Clustering Water, Hopping Protons
Scientists take unique research approaches to fuel cell puzzle
Like skipping stones across a pond, protons hop along water networks in fuel cell membranes – when conditions are just right. Scientists at Pacific Northwest National Laboratory (PNNL) and partners in India and Germany used a novel modeling method for the first time to study protons in fuel cell membranes. This research was featured on a 2010 cover of Journal of Physical Chemistry B. Enlarge Image
Results: Like skipping stones across a pond, protons hop along water networks in fuel cell membranes—when conditions are just right. Scientists at Pacific Northwest National Laboratory (PNNL) and partners in India and Germany used a novel modeling method for the first time to study protons in fuel cell membranes. Quantum hopping molecular dynamics, a new computer simulation model of physical movements of protons in a fuel cell, was combined with percolation theory to better understand how water networks form and sustain proton hopping in a polymer fuel cell membrane.
The results of this research demonstrate the critical point at which isolated water clusters form a network of connected molecules and become a meandering, three-dimensional "conductivity wire" across the membrane. At this critical point, proton transport—and electrical conductivity—becomes possible. This research was featured on the cover of the November 4, 2010, edition of Journal of Physical Chemistry B.
Why it matters: With the potential to revolutionize transportation and portable power, as well as bringing electricity to remote populations off the power grid, fuel cells are among the most promising technologies on the alternate energy horizon. Fuel cells convert chemical energy to electricity, and the only byproducts are heat and water. The key challenge to more widespread commercialization is cost reduction.
Within the fuel cell, water provides the hydration network on which protons "hop" across proton exchange membranes, creating an electric current. However, at temperatures below 100 degrees Celsius, highly expensive platinum catalysts become fouled and increasingly inefficient over time. Above 120 degrees Celsius, catalysts operate cleanly and efficiently, but water molecules evaporate, and with them, the water network required for proton transport.
By developing greater understanding about the water network formation, the optimum hydration level, and the physical models of the network in fuel cells, scientists believe a new generation of fuel cells can be developed to operate at higher temperatures with greater efficiency and lower cost. And at temperatures above 120 degrees Celsius, fuel cells can use an inexpensive metal or alloy in place of platinum, eliminating a significant cost barrier to widespread commercialization.
This research has built a greater understanding of water percolation and the dynamics of proton transfer. It may also help in the design of polymer membrane materials that have lower water uptake, yet offer faster proton transfer and transport.
Methods: "We are focused on the fundamental science of these fuel cell membranes so that we can identify an alternative liquid that can still transport protons, but at high temperatures," said Dr. Ram Devanathan, a PNNL materials scientist. "It is a difficult puzzle, so we approached it from two unique modeling perspectives."
Percolation theory, a widely used research technique that provides a mathematic model to describe the behavior of connected clusters, was used to characterize the water network in fuel cells and network formation across proton exchange membranes. The analysis places the characterization of this complex molecular environment on firm theoretical footing, establishing atomic-level evidence of water network percolation and its effect on proton conductivity.
The team in Germany developed quantum hopping molecular dynamics, a computer simulation of physical movements of protons. This helped researchers calculate the mean residence times, rate constants and activation energies for proton transfer across proton exchange membranes in fuel cells.
Using the computational capabilities of EMSL, DOE's Environmental and Molecular Sciences Laboratory at PNNL, researchers performed a detailed analysis and simulation of water clustering and percolation in various configurations of hydrated Nafion, a widely used fuel cell membrane.
What's next: Using data gained in this work, PNNL researchers are already modeling ionic liquids to see how they can be used in proton exchange membranes at temperatures above 100 degrees Celsius. With new membrane design technology, the goal is to operate above 120 degrees Celsius. At that point, an inexpensive catalyst could be introduced, and fuel cell technology would become much less costly and more suitable for commercialization.
Acknowledgments: This research was funded by the Department of Energy Office of Basic Energy Sciences, Chemical Sciences, Geosceinces and Biosciences Division, and was performed by Ram Devanathan, Roger Rousseau, and Michel Dupuis, Pacific Northwest National Laboratory; Arun Venkatnathan, Indian Institute of Science Education and Research-Pune; Tomaso Frigato, Freie Universität Berlin; and Wei Gu, and Volkhard Helms, University of Saarland.
Reference: Devanathan R, A Venkatnathan, R Rousseau, M Dupuis, T Frigato, W Gu, and V Helms. 2010. "Atomistic Simulation of Water Percolation and Proton Hopping in Nafion Fuel Cell." Journal of Physical Chemistry B 114 (43):13681-13690. DOI: 10.1021/jp103398b.