Organic Proton-Buffer Electrode to Separate Hydrogen and Oxygen Evolution in Acid Water Electrolysis

Article abstract of DOI:10.1002/anie.201814625

Hydrogen production from water via electrolysis in acid is attracting extensive attention as an attractive alternative approach to replacing fossil fuels. However, the simultaneous evolution of H₂ and O₂ requires a fluorine-containing proton exchange membrane to prevent the gases from mixing while using the same space to concentrate the gases, which significantly increases the cost and reduces the flexibility of this approach. Here, a battery electrode based on the highly reversible enolization reaction of pyrene-4,5,9,10-tetraone is first introduced as a solid-state proton buffer to separate the O₂ and H₂ evolution of acidic water electrolysis in space and time, through which the gas mixing issue can be avoided without using any membrane. This process allows us to separately consider H₂ and O₂ production according to the variation in input power (e.g., the renewable energy) and/or the location for H₂ concentration, thus showing high flexibility for H₂ production.

Full text of DOI:10.1002/anie.201814625

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Accepted Article
Title: Organic Proton-buffer-electrode to Separate Hydrogen and
Oxygen Evolution in Acid Water Electrolysis
Authors: Yonggang Wang, Yuanyuan Ma, Zhaowei Guo, Xiaoli Dong,
and Yongyao Xia
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201814625
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COMMUNICATION
Organic Proton-buffer-electrode to Separate Hydrogen and Oxygen
Evolution in Acid Water Electrolysis
Yuanyuan Ma, Zhaowei Guo, Xiaoli Dong, Yonggang Wang*, Yongyao Xia
Abstract: Hydrogen production from water via electrolysis in acid is
The operation of electrolytic water splitting in acid involves
simultaneous H evolution on the cathode and O evolution on the anode,
attracting extensive attention as an attractive alternative approach to
2
2
replacing fossil fuels. However, the simultaneous evolution of H
2
and O
2
which is in parallel with the proton and electron transfer between the
requires a fluorine-containing proton exchange membrane to prevent the
gases from mixing while using the same space to concentrate the gases,
which significantly increases the cost and reduces the flexibility of this
approach. Here, a battery electrode based on the highly reversible
enolization reaction of pyrene-4,5,9,10-tetraone is first introduced as a
anode and cathode (Figure 1a). In theory, if there is proton buffer
electrode between the anode and cathode (Figure 1b), the H
2
evolution
and O evolution could be decoupled. Here, we first introduce a battery
2
electrode based on the enolization reaction (Figure 1c) of pyrene-
4,5,9,10-tetraone (PTO) as a solid-state proton buffer to separate the
HER and OER during acidic water electrolysis in space and time,
providing a flexible and membrane-free architecture for electrochemical
water splitting. In this architecture, the electrode reactions of the
decoupled acid water electrolysis can be summarized as follows:
2 2
solid-state proton buffer to separate the O and H evolution of acidic water
electrolysis in space and time, through which the gas mixing issue can be
avoided without using any membrane. This process allows us to separately
consider H
e.g., the renewable energy) and/or the location for H
showing high flexibility for H production.
2
and O
2
production according to the variation in input power
(
2
concentration, thus
2
Step 1 for O production:
2
ꢁ
ꢂ
Anode: 2H O − 4e → O + 4H
(1)
(2)
ꢀ
ꢀ
ꢁ
ꢂ
Electrochemical water splitting is an emerging technology for
Cathode: PTO + 4e + 4H → PTO-4H
production.[ Alkaline water electrolysis has been well
1-3]
sustainable H
developed for H
2
Step 2 for H production:
2
2
production because of its low cost nature arising from
the use of non-precious catalyst and a porous separator.[ However, the
1-3]
Anode: PTO-4H − 4eꢁ → PTO + 4Hꢂ
Cathode: 4Hꢂ + 4eꢁ → 2Hꢀ
(3)
(4)
2
H production rate of alkaline water electrolysis is still much lower than
conventional reforming method. Therefore, the acid water electrolysis
using proton exchange membrane entered researchers’ spotlight because
In step 1, the anodic OER releases protons and electrons (Eq. 1), and
simultaneously, the PTO stores the protons and electrons through the
cathodic reduction to form PTO-4H (Eq. 2 and Figure 1c). In step 2, the
anodic oxidization of PTO-4H releases protons and electrons (Eq. 3 and
Figure 1c), and at the same time, the cathodic HER consumes the
2
2
it exhibits high H production rate, low kinetic over potential and a
compact design.[ During the past few years, many endeavours have
been made to develop a highly active and low-cost catalyst for the
hydrogen evolution reaction (HER) and oxygen evolution reaction
1-9]
(
OER) in acid,[4-24] which further increases the overall performance of
protons and electrons to produce H (Eq. 4). That is, the reversible
electrolytic water splitting in acid. However, the inherent co-production
of hydrogen and oxygen still limits the operational flexibility and
practical application of this technology. First, an expensive proton
enolization reaction (Eqs. 2, 3) of the solid-state PTO plays the role of
the proton buffer for the OER (Eq. 1) and HER (Eq. 2), through which
the OER and HER can be conducted in different times and spaces (or
places) without using any membrane.
