Hydroxide conductivity

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. 

The theory of negative peaks for sample anions is easy to understand. Suppose wc use 1.0 X 10 M sodium hydroxide as the eluent. The background conductance will be the sum of the sodium and hydroxide conductances and will be relatively high. Injection of a sample will result in the uptake of the anions by the resin column with an equivalent amount of resin hydroxide ion passing into solution. Once the matrix peak is through, the anion concentration of the column effluent will be constant, as fixed by the 1.0 x 10 M eluent concentration. When a sample anion, A , is eluted from the column and passed through the detector, the eluent hydroxide ion will be decreased because a constant anion concentration must be maintained... 

Fuel cell CLs are the key components in the entire fuel cell device because the reactions such as hydrogen—oxidation reaction (HOR) at anode and the ORR at cathode occur inside the CLs. Particularly, in order to carry out the ORR, the catalyst particles inside the cathode CL must be in contact with each other for electrical conductivity and also in contact with protonic conducting (in acidic PEM fuel cells), or hydroxide conducting (in alkaline PEM fuel cells) ionomer for ionic conductivity. In addition, there must be some channels within the CL for transporting the reactants and the products. In other words, the catalyst particles must be in close contact with each other, with the electrolyte, and also with the adjacent diffusion medium (DM). Moreover, the reactants gas O2) and the produced water travel mainly through the voids, so the CL must be porous enough to allow gas to diffuse to the reaction sites and liquid water to wick out. 

Alkaline fuel cells (AFCs) with a hydroxide conducting electrolyte,... 

Yuan C, et al. Synthesis of flexible and porous cobalt hydroxide/conductive cotton textile sheet and its application in electrochemical capacitors. Electrochim Acta 2011 56(19) 6683-7. 

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

Due to no leakage and no freezing of electrolyte solutions and thirmess and compactness of batteries, aU-solid-state batteries with proton- or hydroxide-conductive soUd electrolytes have been developed. However, they are not practically used yet. The most serious issue was that the solid electrolytes had much lower electrical conductivity than alkaline electrol)de solutirms. Since the second half of 1990s, new tyqres of solid electrolytes, hydrogel electrolytes, have been developed by various research groups and applied to aU-soUd-state electrochemical devices like batteries and capacitors. In this section, the hydrogel electrolytes can be put into five categories. 

Wang JH, Li SH, Zhang SB (2010) Novel hydroxide-conducting polyelectrolyte composed of a poly(arylene ether sulfone) containing pendant quaternary guanidinium groups for alkaline fuel cell applications. Macromolecules 43 3890-3896... 

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... 

As ionic conductors, HEMs and HEIs substantially control the performance and durability of HEMFCs through their hydroxide conductivity and chemical/... 

For high-performance HEMFC applications, HEMs and HEIs are required to have high hydroxide conductivity, excellent chemical stability, sufficient physical stability, controlled solubility, and other important properties. 

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. 

The cationic functional group has been the central focus in HEM chemical structure because it dominates hydroxide conductivity through its basicity as well as its density (i.e., ion exchange capacity, lEC). The intrinsic nature of the functional group also determines solubihty and controls chemical stability. Currently, two major types of cationic functional groups are available one type based on nitrogen atoms and the other type based on phosphorus ones. 

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). 

Linkage density is an important parameter to practically tune the properties of cross-linked HEMs. At a given lEC, increasing linkage density translates directly to improved dimensional stability and mechanical strength, but it also reduces water uptake, thereby potentially lowering the hydroxide conductivity. 

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. 

C 4 and C 6 a-olefin sulfonates, demonstration of mobile phase ion chromatography Dionex MPIC-NSl, 4.6 x 250 mm H2O/CH3CN, about 60 40, with the water 0.01 M in NH3 or 0.002 M in tetramethyl- or tetrapropylammonium hydroxide Conductivity 13,14... 

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