Hydrogen Partial Pressure Research Articles

Stress concentration affecting hydrogen-assisted crack in API X70 pipeline base and weld steel under hydrogen/natural gas mixture

A sudden change in the geometrical parameters of a material, such as bends, T-branches, and welded joints may increase the stress concentration, which in turn may enhance the diffusible hydrogen concentration. In this study, quantitative evaluations of the effect of stress concentration factors on hydrogen environmental embrittlement (HEE) will be studied based on the mechanical performance of notched tensile specimens with variations in the stress concentration factors (Kt) of the X70 pipeline base and weld metal. To highlight the role of stress concentration, the tests are performed under a 10 MPa gas mixture condition with a low partial hydrogen pressure. The maximum strengths of the notched specimens were nearly consistent across all Kt values. The degree of HEE susceptibility of the base metal increased with increasing Kt value. However, in the weld metal, prominent reductions in the RA occurred at all Kt values. The complex HEE susceptibility of the weld metal can be attributed to the synergistic action of the microstructure heterogeneity and Kt values.

A thermodynamics study of hydrogen interaction with (1 1 0) transition metal surfaces

An in-depth understanding of the interactions between hydrogen and transition metal catalysts is of great significance in exploring novel heterogeneous hydrogenation reaction mechanisms. Herein we present a comprehensive study of the interactions of hydrogen on active metal surfaces by using a multiscale method. Six different transition metals of Ni-group and Cu-group are considered. Different from two stable (111) and (100) surfaces, the energetic results of hydrogen species diffusing on and permeating into the active (110) surfaces are fully addressed from three-dimensional potential energy surfaces and density functional theory calculations. Ab initio thermodynamics calculations show that a stable adsorption phase diagram with full hydrogen coverage preferably forms with decreasing reaction temperature and increasing hydrogen partial pressure, especially on the (110) surfaces of Ni-group metals without consideration of the reconstruction events. During the evolutions of metal nanoparticles under moderate reaction conditions, the active (110) surface is difficult to be exposed for Ni-group metal nanoparticles, while for Cu-group metal nanoparticles it is easy to get exposed. These important thermodynamic results will contribute to an in-depth understanding of the interactions between hydrogen species and transition metal catalysts in heterogeneous catalysis.

Hydrogenation of ethylene over palladium: evolution of the catalyst structure by operando synchrotron-based techniques.

Palladium-based catalysts are exploited on an industrial scale for the selective hydrogenation of hydrocarbons. The formation of palladium carbide and hydride phases under reaction conditions changes the catalytic properties of the material, which points to the importance of operando characterization for determining the relation between the relative fractions of the two phases and the catalyst performance. We present a combined time-resolved characterization by X-ray absorption spectroscopy (in both near-edge and extended regions) and X-ray diffraction of a working palladium-based catalyst during the hydrogenation of ethylene in a wide range of partial pressures of ethylene and hydrogen. Synergistic coupling of multiple techniques allowed us to follow the structural evolution of the palladium lattice as well as the transitions between the metallic, hydride and carbide phases of palladium. The nanometric dimensions of the particles resulted in the considerable contribution of both surface and bulk carbides to the X-ray absorption spectra. During the reaction, palladium carbide is formed, which does not lead to a loss of activity. Unusual contraction of the unit cell parameter of the palladium lattice in the spent catalyst was observed upon increasing hydrogen partial pressure.

Improved Semi-empirical Model of Proton Exchange Membrane Fuel Cell Incorporating Fault Diagnostic Feature

The membrane water content of the proton exchange membrane fuel cell (PEMFC) is the most important feature required for water management of the PEMFC system. Any improper management of water in the fuel cell may lead to system faults. Among various faults, flooding and drying faults are the most frequent in the PEMFC systems. This paper presents a new dynamic semi-empirical model which requires only the load current and temperature of the PEMFC system as the input while providing output voltage and membrane water content as its major outputs. Unlike other PEMFC systems, the proposed dynamic model calculates the internal partial pressure of oxygen and hydrogen rather than using special internal sensors. Moreover, the membrane water content and internal resistances of PEMFC are modelled by incorporating the load current condition and temperature of the PEMFC system. The model parameters have been extracted by using a quantum lightening search algorithm as an optimization technique, and the performance is validated with experimental data obtained from the NEXA 1.2 kW PEMFC system. To further demonstrate the capability of the model in fault detection, the variation in membrane water content has been studied via the simulation. The proposed model could be efficiently used in prognostic and diagnosis systems of PEMFC fault.

