Highly active and durable carbon support Pt-rare earth catalyst for proton exchange membrane fuel cell

Pt-rare earth catalysts are highly efficient novel electrocatalyst for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) due to their high stability and activity. In this study, we prepare Pt-YO_x/C catalysts using the traditional wet chemical reduction method. The optimal quantity of Y-oxides loaded onto the Pt/C surface is determined based on electrochemical performance using linear sweep voltammetry (LSV) and cyclic voltammetry (CV) methods. After accelerated durability tests (ADT), the remnant electrochemical surface area (ECSA) and mass active (MA) in Pt-YO_x/C catalyst are relatively higher compared to the commercial Pt/C (JM). In the single-cell test, the maximum mass power densities of the MEAs prepared by self-made Pt-YO_x/C and Pt/C (JM) catalysts in cathodes record at 1895 and 1371 mW mg_Pt⁻¹, respectively, which shows a successful increment in platinum utilization. These results indicate that Pt-YO_x/C catalyst can potentially improve the durability and lower the cost of PEMFCs.     

A novel catalyst layer with carbon matrix for Pt nanowire growth in proton exchange membrane fuel cells (PEMFCs)

Novel catalyst layers for proton exchange membrane fuel cells (PEMFCs) were investigated by in-situ growing of Pt nanowires (Pt-NWs) on carbon matrix. The Pt-NWs grew on the matrix along the thickness direction with a length of 10–20 nm and a diameter of 4 nm. In-situ cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and polarization experiments were employed to characterize the electrochemical performance of the Pt-NWs electrodes. The results showed that the predominantly {111}-oriented facets and oxygen access of the Pt-NWs structure contribute to the higher performance in comparison with that of the conventional catalyst layers. This work is advantageous for fuel cell catalyst layer design by allowing the controlled modification of both Pt distribution and pore size.     

Facile synthesis and characterization of highly dispersed platinum nanoparticles for fuel cells

We report a facile synthesis and characterization of highly-dispersed platinum nanoparticles supported on Ketjen carbon black (Pt/C) as electrocatalysts for polymer electrolyte membrane fuel fells (PEMFCs). Pt particles with size of ∼ 2.6 nm were synthesized through adsorption of Pt acetylacetonate on carbon supports and subsequently thermal decomposition. A comparative characterization analysis, including X-ray diffraction (XRD), high resolution transmission electron microscope (HR-TEM), cyclic voltammetry (CV), and hydrodynamic voltammetry measurements, was performed on the synthesized and commercial TKK catalysts. It revealed the details of Pt dispersion on the carbon support, particle size and distribution, electrochemical surface area (ECSA), and oxygen reduction reaction (ORR) activity of the catalysts. It was found that the synthesized Pt/C has similar particle size to that of the TKK catalyst (2.6 nm and 2.7 nm, respectively), but narrower particle size distribution. Accelerated durability tests under potential cycles were performed to study electrochemical degradation of the catalysts in corrosive environments. The synthesized Pt/C displayed significant losses in ECSA and activities after 20 k potential cycles, especially from 5 k to 20 k cycles, though with higher initial values (43% and 79% higher in ECSA and mass activity, respectively).     

Degradation of polymer electrolyte membrane fuel cells repetitively exposed to reverse current condition under different temperature

Effects of operating temperature on performance degradation of polymer electrolyte membrane fuel cells (PEMFCs) were investigated under the repetitive startup/shutdown cycling operation that induced the so-called ‘reverse current condition’. With repeating the startup/shutdown cycle, polarization curves, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), linear sweep voltammetry (LSV) were measured to examine in situ electrochemical degradation of the MEAs. To investigate physicochemical degradation of the MEAs, scanning electron microscopy (SEM), electron probe micro analysis (EPMA), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FT-IR) were employed before and after the startup/shutdown cycling operation. With increasing operating temperature from 40 to 65 and 80 °C under the repetitive reverse-current condition, the cell performance decayed faster since corrosion of the carbon support and dissolution/migration/agglomeration of Pt catalyst were accelerated resulting in increases in ohmic and charge transfer resistance and loss of EAS.     

