Experimental section, discussion on high current density and heat rejection limit, and physical and electrochemical properties of different carbon supports (PDF)

Maintaining high performance after extensive use remains a key challenge for low-Pt proton exchange membrane fuel cells for transportation applications. Strategically improving catalyst durability requires better understanding of the relationship between degradation mechanisms and catalyst structure. To investigate the effects of the carbon support morphology, we compare the electrochemical performance and durability of membrane electrode assemblies (MEAs) using Pt and PtCo x catalysts with a range of porous, solid, and intermediate carbon supports (HSC, Vulcan, and acetylene black). We find that electrochemical surface area (ECSA) retention after a catalyst-targeted durability test tends to improve with increasing support porosity. Using electron microscopy, we investigate microstructural changes in the catalysts and reveal the underlying degradation mechanisms in MEA specimens. Pt migration to the membrane and catalyst coarsening, measured microscopically, together were quantitatively consistent with the ECSA loss, indicating that these were the only two significant degradation pathways. Changes in catalyst particle size, morphology, and PtCo core-shell structure indicate that Ostwald ripening is a significant coarsening mechanism for catalysts on all carbons, while particle coalescence is only significant on the more solid carbon supports. Porous carbon supports thus appear to protect against particle coalescence, providing an effective strategy for mitigating catalyst coarsening.

Ordered intermetallic nanoparticles are promising electrocatalysts with enhanced activity and durability for the oxygen-reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). The ordered phase is generally identified based on the existence of superlattice ordering peaks in powder X-ray diffraction (PXRD). However, after employing a widely used postsynthesis annealing treatment, we have found that claims of “ordered” catalysts were possibly/likely mixed phases of ordered intermetallics and disordered solid solutions. Here, we employed in situ heating, synchrotron-based, X-ray diffraction to quantitatively investigate the impact of a variety of annealing conditions on the degree of ordering of large ensembles of Pt3Co nanoparticles. Monte Carlo simulations suggest that Pt3Co nanoparticles have a lower order–disorder phase transition (ODPT) temperature relative to the bulk counterpart. Furthermore, we employed microscopic-level in situ heating electron microscopy to directly visualize the morphological changes and the formation of both fully and partially ordered nanoparticles at the atomic scale. In general, a higher degree of ordering leads to more active and durable electrocatalysts. The annealed Pt3Co/C with an optimal degree of ordering exhibited significantly enhanced durability, relative to the disordered counterpart, in practical membrane electrode assembly (MEA) measurements. The results highlight the importance of understanding the annealing process to maximize the degree of ordering in intermetallics to optimize electrocatalytic activity.

Relatively large O 2 transport resistance at the ionomer and Pt interface has been thought to be responsible for the large performance loss at high power for a low Pt loading proton-exchange-membrane fuel cell. A facile method to characterize the interface in the fuel cell electrode is needed. In this study, the CO displacement method was explored on polycrystalline Pt and carbon-supported Pt nanoparticles. The displacement charge coverages were used to quantify the adsorption of perchlorate, sulfate, and perfluorosulfonic acid ionomer. Heavy use of platinum in the electrodes of proton-exchange membrane (PEM) fuel cells is a key challenge preventing automotive manufacturers from bringing fuel cell electric vehicles to mass market. Current state-of-the-art fuel cell vehicles use >20 g of Pt per vehicle, which is significantly higher than the internal-combustion engine (ICE) incumbent (<5 g of precious metal per vehicle).1,2 Because heavy use of Pt is needed to obtain high energy conversion efficiency in the fuel cell, improving the activity of Pt-based catalysts has continued to be a high-priority research topic for many years.On the other hand, it was found that at high power density of a low-loaded electrode (<0.10 mg Pt /cm 2 or ∼11 g Pt /vehicle), significant performance losses are observed.3-7 These large performance losses are likely due to the need to deliver more O 2 to a small area of the Pt surface. It was also found that the bulk of the observed O 2 transport resistance occurs at the interface of Pt and electrolyte, 1,4,6 which is surprising because it has generally been seen that the thickness of the ionomer coated on a Pt surface in a well optimized electrode is only a few nanometers. If one calculates the O 2 transport resistance of the thin film using known O 2 permeability of a thick ionomer membrane, 8 it would require an ionomer film with unreasonable thickness (>20 nm) in order to explain the performance loss. Ex-situ measurements on thin-film ionomer performed by several groups have shown that the ionomer nanostructure and its properties such as water uptake, proton conduction, and O 2 permeability can vary substantially depending on its thickness, treatment history, and substrate interaction.9-17 Furthermore, sulfonate groups in the ionomer can adsorb on a Pt surface and reduce the oxygen reduction reaction (ORR) activity. 18,19 Because the adsorption of the acid group immobilizes the ionomer to the Pt surface, [20][21][22] it is surmised that it will also increase O 2 transport resistance. Recent molecular dynamics and density functional theory (DFT) calculations show that ionomers fold onto the Pt surface, leading to a highly dense layer which in turn can reduce the O 2 concentration close to the Pt surface to nearly zero. 23It is also shown that the type of ionomer and operational history can affect the observed performance.1,24 Unfortunately, there is still no characterization method available that will evaluate the ionomer/Pt interface in a fuel cell electrode and in a way that can be re...

