Kinetic analysis of carbon monoxide and methanol oxidation on high performance carbon-supported Pt-Ru electrocatalyst for direct methanol fuel cells

The kinetic parameters of carbon monoxide and methanol oxidation reactions on a high performance carbon-supported Pt-Ru electrocatalyst (HP 20% 1:1 Pt-Ru alloy on Vulcan XC-72 carbon black) have been studied using cyclic voltammetry and rotating disk electrode (RDE) techniques in 0.50 M H₂SO₄ and H₂SO₄ (0.06-0.92 M) + CH₃OH (0.10-1.00 M) solutions at 25.0-45.0 °C. CO oxidation showed an irreversible behaviour with an adsorption control giving an exchange current density of 2.3 × 10⁻⁶ A cm⁻² and a Tafel slope of 113 mV dec⁻¹ (α = 0.52) at 25.0 °C. Methanol oxidation behaved as an irreversible mixed-controlled reaction, probably with generation of a soluble intermediate (such as HCHO or HCOOH), showing an exchange current density of 7.4 × 10⁻⁶ A cm⁻² and a Tafel slope of 199 mV dec⁻¹ (α = 0.30) at 25.0 °C. Reaction orders of 0.5 for methanol and −0.5 for proton were found, which are compatible with the consideration of the reaction between Pt-CO and Ru-OH species as the rate-determining step, being the initial methanol adsorption adjustable to a Temkin isotherm. The activation energy calculated through Arrhenius plots was 58 kJ mol⁻¹, practically independent of the applied potential. Methanol oxidation on carbon-supported Pt-Ru electrocatalyst was improved by multiple potential cycles, indicating the generation of hydrous ruthenium oxide, RuO_xH_y, which enhances the process.     

Structure and electrocatalytic performance of carbon-supported platinum nanoparticles

The structure of Pt nanoparticles and the composition of the catalyst-Nafion films strongly determine the performance of proton exchange membrane fuel cells. The effect of Nafion content in the catalyst ink, prepared with a commercially available carbon-supported Pt, in the kinetics of the hydrogen oxidation reaction (HOR), has been studied by the thin layer rotating disk electrode technique. The kinetic parameters have been related to the catalyst nanoparticles structure, characterized by X-ray diffraction and high-resolution transmission electron microscopy. The size-shape analysis is consistent with the presence of 3D cubo-octahedral Pt nanoparticles with average size of 2.5 nm. The electrochemically active surface area, determined by CO stripping, appears to depend on the composition of the deposited Pt/C-Nafion film, with a maximum value of 73 m² g_Pt⁻¹ for 30 wt.% Nafion. The results of CO stripping indicate that the external Pt faces are mainly (1 0 0) and (1 1 1) terraces, thus confirming the cubo-octahedral structure of nanoparticles. Cyclic voltammetry combined with the RDE technique has been applied to study the kinetic parameters of HOR besides the ionomer resistance effect on the anode kinetic current at different ionomer contents. The kinetic parameters show that H₂ oxidation behaves reversibly with an estimated exchange current density of 0.27 mA cm⁻².     

Polycarbazole as an efficient promoter for electrocatalytic oxidation of formic acid on Pt and Pt-Ru nanoparticles

Electrocatalytic activities of the monometallic Pt and bimetallic Pt-Ru nanoparticles dispersed onto polycarbazole (PCZ) films obtained by the electropolymerization on glassy carbon electrode (GC) (i.e., Pt/PCZ/GC, Pt-Ru/PCZ/GC) towards formic acid oxidation have been investigated using cyclic voltammetry and chronoamperometry methods. As-formed electrodes are characterized by SEM, EDX and electrochemical analysis. Relative to Pt and Pt-Ru deposited on the bare GC (i.e., Pt/GC and Pt-Ru/GC), Pt/PCZ/GC and Pt-Ru/PCZ/GC, respectively, exhibit higher catalytic activity and stronger poisoning-tolerance ability towards formic acid electrooxidation. The enhanced performance is proposed to come from the synergetic effect between metal nanoparticles (Pt, Pt-Ru) and PCZ. At the same time, the results of the stripping voltammograms of CO show that PCZ can weaken largely the adsorption strength of CO on catalysts and can make CO oxidation easier under lower potential, implying further that PCZ can be used as an efficient promoter for electrocatalytic oxidation of formic acid on Pt/PCZ and Pt-Ru/PCZ catalysts.     

