Utilization of the graphene aerogel as PEM fuel cell catalyst support: Effect of polypyrrole (PPy) and polydimethylsiloxane (PDMS) addition

In this study, three-dimensional (3D) graphene aerogel (GA) was synthesized by a self-assembly hydrothermal process as a PEM fuel cell catalyst support. The synthesized GA was also modified with the polypyrrole (PPy) by in-situ chemical oxidative polymerization of the pyrrole monomer (PPy-GA). The electrocatalytic performance of the platinum (Pt) nanoparticles (NPs) supported with both GA and PPy-GA materials towards oxygen reduction reaction (ORR) was investigated. In addition, the hydrophobic polydimethylsiloxane (PDMS) polymer was added to the catalyst ink media in order to enhance the hydrophobic property and durability of the synthesized GA and PPy-GA supported Pt catalysts. Pt NPs were decorated over the support materials with the microwave irradiation technique. Various characterization techniques such as FTIR, Raman Spectroscopy, BET, SEM, EDX, TEM, TGA, contact angle measurements, 3D topography images and four-point probe electrical conductivity measurements were performed in order to analyze the GA based support materials. Electrochemical characterizations were also carried out with cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements. It was observed that PDMS addition to the catalyst ink media increased the electrocatalytic activity and durability of the GA supported Pt catalyst. Otherwise, the performance of the PPy modified GA supported Pt catalyst was negatively affected by the addition of PDMS to the catalyst media.     

Synthesis and characterization of polypyrrole/carbon composite as a catalyst support for fuel cell applications

Polypyrrole (PPy)/Carbon composites were synthesized by in situ chemical oxidative polymerization of pyrrole monomer on carbon black. Effects of polymerization temperature (either 0 °C or 25 °C) and different dopants including p-toluenesulfonic acid (p-TSA) and sodium dodecyl sulfate (SDS) on the properties of the composites were investigated. The synthesized composites were characterized by XRD, FTIR and TGA. Electrical conductivities of the composites were determined by using four-point probe technique. Electrochemical oxidation characteristics of the synthesized PPy/Carbon composites were investigated by cyclic voltammetry via potential holding experiments. The PPy/Carbon composites synthesized at 0 °C and with p-TSA as dopant showed the best oxidation resistance than carbon and other composites.     

Use of cellulose-based carbon aerogels as catalyst support for PEM fuel cell electrodes: Electrochemical characterization

New nanostructured carbons have been developed through pyrolysis of organic aerogels, based on supercritical drying of cellulose acetate gels. These cellulose acetate-based carbon aerogels (CA) are activated by CO₂ at 800 °C and impregnated by PtCl₆²⁻; the platinum salt is then chemically or electrochemically reduced. The resulting platinized carbon aerogels (Pt/CA) are characterized with transmission electron microscopy (TEM) and electrochemistry. The active area of platinum is estimated from hydrogen adsorption/desorption or CO-stripping voltammetry: it is possible to deposit platinum nanoparticles onto the cellulose acetate-based carbon aerogel surface in significant proportions. The oxygen reduction reaction (ORR) kinetic parameters of the Pt/CA materials, determined from quasi-steady-state voltammetry, are comparable with that of Pt/Vulcan XC72R. These cellulose acetate-based carbon aerogels are thus promising electrocatalyst support for PEM application.     

Investigation of the effect of graphitized carbon nanotube catalyst support for high temperature PEM fuel cells

