An ultrasmall Ru2P nanoparticles-reduced graphene oxide hybrid: An efficient electrocatalyst for NH3 synthesis under ambient conditions

Article abstract of DOI:10.1039/c9ta10346e

Industrial NH₃ synthesis highly relies on the Haber-Bosch process which consumes a large amount of energy and emits a massive amount of CO₂. Electrochemical N₂ reduction is an eco-friendly and sustainable approach to realize NH₃ synthesis under ambient conditions, but its implementation requires efficient electrocatalysts for the N₂ reduction reaction. In this work, a hybrid of Ru₂P nanoparticles and reduced graphene oxide is proposed as an efficient electrocatalyst for artificial N₂-to-NH₃ fixation with excellent selectivity under ambient conditions. Electrochemical tests in 0.1 M HCl show that such a hybrid achieves a large NH₃ yield of 32.8 μg h^-1 mg_cat.^-1 and a high faradaic efficiency of 13.04% at -0.05 V vs. the reversible hydrogen electrode. Furthermore, it also exhibits remarkable electrochemical and structural stability. Theoretical calculations reveal that Ru₂P-rGO can efficiently catalyze NH₃ synthesis with a low energy barrier.

Full text of DOI:10.1039/c9ta10346e

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Materials Chemistry A
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An ultrasmall Ru2P nanoparticles–reduced
graphene oxide hybrid: an efficient electrocatalyst
for NH3 synthesis under ambient conditions†
Cite this: J. Mater. Chem. A, 2020, 8, 77
Runbo Zhao, ‡a Chuangwei Liu, ‡b Xiaoxue Zhang,c Xiaojuan Zhu,c Peipei Wei,a
Received 19th September 2019
Accepted 25th November 2019
d
c
a
Lei Ji,a Yanbao Guo,b Shuyan Gao,
Yonglan Luo,
Zhiming Wang
*
*
a
*
and Xuping Sun
DOI: 10.1039/c9ta10346e
rsc.li/materials-a
Industrial NH3 synthesis highly relies on the Haber–Bosch process economical approach is needed for articial NH3 synthesis
which consumes a large amount of energy and emits a massive under mild reaction conditions.
amount of CO2. Electrochemical N2 reduction is an eco-friendly and
Electrochemical N2 reduction is an energy-saving and envi-
sustainable approach to realize NH3 synthesis under ambient condi- ronmentally friendly approach which can replace the Haber–
tions, but its implementation requires efficient electrocatalysts for the Bosch process for NH3 synthesis under ambient conditions;
N2 reduction reaction. In this work, a hybrid of Ru2P nanoparticles and however, efficient electrocatalysts for the N2 reduction reaction
reduced graphene oxide is proposed as an efficient electrocatalyst for (NRR) are needed to break the inert N2 molecule.4–28 Ru
artificial N2-to-NH3 fixation with excellent selectivity under ambient performs efficiently in the traditional Haber–Bosch process.29
conditions. Electrochemical tests in 0.1 M HCl show that such a hybrid Ru is also theoretically predicted to be a good NRR electro-
achieves a large NH3 yield of 32.8 mg hÀ1 mgcat.À1 and a high faradaic catalyst due to its appropriate overpotential and energy for N2
efficiency of 13.04% at À0.05 V vs. the reversible hydrogen electrode. adsorption and activation, which are much lower than those of
Furthermore, it also exhibits remarkable electrochemical and struc- Pt and Pd.30–32 A recent experimental study showed that Ru
tural stability. Theoretical calculations reveal that Ru2P–rGO can nanoparticles catalyze the NRR under ambient conditions with
efficiently catalyze NH3 synthesis with a low energy barrier.
an NH3 yield of 0.55 mg hÀ1 cmÀ2 and a faradaic efficiency (FE)
of 5.4%.33 Theoretical investigation further revealed that *N2Hx
species (0 # x # 2) could accumulate on the surface of Ru due to
the strong binding strength between them, resulting in
a decreased performance toward the NRR.34 Obviously, to
improve the NRR activity, the binding strength should be
weakened. Bonding metal atoms with electronegative
nonmetals is an effective way to weaken the binding strength
through controlling the electron transfer between them.4 Our
recent study veried that the NRR performance of a B catalyst is
greatly improved by alloying with P due to further weakened
N^N bonds.35 Hence, enhanced N2 reduction electrocatalysis is
expected for the Ru catalyst aer alloying with P, which however
has not been explored before.
