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Remarkably Improved Electrochemical Hydrogen Storage by Multi-Walled Dalton Transactions View Article Online PAPER View Journal | View Issue Remarkably improved electrochemical hydrogen storage by multi-walled carbon nanotubes Cite this: Dalton Trans., 2019, 48, 898 decorated with nanoporous bimetallic Fe–Ag/TiO2 nanoparticles Raziyeh Akbarzadeh,a Mehrorang Ghaedi, *a Syamak Nasiri Kokhdana and Daryoosh Vashaee b Nanoporous bimetallic Fe–Ag nanoparticles (NPs) were synthesized using a facile chemical reduction method and used to decorate the surface of multi-walled carbon nanotubes (MWCNTs) for hydrogen sorption and storage. The effect of TiO2 nanoparticles on the hydrogen storage properties of Fe–Ag/ CNTs was further studied in detail. For this purpose, several nanocomposites of nanoporous bimetallic Fe–Ag/TiO2 nanoparticles with different amounts of bimetallic Fe–Ag NPs were prepared via a hydro- thermal method. The hydrogen storage capacity of the as-prepared nanocomposites was studied using electrochemical methods. The Fe–Ag/TiO2/CNT nanocomposite with 0.04 M bimetallic Fe–Ag NPs showed the highest capacity for hydrogen storage, which was ∼5× higher than that of pristine MWCNTs. Received 27th September 2018, The maximum discharge capacity was 2931 mA h g−1, corresponding to a 10.94 wt% hydrogen storage Accepted 3rd December 2018 capacity. Furthermore, a 379% increase in discharge capacity was measured after 20 cycles. These results DOI: 10.1039/c8dt03897j show that Fe–Ag/TiO2/CNT electrodes display superior cycling stability and high reversible capacity, rsc.li/dalton which is attractive for battery applications. 1. Introduction impressive chemical stability, high mechanical ability, low density and high electron-transfer rate.13,14 Hydrogen energy is researched extensively as an alternative to There are two hydrogen adsorption pathways in CNTs. First fossil fuels because of critical advantages such as large heat of is the physical adsorption of hydrogen molecules, which is −1 Published on 04 December 2018. Downloaded by North Carolina State University 9/9/2020 11:08:16 PM. combustion (287 kJ mol ), abundant supply, renewability, favored at a high pressure H2 atmosphere. Second is electro- high efficiency, environmentally compatible by-product (water) chemical adsorption of hydrogen, which is applicable in sec- and greenhouse gas emissions, etc.1,2 However, design and ondary batteries that are beneficial in configuring small-scale development of appropriate hydrogen storage materials with energy storage systems.15 Electrochemical hydrogen storage long-lasting stability is a scientific challenge that is crucial for provides higher capacity at ambient temperature and atmos- – their use in a future hydrogen economy.3 5 Sodium boro- pheric pressure than conventional hydrogen storage techno- 16 hydride (NaBH4) has emerged as a new and promising hydro- logy at low temperature and high pressure. – gen storage material.6 8 Metals, mainly Ni, Ag, Pd and Pt, or metal oxides, such as In recent years, great attention has been devoted to the TiO2 nanocatalysts, decorated onto CNTs lead to an increase development of new porous materials with high hydrogen in hydrogen storage capacity through a very interesting – sorption ability, such as carbon nanotubes (CNTs), graphene mechanism called spillover.7,13,17 20 Metal or metal oxide – and few-layer graphene.9 12 CNTs are a good choice for electro- nanoparticles spill hydrogen over the carbon nanotubes and chemical hydrogen storage due to their unique physical and when their highest content is exposed to hydrogen during chemical properties, such as high specific surface area, the adsorption process the spillover mechanism is achieved.19,21 Moreover, bimetallic catalyst nanostructures are a good choice for achieving selectivity, stability and superior specific a Chemistry Department, Yasouj University, Yasouj 75918-74831, Iran. activity in various applications, such as catalysis,22,23 chemi- E-mail: [email protected], [email protected]; Fax: +98 741 2223048; cal/gas sensors,24 fuel cells,25,26 hydrogen generation27 and Tel: +98 741 2223048 28,29 bElectrical & Computer Engineering Department, North Carolina State University, hydrogen storage, and are more favorable in sustainable Raleigh, NC 27606, USA energy technology than their monometallic counterparts. The 898 | Dalton Trans.,2019,48,898–907 This journal is © The Royal Society of Chemistry 2019 View Article Online Dalton Transactions Paper addition of the second metal may cause considerable changes TiO2 nanoparticles decorated onto the surface of MWCNTs in catalytic properties, activity and selectivity.23 with a composition of 0.04 M Fe–Ag nanoparticles significantly Palladium and platinum, and their bimetallic systems, are enhance hydrogen storage capacity. a