Researchers from New York University
and the Max Planck Institute in Stuttgart
reveal how protons move in phosphoric acid in a Nature Chemistry study
that sheds new light on the workings of a promising fuel cell electrolyte.
Phosphoric acid fuel cells were
the first modern fuel cell types to be used commercially and have found
application as both stationary and automotive power sources. Their high
efficiency as combined power and heat generators make them attractive targets
for further development. In the cell, phosphoric acid functions as the medium
(or “electrolyte”) that transports protons produced in the reaction that
decomposes the fuel across the cell. Indeed, phosphoric acid has the highest
proton conductivity of any known substance, but what makes it work so well as a
proton conductor has remained a mystery.
Efficient proton transport across
a fuel cell is just one of several technical challenges that must be tackled
before this technology can be applied on a massive scale. The key to this
problem is the identification of a suitable electrolyte material. Hydrated
polymers are often employed, but these must operate at temperatures below the
boiling point of water, which limits their utility. Phosphoric acid fuel cells
and other phosphate-based cells, by contrast, can be operated at substantially
higher temperatures.
Chemists have sought a molecular
level understanding of proton conduction phenomena for more than 200 years. The
earliest studies concerned water and can be traced back to a landmark paper in
1806 by the German chemist Theodor von Grotthuss. In this paper, Grotthuss
suggested that excess protons in aqueous acids are not themselves transported,
but rather it is the chemical bonding pattern they create that is transported
via a series of short hops of protons between neighboring water molecules. Such
hops occur through the hydrogen bonds that connect water molecules into a
network.
One can liken this process to an
old-time fire brigade in which each fireman in a long line holds a bucket of
water in his left hand. A fireman at the end of the line receives a new water
bucket in his right hand, so in order to make the transport of water down the
line as efficient as possible, he passes the bucket in his left hand to the
right hand of his neighbor. The neighbor, who now holds buckets in his left and
right hands, passes the bucket in his left hand to the right hand of the next
fireman in the line, and the process continues like this until the person at
the opposite end of the line holds two buckets. Overall, water is transported
down the line, but it is not the same bucket being passed in each transfer.
Of course, the transport of
excess protons in water is not this simple—it involves complex rearrangements
of the hydrogen bonds at each transfer step to accommodate the diffusing
chemical bonding pattern. Because of this, proton transport in water appears to
be a step-wise process. Water faces other limitations—it cannot function as an
intrinsic proton conductor but must have protons added to it to create aqueous
acid solutions before any noticeable proton transport occurs.
The Nature Chemistry
study contrasted proton conduction in phosphoric acid with excess protons in
aqueous solutions. In their work, the researchers carried out a type of
“computerized experiment” or “simulation” in which no prior knowledge of the
chemical processes is required. The only input is the atomic composition of
phosphoric acid (hydrogen, oxygen, and phosphorus). Based on this input, the
atoms’ motion in time is determined from the fundamental laws of physics. In
this way, the proton conduction mechanism can be allowed to unfold and be
discovered directly from the simulation output.
Their results showed that proton
motion in phosphoric acid is a highly cooperative process that can involve as
many as five phosphoric acid molecules at a time serving as a kind of temporary “proton wire” or chain. The basic findings are:
• In contrast to the step-wise
mechanism that operates in water, phosphoric acid transfers protons in a more “streamlined” fashion, in which protons move in a concerted manner along one of
these temporary wires.
• Eventually, it becomes
energetically unfavourable for this wire to sustain this proton motion. Hence,
the system then seeks to resolve this unfavourable condition by breaking one of
the hydrogen bonds in this temporary wire and forming a new wire arrangement
with other nearby phosphoric acid molecules. New wire arrangements persist
until they can no longer sustain the proton motion in them, at which point they
break and new wires are formed. This process of forming and breaking the short
wires allows for a steady proton current and overall high proton conductivity.
Although phosphoric acid has its
advantages in fuel cell applications, phosphoric acid fuel cells still are not
as powerful as other types of cells and, as pure power sources, are not as
efficient. However, an understanding of the basic proton transport mechanism
can help improve the design of such cells or suggest other phosphate based
materials that could serve as the proton carrier.