Efficient methane electrocatalytic conversion over a Ni-based hollow fiber electrode

Article abstract of DOI:10.1016/S1872-2067(20)63548-3

Natural gas and shale gas, with methane as the main component, are important and clean fossil energy resources. Direct catalytic conversion of methane to valuable chemicals is considered a crown jewel topic in catalysis. Substantial studies on processes including methane reforming, oxidative coupling of methane, non-oxidative coupling of methane, etc. have been conducted for many years. However, owing to the intrinsic chemical inertness of CH₄, harsh reaction conditions involving either extremely high temperatures or highly oxidative reactants are required to activate the C–H bonds of CH₄ in such thermocatalytic processes, which may cause the target products, such as ethylene or methanol, to be further converted into coke or CO and CO₂. It is desirable to adopt a new strategy for direct CH₄ conversion under mild conditions. Herein, we report that efficient electrocatalytic oxidation of methane to alcohols at ambient temperature and pressure can be achieved using a NiO/Ni hollow fiber electrode. This work opens a new avenue for direct catalytic conversion of CH₄.

Full text of DOI:10.1016/S1872-2067(20)63548-3

Chinese Journal of Catalysis 41 (2020) 1067–1072
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Communication
Efficient methane electrocatalytic conversion over a Ni-based
hollow fiber electrode
Zhikai Guo a,b, Wei Chen a,*, Yanfang Song a, Xiao Dong a, Guihua Li a, Wei Wei a,c, Yuhan Sun a,c,#
a CAS Key Laboratory of Low-carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences,
Shanghai 201210, China
b University of Chinese Academy of Sciences, Beijing 100049, China
c School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Natural gas and shale gas, with methane as the main component, are important and clean fossil
energy resources. Direct catalytic conversion of methane to valuable chemicals is considered a
crown jewel topic in catalysis. Substantial studies on processes including methane reforming, oxida-
tive coupling of methane, non-oxidative coupling of methane, etc. have been conducted for many
years. However, owing to the intrinsic chemical inertness of CH4, harsh reaction conditions involv-
ing either extremely high temperatures or highly oxidative reactants are required to activate the
C–H bonds of CH4 in such thermocatalytic processes, which may cause the target products, such as
ethylene or methanol, to be further converted into coke or CO and CO2. It is desirable to adopt a new
strategy for direct CH4 conversion under mild conditions. Herein, we report that efficient electrocat-
alytic oxidation of methane to alcohols at ambient temperature and pressure can be achieved using
a NiO/Ni hollow fiber electrode. This work opens a new avenue for direct catalytic conversion of
CH4.
Received 25 September 2019
Accepted 16 November 2019
Published 5 July 2020
Keywords:
Methane
Electrocatalytic conversion
Nickel
Nickel oxide
Hollow fiber
© 2020, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
Natural gas and shale gas, with methane as the main com-
ponent, are important and clean fossil energy resources [1–3].
While unconventional natural gas especially shale gas is ex-
ploited on a large scale, it is potentially more economical and
environmentally friendly to use CH4 directly as a chemical
feedstock [4–6]. However, owing to its perfectly stable sym-
metrical tetrahedral structure, the methane molecule exhibits
high C–H bond strength (439 kJ/mol), negligible elec-
tron/proton affinity, large ionization energy, and low polariza-
bility, resulting in great difficulties in activating methane and
breaking C–H bonds [7–12]. Therefore, conversion of methane
into value-added chemicals remains highly attractive yet chal-
lenging, and is considered a crown jewel topic in catalysis. Un-
like methane reforming [13–15], where methane is converted
only to syngas (CO/H2) as a feedstock for the Fischer-Tropsch
synthesis process [16,17], oxidative coupling of methane
(OCM) can produce C2 hydrocarbons such as ethylene and
ethane and has been studied extensively since the early 1980s
[18–20]. However, the existence of oxygen (O2) at high temper-
atures causes potential explosion risks and irreversible
* Corresponding author. Tel: +86-21-20350954; Fax: +86-21-20350867; E-mail: chenw@sari.ac.cn
# Corresponding author. Tel: +86-21-20608005; Fax: +86-21-20350867; E-mail: sunyh@sari.ac.cn
This work was supported by the National Natural Science Foundation of China (91745114, 21802160), the Ministry of Science and Technology of
China (2016YFA0202800, 2018YFB0604700), Shanghai Sailing Program (18YF1425700), Shanghai Advanced Research Institute Innovation Re-
search Program (Y756812ZZ1, Y756803ZZ1), and Shanghai Functional Platform for Innovation Low Carbon Technology. W. C. also acknowledges the
support from the Hundred Talents Program of Chinese Academy of Sciences.
