Full paperHierarchical Co and Nb dual-doped MoS2 nanosheets shelled micro-TiO2 hollow spheres as effective multifunctional electrocatalysts for HER, OER, and ORR
Graphical Abstract
Introduction
In recent years, many different research efforts have been undertaken in the energy field, especially for renewable energy conversion and storage techniques, such as water splitting, metal-air batteries, and fuel cells, to meet the urgent demand in the near future for green energy. These technologies are primarily realized via the three major HER, OER, and ORR reactions [1], [2]. Unfortunately, the sluggish kinetics and high overpotential of these reactions result in the practical efficiency of the related energy systems becoming much inferior, as compared with the theoretical efficiency [3], [4], and thus efficient electrocatalysts are needed. Even though noble metals (e.g. Pt) or transition metal oxides (e.g. RuO2 or IrO2)-based catalysts have demonstrated excellent catalytic activity, their high cost, insufficient stability, and poor electrical conductivity are critical barriers for large-scale practical application [5], [6]. Accordingly, the design of alternative multifunctional electrocatalysts that meet the requirements of cost-effectiveness, high catalytic efficiency, and good stability for the HER, OER, and ORR is an important task, but so far remains a great challenge.
In recent years, layered molybdenum dichalcogenide (MoS2) has received considerable attention in the catalysis field by reason of its great advantages, such as unique two-dimensional layered graphene-like structure, and excellent electrochemical and mechanical properties [7], [8], [9], [10], [11]. Previous studies have proven that MoS2 not only has high electrocatalytic activity for the HER [8], [11], but also serves as a promising low-cost electrocatalyst for both the ORR and OER [12], [13]. In general, natural semiconducting material with low electrical conductivity and poor exposed active edge sites owing to the irregular reaggregation and agglomeration of MoS2 induces unsatisfactory performance to meet the practical requirements [14], [15], [16]. Therefore, a unique approach is needed to seek an effective route to optimize the catalytic activity of MoS2-based catalysts towards replacing noble metal catalysts.
Among the tremendous efforts that have been made, transition metal doping has been considered an effective pathway to promote the electrochemical properties of MoS2, due to its ability to adjust the electronic structure of MoS2 and optimize the electron density of Mo and S, resulting in the increase of conductivity and new active sites [17], [18]. Shi et al. reported that the doping of transition metals dramatically enhanced the HER activity of MoS2 through exposing more active sites and modifying the 3d electron configuration, leading to a positive shift of about − 0.13 V vs. RHE in the onset overpotential, and far higher turnover (15.44 s−1 at 300 mV) than that of the pure MoS2 reported elsewhere [19]. In another study, Xiong et al. reported significant increase in the HER and OER performances of the MoS2 towards efficient water splitting through Co-doping engineering, demonstrating that effective Co-doping resulted in a minimized bandgap of Co-doped MoS2 (~ 0 eV), which is beneficial to catalyze both the HER and OER reactions [20]. Noticeably, the doping of secondary elements, such as Co, Fe, and Nb, into MoS2 not only effectively regulates the electronic structure towards HER efficiency, but also provides additional OER active sites to boost OER performance [21], [22]. In addition, Wang et al. have performed computational research on the substitutional doping of different transition metals for MoS2, demonstrating that the atoms doping can strongly interact with the S-vacancy, as well as adjust the electronic structure of MoS2, leading to improvement of the ORR activity of MoS2 [23]. Considering the above-mentioned reasons, it is reasonable to expect the development of multifunctional catalysts via transition metal-doped MoS2 materials.
In addition to improving intrinsic activity, the development of heterostructures based on MoS2 and a conductive/stable backbone is also emerging as another effective way to improve catalytic performance. Evidence has been reported that the contactable surface area of the MoS2 for reactants/electrolyte ions could be extended through heterostructural form by alleviating the bulky aggregates. Titanium dioxide (TiO2), with the excellent merits of cheap price, richness, nontoxicity, environmental friendliness, superior chemical stability/durability, and excellent physicochemical properties [24], [25], [26], has been extensively utilized in many different electrochemical applications, especially as a popular catalyst support [27], [28], [29]. Liang et al. has reported that three-dimensional (3D) heterostructured catalysts containing TiO2 and MoS2 nanosheets (NSs), as core and shell parts, respectively, showed dramatic HER performance, in comparison with the pure MoS2 NSs [30]. This study highlighted that the very rough TiO2 layer core not only provided a large amount of favorable sites for the growth of MoS2 without agglomerated form, but during the HER process also acted as an electron transfer layer for smooth charge transfer. Among various morphologies of TiO2 support materials, hollow TiO2 sphere has a large and curved growth surface to benefit MoS2 loading, and offers an effective transport path to improve the mass transfer property of the material [31], [32]. As compared to solid particles, the porous hollow structures possess lower density and mass requirements, and these features are significant for cost reduction [33]. Although there have various efforts to improve the electrocatalytic activities of MoS2, no data has been available on inducing multifunctionality to MoS2 via combining co-doping engineering and constructing MoS2-based heterostructures towards the HER, OER, and ORR applications.
