Optimization of Nafion content in Nafion–polyaniline nano-composite modified cathodes for PEMFC application

In this research several Nafion–Polyaniline nano-composite modified cathodes have been fabricated and evaluated in oxygen reduction reaction (ORR) in order to use in proton exchange membrane fuel cell (PEMFC). Modified cathodes made by the wide range of Nafion content (from 0 to 1.6 mg cm⁻²) and investigated in the acidic solution by different electrochemical techniques at 25 °C. The results indicate the activity of the modified electrodes is increased by employing of Nafion–Polyaniline nano-composite in the reaction layer, but there is an optimum value for Nafion content in the catalyst layer. The modified electrode impregnated by 0.4 mg cm⁻² of Nafion shows the highest activity. Analysis of the surface morphology of the Nafion–polyaniline modified electrodes by scanning electron microscopy and electrochemical data reveal that the existence of polyaniline (PANI) nanofibers in the catalyst layer before adding Nafion solution, improves the homogeneity distribution of the ionomer in catalyst layer, change the morphology of electrode and increase the performance of gas diffusion electrodes (GDEs) in oxygen reduction reaction.     

Performance improvement by a glue-functioned Nafion layer coating on gas diffusion electrodes in PEM fuel cells

To reduce the performance difference of membrane electrode assembles (MEAs) between catalyst coated membrane (CCM) and gas diffusion electrode (GDE) methods, this study presents a novel structure of a glue-functioned Nafion layer coating on the catalyst layer of GDEs to enhance performance. The process of hot pressing is omitted from the membrane electrode assembly (MEA) fabrication in this study. In exploring the effect of the glue-functioned Nafion layer, five MEAs are compared, including one made by the CCM method and four by the GDE method. Loadings of 0, 0.1, 0.3 and 0.5 mg cm⁻² of glue-functioned Nafion layer are coated on the surface of the catalyst layer at room temperature with a condensed Nafion solution (20 wt%). The performance of the 0.3 mg cm⁻² Nafion layer coating improves 55% compared with that without an extra Nafion layer for the peak power density, and the performance difference reduces from 61.2% to 39.4% when compared with the one using the CCM method. However, the performance of the 0.5 mg cm⁻² Nafion layer coating is almost the same as without the Nafion layer. This indicates that increased Nafion cannot guarantee higher performance.     

Study on the membrane electrode assembly fabrication with carbon supported cobalt triethylenetetramine as cathode catalyst for proton exchange membrane fuel cell

An improved fabrication technique for conventional hot-pressed membrane electrode assemblies (MEAs) with carbon supported cobalt triethylenetetramine (CoTETA/C) as the cathode catalyst is investigated. The V-I results of PEM single cell tests show that addition of glycol to the cathode catalyst ink leads to significantly higher electrochemical performance and power density than the single cell prepared by the traditional method. SEM analysis shows that the MEAs prepared by the conventional hot-pressed method have cracks between the cathode catalyst layer and Nafion membrane, and the contact problem between cathode catalyst layer and Nafion membrane is greatly suppressed by addition of glycol to the cathode catalyst ink. Current density-voltage curve and impedance studies illuminate that the MEAs prepared by adding glycol to the cathode catalyst ink have a higher electrochemical surface area, lower cell ohmic resistance, and lower charge transfer resistance. The effects of CoTETA/C loading, Nafion content, and Pt loading are also studied. By optimizing the preparation parameters of the MEA, the as-fabricated cell with a Pt loading of 0.15 mg cm⁻² delivers a maximum power density of 181.1 mW cm⁻², and a power density of 126.2 mW cm⁻² at a voltage of 0.4 V.     

Experimental study of commercial size proton exchange membrane fuel cell performance

Commercial sized (16 × 16 cm² active surface area) proton exchange membrane (PEM) fuel cells with serpentine flow chambers are fabricated. The GORE-TEX® PRIMEA 5621 was used with a 35-μm-thick PEM with an anode catalyst layer with 0.45 mg cm⁻² Pt and cathode catalyst layer with 0.6 mg cm⁻² Pt and Ru or GORE-TEX® PRIMEA 57 was used with an 18-μm-thick PEM with an anode catalyst layer at 0.2 mg cm⁻² Pt and cathode catalyst layer at 0.4 mg cm⁻² of Pt and Ru. At the specified cell and humidification temperatures, the thin PRIMEA 57 membrane yields better cell performance than the thick PRIMEA 5621 membrane, since hydration of the former is more easily maintained with the limited amount of produced water. Sufficient humidification at both the cathode and anode sides is essential to achieve high cell performance with a thick membrane, like the PRIMEA 5621. The optimal cell temperature to produce the best cell performance with PRIMEA 5621 is close to the humidification temperature. For PRIMEA 57, however, optimal cell temperature exceeds the humidification temperature.     

