Electrochemical Splitting of Methane in Molten Salts To Produce Hydrogen

Article abstract of DOI:10.1002/anie.202017243

Industrial hydrogen production based on methane steam reforming (MSR) remains challenges in intensive carbon emissions, retarded hydrogen generation owing to coke deposition over catalysts and huge consumption of water. We herein report an electrochemical splitting of methane (ESM) in molten salts at 500 °C to produce hydrogen in an energy saving, emission-free and water-free manner. Following the most energy-saving route on methane-to-hydrogen conversion, methane is electrochemically oxidized at anode to generate carbon dioxide and hydrogen. The generated anodic carbon dioxide is in situ captured by the melts and reduced to solid carbon at cathode, enabling a spatial separation of anodic hydrogen generation from cathodic carbon deposition. Life-cycle assessment on hydrogen-generation technologies shows that the ESM experiences an equivalent carbon emission much lower than MSR, and a lower equivalent energy input than alkaline water electrolysis.

Full text of DOI:10.1002/anie.202017243

Angewandte
Communications
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How to cite: Angew. Chem. Int. Ed. 2021, 60, 7664–7668
International Edition:
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doi.org/10.1002/anie.202017243
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Hydrogen Production Very Important Paper
Electrochemical Splitting of Methane in Molten Salts To Produce
Hydrogen
Zeyu Fan and Wei Xiao*
Abstract: Industrial hydrogen production based on methane
steam reforming (MSR) remains challenges in intensive carbon
emissions, retarded hydrogen generation owing to coke
deposition over catalysts and huge consumption of water. We
herein report an electrochemical splitting of methane (ESM) in
molten salts at 5008C to produce hydrogen in an energy saving,
emission-free and water-free manner. Following the most
energy-saving route on methane-to-hydrogen conversion,
methane is electrochemically oxidized at anode to generate
carbon dioxide and hydrogen. The generated anodic carbon
dioxide is in situ captured by the melts and reduced to solid
carbon at cathode, enabling a spatial separation of anodic
hydrogen generation from cathodic carbon deposition. Life-
cycle assessment on hydrogen-generation technologies shows
that the ESM experiences an equivalent carbon emission much
lower than MSR, and a lower equivalent energy input than
alkaline water electrolysis.
Electrochemical conversion at 5008C offers a solution to
the aforenamed challenges. Overpotentials facilitate the
activation of CH at lower temperatures to retard TDM.
4
Considering the thermodynamic fact that the conversion of
CH into CO is easier than that to CO (Figure 1a), it is
4
2
possible to tailor overpotentials to avoid CO evolution and
hence to eliminate the Boudouard reaction at 5008C. Carbon
neutrality of electrochemical CH4 conversion could be
enhanced by using electricity from renewable energy which
becomes increasingly available in the electricity grid. Such
a scenario also stores the intermittent renewable energy in the
[
2,14]
form of chemical energy.
Molten salt is an ideal electrolyte
at 5008C, benefitting from the excellent ionic conductivity far
exceeding solid oxide electrolytes at medium temperatures
(Supporting Information, Figure S1a). The oxygen ions in
molten carbonates could swiftly absorb CO2 (CO +
2
Li OÐLi CO ) with high flux, which is disclosed by the
2
2
3
ꢀ
1
negative DG (ꢀ115.2 kJmol at 5008C, HSC software) and
7
M
ethane (CH ) is an important fossil fuel and a starting
the extremely high reaction equilibrium constant (6.1 ꢀ 10 at
4
[
16,17]
point of C1 chemistry. Efficient utilization of CH is an
5008C, HSC software).
This unique feature of molten
4
[1–7]
imperative task.
CH is the most thermodynamically stable
carbonates is of great significance to realize a CH conversion
4
4
alkane and has a first bond dissociation energy as high as
without CO emissions.
Here we propose an electrochemical splitting of methane
(ESM) to produce H2 in molten Li CO -Na CO -K CO -
2
ꢀ
1 [2,8]
4
39.3 kJmol .
