Ambient electrochemical N2 reduction to NH3 under alkaline conditions enabled by a layered K2Ti4O9 nanobelt

Article abstract of DOI:10.1039/c9cc02409c

Using electrocatalytic N₂ reduction to make NH₃ is considered to be an attractive environmentally-friendly alternative to the Haber-Bosch process, which is rather energy-intensive and has heavy CO₂ emissions. However, only limited success has been achieved in identifying metal oxides as effective electrocatalysts for the N₂ reduction reaction (NRR) under alkaline conditions. In this communication, we report that a K₂Ti₄O₉ nanobelt is able to effectively electrocatalyze the ambient NRR for N₂-to-NH₃ fixation at alkaline pH. When tested in 0.1 M KOH, a high NH₃ yield of 22.88 μg h^-1 mg^-1_cat. and a faradaic efficiency of 5.87% were attained. Moreover, K₂Ti₄O₉ also demonstrates good selectivity and high electrochemical stability for NH₃ formation.

Full text of DOI:10.1039/c9cc02409c

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DOI: 10.1039/C9CC02409C.
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DOI: 10.1039/C9CC02409C
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Ambient electrochemical N2 reduction to NH3 under alkaline
conditions enabled by layered K2Ti4O9 nanobelt
Received 00th January 20xx,
Accepted 00th January 20xx
Dan Wu,a Huanbo Wang,b Hong Huang,c Rong Zhang,c Lei Ji,c Hongyu Chen,c,d Yonglan Luo,d Jinmao
You,e Dianping Tang,f Zhonghai Zhanga* and Xuping Sunc*
DOI: 10.1039/x0xx00000x
www.rsc.org/
To make NH3, electrocatalytic N2 reduction is considered as an
attractive environmentally-friendly alternative to the Haber- ambient NH3 synthesis via the N2 reduction reaction (NRR), while
Bosch process that is rather energy-intensive with heavy CO2 he challenge associated with N2 activation underlines the
Electrochemical method offers a promising alternative for
t
emission, but only limited success has been made in requirement of efficient electrocatalysts for NRR.7-9 Noble
identifying metal oxides as effective electrocatalysts for the metals catalysts perform efficiently for the NRR,10-13 but the
N2 reduction reaction (NRR) under alkaline conditions. In this high cost and scarcity limit their wide applications. Much
communication, we report that K2Ti4O9 nanobelt is able to attention has thus focused on developing noble-metal-free
effectively electrocatalyze the ambient NRR for N2-to-NH3 NRR electrocatalysts.14-41 Among such catalysts, metal
fixation at alkaline pH. When tested in 0.1 M KOH, it attains a oxides/hydroxides that can be easily made on a large scale
high NH3 yield of 22.88 μg h-1 mg-1 and a Faradaic efficiency have received more considerable attention for the NRR in
cat.
of 5.87%. Moreover, the K2Ti4O9 also shows good selectivity acidic and neutral media.26-43 Given that oxygen evolution
and high electrochemical stability for NH3 formation.
reaction (OER) also simultaneously occurs at the anode during
the NRR and a variety of earth-abundant OER electrocatalysts
under alkaline reaction conditions are available,44,45 it is also
highly attractive to identify NRR electrocatalysts operating
efficiently in alkaline media while only limited metal oxides
have been reported for such applications like Fe2O3/TiO2
(16.52 μg h−1 mg−1cat.; 0.31%),46 Fe2O3 (0.46 μg h−1 cm−2;
6.04%),47 and Fe2O3-CNT (0.4125 μg h−1 cm−2; 0.028).48
Nitrogen is a fundamental component of living organisms
and plays a key role in controlling primary production in the
biosphere.1 Although N2 is widely available in the atmosphere ,
its fixation to NH3 is a difficult reaction owing to the chemical
inertness of N≡N triple bond.2-4 Industrially, the artificial
nitrogen fixation is carried out by the Haber-Bosch process (N2
+ 3H2 → 2NH3) at harsh conditions, which seriously relies on
consumption of fossil fuels and results in a large amount of
CO2 emission.5,6 Thus, a sustainable and environmentally-benign
approach to produce NH3 is urgently demanded.
