ELECTRICALLY CHARGED METAL-DOPED CARBON AEROGELS

As transportation accounts for 67% of the 20 million barrels of oil our nation uses per day, it would make sense to seek an alternative to the traditional internal combustion engine (ICE). To this end, the Department of Energy (DOE) has set some lofty goals for the introduction of a cost competitive fuel-cell vehicle (FCV). While continuous advances are being made on fuel-cells in terms of costs and materials, hydrogen storage remains a major problem that needs to be addressed adequately. Indeed, "Hydrogen storage is one of the key hurdles in creating a hydrogen-based transportation system," according to James Spearot, director of GM's chemical and environmental sciences laboratory. Pharogen has worked jointly with university researchers to develop novel carbon-based materials utilizing novel storage processes to achieve hydrogen storage capacities > 6wt% and 0.045kg/L that would meet DOE goals for 2010.

Our work applies nanotechnology processes that seek to eliminate the need for high-pressure hydrogen storage. At the moment, several hydrogen storage methods are being tried. However, so far, none have proven effective in meeting DOE goals.

• Metal hydrides: These are promising metal based materials (ie. NaAlH4, KAlH4). Hydrides can currently store between 5 and 9wt % hydrogen, but most still require temperatures between 525°F and 1,650°F to release the hydrogen.
• Carbon based: There are a significant number of carbon-based materials being researched such as carbon nanotubes, “buckyballs”, and carbon aerogels. However in most cases it is necessary to operate at temperatures in the 80K range. Currently, the energy required to operate at these temperatures renders these materials unsuitable for use in a vehicle. Additionally, leakage occurs over a short period of time if the temperature is not stabilized around the 80 K range.
• Other materials: Zeolites, glass microspheres, metal organic compounds, ceramic powders have not yet proven to be effective hydrogen storage materials.
• Chemical hydrides: (ie. NaBH4) The main issue is that of reaction reversibility.
• Compressed gas does not meet DOE gravimetric and volumetric goals. Additionally, customers may be weary of the 5kpsi and 10kpsi tanks.
• Liquified hydrogen gas comes closest to meeting DOE goals. However, the energy necessary for liquefying the hydrogen currently makes this storage medium commercially nonviable.

As you may know, a fuel cell is in essence a battery. However, unlike common batteries, fuel cells can also be thought of as engines. They continuously generate power as long as feedstock is available: Oxygen, from ambient air, and hydrogen. Through a series of catalytic reactions, hydrogen and oxygen combine to generate electric power. In the process, heat and water vapor are emitted as exhaust (see diagrams below). While oxygen can be obtained directly from ambient air, hydrogen must be stored onboard the vehicle through some means. To date, there are no satisfactory means of storing hydrogen onboard vehicles that meet cost, safety, and operational goals, as noted above. As you can imagine, this industry will revolutionize the internal combustion engine era that has been king for over 100 years and pave the way to more environmentally friendly modes of energy consumption.

Figure 1:     a) Fuel Cell schematic, b) Hydrogen driven fuel cell propulsion system.

Metal-Doped Carbon Aerogels (MDCA) capable of meeting the Department of Energy gravimetric and volumetric goals for hydrogen storage to be used onboard fuel-cells vehicles are the primary materials in this work. By using a MDCA in combination with an electric charge process our concept exploits the physical properties of MDCAs and metal nanoclusters. Within the MDCA matrix, we induce dipole moments in hydrogen molecules using electric charge fields that aid in the electrosorption and formation of several hydrogen monolayers, which increase the hydrogen storage capacity of the base material.

CAs were chosen as the base material due to their proven capacity to store H2 through adsorption in the
range of 3-17wt%.1 Table 1 shows the H2 storage potential of non-doped CA.

Table 1: Hydrogen Adsorption at 77k1

Aerogel Composition*	Aerogel Density (g/cm3)	Pyrolisis Temp (°C)	H2 (wt%)	H2 (kg/m3)
RF-derived C	0.149	1050	5.87	9.3
RF-derived C	0.284	1050	3.56	10.5
RF-derived C	0.637	1050	3.19	21.0
PF-derived C	0.422	1050	2.12	9.1
PF-derived C	0.547	1050	2.25	12.6
PF-derived C	0.742	1050	1.46	11.0
RF	0.106	-	16.75	21.3
RF	0.193	-	4.5	9.1
RF	0.411	-	4.4	19.0

*Note: RF-Resorcinol-Furfural; PF-Phenolic-Furfural

Much literature exists on hydrogen adsorption research with carbon nanotubes. Non-reproducible storage results have ranged from 67% to less than 1%.2 Their hydrogen storage capacity is still being studied, but to date CNT hydrogen storage has not been greater than 3-4wt%. From a commercial viewpoint, carbon nanotubes are very expensive to manufacture in comparison to CAs. The high surface areas, 600-1100 m2/g, of CAs provide increased probability of molecular adsorption, primarily within the micropore (≤ 2nm) structure. We believe the physisorption kinetics in both cases are similar, albeit the cross-linked carbon nanoparticle structure in CAs, resulting from the pyrolized organic precusor, provide a better storage medium arising from a combination of mesopore (5-20nm) and micropore volume available for adsorption. We propose that the CA conductivity (20-100 S/cm)11, primarily through a hopping or tunneling mechanism, is an additional characteristic that can be exploited for enhanced hydrogen storage.

