High Direct Methanol Fuel Cells Research Articles

Sulfonated PVdF-HFP/shuttle-like SrBaWO4 nanocomposite membranes for the evolution of high performance and durable DMFCs

Sulfonated poly (vinylidene fluoride-co-hexafluoropropylene) (sPVdF-HFP) nanocomposite membranes with shuttle-like sulfonated barium tungstate (sBaWO4) and strontium barium tungstate (sSrBaWO4) nanofillers are formulated as solid state electrolytes for direct methanol fuel cells (DMFCs). Morphological features of shuttle-like fillers are intact even after their blending with sPVdF-HFP in the form of nanocomposite membranes, which exemplifies their structural and physical integrities, conscripting the high oxidative and thermal stabilities of sPVdF-HFP/sBaWO4/sSrBaWO4 nanocomposite membranes. Owing to the prevalence of oxygen vacancies and the consequent surface reactivities, sBaWO4 and sSrBaWO4 nanofillers augment the water adsorption properties of sPVdF-HFP based nanocomposite membranes, whilst their substantial ion exchange capacities are corroborated with the ionizable functionalities. The elevated oxygen and cation vacancies and hydrogen bonding exerted amid the neighborhood oxygen lattices of SrBaWO4 nanostructures enunciate the swift ionic mobility for sPVdF-HFP/sSrBaWO4 nanocomposite membrane. Hydrogen bonding stimulated between the sulfonic acid moieties of both the filler and host polymer/hydroxyl functionalities of filler with free water molecules and hydrophobic polymeric backbone narrow down the molecular transportation networks, confining the methanol permeation of as-formulated nanocomposite membranes. Thus, the discerning channels implanted for H+ ions and methanol in sPVdF-HFP/sSrBaWO4 nanocomposite membrane enumerate the maximal ionic conductivity/methanol permeation selectivity ratio that enacts the maximum power density of 127 mW cm−2 with uplifted durability over the Nafion 117 membrane, convening it's propriety as a dynamic solid electrolyte in high performance and durable DMFCs.

New Insights into Properties of Methanol Transport in Sulfonated Polysulfone Composite Membranes for Direct Methanol Fuel Cells.

Methanol crossover through a polymer electrolyte membrane has numerous negative effects on direct methanol fuel cells (DMFCs) because it decreases the cell voltage due to a mixed potential (occurrence of both oxygen reduction and methanol oxidation reactions) at the cathode, lowers the overall fuel utilization and contributes to long-term membrane degradation. In this work, an investigation of methanol transport properties of composite membranes based on sulfonated polysulfone (sPSf) and modified silica filler is carried out using the PFG-NMR technique, mainly focusing on high methanol concentration (i.e., 5 M). The influence of methanol crossover on the performance of DMFCs equipped with low-cost sPSf-based membranes operating with 5 M methanol solution at the anode is studied, with particular emphasis on the composite membrane approach. Using a surface-modified-silica filler into composite membranes based on sPSf allows reducing methanol cross-over of 50% compared with the pristine membrane, making it a good candidate to be used as polymer electrolyte for high energy DMFCs.

Fabrication and characterization of gold nanoparticles and fullerene-C60 nanocomposite film at glassy carbon electrode as potential electro-catalyst towards the methanol oxidation

Development of low cost anodic materials and high efficient electro-kinetics of methanol in direct methanol fuel cell (DMFC) has been a promising approach. However it has not been successfully reached to market from laboratory due to its high cost and low kinetic oxidation. Both issues encounter from one of its main components, the catalyst. Therefore, present work focuses upon the development of new catalyst material and optimization of various most significant influencing parameters of a high performance DMFC. We have developed a nanocomposite material employing gold nanoparticles and fullerene-C60 at glassy carbon electrode (AuNP@reduced-fullerene-C60/GCE) as anode for high performance oxidation of methanol. Fullerene-C60 was manually dropped on pre treated GCE and partially electro-reduced in KOH to make it more conductive. Gold nanoparticles (AuNPs) were deposited on reduced-fullerene-C60 modified electrode using cyclic voltammetry (CV). Electrochemical characterization techniques such as CV, electrochemical impedance spectroscopy (EIS) and chronocoulometry were used to characterize modified electrode. Modified electrode was also characterized by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) for morphological properties. The electrochemical behavior of methanol was performed in alkaline medium using CV and chronoamperometry methods. The results revealed good electrocatalytic performance and better stability than previously reported catalysts using AuNP@reduced-fullerene-C60 catalyst, suggesting making promising anodic material for direct methanol oxidation fuel cell.

