现代化工

现代化工 ›› 2025, Vol. 45 ›› Issue (1) : 220-224. DOI: 10.16606/j.cnki.issn0253-4320.2025.01.038

科研与开发

Cu掺杂SnS₂纳米花高效电化学还原CO₂合成甲酸盐的研究

丁笑含 ¹ ,
赵林飞 ¹^,^* ,
袁章福 ¹ ,
田越 ¹ ,
徐秉声 ²

作者信息 +

Research on Cu-doped SnS₂ nanoflowers for efficient electrochemical reduction of CO₂ to synthesize formate

Author information +

文章历史 +

Received		Published
2024-03-08		2025-01-20
Issue Date	Revised Date
2025-01-17	2024-03-08

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摘要

为了提高Sn基材料表面CO₂电化学还原为甲酸盐的法拉第效率,利用一步溶剂热法合成了具有丰富硫空位的Cu掺杂SnS₂纳米花(Cu-SnS_2-_x)催化剂,在较宽的电位窗口实现了CO₂电化学还原制备甲酸盐。结果表明,通过调控催化剂制备过程中Cu和Sn的摩尔比,在-1.1 V vs.RHE电位条件下得到了72.64%的FE_formate,电流密度J_formate达到-14.38 mA/cm²。二维纳米片阵列增加了催化活性位点,Cu掺杂所产生的硫空位能够协同提高催化活性、促进电子转移,从而提高甲酸盐的选择性。

Abstract

To enhance the Faraday efficiency of the surface of Sn-based materials for electrochemical reduction of CO₂ to formate,Cu-doped SnS₂ nanoflower (Cu-Sn $S 2 - x$ ) catalysts enriched with sulfur vacancies are synthesized via a one-step solvothermal method.The prepared catalysts facilitate the electrochemical reduction of CO₂ to formate over a broad potential window.By adjusting the molar ratio of Cu to Sn during the catalyst preparation process,a formate Faraday efficiency of 72.64% is achieved at a potential of -1.1 V vs.RHE,with a formate current density reaching -14.38 mA·cm^-2.Two-dimensional nanosheet array increases the active sites of the catalysts,and the sulfur vacancies induced by Cu doping synergistically enhances catalytic activity and facilitates electron transfer,thereby improving the selectivity of formate.

Graphical abstract

关键词

CO₂还原 / 甲酸 / 溶剂热 / 掺杂 / 电化学

Key words

carbon dioxide reduction / formic acid / solvothermal / dopant / electrochemistry

Author summay

丁笑含(1998-),女,硕士生,研究方向为CO₂电催化,hapd2021@163.com。

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钢铁工业是国民经济的重要支柱性产业,为我国的经济发展提供了巨大动力。然而作为典型的高耗能、高排放行业,钢铁工业也是减碳压力最大的行业之一,面临着碳达峰、碳中和的新挑战^[1⇓-3]。目前,钢铁工业较为主流的减碳手段包括氧气高炉、氢能冶炼和碳捕获、利用与封存技术(Carbon Capture,Utilization and Storage,CCUS)等,还有其他减碳手段如增加废钢比、采取短流程炼钢、增加余热余能回收、降低铁钢比、提高球团比、确定CO₂排放量和低碳技术评价体系等,但仍有许多技术问题尚未解决^[2,4-5,7]。电化学CO₂还原反应(CO₂RR)作为一种CCUS技术,具有转化效率高、反应在常温常压下进行、可有效利用可再生能源和剩余电能等优点,有望实现规模化部署,以钢铁-化工耦合联产的方式变革现有冶金工艺^[6,8-9]。然而,由于CO₂分子中的C=O键合化学稳定,活化和电催化需要较高的能量势垒。同时,CO₂RR过程中伴随着析氢反应(HER),在大多数催化剂上二者反应电位重叠,降低了产物选择性和能量效率^[8⇓⇓-11]。开发高选择性、高活性、高过电位的电催化剂是解决这些问题的关键。

