Evaluation of the stability and durability of Pt and Pt–Co/C catalysts for polymer electrolyte membrane fuel cells

Carbon supported Pt and Pt–Co nanoparticles were prepared by reduction of the metal precursors with NaBH₄. The activity for the oxygen reduction reaction (ORR) of the as-prepared Co-containing catalyst was higher than that of pure Pt. 30 h of constant potential operation at 0.8 V, repetitive potential cycling in the range 0.5–1.0 V and thermal treatments were carried out to evaluate their electrochemical stability. Loss of non-alloyed and, to a less extent, alloyed cobalt was observed after the durability tests with the Pt–Co/C catalyst. The loss in ORR activity following durability tests was higher in Pt–Co/C than in Pt/C, i.e. pure Pt showed higher electrochemical stability than the binary catalyst. The lower stability of the Pt–Co catalyst during repetitive potential cycling was not ascribed to Co loss, but to the dissolution–re-deposition of Pt, forming a surface layer of non-alloyed pure Pt. The lower activity of the Pt–Co catalyst than Pt following the thermal treatment, instead, was due to the presence of non-alloyed Co and its oxides on the catalyst surface, hindering the molecular oxygen to reach the Pt sites.     

Lifetime prediction of a polymer electrolyte membrane fuel cell via an accelerated startup–shutdown cycle test

To expand commercial applications of polymer electrolyte membrane fuel cells (PEMFCs), the evaluation time for their durability must be shortened. This article provides a straightforward accelerated degradation testing (ADT) procedure for PEMFC for easy and quick implementation of the procedure. The ADT procedure includes statistical modeling of degradation patterns of membrane electrode assemblies (MEAs) in PEMFCs under startup–shutdown cycling conditions. For this purpose, we propose a nonparametric degradation model to describe the nonlinear performance degradation paths of PEMFC MEAs. The analysis results indicate that the nonparametric approach provides more accurate estimates of the observed degradation data than other parametric approaches. Based on the nonparametric degradation model, we suggest a method to predict failure-times under normal operating conditions by estimating the time-scale factor under accelerated operating conditions.     

Concentration measurements in lithium/polymer–electrolyte/lithium cells during cycling

We report on in situ and ex situ concentration measurements in lithium/polymer–electrolyte/lithium cells during cycling. We have used three different methods which give complementary results, in good agreement with theoretical predictions and previous concentration measurements by Raman confocal microspectroscopy. Our methods allow to obtain concentration maps in the electrolyte, in particular, when dendrites are observed: from these measurements, we can correlate the onset of dendritic growth with local concentration gradients.     

Electrophoretic deposition of carbon-supported octahedral Pt–Ni alloy nanoparticle catalysts for cathode in polymer electrolyte membrane fuel cells

The advanced electrochemical catalytic activity for oxygen reduction reaction (ORR) based on the octahedral Pt–Ni alloyed catalyst has been demonstrated. However, a means of fabricating catalyst electrodes for use in PEMFCs that is cost-effective, scalable, and maintains the high activity of Pt–Ni_alloy/C has remained out of reach. Electrophoretic deposition (EPD) is a colloidal production process that has a history of successful deployment at the industrial scale. Here, we report on the facile preparation of an effective and active cathode consisting of Pt–Ni alloy loaded on the carbon cloth substrate using the electrophoretic deposition (EPD) technique, in which the optimum applied voltages and suspension pH are systematically investigated to obtain the highly porous Pt–Ni_alloy/C catalyst electrode. In a half cell test, the EPD-made Pt–Ni_alloy/C catalyst electrodes fabricated at 45 V and in a solution with a pH of 9.0 yields the best performances. On the other, as an active cathode, the EPD-made Pt–Ni_alloy/C electrodes deliver a superior performance in single cell test, with the maximum power density reaches 7.16 W/mg_Pt, ～28.1% higher than that of the spray-made Pt/C conventional electrode. The outperformance is attributed to the significantly higher porosity and surface roughness of the EPD-made electrode.     

Integrated carbon composite bipolar plate for polymer–electrolyte membrane fuel cells

The electrical resistance of bipolar plates for polymer–electrolyte membrane fuel cells (PEMFCs) should be very low to conduct the electricity generated with minimum electrical loss. The resistance of a bipolar plate consists of the bulk material resistance and the interfacial contact resistance when two such plates are contacted to provide channels for fuel and air (oxygen) supplies.     

