Interfacial water reorganization as a pH-dependent descriptor of the hydrogen evolution rate on platinum electrodes

TLDR: In this paper, a detailed kinetic study of hydrogen adsorption and evolution on Pt(111) in a wide pH range is presented, highlighting the role of reorganization of interfacial water to accommodate charge transfer through the electric double layer, the energetics of which are controlled by how strongly water interacts with the interfacial field.

Abstract: Hydrogen evolution on platinum is a key reaction for electrocatalysis and sustainable energy storage, yet its pH-dependent kinetics are not fully understood. Here we present a detailed kinetic study of hydrogen adsorption and evolution on Pt(111) in a wide pH range. Electrochemical measurements show that hydrogen adsorption and hydrogen evolution are both slow in alkaline media, consistent with the observation of a shift in the rate-determining step for hydrogen evolution. Adding nickel to the Pt(111) surface lowers the barrier for hydrogen adsorption in alkaline solutions and thereby enhances the hydrogen evolution rate. We explain these observations with a model that highlights the role of the reorganization of interfacial water to accommodate charge transfer through the electric double layer, the energetics of which are controlled by how strongly water interacts with the interfacial field. The model is supported by laser-induced temperature-jump measurements. Our model sheds light on the origin of the slow kinetics for the hydrogen evolution reaction in alkaline media. Despite its role in electrocatalysis and hydrogen generation, a complete understanding of the hydrogen evolution reaction on platinum remains elusive. Here, a detailed kinetic study of hydrogen adsorption and evolution on Pt(111) highlights the role of interfacial water reorganization in the hydrogen adsorption step.

Figures

Figure 2. a) Equivalent electric circuit for H-UPD region, featuring the double layer capacitance, CDL, the capacitance associated to the hydrogen adsorption, CAD, the charge-transfer resistance for hydrogen adsorption, RCT, and the solution resistance, RSOL. b) Admittance Nyquist plots for Pt(111) in 0.1 M solutions for different pH values, measured with frequencies ranging from 10 kHz to 0.1 Hz and an amplitude of 5 mV, at 0.2 VRHE: the dots represent the experimental data points collected, while the solid lines correspond to the fit obtained using the EEC. The arrows point out how the components from the EEC in Fig. 2a are related to each semi-circle. The data for the entire H-UPD region can be found in Figure Supplementary S1. c) Inverse of the charge transfer resistance as a function of the pH value, proving that the rate of H-UPD formation becomes slower in more alkaline solutions. The Rct was obtained from the fitting parameters for the admittance plots, collected for Pt(111) in 0.1 M solutions with different pH value. The different colors represent the potential at which the data was collected (see

Figure 5. a) Cyclic voltammograms for Pt(111) in 0.1 M NaOH under argon atmosphere, registered at a scan rate of 50 mV.s-1: The black line shows the blank of the bare electrode, whereas the blue line shows the Pt(111) electrode decorated with Ni(OH)2, growth with instant deposition from a solution containing 5 mM Ni(NO3)2 (Ni 0) and 3 cycles in the 0.1 M NaOH solution (~13% coverage). A voltammogram of Pt(111) with a higher coverage of Ni(OH)2 is shown in Supplementary Figure S7 b) Laserinduced coulostatic potential transients collected for the Pt(111) electrode. c) Laser-induced coulostatic potential transients collected for the Pt(111) electrode decorated with Ni(OH)2. Beam energy: 1 mJ.

Figure 4. Cyclic voltammetry of Pt (111) in contact with the buffered solution with pH=10: 0.05 M Na2CO3 + 0.05 M NaHCO3.

Figure 3. a) Tafel plots for HER on for Pt(111) at pH 1 and pH 13 and on a Pt(111) electrode decorated with 0.1 ML of Ni(OH)2 in 0.1 M NaOH (pH 13), under argon atmosphere. The polarization curves were registered at a scan rate of 10 mV.s-1. b) The inverse of the charge transfer resistance as a function of potential for Pt(111) (black squares) and Pt(111) electrode decorated with ~10% of Ni(OH)2 in 0.1 M NaOH, pH 13 (blue squares); argon atmosphere. The data shows how the presence of Ni(OH)2 increases the rate of H-UPD formation on Pt(111) in alkaline solution. The Rct was obtained from the fitting parameters for admittance plots (see Supplementary Figure S5).

Figure 1. Cyclic voltammetries for Pt(111) in 0.1 M perchloric acid (pH 1); 0.001 M NaOH + 0.099 M NaClO4 (pH 11); 0.01 M NaOH + 0.09 M NaClO4 (pH 12); 0.1 M NaOH (pH 13). Scan rate: 50 mV.s -1.

