Factors that reduce membrane performance

Concentration polarization is a reversible increase in the concentration of retained solutes at the membrane feed interface. During operation, this effect will achieve a steady-state value and will disappear when the process is turned off. 

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Other system variables that will have an effect on the separation process are temperature and relative humidity of the gas. Increasing the temperature raises most permeabilities by about 10 to 15% per 10°C and has little effect on separation factors. The effect of relative humidity is variable depending upon the membrane used. High relative humidities, greater than 95%, are generally detrimental due to membrane plasticization. Contamination with liquid water has been found to dramatically reduce membrane performance for cellulose acetate ... 

Ammonia (NH3) or ammonium (NH4+) can exist in both the fuel and air streams. The diffusion of ammonium is fast, therefore, the ammonium entering the fuel cell from either side can quickly diffuse to the other side causing the contamination effect on both sides. For instance, for a typical membrane with a thickness of 10 to 100 jim, the estimated characteristic time constant for diffusion is 1 to 100 sec [149]. Ammonia may affect the PEMFC performance in different ways (1) by the reduction of the ionic conductivity of the membrane, which in its ammonium form is a factor of 4 lower than in the protonated form [149-151] (2) by poisoning the cathode catalyst [151] and (3) by poisoning the anode catalyst [149]. Recently, fuel cell tests have shown that the reduced membrane conductivity is not the major reason for performance losses induced by ammonia [149,150]. The effect of ammonia on the HOR was found to be minor at current densities below 0.5 A cm", but would increase with increasing current densities. The current density did not exceed 1 A cm in the presence of ammonia [149]. 

When constmcting biosensors from cells and tissues there can be several factors that can affect the performance of the sensor. The amount of cells immobilized, the type of membrane used, the degree of polymerization, and the amount of cross-linking agent can also influence the performance of the biosensor. Additionally, the pH, composition, and temperature of the solution tested can influence the biosensor performance dramatically. Even small changes in pH can reduce the enzymatic activity, which is often a function of pH, or even irreversibly destroy the cells or tissues. Furthermore, the composition of the solution is important as certain compounds could inhibit the enzymatic function. 

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]. 

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