The effects of ionomer content on PEM water electrolyser membrane electrode assembly performance

In this study, the effects of Nafion^® ionomer content in membrane electrode assemblies (MEAs) of polymer electrolyte membrane (PEM) water electrolyser were discussed. The MEAs were prepared with a catalyst coated membrane (CCM) method. The catalysts inks with Nafion ionomer could form uniform coatings deposited on the membrane surfaces. SEM and area EDX mapping demonstrated that anode catalyst coating was uniformly distributed, with a microporous structure. The contents of Nafion ionomer were optimized to 25% for the anode and 20% for cathode. A current density of 1 A cm⁻² was achieved at terminal voltage 1.586 V at 80 °C in a PEMWE single cell, with Nafion 117, Pt/C as cathode, and Ru_0.7Ir_0.3O₂ as anode.     

Fabrication of a bimetallic Cu/Pt particle-modified carbon nanotube paste electrode and its use for the electrocatalytic oxidation of methanol

In this work, carbon nanotube paste electrode (CNTPE) was used as a substrate for deposition of bimetallic Cu/Pt particles. At first, a Cu film was prepared by electrochemical reduction of Cu ions onto the CNTPE in 0.1 M H₂SO₄ solution. Cu/Pt catalysts were prepared by partial galvanic replacement of Cu with Pt by simply immersion of the Cu-coated CNTPE in 2.0 mM H₂PtCl₆ solution. The nature and surface morphology of the bare CNTPE and fabricated Cu/Pt species were characterized by scanning electron microscopy and energy dispersive X-ray spectrometry. The Cu/Pt-modified CNTPE exhibits remarkable electrocatalytic activity towards methanol oxidation. It has been shown that carbon nanotubes improve the electrocatalytic activity of the catalysts towards oxidation. Then, the influence of various parameters such as Cu source concentration, electrodeposition time, replacement time, and methanol concentration on its oxidation as well as long-term stability of the modified electrode have been investigated by electrochemical methods.     

Investigation of a carbon-supported quaternary PtRuSnW catalyst for direct methanol fuel cells

An investigation was carried out in the electro-oxidation of methanol on a carbon-supported quaternary PtRuSnW catalyst prepared by a liquid-phase reduction method. As derived by X-ray diffraction and X-ray photoelectron spectroscopy, the catalyst was composed of metallic Pt, microcrystalline RuO₂ and SnO₂ phases and amorphous WO₃/WO₂ species. The electrochemical analysis was carried out in half-cell containing sulfuric acid electrolyte as well as in a liquid methanol-fed solid polymer electrolyte single-cell. The activity of catalyst in the half-cell varied as a function of the methanol concentration, it increased with CH₃OH molarity in the activation-controlled region and showed a maximum in 2 M CH₃OH at high currents. IR-free polarization curves showed that the activity of the quaternary catalyst was superior to Pt metal/C samples having the same Pt amount. The presence of semi-insulating metal oxides such as RuO₂, SnO₂ and WO₃ on the electrode surface exhibited a significant uncompensated resistance. The single-cell performance was lower than that predicted by the half-cell experiments mainly due to the methanol cross-over through the Nafion membrane.     

Surface chemistry and physical properties of Nafion/polypyrrole/Pt composite membrane prepared by chemical in situ polymerization for DMFC

We modified Nafion by means of chemical in situ polymerization of pyrrole monomers with platinum (Pt) precursors for an application into an electrolyte of direct methanol fuel cells (DMFCs). SEM and EPMA exhibited the presences of polypyrrole and Pt at the surface region of Nafion, after diffusing and polymerizing pyrrole monomers with Pt precursors. XPS and FT-IR spectra were used to characterize the surface of Naf–Ppy–Pt composite membranes, demonstrating that pyrrolinum groups of polypyrrole were interacted with sulfonic groups or Pt precursors (PtCl₆⁻ or PtCl₄⁻). After in situ polymerization of pyrrole monomers, the morphological reorganization of sulfonic groups in Naf–Ppy–Pt composite membranes occurred via electrostatic interaction. Thermal stability, proton conductivity, methanol permeability, and cell performance of composite membranes were analyzed by TGA, AC impedance, refractometer, and potentiostat. Naf–Ppy–Pt composite membranes had higher thermal stabilities of sulfonic groups and side chains than Nafion and Naf–Ppy as a result of the interaction between Nafion–SO₃⁻?polypyrrole–NH₂⁺ and the presence of thermally stable Pt. The cell performance of Naf–Ppy–Pt 0 0 2 was enhanced significantly compared to that of Nafion under the specific condition, due to more reduction of methanol crossover than that of proton conductivity. Therefore, this synthetic method offers a facile way to improve physical properties of polymer electrolyte for the fabrication of advanced composite membranes.     

