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Mechanical Engineering Department University of Victoria Ferrous Non-Ferrous Materials (Mech473). Metal hydride overview By Ramadan Abdiwe. Experiment. Purpose :
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Mechanical Engineering Department University of VictoriaFerrous Non-Ferrous Materials (Mech473) Metal hydride overview By Ramadan Abdiwe
Experiment Purpose: • Past work has involved the development of a mathematical model that simulates the behaviour inside a metal hydride tank during absorption and desorption. The model has been validated against numerical and experimental work reported in the literature. The first goal of the experiments is to validate the developed model against in-house experimental data. The second goal is to properly characterize the specific metal hydride alloys being used in the lab, so that more accurate modelling can be performed in the future
Role of Team Members Dr. Andrew Rowe:Faculty supervisor of the project Brendan MacDonald:Graduate student in charge of co-ordinating the research and performing the modelling simulations and experiments Fred McGuinness:Undergraduate student who is responsible for designing and manufacturing a metal hydride tank that will allow detailed testing of the behaviour inside the vessel, specifically through the use of internal instrumentation devices Tyler Isaacson:Undergraduate student who is responsible for designing a metal hydride tank which will be used for testing some of the enhanced heat transfer configurations, and also for writing a user’s manual that describes the test apparatus Ramadan Abdiwe:Graduate student who is responsible for assisting with the experimental testing work, specifically through the use of an inert environment glove box which will be used to fill the tanks designed by Fred and Tyler with the various metal hydride alloys utilized in the test runs
All the fossil fuels we are using are running out and burning them increases carbon dioxide in atmosphere which increases the greenhouse effects, causing GLOBAL WARMING Some fossil fuels contain sulphur and when they burn this becomes sulphur dioxide, a poisonous gas which reacts with water in the atmosphere to form sulphuric acid or ACID RAIN Main Problems with fossil fuel
Hydrogen gas is very powerful, It has the highest energy per unit of weight of any chemical fuel Hydrogen is highly abundant element, it is one of the most common substances on earth Hydrogen is environmentally friendly and its oxidation product is water. Properties of hydrogen
Challenges of using hydrogen • Even though, hydrogen is an attractive alternative to hydrocarbon fuels such as gasoline in mobile applications. However, the storage of hydrogen in these applications still remains a problem and and scientists have to come up with applicable, light, affordable, and safe method for storing hydrogen in such applications.
Storage as gas under pressure (250-350 bar) Cryogenic storage as liquid hydrogen (temp –253 0C ) Storage as metallic hydrides Carbon adsorption and glass microsphere storage techniques (under development) Hydrogen storage options
Compressed hydrogen • the most straightforward option at this time • offers the simplest and least expensive method for onboard storage of hydrogen • The refilling time of compressed hydrogen tanks is also similar to that of gasoline tanks.
Problems • Low energy density (One way to increase the fuel stored in the container is to increase pressure, but this requires more expensive storage containers, increasing compression costs ) • hydrogen has a tendency to leak because of its small size. Seals and valves on the containers need to be designed to prevent leaks. If a fuel cell vehicle is stored in a closed garage, hydrogen that has leaked out could accumulate and increase the risk of fire or explosion.in addition to the explosion could happen from cars accidents on the road
Liquefied Hydrogen • Could perform better in an accident • high energy density of liquid hydrogen • Increase available space • Reduce environments effect
Problems • Does not liquefy until (~-253 0C) Which cost energy • 40% of energy can be lost(25 percent of LH2 boiled off during refueling and 1 percent lost per day for onboard storage.) • requires excellent insulation of storage containers; otherwise, left for a period of time, the storage tanks could become depleted
Bonded hydrogen (Metal hydride) • Since heat is required to release the hydrogen, this method avoids safety concerns surrounding leakage that can be a problem with compressed hydrogen and LH2. In fact, metal hydrides are one of the safest methods for storing hydrogen.
