MS Material Sciences 2160-7613 Scientific Research Publishing 10.12677/MS.2022.1211137 MS-58423 MS20221100000_71906945.pdf 化学与材料 二维ZIF8衍生的Fe-N-C纳米片的制备及其氧还原催化性能研究 Preparation of Two-Dimensional Fe-N-C Nanosheets Derived from ZIF8 and Their Catalytic Performance for Oxygen Reduction Reaction 王 晋兿 1 2 刘 倩 1 2 null 天津工业大学，天津 14 11 2022 12 11 1230 1236 © Copyright 2014 by authors and Scientific Research Publishing Inc. 2014 This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/

开发先进的氧还原反应(ORR)电催化剂是提升燃料电池、金属空气电池器件效率的关键，对可再生清洁能源的存储与转换利用具有重要意义。本文首先通过无机盐氯化钾辅助热解制备了二维氮掺杂碳(2D-NC)纳米片，再利用浸渍和两步退火的方法将铁掺杂到2D-NC基质中，得到二维Fe-N-C (2D-Fe-N-C)纳米片，相对于已被广泛报道的三维Fe-N-C，二维Fe-N-C纳米片将更多的活性位点暴露在表面，有利于提升催化活性。结果表明，通过加入氯化钾，2D-NC在较低温度(700℃)下即可碳化，且通过浸渍和两步低温退火实现铁掺杂，避免了铁物种的团聚。通过优化无机盐的用量，得到2D-Fe-N-C-5催化剂在碱性溶液中具有比较优异的催化活性，相对可逆氢电极，起始电位达0.999 V，半波电位为0.808 V，其氧还原反应的动力学电流密度与商业Pt/C相当，本文的研究工作对金属–氮–碳催化剂的制备具有一定参考意义。 The development of advanced electrocatalysts for oxygen reduction reaction (ORR) is the key to improve the efficiency of fuel cells and metal-air cells, and is of great significance for the storage, conversion and utilization of renewable clean energy. In this paper, two-dimensional nitrogen-doped carbon nanosheets (2D-NC) were prepared by inorganic salt KCl assisted pyrolysis, and then iron was doped into the 2D-NC matrix by immersion and two-step annealing to obtain two-dimensional Fe-N-C nanosheets (2D-Fe-N-C). Compared with the widely reported three-dimensional Fe-N-C, two- dimensional Fe-N-C nanosheets expose more active sites on the surface, which is conducive to improving catalytic activity. The results show that 2D-NC can be carbonized at a lower temperature (700℃) by adding KCl, and iron doping can be achieved by immersion and two-step low temperature annealing, avoiding the agglomeration of iron species. By optimizing the amount of inorganic salts, 2D-Fe-N-C-5 catalyst has excellent catalytic activity in alkaline solution. The onset potential is 0.999 V (vs RHE), and the half-wave potential is 0.808 V (vs RHE). The kinetic current density of its oxygen reduction reaction is equivalent to that of commercial Pt/C. The research work in this paper has certain reference significance for the preparation of metal nitrogen carbon catalysts.

 金属–氮–碳，两步退火，氧还原反应，电催化，无机盐辅助热解, Metal-Nitrogen-Carbon Two-Step Annealing Oxygen Reduction Reaction Electrocatalysis Inorganic Salt Assisted Pyrolysis 摘要

开发先进的氧还原反应(ORR)电催化剂是提升燃料电池、金属空气电池器件效率的关键，对可再生清洁能源的存储与转换利用具有重要意义。本文首先通过无机盐氯化钾辅助热解制备了二维氮掺杂碳(2D-NC)纳米片，再利用浸渍和两步退火的方法将铁掺杂到2D-NC基质中，得到二维Fe-N-C (2D-Fe-N-C)纳米片，相对于已被广泛报道的三维Fe-N-C，二维Fe-N-C纳米片将更多的活性位点暴露在表面，有利于提升催化活性。结果表明，通过加入氯化钾，2D-NC在较低温度(700℃)下即可碳化，且通过浸渍和两步低温退火实现铁掺杂，避免了铁物种的团聚。通过优化无机盐的用量，得到2D-Fe-N-C-5催化剂在碱性溶液中具有比较优异的催化活性，相对可逆氢电极，起始电位达0.999 V，半波电位为0.808 V，其氧还原反应的动力学电流密度与商业Pt/C相当，本文的研究工作对金属–氮–碳催化剂的制备具有一定参考意义。

 关键词

金属–氮–碳，两步退火，氧还原反应，电催化，无机盐辅助热解

 Preparation of Two-Dimensional Fe-N-C Nanosheets Derived from ZIF8 and Their Catalytic Performance for Oxygen Reduction Reaction<sup> </sup>

