Methanol Oxidation Reaction Mechanism Research Articles

Elucidating Pathways of Methanol Oxidation on Oxygen-Vacant NiOOH Sites Using a Synergistic in Situ Attenuated Total Reflectance Surface-Enhanced Infrared Absorption Spectroscopy and DFT Study

A thorough comprehension of the mechanism underlying the methanol oxidation reaction (MOR) on Ni-based catalysts is critical for electrocatalysis design and development of Ni-based alloys. However, the mechanism of MOR on these materials remains a matter of controversy. There have been few publications on employing surface enhanced spectroscopy to adequately capture the surface chemistry of MOR on NiOOH while the existing theoretical chemistry studies on the reaction mechanism presents a multitude of inconsistent and incomplete results. Here, we combine in situ surface-enhanced infrared absorption spectroscopy (SEIRAS) and density functional theory (DFT) to determine the mechanism and product distribution of MOR on monometallic Ni-based catalysts in alkaline media. Critically, the DFT calculations include the role of O-vacancy of NiOOH, which is argued to be the active site for methanol oxidation. The SEIRAS results show that two bands corresponding to nas(O=C–O) of HCOO- and nas(C–O) of HCO3 -/CO3 2- appears after the commencement of MOR and varies as a function of potentials. These spectroscopic results are in good agreement with the DFT-based energetics of reaction, suggesting that the MOR mainly proceeds in the following pathway: The early consumption of methanol yields HCOO- as the major product while increasing potentials stimulate further oxidation of adsorbed HCOO* to form HCO3 -/CO3 2-. On the other hand, we show a parallel pathway for the generation of HCO3 -/CO3 2- at high potentials that bypasses the formation of HCOO*. The two main pathways presented in this study are thermodynamically more feasible than the one predominantly reported in literature for MOR on NiOOH. The two former pathways circumvent CHO and/or CO to produce HCO3 -/CO3 2-, whereas the latter involves these species as intermediates. These DFT-based hypotheses are backed up by the spectroscopic evidence that no band associated with CHO and CO can be detected by SEIRAS. This study has the potential to offer invaluable atomic-level insights into MOR on NiOOH as a stepping stone to development of high-performance multi-metallic Ni-based catalysts. Figure 1

Rational one-step synthesis of porous PtAg nanowires for methanol oxidation with a CO-poisoning tolerance: An experimental and theoretical study

Binary Pt-based porous nanowires (PNWs) are highly promising for electrochemical applications owing to their high surface areas, abundant active sites, and immense durability against aggregation, but their simple template-free synthesis for methanol oxidation reaction (MOR) is rarely reported. Herein, we provide a simple and template-free method for the one-step fabrication of PtAgx PNWs via autocatalytic and oriented attachment growth mechanism in an autoclave. This drives the formation of ultra-long PtAg PNWs (∼1.5 µm length and 20 nm width) with agglutinated pores (2–10 nm), high Ag content (22–34 at%), and upshifted d-band center. These merits endowed the MOR mass activity of PtAg PNWs (2.22 mA/µgPt) by 2.26 and 2.81 times than those of Pt PNWs and commercial Pt/C catalyst, respectively, based on equivalent Pt mass besides a superior stability and CO-poisoning tolerance. The MOR activity of PtAg PNWs outperformed previously reported PtAg nanostructures. Density functional theory (DFT) simulations confirmed the bifunctional MOR mechanism with a lower energy barrier for dissociative adsorption of methanol molecules (0.49 eV) and CO oxidation on PtAg PNWs than that on Pt PNWs. This study may allow the formation of Pt-based PNWs alloys with various metals for MOR.

Oxygen Vacancies Alter Methanol Oxidation Pathways on NiOOH.

