A shape-memory V3O7·H2O electrocatalyst for foldable N2fixation

Article abstract of DOI:10.1039/d0ta10510d

Shape-memory materials can retain their functionalities during mechanical deformation, and thus hold great promise for utilizations in versatile, wearable and portable systems. Here, we report a shape-memory V3O7·H2O monolith that works as a new emerging foldable electrocatalyst for nitrogen reduction reaction (NRR). Remarkably, the electrocatalyst has been designed according to our unexpected observation that metal oxides, commonly considered as a class of tough and brittle materials, can show shape-memory properties after anisotropic alignment of their microstructures via an ice-Templated freeze-casting method. We demonstrate the V3O7·H2O electrocatalyst for promoting the NRR characteristic of excellent performances, including an ammonia yield rate of 36.42 μg h-1 mg-1, faradaic efficiency of 14.20% at-0.55 V (vs. RHE), and operation for seven cycles without activity or structural degradation. Remarkably, NRR faradaic efficiencies do not change during electrode deformations, while ammonia yield rates only show a slight decline even after significant foldings. We further elucidate through density function theory that NRR proceeds at vanadium active sites of V3O7·H2O via the associative distal pathway with ?N2 + H+ → ?N2H as the rate-limiting step. This journal is

Full text of DOI:10.1039/d0ta10510d

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Shape-memory V3O7·H2O Electrocatalyst for Foldable N2 Fixation
Received 00th January 20xx,
Accepted 00th January 20xx
Yuntong Sun, Shan Ding, Chen Zhang, Jingjing Duan,* Sheng Chen*
Shape-memory materials can retain their functionalities during mechanical deformation, thus hold great
promise for utilizations in versatile wearable and portable systems. Here we report a shape-memory
V3O7·H2O monolith to works as a new emerging foldable electrocatalyst for nitrogen reduction reaction
(NRR). Remarkably, the electrocatalyst has been designed according to our unexpected observation that
metal oxides, commonly considered as a class of tough and brittle materials, can show shape-memory
properties after anisotropic alignment of their microstructures by using an ice-templated freeze-casting
method. We demonstrate the V3O7·H2O electrocatalyst for promoting NRR characteristic of excellent
performances, including an ammonia yield rate of 36.42 μg h-1 mg-1, Faradic efficiency of 14.20% at -0.55 V
(vs. RHE), and operation for seven cycles without activity or structural degradation. Remarkably, the NRR
Faradic efficiencies do not change during electrode deformations, while ammonia yield rates only show a
slight decline even after significant foldings. We further elucidate through density function theory that NRR
proceeds at vanadium active sites of V3O7·H2O via associative distal pathway with *N2 + H+ → *N2H as the
rate-limiting step.
DOI: 10.1039/x0xx00000x
electrofixation is cathodic nitrogen reduction reaction (NRR, N2
+
6H2O + 6e → 2NH3 + 6OH-), which takes place at complex
N2/electrolyte/electrode interfaces involving a six-electron coupled
with proton transferring process.8 Consequently, various NRR
electrocatalysts, both molecular and heterogeneous, have been
designed to enhance the reaction efficiency. Molecular catalysts
possess the advantage of highly exposed active sites,9 but tend to
de-activate during electrochemical cycling. In contrast,
heterogeneous catalysts (such as metals10 and their oxides11-13 and
heteroatom-doped carbons14) are more structurally stable, and can
easily interface with electrode systems. Among heterogeneous
catalysts, metal oxides (like vanadium oxide8) are promising
candidates for NRR because of their suitable N2 protonation and
ammonia desorption capability.11-12 Several strategies have been
reported to enhance their properties like morphology control,12
downsizing,11 and hybridization with secondary elements.13
Introduction
Known as one of the most critical industrial chemicals, ammonia
(NH3) has attracted increasing attention as a viable energy carrier
with intensive energy densities and high hydrogen percentages.1-2
Generally, ammonia has been produced by the hydrogenation of
nitrogen gas (N2) from atmosphere, which is extremely difficult
because of the inherently tough N≡N triple bondings.3-4 In industry,
this energy barrier has been overcome by Haber-Bosch process
o
under harsh reaction conditions of high-temperature (350-550 C)
and pressure (150-350 atm), which has consumed more than 1% of
annual energy supply in the world.2, 4 Moreover, the Haber-Bosch
process has belched out millions of tonnes of carbon dioxide into
atmosphere every year. Consequently, to alleviate these problems,
it is of great importance to develop alternative pathways for N2
fixation under mild and eco-friendly conditions.
