Reactive Ionic Liquid Enables the Construction of 3D Rh Particles with Nanowire Subunits for Electrocatalytic Nitrogen Reduction

Article abstract of DOI:10.1002/asia.202000008

Until now, the synthesis of Rh particles with unusual three-dimensional (3D) nanostructures is still challenging. A 3D nanostructure enables fast ion/molecule transport and possesses plenty of exposed active surface, and therefore it is of great interest to construct 3D Rh particles catalysts for the N₂ reduction reaction (NRR). Herein, we proposed a reactive ionic liquid strategy for fabricating unusual 3D Rh particles with nanowires as the subunits. The ionic liquid n-octylammonium formate simultaneously worked as reaction medium, reductant and template for the successful construction of 3D Rh particles. The as-prepared 3D Rh particles demonstrated excellent activity for electrocatalytic N₂ fixation in 0.1 M KOH electrolyte under ambient conditions with a high NH₃ yield of 35.58 μg h^?1 mg_cat. ^?1 at ?0.2 V versus reversible hydrogen electrode (RHE), surpassing most of the state-of-the-art noble metal catalysts. Our reactive ionic liquid strategy thus holds great promise for the rational construction of high-performance electrocatalysts toward NRR.

Full text of DOI:10.1002/asia.202000008

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Reactive Ionic Liquid Enables Construction of 3D Rh Particles
with Nanowire Subunits for Electrocatalytic Nitrogen Reduction
Authors: Tingting Chen, Shuai Liu, Hao Ying, zhonghao li, and
Jingcheng Hao
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10.1002/asia.202000008
Chemistry - An Asian Journal
Reactive Ionic Liquid Enables Construction of 3D Rh Particles with
Nanowire Subunits for Electrocatalytic Nitrogen Reduction
Tingting Chen, Shuai Liu, Hao Ying, Zhonghao Li,* Jingcheng Hao
Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, Shandong
University, Jinan, 250100, P. R. China, E-mail: zhonghaoli@sdu.edu.cn
Abstract: Until now, the synthesis of Rh particles with unusual three-dimensional
(3D) nanostructures is still challenging. 3D nanostructure enables fast ion/molecule
transport and possesses plenty of exposed active surface, and therefore it is of great
interest to construct 3D Rh particles catalyst for N2 reduction reaction (NRR). Herein,
for the first time, we proposed a reactive ionic liquid strategy for fabricating unusual
3D Rh particles with nanowires as the subunits. The ionic liquid n-octylammonium
formate simultaneously worked as reaction medium, reductant and template for the
successful construction of 3D Rh particles. The as-prepared 3D Rh particles
demonstrated excellent activity for electrocatalytic N2 fixation in 0.1 M KOH
-1
electrolyte under ambient conditions with a high NH3 yield of 35.58 μg h-1 mgcat. at
-0.2 V versus reversible hydrogen electrode (RHE), surpassing most of the
state-of-the-art noble metal catalysts. Our reactive ionic liquid strategy thus holds
great promise for rational construction of high-performance electrocatalysts toward
NRR.
Introduction
As the main product of nitrogen (N2) fixing, ammonia (NH3) is widely used to
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produce agricultural fertilizers and nitrogen-containing industrial chemicals for a
sustainable modern society.[1] Unfortunately, converting N2 to NH3 is kinetically
complex because of the chemical inertness of N2. The high activation energy of N≡N
triple bond and low polarizability lead to harsh reaction conditions for N2 fixation.[2]
Up to now, industrial-scale procedure is mainly based on Haber-Bosch process
operated at high pressure and temperature.[3] However, the hydrogen feedstock and
energy predominantly depend on limited fossil fuels, causing unavoidable CO2
emission and plenty of energy consumption.[4] Thus, much attention has focused on
exploring sustainable and efficient alternative processes for synthesizing NH3.[5]
Electrocatalytic N2 reduction under the ambient reaction conditions has recently
become a promising candidate for N2 fixation, particularly because N2 and H2O used
as the reactants are naturally abundant, and electrical energy is from renewable energy
sources without fossil fuel consumption.[6] For the nitrogen reduction reaction (NRR),
it is critical to fabricate efficient electrocatalysts for N2 activation. Until now, noble
metal catalysts such as Ru,[7] Pd[8] and Au[9] have been largely investigated for
electrocatalytic NRR. According to the theoretical evaluation of transition metal
electrocatalysts for N2 reduction, Rh might be highly active for NRR as it is on the top
of volcano diagram.[10] Until now, various Rh nanostructures, including spherical
nanoparticles,[11] nanosheets,[12] nanocubes,[13] multipods[14] and nanowires[15] have
been synthesized by different synthetic routes. However, the synthesis of Rh particles
with 3D nanostructures is still challenging. Based on the structure, 3D nanostructure
enables fast ion/molecule transport and possesses plenty of exposed active surface,
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and therefore it is of great interest to construct 3D Rh particles catalyst for NRR.
