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A convenient and realistic ex-situ method for determining the degradation rate of hydroxide-exchange-membranes for fuel cell applications

Kreuer, Klaus-Dieter and Jannasch, Patric LU (2017) Workshop on Ion Exchange Membranes for Energy Applications, EMEA2017
Abstract
The application of anion exchange membranes (AEM) in their hydroxide (OH–) form (sometimes denoted by HEM) as separators in low temperature fuel cells is a matter of ongoing research. OH– conductivity close to the proton conductivity of PEMs (such as the well-established Nafion®) are quite common [i] for high levels of hydration, and the reactivity of OH– with CO2 in air (used as oxidant in fuel cells) may be managed.

The major problem which remains to be solved steams from the mandatory presence of highly nucleophilic hydroxide as conducting ion. OH– tends to react with quaternized ammonium (QA) groups which are commonly used as positive ionic counter-charge within the polymeric structure. As typical leaving groups in organic... (More)
The application of anion exchange membranes (AEM) in their hydroxide (OH–) form (sometimes denoted by HEM) as separators in low temperature fuel cells is a matter of ongoing research. OH– conductivity close to the proton conductivity of PEMs (such as the well-established Nafion®) are quite common [i] for high levels of hydration, and the reactivity of OH– with CO2 in air (used as oxidant in fuel cells) may be managed.

The major problem which remains to be solved steams from the mandatory presence of highly nucleophilic hydroxide as conducting ion. OH– tends to react with quaternized ammonium (QA) groups which are commonly used as positive ionic counter-charge within the polymeric structure. As typical leaving groups in organic chemistry, QAs are well known to react with OH– through nucleophilic substitution, b-elimination, and rearrangement reactions such as Stevens rearrangement in the absence of b-protons. As a consequence, HEMs inherently degrade while losing their ion exchange capacity (IEC) and, as a consequence, also their ionic conductivity.[ii],[iii],[iv], [v],[vi],[vii]

The degradation rate not only depends on the kind of QAs but also on its environment within the polymeric structure. In order to remove this complexity, we had therefore studied the degradation rates of a series of QA-salts in concentrated aqueous solutions of NaOH in order to identify suitable candidates for ionic groups in HEMs.[viii] However, these conditions differ from the conditions provided by aqueous solution of NaOH (KOH) in various ways: i) within a HEM, OH– counter ions are consumed in degradation reactions while the concentration (activity) of OH– in excess NaOH solution is virtually unaffected by membrane degradation. ii) For high molarity, significant co-ion uptake (which corresponds to an uptake of excess NaOH) may affect the degradation rate through the presence of Na+ in the membrane. iii) Even for high NaOH molarity, the molar ratio [H2O]/[OH–] may be higher than for the low hydration conditions which may occur in running fuel cells. Especially the cathode side is expected to dry out as a result of electroosmotic water drag from the cathode to the anode side especially at high ionic (OH–) current density. Since ion solvation (kind of solvent and degree of hydration) is known to heavily affect degradation rates,9 low hydration levels must be put into effect in meaningful HEM degradation studies. In this work, we therefore present a convenient method which allows following HEM degradation at controlled temperature and hydration level. It is making use of a thermal gravimetric analysis technique, which allows recording sample weights under controlled T/RH (relative humidity) conditions.[ix] If commonly accepted, this method may help to resolve the debate about relative durability of hydroxide-exchange-membranes currently developed in many laboratories. (Less)
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Workshop on Ion Exchange Membranes for Energy Applications, EMEA2017
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English
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20ef7d1f-dedd-4347-954d-a11d3f951ce2
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https://www.next-energy.de/en/research-areas/fuel-cells/fuel-cells-workshops/fuel-cells-workshop-emea2017/#1494234098551-ff69c049-9328
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2017-06-29 22:22:38
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@misc{20ef7d1f-dedd-4347-954d-a11d3f951ce2,
  abstract     = {The application of anion exchange membranes (AEM) in their hydroxide (OH–) form (sometimes denoted by HEM) as separators in low temperature fuel cells is a matter of ongoing research. OH– conductivity close to the proton conductivity of PEMs (such as the well-established Nafion®)  are quite common [i] for high levels of hydration, and the reactivity of OH– with CO2 in air (used as oxidant in fuel cells) may be managed.<br/><br/>The major problem which remains to be solved steams from the mandatory presence of highly nucleophilic hydroxide as conducting ion. OH– tends to react with quaternized ammonium (QA) groups which are commonly used as positive ionic counter-charge within the polymeric structure. As typical leaving groups in organic chemistry, QAs are well known to react with OH– through nucleophilic substitution, b-elimination, and rearrangement reactions such as Stevens rearrangement in the absence of b-protons.  As a consequence, HEMs inherently degrade while losing their ion exchange capacity (IEC) and, as a consequence, also their ionic conductivity.[ii],[iii],[iv], [v],[vi],[vii]<br/><br/>The degradation rate not only depends on the kind of QAs but also on its environment within the polymeric structure. In order to remove this complexity, we had therefore studied the degradation rates of a series of QA-salts in concentrated aqueous solutions of NaOH in order to identify suitable candidates for ionic groups in HEMs.[viii] However, these conditions differ from the conditions provided by aqueous solution of NaOH (KOH) in various ways: i) within a HEM, OH– counter ions are consumed in degradation reactions while the concentration (activity) of OH– in excess NaOH solution is virtually unaffected by membrane degradation.   ii) For high molarity, significant co-ion uptake (which corresponds to an uptake of excess NaOH) may affect the degradation rate through the presence of Na+ in the membrane. iii) Even for high NaOH molarity, the molar ratio [H2O]/[OH–] may be higher than for the low hydration conditions which may occur in running fuel cells. Especially the cathode side is expected to dry out as a result of electroosmotic water drag from the cathode to the anode side especially at high ionic (OH–) current density. Since ion solvation (kind of solvent and degree of hydration) is known to heavily affect degradation rates,9 low hydration levels must be put into effect in meaningful HEM degradation studies. In this work, we therefore present a convenient method which allows following HEM degradation at controlled temperature and hydration level. It is making use of a thermal gravimetric analysis technique, which allows recording sample weights under controlled T/RH (relative humidity) conditions.[ix] If commonly accepted, this method may help to resolve the debate about relative durability of hydroxide-exchange-membranes currently developed in many laboratories.},
  author       = {Kreuer, Klaus-Dieter and Jannasch, Patric},
  language     = {eng},
  title        = {A convenient and realistic ex-situ method for determining the degradation rate of hydroxide-exchange-membranes for  fuel cell applications},
  year         = {2017},
}