The chemical inertness of CH make its
4
conversion energy-intensive, reflected by industrial produc-
2
3
2
3
2
3
tion of hydrogen (H ) based on the highly endothermic
3 wt% Li O at 5008C (see the Supporting Information for
2
2
methane steam reforming (MSR, CH + 2H OÐCO + 4H )
experimental details). The most thermodynamically favorable
path for electrochemical oxidation of CH4 in the O -
4
2
2
2
[
6,7,9]
2ꢀ
at temperatures higher than 7008C.
Upon such huge input
energy to active CH (E , see inset of Figure 1a), the weak
containing molten carbonates at 5008C is to generate CO2
4
a1
oxidation product (C H ) with lower activation energy (E ,
and H (red plot in Figure 1c, the calculation details are
2
4
a2
2
see inset of Figure 1a) tends to be further oxidized to CO ,
shown in the Supporting Information). Therefore, CH4 is
oxidized to H and CO at the Ni-YSZ (Ni decorated yttria
2
[10–12]
causing intensive carbon emissions.
CH conversion is
4
2
2
also challenged by coke deposition over catalysts due to the
stabilized zirconia) anode with the assistance of oxygen ion
2
ꢀ
Boudouard reaction (2COÐC + CO ) below 6008C and
(O ) (Figure 1d, reaction 1). CO is in situ captured by the
2
2
2
ꢀ
thermal decomposition of methane (TDM, CH ÐC + 2H )
O
in the melt to form soluble carbonate ion (Figure 1d,
4
2
[
13–15]
above 6008C (Figure 1b).
A possible protocol to elimi-
reaction 2), which diffuses to the nickel cathode and gets
[
18,19]
nate coke deposition is to decrease the conversion temper-
ature to about 5008C while avoiding the generation of CO.
reduced to solid carbon (Figure 1d, reaction 3).
The
2
ꢀ
concomitantly released O participates in the oxidation of
CH4 and in situ capture of CO2 again, guaranteeing the
stability of molten salts. Overall, CH is electrochemically
[*] Z. Fan, Prof. W. Xiao
4
School of Resource and Environmental Sciences, Hubei International
Scientific and Technological Cooperation Base of Sustainable
Resource and Energy, Wuhan University
Wuhan 430072 (P. R. China)
split into H2 at anode and carbon at cathode, achieving
a spatial isolation between carbon deposition and H gen-
2
eration. In the ESM, coke deposition over the H -generation
2
site (that is, the Ni-YSZ anode) is avoided. Carbon atoms in
E-mail: 00030042@whu.edu.cn
CH are totally fixed in the form of solid carbon at cathode,
4
Prof. W. Xiao
avoiding CO emissions. The DH of ESM is lower than that of
2
Hubei Key Laboratory of Electrochemical Power Sources, College of
Chemistry and Molecular Sciences, Wuhan University
Wuhan 430072 (P. R. China)
methane dry reforming (MDR) and MSR (Supporting
Information, Figure S1b), revealing the superiority in energy
consumption of ESM. H O is no more needed, relieving the
additional water stress in arid regions.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202017243.
2
7664
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in the O concentration, the
2
consumption of CH4, and
noticeable H evolution. CO
2
is absent, confirming that the
oxidation of CH4 has little
CO selectivity at 5008C. The
absence of CO indicates that
2
the generated CO2 is com-
2
ꢀ
pletely captured by O in
molten salts to form soluble
2
ꢀ
CO3 . X-ray diffraction pat-
terns (XRD, Figure 2b),
Raman spectra (Figure 2c)
and thermogravimetric anal-
ysis (TGA; Supporting Infor-
mation, Figure S4) confirms
the absence of coke deposi-
tion on the spent Ni-YSZ
anode upon ESM. In con-
trast, the TDM in the similar
conditions results in clear
coke deposition on the spent
Ni-YSZ. In the ESM, the
anodic CO2 is in situ con-
2
ꢀ
verted to CO3 and then gets
reduced to carbon at cathode
(Figure 2d), as evidenced by
the generation of one-dimen-
sional and two-dimensional
carbon on the Ni and Ag
cathodes (Supporting Infor-
mation, Figure S5). Such
reaction mechanisms are
also confirmed by the appear-
ance of CO during ESM in
2
the molten salts without
Li O, while CO2 disappears
2
after adding 3 wt% Li O
2
(Figure 2e). In addition, the
DC of H in the cathode gas
2
chamber during potentio-
static ESM is zero (Support-
ing Information, Figure S6),
confirming that the detected
Figure 1. a) Gibbs free energy change (DG) of several methane conversion reactions. Inset: illustration of an
energy diagram for methane activation. b) DG of two reactions (the Boudouard reaction and TDM reaction)
that cause coking. c) Decomposition voltage of CH in Li CO -Na CO -K CO -3 wt% Li O for various anodic
4
2
3
2
3
2
3
2
products. The calculation ensures that the cathode reaction and electron transfer number of the four
2
ꢀ
reactions are consistent (see the Supporting Information for details). d) Illustration on ESM in O
-
H shown in Figure 2a comes
2
containing molten salts. The left part shows the transfer/conversion route of chemicals and the right
from the anode reaction. The
YSZ plate shows no ESM
activity, and NiO-YSZ offers
much higher activity to OER
denotes the corresponding reactions, and the equivalent energy input and equivalent CO emissions based
2
on the life cycle assessment.
