Comparison of the electrocatalytic performance of PtRu nanoparticles supported on multi-walled carbon nanotubes with different lengths and diameters

To investigate the electrocatalytic performance of PtRu nanoparticles supported on multi-walled carbon nanotubes (MWCNTs) with different lengths and diameters, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and cyclic voltammetry experiments were conducted. It is demonstrated that the length and diameter of MWCNTs play an important role in the electrocatalytic performance of PtRu catalysts. The co-existence of amorphous carbon impurities on the MWCNT10-2 support lowered the accessible surface area of the PtRu nanoparticles, hampered the dispersion of the PtRu nanoparticles, and induced the formation of a low degree of PtRu alloy, thus lowered the electrocatalytic performance of the PtRu/MWCNT10-2 catalyst for methanol oxidation. The highest mass-specific activity of PtRu/MWCNT3050-2 results from a highly accessible PtRu surface and a good dispersion of PtRu particles. Our experimental results also demonstrate that the tube length of MWCNT samples has little effect of the surface area specific activity of the PtRu/MWCNT catalyst, whereas the PtRu nanoparticles supported on the MWCNT samples with large tube diameter tends to exhibit a higher surface area specific activity for methanol oxidation reaction. This result is suggested to be the combined effects of a high degree of PtRu alloying and the high electronic conductivity of these MWCNT samples.     

A simple method for the synthesis of PtRu nanoparticles on the multi-walled carbon nanotube for the anode of a DMFC

We report the synthesis of PtRu nanoparticles on the multi-walled carbon nanotubes (MWCNTs) by a simple sodium borohydride reduction method. Transmission electron microscopy (TEM) analysis indicated that well-dispersed small (2-3 nm) PtRu particles were formed on the MWCNTs. X-ray diffraction (XRD) analysis confirmed the formation of the PtRu alloy on the MWCNTs. X-ray photoelectron spectroscopy (XPS) measurements revealed that 70.4% Pt and 61.0% Ru are present in their metallic states. Cyclic voltammetry (CV) and chronoamperometry results demonstrated that the PtRu/MWCNT synthesized by this method exhibited a higher methanol oxidation current than did the PtRu/MWCNT synthesized by the more complex method using sodium borohydride as the reducing agent and tetraoctyl ammonium bromide as the stabilizer. Finally, the direct methanol fuel cell (DMFC) performance test showed that the PtRu/MWCNT nanocatalyst used at the anode of the fuel cell yielded higher performance than did the commercial E-TEK PtRu/C catalyst.     

Particle size effects of PtRu nanoparticles embedded in TiO2 on methanol electrooxidation

Size-controlled PtRu nanoparticles embedded in TiO₂ were prepared by simultaneous multi-gun sputtering from pure targets of Pt, Ru, and TiO₂. The mean diameter of the PtRu nanoparticles, as confirmed by their high-resolution transmission electron microscopic images, can be varied from ∼1.8 to ∼3.7 nm by changing the RF power ratio of PtRu and TiO₂. The transmission electron diffraction and grazing incidence wide angle X-ray scattering patterns of the PtRu nanoparticles embedded in TiO₂ confirmed that the PtRu exists as a mixed alloy structure consisting of both fcc and hcp phases, whereas the TiO₂ matrix is present as an amorphous phase. The size-controlled PtRu/TiO₂ electrodes were found to exhibit unique electronic properties depending on their size, which affected the potential of zero total charge and methanol oxidation reaction. The mass activity of PtRu/TiO₂ for methanol oxidation was determined by the interplay of the surface electronic factors at the metal-solution-interface and the value of the oxophilicity of the nanoparticles was increased by decreasing the particle size.     

Electrocatalytic oxidation of methanol at 2-aminophenoxazin-3-one-functionalized multiwalled carbon nanotubes supported PtRu nanoparticles

A new process to prepare well-dispersed PtRu nanoparticles on 2-aminophenoxazin-3-one (APZ)-functionalized multiwalled carbon nanotubes (PtRu/APZ-MWCNTs) was reported. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectrometry (FT-IR), UV-vis absorption spectra as well as Raman spectra were used for the catalyst characterization. The Raman spectroscopic results reveal that APZ-functionalized MWCNTs has good integrity and electronic structure than MWCNTs treated with chemical acid. The composite catalyst shows excellent electrocatalytic activity toward methanol oxidation and appears as a promising candidate for use in direct methanol fuel cells. Cyclic voltammetry and chronoamperometry studies indicate that the PtRu/APZ-MWCNTs catalysts exhibited higher electrochemical active surface area, electrocatalytic activity and stability for the electro-oxidation of methanol as compared to that of PtRu electrocatalysts supported on conventional acid-treated MWCNTs and carbon black.     

