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Chapter 1 Introduction to Hydrogen Energy
Hydrogen Energy lish/pro/ren_hsto_idx.jsp magn-mat/hydrogen-storage/
Devlin, P., Public-Private R&D Partnerships Examples, DOE Hydrogen Program, July 14, 2005.
Hydrogen Energy System
Will a hydrogen-based energy economy, with its promise of clean, sustainable energy, become a reality? This is clearly a complex issue involving economic and societal drivers (such as energy independence, energy costs, global warming, pollution) politico-economic decisions (such as infrastructure investment, R&D investment) exogenous developments(such as advancement in the performance of other energy systems, military conflicts) The use of hydrogen as an energy carrier may ultimately hinge upon the performance achieved in hydrogen production distribution, storage, and propulsion systems and components. The performance of those, in turn, is highly dependent on technological advancements, particularly on the properties of the materials used in their manufacture. In other words, materials are key enabling technology to a viable hydrogen economy.
It is clear, though, that alternative energy sources will eventually be needed to satisfy the world’s ever-increasing energy requirement. Since such a transition would be revolutionary, rather than evolutionary, it will require a significant investment in research, development, and infrastructure over a relatively long period. In other words, it is not too soon to pursue the development of alternative fuels. In the transportation sector, in fact, hydrogen could have the greatest impact. For more than 100 years, gasoline- and diesel- fueled internal combustion engines have been used to supply motive power for a wide range of vehicle sizes, shapes, and applications. These vehicles are supplied with fuel by an efficient and pervasive petroleum-based infrastructure that products a fuel with high energy density and consistent performance. The challenge, then, for alternative fuels is to supply equivalent, or nearly equivalent, vehicle performance, vehicle cost, and operating costs.
Furthermore these requirements must be met on a scale sustainable at the levels expected for global automotive use. The major advantage of hydrogen as a transportation fuel, particularly with hydrogen fuel cell vehicles, is that it simultaneously addresses many issues associated with current petroleum-base vehicle technologies, including (1)reduced greenhouse gas emissions (2)reduced pollutant emissions (3)diversification of fuel feedstocks (4)energy independence (5)on-board fuel efficiency Each of the stages in the hydrogen fuel chain—production, distribution, storage, utilization (e.g., fuel cell, internal combustion engines) — employs components and systems that require unique and sometimes extraordinary material properties.
1. Hydrogen Production
Hydrogen is used primarily for petroleum refining and ammonia production with about 3.2 X scf produced in Most of this H 2 was produced by steam methane reforming. There are a number of processes that can produce H 2 by the dissociation of water or steam. These include low- and high-temperature electrolysis, solar and photoelectrochemical processes, and themochemical processes such as the sulfur-iodine processes. The source of the energy to dissociate water is a key to whether these processes will reduce greenhouse gases and dependence on foreign fossil fuels. Nuclear energy as a source of electrical and thermal energy offers a significant opportunity to achieve both goals. Steam methane reforming is performed in a high-temperature, high- pressure reaction chamber typically operating between 1,250 to 1,575 o C at pressures of 20 to 100 atmospheres. Materials issues are the same as those of high-temperature, high-pressure vessels where creep of corrosion-resistant materials is important for the containment vessel and durability of alumina, chromia, or SiC refractory lining materials is critical to the performance of the system.
Electrolytes are a critical material in the performance of electrolyzers. Low-temperature electrolysis of water relies on proton exchange membrane (PEM) cells using sulfonated polymers for the electrolytes. Key issues for all electrolyzers are the kinetics of the system that is controlled by reaction and diffusion rates. Catalysts such as platinum, IrO 2 and RuO 2 are used to improve the reaction kinetics, but they also contributed to the cost of the system, which is also an issue. Steam electrolysis is also a possibility at a temperature of about 1,000 o C using ceramic membranes. Materials issues surround the kinetics of the electrode processes and durability of the interconnect materials in the high-temperature, oxygen- rich environments. Thermochemical water-splitting processes such as the S-I process offer high efficiency when coupled with an efficient source of heat, but have significant issues associated with corrosion of system materials. Materials being considered include Hastelloy B-2, C- 276, Incoloy 800H, SiC, and Si 3 N 4 with and without noble metal coatings.
Use of solar energy to produce H 2 is another route for reducing greenhouse gas emissions form fossil fuels while also reducing our dependence on foreign fossil fuels. Photoelectrochemical and photobiological processes are two examples that are solar energy driven. Photobiological hydrogen production is a process where microorganisms (algae or cyanobacteria) function as photocatalysts. Algae or cyanobacteria use photosynthesis to split water into O 2, protons, and electrons. Materials issues associated with this process are sketchy since this process has not developed beyond the exploratory stage. The low energy density of sunlight will dictate a system that covers a large area, so material costs will be a critical issue in the economics of this process. A concentrating reactor system will require light-transmitting elements from the dish-concentrating collector into the reactor. An overall list of material properties that will be critical to the operation of this type of H 2 production system includes transmittance, outdoor lifetime (i.e., durability to sunlight), biocompatibility, H 2 and O 2 permeation rates, and physical and mechanical properties.
2. Hydrogen Distribution Mintz, et al., Hydrogen Distribution Infrastructure,
The distribution of hydrogen from a central production facility may be done with pipelines, trucks, or other carries, but will very likely involve some off-board storage capability as well. Therefore, the primary materials issue associated with distribution deal with H 2 effects on pipeline and vessel materials. Transport of H 2 in a carrier such as ammonia, a hydrocarbon, or other from or local production of H 2 could alter some of the issues but is not likely to totally eliminate them. The safety of hydrogen distribution is a primary issue that affects material choice. The closer to population centers, the higher the risk and the more conservative the design.
