Iron(III) tetramethoxyphenylporphyrin (FeTMPP-C1) as electrocatalyst for oxygen reduction in direct methanol fuel cells

Iron(III) tetramethoxyphenylporphyrins (FeTMPP-C1) adsorbed on high-area carbons were heat treated in an inert atmosphere at various temperatures ranging from 200–1000 °C to produce catalyst for the electroreduction of oxygen in acid electrolytes. It was found that the specific surface area of the FeTMPP-C1/C catalysts linearly decreases with increase of the FeTMPP-C1 loading on RB carbon. The electrical resistivity of the catalyst decreases with the increase of heat treatment temperature in the range of 200 to 800 °C, then it increases beyond 800 °C. The results obtained with elemental analysis, FTIR spectroscopy and XPS techniques indicate that the onset temperature for partial decomposition for the FeTMPP chelate occurs at temperatures of about 400–500 °C. The surface concentrations of both iron and nitrogen on the carbon support increase as the heat treatment temperature increases, and the maximum occurs at 700 °C. Some possibilities about the nature of active sites in the catalyst are discussed.     

Gas evolution and power performance in direct methanol fuel cells

The use of acrylic cells and a CCTV camera for visually investigating the carbon dioxide gas evolution process inside an operating direct methanol fuel cell environment is demonstrated. Also, the effect of operating parameters on the system gas management, using a series of tests with different gas diffusion layer supporting materials, flow bed designs, cell sizes and exhaust manifold configurations, is studied. Carbon dioxide gas management is an important issue obstructing progress in viable direct methanol fuel cell systems development. Gas evolution mechanisms and gas management techniques are discussed and analysed with reference to several video picture and performance data. The data demonstrate that Toray carbon paper is not a suitable material for DMFCs due to its poor gas removal properties. A type carbon cloth shows relatively good gas removal behaviour. Increasing the liquid phase inlet flow rate is beneficial for gas removal. Increasing the current density results in higher gas production and in the formation of gas slugs, especially at low flow rates, which can lead to blocking of the channels and hence deterioration in the cell performance. A new flow bed design, based on a heat exchanger concept, is affective for gas management and gives a more uniform flow distribution in the flow bed channels. Using the results of this study, and the modelling techniques developed by our group, will are able to determine suitable operating conditions for our prototype 0.5kW cell DMFC stack.     

Influence of the PTFE content in the diffusion layer of low-Pt loading electrodes for polymer electrolyte fuel cells

The influence of the structure and composition of the diffusion layer on polymer electrolyte fuel cell (PEFC) cathode performance was investigated. Electrodes were prepared with different poly-tetrafluoroethylene (PTFE) content in the diffusion layer and maintaining a constant composition for the catalytic layer with a low-Pt loading (0.11 mg cm⁻²). Electrodes were characterized by Hg-intrusion porosimetry, scanning electron microscopy and electrochemical techniques (cyclic voltammetry, galvanostatic polarization and ac-impedance spectroscopy).     

CoPdx oxygen reduction electrocatalysts for polymer electrolyte membrane and direct methanol fuel cells

The electrochemical activity of carbon-supported cobalt-palladium alloy electrocatalysts of various compositions have been investigated for the oxygen reduction reaction in a 5 cm² single cell polymer electrolyte membrane fuel cell. The polarization experiments have been conducted at various temperatures between 30 and 60 °C and the reduction performance compared with data from a commercial Pt catalyst under identical conditions. Investigation of the catalytic activity of the CoPd_x PEMFC system with varying composition reveals that a nominal cobalt-palladium atomic ratio of 1:3, CoPd₃, exhibits the best performance of all studied catalysts, exhibiting a catalytic activity comparable to the commercial Pt catalyst. The ORR on CoPd₃ has a low activation energy, 52 kJ/mol, and a Tafel slope of approximately 60 mV/decade, indicating that the rate-determining step is a chemical step following the first electron transfer step and may involve the breaking of the oxygen bond. The CoPd₃ catalyst also exhibits excellent chemical stability, with the open circuit cell voltage decreasing by only 3% and the observed current decreasing by only 10% at 0.8 V over 25 h. The CoPd₃ catalyst also exhibits superior tolerance to methanol crossover poisoning than Pt.     

