Hydrogen Diffusion Rate Research Articles

Mass transport in the PdCu phase structures during hydrogen adsorption and absorption studied by XPS under hydrogen atmosphere

We investigated mass transports during hydrogen adsorption and absorption processes of PdCu alloys that has advantages of higher hydrogen diffusivity and economically lower-cost than the other Pd-alloys. The research was made with a comparison of the well-known ordered phase of the bcc structure (the B2 phase) and a mixed phase of the fcc and B2 structures. In-situ ultrahigh vacuum X-ray photoelectron spectroscopy (UHV-XPS) and ambient pressure X-ray photoelectron spectroscopy (AP-XPS) using synchrotron radiation were carried out to trace the chemical states of the Pd and Cu atoms as a function of temperatures. It is newly elucidated that the initial adsorption and absorption processes were similar in the two phases, but a hydrogen diffusion rate to the bulk was apparently higher in the ordered phase than in the mixed one. We found the dynamics of the Pd and Cu atoms during the hydrogen adsorption/absorption processes largely depends on temperature. In the hydrogen atmosphere, the Pd atoms segregate at the surface below 373 K and Cu atoms segregate at the surface above 373 K, showing a reversal behavior. The present results agree well with the previous theoretical calculations and, thus, provide appropriate inputs toward developments of the industrial hydrogen permeation materials.

Effect of CrxCy–NiCr coating on the hydrogen embrittlement of 17-4 PH stainless steel using the smooth bar tensile test

A CrxCy–NiCr cermet coating was fabricated using the supersonic plasma spray technique, and its effect on the hydrogen embrittlement of 17-4 PH stainless steel was studied through electrochemical hydrogen charging and slow strain rate tensile testing. The index of hydrogen embrittlement susceptibility was calculated, and the results show that the CrxCy–NiCr coating plays a significant role in preventing hydrogen embrittlement cracking. The hydrogen content analysis demonstrates that the CrxCy–NiCr coating enhances the resistance to hydrogen embrittlement by acting as a hydrogen diffusion barrier. This effect is mainly due to the CrxCy ceramics in the cermet coating having a very low hydrogen diffusion rate. In addition, the strength and ductility of the specimen without the coating decrease gradually with increasing hydrogen charging current density. According to the tensile testing results and the fracture surface analysis, the hydrogen embrittlement phenomenon is explained by a mixed mechanism involving hydrogen-enhanced local plasticity and hydrogen-enhanced decohesion.

Hydrogen oxidation kinetics on platinum-palladium bimetallic thin films for solid acid fuel cells

Solid acid fuel cells (SAFCs) based on the proton-conductive electrolyte CsH2PO4 have shown promising power densities at an intermediate operating temperature of ∼250 °C. However, Pt loadings in SAFCs remain higher than desirable, and the electrocatalysis mechanisms in these devices are still unknown. Here, hydrogen oxidation kinetics on Pt and Pt-Pd bimetallic thin film electrodes on CsH2PO4 have been evaluated to establish the potential for a beneficial role of Pd in SAFC anodes. Symmetric cells fabricated by depositing a metal film on both sides of electrolyte discs are characterized for studying hydrogen electro-oxidation across the gas|metal|CsH2PO4 structure. It was found that Pd reacts with CsH2PO4, forming palladium phosphide at the metal-electrolyte interface. Accordingly, the activity of Pd was examined in a bilayer geometry of Pd|Pt|CsH2PO4|Pt|Pd. The bilayer Pt|Pd films showed much higher activity for hydrogen electro-oxidation than films of Pt alone, as measured by AC impedance spectroscopy. Ex situ low energy ion scattering and scanning transmission electron microscopy revealed that Pd diffused into the Pt layer under operating conditions. The dramatic impact of Pd along with its presence throughout the film suggests that it catalyzes reactions at both the metal-gas and metal-electrolyte interfaces, as well as increasing hydrogen diffusion rates through the films.

