Recent Progress in ZIF-Derived Carbons for Enhanced Oxygen Reduction Reaction Electrocatalysis

The oxygen reduction reaction (ORR) represents a cornerstone for many clean energy conversion technologies such as fuel cells and metal-air batteries. Nevertheless, the commercialization of these technologies is largely impeded by the slow kinetics of ORR, for which active, durable and cost-effective ORR catalysts are needed. In recent years, zeolitic imidazolate framework (ZIF) derived carbon materials emerge as a new class of non-precious metal catalysts (NPMCs) toward ORR, largely benefiting from their high surface area, abundant porosity, tunable chemical/electronic structure, and superior ORR activity which is comparable or even surpasses those state-of-the-art Pt-based ORR catalysts. This review offers a comprehensive overview of the recent advances in ZIF-derived carbons for ORR. The synthesis strategies and the key factors affecting the ORR performance of ZIF-derived carbon materials are discussed. Future research directions and perspectives on exploring ZIF derived carbons as efficient ORR catalysts are highlighted, with a focus on the principles of rationally engineering the coordination structures of active sites.     

部分氧化碳化硅提高钯催化氧还原反应性能

燃料电池具有能量转换效率高的优点,是能量转换与储存的高效器件之一.目前,燃料电池阴极氧还原反应(ORR)动力学缓慢,并且催化ORR大量使用铂碳(Pt/C)催化剂,由于Pt储量少,价格高,载体碳材料易发生碳蚀导致催化剂稳定性降低,限制了其进一步商业化应用.钯(Pd)与Pt为同族元素,具有相似的电子结构和化学性质,其储量是Pt的50倍,同时,Pd具有良好的抗甲醇毒性和抗一氧化碳毒性,因此,被视为燃料电池中阴极Pt催化剂的潜在替代品.但商用Pd/C催化剂的ORR活性较Pt/C差,因此,大量的研究工作集中在提高Pd基ORR催化剂的活性方面:将Pd与具有3d轨道的金属形成合金或将Pd负载到不同的载体上.通过选择合适的载体影响Pd的电子结构,从而提高催化剂活性和稳定性,是一种较简单的、有利于规模化生产Pd基ORR催化剂的方法.碳化硅(SiC)具有良好的电化学稳定性、热稳定性、机械强度和较强的供电子能力,可被用作ORR的金属催化剂载体.然而,由于金属与SiC作用较弱,需要制备特殊形貌的SiC或将SiC表面改性;通常,这些SiC基载体的制备过程复杂并且成本高.而在有氧条件下制备、保存或使用SiC时,其表面不可避免地被氧化,这种在温和条件下生成的表面具有含氧官能团的SiC,由于制备过程简便,可以大规模生产,且与金属有强的相互作用,是一种很有前景的ORR的Pd基催化剂载体.对于用于替代Pt基催化剂的负载型Pd基ORR催化剂的开发和大规模制造来说,对载体表面改性的深入了解是一个重要并且具有挑战性的课题.目前尚未发现关于SiC表面的含氧基团对ORR性能影响的报道.因此,详细考察SiC载体上含氧基团在ORR中的作用对于理解、设计和开发具有优异ORR性能的SiC负载催化剂至关重要.本文采用沉积沉淀法在表面部分氧化的碳化硅(O-SiC)均匀负载了平均直径为5.2 nm的Pd纳米颗粒.与20 wt%商业Pt/C相比,制备的2.5 wt%Pd/O-SiC催化剂显示出较好的ORR活性(半波电位正向移动10 mV),较好的稳定性(10 h后,电流密度损失3.5%vs.34.9%),和较高的抗甲醇毒性.结构表征及密度泛函理论(DFT)计算结果表明,与Pd/C相比,Pd/O-SiC具有优异的ORR性能主要是由于O-SiC载体对Pd纳米颗粒具有电子调控作用,使Pd带负电.富电子Pd增强了ORR关键中间体OOH的吸附,降低了反应的吉布斯自由能,从而提高了ORR活性.另外,O-SiC载体对Pd纳米颗粒具有大的结合能和较好的SiC稳定性,增强了Pd/O-SiC催化剂的抗甲醇毒性及稳定性.DFT计算结果表明,SiC表面部分氧化后,仍保持对Pd的较高结合能,同时大幅增强了催化剂对中间体的吸附,降低了ORR关键电化学步骤吉布斯自由能,从而提高了氧还原性能.因此,本工作明确了SiC表面氧化的作用,同时提供了一种简易大规模制备高效负载型铂基替代ORR催化剂的策略.     

