Synthesis of Diverse Functionalized Quinoxalines by Oxidative Tandem Dual C?H Amination of Tetrahydroquinoxalines with Amines

Article abstract of DOI:10.1002/chem.201903696

The tandem dual C?H amination of tetrahydroquinoxalines with free amines under aerobic copper catalysis conditions has been demonstrated. The synthetic protocol proceeds with good substrate and functional group compatibility, mild reaction conditions, short reaction time, the use of the naturally abundant [Cu]/O₂ catalyst system, excellent chemoselectivity and synthetic efficiency, and with no need for the pre-installation of specific aminating agents, which offers a practical platform for the rapid and diverse synthesis of diaminoquinoxalines. Moreover, this work has shown the potential of single-electron-oxidation-induced C?H functionalization of N-heterocycles, and its application in the development of optoelectronic materials.

Full text of DOI:10.1002/chem.201903696

NanoEnergy58(2019)183–191
Contents lists available at ScienceDirect
Nano Energy
journal homepage: www.elsevier.com/locate/nanoen
Full paper
Pyro-electrolytic water splitting for hydrogen generation
⁎
Yan Zhanga,b, , Santosh Kumarc, Frank Markend, Marcin Krasnya, Eleanor Roakea,
Salvador Eslavac, Steve Dunne, Enrico Da Comof, Chris R. Bowena
a Department of Mechanical Engineering, University of Bath, Bath BA2 7AY, UK
b State Key Laboratory of Powder Metallurgy, Central South University, Changsha, China
c Department of Chemical Engineering, University of Bath, Bath BA2 7AY, UK
d Department of Chemistry, University of Bath, Bath BA2 7AY, UK
e Chemical Engineering, School of Engineering, London South Bank University, 103 Borough Road, London SE1 0AA, UK
f Department of Physics, University of Bath, Bath BA2 7AY, UK
A R T I C L E I N F O
A B S T R A C T
Keywords:
Water splitting
Pyroelectric
External power source
H2 generation
Water splitting by thermal cycling of a pyroelectric element that acts as an external charge source offers an
alternative method to produce hydrogen from transient low-grade waste heat or natural temperature changes. In
contrast to conventional energy harvesting, where the optimised load resistance is used to maximise the com-
bination of current and voltage, for water splitting applications there is a need to optimise the system to achieve
a sufficiently high potential difference for water electrolysis, whilst also maintaining a high current output. For
the thermal harvesting system examined here, a high impedance 0.5 M KOH electrolyte with working electrodes
connected to a rectified pyroelectric harvester produced the highest voltage of 2.34 V, which was sufficient for
H2 generation. In addition to electrolyte concentration, the frequency of the temperature oscillations was ex-
amined and reducing the heating-cooling frequency led to a larger change in temperature to generate increased
pyroelectric charge and a higher potential difference for pyro-water splitting. Finally, in the absence of sacrificial
reagents, cyclic production of H2 (0.654 μmol/h) was demonstrated for the optimised processing parameters of
electrolyte and thermal cycling frequency using the external pyroelectric element as a charge source for water
splitting.
1. Introduction
employ a wide band semiconductor [2,4,6] that can absorb sunlight
and generate photo-excited carriers to electro-chemically reduce and
Carbon dioxide emission from traditional carbon based sources of
energy such as coal, oil and natural gas has been a significant con-
tributor to global warming and climate change. Therefore, interest in
alternative and renewable fuels for energy sources has increased rapidly
worldwide. In this regard, hydrogen fuel can contribute significantly to
the future energy mix, since it has the attractive features of being a
clean and sustainable energy source with a high energy density and no
greenhouse gas emission when burned in the oxygen. However, over
90% of hydrogen is currently produced from fossil fuels and biomass,
which is high cost and the processing route produces further green-
house gases as a by-product [1]. To address these problems, hydrogen
generation by water electrolysis (or water splitting) using electricity or
solar energy has attracted significant attention [2–5].
oxidize water. However, the range of potential materials is limited [7]
and material lifetime and performance remains a challenge. As an al-
ternative approach, exploiting the pyroelectric effect to produce hy-
drogen from time-temperature variations has emerged recently as po-
tential approach to generate hydrogen from transient low-grade waste
heat (< 100 °C) or natural temperature changes [8–13]. The thermo-
electric effect can also be considered for spatial temperature gradients
[14] and a combination of sunlight and wind have also been considered
to generate temperature oscillations in a pyroelectric system [15]. A
pyroelectric material is highly polarised, and when the temperature of
the material is increased the polarisation level falls, which releases
electrical charge on its surface [16]. On cooling the polarisation in-
creases, reversing the electric current flow as charge is attracted to the
more polarised surface.
A common approach to generate hydrogen by water splitting is to
⁎ Corresponding author.
E-mail addresses: y.zhang2@bath.ac.uk, yanzhangmaterial@hotmail.com (Y. Zhang), sk2352@bath.ac.uk (S. Kumar), fm202@bath.ac.uk (F. Marken),
mjk52@bath.ac.uk (M. Krasny), eer25@bath.ac.uk (E. Roake), sef39@bath.ac.uk (S. Eslava), dunns4@lsbu.ac.uk (S. Dunn), edc25@bath.ac.uk (E. Da Como),
msscrb@bath.ac.uk (C.R. Bowen).
https://doi.org/10.1016/j.nanoen.2019.01.030
Received 1 October 2018; Received in revised form 5 January 2019; Accepted 8 January 2019
Availableonline14January2019
2211-2855/©2019ElsevierLtd.Allrightsreserved.