Influence of the structure in low-Pt loading electrodes for polymer electrolyte fuel cells

The influence of the structure and composition of the electrodes on a polymer electrolyte fuel cell (PEFC) performance has been investigated. Electrodes have been prepared by varying the composition of diffusion and/or catalyst layer. Improvements have been obtained by introducing a hydrophobic carbon layer between the carbon paper and the catalyst layer for the gas diffusion backing. High performance has been achieved with low Pt-loading electrodes (0.15 mg/cm²) by including the ionomer Nafion in the catalyst ink. Electrodes have been characterized by SEM-EDX analyses and electrochemical tests in a 50 cm² single cell.     

Effects of the cathode gas diffusion layer characteristics on the performance of polymer electrolyte fuel cells

Polymer electrolyte fuel cell (PEFC) electrodes were prepared by applying different porous gas diffusion half-layers (GDHLs) onto each face of a carbon cloth support, followed by the deposition of a catalyst layer onto one of these half-layers. The performance of PEFCs in H₂/air operation using cathodes with GDHLs presenting different characteristics were compared. The best result was obtained using cathodes with GDHLs having polytetrafluorethylene (PTFE) contents of 30 wt % in the gas side and 15 wt % in the catalyst side. This behaviour was explained in terms of a better water management within the cell.     

Carbon supported Pt–Cr alloys as oxygen-reduction catalysts for direct methanol fuel cells

Electrocatalytic activities of various carbon-supported platinum–chromium alloy electrocatalysts towards oxygen reduction in 1 mol l⁻¹ H₂SO₄ and in 1 mol l⁻¹ H₂SO₄/1–3 mol l⁻¹ CH₃OH, were investigated by means of rotating disc electrode experiments and in solid polymer electrolyte direct methanol fuel cells. The activity of these electrocatalysts for methanol oxidation was evaluated using cyclic voltammetry. It was found that Pt₉Cr/C prepared by reduction with NaBH₄ exhibits the lowest activity for methanol oxidation and the highest activity for oxygen reduction in the presence of methanol, in comparison to commercial Pt/C, Pt₃Cr/C and PtCr/C electrocatalysts.     

Ruthenium selenide catalysts for cathodic oxygen reduction in direct methanol fuel cells

The oxygen reduction reaction in sulphuric acid on commercial carbon supported platinum and ruthenium catalysts as well as on a home-made carbon supported ruthenium selenide catalysts (RuSe _x /C) was investigated. The RuSe _x /C catalysts were synthesised using similar procedures to those found in the literature. A dependency of H₂O₂ formation on the selenium content was found using the thin-film rotating ring disc electrode technique, namely that the H₂O₂ formation in the typical operation range of a Direct Methanol Fuel Cell (0.7–0.4 V) on Pt/C is below 1% and 1–4% on Ru/C and RuSe _x /C catalysts. Finally for comparing the intrinsic activities of the catalysts the electrochemically active surface areas were determined in-situ by means of copper underpotential deposition. Our results indicate a comparable activity of the present RuSe _x /C catalyst to commercial Pt/C if the activities are related to the electrochemical active areas.     

Investigation of carbon-supported Pt and PtCo catalysts for oxygen reduction in direct methanol fuel cells

Carbon-supported Pt and Pt₃Co catalysts with a mean crystallite size of 2.5 nm were prepared by a colloidal procedure followed by a carbothermal reduction. The catalysts with same particle size were investigated for the oxygen reduction in a direct methanol fuel cell (DMFC) to ascertain the effect of composition. The electrochemical investigations were carried out in a temperature range from 40 to 80 °C and the methanol concentration feed was varied in the range 1-10 mol dm⁻³ to evaluate the cathode performance in the presence of different conditions of methanol crossover. Despite the good performance of the Pt₃Co catalyst for the oxygen reduction, it appeared less performing than the Pt catalyst of the same particle size for the cathodic process in the presence of significant methanol crossover. Cyclic voltammetry analysis indicated that the Pt₃Co catalyst has a lower overpotential for methanol oxidation than the Pt catalyst, and thus a lower methanol tolerance. Electrochemical impedance spectroscopy (EIS) analysis showed that the charge transfer resistance for the oxygen reduction reaction dominated the overall DMFC response in the presence of high methanol concentrations fed to the anode. This effect was more significant for the Pt₃Co/KB catalyst, confirming the lower methanol tolerance of this catalyst compared to Pt/KB. Such properties were interpreted as the result of the enhanced metallic character of Pt in the Pt₃Co catalyst due to an intra-alloy electron transfer from Co to Pt, and to the adsorption of oxygen species on the more electropositive element (Co) that promotes methanol oxidation according to the bifunctional theory.     