Full text

1
Interfacial water reorganization as a pH-dependent descriptor of
the hydrogen evolution rate on platinum electrodes
I. Ledezma-Yanez
1
, W.D.Z. Wallace
1
, P. Sebastián-Pascual
2
, V. Climent
2
, J.M. Feliu
2
, M.T.M.
Koper
1
1
Leiden Institute of Chemistry, Leiden University, 2300 RA, Leiden, The Netherlands
2
Instituto de Electroquímica, Universidad de Alicante, Apdo. 99, Alicante, Spain
E-mail: m.koper@chem.leidenuniv.nl
Abstract
The hydrogen evolution on platinum is a milestone reaction in electrocatalysis as well
as an important reaction towards sustainable energy storage. Remarkably, the pH
dependent kinetics of this reaction is not yet fully understood. Here, we present a
detailed kinetic study of the hydrogen adsorption and evolution reaction on Pt(111) in
a wide pH range. Impedance and Tafel slope measurements show that the hydrogen
adsorption and hydrogen evolution are both slow in alkaline media, which is
consistent with the observation of a shift in the rate-determining step for H
2
evolution.
Adding nickel to the Pt(111) surface lowers the barrier for the hydrogen adsorption
rate in alkaline solutions and thereby enhances the hydrogen evolution rate. These
observations are explained by a new model which highlights the role of the
reorganization of interfacial water to accommodate charge transfer through the
electric double layer, the energetics of which is controlled by how strongly water
interacts with the interfacial field. The new model is supported by laser-induced
temperature-jump measurements. Our model sheds new light on the origin of the
slow kinetics for the hydrogen evolution reaction in alkaline media.
Introduction
There is a global call for industrial processes that combine economic progress with
long-term preservation of natural resources. In terms of technological advances for
sustainable energy production
1,2
, there is a recent renewed interest to realize the so-
This is a previous version of the article published in Nature Energy. 2017, 2: 17031. doi:10.1038/nenergy.2017.31

2
called hydrogen economy
3,4
by photocatalytic water splitting or by the combination of
photovoltaics with water electrolysis
5,6
. Methane steam-reforming
7
is currently the
most cost-efficient technology available for hydrogen production, but unsustainable in
the long run as it is still based on the deployment of fossil fuels. In order for the
hydrogen economy to meet our future energy demands
8
, there are, however, various
fundamental bottlenecks to be overcome, such as that related to the efficient
catalysis of the associated multi-proton multi-electron transfer reactions. Essentially,
significant advances are required to lower the high inherent costs of electrocatalysts,
by increasing the efficiency of water oxidation, by the replacement of scarce and
expensive catalyst materials by earth-abundant alternatives, and by maximizing their
durability and lifetime. Substantial efforts have thus been devoted to lowering the
costs of the electrodes necessary for water splitting
9,10
. Recent theoretical works
exposed the mechanistic features of the oxygen evolution reaction (OER)
11,12
, taking
place at the anodes of electrolyzers, in which water is oxidized to produce molecular
oxygen and protons. These insights motivated several reports on earth-abundant and
highly-efficient OER anode materials that typically work at high pH
13,14
. Unfortunately,
the price to be paid for the use of alkaline conditions is a significant overpotential at
the cathode, where the hydrogen evolution reaction (HER) takes place.
It has long been known in the electrochemistry literature that the kinetics of both the
hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) on
platinum are significantly slower in alkaline media than in acidic media
15-17
. The
elucidation of the molecular-level origin of this problem would be of obvious
importance for the further development of alkaline electrolyzers and alkaline fuel
cells. Marković et al. have recently shown
18-20
how the oxophilicity of the interface, as
modified by adsorbing a small amount of Ni(OH)
2
on Pt(111), may improve the
kinetics of the HER/HOR, and ascribed this effect to the favorable interaction of
surface adsorbed OH
ads
with the relevant intermediates. Mechanistic studies on the
HER
18,19,21-24
have traditionally correlated reaction rates with thermodynamic
descriptors, in particular the strength of the bond between hydrogen and the metal
electrode, following the Sabatier principle
25,26
. Gasteiger et al. and Yan et al. have
suggested
22,27
that a pH-dependent H-binding energy lies at the origin of the pH-
dependent HER/HOR kinetics, and therefore they concluded that the H-binding
energy is and remains the sole descriptor for the HER/HOR reaction. Specifically,