Y. Zhang et al.
NanoEnergy58(2019)183–191
If we consider a water splitting application, the amount of available
charge (Q) generated by a temperature change (ΔT) of a pyroelectric is
given by;
separated from different electrode sides, which is beneficial for high
hydrogen production and simplifying hydrogen and oxygen separation.
Moreover, the external pyro-charge provider can be easily collected
compared with the internal powder form.
Q = p·A·ΔT
(1)
Preliminary work has been undertaken on the potential of pyro-
electric materials and geometries for externally pyroelectric water
electrolysis, which demonstrated that thin layers of lead zirconate ti-
tanate (PZT) are promising for H2 generation [30]. This work utilizes
PZT as an external charge source that undergoes hot-cold thermal cy-
cles for pyro-electric water splitting; the work has a focus on the opti-
misation of system parameters, such as electrolyte impedance and
heating-cooling cyclic frequency and its effect on the current, voltage
and generated power were systematically examined. In addition, the
resulting H2 production is detected, quantified and corresponding ef-
ficiency also evaluated. Finally, the resulting H2 generation from the
optimised conditions were explored and measured. This is the first work
to understand and systematically examine the processing parameters
(frequency and electrolyte impedance) for external H2 generation based
on the pyroelectric effect and to measure and quantify the amount of
hydrogen and oxygen generated.
where p is the pyroelectric coefficient (C m−2 K−1) and A is the surface
area (m2) of the material. While the amount of charge generated is
related to the area of the pyroelectric, the potential (V) developed
across a pyroelectric is dependent on its thickness (h), and is given by;
p·h·ΔT
V =
ε0ε33
(2)
where ε33 is the relative permittivity of the material and ε0 is the per-
mittivity of free space. Since Q is proportional to the surface area (Eq.
(1)) and V is proportional to thickness (Eq. (2)), there is potential to
design the optimum geometry of the pyroelectric elements for electro-
lysis; where the thickness can be used to ensure a sufficient potential is
produced to initiate water splitting and the area should be maximized
for harvesting the greatest amount of available surface charge.
An overview of coupling energy harvesting devices to electro-che-
mical systems has been recently provided by Zhang et al. [17]. To date,
pyroelectric energy harvesting using thermal fluctuations and/or tran-
sient waste heat has been utilised for water related electro-chemical
reactions, in terms of water treatment [9,10,18–28]; however for water
splitting applications only a small amount of work has been undertaken
[8,11,12,29,30]. The first experimental evidence of pyroelectric water
splitting was reported by the comparison of bulk lead zirconate titanate
(PZT) and polymer ferroelectrics as an external charge source [30]. The
materials were thermally cycled at a fixed frequency and electrolyte,
and direct measurements of hydrogen and oxygen generation were not
reported. Belitz et al. recently explored pyroelectric water splitting by
placing a crushed and polarised BaTiO3 single crystal powder into di-
rect contact with water, and thermal cycling the mixture from 40 to
70 °C [12]. The advantage of this Internally Positioned Pyroelectric
(IPP) approach is that using finely dispersed pyroelectric particulates
suspended in the electrolyte enables the area of the pyroelectric to be
increased, and therefore the available charge for hydrogen production;
see Eq. (1). In terms of modeling a pyroelectric induced water splitting
process, Kakekhani et al. developed a density functional theory (DFT)
model of a ferroelectric lead titanate (PbTiO3) material and examined
the impact of thermal cycling of the ferroelectric as it is heated and
cooled above below it Curie temperature (Tc) in the presence of water
molecules [8]. The work showed that cycling between the low tem-
perature ferroelectric state and high temperature paraelectric state
provides scope to harvest thermal fluctuations and produce hydrogen.
Xu et al. recently presented experimental data of Ba0.7Sr0.3TiO3 pow-
ders suspended in an electrolyte, achieving a hydrogen production of
46.89 mol per gram of the powder after 36 thermal cycles above and
below its Curie temperature [11]. Pyroelectric two-dimensional black
phosphorene has also been reported as a charge source under thermal
cycling for both hydrogen generation and dye decomposition [10]. The
charge generated by pyroelectrics subjected to hot-cold cycles has also
been used for dye decomposition using BaTiO3 particles [9] and
NaNbO3 nanofibers [20].
2. Experimental section
2.1. Material and heat source
For the pyro-electrolysis water splitting, a commercial dense and
thin PZT sheet (PSI-5H4E, Piezo system, inc. USA) with a thickness of
127 µm and surface area of 49 cm2 was utilised as the external pyro-
electric charge source that is subjected to thermal cycles outside of the
electrolyte [30]. The surface of both sides of the PZT sheet were cov-
ered by the vacuum sputtered nickel electrode. For the electrolyte, KOH
solutions with a variety of concentrations of 0.5, 1, 2, 3 M were em-
ployed for H2 generation to examine the influence of electrolyte im-
pedance on output voltage and current (4 M KOH was also prepared for
the impedance analysis). During the pyroelectric initiated water split-
ting reaction, the PZT sheet was heated by irradiating with an infrared
heat lamp whose maximum light intensity of ~370 mW/cm2, which
was placed at a fixed distance of 13 cm from the PZT surface. Varying
heating cycles at frequencies of 0.05, 0.1, 0.2 and 0.3 Hz were used to
examine the impact of frequency on generated voltage and current. At
the same time a thermoelectric Peltier cooling system was also supplied
to cool down the PZT sheet in a periodic fashion. A Type K thermo-
couple was used to monitor the temperature of the surface of the
pyroelectric element. This combination of heating and cooling was
chosen as it provided continuous and controlled periodic heating and
cooling cycles to achieve a pyroelectric response for the PZT sheet,
while in a harvesting application the material may be subjected to a
range of waste heat or natural thermal transients. Alternative ap-
proaches can be employed to provide a thermal fluctuation, for example
a wind powered fan and light chopper has been coupled to a pyro-
electric energy harvesting system, where heat from the sun was peri-
odically applied to thermally cycle a pyroelectric element [15].
