Activated carbon fiber filter media for proton exchange membrane fuel cell

Abstract

Sulfur dioxide, a major air pollutant, has been found to be poisonous to proton exchange membrane fuel cells and causes degradation of fuel cell systems. Currently used industrial desulfurization systems, such as wet scrubbing, cannot address this problem because they cannot completely clean sulfur dioxide and SO2 with the concentration lower than 30 ppmv escapes. A fuel cell is more sensitive to poisoning than human’s lung and even a trace of sulfur dioxide will cause the irreversible degradation of a fuel cell. Activated carbon fibers (ACFs), one of relatively new adsorbents, are considered to remove sulfur dioxide from air before it is fed into fuel cell systems since ACFs have advantages in adsorption rate in comparison with granular activated carbon due to easy access of adsorbate to adsorption sites in micropores in activated carbon fibers. Activated carbon nanofibers were manufactured in the lab by electrospinning of polyacrylonitrile followed by chemical activation with potassium hydroxide. Micropores were shown to be dominant in the electrospun activated carbon nanofibers. However, their fragility and high manufacturing cost limit their applications in filtration. Pore structures and capabilities of removing sulfur dioxide of novoloid-based and rayon-based activated carbon fibers were analyzed. Surface areas were found to have correlation with pore volume in fibers. The sulfur dioxide capacities of ACFs were found to be proportional to the percentage of micropores in fibers. To improve sulfur dioxide removal efficiency of activated carbon fibers, alkali, metal and oxidants were applied onto ACFs as promoters. KOH (potassium hydroxide), K2CO3 (potassium carbonate), KClO3 (potassium chlorate) and KMnO4 (potassium permanganate) increased the desulfurization efficiency of ACFs greatly. Only KMnO4 modified ACFs are suitable for the wet laying technique with which ACF paper was made for filter device in the current study since the other three promoters can be removed easily by exposure to water. The optimum potassium permanganate weight ratio in ACF15 was found to be in the range of 43~45 wt% which exhibited the maximum sulfur dioxide capacity. The effect of carrier gas on desulfurization was studied which showed that the co-presence of water and oxygen would be preferred. The advantage of KMnO4 modified ACF15 is that the modification process does not need high temperature calcination in inert gas and thus is energy saving. Activated carbon fiber paper (K-ACFP) was manufactured from KMnO4 modified ACF15 and the binder fiber, copolyester (CoPET). The strongest K-ACFP was obtained when the ratio of KMnO4/ACF15 to CoPET is 7:3. Sulfur dioxide breakthrough capacity of the resulting K-ACFP was found to be similar to KMnO4/ACF15 fiber before the wet laying process. Produced K-ACFP can be used as the filter media for fuel cell protection from sulfur dioxide. The desulfurization mechanisms of KMnO4 modified ACFs with different carrier gases were studied. Considering the diffusion of adsorbate within packed bed and single activated carbon fiber and reaction over fiber surfaces, models were developed for different compositions of the challenge gas to predict breakthrough characteristics of KMnO4/ACFs. The predicted curves agree with the experimental results reasonably well. The dependency of sulfur dioxide removal efficiency of KMnO4/ACFs on the location in the packed bed and the elapsed time are predicted with the developed models as well.