3
Yan et al. have considered the pH-dependent shifts of the voltammetric peaks in the
so-called “underpotential deposition” (UPD) hydrogen region of polycrystalline
platinum as evidence for such a pH-dependent H-binding energy. However, the
molecular-level origin of the pH-dependent H-binding energy has remained elusive.
More importantly, we have recently argued
28
that the nature of the “hydrogen” peaks
on polycrystalline platinum are unlikely to be associated with the adsorption of
hydrogen only, but also include the effect of the adsorption of oxygenated species on
(110) and (100) sites. Therefore, their peak potentials are not unambiguous
indicators of H-binding energy. Additionally, on a Pt(111) electrode, there is no
significant shift of the H-UPD with pH, but there is still a very significant pH-
dependence of the HER/HOR kinetics (see below). Therefore, it appears that in spite
of the undeniable success of the traditional models, they overlook important kinetic
details, and that a consistent explanation of why HER/HOR on Pt is slow in alkaline
media is still missing.
In this work, we present a detailed kinetic study of the hydrogen adsorption and
evolution reaction on a Pt(111) single-crystal electrode, in a wide pH range, and in
the absence and presence of a small amount of Ni(OH)
2
promoter. Electrochemical
impedance spectroscopy is used to accurately measure the charge transfer kinetics
of hydrogen adsorption. We present a new model for the rate of hydrogen adsorption
step, based on the idea that the barrier for this reaction depends on how close or
remote the electrode potential is in relation to the potential of zero (free) charge
(pzfc). The laser-induced temperature jump technique is then used to measure the
potential of maximum entropy (pme), which is closely related to the pzfc, to show that
at pH 13, the addition of Ni(OH)
2
to Pt(111) indeed shifts the pme/pzfc closer to the
hydrogen adsorption region. We attribute the impact of the pzfc on the activation
barrier of hydrogen adsorption to the energy penalty associated with the
reorganization of interfacial water to accommodate charge movement through the
double layer. In acid media, the pzc/pme of Pt(111) is close to the hydrogen region,
and the energy of reorganization of the interfacial water to move a proton through the
double layer is relatively small. In alkaline media, the pzc/pme of Pt(111) is far from
the hydrogen region (i.e. close to the OH
ads
region) and the corresponding strong
electric field existing at the electrode/electrolyte interface in the hydrogen region
leads to a large reorganization energy for interfacial water when OH
-
transfers

4
through the double layer. In general terms, our results show how a cost-effective,
earth-abundant metal as nickel, in the form of nickel hydroxide, promotes the
reorganization of water networks at the electrode-electrolyte interface by shifting the
pzc closer to the equilibrium potential of the HER reaction, thereby enhancing the
reaction rate for the hydrogen evolution at high pH values. Our model also suggests
a new strategy for designing new and better electrocatalysts in aqueous media,
highlighting the role of the interfacial solvent reorganization on ion transfer steps.
Results and Discussion
Kinetic measurements on H adsorption and H
2
evolution as a function of pH
Figure 1 illustrates the key observation that we wish to explain in this work,
comparing the cyclic voltammetries for a Pt(111) electrode at pH 11 (blue line), pH 12
(red line), pH 13 (turquoise line) and pH 1 (black, dashed line). All the measurements
were collected using a scan rate of 50 mV.s
-1
. The voltammetric curves are very
similar in current and shape for the alkaline pH, showing the typical features for the
Pt(111) surface orientation: the hydrogen adsorption region, known as H-UPD (0.1 –
0.35 V
RHE
), which remains almost unaltered for the several pH values presented
(versus the RHE), the double layer region (0.35 – 0.55 V
RHE
), and the hydroxyl
adsorption-desorption region (0.6 – 0.8 V
RHE
). The reduction current registered
between 0.05 V
RHE
and -0.1 V
RHE
corresponds to the hydrogen evolution, and the
adsorbed hydrogen formed in this potential window is referred to as H-OPD
(overpotential deposition). We observe that the onset potential for the HER in alkaline
solutions exhibits a shift towards negative potentials as the pH value increases.
Hydrogen evolution clearly has the lowest overpotential at pH 1, even though there is
only a small shift in the H-UPD region in comparison to alkaline media. We conclude
from Figure 1 that a pH-dependent shift in the H-binding energy (if any) cannot
explain the significant pH dependence of the HER overpotential on Pt(111).

