Structural and Electrochemical Characterization of Binary, Ternary, and Quaternary Platinum Alloy Catalysts for Methanol Electro-oxidation

Precision Synthesis of Bimetallic Nanoparticles via Nanofluidics in Nanopipets

Bimetallic nanoparticles often show properties superior to their single-component counterparts. However, the large parameter space, including size, structure, composition, and spatial arrangement, impedes the discovery of the best nanoparticles for a given application. High-throughput methods that can control the composition and spatial arrangement of the nanoparticles are desirable for accelerated materials discovery. Herein, we report a methodology for synthesizing bimetallic alloy nanoparticle arrays with precise control over their composition and spatial arrangement. A dual-channel nanopipet is used, and nanofluidic control in the nanopipet further enables precise tuning of the electrodeposition rate of each element, which determines the final composition of the nanoparticle. The composition control is validated by finite element simulation as well as electrochemical and elemental analyses. The scope of the particles demonstrated includes Cu-Ag, Cu-Pt, Au-Pt, Cu-Pb, and Co-Ni. We further demonstrate surface patterning using Cu-Ag alloys with precise control of the location and composition of each pixel. Additionally, combining the nanoparticle alloy synthesis method with scanning electrochemical cell microscopy (SECCM) allows for fast screening of electrocatalysts. The method is generally applicable for synthesizing metal nanoparticles that can be electrodeposited, which is important toward developing automated synthesis and screening systems for accelerated material discovery in electrocatalysis.

PdRuIr ternary alloy as an effective NO reduction catalyst: insights from first-principles calculation

NO dissociation is an important reaction step in the NO reduction reaction, particularly in the three-way catalyst conversion system for automotive gas exhaust purification. In this study, we used first-principles calculations based on density functional theory to analyze the interaction and dissociation of NO on the PdRuIr ternary alloy. The electronic properties of the atomic combination of the PdRuIr ternary alloy create an effective catalyst that is active for NO dissociation and relatively stable against the formation of volatile RuO_x through a weakened O adsorption. This study also shows that for an alloyed system, the strength of NO adsorption may not necessarily predict the dissociation activity. This tendency is observed in the PdRuIr ternary alloy where Ru top is the active site for NO adsorption albeit not an effective site for dissociation. It is presumed that NO dissociation is mediated by its molecular diffusion to active sites for dissociation, which are usually high Ru- and/or Ir-coordinated hollow or bridge sites. These active sites allow high charge transfer from the surface to NO, which fills the NO anti-bonding state and facilitates dissociation. This therefore assumes that the strength of NO molecular adsorption is not a descriptor for NO dissociation on metal alloys but rather the ability of the surface to transfer charge to NO and homogeneity of the strength of adsorption. Furthermore, O adsorption on the ternary alloy, particularly near the Ru sites, is relatively weaker as compared to the pure Ru surface. This weakened O adsorption is attributed to charge re-distribution through alloying, particularly charge transfer from the Ru atom to the Ir and Pd atoms.

Metal-Based Electrocatalysts for Methanol Electro-Oxidation: Progress, Opportunities, and Challenges

Direct methanol fuel cells (DMFCs) are among the most promising portable power supplies because of their unique advantages, including high energy density/mobility of liquid fuels, low working temperature, and low emission of pollutants. Various metal-based anode catalysts have been extensively studied and utilized for the essential methanol oxidation reaction (MOR) due to their superior electrocatalytic performance. At present, especially with the rapid advance of nanotechnology, enormous efforts have been exerted to further enhance the catalytic performance and minimize the use of precious metals. Constructing multicomponent metal-based nanocatalysts with precisely designed structures can achieve this goal by providing highly tunable compositional and structural characteristics, which is promising for the modification and optimization of their related electrochemical properties. The recent advances of metal-based electrocatalytic materials with rationally designed nanostructures and chemistries for MOR in DMFCs are highlighted and summarized herein. The effects of the well-defined nanoarchitectures on the improved electrochemical properties of the catalysts are illustrated. Finally, conclusive perspectives are provided on the opportunities and challenges for further refining the nanostructure of metal-based catalysts and improving electrocatalytic performance, as well as the commercial viability.

