Synthesis and performance of platinum supported on ordered mesoporous carbons as catalyst for PEM fuel cells: Effect of the surface chemistry of the support

Platinum electrocatalysts supported on ordered mesoporous carbon (CMK-3) have been prepared as alternative catalysts for PEM fuel cells. Their performance has been compared with that of a commercial Pt-carbon black on carbon cloth electrode (E-TEK) for the hydrogen oxidation in a PEM single cell. Ordered mesoporous carbon was synthesized using nanocasting method and then platinum was deposited by incipient wetness impregnation. Before the platinum deposition, carbon support was functionalized using HNO₃ as oxidizing agent to modify its surface chemistry. The characterization study of the electrocatalysts demonstrated that the surface chemistry of the support has an important effect on both the physicochemical and electrochemical properties of electrocatalysts. For this catalyst synthesis method, functionalization did not improve the preparation of the catalyst, since the presence of surface oxygen groups facilitated the aggregation of metal particles. However, the Pt/CMK-3 based electrodes showed a better performance than the commercial one, which could be attributed to the porous structure of the support.     

Facile synthesis of N,P-doped hierarchical porous carbon framework catalysts based on gelatin/phytic acid supermolecules for electrocatalytic oxygen reduction

The design of porous carbon frameworks with high-density distributions of dopant sites, especially those derived from renewable biomaterials, is highly desirable to improve the activity of nonprecious porous carbon catalysts for oxygen reduction reaction (ORR). In this paper, we prepared N, P-doped hierarchical porous carbon catalysts via a direct pyrolysis of a gelatin/phytic acid supermolecular aggregate without extra activation processes. The optimal catalyst materials (NPC1000) showed not only the highest N and P contents but also the highest specific surface area, total pore volume and mesoporous pore volume, which are also higher than those of the contrast sample prepared with a chitosan/phytic acid supermolecular aggregate. Thus, the sample NPC1000 had the best electrocatalytic activity compared with the Pt/C catalyst, which was nearly a direct four-electron reaction. The sample also had higher stability and methanol tolerance than Pt/C. This work provides a simple, economical and environmentally friendly method to prepare nonprecious porous carbon catalysts with the desired properties.     

Natural aloe vera derived Pt supported N-doped porous carbon: A highly durable cathode catalyst of PEM fuel cell

Evolution of highly durable electrocatalyst for oxygen reduction reaction (ORR) is the most critical barrier in commercializing polymer electrolyte membrane fuel cell (PEMFC). In this work, Pt deposited N-doped mesoporous carbon derived from Aloe Vera is developed as an efficient and robust electro catalyst for ORR. Due to its high mesoporous nature, the aloe vera derived carbon (AVC) play a very vital role in supporting Pt nanoparticles (NPs) with N-doping. After doping N into AVC, more defects are created which facilitates uniform distribution of Pt NPs leading to more active sites towards ORR. Pt/N-AVC shows excellent ORR activity when compared with commercial Pt/C and showing a half wave potential (E_1/2–0.87 V Vs. RHE) and reduction potential (E_red ~ 0.72 V Vs. RHE) towards ORR. Even after 30,000 potential cycles, Pt/N-AVC shows in its E_1/2 only ~5 mV negative shift and lesser agglomeration of Pt NPs is seen in the catalyst. In membrane electrode assembly (MEA) fabrication, Pt/N-AVC as a cathode catalyst in a PEMFC fixture and performance were studied. The Pt/N-AVC shows good performance, which proves the potential application of this naturally available bio derived carbon, which serves as an excellent high durable support material in PEMFC. All these features show that the Pt/N-AVC is the most stable, efficient and suitable candidate for ORR catalyst.     

Covalent hybrid of hemin and mesoporous carbon as a high performance electrocatalyst for oxygen reduction

A high performance hemin and mesoporous carbon hybrid electrocatalyst for the oxygen reduction reaction (ORR) is developed by using hemin as the Fe–N-containing precursor to control the chemistry of the metal and the chemical composition of the carbon surface. As a first step, Hemin is used as the Fe–N-containing precursor to prepare the Fe–N-doped mesoporous carbon (H-MC) via a nano-casting process by using sucrose as a carbon source and mesoporous silica as a hard template. Hemin is then used as the Fe–N₄-containing precursor to prepare H-MC supported hybrid catalyst. The Fe-doped and N-doped mesoporous carbons are also prepared and the catalytic properties of the prepared catalysts for ORR in alkaline media are investigated. The results show that as compared with the much more expensive Pt/C catalyst, the hybrid catalyst obtained in this work exhibits not only a higher onset potential, but also a higher current density.     

