Development of Pd and Pd-Co catalysts supported on multi-walled carbon nanotubes for formic acid oxidation

Pd-Co and Pd catalysts were prepared by the impregnation synthesis method at low temperature on multi-walled carbon nanotubes (MWCNTs). The nanotubes were synthesized by spray pyrolysis technique. Both catalysts were obtained with high homogeneous distribution and particle size around 4 nm. The morphology, composition and electrocatalytic properties were investigated by transmission electron microscopy, scanning electron microscopy-energy dispersive X-ray analysis, X-ray diffraction and electrochemical measurements, respectively. The electrocatalytic activity of Pd and PdCo/MWCNTs catalysts was investigated in terms of formic acid electrooxidation at low concentration in H₂SO₄ aqueous solution. The results obtained from voltamperometric studies showed that the current density achieved with the PdCo/MWCNTs catalyst is 3 times higher than that reached with the Pd/MWCNTs catalyst. The onset potential for formic acid electrooxidation on PdCo/MWCNTs electrocatalyst showed a negative shift ca. 50 mV compared with Pd/MWCNTs.     

Pd nanoparticles supported on HPMo-PDDA-MWCNT and their activity for formic acid oxidation reaction of fuel cells

A novel Pd electrocatalyst is developed by self-assembly of Pd nanopartilces on phosphomolybdic acid (HPMo)-poly(diallyldimethylammonium chloride) (PDDA)-functionalized multiwalled carbon nanotubes supports (Pd/HPMo-PDDA-MWCNTs). The as-synthesized Pd/HPMo-PDDA-MWCNTs were characterized by TEM, EDS mapping, Raman spectra, X-ray photoelectron spectroscopy, electrochmeical CO stripping and cyclic voltammetry techniques. Pd nnaoparticles deposited on HPMo-PDDA-MWCNTs are in the range of 3.1 nm with uniform distributon. Pd/HPMo-PDDA-MWCNT catalysts have lower overpotential for CO_ad oxidation manifested as lower peak and onset potentials as compared to acid-treated MWCNTs supported Pd (Pd/AO-MWCNTs) and carbon supported Pd catalysts (Pd/C). Pd/HPMo-PDDA-MWCNTs catalysts also exhibit a much higher electrocatalytic activity and stability for formic acid oxidation reaction as compared to that on Pd/AO-MWCNTs and Pd/C. The high electrocatalytic activities of Pd/HPMo-PDDA-MWCNTs catalysts are most likely related to highly dispersed and fine Pd nanoparticles as well as synergistic effects between Pd and HPMo immobilized on PDDA-functionalized MWCNTs.     

Pd nanoparticles deposited on vertically aligned carbon nanotubes grown on carbon paper for formic acid oxidation

Vertically aligned carbon nanotubes (VACNTs) grown on carbon paper were obtained by the spray pyrolysis method and highly dispersed Pd nanoparticles were deposited on VACNTs by the wet chemical method. For comparison, the entangled carbon nanotubes (ECNTs) and Vulcan XC-72 based electrodes were fabricated by brush painting the corresponding Pd/ECNTs and Pd/XC-72 catalysts on carbon paper, respectively. Compared with Pd deposition on the entangled carbon nanotubes (ECNTs) and Vulcan XC-72 electrodes, the VACNTs electrode exhibited higher activity for formic acid oxidation, which is mainly due to the three-dimensional structure and better conductive paths in the VACNTs electrode, as well as higher Pd utilization.     

Effects of Pt/C, Pd/C and PdPt/C anode catalysts on the performance and stability of air breathing direct formic acid fuel cells

Pt/C, Pd/C and PdPt/C catalysts are potential anodic candidates for electro-oxidation of formic acid. In this work we designed a miniature air breathing direct formic acid fuel cell, in which gold plated printed circuit boards are used as end plates and current collectors, and evaluated the effects of anode catalysts on open circuit voltage, power density and long-term discharging stability of the cell. It was found that the cell performance was strongly anode catalyst dependent. Pd/C demonstrated good catalytic activity but poor stability. A maximum power density of 25.1 mW cm⁻² was achieved when 5.0 M HCOOH was fed as electrolyte. Pt/C and PdPt/C showed poor activity but good stability, and the cell can discharge for about 10 h at 0.45 V (Pt/C anode) and 15 h at 0.3 V (PdPt/C) at 20 mA.     