Endurance study of a solid polymer electrolyte direct ethanol fuel cell based on a Pt–Sn anode catalyst

The performance decay of a solid polymer electrolyte direct ethanol fuel cell (DEFC) based on a Pt₃Sn₁/C anode catalyst during an endurance test has been investigated. The effect of different cell shut-down procedures on the cycled behaviour of the DEFC has been studied. To get specific insights into the degradation mechanism, polarization and ac-impedance spectroscopy studies have been carried out. These analyses have been complemented by post-operation transmission electron microscopy and X-ray diffraction studies. The combination of these techniques has allowed to get information on recoverable and unrecoverable losses. This provides a basis for further improvement of DEFC components.     

Effective transport properties for polymer electrolyte membrane fuel cells – With a focus on the gas diffusion layer

Multi-phase transport of reactant and product species, momentum, heat (energy), electron and proton in the components of polymer electrolyte membrane (PEM) fuel cells forms the three inter-related circuits for mass, heat (energy) and electricity. These intertwined transport phenomena govern the operation and design, hence the performance, of such cells. The transport processes in the cell are usually determined with their respective effective transport properties due to the porous nature of PEM fuel cell components. These properties include the effective diffusion coefficient for the mass transfer, effective thermal conductivity for heat transfer, effective electronic conductivity for electron transfer, effective protonic conductivity for proton transfer, intrinsic and relative permeability for fluid flow, capillary pressure for liquid water transfer, etc. Accurate determination of these effective transport properties is essential for the operation and design of PEM fuel cells, especially at high current density operation. Thus, it is the focus of intensive research in the recent years. In this article, a review is provided for the determination of these effective transport properties through the various PEM fuel cell components, including the gas diffusion layer, microporous layer, catalyst layer and the electrolyte membrane layer. Given the simplicity of the GDL in structure compared to the other components of the cell, much more work in literature is focused on its transport properties. Hence, its review in this paper is more extensive. Various methods used for the determination of the effective transport properties with and without the presence of liquid water are reviewed, including experimental measurements, numerical modeling and theoretical analyses. Correlations are summarized for these transport properties, where available and further work in this area is provided as a direction for future work.     

Effect of fiber orientation on Liquid–Gas flow in the gas diffusion layer of a polymer electrolyte membrane fuel cell

In this study, the lattice Boltzmann method was used to simulate the three-dimensional intrusion process of liquid water in the gas diffusion layer (GDL) of a polymer electrolyte membrane fuel cell (PEMFC). The GDL was reconstructed by the stochastic method and used to investigate fiber orientation's influence on liquid water transport in the GDL of a PEMFC. The fiber orientation can be described by the angle between a single fiber and the in-plane direction; three different samples were simulated for three different fiber orientation ranges. The simulated permeability correlated well with the anisotropic characteristics of reconstructed carbon papers. It was concluded that the fiber orientation had a significant effect on the liquid invasion pattern in the GDL by changing the pore shape and distribution of the GDL. The results indicated that the stochastically reconstructed GDL, taking into account the fiber orientation, better demonstrates the mass transport properties of the GDL.     

Corrosion behavior of polyphenylene sulfide–carbon black–graphite composites for bipolar plates of polymer electrolyte membrane fuel cells

The aim of this work was to study the corrosion behavior of polyphenylene sulfide (PPS) – carbon black – graphite composites regarding their application as bipolar plates of polymer electrolyte membrane (PEM) fuel cells. Electrochemical impedance spectroscopy (EIS), potentiostatic and potentiodynamic polarization tests were used to characterize the electrochemical response of the composites in a simulated PEM fuel cell environment. Cross-sectional views of fractured specimens were observed by scanning electron microscopy (SEM). The results showed that the corrosion behavior depends on the carbon black content incorporated into the composite formulation. There was a trend of decreasing the corrosion resistance for higher carbon black contents. This behavior could be explained based on the porosity and electrical conductivity of the composites.     

