Microwave-assisted Polyol Reduction Method Research Articles

Nano-engineered catalysts for high-performance oxygen reduction reaction

The efficient energy conversion of fuel cells is greatly constrained by the slow oxygen reduction reaction (ORR) kinetics, which necessitates the use of highly active metal catalysts such as platinum (Pt). The critical challenge limiting large-scale usage of Pt is the capital cost that can be addressed through a prototypical approach by embedding metal nanoparticles (NPs), e.g., Pt NPs, in the conductive framework. However, previously reported embedding approaches are sophisticated and suffer from limited yields, leading to higher chemical process costs and remaining distant from commercial viability. Here, we report a facile, cost-effective and time-efficient structural tuning approach to synthesizing ultrafine Pt NPs impregnated within a conductive and highly porous carbon framework via a microwave-assisted polyol reduction method. Pt NPs with a uniform size of ∼2.27 nm can be successfully integrated within the pores of the carbon framework, enabling homogeneous dispersion. Benefiting from these highly dispersed and ultrafine Pt NPs, the electrochemical surface area (ECSA) is improved to 142.98 m²/gPt, 2.25 times higher than that of the commercial counterpart (63.52 m²/gPt). Furthermore, our structurally optimized catalyst composite features a remarkably catalytic activity with a high half-wave potential (E1/2) of 0.895 V and an improved mass activity (MA) of 0.2289 A/mgPt, 2.39-fold improvement compared to the commercial counterpart. In addition, orthogonal experiments were designed to identify the key process parameters for fabricating Pt/C catalysts, offering insights for scaled-up and industrial production.

Oxygen-vacancy-rich tungsten oxide boosted ultrasmall iridium nanoparticles for acidic oxygen evolution

Proton exchange membrane water electrolysis (PEMWE) exhibits the superior benefits including high gas purity, high power density and wide variable loading but is still limited by the prohibitive cost and dearth of iridium (Ir)-based anode catalysts. Herein, we report a low-loading Ir-based catalyst with ultrasmall iridium nanoparticles (1.46 ± 0.01 nm) supported on oxygen-vacancy-rich tungsten oxide nanofibers (WO3-x) through electrospinning combined with surfactant-free microwave-assisted polyol reduction method for active acidic oxygen evolution reaction (OER). According to structural analysis and theoretical calculation, the charge transfer from W to Ir is induced by the strong electronic interaction at the Ir and WO3-x interface, thereby raising the electron density surrounding the Ir sites and favoring to boost OER activity. The resultant Ir@WO3-x exhibits an excellent overpotential of ca. 276 mV@10 mA cm−2 and remarkable durability. This study offers a new path for promoting the activity and durability of electrocatalysts while minimizing the mass loading of Ir towards OER.

Cu@Pt/NCNT preparation and electrochemical performance

In this paper, platinum–copper (Pt–Cu) alloy colloid was prepared using the microwave-assisted polyol reduction method with chloroplatinic acid and copper chloride as precursors. During the preparation, the metal particles were loaded on the nitrogen-doped carbon nanotubes (NCNTs) using the sol–gel method. Catalysts with different metal proportions were prepared to investigate the activity of oxygen reduction reaction (ORR), and commercial Pt/C catalyst was used as control. A series of characterizations including X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy were employed to study the morphology and composition of the catalysts. The results revealed that the Pt–Cu alloy can be well loaded on NCNTs and displayed good ORR catalytic activities, especially CuPt–NCNT1:3 with the mass activities as 0.12A/mgPt; Ik was 0.56 mA/cm2Pt, which was better than those of the commercial Pt/C catalyst. It was also found that the half-wave potential and diffusion-limited current curves were close to those of the commercial ones, which demonstrate a simple and effective way to prepare the catalyst for fuel cell.

Use of palladium nanoparticles dispersed on GNS - modified with 10 wt%CoMoO4 as efficient bifunctional electrocatalysts

Highly dispersed Pd-nanoparticles (NPs) are obtained on CoMoO4-doped graphene nano-sheets (GNS) by microwave-assisted polyol reduction method for use as electrocatalysts for methanol oxidation (MOR) and oxygen reduction (ORR) reactions. The activity of Pd/10 wt%CoMoO4-GNS towards MOR and ORR has been examined by cyclic (CV) and linear sweep (LSV) voltammetries and chronoamperometry measurements in 1 M KOH at 25 °C . The structural characterization of the hybrid catalyst was performed by X-ray diffraction, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. Results have shown that the novel Pd/10 wt%CoMoO4-GNS electrode has the MOR and ORR activities superior to Pd/GNS.