Factors controlling the energetics of the oxygen reduction reaction on the Pd-Co electro-catalysts: insight from first principles

We report here results of our density functional theory based computational studies of the electronic structure of the Pd-Co alloy electrocatalysts and energetics of the oxygen reduction reaction (ORR) on their surfaces. The calculations have been performed for the (111) surfaces of pure Pd, Pd(0.75)Co(0.25) and Pd(0.5)Co(0.5) alloys, as well as of the surface segregated Pd/Pd(0.75)Co(0.25) alloy. We find the hybridization of dPd and dCo electronic states to be the main factor controlling the electrocatalytic properties of Pd/Pd(0.75)Co(0.25). Namely the dPd-dCo hybridization causes low energy shift of the surface Pd d-band with respect to that for Pd(111). This shift weakens chemical bonds between the ORR intermediates and the Pd/Pd(0.75)Co(0.25) surface, which is favorable for the reaction. Non-segregated Pd(0.75)Co(0.25) and Pd(0.5)Co(0.5) surfaces are found to be too reactive for ORR due to bonding of the intermediates to the surface Co atoms. Analysis of the ORR free energy diagrams, built for the Pd and Pd/Pd(0.75)Co(0.25), shows that the co-adsorption of the ORR intermediates and water changes the ORR energetics significantly and makes ORR more favorable. We find the onset ORR potential estimated for the configurations with the O-OH and OH-OH co-adsorption to be in very good agreement with experiment. The relevance of this finding to the real reaction environment is discussed.     

The sum is more than its parts: stability of MnFe oxide nanoparticles supported on oxygen-functionalized multi-walled carbon nanotubes at alternating oxygen reduction reaction and oxygen evolution reaction conditions

Successful design of reversible oxygen electrocatalysts does not only require to consider their activity towards the oxygen reduction (ORR) and the oxygen evolution reactions (OER), but also their electrochemical stability at alternating ORR and OER operating conditions, which is important for potential applications in reversible electrolyzers/fuel cells or metal/air batteries. We show that the combination of catalyst materials containing stable ORR active sites with those containing stable OER active sites may result in a stable ORR/OER catalyst if each of the active components can satisfy the current demand of their respective reaction. We compare the ORR/OER performances of oxides of Mn (stable ORR active sites), Fe (stable OER active sites), and bimetallic Mn_0.5Fe_0.5 (reversible ORR/OER catalyst) supported on oxidized multi-walled carbon nanotubes. Despite the instability of Mn and Fe oxide for the OER and the ORR, respectively, Mn_0.5Fe_0.5 exhibits high stability for both reactions.

     

Mn-N-C催化剂的制备及其在碱性介质中对氧还原反应的电催化性能

选用壳聚糖(CS)为原料制备了壳聚糖水杨醛席夫碱锰配合物(Mn-CS-sal)。将Mn-CS-sal配合物负载于石墨碳上得到碳载配合物(Mn-CS-sal/C),后经高温热处理得到Mn-N-C目标催化剂(Mn-N-C-t,t=200、400、600、800、1 000℃)。采用FT-IR、XRD、XPS和电化学等方法对催化剂的组成和结构进行了表征,对其在氧还原反应中的电催化性能进行了研究。结果表明,所得到的Mn-N-C催化剂对氧还原反应(ORR)具有很好的催化作用,但以600℃热处理制备的催化剂其活性最好。催化剂中Mn-N-C结构是催化ORR的活性位。采用循环伏安法获得了Mn-N-C-t催化ORR的动力学参数,即总的转移电子数n和电子传递系数αnα;具有最佳活性的Mn-N-C-600催化剂的总转移电子数为3.63,说明在此条件下,Mn-N-C-600催化ORR主要以4e转移途径为主,由此提出了可能的氧还原反应的机理。     