Synthesis of PtCoNiRu/C nanoparticles by spray drying combined with reduction sintering for methanol electro-oxidation

The controllable synthesis of carbon-supported platinum-based multicomponent alloys is important for the development and application of direct methanol fuel cells (DMFCs). In this paper, controllable synthesis of carbon-supported PtCoNiRu quaternary alloy is realized by spray drying and reduction sintering. The effects of reduction temperature on the size, morphology and catalytic properties of the metal nanoparticles were investigated. The electrochemical performance of the as-synthesized PtCoNiRu/C catalysts towards methanol electro-oxidation was studied using cyclic voltammetry (CV) and chronoamperometry. The results show that metal nanoparticles with uniform size and dispersity on the carbon surface can be obtained at a suitable sintering temperature, while the catalyst has a higher electrochemical active surface area (ECSA) and shows better catalytic activity and stability for methanol electro-oxidation. The method described in this study provides a new route for the manufacture of Pt alloy nanoparticles with higher catalytic activity and stability.

Synthesis of Hollow Pt-Ni Nanoboxes for Highly Efficient Methanol Oxidation

In direct methanol fuel cell technology, highly stable electrochemical catalysts are critically important for their practical utilization at the commercial scale. In this study, sub ~10 nm hollow Pt-Ni (1:1 at. ratio) nanoboxes supported on functionalized Vulcan carbon (Pt-Ni/C-R2) were synthesized through a facile method for the efficient electrooxidation of methanol. Two reaction procedures, namely, a simultaneous reduction and a modified sequential reduction method using a reverse microemulsion (RME) method, were adopted to synthesize solid Pt-Ni NPs and hollow nanoboxes, respectively. To correlate the alloy composition and surface structure with the enhanced catalytic activity, the results were compared with the nanocatalyst synthesized using a conventional NaBH₄ reduction method. The calculated electroactive surface area for the Pt-Ni/C-R2 nanoboxes was 190.8 m².g^-1, which is significantly higher compared to that of the Pt-Ni nanocatalyst (96.4 m².g^-1) synthesized by a conventional reduction method. Hollow nanoboxes showed 34% and 44% increases in mass activity and rate of methanol oxidation reaction, respectively, compared to solid NPs. These results support the nanoreactor confinement effect of the hollow nanoboxes. The experimental results were supported by Density Functional Theory (DFT) studies, which revealed that the lowest CO poisoning of the Pt₁Ni₁ catalyst among all Pt_m-Ni_n mixing ratios may account for the enhanced methanol oxidation. The synthesized hollow Pt-Ni/C (R2) nanoboxes may prove to be a valuable and highly efficient catalysts for the electrochemical oxidation of methanol due to their low cost, numerous catalytically active sites, low carbon monoxide poisoning, large electroactive surface area and long-term stability.

3D highly branched PtCoRh nanoassemblies: Glycine-assisted solvothermal synthesis and superior catalytic activity for alcohol oxidation

Advanced Pt-based ternary nanocatalysts display dramatically enhanced utilization efficiency of Pt alternative to mono- and bi-counterparts, owing to the synergistic effects of the tri-metals. Herein, multicomponent uniform 3D PtCoRh highly branched nanoassemblies (HBNAs) were prepared by glycine-assisted one-pot solvothermal method in oleylamine (OAm). The effects of the precursor types, reaction time and amount of glycine were critically investigated in this synthesis. The as-prepared PtCoRh HBNAs displayed outstanding electrocatalytic activity and improved stability towards ethanol oxidation reaction (EOR) and methanol oxidation reaction (MOR) in 1 M KOH electrolyte, whose mass/specific activities were 1.75 A mg^-1/4.03 mA cm^-2 and 0.98 A mg^-1/2.34 mA cm^-2, respectively, which were remarkably higher than commercial Pt/C (0.85 A mg^-1/4.03 mA cm^-2 and 0.47 A mg^-1/0.89 mA cm^-2). This study provides some novel guidelines to fabricate advanced multimetallic electrocatalysts for practical applications in direct alcohol fuel cells (DAFCs).