Herein, we have developed a novel multi-shelled hollow structure containing Co and Nb dual-doped MoS2 NSs directly grown on TiO2 hollow microspheres (denoted as Co,Nb-MoS2/TiO2 HSs) as highly active and cost-effective multifunctional electrocatalysts. The spherical hollow structure with ultrathin NSs attached on the surface can enlarge the contact area for electrolyte/reactant, promote the electrolyte/reactant–electrode diffusion/penetration process, and enhance the charge transfer ability. In addition, the Co, Nb-dual doping adjusted the unique electronic structure of the MoS2 towards improving intrinsic catalytic activity and enhanced the electrochemically active surface area (ECSA). Moreover, the strong electronic interactions between the inner TiO2 hollow sphere and outer Co,Nb-MoS2 layer at the interface significantly improved electrical conductivity, thereby providing favorable kinetic for lower overpotential values of electrochemical reactions. Therefore, the Co,Nb-MoS2/TiO2 HSs has the ability to simultaneously catalyze the HER, OER, and ORR with high electrocatalytic performance. The catalyst only required overpotential (η) of 58.8 mV for the HER, and 260.0 mV for the OER, to achieve 10 mA cm−2, which is superior to most of the efficient HER and OER catalysts reported recently. In addition, the fast and favorable kinetics were demonstrated through small Tafel slopes of 40.2 and 65.0 mV dec−1 for HER and OER, respectively. When applied as two electrodes of the electrochemical water splitting device, the Co,Nb-MoS2/TiO2 HSs was capable of driving 10 and 50 mA cm−2 at small operating voltage values of 1.57 and 1.88 V, respectively, and achieved stable gaseous production for 60 h with a retention of 89.2%. The positive onset potential (Eonset) and half-wave potential (E1/2) of + 0.96 and + 0.87 V, respectively, with desirable 4-electron transfer pathway, were also found for the Co,Nb-MoS2/TiO2 HSs towards the ORR. Furthermore, the catalyst showed good stability under the ORR condition with a current density retention of 96.65% after 60,000 s along with excellent alcohol tolerance, which even outperformed the commercial Pt/C catalyst.
Section snippets
Chemical reagents
Sodium lauryl sulfate (CH3(CH2)11OSO3Na, ≥ 98.5%), potassium persulfate (K2S2O8, ≥ 99%), methanol (CH3OH, ≥ 99.9%), styrene (C8H8CH=CH2, ≥ 99%), ethanol (CH3CH2OH, ≥ 99.8%), hexadecylamine (CH3(CH2)15NH2, 98%), ammonium hydroxide (NH4OH, 28.0% NH3 in H2O), titanium isopropoxide (Ti[OCH(CH3)2]4, 99.9%), cobalt nitrate hexahydrate (Co(NO3)2.6H2O, ≥ 98%), ruthenium oxide (RuO2, 99.9%), niobium chloride (NbCl5, 99%), ammonium tetrathiomolybdate ((NH4)2MoS4, 99.97%), potassium hydroxide (KOH, ≥ 85%),
Morphological and structural characterization
FE-SEM and energy-dispersive X-ray (EDAX) analyses were used to verify the morphology and elemental composition of the materials. Fig. S1a and b show SEM images of PS at different magnifications, which reveal the spherical structure, along with the average diameter of ~ 350 nm, together with very smooth surface. A typical EDAX pattern of the PS (Fig. S1c) indicates existence of C and O elements in the material, demonstrating that the obtained microspheres are PS. The SEM images of PS@TiO2 (Fig.
Conclusions
In summary, we have developed a novel Co,Nb-MoS2/TiO2 HSs hybrid material with mesoporous nanosheet-shelled heterostructure via a simple and effective strategy. Benefiting from the synergistic effects of valuable factors, including the unique hollow core–shell structure containing strong electronic interactions between the core part and shell layer, the Co and Nb dual-doping effects, and the ultrathin and mesoporous characteristics of the MoS2 NSs, the catalytic performance of the Co,Nb-MoS2/TiO
CRediT authorship contribution statement
Dinh Chuong Nguyen: Methodology, writing-original draft, validation, validation visualization. Thi Luu Luyen Doan: Methodology, investigation, formal analysis. Sampath Prabhakaran: Density-functional theory calculation. Duy Thanh Tran: Writing- reviewing and editing. Do Hwan Kim: Density-functional theory calculation, writing- reviewing and editing. Joong Hee Lee: Conceptualization, data curation, writing- reviewing and editing, supervision. Nam Hoon Kim: Conceptualization, writing- reviewing
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 research was supported by the Basic Science Research Program (2019R1A2C1004983) and the Regional Leading Research Center Program (2019R1A5A8080326) through the National Research Foundation funded by the Ministry of Science and ICT of Republic of Korea.
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