Effects of surface chemical states of carbon nanotubes supported Pt nanoparticles on performance of proton exchange membrane fuel cells

This study synthesized platinum (Pt) nanoparticles supported on carbon nanotubes (CNTs) using a microwave-assisted polyol method. The oxidation treatment of CNTs introduced primarily -OH and -COOH groups to the CNTs, thereby enhancing the reduction of Pt ionic species, resulting in smaller Pt particles with improved dispersion and attachment properties. The Pt particles supported on oxidized CNTs displayed superior durability to those on pristine CNTs or commercially available Pt/C. These improvements are most likely associated with the percentage of metallic Pt in the particles. After 400 cycles, the losses of electrochemical surface area in Pt nanoparticle supported on oxidized CNTs and pristine CNTs catalysts were 66 and 84%, respectively, of that associated with commercial Pt/C. A single proton exchange membrane fuel cell using Pt supported on oxidized CNTs at the cathode with a total catalytic loading of 0.6 Pt mg cm⁻² exhibited the highest power density of 890 mW cm⁻² and displayed a lower mass transfer loss, compared to Pt/C.     

High efficient electrocatalytic oxidation of formic acid at Pt dispersed on porous poly(o-methoxyaniline)

A Pt-based composite electrode material has been developed by dispersing Pt nanoparticles on a porous poly(o-methoxyaniline) (POMA) film, which was produced via electropolymerization on a glassy carbon (GC) electrode. As-formed Pt/POMA/GC electrode was characterized by SEM, EDX and electrochemical analysis. Furthermore, the composite electrode material was evaluated by its electrocatalytic performance for formic acid oxidation using cyclic voltammetry and chronoamperometry methods. Compared to Pt deposited on bare GC (Pt/GC), Pt/POMA/GC exhibits higher catalytic activity and stronger poisoning-tolerance ability towards formic acid electro-oxidation. The improved performance is attributed to the synergetic effect between Pt and POMA. Also, as demonstrated by CO stripping voltammograms, the interference of CO on Pt/POMA/GC is greatly weakened. These results suggest that the POMA film has great potential to serve as a promising support material for the electrocatalytic oxidation of formic acid.     

Hydrogen co-reduction synthesis of PdPtNi alloy nanoparticles on carbon nanotubes as enhanced catalyst for formic acid electrooxidation

A novel electrocatalyst of PdPtNi ternary alloy nanoparticles supported on multi-walled carbon nanotubes (MWCNTs) for formic acid oxidation (FAO) reaction is synthesized by a simple hydrogen co-reduction process. The as-synthesized catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS). It is found that highly dispersed PdPtNi alloy nanoparticles of ca. 2.56 nm are homogeneously deposited on the MWCNT surface, and the alloying with Pt and Ni alters the electronic structure of Pd atoms with the downshift of Pd d-band center. Studies of cyclic voltammetry and chronoamperometry indicate that the electrocatalytic activity and durability of the PdPtNi/MWCNT for FAO are significantly enhanced as compared with the PdPt/MWCNT and commercial Pd/C catalysts. This study implies that the prepared PdPtNi/MWCNT composite is a promising anode electrocatalyst of direct formic acid fuel cells.     

Effects of temperature and humidity on the cell performance and resistance of a phosphoric acid doped polybenzimidazole fuel cell

Performance and electrochemical impedance spectroscopy (EIS) tests were performed at different temperatures and humidity levels to understand the effects of temperature and humidity on the performance and resistance of a PBI/H₃PO₄ fuel cell.The results of the performance tests indicated that increasing the temperature significantly improved the cell performance. In contrast, no improvement was observed when the gas humidity was increased. On the other hand, the EIS results showed that the membrane resistance was reduced for elevated temperatures. This development can be interpreted by the increase in membrane conductivity, as reflected by the Arrhenius equation. As the formation of H₄P₂O₇ and the self-dehydration of H₃PO₄ start around 130-140 °C, in PBI, they increase the membrane resistance at temperatures that are higher than 130 °C. In addition, the membrane resistance was reduced for elevated gas humidity levels. This is because an increase in humidity leads to an increase of the membrane hydration level.The resistance of the catalyst kinetics mainly contributes to the charge transfer resistance. However, under certain conditions, the interfacial charge transfer resistance is also important. It was concluded that the gas diffusion is the main contributor to the mass transfer resistance under dry conditions while it is the gas concentration under humid conditions.