Morphological influence of graphitic carbon nanofibers by N–F dual-doping on Pt electrocatalytic activity and stability for oxygen reduction reaction in polymer electrolyte membrane fuel cells

Morphology of carbon nanofibers significantly effects Pt nanoparticles dispersion and specific interaction with the support, which is an important aspect in the fuel cell performance of the electrocatalysts. This study emphasizes, the defects creation and structural evolution comprised due to N–F co-doping on graphitic carbon nanofibers (GNFs) of different morphologies, viz. GNF-linearly aligned platelets (L), antlers (A), herringbone (H), and their specific interaction with Pt nanoparticle in enhancing the oxygen reduction reaction (ORR). GNFs–NF–Pt catalysts exhibit better ORR electrocatalytic activity, superior durability that is solely ascribed to the morphological evolution and the doped N–F heteroatoms, prompting the charge density variations in the resultant carbon fiber matrices. Amongst, H–NF–Pt catalyst performed outstanding ORR activity with exceptional electrochemical stability, which shows only 20 mV loss in the half-wave potential whilst 100 mV loss for Pt/C catalyst on 20,000 potential cycling. The PEMFC comprising H–NF–Pt as cathode catalyst with minimum loading of 0.10 mg cm^?2, delivers power density of 0.942 W cm^?2 at current density of 2.50 A cm^?2 without backpressures in H₂–O₂ feeds. The H–NF–Pt catalyst owing to its hierarchical architectures, performs well in PEMFC at the minimized catalyst loading with outstanding stability that can significantly decrease total price for the fuel cell.     

CO tolerance of nano-architectured Pt–Mo anode electrocatalysts for PEM fuel cells

PEM fuel cell membrane electrode assemblies with Nafion electrolytes and commercial Pt-based cathodes were tested with Pt_0.8Mo_0.2 alloy and MoO_x@Pt core–shell anode electrocatalysts for CO tolerance and short-term stability to corroborate earlier thin-film RDE results. Polarization curves at 70 °C for the Pt_0.8Mo_0.2 alloy in H₂ with 25–1000 ppm CO showed a significant increase in CO tolerance based on peak power densities in comparison to PtRu electrocatalysts. MoO_x@Pt core–shell electrocatalysts, which showed extremely high activity for H₂ in 1000 ppm CO during RDE studies, performed relatively poorly in comparison to the Pt_0.8Mo_0.2 and PtRu alloys for the same total catalyst loading on a per area basis in MEA testing. The discrepancy is attributed to residual stabilizer from the core–shell synthesis impacting catalyst-ionomer interfaces. Nonetheless, the MoO_x@Pt electrochemical performance is superior on a per-gram-of-precious-metal basis to the Pt_0.8Mo_0.2 electrocatalyst for CO concentrations below 100 ppm. Due to cross-membrane Mo migration, the stability of the Mo-containing anode electrocatalysts remains a challenge for developing stable enhanced CO tolerance for low-temperature PEM fuel cells.     

Effect of Pt: Pd atomic ratio in Pt–Pd/C electrocatalyst-coated membrane on the electrocatalytic activity of ORR in PEM fuel cells

The influence of Pt: Pd atomic ratios (1:2–1:8) on a carbon support upon its suitability as a cathode for a proton exchange membrane (PEM) fuel cell was evaluated at a constant membrane electrocatalyst loading of 0.15 mg/cm². The results clearly demonstrated that the different Pt: Pd atomic ratios had a significant effect on both the electrocatalyst activity and also on the performance in a H₂/O₂ fuel cell. Decreasing the Pt: Pd atomic ratio led to an increase in the particle size of the electrocatalyst but a decrease in the particle dispersion and electrochemical surface area (ESA). With respect to the performance in a PEM fuel cell, decreasing the Pt: Pd atomic ratio led to a decreased exchange current density (j₀), electrocatalytic activity and also mass activity (M_A), but to an increased total resistance (R) of the cell. The maximum activity of the oxygen reduction reaction (ORR) and the peak power (492 mW/cm²) were obtained with an electrocatalyst with a Pt: Pd atomic ratio of 1:2. Finally, the rotating disk electrode (RDE) analysis showed that the mechanism of oxygen reduction on the prepared Pt–Pd/C electrocatalyst involved a four-electron pathway with high oxygen permeability in the Nafion film.     

