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Pre-Cleaning Separator Sheets

From time to time, there will be a need for membrane pretreatment using various methods.  Membranes are functional materials with functional side groups that are designed to transfer certain ions.  Cation exchange membranes will allow the transfer cations.  Anion exchange membranes will allow the transfer of anions.  Bipolar membranes will allow the transfer of both anions and cations.  Membrane manufacturer fabricate the membrane products with a counter ion that suits best to their production method.  Some membranes would come in the acidic, some in bromide, and some with a mixture of anions and cations.  In order to get the best performance, the counter-ion for the membrane of desire needs to match the major counter ion for the intended application.  This is where the benefit of membrane pre-cleaning separator sheets is seen.  Majority of membrane pre-cleaning or pre-treatment requires submersion of the membrane sheets into a liquid solution that has the desired counter-ion or solvent.  Since membrane sheets are thin plastic-like films, they will tend to stick to the side or bottom of the container that has the immersion solution or if there are multiple membranes in the same immersion bath, then these membranes would stick to each other and prevent or limit the ion exchange process.  In order to have complete ion exchange with the membrane piece, it is essential that the both sides of the membrane are exposed to the immersion liquid.  Making the both sides of the membrane available for ion exchange, the pre-cleaning or pre-treatment process can proceed efficiently.

For example, Nafion membrane is a cation exchange membrane and it is manufactured in the acidic form (H+).  Someone may be curious for the reason.  Low temperature proton exchange membrane fuel cell (PEMFC) hardware requires the presence of protons for the anode and cathode electrochemical reactions to occur. What this means is the following: whatever the fuel species (H2 gas and methanol being the two most popular ones) is supplied to the anode side of PEMFC, protons will be generated from the consumption of the fuel species (via electrochemical oxidation route) and these protons have to travel to the cathode side in order to react with O2 and electrons to generate the by-product water.  In summary, in theory the sole ion that is being transferred from anode to cathode will be the protons.  If the Nafion membrane is in the proton form for its counter ion, then there will not be any resistance for the transfer of protons from anode to cathode.  Let’s assume that the Nafion membrane is not in the proton form and has been dried or contaminated during its shipment, storage, or handling.  Let’s say the counter ion is K+ in this case.  Due to the stronger binding of K+ to the functional sites of Nafion, it becomes very difficult for protons to travel through K+ form of Nafion and this requires pre-cleaning or pre-treatment of Nafion membrane.  To achieve this, a piece of Nafion membrane is placed between two sheets of pre-cleaning separator sheet and then submerged into an acid bath (such as 1M sulfuric acid) for a pre-defined submersion time.  This pre-treatment allows the protons from the acid to move into the Nafion membrane and replace the functional sites that are holding K+ ions.  Once sufficient time is allowed, Nafion membrane will be converted into the acidic form and be ready for use in the PEMFC hardware.  The same immersion process needs to be done with other membranes that arrive in a different counter-ion than the desired one and pre-cleaning separator sheets ensures complete conversion of the membranes with the desired counter-ion as long as the correct immersion bath is used.

Pre-cleaning separator sheets can also be used to re-hydrate the membranes in aqueous solutions.

Fuel Cell Store team supplies two different pre-cleaning separator sheets, namely polypropylene and PTFE based materials.  Polypropylene separator sheets are ideal with most common acids, bases, salt solutions for low temperature pre-treatment of pre-cleaning efforts (<80 deg Celsius).  PTFE, on the other hand, is a high temperature stable material (also called Teflon) and have excellent chemical compatibility with most common acidic, basic, and salt solutions both at low and high temperatures (up to 200 deg Celsius depending on the nature of the immersion bath).


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