Research Article www.acsami.org
Integrated Energy Aerogel of N,S-rGO/WSe2/NiFe-LDH for Both Energy Conversion and Storage Xiaowei Xu,† Hang Chu,† Zhuqing Zhang,† Pei Dong,‡ Robert Baines,‡ Pulickel M. Ajayan,‡ Jianfeng Shen,*,† and Mingxin Ye*,† †
Institute Special Materials and Technology, Fudan University, Shanghai 200433, China Department of Materials Science and NanoEngineering, Rice University, 6100 Main Street, Houston, Texas 77005, United States
‡
S Supporting Information *
ABSTRACT: High-performance active materials for energy-storage and energy-conversion applications require a novel class of electrodes: ones with a structure conducive to conductivity, large specific surface area, high porosity, and mechanical robustness. Herein, we report the design and fabrication of a new ternary hybrid aerogel. The process entails an in situ assembly of 2D WSe2 nanosheets and NiFe-LDH nanosheets on a 3D N,S-codoped graphene framework, accomplished by a facile hydrothermal method and electrostatic self-assembly technology. The obtained nanocomposite architecture maximizes synergistic effects among its three 2D-layer components. To assess the performance of this hybrid material, we deployed it as an advanced electrode in overall water splitting and in a supercapacitor. Results in both scenarios attest to its excellent electrochemical properties. Specifically, serving as a catalyst in an oxygen evolution reaction, our nanocomposite requires overpotentials of 1.48 and 1.59 V to obtain current densities of 10 and 100 mA cm−2, respectively. The hybrid material also efficiently electrocatalyzes hydrogen evolution reactions in base solution, necessitating overpotentials of −50 and −237 mV for current densities of 1.0 and 100 mA cm−2, respectively. The 3D hybrid, when applied to a symmetric supercapacitor device, achieves 125.6 F g−1 capacitance at 1 A g−1 current density. In summary, our study elucidates a new strategy to maximize efficiency via synergetic effects that is likely applicable to other 2D materials. KEYWORDS: N,S-rGO/WSe2/NiFe-LDH, hybrid aerogel, 3D structure, overall water splitting, supercapacitors
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INTRODUCTION Prolific fossil fuel consumption has raised concerns about environment pollution, triggering significant research interest in energy storage and conversion devices. Attractive prospects typify high performance, are eco-friendly, and are cheap to fabricate. Novel electrode materials for energy-conversion technologies, particularly electrocatalytic water splitting, have attracted particular attention.1−3 Electrocatalytic water splitting consists primarily of hydrogen and oxygen evolution reactions (HER and OER).4−9 To date, most of the state-of-art electrocatalysts for water splitting employ precious metals. For instance, Pt is used for HER under acidic conditions, while IrO2 or RuO2 is applicable in OER under basic conditions.10−12 Unfortunately, large-scale application of precious-metal based catalysts is strictly impeded by their high cost and scarcity. Thus, recent years witnessed extensive initiatives dedicated to developing highly efficient, economical, and earth-abundant electocatalysts for HER and OER. Of the catalysts available today, transition-metal dichalcogens (TMDs), such as MoS2, MoSe2, WS2, and WSe2, have great electrocatalytic activity for HER in acidic electrolytes,13−18 while transition-metal-layered double hydroxides (LDHs) demonstrate fair activity and stability for OER in basic electrolytes.19−22 Moreover, the act © 2017 American Chemical Society
