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FUELS – HYDROGEN STORAGE | Glass Microspheres
Glass Microspheres JE Shelby, FC Raszewski, and MM Hall, Alfred University, Alfred, NY, USA & 2009 Elsevier B.V. All rights reserved.
Introduction Hydrogen storage in hollow glass microspheres (HGMSs) was first demonstrated in the 1970s. Scientists at the Lawrence Livermore National Laboratory (LLNL) filled commercial HGMSs with deuterium and tritium under high pressure for use as targets for the inertial confinement fusion programme. Although their goal was not storage of hydrogen per se, the work indicated that small, hollow spheres could serve as containers for highpressure hydrogen and thereby as a lightweight replacement for the heavy, high-pressure cylinders in use at that time. Although this approach to hydrogen storage has not been very actively pursued, recent interest in hydrogen fuel cells as a replacement for the internal combustion engine has reignited interest in the possibilities inherent in the concept.
where c is the concentration of dissolved gas and P the partial pressure of that gas in the surrounding atmosphere. This behavior is in sharp contrast to that found for metals, where hydrogen dissociates on dissolution. Hydrogen remains in molecular form during dissolution in glasses and other insulators unless heated to sufficiently high temperatures for reactions to occur between the dissolved gas and the glass network. At high pressures, the concentration of dissolved gas is no longer directly dependent on the pressure, but varies with the fugacity of the gas in the atmosphere. The fugacity, f, is given by the expression f ¼ fP
½4
where f is the fugacity coefficient. The fugacity coefficient varies effectively from unity at low pressures to a value of o2 at 69 MPa (10 000 psi).
Fundamental Concepts
Historical Perspective
Hydrogen storage in HGMSs is based on the very large temperature dependence of the rate of entry and exit of hydrogen through the glass walls, and on the ability of the spheres to contain high pressures of hydrogen for long times. Gas permeability, K, can be described as the ease with which atoms or molecules pass through a membrane without reacting with the material under steady-state conditions, i.e., the steady-state flow rate of gas through a membrane. The time to reach a steady state can be determined by the diffusion coefficient, D, also called the diffusivity, which is a measure of the rate of movement of individual atoms. The two aforementioned parameters are related by the gas solubility, S, as expressed by
The work undertaken at the LLNL included a study of the isothermal outgassing of helium, deuterium, and tritium from commercial HGMSs that were based on a soda–lime–silica composition (79 SiO2–10 CaO–6.8 Na2O–2 B2O3–0.8 ZnO–0.5 MgO–0.3 P2O5–0.2 Al2O3–0.1 K2O–0.3 others, the numbers refer to mol%) that were sieved to 40–45 mm. Samples were treated in helium at 400 1C and hydrogen or deuterium at 490 1C for 24 h at the desired pressure. Outgassing was performed at 450 1C for helium and 490 1C for hydrogen or deuterium until no further gas was released, for B1–4 h. Although the studies were not intended to suggest that HGMSs could be used for hydrogen storage, the results established the validity of that possibility. The specific concept of hydrogen storage in HGMSs was first proposed by R. J. Teitel in 1977. It was noted that hydrogen permeability is enhanced at elevated temperatures (175–400 1C) and this allows hydrogen to diffuse into the microspheres. At room temperature, however, the permeability is reduced by several orders of magnitude. This effectively ‘traps’ the hydrogen until the HGMSs are reheated to elevated temperatures, where the increase in hydrogen permeability allows hydrogen release at high rates. It was further found that most commercial microspheres could not withstand roomtemperature filling cycles at 40.5 MPa (400 atm), which limited the storage density to o5 wt%, as opposed to the projected value of 10 wt%. The high failure rate was
K ¼ DS
½1
The temperature dependence of the permeability for hydrogen in glasses is given by the Arrhenius expression: Ea K ¼ Ko exp RT
½2
where Ko is a constant, Ea the activation energy for hydrogen permeation, R the universal (molar) gas constant, and T the absolute temperature. At low pressures, the solubility of hydrogen in glasses is given by Henry’s law: c ¼ SP
488
½3
Fuels – Hydrogen Storage | Glass Microspheres
attributed to low wall strengths as the spheres available at that time only had an apparent tensile strength of B344– 517 MPa (50 000–75 000 psi). It was concluded that the glass composition must be tailored to optimize permeability for specific applications while still maintaining strength. A hydrogen supply system was envisioned in which hydrogen-filled HGMSs would be used in conjunction with an undefined metal hydride component to provide a more efficient system. The HGMSs would offer a constant supply of hydrogen that would also regenerate the metal hydride, whereas the metal hydride would be reserved to meet any short-term demands for a very high rate of hydrogen release. Workers at the LLNL subsequently developed a method to produce monosized, defect-free HGMSs with tensile strengths as high as 1.03 GPa (150 000 psi). The spheres increased the pressure limit for hydrogen storage to B68.9 MPa (10 000 psi). Storage densities in a bed of microspheres (B50 mm in diameter) have been calculated to lie in the range of 10–14 wt% for filling pressures of 24.1 MPa (3500 psi) and 62.1 MPa (9000 psi), respectively. Because of the sensitivity of gas permeability to temperature, filling times are reduced from thousands of hours to tens of minutes by increasing the temperature to 300–500 1C.
