Chapter 1

Introduction to Hydrogen Energy

Hydrogen Energy

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Devlin, P., Public-Private R&D Partnerships Examples, DOE Hydrogen Program, July 14, 2005.

Hydrogen Energy System

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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 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, high-pressure 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, 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 Si3N4 with and without noble metal coatings.

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 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 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.

http://hcc.hanwha.co.kr/english/pro/ren_hsto_idx.jsp

http://www.ifw-dresden.de/institutes/imw/sections/21/funct-

magn-mat/hydrogen-storage/

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.

References