Recent Approach of Forensic-Based Investigation Algorithm for Optimizing Fractional Order PID-Based MPPT With Proton Exchange Membrane Fuel Cell

The output power of the fuel cell (FC) is mainly depending on the membrane water content, temperature, the hydrogen and oxygen partial pressures. The polarization curve has one global maximum power to be tracked. Therefore, a robust maximum power point tracking (MPPT) is highly required to follow the optimal operating point under any operating conditions. In this article, a recent approach of forensic-based investigation (FBI) algorithm is proposed to identify the optimal parameters of fractional order PID-based MPPT with proton exchange membrane (PEM) fuel cell. FBI is selected due to its high accuracy and requirement of less computational efforts. The considered objective function to be minimized is the error between the voltage at maximum power (VMP) and the actual one at FC terminals. To prove the robustness of the proposed methodology, four cases of operating conditions are analyzed which are constant membrane water content and temperature, constant membrane water content with variable temperature, variable membrane water content with constant temperature, and variable membrane water content with variable temperature. The obtained results are compared to other approaches such as incremental resistance (INCR), particle swarm optimizer (PSO), invasive weed optimizer (IWO), sin-cosine algorithm (SCA), and artificial ecosystem optimizer (AEO). In case (1), the proposed FBI-FOPID succeeded in achieving maximum power of 5185.101 W. While the minimum objective functions in the second, third, and fourth cases are 2.5736 V, 1.4436 V, and 1.1568 V respectively obtained via the proposed approach. The comparison confirmed the superiority of the proposed FBI-based MPPT compared with other methods.

The effect of H2 : N2 ratio on the NH3 synthesis rate and on process economics over the Co3Mo3N catalyst.

In this study, the process economics of ammonia synthesis over Co3Mo3N was investigated by searching for an optimum feed stoichiometry. From ammonia synthesis rate measurements at atmospheric pressure and 400 °C over Co3Mo3N, it was found that the rate was independent of H2 : N2 stoichiometry for stoichiometries above 0.5 : 1. For H2 : N2 stoichiometries below 0.5 : 1, there was a linear dependency of ammonia synthesis rate on the H2 : N2 stoichiometry. Static measurements of hydrogen adsorption isotherms at 25, 50, and 100 °C revealed that the adsorbed amounts of strongly bound hydrogen over the Co3Mo3N surface were saturated at around 100 Torr hydrogen pressure. This pressure corresponds to the partial pressure of hydrogen when the H2 : N2 stoichiometry is around 0.5 : 1, confirming the role of strongly bound hydrogen in ammonia synthesis. These results were used to modify an existing kinetic expression to be used in a conceptual design, based on a late mixing strategy for the hydrogen stream. This conceptual design and its economic analysis revealed that using low hydrogen stoichiometries can cut the investment and operating costs by a factor of 2.

Performance assessment and evaluation of SOC stacks designed for application in a reversible operated 150 kW rSOC power plant

The coupling of renewable energy sources with reversible operated high temperature solid oxide cells (rSOC) seems to be a promising option to store and supply clean energy in the future. This work provides the results of experimental investigations on the performance of a rSOC stack as a main unit of a large-scale module installed at a real industrial power plant. In order to determine the requirements as well as to propose strategies for safe and stable operation of the large-scale module the stack was tested under system relevant gas mixtures and operational conditions. Hereby, the focus was on steam electrolysis (EC) and fuel cell (FC) operation utilizing a H2/H2O/CO/CO2/N2 gas mixture, predefined by the reformer installed within the module at the power plant. Furthermore, the alternating operation between EC and FC mode was analyzed. The comprehensive analysis of the performance of the stack includes polarization curves, electrochemical impedance spectroscopy (EIS), distribution of relaxation times (DRT), gas analysis and temperature measurements. Thus, numerous combinations of operational parameters are linked to the individual processes within the stack. During EC operation higher hydrogen partial pressures presented significantly lower diffusion losses especially at low current densities, whereby in FC mode low fuel flows presented increased concentration losses. During alternating operation, prolonged voltage stabilization periods with increasing operation time were observed. Additionally, the processes represented by the peaks calculated from the DRT showed enhanced unstable behavior and presented a significant shift towards higher frequencies compared to constant mode operation.