The influence of hydrogen sulphide contamination on platinum catalyst used in polymer electrolyte membrane fuel cells during potential cycling at 0.05–1.05 V vs RHE: An RRDE study

The effect of H₂S contamination on platinum catalyst has been investigated in terms of rotating ring disk electrode measurements (RRDE) in 0.1 M HClO₄. The electrochemical surface area (ECSA) was determined by hydrogen underpotential deposition (HUPD) and CO stripping methods before and after accelerated stress tests (AST). The observed reduced losses of ECSA of the catalyst in the presence of H₂S were associated to adsorbed sulphuric compounds on the catalyst surface which changed electrochemical characteristics of the materials surface. The RRDE experiments revealed that for oxygen reduction reaction (ORR) mass and specific activities were essentially decreased after AST with H₂S contamination which was attributed to the interstitial defect on platinum atom sites due to adsorbed sulphuric compounds. Identical location and high resolution transmission electron microscopy (TEM) analysis have revealed only slightly different catalyst surface morphology and particle sizes before and after the AST indicating rather atomic scale deterioration of the platinum catalyst surface due to the adsorbed sulphur species.     

Ni–Cr Co-implanted 316L stainless steel as bipolar plate in polymer electrolyte membrane fuel cells

A Ni–Cr enriched layer about 60 nm thick with improved conductivity is formed on the surface of austenitic stainless steel 316L (SS316L) by ion implantation. The electrochemistry results reveal that a proper Ni–Cr implant fluence can greatly improve the corrosion resistance of SS316L in the simulated PEMFC environment. The samples after the potentiostatic test are also analyzed by XPS and the ICR values are measured. The XPS results indicate that the composition of the passive film change from a mixture of Fe oxides and Cr oxide to a Cr oxide dominated passive film after the potentiostatic test. Hence, the ICR increases after polarization due to depletion of iron in the passive film. Nickel is enriched in the passive film formed in the simulated PEMFC cathode environment after ion implantation thereby providing better conductivity than that formed in the anode one.     

Evaluation of vacuum heat-treated α-C films for surface protection of metal bipolar plates used in polymer electrolyte membrane fuel cells

Proton exchange membrane fuel cells (PEMFCs) assembled with metal bipolar plates (BPPs) is a promising power source for new energy vehicles. However, metal BPPs have serious corrosion issues and the surface electrical performance degrades in the harsh PEMFC environment. Amorphous carbon (α-C) films exhibit improved properties with both high corrosion resistance and electrical conductivity. Subsequent vacuum heat treatment of the as-coated α-C films can change their phase composition and film structure, modifying their performance. This study prepared α-C films with a titanium interlayer on a SS316L substrate by DC balance magnetron sputtering and then subjected them to vacuum heat treatment at different temperatures (400–700 °C). These treated α-C films are systemically analyzed in terms of surface and cross-sectional morphologies, sp bond hybridization, interfacial electrical conductivity, and corrosion resistance. The results indicate that the conversion of sp³ to sp² and the compact density of α-C films are greatly enhanced with an increase in temperature, greatly improving the corrosion resistance and surface conductivity of the films. These promising results lead to a potential direction for the post-coating treatment of α-C films on metal BPPs in PEMFCs.     

Voltage–time behavior of a polymer electrolyte membrane fuel cell stack at constant current discharge

Empirical equations were developed to describe the voltage–time behavior of polymer electrolyte membrane fuel cell (PEMFC) stacks at constant current discharge. When either ambient temperature or discharge current is too high, the experimental voltage–time curves exhibit rapidly falling cell voltage within a short discharge time. Various experimental voltage–time curves have been fitted very well with empirical equations at different discharge currents and ambient temperatures. The effect of parameters of the empirical equations on the shape of voltage–time curve is also analyzed. Inadequate mass-transfer is likely a reason for the voltage falling down rapidly, and polymer electrolyte membrane dehydration is responsible for the inclination of the voltage–time curves. The empirical equations are helpful for forecasting and explaining the long-term discharge performance of the PEMFC stacks.     

Electrochemical synthesis and characterization of carbon-supported Pt and Pt–Ru nanoparticles with Cu cores for CO and methanol oxidation in polymer electrolyte fuel cells

Pt and Pt–Ru shells on Cu cores supported on Vulcan carbon XC72R have been synthesized and tested as possible anode electrocatalysts for polymer electrolyte fuel cells. Pt(Cu)/C was prepared by Cu electrodeposition on the black carbon support at constant potential followed by Pt deposition on Cu by galvanic exchange, whereas Pt–Ru(Cu)/C was prepared by spontaneous deposition of Ru species on Pt(Cu)/C. The corresponding cyclic voltammograms in 0.5 M H₂SO₄ solution showed the hydrogen adsorption/desorption peaks and no Cu oxidation. The respective CO stripping peak potentials of Pt(Cu)/C and Pt–Ru(Cu)/C were about 0.1 and 0.2 V more negative than those corresponding to Pt/C and Ru-decorated Pt/C. The best conditions for CO oxidation were found for Cu deposition potentials between −0.2 and −0.4 V vs. Ag/AgCl/KCl(sat). The Pt economy of the Pt–Ru(Cu)/C system was proved for the methanol oxidation, with specific currents more than twice those obtained on the Ru-decorated commercial Pt/C catalysts.     

