SOLID POTENTIOMETRIC PH-ELECTRODE

Potentiometric pH-glass electrodes are among the most reliable so far known. Most remaining disadvantages (e.g. limited operating temperature, minimum size) stem from their aqueous component (reference buffer). The purpose of this investigation is therefore to maintain glass as a menbrane with outstandingly good sensing properties but to replace the reference buffer, including the Ag/AgCl electrode, by a solid contact providing a reversible transition from the ionic to the electronic part of the sensor The problems in making such a contact are discussed in terms of thermodynamic and kinetic parameters. The properties of glass electrodes contacted with the materials belonging to the Li, Ag, I ternary system are presented. They are compared with those of conventional pH-electrodes and pH-FETs. The advantages arising from the chemical stability of the contact with the glass membrane are outlined

A practical method for measuring the ion exchange capacity decrease of hydroxide exchange membranes during intrinsic degradation

In this work we present a practical thermogravimetric method for quantifying the IEC (ion exchange capacity) decrease of hydroxide exchange membranes (HEMs) during intrinsic degradation mainly occurring through nucleophilic attack of the anion exchanging group by hydroxide ions. The method involves measuring weight changes under controlled temperature and relative humidity. These conditions are close to these in a fuel cell, i.e. the measured degradation rate includes all effects originating from the polymeric structure, the consumption of hydroxide ions and the release of water. In particular, this approach involves no added solvents or base, thereby avoiding inaccuracies that may arise in other methods due to the presence of solvents (other than water) or co-ions (such as Na+ or K+). We demonstrate the method by characterizing the decomposition of membranes consisting of poly(2,6-dimethyl-1,4-phenylene oxide) functionalized with trimethyl-pentyl-ammonium side chains. The decomposition rate is found to depend on temperature, relative humidity RH (controlling the hydration number λ) and the total water content (controlled by the actual IEC and RH)

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

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

The "Chameleon Surface" : A New Concept for a Solid State Reference Electrode

A new concept for an “all solid state” reference half-cell for use in aqueous solutions based on mixed valent transition metal oxids is presented. XPS-surface analytical measurements demonstrate the adaptation of the surface chemistry to the chemistry of the solution ("Chameleon Surface"). Thus the driving force of the potential formation at the solid/liquid interface is significantly attenuated which may explain the electrochemically observed sub-Nernst sensitivities (~ 10 mV/pH)