Jinyi Wang, Qian Liu*

Tiangong University, Tianjin

Received: Oct. 25th, 2022; accepted: Nov. 20th, 2022; published: Nov. 28th, 2022

 ABSTRACT

The development of advanced electrocatalysts for oxygen reduction reaction (ORR) is the key to improve the efficiency of fuel cells and metal-air cells, and is of great significance for the storage, conversion and utilization of renewable clean energy. In this paper, two-dimensional nitrogen-doped carbon nanosheets (2D-NC) were prepared by inorganic salt KCl assisted pyrolysis, and then iron was doped into the 2D-NC matrix by immersion and two-step annealing to obtain two-dimensional Fe-N-C nanosheets (2D-Fe-N-C). Compared with the widely reported three-dimensional Fe-N-C, two- dimensional Fe-N-C nanosheets expose more active sites on the surface, which is conducive to improving catalytic activity. The results show that 2D-NC can be carbonized at a lower temperature (700℃) by adding KCl, and iron doping can be achieved by immersion and two-step low temperature annealing, avoiding the agglomeration of iron species. By optimizing the amount of inorganic salts, 2D-Fe-N-C-5 catalyst has excellent catalytic activity in alkaline solution. The onset potential is 0.999 V (vs RHE), and the half-wave potential is 0.808 V (vs RHE). The kinetic current density of its oxygen reduction reaction is equivalent to that of commercial Pt/C. The research work in this paper has certain reference significance for the preparation of metal nitrogen carbon catalysts.

Keywords:Metal-Nitrogen-Carbon, Two-Step Annealing, Oxygen Reduction Reaction, Electrocatalysis, Inorganic Salt Assisted Pyrolysis

Copyright © 2022 by author(s) and beplay安卓登录

This work is licensed under the Creative Commons Attribution International License (CC BY 4.0).

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 1. 引言

氢氧燃料电池和金属–空气电池由于其高能量密度、低运行温度和环境友好性，是实现可再生清洁能源的存储与转换利用的关键技术 [ 1 ] [ 2 ]。但这两种器件缓慢的阴极氧还原反应(ORR)动力学在很大程度上限制了其性能，开发高效、经济的ORR的电催化剂是实现氢氧燃料电池、金属–空气电池大规模推广应用的主要挑战之一。迄今为止，最有效的ORR催化剂是铂基金属(PGM)基材料，可以满足实际应用的高活性和耐久性要求 [ 3 ]。然而，PGM的高成本和稀缺性大大阻碍了它们的广泛使用。因此，迫切需要开发低成本、高活性、高稳定性的阴极氧还原电催化剂。

与过渡金属和氮共掺杂的碳材料(M-N-C)，特别是单原子，由于其最大的原子利用率和高固有活性 [ 4 ] [ 5 ] [ 6 ]，是氧还原反应的潜在候选材料。金属有机框架(MOFs)，特别是具有丰富的碳和氮物种的ZIF-8，是构建M-N-C电催化剂的良好平台 [ 7 ]。据报道，具有活性位点的铁单原子催化剂具有较优越的半波电位(E1/2)，可与商业化的Pt/C相媲美 [ 8 ] [ 9 ]。目前已经建立了各种技术策略，将Fe-Nx位点嵌入到催化剂中，以提高其电催化活性。例如，Chen课题组制备了一种高活性和稳定的分离单原子Fe/N掺杂多孔碳(ISA Fe/CN)催化剂 [ 10 ]。通过900℃的高温热解将ZIF-8转化为N掺杂多孔碳，同时笼内的Fe(acac)3被有机连接剂的碳化还原，导致形成锚定在氮物种上的孤立的单个铁原子。该材料体现出了优异的ORR电催化性能(半波电位为0.90 V vs RHE)。煅烧过程可以调节金属离子的配位环境，从而可以调节催化性能，近年来ZIF-8衍生的单原子催化剂的设计策略有很大的进展，但是煅烧温度一般都在800℃以上。在这个温度区域，MOF的晶体结构完全坍塌，大量有关结构演化的信息被隐藏，导致该类型材料的电催化活性降低 [ 11 ] [ 12 ] [ 13 ] [ 14 ]。另外，大多数ZIF-8是使用有机溶剂甲醇制备得到的，这不仅提高了成本，还不利于环保。因此，合理设计ZIF-8衍生的单原子催化剂并降低煅烧温度是制备高性能氧还原催化剂的关键。

近年来，广大研究者们还发现大多数埋藏在大量碳基质中的Fe-N-C位点不能被全面用于催化ORR [ 15 ]。然而，为了进一步提高其催化性能，还需要采用更多的策略。Sun及其同事和Strasser及其同事开发的多步热解技术是有效提高铁基碳材料催化性能的最有前途的方法 [ 16 ] [ 17 ]。Ye课题组在Ar和NH3中用柠檬酸铁铵功能化的ZIF-8的两步热解反应，以提高在碱性和酸性介质中的ORR性能 [ 18 ]。通过改变热解时间，最佳C-ZIF-8 (4/2)催化剂在碱性和酸性介质中表现出较高的ORR活性，在锌空气电池和PEMFC试验中具有较高的性能，其中两步退火最主要的目的是可以进一步地将材料全部碳化为M-Nx [ 19 ]。因此，两步热解为合成ZIF-8衍生的高效ORR催化剂提供了一个有吸引力的途径。