Electrochemical N2 fixation is a viable technique to produce
ammonia, as it can work under ambient temperature and pressure
without involving fossil fuels.5-7 The primary process of nitrogen
Recently, flexible energy systems have received enormous
interest because of their remarkable features such as lightweight,
small-size unit, and shape conformability,15-18 which constitute
promising candidates for the utilizations in versatile foldable,
portable and wearable devices. However, the currently reported
nitrogen fixation systems are generally present in the form of bulky
a. Key Laboratory for Soft Chemistry and Functional Materials (Ministry of
Education), School of Chemical Engineering, School of Energy and Power
Engineering, Nanjing University of Science and Technology, Nanjing, 210094,
China
and heavy architectures, indicating that they are far behind the
19
requirement of flexibility.1,
Traditional NRR electrodes are
prepared by the deposition of powder- or thin film-form
electrocatalysts, such as metal oxides11-13 and others,14onto rigid
substrates (like glassy carbon19 and silicon wafer1), resulting in bulk
E-mail: jingjing.duan@njust.edu.cn (J.D.); sheng.chen@njust.edu.cn (S.C.)
Electronic
Supplementary
Information
(ESI)
available:
See
DOI: 10.1039/x0xx00000x
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and fragile electrochemical devices. Lately, several soft current Consequently, V3O7·H2O was forced to align along the direction of
View Article Online
collectors (such as carbon-fiber paper20-21 and metal foams22-23
)
ice solidification movement, resulting in a highly ordered and
DOI: 10.1039/D0TA10510D
have been employed to make NRR catalyst electrodes by taking anisotropic 3D structure (Fig. S8c). During this process, we find two
advantages of their excellent properties of interconnected porous important synthetic parameters to influence the morphology,
networks, high electrical conductivity, and mechanical stability. including dispersion concentrations and freezing methods. Beyond
Integrating these soft substrates with catalytic active species has the concentration range of 0.75-4.8 mg mL-1, V3O7·H2O cannot form
been achieved by the synthetic strategies of solution-casting or a 3D monolithic architecture (Fig. S3). On the other hand, only
direct growth.12-15 However, owning to the intrinsically fragile disordered V3O7·H2O monoliths or powders were obtained if the
nature of powders19 or thin-films,1 the flexible behaviors of as- freezing method was changed into quick liquid nitrogen freezing
resultant NRR electrodes are still compromised, leading to (rate: 5 oC min-1 mL-1), freezing by a refrigerator (rate: 0.014 oC min-1
considerable activity decay in the process of electrode mL-1), or directly dry in air (Figs. S8a,b).
deformations.
Shape-memory materials represent a category of functional
materials renowned for their excellent mechanical flexibility, which
can maintain the original functionalities after being quasi-plastically
distorted.24-25 The corresponding phenomenon is known to be
shape-memory effect, which is originated from different
mechanisms with respect to the recently reported two subclass of
members, i.e., superelasticity for shape memory alloys and visco-
elasticity for shape memory polymers.24 Remarkably, shape-
memory materials have also exhibited excellent structural features
such as rich meso/macropores and hierarchically arranged
architectures, thus could become potential candidates for flexible
and foldable electrocatalysts. Currently, no shape-memory
materials have been explored for electrocatalytic reactions (such as
NRR) yet.
In this work, the initial plan is to manipulate V3O7·H2O, commonly
considered as a tough material, into an ordered 3D architecture;
but we unexpectedly see the resultant 3D monolith characteristic of
typical shape memory behaviour, i.e., it can quickly restore the
original shape after significant mechanical deformation. To our best
knowledge, this is the first observation of shape-memory effect in
oxide materials. Consequently, we make use of this new emerging
property of V3O7·H2O to fabricate a foldable NRR electrocatalyst,
which has delivered excellent performances both with and without
electrode deformations. We further elucidate the NRR mechanisms
for V3O7·H2O electrocatalyst by theoretical simulations.
