Bis-Aniline-Crosslinked Enzyme–Metal Nanoparticle Composites on Electrodes for Bioelectronic Applications

The electrical contacting of redox enzymes with electrodes is the most fundamental requirement for the development of amperometric biosensors and biofuel cell elements. We describe a novel method to prepare electrically contacted metallic nanoparticles (NPs) or carbon nanotubes (CNTs)/enzyme hybrid composites on electrodes that act as amperometric biosensors or as the constituents of biofuel cells. Au NPs or Pt NPs were modified with thioaniline electropolymerizable groups, and so were the enzymes glucose oxidase (GOx) or bilirubin oxidase (BOD). Electrochemical polymerization of the thioaniline-functionalized Pt NPs and GOx on a thioaniline monolayer-modified Au surface led to the formation of a bis-aniline-bridged Pt NPs/GOx composite electrode that enabled the analysis of glucose through the electrocatalyzed reduction of H₂O₂. Similarly, a Pt NPs/BOD composite-functionalized electrode showed electrocatalytic activity toward the reduction of O₂ to H₂O. Also, a Au NPs/GOx composite-functionalized electrode revealed direct electrical contacting between the enzyme and the electrode through the electrocatalytic reduction of the bis-aniline bridges, and this enabled the bioelectrocatalytic oxidation and the amperometric sensing of glucose. Finally, a biofuel cell consisting of an anode modified with Nile blue/NAD⁺/alcohol dehydrogenase on carbon nanotubes, and a cathode composed of the bis-aniline-crosslinked Pt NPs/BOD composite was constructed. The biofuel cell operates with a power output corresponding to 200 μW cm^-2.     

Controlled platinum nanoparticles uniformly dispersed on nitrogen-doped carbon nanotubes for methanol oxidation

This work presents the synthesis of platinum nanoparticles (Pt NPs) and their subsequent deposition on the nitrogen-doped carbon nanotubes, which have been directly grown on a carbon cloth (CNT-CC electrode). The CNT-CC electrode provides a fast electron-transfer path to the carbon cloth, resulting in energy-loss reduction and enhancing catalytic activity of Pt NPs. The N-dopants in CNT serve as the defect sites to enhance nucleation of Pt particles. The reduction of the Pt precursor salt was carried out in the ethylene glycol solution at an elevated temperature. In order to control the Pt NP size, the pH of the reaction solution was controlled by the addition of NaOH. Zeta potential measurements of the as-prepared sample indicate that a higher zeta potential results in a smaller particle size, due to a stronger electrostatic repulsion between NPs. This serves a powerful tool for size control of the Pt nanoparticle. The Pt NPs dispersed on the CNT-CC have an average size of 2.81 nm (Pt/CNT-CC) prepared using 15 mM NaOH, with high uniformity under electron microscopy. Cyclic voltammetry measurements of the electrocatalytic activity of the Pt/CNT-CC for methanol oxidation indicate that it exhibits excellent electrocatalytic activity and are ideal for direct methanol fuel cell applications.     

Polyaniline‐supported platinum‐hydrous ruthenium oxide nanocomposite as electrocatalyst for methanol oxidation

A novel composite electrode is fabricated through the electrodeposition of hydrous ruthenium oxide (RuO₂·xH₂O) and platinum (Pt) particles into the matrix of polyaniline (PANI). Scanning electron microscopy reveals that RuO₂·xH₂O and Pt particles are homogeneously distributed into the matrix of PANI. A comparison of the sizes of Pt and RuO₂·xH₂O particles incorporated into the PANI film reveals that Pt particles are smaller in sizes as compared with the sizes of RuO₂·xH₂O particles. The catalytic activity of composite electrodes was evaluated for the oxidation of methanol by using cyclic voltammetry and chronoamperometry. A relatively high catalytic current was noticed for the oxidation of methanol (2.37 mA/cm²) at PANI‐Pt‐RuO₂·xH₂O electrode (+0.6 V (V vs. Ag/AgCl) in comparison to oxidation current at PAN‐Pt (1.27 mA/cm²) electrode. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Ethanol electrooxidation on platinum particles dispersed on poly(neutral red) film

