A nickel-borate nanoarray: a highly active 3D oxygen-evolving catalyst electrode operating in near-neutral water

Article abstract of DOI:10.1039/c6cc09893b

The exploration of high-performance and cost-effective water oxidation catalysts operating under mild conditions is still urgent and challenging. In this communication, a nickel-borate nanoarray supported on carbon cloth (Ni-Bi/CC) has been fabricated through oxidative polarization of a nickel oxide nanoarray on CC (NiO/CC) in a borate electrolyte (pH 9.2). As a 3D electrode, this Ni-Bi/CC exhibits superior catalytic activity for water oxidation in 0.1 M potassium borate (K-Bi) solution, yielding a geometrical catalytic current density of 10 mA cm^-2 at an overpotential of 470 mV. Notably, this electrode also demonstrates outstanding long-term electrochemical durability for 25 h with 100% Faradaic efficiency.

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A nickel-borate nanoarray: a highly active 3D
oxygen-evolving catalyst electrode operating
Cite this:DOI: 10.1039/c6cc09893b
in near-neutral water†
Received 13th December 2016,
Accepted 20th February 2017
Xuqiang Ji,ab Liang Cui,ab Danni Liu,a Shuai Hao,a Jingquan Liu,b Fengli Qu,*c
Yongjun Ma,d Gu Du,e Abdullah M. Asirif and Xuping Sun*a
DOI: 10.1039/c6cc09893b
rsc.li/chemcomm
The exploration of high-performance and cost-effective water environment greatly hinder their large-scale application. It is
oxidation catalysts operating under mild conditions is still urgent therefore highly desirable to develop noble-metal-free water
and challenging. In this communication, a nickel-borate nanoarray oxidation catalysts operating under mild pH conditions.
supported on carbon cloth (Ni-Bi/CC) has been fabricated through
In 2010, Nocera and co-workers reported the first use of
oxidative polarization of a nickel oxide nanoarray on CC (NiO/CC) in nickel-borate (Ni-Bi) for water oxidation under near-neutral
a borate electrolyte (pH 9.2). As a 3D electrode, this Ni-Bi/CC conditions, and the catalyst film electrodeposited on indium tin
exhibits superior catalytic activity for water oxidation in 0.1 M oxide needed an overpotential of 425 mV to drive a current density
potassium borate (K-Bi) solution, yielding a geometrical catalytic of 1 mA cmÀ2 in 0.1 M borate buffer.21 Since then, considerable
current density of 10 mA cmÀ2 at an overpotential of 470 mV. research interest has been focused on the development of such
Notably, this electrode also demonstrates outstanding long-term Ni-Bi films as OER catalysts at near-neutral pH.22–25 The most
electrochemical durability for 25 h with 100% Faradaic efficiency.
active catalyst affords 1 mA cmÀ2 at an overpotential of 390 mV
in 0.5 M potassium borate (K-Bi).24 It is well-established that a
Increasing fossil energy consumption and ever-worsening environ- 3D nanoarray catalyst not only facilitates the diffusion of an
mental pollution are driving major international endeavours to electrolyte and gas, but also exposes more active sites, leading to
seek new sources of clean energy.1–4 Hydrogen is considered an enhanced electrocatalytic behaviors.26,27 However, it is still
efficient energy carrier that is free from environmental issues highly challenging to develop such a Ni-Bi catalyst.
