RChahine.pdf

RChahine.pdf
Briefing on the state of the art of Hydrogen Technology (prepared by R.Chahine/Hydrogen
Research Institute, UQTR)
Vision & Transition: A sustainable energy system using electricity and hydrogen as carriers and
providing safe, reliable and secure energy supply. This vision is built on meeting two
expectations: 1- that on the supply side hydrogen can be produced cleanly and economically
from primary energy sources; and 2- that on the demand side hydrogen applications, like fuel
cells for transportation, can effectively compete with the alternatives. Unlike other energy
systems, both expectations should be met; one will not work without the other and there are
major challenges that must be overcome before they become a reality. The transition to the
hydrogen economy will not be simple or straightforward, its course will be dictated on one hand
by our ability to overcome the major economical and technical hurdles spread across the whole
spectrum of the hydrogen economy namely production, storage & distribution and use; and on
the other hand by advancements in competing technologies that are not hydrogen dependent.
Challenges of the hydrogen economy: In a recent exhaustive report 1 published by National
Academy of engineering and emphasizing hydrogen-fuelled transportation, the four most
fundamental technological and economic challenges were resumed as follows:
1. “To develop and introduce cost-effective, durable, safe, and environmentally desirable fuel
cell systems and hydrogen storage systems. Current fuel cell lifetimes are much too short and
fuel cell costs are at least an order of magnitude too high. An on-board vehicular hydrogen
storage system that has an energy density approaching that of gasoline systems has not been
developed. Thus, the resulting range of vehicles with existing hydrogen storage systems is
much too short.
2. To develop the infrastructure to provide hydrogen for the light-duty-vehicle user. Hydrogen
is currently produced in large quantities at reasonable costs for industrial purposes. At a
future, mature stage of development, hydrogen (H2) can be produced and used in fuel cell
vehicles at reasonable cost. The challenge, with today’s industrial hydrogen as well as
tomorrow’s hydrogen, is the high cost of distributing H2 to dispersed locations. This
challenge is especially severe during the early years of a transition, when demand is even
more dispersed. But the transition is difficult to imagine in detail. It requires many
technological innovations related to the development of small-scale production units. Also,
non-technical factors such as financing, siting, security, environmental impact, and the
perceived safety of hydrogen pipelines and dispensing systems will play a significant role.
All of these hurdles must be overcome before there can be widespread use. An initial stage
during which hydrogen is produced at small scale near the small user seems likely. In this
1
The Hydrogen Economy: Opportunities, costs, barriers, and R&D Needs (2004); National Research Council and
National Academy of Engineering of the National Academies, National Academic Press, USA. Available at
http://www.nap.edu/catalog/10922.html.
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case, production costs for small production units must be sharply reduced, which may be
possible with expanded research.
3. To reduce sharply the costs of hydrogen production from renewable energy sources, over a
time frame of decades. Tremendous progress has been made in reducing the cost of making
electricity from renewable energy sources. But making hydrogen from renewable energy
through the intermediate step of making electricity, a premium energy source, requires
further breakthroughs in order to be competitive. Basically, these technology pathways for
hydrogen production make electricity, which is converted to hydrogen, which is later
converted by a fuel cell back to electricity. These steps add costs and energy losses that are
particularly significant when the hydrogen competes as a commodity transportation fuel
suggesting that most current approaches—except possibly that of wind energy—need to be
redirected. The required cost reductions can be achieved only by targeted fundamental and
exploratory research on hydrogen production by photobiological, photochemical, and thinfilm solar processes.
4. To capture and store (“sequester”) the carbon dioxide by-product of hydrogen production
from coal. Coal is a massive energy resource that has the potential for producing costcompetitive hydrogen. However, coal processing generates large amounts of CO2. In order to
reduce CO2 emissions from coal processing in a carbon-constrained future, massive amounts
of CO2 would have to be captured and safely and reliably sequestered for hundreds of years.
Key to the commercialization of a large-scale, coal-based hydrogen production option (and
also for natural-gas-based options) is achieving broad public acceptance, along with
additional technical development, for CO2 sequestration”.
State of the art of the hydrogen Technologies: Several exhaustive reports dealing with
different aspects of the hydrogen economy were published in the last couple of years in USA,
Japan and the European Community. These reports arriving at similar conclusions served as a
basis for establishing major national hydrogen programs in these countries where hydrogen and
fuel cells are considered to be core technologies for the 21st century important for economic
prosperity. A summary of the state of the art of hydrogen technologies detailed in such reports 2
was recently published in Physics Today 3 , relevant excerpts are reproduced here with some
edititing to shorten text:
2
US Department of Energy, Office of Basic Energy Sciences, Basic Research Needs for the Hydrogen Economy,
US DOE, Washington, DC (2004), available at http://www.sc.doe. gov/bes/hydrogen.pdf; Basic Energy Sciences
Advisory Committee, Basic Research Needs to Assure a Secure Energy Future, US DOE, Washington, DC (2003),
available at http://www.sc.doe.gov/bes/reports/files/SEF_rpt.pdf; Committee on Alternatives and Strategies for
Future Hydrogen Production and Use, The Hydrogen Economy: Opportunities, Costs, Barriers, and R&D Needs,
National Research Council, National Academies Press, Washington, DC (2004), available at
http://www.nap.edu/catalog/10922.html.