Fig. 1 Synthesis and morphology of V3O7·H2O monolith. a) Schematic illustration
of two-step synthetic procedure (scale bar: 100 μm). b-d) SEM images of V3O7·H2O
monolith (scale bars are 200 μm, 1 μm, and 200 nm for b, c and d. The inset of (b)
is an optical image. e, f) SEM element mapping of O and V (scale bar, 200 nm). g)
AFM image and corresponding height profile. h-j) TEM images (scale bars are 200,
100, and 5 nm for h, i, and j).
RESULTS AND DISCUSSIONS
Next, the morphology of V3O7·H2O monolith is characterized by
optical image (inset of Figs. 1b,S3), which show a dark green
macroscopic cylinder architecture with a mass density of 4.8 mg cm-
3. The 3D monolith is composed of fluffy and ordered arrays
composed of interwoven nanobelts according to scanning electron
microscope images (SEM, Figs. 1b-d,2f-h). These nanobelts have a
typical size of ~100 nm, thickness of ~15 nm, and length over
several micrometers with very smooth surface and good structural
flexibility (Figs. 1g-i). These features are different from the powder
counterpart obtained from dry in air (Figs. 1a,S1,S2). High-
resolution TEM (HRTEM) image shows well-resolved lattice fringes
with a regular spacing of 0.467 nm, corresponding to the space
between adjacent (020) crystal lines (Fig. 1j). The elements of V, O
are uniformly distributed throughout the nanobelts as revealed by
element mapping and energy dispersive spectroscopy (EDS, Figs.
1e,f,S9).
Synthesis and morphology characterizations of V3O7·H2O
monolith. Different from traditional methods,26-27 we propose a
two-step procedure for preparing shape-memory V3O7·H2O
monolith that involves the crystal growth of V3O7·H2O nanobelts
followed by an ice-templated assembly assisted by freeze casting
(Figs. 1a,S1-S3). In the first step, vanadium acetylacetonate
(VO(acac)2) has been used as the feeding source to hydrolyze in
aqueous solution under hydrothermal treatment. The
corresponding reaction intermediates have been taken out and
examined by Fourier Translation Infrared Spectroscopy (FT-IR, Fig.
S4) and Raman spectroscopy (Fig. S5), which illustrate a typical
“nuclei and crystal growth” mechanism.
In the second step, V3O7·H2O aqueous solution was prepared in a
tube (Figs. S6,S7), and freezed and thawed under different
o
conditions. When liquid nitrogen (-35 C) is slowly poured into the
bottom of the tube, water is frozen into ice at a suitable
temperature gradient from bottom to up (~0.5 oC min-1 mL-1).28-29
2 | J. Name., 2012, 00, 1-3
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Structure and mechanical property characterizations of V3O7·H2O anisotropic architecture of V O ·H O nanobelt during compression
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DOI: 10.1039/D0TA10510D
monolith. The structure of V3O7·H2O monolith is firstly cycles. Further, V3O7·H2O monolith could exhibit excellent cycling
characterized by X-ray diffraction (XRD), showing all the performance with little structural degradation after ten cycles (Fig.
characteristic peaks corresponding to orthorhombic crystal phases 2u). Even if it has been deposited onto a carbon fiber substrate to
of V3O7·H2O (PDF# 85-2401, space group: Pnam, a = 16.9298 Å, b = making electrode (mass loading: 0.2 mg cm-2), it still maintains the
9.3598 Å, c = 3.6443 Å, Fig. 2a).26, 30-31 This result is consistent to structural integrity after applying external forces during electrode
other analytic methods including FT-IR (Fig. 2b), Raman spectra (Fig. deformation (Fig. 2v). All of these results indicate V3O7·H2O
S10), and X-ray photoemission spectroscopy (XPS, Fig. S11), all of monolith is a favorable candidate for making flexible electrodes to
which indicate the material contains V and O components without promote electrocatalytic reactions.
impurities. In addition, XPS V2p is deconvoluted into V2p3/2 at 515.6
and 516.8 eV, and V2p1/2 at 522.6 and 523.9 eV, which correspond
to V4+ and V5+ of V3O7·H2O phase (Fig. 2c). The XPS O1s spectrum at
529.6, 530.9 and 532 eV are attributed to O-V5+, HO-V and O-V4+ of
V3O7·H2O, respectively (Fig. 2d). Further, a remarkable structural
advantage of V3O7·H2O monolith is its high BET surface area of 22.3
m2 g-1 and a pore volume of 0.077 cm3 g-1 (Fig. S12), which
outperforms its powder counterpart (14.3 m2 g-1 and 0.035 cm3 g-1).
