Effects of phosphoric acid as additive in the preparation of activated carbon fibers from poly(p-phenylene benzobisoxazole) by carbon dioxide activation

Poly(p-phenylene benzobisoxazole) (PBO) was impregnated with small amounts of H3PO4, and the effects of this additive on the porosity and other characteristics of chars and activated carbon fibers (ACFs) derived from this polymer were investigated. To this end, PBO-AS impregnated with 5, 10 or 15 wt.% H3PO4 was pyrolyzed at 850 °C, and the resulting chars were physically activated with carbon dioxide at 800 °C to different burn-off (BO) degrees. Thermal analysis techniques only detected minor effects of H3PO4 on PBO pyrolysis. The char yield and char reactivity towards CO2 increased following PBO-AS impregnation with H3PO4. Structural (X-ray diffraction), porous textural (CO2 adsorption) and surface chemical (temperature-programmed desorption, X-ray photoelectron spectroscopy) characterizations of the pyrolysis chars indicated that the increase in char reactivity is probably associated with a higher content of oxygenated functionalities. Following CO2 activation, the surface area and pore volume of the obtained ACFs chiefly depended on the BO degree, but impregnation with H3PO4 restricted the pore size to the micropore and narrow mesopore range, thus producing adsorbents with a slightly narrower pore size distribution than in the absence of H3PO4. The results are compared with those previously obtained under equivalent conditions with other high-crystallinity polymers as precursors for ACFs.