High current density operation

Mr W C Meadowcroft E I DuPont de Nemours Co Inc, Nafion Customer Service Lab, DuPont Fayetteville Works, 22828 NC 87 Highway West, Fayetteville, NC 28306, USA High Current Density Operation of Chlor-alkali Electrolysers - The Standard for the New Millennium. E-mail william.c.meadowcroft usa.dupont.com... 

High Current Density Operation of Chlor-Alkali Electrolysers - the Standard for the New Millennium... 

If all responses to these tests are linear and typical, and all other independent variables remain within normal operating specifications, it can be assumed that the membrane and electrolyser interactions are optimised for operation within the current density range tested in Section 6.3.1. This procedure has been used successfully to diagnose and optimise operating conditions for both standard and high current density operations where unexpected performance issues have arisen. Furthermore, operators... 

AGC has been recently focusing on the development of a new electrolyser and a new membrane for high current density operation, a facility much requested by many users. In July 1998, AGC completed the conversion of its last diaphragm process plant to the then newest Bipolar Electrolyser, the AZEC B-l (hereinafter, B-l) with Flemion F-893 (hereinafter, F-893) membrane and also the then-newest membrane Flemion Fx-8964 (hereinafter, Fx-8964). This conversion was the result of AGC s development efforts. AGC is now on the way to the next stage of its ion-exchange membrane technology, where 6 kA /m-2 operation will be the norm and 8 kA m-2 operation will be made a feasibility. 

AZEC Improved B-1 the high performance bipolar electrolyser for high current density operation... 

The above-mentioned technology and structure provide advantages for the Improved B-l electrolyser in performance and reliability even under high current density. Good electrolyte distribution and no gas stagnation in each chamber, smooth discharge of gas and liquid, and low ohmic drop are necessary to overcome the difficulties of high current density operation. 

AGC has developed the low ohmic resistance membrane (F-8934) for high current density operation up to values of approximately 6 kA m-2. The new arrangement of the sub-structure of the membrane has contributed to wider distribution of the current passing through the membrane. This configuration decreases the actual current density localised over the membrane. Thus, the F-8934 shows 25% lower ohmic resistance than that for the F-893, as is shown in Fig. 19.10, even though the former comprises almost the same materials as the F-893. 

The membranes used in the present cells are expensive and available only in limited ranges of thickness and specific ionic conductivity. There is a need to lower the cost of the present membranes and to investigate lower cost membranes that exhibit low resistivity. This is particularly important for transportation applications where high current density operation is needed. Cheaper membranes promote lower cost PEFCs and thinner membranes with lower resistivities could contribute to power density improvement (29). It is estimated that the cost of current membranes could fall (by one order of magnitude) if the market increased significantly (by two orders of magnitude) (22). 

Along with electronic transport improvements must come attention to substrate transport in such porous structures. As discussed above, introduction of gas-phase diffusion or liquid-phase convection of reactants is a feasible approach to enabling high-current-density operation in electrodes of thicknesses exceeding 100 jxm. Such a solution is application specific, in the sense that neither gas-phase reactants nor convection can be introduced in a subclass of applications, such as devices implanted in human, animal, or plant tissue. In the context of physiologically implanted devices, the choice becomes either milliwatt to watt scale devices implanted in a blood vessel, where velocities of up to 10 cm/s can be present, or microwatt-scale devices implanted in tissue. Ex vivo applications are more flexible, partially because gas-phase oxygen from ambient air will almost always be utilized on the cathode side, but also because pumps can be used to provide convective flow of any substrate. However, power requirements for pump operation must be minimized to prevent substantial lowering of net power output. 

This is an area of much practical importance for research and development in electrolyzer technology more work is currently required for elucidation of the behavior of high-area porous and composite electrode materials with regard especially to the values of Tafel slope and conditions under which low b values can be achieved for H2, O2, and CI2 evolution reactions, thus minimizing activation overpotential energy losses in high current-density operations. 

Thus, at current densities above the limiting current, NaCl concentration at the membrane surface will be zero and discharge of water (or OH- migration) would occur, which results in decreased current efficiency. Hence, it is important to minimize 5 in order to operate the cell at as high a current density as possible, high current density operation being desirable in view of the high cost of the membranes. 

J.T. Keating and H.M.B. Gemer, High Current Density Operation—The Behavior of Ion Exchange Membranes in Chlor-Alkali Electrolyzers. In S. Sealy (ed.). Modem Chlor-Alkali Technology, Vol. 7, Royal. Society of Chemistry, Cambridge (1995), p. 135. 

Results High current density operations require smaller width channels and bipolar plate shoulders, higher porosity electrodes result from increasing electrode area under bipolar plate shoulder, relative humidity in anode gas stream is more important for FC performance than relative humidity in cathode gas stream... 

Kubo, N., and Shinohara, K. (2010) Effects of heat and water transport on the performance of polymer electrolyte membrane fuel cell under high current density operation. Electrochim. Acta, 56, 352-360. 

The various challenges of the improvement in the ORR activity and the durability have been advancing by means of alloying, formation of Pt skin layers, and core-shell preparation methods. Approaches based on non-precious metals are proceeding as well. In addition, the further optimization of the thin CL structure by use of new, durable support materials (e.g., graphitized carbon blacks and conductive ceramic supports) should help to improve the effectiveness of Pt and the durability, as well as specific gas transport problems in the CL under high current density operating conditions. 

Optimization of electrode and MEA performance with new electrode materials (catalysts, catalyst supports, and ionomers [2]), particularly for high-current density operation with low platinum loadings. 

High current density is generally a less degrading condition than low current density due to the reduced cell voltage. However, high current density operation is... 

High rates of water production under high current density operation can increase the water content depending on the water balance, which can accelerate oxidation and corrosion mechanisms (see Sections 6.3.1 and 6.4.2). 

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