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.     

Trimethoxymethane as an alternative fuel for a direct oxidation PBI polymer electrolyte fuel cell

The oxidation of trimethoxymethane (TMM) (trimethyl orthoformate) in a direct oxidation PBI fuel cell was examined by on-line mass spectroscopy and on-line FTIR spectroscopy. The results show that TMM was almost completely hydrolyzed in a direct oxidation fuel cell which employs an acid doped polymer electrolyte to form a mixture of methylformate, methanol and formic acid. It also found that TMM was hydrolyzed in the presence of water at 120°C even without acidic catalyst. The anode performance improves in the sequence of methanol, TMM, formic acid/methanol, and methylformate solutions. Since formic acid is electrochemically more active than methanol, these results suggest that formic acid is probably a key factor for the improvement of the anode performance by using TMM instead of methanol under these conditions.     

Highly methanol-tolerant non-precious metal cathode catalysts for direct methanol fuel cell

This work reports on the oxygen reduction activity of several non-precious metal (non-PGM) catalysts for oxygen reduction reaction (ORR) at the fuel cell cathode, including pyrolyzed CoTPP, FeTPP, H₂TMPP, and CoTMPP. Of the studied catalysts, pyrolyzed CoTMPP (Co-tetramethoxyphenylporphyrin) was found to perform significantly better than other materials. The catalyst underwent a thorough testing in both hydrogen-air polymer electrolyte fuel cell (PEFC) and direct methanol fuel cell (DMFC). It was found that CoTMPP cathode can sustain currents that are only 2-3 times lower than those obtained with a conventional Pt-black cathode in an H₂-air PEFC. DMFC experiments, including methanol crossover and methanol tolerance measurements, indicate high ORR selectivity of the CoTMPP catalyst. Based on results obtained to date, the CoTMPP-based catalyst offers promise for the use in conventional and mixed-reactant DMFCs operating with concentrated methanol feeds. However, hydrogen-air fuel cell life data, consisting of over 800 h of continuous cell operation, indicate that improvement to long-term stability of the CoTMPP catalyst will be required to make it practical.     

A direct methanol fuel cell using acid-doped polybenzimidazole as polymer electrolyte

A direct methanol/oxygen solid polymer electrolyte fuel cell was demonstrated. This fuel cell employed a 4 mg cm^–2 Pt-Ru alloy electrode as an anode, a 4 mg cm^–2 Pt black electrode as a cathode and an acid-doped polybenzimidazole membrane as the solid polymer electrolyte. The fuel cell is designed to operate at elevated temperature (200°C) to enhance the reaction kinetics and depress the electrode poisoning, and reduce the methanol crossover. This fuel cell demonstrated a maximum power density about 0.1 W cm^–2 in the current density range of 275–500 mA cm^–2 at 200°C with atmospheric pressure feed of methanol/water mixture and oxygen. Generally, increasing operating temperature and water/methanol mole ratio improves cell performance mainly due to the decrease of the methanol crossover. Using air instead of the pure oxygen results in approximately 120 mV voltage loss within the current density range of 200–400 mA cm^–2 .     

Performance of a direct methanol fuel cell

The performance of a direct methanol fuel cell based on a Nafion® solid polymer electrolyte membrane (SPE) is reported. The fuel cell utilizes a vaporized aqueous methanol fuel at a porous Pt–Ru–carbon catalyst anode. The effect of oxygen pressure, methanol/water vapour temperature and methanol concentration on the cell voltage and power output is described. A problem with the operation of the fuel cell with Nafion® proton conducting membranes is that of methanol crossover from the anode to the cathode through the polymer membrane. This causes a mixed potential at the cathode, can result in cathode flooding and represents a loss in fuel efficiency. To evaluate cell performance mathematical models are developed to predict the cell voltage, current density response of the fuel cell.     

A Solid-polymer Electrolyte Direct Methanol Fuel Cell with a Methanol-tolerant Cathode and its Mathematical Modelling

Solid-polymer electrolyte direct methanol fuel cells (SPE-DMFCs) employing carbon-supported Pt–Fe as oxygen-reduction catalyst to mitigate the effect of methanol on cathode performance while operating with oxygen or air have been assembled. These SPE-DMFCs provided maximum power densities of 250 and 120 mW cm^–2 at 85 °C on operating with oxygen and air, respectively. The polarization data for the SPE-DMFCs and their constituent electrodes have also been derived numerically employing a model based on phenomenological transport equations for the catalyst layer, diffusion layer and the membrane electrolyte.     

