Thermal and electrochemical stability of tungsten carbide catalyst supports

The thermal and electrochemical stability of tungsten carbide (WC), with and without a catalyst dispersed on it, have been investigated to evaluate the potential suitability of the material as an oxidation-resistant catalyst support. Standard techniques currently used to disperse Pt on carbon could not be used to disperse Pt on WC, so an alternative method was developed and used to disperse Pt on both commercially available WC and on carbon for comparison of stability. Electrochemical testing was performed by applying oxidation cycles between +0.6 V and +1.8 V to the support-catalyst material combinations and monitoring the activity of the supported catalyst over 100 oxidation cycles. Comparisons of activity change with cumulative oxidation cycles were made between C and WC supports with comparable loadings of catalyst by weight, solid volume, and powder volume. WC was found to be more thermally and electrochemically stable than currently used carbon support material Vulcan XC-72R. However, further optimization of the particle sizes and dispersion of Pt/WC catalyst/support materials and of comparison standards between new candidate materials and existing carbon-based supports are required.     

Improved stability of TiO2 modified Ru85Se15/C electrocatalyst for proton exchange membrane fuel cells

The electrocatalytic stability of the carbon supported Ru₈₅Se₁₅ nanoparticles has been improved by the modification of titanium dioxide for proton exchange membrane fuel cells (PEMFCs). Transmission electron microscopy (TEM), X-ray diffraction (XRD) measurements and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) are applied for characterizing Ru₈₅Se₁₅/C and titanium dioxide modified Ru₈₅Se₁₅/C (Ru₈₅Se₁₅/TiO₂/C) electrocatalysts. Electrochemical measurements and single cell tests are conducted for the evaluation of the electrocatalysts. The results indicate that Ru₈₅Se₁₅/TiO₂/C electrocatalyst, presenting similar initial oxygen reduction reaction (ORR) activity with Ru₈₅Se₁₅/C, reveals better electrochemical stability. The final potential of Ru₈₅Se₁₅/TiO₂/C is 137 mV higher than that of Ru₈₅Se₁₅/C at 2 mA cm⁻² after the electrochemical durability test. Moreover, in the single cell stability test Ru₈₅Se₁₅/TiO₂/C also shows comparable initial performance with Ru₈₅Se₁₅/C, but better final performance. Therefore, the Ru₈₅Se₁₅/C is expected to be used as an effective cathode electrocatalyst for PEMFCs by TiO₂ modification on the carbon support.     

Effects of surface chemical states of carbon nanotubes supported Pt nanoparticles on performance of proton exchange membrane fuel cells

This study synthesized platinum (Pt) nanoparticles supported on carbon nanotubes (CNTs) using a microwave-assisted polyol method. The oxidation treatment of CNTs introduced primarily -OH and -COOH groups to the CNTs, thereby enhancing the reduction of Pt ionic species, resulting in smaller Pt particles with improved dispersion and attachment properties. The Pt particles supported on oxidized CNTs displayed superior durability to those on pristine CNTs or commercially available Pt/C. These improvements are most likely associated with the percentage of metallic Pt in the particles. After 400 cycles, the losses of electrochemical surface area in Pt nanoparticle supported on oxidized CNTs and pristine CNTs catalysts were 66 and 84%, respectively, of that associated with commercial Pt/C. A single proton exchange membrane fuel cell using Pt supported on oxidized CNTs at the cathode with a total catalytic loading of 0.6 Pt mg cm⁻² exhibited the highest power density of 890 mW cm⁻² and displayed a lower mass transfer loss, compared to Pt/C.     

