现代化工

现代化工 ›› 2025, Vol. 45 ›› Issue (7) : 88-93. DOI: 10.16606/j.cnki.issn0253-4320.2025.07.016

科研与开发

锰、铁掺杂的Co₃O₄/NCNTs在可逆氧反应中的性能研究

周广远 ,
赵安琪 ^*

作者信息 +

Study on properties of Mn and Fe doped Co₃O₄/NCNTs in reversible oxygen reaction

Guang-yuan ZHOU ,
An-qi ZHAO ^*

Author information +

文章历史 +

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摘要

以氮掺杂碳纳米管为载体,制备锰、铁掺杂的Co₃O₄/NCNTs双功能催化剂,并研究其在可逆氧反应中的催化性能。结果表明,锰掺杂比铁掺杂和未掺杂的催化剂具有更高的氧还原反应(ORR)、氧析出反应(OER)双功能电催化活性,掺杂一定量的锰元素后形成了Mn²⁺和Mn³⁺,提升了催化剂在可逆氧反应中的催化活性。

Abstract

Mn and Fe respectively doped Co₃O₄/NCNTs bifunctional catalysts are synthesized through using nitrogen doped carbon nano tubes (NCNTs) as a support,and their catalytic performance in reversible oxygen reactions is evaluated.The results indicate that the Mn-doped catalysts exhibit superior ORR/OER bifunctional electrocatalytic activity than Fe-doped and non-doped catalysts.After doping with a certain amount of Mn element,Mn²⁺ and Mn³⁺ are formed,which enhances the catalytic activity of the catalysts in reversible oxygen reactions.

Graphical abstract

关键词

锰掺杂 / 氧析出 / 氧还原 / 氮掺杂碳纳米管 / Co₃O₄

Key words

Mn-doped / oxygen evolution / oxygen reduction / nitrogen-doped carbon nanotubes / Co₃O₄

Author summay

周广远(1997-),男,硕士生,研究方向为电催化,664640041@qq.com。

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随着化石能源的不断消耗和环境问题的日益严峻,使得人们对可再生新能源的需求不断增长,促使电化学能的研究和开发一直备受关注^[1-3]。氧还原反应(ORR)和氧析出反应(OER)过程复杂,受反应动力学的限制,需要较高的过电位才能促使反应进行,因此,一直被认为是电化学能量转化和储存技术领域研究的两个关键反应过程,决定了电池的工作效率^[4]。目前,工业上普遍使用的氧还原催化剂是Pt/C,不仅成本高、储量有限,而且其催化活性会随着反应时间的延长而迅速下降^[5]。氧化钌和氧化铱是典型的OER催化剂,与ORR的铂基催化剂类似,储量与价格极大地限制了它们在商业领域中的广泛应用^[6-7]。值得注意的是,高效的ORR催化剂在OER中不一定具有同样的高活性,如铂基催化剂在ORR中具有优异的催化活性,但在OER中的催化活性较低;而氧化钌和氧化铱在OER中具有非常高的活性,但在ORR中的活性较低。因此,开发具有高活性和高稳定性的ORR/OER双功能催化剂至关重要。

在众多非贵金属催化剂中,过渡金属及其氧化物因其储量大、成本低、无污染、活性高等优点引起人们的广泛关注,特别是尖晶石类金属氧化物AB₂O₄(A和B均为过渡金属阳离子),这类氧化物具有独特的结构和良好的稳定性,被认为是一类具有较好应用前景的催化剂^[8-11]。与传统催化剂不同,电催化剂催化的反应过程涉及电子转移,这就要求ORR/OER双功能催化剂具有良好的导电性。对于过渡金属基催化剂而言,其自身较低的导电性在一定程度上限制了催化性能的提升^[9]。尖晶石氧化物具有可控的组成、结构和价态,但其导电性并不十分优异,除了通过增加氧空位^[12]和杂元素掺杂^[10]来提高导电性外,也可以通过引入具有良好导电性的载体如石墨烯、碳纳米管(CNTs)等碳材料来改进氧化物的导电性,提升其在反应过程中的电子转移速率^[13-14]。