DOI: 10.1016/S1872-2067(20)63548-3 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 41, No. 7, July 2020

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Zhikai Guo et al. / Chinese Journal of Catalysis 41 (2020) 1067–1072
overoxidation, producing a large amount of CO2 with low car-
bon utilization in the OCM route [21,22]. Recently,
non-oxidative coupling of methane (NOCM) to olefins and aro-
matics was reported as a promising route [23,24], but the harsh
conditions involving high temperatures (~1000 °C) may limit
its industrial adoption to some extent [25]. To overcome the
disadvantages in the OCM and NOCM routes, it is desired to
develop a new approach for direct methane conversion.
methane must pass through the NiO/Ni interface with numer-
ous pores in a 0.1 mol/L aqueous NaOH solution, as shown in
Scheme 1. The products, methanol and ethanol, are efficiently
generated at the NiO@NiHF anode with faradaic efficiencies for
CH4 oxidation up to 90% at ambient temperature and pressure.
Ni-based hollow fiber electrodes can be prepared via a facile
method that is compatible with existing large-scale production
processes.
Compared to the thermocatalytic routes, the electrocatalytic
route for converting methane into chemicals exhibits obvious
merits [26–28]: (1) mild reaction conditions, since electrocata-
lytic methane conversion reactions may occur even at ambient
temperature and pressure; (2) easy handling of potential and
current, which are key experimental parameters for manipu-
lating reaction kinetics; and (3) the use of renewable electrical
energy. Electrocatalytic conversion of methane can be consid-
ered as the process of storing renewable electricity as chemical
energy. However, electroactive components are crucial for the
electrocatalytic conversion of methane, and Ni-based catalysts
have been widely reported to be active for methane electro-
conversion [29,30]. For instance, Fan [29] reported methanol
production through electrocatalytic methane oxidation with a
Ni(OH)2/CoO(9:1)/Ag/C/CeO2(85/5/10) anode. Mustain et al.
[30] developed a NiO-ZrO2 bifunctional electrocatalyst with
The synthesis of NiO@NiHF composites is mainly divided
into two steps: (1) preparation of Ni hollow fiber (NiHF), which
involves an improved phase-inversion method to make NiHF
with commercial nickel powder as a pristine feedstock (see
Supporting Information (SI) for full details), and (2) active NiO
layer fabrication. The as-prepared NiHFs were calcined in air at
200–300 °C to obtain xNiO@NiHF composites, where x repre-
sents the NiO mass percentage.
Figure 1 displays the porous structures and gas permeance
behaviors of the NiHFs. The scanning electron microscopy
(SEM) images of the inner and outer surfaces of the NiHFs (Fig.
1a and 1b) show the abundant pores dispersed on these sur-
faces. The high-magnification SEM images (the insets in Fig. 1a
and 1b) further indicate an average pore size in the range of
1–2 μm as well as smooth surfaces. The cross-sectional mor-
phology of a whole NiHF (Fig. 1c) showed characteristic fin-
ger-like structures in the outer and inner regions without any
visible large microvoids formed. The high-resolution transmis-
sion electron microscopy (HRTEM) image of the outer surface
of a NiHF (Fig. 1d) shows that a particle with a d-spacing of
2.03 Å corresponds to the (111) lattice fringe of metallic nickel.
The permeances of H2, CH4, N2, and CO2 through a NiHF remain
almost constant at different pressure drops (Fig. 1e), and the
gas permeance is inversely proportional to the square root of
the molecular weight (the inset in Fig. 1e), implying that the gas
transport mechanism through a NiHF is dominated by Knudsen
diffusion.
2−
CO3 anions for CH4 oxidation, producing CH3OH, HCHO, and
other oxygenates. However, the solubility of methane in elec-
trolyte solutions is quite low, which greatly hampers the elec-
trocatalytic performance. Furthermore, the essential
gas-liquid-solid three-phase interfaces cannot be attained in
aqueous methane electrooxidation systems. These problems
are also encountered during liquid-phase electrochemical reac-
tions involving at least one gas-phase reactant, such as electro-
chemical CO2 reduction and oxygen reduction reactions.
Herein, we report a porous nickel-based hollow fiber elec-
trode comprising a NiO active layer on the surface (denoted as
NiO@NiHF) for electrocatalytic methane conversion reactions.