As one of the most produced chemicals, NH3 is widely employed
as a nitrogen source to produce fertilizers, medicaments, plas-
tics, and so on.1 Furthermore, with a considerable hydrogen
content, high energy density and zero carbon emission, NH3 is
also considered an ideal candidate for energy storage and clean
fuel.2 Nowadays, industrial NH3 synthesis is still predominantly
performed by the Haber–Bosch process, which is conducted
under harsh conditions with massive energy consumption and
greenhouse gas emission.3 Therefore,
a sustainable and
aInstitute of Fundamental and Frontier Sciences, University of Electronic Science and
Technology of China, Chengdu 610054, China. E-mail: zhmwang@gmail.com;
xpsun@uestc.edu.cn
Herein, we report the development of an ultrasmall Ru2P
nanoparticles–reduced graphene oxide (Ru2P–rGO) hybrid as an
efficient NRR electrocatalyst for ambient N2-to-NH3 conversion
with excellent selectivity. In 0.1 M HCl, this catalyst is capable of
achieving a large NH3 yield of 32.8 mg hÀ1 mgcat.À1 and a high FE
of 13.04% at À0.05 V vs. the reversible hydrogen electrode
(RHE), superior to those of its Ru–rGO counterpart (6.4 mg hÀ1
mgcat.À1; 1.86%). Furthermore, it also exhibits remarkable
electrochemical and structural stability. Density functional
theory (DFT) calculations reveal that Ru2P–rGO can efficiently
catalyze NH3 synthesis with a low energy barrier of 0.68 eV.
bCollege of Mechanical and Transportation Engineering, China University of
Petroleum, Beijing 102249, China
cChemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, School
of Chemistry and Chemical Engineering, China West Normal University, Nanchong
637002, Sichuan, China. E-mail: luoylcwnu@hotmail.com
dSchool of Materials Science and Engineering, Henan Normal University, Xinxiang
453007, Henan, China
† Electronic supplementary information (ESI) available: Experimental section and
supplementary gures. See DOI: 10.1039/c9ta10346e
‡ These authors contributed equally to this work.
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The X-ray diffraction (XRD) pattern of Ru2P–rGO is shown in
Fig. 1a. The characteristic diffraction peaks at 30.5ꢀ, 38.1ꢀ,
40.6ꢀ, 42.2ꢀ, 47.1ꢀ, and 76.7ꢀ are indexed to the (111), (112),
(211), (103), (020), and (403) planes of Ru2P (JCPDS no. 65-2382).
Transmission electron microscopy (TEM) analysis indicates
that Ru2P nanoparticles with ultrasmall diameters (2–4 nm) are
uniformly anchored on the surface of rGO nanosheets (Fig. 1b).
As shown in Fig. 1c, the high-resolution TEM (HRTEM) image
depicts clear crystallographic fringes of Ru2P–rGO with an
average interplanar distance of 0.357 nm, which corresponds to
the (112) lattice plane of Ru2P. Furthermore, the selected area
electron diffraction (SAED) pattern (Fig. 1d) shows three
diffraction rings, which can be separately assigned to the (112),
(103), and (403) planes of Ru2P. Fig. 1e shows the scanning TEM
(STEM) image and energy-dispersive X-ray (EDX) elemental
mapping images of Ru2P–rGO, which conrm the uniform
distribution of Ru and P species on rGO nanosheet.
