Abundant heterointerfaces in MOF-derived hollow CoS2–MoS2 nanosheet array electrocatalysts for overall water splitting
Introduction
The booming development of modern society has raised an urgent demand for sustainable and economical energy sources beyond conventional fossil fuels [1], [2]. Considering the high calorific value (142 MJ kg−1) and pollution-free advantages, the massive production of clean hydrogen fuel by electric-powered water splitting has drawn extensive attention for future energy supply [3], [4]. Broad adaption of water electrolysis, however, has been restricted by the inefficient anodic oxygen evolution reaction (OER) and the cathodic hydrogen evolution reaction (HER), which demand high-active catalysts to reduce the overpotential requirement [5], [6]. Currently, precious-metal-based materials are still the state-of-the-art electrocatalysts for OER (e.g. IrO2) and HER (e.g. Pt) [7], [8]. However, their scalable utilization faces a few congenital deficiencies such as limited elemental reserves and subsequent rocketing costs [9], [10]. Therefore, tremendous research efforts have been focused on developing various earth-abundant alternatives for OER/HER processes, such as transition metal oxides [11], [12], carbides [13], nitrides [14], chalcogenides [15], [16], [17], [18], and phosphides [19], [20]. However, the widely reported catalysts usually show high OER activity at the expense of the HER activity, and vice versa. Moreover, the working medium for most HER catalysts (such as acidic) is incompatible with that used for OER catalysts (such as alkaline), thus resulting in a moderate water-splitting efficiency. There is still a lack of low-cost and high-efficiency precious-metal-free electrocatalysts to simultaneously catalyze OER/HER processes.
Transition metal chalcogenides (TMCs) are a brilliant class of advanced materials for cutting-edge electrolytic applications [21]. For instance, molybdenum disulfide (MoS2) materials with abundant exposed edge sites and unique two-dimensional structure have been widely deemed to be promising Pt-substituting HER catalyst under acidic conditions [22], [23]. Nevertheless, under alkaline conditions, they are usually inert in HER not to mention in OER, because of their unfavorable water adsorption and dissociation features [24], [25]. To resolve these issues, a number of strategies have been carried out, including morphologies design [26], [27], defect engineering [28], [29], phase regulating [30], [31], and compositions optimizing [32], [33], [34], [35]. Among them, integration with foreign active species represents a smart avenue to stimulate MoS2-based electrocatalysts for both alkaline OER and HER processes. In this regard, cobalt sulfides (e.g. Co9S8, Co3S4, CoS2, CoS) clearly stand out by the virtue of metallic-like conductivity and multiple 3d electrons configuration [36], [37], which makes them as a smart choice to form heterostructures with MoS2 [38]. Along this way, heterostructures of cake-style CoS2@MoS2@RGO [39], Co3S4@MoS2 hollow polyhedron [40], Co9S8@MoS2 core–shell nanoparticle [41], CoS/MoS2 hierarchical nanotube [42], Co3S4@MoS2 nanoboxes [43], and hierarchical MoS2/CoS2 nanotube [44] have been reported over past few years. However, the efficiency of these Co-S/MoS2 heterostructures for overall water splitting is far from that of benchmark IrO2 and Pt couples.