In this communication, we report our recent experimental
finding that K2Ti4O9 nanobelt is capable of effectively
electrocatalyzing the ambient NRR toward N2-to-NH3 fixation
with excellent selectivity in alkaline electrolytes. In 0.1 M KOH,
this catalyst offers a NH3 yield of 22.88 μg h–1 mg–1 and a
cat.
Faradaic efficiency (FE) of 5.87% at –0.50 V vs. reversible
hydrogen electrode (RHE), superior to TiO2 (NH3 yield: 7.46 μg
h–1 mg–1cat.; FE: 1.51%) and reported metal oxides at alkaline
media. It also shows high electrochemical stability.
a
School of Chemistry and Molecular Engineering, East China Normal University,
Dongchuan
Road
500,
Shanghai
200241,
China.
E-mail:
zhzhang@chem.ecnu.edu.cn
b
c
School of Environment and Resource, Southwest University of Science and
Technology, Mianyang 621010, Sichuan, China
Institute of Fundamental and Frontier Sciences, University of Electronic Science
and Technology of China, Chengdu 610054, Sichuan, China. E-mail:
xpsun@uestc.edu.cn
Fig. 1a shows the X-ray diffraction (XRD) pattern for K2Ti4O9
with diffraction peaks at 10.089°, 14.297°, 28.036°, 30.272°,
31.026°, 32.902°, 33.665° and 41.265° indexed to the (200),
(201), (310), (311), (004), (-603), (-313) and (512) planes of
K2Ti4O9 phase (JCPDS No. 32–0861), respectively.49 Fig. 1b
presents a typical scanning electron microscopy (SEM) image
of K2Ti4O9 with the stacks of layered belt-like structure. The
transmission electron microscopy (TEM) image of K2Ti4O9
further evidences the layered nanobelt structure, as shown in
Fig. 1c. High-resolution TEM (HRTEM) image (Fig. 1d) reveals
lattice fringes with an interplanar distance of 0.267 nm
d
Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province,
College of Chemistry and Chemical Engineering, China West Normal University,
Nanchong 637002, Sichuan, China
e
Key Laboratory of Life-Organic Analysis of Shandong Province, College of
Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165,
Shandong, China
f
Key Laboratory for Analytical Science of Food Safety and Biology (MOE & Fujian
Province), State Key Laboratory of Photocatalysis on Energy and Environment,
Department of Chemistry, Fuzhou University, Fuzhou 350108, Fujian, China
† Electronic Supplementary Information (ESI) available: Experimental section and
supplementary figures. See DOI:
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indexed to the (-313) crystal face of K2Ti4O9. TEM and 532.3 eV, 530.9 eV and 529.5 eV were assigned to the
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DOI: 10.1039/C9CC02409C
corresponding energy-dispersive X-ray (EDX) elemental adsorbed oxygen, hydroxyl oxygen and lattice oxygen,
mapping images reveal uniform distribution of K, Ti and O respectively.51 The presence of hydroxyl oxygen was indicative
elements within K2Ti4O9 (Fig. 1e).
of trace amount of water molecules on the surface of sample,
while the lattice oxygen peak observed is indicative of the
existence of a Ti-O-Ti bond in the layered material. Fig. 2d
shows XPS spectrum in the Ti 2p core-level region. The peaks
located at the BEs of 463.6 eV and 457.9 eV, which are
attributed to spin orbit components of 2p1/2 and 2p3/2
,
respectively. A spin orbit splitting energy between Ti 2p1/2 and
Ti 2p3/2 is determined as 5.7 eV, which corresponds to Ti4+ in
K2Ti4O9.52
Fig. 3. (a) LSV curves of K2Ti4O9/CP in Ar- and N2-saturated 0.1 M KOH. (b) Time-
dependent current density curves of K2Ti4O9/CP at various potentials for 2 h in
N2-saturated 0.1 M KOH. (c) UV-V is absorption spectra of the electrolytes
stained with indophenol indicator after NRR electrolysis at a series of potentials
for 2h. (d) NH3 yields and FEs of K2Ti4O9/CP for NRR at a series of potentials.