Electrocatalytic activity of core-shell Au@Pt nanoparticles for the hydrogen oxidation reaction

The hydrogen oxidation reaction (hor) was studied for the first time on core-shell Au@Pt nanoparticles. They were dispersed on a rotating gold disc, with four different values of the active area factor (0.03 < f_aa < 2). The experimental current-overpotential dependences on steady state for the hor were obtained at a rotation rate of 2500 rpm. The elementary kinetic parameters were evaluated and compared with previous results obtained for platinum nanoparticles. It can be concluded that the substitution of the core of a Pt nanoparticle by Au would lead to a slight decrease of the reaction rate of the Tafel step (chemical), but it would not affect the charge transfer steps. Thus, the electrocatalytic activity of the Au@Pt nanoparticles is slightly lower than that of Pt nanoparticles at low overpotentials, but they are equal for η > 0.20 V.     

Novel peapod-like Ni2P@N-doped carbon nanorods array on carbon fiber for efficient electrocatalytic urea oxidation and hydrogen evolution reaction

Designing and optimizing structure is an effective method to enhance electrocatalytic performance of transition metal-based catalysts. In this work, an innovative nanostructured electrode, consisted of peapod-like Ni₂P@N-doped carbon nanorods array coating on carbon fiber (CF@p-Ni₂P@NC), is devised and synthesized. The N-doped carbon layer is crucial for maintaining the peapod-like nanostructure, which allows for multi-channel electrolyte transport and gas product release. And the carbon layer coating Ni₂P nanoparticles also enhance electrical conductivity and stability, thus ensuring fast electron transport from/to active sites and the long-term stability of catalyst during urea oxidation reaction (UOR)/hydrogen evolution reaction (HER). Benefit from the reasonable structure, CF@p-Ni₂P@NC present perfect performance with getting 100 mA cm^?2 at potential/overpotential of 1.417/0.194 V for UOR/HER in 1.0 M KOH containing 0.5 M urea. In addition, the overall urea-electrolysis system using CF@p-Ni₂P@NC bifunctional electrode only requires 1.590 V to obtain 100 mA cm^?2.     

Direct immobilization of Pt-Ru alloy nanoparticles on nitrogen-doped carbon nanotubes with superior electrocatalytic performance

Taking the advantage of the inherent chemical activity arisen from the nitrogen incorporation for nitrogen-doped carbon nanotubes (NCNTs), we have developed a facile strategy for the construction of binary Pt-Ru/NCNT electrocatalysts. Alloyed Pt-Ru nanoparticles have been directly immobilized onto the outer surface of NCNTs without pre-modification due to the nitrogen participation. The Pt-Ru nanoparticles have a high dispersion, a narrow size distribution of 2.5-3.5 nm and tunable chemical composition. These catalysts have been evaluated for methanol oxidation and show good stability and better CO tolerance than the monometallic Pt/NCNT catalyst due to the bifunctional and electronic effects. The Pt₅Ru₅/NCNT catalyst shows superior electrocatalytic performance to the commercial Pt₅Ru₅/C catalyst. The easy fabrication and excellent performance of the NCNT-based Pt-Ru alloy catalysts indicate their potential application in direct methanol fuel cells.     