Ceria-promoted oxygen reduction reaction in Pd-based electrocatalysts

PdCo-ceria electrocatalyst is synthesized on carbon support in a size of a few nm by colloid method. Enhanced oxygen reduction reaction (ORR) kinetics of PdCo is observed in the presence of ceria similarly as confirmed for PtCo-ceria in a half cell experiment. In addition, there appears a positive shift of the ORR onset potential of PdCo-ceria compared to PdCo while PtCo-ceria shows no such an apparent shift of onset potential. These effects of ceria on the ORR onset potential and the ORR kinetics are more remarkable as temperature increases. To get the most of oxygen storage and supply capacity of ceria, a high temperature proton exchange membrane fuel cell (PEMFC) is fabricated using PdCo-ceria as a catalyst at the cathode. Ceria effects on the ORR of PdCo-ceria catalyst are realized in the form of higher OCV and lower Tafel slope compared to PdCo catalyst in the PEMFC single cell performance. Enhancements in both ORR kinetics and ORR onset potential are very attractive features of ceria as a co-catalyst in the development of a non-Pt ORR electrocatalyst.     

Transition metal coordinated framework porphyrin for electrocatalytic oxygen reduction

A series of transition metal coordinated framework porphyrin was evaluated regarding the electrocatalytic oxygen reduction reactivity for an optimized selection of the coordinated metal ion.     

Polyethyleneimine modified AuPd@PdAu alloy nanocrystals as advanced electrocatalysts towards the oxygen reduction reaction

Designing the low cost, active, durable, and alcohol-tolerant cathode catalysts towards the oxygen reduction reaction(ORR) is significant for the large-scale commercialization of direct alcohol fuel cells.Recently, Pd-based nanocrystals have attracted attention as Pt-alternative cathode catalysts towards the ORR in the alkaline electrolyte. Unfortunately, the pristine Pd-based nanocrystals lack the selectivity towards the ORR due to their inherent activity for the alcohol molecule oxidation reaction in the alkaline electrolyte. In this work, polyethyleneimine(PEI) modified Au Pd alloy nanocrystals with Au-rich Au Pd alloy cores and Pd-rich Pd Au alloy shells(AuPd@PdAu-PEI) are successfully synthesized using a traditional chemical reduction method in presence of PEI. The rotating disk electrode(RDE) technique is applied to evaluate the ORR performance of AuPd@PdAu-PEI nanocrystals. Compared with commercial Pd black,AuPd@PdAu-PEI nanocrystals show significantly enhanced activity and durability towards the ORR, and simultaneously exhibit particular alcohol tolerance towards the ORR in the alkaline electrolyte.     

氢钼青铜对铂催化氧还原反应的促进作用

采用循环伏安法在玻碳电极上和硫酸溶液中电沉积制备出铂催化剂(Pt)及铂-氢钼青铜复合催化剂(Pt-HxMoO3), 用旋转圆盘电极研究并比较了它们对硫酸溶液中氧还原反应的催化活性. 研究结果表明, HxMoO3能明显地提高Pt对氧还原反应的电催化活性. 通过对静态电极上氧还原的峰电流与扫描速度的关系以及旋转圆盘电极上氧还原电流与旋转速度的关系的分析发现, HxMoO3提高了铂电极氧还原反应电荷传递步骤的传递系数, 因此加快了氧还原的动力学过程.     

吸电子和亲水性Co-卟啉促进电催化氧还原反应的研究

研究影响电催化氧还原反应活性的因素对于合理设计高效的氧还原反应催化剂至关重要。调节催化剂电子结构通常被用于精确调控电催化氧还原反应活性。然而, 该反应发生在液/气/固界面, 很少有报道调控分子催化剂的亲疏水性来提高其催化活性。在此, 我们报道了两种钴卟啉NO₂-CoP（5,10,15,20-四（4-硝基苯基）钴卟啉）和5F-CoP（5,10,15,20-四（五氟苯基）钴卟啉）并研究了其电催化氧还原反应性能。通过同时调控meso-位取代基的电子结构和亲水性能, NO₂-CoP显示出比5F-CoP更高的电催化氧还原反应活性, 其半波电位向阳极方向移动近60 mV。NO₂-CoP比5F-CoP具有更好的亲水性。理论计算表明, NO₂-CoP比5F-CoP更容易有效地与O₂分子结合形成Co^III-O₂^·-。这项工作提供了一个简单而有效的策略, 通过使用吸电子和亲水取代基来提高钴卟啉的氧还原反应活性。该策略对于设计和开发其他用于电催化的分子催化剂体系也具有重要的启发意义。     

Fe-N-modified multi-walled carbon nanotubes for oxygen reduction reaction in acid