Development of tailored high-performance and durable electrocatalysts for advanced PEM fuel cells

A family of novel carbon materials with intermediate surface area and varying morphology and surface chemistry were used to prepare Pt/C catalysts by two different preparation procedures; a chemical impregnation method and a microwave-assisted polyol method. The catalysts were thoroughly characterized, and their electrochemical performance and stability were investigated with rotating disc electrode (RDE) cyclic voltammetric (CV) measurements. The intermediate-surface-area carbon supports gave catalysts with much greater support stability than a widely used standard catalyst. The novel catalysts had lower electrochemical surface area than the reference, but their specific electrocatalytic activity towards the oxygen-reduction reaction (ORR) was much higher, and some of them also featured higher mass-specific ORR activity than the reference. The series of catalysts prepared by the microwave-assisted polyol method featured smaller Pt nanoparticles and higher activities than those prepared by impregnation. On the other hand, the impregnated catalysts showed better durability of the Pt particles. The most promising catalysts were selected and elaborated in further optimized preparation procedures to obtain quantities sufficient for their use in proton-exchange membrane fuel cells (PEMFCs).     

Combined method to prepare core–shell structured catalyst for proton exchange membrane fuel cells

This paper reports a modified core–shell structured CuPd@Pt/C catalyst, which was synthesized by combining a two-step reduction method and chemical dealloying step using carbon black Vulcan XC-72R as the supporting material. The physical measurements confirmed that the final catalyst has a complete core–shell structure. The interaction between the core and the shell as well as the particle size, particle size distribution, and morphology of the catalyst particles were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results of inductively coupled plasma atomic emission spectroscopy (ICP-AES) and X-ray photoelectron spectroscopy (XPS) indicated that the Pt and Pd on the surface of the catalyst nanoparticles are mainly in the zero valence oxidation state. Cyclic voltammetry (CV) and rotating disk electrode (RDE) tests were used to measure the electrochemical performance, and the results showed that the modified CuPd@Pt/C catalyst has higher electrochemical catalytic activity in catalyzing the oxygen reduction reaction (ORR) than Pt/C, making it possible to reduce the usage of platinum and leading to a promising low-Pt catalyst for proton exchange membrane fuel cells (PEMFCs).     

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.     

Comparison of the performance and degradation mechanism of PEMFC with Pt/C and Pt black catalyst

Proton exchange membrane fuel cell (PEMFC) has been used in supplying power for Unmanned Underwater Vehicle which operate in a closed environment and dead-ended anode and cathode (DEAC) mode is deemed as an effective way to enhance the fuel utilization rate. Catalyst is an important factor that influences the performance and durability of PEMFC, especially in DEAC mode. In this paper, the degradation characteristics of PEMFCs with Pt black and Pt/C catalyst after 100 h operation have been investigated by electrochemical techniques and morphological characterization methods. It's shown that the degradation of Pt black catalyst layer (CL) was more severe than that of Pt/C CL. The difference of performance degradation is due to the dominant decay mechanism of these two catalysts is different. According to SEM, TEM and XPS results, the decay of Pt black catalyst is mainly caused by Pt agglomeration and oxidation, causing a higher ohmic resistance, higher mass transfer resistance and severer degradation of performance. The degradation of Pt/C catalyst is mainly due to the reduction of electrochemical surface area and carbon corrosion because the larger carbon corrosion makes micropores and the thicker supporting structure, resulting in the performance degradation.     

Non-kinetic losses caused by electrochemical carbon corrosion in PEM fuel cells

This paper presented non-kinetic losses in PEM fuel cells under an accelerated stress test of catalyst support. A cathode with carbon-supported Pt catalyst was prepared and characterized during potential hold at 1.2 V vs. SHE in a PEM fuel cell. Irreversible losses caused by carbon corrosion were evaluated using a variety of electrochemical characterizations including cyclic voltammetry, linear sweep voltammetry, electrochemical impedance spectroscopy, and polarization technique. Ohmic losses at the cathode during potential hold were determined using its capacitive responses. Concentration losses in the PEM fuel cell were analyzed in terms of Tafel behavior and thin film/flooded-agglomerate dynamics.     

Analysis of degradation in PEMFC caused by cell reversal during air starvation

The damage caused by cell reversal during proton exchange membrane fuel cells (PEMFCs) operation with air starvation was investigated by a single-cell experiment. Samples from degraded membrane–electrode assemblies (MEAs) were characterized. The loss of electrochemical surface area of the cathode platinum was detected by in situ cyclic voltammetry, and platinum sintering was detected by transmission electron microscopy (TEM) analysis. Degradation at the anode was not detected in the chemical analysis of the anode catalyst layer of MEA samples by energy dispersive X-ray analysis (EDX) and TEM. An obvious decrease in the performance of PEMFC was observed in a sample degraded by cell reversal for 120 min.     

