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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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At least 1,585 records · Page 88
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Computational Modeling of Degradation of Substituted Benzyltrimethyl Ammonium: Preprint

The degradation of cations on the alkaline exchange membranes is the major challenge for alkaline membrane fuel cells. In this paper, we investigated the degradation barriers by density functional theory for substituted benzyltrimethyl ammonium (BTMA+) cations, which is one of the most commonly used cations for alkaline exchange membranes. We found that substituted cations with electron-releasing substituent groups at meta-position of the benzyl ring could result in improved degradation barriers. However, after investigating more than thirty substituted BTMA+ cations with ten different substituent groups, the largest improvement of degradation barriers is only 1.6 kcal/mol. This implies that the lifetime of alkaline membrane fuel cells could increase from a few months to a few years by using substituted BTMA+ cations, an encouraging but still limited improvement for real-world applications.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2015

Catalyst-layer ionomer imaging of fuel cells

Investigation of membrane/electrode assembly (MEA) microstructure has become an essential step to optimize the MEA components and manufacturing processes or to study the MEA degradation. For these investigations, transmission electron microscopy (TEM) is a tool of choice as it provides direct imaging of the different components. TEM is then widely used for analyzing the catalyst nanoparticles and their carbon support. However, the ionomer inside the electrode is more difficult to be imaged. The difficulties come from the fact that the ionomer forms an ultrathin layer surrounding the carbon particles and in addition, these two components, having similar density, present no difference in contrast. In this paper, we show how the recent progresses in TEM techniques as spherical aberration (Cs) corrected HRTEM, electron tomography and X-EDS elemental mapping provide new possibilities for imaging this ionomer network and consequently to study its degradation.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2015

Carbon corrosion in PEM fuel cells during drive cycle operation

One of the major contributors to degradation involves the electrocatalyst, including the corrosion of the carbons used as catalyst supports, which leads to changes in the catalyst layer structure. We have measured and quantified carbon corrosion during drive cycle operation and as a variation of the upper and lower potential limits used during drive cycle operation. The amount of carbon corrosion is exacerbated by the voltage cycling inherent in the drive cycle compared with constant potential operation. The potential gap between upper and lower potentials appears to be more important than the absolute operating potentials in the normal operating potential regime (0.40V to 0.95V) as changes in the measured carbon corrosion are similar when the upper potential was lower compared to raising the lower potential. Catalyst layer thinning was observed during the simulated drive cycle operation which had an associated decrease in catalyst layer porosity. This catalyst layer thinning is not due solely to carbon corrosion, although carbon corrosion likely plays a role; much of this thinning must be from compaction of the material in the catalyst layer. As a result, the decrease in catalyst layer porosity leads to additional performance losses due to mass transport losses.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2015

Experimental results with fuel cell start-up and shut-down. Impact of type of carbon for cathode catalyst support

Separate testing protocols for fuel cell startups and shutdowns were developed to distinguish between their effects on reverse currents and CO 2 evolution. The internal currents and the local potentials were measured with different membrane-electrode assemblies (MEAs): we examined the influence of the type of carbon for cathode catalyst support as well as the mitigating effect of low anode Pt loading. In conclusion, significant differences were observed and the experiments also confirmed previous results that the evolved CO 2 accounts for less than 25% of the total exchanged charge.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2016

Manganese oxide/poly(3,4-ethylenedioxythiophene) hybrid electrocatalysts for the oxygen reduction reaction in alkaline fuel cells