We report a facile synthesis of Fe-N-C catalysts based on the surface functionalization of multi-walled carbon nanotubes (MWCNTs), which show high activity and stability for oxygen reduction reaction (ORR) in acid. Fe-N-MWCNT catalysts, whose ORR mass activities could vary by 3-4 times depending on the choice of Fe precursors, were found to have considerably higher ORR mass activity and higher stability than N-modified MWCNTs (N-MWCNTs). The Fe-N-MWCNT catalyst with a dominant Fe-N(x) moiety (with x ≈ 4) and a surface Fe/C ratio of ～0.004 exhibits the highest ORR mass activity in acid (～0.7 mA mg(-1)(Fe-N-MWCNT) at 0.8 V vs. RHE), where the lower mass activity of other Fe-N-MWCNT catalysts can be attributed to lower Fe/C ratios and Fe-N(x) moieties (with x smaller than 4) as revealed from X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) spectroscopy. Moreover, the enhanced stability of Fe-N-MWCNTs in comparison to N-MWCNTs can be attributed to less H(2)O(2) production during ORR as determined from rotating ring disk electrode (RRDE) measurements, and higher activity for H(2)O(2) electro-reduction by rotating disk electrode (RDE) measurements. The large surface Fe/C ratio and Fe-N(x) moiety corresponding to high ORR activity and stability of Fe-N-MWCNTs demonstrate that surface functionalization can be very helpful to graft active catalytic sites onto carbon nanostructures, and to provide insights into the ORR mechanism of non-noble metal catalysts (NNMCs) for proton exchange membrane fuel cells (PEMFCs).     

Recent development of efficient electrocatalysts derived from porous organic polymers for oxygen reduction reaction

Porous organic polymers (POPs) have recently emerged as promising candidates for catalyzing oxygen reduction reaction (ORR). Compared to conventional Pt-based ORR catalysts, these newly developed porous materials, including both non-precious metal based catalysts and metal-free catalysts, are more sustainable and cost-effective. Their porous structures and large surface areas facilitate mass and electron transport and boost the ORR kinetics. This mini-review will give a brief summary of recent development of POPs as electrocatalysts for the ORR. Some design principles, different POP structures, key factors for their ORR catalytic performance, and outlook of POP materials will be discussed.     

Insights into Nitrogen-doped Carbon for Oxygen Reduction: The Role of Graphitic and Pyridinic Nitrogen Species

Nitrogen-doped carbons (N/Cs) manifest good catalytic performance for oxygen reduction reaction (ORR) for fuel cell systems. However, to date, controversies remain on the role of active sites in N/Cs. In the present study, ORR test was conducted on three N/Cs in O₂-saturated 0.1 M KOH aqueous solution, where apparent linear correlation between graphitic N contents and ORR activity was observed. Theoretical calculations demonstrated that graphitic N doping is energetically more favorable than that of pyridinic N doping for ORR and the pyridinic N leads to more preferential with 2 e^– ORR pathway. These results reveal that graphitic N plays a key role in N/Cs mediated ORR activity. This work lays a solid foundation on identifying the active sites in heteroatom-doped carbons and can be exploited for rational design and engineering of effective carbon-based ORR catalysts.     