Mono- and bimetallic (Ru-Co) polymeric catalysts for levulinic acid hydrogenation

Levulinic acid (LA) is one of the substances, which can be obtained from cellulosic biomass via acid hydrolysis and serves as a precursor in the synthesis of gamma-valerolactone (GVL) – platform chemical for so-called valeric fuels. This work is devoted to the study of catalytic conversion of LA to GVL using mono- (Ru) and bimetallic (Ru-Co) catalysts based on ruthenium-containing nanoparticles immobilized in hyper-crosslinked polystyrene (HPS). It was shown that RuO2 nanoparticles are an active phase of HPS-based Ru catalysts, which allow nearly 100 % yield of GVL at 120 °C and 2 MPa of hydrogen partial pressure for the reaction time 60 min. Introduction of cobalt in catalyst composition in a proper amount (Ru-to-Co weight ratio is 30) was found to result in Ru redistribution inside the polymeric support and, in turn, to the increase of reaction rate of LA hydrogenation.

Synthesis of Hydrogen Getter Zr1-xCox (x=0-1) Alloy Films by Magnetron Co-Sputtering

The Zr1-xCox(x=0, 0.25, 0.53, 0.63, 1) thin films were deposited on quartz substrate using magnetron co-sputtering of Zirconium and Cobalt targets in confocal geometry. A constant pulsed direct current (PDC) on Zirconium and radio frequency (RF) of various powers on Cobalt target were applied to vary the concentration of Co in the Zr1-xCox film. The film composition was quantified using EDX measurements. The hydrogen storage capacity of these films was studied using an in-house developed hydrogen adsorption setup, in which the electrical resistivity of the film was monitored as a function of hydrogen partial pressure and temperature. The films' surface morphology and crystal structure before and after hydrogenation were characterized using atomic force microscopy and grazing incidence X-ray diffraction techniques using synchrotron radiation, respectively. An increase in the particle size after hydrogenation was observed for all the films. An increase in resistivity was also observed due to the absorption of hydrogen in all the compositions. The near stoichiometric film Zr0.47Co0.53 showed the highest hydrogen absorption level at 200 oC at all partial pressures. However, a decrease in the response at temperatures higher than 200 oC was observed in the film containing a Co concentration. The mechanism for the increase in resistivity of the film on hydrogenation is explained

Effect of Cultivation Parameters on Fermentation and Hydrogen Production in the Phylum Thermotogae.

The phylum Thermotogae is composed of a single class (Thermotogae), 4 orders (Thermotogales, Kosmotogales, Petrotogales, Mesoaciditogales), 5 families (Thermatogaceae, Fervidobacteriaceae, Kosmotogaceae, Petrotogaceae, Mesoaciditogaceae), and 13 genera. They have been isolated from extremely hot environments whose characteristics are reflected in the metabolic and phenotypic properties of the Thermotogae species. The metabolic versatility of Thermotogae members leads to a pool of high value-added products with application potentials in many industry fields. The low risk of contamination associated with their extreme culture conditions has made most species of the phylum attractive candidates in biotechnological processes. Almost all members of the phylum, especially those in the order Thermotogales, can produce bio-hydrogen from a variety of simple and complex sugars with yields close to the theoretical Thauer limit of 4 mol H2/mol consumed glucose. Acetate, lactate, and L-alanine are the major organic end products. Thermotagae fermentation processes are influenced by various factors, such as hydrogen partial pressure, agitation, gas sparging, culture/headspace ratio, inoculum, pH, temperature, nitrogen sources, sulfur sources, inorganic compounds, metal ions, etc. Optimization of these parameters will help to fully unleash the biotechnological potentials of Thermotogae and promote their applications in industry. This article gives an overview of how these operational parameters could impact Thermotogae fermentation in terms of sugar consumption, hydrogen yields, and organic acids production.

Parallel mechanism of growth of the oxide and α-Zr(O) layers on Zircaloy-4 oxidized in steam at high temperatures

The kinetic behavior of Zircaloy-4 tube samples was examined during high temperature steam oxidation in order to propose a mechanism with detailed elementary steps. The study revealed that steam and hydrogen partial pressures have no effect on the reaction kinetics. Based on these experiments, the kinetic model established herein takes into account the oxide growth and the oxygen dissolution in the metal. A rate-determining step of diffusion of oxygen vacancies in the oxide governs the kinetics of the reaction in both pre- and post-breakaway regimes meaning that the oxide growth and oxygen dissolution in the metal advance at proportional rates.