As we all know, heterogeneous catalysis belongs to be a surface process, in which multiscale surface engineering of electrocatalysts is able to expedite their catalytic efficiency [45]. The engineering of the multiscale surface mainly includes three aspects: (1) Designing suitable interface within heterostructures to boost electrocatalytic kinetics [46], [47], [48]. For example, Yang et al. [49] demonstrated that the modulated interface in hierarchical MoS2–Ni3S2 heteronanorods was beneficial to local charge redistribution and accordingly favored the activation of hydrogen and oxygen-containing intermediates in HER and OER processes. (2) Nanostructuring surface of catalysts to enlarge the number of catalytically active sites [50], [51], [52]. Taking NiCo2O4 as a example, Gao et al. [53] revealed that the unique features of porous and hollow nanostructures, such as large specific surface areas, high exposure of active sites, shortened ion diffusion length, were vital for achieving superior activity toward electrocatalytic water-splitting. (3) Architecting self-supported nanoarray electrode nanostructures to tailor a reasonable triple-phase interface among gas/solid (catalysts)/liquid (electrolyte) [54], [55]. For instance, benefiting from the enhanced electron migration, mass transfer, and gas release, the Co9S8/Ni3S2 nanowire array exhibited higher electrocatalytic performance as compared to its powder counterpart [56], [57]. To date, there are few research efforts dedicated to design bimetal (Co and Mo) sulfide heterostructures by combining the above design strategies and a featured architecture for overall water splitting.
Banking on the above consideration, we report fabrication of hollow CoS2–MoS2 heteronanosheet arrays on titanium foil (denoted as CoS2–MoS2 HNAs/Ti) through a facile metal–organic framework (MOF) template-directed strategy. When used as bifunctional electrocatalysts for water splitting, the resultant CoS2–MoS2 HNAs/Ti shows multiscale surface advantages. First, the well-exposed heterogeneous interfaces between CoS2 and MoS2 regulate local charge distribution, which dramatically lowers the kinetics barriers for water decomposition. Second, the hollow porous nanostructures allow high exposure of surface-active sites and intimate electrolyte access to active sites. Third, the nanoarrays with a tight connection to the conductive titanium foil can be used as a binder-free electrode, which not only prevents the active materials from delaminating, but also ensures good electric conductivity and charge transfer capability. Given these advantages, CoS2–MoS2 HNAs/Ti yields highly efficient HER and OER electrolysis with small overpotentials of 82 and 266 mV at 10 mA cm−2, respectively. The alkaline electrolyzer based on CoS2–MoS2 HNAs/Ti enables overall water splitting with a current density of 10 mA cm−2 at a cell voltage of 1.56 V, which is even smaller than the integrated IrO2 and Pt/C couples. These inspiring results highlight the importance of a multiscale surface engineering strategy for the exploration of efficient and robust bifunctional electrocatalysts for large-scale alkaline electrolyzers.
Section snippets
Preparation of solid Co-MOF arrays/Ti
In a typical synthesis, 0.65 g Co(NO3)2 and 1.46 g 2-methylimidazole were dissolved in 40 mL water, respectively. Subsequently, the two solutions were quickly mixed under continuous magnetic stirring. After 10 min, a piece of clean Ti foil (2 × 5 × 0.036 cm3) was immersed into the mixture solution at room temperature. After reaction for 4 h, the sample was taken out, washed by deionized water and dried overnight at 60 °C.
Preparation of hollow CoMo-LDH arrays/Ti
0.3 g Na2MoO4 was first dissolved in 60 mL mixed solution (40 mL water and
Results and discussion
The synthesis process of porous and hollow CoS2–MoS2 heteronanosheet arrays on titanium foil is illustrated in Figs. 1(a) and S1. A facile solution method was firstly utilized to fabricate Co-MOF nanosheet arrays on Ti foil, according to our previous works [58] and related literature [20], [59]. The SEM images indicate that the Co-MOF arrays grown on Ti foil present typical 2D leaf-like features with the solid nature and the smooth surface (Fig. 1b and c). XRD results (Fig. S2) confirm the
Conclusions
In summary, we have successfully constructed porous and hollow CoS2–MoS2 heteronanosheet arrays on Ti foil through a facile MOF-based strategy. Systematic investigation indicates that ample heterointerfaces of CoS2–MoS2 HNAs/Ti are capable of tuning local charge distribution, which propels water dissociation and accelerates OER/HER kinetics. At the same time, the self-supported hollow porous 2D architecture displays several advantages including high surface area, intimate electrolyte
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.
Acknowledgment
The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China (NSFC) Grants (51702295).
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