Novel approach to membraneless direct methanol fuel cells using advanced 3D anodes

A conventional membrane electrode assembly (MEA) for a direct methanol fuel cell (DMFC) consists of a polymer electrolyte membrane (PEM) compressed between an anode and cathode electrode. Limitations with this conventional design include: cost, fuel crossover, membrane degradation or contamination, ohmic losses and reduced active triple phase boundary (TPB) sites for catalyst located away from the electrode/membrane interface. In this work, ex situ and in situ characterization of a novel electrode assembly based on a membraneless architecture and advanced 3D anodes was investigated. The approach was shown to be fuel independent and scaleable to a conventional bi-polar fuel cell arrangement. The membraneless configuration exhibits comparable performance to a conventional ambient (25 °C, 1 atm) air-breathing DMFC. However, it has the additional advantages of a simplified design, the elimination of the membrane (a significant component expense) and enhanced fuel and catalyst utilization through the extension of the active catalyst zone.     

Enhanced high-temperature polymer electrolyte membrane for fuel cells based on polybenzimidazole and ionic liquids

Polybenzimidazole (PBI)/ionic liquid (IL) composite membranes were prepared from an organosoluble, fluorine-containing PBI with ionic liquid, 1-hexyl-3-methylimidazolium tri?uoromethanesulfonate (HMI-Tf). PBI/HMI-Tf composite membranes with different HMI-Tf concentrations have been prepared. The ionic conductivity of the PBI/HMI-Tf composite membranes increased with both the temperature and the HMI-Tf content. The composite membranes achieve high ionic conductivity (1.6 × 10⁻² S/cm) at 250 °C under anhydrous conditions. Although the addition of HMI-Tf resulted in a slight decrease in the methanol barrier ability and mechanical properties of the PBI membranes, the PBI/HMI-Tf composite membranes have demonstrated high thermal stability up to 300 °C, which is attractive for high-temperature (>200 °C) polymer electrolyte membrane fuel cells.     

Composite electrocatalysts for anodic methanol and methanol-reformate oxidation

Binary anode electrocatalyst formulations were prepared by adsorption of phthalocyanine and tetraphenylporphyrin complexes of different transition metals on a commercial carbon supported platinum catalyst. Only after pyrolyzing the complexes at 700 °C under nitrogen were catalysts of some activity obtained. A binary Pt/Ni electrocatalyst prepared by this procedure exhibits considerable anodic catalytic activity in the acidic environment of the Nafion^® electrolyte for reformate and direct methanol oxidation for more than 400 h without deterioration. Ternary electrocatalyst formulations Pt/Ru/W = 1/1/y were produced according to the Bönnemann method. The Pt/Ru/W catalyst of 1/1/1.5 (mol/mol/mol) composition is optimal. Compared to the Pt/Ru catalyst, it enhances the performance of reformate (H₂ + 150 ppm CO) fuel cells by 50% and of direct methanol fuel cells (steam/methanol vapour = 50:1 mol/mol) by 80%. Attached to a GC electrode by a thin Nafion^® film, the catalysts were also tested for methanol oxidation in aqueous methanol solutions in half cells by slow potential stepping. This procedure is useful for fast initial screening.     

Effect of operating conditions on energy efficiency for a small passive direct methanol fuel cell

Energy conversion efficiency was studied in a direct methanol fuel cell (DMFC) with an air-breathing cathode using Nafion 117 as electrolyte membrane. The effect of operating conditions, such as methanol concentration, discharge voltage and temperature, on Faradic and energy conversion efficiencies was analyzed under constant voltage discharge with quantitative amount of fuel. Both of Faradic and energy conversion efficiencies decrease significantly with increasing methanol concentration and environmental temperature. The Faradic conversion efficiency can be as high as 94.8%, and the energy conversion efficiency can be as high as 23.9% if the environmental temperature is low enough (10 °C) under constant voltage discharge at 0.6 V with 3 M methanol for a DMFC bi-cell. Although higher temperature and higher methanol concentration can achieve higher discharge power, it will result in considerable losses of Faradic and energy conversion efficiencies for using Nafion electrolyte membrane. Development of alternative highly conductive membranes with significantly lower methanol crossover is necessary to avoid loss of Faradic conversion efficiency with temperature and with fuel concentration.     

