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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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747 records · Page 42
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Final Technical Report: Effects of Impurities on Fuel Cell Performance and Durability

The main objectives of this project were to investigate the effect of a series of potential impurities on fuel cell operation and on the particular components of the fuel cell MEA, to propose (where possible) mechanism(s) by which these impurities affected fuel cell performance, and to suggest strategies for minimizing these impurity effects. The negative effect on Pt/C was to decrease hydrogen surface coverage and hydrogen activation at fuel cell conditions. The negative effect on Nafion components was to decrease proton conductivity, primarily by replacing/reacting with the protons on the Bronsted acid sites of the Nafion. Even though already well known as fuel cell poisons, the effects of CO and NH3 were studied in great detail early on in the project in order to develop methodology for evaluating poisoning effects in general, to help establish reproducibility of results among a number of laboratories in the U.S. investigating impurity effects, and to help establish lower limit standards for impurities during hydrogen production for fuel cell utilization. New methodologies developed included (1) a means to measure hydrogen surface concentration on the Pt catalyst (HDSAP) before and after exposure to impurities, (2) a way to predict conductivity of a Nafion membranes exposed to impurities using a characteristic acid catalyzed reaction (methanol esterification of acetic acid), and, more importantly, (3) application of the latter technique to predict conductivity on Nafion in the catalyst layer of the MEA. H2-D2 exchange was found to be suitable for predicting hydrogen activation of Pt catalysts. The Nafion (ca. 30 wt%) on the Pt/C catalyst resides primarily on the external surface of the C support where it blocks significant numbers of micropores, but only partially blocks the pore openings of the meso- and macro-pores wherein lie the small Pt particles (crystallites). For this reason, even with 30 wt% Nafion on the Pt/C, few Pt sites are blocked and, hence, are accessible for hydrogen activation. Of the impurities studied, CO, NH3, perchloroethylene (also known as tetrachloroethylene), tetrahydrofuran, diborane, and metal cations had significant negative effects on the components in a fuel cell. While CO has no effect on the Nafion, it significantly poisons the Pt catalyst by adsorbing and blocking hydrogen activation. The effect can be reversed with time once the flow of CO is stopped. NH3 has no effect on the Pt catalyst at fuel cell conditions; it poisons the proton sites on Nafion (by forming NH4+ cations), decreasing drastically the proton conductivity of Nafion. This poisoning can slowly be reversed once the flow of NH3 is stopped. Perchloroethylene has a major effect on fuel cell performance. Since it has little/no effect on Nafion conductivity, its poisoning effect is on the Pt catalyst. However, this effect takes place primarily for the Pt catalyst at the cathode, since the presence of oxygen is very important for this poisoning effect. Tetrahydrofuran was shown not to impact Nafion conductivity; however, it does affect fuel cell performance. Therefore, its primary effect is on the Pt catalyst. The effect of THF on fuel cell performance is reversible. Diborane also can significant affect fuel cell performance. This effect is reversible once diborane is removed from the inlet streams. H2O2 is not an impurity usually present in the hydrogen or oxygen streams to a fuel cell. However, it is generated during fuel cell operation. The presence of Fe cations in the Nafion due to system corrosion and/or arising from MEA production act to catalyze the severe degradation of the Nafion by H2O2. Finally, the presence of metal cation impurities (Na+, Ca 2+, Fe3+) in Nafion from MEA preparation or from corrosion significantly impacts its proton conductivity due to replacement of proton sites. This effect is not reversible. Hydrocarbons, such as ethylene, might be expected to affect Pt or Nafion but do not at a typical fuel cell temperature of 80oC. In the presence of large quantities of hydrogen on the anode side, ethylene is converted to ethane which is very nonreactive. More surprisingly, even more reactive hydrocarbons such as formic acid and acetaldehyde do not appear to react enough with the strong Bronsted acid sites on Nafion at such low temperatures to affect Nafion conductivity properties. These results clearly identify a number of impurities which can have a detrimental impact on fuel cell performance, although some are reversible. Obviously, fuel cells exposed to impurities/poisons which are reversible can recover their original performance capabilities once the impurity flow is stopped. Impurities with irreversible effects should be either minimized in the feed streams, if possible, or new catalytic materials or ion conductors will need to be used to minimize their impact.