The result is consistent with the phenomenon shown by SEM (Figs.
1a,b,2f-h,S1).
Fig. 3 Electrocatalytic NRR performance tested by H-cell of V3O7·H2O monolith.
a) Linear sweep voltammetry curves recorded at the scan rate of 5 mV s-1 in N2-
and Ar-saturated 0.1 M Na2SO4 electrolytes. The inset of (a) shows Faradaic
Fig.
2 Structure and mechanical property characterizations of V3O7·H2O
efficiency (FE) and NH3 yield rates as comparison to powder counterpart at -0.55
V (vs. RHE) in N2 atmosphere. b) Chronoamperometry curves at different applied
potentials (vs. RHE). c) FE and NH3 yield rates at different applied potentials. d)
500 MHz 1H NMR spectra analyses at -0.55 V (vs. RHE for 20 hrs) by using 14N2 or
15N2 as N2 feeding sources. e) FE and NH3 yields for recycling test at -0.55 V (vs.
RHE). f) stability test for 20 hrs at -0.55 V (vs. RHE). The inset of (f) shows the LSV
plots before and after stability test.
monolith. a-e) Structural characterizations: a) XRD patterns, showing a similar
cystal structure to V3O7 · H2O powder counterpart. b) FT-IR spectra. c, d) XPS
spectra of V2p and O1s. e-v) mechanical properties: e-t) Real-time optical and
SEM images for a typical compression cycle from 0 to 3 s (The scale bars are 500
μm for f, j, n, r; 300 μm for g, k, o, s; 100 μm for h, l, p, t). u) Repeated
compression cycles up to 50% strain. v) SEM image of V3O7·H2O monolith
electrode after bending (scale bar, 300 μm).
Electrocatalytic nitrogen reduction performances tested by H-cell.
Electrocatalytic NRR was performed by using V3O7·H2O monolith
decorated on carbon paper substrate as working electrode in 0.1 M
Na2SO4 aqueous electrolyte by H-cell. The electrolytic resistance of
overall system is only 6.5 Ω, so all data are presented without iR
compensation. Firstly, linear sweep voltammetry (LSV) is tested in
N2-saturated electrolytes (Fig. 3a), which shows a different current
density as comparison to Ar-saturated counterpart, thus indicating
the occurrence of NRR in electrochemical system.20-21 To quantify
NRR activities, N2 electrolysis was carried out for 2 hrs from -0.35 to
-0.75 V (vs. RHE, Fig. 2b), with their products determined by
indophenol blue and Watt-Chrisp methods (Figs. S13-S21).
Consequently, the maximal NH3 yield rate and Faradic efficiency of
V3O7·H2O monolith are 12.96±0.3 μg h-1 mg-1 and 6.03±0.6% at -0.55
The shape-memory property for V3O7·H2O monolith has been
examined by optical and SEM images in a typical compression and
unloading cycle (mass density is 4.80 mg cm-3, Figs. 2e-t). As
indicted by direction arrows: V3O7·H2O monolith is in fully relaxed
initial state (0 s), applied external forces to 50% strain (1 s), restore
its structure after release external forces (2 s), and complete relax
process (3 s). The corresponding microstructures of shape change
have been further examined by SEM (Figs. 2f-h,j-l,n-p,r-t), which
clearly displays the anisotropic layered structure of V3O7·H2O
nanobelts monolith. The adjacent layer spacing change from 125 to
32 μm after applying 50% strain to the architecture, which restore
to 88 and 125 μm after the external force release within 2 and 3 s.
