A correlation of the hydrogen evolution reaction activity to the defects formed by the decomposition of doped phosphorus species in carbon nanotubes
摘要: 作为一种新型碳基析氢反应催化剂，磷掺杂碳材料近年来已引起了人们极大关注。然而到目前为止，磷掺杂碳材料中的C-P物种对于氢析出活性的作用尚未被揭示。为了探讨碳基催化剂中C-P物种对其氢析出性能的影响，我们制备了四种具有不同石墨、吡啶、吡咯类磷物种分布的磷掺杂碳纳米管，并探讨了这三种磷物种的含量和氢析出活性之间的关系。结果表明在酸性介质中，一种在电流密度为10 mA cm−2 时过电位为0.266 V的磷掺杂碳纳米管展现出较高的氢析出活性。同时，密度泛函理论计算表明较高的氢析出性能主要是由石墨磷分解产生的五元环和九元环缺陷所引起，这为磷掺杂碳基催化剂表面的析氢反应提供一更为深入的理解。Abstract: The phosphorus-doped carbon materials as one of novel carbon catalysts towards the hydrogen evolution reaction (HER) have attracted considerable attention over the past years. However, the role of C-P species palyed in the HER activity is still not clear up to now. Phosphorus-doped carbon nanotubes (P-CNTs) were prepared by chemical vapor deposition and annealed at 900, 1000 and 1200 ℃ to remove all or parts of phosporus species, resulting in four samples with different proportions of graphite-, pyridine- and pyrrole-like P species. The correlations between their HER activity and the contents of three types of P species were investigated. Results showed that the content of graphite-like P decreased with the annealing temperature and no graphite-like P was retained at 1200℃. The HER activity increased with the annealing temperature and the one annealed at 1200 ℃ had the highest HER activity in an acid medium with an overpotential of 0.266 V at a current density of 10 mA/cm−2. Density functional theory calculations revealed that the pentagon- and nine-membered ring defects formed by the destruction of graphite-P species contributed mainly to the HER activity, which gave a deep insight into the active sites for HER.
Figure 5. (a) The HER polarization curves of P-CNTs1, P-CNTs2, P-CNTs3, P-CNTs4 and 40 wt% Pt/C catalysts in a N2-saturated 0.5 M H2SO4 solution, (b) their corresponding overpotentials at the current density of 10 mA cm−2 and (c) tafel plots from (a), (d) the dependence of the HER current densities (at 300 mV overpotential) of P-CNTs1 (■), P-CNTs2 (■), P-CNTs3 (■), P-CNTs4 (■) on the C-P content and the values of ID/IG, (e) impedance diagrams of P-CNTs1, P-CNTs2, P-CNTs3, and P-CNTs4, (f) the HER curves of P-CNTs4 in a 0.5 M H2SO4 medium before and after 1000 cycles.
Table 1. Elemental contents (at. %) in four samples from the XPS.
Samples C O P C—P C3-P＝O C—O—P C—P—O P-CNTs1 93.12 6.52 0.191 0.109 0.032 0.028 P-CNTs2 97.52 2.20 0.131 0.082 0.067 0 P-CNTs3 97.98 1.79 0.119 0.039 0.072 0 P-CNTs4 98.27 1.73 0 0 0 0
Table 2. The contents of sp2 and sp3 carbons, C＝O, C—O, π-π* and the ratios of sp2/sp3 carbon from the C1s spectra.
Samples sp2 sp3 sp3/sp2 C-O π-π* P-CNTs1 69.74 8.77 12.58 10.80 10.69 P-CNTs2 66.82 10.84 16.22 6.14 13.73 P-CNTs3 66.76 11.66 17.46 6.58 12.98 P-CNTs4 66.07 14.40 21.79 5.08 12.71
Table 3. The charge and spin densities of carbon atoms in the new formed defects after the decomposition of the C3-P graphite-like structure and those of the corresponding carbon atoms in the blank.
Carbon atom Charge density (blank) Spin density (blank) Charge density(after pyrolysis) Spin density
1 −0.009 0 −0.026 0 2 0 0 0.038 0 3 0 0 −0.067 0 4 0.018 0 −0.009 0 5 −0.014 0 −0.063 0 6 −0.014 0 0.034 0 7 0.035 0 0.030 0 8 −0.153 0 −0.143 0 9 0.016 0 −0.042 0 10 0.002 0 −0.073 0 11 0.002 0 −0.046 0 12 −0.021 0 0.009 0 13 0.019 0 − −
Table 4. The H+ adsorption energies on carbon atoms in the new formed defects from the decomposition of the C3-P graphite-like structure and those of corresponding carbon atoms in the blank.
Pure CNT (blank) CNT (after pyrolysis) 1 −58.163 −74.825 3 −56.805 −101.009 4 −55.909 −113.664 5 −58.719 −101.588 9 −55.458 −80.907 10 −55.824 −142.826 11 −57.595 −75.465
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