A thorough comprehension of the mechanism underlying the methanol oxidation reaction (MOR) on Ni-based catalysts is critical for future electrocatalytic design and development. However, the mechanism of MOR on these materials remains a matter of controversy. Herein, we combine in situ surface-enhanced infrared absorption spectroscopy (SEIRAS) and density functional theory (DFT) calculations to identify the active sites and determine the mechanism of MOR on monometallic Ni-based catalysts in alkaline media. The SEIRAS results show that formate and (bi)carbonate are formed after the commencement of the MOR with potential-dependent relative distributions. These spectroscopic results are in good agreement with the DFT-computed reaction profiles over an oxygen vacancy, suggesting that the MOR mainly proceeds through the formate-involving pathway, in which the early consumption of methanol yields formate as the major product, while increasing potential drives further oxidation of formate to (bi)carbonate. We also find a parallel pathway for the generation of (bi)carbonate at high potentials that bypasses the formation of formate. The two main pathways are thermodynamically more feasible than the one predominantly reported in the literature for MOR on NiOOH that involves CHO and/or CO as key intermediates. These DFT results are supported by spectroscopic evidence showing that no band associated with CHO or CO can be detected by SEIRAS, which is attributed to the nature of the oxygen vacancies as the active sites, suppressing deep dehydrogenation of CH2O to CHO. This work thus shows the promising role of defect engineering in promoting the electrocatalytic MOR activity and selectivity.

Unraveling the electro-oxidation steps of methanol on a single nanoparticle by in situ nanoplasmonic scattering spectroscopy

Understanding the mechanism of methanol oxidation reaction (MOR) remains a challenge in the development of direct methanol fuel cells. Large-scale investigations of the MOR encounter issues related to mass transfer and averaging effects. To address these limitations, exploring the MOR on the surfaces of individual nanocatalyst and precisely identifying the reaction steps can yield valuable insights into the underlying pathways. In this study, we employed in situ nanoplasmonic resonance scattering spectroscopy to dynamically monitor the MOR process on single gold nanorod particles (GNPs) and Pt-coated gold nanoparticles (Pt-GNPs). We observed the evolution of metal hydroxides, which was assumed as the active species. Notably, the dynamic behavior of the surface atomic layers revealed the rate-determining steps for both the GNPs and Pt-GNPs, indicating competitive adsorption of intermediates on the nanocatalyst surface. The resulting inherent reaction mechanism highlights the thermodynamics-dependent catalysts’ redox processes and their surface adsorptions, which holds significance for advancing highly active MOR catalysts.

In situ growth of Ni/Ni3S2–MoO2 nanocrystals on carbon cloth for the enhanced electrocatalytic oxidation of methanol

The rational design and construction of high-performance Ni-based methanol oxidation catalyst is the key technology for direct methanol fuel cell (DMFC). In this work, Ni/Ni3S2–MoO2@CC composite is constructed by in-situ growth of Ni/Ni3S2–MoO2 nanoparticles on carbon cloth (CC) by a two-step method, which has excellent activity and stability for methanol oxidation reaction (MOR), that is, the current density (j) can reach 240.08 mA cm−2 at the potential of 1.67 V. The potentials at different j are 10 mA cm−2@1.37 V and 100 mA cm−2@1.50 V, respectively. Meanwhile, the retention rate of j is 90.36 % after 12 h CA stability test. Based on density functional theory (DFT) calculation, the excellent properties of Ni/Ni3S2–MoO2@CC composite are mainly attributed to its higher conductivity and higher carrier density accelerate the charge transfer in the MOR process, its Ni and Mo sites facilitate the adsorption of methanol as a reactant and desorption of carbon dioxide as a product, thus promoting the kinetics of MOR. In addition, the reaction mechanism of MOR on the Ni/Ni3S2–MoO2@CC is more likely to follow the formation path of CO* intermediates. This work provides a feasible route for the synthesis of high performance electrocatalysts for the MOR.

CO2 and Formate Pathway of Methanol Electrooxidation at Rhodium Electrodes in Alkaline Media: An In Situ Electrochemical Attenuated Total Refection Surface-Enhanced Infrared Absorption Spectroscopy and Infrared Reflection Absorption Spectroscopy Investigation.

