Post of the Day
April 12, 2000
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Limits to Fuel Cell Efficiency
I admit to being fascinated by the elegance of the concept of fuel cells (like many of you, no doubt!), so when I found out that the meeting of the American Physical Society a couple of weeks ago included an all-day session on fuel cells, I went to learn some more about the science of PEM (polymer electrolye membrane) fuel cells. There was a lot of very interesting information presented, some of which I will try to condense here. It's not too technical and involves some pretty practical issues (especially regarding efficiency) that people should be aware about when evaluating the technology.
All fuel cells use a simple oxidation reaction to generate power. There is nothing special about this, it's just a controlled version of combustion. What is special about fuel cells is that the reactants are physically separated in such a way that in order to complete the reaction, the electrons involved in the oxidation process must travel through an external circuit. This is achieved by separating the electrodes with an electrolyte; in the case of PEM fuel cells, this is a polymer membrane (hence the name�). The task of the PEM is to let ions but not electrons through to the cathode.
A lot of work has gone into finding the best PEM compound. The grand-daddy of PEMs is a co-polymer of Teflon and perfluorosulfonate, where the sulfonate groups are the active elements in H+ ion transport across the membrane and the fluorine helps to increase the conductance. I gather that Ballard uses a more efficient PEM, but details were not given at the conference (presumably it is proprietary).
What happens at the membrane is the following. First a H2 molecule physisorbs and chemisorbs (ie, sticks to the surface) on a catalyst particle (platinum) and dissociates. The two H atoms are reduced (lose an electron) and bond to 2 H2O molecules to form two H3O+ hydronium ions. The hydronium ions migrates across the membrane to the cathode. Here, an O2 molecule dissociates into two O atoms, one of which joins up with the hydronium ions and the two electrons taken at the anode from the H atoms to form 3 H2O molecules: anode: H2 + 2 H2O --> 2 H3O(+) + 2 e(-) cathode: (1/2)O2 + 2 H3O(+) + 2 e(-) --> 3 H2O
The net reaction is therefore the familiar H2 + (1/2)O2 --> H2O. Notice, though, that water is not just a byproduct: it is also a crucial intermediary without which no reaction will take place. Not only does the ionic conductivity of the PEM depend on the water concentration, but so does the catalyst dissociating kinetics. In fact, water management is one of the practical problems that places limits on how efficient fuel cells can be. In order for the fuel cell to keep operating, the water that is carried across the membrane in the hydronium ions must return back to the anode. But it turns out that the hydronium flow towards the cathode typically prevents this from happening easily, so the anode tends to want to dry out, reducing the effectiveness of the fuel cell. When fuel cells are operated at high current densities (which is what is wanted for practical applications), the efficiency of the fuel cell is limited by the transport of the reactants. This is why so much effort has gone into making very thin membranes: so that the back-diffusion of the water will be enhanced, increasing the current density that can be achieved.
As far as efficiency goes, fuel cells are rated in terms of the voltage produced by a single cell compared to the maximum possible open circuit voltage (OCV, the maximum possible voltage that would be produced if no current were being drawn from the fuel cell). Electrochemistry says that 237.1 kJ of energy are released per mole of H (to liquid water product), equivalent to an OCV of 1.229 V. Taking into account factors like temperature and pressure of operation, atmospheric partial pressure of O2, and typically gaseous products, the actual maximum possible OCV is 1.177 V.
Ballard was mentioned as having the most efficient fuel cell stacks available today, producing about 0.7V at typical current densities (higher current reduces the voltage pretty rapidly), for a stack efficiency of about 60%. Hydrogen utilisation can typically be kept at about 95%, while parasitic losses in the system as a whole tend to be about 10%, so we get that Ballard's fuel cells are about 50% efficient, which is phenomenally good.
Unfortunately, this is not the whole story. The problem is that Ballard's design requires high pressure operation in order to manage the water at the anode properly and keep the ionic conductivity of the membrane high. The means that some of the energy of the fuel cell has to be used to run a compressor to pressurise the stack. I was told they get about a 20% loss in efficiency just from this that has to be added on to the figure above, bringing down the total efficiency of the overall system to about 40%. Other designs of PEM fuel cells avoid the use of high pressures and associated power losses by hydrating the anode directly, but this lowers the stack OCV a lot, so that there is not really any gain in efficiency.
I hope this is understandable--the main point is that water management and the tricks used to deal with hydrating the anode are big issues in terms of limiting efficiency of fuel cells, something that I for one did not really appreciate!!
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