The hollow-fiber-type electrode possesses compact
three-dimensional geometry, which provides a large area and
three-phase boundary for methane electrooxidation. A me-
thane gas flow enters a Ni-based hollow fiber from one end
while the other end is sealed by epoxy resin; thus, excurrent
The surface morphologies of xNiO@NiHF composites are
obviously different from those of the NiHFs (Fig. S2). The outer
surface roughness of xNiO@NiHF is much higher than that of a
NiHF, which is attributed to the occurrence of the thermal oxi-
dation of nickel. According to the cross-section image (Fig. 2a),
a thin NiO layer is uniformly grown on the nickel substrate. The
coarse outer surfaces of xNiO@NiHF improve the access to the
Fig. 1. Characterization of NiHFs. SEM images of the (a) inner and (b)
outer surfaces, (c) cross-section of a NiHF, (d) HRTEM image of the
outer surface of a NiHF, and (e) gas permeances measured at different
pressure drops. The inset in (e) shows the permeance (at 2.0 bar) ver-
sus the inverse square root of the molecular weights of individual gases.
Scheme 1. Schematic illustration of electrocatalytic methane conver-
sion using a NiO@NiHF anode.

Zhikai Guo et al. / Chinese Journal of Catalysis 41 (2020) 1067–1072
1069
2p3/2
(b)
(a)
2p1/2
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Ni
NiO
3%NiO@NiHF
1%NiO@NiHF
0.5%NiO@NiHF
NiHF
880
870
860
850
0
1
2
3
Mass of NiO (%)
Binding energy (eV)
Fig. 3. (a) Ni 2p XPS spectra of the NiHFs and xNiO@NiHF composites,
and (b) the relative intensity ratios of NiO/Ni versus the mass percent-
age of NiO.
Fig. 2. Characterization of NiO@NiHF. (a) Cross-section SEM and (b)
HRTEM images of the outer region of 1%NiO@NiHF and (c) XRD pat-
terns of NiHFs and xNiO@NiHF composites.
stepwise up to 1.19 in 3%NiO@NiHF with a relatively slow
rate, implying potential surface/interface configuration inflex-
ion in 1%NiO@NiHF.
According to the electrochemical impedance spectroscopy
(EIS) spectra of the NiHFs and xNiO@NiHF composites (Fig.
4a), the charge transfer resistance (Rct) of the NiHFs is 2.4 Ω,
much smaller than those of xNiO@NiHF. The Rct of 9.6 Ω for
three-phase reaction interfaces, which is beneficial to methane
electrooxidation. HRTEM investigation of 1%NiO@NiHF
showed that one particle with a d-spacing of 2.09 Å assigned to
the NiO (200) plane is adjacent to the metallic Ni substrate,
which possesses a d-spacing of 2.03 Å corresponding to the
metallic Ni (111) lattice fringe (Fig. 2b).
0.5%NiO@NiHF monotonically increased to 11.4
Ω for
1%NiO@NiHF and 19.1 Ω for 3%NiO@NiHF with increasing
NiO contents. These indicate that the presence of NiO content
significantly hampers the charge transfer owing to its dielectric
property. The electrochemically active surface areas (ECSAs) of
the NiHFs and xNiO@NiHF composites were determined by
using a cyclic voltammetry (CV) method based on their dou-
ble-layer capacitances. All CV profiles (Fig. S3 and the inset in
Fig. 4b) exhibit nearly symmetrical rectangular shapes in the
range of 1.21–1.31 V (vs. RHE), indicating the ideal capacitive
behaviors of double-layer capacitance. The cathodic and anodic
current densities were obtained from the double-layer
charge/discharge CV curves at 1.26 V (vs. RHE). The dou-
ble-layer capacitance (Cdl), which is directly proportional to the
ECSA, was then calculated by averaging the absolute values of
the cathodic and anodic slopes of the linear fits. The Cdl values
calculated were 0.4, 16.4, 24.5, and 22.8 mF/cm2 for NiHFs,
0.5%NiO@NiHF, 1%NiO@NiHF, and 3%NiO@NiHF, respec-
tively (Fig. 4b). This implies that the ECSA of 1%NiO@NiHF is
the largest, more than sixty times that of the NiHFs. The ECSAs
of 0.5%NiO@NiHF and 3%NiO@NiHF were about 67% and
93% of that of 1%NiO@NiHF, respectively.
Potentiostatic CH4 electrooxidation was carried out in
CH4-saturated 0.1 mol/L NaOH (see SI for details), and no
product was detected when the NiHFs were used. Both metha-
nol and ethanol were the main products at 1.40–1.50 V (vs.
