High methanol oxidation activity of electrocatalysts supported by directly grown nitrogen-containing carbon nanotubes on carbon cloth

The microstructure and electrochemical activity of the Pt-Ru supported by nitrogen-containing carbon nanotubes (CN_x NTs) directly grown on the carbon cloth have been investigated. The CN_x NTs directly grown on the carbon cloth (CN_x NTs-carbon cloth composite electrode) were synthesized using microwave-plasma-enhanced chemical vapour deposition first and then use as the template to support the Pt-Ru nanoclusters subsequently sputtered on. The ferricyanide/ferrocyanide redox reaction in cyclic voltammetry (CV) measurements showed a faster electron transfer on the CN_x NTs-carbon cloth composite electrode than the one with carbon cloth alone. Comparing their methanol oxidation abilities, it is found that the Pt-Ru nanoclusters supported by the CN_x NTs-carbon cloth composite electrode have considerably higher electrocatalytic activity than the carbon cloth counterpart. This result suggests high performance of the CN_x NTs-carbon cloth composite electrode, and demonstrates its suitability for direct methanol fuel cell applications.     

MXene (Ti3C2Tx) supported electrocatalysts for methanol and ethanol electrooxidation: A review

MXene is a growing class of two-dimensional layered material that has been showed promising application in various energy storage and conversion technologies, due to its unique electrical, mechanical, surface and electrocatalytic properties. One of the promising applications of MXene is, its definite role as durable catalyst support material, for fuel cell anodic/cathodic reactions. Very recently, MXene supported catalysts have been identified as potential and stable support for various Pt and non-Pt metals designed for cathodic oxygen reduction reaction and it has been proved as excellent material to enhance the oxygen reduction kinetics. This had motivated the researchers to explore MXene as catalyst support for anodic methanol/ethanol oxidation reactions. In this review, recent developments in experimental and theoretical research on MXene-based electrocatalysts for the methanol/ethanol oxidation reactions are examined and overviewed. After careful examination of the available literature, we presume that MXene supported electrocatalyst showed enhanced methanol/ethanol oxidation kinetics and therefore believed to have tremendous opportunities as durable catalyst support for alcohol fuel cells. However, researchers also consider several limitations and challenges that must be addressed for efficient application of MXenes as catalyst supports. Therefore, current research on MXenes with respect to the methanol/ethanol oxidation are discussed, with a focus on synthesis strategies, oxidation kinetics, and factors responsible for enhancing the electrocatalytic performance. Several strategies for the further development of efficient and durable MXene supported catalysts are also proposed.     

Quaternary Pt-based electrocatalyst for methanol oxidation by combinatorial electrochemistry

The demonstration to apply the combinatorial method using a repeated cyclic voltammetry is reported to find the anodic material for DMFC that shows a higher electrocatalytic activity and that can replace a portion of precious metals with cheap ones. The activity of newly found electrocatalyst whose composition was determined through high-throughput screening was compared with that of commercially available Johnson–Matthey Pt(50)Ru(50). It was found Pt(77)Ru(17)Mo(4)W(2) was more active and stable than Pt(50)Ru(50) in methanol electro-oxidation. The repeated cyclic voltammetry makes the combinatorial method expand into a screening tool to find the electrocatalyst that not only showing an initial excellent performance but also being active in the long-run reaction.     

Nickel-decorated MoS2/MXene nanosheets composites for electrocatalytic oxidation of methanol

Nickel incorporated on MoS₂/MXene composites (NiMoS₂/MXene) via a wet impregnation method is used as an anode electrode material for methanol electro-oxidation. X-ray diffraction, X-ray photoelectron spectra, and scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy techniques were used for the confirmation of MoS₂/MXene formation. Textural properties of catalysts were obtained in N₂ adsorption-desorption analysis. Electrochemical measurements in 0.1 M KOH demonstrated the better electrocatalytic activity of NiMoS₂/MXene catalysts. The NiMoS₂/MXene system exhibited enhanced electrocatalytic activity for methanol oxidation due to low onset potential offered by Ni, high tolerance toward CO poisoning by MoS₂, and high conductivity and high mechanical stability of MXene. NiMoS₂-3/MXene catalysts exhibited high current density, electrochemical active surface area, long-term stability, and low Rct value. Based on the electrochemical results, NiMoS₂/MXene catalysts is a highly electroactive anode material. Hence can be utilized in fuel cell applications such as Direct Methanol Fuel Cell (DMFC).     

