Department of Chemical & Biological Engineering and Center for Micro-Engineered Materials (CMEM) , The University of New Mexico, Advanced Materials Laboratory , Albuquerque , New Mexico 87131 , United States.
ACS Appl Mater Interfaces. 2018 Apr 11;10(14):11623-11632. doi: 10.1021/acsami.7b18651. Epub 2018 Mar 27.
This study elucidates the synthesis-structure-property correlations of nitrogen moieties present in nitrogen-functionalized graphene nanomaterials toward oxygen reduction reactions (ORRs) and their electrochemical pathways in acidic and alkaline electrolytes. Porous three-dimensional nitrogen-doped graphene nanosheets (N/3D-GNSs) were fabricated using the sacrificial support method and doped with nitrogen using 10 atom % NH under thermal pyrolysis at T = 650, 850, and 1050 °C for evaluating the nitrogen species formed under different temperatures. The abundances of the various nitrogen species formed under pyrolytic conditions were evaluated with X-ray photoelectron spectroscopy. Using rotating ring-disk electrode, we analyzed the role played by the nitrogen moieties influencing the electrochemical activity of the N/3D-GNS supports for oxygen reduction reactions (ORRs) in both acidic and alkaline media. It was demonstrated that the concentrations of the nitrogen moieties: graphitic-N, quaternary, hydrogenated-N (hydrogenated nitrogen combined pyrrolic nitrogen and hydrogenated pyridine) and pyridinic-N varied considerably with pyrolysis temperatures. A decrease in graphitic-N content and an increase in the ratio of hydrogenated-N/pyridinic-N significantly improved the activity of the material. The half-wave and onset potentials as well as the current densities and hydrogen peroxide (HO)/(HO) yields of the N/3D-GNS materials also varied between acidic and alkaline electrolytes but followed the general trend in terms of pyrolysis temperatures and abundance of the nitrogen moieties. Among the synthesized materials, the 3D-graphene nanosheets that were doped with nitrogen at 850 °C, optimized to have the highest hydrogenated-N and lowest pyridinic-N as well as better catalyst-ionomer integration, showed the highest ORR performance. This strategy for the tunable synthesis of nitrogen-doped graphene materials with controlled nitrogen functionalization offers a platform for developing active supports or catalytic nanomaterials for fuel cell applications.
本研究阐明了氮官能化石墨烯纳米材料中氮基团在氧气还原反应（ORR）中的合成-结构-性能关系，以及它们在酸性和碱性电解液中的电化学途径。通过牺牲支撑法制备了多孔三维氮掺杂石墨烯纳米片（N/3D-GNS），并在 T = 650、850 和 1050°C 下使用 10 原子% NH3 进行热解，以评估不同温度下形成的氮物种。使用 X 射线光电子能谱评估了热解条件下形成的各种氮物种的丰度。通过旋转环盘电极，我们分析了氮基团在酸性和碱性介质中对 N/3D-GNS 支持物的氧还原反应（ORR）电化学活性的影响。结果表明，氮基团的浓度：石墨-N、季氮、氢化-N（氢化氮与吡咯氮和氢化吡啶结合）和吡啶-N 随热解温度有很大变化。石墨-N 含量降低和氢化-N/吡啶-N 比例增加显著提高了材料的活性。N/3D-GNS 材料的半波电位和起始电位以及电流密度和过氧化氢（HO）/（HO）产率在酸性和碱性电解液之间也有所不同，但遵循热解温度和氮基团丰度的一般趋势。在所合成的材料中，在 850°C 下掺杂氮的 3D 石墨烯纳米片优化为具有最高氢化-N 和最低吡啶-N 以及更好的催化剂-离聚物集成，表现出最高的 ORR 性能。这种用于可调谐合成具有受控氮官能化的氮掺杂石墨烯材料的策略为开发用于燃料电池应用的活性支持物或催化纳米材料提供了一个平台。