近年来,锡基催化剂因其价格低、丰富、无毒和环境友好、适合大规模应用的优点,受到越来越多的关注^[10,19-20]。研究发现锡基材料在催化CO₂RR生成甲酸盐方面很有前景^[11-12]。原始Sn在CO₂RR过程中主要生成HCOOH/HCOO^-,这是由于其与 *C

O 2 -

的结合较弱,并且在随后的还原过程中优先形成*HCOO中间体。目前主要问题在于选择性不高和耐久性不足的窄电位窗口^[10,21]。二维(2D)纳米材料由于其横向尺寸、厚度以及较大的比表面积,是催化剂十分合适的结构平台^[13-14]。作为二维材料硫化物,SnS₂具有高比表面积和优异的电化学性质,在许多电解质中应用并已被广泛研究^[15-16]。通过理论和实验研究发现,硫作为表面改性剂可以降低催化CO₂RR合成甲酸盐的金属活性中心的自由能^[17,20]。在基体金属中加入第二金属形成金属合金,可以诱导电荷转移,从而调节基体金属的d带中心,有效改善反应的活性和选择性^[18,22]。

笔者结合空位工程和杂原子掺杂,通过调整Cu掺杂含量来调节SnS₂的表面结构,使得自发产生富集的S空位。所开发的具有丰富硫空位的Cu掺杂的SnS₂(Cu-SnS_2-_x)同时具有丰富的活性位点和Cu与硫空位之间的协同作用的特征,从而提升CO₂至甲酸盐的转化效率。CuSnS-3表现出显著增强的CO₂RR活性,在高过电位(-1.1 V vs.RHE)下甲酸盐生成的法拉第效率约为72.64%,电流密度达到 -14.38 mA/cm²,控制Cu掺杂量进一步调节了电子分布。

1 实验材料和方法

1.1 材料

五水四氯化锡(SnCl₄·5H₂O,≥99.9%)、五水氯化铜(CuCl₂·2H₂O,≥99.9%)、硫代乙酰胺、碳酸氢钾(KHCO₃,≥99.5%)、乙醇(CH₃CH₂OH,≥99.8%)、Nafion溶液(5%),阿拉丁试剂公司生产;碳纸,TGP-H-060型,日本东丽公司生产,使用前在丙酮、乙醇和去离子水中超声清洗30 min。所有化学品均从商业供应商处获得,未经进一步纯化。在催化剂的制备和性能测试整个过程中都使用了去离子水。

1.2 合成Cu-SnS_2-_x纳米花

将适量CuCl₂·2H₂O和SnCl₄·5H₂O(Cn和Sn摩尔比分别为1∶1、1∶2、1∶5和1∶11,总摩尔量保持在0.9 mmol)及1.8 mmol硫代乙酰胺依次加入 25 mL乙醇中,剧烈搅拌30 min形成均匀溶液。然后,将溶液转移到50 mL内衬特氟龙不锈钢高压反应釜中,适当密封放入烘箱在160℃下保持12 h。待其自然冷却至室温,以转速4 500 r/min离心 5 min得到催化剂粉末,用乙醇和去离子水洗涤3次,并在80℃下真空干燥过夜。将得到的催化剂根据Cn和Sn摩尔比(1∶1、1∶2、1∶5、1∶11)的不同分别命名为CuSnS-1、CuSnS-2、CuSnS-3、CuSnS-4。

1.3 合成原始SnS₂纳米花

在不添加CuCl₂·5H₂O的情况下,采用与Cu-SnS_2-_x相同的合成步骤制备了原始SnS₂纳米花。

1.4 催化剂的表征

利用X射线衍射仪(XRD,理学Miniflex 600,Cu Kα辐照,λ=1.514 84 Å)对催化剂的晶相结构进行表征,2θ为10~80°,扫描速率为10°/min。利用场发射电子显微镜(ZEISS Gemini 500,德国)观察和测量催化剂的形态并进行EDX元素分析。利用具有单色Al-Kα X射线源(hv=1 486.6 eV)的Thermo Scientific K-Alpha X射线光电子能谱仪(XPS)对催化剂进行表征。所有结合能均由284.8 eV处的 C 1s峰值校准。

