Download Seo Powersuite Full Crack Membrane
Vida Hubbert
To attain high cell efficiency, the membrane must satisfy several criteria: high ionic conductivity to provide high currents with minimal resistive losses and minimal or no electronic conductivity, good mechanical strength and stability, chemical and electrochemical stability under operating conditions, adequate moisture, extremely low fuel or oxidant permeability to maximize coulombic efficiency, and cost-effectiveness [37,38]. Still, it is necessary to make a compromise of properties to fulfill the requirement for low internal resistance, good separation as well as adequate physical strength. It has been shown that the membrane thickness has a large impact on cell performance with peak power density increasing with the increase of the membrane thickness [34]. Yet, this influence is rather complex, as thicker membranes will have reduced reactants crossover, i.e., lower NaBH4 penetrability, but also higher ionic resistance [38,39,40].
Membrane stability under fuel cell operation conditions affects the lifetime and the cost of DBPFCs. In general, both anion-exchange membranes (AEMs) and cation-exchange membranes (CEMs) may be considered as separators for DBPFCs. Membranes operate according to the principle of Donnan exclusion [41], i.e., only transfer of oppositely charged ions is allowed (solid lines in Figure 1), while the transfer of ions of the same charge as the immobilized membrane group is mostly blocked (dotted lines in Figure 1).
Schematic illustration of the major migrative and diffusive fluxes across (a) anion and (b) cation exchange membranes used in direct borohydride/peroxide fuel cells (DBPFCs).
Figure 2c,d shows the SEM micrographs of the cross sections of AMI-7001S and CMI-7000S membranes, respectively. Presence of densely-packed microfiber in the membranes structure can be observed and, in the case of the anion-exchange membrane, presence of filaments among the microfibers.
Main properties of the studied membranes, anion-exchange AMI-7001S and cation-exchange CMI-7000S membrane, are summarized in Table 1. Both AMI-7001S and CMI-7000S are heterogeneous membranes, based on polystyrene gel cross linked with divinylbenzene, but with different functional groups, i.e., quaternary ammonium and sulfonic acid, respectively. As previously mentioned, membranes are charge selective, AMI-7001S preferably allows migration of anions and CMI-7000S preferably allows migration of cations including H+. Limited migration of cations (AEM) and anions (CEM) into the opposite direction can take place as well.
Recorded electrode potentials for the two DBPFCs do not show a very different behavior. Therefore, the better power performance of the DBPFC employing CMI-7001S membrane separator in comparison with that of DBPFC with AMI-7001S could be attributed to the higher ohmic resistance of the AEM.
The observed continuous falling of the DBPFCs voltage during cell operation is due to the overpotentials generated inside the cell. These include activation losses due to slow kinetics of the electrochemical reactions at the electrodes, ohmic losses due to resistance of the membrane separator, cell components and interconnects, mass transport losses due to insufficient concentrations of reactants at the electrode/electrolyte interface at high load current condition, and crossover losses due to the crossover of fuel (NaBH4) and oxidant (H2O2) through the membrane.
The long-term durability of Pt, NaBH4/commercial membrane separator/H2O2, Pt fuel cells using AMI-7001S or, alternatively, CMI-7000S membrane separators was studied continuously for ca. 90 h at no current flow and at the temperature of 20 C (Figure 6).
The voltage decrease for DBPFCs with AMI-7001S and CMI-7000S was less pronounced than in the case of DBPFCs with the studied Nafion membranes, where cell voltages decrease to a value of about 1.0 V within the first few hours of the test. The slower and less pronounced cell voltage decrease in the case of DBPFCs with AMI-7001S and CMI-7000S membranes is, again, most likely due to the higher thickness of these two membranes when compared to the Nafion ones. Comparing DBPFCs studied herein with the previously explored IONAC membranes, AMI-7001S and CMI-7000S exhibited performance similar to IONAC MC-3470 and somewhat worse than IONAC MC-3475, which sustained a relatively constant cell voltage of 1.6 V during a 50 h test period [32].
Tests performed within this study have shown that CMI-7000S membrane-based DBPFC is able to achieve somewhat higher power density than AMI-7001S-based DBPFC as well as better performance stability and durability. Thus, this CEM separator can be used in DBPFCs to increase power, stability and efficiency, and decrease cost at the same time.
The authors acknowledge Membranes International Inc. (Ringwood, NJ, USA) for supplying the membrane separator samples. A.L. Morais, D.M.F. Santos and B. Šljukić would like to thank Fundação para a Ciência e a Tecnologia (FCT, Portugal) for PhD grant No. SFRH/BD/72968/2010 and postdoctoral research grants No. SFRH/BPD/63226/2009 and SFRH/BPD/77768/2011, respectively.
There are various fuel cell technologies that are commercially available, though research to improve efficiency, safety and performance is ongoing. Some examples include proton exchange membrane (PEM), phosphoric acid, direct methanol and solid oxide fuel cell (SOFCs). There are also emerging technologies under development, such as microbial fuel cells. Regardless of the chemistry involved, all fuel cells include an anode, cathode and electrolyte.
The research in this area often begins with selecting the ideal anode or cathode and catalyst design, which typically involves the use of a rotating disk electrode (RDE) or rotating ring disk electrode (RRDE) to evaluate the kinetics and mechanism of the fuel conversion reaction. Other considerations include electrolyte/fuel composition and purity, cell geometry, membrane design, operating temperature and more.
If that escape point is where the membranes are situated, these stray currents could interfere with the electrochemical processes occurring between the layers of the membranes, causing a loss of efficiency. If that escape point is only through the steel filter casings or steel piping, and currents are forced to travel great distances before they can find a path-to-ground, that longitudinal flow of current can dramatically increase the rate of corrosion on the steel structural members of the RO plant.
So, what have we learned? First, it is very important to install electrical measures at the VFD to reduce the amount of objectionable and stray currents entering the water system during the initial pumping stage. Second, a sound well-bonded grounding system will remove the remaining currents, helping to improve the efficiency of the membranes and to reduce the rates of corrosion.
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