Development of gas diffusion electrodes for low relative humidity proton exchange membrane fuel cells

A series of polyaniline nanofibers (PANFs) were synthesized and incorporated into gas diffusion electrodes (GDE) of proton exchange membrane fuel cells (PEMFC) to improve their performances at low relative humidity (RH) conditions. Three different placements to incorporate the PANFs in the anodes include (1) placing a PANFs layer between catalyst layer (CL) and membrane, (2) coating the CL with PANFs and catalyst mixed slurry, and (3) placing a PANFs layer between the CL and gas diffusion layer (GDL). Fuel cell performance data indicates that the last method is superior to the others and is adopted as incorporation method thereafter. Extensive studies on single cell performances have been conducted to compare the membrane electrode assemblies with and without the incorporation of PANFs in both anode and cathode. Polarization curves show the incorporation of H₂SO₄-doped PANFs is highly effective in improving the hydrophilic characteristic of the electrodes and thus can promote the PEMFC performance at low RH conditions. For example, with a lowering of reactant RH from 100 to 70%, the electrode with H₂SO₄-doped PANFs layer exhibits an increase in power density from 0.57 to 0.7 W cm⁻². On the other hand, a traditional carbon-supported platinum electrode exhibits a decline of performance from 0.73 to 0.55 W cm⁻².     

Enhancement of water retention in the membrane electrode assembly for direct methanol fuel cells operating with neat methanol

It is desirable to operate a direct methanol fuel cell (DMFC) with neat methanol to maximize the specific energy of the DMFC system, and hence increasing its runtime. A way to achieve the neat-methanol operation is to passively transport the water produced at the cathode through the membrane to the anode to facilitate the methanol oxidation reaction (MOR). To achieve a performance of the MOR similar to that under the conventional diluted methanol operation, both the water transport rate and the local water concentration in the anode catalyst layer (CL) are required to be sufficiently high. In this work, a thin layer consisting of nanosized SiO₂ particles and Nafion ionomer (referred to as a water retention layer hereafter) is coated onto each side of the membrane. Taking advantage of the hygroscopic nature of SiO₂, the cathode water retention layer can help maintain the water produced from the cathode at a higher concentration level to enhance the water transport to the anode, while the anode retention layer can retain the water that is transported from the cathode. As a result, a higher water transport rate and a higher water concentration at the anode CL can be achieved. The formed membrane electrode assembly (MEA) with the added water retention layers is tested in a passive DMFC and the results show that this MEA design yields a much higher power density than the MEA without water retention layers does.     

Design of a MEA with multi-layer electrodes for high concentration methanol DMFCs

According to the conventional MEA test, methanol and water crossover are the main factors to determine performance of a passive DMFC. Thus, to ensure the high cell performance of a passive DMFC using high concentration methanol of 50–95 vol%, the MEA in this study introduces the barrier layer to limit the crossover of high concentration methanol, a hydrophobic layer to reduce water crossover, and a hydrophilic layer to enhance the water recovery from the cathode to the anode. The functional layers of the MEA have the effect of improving the performance of the passive DMFC by decreasing the methanol and water crossover. In spite of the operation with 95 vol% methanol, the MEA with multi-layer electrodes for high concentration methanol DMFCs shows a maximum power density of 35.1 mW cm⁻² and maintains a high power density of 30 mW cm⁻² (0.405 V) under constant current operation.     

Bubble behaviors in direct methanol fuel cell anode with parallel design

In this paper, the Volume of Fluid (VOF) method for tracking the gas-liquid interface is employed to investigate the carbon dioxide (CO₂) behaviors inside the anode of a direct methanol fuel cell (DMFC). The CO₂ bubble emergence processes from the catalyst layer (CL) to the gas diffusion layer (GDL) and then to the flow channels are studied with two different strategies. In the first strategy, the CL and GDL are modeled as a uniform porous layer; in the second strategy, they are modeled as a well-ordered-path GDL and a uniform CL. The simulation results show that the second modeling strategy can better capture and match the fundamental phenomena of CO₂ bubble formation and evolvement observed from the experiments inside a DMFC anode.     

