材料研究学报, 2026, 40(7): 507-518 DOI: 10.11901/1005.3093.2026.110

研究论文

NiFe-LDH/Co, La-CeO2 催化剂的制备及其性能

王俊1, 刘雯,1, 籍伟花,2, 苗洋,2,3, 王一杰,4, 宗振昊1, 马浩铭1, 周玲玲5

1.太原科技大学化学工程与技术学院 太原 030024

2.太原理工大学材料科学与工程学院 太原 030024

3.山西省煤基固废资源化利用创新技术中心 太原 030021

4.北京科技大学 北京材料基因工程高精尖创新中心 新金属材料国家重点实验室 北京 100083

5.中国科学院力学研究所 非线性力学国家重点实验室 北京 100190

Effect of Lattice-interface Synergy of NiFe-layered Double Hydroxide /Co, La-doped CeO2 on Efficient Oxygen Evolution Reaction

WANG Jun1, LIU Wen,1, JI Weihua,2, MIAO Yang,2,3, WANG Yijie,4, ZONG Zhenhao1, MA Haoming1, ZHOU Lingling5

1.College of Chemical Engineering and Technology, Taiyuan University of Science and Technology, Taiyuan 030024, China

2.College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China

3.Shanxi Innovation Technology Center for Coal-Based Solid Waste Resource Utilization, Taiyuan 030021, China

4.Beijing Advanced Innovation Center for Materials Genome Engineering, State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China

5.State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China

通讯作者: 刘雯,教授,13700509372@163.com,研究方向为纳米碳材料的制备及微观界面结构;籍伟花,讲师,jiweihua@tyut.edu.cn,研究方向为电催化析氧反应;苗洋,教授,miaoyang198781@163.com,研究方向为无机非金属;王一杰,博士,wangyijie_2016@126.com,研究方向为电化学模拟

责任编辑: 黄青

收稿日期: 2026-01-26   修回日期: 2026-03-27  

基金资助: 国家自然科学基金(52472075)
山西省青年科学基金(202403021212037)

Corresponding authors: LIU Wen, Tel: 13700509372, E-mail:13700509372@163.com;JI Weihua, Tel: 18800185926, E-mail:jiweihua@tyut.edu.cn;MIAO Yang, Tel: 18636918826, E-mail:miaoyang198781@163.com;WANG Yijie, Tel: 18811398701, E-mail:wangyijie_2016@126.com

Received: 2026-01-26   Revised: 2026-03-27  

Fund supported: National Natural Science Foundation of China(52472075)
Natural Science Foundation for Young Scientists of Shanxi Province(202403021212037)

作者简介 About authors

王 俊,男,1999年生,硕士生

摘要

用一锅水热法合成Co,La-CeO2基底,引入了可控的晶格应变和丰富的氧空位(Ov),使电子传导能力和结构稳定性显著提高。NiFe-LDH与Co, La-CeO2之间形成的强界面耦合,提高了电荷传递效率和优化了含氧中间体的吸附行为。这种NiFe-LDH/Co, La-CeO2/CF复合电极在1.0 mol/L KOH溶液中的OER催化性能优异,电流密度为50 mA·cm-2所需过电位仅为230 mV,Tafel斜率则为74.65 mV·dec-1,连续运行50 h其初始电流密度仍保持在97.14%。表明在电催化材料中晶格掺杂改性与界面的协同效应起了关键作用。

关键词: 复合材料; 异质结; 晶格应变; 氧空位; 析氧反应; 铈基催化剂

Abstract

A novel heterojunction electrocatalyst of NiFe-layered double hydroxide/Co, La doped CeO2 was prepared by electrodepositing ultrathin NiFe-LDH nanosheets onto a Co, La co-doped CeO2 (Co, La-CeO2) support on copper foam (CF) according to the "lattice-interface synergistic regulation" strategy of our group. The support of Co, La co-doped CeO2 on CF was first synthesized via a one-pot hydrothermal method, while the controllable lattice strain and abundant oxygen vacancies (Oᵥ) were introduced, which may significantly enhance the intrinsic electronic conductivity and structural stability of the support. Furthermore, the strong interfacial coupling between NiFe-LDH and Co, La co-doped CeO2 may facilitate the charge transfer and optimize the adsorption of oxygen-containing intermediates. As a result, the NiFe-LDH/Co, La co-doped CeO2/CF composite exhibits superior performance of oxygen evolution reaction in 1.0 mol/L KOH, requiring an overpotential of only 230 mV to reach 50 mA·cm-2 with a Tafel slope of 74.65 mV·dec-1. It also retains 97.14% of its initial current density after 50 h of continuous operation. This study highlights the importance of lattice doping and interfacial synergy, providing a generalizable design framework for high-efficiency non-noble metal electrocatalysts for oxygen evolution reaction.