Efficacious Electrochemical Oxygen Evolution from a Novel Co(II) Porphyrin/Pyrene-Based Conjugated Microporous Polymer

Oxygen evolution reaction (OER) is energetically challenging from the platform of making many photovoltaic devices such as metal-air batteries and water splitting systems because of its poor kinetics even when precious metals are used. Herein, a Co(II)-porphyrin/pyrene-comprised conjugated microporous polymer Co-MPPy-1 has been developed which shows efficient OER in alkaline medium. The material was characterized by Fourier transform infrared, solid-state ¹³C cross-polarization magic angle spinning nuclear magnetic resonance, N₂ volumetric adsorption/desorption analysis, scanning electron microscopy, ultra high resolution-transmission electron microscopy, X-ray photoelectron spectroscopy, and other physical studies. Co-MPPy-1 showed Brunauer-Emmett-Teller surface area of ∼501 m² g^-1. Co-MPPy-1 achieved a current density of 1 and 10 mA/cm^-2 at 340 and 420 mV, respectively. The turnover frequency calculated for the OER is 0.43 s^-1. The heterogeneity of this electrocatalyst was tested by chronoamperometric measurement and 1000 cycle recyclability test with retainment of the excellent electrochemical catalytic activity. This can be attributed to the presence of high density of Co(II) porphyrin unit and efficient charge transport in the π-conductive conjugated polymeric backbone.

A graphitic carbon nitride–titania nanocomposite as a promising catalyst support for electro-oxidation of methanol

A graphitic carbon nitride–titania nanocomposite has been synthesized as a catalyst support for Pt which enhanced the electrocatalytic methanol oxidation reaction.

Trimetallic PtAuNi alloy nanoparticles as an efficient electrocatalyst for the methanol electrooxidation reaction

Platinum is an excellent electrocatalyst. However, the disadvantages of Pt as an electrocatalyst lie in its poor earth abundance, high cost, and poor stability due to surface poisoning. Thus it remains a challenge to find suitable alternative electrocatalysts and/or reduce the use of Pt. Herein, we report the solvothermal synthesis of bimetallic (PtAu and PtNi) and trimetallic (PtAuNi) alloy nanoparticles (NPs) with controlled percentages of individual metals. With an optimized Ni content, the trimetallic (Pt₆₆Au₁₁Ni₂₃) alloy NPs show superior electrocatalytic activity (in terms of lower onset oxidation potential and higher mass activity) not only to bimetallic alloy NPs (PtAu and PtNi) but also to commercial Pt/C (20% Pt loading) for methanol electrooxidation (MEO). This enhanced electrocatalytic activity is due to the synergistic role of different metals in MEO catalysis. In particular, the catalytic activity is found to be controlled by the balance between the adsorption of methanol species on Pt and the removal of carbonaceous species from the catalyst surface by Au and Ni, as demonstrated here. The multimetallic alloy of optimal individual content thereby not only reduces the Pt content in the catalyst but also exhibits higher electrocatalytic activity than Pt/C for MEO that is desirable for fuel cell applications.

