Multichelate-functionalized carbon nanospheres used for immobilizing Pt catalysts for fuel cells

We have developed a covalent/coordinate strategy to immobilize Pt nanocrystals (average diameter 3.5 nm) on multichelate-functionalized carbon nanospheres (CNS). The method involves the covalent grafting of triethylenetetramine onto the CNS’ surface and the coordination of well-structured ethylenimine chains to Pt ions or atoms. The Pt-CNSs interface is probed with X-ray photoelectron spectroscopy to elucidate the nature of the chemical binding of ethylenimine to Pt. The Pt particle deposition can be easily controlled; to form separated and uniform Pt nanocrystals, or densely loaded Pt particles, depending on the molar ratio of Pt to amine groups. The Pt particle layer covered carbon exhibits significantly higher activity toward methanol oxidation (0.75 A mg⁻¹ cm⁻²) than commercial E-TEK 40% Pt loaded carbon with the corresponding data of 0.51 A mg⁻¹ cm⁻².     

Preparation and characterization of Pt supported on graphene with enhanced electrocatalytic activity in fuel cell

Pt nanoparticles are deposited onto graphene sheets via synchronous reduction of H₂PtCl₆ and graphene oxide (GO) suspension using NaBH₄. Lyophilization is introduced to avoid irreversible aggregation of graphene (G) sheets, which happens during conventional drying process. Pt/G catalysts reveal a high catalytic activity for both methanol oxidation and oxygen reduction reaction compared to Pt supported on carbon black (Pt/C). The performance of Pt/G catalysts is further improved after heat treatment in N₂ atmosphere at 300 °C for 2 h, and the peak current density of methanol oxidation for Pt/G after heat treatment is almost 3.5 times higher than Pt/C. Transmission electron microscope (TEM) images show that the Pt particles are uniformly distributed on graphene sheets. X-ray photoelectron spectroscopy (XPS) results demonstrate that the interaction between Pt and graphene is enhanced during annealing. It suggests that graphene has provided a new way to improve electrocatalytic activity of catalyst for fuel cell.     

Effect of heat treatment on carbon in multicrystalline silicon

Effect of heat treatment on carbon in cast multicrystalline silicon (mc-Si) has been studied by means of Fourier Transmission Infrared Spectroscopy. Carbon is found to be involved in the formation of as-grown precipitates in mc-Si with higher oxygen content. The experimental results reveal that carbon is difficult to precipitate in mc-Si with lower oxygen or higher nitrogen concentration during annealing in the temperature range from 450°C to 1150°C. Carbon can enhance the nucleation of oxygen precipitates at lower temperature (<850°C). Although carbon does not affect the amount of oxygen precipitates at higher temperature (>950°C), it is suggested that carbon diffuses into oxygen precipitates by the enhancement of silicon self-interstitials. The experiments point out that preannealing at 750°C enhances the decrease of substitute carbon concentration during subsequent annealing at 1050°C. Dislocations and grain boundaries in mc-Si do not affect carbon thermal treatment properties.     

High catalytic performance and stability of Pt/C using acetic acid functionalized carbon

Carbon (Vulcan XC-72, Cobat Corp.) is pretreated using acetic acid (HAC) before the Pt deposition by microwave assisted glycol method. TEM and XRD results indicate that 3 nm Pt nano-particles are uniformly dispersed on the surface of modified XC-72. In order to examine the interaction between Pt nano-particles and carbon, Pt/C-HAC and commercial Pt/C (Johnson Matthey Corp.) are calcined at 500 °C for 2 h under nitrogen atmosphere. The average Pt particle size of Pt/C-HAC after calcination is only 10–12 nm in diameter while commercial Pt particles grow up to 25–35 nm with a broad size distribution. Meanwhile, electrochemical studies of Pt/C-HAC reveal higher activity and stability for both methanol oxidation and oxygen reduction than that of Pt/C-JM. The pore structure and surface composition are investigated by BET and XPS, which implies that much microporous structure and carbonyl functional groups on carbon surface are obtained after HAC treatment. The high catalytic performance and stability might mainly be due to the strong interaction between Pt nano-particles and carbon by carbonyl functional groups. Therefore, HAC treatment is proved to be a facile and effective method for carbon as the support for Pt as fuel cell catalyst.     

