mechanical engineering department university of victoria ferrous non-ferrous materials (mech473)

Mechanical Engineering Department University of Victoria

Ferrous Non-Ferrous Materials (Mech473)

Metal hydride overview

By

Ramadan Abdiwe

July 24, 2006 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


Main Problems with fossil fuel

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


Properties of hydrogen

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.


Hydrogen Production


Direct Energy Conversion using H2


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.


Hydrogen storage options

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)


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.


Problems

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


Vehicles requirements of H2

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)


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


Metal Hydride Alloys

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


Metallic hydride

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 behavior for 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


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 Ni with 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


PCT and cost properties of selected 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


PCT and cost properties of selected AB2

hydrides


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


PCT and cost properties of selected TiFe-type hydrides


A2B Intermetallic compounds

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


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


Practical Problems

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 .


Conclusion

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

The End

mechanical engineering department university of victoria ferrous non-ferrous materials (mech473)

Documents

ramadan abdiwechallenges

storage of hydrogen

metal hydride tank

ramadan abdiwemain problems

various metal hydride

specific metal hydride

undergraduate student

experimental testing