2.2. Electrical characterisation and H2 detection
However, potential challenges for pyroelectric water splitting using
a material in powder form that is suspended in the electrolyte for
practical large-scale applications is the need to collect the dispersed
powder and separate the hydrogen and oxygen gas when formed,
leading to low efficiency of the water splitting process. In addition, due
to the hydrolysis of the ceramic powder, pH changes during the process
can affect kinetic behavior during water splitting. In contrast, the use of
an externally positioned pyro-electrolysis (EPP) is accompanied by the
flow of a rectified electric current through an external circuit in the
water splitting system. Compared to water splitting using suspensions
in an electrolyte, the EPP system has the advantage that there is no need
for gas separation because the generation of H2 and O2 is spatially
The polarisation-field hysteresis loop of the PZT sheet was measured
by a Radiant RT66B-HVi ferroelectric test system to confirm its ferro-
electric response. The impedance of the KOH electrolytes from 1 Hz to
1 MHz was measured by an impedance analyzer (Solartron 1260,
Hampshire, UK) at room temperature. The current levels produced by
different concentrations of the electrolytes under applied DC voltages
from −4 V to 4 V were performed with a commercial potentiostat
(Compact Stat, Ivium Technologies) to characterise the cell used for
water splitting. The system consisted of a counter electrode, working
electrode, the I/E converter, the control amplifier, and the signal.
During thermal cycling of the pyroelectric element for water
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Fig. 1. (A) Schematic of pyroelectric as an external source for water splitting, (B) photograph of the PZT film, (C) polarisation-electric field loop of PZT sheet and
schematic of the surface pyroelectric charges.
splitting an ac-dc converter composed of a rectifier bridge circuit was
used to provide a unipolar output so that hydrogen and oxygen were
produced at the cathode and anode respectively. An electrometer
(Model 6517B, Keithley Instruments, Cleveland, OH) was used to
measure the voltage and current in the circuit accurately. Before ap-
plying the thermal fluctuation, ultra-pure N2 gas was introduced to
purge the electrolysis cell by bubbling through the electrolyte for 0.5 h
to remove residual air. The produced hydrogen and oxygen gases were
evolved and analysed by the gas chromatography (GC) system (GC,
Vairan3800), using ultra-pure Ar (99.9995 vol%) as carrier gas and a
thermal conductivity detector (TCD).
to act as the DC input to the electrolysis cell where electrons move to
the cathode in the cell. The H+ in the water is then reduced to produce
H
2 on the cathode surface, where O2 is simultaneously produced at the
anode. The change of the polarisation (dP/dt) of the ferroelectric during
thermal cycling with time is the driving force for the pyroelectric
charge generation during hot-cold fluctuations. Fig. 1(B) shows the PZT
sheet used in this experiment whose remnant polarisation, saturation
polarisation and coercive field are ~20 μC/cm2, 28 μC/cm2 and 26 kV/
cm observed from the hysteresis loop shown in Fig. 1(C). This high
polarisation is the primary reason for the excellent piezo-, pyro- and
ferro-electric properties of PZT compared with ferroelectric polymers
and lead-free materials [31]. This is a widely-used piezo-material,
which is a Type 5 H (Navy Type VI) piezoelectric ceramic, with typical
pyroelectric coefficients of ~450 μC m−2 K−1 [32]. In this work, under
cyclic hot-cold fluctuations at a frequency of 0.1 Hz, the change of the
temperature ΔT on the surface of the PZT sheet was ~3 °C, with a
corresponding rate of change of temperature with time, dT/dt ~
0.16 °C/s. The generated bipolar voltage and current outputs are shown
in Fig. S1(C) and (E). After rectification, the resultant unipolar peak-
peak voltage and current were 8.36 V and 11.25 μA, while the peak
voltage and current were 9.58 V and 13.21 μA, respectively, as shown
in Fig. S1(D) and (F). Moreover, a range of PZT sheets with a constant
thickness of 127 µm but different surface areas (A) were utilised to
3. Results and discussion
3.1. Basic mechanism involved in pyroelectric water splitting
Fig. 1(A) shows a schematic of the setup utilised to carry out the
water splitting reaction with the pyroelectric material externally posi-
tioned. The mechanism consists of three main steps where, firstly,
electrical charge with negative and positive signs are separately gen-
erated on the surface of the pyroelectric element that is subjected to
thermal fluctuations (inset of Fig. 1(A)). The AC electrical output from
the pyroelectric generator is then rectified through an external circuit
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explore the current in the electrolyte and the voltage between cathode
and anode produced from PZT sheet, shown in Fig. S2. With the in-
crease of the surface area, the voltage was unchanged at ~2.3 V, since
the thickness is unchanged (see Eq. (2)). However, the peak current
increased almost linearly from 1.75 µA to 14.21 µA on increasing the
surface area from 12.25 cm2 to 98 cm2, due to the linear relationship
between the pyroelectric current (I) and surface area (A) of I=p·A·dT/dt
[33] at a specific rate of the temperature change dT/dt.
