88 Z.R. Yue et al. /Carbon 37(1999)1785-179 Partial decarboxylation with the resultant weight loss led to increased fiber surface fac roughness. The shape of the weight loss versus the extent of electrochemical oxidation curve(Fig. 1)shows that the morphology/pore structure continually changes during 020 oxidation and CO, evolution. The number and type of active sites change with increasing extent of electrochemi- cal el keygen- containing functions per gram of fiber and a higher surface area due to continually developing ultramicroporosity below the outer fiber surface. Our previous studies [20] demonstrated that a large increase in acidic functions (to 1-1. 1 mmol/g of fiber), measured by NaoH uptake occurred as electrochemical oxidation proceeded to 6360 C/g. To accommodate this number of titratable groups Fig. 2. XPS O Is/C Is and N Is/c Is atomic ratios of (a)the there must be a large increase in surface area even if every as-received carbon fiber and fibers electrochemically oxidized in o wt KNO, solution at levels of(b) 133 C/g;(c)1060 C/g;(d) surface carbon atom is oxygenated. The only way that such 4240Clg(e)5300c/g(f6360c/gand(g)10600C/g a large surface could form is by the generation of a small diameter pore/slit interconnected network below the outer surface of the fibers. However, nitrogen BET measure- appears in Table 1. Fig. 2 shows the O Is/c Is and n ments were only able to detect a small fraction of this new ls/c Is atomic ratios obtained from high resolution XPS porosity. Thus, the majority of the pores/slits etc. must be The as-received fibers display a smaller O Is/C Is ratio very small. Such very small pores require thermal activa-(0. 15)while electrochemically oxidized samples show tion to effect nitrogen filling. Specific surface areas were higher O Is/C Is ratios (0.23-0.27). The N 1s/c Is more effectively measured by DR/CO2 adsorption at 273K atomic ratios remained below 0.04 at all levels of oxidation and interpreted with the aid of density functional theory indicating no specific nitrogen incorporation occurred from [191. DR/CO, measurements were able to account for KNO, or dissolved nitrogen during electrochemical oxida most of the surface area which had to exist based on tion. These values reflect the integrated o/c and N/c existing titratable acidic functions. A large fraction of the ratios only over the sampling depth of-50 A from which pores were found to be very small(diameters of 4-6A). ejected electrons are able to escape when probed by XPS Thus, the 1 mmol/g of total acidic groups per gram of using a 30 electron take off angle. The surface oxygen carbon fiber( formed after 6360 C/g of electrochemical concentration rose rapidly to 24% after initial electro- oxidation) were occupying 67 m /g of surface area mainly chemical oxidation at 133 C/g and then remained at this composed of 4, 5 and 6 A average diameter ultramicro- level (or rose somewhat)with an increase in the extent of oxidation up to 10 600 C/g. The total amount of acidic functions(detected by NaoH titration) increased from 3 3. 1. 2. Studies by X-ray photoelectro umol/g(as-received )to 2476 umol/g(10 600 C/g)[601 XPS experiments were performed on both as-received This large (838-fold) increase in acidic functions and selected electrochemically oxidized carbon fibers. A COOH and phenolic -OH) which accompanies fiber weight summary of the fiber treatments and their designations loss(e.g, loss of carbon and nitrogen from oxidized fibers) Table I Treatments of carbon fibers used in XPS analysis Applied current Residence time Extent of electro (Amps) xidation(C/g) b)ECF-40-0.05 44.2 (c)ECF-10-0.1 d)ECF-15-0.6 117.8 (e)ECF-10-0.5 (fECF-10-0.6 (g)ECF-10-1.0 10,600 All electrochemical oxidations are referenced eceived fiber1788 Z.R. Yue et al. / Carbon 37 (1999) 1785 –1796 Partial decarboxylation with the resultant weight loss led to increased fiber surface area and