2 2
exchange membrane is required to prevent H /O mixing, which
increases the system cost and electrolysis resistance.[ Second, the high
operating pressures and the pressure difference between H and O
theoretical H :O
= 2:1) decreases the lifetime of the membrane.[
1-3]
2
2
1,2,25]
(
2
2
Third, the reactive oxygen species (ROS) yielded during the OER also
aggravates the degradation of the membrane.[26-29] Finally, the inherent
co-production leads to the concentration of H and O at the same time
2 2
and place, which aggravates the safety issue of explosive gas mixtures
during large-scale application.[25,30-32] Therefore, it is highly desired to
2
develop the membrane-free acid water electrolysis, in which the H and
O
2
evolution is decoupled in time and space.
[*]
Y. Ma, Z. Guo, X. Dong, Prof. Y. Wang, Prof. Y. Xia
Department Department of Chemistry and Shanghai Key Laboratory of
Molecular Catalysis and Innovative Materials, Institute of New Energy,
iChEM (Collaborative Innovation Center of Chemistry for Energy
Materials), Fudan University, Shanghai 200433, China.
E-mail: ygwang@fudan.edu.cn
Figure 1. Schematic illustration of (a) the conventional water electrolysis system, (b)
the decoupled water electrolysis system using a proton buffer electrode, and (c) reaction
mechanism of PTO electrode showing the function of proton buffer electrode.
PTO was synthesized by the oxidation reaction of pyrene (see
Supporting Information).[33] The morphological structure of PTO was
Supporting information for this article is given via a link at the end of the
document.
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COMMUNICATION
Figure 2. (a) TEM image of PTO power. (b) CV curve of the PTO electrode (black line), LSV data of the HER electrode (red line) and the LSV data of the OER electrode (blue
line) at a scan rate of 5 mV s- in 0.5 M H
1
SO
2
4
. (c) Relationship between the peak current and scan rate of PTO film electrode. (d) Rate performance of PTO electrode for selected
current densities in 0.5 M H
approximately 3 mgPTO cm .
2
SO
4
. The current densities and specific capacities were calculated based on the mass loading of PTO. The mass loading of the PTO electrode is
-2
characterized by scanning electron microscopy (SEM) images (Figure
S1) and transmission electron microscopy (TEM) images (Figure 2a).
The SEM and TEM images reveal that PTO has a rod-like morphology
and grows as an aggregation, with an average diameter of ~400 nm. The
kinetics of PTO (see Figure S3b for details). Galvanostatic charge-
discharge tests in the potential range of 0-0.7 V (versus Ag/AgCl) were
employed to characterize the proton storage capacity of the PTO
electrode. As shown in Figure 2d, the PTO electrode displays a
reversible capacity of 156 mAh g-1 at a current density of 0.2 A g-1,
which is much higher than the theoretical storage capacity of soluble
1
molecular structure of the prepared PTO was confirmed by H NMR
spectroscope (Figure S2a) and FT-IR spectroscopy (Figure S2b). Prior
to demonstrating the hypothesis shown in Figure 1b, we investigated
[25,34,35]
electron-coupled proton buffers of previous reports.
With the
2
the electrochemical behaviours of the PTO electrode (1 cm ; 3 mgPTO
increase of the current density, the specific capacity of the PTO slowly
-2
-1
cm ) in an acidic electrolyte solution (0.5 M H
electrode system. The cyclic voltammetry (CV) curve of the PTO
electrode tested at a scan rate of 5 mV s-1 in 0.5 M H
SO electrolyte is
2
SO
4
) with a three-
decreases. Even at a high current density of 20 A g , a reversible
capacity of 107 mAh g-1 can be still achieved, indicating an excellent
rate ability (i.e., a fast rate for accepting/releasing protons and electrons).