Capillary microreactor with PdZn/(Ti, Ce)O2 coating for selective hydrogenation of 2-methyl-3-butyn-2-ol

The catalytic properties of Pd/Ti0.95Ce0.05O2, Pd80Zn20/Ti0.95Ce0.05O2 and Pd/TiO2 coatings on the inner wall of capillary microreactor were studied in the selective hydrogenation of 2-methyl-3-butyn-2-ol (MBY). The effect of initial MBY concentration, hydrogen partial pressure, reaction temperature on activity and selectivity to MBE was studied for three microcapillaries coated with different catalysts. The most noticeable impact was observed for the catalyst composition such as Ce-doping and Zn addition to Pd nanoparticles and can be explained by the different strengths of MBY adsorption. The catalyst activity increases in the series: Pd/TiO2 < Pd80Zn20/Ti0.95Ce0.05O2 < Pd/Ti0.95Ce0.05O2, while the adsorption constant of MBY and selectivity to 2-methyl-3-buten-2-ol (MBE) decline. The catalytic performance also depends on test duration and oxidation-reduction treatments. The highest yield of MBE (95.8 %) at 99 % conversion on Ce-doped coatings was obtained on Pd50Zn50/Ti0.95Ce0.05O2 coating after 68 h on stream.

Thermodynamic Analysis of Methylcyclohexane Dehydrogenation and Solar Energy Storage via Solar-Driven Hydrogen Permeation Membrane Reactor.

A novel methylcyclohexane (MCH) dehydrogenation system driven by solar energy with a hydrogen permeation membrane (HPM) reactor is proposed in this study. It is a promising method, via this novel system, to generate pure hydrogen and store intermittent solar energy. In this research, the thermodynamic analysis of MCH dehydrogenation via the HPM reactor was conducted based on numerical simulation. The conversion rates and thermodynamic efficiencies under different temperatures (150–350 °C), permeate pressures from 0.001 to 0.5 bar, and solar irradiation in the four seasons were studied and analyzed. Under a hydrogen partial pressure difference, HPM can separate hydrogen and shift the reaction equilibrium forward for a higher conversion rate of MCH, which can reach nearly 99.7% in this system. The first-law of thermodynamic efficiency, the solar-to-fuel efficiency, and the exergy efficiency are up to 95.58%, 38.65%, and 94.22%, respectively. This study exhibits the feasibility and potential of MCH dehydrogenation via the HPM reactor driven by solar energy and provides a novel approach for solar energy storage.