受上述工作的启发，我们探索了将无机盐辅助热解和两步退火结合起来的方法来提高活性位点的利用率和促进传质。我们首先通过KCl辅助热解法在较低的碳化温度下制备了二维氮掺杂碳基底(2D-NC)。利用2D-NC作为载体，以六水三氯化铁为铁源，通过两步退火处理合成了形貌良好的二维片状Fe-N-C电催化剂(2D-Fe-N-C)。通过对KCl用量的优化，发现用5 g KCl制备的氮掺杂碳再经过两步退火而得到的2D-Fe-N-C-5催化剂在0.1 mol/L KOH中展现出比较优异的催化活性(半波电位E1/2= 0.808 V)。

 2. 实验部分 2.1. 试剂

六水合硝酸锌［Zn(NO3)2∙6H2O］、六水合三氯化铁(FeCl3∙6H2O)、2-甲基咪唑(C4H6N2)、氯化钾(KCl)、盐酸(HCl)、无水乙醇(C2H6O)，均为分析纯。实验中所用的试剂未经任何额外处理，实验用水均为去离子水。

 2.2. 2D-NC的制备

称取1.275 g Zn(NO3)2∙6H2O和2.915 g 2-甲基咪唑分别溶解在100 mL去离子水中，然后快速混合两种水溶液，并剧烈搅拌2 h。将所产生的白色沉淀静置12 h。离心收集产物，然后用去离子水和无水乙醇洗涤，在80℃下干燥过夜，得到2D ZIF-8。取0.5 g 2D ZIF-8与一定量的KCl溶解于40 mL的去离子水中，然后旋转蒸发干燥。在干燥(120℃)过夜后，将KCl插入的粉末在N2发酵中加热至700℃ (加热速率为5℃/min)，维持5 h，分别用HCl (2 M)、去离子水、无水乙醇洗涤材料，然后在80℃下干燥过夜，最后得到二维氮掺杂碳基底，命名为2D-NC。改变KCl的用量，用5 g和10 g KCl制备的氮掺杂碳分别标记为2D-NC-5和2D-NC-10。

 2.3. 2D-Fe-N-C的制备

将174 mg FeCl3∙6H2O与100 mg 2D-NC分散在20 mL无水乙醇溶液中，超声10 min，首先旋转蒸发干燥，然后在80℃的烘箱中干燥。首先在N2流中加热到300℃ (加热速率为5℃/min)，维持5 h；使用去离子水-无水乙醇混合物彻底洗涤后，干燥的粉末(80℃)在N2流下以550℃ (加热速率为5℃/min)，维持5 h。得到最终的催化剂二维片状Fe-N-C电催化剂，命名为2D-Fe-N-C-x (x表示为加入无机盐KCl的用量)。

 2.4. 样品结构和电化学性能测试

本文通过扫描电子显微镜(SEM)研究了样品的微观形貌并且通过X射线衍射(XRD)研究了样品的晶体结构。

电化学测试均使用电化学工作站(CHI 760E)，选择三电极体系进行相关测试，以旋转圆盘电极(RDE)为工作电极，对电极和参比电极分别为石墨棒电极和Ag/AgCl电极，电解液为0.1 mol/L KOH溶液。测试中的所有电位都转化为可逆氢电极(RHE)电位，转换公式：

ERHE= EAg/AgCl+ 0.0591 × pH + 0.964 (1)

其中，ERHE为相对于可逆氢电极的电势(V)，EAg/AgCl为实验测得的相对于Ag/AgCl参比电极的电位(V)，pH由电解液的pH测得。

 3. 结果与讨论

图1(a)为2D ZIF-8前驱体的二维纳米片状形貌。通过旋转蒸发干燥，将Fe源掺杂之后得到的2D-Fe-N-C-5仍然具有二维纳米片的形貌(图1(b))。其中，2D-Fe-N-C-5的表面较为平滑，且无金属颗粒生成。

图1. (a) 2D ZIF-8和(b) 2D-Fe-N-C-5的SEM图

由扫描电镜可以看出，在合成2D-Fe-N-C时，通过添加KCl，催化剂的形貌没有发生明显的变化，但是尺寸相对变小。值得注意的是，扫描电镜图中没有看到明显的金属颗粒/团簇，且2D-Fe-N-C-5样品主要呈现碳材料的二维形貌特征，Fe元素可能是以单分散的Fe-Nx位点分布在碳基底中，而这种高密度的单分散催化活性中心有望在电催化ORR中显现出相对优异的性能。而且相对于已被广泛报道的三维Fe-N-C，二维Fe-N-C纳米片将更多的活性位点暴露在表面，有利于提升催化活性。

