High-current-density performance

High current density performance of PEFCs is known to be limited by transport of reactants and products. In addition, at high current densities, excess water is generated and condenses, filling the pores of electrodes with liquid water and hence limiting the reactant transport to catalyst sites. This phenomenon known as flooding is an important limiting factor of PEFC performance. A fundamental... 

Developed a unique flow field design via modeling and basic principles of gas transport that optimizes the uniformity of hydrogen and air mass flow velocities for the matched EB media and increases high current density performance over conventional designs. 

Platinised-titanium anodes may also be used in soils when surrounded by a carbonaceous backfill. Warne and Berkeley " have investigated the performance of platinised-titanium anodes in carbonaceous backfills and conclude that the anodes may be successfully operated in this environment at a current density of up to 200 AmThis also supplements the findings of Lewis, who states that platinised-titanium anodes may be used in carbonaceous backfill without breakdown of the titanium oxide film. Success with platinised-titanium anodes has been reported with anodes operating at a few tens of Am and failures of anodes have often been attributed to operation at high current densities . 

Electropolishing is performed in concentrated mixtures of acids (sulfuric, phosphoric, chromic, etc.). Often, organic acids and glycerol are added. It is somewhat inconvenient that almost all metals and alloys require their own solution composition. For electropolishing, intermediate and high current densities are used, between about 0.1 and 500 mA/cm. Depending on current density, the process requires between 30 s and 20 to 30 min. Usually, a metal layer 2 to 5 pm thick is removed under these conditions. 

Electrochemical machining is performed in concentrated solntions of salts alkali chlorides, snlfates, or nitrates. Very high current densities are nsed hundreds or thousands of kA/m when referring to the surface area of the anodic working sections. At a current density of 10" mA/cm, the rate of iron dissolution is about 0.15 mm/min. This should also be the rate of advance of the cathode in the direction of the anode. High rates of solution flow through the working gap are used to eliminate the reaction products and heat evolved (e.g., flow rates of 10" cm/s). 

At the electrochemical performance level, these novel natural graphite-based materials surpass mesophase carbon s characteristics as related to cell/battery safety performance, low irreversible capacity loss, and good rate capability even at high current densities. 

The main contributions to this development stem from by B.C. Research (BCR) [129,130] for Pacific Northern Gas Ltd. and W.R.-Grace/Hydro-Quebec [129-131] in the period 1980 till 1992. The economic viability of this process depends largely on the performance of the electrochemical reactors for the generation of Ce4+, which should produce concentrated solutions of Ce4 + ( > 0.3 M) at high current density ( > 1 kA/m2), high current efficiency ( > 70%) and long life ( > 2 years). 

DuPont research into high current density and the associated effect on membrane and electrolyser performance has been underway for a decade. It has been the area of greatest concentration for the company during the last 5 years. Studies at the DuPont Experimental Station and Fayetteville Nafion Customer Service Laboratories resulted in polymer innovation and new membrane designs. This work has also identified interactions between membranes and electrolysers... 

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... 

The development of high current density electrolysis technology is a continuing effort. Asahi Chemical s focus is currently on the confirmation of stable long-term performance and reliability, in preparation for the supply of this process equipment and technology to chlor-alkali producers. 

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. 

A commercial-scale Improved B-1, with a capacity of 10 000 metric tonnes per year (caustic base), came into operation successfully at the AGC Kashima factory in July 1999. Since then, the plant has demonstrated a long-run operation to prove its stable performance and durability under high current density. For the first 3 months, the electrolyser was operated at 5kA m-2. Then the current density was increased step by step up to 7kA m-2. An Fx-8964 membrane, which had just been developed as one of the optional membranes for high current density, was installed. The operation was observed to be stable for each current density. As is shown in Fig. 19.7, the power consumption at present is approximately 2300 (d.c.) kWh tonne-1 at 7 kA m-2. 

In the current-voltage curve in Fig. 14.15, three different regions can be discerned. At low current densities, the performance is kinetically limited. In the linear part, ohmic losses are significant. At high current densities, mass transport losses dominate. 

The improvement in cell performance at higher pressure and high current density can be attributed to a lower diffusion polarization at the cathode and an increase in the reversible cell potential. In addition, pressurization decreases activation polarization at the cathode because of the increased oxygen and water partial pressures. If the partial pressure of water is allowed to increase, a lower acid concentration will result. This will increase ionic conductivity and bring about a higher exchange current density. The net outcome is a reduction in ohmic losses. It was reported (33) that an increase in cell pressure (100% H3PO4, 169°C (336°F)) from 1 to 4.4 atm (14.7 to 64.7 psia) produces a reduction in acid concentration to 97%, and a decrease of about 0.001 ohm in the resistance of a small six cell stack (350 cm electrode area). 

Japan The Kansai Electric Company has tested a four-cell article and accumulated 10,529 hours of operation at high current densities and completed 101 thermal cycles. Tokyo Gas started research and development of a planner SOFC in 1993. They conducted a 1.7 kW module test with stable performance. 

However, the performance of a fuel cell with these carbon aerogels as DLs was around a factor of six lower than the performance of commercial electrodes. This was due mainly to the fact that the authors did not use additional electrolyte when depositing the catalytically active layer, thus causing reduced ionic conductivity between the catalyst (Pt particles) and the membrane. In addition, the MEAs with carbon aerogels performed poorly at high current densities because the Pt particles used were 10 times larger than the ones normally used [20]. 

ITottinen et al. [44,45] used htanium sintered meshes as DEs on the cathode side of a PEMEC because the porosity of these metal sheets does not reduce when in compression. It was demonstrated that in order ter the cell to achieve the required performance, the sintered meshes had to be coated with platinum. However, the results showed that a cell with CEP (SIGRACET GDEIO-BB) as the DE shll performed slightly better (especially at high current densities) than the cell with the Pt-coated sintered Ti mesh. Cisar et al. [46 presented another example in which a DE consisting of sintered metal fibers was used on the cathode side of a PEMEC. Once put together, these fibers were rmified or bonded to the EE plate (made out of metal) in order to combine the two components into one. 

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. 

In order to improve the performance of fuel cells, Wilkinson and St-Pierre [92] and Johnson et al. [93] compared typical CFP cathode DLs with modified DLs (from CFP or CC) that improved the mass transport at high current densities. Figure 4.14 shows the different DLs that were used to improve the cell s performance. Similar strategies can be implemented in other types of DLs, such as metallic or engineered. 

In air-breathing PEM fuel cells, Jeong et al. [113] were able to demonstrate that with high PTFE content, the fuel cell performed poorly at high current densities because the high amount of PTFE lowered the porosity of the DL, as discussed previously. They concluded that cathode DLs (Toray CFPs) with 5-10 wt% PTFE performed the best (the PTFE content of the anode DL was kept constant at 20 wt% PTFE). 

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