08 HYDROGEN↗
DOE OSTI2012

WaterTransport in PEM Fuel Cells: Advanced Modeling, Material Selection, Testing and Design Optimization

Water management in Proton Exchange Membrane, PEM, Fuel Cells is challenging because of the inherent conflicts between the requirements for efficient low and high power operation. Particularly at low powers, adequate water must be supplied to sufficiently humidify the membrane or protons will not move through it adequately and resistance losses will decrease the cell efficiency. At high power density operation, more water is produced at the cathode than is necessary for membrane hydration. This excess water must be removed effectively or it will accumulate in the Gas Diffusion Layers, GDLs, between the gas channels and catalysts, blocking diffusion paths for reactants to reach the catalysts and potentially flooding the electrode. As power density of the cells is increased, the challenges arising from water management are expected to become more difficult to overcome simply due to the increased rate of liquid water generation relative to fuel cell volume. Thus, effectively addressing water management based issues is a key challenge in successful application of PEMFC systems. In this project, CFDRC and our partners used a combination of experimental characterization, controlled experimental studies of important processes governing how water moves through the fuel cell materials, and detailed models and simulations to improve understanding of water management in operating hydrogen PEM fuel cells. The characterization studies provided key data that is used as inputs to all state-of-the-art models for commercially important GDL materials. Experimental studies and microscopic scale models of how water moves through the GDLs showed that the water follows preferential paths, not branching like a river, as it moves toward the surface of the material. Experimental studies and detailed models of water and airflow in fuel cells channels demonstrated that such models can be used as an effective design tool to reduce operating pressure drop in the channels and the associated costs and weight of blowers and pumps to force air and hydrogen gas through the fuel cell. Promising improvements to materials structure and surface treatments that can potentially aid in managing the distribution and removal of liquid water were developed; and improved steady-state and freeze-thaw performance was demonstrated for a fuel cell stack under the self-humidified operating conditions that are promising for stationary power generation with reduced operating costs.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2022

Recreating Fuel Cell Catalyst Degradation in Aqueous Environments for Identical-Location Scanning Transmission Electron Microscopy Studies

The recent surge in interest of proton exchange membrane fuel cells (PEMFCs) for heavy-duty vehicles increases the demand on the durability of oxygen reduction reaction electrocatalysts used in the fuel cell cathode. This prioritizes efforts aimed at understanding and subsequently controlling catalyst degradation. Identical-location scanning transmission electron microscopy (IL-STEM) is a powerful method that enables precise characterization of degradation processes in individual catalyst nanoparticles across various stages of cycling. Recreating the degradation processes that occur in PEMFC membrane electrode assemblies (MEAs) within the aqueous cell used for IL-STEM experiments is vital for generating an accurate understanding of these processes. In this work, we investigate the type and degree of catalyst degradation achieved by cycling in an aqueous cell compared to a PEMFC MEA. While significant degradation is observed in IL-STEM experiments performed on a traditional Pt catalyst using the standard accelerated stress test potential window (0.6-0.95 VRHE), degradation of a PtCo catalyst designed for heavy-duty vehicle use is very limited compared to that observed in MEAs. We therefore explore various experimental parameters such as temperature, acid type, acid concentration, ionomer content, and potential window to identify conditions that reproduce the degradation observed in MEAs. We find that by extending the cycling potential window to 0.4-1.0 V RHE in an electrolyte containing Pt ions, the degraded particle size distribution and alloy composition better match that observed in MEAs. In particular, these conditions increase the relative contribution of Ostwald ripening, which appears to play a more significant role in the degradation of larger alloy particles supported on high-surface-area carbons than coalescence. Results from this work highlight the potential for discrepancies between ex situ aqueous experiments and MEA tests. While different catalysts may require a unique modification to the AST protocol, strategies provided in this work enable future in situ and identical-location experiments that will play an important role in the development of robust catalysts for heavy-duty vehicle applications.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2023

