V. Antonucci

DMFCs: From Fundamental Aspects to Technology Development

This review paper describes recent developments in both the fundamental and technological aspects of direct methanol fuel cells (DMFCs). Most previous studies in this field have dealt with fundamental aspects, whereas in recent years, the technology of these devices has become the object of significant interest. This is mainly due to the fact that a probable application of DMFCs in portable power sources and in hybrid electrical vehicles has only recently been envisaged. The section on fundamentals is particularly focused on the electrocatalysis of the methanol oxidation reaction and oxygen electroreduction. In this regard, particular relevance is given to the interpretation of the promoting effect on Pt of additional elements and some aspects of the electrocatalysis of oxygen reduction in the presence of methanol crossover have been treated. The technology section deals with the development of both components and devices. Particular emphasis is given to the development of high surface area electrocatalysts and alternative electrolyte membranes to Nafion, also the fabrication methodologies for the M&E assembly have been discussed. The last part of the paper describes the recent efforts in developing DMFC stacks for both portable and electro-traction applications. The current status of the technology in this field is presented and some important technical and economical challenges are been discussed.

Investigation of a direct methanol fuel cell based on a composite Nafion®-silica electrolyte for high temperature operation

Abstract Operation of a liquid-fed Direct Methanol Fuel Cell (DMFC) working at 145°C was demonstrated by using a composite membrane made of Nafion ® ionomer and silica. The enhanced humidification conditions of the membrane–electrode (ME the measured methanol cross-over rate was 4×10 −6 moles min −1 cm −2 . A physico-chemical characterization of the cell components is also reported.

Hybrid Nafion–silica membranes doped with heteropolyacids for application in direct methanol fuel cells

Nafion–silica composite membranes doped with phosphotungstic and silicotungstic acids have been investigated for application in direct methanol fuel cells at high temperature (145°C). The phosphotungstic acid-based membrane showed better electrochemical characteristics at high current densities with respect to both silicotungstic acid-modified membrane and silica–Nafion membrane. A maximum power density of 400 mW cm−2 was obtained at 145°C in the presence of oxygen feed, whereas the maximum power density in the presence of air feed was approaching 250 mW cm−2. The results indicate that the addition of inorganic hygroscopic materials to recast Nafion extends the operating range of a direct methanol fuel cell. Operation at high temperatures significantly enhances the kinetics of methanol oxidation.

Sulfonated polysulfone ionomer membranes for fuel cells

Abstract Sulfonated polysulfone (SPSU) membranes with different sulfonation levels have been prepared and evaluated as proton exchange membranes in polymer electrolyte fuel cells (PEFC). The membranes have been characterized by ion-exchange capacity (IEC), thermal analysis, proton conductivity and single cell performance. The introduction of sulfonic groups in the base polymer produces an increase in glass transition temperature ( T g ) from 190 to about 200–220°C and a thermal decomposition of polymer at lower temperature. Proton conductivity of 4.3×10 −2 S·cm −1 at 80°C on a SPSU membrane with IEC=1.25 meq/g was obtained. Membrane/electrodes assemblies (MEAs) prepared with SPSU gave, in single cell tests at 80°C, power densities of 400 and 500 mW·cm −2 in H 2 /air and H 2 /O 2 , respectively.

Analysis of the Electrochemical Characteristics of a Direct Methanol Fuel Cell Based on a Pt‐Ru/C Anode Catalyst

A vapor-feed direct methanol fuel cell based on a Nafion 117{reg_sign} solid polymer electrolyte was investigated. Pt-Ru/C and Pt/C catalysts were employed for methanol oxidation and oxygen reduction, respectively. The structure, surface, and morphology of the catalysts were investigated by X-ray powder diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Crystalline face-centered cubic phases were found in the Pt and Pt-Ru catalysts. The alloy composition in the Pt-Ru/C catalyst was different from the nominal composition, probably due to the formation of surface RuO{sub x} species, as indicated by X-ray photoelectron spectroscopy. Transmission electron microscopy observation showed an increase of the average particle size and particle agglomeration in the Pt-Ru/C catalyst compared to the Pt/C catalyst. The membrane/electrode assembly was prepared by using a paste process method. Scanning electron microscopy and energy dispersive X-ray analyses showed good adhesion of catalyst layers to the membrane and a homogeneous distribution of the ionomer inside the catalyst. AC-impedance and galvanostatic steady-state polarization techniques were used to investigate the electrochemical performance of the direct methanol fuel cell.

Influence of the acid-base characteristics of inorganic fillers on the high temperature performance of composite membranes in direct methanol fuel cells

Various recast Nafion® composite membranes containing ceramic oxide fillers with different surface characteristics (SiO2, SiO2–PWA, Al2O3, ZrO2) have been investigated for application in high temperature direct methanol fuel cells (DMFCs). Cell resistance at 145 °C increases as a function of the pH of slurry of the inorganic filler indicating a strong influence of the acid–base characteristics on the electrolyte conductivity. This effect has been attributed to the different water retention capabilities of the various membranes. Fuel cell performance at 145 °C, expressed as both maximum power density and current density at 0.5 V cell potential, increases almost linearly as the pH of slurry of the oxide materials decreases. Appropriate selection of the surface properties for the inorganic fillers allows to enhance the proton conductivity and extends the operating temperature range of composite membranes. The influence of fuel cell operating pressure on the humidification properties of these electrolytes at high temperature has been also investigated.

Investigation of a carbon-supported quaternary PtRuSnW catalyst for direct methanol fuel cells

Abstract 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.

Influence of flow field design on the performance of a direct methanol fuel cell

Serpentine (SFF) and interdigitated (IFF) flow fields were investigated with regard to their use in a direct methanol fuel cell (DMFC). The DMFC equipped with SFFs showed lower methanol cross-over, higher fuel utilisation and slightly larger voltage efficiency at low current densities. IFFs enhanced mass transport and membrane humidification allowing to achieve high power densities of 450 and 290 mW cm−2 in the presence of oxygen and air feed, respectively, at 130°C. A fuel efficiency of 90% was obtained with the IFFs in the presence of 1 M methanol feed at 130°C and a current load of 500 mA cm−2.

Effect of carbon-supported and unsupported Pt–Ru anodes on the performance of solid-polymer-electrolyte direct methanol fuel cells

The structure, chemistry and morphology of commercially available carbon-supported and unsupported Pt–Ru catalysts are investigated by X-ray diffraction, energy-dispersive analysis by X-rays and electron microscopy. The catalytic activities of these materials towards electrooxidation of methanol in solid-polymer-electrolyte direct methanol fuel cells have been investigated at 90∘C and 130∘C with varying amounts of Nafion ionomer in the catalytic layer. The unsupported Pt–Ru catalyst exhibits higher performance with lower activation-control and mass-polarization losses in relation to the carbon-supported catalyst.

Investigation of direct methanol fuel cells based on unsupported Pt-Ru anode catalysts with different chemical properties

The structure, chemistry and morphology of commercial unsupported Pt–RuOx and in-house prepared Pt–Ru catalysts were analysed. The catalytic activity of these materials towards electro-oxidation of methanol in solid-polymer-electrolyte direct methanol fuel cells have been investigated at 130°C. The in-house prepared unsupported Pt–Ru catalyst showed higher performance with lower activation control and mass polarisation losses in relation to the Pt–RuOx catalyst. An enhancement of mass-transport properties was obtained in the presence of interdigitated flow-fields. Maximum power densities of about 0.45 and 0.29 W cm−2 in the presence of oxygen and air-feed, respectively, were obtained at 130°C. Methanol cross-over rates and fuel efficiency were determined under various conditions.