Ni₃ZnC_0.7 nanodots decorating nitrogen-doped carbon nanotube arrays as a self-standing bifunctional electrocatalyst for water splitting

Abstract

Developing of inexpensive, high-efficient, and earth-abundant bifunctional catalysts for water splitting is of great significance for green and sustainable energy development. Herein, a bifunctional hybrid electrocatalyst of Ni₃ZnC_0.7 nanodots in-situ grown on nitrogen-doped carbon nanotube (Ni₃ZnC_0.7/NCNT) arrays is synthesized by a one-step template strategy with 1, 3, 5-triamino-2, 4, 6-trinitrobenzene serving as carbon/nitrogen sources, ZnO nanorods as template and zinc source, and nickel foam as substrate and nickel source. Benefiting from the introduction of Ni₃ZnC_0.7 nanodots and nitrogen doping to the carbon nanotubes, the Ni₃ZnC_0.7/NCNT-700 arrays exhibit superior hydrogen evolution reaction and oxygen evolution reaction catalytic activity in terms of low overpotential (203 mV and 380 mV vs RHE at 10 mA cm⁻² for hydrogen evolution reaction and oxygen evolution reaction, respectively). When the Ni₃ZnC_0.7/NCNT-700 is served as both anode and cathode catalysts for overall water splitting, a potential of 1.66 V is needed to deliver a current density of 10 mA cm⁻², and it also displays negligible degradation after 24 h of operation in alkaline solution. The present work not only provides an efficient bifunctional electrocatalyst for overall water splitting, but also offers a new strategy to design and synthetize the bimetallic carbide.

Introduction

As increasing in energy demand worldwide, rapid depletion of fossil fuels, and continued negative environmental impacts, it is urgent and necessary to develop renewable and sustainable energy sources [1]. Electrochemical water splitting provides an attractive and sustainable approach to produce environmental friendly and high-purity hydrogen fuels [[2], [3], [4]]. However, the high overpotential and low energy conversion efficiency are the biggest obstacles for efficient electrochemical water splitting [5]. To address this issue, highly active electrocatalysts are desired to accelerate reaction kinetics of both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) [[6], [7], [8]]. It is known that noble metal (e.g., Pt, Pd) and metal oxide (e.g., RuO₂, IrO₂) are the state-of-the-art catalysts for HER and OER, respectively [[9], [10], [11]], but they are still away from industrial applications owing to their high price, scarcity and poor stability. On the other hand, from the perspective of practical applications, the catalysts should be capable of catalyzing both HER and OER reactions and work in a same electrolyte, which will not only simplify the overall system design, but also lower the cost of water splitting [4,5]. However, the traditional OER or HER catalysts usually work in electrolyte with different pH range [12,13]. Therefore, the development of bifunctional catalysts with low-cost, earth-abundant and high-efficient is a great challenge for the water splitting.

In recent years, a large number of transition metal compounds, such as transition metal phosphides [[14], [15], [16]], oxides [17,18], nitrides [19,20], sulfides [21] and carbides [8,22] have been employed as bifunctional catalysts for both HER and OER. Among these materials, transition-metal carbides (TMCs) have received increasing research interest as bifunctional catalysts for water splitting due to their outstanding physicochemical properties, such as high electrical conductivity, low cost and excellent chemical stability [8,23]. The typical materials include Fe₃C, Co₃C and Ni₃C, which consist of densely packed metal lattice with small carbon atoms filling in its interstitial voids and show the interstitial alloy properties [24,25]. Nevertheless, their catalytic activity and stability are still far from the desirable performance, preventing them from practical applications. Tremendous efforts have been devoted to modulate the valence of active sites of TMCs catalysts for improving their catalytic performance. For example, Zn cation modulated electrocatalyst Ni₃ZnC_0.7 is beneficial to both of HER and OER under alkaline conditions [26]. Fe doping also greatly improved the HER and OER performance of Ni₃C in a KOH solution [24]. What's more, it is particularly difficult to synthetize phase-pure TMCs, since the high temperature and reducing conditions often result in the formation of metal or mixed metal/metal carbides products [25]. Although various alternative synthesis methods, including hot filament chemical vapor deposition [25], spry-pyrolysis [27] and arc-discharge [28] were developed for obtaining phase-pure TMCs, the synthesis of phase-pure TMCs are often agglomerated showing low electrocatalytic activity. Thus, to maximize their catalytic activity, the TMCs were dispersed on large surface areas of carbon materials, such as graphene nanoribbons, CNT, carbon nanosheets, etc. In such hybrids, the interplay between TMCs and carbon support is so strong that it can promote the catalytic activity on the carbon surface arising from electron penetration from the encapsulated TMCs [25,29]. However, more progress is still needed to optimize synthesis methods and improve the activity of the bifunctional catalysts.

In this work, a bifunctional hybrid electrocatalyst with Ni₃ZnC_0.7 nanodots in-situ grown on highly nitrogen-doped carbon nanotube (Ni₃ZnC_0.7/NCNT) arrays is designed and prepared by a one-step template strategy with 1, 3, 5-triamino-2, 4, 6-trinitrobenzene (TATB) serving as carbon/nitrogen sources and ZnO nanorods supported on nickel foam as template. There is no need to add nickel and zinc sources and remove template by etching agent in the synthesis process. The resultant Ni₃ZnC_0.7 nanodots are homogenously disposed on the NCNT, which is grown on nickel foam forming a self-supporting electrode for water splitting. The hollow architecture of the carbon nanotubes not only supplies more catalytic active sites for HER and OER reaction, but also provides effective releasing pathways for bubbles [[29], [30], [31], [32]]. Benefiting from collaborative advantages of unique hollow structure, bimetallic composite, high nitrogen doping and self-supporting, the resulting Ni₃ZnC_0.7/NCNT-700 exhibits excellent activity for both HER and OER with low overpotential (203 mV for HER and 380 mV for OER at 10 mA cm⁻²) and relatively small Tafel slopes (91 mV·dec⁻¹ for HER and 89 mV·dec⁻¹ for OER), as well as the extremely good durability of overall water splitting with 10 mA cm⁻² at 1.66 V in an alkaline electrolyzer. Such superior electrocatalytic activity enables Ni₃ZnC_0.7/NCNT-700 to be used as a catalyst for large-scale water electrolysis in practical.