3
George W. Crabtree, Mildred S. Dresselhaus, and Michelle V. Buchanan, The Hydrogen Economy, Physics Today,
December 2004. Available at http://www.physicstoday.org/pt/vol-57/iss-12/p39.html
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Hydrogen as energy carrier: Hydrogen does not occur in nature as the fuel H2. Rather, it occurs
in chemical compounds like water or hydrocarbons that must be chemically transformed to yield
H2. Hydrogen, like electricity, is a carrier of energy, and like electricity, it must be produced
from a natural resource. At present, most of the world's hydrogen is produced from natural gas
by a process called steam reforming. However, producing hydrogen from fossil fuels would rob
the hydrogen economy of much of its raison d'être: Steam reforming does not reduce the use of
fossil fuels but rather shifts them from end use to an earlier production step; and it still releases
carbon to the environment in the form of CO2. Thus, to achieve the benefits of the hydrogen
economy, we must ultimately produce hydrogen from non−fossil resources, such as water, using
a renewable energy source.
Figure 1. The hydrogen economy as a
network of primary energy sources
linked to multiple end uses through
hydrogen as an energy carrier.
Hydrogen adds flexibility to energy
production and use by linking naturally
with fossil, nuclear, renewable, and
electrical energy forms: Any of those
energy sources can be used to make
hydrogen.
Figure 1 depicts the hydrogen economy as a network composed of three functional steps:
production, storage, and use. There are basic technical means to achieve each of these steps, but
none of them can yet compete with fossil fuels in cost, performance, or reliability. Even when
using the cheapest production method—steam reforming of methane—hydrogen is still four
times the cost of gasoline for the equivalent amount of energy. And production from methane
does not reduce fossil fuel use or CO2 emission. Hydrogen can be stored in pressurized gas
containers or as a liquid in cryogenic containers, but not in densities that would allow for
practical applications—driving a car up to 500 kilometers on a single tank, for example.
Hydrogen can be converted to electricity in fuel cells, but the production cost of prototype fuel
cells remains high: $3000 per kilowatt of power produced for prototype fuel cells (mass
production could reduce this cost by a factor of 10 or more), compared with $30 per kilowatt for
gasoline engines.
The gap between the present state of the art in hydrogen production, storage, and use and
that needed for a competitive hydrogen economy is too wide to bridge in incremental
advances. It will take fundamental breakthroughs of the kind that come only from basic
research.
Beyond reforming: Almost all of the hydrogen currently produced is by reforming natural gas.
The challenge is to find inexpensive and efficient routes to create hydrogen in sufficient
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quantities from non−fossil natural resources. The most promising route is splitting water, which
is a natural carrier of hydrogen. It takes energy to split the water molecule and release hydrogen,
but that energy is later recovered during oxidation to produce water. To eliminate fossil fuels
from this cycle, the energy to split water must come from non−carbon sources, such as the
electron−hole pairs excited in a semiconductor by solar radiation, the heat from a nuclear reactor
or solar collector, or an electric voltage generated by renewable sources such as hydropower or
wind.
The direct solar conversion of sunlight to H2 is one of the most fascinating developments in
water splitting. Established technology splits water in two steps: conversion of solar radiation to
electricity in photovoltaic cells followed by electrolysis of water in a separate cell. The two
processes, however, can be combined in a single nanoscale process: Photon absorption creates a
local electron−hole pair that electrochemically splits a neighboring water molecule. The
efficiency of this integrated photochemical process can be much higher, in principle, than the
two sequential processes. The technical challenge is finding robust semiconductor materials that
satisfy the competing requirements of nature.
Water can be split in thermochemical cycles operating at elevated temperatures to facilitate the
reaction kinetics. Heat sources include solar collectors operating up to 3000°C or nuclear
reactors designed to operate between 500°C and 900°C. More than 100 types of chemical cycles
have been proposed. At high temperatures, thermochemical cycles must deal with the tradeoff
between favorable reaction kinetics and aggressive chemical corrosion of containment vessels.
Separating the reaction products at high temperature is a second challenge: Unseparated mixtures
of gases recombine if allowed to cool. But identifying effective membrane materials that
selectively pass hydrogen, oxygen, water, hydrogen sulfate, or hydrogen iodide, for example, at
high temperature remains a problem. Dramatic improvements in catalysis could lower the
operating temperature of thermochemical cycles, and thus reduce the need for high−temperature
materials, without losing efficiency.