Highly active PtRu catalysts supported on carbon nanotubes prepared by modified impregnation method for methanol electro-oxidation

A simple and rapid synthesis method (denoted as modified impregnation method, MI) for PtRu/CNTs (MI) and PtRu/C (MI) was presented. PtRu/CNTs (MI) and PtRu/C (MI) catalysts were characterized by transmission electron microscopy (TEM) and X-ray diffractometry. It was shown that Pt-Ru particles with small average size (2.7 nm) were uniformly dispersed on carbon supports (carbon nanotubes and carbon black) and displayed the characteristic diffraction peaks of Pt face-centered cubic structure. Cyclic voltammetry and chronoamperometry showed that the Pt-Ru/CNTs (MI) catalyst exhibited better methanol oxidation activities than Pt-Ru/C (MI) catalyst and commercial Pt-Ru/C (E-TEK) catalyst. The single cells with Pt-Ru/CNTs (MI) catalyst exhibited a power density of 61 mW/cm², about 27% higher than those single cells with commercial Pt-Ru/C (E-TEK) catalyst.     

Electrochemical characterization of the electrooxidation of methanol,ethanol and formic acid on Pt/C and PtRu/C electrodes

Methanol, ethanol and formic acid electrooxidations in acid medium on Pt/C and PtRu/C catalysts were investigated. The catalysts were prepared by a microwave-assisted polyol process. Cyclic voltammetry and chronoamperometry were employed to provide quantitative and qualitative information on the kinetics of methanol, ethanol and formic acid oxidations. The PtRu/C catalyst showed higher anodic current densities than the Pt/C catalyst and the addition of Ru reduced the poisoning effect.     

Oxygen functionalization of multiwall carbon nanotubes by Ar/H2O plasma treatment

To increase the applicability of multiwall carbon nanotubes (MWCNTs), oxygen-containing functional groups were introduced on the surfaces of MWCNTs by using microwave-excited Ar/H₂O surface-wave plasma. X-ray photoelectron spectroscopy and Raman spectroscopy were used to determine dependencies of Ar/H₂O gas partial pressure, treatment time and microwave power. The oxygen functionalization of MWCNTs by plasma can be achieved very rapidly, about 10 min. The C-O and O-C═O fractions firstly increase and then decrease with increasing Ar partial pressure. The C-O and O-C═O fractions increase with increasing microwave power from 400 W to 700 W. A slight increase of the R (I_D/I_G ratio) value for the treated MWCNTs indicated disordering in the surface microstructure of MWCNTs coincident with the introduction of surface oxygen. The oxygen-containing groups introduced on the surfaces of MWCNTs by plasma treatment are hydrophilic. The dispersion of plasma treated MWCNTs is therefore improved.     

Characterization and electrocatalytic properties of PtRu/C catalysts prepared by impregnation-reduction method using Nd2O3 as dispersing reagent

A simple impregnation-reduction method introducing Nd₂O₃ as dispersing reagent has been used to synthesize PtRu/C catalysts with uniform Pt-Ru spherical nanoparticles. X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis have been used to characterize the composition, particle size and crystallinity of the catalysts. Well-dispersed catalysts with average particle size about 2 nm are achieved. The electrochemically active surface area of the different PtRu/C catalysts is determined by the CO_ad-stripping voltammetry experiment. The electrocatalytic activities of these catalysts towards methanol electrooxidation are investigated by cyclic voltammetry measurements and ac impedance spectroscopy. The in-house prepared PtRu/C catalyst (PtRu/C-03) in 0.5 M H₂SO₄ + 1.0 M CH₃OH at 30 °C display a higher catalytic activity and lower charge-transfer resistance (R_t) than that of the standard PtRu/C catalyst (PtRu/C-C). It is mainly due to enhanced electrochemically active specific surface, higher alloying extent of Ru and the abundant Pt⁰ and Ru oxides on the surface of the PtRu/C catalyst.     

