Debunking the CO2 Myth
Finding your way through the disinformation about alkaline fuel cells
For quite some time now there has been the misconception that the Alkaline fuel cell suffers from CO2 poisoning. The facts are rather different from those promoted by various mis-informed parties.
This fable of misconception has become so ingrained in the fuel cell folklore that we have decided to devote a separate section to this aspect and help to put the perceived problem in its proper context.
Contamination
The Alkaline fuel cell functions so efficiently because of its highly conductive electrolyte known as KOH (Potassium Hydroxide). In this electrolyte the Hydroxil-ions can move around fairly freely, unlike for example the solid polymer electrode types used in PEM fuel cells which provide for a much greater resistance and thus are less efficient at transferring the transient matter from Anode to Cathode.
The electrolyte can also be used very efficiently for the purpose of cooling the fuel cell. It is also excellent as a medium to absorb the water produced by the reaction of splitting the H2 atoms at the active surface.
It is true that KOH will absorb CO2, this is a fact and should be taken into account when designing and constructing an Alkaline fuel cell.
However, it is NOT true that CO2 poisons the Alkaline fuel cell.
Poisoning is a term used to describe an irreversible destructive process, such as for example CO poisoning of the catalyst in a PEM fuel cell which is an irreversible termination of the active process on the electrode.
Unlike PEM cells which do get permanently damaged from CO poisoning, accumulated CO2 in the electrolyte of an Alkaline fuel cell is simply removed with a change of liquid, just like an oil change in a car, only KOH does not need to be replaced nearly as often.
The CO2 that accumulates in the electrolyte will reduce the conductivity. In the same manner as a blocked air filter will reduce the performance and fuel consumption of the engine in your car. The air filter is part of the regular maintenance cycle and once replaced, the engine will perform as it did before.
The very same is true of the electrolyte in the Alkaline fuel cell. KOH (the electrolyte) is almost cheaper than water these days and its replacement is easily achieved.
As a fuel cell company we have run thousands and thousands of hours of system and electrode tests. In some instances we have run comparison tests to ascertain the severity of the problem of CO2 contamination in KOH. We have run several 6.5 kW systems with and without for many thousands of carefully monitored hours of running both under load and in a variety of conditions.
We have not found there to be any very significant influence of CO2 until well beyond 3000hrs when a small reduction (typically in the region of 8-10 percent can be detected.) in the system running without CO2 scrubber protection. New electrolyte immediately restores the performance of the system.
Systems with protection can achieve any number of hours given that the scrubber efficiency is in the region of 50 PPM capability. We have also run electrode tests with and without CO2 in the atmosphere, and in many cases with our new generation electrode, degradation caused by CO2 poisoning could not be detected. Tests run by Surrey University have shown no detectable degradation despite running all the electrodes without scrubbing the air stream feed.
There is also a wealth of independent evidence to support our findings and this can be found in the following articles:
Ref: Fuel Cell Review Issue February/March 2006 ‘ A renaissance for alkaline fuel cells’. PDF - 5.4mb
Ref: Journal of Power sources Elsevier 127 (2004) 243-251 ‘Long term operation of AFC electrodes with CO2 containing gases’. PDF - 500kb
Ref: International Journal of Hydrogen Energy 27 (2002) 507-526 ‘An assessment of AFC technology’
Ref: Journal of Power sources Elsevier 153 (2006) 1-10 ‘Quantification of carbon dioxide poisoning in air breathing AFC’s.