NiO_x nanoparticles supported on polyethylenimine functionalized CNTs as efficient electrocatalysts for supercapacitor and oxygen evolution reaction

Highlights

• Nano-sized NiO_x particles are successfully deposited on PEI functionalized CNTs.

• NiO_x nanoparticles show uniform dispersion with average particle size in the range of 2–3 nm.

• NiO_x/PEI-CNTs show high-performance as supercapacitor, achieving 1728 F g⁻¹ based on active material loading.

• NiO_x/PEI-CNTs show high activity for OER, achieving 100 A g⁻¹ at η = 0.35 V in 1 M KOH.

Abstract

Ni oxide based nanoparticles (NPs) have been widely used as electrocatalysts in the electrochemical energy storage and conversion applications. In this paper, NiO_x NPs are successfully synthesized by the self-assembly of Ni precursor onto polyethylenimine functionalized carbon nanotubes (PEI-CNTs) assisted with microwave radiation. NiO_x NPs with size around 2–3 nm are homogenously dispersed on the PEI-CNTs supports with no aggregation. The electrochemical activity of NiO_x NPs on PEI-CNTs, NiO_x/PEI-CNTs, as effective electrocatalysts is studied for supercapacitor and oxygen evolution reaction in alkaline solutions. NiO_x/PEI-CNTs show a capacitance of 1728 and 1576 F g⁻¹ based on active material, and 221 and 394 F g⁻¹ based on total catalyst loading on 12.5% and 25% NiO_x/PEI-CNTs, respectively, which is substantially higher than 152 F g⁻¹ of unsupported NiO. The NiO_x/PEI-CNTs electrodes exhibit reversible and stale capacitance of ∼1200 F g⁻¹ based on active materials after 2000 cycles at a high current density of 10 A g⁻¹. NiO_x/PEI-CNTs also exhibit significantly higher activities for oxygen evolution reaction (OER) of water electrolysis, achieving a current density of 100 A g⁻¹ at an overpotential of 0.35 V for 25% NiO_x/PEI-CNTs. It is believed that the uniformly dispersed nano-sized NiO_x NPs and synergistic effect between the NiO_x NPs and PEI-CNTs is attributed to the high electrocatalytic performance of NiO_x/PEI-CNTs electrocatalysts. The results demonstrate that NiO_x NPs supported on PEI-CNTs are highly effective electrocatalysts for electrochemical energy storage and conversion applications.

Introduction

Transition metal oxides-based materials have been extensively studied as electrocatalysts in the areas of energy storage and conversion technologies such as fuel cells [1], [2], catalysts [3], lithium ion batteries [4], supercapacitors [5], [6], and hydrogen generation and storage systems [7], [8]. However, the electrochemical activity of the transitional metal oxides depends strongly on the microstructure of the catalysts. Nano-structured catalysts can significantly improve the electrocatalytic properties due to the fact that the nanoscale dimension offers the materials with high aspect ratios, enhances the physical and electrochemical properties and favors the transport processes [7], [9]. The electrical conductivity and long-term stability of nano-structured transition metal oxide based catalysts can be further enhanced by employing high surface area and electronically conducting carbon materials to provide physical support for nano-structured catalysts as well as the conducting path for the charge transport process [10], [11].

Carbon nanotubes (CNTs) and graphene have been extensively studied as catalysts support due to their large specific surface area, excellent mechanical and electrical properties and structural stability [8], [10], [12], [13], [14]. Wu et al. [15] prepared Co₃O₄ particles with size around 6 nm supported on single-walled carbon nanotubes (SWNTs) through a noncovalent functionalization route, and yielded a current density of 66 A g⁻¹ at overpotential of 0.37 V in 1 M KOH solution for oxygen evolution reaction (OER), significantly better than unsupported Co₃O₄ nanoparticles. The enhanced catalytic activity for OER has been considered to be the synergistic effect of SWNTs and Co₃O₄. Nano-structured MnO₂ [16], NiO [17], Co₃O₄ [18] and Fe₂O₃ [19] supported on CNTs or graphene have also been shown to be the promising electrocatalysts for high-performance supercapacitors.

Nickel based materials such as nickel, nickel oxides, hydroxides as well as oxyhydroxides exhibit excellent electrochemical properties for applications such as fuel cell, supercapacitor and water electrolysis [20], [21], [22]. Shape control of nickel based nanomaterials, such as nanorod, nanowire, nanotube or nanoribbon can be fabricated using chemical routes by controlling the solvents, temperature, surfactant templates, etc [23], [24]. However, it is a significant challenge to synthesize nano-structured nickel oxides-based electrocatalysts with particle size less than 3 nm. Polyethylenimine (PEI) is an amino-rich highly hydrophilic cationic polyelectrolyte and recently we have shown that PtRu NPs homogenously supported on PEI functionalized CNTs catalysts show high electrochemical surface area, high CO tolerance, enhanced activity and durability for methanol electrochemical oxidation [25]. Here we demonstrated that NiO_x NPs with size around 2–3 nm can be homogenously dispersed on the PEI functionalized CNTs (PEI-CNTs) assisted with a facile microwave radiation method. The results indicate that the NiO_x supported on PEI-CNTs exhibit high capacitance and excellent activity for OER of water electrolysis in alkaline solutions.