Optimized purification methods for metallic contaminant removal from directly recycled Li-ion battery cathodes

Metallic contaminants pose a significant challenge to the viability of directly recycling Li-ion batteries. To date, few strategies exist to selectively remove metallic impurities from mixtures of shredded end-of-life material (black mass; BM) without concurrently damaging the structure and electrochemical performance of the target active material. We herein present tailored methods to selectively ionize two major contaminants—Al and Cu—while retaining a representative cathode (LiNi 0.33 Mn 0.33 Co 0.33 O 2 ; NMC-111) intact. This BM purification process is conducted at moderate temperatures in a KOH-based solution matrix. We rationally evaluate approaches to increase both the kinetic corrosion rate and the thermodynamic solubility of Al 0 and Cu 0 , and evaluate the impact of these treatment conditions on the structure, chemistry, and electrochemical performance of NMC. Specifically, we explore the impacts of chloride-based salts, a strong chelating agent, elevated temperature, and sonication on the rate and extent of contaminant corrosion, while concurrently evaluating the effects on NMC. The reported BM purification process is then demonstrated on samples of “simulated BM” containing a practically relevant 1 wt% concentration of Al or Cu. Increasing the kinetic energy of the purifying solution matrix through elevated temperature and sonication accelerates the corrosion of metallic Al and Cu, such that ∼100% corrosion of 75 μm Al and Cu particles is achieved within 2.5 hr. Further, we determine that effective mass transport of ionized species critically impacts the efficacy of Cu corrosion, and that saturated Cl – hinders rather than accelerates Cu corrosion by increasing solution viscosity and introducing competitive pathways for Cu surface passivation. The purification conditions do not induce bulk structural damage to NMC, and electrochemical capacity is maintained in half-cell format. Testing in full cells suggests that a limited quantity of residual surface species are present after treatment, which initially disrupt electrochemical behavior at the graphite anode but are subsequently consumed. Process demonstration on simulated BM suggests that contaminated samples—which prior to treatment show catastrophic electrochemical performance—can be recovered to pristine electrochemical capacity. The reported BM purification method offers a compelling and commercially viable solution to address contamination, particularly in the “fine” fraction of BM where contaminant sizes are on the same order of magnitude as NMC and where traditional separation approaches are unfeasible. Thus, this optimized BM purification technique offers a pathway towards viable direct recycling of BM feedstocks that would otherwise be unusable.

25 ENERGY STORAGE↗
DOE OSTI2026

Spin Polarization Enhanced Ethanol Selectivity in Electrocatalytic CO 2 Reduction on the Paramagnetic CuO Surface

We report an electrochemical CO 2 reduction reaction catalyzed by a paramagnetic and conductive CuO/Cu interface with spins polarized by a moderate external magnetic field (MF) of similar to 800 gauss, achieving a similar to 30% increase in CO 2 -to-C 2+ Faradaic efficiency (FE) compared to that in the absence of the MF in a flow cell electrolyzer. At a current density of 400 mA/cm 2 , the CO 2 -to-C 2+ FE reached 86.7 ± 2.7% with 47.9 ± 1.4% cathodic energy efficiency (EE) in contrast to the CO 2 -to-C 2+ FE of 67.6% with 36.4% of EE in the absence of MF. Notably, ethanol production exhibits a much higher response to the MF (similar to 55.6% increase in FE) than ethylene (similar to 6.4% increase in FE) at 400 mA/cm 2 . In situ surface-enhanced Raman spectroscopy (SERS) captured magnetic-field-enhanced *CO coverage and ethanol-forming C 2 intermediates on CuO/Cu, providing direct spectroscopic evidence of spin-modulated pathway selection. Here, computational study suggests that the enhancement of ethanol selectivity is due to the reduced reaction kinetic barrier under MF, while the ethylene selectivity is less affected, mainly due to the insensitivity of the kinetic barriers under MF.

