γ 射线辐照合成氮掺杂石墨单炔/ 铁催化剂及其性能
近年来,使用清洁可再生能源高效发电的燃料电池日益引起重视,不过电池负极上氧还原反应动力学迟缓,影响了其整体性能。所以,设计经济有效的电催化剂是增加金属氧电极反应催化活性的核心。贵金属催化剂因其较高的催化性能,是实际应用中的主要催化剂[1-3]。然而,贵金属的稀缺性、高成本和低稳定性限制了贵金属催化剂的应用[4-6],迫使人们开发可替代材料,尽管目前已经研制出性能良好的非贵金属催化剂、合金催化剂等,但也面临着一定问题。因为金属与碳材料之间的电子吸附能力较低,而且金属纳米粒子在化学反应中非常容易团聚或脱落,所以选用理想的载体至关重要。碳基纳米材料因其特殊的结构和化学物理性质引起了人们的广泛关注,包括碳纳米纤维[7-8]、碳纳米管[9-10]、石墨烯[11-12]、石墨氮化碳[13-14]和生物炭[15]等。与其他材料相比,碳基催化剂在稳定性和耐久性方面具有更明显的优势,这可归因于两点:1)导电碳基体上负载纳米粒子(NPs)可以避免金属纳米粒子严重团聚,从而增强催化剂的导电性;2)碳层可以阻止内部纳米颗粒渗入电解液,避免因外部因素引起的腐蚀和氧化,从而赋予催化剂较高的耐久性[7-8]。
石墨炔是sp和sp2杂化碳原子按照一定规则组成的新型二维碳材料[16-17]。特殊的sp和sp2杂化网络赋予石墨炔优异的性能,包括分布均匀的孔隙结构、大的比表面积、可调的电子性能和优异的电子导电性[17-18]。上述这些特性使得石墨炔在催化方面具有巨大的潜力[19-21]。例如,由乙炔键形成的石墨炔其特殊的大环结构为其作为催化剂提供了多孔结构和更多暴露的活性中心,实现快速传质[22-23]。此外,这些结构为作为支撑材料的石墨炔提供了大的比表面积。理论计算表明,与活性炭(2 000 m2/g)和石墨烯(实验值:1 500 m2/g,理论值:2 620 m2/g)相比,石墨炔(3 440 m2/g)具有更高的比表面积[24-25],有利于催化剂纳米粒子在石墨炔上均匀分配,降低因颗粒聚集造成的催化活性损失[26]。与其他碳材料不同,sp-和sp2-C的共存使sp碳原子表现出正电特性[27-28],有利于O2分子的吸附,从而促进电催化氧还原反应。此外,具有特殊炔键和高度共轭π-系统的杂化碳网络赋予石墨炔更高的性能,同时其由12个碳原子组成的六角环形成的孔是金属纳米颗粒最稳定的吸附位点,使得石墨炔成为碳基负载非贵金属复合材料的潜在碳载体。
然而,理论和实验研究已经共同表明,原始石墨单炔的高O2屏障使其在氧还原反应中活性较低,并且材料缺少活性位点,不太适于直接作为燃料电池阴极催化剂。本课题组已经利用γ射线辐照的方法成功制备了氟掺杂石墨炔基铂/钯纳米颗粒复合材料,并将其应用于燃料电池阴极催化剂[29];还通过sp-N掺杂和γ射线辐照的协同调控,研究了石墨单炔作为碳材料基底负载铂纳米颗粒及其氧还原反应性能[30]。为深入探索γ辐照对石墨单炔活性位点与负载性能的影响,本文选用高能量、强穿透力、环保及在室温下反应的γ射线辐照N掺杂石墨单炔作为碳基负载体,成功制备了具有优异电化学活性和长期稳定性的管状石墨炔催化剂,并探究了γ辐照对N掺杂石墨单炔负载铁纳米粒子催化剂结构和氧化还原反应催化性能的影响。
1 材料与方法
1.1 原料与试剂
碳化钙、六溴苯、氯铂酸、甲醇、己烷、异丙醇、聚氧乙烯月桂醚(Brij30®)等均为分析纯;无水乙醇,高纯;氢氧化钾,优级纯;以上均购自天津市科密欧化学试剂有限公司。浓硝酸,分析纯,天津市北方天医试剂厂。三聚氰胺、六水合氯化铁,分析纯,上海麦克林生化科技有限公司。超纯水,一级纯,天津市蓝水晶公司。商用铂碳,分析纯,中科科创新能源有限公司。氮气、氧气,高纯,天津六方气体有限公司。全氟磺酸型聚合物溶液、Nafion,分析纯,杜邦中国有限公司。
1.2 制备过程
1.2.1 石墨单炔的制备
本研究采用机械化学合成法(球磨法)制备石墨炔(GY),实验步骤如下:(1)将块状碳化钙在手套箱中研磨至粒径小于0.15 mm备用;(2)将0.434 g六溴苯、0.503 9 g碳化钙和37.5 g球磨珠放入球磨罐中,将氩气冲入前体化合物中,在氩气的气氛下球磨处理12 h;(3)将球磨后获得的中间产物放入高温管式炉,并保持在氩气介质中,设定升温速率为5 ℃/min,在450 ℃下煅烧处理2 h;(4)将高温处理后的中间产物使用1 mol/L的稀硝酸、超纯水,采用离心机多次洗涤以去除未反应的碳化钙和溴化钙的副产物;(5)放入80 ℃真空烘箱中进行干燥处理,最终所得黑色粉末即为GY样品。
1.2.2 γ辐照氮掺杂石墨单炔负载铁NPs催化剂的制备