Current Status of Concept Storage Capacity Storage capacity is the most important criteria for determining the potential of any hydrogen storage method for practical application. Examples of the storage capacity of a few proposed methods are listed in Table 1. The storage capacity of microspheres can be expressed as either a volumetric fraction or a mass fraction. In each case, the maximum storage capacity is determined by the maximum pressure that can be contained without fracture of the microspheres. This pressure, in turn, is determined by the inherent strength of the glass and by the ratio of the wall thickness to the outer diameter of the individual microspheres. The inherent strength of glasses does not vary over a wide range as a function of glass composition, which implies that the fill pressure would be approximately the same for any number of different glasses. The maximum pressure Table 1
Examples of hydrogen storage capacities
Storage technique
Storage capacity (wt%)
Hollow glass microspheres Traditional cylinders MgH2 NiH2 PdH2
6–10 1 7.7 3.3 1.9
489
difference, DP, that can be applied to the spheres without fracture is given by the expression DP ¼ 4x
d d
½5
where x, d, and d are the burst strength, wall thickness, and diameter of the spheres, respectively. This simple model suggests that the fill pressure would continue to increase as the ratio of the wall thickness to the sphere diameter increases. Unfortunately, however, increasing the wall thickness leads to a decrease in glass strength once the wall thickness surpasses the length of a critical (Griffith) flaw, namely, in the range of 1–2 mm. This effect has not been recognized in many analyses of hydrogen storage in HGMSs because of a lack of understanding of the influence of glass thickness on strength. A second effect also acts to decrease the storage capacity as the wall thickness to diameter ratio increases. The increase in strength obtained by increases in wall thickness for a constant sphere diameter results in a decrease in internal sphere volume and an increase in the bulk density of the spheres. The void fraction, V, for a hollow sphere is given by the expression 3 d V ¼ 1 2 d
½6
The void fraction decreases rapidly with increasing d/d ratio and thereby decreases the space available for the containment of hydrogen. As a result, there is a maximum in storage density as a function of microsphere characteristics. One published estimate indicates that this maximum occurs at a d/d ratio of B0.12. If the microspheres are filled at an elevated temperature in the range of 300–500 1C to accelerate the process, as is normally proposed, the fill pressure at a nominal ambient temperature of 25 1C will be reduced by a factor of 2–2.5 by the reduction of gas pressure as the gas is cooled. As the pressure during filling cannot be allowed to exceed the burst pressure of the spheres, the storage density will be considerably reduced by filling at elevated temperatures. The filling time is controlled by the hydrogen permeability of the glass and by the wall thickness of the spheres. The hydrogen permeability increases exponentially with increasing temperature, but decreases inversely with the wall thickness. The hydrogen permeability is also a strong function of glass composition. Increasing the fraction of glass-forming oxides in a glass will increase the permeability by several orders of magnitude. Unfortunately, however, an increase in permeability at elevated temperature will also increase the permeability at ambient storage temperatures. Again, this results in a trade-off between increasing ambient storage pressure by decreasing the fill temperature and
490
Fuels – Hydrogen Storage | Glass Microspheres
increasing the rate of hydrogen outgassing of the filled spheres during storage. The recent discovery of photoinduced hydrogen permeation in glasses at Alfred University offers a considerable improvement in storage capacity by reducing the temperature for filling the spheres that increases the storage pressure at ambient temperature. Although the HGMSs reach only 150 1C, the filling rate obtained is equal to that for HGMSs heated to 400 1C using other thermal methods. The reduction in filling temperature will increase the ambient fill pressure by a factor of 1.5 for the same burst pressure, i.e., maximum pressure to which the spheres can be exposed. Kinetics The pressure, P, inside a microsphere is described by the expression h t i P ¼ Pi 7DP 1 exp t
½7
where Pi is the pressure at time zero, DP the change in pressure after time t, and t a time constant. The7sign indicates that the internal pressure will either increase on filling or decrease on hydrogen release from the sphere. The time constant is given by t¼
dd 6RTK
½8