caused by fossil fuels. Electrochemical water splitting, a process
In this communication, we describe the first development of
that couples anodic water oxidation with cathodic hydrogen a Ni-Bi nanoarray supported on carbon cloth (Ni-Bi/CC)
reduction, serves as an effective method for large-scale hydrogen through oxidative polarization of a nickel oxide nanoarray on
production.5–7 However, oxygen evolution reaction (OER) at the CC (NiO/CC) in a borate electrolyte (pH 9.2). As a 3D electrode,
anode greatly limits the water splitting procedure due to the this Ni-Bi/CC exhibits superior catalytic activity for water oxida-
multistep four-electron oxidation and sluggish kinetics.8,9 tion in 0.1 M K-Bi, yielding a geometrical catalytic current density
An effective electrocatalyst is thus required to reduce the OER of 10 mA cmÀ2 at an overpotential of 470 mV. In 0.5 M K-Bi, this
overpotential.10–20 IrO2 and RuO2 have been verified to be the electrode needs an overpotential of only 390 mV for the same
most active OER catalysts in acidic or alkaline solutions. But current density. Notably, it also shows strong long-term electro-
their scarcity, high cost and requirement for an extreme pH chemical durability to retain its catalytic activity for 25 h with
100% Faradaic efficiency (FE).
Ni-Bi/CC was synthesized by oxidative polarization of NiO/CC
(Fig. S1, see the ESI† for details). Fig. 1a presents the
XRD patterns for bare CC, NiO/CC and Ni-Bi/CC. It is seen that
a College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China.
E-mail: sunxp@scu.edu.cn; Fax: +86-28-85412198; Tel: +86-28-85412198
b College of Materials, Qingdao University, Qingdao 266071, China
NiO/CC exhibits three peaks at 37.31, 43.41 and 631, which
correspond to the (111), (200) and (220) planes of NiO, respec-
tively (JCPDS No. 75-0197).28 However, Ni-Bi/CC shows only
diffraction peaks of CC (JCPDS No. 75-2078), implying the
amorphous nature of the product. Further, inductively coupled
plasma mass spectrometry (ICP-MS) analysis concludes the
existence of Ni, B, and O elements with a Ni/B/O molar ratio of
1.06: 1.44 : 6.23. X-ray photoelectron spectroscopy (XPS) spectra
c College of Chemistry and Chemical Engineering, Qufu Normal University,
Qufu 273165, Shandong, China. E-mail: fengliquhn@hotmail.com
d Analytical and Test Center, Southwest University of Science and Technology,
Mianyang 621010, China
e Chengdu Institute of Geology and Mineral Resources, Chengdu 610064, China
f College of Chemical and Environ Department, Faculty of Science,
King Abdulaziz University, Jeddah 21589, Saudi Arabia
† Electronic supplementary information (ESI) available: Experimental section
and supplementary figures. See DOI: 10.1039/c6cc09893b
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Fig. 1 (a) XRD patterns of bare CC, NiO/CC and Ni-Bi/CC. XPS spectra of
Ni-Bi/CC in the (b) Ni 2p, (c) B 1s, and (d) O 1s regions.
further confirm the presence of these three elements (Fig. 1b–d).
In the Ni region (Fig. 1b), the peaks at 856.2 and 874.3 eV correspond
to the signals of Ni 2p3/2 and Ni 2p1/2, respectively, implying the
existence of Ni with a high oxidation state (Ni2+ or Ni3+).29 Obvious
shakeup satellite peaks (Sat.) are also observed at 881.7 and
863.1 eV for Ni 2p1/2 and Ni 2p3/2, respectively.30 Moreover, the
binding energies of 192.1 eV in the B 1s region (Fig. 1c) and
531.9 eV in the O 1s region (Fig. 1d) correspond to the core
levels of the central boron and oxygen atoms in the borate
species, respectively.31
Fig. 2 SEM images of (a and b) NiO/CC and (c and d) Ni-Bi/CC. TEM and
HRTEM images of (e and f) NiO and (g and h) Ni-Bi nanosheet.