Synthesis and characterization of methanol tolerant Pt/TiOx/C nanocomposites for oxygen reduction in direct methanol fuel Cells

Pt/TiO_x/C nanocomposites have been synthesized by depositing hydrated titanium oxide on carbon-supported Pt (Pt/C), reducing H₂PtCl₆ with sodium formate on carbon-supported hydrated titanium oxide (TiO₂/C), and simultaneously depositing hydrated titanium oxide and reducing H₂PtCl₆ with formate on carbon support, followed by heat treatment at 500 and 900 °C in 90% Ar-10% H₂ mixture. The catalytic activity for oxygen reduction was evaluated in half cells with sulfuric acid electrolyte and in single direct methanol fuel cells (DMFC). Tolerance to methanol was studied with half cells containing sulfuric acid mixed with methanol. Charge transfer resistance and electrochemical active surface area of the Pt/TiO_x/C catalysts were studied with impedance and cyclic voltammetry measurements. Both the synthesis methods and heat treatments influence the catalytic activity, and some of the Pt/TiO_x/C composites exhibit higher catalytic activity than Pt/C. The Pt/TiO_x/C catalysts also exhibit better methanol tolerance than Pt/C. The mechanism for the enhanced catalytic activity of Pt/TiO_x/C is discussed.     

Methanol-tolerant electrocatalysts for oxygen reduction in a polymer electrolyte membrane fuel cell

Heat-treated -oxo-iron(iii) tetramethoxy phenyl porphyrin (Fe-TMPP)2O and iron(iii) tetramethoxy phenyl porphyrin (FeTMPP-Cl) as well as iron(iii) octaethyl porphyrin (FeOEP-Cl) adsorbed on high-area carbons such as deashed and un-deashed RB carbon (Calgon) and Black Pearls-2000 (Cabot) have been found to exhibit stable and very high oxygen reduction rates. Experiments done over a period of 24h showed no performance degradation. Measured performances were very similar to supported platinum (E-Tek), when tested in 85% H3PO4-equilibrated Nafion 117 membrane at 125°C and hydrated-Nafion membrane at 60°C in a minifuel cell. The macrocycle cathodes are insensitive to the presence of methanol whereas the platinum cathodes are very sensitive and show degradation in the oxygen reduction performance.     

金属卟啉负载炭黑电催化剂氧还原性能

合成了四甲氧基苯基钴卟啉（CoTMPP）和四甲氧基苯基铁卟啉（FeTMPP）配合物,分别负载于经过双氧水和硝酸预处理且掺杂了MnO_x的炭载体,用于质子交换膜燃料电池阴极氧还原反应电催化剂.讨论了不同中心金属离子、不同载体、不同预处理方法和不同焙烧温度对催化剂催化活性的影响.通过旋转圆盘电极技术（RDE）和紫外可见光谱（UV-vis）测试,利用循环伏安曲线（CV）和Koutevky-Levich关系式评价了电催化剂对氧还原反应的电催化性能.研究表明,CoTMPP负载于双氧水处理过的炭载体BP 2000上活性最好,焙烧的最佳温度是900℃,同时发现在载体中掺杂MnO_x并没有达到预期效果.     

