A free-standing three-dimensional
(3D) electrocatalyst is demonstrated
as an example of fine-tuning the hydrogen evolution reaction (HER)
and oxygen evolution reaction (OER) activities through a simple electrodeposition
method. The Ni3S2 nanosheets are electrodeposited
onto the highly conducting Au dendrites that are grown over 3D-graphene
(3DG) framework. The highest electrochemical surface area has been
achieved to enhance exposure of active sites by the integration of
hierarchical Au dendrites with a sufficient amount of Ni3S2 nanosheets. The optimized electrocatalyst (3DG–Au-Ni3S2-15c) delivers the current density of 10 mA cm–2 at an overpotential of 140 mV and a lowest Tafel
slope of 93 mV dec–1 for HER. The higher HER activity
is supported by the smaller charge-transfer resistance of 1.09 Ω,
which confirms the faster interfacial electron transfer between the
electrode and electrolyte phases. The OER experiments suggest that
3DG–Au-Ni3S2 (15c) has a higher current
density of 91.2 mA cm–2 at a potential of 1.6 V
and a lower Tafel slope of 148 mV dec–1 in alkaline
media. Furthermore, X-ray photoelectron spectroscopy reveals the conversion
of Ni3S2 surface into the highly OER active
hydrated nickel oxide during polarization experiment. With 3DG–Au-Ni3S2 (15c) as bifunctional electrocatalysts, an alkali
electrolyzer delivers the current density of 10 mA cm–2 at a low cell voltage of 1.63 V. Furthermore, this electrolyzer
gives a smooth line with a very small deviation of 4% from the initial
potential value for 19 h, suggesting the excellent long-term stability
of 3DG–Au-Ni3S2 (15c).
Hierarchical Co 9 S 8 hollow microspheres with nanosheet building units are synthesized via a one-step hydrothermal method. The presence of glucose in the synthesis is essential to the formation of the hollow structure and influences the size and morphology of the microspheres. Benefiting from the advantages of a hollow architecture, a conductive carbon coating and Co 9 S 8 nanosheet units, the composite exhibits outstanding electrochemical properties. When the composite electrode is evaluated as the cathode material for supercapacitors, it exhibits a high specific capacitance of 514 F g À1 at 1.0 A g À1 and a capacitance retention of circa 88 % over 1000 cycles at 8 A g À1 .On the other hand, the nanosheet-constructed hollow structure can efficiently promote the electrocatalytic activity of Co 9 S 8 toward the oxygen evolution reaction. Due to the fact that Co 9 S 8 nanosheets impart largely exposed active sites, the composite exhibits a low Tafel slope of 66 mV dec À1 and no obvious degradation after 12 h water oxidation.
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