Rohit Makharia

Instability of Pt ∕ C Electrocatalysts in Proton Exchange Membrane Fuel Cells A Mechanistic Investigation

Equilibrium concentrations of dissolved platinum species from a Pt/C electrocatalyst sample in 0.5 M H2SO4 at 80°C were found to increase with applied potential from 0.9 to 1.1 V vs reversible hydrogen electrode. In addition, platinum surface area loss for a short-stack of proton exchange membrane fuel cells PEMFCs operated at open-circuit voltage 0.95 V was shown to be higher than another operated under load 0.75 V. Both findings suggest that the formation of soluble platinum species such as Pt 2+ plays an important role in platinum surface loss in PEMFC electrodes. As accelerated platinum surface area loss in the cathode from 63 to 23 m 2 /gPt in 100 h was observed upon potential cycling, a cycled membrane electrode assembly MEA cathode was examined in detail by incidence angle X-ray diffraction and transmission electron microscopy TEM to reveal processes responsible for observed platinum loss. In this study, TEM data and analyses of Pt/C catalyst and cross-sectional MEA cathode samples unambiguously confirmed that coarsening of platinum particles occurred via two different processes: i Ostwald ripening on carbon at the nanometer scale, which is responsible for platinum particle coarsening from 3t o6 nm on carbon, and ii migration of soluble platinum species in the ionomer phase at the micrometer scale, chemical reduction of these species by crossover H2 molecules, and precipitation of platinum particles in the cathode ionomer phase, which reduces the weight of platinum on carbon. It was estimated that each process contributed to 50% of the overall platinum area loss of the potential cycled electrode.

Measurement of Catalyst Layer Electrolyte Resistance in PEFCs Using Electrochemical Impedance Spectroscopy

In this paper, electrochemical impedance spectroscopy (EIS) is used to resolve various sources of polarization loss in a pure hydrogen-fueled polymer electrolyte fuel cell (PEFC). EIS data are fitted to a fuel cell model in which the catalyst layer physics are accurately represented by a transmission line model. Extracted parameters include cell ohmic resistance, catalyst layer electrolyte resistance, and double-layer capacitance. The results showed that the catalyst layer electrolyte resistance for a state-of-the-art electrode (47 wt % Pt on Vulcan XC-72 carbon, 0.8 Nation (1100EW)-to-carbon weight ratio, 13 μm thick) at 80°C and fully humidified conditions was approximately 100 mΩ-cm 2 ; this translates to a dc voltage loss of about 33 mV at a current density of 1 A/cm 2 . Similar results were obtained for two experimental methods, one using H 2 (anode) and O 2 (cathode gas feed) and another with H, and N 2 supplies, and for two cell active areas, 5 and 50 cm 2 . The measured catalyst layer electrolyte resistance increased with decreasing ionomer concentration in the electrode, as expected. We also observed that the real impedance measured at 1 kHz, often interpreted as the ohmic resistance in the cell, can include contributions from the electrolyte in the catalyst layer.

The Impact of Carbon Stability on PEM Fuel Cell Startup and Shutdown Voltage Degradation

Conventional carbon MEAs and graphitized carbon MEAs were evaluated for the resistance to carbon corrosion and startup/shutdown durability in this paper. Graphitized carbon MEAs show higher resistance to carbon corrosion than conventional carbon MEAs by a factor of 35 at a point where 5% weight loss had occurred. A graphitized carbon MEA yielded lower degradation rate than that of a conventional carbon MEA by a factor of 5 after 1,000 startup/shutdown cycles. The kinetics of carbon corrosion over both conventional carbon MEAs and graphitized carbon MEAs were measured, and carbon corrosion during startup/shutdown was explained and modeled. The model results correlate to what we have measured from our startup/shutdown durability test. Overall, MEAs with corrosion resistant carbon supports are one of major materials approaches to mitigate cell voltage degradation due to fuel cell startup/shutdown. We believe that a combination of corrosion resistant materials and system operating mitigation strategies is the path to attain the strict automotive durability targets.

Atomic-Resolution Spectroscopic Imaging of Ensembles of Nanocatalyst Particles Across the Life of a Fuel Cell

The thousand-fold increase in data-collection speed enabled by aberration-corrected optics allows us to overcome an electron microscopy paradox: how to obtain atomic-resolution chemical structure in individual nanoparticles yet record a statistically significant sample from an inhomogeneous population. This allowed us to map hundreds of Pt-Co nanoparticles to show atomic-scale elemental distributions across different stages of the catalyst aging in a proton-exchange-membrane fuel cell, and relate Pt-shell thickness to treatment, particle size, surface orientation, and ordering.

Catalyst Development Needs and Pathways for Automotive PEM Fuel Cells

Mass production of fuel cell light-duty vehicles at competitive cost requires cathode (oxygen reduction reaction [ORR]) kinetic mass activities 4-fold higher than those of current state-of-the-art platinum / carbon black catalysts. 1 A catalyst-related cell voltage loss less than 50 mV over the entire current density range is sought over a >10 year automotive lifetime including ~300,000 large load cycles and ~30,000 start-stop cycles. While the materials set used in current demonstration vehicles falls short of these goals, pathways to these challenging targets are visible via increased attention to the structural details of Pt-containing multicomponent catalysts and through development of catalyst and support as a single system.