One-pot synthesis of Pt/CeO2/C catalyst for improving the ORR activity and durability of PEMFC

Oxygen reduction reaction (ORR) activity and durability of Pt catalysts should be both valued for successful commercialization of proton exchange membrane fuel cells (PEMFCs). We offer a facile one-pot synthesis method to prepare Pt/CeO₂/C composite catalysts. CeO₂ nanoparticles, with high Ce³⁺ concentration ranging from 30.9% to 50.6%, offers the very defective surface where Pt nanoparticles preferentially nuclear and growth. The Pt nanoparticles are observed sitting on the CeO₂ surface, increasing the PtCeO₂ interface. The high concentration of oxygen vacancies on CeO₂ surface and large PtCeO₂ interface lead to the strong PtCeO₂ interaction, effectively improving the ORR activity and durability. The mass activity is increased by up to 50%, from 36.44 mA mg^?1 of Pt/C to 52.09 mA mg^?1 of Pt/CeO₂/C containing 20 wt.% CeO₂. Pt/CeO₂/C composite catalysts containing 10–30 wt.% CeO₂ loss about 80% electrochemical surface area after 10,000 cycles, which is a fivefold enhancement in durability, compared to Pt/C losing 79% electrochemical surface area after 2000 cycles.     

Effects of anatase TiO2 with different particle sizes and contents on the stability of supported Pt catalysts

Pt/TiO₂-C catalyst with TiO₂ and carbon black as the mixed support has been synthesized by the microwave-assisted polyol process (MAPP). Effects of anatase TiO₂ with different particle sizes and contents on the stability of supported Pt catalysts have been systematically studied. X-ray diffraction (XRD), transmission electron microscopy (TEM), cyclic voltammograms (CV), and accelerated potential cycling tests (APCT) have been carried out to present the influence degree. The experimental results indicate that the original electrochemically active specific surface areas (ESA) of the catalysts decrease with the increase of mean particle sizes of TiO₂ and TiO₂ contents. However, the activity of Pt/TiO₂-C-20 is very close to that of Pt/TiO₂-C-5 and the stability of Pt/TiO₂-C-20 is the best after 1000 cycles APCT, illustrating that the optimized particle size of TiO₂ in Pt/TiO₂-C catalyst is 20 nm. Furthermore, the stability of the catalysts increase with the increase of TiO₂ contents in the mixed support. Taking into account both the activity and stability of various Pt/TiO₂-C catalysts, the optimized particle size of TiO₂ is 20 nm and the optimal TiO₂ content existed in the mixed support is 40%.     

Controlling Pt loading and carbon matrix thickness for a high performance Pt-nanowire catalyst layer in PEMFCs

Pt-nanowire (Pt-NW) catalyst layers were prepared by in-situ growing Pt nanowires onto carbon matrix coated on electrolyte membrane surface and used as PEMFC cathodes. The performances of the catalyst layer with various catalyst loadings and carbon matrix thicknesses were evaluated. Scanning electron microscopy (SEM) was employed to observe the morphology and thickness of the Pt-NW catalyst layers, energy-dispersive X-ray spectroscopy (EDS) was used to investigate the Pt distribution along the layers. The polarization curve, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) were performed in fuel cells to check the practical electrochemical activity of the Pt-NW catalyst layers. The results showed that the electrochemical surface area (ECSA) and the cell performance exhibited a volcano-type curve with different Pt loadings, while the carbon matrix thickness had an influence on the Pt-NW gradient distribution, the mass diffusion, and the charge transfer in the electrodes.     

Effect of carbon black support corrosion on the durability of Pt/C catalyst

The work intends to clarify the effect of carbon black support corrosion on the stability of Pt/C catalyst. The corrosion investigations of carbon blacks with similar structures and characteristics were analyzed by cyclic voltammograms (CV) and X-ray photoelectron spectroscopy (XPS). The results indicate that a higher oxidation degree appears on the Black Pearl 2000 (BP-2000) support, i.e. BP-2000 has a lower corrosion resistance than Vulcan XC-72 (XC-72). The durability investigation of Pt supported on the two carbon blacks was evaluated by a potential cycling test between 0.6 and 1.2 V versus reversible hydrogen electrode (RHE). A higher performance loss was observed on the Pt/BP-2000 gas diffusion electrode (GDE), compared with that of Pt/XC-72. XPS analysis suggests that higher Pt amount loss appeared in the Pt/BP-2000 GDE after durability test. X-ray diffraction (XRD) analysis also shows that Pt/BP-2000 catalyst presents a higher Pt size growth. The higher performance degradation of Pt/BP-2000 is attributed significantly to the less support corrosion resistance of BP-2000.     