Microstructural evaluation of hydrogen embrittlement and successive recovery in advanced high strength steel

Advanced high strength steels (AHSS) have high susceptibility to hydrogen embrittlement, and are often exposed to hydrogen environments in processing. In order to study the embrittlement and recovery of steel, tensile tests were conducted on two different types of AHSS over time after hydrogen charging. Concentration measurements and hydrogen microprinting were carried out at the same time steps to visualize the hydrogen behavior during recovery. The diffusible hydrogen concentration was found to decay exponentially, and empirical relations were found for both types of steel. Hydrogen concentration decay rates were determined to be -0.355 /hr in TBF-980, and -0.225 /hr in DP-980. Hydrogen concentration thresholds for embrittlement were found to be 1.04 mL/100 g for TBF, and 0.87 mL/100 g for DP steel. TBF steel is predicted to recover from embrittlement within 4.2 h, compared to 7.2 h in DP steel. A two-factor method of evaluating recovery from embrittlement, requiring hydrogen concentration threshold and decay rate, is explained for use in predicting recovery after exposure to hydrogen. Anisotropic hydrogen diffusion rates were also observed on the surface of both steels for a short time after charging, as hydrogen left the surface through <001> and <101> grains faster than grains with <111> orientations. This could be explained by differences in surface energies between the different orientations.

Enhanced electrochemical properties of LaFeO3 with Ni modification for MH–Ni batteries

In this work, we synthesized LaFeO3–xwt%Ni (x = 0, 5, 10, 15) composites via a solid-state reaction method by adding Ni to the reactants, La2O3 and Fe2O3. Field-emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectroscopy (EDS) results revealed that Ni powders evenly dispersed among the LaFeO3 particles and apparently reduced their aggregation, which imparted the composites with a loose structure. Moreover, the Ni formed a conductive network, thus improving the conductivity of the composites. The maximum discharge capacity of the LaFeO3 electrodes remarkably increased from 266.8 mAh·g–1 (x = 0) to 339.7 mAh·g–1 (x = 10). In particular, the high-rate dischargeability of the LaFeO3–10wt%Ni electrode at a discharge current density of 1500 mA·g−1 reached 54.6%, which was approximately 1.5 times higher than that of the pure LaFeO3. Such a Ni-modified loose structure not only increased the charge transfer rate on the surface of the LaFeO3 particles but also enhanced the hydrogen diffusion rate in the bulk LaFeO3.

SCC susceptibility of solution-annealed 316L SS in hydrogenated hot water below 288 °C

Stress corrosion cracking (SCC) susceptibility of solution-annealed 316L stainless steel was evaluated by slow strain rate test in oxygenated, deaerated and hydrogenated water below 288 °C. At 288 °C, no SCC was found in the specimens tested in oxygenated water, while in hydrogenated water, SCC occurred irrespective of dissolved-hydrogen (DH) content in the late stage of deformation where cold-working was followed by necking. The SCC susceptibility diminishes as temperature decreased from 288 °C to RT, which was accompanied by a reduction of oxide film thickness. At 220 °C, SCC occurred at the highest DH content only, which suggests a combined effect of DH content and test temperature on the SCC susceptibility. The combined effect can be explained by anodic reaction rate, hydrogen diffusion rate and hydrogen trapping ability. The SCC susceptibility appeared to increase with DH content, which was reasonably explained by suppression of the protective oxide film formation on specimen surface and increasing hydrogen content in the specimen. Based on an SCC model that hydrogen reduces cohesive force in austenitic stainless steel lattice, the SCC behavior can be interpreted in terms of hydrogen-assisted SCC (HASCC) where corrosion-induced hydrogen production, suppression of the formation of protective oxide film caused by hydrogen, work-hardening, and hydrogen-induced reduction of cohesive force of the lattice atoms are necessary.

Effect of substituting Y with Mg on the microstructure and electrochemical performance of LaY2Ni9 hydrogen storage alloy