Keywords: composite; heterojunction; lattice strain; oxygen vacancies; oxygen evolution reaction; cerium-based catalysts

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本文引用格式

王俊, 刘雯, 籍伟花, 苗洋, 王一杰, 宗振昊, 马浩铭, 周玲玲. NiFe-LDH/Co, La-CeO2 催化剂的制备及其性能[J]. 材料研究学报, 2026, 40(7): 507-518 DOI:10.11901/1005.3093.2026.110

WANG Jun, LIU Wen, JI Weihua, MIAO Yang, WANG Yijie, ZONG Zhenhao, MA Haoming, ZHOU Lingling. Effect of Lattice-interface Synergy of NiFe-layered Double Hydroxide /Co, La-doped CeO2 on Efficient Oxygen Evolution Reaction[J]. Chinese Journal of Materials Research, 2026, 40(7): 507-518 DOI:10.11901/1005.3093.2026.110

化石燃料的过度消耗已引起变暖和能源危机[1]。电催化全解水制氢是规模化生产清洁能源的重要途径,但是析氧反应(OER)的多步质子-电子耦合过程繁琐,使反应缓慢且所需过电位较高[2,3]。因此,急需开发高效、稳定的(OER)催化剂[4,5]。当前Ir、Ru及其氧化物(IrO2、RuO2)等贵金属催化剂的OER活性优异,但是其储量过低成本高昂[6~8]。因此,成本低廉的过渡金属(如Ni、Co、Fe)氧化物和氢氧化物逐渐受到关注[9~15]。镍铁层状双氢氧化物(NiFe-LDHs)金属离子间产生协同效应,是碱性条件下较有潜力的OER催化剂[16,17]。Liu等用镍(II)-次氮基三乙酸阴离子作为导向剂制备的非晶态NiFe-LDH,电流密度为10 mA/cm2时过电位仅为241 mV[18]。但是,NiFe-LDH固有的层间堆叠效应和较低的本征导电性使其活性位点暴露和电子传输效率降低,在大电流密度下的催化活性和稳定性难以满足使用要求[19,20]。为了解决上述问题,可构建NiFe-LDH与功能材料复合的异质结构[21~23]。在载体材料中,化学稳定性和储放氧性能优异的稀土氧化物CeO2应用潜力较大[24~26]。Ding等的研究表明,CeO2纳米颗粒能诱导强烈的界面电子相互作用,用其构建的CoP/CeO2异质结具有优于单组分CoP和贵金属RuO2的较低过电位(257 mV)[27]。虽然Ce3+/Ce4+氧化还原有利于电子转移[28~31],但是CeO2本征电导率较低且易形成大颗粒,使其电催化性能降低[32]。在材料中掺杂适量元素引入晶格应变和产生氧空位(Ov),可调控中间体的吸附能和提高其导电性[33~36]。Swathi等发现,掺杂2%的Gd使CeO2电极的电导率提高[37]。但是,掺杂元素的半径与本体差异过大引起的电荷补偿产生晶格畸变和缺陷结构,使材料的结构稳定性和耐久性降低[38~40]。针对上述问题,本文用过渡金属Co和稀土元素La作为共掺杂剂,提出一种“晶格界面”协同调控策略,用一锅水热法合成Co, La-CeO2基底,再在其表面制备NiFe-LDH/Co, La-CeO2/CF异质结电催化材料并研究其性能。