Facile synthesis of flower-like Au@AuPd nanocrystals with highly electrocatalytic activity for formic acid oxidation and hydrogen evolution reactions

Herein, a one-pot co-reduction method was developed to prepare flower-like Au@AuPd core-shell nanocrystals (Au@AuPd NCs) under the guidance of 2,4-diamino-6-hydroxypyrimidine (DAHP). The product was mainly characterized by microscopic measurements, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analysis, and its formation mechanism was discussed in details. The architectures showed much larger electrochemical active surface area (62.68 m² g^?1_Pd) than commercial Pd black (8.23 m² g^?1_Pd), together with the higher mass activity (1250 mA mg^?1) for formic acid oxidation reaction (FAOR). Besides, the catalyst displayed improved catalytic features for hydrogen evolution reaction (HER) relative to Pd black and Pt/C catalysts. These indicate the potential applications of the catalyst in energy storage and transformation.     

Enhancement of electrocatalytic activity towards hydrogen evolution reaction by boron-modified nickel nanoparticles

Nanosized nickel particles have been synthesized by three different routes: polyol, microemulsion and precipitation/reduction methods. Nickel nanoparticles have been evaluated as electrocatalysts for the hydrogen evolution reaction (HER). The electrocatalysts have been characterized by using X-ray diffraction (XRD), thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). Their electrocatalytic performance in the hydrogen evolution reaction has been evaluated by means of the Tafel curves recorded in alkaline medium. The activity for the hydrogen evolution reaction increases with the increasing amount of reduced Ni in the electrocatalysts. Remarkably, the formation of a nickel-boride alloyed phase (Ni₃B) is responsible for the higher activity of the sample prepared by the precipitation/reduction method for the HER. The crystalline phase Ni₃B appears to be responsible for the very high activity in hydrogen production.     

Enhancement of electrocatalytic activity of platinum for hydrogen oxidation reaction by sonochemically synthesized WC1−x nanoparticles

Two types of composite materials composed of Pt and WC_1−x nanoparticles supported on multiwalled carbon nanotubes (MWNT) are synthesized and evaluated in terms of their electrochemical properties, especially for the hydrogen oxidation reaction (HOR). The Pt nanoparticles are prepared by reduction of H₂PtCl₆ with NaBH₄, and the WC_1−x nanoparticles by a sonochemical method with a W(CO)₆ precursor. One of the composites is synthesized by forming WC_1−x nanoparticles on Pt-loaded MWNT and the other by physically mixing Pt-loaded MWNT with WC_1−x-loaded MWNT. The sonochemical synthesis of WC_1−x on Pt-loaded MWNT forms WC_1−x preferentially on Pt nanoparticles, which makes intimate contact between WC_1−x and Pt nanoparticles. The cyclic voltammograms of these composite materials show evidences for H⁺-spill-over from Pt to WC_1−x, thereby increasing the electrochemically active surface area (ECA). The composite in which WC_1−x is deposited on Pt shows a remarkable increase in ECA probably because the intimate contact between WC_1−x and Pt enhances the H⁺-spill-over. These materials exhibit enhanced HOR characteristics with Pt-specific mass activities about twice that of pure Pt nanoparticles.     

Kinetic study of the hydrogen oxidation reaction on sub-stoichiometric titanium oxide-supported platinum electrocatalyst in acid solution

The kinetics and mechanism of the hydrogen oxidation reaction were studied in 0.5 mol dm⁻³ HClO₄ solution on an electrode based on titanium oxide with Magneli phase structure-supported platinum electrocatalyst applied on rotation Au disk electrode. Pt catalyst was prepared by impregnation method from 2-propanol solution of Pt(NH₃)₂(NO₂)₂ and sub-stoichiometric titanium oxide powder. Sub-stiochiometric titanium oxide support was characterized by X-ray diffraction and BET techniques. The synthesized catalyst was analyzed by TEM technique. Based on Tafel-Heyrovsky-Volmer mechanism the corresponding kinetic equations were derived to describe the hydrogen oxidation current-potential behavior on RDE over the entire potential region. The polarization RDE curves were fitted with derived polarization equations according to proposed model. The fitting shows that the HOR on Pt proceeds most likely via the Tafel-Volmer (TV) pathway in the lower potential region, while the Heyrovsky-Volmer (HV) pathway is operative in the higher potential region. It is pointed out that Tafel equation that has been frequently used for the kinetics analysis in the HOR, can not reproduce the polarization curves measured with high mass-transport rates. Polarization measurements on RDE revealed that the Pt catalyst deposited on titanium suboxide support showed equal specific activity for the HOR compared to conventional carbon-supported Pt fuel cell catalyst.     

