial dynamics on the current transient
profile of a single Pt NP collision via the ECA towards the HER. Firstly, Pt NPs aggregation
as a function of proton concentration was studied to achieve a genuinely single NP collision.
It is found that the 70 nm diameter Pt NPs remain stable against aggregation for a long
duration (≥ 30 min) at the proton concentrations ≤ 1.0 mM. Afterwards, the
chronoamperograms were collected to track single NP collisions at different electrode
potentials under varied atmospheres. It is found that current transient evolves from spike to
staircase with applying more negative potentials in He-saturated solution. Thermodynamics
analysis indicates that the shape of the chronoamperogram depends on the interfacial
dynamics, which results in a drift in equilibrium potential and slower HER kinetics than mass
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transfer under He atmosphere at insufficient overpotentials. In comparison, the current signal
is staircase-like at all investigated potentials in H2-containing solution, but at least five times
lower in magnitude than expected. This difference is supposed to originate from the
passivation of the HER electrocatalyst by the hydrogen absorption into the crystal lattice of Pt
NPs,[29] as suggested earlier by Bard et al.[16a, 16b] The calculated Pt NP’s size distribution via
the electrochemical method in He-saturated solution correlates well with both the DLS and
the SEM results. Our work presented herein sheds light on the nanoelectrode electrochemistry
via ECA, which is likely to be a cornerstone to study electrochemistry at a genuinely single
nanoparticle level.
Acknowledgments
The work described in this paper was jointly supported by the National Natural Science
Foundation of China (Nos. 21676106, 21476087), Science and Technology Program of
Guangzhou (201704030065), Science and Technology Program of Guangdong
(2017A050506015). D.M. acknowledges the support by the Israeli Ministry of Science and
Technology (Contract No. 3-12610). H.D. acknowledges the financial support from the PBC
Fellowship jointly sponsored by the Planning and Budgeting Committee of the Council for
Higher Education in Israel and the Hebrew University of Jerusalem and also appreciates Prof.
Daniel Mandler for hosting his postdoctoral studies. P.P. acknowledges the financial support
from the Swiss National Science Foundation, grant Ambizione Energy 160553.
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Entry for the Table of Contents
Evolution or stability? Under helium atmosphere: electrochemical catalytic current profile of
hydrogen evolution reaction on a single platinum nanoparticle evolves from a spike to a
staircase with polarizing more negative potentials on the underlying inert ultramicroelectrode;
however, under hydrogen atmosphere: the current profile stabilizes as a staircase at all
polarizedpotentials but at least 5 times lower than that in helium atmosphere.
12
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