High surface area synthesis,electrochemical activity,and stability of tungsten carbide supported Pt during oxygen reduction in proton exchange membrane fuel cells

The oxidation of carbon catalyst supports to carbon dioxide gas leads to degradation in catalyst performance over time in proton exchange membrane fuel cells (PEMFCs). The electrochemical stability of Pt supported on tungsten carbide has been evaluated on a carbon-based gas diffusion layer (GDL) at 80 °C and compared to that of HiSpec 4000™ Pt/Vulcan XC-72R in 0.5 M H₂SO₄. Due to other electrochemical processes occurring on the GDL, detailed studies were also performed on a gold mesh substrate. The oxygen reduction reaction (ORR) activity was measured both before and after accelerated oxidation cycles between +0.6 V and +1.8 V vs. RHE. Tafel plots show that the ORR activity remained high even after accelerated oxidation tests for Pt/tungsten carbide, while the ORR activity was extremely poor after accelerated oxidation tests for HiSpec 4000™. In order to make high surface area tungsten carbide, three synthesis routes were investigated. Magnetron sputtering of tungsten on carbon was found to be the most promising route, but needs further optimization.     

Self-humidification of a PEM fuel cell using a novel Pt/SiO2/C anode catalyst

In this work, a membrane electrode assembly (MEA) for proton exchange membrane fuel cell (PEMFC) operating under no external humidification has been successfully fabricated by using a composite Pt/SiO₂/C catalyst at the anode. In the composite catalyst, amorphous silica, which originated from the hydrolysis of tetraethyl orthosilicate (TEOS), was immobilized on the surface of carbon powder to enhance the stability of silica and provide a well-humidified surrounding for proton transport in the catalyst layer. The characteristics of silica in the composite catalyst were investigated by XRD, SEM and XPS analysis. The single cell tests showed that the performance of the novel MEA was comparable to MEAs prepared using a standard commercial Pt/C catalyst with 100% external humidification, when both were operated on hydrogen and air. However, in the absence of humidification, the MEA using Pt/SiO₂/C catalyst at the anode continued to show excellent performance, while the performance of the MEA containing only the Pt/C catalyst rapidly decayed. Long-term testing for 80 h further confirmed the high performance of the non-humidified MEA prepared with the composite catalyst. Based on the experimental data, a possible self-humidifying mechanism was proposed.     

Self-humidifying nanocomposite membranes based on sulfonated poly(ether ether ketone) and heteropolyacid supported Pt catalyst for fuel cells

In the present study, the self-humidifying nanocomposite membranes based on sPEEK and Cs_2.5H_0.5PW₁₂O₄₀ supported Pt catalyst (Pt-Cs_2.5H_0.5PW₁₂O₄₀ catalyst or Pt-Cs_2.5) and their performance in proton exchange membrane fuel cells with dry reactants has been investigated. The XRD, FTIR, SEM-EDXA and TEM analysis were conducted to characterize the catalyst and membrane structure. The ion exchange capacity (IEC), water uptake and proton conductivity measurements indicated that the sPEEK/Pt-Cs_2.5 self-humidifying nanocomposite membranes have higher water absorption, acid and proton-conductive properties compared to the plain sPEEK membrane and Nafion-117 membrane due to the highly hygroscopic and acidy properties of Pt-Cs_2.5 catalyst. The single cells employing the sPEEK/Pt-Cs_2.5 self-humidifying nanocomposite membranes exhibited higher cell OCV values and cell performances than those of plain sPEEK membrane and Nafion-117 membrane under dry or wet conditions. Furthermore, the sPEEK/Pt-Cs_2.5 self-humidifying nanocomposite membranes showed good water stability in aqueous medium. After investigation of several membranes such as sPEEK and sPEEK/Pt-Cs_2.5 membranes, the self-humidifying nanocomposite membrane with sulfonation degree of 65.12% for its sPEEK and 15 wt.% of catalyst with 1.25 wt.% Pt within catalyst was found to be the best proton exchange membrane for fuel cell applications. This self-humidifying nanocomposite membrane has a higher single cell performance than the Nafion-117 which was frequently used as a proton exchange membrane for fuel cell applications.     