The conductive polymer poly(neutral red) polymerized on a graphite electrode (PNR/graphite) as a support material was used for catalytic oxidation of ethanol in acidic solution and investigated by electrochemical methods. Pt particles loaded on the surface of PNR/graphite electrode exhibited higher electrocatalytic activity for ethanol oxidation in comparison with Pt particles supported directly on graphite. With the equivalent loading mass of Pt catalyst, the special activity (S _A ) at peak a of the Pt/PNR/graphite electrode polymerized for 10 cycles in 5 × 10⁻⁴ M NR + 0.5 M H₂SO₄ solution is 3,478 A C⁻¹ and about 2.20 times higher than that of the Pt/graphite electrode (1,582 A C⁻¹). The results show that the electrochemical performance of Pt catalyst for ethanol oxidation is improved by the addition of PNR     

Electrocatalytic oxidation of methanol and ethanol at carbon ceramic electrode modified with platinum nanoparticles

The use of carbon ceramic electrode (CCE) modified with platinum particles was studied for the electrocatalytic oxidation of methanol and ethanol by cyclic voltammetry and chronoamperometry. After preparation of a carbon ceramic as an electrode matrix by sol–gel technique, its surface was potentiostatically coated with Pt nanoparticles at −0.2 V vs. SCE in an aqueous solution of 0.1 M H₂SO₄ containing 0.002 M H₂PtCl₆. The electrocatalyst was characterized by XRD, SEM and cyclic voltammetry. The effective parameters on electrocatalytic oxidation of the alcohols, i.e. the amount Pt loadings, medium temperature and working potential limit in anodic direction were investigated and the results were discussed. This modified electrode showed an enhanced current density over the other Pt-modified electrodes making it more attractive for fuel cell applications.     

The catalysts supported on metallized electrospun polyacrylonitrile fibrous mats for methanol oxidation

Polyacrylonitrile nanofibrous mats coated with continuous thin gold films (Au-PAN) have been fabricated by combining the electrospinning and electroless plating techniques. The Pt particles are electrodeposited on the Au-PAN fibers surface by multi-cycle CV method, and the Au-PAN decorated with Pt (Pt/Au-PAN) shows higher activity toward methanol electro-oxidation. The catalytic peak current for methanol oxidation on the optimum Pt/Au-PAN electrode can reach about 450 mA mg⁻¹ Pt which is much larger than the catalytic peak current for methanol oxidation (118.4 mA mg⁻¹ Pt) on the electrode prepared by loading commercial Pt/C on Au-PAN (Pt/C/Au-PAN). Further experiments reveal that the Pt/Au-PAN electrodes exhibit better stability and smaller charge transfer resistance than Pt/C/Au-PAN electrodes, which indicates that the Au-PAN may be developed as supporting material for catalyst. The microscopy images of the electrodes show that the Pt particles deposited on Au-PAN conglomerate into larger particles, and that the Pt/C catalyst loaded on the Au-PAN also exhibits conglomeration after stability test. The hydrogen adsorption-desorption experiments indicate that the electrochemical surface area of the Pt particles for the both kinds of electrodes has decreased after stability test.     

Porous-microelectrode study on Pt/C catalysts for methanol electrooxidation

We have developed a porous-microelectrode (PME) to investigate the electroactivity of catalyst particles for proton exchange membrane fuel cells. The cavity at the tip of the PME was filled with Pt/C catalysts prepared by impregnation method. Cyclic voltammograms (CVs) recorded in 1 N H₂SO₄ aqueous solution revealed that the active area of the stacked catalysts exist not only at the surface but also inside of the stack. For methanol electrooxidation, 30 wt.% Pt/C exhibited the highest electroactivity, whereas the 50 wt.% Pt/C showed extremely small current. The small current is considered as a result of a small active-surface area. Methanol oxidation peak potential shifted toward cathodic direction as Pt-loading decreased, which agrees well with the Pt-oxide formation potential. The activation energy for methanol oxidation was assessed to be 44±3 kJ mol⁻¹ for all Pt/C catalysts and Pt-disc electrode.     

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.     

Selective Electrocatalysis on Platinum Nanoparticles with Preferential (100) Orientation Prepared by Cathodic Corrosion

The “cathodic corrosion” method for nanoparticle synthesis has been used to produce Pt nano-crystals with a preferential (100) orientation. These particles are surfactant-free and electrochemically clean, featuring a significant percentage of (100) terrace sites, as confirmed by electrochemical characterization tests to establish the amount of (100) sites: blank voltammetry in H₂SO₄, adsorbed CO stripping, and Ge irreversible adsorption. The high catalytic activity of these preferentially oriented particles is confirmed for reactions preferring (100) sites, such as dimethyl ether oxidation, ammonia oxidation, and nitrite reduction in alkaline media. In the case of nitrite reduction it is demonstrated that, similarly to (100) terraces of a well-ordered Pt single crystal electrode, the (100) facets of the nano-crystals can steer the reaction towards the selective formation of N₂. The use of an inexpensive preparation method to obtain nano-electrocatalysts that can perform selective electrocatalytic reactions such as ammonia oxidation and nitrite reduction, can pave the way for a new generation of practical catalysts for environmental and energy purposes.     