In this study, it is aimed to investigate the graphitization effect on the performance of the multi walled carbon nanotube catalyst support for high temperature proton exchange membrane fuel cell (HT-PEMFC) application. Microwave synthesis method was selected to load Pt nanoparticles on both CNT materials. Prepared catalyst was analyzed thermal analysis (TGA), Transmission Electron Microscopy (TEM) and corrosion tests. TEM analysis proved that a distribution of Pt nanoparticles with a size range of 2.8–3.1 nm was loaded on the Pt/CNT and Pt/GCNT catalysts. Gas diffusion electrodes (GDE) were manufactured by an ultrasonic spray method with synthesized catalyst. Polybenzimidazole (PBI) membrane based Membrane Electrode Assembly (MEA) was prepared for observe the performance of the prepared catalysts. The synthesized catalysts were also tested in a HT-PEMFC environment with a 5 cm² active area at 160 °C without humidification. This study demonstrates the feasibility of using the microwave synthesis method as a fast and effective method for preparing high performance Pt/CNT and Pt/GCNT catalyst for HT-PEMFC. The HT-PEMFC performance evaluation shows current densities of 0.36 A/cm²0.30 A/cm² and 0.20 A/cm² for the MEAs prepared with Pt/GCNT, Pt/CNT and Pt/C catalysts @ 0.6 V operating voltage, respectively. AST (Accelerated Stress Test) analyzes of MEAs prepared with Pt/GCNT and Pt/CNT catalysts were also performed and compared with Pt/C catalyst. According to current density @ 0.6 V after 10,000 potential cycles, Pt/GCNT, Pt/CNT and Pt/C catalysts can retain 61%, 67% and 60% of their performance, respectively.     

Polypyrrole-modified hydrophobic carbon nanotubes as promising electrocatalyst supports in polymer electrolyte membrane fuel cells

As an alternative to oxidative acid treatment, a hydrophobic graphitized carbon nanotube (CNT) was functionalized with 1-4 nm thick polypyrrole (PPy) prior to application as catalyst supports in polymer electrolyte membrane (PEM) fuel cells. Unlike oxidative acid treatment, the PPy coating method converts the hydrophobic surface of a CNT to a hydrophilic one without creating defects on the surface of the CNT. As a result, Pt nanoparticles deposited on the PPy-coated CNTs showed an improved distribution, which significantly enhanced the fuel cell performance while preserving the intrinsic properties of the CNTs, i.e., resistance to electrochemical carbon corrosion. An additional advantage of PPy coating is that it prevents Pt nanoparticles from agglomerating on the CNT surface. These results indicated that PPy-coated CNTs are a promising catalyst support to improve both the performance and durability of PEM fuel cells.     

Influence of carbon nanofiber properties as electrocatalyst support on the electrochemical performance for PEM fuel cells

Novel carbonaceous supports for electrocatalysts are being investigated to improve the performance of polymer electrolyte fuel cells. Within several supports, carbon nanofibers blend two properties that rarely coexist in a material: a high mesoporosity and a high electrical conductivity, due to their particular structure. Carbon nanofibers have been obtained by catalytic decomposition of methane, optimizing growth conditions to obtain carbon supports with different properties. Subsequently, the surface chemistry has been modified by an oxidation treatment, in order to create oxygen surface groups of different nature that have been observed to be necessary to obtain a higher performance of the electrocatalyst.     

Nanostructured polypyrrole/carbon composite as Pt catalyst support for fuel cell applications

A novel catalyst support was synthesized by in situ chemical oxidative polymerization of pyrrole on Vulcan XC-72 carbon in naphthalene sulfonic acid (NSA) solution containing ammonium persulfate as oxidant at room temperature. Pt nanoparticles with 3–4 nm size were deposited on the prepared polypyrrole–carbon composites by chemical reduction method. Scanning electron microscopy and transmission electron microscopy measurements showed that Pt particles were homogeneously dispersed in polypyrrole–carbon composites. The Pt nanoparticles-dispersed catalyst composites were used as anodes of fuel cells for hydrogen and methanol oxidation. Cyclic voltammetry measurements of hydrogen and methanol oxidation showed that Pt nanoparticles deposited on polypyrrole–carbon with NSA as dopant exhibit better catalytic activity than those on plain carbon. This result might be due to the higher electrochemically available surface areas, electronic conductivity and easier charge-transfer at polymer/carbon particle interfaces allowing a high dispersion and utilization of deposited Pt nanoparticles.     