of a series RC circuit where the pyroelectric element can be regarded as
the power source and the capacitance (C), while the electrolyte acts as
the external resistance load (R). The voltage created by the pyroelectric
generator in response to a temperature change can be divided into two
parts, where one part is used for triggering the electrolysis process for
H2 and the other is acting as a normal resistor in the circuit. The critical
voltage for the oxidation potential of H2O to O2 is ~1.23 V, where an
overpotential is required to overcome the kinetic barrier for the hy-
drogen evolution reaction [34]. When subjected to the thermal ex-
citation, the speed of the thermal cycling (frequency) will influence the
When an externally positioned pyro-electrolysis process is utilised
for H2 generation, the whole system can be considered as an analogue
Fig. 2. (A) Current-voltage characteristics with different concentrations of KOH, (B) a zoom-in current-voltage curve of (A), impedance of the KOH solutions (C) with
the frequency sweeping from 0.1 Hz to1 MHz and (D) fixed at 0.1 Hz, (E) peak voltage and current measured from different concentrations of KOH produced by the
pyroelectric PZT sheet, (F) the corresponding peak power and peak power density calculated based on (E).
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Fig. 3. Effect of different concentrations on the rectified current (A) 0.5 M, (C) 1.0 M, (E) 2.0 M, (G) 3.0 M, and voltage (B) 0.5 M, (D) 1.0 M, (F) 2.0 M, (H) 3.0 M of
the KOH electrolyte, with periodic heating and cooling at a frequency of 0.1 Hz.
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Fig. 4. Change of temperature (ΔT) with different heating-cooling frequencies in terms of (A) 0.05 Hz, (B) 0.1 Hz, (C) 0.2 Hz, and (D) 0.3 Hz, and (E) the relationship
between ΔT and frequency, (F) produced peak current and voltage.
total temperature change (ΔT) of the pyroelectric material that leads to
a change of polarisation, since dP = p·ΔT. In this case, the higher the
change in polarisation, the larger the driving force to generate a po-
tential difference (see Eq. (2)). Moreover, a larger ΔT is advantageous to
achieve more charge based on the Eq. (1). Therefore, in this EPP system
for H2 generation, there is a balance between the high voltage necessary
to split the water molecule (a high ΔT) and a high current (high dT/dt
and more total charge) for hydrogen ion (H+) reduction. This can be
adjusted by changing either the concentration of the electrolyte or the
frequency of the hot-cold fluctuation, which will be discussed later in
detail.
Fig. 2(A) shows the current-voltage (I-V) curves of different con-
centrations of KOH electrolytes, without a pyroelectric element or
rectifier attached to the cell. A plateau exists at all the ranges of KOH
concentrations with a potential difference of approximately −1.2 V to
1.2 V where there is no current flow between the electrodes. When the
absolute value of voltage was higher than 1.2 V, the current increased
where small potential changes lead to large changes in current and
bubble formation was observed at each electrode. A rapid increase in
current is especially observed when the potential was higher than 2.2 V,
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as shown in Fig. 2(B), demonstrating the electrolysis of water. Clearly,
any pyroelectric generator must also generate such potentials (> 2.0 V)
to achieve water splitting. Fig. 2(C) shows the frequency dependence of
electrical impedance of different concentrations of KOH electrolytes
(0.5, 1, 2, 3 M). The impedance of the KOH solutions for all con-
centrations decreased with an increase of frequency, due to con-
centration polarisation at low frequencies. As expected, the electrolyte
impedance decreased with increasing KOH concentration at a particular
frequency; since the thermal cycles to be employed later in this paper
are typically ~0.1 Hz the gradual reduction in impedance with in-
creasing KOH concentration is shown in Fig. 2(D). When the pyro-
electric generator is coupled to the electrochemical cell and subjected
to thermal cycling, the resulting peak voltage and current obtained
during periodic heating are summarised in Fig. 2(E). Examples of ty-
pical generated voltage and current profiles with time obtained for the
different concentrations of KOH are shown in Fig. 3. Fig. 3(B), (D), (F),
(H) show that when there is a decrease in KOH concentration, the peak
voltage increases since it is more difficult for the pyroelectric element to
discharge due to the higher impedance of the electrolyte. This also
resulted in the current decreasing with decreasing KOH concentration;
see Fig. 3(A), (C), (E), (G). In theory, when the energetic requirements
are met, namely the critical potential of ~1.23 V, the water splitting
reaction will occur. However, the practical potential will be higher due
to the existence of overpotential and/or other system losses [35]. Based
on the observation from the I-V curves presented in Fig. 2(A) and (B), a
voltage higher than 2.2 V was beneficial to achieve water splitting, and
a voltage of 2.34 V was generated by the pyroelectric element coupled
to the higher impedance 0.5 M KOH solution.