Fu等^[15]通过控制晶相和成分制备了尺寸超小的Co_xMn_3-_xO₄颗粒,引入碳材料后解决了尖晶石氧化物导电性差的问题,大大提高了其催化活性。研究也发现CoFe₂O₄负载在碳纳米管上的电催化性能明显高于CoFe₂O₄与碳纳米管的混合物,证明了尖晶石氧化物与碳纳米管之间的强耦合作用提高了该材料的催化活性与稳定性^[16]。Prabu等^[17]制备了CoMn₂O₄负载在氮掺杂还原氧化石墨烯(N-rGO)上的负载型电催化剂,证实CoMn₂O₄和N-rGO之间的协同效应使负载在N-rGO上的CoMn₂O₄纳米粒子比非负载的CoMn₂O₄纳米粒子具有更优异的电催化性能。由于尖晶石氧化物与载体之间的协同效应使MnCo_xO₄/NCNT、ZnCoMnO₄/N-rGO和NiFe₂O₄/CNTs等材料的催化性能均得以提升^[18-20]。

因此,以氮掺杂碳纳米管(NCNTs)为载体,采用浸渍法制备了具有尖晶石结构的Co₃O₄/NCNTs和锰或铁掺杂的Co₃O₄/NCNTs催化剂,通过材料表征和电化学分析研究不同元素掺杂的催化剂在ORR和OER中的电催化活性。

1 试剂与仪器

1.1 试剂

硝酸钴、硝酸锰、硝酸铁,分析纯,Macklin生产;氨水;碳纳米管;乙醇;Nafion溶液(质量分数5%),杜邦公司生产;KOH溶液,上海国药集团生产。

1.2 仪器

电子天平(JA5203),集热式磁力搅拌器(DF-101S),超声波清洗机(CH-060S),旋转蒸发仪(RE-52CS),管式炉(BZG-80-12K),电化学工作站(CHI760E),旋转圆盘电极(IPS-RRDE/RDE),透射电子显微镜(JEM-2200FS),X射线衍射仪(D8 Advance),X射线光电子能谱(K-Alpha)。

2 实验方法

2.1 催化剂的制备

将酸洗氧化后的碳纳米管与氨水按体积比1∶3混合于50 mL的离心管中密封,30℃恒温水浴中超声30 min后抽滤,洗至中性干燥,得到的样品标记为NCNTs。称取一定量的硝酸钴和NCNTs按摩尔比3∶10加入到去离子水中,超声10 min使其充分混合,移至旋转蒸发仪中60℃、8 r/min旋蒸至液体被完全蒸出,取出后在管式炉中氮气气氛下400℃煅烧2 h,得到的样品标记为Co₃O₄/NCNTs。当添加一定量的硝酸锰和硝酸铁时,制备得到的样品分别标记为MnCo₂O₄/NCNTs、FeCo₂O₄/NCNTs、CoMn₂O₄/NCNTs和CoFe₂O₄/NCNTs。

2.2 电化学测试

称取5 mg催化剂加入到乙醇(490 μL)、去离子水(490 μL)和Nafion(20 μL)的混合溶液中,超声5 min,取10 μL分散液滴在旋转圆盘电极(5 mm)上,室温下自然晾干,催化剂担载量为0.119 mg/cm²。

线性扫描曲线测试均采用典型的三电极体系,担载催化剂的玻碳圆盘电极作为工作电极、Pt片作为对电极、Ag/AgCl/3 mol/L KCl作为参比电极,氮气或氧气饱和的0.1 mol/L KOH溶液作为电解液。所有电势均依据能斯特方程转换为可逆氢电极(RHE)电势。

ORR中的电子转移数可基于K-L方程式计算:

1 / j = 1 / j k + 1 / j d = 1 / j k + 1 / (0.20 n F C 0 D 0 2 / 3 v - 1 / 6 w 1 / 2)

其中:j为实验测得的电流密度,A/cm²;j_k为动力学控制电流密度,A/cm²;j_d为极限扩散电流密度,A/cm²;n为ORR电子转移数;F为法拉第常数,96 500 C/mol;C₀为0.1 mol/L KOH溶液中的氧气浓度,1.2×10^-3 mol/L;D₀为氧气分子在0.1 mol/L KOH电解液中的扩散率,1.9×10^-5 cm²/s;v为电解液的动力学黏度,0.01 cm²/s;w为旋转圆盘转速,r/min。