RHE) when the xNiO@NiHF composites were used, while be-
yond this potential range, the amount of product from CH4
electrooxidation was negligible owing to possible further oxi-
dation of alcohols at higher potentials. As shown in Fig. 5, the
faradaic efficiencies (FEs) for methanol generation increased
quickly from 1.40 V and reached a maximum at 1.44 V, followed
by dramatic fading at higher potentials. Meanwhile, the ethanol
X-ray diffraction (XRD) patterns of the NiHFs and
xNiO@NiHF composites in the range of 30°–80° are displayed
(Fig. 2c). Diffraction peaks corresponding to the (111), (200),
and (220) planes of cubic metallic Ni with a space group of
Fm-3m (JCPDS No. 04-0850) are present for all samples. No
NiO-related peak was manifested for 0.5%NiO@NiHF, implying
only slight thermal oxidation of NiHFs at 200 °C, resulting in the
NiO content being too scarce to be detected by XRD. For
1%NiO@NiHF, in addition to the metallic Ni peaks, a new peak
assigned to the cubic NiO(200) plane with a space group of
Fm-3m (JCPDS No. 47-1049) appeared, consistent with the
HRTEM observations. With a higher calcination temperature,
the intensity of the NiO(200) peak increased and the other two
NiO peaks corresponding to the (111) and (220) planes also
appeared for 3%NiO@NiHF.
The surface electronic states of the NiHFs and xNiO@NiHF
composites were studied by X-ray photoelectron spectroscopy
(XPS), as shown in Fig. 3a. The Ni 2p spectrum of the NiHFs
shows the main Ni 2p3/2 and Ni 2p1/2 core peaks at binding
energies of 852.7 and 870.0 eV, respectively, indicating metallic
Ni characteristics. A Ni shake-up satellite 2p3/2 peak at 858.6 eV
also appeared for the NiHFs. As for the xNiO@NiHF composites,
in addition to the metallic Ni peaks, new split peaks located at
854.2, 856.3 (Ni 2p3/2), and 871.7 eV (Ni 2p1/2) are the charac-
teristic NiO peaks along with the NiO satellite 2p3/2 peak at
861.6 eV. Variations of the relative intensity ratios of NiO/Ni
with respect to the mass percentage of NiO in the NiHFs and
xNiO@NiHF composites are displayed in Fig. 3b. The NiO/Ni
ratio was as high as 0.71 in 0.5%NiO@NiHF while its NiO con-
tent could not be detected by XRD. Moreover, the NiO/Ni ratio
continuously increased to 1.09 in 1%NiO@NiHF, but then

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Zhikai Guo et al. / Chinese Journal of Catalysis 41 (2020) 1067–1072
0.25
0.2
1 mV/s
2 mV/s
4 mV/s
6 mV/s
8 mV/s
(b)
0.1
0.0
0.20
-0.1
-0.2
0.15
1.20 1.24 1.28 1.32
Potential (V vs. RHE)
0.10
NiHF
0.5% NiO@NiHF
1% NiO@NiHF
3% NiO@NiHF
0.05
0.00
0
2
4
6
8
Scan rate (mV S-1)
Fig. 4. (a) EIS Nyquist plots of NiHFs and xNiO@NiHF composites at 1.46 V (vs. RHE) in CH4-saturated 0.1 mol/L NaOH. The inset in (a) shows the
fitting equivalent circuit. (b) ECSA comparisons for NiHFs and xNiO@NiHF composites based on their double-layer capacitances using CV at different
scan rates from 1 to 8 mV/s in CH4-saturated 0.1 mol/L NaOH. The inset in (b) shows the CV curves of 1%NiO@NiHF in the range of 1.21–1.31 V (vs.
RHE).
FEs reached a maximum at 1.46 V and then faded rapidly. With
increasing potentials, the current densities monotonically in-
creased mildly from 1.40 to 1.46 V and exponentially after 1.46
V. A possible reason is the oxidation of the nickel substrate at
higher potentials. Among the xNiO@NiHF composites,
1%NiO@NiHF exhibited the highest methanol FE of 54% at
1.44 V and the highest ethanol FE of 85% at 1.46 V. The elec-
trocatalytic performance of 1%NiO@NiHF remained relatively
stable in potentiostatic CH4 oxidation for 180 min (Fig. S4). This
implies that the appropriate NiO/Ni configuration is crucial for
efficient electrochemical CH4 conversion, which requires a bal-
ance between the charge transfer and active oxide sites. In ad-
dition, adsorbed CH4 dissociation to form CH3* at the NiO/Ni
interface is considered as the initial step for CH4 electrooxida-
tion to methanol and ethanol, as stated in previous reports
[31,32]. The coupling of CH3* with its dehydrogenated species
CH2* may be the important step for forming ethanol in an alka-
line solution. The related theoretical calculations for specula-
tions on the formation mechanisms of methanol and ethanol
through CH4 electrooxidation are underway.