Ionic liquids (ILs), with rich tunable chemical and physical properties, have great
application in inorganic materials community.[16] The ILs have recently been
demonstrated as powerful reaction media to fabricate various nanostructured
materials.[17] Specifically, inorganic nanomaterials with unexpected morphologies and
properties could be achieved in ILs,[18] which brings more opportunities for
constructing advanced catalysts. As a result, the ionic liquid reaction medium might
be a good solvent platform toward designing new nanostructured Rh catalyst for NRR.
However, the electrocatalyst toward NRR is rarely fabricated in ILs up to date.[19]
Herein, we, for the first time, proposed a one-step route for the synthesis of 3D
Rh particles toward electrocatalytic NRR in a reactive ionic liquid, n-octylammonium
formate. The ionic liquid acted as reductant and structure-directing agent as well as
reaction medium during the growth of 3D Rh particles with nanowire subunits. When
tested for electrocatalytic NRR activity in 0.1 M KOH electrolyte under ambient
conditions, the Rh particles achieved a high NH3 yield of 35.58 μg h-1 mgcat.-1 at -0.2 V
(vs. RHE), which outperformed most of the state-of-the-art noble metal catalysts.
Additionally, the as-prepared 3D Rh particles catalyst demonstrated good
electrochemical stability and selectivity for NRR. Therefore, our ionic liquid-based
strategy brings more opportunities for constructing advanced catalysts toward NRR.
Results and discussion
The ionic liquid n-octylammonium formate was obtained by an equimolar reaction of
formic acid and n-octylamine in an ice bath, which was a simple acid-base reaction
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(Scheme 1). The broad band between 3200 and 2100 cm-1 in Fourier transform
infrared (FTIR) spectrum (Figure S1) agreed with the stretching vibrations of
+ [20]
functional ammonium group n-C8H17NH3 ,
indicating the successful synthesis of
ionic liquid n-octylammonium formate.
Scheme 1. Synthesis of the ionic liquid.
The X-ray diffraction (XRD) pattern of Rh particles synthesized in ionic liquid
(denoted as Rh- IL) showed four peaks located at 41.05°, 47.78°, 69.87° and 84.39°
(Figure 1a), which could be assigned to (111), (200), (220) and (311) reflections of the
standard Rh crystal, respectively.[21] Figure 1b showed a scanning electron
microscopy (SEM) image of the synthesized Rh particles. It was found that the size of
the particles was 156.8 ± 43.4 nm (Figure S2). The low magnification transmission
electron microscopy (TEM) image was presented in Figure 1c. Figure 1d and 1e
displayed the close view of high magnification TEM images for the as-prepared Rh
particles. It was clear that the particle was composed of many nanowires with
diameter of 1.6 ± 0.2 nm (Figure S3), resulting in a porous 3D nanostructure.
