Hydroxide-conducting membrane

Yao W, Tsai T, Chang YM, Chen M (2001) Polymer-based hydroxide conducting membranes. US Patent 6,183,914... 

Xu, S. Zhang, G. Zhang, Y Zhao, C. Zhang, L. Li, M. Wang, J. Zhang, N. Na, H., Cross-linked hydroxide conductive membranes with side chains for direct methanol fuel cell applications. Journal of Materials Chemistry 2012, 22(26), 13295. 

This type of electrolytic cell consists of anodes and cathodes that are separated by a water impermeable ion-conducting membrane. Brine is fed through the anode where chlorine gas is generated and sodium hydroxide solution collects at the cathode. Chloride ions are prevented from migrating from the anode compartment to the cathode compartment by the membrane and this, consequently, leads to the production of sodium hydroxide, free of contaminants like salts. The condition of the membrane during operation requires more care. They must remain stable while being exposed to chlorine and strong caustic solution on either side they must allow, also, the transport of sodium ions and not chloride ions. 

A second class of fuel cells employs hydroxide-conducting (alkaline) electrolytes, again either in form of a solid membrane (alkaline membrane fuel cells) or a liquid electrolyte (alkaline fuel cells). While the modem era of fuel cells began with the latter type, the former type is under intense research today because a stable, highly conducting alkaline membrane with good C02 tolerance has remained elusive to date. 

AFCs can work at rather low temperatures of 80-100°C. They are made of hydroxide ion (OH ) conducting membrane. The reactions are as follows ... 

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... 

Hibbs MR, Hickner MA, Alam TM, McIntyre SK, Fujimoto CH, Cornelius CJ (2008) Transport properties of hydroxide and proton conducting membranes. Chem Mater 20 2566-2573... 

Clark TJ, Robertson NJ, Kostalik HA IV, Lobkovsky EB, Mutolo PF, Abruna HD, Coates GW (2009) A ring-opening metathesis polymerization route to alkaline anion exchange membranes development of hydroxide-conducting thin films from an ammonium— functionalized monomer. J Am Chem Soc 131 12888-12889... 

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. 

Besides increasing hydroxide conductivity, another strategy to lower resistance of the membrane is to reduce its thickness. HEMFCs can operate with even thinner membranes (e.g., 10/28 pm for Tokuyama Co. s A901/A201 products [6]), largely because HEMFCs have, in principle, lower fuel (e.g., H2) crossover than PEMFCs. In PEMFCs, ions flow from anode to cathode, in the same direction as fuel crossover in HEMFCs, ions flow from cathode to anode, in the opposite direction. Even if its ionic conductivity is only half as high, an HEM that is half as thick as a PEM will hold the same membrane resistance. 

Tris (2,4,6-trimethoxyphenyl) benzyl phosphonium hydroxide shows the highest basicity ever reported. Its HEM has the highest specific hydroxide conductivity among all reported cationic functional group-based HEMs, typically about twice that of trimethyl benzyl ammonium and more than foiu- times that of methyl imidazolium (39 [5], 19 [31], and 8.4mSgcm mmor [32] respectively, with the same polysulfone polymer matrix and homogeneous membrane structure in each case) (Table 6.2). 

Hydroxide-Ion-Conduction Membrane Used for Electrochemical Conversion of Carbon Dioxide in... 

The electrochemical reduction of carbon dioxide to produce fuel has often been termed by some as artificial photosynthesis. Compared to the traditional researches using alkaline solution electrolytes, porous separators, and solid metallic electrode structures, there are numerous benefits to using a cell design based on a solid polymeric ion-conduction MEA with porous catalytic electrodes. In this part, we will introduce a typical example of hydroxide-ion-conduction membrane used for electrochemical conversion of carbon dioxide in alkaline PEM cells, conducted by Valdez and coworkers [132],... 

The hydroxide-ion-conduction membrane (AMI-7001S) was obtained from American Membranes International Inc. (Ringwood, NJ). The AMI (Applied Membranes, Inc.) membrane consisted of PS with quaternary amine side groups, and the polymer had been cross-linked with divinylbenzene similar to the anion-exchange resins and electrodialysis membranes. The AMI membrane had a thickness of 450 pm. The as-received membranes from AMI contained chloride ions. Therefore, these membranes were soaked overnight in a solution of I M sodium bicarbonate to exchange the chloride ions for hydroxide ions (Figure 10.12). 

The performances of the Nafion and the hydroxide-ion-conduction membrane cells were compared for two cathode solutions (1) D1 water (18 Mil cm) saturated with carbon dioxide by continuous bubbling of the pure gas at a pressure of 1 atm and (2) 1 M sodium bicarbonate solution. As shown in Figure 10.13, the cell voltages with the hydroxide-ion membrane were found to decrease substantially upon changing the electrolyte from C02-saturated D1 water to 1 M sodium bicarbonate. However, the performance of the Nalion-based cell did not alter significantly for the same change in the cathode solution. This phenomenon can be ascribed to differences in ionic contact at the electrode/manhrane interface, as hydroxide-ion membrane is cross-linked and could not be hot pressed to improve the ionic contact. Thus, the use of a liquid electrolyte could substantially improve the ionic contact between the catalyst layer and the hydroxide-ion-conduction manbrane. 

FIGURE 10.13 Performance of cells with the alkaline Nafion membrane and hydroxide-ion-conduction membrane using various cathode solutions indicated. (From Narayanan, S.R. 

FIGU RE 10.14 Comparison of the cumulative efficiency of formate production in the Nafion membrane cell and hydroxide-ion-conduction membrane cell with the sodium bicarbonate solution in the cathode. (From Narayanan, S.R. et al., J. Electrochem. Soc., 158, A167, 2011.)... 

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