class of interesting materials for storing hydrogen due to their fast adsorption/desorption rates, but their application for hydrogen storage is restricted because of their high 2. Experimental – cost.17,29 31 The conspicuous physicochemical catalytic properties, The chemicals used in this investigation were iron(III) nitrate strong structural characteristics and relatively low cost of silver nonahydrate (Fe(NO3)3·9H2O), silver nitrate (AgNO3), tri- make it a noteworthy material for hydrogen storage. Silver can sodium citrate, titanium isopropoxide (TTIP) and multi-walled – combine with some other metals, such as iron, to form bi- carbon nanotubes (MWCNTs, outer diameter 10 20 nm, metallic systems.32,33 The effect of iron on improving the length 20 µm) purchased from Sigma-Aldrich. Polyvidone 25, effective storage capacity of such hydrogen storage alloys has sodium borohydride (NaBH4) and acetic acid were obtained been investigated.34,35 In particular, bimetallic Fe–Ag particles from Merck. Absolute ethanol and deionized water were used exhibited higher hydrogen storage compared to monometallic during the preparation of powders. All chemicals were used Fe or Ag particles. This increase in storage capacity is attribu- without further purification. ted to the strong metal–metal interaction which enhances the 2.1. Materials characterization hydrogen spillover effect by adjustment of the electronic struc- ture of Ag using Fe. The size and surface site distribution of The powder X-ray diffraction (XRD) patterns of the products the particles has a significant influence on the electrochemical were determined using a Phillips X’Pert PRO equipped with a response of hydrogen in hydrogen storage materials.29,36,37 Cu Kα source having a scanning range of 0–80° Bragg angle. Fast hydrogen diffusion in nanostructured materials signifi- The morphological characterization was obtained using a cantly improves the kinetics of hydrogen adsorption and Philips XL-30FESEM field emission scanning electron micro- – desorption.38 40 Therefore, CNTs decorated with bimetallic scope (FESEM) equipped with an energy dispersive X-ray analy- materials are expected to have the ability of electrochemical zer (EDS). Elemental distribution of the samples was revealed hydrogen storage.29,31 Additionally, metal oxides are attractive, using a HAADF-STEM-FEI Titan 80–300 high-angle annular low toxicity and economical energy storage materials for dark-field scanning transmission electron microscope electrochemical capacitor applications.19 Among the metal (HAADF-STEM) equipped with EDS. EDS spectroscopy and 41 42 oxides, TiO2 with anatase and rutile phases has attracted elemental mapping were used for compositional analysis and much interest and is a promising material for energy storage to investigate the homogeneity of the elemental distribution. because of its excellent charge storage capability, high redox Raman spectra were measured with a Takram P50C0R10 activity, low cost and superior electrochemical properties.43,44 Teksan dispersive Raman spectrometer equipped with a To date, various composites have been developed based on 532 nm Nd:YLF laser as the excitation source. TiO2 and conductive components such as carbon nanotubes – – (CNTs), graphene and carbon black.45 47 Research results have 2.2. Preparation of nanoporous bimetallic Fe Ag nanoparticles and Fe–Ag/CNT nanocomposites Published on 04 December 2018. Downloaded by North Carolina State University 9/9/2020 11:08:16 PM. demonstrated that TiO2 nanocatalyst-coated CNTs have better electrochemical storage performance, which enhances their The nanoporous bimetallic Fe–Ag NPs were prepared as 45,48,49 hydrogen storage capacity, than pure CNTs. follows: 5 ml 0.02 M Fe(NO3)3·9H2O, 5 ml 0.034 M tri-sodium Inspired by these findings, we have studied low cost but citrate and 10 ml 0.5 g polyvidone 25 (which all dissolved in effective materials with a high capacity for hydrogen storage. deionized water) were diluted by 20 ml of deionized water. The In this work, a Fe–Ag/CNT nanocomposite with high hydrogen mixed solution was stirred for 15 min at ambient temperature storage capacity was prepared by coating nanoporous bi- under an argon atmosphere and subsequently 0.3 g sodium metallic Fe–Ag nanoparticles, synthesized by a simple chemi- borohydride in 20 ml deionized water was added dropwise. cal reduction method, onto the surface of oxidized MWCNTs. After 1 h stirring, 5 ml 0.01 M AgNO3 and 10 ml sodium boro- Moreover, nanoporous bimetallic Fe–Ag/TiO2 nanoparticles hydride solution (0.2 g NaBH4) were added dropwise into the with different amounts of Fe–Ag were synthesized using a solution. After 2 h of stirring, the colloidal solution obtained facile hydrothermal method, and subsequently the effect of was centrifuged at 12 000 rpm for
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