Magnetron sputtering of platinum nanoparticles onto vertically aligned carbon nanofibers for electrocatalytic oxidation of methanol

The electrochemical activities of Pt-sputtered electrodes based on vertically aligned carbon nanofibers (Pt/VACNFs) directly grown on the carbon paper are investigated. Different Pt loading (0.01 mg cm⁻², 0.025 mg cm⁻² and 0.05 mg cm⁻²) electrodes are developed. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results show that the Pt nanoparticles are homogeneously dispersed on the surface of vertically aligned carbon nanofibers. TEM and X-ray diffraction (XRD) results reveal the Pt nanoparticles diameter increase with increasing Pt loading. The Pt/VACNFs electrodes show good electrochemical active surface area, methanol oxidation peak current density and CO tolerance. The electrochemical catalyst activities weaken as the diameter grows larger. Compared to common electrodes prepared by commercial catalyst in a conventional ink-process, the performance improvement suggests that unique structure of Pt/VACNFs electrode ensures the electronic pathway and Pt nanoparticles exposed to three-phase boundary, which leads to a significant improvement of the Pt utilization and a potential application in direct alcohol fuel cells.     

The use of the heteropoly acids, H3PMo12O40 and H3PW12O40, for the enhanced electrochemical oxidation of methanol for direct methanol fuel cells

Polarization and electrochemical impedance spectroscopy experiments were performed on a direct methanol fuel cell (DMFC) incorporating the heteropoly acids (HPAs) phosphomolybdic acid, H₃PMo₁₂O₄₀, (HPMo) or phosphotungstic acid, H₃PW₁₂O₄₀, (HPW) in the anode Pt/C catalyst layer. Both HPW-Pt and HPMo-Pt showed higher performance than the Pt control at 30 psig of backpressure and at ambient pressure. Anodic polarizations were also performed, and Tafel slopes were extracted from the data between 0.25 V and 0.5 V. At 30 psig, Tafel slopes of 133 mV/dec, 146 mV/dec, and 161 mV/dec were found for HPW-Pt, HPMo-Pt and the Pt control, respectively. At 0 psig, the Tafel slopes were 172 mV/dec, 178 mV/dec, and 188 mV/dec for HPW-Pt, HPMo-Pt and the Pt control. An equivalent circuit model, which incorporated constant phase elements (CPEs), was used to model the impedance data. From the impedance model it was found that the incorporation of HPAs into the catalyst layer resulted in a reduction in the resistances to charge transfer. This shows that these two heteropoly acids do act as co-catalysts with platinum for methanol electrooxidation.     

Effect of NaBH4 concentration on the characteristics of PtRu/C catalyst for the anode of DMFC prepared by the impregnation method

PtRu/C catalysts were prepared using an aqueous co-impregnation method with NaBH₄ as a reducing agent. In order to investigate the effect of the reducing agent concentration, metal ions were reduced in different NaBH₄ concentrations for which the molar ratios of NaBH₄ to metal ions were controlled to 1, 2, 5, 15, 50, and 250. The electrochemical properties were studied by cyclic voltammetry in a 0.5 M H₂SO₄ solution. The surface compositions and oxidation states of the catalysts were observed by X-ray photoelectron spectroscopy (XPS). According to the X-ray diffraction (XRD) results, Pt (fcc) peak shifts were observed and crystal sizes were calculated. The electro-catalytic activities of the prepared catalysts for methanol electro-oxidation were estimated using linear sweep voltammetry. Unit cell tests were carried out to compare the direct methanol fuel cell performances. The NaBH₄ concentration was found to affect the dispersion and the surface composition of the prepared PtRu particles. Optimum molar ratios of NaBH₄ to metal ions were 5 and 15 for methanol electro-oxidation.     

Preparation and characterization of a PtRu/C nanocatalyst for direct methanol fuel cells

PtRu/C nanocatalysts were prepared by changing the molar ratio of citric acid to platinum and ruthenium metal salts (CA:PtRu) from 1:1, 2:1, 3:1 to 4:1 using sodium borohydride as a reducing agent. Transmission electron microscopy analysis indicated that well-dispersed smaller PtRu particles (2.6 nm) were obtained when the molar ratio was maintained at 1:1. X-ray diffraction analysis confirmed the formation of PtRu alloy; the atomic percentage of the alloy analyzed by the energy dispersive X-ray spectrum indicated an enrichment of Pt in the nanocatalyst. X-ray photoelectron spectroscopy measurements revealed that 83.34% of Pt and 79.54% of Ru were present in their metallic states. Both the linear sweep voltammetry and chronoamperometric results demonstrated that the 1:1 molar ratio catalyst exhibited a higher methanol oxidation current and a lower poisoning rate among all the other molar ratios catalysts. The CO stripping voltammetry studies showed that the E-TEK catalyst had a relatively higher CO oxidation current than did the 1:1 molar ratio catalyst. Testing of the PtRu/C catalysts at the anode of a direct methanol fuel cell (DMFC) indicated that the in-house PtRu/C nanocatalyst gave a slightly higher performance than did the E-TEK catalyst.     