In summary, this work demonstrated the fabrication of a
hollow-fiber-type NiO@Ni porous electrode to achieve a
methanol FE of 54% at a current density of 20 µA/cm2 and a
potential of 1.44 V (vs. RHE) as well as an ethanol FE as high as
85% at a current density of 40 µA/cm2 and a potential of 1.46 V
(vs. RHE) for electrocatalytic CH4 conversion. Further experi-
ments and theoretical calculations are underway to explain the
C–C coupling at the NiO/Ni interface of the hollow fiber elec-
trode. The study provides a new pathway for engineering po-
rous electrodes for developing low-cost and highly efficient
electrocatalysts for direct methane electrooxidation.
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Graphical Abstract
Chin. J. Catal., 2020, 41: 1067–1072 doi: 10.1016/S1872-2067(20)63548-3
Efficient methane electrocatalytic conversion over a Ni-based hollow
fiber electrode
Zhikai Guo, Wei Chen *, Yanfang Song, Xiao Dong, Guihua Li, Wei Wei,
Yuhan Sun *
Shanghai Advanced Research Institute, Chinese Academy of Sciences;
University of Chinese Academy of Sciences; ShanghaiTech University
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镍基中空纤维催化剂实现甲烷高效电催化转化
郭志凯a,b, 陈 为a,*, 宋艳芳a, 董 笑a, 李桂花a, 魏 伟a,c, 孙予罕a,c,#
a中国科学院上海高等研究院, 中国科学院低碳转化科学与工程重点实验室, 上海201210
b中国科学院大学, 北京100049
c上海科技大学物质科学与技术学院, 上海201210
摘要: 随着天然气以及页岩气为代表的非常规天然气的大规模开采, 甲烷作为化工原料的直接转化利用受到了越来越多
的关注. 然而, 甲烷分子具有极其稳定的正四面体结构, 其物理化学性质非常稳定, 如具有高达439 kJ/mol的C–H键能、极弱
的电子亲和力、相当大的离子化能量和低的极化率, 这都使得甲烷分子C–H键的活化相当困难. 如何实现甲烷直接高效催
化转化被誉为催化领域的“皇冠式”课题. 与经甲烷重整制合成气, 然后通过F-T合成获取化学品的间接转化法相比, 甲烷直
接转化无论在物料、能量转换效率还是在设备成本、环境保护等方面都有着非常明显的优势. 以甲烷氧化偶联以及非氧
化偶联(如无氧芳构化等)为典型代表的甲烷直接转化研究不断取得突破, 但其各自都存在一定的局限性.
相比于热催化转化路径, 电催化转化路径在许多方面存在着十分明显的优势: (1)反应条件温和,甚至在常温常压条件
下也能实现甲烷电催化转化反应的发生; (2) 可调控程度高, 仅需调节关键实验参数如电压和电流等, 就能实现对反应过程
热力学以及动力学的调控; (3) 能够利用可再生电能驱动甲烷转化反应的发生, 可将低品阶的电能转化并存储为化学能.
本文采用Ni中空纤维作为基底，在其表面构筑NiO活性层, 将NiO@Ni中空纤维作为电极, 实现了常温常压条件下的甲
烷电催化转化. 通过X射线衍射、扫描电镜、透射电镜等表征手段, 确定了中空纤维特有的多孔三维结构、气体传输规律、
NiO活性层分布状态等物化性质. 通过电化学交流阻抗与循环伏安等测试手段, 获得了电荷传递、电化学活性比表面积等

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Zhikai Guo et al. / Chinese Journal of Catalysis 41 (2020) 1067–1072
电化学性质. 恒电压电氧化甲烷研究发现, 1%NiO@Ni中空纤维具有最优的催化活性, 分别在1.44 V与1.46 V (vs. RHE)电
势下获得54%的甲醇法拉第效率和85%的乙醇法拉第效率.
关键词: 甲烷; 电催化转化; 镍; 氧化镍; 中空纤维
收稿日期: 2019-09-25. 接受日期: 2019-11-16. 出版日期: 2020-00-05.
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#通讯联系人. 电话: (021)20608005; 传真: (021)20350867; 电子信箱: sunyh@sari.ac.cn
基金来源: 国家自然科学基金(91745114, 21802160); 国家重点研发计划(2016YFA0202800, 2018YFB0604700); 中国科学院百人
计划; 上海市青年科技英才扬帆计划(18YF1425700); 中国科学院上海高等研究院创新基金(Y756812ZZ1 (172002), Y756803ZZ1
(171003)); 上海市低碳技术创新功能型平台.
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).