Because the permeability, K, is exponentially dependent on the temperature, the value of t will change by several orders of magnitude between 25 1C and 300–500 1C. As a result, a filling time at elevated temperatures of o1 h can easily be obtained, while an acceptable loss rate at ambient temperatures can also be achieved. Experimentally, it has been found that Ko in eqn [2] varies only by a small amount with glass composition. Changes in the activation energy for permeation are far more important in determining the effect of changes in glass composition on the permeability at filling and release temperatures as opposed to storage temperatures. Ideally, the filling/release time should be minimized, whereas the storage time should be maximized. The desired effect can be obtained using glasses with high activation energies for permeation, which lead to a greater difference between filling/release time and storage time for any given set of these temperatures. Fortunately, the commercial hollow glass microspheres currently available fall into the range of desirable activation energies for optimum filling/release and storage times. Equation [8] indicates that increases in both sphere diameter and wall thickness will increase the relaxation time for any given temperature and permeability. Minimum filling and release times will thus be obtained using small-diameter, thin-walled spheres. This combination,
however, will lead to short storage times. On the contrary, large-diameter, thick-walled spheres will cause an undesirable increase in filling/release times while improving the retention of hydrogen at ambient storage temperatures. It follows that an optimum combination of diameter and wall thickness exists as a compromise between desired filling/release and storage times. As the sphere diameter and wall thickness also affect the strength, i.e., maximum fill pressure, determination of the ideal physical characteristics of the spheres is quite complex. The shelf-life of the filled microspheres is dependent on the physical characteristics of the spheres and the permeability of the glass. Increasing the wall thickness will increase the shelf-life, i.e., minimize the loss of hydrogen at ambient temperature, at the expense of the required fast filling and release times. A low permeability at room temperature will also improve the shelf-life but, depending on the activation energy for permeation, may result in an unacceptable increase in filling and release times. Studies at Alfred University have shown that 2–15 wt% of the hydrogen is lost in 5 weeks from a variety of soda lime borosilicate microspheres of slightly different compositions at 25 1C, whereas as much as 5–36 wt% of the hydrogen will be lost from the same materials in 5 weeks at 50 1C. The variation among the samples appears to be primarily controlled by differences in wall thicknesses and size distributions of the HGMSs. Kinetics of Photoinduced Hydrogen Permeation The discovery of photoinduced hydrogen permeation has dramatically altered the potential for hydrogen storage in HGMSs. Previous concepts have relied on heating the material to temperatures above 300 1C for both filling and fast release of hydrogen on demand. Control of the precise release rate requires rapid heating and cooling of the spheres as the demand for hydrogen fluctuates during service. As HGMSs are widely used as thermal insulation, it is obvious that rapid heating and cooling of a large mass of these spheres is very difficult. Photoinduced hydrogen permeation does not rely on changing the temperature of the spheres to alter the permeation rate of hydrogen into and out of the material. Exposure to light in the range of 1500–2200 nm results in almost instantaneous changes in the permeability of the material. While heating of the spheres to 100–150 1C does eventually occur, the change in temperature lags the change in hydrogen flow rate through the sphere wall by a significant amount. Results of multiple studies indicate that photoinduced changes in hydrogen permeation is not simply due to heating or cooling of the spheres, but is caused by a process with much faster reaction times. Precise control of the hydrogen release rate can be obtained either by varying the intensity of the light or by
Fuels – Hydrogen Storage | Glass Microspheres
pulsing the output of the light source. The response of the hydrogen release rate to variations in light intensity is virtually instantaneous, whereas the response to temperature differences is much slower. Thus the effect is not solely due to heating and cooling of the spheres. The photoinduced hydrogen permeation effect is not observed when using the HGMSs currently available. The glass must contain iron, nickel, or cobalt ions for the effect to occur. Addition of these ions to the soda lime borosilicate glass used in commercial HGMSs is easily done; it requires relatively little additional processing of the frit used to produce the spheres. The application of photoinduced hydrogen permeation will also increase the storage capacity of the microspheres by reducing the filling temperature, as discussed above. The time required for photoinduced and thermally driven filling is approximately