10 SYNTHETIC FUELS↗
DOE OSTI2022

High Performance SOFCs with a Superior Stability for Reliable and Durable Power Systems

Next generation of fuel cells, electrolyzers, and batteries requires higher power, faster kinetics, and larger energy density, which necessitate the use of compositionally complex oxides to achieve multifunctionalities and activity. These compositionally complex oxides may change their phases and structures during an electrochemical process – a so-called “electrochemically driven phase transformation”. The origin for such a phase change has remained obscure. More importantly, there is a need to develop high performance solid oxide fuel cells with an enhanced stability. In this work, the La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF) cathode surface is modified by infiltration of Pr 6 O 11 and the power density at 0.8V and 750 °C is improved by 21%. In addition, by replacing the traditional barrier layer Gd 0.2 Ce 0.8 O 1.9 with mixed conducting Pr 0.1 Gd 0.1 Ce 0.8 O 1.9 , the power density increases by 38%. The different mechanism of promotions was investigated by electrochemical impedance spectroscopy. The ohmic resistance is dramatically reduced by applying the PGCO interlayer, and the distribution of relaxation time was used to analyze the mechanism for which the polarization resistance was decreased attributing to the mixed conduction nature in PrO x . An increase of power density at 0.8 V of 0.358 W/cm 2 (71%) is achieved with the implementation of both surface modification and buffer layer engineering. An experimental study and a theoretical analysis were then carried out on phase evolution in praseodymium nickelates. Nickelate-based electrodes show up to 60× greater phase transformation during operation when compared to thermally annealed ones. Theoretical analysis suggests that the presence of a reduced oxygen partial pressure at the interface between the oxygen electrode and the electrolyte is the origin for the phase change in an oxygen electrode. Guided by the theory, an addition of the electronic conduction in the interface layer leads to the significant suppression of phase change, while improving cell performance and performance stability. When an oxygen electrode is under polarization, the oxygen partial pressure at the interface between the oxygen electrode and the electrolyte is lower than that of incoming oxidant. Under a high polarization, the environment at the aforementioned interface may lead to phase transformation of the oxygen electrode. The local oxygen partial pressure is determined by the transport properties at the interfaces. An addition of the electronic conduction in the interface layer, for instance using (Pr,Gd)-doped ceria to replace Gd-doped ceria, results in improved cell performance and performance stability, while the phase transformation is significantly suppressed. This work provides a fundamental understanding of the origin for phase transformation in oxygen electrodes during operation and use this knowledge to develop a high-performance electrode that exhibits improved performance stability.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2020

Materials and Approaches for Mitigation of SOFC Cathode Degradation in SOFC Power Systems

Perovskites such as doped lanthanum manganite and cobaltite have been used successfully as a cathode material in SOFC power systems operating between 700-1000°C. The electrodes, however, remain prone to degradation in presence of Cr containing gas atmosphere under systems operating conditions. Cathode in SOFCs is considered as one of the largest contributors of electrochemical performance degradations, leading to significant increase in ohmic and non-ohmic losses as well as interfacial compound formation. These degradations are mostly irreversible due to permanent changes to chemical and morphological alterations in the electrodes, resulting in the blockage of triple phase boundary (TPB) sites and decrease in oxygen reduction reactions (ORR). Fundamental mechanistic understanding has been developed to elucidate the reaction pathways of electrode poisoning in electronically conducting electrodes (LSM) as well as mixed ionic and electronically conducting electrodes (LSCF). The overall goal of this program has been identification and development of cost effective solutions to mitigate electrode degradations under “real world” systems operating conditions. The overall technological innovations and impact from this work are: The research provides an understanding of the degradation mechanism and cost-effective approaches for implementation in SOFC systems; Develop mitigation processes utilizing low cost getters using conventional synthesis and fabrication route; Develop tailored high surface area powders and coatings for high Cr capture capacity under systems operating conditions; >Developed computational models to understand mechanistic behavior; Experimentally measure rate of Cr evaporation from chromia and alumina forming alloys under SOFC operating conditions; Developed conditions to minimize Cr evaporation from balance-of-plant alloys; This innovation will support the development of getter platform to capture both intrinsic and extrinsic impurities present in air; and, Applications under wide operating temperature range (500-900°C) as well for wide high-temperature electrochemical systems (SOFC, SOEC, and OTM). The work has been successfully developed, validated experimentally and implemented. Successful technology transfer of this knowledge with industrial partners has been accomplished and the getters have been independently tested and validated under their systems operating conditions. The innovation will also find application in related high temperature electrochemical systems such as OTM and SOEC for the prevention of Cr assisted performance degradation. The proposed approach for Cr capture can also be applied to oxy-combustion and other advanced combustion techniques for the reduction of Cr vapor in the exhaust gas stream. This fundamental knowledge gained also has led to the path forward for developing advanced getters for capturing more than one contaminant in air.