In conventional energy harvesting systems coupled to an electrical
load, such as a simple resistor, it is common to calculate the peak power
and match the electrical load (RL) to the capacitive impedance of the
energy harvester; for example by achieving the condition 2πfCp= 1/RL
where Cp is the capacitance of the pyroelectric element and f is the
frequency. The peak power (Pout) of the pyroelectric element was cal-
culated with the standard power relationships (Pout = VI) of voltage
and current, shown in Fig. 2(E), where the resistive load is the elec-
trolyte resistance. It can be seen in Fig. 2(F) that as the KOH con-
centration increased from 0.5 M to 3 M, the peak power and peak power
density increased from 16.9 μW (27.2 μW/cm3) to 19.2 μW (30.8 μW/
cm3) followed by a decrease to 16.9 μW (27.2 μW/cm3) at the highest
concentration 4 M KOH; where a heating-cooling frequency of 0.1 Hz
was utilised. In an R-C circuit, by matching the load and generator
impedances at 2πfCp= 1/RL the power generated reaches a maximum
value since this provide a good combination of voltage and current.
Based on a frequency of 0.1 Hz and a capacitance of 650 nF for the
pyroelectric element the optimum resistance was calculated to be ap-
proximately 2.5 MΩ, which is comparable to that of the 3 M KOH
electrolyte (see Fig. 2(D)). However, for a harvester coupled to an
electrochemical system, optimisation of the system is different to a
simple electrical load. While the peak power was obtained at 3 M KOH
(Fig. 2(F)) with a rectified peak voltage of 1.58 V (Fig. 2(E)) and high
current of ~12 μA (Fig. 2(B)), the need for a sufficiently high voltage
Fig. 5. (A) H2 evolution from external pyroelectric water splitting with sampling time ranging from 1 to 6 h, (B) the amount and evolution rate of H2 and O2 after 6 h
detected from the gas chromatography and (C) comparison of the theoretical results and experimental data on the H2 production generated with the 0.5 M of the KOH
electrolyte and 0.1 Hz of the cyclic thermal fluctuation.
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plays a more important role for water splitting (Fig. 2(A)). Therefore,
the 0.5 M KOH with the ability to generate the highest voltage (2.34 V)
and micro-level current (~7 μA) from the PZT during thermal cycling
was chosen as the electrolyte for periodic heating-cooling frequency
exploration. This also correlated with visual observation of hydrogen
bubbles generated at the electrodes within the cell during thermal cy-
cling.
temperature. As a result, in this work we have selected a relatively low
frequency of 0.1 Hz since it is the highest frequency that is able to
achieve a change in temperature which is sufficient to produce a vol-
tage > 2 V for water splitting. Lower frequencies e.g. 0.05 Hz (see
Fig. 4) could achieve only slightly higher temperature changes (and
therefore charge per cycle) but at this low frequencies the total charge
over time is lower. An example of real-time water splitting process
during thermal cycling is shown in the video (Video. S1). As can be seen
from Fig. 5(A), the degree of H2 evolution increased from 0.38 μmol to
3.93 μmol with increasing build-up time (1–6 h). The H2 evolution
production increased slowly from 1 to 3 h with the slope between the
H2 evolution and time of 0.38 µmol/h, with an increase of the H2
production with a slope of 1.06 µmol/h was achieved at longer times
from 4 to 6 h. The change in production rate is thought to result from a
surface conditioning phenomenon where the physical roughness, ad-
sorbed impurities and surface oxides on the surface of the electrode are
changing with time, which is commonly observed for plantinum elc-
trodes [37]. The production of continuous bubbles of hydrogen and
oxygen during each thermal cycle from the electrodes and the corre-
sponding amount of H2 and O2 and evolution rate after 6 h is presented
in Fig. 5(B); the ratio of H2 and O2 is approximately 2:1, as would be
expected for the reaction 2H2O → 2H2 + O2. In this system, the
pyroelectric induced water-splitting produced both hydrogen and
oxygen simultaneously on separate electrodes, without the consump-
tion of the sacrificial reagents or hole scavengers [10,11]. According to
Fig. 4 shows the change of the temperature (ΔT) on the PZT surface
and the resultant peak voltage and current in the 0.5 M KOH electrolyte
with periodic heating-cooling frequencies ranging from 0.05 to 0.3 Hz.
As can be seen from Fig. 4(A)–(D), the change of temperature decreased
with an increase in frequency. It is noted that the ΔT obtained at 0.1 Hz
(2.9 °C) is only slightly smaller compared to that at 0.05 Hz (3 °C), while
a larger decrease in ΔT was observed at 0.2 Hz (2.1 °C) and 0.3 Hz
(1.3 °C), shown in Fig. 4(E). At a low frequency, the pyroelectric ele-
ment was exposed to the heat source for a longer period of time, al-
lowing greater heat transfer through its thickness, which leads to a
more homogeneous temperature distribution. From the Eqs. (1) and (2),
the larger the temperature change (ΔT) of the pyroelectric, the greater
the generated charge (Q) and potential difference (V) produced, leading
to the more electrons to reduce the H+ in the electrolyte and higher
potential as the driving force to split water. This was in agreement with
the obtained peak voltage and current in the 0.5 M KOH electrolyte,
shown in Fig. 4(F). A small decrease of the peak voltage (2.35 V/
2.34 V) and current (7.27 μA/7.23 μA) was found on increasing the
frequency from 0.05 Hz to 0.1 Hz, while a sharp drop was observed for
the 0.5 M KOH electrolyte on increasing the frequency from 0.2 Hz to
0.3 Hz, e.g. a fall to 1.11 V and 2.21 μA when an oscillation frequency of
0.3 Hz applied. The voltage/current generation profiles obtained via a
range of periodic heating-cooling frequencies are shown in Fig. S3 and
the corresponding power and power density are shown in Fig. S4. Un-
derstanding the frequency dependence of change in polarisation would
be of interest for further studies for this application, for example by
laser intensity modulation method (LIMM) which has been used to
characterise pyroelectric materials [36]. In this work we have selected a
relatively low frequency of 0.1 Hz for the final detection of H2 an
quantification since it is the highest frequency that is able to achieve a
change in temperature which is sufficient to produce a voltage > 2 V
for water splitting. There is also a small difference of the generated
voltage and current in the electrolyte between the frequencies of 0.05
and 0.1 Hz, see Fig. 4(F).