Characteristics of water transport through the membrane in direct methanol fuel cells operating with neat methanol

The water required for the methanol oxidation reaction in a direct methanol fuel cell (DMFC) operating with neat methanol can be supplied by diffusion from the cathode to the anode through the membrane. In this work, we present a method that allows the water transport rate through the membrane to be in-situ determined. With this method, the effects of the design parameters of the membrane electrode assembly (MEA) and operating conditions on the water transport through the membrane are investigated. The experimental data show that the water flux by diffusion from the cathode to the anode is higher than the opposite flow flux of water due to electro-osmotic drag (EOD) at a given current density, resulting in a net water transport from the cathode to the anode. The results also show that thinning the anode gas diffusion layer (GDL) and the membrane as well as thickening the cathode GDL can enhance the water transport flux from the cathode to the anode. However, a too thin anode GDL or a too thick cathode GDL will lower the cell performance due to the increases in the water concentration loss at the anode catalyst layer (CL) and the oxygen concentration loss at the cathode CL, respectively.     

Passive direct methanol fuel cells fed with methanol vapor

A vapor fed passive direct methanol fuel cell (DMFC) is proposed to achieve a high energy density by using pure methanol for mobile applications. Vapor is provided from a methanol reservoir to the membrane electrode assembly (MEA) through a vaporizer, barrier and buffer layer. With a composite membrane of lower methanol cross-over and diffusion layers of hydrophilic nanomaterials, the humidity of the MEA was enhanced by water back diffusion from the cathode to the anode through the membrane in these passive DMFCs. The humidity in the MEA due to water back diffusion results in the supply of water for an anodic electrochemical reaction with a low membrane resistance. The vapor fed passive DMFC with humidified MEA maintained 20–25 mW cm⁻² power density for 360 h and performed with a 70% higher fuel efficiency and 1.5 times higher energy density when compared with a liquid fed passive DMFC.     

Low methanol-permeable polyaniline/Nafion composite membrane for direct methanol fuel cells

Protonated polyaniline (PANI) is directly polymerized on Nafion 117 (N117), forming a composite membrane, to act as a methanol-blocking layer to reduce the methanol crossover in the direct methanol fuel cell (DMFC), which is beneficial for the DMFC operating at high methanol concentration. The PANI layer grown on the N117 with a thickness of 100 nm has an electrical conductivity of 13.2 S cm⁻¹. The methanol permeability of the PANI/N117 membrane is reduced to 59% of that of the N117 alone, suggesting that the PANI/N117 can effectively reduce the methanol crossover in the DMFC. Comparison of membrane-electrode-assemblies (MEA) using the conventional N117 and the newly developed PANI/N117 composite shows that the PANI/N117-based MEA outputs higher power at high methanol concentration, while the output power of the N117-based MEA is reduced at high methanol concentration due to the methanol crossover. The maximum power density of the PANI/N117-based MEA at 60 °C is 70 mW cm⁻² at 6 M methanol solution, which is double the N117-based MEA at the same methanol concentration. The resistance of PANI/N117 composite membrane is reduced at elevated methanol concentration, due to the hydrogen bonding between methanol and PANI pushes the polymer chains apart. It is concluded that the PANI/N117-based MEA performs well at elevated methanol concentration, which is suitable for the long-term operation of the DMFC.     

Advanced air-breathing direct methanol fuel cells for portable applications

A novel MEA is fabricated to improve the performance of air-breathing direct methanol fuel cells. A diffusion barrier on the anode side is designed to control methanol transport to the anode catalyst layer and thus suppressing the methanol crossover. A catalyst coated membrane with a hydrophobic gas diffusion layer on the cathode side is employed to improve the oxygen mass transport. It is observed that the maximum power density of the advanced DMFC with 2 M methanol solution achieves 65 mW cm⁻² at 60 °C. The value is nearly two times more than that of a commercial MEA. At 40 °C, the power densities operating with 1 and 2 M methanol solutions are over 20 mW cm⁻² with a cell potential at 0.3 V.     

Performance of an integrated composite membrane electrode assembly in DMFC

We report here the performance of a metal-based integrated composite membrane electrode assembly (IC-MEA) in direct methanol fuel cell (DMFC). The IC-MEA integrates the multi-functions of a conventional MEA, gas diffusion layer (GDL) and current collector. It was fabricated by impregnating Nafion electrolyte into a sandwiched structure containing expanding-Polytetrafluoroethylene (e-PTFE) and porous titanium sheets and subsequently coating with catalyst layer and microporous layer (MPL). While operating with air and 2 M methanol under ambient pressure, the IC-MEA in DMFC can yield a maximum power density of 19 mW cm⁻² at 26 °C, higher than a in-house made Nafion 115 MEA under the same working conditions. The IC-MEAs has been successfully applied to planar multi-cell stacks.     