Therefore, 3D Rh particles with nanowires as subunits were successfully synthesized
in ionic liquid n-octylammonium formate. For comparison, the control experiment
with the mixture of formic acid and water (FA/water) as solvent was performed for
the synthesis of Rh particles (denoted as Rh- FA/water). As shown in Figure S4, only
nanoparticle aggregates were obtained in the control experiment. As a result, IL
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played an important role in forming the 3D nanostructured particles with nanowire
subunits. From the FTIR of Rh-IL in Figure S1, a weak peak at 1630 cm-1
corresponding to carbonyl stretching vibrations was observed, suggesting there was a
little amount of IL adsorbed on the particles. This further demonstrated the unique 3D
nanostructured Rh particles could be attributed to the template role of ionic liquid
n-octylammonium formate solvent. The template role for various ionic liquids has
been largely reported toward nanoparticle synthesis.[22] During the growth of Rh
particles, the n-octylammonium formate simultaneously worked as solvent, reductant
and template. Rh3+ was initially reduced by formic anion of ionic liquid and then the
template role of ionic liquid solvent mediated the final formation of 3D nanostructure
with nanowire subunits. X-ray photoelectron spectroscopy (XPS) characterization was
further carried out to study the chemical state of as-prepared 3D Rh particles. As
shown in Figure 1f, the Rh 3d signal exhibited two main peaks located at 307.45 eV
and 312.27 eV, corresponding to 3d5/2 and 3d3/2 of metallic Rh, respectively.[23] The
peaks at 308.54 and 313.8 eV were corresponding to 3d5/2 and 3d3/2 of of Rh3+.[24] The
little amount of oxidation state (Rh3+) could be attributed to the partly oxidation of
metallic Rh by the oxygen in the air.
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Figure 1. Characterizations of 3D Rh particles with nanowire subunits. a) XRD
pattern; b) SEM image; c-e) low and high magnification TEM images; f) Rh 3d
high-resolution XPS spectrum.
The electrocatalytic NRR performance of 3D Rh particles was studied in an
H-type cell under 0.1 M KOH electrolyte. The cell was separated by a Nafion 211
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membrane in order to prevent NH3 from diffusing to anode to be oxidized (Figure 2).
All potentials in this work were calibrated to RHE. In order to avoid NH3 loss, an
absorber with 0.001 M H2SO4 was connected to cell. Indophenol blue method was
used to determine the concentration of NH3.[25] Similarly, Watt-Chrisp method was
used for the detection of possible N2H4 by-product.[26] Figure S5 and Figure S6
displayed the corresponding calibration curves for NH3 and N2H4.
Figure 2. Schematic illustration of electrochemical setup for NRR in 0.1 M KOH.
The NRR activity of 3D Rh particles was preliminarily investigated via linear
sweep voltammetry (LSV) measurements in Ar- and N2-saturated 0.1 M KOH
electrolyte, respectively. As displayed in Figure 3a, two curves were similar in shape
but different from current density. There were distinct higher current densities in N2
atmosphere from 0 V to -0.4 V, indicating N2 was converted to NH3 catalyzed by Rh
particles. The chronoamperometry tests were investigated at different selected
potentials in N2 atmosphere (Figure 3b). After 2 h electrolytic reaction at constant
potentials, the UV-Vis absorption spectra were obtained by adding indophenol
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indicators to corresponding cathodic electrolytes (Figure 3c). Figure S7 displayed the
UV-Vis absorption spectra of solutions obtained from absorber stained with the same
method. Compared to the solution in cathodic compartment, there was a little NH3 in
absorber. The total NH3 yields and Faradaic efficiencies (FE) were thus calculated
based on the calibration curves of NH3 (Figure 3d and Table S1). From Figure 3d, the
average NH3 yield rose as negative potential increased until −0.2 V, where the highest
NH3 yield of 35.58 μg h-1 mgcat.-1 was achieved, outperforming most electrocatalysts
reported recently under ambient conditions (Table S2). The highest Faraday efficiency
of 1.2% was obtained at 0 V, while at potential of -0.2 V, the FE became 0.52%. When
the negative potential continued to shift to -0.4 V, the FE decreased to 0.12%. The low
Faraday efficiency at more negative potential was mainly caused by the competitive
hydrogen evolution reaction (HER), in which active sites of catalyst surface were
occupied by a large number of hydrogen species in solution, thus, the adsorption of
nitrogen was hindered. Taking both NH3 yield and Faraday efficiency into
consideration, the optimal potential of Rh particles for NRR should be -0.2 V.