Faraday's law, m = QM , where m is the mass of the substance altered at
Fz
an electrode which is H2 in this work, Q is the total electric charge
passed through the substance, F = 96.485 C mol−1 is the Faraday
constant, M is the molar mass of the substance, and z is the valance
number of ions of the substance, the theoretical mole (m/M) of gen-
erated H2 can be predicted based on the charge generated from the
pyroelectric PZT sheet during thermal fluctuations, and a comparison
with the experimental measured data is shown in Fig. 5(C). The Faradic
efficiency of pyro-electrolytic water splitting was calculated as ~10%,
which was enhanced to ~15% at six hours due to surface conditioning,
shown in Fig. 5(A). Table 1 summarizes the main parameters and final
H2 production between the internal (suspended pyroelectric particles in
the electrolyte) and the external pyroelectric water splitting of this
work. In this work, thermal fluctuations were conducted well below the
Curie temperature of 350 °C to avoid the loss of remnant polarisation
and to generate a continuous voltage and current, e.g. as in Fig. 3.
Despite a bulk material being used, with a total PZT surface area much
smaller than nano-powders [38], and a relatively small change of
temperature (ΔT ~3 °C; see Table 1), pyroelectric induced water split-
ting using an externally source had comparable [10] or higher [11] H2
generation performance than using the internal positioned route. Since
the charge Q and potential V for pyro-water splitting are directly pro-
portional to the area and thickness (Eqs. (1) and (2)), future work could
be focused on pyroelectric sheet with high pyroelectric response, high
thermal conductivity or high heat transfer configurations, reduced
permittivity and low Curie temperature for the external pyro-electro-
lytic water splitting.
Fig. 5 shows the production of oxygen and hydrogen from the
pyroelectric induced electro-chemical reaction using
a KOH con-
centration of 0.5 M, with periodic heating and cooling at a frequency of
0.1 Hz. In terms of the charge Q, the higher the temperature the more
charge that will be produced (see Eq. (1)). However, the total charge for
a given time is also related to the frequency of the temperature fluc-
tuations and the output voltage. Therefore, when thermal cycling via a
lamp, there is a balance between the frequency of thermal cycles, the
achievable temperature change and developing a sufficient potential
difference for water splitting (2.34 V). For example, see Fig. 4 where
increasing the frequency leads to
a reduction in the achievable
Table 1
Comparison of the internal and external positioned pyroelectric for water splitting.
Material
Dimension
ΔT (K) Time
Sacrificial reagent H2 evolution rate
Ref.
BaTiO3 single crystals
Nano Ba0.7Sr0.3TiO3 powders
< 100 µm
30
25
Few cycles (2 min/
cycle)
6 h
No
300 vol.-ppb (part per
billion)
[12]
10 mg, 200 nm, cubed shape
Yes
No
Yes
0.075 μmol/ha
1.25 × 10−3 μmol/ha
22.5 μmol/g
[11]
Nano 2D black phosphorene powders 1 mg, thickness of 0.35 nm, surface area 50
of 9 cm2
PZT sheet
1 thermal cycle
6 h
[10]
4.85 g, thickness of 170 µm, surface
3
No
0.654 μmol/h
This work
area of 49 cm2
a
Calculated data based on relative parameter reported in the reference.
190

Y. Zhang et al.
NanoEnergy58(2019)183–191
Supplementary material related to this article can be found online at
doi:10.1016/j.nanoen.2019.01.030.
[10] H. You, Y. Jia, Z. Wu, F. Wang, H. Huang, Y. Wang, Room-temperature pyro-cat-
alytic hydrogen generation of 2D few-layer black phosphorene under cold-hot al-
ternation, Nat. Commun. 9 (1) (2018) 2889.
[11] X. Xu, L. Xiao, Y. Jia, Z. Wu, F. Wang, Y. Wang, N.O. Haugen, H. Huang, Pyro-
catalytic hydrogen evolution by Ba0.7Sr0.3TiO3 nanoparticles: harvesting cold–hot
alternation energy near room-temperature, Energy Environ. Sci. (2018).
[12] R. Belitz, P. Meisner, M. Coeler, U. Wunderwald, J. Friedrich, J. Zosel, M. Schelter,
S. Jachalke, E. Mehner, Waste heat energy harvesting by use of BaTiO3 for pyro-
electric hydrogen generation, Energy Harvest. Syst. (2017) 107.
[13] A.L. Cottrill, A.T. Liu, Y. Kunai, V.B. Koman, A. Kaplan, S.G. Mahajan, P. Liu,
A.R. Toland, M.S. Strano, Ultra-high thermal effusivity materials for resonant am-
bient thermal energy harvesting, Nat. Commun. 9 (1) (2018) 664.