Optimization of operating parameters of a direct methanol fuel cell and physico-chemical investigation of catalyst–electrolyte interface

A physico-chemical investigation of catalyst–Nafion® electrolyte interface of a direct methanol fuel cell (DMFC), based on a Pt–Ru/C anode catalyst, was carried out by XRD, SEM-EDAX and TEM. No interaction between catalyst and electrolyte was detected and no significant interconnected network of Nafion micelles inside the composite catalyst layer was observed. The influence of some operating parameters on the performance of the DMFC was investigated. Optimal conditions were 2 M methanol, 5 atm cathode pressure and 2–3 atm anode pressure. Power densities of 110 and 160 mW cm⁻² were obtained for operation with air and oxygen, respectively, at temperatures of 95–100°C and with 1 mg cm⁻² Pt loading.     

Investigation on cathode degradation of direct methanol fuel cell

The cathode degradation of a direct methanol fuel cell (DMFC) was investigated after a 240 h discontinuous galvostatic operation at 80 °C. The catalyst coated membrane (CCM) and the cathode diffusion layer were not combined so as to isolate electrochemical and mass transport processes. It was indicated by the EDS and SEM tests that the loss of the cathode electrochemical surface area (ESA) was associated with the decays of the Pt/C catalyst and the interfacial contact. Furthermore, Ru crossover and higher methanol crossover resulting from the anode failure aggravated the degradation of the cathode. On the other hand, the change of the pore structure led to a higher wettability of the cathode microporous layer. Therefore, the oxygen transport was suppressed due to the decrease of hydrophobic passages.     

The influence of a new fabrication procedure on the catalytic activity of ruthenium-selenium catalysts

A new procedure has been introduced to enhance catalytic activity of ruthenium-selenium electro-catalysts for oxygen reduction, in which materials are treated under hydrogen atmosphere at elevated temperatures. The characterisation using scanning electron microscopy, energy dispersive spectroscopy or energy dispersive X-ray spectroscopy exhibited that the treatment at 400 °C made catalysts denser while their porous nature remained, led to a good degree of crystallinity and an optimum Se:Ru ratio. The half cell test confirms feasibility of the new procedure; the catalyst treated at 400 °C gave the highest reduction current (55.9 mA cm⁻² at −0.4 V) and a low methanol oxidation effect coefficient (3.8%). The direct methanol fuel cell with the RuSe 400 °C cathode catalyst (2 mg RuSe cm⁻²) generated a power density of 33.8 mW cm⁻² using 2 M methanol and 2 bar oxygen at 90 °C. The new procedure produced the catalysts with low decay rates. The best sample was compared to the Pt and to the reported ruthenium-selenium catalyst. Possible reasons for the observations are discussed.     

Carbon supported Pt–Co alloys as methanol-resistant oxygen-reduction electrocatalysts for direct methanol fuel cells

The electrocatalysis of the oxygen reduction reaction on carbon supported Pt and Pt–Co (Pt/C and Pt–Co/C) alloy electrocatalysts was investigated in sulphuric acid (both in the absence and in the presence of methanol) and in direct methanol fuel cells (DMFCs). In pure sulphuric acid Pt–Co/C alloys showed improved specific activity towards the oxygen reduction compared to pure platinum. In the methanol containing electrolyte a higher methanol tolerance of the binary electrocatalysts than Pt/C was observed. The onset potential for methanol oxidation at Pt–Co/C was shifted to more positive potentials. Accordingly, Pt–Co/C electrocatalyts showed an improved performance as cathode materials in DMFCs.     

Development and operation of a 150 W air-feed direct methanol fuel cell stack

A five-cell 150 W air-feed direct methanol fuel cell (DMFC) stack was demonstrated. The DMFC cells employed Nafion 117^® as a solid polymer electrolyte membrane and high surface area carbon supported Pt-Ru and Pt catalysts for methanol electrooxidation and oxygen reduction, respectively. Stainless steel-based stack housing and bipolar plates were utilized. Electrodes with a 225 cm² geometrical area were manufactured by a doctor-blade technique. An average power density of about 140 mW cm^–2 was obtained at 110 °C in the presence of 1 M methanol and 3 atm air feed. A small area graphite single cell (5 cm²) based on the same membrane electrode assembly (MEA) gave a power density of 180 mW cm^–2 under similar operating conditions. This difference is ascribed to the larger internal resistance of the stack and to non-homogeneous reactant distribution. A small loss of performance was observed at high current densities after one month of discontinuous stack operation.     

Reforming methanol to electricity in a high temperature PEM fuel cell

In this paper we demonstrate for the first time a compact power unit, where a methanol reforming catalyst is incorporated into the anode of a PEMFC. The proposed internal reforming methanol fuel cell (IRMFC) mainly comprises: (i) a H₃PO₄-imbibed polymer electrolyte based on aromatic polyethers bearing pyridine units, able to operate at 200 °C and (ii) a 200 °C active and with zero CO emissions Cu–Mn–O methanol reforming catalyst supported on copper foam. Methanol is being reformed inside the anode compartment of the fuel cell at 200 °C producing H₂, which is readily oxidized at the anode to produce electricity. The IRMFC showed promising electrochemical behavior and no signs of performance degradation for more than 72 h.     