01 COAL, LIGNITE, AND PEAT↗
DOE OSTI2013

Theory Aided Design of Active and Durable Nanoscale Cathode Catalysts (Final Report)

One of the greatest technological hurdles to deployment of fuel cells relates to the sluggish activity, low durability and the high cost of the catalysts that are currently employed. For automotive PEM fuel cells to become commercially viable, the Pt-specific power density would need to be reduced to less than 0.2gPt/kW (at cell voltages >0.65 V). This would require the Pt loadings to be less than 0.15 mg Pt /cm 2 MEA within the membrane electrode assembly. This could be achieved by enhancing the catalytic activity at the cathode, thus lowering its overpotential. Various different Pt-alloys have shown 2-4 times enhanced activities over Pt alone but still suffer some of the same durability issues as that of the pure Pt. There is a general loss of active Pt due to dissolution and sintering. While there have been a number of elegant fundamental experimental and theoretical studies on ideal single crystal Pt and Pt alloy surfaces which have helped to elucidate the factors that control the activity, there have been very few fundamental studies focused on the stability, reactivity and durability of well-defined Pt nanoparticles. We carried out ab initio density functional theory together with a novel double reference method that we developed to simulate constant potential electrochemical systems in order to model the electrocatalytic reduction of oxygen over model Pt alloy surfaces and nanoparticles. These simulations were used to probe the factors that control the electrocatalytic activity, guide the potential selection of new materials and test their stability under reaction conditions. Ab initio calculations were used to determine the reaction energies and activation barriers for a comprehensive array of different elementary adsorption, desorption, surface reaction and diffusion steps over Pt and Pt alloys involved in the electroreduction of oxygen as a function of surface coverage, the electrochemical potential and temperature. The calculations were used to simulate the potential dependent adsorption and surface reaction energies along with activation barriers in order to determine the kinetics for different surface structures and structural features (step edge and corner sites) to provide the necessary input for kinetic Monte Carlo simulations to follow the rates of reaction. More coarse-grained simulated annealing methods were used to help establish the lowest energy structures and morphologies for different Pt and Pt-alloy nanoparticles that form. The results from the DFT calculations were used to establish an ab initio-derived kinetic database that was used in both 2D and 3D kinetic Monte Carlo simulations that were used to follow the electrocatalytic performance over different particle sizes, shapes and compositions. The ab initio calculations together with the kinetic Monte Carlo simulations were used to complete the following objectives: 1) Determine the controlling elementary reaction pathways and intrinsic kinetics involved in ORR and their potential dependent behavior; 2) Establish the influence of the extrinsic reaction environment including surface structure, alloy composition and spatial arrangement and the humidity on ORR kinetics; 3) Elucidate the effects of particle size and morphology as well as the atomic structure and composition of nanoparticles of Pt, Pt 3 Co, Pt 3 Ni, Pt 3 Fe and other Pt-alloys on the electrocatalytic activity and reaction selectivity; 5) Construct surface Pourbaix phase diagrams for different Pt and Pt-alloys in order to map out the most stable surface phases for different material compositions and reaction conditions; 6) Elucidate the mechanisms that control dissolution and re-deposition of Pt.