The chemistry to make the carbon aerogel precursors is well known and easily accomplished in the laboratory. Furfural and resorcinol are the reactants, HCl is used as a catalyst, and the solvent used is isopropanol.3 Such method eliminates two processing steps from a previously established process whereby H20 was a solvent that had to be exchanged with acetone prior to supercritical drying. Subsequently, the dried gels are carbonized by heating at elevated temperature (≥800°C) in an inert atmosphere to remove any remaining moisture and hydrogen. The resultant carbon aerogels are composed of an interlinked matrix of carbon nanoparticles, which creates macropores (>50nm), mesopores, and micropores. The CA structure, thus the BET surface area, depends on organic precursor densities, pyrolisis temperatures (Tpy), and gelation temperatures. The metal particle size diameter is a function of pyrolisis temperature as well. In general, the BET surface area decreases with
increasing pyrolisis temperature-up to 900 °C after which it increases slightly-while the micropore volume peaks at around 800 °C, then decreases with increasing Tpy. Conversely, the micropore diameter increases with increasing Tpy, but decreases with increasing aerogel density.13 Table 2 lists some processing parameters relationships obtained with the isopropanol solvent method of CA preparation:

Table 2: Effects of processing parameters on Carbon Aerogels12

Mass content of reactant (wt%)	Density (mg/ml)	Gelation Temp (oC)	Micropore Volume (ml/g)	BET surface area (m2/g)
16.7	479	70	0.171	557.1
11.5	424	60	0.186	615.3
5.9	231	60	0.189	630.4
5.9	148	70	0.202	655.6
Tpy=800-900 °C; Micropore width ~ 0.5nm

One metal-doping method is to immerse the carbon aerogel in aqueous solutions of metal salts for several days then drive off the moisture in a low temperature oven. A second method is one in which the organic precursor contains ion exchange sites that are subsequently interchanged with metallic ions.This is a preferred method as dopants are more uniformly dispersed within the carbon framework. In both cases, metal species are created upon carbonization.

For our preliminary research, we immersed commercially available carbon aerogels in two separate solutions of
Ni(NO3)2 •6H2O Nickel(ous) Nitrate: One at a 0.5M concentration , the other at 1.0M to generate Nickel nanoparticles within the CA. We found that that the dopant is not homogeneously distributed within the carbon framework using this method. Figures 2 and 3 show SEM and X-ray scattering analyses. There are wide areas void of nickel particles, including the area studied under X-ray scattering. Many fissures were also discovered throughout the CA sample, pointing to manufacturing processes that could be optimized. Such fissures are not conducive to H2 storage. Table 3 presents particle size dependence on pyrolisis temperature. References are in superscripts.

Table 3: Effects of pyrolisis temperature on metal particle size.

Metal	Tpy (°C)	Particle Size (nm)
Ni 14	400	2-4
	600	2-4 w/increased Ni density
	1050	200-400
Co 5	500	<2
	600	20
	850	30
	1050	80-100
Cu 4	1050	10-50

Calculations have shown that the addition of alkali metal dopants to carbon nanotubes enhance H2 adsorption volume by 30% .6 That increase is due to a charge transfer of electrons from the metal cluster to the carbon atoms in single wall nanotubes (SWNT). This charge transfer polarizes the H2 molecule causing a dipole interaction between the alkali metal and the carbon atoms, characterizing the H2 physisorption on the SWNT.7 Increased H2 adsorption, as a result of carbon-metal bonds, is not
limited to alkali metals. Yildrim and Ciraci demonstrated, through a first-principals total-energy study, that a total of four H2 monolayers are possible in SWNT doped with titanium, a transition metal, that corresponds to 8wt% hydrogen storage.15 We believe a similar behavior is inherent in MDCAs, which makes use of transition metals engrained within a CA matrix. We suggest that the hydrogen will be primarily stored within the micropores, which are of approximately 20 angstroms (~20 Å) in width. In separate research, Yildrim and Ciraci found that, in metal-organic frameworks (MOF), hydrogen molecules form nano-clusters with intermolecular distances of 3Å instead of the longer 3.6Å distances formed in pure hydrogen.16 If this is also the case with MDCAs, three to four H2 layers can be stored within a micropore width range of 15-21Å.

A final step to our concept is to apply and electrical current to optimized MDCAs. Grand Canonical Monte Carlo computer simulation studies have shown that a quadrupole moment and induced dipole interaction of H2 with charged SWNT lead to an increase in adsorption relative to the uncharged tubes by 10-20% for 298K and 15-30% for 77K.8 We anticipate similar carbon-metal interactions in MDCA particularly since current flows through a tunneling mechanism in both CAs, as well as metal nanoclusters.9, 10 It is expected that the asymmetric charge distributions responsible for inducing the dipole moments will carry
over to a second and third H2 monolayer, thus improving hydrogen adsorption capacity.

(a)	(b)
Figure 2: (a) SEM picture of a Nickel-doped carbon aerogel sample immersed in a 0.5M Ni(NO3)2 •6H2O Nickel(ous) Nitrate solution, (b) Different area of the same sample.

(a)	(b)
Figure 3: (a) X-ray scattering analysis within (b) probe area represented by picture.

REFERENCES

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in Metal-Organic Frameworks (MOF)”, cond-mat/0507220 v1 9 July 2005.