Fe–Mn bimetallic oxides-catalyzed oxygen reduction reaction in alkaline direct methanol fuel cells

Two Fe–Mn bimetallic oxides were synthesized through a facile solvothermal method without using any templates. Fe₂O₃/Mn₂O₃ is made up of Fe₂O₃ and Mn₂O₃ as confirmed via XRD. TEM and HRTEM observations show Fe₂O₃ nanoparticles uniformly dispersed on the Mn₂O₃ substrate and a distinct heterojunction boundary between Fe₂O₃ nanoparticles and Mn₂O₃ substrate. MnFe₂O₄ as a pure phase sample was also prepared and investigated in this study. The current densities in CV tests were normalized to their corresponding surface area to exclude the effect of their specific surface area. Direct methanol fuel cells (DMFCs) were equipped with bimetallic oxides as cathode catalyst, PtRu/C as the anode catalyst and PFM as the electrolyte film. CV and DMFC tests show that Fe₂O₃/Mn₂O₃(3 : 1) exhibits higher oxygen reduction reaction (ORR) activity than Fe₂O₃/Mn₂O₃(1 : 1), Fe₂O₃/Mn₂O₃(1 : 3), Fe₂O₃/Mn₂O₃(5 : 1) and MnFe₂O₄. The much superior catalytic performance is due to its larger surface area, the existence of numerous heterojunction interfaces and the synergistic effect between Fe₂O₃ and Mn₂O₃, which can provide numerous catalytic active sites, accelerate mass transfer, and increase ORR efficiency.

Heterojunction interfaces and synergistic effect between Fe₂O₃ and Mn₂O₃ play a key role in Fe₂O₃/Mn₂O₃-catalyzed ORR.

Evaluation of the Optimum Composition of Low-Temperature Fuel Cell Electrocatalysts for Methanol Oxidation by Combinatorial Screening

Combinatorial chemistry and high-throughput screening represent an innovative and rapid tool to prepare and evaluate a large number of new materials, saving time and expense for research and development. Considering that the activity and selectivity of catalysts depend on complex kinetic phenomena, making their development largely empirical in practice, they are prime candidates for combinatorial discovery and optimization. This review presents an overview of recent results of combinatorial screening of low-temperature fuel cell electrocatalysts for methanol oxidation. Optimum catalyst compositions obtained by combinatorial screening were compared with those of bulk catalysts, and the effect of the library geometry on the screening of catalyst composition is highlighted.

Silver-induced electronic drift in AgPd bimetallics: rationale for enhanced electrocatalytic activity of ethanol oxidation reaction

Enhanced catalytic ethanol oxidation is mechanistically driven by the Ag-induced electronic drift from Pd in an AgPd bimetallic system.

Electrochemical synthesis of dimethyl carbonate from CO2 and methanol over carbonaceous material supported DBU in a capacitor-like cell reactor

Electro-assisted synthesis of dimethyl carbonate from CO₂ and methanol under solvent-free conditions in a capacitor-like cell reactor.

Electro-catalysis of carbon black or titanium sub-oxide supported Pd–Gd towards formic acid electro-oxidation

Carbon black supported Pd–Gd catalysts (Pd–xGd/C, x is weight percent in catalyst) with different amounts of Gd were prepared by a simultaneous reduction reaction with sodium borohydride in aqueous solution.

Nickel foam supported mesoporous NiCo2O4 arrays with excellent methanol electro-oxidation performance

Schematic of the electro-oxidation reaction process of NiCo₂O₄ nanocloth arrays on nickel foam. The unique morphological features can afford efficient 3D electron transport during the redox reaction.

Combinatorial high-throughput optical screening of high performance Pd alloy cathode for hybrid Li-air battery

Combinatorial high-throughput optical screening method was developed to find the optimum composition of highly active Pd-based catalysts at the cathode of the hybrid Li-air battery. Pd alone, which is one-third the cost of Pt, has difficulty in replacing Pt; therefore, the integration of other metals was investigated to improve its performance toward oxygen reduction reaction (ORR). Among the binary Pd-based catalysts, the composition of Pd-Ir derived catalysts had higher performance toward ORR compared to other Pd-based binary combinations. The composition at 88:12 at. % (Pd: Ir) showed the highest activity toward ORR at the cathode of the hybrid Li-air battery. The prepared Pd(88)Ir(12)/C catalyst showed a current density of -2.58 mA cm(-2) at 0.8 V (vs RHE), which was around 30% higher compared to that of Pd/C (-1.97 mA cm(-2)). When the prepared Pd(88)Ir(12)/C catalyst was applied to the hybrid Li-air battery, the polarization of the cell was reduced and the energy efficiency of the cell was about 30% higher than that of the cell with Pd/C.