Pt–Ru nanoparticles supported on functionalized carbon as electrocatalysts for the methanol oxidation

Platinum–ruthenium alloy electrocatalysts, for methanol oxidation reaction, were prepared on carbons thermally treated in helium atmosphere or chemically functionalized in H₂O₂, or in HNO₃ + H₂SO₄ or in HNO₃ solutions. The functionalized carbon that is produced using acid solutions contains more surface oxygenated functional groups than carbon treated with H₂O₂ solution or HeTT. The XRD/HR-TEM analysis have showed the existence of a higher alloying degree for Pt–Ru electrocatalysts supported on functionalized carbon, which present superior electrocatalytic performance, assessed by cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy, as compared to electrocatalysts on unfunctionalized carbon. It also was found that Pt–Ru alloy electrocatalysts on functionalized carbon improve the reaction rate compared to Pt–Ru on carbons treated with H₂O₂ solution and thermally. A mechanism is discussed, where oxygenated groups generated from acid functionalization of carbon and adsorbed on Pt–Ru electrocatalysts are considered to enhance the electrocatalytic activity of the methanol oxidation reaction.     

Enhanced electrochemical activity of Pt nanowire network electrocatalysts for methanol oxidation reaction of fuel cells

In this work, Pt nanowire networks supported on high surface area carbon (Pt NWNs/C) are synthesized as electrocatalysts for direct methanol fuel cells (DMFCs). The electrocatalytic behavior of Pt NWNs/C catalysts for the methanol and adlayer CO oxidation reactions is investigated and the results are compared with the Pt nanoparticles (NPs) supported on carbon (Pt NPs/C). The results indicate that Pt NWNs are characterized by interconnected nanoparticles with large number of grain boundaries, downshifted d-band center and reduced oxophilicity, which results in the enhanced surface mobility of oxygen-containing species such as CO_ads and OH_ads. The enhanced surface mobility of CO_ads and OH_ads in turn facilitates the removal of intermediate CO species during the methanol oxidation. The activity of the Pt NWNs/C electrocatalyst for the methanol oxidation reaction and electrooxidation of adsorbed CO is also evaluated by cyclic voltammetry, CO stripping, and kinetic analysis. The results show that Pt NWNs/C catalysts have a significantly higher electrocatalytic activity for the methanol oxidation reaction as compared to Pt NPs/C catalysts. The enhanced electrocatalytic activity of Pt NWNs/C catalysts is mainly due to the existence of large number of the grain boundaries of the interconnected nanoparticles of the unique Pt NWN structure.     

Hollow graphitic carbon spheres for Pt electrocatalyst support in direct methanol fuel cell

Hollow graphitic carbon spheres (HGCSs) with a high surface area are produced by the carbonization of hollow polymer spheres obtained by the polymerization of core/shell-structured pyrrole micelles. HGCSs are employed as a carbon support material in a direct methanol fuel cell catalyst, and their effect on the electrocatalytic activity toward methanol oxidation is investigated. Pt catalyst supported on HGCSs shows a better electrocatalytic activity compared to that on Vulcan XC-72, which has been commonly used in fuel cell catalysts. The observed enhancement in the electrocatalytic activity is attributed to the improved electronic conductivity and high surface area of HGCSs.     

Kinetic study of methanol oxidation on carbon-supported PtRu electrocatalyst

Methanol electrooxidation was investigated on the carbon-supported PtRu electrocatalyst (1:1 atomic ratio) in acid media. X-ray diffraction measurement indicated alloying of Pt and Ru. Cyclic voltammetry of the sample reflects the amount of Ru in the catalyst and its ability to adsorb OH radicals. Tafel plots for the oxidation of 0.02-3 M methanol in the solutions containing 0.05-1 M HClO₄ and in the temperature range 27-40 °C showed reasonably well-defined linear region with the slope of about 115 mV dec⁻¹ at the low currents, irrespective of the experimental conditions employed. Reaction order with respect to methanol was found to be 0.5. A correlation between methanol oxidation rate and pseudocapacitive current of OH adsorption on Ru sites was established. It was proposed that bifunctional mechanism is operative with the reaction between methanol residues adsorbed on Pt sites and OH radicals adsorbed on Ru sites as the rate-determining step.     