Heavy weight. (One major obstacle to this method is that the metal compounds used to attract the hydrogen tend to be very heavy resulting in only 1.0 to 1.5 percent hydrogen by weight) Some of the metals used for hydrides are very expensive. There are less expensive options but they are impractical for use in fuel cell vehicles as these cheaper metals require extremely high temperatures to release the hydrogen Problems
A modern, commercially available car optimized for mobility and not prestige with a range of 400km burns about 24 kg of petrol in a combustion engine; to cover the same rage by electric car with a fuel cell 4kg hydrogen is needed (Louis Schlapbach & Andreas Zuttel) Vehicles requirements of H2
Volume of 4kg hydrogen compacted in different ways, with size relative to the size of a car
Background of metal hydride • the last century, scientists discover that Pd metal occluded large amount of H2 at ambient pressure and temperature. However, it was not very useful because of issues associated with the cost and low capacity of hydrogen storage. The recent discovery of hydrogen sorption by intermetallic compounds created great hopes and stimulated research and development worldwide of using the metal hydrides as a new alternative for storing and delivering pure hydrogen that can be very useful for fuel cell technology
Mechanism of H2 movement through the metal crystal lattice • The H2 molecule is first weakly physisorbed on the surface and then dissociatively chemisorbed as strongly bound, individual H-atoms • the size of the hydrogen atoms is lighter and smaller than the metal atoms; therefore, they diffuse quickly from the surface into the periodic sites in the metal crystal lattice hydrogen adsorption/ dissociation and hydride formation
PCT curves • Basically, PCT is determined by keeping an alloy sample at constant temperature and measuring the pressure change as hydrogen is absorbed. The hydride/dehydride cycling causes a change of the intermetallic compounds volume that cause cracking for the particles. This cracking of the particles cause increasing of the surface area, which leads to an increase of the hydrogen reactivity • Most metal have high attraction for hydrogen and there are also few have poor attraction for hydrogen and the reaction between the metal and hydrogen can be exothermic or endothermic respectively. The process of absorption and desorption is best illustrated by the pressure compostion-tempreture profiles (PCT curves)
Thermodynamic Behavior of M-H Reaction • The thermodynamic behavior of metal hydride formation is illustrated in the figure beside, which is a PCT curve. The line of the upper pressure represents the absorption process and the lower line represents the desorption process and the flat part of both lines is called Plateau. And the difference in equilibrium pressures between the absorption and desorption reactions is called Hystersis Schematic isothermal pressure-composition hysteresis loop
The chemical reaction associated with absorption and desorption is as follows: M + (x/2) H2 MHx+ Heat The heat of the reaction in the absorption process could be very useful in using metal hydride for heat pumps and thermal storage. But this heat will be a problem when using metal hydride in vehicles. Also it should be mentioned that higher heat of reactions, have lower equilibrium pressures at a given temperature. The fundamental connection between pressure and temperature is given by the Van’t Hoff equation lnP = ΔH/RT - ΔS/R R is the gas constant. It should be mention that ΔH vary widly from metal to metal because it is a measure of the strength of the M-H bond. Where ΔS doesn’t vary much
Metal hydrides material • Hydrogen is a highly reactive element and has been shown to form hydrides and solid solutions with thousands of metals and alloys. Figure beside shows the family tree of hydriding alloys and complexes Family tree hydriding alloys and complexes
Metal hydride elements • For PEM fuel cell vehicular applications the range is 0-100 0C and 1-10 atm. This is based on the possibility of using the waste heat from the fuel cell to release the hydrogen from the metal hydride. In the last century Pd was used to store H2. However, it is no longer used because it is very expensive, doesn’t hold much hydrogen, and requires heating above 100 0C to release that hydrogen. Today Vanadium (V) and Niobium (Nb) are well known elements for storing hydrogen in the range of practical applications Van’t Hoff lines (desorption) for elemental hydrides. Box indicates 1–10 atm, 0–100°C ranges
Scientists have been required to combine strong hydride forming elements (A) with weak hydride elements (B) to form alloys (especially intermetallic compounds) that have the desired intermediate thermodynamic affinities for hydrogen there are some conditions should be considered of choosing these alloys as metal hydrides: 1-Temperature and pressure of hydride and dehydride 2-Hydrogen storage capacity of the alloys 3-Rate of absorption and desorption 4-Ease of activation 5-Poisoning by impurities 6-Cost and availability Typically there are three type of bonding between metals and hydrogen: Ionic, Covalent, and Metallic Metal Hydride Alloys