Alternative platinum electrocatalyst supporter with micro/nanostructured polyaniline for direct methanol fuel cell applications

In this study, a series of micro/nanostructured polyanilines were synthesized and their morphology-dependent electrochemical properties for acting as a catalyst supporter for direct methanol fuel cell (DMFC) applications were investigated. These micro/nanostructures include submicron spheres, hollow microspheres, nanotubes, and nanofibers. Among the four micro/nanostructures, polyaniline nanofibers (PANF) manifest their superiority in high electrochemical active surface. Accordingly, PANF is adopted as the catalyst supporter thereafter. To couple with the use of the alternative catalyst supporter, this study also investigates the effect of reductant type on morphology and electrocatalytic properties of the PANF-supported Pt particles through a chemical reduction reaction. TEM images indicate that formic acid as a reductant results in well-dispersed Pt particles on the PANF surface. On the other hand, aggregations of Pt are observable when NaBH₄ is selected as a reductant. Moreover, the methanol oxidation current density measured with the Pt/PANF electrode being prepared by using formic acid is double that by using NaBH₄. Compared with Pt/XC-72, the Pt/PANF electrode possesses higher electrocatalytic activity and exhibits double power density. Moreover, Pt/PANF is superior to Pt/XC-72 in the aspect of operation stability based on a continuous discharge for 5 h.     

Pt-WO3 supported on carbon nanotubes as possible anodes for direct methanol fuel cells

The carbon nanotube (CNT) synthesised by the template carbonisation of polypyrrole on alumina membrane has been used as the support for Pt-WO₃, Pt-Ru, and Pt. These materials have been used as the electrodes for methanol oxidation in acid medium in comparison with E-TEK 20 wt% Pt and Pt-Ru on Vulcan XC72R carbon. The higher electrochemical surface of the carbon nanotube (as evaluated by cyclic voltammetry) has been effectively used to disperse the catalytic particles. The morphology of the supported and unsupported CNT has been characterised by scanning electron micrograph and high-resolution transmission electron micrograph. The particle size of Pt, Pt-Ru, and Pt-WO₃ loaded CNT was found to be 1.2, 2, and 5 nm, respectively. The X-ray photoelectron spectra indicated that Pt and Ru are in the metallic state and W is in the +VI oxidation state. The electrochemical activity of the methanol oxidation electrode has been evaluated using cyclic voltammetry. The activity and stability (evaluated from chronoamperometric response) of the electrodes for methanol oxidation follows the order: GC/CNT-Pt-WO₃-Nafion>GC/E-TEK 20% Pt-Ru/Vulcan Carbon-Nafion>GC/CNT-Pt-Nafion>GC/E-TEK 20% Pt/Vulcan carbon-Nafion>Bulk Pt. The amount of nitrogen in the CNT plays an important role as observed by the increase in activity and stability of methanol oxidation with N₂ content, probably due to the hydrophilic nature of the CNT.     

High Methanol Oxidation Activity of Well-Dispersed Pt Nanoparticles on Carbon Nanotubes Using Nitrogen Doping

Pt nanoparticles (NPs) with the average size of 3.14 nm well dispersed on N-doped carbon nanotubes (CNTs) without any pretreatment have been demonstrated. Structural properties show the characteristic N bonding within CNTs, which provide the good support for uniform distribution of Pt NPs. In electrochemical characteristics, N-doped CNTs covered with Pt NPs show superior current density due to the fact that the so-called N incorporation could give rise to the formation of preferential sites within CNTs accompanied by the low interfacial energy for immobilizing Pt NPs. Therefore, the substantially enhanced methanol oxidation activity performed by N-incorporation technique is highly promising in energy-generation applications.     

