Metal-free carbon materials functionalized with pyridinic nitrogen groups exhibit promising electrocatalytic activity for the oxygen reduction reaction (ORR). However, not all pyridinic nitrogen groups are equally active for the ORR, which remains ambiguous and requires rigorous quantification and differentiation by their basicity. Here, we introduce the potentiometric titration method for identifying and quantifying nitrogen-containing groups on carbon materials by their Lewis basicity and reactivity in characteristic tests. Various carbon materials are functionalized with nitrogen heteroatoms. Potentiometric titration, X-ray photoelectron spectroscopy (XPS), and elemental analysis suggest that a significant amount of pyridinic nitrogen groups are buried within the bulk structures and cannot be accessed by protons and oxygen molecules. Besides, pyridinic nitrogen functions located adjacent to other nitrogen atoms exhibit weaker basicity due to strong inductive or resonance effects, resulting in a negligible contribution to the ORR activity. ORR measurements under alkaline conditions suggest that the titratable pyridinic nitrogen groups are essential for the active site (or site pair), and kinetic current density is directly proportional to the density of titratable pyridinic nitrogen groups. Furthermore, the turnover frequency for the ORR increases with the Lewis basicity of the pyridinic nitrogen groups for all investigated carbon materials in alkaline and acidic conditions. Density functional theory (DFT) calculations suggest that the ORR occurs on the carbon atoms adjacent to pyridinic nitrogen groups. Pyridinic nitrogen with a higher Lewis basicity can affect adjacent carbon atoms more efficiently, which stabilizes the key intermediates for the ORR and decreases the activation barrier. This work provides an informative and convenient way for characterizing nitrogen-containing groups on carbon materials, especially in quantifying the active pyridinic nitrogen sites for the ORR.

中文翻译：

---

氧还原反应对吡啶氮功能化碳：活性位点定量和路易斯碱度的影响


用吡啶氮基团官能化的无金属碳材料对氧还原反应 （ORR） 表现出有希望的电催化活性。然而，并非所有吡啶氮基团对 ORR 的活性都相同，ORR 仍然不明确，需要根据其碱度进行严格的定量和区分。在这里，我们介绍了电位滴定法，用于在特性测试中通过路易斯碱度和反应性来识别和量化碳材料上的含氮基团。各种碳材料都用氮杂原子进行功能化。电位滴定、X 射线光电子能谱 （XPS） 和元素分析表明，大量的吡啶氮基团埋藏在体结构中，质子和氧分子无法进入。此外，由于强烈的感应或共振效应，位于其他氮原子附近的吡啶氮官能团表现出较弱的碱度，因此对 ORR 活性的贡献可以忽略不计。碱性条件下的 ORR 测量表明，可滴定的吡啶氮基团对于活性位点（或位点对）至关重要，并且动力学电流密度与可滴定的吡啶氮基团的密度成正比。此外，在碱性和酸性条件下，对于所有研究的碳材料，ORR 的周转频率随着吡啶氮基团的 Lewis 碱度而增加。密度泛函理论 （DFT） 计算表明，ORR 发生在与吡啶氮基相邻的碳原子上。 具有较高路易斯碱度的吡啶氮可以更有效地影响相邻的碳原子，从而稳定 ORR 的关键中间体并降低活化屏障。这项工作为表征碳材料上的含氮基团提供了一种信息丰富且方便的方法，尤其是在量化 ORR 的活性吡啶氮位点方面。