Additionally, Figure S8a and S8b showed no hydrazine was detected in cathodic
electrolytes and corresponding solutions obtained from absorber at given potentials,
indicating 3D Rh particles had good selectivity for NRR.
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Figure 3. Electrocatalytic performances of the 3D Rh particles. a) LSV curves in 0.1
M KOH electrolyte under N2 and Ar atmosphere; b) chronoamperometric curves in
N2-saturated 0.1 M KOH electrolyte under potentials from 0 V to -0.4 V; c) UV-Vis
absorption spectra of corresponding cathodic electrolytes after staining with
indophenol indicator; d) average NH3 yields and Faradaic efficiencies at selected
potentials.
The stability of 3D Rh catalyst was further studied by cycling experiment at -0.2
V every 2 h. As observed in Figure 4a, there were slight decay of average NH3 yield
and negligible loss of Faradaic efficiency in four-time consecutive cycles. Additional,
the current density of chronoamperometric curve was almost steady during the
long-term electrolysis for 14 h (Figure S9). Furthermore, the catalyst retained its
original crystalline phase proved by XRD pattern (Figure S10) and morphology of 3D
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particles with nanowire subunits shown in TEM image (Figure S11) after 14 h
electrolysis, further confirming the 3D Rh particles had excellent durability for NRR.
The effect of N2 flow rate on Rh particles for NRR was also studied at optimal
potential (-0.2 V). From Figure 4b, the average NH3 yields and Faradaic efficiencies
were approximately the same at various rates of 20, 30, 40, 50 and 60 cm3 min-1,
revealing the rate of electrocatalytic NRR was irrelevant to gas-solid interface.
Figure 4. a) Average NH3 yields and FEs of cycling stability tests at -0.2 V. b)
Average NH3 yields and FEs with various N2 flow rates.
To further explore the high electrocatalytic activity of 3D Rh particles with
nanowire subunits, the electrocatalytic performance of Rh-FA/water was also studied
at -0.2 V. In sharp contrast, the Rh-FA/water exhibited poor activity for the NRR with
-1
NH3 yield of 19.34 μg h-1 mgcat. and FE of 0.13% (Figure 5a). However,
Rh-FA/water also had good selectivity with no N2H4 byproduct during the electrolysis
(Figure S12). Additionally, the electrochemical surface areas (ECSAs) were evaluated
to better understand the NRR catalytic activity, using double-layer capacitances (Cdl)
due to their proportional relationship. Figure S13 presented cyclic voltammetry (CV)
curves in the potential region between 0.3 V to 0.4 V where there were no peaks of
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redox or other reactions. From Figure 5b, Rh-IL exhibited a higher Cdl value than
Rh-FA/water or blank carbon cloth. As a result, Rh-IL had the highest ECSA value
(150.25 cm2), compared with of Rh-FA/water (97.25 cm2) and blank carbon cloth
(14.75 cm2). The specific ECSA was further calculated based on the loading amount
of 3D Rh particles on the electrode. The high specific ECSA of 75.13 m2 g-1 for 3D
Rh particles further demonstrated abundant electrochemical active sites were exposed
due to the interconnected porous nanostructures.
Figure 5. Comparison of NRR activity of Rh-IL and Rh-FA/water. a) Corresponding
NH3 yields and Faradaic efficiencies at -0.2 V; b) the double-layer capacitances.
In order to identify the nitrogen origination of ammonia, LSV measurements of
blank carbon cloth were performed firstly under Ar and N2 atmosphere in 0.1 M KOH.