Preparation of dispersible double-walled carbon nanotubes and application as catalyst support in fuel cells

Dispersion of double-walled carbon nanotubes (DWCNTs) in ethylene glycol (EG) medium by a simple ultrasonication method is investigated. Excellent dispersion of DWCNTs in EG without addition of a surfactant is found. Surface structure and crystallinity of the DWCNTs undergo little change. The dispersion state of DWCNTs is found to be very important for deposition of Pt nanoparticles on them. The Pt particles prepared in the homogenous dispersion system has a small size and uniform distribution. As a result, the electrochemical activity of the Pt catalyst is much higher than that prepared in the nondispersible system. In terms of the good dispersion in EG medium achieved by a simple method, the DWCNT solutions could also be widely used in energy, biology, medicine and other fields.     

Sea urchin-like mesoporous carbon material grown with carbon nanotubes as a cathode catalyst support for fuel cells

A sea urchin-like carbon (UC) material with high surface area (416 m² g⁻¹), adequate electrical conductivity (59.6 S cm⁻¹) and good chemical stability was prepared by growing carbon nanotubes onto mesoporous carbon hollow spheres. A uniform dispersion of Pt nanoparticles was then anchored on the UC, where the Pt nanoparticles were prepared using benzylamine as the stabilizer. For this Pt loaded carbon, cyclic voltammogram measurements showed an exceptionally high electrochemically active surface area (EAS) (114.8 m² g⁻¹) compared to the commonly used commercial E-TEK catalyst (65.2 m² g⁻¹). The durability test demonstrates that the carbon used as a support exhibited minor loss in EAS of Pt. Compared to the E-TEK (20 wt%) cathode catalyst, this Pt loaded UC catalyst has greatly enhanced catalytic activity toward the oxygen reduction reaction, less cathode flooding and considerably improved performance, resulting in an enhancement of ca. 37% in power density compared with that of E-TEK. Based on the results obtained, the UC is an excellent support for Pt nanoparticles used as cathode catalysts in proton exchange membrane fuel cells.     

Polyaniline and polypyrrole coatings on aluminum for PEM fuel cell bipolar plates

The conducting polymers polypyrrole and polyaniline were deposited on 6061 aluminum using cyclic voltammetry and painting, respectively. These samples are intended for proton exchange membrane fuel cell applications where surface contact resistance as well as bulk corrosion resistance are requirements for the bipolar plates that separate the cells. Corrosion current and voltage were measured on the samples as well as contact resistance between coated samples as a function of contact pressure. The polypyrrole samples showed neither improved corrosion resistance nor acceptable contact resistance. The painted polyaniline samples, however, showed about an order of magnitude reduction in corrosion current with only a minor increase in contact resistance. It is believed that in the more acidic environment of a fuel cell, the polyaniline will become even more conductive and that further reduction in contact resistance should be possible.     

Graphene based catalyst supports for high temperature PEM fuel cell application

In this study, the effect of graphene nanoplatelet (GNP) and graphene oxide (GO) based carbon supports on polybenzimidazole (PBI) based high temperature proton exchange membrane fuel cells (HT-PEMFCs) performances were investigated. Pt/GNP and Pt/GO catalysts were synthesized by microwave assisted chemical reduction support. X-ray diffraction (XRD), Thermogravimetric analysis (TGA), Brauner, Emmet and Teller (BET) analysis and high resolution transmission electron microscopy (HRTEM) were used to investigate the microstructure and morphology of the as-prepared catalysts. The electrochemical surface area (ESA) was studied by cyclic voltammetry (CV). The results showed deposition of smaller Pt nanoparticles with uniform distribution and higher ECSA for Pt/GNP compared to Pt/GO. The Pt/GNP and Pt/GO catalysts were tested in 25 cm² active area single HT-PEMFC with H₂/air at 160 °C without humidification. Performance evaluation in HT-PEMFC shows current densities of 0.28, 0.17 and 0.22 A/cm² for the Pt/GNP, Pt/C and Pt/GO catalysts based MEAs at 160 °C, respectively. The maximum power density was obtained for MEA prepared by Pt/GNP catalyst with H₂/Air dry reactant gases as 0.34, 0.40 and 0.46 W/cm² at 160 °C, 175 °C and 190 °C, respectively. Graphene based catalyst supports exhibits an enhanced HT-PEMFC performance in both low and high current density regions. The results indicate the graphene catalyst support could be utilized as the catalyst support for HT-PEMFC application.     