LaY2Ni9 showed a much lower discharge capacity than theoretical electrochemical capacity due to the hydrogen induced amorphization (HIA). To improve its electrochemical performance, Mg was added into LaY2Ni9 alloy and the effects of Mg on microstructure and electrochemical performance of LaY2Ni9 alloy were investigated. LaY2-xMgxNi9 (x = 0, 0.25, 0.50, 0.75, and 1.00) hydrogen storage alloys were prepared by vacuum induction melting followed by annealing at 1173 K for 12 h. The X-ray diffraction (XRD) analysis and Rietveld refinement showed that Mg addition has significant effects on the phase component. A small amount of Mg addition (x ≤ 0.50) can lead to a transformation between C15 and C14 phases. Further increase of Mg content (x = 0.75) is responsible for the formation of Gd2Co7-type phase. Electrochemical measurements indicated that the maximum discharge capacity and high rate discharge ability (HRD) of the alloys can benefit from a suitable amount of Mg addition. The maximum discharge capacity of LaY2-xMgxNi9 increases from 267.3 mA h g−1 for LaY2Ni9 to 366.4 mA h g−1 for LaY1.5Mg0.5Ni9, which is due to the suppression effect of Mg on HIA of LaY2Ni9 phase. HRD1500 (70.82%) is the best when x = 0.75, which contributes to the existence of Gd2Co7-type phase by accelerating the hydrogen diffusion rate.

Microstructure and Electrochemical Performance of CeMg12/Ni/TiF3 Composites for Hydrogen Storage

Ball milling was used to prepare CeMg12/Ni/TiF3 composite alloys in this study. Microstructures of experimental alloys were analyzed by scanning electron microcopy (SEM), x-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM) with electron diffraction (ED). The electrochemical and kinetic characteristics of the ball-milled composite alloys were further evaluated based on their galvanostatic charge–discharge measurement, high-rate dischargeability (HRD), hydrogen diffusion behavior, as well as electrochemical impedance spectrum (EIS). Results indicated that the ball-milled alloy with 3 wt.% TiF3 exhibited the best electrochemical discharge capacity, which was not only related to the formation of amorphous and/or nanocrystalline phase but to the formation of MgF2 because it can effectively reduce the thermodynamic stability of hydride. With increasing TiF3, the cycle degradation rates of the milled alloys were ameliorated remarkably. This improvement was attributed to the formation of an amorphous phase (which possesses strong anticorrosive and antioxidation abilities) as well as the formation of MgF2 and TiNi secondary phases. The ball-milled alloy with 3 wt.% TiF3 additive exhibited the strongest electrochemical kinetic properties, related to the highest hydrogen diffusion rate in the alloy (which was associated with multiple defects and grain boundary of amorphous and/or nanocrystalline phases) and the electrochemical reaction on the surface of alloy (which was related to the lowest apparent activation energy).

Hydrogen diffusion and vacancy clusterization in iron

Molecular statics and molecular dynamics simulations were performed to study hydrogen diffusion and vacancy clustering in alpha iron. In particular, it was found that hydrogen atom binds very strongly with vacancies, rather than other hydrogen atoms. The monovacancies were inclined to form the VH4, VH3, VH2 and VH1 complexes, rather than VH6 in the range of our simulated temperatures. The rate of hydrogen diffusion was apparently reduced in the presence of vacancies, while the vacancy trap effect was gradually weakened with increasing temperature. The presence of vacancies changes the diffusion mechanism of H atoms. Moreover, we found that vacancy clusters tended to be formed at the moderate range of temperatures, and fewer clusters were observed at either low or high temperatures. The number of vacancy clusters reduced, while hydrogen-vacancy clusters were gradually created with the increase of hydrogen concentration.

Hydrogen diffusion in titanium dihydrides from first principles

Transition metal hydrides are of great practical interest due to high volume concentration of hydrogen. Titanium dihydride is a typical representative of such materials. Safe and reliable application of the dihydride requires established values of hydrogen diffusion rates. Experimental studies of the matter provide somewhat controversial results, therefore we investigate the process from first principles. We calculate hydrogen diffusion rates in representative compounds (TiH1.75 and TiH2.0) and demonstrate that the dominant diffusion mechanism depends on temperature and composition.