Preparation of Pt/C via a polyol process - Investigation on carbon support adding sequence

To investigate the effect of carbon support adding sequence on Pt particle sizes and Pt utilizations during the polyol process of the electrocatalyst preparation, a series of Pt/C electrocatalysts (different Pt loadings, two kinds of carbon supports−Vulcan XC and Black Pearls 2000) were prepared, namely, Pt/C-a (first reduced Pt ions to form Pt nanoparticles, which subsequently deposited on carbon supports) and Pt/C-b (Pt precursors were first impregnated with carbon supports, then reduced to Pt nanoparticles). The physical properties of the electrocatalysts were characterized by X-ray diffraction, transmission electron microscopy and nitrogen adsorption. The catalytic activities of the electrocatalysts toward the oxygen reduction reaction and the methanol oxidation reaction were characterized by potentiodynamic measurements. The results show that the carbon support adding sequence has significant effects on the Pt particle sizes, especially for the carbon supports with large amount of micropores, as a result, leading to different catalytic activities.     

High-performance anode for solid acid fuel cells prepared by mixing carbon substances with anode catalysts

Optimization of Pt-based electrode structure is a key to enhance power generation performance of fuel cells and to reduce the Pt loading. This paper presents a new methodology for anode fabrication for solid acid fuel cells (SAFCs) operating at ca. 200 °C. Our membrane electrode assembly for SAFCs consisted of a CsH₂PO₄/SiP₂O₇ composite electrolyte and Pt-based electrodes. To obtain the anode, a commercial Pt/C catalyst and carbon substance, such as carbon black and carbon nanofiber, were mixed. The composite anode with Pt loading = 0.5 mg cm⁻² demonstrated superior current-voltage characteristics to a benchmark Pt/C anode with Pt loading = 1 mg cm⁻². We consider that the mixing of Pt/C catalyst and carbon substrate facilitated H₂ mass transfer and increased the number of active sites.     

Evaluation of assemblies based on carbon materials modified with dendrimers containing platinum nanoparticles for PEM-fuel cells

Polyamidoamine (PAMAM) dendrimer-encapsulated Pt nanoparticles (G4OHPt) are synthesized by chemical reduction and characterized by transmission electronic microscopy. An H₂–O₂ fuel cell has been constructed with porous carbon electrodes modified with the dendrimer nanocomposites. Electrochemical and physical impregnation methods of electrocatalyst immobilization are compared. The modified surfaces are used as electrodes and gas-diffusion layers in the construction of three different membrane-electrode assemblies (MEAs). The MEAs have been tested in a single polymer-electrolyte membrane-fuel cell at 30 °C and 20 psig. The fuel cell is, then characterized by electrochemical impedance spectroscopy and cyclic voltammetry, and its performance evaluated in terms of polarization curves and power profiles. The highest fuel cell performance is reached in the MEA constructed by physical impregnation method. The results are compared with a 32 cm² prototype cell using commercial electrocatalyst operated at 80 °C, obtaining encouraging results.     

Redeposition of electrochemically dissolved platinum as nanoparticles on carbon

Electrochemical dissolution of platinum has been proposed by several research groups as an environmentally friendly way to recover platinum from catalytic structures such as fuel cell electrodes. For the case of electrochemical dissolution of platinum in hydrochloric acid electrolyte, the present communication reports a simple chemical method for reprecipitating platinum as nanoparticles of reasonable particle size on a carbon substrate without intermediary separation and handling of solid platinum salt.     