of hybridizing heteroatom-doped graphene with HER or OER catalysts represents a recent school of thought that could further enhance electrocatalytic performance by effectively combining reaction kinetics.23,24 Heteroatom-doped graphenebased nanocomposites have been used for supercapacitor applications as well.25,26 Such graphene-based nanocomposites have the potential to ameliorate energy storage devices by leveraging the ample specific surface area of graphene and Faradaic redox reactions of TMDs or LDHs.27−32 Despite the progress investigating graphene-based nanocomposites as electrocatalysts achieved to date, the overall performance of the composite electrodes needs to be optimized to be comparable to commercially available electrode materials. Furthermore, the electrode materials for HER or OER are disparate, usually only working well under acidic or basic medium. Performance and versatility are valuable qualities, but they are challenging to attain for new composite electrode materials, especially multicomponent materials for both energystorage and energy-conversion systems. Received: July 7, 2017 Accepted: September 7, 2017 Published: September 7, 2017 32756
DOI: 10.1021/acsami.7b09866 ACS Appl. Mater. Interfaces 2017, 9, 32756−32766
Research Article
ACS Applied Materials & Interfaces
was stirred at 35 °C in a water bath for 1 h followed by addition of H2O (92 mL). The color of the solution turned yellow after additional stirring at 98 °C for 1 h. Then the solution was further diluted with distilled water (300 mL), and H2O2 (30%, 30 mL) was added. After being settled overnight, the remaining product was centrifuged and washed with distilled water until the pH became neutral. The product was dried in vacuo to obtain the brown flake GO. Preparation of 2D WSe2 Nanosheets. WSe2 nanosheets were synthesized by a liquid-phase exfoliation process. Typically, bulk WSe2 (20 mg) was added to 4 mL of isopropanol (IPA)/water cosolution with IPA volume fraction of 60%. The mixture was batch sonicated in an SB-5200DTD sonicator at a power of 200 W and a frequency of 40 kHz for 4 h with circulating cooling water to keep the ambient temperature during sonication. Subsequently, the dispersion was centrifuged at 4000 rpm for 20 min to remove the remaining unexfoliated bulk materials. The collected supernatant possessed a concentration of about 0.5 mg mL−1 Preparation of NiFe-LDH Nanosheets. NiFe-LDH was synthesized according to the previously reported method. In a typical experiment, 0.27 g of Ni(NO3)2·6H2O, 0.09 g of Fe(NO3)3·9H2O, 0.12 g of urea, and 0.03 g of trisodium citrate were dispersed in 75 mL of distilled water and sonicated for about 30 min until clear. The resultant solution was transferred into a Teflon-lined stainless steel autoclave, sealed, and heated at 150 °C for 20 h. After being cooled to room temperature, the solid product was collected by filtration, washed with distilled water and ethanol three times each, and dried in vacuum at 60 °C for 8 h. The layered NiFe-LDH nanosheets were obtained by a liquid phase exfoliation process via sonication in a degassed formamide solution. Subsequently, the dispersion was centrifuged at 4000 rpm for 20 min to remove the remaining unexfoliated bulk materials. The collected supernatant possessed a concentration of about 1 mg mL−1. Preparation of N,S-rGO/WSe2. L-Cysteine (50 mg) and 300 μL of NH3·H2O (27 wt %) were gradually added to a mixture solution of 5 mL of GO (2.0 mg mL−1) and 5 mL of WSe2 (0.5 mg mL−1), followed by an ultrasonic treatment. The resulting mixture solution was transferred to a Teflon-lined stainless steel autoclave, sealed, and heated at 180 °C for 3 h. The autoclave was cooled down naturally to room temperature to obtain N,S-rGO/WSe2 hydrogels. Subsequently, the N,S-rGO/WSe2 hydrogels were washed several times with distilled water and then freeze-drying to obtain N,S-rGO/WSe2 aerogels. Similarly, the N,S-rGO aerogels were synthesized using the same method without the adding of WSe2 solution, and the synthesis of NrGO aerogels was similar to N,S-rGO aerogels just using ascorbic acid (Vc) instead of L-cysteine. Preparation of N,S-rGO/WSe2/NiFe-LDH. The obtained N,S-rGO/ WSe2 aerogels were immerged into a layered NiFe-LDH nanosheets dispersion (1 mg mL−1) for 24 h to reach electrostatic self-assembly equilibrium of the layered NiFe-LDH nanosheets on N,S-rGO/WSe2 aerogels. Thereafter, the obtained hybrid hydrogels of N,S-rGO/ WSe2/NiFe-LDH were washed several times with distilled water and then freeze-dried to obtain N,S-rGO/WSe2/NiFe-LDH aerogels. Similarly, the N-rGO/WSe2/NiFe-LDH aerogels were synthesized using the same method using