Liquid assets

Although the widespread use of fuel cells in vehicles and the home is getting closer, there is still a great deal of research to be done on how the devices can be used safely and practically.



At the US’s Sandia National Laboratory in New Mexico, Ken Chen and Mike Hickner are using 2D and 3D modelling techniques to determine how water behaves inside fuel cells, and how it should be managed to produce the best energy output.



The work is focusing on polymer electrolyte membrane (PEM) cells which, because of their relatively high output and compact size, are generally believed to be the most versatile cell type for both domestic and automotive applications.



In these cells, the cathode and anode are positioned on either side of a polymer membrane. At the anode, electrons are stripped from the hydrogen atom, and the membrane only allows protons — hydrogen ions — through from anode to cathode, where they combine with oxygen ions to form water.



The electrons must flow through an external circuit to get to the cathode, which creates the electrical current. The membrane also prevents gases from permeating from one side of the cell to the other.



‘Water is a natural by-product of using hydrogen and oxygen to produce electricity in a PEM cell, with waste heat being the other,’ said Chen. ‘One challenge is maintaining the proper amount of water — a sufficient amount is needed in the membrane to maintain its conductivity, whereas too much can result in flooding the cathode gas diffusion layer. This prevents oxygen from reaching catalytic sites and causes performance deterioration.’



Chen is using computer modelling techniques to determine how the water is formed inside the cell, how it is transported around the system, and the best ways to remove it efficiently.



Hickner, meanwhile, is carrying out physical experiments to support and test Chen’s conclusions. The goal is to build a computational tool which can be used to design cells without experimenting and testing every component.



Chen’s software suite includes a multi-dimensional and multiphysics finite element analysis program called GOMA, developed at Sandia, which he uses to develop 2D models of the cells; and the proprietary computational fluid dynamics system FLUENT for producing 3D models of gas and liquid flow patterns.



‘Our approach to combining computational models with experiments is unique,’ said Chen. ‘Typically, Mike would perform discovery experiments to gain physical insights. I would then develop a model to describe the observation or data that he has obtained. He would then perform further experiments so I can validate the model I have developed.’



Until now, the two have focused on water, but the next phase of the research will see them looking at how the cells’ performance changes over time.



This will look at the degradation of performance, both under normal conditions and when the ambient temperature is extremely low. This is a major barrier to the use of fuel cells in vehicles, as many systems will not start up in sub-zero temperatures.