Mass balance research for high electrochemical performance direct methanol fuel cells with reduced methanol crossover at various operating conditions

Mass balance research in direct methanol fuel cells (DMFCs) provides a more practical method in characterizing the mass transport phenomena in a membrane electrode assembly (MEA). This method can be used to measure methanol utilization efficiency, water transport coefficient (WTC), and methanol to electricity conversion rate of a MEA in DMFCs. First, the vital design parameters of a MEA are recognized for achieving high methanol utilization efficiency with increased power density. In particular, the structural adjustment of anode diffusion layer by adding microporous layer (MPL) is a very effective way to decrease WTC with reduced methanol crossover due to the mass transfer limitation in the anode. On the other hand, the cathode MPL in the MEA design can contribute in decreasing methanol crossover. The change of structure of cathode diffusion layer is also found to be a very effective way in improving power density. In contrast, the WTC of DMFC MEAs remains virtually constant in the range of 3.4 and 3.6 irrespective of the change of the cathode GDL. The influence of operating condition on the methanol utilization efficiency, WTC, and methanol to electricity conversion rate is also presented and it is found that these mass balance properties are strongly affected by temperature, current density, methanol concentration, and the stoichiometry of fuel and air.     

Proton-conducting membranes with high selectivity from cross-linked poly(vinyl alcohol) and poly(vinyl pyrrolidone) for direct methanol fuel cell applications

A series of hydrocarbon membranes consisting of poly(vinyl alcohol) (PVA), sulfosuccinic acid (SSA) and poly(vinyl pyrrolidone) (PVP) were synthesized and characterized for direct methanol fuel cell (DMFC) applications. Fourier transform infrared (FT-IR) spectra confirm a semi-interpenetrating (SIPN) structure based on a cross-linked PVA/SSA network and penetrating PVP molecular chains. A SIPN membrane with 20% PVP (SIPN-20) exhibits a proton conductivity value comparable to Nafion^® 115 (1.0 × 10⁻² S cm⁻¹ for SIPN-20 and 1.4 × 10⁻² S cm⁻¹ for Nafion^® 115). Specifically, SIPN membranes reveal excellent methanol resistance for both sorption and transport properties. The methanol self-diffusion coefficient through a SIPN-20 membrane conducted by pulsed field-gradient nuclear magnetic resonance (PFG-NMR) technology measures 7.67 × 10⁻⁷ cm² s⁻¹, which is about one order of magnitude lower than that of Nafion^® 115. The methanol permeability of SIPN-20 membrane is 5.57 × 10⁻⁸ cm² s⁻¹, which is about one and a half order of magnitude lower than Nafion^® 115. The methanol transport behaviors of SIPN-20 and Nafion^® 115 membranes correlate well with their sorption characteristics. Methanol uptake in a SIPN-20 membrane is only half that of Nafion^® 115. An extended study shows that a membrane-electrode assembly (MEA) made of SIPN-20 membrane exhibits a power density comparable to Nafion^® 115 with a significantly higher open current voltage. Accordingly, SIPN membranes with a suitable PVP content are considered good methanol barriers, and suitable for DMFC applications.     

Cross-linked poly(vinyl alcohol) and poly(styrene sulfonic acid-co-maleic anhydride)-based semi-interpenetrating network as proton-conducting membranes for direct methanol fuel cells

A series of semi-interpenetrating network (SIPN) membranes was synthesized by using poly(vinyl alcohol) (PVA) with sulfosuccinic acid (SSA) as a cross-linking agent and poly(styrene sulfonic acid-co-maleic acid) (PSSA-MA) as a proton source for direct methanol fuel cell (DMFC) application. A bridge of SSA between PVA molecules not only reinforced the network but also provided extra proton-conducting paths. PSSA-MA chains trapped in the network were the major proton conduction path of the membrane. The SIPN membranes with 80% PSSA-MA (SIPN-80) exhibited a higher proton conductivity value of 2.59 × 10⁻² S cm⁻¹ and very low methanol permeability (4.1 × 10⁻⁷ cm² s⁻¹). More specifically, the SIPN membranes exhibited very high selectivity (proton conductivity/methanol permeability). Membrane characteristics such as water uptake, proton conductivity and methanol permeability were evaluated to determine the effect of PVA molecular weights. The SIPN membranes with higher PVA molecular weight were also evaluated using methanol and oxygen gas in a single cell fuel cell at various temperatures. Power density value of over 100 mW cm⁻² was obtained for SIPN membrane-based membrane electrode assembly at 80 °C and using commercial binary alloy anode catalysts and 2 M methanol.     