The current densities and specific capacities are calculated based on the
mass of PTO unless otherwise stated. In addition, the PTO electrode also
shows a high stability during 200 cycles, with a capacity retention of
2
4
shown in Figure 2b (black line), where the two pairs of symmetric redox
peaks located at 0.34 V/0.2 V and 0.46 V/0.32 V (vs. Ag/AgCl) are
clearly observed, showing the reversible conversion between PTO and
-1
PTO-4H (Eqs. 2, 3). The HER and OER potentials of the commercial
approximately 80% at a current density of 1 A g (Figure S3c). The
results shown in Figures 2b-d indicate that PTO could act as the proton
buffer electrode to split the acid water electrolysis process into two steps.
After the characterization of the PTO electrode, we constructed an
acid water electrolysis cell to demonstrate the hypothesis shown in
Figure 1b. As shown in Figure S4a, this cell involves a commercial Pt-
2
Pt-coated Ti-mesh electrode (1 cm ) and commercial RuO
2
/IrO
2
-coated
2
Ti-mesh electrode (1 cm ) were also measured for comparison. The
corresponding linear sweep voltammetric (LSV) data tested at a sweep
rate of 5 mV s for the HER (red line) and OER (blue line) are also given
in Figure 2b. Examination of Figure 2b indicates that the potential
window of the PTO redox couple is located between the onset potentials
of the HER and OER. The kinetics of the PTO electrode were
investigated by CV measurements at different scan rates from 0.5 mV s
to 30 mV s-1 (Figure S3a). The CV response of the PTO electrode at
various scan rates can be summarized by i
-1
2
coated Ti-mesh electrode (1×2 cm ) for the HER, a commercial
2
RuO
2
/IrO
2
-coated Ti-mesh electrode (1 ×2 cm ) for the OER and a PTO
-
film electrode (1.5 ×2 cm ; the mass loading of PTO is 33.3 mgPTO cm-
2) in between. The performance of the decoupled water electrolysis in
2
1
b
p
= av (i
p
is the peak current,
2 4
0.5 M H SO was investigated by chronopotentiometry measurements
a and b are parameters), showing a power law relationship between the
CV response and the potential scan rate (v). As shown in Figure 2c, the
b value of the PTO electrode is 0.77 (that is higher than 0.5), indicating
that the fast kinetics of the PTO electrode are not controlled by the
diffusion process. Furthermore, electrochemical impedance
spectroscopy (EIS) measurements were also carried out to clarify the
with the applied currents varying from 5 to 1000 mA. The selected
chronopotentiometry data (voltage versus time) of the electrolytic cell
achieved at 5 mA, 50 mA and 500 mA are given in Figures 3a-c,
respectively. The corresponding chronopotentiometry data (potential
versus time) of the anode and cathode were also recorded during the
electrolysis process and are provided in Figure 3. As shown in Figure
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COMMUNICATION
of the chronopotentiometry data (Figure S6) shows that OER is the rate
limiting step for the decoupled process. Cycle performance of the
hydrogen and oxygen generation is shown in Figure S7. After 300
consecutive cycles under different applied currents (5 mA, 50 mA and
5
00 mA), the cell voltages in step 1 (or step 2) were remain constant,
showing the high stability of the alternating H production and O
2
2
production. It should be noted that the mass loading of PTO determines
the duration of each step, which is a drawback. To some extent, such
drawback (that is O
the intermitted nature of renewables, which might facilitate the
renewables-to-hydrogen”. This assumption will be clarified later. Cell
2 2
/H discontinuous production) matches well with
“
voltages of the decoupled process tested by chronopotentiometry
measurements at different currents are compared with the conventional
acid water electrolysis (Figure S8).
Figure 3. Chronopotentiometry cures (cell voltage versus time) of the decoupled cell at
a current of (a) 5 mA, (b) 50 mA and (c) 500 mA with a step time of 6000 s, 600 s and
7
2 s, respectively. The voltages for O
step 2) are labelled by the black and red lines, respectively. Chronopotentiometry data
potential versus time) of the OER electrode (magenta line), HER electrode (green line)
and PTO electrode (blue line) are provided. The mass loading of PTO electrode is
2 2
production (i.e. step 1) and H production (i.e.
(
-2
approximately 33.3 mgPTO cm .
3
2
a, step 1 (i.e., the O production process with an applied current of 5
mA and a fixed electrolysis time of 6000 s) exhibits an average cell
voltage of ~1.1 V, which is based on the potential difference between
2
the average anodic potential of 1.38 V (vs. Ag/AgCl) of the H O
+
-
oxidization (2H
V (vs. Ag/AgCl) of the PTO reduction (PTO + 4H + 4e → PTO-4H).