Nanostructured Transition Metal Nitride (MN) As a Potential Support for Pt(Ru) Anode Electro-Catalyst for Direct Methanol Fuel Cells (DMFCs)

The rapid depletion of fossil fuels and increased environmental pollution due to vast fossil-fuel consumption is a thrust for efficient use of energy and exploration of renewable and clean energy sources.1 Generation of electricity from renewable energy sources, such as solar, wind without producing carbon dioxide-an undesired green house pollutant, offers enormous potential for meeting future energy demands. In this direction, fuel cell technology has gained special attention as it provides promising and sustainable approach for the production of continuous power with reduced greenhouse gas emissions and higher efficiencies compared to combustion based technologies. In particular, direct methanol fuel cells (DMFCs) have got attention as power sources due to advantages like use of methanol as fuel which has higher energy density (3800 kcal/l) than hydrogen (658 kcal/l, used as a fuel in proton exchange membrane fuel cells), safety and ease of handling the fuel (than H2) and low operating temperature. This offers quick start-up and extended durability of system components. The simple system design would be reflected as an ease in operation, reduced cost and high reliability. Hence, they are considered to be an ideal energy source for applications like automobile, portable devices, stationary systems, etc.2One of the major constraints in the development of high performance DMFCs is the low power density of the system due to poor catalytic performance at the electrodes and high capital cost due to use of expensive noble metal electro-catalysts. Hence, it is important to explore new electro-catalyst with reduced noble metal content at both anode and cathode to enhance the efficiency and power density of DMFC. Currently, Pt(Ru) alloy has been identified as a most active anode electro-catalyst for methanol electro-oxidation in DMFC. The synthesis of high surface area Pt(Ru) electro-catalyst is important to improve the reaction kinetics. In addition, efficient utilization of electro-catalyst by achieving high electrochemical active surface area is important, which can be accomplished by dispersing Pt(Ru) electro-catalyst on support with high surface area and superior electrical conductivity. The present study explores highly conductive nanostructured transition metal nitride (MN) as a potential support for Pt(Ru) electro-catalyst for methanol eletcro-oxidation. High surface area Pt(Ru)/MN (~125 m2/g) has been synthesized by wet chemical approach employing non-halide precursors of Pt and Ru. The electrochemical characterization has been carried out in 1 M methanol and 0.5 M sulfuric acid, used as fuel and electrolyte, using Pt wire as counter electrode and Hg/Hg2SO4 as reference electrode (+0.65 V with respect to NHE) and Pt loading of 0.3 mg/1cm2 at 400C. Pt(Ru)/MN shows promising performance for methanol oxidation, showing ~52% improved catalytic activity at ~0.65 V (vs NHE) in half-cell configuration and ~56% higher maximum power density in single DMFC study (Fig. 1) than that of commercial JM-Pt(Ru). This is attributed to well-dispersion of Pt(Ru) on MN (support of surface area of ~125 m2/g) offering high utilization of noble metals Pt and Ru, along with superior electrochemical active surface area and low charge transfer resistance for Pt(Ru)/MN than that of JM-Pt(Ru). Pt(Ru)/MN also shows superior electrochemical stability in half-cell configuration and single DMFC study than JM-Pt(Ru) electro-catalyst. Thus, the present study demonstrates the potential of MN as a support in improving the electrochemical activity of Pt(Ru) electro-catalyst, in the aim of achieving superior electrochemical performance and stability with reduced noble metal content for the electro-catalyst of DMFC. This portends significant reduction in the overall capital cost of DMFC system. Results of the synthesis, structural characterization and electrochemical activity of these electro-catalysts will be presented and discussed.

