Low current density operation

There have been various reports comparing costs of polymer-modified asbestos diaphragms and non-asbestos diaphragms. The non-asbestos diaphragm is not economical for low current density operations. At higher current densities, the life of the diaphragm should be substantially more than three years to be economically attractive, as evident from the cost comparison developed by PPG [85], and presented in Table 4.7.6. 

Low current density operation is of particular concern for PEM fuel cell durability. There are two key stressors that become more important under low current density. The first is increased cell voltage, e.g. >0.8 V, providing increased electrochemical energy for degradation mechanisms. The second is associated with the fact that the cell design may be optimized for high-power conditions, wherein operation at... 

Fig. 10 Cole-Cole plot (a) and Bode plot (b) of electrochemical impedance spectroscopy (EIS) analyses of the MEA represented in Fig. 9 after low current density operation before dry treatment. Anode/cathode, pure H, or H -CO (10ppm)/Oj. Current density, 200mA...

As can be seen from Eigure 11b, the output voltage of a fuel cell decreases as the electrical load is increased. The theoretical polarization voltage of 1.23 V/cell (at no load) is not actually realized owing to various losses. Typically, soHd polymer electrolyte fuel cells operate at 0.75 V/cell under peak load conditions or at about a 60% efficiency. The efficiency of a fuel cell is a function of such variables as catalyst material, operating temperature, reactant pressure, and current density. At low current densities efficiencies as high as 75% are achievable. 

Plating variables for this process maybe summarized as higher (87°C) operating temperatures enable the oxygen content of the metal to be reduced to 0.01% the CrO iSO ratio should be below 100 to obtain low oxygen metal current efficiencies >8% are associated with high oxygen contents and better current efficiencies are obtained at low current densities. 

It follows from the figures and also from an analysis of Eq. (6.40) that in the particular case being discussed, electrode operation is almost purely diffusion controlled at all potentials when flij>5. By convention, reactions of this type are called reversible (reactions thermodynamically in equilibrium). When this ratio is decreased, a region of mixed control arises at low current densities. When the ratio falls below 0.05, we are in a region of almost purely kinetic control. In the case of reactions for which the ratio has values of less than 0.02, the kinetic region is not restricted to low values of polarization but extends partly to high values of polarization. By convention, such reactions are called irreversible. We must remember... 

In Figure 11, we have presented the AE - I curves of air electrodes with catalysts from active carbon and active carbon promoted with different amounts of silver (from Figure 5). It is seen that the transport hindrances in the electrodes with catalysts from pure active carbon and with active carbon promoted with 5% of silver are near to each other. The transport hindrances in the air electrodes with catalyst containing 30% of silver are much higher. That s why catalysts containing large amount of silver are suitable to be used in air electrodes operating at comparatively low current densities. 

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. 

In the Improved B-1 at the Kashima factory, low-oxygen content chlorine gas can be obtained by adding hydrochloric acid to the feed brine. Figure 19.8 shows the dependence of the oxygen content in the chlorine gas upon the content of the hydrochloric acid in the feed brine at 6 kA m 2 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. 

Most electrochemical experiments need an electrical contact of some kind to the silicon substrate. Because of the semiconducting nature of silicon a metallic tip or clip attached to the surface will not produce an ohmic contact but constitutes a Schottky junction. However, for some applications, like the ELYMAT (Section 10.3), where the contact is only operated under forward conditions at low current densities, such a contact is sufficient. For silicon samples with a doping concentration in excess of 1019 cnT3 the contact to a metal becomes ohmic. An ohmic contact to a silicon sample with a doping concentration below 1019 cm-3 can be achieved in different ways ... 

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). 

One further degradation mode related to catalysis is a consequence of operating at low current densities typical of portable power application. Under these conditions, overoxidation of the Pt cathode catalyst occurs, reducing cathode and overall MEA performance. Zelenay has shown that starving the cathode of air flow lowers the cathode potential to low values, causing reduction of Pt oxides and restoring cathode activity. ... 

In PEMFCs, Ralph et al. [86] tested a Ballard Mark V single cell with two different DLs a carbon cloth (Zoltek PWB-3) and a carbon fiber paper (Toray TGP-090) all the other operating conditions stayed the same for bofh cases. It was observed that the carbon cloth demonstrated a distinct advantage over the CFP at high current densities (>600 mA/cm ), while at low current densities both DLs performed similarly. If was claimed fhaf this was because the CC material enhanced mass transport properties and improved the water management within the cell due to its porosity and hydrophobicity. 

Various research groups have been able to demonstrate that the best PTFE loading in the MPL is around 20 wt% when a fuel cell is operated at fairly high humidity conditions [109,136,137,155,157]. It is important to note that in most cases, at low current densities (<0.2 A cm ), differences due to PTFE... 

MgO, Y2O3, and AIN powder-based separators are substantially less expensive. Cell assembly with such powder separators is simple and amenable to mass production. Although powder separators were shown to maintain their integrity and conform to small dimensional changes of electrodes, their porosity ( 50%) is considerably lower than that of BN felt and this drawback can limit their application to stationary load-leveling systems operating at low current densities. ... 

For laboratory scale conversions a simple beaker-type cell (Fig. 1) is convenient When lower current densities have to be applied or in big scale operations 21,26) the Swiss-roll cell is of advantage. The latter cell (Fig. 2) contains a rolled-up sandwich, consisting of an anode and cathode sheet and a separator net. This allows a high electrode area applied in a small cell volume, which results in the low current densities necessary for efficient electrolyses at the nickel hydroxide electrode. 

There are two ways to recover the anode surface after the occurrence of the anode effect. One is the application of a high voltage, such as 50 V, a short period after the exchange of polarity between the anode and cathode. The other is by grinding the anode surface with a sander after taking out the polarized electrode from the bath. In general, the cell is operated at low current density such as 5 8 A dm-2 to avoid the occurrence of the anode effect. 

Consequently, working temperatures of metal-winning and metal-plating electrolyses with anodic 02 evolution must be operated below 40°C if titanium anodes are intended to be used. Fortunately usually these types of electrolyses are operated at relatively low current densities allowing for relatively low process temperatures. 

On crystallisation, copper selenate separates, contaminated with about 1 per cent, of cupric chloride. The latter may be removed by extraction with acetone, in which it is readily soluble, whereas the selenate is only very slightly soluble after this operation the copper selenate is finally purified by recrystallisation from water. The copper may then be removed by electrolysis,1 using low current density, when selenic acid free from selenious acid and chlorine remains in the electrolyte. The solution may be concentrated until it contains about 82 per cent, of the acid by evaporating at 95° C. under reduced pressure. 

In order to study cathode flooding in small fuel cells for portable applications operated at ambient conditions, Tuber et al.81 designed a transparent cell that was only operated at low current densities and at room temperature. The experimental data was then used to confirm a mathematical model of a similar cell. Fig. 4 describes the schematic top and side view of this transparent fuel cell. The setup was placed between a base and a transparent cover plate. While the anodic base plate was fabricated of stainless steel, the cover plate was made up of plexiglass. A rib of stainless steel was inserted into a slot in the cover plate to obtain the necessary electrical connection. It was observed that clogging of flow channels by liquid water was a major cause for low cell performance. When the fuel cell operated at room temperature during startup and outdoor operation, a hydrophilic carbon paper turned out to be more effective compared with a hydrophobic one.81... 

Three dimensional packed bed electrodes are generally considered for reactions which operate with low current densities in order to increase localized mass transfer rates and/or increase overall current per unit cell volume. The maximum current density at any position in the electrode structure is limited by the prevailing conditions of mass transfer. The limiting current thus can also have... 

---