1 实验方法

1.1 实验用化学试剂

实验用金属前驱体包括:六水合硝酸铈(III) (Ce(NO3)3·6H2O)、四水合乙酸钴(II) (Co(CH3COO)2·4H2O)、六水合硝酸镧(III) (La(NO3)3·6H2O)、六水合氯化镍(II) (NiCl2·6H2O)以及六水合氯化铁(III) (FeCl3·6H2O)。六亚甲基四胺(HMTA)用作形貌调节剂,乙二醇(EG)和乙醇(EtOH)为溶剂,氢氧化钾(KOH)为电解液。泡沫铜(Copper Foam, CF)需在使用前进行表面预处理,其余化学试剂均为分析纯(AR级)。实验用水为去离子水。

1.2 催化剂的合成

1.2.1 Co, La-CeO2/CF的合成

先对泡沫铜(CF)基底进行预处理:将其依次在10%HCl、无水乙醇和去离子水中各超声清洗5 min以去除表面氧化层,随后在60 ℃真空烘箱中充分干燥。将Ce(NO3)3·6H2O (1.5 mmol)、(CH3COO)2Co·4H2O (0.5 mmol)、La(NO3)3·6H2O (0.5 mmol)以及HMTA (0.125 g)溶解于15 mL乙二醇中,将其磁力搅拌直至形成均匀透明混合溶液(即反应液)。将混合溶液转移至聚四氟乙烯内衬的不锈钢高压釜中,并将CF垂直浸入反应液中。将反应釜密封后置于170 ℃烘箱中反应8 h。烘箱自然冷却至室温后,取出样品用无水乙醇反复洗涤后在60 ℃下干燥,得到前驱体。将前驱体置于马弗炉中,在空气氛围下以2 ℃·min-1的升温速率加热至 400 ℃并保温2 h,自然冷却后得到Co, La-CeO2/CF。作为对照,单金属或双金属掺杂样品除改变金属盐配比外,其余制备步骤保持一致。

1.2.2 NiFe-LDH/Co, La-CeO2/CF的合成

在CHI760e电化学工作站采用标准三电极体系进行电沉积。制备出的Co, La-CeO2/CF为工作电极,石墨棒和Ag/AgCl (饱和KCl)分别作为对电极和参比电极。电解液是150 mL含有30 mmol/L NiCl2·6H2O和10 mmol/L FeCl3·6H2O的混合水溶液。沉积时的恒定电位为-1.4 V (vs. Ag/AgCl),沉积时间为600 s。沉积结束后,将所得电极用去离子水反复冲洗以去除表面残留的电解质,并在室温下自然干燥过夜,将得到的样品记为NiFe-LDH/Co, La-CeO2/CF。为了进行对比研究,在相同实验条件下直接将NiFe-LDH沉积于预处理后的泡沫铜上,制得对照样品记为NiFe-LDH/CF。

1.3 样品的表征

用扫描电子显微镜(SEM)和透射电子显微镜(TEM)表征催化剂的表面形貌和微观结构,测试能量色散X射线光谱(EDS)以分析其化学成分。测定粉末样品的XRD谱(Cu Kα辐射源)以表征其晶体结构。使用Horiba LabRAM HR光谱仪测定Raman光谱,激发波长为632.8 nm。用配有单色Al Kα辐射源的X射线光电子谱仪测定XPS谱以分析样品的表面化学状态。

用标准三电极体系测试OER性能,电解液为1.0 mol/L KOH溶液。负载催化剂的泡沫铜为工作电极,石墨棒为对电极,Hg/HgO为参比电极。所有测量电位均依据公式ERHE = EHg/HgO + 0.059 × pH + 0.098转换为可逆氢电极(RHE)标度。在测试线性扫描伏安(LSV)曲线前,先以50 mV·s-1的扫速进行多圈循环伏安(CV)扫描直到曲线稳定。在5 mV·s-1扫速下采集LSV曲线,并进行90%的iR补偿。按η = ERHE-1.23 V计算过电位(η)。测试电化学阻抗谱(EIS)的频率范围为105~10-2 Hz。根据在非法拉第区测试CV曲线计算双电层电容(Cdl)。由方程η = a + b·lg|j|拟合得到Tafel斜率。

使用Vienna Ab initio模拟软件包完成所有自旋极化密度泛函理论计算。用投影级加波方法描述离子实与价电子间的相互作用,在广义梯度近似下用 Perdew-Burke-Ernzerhof泛函处理交换关联能则。平面波截断能设定为450 eV。能量与力的收敛标准分别设为10-5 eV和0.2 eV/nm。

