Solid: Protons pdf
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Solid: Protons
J Tolchard, Norwegian University of Science and Technology, Trondheim, Norway
& 2009 Elsevier B.V. All rights reserved.
Introduction
It may seem something of a truism to describe an
element as unique, but hydrogen is certainly one of the
more unusual elements in the periodic table. It is the
lightest element and consists of only a proton and an
electron and is thus the only element not to contain a
neutron in its main isotope (
1
H). In its stable molecular
form (H
2
), it is also the least dense and the most abun-
dant in the universe. Perhaps surprisingly, given that its
low density results in gaseous hydrogen being almost
nonexistent in the Earth’s lower atmosphere, it is also the
third most abundant element on the Earth’s surface. The
key to this of course lies in its chemistry, and this is where
its uniqueness is truly apparent: In compounds, hydrogen
shows remarkable flexibility in its ability to bond to both
‘covalent’ and ‘ionic’ systems. It also demonstrates a
unique bonding capability in the form of the hydrogen
bond, which, although weak, is crucial in properties as
seemingly unrelated as the boiling point of water, the
structure of DNA, and, of course the focus of this work,
proton conductivity.
So why are proton-conducting electrolytes so inter-
esting? Basically it comes down to the reaction
H
2
þ
1
2
O
2
-H
2
O ½I
This simple reaction releases a significant quantity of
energy (DG
f
¼À237 kJ mol
À1
at 25 1C) with the only
by-product being pure water. Hydrogen is thus an ex-
tremely attractive replacement to the hydrocarbon fuels
widely used today. However, to achieve this replacement, a
means to both generate large quantities of H
2
and utilize it
in a safe and efficient manner is required. There are several
ways to achieve both these goals, but probably the most
promising, and the most efficient in terms of utilization, is
electrochemical conversion. Electrochemical conversion
processes offer the ability to directly convert the energy
released in reaction [I] to electricity, thus avoiding the
inefficiencies of the combustion–mechanical conversion
systems used presently. They also offer a further advantage
over combustion technologies in that reaction [I] is re-
versible and by inputting energy to the reactor rather than
extractingit,itispossibletogeneratelargequantitiesof
hydrogen from one of the most abundant compounds on
Earth – water. From a cost and development perspective,
both the generation and utilization devices can be built
around largely the same materials and technologies. At the
core of these electrochemical devices is an electrolyte – the
reason for our interest again.
What Is an Electrolyte?
An electrolyte is defined by the basic property that it
conducts electrical charge in the form of ions (charged
atoms) but not in the form of electrons. That is, it is
ionically conducting but electronically insulating. It is
also generally the case that such a material will show a
high degree of specificity in the ions that are transported.
In application, the function of an electrolyte is to sep-
arate the electrodes of an electrochemical device,
blocking electronic conduction but allowing charge to
pass in the form of selected ions (e.g., protons). There are
numerous examples of electrolytes (in similarly numer-
ous applications) which could be used for demonstrative
purposes, but the most familiar tend to be liquids (such as
a molten or aqueous metal salt) and thus fall outside the
scope of this article. Also, in many of these examples,
there is a complication in that the function of the elec-
trolyte can vary slightly between applications. For ex-
ample, in electroplating the electrolyte may perform the
dual function of being both an ionic conductor and a
reactant, whereas in a rechargeable lithium battery the
electrolyte functions solely as a transport medium, being
inert with respect to the overall reaction occurring. The
solid proton conductors fall into the latter category, with
the electrolyte material itself not being consumed in the
electrochemical cell reactions. An excellent example of
this function, given in Figure 1, is of a generic fuel cell
based around a proton-conducting electrolyte. It has to
be noted here that such a device can also perform re-
action [I] via an oxide ion (O
2À
) conductor. However, a
proton conductor offers an advantage over this in that the
migration kinetics are rather faster, and so the cell can
run at lower temperatures.
The basic operation of the cell is relatively simple:
Hydrogen is oxidized at the anode to give protons and
electrons. The protons then travel through the electro-
lyte to recombine with the electrons, which travel via an
external circuit, and react with oxygen to produce water.
The useful energy (i.e., work) is extracted from the sys-
tem directly via the electrical circuit. The driving force
for this process is the difference in chemical potential or
activity between hydrogen at the anode and that at the
cathode, and for an ideal electrochemical system (i.e., no
losses), this can be related to the cell voltage via the
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