08 HYDROGEN↗
DOE OSTI2016

Development of Novel Non-Pt Group Metal Electrocatalysts for PEM Fuel Cell Applications

The objective of this multi-institutional effort was to comprehensively pursue the goal of eliminating noble metal (Pt group metals, PGM) from the cathodic oxygen reduction reaction (ORR) electrode thereby providing a quantum leap in lowering the overall PGM loading in a polymer electrolyte fuel cell (PEMFC). The overall project scope encompassed (a) comprehensive materials discovery effort, (b) a concomitant effort to scale up these materials with very high ( ±5%) reproducibility, both intra and inter, (c) understanding mass transport in porous medium both in gas diffusion and micro-porous layers for enhanced areal activity, (d) understanding mechanistic aspects of active site structure and ORR electrocatalytic pathway. Overall project milestones and metrics were (a) first phase effort based on performance in oxygen where the project’s Go/No-Go decision point milestone of 100 mA/cm 2 at 0.8 V (internal resistance-free, iR-free) at 80°C, pure H 2 /O 2 , with 1.5 bar total pressure was met. Subsequently, the principle objectives were to (a) transition the project from H 2 /O 2 to H 2 /Air with slated target of exceeding 30 mA/cm 2 @ 0.8 V, 2.5 bar total pressure and an end of the project target of 1 A/cm 2 @ 0.4 V (same total pressure), both under 100% relative humidity. The target for catalyst material scale up was to achieve 100 g batch size at the end of the program. This scale up target had a quality control milestone of less than 5% variation of activity measured with H 2 /Air (2.5 bar total pressure) at 0.8 V. In addition, the project also aimed at arriving at a unified understanding of the nature of active sites in these catalysts as well as some preliminary understanding of the mechanistic pathway. Also addressed is the development of an integrated method for determination of mass transport parameters using a combination of Helox experiments and modeling of the gas diffusion media, especially the micro-porous layer on the gas diffusion electrode (GDE). Detailed aspects of technical metrics and milestones are provided in Table 1 of the final report. Besides the success in meeting the DOE milestones in areal activities for oxygen and air described above one of the key successes of this effort was in understanding the nature of the active site(s) and aspects of the ORR pathway. In this it should be noted that the materials discovery effort provided for use of unconventional approaches, some of which led to very active catalysts. This aspect is described in detail in the final report. From a mechanistic perspective, a combination spectroscopic techniques confirmed that the high activity observed for most pyrolyzed Fe-based catalysts, irrespective of the precursors materials (macrocycles or individual Fe, N, and C precursors), the synthesis method (wet chemical impregnation or SSM), and final Fe-species (with or without inorganic iron species), can mainly be attributed to a single active site: non-planar Fe-N 4 moiety embedded in distorted carbon matrix characterized by a high potential for the Fe 2+/3+ redox transition in acidic electrolyte/environment, which is likely formed via the covalent incorporation of distorted Fe-N4 moieties in the defective centers on the carbon basal plane or in armchair edges of two adjacent graphene layers. This Fe 2+ -N 4 active site at 0.3 V undergoes redox transition to a pentacoordinate HO₋Fe 3+ ₋N 4 at 0.90 V, and the adsorption of the *OH trigged by the Fe 2+ /Fe 3+ redox transition poisons the active sites, thereby providing experimental evidence of the redox mechanism. Moreover, a highly active MOF-based catalyst devoid of any Fe-N moieties was also developed, and the active sites were identified as nitrogen-doped carbon fibers with embedded iron particles that are not directly involved in the oxygen reduction pathway. The high ORR activity and durability of catalysts involving this site in fuel cells are attributed to the high density of active sites and the elimination or reduction of Fenton-type processes. The latter are initiated by hydrogen peroxide but are known to be accelerated by iron ions exposed to the surface, resulting in the formation of damaging free-radicals. We expect that the comprehensive understanding of the synthesis-products correlations, nature of active sites, and the reaction mechanisms acquired here by systematically studying a broad variety of M-N-C materials under in situ conditions will provide guidelines to rational design of this type of non-PGM catalysts.

08 HYDROGEN↗