将GY和三聚氰胺分散在100 mL乙醇中,充分混合后,将上述悬浮液超声处理30 min;将上述溶液在真空烘箱中干燥20 h得到前驱体;将上述前驱体加入到100 mL微乳液中(微乳液由Brij30®、己烷和异丙醇以9.5%、54.0%和36.5%的体积比配制而成)。然后,将上述溶液超声处理30 min,将六水合氯化铁添加到混合溶液中(铁源和碳源GY的质量总和为15 mg),加水直至 200 mL,并在室温连续搅拌24 h充分进行离子交换。最后,将所得的悬浮液倒入棕色试剂瓶中,通入氩气约30 min排除空气,最后密封试剂瓶。采用γ射线辐照试剂瓶至总吸收剂量为150 kGy。辐照完成后,用超纯水将样品离心数次以去除杂质离子(离心速度设置为10 000 r/min,10 min/次),最后,在内部压力小于10 Pa,冷凝温度为-50 ℃,冷冻干燥获得催化剂NGY-Fe。
1.3 表征方法
1.3.1 扫描电子显微镜测试
采用Gemini SEM 500扫描电子显微镜(SEM)测试表征制备样品的形貌、颗粒大小及分布情况。
1.3.2 透射电子显微镜测试
采用透射电子显微镜(TEM)表征材料的微观结构和形貌。采用TEM与X射线能谱仪联用表征样品表面的元素分布、掺杂及含量。利用Digital Micrograph软件对获得的图像进行数据处理,得到材料的晶面间距、晶格信息和衍射环。
1.3.3 X射线光电子能谱表征
采用型号为Thermo ESCALAB-250Xi的X射线光电子能谱(XPS)表征催化剂表面的元素成分、存在状态及相对含量。
1.3.4 激光共焦拉曼光谱表征
使用激光共焦拉曼光谱仪(产自日本,规格是XploRA PLUS)(Raman),波长λ=532 nm表征催化剂样品的缺陷结构和石墨化程度。
1.3.5 比表面积及孔径分布表征
利用BET(Brunauer-Emmett-Teller)和BJH(Barrett-Joyner-Halenda)方法对氮气吸脱附等温曲线进行分析,获得材料的比表面积、孔体积和孔径分布等信息。
1.3.6 催化剂电化学性能测试
使用上海辰华(型号为CHI760E)的电化学工作站进行电化学测试;使用标准三电极体系,工作电极为催化剂薄膜涂层旋转圆盘-环盘电极(RRDE,直径4 mm,GCE,即玻碳电极),对电极为铂丝电极,参比电极为Hg/HgO;电解池使用双层五口电解池(美国Pine型旋转圆盘-环电极仪器)。
2 结果与讨论
2.1 辐照催化剂的结构与形貌表征
六水合氯化铁与GY以质量比为1∶3、2∶3、1∶2、4∶5和1∶1制备的样品分别命名为NGY-Fe1/3、NGY-Fe2/3、NGY-Fe1/2、NGY-Fe4/5和NGY-Fe1/1。
图 1. (a)NGY-Fe催化剂制备流程图;经过γ辐照后,NGY-Fe样品的(b)SEM图像和(c)TEM图像;(d)通过SEM内置的EDS得到的NGY-Fe样品的元素(C、N和Fe)分布图
Fig. 1. (a) Flow chart of NGY-Fe catalyst preparation; (b) SEM image and (c) TEM image of NGY-Fe sample prepared after γ irradiation; (d) elemental distribution (C, N and Fe) plots of NGY-Fe samples obtained by SEM's built-in EDS
从
通过XPS测试(
图 2. (a)γ射线辐照后NGY-Fe样品和GY样品的高分辨率核心级XPS全谱图;(b)NGY-Fe样品的Raman谱图;(c)C1s、(d)Fe2p和(e)N1s的高分辨率核心级XPS分峰光谱
Fig. 2. (a) High-resolution core-grade XPS full spectra of NGY-Fe samples and GY samples after γ irradiation; (b) Raman spectra of NGY-Fe samples; high-resolution core-level XPS peak splitting spectra of (c) C1s, (d) Fe2p and (e) N1s
此外,通过Raman光谱研究了催化剂的结构变化和石墨化程度,结果如
表 1. 制备催化剂和商业Pt/C催化剂的结构性质
Table 1. Structural properties of the prepared catalysts and commercial Pt/C catalysts
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2.2 辐照催化剂的电化学氧还原性能
在碱性溶液中对γ辐照制备的催化剂进行了旋转圆盘电极(RDE)和旋转环盘电极(RRDE)电催化测试。在氧气饱和的0.1 mol/L氢氧化钾电解液中,采用RDE对NGY-Fe催化剂进行了氧还原反应的循环伏安测试(
