1 3
Topics in Current Chemistry (2018) 376:42
for the electrolyte ions to transfer on the inner surface of layer. In addition, Liang
et al. [88] mentioned that Ni(OH) 2 monolayers separated from its matrix can provide
more contact surfaces for the molecules and catalysts, thus exhibiting a higher electrochemical performance. However, α-Ni(OH) 2 was unstable in basic environments,
which was simply changed to β-Ni(OH) 2 according to the research of Kamath et al.
[89]. The closely arranged internal β-Ni(OH) 2 layers with minimal interlayer spacing (surface area) hinder the liquid electrolyte from entering the β-Ni(OH) 2 layer,
therefore causing the low electrochemical activity. From the above principle,
although much attention has been paid to the structure and morphology, the crystal
form of Ni(OH) 2 is also worth investigating [90].
Wang et al. [91] successfully fabricated two-dimensional Ni(OH) 2 nanosheets
catalyst by polishing layered Ni(OH) 2 , the size of which range from several hundred nanometers to 1 μm in length and 1 nm in thickness. XRD results showed that
the layered sheets with an interlayer distance of 2.67 nm were arranged along the
c axis, judging as α-Ni(OH) 2 phase. From the electrochemical measurements, the
α-Ni(OH) 2 nanosheets reduced the OOP by 100 mV and enhanced the oxidation current by 154 mA cm
−2
mg
−1
relative to bulk Ni(OH) 2 , whereas this synthetic strategy
has a low yield that needs to be ameliorated. Wang et al. prepared one-dimensional
(1D) Ni(OH) 2 nanoribbons by convenient hydrothermal strategy [92]. The as-synthesized nanoribbons displayed a thickness of 15–20 nm with several micrometers
in length and a surface area of 54.9 m
2
g
−1
via BET test. From the XRD pattern, it
was seen that the nanoribbon belong to β-Ni(OH) 2 phase. The current density was
ca. 7 mA cm
−2
mg
−1
, greatly lower than the α phase. These differences demonstrate
that the crystal form of Ni(OH) 2 has a great influence on the urea electro-oxidation
performance.
Low-dimensional nanomaterials have been recognized to exhibit superior physicochemical properties, which can greatly reduce the over-potential and increase the
current density. Ji et al. [93, 94] fabricated open-ended Ni(OH) 2 nanotubes via electrochemical deposition method using hexagonal zinc oxide sphere templates. This
kind of nanostructure with openness and connectivity make it easier to transport
electrons and urea molecules. Its electro-oxidation current reached a competitive
value of 100 mA cm
−2
mg
−1
while the OOP was lowly obtained at 0.29 V. Recently,
Ye et al. [95] successfully prepared 3D Ni(OH) 2 catalysts on Ni foam substrate with
different nanostructures (Fig. 10) through a facile template-free strategy. Remarkably, nano-sheet Ni(OH) 2 with ultrathin thickness exhibited special 3D structures,
allowing the reactants to touch the surface of the electro-catalysts. The oxidation current density of nano-sheet Ni(OH) 2 catalyst reaches the highest current of
337 mA cm
−2
at 0.45 V in 5 mol l
−1
KOH solutions containing 0.6 mol l
−1
urea
(Fig. 11a). The DUPFC employing nano-sheet Ni(OH) 2 /Ni foam as anode and Pd
NP/C@TiC as cathode displays an OCV at 0.86 V and the highest power density at
19.7 mW cm
−2
among the obtained Ni(OH) 2 anodic catalysts (Fig. 11b).
4.2.2 Doped Nickel‑Based Hydroxide
Based on the excellent electrochemical activity of nickel-based bimetallic or polymetallic catalysts, the introduction of doped nickel-based hydroxide is aimed at the
Reprinted from the journal
59
Topics in Current Chemistry (2018) 376:42
for the electrolyte ions to transfer on the inner surface of layer. In addition, Liang
et al. [88] mentioned that Ni(OH) 2 monolayers separated from its matrix can provide
more contact surfaces for the molecules and catalysts, thus exhibiting a higher electrochemical performance. However, α-Ni(OH) 2 was unstable in basic environments,
which was simply changed to β-Ni(OH) 2 according to the research of Kamath et al.
[89]. The closely arranged internal β-Ni(OH) 2 layers with minimal interlayer spacing (surface area) hinder the liquid electrolyte from entering the β-Ni(OH) 2 layer,
therefore causing the low electrochemical activity. From the above principle,
although much attention has been paid to the structure and morphology, the crystal
form of Ni(OH) 2 is also worth investigating [90].
Wang et al. [91] successfully fabricated two-dimensional Ni(OH) 2 nanosheets
catalyst by polishing layered Ni(OH) 2 , the size of which range from several hundred nanometers to 1 μm in length and 1 nm in thickness. XRD results showed that
the layered sheets with an interlayer distance of 2.67 nm were arranged along the
c axis, judging as α-Ni(OH) 2 phase. From the electrochemical measurements, the
α-Ni(OH) 2 nanosheets reduced the OOP by 100 mV and enhanced the oxidation current by 154 mA cm
−2
mg
−1
relative to bulk Ni(OH) 2 , whereas this synthetic strategy
has a low yield that needs to be ameliorated. Wang et al. prepared one-dimensional
(1D) Ni(OH) 2 nanoribbons by convenient hydrothermal strategy [92]. The as-synthesized nanoribbons displayed a thickness of 15–20 nm with several micrometers
in length and a surface area of 54.9 m
2
g
−1
via BET test. From the XRD pattern, it
was seen that the nanoribbon belong to β-Ni(OH) 2 phase. The current density was
ca. 7 mA cm
−2
mg
−1
, greatly lower than the α phase. These differences demonstrate
that the crystal form of Ni(OH) 2 has a great influence on the urea electro-oxidation
performance.
Low-dimensional nanomaterials have been recognized to exhibit superior physicochemical properties, which can greatly reduce the over-potential and increase the
current density. Ji et al. [93, 94] fabricated open-ended Ni(OH) 2 nanotubes via electrochemical deposition method using hexagonal zinc oxide sphere templates. This
kind of nanostructure with openness and connectivity make it easier to transport
electrons and urea molecules. Its electro-oxidation current reached a competitive
value of 100 mA cm
−2
mg
−1
while the OOP was lowly obtained at 0.29 V. Recently,
Ye et al. [95] successfully prepared 3D Ni(OH) 2 catalysts on Ni foam substrate with
different nanostructures (Fig. 10) through a facile template-free strategy. Remarkably, nano-sheet Ni(OH) 2 with ultrathin thickness exhibited special 3D structures,
allowing the reactants to touch the surface of the electro-catalysts. The oxidation current density of nano-sheet Ni(OH) 2 catalyst reaches the highest current of
337 mA cm
−2
at 0.45 V in 5 mol l
−1
KOH solutions containing 0.6 mol l
−1
urea
(Fig. 11a). The DUPFC employing nano-sheet Ni(OH) 2 /Ni foam as anode and Pd
NP/C@TiC as cathode displays an OCV at 0.86 V and the highest power density at
19.7 mW cm
−2
among the obtained Ni(OH) 2 anodic catalysts (Fig. 11b).
4.2.2 Doped Nickel‑Based Hydroxide
Based on the excellent electrochemical activity of nickel-based bimetallic or polymetallic catalysts, the introduction of doped nickel-based hydroxide is aimed at the
Reprinted from the journal
59