Hydrogen Visualization in a Fuel Cell Anode By Raman Imaging

Development of durable commercial/heavy fuel cell (FC) vehicles is required to decrease carbon dioxide emission from commercial/heavy vehicles. The mileage of heavy vehicles is far higher than that of passenger vehicles. Because FC stacks in vehicles deteriorate during long-term operation, their lifetime should be maximized by increasing durability. To improve FC stack durability, we must avoid catalyst degradation. When the hydrogen supply on the anode catalyst surface is inadequate, carbon of the cathode catalyst is oxidized with water. This occurs where there is insufficient hydrogen, such as at anode outlet during cell starting or water flooding. It is therefore important to prevent deterioration of FC operation caused by inadequate hydrogen. In stacks where the anode gas is circulated, nitrogen permeates and remains at the anode until it is exhausted, which lowers hydrogen concentration. Moreover, hydrogen is consumed in the piling direction, so its concentration is lower downstream. This leads to degradation at the anode outlet, so a high hydrogen flow rate is required during operation.To optimize hydrogen supply through feedback control, it is beneficial to measure the local hydrogen partial pressure. Hydrogen distributions exist in the reactive plane, gas manifolds, and piping system. To avoid FC deterioration caused by insufficient hydrogen, hydrogen concentration needs to be measured in the in-plane direction. However, it is difficult to measure hydrogen distribution in the in-plane direction. Some commercial sensors may affect FC materials. General hydrogen sensors are unsuitable for hydrogen concentrations over 4% inside FC systems.Raman spectroscopy is a useful hydrogen detection method. Generally, leaked hydrogen gas is intermittently detected with Raman spectroscopy. The intensity of Raman scattering is about 10−4 times that of Rayleigh scattering, and low-density gas scattering is weak compared with that of a liquid or solid. However, the Raman shift of hydrogen vibration (4160 cm−1) is far larger than that of other materials and does not require high wavelength resolution. Moreover, the hydrogen partial pressure in FC systems is high and easily detected.We developed a new method to measure the hydrogen Raman scattering from the reacting surface of an operating FC. During operation, hydrogen is consumed in the anode channel and its partial pressure drops near the outlet. To confirm this phenomenon with Raman spectroscopy, we needed to detect scattering light from the anode channel during cell operation. This measurement also required optics able to resolve very weak hydrogen Raman scattering. For these reasons, we fabricated a visualization cell that transmitted pump and scattered light without gas leakage. Pump light was introduced into a flow field in a channel with a depth of 1 mm, so the channel worked as an optical path . The optical path was from a mirror on the cathode flow field toward the anode to avoid interference with sealing materials. Sapphire windows were included in the anode flow field to allow visualization of scattered light.Using this cell and an Intensified Charge-Coupled Device (ICCD ) camera, we vertically observed Raman scattering from hydrogen. We included a bandpass filter with a center wavelength of 420-nm and halfwidth of 10 nm inside the camera lens to capture images of 417-nm scattered light from hydrogen excited by 355-nm pump light through the filter. A 355-nm notch filter was also added to remove the optical noise from Rayleigh scattering, allowing visualization of only hydrogen.Before the measurement, scattering intensity was calibrated using 100% hydrogen and nitrogen to clarify the relation between scattering intensity and partial pressure. The flow field was filled with 100% hydrogen (100 kPa) and an intensity distribution was obtained. Pure nitrogen was also measured to evaluate the dark noise and the correlation between intensity and hydrogen partial pressure. Local partial pressure was determined from the intensity at a desired point.We then measured the scattered light during cell operation at 100 kPaG. Hydrogen with a stoichiometric ratio of 1.1 diluted 45% by nitrogen was supplied to the cell to mimic stack operation. The cell operated at 0.5 A/cm2. The scattering distribution was measured at 10 Hz and ten measurements were integrated to determine hydrogen partial pressure distribution from the scattered light intensity . Hydrogen partial pressure decreased from 80 to 40 kPa along the in-plane flow, consistent with our rough predictions of 90 and 38 kPa, respectively. This data was thought to be affected by the sensitization noise from the photomultiplier tube elements in the ICCD camera. Suppressing optical and electronic noise or conducting an imaging process with dispersion treatment will allow detection of various gases with this Raman method.

Pressure Dependency of the Hydrogen Oxidation and Evolution Reaction Kinetics on Carbon Supported Pt Catalysts Using a PEMFC Based Hydrogen Pump Approach

The hydrogen oxidation and evolution reaction (HOR/HER) has been widely investigated in electrocatalysis due to its importance for a broad variety of applications especially in electrolysis and fuel cells (FC). While the HOR/HER reaction rates on platinum and platinum alloy catalysts in alkaline environment can be determined precisely with rotating disk electrode (RDE) measurements,1-3 this is not possible at low pH due to the two orders of magnitude higher reaction rates that far exceed the slow mass transport limited rates afforded by the RDE configuration.4 As a result, meaningful measurements are limited to setups with much faster mass transport rates, such as floating electrodes or proton exchange membrane (PEM) fuel cell based electrochemical H2-pumps,4, 5 but the effect of hydrogen partial pressure and kinetic limitations at higher overpotentials are not yet fully understood.Here we report detailed kinetic investigations on the temperature and pressure dependence of the HOR/HER kinetics on carbon supported platinum (Pt/C) using the PEMFC based H2-pump approach. The measurements were performed on assymmetric membrane electrode assemblies (MEAs), where the working electrode has an ultra-low loading of only 1-2 µgPt cmMEA -2 (resulting in roughness factors of ≈1.5 cmPt 2 cmMEA -2), while the counter electrode has a much higher loading of 0.4 mgPt cmMEA -2. Using a fuel cell test station that allows for the feed of humidified hydrogen on both the anode and the cathode, we were able to show that the apparent activation energy of the HOR/HER on platinum increases with increasing partial pressure of hydrogen, which we ascribe to a decreasing effect of the hydrogen adsorption enthalpy with increasing coverage by adsorbed hydrogen atoms on the Pt surface. Consequently, the HOR/HER reaction order with respect to H2 partial pressure is also depending on the temperature. We further observed that the HOR quickly reaches a limiting current with increasing HOR overpotential, ηHOR (orange area in Fig. 1), which we ascribe to a change in the rate-determining step (rds) from the Volmer reaction (i.e., the electrochemical oxidation of adsorbed H) at low ηHOR to the Tafel reaction (i.e., the dissociative adsorption of H2 on the Pt surface) at higher ηHOR. The Tafel-limited current shows a pressure-independent activation energy and a direct proportionality to the partial pressure of hydrogen, which is exactly what one would expect for the dissociative Langmuir adsorption as the rds. Finally, the HOR/HER reaction rates on a Pt3Co alloy and a Ru@Pt core-shell catalyst will be compared to that on a Pt/C catalyst.

