Fuel Cell Relevant Conditions Research Articles

Influence of the shell thickness on the degradation of Ru@Pt core-shell catalysts in PEM fuel cells

Polymer electrolyte membrane fuel cells, which are fueled by reformate, are reliant on highly active, CO tolerant Ru@Pt core-shell anode catalysts. As little is known of the degradation behaviour of these catalysts, the herein presented study investigated membrane electrode assemblies (MEAs), which were stressed under fuel cell relevant conditions, via electrochemical and physical methods. Two catalysts with different shell thicknesses (Ru@1 Pt and Ru@2 Pt) were incorporated in the MEAs to investigate how the shell thickness influences the degradation behaviour. While the MEA incorporating Ru@1 Pt as anode catalyst was higher in performance than Ru@2 Pt MEA under reformate conditions in the beginning, it strongly degraded during stress testing to end up with a lower performance than Ru@2 Pt afterwards. Due to cyclic voltammetry experiments, it was evidenced that for both catalysts Ru was leached from the anode and crossed over to the cathode – but to a significantly lower extent for Ru@2 Pt. This was further quantified by advanced corrected (scanning) transmission electron microscopy coupled with energy dispersive spectroscopy detector. Additionally, due to imaging local degradation effects were observed for the anode, while Ru poisoning the Pt catalyst was found to be a global problem in the cathode.

(Invited) Polymers for Proton and Anion Exchange Membrane Fuel Cells

The polymers used for polymer electrolyte fuel cells have many demands on them. Fundamentally these materials are ion conductors and should have ionic conductivities >100 mS cm-1under the conditions of operation. To date polymer electrolytes still achieve practical ionic conductivities by water mediated ion conduction. So as membranes the material needs to be thin to enable a low area specific resistance (ASR) and to facilitate water diffusion from the anode to the cathode so that external humidification of inlet streams is not necessitated. In addition, the material must be an electrical insulator, be a barrier to fuel and oxidant and be mechanically and chemically stable under the transient operation of a typical fuel cell. For proton exchange membranes (PEMs) the primary chemical degradation mechanism is through attack by oxygenated radicals and so current PEMs are pefluorinated sulfonic acid (PFSA) polymers. For anion exchange membranes (AEMS) the primary chemical degradation mechanism is through attack by the hydroxyl anion, a strong nucleophile, and so these materials tend to be wholly hydrocarbon based with advanced cations designed to resist nucleophilic attack. The water transport in AEM fuel cells becomes more critical as water is both a product and a reactant in these systems. For both situations the polymer should be scalable, processable and manufacturable if membrane at the correct price point for market adoption is to be achieved. Th polymer electrolyte in the catalyst layer, the ionomer, has subtly different properties while it must still be a good ion conductor and it no longer needs to be mechanically stable it must now not be a barrier to the product and reactants. Through many years of research for PEMS it is often a low equivalent weight PFSA that can be added to the catalyst layer that enhances ion transport and when thin does not inhibit product and reactant transport. For AEM fuel cells the development of ionomers is still at an early stage, and it is not obvious that cathode and anode ionomers will have the same chemistry. In addition, as many AEM polymers are extremely insoluble finding polymers that can be dispersed for application of the catalyst layer is a non-trivial task. Some groups have even reported god performance in fuel cells by the application of a ground powder of the AEM polymer to the fuel cell electrode. To date a vast number of PEM and AEM polymer chemistries have been proposed. Generally, for PEMs these are PFSAs with bicontinuous phase separated morphologies. For AEMs the variety of morphologies has been quite astonishing. Because of the ease of synthesis of hydrocarbon polymers, and especially block co-polymers, almost all morphologies that can be synthesized have been studied. In this paper we will briefly review the literature and draws some general conclusions. We will then use of own work to illustrate how an experimental polymer is scaled up and processed into device ready films. Using developer feedback, we will the show how modifications to the polymer can tailor the material for specific devices. An interplay between cross-linking, ion exchange capacity, and polymer chemistry and morphology becomes apparent to achieve a successful outcome. Our group makes much use of environmental SAXS and WAXS to study membranes under fuel cell relevant conditions and we have recently begun to expand these studies to environmental GISAXS for catalyst layer relevant work.