1.5 电极的制备

将10 mg催化剂粉末、5 mg导电炭黑和50 μL 5% Nafion溶液分别加入450 μL异丙醇中,超声分散90 min,形成均匀稳定的催化剂ink。分2次各取25 μL ink滴涂在有效尺寸为1 cm×1 cm的碳纸上,催化剂负载量为1.0 mg/cm²,在室温下空气中干燥过夜后进行电化学测试。

1.6 CO₂电化学测试

电催化CO₂RR测量在CO₂饱和0.1 mol/L KHCO₃电解质(pH=6.8)及Nafion 117膜分离的三电极H型电解池中进行。分别以Ag/AgCl和Pt箔为参比电极和对电极。工作电极和参比电极放置在阴极室中,对电极放置在阳极室中。在电化学工作站(CHI760E,上海辰华仪器有限公司生产)上测量电流电位。所有电位均相对Ag/AgCl参比电极进行测量,并使用E_RHE=E_Ag/AgCl+0.197+0.059 1×pH和iR补偿转换为RHE参考标度。将高纯CO₂以20 mL/min流速通入阴极室30 min,直至达到饱和CO₂,确保电解前电解液完全除去氧气。以20 mV/s的扫描速率测量线性扫描伏安(LSV)曲线。在每个电位处进行计时电流法连续测量1 h以收集产物。用二甲基亚砜(DMSO)作内标,利用核磁共振波谱仪(NMR,Bruker AVANCE Ⅲ 400 MHz,德国)分析液体产物。

2 结果与分析

2.1 CuSnS-3物相分析

CuSnS-3的SEM图及EDX元素分布图如图1所示。

从图1(a)中可以看出,扫描电子显微镜(SEM)图像显示反应生成了由二维(2D)纳米片组装而成的纳米花结构,尺寸分布在1~5 μm之间,多数为 2 μm。从图1(b)~图1(e)中可以看出,Cu元素均匀分散在SnS₂纳米花的表面。

溶剂热法制备了不同比例Cu掺杂的SnS₂催化剂。CuSnS-3粉末XRD图谱及XPS谱图如图2所示。从图2(a)中可以看出,样品CuSnS-3的峰值与SnS₂(100)、(110)晶面相对应并且强度发生变化,说明通过12 h溶剂热反应,Cu元素成功掺杂在SnS₂纳米花结构内并改变其结晶度。

X射线光电子能谱(XPS)用于评估材料的化学状态。从图2(b)~图2(e)中可以看出,对比原始SnS₂与掺杂后CuSnS-3,CuSnS-3总谱中出现了新峰,且Cu 2p能谱中出现的双峰[930.7 eV(Cu 2p_3/2)和952 eV(Cu 2p_1/2)]证实Cu元素成功引入SnS₂中并以Cu⁺形式存在。此外,在946 eV处观察到弱峰,为Cu⁺的卫星峰。而Sn 3d能谱中484.85 eV(Sn 3d_5/2)和493.3 eV(Sn 3d_3/2)及S 2p能谱中159.8 eV(S 2p_3/2)和160.85 eV(S 2p_1/2)与原始SnS₂相比均发生了不同程度的负偏移,表明掺杂使Cu_xSn_2-_xS中出现价态降低,电子增多,进一步证实了Cu元素和S空位的存在。