Effect of Pt-loaded carbon support nanostructure on oxygen reduction catalysis

This work demonstrates the impact of the nanostructure (pore size, wall thickness and wall crystallinity) of several carbon materials on their performance as oxygen reduction reaction (ORR) catalyst supports in PEM fuel cell applications. Two different mesoporous carbons [a surfactant-templated ordered mesoporous carbon (OMC) with 1.6 and 3.3 nm pores, and a silica colloid-imprinted carbon (CIC) with a 15 nm pore size], selected as being the most active in their class, were compared with microporous Vulcan Carbon. After loading with 20 and 40% Pt, both 3D transmission electron microscopy and electron tomography revealed that the Pt nanoparticles reside inside the majority of the pores of the OMC and CIC, but are located only on the outer surface of the VC particles. ORR performance studies on a rotating glassy carbon disc electrode in O₂-saturated 0.5 M H₂SO₄ showed that the Pt-loaded CIC outperforms both Pt-loaded OMC and VC. This is attributed to the higher electronic conductivity (due to the thicker and more crystalline walls, seen by both X-ray diffraction and thermogravimetric analysis) and facilitated mass transport in the larger pores of the CIC support.     

Improvement in the durability of carbon black-supported Pt cathode catalysts by silica-coating for use in PEFCs

Commercially available Pt metal catalysts supported on carbon black (Pt/CB) for polymer electrolyte fuel cell (PEFC) cathodes were covered with silica layers to improve their durability under the severe cathode operating conditions. The Pt metal particles in the Pt/CB catalyst grew in size during the accelerated durability tests (potential cycling between 0.6 and 1.0 V vs. RHE in an aqueous HClO₄ electrolyte). Thus, the Pt/CB catalyst was seriously deactivated for the oxygen reduction reaction over the course of the durability tests. In contrast, the silica layers, which wrapped around the Pt metal particles in the silica-coated Pt/CB catalyst, prevented the migration of the Pt metal particles on the carbon supports and the diffusion of Pt cations out of the silica layers. Thus, the silica-coated Pt/CB catalysts maintained a high activity for the oxygen reduction reaction over the course of the durability tests. In addition, the silica-coated Pt/CB prepared from methyltriethoxysilane showed a higher activity than that prepared from tetraethoxysilane. The porous structures and hydrophobicity of silica prepared from methyltriethoxysilane promoted the diffusion of oxygen and water molecules in the silica layers of the silica-coated Pt catalysts.     

Facile synthesis of highly conducting and mesoporous carbon aerogel as platinum support for PEM fuel cells

We report novel method for synthesis of carbon aerogel as platinum support for PEM fuel cells applications. The sol gel polymerization has been carried out using resorcinol and furfuraldehyde in non-aqueous medium followed by gelation at high pressure. This resulted in highly conducting and mesoporous carbon aerogel under ambient drying conditions. Platinum nano-particles are impregnated in the mesoporous carbon aerogel using microwave assisted polyol process. The support material and the catalyst are characterized by different analytical techniques like surface area analyzer, X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. Cyclic voltammetry and linear sweep voltammetry are used to evaluate the electro–catalytic activity of the Pt/carbon aerogel catalyst using rotating disk electrode technique. Well dispersed Pt nano-particles of size ∼3 nm on carbon aerogel showed good catalytic activity with onset potential of 964 mV and half wave potential of 814 mV towards oxygen reduction reaction kinetics. A membrane electrode assembly fabricated with the prepared Pt/carbon aerogel catalyst as a cathode and anode is tested in PEMFCs (H₂O₂) single cell, the power density of 536 mW cm⁻² at 0.6 V is obtained at 60 °C under atmospheric pressure.     

Noncovalently functionalized graphitic mesoporous carbon as a stable support of Pt nanoparticles for oxygen reduction