The Importance of Ion Selectivity of Perfluorinated Sulfonic Acid Membrane for the Performance of Proton Exchange Membrane Fuel Cells

Nafion 212 membrane was subjected to swelling–dehydration (SD) cycles, as a relevant operation condition for direct methanol fuel cells (DMFCs). The major degradation mechanism due to the treatment was found to be sulfonic group contamination with trace ion, rather than formation of sulfonic anhydride, which is a well-known degradation mechanism for Nafion® membranes under hydrothermal (HT) aging condition. The consequences of the degradation include decreasing water content, thickness, and surface fluoride and increasing resistance, dry weight, and a changed surface morphology. Ion selectivity of the sulfonic group was studied toward different fuel cell relevant conditions. It turned out that the sulfonic groups have much higher selectivity toward cations rather than neighbor sulfonic groups. Trace impurities in the liquid methanol feed in DMFC may therefore represent an important contamination source.

Carbon monoxide and ethanol oxidation on PtSn supported catalysts: Effect of the nature of the carbon support and Pt:Sn composition

This paper presents the behavior of 20wt.% PtSn supported catalysts as anodes for direct ethanol fuel cell (DEFC), which is fed with an aqueous ethanol solution. PtSn nanoparticles with different Pt:Sn ratios (1:1 and 3:1) were supported on carbon black and carbon nanofibers. Physicochemical characterization showed high particle dispersion onto the carbon supports and similar particle size (around 4.0nm). XRD analysis indicated the presence of SnO2 and similar lattice parameter for PtSn (1:1) to those of pure Pt3Sn1. Prepared catalysts were electrochemically studied in acidic medium by carbon monoxide (CO) stripping voltammetry, cyclic voltammetry, lineal sweep voltammetry and chronoamperometry in three-electrode cell experiments. In addition, cell voltage vs. current density curves were recorded in a DEFC. It was found that PtSn anodes exhibit good activity toward ethanol electrooxidation in half-cell experiments under fuel cell relevant conditions. Specifically, PtSn supported catalysts on carbon black showed higher catalytic activity compared to those supported on carbon nanofibers. The low activity for ethanol adsorption/deprotonation and CC cleavage appear as the main causes for the low performance for ethanol oxidation reaction on catalysts supported on carbon nanofibers. A similar behavior was observed in single fuel cell experiments.

Iridium-Titanium Oxide as Support for Pt Catalyst in PEFC Cathodes

Platinum supported on iridium-titanium oxide was investigated as cathode catalyst under polymer electrolyte fuel cell relevant conditions. Degradation of the cathode catalyst layer during automotive fuel cell operation was simulated by performing 1000 cyclic voltammetry scans between 0.5 V vs. RHE and 1.5 V vs. RHE in 0.1 M perchloric acid at room temperature. The electrochemically active platinum surface area was determined by CO stripping voltammetry with a novel method of baseline subtraction, and was found to reduce by 42% under these conditions. The activity of the metal oxide supported platinum catalyst towards oxygen reduction reaction was determined before and after the degradation test by rotating disk electrode measurements. The absolute kinetic current suffered a loss of 36% during the start / stop cycling. In contrast, the specific kinetic current was found to increase, which could be explained with a particle size effect due to platinum nanoparticle growth during potential cycling.

Active Area Determination for Porous Pt-Electrodes used in PEM Fuel Cells - Temperature And Humidity Effects

This paper aim to discuss a proper measure of the electrochemical active area of carbon supported Pt nanoparticle catalyst used in polymer electrolyte membrane (PEM) fuel cells. The cyclic voltammetric determination of hydrogen under potential deposition (Hupd) and carbon monoxide monolayer oxidation (CO stripping) performed in a fuel cell at fuel cell relevant conditions are compared and the influences of operation temperature (25-80 {degree sign}C) and relative humidity (RH) (40-90% RH) are discussed. The results show that both the shape and the charge of the Hupd are strongly dependent on operating conditions. However, for the CO stripping experiments only a negative shift in potential with increasing temperature or humidity was observed in accordance with results from aqueous electrolytes, whereas the charges for CO monolayer oxidation were almost constant for the temperatures investigated.

A thermostatic cell with gas diffusion electrode for oxygen reduction reaction under fuel cell relevant conditions

A thermostatic cell with gas diffusion electrode (GDE) as working electrode has been developed, where gas reactant is directly supplied to the catalysts surface through the porous GDE. The setup greatly eliminates the interferences of temperature-dependent change in mass transport (solubility, diffusion coefficient) of gaseous reactants on the kinetic analysis, as demonstrated by results from the cyclic voltammetric measurements of oxygen reduction reaction (ORR) on Pt-GDE at various temperatures. In contrast to the previous studies using rotating disk electrode or flow cell systems, a monotonically increase in the ORR current with decrease in potential from 1.0 to 0.3 V or with the increase in temperature from 5 to 70 °C is observed. The versatile applicability of such setup for the kinetic studies of reactions with gaseous reactants is discussed.