Nitrogen-doped graphene supported Cu-Ag2.9 nanoparticles as efficient methanol tolerant cathode for oxygen reduction

Highly dispersed nanoparticles (NPs) of Cu, Ag and their composites are obtained on nitrogen-doped graphene nanosheets (N-GNS) using a microwave-assisted polyol reduction method. Structural, electrochemical and electrocatalytic surface characterization of materials have been carried out by TEM, XRD, XPS, cyclic (CV) and linear sweep (LSV) voltammetries. The study shows that the crystallites of the Cu-Ag composite follow the face-centered cubic (fcc) crystal structure. Further, the catalytic activities of Cu/N-GNS and Ag/N-GNS towards oxygen reduction reaction (ORR) are greatly enhanced when Cu and Ag are dispersed on the N-GNS surface in 1:2.9 M ratio. The active composite material does not oxidize methanol practically and has outstanding stability under ORR condition.

1D-2D carbon heterostructure with low Pt loading as a superior cathode electrode for dye-sensitized solar cell

Cost-effective counter electrode (CE) with high electrocatalytic performance is very much essential for the wide application of dye-sensitized solar cells (DSSC). The 1D-2D carbon heterostructure (Pt/GR@CNT) with low platinum (Pt) loading has been synthesized by a facile in situ microwave-assisted polyol-reduction method. The excellent electrocatalytic activity as well as photovoltaic performance was achieved due to the combination of 2D graphene nanoribbons (GR) and 1D multi-walled carbon nanotubes (CNT) with high catalytically active Pt nanoparticles. Microwave-assisted longitudinal unzipping of few outer layers of CNTs along with co-reduction of Pt nanoparticles is an effective method to create electrochemically active defective edge sites, which have a crucial role in enhancing electrochemical performance. Synergistic effect of ultra-fine Pt nanoparticles, partially unzipped graphene nanoribbons and inner core tubes of CNTs modulates the power conversion efficiency of solar cell to 5.57% ± 0.03 as compared with 4.73% ± 0.13 of CNTs. Pt/GR@CNT CE even with low Pt loading of 14 μg cm−2 showcases equivalent performance with that of pure Pt counter electrode.

Functional separation of oxidation–reduction reactions and electron transport in PtRu/ND and conductive additive hybrid electrocatalysts during methanol oxidation

Undoped nanodiamond (ND) supported PtRu (PtRu/ND) electrocatalyst for methanol oxidation reactions (MOR) in direct methanol fuel cells was prepared by a microwave-assisted polyol reduction method. Sp3-bonded ND possesses high electrochemical stability but low conductivity, while sp2-bonded carbon nanomaterials with high conductivity are prone to oxidation. Therefore, the functions of the supporting material were separated in this study. ND (sp3), as a support, and AB or CNTs (sp2), as a conductive additive, were combined to form the hybrid electrocatalysts PtRu/ND+AB and PtRu/ND+CNT for MOR. The morphology of the electrocatalysts was characterized by scanning electron microscopy and electrochemical measurements were performed using an electrochemical workstation. The results indicated that the electrocatalytic activity of PtRu/ND for MOR was improved with the addition of AB or CNTs as a conductive additive. Moreover, adding CNTs to PtRu/ND as a conductive additive showed better electrocatalytic activities than adding AB, which can be ascribed to the better electron-transfer ability of CNTs.

Fabrication of graphene supported SnO2 nanoparticles and their sodium storage properties

The graphene supported SnO2 nanoparticles (SnO2/graphene nanocomposites) were synthesized by a facile microwave-assisted polyol reduction method. The Na-storage performance of the as-prepared SnO2/graphene nanocomposites was evaluated as an anode material for sodium-ion batteries (SIBs), presenting a high initial reversible capacity of ~220mAhg−1 at a current density of 0.1Ag−1 and an acceptable rate capability. This work provides an effective approach on synthesizing the SnO2/graphene nanocomposites and a potential anode material for SIBs.