Synthesis of well-dispersed Pt/carbon nanotubes catalyst using dimethylformamide as a cross-link

In synthesizing carbon nanotubes supported catalysts, a significant challenge is how to deposit metal nanoparticles uniformly on the surface of carbon nanotubes due to the inherent inertness of carbon nanotube walls. This study reports a facile procedure using N,N-dimethylformamide as a dispersant, ligand and reductant, with which Pt nanoparticles can be deposited uniformly on pristine carbon nanotubes. X-ray photoelectron spectroscopy measurements reveal that metallic Pt nanoparticles are successfully prepared with this method. Transmission electron microscopy and X-ray diffraction analyses confirm the formation of face-centered cubic crystal Pt particles with a size that ranges from 2.0 to 4.0 nm. The support-dependent catalytic properties of the prepared Pt catalyst are characterized by cyclic voltammetric studies of the formic acid electro-oxidation reaction. The results show that a pristine carbon nanotubes supported Pt catalyst has a higher catalytic activity than both carbon powder and modified carbon nanotubes supported catalysts.     

Surface modification of carbon black by nitrogen and allylamine plasma treatment for fuel cell electrocatalyst

Carbon black used for fuel cell catalyst support system is modified using nitrogen and allylamine plasma and its effect on the carbon surface and fuel cell performance are reported. Custom designed radio frequency tumbling plasma reactor is used to surface modify the carbon black. Boehms Titration method, XRD and TEM are performed to confirm and analyze the effects of plasma treatment on the carbon surface. In the fuel cell electrochemical study both the nitrogen and allylamine modified catalyst support system exhibited better discharge performance than the control system. Nitrogen moieties on the carbon surface helped to decrease the particle size of catalytically active sites and provided good anchoring of Pt to the surface thereby resulted in increased electrochemical performance in the fuel cell evaluation.     

Effect of chemical oxidation of CNFs on the electrochemical carbon corrosion in polymer electrolyte membrane fuel cells

Effect of chemical oxidation of carbon nanofibers (CNFs) on the electrochemical carbon corrosion in polymer electrolyte membrane (PEM) fuel cells is examined. With increasing time of chemical oxidation treatment using an acidic solution, more oxygen functional groups are formed on the surface of CNF resulting in an increasingly hydrophilic carbon surface. This effect contributes to improvements in Pt loading and the distribution of Pt particles on carbon supports. However, the chemical oxidation treatment is found to accelerate electrochemical carbon corrosion. The oxygen functional group and the hydrophilic nature of CNFs after chemical oxidation treatment are believed to encourage the formation of CO₂, which is a product of carbon corrosion. From the observed results, it can be concluded that the chemical oxidation of CNFs is beneficial for catalyst loading and distribution. On the other hand, however, it reduces the durability of the PEM fuel cells caused by the electrochemical carbon corrosion.     

Influence of different carbon nanostructures on the electrocatalytic activity and stability of Pt supported electrocatalysts

Commercially available graphitized carbon nanofibers and multi-walled carbon nanotubes, two carbon materials with very different structure, have been functionalized in a nitric–sulfuric acid mixture. Further on, the materials have been platinized by a microwave assisted polyol method. The relative degree of graphitization has been estimated by means of Raman spectroscopy and X-ray diffraction while the relative concentration of oxygen containing groups has been estimated by X-ray photoelectron spectroscopy, which resulted in a graphitic character trend: Pt/GNF > Pt/F-GNF ? Pt/MWCNT > Pt/F-MWCNT. Transmission electron microscopy showed that the Pt particle size is around 3 nm for all samples, which was similar to the crystallite size obtained by X-ray diffraction. The activity towards electrochemical reduction of oxygen has been quantified using the thin-film rotating disk electrode, which has shown that all the samples have a better activity than the commercially available electrocatalysts. The trend obtained for the graphitic character maintained for the electrochemical activity, while the reverse trend has been obtained for the accelerated ageing test. Long-term potential cycling has demonstrated that the functionalization improves the stability for multi-walled carbon nanotubes, at the cost of decreased activity.     