the same. Safety Hollow glass microspheres can be considered to be extremely small, high-pressure vessels. As a result, each microsphere contains a minute amount of hydrogen. Tests indicate that microspheres filled with high-pressure hydrogen can be ignited and can propagate a flame, with an ignition temperature similar to that of gaseous hydrogen. A dust cloud of filled spheres yields an explosion pressure in the range of 70–140 kPa, as compared with an explosion pressure of approximately 700 kPa for a hydrogen–air mixture. Similar spheres packed into a ceramic tube can be ignited by a hot wire and will propagate a flame. Crushing of a significant volume of spheres will release a measurable amount of hydrogen gas, with the usual attendant safety issues. Exposure to temperatures in excess of 700 1C has been shown to ignite a mass of filled spheres, as has a static electric shock of sufficient energy. On the contrary, a mass of spheres can be swept up without release of a significant amount of hydrogen, so that spills present a less likely source of explosion or fire than the release of the same amount of gaseous hydrogen. In general, storage of hydrogen in microspheres should be less likely to produce combustion explosions than storage in large, high-pressure vessels. The potential for noncombustion explosions, i.e., those associated with any high-pressure gas, including inert gases, is far less than that of traditional storage vessels. The risk associated with filled microspheres appears to be no greater than that for storage in metal hydrides and less than that of combustible compounds such as methane. The chemical durability of the glasses typically used to make these spheres is excellent and this indicates that HGMS can be recycled many times without degradation. The small size of the HGMSs also presents some potential health hazards. Efforts must be made to avoid eye contact because of the possibility of corneal abrasion.
491
Inhalation may irritate the upper respiratory tract and thereby cause coughing, sneezing, and possibly nose and throat pain. Prolonged exposure can result in microspheres entering the lungs. Ingestion can lead to nausea, stomach pain, and vomiting. As the anticipated compositions are nontoxic, with glasses consisting primarily of silica, soda, and lime, the safety issue is primarily due to the size of the microspheres and not to their composition. Cost Hollow glass microspheres are available as commercial products in large quantities at cost of less than US$2 per kilogram, which would be expected to decrease with economies of scale. The raw materials are relatively inexpensive, the production processes are well established, and the durability of the spheres is such that they can withstand a large number of filling/outgassing cycles. The materials can easily be recycled to produce new spheres to replace those broken during service. As a result, HGMSs appear to be the cheapest of all current materials for the solid-state storage of hydrogen.
Nomenclature Symbols and Units c d D Ea f K Ko P Pi R S T V x d DP s /
gas concentration (molecules cm 3) diameter of sphere (cm) diffusion coefficient (diffusivity) (cm 2 s 1) activation energy for gas permeation (kJ mol 1) fugacity (atm) gas permeability (molecules s 1 cm 1 atm 1) preexponential constant (molecules s 1 cm 1 atm 1) partial pressure of gas in surrounding atmosphere (Pa) pressure at time zero universal gas constant (=8.3145 J K 1 mol 1) gas solubility (molecules cm 3) absolute temperature (K) void fraction burst strength of sphere (MPa) wall thickness of sphere (cm) pressure differential (Pa) time constant, relaxation time (s) fugacity coefficient (unitless)
Abbreviations and Acronyms HGMS LLNL
hollow glass microsphere Lawrence Livermore National Laboratory
492
Fuels – Hydrogen Storage | Glass Microspheres
See also: Fuels – Hydrogen Storage: Carbon Materials; Chemical Carriers; Compressed; High Temperature Hydrides; Hydrides; Liquid; Metal–Organic Frameworks; Zeolites.
Further Reading Bratt PW, Cunnion JP, and Spivack BD (1983) Mechanical testing of glass hollow microspheres. In: Rossington DR, Condrate RA, and Snyder RL (eds.) Advances in Materials Characterization, pp. 441--447. New York: Plenum Press.
Hendricks CD (1991) Glass spheres. In: Schneider SJ (ed.) Ceramics and Glasses, vol. 4, pp. 418--422. Materials Park, OH: ASM International. Rapp DB and Shelby JE (2004) Photo-enhanced hydrogen outgassing of glass. Journal of Non-Crystalline Solids 349: 254--259. Raszewski FC (2007) Hydrogen Storage in Hollow Glass Microspheres. PhD Thesis, Alfred University. Shelby JE (1996) Handbook of Gas Diffusion in Solids and Melts. Materials Park, OH: ASM International. Shelby JE (2005) Introduction to Glass Science and Technology. Cambridge: Royal Society of Chemistry. Tsugawa RT, Moen I, Roberts PE, and Souers PC (1976) Permeation of helium and hydrogen from glass-microsphere laser targets. Journal of Applied Physics 47: 1987--1993.