Bio−inspired processes offer stunning opportunities to approach the hydrogen production
problem anew. The natural mechanisms for producing hydrogen involve elaborate protein
structures that have only recently been partially solved. …The hope is that researchers can
capitalize on nature's efficient manufacturing processes by fully understanding molecular
structures and functions and then imitating them using artificial materials.
Storing hydrogen: Storing hydrogen in a high−energy−density form that flexibly links its
production and eventual use is a key element of the hydrogen economy. The traditional storage
options are conceptually simple—cylinders of liquid and high−pressure gas. Industrial facilities
and laboratories are already accustomed to handling hydrogen both ways. These options are
viable for the stationary consumption of hydrogen in large plants that can accommodate large
weights and volumes. Storage as liquid H2 imposes severe energy costs because up to 40% of its
energy content can be lost to liquefaction.
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Figure 2. The energy densities of hydrogen fuels stored
in various phases and materials are plotted, with the
mass of the container and apparatus needed for filling
and dispensing the fuel factored in. Gasoline
significantly outperforms lithium−ion batteries and
hydrogen in gaseous, liquid, or compound forms. The
proposed DOE goal refers to the energy density that the
US Department of Energy envisions as needed for viable
hydrogen−powered transportation in 2015.
For transportation use, the on−board storage of hydrogen is a far more difficult challenge. Both
weight and volume are at a premium, and sufficient fuel must be stored to make it practical to
drive distances comparable to gas−powered cars. Figure 2 illustrates the challenge by showing
the gravimetric and volumetric energy densities of fuels, including the container and apparatus
needed for fuel handling. For hydrogen, that added weight is a major fraction of the total. For
on−vehicle use, hydrogen need store only about half of the energy that gasoline provides because
the efficiency of fuel cells can be greater by a factor of two or more than that of internal
combustion engines. Even so, the energy densities of the most advanced batteries and of liquid
and gaseous hydrogen pale in comparison to gasoline.
Meeting the volume restrictions in cars or trucks, for instance, requires using hydrogen stored at
densities higher than its liquid density. The volume density of hydrogen stored in several
compounds and in some liquid hydrocarbons is higher than the liquid or the compressed gas at
10 000 psi (700 bar).
The two challenges for on−vehicle hydrogen storage and use are capacity and cycling
performance under the accessible on−board conditions of 0−100°C and 1−10 bars. To achieve
high storage capacity at low weight requires strong chemical bonds between hydrogen and
light−atom host materials in stable compounds, such as lithium borohydride (LiBH4). But to
achieve fast cycling at accessible conditions requires weak chemical bonds, fast kinetics, and
short diffusion lengths, as might be found in surface adsorption. Thus, the high−capacity and
fast−recycling requirements are somewhat in conflict. Many bulk hydrogen−storage compounds
contain high volumetric hydrogen densities but require temperatures of 300°C or more at 1 bar to
release their H2. Compounds with low−temperature capture and release behavior have low
hydrogen−mass fractions and are thus heavy to carry. (Moreover hydrogenation/dehydrogenation
is energy intensive and require high capacity heat exchange systems). Hydrogen absorption on
high surface area materials is a potential route to fast cycling, but up until now results show that
the hydrogen uptake is very small at ambient conditions.
Hydrogen Use: A major attraction of hydrogen as a fuel is its natural compatibility with fuel
cells. The higher efficiency of fuel cells—currently 60% compared to 22% for gasoline or 45%
for diesel internal combustion engines—would dramatically improve the efficiency of future
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energy use. Coupling fuel cells to electric motors, which are more than 90% efficient, converts
the chemical energy of hydrogen to mechanical work without heat as an intermediary.
Although fuel cells are more efficient, there are also good reasons for burning hydrogen in heat
engines for transportation. Jet engines and internal combustion engines can be rather easily
modified to run on hydrogen instead of hydrocarbons. Internal combustion engines run as much
as 25% more efficiently on hydrogen compared to gasoline and produce no carbon emissions.
BMW, Ford, and Mazda are road− testing cars powered by hydrogen internal combustion
engines that achieve a range of 300 kilometers, and networks of hydrogen filling stations are
being implemented in some areas of the US, Europe, and Japan. Such cars and filling stations
could provide an early start and a transitional bridge to hydrogen fuel−cell transportation.
The versatility of fuel cells makes them workable in nearly any stationary or mobile application
where electricity is useful. Europe already has a demonstration fleet of 30 fuel−cell buses
running regular routes in 10 cities, and Japan is poised to offer fuel−cell cars for sale. Fuel−cell
power for consumer electronics like laptop computers, cell phones, digital cameras, and audio
players provide more hours of operation than batteries at the same volume and weight. Although
the cost per kilowatt is high for these small units, the unit cost can soon be within an acceptable
consumer range. Electronics applications may be the first to widely reach the consumer market,
establish public visibility, and advance the learning curve for hydrogen technology.