Novel Pt-Ru nanoparticles formed by vapour deposition as efficient electrocatalyst for methanol oxidation: Part II. Electrocatalytic activity

The methods developed and described in paper—part I are employed to prepare nanometer size Pt-Ru particles on a Vulcan^® XC72R substrate with controlled metal loading. Transmission Electron Microscopy (TEM) confirmed uniform particles size (average diameter 2 nm) and homogeneous dispersion of the particles over the substrate. Energy Dispersive X-ray absorption (EDX) analysis confirmed the compositional homogeneity. The catalytic activity of these supported nanoparticles with regard to methanol electrooxidation is investigated using cyclic voltammetry (CV), chronoamperometry (CA) and CO-stripping voltammetry techniques at temperatures between 25 and 60 °C. Such investigation concerns supported catalysts prepared with ca. 10 and 18 wt.% overall metal loading (Pt + Ru) onto the Vulcan^® XC72R substrate. Comparative testing of our catalysts and a commercial Pt-Ru/Vulcan reveals markedly superior activity for our catalysts. In fact, we observe for the latter a five-fold increase of the oxidation current as compared to a commercial Pt-Ru/Vulcan with equal metal loading. One of the reasons for the greater activity is found to be the very high dispersion of the metals over the substrate, i.e. the large surface area of the active phase. Other reasons are plausibly ascribable to the varied Pt/Ru composition and/or reduced presence of contaminants at the catalyst surface.     

Structure and chemical composition of supported Pt-Sn electrocatalysts for ethanol oxidation

Carbon supported PtSn alloy and PtSnO_x particles with nominal Pt:Sn ratios of 3:1 were prepared by a modified polyol method. High resolution transmission electron microscopy (HRTEM) and X-ray microchemical analysis were used to characterize the composition, size, distribution, and morphology of PtSn particles. The particles are predominantly single nanocrystals with diameters in the order of 2.0-3.0 nm. According to the XRD results, the lattice constant of Pt in the PtSn alloy is dilated due to Sn atoms penetrating into the Pt crystalline lattice. While for PtSnO_x nanoparticles, the lattice constant of Pt only changed a little. HRTEM micrograph of PtSnO_x clearly shows that the change of the spacing of Pt (1 1 1) plane is neglectable, meanwhile, SnO₂ nanoparticles, characterized with the nominal 0.264 nm spacing of SnO₂ (1 0 1) plane, were found in the vicinity of Pt particles. In contrast, the HRTEM micrograph of PtSn alloy shows that the spacing of Pt (1 1 1) plane extends to 0.234 nm from the original 0.226 nm. High resolution energy dispersive X-ray spectroscopy (HR-EDS) analyses show that all investigated particles in the two PtSn catalysts represent uniform Pt/Sn compositions very close to the nominal one. Cyclic voltammograms (CV) in sulfuric acid show that the hydrogen ad/desorption was inhibited on the surface of PtSn alloy compared to that on the surface of the PtSnO_x catalyst. PtSnO_x catalyst showed higher catalytic activity for ethanol electro-oxidation than PtSn alloy from the results of chronoamperometry (CA) analysis and the performance of direct ethanol fuel cells (DEFCs). It is deduced that the unchanged lattice parameter of Pt in the PtSnO_x catalyst is favorable to ethanol adsorption and meanwhile, tin oxide in the vicinity of Pt nanoparticles could offer oxygen species conveniently to remove the CO-like species of ethanolic residues to free Pt active sites.     

Aspects of the anodic oxidation of methanol

This paper describes some aspects of recent investigations into the anodic oxidation of methanol. Methanol has long been proposed as an anode fuel for a fuel cell, chiefly because of its ease of carriage, distribution and manipulation. However, methanol is very much more difficult to oxidise anodically than hydrogen, the more conventional anode fuel, and this has hampered development of commercial direct methanol fuel cells. Platinum-ruthenium catalysts are the most active discovered to date. Some advances in electrocatalysis of the methanol reaction by non-noble materials are discussed.     