Fig. 2 (a) XPS survey spectrum of Ru2P–rGO. XPS spectra of Ru2P–
rGO in the (b) C 1s and Ru 3d, (c) Ru 3p, and (d) P 2p regions.
Fig. 2a shows the X-ray photoelectron spectroscopy (XPS)
survey spectrum of Ru2P–rGO, which reveals the existence of Ru,
P, C and O elements in the hybrid with Ru2P about 38.69%. As
shown in Fig. 2b, the peaks at 284.5 and 280.6 eV are assigned to consistent with the P 2p1/2 and P 2p3/2 peaks, respectively.40
the binding energies (BEs) of Ru 3d3/2 and Ru 3d5/2,
36,37 while the Additionally, the BE at 134.1 eV is ascribed to the P–O bond,
peaks at 288.7, 285.8, and 284.9 eV are attributed to the O–C]O, which is formed by surface oxidation aer exposure to air.39 It is
C]O, and C]C bonds, respectively.38 In the Ru 3p region worth noting that the BE of Ru 3d5/2 (280.6 eV) exhibits a positive
(Fig. 2c), the peaks at 484.3 and 462.1 eV are assigned to the BEs shi compared with that of metallic Ru (279.8 eV),41 while the BE
of Ru 3p1/2 and Ru 3p3/2 peaks.39 Note that the peaks at 486.9 and of P 2p3/2 (129.7 eV) shows a negative shi compared with that of
464.3 eV can be ascribed to the oxidized RuPx species.39 In the elemental P (130.2 eV).42 These shis reveal that Ru possesses
P 2p region (Fig. 2d), the peaks located at 130.7 and 129.7 eV are a partially positive charge, while P has a partially negative charge,
implying that the electron density is transferred from Ru to P.38
Ru2P–rGO was loaded on carbon paper (Ru2P–rGO/CP) with
a loading of 0.1 mg cmÀ2 as the working electrode to evaluate
the electrocatalytic NRR performance of this electrocatalyst in
0.1 M HCl. All potentials in the electrocatalytic NRR process
were calibrated to the RHE scale. Chronoamperometry was
performed at certain potentials ranging from 0.00 to À0.20 V for
2 h to obtain enough target product (NH3) for quantitative
evaluation of the catalytic activity of Ru2P–rGO. As depicted in
Fig. 3a, the current density increases with the increasing
working potential. Aer electrolysis, NH3 and the possible by-
product (N2H4) were determined by the indophenol blue43 and
Watt–Chrisp spectrophotometry methods,44 respectively. The
corresponding calibration curves for NH3 and N2H4 are shown
+
in Fig. S1 and S2,† respectively. Notably, the NH4 calibration
solution was prepared in 0.1 M HCl. The UV-vis absorption
spectra of the electrolytes dyed with the indophenol indicator
aer electrolysis for 2 h are displayed in Fig. 3b and employed to
quantify the produced NH3. The corresponding NH3 yields and
FEs arÀe1depicted in Fig. 3c. The largest NH3 yield of 32.8 mg hÀ1
mgcat.
and FE of 13.04% are achieved at À0.05 V under
ambient conditions and are comparable to those of most noble-
metal electrocatalysts under ambient conditions in aqueous
electrolytes (Table S1†). Notably, when the applied potential is
below À0.05 V, both the NH3 yield and FE decrease, which is
probably caused by the gradually enhanced competitive
hydrogen evolution reaction.45
Fig. 1 (a) XRD pattern for Ru2P–rGO. (b) TEM (inset: the particle size
distribution of Ru2P) and (c) HRTEM image of Ru2P–rGO. (d) SAED
pattern of Ru2P–rGO. (e) STEM and EDX elemental mapping images of
Ru2P–rGO.
