Hydroxide exchange membrane fuel cells

GuS, CaiR, LuoT, Chen Z, Sun M, Liu Y, He G, Yan Y (2009) A soluble and highly conductive ionomer for high-performance hydroxide exchange membrane fuel cells. Angew Chem 121 6621-6624. doi 10.1002/ange.200806299... 

An HEM is a membrane-form polymer electrolyte capable of conducting hydroxide anions (OH ), and an HEI is a binder-form polymer electrolyte capable of not only conducting hydroxide anions but also creating triple-phase boundary in the electrode catalyst layer. HEMs and HEIs are already used in hydroxide exchange membrane fuel cells (HEMFCs) and can also be used in many other electrochemical energy conservation and storage devices. 

Unlu M, Zhou J, Kohl P (2009) Anion exchange membrane fuel cells experimental comparison of hydroxide and carbonate conductive ions fuel cells and energy conversion. Electrochem Solid State Lett 12(3) B27-B30... 

Since the type of electrolyte material dictates operating principles and characteristics of a fuel cell, a fuel cell is generally named after the type of electrolyte used. For example, an alkaline fuel cell (AFC) uses an alkaline solution such as potassium hydroxide (KOH) in water, an acid fuel cell such as phosphoric acid fuel cell (PAFC) uses phosphoric acid as electrolyte, a solid polymer electrolyte membrane fuel cell (PEMFC) or proton exchange membrane fuel cell uses proton-conducting solid polymer electrolyte membrane, a molten carbonate fuel cell (MCFC) uses molten lithium or potassium carbonate as electrolyte, and a solid oxide ion-conducting fuel cell (SOFC) uses ceramic electrolyte membrane. 

As with the conventional proton exchange membranes (PEMs) whose chief application is in proton exchange membrane fuel cells (PEMFCs), HEMs are thin-membrane polymer electrolytes, and thus HEMECs can achieve high energy density and power density. The difference is in the conducting ion HEMs conduct hydroxides PEMs conduct protons. 

Yan X, He G, Gu S, Wu X, Du L, Wang Y (2012) Imidazolium-functionalized polysulfone hydroxide exchange membranes for potential applications in alkaline membrane direct alcohol fuel cells. Int J Hydrogen Energ 37 5216-5224... 

Hydroxide Exchange Membrane Alkali Fuel Cells (HEMFCs)... 

Xu, S. Zhang, G. Zhang, Y Zhao, C. Ma, W. Sun, H. Zhang, N. Zhang, L. Jiang, H. Na, H., Synthesis and properties of a novel side-chain-type hydroxide exchange membrane for direct methanol fuel cells (DMFCs). Journal of Power Sources 2012, 209, 228-235. 

A proton exchange membrane (PEM) separates the two halves of a fuel cell. This membrane allows protons, formed by the oxidation of hydrogen gas, to pass through and react with hydroxide ions, produced by reduction of oxygen gas at the cathode, to form water. The waste product of this fuel cell is simply water vapor that can be condensed for use, for instance, by astronauts. 

As membrane material for their direct ammonia-oxygen fuel cells, Lan and Tao used a blend of the anion-exchange resin Amberlite IRA 400 (hydroxide form) and poly(vinyl alcohol). As cathode material, Mn02 deposited on carbon materials was used. In different cell versions the anodes were prepared from Pt-Ru-C and from chrom-decorated nanosized nickel (size about 6 nm). Experiments with such cells at room temperature showed maximal power densities in the range 12 to 16 mW/cm. In some cases the power densities for ammonia-fed cells were higher than those for hydrogen-fed cells. The authors note that the development of direct ammonia fuel cells with alkaline membranes and inexpensive catalysts is still at an early stage. 

The PVDF-based membrane [104,116-119] obtained after amination and the alkaline exchange process is a very brittle material due to a physical degradation of the backbone. Moreover, these materials exhibited low lECs (0.7 x 10 equiv./g) and would be unsuitable for use in any kind of fuel cell or electrochemical device. However, membranes based on FEP, ETFE-co-FEP, and ETFE obtained high enough conductivities to be tested for AFC application. FEP-based AEMs showed conductivities on the order of 10-20 X10 S/cm at room temperature [104,116,124]. Fuel cell test data obtained with FEP-based MEAs (with 0.5 x 10 g/cm Pt/C (20wt%) electrodes) show a peak power density of 55 x 10 W/cm at 0.5 V at 50 °C and 100% relative humidity [104,126]. AEM based on FEP exhibited a conductivity of 30 X 10 S/cm when fully hydrated [127]. This result represents a high level of conductivity for a solid alkaline polymer without incorporation of metal hydroxide species. Unfortunately, at lower humidity, the conductivity of these membranes drops considerably and even if they are operational with low-humidity gases, they exhibit low efficiency. 

A dynamic method called Hittorf s method allows determining transport numbers under migration, a method comparable with a fuel cell under operation. The two compartments in the cell are separated by an ion exchange membrane and both compartments are filled with the same KOH solution (same concentration). A current is applied between the two electrodes on either side of the compartments. At the cathode, hydrogen and hydroxide are produced, while at the anode hydroxide is consumed and oxygen is produced. To maintain electroneutrality, the cations (K" ) and anions (OH ) present in the solution migrate through the membrane from one compartment to the other. Anionic and cationic transport numbers f and f ) can subsequently be determined [20,205,206] ... 

---