Non-noble metal oxygen reduction electrocatalysts based on carbon nanotubes with controlled nitrogen contents

Nitrogen-doped carbon nanotubes (CN_x) were prepared via a floating catalyst chemical vapor deposition method using precursors consisting of ferrocene and melamine to control the nitrogen content. Structure, morphology and composition of all CN_x catalysts were characterized by SEM, TEM, and XPS. These results indicated that the surface nitrogen content (up to 7.7 at.%) increases with the increase of melamine used. Electrochemical methods were used to study the correlation between surface structure and the activity of oxygen reduction reaction (ORR) in acid and alkaline solutions. Electrochemical data indicated that the higher the nitrogen content is, the higher the oxygen reduction activity. Especially, the results from the rotating ring disk electrode technique demonstrated that CN_x (7.7%) has similar ORR activity and selectivity with commercial Pt/C in alkaline solution.     

Highly stable nanocarbon supported Pt catalyst for fuel cell via a molten salt graphitization strategy

Proton exchange membrane fuel cells (PEMFCs) durability has been severely hindered by carbon support poor stability in the cathodic Pt-based catalyst. Herein, a high-surface-area nitrogen-doped graphitic nanocarbon (N-G-CA) with mesopores is developed as Pt support to address PEMFCs durability challenge. Resorcinol-formaldehyde aerogel pyrolyzed carbon aerogel is selected as N-G-CA raw material. Nitrogen atoms are introduced into carbon aerogel via NH₃ heat treatment. Then, nitrogen-doped carbon aerogel is transferred into N-G-CA via heating together with transition-metal salts (one of FeCl₃, FeCl₂, CoCl₂, or MnCl₂, etc.) at 1200 °C. As ORR catalyst, Pt/N-G-CA half-wave potential only lost 10 mV, after 30, 000 cycles accelerated aging test in the rotating-desk-electrode. Only 12 mV voltage loss at 1.5 A/cm2 is observed, after 5, 000 cycles for membrane electrode. Pt/N-G-CA exhibits superior durability and activity than commercial Pt/C. High durability of Pt/N-G-CA is due to N-G-CA high graphitization extent, as well as the interactions between doping nitrogen and Pt. N-G-CA is promising as stable support for durable Pt-based catalysts in PEMFCs, thanks to enhanced carbon corrosion resistance, uniformly dispersed Pt, and strong support-metals interaction.     

High loading and monodispersed Pt nanoparticles on multiwalled carbon nanotubes for high performance proton exchange membrane fuel cells

Composite electrodes consisting of Pt nanoparticles-supported on multiwalled carbon nanotubes grown directly on carbon paper (Pt/CNTs/carbon paper) have been synthesized by a new method using glacial acetic acid as a reducing agent. Transmission electron microscopy (TEM) images show that the Pt nanoparticles with high density and relative small in size (2–4 nm) were monodispersed on the surface of CNTs. X-ray photoelectron spectroscopy (XPS) analysis indicates that the glacial acetic acid acts as a reducing agent and has the capability of producing a high density of oxygen-containing functional groups on the surface of CNTs that leads to high density and monodispersion of Pt nanoparticles. Compared with standard Pt/C electrode, the Pt/CNT/carbon paper composite electrodes exhibit higher electrocatalytic activity for methanol oxidation reaction and higher single-cell performance in a H₂/O₂ fuel cell.     

