Synergistic cerium doping and MXene coupling in layered double hydroxides as efficient electrocatalysts for oxygen evolution
Graphical Abstract
A novel 2D/2D nanohybrid consisting of NiFeCe-LDH nanoflakes vertically grown on the MXene surface is synthesized, which delivers a superb OER activity due to the synergistic effect of Ce doping and MXene coupling.
Introduction
With the depletion of fossil fuels and environmental crisis, exploration of renewable energy technologies is attracting much research attention [1,2]. Electrochemical water splitting is an ideal strategy to generate pure oxygen and hydrogen, which both satisfies the demands for renewable energy resources and reduces the emission of greenhouse gases [3,4]. However, the anodic oxygen evolution reaction (OER) is always the determining step of the overall water splitting process because of its sluggish kinetics, which requires four proton-coupled electron transfer and the formation of oxygen-oxygen bond [5], [6], [7]. To this end, suitable electrocatalysts are needed to expedite the reaction, making electrochemical water splitting a scalable technology for energy systems [8].
Up to date, noble metals or their oxides, such as RuO2 and IrO2, are the state-of-the-art electrocatalysts for OER, but their high price and natural scarcity greatly impede their large-scale productions [9,10]. Besides, these noble metal-based catalysts are unstable and might suffer from oxidation in alkaline electrolytes, forming RuO4 and IrO3 at high anodic potentials and thus being dissolved in the electrolytes [6]. In this respect, extensive effort has been devoted to exploring the alternatives with features of high efficiency, stability, low cost and earth-abundance [11,12].
Layered double hydroxides (LDH) are a family of layered hydroxides, consisting of positively charged host layers, charge-balancing anions and interlayer water molecules [13,14]. Its formula is described as [M2+1-xM3+x(OH)2][An−x/n·mH2O], where M2+/M3+ stands for the divalent/trivalent metal ions, and An− for the anions [4,15]. Their metal composition (x = 0.2–0.33), exchangeable anions and interlayer spacing can be readily tuned, endowing LDH with attractive properties tailoring for various applications [16]. Nevertheless, LDH faces great challenges in their large-scale production as an OER catalyst. The main obstacle is the deficient catalytic activity in the bulk LDH, which hinders the exposure of sufficient electrocatalytically active metal ions in the bulk phases [17]. While, the nano-sized LDH is prone to aggregate together, in addition to the insulating and unstable nature, which also enlarges the mass diffusion and charge transfer pathway during the reaction [18]. Furthermore, despite the high intrinsic activity of LDH, its low electrical conductivity is still one of major problems [6]. With regard to this, it is of great interest to design new LDH nanostructures to address the above-discussed issues.
As aforementioned, the tunable composition makes LDH attractive for specific features [16]. The incorporation of foreign metal ions would modify the electronic structure of pristine LDH and enhance interactions between doped metals and the host metals, significantly catalyzing the reaction activity [16,19]. Besides, coupling LDH with a conductive support is another approach to prompt the catalytic performance by shortening charge transfer pathway in the catalyst [5]. MXene is a new group of two-dimensional nanosheets (early transition metal carbides/carbonitrides), characterized by its excellent electrical conductivity and surface hydrophilicity [20,21]. Previous reports have demonstrated the improved catalytic activity when coupling MXene with the electrocatalytically active phases, such as graphitic C3N4/MXene hybrid film [22], Co-BDC/MXene [23], MoS2/MXene [24], FeNC/MXene [25] and ZIF-67/MXene hybrids [26]. Nevertheless, the development of MXene-based LDH hybrid systems is still in a preliminary stage compared with the prosperity of carbon-supported LDH composites. Hence, a hybrid of foreign-metal-doped LDH and MXene should become an intriguing strategy to pursue.
Herein, we adopt a facile route to synthesize a novel class of MXene-supported ternary transition metal LDH (denoted as NiFeCe-LDH/MXene) hybrid. The corresponding merits are summarized as follows: (i) MXene nanosheets that act as the conductive substrate not only render the hybrid with rapid electron and ion transport, but also hinder the aggregation of the LDH nanoflakes, producing a hierarchical nanoporous structure; (ii) doping Ce3+ into NiFe-LDH may alter the topology of the LDH and cause electron transfer between metal ions due to their different electronegativity; (iii) strong interaction between LDH and MXene, bringing about apparent charge transfer at the interfacial junctions, further promote the catalytic activity. As a result, the obtained NiFeCe-LDH/MXene hybrid exhibited a remarkable OER activity, fast reaction kinetics and superb durability compared to its pure LDH counterparts and the IrO2 catalyst.
Section snippets
Synthesis of Ti3C2Tx MXene
Ti3AlC2 MAX phase was synthesized according to our previous work [27], sieved with a 400 mesh sieve before use. The Ti3C2Tx MXene was obtained by etching Ti3AlC2 (1 g) in a mixture solution of lithium fluoride (1 g) and hydrochloric acid (20 mL, 6 M) for 24 h at 35 °C, followed by several centrifugation-rinsing cycles using deionized water. Subsequently, the resultant product was intercalated using tetrapropylammonium hydroxide solution (TPAOH, 10 mL, 25 wt%) for 24 h at 60 °C, and exfoliated
Results and discussion
The synthesis route of NiFeCe-LDH/MXene hybrid is schematically illustrated in Fig. 1. In brief, Ti3C2Tx MXene nanosheets were first prepared by a LiF-HCl etching method from its MAX phase, followed by an TPAOH-assisted exfoliation treatment. During the etching process, Al layers in the MAX phase were removed, leaving Ti3C2Tx layers terminated with negatively charged surface groups (including –O, –OH, and –F), which would benefit the absorption and anchoring of positively charged ions (such as
Conclusions
In summary, a novel NiFeCe-LDH/MXene hybrid catalyst, consisting of ternary NiFeCe-LDH nanoflakes coupled with MXene nanosheets, was prepared via an in-situ coprecipitation method. It is revealed that the synergistic effect of Ce doping and MXene coupling in the hybrid produced a hierarchical nanostructure, enhanced electrical conductivity and strong interfacial junction in the heterostructure, making it an excellent electrocatalyst. Consequently, the NiFeCe-LDH/MXene hybrid catalyst exhibited
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the Science Foundation of China University of Petroleum, Beijing (No. 2462017YJRC013).
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