3 结果与分析

3.1 物相分析和形貌表征

样品Co₃O₄/NCNTs、MnCo₂O₄/NCNTs、FeCo₂O₄/NCNTs、CoMn₂O₄/NCNTs和CoFe₂O₄/NCNTs的XRD分析如图1所示。从XRD谱图中可以看出,所有样品均在26.3°处出现一个较宽的衍射峰,对应碳纳米管的(002)晶面。19.0°、31.3°、36.9°、44.8°、55.8°、59.3°和65.2°处的特征衍射峰分别对应Co₃O₄(PDF#43-1003)的(111)、(220)、(311)、(400)、(422)、(511)和(440)晶面^[21],其余衍射峰归属CoO(PDF#48-1719),上述结果基本表明已制备了Co₃O₄/NCNTs,且样品中含有少量CoO。FeCo₂O₄/NCNTs、MnCo₂O₄/NCNTs、CoFe₂O₄/NCNTs和CoMn₂O₄/NCNTs的XRD图谱均与标准谱图一致,证明已制备出相应的尖晶石氧化物催化剂。从图1(a)和1(b)中均可以看出,无论在尖晶石结构中的A位置还是B位置,随着Fe和Mn替代Co,其衍射峰均向小角度方向移动,与文献[22-23]中的相关报道一致。

图2为5种催化剂的TEM和HRTEM图。从图2(a)~(e)中可以清晰地观察到碳纳米管的结构,黑色的颗粒分布在碳纳米管上。从图2(f)~(j)中可以观察到黑色颗粒的晶格条纹,测量其晶面间距分别为0.248 2、0.248 9、0.212 5、0.253 5 nm和0.246 0 nm,依次对应Co₃O₄(PDF#43-1003)的(311)晶面、FeCo₂O₄(PDF#74-2120)的(311)晶面、MnCo₂O₄(PDF#01-1130)的(400)晶面、CoFe₂O₄(PDF#22-1086)的(311)晶面和CoMn₂O₄(PDF#77-0471)的(211)晶面,表明在碳纳米管上负载的黑色纳米颗粒分别为Co₃O₄、FeCo₂O₄、MnCo₂O₄、CoFe₂O₄和CoMn₂O₄,其结果与XRD一致。

3.2 电化学性能测试

采用线性扫描曲线分析5种催化剂在ORR中的电催化活性,结果如图3所示,其中,OCNTs为氧化后的碳纳米管,20% Pt/C为商业催化剂,OCNTs、NCNTs和20% Pt/C均为对比样。从图3(a)中可以发现,OCNTs在ORR中的催化活性最低,NCNTs具有比OCNTs更高的起始电位和更大的电流密度,表明在碳纳米管表面掺杂一定的氮原子有利于ORR催化活性的提升。以NCNTs为载体负载过渡金属氧化物颗粒后,其在ORR中表现出比NCNTs更优异的电催化活性。MnCo₂O₄/NCNTs的起始电位为0.91 V,明显高于Co₃O₄/NCNTs(0.88 V)和FeCo₂O₄/NCNTs(0.85 V),且具有最快的反应速率和最大的电流密度。从图3(b)中可发现CoMn₂O₄/NCNTs的起始电位和电流密度高于Co₃O₄/NCNTs和CoFe₂O₄/NCNTs,过渡金属氧化物催化剂的起始电位均低于20% Pt/C,但电流密度明显高于后者。由图3(c)可知,随转速增加,电流密度增大,不同电位下的K-L曲线呈线性且相互平行。基于K-L方程计算了催化剂在ORR中的电子转移数,NCNTs的平均电子转移数为2.6,ORR以2e路径为主,当过渡金属氧化物颗粒负载在NCNTs上后,催化剂在ORR中的电子转移数均为4.0,反应遵循了高效的4e路径。从图3(d)的Tafel曲线可以看出,MnCo₂O₄/NCNTs和CoMn₂O₄/NCNTs的电催化活性优于 FeCo₂O₄/NCNTs和CoFe₂O₄/NCNTs,CoMn₂O₄/NCNTs的Tafel斜率是56 mV/dec。