The current densities shown in Figure S14 were almost the same, demonstrating the
blank electrode had no NRR activity. Then some control experiments were further
tested in 0.1 M KOH electrolyte at -0.2 V, including (i) using blank carbon cloth as
working electrode; (ii) introducing N2 into cathodic compartment at open-circuit; (iii)
replacing N2 with Ar. From their UV-Vis absorption spectra presented in Figure S15,
it was clear that no NH3 was detected in above cases, implying NH3 was only
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generated from electrocatalytic N2 reduction. In addition, 15N isotopic labeling
1
experiment was further carried out at -0.2 V. As shown in Figure S16, the H NMR
+
spectra showed a doublet coupling for 15NH4 sample while a triplet coupling for
+
14NH4 NRR sample, further confirming detected NH3 was produced by
electroreduction of N2 on 3D Rh particles catalyst.
Furthermore, NRR activity of 3D Rh particles was also investigated in acidic
media and neutral media. Figure S17 and Figure S18 presented the corresponding
calibration curves of NH3 and possible N2H4 by-product. As observed in Figure 6, the
average NH3 yield can get the value of 16.65 μg h-1 mgcat.-1 with FE of 0.20% in 0.1 M
HCl electrolyte at -0.2 V, which was much lower than the value in 0.1 M KOH
electrolyte. For 0.1 M Na2SO4 electrolyte, the NH3 yield of 7.67 μg h-1 mgcat.-1 was
much lower than that in alkaline medium (0.1 M KOH), although the FE of 7.16%
was higher. The higher FE could be caused by the suppressed competitive HER in
neutral media, as proved by LSV curves of 3D Rh particles in Ar-saturated three
electrolytes in Figure S19, where the current density was much lower in 0.1 M
Na2SO4 than that in 0.1 M HCl or 0.1 M KOH at the same potential. Similar to
alkaline medium, hydrazine was absent in both acidic and neutral media (Figure S20),
demonstrating 3D Rh particles had good selectivity in different media.
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Figure 6. Average NH3 yields and FEs of 3D Rh particles in different electrolytes at
-0.2 V.
Typically, the electrocatalytic NRR mechanisms include associative and
dissociative pathways.[5b] For dissociative mechanism, hydrogenation process occurs
after N≡N triple bond is broken which requires harsh reaction conditions. According
to the different ways of adding protons, the associative mechanism is divided into
alternating and distal pathway. In the associative alternating pathway, two N atoms are
protonated in turn before final N–N bond is broken, thus by-product of N2Hy may be
produced together with NH3. In contrast, the distal N atom from catalyst surface takes
place consecutive hydrogenation preferentially in associative distal pathway. Protons
are added to proximal N atom successively after the first NH3 was formed.
Considering no by-product of N2H4 was formed during the electrolysis process, the
associative distal pathway could be suitable on 3D Rh particles as N2 → *N2 →
*NNH → *NNH2 → *NNH3 → *N + NH3 → *NH → *NH2 → *NH3 →NH3.[27] The
interconnected porous nanostructures of 3D Rh particles exposed more
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electrochemical active sites to absorb the N2 molecule due to unsaturated coordination,
and then an activated H in electrolyte preferentially combined with the distal N atom
to break N≡N triple bond, followed by consecutive addition of two other protons to
the N atom until one NH3 was generated. After that, the remaining Rh nitrido (Rh≡N)
bond will occur successive hydrogenation process to form another NH3 molecule.
Conclusions
In summary, for the first time, we have demonstrated a reactive ionic liquid-based
strategy for fabricating 3D Rh particles with nanowires as the subunits. The ionic
liquid n-octylammonium formate simultaneously worked as solvent, reductant and
template for the construction of 3D Rh particles with interconnected porous
nanostructures. The as-prepared 3D Rh particles demonstrated excellent activity for
electrocatalytic N2 fixation in 0.1 M KOH electrolyte under ambient conditions with a
-1
high NH3 yield of 35.58 μg h-1 mgcat. at -0.2 V, outperforming most of the
state-of-the-art noble metal catalysts. Our reactive ionic liquid-based strategy thus
may pave the way for rational design of highly active electrocatalysts toward NRR,
holding great interest for construction of advanced catalysts.