We report a durable electrocatalyst support, highly graphitized mesoporous carbon (GMPC), for oxygen reduction in polymer electrolyte membrane (PEM) fuel cells. GMPC is prepared through graphitizing the self-assembled soft-template mesoporous carbon (MPC) under high temperature. Heat-treatment at 2800 °C greatly improves the degree of graphitization while most of the mesoporous structures and the specific surface area of MPC are retained. GMPC is then noncovalently functionalized with poly(diallyldimethylammonium chloride) (PDDA) and loaded with Pt nanoparticles by reducing Pt precursor (H₂PtCl₆) in ethylene glycol. Pt nanoparticles of ∼3.0 nm in diameter are uniformly dispersed on GMPC. Compared to Pt supported on Vulcan XC-72 carbon black (Pt/XC-72), Pt/GMPC exhibits a higher mass activity towards oxygen reduction reaction (ORR) and the mass activity retention (in percentage) is improved by a factor of ∼2 after 44 h accelerated degradation test under the potential step (1.4-0.85 V) electrochemical stressing condition which focuses on support corrosion. The enhanced activity and durability of Pt/GMPC are attributed to the graphitic structure of GMPC which is more resistant to corrosion. These findings demonstrate that GMPC is a promising oxygen reduction electrocatalyst support for PEM fuel cells. The approach reported in this work provides a facile, eco-friendly promising strategy for synthesizing stable metal nanoparticles on hydrophobic support materials.     

Fabrication of a mesoporous Pt-carbon catalyst by the direct templating of mesoporous Pt-alumina for the methanol electro-oxidation

Mesoporous Pt-carbon catalysts were directly fabricated using mesoporous Pt-alumina as a template with a metal source and using poly(divinylbenzene) as a carbon precursor. Two types of mesoporous Pt-alumina templates were prepared by employing different calcination conditions (PtAl-A and PtAl-N were produced by the calcination in a stream of air and nitrogen, respectively). Both the mesoporous Pt-aluminas served as efficient templates for the fabrication of replicated Pt-carbon catalysts (PtC-A and PtC-N). The PtC catalysts showed high surface area with a narrow pore size distribution centered at ca. 4.0 nm. Together with pore-confined metal growth, the characteristic feature of the template, such as a strong interaction of metal species with the support was beneficial for the formation of highly dispersed Pt particles on the replicated mesoporous carbon catalysts. The mesoporous Pt-carbon (PtC) catalysts exhibited a higher metal dispersion than Pt catalyst impregnated on CMK-3 (Pt/CMK-3). Futhermore, the PtC-N catalyst exhibited a higher metal dispersion than the PtC-A catalyst. Methanol electro-oxidation experiments revealed the catalytic performance was closely related to the metal dispersion in the supported catalysts. The PtC-N catalyst with the highest metal dispersion exhibited the best catalytic performance in methanol electro-oxidation.     

Methanol tolerant core-shell RuFeSe@Pt/C catalyst for oxygen reduction reaction

Pt decorated RuFeSe/C catalyst is prepared by reduction of Pt precursor on pre-formed RuFeSe/C for oxygen reduction reaction (ORR). The catalyst is characterized by X-ray diffraction (XRD), energy dispersive spectrometer (EDS), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The catalyst particles are found to disperse on the carbon support with an average particle size of 2.8 nm. Physical characterizations and electrochemical tests confirm that Pt is deposited on the surfaces of RuFeSe particles and RuFeSe@Pt/C catalyst has a core-shell structure. The as-prepared catalyst has high durability and shows high ORR activity through a four-electron transfer process. RuFeSe@Pt/C exhibits 1.3-fold greater specific activity and 1.4-fold greater mass activity for ORR than Pt/C. More importantly, it has excellent tolerance to methanol. Consequently, RuFeSe@Pt/C may be used as fine cathode catalyst in direct methanol fuel cells (DMFCs).     

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.     

Electrochemical activity and durability of platinum nanoparticles supported on ordered mesoporous carbons for oxygen reduction reaction

A facile procedure for synthesizing platinum nanoparticles (NPs) studded in ordered mesoporous carbons (Pt–OMCs) based on the organic–organic self-assembly (one-pot) approach is reported. These Pt–OMCs, which can be easily fabricated with controllable Pt loading, were found to possess high surface areas, highly accessible and stable active sites and superior electrocatalytic properties pertinent as cathode catalysts for hydrogen–oxygen fuel cells. The enhanced catalytic activity and durability observed for the Pt–OMC electrocatalysts are attributed to the strengthened interactions between the Pt catalyst and the mesoporous carbon that effectively precludes migration and/or agglomeration of Pt NPs on the carbon support.     

Nanowire-based three-dimensional hierarchical core/shell heterostructured electrodes for high performance proton exchange membrane fuel cells

In order to effectively utilize expensive Pt in fuel cell electrocatalyst and improve the durability of PEM fuel cells, new catalyst supports with three-dimensional (3D) open structure are highly desirable. Here, we report the fabrication of a 3D core/shell heterostructure consisting tin nanowire core and carbon nanotube shell (SnC) grown directly onto fuel cell backing (here carbon paper) as Pt catalyst support for PEM fuel cells. Compared with the conventional Pt/C membrane electrode assembly (MEA), SnC nanowire-based MEA shows significantly higher oxygen reaction performance and better CO tolerance as well as excellent stability in PEM fuel cells. The results demonstrate that the core/shell nanowire-based composites are very promising supports in making cost effective and electrocatalysts for fuel cell applications.     

