Co, N, S Tri-Doped Carbon as an Effective Electrocatalyst for Oxygen Reduction Reaction Prepared from High-Sulfur Raw Coal

Abstract

A class of non-precious metal Co, N, S tri-doped carbon (Co/NSC) electrocatalyst for oxygen reduction reaction (ORR) is synthesized from earth-abundant high-sulfur inferior raw coal as the single N, S and C precursor by a two-step co-pyrolysis method. High-sulfur raw coal derived N, S co-doped carbon (NSC) and direct co-pyrolysis coal with CoCl 2 (coal-Co) are also prepared for comparison. The as-prepared Co/NSC has an amorphous carbon structure and is doped with heteroatoms N, S and Co nanoparticles in the carbon matrix. Furthermore, compared with NSC, S content decreases obviously and N content mainly in the form of pyrrolic-N changes little in Co/NSC. The as-prepared Co/NSC exhibits a more excellent ORR electrocatalytic activity than that of NSC and coal-Co in terms of the onset potential, ORR peak potential, the half-wave potential, the limiting-diffusion current density and the predominantly 4-electron transfer pathway in 0.1 M KOH. Based on these results, some possibilities attributing to the enhanced performance of Co/ NSC are also explained. This two-step co-pyrolysis process is also demonstrated a green utilization of high-sulfur raw coal.

Metathesis Reaction to Form Nanosheet-Structured Co(OH)2 Deposited on N-Doped Carbon as Composite Electrocatalysts for Oxygen Reduction

“…As shown in Figure 2b, each sample displays a peak around 1340 cm −1 (called as the D band, meaning disorder carbon) and a peak near 1590 cm −1 (named the G band, meaning graphited carbon). 30 The I D /I G values for these samples are all greater than 1.0, meaning a high degree of defects. Furthermore, the peak at 669 cm −1 can be assigned to the A 1g symmetric stretching mode of Co−O and the other two weak peaks at 474 and 510 cm −1 are attributed to the E g and F 2g modes of Co(OH) 2 , 19,31,32 respectively, demonstrating the existence of Co(OH) 2 .…”

“…The composition of these samples is further confirmed by Raman spectra. As shown in Figure b, each sample displays a peak around 1340 cm –1 (called as the D band, meaning disorder carbon) and a peak near 1590 cm –1 (named the G band, meaning graphited carbon) . The I D / I G values for these samples are all greater than 1.0, meaning a high degree of defects.…”

“…Its survey XPS analysis shows the element contents of C (89.80 at %), O (6.69 at %), N (1.24 at %), and Co (2.27 at %) (Figure S3). The high-resolution spectrum of C 1s in Figure a is deconvoluted into three peaks: the C–C peak (284.6 eV), C–N peak (285.6 eV), and C–O peak (289.8 eV). , In Figure b, the deconvoluted N 1s spectrum shows four peaks: pyridinic N (398.4 eV, 50%), pyrrolic N (400.3 eV, 15%), graphitic N (401.3 eV, 28%), and oxidized N (403.2 eV, 7%) in Co­(OH) 2 /NC-KOH. , In terms of N-doped carbon, it is generally accepted that pyridinic N located at the edge of the graphitic layers is an effective electrocatalytic site for ORR as it has the ability to reduce the energy barrier for adsorption of O 2 on adjacent carbon atoms and accelerate the first-electron transfer process during the ORR. , Moreover, in Figure c, three peaks around 529.9, 531.4, and 533.0 eV are for O 1s, assigned as Co–O, Co–OH, and chemisorbed/structural water (H–O–H), respectively. ,,, For the high-resolution XPS spectrum of Co 2p in Figure d, there are peaks at 780.2 eV for Co 2p 3/2 and 795.7 eV for Co 2p 1/2 , which are attributed to Co 2+ . The binding energy distance (Δ E b ) between these two Co 2p peaks is 15.5 eV, showing that Co 2+ exists in the form of a Co­(OH) 2 state. ,,,, Two additional satellite peaks at 784.3 and 802.0 eV auxiliary indicate that Co mainly exists in a Co 2+ state. ,, By the way, there is no peak around 778 eV for metallic Co.…”

“…The high-resolution spectrum of C 1s in Figure 5a is deconvoluted into three peaks: the C−C peak (284.6 eV), 21 C−N peak (285.6 eV), and C−O peak (289.8 eV). 30,35 In Figure 5b, the deconvoluted N 1s spectrum shows four peaks: pyridinic N (398.4 eV, 50%), pyrrolic N (400.3 eV, 15%), graphitic N (401.3 eV, 28%), and oxidized N (403.2 eV, 7%) in Co(OH) 2 /NC-KOH. 30,35 In terms of N-doped carbon, it is generally accepted that pyridinic N located at the edge of the graphitic layers is an effective electrocatalytic site for ORR as it has the ability to reduce the energy barrier for adsorption of O 2 on adjacent carbon atoms and accelerate the first-electron transfer process during the ORR.…”

“…30,35 In Figure 5b, the deconvoluted N 1s spectrum shows four peaks: pyridinic N (398.4 eV, 50%), pyrrolic N (400.3 eV, 15%), graphitic N (401.3 eV, 28%), and oxidized N (403.2 eV, 7%) in Co(OH) 2 /NC-KOH. 30,35 In terms of N-doped carbon, it is generally accepted that pyridinic N located at the edge of the graphitic layers is an effective electrocatalytic site for ORR as it has the ability to reduce the energy barrier for adsorption of O 2 on adjacent carbon atoms and accelerate the first-electron transfer process during the ORR. 36,37 Moreover, in Figure 5c, three peaks around 529.9, 531.4, and 533.0 eV are for O 1s, assigned as Co−O, Co−OH, and chemisorbed/structural water (H−O−H), respectively.…”