Pt–Sn/C electrocatalysts for methanol oxidation synthesized by reduction with formic acid

Carbon supported Pt–Sn alloy catalysts were prepared by reduction of Pt and Sn precursors with formic acid, and their electrocatalytic activity for methanol oxidation was compared with commercial Pt/C and Pt₇₅Sn₂₅/C electrocatalysts. By X-ray diffraction analysis it was found that the Pt lattice parameter increases with the addition of Sn, indicative of alloy formation. It was confirmed that Sn exhibits cocatalytic activity for CO oxidation. The onset potential for the methanol oxidation reaction of the Pt–Sn electrode was approximately 0.1 V smaller than that on Pt both at room temperature and at 90 °C. The best performance in a direct methanol fuel cell was obtained using the Pt₇₅Sn₂₅/C alloy catalyst prepared by the formic acid method as the result of an optimal balance of Sn content, degree of alloying and metal particle size.     

Growth of binder free mesoporous 3D-CuCo2O4 electrocatalysts with high activity and stability for electro-oxidation of methanol

Designing an active and reliable electrocatalyst is the urgent need for the desirable improvement in direct methanol fuel cells (DMFCs). In recent time, binary metal oxides have shown much attention as possible electrocatalysts for the future DMFCs. Herein, we have reported direct growth of 3D-CuCo₂O₄ on Nickel foam (NF) by hydrothermal route for electrochemical methanol oxidation. The electrochemical performance was examined by cyclic voltammetry (CV), chronoamperommetry (CA) and electrochemical impedance spectroscopy (EIS) techniques. Electrochemical analysis of 3D-CuCo₂O₄ exhibits high current density of 112 A g^?1 at scan rate of 10 mV s^?1 and retains 91% of initial current density after 1000 C V cycles. The high electrocatalytic activity of mesoporous 3D-CuCo₂O₄ is mainly ascribed to the synergetic effect of bimetallic element (Cu and Co), high surface area and enhanced charge-transfer because of direct growth of catalyst on NF. The present synthesis strategy and use of spinel oxides can offer promising feature for the development of non-precious catalysts for DMFCs.     

Composite electrocatalysts for anodic methanol and methanol-reformate oxidation

Binary anode electrocatalyst formulations were prepared by adsorption of phthalocyanine and tetraphenylporphyrin complexes of different transition metals on a commercial carbon supported platinum catalyst. Only after pyrolyzing the complexes at 700 °C under nitrogen were catalysts of some activity obtained. A binary Pt/Ni electrocatalyst prepared by this procedure exhibits considerable anodic catalytic activity in the acidic environment of the Nafion^® electrolyte for reformate and direct methanol oxidation for more than 400 h without deterioration. Ternary electrocatalyst formulations Pt/Ru/W = 1/1/y were produced according to the Bönnemann method. The Pt/Ru/W catalyst of 1/1/1.5 (mol/mol/mol) composition is optimal. Compared to the Pt/Ru catalyst, it enhances the performance of reformate (H₂ + 150 ppm CO) fuel cells by 50% and of direct methanol fuel cells (steam/methanol vapour = 50:1 mol/mol) by 80%. Attached to a GC electrode by a thin Nafion^® film, the catalysts were also tested for methanol oxidation in aqueous methanol solutions in half cells by slow potential stepping. This procedure is useful for fast initial screening.     

Iron-chromium-molybdenum oxide catalysts for methanol oxidation

The activity and selectivity of a precipitated iron-chromium-molybdenum oxide catalyst (Mo/(Fe + Cr) = 2.5/(0.5 + 0.5)) towards methanol mild oxidation have been studied by a flow-circulation method. Commensurable activity and selectivity with those of the industrial Fe₂ (MoO₄)₃-MoO₃ catalysts as well as an enhanced stability have been found. The Mössbauer spectra of fresh and tested catalysts show that during the catalytic reaction a partial reduction occurs and a steady state composition differing from the initial one is formed.     

RuAg/TiO2-C催化剂的制备及对甲醇的电催化氧化性能

采用改性溶胶凝胶法和水热合成法制备了掺C多孔纳米TiO2,并以其为载体制备了一种RuAg/TiO2-C甲醇催化剂。采用X射线衍射（XRD）、透射电镜（TEM）、X射线能谱（EDS）和X射线光电子能谱（XPS）等对催化剂进行了表征,测定了其对甲醇的电催化氧化性能。实验结果表明,RuAg的负载和C的掺杂能提高TiO2对甲醇的电催化性能,RuAg/TiO2-C对甲醇电催化的循环伏安曲线中未见甲醇氧化中间产物的氧化峰,0.544 V处有一个较大的甲醇氧化峰,其峰电流密度5.8 mA/cm2,RuAg/TiO2-C比商用PtRu/C催化剂具有更高的催化活性和抗毒性,RuAg合金的负载以及RuAg合金与掺C多孔纳米TiO2载体之间较强的相互作用是其对甲醇催化性能提高的主要因素。     

