Manipulating the morphology and the electronic structures of nickel-cobalt selenides@N-doped carbon for aqueous alkaline batteries

Highlights

• (Ni,Co)Se₂@N-doped carbon nanocubes and nanocages were synthesized.

• The morphology and electronic structure were manipulated by chemical etching.

• The effect of chemical etching on the electrochemical properties was investigated.

• Higher specific capacity was achieved in (Ni,Co)Se₂@N-doped carbon nanocubes.

Abstract

Morphology engineering and surface modification are efficient approaches to enhance the electrochemical performance of transition metal chalcogenides. In the present work, (Ni,Co)Se₂@N-doped carbon with different morphologies derived from Prussian blue analogous were synthesized. The influence of chemical etching on the morphology, electronic structure and electrochemical performance and activity of (Ni,Co)Se₂@N-doped carbon were investigated. The results demonstrated that the chemical etching not only resulted in increased proportion of metal ions with low oxidation states, but also caused the reduction of the nickel and cobalt species on the surface. Therefore, better reversibility and enhanced reaction kinetics were observed in (Ni,Co)Se₂@N-doped carbon nanocages despite that the resultant nanocubes showed higher specific capacity (432.1 C g⁻¹ at 0.5 mA cm⁻²). Moreover, an aqueous alkaline battery was assembled by integrating commercial activated carbon and (Ni,Co)Se₂@N-doped carbon nanocubes as negative and positive electrode, respectively, which could reach a working voltage of 1.65 V and achieve an energy density of 28.2 W h kg⁻¹ at 307 W kg⁻¹ and remain 18.5 W h kg⁻¹ at 3.07 kW kg⁻¹. Our study on (Ni,Co)Se₂@N-doped carbon provides helpful inspiration to design nickel-cobalt selenides with excellent electrochemical performance as active materials for aqueous alkaline batteries.

Introduction

Recently, energy storage devices employing aqueous electrolyte (e.g., Ni-Cd, Pb-acid, aqueous rechargeable metal-ion batteries and hybrid supercapacitors) have attracted increasing attentions in virtue of its higher ionic conductivity, low cost, environmentally friendly, high safety and machinability, which can tackle the drawbacks of conventional organic electrolyte and meet the requirement of durability, high power density and energy efficiency [1], [2], [3]. However, it also suffers from low energy density compared to the state-of-art lithium-ion batteries. Much efforts have been devoted to developing electrode materials with high electrochemical performance.

Transition metal chalcogenides (TMCs) have received tremendous attentions for the past decade owing to their unique electronic structure and remarkable physicochemical properties, making them advanced active materials for metal-ion batteries, supercapacitors, water splitting, and fuel cells, etc [4], [5], [6], [7]. Selenium and sulfur elements belong to the same main group but different periods, which endows transition metal selenides and sulfides with similar characteristics. Nevertheless, due to the lower electronegativity of selenium, transition metal selenides are granted to possess higher conductivity and richer valence states, which is conducive to electron transfer and facilitating the electrochemical process [8], [9], [10]. In addition, compared with monometallic selenides, bimetallic transition metal selenides are generally considered to be a superior candidate by harnessing of the synergistic effect between two metals, which can offer rich and multi-electron reactions and improve the electrochemical performance [10], [11]. More importantly, the electrochemical properties of bimetallic transition metal selenides can be effectively manipulated and optimized by regulating their compositions, morphologies, microstructures and electronic configuration [4], [12], which greatly promotes the application and development of bimetallic transition metal selenides in the fields of energy conversion and storage.

Among various reported bimetallic selenides, nickel-cobalt selenides are extensively investigated as an attractive electrode material for aqueous alkaline batteries due to its good electronic conductivity, excellent electrochemical activity and high theoretical capacity [13], [14]. However, the electrochemical performance of nickel-cobalt selenides is still far from satisfactory due to its poor reversibility and dissolution in aqueous solution. It is reported that the electrochemical performance of nickel-cobalt selenides heavily relies on its microstructure and electronic configuration. Among many optimization methods, morphology and composition engineering have proven to be advantageous strategies to enhance its electrochemical performance. For instance, tremella-like Ni-Co selenides prepared by hard template method exhibited a capacity of 636.2 C g⁻¹. Nickel-cobalt nanotubes with an optimized Ni/Co ratio of 0.67:0.33 showed a specific capacity of 1157 F g⁻¹ [13]. Besides, the core-shell nickel-cobalt selenides with a high content of selenium provided a capacity of 164.4 mA h g⁻¹ and an energy density of 37.5 W h kg⁻¹ was achieved when assembled with activated carbon [15]. Particularly, nickel-cobalt selenides derived from metal-organic framework (MOFs) exhibit incredible tunability with uniform size distribution and diverse compositions, giving MOFs instinct superiority as precursors or templates to prepare hollow porous nanostructures with abundant active sites, such as hollow (Ni,Co)Se₂ nanocubes [10], double-shelled CoSe₂/(NiCo)Se₂ hollow nanobox [16], triangle-like hollow (Ni,Co)Se₂ arrays [11], rhombohedral Co(Ni)Se₂/N-doped carbon [17], etc. Generally, hollow structures can usually afford more exposed active sites, alleviate volumetric expansion, reduce mass transport length and promote electrolyte ion diffusion [18], [19], [20], [21], [22]. Hence, benefiting from the structural merits, the electrochemical performance could be greatly improved. Apart from the above strategies, surface modification, including doping [14], [23], defect engineering [24], [25], [26], incorporating with carbon or polymer materials [10], [27], is another efficient approach to improve the electrochemical performance of transition metal chalcogenides. For instance, Mn-Co-Ni sulfide nanotube arrays with internal and external defects introduced by chemical reduction showed enhanced electrochemical performance (from 2944 F g⁻¹ to 3794 F g⁻¹) [26]. It is believed that doping and defect engineering could effectively increase the electron densities, and thereby significantly improve the reaction kinetics and facilitate faradic redox reactions.

Inspired by the above researches, in this work, we constructed nickel-cobalt selenides@N-doped carbon with two optimized morphologies via a three- or four- steps method. The influence of chemical etching on the morphology, electronic structure and electrochemical performance of samples are investigated. The as-prepared nickel-cobalt selenides@N-doped carbon nanocubes exhibited high specific capacity (432.1 C g⁻¹, 0.5 mA cm⁻²) and good rate capability (57.8 %, 20 mA cm⁻²), while the nanocages demonstrated improved cycling stability. Moreover, the assembled asymmetric aqueous alkaline batteries delivered a maximum energy density of 28.2 W h kg⁻¹ at 307 W kg⁻¹. Our study on nickel-cobalt selenides@N-doped carbon provides helpful inspiration to design nickel-cobalt selenides with excellent electrochemical performance for aqueous alkaline batteries.