Fig. 2a and b present the scanning electron microscopy
(SEM) images of NiO/CC, demonstrating the formation of using a typical three-electrode cell setup with a scan rate of
vertically aligned NiO nanoplates interconnected with each 1 mV sÀ1. For comparison, NiO/CC (NiO loading: B1.8 mg cmÀ2),
other to form a 3D network structure on CC. Note that the RuO2 on CC (RuO2/CC, RuO2 loading: B2.3 mg cmÀ2) and bare CC
resulting Ni-Bi/CC possesses nearly the same morphology but were also measured. The ohmic potential drop (iR) losses due
with a roughened sheet surface (Fig. 2c and d). The cross- to solution resistance were applied to all initial data unless
sectional photograph of Ni-Bi/CC further verifies that the specifically stated and all potentials were reported on a
thickness of the vertically grown Ni-Bi layer is about 3 mm reversible hydrogen electrode (RHE) scale. Fig. 3a shows the
(Fig. S2, ESI†). Transmission electron microscopy (TEM) analy- polarization curves for Ni-Bi/CC, NiO/CC, RuO2/CC and bare CC
sis provides another piece of evidence to support the chemical in 0.1 M K-Bi (pH 9.2). Bare CC has no activity for the OER,
conversion of the crystalline NiO nanoplate (Fig. 2e and f) into a while RuO2/CC is a highly active OER catalyst with the need for
porous amorphous Ni-Bi nanoplate (Fig. 2g and h) after oxida- an overpotential of 380 mV to drive 10 mA cmÀ2. Ni-Bi/CC is
tive polarization. This topotactic conversion resulting in larger also efficient for the OER with the need for an overpotential of
thickness for Ni-Bi nanosheets could be explained as follows. 470 mV to reach 10 mA cmÀ2. This overpotential is much lower
Oxidative polarization leads to electrochemical oxidation of Ni than those of NiO/CC (580 mV) and other Ni-based OER
at NiO to form Ni anions firstly. Then the electrostatic attractive catalysts (Table S1, ESI†). In addition, the oxidation peak of
interactions between Ni anions and borate anions drive in situ Ni-Bi/CC is observed due to the conversion of Ni3+ to Ni4+,
deposition of Ni-Bi on the NiO nanosheet. This deposition consistent with a recent report.25 Tafel analysis is also used to
process proceeds repeatedly until the complete conversion of estimate the OER kinetics for the catalyst electrodes. As pre-
crystallized NiO into the amorphous Ni-Bi nanoplate takes sented in Fig. 3b, the Tafel slopes for Ni-Bi/CC and NiO/CC are
place. The energy-dispersive X-ray (EDX) elemental mapping 107 and 126 mV decÀ1, respectively, indicating a more favorable
demonstrates the uniform distribution of Ni, B and O in Ni-Bi/CC OER kinetics on Ni-Bi/CC. Compared with the reported Ni-Bi21
(Fig. S3, ESI†).
and NiO32 catalysts, the slightly larger Tafel slopes for Ni-Bi/CC
The electrocatalytic OER performances of Ni-Bi/CC (Ni-Bi and NiO/CC could be due to the intrinsic activity from the
loading: B2.3 mg cmÀ2) were further investigated in 0.1 M K-Bi synergistic effect of substrate, morphology and adsorbates.33,34
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potential between +0.7 and +1.2 V vs. Ag/AgCl at a scan rate of
100 mV sÀ1, are conducted to test the stability of Ni-Bi/CC. After
1000 CV cycles in 0.1 M K-Bi, the polarization curve of Ni-Bi/CC
shows negligible loss of anodic current compared with the
initial one (Fig. 3d). In Fig. 3e, a stable current driven at
1.0 V vs. Ag/AgCl is observed for 25 h, further indicating the
good durability of the catalyst. Notably, Ni-Bi/CC still preserves
its nanoarray feature after OER electrolysis (Fig. S5, ESI†). The
high electrocatalytic performance of Ni-Bi/CC may be attributed
to the following possible facts. (1) Direct growth of the catalyst
layer on CC without the need for polymer binders significantly
enhances the mechanical stability and the electron transport of
Ni-Bi/CC. (2) Nanoarray morphology contributes to the mass
transport within electrodes and increases the interfacial contact
between active sites and the electrolyte.