Rh catalysts exhibit unexpected high activity for the methanol oxidation reaction (MOR) in alkaline conditions, making them potential anodic catalysts for direct methanol fuel cells (DMFCs). Nevertheless, the MOR mechanism on Rh electrodes has not been clarified thus far, which impedes the development of high-efficiency Rh-based MOR catalysts. To investigate it, a combination of in situ electrochemical techniques called attenuated total refection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and infrared reflection absorption spectroscopy (IRAS) is used. Cyclic voltammograms of MOR at Rh electrodes show considerable activity in alkaline media rather than acidic media, although the real-time ATR-SEIRA spectral results demonstrate that methanol can rarely self-decompose on Rh at open-circuit conditions. Meanwhile, in combination of ATR-SEIRAS and IRAS results, CO2 and formate are thought to be MOR products, suggesting a dual-pathway mechanism ("CO2 pathway" and "formate pathway"). Specifically, COad species, which are the major intermediates in the CO2 pathway, can produce at lower potentials and be oxidized into CO2 at a potential of 0.5-0.75 V. Concurrently, the formate can be produced from 0.5 V and diffuse into the bulk electrolyte to become one of the MOR products, while the further electrochemical conversion of formate to CO2 is essentially negligible. More directly, the apparent selectivity (r) of the CO2 pathway is estimated to reach ca. 0.63 at 0.9 V, confirming the potential-dependent selectivity of MOR at Rh surfaces. This study might provide fresh insights into the design and fabrication of effective Rh-based catalysts for MOR.

Tubular Nickel Hydroxide Embedded in Zeolitic Cobalt Oxide for Methanol Oxidation Reaction

Non-noble metal-based electrocatalysts have been designed for effective electrochemical oxidation of methanol in basic medium. The catalyst consisting of nickel (Ni) and cobalt (Co) metals was synthesized with a zeolitic imidazolate framework (ZIF) template approach. Such a method leads to the decoration of cylindrically shaped nickel hydroxides, Ni(OH)2, in nanodimensions within the zeolitic crystals of spinel cobalt oxides, Co3O4. The mixed metal hydroxide and oxide, Ni(OH)2-Co3O4, represented good activity toward electrochemical oxidation of methanol in 1 M KOH at a low onset potential. When combined with a carboxylate-functionalized multiwalled carbon nanotube (COOH-MWCNT), the same material, Ni(OH)2-Co3O4, exhibited superior MOR (methanol oxidation reaction) activity, giving a peak current density of 4.2 Amg–1 at similar conditions. The MWCNT-modified catalyst, Ni(OH)2-Co3O4-MWCNT, also showed high stability up to 500 cycles and 25000 s without a significant loss in the current density. The linear dependency of the current density against the square root of the scan rate indicated a diffusion-controlled MOR process. The decrease in onset potential with increasing scan rate also predicted a kinetically favorable MOR process. Fourier transform infrared and Raman analyses suggested that the MOR mechanism proceeded through the adsorption of methanol (CH3OH) on the catalyst surface, and its deprotonation formed the methoxide ion (CH3O–) which in the later course decomposed to CO2 and H2O. The Raman study also showed that, during the electrochemical oxidation process, Co2+ species in Co3O4 transformed to CoOOH and thereby favored the MOR.

Self-standing three-dimensional PdAu nanoflowers for plasma-enhanced photo-electrocatalytic methanol oxidation with a CO-free dominant mechanism

Precise design of high efficacious catalysts and the insight into the mechanism for photo-electrocoupling catalytic methanol oxidation reaction (MOR) are two major issues for the development and practical application of direct methanol fuel cells (DMFCs). Herein, a novel self-standing three-dimensional nanosheet assembly PdAu nanoflower with local surface plasmon resonance effect is fabricated to acquire excellent catalytic performance and explore the photo-electrocatalytic mechanism for MOR. Interestingly, the Pd1Au1 nanoflower electrocatalyst exhibits superior mass activity than pure Pd and Pd/C catalysts thanks to the abundant active sites and efficacious charge transfer. Further on, with the assistance of LSPR effect, the catalytic activity for MOR of Pd1Au1 catalyst (4179.04 mA mg−1Pd) under visible light illumination achieved 2.41-fold than dark conditions (1731.42 mA mg−1Pd). Moreover, the long-term durability of Pd1Au1 catalysts with visible light is also improved compare to dark condition and other mentioned Pd catalyst. More significantly, a photo-electrocoupling CO-free dominant mechanism is proposed to in-depth understand the promotion of catalytic activity and durability for MOR. This contribution provides the rational design of plasma-enhanced high-effective photo-electrocatalyst and reveals a CO-free dominant MOR mechanism for the progress of future liquid direct fuel cells.