Novel Nanoscale Ceria–Platinum Composite Electrodes for Direct Alcohol Electro-Oxidation

A nanocrystalline Pt/CeO₂ composite electrode is fabricated to study the electrochemical oxidation of methanol and ethanol. Three different methods have been developed for the preparation of the composite electrode and are compared in terms of their oxidation efficiencies. In general, all the electrode preparation methodologies show a great enhancement of the oxidation current for the alcohol. However, methods based on the electrodeposition of a ceria rich Pt matrix show the greatest effectiveness of alcohol oxidation. The enhancement of the anodic current is dependent on the particle loading and size distribution in the composite matrix. In general, ceria appears to be an effective alternative to ruthenium in direct alcohol fuel cells.     

Electrocatalytic activity of Pt nanoparticles deposited on porous TiO2 supports toward methanol oxidation

Porous TiO₂ thin films were prepared on the Si substrate by hydrothermal method, and used as the Pt electrocatalyst support for methanol oxidation study. Well-dispersed Pt nanoparticles with a particle size of 5–7 nm were pulse-electrodeposited on the porous TiO₂ support, which was mainly composed of the anatase phase after an annealing at 600 °C in vacuum. Cyclic voltammetry (CV) and CO stripping measurements showed that the Pt/TiO₂ electrode had a high electrocatalytic activity toward methanol oxidation and an excellent CO tolerance. The excellent electrocatalytic performance of the electrode is ascribed to the synergistic effect of Pt nanoparticles and the porous TiO₂ support on CO oxidation. The strong electronic interaction between Pt and the TiO₂ support may modify CO chemisorption properties on Pt nanoparticles, thereby facilitating CO oxidation on Pt nanoparticles via the bifunctional mechanism and thus improving the electrocatalytic activity of the Pt catalyst toward methanol oxidation.     

Synthesis of Ultrafine Size Platinum Nanoparticles on Defective Graphene with Enhanced Performance Towards Methanol Electro‐Oxidation

Graphene nanosheets (GS) were formed by the thermal‐expansion method. Large micropores about 1–2 nm were produced, which might provide abundant anchor sites for fixing catalyst. Platinum nanoparticles (NPs) supported on exfoliated GS (Pt/GS) were synthesized through an improved impregnation approach and mixture gas (5% H₂ in N₂) reduction. SEM and TEM images indicated the simple and clean method can effectively synthesize Pt with uniform dispersion and small size (below 3 nm) on the 2D specific and stratiform GS. The different amounts of Pt loaded on carbon carriers have been investigated respectively to evaluate the preferable electrocatalyst. Experimental results showed that Pt/GS of 20 wt.% initiated CO oxidation at the lowest onset potential in comparison with the commercial Pt/C (JM), indicating a higher CO tolerance of Pt/GS catalysts. In addition, Pt/GS of 20 wt.% exhibited enhanced electrocatalytic activity and high durability towards methanol oxidation. The high performance is exclusively attributed to synergistic effects of exfoliated GS and ultrafine size Pt NPs. Combining a melt‐diffusion strategy with the effective reduction of Pt precursors by the hydrogen gas, this present method is easy to scale up and possesses a significant potential for synthesizing anode electro‐catalyst of direct methanol fuel cells.     

铂微粒修饰聚苯胺膜电极对甲酸电催化氧化的研究

采用循环伏安法研究Pt盘电极 (Pt)、铂微粒修饰Pt盘电极 [Pt(Pt) ]和Pt微粒修饰聚苯胺膜电极 [PAN(Pt) ]对甲酸电催化氧化行为的影响 ,比较了它们对甲酸电催化氧化的活性 ,发现PAN(Pt)电极对甲酸电催化氧化的表观电流密度为 3 79× 10 2 mA·cm-2 ,分别比Pt、Pt(Pt)和Pt-PDMA/Pt电极约高 2 35、2 5和 6 3倍。峰电位比Pt PDMA/Pt电极约低 0 16V。     

Potential and current oscillations during formaldehyde oxidation on platinum particles dispersed in three-dimensional pore networks of TiOx/Ti

