Chapter 1 Introduction

Chapter 1
Introduction to Hydrogen Energy
Hydrogen Energy
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
 economic and societal drivers (such as energy independence, energy
costs, global warming, pollution)
 politico-economic decisions (such as infrastructure investment, R&D
 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 dieselfueled 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
 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 1012 scf produced in 2003. Most of this H2
was produced by steam methane reforming.
 There are a number of processes that can produce H2 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, highpressure reaction chamber typically operating between 1,250 to 1,575 oC
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,
IrO2 and RuO2 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 oC using
ceramic membranes.
 Materials issues surround the kinetics of the electrode processes and
durability of the interconnect materials in the high-temperature, oxygenrich 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, C276, Incoloy 800H, SiC, and Si3N4 with and without noble metal
 Use of solar energy to produce H2 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 O2, 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
H2 production system includes transmittance, outdoor lifetime (i.e.,
durability to sunlight), biocompatibility, H2 and O2 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 H2 effects on pipeline and vessel materials.
 Transport of H2 in a carrier such as ammonia, a hydrocarbon, or other
from or local production of H2 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 H2 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 H2 uptake and modifying the steel to reduce its susceptibility.
 The addition of impurity concentrations of O2 is one option for reducing
H2 uptake, while manganese and silicon additions to the steel are
possible routes for reducing the susceptibility of gas pipeline steels to H2
 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
 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 2017. 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 TaO0.92, N1.05ZrOx, 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 H2
and O2, 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 oC, while PEM fuel cells operate at round 100 oC 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 CeO2 is a leading candidate for operating
temperatures below 600 oC.
 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 oC. CeO2 materials have a higher ionic conductivity than YSZ
and can operate in the temperature range of 500 to 700 oC 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)3O4
spinels look promising but need further development.
 Devlin, P., Public-Private R&D Partnerships Examples, DOE
Hydrogen Program, July 14, 2005.
 Materials for the Hydrogen Economy, Jones, R. H. and
Thomas, G. J., ed., CRC Press, Boca Raton, 2008.