September 28, 2012
Researchers from NYU 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.
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.
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 unfavorable for this wire to sustain this proton motion. Hence, the system then seeks to resolve this unfavorable 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.
The study’s authors were: Mark Tuckerman, a professor in NYU’s Department of Chemistry, as well as Linas Vilciauskas, Gabriel Bester, and Klaus-Dieter Kreuer from the Max Planck Institute, and Stephen J. Paddison of the University of Tennessee, Knoxville.