| Electrocatalysis has attracted growing interest in the areas of pollution control (such as persistent organic pollutant degradation and CO2 reduction) and renewable energy (such as oxygen reduction and hydrogen production) due to its advantages of environmental compatibility, easy handling and mild reaction condition. For electrocatalysis, its performance mainly depends on the electrocatalysts. However, the traditional electrocatalysts suffered from low activity of non-precious metal materials, high cost and limited supply of noble metals, which hinder their applications. Besides, their narrow electrochemical potential windows led to low efficiency for reactions occurred at very negative potential. To solve these issues, doped diamond and porous carbon were designed as alternative electrocatalysts for persistent organic pollutant degradation, oxygen reduction, hydrogen production, CO2 or Nt reduction. Heteroatom doping and creating catalytic sites were explored to enhance their electrocatalytic performances. Meanwhile, we also uncovered the mechanisms of these reactions. The works more detailed as follows:(1) N-doped nanodiamond/Si rod array (NDD/Si RA) electrode was prepared by MPECVD. Its hygrogen evolution potential and electrocatalytic reduction activity were tuned by changing N content. To evaluate its performance for electrocatalytic reduction, BDE-47, PFOA and PFOS degradation were conducted. As a result, NDD/Si RA with N content of 2.26 at% had a hydrogen evolution potential of-1.5 V (vs Ag/AgCl), which was effcient for electrocatalytic reduction of BDE-47, PFOA and PFOS. At-1.4 V, its kinetic constant for BDE-47 (20 mg/L) degradation was 2.3-8.7 times as great as those of Pd film, Pt wafer, graphite and BDD/Si RA electrodes. Besides, NDD/Si RA was stable and reusable after successive electroreduction of BDE-47 for 28 times. Its superior performance for electroreduction has been confirmed by PFOA and PFOS degradation. Mechanism analysis revealed these pollutants were degraded through dehalogenation, where the halogen atom was substituted by hydrogen atom.(2) NDD/Si RA electrode was proposed for electrocatalytic CO2 reduction. To enhance its electrocatalytic performance, its N species were adjusted by altering systhesis temperature, and the correlation between N species and CO2 reduction activity was investigated. The results showed N-sp3C species could be easily formed at low temperature, while N-sp2C species tended to form at high temperature. Although NDD/Si RA with N-sp3C or N-sp2C rapidly converted CO2 to acetate over formate, NDD/Si RA dominated by N-sp3C presented higher CO2 reduction activity. Its production rate and current efficiency of acetate were 1.7-1.9 and 5.0-5.7 times as high as those of formate, overcoming the usual limitation of low selectivity for C2 products. Moreover, it had high faradic efficiency (91.8%) and good stability for electrocatalytic CO2 reduction. Electrokinetic data and in situ infrared spectrum indicated the main pathway for acetate production on NDD/Si RA was CO2 ? CO2·-?> (COO)2·? CH3COO-.(3) B/N codped nanodiamond (BND)/Si RA was prepared by MPECVD. Its performance for electrocatalytic oxidation and reduction was evaluated by methanol oxidation and BDE-47 reduction. Meanwhile, its electrocatalytic oxidation and reduction activity were tuned by altering the B and N contents. BND/Si RA was found to be advantageous over BDD/Si RA and NDD/Si RA for electrocatalysis, indicating the the synergetic effect between codoped B and N. Its electrooxidation activity was enhanced by increasing B or N contents. More importantly, BND/Si RA with 3.78 at% of B and 0.96 at% of N presented superior performance for both electrocatalytic oxidation and electrocatalytic reduction. Its potential for methanol electrooxidation was 0.1 V more positive than that of Pt/C, while its kinetics and effective current ratio for BDE-47 electroreduction were 1.5 times and 2.3 times as high as those of Pd film. Intermediate analysis revealed BDE-47 could be completely debrominatd by BND/Si RA. Based on these results, the performance of BND for electrocatalytic oxygen reduction was further studied. When the B content was 1.73 at% and N content was 1.94 at%, oxygen reduction catalyzed by BND occurred via an efficient four electron process. Its kinetic current density for oxygen reduction was 2.2-2.3 times as high as that of commercial Pt/C, and its stability towards oxygen reduction was 7.1 times higher than that of Pt/C. The zinc-air battery with BND as air cathode presented 10% higher power density and better charge-discharge ability than that with Pt/C.(4) N-doped hexagonal carbon (NHC), composed of hexagonal nanodiamond and graphite, was prepared by MPECVD and used for hydrogen production from water electrolysis. The factors affecting its hydrogen evolution performance were investigated as well as the origin of electrocatalytic activity. When N content was 1.10 at% and the ratio of hexagonal nanodiamond and graphite was 1.35, NHC was highly active for hydrogen production with an overpotential of 65 mV, a hydrogen production rate of 20.8 mL/(cm2·h) at-0.35 V and an exchange current density of 0.057 mA/cm2. Its performance for hydrogen generation is comparable or even better than that of nonnoble electrcatalysts reported recently. The excellent electrocatalytic activity of NHC for hydrogen production was benifited from the intrinsic electrocatalytic property of hexagonal nanodiamond, the rapid charge transfer and abundance of electrocatalytic sites after N doping.(5) Hierarchically porous carbon (HPC) was derived from MOF-5 carbonization, which was proposed for electrosynthesis of H2O2 from O2 reduction. Since HPC had high contents of sp3-C and defects, their correlation with H2O2 production were elucidated. HPC exhibited high activity for O2 reduction and good H2O2 selectivity. High-yield H2O2 generation has been achieved on HPC with H2O2 production rates of 395.7?110.2 mmol/(h·g) at -0.5 V and pH 1?7, which was 1?2 orders of magnitude higher than those of recently reported electrocatalysts. The exceptional performance of HPC for H2O2 electrosynthesis could be attributed to its high contents of sp-C and defects, large electroactive area and fast mass transfer. Based on its high H2O2 yield, an electro-Fenton system with HPC as cathode was employed for PFOA degradation, which was intended to explore the capability of electro-Fenton for perfluorochemical mineralization as well as the mineralization mechanism. The results showed PFOA (50 mg/L) was effectively mineralized by electro-Fenton with a total organic carbon removal efficiency of 90.7?70.4%?in 4 h (-0.4 V, pH 2?6). Moreover, the current efficiency of electro-Fenton for PFOA degradation was one order of magnitude higher than those of electrochemical oxidation. On the basis of intermediate analysis, a possible mechanism for PFOA degradation was proposed:PFOA lost one electron to the anode and got decarboxylated. The formed perfluoroalkyl radical was oxidized by·OH, resulting in the formation of shorter chain perfluorocarboxylic acid, which followed the same reaction cycle as PFOA until it was mineralized.(6) N-doped porous carbon (NPC) was prepared by ZIF-8 carbonization. Its performance for NH3 electrosynthesis from N2 reduction was investigated as well as the correlation between N contents, N species and N2 reduction activity. Results showed the electrocatalytic activity of NPC for NH3 synthesis was enhanced by increasing its total N content or pyridinic N content. At-1.2 V, the NH3 synthesis rate of NPC was 2.42 mmol/(g·h), much higher than those of electrocatalysts and photocatalysts reported.In summary, doped nanodiamond and porous carbon were highly active and stable nonmetallic electrocatalysts for pollutant degradation or energy conversion. Their electrocatalytic performance could be enhanced by tuning the dopant content, altering N species or introducing sp3-C and defects. These results along with the uncovered reaction mechanism are valuable in exploring cost-effective metal-free or all-carbon based electrocatalysts for environmental pollution control and renewable energy. |
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