Structure and electrocatalytic performance of carbon-supported platinum nanoparticles

The structure of Pt nanoparticles and the composition of the catalyst-Nafion films strongly determine the performance of proton exchange membrane fuel cells. The effect of Nafion content in the catalyst ink, prepared with a commercially available carbon-supported Pt, in the kinetics of the hydrogen oxidation reaction (HOR), has been studied by the thin layer rotating disk electrode technique. The kinetic parameters have been related to the catalyst nanoparticles structure, characterized by X-ray diffraction and high-resolution transmission electron microscopy. The size-shape analysis is consistent with the presence of 3D cubo-octahedral Pt nanoparticles with average size of 2.5 nm. The electrochemically active surface area, determined by CO stripping, appears to depend on the composition of the deposited Pt/C-Nafion film, with a maximum value of 73 m² g_Pt⁻¹ for 30 wt.% Nafion. The results of CO stripping indicate that the external Pt faces are mainly (1 0 0) and (1 1 1) terraces, thus confirming the cubo-octahedral structure of nanoparticles. Cyclic voltammetry combined with the RDE technique has been applied to study the kinetic parameters of HOR besides the ionomer resistance effect on the anode kinetic current at different ionomer contents. The kinetic parameters show that H₂ oxidation behaves reversibly with an estimated exchange current density of 0.27 mA cm⁻².     

Contribution of Nafion loading to the activity of catalysts and the performance of PEMFC

Different amounts of Nafion loadings (in the range of 0–2.0 mg cm⁻²) were added to a catalyst containing 0.5 mg cm⁻² of Pt; these were prepared by spraying a Nafion solution on an electrode surface. The effect of Nafion loading on the activity of the catalyst and the performance of a proton exchange membrane fuel cell (PEMFC) was investigated by using electrochemical methods such as direct current polarization (using an I–V curve), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and linear scan voltammetry (LSV). The results of the I–V and EIS were compared in order to resolve the ohmic resistance (R_Ω, calculated from the I–V curve) into interfacial and internal resistances (R_if and R_s, simulated from the EIS). The analysis of the electrochemical data revealed that the interfacial resistance (R_if) is closely related to the reactive region of three-phase zones (interfaces among the reactants, electrolyte and catalyst), and it provides a major parameter for diagnosing the activity of the catalysts and the performance of the PEMFC.     

Structured multilayered electrodes of proton/electron conducting polymer for polymer electrolyte membrane fuel cells assembled by spray coating

Membrane electrode assemblies (MEAs) for fuel cell applications consist of electron conductive support materials, proton conductive ionomer, and precious metal nanoparticles to enhance the catalytic activity towards H₂ oxidation and O₂ reduction. An optimized connection of all three phases is required to obtain a high noble metal utilization, and accordingly a good performance. Using polyaniline (PANI) as an alternative support material, the generally used ionomer Nafion^® could be replaced in the catalyst layer. PANI has the advantage to be electron and proton conductive at the same time, and can be used as a catalyst support as well. In this study, a new technique building up alternating layers of PANI supported catalyst and single-walled carbon nanotubes (SWCNT) supported catalyst is introduced. Multilayers of PANI and SWCNT catalysts are used on the cathode side, whereas the anode side is composed of commercial platinum/carbon black catalyst and Nafion^®, applied by an airbrush. No additional Nafion^® ionomer is used for proton conductivity of the cathode. The so called spray coating method results in high power densities up to 160 mW cm⁻² with a Pt loading of 0.06 mg cm⁻² at the cathode, yielding a Pt utilization of 2663 mW mg_Pt⁻¹. As well as PANI, supports of SWCNTs have the advantage to have a fibrous structure and additional, they provide high electron conductivity. The combination of the new technique and the fibrous 1-dimensional support materials leads to a porous 3-dimensional electrode network which could enhance the gas transport through the electrode as well as the Pt utilization. The spray coating method could be upgraded to an in-line process and is not restricted to batch production.     

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.     