Development of PtGe and PtIn anodic catalysts supported on carbonaceous materials for DMFC

Bimetallic PtGe and PtIn catalysts (Ge/Pt and In/Pt molar ratios = 0.33, 17 wt.% Pt loading) were prepared over carbonaceous supports (carbon Vulcan, carbon nanotubes and structured mesoporous carbon). The liquid phase deposition–reduction method by using 0.4 M sodium borohydride as a reducing agent was employed. The electrocatalytic activity for methanol oxidation was compared with a commercial Pt/CV E-TEK catalyst. The onset potential of the CO oxidation was shifted to less positive values for carbon Vulcan and carbon nanotubes supported catalysts and with a smaller effect in the case of mesoporous carbon supported ones. The best electrocatalytic performance was obtained by using carbon Vulcan as support of bimetallic catalysts, followed by carbon nanotubes. The performance of mesoporous carbon as a support was not adequate. PtGe/CV, PtGe/NT and PtIn/CV catalysts displayed the best performance in DMFC. The good performance of these catalysts could be due to the presence of small particle sizes with a narrow distribution, and to geometric effects (observed from different characterization techniques) related to a probable decoration of Ge or In around the small Pt particles.     

Methanol electro-oxidation on Pt/C modified by polyaniline nanofibers for DMFC applications

In the present study, in order to achieve an inexpensive tolerable anode catalyst for direct methanol fuel cell applications, a composite of polyaniline nanofibers and Pt/C nano-particles, identified by PANI/Pt/C, was prepared by in-situ electropolymerization of aniline and trifluoromethane sulfonic acid on glassy carbon. The effect of synthesized PANI nanofibers in methanol electrooxidation reaction was compared by bare Pt/C by different electrochemical methods such as; cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronoamperometry. Scanning electron microscopy (SEM) was also employed to morphological study of the modified catalyst layer. The test results reveal that introduction of PANI nanofibers within catalyst layer improves the catalyst activity in methanol oxidation, hinders and prevents catalyst from more poisoning by intermediate products of methanol oxidation and improves the mechanical properties of the catalyst layer. SEM images also indicate that PANI nanofibers placed between platinum particles and anchor platinum particles and alleviate the Pt migration during methanol electrooxidation.     

Hydrogen production via the aqueous phase reforming of ethylene glycol over platinum-supported ordered mesoporous carbon catalysts: Effect of structure and framework-configuration

Structurally and framework-configurationally different kinds of ordered mesoporous carbon (OMC) supported platinum catalyst were applied to aqueous phase reforming (APR) of ethylene glycol (EG) for hydrogen production. Wide-angle XRD patterns, CO chemisorption results, and TEM analyses clearly provide evidence that Pt nanoparticles on 3-dimensional (3-D) OMC support (CMK-8, CMK-9) are less sintered than Pt nanoparticles on 2-dimensional (2-D) OMC support (CMK-3, CMK-5). In addition, due to the large surface area caused by the carbon microporosity and the additional mesopores of hollow-type OMC support, OMC with a hollow-type framework-configuration (CMK-5, CMK-9) provides Pt metal nanoparticles smaller than those of the rod-type OMC (CMK-3, CMK-8). Therefore, the Pt/CMK-9 catalyst with a 3-D OMC and hollow-type framework configuration exhibits the best hydrogen production in the APR with EG due to both the synergetic effect of having a low amount of metal sintering during the reaction process and the more favorable transport and diffusion of the reactants and products.     

Carbon-supported Pd-Pt cathode electrocatalysts for proton exchange membrane fuel cells

A series of carbon-supported Pd-Pt alloy (Pd-Pt/C) catalysts for oxygen reduction reaction (ORR) with low-platinum content are synthesized via a modified sodium borohydride reduction method. The structure of as-prepared catalysts is characterized by powder X-ray diffraction (XRD) and transmission electron microscope (TEM) measurements. The prepared Pd-Pt/C catalysts with alloy form show face-centered-cubic (FCC) structure. The metal particles of Pd-Pt/C catalysts with mean size of around 4-5 nm are uniformly dispersed on the carbon support. The electrocatalytic activities for ORR of these catalysts are investigated by rotating disk electrode (RDE), cyclic voltammetry (CV), single cell measurements and electrochemical impedance spectra (EIS) measurements. The results suggest that the electrocatalytic activities of Pd-Pt/C catalysts with low platinum are comparable to that of the commercial Pt/C with the same metal loading. The maximum power density of MEA with a Pd-Pt/C catalyst, the Pd/Pt mass ratio of which is 7:3, is about 1040 mW cm⁻².     