Fig. 1. (a) XRD pattern for K2Ti4O9. (b) SEM image of K2Ti4O9. (c) TEM image for
K2Ti4O9. (d) HRTEM image for K2Ti4O9. (e) TEM and corresponding EDX elemental
mapping images of K, Ti and O for K2Ti4O9.
The NRR catalytic performance was tested using a three-
electrode system comprising of a graphite rod as a counter
electrode, Ag/AgCl as a reference electrode and K2Ti4O9
deposited on carbon paper (CP) electrode as a working
electrode (K2Ti4O9/CP; K2Ti4O9 loading: 0.1 mg cm-2). The
potentials for NRR are reported based on an RHE scale. Both
produced NH3 and possible by-product hydrazine (N2H4) were
spectrophotometrically determined by the indophenol blue
method53 and Watt and Chrisp method,54 respectively. The
corresponding calibration curves of NH3 and N2H4 are shown in
Fig. S1 and Fig. S2. Fig. 3a shows the linear sweep voltammetry
(LSV) curves under N2- and Ar-saturated 0.1 M KOH solutions.
A distinct current density enhancement under N2 atmosphere
reveals that K2Ti4O9/CP has catalytic activity for the NRR. Fig.
S3 shows the cyclic voltammetry curves for K2Ti4O9/CP and CP
in Ar- and N2-saturated 0.1 M KOH solution, respectively. It is
clearly seen that K2Ti4O9/CP attains higher current density in
N2-saturated solution, which is consistent with the results of
LSV curves. As shown in Fig. 3b, the currents of
chronoamperometry curves at different potentials reveal good
stability for 2 h electrolysis under N2-saturated electrolytes.
After electrolysis, the collected electrolytes were colored with
indophenol indicator and measured by ultraviolet
spectrophotometer. The corresponding UV-Vis absorption
spectra are shown in Fig. 3c. Clearly, the electrocatalytic N2
reduction over K2Ti4O9/CP occurs at potential ranges from –
0.40 V to –0.60 V. The average NH3 yields and corresponding
Fig. 2. (a) XPS survey spectrum for K2Ti4O9. XPS spectrum in the (b) K 2p, (c) O 1s
and (d) Ti 2p regions for K2Ti4O9.
X-ray photoelectron spectroscopy (XPS) was employed to
explore the surface chemical components and corresponding
chemical bonds of K2Ti4O9. Fig. 2a shows the XPS survey
spectrum of K2Ti4O9, confirming the presence of K, Ti and O
elements. In the K 2p spectrum (Fig. 2b), the binding peaks at
295.2 eV and 292.4 eV can be assigned to K 2p1/2 and K 2p3/2
,
respectively.50 The XPS peaks of O 1s for the sample is shown
in Fig. 2c. The peaks centered at the binding energies (BEs) of
2 | J. Name., 2012, 00, 1-3
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FEs are shown in Fig. 3d. As observed, NH3 yields and FEs image shows the preservation of nanobelt feature for K2Ti4O9
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increase with more negative potential until –0.50 V, where the (Fig. S10) after NRR electrolysis. XRDDOI:a1n0a.1l0y3s9is/C9(FCiCg0. 24S0191C)
maximum NH3 yield of 22.88 μg h–1 mg–1 and a FE of 5.87% confirms that K2Ti4O9 has no obvious change in crystalline
cat.
are achieved. The values are higher than those for most phase after stability test. The XPS spectra of K2Ti4O9 almost
reported NRR electrocatalysts (Table S1). The NH3 yield and FE remain unchanged after long-term NRR electrolysis, as shown
of K2Ti4O9/CP increase initially from −0.40 V to −0.50 V. in Fig. S12. Those results imply the high structure stability.