Performance of a miniaturized silicon reformer-PrOx-fuel cell system

A fuel cell made with silicon is operated with hydrogen supplied by a reformer and a preferential oxidation (PrOx) reactor those are also made with silicon. The performance and durability of the fuel cell is analyzed and tested, then compared with the results obtained with pure hydrogen. Three components of the system are made using silicon technologies and micro electro-mechanical system (MEMS) technology. The commercial Cu-ZnO-Al₂O₃ catalyst for the reformer and the Pt-Al₂O₃ catalyst for the PrOx reactor are coated by means of a fill-and-dry method. A conventional membrane electrode assembly composed of a 0.375 mg cm⁻² PtRu/C catalyst for the anode, a 0.4 mg cm⁻² Pt/C catalyst for the cathode, and a Nafion™ 112 membrane is introduced to the fuel cell. The reformer gives a 27 cm³ min⁻¹ gas production rate with 3177 ppm CO concentration at a 1 cm³ h⁻¹ methanol feed rate and the PrOx reactor shows almost 100% CO conversion under the experimental conditions. Fuel cells operated with this fuel-processing system produce 230 mW cm⁻² at 0.6 V, which is similar to that obtained with pure hydrogen.     

Optimization of the performance of polymer electrolyte fuel cell membrane-electrode assemblies: Roles of curing parameters on the catalyst and ionomer structures and morphology

In order to understand the origin of performance variations in polymer electrolyte membrane fuel cells (PEMFCs), a series of membrane-electrode assemblies (MEAs) with identical electrode layer compositions were prepared using different electrode curing conditions, their performances were evaluated, and their morphologies determined by scanning electron microscopy (SEM). The polarization curves varied markedly primarily due to differences in morphologies of electrodes, which were dictated by the curing processes. The highest performing MEAs (1.46 W cm⁻² peak power density at 3.2 A cm⁻² and 80 °C) were prepared using a slow curing process at a lower temperature, whereas those MEAs prepared using a faster curing process performed poorly (0.1948 W cm⁻² peak power density at 440 mA cm⁻² and 80 °C). The slowly cured MEAs showed uniform electrode catalyst and ionomer distributions, as revealed in SEM images and elemental maps. The relatively faster cured materials exhibited uneven distribution of ionomer with significant catalyst clustering. Collectively, these results indicate that to achieve optimal performance, factors that affect the dynamics of the curing process, such as rate of solvent evaporation, must be carefully controlled to avoid solvent trapping, minimize catalyst coagulation, and promote even distribution of ionomer.     

Platinum-sputtered electrode based on blend of carbon nanotubes and carbon black for polymer electrolyte fuel cell

A novel Pt-sputtered electrode based on a blend layer of carbon black (CB) and carbon nanotubes (CNTs) is developed for polymer electrolyte fuel cells. The Pt is sputtered on the surface of the blend to form a catalyst layer. The CNTs generate a pore in the blend layer, and the CB provides a high surface roughness for the blend layer. At a CNT content of 50 wt.%, the maximum value (20.6 m² g⁻¹) for the electrochemical area of the Pt is obtained, which indicates that the surface area of the blend layer exposed for Pt deposition is the largest. The power density of a membrane-electrode assembly (MEA) employing the Pt-sputtered electrodes shows a linear increase with electrochemical area. The mass activity of the optimized Pt-sputtered electrode with a Pt loading of 0.05 mg cm⁻² is 8.1 times that of an electrode with a Pt loading of 0.5 mg cm⁻² prepared using a conventional screen-printing technique. Excellent mass transfer is obtained with the Pt-sputtered electrode.     

Performance of an ultra-low platinum loading membrane electrode assembly prepared by a novel catalyst-sprayed membrane technique

Membrane electrode assemblies (MEAs) with ultra-low platinum loadings are attracting significant attention as one method of reducing the quantity of precious metal in polymer electrolyte membrane fuel cells (PEMFCs) and thereby decreasing their cost, one of the key obstacles to the commercialization of PEMFCs. In the present work, high-performance MEAs with ultra-low platinum loadings are developed using a novel catalyst-sprayed membrane technique. The platinum loadings of the anode and cathode are lowered to 0.04 and 0.12 mg cm⁻², respectively, but still yield a high performance of 0.7 A cm⁻² at 0.7 V. The influence of Nafion content, cell temperature, and back pressures of the reactant gases are investigated. The optimal Nafion content in the catalyst layer is ca. 25 wt.%. This is significantly lower than for low platinum loading MEAs prepared by other methods, indicating ample interfacial contact between the catalyst layer and membrane in our prepared MEAs. Scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS) measurements reveal that our prepared MEA has very thin anode and cathode catalyst layers that come in close contact with the membrane, resulting in a MEA with low resistance and reduced mass transport limitations.     

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.     

