voltage is lost in order to compensate
for the lack of electro-catalytic activity.
As you can see in Figure 1, this is the
steep part of the voltage-current curve
on the left. Due to the chemical
energy barrier, the internal resistance
at the very beginning is very high
which accounts for the low current.
Once this barrier is overcome, the
resistance drops allowing for more
current flow and voltage drop. It’s
kind of like overcoming the starting
friction in a mechanical system,
except on a chemical basis.
Our experiments will confirm this
part of the curve in a slightly different
manner since the REEL Power software
does not plot voltage against current,
but rather voltage, current, and
resistance against time. Nevertheless,
it is interesting to see just how this
part of the fuel cell operates.
Ohmic polarization occurs due
to resistive losses in the cell. Since
the MEA obeys Ohm’s law (V = IR),
the amount of voltage lost in order to
force conduction varies linearly
throughout this region. The Ohmic
region is really the working area of
the fuel cell where — with enough
hydrogen and oxygen fuel — the
voltage and currents remain constant
for a given load. This is the flat part
of the curve in Figure 1, and just like
a modern lithium-ion or nickel metal
hydride battery (Part 3), the output
voltage and current remain stable
over time as long as the load remains
the same and fuel is available. Again,
our experiments will illustrate and
confirm this performance area.
In this region, the reactants
become consumed at greater rates
than they can be supplied as the fuel
begins to run out. Ultimately, these
effects inhibit further reaction
altogether, and the cell voltage and
current drop to zero very rapidly.
There’s a lot more chemistry in all
three of these processes, but it’s beyond
the scope of this article. The main points
to realize are that fuel cell behavior
resembles a modern rechargeable
battery and that its usefulness for
powering electrical systems is
generally equivalent to batteries.
The Fuel Cell’s
Another interesting thing about
the fuel cell’s performance is the
power curve. Figure 1 shows it to be
nearly linear right up to when the
fuel runs out, which means that the
fuel cell can supply a load (an
electric motor, for instance) strictly
based on demand. No power is lost
or expended “getting to” the
maximum power point. In contrast,
internal combustion engines operate
most efficiently only at full load and
exhibit a rapid decrease in efficiency
at partial loading. This fact alone may
create the basis for the popularity of
the electric car once it has a chance
to mature in the marketplace —
whether it’s powered by batteries or
a fuel cell, or a combination of both.
Fuel Cell Efficiencies
A fuel cell’s efficiency — basically
the ratio of power out versus power in,
or fuel consumed versus fuel supplied
— can be defined and measured in two
ways. As mentioned previously, one is
called Energy Efficiency and the other
is Faraday Efficiency. The symbol
used to express efficiency is generally
the Greek lower-case letter, eta (η).
We’ll measure both efficiencies in
our experiments, but here’s a
preview of what we’ll be measuring.
The Energy Efficiency of a fuel
cell is the ratio of the electricity
produced by the consumed hydrogen,
compared with the calculated
theoretical energy contained in the
consumed hydrogen; this is
expressed as a percentage. To figure
this out, we need to express a few
equations in chemical nomenclature
in order to determine the theoretical
energy contained in hydrogen. So,
get out your high school chemistry
book if you’re not familiar with some
of the following chemistry terms.
Breakthrough Producing Hydrogen from Water and Sunlight
Sunlight + Water = Hydrogen Gas
Led by Dr. Thomas Nann, scientists at the University of East Anglia report a breakthrough in
the production of hydrogen from water using the energy of sunlight. Amidst all the hype about
a potential hydrogen economy, one of the big questions has been whether sufficient hydrogen
can be produced without using yet more energy to create it (the main stumbling block thus
far). Typical production methods include stripping hydrogen from other fuels like methane
(called reforming) or using electrolysis to split the hydrogen out of water (as we have done).
But with efficiencies between 20% and 40% for producing energy from traditional photovoltaic
processes, the hydrogen economy cannot be solar powered. Or can it?
Dr. Namm’s group has discovered a more efficient way to do this using a gold electrode coated with nanoclusters of indium
phosphide to absorb incoming photons of light (that is the wavy line marked “hv” in the image). The nanoclusters then pass electrons
liberated by the sun’s energy into an iron-sulfur complex which acts like a match-maker between the negatively charged electron and a
hydrogen proton in the surrounding water molecules. Gaseous hydrogen is generated and the net result is a 60% efficiency for a
process in which hydrogen is produced from water by the photons in light that strike a specially designed submersed electrode. The
next step is to demonstrate the process with cheaper materials. The scientists report there is no special reason to use gold or platinum
— which was used as the second electrode to complete the circuit — other than these noble metals happened to be lying about the
lab (some lab!).
Source: Christine Lepisto, TreeHugger.com with edits by John Gavlik.
June 2010 47