Electrocatalytic activity and durability study of carbon supported Pt nanodendrites in polymer electrolyte membrane fuel cells

In this paper, Pt nanodendrites are synthesized, and their use as an oxygen reduction catalyst in polymer electrolyte membrane fuel cells is examined. When the Pt nanoparticles are shape-controlled in a dendritic form, the Pt nanoparticles exhibit a high mass activity that is nearly twice as high as the commercial Pt/C catalyst for the oxygen reduction reaction. This high activity is only achieved when the Pt nanodendrites are supported on carbon. The unsupported Pt nanodendrites exhibit very poor catalytic activity due to the limited accessibility of the active sites in the catalyst layer of the fuel cells. Based on the durability study of Pt nanodendrites, however, the dendritic structure is not stable during repeated potential cycling test and its structure collapse is the primary reason for the performance loss in the fuel cells.     

Oxygen reduction on carbon supported Pt-W electrocatalysts

The catalytic activity of Pt-W electrocatalysts towards oxygen reduction reaction (ORR) was studied. Pt-W/C materials were prepared by thermolysis of tungsten and platinum carbonyl complexes in 1-2 dichloro-benzene during 48 h. The precursors were mixed to obtain relations of Pt:W: 50:50 and 80:20%w, respectively. The Pt carbonyl complex was previously synthesized by bubbling CO in a chloroplatinic acid solution. The synthesized materials were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), cyclic voltammetry (CV) and a rotating disk electrode (RDE). The results show that both materials (Pt₅₀W₅₀/C and Pt₈₀W₂₀/C) have a crystalline phase associated with metallic platinum and an amorphous phase related with tungsten and carbon. The particle size of the electrocatalysts depends on the relationship between platinum and tungsten. Finally, both materials exhibit catalytic activity for oxygen reduction.     

Pt overgrowth on carbon supported PdFe seeds in the preparation of core-shell electrocatalysts for the oxygen reduction reaction

In this study, a novel core-shell structured Pd₃Fe@Pt/C electrocatalyst, which is based on Pt deposited onto carbon supported Pd₃Fe nanoparticles, is prepared for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). The carbon supported Pd₃Fe nanoparticles act as seeds to guide the growth of Pt. The formation of the core-shell structured Pd₃Fe@Pt/C is confirmed by transmission electron microscopy (TEM), X-ray diffraction (XRD) and electrochemical characterization. The higher surface area of the synthesized catalyst suggests that the utilization of Pt in the Pd₃Fe@Pt/C catalyst is higher than that in Pt/C. Furthermore, better electrocatalytic performance than that of Pt/C and Pd₃Fe/C catalyst is observed in the ORR which follows a four-electron path. Consequently, the results indicate that the Pd₃Fe@Pt/C catalyst could be used as a more economically viable alternative for the ORR of PEMFCs.     

Enhanced electrocatalytic activity of platinum supported on nitrogen modified ordered mesoporous carbon

Nitrogen-modified ordered mesoporous carbon is synthesized via the 900 °C carbonization of polyaniline-coated mesoporous carbon. The electronic states of nitrogen atoms are investigated by XPS technique. Pyridinic nitrogen and quaternary nitrogen generate disorders and curvatures on the surface of graphitic carbon layers with nitrogen atoms replacing carbon atoms at the edges and the interior of carbon stacking, and thus offering beneficial anchoring sites for PtCl₆²⁻ ions. Pyridinic nitrogen and pyrrolic nitrogen offer p electrons to the sp² hybridized graphitic carbon layers, decreasing the inner electrical resistance of the catalytic carbon layer, enhancing the rate of proton diffusion, and transporting more free electrons to oxidative platinum. Due to the advantageous modification of the electronic structure of carbon atoms, platinum nanoparticles with a narrow size distribution are homogenously dispersed onto the surface of nitrogen-modified ordered mesoporous carbon, as evidenced by TEM images. Electrochemical tests show that the samples loaded platinum calcined at the 900 °C exhibit the optimum loading performance among as-made catalysts and a gradually decreased decay in electro-catalytic activity with time, with the current density stabilized at 3.64 mA cm⁻², which is far higher than that of mesoporous carbon (0.15 mA cm⁻²).     