Green bio-synthesis of Ni/montmorillonite nanocomposite using extract of Allium jesdianum as the nano-catalyst for electrocatalytic oxidation of methanol

In this work, synthesis of Ni nanoparticles was carried out successfully by water extract of Allium jesdianum as a biochemical reducing agent in the presence of montmorillonite clay (MMT) as a natural solid support for the first time. Then the electrochemical activity of the synthesized nanocomposite was investigated in methanol electrocatalytic oxidation. MMT with high cation exchange capacity and nano layer structure was exposed to ion exchange conditions in nickel solution. Then Ni²⁺ ion exchanged form was used in this process as a source of ions and also capping agent. Water extract of Allium jesdianum used as a reducing agent due to abundant availability of phenolic and flavonoid contents. The synthesized Ni/MMT nanocomposite was characterized using UV–Vis spectroscopy (UV–Vis), Fourier Transform Infrared Spectroscopy (FT-IR), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission electron microscopy (TEM) and Energy-dispersive X-ray spectroscopy (EDX). The surface of prepared modified electrode has been characterized using SEM to evaluate the morphology, showing uniform dispersion of Ni nanoparticles with mean diameter of 12 to 20 nm. The modified carbon paste electrode was then used in methanol electrocatalytic oxidation reaction. Methanol oxidation on the proposed modified electrode surface occurs at 0.6 V and 0.3 V in alkaline and acidic medium respectively. Also, the results showed the better performance of modified electrode toward methanol electrocatalytic oxidation in comparison with carbon paste electrode that is modified by ion exchanged MMT. Charge transfer coefficients and apparent charge transfer rate constant for the modified electrode in the absence of methanol in alkaline medium were respectively found as: α_a = 0.53, α_c = 0.37 and k_s = 1.6 × 10⁻¹ s⁻¹. Also, the average value of catalytic rate constant for the electrocatalytic oxidation of methanol by the prepared nano-catalyst was estimated to be about 0.9 L·mol^-1·s^-1 by chronoamperometry technique. The prepared electrode was also effective for electrocatalytic oxidation of ethanol and formaldehyde in alkaline medium.     

Pt-Co supported on single-walled carbon nanotubes as an anode catalyst for direct methanol fuel cells

Catalyst of Pt-Co supported on single-walled carbon nanotubes (SWCNTs) is prepared using mixed reducing agents. The SWCNTs were pretreated in a microwave oven to enable surface modification. Pt-Co nanoparticles with narrow particle size distribution around 5.4 nm were uniformly deposited onto the SWCNTs. Under same Pt loading mass and experimental conditions, the SWCNTs-Pt-Co catalyst shows higher electrocatalytic activity and improved resistance to CO poisoning than the SWCNTs-Pt catalyst.     

Rationally designed CdS/Ti3C2 MXene electrocatalysts for efficient CO2 reduction in aqueous electrolyte

MXene-based catalysts have shown excellent activities in various electrocatalytic reactions due to the two-dimensional structure, good electrical conductivity and abundant surface functional groups. However, because of the competitive reactions in aqueous electrolytes, the application of MXene materials in CO₂ electroreduction still remains a challenge. Herein, a simple strategy was developed for the design of high efficient and stable CO₂ electroreduction catalysts in aqueous electrolyte. A series of MXene composite catalysts were successfully synthesized by densely coating sulfur vacancy-rich CdS nanoparticles on Ti₃C₂. The two-dimensional MXene skeleton with good conductivity delivers fast electron transfer, improves the electrolyte infiltration and increases the electrochemical surface area. CdS nanoparticles with abundant sulfur vacancies are attached on Ti₃C₂ MXene surface, providing active sites for CO₂ reduction. Faraday efficiency of the by-product hydrogen could be significantly reduced by minimizing the surface-exposed Ti of the catalyst. Benefited from these merits, the optimal CdS/Ti₃C₂ possesses fast CO₂ electroreduction reaction kinetics, exhibiting a high CO Faraday efficiency of 94% at -1.0 V vs. reversible hydrogen electrode. This work provides a feasible pathway for the design of MXene-based catalysts of CO₂ electroreduction.