Multiple Wall-Jet Flow Cell Setup for Quantitative Parallel Evaluation of Carbon-Supported Electrocatalysts under Fuel Cell Relevant Conditions

We describe a multiple wall-jet flow cell setup for parallel evaluation of the electrocatalytic activity of carbon-supported electrocatalysts under relevant reaction conditions (i.e., under enforced and controlled mass transport and at elevated temperatures). The performance of the setup is illustrated using the oxygen reduction reaction (ORR) as a test reaction and a four-cell setup for demonstration. The identical current voltage characteristics obtained on four equally prepared thin-film electrodes at reaction temperatures between 30 and demonstrate similar flow conditions in the different cells, as evidenced by the same mass transport limited ORR currents, a high-temperature stability, and a high reproducibility of the measurements and in the fabrication of the thin-film electrodes. Parallel multiple-cell measurements and (sequential) measurements in a single-cell setup, using a set of four different commercial (E-TEK, Inc.) carbon-supported catalysts (, , , and ), result in similar reaction kinetics in both setups. The potential of the setup for parallel catalyst screening under fuel cell relevant reaction conditions is discussed.

Detection of Ptz + Ions and Pt Nanoparticles Inside the Membrane of a Used PEMFC

The physical and chemical state of proton exchange membrane fuel cell (PEMFC) electrocatalysts were investigated after of operation under fuel cell relevant conditions (, ) and of rest time under inert atmosphere . Upon aging, pronounced corrosion of the cathode electrocatalyst (carbon-supported platinum nanoparticles: ) was evidenced by field-emission gun scanning electron microscopy (FEG-SEM), high-resolution transmission electron microscopy (HRTEM), and electrochemical techniques. Carbon corrosion was witnessed by the decrease of the cathode thickness and by the presence of nonsupported Pt particles inside the electrode. At the cathode, the corrosion of Pt nanoparticles produces ions which diffuse in the ionomer phase or in solution [ stands for ionic species present in the ionomer phase or in solution]. These ions are highly mobile inside the membrane electrode assembly (MEA) and may cross over from the cathode to the anode through the PEM. The driving force for that is the electro-osmotic drag (if ions combine with anions and carry a negative charge) and the chemical diffusion (concentration gradient of oxidized platinum species). Consequently, ions were detected by ultraviolet (UV) spectroscopy in the PEM. Due to the high mobility of ions, FEG-SEM and HRTEM analysis of the cross sections of MEAs revealed a pronounced change of Pt distribution after operation. Size distributions of both anode and cathode electrocatalysts evidenced an increase of the mean particle size, tailing toward large particle sizes and particle agglomeration. Nonspherical Pt nanoparticles were detected inside the PEM, the size and shape distribution of which strongly depend on the distance from the cathode. We believe ions are reduced chemically inside the membrane by crossing over the PEM and both chemically and electrochemically at the anode/cathode.

Is carbon-supported Pt-WO x composite a CO-tolerant material?

Pt-WO x /C composite materials elaborated via a two-step impregnation/electrochemical reduction method have been characterized and tested for the electrooxidation of CO/H 2 mixtures. TEM and EDS measurements revealed that WO x covered imperfectly the C particles. Nanometer-sized or agglomerated Pt particles were found on the WO x /C surface. XRD measurements revealed the absence of diffraction peaks characteristic of crystalline WO x and could indicate that this material is amorphous. No evidence of alloying between the Pt and W was observed. A significant improvement toward the electrooxidation of a CO ads monolayer was observed for the composite material compared to pure Pt/C electrocatalyst, which is evidenced by a new electrooxidation peak at 0.55 V versus RHE ( v = 0.02 V s − 1 ). As the electrical charge below this electrooxidation peak is sweep rate dependant, it is probably associated to the electrooxidation of CO ads on Pt sites at the interface with the WO x /C support. The performance of the Pt-WO x /C material for the electrooxidation of CO/H 2 mixtures was tested by polarization curves under steady-state conditions (0.001 V s −1) or potentiostatic measurements under fuel cell relevant conditions and compared with that of commercial 20 wt% Pt/C and Pt-Ru/C materials.