Selective transformation of aqueous methanol to value-added formic acid and hydrogen on bifunctional Mo2P monolayers in fuel cells

Direct methanol fuel cell (DMFC) running on hydrogen is an attractive alternative power source for a wide range of applications. Importantly, the required hydrogen can be in situ released from the stable liquid, which ensures its safe transformation and storage. However, the catalyst in DMFC is usually poisoned by the generated CO species, thus it is challenging to find an efficient and durable catalyst for practical applications. Here, the mechanism of methanol oxidation reaction (MOR) on the Mo2P monolayer is first studied using density functional theory. Compared with the initial C–H and C–O bond scissions, the initial O–H bond scission is found to be the most favorable. The reason is systematically analyzed based on steric effect, activation Gibbs free energy (Gact) decomposition and the Brønsted − Evans − Polanyi (BEP) relationships. The further dehydrogenation of CH2OH via the O–H bond scission has a significantly lower Gact than that of CH3O, thus the methanol decomposition on the Mo2P monolayer may proceed via two competitive pathways, i.e., CH3OH→CH3O→CH2O→CHO→CO and CH3OH→CH2OH→CH2O→CHO→CO. Then, through overcoming a relatively low barrier, the generated CO species can be combined with hydroxyl group from water dissociation on the Mo2P monolayer to produce formic acid, which can be easily desorbed from the catalyst surface to avoid the block of active sites. This study provides a better understanding of the MOR mechanism and paves the way for further advancing the catalyst in DMFCs.

Alkali-induced Transformation of Ni-MOF into Ni(OH)2 Nanostructured Flowers for Efficient Electrocatalytic Methanol Oxidation Reaction.

Post treatment of metal-organic frameworks (MOFs) is widely employed to develop efficient electrocatalysts with better catalytic properties. But the complex processes of post treatment generally led to the collapse of the original structures of MOFs, making the preservation of their pristine hierarchical porous structure a great challenge. Herein, we propose the strategy of alkali treatment of Ni-MOF to transform it into Ni(OH)2 with similar morphology and enhanced electrocatalytic properties for methanol oxidation reaction (MOR). The structure and electrocatalytic properties of as-obtained Ni(OH)2 nanostructured flowers were seriously depended on the alkali concentrations. As the result, Ni(OH)2 obtained from Ni-MOF treated by 0.25 M NaOH (noted as Ni(OH)2 -0.25) performs 1.5 and 2.5 times larger current density than those of Ni(OH)2 -0.025 and Ni(OH)2 -0.5 for MOR. Moreover, the electrocatalytic process and mechanism of MOR on the catalyst of Ni(OH)2 -0.25 are also revealed. Hence, this ex situ conversion strategy of alkali treatment for Ni-MOF uncovered the transformation of MOFs in alkaline solution and develops robust electrocatalyst for practical application of methanol fuel cells.

Kinetic UV-Vis Spectroscopic and DFT Mechanistic Study of the Redox Reaction of [OsVIIIO4(OH)n]n- (n = 1, 2) and Methanol in a Basic Aqueous Matrix.