图 3. (a)NGY-Fe催化剂和商用Pt/C催化剂在0.1 mol/L KOH电解液中的循环伏安曲线;(b)NGY-Fe催化剂和商用Pt/C催化剂在转速为1 600 r/min时的极化曲线(彩色见网络版)
Fig. 3. (a) Cyclic voltammetry curves of NGY-Fe catalysts and commercial Pt/C catalysts in 0.1 mol/L KOH electrolyte; (b) polarisation curves of NGY-Fe catalyst and commercial Pt/C catalyst at 1 600 r/min (color online)
所有NGY-Fe催化剂都显示出比商用Pt/C催化剂更高的正氧化物脱附峰电位,具有更强的电正性(
表 2. 制备NGY-Fe催化剂和商用Pt/C催化剂的氧还原催化活性
Table 2. Oxygen reduction catalytic activity of prepared NGY-Fe catalysts and commercial Pt/C catalysts
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图 4. (a)最优样品NGY-Fe1/2在不同转速下的LSV曲线;(b)最优样品NGY-Fe1/2在不同电位下对应的K-L曲线;(c)样品NGY-Fe1/2和商用Pt/C催化剂的奈奎斯特图(彩色见网络版)
Fig. 4. (a) LSV curves of the optimal sample NGY-Fe1/2 at different rotational speeds; (b) K-L curves corresponding to the optimal sample NGY-Fe1/2 at different potentials; (c) nyquist plots of samples NGY-Fe1/2 and commercial Pt/C catalyst (color online)
图 5. 根据RRDE实验测试得样品NGY-Fe1/2和商用Pt/C催化剂的(a)转移电子数(n);(b)氧还原反应过程中H2O2产率
Fig. 5. Number of (a) transferred electrons (n) of samples NGY-Fe1/2 and commercial Pt/C catalysts obtained according to RRDE experimental tests; (b) H2O2 yield during oxygen reduction reaction
为了进一步验证四电子过程,通过RRDE计算了反应过程中的过氧化氢产率。如
通过观察到的塔菲尔斜率可以揭示反应动力学。如
图 6. 根据NGY-Fe1/2催化剂和Pt/C催化剂的氧还原反应极化数据计算得到的塔菲尔斜率
Fig. 6. Tafel slope calculated based on the polarization data of oxygen reduction reaction of NGY-Fe1/2 catalyst and Pt/C catalyst
采用计时电流法测试NGY-Fe1/2催化剂对甲醇的耐受性,测试进行200 s时将甲醇注入O2饱和KOH电解液中。从
图 7. (a) NGY-Fe1/2催化剂和Pt/C催化剂的I-t曲线;(b) NGY-Fe1/2催化剂和Pt/C催化剂甲醇耐受性曲线
Fig. 7. (a) I-t curves of NGY-Fe1/2 catalyst and Pt/C catalyst; (b) Methanol tolerance curves for NGY-Fe1/2 catalyst and Pt/C catalyst
3 结论
综上所述,我们通过γ辐照与氮掺杂协同调控改性制备出NGY-Fe管状催化剂,实现了石墨炔从二维层状结构到一维管状结构的转变,制备的NGY-Fe1/2催化剂具有高度互连的导电网络和充足的缺陷表面活性中心。碱性条件下,NGY-Fe1/2催化剂具有比市售Pt/C催化剂更高的催化性能。尤其是NGY-Fe1/2催化剂,具有最优的催化活性,其含氧化合物脱附峰的电位为0.787 V,起始电势为0.903 V,半波电势为0.782 V,极限扩散电流密度为 5.32 mA/cm2,高于其他NGY-Fe催化剂。除此之外,NGY-Fe1/2催化剂在抗甲醇毒性和稳定性上远高于Pt/C催化剂。
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Article Outline
董英杰, 石海婷, 王硕, 闵春英, 王道喜, 邵瑞琪, 徐志伟.