2 结果和讨论

2.1 NiFe-LDH/Co, La-CeO2/CF的合成

通过水热反应和在空气中煅烧在泡沫铜(CF)表面原位生长Co, La共掺杂的CeO2纳米结构,制备出高比表面积的载体。再以此为骨架,进行恒电位电沉积将超薄NiFe-LDH纳米片均匀锚定在Co, La-CeO2表面。图1给出了制备流程示意图。

图1

图1   NiFe-LDH/Co, La-CeO2/CF制备过程的示意图

Fig.1   Schematic illustration of the preparation process for the NiFe-LDH/Co, La-CeO2/CF composite


图2可见,Co掺杂使CeO2的表面形貌显著改变,形成了更为多孔的枝状结构。这种结构,增大了活性表面积。

图2

图2   CeO2/CF和Co-CeO2/CF的SEM照片

Fig.2   SEM images of the synthesized CeO2/CF (a) and Co-CeO2/CF (b)


图3a~c可见,Co, La-CeO2/CF演化成致密纳米片组装成的球状纳米花形貌。这种独特的三维分级结构不仅大幅度增大了比表面积,还提高了骨架的结构稳定性。NiFe-LDH的引入使电沉积后的NiFe-LDH/Co, La-CeO2/CF保留了花状骨架,表面还紧密包裹了一层相互交错的纳米片网络(图d~f),不但增大了纳米片的厚度还形成了更多的网状孔隙结构。这种结构提高了活性位点的暴露密度,还提高了孔隙率,使电解液更好地浸润和使反应气泡(O2)高速脱附,从而使OER动力学性能大幅度提高。从图3g中的SEM-EDS能谱和元素面分布可见,Ni、Fe、Co、La、Ce、O等元素在材料表面均匀分布,表明已经成功构建出NiFe-LDH与Co, La-CeO2异质结。

图3

图3   Co, La-CeO2/CF和NiFe-LDH/Co, La-CeO2/CF的SEM照片、NiFe-LDH/Co, La-CeO2/CF的EDS元素分布以及NiFe-LDH/Co, La-CeO2的XRD谱

Fig.3   SEM images of as-synthesized Co, La-CeO2/CF (a-c), NiFe-LDH/ Co, La-CeO2/CF (d-f), EDS elemental mapping of NiFe-LDH/Co, La-CeO2/CF (g), XRD patterns of NiFe-LDH/Co, La-CeO2 (h)


图3h给出了NiFe-LDH/Co,La-CeO2样品的XRD谱。可以看出,谱中CeO2的所有衍射峰均与立方萤石结构的标准卡片(PDF#34-0394)匹配,表明样品具有高纯度和高结晶性。引入Co和La使CeO2的特征主峰向高角度轻微偏移,是离子半径不同引起的晶格收缩和晶格应变所致。在NiFe-LDH/Co, La-CeO2复合材料的谱中出现了新的特征衍射峰。位于2θ = 11.5°和23.3°等处的强峰分别对应NiFe-LDH (PDF#40-0215)的(003)和(006)晶面,位于34.6°,39.0°,46.4°,60.3°,和61.3°的衍射峰则分别对应(012)、(015)、(018)、(110)和(113)晶面[41]。这表明,已经成功地构建出LDH结构。XRD谱表明,NiFe-LDH相在掺杂后的CeO2载体表面成功生成,在复合过程载体的本征结构没有受到破坏。

用TEM分析了复合材料的微观精细结构。如图4a~c所示,与纯CeO2相比,Co, La共掺杂样品的晶面间距稍有变化,CeO2 (111)晶面的晶格条纹的0.312 nm和0.324 nm两种间距,表明发生了晶格畸变。从图4d, e可见,NiFe-LDH/Co, La-CeO2继承了前驱体的三维骨架形貌,LDH纳米片均匀地覆在载体表面且未出现显著的团聚。从高分辨透射电镜(HRTEM)图像(图4f)清晰可见两种不同的晶格条纹:间距为0.315 nm的条纹对应CeO2的(111)晶面,间距为0.260 nm的条纹则对应NiFe-LDH的(012)晶面。这一结果,与XRD谱给出的结果一致。同时,HRTEM表明,CeO2与NiFe-LDH之间紧密结合的异质界面,证明已经成功地构建出n-n型异质结。