Development of an advanced MEA to use high-concentration methanol fuel in a direct methanol fuel cell system

Despite serious methanol crossover issues in Direct Methanol Fuel Cells (DMFCs), the use of high-concentration methanol fuel is highly demanded to improve the energy density of passive fuel DMFC systems for portable applications. In this paper, the effects of a hydrophobic anode micro-porous layer (MPL) and cathode air humidification are experimentally studied as a function of the methanol-feed concentration. It is found in polarization tests that the anode MPL dramatically influences cell performance, positively under high-concentration methanol-feed but negatively under low-concentration methanol-feed, which indicates that methanol transport in the anode is considerably altered by the presence of the anode MPL. In addition, the experimental data show that cathode air humidification has a beneficial effect on cell performance due to the enhanced backflow of water from the cathode to the anode and the subsequent dilution of the methanol concentration in the anode catalyst layer. Using an advanced membrane electrode assembly (MEA) with the anode MPL and cathode air humidification, we report that the maximum power density of 78 mW/cm² is achieved at a methanol-feed concentration of 8 M and cell operating temperature of 60 °C. This paper illustrates that the anode MPL and cathode air humidification are key factors to successfully operate a DMFC with high-concentration methanol fuel.     

Performance and design analysis of tubular-shaped passive direct methanol fuel cells

To achieve the maximum performance from a Direct Methanol Fuel Cell (DMFC), one must not only investigate the materials and configuration of the MEA layers, but also consider alternative cell geometries that produce a higher instantaneous power while occupying the same cell volume. In this work, a two-dimensional, two-phase, non-isothermal model was developed to investigate the steady-state performance and design characteristics of a tubular-shaped, passive DMFC. Under certain geometric conditions, it was found that a tubular DMFC can produce a higher instantaneous Volumetric Power Density than a planar DMFC. Increasing the ambient temperature from 20 to 40 °C increases the peak power density produced by the fuel cell by 11.3 mW cm⁻² with 1 M, 16.3 mW cm⁻² with 2 M, but by only 8.4 mW cm⁻² with 3 M methanol. The poor performance with 3 M methanol at a higher ambient temperature is caused by increased methanol crossover and significant oxygen depletion along the Cathode Transport Layer (CTL). For a 5 cm long tubular DMFC to maintain sufficient Oxygen transport, the thickness of the CTL must be greater than 1 mm for 1 M operation, greater than 5 mm for 2 M operation, and greater than 10 mm for 3 M or higher operation.     

Factors affecting methanol transport in a passive DMFC employing a porous carbon plate

The effect of the pore structure and thickness of the porous carbon plate, PCP, as well as the gas barrier thickness on the methanol transport and the performance of a passive DMFC under the different cell voltages of 0.1, 0.2 and 0.3 V using different methanol concentrations was investigated. As a result of the mass transfer restrictions by employing the PCP, high methanol concentrations over 20 M could be efficiently used to produce the relatively high power density of 30 mW cm⁻² for more than 10 h. The DMFC was operated under limiting current conditions in all the PCPs at 0.1 and 0.2 V to more than 20 M. The main factors for controlling the methanol transport were the barrier of the gas layer with CO₂, which was formed between the anode surface and the PCP and the properties of the PCP. At the low current densities of less than 60 mA cm⁻², when no CO₂ bubbles are emitted, both the pore structure and thickness of the PCP did not affect the methanol transport and the current voltage relationship. At the higher current densities, CO₂ bubbles were evolved through the PCP and different resistances to the methanol transport were observed depending on the PCP pore structure and thickness. The CO₂ gas layer between the MEA and the PCP caused a major resistivity for the methanol transport, and its resistivity increased with its thickness increasing. By using the PCP at 0.1 V, the energy density of the passive DMFC was significantly increased, e.g., more than seven times.     