Fabrication of cost-effective non-noble metal supported on conducting polymer composite such as copper/polypyrrole graphene oxide (Cu2O/PPy–GO) as an anode catalyst for methanol oxidation in DMFC

Cost-effective non-noble metal catalysts are of key significance to the successful use of direct methanol fuel cells (DMFCs) for electricity generation. Herein, cuprous oxide nanoparticles (Cu₂O NPs) supported graphene oxide (GO), polypyrrole (PPy) and polypyrrole–graphene oxide (PPy–GO) matrices were prepared using borohydride reduction method. The prepared catalysts were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–Vis spectra, Zeta potential and transmission electron microscopy (TEM). The elemental analysis of the composites was done by energy dispersive X-ray spectroscopy (EDX). Cu₂O NPs were homogeneously dispersed and strongly anchored on the PPy grafted GO matrix and this was examined through morphological analysis. The Cu₂O/PPy–GO (80:10:10) NPs exhibited noticeable improvement in electrochemical performance in comparison to pure graphene oxide (GO) and pure PPy supported Cu₂O NPs catalyst and revealed the peak current density of 300 μA cm^?2 at +0.68 V. The Cu₂O/PPy–GO system demonstrated higher current density and also exhibited greater stability in comparison to the commercial Pt–Ru/C catalyst as characterized by chronoamperometry (CA) analysis. This prospective nano-catalyst showed higher I_F/I_B ratio (26%, 8.6% and 19%) compared to the corresponding catalyst systems of Cu₂O/GO, Cu₂O/PPy and Pt–Ru/C. In direct methanol fuel cell (DMFC), the efficiency of Cu₂O/PPy–GO nano-catalyst system as an anode catalyst for methanol oxidation reaction (MOR) was investigated and the result revealed a maximum current density of 155 mA cm^?2 at +0.2 V and power density of 31 mW cm^?2. Hence, Cu₂O/PPy–GO NPs are a cost-effective alternative for Pt–Ru/C system to execute practical application in DMFC.     

The performance and mechanism of multi-step activation of MEA for DMFC

Membrane electrode assembly (MEA) after hot-pressing could not achieve its best performance before activated properly because the passages for protons, electrons, reactants and products in the MEA are not constituted completely. In this research, effective multi-step activation for MEA was proposed and the mechanism was discussed. The process includes three steps: methanol activation at room temperature, forced activation at intermediate temperature and proton activation at intermediate temperature. The performance of the MEA was correspondingly improved and reached respectively 54 mW cm⁻², 61 mW cm⁻², 78 mW cm⁻², after each step of the activation. Apparently the performance was greatly enhanced by multi-step activation. The activation process of MEA is not only a process of the humidification of the proton exchange membrane but also a complex process for the establishment of the channels for electrons, ions, gases and liquids. Furthermore the conduction of electrons, ions and the transportation of the reactants and products are enhanced while the micro-structure of the MEA is kept stable.     

Carbon nanotubes based gas diffusion layers in direct methanol fuel cells

A carbon nanotube based gas diffusion layer (CNT-GDL) for direct methanol fuel cells (DMFCs) was prepared by sintering the wet carbon paper made of CNT, polyacrylonitrile-based carbon fibre (PCF) and PTFE, completely replacing the commercial carbon paper based GDL (Toray-GDL). The results of X-ray diffraction (XRD), scanning electron microscope (SEM) and mercury intrusion porosimeter (MIP) showed that CNT-GDL is of higher graphitization degree, more abundant pores in the pore diameter range of 1000–3000 nm and better mass transfer ability, compared with those of Toray-GDL. Electrochemical impedance spectroscopy (EIS) results display that the cell with CNT-GDL had better electrical conductivity and mass transfer ability than those of Toray-GDL. As a result, the limiting current density and peak power density of DMFC single cell with CNT-GDL are improved by 40% and 27%, respectively.     