Electrocatalysis of oxygen reduction on a carbon supported platinum-vanadium alloy in polymer electrolyte fuel cells

The electrocatalysis of the oxygen reduction reaction (ORR) on carbon supported Pt:V 1:1 catalyst in polymer electrolyte fuel cells (PEFC) was investigated. At an oxygen pressure of 1 atm results indicate a lower electrocatalytic activity for the ORR in the presence of vanadium. However, at an O₂ pressure ≥2 atm an enhanced electrocatalytic property of PtV/C compared with Pt/C is revealed. This result indicates the occurrence of a different electrocatalytic mechanism for the ORR on Pt/C and PtV/C. An increase of mass transport overpotentials is observed for the PtV/C catalyst, and this was related to the presence of vanadium oxide. Indeed, XRD analysis revealed that only about 30% of V present in the catalyst is alloyed with Pt, forming a face centred cubic (fcc) Pt₃V solid solution. A thermal treatment at 850 °C under reducing atmosphere leads to the formation of an ordered fcc Pt₂V phase. After this, the ORR activity of PtV/C at O₂ pressure 1 atm is higher than that of Pt/C.     

Electrodes modified by electrodeposition of CoTAA complexes as selective oxygen cathodes in a direct methanol fuel cell

The oxygen reduction reaction (ORR) at cobalt tetraazaanulene (CoTAA) modified electrodes was investigated. As a first approach, modified electrodes were prepared by electrodeposition of CoTAA on glassy carbon (GC). The modification of the GC surface was monitored by u.v.–vis. differential reflectance spectroscopy (UVDRS). The recorded spectra (i.e., absorbance as a function of wavelength and time) showed that the electrodeposition of CoTAA at 0.8 V vs Ag|AgCl, that is, at a potential where the TAA ligand is oxidized to TAA⁺, seems to produce a thin polymer film. Starting from these preliminary results, porous rotating disc electrodes (RDEs) were prepared by electrodeposition of CoTAA (0.8 V vs Ag|AgCl, 1 min) on graphite powder embedded in a recast Nafion^® film. The use of a porous RDE allowed comparison of the activity and selectivity of Pt nanoparticles and CoTAA for the ORR under experimental conditions close to those of a fuel cell cathode, that is, at the catalyst|Nafion^® interface. The activity towards the ORR of a porous electrode modified by electrodeposition of CoTAA is not affected when methanol is present in the electrolyte phase, whereas a noticeable decrease in the activity of Pt-based oxygen cathodes was observed under the same conditions. Half-cell life tests showed that CoTAA-modified electrodes and Pt-based electrodes have a comparable stability over a period of 90 min.     

Carbon nanocapsules as an electrocatalyst support for the oxygen reduction reaction in alkaline electrolyte

Carbon Nanocapsules (CNCs) were investigated for their electrocatalytic performances for the oxygen reduction reaction in alkaline electrolyte. With an average diameter of 10–30 nm, the CNCs are composed of graphene layers encapsulating a hollow core. A gas diffusion electrode (GDE) made of CNCs revealed a much enhanced i–V polarization response than that of Vulcan XC72. However, its performance was moderately lower than that of Black Pearls 2000. In addition, the CNCs were impregnated with nanoparticles of Ag, MnO _x and CoO _x . The i–V and galvanostatic results of the catalyzed CNCs indicated significant improvements over that of noncatalyzed CNCs. For example, a Ag–CNC derived GDE was capable of delivering 1.03 and 0.88 V at current densities of 100 and 200 mA cm⁻², respectively. Our study offers direct evidence that the CNCs not only exhibit unique electrocatalytic abilities but also function superbly as an electrocatalyst support.     

On a new degradation mode for high-temperature polymer electrolyte fuel cells: How bipolar plate degradation affects cell performance

The contribution of the bipolar plate material to the overall degradation of a high temperature membrane electrode assembly (HT MEA) for polymer electrolyte fuel cells (PEFCs) is studied in terms of performance decrease, phosphoric acid uptake in the bipolar plates and change of surface morphology of the bipolar plates. Two different high temperature graphite composites, a surface treated graphite and a gold coated stainless steel flowfield and the respective MEAs are compared after operation at 180 °C. Both graphite surface treatment and gold coating lead to negligible uptake of the electrolyte and ensure low degradation rates, whereas the composite plates exhibit high uptake of acid from the MEA into the surface near bulk. Apparent MEA degradation caused by acid redistribution from the MEA to the increasingly porous plates is observed in terms of increased ohmic cell resistances and reduction of catalyst utilization as consequence of acid loss from the catalyst layers.     