氮掺杂的碳材料中石墨化氮和吡啶氮对氧还原反应的催化特性

目前,开发高效的阴极氧还原反应(ORR)电催化剂是实现燃料电池和金属-空气电池商业化发展急需完成的目标。在过去的几十年中,人们在探索廉价高效的 ORR电催化剂(如 N掺杂的非金属及非铂电催化剂)领域做了广泛的研究。在 N掺杂的碳基 ORR催化剂中,已知的 N基活性位点主要分为四类,即吡啶类氮(P-N)、吡咯类氮(Py-N)、石墨化氮(G-N)和氧化类氮(O-N)。尽管人们对这四种类型氮的活性位点做了大量的研究,但是它们在催化反应中起到的 ORR催化作用以及催化机理和活性位点本身结构的关系仍不够明确。早期的研究中有人认为 P-N或者 Py-N是 ORR催化活性位点,也有人认为是 G-N起作用。最近也有研究表明, P-N和 G-N都是 ORR催化活性位点,只是在 ORR中所起的催化能力不同。因此,很有必要认清这些问题。
　　本文通过 Hummer法酸性氧化一次和两次碳黑 Vulcan XC-72(VXC-72)以及随后高温热处理,制备了一系列 ORR催化剂 VXCO-1, VXCO-2, VXCO-1(900)和 VXCO-2(900),采用场发射扫描电子显微镜(SEM), N2吸附脱附法,元素分析仪(EA), X射线光电子能谱(XPS),拉曼光谱仪(Raman), X射线衍射能谱(XRD),电化学循环伏安法和线性伏安法测试等手段研究 Hummers法酸氧化和高温热处理对 VXC-72形貌组成的影响,以及这些碳基中成分和其催化 ORR能力的关系。
　　 SEM结果表明, Hummer法酸性氧化处理 VXC-72一次和两次后可以逐层剥落其最外边的碳层结构,最终得到表面光滑的类片层状结构的碳材料(VXCO-1和 VXCO-2)。这种表面光滑的类片层状结构的碳材料比表面积大于处理前的 VXC-72,而高温热处理之后的碳材料(VXCO-1(900)和 VXCO-2(900))由于类石墨层碎片结构蒸发损失暴露出更多内部的微孔和介孔结构使比表面积增加。 Raman和 XRD结果表明,氧化处理使碳材料的石墨化程度增加,而高温热处理则降低了其石墨化程度。
　　 EA和 XPS结果表明, Hummer法酸性氧化处理可以使在碳材料中掺入的 N以石墨化的为主,高温热处理却使得石墨化氮转变为吡啶类的氮。 ORR结果发现,活性的石墨化氮倾向于使 ORR反应经历两电子过程,从而生成 H2O2为主要产物;而吡啶类氮的活性位点更倾向于使 ORR反应经过四电子过程,主产物是水。该结果有助于新型碳基氧还原催化剂的设计和分析。     

CoPy/C催化氧还原的碱效应及活性位研究

以碳黑(Vulcan XC-72R)为载体, 硫酸钴(CoSO₄ · 7H₂O)和吡啶(Py)作为催化剂前躯体, 经溶剂分散及800℃热处理可制备出高效催化氧还原反应(ORR)的碳载钴吡啶复合催化剂(15%Co25%Py/C, 质量分数). 采用红外光谱(IR)和X射线光电子能谱(XPS)等对催化剂的结构进行表征. 运用旋转圆盘电极(RDE)技术研究了不同浓度的KOH溶液(0.05~12.0 mol/L)对CoPy/C催化氧还原活性的影响. 结果表明, 不同浓度的KOH溶液对CoPy/C催化剂催化氧还原反应(ORR)的性能影响很大, 在0.05和0.1 mol/L KOH溶液中催化剂活性最高. 以其制备的气体扩散电极在0.05 mol/L KOH溶液(O₂气氛)中的半波电位为-0.138 V, 起峰电位为0.10 V, 同时表现出明显的极限扩散电流. 在-0.381 V时电流密度达到最大值(4.39 mA/cm²). 随着KOH溶液浓度的增加(pH值下降), 起始电压沿负方向移动, 同时动力学、 混合动力学和扩散区的电流密度均下降. RDE研究结果表明, 在0.05和0.1 mol/L KOH溶液中, O₂在CoPy/C电极上的还原主要经4e^-过程还原成H₂O. XPS研究结果表明, 吡啶作为小分子富氮源对提高催化剂的活性具有重要作用, 所制备催化剂经800℃高温热处理形成了石墨N, 吡啶N以及部分氧化态的氮结构, 其中石墨N和吡啶N作为催化剂的活性中心, 提供氧还原活性位, 从而使该类催化剂对氧还原表现出很好的电催化性能和选择性.     

A computational exploration of the oxygen reduction reaction over a carbon catalyst containing a phosphinate functional group

Oxygen reduction reaction (ORR) over a carbonaceous catalyst (1) with a phosphinate (>P(=O)OH) moiety was explored computationally. Under the acidic environment of a fuel cell, 1 could be active for ORR and be converted to 2 with a >P(OH)(2) moiety. An edge phosphinate could be active for both 2- and 4-electron ORR.     