The above phenomenon confirms the full recovery of ordered and
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V (vs. RHE, Figs. 3b,c,S22). The activities of V3O7·H2O monolith are
Consequently, as shown in Fig. 4b,S30, the maximal NH3 yield
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DOI: 10.1039/D0TA10510D
4.05 and 1.37 times higher than its powder counterpart (3.82 μg h-1 rate and Faradic efficiency for V3O7·H2O monolith in flow-cells are
mg-1 and 4.39%, the inset of Figs. 3a,S23,S24). In addition, the 36.42 μg h-1 mg-1 and 14.20% at -0.55 V (vs. RHE). These values are
charge transfer resistances (Rct) of V3O7·H2O monolith and powder significantly exceed corresponding performances obtained from H-
electrodes have been evaluated by electrochemical impedance type electrochemical cell with ammonia yield of 12.96 μg h-1 mg-1
spectroscopy (EIS). As shown in Fig. S25, V3O7·H2O monolith shows and Faradaic efficiency of 6.03% (Fig. 3c). In addition, V3O7·H2O
a semicircle comparable to V3O7·H2O powder in the Nyquist powder in flow-cells are also characteristic of increased NH3 yield
diagram, indicating the similar Rct for these electrodes (3.1 vs. 2.5 rate of 16.52 μg h-1 mg-1 and Faradic efficiency of 8.11% at -0.55 V
Ω). However, the electrochemical specific surface area (ECSA) of (vs. RHE, Figs. 4b,S31). As comparison to recently reported NRR
V3O7·H2O monolith is 87.14 cm-2, which is twice of V3O7·H2O powder catalysts, our V3O7·H2O monolith is one of the most active
(42.57 cm-2, Fig. S26). The above results indicate the remarkable electrocatalysts in the literature (Table S2).
structural advantages of 3D monolithic architecture.
Importantly, the electrochemical testing in flow-cell has
Next, the source of N2 during NRR was elucidated by 15N2 isotope maintained excellent stability with little activity degradation in NH3
experiments at the applied potential of -0.55 V (vs. RHE). As shown yield rates and Faradaic efficiencies after seven cycles (Fig. 4c). This
in Fig. 2d, 500 MHz nuclear magnetic resonance (NMR) analysis result is consistent to the analyses of SEM, element mapping, XRD,
shows a distinct double peak signal from 15NH3 rather than a triplet FT-IR and XPS that shows negligible changes of morphology,
from 14NH3, thus indicate ammonia produced from nitrogen rather structure and chemical state for the electrode after cycling test (Fig.
than external contamination. In addition, the electrolytic system S32-36). Further, the isotope labelling experiment with 15N2 feeding
has shown excellent selectivity and durability, which produce little gas show obvious characteristic doublet signals for 15NH4+ (the inset
N2H4 by-product as determined by Watt-Chrisp methods (Fig. S27). of Fig. 4c), which thus verified NRR process sourced from N2 gas
The NH3 yield rate and FE of V3O7·H2O monolith-based electrode did precursor rather than external pollutions.
not change significantly for five electrochemical cycling tests at -
0.55V (vs. RHE, Figs. 2e,S28) and the electrode can work
continuously for at least 20 hrs without performance degradation
(Figs. 2f).
Fig.
4 Electrocatalytic NRR performance tested in flow-cells for V3O7·H2O
monolith. a) Schematic illustration of a flow cell system. b) the optimal NH3 yield
rates and FEs of V3O7·H2O monolith and powder in flow-cells as comparison to H-
cells at -0.55 V (vs. RHE) in N2 atmosphere. c) Chronoamperometry curves of
V3O7·H2O monolith in flow-cells at -0.55 V (vs. RHE) for seven cycles; inset of (c)
are NH3 yield rates, FEs for cycling test at -0.55 V (vs. RHE) and 1H NMR spectra
analyses at -0.55 V (vs. RHE) by using 15N2 as gas feeding source.
Fig. 5 NRR performance of V3O7·H2O monolith and powder during electrode
deformations. a-e) V3O7·H2O monolith: a) Chronoamperometry curves for
different electrode foldings at -0.55 V (vs. RHE). The inset of (a) shows the optical
images of electrode foldings. b-e) The corresponding utraviolet (UV) absorption
spectra, total charge analyses, NH3 yield rates and FEs at -0.55 V (vs. RHE),
respectively. f,g) V3O7·H2O powder: total charge analyses and NH3 yield rates at -
0.55 V (vs. RHE) for different electrode foldings. h) Comparison total Charge
analyses and NH3 yield rates for different electrode foldings.