图4

图4   Co, La-CeO2和NiFe-LDH/Co, La-CeO2的TEM和HRTEM图像

Fig.4   TEM (a, b, d, e) and HRTEM (c, f) images of Co, La-CeO2 (a-c) and NiFe-LDH/Co, La-CeO2 (d-f)


测试Raman光谱研究了样品的局部结构演变和缺陷密度。如图5所示,在CeO2的谱中463 cm-1处明显的特征峰,对应萤石立方结构的F2g对称振动模式。掺杂Co和La使该特征峰轻微展宽和红移,表明异质原子的掺入引起晶格畸变并使结构对称性降低。在580 cm-1处出现了一个新的宽峰(标记为Ov),该峰也出现在Co, La-CeO2和NiFe-LDH/Co, La-CeO2样品的谱中。此峰是缺陷诱导模式(D峰)所致,与电荷补偿所需的外源性氧空位相关。与Co, La-CeO2相比,NiFe-LDH/Co, La-CeO2复合材料中缺陷峰的强度I(Ov)显著高于F2g峰的强度I(F2g)。这进一步表明,NiFe-LDH纳米片与掺杂氧化物载体之间的强界面相互作用促进了氧空位的形成。这些丰富的氧空位有望优化材料的电子结构,并作为吸附反应物的活性位点使材料的电催化性能提高。

图5

图5   NiFe-LDH/Co, La-CeO2、Co, La-CeO2和CeO2的Raman光谱

Fig.5   Raman spectra of NiFe-LDH/Co, La-CeO2, Co,La-CeO2 and CeO2


根据X射线光电子能谱分析了NiFe-LDH/Co,La-CeO2的表面化学状态。如图6a所示,XPS全谱证实样品中只有Ce、Ni、Fe、Co、La、O及C元素。高分辨C 1s谱(图6b)表明其主峰位于284.8 eV,以此为基准的校正排除了荷电效应的干扰。

图6

图6   NiFe-LDH-Co, La-CeO2/CF的XPS全谱和NiFe-LDH/Co, La-CeO2/CF中C 1s的XPS谱

Fig.6   XPS survey spectrum of NiFe-LDH-Co, La-CeO2/CF (a) and XPS spectrum of C 1s in NiFe-LDH/Co, La-CeO2/CF (b)


图7a给出了Ce 3d 的XPS谱,在谱中885.5 eV、898.8 eV和908.0 eV附近CeO2、Co, La-CeO2和NiFe-LDH/Co, La-CeO2的特征峰,可归属为Ce3+和Ce4+物种[42]。Co, La-CeO2样品Ce3+峰的强度显著高于CeO2样品Ce3+峰的强度,表明Co和La掺杂促进了CeO2的部分还原,产生了更多的Ce3+和氧空位[43]。在NiFe-LDH/Co, La-CeO2复合材料的谱中,除了Ce的主要信号在879.8 eV和873.6 eV处还出现两个额外峰。这表明,NiFe-LDH与CeO2之间的强界面相互作用提高了Ce3+的含量[44]。Ce价态的变化为氧空位和表面活性氧的生成提供更多的反应位点,从而加速了反应动力学过程。与纯CeO2相比,共掺杂Co和La使Co, La-CeO2中Ov和Oads的相对面积显著增加(图7b)。与NiFe-LDH构建异质界面使这些组分进一步增强,表明异价掺杂(La3+、Co2+/3+)的电荷补偿与界面电子再分布协同促进了更多氧空位和表面活性氧物种的生成。同时,NiFe-LDH/Co, La-CeO2的O 1s结合能发生负移,是Ce-O-Ni结构引起的晶格氧电子态的变化所致[45]。这种调控优化了OH-/O-/OOH-在Ni位点的吸附能,通过晶格氧机制促进表面氧循环并可逆调节Ce4+/Ce3+-Ov,从而从机理上提高了复合材料的OER活性且不降低其结构稳定性。