Functionalizing carbon nanotubes for proton exchange membrane fuel cells electrode

In recent years, carbon nanotubes (CNTs) have been increasingly considered as an advanced metal catalyst support for proton exchange membrane fuel cells (PEMFCs), owing to their outstanding physical and mechanical characteristics. However, the effective attachment of metal catalysts, uniformly dispersed onto the CNT surface, remains a formidable challenge because of the inertness of the CNT walls. Therefore, the surface functionalization of CNTs seems necessary in most cases in order to enable a homogeneous metal deposition. This review presents the different surface functionalization approaches that provide efficient avenues for the deposition of metal nanoparticles on CNTs, for the application of catalyst supports in PEMFCs with improved reactivity.     

Carbon support oxidation in PEM fuel cell cathodes

Oxidation of the cathode carbon catalyst support in polymer electrolyte fuel cells (PEMFC) has been examined. For this purpose platinum supported electrodes and pure carbon electrodes were fabricated and tested in membrane-electrode-assemblies (MEAs) in air and nitrogen atmosphere. The in situ experiments account for the fuel cell environment characterized by the presence of a solid electrolyte and water in the gas and liquid phases. Cell potential transients occurring during automotive fuel cell operation were simulated by dynamic measurements. Corrosion rates were calculated from CO₂ and CO concentrations in the cathode exhaust measured by non-dispersive infrared spectroscopy (NDIR). Results from these potentiodynamic measurements indicate that different potential regimes relevant for carbon oxidation can be distinguished. Carbon corrosion rates were found to be higher under dynamic operation and to strongly depend on electrode history. These characteristics make it difficult to predict corrosion rates accurately in an automotive drive cycle.     

Multi walled carbon nanotubes based micro direct ethanol fuel cell using printed circuit board technology

Multi walled carbon nanotubes (MWNTs) have been synthesized by chemical vapour deposition technique using AB₃ alloy hydride catalyst. Platinum supported MWNTs (Pt/MWNTs) and platinum-tin supported MWNTs (Pt–Sn/MWNTs) electrocatalysts have been prepared by chemical reduction method. MWNTs and electrocatalysts have been characterized by powder X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), high resolution TEM (HRTEM) and Energy dispersive X-ray analysis (EDAX). The anode and cathode electrodes for DEFC have been fabricated using Pt–Sn/MWNTs and 1:1 Pt/MWNTs + Pt/C electrocatalyst respectively. Performances of Direct Ethanol Fuel Cell (DEFC) with these electrodes have been studied at different temperatures of the membrane electrode assembly at ambient fuel conditions and the results have been discussed. A maximum power density of 38.6 mW/cm² at a current density of 130 mA/cm² is obtained. A six cell planar Micro Direct Ethanol Fuel Cell (μ-DEFC) stack was also constructed using these electrocatalysts and etched printed circuit boards as anode and cathode current collectors. A maximum power density of 2 mW/cm² was achieved when the μ-DEFC was operated in air breathing mode at room temperature. This enhancement of the performance may be attributed to dispersion and accessibility of MWNTs support and Pt–Sn in the electrocatalyst mixture for ethanol oxidation reaction.     