Electrocatalytic nitrogen reduction performances in flow-cells. To
further improve the NRR performances, we adopted a newly
designed flow-cell (Figs. 4a,S29). We use a gas-diffusion layer with
smart components that can promote the reaction kinetics at three-
phase boundary. Also, the novel compact cell design can decrease
the transport resistance. Further, electrolytes have been cycled by
using an external pump, which can bring large quantities of N2
feedstock and take away the NH3 products. According to the theory
from physical chemistry, the reaction equilibrium can be shifted
right (2N2 + 6H2O  4NH3 + 3O2); that is, more ammonia can be
produced.
Shape memory V3O7·H2O monolith for foldable NRR. To see if
V3O7·H2O electrode can satisfy the flexibility demands, its NRR
performance was measured with different electrode foldings and
compared with V3O7·H2O powder sample. Fig. 5a shows the 2 hr-
chronometric response plots of the electrodes with one to three
folding, which demonstrated current decrease of only 5.7%, 16.1%,
and 32.9%, respectively. Correspondingly, the ammonia yield rate
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declines slightly from 12.96 (no folded) to 12.06 (one fold), 11.21 begins with moderate physisorption of N2 gas mVoileewcAurlteicsle Oonnlitnoe
DOI: 10.1039/D0TA10510D
(two folds) and 8.51 μg h-1 mg-1 (three folds, Figs. 5b-d). In great electrode surface with a small Gibbs free energy of 0.21 eV, which
contrast, the powder V3O7·H2O-based electrodes can only maintain enables N2 activation for subsequent hydrogenation steps.
25% of NH3 yield rate after three folding, which shows a much Secondly, the Gibbs free energy for the first hydrogenation step of
worse activities as comparison to shape-memory V3O7·H2O NRR (i.e., *NN + H+ → *NNH) is 2.28 eV, followed by the second
monolith (Figs. 5f-h,S37,S38). In addition, the FEs of V3O7·H2O proton-electron pair transfer proceeding on an un-hydrogenated
monolith and powder did not change with the number of electrode nitrogen atom, which leads to the formation of *NHNH
foldings (Fig. 5e,S39), which indicate that the intrinsic activity of the intermediate with a Gibbs free energy of 0.29 eV. Next, the relative
electrode material was not reduced. This result is consistent to SEM Gibbs free energies for the third and fourth proton-electron pair
image of V3O7·H2O monolith-based electrode that can still maintain transfer gains to produce *NH and *NH2 with the Gibbs free energy
structural integrity after applying external forces during of 0.97 and 0.08 eV, respectively. Consequently, the first ammonia
deformation (Fig. 2w). The afore-mentioned results suggest that (NH3) molecule has formed after the fifth proton-electron pair
V3O7·H2O monolith possesses excellent physical flexibility, durability, transfer step with another hydrogenation on *NNH2 moiety with a
and mechanical integrity towards folding treatments during Gibbs free energy of 1.57 eV. The electrocatalytic reaction is finally
electrocatalytic processes.
closed by the release of second NH3 molecule with an energy input
of -0.81 eV.
Further, the significantly enhanced NRR performance of V3O7·H2O
electrode is attributable to its remarkable structural characteristics.
Firstly, V3O7·H2O has one-dimensional nanobelt structure with a
typical thickness of only ~15 nm and length up to several
micrometers, which has ordered aligned into a 3D anisotropic
architecture driven by ice-templated method. Therefore, the
aggregation of 1D nanobelts can be effective prevented, and the 3D
monolith can exhibit a high specific surface area of 22.3 m2 g-1 as
comparison to its powder counterpart (14.3 m2 g-1, Fig. S12). This
result is consistent to intrinsic activities of V3O7·H2O monolith and
powder normalized according to ECSA as shown in Supplementary
Figs. S26. The intrinsic NH3 yield rate of V3O7·H2O monolith is 190.63
μg h-1 m-2, which is roughly 1.5 times of that for V3O7·H2O powder
(125.46 μg h-1 m-2, Fig. S44). Therefore, V3O7·H2O monolith can
provide a large number of accessible active sites as catalytic centers
for promoting NRR. Secondly, the ordered assembly of 1D V3O7·H2O
nanobelts has generated hierarchical porosity that can increase the
transport of electrolyte and gas diffusion within the electrode. We
can see from the SEM images (Figs. 1,2) that V3O7·H2O nanobelts
have formed ultra-large pore over tens of micrometers between the
ordered nanobelts array, in addition to a wide range of pores
around tens to hundreds of nanometers between adjacent
nanobelts. Also, N2 isotherm displays the formation of mesoporous
in the range of 5-50 nm and centered at 28 nm (Fig. S12), which
shows a porous volume of 0.077 m2 g-1 for V3O7·H2O monolith as
comparison to 0.033 m2 g-1 for the powder counterpart.