Porous N-doped Mo2C@C nanoparticles for high-performance hydrogen evolution reaction

Exploration for an earth-rich and competent electrocatalyst for the hydrogen evolution reaction (HER) is a significant and challenging approach to confronting the resources shortage and environmental crisis. Porous N-doped Mo₂C@C (N-Mo₂C@C) nanoparticles self-encapsulated in nanospheres are presented as a high-performing HER electrocatalyst fabricated through a one-pot solvothermal method followed by hydrogen calcination. Structural analyses show that acetamide can regulate the size of the nanospheres, provide a N source for doping and form porous structures composed of Mo₂C, which suggests the exposure of extensive active sites as well as the contact and diffusion among the medium, electrodes, and gas. Theoretical calculations show that the N doping can enhance the activity of the Mo-C bond, reduce the energy of capturing hydrogen intermediates, and increase the catalytic conductivity. This work offers a simple and promising strategy to understand the catalytic mechanism required to optimize the activity of Mo-based electrocatalysts via N doping.     

Sn-modified carbon-supported Pt nanoparticles synthesized using spontaneous deposition as electrocatalysts for direct alcohol fuel cells

Sn-modified carbon-supported Pt nanoparticles (Sn(Pt)/C electrocatalysts) were prepared by spontaneous deposition. Sn species were deposited on Pt/C by immersion in 2.0 × 10⁻⁴ M SnCl₂ + 0.1 M HClO₄ for different times, which allowed achieving an adequate control of the coverage (θ). Cyclic voltammetry (CV) in 0.5 M H₂SO₄ was carried out to determine θ and to evaluate the Sn(Pt)/C performance. The activity towards the oxidation reactions of methanol (MOR) and ethanol (EOR) was analyzed using CV in 0.5 M H₂SO₄ + 1.0 M alcohol. A promotional effect for the MOR and the EOR after the partial coverage by the Sn species was shown, as indicated by the significant reduction of the overpotential and the higher oxidation currents in both cases. This activation was explained by the formation of hydroxylated species on the tin deposits, thus facilitating the removal of the adsorbed intermediates. The best performance was achieved for θ ≈ 0.3 in the case of the MOR and for θ ≈ 0.5 in the case of the EOR. The reaction pathway for both alcohols was analyzed according to the obtained kinetic parameters, which significantly depended on the coverage.     

Enhanced electrocatalytic performance for hydrogen oxidation reaction on gold nanoparticles supported on tungsten oxide (VI) modified carbon

We report on a hydrogen oxidation reaction (HOR) catalyst system composed of gold nanoparticles (Au NPs) and tungsten oxide (WO₃). Previously, we reported that Au NPs could be activated for HOR by sonochemical heating and quenching. However, we also found that the activated Au NPs were poisoned by protons, the HOR product. In order to further improve the catalytic behavior of Au NPs, we employed tungsten oxide as a part of the support and a co-catalyst, by which proton spillover could be achieved. Au NPs supported on WO₃/C were synthesized. The intermediates and final product were characterized by powder X-ray diffraction, energy dispersive X-ray spectroscopy, and transmission electron microscopy. Electrocatalytic activity of the samples for HOR was investigated by the linear sweep voltammetry with rotating disk electrode technique, which showed the disappearance of the proton poisoning of Au NPs in contact with WO₃. Therefore, with sonication treatment, the Au NPs and WO₃ composite showed a very high and stable activity for HOR.     