Estimation of Surface Coverages on H2/H2o Electrode in Reversible Operation of Solid Oxide Fuel Cell/ Electrolysis Cell Using Electrochemical Kinetics with Optimized Parameter By Genetic Algorithm

Solid oxide fuel cell / electrolysis cell (SOFC / EC) can be expected as energy converters with high efficiency. In SOFC / EC material developments, the coverages of the chemical species at the three-phase boundary (TPB) are directly linked to the electrode performance. Accordingly, it is important to estimate the surface coverages at TPB to develop a new electrode. However, it is difficult to directly analyze and evaluate the coverages at the reaction field of the oxide / metal composites at high temperature (> 600 °C). Therefore, it is required to construct electrochemical kinetics with the various parameters, such as reaction constants and adsorption equilibrium constants.In order to determine the coverages at the TPB, it has demands not only to construct an appropriate electrochemical model but also both to develop a method for fitting a large number of parameters included in the model and to acquire a series of various experimental data sufficient for fitting. These requirements are not fully satisfied in previous researches [1-2]. Therefore, the chemical properties of the electrode surface have not been clarified while the reaction mechanism on the H2 electrode has been understood in FC operation.In this study, the determination method of surface coverage using an electrochemical reaction model for H2 / H2O porous electrodes composed of Ni / YSZ was established. We considered that the local equilibrium between oxygen and oxide ions is achieved in the solid electrolyte, and proposed a H2 electrode reaction model consisting of a competitive adsorption reaction as Langmuir-Hinshelwood type at TPB. Using the model, a relational expression between the electric current density and the coverage could be obtained.The equation includes the parameters of current density (i), oxygen activity (a O), the partial pressures of hydrogen (P H2) and water (P H2O), the kinetics constant of the water generation reaction at FC operation (k a), and the adsorption equilibrium constant of H, O, OH and H2O (K H, K O, K OH and K H2O). a O(eq) is the oxygen activity at equilibrium potential. The i, P H2, P H2O, a O and a O(eq) could be obtained by experimental conditions and data while ka , K H, K O, K OH and K H2O are unknown constants without appropriate fitting. The simple optimization method cannot approximate optimal solutions and determine the coverages because this model is multivariate function, which has 5 parameters.As a method for quantifying the coverage of chemical species by fitting the experimental data to the reaction model, the method using a genetic algorithm was applied. The genetic algorithm is one of the multivariable optimization methods, which efficiently approximates the optimized value of the parameters by alternation of generations created by operations composed of selection, crossover, and mutation [3]. By taking advantage of the characteristics of the genetic algorithm that randomly generates and converges initial value candidates during optimization, multiple optimal solutions can be efficiently obtained with preventing the drop to local optimum by increasing the number of trials. The most effective optimal solution could be determined from them.As the various electrochemical data for estimating the surface coverage at a temperature of 900 °C, the electrochemical data of both FC / EC under 12 conditions, in which P H2 and P H2O were changed were got using the same cell. The current interruption method was used to separate the overvoltage and ohmic voltage loss of the H2/H2O electrode during power generation and electrolysis under constant current, and then, the oxygen activity was calculated from the ohmic free-anode potential (vs. 1 atom O2). Using genetic algorithm, optimization schemes were attempted more than 50,000 times and searched for the parameter set with the smallest error between the experimental value and the estimated value from the obtained optimal solutions. The outline of the optimization process was shown in Fig. 1.As a result of fitting various reaction constants to the model with optimal solutions, the relationship between i and a O could be roughly expressed by multiple reaction rate constants and equilibrium constants, and the coverages could be quantified. Furthermore, it was possible to draw the chemical state at TPB of Ni / YSZ electrodes during FC operation.[1] W. G. Bessler et al., Phys. Chem. Chem. Phys., 12, 13888 (2010).[2] M. Ihara et al., J. Electrochem. Soc., 148(3), A209 (2001).[3] M. Srinivas et al., IEEE, 27(6), 17 (1994). Figure 1

