Freestanding and Hierarchically Structured Au-Dendrites/3D-Graphene Scaffold Supports Highly Active and Stable Ni₃S₂ Electrocatalyst toward Overall Water Splitting

Abstract

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).

Single Step Grown NiFe Sponges as Efficient Water Splitting Electrocatalysts in Alkaline Medium

“…The NiFe15 electrode again possesses the lowest Tafel slope of 56 mV dec −1 in comparison to bare Ni (91.7 mV dec −1 ), NiFe5 (77.6 mV dec −1 ), NiFe10 (74.3 mV dec −1 ), NiFe20 (64 mV dec −1 ), and even the benchmark IrO 2 /C (68.9 mV dec −1 ), suggesting the outstanding catalytic reaction kinetics of the NiFe15 catalyst towards OER. It is also to be noted that η 10 and Tafel slope of NiFe15 are not only lower than the benchmarked IrO 2 catalyst but also very much comparable or superior to many other transition metal catalysts reported in the literature (Table ),‐ implying an outstanding OER catalytic performance of the prepared material.…”

“…The polarization curve in Figure b shows that the NiFe15//NiFe5 catalyst require voltage of only 1.62 V to generate the overall water splitting current density of 10 mA cm −2 , which is on par with other recently reported NiFe electrocatalysts such as NiFe‐NC// NiFeNC (1.67 V), Ni 1‐x Fe x /NC // Ni 1‐x Fe x (1.58 V), and NiFe@OCC // NiFe@OCC (1.70 V) . Moreover, the electrocatalytic overall water‐splitting activity of NiFe sponge is also on par to that of others bifunctional catalysts recently studied (Table ) …”

“…The NiFe5 sponge exhibits the lowest Tafel slope of 82 mV dec −1 among the various NiFe sponge and close to the value of the Pt/C catalyst (52 mV dec −1 ), indicating a faster HER catalytic kinetics. The Tafel slope and η 10 values of NiFe5 electrode are also on par with the several recently reported HER electrocatalysts (Table ) . Further, in order to evaluate the electron‐transfer kinetics of the electrodes, EIS measurements were performed.…”

“…Such bi‐metallic sponges having mixed but reproducible phases of oxides (NiO and Fe 2 O 3 ) and pure metal (Ni), and showed varying HER and OER electrocatalytic properties in alkaline medium (1 M KOH). The OER performance of the best bi‐metallic sponge is found to be from NiFe15, where its overpotential for the 10 mAcm −2 current (280 mV), tafel slope (56 mV dec −1 ), and charge transfer resistance are found to be better than the benchmarked IrO 2 (335 mV and 68.9 mV dec −1 ), and other metallic single phasic and/or multi‐phasic catalysts reported recently . While developing a complete water electrolysis cell devoid of precious metals such as Pt, Ir, Co etc., the HER of NiFe5 is found to be comparable (slightly lower) to that of benchmarked Pt.…”

Developments and Perspectives on Robust Nano‐ and Microstructured Binder‐Free Electrodes for Bifunctional Water Electrolysis and Beyond

“…In order to enhance the electrocatalytic performance of 3DGF, several researchers have used a doping strategy. [ 186,401,402 ] In this regard, Zhou et al. [ 403 ] prepared Ar plasma pretreated and N, S co‐doped 3DGF (3DGF‐Ar‐NS).…”