Ionic, Covalent types of bonding are not very practical in mobile applications because they require extremely high temperature in order to liberate H2 . Where as the metallic type bond offers the necessary behaviorfor hydrogen storage systems (In the metallic hydrides, the hydrogen acts an electron accepter, the hydrogen atom accept the electron from the conduction band of the metal and fill its first orbital ) The conventional metal hydride alloy families to be described here are the AB5, AB2, and AB intermetallic compounds Metallic hydride
AB5 Intermetallic compounds • AB5 intermetallic compounds have a hexagonal crystal structure and also have an extraordinary versatility because many different elements species can be substituted (at least partially) into the A and B lattice sites. Element A mostly is one or more lanthanides with atomic number between 57-71 and element B is mostly based on Niwith other substitutional elements such as Co, Al, Mn, Fe, Cu, Sn, Si [2]. Figure beside show the PCT properties of some of AB5 family alloys Van’t Hoff lines for various AB5 hydrides
There are some advantages of this type of alloy compared to the other conventional ones. These advantages are: low hysteresis, easy to activate; (doesn’t require any heat), good tolerance to impurities such as O2 or H2O, and good intrinsic kinetics. On the other hand, there are some disadvantages due to relatively low hydrogen capacity (~1.3 wt %) and also high cost of the raw material compared to AB2 type alloys
AB2 Intermetallic compounds • There is a large number of this type of alloys (~500 alloys). AB2 alloys have two types of crystal structures: hexagonal and cubic. Elements A in this family are often IVA group (Ti, Zr, Hf) and/or rare earth series (at atomic number from 57 to 71) and B elements are from elements with atomic number 23 to 26 such as V, Cr, Mn, Fe. Figure beside show the properties of some of AB2 alloys
There are two main advantages in this type of alloys: they have higher capacity compared to AB5 and lower cost especially if A elements are mainly Ti (because Zr and V are very expensive in comparison to Ti). However, this type of alloy suffers from difficulty in activation, and sensitivity to impurities
AB Intermetallic compounds • Fig.7 and table.4 show the properties of the most popular composition in this family: TiFe, TiFe 0.85 Mn 0.15 ,and TiFe 0.8 Ni 0.2. . In summary, Tife-based AB alloys have good PCT properties, good H-capacities and low raw materials costs, but there are problems associated with activation, gaseous impurities and upper plateau instabilities, which make them unattractive for mobile storage
Various crystal structures are possible. In one subfamily, A is typically of the group IVA elements Ti, Zr or Hf and B is a transition metal, typically Ni and the family is based on Mg2Ni. H-capacity and cost properties of Mg2Ni are attractive, but desorption temperatures are too high for most applications A2B Intermetallic compounds
Problems of metal hydrides • Low mass density is the general weakness of all known metal hydrides working near room temperature. There are intermetallic compounds and alloys can form hydrides up to 9 mass% hydrogen but they are not reversible within the required range of temperature and pressure. Table beside shows the mass density of the well-known compounds and alloys Intermetallic compounds and their hydrogen-storage properties
The other common problem of storing hydrogen is the impurity and the most problematic impurities are: CO, CO2, NH3, H2S, CH4 and O2. These impurities reduce the storage capacity and also could cause poisoning, retardation, or reaction. Table beside shows the effect of impurities on metal hydride • Poisoning = Rapid loss of H-capacity with cycling • Retardation = Reduction in absorption/desorption kinetics without significant loss in the ultimate capacity • Reaction = Bulk corrosion was leading to irreversible capacity loss
There are also some practical problems with using metal hydrides; the container (tank) is one of these problems. During the hydriding cycle, the particles of the hydrides become very small which leads to entrainment of the hydride fines in the gas streams. Therefore, the container system must provide for these small particles and resulting expansion. most of the hydrides powder (beds) have poor thermal conductivity. Therefore, the most important parameter of designing metal hydrides containers is the thickness of the hydride bed, and most studies done in this field conclude that in order to have the highest reaction, the heat resistance of the hydride bed should be decreased and the best way to design containers with small hydride bed resistance is to keep the bed thickness as small as possible . Practical Problems
With the present status of technology, the three main options of storing hydrogen are: compressed gas at high pressure (300-500 bar), liquid hydrogen (-253 0C), and metal hydride. One of the major issues of using metal hydrides in vehicular applications is the weight of the storage system. Therefore, for the metal hydride to appear as suitable option, the weight has to be lowered drastically. Heat resistance of the metal hydride powder (bed) affect the hydriding / dehydriding cycles and control the rate of hydrogen uptake or removal and this is due to the low conductivity of the powder and also the poor heat transfer between the particles and the wall of the container. Therefore, the best way to design containers with small hydride bed resistance is to keep the bed thickness as small as possible Conclusion