Introduction Hydrogen storage in hollow glass microspheres (HGMSs) was first demonstrated in the 1970s. Scientists at the Lawrence Livermore National Laboratory (LLNL) filled commercial HGMSs with deuterium and tritium under high pressure for use as targets for the inertial confinement fusion programme. Although their goal was not storage of hydrogen per se, the work indicated that small, hollow spheres could serve as containers for highpressure hydrogen and thereby as a lightweight replacement for the heavy, high-pressure cylinders in use at that time. Although this approach to hydrogen storage has not been very actively pursued, recent interest in hydrogen fuel cells as a replacement for the internal combustion engine has reignited interest in the possibilities inherent in the concept.
where c is the concentration of dissolved gas and P the partial pressure of that gas in the surrounding atmosphere. This behavior is in sharp contrast to that found for metals, where hydrogen dissociates on dissolution. Hydrogen remains in molecular form during dissolution in glasses and other insulators unless heated to sufficiently high temperatures for reactions to occur between the dissolved gas and the glass network. At high pressures, the concentration of dissolved gas is no longer directly dependent on the pressure, but varies with the fugacity of the gas in the atmosphere. The fugacity, f, is given by the expression f ¼ fP
½4
where f is the fugacity coefficient. The fugacity coefficient varies effectively from unity at low pressures to a value of o2 at 69 MPa (10 000 psi).
Fundamental Concepts
Historical Perspective
Hydrogen storage in HGMSs is based on the very large temperature dependence of the rate of entry and exit of hydrogen through the glass walls, and on the ability of the spheres to contain high pressures of hydrogen for long times. Gas permeability, K, can be described as the ease with which atoms or molecules pass through a membrane without reacting with the material under steady-state conditions, i.e., the steady-state flow rate of gas through a membrane. The time to reach a steady state can be determined by the diffusion coefficient, D, also called the diffusivity, which is a measure of the rate of movement of individual atoms. The two aforementioned parameters are related by the gas solubility, S, as expressed by
The work undertaken at the LLNL included a study of the isothermal outgassing of helium, deuterium, and tritium from commercial HGMSs that were based on a soda–lime–silica composition (79 SiO2–10 CaO–6.8 Na2O–2 B2O3–0.8 ZnO–0.5 MgO–0.3 P2O5–0.2 Al2O3–0.1 K2O–0.3 others, the numbers refer to mol%) that were sieved to 40–45 mm. Samples were treated in helium at 400 1C and hydrogen or deuterium at 490 1C for 24 h at the desired pressure. Outgassing was performed at 450 1C for helium and 490 1C for hydrogen or deuterium until no further gas was released, for B1–4 h. Although the studies were not intended to suggest that HGMSs could be used for hydrogen storage, the results established the validity of that possibility. The specific concept of hydrogen storage in HGMSs was first proposed by R. J. Teitel in 1977. It was noted that hydrogen permeability is enhanced at elevated temperatures (175–400 1C) and this allows hydrogen to diffuse into the microspheres. At room temperature, however, the permeability is reduced by several orders of magnitude. This effectively ‘traps’ the hydrogen until the HGMSs are reheated to elevated temperatures, where the increase in hydrogen permeability allows hydrogen release at high rates. It was further found that most commercial microspheres could not withstand roomtemperature filling cycles at 40.5 MPa (400 atm), which limited the storage density to o5 wt%, as opposed to the projected value of 10 wt%. The high failure rate was
K ¼ DS
½1
The temperature dependence of the permeability for hydrogen in glasses is given by the Arrhenius expression: Ea K ¼ Ko exp RT
½2
where Ko is a constant, Ea the activation energy for hydrogen permeation, R the universal (molar) gas constant, and T the absolute temperature. At low pressures, the solubility of hydrogen in glasses is given by Henry’s law: c ¼ SP
488
½3
Fuels – Hydrogen Storage | Glass Microspheres
attributed to low wall strengths as the spheres available at that time only had an apparent tensile strength of B344– 517 MPa (50 000–75 000 psi). It was concluded that the glass composition must be tailored to optimize permeability for specific applications while still maintaining strength. A hydrogen supply system was envisioned in which hydrogen-filled HGMSs would be used in conjunction with an undefined metal hydride component to provide a more efficient system. The HGMSs would offer a constant supply of hydrogen that would also regenerate the metal hydride, whereas the metal hydride would be reserved to meet any short-term demands for a very high rate of hydrogen release. Workers at the LLNL subsequently developed a method to produce monosized, defect-free HGMSs with tensile strengths as high as 1.03 GPa (150 000 psi). The spheres increased the pressure limit for hydrogen storage to B68.9 MPa (10 000 psi). Storage densities in a bed of microspheres (B50 mm in diameter) have been calculated to lie in the range of 10–14 wt% for filling pressures of 24.1 MPa (3500 psi) and 62.1 MPa (9000 psi), respectively. Because of the sensitivity of gas permeability to temperature, filling times are reduced from thousands of hours to tens of minutes by increasing the temperature to 300–500 1C.