This combined experimental and computational study builds on our previous studies to elucidate the reaction mechanism of methanol oxidation by OsVIII oxido/hydroxido species (in basic aqueous media) while accounting for the simultaneous formation of OsVII species via a comproportionation reaction between OsVIII and OsVI. UV-Vis spectroscopy kinetic analyses with either CH3OH or the deuterated analogue CD3OH as a reducing agent revealed that transfer of α-carbon-hydrogen of methanol is the partial rate-limiting step. The resulting relatively large KIE value of approximately 11.82 is a combination of primary and secondary isotope effects. The Eyring plots for the oxidation of these isotopologues of methanol under the same reaction conditions are parallel to each other and hence have the same activation enthalpy [Δ⧧H° = 14.4 ± 1.2 kcal mol-1 (CH3OH) and 14.5 ± 1.3 kcal mol-1 (CD3OH)] but lowered activation entropy (Δ⧧S°) from -12.5 ± 4.1 cal mol-1 K-1 (CH3OH) to -17.1 ± 4.4 cal mol-1 K-1 (CD3OH). DFT computational studies at the PBE-D3 level with QZ4P (Os) and pVQZ (O and H) basis sets provide clear evidence to support the data and interpretations derived from the experimental kinetic work. Comparative DFT mechanistic investigations in a simulated aqueous phase (COSMO) indicate that methanol and OsVIII first associate to form a noncovalent adduct bound together by intermolecular H-bonding interactions. This is followed by spin-forbidden α-carbon-hydrogen transfer (not O-H transfer) from methanol to OsVIII by means of HAT, which is found to be the partial rate-limiting step. Without the organic and inorganic fragments dissociating from each other during the entire stepwise redox reaction (in order to avoid formation of highly energetically unfavorable monomer species), the HAT step is followed by PT and then ET before the final product monomers formaldehyde and OsVI dissociate from each other. DFT-calculated Δ⧧H° is within 5 kcal mol-1 of the experimentally obtained value, while the DFT Δ⧧S° is three times larger than that found from the experiment.

The anode oxidation reaction in fuel cell: A DFT study

Although fuel cell (FC) has been regarded as promising “green” power generator, the Pt-based catalysts in the FC hampered their further development for its high cost and scarcity. Direct methanol fuel cell (DMFC) as another kind of proton exchange membrane fuel cell (PEMFC) has been manifested that Pd also performs a certain activity for methanol oxidation reaction (MOR). To better know the mechanism of MOR, we present a DFT study on the first step reaction of MOR on the Pd(111). The results show that methanol prefers to physically adsorb on the Pd(111) through oxygen atom, while the dehydrogenated hydrogen atoms can adsorb either on face-cantered cubic (FCC) or hexagonal close packed (HCP) sites. The intermediate products will form a much stronger interaction with the Pd(111) since they contain more unsaturated bonds. The energy barrier of O-H bond scission is most favourable, while the C-O bond is unfavourable in the first step of MOR.

Theoretical study of the mechanism of methanol oxidation on PtNi catalyst

The anodic methanol oxidation reaction (MOR) generates a toxic intermediate CO in direct methanol fuel cell (DMFC), where the Pt catalyst is easily deactivated and the catalytic activity will be inevitably lowered. Alloying modification is an important approach to improve the anti-CO ability of Pt catalyst. In this work, MOR mechanism on PtNi catalyst is theoretically studied on four crystal planes doped with one-to-three Ni atoms. The main path of CO poisoning on the catalyst surface: CH3OH → CH2OH → CHOH → CHO → CO was studied. Meanwhile, the adsorption of PtNi catalyst on the reaction intermediate was carried out. Based on the relative energy ΔE, the four optimal catalysts were compared for MOR. Building on the above comprehensive evaluations the appropriate catalysts were finally selected by the activation effect. It is proved by calculation that the selected 1,2,3Ni-Pt (200) reduces the relative energy of the overall reaction path, possessing good catalytic effect on MOR with certain anti-CO toxicity.