Analytical Model for Effect of Polymer Composite Membrane Properties on Direct Methanol Fuel Cell Performance

Abstract. Performance of direct methanol fuel cell (DMFC), using polymer composite membrane, can be directly affected by membrane properties, including permeability, proton conductivity and membrane thickness. In order to obtain high DMFC performance, methanol permeability of the membrane should be low, while keeping high proton conductivity. This may be achieved by modification of incorporating inorganic filler into high proton conducting membrane. In this work, the analytical modeling for DMFC performance prediction was developed to be a guideline for Nafion based membrane improvement. Methanol permeability and proton conductivity were set at 0.30 – 5.60 cm2×S-1 and 0.08 – 0.15 S×cm-1 with the thickness of 25 – 1000 μm. The results show that DMFC performance strongly depends on the methanol permeability especially with thin membrane giving maximum power density of 1034 and 100mW×cm-2 at the permeability of 0.30 and 5.60 cm2×S-1, respectively, with thickness of 45 μm, while with thick membrane the permeability has negligibly effect. However, the proton conductivity mainly affects DMFC performance only with thick membrane as a result of ohmic resistance.

Polarization and Electrocatalyst Selection for Polybenzimidazole Direct Methanol Fuel Cells

High temperature direct methanol fuel cells (DMFCs) using polybenzimidazole (PBI) membranes could improve the energy density of portable power sources. This study examines the polarization of vapor phase PBI DMFCs constructed with commercial membranes manufactured by a sol-gel method. The polarization of the high temperature DMFCs is compared to similar low temperature membrane electrode assemblies (MEAs) using Nafion® membranes. The results showed that the cathode of the PBI DMFC had higher kinetic losses that are likely due to phosphate poisoning of the Pt electrocatalyst. At the tested conditions, the membrane conductivity of the PBI MEAs was comparable to the Nafion® MEA even with no humidification. Higher cell temperatures significantly improved PBI DMFC performance for Pt electrocatalyst electrodes. In full cell tests, the PBI DMFC MEAs had higher performance than Nafion® MEAs with similar catalyst loadings. The Pt and PtRu catalysts were tested for methanol oxidation and oxygen reduction activity by a rotating disk electrode (RDE) under 0.5 M H2SO4 and 0.5 M H3PO4. The combination of the polarization and RDE results for the PBI and Nafion® DMFCs suggest that Pt is a more active electrocatalyst for methanol oxidation in PBI than in Nafion®.

Characterisation of PAMPS–PSS pore-filling membrane for direct methanol fuel cell

A novel, low cost polymer electrolyte membrane for direct methanol fuel cell (DMFC) has been fabricated using the pore-filling technique with copolymer composed of poly-2-acrylamide-2-methylpropanesulphonic acid (PAMPS) and poly-styrene sulphonic acid (PSS) impregnated in a porous polyimide (PI) substrate. The pore-filling membranes, as well as a Nafion membrane as a reference, were subjected to proton conductivity, methanol permeability and DMFC performance tests at various methanol concentrations to understand the effect of ion-conductive copolymer component towards the DMFC applications. PAMPS–PSS pore-filling membranes showed a decrease in methanol permeability whilst they retained their proton conductivity, consequently improving the DMFC performance compared to commercial Nafion membrane. The steric and hydrophobic properties of PSS allow to improve methanol crossover, achieving high DMFC performance in methanol concentration up to 5molL−1, demonstrating that PAMPS–PSS pore-filling membranes are attractive for the DMFC applications.

Degradation mechanisms and mitigation strategies of metal cations in recycled fuel for direct methanol fuel cell membrane electrode assembly

Some metal contaminants, such as Al3+, Ni+2, Fe2+ and Cr3+, are produced during reactions in heat exchangers, stacks, and other fuel/water management system components. Due to the gradual build-up of these contaminants generated in the system, direct methanol fuel cell (DMFC) membrane electrode assemblies (MEAs) deteriorate steadily with increasing operation time. Hence, this study systematically investigates the effects of metal cations by supplying various concentrations of metal solutions to the fuel stream at constant-current densities, with the aim of understanding the mechanism and influence of metal contamination on a DMFC MEA. Various electrochemical diagnostic techniques are used to determine the main cause of MEA degradation, including electrochemical impedance spectroscopy, electrode polarization, and methanol stripping voltammetry. In addition, the critical concentration of metal cations in methanol fuel is investigated for high DMFC MEA stability. Further, various novel methods for mitigating the influence of the metal contaminants on the performance of a DMFC are suggested and verified.