Nontrivial role of carbon nanofibers morphology in binderless Pt nanocatalyst supported electrode

Similar to conventional composite electrodes, developing binderless-based carbon nanostructured (CNs) electrodes for fuel cells requires particularly the optimisation of both the morphology and the density of the CNs. In this work, carbon nanofibers (CNFs) have been optimised and used as catalyst support for Pt nanoparticles (NPs). The nontrivial role of the CNFs on the catalytic behavior is clearly demonstrated. We have shown that for a similar amount, morphology and dispersion of the Pt NPs fabricated onto CNFs, the density of the latter and to a lesser extent their diameter are the main factors influencing the catalytic activity. For the particular case of CNFs considered in this work, an optimum activity toward methanol fuel cell reaction was obtained when Pt NPs were supported with CNFs synthesized with a C₂H₂/Ar ratio of 0.31.     

Photo-generation of ultra-small Pt nanoparticles on carbon-titanium dioxide nanotube composites: A novel strategy for efficient ORR activity with low Pt content

Despite extensive work on Pt/C based catalyst materials for fuel cells towards oxygen reduction reaction, the high cost of catalyst is still a major problem for efficient use of fuel cells in daily life. This demands at designing a new electrocatalyst with high catalytic activity for oxygen reduction reaction. In this regard, we present a novel composite material consisting of functionalized acetylene black and TiO₂ nanotube (FAB/TNT) as the substrate for Pt as catalyst. Using this novel composite, Pt was decorated using photo-reduction process. Due to the presence of photo-electrons all over the conducting part of the material, H₂PtCl₆ was photo-reduced to Pt nanoparticles with extremely small size ∼1.6 nm. The material demonstrated that even with as less as 3.5 wt% of Pt, mass activity was found to be 37.5 A/g and specific actitvity was found to be 0.75 A/m² which is higher than commercial catalyst, Pt-Vulcan XC-72 (mass activity and specific activity was found to be 10.7 A/g and 0.81 A/m², respectively) at 0.85 V vs RHE.     

In situ synthesis of CoFe2O4 nanoparticles embedded in N-doped carbon nanotubes for efficient electrocatalytic oxygen reduction reaction

The development of electrocatalyst possessing superior oxygen reduction reaction (ORR) activity is highly desirable due to the low sluggish kinetics at the cathode of fuel cell. Here, CoFe₂O₄ nanoparticles embedded in N-doped carbon nanotubes electrocatalyst (denoted as CoFe₂O₄-NC) is synthesized via polymerization of pyrrole, absorbing metal ion and annealing under Ar/NH₃ atmosphere. By in situ integrating the catalytically active CoFe₂O₄ nanoparticles with the N-doped carbon nanotubes and enhancing electrical conductivity, the as-obtained electrocatalyst exhibits excellent ORR activity and long-term stability with a half-wave potential of 0.86 V and 10 h continuous cycling, outperforming the reported similar catalysts. This work opens a new path for the expansion of low cost and efficient ORR electrocatalysts to substitute Pt-based metals for energy storage and conversion devices.     

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.     

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.     

Ultrathin nitrogen doped carbon layer stabilized Pt electrocatalyst supported on N-doped carbon nanotubes

