Phosphoric acid porosity development

Fig. 19 shows the adsorption capacity attained with other precursors such as a bituminous and a subbituminous coal, vs. the heat treatment temperature and compared with wood [46]. It is clear that phosphoric acid activation must only be applied to lignocellulosic materials since the coals hardly develop any porosity. 

In previous sections, it was explained how different precursors can be used to produce activated carbons and the type of porosity developed depending on the type of activation method applied. Thus, thermal activation imrmally yields adsorbents with a medium to liigh adsorption capacity, a medium micropore size distribution and no mesopore formation (except in the case of high bum-off ratios, where micropores may be of a large size). Phosphoric acid activation yields a carbon with a higher adsorption capacity than thermal activation and a wider micropore size distribution (even in the low mesopore range), whereas KOH yields extremely narrow microporous carbons. 

With phosphoric acid activation, the porosity development is different because essentially microporous carbons can be prepared. However, the use of higher concentrations of this chemical also develops meso- and macroporosity. Further, a controlled process leads to carbon monoliths in which the internal... 

All these factors make it difficult to develop a cost effective solution for the separator plates. Initial trials were with graphite polymer molded composites. The polymers that were tried includes polyvinylidenefluoride (PVDF), polyetheretherketone (PEEK), perfluoroalkoxy (PEA) etc. Although most of the graphite polymer composite passed the 200°C phosphoric acid test, few survived more than few thousand hours, when they are maintained in 200°C phosphoric acid with a potential of 1.0 V w.r.t. SHE. Of the various polymers, PFA was found to be the most suitable. However, it was observed that after long exposure to PAFC temperature, the polymers got relaxed, causing an increase in the plate electrical resistance and porosity. However, all these plates exhibited excellent gas crossover resistance at least in the initial phase. 

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