Corrosion of carbon support for PEM fuel cells by electrochemical quartz crystal microbalance

During the voltammetry of carbon supports for proton exchange membrane fuel cells (PEMFCs), including commercial carbon blacks, graphitized carbon black and multi-wall carbon nanotubes (MWNTs), in deaerated 0.5 M H₂SO₄ solution results in mass changes as observed by using in situ electrochemical quartz crystal microbalance (EQCM). The mass change and corrosion onset potential during electrochemical carbon corrosion indicate that oxides are formed and accumulated on the carbon surface, leading to an increase in mass. A decrease in the mass is associated with carbon loss from the gasification of carbon surface oxides into carbon dioxide. High BET surface area carbon blacks ECP600 and ECP 300 have a carbon loss of 0.0245 ng cm⁻² s⁻¹ and 0.0144 ng cm⁻² s⁻¹ and as compared to 0.0115 ng cm⁻² s⁻¹ for low surface area support XC-72 and so they are less resistant to corrosion. Graphitized XC-72 and MWNTs, with higher graphitization have higher carbon corrosion onset potential at 1.65 V and 1.62 V and appear to be more intrinsically resistant to corrosion.     

Assessment of natural gas/hydrogen blends as an alternative fuel for industrial heat treatment furnaces

The present study investigates the application of natural gas/hydrogen blends as an alternative fuel for industrial heat treatment furnaces and their economic potential for decreasing carbon dioxide emissions in this field of application. Doing so, a detailed technological analysis of several influencing parameters on the heating system was performed as well as a consideration of furnace heating technology challenges. Starting with an evaluation of the main thermophysical properties of the blends and their corresponding flue gases, requirements for the heating systems were identified. Potential ways of decreasing flue gas losses and increasing the heat transfer are shown. In the radiant tube application, an increased overall combustion efficiency of about 1.2% was measured at 40 vol% hydrogen in the fuel gas. Influences on the air to gas ratio control system of the furnace is a further important point, which was considered in this study. Two commonly used control systems were evaluated concerning their capabilities to regulate the gas flow rates of blends with varying hydrogen contents and combustion properties, such as Wobbe Index. This is important, since it shows the capability to retrofit existing furnaces. Two types of burners were tested with different natural gas/hydrogen blends. This includes an open jet burner with air-staged and flameless combustion operation modes. A recuperative burner for radiant tube application was considered as well in these tests. Doing so, the nitrogen oxide formation of both systems under different operating conditions and different fuel blends were evaluated. An increase by about 10% at air-staged combustion and about 100% at flameless combustion was measured by a hydrogen content of 40 vol% in comparison to pure natural gas firing. Finally, the additional fuel costs of natural gas hydrogen blends and different cases are presented in an economic analysis. The driving force for the use of hydrogen as a fuel is the price of the CO2 certificates, which are considered in the analysis at a current price of 25.2 €/t CO2.     

Carbon and non-carbon support materials for platinum-based catalysts in fuel cells

Carbon and other platinum-supporting materials have been studied as electrode catalyst component of low-temperature fuel cells. Platinum (Pt) is commonly used as the catalyst due to its high electro-catalytic activity. Current research is now focusing on using either modified carbon-based or non-carbon-based materials as catalyst supports to enhance the catalytic performance of Pt. In recent years, Pt and Pt-alloy catalysts supported on modified carbon-based and non-carbon-based materials have received remarkable interests due to their significant properties that can contribute to the excellent fuel cell performance. Thus, it is timely to review this topic, focusing on various modified carbon-based supports and their advantages, limitations and future prospects. Non-carbon-based support for Pt and Pt-alloy catalysts will also be discussed. Firstly, this review summarises the progress to date in the development of these catalyst support materials; from carbon black to the widely explored catalyst support, graphene. Secondly, a comparison and discussion of each catalyst support in terms of morphology, electro-catalytic activity, structural characteristics, and its fuel cell performance are emphasized. All the catalyst support materials reviewed are considered to be promising, high-potential candidates that may find commercial value as catalyst support materials for fuel cells. Finally, a brief discussion on cost relating Pt based catalyst for mass production is included.     