just N-rGO/WSe2 aerogels instead of N,S-rGO/WSe2 aerogels. Preparation of N,S-rGO/NiFe-LDH. The obtained N,S-rGO aerogels were immerged into a layered NiFe-LDH nanosheet solution (1 mg mL−1) for 24 h to reach electrostatic self-assembly equilibrium of the layered NiFe-LDH nanosheets on N,S-rGO aerogels. Thereafter, the obtained hybrid hydrogels of N,S-rGO/NiFe-LDH were washed several times with distilled water and then freeze-dried to obtain N,S-rGO/NiFe-LDH aerogels. Materials Characterization. The morphology and microstructural of the samples were studied by scanning electron microscopy (SEM, Tescan MAIA3 XMH) and transmission electron microscopy (TEM, JEOL 2010). The crystal structures of the samples were determined by XRD (D/max-γB diffractometer) with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) was conducted to characterize the elemental compositions of the samples on XR 5 VG (UK). Raman spectra were conducted on a Dilor LABAM-1B multichannel confocal microspectrometer with 514 nm laser excitation. The energy
The fabrication of ternary composite electrodes with tailored nanostructure offers a step toward improving electrocatalysis or supercapacitor performance. For instance, 3D ternary hybrids based on EG/Co0.85Se/NiFe-LDH demonstrate excellent electrocatalytic activity for overall water splitting due to the strong and stable coupling effect among its components.19 Likewise, the exceptional supercapacitor performance of ternary CoNi2S4-graphene-2D-MoSe2 can be ascribed to its welldesigned nanostructure that maximizes synergistic effects among constituent parts.33 Recently, tungsten selenide (WSe2) nanosheets, as representative TMDs, have attracted critical attention in the field of energy storage.34 Nickel−ironlayered double hydroxide (NiFe-LDH) materials not only exhibit high OER activity but also demonstrate admirable supercapacitor performance.35,36 To this end, integrating 2D WSe2 nanosheets and NiFe-LDH with graphene is a promising strategy to optimize interfacial interactions due to surface area and conductivity benefits. The integrated composites may serve as multifunctional electrode materials able to be applied in water splitting and supercapacitors under basic conditions. Strong yet stable interfacial contact is the key for enhancing performance of ternary composites. This being said, 3D graphene frameworks have advantages over 2D graphene nanosheets, such as a hierarchically porous structure, multidimensional electron transport pathways, and superior conductivity, all of which benefit electrocatalysis and supercapacitor performance.37,38 Among the promising results of graphene-based aerogel materials for energy applications,39−41 there have been no reports on controllable integration of WSe2 nanosheets and NiFe-LDH into 3D N,S-codoped graphene frameworks. Tuning the structure to a desired synergetic activity lends itself well to overall water splitting and supercapacitor use. Herein, we report the design and fabrication of a novel ternary hybrid aerogel based on N,S-codoped graphene (N,SrGO), WSe2 nanosheets, and NiFe-LDH. First, an aerogel based on N,S-codoped graphene and WSe2 nanosheets was synthesized by a hydrothermal method. During the formation of aerogel, graphene oxide was reduced to graphene and simultaneously doped with N and S by L-cysteine. Then, the aerogel was immerged into a NiFe-LDH nanosheet suspension. This yielded ternary hybrid aerogel via electrostatic attraction between oppositely charged nanosheets. The N,S-rGO/WSe2/ NiFe-LDH hybrid materials have an optimal architecture for use as an electrode in both water splitting and supercapacitors. We then thoroughly investigate the electrochemical properties of the hybrid material and propose a possible mechanism by which it operates. Lastly, we attribute its excellent electrochemical performance to the notable synergistic effects of efficient charge separation promoted by graphene, superior HER activity coming from WSe2, and excellent OER activity from NiFe-LDH.
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EXPERIMENTAL SECTION
Materials Preparation. Graphite (