An improved DNA biosensor built by layer-by-layer covalent attachment of multi-walled carbon nanotubes and gold nanoparticles

In this work, we synthesized a type of compound, MWCNTs-CONH-(CH₂)₂-SH, via carboxylation, and investigated a thickness-tunable multilayer films DNA biosensor built by layer-by-layer (LBL) covalent attachment of gold nanoparticles (GNPs) and multi-walled carbon nanotubes (MWCNTs) on an Au electrode. Fourier transform infrared (FT-IR) spectra were used to identify the products formed in the step of carboxylation; scanning electron microscopy (SEM) and cyclic voltammetry (CV) were used to study the film assembly process. The hybridization events were monitored through measurement the signal of intercalated doxorubicin by differential pulse voltammetry (DPV), and the oxidation peak currents show a good linear relationship with the logarithm of the concentration of target DNA from 5.0 × 10⁻¹⁰ to 1.0 × 10⁻¹¹ M with a detection limit of 6.2 pM. The improved DNA biosensor has a good stability and reproducibility.     

Nano-composite of PtRu alloy electrocatalyst and electronically conducting polymer for use as the anode in a direct methanol fuel cell

Nano-composites comprised of PtRu alloy nanoparticles and an electronically conducting polymer for the anode electrode in direct methanol fuel cell (DMFC) were prepared. Two conducting polymers of poly(N-vinyl carbazole) and poly(9-(4-vinyl-phenyl)carbazole) were used for the nano-composite electrodes. Structural analyses were carried out using Fourier transform nuclear magnetic resonance spectroscopy, AC impedance spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM). Electrocatalytic activities were investigated by voltammetry and chronoamperometry in a 2 M CH₃OH/0.5 M H₂SO₄ solution and the data compared with a carbon-supported PtRu electrode. XRD patterns indicated good alloy formation and nano-composite formation was confirmed by TEM. Electrochemical measurements and DMFC unit-cell tests indicate that the nano-composites could be useful in a DMFC, but its performance would be slightly lower than that of a carbon-supported electrode. The interfacial property between the PtRu-polymer nano-composite anode and the polymer electrolyte was good, as evidenced by scanning electron microscopy. For better performance in a DMFC, a higher electric conductivity of the polymer and a lower catalyst loss are needed in nano-composite electrodes.     

Electrochemistry and electrocatalysis of hemoglobin on multi-walled carbon nanotubes modified carbon ionic liquid electrode with hydrophilic EMIMBF4 as modifier

The direct electrochemistry of hemoglobin (Hb) on multi-walled carbon nanotubes (MWCNTs) modified carbon ionic liquid electrode (CILE) was achieved in this paper. By using a hydrophilic ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF₄) as the modifier, a new CILE was fabricated and further modified with MWCNTs to get the MWCNTs/CILE. Hb molecules were immobilized on the surface of MWCNTs/CILE with polyvinyl alcohol (PVA) film by a step-by-step method and the modified electrode was denoted as PVA/Hb/MWCNTs/CILE. UV-vis and FT-IR spectra indicated that Hb remained its native structure in the composite film. Cyclic voltammogram of PVA/Hb/MWCNTs/CILE showed a pair of well-defined and quasi-reversible redox peaks with the formal potential (E^0′) of −0.370 V (vs. SCE) in 0.1 mol/L pH 7.0 phosphate buffer solution (PBS), which was the characteristic of the Hb heme Fe^III/Fe^II redox couples. The redox peak currents increased linearly with the scan rate, indicating the direct electron transfer was a surface-controlled process. The electrochemical parameters of Hb in the film were calculated with the results of the electron transfer coefficient (α) and the apparent heterogeneous electron transfer rate constant (k_s) as 0.49 and 1.054 s⁻¹, respectively. The immobilized Hb in the PVA/MWCNTs composite film modified CILE showed excellent electrocatalytic activity to the reduction of trichloroacetic acid (TCA) and hydrogen peroxide. So the proposed electrode showed the potential application in the third generation reagentless biosensor.     