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.     

Carbon corrosion behaviors and the mechanical properties of proton exchange membrane fuel cell cathode catalyst layer

Carbon corrosion-induced catalyst layer destruction is the primary reason to the performance decay of proton exchange membrane fuel cells (PEMFCs). In this study, the accelerated stress test (AST) on carbon corrosion was conducted, and real-time CO₂ evolution was measured in-situ by non-dispersive infrared (NDIR) analysis. The performance degradation was investigated by the reduction of the current density and the loss of electrochemical active surface area (ECSA) of Pt. The loss of catalyst layer porosity and increase of mass transport resistance were investigated by the visible reduction of porosity and thickness in the cathode catalyst layer (CCL). Further mechanical tensile tests showed that the elastic modulus of CCL remained unchanged initially, and then increased probably due to the compaction of CCL. In the final step, it decreased due to the complete failure of the material. Thus, carbon corrosion was proved to alter the mechanical strength of CCL.     

Pt-based trimetallic nanocrystals with high proportions of M (M=Fe,Ni) metals for catalyzing oxygen reduction reaction

For boosting oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs), a new type of multi-functional catalyst with high activity, high stability, and low cost has been designed and prepared by introducing high proportions of M (M = Fe, Ni) metals in Pt-based nanoparticles through a microwave-assisted polyol method, followed by thermal annealing process. A uniform dispersion of nanoparticles (5 nm) and a face-centered tetragonal (fct) phase improve the activity and stability of the Pt–Fe–Ni/C catalyst. Owing to differences in the surface energies of the alloying elements, Pt atoms with low surface energy have a tendency to segregate from the subsurface to the surface during the annealing. This tendency exposes the internal Pt atoms to the surface of the nanoparticles in the existence of high proportions of M metals, significantly improving the utilization of Pt. As a cathode catalyst, the Pt–Fe–Ni/C catalyst annealed at 675 °C with a mass activity of 0.73 A/mg_Pt, which is 3.5 times higher than that of the commercial Pt/C catalyst, exhibits an excellent half-cell performance. An accelerated durability test demonstrates that the prepared Pt–Fe–Ni/C-675 catalyst is more stable than the commercial Pt/C. The proposed multi-functional catalyst has great potential for PEMFCs and other applications.     

Performance of carbon-supported PtPd as catalyst for hydrogen oxidation in the anodes of proton exchange membrane fuel cells

In this work, the replacement of platinum by palladium in carbon-supported catalysts as anodes for hydrogen oxidation reaction (HOR), in proton exchange membrane fuel cells (PEMFCs), has been studied. Anodes with carbon-supported Pt, Pd, and equiatomic Pt:Pd, with various Nafion^® contents, were prepared and tested in H₂|O₂ (air) PEMFCs fed with pure or CO-contaminated hydrogen. An electrochemical study of the prepared anodes has been carried out in situ, in membrane electrode assemblies, by cyclic voltammetry and CO electrooxidation voltammetry. The analyses of the corresponding voltammograms indicate that the anode composition influences the cell performance. Single cell experiments have shown that platinum could be replaced, at least partially, saving cost with still good performance, by palladium in the hydrogen diffusion anodes of PEMFCs. The performance of the PtPd catalyst fed with CO-contaminated H₂ used in this work is comparable to Pt, thus justifying further work varying the CO concentration in the H₂ fuel to assert its CO tolerance and to study the effect of the Pt:Pd atomic ratio.     

Effect of gas composition on Ru dissolution and crossover in polymer-electrolyte membrane fuel cells

Pt-Ru-based anodes are commonly used in polymer-electrolyte membrane fuel cells (PEMFCs) to provide improved CO tolerance for reformate fuel applications. However, Ru crossover from the anode to the cathode has been identified as a critical durability problem that has severe performance implications. In the present study, an anode accelerated stress test (AST) was used to simulate potential spikes that occur during fuel cell start-ups and shutdowns to induce Ru crossover. The effects of fuel gas composition, namely hydrogen and carbon dioxide concentrations, on Ru dissolution and crossover were investigated. The cell performance losses were correlated with the degree of Ru crossover as determined by the changes in cathode cyclic voltammetry (CV) characteristics and neutron activation analysis (NAA). It was found that higher hydrogen concentration in the fuel accelerated Ru crossover and that the presence of carbon dioxide hindered Ru crossover. In particular, the injection of 20 vol.% carbon dioxide during potential cycling resulted in very minor Ru crossover, which showed essentially identical performance losses and CV characteristic changes as a fuel cell composed of a Ru-free anode. The experimental results suggest that the Ru species in our Pt-Ru metal oxide catalysts need to go through a reduction step by hydrogen before dissolution. The presence of carbon dioxide may play a role in hindering the reduction step.     