Highly alloyed PtRu nanoparticles confined in porous carbon structure as a durable electrocatalyst for methanol oxidation

The state-of-the-art carbon-supported PtRu catalysts are widely used as the anode catalysts in polymer electrolyte fuel cells (PEMFCs) but suffer from instability issues. Severe ruthenium dissolution occurring at potentials higher than 0.5 V vs NHE would result in a loss of catalytic activity of PtRu hence a worse performance of the fuel cell. In this work, we report an ultrastable PtRu electrocatalyst for methanol oxidation by confining highly alloyed PtRu nanoparticles in a hierarchical porous carbon structure. The structural characteristics, e.g., the surface composition and the morphology evolution, of the catalyst during the accelerated degradation test were characterized by the Cu-stripping voltammetry and the TEM/SEM observations. From the various characterization results, it is revealed that both the high alloying degree and the pore confinement of PtRu nanoalloys play significant roles in suppressing the degradation processes, including Ru dissolution and particle agglomeration/migration. This report provides an opportunity for efficient design and fabrication of highly stable bimetallic or trimetallic electrocatalysts in a large variety of applications.

Enhancing the electrocatalytic property of hollow structured platinum nanoparticles for methanol oxidation through a hybrid construction

The integration of different components into a hybrid nanosystem for the utilization of the synergistic effects is an effective way to design the electrocatalysts. Herein, we demonstrate a hybrid strategy to enhance the electrocatalytic property of hollow structured Pt nanoparticles for methanol oxidation reaction. This strategy begins with the preparation of bimetallic Ag-Pt nanoparticles with a core-shell construction. Element sulfur is then added to transform the core-shell Ag-Pt nanostructures into hybrid nanodimers consisting of Ag₂S nanocrystals and remaining Pt domains with intact hollow interiors (Ag₂S-hPt). Finally, Au is deposited at the surface of the Ag₂S domain in each hetero-dimer, resulting in the formation of ternary Ag₂S-Au-hPt nanocomposites with solid-state interfaces. The ternary nanocomposites exhibit enhanced electrocatalytic property toward methanol oxidation due to the strong electronic coupling between Pt and other domains in the hybrid particles. The concept might be used toward the design and synthesis of other hetero-nanostructures with technological importance.

Long-term, stable, and improved oxygen-reduction performance of titania-supported PtPb nanoparticles

Anatase-type titania-supported intermetallic PtPb nanoparticles synthesized through a wet-chemical route showed a long-term, stable, and improved oxygen reduction reaction performance.

Combinatorial high-throughput screening for highly active Pd-Ir-Ce based ternary catalysts in electrochemical oxygen reduction reaction

A combinatorial library having 66 different ternary compositions of Pd-Ir-Ce was prepared via the impregnation method to find the optimum ternary composition with the highest performance toward oxygen reduction reaction (ORR) in acid media. Its performance in ORR activity of the combinatorial array was evaluated through two different combinatorial high-throughput screening methods to gain validity: (1) multielectrode half-cell method and (2) optical screening method. From the combinatorial results, the spot at 79:12:9 for Pd-Ir-Ce (at. %) in the array showed the highest ORR activity. The electrochemical characterizations of the single catalyst demonstrates that the optimized Pd79Ir12Ce9/C catalyst shows 1.5 times the ORR activity compared to that of Pd/C catalyst at 0.85 V (vs. RHE). In the Pd-Ir-Ce based catalysts, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) results reveal that Ir and Ce are present in the form of IrO2 and CeO2, respectively, and the electron configuration of Pd is effectively modified through the decoration with IrO2 and CeO2. From the results, we suggest that the electro-modification of Pd through strong metal-metal oxide interaction with IrO2-CeO2 was a reason for the enhanced ORR activity.