Manganese oxide/poly(3,4-ethylene-dioxythiophene) (MnO x / PEDOT) nanostructured hybrid thin films were prepared using a simple anodic electrodeposition process from aqueous solution, and then tested for oxygen reduction reaction (ORR) activity in alkaline electrolyte using rotating disk electrode and rotating ring disk electrode methods. MnO x /PEDOT provided improvements over MnO x -only and PEDOT-only control films, with > 0.2 V decrease in onset and half-wave overpotentials, and > 1.5 times increase in terminal current density. The MnO x /PEDOT film exhibited only a slightly lower n value (n = 3.86-3.92) than the 20% Pt/C benchmark electrocatalyst (n = 3.98) across all potentials. MnO x /PEDOT also displayed a more positive half-wave potential and superior electrocatalytic selectivity for the ORR upon methanol exposure than 20% Pt/C. Here, the high activity and synergism of MnO x /PEDOT towards the ORR is attributed to effective intermixing/dispersion of the two materials, intimate substrate contact with improved charge transfer processes attained by co-electrodepositing MnO x with PEDOT and due to the increase in Mn 3+ content at the surface of the oxide.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2016

Cobalt phosphide based nanostructures as bifunctional electrocatalysts for low temperature alkaline water splitting

Cobalt phosphide based thin films and nanoparticles were prepared by the thermal phosphidation of spinel Co 3 O 4 precursor films and nanoparticles, respectively. CoP films were prepared with overall retention of the Co 3 O 4 nanoplatelet morphology while the spherical/cubic Co 3 O 4 and Ni 0.15 Co 2.85 O 4 nanoparticles were converted to nanorods or nanoparticles, respectively. The inclusion of nickel in the nanoparticles resulted in a 2.5 fold higher surface area leading to higher gravimetric performance. In each case high surface area structures were obtained with CoP as the primary phase. All materials were found to act as effective bifunctional electrocatalysts for both the HER and the OER and compared well to commercial precious metal benchmark materials in alkaline electrolyte. As a result, a symmetrical water electrolysis cell prepared from the CoP-based film operated at a low overpotential of 0.41-0.51 V.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2017

(Invited) Effect of Aging on Mechanical Properties of Lithium Ion Cell Components

The mechanical properties of aged and fresh lithium ion cell components are evaluated in this paper. Cells components were obtained from destructive physical analysis of 40Ah NMC/Graphite-based pouch cells before and after cycling and were subjected to mechanical testing. The aging tests comprised of cycling the cell across a voltage window of 4.1V to 3.0V at room temperature (25?). Using a 2C charging rate and 1C discharging rate, the cells were subjected to over 5600 cycles before a 80% drop in the name-plate capacity was observed. Mechanical tests, including compression test, tensile test and indentation test, were conducted on the cell components to investigate differences in the mechanical performance. Comparison of the fresh and aged cells components shows that cycling the cells has different degrees of impact on the different cell components. Anodes suffered the most serious deterioration in mechanical properties while separators remained intact under the test condition investigated.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2017

Synthesis and Characterization of Perfluorinated Anion Exchange Membranes

Anion exchange membranes (AEMs) are of high interest for a number of electrochemical device applications including fuel cells, electrolyzers, and flow batteries. Perfluorinated sulfonic acid polymers have been the standard polymer used in the much more established area of proton exchange membrane based devices due to specific advantageous attributes including chemical stability, high conductivity, high water mobility, and the ability to create high performance electrodes. These attributes would make for desirable AEMs, but synthesizing perfluorinated AEMs has been limited and has significant challenges. Here, we report our efforts to develop novel synthesis routes to sulfonamide-linked alkyl ammonium perfluorinated AEMs. We have demonstrated the ability to achieve both high levels of ion exchange and membrane conductivity. We have achieved improved durability by extending the length of the alkyl tether from 3 to 6 carbons, and we have demonstrated the ability to process these polymers into membranes, ionomer solutions/dispersions, and fuel cells with reasonable performance.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2018

Reduction Reaction Activity on Pt-Monolayer-Shell PdIr/Ni-core Catalysts

Platinum monolayer oxygen reduction reaction catalysts present promising way of reducing the Pt content without scarifying its fuel cell performance. We present a facile way of preparing Pt monolayer shell PdIr-based core catalysts, which showed much higher activity for oxygen reduction reaction than that of TKK 46.6% Pt/C catalyst. Among tested samples, PtMLPd2Ir/Ni/C performs the best with Pt and Platinum Group Metal mass activity around 9 and 0.25 times higher of that of TKK 46.6% Pt/C. In addition, accelerated aging test indicates its excellent durability.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2018