Current Status of Concept Storage Capacity Storage capacity is the most important criteria for determining the potential of any hydrogen storage method for practical application. Examples of the storage capacity of a few proposed methods are listed in Table 1. The storage capacity of microspheres can be expressed as either a volumetric fraction or a mass fraction. In each case, the maximum storage capacity is determined by the maximum pressure that can be contained without fracture of the microspheres. This pressure, in turn, is determined by the inherent strength of the glass and by the ratio of the wall thickness to the outer diameter of the individual microspheres. The inherent strength of glasses does not vary over a wide range as a function of glass composition, which implies that the fill pressure would be approximately the same for any number of different glasses. The maximum pressure Table 1
Examples of hydrogen storage capacities
Storage technique
Storage capacity (wt%)
Hollow glass microspheres Traditional cylinders MgH2 NiH2 PdH2
6–10 1 7.7 3.3 1.9
489
difference, DP, that can be applied to the spheres without fracture is given by the expression DP ¼ 4x
d d
½5
where x, d, and d are the burst strength, wall thickness, and diameter of the spheres, respectively. This simple model suggests that the fill pressure would continue to increase as the ratio of the wall thickness to the sphere diameter increases. Unfortunately, however, increasing the wall thickness leads to a decrease in glass strength once the wall thickness surpasses the length of a critical (Griffith) flaw, namely, in the range of 1–2 mm. This effect has not been recognized in many analyses of hydrogen storage in HGMSs because of a lack of understanding of the influence of glass thickness on strength. A second effect also acts to decrease the storage capacity as the wall thickness to diameter ratio increases. The increase in strength obtained by increases in wall thickness for a constant sphere diameter results in a decrease in internal sphere volume and an increase in the bulk density of the spheres. The void fraction, V, for a hollow sphere is given by the expression 3 d V ¼ 1 2 d
½6
The void fraction decreases rapidly with increasing d/d ratio and thereby decreases the space available for the containment of hydrogen. As a result, there is a maximum in storage density as a function of microsphere characteristics. One published estimate indicates that this maximum occurs at a d/d ratio of B0.12. If the microspheres are filled at an elevated temperature in the range of 300–500 1C to accelerate the process, as is normally proposed, the fill pressure at a nominal ambient temperature of 25 1C will be reduced by a factor of 2–2.5 by the reduction of gas pressure as the gas is cooled. As the pressure during filling cannot be allowed to exceed the burst pressure of the spheres, the storage density will be considerably reduced by filling at elevated temperatures. The filling time is controlled by the hydrogen permeability of the glass and by the wall thickness of the spheres. The hydrogen permeability increases exponentially with increasing temperature, but decreases inversely with the wall thickness. The hydrogen permeability is also a strong function of glass composition. Increasing the fraction of glass-forming oxides in a glass will increase the permeability by several orders of magnitude. Unfortunately, however, an increase in permeability at elevated temperature will also increase the permeability at ambient storage temperatures. Again, this results in a trade-off between increasing ambient storage pressure by decreasing the fill temperature and
490
Fuels – Hydrogen Storage | Glass Microspheres
increasing the rate of hydrogen outgassing of the filled spheres during storage. The recent discovery of photoinduced hydrogen permeation in glasses at Alfred University offers a considerable improvement in storage capacity by reducing the temperature for filling the spheres that increases the storage pressure at ambient temperature. Although the HGMSs reach only 150 1C, the filling rate obtained is equal to that for HGMSs heated to 400 1C using other thermal methods. The reduction in filling temperature will increase the ambient fill pressure by a factor of 1.5 for the same burst pressure, i.e., maximum pressure to which the spheres can be exposed. Kinetics The pressure, P, inside a microsphere is described by the expression h t i P ¼ Pi 7DP 1 exp t
½7
where Pi is the pressure at time zero, DP the change in pressure after time t, and t a time constant. The7sign indicates that the internal pressure will either increase on filling or decrease on hydrogen release from the sphere. The time constant is given by t¼
dd 6RTK
½8