Catalyst Degradation in Polymer Electrolyte Fuel Cells with Multi-Modal Techniques: Understanding Phenomena Under Varied Gas and Relative Humidity

For widespread commercialization of polymer electrolyte fuel cells (PEFCs), cost and durability need to be addressed. Reduction in cost should not negatively affect PEFC durability and performance. At high volume production, platinum (Pt) electro catalyst contributes to 41% of the total fuel cell stack cost for light duty vehicles. This cost contribution increases to 53% for middle and heavy-duty vehicles. When membrane electrode assemblies (MEAs) are designed for durability, this cost will increase further. A complex balance exists between cost, performance and durability. With smaller nanoparticle size better Pt dispersion is achieved which gives improved performance and reduces Pt loading and cost. But this negatively affects durability, as smaller nanoparticles have higher surface energy which causes faster electrocatalyst degradation1. During an actual drive cycle, PEFCs need to operate over a wide range of operating conditions such as temperature (-40C to 120C), potential (0V to 1.5V) and relative humidity which can further accelerate Pt dissolution Under load cycling during typical drive cycle, repeated oxidation and reduction of Pt surface leads to Pt dissolution2. Dissolved Pt ions redeposit on nearby nanoparticles effectively increasing their particle size2. This electrochemical surface area (ECSA) loss via increase in particle size is well known as Ostwald ripening. Some Pt ions are reduced in the electrolyte membrane by crossover hydrogen which leads to formation of a Pt band, usually at the membrane and cathode catalyst layer interface3. At > 1V, carbon corrosion can lead to Pt detachment2. This can happen during start-up/shut down cycles when anode is exposed to hydrogen and air. Such degradation mechanisms need to be studied systematically with effect of operating conditions like gas flow rates, temperature and relative humidity under dynamic load cycling.In this study, we investigate the effect of cathode gas environment and gas flow rates on Pt particle size during aging experiment. A DOE adopted accelerated stress test (AST) protocol simulating the drive cycle operating conditions is used. It consists of a 0.6 V – 0.95 V square wave with 3s hold time at each potential. In situ electrochemical characterization at regular stages is used to identify ECSA loss trends and polarization performance. Figure 1 shows normalized ECSA loss for the four cells cycled in different environments during catalyst AST. Measurements were taken beginning of life (BOL), after 1,000, 5,000, 15,000 and 30,000 cycles. We observed that the cell that was cycled in N2 and 100 % RH degraded the most , having 25 % of ECSA left after 30,000 cycles compared to BOL. Electrochemical measurements are supplemented by detailed post-mortem ex-situ x-ray characterization by using micro x-ray fluorescence, x- ray computed tomography and micro x-ray diffraction, SEM-EDS. 1cm2 active areas of the catalyst layer at the inlet, middle and outlet of the flow field were mapped using micro x-ray diffraction to understand Pt particle size distribution under land/channel. Catalyst layer loading before and after the AST were confirmed using micro x-ray fluorescence.References(1) Cherevko, S.; Kulyk, N.; Mayrhofer, K. J. J. Durability of Platinum-Based Fuel Cell Electrocatalysts: Dissolution of Bulk and Nanoscale Platinum. Nano Energy 2016, 29, 275–298. https://doi.org/10.1016/j.nanoen.2016.03.005.(2) Meier, J. C.; Galeano, C.; Katsounaros, I.; Witte, J.; Bongard, H. J.; Topalov, A. A.; Baldizzone, C.; Mezzavilla, S.; Schüth, F.; Mayrhofer, K. J. J. Design Criteria for Stable Pt/C Fuel Cell Catalysts. Beilstein J. Nanotechnol. 2014, 5 (1), 44–67. https://doi.org/10.3762/bjnano.5.5.(3) Zhang, J.; Litteer, B. A.; Gu, W.; Liu, H.; Gasteiger, H. A. Effect of Hydrogen and Oxygen Partial Pressure on Pt Precipitation within the Membrane of PEMFCs. J. Electrochem. Soc. 2007, 154 (10), B1006. https://doi.org/10.1149/1.2764240. Figure 1