Hydrogen storage and transport in steel pipelines have been done successfully in the industrial gas and petroleum industries. A key difference will be the gas pressures needed for commercial distribution of H 2 for the hydrogen economy. Materials are more susceptible to hydrogen effects with increasing pressure. Hence, there will be key issues related to safety and economy. Yet it is well known that steels can be susceptible to hydrogen-induced crack growth and embrittlement. Methods to reduce these effects include modifying the gas composition to reduce H 2 uptake and modifying the steel to reduce its susceptibility. The addition of impurity concentrations of O 2 is one option for reducing H 2 uptake, while manganese and silicon additions to the steel are possible routes for reducing the susceptibility of gas pipeline steels to H 2 effects. Considerable effort is needed to verify that these changes can be done effectively and that they provide the needed operational safety.
3. Hydrogen Storage
A key technical impediment to the deployment of hydrogen as a transportation fuel is the relatively low energy density for on-board hydrogen storage systems. Physical approaches, such as compressed gas and liquid hydrogen systems, are the only near-term options available, but these have limitations in terms of volumetric energy density or cryogenic requirements. In the long term, better storage alternatives will be needed, and current research efforts are focused on materials and chemical approaches, where the chemical bonding between hydrogen and other elements increases the volumetric density beyond the liquid state. With the recent launch by the U.S. DOE (Department of Energy) of a national “Grand Challenge” for hydrogen storage development, a number of exciting new research directions have appeared that have shown good progress over the last few years.
In contrast to the earlier development work in the 1970s, where intermetallic hydrides were intensively studied, recent work has focused on materials with high hydrogen capacity. The FreedomCAR and Fuel Partnership (an industry-government partnership) has established very challenging system-level performance targets for storage, for example, gravimetric energy density targets of 4.5 wt % for 2010 and 5.5 wt % for Since these targets include the mass of system components, the storage materials must have even higher hydrogen capacities. System-level volumetric targets are equally as challenging. Generally speaking, high-capacity materials often have thermodynamic properties (e.g., enthalpy of formation, operating temperature, stability, reversibility) or kinetic properties (e.g. absorption, desorption rates) that render them unsuitable for use in storage systems. Thus, research efforts are directed at (1) searching for new storage materials using rapid combinatorial screening methods and computational techniques; (2) improving the performance of storage materials through alloying, using catalysts and nano- or mesoscale structural modifications; and (3) examining alternate reaction pathways to overcome thermodynamic barriers.
4. Hydrogen Fuel Cells
Proton exchange membrane (PEM) fuel cells are the primary choice for transportation systems, but they can be useful for stationary power production or local hydrogen production. Most of the challenges of PEM fuel cell commercialization center around cost and materials performance in an integrated system. Some specific issues are the cost of catalyst materials, electrolyte performance, i.e., transport rates, and water collection in the gas diffusion layer (GDL). The anode and cathode electrodes currently consist of Pt or Pt alloys on a carbon support. Two low-cost, nonprecious metal alternative materials for anode catalysts are WCx and WOx. Pt alloyed with W, Sn, or Mo has also been evaluated for anode catalyst materials. Some non-Pt cathode catalysts that are being evaluated include TaO 0.92, N 1.05 ZrOx, pyrolyzed metal porphyrins such as Fe- or Co-Nx/C and Co-polypyrrole-carbon. However, none of these have matched the catalytic performance of Pt.
The electrolyte membrane presents critical materials issues such as high protonic conductivity over a wide relative humidity (RH) range, low electrical conductivity, low gas permeability, particularly for H 2 and O 2, and good mechanical properties under wet-dry and temperature cycles; has stable chemical properties under fuel cell oxidation conditions and quick start-up capability even at subfreezing temperatures; and is low cost. Polyperfluorosulfonic acid (PFSA) and derivatives are the current first-choice materials. A key challenge is to produce this material in very thin form to reduce ohmic losses and material cost. PFSA ionomer has low dimensional stability and swells in the presence of water. These properties lead to poor mechanical properties and crack growth.
Solid – oxide fuel cells (SOFCs) are being developed for distributed power such as home power units and large power production units. They are not being considered for transportation, although that is conceivable with some difficulties. SOFC electrolytes are ceramic and operate at temperatures of up to 1,000 o C, while PEM fuel cells operate at round 100 o C or less. A key to the power production with SOFCs, as with PEM fuel cells, is the ability to produce thin electrolyte layers. Considerable development effort has resulted in cost-effective methods for producing thin and dense layers of ytrria stabilized zirconia (YSZ) that exhibit sufficient stability in the air/fuel environment. Doped CeO 2 is a leading candidate for operating temperatures below 600 o C.
A primary limitation of YSZ is its low ionic conductivity. To overcome this, thinner electrolyte layers have been developed and yttria has been replaced with other acceptors. Even with these developments, the electrolytes must operate at temperatures exceeding 600 o C. CeO 2 materials have a higher ionic conductivity than YSZ and can operate in the temperature range of 500 to 700 o C but suffer from structural instability in the reducing atmosphere of the cell. Interconnects are used to electrically connect adjacent cells and to function as gas separators in cell stacks. High-temperature corrosion of interconnects is a significant issue in the development of SOFCs. Ferritic stainless steels have many of the desired properties for interconnects but experience stability issues in both the anode and cathode environment. The dual environments cause an anomalous oxidation for which a mechanistic understanding has yet to be determined. Protective coatings from non-chromium-containing conductive oxides such as Mn, Co) 3 O 4 spinels look promising but need further development.
magn-mat/hydrogen-storage/ Devlin, P., Public-Private R&D Partnerships Examples, DOE Hydrogen Program, July 14, Materials for the Hydrogen Economy, Jones, R. H. and Thomas, G. J., ed., CRC Press, Boca Raton, References