Because the permeability, K, is exponentially dependent on the temperature, the value of t will change by several orders of magnitude between 25 1C and 300–500 1C. As a result, a filling time at elevated temperatures of o1 h can easily be obtained, while an acceptable loss rate at ambient temperatures can also be achieved. Experimentally, it has been found that Ko in eqn [2] varies only by a small amount with glass composition. Changes in the activation energy for permeation are far more important in determining the effect of changes in glass composition on the permeability at filling and release temperatures as opposed to storage temperatures. Ideally, the filling/release time should be minimized, whereas the storage time should be maximized. The desired effect can be obtained using glasses with high activation energies for permeation, which lead to a greater difference between filling/release time and storage time for any given set of these temperatures. Fortunately, the commercial hollow glass microspheres currently available fall into the range of desirable activation energies for optimum filling/release and storage times. Equation [8] indicates that increases in both sphere diameter and wall thickness will increase the relaxation time for any given temperature and permeability. Minimum filling and release times will thus be obtained using small-diameter, thin-walled spheres. This combination,
however, will lead to short storage times. On the contrary, large-diameter, thick-walled spheres will cause an undesirable increase in filling/release times while improving the retention of hydrogen at ambient storage temperatures. It follows that an optimum combination of diameter and wall thickness exists as a compromise between desired filling/release and storage times. As the sphere diameter and wall thickness also affect the strength, i.e., maximum fill pressure, determination of the ideal physical characteristics of the spheres is quite complex. The shelf-life of the filled microspheres is dependent on the physical characteristics of the spheres and the permeability of the glass. Increasing the wall thickness will increase the shelf-life, i.e., minimize the loss of hydrogen at ambient temperature, at the expense of the required fast filling and release times. A low permeability at room temperature will also improve the shelf-life but, depending on the activation energy for permeation, may result in an unacceptable increase in filling and release times. Studies at Alfred University have shown that 2–15 wt% of the hydrogen is lost in 5 weeks from a variety of soda lime borosilicate microspheres of slightly different compositions at 25 1C, whereas as much as 5–36 wt% of the hydrogen will be lost from the same materials in 5 weeks at 50 1C. The variation among the samples appears to be primarily controlled by differences in wall thicknesses and size distributions of the HGMSs. Kinetics of Photoinduced Hydrogen Permeation The discovery of photoinduced hydrogen permeation has dramatically altered the potential for hydrogen storage in HGMSs. Previous concepts have relied on heating the material to temperatures above 300 1C for both filling and fast release of hydrogen on demand. Control of the precise release rate requires rapid heating and cooling of the spheres as the demand for hydrogen fluctuates during service. As HGMSs are widely used as thermal insulation, it is obvious that rapid heating and cooling of a large mass of these spheres is very difficult. Photoinduced hydrogen permeation does not rely on changing the temperature of the spheres to alter the permeation rate of hydrogen into and out of the material. Exposure to light in the range of 1500–2200 nm results in almost instantaneous changes in the permeability of the material. While heating of the spheres to 100–150 1C does eventually occur, the change in temperature lags the change in hydrogen flow rate through the sphere wall by a significant amount. Results of multiple studies indicate that photoinduced changes in hydrogen permeation is not simply due to heating or cooling of the spheres, but is caused by a process with much faster reaction times. Precise control of the hydrogen release rate can be obtained either by varying the intensity of the light or by
Fuels – Hydrogen Storage | Glass Microspheres
pulsing the output of the light source. The response of the hydrogen release rate to variations in light intensity is virtually instantaneous, whereas the response to temperature differences is much slower. Thus the effect is not solely due to heating and cooling of the spheres. The photoinduced hydrogen permeation effect is not observed when using the HGMSs currently available. The glass must contain iron, nickel, or cobalt ions for the effect to occur. Addition of these ions to the soda lime borosilicate glass used in commercial HGMSs is easily done; it requires relatively little additional processing of the frit used to produce the spheres. The application of photoinduced hydrogen permeation will also increase the storage capacity of the microspheres by reducing the filling temperature, as discussed above. The time required for photoinduced and thermally driven filling is approximately the same. Safety Hollow