Investigation of tungsten carbide supported Pd or Pt as anode catalysts for PEM fuel cells

In this contribution, we present results of electrochemical characterization of prepared tungsten carbide supported palladium and platinum and Vulcan XC-72 supported palladium. These catalysts were employed as anode catalysts in PEMFC and results are compared to commercial platinum catalyst. Platinum seems to be irreplaceable as a proton exchange membrane fuel cell (PEMFC) catalyst for both the anode and the cathode, yet the high price and limited natural resources are holding back the commercialization of the PEMFCs. Tungsten carbide is recognized as promising catalyst support having the best conductivity among interstitial carbides. Higher natural resources and significantly lower price make palladium good candidate for replacement of the platinum catalyst. The presented results show that all prepared catalysts are very active for the hydrogen oxidation reaction. Linear sweep voltammetry curves of Pd/C and Pd/WC show existence of peaks at 0.07 V vs. RHE, which is assigned to absorbed hydrogen. H₂|Pd/WC|Nafion117|Pt/C|O₂ fuel cell has almost the same efficiency and similar power output as commercial platinum catalyst.     

The electrochemical oxidation of methanol on a Pt/TNTs/Ti electrode enhanced by illumination

A Pt/TNTs/Ti electrode is prepared by electrochemically depositing Pt using the modulated pulse current method onto high density, well ordered and uniformly distributed TiO₂ nanotubes (TNTs) on a Ti substrate. The results show that the performance and anti-poison ability of the Pt/TNTs/Ti electrode for methanol electro-oxidation under illumination is remarkably enhanced and is even better than the best bi-metallic Pt-Ru catalysts. CO poisoning is no longer a problem during methanol electro-oxidation with the Pt/TNTs/Ti electrode under illumination.     

Thin film Pt/TiO2 catalysts for the polymer electrolyte fuel cell

Thin film Pt/TiO₂ catalysts are evaluated in a polymer electrolyte electrochemical cell. Individual thin films of Pt and TiO₂, and bilayers of them, were deposited directly on Nafion membranes by thermal evaporation with varying deposition order and thickness (Pt loadings of 3–6 μg cm⁻²). Structural and chemical characterization was performed by transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Oxygen reduction reaction (ORR) polarization plots show that the presence of a thin TiO₂ layer between the platinum and the Nafion increases the performance compared to a Pt film deposited directly on Nafion. Based on the TEM analysis, we attribute this improvement to a better dispersion of Pt on TiO₂ compared to on Nafion and in addition, substantial proton conduction through the thin TiO₂ layer. It is also shown that deposition order and the film thickness affects the performance.     

Displacement triangular Ag/Pd nanoplate as methanol- tolerant electrocatalyst in oxygen reduction reaction

Porous triangular Ag/Pd nanoplates with different alloy ratios, including Ag₁₈Pd₁, Ag₁₈Pd_1.5, and Ag₁₈Pd₂, were successfully prepared by a galvanic displacement reaction. These alloy nanoplates were then used as methanol-tolerant electrocatalysts in an alkaline oxygen reduction reaction (ORR). Electrochemical measurements were conducted using an ultrathin film rotating ring-disk electrode. The mass activity was found to decrease in the order Ag₁₈Pd₁ > Ag₁₈Pd₂ > Ag₁₈Pd_1.5 > Pt nanoparticles > Pd nanoparticles, similar to an observation made in a past analysis of the nanoplates in an electrolyte of free methanol; this indicates that these nanoplate catalysts are more economical than Pd nanoparticles, and even Pt nanoparticles. Additionally, compared to the reactive direction in the case of Pt and Pd nanoparticles toward methanol oxidation in an ORR electrolyte with methanol, all Ag/Pd nanoplate catalysts experienced cathodic currents, which indicate that ORRs occurred even in the presence of methanol. Despite working in a methanol-tolerant solution, the prepared alloy nanoplates still exhibited high electroactivity.     