Operating Temperature Dependency on Performance Degradation of Direct Methanol Fuel Cells

AbstractThe effect of operating temperature on performance degradation of direct methanol fuel cell (DMFCs) is examined to disclose the main parameter of the degradation mechanism and the degradation pattern in the membrane electrode assemblies (MEAs). The DMFC MEA degradation phenomenon is explained through the use of various electrochemical/physicochemical tools, such as electrochemical impedance spectroscopy, electrode polarization, methanol stripping voltametry, field emission‐scanning electron microscopy, X‐ray diffraction, inductively coupled plasma‐atomic emission spectroscopy, and X‐ray photoelectron spectroscopy analysis. The operation of DMFC under high temperature accelerates the degradation process of the DMFC. The higher degradation rate under high temperature DMFC operation is mainly attributed to the formation of membrane pinhole with interfacial delamination and cathode degradation. A high operating temperature may result in more considerable thermal and mechanical stress of the polymeric membrane continuously due to frequent dry–wet cycling mode and substantial uneven distribution of water between the anode and the cathode during a long period of DMFC operation. On the other hand, the electrochemical surface area deterioration by Pt coarsening and ionomers loss is not directly related to the larger DMFC performance decay at high temperature.

The Influence of Pt Loading, Support and Nafion Content on the Performance of Direct Methanol Fuel Cells: Examined on the Example of the Cathode

AbstractNano‐sized Pt colloids were prepared using the polyol method and supported on Ketjen black EC 600J (KB), Vulcan XC‐72 (VC) and high surface area graphite 300 (HG). The effects of the Nafion ionomer content, and the Pt loading of the cathode catalyst layer as well as the Pt loading on the support on the performance of direct methanol fuel cells (DMFCs), were studied. The membrane electrode assemblies (MEAs) were analysed using current–voltage curves, cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and adsorbed CO stripping voltammetry. Optimum Nafion to carbon (N/C) ratios (N/C being defined as the weight ratio of the Nafion ionomer to the carbon) were determined. The optimum N/C ratios were found to depend on the support as follows, 1.4, 0.7 and 0.5 for Pt/KB, Pt/VC and Pt/HG, respectively and to be independent of the Pt/C loading range of 20–80 wt% tested in this work. The highest DMFC performances, as well as the highest electrochemical active surface areas, and improved gas diffusivities, were achieved using these ratios. For the catalysts prepared in this work, the average Pt crystallite size was found to decrease with increasing surface area of the support for a particular Pt loading. MEAs made using KB as support and the optimal N/C ratio of 1.4 showed the best performances, i.e. higher than the VC and HG supports for any N/C ratio. The highest DMFC performance was observed using 60 wt% Pt on KB cathode electrodes of 1 mg Pt cm–2 loading and an N/C value of 1.4. For all three supports studied, the 60 wt% Pt on carbon loading resulted in the best DMFC performance. This may be linked to the Pt particle size and catalyst preparation method used in this work. In comparison to literature results, high DMFC performances were achieved using relatively ‘low' Pt and Ru loadings. For example, a maximum power density of >100 mW cm–2 at 60 °C was observed using a 1 mg Pt cm–2 cathode loading and a 2 mg PtRu cm–2 anode loading.

Sulfonated polyimides bearing benzimidazole groups for direct methanol fuel cell applications

Sulfonated polyimides (SPIs) bearing benzimidazole groups in the main chains were synthesized from 1,4,5,8-naphthalenetetracarboxiylic dianhydride, 4,4′-bis(4-aminophenoxy)biphenyl-3,3′-disulfonic acid and 2-( p- or m-aminophenyl)-5-aminobenzimidazole in m-cresol. The resulting polymer solutions were not precipitated to gain the solid polymers but directly cast into films because the precipitated SPIs became insoluble in common organic solvents. The strong interaction between sulfonic acid and benzimidazole groups reduced the water uptake and methanol uptake. The SPI membranes with ion exchange capacities (IECs) of 1.9–2.0 mequiv./g displayed reasonably high mechanical strength, thermal stability, water stability and proton conductivity. They showed anisotropic membrane swelling in water with 2.8 times larger swelling in thickness direction than in plane one and anisotropic proton conductivity with 25% smaller conductivity in thickness direction than in plane one. They suppressed methanol crossover in direct methanol fuel cell (DMFC) operation and displayed fairly high DMFC performances even at high methanol concentrations. The Faraday's efficiency and overall DMFC efficiency at 60 °C and 200 mA/cm 2 for SPI membrane with IEC of 1.9 mequiv./g were 75 and 23%, respectively, at 5 wt% methanol feed concentration and 39 and 10.2%, respectively, at 20 wt%. The SPI membranes have high potential for DMFC applications at mediate temperatures (40–80 °C).