Carbon nano-tube supported Pt–Pd as methanol-resistant oxygen reduction electrocatalyts for enhancing catalytic activity in DMFCs

This work tries to study the problem of methanol crossover through the polymer electrolyte in direct methanol fuel cells (DMFCs) by developing new cathode electrocatalysts. For this purpose, a series of gas diffusion electrodes (GDEs) were prepared by using single-walled carbon nanotubes (SWCNTs) supported Pt–Pd (Pt–Pd/SWCNT) with different Pd contents at the fixed metal loading of 50 wt%, as bimetallic electrocatalysts, in the catalyst layer. Pt–Pd/SWCNT was prepared by depositing the Pt and Pd nanoparticles on a SWCNTs support. The elemental compositions of bimetallic catalysts were characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES) system. The performances of the GDEs in the methanol oxidation reaction (MOR) and in the oxygen reduction reaction with/without the effect of methanol oxidation reaction were investigated by means of electrochemical techniques: cyclic voltammetry (CV), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS). The results indicated that GDEs with Pt–Pd/SWCNT possess excellent electrocatalytic properties for oxygen reduction reaction in the presence of methanol, which can originate from the presence of Pd atoms and from the composition effect.     

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.     

Effect of heat treatment on stability of gold particle modified carbon supported Pt-Ru anode catalysts for a direct methanol fuel cell

Carbon supported Au-PtRu (Au-PtRu/C) catalysts were prepared as the anodic catalysts for the direct methanol fuel cell (DMFC). The procedure involved simple deposition of Au particles on a commercial Pt-Ru/C catalyst, followed by heat treatment of the resultant composite catalyst at 125, 175 and 200 °C in a N₂ atmosphere. High-resolution transmission electron microscopy (HR-TEM) measurements indicated that the Au nanoparticles were attached to the surface of the Pt-Ru nanoparticles. We found that the electrocatalytic activity and stability of the Au-PtRu/C catalysts for methanol oxidation is better than that of the PtRu/C catalyst. An enhanced stability of the electrocatalyst is observed and attributable to the promotion of CO oxidation by the Au nanoparticles adsorbed onto the Pt-Ru particles, by weakening the adsorption of CO, which can strongly adsorb to and poison Pt catalyst. XPS results show that Au-PtRu/C catalysts with heat treatment lead to surface segregation of Pt metal and an increase in the oxidation state of Ru, which militates against the dissolution of Ru. We additionally find that Au-PtRu/C catalysts heat-treated at 175 °C exhibit the highest electrocatalytic stability among the catalysts prepared by heat treatment: this observation is explained as due to the attainment of the highest relative concentration of gold and the highest oxidation state of Ru oxides for the catalyst pretreated at this temperature.     

Mass transport of direct methanol fuel cell species in sulfonated poly(ether ether ketone) membranes

Homogeneous membranes based on sulfonated poly(ether ether ketone) (sPEEK) with different sulfonation degrees (SD) were prepared and characterized. In order to perform a critical analysis of the SD effect on the polymer barrier and mass transport properties towards direct methanol fuel cell species, proton conductivity, water/methanol pervaporation and nitrogen/oxygen/carbon dioxide pressure rise method experiments are proposed. This procedure allows the evaluation of the individual permeability coefficients in hydrated sPEEK membranes with different sulfonation degrees. Nafion^® 112 was used as reference material. DMFC tests were also performed at 50 °C. It was observed that the proton conductivity and the permeability towards water, methanol, oxygen and carbon dioxide increase with the sPEEK sulfonation degree. In contrast, the SD seems to not affect the nitrogen permeability coefficient. In terms of selectivity, it was observed that the carbon dioxide/oxygen selectivity increases with the sPEEK SD. In contrast, the nitrogen/oxygen selectivity decreases. In terms of barrier properties for preventing the DMFC reactants loss, the polymer electrolyte membrane based on the sulfonated poly(ether ether ketone) with SD lower or equal to 71%, although having slightly lower proton conductivity, presented much better characteristics for fuel cell applications compared with the well known Nafion^® 112. In terms of the DMFC tests of the studied membranes at low temperature, the sPEEK membrane with SD = 71% showed to have similar performance, or even better, as that of Nafion^® 112. However, the highest DMFC overall efficiency was achieved using sPEEK membrane with SD = 52%.