A host of fundamental performance problems remain to be solved before hydrogen in fuel cells
can compete with gasoline. [Technological breaktroughs are needed for simultaneously
improving fuel cell performance, reliability and cost]. The heart of the fuel cell is the ionic
conducting membrane that transmits protons or oxygen ions between electrodes while electrons
go through an external load to do their electrical work. Each of the half reactions at work in that
circuit requires catalysts interacting with electrons, ions, and gases traveling in different media.
Designing nanoscale architectures for these triple percolation networks that effectively
coordinate the interaction of reactants with nanostructured catalysts is a major opportunity for
improving fuel−cell performance. The trick is to get intimate contact of the three phases that
coexist in the cell—the incoming hydrogen or incoming oxygen gas phase, an electrolytic
proton−conducting phase, and a metallic phase in which electrons flow into or from the external
circuit.
A primary factor limiting proton−exchange−membrane (PEM) fuel−cell performance is the slow
kinetics of the oxygen reduction reaction at the cathode. Even with the best platinum−based
catalysts, the sluggish reaction reduces the voltage output of the fuel cell from the ideal 1.23 V to
0.8 V or less when practical currents are drawn. This voltage reduction is known as the oxygen
overpotential. The causes of the slow kinetics, and solutions for speeding up the reaction, are
hidden in the complex reaction pathways and intermediate steps of the oxygen reduction
reaction. It is now becoming possible to understand this reaction at the atomic level using
sophisticated surface−structure and spectroscopy tools combined with equally powerful and
impressive computational quantum chemistry.
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Beyond the oxygen reduction reaction, fuel cells provide many other challenges. The dominant
membrane for PEM fuel cells is perfluorosulfonic acid (PFSA), a polymer built around a C−F
backbone with side chains containing sulfonic acid groups (SO3−) (for example, Nafion). Beside
its high cost, this membrane must incorporate mobile water molecules into its structure to enable
proton conduction. That restricts its operating temperature to below the boiling point of water. At
this low temperature—typically around 80°C— expensive catalysts like platinum are required to
make the electrochemical reactions sufficiently active, but even trace amounts of carbon
monoxide in the hydrogen fuel stream can poison the catalysts. A higher operating temperature
would expand the range of suitable catalysts and reduce their susceptibility to poisoning.
Promising research directions for alternative proton−conducting membranes that operate at
100−200°C include sulfonating C−H polymers rather than C−F polymers, and using inorganic
polymer composites and acid−base polymer blends.
Solid oxide fuel cells (SOFCs) require O−2 transport membranes, which usually consist of
perovskite materials containing specially designed defect structures that become sufficiently
conductive only above 800°C. The high temperature restricts the construction materials that can
be used in SOFCs and limits their use to special environments like stationary power stations
where adequate thermal insulation and safety can be ensured. Finding new materials that conduct
O−2 at lower temperatures would significantly expand the range of applications and reduce the
cost of SOFCs.
Safety: The public acceptance of hydrogen depends not only on its practical and commercial
appeal, but also on its record of safety in widespread use. The special flammability, buoyancy,
and permeability of hydrogen present challenges to its safe use that are different from, but not
necessarily more difficult than, those of other energy carriers. Researchers are exploring a
variety of issues: hydrodynamics of hydrogen−air mixtures, the combustion of hydrogen in the
presence of other gases, and the embrittlement of materials by exposure to hydrogen, for
example. Key to public acceptance of hydrogen is the development of safety standards and
practices that are widely known and routinely used—like those for self−service gasoline stations
or plug−in electrical appliances. The technical and educational components of this aspect of the
hydrogen economy need careful attention.
Technical progress will come in two forms. Incremental advances of present technology provide
low−risk commercial entry into the hydrogen economy. Those advances include improving the
yield of natural−gas reforming to lower cost and raise efficiency; improving the strength of
container materials for high−pressure storage of hydrogen gas; and tuning the design of internal
combustion engines to burn hydrogen. To significantly increase the energy supply and security,
and to decrease carbon emission and air pollutants, however, the hydrogen economy must go
well beyond incremental advances. Hydrogen must replace fossil fuels through efficient
production using solar radiation, thermochemical cycles, or bio−inspired catalysts to split water.
Hydrogen must be stored and released in portable solid−state media, and fuel cells that convert
hydrogen to electrical power and heat must be put into widespread use. Achieving these
technological milestones while satisfying the market discipline of competitive cost, performance,
and reliability requires technical breakthroughs that come only from basic research.
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