图7

图7   NiFe-LDH/Co, La-CeO2/CF、Co, La-CeO2/CF和CeO2/CF的XPS谱

Fig.7   XPS of NiFe-LDH/Co, La-CeO2/CF, Co, La-CeO2/CF and CeO2/CF (a) Ce 3d, (b) O 1s, (c) Co 2p, (d) La 3d, (e) Ni 2p, (f) Fe 2p


图7c给出的Co 2p谱中,778.75 eV和793.43 eV处的峰对应Co3+,而781.15 eV和796.58 eV处的峰则对应Co2+ [46]。与Co, La-CeO2相比,NiFe-LDH/Co,La-CeO2中Co2+/Co3+的比例提高,表明NiFe-LDH的引入调控了Co的电子密度并促进了电子转移。在La 3d的谱中(图7d) 834 eV和851 eV处出现特征峰[47],表明La3+成功掺入Co, La-CeO2晶格。在NiFe-LDH/Co, La-CeO2的谱中该峰展宽并轻微偏移,表明组分间的相互作用改变了La的电子环境。这种电子调控,可能有助于提高材料的电催化性能。在Ni 2p区(图7e),873.6 eV与855.9 eV处的特征峰分别对应Ni2+的Ni 2p1/2和Ni 2p3/2[48],而在880.1 eV和861.8 eV处还出现了典型的伴峰。在Fe 2p谱(图7f)中的725.9 eV、718.2 eV与712.9 eV处出现的峰分别对应Fe 2p1/2、伴峰及Fe 2p3/2,表明复合材料中的Fe主要以Fe3+形式存在[49]

2.2 电化学性能

使用标准三电极体系在1.0 mol·L-1 KOH电解液中测试了NiFe-LDH/Co, La-CeO2/CF电极的电化学性能。为了消除初始表面状态差异的影响,先对电极进行50圈CV扫描使其活化。同时,为了比较,在相同的条件下,测试了CeO2/CF、Co, La-CeO2/CF、NiFe-LDH/CF及RuO2/CF等样品。如图8a所示,LSV曲线表明,NiFe-LDH/Co, La-CeO2/CF的OER催化活性最优,电流密度达到50 mA·cm-2只需230 mV的过电位。此过电位明显比NiFe-LDH/CF (320 mV)、Co, La-CeO2/CF (484 mV)、CeO2/CF (496 mV)及RuO2/CF (438 mV)的低,表明NiFe-LDH与Co, La-CeO2的复合异质结构较强的协同效应提高了材料的催化性能。同时,NiFe-LDH/Co, La-CeO2/CF的活性优于多种已报道的NiFe-LDH基异质结构和Ce基催化剂(表1),表明这种结构设计提高OER性能的可行性。

图8

图8   催化剂的OER电化学测试

Fig.8   OER LSV curves of samples (a), overpotential histograms of various samples at 10 mA·cm-2, 50 mA·cm-2, and 100 mA·cm-2 current densities (b), associated tafel plots (c), Cdl plots for the relevant (d), EIS Nyquist plots (e) and stability test of NiFe-LDH/Co, La-CeO2/CF at 100 mA·cm-2 (f)


表1   NiFe-LDH/Co, La-CeO2/CF与已报道电催化剂的OER性能对比

Table 1  Comparison of OER performance of NiFe-LDH/Co, La-CeO2/CF with reported electrocatalysts

Catalystj / mA·cm-2η / mVElectrolystsReference
NiFe-LDH/Co, La-CeO2/CF502301 mol/L KOHThis work
10172
NiFe-LDH/Co/C@NF502441 mol/L KOH[52]
10
NiFeLDH/NiCoP@NC/NF501 mol/L KOH[53]
10210
Ce-CoP@CC501 mol/L KOH[54]
10240
NiFeCe-LDH@CP502551 mol/L KOH[55]
10232
Co3O4@NiFe-LDH/NF502701 mol/L KOH[56]
10
CoP/CeO2-20502981 mol/L KOH[27]
10257
2%Gd-CeO2501 mol/L KOH[37]
10369
Ce(OH)3@NiFe LDH501 mol/L KOH[57]
10220
Ce-Ni3S2/MnS/NF502451 mol/L KOH[58]
10194
S-FeCoNiO x501 mol/L KOH[59]
10221