The enhancements in fuel cell performance and durability are crucial for the commercialization of polymer electrolyte fuel cells (PEFCs). Here, we deposit platinum nanoparticles on nitrogen doped carbon nanotubes (N-CNT) and continuously coat the electrocatalyst with nitrogen doped carbon (NC) layer derived from the carbonization of poly(vinyl pyrrolidone) (PVP). The NC-coated electrocatalyst shows stable electrochemical surface area (ECSA) during the potential cycling from 0.6 V to 1.0 V vs. RHE; while, the commercial and non-coated electrocatalysts lose 50% and 33% of initial ECSAs, respectively. Moreover, the NC-coated electrocatalyst shows higher oxygen reaction reduction (ORR) activity compared to non-coated electrocatalyst due to the additional nitrogen atoms in the electrocatalyst. The maximum power density of the coated electrocatalyst reaches 676 mW cm^?2 with Pt loading of 0.1 mg cm^?2, indicating that the mass power density of the electrocatalyst is one of the highest values in recently published literature. The NC layer is significantly important for simultaneous enhancements in durability and fuel cell performance.     

Improved thermal stability,properties, and electrocatalytic activity of sol-gel silica modified carbon supported Pt catalysts

An improvement in the thermal properties of catalysts used in PEM type fuel cells was achieved by siliceous additions. A sol-gel method was developed to deposit controlled amounts of a siliceous composition on Vulcan XC72 carbon (VC). The catalysts designated as Pt/(VC–SiO₂) were obtained by reduction of H₂PtCl₆ in an aqueous solution with NaBH₄. A nanoscale Pt particle formation was observed on the support materials having a range 2.7–5.1 nm. The thermal stability study for Pt/(VC–SiO₂) catalysts demonstrated that the presence of a siliceous phase conferred an increased resistance to Pt nanoparticle agglomeration at 600 °C. In addition, a decrease in low temperature mass loss was observed. Electrochemical properties evaluated by cyclic voltammetry coupled with a rotating disk electrode (RDE) showed improvements with moderate SiO₂ addition. The synthesized catalysts performance was as good as the performance of the control catalyst (46 wt% Pt/VC, Tanaka). Unfortunately, fuel cell performance experiments showed an unwanted hydrophillic behavior of carbon-silica composite aerogel supports at high current density values. The C–SiO₂ aerogel composite catalyst support seems suitable for low current applications.     

Flame synthesis of carbon nanostructures on stainless steel anodes for use in microbial fuel cells

Microbial fuel cells (MFCs) offer a promising alternative energy technology, but suffer from low power densities which hinder their practical applicability. In order to improve anodic power density, we deposited carbon nanostructures (CNSs) on an otherwise plain stainless steel mesh (SS-M) anode. Using a flame synthesis method that did not require pretreatment of SS-M substrates, we were able to produce these novel CNS-enhanced SS-M (CNS-M) anodes quickly (in a matter of minutes) and inexpensively, without the added costs of chemical pretreatments. During fed batch experiments with biomass from anaerobic digesters in single-chamber MFCs, the median power densities (based on the projected anodic surface area) were 2.9 mW m⁻² and 187 mW m⁻² for MFCs with SS-M and CNS-M anodes, respectively. The addition of CNSs to a plain SS-M anode via flame deposition therefore resulted in a 60-fold increase in the median power production. The combination of CNSs and metallic current collectors holds considerable promise for power production in MFCs.     

Pd nanoparticles supported on PDDA-functionalized carbon black with enhanced ORR activity in alkaline medium

Pd/C catalyst with small particle size, high dispersion and high wt.% of metal was in situ synthesized by a simple aqueous phase reduction method. Poly(diallyldimethylammonium chloride) PDDA-functionalized carbon black was used as a support material for the in situ deposition of Pd nanoparticles by means of electrostatic attraction. The catalysts were characterized by transmission electron microscopy, X-ray diffractometry and X-ray photoelectron spectroscopy, cyclic voltammetry and rotating disc electrode test. The results indicated that Pd nanoparticles with an average size of 2.09 nm were uniformly dispersed onto the carbon black with a metal weight percentage of ∼30 wt.%. The prepared Pd/C catalyst has showed remarkably larger electrochemical surface area and higher and more stable ORR activity as compared to commercial Pd/C catalyst and commercial Pt/C catalyst in alkaline media, which was believed to be a promising alternative to Pt-based catalyst used in alkaline fuel cell.     