Carbon nanotube supported Pt electrodes for methanol oxidation: A comparison between multi- and single-walled carbon nanotubes

This work presents a detailed comparison between multi-walled (MWNT) and single-walled carbon nanotubes (SWNT) in an effort to understand which can be the better candidate of a future supporting carbon material for electrocatalyst in direct methanol fuel cells (DMFC). Pt particles were deposited via electrodeposition on MWNT/Nafion and SWNT/Nafion electrodes to investigate effects of the carbon materials on the physical and electrochemical properties of Pt catalyst. The crystalloid structure, texture (surface area, pore size distribution, and macroscopic morphology), and surface functional groups for MWNT and SWNT were studied using XRD, BET, SEM and XPS techniques. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to characterize the electrochemically accessible surface area and charge transfer resistances of the MWNT/Nafion and SWNT/Nafion electrodes. CO stripping voltammograms showed that the onset and peak potentials on Pt-SWNT/Nafion were significantly lower that those on the Pt-MWNT/Nafion catalyst, revealing a higher tolerance to CO poisoning of Pt in Pt-SWNT/Nafion. In methanol electrooxidation reaction, Pt-SWNT/Nafion catalyst was characterized by a significantly higher current density, lower onset potentials and lower charge transfer resistances using CV and EIS analysis. Therefore, SWNT presents many advantages over MWNT and would emerge as an interesting supporting carbon material for fuel cell electrocatalysts. The enhanced electrocatalytic properties were discussed based on the higher utilization and activation of Pt metal on SWNT/Nafion electrode. The remarkable benefits from SWNT were further explained by its higher electrochemically accessible area and easier charge transfer at the electrode/electrolyte interface due to SWNT's sound graphitic crystallinity, richness in oxygen-containing surface functional groups and highly mesoporous 3D structure.     

Pt nanodendrites anchored on bamboo-shaped carbon nanofiber arrays as highly efficient electrocatalyst for oxygen reduction reaction

In order to improve the Pt utilization and enhance their catalytic performance in fuel cells, a novel composite electrode composed of single-crystalline Pt nanodendrites and support constructed by bamboo-shaped carbon nanofiber arrays (CNFAs) on carbon paper, is reported. This electrode is designed by growing vertically CNFAs on carbon paper via plasma enhanced chemical vapor deposition, followed by the direct synthesis of Pt nanodendrites using a simple surfactant-free aqueous solution method. Electron microscopy studies reveal that the Pt nanodendrites are uniformly high dispersed and anchored on the surface of CNFAs. Electrochemical measurements demonstrate that the resultant electrode exhibits higher electrocatalytic activity and stability for oxygen reduction reaction than commercial Pt/C catalyst, suggesting its potential application in fuel cells.     

Carbon nanotubes-supported Pt catalysts for decalin dehydrogenation to release hydrogen: A comparison between nitrogen- and oxygen-surface modification

The effect of carbon nanotubes (CNT) surface chemistry on supported Pt catalytic performance in decalin dehydrogenation to release hydrogen was investigated through modifying CNT surface with nitrogen and oxygen functional groups. The results indicate that the mechanism of improving Pt dispersion is different for nitrogen and oxygen groups. Although oxygen groups can improve Pt dispersion, their electron donating feature inhibits the electron transfer from Pt to carbon support. DFT calculations have clarified the negative effect of oxygen group on decalin dehydrogenation in aspect of adsorption energy. Moreover, the abundant oxygen species on catalyst surface lead to a bad wettability of catalyst in non-polar reaction liquid, which significantly weaken the approaching of reactant onto catalyst. Therefore, the introduction of oxygen groups is unfavorable to the catalyst activity. In contrast, the highly dispersed Pt nanoparticles anchored by nitrogen groups may be one reason for the high activity of Pt supported on CNT modified with nitrogen groups. Furthermore, nitrogen groups promote the electron transfer from Pt to CNT, leading to lower Pt d-band and weaker naphthalene adsorption. Meanwhile little change happened on the non-polar characteristics of CNT during nitrogen doping treatment. Therefore, the introduction of nitrogen functional groups is conducive to the catalytic dehydrogenation.