To further verify the superior catalytic performance of Ru2P–
rGO, a series of control experiments with catalysts such as Ru–
78 | J. Mater. Chem. A, 2020, 8, 77–81
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Na2SO4. The results indicate that the electrocatalytic NRR
performance of Ru2P–rGO in 0.1 M Na2SO4 is inferior to that in
0.1 M HCl (Fig. S7†).
The optimized geometries and Bader charge results before
and aer nitrogen molecule adsorption on Ru2P–rGO are shown
in Fig. 4a and b, respectively. P atoms have negative-mass
electrons, which cover the positive charge of Ru, indicating
net electron transfer from rGO to Ru2P. Fig. 4b shows the
detailed distribution of Bader charges aer nitrogen adsorp-
tion. Clearly, the net charge on Ru2P is slightly changed, due to
donating 0.50 eÀ to N2. Interestingly, the two N atoms show
slightly different charge values. To further understand the NRR
catalytic site and mechanism of the Ru2P–rGO, elementary steps
associated with distal and alternating possible pathways for
four potential N2-adsorption catalytic routes have been studied
by DFT calculation (calculation details in the ESI†), including
a single Ru site (1), two Ru sites (2), one Ru and one P site (3),
and a single P site (4). The (3) and (4) possible catalytic sites are
excluded aer initial screening in this calculation, due to the
fact that the P atom cannot adsorb a N2 molecule. Our calcu-
lations show that the dinitrogen molecule would indeed be well
captured by the Ru atom, because the adsorption energies are
Fig. 3 (a) Chronoamperometry curves for Ru2P–rGO/CP at different
given potentials in 0.1 M HCl. (b) Corresponding UV-vis absorption negative on both the (1) and (2) possible catalytic sites. More-
spectra of the electrolytes stained with the indophenol indicator after
NRR electrolysis at different potentials for 2 h. (c) NH3 yields and FEs of
Ru2P–rGO/CP. (d) Amount of NH3 produced using different elec-
trodes at À0.05 V after 2 h of electrolysis under ambient conditions. (e)
Recycling tests of Ru2P–rGO/CP at À0.05 V. (f) Chronoamperometry
˚
over, the N–N distance increases from 1.10 A in the free indi-
˚
˚
vidual nitrogen molecule to 1.14 A and 1.16 A on the (1) and (2)
possible catalytic sites, respectively, indicating that N2 is acti-
vated. The real challenge comes from the rst hydrogenation
curve for Ru2P–rGO/CP at À0.05 V in a N2-saturated electrolyte for step of N*2 to N2H*, which requires a Gibbs free energy of about
24 h.
1.30 and 0.68 eV on both possible pathways, respectively. The
subsequent reduction steps of route 2 are the most exothermic,
but the last desorption step (NH3* to NH3) requires energy input
rGO/CP, rGO/CP and blank CP were performed at À0.05 V for
to complete the reaction (0.47 eV) (Fig. 4c and d). But all input
energies are smaller than the rst hydrogenation step energy,
2 h. As depicted in Fig. 3d, CP and rGO/CP exhibit low catalytic
activity toward NH3 synthesis. The better catalytic activity of
Ru2P–rGO/CP than of Ru–rGO/CP may be caused by the critical
function of P in boosting the performance toward the NRR.35 To
verify the origin of the generated NH3, NRR experiments using
Ru2P–rGO/CP under a N2 atmosphere at open circuit and in Ar
gas at À0.05 V were carried out. As shown in Fig. S3,† the weak
UV-vis spectra indicate that the produced NH3 stems from N2
electroreduction. Notably, N2H4 was not detected in the nal
product (Fig. S4†), revealing that Ru2P–rGO/CP possesses good
selectivity for NH3 synthesis by the NRR process.