Synthesis and characterization of Pt nanoparticles on sulfur-modified carbon nanotubes for methanol oxidation

Pt nanoparticles supported on carbon nanotubes (Pt/CNTs) have been synthesized from sulfur-modified CNTs impregnated with H₂PtCl₆ as Pt precursor. The dispersion and size of Pt nanoparticles in the synthesized Pt/CNT nanocomposites are remarkably affected by the amount of sulfur modifier (S/CNT ratio). The results of X-ray diffraction and transmission electron microscopy indicate that an S/CNT ratio of 0.3 affords well dispersed Pt nanoparticles on CNTs with an average particle size of less than 3 nm and a narrow size distribution. Among different catalysts, the Pt/CNT nanocomposite synthesized at S/CNT ratio of 0.3 showed highest electrochemically active surface area (88.4 m² g⁻¹) and highest catalytic activity for methanol oxidation reaction. The mass-normalized methanol oxidation peak current observed for this catalyst (862.8 A g⁻¹) was ∼ 6.5 folds of that for Pt deposited on pristine CNTs (133.2 A g⁻¹) and ∼ 2.3 folds of a commercial Pt/C (381.2 A g⁻¹). The results clearly demonstrate the effectiveness of a relatively simple route for preparation of sulfur-modified CNTs as a precursor for the synthesis of Pt/CNTs, without the need for tedious pretreatment procedures to modify CNTs or complex equipments to achieve high dispersion of Pt nanoparticles on the support.     

High stability of oxidation of methanol catalyzed by Pt supported by oxygen-incorporated bamboo-shaped CNTs grown directly on carbon cloth

Bamboo-shaped CNTs in which oxygen was incorporated were directly grown on carbon cloth (O-BCNT-CC) by microwave plasma-enhanced chemical vapor deposition. Mixed precursors CH₄/H₂/N₂/O₂ were introduced during the growth process. For comparison, bamboo-shaped CNTs without incorporated oxygen were prepared herein (BCNT-CC). Then, platinum catalysts were prepared on the as-grown O-BCNT-CC (Pt/O-BCNT-CC) and the as-grown BCNT-CC (Pt/BCNT-CC). According to TEM-EELS oxygen mapping, O atoms were uniformly distributed on the O-BCNT surface. Methanol oxidation was performed using Pt/O-BCNT-CC and Pt/BCNT-CC in 1 M methanol and 1 M sulfuric acid by cyclic voltammetry. In the initial cycle, the peak current density of Pt/O-BCNT-CC was almost equal to that of Pt/BCNT, indicating that both had nearly equal activities in the beginning. After 300 cycles, the peak current of Pt/BCNT-CC was reduced to half of the initial peak current owing to platinum-poisoning; however, the peak current of Pt/O-BCNT-CC decayed less. In Pt/O-BCNT-CC, the oxygen-containing functional groups affect the orientation index of crystalline Pt, providing a means of oxidizing methanol and stabilizing Pt catalysts for the long-term.     

A robust electrocatalytic activity and stability of Pd electrocatalyst derived from carbon coating

Palladium (Pd) as an efficient anodic catalyst has been extensively investigated in direct formic acid fuel cells (DFAFCs); while, Pd catalyst is electrochemically unstable in acidic electrolyte resulting in low stability retarding the widespread application of DFAFCs. In this study, a new method is invented to prevent the Pd nanoparticles from rapid dissolution by carbon layer originated from the carbonization of glucose. Ascribing to the presence of carbon layer, Pd electrocatalyst demonstrates much higher stability in comparison with Pd electrocatalyst without carbon layer in the course of stability tests. Robust electrocatalytic activities toward formic acid and methanol/ethanol oxidation are observed for carbon-stabilized Pd electrocatalyst resulted from the higher content of metallic Pd atoms coming from the carbonization process, in which Pd (II) species are further reduced. Moreover, the fuel cell performance of carbon-stabilized Pd electrocatalyst reaches 90 mW cm_Pd⁻² measured with 1 M formic acid; while, power density of bare Pd electrocatalyst is only 74 mW cm_Pd⁻². This work highlights that carbon layer carbonized from glucose improves not only the stability of Pd electrocatalyst, but also the electrocatalytic activity.     