Note that electrolyte concentration has a heavy influence on
the OER activity of the Ni-Bi/CC catalyst electrode. As shown in
Fig. 3f, the catalytic activity of Ni-Bi/CC is improved with
increased electrolyte concentration from 0.1 M to 0.5 M and a
much smaller overpotential of only 390 mV is needed to drive
10 mA cmÀ2 in 0.5 M K-Bi.
As another crucial parameter to evaluate the catalyst, FE was
calculated by comparing the quantity of experimentally evolved
oxygen with the theoretically calculated one. The experimentally
evolved O2 was measured quantitatively by monitoring the
pressure change via a calibrated pressure sensor in the anode
compartment of an H-type electrolytic cell.26 As shown in Fig. 4,
the consistency of experimental and theoretical values indicates
a 100% FE for Ni-Bi/CC toward water oxidation.
In conclusion, a Ni-Bi nanoarray has been successfully
transformed from a NiO nanoarray via oxidative polarization
in borate buffer. The 3D Ni-Bi/CC electrode is superior in catalytic
activity and durability toward water oxidation, and it demands
overpotentials of only 470 and 390 mV to deliver 10 mA cmÀ2 in
Fig. 3 (a) LSV curves of NiO/CC, Ni-Bi/CC, RuO2/CC and bare CC for
OER. (b) Corresponding Tafel plots for Ni-Bi/CC and NiO/CC. (c) Multi-
current process of Ni-Bi/CC. The current density started at 10 mA cmÀ2
at 0.95 V vs. Ag/AgCl and ended at 40 mA cmÀ2, with an increment of
B3.3 mA cmÀ2 per 500 s without iR correction. (d) LSV curves for Ni-Bi/CC
before and after 1000 CV cycles. (e) Time-dependent current density curve
at 1.0 V vs. Ag/AgCl without iR correction. All experiments are conducted in
0.1 M K-Bi. (f) LSV curves of Ni-Bi/CC in 0.1, 0.3, and 0.5 M K-Bi for OER.
To further illustrate the intrinsic activity of Ni-Bi/CC, the 0.1 and 0.5 M K-Bi, respectively, with 100% FE. This work not only
turnover frequency (TOF) under a certain overpotential was provides us with an attractive Ni-Bi nanoarray as a high-activity
calculated.35,36 The active sites for Ni-Bi/CC were quantified
by electrochemistry (Fig. S4, see the ESI† for details). And we
assume all the Ni centers in Ni-Bi/CC to perform a one-electron
oxidation process. The TOF, as the product evolving rate per
mole of active sites, is calculated to be 0.45 mol O2 sÀ1 at an
overpotential of 600 mV, larger than those of the reported
noble-metal-free OER catalysts like Ni-Bi (0.01 mol O2 sÀ1
,
Z = 600 mV),21 Fe-Bi (0.17 mol O2 sÀ1, Z = 600 mV)37 and NiOx
(0.02 mol O2 sÀ1, Z = 610 mV).14
Fig. 3c represents a multi-step chronopotentiometric curve
of Ni-Bi/CC in 0.1 M K-Bi with the current changing from
10 to 40 mA cmÀ2 (an increment of 3.3 mA cmÀ2 per 500 s).
In the experiment, the potential immediately levels off at 0.95 V
vs. Ag/AgCl at the start current value, and remains constant for
the remaining 500 s. Similar results are also observed for other
current values, demonstrating the excellent mass transport
properties, conductivity and mechanical robustness of the
Ni-Bi/CC electrode.38 Aside from the strict requirement for high
activity, long-term electrochemical stability is also an important
factor for all catalysts. Cyclic voltammetry (CV) scans, with a
Fig. 4 Amount of oxygen theoretically calculated and experimentally
measured versus time for Ni-Bi/CC in 0.1 M K-Bi.
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OER catalyst in water-splitting devices for production of hydrogen
fuel at mild pH, but also would trigger a new research direction to
electrochemical topotactical fabrication of Ni-based 3D catalyst
electrodes from their oxide/hydroxide precursors for applications.
This work was supported by the National Natural Science
Foundation of China (No. 21575137 and 21375076).
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