Stimulate the hidden catalysis potential and exposure of nickel site in NiSe@CNTs result in ultra-high HER/OER activity and stability

Highlights

• The ultra-high activity for HER (27 mV at 10 mA cm^-2) and OER (145 mV at 100 mA cm^-2) and outstanding long-term durability (730 h).

• Internal and external means to enhance catalytic activity.

• The interfacial effect between NiSe and CNTs in the heterostructure exacerbates the further π-electrons delocalization of CNTs.

Abstract

We present an electrodeposition strategy to construct heterostructures nickel selenide (NiSe) and carbon nanotubes (CNTs), namely NiSe@CNTs, to tune the electronic structure of the Ni sites, thus stimulating the hidden catalytic potential of the Ni sites. As expected, the optimized NiSe@CNTs exhibits unprecedentedly ultra-high activity for HER (27 mV at 10 mA cm⁻²) and OER (145 mV at 100 mA cm⁻²) and outstanding long-term durability (730 h). Based on the systematic characterization and DFT results, the Ni sites with increased electron density induced by π-electron off-domain of CNTs not only greatly optimize the H* adsorption kinetics but also reduce the energy barrier of the *O→*OOH step, demonstrating the enhanced intrinsic catalytic activity. Furthermore, the introduced CNTs not only avoid aggregation of nanoparticles but also reduce particle size, thus increasing the number of exposed Ni active sites. This study provides guidance for the exploration of catalysts with ultra-high activity and durability.

Introduction

Currently, hydrogen is considered an ideal alternative energy source due to its high energy density, storability, renewability and zero pollution [1], [2], [3], [4]. To achieve highly efficient electrochemical water splitting for hydrogen production, it is an inevitable trend to construct cost-effective, highly active and stable electrocatalysts [5], [6], [7]. Nickel chalcogen (Group VIA elements) compounds are considered highly promising bifunctional nanomaterials owing to their tunable electronic structures, high-density state of d-electrons and high covalent degree of metal-anion bonding [8], [9], [10], [11]. Among them, selenium with the electronic structure 4 s² 4p⁴ and empty 4d orbitals presents a stronger metallicity and smaller ionization energy due to its lower electronegativity than oxygen and sulfur, so nickel selenides usually exhibit higher electrocatalytic efficiency than nickel oxides and nickel sulfides [12]. However, the electron-deficient Ni sites in pristine NiSe have insufficient adsorption and desorption capacity for hydrogen protons and oxygen-containing species, resulting in less-than-optimal activity [13]. Therefore, improving the electron density of the Ni sites is essential to enhance the catalytic activity of NiSe.

Considering the above problems, the construction of heterojunctions from NiSe and another phase can not only compensate for the shortcomings of single-component NiSe, but also regulate the active site and electronic structure, thus optimizing the adsorption and desorption capacity for reaction intermediates on the catalyst surface [14], [15]. Therefore, the constructing heterostructures strategy is considered an effective way to solve the lack of intrinsic activity for NiSe. For example, Lee et al. prepared a hierarchical heterostructure of CoFe–LDH@NiSe on NF, which displayed remarkable catalytic activity toward the overall water-splitting due to interfacial coupling [12]. Moreover, the synthesis of MoSe₂/NiSe-1 heterojunction nanocomposites greatly enhanced HER catalytic activity due to higher electron density and conductivity via the strong electronic synergistic interaction among the different interfaces [16]. Ray and coworkers constructed Ni₃S₂ @NiSe/NF catalyst promoted better electron localization and faster charge transfer of the adsorbed charged species, resulting in excellent overall water splitting performance [17]. Although the aforementioned studies have affirmed the contribution of constructing heterostructures to improve the electrocatalytic performance of NiSe, the catalysis potential of the Ni sites was not fully exploited, resulting in its catalytic performance remaining below expectations. Therefore, the choice of suitable coordination material that can provide a large number of free electrons plays a crucial role in further exciting the full potential of the Ni sites.

In addition to the optimization of intrinsic activity, the modification of insufficient active sites for NiSe should not be neglected [18]. The severe aggregation between nanoparticles not only prevented adequate contact between the active sites and electrolytes but also retarded the mass diffusion, leading to a decrease in electrocatalytic activity [19], [20], [21]. Thus, the study of how to avoid the aggregation of nanoparticles is also a challenge that we have to face. For CNTs, the P-electrons of carbon atoms form a large range of off-domain π-bonds with significant conjugation effects, and the electrons are free to move over a large range independent of individual carbon atoms, resulting in high charge mobility [22], [23]. The heterostructures constructed based on CNTs can provide a convenient pathway for mass diffusion and fast electron transport, thus exhibiting high activity and stable electrocatalytic performance [24], [25], [26].

Herein, we establish monodisperse NiSe nanoparticles with a much small size of 10–15 nm supported on self-made CNTs for efficient electrochemical water splitting. Compared with the single-component NiSe nanoparticles, the introduced CNTs not only avoid the agglomeration of nanoparticles but also reduce their particle size, which helps to stabilize and increase the number of exposed active sites, thus improving the catalytic stability and activity. The interfacial effect between NiSe and CNTs in the heterostructure exacerbates the further π-electrons delocalization of CNTs, which leads to the localization of d-electrons at the Ni sites, making the local electron enrichment at the Ni sites. The increased electron density excites the hidden catalysis potential of the Ni sites in NiSe@CNTs, which greatly compensates for the lack of NiSe adsorption capacity for hydrogen protons and desorption capacity for all three oxygen-containing species, respectively, thus improving the intrinsic activity. Benefit from the further release of the intrinsic catalytic activity potential of the nickel sites and exposure of more active sites, the designed NiSe@CNTs exhibits ultra-high HER activity with a low overpotential of only 27 mV at 10 mA cm⁻² and unbelievable activity toward OER (overpotential of 145 mV to achieve 100 mA cm⁻²), which is superior to that of precious metals and almost all of the recently reported advanced catalysts. Moreover, it exhibits strong long-term stability of more than one month (730 h). Particularly, the NiSe@CNTs electrocatalyst demonstrates efficient water splitting in a two-electrode system with a cell voltage as low as 1.43 V at 10 mA cm⁻² with outstanding long-term stability for 350 h. The constructed catalyst can greatly improve the water splitting efficiency, which not only fundamentally solves the high energy consumption problem in the process of hydrogen from water splitting, but also promotes the process of industrial application.