Fuel cells
The whole history of mankind has been based on the
utilization of the energy stored in the form of the
chemical energy of fuels: wood, coal, oil, gas. Energy
is critical to global human development and secure
long-term energy supply is one of the largest
challenges for mankind in the 21st century. In this
context, the development of fuel cells may appear as a
good solution for the Future.
Introduction
In addition to the slowly diminishing
availability of fossil fuels and concerted price
rise, environmental and climate concerns play
increasingly important roles and call for
concerted efforts to change the present energy
system to a sustainable one. In order to do so,
the human race needs to learn how to replenish
the natural energy resources using renewable
energy sources such as solar light, wind,
biomass, etc. We must be aware of the fact that
within the next century natural resources of oil
and gas will be greatly diminished, if not fully
consumed, and must develop new infrastructure
based on renewable energy. This energy must be
converted in the chemical energy of a suitable
fuel which can then be utilized on demand,
heating our houses, powering large industrial
installations, vehicles and portable electronic
devices such as laptops, cellular phones, etc. An
essential element in this sustainable energy
system is a fuel cell, which is a device converting
chemical energy of a fuel into electrical energy.
will soon place the Siemens generator in the
museum.” Due to easily accessible and large
amounts of oil and the invention of the
combustion engine, fuel cells were unfortunately
forgotten until the middle of the 20th century. In
the US Apollo space program, fuel cells
exhibited their first renaissance in the 1960’s.
Principle of a fuel cell
The principle of a fuel cell is explained in the
Figure showing a polymer electrolyte membrane
(PEM) fuel cell fed with hydrogen. The heart of
a fuel cell consists of a membrane-electrode
assembly (MEA), which is a sandwich consisting
of an ion conducting membrane, and two
catalytic and gas diffusion layers (Figure).
e-
H2
O2
+
H+
+
+
H2O
+
+
+
History of fuel cells
The fuel cell effect was discovered in 1838 by
Professor Christian Friedrich Schoenbein from
the University of Basel, who observed electrical
current caused by combining hydrogen and
oxygen. This discovery was documented in the
"Philosophical Magazine" in January 1839.
Schoenbein was in correspondence with his
friend Sir William Robert Grove, British jurist
and amateur physicist who, inspired by the idea
of Schoenbein, continued experiments. The first
fuel cell power generator was presented by
Grove in 1843/1845. The term “fuel cell” was
coined later in 1889 by Ludwig Mond and
Charles Langer, who attempted to build the first
practical device using air and industrial coal gas.
Wilhelm Ostwald (later a Nobel prize winner)
said in 1884: “The fuel cell is a larger invention
for the civilisation than the steam machine and
Nafion®
carbon particles
Pt particles
Cathode: O2 + 4H+ + 4e- → 2H2O
Anode:
H2 → 2H+ + 2e-
Cell reaction: 2H2 + O2 → 2H2O
Figure. Schematic representation of a polymer
electrolyte membrane (PEM) fuel cell
Hydrogen is ionized at the anode to produce
protons and electrons. These flow to the
cathode: the electrons through the external
electric circuit, while the protons through a
membrane, separating the anode and the
cathode compartments. At the cathode catalytic
layer (also composed of Pt/C) an oxygen
molecule accepts four electrons and two
protons, producing water.
This allows the most efficient transformation of
the energy of the fuel into the electrical energy.
Other advantages of fuel cells are environmental
friendliness (for example, a hydrogen fuel cell
does not produce any pollutants, its only
product being water vapor), quiet operation
(which is especially important in large cities,
where noise becomes one of the serious risk
factors).
Fuel cell types and components
The catalytic layers of the state of the art PEM
fuel
cells
are
multi-component
media
comprising a catalyst, accelerating the rates of
electrochemical reactions; ionomer, providing
flow of protons through the MEA; and gas- and
liquid-filled pores, providing access of the
reagents to and the products from the catalyst
surface. The catalytic layers of the early PEM
fuel cells were prepared from noble metal blacks
and thus contained very high metal loadings per
geometric area. Later, new generation of PEM
fuel cells emerged, based on carbon supported
precious metal catalysts (usually Pt or Pt-based
alloys). State of the art PEM fuel cells utilize
proton conducting polymer membranes such as
Nafion®. Novel catalytic and membrane
materials are under development.
Since the early days of fuel cells various
concepts have been developed and laid ground
to the appearance of phosphoric acid, alkaline,
PEM, solid oxide and other types of fuel cells.
Depending on the type of electrolyte they
operate in different temperature windows and
thus have various niche applications. Thus, PEM
fuel cells fed by hydrogen are considered as
potential power sources for vehicle propulsion,
direct methanol fuel cells (DMFCs) fed by
methanol are ideal for portable applications.
They are much more convenient and durable
power sources compared to batteries. Sold oxide
fuel cells are very interesting for stationary
generation of electricity at large installations and
co-generation of the electricity and heat.
Present status and future
developments
Numerous research institutions and industrial
companies are nowadays involved in the
research and development of novel efficient
materials for fuel cells. This resulted in a
spectacular improvement of the efficiency of the
electricity production in fuel cells over the last
decades. There is still room for improvement in
terms of the power output per gram of noble
metal, the overall price of fuel cell stacks and
their durability. However, it is important to
mention that fuel cells have overcome the
commercialization barrier and have entered the
market. Today a consumer may buy, for
example, a direct methanol fuel cell stack and
power her/his recreation vehicle in a remote
place for a few weeks. Most of the car
manufacturers have developed car prototypes
powered
by
PEM
fuel
cells.
Wide
commercialization of fuel cells, however,
critically depends on the following factors:
-
-
-
Public awareness of the problems concerned
with the exhaustion of fossil fuels and
pollution caused by their burning in internal
combustion engines;
Political decisions and requirements to
decrease emission of greenhouse gases and
pollutants;
vailability of novel materials allowing
cheap, efficient and durable energy
transformation and storage.
In fact, energy storage is one of the key issues
for fuel cell commercialization. While hydrogen
appears to be promising ecologically safe fuel,
its storage is a big issue. Novel materials are in
need which allows reversible, efficient, cost
effective storage of hydrogen. Various materials
are nowadays under investigation, including
metal hydrides, metal organic frameworks, etc.
References
1. Handbook of Fuel Cells. Fundamentals,
Technology and Applications, W. Vielstich, A.
Lamm, H.A. Gasteiger, Editors, 2003, John Wiley
& Sons, Chichester.
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