我们还通过XRD研究了样品的晶体结构。由图2所示，2D-Fe-N-C-5催化剂在25˚和43˚处有两个明显的峰(图2)，它们被鉴定为一定程度的石墨化碳结构，分别对应于石墨碳的(100)和(002)平面。除了两个石墨化的碳峰外，没有观察到其他属于铁种类的峰，这表明所合成的纳米片中可能不存在铁单质颗粒或氧化铁颗粒。

我们在O2饱和的0.1 M KOH电解液中评估了所有催化剂的活性。通过线性扫描伏安法(LSV)进一步研究所有样品的催化活性，扫描速率和旋转速度分别设置为5 m∙V∙s−1和1600 rpm。如图3(a)所示，2D-Fe-N-C-5表现出较优的ORR催化活性，其起始电位(Eonset)为0.999 V，半波电位(E1/2)为0.808 V，极限电流密度(jk)为5.06 mA∙cm−2；超过了2D-Fe-N-C-10 (Eonset= 1 V，E1/2= 0.725 V，jk= 3.9635 mA∙cm−2)，且高于文献中报道的不添加KCl的NC (E1/2= 0.77 V) [ 20 ]。显然，添加适量的KCl对提高Fe-N-C的催化活性有一定的帮助，可以归因于纳米片的结构有助于活性位点的暴露，加速反应物的传输。随着KCl用量的进一步增加，导致了ORR催化性能的下降。因此，本研究中KCl的最佳用量为5 g。

图2. 2D-Fe-N-C-5和2D-NC-5的XRD光谱

图3. 2D-Fe-N-C-5催化剂的ORR测试性能。(a) 2D-Fe-N-C-5、2D-Fe-N-C-10及20% Pt/C催化剂的极化曲线；(b) 2D-Fe-N-C-5在不同转速下的极化曲线；(c) 2D-Fe-N-C-5的电子转移数；(d) 2D-Fe-N-C-5、2D-Fe-N-C-10及20% Pt/C催化剂的Tafel斜率

为了进一步理解ORR的动力学信息，我们还利用LSV曲线研究了2D-Fe-N-C-5在不同速度下的电子传递动力学。2D-Fe-N-C-5电极的极限电流密度随着转速的增加而增加，导致更快的氧通量到达电极表面，从而加快ORR的反应动力学过程(图3(b))。如图3(c)所示，电子转移数(n)是ORR催化剂的另一个关键参数。根据K-L方程计算，在0.3~0.55 V的电位范围内，2D-Fe-N-C-5的转移电子数在3.68~3.82之间，说明2D-Fe-N-C-5催化ORR是一个以四电子转移反应为主的过程。为了进一步地验证反应动力学，根据LSV得到了Tafel斜率，如图3(d)所示。在合成氮掺杂碳的过程中，加入KCl辅助热解，使得Tafel斜率逐渐减小，2D-Fe-N-C-5的最终值仅为77.46 mV∙dec−1，低于Pt/C (113.81 mV∙dec−1)。说明KCl辅助热解和两步退火都在ORR动力学方面具有优异的优势。

 4. 结论

综上所述，我们通过KCl辅助热解ZIF-8制备了2D-NC，之后又经过两步退火处理合成了形貌良好的二维片状Fe-N-C电催化剂(2D-Fe-N-C)。在合成2D-NC时，加入无机盐KCl，降低了碳化温度(700℃)，且形貌也没有发生坍塌和破碎；在Ar气氛中的第一步热解温度低于金属前驱体的分解温度，所以不能全部碳化为单原子，会有团簇或者氧化物附着在表面；再进行一次热解，就可以全部碳化为Fe-Nx，使其成分较均匀化，石墨化程度增加。得益于电子结构和形貌的强协同作用，所得到的催化剂2D-Fe-N-C-5在增强的ORR活性中表现出0.808 V的半波电位，且2D-Fe-N-C-5也显示出了接近四电子过程。因此，KCl辅助热解策略和两步退火方法都可以提高其电催化活性，这对其它氧还原催化剂制备具有一定借鉴作用。

 致谢

本文作者感谢天津市教委科研计划项目(No.2017KJ098)对本论文的资助。

 文章引用

王晋兿,刘 倩. 二维ZIF8衍生的Fe-N-C纳米片的制备及其氧还原催化性能研究Preparation of Two-Dimensional Fe-N-C Nanosheets Derived from ZIF8 and Their Catalytic Performance for Oxygen Reduction Reaction[J]. 材料科学, 2022, 12(11): 1230-1236. https://doi.org/10.12677/MS.2022.1211137

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