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图8b比较了电流密度为10、50和100 mA·cm-2时的过电位。可以看出,电流密度为10和100 mA·cm-2时NiFe-LDH/Co, La-CeO2/CF只需172 mV和273 mV的过电位,优于NiFe-LDH/CF (208, 350 mV)、Co, La-CeO2/CF (379, 538 mV)、CeO2/CF (388, 565 mV)和RuO2/CF (330, 500 mV),表明其在碱性条件下优异的催化效率。根据LSV曲线的Tafel斜率评估了反应动力学。图8c给出了不同样品Tafel斜率的对比。可以看出,NiFe-LDH/Co, La-CeO2/CF电极的Tafel斜率(74.65 mV·dec-1)最低,而CeO2/CF (158.77 mV·dec-1)、Co, La-CeO2/CF (97.99 mV·dec-1)、NiFe-LDH/CF (87.12 mV·dec-1)和RuO2/CF (110.2 mV·dec-1)的Tafel斜率都比较高,表明其析氧反应动力学显著优于其他电极,证实了NiFe-LDH/Co, La-CeO2/CF在电催化过程中的电子传输速率较高和能量势垒较低。电化学活性表面积(ECSA)是评价催化性能的重要参数,可根据双电层电容(Cdl)确定[50]Cdl与ECSA值成正比,对不同扫描速率的循环伏安曲线进行线性拟合,比较了各样品的Cdl值。如图8d所示,NiFe-LDH/Co, La-CeO2/CF的Cdl值为36 mF·cm-2,显著比NiFe-LDH/CF (14.79 mF·cm-2)、Co, La-CeO2/CF (8.23 mF·cm-2)和CeO2/CF (3.18 mF·cm-2)的高。测试结果表明,富含缺陷的Co, La-CeO2结构与NiFe-LDH形成的异质结构能暴露出更多的活性位点,从而使材料的催化活性显著提高。在电化学反应中,材料的导电性起着至关重要的作用。因此,使用电化学阻抗谱评估了样品的电荷转移动力学,其中催化剂的电荷转移电阻对应Nyquist图中半圆的直径[51]。如图8e中的等效电路Nyquist图所示,与NiFe-LDH/CF、Co, La-CeO2/CF和CeO2/CF相比,NiFe-LDH/Co, La-CeO2/CF的半圆直径小得多,表明其电荷转移电阻较低,即电子转移更快。表2列出了所有样品EIS数据的总结,其中NiFe-LDH/Co, La-CeO2/CF的Rct为0.47 Ω·cm2,显著比NiFe-LDH/CF (1.94 Ω·cm2)、Co, La-CeO2/CF (3.27 Ω·cm2)和CeO2/CF (7.57 Ω·cm2)的低。这些结果表明,NiFe-LDH/Co, La-CeO2/CF复合催化剂在OER过程中显著加速了电荷转移过程,从而使其催化效率提高。

表2   OER测试中各催化剂等效电路的电阻值

Table 2  Resistance values of the equivalent circuits for various catalysts during OER tests

SampleRs / Ω·cm2Rct / Ω·cm2
NiFe-LDH/Co, La-CeO2/CF1.20.47
Co, La-CeO2/CF1.183.27
NiFe-LDH/CF1.281.94
CeO2/CF1.377.57

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除了催化活性,耐久性也是评估催化剂性能的重要参数。在1 mol/L KOH溶液中,在电流密度为100 mA·cm-2的条件下测试了材料的稳定性(图8f)。同时,为了验证材料的本征稳定性,对比了测试前后的LSV曲线(图 8f中的插图)。持续电解50 h,NiFe-LDH/Co, La-CeO2/CF的电流密度只下降2.86%,LSV曲线几乎与原始曲线重合,表明其稳定性极高。与传统催化剂相比,这种催化剂在高电流密度下的耐久性优异,表明其在长周期电解过程中的结构稳定性较高。