5
Figure 1. Cyclic voltammetries for Pt(111) in 0.1 M perchloric acid (pH 1); 0.001 M NaOH + 0.099 M NaClO
4
(pH 11); 0.01 M
NaOH + 0.09 M NaClO
4
(pH 12); 0.1 M NaOH (pH 13). Scan rate: 50 mV.s
-1
.
To demonstrate that the pH also affects the rate of hydrogen adsorption in the H-
UPD region, we studied the kinetics of hydrogen adsorption in the 0.1-0.3 V potential
window. Figure 2 summarizes the measurements made by means of electrochemical
impedance spectroscopy (EIS) as a function of potential and pH. The experimental
admittance spectra were fitted using the equivalent electric circuit (EEC
17,29
)
presented in Figure 2a. This EEC corresponds to a simple heterogeneous adsorption
step, forming an adsorbed intermediate without diffusion limitation. The charge
transfer resistance, R
ct
,
is inversely proportional to the rate of the corresponding
hydrogen adsorption reaction, i.e. H
+
+ e
-
→ H
ads
in acidic media, and H
2
O + e
-
→
H
ads
+ OH
-
in alkaline media. R
sol
stands for the resistance of the solution, whereas
C
DL
and C
AD
represent the capacitance of the double layer and the pseudo-
capacitance of the adsorbed hydrogen, respectively. Figure 2b shows the admittance
plots, measured at 0.1 V
RHE
in solutions of 0.1 M ionic strength at different pH values.
The data collected for the entire H-UPD region were fitted to the EEC shown in
Figure 2a and can be found in Supplementary Figure 1. From the Nyquist plots
presented in Figure 2b we notice that for alkaline pH we observe two semicircles.
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
-400
-300
-200
-100
0
100
j / µA.cm
-2
E / V
RHE
pH 1
pH 11
pH 12
pH 13

Frequently asked questions

Q1. What are the contributions in "Interfacial water reorganization as a ph-dependent descriptor of the hydrogen evolution rate on platinum electrodes" ?

Here, the authors present a detailed kinetic study of the hydrogen adsorption and evolution reaction on Pt ( 111 ) in a wide pH range. 

Q2. How did Pecina and Schmickler calculate the activation energy for proton transfer?

Using Monte Carlo simulations to sample water configurations near a negatively charged surface, as it is the case in alkaline solutions, they calculated the activation energy for proton transfer through the interfacial water layer. 

Q3. What is the onset potential for the HER in alkaline solutions?

The authors observe that the onset potential for the HER in alkaline solutions exhibits a shift towards negative potentials as the pH value increases. 

Q4. What is the Tafel slope for HER in acidic media?

A Tafel slope of ca. 40-30 mV.dec-1 implies that either the second ET step is the RDS or the RDS is a chemical step preceded by two ET steps, indicating that in acidic media the mechanism involves the Volmer step H+ + e- ↔ 

Q5. What are the fundamental bottlenecks to be overcome?

In order for the hydrogen economy to meet their future energy demands8, there are, however, various fundamental bottlenecks to be overcome, such as that related to the efficient catalysis of the associated multi-proton multi-electron transfer reactions. 

Q6. What was the procedure used to remove impurities from the cell?

The argon was bubbled through a 3 M KOH trap before it entered the cell, inorder to remove impurities which may be present from the tubing. 

Q7. What is the role of Ni(OH)2 in the HER?

Together with the data in Fig.3b, this suggests that the role of Ni(OH)2 is to lower the barrier for the hydrogen adsorption reaction, rather than to change the energetics of the hydrogen intermediate or the mechanism. 

Q8. What is the pzfc of Pt(111) in alkaline solutions?

Since the H-UPD region is far from the pzfc of Pt(111) in alkaline media, thehydrogen adsorption reaction is slow in alkaline solutions. 

Q9. What is the effect of the classical model potentials on the water?

The water in the work of Pecina and Schmickler was modeled by classical model potentials but it is not expected that the qualitative effect will change if more accurate potentials, such as provided by first-principles density functional theory calculations, would have been used. 

Q10. What is the effect of Ni(OH)2 on the pme?

This shift in pme/pzfc is in accordance with their model for the enhancement of the H-UPD and hydrogen evolution by the presence of Ni(OH)2 on the surface as proposed above, relating the hydrogen adsorption and hydrogen evolution rates to the energy required for the water reorganization in the electrode interface. 

Q11. What is the pme of Pt(111) in alkaline solutions?

Details regarding the laser-induced temperature jump method can be found in the literature43, and a detailed study of the pme of Pt(111) in alkaline solutions will be given in a separate publication. 

Q12. What is the effect of Ni(OH)2 on the interfacial water network?

These simulations correlate well with the change in the pzfc of the Pt(111) in presence of small amounts of Ni(OH)2, since the presence of Ni(OH)2 lowers the interfacial electric field and thereby lowers the energetic barrier for the reorganization of the interfacial water network. 

Q13. What is the inverse of the charge transfer resistance?

The charge transfer resistance, Rct, is inversely proportional to the rate of the corresponding hydrogen adsorption reaction, i.e. H+ + e- →