In step 2 (that is the H production process with an applied current of 5
2 2
O → O +4H + 4e ) and the cathodic potential of 0.28
Figure 4. High pure H
2 2
and O production demonstrated by the in situ DEMS analysis
+
-
(a) and the corresponding chronopotentiometry curves (voltage versus time) of the
2
electrolytic cell at an applied current of 50 mA (b).
mA), the cell voltage is ~0.56 V, which comes from the anodic potential
of PTO-4H oxidization (PTO-4H → PTO + 4H+ + 4e , 0.37 V vs.
-
The in situ differential electrochemical mass spectrometry (DEMS)
was used to analyse the gas produced by the decoupled acid water
electrolysis at a constant applied current of 50 mA (see Supporting
Information and Figure S9 for detailed information). The achieved
DEMS data and the corresponding chronopotentiometry data of the
electrolytic cell are given in Figure 4a and Figure 4b, respectively. In
this experiment, a pure Ar gas stream was used as both the purge gas and
carrier gas. As shown in Figure 4a, in parallel with the beginning of step
+
-
Ag/AgCl) and the cathodic potential of H
2 2
O reduction (4H + 4e → 2H ,
-
0.19 V vs. Ag/AgCl). It should be noted that the cell voltage of step 2
increases sharply when the electrolysis time of step 2 reaches 6000 s.
This phenomenon should be attributable to that all of the PTO-4H has
+
been oxidized into PTO with the release of H . Herein, the equal
electrolysis times of steps 1 and 2 clearly indicate a Coulombic
efficiency of 100%. Figures 3b and c show the results when higher
currents of 50 mA and 500 mA were applied to this electrolysis cell with
step times of 600 s and 72 s, respectively. Inspection of Figure 3b (or
1 (i.e., O
analysis record, whereas the H
indicating no H
step of 25 min was performed with a pure Ar stream to eliminate the
remnant O in the system, showing a hysteresis of O . Then, H evolution
is observed in the online analysis record of DEMS during step 2 but
without any O evolution. The results (Figure 4a) clearly demonstrate
the purities of O and H produced by the decoupled process. A drainage
method was employed to quantify the H /O generation over a specific
electrolysis time, and the achieved result indicates that the Faraday
efficiency for H production is approximately 98.7% with a H to O
2
production step), the O
2
evolution was detected in the online
2
remained at the background level,
3
c) reveals that the cell voltages of step 1 and step 2 are 1.27 V (or 1.84
2
generation during step 1. After the end of step 1, a rest
V) and 0.6 V (or 1.15 V), respectively. The water electrolysis data
investigated at other currents of 80 mA, 100 mA, 300 mA, 800 mA and
2
2
2
1
000 mA are shown in Figures S4b-f to further illustrate the operational
flexibility of this device. In addition, photo profiles of step 1 and step 2
under the applied current of 100 mA are shown in Figure S5,
respectively, to characterize the decoupled steps. The corresponding
2
2
2
2
2
videos are also given to further clarify the separated O
2 2
and H
generation of step 1 and step 2, respectively (Movies S1, S2). Analysis
2
2
2
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Angewandte Chemie International Edition
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COMMUNICATION
ratio of 2:1 (see Supporting Information). The separated H
production at different times facilitates the flexible utilization of
renewable resources. For example, as shown in Figure S10, the wind
power (or valley electricity) at night can be used to drive the O
2
and O
2
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production, and the solar energy in the daytime can be used only for H
2
1
054.
production (see Figure S10 and corresponding discussion). The
decoupled H and O production could potentially facilitate the H
transfer and centralized hydrogen storage. As demonstrated in Movies
S3 and S4, the O production driven by a solar cell can be performed
outdoors, and then, the PTO-4H is transferred indoors for H production
using an electrochemical work station as the power source. Furthermore,
enolization reaction of organic solids are being widely used to build
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could also be used to decouple the acid water electrolysis, which needs
further investigation. Furthermore, non-precious OER/HER electrodes
can also be directly applied in the PTO-based decoupled process, which
is clarified by Figure S11 and Movies S5, S6.
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using a solid-state proton buffer electrode as a redox mediator. The
2 2
decoupled H and O production in time and space not only avoids the
cost of using an expensive ion exchange membrane but also facilitates
the use of flexible renewable resources to generate hydrogen. It is also
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
A battery electrode of pyrene-4,5,9,10-
tetraone is used as a solid-state proton-
buffer to separate the HER and OER
during acidic water electrolysis in space
and time, providing a flexible and
membrane-free architecture for
Yuanyuan Ma, Zhaowei Guo, Xiaoli Dong,
Yonggang Wang*, Yongyao Xia
Page No. – Page No.
Organic Proton-buffer-electrode to
Separate Hydrogen and Oxygen
Evolution in Acid Water Electrolysis
electrochemical water splitting.
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