October 1, 2006
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Research on fuel cells and hydrogen storage has captured the attention of engineers, scientists, governments and the global population. The promise of clean and-more importantly-renewable energy has been gaining momentum for more than a generation. Global consumption of fossil fuels continues to increase, and the increase is likely to accelerate as the economies of China and India continue their development as industrial and economic powers. Unfortunately, the discovery and production of fossil fuels (mainly coal and oil) has not maintained pace with demand. The use of hydrogen in fuel cells provides the promise of clean energy, and, with advances in materials and technology, hydrogen may become a significant source of energy.

A key component of fuel cell research and development programs is the discovery and characterization of new materials that will become the components of the power generation system. As with any research program, the ability to understand the performance and properties of a new material is dependent on the available analytical tools.

Several facets of analytical tools and materials characterization must be addressed for successful implementation in a research program. Appropriate hardware must be selected, and consideration needs to be given to the area of investigation. For example, the system can be broken into two key components-the fuel storage system and the fuel cell or energy generation unit. A successful system must incorporate both energy storage and generation.

Adsorbent Characterization for Hydrogen Storage

Research tools need to be focused on finding relevant properties that can be used to differentiate performance, establish a simple method for ranking materials that correlates to performance, and accurately measure the hydrogen (H2) storage capacity. Hydrogen storage provides unique challenges for determining the best storage media. Several common techniques include pressurized tanks, adsorbents, solid storage and chemical storage.

Pressurized tanks offer simplicity because the capacity of the tank is easily specified by using an equation of state to predict the pressure-volume-temperature relationship for hydrogen. Unfortunately, these vessels may be required to accommodate up to 10,000 psi (700 bar) of pressure to provide an efficient or substantial quantity of hydrogen. For transportation applications like buses and automobiles, these high-pressure storage vessels may present significant safety concerns.

Adsorbents are a second choice. High-surface-area carbons and novel materials like carbon nanotubes adsorb hydrogen, and larger quantities of hydrogen can be stored in smaller volumes at lower pressures. The challenge of using adsorbents involves understanding the structure of the material and then measuring the hydrogen storage capacity.

Standard technologies, such as surface area, pore size distribution and pore volume, can be used to help understand the material's structure. For surface area, we turn to nitrogen adsorption and employ the models developed by Brunauer, Emmett and Teller (BET surface area).1 This is a mature technique, and the analysis is rapid and can be fully automated. Table 1 provides a summary of BET surface area measurements for several adsorbents. The analysis' speed and accuracy is due to the high level of software control and standard data analysis protocols. Both the International Standards Organization (ISO) and ASTM International have documented these into standard procedures.

Similarly, several techniques are available for determining the sample's pore size, pore volume and micro-porosity. All of these properties can be calculated from the same isotherm data that was used for the surface area modeling. The porosity calculations, which can be difficult to perform via a spreadsheet, often require sophisticated programming to quickly and properly determine the pore size and area distribution. Fortunately, these calculations can be easily programmed to provide a graphical representation of the sample's porosity.

To determine the hydrogen storage capacity of the adsorbents listed in Table 1, techniques similar to BET surface area were employed. Each sample was prepared by heating it in an inert environment or in a vacuum. This sample preparation method is commonly referred to as degassing, and it is used to remove moisture or other adsorbed contaminants from the surface of the adsorbent.

Ready for characterization, each prepared (degassed) sample was evacuated to less than one milli torr of residual pressure. The sample was then exposed to hydrogen, and the quantity adsorbed was determined by using common equations of state for ideal and real gases. The quantity adsorbed on the sample material was determined by calculating the quantity dosed onto the sample minus any residual or unabsorbed hydrogen gas. Pressure, volume, and temperature measurements were used to determine both the quantity dosed and any residual hydrogen after the adsorption was complete.

Figure 1. Hydrogen adsorption isotherms at 77 K on super carbon, activated carbon, carbon nanotubes and boron nitride.

The quantity adsorbed and the residual hydrogen pressure, often referred to as the equilibrium pressure, were recorded and used to construct an isotherm that illustrates the quantity adsorbed as a function of hydrogen pressure. Hydrogen adsorption isotherms for the materials listed in Table 1 are given in Figure 1. It is apparent that the high-surface-area "super carbon" has a superior adsorption capacity when compared to activated carbon, carbon nanotubes and boron nitride. The super carbon provides more capacity-by two orders of magnitude-than the boron nitride.