Synthesis and characterization of carbon nanotubes supported platinum nanocatalyst for proton exchange membrane fuel cells

Multi-walled carbon nanotubes (MWCNTs) were used as catalyst support for depositing platinum nanoparticles by a wet chemistry route. MWCNTs were initially surface modified by citric acid to introduce functional groups which act as anchors for metallic clusters. A two-phase (water-toluene) method was used to transfer PtCl₆²⁻ from aqueous to organic phase and the subsequent sodium formate solution reduction step yielded Pt nanoparticles on MWCNTs. High-resolution TEM images showed that the platinum particles in the size range of 1-3 nm are homogeneously distributed on the surface of MWCNTs. The Pt/MWCNTs nanocatalyst was evaluated in the proton exchange membrane (PEM) single cell using H₂/O₂ at 80 °C with Nafion-212 electrolyte. The single PEM fuel cell exhibited a peak power density of about 1100 mW cm⁻² with a total catalyst loading of 0.6 mg Pt cm⁻² (anode: 0.2 mg Pt cm⁻² and cathode: 0.4 mg Pt cm⁻²). The durability of Pt/MWCNTs nanocatalyst was evaluated for 100 h at 80 °C at ambient pressure and the performance (current density at 0.4 V) remained stable throughout. The electrochemically active surface area (64 m² g⁻¹) as estimated by cyclic voltammetry (CV) was also similar before and after the durability test.     

Effect of platinum amount in carbon supported platinum catalyst on performance of polymer electrolyte membrane fuel cell

The performance of polymer electrolyte membrane fuel cells fabricated with different catalyst loadings (20, 40 and 60 wt.% on a carbon support) was examined. The membrane electrode assembly (MEA) of the catalyst coated membrane (CCM) type was fabricated without a hot-pressing process using a spray coating method with a Pt loading of 0.2 mg cm⁻². The surface was examined using scanning electron microscopy. The catalysts with different loadings were characterized by X-ray diffraction and cyclic voltammetry. The single cell performance with the fabricated MEAs was evaluated and electrochemical impedance spectroscopy was used to characterize the fuel cell. The best performance of 742 mA cm⁻² at a cell voltage of 0.6 V was obtained using 40 wt.% Pt/C in both the anode and cathode.     

Development of high performance carbon composite catalyst for oxygen reduction reaction in PEM Proton Exchange Membrane fuel cells

Highly active and stable carbon composite catalysts for oxygen reduction in PEM fuel cells were developed through the high-temperature pyrolysis of Co–Fe–N chelate complex, followed by the chemical post-treatment. A metal-free carbon catalyst was used as the support. The carbon composite catalyst showed an onset potential for oxygen reduction as high as 0.87 V (NHE) in H₂SO₄ solution, and generated less than 1% H₂O₂. The PEM fuel cell exhibited a current density as high as 0.27 A cm⁻² at 0.6 V and 2.3 A cm⁻² at 0.2 V for a catalyst loading of 6.0 mg cm⁻². No significant performance degradation was observed over 480 h of continuous fuel cell operation with 2 mg cm⁻² catalyst under a load of 200 mA cm⁻² as evidenced by a resulting cell voltage of 0.32 V with a voltage decay rate of 80 μV h⁻¹. Materials characterization studies indicated that the metal–nitrogen chelate complexes decompose at high pyrolysis temperatures above 800 °C, resulting in the formation of the metallic species. During the pyrolysis, the transition metals facilitate the incorporation of pyridinic and graphitic nitrogen groups into the carbon matrix, and the carbon surface doped with nitrogen groups is catalytically active for oxygen reduction.     

Characterization and single cell testing of Pt/C electrodes prepared by electrodeposition

Electrodes for proton exchange membrane fuel cells (PEMFC) have been prepared by the electrodeposition method. For this task, the electrodeposition of platinum is carried out on a carbon black substrate impregnated with an ionomer, proton conducting, medium. Before electrodeposition, the substrate is submitted to an activation process to increase the hydrophilic character of the surface to a few microns depth.Electrodeposition of platinum takes place inside the generated surface hydrophilic layer, resulting in a continuous phase covering totally or partially carbon substrate grains. Cross sectional images show a decay profile of platinum towards the interior of the substrate, reflecting a deposition process limited by diffusion of PtCl₆²⁻ through the porous substrate. Electrodes with different platinum loads have been prepared, and membrane electrode assemblies (MEA) have been mounted with the electrodeposited electrodes as cathode and other standard components (commercial anode and Nafion^R 117 membrane). The electrochemically active surface area determined from hydrogen underpotential deposition charge, is lower on the electrodeposited electrodes than on standard electrodes. However, single cell testing shows higher mass specific activity on electrodeposited cathodes with low and intermediate Pt load (below 0.05 mg Pt cm⁻²).     