The experimental set-up is illustrated in the Supporting
Information, Figure S2. Pre-electrolysis is performed before
than H evolution (Figure 2 f; Supporting Information, Fig-
ures S7, S8, S9). The Ni-YSZ shows an apparent turnover
2
ESM until no H can be detected, to eliminate the interfer-
frequency (TOF) of H generation 17 times higher than that
2
2
ence of residual water (Supporting Information, Figure S3). A
Ni-YSZ and a Ni wire are used as the anode and cathode for
the galvanostatic ESM at 27 mAcm . With the absence of
of NiO-YSZ (Figure 2 f), in accordance with the much lower
TOF of unwanted O for Ni-YSZ. The results indicate that the
2
ꢀ
2
active sites of ESM are metallic Ni.
CH , the oxygen evolution reaction (OER) occurs in anode
Electrochemical oxidation of CH competes with OER at
Ni-YSZ anode. Under 5 and 10 mAcm (Figure 3; Support-
4
4
ꢀ2
and only O is detected in the outlet gas (Figure 2a). Upon
2
introducing CH , the anodic process changes from OER to
ing Information, Figure S10), H is dominant up to 100 vol%
and O2 is not detected in gas products, with the current
4
2
2
ꢀ
ꢀ
CH + 2O ÐCO + 2H + 4e , as evidenced by a sharp drop
4
2
2
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ꢀ2
Figure 2. a) The concentration variation (DC) of gases during galvanostatic ESM in molten salts at 27 mAcm with Ni-YSZ plate as the anode
and tangled Ni wire as the cathode. XRD patterns (b) and Raman spectra (c) of the fresh Ni-YSZ plate (grey lines), Ni-YSZ plates after ESM
(
orange lines) and Ni-YSZ plates after TDM (blue lines) tests for 5 h. In (b), the JCPDS card for Ni, NiO and YSZ are 04-0850, 47-1049, and 48-
224. (d) XRD pattern of the cathode products and optical cathode images. The cathode, anode and reference electrode are tangled Ni wire, Ni-
YSZ plate and Ag/Ag SO , and the anode potential is 0.8 V vs. Ag/Ag SO in (b)–(d). e) Average concentration changes (DC) of H , O , and CO
2
0
2
4
2
4
2
2
during electrolysis at 0.8 V vs. Ag/Ag SO for 2 h under different conditions with Ni sheets as the anodes. f) Apparent turnover frequency (TOF)
2
4
and current efficiency of different anodes (Ni sheet, NiO-YSZ plate and NiO-YSZ plate) for ESM at 0.8 V vs. Ag/Ag SO . The error bars represent
2
4
the standard deviation.
Figure 3. a) The concentration variation (DC) of H and O during galvanostatic ESM at different anode current density. b) Gas product
2
2
distribution and current efficiency in 5 h. c) Average rates of H , O generation and CH consumption in 5 h. d) Methane conversion and
2
2
4
ꢀ2
selectivity of H in 5 h. e) Long-term performance of ESM at 10 mAcm . The cathode, anode and reference electrode are tangled Ni wire, Ni-YSZ
2
plate and Ag/Ag SO . The error bars represent the standard deviation.