Low temperature decal transfer method for hydrocarbon membrane based membrane electrode assemblies in polymer electrolyte membrane fuel cells

Decal transfer is an effective membrane electrode assembly (MEA) fabrication method known for its low interfacial resistance and suitability for mass processing. Previously decal transfer for hydrocarbon membranes was performed at temperatures above 200 °C. Here a novel low temperature decal transfer (LTD) method for hydrocarbon membranes is introduced. The new method applies a small amount (2.2 mg cm⁻²) of liquid (1-pentanol) onto the membrane separator before decal transfer to lower the T_g of the membrane and achieves complete decal transfer at 110 °C and 6 MPa. Nafion binder amount in the catalyst layer and catalyst layer annealing temperature is controlled to optimize the fuel cell performance. Compared to conventional decal transfer (CDT), the novel LTD method shows enhancement in energy efficiency, simplicity in the process scheme, and improvement in fuel cell performance.     

Laser-perforated anode gas diffusion layers for direct methanol fuel cells

Novel anode gas diffusion layers (AGDLs) with both hydrophobic and hydrophilic pathways are created to enhance transfer of both methanol and CO₂. Such AGDLs are created by perforating PTFE-treated AGDLs with laser, so that the original pores/pathways in the AGDL are hydrophobic and the laser perforations are hydrophilic, thus providing easy transport paths for both the liquid methanol solution and CO₂. One of the novel AGDLs has increased the cell performance by 32% over the non-perforated AGDL. Results of electrochemical impedance spectroscopy (EIS) show that the main reason for the performance enhancement is due to the reduction in mass transfer resistance. Additionally, there is a reduction in charge transfer resistances due to the enhanced methanol transfer to the catalyst layer. The results of linear sweep voltammetry (LSV) show that the perforations increase methanol crossover, thus if perforation density of the AGDL is too high, the cell performances are lower than that of the virgin AGDL.     

Water-transport in outer micro-porous layers for direct methanol fuel cells

The outer micro-porous layer (MPL) between the gas diffusion layer and channel of the bipolar plate was studied for both sides of the electrodes in DMFC, with particular attention to the effects of the hydrophobicity of the MPL on mass transport as well as cell performance. Water-transport behavior from the electrodes to the channel was observed through the transparent window of the single cell with membrane-electrode assemblies (MEAs) including three combinations of outer MPLs. The crossover amount of methanol as well as water through the membrane was measured, and the mass balance, based on the measured flux, was established to understand the mass transport in MEAs. The design of outer MPLs is discussed for the best cell performance.     

Fabrication and characterization of poly(vinyl alcohol)/montmorillonite/poly(styrene sulfonic acid) proton-conducting composite membranes for direct methanol fuel cells

A high performance poly(vinyl alcohol)/montmorillonite/poly(styrene sulfonic acid) (PVA/MMT/PSSA) proton-conducting composite membrane was fabricated by a solution casting method. The characteristic properties of these blend composite membranes were investigated by using thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, methanol permeability measurement, and the AC impedance method. The ionic conductivities for the composite membranes are in the order of 10⁻³ S cm⁻¹ at ambient temperature. There are two proton sources used on this novel composite membrane: the modified MMT fillers and PSSA polymer, both materials all contain the -SO₃H group. Therefore, the ionic conductivity was greatly enhanced. The methanol permeabilities of PVA/MMT/PSSA composite membranes is of the order of 10⁻⁷ cm² s⁻¹. It is due to the excellent methanol barrier properties of the PVA polymer. The peak power densities of the air-breathing direct methanol fuel cells (DMFCs) with 1M, 2M, 4M CH₃OH fuels were 14.22, 20.00, and 13.09 mW cm⁻², respectively, at ambient conditions. The direct methanol fuel cell with this composite polymer membrane exhibited good electrochemical performance. The proposed PVA/MMT/PSSA composite membrane is therefore a potential candidate for future applications in DMFC.     