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.     

A facile approach of fabricating proton exchange membranes by incorporating polydopamine-functionalized carbon nanotubes into chitosan

Recently, fabricating low-cost and high-performance proton exchange membranes (PEMs) with carbon nanotubes (CNTs) and chitosan (CS) have been extensively studied. Herein, we reported a facile but effective method to prepare CS-based PEM with polydopamine (PDA) functionalized CNTs (PDA@CNTs). The CS/PDA@CNTs composite membranes were prepared by incorporating PDA@CNTs into CS matrix followed by ion cross-linking with sulfuric acid. Due to the hydrogen bonding between the CS matrix and PDA coating, the thermal, mechanical and oxidation stability of the CS/PDA@CNTs composite membranes were improved. Moreover, the proton conductivity was also improved owing to the improved dispersion of PDA@CNTs in the CS matrix and the electrostatic interaction between PDA@CNTs and CS matrix. The maximum proton conductivity can reach to as high as 0.028 S cm⁻¹ under 80 °C with 2 wt% loading of PDA@CNTs. These results indicate that the CS/PDA@CNTs composite membranes have great potential to be used as PEMs in fuel cells.     

New nitrogen-containing carbon supports with improved corrosion resistance for proton exchange membrane fuel cells

In the present study the effect of the modification of commercial carbon black KetjenBlack DJ-600 with nitrogen and carbon containing compounds on the electrochemical stabilities has been investigated. The modification has been carried out by high temperature decomposition of acetonitrile, pyridine vapors and ethylene on the support surface. The corrosion stability of the KetjenBlack DJ-600 carbon black has been found to be improved upon modification.     

Porous carbon polyhedrons with exclusive Cu-Nx moieties as highly effective electrocatalysts for oxygen reduction reactions

The design and development of robust and efficient non-Fe doped transition metal-nitrogen doped carbon (TM-N/C) catalysts for oxygen reduction reaction is significant for promoting the applications of TM-N/C based catalysts in sustainable energy devices such as metal-air batteries and fuel cells. In this study, exclusive Cu-N_x moieties implanted in metal-organic framework (MOF) polyhedrons are successfully fabricated by an adsorption-calcination approach. The optimal electrocatalyst (CuNC-1100) exhibits much higher stability and satisfactory catalytic activity with a half-wave potential (E_1/2) of 0.885 V, which is 40 mV higher than commercial Pt/C in 0.1 M KOH solution, and exhibits a dominant 4-electron direct reaction path. The comprehensive characterization data reveals that the high activity of the catalyst originates from the porosity and high surface area, high dispersion of Cu atoms, and high density of graphitic nitrogen content and Cu-N_x species.     

Electrochemical activity and stability of Pt catalysts on carbon nanotube/carbon paper composite electrodes

This article reports a facile microwave-assisted approach to synthesize Pt catalysts on carbon nanotube (CNT)/carbon paper (CP) composite through catalytic chemical vapor deposition. The Pt deposits, with an average size of 3–5 nm were uniformly coated over the surface of oxidized CNTs. The electrochemical activity and stability of the Pt–CNT/CP electrode were investigated in 1 M H₂SO₄ using cyclic voltammetry (CV) and ac electrochemical impedance spectroscopy. The Pt catalysts showed not only fairly good electrochemical activity (electrochemically active surface area) but also durability after a potential cycling of >1000 cycles. The analysis of ac impedance spectra associated with equivalent circuit revealed that the presence of CNTs significantly reduced both connect and charge transfer resistances, leading to a low equivalent series resistance ˜0.22 Ω. With the aid of CNTs, well-dispersed Pt catalysts enable the reversibly rapid redox kinetic since electron transport efficiently passes through a one-dimensional pathway. Thus, the CNTs do not only serve as carbon support, they also charge transfer media between the Pt catalysts and the gas diffusion layer. The results shed some light on the use of CNT/CP composite, offering a promising tool for evaluating high-performance gas diffusion electrodes.