However, they start to decrease as the potential is negatively
shifted to −0.60 V, which is ascribed to the competitive
adsorption of nitrogen and hydrogen species on the electrode
surface.27 Beyond this negative potential, the reduction rate
and FE decrease obviously, which is due to the competition of
the hydrogen evolution reaction on the electrode surface.55,56
Also note that TiO2 offers a NH3 yield of 7.46 μg h–1 mg–1cat. and
a FE of 1.51% at –0.50 V (Fig. S4). NRR on the surface of
K2Ti4O9 may follow the associative mechanism where the
nitrogen molecules are step-by-step reduced with protons and
electrons.57 Alkali metal ions, which can bind with N2, have
been utilized in electrolyte to enhance the NRR performances
because such cations electrostatically enriched in the stern
layer interact with the dissolved N2 molecules leading to
higher concentrations of N2 at the electrode surface than in
the electrolyte.58 The enhanced N2 adsorption would then
boost the N2 reduction activity and selectivity on K2Ti4O9
catalyst. That can explain why K2Ti4O9 rich with K+ has higher
NRR activity compared with TiO2. It is expected that other K-Ti-
Fig. 4. (a) NH3 yields and FEs for K2Ti4O9/CP with alternating 2 h cycles between
N2-saturated and Ar-saturated electrolytes. (b) Amount of produced NH3 for
K2Ti4O9/CP, TiO2/CP and bare CP after 2 h electrolysis. (c) Recycling test for
K2Ti4O9/CP at –0.50 V for 6 times. (d) Time-dependent current density curve for
K2Ti4O9/CP at –0.50 V for 24 h.
In summary, K2Ti4O9 has been experimentally proven as a
high-active catalyst for ambient N2-to-NH3 fixation with
excellent selectivity. This catalyst achieves a high NH3 yield of
22.88 μg h–1mg–1cat. with a FE of 5.87% at –0.50 V vs. RHE in 0.1
M KOH. This study not only offers us an attractive earth-
abundant catalyst for electrochemical NH3 synthesis in alkaline
media, but it would open up new avenues to the rational
design of titanate–based materials for N2 fixation applications.
O
catalysts with various compositions or Na-Ti-O
nanomaterials are also capable of effectively catalyzing the
NRR.
To verify that the detected NH3 was indeed generated from
electrocatalytic NRR over K2Ti4O9, control experiments were
conducted in Ar-saturated solution at –0.50 V and in N2-
saturated solutions at open circuit potential (Fig. S5). Clearly,
almost no NH3 is detected in either case. In addition, we
performed experiments with alternating 2 h cycles between
N2- and Ar- saturated solutions at –0.50 V (Fig. 4a). Clearly,
NH3 was only obtained in N2-saturated solution. There is a tiny
amount of NH3 in Ar-saturated solution that may come from
the trace amount of NH3 in air.59 Moreover, the amount of
detected NH3 increases roughly linearly during a total reaction
time of 4 h (Fig. S6). These results declare that the N2 fixation
process takes place by K2Ti4O9. No by-product N2H4 is detected
in final electrolytes at all potentials (Fig. S7), indicating that
K2Ti4O9 has good selectivity for NRR. For comparison, we
investigated the NRR performance of CP and TiO2, the
precursor of K2Ti4O9, at –0.50 V for 2 h electrolysis at ambient
conditions (Fig. 4b). The relevant UV-vis absorption spectra are
displayed in Fig. S8. Obviously, CP has almost no catalytic
activity for the NRR. TiO2/CP shows much poorer NRR
performance than those of K2Ti4O9/CP. Stability is also critical
to the catalyst for practice. The corresponding UV-Vis
absorption spectra are plotted in Fig. S9 and the NH3 yield
rates as well as the FEs shown in Fig. 4c, NH3 yields and FEs
shows small changes during consecutive tests for 6 times at –
0.50 V, indicating the high stability of K2Ti4O9. In addition, the
long-term electrochemical stability of K2Ti4O9 was also tested
at –0.50 V in N2-saturated electrolyte, revealing a stable
current profile for the testing period of 24 h (Fig. 4d). SEM
Conflicts of interest
There are no conflicts to declare.
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DOI: 10.1039/C9CC02409C
Table of Contents Entry
K2Ti4O9 nanobelt effectively electrocatalyzes the ambient N2-to-NH3 fixation. In 0.1
M KOH, it attains a NH3 yield of 22.88 μg h-1 mg-1 and a Faradaic efficiency of
cat.
5.87%, with good selectivity and high electrochemical stability.