Immobilization of Ni3Co Nanoparticles into N‐Doped Carbon Nanotube/Nanofiber Integrated Hierarchically Branched Architectures toward Efficient Overall Water Splitting

Abstract Exploring cost‐effective and high‐performance bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is of paramount importance for the advancement of H2 production technology, yet remains a huge challenge. Herein, a simple electrospinning–pyrolysis strategy is developed to directly immobilize uniform Ni3Co nanoparticles into a hierarchical branched architecture constructed by in situ formed N‐doped carbon‐nanotube‐grafted carbon nanofibers. The elaborate construction of such hybrid hierarchical architecture can effectively modulate the electronic structure of the active sites, enlarge the exposure of active sites, and facilitate the electron transfer and mass diffusion, favoring both the HER and OER. As a result, the optimized catalyst requires relatively low overpotentials of 114 and 243 mV for HER and OER, respectively, to deliver a current density of 10 mA cm−2 in 0.1 m KOH electrolyte. When employed as a bifunctional catalyst for overall water splitting, the resultant catalyst shows a low cell voltage of 1.57 V to achieve a current density of 10 mA cm−2, along with an impressive stability without noticeable attenuation even after 27 h. This work presents a successful demonstration in optimizing the electrocatalytic performance of Ni‐based bifunctional electrocatalysts by simultaneously considering modulation of electronic structure, hybridization with carbon substrate, and nanostructuring through a facile synthetic strategy, which provides a new avenue to the design of a rich variety of robust transition‐metal‐based electrocatalysts for large‐scale water electrolysis.


Synthesis of Ni 1.5 Co 0.5 @N-C NT/NFs
For the preparation of the standard Ni 1.5 Co 0.5 @N-C NT/NFs, 1.0 g of PVP was initially dissolved in 6 mL of DMF and 6 mL of ethanol to form transparent viscous solution. After that, 1.5 mmol of Ni(NO 3 ) 2 •6H 2 O and 0.5 mmol of Co(NO 3 ) 2 •6H 2 O were introduced into the above solution to form homogeneous chocolate sol after stirring for ~12 h. The resultant sol was then transferred into a plastic syringe equipped with a 21-gauge needle at the tip. The distance between the needle tip and fiber collector (aluminum foil) was set as 18 cm, and a high voltage of 20 kV was applied. The feeding rate of the sol was maintained at 0.6 mL h -1 with the assistance of a syringe pump. The as-spun fibers were subsequently stabilized in air at 200 o C for 3 h and further underwent calcination at 700 o C under a flow of 5% H 2 /95% Ar for 6 h, leading to the formation of Ni 1.5 Co 0.5 @N-C NT/NFs. For comparison, a range of reference samples were also synthesized according to the similar protocol of the standard product, except adjusting the synthetic parameters, including pyrolysis temperature, feeding ratio and amount of metal precursors. To unravel the influence of pyrolysis temperature, the as as-spun fibers were also calcinated at 600, 650, and 750 o C, respectively, while other synthetic parameters were kept unchanged. To investigate the effect of feeding ratio of the metal precursors, only 2.0 mmol of Ni(NO 3 ) 2 led to the formation of Ni@N-C NT/NFs and only introduction of 2.0 mmol of Co(NO 3 ) 2 resulted in the generation of Co@N-CNFs. Metal precursors containing 0.5 mmol of Ni 2+ /1.5 mmol of Co 2+ and 1.0 mmol of Ni 2+ /1.0 mmol of Co 2+ generate Ni 0.5 Co 1.5 @N-CNFs and Ni 1.0 Co 1.0 @N-CNFs, respectively. Moreover, the influence of total amount of metal precursors was also examined.
1.0 mmol of Ni 2+ /0.33 mmol of Co 2+ and 2.0 mmol of Ni 2+ /0.67 mmol of Co 2+ brought about the formation of Ni 1.0 Co 0.33 @N-C NT/NFs and Ni 2.0 Co 0.67 @N-CNFs, respectively. N-doped carbon nanofibers (N-CNFs) were prepared by the direct pyrolysis of PVP nanofibers, without involving any metal precursors. X-ray photoelectron spectroscopy (XPS) was analyzed on a Thermo VG Scientific ESCALAB 250 spectrometer with an Al Kα radiator. Raman spectra were obtained on a Raman spectrometer (Lab RAM HR800, λ = 514 nm). N 2 sorption isotherms and Brunauer-Emmett-Teller (BET) surface areas were measured on Micromeritics ASAP 2050 instrument.

Electrochemical Measurements
The electrochemical measurements for HER and OER were performed on a CHI 760E workstation in a conventional three-electrode system. For the configuration of the three-electrode system, a catalyst-modified glassy carbon electrode (d = 3 mm), a saturated calomel electrode (SCE) and a graphite rod were used as the working electrode, the reference electrode and the counter electrode, respectively. To prepare the catalyst ink, 4.0 mg of the obtained catalyst was ultrasonically dispersed in a mixed solvent containing 1.5 mL H 2 O, 0.5 mL C 2 H 5 OH and 5 μL Nafion (5 wt%) solution for 30 min. Afterwards, 20 μL of the above suspension was dropped onto the glassy carbon electrode surface and then dried at room temperature, and the mass loading density of the active material in glassy carbon electrode was ~0.56 mg/cm 2 . The linear-sweep voltammograms (LSV) were carried out at a scan rate of 5 mV s -1 in N 2 -saturated electrolyte for HER and O 2 -saturated electrolyte for OER in 0.1 M KOH solution. All potentials in this work were referenced to reversible hydrogen electrode (RHE), using the following equation: The evaluation of overall water splitting was carried out in a two-electrode system, and two pieces of nickel foams (3 cm ×1 cm) were employed to support the Ni 3 Co@N-C NT/NF catalyst. Before the immobilization of catalyst, the nickel foams were thoroughly cleaned with acetone, 6.0 M HCl and copious amount of water. The pre-cleaned nickel foam piece was wrapped with Teflon tape at the 2 cm ×1 cm area from one end, leaving the rest 1 cm × 1 cm area exposed for the deposition of catalyst. The catalyst ink was prepared by dispersing 5 mg of the catalyst into 0.1 mL of ethanol, 0.1 mL of H 2 O, and 50 μL of 5 wt% Nafion solution under sonication for 1 h. Subsequently, 100 μL of the catalyst ink was drop-cast onto the exposed area of the nickel foam piece and air-dried under ambient conditions. The loading mass of the catalyst on the nickel foam substrate is ~2 mg/cm 2 . The catalyst-modified nickel foam pieces were assembled as the negative electrode for HER and the positive electrode for OER, respectively. The overall water splitting was performed in 1.0 M KOH electrolyte at a scan rate of 5 mV s -1 .               Table S1. Comparison of HER performance of Ni 1.5 Co 0.5 @N-C NT/NFs with some previously reported non-noble catalysts in alkaline solution.
Catalysts Overpotential @ 10 mA cm -2 (mV) Tafel slope (mV dec -1 ) Table S3. Comparison of the electrochemical performance of Ni 1.5 Co 0.5 @N-C NT/NFs with some previously reported non-noble catalysts for overall water splitting in alkaline solution.
NiCo@N-C NT/NFs 1.57 This work