[14] S.W. Lee, Y. Yang, H.-W. Lee, H. Ghasemi, D. Kraemer, G. Chen, Y. Cui, An elec-
trochemical system for efficiently harvesting low-grade heat energy, Nat. Commun.
5 (2014) 3942.
[15] S.H. Krishnan, D. Ezhilarasi, G. Uma, M. Umapathy, Pyroelectric-based solar and
wind energy harvesting system, IEEE Trans. Sustain. Energy 5 (1) (2014) 73–81.
[16] C.R. Bowen, J. Taylor, E. LeBoulbar, D. Zabek, A. Chauhan, R. Vaish, Pyroelectric
materials and devices for energy harvesting applications, Energy Environ. Sci. 7
(12) (2014) 3836–3856.
[17] Y. Zhang, M. Xie, V. Adamaki, H. Khanbareh, C.R. Bowen, Control of electro-che-
mical processes using energy harvesting materials and devices, Chem. Soc. Rev. 46
(24) (2017) 7757–7786.
[18] J. Wu, N. Qin, B. Yuan, E. Lin, D. Bao, Enhanced pyroelectric catalysis of BaTiO3
nanowires for utilizing waste heat in pollution treatment, ACS Appl. Mater.
Interfaces 10 (44) (2018) 37963–37973.
[19] X. Xu, D. Chen, Z. Yi, M. Jiang, L. Wang, Z. Zhou, X. Fan, Y. Wang, D. Hui,
Antimicrobial mechanism based on H2O2 generation at oxygen vacancies in ZnO
crystals, Langmuir 29 (18) (2013) 5573–5580.
[20] H. You, X. Ma, Z. Wu, L. Fei, X. Chen, J. Yang, Y. Liu, Y. Jia, H. Li, F. Wang,
H. Huang, Piezoelectrically/pyroelectrically-driven vibration/cold-hot energy har-
vesting for mechano-/pyro- bi-catalytic dye decomposition of NaNbO3 nanofibers,
Nano Energy 52 (2018) 351–359.
[21] T. Zhao, W. Jiang, D. Niu, H. Liu, B. Chen, Y. Shi, L. Yin, B. Lu, Flexible pyroelectric
device for scavenging thermal energy from chemical process and as self-powered
temperature monitor, Appl. Energy 195 (2017) 754–760.
[22] W. Qian, Z. Wu, Y. Jia, Y. Hong, X. Xu, H. You, Y. Zheng, Y. Xia, Thermo-electro-
chemical coupling for room temperature thermocatalysis in pyroelectric ZnO na-
norods, Electrochem. Commun. 81 (2017) 124–127.
[23] J. Wu, W. Mao, Z. Wu, X. Xu, H. You, A.X. Xue, Y. Jia, Strong pyro-catalysis of
pyroelectric BiFeO3 nanoparticles under a room-temperature cold-hot alternation,
Nanoscale 8 (13) (2016) 7343–7350.
[24] H. Zhang, S. Zhang, G. Yao, Z. Huang, Y. Xie, Y. Su, W. Yang, C. Zheng, Y. Lin,
Simultaneously harvesting thermal and mechanical energies based on flexible hy-
brid nanogenerator for self-powered cathodic protection, ACS Appl. Mater.
Interfaces 7 (51) (2015) 28142–28147.
[25] A. Benke, E. Mehner, M. Rosenkranz, E. Dmitrieva, T. Leisegang, H. Stöcker,
W. Pompe, D.C. Meyer, Pyroelectrically driven •OH generation by barium titanate
and palladium nanoparticles, J. Phys. Chem. C 119 (32) (2015) 18278–18286.
[26] Y. Yang, H. Zhang, S. Lee, D. Kim, W. Hwang, Z.L. Wang, Hybrid energy cell for
degradation of methyl orange by self-powered electrocatalytic oxidation, Nano Lett.
13 (2) (2013) 803–808.
4. Conclusions
Hydrogen production via pyro-electrolytic water splitting is a pro-
mising green process where fluctuating thermal energy, such as tran-
sient low-grade waste heat and natural temperature changes, can be
converted into chemical energy. In this study, the effects of the elec-
trolyte concentration and heating-cooling frequency were investigated
to explore the pyroelectric H2 generation performance. The low im-
pedance KOH electrolyte with a concentration of 0.5 M had the max-
imum voltage (2.34 V) and micro-level current (~7 μA) among all the
concentrations explored for a PZT element with an area of 49 cm2; the
differences in optimisation of the electrochemical system compared to
conventional electrical systems are discussed. With an increase of the
heating-cooling frequency from 0.05 to 0.3 Hz, the change of the sur-
face temperature of the pyroelectric decreased from 2.9 to 1.3 °C.
Without the aid of the sacrificial reagents, the pyro-electrolytic water
splitting with the pyroelectric externally positioned configuration was
able to produce continuous H2 gas, which was detected as 3.93 μmol in
6 h. Future work can focus on the formation of pyroelectric nano-
structures to increase the surface area of the pyroelectric element,
which is proportional to the generated charge or employing ferro-
electric geometries with high heat transfer rates to increase both the
magnitude and rate of temperature change.