Figure 2. Hydrogen adsorption isotherms normalized for adsorbent surface area to illustrate the quantity of hydrogen adsorbed per unit surface area.

Traditionally, isotherms have been reported by showing the quantity adsorbed per unit mass vs. pressure, which is quite convenient for comparing materials. However, by using the surface area results from Table 1 and combining this data with the isotherms in Figure 1, the translated isotherms in Figure 2 show a new trend for the samples as we view quantity adsorbed per unit surface area vs. pressure. Unlike Figure 1, the transformed isotherms in Figure 2 identify the carbon nanotubes as the adsorbent with the larger hydrogen adsorption capacity, and all of the adsorbents now fall within the same order of magnitude for performance.

Simply presenting the data on an alternate basis provides unique insight to the hydrogen storage problem. While specially prepared super-high-surface-area carbons and carbon nanotubes have been shown to have higher hydrogen adsorption capacity, their performance is not orders of magnitude better than low-cost and readily-available activated carbon.

Isotherm data may also be used to determine the heat of adsorption. Heat is released as hydrogen adsorbs to the surface during the adsorption process. To determine the heat of adsorption, several isotherms are measured at different temperatures, and this data can then be used with the Clausius-Clapyeron2 equation to calculate the heat of adsorption.

Figure 3. Hydrogen adsorption isotherms collected at 77 and 87 K for carbon nanotubes. The isosteric heat of adsorption was calculated using the Clausius Clapyeron equation.

Adsorption for hydrogen on carbon nanotubes is presented in Figure 3, along with the heat of adsorption as a function of surface concentration. The greatest quantity of heat is released at low hydrogen concentration on the surface. As the surface becomes more crowded (the hydrogen concentration increases), the quantity of heat released lessens. This is a typical profile for a heterogeneous surface, and the isosteric heat of adsorption is a valuable tool for understanding the surface heterogeneity.

Figure 4. Hydrogen chemisorbed on platinum at 25°C (308 K).

The anode, cathode and electrolyte must be evaluated for energy generation (the fuel cell). The anode must be able to efficiently adsorb hydrogen and then dissociate the molecular hydrogen to atomic hydrogen. The anode materials are typically carbon with a coating of platinum, palladium, or other metals that are effective for adsorbing and dissociating hydrogen.

The adsorption of gases on metals can be traced back to the early 20th century. Hydrogen adsorption on Nobel metals also enjoys a long scientific history due to the high value of hydrogen in the refining industry.3 Nobel metals supported on oxides are commonly used for hydrogen generation in petroleum refining. Additionally, Nobel metals supported on carbon are commonly used in chemical processing for catalyzing hydrogenation reactions.

Hydrogen adsorption on metals is often classified as chemical adsorption or chemisorption. Chemisorption is an activated process, meaning that it usually occurs at elevated temperatures, and is also very specific and sensitive to system chemistry. For example, hydrogen readily adsorbs to platinum and may also react with platinum oxides, but it does not adsorb to copper or silver under typical conditions. In addition, a hydrogen adsorption experiment may be confounded by the other elements present, such as nickel, cobalt and other hydrogen adsorbing materials.

Another area of difficulty is often referred to as hydrogen spillover, which is the unusual tendency of hydrogen to adsorb on metal atoms and then migrate to the carbon surface. This phenomenon exemplifies why the application must be considered during characterization. In particular, an analytical technique must be devised to help determine the quantity of hydrogen adsorbed on the metal and to try to gain some understanding regarding the quantity of hydrogen adsorbed on the carbon.

To separate the quantity of hydrogen adsorbed on the metal from the hydrogen adsorbed on the surface, we can use some basic understanding of the nature of chemical adsorption. Hydrogen dissociates and strongly bonds to the platinum, and chemical adsorption is often considered to be irreversible. An inert purge or simple evacuation will not remove strongly chemisorbed hydrogen from platinum. The hydrogen can be removed from the sample by increasing the temperature to promote its desorption from the platinum surface. However, the hydrogen that spills over to the carbon surface is not strongly bound and can be removed by simple evacuation or an inert purge.