Electrochemical oscillations during the anodic oxidation of formaldehyde (HCHO) were studied on a modified electrode of platinum particles highly dispersed in the three-dimensional pore networks of TiO_x/Ti (Pt-TiO_x/Ti). Under conditions of room temperature and stationary electrode, not only current oscillations under both cyclic voltammetric and potentiostatic conditions but also potential oscillations under galvanostatic conditions were obtained. The intensity of current oscillations strongly depends on the concentration of HCHO or H₂SO₄, upper potential limit (upl) of cyclic voltammetry, applied constant potential and duration of time (t_d) at constant potential. Potential oscillations exhibit various patterns such as periodic, quasi-periodic, mixed-mode oscillations and other different bifurcations, which are greatly effected by the applied constant current and the concentration of HCHO or H₂SO₄. Meanwhile, the oscillatory system has a bistable characteristic with stable states at both low and high potentials. The observed potential and current oscillations are caused by the cyclic formation/removal of intermediate poison CO from the electrode surface during HCHO oxidation. The highly dispersed Pt particles on the surface of Pt-TiO_x/Ti electrode improve the electrocatalytic activity of the electrode, which greatly facilitates the formation of CO by the oxidation of HCHO and the removal of CO by its reaction with hydroxyl radicals (OH). Furthermore, the three-dimensional pore networks of the electrode's TiO_x/Ti support are favorable to the adsorption/desorption of reactants or intermediate product and thus increase the rate of reactions giving rise to electrochemical oscillations.     

Graphite Electrodes Modified with Platinum‐Nickel Nano‐Particles for Methanol Oxidation

Methanol electro‐oxidation is investigated at graphite electrodes modified with various platinum and nickel nano‐particle deposits using cyclic voltammetry. The modified electrodes are prepared by the simultaneous electrodeposition of metals from their salt solutions using potentiostatic and galvanostatic techniques. They show enhanced catalytic activity towards methanol oxidation in KOH solution. The catalytic activity of platinum nano‐particles is found to be significantly affected by the presence of relatively small amounts of nickel deposits. A comparison is made between the electrocatalytic activity of Pt/C and (Pt‐Ni)/C electrodes. The results show that the methanol electro‐oxidation current increases with an increase in the nickel content. In particular, the highest catalytic activity is achieved for platinum to nickel deposits of 95%:5% (wt.‐%), in other cases the catalytic activity decreases. It is found that Ni enhances the catalytic activity of Pt by increasing the number of active sites, as well as through an electron donation process from Ni to Pt. This process takes place once the nickel hydroxide (Ni(OH)₂)/nickel oxy‐hydroxide (NiOOH) transformation begins. The effect of the methanol concentration on the methanol oxidation reaction is investigated. The order of reaction, with respect to methanol, at the modified (Pt‐Ni)/C electrode is found to be 0.5.     

Composite electrodes consisting of platinum particles and polyaniline nanowires as electrocatalysts for methanol oxidation

Polyaniline (PANI) with nanowire (PANI‐(NW)) network structure (mean diameter 10–20 nm) was successfully deposited on a stainless steel (SS) electrode by a galvanostatic process. Platinum particles were deposited into the PANI nanowire network structure to result the PANI(NW)‐Pt composite electrode. The PANI(NW)‐Pt electrode was used as electrocatalysts for the electrochemical oxidation of methanol. The PANI nanowires and PANI(NW)‐Pt nanocomposite were characterized by scanning electron microscopy (SEM), X‐ray photoelectron spectroscopy (XPS), and UV–vis absorption spectroscopy. Nanowire morphology with an average diameter of 10–20 nm could be seen from scanning electron micrograph. Small amount (70 mμm) of spherical Pt particles could be deposited into the PANI(NW). Catalytic activity for the oxidation of methanol was studied by using cyclic voltammetry (CV). For comparative purposes, bulk Pt (deposited Pt on SS) and PANI nanowires based electrodes were tested. The PANI(NW)‐Pt nanocomposite electrode exhibited excellent catalytic activity for the electrooxidation of methanol in comparison to bulk Pt electrodes, which reveals that the PANI(NW)‐Pt nanocomposite electrodeis more promising for application in electrocatalyst as a support material. POLYM. COMPOS., 28:650–656, 2007. © 2007 Society of Plastics Engineers     

Structural and electrochemical features of 3D nanoporous platinum electrodes

The morphologies, roughness factors, and thicknesses of 3D nanoporous Pt (3D-npPt) films were investigated in terms of electroplating conditions. The electrochemical behaviors of 3D-npPt films with regard to electrochemical glucose oxidation, O₂ reduction, and H₂O₂ reduction were investigated as a function of roughness factors (Rf). Close comparison of glucose oxidation on 3D-npPt and 1D nanoporous Pt (1D-npPt) showed that the overall electrode activity of 3D-npPt is significantly higher than that of 1D-npPt. Electrochemical impedance analysis based on transmission line theory confirmed a substantially low pore resistance of 3D-npPt, which may account for the superior electrode response of this material.