As one of the important parameters to evaluate the perfor-
mance of electrocatalysts, the stability should also be consid-
ered in the NRR process. Consecutive recycling tests were
performed at À0.05 V in a N2-saturated electrolyte 6 times and
the corresponding results are shown in Fig. S5.† The well
maintained NH3 yields and FEs (Fig. 3e) reveal that Ru2P–rGO/
CP is a stable electrocatalyst. In addition, the current density of
Ru2P–rGO/CP without obvious variation in 24 h of NRR elec-
trolysis also suggests its excellent long-term stability as shown
in Fig. 3f. Furthermore, the photographs of pH test papers for
the HCl aqueous solution before and aer 24 h of electrolysis
are shown in Fig. S6,† which indicate that there is almost no
change in pH in our experiment. In addition, we also analyzed
the electrocatalytic NRR performance of Ru2P–rGO/CP in 0.1 M
Fig. 4 Optimized geometries and Bader charge results (a) before and
(b) after N2 adsorption on Ru2P–rGO. Calculated energy profiles for
the NRR catalyzed by Ru2P–rGO: (c) distal and (d) alternating
mechanisms.
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Ru atoms (route 2) is the favorable reaction route for the NRR on
Ru2P–rGO, and the energy barrier of the NRR is equal to 0.68 eV
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In conclusion, Ru2P–rGO is experimentally and theoretically
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16 L. Zhang, X. Xie, H. Wang, L. Ji, Y. Zhang, H. Chen, T. Li,
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articial N2-to-NH3 conversion under ambient conditions. In 17 J. Wang, L. Yu, L. Hu, G. Chen, H. Xin and X. Feng, Nat.
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mgcat.À1 and a high FE of 13.04% at À0.05 V. In addition, it also 18 Z. Xing, W. Kong, T. Wu, H. Xie, T. Wang, Y. Luo, X. Shi,
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synthesis with a low energy barrier of 0.68 eV and that the rst
hydrogenation step is the PDS. This investigation not only
G. Wang, H. Zhang and H. Zhao, Adv. Energy Mater., 2019,
9, 1803935.
provides us with an efficient electrocatalyst toward NH3 20 Y. Zhang, H. Du, Y. Ma, L. Ji, H. Guo, Z. Tian, H. Chen,
synthesis under ambient conditions, but also offers a path to
design noble-metal-based phosphides for NRR applications.46
H. Huang, G. Cui, A. M. Asiri, F. Qu, L. Chen and X. Sun,
Nano Res., 2019, 12, 919–924.
21 H. Jin, L. Li, X. Liu, C. Tang, W. Xu, S. Chen, L. Song,
Y. Zheng and S. Qiao, Adv. Mater., 2019, 31, 1902709.
22 J. Yu, C. Li, B. Li, X. Zhu, R. Zhang, L. Ji, D. Tang, A. M. Asiri,
X. Sun, Q. Li, S. Liu and Y. Luo, Chem. Commun., 2019, 55,
6401–6404.
Conflicts of interest
There are no conicts to declare.
23 R. Zhang, H. Guo, L. Yang, Y. Wang, Z. Niu, H. Huang,
H. Chen, L. Xia, T. Li, X. Shi, X. Sun, B. Li and Q. Liu,
ChemElectroChem, 2019, 6, 1014–1018.
24 S. Zhang, C. Zhao, Y. Liu, W. Li, J. Wang, G. Wang, Y. Zhang,
H. Zhang and H. Zhao, Chem. Commun., 2019, 55, 2952–
2955.
25 H. Huang, F. Gong, Y. Wang, H. Wang, X. Wu, W. Lu,
R. Zhao, H. Chen, X. Shi, A. M. Asiri, T. Li, Q. Liu and
X. Sun, Nano Res., 2019, 12, 1093–1098.
26 S. Chen, S. Perathoner, C. Ampelli, C. Mebrahtu, D. Su and
G. Centi, Angew. Chem., Int. Ed., 2017, 56, 2699–2703.
27 G. Yu, H. Guo, W. Kong, T. Wang, Y. Luo, X. Shi, A. M. Asiri,
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 21575137).
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