Effect of conditioning method on direct methanol fuel cell performance

We investigated the effect of the conditioning methods on improving the direct methanol fuel cell (DMFC) performance. The DMFC performance after the conditioning was measured using a newly developed single cell having an Ag/Ag 2SO 4 reference electrode, which is not influenced by methanol. As a result, we succeeded in developing an original two-step conditioning method in which the conditioning by fueling H 2 gas is conducted prior to a conventional DMFC conditioning. The anode and cathode characteristics after the two-step conditioning were measured with respect to a reference electrode. Based on the obtained i– E curves, the two-step conditioning is found to improve the methanol oxidation performance at the anode and also suppress the decline of the O 2 reduction performance at the cathode. The high DMFC performance based on the two-step conditioning is well explained by the anode and cathode characteristics.

The preparation of high activity DMFC Pt/C electrocatalysts using a pre-precipitation method

It is suggested that a Pt/C cathodic catalyst for the direct methanol fuel cell (DMFC) can be prepared with a pre-precipitation method, in which, H 2PtCl 6 is precipitated onto the carbon black as (NH 4) 2PtCl 6 before H 2PtCl 6 is reduced to Pt. The electrocatalytic activity of this Pt/C-A catalyst for oxygen reduction is excellent because the Pt/C catalyst prepared with this pre-precipitation method possesses a small average particle size, low relative crystalinity and a large electrochemically active surface area. In addition, the pre-precipitation method is simple and economical and it can be used to prepare a Pt/C catalyst on a large scale.

Fabrication of a micro DMFCs array made of photosensitive glass

We propose a novel design for a direct methanol fuel cell (DMFC) by using micromachining technologies. Thirteen micro DMFCs were fabricated on a photosensitive glass substrate containing a micro-holes array (diameter = 50–200 μm). Our concept is that a high voltage DMFC can be achieved by the integration of the micro DMFC array on one substrate. The first purpose of the experimental trials was to verify the feasibility of the novel structure. Preliminary testing results show that our new DMFC design is useful as a miniature DMFC.

DMFC performances using a pore-filling polymer electrolyte membrane for portable usages

To develop portable direct methanol fuel cells (DMFCs) with high power density, a high concentration of methanol in aqueous solution must be used as the fuel. We have developed a new type of membrane, namely the pore-filling membrane, and its membrane–electrode assembly (MEA) gives high DMFC performance with high concentration methanol fuel (around 10 M). The results were compared with a commercial Nafion membrane MEA.

Zeolite-based composite membranes for high temperature direct methanol fuel cells

Composite Nafion membranes containing three natural zeolites (Mordenite, Chabazite and Clinoptilolite) were prepared by using a recast procedure for application in high temperature Direct Methanol Fuel Cells (DMFCs). The Nafion-zeolite membranes have shown good properties for high temperature DMFC application, due to their improved water retention characteristics. A maximum power density of 390 mW cm−2 was achieved at 140 °C with the mordenite-based composite membranes in the presence of oxygen feed. The electrochemical behaviour of the composite membranes was interpreted in the light of surface properties and acidic characteristics of the fillers.

Nafion–TiO 2 composite DMFC membranes: physico-chemical properties of the filler versus electrochemical performance

TiO 2 nanometric powders were prepared via a sol–gel procedure and calcined at various temperatures to obtain different surface and bulk properties. The calcined powders were used as fillers in composite Nafion membranes for application in high temperature direct methanol fuel cells (DMFCs). The powder physico-chemical properties were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and pH measurements. The observed characteristics were correlated to the DMFC electrochemical behaviour. Analysis of the high temperature conductivity and DMFC performance reveals a significant influence of the surface characteristics of the ceramic oxide, such as oxygen functional groups and surface area, on the membrane electrochemical behaviour. A maximum DMFC power density of 350 mW cm −2 was achieved under oxygen feed at 145 °C in a pressurized DMFC (2.5 bar, anode and cathode) equipped with TiO 2 nano-particles based composite membranes.