Niobium diffusion modified austenitic stainless steel bipolar plate for direct methanol fuel cell

An austenitic stainless steel with a niobium diffusion protective layer is evaluated for bipolar plate of direct methanol fuel cell (DMFC). Corrosion resistance of niobium diffusion modified AISI 304 stainless steel (niobized 304 SS) is investigated in simulated DMFC cathodic environment (0.5 M H₂SO₄ + 2 ppm HF + 0.1 M methanol solution at 50 °C) and anodic environment (0.5 M H₂SO₄ + 2 ppm HF + (1 M, 10 M and 20 M) methanol solution at 50 °C), respectively. Potentiodynamic, potentiostatic as well as electrochemical impedance spectroscopy tests show that, comparing with a bare 304 SS, the corrosion current density of niobized 304 SS is reduced greatly while the polarization resistance is raised in the simulated DMFC cathodic environment. Corrosion tests in the simulated anodic environment are applied to examine the effect of methanol on the corrosion behaviour of niobized 304 SS. It is interesting to find that the niobized 304 SS shows better corrosion resistance in the higher methanol concentration solutions.     

Effect of the porous carbon layer in the cathode gas diffusion media on direct methanol fuel cell performances

The effect of cathode gas diffusion media with microporous layers (MPLs) on direct methanol fuel cell (DMFC) performances is studied by combining electrochemical analysis and physicochemical investigation. The membrane electrode assemblies (MEAs) using MPL-modified cathode gas diffusion layers (GDLs, GDL-1) showed slightly better performances (117 mW cm⁻²) at 0.4 V and 70 °C than commercial GDL (SIGRACET^® product version: GDL-35BC, SGL Co.) DMFC MEAs (110 mW cm⁻²). This might be due to high gas permeability, uniform pore distributions, and low water transport coefficient including methanol crossover. For GDL-1, the air permeability was 31.0 cm³ cm⁻² s⁻¹, while the one for SGL 35BC GDLs was 21.7 cm³ cm⁻² s⁻¹. Also, the GDL-1 in the pore-size distribution diagrams had distinct peaks due to more uniform distributions of macropores and micropores with smaller holes between aggregates of carbon particles compared to GDL-35 BC as confirmed by SEM images. Furthermore, the MEA using GDL-1 for the cathode had a lower water transfer coefficient compared to an MEA with a commercial 35 BC GDL.     

Suppressing methanol crossover with a deposited quaternary Pt-based catalyst on the Nafion surface

A Pt₄₉–Ru₃₅–Ir₆–Os₁₀ alloy layer is deposited on the Nafion membrane surface using the impregnation-reduction (IR) method to mitigate methanol crossover. The methanol crossover in a membrane electrode assembly (MEA) with a deposited Pt–Ru–Ir–Os layer is compared with a MEA without any layer on the proton exchange membrane (PEM). The deposited Pt₄₉–Ru₃₅–Ir₆–Os₁₀ layer functions like a catalytically active layer, a methanol barrier, and an electrode all at the same time. This layer yields up to a 30% suppression of methanol crossover and a 15% improvement in fuel cell voltage performance (@170 mA cm⁻²) at 80 °C. The porous metal alloy layer with a high surface area of the Pt–Ru layer suppresses methanol crossover by the catalytic activity of the deposited layer. The presence of the solid Pt₄₉–Ru₃₅–Ir₆–Os₁₀ layer on the Nafion membrane surface reduces the proton conductivity of the PEM (from 10.75 to 4.22 mS cm⁻¹), and degrades the output of the cell voltage performance (from 0.350 to 0.335 V at 90 mA cm⁻² of current density) at 60 °C, even though methanol crossover is reduced (from 6928 ppm to 4415 ppm (CO₂ concentration at cathode exhaust is proportional to methanol crossover)).     

Development of micro direct methanol fuel cells for high power applications

A micro direct methanol fuel cell (μDMFC) with active area of 1.625 cm² has been developed for high power portable applications and its electrochemical characterization carried out in this study. The fragility of the silicon wafer makes it difficult to compress the cell for good sealing and hence to reduce contact resistance in the Si-based μDMFC. We have instead used very thin stainless steel plates as bipolar plates with the flow field machined by photochemical etching technology. For both anode and cathode flow fields, widths of both the channel and rib were 750 μm, with a channel depth of 500 μm. A gold layer was deposited on the stainless steel plate to prevent corrosion. This study used an advanced MEA developed in-house featuring a modified anode backing structure with a compact microporous layer. Maximum power density of the micro DMFC reached 62.5 mW cm⁻² at 40 °C, and 100 mW cm⁻² at 60 °C at atmospheric pressure, which almost doubled the performance of our previous Si-based μDMFC.     

