Carbon-based metal-free nanomaterials for the electrosynthesis of small-molecule chemicals: A review
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摘要: 电催化是各种以化学品形式储存可再生电力能源技术的核心。目前,贵金属基催化剂广泛应用于提高电催化转化效率。然而,成本高、稳定性差等缺点严重阻碍了其在电合成和可持续能源器件中的大规模应用。碳基无金属催化剂(CMFCs)在提高催化性能方面展现出巨大潜力,且受到越来越多的关注。本文概述了用于电催化合成的CMFCs的最新研究进展,并讨论了其催化机理和设计策略。此外,简要总结了电催化合成过氧化氢、氨、氯以及各种碳基和氮基化合物的研究现状,并重点阐述了CMFCs目前面临的挑战和未来前景。Abstract: Electrocatalysis is a key component of many clean energy technologies that has the potential to store renewable electricity in chemical form. Currently, noble metal-based catalysts are most widely used for improving the conversion efficiency of reactants during the electrocatalytic process. However, drawbacks such as high cost and poor stability seriously hinder their large-scale use in this process and in sustainable energy devices. Carbon-based metal-free catalysts (CMFCs) have received growing attention due to their enormous potential for improving the catalytic performance. This review gives a concise comprehensive overview of recent developments in CMFCs for electrosynthesis. First, the fundamental catalytic mechanisms and design strategies of CMFCs are presented and discussed. Then, a brief overview of various electrosynthesis processes, including the synthesis of hydrogen peroxide, ammonia, chlorine, as well as various carbon- and nitrogen-based compounds is given. Finally, current challenges and prospects for CMFCs are highlighted.
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Figure 1. (a) Calculated charge density distribution of N-doped CNTs and the corresponding adsorption modes of oxygen molecule[27]. Reproduced with permission from AAAS. (b) Reaction process between pyridinic N and OH species. (c) Proposed mechanism for ORR on nitrogen-doped carbon materials[59]. Reproduced with permission from AAAS. (d) Heteroatom-doping mechanism in CMFCs[60]. Reproduced with permission from Wiley-VCH
Figure 2. (a) Heteroatoms-doping strategies and the electronegativity of various elements[26]. Reproduced with permission from American Chemical Society. (b) Schematic diagram of preparation for controlled N-doped graphene[81]. Reproduced with permission from The Royal Society of Chemistry. (c, d) Design principles for various CMFCs[87]. Reproduced with permission from Wiley-VCH
Figure 3. (a, b) TEM and HRTEM images of edge-rich graphitic ordered mesoporous carbon (GOMC), (c) The stability test of o-GOMC-1 for H2O2 electrosynthesis[96]. Reproduced with permission from Wiley-VCH. (d) O 1s spectrum of O-CNTs. (e) ORR polarization curves for SP and AB-based CMFCs. (f) Calculated two-electron ORR-related volcano plot via the function of ΔGOOH*[62]. Reproduced with permission from Nature Publishing Group. (g) The mechanism of H2O2 electrosynthesis for N-doped CNH[98]. Reproduced with permission from Elsevier Inc. (h) Free energy profile of two- and four-electron ORR pathways for different N-doped CMFCs[63]. Reproduced with permission from Nature Publishing Group. (i) The possible two-electron ORR mechanism of O-DG-30[64]. Reproduced with permission from Nature Publishing Group
Figure 4. (a) Schematic illustration of reaction pathways for different acetylene-based polymer, (b, c) WOR polarization curves of different acetylene-based polymer, the potential of Pt ring were set to -0.23 V and 0.6 V vs. Ag/AgCl to detect O2 and H2O2, respectively[71]. Reproduced with permission from Wiley-VCH. (d) The possible reaction pathway of PTFE-coated carbon electrode. (e) The data points depict *OH binding energies on different carbon-based metal-free nanomaterials. (f) The coverages of oxygen atoms in different carbon-based defected structures surface[99]. Reproduced with permission from Nature Publishing Group
Figure 5. (a, b) The synthesis illustration of N-doped CMFCs and the corresponding catalytic ability for CO2RR[76]. Reproduced with permission from Wiley-VCH. (c) Schematic diagram of the structure of NSHCF. (d-f) Polarization curves, FE, and CO2RR free energy of NSHCF900 catalyst[102]. Reproduced with permission from Wiley-VCH
Figure 6. (a, b) Schematic illustration of synthesis procedures, the stability and corresponding formate FE of N-C61 catalyst. (c) Tafel plots comparison for N-C61-800 and pristine C61. (d) The proposed mechanism of N-C61 electrocatalysts for CO2RR[104]. Reproduced with permission from Royal Society of Chemistry. (e) TEM and HRTEM images, (f, g) FE of CH4 and
$j_{{{\rm{CH}}}_4}$ for GQD-NH2-H and GQD-NH2-L electrocatalysts[105]. Reproduced with permission from Nature Publishing GroupFigure 7. (a, b) The performance of ammonia production for different N-doped porous carbons[18]. Reproduced with permission from American Chemical Society. (c) Structural illustration of B-doped graphene and the corresponding BC3 sites for adsorbing N2[19]. Reproduced with permission from Elsevier Inc. (d, e) NH3 yields, FEs and DFT calculations for NRR on the B4C (110) surface[113]. Reproduced with permission from Nature Publishing Group. (f, g) NH3 yields of PCN-based catalysts. (h-j) N2 adsorption model and charge density difference on “N” vacancy of PCN. (k) Free energy diagram toward NRR for PCN[114]. Reproduced with permission from Wiley-VCH
Figure 8. (a) The Cl adsorption structure at different O sites[120]. Reproduced with permission from Wiley-VCH. (b, c) Schematic illustration and performance of urea synthesis on the F-CNT[121]. Reproduced with permission from Elsevier Inc. (d) The FE of electrochemical formaldehyde reduction for various metal/metal-free catalysts[123]. Reproduced with permission from Nature Publishing Group
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