Characterization of direct methanol fuel cell (DMFC) applications with H2SO4 modified chitosan membrane

Chitosan (Chs) flakes were prepared from chitin materials that were extracted from the exoskeleton of Cape rock lobsters in South Africa. The Chs flakes were prepared into membranes and the Chs membranes were modified by cross-linking with H₂SO₄. The cross-linked Chs membranes were characterized for the application in direct methanol fuel cells. The Chs membrane characteristics such as water uptake, thermal stability, proton resistance and methanol permeability were compared to that of high performance conventional Nafion 117 membranes. Under the temperature range studied 20-60 °C, the membrane water uptake for Chs was found to be higher than that of Nafion. Thermal analysis revealed that Chs membranes could withstand temperature as high as 230 °C whereas Nafion 117 membranes were stable to 320 °C under nitrogen. Nafion 117 membranes were found to exhibit high proton resistance of 284 s cm⁻¹ than Chs membranes of 204 s cm⁻¹. The proton fluxes across the membranes were 2.73 mol cm⁻² s⁻¹ for Chs- and 1.12 mol cm⁻² s⁻¹ Nafion membranes. Methanol (MeOH) permeability through Chs membrane was less, 1.4 × 10⁻⁶ cm² s⁻¹ for Chs membranes and 3.9 × 10⁻⁶ cm² s⁻¹ for Nafion 117 membranes at 20 °C. Chs and Nafion membranes were fabricated into membrane electrode assemblies (MAE) and their performances measure in a free-breathing commercial single cell DMFC. The Nafion membranes showed a better performance as the power density determined for Nafion membranes of 0.0075 W cm⁻² was 2.7 times higher than in the case of Chs MEA.     

Fabrication and impedance studies of DMFC anode incorporated with CNT-supported high-metal-content electrocatalyst

In this study, the fabrication of a direct methanol fuel cell (DMFC) anode with the incorporation of a multiwalled carbon nanotube (CNT)-supported high-metal-content Pt/Ru electrocatalyst, i.e., 40 wt%Pt-20 wt%Ru/CNT, using a novel approach and the resultant DMFC performances were investigated. Employing a vacuum filtration method, we were able to successfully fabricate the DMFC anode with a good electrode structure using an in-house prepared Pt-Ru/CNT electrocatalyst. The catalyst layer was formed directly on a Teflon-treated carbon cloth having a buckypaper texture with a catalyst loading of 4.0 mg cm⁻². From single-cell tests, excellent cell performances were obtained. At 80 °C, the power density was found to be as high as >100 mW cm⁻². This can be attributed to a thinner catalyst layer formed with a more efficient utilization of the catalyst than that using a low-metal-content counterpart, i.e., 20 wt%Pt-10 wt%Ru/CNT, as reported in an earlier study. However, the Nafion^® ionomer content in the catalyst layer played a key role in the anode fabrication to obtain a good cell performance. In addition, the electrochemical impedance spectroscopy (EIS) with a constant phase element (CPE)-based equivalent-circuit model was employed to analyze the fabricated anode. It distinctively revealed some specific characteristics in the resistances and the interface properties. Overall, the obtained impedance results are somewhat different from those of a conventional DMFC anode with the catalyst layer coated onto a porous gas diffusion layer (GDL) on a carbon backing material. Based on the experimental results and the impedance analyses, the high-metal-content Pt-Ru/CNT catalyst was found to be much more favorable and suitable for use as a DMFC anode catalyst.     

Performance of laboratory polymer electrolyte membrane hydrogen generator with sputtered iridium oxide anode

The continuous improvement of the anode materials constitutes a major challenge for the future commercial use of polymer electrolyte membranes (PEM) electrolyzers for hydrogen production. In accordance to this direction, iridium/titanium films deposited directly on carbon substrates via magnetron sputtering are operated as electrodes for the oxygen evolution reaction interfaced with Nafion 115 electrolyte in a laboratory single cell PEM hydrogen generator. The anode with 0.2 mg cm⁻² Ir catalyst loading was electrochemically activated by cycling its potential value between 0 and 1.2 V (vs. RHE). The water electrolysis cell was operated at 90 °C with current density 1 A cm⁻² at 1.51 V without the ohmic contribution. The corresponding current density per mgr of Ir catalyst is 5 A mg⁻¹. The achieved high efficiency is combined with sufficient electrode stability since the oxidation of the carbon substrate during the anodic polarization is almost negligible.