Carbon-Support Requirements for Highly Durable Fuel Cell Operation

Owing to its unique electrical and structural properties, high surface area carbon has found widespread use as a catalyst support material in proton exchange membrane fuel cell (PEMFC) electrodes. The highly dynamic operating conditions in automotive applications require robust and durable catalyst support materials. In this chapter, carbon corrosion kinetics of commercial conventional-carbon-supported membrane electrode assemblies (MEAs) are presented. Carbon corrosion was investigated under various automotive fuel cell operating conditions. Fuel cell system start/stop and anode local hydrogen starvation are two major contributors to carbon corrosion. Projections from these studies indicate that conventional-carbon-supported MEAs fall short of meeting automotive the durability targets of PEMFCs. MEAs made of different carbon support materials were evaluated for their resistance to carbon corrosion under accelerated test conditions. The results show that graphitized-carbon-supported MEAs are more resistant to carbon corrosion than nongraphitized carbon materials. Fundamental model analyses incorporating the measured carbon corrosion kinetics were developed for start/stop and local hydrogen starvation conditions. The combination of experiment and modeling suggests that MEAs with corrosion-resistant carbon supports are promising material approaches to mitigate carbon corrosion during automotive fuel cell operation.

Modeling of Membrane-Electrode-Assembly Degradation in Proton-Exchange-Membrane Fuel Cells - Local H2 Starvation and Start-Stop Induced Carbon-Support Corrosion

Carbon-support corrosion causes electrode structure damage and thus electrode degradation. This chapter discusses fundamental models developed to predict cathode carbon-support corrosion induced by local H2 starvation and start–stop in a proton-exchange-membrane (PEM) fuel cell. Kinetic models based on the balance of current among the various electrode reactions are illustrative, yielding much insight on the origin of carbon corrosion and its implications for future materials developments. They are particularly useful in assessing carbon corrosion rates at a quasi-steady-state when an H2-rich region serves as a power source that drives an H2-free region as a load. Coupled kinetic and transport models are essential in predicting when local H2 starvation occurs and how it affects the carbon corrosion rate. They are specifically needed to estimate length scales at which H2 will be depleted and time scales that are valuable for developing mitigation strategies. To predict carbon-support loss distributions over an entire active area, incorporating the electrode pseudo-capacitance appears necessary for situations with shorter residence times such as start–stop events. As carbon-support corrosion is observed under normal transient operations, further model improvement shall be focused on finding the carbon corrosion kinetics associated with voltage cycling and incorporating mechanisms that can quantify voltage decay with carbon-support loss.

Catalyst Degradation Mechanisms in PEM and Direct Methanol Fuel Cells

While much attention has been given to optimizing initial fuel cell performance, only recent research has focused on the various materials degradation mechanisms observed over the life-time of fuel cells under real-life operating conditions. This presentation will focus on fuel cell durability constraints produced by platinum sintering/dissolution, carbon-support oxidation, and membrane chemical and mechanical degradations. Over the past 10 years, extensive R&D efforts were directed towards optimizing catalysts, membranes, and gas diffusion layers (GDL) as well as combining them into improved membrane electrode assemblies (MEAs), leading to significant improvements in initial performance of H2/air-fed proton exchange membrane fuel cells (PEMFCs) and methanol/air-fed direct methanol fuel cells (DMFCs). 3 While the required performance targets have not yet been met, current PEMFC and DMFC performance are close to meeting entry-level applications and many prototypes have been developed for field testing. This partially shifted the R&D focus from performance optimization to more closely examining materials degradation phenomena which limit fuel cell durability under real-life testing conditions. The predominant degradation mechanisms are sintering/dissolution of platinum-based cathode catalysts under highly dynamic operating conditions, dissolution of ruthenium from DMFC anode catalysts, the oxidation of carbon-supports of the cathode catalyst during fuel cell startup and shutdown, and the formation of pinholes in proton exchange membranes if

One Million EEL Spectra Acquisition with Aberration-Corrected STEM: 2-D Chemical Investigation of a Statistically Significant Ensemble of Nanocatalysts

Aberration-corrected scanning transmission electron microscopes (STEM) can achieve high probing currents while maintaining an Ångstrom-scale probe for analytical mapping. If aided by another correcting module for improved coupling of the inelastic signal into an electron energy loss spectrometer, the collection efficiency on a thin specimen can be well over 90%. This large usable current allows an electron energy loss spectrum (EELS) with identifiable compositional and bonding information to be recorded in just a few tens of milliseconds and a 64x64-pixel spectroscopic map to be acquired in just a few minutes or less. Using an EELS-optimized system, a 100KeV Nion UltraSTEM, enabled us to collect more than one million EEL spectra providing chemical maps of hundreds of Pt-Co core-shell structured nanocatalysts before and after fuel cell operations. The unprecedented quantity of data allows us to draw statistically significant conclusions about the chemical microstructure and correlations among microscopic degrees of freedom. We show for example, the preferential segregation of a single atomic layer Pt skin on the {111} facets of annealed Pt-Co nanoparticles. However, the complexity of the large and spatially-correlated data sets poses challenges in collection, analysis and presentation. We discuss new and borrowed strategies for elegantly condensing information and providing new opportunities to chemically explore an ensemble of heterogeneous nanomaterials at atomic resolution.