On the Role of Reactant Transport and (Surface) Alloy Formation for the CO Tolerance of Carbon Supported PtRu Polymer Electrolyte Fuel Cell Catalysts

AbstractThe role of atomic scale intermixing for the electrocatalytic activity of bimetallic PtRu anode catalysts in reformate operated polymer electrolyte fuel cells (PEFC) was investigated, exploiting the specific properties of colloid based catalyst synthesis for the selective preparation of alloyed and non‐alloyed bimetallic catalysts. Three different carbon supported PtRu catalysts with different degrees of Pt and Ru intermixing, consisting of (i) carbon supported PtRu alloy particles (PtRu/C), (ii) Pt and Ru particles co‐deposited on the same carbon support (Pt+Ru/C), and (iii) a mixture of carbon supported Pt and carbon supported Ru (Pt/C+Ru/C) as well as the respective monometallic Pt/C and Ru/C catalysts were prepared and characterized by electron microscopy (TEM), X‐ray absorption spectroscopy, and CO stripping. Their performance as PEFC anode catalysts was evaluated by oxidation of a H2/2%CO gas mixture (simulated reformate) under fuel cell relevant conditions (elevated temperature, continuous reaction and controlled reactant transport) in a rotating disk electrode (RDE) set‐up. The CO tolerance and H2 oxidation activity of the three catalysts is comparable and distinctly different from that of the monometallic catalysts. The results indicate significant transport of the reactants, COad and/or OHad, between Pt and Ru surface areas and particles for all three catalysts, with only subtle differences from the alloy catalyst to the physical mixture. The high activity and CO tolerance of the bimetallic catalysts, through the formation of bimetallic surfaces, is explained, e.g., by contact formation in nanoparticle agglomerates or by material transport and subsequent surface decoration/surface alloy formation during catalyst preparation, conditioning, and operation. The instability and mobility of the catalysts under these conditions closely resembles concepts in gas phase catalysis.

Activity of PtRuMeO x (Me = W, Mo or V) catalysts towards methanol oxidation and their characterization

The activity of unsupported ternary PtRuMeO x (Me=W, Mo, V) high surface area catalysts towards methanol oxidation under fuel cell relevant conditions, at constant potential and elevated temperature of 60 °C, was investigated by the thin-film electrode method. CO-stripping experiments for actual surface area detection, using differential electrochemical mass spectrometry (DEMS) to evaluate the amount of adsorbed CO, show that the electrochemically active area of the particle surface is significantly smaller than the total particle surface area determined by N 2 BET measurements, indicating that part of their surface consists of MeO x . The specific surface activity of the catalysts towards methanol oxidation at 60 °C, normalized to the current density per square centimeter of electrochemically active noble metal surface, decreases in the order PtRuVO x >PtRuMoO x >PtRu>PtRuWO x =PtRu (E-TEK). Apparently, the surface oxide leads to an acceleration of the methanol oxidation reaction, which is, however, partly compensated by a decrease in active surface area.

Influence of selenium on the catalytic properties of ruthenium-based cluster catalysts for oxygen reduction

The favourable influence of selenium on the catalytic properties of Ru-based catalysts for the oxygen reduction reaction in acid electrolytes has been investigated by rotating disk electrode measurements. Compared to the oxygen reduction of selenium-free Ru-based catalysts, the overpotential at low current densities (ca. 10 μA cm−2) is not affected by the presence of selenium whereas selenium-containing catalysts show higher current densities under fuel cell relevant conditions. The kinetically controlled current density at 0.6 V versus SHE increases 4–5 fold with increasing selenium content. A maximum value is obtained at about 15 mol% Se. This effect is tentatively explained by a modification of the catalytic active centre, which is assumed to consist of RuCCO complexes. IR spectroscopic investigations indicate a reaction of selenium with these complexes. This model is also supported by the study of the electrooxidation of CO. In contrast to the selenium-free catalyst, no CO oxidation is observed on the selenium-containing catalyst. Additional effects of selenium are an enhanced stability towards electrochemical oxidation and a lower amount of Ru oxides formed during synthesis, as evidenced from XRD investigations. Direct four electron oxygen reduction to water is efficient and H2O2 production of these catalysts is small (about 5% at potentials <0.3 V vs. SHE ).