Ru effect on the catalytic performance of Pd@Ru/C catalysts for methanol electro-oxidation

Pd@Ru bimetallic nanoparticles deposited on carbon black electro-catalysts have been fabricated by microwave-assisted polyol reduction method and investigated for methanol electro-oxidation (MEO). The structure and electro-catalytic properties of the as-prepared catalysts were characterized by XRD, SEM, TEM and cyclic voltammetry (CV) techniques. The results showed that the introduction of Ru element (2–10 wt%) into Pd 20 wt%/C (hereafter, denoted as Pd/C) produced a series of core-shell structured binary catalysts. Pd@Ru 5 wt%/C (hereafter, denoted as Pd@Ru5/C) catalyst displayed the highest catalytic activity towards MEO. And the mass activity of Pd@Ru5/C electrode catalyst at E = −0.038 V (vs. Hg/HgO) was 1.42 times higher than that of Pd/C electrode catalyst. In addition, the relationship between the catalytic stability for MEO on Pd@Ru/C catalysts and the value of Jbp/Jfp (the ratio of MEO peak current density in the negative scan and positive scan) were also investigated. The result demonstrated that Pd@Ru5/C offering the smallest value of Jbp/Jfp displayed the best stable catalytic performance.

Use of graphene-supported manganite nano-composites for methanol electrooxidation

Binary nano-composites of palladium and a metal (Fe or Cu) manganite on graphene nanosheets (GNS) have been prepared by a microwave-assisted polyol reduction method and investigated as electrocatalysts for the methanol oxidation reaction (MOR) in 1 M KOH at 25 °C. Structural and electrocatalytic surface characterizations of composites are carried out by X-ray diffraction, transmission electron microscopy, X-ray photoelectron microscopy, cyclic voltammetry and chronoamperometry. Results show that new composite catalysts, particularly 40 wt%Pd–x wt%FeMn2O4/GNS (where x = 5, 8, 10 & 15) are MOR active and that the activity is the greatest with the catalyst containing 8 wt% of the oxide. The composite, Pd–8 wt%FeMn2O4/GNS, exhibits much superior catalytic activity as well as stability compared to the base (Pd/GNS) electrode. The enhanced catalytic activity and stability of the Pd/GNS catalyst in presence of the oxide can be ascribed to increased population of adsorbed OH− ions/OH radicals at the electrode surface.

Electro-oxidation of methanol on SnO2-promoted Pd/MWCNTs catalysts in alkaline solution

The methanol electro-oxidation (MEO) on Pd–SnO2/MWCNTs catalysts prepared by microwave-assisted polyol reduction method has been investigated. The structure, morphology and electro-catalytic performances of the catalysts were characterized with XRD, TEM and cyclic voltammetry (CV). The results showed that the highly dispersed Pd nano-particles (PdNPs) with a narrow size distribution on MWCNTs were successfully synthesized. The catalytic activity of Pd–SnO2/MWCNTs for MEO was up to 778.8 mA/mg Pd in 0.1 M KOH solution containing 1 M methanol, which was significant higher than that of Pd/C (414.2 mA/mg Pd) or Pd–SnO2/C (566.7 mA/mg Pd). Moreover, the MEO on Pd–SnO2/MWCNTs electrode displayed an irreversible behavior under a diffusion control giving an exchange current density (j0) of 3.76 × 10−4 A cm−2 and a Tafel slope of 149 mV dec−1 (α = 0.56) at 25 °C, which indicates that Pd–SnO2/MWCNTs catalyst has a high electro-catalytic performance for the MEO in alkaline media.

Preparation of Bimetallic Pd-Co Nanoparticles on Graphene Support for Use as Methanol Tolerant Oxygen Reduction Electrocatalysts

Graphene-supported (40-x) wt% Pd x wt% Co (0≤x≤13.33) alloys/composites have been prepared by a microwave-assisted polyol reduction method and been investigated for their structural and electrocatalytic properties for the oxygen reduction reaction (ORR) in 0.5 M H2SO4 at 298 K. The study demonstrated that the bimetallic Pd-Co composite nanoparticles are, in fact, alloy nanoparticles with fcc crystalline structure. Partial substitution of Pd by Co (from 3.64 to 13.33 wt%) in 40 wt% Pd/graphene decreases the lattice parameter as well as the crystallite size and increases the apparent catalytic activity, the latter, however, being the greatest with 8 wt% Co. The ORR activity of the active 32 wt% Pd 8wt% Co is found to be considerably low when it was deposited on the support multiwall carbon nanotubes under similar conditions. The rotating disk electrode study indicated that the ORR on 32 wt% Pd 8 wt% Co/GNS in 0.5 M H2SO4 follows approximately the four-electron pathway.