Consequently, V3O7·H2O monolith has delivered a much larger NH3
yield rate (36.42 vs. 16.52 μg h-1 mg-1) and FE (14.20% vs. 8.11%)
than the powder counterpart.
Importantly, the distinguished hierarchical structure of V3O7·H2O
monolith gives rise to high shape-memory properties. According to
the literature,26 metal oxides are conventionally considered as a
class of tough materials that are prone to fail in a brittle manner
when subjected to macroscopic deformation. When metal oxide
monoliths are severely compressed, the inter-sheet van der Waals
adhesion would overwhelm the elastic energy stored, preventing
elastic recovery. Indeed, shape memory has not been observed in
randomly structured V3O7·H2O architecture or powders. In great
contrast, our proposed ice-templated method can enable the
capability of ordered and anisotropic alignment of metal oxide
nanostructures, and introduce a new emerging property of 3D
Fig. 6 NRR Mechanism study for V3O7·H2O monolith. a, b) Optimized structures
for hydrogen atom at V3O7·H2O monolith and corresponding Gibbs free energy
change. c) Optimized structures for associative alternative pathway for V3O7·H2O
monolith. d) Gibbs free energy changes for three possible NRR pathways.
Mechanism study. To elucidate the active sites for NRR at V3O7·H2O
electrode, computation calculations were performed by using
DFT.32-37 According to XRD and XPS analyses (Figs. 2a,c,d), a
structural model is built by cleaving V3O7·H2O surface across (200)
crystal direction that form a slab with (2×2) unit cell based on Pnam
space group. Generally, NRR takes place via associative-alternate,
38-39
associative-distal, or dissociative pathways,4,
while our
calculations suggest that V3O7·H2O promote NRR favorably through
associative-alternate pathway with the lowest energy barrier of
2.28 eV than other pathways (Figs. 6d,S40-S43). Further, the
reaction active sites have been identified to be vanadium atoms
characteristic of a smaller Gibbs free energy as comparison to
oxygen atoms inside V3O7·H2O (2.28 vs. 3.86 eV, Fig. S43). This
result is further confirmed by the calculated Gibbs free energy of
hydrogen adsorption (Figs. 6a,b), which reveals the vanadium active
sites have a favorable hydrogen adsorption capability than oxygen
active sites (-3.26 vs. -4.46 eV) thus can access to more hydrogen
from water to promote NRR (N2 + 6H2O + 6e → 2NH3 + 6OH-).
Next, each intermediate step of associative-alternate pathway at
V3O7·H2O electrode has been discussed (Fig. 6d). The NRR process
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Journal of Materials Chemistry A
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Journal of Materials Chemistry A
6. S. Z. Andersen, V. Čolić, S. Yang, J. A. Schwalbe, A. C.
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alloys owning to “superelasticity” mechanism.18 Consequently,
metal oxides-based electrodes can become long-term compressible
and foldable, and thus can be more conveniently applied to actual
electrocatalytic nitrogen fixation and other energy conversion
processes. Therefore, this work has opened up enormously
opportunities for applying versatile metal oxides in the form of
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Conclusions
In conclusion, we have demonstrated a shape-memory V3O7·H2O
monolith characteristic of ordered and anisotropic microstructure
through an ice-templated freeze-casting method. With low mass
density, superelasticity, high specific surface, and hierarchical
porosity all combined together, our V3O7·H2O material has been
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delivered good NRR performances with high NH3 yield rates and FE,
and flexibility for electrocatalytic reactions. We would expect that
such exceptional metal oxides-based materials open up enormous
opportunities for a broad range of technological applications. In
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and applications of metal oxides in structurally adaptive and 3D
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Conflicts of interest
The authors declare no competing financial interests.
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Acknowledgements
The authors would like to acknowledge the financial support
from Jiangsu Natural Science Foundation (No. BK20190460),
Jiangsu innovative/entrepreneurial talent program, and the
Research Innovation Program for College Graduates of Jiangsu
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