Ni–Pt/C as anode electrocatalyst for a direct borohydride fuel cell

In this study, a series of Ni–Pt/C and Ni/C catalysts, which were employed as anode catalysts for a direct borohydride fuel cell (DBFC), were prepared and investigated by XRD, TEM, cyclic voltammetry, chronopotentiometry and fuel cell test. The particle size of Ni₃₇–Pt₃/C (mass ratio, Ni:Pt = 37:3) catalyst was sharply reduced by the addition of ultra low amount of Pt. And the electrochemical measurements showed that the electro-catalytic activity and stability of the Ni₃₇–Pt₃/C catalysts were improved compared with Ni/C catalyst. The DBFC employing Ni₃₇–Pt₃/C catalyst on the anode (metal loading, 1 mg cm⁻²) showed a maximum power density of 221.0 mW cm⁻² at 60 °C, while under identical condition the maximum power density was 150.6 mW cm⁻² for Ni/C. Furthermore, the polarization curves and hydrogen evolution behaviors on all the catalysts were investigated on the working conditions of the DBFC.     

Numerical investigation of tridirectionally synergetic Nafion® ionomer gradient cathode catalyst layer for polymer electrolyte membrane fuel cells

Pervious studies have compressively examined a single direction ionomer gradient cathode catalyst layer (CCL) for enhancement of polymer electrolyte membrane (PEM) fuel cells. However, a tridirectionally synergetic (TS) ionomer gradient CCL has not been investigated to date. It is reported that the design of multi-directional ionomer gradient CCL is significant for fuel cells. Therefore, this study proposes a novel TS gradient CCL for fuel cells. By implementing a three-dimensional multiphase fuel cell model, the novel TS gradient CCL is evaluated regarding internal physicochemical quantity distributions and overall cell performance. Numerical results indicate that in comparison with CCL with uniform and a single direction through-plane (TP) or in-plane (IP) ionomer gradient CCL with an identical ionomer content, the novel TS gradient CCL can enhance oxygen diffusion and proton conductivity within porous electrodes, thus improving the maximum power density by approximately 6.8%. Moreover, the optimal TS ionomer gradient CCL contributes to a reduction in coefficient variation of current density within CCL by 10.3% and 17.1% compared to that of uniform and TP(X) gradient CCLs, leading to more homogeneous internal physical quantity profiles, ultimately benefiting the stable operation and durability of fuel cells. The findings here can provide a new insight for guiding the design of ionomer gradient CCL.     

Development of ultra–low highly active and durable hybrid compressive platinum lattice cathode catalysts for polymer electrolyte membrane fuel cells

A highly active and stable catalyst/support system is developed by using a two-step process. In the first step, activated carbon composite support (ACCS) is synthesized that retains its activity after accelerated stress test (AST). A 30% Pt/ACCS catalyst shows no loss of mass activity and power density after 5000 cycles at 1.0–1.5 V while the commercial Pt/C and Pt/290G catalysts show drastic mass activity losses (57.5% and 66.2%, respectively) and power density losses (88.7% and 84.0%, respectively). In the second step, Pt catalyst with a compressive Pt lattice (Pt¹) is synthesized through a USC-developed annealing procedure in which Co atoms previously embedded in the support diffuse into Pt. The 30% Pt¹/ACCS shows high initial power density (rated) of 0.174 g_Pt kW^?1 and high stability of 24 mV loss at 0.8 A cm^?2 with an electrochemical active surface area (ECSA) loss of 42% after 30,000 cycles (0.6–1.0 V). The support stability under 1.0–1.5 V potential cycling shows potential loss of 8 mV at 1.5 A cm^?2 and ECSA loss of 22% after 5000 cycles. Improved stability and activity of Pt*/ACCS catalyst are due to synergistic effect of catalytic activity and stability of ACCS and formation of compressive Pt lattice catalyst.