Metal–Organic Frameworks Based Electrocatalysts for the Oxygen Reduction Reaction

In view of the clean and sustainable energy, metal–organic frameworks (MOFs) based materials, including pristine MOFs, MOF composites, and their derivatives are emerging as unique electrocatalysts for oxygen reduction reaction (ORR). Thanks to their tunable compositions and diverse structures, efficient MOF-based materials provide new opportunities to accelerate the sluggish ORR at the cathode in fuel cells and metal–air batteries. This Minireview first provides some introduction of ORR and MOFs, followed by the classification of MOF-based electrocatalysts towards ORR. Recent breakthroughs in engineering MOF-based ORR electrocatalysts are highlighted with an emphasis on synthesis strategy, component, morphology, structure, electrocatalytic performance, and reaction mechanism. Finally, some current challenges and future perspectives for MOF-based ORR electrocatalysts are also discussed.     

Metal–Organic Frameworks Based Electrocatalysts for the Oxygen Reduction Reaction

In view of the clean and sustainable energy, metal–organic frameworks (MOFs) based materials, including pristine MOFs, MOF composites, and their derivatives are emerging as unique electrocatalysts for oxygen reduction reaction (ORR). Thanks to their tunable compositions and diverse structures, efficient MOF‐based materials provide new opportunities to accelerate the sluggish ORR at the cathode in fuel cells and metal–air batteries. This Minireview first provides some introduction of ORR and MOFs, followed by the classification of MOF‐based electrocatalysts towards ORR. Recent breakthroughs in engineering MOF‐based ORR electrocatalysts are highlighted with an emphasis on synthesis strategy, component, morphology, structure, electrocatalytic performance, and reaction mechanism. Finally, some current challenges and future perspectives for MOF‐based ORR electrocatalysts are also discussed.     

Electrochemical performance of annealed cobalt-benzotriazole/CNTs catalysts towards the oxygen reduction reaction

One of the major limitations yet to the global implementation of polymer electrolyte membrane fuel cells (PEMFCs) is the cathode catalyst. The development of efficient platinum-free catalysts is the key issue to solve the problem of slow kinetics of the oxygen reduction reaction (ORR) and high cost. We report a promising catalyst for ORR prepared through the annealing treatment under inert conditions of the cobalt-benzotriazole (Co-BTA) complex supported on carbon nanotubes (CNTs). The N-rich benzotriazole precursor was chosen based on its ability to complex Co(II) ions and generate under annealing highly reactive radicals able to tune the physicochemical properties of CNTs. X-Ray photoelectron spectroscopy (XPS) was used to follow the surface structure changes and highlight the active electrocatalytic sites towards the ORR. To achieve further evaluation of the catalysts in acidic medium, voltamperometry, rotating disk electrode (RDE), rotating ring-disk electrode (RRDE) and half-cell measurements were performed. The resulting catalysts (Co/N/CNTs) all show catalytic activity towards the ORR, the most active one resulting from annealing at 700 °C. The overall electron transfer number for the catalyzed ORR was determined to be ～3.7 with no change upon the catalyst loading, suggesting that the ORR was dominated by a 4e(-) transfer process. The results indicate a promising alternative cathode catalyst for ORR in fuel cells, although its performance is still lower (overpotential around 110 mV evaluated by RDE and RRDE) than the reference Pt/C catalyst.     

不同氮掺杂石墨烯氧还原反应活性的密度泛函理论研究

N掺杂石墨烯作为一种具有较高活性和稳定性的氧还原反应(ORR)催化剂,受到人们的广泛关注。然而不同的N掺杂类型对氧还原活性的影响一直存在争议。本文通过密度泛函理论分别对石墨型和吡啶型两种N掺杂石墨烯的ORR活性进行比较研究。能带结构分析表明,石墨氮掺杂石墨烯(GNG)的导电性随掺N量的增加而降低;吡啶氮掺杂石墨烯(PNG)的导电性则随掺N量的增加先提高后降低。当N掺杂浓度达到4.2%(原子分数)时,PNG具有最优导电性。且当N掺杂浓度大于1.4%时,PNG的导电率总是高于GNG。氧还原自由能阶梯曲线发现O₂的质子化是整个氧还原过程的潜在控制步骤。在同等氮掺杂浓度下,O₂的质子化自由能能变在GNG上低于在PNG上,意味着若在同等电子传输能力的情况下,GNG具有比PNG更优异的催化活性。进一步分析发现：当N掺杂浓度在低于2.8%时,GNG和PNG导电性差异小,其催化ORR活性由O₂质子化反应难易程度决定,GNG的催化活性优于PNG;当N掺杂浓度高于2.8%时,氮掺杂石墨烯的电子传输性能(导电性)成为决定催化剂ORR活性的主要因素,因此PNG表现出较GNG更高的活性。