Component-dependent activity of bimetallic PdCu and PdNi electrocatalysts for methanol oxidation reaction in alkaline media

In the work, the catalytic performance and reaction mechanism of methanol oxidation reaction(MOR) are studied for the bimetallic PdxCuy and PdxNiy catalysts in alkaline solution. The results reveal that the Pd1Ni1/C and Pd1Cu1/C bimetallic catalysts exhibit high catalytic activity 4.1 and 3.9 times higher than that of the commercial Pd/C catalyst. Combined with the X-ray diffraction (XRD), High-resolution transmission electron microscopy(HRTEM) and X-ray photoelectron spectroscopy (XPS) analysis, the charge transfer between Pd and Cu(or Ni) alters the electron density of Pd and enhances the anti-CO poisoning ability, which is confirmed by DFT (Density Functional Theory) calculations. The CO binding energy is weakened on various bimetallic surfaces compared with that on Pd(111), consistent with experimental results. A linear relationship exists between the CO binding energy and the catalytic activity on the bimetallic catalysts, indicating that the CO binding energy may be the potential descriptor for MOR in alkaline media.

Elucidating the role of Ni to enhance the methanol oxidation reaction on Pd electrocatalysts

Amongst promising available technologies enabling the transition to renewable energy sources, electrochemical oxidation of alcohols, in a direct fuel cell or in an electrolysis reaction (H2 production), can be an economically and sustainable alternative to currently used technologies. In this work, we highlight the advantages of a Pd-Ni bimetallic electrocatalyst for methanol electrooxidation - a convenient choice due to the low cost of Ni combined with the observed acceptable catalytic performance of Pd. We report a synergistic effort between experiments and theoretical calculations based on density functional theory to provide an in-depth understanding - at the atomistic level - of the origin of the enhanced electrochemical activity of methanol electrooxidation using the bimetallic catalysts Pd3Ni and PdNi over pure Pd. Cyclic voltammograms and High-Performance Liquid Chromatography (HPLC) demonstrate higher activity towards methanol electrooxidation with increased Ni concentration and, furthermore, higher selectivity for CO2. These effects are understood by: 1) changes in the methanol oxidation reaction mechanism. 2) Mitigation or suppression of CO poisoning on the Pd-Ni alloys as compared to the pure Pd catalyst. 3) A stronger tendency towards highly oxidized intermediates for the alloys. These findings elucidate the effects of a bimetallic electrocatalyst for alcohol electrooxidation as well as unambiguously suggest PdNi as a more cost-effective alternative electrocatalyst.

Bi‐Co‐Cu Metal Oxide Foam as Significant Electrocatalyst for Methanol Electrooxidation

AbstractTransition metals and metal oxides are carrying out vibrant role in electrooxidation of methanol. Unfortunately, the unclear cognition of methanol oxidation reaction (MOR) mechanism hampers the development of high performance electrocatalysts. Thus, in this report, a fascinating MOR mechanism has been studied using CoCuBi oxide foam structure, where the (220) plane of CoCuBi oxides are playing an exciting role in an unprecedented mechanism. Interestingly, the (220) plane belongs to the families of plane, where transition metal oxides prefer Eley‐Rideal (ER) mechanism with concomitant reduction in the poisoning effect due to CO. The synergetic effect of these planes triggers the excellent current density of 118 mA/cm2 with 50% retention of activity even after 10 h stability test. X‐ray photoelectron spectroscopy (XPS) and X‐ray diffraction (XRD) studies revealed different oxidation states of metal ions indicating formation of mixed oxides along with different phase formation in CoCuBi foam structure. More excitingly, CoCuBi nanocomposites show remarkable electrochemical methanol oxidation to form specific product such as formate, which is strongly confirmed by proton NMR spectroscopy.

Methanol Oxidation Using Ternary Ordered Intermetallic Electrocatalysts: A DEMS Study

The sluggish methanol oxidation reaction (MOR) remains the biggest challenge for direct methanol fuel cells. To advance our understanding of the MOR mechanism, we have employed dual thin-layer diff...