Graphitized carbon nanocages/palladium nanoparticles: Sustainable preparation and electrocatalytic performances towards ethanol oxidation reaction

Graphitized carbon (GC) nanocages have been successfully prepared via a sustainable carbon powder buried-type Ni catalysis-growth technology from Tween-80 molecule precursor. The GC nanocages are used as support for the further construction of GC/Pd electrocatalyst towards ethanol oxidation reaction. The material structures and surface morphologies are studied by XRD, SEM and TEM techniques. The electrochemical properties are investigated by CV, LSV, EIS and CP techniques. The results showed that GC nanocages have good graphited structure and plentiful opening gaps, and the Pd nanoparticles were evenly distributed on the inner and outer surfaces of GC nanocages. The GC/Pd electrocatalyst exhibits excellent electrocatalytic performance towards ethanol oxidation. The positive scanning peak current density of GC/Pd electrode is up to 1612 A/g Pd in 1.0 mol/L NaOH +1.0 mol/L ethanol electrolyte, which is much higher than those (500–1100 A/g Pd) of traditional Pd electrodes supported with carbon nanotubes or graphene nanosheets.     

Nano-sized RuO2 electrocatalyst improves the electrochemical performance for hydrogen oxidation reaction

Hydrogen oxidation reaction (HOR) can be applied to proton exchange membrane fuel cells to generate electrical energy and anode discharge. Due to its special properties, RuO₂ has been applied to supercapacitors, phenolic wastewater, textile industry wastewater, and degrading organic substances. However, there is few reports on the application of the RuO₂ catalyst to hydrogen oxidation reaction (HOR). In this study, we successfully obtained RuO₂ NPs using a simple and eco-friendly hydrothermal method. Furthermore, the electrochemical activity of RuO₂ NPs prepared at different concentration (0.15 M, 0.20 M) and different hydrothermal temperature (150 °C, 160 °C, and 170 °C) was evaluated by the hydrogen oxidation reaction. The particle size, composition, dispersion and morphology of the obtained RuO₂ catalysts were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). In addition, cyclic voltammograms (CV) were utilized to investigate the electrochemical activity of the RuO₂ catalysts. The results showed that the obtained catalyst at a hydrothermal temperature of 160 °C and a concentration of 0.15 M displayed a Brunauer-Emmett-Teller (BET) surface area of 26.74 m² g⁻¹. Meanwhile, the catalyst had a uniform distribution. The hydrogen oxidation current density of the obtained RuO₂ catalysts is upto 6 mA cm⁻², showing a good electrochemical activity for hydrogen oxidation reaction.     

Reduced graphene oxide-supported ruthenium nanocatalysts for highly efficient electrocatalytic hydrogen evolution reaction

Hydrogen energy has received great attention because of its advantages such as large energy density and not producing carbon dioxide, and it is currently considered to be one of the most valuable green energy sources. Therefore, the development of efficiently hydrogen production is of great importance. Hydrogen production from water electrolysis has large application prospects due to its cleanliness and no pollution. However, how to prepare an efficient, stable and low-cost electrocatalyst for this process is still challenging. Here, we develop a reduced graphene oxide-supported ruthenium (Ru) nanoparticle electrocatalyst synthesized by a simple method. The ruthenium precursors are encapsulated and isolated with N,N-dimethylformamide (DMF) (Ru³⁺-DMF), which effectively inhibits the further agglomeration growth of ruthenium. After Ru³⁺-DMF being loaded on graphene oxide, Ru is supported on reduced graphene oxide (Ru/rGO) by the liquid phase chemical reduction method and the remaining organic solvent could be removed by calcination to form a well-dispersed Ru-based electrocatalyst. Ru/rGO shows excellent electrocatalytic activity and long-term stability for hydrogen evolution reaction (HER). In a solution of 1.0 M KOH, the overpotential of 3.0 wt%Ru/rGO for the HER at 100 mA cm^?2 is only 111.7 mV, and the Tafel slope is 31.5 mV dec^?1. It exhibits better HER performance compared to commercial Pt/C and other Ru/rGO catalysts with different Ru loadings. The work could give a new strategy for the synthesis of efficient electrocatalysts.     