Temperature-Dependent Kinetic Study of Ammonia Oxidation Reaction on Gas Diffusion Electrodes in NH 3 -Saturated 1 M KOH Solutions

Ammonia oxidation reaction (AOR) is sluggish, especially at ambient temperature. To make kinetic study in electrochemical cell more informative and relevant to the catalysts’ performance in direct ammonia fuel cells (DAFCs) operating at about 100°C, it is desirable to study the AOR kinetics at elevated temperatures. However, ammonia evaporation accelerates with increasing temperature causing decrease of ammonia concentration with time. Here, we show a feasible solution to this issue - let argon gas bubble through concentrated ammonia before entering the electrochemical cell so that the solution can be kept ammonia saturated and oxygen free. Repeatable AOR polarization curves were obtained at temperatures up to 60°C. The AOR activities are characterized by the average currents at 0.5 V versus RHE measured at 20 mV s-1 in potential cycles below the potential of peak current. For PtIr/C, the PGM mass activities are 25 A g-1 at 25°C and 225 A g -1 at 60°C. The results for Pt/C and Ir/C and discussion of the causes for their distinct kinetic behavior will be presented at the meeting.

25 ENERGY STORAGE↗
DOE OSTI2018

Strategies for Reducing the PGM Loading in High Power AEMFC Anodes

Anion Exchange Membrane Fuel Cells (AEMFCs) have experienced a significant rise in attention in recent years, largely motivated by the potential to overcome the costs that have plateaued for proton exchange membrane fuel cells. However, despite significant advances in power generation, membrane conductivity, membrane stability, and catalyst activity, the vast majority of high performing AEMFCs are fabricated with a high PGM loading (0.4-0.8 mg cm -2 ). This work demonstrates an electrode fabrication method that reduces the anode catalyst loading by 85 % while still achieving performance ca. 1 W cm -2 - accomplished by designing a multi-layered electrode comprised of an optimized ionomer:carbon:PGM ratio catalyst layer coupled with a hydrophobic microporous layer. If paired with a high-performing PGM-free cathode, this new anode can meet existing DOE PGM loading and performance targets.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2018

Microwatt Fuel Cell for Long-Term and Wide Ambient Temperature Range Operation

A small microwatt fuel cell stack and system was designed, fabricated, and tested for passive operation with pure H 2 and O 2 to provide continuous power for multiple decades with uncontrolled fluctuating ambient conditions. The stack was designed to operate with dead ended gas flows with water removal via passive membrane diffusion to notches in the bipolar plates. Additional requirements for decades long operation is minimizing H 2 and O 2 consumption, specifically membrane cross-over; to minimize cross-over, stacks were built with layered membranes of 100 µm and 400 µm. Here, the stack spatial water concentration level was monitored during operation by in situ neutron imaging to evaluate the passive water management design. Operation was verified by testing at temperatures ranging from -55 °C to 80 °C, including periodic pulses in power.

08 HYDROGEN↗
DOE OSTI2018

Heteropoly Acid Functionalized Membranes for High Performance and Chemically Stable Fuel Cell Operation

Chemical degradation of perfluorinated sulfonic acid membranes, such as Nafion(R) via radical attack represents one of the current challenges with fuel cell durability. Ce and Mn have been used to decompose radicals, but migration of these species results in loss of chemical stability efficacy. Herein we report on a recent breakthrough in chemical durability that has been achieved through using heteropoly acids (HPAs) as both the proton conducting acid and the radical decomposition catalyst. Exceptional chemical durability is reported for a thin film and a mechanism is purposed where the W atoms in the HPA reversibly change oxidation state from W(VI) to W(V) while decomposing radical species. It is concluded that covalently tethered HPA should be considered as a next generation chemical stabilization strategy for polymer electrolyte fuel cells.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2018