Intermittent renewable energy sources like wind and solar require energy storage to be fully exploited. The H 2 -Br 2 flow battery has been identified as a great candidate for this application because of its high energy conversion efficiency and power density capability. To evaluate the performance of this system a fuel cell with a membrane electrode assembly made of a Nafion 212 membrane and electrodes with 0.55 mg Pt/cm 2 loading was tested at 22 • C under the H 2 -H 2 , H 2 -O 2 , H 2 -Br 2 modes to compare the performance between these systems. The exchange current density of the Br − /Br 2 reactions on platinum was found to be about a quarter of that of the hydrogen reactions on the same substrate (0.3 mA/cm 2 Pt versus 1.2 mA/cm 2 Pt). Peak power density during discharge was 0.30 W/cm 2 at a voltage efficiency of 70% for the H 2 -Br 2 mode versus 0.17 W/cm 2 at 40% for the H 2 -O 2 mode under similar conditions. The performance of the H 2 -Br 2 fuel cell is determined mainly by the ohmic and mass transport resistances in the cell, which could be improved by using higher reactant concentrations, higher operating temperatures, more conductive membranes, and electrode and cell designs that enhance transport.

Reducing the use of precious metal in a proton exchange membrane fuel cell is a key challenge in mass commercialization of fuel cell electric vehicles. However, as Pt loadings are reduced, fuel cell performance losses due to mass transport phenomena become more localized to the Pt and ionomer interface. In this paper, we provide an overview of how we use in-situ electrochemical diagnostics and modeling to understand the performance of low-Pt electrodes, identify their key limiting factors, and guide our catalyst layer development. In particular, diagnostics used to quantify the local-Pt oxygen transport resistance and the ionomer adsorption on the Pt surface will be discussed.

Development of electrocatalysts with higher activity and stability is one of the highest priorities in enabling cost-competitive hydrogen-air fuel cells. Although the rotating disk electrode (RDE) technique is widely used to study new catalyst materials, it has been often shown to be an unreliable predictor of catalyst performance in actual fuel cell operation. Fabrication of membraneelectrode assemblies (MEA) for evaluation which are more representative of actual fuel cells generally requires relatively large amounts (>1 g) of catalyst material which are often not readily available in early stages of development. In this study, we present two MEA preparation techniques using as little as 30 mg of catalyst material, providing methods to conduct more meaningful MEA-based tests using research-level catalysts amounts.

The hydrogen/bromine flow battery is a promising candidate for large-scale energy storage due to fast kinetics, highly reversible reactions and low chemical costs. However, today's conventional hydrogen/bromine flow batteries use membrane materials (such as Nafion), platinum catalysts, and carbon-paper electrode materials that are expensive. In addition, platinum catalysts can be poisoned and corroded when exposed to HBr and Br 2, compromising system lifetime. To reduce the cost and increase the durability of H 2 /Br 2 flow batteries, new materials are developed. The new Nafion/ polyvinylidene fluoride electrospun composite membranes have high perm-selectivity at a fraction of the cost of Nafion membranes; the new nitrogen-functionalized platinum-iridium catalyst possesses excellent activity and durability in HBr/Br 2 environment; and the new carbon-nanotube-based Br 2 electrodes can achieve equal or better performance with less materials when compared to baseline electrode materials. Preliminary cost analysis shows that the new materials reduce H 2 /Br 2 flow-battery energy-storage system stack and system costs significantly. The resulting advanced H 2 /Br 2 flow batteries offer high power, high efficiency, substantially increased durability, and expected reduced cost. The active reactant material, hydrobromic acid (HBr) is also used as the supporting electrolyte. If the energy-storage system is commissioned in the discharged state, which is the most common case, HBr is the only chemical that is required. During charge, hydrobromic acid is electrolyzed to generate hydrogen and bromine, which are stored in separate tanks. Bromine has a moderate solubility in water which can be greatly enhanced by the presence of Br − via complexation to form Br 3 − or Br 5 − . 8,9 The gas phase H 2 electrode also simplifies the separation and recovery of crossover catholyte, which can be returned back to the catholyte tank.The H 2 /Br 2 flow battery technology has been under investigation since the 1960s. Brief literature reviews can be found in recent publications by Cho et al., 4 Kreutzer et al. 5 and Tolmachev. 6 H 2 /Br 2 flow batteries share the same cell architecture as proton-exchange-membrane fuel cells (PEMFCs). Therefore, H 2 /Br 2 flow batteries are also referred to as regenerative or reversible H 2 /Br 2 fuel cells. Similar to PEMFCs, * Electrochemical Society Active Member. * * Electrochemical Society Student Member. * * * Electrochemical Society Fellow.z E-mail: gygylin@gmail.com membrane-electrode assemblies (MEAs) are the most crucial components in the H 2 /Br 2 flow batteries. In today's state-of-the-art H 2 /Br 2 flow batteries, MEAs are commonly made of commercial perfluorosulfonic acid (PFSA) membranes such as Nafion, platinum catalysts, and plain carbon papers. The PFSA membrane in a H 2 /Br 2 flow battery is used to physically separate the positive and negative electrodes, and prevent mixing of hydrogen and bromine/bromides while allowing proton transport between the electrodes. The membrane resistance has ...