Polyol synthesis of reduced graphene oxide supported platinum electrocatalysts for fuel cells: Effect of Pt precursor,support oxidation level and pH

In this work, a comprehensive study on the polyol synthesis of platinum supported on reduced graphene oxide (Pt/rGO) catalysts, including both ex-situ and in-situ characterizations of the prepared Pt/rGO catalysts, was performed. The polyol synthesis was studied considering the influence of the platinum precursor, oxidation level of graphite oxide and pH of reaction medium. The as-prepared catalysts were analyzed using thermo-gravimetric (TG) analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and cyclic voltammetry (CV). The best results in terms of platinum particle size and distribution were obtained when the synthesis was performed in acidic medium, using chloroplatinic acid as precursor and using graphene oxide with high oxidation level. The most promising graphene-supported catalyst was used to prepare a polymer electrolyte membrane fuel cell electrode. The membrane electrode assembly (MEA) prepared with graphene-based electrode was compared with a MEA prepared with catalyst based on commercial platinum supported in carbon black (Pt/C). Single cell characterization included polarization curves and in-situ electrochemical impedance spectroscopy (EIS). The graphene-based electrode presented promising albeit unstable electrochemical performance due to water management issues. Additionally, EIS measurements revealed that the MEA made with Pt/rGO catalyst presented a lower mass transport resistance than the commercial Pt/C.     

Pt modified TiO2 nanotubes electrode: Preparation and electrocatalytic application for methanol oxidation

Pt nanoparticles decorated TiO₂ nanotubes (Pt/TiO₂NTs) modified electrode has been successfully synthesized by depositing Pt in TiO₂NTs, which were prepared by anodization of the Ti foil. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and electrochemical methods were adopted to characterize their structures and properties. The Pt/TiO₂NTs electrode shows excellent electrocatalytic activity toward methanol oxidation reaction (MOR) in alkaline electrolyte without UV irradiation.     

Study of the influence of Nafion/C composition on electrochemical performance of PEM single cells with ultra-low platinum load

The electrochemical performance of membrane electrode assemblies (MEAs) with ultra-low platinum load (0.02 mg_Pt cm^?2) and different compositions of Nafion/C in the catalytic layer have been investigated. The electrodes were fabricated depositing the catalytic ink, prepared with commercial catalyst (HiSPEC 2000), onto the gas diffusion layers by wet powder spraying. The MEAs were electrochemically tested using current-voltage curves and electrochemical impedance spectroscopy measurements. The experiments were carried out at 70 °C in H₂/O₂ and H₂/air as reactant gases at 1 and 2 bar pressure and 100% of relative humidity. For all MEAs tested, power density increases when the gasses pressure is increased from 1 to 2 bar. On the other hand, power density also increased when oxygen is used instead of air as oxidant gas in cathode. The lower power density (34 mW cm^?2) and power per Pt loading (0.86 kW g_Pt^?1) corresponds to the MEA prepared without Nafion in anode and cathode catalytic layers working with hydrogen and air at 1 bar pressure as reactants gas. The MEA with 30% wt Nafion/C reached the highest power density (422 mW cm^?2) and power per Pt loading (10.60 kW g_Pt^?1) using hydrogen and oxygen at 2 bar pressure. Finally, electrode surface microstructure and cross sections of MEAs were analyzed by Scanning Electron Microscopy (SEM). Examination of the electrodes, revealed that the most uniform ionomer network surface corresponds to the electrode with 40 wt% Nafion/C, and MEA ionomer-free catalytic layer shows delamination, it leads to low electrochemical performance.     

Catalytic activity of platinum/tungsten oxide nanorod electrodes towards electro-oxidation of methanol

Tungsten oxide (WO₃) nanorods are synthesized using an Anodisc alumina membrane as a template and platinum nanoparticles are supported on the nanorods. The nanorods, serving as platinum catalyst supports, are characterized by electron microscopy and by electrochemical analysis. Methanol oxidation on the prepared electrodes is studied by means of cyclic voltammetry and chronopotentiometry. A film of Pt/WO₃ nanorods on a glassy carbon electrode exhibits good electrocatalytic activity towards the oxidation of methanol. High electrocatalytic activities and good stabilities are attributed to a synergistic effect between Pt and WO₃ that avoids poisoning of the electrodes.     