2.2 不同掺杂比Cu_xSnS_2-_x电极的电化学测试

在市售的密封双室H电池中评估了不同掺杂比的Cu_xSnS_2-_x电极的CO₂RR性能,通过核磁共振(¹HNMR)对CO₂RR过程中的所有液体产物进行定量分析。不同掺杂比Cu_xSnS_2-_x电极的电化学测试结果如图3、表1所示。结果表明,在CO₂饱和的0.1 mol/L KHCO₃电解质中,甲酸盐是唯一的液体产物。从表1中可以看出,随着过电位的增大,Cu_xSnS_2-_x电极的甲酸盐选择性显著增强。特别在低过电位(-0.7 V vs.RHE)下,仅有CuSnS-3反应得到微量甲酸盐,法拉第效率(FE_formate)低至9.36%;而在高过电位(-1.1 V vs.RHE)下最高可获得72.64% FE_formate,提升将近6.8倍。Cu含量的降低同样影响Cu_xSnS_2-_x电极的甲酸盐选择性,并呈现火山型趋势,在高过电位下,CuSnS-3电极选择性明显优于其他含量催化剂。

CuSnS-3电极同样在CO₂RR活性上表现出最佳的性能。在饱和CO₂电解质中以20 mV/s扫速测试了不同掺杂比Cu_xSnS_2-_x电极的线性伏安(LSV)曲线。从图3(a)、图3(b)中可以看出不同掺杂比的Cu_xSnS_2-_x电极在-0.7~-1.2 V vs.RHE宽电位窗口内的电流密度J和部分电流密度J_formate。显然CuSnS-3电极在相同电位下观察到更大的电流密度,在-1.1 V vs.RHE下J_formate最高可达-14.38 mA/cm²,有效还原CO₂获得甲酸盐,改善催化活性。

在恒电位-1.1 V vs.RHE下连续4 h测量,每间隔1 h更换饱和电解液,结果如图3(c)所示。从图3(c)中可以看出,CuSnS-3电极仅在前2 h具有稳定的电流密度约-13 mA/cm²,FE_formate在45%上下波动。2.5 h后出现异常,电流密度迅速下降,FE_formate不降反增。或许可通过溶剂热后低温退火增强其稳定性,这有待实验进一步验证。

2.3 Cu掺杂对Cu_xSnS_2-_x电极增强CO₂RR性能的机理分析

为了探究Cu掺杂对Cu_xSnS_2-_x电极增强CO₂RR性能的潜在机制,采用电化学双电层电容法测量了电化学活性表面积(ECSA),结果如图4(a)所示。从图4(a)中可以看出,CuSnS-3电极的双电层电容(Cdl)值为11.43 mF/cm²,大于CuSnS-1(9.43 mF/cm²)、CuSnS-2(6.74 mF/cm²)和CuSnS-4(6.71 mF/cm²),表明CuSnS-3电极具有相对大量的活性位点,一定量Cu掺杂与S空位协同作用促进CO₂快速转化为甲酸盐。模拟的等效电路图和电化学阻抗谱(EIS)测试结果如图4(b)、图4(c)所示,其中R_Ω、Rct、Zw和Cdl分别表示欧姆电阻、电荷转移电阻、传质阻抗和双电层电容。与其他Cu含量电极相比,CuSnS-3电极显示出更小的电荷转移电阻,表明反应速率更快。这是因为控制Cu含量可以增强Cu掺杂和S空位的协同作用,降低还原反应过电位,特别是在高电位下,实现了更高的CO₂RR产物选择性和部分电流密度。

3 结论

采用简单的一步溶剂热法合成了具有丰富硫空位的Cu掺杂SnS₂纳米花(Cu-SnS_2-_x),在较宽的电位窗口实现了CO₂电还原制备甲酸盐。Cu∶Sn掺杂比为1∶5时,Cu-SnS_2-_x催化剂具有更为优异的电化学性能,在高过电位(-1.1 V vs.RHE)下最高可获得72.64%法拉第效率,部分电流密度达到-14.38 mA/cm²。控制Cu含量在一定值,促进Cu掺杂和S空位的协同作用,加强CO₂RR在催化剂表面发生的电子转移,特别是在高过电位下,可改善生成甲酸盐的选择性和活性。

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