Review in Applied Electrochemistry. Number 54 Recent Developments in Polymer Electrolyte Fuel Cell Electrodes

Since the 1980s there has been a significant lowering of the platinum loading of polymer electrolyte fuel cell electrodes from about 4–10 mg cm^–2(platinum black) to about 0.4 mg cm^–2 or even less (carbon supported platinum), by the introduction of ionomer (liquid Nafion^®) impregnated gas diffusion electrodes, extending the three-dimensional reaction zone. From the 1990s to the present studies have been carried out to decrease the loss of performance during cell operation due both to the presence of liquid water causing flooding of the catalyst layer and mass transport limitations and to the poisoning of platinum by the use of reformed fuels. This review deals with the developments in electrode configuration going from dual layer to three layer electrodes. The preparation methods, the characteristics and the optimal composition of both diffusion and reactive layers of these electrodes are described. The improvement in the performance of both CO tolerant anodes and cathodes with enhanced oxygen reduction by Pt alloying is also discussed.     

Carbon incorporated FeN/C electrocatalyst for oxygen reduction enhancement in direct methanol fuel cells: X-ray absorption approach to local structures

The C-containing iron nitride electrocatalyst is fabricated by chelating N-containing species and Fe²⁺ with a carbon support under heat treatment in an NH₃ atmosphere, which induces the oxygen reduction reaction activity. This is the first demonstration of forming Fe_xC species on iron nitride materials. The correlation between the electrochemical properties and structures are aided to elucidate their features under investigation by using X-ray absorption spectroscopy. A rotating ring disk electrode test is conducted in sulfuric acid solution and the results reveal the low H₂O₂ yield and approximately 4e⁻ transfer process of the carbon-containing FeN/C electrocatalyst.     

Preparation of supported PtRu/C electrocatalyst for direct methanol fuel cells

In this work, high-surface supported PtRu/C were prepared with Ru(NO)(NO₃)₃ and [Pt(H₂NCH₂CH₂NH₂)₂]Cl₂ as the precursors and hydrogen as a reducing agent. XRD and TEM analyses showed that the PtRu/C catalysts with different loadings possessed small and homogeneous metal particles. Even at high metal loading (40 wt.% Pt, 20 wt.% Ru) the mean metal particle size is less than 4 nm. Meanwhile, the calculated Pt crystalline lattice parameter and Pt (2 2 0) peak position indicated that the geometric structure of Pt was modified by Ru atoms. Among the prepared catalysts, the lattice parameter of 40-20 wt.% PtRu/C contract most. Cyclic voltammetry (CV), chronoamperometry (CA), CO stripping and single direct methanol fuel cell tests jointly suggested that the 40-20 wt.% PtRu/C catalyst has the highest electrochemical activity for methanol oxidation.     

Sulfonated poly(ether imide) and poly(ether sulfone) blends for direct methanol fuel cells. I. Sulfonation of PEI and characterization of the products

This investigation examines characteristics of sulfonated polyether imides (SPEI) with various ion exchange capacity values (IEC) and completes previous work to enable its blends to be adopted as polyelectrolyte in direct methanol fuel cells (DMFC). Polyether imides (PEI) were sulfonated by using chlorosulfonic acid as the sulfonating agent and chloroform as the solvent. The structure of SPEI was observed by FTIR and ¹H NMR. The sulfonate or sulfonic acid content of the polymers, expressed as a number per repeat unit of the polymer, was accurately determined by elemental analysis and conductometric titration. Physical properties such as solubility, intrinsic viscosities, thermal stability, and glass transition temperature (T_g) were studied for both PEI and SPEI. TGA‐FTIR verified that sulfonic groups, attached to the aromatic ring in the PEI backbone, are split at 230–350°C, but the main‐chain splitting temperature of SPEI is similar to that of pure polymer. The sulfonated samples exhibited good solubilities and increased glass transition temperatures (T_g values) as degree of sulfonation (DS) increased; two T_g values were detected when IEC was sufficiently high. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008