Studying of electrochemical discharging and kinetic properties of Ni-TiF3-CeMg12 composite materials with nanocrystalline and amorphous structure

The ball milling technology was used for fabricating CeMg12/Ni/TiF3 hydrogen materials. The effect of TiF3 content on microstructures and electrochemical performances of the milling alloys was investigated in detail. The microstructures were characterized by scanning electron microcopy (SEM), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM). The electrochemical hydrogen storage properties were tested by discharging capacity and cycling stability. The electrochemical kinetic characteristics were further analyzed with high rate discharging ability (HRD), electrochemical impedance spectroscopy (EIS) technique, hydrogen diffusion behavior (the diffusion coefficient was denoted by D) and apparent activation energy (ΔE). The results reveal that the addition of TiF3 notably enhances the glass forming ability of alloy sample. The maximum discharge capacity of experimental alloy first goes upward from 1060 to 1250 mA h/g then decreases to 1050 mA h/g with TiF3 content is increased from 0 to 5 wt.%. The milling CeMg12-Ni-TiF3 (5 wt.%) alloy shows the best electrochemical cycle stability. The milled CeMg12-Ni-TiF3 (3 wt.%) alloy has optimum electrochemical kinetic performances, which is responsible for the smallest surface activation energy and the fastest hydrogen diffusion rate.

Effect of Hydrogen Trapping and Poisons on Diffusion Behavior of Hydrogen in Low Carbon Steel

Effect of hydrogen trapping and poisons on diffusion behavior of hydrogen in commercial cold-rolled low carbon steel was investigated by means of electrochemical hydrogen permeation techniques. The experimental results reveal that diffusion rate and diffusion flux of hydrogen in the materials gradually increase with increasing the number of hydrogen charging and outgassing, and lag time significantly shortens with them, therefore, hydrogen trapping impede diffusion behavior of hydrogen in the materials. Different poisons in the hydrogen charging solution have also resulted in a certain influence on the assessment of hydrogen diffusion behavior.

Reduction Mechanisms of Cu2+-Doped Na2O-Al2O3-SiO2 Glasses during Heating in H2 Gas.

Controlling valence state of metal ions that are doped in materials has been widely applied for turning optical properties. Even though hydrogen has been proven effective to reduce metal ions because of its strong reducing capability, few comprehensive studies focus on practical applications because of the low diffusion rate of hydrogen in solids and the limited reaction near sample surfaces. Here, we investigated the reactions of hydrogen with Cu2+-doped Na2O-Al2O3-SiO2 glass and found that a completely different reduction from results reported so far occurs, which is dominated by the Al/Na concentration ratio. For Al/Na < 1, Cu2+ ions were reduced via hydrogen to metallic Cu, distributing in glass body. For Al/Na > 1, on the other hand, the reduction of Cu2+ ions occurred simultaneously with the formation of OH bonds, whereas the reduced Cu metal moved outward and formed a metallic film on glass surface. The NMR and Fourier transform infrared results indicated that the Cu2+ ions were surrounded by Al3+ ions that formed AlO4, distorted AlO4, and AlO5 units. The diffused H2 gas reacted with the Al-O-···Cu+ units, forming Al-OH and metallic Cu, the latter of which moved freely toward glass surface and in return enhanced H2 diffusion.

The influence of hydrogen desorption on micromechanical properties and tribological behavior of iron and carbon steels

The influence of the previous electrolytic hydrogenation on the micromechanical properties and tribological behavior of the surface layers of iron and carbon steels has been studied.The concentrations of diffusion-moving and residual hydrogen in steels are determined depending on the carbon content. It is shown that the amount of sorbed hydrogen is determined by the density of dislocations and the relative volume of cementite. After desorption of diffusion-moving hydrogen the microhardness increases and materials plasticity decreases. The change of these characteristics decreases with the increase of carbon content in the steels. Internal stresses increase and redistribute under hydrogen desorption. Fragmentation of ferrite and perlite occurs as a result of electrolytic hydrogenation. Ferrite is characterized by the structure fragmentation and change of the crystallographic orientation of planes. The perlite structure shows the crushing of cementite plates and their destruction. The influence of hydrogen desorption on the microhardness of structural components of ferrite-perlite steels is shown. Large scattering of microhardness ​​is found in perlite, due to different diffusion rates of hydrogen because of the unequally oriented cementite plates. It was found that the tendency of materials to blister formation is reduced with the increase of carbon content. The influence of hydrogen on the tribological behaviour of steels under dry and boundary friction has been studied. It is shown that hydrogen desorption intensifies the materials wear. After hydrogen desorption, tribological behaviour is determined by the adhesion interaction between the contacting pairs.

Reduced Graphene Oxide Coating with Anticorrosion and Electrochemical Property-Enhancing Effects Applied in Hydrogen Storage System.