Gold Nanoparticle Decorated Multi-Walled Carbon Nanotubes as Counter Electrode for Dye Sensitized Solar Cells

A novel counter electrode material for dye sensitized solar cells (DSSCs) composed of nanostructured Au particles decorated on functionalized multi-walled carbon nanotubes (f-MWNTs) is demonstrated for the first time. MWNTs synthesized by catalytic chemical vapor deposition technique are purified and functionalized by treating with concentrated acids. Au nanoparticles are decorated on f-MWNTs by a rapid and facile microwave assisted polyol reduction method. The materials are characterized by X-ray diffractometry, Fourier transform infra red spectroscopy and electron microscopy. The DSSC fabricated with Au/f-MWNTs based counter electrode shows enhanced power conversion efficiency (eta) of 4.9% under AM 1.5G simulated solar radiation. In comparison, the reference DSSCs fabricated with f-MWNTs and Pt counter electrodes show eta of 2.1% and 4.5%. This high performance of Au/f-MWNTs counter electrode is investigated using electrochemical impedance spectroscopy and cyclic voltammetry studies.

Graphene-supported Pd–Ru nanoparticles with superior methanol electrooxidation activity

Pd–Ru bimetallic nanoparticles dispersed on graphene nanosheets (GNS) have been obtained by a microwave-assisted polyol reduction method and investigated for methanol electrooxidation in 1M KOH+1M CH3OH at 25°C. Structural and electrochemical characterizations of electrocatalysts are carried out by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, cyclic voltammetry, CO stripping voltammetry and chronoamperometry. The study shows that introduction of Ru (1–10 wt.%) into 40wt.%Pd/GNS produces an alloy of Pd and Ru with the face centered cubic crystal structure. The electrocatalytic activity increased with increasing percentage of Ru in the Pd–Ru alloy showing maximum with 5wt.%Ru. The electrocatalytic activity of the 40wt.%Pd–5wt.%Ru/GNS electrode at E=−0.10V vs. Hg/HgO was ∼2.6 times greater than that of the base (40wt.%Pd/GNS) electrode. Based on the methanol oxidation current, measured at 1h during the chronoamperometry tests at E=−0.10V vs. Hg/HgO, the active 40wt.%Pd–5wt.%Ru/GNS electrode exhibited ∼72% and ∼675% higher poisoning tolerance as compared to 40%Pd/GNS and 40%Pd/multiwalled carbon nanotube electrodes, respectively.

An optically transparent cathode for dye sensitized solar cells based on cationically functionalized and metal decorated graphene

Optically transparent counter electrodes outperforming the conventional Pt counter electrodes in dye sensitized solar cells (DSSCs) are seldom reported. In this work, a highly transparent (∼88% at 550nm) and catalytic (charge transfer resistance <1Ωcm2) counter electrode based on a novel Pt—polyelectrolyte functionalized graphene composite is demonstrated. Graphene sheets (HEG) are synthesized by hydrogen induced simultaneous exfoliation–reduction of graphite oxide. HEG is functionalized by a cationic polyelectrolyte, poly-diallyl-diammonium chloride (PDDA) which causes minimum damage to graphene and assists in uniform tethering of nanoparticles. Pt nanoparticles are decorated on PDDA-HEG by a simple microwave assisted polyol reduction method. Electrochemical impedance measurements and cylic voltammetry measurements show that the higher catalytic activity of Pt/PDDA-HEG is due to the presence of positive charges and nanostructured Pt. As a consequence, the DSSCs fabricated with Pt/PDDA-HEG counter electrode exhibits an enhanced power conversion efficiency of ∼5.7% as compared to ∼4.5% for reference cells with Pt counter electrode.

Toward the decoration of Pt nanoparticles supported on carbon nanotubes with Fe oxides and its effect on the catalytic reaction

Deposition of metallic oxides on noble metal catalysts improves their catalytic performance dramatically due to the highly active sites formed at the interface of two metals (nano-contact). In this work, Fe oxides were deposited on preformed Pt nanoparticles by using a novel microwave assisted polyol reduction method. The facile synthesis is performed in liquid phase at 438K, which preserves the growth of large Pt nanoparticles and precludes the formation of Pt–Fe alloy. The Fe promoted Pt catalyst exhibited high activity (6340h−1 in turnover frequency) and selectivity (89.1% for cinnamal alcohol) in the hydrogenation of cinnamaldehyde, which was attributed to the uniform distribution of Fe promoters on Pt surfaces. The characteristics of Fe oxides on Pt nanoparticles were identified by employing electrochemical analysis and X-ray photoelectron spectroscopy.