图4是0.1 mol/L KOH溶液中1 600 r/min下测试的OER极化曲线。从图4(a)可以看出,Co₃O₄/NCNTs、FeCo₂O₄/NCNTs和MnCo₂O₄/NCNTs的起始电位分别为1.08、1.44 V和1.03 V,电流密度为10 mA/cm²时,其电位分别为1.78、1.76 V和1.75 V。与NCNTs的OER催化活性对比发现,负载过渡金属氧化物后,催化剂的OER活性明显增强,且MnCo₂O₄/NCNTs具有最佳的OER活性。如图4(b)所示,在OER中CoMn₂O₄/NCNTs和Co₃O₄/NCNTs的催化活性明显优于CoFe₂O₄/NCNTs。与商业RuO₂相比,CoMn₂O₄/NCNTs的起始电位略高,在10 mA/cm²电流密度时,其电位与RuO₂接近。MnCo₂O₄/NCNTs和CoMn₂O₄/NCNTs在ORR和OER中均表现出优于Co₃O₄/NCNTs、FeCo₂O₄/NCNTs和CoFe₂O₄/NCNTs的电催化活性,表明Co₃O₄中掺杂一定量的锰元素后,提升了催化剂在可逆氧反应中的催化活性。

3.3 XPS分析

图5为MnCo₂O₄/NCNTs、CoMn₂O₄/NCNTs、Co₃O₄/NCNTs、FeCo₂O₄/NCNTs和CoFe₂O₄/NCNTs的XPS谱,5种催化剂中过渡金属元素原子含量见表1。从图5(a)的Co 2p谱中可观察到,780.38 eV和795.66 eV处的峰分别归属于Co 2p_3/2和Co 2p_1/2,786.61 eV和803.57 eV处的峰为相应的振动卫星峰^[24-26]。在5种负载型过渡金属氧化物催化剂中,钴元素均以二价钴和三价钴形式存在。CoMn₂O₄/NCNTs和CoFe₂O₄/NCNTs中的三价钴与二价钴含量接近,而在Co₃O₄/NCNTs、FeCo₂O₄/NCNTs和 MnCo₂O₄/NCNTs中,三价钴含量较高。通过对比图5(a)和表1发现,随着金属氧化物中钴含量的增加,其氧化物中Co³⁺/Co²⁺比值增大。

图5(b)是FeCo₂O₄/NCNTs和CoFe₂O₄/NCNTs的Fe 2p谱,图5(c)是MnCo₂O₄/NCNTs和CoMn₂O₄/NCNTs的Mn 2p谱。711.27 eV和724.44 eV处的峰分别归属于Fe 2p_3/2和Fe 2p_1/2^[27-28],641.90 eV和653.37 eV处的峰分别归属于Mn 2p_3/2和Mn 2p_1/2^[29-30]。结果表明,无论是ACo₂O₄/NCNTs还是CoB₂O₄/NCNTs,掺杂的铁和锰均存在二价和三价形式,其结构为A²⁺Co²⁺(A³⁺Co³⁺)₂O₄和Co²⁺B²⁺(Co³⁺B³⁺)₂O₄,且随着金属氧化物中过渡金属铁和锰含量的增加,其氧化物中三价金属的比例增加。

4 结论

以NCNTs为载体,采用浸渍法制备了具有尖晶石结构的Co₃O₄/NCNTs催化剂,并通过锰、铁掺杂制备了FeCo₂O₄/NCNTs、MnCo₂O₄/NCNTs、CoFe₂O₄/NCNTs和CoMn₂O₄/NCNTs催化剂,在ORR和OER中均表现出优异的催化活性。在可逆氧反应中,MnCo₂O₄/NCNTs和CoMn₂O₄/NCNTs具有最优异的ORR/OER活性。研究发现,在ACo₂O₄/NCNTs和CoB₂O₄/NCNTs中,无论是在A还是B,Mn掺杂后的双功能催化活性高于Fe掺杂和未掺杂的催化剂,且在形成的金属氧化物催化剂中过渡金属均含有二价和三价形态,随过渡金属含量增加,三价金属比例增加。本文为可逆氧电极催化剂的制备提供了一种简单、易工业化的制备方法,并研究了不同过渡金属成分对可逆氧催化剂活性的影响,为催化剂的设计提供了思路。

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