Performance of polymer electrolyte membrane fuel cells with carbon nanotubes as oxygen reduction catalyst support material

Platinum/carbon nanotubes (Pt/CNT) electrocatalysts are prepared. The CNTs are pre-treated in order to obtain reactive sites for the adherence of Pt metal particles. The electrocatalysts are characterized by scanning electron micrograph (SEM), transmission electron micrograph (TEM) and X-ray photoelectron spectrum (XPS) measurements. It is found that the catalysts contain both Pt(0) and Pt(IV) species. A high Pt loading of 32.5% on CNTs is obtained when the catalysts are prepared with ethylene glycol and Pt salt. The electrocatalysts are used for the oxygen reduction reaction in polymer electrolyte membrane fuel cells (PEMFCs) and the performance of PEMFC is analyzed with respect to catalyst synthesis and Pt loading. Cyclic voltammetric studies show that the Pt utilization in the fuel-cell electrodes is around 44%. Catalysts obtained with mild nitric acid-treated CNTs give a better performance of 680 mV at 500 mA cm⁻² and 600 mV at 800 mA cm⁻² than catalysts prepared with ethylene glycol and Pt salt.     

Highly porous PEM fuel cell cathodes based on low density carbon aerogels as Pt-support: Experimental study of the mass-transport losses

Carbon aerogels exhibiting high porous volumes and high surface areas, differentiated by their pore-size distributions were used as Pt-supports in the cathode catalytic layer of H₂/air-fed PEM fuel cell. The cathodes were tested as 50 cm² membrane electrode assemblies (MEAs). The porous structure of the synthesized catalytic layers was impacted by the nanostructure of the Pt-doped carbon aerogels (Pt/CAs). In this paper thus we present an experimental study aiming at establishing links between the porous structure of the cathode catalytic layers and the MEAs performances. For that purpose, the polarization curves of the MEAs were decomposed in 3 contributions: the kinetic loss, the ohmic loss and the mass-transport loss. We showed that the MEAs made with the different carbon aerogels had similar kinetic activities (low current density performance) but very different mass-transport voltage losses. It was found that the higher the pore-size of the initial carbon aerogel, the higher the mass-transport voltage losses. Supported by our porosimetry (N₂-adsorption and Hg-porosimetry) measurement, we interpret this apparent contradiction as the consequence of the more important Nafion penetration into the carbon aeorogel with larger pore-size. Indeed, the catalytic layers made from the larger pore-size carbon aerogel had lower porosities. We thus show in this work that carbon aerogels are materials with tailored nanostructured structure which can be used as model materials for experimentally testing the optimization of the PEM fuel cell catalytic layers.     

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.     

Evaluation of electrochemical performance for surface-modified carbons as catalyst support in polymer electrolyte membrane (PEM) fuel cells

The electrochemical performance of platinum (Pt) catalyst deposited on various functionalized carbon supports was investigated and compared with that of a commercial catalyst, Pt on Vulcan XC-72 carbon. The supports employed were graphitic or amorphous with a wide range of surface areas. Cyclic voltammetry (CV) and rotating disk electrode (RDE) studies on the supported catalysts indicated equivalent platinum catalyst activities. Fuel cell performance was determined for membrane electrode assemblies (MEA) fabricated from the supported catalysts. The use of high surface area supports did not necessarily translate into a higher electrochemical utilization of platinum. Electrochemical impedance spectroscopy (EIS) measurements indicated lower ohmic losses for low surface area carbon MEAs. This is explained by the supported catalyst electrode microstructures and their intrinsic resistivities. Correlation of all data indicates that for low surface carbons, nature of the support does not significantly affect the Pt catalytic activity. The influence of the support is more critical when high surface area carbons are used because of the vastly different electrode morphology and resistivity.     

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.     

Graphitic mesoporous carbon as a durable fuel cell catalyst support

Highly stable graphitic mesoporous carbons (GMPCs) are synthesized by heat-treating polymer-templated mesoporous carbon (MPC) at 2600 °C. The electrochemical durability of GMPC as Pt catalyst support (Pt/GMPC) is compared with that of carbon black (Pt/XC-72). Comparisons are made using potentiostatic and cyclic voltammetric techniques on the respective specimens under conditions simulating the cathode environment of PEMFC (proton exchange membrane fuel cell). The results indicate that the Pt/GMPC is much more stable than Pt/XC-72, with 96% lower corrosion current. The Pt/GMPC also exhibits a greatly reduced loss of catalytic surface area: 14% for Pt/GMPC vs. 39% for Pt/XC-72.