Experimental and Numerical Analyses of Mass Transfer in Solid Oxide Cells

In solid oxide cells including solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs), spatial reactant/product concentration variations cause current and temperature distributions over the electrodes. Design optimization of the cell geometry and gas flow configurations, which include the design of separator (interconnector) ribs and flow channels for a planar cell, improves the efficiency and chemical/thermo-mechanical durabilities of practical stacks. Numerical models validated by in-situ measured current distributions are powerful tool for such optimization. In the present study, current and hydrogen partial pressure distributions have been modeled for a tubular SOFC [1] and planar SOFCs [2] using finite element modeling (COMSOL Multiphysics) on the basis of theoretical consideration of the concentration overpotential and the Nernst loss from local exchange current density determined by local reactant/product concentrations [3, 4] so that the models agree with in-situ measurements using segmented cathodes [5-7], determining the exchange current densities, electrode porosities, electrolyte ion conductivities, and electrode ion/electron conductivities. Anode-supported honeycomb SOFCs [8] and cathode-supported honeycomb SOECs [9] were also modeled for three-dimensional gas transports in agreement with their experimental current-voltage characteristics.[1] Ö. Aydın and H. Nakajima, Journal of the Electrochemical Society, 165 (5), F365-F374 (2018).[2] Ö. Aydın, T. Ochiai, H. Nakajima, T. Kitahara, K. Ito, Y. Ogura, and J. Shimano, International Journal of Hydrogen Energy, 43 (36), 17420-17430 (2018).[3] H. Nakajima and T. Kitahara, Marine Engineering, 53(2), 230-236 (2018).[4] H. Nakajima, ECSarXiv, in press.[5] Ö. Aydın, T. Koshiyama, H. Nakajima, and T. Kitahara, Journal of Power Sources, 279, 218-223 (2015).[6] T. Ochiai, H. Nakajima, T. Karimata, T. Kitahara, K. Ito, and Y. Ogura, ECS Transactions, 78, 1, 2203-2209 (2017).[7] H. Nakajima, T. Kitahara, and E. Tsuda, ECS Transactions, 78 (1), 2109-2113 (2017).[8] H. Nakajima, S. Murakami, S. Ikeda, and T. Kitahara, Heat and Mass Transfer, 54 (8), 2545-2550 (2018).[9] Y. Iwanaga, H. Nakajima, and K. Ito, ECS Transactions, 91 (1), 2707-2712 (2019).

Tritium adsorption and desorption on/from nuclear graphite edge by a first-principles study

The removal of tritium from irradiated nuclear graphite is one of the key targets of nuclear graphite decontamination. Here, we employ first principle density functional theory (DFT) to study its adsorption and molecular desorption on the most common edges and their reconstructions. The calculated adsorption energy ranges from −2.6 to - 5 eV, depending on the edge structure and the hydrogenation level, and are much larger than on the bulk of graphite. The hydrogenation level increases with the increase of hydrogen partial pressure, and drops rapidly at high temperature. The pathways to fully saturated edges are then determined for each edge variant and the activation energy (Eac) for molecular desorption computed by properly accounting for vibrational zero-point energy corrections (ΔEZPE). Our results showed that, there are three different stages for the desorption of hydrogen isotopes from nuclear graphite, namely stage 1 (200–300 °C), stage 2 (500–700 °C), stage 3 (1000–1100 °C). Our results can provide a theoretical basis for the tritium removal experiment from nuclear graphite.

Projection algorithm-based nonlinear observation of internal states in fuel delivery systems with gas diffusion

In this paper, a nonlinear observer design to estimate different gas species profiles and partial pressures in a fuel delivery system (FDS) with gas permeation is presented. Precise knowledge of critical internal states is of great importance to ensure reliable and efficient operation for FDS. First, a nonlinear isothermal dynamic model including fuel supply, hydrogen consumption, mixture recirculation and exhaust is formulated. Then a nonlinear observer based on projection algorithm is proposed, giving detailed information on internal conditions of FDS online. Without calculation of inverse matrix, the stack output voltage is treated as the only measurable observer input. The voltage estimation error is obtained from the difference between the prediction and the actual measurement, and nonlinear error injection matrix and projection term are employed as correction inputs. Finally, the proposed dynamic state observer is validated and compared with second order sliding mode (SOSM) observer on RT-LAB platform, the results indicate that the observer shows better real-time estimation performance and robustness. Furthermore, the observer demonstrates good robustness in terms of hydrogen partial pressures observation, but less robustness in nitrogen partial pressures estimation. Moreover, the sequence of key system parameters effecting the estimations are analyzed deeply and some conclusions are extracted.