The sample is activated and the adsorption isotherm measured to determine the total quantity adsorbed. Nobel metals can be activated by heating the sample in a reducing environment to transform the metal oxide to pure metal crystallites. For example, platinum supported on carbon can be heated to 400°C in a hydrogen-rich environment. The platinum oxide is rapidly converted to platinum metal crystallites, which readily adsorb hydrogen, oxygen, ethylene and carbon monoxide. After the sample is reduced to provide the active surface, the excess hydrogen is removed from the system via evacuation or a gas purge. The clean active sample is cooled to ambient (or near ambient) temperature so that the hydrogen adsorption isotherm can be measured.

Similar to the previous techniques for determining storage capacity, the H2 adsorption isotherm is a composition diagram that represents the quantity of hydrogen adsorbed on a material as a function of the hydrogen pressure. Both the hydrogen adsorbed on the metal surface and any hydrogen that spills over to the carbon or support material can be determined. First, the sample is activated and the adsorption isotherm is measured. The sample is evacuated to remove any weakly bound hydrogen, and the adsorption isotherm is then repeated to reproduce (repeat) the weakly bound hydrogen. (The strongly adsorbed hydrogen for the first analysis will not be removed via evacuation. Heating the sample to elevated temperature is required to remove the strongly bound hydrogen.) The initial isotherm minus the repeat isotherm yields the quantity of hydrogen adsorbed on the platinum surface (see Figure 4).

Several key parameters can be estimated from the quantity of strongly adsorbed hydrogen on the platinum surface: the percent dispersion, the metal surface area and the average metal crystallite size. The percent dispersion is a useful index that gives the percentage of metal atoms available for chemisorption. For example, a 0.5 weight percent platinum on carbon may have 35% dispersion. We can ascertain that only 0.175 weight percent of the platinum is available for adsorption, not the full 0.5 weight percent.

The metal surface area is often used to compare materials that contain different compositions, since the metal surface area often relates directly to adsorption capacity. And finally, we can use the quantity adsorbed to estimate the average crystallite size. It is easy to achieve high metal surface areas and small crystallite sizes (less than 5 nm) by using low percentages of platinum supported on carbon or oxides. However, small platinum crystallite (less than 1 nm) may not effectively adsorb or dissociate hydrogen. During any experimental program, it is important to relate performance to properties.

Figure 5. Isosteric heat of adsorption for hydrogen chemisorbed on platinum.

The heat of adsorption for hydrogen chemisorbing on platinum can be calculated using the same techniques previously described for the carbon nanotubes-the Clausius-Clapyeron equation and calculation of the isosteric heat of adsorption.4 The heat of adsorption has a very practical use. As hydrogen adsorbs on platinum, heat typically greater than 100 kj/mol is released (Figure 5). This high energy adsorption creates the strongest, most active platinum atoms with the greatest reactivity and potential to adsorb and dissociate hydrogen.

Tools of the Trade

Advanced software and convenient reporting have become essential tools for hydrogen storage and fuel cell researchers. These software tools must include accepted techniques for material characterization to understand the physical properties, and they must also include techniques to directly measure the performance of the material, such as the hydrogen adsorption capacity and the heat of adsorption. The benefit of having these tools available in software is that the isotherm data may be used in several ways and is not just limited to capacity measurements.

For hydrogen storage applications, understanding the heat of adsorption also has a practical application. While the adsorbent is recharged with hydrogen, heat is released. It is critical to remove the heat generated while the material adsorbs hydrogen. An increase in temperature will result in a decrease in adsorbent capacity. The heat of adsorption may be used to specify the cooling requirements of an adsorbent system and thus maintain the highest capacity for hydrogen storage.

Similarly, in fuel cells, for anodes comprised of platinum supported on carbon, the heat of adsorption provides a direct indication of the active capacity of the anode. High energy sites are the most efficient for adsorption of hydrogen and its dissociation to atomic H2.

For additional information regarding analytical tools for hydrogen storage and fuel cells, contact Micromeritics Instrument Corp., One Micromeritics Dr., Norcross, GA 30093-1877; (770) 662-3633; fax (770) 662-3696; e-mail ussales@micromeritics.com ; or visit http://www.micromeritics.com .

Editor's note: Data for the figures in this article was gathered using Micromeritics Instrument Corp. ASAP 2020 gas adsorption analyzers (with the chemisorption option when needed) and software.


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