One-step synthesis in deep eutectic solvents of PtV alloy nanonetworks on carbon nanotubes with enhanced methanol electrooxidation performance

We report a simple one-step chemical reduction strategy in deep eutectic solvents (DESs) for the fabrication of a PtV alloy nanonetwork (ANN)/multiwalled carbon nanotube (MWCNT) nanohybrid, which exhibits excellent electrocatalytic performance in both activity and stability for the methanol oxidation reaction (MOR). The as-synthesized nanohybrid was characterized by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy, confirming the formation of a porous nanonetwork structure composed of smaller PtV alloy nanoparticles (~3.8 nm) and the presence of strong electronic transfer interactions between Pt and alloyed V. The electrochemical properties of catalysts for the MOR were evaluated by using cyclic voltammetry and chronoamperometry techniques. The electrocatalytic activity, durability and CO tolerance ability of PtV ANNs/MWCNTs toward the MOR are found to be considerably higher than those of the Pt/MWCNT and commercial Pt/C catalysts. This investigation of the effect of several reaction parameters (e.g., scan rate and methanol concentration) indicates that the electrocatalytic oxidation of methanol on PtV ANNs/MWCNTs is a diffusion-controlled electrochemical process. The performance enhancement mechanism of MOR on the PtV ANN/MWCNT catalyst is analyzed based on the structure and electrochemical studies.

New Insights into the Electrocatalytic Mechanism of Methanol Oxidation on Amorphous Ni-B-Co Nanoparticles in Alkaline Media

Despite an increased interest in sustainable energy conversion systems, there have been limited studies investigating the electrocatalytic reaction mechanism of methanol oxidation on Ni-based amorphous materials in alkaline media. A thorough understanding of such mechanisms would aid in the development of amorphous catalytic materials for methanol oxidation reactions. In the present work, amorphous Ni-B and Ni-B-Co nanoparticles were prepared by a simple chemical reduction, and their electrocatalytic properties were investigated by cyclic voltammetry measurements. The diffusion coefficients (D0) for Ni-B, Ni-B-Co0.02, Ni-B-Co0.05, and Ni-B-Co0.1 nanoparticles were calculated to be 1.28 × 10−9, 2.35 × 10−9, 4.48 × 10−9 and 2.67 × 10−9 cm2 s−1, respectively. The reaction order of methanol in the studied transformation was approximately 0.5 for all studied catalysts, whereas the reaction order of the hydroxide ion was nearly 1. The activation energy (Ea) values of the reaction were also calculated for the Ni-B and Ni-B-Co nanoparticle systems. Based on our kinetic studies, a mechanism for the methanol oxidation reaction was proposed which involved formation of an electrocatalytic layer on the surface of amorphous Ni–B and Ni-B-Co nanoparticles. And methanol and hydroxide ions could diffuse freely through this three-dimensional porous conductive layer.

Enhanced electrocatalytic performance for the oxidation of methanol by hierarchical NiS/Ni(OH)2@polypyrrole/graphene oxide nanosheets

Novel hierarchical nanostructures composed of two-dimensional inorganic NiS/Ni(OH)2 and organic polypyrrole/graphene oxide (PPy/GO) nanosheets are successfully prepared by a simple solvothermal method. Due to the coordination interaction between Ni2+ and -NH- segments in PPy chains, lamellar NiS/Ni(OH)2 with the average thickness less than 10 nm are well immobilized and uniformly distributed on the surface of PPy/GO nanosheets by an in-situ growth, and the obtained hierarchical NiS/Ni(OH)2@PPy/GO nanosheets exhibit the excellent electrocatalytic activity towards methanol oxidation reaction (MOR) due to synergistic effects of lamellar GO, conductive PPy, electrocatalyticallyactive Ni(OH)2 and NiS, which may be further enhanced by the synergistic effect of the hierarchical nanostructures. Therefore, the hierarchical NiS/Ni(OH)2@PPy/GO nanosheets can act as good candidates with the excellent catalytic activity for the substitution of Pt to the electrooxidation of methanol in alkaline media and reveal a potential application to direct methanol fuel cells (DMFCs). The mechanism of MOR on the surface of NiS/Ni(OH)2@PPy/GO modified glassy carbon electrode (GCE) is also investigated in detail, where methanol is mainly oxidized on Ni(III) oxide surface through direct electrooxidation and Ni(III) is used as active surface for MOR.