The influence of anode gas diffusion layer on the performance of low-temperature DMFC

The influence of the anode gas diffusion layers (GDLs) on the performances of low-temperature DMFCs, and the properties of mass transport and CO₂ removal on these anode GDLs were investigated. The membrane electrode assembly (MEA) based on the hydrophilic anode GDL, which consisted of the untreated carbon paper and hydrophilic anode micro-porous layer (comprised carbon black and 10 wt.% Nafion), showed the highest power density of 13.4 mW cm⁻² at 30 °C and ambient pressure. The performances of the MEAs tended to decline with the increase of the PTFE content in the anode GDLs due to the difficulty of methanol transport. The contact angle measurements revealed that the wettabilities of the anode GDLs decreased as the increase of PTFE content. The wettabilities of the GDLs were improved by addition of hydrophilic Nafion ionomer to the GDLs. From the visualizations of CO₂ gas bubbles dynamics on the anodes using a transparent cell, it was observed that uniform CO₂ gas bubbles with smaller size formed on hydrophilic anode GDLs. And bubbles with larger size were not uniform over the hydrophobic anode GDLs. It was believed that adding PTFE to the anode GDL was not helpful for improving the CO₂ gas transport in the anode GDL of the low-temperature DMFC.     

Improved fuel use efficiency in microchannel direct methanol fuel cells using a hydrophilic macroporous layer

We demonstrate state-of-the-art room temperature operation of silicon microchannel-based micro-direct methanol fuel cells (μDMFC) having a very high fuel use efficiency of 75.4% operating at an output power density of 9.25 mW cm⁻² for an input fuel (3 M aqueous methanol solution) flow rate as low as 0.55 μL min⁻¹. In addition, an output power density of 12.7 mW cm⁻² has been observed for a fuel flow rate of 2.76 μL min⁻¹. These results were obtained via the insertion of novel hydrophilic macroporous layer between the standard hydrophobic carbon gas diffusion layer (GDL) and the anode catalyst layer of a μDMFC; the hydrophilic macroporous layer acts to improve mass transport, as a wicking layer for the fuel, enhancing fuel supply to the anode at low flow rates. The results were obtained with the fuel being supplied to the anode catalyst layer via a network of microscopic microchannels etched in a silicon wafer.     

Double microporous layer cathode for membrane electrode assembly of passive direct methanol fuel cells

A novel membrane electrode assembly (MEA) is described that utilizes a double microporous layer (MPL) structure in the cathode of a passive direct methanol fuel cell (DMFC). The double MPL cathode uses Ketjen Black carbon as an inner-MPL and Vulcan XC-72R carbon as an outer-MPL. Experimental results indicate that this double MPL structure at the cathode provides not only a higher oxygen transfer rate, but enables more effective back diffusion of water; thus, leading to an improved power density and stability of the passive DMFC. The maximum power density of an MEA with a double MPL cathode was observed to be ca. 33.0 mW cm⁻², which is found to be a substantial improvement over that for a passive DMFC with a conventional MEA. A. C. impedance analysis suggests that the increased performance of a DMFC with the double MPL cathode might be attributable to a decreased charge transfer resistance for the cathode oxygen reduction reaction.     

Sulfonated poly(ether ether ketone) as an ionomer for direct methanol fuel cell electrodes

Sulfonated poly(ether ether ketone) has been investigated as an ionomer in the catalyst layer for direct methanol fuel cells (DMFC). The performance in DMFC, electrochemical active area (by cyclic voltammetry), and limiting capacitance (by impedance spectroscopy) have been evaluated as a function of the ion exchange capacity (IEC) and content (wt.%) of the SPEEK ionomer in the catalyst layer. The optimum IEC value and SPEEK ionomer content in the electrodes are found to be, respectively, 1.33 meq. g⁻¹ and 20 wt.%. The membrane-electrode assemblies (MEA) fabricated with SPEEK membrane and SPEEK ionomer in the electrodes are found to exhibit superior performance in DMFC compared to that fabricated with Nafion ionomer due to lower interfacial resistance in the MEA as well as larger electrochemical active area. The MEAs with SPEEK membrane and SPEEK ionomer also exhibit better performance than that with Nafion 115 membrane and Nafion ionomer due to lower methanol crossover and better electrode kinetics.