Low-capacity retention is the most prominent problem of the magnesium nickel alloy (Mg2Ni), which prevents it from being commercially applied. Here, we propose a practical method for enhancing the cycle stability of the Mg2Ni alloy. Reduced graphene oxide (rGO) possesses a graphene-based structure, which could provide high-quality barriers that block the hydroxyl in the aqueous electrolyte; it also possesses good hydrophilicity. rGO has been successfully coated on the amorphous-structured Mg2Ni alloy via electrostatic assembly to form the rGO-encapsulated Mg2Ni alloy composite (rGO/Mg2Ni). The experimental results show that ζ potentials of rGO and the modified Mg2Ni alloy are totally opposite in water, with values of -11.0 and +22.4 mV, respectively. The crumpled structure of rGO sheets and the contents of the carbon element on the surface of the alloy are measured using scanning electron microscopy, transmission electron microscopy, and energy dispersive spectrometry. The Tafel polarization test indicates that the rGO/Mg2Ni system exhibits a much higher anticorrosion ability against the alkaline solution during charging/discharging. As a result, high-capacity retentions of 94% (557 mAh g-1) at the 10th cycle and 60% (358 mAh g-1) at the 50th cycle have been achieved, which are much higher than the results on Mg2Ni capacity retention combined with the absolute value reported so far to our knowledge. In addition, both the charge-transfer reaction rate and the hydrogen diffusion rate are proven to be boosted with the rGO encapsulation. Overall, this work demonstrates the effective anticorrosion and electrochemical property-enhancing effects of rGO coating and shows its applicability in the Mg-based hydrogen storage system.

Enhanced electrochemical kinetics of magnesium-based hydrogen storage alloy by mechanical milling with graphite

Magnesium nickel alloy (Mg2Ni) which used as the negative electrode material in the nickel-metal hydride (Ni/MH) secondary battery is modified by graphite via mechanical milling. The effects of graphite on the Mg2Ni are systematically investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) and a series of electrochemical tests. The results show that the cycle stability of the Mg2Ni alloy is improved with the addition of 10 wt.% graphite and the discharge capacity at the 20th cycle increase from 116.9 mA g−1 to 178.5 mA g−1. The Tafel polarization test indicates better corrosion resistance of the Mg2Ni/graphite composite. Meanwhile, the results of electrochemical tests indicate that both the charge-transfer reaction rate on the surface of the alloy and the hydrogen diffusion rate inside the bulk of alloy are boosted with the introduction of graphite.

Hydrogen Diffusion Transport in an All-Iron Flow Battery Rebalance System

All-iron flow batteries (IFBs) are being developed for low-cost large-scale energy storage applications. IFBs use abundantly available, non-toxic and low cost electrolyte materials[1]. The all-iron flow battery involves plating and stripping of metallic iron in the negative half-cell and utilizes the Fe II/III (ferrous/ferric) redox couple reaction at its positive electrode[2-3]. Hydrogen evolution occurs as the iron deposits at the negative electrode during the flow battery charging and even if the amount might be small, there will be an impact on the electrolyte balance over many cycles. Recently, Selverston et al.[3] demonstrated a concept of hydrogen rebalance in an all-iron flow battery through designing a capillary-action galvanic reactor (CGR) to consume hydrogen and reduce excess ferric ions in the positive electrolyte reservoir. This design can maintain the balance of protons and reduce undesired precipitation reactions. A tube connects the negative tank headspace where hydrogen collects to the positive tank headspace where the CGR reactor is placed. An understanding of design factors on hydrogen transport through the tube between headspaces is desired and mathematical modeling[4-7] is the approach we took to achieve this. In this poster, we report a mathematical model of hydrogen diffusion between the headspaces of the negative electrolyte tank and the positive electrolyte tank. The effects of dimensional parameters, including diameter of tank, height of tank, diameter of tube, length of tube and diffusion coefficient on hydrogen diffusion rates and partial pressure are investigated during both battery charge and battery standby cases. In Figure 1, we demonstrate that the mathematical model is reasonably validated by the previous published experimental data for a specific all-iron flow battery rebalance device. Acknowledgements This work is supported by the all-iron flow battery project (grant number: DE-AR0000352) funded by Department of Energy (DOE) of the United States.