Microstructure effect of carbon nanofibers on Pt/CNFs electrocatalyst for oxygen reduction

Pt nanoparticles supported on microstructure controllable carbon nanofibers (CNFs), i.e. platelet CNFs (p-CNFs), fish-bone CNFs (f-CNFs) and tubular CNFs (t-CNFs), are synthesized, and CNF microstructure effect on physio-chemical and oxygen reduction reaction (ORR) properties of Pt/CNFs is investigated. The physio-chemical properties of different Pt/CNFs electrocatalysts are characterized by high resolution transmission electron microscope (HRTEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). HRTEM results exhibit Pt nanoparticles are uniformly dispersed on CNF surface, and Pt/p-CNFs shows a smaller particle size compared with those other catalysts. XPS results reveal that CNF microstructure can influence the metal–support interaction, and Pt/p-CNFs have a higher binding energy compared with Pt/t-CNFs. From cyclic voltammetric studies, it is found that Pt/p-CNFs performed a higher ORR activity than Pt/f-CNFs and Pt/t-CNFs, which may be resulted from the smaller Pt particle size and the stronger metal–support interaction of Pt/p-CNFs. Furthermore, CNF microstructure can influence the reaction process. ORR on Pt/p-CNFs or Pt/f-CNFs is controlled by diffusion process, while on Pt/t-CNFs is surface reaction controlled.     

Electrochemical activity and stability of dealloyed Pt–Cu and Pt–Cu–Co electrocatalysts for the oxygen reduction reaction (ORR)

A comparative study of the electrochemical stability of Pt₂₅Cu₇₅ and Pt₂₀Cu₂₀Co₆₀ alloy nanoparticle electrocatalysts in liquid electrolyte half-cell environment was conducted. The aforementioned catalysts were shown to possess improved resistance to electrochemical surface area (ECSA) loss during voltage cycling relative to commercially available pure Pt electrocatalysts. The difference in ECSA loss was attributed to their initial mean particle size, which varied depending on the temperature at which the alloy catalysts were prepared (e.g. 600, 800 and 950 °C). Higher preparation temperatures resulted in larger particles and lead to lower ECSA loss. Liquid electrolyte environment short-term durability testing (5000 voltages cycles) revealed the addition of cobalt to be beneficial as ternary compositions exhibited stability advantages over binary catalysts.     

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.     

Mn3O4 nanosheets coated on carbon nanotubes as efficient electrocatalysts for oxygen reduction reaction

MnO-MnC_x coated carbon nanotubes (MnO/MnC_x/CNTs) nanocomposites were prepared by a one-pot deposition method. The coating consisted of MnO, Mn₅C₂, Mn₁₅C₄ and Mn₂₃C₆ was formed on the surface of CNTs by heating a mixture of Mn particles and CNTs at 600 °C for 40 min under vacuum. Then after heated MnO/MnC_x/CNTs in air at 350 °C for 2 h, MnO nanoparticles were partially converted to Mn₃O₄ nanosheets. Then Mn₃O₄-MnC_x coated carbon nanotubes (Mn₃O₄/MnC_x/CNTs) composed of interconnected nanosheets structure were successfully synthesized by a two-step method of one-pot deposition and heat post-treatment. The Mn₃O₄/MnC_x/CNTs showed better oxygen reduction reaction performance in alkaline condition than MnO/MnC_x/CNTs and pristine CNTs. Besides, the formed MnC_x (Mn₅C₂ and Mn₂₃C₆) by one-pot deposition method provided a strong interface bonding between Mn₃O₄ and CNTs, leading to improved stability of Mn₃O₄/MnC_x/CNTs as an electrode material.