Carbon xerogels as Pt catalyst supports for polymer electrolyte membrane fuel-cell applications

Carbon xerogels prepared by the resorcinol-formaldehyde (RF) sol-gel method with ambient-pressure drying were explored as Pt catalyst supports for polymer electrolyte membrane (PEM) fuel cells. Carbon xerogel samples without Pt catalyst (CX) were characterized by the N₂ sorption method (BET, BJH, others), and carbon xerogel samples with supported Pt catalyst (Pt/CX) were characterized by thermogravimetry (TGA), powder X-ray diffraction (XRD), electron microscopy (SEM, TEM) and ex situ cyclic voltammetry for thin-film electrode samples supported on glassy carbon and studied in a sulfuric acid electrolyte. Experiments on Pt/CX were made in comparison with commercially obtained samples of Pt catalyst supported on a Vulcan XC-72R carbon black support (Pt/XC-72R). CX samples had high BET surface area with a relatively narrow pore size distribution with a peak pore size near 14 nm. Pt contents for both Pt/CX and Pt/XC-72R were near 20 wt % as determined by TGA. Pt catalyst particles on Pt/CX had a mean diameter near 3.3 nm, slightly larger than for Pt/XC-72R which was near 2.8 nm. Electrochemically active surface areas (ESA) for Pt as determined by ex situ CV measurements of H adsorption/desorption were similar for Pt/XC-72R and Pt/CX but those from CO stripping were slightly higher for Pt/XC-72R than for Pt/CX. Membrane-electrode assemblies (MEAs) were fabricated from both Pt/CX and Pt/XC-72R on Nafion 117 membranes using the decal transfer method, and MEA characteristics and single-cell performance were evaluated via in situ cyclic voltammetry, polarization curve, and current-interrupt and high-frequency impedance methods. In situ CV yielded ESA values for Pt/XC-72R MEAs that were similar to those obtained by ex situ CV in sulfuric acid, but those for Pt/CX MEAs were smaller (by 13-17%), suggesting that access of Nafion electrolyte to Pt particles in Pt/CX electrodes is diminished relative to that for Pt/XC-72R electrodes. Polarization curve analysis at low current density (0.9 V cell voltage) reveals slightly higher intrinsic catalyst activity for the Pt/CX catalyst which may reflect the fact that Pt particle size in these catalysts is slightly higher. Cell performance at higher current densities is slightly lower for Pt/CX than the Pt/XC-72R sample, however after normalization for Pt loading, performance is slightly higher for Pt/CX, particularly in H₂/O₂ and at lower cell temperatures (50 °C). This latter finding may reflect a possible lower mass-transfer resistance in the Pt/CX sample.     

Platinum decorated Ru/C: Effects of decorated platinum on catalyst structure and performance for the methanol oxidation reaction

Platinum decorated Ru/C catalysts are prepared by successive reduction of a platinum precursor on pre-formed Ru/C. Pt:Ru atomic ratios are varied from 0.13:1 to 0.81:1 to investigate the platinum decoration effects on the catalyst's structure and electrochemical performance towards the methanol oxidation reaction (MOR) at room temperature. The catalysts are extensively characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Ru@Pt/C catalysts show enhanced mass-normalized activity and specific activity for the MOR relative to Pt/C. For the anodic oxidation of methanol, the ratio of forward to reverse oxidation peak current R (I_f/I_b) varies considerably: R decreases from 5.8 to 0.8 when the Pt:Ru ratio increases from 0.13:1 to 0.81:1. When the ratio of Pt:Ru is 0.42:1, R reaches 0.99 (close to that of Pt/C), and further increase of the Pt:Ru ratio leads to almost no decrease in R. Coincidentally, maximum mass-normalized activity is also obtained when Pt:Ru is 0.42:1.     

New membranes for direct methanol fuel cells

The performance of direct methanol fuel cells (DMFC) is limited by the cross-over of methanol through the electrolyte. Electrolyte membranes prepared by blending of sulfonated arylene main-chain polymers like sulfonated PEEK Victrex (sPEEK) or sulfonated PSU Udel (sPSU) with basic polymers like poly(4-vinylpyridine) (P4VP) or polybenzimidazole (PBI) show excellent chemical and thermal stability, good proton-conductivity, and good performance in H₂ PEM fuel cells. Furthermore, these materials have potentially lower methanol cross-over when compared to standard Nafion-type membranes.In this work, membrane electrode assemblies (MEAs) have been prepared from such membranes according to the thin-film method. The catalyst layer was spray-coated directly on the heated membrane using an ink consisting of an aqueous suspension of catalyst powder and Nafion solution. Unsupported catalysts were used for anode and cathode. A rather high catalyst loading was chosen in order to minimize the effects of limited catalyst utilization due to flooding conditions at both electrodes.     