Efficient synthesis of Pt3Co/NC alloy catalysts with enhanced durability and activity for the oxygen reduction reaction

Lack of catalytic performance, short life, and high cost are three main problems related to JM-Pt/C catalysts for proton exchange membrane fuel cells. The introduction of cheap transition metals improves catalytic performance while significantly reducing the cost of the catalysts. Here, we report the synthesis of Pt₃Co/NC alloy catalysts via coating and pyrolysis treatment. The agglomeration of nanoparticles during the high-temperature alloying process is significantly inhibited by coating with PANI. Remarkably, the obtained Pt₃Co/NC alloy catalysts exhibit excellent ORR catalytic performance and structural stability in 0.1 mol/L HClO₄. After 30,000 potential cycles, the mass activity and area-specific activity of Pt₃Co/NC alloy catalysts are 1.949 and 3.936 times higher, respectively, than that of JM-Pt/C with negligible performance loss. The strong metal-support interaction between N and Pt and the Pt-rich surface restrict the dissolution of Pt and Co, resulting in excellent stability. This synthesis approach provides an effective way to develop active and stable Pt alloy catalysts.     

Effect of sulfonated carbon nanofiber-supported Pt on performance of Nafion-based self-humidifying composite membrane for proton exchange membrane fuel cell

In the present study, the Nafion^®-based self-humidifying composite membrane (N-SHCM) with sulfonated carbon nanofiber-supported Pt (s-Pt/CNF) catalyst, N-s-Pt/CNF, is successfully prepared using the solution-casting method. The scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) images of N-s-Pt/CNF indicate that s-Pt/CNF is well dispersed in the Nafion^® matrix due to the good compatibility between Nafion^® and s-Pt/CNF. Compared with those of the non-sulfonated Pt/CNF-containing N-SHCM, N-Pt/CNF, the properties of N-s-Pt/CNF, including electronic resistivity, ion-exchange capacity (IEC), water uptake, dimensional stability, and catalytic activity, significantly increase. The maximum power density of the proton exchange membrane fuel cell (PEMFC) fabricated with N-s-Pt/CNF operated at 50 °C under dry H₂/O₂ condition is about 921 mW cm⁻², which is approximately 34% higher than that with N-Pt/CNF.     

Evaluation of TiO2 as catalyst support in Pt-TiO2/C composite cathodes for the proton exchange membrane fuel cell

Anatase TiO₂ is evaluated as catalyst support material in authentic Pt-TiO₂/C composite gas diffusion electrodes (GDEs), as a different approach in the context of improving the proton exchange membrane fuel cell (PEMFC) cathode stability. A thermal stability study shows high carbon stability as Pt nanoparticles are supported on TiO₂ instead of carbon in the Pt-TiO₂/C composite material, presumably due to a reduced direct contact between Pt and C. The performance of Pt-TiO₂/C cathodes is investigated electrochemically in assembled membrane-electrode assemblies (MEAs) considering the added carbon fraction and Pt concentration deposited on TiO₂. The O₂ reduction current for the Pt-TiO₂ alone is expectedly low due to the low electronic conductivity in bulk TiO₂. However, the Pt-TiO₂/C composite cathodes show enhanced fuel cell cathode performance with growing carbon fraction and increasing Pt concentration deposited on TiO₂. The proposed reasons for these observations are improved macroscopic and local electronic conductivity, respectively. Electron micrographs of fuel cell tested Pt-TiO₂/C composite cathodes illustrate only a minor Pt migration in the Pt-TiO₂/C structure, in which anatase TiO₂ is used as Pt support. On the whole, the study demonstrates a stable Pt-TiO₂/C composite material possessing a performance comparable to conventional Pt–C materials when incorporated in a PEMFC cathode.     

The effect of carbon support treatment on the stability of Pt/C electrocatalysts

A potential cycling test was conducted to evaluate the effect of carbon support treatment on the stability of Pt/C electrocatalysts. The FTIR spectra show that after oxidative treatments, the carbon support became rich in oxygen-containing functional groups. Oxidative treatments of the carbon support increase the interaction between the metal particle and the support, resulting in an improved electrochemical stability of Pt/C catalysts. The Pt/C catalyst prepared from the H₂O₂-treated carbon support exhibits a higher stability than that prepared from the HNO₃-treated carbon support.     