Enhanced efficiency in dye sensitized solar cells with nanostructured Pt decorated multiwalled carbon nanotube based counter electrode

Nanostructured Pt decorated functionalized multiwalled carbon nanotubes (Pt/f-MWNTs) cast on fluorine doped tin oxide coated glass (FTO) is demonstrated as a novel counter electrode (CE) for dye sensitized solar cells (DSSCs). Pt particles (20wt.%, size <5nm) are uniformly decorated on f-MWNTs using a facile microwave assisted polyol reduction method. The DSSC fabricated with Pt/f-MWNTs CE (effective Pt loading ∼10μgcm−2) exhibits an enhanced efficiency of ∼5.4% (under an illumination of AM 1.5G, 100mWcm−2), compared to ∼2.3% and 4.5% for f-MWNTs and Pt CE based reference DSSCs. The CEs are fabricated without using conventional binders using a simple doctor blade method. A new equivalent circuit is proposed to model the electrochemical impedance behavior of porous f-MWNTs based CE. On the other hand, the behavior Pt/f-MWNTs based electrode is similar to that of Pt nanoparticles but with higher catalytic activity. Cyclic voltammetry studies reveal that Pt/f-MWNTs counter electrode offers greater catalytic surface area and facilitates faster reaction at its surface.

Synthesis and structural characterization of a ternary palladium–ruthenium–tin nanoalloy supported on graphene nanosheets for methanol electrooxidation in alkaline medium

A graphene-supported ternary Pd–Ru–Sn nanoalloy with a face centered cubic structure has been synthesized by a microwave-assisted polyol reduction method and investigated for electrooxidation of methanol in 1 M KOH at 25 °C. The electrocatalytic activity of the novel ternary alloy electrode is more than 2 times higher than that of the Pd/GNS (base) electrode. It is observed that the percentage increase in catalytic activity of the ternary alloy electrode in comparison with the base electrode increases with the electrolysis time during the chronoamperometry study.

Carbon nanotube-supported Pt-based bimetallic catalysts prepared by a microwave-assisted polyol reduction method and their catalytic applications in the selective hydrogenation

Platinum-based bimetallic catalysts supported on multi-walled carbon nanotube (CNT) were prepared by a facile microwave-assisted polyol reduction method (MAPR) and chemically probed in the selective hydrogenation of cinnamaldehyde. These bimetallic catalysts outperformed the catalysts prepared by the conventional impregnation method. CNT surface properties, selection of solvent and transition metal promoter were identified to be of great significance in tuning the catalytic performance. The best catalytic results were obtained on the Fe- and Co-modified Pt catalysts in the presence of ethyl acetate as the solvent after removing the oxygen-containing groups from CNT surfaces. Electrochemistry measurements including cyclic voltammetry and CO stripping, competitive hydrogenations of toluene and benzene, X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy were employed to gain insights into the physicochemical properties of catalysts. Both catalytic reactions and characterizations revealed that the MAPR method afforded close contact between promoters and platinum active sites, leading to superior catalytic properties.

Microwave assisted synthesis of surfactant stabilized platinum/carbon nanotube electrocatalysts for direct methanol fuel cell applications

Platinum electrocatalysts deposited on multi-walled carbon nanotubes (CNT) with high loading were prepared using a microwave-assisted polyol reduction method and employed for direct methanol fuel cells (DMFC). A zwitterionic surfactant was used as a stabilizing agent for the formation of Pt nanoparticles. A uniform and narrow size distribution of highly dispersed Pt nanoparticles could be achieved by adjusting the weight ratio of surfactant to Pt precursor allowing for Pt loadings of up to 60 wt%. The heating time and the temperature for the ethylene glycol (EG) oxidation were found to be the key factors for depositing Pt nanoparticles homogeneously on carbon nanotubes. The smallest average particle diameter of 1.8 nm was obtained through microwave heating to 140 °C in 50 s. The structure, amount and morphology of the electrocatalysts were characterized with XRD, TGA, and TEM, respectively. Single cell DMFC measurements were performed in a membrane-electrode assembly (MEA) with 5 cm 2 active area and very low catalyst loading (0.25 mg cm −2 of noble metal on both anode and cathode). The DMFC performance of the surfactant stabilized cathode catalyst obtained by the new method described here revealed that the power density was three times higher than for a commercial catalyst used for comparison and two times higher than for an unstabilized CNT supported catalyst.