High stability of oxidation of methanol catalyzed by Pt supported by oxygen-incorporated bamboo-shaped CNTs grown directly on carbon cloth

Bamboo-shaped CNTs in which oxygen was incorporated were directly grown on carbon cloth (O-BCNT-CC) by microwave plasma-enhanced chemical vapor deposition. Mixed precursors CH₄/H₂/N₂/O₂ were introduced during the growth process. For comparison, bamboo-shaped CNTs without incorporated oxygen were prepared herein (BCNT-CC). Then, platinum catalysts were prepared on the as-grown O-BCNT-CC (Pt/O-BCNT-CC) and the as-grown BCNT-CC (Pt/BCNT-CC). According to TEM-EELS oxygen mapping, O atoms were uniformly distributed on the O-BCNT surface. Methanol oxidation was performed using Pt/O-BCNT-CC and Pt/BCNT-CC in 1 M methanol and 1 M sulfuric acid by cyclic voltammetry. In the initial cycle, the peak current density of Pt/O-BCNT-CC was almost equal to that of Pt/BCNT, indicating that both had nearly equal activities in the beginning. After 300 cycles, the peak current of Pt/BCNT-CC was reduced to half of the initial peak current owing to platinum-poisoning; however, the peak current of Pt/O-BCNT-CC decayed less. In Pt/O-BCNT-CC, the oxygen-containing functional groups affect the orientation index of crystalline Pt, providing a means of oxidizing methanol and stabilizing Pt catalysts for the long-term.     

Noble metal nanowires incorporated Nafion membranes for reduction of methanol crossover in direct methanol fuel cells

We electrodeposited noble metal (palladium, platinum) nanowires into the hydrophilic pores of Nafion membrane for mitigating the problem of methanol crossover in direct methanol fuel cells (DMFCs). The DMFC performance result shows that the composite membranes yield lower rate of methanol crossover and better cell performance than the pure Nafion^® membrane. At low current densities, the Pd nanowire incorporated Nafion membrane shows the best performance. In comparison, the highest performance is achieved at higher current densities with the Pt nanowire modified Nafion membrane. Based on the above findings, we suggest that for the Pd nanowire incorporated Nafion membrane, the mechanism for the suppression of the methanol crossover is mainly the blocking effect due to the ‘narrowed’ hydrophilic channels in Nafion membrane. For the Pt nanowire modified Nafion membrane, the mechanism includes both increasing the membrane tortuosity and so-called ‘on-way consumption’ of methanol on the Pt nanowires deposited into the Nafion membrane when the fuel cell is discharging.     

Performance of carbon-supported PtPd as catalyst for hydrogen oxidation in the anodes of proton exchange membrane fuel cells

In this work, the replacement of platinum by palladium in carbon-supported catalysts as anodes for hydrogen oxidation reaction (HOR), in proton exchange membrane fuel cells (PEMFCs), has been studied. Anodes with carbon-supported Pt, Pd, and equiatomic Pt:Pd, with various Nafion^® contents, were prepared and tested in H₂|O₂ (air) PEMFCs fed with pure or CO-contaminated hydrogen. An electrochemical study of the prepared anodes has been carried out in situ, in membrane electrode assemblies, by cyclic voltammetry and CO electrooxidation voltammetry. The analyses of the corresponding voltammograms indicate that the anode composition influences the cell performance. Single cell experiments have shown that platinum could be replaced, at least partially, saving cost with still good performance, by palladium in the hydrogen diffusion anodes of PEMFCs. The performance of the PtPd catalyst fed with CO-contaminated H₂ used in this work is comparable to Pt, thus justifying further work varying the CO concentration in the H₂ fuel to assert its CO tolerance and to study the effect of the Pt:Pd atomic ratio.     

Influence of metal oxides on Pt catalysts for methanol electrooxidation using electrochemical impedance spectroscopy

Carbon nanotubes used as supports for platinum catalysts deposited with metal oxides (CeO₂, TiO₂, and SnO₂) were prepared for their application as anode catalysts in a direct methanol fuel cell. Cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy measurements were carried out in a solution of 0.5 M CH₃OH and 0.5 M H₂SO₄. Catalysts with the addition of CeO₂, TiO₂, and SnO₂ presented higher catalytic activity than pure platinum catalysts, and the catalysts with CeO₂ were the best among them. Electrochemical impedance spectra indicated that methanol electrooxidation on these catalysts had different impedance behaviors at different potential regions. The mechanism of methanol electrooxidation changed with increases of the potential. The promotion effect of the metal oxides lies in the oxidation of intermediate CO_ads on Pt at low potential regions.