Experimental section
Materials and chemicals
Sodium hypochlorite solution (NaClO), n-octylamine (C8H17NH2), rhodium chloride
hydrate (RhCl3·3H2O) and sodium nitroferricyanide dihydrate (C5FeN6Na2O·2H2O)
were obtained from Shanghai Macklin Biochemical Co., Ltd. Hydrazine monohydrate
(N2H4·H 2O), para-(dimethylamino) benzaldehyde (p-C9H11NO) and salicylic acid
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(C7H6O3) were purchased from Aladdin Chemistry Co., Ltd. Nafion solution (5 wt%)
was obtained from Sigma Co., Ltd. Formic acid (HCOOH), ammonium chloride
(NH4Cl) and sodium citrate dehydrate (C6H5Na3O7·2H2O) were provided by
Sinopharm Chemical Reagent Co., Ltd. Nafion 211 membrane was provided by
Wuhan GaossUnion technology Co., Ltd. Carbon cloth was obtained from Changsha
15
Lyrun Material Co., Ltd. N2 gas was purchased from Wuhan Newradar Special Gas
Co., Ltd. Nitrogen (N2) and argon (Ar) were provided by Jinan Deyang Special gas
Co., Ltd.
Preparation of ionic liquid
The IL was synthesized by reacting equimolar formic acid and n-octylamine at 0 °C in
N2 atmosphere basing on the reported procedure.[20] Typically, 3.68 g n-octylamine
(0.0282 mol) was placed in a flask at an ice bath. Then 1.32 g formic acid (0.0282
mol) was added dropwise to the flask in N2 atmosphere under vigorous stirring to
obtain 5 g IL, n-octylammonium formate. Then the sample was placed in a vacuum
oven for future use. The melting point of obtained IL was 34 °C . [28]
Synthesis of 3D Rh particles in ionic liquid
In a typical procedure, 10 mg RhCl3∙3H2O was dissolved into 5 g ionic liquid at 90 oC
with continuous stirring for 40 min. Subsequently, the as-prepared solution was added
to a teflon-lined stainless autoclave and maintained at 180 °C for 24 h in an oven.
After that, the autoclave was cooled down to room temperature naturally. Then the
obtained sample was washed thoroughly by ethanol and water for several times.
Finally, the sample was dried in a vacuum oven for 12 h.
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Synthesis of Rh nanoparticles (Rh NPs) in formic acid/water
5 g FA/water mixture containing 1.32 g formic acid (0.0282 mol) was served as
solvent. Then the synthesis procedure of Rh NPs in FA/water was similar to the
synthesis of Rh particles in IL.
Material Characterizations
The morphologies were observed by TEM on a JEM 1400 transmission electron
microscope as well as SEM using G300 field-emission microscope. The XRD
characterization was performed using a Rigaku Dmax-rc X-ray diffractometer with
Cu Kα radiation. FT-IR spectrum was obtained from a VERTEX-70/70V FT-IR
spectrometer. XPS measurement was conducted on using an ESCALAB 250 X-ray
photoelectron spectrometer. The SHIMADZU UV-2600 UV-Vis spectrophotometer
1
was used to record absorption spectra. H NMR spectra were carried on a Bruker
Avance 400 spectrometer.
Electrochemical measurements
The Nafion 211 membrane was treated in ultrapure water and 5% H2O2 aqueous
solution for 1 h at 80 oC, respectively. Then the pretreated Nafion 211 membrane was
further treated in 0.5 M H2SO4 solution for 3 h and in ultrapure water for 6 h at 80oC.
Working electrode was obtained by the following procedure. Typically, 3 mg sample
and 5 wt% Nafion solution (20 μL) were added to 580 μL absolute ethanol. After
forming a homogeneous ink by sonication, 40 μL catalyst dispersion was taken out
and dropped on a carbon cloth (1 × 1 cm2). Then the modified carbon cloth was put in
a vacuum oven for drying. Electrochemical workstation (CHI 760E) was used to
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perform electrochemical experiment with a three-electrode system using the carbon
cloth modified with Rh particles catalyst as working electrode, a platinum foil as
counter electrode and a Ag/AgCl eletrode as reference electrode. The potentials versus
Ag/AgCl reference electrode were calibrated to RHE by the following equation:
ERHE=EAg/AgCl+ 0.059 × pH + 0.197 V
(1)
An absorber with 0.001 M H2SO4 solution was connected to the H-type cell to gather
NH3 overflowing from cathodic electrolyte. The total amount of NH3 was the sum of
that detected from cathodic compartment and absorber. All experiments were
performed at about 25 oC. The electrolyte was pre-saturated by bubbling with N2 for
30 min before each electrolysis, and then purged with a continuous N2 flow during the
whole NRR process. LSV measurements were firstly performed to probe nitrogen
electrocatalytic performance of synthesized catalysts in the range of -0.6-0.1 V at 50
mV s-1. Double-layer capacitances were determined by CV experiments in the
potential region of 0.3−0.4 V with scanning rates of 20, 30, 40, 50, 60 and 70 mV s−1.