Degradation mechanism study of PTFE/Nafion membrane in MEA utilizing an accelerated degradation technique

Nafion membranes are widely used for commercial membrane electrode assemblies (MEAs) in proton exchange fuel cells (PEMFCs). The polytetrafluoroethylene (PTFE)/Nafion (PN) composite membrane has the advantages of being low in cost, high in mechanical strength, and does not swell excessively. This study focuses on the properties of PTFE/Nafion membranes and PTFE/Nafion MEAs by comparing the durability and performance of the PN MEAs to commercial Nafion 211 MEAs. In an accelerated degradation test (ADT), the characterization of PTFE/Nafion and Nafion MEAs were analyzed using in-situ electrochemical methods such as polarization curves, AC impedance, cyclic voltammetry (CV), and linear sweep voltammetry (LSV). The results demonstrate an increase in the internal resistance on the PTFE/Nafion MEA only. The three mechanisms behind this unique result were proposed to be: (a) Separation of the catalyst layer from the membrane due to creep deformation; (b) Separation of the outer Nafion layer film from the core PTFE/Nafion membrane due to creep deformation; (c) Degradation of the Nafion plane (or Nafion dissolution) from the PTFE surface.     

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.     

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.     

Analysis of carbon-supported platinum through potential cycling and potential-static holding

The stability of platinum and carbon support in catalyst-coated membrane (CCM) was investigated by a potential cycling between 0.7 and 0.9 V and a potential-static holding at 1.2 V, 1.3 V and 1.5 V in single cells. Clear cell performance deterioration can be observed by polarization curves during accelerated stress tests, along with electrochemical surface area (ESA) loss of Pt catalysts by cyclic voltammogram (CV). The X-ray diffraction (XRD) results of CCM before and after tests show that a distinct Pt agglomeration occurred from approximate 3 nm–8 nm in diameter, which is in accord with the observation of Pt/C by transmission electron microscopy (TEM). It is also interesting to note that, redeposited Pt particles in the membrane could be as large as hundreds of nanometers from TEM images of CCM microtomy. X-ray photoelectron spectroscopy (XPS) of carbon 1S indicates that the corrosion of carbon support is highly dependent on the holding potential, and enormous surface groups, such as carboxyl, lactones and ether were generated after tests. Meanwhile, a severer ESA loss of Pt after carbon corrosion under high potential holdings happens than that of potential cycling. The results indicate that both Pt and carbon support in the catalyst are important to maintain a long-term stable operation for fuel cells.     

The effect of CuO on a Pt−Based catalyst for oxidation in a low-temperature fuel cell

Electrocatalytic oxidation of methanol, ethanol, and formic acid has currently attracted research attention for low-temperature fuel cells. However, the efficiencies of these fuel cells mainly depend on the electrocatalytic activities of Pt-based anodic catalysts due to the problems of low kinetics for small organic molecule electro-oxidation. An anode catalyst can be developed by the addition of some metal oxides into a Pt-based catalyst, which can effectively promote the electro-oxidation of fuels based on small organic molecules. In this work, a nanocomposite catalyst consisting of multi-wall carbon nanotubes (CNTs), copper oxide (CuO) and Pt nanoparticles was synthesized and used to improve fuel cell oxidation. Due to its low cost and oxophilic character, the metal oxide can play a major role in the oxidation of CO. The synthesis of xPt?yCuO/CNT electrocatalysts was executed through two steps: supporting of CuO nanoparticles on CNTs by the alcothermal method followed by Pt loading onto the prepared CuO/CNT by chemical reduction. The as-prepared catalysts were physically characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, and electrochemical measurements. The results demonstrate that CuO is well dispersed onto the CNTs and that this oxide can further interact with the active Pt present on the as-prepared catalyst composites. The activity of various xPt?yCuO/CNT electrocatalysts was determined by cyclic voltammetry (CV), where x and y are the mass ratios of Pt and CuO, respectively. The presence of CuO was found to significantly contribute to enhanced electroactivity towards oxidation reactions. The 1Pt–3CuO/CNT electrocatalyst is a capable catalyst for improving low-temperature fuel cell applications.