Kinetic Equilibrium Study of CO 2 Poisoning Observed in Anion Exchange Membranes When Exposed to Ambient Air and Varying Levels of CO 2 ppm

One of the hurdles in Anion exchange membrane fuel cell technology is weaker understanding of CO 2 poisoning observed when they are exposed to air. Our previous work suggests that OH- ions depletes in approximately 15 min to form CO 3 2- and HCO 3 - ions when exposed to ambient air at 30 degrees C and 95%RH (1). However, an actual fuel cell device is operated at a much higher temperature. In this study we have reported the CO 2 absorption data at different temperatures and different CO 2 concentration environment. Our data suggest that the air equilibrated AEM concentration of CO 3 2- increases at higher temperatures whereas HCO 3 - concentration follows a decreasing trend. The OH - ion has a very insignificant change at higher temperatures but we observe that the equilibrium OH - concentration is higher at lower ppm level of CO 2 environment. This indicates that the CO 2 reaction is dependent on CO 2 concentration in air.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2019

Effective Electrode Edge Protection for Proton Exchange Membrane Fuel Cell Drive Cycle Operation

Drive cycle (DC) tests employ rapid load cycling which will result in rapidly changing local operating conditions and consequently high non-uniform mechanical stress at the electrode perimeter. In order to better investigate the impact of electrode irregularities on the long-term behavior of the cell, it is necessary to exclude the edge effects of the membrane electrode assemblies (MEAs) as a failure mode. Therefore, an effective electrode edge protection technique using thin protective gaskets and a hot-pressing procedure was developed which dramatically prolonged lifetime. Open circuit voltage (OCV), air polarization curve, and hydrogen crossover limiting current density were monitored during the DC tests. For post-DC ex-situ analysis, an in-house developed pinhole detection apparatus was employed to analyze quantity, size, and location of the failure points of MEAs with and without edge protection. Non-protected MEAs typically developed tears at the electrode perimeter, while the longer-lasting protected MEAs exhibited seemingly random pinhole development.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2019

Thin Film Morphological Characteristics of a Perfluorinated Anion Exchange Membrane

Anion exchange membranes (AEMs) are a promising technology for fuel cell applications. Most of the research done to date was focused on membrane development, electrode fabrication, or cell performance. Studies of the polymer catalyst interface are needed in order to advance the science of these devices and achieve further improvements. In this work we have studied these interfacial properties using grazing-incidence small-angle x-ray scattering (GISAXS). Thin films of a perfluorinated anion exchange polymer were spin cast onto bare, or silver or platinum coated flat silicon substrates. From the GISAXS data, we can conclude that the polymer chains align parallel to the plain silicon or silver coated substrate at higher thicknesses, >17 nm and, >13 nm respectively. However, on the coated platinum substrate the alignment is somewhere between parallel and perpendicular. The in-plane alignment of ionomer on the surface constrains the swelling of water channels which affects the transport of reactants and products.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2019

High-Throughput Anion Exchange Membrane Characterization at NREL

Anion exchange membrane fuel cells (AEMFCs) are of high interest because they utilize lower cost catalyst and separator materials relative to proton exchange membrane fuel cells (PEMFCs). AEMFCs require anion exchange membranes (AEMs) that have high alkaline chemical stability, high hydroxide ion conductivity, and adequate thermal/mechanical properties. We report NREL has developed protocols to allow for high throughput, parallel characterization of AEM materials. More specifically, NREL has received membranes from more than 10 partners and has tested more than 50 membranes using these approaches. In many cases these ex-situ results can be compared to fuel cell performance and durability. Technical investigations include conductivity, ion exchange capacity, mechanical performance, chemical stability, thermal properties, water uptake, and swelling. Structure-property relationships that affect membrane performance and durability will be discussed, lending insight into optimal backbone and cation strategies to employ in development of AEMs.

30 DIRECT ENERGY CONVERSION↗