Topics in Current Chemistry (2018) 376:42
1 3
nickel-based compounds, respectively. The detailed classification is summarized as
follows.
4.1 Nickel‑Based Metals
4.1.1 Single Nickel Metal
At the beginning, the work on nickel-based catalysts starts from the pure nickel
metal. Kaulen et al. [78] proposed that nickel metal with its surface oxidant will
form Ni(OH) 2 in basic solution and further converted to NiOOH as the applied
potential gradually shifts to positive. Simultaneously, the morphology and structure
of the catalyst affect the catalytic performance in case that the urea electro-oxidation reaction takes place on the three-phase interface of catalyst. Low-dimensional
nanostructures (such as nanowires or nanosheets) have a unique exposed crystal surface and high specific surface area, so as to provide more active sites and exhibit
higher catalytic activity. Tao et al. [47] synthesized nickel nanoparticles with a
size of 2–3 nm using KBH 4 reduction method, which is the smallest nickel particle reported in the literature. Compared to commercial nickel catalyst (particle size
reaches 4–10 μm), their anodic nickel nano-catalysts had a much smaller size and
larger surface area, resulting in a peak power density of 14.2 mW cm
−2
in DUFCs
when operated at 60 °C with 1 mol l
−1
urea as fuels and humidified air as oxidants.
Yan et al. [79] and Cao et al. [75] successfully synthesized nickel nanowires with a
length of several micrometers and a diameter of several tens of nanometers by means
of electrodeposition method with the anodized aluminum oxide template and polycarbonate template, respectively. The nickel nanowires prepared by Yan et al. have
a regular structure with a diameter of 90 nm and electrochemically active surface
area (EASA) of 79.1 cm
2
mg
−1
, which performed a current density of 40 mA cm
−2
at 0.55 V (vs. Hg/HgO). In addition, the nickel nanowire arrays prepared by Cao
et al. behave a diameter of 50 nm for a single wire and an active surface area of
25.21 cm
2
. The typical SEM images of the nickel nanowire electrodes at different
magnifications are shown in Fig. 7. All nanowires stand on the surface of the nickel
Fig. 7 SEM images of wheat-spike nickel nanowire arrays electrode at a low magnification and b high
magnification Reproduced with permission from Ref. [75]
Reprinted from the journal
54
1 3
nickel-based compounds, respectively. The detailed classification is summarized as
follows.
4.1 Nickel‑Based Metals
4.1.1 Single Nickel Metal
At the beginning, the work on nickel-based catalysts starts from the pure nickel
metal. Kaulen et al. [78] proposed that nickel metal with its surface oxidant will
form Ni(OH) 2 in basic solution and further converted to NiOOH as the applied
potential gradually shifts to positive. Simultaneously, the morphology and structure
of the catalyst affect the catalytic performance in case that the urea electro-oxidation reaction takes place on the three-phase interface of catalyst. Low-dimensional
nanostructures (such as nanowires or nanosheets) have a unique exposed crystal surface and high specific surface area, so as to provide more active sites and exhibit
higher catalytic activity. Tao et al. [47] synthesized nickel nanoparticles with a
size of 2–3 nm using KBH 4 reduction method, which is the smallest nickel particle reported in the literature. Compared to commercial nickel catalyst (particle size
reaches 4–10 μm), their anodic nickel nano-catalysts had a much smaller size and
larger surface area, resulting in a peak power density of 14.2 mW cm
−2
in DUFCs
when operated at 60 °C with 1 mol l
−1
urea as fuels and humidified air as oxidants.
Yan et al. [79] and Cao et al. [75] successfully synthesized nickel nanowires with a
length of several micrometers and a diameter of several tens of nanometers by means
of electrodeposition method with the anodized aluminum oxide template and polycarbonate template, respectively. The nickel nanowires prepared by Yan et al. have
a regular structure with a diameter of 90 nm and electrochemically active surface
area (EASA) of 79.1 cm
2
mg
−1
, which performed a current density of 40 mA cm
−2
at 0.55 V (vs. Hg/HgO). In addition, the nickel nanowire arrays prepared by Cao
et al. behave a diameter of 50 nm for a single wire and an active surface area of
25.21 cm
2
. The typical SEM images of the nickel nanowire electrodes at different
magnifications are shown in Fig. 7. All nanowires stand on the surface of the nickel
Fig. 7 SEM images of wheat-spike nickel nanowire arrays electrode at a low magnification and b high
magnification Reproduced with permission from Ref. [75]
Reprinted from the journal
54