Long-term durability test for direct methanol fuel cell made of hydrocarbon membrane

A long-term durability test has been conducted for a direct methanol fuel cell (DMFC) using the commercial hydrocarbon membrane and Nafion ionomer bonded electrodes for 500 h. Membrane electrode assembly (MEA) made by a decal method has experienced a performance degradation about 34% after 500 h operation. Cross-sectional analysis of the MEA shows that the poor interfacial contact between the catalyst layers and membrane in the MEA has further deteriorated after the durability test. Therefore, the internal resistance of a cell measured by electrochemical impedance spectroscopy (EIS) has considerably increased. The delamination at the interfaces is mainly attributed to incompatibility between polymeric materials used in the MEA. Furthermore, X-ray diffraction (XRD) analysis reveals that the catalyst particles have grown; thereby decreasing the electrochemical surface area. Electron probe micro analysis (EPMA) shows a small amount of Ru crossover from anode to cathode; and its effect on the performance degradation has been analyzed.     

Impact of the patterned membrane morphology on PEMFC performances of ultra-low platinum loaded MEAs

Two types of patterns were achieved on the surface of NR211 membranes: holes and knolls. These patterns were produced on only one side of the membrane at the cathode. For equivalent specific surfaces, these two membrane patterning morphologies were tested in fuel cell conditions with very low platinum loading (30 μg/cm²). Catalyst loading was achieved by magnetron plasma deposition either on the microporous carbon electrode or directly on the patterned membrane. The fuel cell performances in dry conditions were found to be highly dependent on the morphology of the membrane surface. Results showed that the knoll morphology gave better fuel cell performances than the hole morphology. For the knoll morphology the current density increased by a factor of 1.78 at 0.7 V versus a pristine membrane, whereas the hole morphology appeared in some cases to deteriorate the fuel cell performances, despite an increase in the specific surface by a factor of 1.87 versus a pristine membrane. The concepts of top level (extreme surface of the patterned membranes) and bottom level (the pattern base) were introduced to highlight the phenomena of micro water management on the areas of the micro-patterns which act on the local conductivity of the protonic membranes. These results suggest that it is necessary to choose the morphology of the patterns very carefully before simply increasing the specific surface of protonic membranes.     

Characterization of electrode structures and the related performance of direct methanol fuel cells

The performance of membrane electrode assemblies (MEAs) in fuel cells is substantially affected by the structures of the electrodes. An increase of about 25% in power performance was achieved merely by controlling the pressure of hot press in the MEA fabrication process for a given Pt loading, instead of by employing pore formers and heat treatment-a widely accepted method-to modify the structures of the electrode. The microstructures of the different hot-pressed electrodes were examined by transmission electron microscopy, scanning electron microscopy, and small angle X-ray scattering to assess the effect of the pressure on the structures of the electrodes. Based on experimental observations, the improved performance of the MEA is attributed to the porosity of the cathode electrode, in which a network of macrofissures and sub-microfissures allows air to penetrate the electrode. Emphasis is also placed on the relationship between the total porosity of the electrodes and the MEA performance. Results of this study demonstrate that the specific power density nearly doubles when the total porosity increased from 57% to 76%. Also, the MEAs mounted in an air-breathing DMFC small pack were fabricated in-house to supply power for a mobile phone.     

Investigation on the durability of direct methanol fuel cells

This study addresses the durability of direct methanol fuel cells (DMFCs). Three performance indices including permanent degradation, temporary degradation and voltage fluctuation are proposed to qualify the durability of DMFC. The decay rate, associated with permanent degradation, follows from such failure mechanisms as dissolution, growth and poisoning of the catalyst, while temporary degradation reflects the elimination of the hydrophobic property of the gas diffusion layer (GDL). However, voltage fluctuations reveal different results which cannot stand for degradation phenomenon exactly. In this investigation, such methods of examination as scanning electron microscope (SEM), and X-ray diffraction (XRD) are employed to check the increase in the mean particle size in the anode and cathode catalysts, and the degree is higher in the cathode. The Ru content in the anode catalyst and the specific surface area (SSA) of the anode and cathode catalysts decrease after long-term operation. Moreover, the crossover of Ru from the anode side to the cathode side is revealed by energy dispersive X-ray (EDX) analysis. Electro-catalytic activity towards the methanol oxidation reaction (MOR) at the anode is verified to be weaker after durability test by cyclic voltammetry (CV). Also, the electrochemical areas (ECAs) of the anode and cathode catalysts are evaluated by hydrogen-desorption. SSA loss simply because of agglomeration and growth of the catalyst particles, of course, is lower than ECA loss. The observations will help to elucidate the failure mechanism of membrane electrode assembly (MEA) in durability tests, and thus help to prolong the lifetime of DMFC.     