There were no peaks of redox or other reactions in the potential window. The
capacitive currents (ΔJ = Ja - Jc) at 0.35 V of different scanning rates were made a
straight line, where Cdl is half of that linear slope. ECSA was calculated according to
the equation ECSA = Cdl / Cs, where Cs is the specific capacitance of 1cm2 atomically
smooth standard electrode (here Cs = 0.040 mF cm−2 ).[29] Specific ECSA = Cdl / Cs /
mcat, where mcat represented the mass amount of catalyst on the electrode.
Determination of Ammonia
The determination of NH3 concentration was performed by UV-Vis
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spectrophotometry using indophenol blue method. Specifically, 1 M NaOH solution
(2 mL) containing salicylic acid (5 wt%) as well as sodium citrate (5 wt%) was add to
the electrolyte (2 mL) taken from cathodic compartment or absorber, followed by
adding 0.05 M NaClO (1 mL) as well as sodium nitroferricyanide solution (1 wt%,
0.2 mL). After standing in the dark for 2 h, the solution was performed with the
absorbance measurement at λ = 655 nm on an UV-Vis spectrophotometer. The
concentration-absorbance calibration curve was made by measuring absorbance of a
serious of standard NH4Cl solutions with NH3 concentration of 0.0, 0.2, 0.4, 0.6, 0.8,
1.0 μg mL-1. The fitting curve showed a good linear relation for absorbance value
with NH3 concentration. The average NH3 yield was calculated by the following
equation:
rNH = (CNH × V) / (t × mcat.)
(2)
3
3
Faradaic efficiency was the ratio of the amount of charge used for synthesizing NH3
to the total charge consuming during whole electrolysis process. Thus, it was
calculated by the equation as follows:
FE = (3F × CNH × V) / (17 × Q)
(3)
3
where CNH is the concentration of NH3 calculated according to calibration curve, V
3
is the volume of cathodic electrolyte or the solution in absorber, t is the electrolysis
time, m is catalyst mass loading, F is Faraday constant, Q is total charge consuming
during electrolysis.
Determination of Hydrazine
The concentration of N2H4 was quantified using Watt and Chrisp method. A mixture
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of para-(dimethylamino) benzaldehyde (0.599 g), HCl (12 M, 3 mL) and ethanol (30
mL) was used as the color reagent. Typically, 5 mL electrolyte removed from cathodic
compartment or absorber was mixed with 5 mL color reagent. Then the solution was
measured by an UV-Vis spectrophotometer. The absorbance (λ = 455 nm) was used to
make concentration-absorbance calibration curve. A serious of standard N2H4
solutions with different concentration (0.0, 0.1, 0.5, 1.0, 1.5, 2.0 μg mL-1) were used
to plot the calibration curve.
15N isotope labeled experiment
15
We used N2 gas as the feeding gas to identify the origination of ammonia in 0.1 M
1
KOH electrolyte. After electrolysis, the resulting solution was performed using H
NMR spectroscopy.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work is supported by National Natural Science Foundation of China (Grant No.
21673128).
Keywords: Solvent • Ionic liquid • Metal • Catalyst • N2 fixation
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Table of Contents
A reactive ionic liquid strategy was demonstrated for fabricating unusual 3D Rh
particles toward efficient electrocatalytic N2 fixation.
24
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