Copper-N-SiO2 nanoparticles catalyst for hydrogen evolution reaction

Herein, we report the Cu(0)-based nanoparticles film generated by in situ electrochemical reductions of Cu(II) ions modified silica exhibits a high activity and durable HER catalyst in acid solution. Copper ions were attached to silica surface using chemical modification with propyl ethylene diamine (PEDA) linker followed by treating with copper sulfate solution to form Cu(II)-PEDA/silica complex. Copper nanoparticles then were obtained by electrochemical reduction of the silica immobilized Cu(II) ions in sulfuric acid solution. The physicochemical properties of the resulted from copper nanoparticles incorporated silica were investigated and analyzed by Energy Dispersive x-ray Spectroscopy (EDS), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), X-ray powder diffraction XRD, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM). The electrochemical characterizations confirm that the Cu(0) nanoparticles supported on silica substrate combining both high activity and stability for hydrogen evolution reaction with overpotential(η), of 200 mV and Tafel slope of 67 mV/dec could serve as Cu-based electrocatalysts in practical applications for hydrogen production in 0.5 M of H₂SO₄ solution.The catalyst exhibited respectable stability and steadily produced hydrogen at several potentials. The catalyst has the perspective to expressively lower the cost of manufacturing hydrogen fuel, thus helping to spread the use of hydrogen fuel which does not harm the environment.     

Porous flower-like nickel nitride as highly efficient bifunctional electrocatalysts for less energy-intensive hydrogen evolution and urea oxidation

Finding a suitable replacement for the high potential of anodic water electrolysis (oxygen evolution reaction (OER)) is significant for hydrogen energy storage and conversion. In this work, a simple and scalable method synthesizes a structurally unique Ni₃N nanoarray on Ni foam, Ni₃N-350/NF, that provides efficient electrocatalysis for the urea oxidation reaction (UOR) that transports 10 mA cm⁻² at a low potential of 1.34 V. In addition, Ni₃N-350/NF exhibits electro-defense electrocatalytic performance for hydrogen evolution reaction, which provides a low overpotential of 128 mV at 10 mA cm⁻². As proof of concept, all-water-urea electrolysis measurement is carried out in 1 M KOH with 0.5 M Urea with Ni₃N-350/NF as cathode and anode respectively. Ni₃N-350/NF||Ni₃N-350/NF electrode can provide 100 mA cm⁻² at a voltage of only 1.51 V, 160 mV less than that of water electrolysis, which proves its commercial viability in energy-saving hydrogen production.     

One-pot solvothermal synthesis of PdCu nanocrystals with enhanced electrocatalytic activity toward glycerol oxidation and hydrogen evolution

The catalytic capability of bimetallic nanocatalysts is closely correlated with their size, shape, and crystal structures. Herein, a facile one-pot solvothermal strategy is designed to fabricate uniform spherical PdCu nanocrystals (NCs) assembled by many smaller grains. Oeylamine (OAm) is acted as the reaction solvent and reductant. KBr and hexadecylpyridinium chloride monohydrate (HDPC) are used as the capping agent and surfactant to avoid the aggregation, respectively. The architectures possess larger electrochemically active surface area (ECSA) of 13.6 m² g^?1 than commercial Pd black (4.4 m² g^?1), showing the improved catalytic ability for glycerol oxidation reaction (GOR) in alkaline electrolyte in contrast with Pd black. Besides, the obtained catalyst exhibits more positive onset potential (?56 mV) and Tafel slope (51 mV decade^?1) toward hydrogen evolution reaction (HER) in acidic media relative to commercial Pd/C, albeit with their performance bellow commercial Pt/C catalyst.     

Phase transformation of iron phosphide nanoparticles for hydrogen evolution reaction electrocatalysis

Transition metal phosphides have emerged as alternative electrocatalysts for hydrogen evolution reaction (HER) due to their high activity and low cost compared to the conventional HER electrocatalysts such as Pt. However, the dependency of HER activity on different crystal phases is not well-understood. Here, we synthesized iron phosphide nanoparticles with two distinct phases via chemical transformation from iron metal to iron phosphides. During the development of iron phosphide phases by varying the synthesis conditions such as reaction temperature and time, the HER activities of the nanoparticle were examined. The HER activities of the iron phosphide nanoparticles were found to be phase-dependent.