In operando neutron diffraction study of LaNdMgNi9H13 as a metal hydride battery anode

La2MgNi9-related alloys are superior metal hydride battery anodes as compared to the commercial AB5 alloys. Nd-substituted La2-yNdyMgNi9 intermetallics are of particular interest because of increased diffusion rate of hydrogen and thus improved performance at high discharge currents. The present work presents in operando characterization of the LaNdMgNi9 intermetallic as anode for the nickel metal hydride (Ni-MH) battery. We have studied the structural evolution of LaNdMgNi9 during its charge and discharge using in situ neutron powder diffraction. The work included experiments using deuterium gas and electrochemical charge-discharge measurements. The alloy exhibited a high electrochemical discharge capacity (373 mAh/g) which is 20% higher than the AB5 type alloys. A saturated β-deuteride synthesized by solid-gas reaction at PD2 = 1.6 MPa contained 12.9 deuterium atoms per formula unit (D/f.u.) which resulted in a volume expansion of 26.1%. During the electrochemical charging, the volume expansion (23.4%) and D-contents were found to be slightly reduced. The reversible electrochemical cycling is performed through the formation of a two-phase mixture of the α-solid solution and β-hydride phases. Nd substitution contributes to the high-rate dischargeability, while maintaining a good cyclic stability. Electrochemical Impedance Spectroscopy (EIS) was used to characterize the anode electrode on cycling. A mathematical model for the impedance response of a porous electrode was utilized. The EIS showed a decreased hydrogen transport rate during the long-term cycling, which indicated a corresponding slowing down of the electrochemical processes at the surface of the metal hydride anode.

Evaluation of mechanical properties of an experimental microalloyed steel subjected to tempering heat treatment and its effect on hydrogen embrittlement

The aim of this study is to evaluate the hydrogen effect on the mechanical properties of experimental microalloyed steel subjected to post-tempering treatments at different temperatures (200–600 °C) and residence times (1–10 min). The results obtained that at higher hydrogen concentrations in the un-tempered steel condition their strength showed from 700 to 900 MPa and the ductility was from 9 to 11% elongation, which represents the highest embrittlement index (%EI), this results clearly presents that the un-tempered conditions are more sensitive to hydrogen embrittlement (HE). The tempered steels showed remarkable variations on the same properties ranging from 650 to 850 MPa, possibly due to the presence of microalloying elements, co-precipitation and dislocation movements. Permeability tests were carried out to evaluate the microalloyed steel hydrogen trapping and diffusion rate under the same post-tempering conditions. The diffusion coefficient (Deff) was higher for the steel in its initial condition showing values of 2.8 × 10−6 cm2/s; and the lowest Deff values was presented at 200 °C–1 min, its value was 1 × 10−6 cm2/s. The apparent hydrogen concentration (Capp) was higher at the tempering condition of 200 °C–3 min, presenting values of 1.95 ppm H, and the lowest Capp value was obtained at 400 °C–10 min (0.76 ppm H). The steel samples with lower Capp showed a lower effect in their ultimate tensile strength (UTS) and yield strength (YS) values, this means that the hydrogen traps are less massive.

Penetration and diffusion of hydrogen in Mg2Ni: A first-principles investigation

First-principles calculations have been used to study the hydrogen adsorption and diffusion in both hexagonal Mg2Ni surface and bulk Mg2Ni, and the hydrogen diffusion rate in bulk Mg2Ni is evaluated. The calculation shows that the adsorption energies on Mg2Ni (100) surface are much higher than in bulk Mg2Ni. For hydrogen diffusing from Mg2Ni (100) surface to subsurface, three diffusion pathways have been studied, and the lowest energy barrier is found to be 0.64 eV. To study the hydrogen atom diffusion in bulk Mg2Ni, nine possible diffusion paths have been considered. The results show hydrogen diffusion from NiNi bridge sites to the most nearby MgNi bridges site is energetically more favorable than the other paths. The energy barrier of this process is 0.34eV, which is close to experimental data of 0.28 ± 0.04 eV. The temperature dependence of the jump rates has been investigated in some details using harmonic transition state theory and semi-classically corrected harmonic transition state theory. The calculated diffusion constant is in good agreement with the experimental data.