increased surface roughness. The shape of the weight loss versus the extent of electrochemical oxidation curve (Fig. 1) shows that the morphology/pore structure continually changes during oxidation and CO evolution. The number and type of 2 active sites change with increasing extent of electrochemi￾cal oxidation, giving a higher total number of oxygen￾containing functions per gram of fiber and a higher surface area due to continually developing ultramicroporosity below the outer fiber surface. Our previous studies [20] demonstrated that a large increase in acidic functions (to 1–1.1 mmol/g of fiber), measured by NaOH uptake, occurred as electrochemical oxidation proceeded to 6360 Fig. 2. XPS O 1s/C 1s and N 1s/C 1s atomic ratios of (a) the C/g. To accommodate this number of titratable groups as-received carbon fiber and fibers electrochemically oxidized in there must be a large increase in surface area even if every 1% wt KNO solution at levels of (b) 133 C/g; (c) 1060 C/g; (d) 3 surface carbon atom is oxygenated. The only way that such 4240 C/g; (e) 5300 C/g; (f) 6360 C/g and (g) 10 600 C/g. a large surface could form is by the generation of a small diameter pore/slit interconnected network below the outer surface of the fibers. However, nitrogen BET measure- appears in Table 1. Fig. 2 shows the O 1s/C 1s and N ments were only able to detect a small fraction of this new 1s/C 1s atomic ratios obtained from high resolution XPS. porosity. Thus, the majority of the pores/slits etc. must be The as-received fibers display a smaller O 1s/C 1s ratio very small. Such very small pores require thermal activa- (0.15) while electrochemically oxidized samples show tion to effect nitrogen filling. Specific surface areas were higher O 1s/C 1s ratios (0.23–0.27). The N 1s/C 1s more effectively measured by DR/CO adsorption at 273K atomic ratios remained below 0.04 at all levels of oxidation 2 and interpreted with the aid of density functional theory indicating no specific nitrogen incorporation occurred from [19]. DR/CO measurements were able to account for KNO or dissolved nitrogen during electrochemical oxida- 2 3 most of the surface area which had to exist based on tion. These values reflect the integrated O/C and N/C ˚ existing titratable acidic functions. A large fraction of the ratios only over the sampling depth of |50 A from which pores were found to be very small (diameters of 4–6 A). ejected electrons are able to escape when probed by XPS ˚ Thus, the 1 mmol/g of total acidic groups per gram of using a 308 electron take off angle. The surface oxygen carbon fiber (formed after 6360 C/g of electrochemical concentration rose rapidly to 24% after initial electro- 2 oxidation) were occupying 67 m /g of surface area mainly chemical oxidation at 133 C/g and then remained at this ˚ composed of 4, 5 and 6 A average diameter ultramicro- level (or rose somewhat) with an increase in the extent of pores. oxidation up to 10 600 C/g. The total amount of acidic functions (detected by NaOH titration) increased from 3 3.1.2. Studies by X-ray photoelectron spectroscopy mmol/g (as-received) to 2476 mmol/g (10 600 C/g) [60]. XPS experiments were performed on both as-received This large (838-fold) increase in acidic functions (- and selected electrochemically oxidized carbon fibers. A COOH and phenolic -OH) which accompanies fiber weight summary of the fiber treatments and their designations loss (e.g., loss of carbon and nitrogen from oxidized fibers) Table 1 Treatments of carbon fibers used in XPS analysis a Fiber notation Surface treatment Applied current Residence time Extent of electro- (Amps) (min) oxidation (C/g) (a) as-received 0 0 0 (b) ECF-40-0.05 0.05 44.2 133 (c) ECF-10-0.1 0.1 176.7 1060 (d) ECF-15-0.6 0.6 117.8 4240 (e) ECF-10-0.5 0.5 176.7 5300 (f) ECF-10-0.6 0.6 176.7 6360 (g) ECF-10-1.0 1.0 176.7 10,600 a All electrochemical oxidations are referenced to the as-received fiber