Acknowledgement
Dr. Y. Zhang, Mr M. Krasny and Prof. C. R. Bowen would like to
acknowledge the funding from the European Research Council, United
Kingdom under the European Union's Seventh Framework Programme
(FP/2007–2013)/ERC Grant Agreement No. 320963 on Novel Energy
Materials, Engineering Science and Integrated Systems (NEMESIS). The
Leverhulme Trust, United Kingdom (project No. RPG-2018-290) is ac-
knowledged by Prof. Bowen. Dr Y. Zhang also would like to acknowl-
edge the project (Grant No. 621011812) supported by State Key
Laboratory of Powder Metallurgy, Central South University, China
Changsha, China.
[27] E. Gutmann, A. Benke, K. Gerth, H. Böttcher, E. Mehner, C. Klein, U. Krause-
Buchholz, U. Bergmann, W. Pompe, D.C. Meyer, Pyroelectrocatalytic disinfection
using the pyroelectric effect of nano- and microcrystalline LiNbO3 and LiTaO3
particles, J. Phys. Chem. C 116 (9) (2012) 5383–5393.
Appendix A. Supporting information
Supplementary data associated with this article can be found in the
online version at doi:10.1016/j.nanoen.2019.01.030.
[28] T.F. Wiesner, Pyroelectrically-mediated Reduction-oxidation Reactions, Texas Tech
University, 2010.
[29] A. Kakekhani, S. Ismail-Beigi, E.I. Altman, Ferroelectrics: a pathway to switchable
surface chemistry and catalysis, Surf. Sci. 650 (2016) 302–316.
[30] M. Xie, S. Dunn, E.L. Boulbar, C.R. Bowen, Pyroelectric energy harvesting for water
splitting, Int. J. Hydrog. Energy 42 (37) (2017) 23437–23445.
[31] J. Briscoe, S. Dunn, Piezoelectric nanogenerators – a review of nanostructured
piezoelectric energy harvesters, Nano Energy 14 (2015) 15–29.
References
[1] R.M. Navarro, M.A. Peña, J.L.G. Fierro, Hydrogen production reactions from carbon
Feedstocks: fossil fuels and biomass, Chem. Rev. 107 (10) (2007) 3952–3991.
[2] M.B. Starr, X. Wang, Coupling of piezoelectric effect with electrochemical pro-
cesses, Nano Energy 14 (2015) 296–311.
[3] S.A. Nahian, R.K. Cheedarala, K.K. Ahn, A study of sustainable green current gen-
erated by the fluid-based triboelectric nanogenerator (FluTENG) with a comparison
of contact and sliding mode, Nano Energy 38 (Suppl. C) (2017) S447–S456.
[4] X. Yu, X. Han, Z. Zhao, J. Zhang, W. Guo, C. Pan, A. Li, H. Liu, Z. Lin Wang,
Hierarchical TiO2 nanowire/graphite fiber photoelectrocatalysis setup powered by
a wind-driven nanogenerator: a highly efficient photoelectrocatalytic device en-
tirely based on renewable energy, Nano Energy 11 (2015) 19–27.
[5] W. Tang, Y. Han, C.B. Han, C.Z. Gao, X. Cao, Z.L. Wang, Self-powered water
splitting using flowing kinetic energy, Adv. Mater. 27 (2) (2015) 272–276.
[6] Pena Pereira, Francisco, Miniaturization in Sample Preparation, De Gruyter, 2014.
[7] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor
electrode, Nature 238 (1972) 37.
[32] 'H.H.S. Chang, R.W. Whatmore, Z. Huang, Pyroelectric effect enhancement in la-
minate composites under short circuit condition, J. Appl. Phys. 106 (2009) 114110.
[33] C.R. Bowen, J. Taylor, E. LeBoulbar, D. Zabek, A. Chauhan, R. Vaish, Pyroelectric
materials and devices for energy harvesting applications, Energy Environ. Sci. 7
(12) (2014) 3836–3856.
[34] T. Shinagawa, K. Takanabe, Towards versatile and sustainable hydrogen production
through electrocatalytic water splitting: electrolyte engineering, ChemSusChem 10
(7) (2017) 1318–1336.
[35] L.J. Minggu, W.R. Wan Daud, M.B. Kassim, An overview of photocells and photo-
reactors for photoelectrochemical water splitting, Int. J. Hydrog. Energy 35 (11)
(2010) 5233–5244.
[36] S.B. Lang, Laser intensity modulation method (LIMM): experimental techniques,
theory and solution of the integral equation, Ferroelectrics 118 (1) (1991) 343–361.
[37] A.J. Bard, Electroanalytical Chemistry: A Series of Advances, 1st ed., CRC Press,
1988.
[8] A. Kakekhani, S. Ismail-Beigi, Ferroelectric oxide surface chemistry: water splitting
via pyroelectricity, J. Mater. Chem. A 4 (14) (2016) 5235–5246.
[38] W. Wohlleben, J. Mielke, A. Bianchin, A. Ghanem, H. Freiberger, H. Rauscher,
M. Gemeinert, V.-D. Hodoroaba, Reliable nanomaterial classification of powders
using the volume-specific surface area method, J. Nanopart. Res. 19 (2) (2017) 61.
[9] Y. Xia, Y. Jia, W. Qian, X. Xu, Z. Wu, Z. Han, Y. Hong, H. You, M. Ismail, G. Bai,
L. Wang, Pyroelectrically induced pyro-electro-chemical catalytic activity of batio3
nanofibers under room-temperature cold–hot cycle excitations, Metals 7 (4) (2017)
122.
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