2
4
efficiency of H formation close to 100% and anode potential
conversion, with a relatively stable H selectivity close to
2
2
stable between 0.6 and 0.7 V vs. Ag/Ag SO (Figure 3b;
100%, and also causes the evolution of undesired O (Fig-
2
4
2
Supporting Information, Figure S11). Further increase of
ure 3c,d). Under the ESM, CO, CO and C species are absent
2
2
current density promotes the H production and the methane
(Supporting Information, Figure S12). The unreacted CH4
2
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can be recycled for further conversion reaction in practical
operation. Therefore, a low current density is necessary to
avoid OER on the Ni-YSZ anode. At 10 mAcm , the anode
1.0 to 0.8 V vs. Ag/Ag SO , a jump of H production rate, as
2 4 2
well as the restrained evolution of O when the input gas is
2
ꢀ
2
switched from N2 to CH4 (Supporting Information, Fig-
potential and H yield remain stable for 24 hours (Figure 3e).
ure S14b). The fall of anode potential after CH introduction
2
4
The methane conversion and specific energy consumption
indicates a decrease in electric energy consumption (proved
by thermodynamic calculations; Supporting Information,
Figure S15), and more importantly, an improved survival
rate of anode. As can be seen from the Supporting Informa-
tion, Figure S16, the metallic Ni in Ni-YSZ anode is converted
ꢀ
3
ꢀ2
(
SEC, 3.4 kWhNm at 10 mAcm , see the Supporting
Information for the calculation details) of the ESM is
comparable to other H -generation methods (Supporting
2
Information, Figure S13).
The present study demonstrates the proof of concept on
ESM for CH -to-H conversion. Despite the merits on low
to NiO after polarization at 0.8 V for 5 h without CH . With
4
the presence of CH in the similar polarizations, metallic Ni is
4
2
4
operating temperature, no CO emissions, low SEC and high
still predominant. Thermodynamically, the equilibrium
potential of CH4 oxidation is 1.31 V more negative than
that of OER at 5008C (Supporting Information, Figure S17).
Therefore, implementation of other catalysts to accelerate
2
H selectivity, the ESM is still in its infancy. The low space-
2
time yield at low current density and O contamination at high
2
current density are yet to be addressed. The main barriers lie
in the sluggish kinetics of CH oxidation. The onset potential
kinetics of CH oxidation is a central focus of future study,
4
4
of CH oxidation on Ni-YSZ is about 0.2 V more negative
aiming to enhancing durability and eliminating oxygen
contamination at high current density. To enhance space–
time yield, a configuration of gas diffusion electrode could be
considered.
4
than that of OER (Supporting Information, Figure S14a). The
above results are further verified by the recorded anode
potential (vs. Ag/Ag SO ) during galvanostatic ESM at
2
4
ꢀ2
6
7 mAcm , as manifested by a fall of anode potential from
Figure 4. Equivalent CO emissions of this work (a), AWE (b), and MSR process (c); d) Equivalent energy input of AWE and ESM. Wind turbine-
2
generated electricity is used as the source of electricity and heating. The calculation details are provided in the Supporting Information, Figure S18
and Tables S1–3.
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The CO equivalent emissions of three typical technolo- Conflict of interest
2
gies for H production, namely ESM for future H generation,
2
2
alkaline water electrolysis (AWE) with emerging deployment
and MSR for currently industrial applications are analyzed
and compared based on the life cycle assessment (LCA)
method. The details on LCA calculations are shown in
Supporting Information, Figure S18 and Tables S1–S3. The
The authors declare no conflict of interest.
Keywords: CH oxidation · hydrogen production ·
4
in situ CO conversion · methane conversion ·
2
molten salt electrochemistry
ꢀ
1
total CO2 equivalent emission of ESM is 1.525 tCO2 tH2
Figure 4a), which is comparable to that of industrial AWE
(
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Acknowledgements
2
The authors acknowledge the funding support from the
National Natural Science Foundation of China (92045302,
2
5
1722404, and 91845113) and the National Key Research and
Manuscript received: December 28, 2020
Accepted manuscript online: January 11, 2021
Version of record online: March 3, 2021
Development Program of China (2018YFE0201703).
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