glass microspheres can be considered to be extremely small, high-pressure vessels. As a result, each microsphere contains a minute amount of hydrogen. Tests indicate that microspheres filled with high-pressure hydrogen can be ignited and can propagate a flame, with an ignition temperature similar to that of gaseous hydrogen. A dust cloud of filled spheres yields an explosion pressure in the range of 70–140 kPa, as compared with an explosion pressure of approximately 700 kPa for a hydrogen–air mixture. Similar spheres packed into a ceramic tube can be ignited by a hot wire and will propagate a flame. Crushing of a significant volume of spheres will release a measurable amount of hydrogen gas, with the usual attendant safety issues. Exposure to temperatures in excess of 700 1C has been shown to ignite a mass of filled spheres, as has a static electric shock of sufficient energy. On the contrary, a mass of spheres can be swept up without release of a significant amount of hydrogen, so that spills present a less likely source of explosion or fire than the release of the same amount of gaseous hydrogen. In general, storage of hydrogen in microspheres should be less likely to produce combustion explosions than storage in large, high-pressure vessels. The potential for noncombustion explosions, i.e., those associated with any high-pressure gas, including inert gases, is far less than that of traditional storage vessels. The risk associated with filled microspheres appears to be no greater than that for storage in metal hydrides and less than that of combustible compounds such as methane. The chemical durability of the glasses typically used to make these spheres is excellent and this indicates that HGMS can be recycled many times without degradation. The small size of the HGMSs also presents some potential health hazards. Efforts must be made to avoid eye contact because of the possibility of corneal abrasion.
491
Inhalation may irritate the upper respiratory tract and thereby cause coughing, sneezing, and possibly nose and throat pain. Prolonged exposure can result in microspheres entering the lungs. Ingestion can lead to nausea, stomach pain, and vomiting. As the anticipated compositions are nontoxic, with glasses consisting primarily of silica, soda, and lime, the safety issue is primarily due to the size of the microspheres and not to their composition. Cost Hollow glass microspheres are available as commercial products in large quantities at cost of less than US$2 per kilogram, which would be expected to decrease with economies of scale. The raw materials are relatively inexpensive, the production processes are well established, and the durability of the spheres is such that they can withstand a large number of filling/outgassing cycles. The materials can easily be recycled to produce new spheres to replace those broken during service. As a result, HGMSs appear to be the cheapest of all current materials for the solid-state storage of hydrogen.
Nomenclature Symbols and Units c d D Ea f K Ko P Pi R S T V x d DP s /
gas concentration (molecules cm 3) diameter of sphere (cm) diffusion coefficient (diffusivity) (cm 2 s 1) activation energy for gas permeation (kJ mol 1) fugacity (atm) gas permeability (molecules s 1 cm 1 atm 1) preexponential constant (molecules s 1 cm 1 atm 1) partial pressure of gas in surrounding atmosphere (Pa) pressure at time zero universal gas constant (=8.3145 J K 1 mol 1) gas solubility (molecules cm 3) absolute temperature (K) void fraction burst strength of sphere (MPa) wall thickness of sphere (cm) pressure differential (Pa) time constant, relaxation time (s) fugacity coefficient (unitless)
Abbreviations and Acronyms HGMS LLNL
hollow glass microsphere Lawrence Livermore National Laboratory
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Fuels – Hydrogen Storage | Glass Microspheres
See also: Fuels – Hydrogen Storage: Carbon Materials; Chemical Carriers; Compressed; High Temperature Hydrides; Hydrides; Liquid; Metal–Organic Frameworks; Zeolites.
Further Reading Bratt PW, Cunnion JP, and Spivack BD (1983) Mechanical testing of glass hollow microspheres. In: Rossington DR, Condrate RA, and Snyder RL (eds.) Advances in Materials Characterization, pp. 441--447. New York: Plenum Press.
Hendricks CD (1991) Glass spheres. In: Schneider SJ (ed.) Ceramics and Glasses, vol. 4, pp. 418--422. Materials Park, OH: ASM International. Rapp DB and Shelby JE (2004) Photo-enhanced hydrogen outgassing of glass. Journal of Non-Crystalline Solids 349: 254--259. Raszewski FC (2007) Hydrogen Storage in Hollow Glass Microspheres. PhD Thesis, Alfred University. Shelby JE (1996) Handbook of Gas Diffusion in Solids and Melts. Materials Park, OH: ASM International. Shelby JE (2005) Introduction to Glass Science and Technology. Cambridge: Royal Society of Chemistry. Tsugawa RT, Moen I, Roberts PE, and Souers PC (1976) Permeation of helium and hydrogen from glass-microsphere laser targets. Journal of Applied Physics 47: 1987--1993.