PtFe alloy catalyst supported on porous carbon nanofiber with high activity and durability for oxygen reduction reaction

To reduce the high cost of oxygen reduction reaction (ORR) catalyst and improve the performance of the proton exchange membrane fuel cell (PEMFC), low-Pt or non-Pt catalysts have been studied in recent years. In this paper, PtFe alloy nanoparticles are loaded on porous carbon nanofiber (PCNF) via one-step modified glycol reduction method by adjusting solution pH. On the surface of PCNF, PtFe alloy nanoparticle can be uniformly dispersed with a narrow particle size distribution. The catalyst Pt_4.8Fe/PCNF prepared in pH = 7 solution with PCNF as carbon support exhibits better ORR performance, which shows even 18 mV higher onset potential than that of commercial catalyst Pt/C (Johnson Matthey, JM20). Moreover, comparable durability is also obtained through accelerated durability test (ADT) test after 2000 cycles. The excellent performance of Pt_4.8Fe/PCNF catalyst may attribute to the structural and electronic effects of transition metal in the PtFe alloy. The rough surface and porous structure of PCNF is also supposed to be beneficial for performance improvement.     

Stable support based on highly graphitic carbon xerogel for proton exchange membrane fuel cells

Highly graphitic carbon xerogel (GCX) is prepared by the modified sol-gel polymerization process using cobalt nitrate as the catalyst, followed by high temperature treatment at 1800 °C. The as-prepared GCX is explored as a stable support for Pt in proton exchange membrane fuel cells. The results of N₂ sorption measurement and X-ray diffraction analysis (XRD) reveal that GCX has a better mesoporous structure and a preferably higher degree of graphitization, compared with the commercial XC-72 carbon black. The transmission electron microscopy (TEM) image indicates that Pt nanoparticles are well dispersed on GCX and exhibit relatively narrow size distribution. Accelerated aging test (AAT) based on potential cycling is used to investigate the durability of the as-prepared Pt/GCX in comparison with the commercial Pt/C. Electrochemical analysis demonstrates that the catalyst with GCX as a support exhibits an alleviated degradation rate of electrochemical active surface area (39% for Pt/GCX and 53% for Pt/C). The results of single cell durability tests indicate that the voltage loss of Pt/GCX at 100 mA cm⁻² is about 50% lower than that of Pt/C. GCX is expected to be a corrosion resistant electrocatalyst support.     

Optimization of ionomer-free ultra-low loading Pt catalyst for anode/cathode of PEMFC via magnetron sputtering

In this study, thin-film Pt catalysts with ultra-low metal loadings (ranging from 1 to 200 μg cm⁻²) were prepared by magnetron sputtering onto various carbon-based substrates. Performance of these catalysts acting as anode, cathode, or both electrodes in a proton exchange membrane fuel cell (PEMFC) was investigated in H₂/O₂ and H₂/air mode. As base substrates we used standard microporous layers comprising carbon nanoparticles with polytetrafluoroethylene (PTFE) or fluorinated ethylene propylene (FEP) supported on a gas diffusion layer. Some substrates were further modified by magnetron sputtering of carbon in N₂ atmosphere (leading to CN_x) followed by simultaneous plasma etching and cerium oxide deposition. The CN_x structure exhibits higher resistance to electrochemical etching as compared to pure carbon as was determined by mass spectrometry analysis of PEMFC exhaust at different cell potentials for both sides of PEMFC. The role of platinum content and membrane thickness was investigated with the above four different combinations of ionomer-free carbon-based substrates. The results were compared with a series of benchmark electrodes made from commercially available state-of-the-art Pt/C catalysts. It was demonstrated that the platinum utilization in PEMFC with magnetron sputtered thin-film Pt electrodes can be up to 2 orders of magnitude higher than with the standard Pt/C catalysts while keeping the similar power efficiency and long-term stability.