G Model
CCLET-6097; No. of Pages 4
J. Wang, S. Kang, X. Zhu et al.
Chinese Chemical Letters xxx (xxxx) xxx–xxx
method. After thermal treatment in air, the as-fabricated Nb2O5-
NCF with pseudo hexagonal phase contains rich oxygen vacancy
defects, as the free-standing electrode, displaying high electro-
catalytic NRR activity with an NH3 yield rate of 2.52 Â10À10 mol
cm-2 s-1 and an FE of 9.81% at À0.4 V (vs. RHE) in 0.1 mol/L Na2SO4
solution (pH 3.2). During electrocatalytic NRR, it was found that the
crystalline phase transformation of Nb2O5-NCF from pseudo
hexagonal phase to hexagonal phase resulted in the electro-
chromism (EC) phenomenon and the decrease of NRR activity. This
is mainly due to the decrease of Vo in hexagonal phase Nb2O5. The
used Nb2O5-NCF electrode can be readily regenerated by low-
temperature thermal treatment or applying an anodic potential,
regaining high NH3 yield rate and FE with superior recycling
reproducibility.
After anodization, the surface of niobium (Nb) foil showed dark
brown and the surface colour of Nb foil became white after further
treating in air at 450 ꢁC (Fig. S1 in Supporting information). The
XRD patterns (Fig. 1a) of the thermally treated sample exhibit a
pseudo hexagonal phase (TT-phase) Nb2O5 (JCPDS No. 28-0317)
[28]. The SEM images (Figs. 1b and c) show that the surface of the
sample possesses porous structure, and a highly ordered nano-
Fig. 2. (a) LSV curves under Ar- or N2-saturated 0.1 mol/L Na2SO4 solution. (b) NH3
yield rate and FE of Nb2O5-NCF at different potentials. (c) UV–vis absorption spectra
using the indophenol blue determination method under different conditions. (d) 1
H
NMR spectra for 15NH4 in electrolyte using 15N2 as the feeding gas.
+
channel film with a thickness of ꢀ2.0
mm forms on the Nb foil
substrate (denoted as Nb2O5-NCF). The TEM image and selected
area electron diffraction (SAED) patterns of the sample (Fig. 1d)
confirm that the nanochannel walls are polycrystalline structure
and the lattice fringes with interplanar distance of 3.12 Å can be
ascribed to the (100) plane of TT-phase Nb2O5. The surface survey
XPS spectrum of Nb2O5-NCF sample was provided in Fig. S2a
(Supporting information), exhibiting Nb and O elements. The high-
resolution Nb 3d XPS spectrum (Fig. S2b in Supporting informa-
tion) indicates that the Nb 3d peaks at 209.6 and 206.9 eV can be
assigned to Nb2O5 [31,32].
In this work, the fabricated Nb2O5-NCF was directly used as the
cathodic electrode for NRR evaluation in 0.1 mol/L Na2SO4 solution
(pH 3.2) using an H-type electrochemical cell. The linear sweep
voltammograms (LSV) results (Fig. 2a) show that the cathodic
current density of Nb2O5-NCF electrode in N2-saturated electrolyte
is obviously larger than that obtained in Ar-saturated electrolyte
when increasing applied potential from À0.3 V to À0.9 V,
confirming good NRR activity of Nb2O5-NCF. Next, we investigated
the influence of applied potential on the NRR performance of
Nb2O5-NCF. During electrocatalytic NRR, the produced NH3 and/or
N2H4 were determined by the indophenol blue [33] and Watt/
Chrisp method [34], respectively. In this work, only NH3 product
can be detected (Fig. S3 in Supporting information) while N2H4 is
undetectable (Fig. S4 in Supporting information). Fig. 2b shows the
NH3 yield rate and Faradaic efficiency (FE) of Nb2O5-NCF at
different potentials, and corresponding chronoamperometric
curves at each potential are shown in Fig. S5 (Supporting
information). According to the UV–vis absorption spectra analysis
(Fig. S3a) of NRR samples, the largest NH3 yield rate can reach
2.52 Â10À10 mol cm-2 s-1 with an FE of 9.81% at À0.4 V (vs. RHE)
with the NRR time of 60 min (Fig. 2b). When the applied potential
is more negative than À0.4 V (vs. RHE), the NRR performance of
Nb2O5-NCF obviously decreases. This is mainly due to the
competitive hydrogen evolution reaction (HER) concurrently
happened on Nb2O5-NCF at more negative potentials [27]. To
eliminate the environmental interference on the NH3 yielded
during NRR, several control experiments were performed. The
results (Fig. 2c) show that no NH3 product can be detected in
0.1 mol/L Na2SO4 solution without catalyst, with Nb2O5-NCF but
without applied potential, or Ar-saturated electrolyte with Nb2O5-
NCF at À0.4 V (vs. RHE), indicating no environmental interference
on the produced NH3 by NRR process. In contrast, the UV–vis
absorption spectra show stronger absorption peak at ꢀ695 nm
ascribed to the yielded NH3 determined by the indophenol blue
method. To further confirm this, the isotopic labelling experiments
using 15N2 as the feeding gas was also conducted. The 1H nuclear
magnetic resonance (NMR) spectra (Fig. 2d) reveal that the
chemical shift of doublet coupling can be observed for the NRR
sample, due to 15N in 15NH4+, suggesting the formed NH3 indeed
originated from the Nb2O5-NCF catalyzed NRR process.
Subsequently, we evaluated the durability of Nb2O5-NCF
electrode for NRR. It was found that the yielded NH3 amount
during NRR initially rapidly increased. After 20 min, the increase
trend of NH3 yield became slow (Fig. S6 in Supporting informa-
tion). This results in an initial increase then decrease of NH3 yield
rate, and the largest NH3 yield rate was obtained to be 3.38 Â 10À10
mol cm-2 s-1 with an FE of 11.2% at -0.4 V (vs. RHE) (Fig. 3a) when
NRR time of 20 min. The above experimental results suggested that
the NRR activity of Nb2O5-NCF decreased after 20 min of reaction,
which deserves a further investigation. During electrocatalytic
NRR, the Nb2O5 electrode was found to transform to NbO2 species
from XRD results (Fig. 3b), and electrochromism (EC) phenomenon
was also observed, namely, the electrode surface color changed
from white to dark with NRR time (Fig. 3c). This is mainly
Fig.1. (a) XRD pattern of Nb2O5-NCF. (b) Surface and (c) cross-sectional SEM images
of Nb2O5-NCF. (d) TEM image and selected area electron diffraction (SAED) patterns
for Nb2O5-NCF.
2