3 密度泛函数(DFT)的计算

计算密度泛函数(DFT),深入研究了Co, La-CeO2调控NiFe-LDH的电子结构并提高其OER活性。Gibbs自由能分布(图9a)表明,纯NiFe-LDH的速控决定步骤是OH氧化为O的过程,能垒为2.01 eV。而NiFe-LDH/Co, La-CeO2体系的速控决定步骤转变为O到OOH的生成,其能垒大幅度降低到1.57 eV,反应活化性能明显提高。需说明的是,DFT计算的决速步能垒(1.57 eV)与实验过电位(230 mV)存在数值偏差。其原因是,计算模型简化了实际反应中复杂的溶剂化效应和电双层结构的影响,实验采用的三维泡沫铜基底较大的活性面积也使宏观表观过电位降低。虽然数值不同,但是DFT计算出掺杂Co, La使能垒显著降低,与实验中观测到的活性提高规律一致。计算催化剂中Fe和Ni位点的态密度,研究了Co, La-CeO2对电子性质的影响(图9b~d)。与NiFe-LDH中Ni位点的d带中心相比,复合结构中Ni位点的d带中心为-1.93 eV,而纯NiFe-LDH中的为-2.04 eV。显然,前者中Ni位点的d带中心更接近费米能级。Fe位点的d带中心也轻微上移,从NiFe-LDH中的-1.42 eV变为复合结构中的-1.38 eV。根据d带中心理论,Fe和Ni位点d带中心的上移提高了氧相关中间体的吸附强度,从而促进了OER过程。同时,Ni位点d带中心更显著的上移,表明Ni与载体氧化物之间存在较强的电子相互作用,Ni位点可能作为主要的活性中心对提高NiFe-LDH/Co, La-CeO2的OER活性起着关键作用。

图9

图9   NiFe-LDH与NiFe-LDH/Co, La-CeO2上OER四步反应的Gibbs自由能、NiFe-LDH与NiFe-LDH/Co, La-CeO2的总态密度和态密度

Fig.9   Gibbs free energy diagram for the four steps of OER (a) on NiFe-LDH and NiFe-LDH/Co, La-CeO2, total density of states (b) and density of states (DOS) (c, d) for NiFe-LDH and NiFe-LDH/Co, La-CeO2


4 结论

将水热法与电沉积工艺结合,可构建具有纳米花结构的NiFe-LDH/Co, La-CeO2复合催化剂。这种催化剂的OER活性和耐久性优异,其原因是:(1) 多孔纳米花结构为反应提供了丰富的活性位点,增大了活性表面积;(2) Co、La共掺杂引发的晶格畸变产生了更多的氧空位,促进了水的解离和提高了反应中间体的结合能力;(3) NiFe-LDH与CeO2载体之间紧密的界面耦合,提高了电荷传输效率并促进了产物中气体的脱附。

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As an alternative for depleting fossil fuel energy, hydrogen economy desires low-cost and efficient hydrogen production from water splitting. In order to explore a cheap, abundant, active, and durable catalyst for the electrocatalytic hydrogen evolution reaction (HER), two-dimensional (2D) ceria nanosheets are produced through a thermal decomposition exfoliation method from CeCOOH with a layer-stacked structure. The additional cobalt dopant promotes formation of oxygen vacancies in ceria nanosheets and, in turn, optimizes hydrogen binding/water dissociation and increases the active sites. As a result, the 2D Co-doped CeO nanosheets exhibit an excellent catalytic performance in alkaline HER such that the overpotential is as low as 132 and 215 mV to deliver a high current density of 100 and 500 mA cm, respectively, outperforming Pt. Such 2D Co-doped CeO nanosheets are also durable HER electrocatalysts, as the activity loss during an extended period of operation is nearly negligible.

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Highly active and low-cost electrocatalysts for water oxidation are required due to the demands on sustainable solar fuels; however, developing highly efficient catalysts to meet industrial requirements remains a challenge. Herein, we report a monolayer of nickel-vanadium-layered double hydroxide that shows a current density of 27 mA cm(-2) (57 mA cm(-2) after ohmic-drop correction) at an overpotential of 350 mV for water oxidation. Such performance is comparable to those of the best-performing nickel-iron-layered double hydroxides for water oxidation in alkaline media. Mechanistic studies indicate that the nickel-vanadium-layered double hydroxides can provide high intrinsic catalytic activity, mainly due to enhanced conductivity, facile electron transfer and abundant active sites. This work may expand the scope of cost-effective electrocatalysts for water splitting.

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