Enhanced electrocatalytic oxidation of methanol on Pd/polypyrrole–graphene in alkaline medium

The new electrocatalyst of Pd nanoparticles supported on polypyrrole-functionalized graphene (Pd/PPy–graphene) was reported. Microstructure, morphology and crystallinity of the synthesized materials were investigated by Fourier transform infrared spectrometry (FT-IR), scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The elements of the composite were characterized using energy dispersive X-ray spectroscopy (EDS). The results showed that the polypyrrole-wrapped graphene support successfully discrete Pd nanoparticles with the crystallite size of about 6 nm. Catalyst activity for methanol electro-oxidation in fuel cells was investigated by cyclic voltammetry (CV) and chronoamperometry. The fundamental electrochemical test results indicated that the electrocatalytic activity of Pd/PPy–graphene is much better than that of commercial catalyst for methanol oxidation.     

Formic acid electro-oxidation on carbon supported PdxPt1−x (0 ≥ x ≥ 1) nanoparticles synthesized via modified polyol method

Carbon supported nanoparticle catalysts of Pd_xPt_1−x (0 ≥ x ≥ 1) were synthesized using a modified polyol method and poly(N-vinyl-2-pyrrolidone) (PVP) as a stabilizer. Resulting nanoparticles were characterized by X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV) and chronoamperommetry (CA) study for formic acid electro-oxidation. Surface composition of the synthesized nanoparticles found from XPS revealed the Pt surface segregation even for the Pd-rich compositions. It is suggested that the surface segregation behavior in PdPt nanoparticles supported on carbon may be influenced, in addition to the difference in Pd and Pt surface energies, by particle size and particle interaction with the support. According to CA, the carbon supported Pd nanoparticles show the highest initial activity towards formic acid electro-oxidation at the potential of 0.3 V (RHE), due to the promotion of the direct dehydrogenation mechanism. However its stability is quite poor resulting in the fast deactivation of the Pd surface. Addition of Pt considerably improves the steady-state activity of Pd in 12 h CA experiment. CA measurements show that the most active catalyst is Pd_0.5Pt_0.5 of 4 nm size, which displays narrow size distribution and Pd to Pt surface atomic ratio of 27-73.     

Electrodeposited porous Pb electrode with improved electrocatalytic performance for the electroreduction of CO2 to formic acid

A porous Pb foam was fabricated electrochemically at a copper substrate and then used as the cathode for the electroreduction of CO₂. The surface morphology and composition of the porous Pb electrode was characterized by scanning electron microscopy, X-ray diffraction, transmission electron microscopy and selected area electron diffraction. The honeycomb-like porous structure was composed of needle-like Pb deposits. Cyclic voltammetry studies demonstrated that the porous Pb electrode had better electrocatalytic performance for the formation of formic acid from CO₂ compared with a Pb plate electrode. The increase in current density was due to the large surface area of the porous Pb electrode, which provides more active sites on the electrode surface. The improved formic acid selectivity was due to the morphology of the roughened surface, which contains significantly more low-coordination sites which are more active for CO₂ reduction. The highest current efficiency for formic acid was 96.8% at -1.7 V versus saturated calomel electrode at 5 °C. This porous Pb electrode with good catalytic abilities represents a new 3D porous material with applications for the electroreduction of CO₂.     

Electrocatalytic activity of 2,3,5,6-tetrachloro-1,4-benzoquinone/multi-walled carbon nanotubes immobilized on edge plane pyrolytic graphite electrode for NADH oxidation

This work reports the electrocatalytic activity of 2,3,5,6-tetrachloro-1,4-benzoquinone (TCBQ)/multi-walled carbon nanotubes (MWCNT) immobilized on an edge plane pyrolytic graphite electrode for nicotinamide adenine dinucleotide (NADH) oxidation. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) were used to confirms the presence of chloro after the nanotube modification with 2,3,5,6-tetrachloro-1,4-benzoquinone. The surface charge transfer constant, k_s, and the charge transfer coefficient for the modified electrode, α, were estimated as 98.5 (±0.6) s⁻¹ and 0.5, respectively. With this modified electrode the oxidation potential of the NADH was shifted about 300 mV toward a less positive value, presenting a peak current much higher than those measured on an unmodified edge plane pyrolytic graphite electrode (EPPG). Cyclic voltammetry and rotating disk electrode (RDE) experiments indicated that the NADH oxidation reaction involves 2 electrons and a heterogenous rate constant (k_obs) of 3.1 × 10⁵ mol⁻¹ l s⁻¹. The detection limit, repeatability, long-term stability, time of response and linear response range were also investigated.