Fabrication of MEA with hydrocarbon based membranes using low temperature decal method for DMFC

The membrane electrode assembly (MEA) with hydrocarbon (HC) based membranes made by a low temperature decal method has been investigated for the direct methanol fuel cells (DMFCs). The conventional low temperature decal (LTD) transfer method (comprised of three layers; viz., carbon, Nafion bonded electrodes and outer ionomer layers over the decal Teflon substrates) meant for the MEAs made of Nafion type membranes is suitably modified to use with hydrocarbon (HC) based membranes. The modification of conventional LTD method is effected by means of modulating the three-layered structure and optimizing other parameters to facilitate complete transfer of catalyst layers onto the HC membranes. The MEAs prepared by the modified LTD method have yielded 21 % higher DMFC performance compared to that of the MEAs produced by conventional LTD method. The structure and electrochemical properties of the MEAs have been analyzed by the field-emission scanning electron microscopy (FE-SEM) and the electrochemical impedance spectroscopy (EIS).     

Conceptual design and statistical overview on the design of a passive DMFC single cell

A conceptual design and statistical overview about passive direct methanol fuel cells that have been fabricated from 2002 to 2013, is performed [1–70]. The major components of passive DMFCs such as: active area, type of Nafion, catalyst loading on the anode and cathode side, characteristics and designs of current collectors (CC), and also the optimum methanol concentration which resulted in the best performance are categorized and studied statistically and individually. Finally, the best combination for the design and fabrication of a reliable passive DMFC is recommended. Obtained results indicated that a MEA with 4 cm² active area, Nafion 117 and 4 mg/cm² Pt/Ru at the anode and 2 mg/cm² Pt black at the cathode (or 4 mg/cm² Pt/Ru at the anode – 4 mg/cm² Pt black at the cathode) as the catalyst loading, which is sandwiched between two stainless steel perforated current collectors that are coated by Pt (or Gold) could be a reliable design for a passive direct methanol single cell.     

A theoretical model of the membrane electrode assembly of liquid feed direct methanol fuel cell with consideration of water and methanol crossover

An algebraic model of the membrane electrode assembly of the direct methanol fuel cell is developed, which considers the simultaneous liquid water and methanol crossover effects, and the associated electrochemical reactions. The respective anodic and cathodic polarization curves can be predicted using this model. Methanol concentration profile and flux are correlated explicitly with the operating conditions and water transport rate. The cathode mixed potential effect induced by the methanol crossover is included and the subsequent cell voltage loss is identified. Water crossover is influenced by the capillary pressure equilibrium and hydrophobic property within the cathode gas diffusion layer. The model can be used to evaluate the cell performance at various working parameters such as membrane thickness, methanol feed concentration, and hydrophobicity of the cathode gas diffuser.     

A prediction model of degradation rate for membrane electrode assemblies in direct methanol fuel cells

This article proposes a new prediction model to describe the nonlinear performance degradation paths of membrane electrode assemblies (MEAs) in direct methanol fuel cell (DMFC): a bi-exponential model with random coefficients. The bi-exponential model is constructed on a mathematical basis representing second-order kinetics. Performance variation between MEAs is incorporated by random coefficients in the proposed model. A likelihood ratio test is sequentially executed to select random effects in the nonlinear random-coefficients model. Analysis results indicate that the reliability estimation can be substantially improved by using the nonlinear random-coefficients model to incorporate two heterogeneous degradation characteristics of MEA performance during continuous operation of DMFC. Confidence intervals of failure-time distributions are obtained by the parametric bootstrap method.